List of Abbreviations and Nomenclature

2.1 Introduction

Petroleum covers approximately 40% of the worldwide energy requirements (Al-Des et al., 2016). The worldwide depletion of high quality low sulfur content crude oil reserves comes in parallel with the more stringent environmental regulations in every country around the world towards the reduction of all forms of greenhouse gas emissions, including the emissions of sulfur oxides (SO_x). Thus, refineries are facing many challenges, including the influx of heavy crude oils, increased fuel quality standards in terms of a severely diminished and regulated sulfur content to approximately 10 ppm for gasoline and diesel fuels (Bandosz et al., 2006; Khamis and Palichev, 2012; Al-Degs et al., 2016), and the need to reduce all forms of emissions to meet air pollution regulations. Babich and Moulijn (2003) reported that approximately $10–15 billion and up to $16 billion are estimated to be invested by European, US, and Canadian refineries, respectively, to meet the new restricted environmental clean-fuel legislation.

Ancheyta (2011) has reported that the range of sulfur-content in crude oils varies according to its classification: extra-light crude oil (0.02–0.2 wt.%), light crude oil (0.05–4 wt.%), heavy crude oil (0.1–5 wt.%), and, finally, extra–heavy crude oil (0.8–6 wt.%).

Based on the SARA compositions of crude oils (Figure 2.1), light crude oil is rich in light distillates, while heavy crude oil is rich in high molecular weight distillates and more polar residuum, where, the asphaltenes content with its high concentrations of polyromantic heterocyclic compounds and heavy metals increases with the decrease of the crude oil quality. According to Ancheyta (2011), the asphaltene content varies as follows: extra-light crude oil (0–<2 wt.%), light crude oil (<0.1–12 wt.%), heavy crude oil (11–25 wt.%), and, finally, extra–heavy crude oil (15–40 wt.%). Moreover, the sulfur content in crude oils, increases with the increment of asphaltene content.

The higher the sulfur content in crude oil, the higher the sulfur content in its distillates. Generally, the sulfur concentration and types of sulfur compounds vary over the boiling range of the petroleum distillates (Table 2.1). Aitani et al. (2000) reported that sulfur is present mainly as thiophenic compounds (i.e. thiophene Th, benzothiopene BT, dibenzothiophene DBT, etc.) and their derivatives. They are mainly concentrated on resins fractions (approximately 70%) and the rest is found in the other fractions, but to higher extent in the asphaltenes. The main liquid fuels coming out from petroleum are gasoline range (naphtha and fluid catalytic cracking (FCC) – naphtha), jet fuel range (heavy cut naphtha (HCN) and middle distillate), diesel fuel range (middle distillate and light cycle oil (LCO)), and boiler fuel feeds (heavy oils and distillation resides) (Song, 2003). Mustafa et al. (2010) reported that alkyl sulfur compounds are concentrated in gasoline, while the high molecular weight aromatic organosulfur compounds are concentrated in diesel oil. Figure 2.2 illustrates some of the predominant sulfur compounds in gasoline and diesel oil. It has been reported that DBT and its derivatives constitute up to 50% of the sulfur content in diesel oil (Lee et al., 2002). The recalcitrance of the sulfur compounds increases with the increase of the boiling points of the distillates. Thus, the desulfurization of thiols is much easier than the desulfurization of polyaromatic sulfur heterocyclic (PASH) compounds.

There are various reported desulfurization methods to remove sulfur from petroleum and its fractions (Figure 2.3). The most developed and commercialized technologies are those which catalytically convert organosulfur compounds with sulfur elimination. Such catalytic conversion technologies include conventional hydro-treating, hydro-treating with advanced catalysts and/or reactor design, and a combination of hydrotreating with some additional chemical processes to maintain fuel specifications. The main feature of the technologies of the second type is the application of physio-chemical processes different in nature from catalytic hydrodesulfurization (HDS) to separate and/or to transform organosulfur compounds from refinery streams. Such technologies are included as a key step in distillation, alkylation, oxidation, extraction, adsorption, or combination of these processes. Among these desulfurization techniques, HDS is currently considered as the most important one. However, HDS has several disadvantages in that it is energy intensive, costly to install and to operate, and does not work well on recalcitrant high molecular weight PASH compounds. Recent research has therefore focused on improving HDS catalysts and processes and also on the development of alternative technologies. Among these technologies are selective or reactive adsorptive desulfurization (ADS), oxidative desulfurization (ODS) combined with extractive desulfurization (EDS) or ADS, and biodesulfurization (BDS), which are advantageous because they can be operated in conditions that require less energy and hydrogen. They can also operate at ambient temperature and pressure with high selectivity, resulting in decreased energy costs, low emission, and no generation of undesirable side products. Moreover, the incorporation of non-HDS with HDS, will reduce the H₂ consumption required for the deep-HDS process (Rang et al., 2006; Srivastava, 2012; Mužic and Sertić -Bionda, 2013).

Distillation range °C	Distillate	S-content & recalcitrance towards desulfurization	S-compounds distribution
IBP-71	Light straight-run naphtha		Thiols, sulfides & traces of thiophenes
71–177	Medium straight-run naphtha
177–204	Heavy straight-run naphtha
204–274	Jet fuel
274–316	Kerosene		Mainly; thiols, sulfuides & thiophenes. Low concentration of benzothiophenes, dibenzothiophenes & heavy sulfides.
316–343	Straight-run gas oil		Mainly; thiophenes, benzothiophenes, dibenzothiophenes & heavy sulfides. Low concentration of thiols & sulfides.
343–454	Light vacuum gas oil		Traces of thiols & sulfides. Lower concentration of thiophenes. Higher concentration of benzothiophenes, dibenzothiophenes & heavy sulfides.
454–538	Heavy vacuum gas oil
>538	Vacuum residue		Mainly; benzothiophenes, dibenzothiophenes & heavy sulfides. Traces of thiols, sulfides & thiophenes.

2.2 Hydrodesulfurization

The conventional desulfurization process which is widely practiced for crude oil and its distillates (gasoline, kerosene and diesel oil) is hydrodesulfurization (HDS) (Heidarinasab et al., 2016; Mohammed et al., 2016). The terms hydrotreating, hydroprocessing, hydrocracking (HCR), and hydrodesulfurization (HDS) are industrially used referring to HDS and HCR. That is because the desulfurization and cracking processes occur simultaneously and are relative as to which is predominated. The main purpose of the HCR process is to reduce the boiling range of a feed to products with lower boiling ranges, while hydrotreating is a process to catalytically stabilize petroleum products by converting olefins to paraffins and/or removing contaminants such as nitrogen, oxygen, sulfur, halides, and trace metals by making them react with hydrogen (El-Gendy and Speight, 2016).

HDS is a technology which converts organic sulfur compounds (such as thiols, sulfides, and thiophenes) to hydrogen sulfuide (H₂S) and other inorganic sulfuides, under high temperature (200 to 455 °C) and high pressure (150 to 3000 psi) and uses hydrogen gas in the presence of sulfidized metal catalysts (e.g. CoMo/Al₂O₃ or NiMo/Al₂O₃₎ (Lecrenary et al., 1997; Rana et al., 2007; Eswaramoorthi et al., 2008; Al-Degs et al., 2016). The produced H₂S is then catalytically air oxidized to elemental sulfur in Claus plants (El-Gendy and Speight, 2016).

Bose (2015) reported that by 2005 approximately 64 million metric tons of sulfur were produced worldwide as byproducts form petroleum refineries and hydrocarbon processing plants. Figure 2.4 illustrates a simple HDS process. Sulfur is used for manufacturing sulfuric acid, medicine, cosmetics, fertilizers, and rubber products. Elemental sulfur is used as fertilizer and pesticide.

The HDS conditions depend on the required desulfurization degree and the type of sulfur compounds in the feed to be treated. The aliphatic S-compounds are known to be very reactive and can be completely removed during the conventional HDS process.

However, S contained in thiophenes is more difficult to be removed since the lone pair electrons from S-atoms participates in the π-electron structure of the conjugated C=C system. Thus, resonance stabilization is around 120–130 kJ/mol, which is less than that of the benzene ring, which is 160–170 kJ/mol, but it is still sufficient to make the HDS energetically demanding (Gronowitz, 1985). The HDS of thiophenic compounds occurs through two pathways: the hydrogenation pathway (hydrogenation followed by hydrogenolysis) and the direct hydrogenolysis pathway (direct elimination of S atom via C–S bond cleavage) and they occur at different active sites. The least hydrogen utilization pathway is direct hydrogenolysis, but the resonance stabilization of sulfur in the thiophene ring makes this process difficult and obligates the HDS-pathway towards the hydrogenation-pathway, where aromatic ring saturation occurs before the occurrence of desulfurization. However, the PAHs are known to be the main inhibitors towards the hydrogenation pathway. Moreover, the equilibrium concentration of the hydrogenated product is low since there is a significant driving force for aromatization by dehydrogenation. Furthermore, the resonance stabilization of thiophene prevents cracking, which leads to the formation of coke during the FCC [Phillipson, 1971; Hatanaka et al., 1998; Reddy et al., 1998; IFP, 2000; Kaufmann et al., 2000; Miller et al., 2000; Nocca et al., 2000; Corma et al., 2001; ExxonMobi, 2001; Halbert et al., 2001; Okamoto et al., 2002; Song and Ma, 2003; Hatanaka, 2005; Shu and Wormsbecher, 2007; Javadli and de Klerk, 2012a).

Nickel–molybdenum (NiMo) catalysts remove contaminants by hydrogenation, while cobalt–molybdenum (CoMo) predominantly removes contaminants by direct hydrogenolysis (Ferreira et al., 2013). Thus, CoMo-catalysts are preferred for the HDS of unsaturated hydrocarbon streams such as fluid catalytic cracking (FCC). It is also more efficient in batch reactors (Lecrenay et al., 1997), but NiMo-catalysts are preferred for the HDS of recalcitrant PASHs, such as 4,6-dimethydibenzothiophene (4,6-DMDBT), thus it is applied more often in the HDS of fractions that require extreme conditions (Bataille et al., 2000; Kim et al., 2003). However, Wang et al. (2004) has reported that the relative significance of hydrogenolysis and hydrogenation routes depend mainly on Co/Mo ratios. Furthermore, the two routes could also be substantially affected by the presence of naphthalene or H₂S for the HDS of DBT or 4,6-DMDBT (Farag et al., 1999a,b). Moreover, Ali and Siddiqui (1997) have reported that the catalysts’ type has a critical influence on the composition and properties of the hydrotreated product. Furthermore, the interactions between the reactants and catalysts and effects of reaction conditions have a very significant effect on desulfurization efficiency (Al-Zeghayer and Jibril, 2006). One of the reasons that the large size of alkyl groups is difficult to desulfurize is that C-S bonds in the aromatic rings are more stable than the others (Tang and Shi, 2011).

Oil refiners depend on such a costly, extreme chemical process. Aitani et al. (2000) have reported that there are more than 35 HDS units all over the world, with a total installed capacity of 1.5 million bbl/day. Moreover, Gupta and Roychoudhury (2005) reported worldwide petroleum refineries use the conventional HDS technology to desulfurize approximately 20 million bbl crude oil per day. Although HDS can easily remove inorganic sulfur or simple organic sulfur compounds (such as thiols, sulfides and disulfides, Figure 2.2), it is not effective for removing complicated PASHs such as DBT-derivatives and alkylated-DBT derivatives (Cx-DBTs) (Babich and Moulijn, 2003; Franchi et al., 2003). Complete HDS of such recalcitrant compounds needs higher temperature, pressure, and more H₂ consumption, which has a negative impact on the octane number of gasoline and cetane index of diesel oil. That is due to the hydrogenation of olefins and PAHs (Scheme 2.1). Knowing that, the PAHs represent approximately 20% of diesel oil composition (Park et al., 2011). As mentioned before, due to the resonance stabilization of sulfur in thiophene rings, a direct pathway is difficult to be achieved. Thus, consequently, HDS takes place through the hydrogenation (i.e. saturation) of the aromatic rings before the removal of sulfur (Scheme 2.1). However, Schulz et al. (1999) reported that, the presence of alkyl substituents on BTs and DBTs molecules might favour one of the possible HDS routes, which depends on the alkyl substituent position and, thus, to what extent the electron density is altered by the electron donating effect of alkyl groups. In addition, substituents in the vicinity of the sulfur atom cause steric hindrance and influence the HDS route (Kabe et al., 1992). The DBT and alkyl derivatives substituted adjacent to the sulfur atom are refractory to HDS using conventional catalysts. The key sulfur compounds present in diesel oil fractions after conventional HDS are 4-MDBT and 4,6-DMDBT.

The quantum chemical calculation on the conformation and electron property of various sulfur compounds and their HDS intermediates shows that the hydrogenation pathway favours desulfurization of the refractory sulfur compounds by both decreasing the steric hindrance of methyl groups and increasing the electron density on the sulfur atom in the sulfur compounds. This steric hindrance increases with the increasing size of the alkyl groups (methyl < ethyl < propyl). The steric hindrance increases, as the alkylation is nearer to the S-atom, and, consequently, the recalcitrance moves towards the HDS process (Milenkovic et al., 2000). The inhibition of the coexistent aromatics towards the HDS of the refractory sulfur compounds by competitive adsorption on the hydrogenation active sites becomes stronger in deep HDS (IFP, 2000; Westervelt, 2001; Golden et al., 2002). Moreover, the produced H₂S from the reactive sulfur compounds in the early stage of the reaction is another inhibitor for HDS of the unreactive species (Vasudevan and Fierro, 1996). Thus, it should be removed from the recycle stream by scrubbing. Haldor–Topsoe indicates that decreasing the concentration of H₂S at the inlet to a cocurrent reactor by 3–6 vol.% can decrease the average temperature needed to achieve a specific sulfur reduction by 15–20 °C, and reduce final sulfur levels by more than two-thirds (Swain, 1991). More refractory sulfur compounds would require lower space velocity for achieving deeper HDS, as it would increase the reactant-catalyst contact time. Higher temperature facilitates more of the high activation–energy reactions, thus increasing the rate of HDS. But, the increase in temperature lowers the catalyst life time, which would consequently increase the overall cost as more catalyst will be needed. It will also affect the production stream while the unit is down for the catalyst change (the current catalyst life ranges from 6 to 60 months). High pressure also enhances the rate of HDS. Improvement of vapour–liquid contact increases the surface area of the catalyst. Akzo Nobel estimates that an improved vapour–liquid distributor can reduce the temperature necessary to meet a 50 ppmw sulfur level by 10 °C, which in turn would increase catalyst life and allow an increase in cycle length from 10 to 18 months. More volume of catalyst can enhance the HDS process and can be achieved through the expansion of catalyst bed volume or denser packing. Increase in reactor volume can enhance desulfurization. UOP assigns that doubling the reactor volume would reduce sulfur from 120 to 30 ppmw (Swain, 1991).

Generally, low hydrogenation efficiency is desirable during the HDS of FFC fractions for gasoline production, while high hydrogenation efficiency is desirable for HDS of refractory light cycle oil (LCO), which is rich in aromatics for diesel oil production (Leliveld and Eijsbouts, 2008). However, the main advantage of the HDS process is that it can desulfurize different oil feeds (such as gasoline, diesel oil, and jet fuel) with high sulfur content (10,000 ppm) to less than 100 ppm in one step (Al-Degs et al., 2016).

The recalcitrance of sulfur compounds towards the HDS reaction can be ranked as follows (El-Gendy and Speight, 2016):

Thiophene < alkylated thiophene < benzothiophene < alkylated benzothiophene < dibenzothiophene and alkylated dibenzothiophene¹ < alkylated dibenzothiophene² < dibenzothiophene and alkylated dibenzothiophene³

 

¹Without substituents at the 4 and 6 positions.

²With one substituent at either the 4 or 6 position.

³With alkyl substituents at the 4 and 6 positions.

One of the reasons that large size alkyl groups are difficult to desulfurize is that C-S bonds in the aromatic rings are more stable than the others (Tang et al., 2013a). Thus, as mentioned above, the type of S-compounds affects the degree of HDS. For example, mercaptans and tetrahydrothiophene in FCC-naphtha are totally removed by using nickel and vanadium catalysts while Ths and BTs are more difficult to be removed. Thus, 4-MDBT and 4,6-DMDBT are known to be the most appropriate compounds for investigations of catalyst activity and proposed reaction mechanisms (Gates and Topsoe, 1997). These recalcitrant S-compounds should be promoted by hydrotreating and/or hydrocracking, where the unsaturated molecules are firstly saturated, then S, O, N, and metal atoms are taken away from hydrogenated oil by cracking (Myrstad, 2000). The commercial, binary-transition-metal catalysts are the most recommended and widely used catalysts in the worldwide refineries. For example, there is a synergetic effect between the active sites of molybdenum (Mo) or tungsten (W) and the Co or Ni as a promoter. In the Co/Mo/Al₂O₃ catalyst, the Co helps the Mo monolayer increase its stability and Co-Mo-S phase formation makes Co-Mo catalysts more active than Mo catalysts alone (Chianelli et al., 2002). Hydrogen atoms generated from Co or Ni promoters under the working condition remove the sulfur atoms on Mo active sites, which would result in the unsaturated coordination of Mo atoms which are the active phase in catalytic desulfurization. These relations make the coordinative unsaturated site facilitate the adsorption of sulfur-containing reactants and the desorption of produced H₂S (Pimerzin et al., 2013). However, Abrahamson (2015) reported that Co-promotor catalysts have higher HDS activity than Ni-promotor ones.

It has been reported that the application of the HDS process alone, to meet the new international standards for transportation fuels, is estimated to require a 3-fold increase in the catalyst volume/reactor size, which is non-economic and has a negative impact on the quality of the treated feed (Song and Ma, 2003). The desulfurization efficiency and selectivity depend on many factors: catalyst type, concentration of catalyst active species, support properties, catalyst synthesis route, the reaction conditions (i.e. the sulfidizing protocol, temperature, and the partial pressure of H₂ and H₂S), the types and concentration of S-compounds in the feed stream, and finally the reactor and process design (Babich and Moulijn, 2003).

Noble metals and new carrier materials (such as amorphous silica-alumina ASA) with high desulfurization efficiency have been developed (Babich and Moulijn, 2003). However, the noble-metals-catalysts are recommended to be used after most of the organosulfur compounds and H₂S are removed, as they are susceptible for sulfur-poisoning. Thus, HDS with noble catalysts is recommended to be applied in a multi-step desulfurization process (Figure 2.5).

The commercial approaches applying two-stage or multi-stage hydrotreating processes are the Shell middle distillate hydrogenation process (Lucien et al., 1994; Stork, 1996), the two-stage Haldor-Topose process (HDS/HDAr) in which the first stage uses their base metal HDS-catalyst TK-573 and the second stage uses their noble dearomatization TK-907/TK-908 or TK-915 catalysts (Cooper et al., 1994), the IFP Prime D hydrotreating process reported to produce SRLGO with 10 ppmw S-content (Marchal et al., 1994; EPA-Diesel RIA, 2000), United Catlysts and Sud-Chemie AG, which reported that the application of their ASAT catalyst in the treatment of feed distillate with 400 ppmw S, 127 ppmw N and 42.5 vol.% aromatics, managed to reduce the S to 8–9 ppm, eliminated N and reduced the aromatics to 2–5 vol.%, with hydrogen consumption of 800–971 standard ft³/bbl (EPA-Diesel RIA, 2000) and the Criterion/ABB Lummus licensed their SynTechnology, which includes SynHDS for ultradeep desulfurization and SynSat/SynShift for cetane improvement, aromatics saturation and density/T9 reduction (Suchanek, 1996; EPA-Diesel RIA, 2000; ABB Lummus Global, 2002).

The γ-Al₂O₃ is known to be the most widely used catalyst support because of its appropriate pore size distribution, large specific surface area, and its ability to provide high dispersion of active metal components (Salcedo, 2015; El-Gendy and Speight, 2016). The γ-Al₂O₃ supported molybdenum oxide catalysts promoted with cobalt or nickel have been widely used in conventional HDS processes (Segawa et al., 2000). Active sites are formed when MoO₃ changes to MoS₂ by sulfurization (Arnoldy et al., 1985). The hydrogenation route is the most important pathway in the HDS of DBT molecules with substituents on the 4- and 6-postition (Kabe et al., 1993). The direct hydrogenolysis route is less favourable due to the steric hindrance (Robinson et al., 1999). The molecule becomes more flexible upon hydrogenation of the aromatic rings and when the steric hindrance is relieved (Kabe et al., 1993; Landau et al., 1996). Consequently, catalysts with a relatively high hydrogenation activity must be considered. The Ni-promoted mixed sulfuide catalysts are known for their high hydrogenation activity (Van Veen et al., 1993), but γ-Al₂O₃ has some disadvantages; the activity of some catalysts supported on alumina is lowered due to the occurrence of numerous chemical interactions between the alumina and the transition metal oxides. The formed species are very stable and completely resist sulfidizing. In addition, the coke formation is another disadvantage in the alumina-supported catalytic HDS-process, which deactivates and shortens the lifetime of the catalysts (Amini et al., 2010). Moreover, nowadays, the application of nanocatalysts in the desulfurization process is very attractive because of their high stability and large specific surface area, thus, there are more active surface sites. Multiwall carbon nanotubes (MWCNTs) have been reported as a novel support material for desulfurizing catalysts (Mohammed et al., 2016).

The concept of bi-functional catalysts has been proposed to increase the sulfur resistance of noble metal hydro-treating catalysts (Song, 1999). It combines catalyst supports with bimodal pore size distribution and two types of active sites. The first type of sites, placed in large pores, is accessible for organosulfur compounds and is sensitive to sulfur inhibition. The second type of active sites, placed in small pores, is not accessible for large S-containing molecules and is resistant to poisoning by H₂S. Since hydrogen can easily access the sites located in small pores, it can be adsorbed and transported within the pore system to regenerate the poisoned metal sites in the large pores. The practical applications of this concept have not been demonstrated yet. The Pd/Pt supported catalysts have been reported to possess high activity in the HDS of DBT and 4,6-DMDBT with a better selectivity to hydrogenation and higher thiotolerance than CoMo and NiMo catalysts (Kabe et al., 2000; Barrio et al., 2003; Niquille-Röthlisberger and Prins 2006; Niquille-Röthlisberger and Prins (2007); Baldovino-Medrano et al., 2009). The Pt and Pt/Pd catalysts have also been reported as very active in the deep HDS of pre-hydrotreated straight-run gas oil (SRGO) under industrial conditions to 6 ppm-sulfur content, while simultaneously reducing the aromatics to 75% of their initial amount (Reinhoudt et al., 1999). Bowker (2011) reported that silica-supported rhodium phosphide (Rh₂P/SiO₂) has a higher DBT-HDS activity than either silica-supported rhodium metal (Rh/SiO₂) or rhodium sulfide (sulf. Rh/SiO₂) and is also more active than the commercial Ni-Mo/Al₂O₃ catalyst, but the silica-supported palladium phosphide (Pd₃P/SiO₂, Pd₅P₂/SiO₂) catalysts are less active than commercial Ni-Mo/Al₂O₃ catalysts, due to their limited active site densities. However, they have higher DBT-HDS efficiency and more S-resistance than Pd/SiO₂. Kaluža and Gulková (2016) reported that the transition metal governed the ranking of the BT- HDS efficiency as follows:

CoMo ≈ NiMo > PtMo ≈ RhMo > PdMo > IrMo > RuMo.

Trakarnpruk et al. (2008) reported the application of an unsupported MoS₂ catalyst in the HDS of two types of diesel oils; the SRGO directly obtained from atmospheric distillation of crude oil and LCO produced in the FCC unit with a total sulfur content of 6100 and 310 ppm, respectively, in a high pressure batch reactor at 350 °C and 30 atm. Seventy percent desulfurization has been achieved in both oil feeds within 90 minutes. However, the reactivity of the refractory sulfur compound in the two feeds were ranked as follows: for LCO BT > DBT > 4,6-DMDBT, while for SRGO BT > 4,6-DMDBT > DBT. The SRGO used in that study contained a total nitrogen of 80 ppm and higher aromatics content than that of LCO (29.2 and 5 wt.%, respectively).

Akzo Nobel produced the trimetallic catalysts KF901 and KF902 for FCC-feed hydrotretament which maintains the hydrodenitrogenation (HDN) and aromatics saturation activity of the conventional Ni-Mo catalyst, while enhancing the HDS activity of the conventional Co-Mo ones at the same operating conditions and life cycle, but they are more expensive than conventional Ni-Mo catalysts since Co is more expensive (Reid, 2000).

However, various supports have been used to enhance the catalytic activity in HDS, including carbon (Farag et al., 1999a,b), silica (Cattaneo et al., 1999), zeolites (Breysse et al., 2002), titania (Morales-Ortuño et al., 2016), zirconia (Afanasiev et al., 2002; Kaluža and Gulková, 2016), and silica-alumina (Qu et al., 2003). The zeolite-based supported HDS catalysts are characterized by high hydrotreating (HDT) capacity, good distribution of products, and high catalytic activity for the removal of sterically hindered sulfur compounds, such as 4,6-DMDBT, due to the strong acidity, large surface area, and high hydrothermal/chemical stability of the zeolite-based supports (Wang et al., 2014). However, it has been reported that the HY-zeolite is deactivated faster than CoMo/Al₂O₃ (Pawelec et al., 1997). It has been reported that NiW/Al₂O₃ is less active than NiMo/Al₂O₃ in DBT HDS, but more active in 4-ethyl, 6-methyl-dibenzothiphnene (4E6MDBT) HDS because of its greater hydrogenation activity (Robinson et al., 1999). Xu and Liu (2004) reported HDS of DBT and 4-MDBT by CoMo/γ-Al₂O₃. Moreover, at high sulfur levels, the ASA supported that noble metal catalysts are poisoned by sulfur and NiW/ASA catalysts become preferable for deep HDS and dearomatization (Robinson et al., 1999). However, Reinhoudt et al. (1999) showed that ASA supported Pt/Pd catalysts are very promising to apply in deep desulfurization, provided that H₂S is removed efficiently. A major drawback of this though is the price of the noble metals. Moreover, the PtPd/ASA catalysts are excellent for streams with low or medium S-content and low aromatics, while the Pt/ASA catalysts are better for streams with higher aromatic contents. The TiO₂ supported Co or CoMo catalysts have higher activities than the Al₂O₃ supported ones (Abrahamson, 2015), but they are not commercialized because of their thermal instability, low surface area and poor mechanical properties (Dhar et al., 2003). However, the TiO₂–Al₂O₃ supports have been extensively studied because of their commercial prospects. Yeetsorn and Tungkamani (2014) reported that the CoMo catalysts supported on titanium-rich (Al₂O₃–TiO₂) and pure titania carriers have higher HDS activity than those supported on pure alumina since the TiO₂ is acting as an electronic promoter in catalysts. But, to a certain limitation, the optimum concentration of tiania that expressed the highest HDS activity was estimated to be 1:0.75 (w:w) Al₂O₃–TiO₂. The ZrO₂–Al₂O₃ supported molybdenum catalysts promoted by Co and Ni received considerable attention for the HDS of Th, 4,6-DNDBT and real feed streams. The CoMo/ZrO₂–Al₂O₃ catalysts are reported to have higher activities than γ-Al₂O₃ supported catalysts (Flego et al., 2001; Zhao et al., 2001). Also, Lecrenay et al. (1998) reported that NiMo/ZrO2–Al₂O₃ has a higher HDS efficiency than NiMo/γ-Al₂O₃ for 4,6-DMDBT, gas, oil, and LCO. Deng et al. (2010) studied the effect of catalyst loading, hydrogen flow rate, and the operating temperature and pressure on the HDS of diesel oil in a slurry reactor using NiMoS/Al₂O₃ in a high pressure autoclave where the HDS efficiency increased proportionally with catalyst loading, increased temperature, pressure, and hydrogen flow rate. Xu et al. (2017) reported the preparation of the bimetallic catalyst, NiMo/SiO₂-Al₂O_3, using gemini surfactant that has a high HDS efficiency towards 4,6-DMDBT and FCC-diesel oil. Krivtcova et al. (2015) reported that in the application of an aluminum-cobalt-molybdenum catalyst (GKD-202) for the HDS of diesel oil feed with total sulfur content of 1.4 wt.%, the rate of HDS of the sulfur compounds has been found to be ranked in the following decreasing order sulfides > BTs > DBTs. Moreover, the rate of HDS decreases with the increase of the alkyl substituents concentrations. The zeolite-alumina supported catalysts have higher activity than alumina supported catalysts (Ali et al., 2002; Marin et al., 2004; Wan et al., 2010; Sankaranarayanan et al., 2011; Nakano et al., 2013; Wang et al., 2014). The noble-ASA/mixed zeolite supported catalysts, for example Pt–Pd/HY-MCM-41, exhibits high HDS efficiency with good S-tolerance (Zhang et al., 2007). Moreover, the NiMo/HY-MCM-41 catalysts are known to have higher HDS activity than alumina supported catalysts (Ren et al., 2008; Li et al., 2009a). It has been reported that the addition of a small amount of HY zeolite to Al₂O₃ increases the HDS activity by 1.2 times compared with the Al₂O₃ supported CoMo catalyst (Ren et al., 2008). Noble metals, especially platinum (Pt), supported on zeolites are reported to have high and stable activity in the HDS of thiophene (Kanda et al., 2009) that is attributed to the enhanced adsorption of the reactant Th on these high specific surface area zeolites and the hydrocracking due to the high acidity of the zeolite substrate. Moreover, upon applying transition metals, an enhancement of hydrogenolysis reactivity over the metal occurs (Boukoberine and Hamada, 2016).

Recently, hierarchical zeolites have attracted much attention due to the presence of additional mesopores and/or macropores which increase the active phase and the access of large reactants as well as diffusion improvement by shortening the mass transfer pathway (Xiao and Meng, 2011; Chen et al., 2012). It has been reported that noble metals supported on hierarchical zeolites are more efficient than those supported on microporous zeolites and Al₂O₃ (Sun and Prins, 2008).

Most of the carriers (i.e. the supports) are acidic in nature, however, MgO as an alkaline active support has been reported (Klicpera and Zdrazil, 2002; Heidarinasab et al., 2016). Moreover, the higher resistance of MgO-support catalysts towards coking would facilitate its application at low or moderate pressure, but to overcome the limitation of its low specific surface area and low textural stability, the introduction of MgO on the formulation of Al₂O₃ has been suggested (Wu et al., 2009). Not only this, but it will hinder the hydrogenation of the unsaturated compounds which have a great value in petroleum products (Zdrazil and Klicpera, 2001). The Mo/P₂O₅-promoted γ–Al₂O₃ favors the DBT-hydrogenation pathway, while the Mo/MgO favors the direct DBT-HDS pathway. Moreover, the Mo/MgO was reported to express higher DBT-desulfurization activity with an extremely low cooking than that of Mo/P₂O₅-promoted γ-Al₂O₃ in a fixed bed reactor at 320 °C under the atmospheric pressure (Heidarinasab et al., 2016). In addition, the physical mixing of the two catalysts promoted Th reactivity. Although, the Th molecule is known to have a lower electron density over its sulfur heteroatom, thus it has an extremely low HDS reactivity under atmospheric pressure. The only possibility to activate Th is throughout its hydrogenation or, at least, its partial hydrogenation (Ma et al., 1995) where the good synergism between MgO and γ–Al₂O₃ is the main reason for gaining the advantages of the dual-function catalyst (Trejo et al., 2008; Heidarinasab et al., 2016). The use of hydrotreating catalysts supported on transition metals containing (Ag⁺, Cu²⁺ and Ni²⁺) micro- and mesoporous materials are reported to have an excellent deep HDS activity (Yang, 2003; Hernández-Maldonado and Yang, 2004a; Gong et al., 2009; Boukoberine and Hamada, 2016). Moreover, for the HDS of heavy and residual distillates containing recalcitrant S-compounds, the use of catalysts with an appropriate balance between cracking and hydrogenolysis functions and lengthy thermal stability are required (Boukoberine and Hamada, 2016).

A trickle-bed reactor is usually used in large refineries for the HDS process (Srivastava, 2012). Improved HDS efficiency can be achieved by applying the countercurrent process (Babich and Moulijn, 2003). ABB Lummus estimates that the counter-current design can reduce the catalyst volume needed to achieve 97% desulfurization by 16% relative the conventional cocurrent design with a cetane improvement (EPA-Diesel RIA, 2000). The SynAlliance (ABB Lummus, Criterion Catalyst Corp., and Shell Oil Co.) has patented a counter-current reactor design called SynTechnology where in a single reactor design, the initial portion of the reactor will follow a co-current design, while the last portion of the reactor will be countercurrent (Figure 2.6, Swain, 1991). This is very helpful in the application of very active, but S-sensitive noble-metal catalysts. The Scanraff’s SynSat gas oil hydrotreating unit in Sweden uses noble-metal catalysts in the second countercurrent stage of the process. In the SynSat technologies, the countercurrent approach not only removes sulfur, but it also removes nitrogen and aromatics, where ULS-level (≈1 ppm) and aromatics level of 4 vol.% could be reached (van der Linde et al., 1999).

The ebullated catalyst bed HDS process is a promising new advanced HDS process where the catalyst, the feed, and hydrogen are a good mix since they are in a fluidized state. Moreover, it is characterized by very good heat transfer efficiency, thus the overheating of the catalyst carrier and the formation of coke are minimized. The reactor operates in an isothermal condition with constant, rather small, pressure drops. The clogging and erosion of the catalyst are minimized. There is a good control on the catalyst activating in an ebullated bed HDS reactor throughout, with the flexibility of adding and removal of catalysts (Song, 2003). The T-Star IFP process uses the ebullated bed to desulfurize heavy feedstocks such as deep cut heavy vacuum gas oils, coker gas oils, and some residues. Integrating the T-Star process with hydrotreating process would produce diesel with an S-content of <50 ppm and gasoline with an S-content of 30–50 ppm (Billon et al., 2000).

Reactors using monolithic catalyst supports are an attractive alternative to conventional multi-phase reactors (Kapteijn et al., 2001), where instead of the catalyst trickle-bed, monolithic channels are present where bubble-train (or Taylor) flow occurs. Gas bubbles and liquid slugs move with constant velocity through the monolith channels approaching plug flow behaviour. Gas is separated from the catalyst by a very thin liquid film and during their travel through the channels the liquid slugs show internal recirculation. It has the advantages of optimal mass transfer properties and the achievement of sharp residence time distributions for gas and liquid compared to trickle flow (Kapteijn et al., 2001; Nijhuis et al., 2001). Larger channel geometries (internally finned monolith channels) might allow counter current flow at a relevant industrial scale. Monolithic catalysts can be prepared in different ways. They can be produced by direct extrusion of support material (often cordierite is used, but different types of clays or typical catalyst carrier materials such as alumina are also used), of a paste also containing catalyst particles (e.g. zeolites, V-based catalysts), or with a precursor of catalyst active species (e.g. polymers for carbon monoliths). The catalyst loading of the reactor, in this case, can be high (Kapteijn et al., 2001). Alternatively, catalysts, supports, or their precursors can be coated into a monolith structure by a wash-coating (Beers et al., 2000).

