10.3.5 Reforming Catalysts
We now consider a dual-site mechanism, which is a reforming reaction found in petroleum refining to upgrade the octane number of gasoline.
Side Note: Octane Number. Fuels with low octane numbers can produce spontaneous combustion in the car cylinder before the air/fuel mixture is compressed to its desired value and ignited by the spark plug. The following figure shows the desired combustion wave front moving down from the spark plug and the unwanted spontaneous combustion wave in the lower right-hand corner. This spontaneous combustion produces detonation waves, which constitute engine knock. The lower the octane number, the greater the chance of spontaneous combustion and engine knock.
The octane number of a gasoline is determined from a calibration curve relating knock intensity to the % iso-octane in a mixture of iso-octane and heptane. One way to calibrate the octane number is to place a transducer on the side of the cylinder to measure the knock intensity (K.I.) (pressure pulse) for various mixtures of heptane and iso-octane. The octane number is the percentage of iso-octane in this mixture. That is, pure iso-octane has an octane number of 100, 80% iso-octane/20% heptane has an octane number of 80, and so on. The knock intensity is measured for this
80/20 mixture and recorded. The relative percentages of iso-octane and heptane are changed (e.g., 90/10), and the test is repeated. After a series of experiments, a calibration curve is constructed. The gasoline to be calibrated is then used in the test engine, where the standard knock intensity is measured. Knowing the knock intensity, the octane rating of the fuel is read off the calibration curve. A gasoline with an octane rating of 92 means that it matches the performance of a mixture of 92% iso-octane and 8% heptane. Another way to calibrate the octane number is to set the knock intensity and increase the compression ratio. A fixed percentage of iso-octane and heptane is placed in a test engine and the compression ratio (CR) is increased continually until spontaneous combustion occurs, producing an engine knock. The compression ratio and the corresponding composition of the mixture are then recorded, and the test is repeated to obtain the calibration curve of compression ratio as a function of % iso-octane. After the calibration curve is obtained, the unknown is placed in the cylinder, and the compression ratio (CR) is increased until the set knock intensity is exceeded. The CR is then matched to the calibration curve to find the octane number.
The more compact the hydrocarbon molecule, the less likely it is to cause spontaneous combustion and engine knock. Consequently, it is desired to isomerize straight-chain hydrocarbon molecules into more compact molecules through a catalytic process called reforming.
One common reforming catalyst is platinum on alumina. Platinum on alumina (Al2O3) (see SEM photo below) is a bifunctional catalyst that can be prepared by exposing alumina pellets to a chloroplatinic acid solution, drying, and then heating in air at 775 K to 875 K for several hours. Next, the material is exposed to hydrogen at temperatures around 725 K to 775 K to produce very small clusters of Pt on alumina. These clusters have sizes on the order of 10 Å, while the alumina pore sizes on which the Pt is deposited are on the order of 100 Å to 10,000 Å (i.e., 10 nm to 1000 nm).
Figure 10-19. Platinum on alumina.
Figure from R.I. Masel, Chemical Kinetics and Catalysis. New York: Wiley, 2001, p. 700.
As an example of catalytic reforming we shall consider the isomerization of n-pentane to i-pentane:
Normal pentane has an octane number of 62, while iso-pentane, which is more compact, has an octane number of 90! The n-pentane adsorbs onto the platinum, where it is dehydrogenated to form n-pentene. The n-pentene desorbs from the platinum and adsorbs onto the alumina, where it is isomerized to i-pentene, which then desorbs and subsequently adsorbs onto platinum, where it is hydrogenated to form i-pentane. That is,
We shall focus on the isomerization step to develop the mechanism and the rate law:
The procedure for formulating a mechanism, rate-limiting step, and corresponding rate law is given in Table 10-4.
Table 10-4. Algorithm for Determining the Reaction Mechanism and Rate-Limiting Step
Isomerization of n-pentene (N) to i-pentene (I) over alumina
- Select a mechanism. (Mechanism Dual Site)
Adsorption:
Surface reaction:
Desorption:
Treat each reaction step as an elementary reaction when writing rate laws.
- Assume a rate-limiting step. Choose the surface reaction first, because more than 75% of all heterogeneous reactions that are not diffusion-limited are surface-reaction-limited. We note that the PSSH must be used when more than one step is limiting (see section 10.3.6).
The rate law for the surface reaction step is
- Find the expression for concentration of the adsorbed species Ci · S. Use the other steps that are not limiting to solve for Ci · S (e.g., CN · S and CI · S). For this reaction,
- Write a site balance.
Ct = Cυ + CN · S + CI · S
- Derive the rate law. Combine Steps 2, 3, and 4 to arrive at the rate law:
- Compare with data. Compare the rate law derived in Step 5 with experimental data. If they agree, there is a good chance that you have found the correct mechanism and rate-limiting step. If your derived rate law (i.e., model) does not agree with the data:
a. Assume a different rate-limiting step and repeat Steps 2 through 6.
b. If, after assuming that each step is rate-limiting, none of the derived rate laws agrees with the experimental data, select a different mechanism (e.g., a single-site mechanism):
and then proceed through Steps 2 through 6.
The single-site mechanism turns out to be the correct one. For this mechanism the rate law is
c. If two or more models agree, the statistical tests discussed in Chapter 7 (e.g., comparison of residuals) should be used to discriminate between them (see the Supplementary Reading).
Table 10-5 gives rate laws for different reaction mechanisms that are irreversible and surface-reaction-limited.
Table 10-5. Irreversible Surface-Reaction-Limited Rate Laws
Single site
Dual site
Eley–Rideal
We need a word of caution at this point. Just because the mechanism and rate-limiting step may fit the rate data does not imply that the mechanism is correct.13 Usually, spectroscopic measurements are needed to confirm a mechanism absolutely. However, the development of various mechanisms and rate-limiting steps can provide insight into the best way to correlate the data and develop a rate law.