21

GREEN CHEMISTRY

Sustainable development meets the needs of present generations

without compromising the possibility of future generation

to attend their needs and aspirations.

21.1 INTRODUCTION

Green chemistry or sustainable chemistry or environmentally benign chemistry is the frontiers of science with the utilisation of set of principles that attempts to reduce or eliminates the use or generation of hazardous substances in the design and manufacture of environmentally and economically sustainable products. The growing attention to sustainable development includes the reduction of environmental impact and the effective utilization of renewable resources. It can help to solve large global problems such as climate change, energy consumption of effective utilization of natural resources, especially renewable resources.

The terms ‘green chemistry’ and ‘sustainable chemistry’ have been often used in several areas to introduce the efforts of academics, scientists and industrialists towards the development of new efficient chemical processes accounting for their environmental impact. Green chemistry may be considered as a modern scientific platform where the common efforts of academia, industry and government converge to develop a sustainable civilization, and in this context, it is certainly clear that fundamental chemistry has an important role to play.

Probably one of the most important steps in the evolution of green chemistry is closely related to U.S. Environmental Protection Agency (USEPA). In the 1980s, a significant change began in the execution of environmental regulations, and pollution became the priority instead of end-of-pipeline control. In 1990, the pollution Prevention Act, approved by the American Congress in the United States, initiated to create a modus operandi for dealing with pollution in an innovative and sustainable way and paved the way to green chemistry concept. Green chemistry takes advantage for technological advancement chemical processing to achieve environmental efficiency.

21.2 TWELVE PRINCIPLES OF GREEN CHEMISTRY

Paul Anastas and John Warner of the U.S. Environmental Protection Agency coined the two-letter word “green chemistry” and formulated 12 principles of green chemistry. They are as follows:

1. Waste prevention instead of remediation: Waste prevention is better than treating or cleaning up after it is formed.
2. Atom economy (or) efficiency: Safer synthetic methods should be designed to give a good yield of the final product.
3. Use of less hazardous and toxic chemical synthesis: Synthetic methodologies should be designed to use and generate environmentally benign, which possess little or no toxicity.
4. Designing of safer chemicals/products: Chemical products should be designed to preserve efficacy of function while reducing toxicity.
5. Innocuous solvents and auxiliaries: The use of auxiliary substances, such as solvent and separating agents, should be made unnecessary wherever possible and innocuous when used.
6. Design for energy efficiency: Energy requirements of chemical processes should be recognised for their environmental and economic impacts and should be minimised. Preferably, synthetic methods should be conducted at ambient temperature and pressure.
7. Use of renewable feedstock or raw material: A raw material or feedstock should be renewable, rather than depleting the environment, whenever used technically, and economically possible.
8. Reduce derivatives (or) shorten synthetic routes: Unnecessary derivatization, such as use of blocking groups, protection/deprotection, temporary modification of physical or chemical processes, should be minimised or avoided because such steps require additional reagents and can also generate waste.
9. Catalysis rather than stoichiometric reagents: Catalytic reagents are superior to stoichiometric reagents.
10. Design for degradation: Chemical products should be designed to degrade, so that at the end of their function, they break down into innocuous degradation products and do not remain in the environment.
11. Real-time analysis for pollution prevention: Analytical methodologies are required to be further developed to allow for real-time, in-process monitoring and control prior to the formation of pollution.
12. Inherently safer processes for accident prevention: Substances, derivatives and products in chemical process should be chosen to minimise the potential for chemical accidents including releases, explosions and fires.

21.3 IMPORTANCE OF GREEN SYNTHESIS

Green chemistry addresses many challenges by opening a wide and multifaceted research scope and allowing the invention of novel reacting chemicals that can maximise the desired products and minimise the waste products, as well as the design of new synthetic schemes that are inherently, environmentally and ecologically benign.

The following are the importance of green synthesis:

1. This prevents pollution at the molecular level by atom economy.
2. It gives innovative scientific solutions to real-world environmental problems.
3. It provides alternative synthetic routes for feedstock and starting material.
4. Biocatalysis and bioleaching are prominent applications in green chemistry.
5. Utilisation of carbon dioxide as a green solvent.
6. Biosorption is one of the important phenomena based on 12 principles of green chemistry. In biosorption process, number of agricultural materials such as wool, palm kernel husk, apple residues, banana husk and sawdust are being used to remove toxic metals from waste water.
7. Effective utilisation of renewable resources as alternate energy sources such as solar energy, wind energy, hydro energy, etc.

21.3.1 Methods for Green Synthesis

Over the past decades, green chemistry has convincingly demonstrated how fundamental scientific methodologies can be devised and applied to protect human health and the environment in an eco-friendly and economically beneficial manner.