The polar nature of S- N- and O- containing compounds makes them have relatively high dielectric constants and dipole moments and more sensitive to microwave irradiation. The application of microwave irradiation in petroleum refinery processes (catalytic reforming, catalytic cracking, catalytic hydrocracking, hydro-dealkylation, and catalytic polymerization) was developed in the 1990s (Shang et al., 2013). Microwave energy can preferentially heat these heterocyclic compounds and induce, for example, the desulfurization assisted by catalysts or solvents. The theory of use of microwaves in the desulfurization of petroleum streams at a relatively lower temperature is supported by the concept “hydrocarbon molecules are more transparent to microwaves than organo-sulfur or organo-sulfur-mettalic compounds” (Shang et al., 2013). Thus, microwave energy would preferentially activate the sulfur compounds. The application of microwaves in HDS would achieve sustainable savings in capital and operating costs because catalytic reactions can be accelerated by the microwave energy, performed under less severe conditions (i.e. lower temperature and pressure), and shorter catalyst contact time. These make it possible to use smaller reaction vessels with reduced catalyst consumption (Loupy, 2006). Moreover, the microwave process is reported to be generally preferred to non-destructive desulfurization when no sensitizers or catalysts are used due to low microwave absorption. However, destructive desulfurization occurs upon their usage (Shang et al., 2013). Furthermore, the pulsed mode microwave input is reported to be better than a continuous one for HDS reactions (Purta et al., 2004).

Several catalysts are reported in microwave HDS processes, including powdered iron, charcoal on iron, palladium oxide-silica based material, calcium oxide, alkali metal oxide catalysts, and traditional hydro-treating catalysts (e.g. molybdenum sulfuide supported on porous γ-alumina promoted by cobalt or nickel (Co-Mo/Al₂O₃, Ni-Mo/Al₂O₃)). In addition, additives like boron, phosphorous, or silica can be used too. Unsupported catalysts like metal hydrides and metal powder are proved to be effective in the microwave HDS process, acting as hydrogen donor. Mutyala et al. (2010) reported the effectiveness of metal powders in the desulfurization of coal pitches and metal hydride catalysts in the desulfurization of hydrocracked petroleum pitch from Athabasca bitumen by using microwave heating. Wan and Kriz (1985) also reported the effectiveness of iron and copper catalysts in removing 70% of sulfur using microwave HDS in hydrocracked pitch. Activated carbon is reported to enhance the microwave absorption and, consequently, the HDS of high density crude oil (986.5 kg/m³), where sulfur removal of approximately 65%, was recorded with a reduced coke yield, reduction in viscosity, and the treated crude met the pipeline regulations without dilution (Jackson and Soveran, 1995). Microwave sensitizers such as diethanolamine, silicon carbide, activated charcoal and serpentine are commonly used to improve microwave absorption (Shang et al., 2013). However, when Miadonye et al. (2009) used activated charcoal and serpentine as microwave sensitizers, palladium oxide as catalyst, and ethanolamines as polar additives and hydrogen donors for HDS of Saudi Arabia heavy crude oil, the microwave absorption by crude oil was enhanced four-fold and doubled with charcoal and polar solvents, respectively, with no recorded significant changes upon using serpentine due to its poor heat transfer properties. Moreover, the HDS improved from 2.3 to 33.8%, upon using ethanolamine as a hydrogen donor within a short contact time (25 min). Furthermore, the HDS was higher for lighter fractions than heavier ones. Also, Khan and Al-Shafel Emad (2013) reported the increase of HDS efficiency of crude oil from 2.3 to 39% upon the application of microwaves.

However, the challenge is to apply microwave assisted HDS in the petroleum industry on a commercial scale since the reactor materials are either polytetrafluoroethylene (PTFE) or glass/quartz, which limit the maximum operating temperature and pressure required for HDS processes to achieve ultra-low sulfur (ULS) levels.

Generally, gasoline is made up of different fractions coming from reforming, isomerization, and FCC units (i.e., blending of straight run naphtha isomerate, reformate and alkylate products, FCC naphtha, and coker naphtha). Those coming from the reforming and isomerization units are produced from distillation cuts and, consequently, contain little or no sulfur since the S-containing compounds present in crude petroleum have generally high boiling points and the feedstocks used in the isomerization and reforming units are generally hydrotreated (Zhao et al., 2010). On the other hand, the atmospheric residues or the vacuum distillates which constitute FCC feedstocks contain significant amounts of sulfur (approximately 0.5–2.5 wt.%). Since 2009, the EU, North American countries, and some Asian countries aimed successively to reach gasoline with a sulfur content <10 ppm. Depending on feed type, the typical sulfur content of FCC gasoline ranges from 150 ppm to 3,500 ppm. FCC gasoline sulfur is the biggest contributor to the gasoline pool (approximately 30–40%) and it is the major source of sulfur in gasoline (approximately 85–95%) (Song, 2003; Siddiqui and Aitani, 2007). The three main approaches for desulfurization of FCC-naphtha are the post-treatment of the FCC-naphtha products, the pre-treatment of the FCC-naphtha feed, and the in-situ HDS during the FCC-operation (Song, 2003). The sulfur-reduction in the FCC feed before its FCC-catalytic cracking reactor reduces the sulfur in the FCC-naphtha and LCO. However, not all refineries have the capability to carry out the FCC-feed deep hydrotreating as it needs more severe conditions including higher temperature and pressure (Shiflett, 2002; Reid, 2000). For achieving a 10 ppm ULS-gasoline target, most refineries install FCC-selective HDS-units as main countermeasures (Zhao et al., 2010; Lee et al., 2013). Shiflett and Krenzke (2001) reported that for the current commercial hydrotreater operations each 1 wt% sulfur removal results in about 18–20 Nm³/m³ feed (110–120 SCFB) of H₂ consumption, each 1000 ppm nitrogen removal results in about 5.9–6.1 Nm³/m³ (35–36 SCFB) of H₂ consumption, each 1 wt% aromatics removal yields about 5.0–8.4 (use half of these numbers if aromatics are reported as volume %), and each one unit increase in °API gravity requires about 17 Nm³/m³ feed (100 SCFB) of H₂ consumption as does each one unit increase in cetane number for diesel stocks. According to Imhof (2004), the investment required to desulfurize FCC feed ranges between $3,000–$6,000/bbl-stream-day, while Lesemann and Schult (2003) reported that the cost of the FCC additive option was estimated at $0.14/barrel of gasoline for 15% sulfur reduction compared to $1.3/bbl for the selective HDS at 90% sulfur removal, where the FCC additives are mainly supported by metal oxides having Lewis acid properties, such as Zn, Zr, Co, Ni, or Mn impregnated on alumina, hydrotalcite, titania, or Mg(Al)O, taking into consideration that the most successful additives are the ones combining high acidity and accessibility. Although there is a high cost for FCC additives, they are very recommendable because of their flexibility and ease of use. They can reduce sulfur in gasoline by the in-situ selective cracking of sulfur compounds into H₂S in the FCC riser (Bhore et al., 2001; Roberie et al., 2002; Chester et al. 2003; Zhao et al., 2003). The activity of different additives for reduction of gasoline sulfur can be ranked in the following decreasing order: Zn/hydrotalcite > ZrO/alumina > Zn/titania > Mn/alumina. Andersson et al., (1999) found that the Zn/hydrotalcite has a value of 80% reduction of sulfur in sulfur-spiked gasoline at the microactivity test level. Shan et al. (2002a,b) reported an additive comprising USY-zeolite/ZnO/alumina with excellent sulfur removal effects, where the optimum temperature for the desulfurization of Th and alky-Ths was found to be 400 °C. The Akzo Nobel’s Resolve (Resolve 700, 750, 800 and 850) and the Grace Davison’s (Saturn GSR6.1, SuRCA™, SATURN™, D-PriSM™ and RFG™) are examples of the commercial FCC catalyst additives converting organic sulfur into H₂S during the FCC-operation, thus reducing the S-content in liquid products (e.g. naphtha and LCO) (Skocpol, 2000; Purnell et al., 2002; Zhao et l., 2002; Kuehler and Humphries, 2003; Krishnaiah and Balko, 2003).

On the other hand, Atlas et al. (2001) reported that the cost of lowering the sulfur content from 200 to 50 ppm has been estimated to be four times higher that reducing the sulfur content of a product from 500 to 200 ppm. Nevertheless, further reduction of sulfur concentration by HDS to <1 ppm will be necessary for future fuels and remains a challenging research and economic target.

As mentioned above, the main S-compounds in FCC-gasoline are thiols, sulfuides, tetrahydrothiophene, thiophenols, Th and its derivatives, and BT (Lappas et al., 2002). Although most of these compounds are not present in FCC-feedstocks, they could either result from the direct transformation of the S-compounds present in the feedstock, throughout the cracking of long alkyl chain thiophenes, and the addition of H₂S to olefins or diolefins followed by cyclization, or they could come from the recombination of FCC-products (Leflaive et al., 2002). However, the thiophene-sulfur represent approximately >60% of the total S-content in FCC-gasoline. Thus, alkyl-Ths are considered as representative models for S-compounds in FCC-gasoline (Yin et al., 2002a,b; Brunet et al., 2005). Scheme 2.2 summarizes the possible pathways for desulfurization of 2-methylthiophene over commercial sulfided CoMo catalysts. It is worthy to know that, relative to the diesel sulfur problem, it is not very difficult to remove sulfur from gasoline by HDS. However, the challenge is to reach to ULS-gasoline, while keeping the octane number.

The HDS of straight-run kerosene that is used for making jet fuels is reported to be more difficult than that from naphtha, but less difficult compared to that from gas oil.

Diesel oil is formed by the blending of straight run diesel, light cycle oil (LCO) from the FCC unit, hydrocracker diesel, and coker diesel. The LCO from FFC is one of the major diesel oil pools. It is characterized by high sulfur and aromatic contents, especially the recalcitrant S-compounds, for example DBT, 4-MDBT, and 4,6-DMDBT. The ultra-deep desulfurization aims to reduce sulfur to <15 ppm in ULS-diesel oil. The problem comes from the low reactivity of 4,6-DMDBT due to the steric hindrance caused by the methyl groups. This prevents the interaction between the S-atom and the active sites of the catalysts. In addition to the inhibiting effects of PAHs and nitrogen compounds found in the SRGO and LCO diesel blends feedstock, PAHs compete with the PASHs on the surface of the hydrotreating catalysts. Moreover, H₂S competes with S-compounds which affects the direct C-S hydrogenolysis route. Much research has been performed on catalyst formylation to enhance hydrogenation of the aromatic ring in 4,6-DMDBT to induce isomerization of methyl-groups away from 4- and 6-positions by incorporating acidic features in the catalyst, removing the inhibiting H₂S, and controlling the reaction conditions for specific catalytic functions (Song, 2003). The general approach for pre-treating FFC feed and the in situ desulfurization during the FCC for reducing S-content in diesel feedstock are as in the case of naphtha desulfurization.

The known commercial processes for the HDS of FCC-gasoline are the selective HDS preserving octane number (Intitute Francais du Petrole (IFP’s) Prime G+, Exxon Mobil’s Selective Cat Naphtha hydrofining (SCANfining), catalytic distillation (CDHydro/CDHDS), RIPP’s RSDS-I/II and OCT-M/OCT-MD, and deep desulfurization associated to octane recovery through alkane isomerization (ExxonMobil’s Oct-Gain and UOP-INTEVEP’s ISAL™). They are the main two strategies applied in the worldwide refineries for the desulfurization of FCC-gasoline. However, the selective HDS processes which are based on conventional catalytic fixed-bed technology and need low cost investments are, so far, the most attractive for the industry (Song, 2002, 2003; Brunet et al., 2005; Lee et al., 2013; El-Gendy and Speight, 2016).

For example, SCANfining (Figure 2.7) uses an RT-225 catalyst system which was jointly developed by ExxonMobil and Akzo Nobel specifically for the selective removal of sulfur from FCC naphtha by HDS with minimum hydrogenation of olefins and preservation of octane. It offers the ability to eliminate FCC naphtha product splitting towers and reduce hydrogen consumption to 30–50% less than in conventional hydro-finishing. This results in significant investment and operating cost savings (Kaufmann et al., 2000; ExxonMobil, 2001; Halbert et al., 2001; Song, 2003).

The Prime G+ (Figure 2.8) is less severe and has been commercially demonstrated for several years in many worldwide refineries. It is based on a combination of a selective hydrogenation unit which removes diolefins and light mercaptans, a splitter, and a selective HDS of mid and HCN cut through a dual catalytic system (HR-806 and HR-841) which were developed by IFP and commercialized by Axens. The HR-806 achieves the bulk of desulfurization, while HR-841 is a polishing catalyst which reduces sulfur and mercaptans with no activity for olefin hydrogenation (Nocca et al., 2000; IFP 2000, 2001; Baco et al., 2002).

The OCTGain process (Figure 2.9) uses a fixed-bed reactor to totally remove sulfur, saturate olefins, and then restore the octane to economically needed levels, producing products with <5 ppm sulfur and <1% olefins (Shih et al., 1999; ExxonMobil, 2001). Qatar Petroleum OCTGain unit reported that to produce an ultra-low-sulfur (ULS <1 ppm) and low olefins gasoline, the process flexibility allows for the adjustment of an octane number between –2 and +2 RON of the feed (Chitnis et al., 2003). The UOP-INTER-VEP’s ISAl and ExxonMobil’s OCTGain process are similar in the concept of process design and processing schemes, but the processing conditions and used catalysts are different. The ISAL™ process, which was jointly developed by INTEVEP SA and UOP, is designed as a low-pressure fixed-bed hydroprocessing technology for desulfurizing gasoline range feedstocks and selectively reconfigures lower octane components to restore product octane number. The catalyst used in this process is typically a combination of an HDS catalyst such as Co–Mo–P/Al₂O₃ and an octane-enhancing catalyst such as Ga–Cr/H-ZSM-5 catalysts in two beds (Salazar et al., 1998; Babich and Moulijn, 2003).

The catalytic distillation desulfurization process developed by CDTech is significantly different from conventional hydrotreating (Rock, 2002; Rock and Shorey, 2003), where hydrotreating requires a distillation column after the fixed-bed hydrogenation unit while the CDTech eliminates the fixed-bed unit by the incorporation of the catalyst in the distillation column. The most important portion of the CDTech desulfurization process (Figure 2.10) is a set of two distillation columns loaded with a desulfurization catalyst in a packed structure. In this process, the LCN, MCN, and HCN are treated separately under optimal conditions for each. The first column, called CDHydro, treats the lighter compounds of FCC gasoline and separates the heavier portion of the FCC gasoline for treatment in the second column. Catalytic distillation essentially eliminates catalyst fouling because the fractionation removes heavy coke precursors from the catalyst zone before coke can be formed and foul the catalyst bed. This prevents the catalyst from poisoning. Moreover, the clean hydrogenated reflux continuously washes the catalyst zone, leading to a longer catalyst life-time. In addition, mercaptans can react with diolefins at the bottom of the catalyst zone, forming heavy and thermally stable higher boiling point sufides (>C5 fraction), which can be easily fractionated to the bottom products. That, consequently, eliminates the separate mercaptans removal step (CDTech, 2000; Rock, 2002). The second column, called CDHDS, catalytically removes the sulfur from the heavier compounds of FCC gasoline (MCN and HCN) in two separate zones. The CDHDS is used in combination with CDHyrdro to selectively desulfurize gasoline with minimum octane loss. The HDS-conditions are optimized for each cut for maximum HDS efficiency with a minimum olefin saturation. Olefines go to the column-top where conditions are mild, while the concentrated sulfur moves to the column-bottom where severe conditions are present for higher HDS-efficiency (CDTech, 2000). The temperature and pressure of the CDTech process columns are lower than fixed-bed hydrotreating processes, particularly in the upper section of the distillation column, which is where most of the olefins are located. These operating conditions minimize yield and octane loss (Rock and Shorey, 2003). It has been estimated that the CDTech is approximately 25% less expensive than the conventional HDS process (Babich and Moulijn, 2003). Thus, it is very attractive for refineries and it has been demonstrated at Motiva’s Port Arthur, Texas Refinery.

Moreover, Philips Petroleum Co., has proposed a combination between the pre-aromatization of FCC gasoline streams and conventional HDS where the aromatics content increases in the end product by approximately 68%, but the olefins are almost completely saturated and the S-content decreases from 300 to 10 ppm, while the octane number increases from 89 to 100 (Drake and Love, 2000), but due to the increased environmental limitations for the aromatics content in the gasoline, that technology is not widely applied.

The worldwide demand for diesel oil is expected to significantly increase. However, the worldwide reserve of high quality low sulfur content crude oil decreases. Thus, the problem of deep desulfurization of diesel fuel has become more serious due to the lower and lower limit of sulfur content in finished fuel products by a worldwide update in strict new regulatory specifications and the higher and higher sulfur contents in the available crude oils. The known approaches for deep desulfurization to produce ULS-diesel oil are ultra-deep HDS of middle distillates [MAKFining Premium Distillates Technology (PDT) by Akzo Noble, Exxon Mobil and Kellog Brown], ultra-deep HDS (SynHDS) and hydroaromatization (SynSat) and cetane improvement by ring opening (SynShift) of middle distillate [SynTehnology includes; new reactor design by SynAlliance including ABB Lummus, Criterion catalyst and Shell Global], two stage hydrotreating for ULS diesel fuel using industrially proven high-activity TK catalysts [Haldor Topose], ultra-deep HDS, HDN and hydrogenation of distillate fuels [Unionfining by UOP], hydrodearomatization (HDAr) of middle distillate in the second stage [PDT, by Akzo Noble, Exxon Mobil and Kellog Brown], FCC feed deep hydrotreating to remove sulfur before catalytic cracking which produces naphtha and LCO, and, finally, undercutting of S-rich narrow fraction of LCO from FCC [to remove a narrow boiling point range that is rich in refractory sulfur compounds and use it for off-road distillate fuels] (Turaga and Song, 2001; Song, 2002, 2003; Song and Ma, 2003).

There are many reported commercial catalysts for diesel oil-HDS that are marketed by Akzo Nobel (KF 752, KF 756, KF 757 and KF 848), Criterion (Century and Centinel), Haldor–Topsoe (TK-554 catalyst is analogous to Akzo Nobel’s KF 756 catalyst and the more active TK-574 catalyst), United Catalyst/Sud-Chemie, and ExxonMobil. For example, Akzo Nobel reported the application of the Super Type II Active Reaction Sites (STARS) catalyst for desulfurizing diesel fuel at the BP Amoco refinery in Orangemouth, UK. The original unit was designed to produce 35,000 bbl/day diesel fuel with an S-content of 500 ppmw from straight-run gas oil (SRGO) feed and LCO. However, the CoMo STARS catalysts are preferred for feed with high S-content (100–500 ppm) and works better at low pressures, while NiMo STARS catalysts are more suitable for low S-content feeds (<100 ppm) and work better at high pressures. They can work for a long-term run up to 400 days (Ma et al., 1996; Topsoe et al., 1999). The application of KF 757 catalysts in a reactor with a capacity 45,000 bbl/day day diesel fuel with an S-content of 10–20 ppmw has also been reported. KF 756 is reported to be widely used in Europe (≈20% of all distillate hydrotreaters operating by it), while KF 757 has been commercially used in at least three hydrotreaters (Swain, 1991; Turk et al., 2002). Century and Centinel are reported to be 40–70 and 80% more active than the conventional HDS catalysts (Swain, 1991). They are claimed to combine superior hydrogenation activity and selectivity. However, the CoMo-Centinel catalysts are preferred for high S-content streams and work better at lower H₂ pressures, but the NiMo-Centinel catalysts are preferred for low S-content streams (<50 ppm) and work better at higher H₂ pressures. This occurred due to the better dispersion of the active metal on the catalyst substrate. In 2001, ExxonMobil, Akzo Nobel, and Nippon Ketjen introduced a new unsupported catalyst called NEBULA. It is a Mo-W-Ni trimetallic catalyst which is reported to be three-times better than any other hydrotreating catalysts available (Soled et al., 2001). Haldor–Topsoe reported that the increase in temperature by 14 °C, using its advanced TK-574 CoMo catalyst in the processing of a mixture of SRGO and LCO, reduced the S-content from 120 to 40 ppmw (Swain, 1991).

During the FFC-process, the resonance stabilization of Th prevents cracking. Thus, most of the thiophenic sulfur compounds end up forming coke (Corma et al., 2001). Aromatic hyrogenation during HCR would facilitate the desulfurization of heavy oils by cracking and/or hydrogenation. That would facilitate the selective ring opening and, consequently, improve the distillate quality (Santana et al., 2006). Although HDS is used for the upgrading of heavy oil, its efficiency is limited by the properties of heavy oil and the high heavy metal content that deactivates the catalyst and causes deposit formation, coking, and fouling propensity which also causes catalyst deactivation, and high molecular size that causes mass transfer limitation and limits the access into the small catalyst pores. Moreover, the alkylated-thiophenic compounds increase the steric protection of the thiophenic sulfur and thus limits the adsorption to the active catalyst sites for HDS (Javadli and Klerk, 2012a).

Transportation fuels, such as gasoline, jet fuel, and diesel, are ideal fuels due to their high energy density, ease of storage and transportation, and established distribution network. One of the main advantages of HDS is versatility in treating different oil feed, such as gasoline, diesel, and jet fuel, with initial high S-content. For example, it can directly decrease the initial S-content of fuel oil from 21,900 ppmw by approximately 57% and the oil feed of 10,000 ppmw to less than 100 ppmw in one step (Al-Degs et al., 2016). However, to meet the new international environmental regulations for producing ultra-clean transportation fuels with a low concentration of sulfur (<15 ppm) or near-zero sulfur products and to prevent the deactivation of catalysts in reforming process and electrodes in fuel cell system, higher temperatures and pressures with the developing of new expensive metal catalysts and reactors are required. This will consequently increase the energy consumption, the operations, and capital costs, as well as create more carbon dioxide emissions (Castorena et al., 2002; Wild et al., 2006; Yi et al., 2013). In addition to desulfurization, this will also result in demetallization, carbon residue reduction, some denitrogenation, hydrocracking, and coking (Song, 2003).

Heavy oils, which are characterized by high S-content, can be treated through hydrocracking before the HDS process. Since hydrocracking facilitates aromatic hydrogenation, it enables desulfurization by both cracking and hydrogenation, but the main aim of hydrocracking is not the HDS of heavy oil, but the selective ring opening to improve the quality of its distillates (Santana et al., 2006). There are less restrictions for S-levels in non-transportation fuels which are formed from vacuum gas oils and residual fractions from coking and FCC units, than that for transportation fuels since industrial fuels are used in stationary applications where sulfur emissions can be avoided by combustion gas cleaning processes. Deep desulfurization of the transportation fuels implies that more and more of the least reactive sulfur compounds must be converted. Keeping this in consideration, the increase of octane number and decrease of the aromatic and S- contents of the treated gasoline increase the cetane number and improve the density and polyaromatic content of diesel oil. The refining industry has made a great deal of progress towards developing more active catalysts and more economical processes to remove sulfur from gasoline and diesel fuel (oil) (Okada et al., 2002a,b; Egorova and Prins, 2004; La Paz Zavala and Rodriguez, 2004). For example, California refineries are producing gasoline which contains 29 ppm sulfur on average (US EPA 2000) despite the high sulfur content of California and Alaska crude oils used as feed stocks up to 11,000 ppm. Due to incentives and regulations, 10 ppm sulfur diesel has been commercially available in Sweden for several years. But, although this process tends to improve diesel quality by raising cetane number, it decreases gasoline quality by lowering its octane number (Babich and Moulijn, 2003; Song and Ma, 2003). In Bulgaria, since 2009, the FCC based refinery, Lukoil Nentochim Bourgas (LNB), has been producing near zero sulfur gasoline (NZSG) (<10 ppm) by applying FCC feed hydrotreating. Later in 2010 by applying also Prime G FCC gasoline post treatment technology, the FCC gasoline (15 ppm S-content) is the cap for production of NZSG in the LNB refinery (Yankov et al., 2011; Khamis and Palichev, 2012).

Briefly, worldwide, meeting the requirements of reaching to ultra-low sulfur fuels is believed to be the main challenge for the petroleum industry. As most of the gasoline in markets is coming from the cracked feeds with high concentrations of aromatics and olefins, that increases the difficulty of S-removal. Nowadays, two-stage HDS of diesel oil is used to reach to <10 ppm S-content. The first stage usually applies a conventional hydrogenating unit with moderate adjustments to the operating parameters to reach an S-content of approximately 50 ppm. However, the second stage requires higher hydrogen flow rate and purity, reducing space velocity and the choice of more expensive catalysts. Not only this, but it will add another problem, which is the increased production of H₂S, which would increase the stress on the Claus plant capacity, which produces hydrogen for the refineries. Accordingly, refineries have started to establish new complementary routes in addition to the HDS process, such as the incorporation of non-HDS techniques with HDS that will reduce hydrogen consumption and create the required high temperatures and pressures and expensive catalysts for the HDS of the recalcitrant S-compounds.

2.3 Oxidative Desulfurization

Oxidative desulfurization (ODS) is an innovative technology that can be used to reduce the cost of producing ultra-low sulfur diesel (ULSD). This technique is based on the polarity of sulfur compounds. It is well known that the electronegativity of sulfur is similar to that of carbon, consequently, the carbon-sulfur bond is relatively non-polar and the organo-sulfur compounds exhibit properties quite similar to their corresponding organic compounds, so their solubility in polar and non-polar solvents are nearly the same. But, upon oxidation of refractory organosulfur compounds, the polarity of the produced sulfoxides/sulfones increases and, consequently, their solubility in polar solvents increases and can be easily separated by extraction or adsorption distillation and thermal decomposition (Ito and van Veen, 2006; Nanoti et al., 2009; Zongxuan et al., 2011; Mužic and Sertić-Bionda, 2013; Al-Degs et al., 2016). If the ODS is combined with distillation, the process scheme would be similar to the catalytic distillation desulfurization (CDTech) (Babich and Moulijn, 2003). The ODS occurs at relatively low temperatures and atmospheric pressures in presence of oxidizing reagents such as aqueous hydrogen peroxide, ozone, tert-butyl hydroperoxide, oxygen/aldehyde, potassium ferrate, etc. The ODS applying an oxidizing agent alone, such as H₂O₂, is slow. Thus, the reaction requires the presence of a homogenous catalyst such as organic acids (for example formic acid, acetic acid), cobalt-manganese-nickel acetates, polyoxometallic acids and their salts in an aqueous solution, or heterogeneous catalyst such as tungsten/zirconia, titanium/mesoporous silica, peroxy-carboxylic acid, functionalized hexagonal mesoporous silica, molybdenum-vanadium oxides supported on alumina, titania, ceria, niobia, silica, or cobalt-manganese-nickel oxides supported on alumina (Javadli and de Klerk, 2012b). The ODS efficiency of organic acids decreases approximately in the following order: formic acid > acetic acid > propanoic acid. Since the longer C-chain decreases the ODS-efficiency, due to its lower solubility in H₂O₂, it causes lower peroxyacid production and, consequently, decreases sulfur removal. Although formic acid can promote a higher removal in some cases, it is an unstable and corrosive reagent (de A. Mello et al., 2009). Thus, in most cases, acetic acid is considered the best choice.

Ozone, as a highly reactive oxidant, was used for the ODS of LGO containing 150 ppm DBT and 145 ppm 4,6-DMDBT in a batch reactor where the DBTs were quantitatively converted to DBT sulfones and the 4,6-DMDBT was more readily oxidized than DBT (Otsuki et al., 1999). Wang et al. (2009a) investigated ozone-mediated ODS of oil containing high thiophene concentrations using SO₄^2–/ZrO₂ (SZ) at ambient temperature and pressure. Kazakov et al. (2016) reported the ODS of straight-run residual fuel oil (with 2.86 wt. % total sulfur content) from Astrakhan gas condensate by ozonized air where the S-content reached 0.48 wt.% at optimum operating conditions of an ozonization temperature of 60 °C, rate of ozonized air circulation of 1222 nm³/m³, residual fuel oil-ozonized air contact time of 0.05 h, ozonized residual fuel oil thermolysis temperature of 300 °C, and thermolysis time of 2 h, whereupon hydrogen sulfide is removed from the residual fuel oil completely and the combustion heat increases. The increase of desulfurization degree with ozonization temperature was attributed to the decrease in viscosity of the reduced residual fuel oil which facilitates its mixing with ozonized air and increases the reaction of ozone with sulfur compounds. An increased rate of ozonized air circulation contributes to greater desulfurization due to an increased degree of oxidation reactions. With the increase of reaction time, the degree of desulfurization increases and is attributable to a higher degree of oxidation of sulfur compounds and their complete oxidation to sulfones.

S-compound	Electron density	Structure and reactivity
Methy phenyl sulfide	5.915
Thiophenol	5.902
Diphenyl sulfide	5.860
4,6-DMDBT	5.760
4-MDBT	5.759
DBT	5.758
BT	5.739
2,5-Dimethyl thiophene (2,5-DMT)	5.716
2-Methhyl thiophene (2-MT)	5.706
Thiophene	5.696

Otsuki et al. (2000) reported that the higher the electron density on the S-atom, the higher the oxidative reactivity of the S-containing compound using H₂O₂ and formic acid. The electron densities on the S-atom of different S-compounds, as estimated by molecular orbital calculations, are summarized in Table 2.2. It is obvious, from Table 2.2, that the oxidative reactivity of S-compounds is opposite to that of hydrodesulfurization reactivity. Yao et al. (2004) confirmed the criteria where conversion of 4,6-DMDBT, DBT, and BT in n-heptane model oil was 100, 96, and 58%, respectively, using formic acid as a catalyst and H₂O₂ as an oxidant, since the electron donation of a methyl group increases the electron density on the S-atoms (Otsuki et al., 2001). Ali et al. (2009) reported the ODS of a model system consisting of a toluene/hexane solvent containing 500 ppm DBT as a source of polyaromatic S-heterocyclic compounds in the presence of H₂O₂ and acetic acid/H₂SO₄ (2:1 molar ration) at 80 °C and under atmospheric pressure. The ODS of two real oil feeds was also performed, being FCC gasoline and hydro treated diesel, where the S-content decreased from 670 and 1045 ppm to 109 and 85 ppm, respectively. The oxidation process did not show any deleterious effects on the distillation profile and other characteristics of diesel fuel. Mamaghani et al. (2013) added sulfuric acid to increase the acidity of the sulfur compounds and catalytic activity of formic acid where a complete desulfurization of a model oil of BT, DBT, and 4,6-DMDBT was achieved by ODS to sulfone using H₂O₂ and formic acid, followed by liquid-liquid extraction using acetonitrile where the oxidation rates of benzo- and dibenzothiophene homologues depended on the amount and position of alkyl substituents. Farshi and Shiralizadeh (2015) reported that the ODS of heavy oil using an H₂O₂/acetic acid system is better than that performed using an H₂O₂/formic acid system where the S-content decreased from 2.75 wt.% to 1.14 wt.% at 60 °C under atmospheric pressure. The hydrogen peroxide/acetic acid system also proved to be efficient for the ODS of tire pyrolysis oil to be blended as heating fuel (Ahmad et al., 2016).

The mechanism of the ODS using an H₂O₂/organic acid oxidative system can be summarized as in Scheme 2.3 (Haw et al., 2010).

There are two competitive reactions for H₂O₂. The first reaction, which is undesirable because it increases the amount of water in the system and reduces the concentration of the oxidant, is undertaken through the thermal decomposition of H₂O₂. Thus, although the stoichiometric n_O/n_S ratio is 2, the oxidant is usually used in excess of the stoichiometric ratio because of the transport limitation in two-liquid-phase reaction systems and the unproductive decomposition of H₂O₂ to water and oxygen. Therefore, high temperatures are not recommended. The second reaction occurs through the production of strong oxidizing hydroxyl radicals (HO^•). DBT is then oxidized to its corresponding sulfoxide or sulfones. In presence of acid (e.g., formic acid), H₂O₂ and acid interact and produce performic acid (CH₂O₃). Then, the performic acid gives its oxygen to DBT producing sulfoxide and sulfone.

The most widely used oxidants in the ODS process are reported to be the hydrogen peroxide and the t-butyl-hydroperoxide. However, H₂O₂ is more applied in the ODS as it is green and inexpensive. It is relatively cheap and readily available, with a worldwide annual production of approximately 4.3 million tonnes (100% H₂O₂), mainly for bleaching, pollution, and water treatment. Moreover, there is no production of undesirable by-products or liquid degradation products during the ODS process, other than water.

ODS using H₂O₂/organic acid oxidative system is characterized by an efficient and high desulfurization rate under mild operating conditions (60–65 °C and atmospheric pressure). However, oil-soluble liquid organic acids impact the treated fuel quality. Moreover, these liquid acids are unrenewable and have a high reclaiming cost. Not only this, but it is also a biphasic catalytic system where the S-containing compounds are present in the diesel phase, while the catalyst and H₂O₂ are present in the aqueous phase. It is well known that the reactions in a biphasic system are relatively slow due to the mass transfer limitations across the interface, where the reaction would take place at the interface or in the bulk of one of the phases. Thus, the process rate would be determined either by the reaction rate or the diffusion rate, so the use of surfactants (Krotz et al., 2005), ultrasonic (Mei et al., 2003; Mello et al., 2009; Zhao and Sun, 2009), or increased mixing speed (Sharipov and Nigmatullin, 2005) were suggested as a solution, but this would increase the overall cost of the process, thus the application of solid catalysts is recommendable.

When a catalyst is employed, which may be homogeneous or heterogeneous, the relative oxidation reactivity is different: DBT > 4-MDBT > 4,6-DMDBT > BT. The same trend was reported applying H₂O₂/ionic liquids systems (Li et al., 2009b). The relative reactivity of DBT and substituted DBT compounds appear to be related to the steric hindrance of the alkylsubstitutions at positions 4 and 6 in the DBT molecule (Campos-Martin et al., 2004; 2010). Moreover, Zongxuan et al. (2011) reported that the aromaticity of the organic solvents also influences the oxidation rate.

However, upon a study performed by Krivtsov and Golovko (2014), the ODS of straight run diesel fraction (200–360 °C) with a high initial sulfur content (1.19%) included a sulfide sulfur of 0.26 wt.% and thiophenes (BTs and DBTs) 0.93 wt.%, and saturates, mono-, di-, and tri- aromatic hydrocarbons of 53.4, 28.7, 8.2 and 7.9 wt.%, respectively. A 96% S-removal was achieved at 35 °C within 8 h ODS using H₂O₂ and formic acid followed by the adsorption of oxidized products. Sulfides were found to be completely removed, while the rate of ODS of the BT and DBT homologue decreased with the increasing number and size of alkyl-substituents due to the enhancement of steric hindrances around the sulfur atom electron pairs with an increasing number of the alkyl substituents. But, compounds containing substituents in the 4- and 4,6-positions (dibenzothiophene homologues which are the most recalcitrant in HDS-process) were almost completely removed using the hydrogen peroxide–formic acid mixture since the combined positive inductive effect of the methyl groups in 4,6-DMDBT dominate over the steric hindrances to the electrophilic attack at a lone electron pair of the S-atom, as in the case of 4-MDBT. However, the rate of ODS of 4,6-DMDBT was lower than that of 4-MDBT because of the enhanced hindrance of the sulfur atom (by the second methyl group of 4,6-DMDBT). The rate of oxidation of the other identified DMDBT isomers was lower than that of DBT. Note that the isomers containing one of the methyl groups in the 4- or 6-position were characterized by higher values of the effective oxidation rate constants. That was attributed to a positive inductive effect (+I) which extends over the C–C bond chain and leads to an increase of electron density in the conjugated aromatic system, thereby facilitating electrophilic addition reactions. This is due to the significant predominance of the +I effect over the steric hindrance created by the methyl group, although 2-ethyl-DBT expressed the highest oxidation rate constant. The rate constant of 4-ethyl-DBT oxidation was almost two times below that of DBT because the steric effect of the ethyl group in the 4-position dominates over its +I effect (in contrast to the methyl group). In conclusion, the relatively low effective rate constants for oxidation by the H₂O₂–HC00H system are due to the fact that the reaction occurs predominantly through the protonation step in which the polar transition state is formed, not through the formation of performic acid (Scheme 2.4). In the absence of compounds that facilitate phase transfer of the interacting components, (Scheme 2.4) what probably occurs most is formic acid partially dissolves in the diesel fraction and an aromatic compound (hydrocarbon or sulfur compound) is protonated. The intermediate product (charge transfer complex), due to its polarity, is pushed to the aqueous solution/diesel fraction interface at which it is oxidized by a hydrogen peroxide molecule (Zhao et al., 2007; di Giuseppe et al., 2009; de Filippis et al., 2010).

The methyl groups of 4-MDBT and 4,6-DMDBT significantly decrease the rate of HDS of these compounds. This is due to the quite close location of the alkyl groups, which sterically hinder the coordination of the DBT molecule through the lone electron pair of the sulfur atom to the catalyst active site. However, the +I effect plays a significant role in the ODS; the closer the alkyl substituent to the sulfur atom, the stronger the effect. Briefly, the interplay of the +I effect and steric hindrances due to the alkyl group size determines the reactivity of DBT homologues in their ODS. In many cases, the presence of alkyl substituents enhances the reactivity of a DBT homologue.

Te et al. (2001) reported the oxidative desulfurization (ODS) for recalcitrant sulfur compounds using a polyoxometalate/H₂O₂ decreases in the following order: DBT > 4-MDBT > 4,6-DMDBT. This is similar to HDS and is attributed to methylation and the steric hindrance effect where the activation energy of DBT, 4-MDBT, and 4,6-DMDBT oxidation were 53.8, 56, and 58.7 kJ/mole, respectively. Otsuki et al. (2001) reported that the γ-Al₂O₃ support itself can enhance the ODS where, in a flow reactor, more than 90% of 150 ppm DBT was oxidized in a decahydronaphthalene (decalin) solution using t-butyl hypochlorite (t-BuOCl) under ambient pressure at 50 °C. When Al₂O₃ was treated with KOH, the conversion of DBT decreased because the number of the acid sites on Al₂O₃ also decreased. Moreover, the ODS decreased with basic metal catalysts in the following order: γ-Al₂O₃ TiO₂ ≈ Diaion > SiO₂ > ZnO >> MgO (Otsuki et al., 2001). The ODS of DBT is also reported to be inhibited by other compounds found in LGO and the extent of retardation is decreased in the order: nitrogen compounds > olefins > aromatics and paraffins (Otsuki et al., 2001). The catalytic ODS of model oil DBT in decalin was performed using oil-soluble oxidant cyclohexanone peroxide (CYHPO) with a molybdenum oxide (MoO₃) catalyst supported on macroporous polyacrylic cationic exchange resin, D113, of weak acid series, where 100% ODS of DBT to DBTO₂ occurred at 100 °C in 40 min (Zhou et al., 2007).

Wang et al. (2003) reported the complete desulfurization of hydrotreated oil feed with a total S-content of 39 ppm throughout the application of ODS using t-butyl hydroperoxide in the presence of a MoO₃/Al₂O₃ catalyst. However, the major drawback of this process is the high price of t-butyl-hydroperoxide and the waste treatment of t-butyl alcohol and sulfone. The t-butyl alcohols can be used as potential octane improving compounds for gasoline. Moreover, they can also be converted to MTBE, which can be used as fuel within the refinery while the sulfone stream could be transferred to a coker or bio-processor (Stanislaus et al., 2010).

Chica et al. (2006) reported the complete conversion of S-compounds throughout the ODS of a partially hydrotreated LCO with a total S-content of 330 ppm in a continuous fixed bed reactor using a Ti-MCM-41S (sililated) catalyst in the presence of t-butyl hydroperoxide under atmospheric pressure at 373 K. Sampanthar et al. (2006) reported the ODS of diesel fuel of 430 ppm S-content using molecular oxygen in the air in the presence of Mn- and Co- containing oxide catalysts, supported on γ-Al₂O₃, at a temperature range of 130–200 °C and atmospheric pressure, followed by extraction using polar solvent, produced diesel fuel with a 40–60 ppm S-content. Binary mixed metal-oxide catalysts (for example 5%MnO₂/3%Co₃O₄/γ-Al₂O₃) showed better oxidation activity. To reach diesel oil with a lower S-content of ≈ 10–15 ppm, the obtained treated diesel (i.e. the oxidized and solvent extracted diesel) was passed into an activated basic γ-Al₂O₃ adsorbent-bed at room temperature. The ODS of model refractory S-compounds was found to decrease in the following order: trialkylsubstituted dibenzothiophene > dialkyl-substituted dibenzothiophene > monoalkyl-substituted dibenzothiophene > dibenzothiophene. This was attributed to the increased electron density of the sulfur atoms in di-substituted thiophenes which overcompensated for the steric hindrance of the C4 and C6 alkyl groups in the oxidative process. Moreover, the olefin content of the diesel was increased, while the aromatic content of the diesel was reduced substantially. The cetane index was also increased by approximately 20%. Density and other parameters were within the required limits. Furthermore, the lubricity of the treated diesel was increased by a substantial amount, which added advantageously to the applied catalyst and inexpensive molecular oxygen found in the air. In addition, the use of air as an oxidant also eliminates the need to carry out any oxidant recovery process that is usually required upon application of liquid oxidants. Furthermore, this allows for ease of integration into any existing refinery without major changes in the infrastructure.

The hydrothermally stable Ti-HMS, a hexagonal mesoporous molecular sieve with a pore size of about 3 nm, as a catalyst, was compared with Ti-MSU (long, worm-like pores) and TS-1 in the ODS of a model fuel (Th, BT or DBT in n-octane) in presence of H₂O₂ (Cui et al., 2007). TS-1 could not oxidize BT or DBT since these molecules hardly penetrate the small pores of TS-1, but BT and DBT can penetrate the mesopores of Ti-MSU and Ti-HMS and oxidize. The Ti-HMS was the most effective catalyst for the oxidation of both BT and DBT with H₂O₂ where the ODS of DBT increased with the increase of the titanium content in the catalyst recording approximately 80% DBT conversion over Ti-HMS with a TiO₂/SiO₂ ratio of 0.056 and 60 °C, within 2 h reaction time.

The method of treatment by direct oxidation of sulfur-containing hydrocarbons into sulfur trioxide, on a heterogeneous catalyst with atmospheric oxygen, which can actively react with water forming sulfuric acid, as an environmentaly friendly process has been also reported (Boikov et al., 2008).

García-Gutiérrez et al. (2008) prepared different Mo/γ-Al₂O₃ catalysts and investigated ODS activity for diesel fuel using H₂O₂ as the oxidizing reagent. The ODS was found to be highly affected by the presence of hepta- and octa- molybdate species on the supported catalyst with the use of a polar aprotic solvent. Moreover, the presence of phosphate markedly enhances ODS where the S-level decreased from 320 ppm to < 10 ppm at 60 °C under atmospheric pressure. Cedneňo-Caero et al. (2008) reported the efficient ODS of commercial diesel with an initial S-content of 10005 ppm, using H₂O₂/V₂O₅, supported on niobia, with acetonitrile as extraction solvent.

In the view of the disadvantage of zeolites, Si et al. (2008) reported the preparation of different noble metal-loaded Ti-MWW (Au/Ti-MWW) catalysts, by impregnation method, then studied their ODS activity on model organic sulfur compounds in iso-octane (1,000 µg/mL of sulfur), 10 mL acetonitrile, and hydrogen peroxide (H₂O₂/sulfur molar ratio of 2:1–4:1) in an electrothermostatic glass flask equipped with a magnetic stirrer and a condenser. In this context, it was found that Au/Ti-MWW with different gold loading has higher ODS activity for DBT than Ti-MWW. Among these catalysts, Au/Ti-MWW with 0.05% gold gives the highest ODS activity, reaching 99% of 1000 ppm DBT, which was 26% higher than Ti-MWW at 323 K with an H₂O₂/S ratio of 3. Further, the activity of the 0.05% Au/Ti-MWW at 313 K was studied on different S-compounds which also expressed higher activity than Ti-MWW, where the conversion of TH, BT, DBT, and 4,6-DMDBT increased from 55% to 71%, 68% to 90%, 60% to 84% and 50% to 68%, respectively (Si et al., 2008).

Rao et al. (2011) reported the ODS of hydrodesulfurized diesel using aldehyde and molecular oxygen in the presence of a cobalt phthalocyaninetetrasulfonamide catalyst. The reactivity of 4,6-DMDBT was higher than that of 4-MDBT. The S-content in diesel was reduced from 448 to 41 ppm, then to 4 ppm by oxidation, then extracted with acetonitrile, followed by adsorption by passing through a silica gel column.

Cui et al. (2012) reported the ODS of DBT using hydrogen peroxide (H₂O₂) by a recyclable amphiphilic catalyst with magnetic silica nanospheres covered with complexes between 3-(trimethoxysilyl)-propyldi-methyloctadecyl ammonium chloride and phosphotungestic acid, under mild reaction conditions (600 rpm and 50 °C), where the amount of sulfur was decreased from 487 ppm to less than 0.8 ppm. The catalysts with oxidized products have been separated by external magnetic field and simply recycled by acetone eluent, then successfully reused three times with almost constant activity.

Li et al. (2009c) reported the preparation of three decatungstates with short carbon chains as the cations: tetrabutylammonium decatungstate ([(C₄H₉)₄N]₄W₁₀O₃₂), tetramethylammonium decatungstate ([(CH₃)₄N]₄W₁₀O₃₂), and benzyltriethylammonium decatungstate ([(C₂H₅)₃NC₇H₇]₄W₁₀O₃₂). These were then applied as catalysts in the [Bmim]PF₆ IL/H₂O₂ extractive catalytic oxidative desulfurization (ECODS) system. The sulfur level in the model oil (1000 ppm S) decreased to 8 ppm. The system could be recycled five times without losing its activity, then the sulfur removal decreased sharply. Moreover, the advantage of applying Ils is the elimination of the extra extraction step since the Ils act as extractants for all possible by-products (Al-degas et al., 2016).

Yun and Lee (2013) reported the preparation of the amphiphilic phosphotungstic acid catalyst (A-PTA) as an example of polyoxometalate, with quaternary ammonium salt and its application in the ODS of light cycle oil (LCO). It expressed high activity, with an ODS conversion of 95% at an H₂O₂/S ratio of 10 and 353 K. That proved, the presence of polycyclic aromatic compounds in LCO feed plays a beneficial role in ODS by minimizing the deposits of oxidized products on the catalyst surface resulting in good ODS reactivity over the catalyst. The ODS of different refractory S-compounds in model feed oil was in the following decreasing order: DBT > 4-MDBT > 4,6-DMDBT > BT. These results were explained by the electron density on sulfur atoms and structural properties of the sulfur compounds where BT has the lowest electron density on its S-atom (Table 2.2), thus the lowest reactivity. Although 4,6-DMDBT and 4-MDBT have higher electron density on their S-atoms than that of DBT (Table 2.2), they expressed lower reactivity than DBT. That proved, the methyl groups become an obstacle for the approach of the sulfur atom to catalytic active sites. The nitrogen compounds reduced the overall ODS in the model oil feed. Sulfur and nitrogen compounds are competitive in oxidation and nitrogen compounds have higher reactivity than sulfur compounds in this reaction and the oxidized nitrogen compounds might reside on the catalyst surface and inhibit the following ODS, while the introduction of 1-methylnaphthalene fully enhanced the ODS activity and was attributed to the high solubility of the oxidized S or N compounds in the 2-ring aromatics. The poor solubility of sulfone in the aliphatic solvent thus proved lower activity of A-PTA in octane. However, the aromatic solvents improved the solubility of ODS products and increased ODS activity by minimizing the deposit of sulfones on the catalyst. Moreover, the type of aromatic solvents also affects ODS activity. For example, the ODS of BT was promoted in the presence of 1-ring aromatics of tetralin, while DBT-derivatives reacted faster upon the introduction of 2-ring aromatics of 1-methylnaphthalene, but, generally, the solubility of the oxidized compounds follows the order: 1-methylnaphthalene > tetralin > n-octane and, consequently, so does the ODS reactivity.

José da Silva and Faria dos Santos (2013) reported the ODS of model oil of DBT into iso-octane using keggin heterpolyacid (HPA) with an alumina catalyst where the model oil was oxidized to sulfones and extracted by acetonitrile with an S-decrease from 1000 ppm to < 1 ppm within 180 min at 60 °C. Tang et al. (2013b) applied H₃PW₁₂O₄₀, an HPA-catalyst with kegging structure incorporated into mesoporous TUD-1 materials, for the ODS of model oil DBT (500 ppm in n-octane) using H₂O₂ as an oxidant, where approximately 98% desulfurization occurred at 60 °C within 180 min using the catalyst 20HPW-TUD-1. It displayed excellent catalytic activity and recovering ability for ODS and has also been used three times without losing its activity.

Yu and Wang (2013) reported effective ODS by the heteropolyanion substituted hydrotalcite-like compound Mg₉Al₃(OH)₂₄[PMo₁₂O₄₀] (MgAl-PMo₁₂) in a biphasic system using H₂O₂ as an oxidant and acetonitrile as extractant under atmospheric pressure at 60 °C. The S-content of diesel oil decreased from 493 ppm to 9.12 ppm, where the oxidative reactivity of different S-compounds decreased in the following order: DBT > 4,6-DMDBT > BT > Th because of the influence of electron density and spatial steric hindrance. The catalyst can be used for five successive cycles without losing its activity. Yu and Wang (2013) also reported the ODS activity of different heteropolyanion substituted hydrotalcite-like compounds to decrease in the following order: MgAl-PMo₁₂ > MgAl-PW₁₂ > MgAl-SiW₁₂. This was attributed to the central atom of phosphorous in the HPA-HTLcs being better than silicon. Moreover, coordinated molybdenum atoms are another important factor which express higher activity than that of tungsten.

Long et al. (2014) reported the ODS of DBT in model gasoline (DBT in n-octane, with an initial S-content of 400 ppm) using catalyst W/D152 which was prepared by depositing tungsten on resin D152, a macroporous polyacrylic cationic resin, in presence of oil-soluble cyclohexanone peroxide (CYHPO) as an oxidant, followed by washing with DMF to obtain the refined gasoline where the DMF was reused after regeneration. The conversion of DBT occurred and the sulfur content reached 99.1% and 3.52 ppm, respectively, at the optimal catalytic conditions of 100 °C, mass ratio of model gasoline to catalyst W/D152 of 100, molar ratio of CYHPO/DBT of 2.5, and reaction time of 40 min. The catalyst could be reused 7 times before the total sulfur content of the treated model gasoline was higher than 10 ppm. Increase in reaction temperature (>100 °C) decomposes the CYHPO and the catalyst is destroyed at 120 °C. The yield decreased at a longer reaction time (>40 min) due to the volatilization of gasoline in presence of CYHPO without the catalyst, where the ODS was recorded at 86.5%, while in presence of catalyst without CYHPO, the ODS recorded only 45.3%. Thus, the high ODS efficiency recorded for W/D152 is due to the good immobilization of tungsten on resin D152 and the role of the tungstate in the catalyst was to activate a CYHPO molecule so DBT could be rapidly oxidized to sulfones at the presence of catalyst W/D152. As a conclusion from that study, the W/D152-CYHPO reaction system overcomes some of the drawbacks and disadvantages of existing technologies and possesses good catalytic ODS potential to be applied for industrial applications.

Abu Bakar et al. (2015) reported the ODS of model diesel fuel by 4.35% WO₃/16.52% MoO₃/γ-Al₂O₃ at constant temperature of 60–65 °C under atmospheric pressure, followed by extraction using dimethylformamide. That removed 100% of DBT and 4,6-DMDBT and 92.5% of Th.

A solid superacid has advantages, such as acidity stronger than 100 % H₂SO₄ (H₀ ≤ –12) (Gillespise and Peel, 1973), easy separation from the reaction mixture, non-corrosion to reactors, environmental friendliness, and high activity in many organic reactions including esterification (Li et al., 2009d), isomerization of n-alkanes (Song et al., 2010), acylation (Guo et al., 2008) and alkylation (Zhang et al., 2005).

Song et al. (2015) reported a novel procedure for ODS of a simulated light fuel oil (200 mg thiophene/L petroleum ether) and straight-run gasoline (169.5 mg/L initial S-content) using a K₂FeO₄ over SO₄^2–/ZrO₂ (SZ) superacid solid support which was prepared by impregnation. The presence of highly dispersed acidic sites over the support, a huge area of catalyst, and the formation of a tetragonal structure play an important role in ODS using K₂FeO₄, where the catalyst subjected to impregnation with 0.8 mol/L H₂SO₄, supported on ZrO₂, and calcined at 600 °C exhibited the best performance. At optimum conditions of 30 °C, 0.2 g SZ, 0.2 g K₂FeO₄, and 30 min reaction time, followed by methanol extraction at 15 °C for 10 min with the volume ratio of solvent/oil of 1, the straight-run gasoline desulfurization rate reached 89.2 % and the residual sulfur content was 18.3 mg/L. At a higher calcination temperature (>600 °C), a large amount of the zirconium oxide tetragonal phase was converted to the monoclinic phase, leading to poorer catalytic activity that may be due to the decomposition of a loaded sulfate at higher temperatures. Not only this, but sintering may also occur at excessive temperatures which would probably destroy the microporous structure of the catalyst and decrease its surface area. A lower sulfuric acid concentration (<0.8 mol/L) usually leads to lower SZ acid strength, resulting in poor catalytic performance, but a higher sulfuric acid concentration (>0.8 mol/L) would lead to oversaturation of the sulfate and would intensify sulfate agglomeration on the surface and cover some of the SZ active sites leading to lower catalytic activity. Furthermore, the huge surface area of SZ likely enhances contact between thiophene and K₂FeO₄ (Scheme 2.5), where only 37.9 % of the model fuel desulfurization rate was obtained in the presence of K₂FeO₄ without SZ, due to the low solubility of K₂FeO₄ in the organic phase, which led to poor contact between K₂FeO₄ and thiophene and, consequently, low ODS. The oxidation reaction takes place in two steps where oxidation of thiophene is considered to be a consecutive nucleophilic addition, during which thiophene first reacts with K₂FeO₄ to produce thiophene sulfoxide, which further reacts with another K₂FeO₄ molecule to form thiophene sulfone.

Rafiee and Rahpeyma (2015) reported the immobilization of 12-tunstophosphoric acid on silica-coated cobalt-ferrite (CoFe₂O₄) nanoparticles that can be magnetically separated. The ODS- efficiency of that catalyst, using H₂O₂ as an oxidant, decreased in the presence of N-compounds and the inhibition of indole was higher than that of quinoline. Due to the competitive inhibition, since N-compounds oxidizes faster than the S-compounds, and the higher electron density on the N-atom of indole, it is oxidized faster than quinoline under mild operating conditions. The prepared catalyst has also been used efficiently for four successive cycles.

Zhang et al. (2016) reported the preparation of a series of SO₄^2– promoted metal oxide solid superacids, including SO₄^2–/ZrO₂-WO_3, SO₄^2–/ZrO₂-MoO₂, and SO4^2–/ZrO₂-Cr₂O₃ by incrorporating different transition metals into a (SZ) superacid solid support which was prepared by impregnation. They were named as (SZW, SZM, and SZC, respectively) and then their ODS activity was tested on model oil (500 ppm S, DBT in n-octane) at 70 °C in the presence of hydrogen peroxide (nH₂O₂:Ns = 20) with catalyst dosage of 0.015 g/L in the oil, followed by extraction using DMF. Their acid strength is in the following decreasing order: SZC > SZM > SZW > SZ. Thus, the SZC expressed the highest ODS activity, recording approximately 91%, as the ODS is related to catalyst acid strength. Moreover, SZC can be reused successfully for five successive cycles.

Figueiredo dos Santos et al. (2016) reported the hydrothermal synthesis of ZSM-12 zeolite, which is impregnated onto TiO₂. The prepared TiO₂/ZSM-12 with 15% of titanium expressed 60% ODS for a model mixture of n-heptane with 5000 ppm thiophene in presence of H₂O₂, with acetonitrile as extractant, at 70 °C within 60 min and under atmospheric pressure.

Jiang et al. (2016) reported the preparation of a series of Brønsted acidic ionic liquids having a protonated amide- or lactam-based cation and investigated their activity as extractants and catalysts in extraction, combined with oxidative desulfurization (ECODS) of both model oil and diesel fuel, with H₂O₂ (30 wt%) as an oxidant. Each of them showed the obvious removal of BT and DBT in model oil. Among them, [HCPL][TFA] exhibited the best performance by completely removing BT and DBT in a short time, since the HCPL+ cation was verified to exist in an enol form which was supposed to contribute to high desulfurization performance by improving the formation of peroxides. [HCPL][TFA] decreased the S-content of real hydrogenated diesel and straight-run diesel fuel from 659.7 and 11,034 ppm to 8.62 and 89.36 ppm, respectively.

Heteropolyacids, particularly the Keggin-type phosphomolybdic acids (HPMo), exhibit high acidic strengths and have good potential in ODS. However, HPMo suffers from some drawbacks such as low reactivity due to its small surface area and its difficulty to be recycled. To overcome these problems, it is usually loaded on an appropriate carrier such as silica, alumina, carbon, titania, zirconia, or mesoporous silica (MS), which facilitates increasing active sites on the surface and enhances catalytic activity. Mesoporous silica (MS) has attracted a great industrial interest for its low-cost and higher surface area. Yao et al. (2016) reported the preparation of MS support by hydrothermal method, then Eu-HPMo/MS catalyst with a specific surface area and pore volume of 309.7 m²/g and 0.389 cm³/g, respectively, that was prepared by impregnation method. The activity of the prepared catalyst was tested for the ODS of model oil (320 ppmw DBT in n-octane) in the presence of H₂O₂. The desulfurization rate reached approximately 100%, in the presence of 0.1 g catalyst, 3 n(H₂O₂)/n(S), within a 60 min reaction time at 323 K. The HPMo leads to the conversion of DBT for its high acidic strengths. The rare earth metal Eu is one of the most available promoters for facilitating the storage and release of oxygen. The high surface areas of the support also promote ODS activity. The catalyst can also be used five successive times. The elucidated mechanism of the ODS, using the Eu-HPMo/MS catalyst (Scheme 2.6), revealed that the active peroxo-species are generated from the reaction of Mo with H₂O₂. Then, the oxidation of the sulfur atom of DBT occurs by the nucleophilic attack of the active peroxo-species to form sulfoxide when DBT reacted with H₂O₂ at the interface. The Mo=O was lengthened by Eu, which facilitated the storage and release of oxygen.

Huang et al. (2017) reported the preparation of expanded vermiculite (EVM) that was treated with acid (aEVM) and then incorporated into H₃PW₁₂O₄₀ (HPW) by an ordinary impregnation method. The prepared HPW/aEVM catalyst expressed high ODS activity, reaching approximately 100% of 100 ppm S-content model oil (mixture of Th, BT, DBT, and 4,6-DMDBT in n-octane), at optimum operating conditions of 333 K, 6 O/S, a catalyst dosage of 0.04 g/10 mL, and reaction time of 0.5 h. The prepared catalyst can be reused for seven cycles with a slight decrease in its catalytic activity of approximately 1.89%. The ODS activity followed the order DBT > 4,6-DMDBT > Th > BT, which is related to the electron density on S-atom and molecular size of the S-compound.

Rafiee et al. (2017) reported the immobilization of different polyoxometalates (POMs), including H₃PMo₁₂O₄₀ (PMo₁₂), H₅PMo₁₀V₂O₄₀ (PMo₁₀V₂), H₆PMo₉V₃O₄₀ (PMo₉V₃), H₇PMo₈V₄O₄₀ (PMo₈V₄), H₃PW₁₂O40 (PW), and H₄SiW₁₂O₄₀ (SiW) on carbon catalyst supports (CCS) prepared from naturally abundant potatoes which were used for the oxidative desulfurization ODS of model oils (DBT, BT and Th in 1/1 ethanol:n-heptane) in the presence of H₂O₂ at room temperature. The initial total S-concentration of the model oils varied between 250 and 1000 ppm. Moreover, the prepared catalysts were tested on real, light cycle oil with an initial S-content of 530 ppm. The natural potato, as a green and cheap source, provides a good support to design a composite nanorod structure via a simple hydrothermal method. Those green catalysts proved to be a unique, effective, and eco-friendly catalyst for selective oxidation sulfides using 30% aq. H₂O₂ as the co-oxidant in the ethanol:n-heptane solvent system, in model and real oil due to excellent yields, very short reaction time, room temperature reaction condition, and using air atmosphere, in addition to simple regeneration methods and reusability, and are all desirable advantages from the stand points of green sustainable chemistry. Moreover, the catalyst could be recovered and reused at least four times without a significant decrease in catalytic activity at an oxidation reaction with a negligible leaching of PMo₁₀V₂.

Guoxian et al. (2007) studied the ODS of model oil (DBT in n-octane) with hydrogen peroxide catalyzed by activated carbon-formic acid. The strong oxidizing agents and hydroxyl radicals generated from hydrogen peroxide can be produced on the surface of AC and the short life radicals produced during the surface reactions are likely to be resonance-stabilized on the carbon surfaces. This was attributed to the presence of Pt, Ag, and Pb metal ions such as Fe³⁺, Cr³⁺, Cu²⁺, and Ag+ and metal oxides such as MnO₂ and Fe₂O₃ in AC, which can catalyse hydrogen peroxide to produce free radicals with very high oxidation potential (OH^• and HO₂). This results in the oxidation of DBT to form SO₄^2–, where activated carbon (AC) can catalyse hydrogen peroxide to produce hydroxyl radicals when pH is less than 7.0 which, consequently, oxidizes the adsorbed DBT on AC. Further, AC can disperse in the oil phase and increase the collision probability of DBT with the active oxygen species, accelerating the oxidation reaction. Thus, the carbon-formic acid systems have good catalytic activities in the oxidative removal of DBT with hydrogen peroxide. However, the catalytic performances of the carbons differ; the wood carbons have better catalytic performances than coal carbon, where the DBT conversion reaches 91.6%.

In the presence of formic acid, the ODS was enhanced due to presence of both hydroxyl radicals and the performic acid in the reaction medium, thus the ODS of DBT reached 100% in presence of 12 mol/L formic acid within 30 min. Wang et al. (2017) also reported a single-stage reactor, in which the dehydrogenation of 2-propanol under alkaline conditions with in-situ production of H₂O₂, oxidation of thiophenes and adsorption of the produced sulfones from model fuel (250 ppm S) are demonstrated in one single step at mild temperature and under atmospheric pressure, using a palladium/activated carbon (Pd/C) catalyst (Scheme 2.7). It involves the dehydrogenation of 2-propanol, generation of an oxygen layer on the catalyst surface, stripping of the adsorbed acetone, releasing of H₂O₂ into the alkaline medium, and, finally, the ODS reaction of thiophenic compounds. The advantages of this process are the in-situ production of H₂O₂ and its rapid diffusion into the aqueous phase. The large specific surface area and pore volume of activated carbon guarantee the easy adsorption of the produced sulfones by simultaneous adsorption on activated carbon, eliminating the subsequent separation operation after the ODS-step. The accumulative sulfur on the activated carbon was also resolved by simple washing with anhydrous ethanol under mild operating conditions, without losing the catalytic adsorption efficiency of Pd/C. Thus, this system simplifies the ODS-process and decreases the overall operational cost.

One of the obstacles that faces the HDS-process, especially in catalytically cracked gasoline, is the high concentrations of olefins which could be hydrogenated, leading to a decrease in octane number (Song, 2003). Taking into consideration that olefin hydrocarbons can be produced not only by catalytic cracking, but also in the pyrolysis of shale oil, this will also cause difficulties during the desulfurization of oil fraction as well. One of the ways of coping with the problem of desulfurization of gasolines with increased olefin content can be preliminary reduction in the total sulfur content via hydrogen-free methods (Matsuzawa et al., 2002; Wan and Yen, 2007), for example by ODS without the use of acid catalysts as pretreatment step before an HDS-unit. This will make it possible to conduct further hydrotreating under more mild conditions without a notable loss in octane number.

Akopyan et al. (2015) studied the effect of olefins (e.g. cyclohexene) and ether (e.g. dibutyl ether) on the ODS of model oil (Methylphenyl sulfide (MPS) or benzothiophene (BT) in n-octane) with an initial S-content of 1770 and 1880 ppmw, respectively, using different catalysts, including sodium molybdate, sodium tungstate and formic acid, H₂O₂, and Dacamid surfactant. The presence of the olefin decreases the oxidation of sulfides in all the cases, but the presence of the ether increases the ODS. The ODS of MPS is higher than that of BT, but the inhibition effect of olefins on the ODS of BT is higher than in case of MPS. Upon the ODS of 40–185 °C fraction of catalytically cracked gasoline that contains methylthiophene, alkylthiophenes, dimethylthiophene, benzothiophene, methyl-, dimethyl, and ethyl-benzothiophene, and dimethyl sulfuide, approximately 20% of olefins and an initial S-content of 2500 ppmw, ODS was very effective using sodium molybdate at a low temperature of 20 °C and a long reaction time of 12 h. Since the oxidation reaction oxidation is less selective at a higher temperature and the rates of side reactions increase, the oxidant is inefficiently consumed for the oxidation of olefins to epoxides and alcohols. In conclusion of this study, it is preferable to use neutral catalysts based on transition metal salts for ODS rather than catalysts of acidic nature.

Another obstacle is during the processing of high-boiling petroleum fractions by catalytic cracking, catalyst deactivation occurs due to high carbon residue value, which mainly occurs due to the presence of PASHs such as benzothiophene, dibenzothiophene, and naphthothiophene derivatives (Song, 2003). Applying ODS as a pretreatment step for high-sulfur heavy petroleum fractions (Sharipov and Nigmatullin, 2005; Zongxuan et al., 2011; Javadli and de Klerk, 2012a,b) can promote the enhancement of the yield and quality of the products of catalytic cracking. Despite the low activity of peroxo complexes in the ODS of individual benzothiophenes, their use for ODS in biphasic technology is recommendable due to the use of low percentage hydrogen peroxide solutions, the ease of isolation of oxidation products from the oxidation system, and the possibility of reusing the metal compound without its regeneration. Hydrogen peroxide, as an oxidant, presents itself in the aqueous phase, the substances to be oxidized present in the organic phase, and the peroxo-complex, which is formed due to the reaction of a metal salt with hydrogen peroxide, is transferred by a surfactant from the aqueous phase to the organic phase where oxidation can take place.

Guseinova et al. (2012) reported the ODS of catalytically cracked gasolines produced from a blend of Baku crude oils with hydrogen peroxide in formic acid medium under phase-transfer catalysis conditions, using pyridine and acetonitrile as ligands, in the presence of heterogeneous catalysts containing ions of Group VI–VIII transition metals. For example, Mo and Co ions decrease the S-content in gasolines from 200 to 33–40 ppm. The best results were obtained upon the usage of the Mo/Al₂O₃ catalyst used in an acidic medium. The data obtained in that study show that a unit volume of feedstock requires the oxidizing phase to contain 0.0025 g of Mo/Al₂O₃ catalyst, 0.0025 mL of 1:1 H₂O₂/CO(NH₂)₂ complex, and 0.2 mL of formic acid which facilitates lowering the total sulfur content of the catalytically cracked gasoline from 200 to 33 ppm within 6–7 h.

Rakhmanov et al. (2013) studied the ODS of a wide cut catalytically cracked gasoline fractions with an initial boiling point of < 205 °C, at 50 °C, containing 30% H₂O₂ as an oxidant with different surfactants, including lauryldimethylbenzylammonium chloride or N,N-diethanolamide of lauric acid C₁₁H₂₁C(O)N(CH₂CH₂OH)₂ and crown ethers as catalysts, where the following transition metal salts were used: Na₂MoO₄, Na₂WO₄, VOSO₄, or [Cu(NH₃)₄]CO₃, within a 2 h reaction time in presence of ethanol/water (1:9). After completion of the oxidation, the treated fuel stream was subjected to distillation to collect the fraction with an initial boiling point (IBP) of 70 °C. The best desulfurization results were achieved with sodium molybdate and an ammonia complex of copper in the presence of lauryldimethylbenzylammonium chloride, whereas crown ethers had a low activity. The main gain of this study was that relatively small amounts of the used oxidant hydrogen peroxide are best (0.123–0.38 mL or 0.001–0.003 mmol) because the economic performance of the process, along with the desulfurization characteristics, will play an important role in the industrial implementation of the oxidation method. The addition of organic compounds, in particular nitrogen bases, during the preparation of peroxo-complexes enhances their activity in oxidation reactions by virtue of ligand insertion in the inner sphere of the complex. Lauryldimethylbenzylammonium chloride can act as a ligand of this kind for the enhancement of the process of ODS. The total sulfur content of the IBP–70 °C fraction before oxidation was 355 ppm. The ODS and subsequent adsorption treatment of the fraction on alumina reduced the total sulfur content to 16 ppm upon the application of two successive ODS steps: in the first step, 1.86 mg [Cu(NH₃)₄]CO_3, 0.123 mL 30% H₂O₂, 25% 6 µL NaOH were used and in the second step, 0.62 mg Na₂MoO₄·2H₂O, 0.38 mL 30% H₂O₂ were used. Both steps were conducted for 2 h at 50 °C in presence of and lauryldimethylbenzylammonium chloride as a surfactant. The desulfurization percentage reached 96% after the two successive ODS steps followed by subsequent adsorption treatment on the alumina.