Alternative Synthetic Route for Feedstocks and Starting Materials

Example: Production of dimethyl carbonate (DMC)

Dimethyl carbonate is a versatile and environmentally friendly material for chemical industry. Due to high oxygen content and blending properties, it is used as a good component of fuel.

Traditional Method

In this method, phosgene (COCl₂) and methanol (CH₃OH) are used to produce DMC.

Here phosgene (reactant) and hydrochloride acid (by-product) are environmentally harmful.

Greener Method (Alternative Method)

This method makes use of copper chloride, methanol, oxygen and carbon monoxide.

Here copper chloride further comes as a by-product, and usage of CO in this method is cheap and indirectly decreases the pollution.

Biocatalysis or Bioleaching

Bioleaching is the extraction of specific metals from their ores by using microorganisms such as bacteria.

Example: Extraction of gold

Traditional Method

Heap leaching method is the traditional method employed for the extraction of gold using cyanide. Here, cyanide is hazardous to health and environment.

Greener Method

In this method, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans bacteria are used to oxidise ferrous and sulphur. The gold will be easily separated from the ore and solution. This method is much cleaner than the traditional heap leaching method.

Catalysis

Catalytic methods are superior than stoichiometric methods.

Example: Synthesis of adipic acid

Adipic acid is a monomer for nylon and starting material for cathode.

Traditional Method

In the past, for the production of adipic acid, benzene is used as a starting material. However, it is highly carcinogenic and causes leukaemia. Afterwards, the starting material became cyclohexanone or a mixture of cyclohexanone and cyclohexanol. In oxidation with nitric acid, it produces toxic fumes of nitric oxides, which are contributors to the greenhouse effect, acid rain and the destruction of ozone layer.

Greener Method

In this method, cyclohexane is oxidised by 30 per cent of hydrogen peroxide in the presence of a catalyst. The catalyst used is a salt of the metal wolfram and dissolved in an organic solvent such as aliquot 336.

Oxidation with H₂O₂ is very effective and environmentally benign. Scientists improved the reaction with other metal catalysts such as tungsten and molybdenum. The process also promoted towards biocatalytic method by using genetically transgenic bacteria like Klebsiella pneumoniae, a non-toxic strain of E. coli. Dr Karen M. Draths and Professor John W. Frost were awarded the “Presidential Green Chemistry Challenge Award” in 1998 in the USA for this achievement.

21.3.2 Applications of Green Synthesis

Green synthesis has wide applications in many fields. Few of them are as follows:

1. Preparation of antibacterial products that are alternative for traditional chlorine or tin containing antibacterial agents such as bandages, sutures, hospital gowns, acne medication, toothpastes, air filters, antiviral agents, etc.
2. Used for cleaning clothes such as the following:
   1. Tetraamidomacrocyclic ligand (TAML) catalyst activates hydrogen peroxide that inhibits dye transfer and is good for washing machines which use less water.
   2. Total Impact Programme (TIP) as laundry formulation incorporates neutral pH, detergents, enzymes, surfactants, oxygen bleach and biodegradable softness.
   3. Dry cleaning with liquid carbon dioxide, which is non-flammable, non-toxic and a renewable substance.
   4. Use of sodium iminodisuccinate for cleaning clothes, which is a biodegradable and environment friendly chelating agent.

1. Used for cleaning water by using the following:
   1. Chlorine disinfection which is toxic to aquatic life but important for preventing diseases.
   2. Polymer technology for manufacture of high molecular weight, water-soluble polymers in aqueous salt solution.
2. Used for industrial cleaning by using simple green, non-toxic, biodegradable surfactants, thereby replacing traditional organic solvents.
3. Use of carbon dioxide blowing agent for polystyrene foam production; polystyrene foam is used in packing and food transportation.
4. The conversion of waste glycerine from biodiesel production to propylene glycol.
5. Synthesis of nanoparticles of metals. The stabilization of small particles is done by using polymers, ligands, solid matrix and surfactants. The preparation of nanoparticles in green solvent such as water and other non-toxic solvents is very popular nowadays.

21.4 GREENHOUSE CONCEPTS

A greenhouse is also known as a glasshouse. In the greenhouse, plants are grown in a building or other complexes. Building of the greenhouse is made up of either glass or plastic material. In a greenhouse, the incoming UV light is absorbed consequently the temperature of air inside the greenhouse increases and retained in the building by the roof and wall, and air is warmed near the ground and flowing within the complex.

21.4.1 Types of Greenhouse

A greenhouse is divided into two types:

1. Glass greenhouse
2. Plastic greenhouse

In both types of greenhouses, the plastics used are polyethylene, polycarbonate or PMMA glass.