Rakhmanov et al. (2014a) also studied the ODS of straight run, nonhydrotreated diesel fractions (boiling range 178–342 °C, with initial S-content of 7260 ppm), that contain benzothiophene, dibenzothiophene, their alkyl-substituted derivatives, and thioxanthene, using hydrogen peroxide in the presence of different transition metal compounds (Na₂MoO₄, Na₂WO₄, NaVO₃, WO₃, tungstic acid, and heteropoly tungstate/molybdate H₃PMo₆W₆O₄₀ (HPTM)) in a biphasic system followed by the extraction of the oxidation products with dimethylformamide (DMF). The ligand in the inner coordination sphere of the peroxo complexes can be water or a specially added organic compound. The addition of an organic compound, in particular a nitrogen base, during the preparation of a peroxo complex enhances the activity of the complex in oxidative reactions, owing the incorporation of the ligand into its inner sphere. It was found that the addition of pyridine as a ligand intensifies the ODS with hydrogen peroxide in the presence of three different salts of transition metals and benzyllauryldimethylammonium chloride. However, unlike the case of Mo compounds, the addition of pyridine to the reaction mixture containing a W or V compound reduced the ODS. This was attributed to the change associated with the higher stability of peroxo-complexes of these metals with pyridine ligands compared to the other peroxo-complexes in which water serves as a ligand. The tested tungsten compounds (tungstic acid in the form of W(VI) oxide monohydrate and mixed HPTM) exhibited the highest activity, reducing the total sulfur content of the diesel fraction almost four- and six- fold, respectively. This was attributed to the fact that the formed peroxo-complexes are more reactive than in the case of tungstic acid salts. The high temperature and long reaction time were not favourable due to the decomposition of H₂O₂. The highest ODS, of approximately 82%, was performed using heteropoly tungstate/molybdate at 50 °C, for 6 h, with a sulfur:hydrogen peroxide:metal:pyridine ratio = 1:5:0.07:0.02 (mol), with surfactant benzyllauryldimethylammonium chloride. Rakhmanov et al. (2014b) reported that complexes of azacrown ethers with NbCl₅ lead to a decrease in the total sulfur content in the model mixtures to 13% of the initial amount. The structure of the azacrown ether used has little effect on the extent of desulfurization of the model mixture. The diesel fuel was simulated using mixtures containing methyl phenyl sulfide, benzothiophene, dibenzothiophene, dibenzyl sulfide, and thianthrene (1.4% S-content) dissolved in a mixture of tri-, hexa-, and hepta-decanes or individual hydrocarbons under the action of hydrogen peroxide in the presence of compounds of NbCl₅ and various crown ethers, where these compounds are oxidized to respective sulfoxides and sulfones. The most efficient oxidation occurs at 80 °C within 8 h. After the reaction, the sulfur content in the model mixture decreased to 40%. This was attributed to the possible complexation between the azacrown ether and NbCl₂, but further increase in temperature was inappropriate due to the decomposition of hydrogen peroxide. In the function of the complexing ligand, azacrown ethers play the role of a surfactant which facilitates the transfer of the resulting peroxo-complex from the aqueous phase to the organic phase, which form a two-phase system that consequently enhances the ODS. The data of that study suggested that an increase in the number of nitrogen atoms in the macrocycle, in general, contributes to an increase in the degree of oxidation of the sulfide components of the model mixture and has little effect on the oxidation of benzothiophene and dibenzothiophene.

In an another study, Rakhmanov et al. (2016) reported the ODS of a model oil mixture (a non-hydrotreated vacuum gas oil from TAIF-NK and a commercial diesel fuel of the Euro-5 standard from a Lukoil filling station with a sulfur content below10 ppm) with an initial S-content of 6300 ppm. The technology of a two-phase system with a phase-transfer catalyst has been employed for the desulfurization, using H₂O₂ and formic acid. The use of DMF for extraction makes it possible to extract up to 40% sulfur compounds, while the extraction with water is inefficient because of the low solubility of the oxidation products (dibenzothiophene sulfones and naphthothiophene sulfones); the degree of desulfurization did not exceed 3% in the case of its use. The optimum reaction time is 6 h and the hydrogen peroxide:sulfur molar ratio is 4:1, which led to approximately 74% desulfurization efficiency, while upon three successive ODS steps, 90% of total sulfur was removed.

A reduction in reserves of conventional crude oil necessitates a search for new hydrocarbon sources, of which oil shales are of particular interest because their reserves are comparable with those of conventional oil, therefore the development of a technology for shale processing into synthetic crude oil is a demanding task. An important step of this processing is the purification of the resulting synthetic crude oil for the removal of sulfur compounds, as it adversely affects many processes of oil refining, as discussed in chapter one. Akopyan et al. (2016) studied the ODS of shale oil, obtained by thermal extraction of organic matter from shale rock, using an oxidative catalytic system composed of 50% hydrogen peroxide, a molybdenum salt, and acids of different natures. The model mixture used in that study was prepared by dissolving shale oil in hydrofined straight-run gasoline (fraction 40–160 °C) in a shale oil; the gasoline had a ratio mass of 1:10 under vigorous stirring for a day with an initial S-content of 1375 ppm. Then, after the ODS reaction, the produced stream was applied to extractive desulfurization by DMF. It has been shown that the application of this method, in combination with extraction of the oxidation products of organic sulfur compounds, makes it possible to remove up to 94% of total sulfur from synthetic oil. The presence of H₂SO₄ is better that H₃PO₄ due to the propensity of sulfuric acid to form a strong peracid, which is an efficient oxidant of sulfur compounds. The presence of molybdenum salt is important for the in-situ generated molybdenum peroxo-complex, which accelerates the direct oxidation of sulfur compounds and acts as a catalyst for the formation of the peracid. The application of formic, sulfuric, and trifluoroacetic acids showed the best results, where trifluoroacetic showed a reduction of the sulfur content in desulfurized oil by almost six times. The maximal degree of removal of sulfur compounds was achieved by the application of the oxidizing catalytic system consisting of sodium molybdate, trifluoroacetic acid, and hydrogen peroxide in a Mo : S : H₂O₂ : CF₃COOH molar ratio of 1 : 100 : 600 : 100 in a reaction carried out for two hours at a temperature of 60 °C. In this case, the residual content of sulfur in the oil was 89 ppm with a degree of sulfur removal at 94%.

Compared to hydrodesulfurization (HDS), oxidative desulfurization (ODS) has several advantages: (i) refractory sulfur compounds, such as alkylated dibenzothiophene derivatives, are easily oxidized under low operating temperature and pressure; (ii) there is no use of expensive hydrogen so the process is safer and can be applied in small and medium size refineries, isolated refineries, and those located away from hydrogen pipelines; (iii) ODS avoids aromatic and olefin saturation and, thus, a low octane number; (iv) the overall capital cost and requirements for an ODS unit is significantly less than the capital cost for deep hydrodesulfurization-units; (v) like BDS, some of the sulfones can be converted to surfactants which could be sold to the soap industry, offsetting some of the overall cost of the ODS process (Gore, 2001; Guo et al., 2011; Srivastava, 2012; Mužic and Setrić-Bionda, 2013).

However, the efficiency and economics of an ODS process is strongly dependent on the methods used for oxidizing sulfur compounds and, successively, the methods used for separating the sulfone and/or the sulfoxide derivatives from the oxidized fuels. Furthermore, the ODS process has some technological and economical problems. Solvent extraction of the sulfone-sulfoxide derivatives using polar solvents, such as γ-butyrolactone, n-methyl pyrrolidone (NMP), methanol, dimethylformamide (DMF), acetonitrile, dimethylsulfoxide (DMSO), sulfolane, and furfural are commonly used, which can be recovered and reused through a distillation process (Liu et al., 2008). However, one of the major drawbacks of the solvent extraction method is the appreciable solubility of hydrocarbon fuels in polar solvents which leads to significant losses of usable hydrocarbon fuel. Such a loss is completely unacceptable on a commercial basis. In addition, sulfone derivatives are polar compounds and form strong bonding with polar solvents and it is difficult to remove them from the solvents to a level below 10 ppmw. This causes a build-up of sulfone derivatives in the solvent during solvent recovery (Nanoti et al., 2009). Thus, ODS is a source of sulfonic waste which requires special treatment. Moreover, the homogeneous catalysts are difficult to separate from the reaction products and this limits their recycling. Furthermore, the preparation of new supported catalysts is the most desirable improvement of the ODS process.

On the other hand, the removal of sulfoxide- sulfone derivatives could be achieved using an adsorption technique using adsorbents such as silica gel, activated carbon, bauxite, clay, coke, alumina, silicalite (polymorph of silica), ZSM-5, zeolite β, zeolite x, and zeolite y, in addition to the mesoporous oxide-based materials. These have attracted much attention in recent years due to their large pore sizes and controlled pore size distribution which may be beneficial in allowing accessibility of large molecular size sulfone derivatives to surface-active sites. Stanislaus et al. (2010) reported that silica-alumina and silica gel are more effective for the adsorption of sulfones after the ODS of LGO than activated carbon, molecular sieves, γ-Al₂O_3, zeolites, and ZSM-5, where the S-content in LGO was decreased from 730 to < 10 ppm applying ODS, followed by ADS using SiO₂-γ-Al₂O₃ or SiO₂. Ma et. (2007) reported the ODS of a model jet fuel (412 ppmw S) and a real jet fuel (JP-8, 717 ppmw S) in a batch system in the presence of molecular oxygen with free Fe(III) nitrate, Fe(III) bromide (denoted Fe-Fe), and carbon supported catalysts (Fe-Fe/ACMB) where the thiophenic compounds in the fuel were converted to their corresponding sulfone and/or sulfoxide compounds at 25 °C under atmospheric pressure. The Fe–Fe/ACMB catalysts expressed higher ODS activity and higher S-adsorptive capacity, which was 4 times higher than that for ACMB and about 1.4 times higher than that for Fe–Fe alone. The alkyl benzothiophenes with more alkyl substituents recorded higher oxidation reactivity and the ODS reactivity of the S-compounds decreased in the order of 2-methylbenzothiophene > 5-methylbenzothiophene > benzothiophene > dibenzothiophene. But, in a real JP-8 fuel, alkylated thiophenic compound, 2,3-dimethylbenzothiophene, was found to be the most refractory sulfur compound oxidized, while among the trialkylated–BTs, 2,3,7-trimethyl benzothiopene was found to be the most refractory. This was attributed to both the electronic and steric effects of the methyl groups which play an important role in determining oxidation reactivity. The ODS enhanced the adsorption onto commercial activated carbons (ACC and ACMB). Thus, upon the ODS on real jet fuel (JP-8, 717), using Fe–Fe/ACMB, the S-content decreased from 717 ppm to 279 ppm. Then, in the second ADS step, the S-content decreased from 279 ppm to 126 ppm. The main advantage of that method is its occurrence in presence of O₂ at ambient conditions without using peroxides and aqueous solvents and without involving the biphasic oil–aqueous-solution system.

However, one of the major drawbacks of the adsorption technique is the amount of oil treated per unit weight of adsorbent is low (Babich and Moulijn, 2003).

Nowadays, different catalysts (Wang and Yu, 2013; Kadijani et al., 2014; Mjalli et al., 2014), photochemical, and ultrasound oxidative desulfurization (Wu and Ondruschka, 2010; Liu et al., 2014a; Wittanadecha et al., 2014) are under research and development (R&D) to improve reaction efficiency.

The introduction of regulations stipulating ultra-low sulfur (ULS) content in fuels caused peroxides, such as hydrogen peroxide (H₂O₂), to become the most used oxidizing agents (Joskić et al., 2014) and molecular oxygen (Murata et al., 2003). The use of molecular oxygen may be appealing to refineries that already have the infrastructure for an oxidation facility to prepare blown asphalt. Other oxidizing agents include in-situ formed per-acids, organic acids, phosphate acid and hetero-polyphosphate acids, Fe-tetra amido macrocyclic ligand (Fe-TAML), Fenton and Fenton-like compounds, as well as solid catalysts such as those based on titanium-silica (tungsten-vanadium-titania, W-V-TiO₂), solid bases such as magnesium and lanthanum metal oxides or hydro-talcite compounds, iron oxides and oxidizing catalysts based on monoliths, and tert-butyl-hydroperoxide with catalyst supports that can effectively oxidize organic sulfur compounds into sulfones with less residue formation (Abdul Jalil and Falah Hassan, 2012; Mužic and Sertić-Bionda, K. 2013). The presence of a sulfone extracting agents such as acetonitrile in the oxidation phase has been also reported to enhance the ODS process (Hulea et al., 2001; Yazu et al., 2003; Ramírez-Verduzco et al., 2004).

There are five processes based on the ODS technique used to reach ULS-fuels that have been reported to reach the commercialization stage.

In 1996, PetroStar Inc. developed a technique combining conversion and extractive desulfurization (Figure 2.11 CEDS) to remove sulfur from diesel fuel (Chapados et al., 2000). Briefly, the diesel fuel is oxidized by mixing with peroxyacetic acid (i.e. H₂O₂/acetic acid) at a low temperature, <100 °C, under atmospheric pressure. Then, liquid-liquid extraction takes place, producing low sulfur content diesel oil, followed by adsorption treatment to yield ultra-low sulfur diesel fuel. Recycling of extract solvent for re-use takes place and the concentrated extract is further processed to remove sulfur. In a bench scale pilot plant unit, the desulfurization of high S-content diesel oil (3500 ppm), applying the PetroStar-CEDS process, produced a product with <20 ppm S-content. Stanislaus et al. (2010) reported that further development is still required before the licensing and commercialization of the PetroStar-CEDS process.

Unipure Inc. and Texaco have jointly developed an ODS process (called ASR-2) based on peroxide (H₂O₂) oxidation in the presence of formic acid (HCO₂H) within a short residence time (5 minutes), mild temperature (120 °C), and at approximately atmospheric pressure where the oxidized fuel is first separated by a phase separator, washed and dried, and then passed over an alumina bed for the separation of sulfones by adsorption. The sulfone- loaded alumina bed can be regenerated by washing with methanol (Levy et al., 2002; Ito and van Veen, 2006). ASR-2 plants, solely, are estimated to produce ULS-fuels from feeds with an S-conent of 50–1500 ppm. This process can desulfurize diesel oil or gasoline with an S-content of 1500 ppm to ≤5 ppm sulfur, at a cost approximately 50% less than the cost of a hydrotreater with an improvement in cetane number and API gravity (Levy, 2003). Unipure is undergoing research to develop an extension to its ASR-2 process that can cost-effectively desulfurize oil feeds with a high S-content (>1%). A demonstration plant, based on the Unipure ASR-2 process which can oxidatively desulfurize diesel oil (500 ppm S-content) to <8 ppm S-content, is in operation at Valero Energy Corp’s Krotz Spring, LA refinery.

Unlike hydrogen peroxide, t-butyl-hydroperoxide (TBHP) is completely fuel soluble and would enhance the ODS process. Lyondell Chemical Technology, L.P. has developed ODS using t-butyl-hydroperoxide (TBHP) followed by extraction for sulfone separation (Karas et al., 2008). This single liquid system is beneficial for simple reactor engineering, enabling the application of a fixed bed column, under mild temperature and pressure. The ODS takes place in less than 10 min, with a near quantitative conversion of thiophenes to sulfones, with the production of t-butyl alcohol as a co-product that can be easily removed from the treated fuel. This process was reported to be in a continuous operation for over 5 months, producing diesel with <10 ppm S (Liotta and Han, 2003). However, the process is still not available for licensing (Stanislaus et al., 2010).

Uop Llc, in cooperation with EniChem S.P.A. (Gosling et al., 2007), has developed a process for the autocatalytic (i.e. without a catalyst) production of organic hydroperoxides and ultra-low sulfur (ULS) diesel boiling range hydrocarbons. The autocatalytic production of hydroperoxides is believed to be initiated in the presence of organic compounds and oxygen by the disassociation of oxygen to produce free radicals which then proceed to react with the organic compounds to produce sulfones. Therefore, no solid catalyst is used in the production of organic hydroperoxides. The ENI-UOP ODS process takes place through three steps: an oxidant supply section (i.e. organic peroxide), sulfur reaction section, and a sulfone separation section (Figure 2.12). The first step is a circulating reaction loop in which the diesel feed is mixed with air to produce a hydroperoxide-containing stream, in the presence of an organic initiator, at 130 °C and 70 atm, in the absence of a catalyst. An induction period may be required during the initial preparation of the organic hydroperoxide in order to achieve the desired concentration of the organic hydroperoxide in the reactor and the recycle stream. In this regard, induction periods may also be essentially eliminated by the addition of a small amount of hydroperoxide other than the hydroperoxide product expected. In this context, the added hydroperoxide is called an initiator. Hydroperoxides that may be suitable initiators are those which decompose under the reaction conditions quickly enough to reduce the induction period.

Examples of suitable initiators include cumene hydroperoxide and cyclohexybenzene hydroperoxide. Generally, hydroperoxide initiators are effective in amounts within a range of 0.2 - 1.5 wt.% of the fresh feedstock. The main advantage of this step is the in-situ production of the alkylhydroperoxide that would decrease the costs of storage, handling, and the use of conventional oxidants. In the second step, the partially oxidized stream, containing about 2000 ppm oxygen as peroxide, is further oxidized to sulfone at a low temperature (<200 °F) and pressure (<100 psig). This occurs in a fixed bed reactor in the presence of a heterogeneous catalyst. The main advantages of this step are a more than 98% conversion of organic S-compounds occurring and the usage of organic peroxide omitting the need to recycle the conventional corrosive organic acid catalysts upon the application of H₂O₂. Finally, in the third step, the separation of the polar sulfones by extraction or adsorption occurs. The adsorption technique is recommended more often as it is more cost-effective and removes any trace of undesirable by-products that might be formed during the oxidation and reaction steps.

The activation of H₂O₂ with polyoxometallates with a Keggin structure, such as H₃PM₁₂O₄₀ [where, M is Mo(VI) or W(VI)], produces more effective and selective oxidants as polyoxoperoxo complexes, such as PO₄[MO(µ-O₂)(O₂)₂]₄^3–, for the oxidation of nucleophiles, such as sulfides and sulfur compounds with less nucleophilicity, such as DBT, under mild conditions to sulfoxides or sulfones in high yields, but the slow rate and excessive decomposition of H₂O₂ withdraw its application on an industrial scale. The use of ultrasound can significantly improve the ODS efficiency under phase transfer conditions (Javadli and de Klerk, 2012b; Srivastava, 2012; Al-Degas et al., 2016). That would occur, by emulsification, throughout the improvement of the liquid-liquid interfacial area for viscous films containing gas-filled bubbles and cavitation bubbles. Thus, increases the interfacial area available, for the reaction, by increasing the effective local concentration of reactive species and enhancing the mass transfer in the interfacial region. Moreover, the ultrasonic irradiation can significantly improve the reaction efficiency, mainly due to cavitation when mechanical vibrations are produced and transmitted into the liquid as ultrasonic waves. This phenomenon involves the formation, growth, and implosive collapse of bubbles in liquids irradiated with high intensity ultrasound, creating shock waves, providing a unique set of conditions to promote chemical reactions and, thus, increasing the chemical reactivity in such systems. When the compression of bubbles occurs during cavitation, short-lived localized hot spots can be generated. Cavitation causes bubble collapses rapidly and violently, providing temperatures of about 5000 K and pressures of about 1000 atm, with heating and cooling rates above 1010 K/s. This microenvironment, with extreme local conditions, is favourable to create active intermediates allowing the reaction to proceed instantaneously. The ultrasound assisted ODS would take place in four successive steps (Scheme 2.8). In the first, the metal precursor is peroxidized and disaggregated to form an anionic peroxometal complex in excess of H₂O₂. In the second step, quaternary ammonium salts, such as Oc₄N⁺Br^- with large lipophilic cation function, act as phase transfer agents (PTA) transferring the peroxometal anion into the organic phase. The phase transfer agents are surface-active species that lower surface tension and permit easy formation of microbubbles under ultrasound. In the third step, the S-compounds, such as DBT, are efficiently and selectively oxidized by the peroxometal complex. Finally, in the fourth step, the reduced oxo-species, that would have been dissociated with PTA, returns to the aqueous phase to restore the catalytic cycle (Mei et al., 2003; Wan and Yen,2007).

Ultrasonic-assisted ODS of fuels has many advantages compared to HDS; it can be operated under atmospheric pressure and at relatively low temperatures. Furthermore, the advantage of ultrasonic-assisted ODS, rather than the conventional ODS, is the higher rate of the former technique that can lead to complete S-removal within a few minutes. It can also be performed without the addition of metallic catalysts (de A. Mello et al., 2009; Duarte at al., 2011; Hosseini, 2012). Ultrasonic-assisted catalytic ozonation combined with the extraction process exhibits high catalytic efficiency for the removal of dibenzothiophene from simulated diesel oil (Zhao and Wang, 2013). Palaić et al. (2015) reported that the ultrasonic-assisted ODS of real diesel fuel revealed a significant reduction in S-content in a shorter reaction time relative to the ODS in a mechanically stirred system where ultrasonic-assisted ODS of diesel fuel with 4000 mg/kg S-content, in a batch reactor, using H₂O₂ as an oxidant and acetic acid as a catalyst, and N,N-dimethylformamide as extractant at solvent/oil ratio of 1.0, produced diesel fuel with a total S-content of 3 mg/kg within 30 min of oxidation at 70 °C and under atmospheric conditions. Kadijani et al. (2016) reported the application of response surface methodology based on central composite face-centered designs of experiments to optimize and study the effect of ultrasonic-assisted ODS of gasoil using tungstophosphoric acid catalyst and tetraoctylammonium bromide as a phase transfer agent in the presence of hydrogen peroxide as an oxidant. The results revealed that a mass of catalyst, mass of PTA, and ultrasonic wave amplitude affected the sulfur conversion positively, while the volume of oxidant could raise the sulfur conversion until a special point. Afterwards, every rise in the oxidant volume led to a decline in sulfur conversion where a maximum sulfur conversion of 95.92% has been achieved in the presence of 21.96 mL of oxidant, 1 g of catalyst, and 0.1 g of phase transfer agent, followed by liquid-liquid extraction using a polar acetonitrile solvent. Khodaei et al. (2016a) also reported that the application of RSM based on the Box–Behnken design has been employed to optimize the ultrasound-assisted oxidative desulfurization of non-hydrotreated kerosene (2490 ppmw S-content) in the presence of formic acid and H₂O₂, where 95.46 % S-removal from kerosene has been achieved in a sonication time of 10.5 min under optimal oxidation conditions (15.02 oxidant-to-sulfur molar ratio (no/ns), 107.8 formic acid-to-sulfur molar ratio nacid/ns, and 7.6 W/mL ultrasound power/fuel oil volume) and 90% of kerosene recovery occurred after the liquid–liquid extraction of the oxidized stream with acetonitrile that occurred with a decrease in the aromatic content of the treated kerosene from 15.9 to 10.9 vol%. Thus, the naphthene and olefin contents of the treated kerosene are slightly increased. There was a negligible increase in the water content of the kerosene after ultrasonic-assisted ODS treatment. In another study, Khodaei et al. (2016b) applied RSM based on the Box–Behnken design to optimize the ultrasound-assisted-ODS of a model light fuel oil 500 ppmw S (that is, 2.50125 g DBT/L toluene) using H₂O₂–HCOOH as the oxidation system. More than 97% sulfur conversion was achieved under the optimum conditions (oxidant to sulfur molar ratio of 26.7, acid to sulfur molar ratio of 74.6, and ultrasound power/fuel oil volume of 7 W/cm³, at 50 °C within 80 s of sonication). The high conversion was attributed to the ultrasound irradiation which can enhance the overall oxidation rate due to an increase in the interphase mass transfer rate and cavitation, which creates a very fine emulsion between the aqueous and organic phases. The remarkably low reaction time emphasizes the application of this oxidation system in industrial applications. The ODS of DBT takes place in four successive steps (Scheme 2.9). Hydrogen peroxide (H₂O₂) reacts with the carbonyl group of formic acid. Then, the π bond between carbon and oxygen in the carbonyl group is broken and an unstable intermediate is formed (step 1), which is further decomposed to peroxyformic acid and water (step 2). DBT is oxidized to dibenzothiophene sulfoxide (DBTO) (step 3) and dibenzothiophene sulfone (DBTO₂) (step 4) and the formic acid is formed again in the aqueous phase.

SulphCo Inc. has developed ODS (Gunnerman, 2003) applying ultrasound power during sulfur oxidation by hydrogen peroxide, using tungsten phosphoric acid at 70 to 80 °C and atmospheric pressure, within a residence time of 1 min, where a desulfurization efficiency on the order of 80 and 90% occurred for crude oil and diesel oil, respectively (Dai et al., 2008). Upon applying the SulphCo technology for upgrading of crude oil, a 50% decrease in S-content occurred with an increase in the API gravity up to 3 points and a reduction in viscosity by approximately 15%, has been reported (SulphCo, 2009). SulphCo has successfully used a 5000 barrel/day mobile “sonocracking” unit to duplicate, on a commercial scale, its proprietary process that applies ultrasonics at relatively low temperatures and pressures (Wu and Ondruschka, 2010). Stanislaus et al. (2010) reported that by the preliminary estimation of Betchel Corp., the SulphCo unit would cost 50% of what an equivalent hydrotreater would cost.

The first ultrasonic desulfurization unit has been installed at the IPLOM petroleum refinery near Genoa in Italy where it showed continuous desulfurization of diesel fuel to <10 ppm at a rate of up to 350 bbl/day (Stanislaus et al., 2010).

Based on the same concept of acceleration of ODS reaction throughout the formation of cavitation, Suryawanshi et al. (2016) proposed a novel ODS process without the use of a catalyst (Figure 2.13). Briefly, the oil feed and aqueous phases (water) are mixed, under ambient temperature and pressure, then passed through a cavitating device (for example a vortex diode). The formed vapor cavities are then collapsed generating very high localized pressure (~ 1000 atm) and temperature (~ 10,000 K), as well as in-situ oxidizing species (hydroxyl radicals) which react with the S-moiety of thiophenic compounds in the organic phase. In the developed method, the removal of organic sulfur can be occurr by both mineralization, as well as oxidation, producing sulfones. But, due to the absence of acid catalysts, the latter mechanism may not be significant. Hydrodynamic cavitation generates hydroxyl radicals through the cleavage of water molecules to active oxidants, which would oxidize the S-compound to sulfones that would go to the aqueous phase. Alternatively, attacking the S-bond releases SO₂, HSO₃, H₂SO₄, and further mineralization of the organic skeleton to CO₂ and H₂O would also occur (Scheme 2.10). When applying this technique on commercial diesel oil with 30 ppm S-content, mainly of refractory S-compounds, a complete S-removal occurred.

The main advantages of this process are lower operational cost, significant ease of operation, and its effective deep desulfurization without using any catalyst, under ambient temperature and atmospheric pressure, at a low pressure drop of just 0.5 bar across the cavitating device. The aqueous phase can be recycled and reused after removing a purge stream (with corresponding make-up water). Hydrodynamic cavitation usually improves performance with scale-up. Thus, the proposed technique can be implemented effectively in large scale S-removal.

Recently, photo-oxidative desulfurization has attracted researchers. This occurs in a specially designed photoreactor, under mild reaction conditions (Javadli and de Klerk, 2012b) where the S-containing hydrocarbons are suspended in an aqueous-soluble solvent and then subjected to UV or visible irradiation, in presence of photo-catalysts such as TiO₂ (Tao et al., 2009), ferric oxide (Fe₂O₃) (Zaki et al., 2013), etc. The photochemical reaction can be assisted by a photosensitizer such as 9,10-dicyanoanthracene (DCA) (Yazu et al., 2001). The polar oxidized S-compounds are extracted directly into aqueous-soluble solvents such as acetonitrile (Ibrahim et al., 2003). Finally, the treated hydrocarbon feed and the solvent phases are separated as in the extractive desulfurization.

Thus, to increase the yield and enhance the economic efficiency, the aromatics should be extracted from the solvent by liquid-liquid extraction using paraffinic solvents to be re-blended into the desulfurized fuel stream. The recovery of the photosensitizer from the solvent should be also done, applying the adsorption technique by using, for example, silica gel as an adsorbent, then recovered by desorption using the aqueous solvent (i.e, the acetonitrile). However, Tao et al. (2009) reported the rate of photo-oxidative desulfurization of sulfuides in kerosene is 100 times greater than that of DBTs.

The photo-oxidative desulfurization pathway for BT, DBT, and their derivatives was elucidated by Shiraishi et al. (1999a) as follows: BT is converted, first, to benzothiophene-2,3-dione, followed by hydrolysis, loss of carbon monoxide, and oxidation of the sulfur atom to be converted, finally, to 2-sulfobenzoic acid. 3-Methyl BT and 2,3-dimethyl BT produce 2-sulfobenzoic acid, benzenesulfonic acid, and 2-acetylbenzenesulfonic acid. DBT is first photooxidized to DBT sulfoxide, which is then further oxidized to form DBT sulfone or dibenz[c,e][1,2]oxathiin-6-oxide. The latter is then oxidized and converted to 2-sulfobenzoic acid via dibenz[c,e][1,2] oxathiin-6,6-dioxide. Benzothiophene-2,3-dicarboxylic acid is likely to be formed directly by the oxidation of DBT by a singlet oxygen, generated by photosensitization with DBT sulfone, while 4-MDBT and 4,6-DMDBT were found to produce the corresponding sulfobenzoic acid, dicarboxylic acid, and sulfone.

The order of reactivity of recalcitrant sulfur compounds in photo-oxidative desulfurization is reported to be the opposite of HDS:

dibenzothiophene < 4-methyl dibenzothiophene < 4,6-dimethyl dibenzothiophene.

This represents a beneficial advantage for the application of photo-oxidative desulfurization (Hirai et al., 1996), but there are some problems that should be solved to make it technically and economically feasible for its application on an industrial scale, for instance the need for better solvents to increase S-compound solubility and aromatic rejection and appropriate commercial techniques for solvent recovery. Not only this, but the order of photo-oxidative desulfurization was found to be dependent on the catalyst used, thus, it is not universal. For example, Zhu et al. (2014) reported the photocatalytic desulfurization of different sulfur compounds in model oil to be decreased in the following order:

DBT > BT > dodecanethiol (RSH) > 4,6-DMDBT.

This is attributed to the steric hindrance and electron density around the S-atom since the S-conversion increases with the increase of the aromatic electron density. Although, the electron density on the S-atom records 5.758, 5.739, and 5.76 for DBT, BT, and 4,6-DMDBT, respectively. But, the photo-oxidation of 4,6-DMDBT was the lowest due to the predominance of the steric hindrance effect. That was also obvious in the case of RSH, where its long alkyl chain was an obstacle for the approach of its S-atom to the catalystic active sites.

Moreover, to increase the photo-transformation of S-compounds, a combination of a solvent and a photosensitizer should be optimized. Also, improvement of the separation processes and photosensitizer recovery is required. Babich and Moulijn (2003) reported that the stabilization of the photosensitizer on solid carriers would simplify the process and eliminate the process of photo-oxidant recovery from the fuels and the solvent.

Hirai et al. (1996) reported the desulfurization of DBT, 4-MDBT, and 4,6-DMDBT in one step under UV-irradiation (λ > 280 nm, to avoid the cleavage of C-C bond), using a high pressure mercury lamp in the presence of H₂O₂ where the sulfur was removed as sulfate in the water phase, under room temperature and atmospheric pressure. Moreover, the addition of 4-phenylbenzophenone is reported to enhance the photooxidation of S-compounds, in a light fuel, under UV-irradiation in the presence of 30% of H₂O₂ solution where the sulfur content of a light fuel was reduced from 0.2 wt.% to < 0.05 wt.% with a yield product of approximately 75% after 24 h of photo-irradiation (Hirai et al., 1997). The total sulfur content in a FCC gasoline was reported to decrease from 309 to 68 ppm applying liquid-liquid extraction with acetonitrile and photooxidation with ultraviolet light from a high-pressure mercury lamp, with a total yield 90–96%. FCC gas oil with an S-content of 1800 ppm was desulfurized by liquid-liquid extraction, using acetonitrile, followed by a photochemical reaction in the presence of an oxidizing agent, to reach 508 ppm-sulfur with a final yield of approximately 90–95% (Ibrahim et al., 2004).

To overcome the drawbacks of titania as a photo-catalyst and increase its photo-oxidative efficiency, titania coating of a multi wall carbon nano tube (MWCNT) by sol-gel method has been used to improve the photo-catalytic removal efficiency of DBT from an n-hexane solution using a 9 W UV lamp (Barmala et al., 2015). Since carbon nanotubes (CNT) have chemical stability, high specific surface areas, and high electrical conductivity, they have electrical conductivity similar to that of copper, which makes them suitable electron acceptors when combined with semiconductors like titatnia. Moreover, the multi wall carbon nano tubes (MWCNTs) decrease the crystallite size of composite recombination chance. However, CNTs are black and a large amount in a CNT/semiconductor composite would disperse light and, consequently, prevent it from reaching the bulk solution, preventing light absorption by the solution and decreasing the efficiency of the process. Thus, it is mandatory to reach for an optimal amount of CNTs in a structure. By contrast, a low dosage of MWCNT causes recombination of the electron holes which also decreases the DBT removal rate. Furthermore, large amounts of catalyst in a solution also decreases the photo-catalytic efficiency for the same reason. It has been noticed, from that study, that by decreasing the MWCNT content in a composite, the ability of a composite to adsorb DBT decreases due to the reduction of specific surface area. However, increasing the MWCNT content in the composite decreases the time required to reach equilibrium. This, again, is due to increased surface area and adsorption ability. In conclusion, the DBT removal rate versus MWCNT content is found to follow a bimodal pattern related to the simultaneous influence of DBT adsorption on the catalyst surface and oxidation by the electron hole. The DBT molecules are easily adsorbed onto the composite, come into contact with the TiO₂ molecules, and are oxidized. As the MWCNT content increases, molecules adsorbed onto the surface are no longer in good contact with TiO₂ and photo-catalytic ability decreases. Thus, the two factors, adsorption and oxidation, influence the photo-catalytic process so the optimum MWCNT contents in the composite are found to be 0.25 g and 0.75 g MWCNT per 80 mL of sol. The rate of DBT removal was doubled in the presence of titania/MWCNT composite, relative to that in presence of titania alone.

Desulfurization of oil feeds, using liquid-liquid extraction by ionic liquids, cannot achieve the required ULS-fuels (Al-Degas et al., 2016). However, ionic liquid ([Bmim]PF6) was reported to be used as the extractant and photochemical reaction medium to promote the oxidation of dibenzothiophene (DBT) in the presence of H₂O₂ where 99.5% removal of DBT from n-octane was achieved under mild conditions of room temperature and atmospheric pressure and the ionic liquid [Bmim]PF₆ was recycled eight times with a slight decrease in desulfurization efficiency (Zhao et al., 2008). Zhu et al. (2014) reported that the desulfurization of DBT-model oil with amorphous TiO₂ was superior to that with anatase TiO₂ and anatase – rutile TiO₂, recording 96.6, 23.6, and 18.2%, respectively, in presence of H₂O₂ and the ionic liquid [Bmim]BF₄. Abid (2015) reported the photo-oxidative desulfurization of DBT in a Y-shape catalytic microreactor using solar incident energy, TiO_2, and an H₂O₂ solution. It was also reported that the Cu-Fe co-catalyst was able to enhance the photo-activity of anatase TiO₂ under visible light illumination, where the photo-desulfurization of 100 ppm DBT, in model oil, in the presence of H₂O₂ reached 82.36 %.

Although, several studies proved the high selectivity to remove sulfur from different oil feeds, such as light oils, catalytic-cracked gasoline, and vacuum gas oils (Shiraishi et al., 1998; 1999a,b; Ibrahim et al., 2003; Zhao et al., 2008). However, this technique is rather far from being applied on an industrial scale.

In conclusion, the efficiency and economics of the ODS process is strongly dependent on the methods used for oxidizing the sulfur compounds and, successively, the methods used for separating the sulfoxide and sulfone derivatives from the oxidized fuels. In addition, the key to successful implementation of ODS technology in most refinery applications is to effectively integrate the ODS unit with the existing diesel hydrotreating unit in a revamp situation.

In recent years, ODS has gained importance and is regarded as an excellent option after the HDS process since the Cx-DBTs derivatives are easily oxidized under low temperature and pressure conditions to form the corresponding sulfoxide derivatives and sulfone derivatives. While sulfur compounds, such as disulfuide derivatives, are easy to be hydrodesulfurized, they are oxidized slowly. For this reason, ODS can be utilized as a second stage after existing HDS units, taking a low sulfur diesel (500 ppm) down to ultra-low sulfur diesel ULSD (<10 ppm) levels. Thus, this would lower the consumption of expensive hydrogen.

British Petroleum used hydrogen peroxide and phosphotungstic acid as a catalyst and tetraoctylammoniumbromide as the phase transfer agent in a mixture of water and toluene and applied this oxidative desulfurization process after the hydrodesulfurization process (Figure 2.14) (Collins et al., 1997).

2.4 Selective Adsorption

Desulfurization by adsorption is a green technology wherein sulfur compounds are selectively removed through adsorption on the solid adsorbent leaving behind sulfur free fuel.

Adsorptive desulfurization (ADS) is considered as a promising approach to produce fuel cell grade gasoline and diesel at both a relatively low temperature and pressure, without using hydrogen gas which is advantageous compared to the conventional HDS method that uses both high temperature and hydrogen pressure. Moreover, it is characterized by easy operation, low cost, no pollution, and leads to deep desulfurization. The sulfur compounds can be removed from commercial fuels either via reactive adsorption by chemisorption, such as π-complexation, i.e. activated adsorption through interaction between adsorbent and adsorbate, or physical adsorption, such as van der Waals and electrostatic interactions, i.e. adsorption through intermolecular forces of attraction between molecules of the adsorbent and the adsorbate (Yang et al., 2014). Physisorbed sulfur compounds can be easily removed from adsorbents by heating or decreasing pressure (Hernández-Maldonado and Yang, 2004a), so it is easy to regenerate the adsorbents. The zeolite-based adsorbents have been used as promising materials for selectively removing sulfur derivatives from diesel. Metal ion exchanged Y zeolites showed high selectivity and capacity for sulfur compounds using π-complexation between metal ion and sulfur compounds (Hernández-Maldonado and Yang, 2004b). The selectivity of the zeolite-based adsorbents varies according to fuel composition, such as aromatic and moisture concentrations (Bhandari et al., 2006). The π–complexation adsorption is the most promising ADS, as it does not suffer from the steric hindrance that would occur by high molecular weight PASHs, such as 4,6-DMDBT (Hernández-Maldonado and Yang, 2004c). Molecular orbital (MO) calculations have shown that the π-complexation bonds between Cu or Ag and thiophene are stronger than those with benzene. Thus, π-complexation sorbents are selective for sulfur removal from transportation fuels (Yang et al., 2001; Yang, 2003; Hernández-Maldonado and Yang, 2003a,b). The MO-calculations and experiments have shown that the refractory compounds (MDBT and DMDBT) bind strongly through π-complexation due to a better electron donation/back-donation ability. Within the π–complexation mechanism, the cations can form the usual σ bonds with their s-orbitals while their d-orbitals can back-donate electron density to the antibonding π–orbitals of the sulfur rings. Methyl groups should not have much effect on π-complexation because they would enhance adsorption through increased polarizabilities (hence, van der Waals interactions). Although the π-complexation bonds are stronger than those formed by van der Waals interactions, they are also weak enough to be broken by traditional engineering means, such as increasing temperature and/or decreasing pressure (King, 1987). This increases the opportunities for tailoring and developing new adsorbents for selective fuel desulfurization processes. The metals that can form strong π–complexation bonding are those that possess empty s-orbitals and available electron densities at the d-orbitals necessary for back donation, for example, the copper Cu(I) electronic configuration 1s²2s²2p⁶3s²3p⁶3d¹⁰4s⁰ (Hernández-Maldonado and Yang, 2004a,b,c; Baeza et al., 2008).

Adsorbents, used industrially, are generally synthetic micro porous solids such as: (i) activated carbon, (ii) molecular sieve carbon, (iii) activated alumina, (iv) silica gel, (v) zeolite derivatives, (v) metal oxides, and (vi) clay. They are usually agglomerated with binders in the form of beads, extrudates, and pellets of a size consistent with the application that is considered (Song and Ma, 2003). The efficiency of adsorptive desulfurization (ADS) is mainly influenced by the adsorbents’ properties, such as capacity, selectivity, stability, and ability to regenerate (Babich and Moulijn, 2003). Moreover, it is highly affected by its specific surface area which preferentially ranges between 200 to 2000 m²/g (Alavi and Hashemi, 2014). The good potential of activated manganese zinc oxides in the ADS of crude oil was reported (Adekanmi and Folorunsho, 2012). Activated carbon, carbon fibers, and carbon nanotube can be used in ADS processes where the adsorption would be physical and depended on the size and volume of the pores or chemical that depends on the chemisorption surface properties (Bagreev et al., 2004; Aparicio et al., 2013; Gawande and Kaware, 2016).

Mužic et al. (2010) reported a comparative study for ADS diesel fuel with activated carbon (AC) and activated aluminium oxide as adsorbents in a LAM A1 batch adsorption apparatus at 50°C under atmospheric pressure. A 2³ factorial design was applied to optimize and study the effect of three experimental parameters: time (t, min), initial sulfur concentration (C_o, mg/kg), and mass of activated carbon (m_AC, g). The ADS of AC was higher than that of activated aluminium oxide, where the maximum ADS efficiency of approximately 52.5% was obtained within 100 min, 16 mg/kg initial sulfur concentration, and 4 g AC/50 cm³ diesel fuel. Mužic et al. (2010) reported in another study the ADS of diesel fuel, applying two Design of Experiments (DOE) methods. The experiments were carried out in a batch adsorption system using Chemviron Carbon SOLCARB^TMC3 activated carbon as the adsorbent. The first DOE method employed was a full factorial with three factors on two levels and five center points and the second was a Box-Behneken design with the same three factors, but on three levels. Response surface methodology (RSM) was applied to estimate the effects of the individual factors of time (t, min), initial sulfur concentration (C_o, mg/kg), and mass of activated carbon (m_AC, g), their interactions on sulfur concentration and sorption capacity were determined, and statistical models of the process were developed. The effects of time and adsorbent mass on output sulfur concentration recorded have very little effect at a lower level of initial S-concentrations, while at high initial S-concentrations they recorded a greater effect.

Desulfurization of a commercial diesel fuel by different adsorbents was studied in a fixed-bed adsorber operated at ambient temperature and pressure (Hernández-Maldonado and Yang, 2004c). Copper (auto-reduced) type-Y zeolites are found to be superior adsorbents for the removal of all sulfur compounds from commercial diesel fuels where the ADS capacity of the tested adsorbents decreased in the following order: AC/Cu(I)-Y > Cu(I)-Y > Selexsorb CDX (alumina) > CuCl/γ-Al₂O₃ > activated carbon > Cu(I)-ZSM-5. The GC/FPD analysis proved that the π-complexation sorbents selectively adsorbed highly substituted thiophenes, benzothiophenes, and dibenzothiophenes from diesel, which is not possible with conventional HDS reactors. The AC/Cu(I)-Y can produce 30 cm³/g of diesel fuel with a total sulfur content of 150 ppbw and 20 cm³/g of diesel fuel with a sulfur content of equal or less than 60 ppbw. This can be applied to a fuel cell which needs ULS-diesel oil. Furthermore, although the AC/Cu(I)-Y has selectivity towards the refractory S-compounds, the Cu(I)-ZSM-5 zeolite is promising for applications when selectivity toward small thiophene molecules is preferred. The Cu supported on zirconia is also reported to be a prominent adsorbent in the desulfurization of sulfur organic compounds. The adsorption capacity of Cu/ZrO₂ system would increase by increasing the surface area of the zirconia through the use of a lower calcination temperature (Baeza et al., 2012).

Commercial diesel fuel liquid desulfurization, via adsorption under mild conditions, using commercial high specific surface area activated carbon (AC) was performed on both laboratory- and pilot-scale experiments in fixed-bed setups. Under laboratory-scale conditions, maximum sulfur removal exceeded 90%, while according to breakthrough curves the total sulfur content remained below 2 ppmw for up to 20–22 mL of processed diesel/g sorbent. Process scaling-up by a factor of 15 showed a moderately negative effect, with the respective breakthrough fuel amount (total sulfur ≤2 ppmw) being ~15–17 mL processed fuel/g sorbent. Several sorbent regeneration strategies were studied under laboratory-scale conditions. The one with the highest restoration of initial (i.e., fresh state) AC performance involved heating under a vacuum (200 mbarA) up to 200 °C and subsequent washing of the material with a binary organic solvent. The amount of solvent required was 50–55 mL/g sorbent where, from the second and up to the seventh cycle, desulfurization efficiency of the material was essentially stable, but from the eighth cycle and on, further performance depletion in desulfurization efficiency was recorded. Based on fresh/regenerated sorbent post-analysis, it was found that cycle-to-cycle degradation was due to a gradual decrease of the sorbent’s surface area, mainly attributed to residual amounts of diesel-derived species remaining in its structure, thereby partially blocking its porosity. The most important advantage of this process is that the main properties of processed fuel remained essentially unaffected, although the removal of di- and polyaromatic compounds was notable (Baltzopoulou et al., 2015). The ADS on activated carbons, natural clays, and other adsorbents prepared from natural sources will be discussed in detail in Chapter 5.

In an attempt to remove the recalcitrant and bulky PASHs from diesel fuel, Ganiyu et al. (2017) reported the modification of commercial activated carbon with different concentrations (0.5–10 wt.%) of boric acid to produce adsorbents with good physico-chemical properties and tested its activity for the ADS of model oil (4,6-DMDBT in iso-ocatne) at room temperature. Boron-doped adsorbents exhibited better adsorption capacity at low boron loadings of 0.5–2.5 wt.%, largely due to their preserved surface area compared to those with higher boron loadings (5–10 wt.%) and enhanced surface acidity compared to the unmodified AC. Since doping AC with metal improved the surface acidity, but at the same time reduced the surface area, which is as also an important property, it would have to be large enough to address the mass transport of a bulky adsorbate like 4,6-DMDBT across the pores of the adsorbent. The maximum ADS capacity of (8.50 mg/g), that was approximately 85% removal from 100 ppmw-S containing 4,6-DMDBT, was achieved using an adsorbent with 1 wt.% of boron loading (1 BDAC), within 5 min contact time. The prepared adsorbent 1BDAC expressed a great selectivity for 4,6-DMDBT in the presence of naphthalene. This was attributed to the ability of 4,6-DMDBT to be preferentially adsorbed on the active site of modified adsorbent through acid-base interaction resulting from the contribution of an oxygen containing functional group of AC and sulfur in 4,6-DMDBT, as well the boron-sulfur interaction, relevant to naphthalene (Yang et al., 2007; Zhou et al., 2009). A remarkable regeneration performance was exhibited by the 1BDAC adsorbent with only 7% loss in ADS capacity after five successive regenerations cycles.

For the use of wastes and a decreased cost of the activated carbon, waste rubber tires can be recycled to activated carbon via heating at 300 °C to isolate the produced oil, distilled diesel oil, and black tire crude oil, followed by carbonization in a muffle furnace at 500 °C for five hours to remove the ash as well as the carbon black. The produced char is then treated by an H₂O₂ solution to oxidize adhering organic impurities and, finally, calcined at 900 °C for five hours yielding a 473 m²/g surface area AC, with a pore volume of 0.77 cm³/g, and pore size of about 6 nm (Saleh and Danmaliki, 2016a,b). Upon its application for the ADS of model oil (150 ppm DBT in 85:15 n-hexane:toluene) in a fixed bed column, the best results were obtained at higher column length and dosage because the longer the column, the better the interaction between the PASHs and the adsorbents, while the higher the dosage the more active sites will be present for ADS. On the contrary, the lower the concentration and flow rates, the better the ADS% since the adsorbent can only take the maximum S-compounds at any given time before reaching saturation. Moreover, the lower the flow rate, the better the interaction of the model fuel with the adsorbent. Contact time did not show any significant effect and the ADS of the S-compounds on the AC was fast, reaching equilibrium within 5–10 min. Danmaliki and Saleh (2017) studied the effect of metal loading on AC for the ADS of recalcitrant S-compounds in both batch and fixed bed models. A model diesel fuel was formed with a mixture of 50 ppm thiophene, 50 ppm BT, and 50 ppm DBT in a solvent of 85% hexane and 15% toluene where the produced AC from waste tire was acid treated with HNO₃ and then loaded with10% cerium, iron, or cerium and iron to produce bimetallic composites by thermal co-precipitation method. The three composites named prepared namely AC/Ce, AC/Fe, and AC/Ce/Fe. Although, the AC had the highest surface area and pore volume of 460.27 m²/g and 0.71 cm³/g, respectively, and the highest surface oxygen-containing groups, it performed the least ADS efficacy. Moreover, although the AC/Ce/Fe recorded the least surface area and pore volume of 430.44 m²/g and 0.64 cm³/g, it performed the best in the adsorption of thiophene (31%), BT (30%), and DBT (75%). This was attributed to the changes in the chemical composition of the adsorbent, the acidic nature of cerium, and the crystal nature of iron. The ADS efficiency was in the following decreasing order: AC/Ce/Fe > AC/Ce > AC/Fe > AC. This confirmed the hypothesis of bimetallic modification of carbon’s surface enhancing its selectivity and adsorptive capacity to refractory sulfur compounds. The adsorption of sulfur compounds and the breakthrough in all the adsorbents followed the order: DBT > BT > Th. Batch and column experiments carried out using AC/Ce/Fe revealed a high absorptive capacity and breakthrough for a DBT of 16 mg/g. The kinetic and isothermic analyses showed a synergistic effect of surface adsorption and intraparticle diffusion occurring concurrently and chemisorption and heterogeneous adsorption occurring on the surface. Thermal regeneration experiments carried out on AC/Ce/Fe showed stable efficiency in the adsorptive desulfurization after three regeneration cycles. In another study by Saleh et al. (2017), activated carbon prepared from waste rubber tires was successfully treated with manganese oxide to further improve their surface properties for ADS. The activated carbon-manganese oxide nanocomposite containing 10% optimum metal loading, of 160.98 m²/g specific surface area, total pore volume of 0.141 cm³/g, and average pore diameter of 9.27 nm, showed significant ADS efficiency within short time process for model oil (mixture of TH, BT and DBT in 85:15 n-hexane:toluene, with initial S-concentration of approximately 150 ppm). Adsorption capacities of 4.5 mg/g, 5.7 mg/g, and 11.4 mg/g were obtained for simultaneous adsorption of Th, BT, and DBT on the as-synthesized adsorbent in a batch process and the same trend was retained in the fixed bed model. The adsorbents performed excellently in the removal of DBT probably because of their structure containing more π-systems that provide more possible interactions, such as π-interaction with the active sites on the surfaces. Thus, dispersive interactions between the delocalized π-electrons, within the benzene rings of DBT and the electron-rich region on the nano-porous carbon aromatic ring, play a significant role in adsorption. In addition to the above mechanism, a feature common for the three S-compounds is the possible coordination with the metal species through S–M interactions. Several coordination configurations can be formed between the sulfur compounds and the metal including the n-type donor donating lone pairs of electrons that lie in the plane of the ring to the nanoparticles and the interaction between S and the oxygen atom on metal oxide nanoparticles.

Ni phosphides, supported on high surface area silica, have been reported for the ADS of diesel fuel to ultra-low-S-levels (Landau et al., 2009). The zeolites are crystalline aluminosilicate materials, composed of tetrahedral units of SiO₄ and AlO₄ They have a stable and regular three-dimensional crystalline framework. Natural zeolites are reported to express high ADS capacity for OSCs from diesel oil, that reached 7.15 mg/g, with the liberation of H₂S (Scheme 2.11) (Mustafa et al., 2010). Cheng et al. (2009) reported the deep desulfurization of FCC gasoline by selective absorption on various ion-exchanged nano-sized Y zeolites where the metal ion, as well as calcination conditions, have been found to significantly influence the ADS performance where the highest S absorption capacity was obtained at 100 °C on a HCuCeY sample calcined at 450 °C for 6 h in N₂ atmosphere. This was attributed to the interaction between Ce and Cu. Zeolite derivatives are efficiently used as adsorbents for selective adsorption of polar compounds and heterogenic organic compounds; they are hydrophilic and have large voids inside their structures (Mužic et al., 2010). The ADS of gasoline with Ni-loaded Y type zeolites was found to decrease with an increase in temperature from 25 to 60 °C, where the adsorption capacity of the adsorbent decreased from 0.55 to 0.65 mg S/g adsorbent that was attributed to the exothermic nature of the process (Majid and Seyedeyn-Azad, 2010). The modified Y-type zeolite was popularly used as an adsorbent to remove sulfur from fuels via π-complexation, and copper (Cu⁺) and silver-exchanged Y-type zeolites were effective to remove sulfur compounds from naphtha (Yang et al., 2001; Takahashi et al., 2002; Hernández-Maldonado and Yang, 2004a,b). Gallium-modified Y-zeolites are also reported for selective ADS of 4,6-DMDBT (Tang et al., 2008).

Numerous oxides and hydroxides of Pb, Hg(II), and Ba have been found to efficiently perform reactive ADS with thiols producing solid metal thiolates (Nehlsen et al., 2003). Shakirullah et al. (2009) studied the ADS-activity of different metal oxides, such as MnO₂, PbO₂, arsenic trioxide (ArO₃), Al₂O₃, MgO₂, ZnO₂ and silica on Jhal Magsi crude oil (collected from the Oil and Gas Development Corporation, Limited (OGDCL), Pakistan) and its distillate fractions, kerosene and diesel oil. Lead oxide (PbO₂) expressed the highest efficiency at 60.28%, 54.54%, and 52.57%, in the case of crude oil, diesel oil, and kerosene within 1 h, respectively. While the manganese oxide (MnO₂) performed maximum desulfurization activity of 58.55%, 49.38%, and 53.30% in the case of crude oil, diesel oil, and kerosene occurred within a longer time of 3 h. The reactivity of the metals with thiols depends on the metals’ heat reaction, where a metal with small exothermic heat formation is the most reactive (Salem and Hamid, 1997). Since lead has low heat formation (ΔHrexn –121.4 kJ/mol), it expressed the fastest and maximum reactive-ADS. But, when the heat formation of metal thiolates is high (≥ –60 kJ/mol), the reaction rate is slow and, consequently, the reactive-ADS needs more time, taking into consideration that metal thiolates have relatively low melting points. Therefore, a descending desulfurization activity may occur with longer reaction time due to the decomposition of the formed metal thiolates formed. Kerosene has a high concentration of low molecular weight thiols and expressed higher desulfurization than that of diesel oil, throughout the faster reaction of thiols with the studied metals that was characterized by the high heat formation. Although PbO₂ expressed better performance than MnO₂, the use of MnO₂ is preferred over lead oxide since lead can be retained in the hydrocarbon stream which may contribute to engine fouling, in addition to environmental pollution in the form of lead particulates. These drawbacks will not occur with the application of MnO₂. This process is simple, employing inexpensive material and completes with minimum energy requirements compared to conventional HDS. Thiols react with metal ions, producing forming metal thiolates and metal oxides can be recovered by dilute-acid extraction. The choice of acid depends upon the desired metal salt (Scheme 2.12).

Adeyi et al. (2015) reported a 53% reduction from the initial S-content (0.175 wt.%) of diesel oil in an ADS batch process using activated manganese dioxide as the adsorbent.

McKinley and Angelici (2003) reported the selective removal of DBT and 4,6-DMDBT from simulated feedstock with Ag⁺/SBA-15 and Ag⁺/SiO₂ as adsorbents. The selective adsorption process for removing sulfur (SARS) at ambient temperature to achieve ultra-clean diesel and gasoline with an Ni-exchanged Y zeolite was reported (Velu et al., 2005). Adsorbents are usually comprised of transition metals supported on base oxides, such as Ni/ZnO and Ni/Al₂O₃–SiO₂, where the Ni acts as the ADS-site (Scheme 2.13). The selective adsorption of various compounds on the nickel-based sorbent at room temperature is reported to increase in the following order: Naphthalene ≈ 1-methylnaphthalene < 4,6-DMDBT < DBT < quinoline < indole, while that of activated alumina follows: naphthalene ≈ 1-methylnaphthalene < 4,6-DMDBT ≈ DBT < indole < quinoline, while that of activated carbon follows: Naphthalene < 1-methylnaphthalene < DBT < 4,6-DMDBT < quinoline < indole (Kim et al., 2006).

Song (2003) reported a process where the selective adsorption for the removal of sulfur compounds (SARS) is carried out and then followed by the HDS of concentrated sulfur (HDSCS) compounds (Figure 2.15) using high-activity catalysts, such as CoMo/MCM-41. Such a technique is much easier than the conventional HDS of diesel streams for two reasons: (1) it is more concentrated and reactor utilization is more efficient and (2) the rate of the HDS reaction is faster related to the removal of aromatics that inhibit the HDS by competitive adsorption in the hydrogenation sites (Ma et al., 2002). Spontaneous monolayer dispersion was used to prepare MoO₃–MCM 41 and its ADS capacity has been tested at room temperature on gasoline with an initial S-content of 300 mg/kg (Shao et al., 2012).

Moreover, acid activated alumina has more selective adsorption towards BT, DBT, and 4,6-DMDBT than zirconia and magnesia (Larrubia et al., 2002; Srivastava and Srivastava, 2009). However, the steric hindrance expressed by the large molecular weight 4,6-DMDBT limited its ADS.

Removal of sulfur compounds can take place at elevated temperatures under a hydrogen atmosphere without the hydrogenation of aromatics, where the reactive ADS takes place under the presence of hydrogen in order to accelerate the reaction between the sulfur compounds and the adsorbing agent, where the S converts to H₂S and is subsequently adsorbed by the adsorbent, but less hydrogen is consumed than is the case of HDS processes. Research and development work has focused on different methods including fluidized bed, moving bed, and slurry for conveying the adsorbing agent through the reaction column and regeneration column. Materials that contain a combination of transition metal catalysts and sorbents are usually used for the reactive adsorption of S-compounds, for example, Cu-ZnO, Ni-Al₂O₃, Ni-ZnO, Ni/Al-SiO₂, NiP/SiO₂, Ni-SiO₂, Ni-SBA-15 (Babich and Moulijn, 2003; Stanislaus et al., 2010).

Park et al. (2008) reported the reactive ADS of diesel oil by NiNPs supported on mesoporous silica materials (SBA-15 and KIT-6). The S-content decreased from 240 ppm to <10 ppm using 30% Ni/SBA-15 with an adsorption value of 1.7 mg/g.

In the USA, Conoco-Phillips Petroleum Company has developed the S-Zorb SRT process for the reactive-ADS of diesel oil. This process is based on fluidized bed technology, where it can decrease sulfur in feedstock containing greater than 2 mg/mL of sulfur to a very low level less than 0.005 mg/mL in the presence of hydrogen within an operating temperature and pressure of 300 to 400 °C and 1.9 to 3.45 MPa, respectively, where the sulfur compounds adsorbed on the adsorbent reacted with reduced metals, producing metal sulfides and newly formed hydrocarbon that would be released into the main stream. The spent adsorbent is continuously removed from the reactor and transported into a regeneration chamber. The sulfur is removed from the surface of the adsorbent by burning and the formed sulfur dioxide is sent to the sulfur plant. The adsorbent is then reduced with hydrogen and recycled back to the reactor (Hernández-Maldonado and Yang, 2004a; Mužic and Sertić-Bionda, 2013). H₂S is not released and the critical diesel fuel properties are unaffected. The process consumes relatively low H₂ and produces relatively lower CO₂ and NO_x. This process is estimated to decrease the overall operating and capital costs upon reaching the required product quality. A 6000 Barrels Per Stream Day (BPSD) gasoline unit has been in operation at the ConocoPhillips Borger refinery since April 2001.

In a study performed by Hagiwara and Echizen (Hagiwara and Echizen, 2001) for evaluation of different applied FCC gasoline desulfurization processes, including hydrodesulfurization processes Octgain 125, Octgain 220, and Scanfining from Exxon-Mobil, Prime G from IFP, CDHydro, and CD-HDS from CDTech, and S-Zorb from Conoco Phillips (Table 2.3), it was found that the S-Zorb ADS-process from ConocoPhillips and the CDTech HDS- process are better than other candidates for commercial implementation.

The Irvine Robert Varraveto Adsorption Desulfurization (IRVAD) process (Irvine, 1998) uses slurry and consumes very little (if any) hydrogen. The adsorption mechanism is based on the polarity of sulfur compounds. An alumina-based selective adsorbent counter-current contact liquid hydrocarbon stream up to 240 °C in a multistage adsorber was used. The adsorbent is regenerated in a continuous cross-flow reactivator using heated reactivation gas. The process operates at lower pressure, does not consume hydrogen or saturate olefins, and can effectively remove sulfur in liquid fuels and satisfy the demand for ultra-low sulfur fuels. However, the adsorbent needs to be regenerated and recycled frequently (Sano et al., 2005).

Xu et al. (2008) reported the RADS of FCC gasoline over Ni/ZnO of approximately 25.4 mg S/g adsorbent, with a 1.1 loss in octane number. Metal halides, such as PdCl₂ supported on SBA-15 and MCM-41, have also been found to be effective for the RADS desulfurization of jet fuel and Cu₂O supported on MCM-41 has been also reported to be more effective for ADS of jet fuel than that supported on SBA-15. All these adsorbents could be regenerated by heating in air and reused (Wang et al., 2008a,b). Huang et al. (2010) studied the RADS of diesel oil over Ni/ZnO adsorbent under nitrogen and hydrogen, which followed two different mechanisms. The results indicated that ADS, in the presence of N₂, is achieved through physical and chemical adsorption and a severe decrease in the desulfurization activity of Ni/ZnO is observed with time on stream where the desulfurization capacity is very low. While in presence of H₂, the organic sulfur compounds in the diesel oil are first decomposed on the surface Ni of Ni/ZnO to form Ni₃S₂, followed by the reduction of Ni₃S₂ to form H₂S and then H₂S is stored in the adsorbent accompanied by the conversion of ZnO into ZnS (Scheme 2.14). The S-content decreased from 560 ppm to 2.6 ppm with an S-adsorption capacity of 0.333 g-S/g-NiO/ZnO, but the S-content in the adsorbent increased steadily with the RADS time on stream. Thus, after 122 h, certain amounts of sulfur-containing compounds (mainly4,6-DMDBT and 2,4,6-TMDBT) are still present in diesel oil which is attributed to the decrease of RADS activity of Ni/ZnO, along with the increase of Ni₃S₂ and ZnS contents in the adsorbent increment. Meanwhile, the desulfurization activity, as well as the acceptance ability of the adsorbent, is also decreased. As a result, at the later stage, the RADS efficiency decreases gradually and certain amounts of H₂S are released to the effluent rather than stored in the adsorbent.

Zhang et al. (2012) reported the preparation of special morphology ZnO materials with smaller crystallite size by the hydrothermal homogeneous precipitation method and the corresponding Ni/ZnO adsorbents were prepared by the incipient impregnation method. Reactive adsorption desulfurization (RADS) of model gasoline, using thiophene as model sulfur-containing compounds over Ni/ZnO adsorbent, was carried out in a fixed bed reactor in the presence of hydrogen. The results showed that the Ni/ZnO adsorbent using ZnO with a larger surface area and smaller crystal grains as an active component shows higher desulfurization activity and stability. During the adsorption process of thiophene on Ni/ZnO adsorbent, sulfur is trapped by zinc oxide and converted to zinc sulfide. Yin et al. (2012) reported the preparation of high-dispersed CeO₂ on mesoporous silica (SBA-15), where an adsorbent with 1.5 mol% Ce was able to remove 0.200 mmol S/g adsorbent from a model oil of thiophene and it also showed good ADS for BT and DMDBT. Sarda et al. (2012) investigated the selective ADS of a diesel fuel containing 325 mg S/kg, using adsorbents with different contents of Ni and Cu, supported on alumina and ZSM-5 zeolite, prepared by wet impregnation and ion exchange, respectively. The Ni/Al₂O₃ adsorbent was found to be more effective than the Cu/Al₂O₃ adsorbent. Up to 93.47% ADS occurred for a jet-A fuel with an initial total S-content of 949.03 mg/kg with a new adsorbent, Ni–Ce/Al₂O₃–SiO₂., which was prepared by extrusion molding, calcined at 600 °C for 3 hours. the main advantage of that method is that it was carried out at ambient temperature and pressure, under static condition (Shen et al., 2012). Kong et al. (2013) reported the RADS of a model gasoline (1000 ppmw thiophene in a mixture of 35 wt.% of isopentene as olefins, 15 wt.% toluene as aromatic compounds, 10 wt.% of cyclohexane to mimic the cycloalkane and 40 wt.% n-heptane) where the presence of olefins decreased the desulfurization capacity from 360 mg S/g sorbent in a thiophene/n-heptane model gasoline to 322 mg S/g sorbent in a thiophene/mixture model gasoline, with slight loss of octane number, in a fixed bed reactor, at optimum operating conditions of 400 °C, 60 h^–1 LHSV (volume hourly space velocity) and a pressure of 1.0 MPa. The RADS occurred through the reaction of active Ni metal with the S-containing compound in the presence of H_2, producing metal sulfuide which could react with ZnO forming ZnS. This would lead to deep desulfurization with a low loss in octane number.

Wang et al. (2012) reported the RADS of 4,6-DMDBT from three different model diesel oils (1000 mg/kg 4,6-DMDBT in 100% n-dodecane, as aliphatic fuel (ALF), 10% benzene and 80% n-dodecane plus 10% naphthalene, as mixed fuel (MXF), and 90% benzene plus 10% naphthalene as aromatic fuel (ARF)) in a fixed-bed reactor packed with a reactive adsorbent that is composed of formaldehyde, the environmentally-friendly heteropoly acid, phosphotungstic acid, and mesoporous silica gel (19.1, 33.3, and 47.6 wt.%, respectively), at 80 °C under atmospheric pressure. The mechanism of RADS was suggested to depend on the condensation reaction of 4,6-DMDBT with formaldehyde using the phosphotungstic acid as a catalyst in pore spaces (Scheme 2.15). The presence of formaldehyde led to 100% removal of 4,6-DMDBT within 90 min compared to 8.14% within three hours in its absence. Further, an optimum load of heteropoly acid is necessary, as it has a dramatic effect on the desulfurization rate. The rapidity of the reaction requires more heteropoly acid, which would result in a shrinkage of the specific surface area, pore diameter, and pore volume. The corresponding reduction of adsorbed formaldehyde brought about a decrease of the desulfurization rate. The sulfur breakthrough capacities decreased as follows: ALF > MXF > ARF due to the inhibitive effect of aromatics, which can competitively adsorb on active sites. Moreover, the sulfur breakthrough capacity per gram of substrate changed from 4.64 mg S to 4.59 mg S and 4.51 mg S when the aromatics went up from 0% to 20% to 100%. Thus, it is evident that the aromatics have less of an adverse effect on the reactive adsorbent than on an adsorbent based on π-complexation in the desulfurization of ARF. The capacity of the regenerated reactive adsorbent is almost completely recovered and the adsorbent sis reported to have almost the same high efficiency after being recycled twice.

It is known that an adsorbent is the carrier of the reaction characteristic for sulfur compounds, and that reagents and catalysts must be loaded on them before the desulfurization process. Thus, the adsorbent must match the catalyst, to guarantee the maximum desulfurization capacity, for example loading H₂SO₄ on carbonaceous material (Dai et al., 2008) and HCl or phosphomolebdic acid (PMA) on silica materials (Wang et al., 2008c, 2009b). As mentioned above, the reagent of the condensation reaction is formaldehyde (FA), which can be pre-loaded in the adsorbent via vapour adsorption. H₂SO₄ or HCl may be then dropped in the adsorbent, while PMA can be loaded by soaking method before loading FA. Silica gel is usually used to load peracetic acid for the oxidation reaction, which is used for commercial fuels following the condensation reaction. The weight ratio of peracetic acid to silica gel is reported to be usually 0.4/1.0.

The preparation of nano-ferrites (for example: Fe₃O₄, NiFe₂O₄, CuFe₂O₄ and MnFe₂O₄) by a reverse (water/oil) micro-emulsion method, containing cetyltrimethylammonium bromide, 1-butanol, cyclohexane, and a metal salt solution, using ammonium hydroxide as a co-precipitating agent, has also been reported (Zaki et al., 2013). The complex nickel-iron oxide (NiFe₂O₄) expressed the highest adsorption capacity towards dibenzothiophene (166.3 µmol dibenzothiophene/g adsorbent) due to the high specific surface area of the oxide (233.4 m²/g), large pore volume of approximately 0.3 cm³/g, mesoporous framework, and strong acid sites.