Uses of Glasshouse

1. In a greenhouse, temperature, level of light and shade are maintained.
2. In a greenhouse, growth of plants is controlled by controlling temperature and lighting of the house.
3. It is also used to improve the qualities of the land and also to improve food production by providing good environment.
4. It is also used for growing flowers, vegetables, fruits and transplantation of specific plants.
5. In addition to all of these, the greenhouse is also used to produce solar fields which produce steam for solar-enhanced oil.

21.5 GREENHOUSE GASES AND GREENHOUSE EFFECT

In greenhouse effect, UV radiations from sun of short wavelength are absorbed through a transparent medium, but IR radiations of longer wavelength are unable to pass back from that medium. As a result, inside temperature increases. Such effect is called the greenhouse effect. This effect is shown due to absorption of excess heat by carbon dioxide present in it.

The greenhouse effect is a natural process that produces a relatively warm environment near the earth’s surface conducive to life on earth; this is broadly of two types: natural and enhanced greenhouse effects.

21.5.1 Natural Greenhouse Effect

The natural greenhouse effect occurs naturally by greenhouse gases present in the earth’s atmosphere; the main natural greenhouse gases are carbon dioxide, methane, nitrous oxide and water vapour.

21.5.2 Enhanced Greenhouse Effect

The enhanced greenhouse effect occurs by human activities which release greenhouse gases into the atmosphere. The main anthropogenic or human-induced greenhouse gases are carbon dioxide, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), methane and sulphur hexafluoride. The enhanced greenhouse effect is responsible for the increase in global temperature, that is, global warming.

Sources of greenhouse gases, properties and their impacts are shown in Table 21.1.

Table 21.1 Greenhouse gases their sources, properties and impacts

21.5.3 Greenhouse Gas Effect

Greenhouse gases in the earth’s atmosphere absorb infrared radiation from the ground. This radiation is re-emitted in all directions and radiated back towards the earth’s surface, thereby leading to the warming of the earth’s surface. Scientists have been able to determine carbon dioxide concentrations on earth over time and the temperature variation by using data from ice cores and other sources.

Since middle of 20th century, temperature variations and climate changes have been observed due to manmade sources of greenhouse gases but not on the basis of the presence of natural greenhouse gases.

During the 21st century, the earth is likely to experience the following greenhouse effects:

1. Higher maximum temperatures and more hot days
2. Higher minimum temperatures and fewer cold days
3. More intense precipitation events like rain over many areas
4. Melting of polar ice caps resulting in rising sea levels
5. Unseasonal rains, flowering and shedding of trees
6. Unexpected floods and droughts

Suggestions for Combating Global Warming

1. Reduction of fossil fuel combustion and sourcing new fuel alternatives
2. Reducing deforestation, replanting of destroyed forests and afforestation
3. Reducing or eliminating the use of HFCs, CFCs, perfluorocarbons, and sulphur hexafluoride
4. Carbon dioxide sequestration

Finding alternatives to these greenhouse gases in chemical and industrial processes have led to the development of a new branch of chemistry called green chemistry.

Example of Greenhouse Effect

1. Bright sun light warms a car on a cold, clear day by the greenhouse effect
2. Global warming
3. Increasing atmospheric carbon dioxide level

21.5.4 Requirements for Greenhouse

1. Greenhouse carbon dioxide: The presence of carbon dioxide in greenhouse is responsible for absorption of radiations by it. Carbon dioxide in greenhouse is also responsible for enhanced plant growth.
2. Greenhouse heating: Heating is most important in colder climates. For good heating, the heat lost by solid opaque wall is prevented by providing greenhouse design.
3. Greenhouse ventilation: For a good greenhouse, and for a good plant, proper ventilation is an important factor. The main aims of ventilation are to regulate the temperature and humidity in a greenhouse. This also ensures a fresh air supply for photosynthesis and plant respiration. Ventilation in a greenhouse is controlled either automatically via a computer or recirculation fans.
4. Green synthesis: Green chemistry is also called sustainable chemistry. Main aim of Green Synthesis is minimize the usage of hazard material and maximizing the efficiency of chemical for the synthesis of particular product.Green synthesis is identified as development in the following fields:
   1. Use of supercritical carbon dioxide as a solvent called green solvent
   2. Oxidant used as an aqueous hydrogen peroxide
   3. Use of hydrogen in asymmetric synthesis

Green synthesis methods have a good impact on the environment.

In these processes, there is less exposure to pollutants, waste reduction and less energy consumption. In research, green synthesis methods include the use of microwave reactors to minimize energy needs and use of microfluidic reactors to minimize solvent waste.

Main aim of Green chemistry the design of chemical products and processes that reduce or minimize the hazardous effects to living beings and the environment.