Rare earth metal–organic framework materials composed of a metal ion center connect to a carboxyl group of the organic ligand (Lin et al. 2012). These metal organic frameworks (MOFs) are characterized by large pores, very high chemical and hydrothermal stability, and a Langmuir surface area which surpasses 500 m²/g. Moreover, rare earth metals have a unique electronic structure where the 4f electron shell is not completely filled, with a different 4f electronic number that has a relatively high coordination number which provides a way to get new structure. This encourages the application of MOFs in ADS (Gustafsson et al., 2010). Liu et al. (2014b) reported the preparation of the rare earth metal-organic frameworks (Ln-MOFs) materials, Ln(TMA)(H₂O)•(DMF), using the rare earth metal (Ln=Sm, Eu, Tb, Y) and 1,3,5-trimesic acid (TMA) as a metal ion center and ligand, respectively. That expressed good ADS capabilities for a thiophene/n-octane model oil with excellent reusability. The yttrium metal–organic framework material, Y(TMA)(H₂O)•(DMF), expressed a comparatively better activity for ADS with a desulfurization rate up to 80.7% and a sulfur adsorption capacity of 30.7 mgS/g(Y-MOFs). Habimana et al. (2016) reported the preparation of europium metal organic framework (Eu-MOF) using Europium as a metal ion center and 1,3,5-trimesic acid (TMA) as organic ligands under hydrosolvothermal conditions. Its ADS efficiency was tested using a thiophene/n-octane (1000 µg/g) model oil where the adsorption rate reached 64.70%, with an adsorption capacity of 24.59 mg S/g MO, under optimum operating conditions of 1:100 (m_absorbent:m_{model oil}), 30 °C, and 4 h. The adsorption of thiophene over Eu-MOF absorbent occurs simultaneously through the monolayer physical adsorption and the multilayer chemical adsorption. The adsorbent can be regenerated and reused for 5 successive cycles with a slight decrease in its activity, from 24.59 to 22.43 mg S/g, with a reduction in desulfurization rate from 64.70 to 59.03%.

It is reported that the film-shear reactor, as a flow process and in a batch reactor, was both very effective in the ADS of various recalcitrant thiophenes (80–90 % removal) in a model diesel fuel (thiophenes in mixture of 85:15 n-hexane/toluene) using a carbon nanotube/TiO₂

(CNT/TiO₂) as an adsorbent, but reactions in the film-shear reactor are faster and require far smaller TiO₂ concentrations to be effective (0.01–0.1 wt./vol% vs. 1–10% in the batch experiments). This is attributed to the intimate mix of the fuels and the CNT/TiO₂ adsorbent in the film-shear reactor. Moreover, the ADS increases as follows: Th < BT < DBT, that is attributed to the increased number of π–π dispersive interactions between the aromatic rings and the nanotubes as the number of aromatic rings increases. Further, the regeneration of CNT/TiO₂ can be achieved by heating at 400 °C without losing its activity (Siddiqui et al., 2016).

Deep desulfurization of liquid fuel by molecular imprinting technology (MIT) is expected to find wide application since MIT is based on the application of biosensors, separation media, and affinity supports for the recognition of target molecules (Liu et al., 2006). MIT has a unique predeterminative characteristic, specificity, and practicability since it is based on the creation of specific molecular recognition sites in polymers to identify template molecules. The prepared polymer is known as a molecularly imprinted polymer (MIP) because it is complimentary to the template in space structure and binding sites. Consequently, based on the selectivity mechanism of MIP towards the sulfur compounds in fuel oils, the prepared MIP using BT, DBT, and DBT-sulfone as templates would remove thiophene-like organosulfur compounds from fuel oils (Yang et al., 2014).

Methacrylic acid based MIP with divinylbenzene as a cross-linker is reported to have a maximum retention capacity of 14.8 mgDBT/g at 25 °C (Castro et al., 2001). Chang et al. (2003) reported preparation of MIP by bulk polymerization using DBT as the template, 4-vinyl pyridine as functional monomer, ethylene glycol dimethylacrylate EDMA as the cross-linker, and toluene as porogen with a maximum binding capacity of 48.3 mg/g at 20 °C and good removing capacity towards BTs and DBTs.

Surface molecular imprinting polymer (SMIP) technology is very promising in the deep desulfurization of liquid fuel oils since SMIP has high selectivity, fast adsorption, and good mechanical and thermal stability. SMIP can be prepared by grafting a thin imprinted polymer film with a large amount of binding sites onto inorganic supports, e.g. silica gel, TiO₂, K₂Ti₄O₉, and carbon microspheres. Silica gel is usually used for its high porosity, large surface area, good computability, mechanical property, and stability (Yang et al., 2014). Hu et al. (2010) reported grafting polymerization using BT as a template, silica gel modified by KH-550 as support, methacrylic acid MAA as monomer, EDMA as cross-linker, azoisobutyrontrile AIBN as initiator, and toluene as solvent. This showed an adsorptive desulfurization capacity of 57.4 mg/g in gasoline at 25 °C and remained unchanged for 30 cycles. Liu et al. (2014c) reported the preparation of a double-template molecularly imprinted polymer (D-MIP) supported on carbon microspheres (CMSs), using benzothiophene (BT) and dibenzothiophene (DBT) as the template molecules for the removal of benzothiophene sulfides from fuel gasoline. The prepared SMIP was selectively able to bind BT and DBT at the same time. The D-MIP/CMSs had five times higher a binding capacity towards BT and DBT in simulated gasoline compared to non-imprinted polymers (D-NIP/CMSs). The adsorption equilibrium of D-MIP/CMSs was achieved within 90 min and the adsorption capacity reached 57.16 and 67.19 mg/ g for BT and (DBT), respectively. It can be also used for 10 times without significant loss in adsorption capacity. Moreover, its good efficiency in real oil (gasoline) promoted the application of D-MIP/CMSs as a new material for deep desulfurization of fuel oils.

As a brief of this review, ADS (Figure 2.16) has some problems to be solved. There are high proportions of aromatics compared to the amounts of sulfur compounds in middle distillates, such as kerosene and diesel and the aromatics can also be adsorbed on the desulfurization adsorbents. Thus, the adsorbents should be well designed to achieve suitable selectivity; when the selectivity is low, the adsorbents are easily regenerated. However, this can lead to heat loss because of the comparative adsorption. As the selectivity increases, the spent adsorbents become more and more difficult to be regenerated (Hernández-Maldonado and Yang, 2004a). Consequently, the major challenge in ADS is to design an adsorbent material which can selectively adsorb the sulfur compounds from fuel without altering the aromatic content.

One of the new approaches for ADS applications is the ADS of methanthiol as one of S-impurities in propylene produced from bioethanol by ethanol-to-olefin (ETO) conversion. Depending on the fermentation route of a raw material of bioethanol, it contains dimethylsulfide and/or dimethylsulfoxide as sulfur impurities that are converted to methanethiol, ethanethiol, or hydrogen sulfuide during the ETO conversion. Poisoning of a metal catalyst, that is used for the polymerization of olefin, even by a trace amount of sulfur impurities can occur, thus desulfurization of olefin derived from bioethanol is mandatory (Bartholomew 2001). Among various porous materials, a hydrophilic zeolite with a low Si/Al ratio is one of the most suitable adsorbents for the desulfurization of olefin derived from bioethanol, since strong Lewis acid/base interaction between an acidic site (Al atom) and a sulfur compound is expected (Cui et al., 2009). The breakthrough curve (BTC) for adsorption of methanethiol from bioethanol, in the presence of propylene, using a fixed-bed column packed with the LTA zeolite was reported, where the competitive adsorption of propylene on the LTA zeolite was found to be strongly dependent on a cation species exchanged in the micropores of the zeolite. The bivalent cation of zinc (Zn²⁺) was proved to be the most effective one to increase the amount of methanethiol adsorbed on the LTA zeolite under the presence of propylene (Yamamoto et al., 2011).

Solvent extraction and oxidation in the air are two methods to regenerate the desulfurization adsorbents, but there are disadvantages inherent in these two methods. The solvent extraction method suffers from the disadvantage that it is difficult to separate sulfur compounds from organic solvents and reuses these solvents. On the other hand, in the calcination method, sulfur compounds and aromatics are burned out which can cause a loss in the heat value of fuels. Recently, bioregeneration of the adsorbents is recommended (Li et al., 2005, 2009e).

In conclusion, reactive adsorption desulfurization (RADS) is superior to ordinary physical adsorption as it involves the π-complexation between aromatic sulfur compounds and the adsorbent, which is stronger than van der Waals interaction. However, the π-complexation can be broken easily by heating or decreasing pressure, thereby it is easy to regenerate the adsorbent. So, briefly the S-Zorb SRT (Sulfur Removal Technology) is a representative of the hydrogenation-based ADS techniques and has advantages in less loss of octane number and relatively high desulfurization efficiency. However, it is not a cost efficient technology since the fuel has to be vaporized and reacted at 380–420 °C. Although, the ADS-techniques which are based on physical adsorption seem non-applicable due to the low selectivity for thiophenic compounds. Prominent success was reported in chemical adsorption and the IRVAD (Irvine Robert Varraveto Adsorption Desulfurization) method, the π-complexion method, and the SARS (selective adsorption for removing sulfur) method are well known representatives. The IRVAD method, as mentioned before, is reported to be suitable for many liquid hydrocarbons and more than 90% sulfur can be removed. However, the sulfur capacity of adsorbents is low and the adsorbents need to be frequently regenerated, but,the sulfur capacity of adsorbents can be considerably increased depending on the principle of π-complexion. However, the adsorption performance declines greatly where aromatic compounds, oxidants, and moisture are present in fuels. Furthermore, the SARS method depends on the preloading of transition metals on a supporting adsorbent, for example, Ni/SiO₂–Al₂O₃, and reached the same or a little higher sulfur capacity in comparison with the π-complexion method.

2.5 Biocatalytic Desulfurization

Microorganisms need sulfur to fulfil both growth and biological activity, as it forms around 1% of the dry weight of the bacterial cell. Some microorganisms have the ability to supply their needed sulfur from various sources due to the fact that sulfur exists in the structure of some enzymes cofactors, amino acids, and proteins. Some microorganisms have the ability to consume sulfur in thiophenic compounds and reduce the sulfur content in fuel (El-Gendy and Speight, 2016).

The concept of microbial desulfurization is not new. The first U.S. patents were issued in the 1950s (Strawinski, 1950; ZoBell, 1953). The two general approaches to microbial desulfurization are aerobic and anaerobic reactions (Monticello, 1994).

References

ABB Lummus Global, Inc. (2002) Hydrotreating-aromatic saturation. Hydrocarbon Processing. 81: 127.

Abdul Jalil, T., Falah Hasan, L. (2012) Oxidative desulfurization of gas oil using improving selectivity for active carbon from rice husk. Diyala Journal for Pure Sciences. 8(3): 68–81.

Abid, M.F. (2015) Desulfurization of gas oil using a solar photocatalytic microreactor. Energy Procedia. 74: 663–678.

Abrahamson J.P. (2015) Effects of catalyst properties on hydrodesulfurization of fluid catalytic cracking decant oils as feedstock for needle coke. MSc. Thesis, The Graduate School, Department of Energy and Mineral Engineering, The Pennsylvania State University, Old Main, State College, PA 16801, USA.

Abu Bakar, W.A.W., Ali, R., Abdul Kadir, A., Mokhtar, W.N.A.W. (2015) The role of molybdenum oxide based catalysts on oxidative desulfurization of diesel fuel. Modern Chemistry and Applications. 3(2): 1000150. http://dx.doi.org/10.4172/2329–6798.1000150.

Aburto, J., Borgne, S.L. (2004) Selective adsorption for dibenzothiophene sulfone by an imprinted and stimuli-responsive chitosan hydrogel. Macromolecules 37: 2938–2943.

Adekanmi, A.A., Folorunsho, A. (2012) Comparative analysis of adsorptive desulphurization of crude oil by manganese dioxide and zinc oxide. Research Journal of Chemical Sciences. 2(8): 14–20.

Adeyi, A., Giwa, A., Adeyi, V.A. (2015) Kinetics analysis of manganese dioxide adsorbent on desulphurization of diesel oil. International Journal of Scientific and Engineering Research. 6(7): 650–658.

Afanasiev P., Cattenot M., Geantet C., Matsubayashi N., Sato K., and Shimada S. (2002) (Ni)W/ZrO2 hydrotreating catalysts prepared in molten salts. Applied Catalysis A: General. 237(1/2): 227–237.

Ahmad, S., Ahmad, M.I., Naeem, K., Humayun, M., Zaeem, S.E., Faheem, F. (2016) Oxidative desulfurization of tire pyrolysis oil. Chemical Industry and Chemical Engineering Quaternary. 22(3): 249–254.

Aitani, A.M., Ali, M.F., Al-Ali, H.H. (2000) A review of non-conventional methods for the desulfurization of residual fuel oil. Petroleum Science and Technology. 18(5/6): 537–553.

Akopyan, A.V., Ivanov, E.V., Polikarpova, P.D., Tarakanova, A.V., Rakhmanov, E.V., Polyakova, O.V., A.V. Anisimov, Vinokurov, V.A. (2015) Oxidative desulfurization of hydrocarbon fuel with high olefin content. Petroleum Chemistry. 55(7): 571–574.

Akopyan, A.V., Kardasheva, Y.S., Eseeva, E.A., Plotnikov, D.A., Vutolkina, A.V., Kardashev, S.V., Rakhmanov, E.V., Anisimov, A.V., Karakhanov, E.A. (2016) Reduction of sulfur content in shale oil by oxidative desulfurization. Petroleum Chemistry. 56(8): 771–773.

Al Zubaidy I.A.H., Tarsh F.B., Darwish N.N., Majeed B.S.S.A., Sharafi A.A., Chacra L.A. (2013) Adsorption process of sulfur removal from diesel oil using sorbent materials. Journal of Clean Energy Technologies (JOCET) 1(1): 66–68.

Alavi, S.A., Hashemi, S.R. (2014) A review on diesel fuel desulfurization by adsorption process. Proceedings of International Conference on Chemical, Agricultural, and Biological Sciences (ICCABS’2014) Oct. 9–10, 2014 Antalya (Turkey). Pp. 33–36.

Al-Degs, Y.S., El-Sheikh, A.H., Al-Bakain, R.Z., Newman, A.P., Al-Ghouti, M.A. (2016) Conventional and upcoming sulfur-cleaning technologies for petroleum fuel: A review. Energy Technology. 6: 679–699.

Al-Ghouti M.A., and Al-Degs Y.S. (2014) Manganese-loaded activated carbon for the removal of organosulfur compounds from high-sulfur diesel fuels. Energy Technology. 2: 802–810.

Ali, M., Tatsumi T., Masuda T. (2002) Development of heavy oil hydrocracking catalysts using amorphous silica-alumina and zeolites as catalyst supports. Applied Catalysis A: General. 233(1/2): 77–90.

Ali, M.F., Al-Malki, A., and Ahmed, S. (2009) Chemical desulfurization of petroleum fractions for ultra-low sulfur fuels. Fuel Processing Technology. 90: 536–544.

Ali, S.A., Siddiqui, M.A.B. (1997) Dearomatization, cetane improvement and deep desulfurization of diesel feedstock in a single-stage reactor. Reaction Kinetics and Catalysis Letters. 61(2): 363–368.

Al-Zeghayer, Y.S., Jibril, B.Y. (2006) Kinetics of hydrodesulfurization of dibenzothiophene on sulfided commercial Co-Mo/γ-Al₂O₃ catalyst. The Journal of Engineering Research. 3(1): 38–42.

Amini, B., Jafari, J.K., Kalbasi, M., Khorami, P., Mohajeri, A., Parviz, D., Rashidi, A. (2010) Hydrodesulphurization nanocatalyst, its use and a process for its production. European Patent EP-2196260 A1.

Ancheyta, J. (2011). Modeling and Simulation of Catalytic Reactors for Petroleum Refining. 1^st Ed., John Wiley & Sons, Inc., Publication, Hoboken, New Jersey, USA.

Andersson, P., Pirjamali, M., Jaras, S., Kizling, M. (1999). Cracking catalyst additives for sulfur removal from FCC gasoline. Catalysis Today. 53: 565–573.

Aparicio, F., Camú, E., Villarroel, M., Escalona, N., Baeza, P. (2013) Deep desulfurization by adsorption of 4,6-dimethyldibenzothiophene, study of adsorption on different transition metal oxides and supports. 58(4): 2057–2060.

Arnoldy, P., Van den Heikant, J.A.M., De Bok, G.D., Moulijn, J.A. (1985) Temperature-programmed sulfiding of MoO₃Al₂O₃ catalysts. Journal of Catalysis 92: 35–55.

Atlas, R.M., Boron, D.J., Deever, W.R., Johnson, A.R., McFarland, B.L., Meyer, J.A. (2001) Method for removing organic sulfur from heterocyclic sulfur-containing organic compounds. US patent number USH1986 H1.

Babich, I.V., Moulijn, J.A. (2003) Science and technology of novel processes for deep desulfurization of oil refinery streams: A review. Fuel. 82(6): 607–631.

Baco, F., Debuisschert, Q., Marchal, N., Nocca, J.-L., Picard, F., Uzio, D. (2002). Prime G+ process, desulfurization of FCC gasoline with minimized octane loss, in: Proceedings of the Fifth International Conference on Refinery Processing, AIChE2002 Spring National Meeting, New Orleans, LA, March11–14, 2002, pp. 180–188.

Baeza, P., Aguila, G., Gracia, F., Araya, P. (2008) Desulfurization by adsorption with copper supported on zirconia. Catalysis Communications. 9(5): 751–755.

Baeza, P., Aguila, G., Vargas, G., Ojeda, J., Araya, P. (2012) Adsorption of thiophene and dibenzothiophene on highly dispersed Cu/ZrO₂ adsorbents. Applied Catalysis B: Environmental. 111–112: 133–140.

Bagreev, A., Menendez, J.A., Dukhno, I., Tarasenko, Y., Bandosz, T.J. (2004) Bituminous coal-based activated carbons modified with nitrogen as adsorbents of hydrogen sulfide. Carbon. 42(3): 469–476.

Baldovino-Medrano, V.G., Giraldo, S.A., Centeno A. (2009) Highly HYD selective Pd–Pt/support hydrotreating catalysts for the high pressure desulfurization of DBT type molecules. Catalysis Letters. 130: 291–295.

Baltzopoulou, P., Kallis, K.X., Karagiannakis, G., Konstandopoulos, A.G. (2015) Diesel fuel desulfurization via adsorption with the aid of activated carbon: laboratory- and pilot-scale studies. Energy and Fuels. 29 (9): 5640–5648.

Bandosz, T.J. (2006) Desulfurization on activated carbons. In: Interface Science and Technology, Bandosz T.J. (Ed.) Elsevier, UK. 7: 231 –292.

Barmala, M., Moghadam, A.Z., Omidkhah, M.R. (2015) Increased photo-catalytic removal of sulfur using titania/MWCNT composite. Journal of Central South University of Technology. 22: 1066−1070.

Barrio, V.L., Arias, P.L., Cambra, J.F., Güemez, M.B., Pawelec, B., Fierro, J.L.G. (2003) Hydrodesulfurization and hydrogenation of model compounds on silica–alumina supported bimetallic systems. Fuel 82:501–509.

Bartholomew, C.H. Mechanisms of catalyst deactivation (2001). Applied Catalysis A: General. 212: 17–60.

Bataille, F., Lemberton, J.L., Michaud, P., Perot, G., Vrinat, M., Lemaire, M., Schulz, E., Breysse, M., Kasztelan, S. (2000) Alkyldibenzothiophenes hydrodesulfurization: Promotor effect, reactivity, and reaction mechanism. Journal of Catalysis. 191: 409–422.

Beers, A.E.W., Hoek, I., Nijhuis, T.A., Downing, R.S., Kapteijn, F., Moulijn, J.A., (2000) Structured catalysts for the acylation of aromatics. Topics in Catalysis. 13(3): 275–280.

Bhandari, V.M., Ko, C.H., Park, J.G., Han, S.S., Cho, S.H., Kim, J.N. (2006) Desulfurization of diesel using ion-exchanged zeolite. Chemical Engineering Science. 61: 2599–2608.

Bhore N.A., Chester A.W., Liu K., Timkin H.K.C. (2001). Reducing gasoline sulfur in FCC. World Patent WO 0121733 A1. Assigned to Mobil Oil Corporation.

Billon, A., Colyar, J.J., Kressmann, S., Morel, F. (2000) Deep conversion of heavy oils via combination of catalytic hydrocracking and deasphalting. World Petroleum Congress 2000, Calgary, Canada, June 2000.

Boikov, E.V., Vakhrushin, P.A., Vishnetskaya, M.V. (2008) Oxidative desulfurization of hydrocarbon feedstock. Chemistry and Technology of Fuels and Oils. 44(4): 271–274.

Bose D. (2015) Design parameters for hydrodesulfurization (HDS) unit for petroleum naphtha at 3500 barrels per day. World Science News. 9: 99–111.

Boukoberine, Y., Hamada, B. (2016) Thiophene hydrodesulfurization over CoMo/Al₂O₃-CuY catalysts: Temperature effect study. Arabian Journal of Chemistry. 9: S522 –S527.

Bowker, R.H. (2011) Hydrodesulfurization and hydrodeoxygenation over noble metal phosphide catalysts. MSc. Thesis, Western Washington University, 516 High Street, Bellingham, WA 98225, USA.

Breysse, M., Furimsky, E., Kasztelan, S., Lacroix, M., Perot, G. (2002) Hydrogen activation by transition metal sulfides. Catalysis Reviews-Science and Engineering. 44: 651–735.

Brunet, S., Mey, D., Pérot, G., Bouchy, C., Diehl, F. (2005). On the hydrodesulfurization of FCC gasoline: a review. Applied Catalysis A: General. 278: 143–172.

Calzada, J., Alcon, A., Santos, V.E., Garcia-Ochoa, F. (2011) Mixtures of Pseudomonas putida CECT 5279 cells of different ages: Optimization as biodesulfurization catalyst. Process Biochemistry. 46: 1323–1328.

Campos-Martin, J.M., Capel-Sanchez, M.C., and Fierro, J.L.G. (2004) Oxidative processes of desulfurization of liquid fuels. Journal of Chemical Technology and Biotechnology. 85: 879–890.

Campos-Martin, J.M., Capel-Sanchez, M.C., Perez-Presas, P. and Fierro, J.L.G. (2010) Highly efficient deep desulfurization of fuels by chemical oxidation. Green Chemistry. 6: 557–562.

Caro, A., Boltes, K., Letón, P., García-Calvo, E. (2007) Dibenzothiophene biodesulfurization in resting cell conditions by aerobic bacteria. Biochemical Engineering Journal. 35(2): 191 -197.

Castorena, G., Suarez, C., Valdez, I., Amador, G., Fernandez, L., Le Borgne, S. (2002) Sulfur-selective desulfurization of dibenzothiophene and diesel oil by newly isolated Rhodococcus sp. Strains. FEMS Microbiolgy Letters. 215(1): 157–161.

Castro, B., Whitcombe, M., Vulfson, E., Vazquez-Duhalt, R., Bárzana, E. (2001) Molecular imprinting for the selective adsorption of organosulphur compounds present in fuels. Analytica Chimica Acta. 435(1): 83–90

Cattaneo, R., Shido, T., Prins, R. (1999) Hydrotreatment and Hydrocracking of Oil Fractions. Elsevier, Amsterdam.

CDTech (2000) Hydrogenation. Hydrocarbon Processing. 79: 122–123.

Cedneňo-Caero, L., Gomez-Bernal, H., Fraustro-Cuevas, A., Guerra-Gomez, D.H., Cuevas-Garcia, R. (2008) Oxidative desulfurization of synthetic diesel using supported catalysts: Part III. Support effect on vanadium-based catalysts. Catalysis Today. (133/135): 244–254.

Chang, Y.H., Liu, B., Ying, H.J., He, M.F. (2003) Solid-phase extraction sorbent for organosulfur compounds present in fuels made by molecular imprinting. Ion Exchange and Adsorption 19(5): 450–456.

Chang, Y.H., Zhang, L., Ying, H.J., Li, Z.J., Lv, H., Ouyang, P.K. (2010) Desulfurization of Gasoline using Molecularly Imprinted Chitosan as selective adsorbents. Applied Biochemistry and Biotechnology. 160: 593–603.

Chapados, D., Bonde, S.E., Chapados, D., Gore, W.L., Dolbear, G., Skov, E. (2000) Desulfurization by selective oxidation and extract of sulfur-containing compounds to economically achieve ultra-low proposed diesel fuel sulfur requirements. Proceedings. NPRA Annual Meeting AM-00–25, San Antonio, Texas. March 26–28.

Chen, L., Li, X., Rooke, J.C., Zhang, Y., Yang, X., Tang, Y., Xiao, F., Su, B. (2012) Hierarchically structured zeolites: synthesis, mass transport properties and applications. Journal of Materials Chemistry. 22: 17381–17403.

Cheng, Z., Liu, X., Lu, J., Luo, M. (2009) Deep desulfurization of FCC gasoline by selective adsorption over nanosized zeolite-based adsorbents. Reaction Kinetics and Catalysis Letters. 97: 1–6.

Chester, A., Timken, H., Roberie T., Ziebarth, M. (2003). Gasoline sulfur reduction in FCC. U.S. Patent 20030089639 A1.

Chianelli, R.R., Berhault, G., Raybaud, P., Kasztela, S., Hafnere, J., Toulhoatf, H. (2002) Periodic trends in hydrodesulfurization: in support of the Sabatier principle. Applied Catalysis A: General. 227: 83–96.

Chica, A., Corma, A., and Domine, M.E. (2006) Catalytic oxidative desulfurization (ODS) of diesel fuel on a continuous fixed0bed reactor. Journal of Catalysis. 242: 2999–308.

Chitnis, G.K., Richter, J.A., Hilbert, T.L., Dabkowski, M.J., Al Kuwari, I., Al Kuwari, N., Sherif, M. (2003) Commercial OCTGain’s unit provides ‘zero’ sulfur gasoline with higher octane from a heavy cracked naphtha feed, AM-03–125, in: Proceedings of the NPRA 2003 Annual Meeting, San Antonio, TX, March 23–25, 2003.

Collins, F.M., Lucy, A.R., Sharp, C.J. (1997) Oxidation desulfurization of oils via hydrogen peroxide and heteropolyanion catalysis. Journal of Molecular Catalysis A: Chemical. 117: 397–403.

Cooper, B.H., Sogaard-Anderson, P., Nielsen-Hannerup, P. (1994) Production of Swedish class I diesel using dual-stage process, in: M.C. Oballa, S.S. Shih (Eds.), Catalytic Hydroprocessing of Petroleum and Distillates, Marcel Dekker, New York, 1994, pp. 279–290.

Corma, A., Martinez, C., Ketley, G., Blair, G. (2001) On the mechanism of sulfur removal during catalytic cracking. Applied Catalysis A: General. 208(1/2): 135–152.

Cui, H., Turn, S.Q., Reese, M.A. (2009) Removal of sulfur compounds from utility pipelined synthetic natural gas using modified activated carbons. Catalysis Today. 139: 274–279.

Cui, S., Ma, F., Wang, Y. (2007) oxidative desulfurization of model diesel oil over Ti-containing molecular sieves using hydrogen peroxide. Reaction Kinetics and Catalysis Letters. 92(1): 155−163.

Cui, X., Yao, D., Li, H., Yang, J., and Hu, D. (2012) Non-magnetic particles as multifunctional microreactor for deep desulfurization. Journal of Hazardous Materials. (205/206): 17–23.

Dai, W., Zhou, Y., Wang, S., Su, W., Sun, Y., Zhou, L. (2008) Desulfurization of transportation fuels targeting at removal of thiophene/benzothiophene. Fuel Processing Technology. 89(8): 749–755.

Danmaliki, G.I., Saleh, T.A. (2017) Effects of bimetallic Ce/Fe nanoparticles on the desulfurization of thiophenes using activated carbon. Chemical Engineering Journal. 307: 914–927.

de A. Mello, P., Duarte, F.A., Nunes, M.A.G., Alencar, M.S., Moreira, E.M., Korn, M., Dressler, V.L., and Flores, E.M.M. (2009) Ultrasound-assisted oxidative process for sulfur removal from petroleum product feedstock. Ultrasonics Sonochemistry. 16: 732–736.

de Filippis, P., Scarsella, M., Verdone, N. (2010) Oxidative desulfurization I: peroxyformic acid oxidation of benzothiophene and dibenzothiophene. Industrial and Engineering Chemistry Research. 49(10): 4594–4600.

Deng, Z., Wang, T., Wang, Z. (2010) Hydrodesulfurization of diesel in a slurry reactor. Chemical Engineering Science 65: 480–486.

Dhar, G.M., Srinivas, B.N., Rana, M.S., Kumar, M., Maity, S.K. (2003) Mixed oxide supported hydrodesulfurization catalysts—a review. Catalysis Today 86: 45–60.

di Giuseppe, A., Crucianelli, M., de Angelis, F., Crestini, C., Saladino, R. (2009) Efficient oxidation of thiophene derivatives with homogeneous and heterogeneous MTO/H₂O₂ systems: A novel approach for, oxidative desulfurization (ODS) of diesel fuel. Applied Catalysis B: Environmental. 89(1/2): 239–245.

Dinamarca, M.A., Ibacache-Quiroga, C., Baeza, P., Galvez, S., Villarroel, M., Olivero, P., Ojeda, J. (2010) Biodesulfurization of gas oil using inorganic supports biomodified with metabolically active cells immobilized by adsorption. Bioresource Technology. 101: 2375–2378.

Doyle, A., Tristao, M.L.B., Felcman, J. (2006) Study of fuel insolubles: Formation conditions and characterization of copper compounds. Fuel. 85(14/15): 2195–2201.

Drake, C.A., Love, S.D. (2000) Process for desulfurizing and aromatizing hydrocarbons. US Patent 6,083,379. Assigned for Phillips Petroleum Company.

Duarte, F.A., de A. Mello, P., Bizzi, C.A., Nunes, M.A.G., Moreira, E.M., Alencar, M.S., Motta, H.N., Dressler, V.L., and Flores, E.M.M. (2011) Sulfur removal from hydrotreated petroleum fractions using ultrasound-assisted oxidative desulfurization process. Fuel. 90: 2158–2164.

Egorova, M., Prins, R. (2004) Mutual influence of the HDS of dibenzothiophene and HDN of 2-methylpyridine. Journal of Catalysis. 221: 11–19.

El-Gendy, N.Sh., Speight, J.G. (2016) Handbook of Refinery Desulfurization. CRC Press, Taylor and Francis Group, LLC, 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487–2742, USA.

EPA-Diesel RIA, Regulatory Impact Analysis (2000): Heavy-duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements, United States Environmental Protection Agency, Air and Radiation, EPA420-R-00–026, December 2000.

Eswaramoorthi I., Sundaramurthy V., Das N., Dalai A.K., and Adjaye J. (2008) Application of multi-walled carbon nanotubes as efficient support to NiMo hydrotreating catalyst. Applied Catalysis A: General. 339(2): 187–195.

ExxonMobil, December 2001. http://www.exxonmobil.com/refiningtechnologies/fuels/mnscanfining.html.

Farag, H., Sakanishi, K., Mochida, I., Whitehurst, D.D. (1999a) Kinetic Analyses and inhibition by naphthalene and H₂S in hydrodesulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) over CoMo-based carbon catalyst. Energy and Fuel. 13(2): 449–453.

Farag, H., Whitehurst, D.D., Sakanishi, K., Mochida, I. (1999b) Carbon versus alumina as a support for Co–Mo catalysts reactivity towards HDS of dibenzothiophenes and diesel fuel. Catalysis Today. 50: 9–17.

Farshi, A., Shiralizadeh, P. (2015) Sulfur reduction of heavy oil by oxidative desulfurization (ODS) method. Petroleum and Coal. 57(3): 295–302.

Ferreira, A.S., Nicoletti, M.C., Bertini, Jr. J.R., Giordano, R.C. (2013) Methodology for inferring kinetic parameters of diesel oil HDS reactions based on scarce experimental data. Computers and Chemical Engineering. 48: 58–73.

Figueiredo dos Santos, M.R., Pedrosa, A.M.G., and Barros de Souza, M.J. (2016) Oxidative desulfurization of thiophene on TiO₂/ZSM-12 zeolite. Materials Research. 1991): 24–30.

Finnerty, W.R. (1993) Symposium on Bioremediation and bioprocessing presented before the division of petroleum chemistry, Inc. 205^th National Meeting, American Chemical Society, Denver, Co. Pp. 282–285.

Flego, C., Arrigoni, V., Ferrari, M., Riva, R., Zanibelli, L. (2001) Mixed oxides as a support for new CoMo catalysts. Catalysis Today. 65: 265–270.

Franchi, E., Rodriguez, F., Serbolisca, L., Ferra, F. (2003) Vector development isolation of new promoters enhancement of catalytic activity of DSZ enzyme complex in Rhodococcus. sp. strain. Oil & Gas Science and Technology – Rev. IFP. 58(58): 515–520.

Ganiyu, S.A., Ajumobi, O.O., Lateef, S.A., Sulaiman, K.O., Bakare, I.A., Qamaruddin, M., Alhooshani, K. (2017) Boron-doped activated carbon as efficient and selective adsorbent for ultra-deep desulfurization of 4,6-dimethyldibenzothiophene. Chemical Engineering Journal. 321: 651–661.

García-Gutiérrez, J.L., Fuentes, G.A., Hernandez-Teran, M.E., Garcıa, P., Murrieta-Guevara, F., and Jimenez-Cruz F. (2008) Ultra-deep oxidative desulfurization of diesel fuel by the Mo/Al₂O₃-H₂O₂ system: The effect of system parameters on catalytic activity. Applied Catalysis A: General 334: 366- 373.

Gates, B.C., Topsoe, H. (1997) Reactivities in deep catalytic hydrodesulfurization: challenges, opportunities, and the importance of 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene. Polyhedron. 16(18): 3213–3217.

Gawande, P.R., Kaware, J. (2014) A review on desulphurization of liquid fuel by adsorption. International Journal of Science and Research. 3(7): 2255–2259.

Gawande, P.R., Kaware, J.P. (2016) Removal of sulphur from liquid fuels using low cost activated carbon -A review. International Journal of Scientific Engineering and Applied Science. 2(10): 64–69.

Gillespise, R.J., Peel, T.E. (1973) Hammett acidity function for some superacid systems. II. Systems sulfuric acid-[fsa], potassium fluorosulfate-[fsa], [fsa]-sulfur trioxide, [fsa]-arsenic pentafluoride, [sfa]-antimony pentafluoride and [fsa]-antimony pentafluoride-sulfur trioxide. Journal of the American Chemical Society 95(16): 5173–5178.

Golden, S.W., Hanson, D.W., Fulton, S.A. (2002) Use better fractionation to manage gasoline sulfur concentration. Hydrocarbon Processing. 81(2): 67–72.

Gong, Y., Dou, T., Kang, S., Li, Q., Hu, Y. (2009) Deep desulfurization of gasoline using ion-exchange zeolites: Cu(I)- and Ag(I)-beta. Fuel Processing Technology. 90(1): 122–129.

Gore, W. (2001) Method of desulfurization of hydrocarbons. US Patent 6274785.

Gosling, C.D., Gatan, R.M., and Barger, P.T. (2007) Process for producing hydrogen peroxides. US Patent 7297253.

Gosling, C.D., Gatan, R.M., Cavanna, A., Molinari, D. (2004) NPRA, Annual meeting, San Antonio, Technical paper AM04–48.

Gronowitz, S. (1985) Thiophene and its derivatives. Part one, Weissberger, A. and Taylor, E.C. Volume 44. In: The Series “The Chemistry of Heterocyclic Compounds” John Wiley and Sons, Inc., New York, USA.

Gunnerman, R.W. (2003) Continuous process for oxidative desulfurization of fossil fuels with ultrasound and products thereof. US patent 20030014911.

Guo, H.F., Yan, P., Hao, X.Y., Wang, Z.Z. (2008) Influences of introducing Al on the solid super acid SO₄^2–/SnO₂. Materials Chemistry and Physics. 112(3): 1065- 1068.

Guo, W., Wang, C., Lin, P., and Lu, X. (2011) Oxidative desulfurization of diesel with TBHP/Isobutyl aldehyde/air oxidation system. Applied Energy. 88: 175–179.

Guoxian, Y.U., Hui, C., Shanxiang, L.U., Zhongnan, Z. (2007) Deep desulfurization of diesel fuels by catalytic oxidation. Frontiers of Chemical Engineering in China. 1(2): 162–166.

Gupta, N., Roychoudhury, P. K. (2005) Biotechnology of desulfurization of diesel: Prospects and challenges. Applied Microbiology and Biotechnology. 66: 356–66.

Guseinova, A.D., Mirzoeva, L.M., Yunusov, S.G., Abbasov, M.F., Guseinova, I.S. (2012) Oxidative desulfurization of catalytically cracked gasolines produced from blended Baku oils. Petroleum Chemistry. 52(2): 126–129.

Gustafsson, M., Bartoszewicz, A., Martı´n-Matute, B., Sun, J., Grins, J., Zhao, T., Zou, X. (2010) A family of highly stable lanthanide metal–organic frameworks: structural evolution and catalytic activity. Chemistry of Materials. 22: 3316–3322.

Habimana, F., Huo, Y., Jiang, S., Ji, S. (2016) Synthesis of europium metal–organic framework (Eu-MOF) and its performance in adsorptive desulfurization. Adsorption. 22: 1147–1155.

Halbert, T.R., Stuntz, G.F., Brignac, G.B., Greeley, J.P., Ellis, E.S., Davis, T.J., Kamienski, P., Mayo, S. (2001). Akzo nobel catalyst symposium on SCANfining: A commercially proven technology for low sulfur gasoline. Noordwijkaan Zee, The Netherlands, June 10–13, 2001. http://www.prod.exxonmobil.com/refiningtechnologies/pdf/AkzoSCANfining2001.pdf.

Hatanaka, S. (2005) Hydrodesulfurization of selective catalytic cracked gasoline. Catalysis Surveys from Asia. 9(2): 87–93.

Hatanaka, S., Yamada, M., Sadakane, O. (1998) Hydrodesulfurization of catalytic cracked gasoline. 3. Selective catalytic cracked gasoline hydrodesulfurization on the Co–Mo/-Al₂O₃ catalyst modified by coking pretreatment. Industrial and Engineering Chemistry Research. 37(5): 1748–1754.

Haw, K.-G., Abu Bakar, W.A.W., Ali, R., Chong, J.-F., and Abdul Kadir, A.A. (2010) Catalytic oxidative desulfurization of diesel utilizing hydrogen peroxide and functionalized-activated carbon in a biphasic diesel-acetonitrile system. Fuel Processing Technology. 91(9): 1105–1112.

Heidarinasab, A., Soltanieh, M., Ardjmand, M., Ahmadpanahi, H., Bahmani, M. (2016) Comparison of Mo/MgO and Mo/γ-Al₂O₃ catalysts: impact of support on the structure and dibenzothiophene hydrodesulfurization reaction pathways. International Journal of Environmental Science and Technology. 13: 1065–1076.

Hernández-Maldonado, A.J., Yang, R.T. (2003a) Desulfurization of liquid fuels by adsorption via π-complexation with Cu(I)-Y and Ag-Y zeolites. Industrial and Engineering Chemistry Research. 42(1): 123- 129.

Hernández-Maldonado, A.J., Yang, R.T. (2003b) Desulfurization of commercial liquid fuels by selective adsorption via π-complexation with Cu(I)-Y Zeolite. Industrial and Engineering Chemistry Research. 42(13): 3103–3110.

Hernández-Maldonado, A.J., Yang, R.T. (2004a) Desulfurization of transportation fuels by adsorption. Catalysis Reviews - Science and Engineering. 46(2): 111–150.

Hernández-Maldonado, A.J., Yang, R.T. (2004b). Desulfurization of Diesel Fuels by Adsorption via π-Complexation with Vapor-Phase Exchanged Cu(I)−Y Zeolites Journal of the American Chemical Society. 126(4): 992–993.

Hernández-Maldonado, A.J., Yang, R.T. (2004c) New sorbents for desulfurization of diesel fuels via π-complexation. AIChE Journal. 50(4): 791–801.

Hirai, T., Ogawa, K., and Komasawa, I. (1996) Desulfurization process for dibenzothiophenes from light oil by photochemical reaction and liquid−liquid extraction. Industrial and Engineering Chemistry Research. 35(2): 586–589.

Hirai, T., Shiraishi, Y., Ogawa, K., and Komasawa, I. (1997) Effect of photosensitizer and hydrogen peroxide on desulfurization of light oil by photochemical reaction and liquid−liquid extraction. Industrial and Engineering Chemistry Research. 36(3): 530–533.

Hosseini, H. (2012) Novel methods for desulfurization of fuel oils. International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering. 6(11): 1072–1074.

Houalla M., Nag N.K., Sapre A.V., Broderick D.H., and Gates B.C. (1978) Hydrodesulfurization of dibenzothiophene catalyzed by sulfided CoO-MoO₃γ-Al₂O₃: The reaction network. AIChE Journal. 24: 1015–1021.

Hu, T.p., Zhang, Y.m., Zheng, L.H., Fan, G.z. (2010) Molecular recognition and adsorption performance of benzothiophene imprinted polymer on silica gel surface. Journal of Fuel Chemistry and Technology. 38(6): 722–729.

Huang, L., Wang, G., Qin, Z., Du, M., Dong, M., Ge, H., Wu, Z., Zhao, Y., Ma, C., Hu, T., Wang, J. (2010) A sulfur K-edge XANES study on the transfer of sulfur species in the reactive adsorption desulfurization of diesel oil over Ni/ZnO. Catalysis Communications. 11(7): 592–596

Huang, P., Luo, G., Kang, L., Zhu, M., and Dai, B. (2017) Preparation, characterization and catalytic performance of HPW/aEVM catalyst on oxidative desulfurization. RSC Advances. 7: 4681–4687.

Hulea, V., Fajula, F., and Bousquet, J. (2001) Mild oxidation with H2O2 over Ticontaing molecular sieves-a very efficient method for removing aromatic sulfur compounds from fuels. Journal of Catalysis. 198: 179–186.

Ibrahim, A., Xian S.B., and Wei, Z. (2003) Desulfurization of FCC gasoline by solvent extraction and photooxidation. Petroleum Science and Technology. 21: 1555–1573.

Ibrahim, A., Xian S.B., and Wei, Z. (2004) Desulfurization of FCC gas oil by solvent extraction, photooxidation, and oxidizing agents. Petroleum Science and Technology. 22(3/4): 287–301.

IFP, December 2001. http://www.ifp.fr/INTF/BI000GF2.html.

Imhof P. (2004). Economics of FCC additives for sulfur removal and emission control. Presented at Akzo Nobel Scope 2004 Symposium, Florence, Italy.

Institute Francais du Petrole IFP (2000) Gasoline desulfurization, ultra-deep. Hydrocarbon Processing. 79(9): 114

Ito, E., van Veen, J.A. (2006) On novel processes for removing sulphur from refinery streams. Catalysis Today. 116(4): 446–460.

Jackson, C., Soveran, D. (1995) Desulfurization of crude oils using microwave energy. P-110–300-C-95.

Javadli R., de Klerk A. (2012a) Desulfurization of heavy oil. Applied Petrochemical Research. 1: 3–19.

Javadli, N., de Klerk, A. (2012b) desulfurization of heavy oil-oxidative desulfurization (ODS) as potential upgrading pathway for oil sands derived bitumen. Energy and Fuels. 26: 594–602.

Jiang, B., Yang, H., Zhang, L., Zhang, R., Sun, Y., Huang, Y. (2016) Efficient oxidative desulfurization of diesel fuel using amide-based ionic liquids. Chemical Engineering Journal. 283(1): 89–96.

José da Silva, M., and Faria dos Santos, L. (2013) Novel oxidative desulfurization of a model fuel with H₂O₂ catalyzed by AlPMo₁₂O₄₀ under phase transfer catalyst-free conditions. Journal of Applied Chemistry. Volume 2013, ArticleID147945, http://dx.doi.org/10.1155/2013/147945.

Joskić, R., Dunja, M., and Sertić-Bionda, K. (2014) Oxidative desulfurization of model diesel fuel with hydrogen peroxide. Goriva i Maziva. 53(1): 11–18.

Kabe, T., Ishihara, A., Tajima, H. (1992) Hydrodesulfurization of sulfur-containing polyaromatic compounds in light oil. Industrial & Engineering Chemistry Research. 31: 1577–1580.

Kabe, T., Ishihara, A., Zhang, Q. (1993) Deep desulfurization of light oil. Part 2: hydrodesulfurization of dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene. Applied Catalysis A: General. 97: L1-L9.

Kabe, T., Qian, W., Hirai, Y., Li, L., Ishihara, A. (2000) Hydrodesulfurization and hydrogenation reactions on noble metal catalysts: I. Elucidation of the behavior of sulfur on alumina-supported platinum and palladium using the 35S radioisotope tracer method. Journal of Catalysis. 190:191–180.

Kadijani, J.A., Narimani, E., and Kadijani, H.A. (2014) Oxidative desulfurization of organic sulfur compounds in the presence of molybdenum complex and acetone as catalysts. Petroleum and Coal 56(1): 116–123.

Kadijani, J.A., Narimani, E., Kadijani, H.A. (2016) Using response surface methodology to optimize ultrasound-assisted oxidative desulfurization. Korean Journal of Chemical Engineering. 33(4): 1286–1295

Kaluža, I., Gulková, D. (2016) Effect of promotion metals on the activity of MoS₂/ZrO₂ catalyst in the parallel hydrodesulfurization of 1-benzothiophene and hydrogenation of 1-methyl-cyclohex-1-ene. Reaction Kinetics, Mechanisms and Catalysis. 118: 313–324.

Kanda, Y., Iwamoto, H., Kobayashi, T., Uemichi, Y., Sugioka, M., (2009) Preparation of highly active alumina-pillared clay montmorillonite-supported platinum catalyst for hydrodesulfurization. Topics in Catalysis. 52(6/7): 765–771.

Kapteijn, F., Nijhuis, T.A., Heiszwolf, J.J., Moulijn, J.A. (2001) New non-traditional multiphase catalytic reactors based on monolithic structures. Catalysis Today. 66: 133–144.

Karas, L.J., Grey, R.A., and Lynch, M.W. (2008) Desulfurization process. US Patent 20080047875.

Kareem, S.A. (2016a) Anaerobic biodesulfurization of kerosene part I: Identifying a capable microorganism. IOSR Journal of Biotechnology and Biochemistry. 2(5): 46–53.

Kareem, S.A. (2016b) Anaerobic biodesulfurization of kerosene part II: Investigating its kinetics. IOSR Journal of Biotechnology and Biochemistry. 2(5): 37–45.

Kaufmann, T.G., Kaldor, A., Stuntz, G.F., Kerby, M.C., Ansell, L.L. (2000) Catalysis science and technology for cleaner transportation fuels. Catalysis Today. 62(1): 77–90.

Kazakov, A.A., Tarakanov, G.V., Ionov. N.G. (2016) Mechanisms of oxidative desulfurization of straight-run residual fuel oil using ozonized air. Chemistry and Technology of Fuels and Oils. 52(1): 33–37.

Khalili, F.I., Sultan, M., Robl, C., Al-Ghouti, M. (2015) Insights into the remediation characterization of modified bentonite in minimizing organosulphur compounds from diesel fuel. Journal of Industrial and Engineering Chemistry. 28: 282–293.

Khamis, F.S., Palichev, T.V. (2012) Production of ultra-low sulfur gasoline and assessment of the efficiency of ferrocene antiknock additive. International Journal of Engineering and Applied Sciences. 1(1): 11–16.

Khan, M.R., Al-Shafel, E.N. (2013) Microwave-promoted desulfurization of crude oil. US Patent 20130341247 A1.

Khodaei, B., Sobati, M.A., Shahhosseini, Sh. (2016a) Optimization of ultrasound-assisted oxidative desulfurization of high sulfur kerosene using response surface methodology (RSM). Clean Technologies and Environmental Policy. 18: 2677–2689.

Khodaei, B., Sobati, M.A., Shahhosseini, Sh. (2016b) Rapid oxidation of dibenzothiophene in model fuel under ultrasound irradiation. Monatshefte für Chemie - Chemical Monthly. DOI 10.1007/s00706–016-1801-z.

Kim, J.H., Ma, X., Song, C., Oyama, S.T., Lee, Y.-K. (2003) Kinetic Study of 4,6-Dimethyldibenzothiophene hydrodesulfurization over Ni phosphide, NiMo and CoMo sulfide catalysts. Fuel Chemistry Division Preprints. 48(1): 40–41.

Kim, J.H., Ma, X., Zhou, A., Song, C. (2006) Ultra-deep desulfur- ization and denitrogenation of diesel fuel by selective adsorption over three different adsorbents: a study on adsorptive selectivity and mechanism. Catalysis Today, 111: 74–83.

Kim, T.S., Kim, H.Y., Kim, B.H. (1990) Petroleum desulfurization by Desulfovibrio desulfuricans M6 using electrochemically supplied reducing equivalentBiotechnology Letters. 12(10): 757–760.

King, C.J. (1987) Separation Processes Based on Reversible Chemical Complexation. In: Handbook of Separation Process Technology. Rousseau, R.W., ed., Wiley, New York, USA.

Klicpera, T., Zdrazil, M. (2002) Preparation of High-activity MgOsupported Co–Mo and Ni–Mo sulfide hydrodesulfurization catalysts. Journal of Catalysis. 206: 314–320.

Kohler, M., Genz, I.L., Schicht, B., Eckart, V. (1984) Microbial desulphurization of petroleum and heavy petroleum fractions: anaerobic desulphurization of organic sulphur compounds of petroleum. Zentralblatt für Mikrobiologie. 139: 239–247.

Kong, A., Wei, Y. and Li, Y. (2013) Reactive adsorption desulfurization over a Ni/ZnO adsorbent prepared by homogeneous precipitation. Frontiers of Chemical Science and Engineering. 7: 170–176.

Krishnaiah, G., Balko, J. (2003) Reduce ultra-low sulfur gasoline compliance costs with Davison clean fuels technologies, S-BraneTM membrane process for sulfur removal and FCC catalyst technologies for clean fuels, AM-03–121, in: Proceedings of the NPRA 2003 Annual Meeting, San Antonio, TX, March 23–25, 2003.

Krivtcova, N.I., Tataurshikov, A.A., Ivanchina, E.D., Krivtsov, E.B. (2015) Mathematical modelling of diesel fuel hydrodesulfurization kinetics. Procedia Chemistry. 15: 180–186.

Krivtsov, E.B., and Golovko, A.K. (2014) The kinetics of oxidative desulfurization of diesel fraction with a hydrogen peroxide-formic acid mixture. Petroleum Chemistry. 54(1): 51–57.

Krotz, A.H., Mehta, R.C., and Hardee, G.E. (2005) Peroxide-mediated desulfurization of phosphorothioate oligonucleotides and its prevention. Journal of Pharmaceutical Sciences. 94(2): 341–352.

Kuehler, C.W., Humphries, A. (2003) Meeting clean fuels objectives with the FCC, AM-03–57, in: Proceedings of the NPRA 2003. Annual Meeting, San Antonio, TX, March 23–25, 2003.

Kurita, S., Endo, T., Nakamura, H., Yagi, T., Tamuja, N.J. (1971) Decomposition of some organic sulfur compounds in petroleum by anaerobic bacteria. The Journal of General and Applied Microbiology. 17: 185–198.

La Paz Zavala, C., Rodriguez, J.E. (2004) Practical Applications of a Process Simulator of Middle Distillates Hydrodesulfurization. Petroleum Science and Technology. 22(1/2): 61–71.

Landau, M.V., Berger, D., Herskowitz, M. (1996) Hydrodesulfurization of Methyl-Substituted Dibenzothiophenes: Fundamental Study of Routes to Deep Desulfurization. Journal of Catalysis 159: 236–245.

Landau, M.V., Herskowitz, M., Hoffman, T., Fuks, D., Liverts, E., Vingurt, D., Froumin, N. (2009) Ultradeep hydrodesulfurization and adsorptive desulfurization of diesel fuel on metal-rich nickel phosphides. Industrial and Engineering Chemistry Research. 48(11): 5239–5249.

Lappas, A., Valla, J., Vasalos, I., Kuehler, C., Francis, J., O’Connor, P., Gudde, N. (2002). Sulfur reduction in FCC gasoline. American Chemical Society Preprints-Division Petroleum Chemistry. 47:50–52.

Larrubia, M.A., Gutierrez-Alejandre, A., Ramirez, J., Busca, G. (2002) A FT-IR Study of the Adsorption of indole, carbazole, benzothiophene, dibenzothiophene, and 4,6-dibenzothiophene over solid adsorbents and catalysts. Applied Catalysis A: General. 224: 167–178.

Le Borgne, S., Quintero, R. (2003) Biotechnological processes for the refining of petroleum. Fuel Processing Technology. 81(2): 155–169.

Lecrenay, E., Sakanishi, K., Mochida, I. (1997) Catalytic hydrodesulfurization of gas oil and model sulfur compounds over commercial and laboratory made CoMo and NiMo catalysts: Activity and reaction scheme. Catalysis Today. 39(1/2): 13–20.

Lecrenay, E., Sakanishi, K., Mochida, I., Suzuka, T. (1998) Hydrodesulfurization activity of CoMo and NiMo catalysts supported on some acidic binary oxides. Applied Catalysis A: General. 175: 237–243.

Lee, M.-C., Cheng, S.-H., Hong, C.-T. (2013) Review and comparison of FCC gasoline selective hydrodesulfurization process. Journal of Petroleum. 49(4): 45–54.

Lee, S.H.D., Kumar, R., Krumpelt, M. (2002). Sulphur removal from diesel fuel contaminated methanol. Separation and Purification Technology. 26: 247 –258.

Leflaive, P., Lemberton, J.L., Pérot, G., Mirgain, C., Carriat, J.Y., Colin, J.M. (2002) On the origin of sulfur impurities in fluid catalytic cracking gasoline–Reactivity of thiophene derivatives and of their possible precursors under FCC conditions. Applied Catalysis A: General. 227: 201–215.

Leliveld, R.G., Eijsbouts, S.E. (2008) Howa70-year-oldcatalyticrefineryprocess is still ever dependent on innovation. Catalysis Today. 130:183–189.

Lesemann, M., Schult, C. (2003). Non-capital intensive technologies reduce FCC sulfur content. Hydrocarbon Processing. 2:69–76.

Levy, R.E. (2003) Refining: the oxidative route to ULS production. Petroleum Technology Quaternary. 8: 53–58.

Levy, R.E., Rappas, A.S., Nero, V.P., DeCanio, S.J. (2002) UniPure’s breakthrough technologies for cost effective desulfurization of crudes and refined products. World Petroleum Congress Proceeding. 3: 339–340.

Li, H., He, L., Li, J., Zhu, W., Jiang, X., Wang, Y., Yan, Y. (2009d) Fatty acid methyl ester synthesis catalyzed by solid superacid catalyst SO₄^2–/ZrO₂–TiO₂/La³⁺. Applied Energy. 87(1): 156–159.

Li, H., He, L., Li, J., Zhu, W., Jiang, X., Wang, Y., Yan, Y. (2009b) Deep oxidative desulfurization of fuels catalyzed by phosphotungstic acid in ionic liquids at room temperature. Energy and Fuels. 23(3): 1354–1357.

Li, H.M., Jiang, X., Zhu, W.S., Lu, J.D., Shu, H.M., and Yan, Y.S. (2009c) Deep oxidative desulfurization of fuel oils catalyzed by decatungstates in the ionic liquid of [Bmim]PF₆. Industrial and Engineering Chemistry Research. 48(19): 9034–9039.

Li, W., Tang, H., Liu, Q., Xing, J., Li, Q., Wang, D., Yang, M., Li, X., Liu, H. (2009e). Deep Desulfurization of Diesel by Integrating Adsorption and Microbial Method. Biochemical Engineering Journal. 44: 297–301.

Li, W.L., Xing, J.M., Xiong, X.C., Liu, H.Z. (2005) Bio-regeneration of π-complexation desulfurization adsorbents. Science in China Series B: Chemistry 48(6): 538–544.

Li, X., Zhou, F., Wang, A., Wang, L., Hu, Y. (2009a) Influence of templates on the overgrowth of mcm-41 over HY and the hydrodesulfurization performances of the supported Ni−Mo catalysts. Industrial and Engineering Chemistry Research. 48(6): 2870–2877.

Lin, Z.J., Yang, Z., Liu, T.F., Huang, Y.B., Cao, R. (2012) Microwave assisted synthesis of a series of lanthanide metal–organic frameworks and gas sorption properties. Inorganic Chemistry. 51: 1813–1820.

Liotta, F.J., Han, Y. (2003) Production of ultra-low sulfur fuels by selective hydroperoxide oxidation. Lyondell Chemical Compant. NPRA Annual Meeting, Newtown Square, PA, USA.

Liu, L., Zhang, Y., Tan, W. (2014a) Ultrasound-Assisted oxidation of dibenzothiophene with phosphotungstic acid supported on activated carbon. Ultrasonics Sonochemistry. 21(3): 970–974.

Liu, S., Wang, B., Cui, B., Sun, L. (2008) Deep desulfurization of diesel oil oxidized by Fe(vi) systems. Fuel. 87: 422–428.

Liu, W., Liu, X., Yang, Y., Zhang, Y., Xu, B. (2014c) Selective removal of benzothiophene and dibenzothiophene from gasoline using double-template molecularly imprinted polymers on the surface of carbon microspheres. Fuel. 117: 184–190.

Liu, X., Wang, J.Y., Li, Q.Y., Jiang, S., Zhang, T., Ji, S.F. (2014b) Synthesis of rare earth metal-organic frameworks (Ln-MOFs) and their properties of adsorption desulfurization. Journal of Rare Earths. 32:189–194.

Liu, X.J., Ouyang, C.B., Zhao, R., Shangguan, D.H., Chen, Y., Liu, G.Q. (2006) Monolithic molecularly imprinted polymer for sulamethoxazole and molecular recognition properties in aqueous mobile phase. Analytica Chimica Acta. 571(2): 235–241.

Long, Z., Yang, C., Zeng, G., Peng, L., Dai, C., He, H. (2014) Catalytic oxidative desulfurization of dibenzothiophene using catalyst of tungsten supported on resin D152. Fuel. 130: 19–24.

Loupy A. (2006) “Microwaves in organic synthesis”, second ed., WILEY-VCH Verlag GmbH & Co kGaA, Weinheim.

Lucien, J.P., van den Berg, J.P., Germaine, G., van Hooijdonk, H.M.J.H., Gjers, M., Thielemans, G.L.B. (1994) Shell middle distillate hydrogenation process, in: M.C. Oballa, S.S. Shih (Eds.), Catalytic Hydroprocessing of Petroleum and Distillates, Marcel Dekker, New York, 1994, pp. 291–313.

Ma, X., Sakanishi, K., Isoda, T., Mochida, I. (1995) Quantum chemical calculation on the desulfurization reactivities of heterocyclic sulfur compounds. Energy & Fuels. 9: 33–37.

Ma, X., Sakanishi, K., Isoda, T., Mochida, I. (1996) Comparison of sulfided CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts, in: M.L. Occelli, R. Chianelli (Eds.), Hydrodesulfurization of Gas Oil Fractions and Model Compounds, in Hydrotreating Technology for Pollution Control, Marcel Dekker, New York. Pp. 183.

Ma, X., Sun, L., Song, C. (2002) A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell applications. Catalysis Today. 77: 107–116.

Ma, X., Zhou, A., Song, C. (2007) A novel method for oxidative desulfurization of liquid hydrocarbon fuels based on catalytic oxidation using molecular oxygen coupled with selective adsorption. Catalysis Today. 123: 276–284.

Majid, D., Seyedeyn-Azad, F., (2010) Desulfurization of gasoline over nanoporous nickel-loaded Y-type zeolite at ambient conditions. Industrial and Engineering Chemistry Research. 49: 11254–11259.

Mamaghani, A.H., Fatemi, S., Asgari, M. (2013) Investigation of influential parameters in deep oxidative desulfurization of dibenzothiophene with hydrogen peroxide and formic acid. International Journal of Chemical Engineering. Volume 2013, Article ID 951045, http://dx.doi.org/10.1155/2013/951045.

Marchal, N., Kasztelan, S., Mignard, S. (1994) A comparative study of catalysts for the deep aromatic reduction in hydrotreated gas oil, in: M.C. Oballa, S.S. Shih (Eds.), Catalytic Hydroprocessing of Petroleum and Distillates, Marcel Dekker, New York, 1994, pp. 315–327.

Marin, C., Escobar, J., Galvan, E., Murrieta, F., Zarate, R., Vaca, H. (2004) Light straight-run gas oil hydrotreatment over sulfided CoMoP/Al₂O₃-USY zeolite catalysts. Fuel processing Technology. 86(4): 391–405.

Matsuzawa, S., Tanaka, J., Sato, S., Takashi, I. (2002) Photocatalytic oxidation of dibenzothiophenes in acetonitrile using TiO₂: effect of hydrogen peroxide and ultrasound irradiation. Journal of Photochemistry and Photobiology A: Chemistry. 149(1/3): 183–189.

McKinley, S.G., Angelici, R.J. (2003) Deep desulfurization by selective adsorption of dibenzothiophenes on Ag⁺/SBA-15 and Ag^+/SiO₂. Chemical Communications. 20: 2620–2621.

Mei, H., Mei, B.W., Yen, T.F. (2003) A new method for obtaining ultra-low sulfur diesel fuel via ultrasound assisted oxidative desulfurization. Fuel. 82(4): 405–414.

Mello, P.D.A., Duarte, F.A., Nunes, M.A.G., Alencar, M.S., Moreira, E.M., Korn, M., Gressler, V.L., Flores, E.M.M. (2009) Ultrasound-assisted oxidative process for sulfur removal from petroleum product feedstock. Ultrasonics Sonochemistry. 16(6): 732–736.

Miadonye, A., Snow, S., Irwin, D.J.G., Rashid Khan, M. (2009) Desulfurization of heavy crude oil by microwave irradiation. WIT Transactions on Engineering Sciences. 63: 455–465.

Mikhail, S., Zaki, T., Khalil, L. (2002) Desulfurization by an economically adsorption technique. Applied Catalysis A: General. 227(1/2): 265–278.

Milenkovic, A., Macaud, M., Schulz, E., Koltai, T., Loffreda, D., Vrinat, M., Lemaire, M. (2000) How could organic synthesis help the understanding of the problems of deep hydrodesulfurization of gas oils? Comptes Rendus de l’Académie des Sciences - Series IIC - Chemistry. 3 (6): 459–463.

Miller, J.T., Reagan, W.J., Kaduk, J.A., Marshall, C.L., Kropf, A.J. (2000) Selective hydrodesulfurization of FCC naphtha with supported MoS₂ catalyst. The role of cobalt. Journal Catalysis. 193(1): 123–131.

Miller, K.W. (1992) Reductive desulfurization of dibenzyldisulfide. Applied and Environmental Microbiology. 58(7): 2176–2179.

Mjalli, F.S., Ahmed, A.U., Al-Wahaibi, T., Al-Wahaibi, Y., AlNashef, I.M. (2014) Deep oxidative desulfurization of liquid fuels. Reviews in Chemical Engineering. 30(4): 337–378.

Mohammed, M.I., Abdul Razak, A.A., Shehab, M.A. (2016) Synthesis of nanocatalyst for hydrodesulfurization of gasoil using laboratory hydrothermal rig. Arabian Journal of Science and Engineering. doi:10.1007/s13369–016-2249–5.

Mohebali, G., Ball, A.S. (2008) Biocatalytic desulfurization (BDS) of petrodiesel fuels. Microbiology. 154, 2169–2183.

Monticello, D.J. (1994) Biocatalytic desulfurization. Hydrocarbon Processing. 39: 39–44.

Monticello, D.J. (1996) Multistage process for deep desulfurization of a fossil fuel. US Patent 5,510,265.

Monticello, D.J. (2000) Biodesulfurization and the upgrading of petroleum distillates. Current Opinion in Biotechnology. 11: 540–546.

Morales-Ortuño, J.C., Ortega-Domínguez, R.A., Hernández-Hipólito, P., Bokhimi, X., Klimova, T.E. (2016) HDS performance of NiMo catalysts supported on nanostructured materials containing titania. Catalysis Today.271: 127–139

Murata, S., Murata, K., Kidena, K., and Nomura, M. (2003) Oxidative desulfurization of diesel fuels by molecular oxygen. Preprints of Papers- American Chemical Society, Division of Fuel Chemistry. 48(2): 531.

Mustafa, F., Al-Ghouti, M.A., Khalili, F.I., Al-Degas, Y.S. (2010) Characteristics of organosulphur compounds adsorption onto Jordanian zeolitic tuff from diesel fuel. Journal of Hazardous Materials. 182(1/3): 97–107.

Mutyala, S., Fairbridge, C., Pare, J.R.J., Belanger, J.M.R., Hawkins, S.Ng.R. (2010) Microwave applications to oil sands and petroleum: A review. Fuel Processing Technology. 91: 127–135.

Mužic, M., Sertić-Bionda, K. (2013) Alternative processes for removing organic sulfur compounds from petroleum fractions. Chemical and Biochemical Engineering. Q. 27(1): 101–108.

Mužic, M., Sertić-Bionda, K., Adžamić, T., Gomiz, Z. (2010) A design of experiments investigation of adsorptive desulfurization of diesel fuel. Chemical and Biochemical Engineering Quarterly. 24(3): 253–264.

Myrstad, T. (2000). Effect of nickel and vanadium on sulphur reduction of FCC naphtha. Applied Catalysis A: General. 192 (2): 299–305.

Nakano, K., Ali, S.A., Kim, H., Kim, T., Alhooshani, K., Park, J., Mochida, I. (2013) Deep desulfurization of gas oil over NiMoS catalysts supported on alumina coated USY-zeolite. Fuel Processing Technology. 116: 44–51.

Nanoti, A., Dasgupta, S., Goswami, A.N., Nautiyal, B.R., Rao, T.V., Sain, B., Sharma, Y.K., Nanoti, S.M., Garg, M.O., and Gupta, P. (2009) Mesoporous silica as selective sorbents for removal of sulfones from oxidized diesel fuel. Microporous and Mesoporous Materials. 124: 94–99.

Nehlsen, J.P.; Benziger, J.B.; Kevrekidis, I.G. (2003) Removal of alkanethiols from a hydrocarbon mixture by a heterogeneous reaction with metal oxides. Industrial and Engineering Chemistry Research. 42(26): 6919–6923.

Nijhuis, T.A., Kreutzer, M.T., Romijn, A.C.J., Kapteijn, F., Moulijn, J.A. (2001) Monolithic catalysts as more efficient three-phase reactors. Catalysis Today. 66: 157–165.

Niquille-Röthlisberger A., and Prins R. (2007) Hydrodesulfurization of 4,6-dimethyldibenzothiophene over Pt, Pd, and Pt–Pd catalysts supported on amorphous silica–alumina. Catalysis Today. 123:198–207.

Niquille-Röthlisberger, A, Prins, R (2006) Hydrodesulfurization of 4,6-dimethyldibenzothiophene and dibenzothiophene over alumina-supported Pt, Pd, and Pt-Pd catalysts. Journal of Catalysis. 242:207–216.

Nocca, J.-L., Cosyns, J., Debuisschert, Q., Didillon, B. (2000) The domino interaction of refinery processes for gasoline quality attainment, AM-00–61, in: Proceedings of the NPRA Annual Meeting, March 2000, San Antonio, TX.

Ohshiro, T., Izumi, Y. (1999) Microbial desulfurization of organic sulfur compounds in petroleum. Bioscience, Biotechnology and Biochemistry. 63(1): 1–9.

Okada, H., Nomura N., Nakahara T. and Maruhashi K. (2002a) Analyses of substrate specificity of the desulfurizing bacterium Mycobacterium sp. G3. Journal of Bioscience and Bioengineering. 93(2): 228–233.

Okada, H., Nomura N., Nakahara T. and Maruhashi K. (2002b) Analysis of dibenzothiophene metabolic pathway in Mycobacterium strain G3. Journal of Bioscience and Bioengineering. 93(5): 491–497.

Okamoto, Y., Ochiai, K., Kawano, M., Kobayashi, M., Kubota, T. (2002) Effects of support on the activity of Co–Mo sulfide model catalysts. Applied Catalysis A: General. 226(1/2): 115–161.

Otsuki, S., Nonaka, T., Qian, W., Ishihara, A., Kabe, T. (1999) Oxidative desulfurization of middle distillate using ozone. Sekiyu Gakkaishi (Journal of the Japan Petroleum Institute). 42(5): 315–320.

Otsuki, S., Nonaka, T., Qian, W.H., Ishihara, A., Kabe, T. (2001) Oxidative desulfurization of middle distillate-oxidation of dibenzothiophene using t-butyl hypochlorite. Sekiyu Gakkaishi. (Journal of the Japan Petroleum Institute). 44(1): 18–24.

Otsuki, S., Nonaka, T., Takashima, N. (2000) Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction. Energy and Fuels. 14(6): 1232–1239.

Pacheco, M.A. (1999) Recent advances in biodesulfurization (biodesulfurization) of diesel fuel. Paper presented at the NPRA Annual Meeting, San Antonio, TX, 21–23 March 1999.

Palaić, N., Sertić-Bionda, K., Margeta, D., and Podolski, Š. (2015) Oxidative desulfurization of diesel fuels. Chemical And Biochemical Engineering Quarterly. 29(3): 323–327.

Park T.H., Cychosz K.A., Wong-Foy A.G., Dailly A., and Matzger A.J. (2011) Gas and liquid phase adsorption in isostructural Cu₃[biaryltricarboxylate]₂ microporous coordination polymers. Chemical Communications. 47: 1452–1454.

Park, J.G., Ko, C.H., Yi, K.B., Park, J.H., Han, S.S., Cho, S.H., Kim, J.N. (2008) Reactive adsorption of sulfur compounds in diesel on nickel supported on mesoporous silica. Applied Catalysis B: Environmental. 81: 244–250.

Pawelec, B., Navarro, R., Fierro, J. L.G., Cambra, J. F., Zugazaga, F., Guemez, M.B., Arias, P.L. (1997) Hydrodesulfurization over PdMo/HY zeolite catalysts. Fuel. 76(1): 61–71.

Phillipson, J.J. (1971) Kinetics of hydrodesulfurization of light and middle distillates: Paper Presented at the American Institute of Chemical Engineers Meeting, Houston, TX. USA.

Pimerzin, Al.A., Nikul’shin, P.A., Mozhaev, A.V., Pimerzin, A.A. (2013) Effect of surface modification of the support of hydrotreating catalysts with transition metal oxides (sulfides) on their catalytic properties. Petroleum Chemistry. 53(4): 245–254.

Purnell, S.K., Hunt, D.A., Leach, D. (2002) Catalytic reduction of sulfur and olefins in the FCCU: commercial performance of Davison catalysts and additives for clean fuels, AM-02–37, in: Proceedings of the NPRA 2003 Annual Meeting, SanAntonio, TX, March 17–19, 2002.

Purta, D.A., Portnoff, M.A., Pourarian, F., Nasta, M.A., Zhang, J. (2004) Catalyst for the treatment of organic compounds. US 2004077485.

Qu, L., Zhang, W., Kooyman, P.J., Prins, R. (2003) MAS NMR, TPR, and TEM studies of the interaction of NiMo with alumina and silica–alumina supports. Journal of. Catalysis. 215: 7–13.

Rafiee, E., Joshaghani, M., Abadi, P.G.-S. (2017) Oxidative desulfurization of diesel by potato based - carbon as green support for H₅PMo₁₀V₂O₄₀: Efficient composite nanorod catalyst. Journal of Saudi Chemical Society. 21: 599–609.

Rafiee, E., Rahpeyma, N. (2015) Selective oxidation of sulfurs and oxidation desulfurization of model oil by 12-tunstophosphoric acid on cobalt-ferrite nanoparticles as magnetically recoverable catalyst. Chinese Journal of Catalysis. 36: 1342–1349.

Rakhmanov, E.V., Anisimov, A.V., Tarakanova, A.V., Baleeva, N.S., Guluzade, D. (2013) Oxidative desulfurization of catalytically cracked gasoline with hydrogen peroxide. Petroleum Chemistry. 53(3): 201–204.

Rakhmanov, E.V., Baranova, S.V., Zixiao, W., Tarakanova, A.V., Kardashev, S.V., Akopyan, A.V., Naranov, E.R., Oshchepkov, M.S., Anisimov, A.V. (2014b) Hydrogen peroxide oxidative desulfurization of model diesel mixtures using azacrown ethers. Petroleum Chemistry. 54(4): 316–322.

Rakhmanov, E.V., Domashkin, A.A., Myltykbaeva, Zh.K., Kairbekov, Zh., Shigapova, A.A., Akopyan, A.V., Anisimov, A.V. (2016) Peroxide oxidative desulfurization of a mixture of nonhydrotreated vacuum gas oil and diesel fraction. Petroleum Chemistry. 56(8): 742–744.

Rakhmanov, E.V., Tarakanova, A.V., Valieva, T., Akopyan, A.V., Litvinovaa, V.V., Maksimov, A.L., Anisimov, A.V., Vakarin, S.V., Semerikova, O.L., Zaikov, Y.P. (2014a) Oxidative desulfurization of diesel fraction with hydrogen peroxide in the presence of catalysts based on transition metals. Petroleum Chemistry. 54(1): 48–50.

Ramírez-Verduzco, L.F., Torres-García, E., Gómez-Quintana, R., Gonzálezpeňa, V., Murrieta-Guevara, F. (2004) Desulfurization of diesel by oxidation/extraction scheme: influence of the extraction solvent. Catalysis Today. 98: 289–294.

Rana, M.S., Samano, V., Ancheyta, J., Diaz, J. (2007) A review of recent advances on process technologies for upgrading of heavy oils and residua. Fuel. 86(9): 1216–1231.

Rang, H., Kann, J., Oja, V. (2006) Advances in desulfurization research of liquid fuel. Oil Shale. 23(2): 164–176.

Rao, T.V., Krishna, P.M., Paul, D., Nautiyal, B.R., Kumar, J., Sharma, Y.K., Nanoti, S.M., Sain, B., Garg, M.O. (2011). The oxidative desulfurization of HDS diesel: using aldehyde and molecular oxygen in the presence of cobalt catalysts. Petroleum Science and Technology. 29(6): 626–632.

Reddy, K.M., Wei, B., Song, C. (1998) Mesoporous molecular sieve MCM-41 supported Co–Mo catalyst for hydrodesulfurization of petroleum resids. Catalysis Today. 43(3/4): 261–272.

Reid, T.A. (2000) Hydrotreating Approaches to Meet Tier 2 Gasoline Regulations, Catalyst Courier No. 42, Akzo Nobel. Pp. 1–6.

Reinhoudt, H.R., Troost, R., Van Langeveld, A.D., Sie, S.T., Van Veen, J.A.R., Moulijn, J.A. (1999) Catalysts for second-stage deep hydrodesulfurisation of gas oils. Fuel Processing Technology. 61: 133–147.

Ren, J., Wang, A., Li, X., Chen, Y., Liu, H., Hu, Y. (2008) Hydrodesulfurization of dibenzothiophene catalyzed by Ni-Mo sulfides supported on a mixture of MCM-41 and HY zeolite. Applied Catalysis A: General. 344: 175–182.

Roberie, T.G., Kumar, R., Ziebarth, M.S., Cheng, W.-C., Zhao, X., Bhore, N. (2002). Gasoline sulfur reduction in FCC. U.S. Patent 6,482,315 B1. Assigned to W.R. Grace & Co.-Conn.

Robinson, W.R.A.M., van Veen, J.A.R., De Beer, V.H.J., van Santen, R.A. (1999) Development of deep hydrodesulfurization catalysts: II. NiW, Pt and Pd catalysts tested with (substituted) dibenzothiophene. Fuel Processing Technology. 61: 103–116.

Rock, K.L. (2002) Ultra-low sulfur gasoline via catalytic distillation, in: Proceedings of the Fifth International Conference onRefinery Processing, AIChE 2002 Spring National Meeting, New Orleans, LA, March 11–14, 2002, pp. 200–205.

Rock, R., Shorey, S. (2003) Producing low sulfur gasoline reliably, AM-03–122, in: Proceedings of the NPRA 2003 Annual Meeting, San Antonio, TX, March 23–25, 2003.

Ryan, G.H., Xie, X.J., Sofia, R.P., Isabel, M., Edward, I.S. (2012) Analysis of resonance Raman data on the blue copper site in pseudoazurin: Excited state π and σ charge transfer distortions and their relation to ground state reorganization energy Journal of Inorganic Biochemistry. 115: 155–162.

Salazar, J.A., Cabrera, L.M., Palmisano, E., Garcia, W.J., Solari, R.B. (1998) US Patent 5,770,047.

Salcedo, S.T.G. (2015) Hydrotreating of synthetic fuels with bulk and supported Ni-Mo(W) sulfide catalysts. MSc. Thesis, Technische Universität München, Arcisstraße 21, 80333 München, Germany.

Saleh, T.A., Danmaliki, G.I. (2016a) Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes. Journal of the Taiwan Institute of Chemical Engineering. 60: 460–468.

Saleh, T.A., Danmaliki, G.I. (2016b) Adsorptive desulfurization of dibenzothiophene from fuels by rubber tyres-derived carbons: Kinetics and isotherms evaluation. Process Safety and Environmental Protection. 102: 9–19.

Saleh, T.A., Sulaiman, K.O., AL-Hammadi, S.A., Dafalla, H., Danmaliki, G.I. (2017) Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation. Journal of Cleaner Production. 154: 401–412.

Salem, A.S., Hamid, H.S. (1997) Removal of Sulfur Compounds from Naphtha solutions using solid adsorbents. Chemical Engineering Technology. 20(5): 342–347.

Sampanthar, J.T., Xiao, H., Dou, J., Yin Nah, T., Rong, X., and Kwan, W.P. (2006) A novel oxidative desulfurization process to remove refractory sulfur compounds from diesel fuel. Applied Catalysis B: Environment. 63(1/2): 85–93.

Sankaranarayanan, T.M., Banu, M., Pandurangan, A., Sivasanker, S. (2011) Hydroprocessing of sun flower oil gas oil blends over sulfide Ni-Mo-AL-zeolite beta composites. Bioresource Technology. 102: 10717–10723.

Sano, Y., Kazomi, S., Ki-Hyonk, C., Yozo, K., Isao, M. (2005) Two-step adsorption process for deep desulfurization of diesel oil. Fuel. 84: 903–910.

Santana, R.C., Do P.T., Santikunaporn, M., Alvarez, W.E., Taylor, J.D., Surghrue, E.L., Resasco, D.E. (2006) Evaluation of different reaction strategies for the improvement of cetane number in diesel fuels. Fuel. 85(5/6): 643–656.

Sarda, K.K., Bhandari, A., Pant, K.K. Jain, S. (2012) Deep desulfurization of diesel fuel by selective adsorption over Ni/Al₂O₃ and Ni/ZSM-5 extrudates. Fuel. 93, 86–91.

Schulz, H., Böhringer, W., Ousmanov, F., Waller, P. (1999) Refractory sulfur compounds in gas oils. Fuel Processing Technology. 61: 5–41.

Segawa, K., Takahashi, K., Satoh, S. (2000) Development of new catalysts for deep hydrodesulfurization of gas oil. Catalysis Today. 63: 123–131.

Shakirullah, M., Ahmad, I. Ishaq, M., Ahmad, W. (2009) Study on the role of metal oxides in desulphurization of some petroleum fractions. Journal of the Chinese Chemical Society. 56: 107–114.

Shan, H., Li, C., Yuan, M., Yang, H. (2002a). Evaluation of a sulfur reduction additive for FCC gasoline. Preprints American Chemical Society- Division of Petroleum Chemistry. 47:55–57.

Shan, H., Yang, C., Zhao, H., Zhang, J. (2002b). Mechanistic studies on thiophene cracking over USY zeolite. Catalysis Today. 77:117–126.

Shang, H., Du, W., Liu, Z., Zhang, H. (2013) Development of microwave induced hydrodesulfurization of petroleum streams: A review. Journal of Industrial and Engineering Chemistry. 19: 1061–1068.

Shao, X., Zhang, X., Yu, W., Wu, Y., Qin, Y., Sun, Z., Song, L. (2012) Effects of surface acidities of MCM-41 modified with MoO3 on adsorptive desulfurization of gasoline. Applied Surface Science. 263: 1–7.

Sharipov, A.K., Nigmatullin, V.R. (2005) Oxidative desulfurization of diesel fuel (a review). Petroleum Chemistry. 45(6): 371–377.

Shen, Y., Xu, X. and Li, P. (2012) A novel potential adsorbent for ultra deep desulfurization of jet fuels at room temperature. RSC Advances. 2: 6155–6160.

Shiflett, W.K.A. (2002) User’s guide to the chemistry, kinetics and basic reactor engineering of hydroprocessing, Proceedings of the Fifth International Conference on Refinery Processing,

AIChE 2002 Spring National Meeting, New Orleans, March 11–14, 2002. Pp. 101–122.

Shiflett W.K. and Krenzke, L.D. (2001) consider improved catalyst technologies to remove sulfur, Hydrocarbon Processing. 81:41–43.

Shih, S.S., Owens, P.J., Palit, S., Tryjankowski, D.A. (1999) Mobil’s OCTGain process: FCC gasoline desulfurization reaches a new performance level, AM-99–30, in: Proceedings of the NPRA 1999 Annual Meeting, March 21–23, 1999.

Shiraishi, Y., Hirai, T., and Komasawa, I. (1998) A deep desulfurization process of light oil by photochemical reaction in an organic two-phase liquid-liquid extraction system. Industrial and Engineering Chemistry Research. 37(1): 203–211.

Shiraishi, Y., Hirai, T., Komasawa, I. (1999a) Identification of desulfurization products in the photochemical desulfurization process for benzothiophenes and dibenzothiophenes from light oil using an organic two-phase extraction system. Industrial and Engineering Chemistry Research. 38(9): 3300–3309.

Shiraishi, Y., Taki, Y., Hirai, T., and Komasawa, I. (1999b) Photochemical desulfurization of light oils using oil hydrogen peroxide aqueous solution extraction system: application for high sulfur content straight-run light gas oil and aromatic light cycle oil. Journal of Chemical Engineering of Japan. 32(1): 158–161.

Shu, Y., Wormsbecher, R. (2007) Catalytic FCC Gasoline: Sulfur Reduction Mechanism Excerpted from The Journal of Catalysis. Catalagram 102: 31–36. www.e-catalysts.com.

Si, X., Cheng, S., Lu, Y., Gao, G., He, M.-Y. (2008) Oxidative desulfurization of model oil over Au/Ti-MWW. Catalysis Letters. 122: 321–324.

Siddiqui, M.A.B., Aitani, A.M. (2007) FCC gasoline sulfur reduction by additives:

A review. Petroleum Science and Technology. 25: 299–313.

Siddiqui, M.N., Saleh, T.A., Anee, M.H., Basheer, M.C., Al-Arfaj, A.A., Tyler, D.R. (2016) Desulfurization of model fuels with carbon nanotube/TiO₂ nanomaterial adsorbents: comparison of batch and film-shear reactor processes. Journal of Inorganic and Organometallic Polymers and Materials. 26: 572–578.

Skocpol, R.C. (2000) RESOLVE prove its worth. http://www.akzonobel-catalysts.com/html/news/Newslinkold/2000/RESOLVE.htm.

Soled, S.L, Miseo, S., Krycak, R., Vroman, H., Ho, T.C., and Riley, K.L. (2001) Nickel molybodtungstate hydrotreating catalysts (law444). U.S. Patent No. 6299760 B1.

Song, C. (1999) Designing sulfur-resistant, noble-metal hydrotreating catalysts. Chemtech. 3: 26–30.

Song, C. (2002) New approaches to deep desulfurization for ultra-clean gasoline and diesel fuels: an overview. American Chemical Society-Fuel Chemistry Division Preprints. 47(2): 438–444.

Song, C., Ma, X. (2003) New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Applied Catalysis B: Environmental. 41(1–2): 207–238.

Song, C.S. (2003) An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catalysis Today. 86: 211–263.

Song, H., Cui, X., Jiang, B., Song, H., Wang, D., Zhang, Y. (2015) Preparation of SO₄^2-/ZrO₂ solid superacid and oxidative desulfurization using K₂FeO₄. Research on Chemical Intermediates. 41: 365–382.

Song, H., Dong, P.F., Zhang, X. (2010) Effect of Al Content on the n-Pentane Isomerization of the Solid Superacid Pt-SO₄^2-/ZrO₂-Al₂O₃. Acta Physico-Chimica Sinica. 26(8): 2229–2234.

Srivastava, A., Srivastava, V.C. (2009) Adsorptive desulfurization by activated alumina. Journal of Hazardous Materials. 170(2/3): 1133–1140.

Srivastava, C.C. (2012) An evaluation of desulfurization technologies for sulfur removal from liquid fuels. RSC Advances. 2: 759–783.

Stanislaus, A., Marafi, A., Rana, M.S. (2010) Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production. Catalysis Today. 153: 1–68.

Stork, W.H.J. (1996) Performance testing of hydroconversion catalysts. Chapter 28. American Chemical Society Symposium Series. 634: 379–400.

Strawinski, R.J. (1950) Method of desulfurizing crude oil. US Patent 2521761 A.

Suchanek, A. (1996) How to make low-sulfur, low aromatics, high cetane diesel fuel? American Chemical Society Preprints-Division Petroleum Chemistry. 41: 583.

SulphCo Presentation (2009) “Oxidative Desulfurization”. IAEE Houston Chapter, June 11, 2009.

Sun, Y., Prins, R. (2008) Hydrodesulfurization of 4,6-dimethyldibenzothiophene over noble metals supported on mesoporous zeolites. Angewandte Chemie International Edition. 47: 8478–8481.

Suryawanshi, N.B., Bhandari, V.M., Sorokhaibam, L.G., and Ranade, V.V. (2016) A non-catalytic deep desulfurization process using hydrodynamic cavitation. Scientific Reports. 6: 33021. DOI: 10.1038/srep33021.

Swain, E.J. (1991) US crude slate gets heavier, higher in sulfur. Oil and Gas Journal. 89(36): 59–61.

Swaty, T.E. (2005) Global refining industry trends: The present and future. Hydrocarbon Processing, September: 35–46.

Takahashi, A., Yang, F.H., Yang, R.T. (2002) New sorbents for desulfurization by -complexation: thiophene/benzene adsorption. Industrial and Engineering Chemistry Research. 41: 2487–2496.

Tang, K., Song, K.L., Duan, L., Li, X., Gui, J., Sun, Z. (2008) Deep desulfurization by selective adsorption on a heteroatoms zeolites prepared by secondary synthesis. Fuel Processing Technology. 89(1): 1–6.

Tang, L., Luo, G., Zhu, M., Kang, L., Dai, B. (2013b) Preparation, characterization and catalytic performance of HPW-TUD-1 catalyst on oxidative desulfurization. Journal of Industrial and Engineering Chemistry. 19(2): 620–626.

Tang, T., Zhang, L., Fu, W., Ma, Y., Xu, J., Jiang, J., Fang, G., Xiao, F.-S. (2013a) Design and synthesis of metal sulfide catalysts supported on zeolite nanofiber bundles with unprecedented hydrodesulfurization activities. Journal of the American Chemical Society. 135(31): 11437–11440.

Tang, X.-L., Meng, X., Shi, L. (2011) Desulfurization of model gasoline on modified bentonite. Industrial and Engineering Chemistry Research. 50: 7527–7533.

Tao, H., Nakazato, T., Sato, S. (2009) Energy-Efficient Ultra-Deep Desulfurization of Kerosene Based On Selective Photooxidation and Adsorption. Fuel. 88: 1961−1969.

Te, M., Fairbridge, C., Ring, Z. (2001) Oxidation reactivities of dibenzothiophenes in polyoxometalate/H₂O₂ and acid/H₂O₂ systems. Applied Catalysis A: General, 2001, 219(1/2): 267–280.

Thaligari, S.K., Srivastava, V.C., Prasad, B. (2016) Adsorptive desulfurization by zinc-impregnated activated carbon: characterization, kinetics, isotherms, and thermodynamic modeling. Clean Technologies and Environmental Policy.18(4): 1021–1030.

Tilstra, L., Eng, G., Olsonn, G.J., Wang, F.W. (1992) Reduction of sulphur from polysulphidic model compounds by the hyperthermophilic archaebacterium Pyrococcus furiosus. Fuel. 71(7): 779–783.

Topsoe, H., Knudsen, K.G., Byskov, L.S., Norskov, J.K., Clausen, B.S. (1999) Advances in deep desulfurization. Studies in Surface Science and Catalysis. 121: 13–22.

Trakarnpruk, W., Seentrakoon, B., Porntangjitlikit, S. (2008) Hydrodesulfurization of diesel oils by MoS₂ catalyst prepared by in situ decomposition of ammonium thiomolybdate. Silpakorn University Science and Technology Journal 2(1): 7–13.

Trejo, F., Rana, M.S., Ancheyta, J. (2008) CoMo/MgO–Al₂O₃ supported catalysts: an alternative approach to prepare HDS catalysts. Catalysis Today. 130: 327–336.

Turaga, U.T., Song, C. (2001) Deep hydrodesulfurization of diesel and jet fuels using mesoporous molecular sieve-supported Co-M0/MCM-41 catalysts. American Chemical Society Preprints-Division Petroleum Chemistry. 46: 275–279.

Turk, B.S., Gupta, R.P., Arena, B.A. (2002) Continuous catalytic process for diesel desulfurization. vols. 79–83. PTQ Summer, 2002.

United States Environmental Protection Agency US-EPA (2000) “Heavy-duty engine and vehicle standards and highway diesel fuel sulfur control requirements”. EPA 420-F-00–057.

van der Linde, B., Menon, R., Davé, D., Gustas, S. (1999) Asian refining technology conference, DD-ARTC-99. Singapore, 21^st–23^rd April 1999. Pp. 24.

van Veen, J.A.R., Colijn, H.A., Hendriks, P.A.J.M., van Welsenes, A.J. (1993) On the formation of type I and type II NiMoS phases in NiMo/Al₂O₃ hydrotreating catalysts and its catalytic implications. Fuel Processing Technology. 35: 137–157.

Vasudevan, P.T., Fierro, J.L.G. (1996) A review of deep hydrodesulfurization catalysis. Catalysis Reviews-Science and Engineering. 38(2): 161–188.

Velu, S., Ma, X., Song, C. (2005) Desulfurization of JP-8 jet fuel by selective adsorption over a Ni-based adsorbent for micro solid oxide fuel cells. Energy and Fuels 19: 1116–1125.

Wan, G., Duan, A., Zhan, Y., Zhao, Z., Jiang, G., Zhang, D., Gao, Z (2010) Zeolite beta synthesized with acid-treated metakaolin and its application in diesel hydrodesulfurization. Catalysis Today. 149(1/2): 69–75.

Wan, J.K.S., Kriz, J.F. (1985) Hydrodesulphurization of hydrocracked pitch. US Patent 4545879.

Wan, M.W., Yen, T.F. (2007) Enhanced efficiency of tetraoctylammonium fluoride applied to ultrasound-assisted oxidative desulfurization (UAOD) process. Applied Catalysis A: General. 319: 237–245.

Wang, B., Zhu, P.J., Ma, H.Z. (2009a) Desulfurization from thiophene by SO₄^2−/ZrO₂ catalytic oxidation at room temperature and atmospheric pressureJournal of Hazardous Materials. 164(1): 256–264.

Wang, D., Qian, E.W., Amano, H., Okata, K., Ishihara, A., Kabe, T. (2003) Oxidation of dibenzothiophenes using tert-butyl hydroperoxide. Applied Catalysis A. 253: 91–99.

Wang, Q., Wang, S., and Yu, H. (2017) Oxidative desulfphurization of model fuel by in situ produced hydrogen peroxide on palladium/active carbon. The Canadian Journal of Chemical Engineering. 95: 136–141.

Wang, R., Yu, F (2013) Deep oxidative desulfurization of dibenzothiophene in simulated oil and real diesel using heteropolyanion-substituted hydrotalcite-like compounds as catalysts. Molecules. 18(11): 13691–13704.

Wang, S., Wang, R., Yu, H. (2012) Deep removal of 4,6- dimethyldibenzothiophene from model transportation diesel fuels over reactive adsorbent. Brazilian Journal of Chemical Engineering. 29(2): 421–428.

Wang, S.Q., Zhou, L., Long, L., Dai, W., Zhou, Y.P. (2008c) Thiophene capture with silica gel loading formaldehyde and hydrochloric acid. Industrial and Engineering Chemistry Research. 47: 2356–2360.

Wang, S.Q., Zhou, L., Su, W., Sun, Y., Zhou, Y.P. (2009b) Deep desulfurization of transportation fuels by characteristic reaction resided in adsorbents. AICHE Journal. 55(7): 1872–1881.

Wang, Y., Sun, Z., Wang, A., Ruan, A.L., Lifeng, L., Ren, J., Li, X., Li, C., Hu, Y., Yao, P. (2004) Kinetics of hydrodesulfurization of dibenzothiophene catalyzed by sulfided Co-Mo/MCM-41. Industrial and Engineering Chemistry Research. 43(10): 2324- 2329.

Wang, Y., Wang, B., Rives, A., Sun, Y. (2014) Hydrodesulfurization of transportation fuels over zeolite-based supported catalysts. Energy and Environment Focus. 3: 1–8.

Wang, Y., Yang, R.T., Heinzel, J.M. (2008a) Desulfurization of jet fuel by-complexation adsorption with metal halides supported on MCM-41 and SBA-15 mesoporous materials. Chemical Engineering Science. 63: 356–365.

Wang, Y., Yang, R.T., Heinzel, J.M. (2008b) Desulfurization of jet fuel JP-5 light fraction by MCM-41 and SBA-15 supported cuprous oxide for fuel cell applications. Industrial and Engineering Chemistry Research. 48: 142–147.

Westervelt, R. (2001) Suppliers benefits from an oil refining boom. Chemical Week. 163(11): 30–32.

Wild, P.J., Nyqvist, R.G., Bruijn, F.A., Stobbe, E.R. (2006) Removal of sulphur-containing odorants from fuel gases for fuel cell-based combined heat and power applications. Journal of Power Sources. 159: 995–1002.

Wittanadecha, W., Laosiripojana, N., Ketcong, A., Ningnuek, N., Praserthdam, P., and Assabumrungrat, S. (2014) Synthesis of Au/C catalysts by ultrasonic-assisted technique for vinyl chloride monomer production. Engineering Journal. 8(3): 65–71.

Wu, L., Jiao, D., Wang, J., Chen, L., Cao, F (2009) The role of MgO in the formation of surface active phases of CoMo/Al₂O₃–MgO catalysts for hydrodesulfurization of dibenzothiophene. Catalysis Communications. 11: 302–305.

Wu, Z., and Ondruschka, B. (2010) Ultrasound-assisted oxidative desulfurization of liquid fuels and its industrial application. Ultrasonics Sonochemistry. 17(6):1027–1032.

Xiao, F., Meng, X. (2011) Hierarchically structured porous materials: from nanoscience to catalysis, separation, optics, energy, and life science. edited by B.-L. Su, C. Sanchez, and X.-Y. Yang, Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany.

Xu, J., Huang, T., Fan, Y. (2017) Highly efficient NiMo/SiO₂-Al₂O₃ hydrodesulfurization catalyst prepared from gemini surfactant-dispersed Mo precursor. Applied Catalysis B: Environmental. 203: 839–850.

Xu, W.Q, Xiong, C.Q, Zhou, G.L, Zhou, H.J. (2008) Removal of sulfur from FCC gasoline by using Ni/ZnO as adsorbent. Acta Petrolei Sinica. 24(6): 739–743. (Petroleum Processing Section).

Xu, Y., Liu, C. (2004) The hydrodesulfurization of 4-methyldibenzothiophene and dibenzothiophene over sulfided Mo/γ-Al₂O₃. Preprints of Papers-American Chemical Society, Division of Fuel Chemistry. 49(2): 990–993.

Yamada, K.O., Morimoto, M., Tani, Y. (2001) Degradation of dibenzothiophene by sulfate-reducing bacteria cultured in the presence of only nitrogen gas. Journal of Bioscience and Bioengineering. 91(1): 91–93.

Yamamoto, T., Chaichanawong, J., Thongprachan, N., Ohmori, T., Endo, A. (2011) Adsorptive desulfurization of propylene derived from bio-ethanol. Adsorption. 17: 17–20.

Yang, R.T. (2003). Adsorbents: Fundamentals and Applications. John Wiley & Sons, Inc., Hoboken, New Jersey, USA.

Yang, R.T., Takahashi, A., Yang, F.H. (2001) New sorbents for desulfurization of liquid fuels by π-complexation. Industrial and Engineering Chemistry Research. 40(26): 6236- 6239.

Yang, Y., Lu, H., Ying, P., Jiang, Z., Li, C. (2007) Selective dibenzothiophene adsorption on modified activated carbons. Carbon. 45(15): 3042–3044.

Yang, Y-Z., Liu, X-G., Xu B-S. (2014) Recent advances in molecular imprinting technology for the deep desulfurization of fuel oils. New Carbon Materials. 29(1): 1–13.

Yankov, V., Stratiev, D., Yalamov, A. (2011) Integration of The Processes FCC Feed Hydrotreatment and FCC Gasoline Posttreatment Through the Prime G Process – Opportunity for Production of Euro V Gasolines at Increased Profitability. 45^th International Petroleum Conference, June 13, 2011, Bratislava, Slovak Republic.

Yao, X.-Q., Wang, S.-J., Ling, F.-X., Li, F.-F., Zhang, J., Ma, B. (2004) Oxidative desulfurization of simulated light oil. Journal of Fuel Chemistry and Technology. 32(3): 318–322.

Yao, Y., Zhang, H., Lu, Y., Zhi, Y., Lu, S. (2016) Catalytic performance of supported Eu/phosphomolybdic acid modified mesoporous silica in the oxidative desulfurization of dibenzothiophene. Reaction Kinetics, Mechanisms and Catalysis. 118: 621–632.

Yazu, K., Yamamoto, Y., Furuya, T., Miki, K., Ukegawa, K (2001) Oxidation of dibenzothiophenes in an organic biphasic system and its application to oxidative desulfurization of light oil. Energy Fuels. 15(6): 1535–1536.

Yazu, K., Yamamoto, Y., Furuya, T., Mild, K., Ukegawa, K. (2003) Oxidation of dibenzothiophenes in an organic biphasic system and its application to oxidative desulfurization of light oil. Energy anf Fuels. 15: 1535–1536.

Yeetsorn R., Tungkamani S. (2014) Potential activity evaluation of CoMo/Al₂O₃-TiO₂ catalysts for hydrodesulfurization of coprocessing bio-oil. KMUTNB: International Journal of Applied Science and Technology. 7(4): 35–45.

Yi D., Huang H., Shi L. (2013) Desulfurization of model oil via adsorption by copper (II) modified bentonite. Korean Chemical Society. 34(3): 777–782.

Yin C., Zhu G., Xia D. (2002a) Determination of organic sulfur compounds in naphtha. Part I. Identification and quantitative analysis of sulfides in FCC and RFCC naphthas. American Chemical Society Preprints-Division Petroleum Chemistry. 47: 391–395.

Yin C., Zhu G., Xia D. (2002b) Determination of organic sulfur compounds in naphtha. Part II. Identification and quantitative analysis of thiophenes in FCC and RFCC naphthas. American Chemical Society Preprints-Division Petroleum Chemistry. 47: 398–401

Yin, Y., Xue, D.-M., Liu, X.-Q., Xu, G., Ye, P., Wu, M.-Y., Su, L.-B. (2012) Unusual ceria dispersion formed in confined space: a stable and reusable adsorbent for aromatic sulfur capture. Electronic Supplementary Material (ESI) for Chemical Communications. S1-S18.

Yu, F., and Wang, R. (2013) Deep oxidative desulfurization of dibenzothiophene in simulated oil and real diesel using heteropolyanion-substituted hydrotalcite-like compounds as catalysts. Molecules. 18: 13691–13704.

Yun, G.-N., Lee, Y.-K. (2013) Beneficial effects of polycyclic aromatics on oxidative desulfurization of light cycle oil over phosphotungstic acid (PTA) catalyst. Fuel Processing Technology. 114: 1–5.

Zaid, H.F.M., Kait, C.F., Abdul Mutalib, M.I. (2016) Preparation and characterization of Cu-Fe/TiO₂ photocatalyst for visible light deep desulfurization. Malaysian Journal of Analytical Sciences. 20(4): 713 – 725.

Zaki T., Saed D., Aman D., Younis S.A., Moustafa Y.M. (2013) Synthesis and characterization of MFe₃O₄ sulfur nanoadsorbants. Journal of Sol-Gel Science and Technology. 65: 269–276.

Zdrazil, M., Klicpera, T. (2001) High surface area MoO₃/MgO: preparation by reaction of MoO₃ and MgO in methanol or ethanol slurry and activity in hydrodesulfurization of benzothiophene. Applied Catalysis A: General. 216: 41–50.

Zhang, C., Zhang, J., Zhao, Y., Yonggeng, J., Sun, J., Wu, G. (2016) Study on the preparation and catalytic activities of SO₄²⁻ promoted metal oxide solid superacid catalysts for model oil desulfurization. Catalysis Letter. 146(7): 1256–1263.

Zhang, H., Meng, X., Li, Y., Lin, Y.S. (2007) MCM-41 overgrown on Y composite zeolite as support of Pd-Pt catalyst for polyaromatic compounds hydrogenation Industrial and Engineering Chemistry Research. 46(12): 4186–4192.

Zhang, Q., Jiao, Q.Z. Min, E. (2005) Iso-butane/1-butene alkylation reaction and deactivation of superacid (SO₄²⁻/ZnO₂) catalysts. Chemical Journal of Chinese Universities. 26: 1130–1132.

Zhang, Y., Yang, Y., Han, H., Yang, M., Wang, L., Zhang, Y., Jiang, Z., Li, C. (2012) Ultra-deep desulfurization via reactive adsorption on Ni/ZnO: The effect of ZnO particle size on the adsorption performance. Applied Catalysis B: Environmental. 119–120: 13–19.

Zhao L., Chen Y., Gao J., and Chen Y. (2010) Desulfurization mechanism of FCC gasoline: A review. Frontiers of Chemical Engineering in China. 4(3): 314–321.

Zhao, D., Liu, R., Wang, J., Liu, B. (2008) Photochemical oxidation−ionic liquid extraction coupling technique in deep desulphurization of light oil. Energy Fuels. 22(2): 1100–1103.

Zhao, D., Ren, H., Wang, J., Yang, Y., Zhao, Y. (2007) Kinetics and mechanism of quaternary ammonium salts as phase-transfer catalysts in the liquid−liquid phase for oxidation of thiophene. Energy and Fuels. 21(5): 2543–2547.

Zhao, D.Z., and Sun, M.Z. (2009) Oxidative desulfurization of diesel fuel using ultrasound. Petroleum Science and Technology. 27 (17): 1943–1950.

Zhao, R., Liu, C., Yin, C., Liang, W. (2001) Support and CoPcTS effects on the catalytic activity and properties of molybdenum sulfide catalysts. Petroleum Science and Technology. 19(5/6): 495–502.

Zhao, X., Cheng, W.-C, Rudesill, J.A. (2002) FCC bottoms cracking mechanisms and implications for catalyst design for resid applications, AM-02–53, in: Proceedings of the NPRA 2003 Annual Meeting, San Antonio, TX, March 17–19, 2002.

Zhao, X., Cheng, W.-C., Rudesill, J.A., Wormsbecher, R.F., Deitz, P.S. (2003). Gasoline sulfur reduction catalyst for fluid catalytic cracking process. U.S. Patent 6,635,168. Assigned to W.R. Grace & Co.-Conn.

Zhao, Y., Wang, R. (2013) Deep desulfurization of diesel oil by ultrasound-assisted catalytic ozonation combined with extraction process. Petroleum and Coal. 55(1): 62–67.

Zhou, A., Ma, X., Song, C. (2009) Effects of oxidative modification of carbon surface on the adsorption of sulfur compounds in diesel fuel. Applied Catalysis B: Environmental. 87: 190–199.

Zhou, X., Zhao, C., Yang, J., Zhang, S. (2007) Catalytic oxidation of dibenzothiophene using cyclohexanone peroxide. Energy and Fuels. 21(1): 7–10.

Zhu, W., Xu, Y., Li, H., Dai, B., Xu, H., Wang, C., Chao, Y., Liu, H. (2014) Photocatalytic oxidative desulfurization of dibenzothiophene catalyzed by amorphous TiO₂ in ionic liquid. Korean Journal of Chemical Engineering. 31(2): 211–217.

ZoBell, C. E. (1953) Process of removing sulfur from petroleum hydrocarbons and apparatus. US Patent 2641564.

Zongxuan, J., Hongying, L., Yonga, Z., Can, L. (2011) Oxidative desulfurization of fuel oils. Chinese Journal of Catalysis. 32(5): 707–715.