21.6 CARBON SEQUESTRATION

Carbon sequestration is the process of carbon capture from burning fossil fuels and the long term storage of CO₂ safely before releasing into the atmosphere.

Terrestrial or biologic sequestration is the storage of carbon via agricultural and forestry practices. Geologic sequestration involves injecting carbon dioxide to deep underground where it stays permanently.

Carbon dioxide capture and sequestration (CCS) is a set of three step processes that can greatly reduce CO₂ emissions from new and existing coal- and gas-fired power plants and large industrial sources. The processes are as follows:

1. Capture of CO₂ from power plants or industrial processes.
2. Transport of the captured and compressed CO₂ commonly through pipeline but can also be transported by train, truck, ship, etc.
3. Underground injection and geologic sequestration of the CO₂ into deep underground rock formations of depleted oil or gas fields, deep coal seams, etc.

21.6.1 Importance of Carbon Sequestration

Carbon dioxide (CO₂) capture and sequestration (CCS) can significantly reduce emissions from large stationary sources of CO₂, which include power plants using coal and natural gas, as well as industries such as ethanol processing plants, natural gas processing plants and cement industries.

Carbon dioxide capture and sequestration play an important role in reducing greenhouse gas emissions dramatically and enabling low-carbon electricity generation from power plants and industries that burn fossil fuels. With this process, about 80–90% CO₂ emission may reduce from the power plants and is equal to:

1. Planting more than 62 million trees and waiting at least 10 years for them to grow.
2. Avoiding annual electricity-related emissions from more than 300,000 homes.

21.7 WHY CARBON DIOXIDE IS A MAJOR PROBLEM

In pre-industrial age, every million molecules of air contained about 280 molecules of carbon dioxide, and now, that proportion exceeds 380 molecules per million and it continues to increase. Evidence is mounting that carbon dioxide’s heat-trapping power has already started to boost average global temperatures. If carbon dioxide levels continue to increase, then further atmospheric warming occurs and its consequences result in rising of sea levels, agriculture disruptions, stronger storms (e.g., hurricanes) striking more often, etc.

However, reducing carbon dioxide emission does not have a simple solution. Fossil fuels, which provide about 85 per cent of the world’s energy, are made of hydrocarbons, and burning them releases huge quantities of carbon dioxide. Fossil-fuel burning will remain substantial, even renewable energy sources emerge. In fossil fuels, coal is the worst carbon dioxide emitter per unit of energy produced. A grand challenge for the 21st century’s engineers will be developing systems for capturing the carbon dioxide produced by burning fossil fuels and sequestering it safely away from the atmosphere.

21.8 REVIEW QUESTIONS

21.8.1 Fill in the Blanks

1. In 1990, the Pollution Prevention Act was approved by the __________ in the United States.

   [Ans.: American congress]

2. __________ of the U.S. Environmental Protection Agency coined the two-letter word “green chemistry”.

   [Ans.: Paul Anastas and John Warner]

3. Waste __________ is better than treating or cleaning up after it is formed.

   [Ans.: prevention]

4. A raw material or feedstock should be __________ rather than depleting the environment whenever technically and economically possible.

   [Ans.: renewable]

5. The __________ occurs by human activities which release greenhouse gases into the atmosphere.

   [Ans.: enhanced greenhouse effect]

6. Higher maximum temperatures and more hot days can be observes due to __________.

   [Ans.: Greenhouse effect]

21.8.2 Multiple-choice Questions

1. Who framed 12 principles of green chemistry
   1. Paul Anastas
   2. John Warner
   3. PPA
   4. Both (a) and (b)

   [Ans.: d]

2. The enhanced greenhouse effect is caused by
   1. N₂O
   2. SF₆
   3. PFCs
   4. All of these

   [Ans.: d]

21.8.3 Short Answer Questions

1. Define green chemistry.

   Ans.: Green chemistry is the frontier of science with the utilization of set of principles that attempts to reduce or eliminate the use or generation of hazardous substances in the design and manufacture of environmentally and economically sustainable products.

2. Define biocatalysis or bioleaching, Explain with one example.

   Ans.: Bioleaching is the extraction of specific metals from their ores by using microorganisms such as bacteria.

   Example: Extraction of gold

   Traditional method: Heap leaching method by using cyanide.

   Greener method: Acidithiobacillus ferocious and Acidithiobacillus thiooxidans bacteria are used.

3. Explain greener route for synthesis of adipic acid.

   Ans.: In greener route, cyclohexane is oxidized by 30 per cent of hydrogen peroxide in presence of catalyst.

   

21.8.4 Descriptive Questions

Q.1 Give brief note on Carbon sequestration and its importance.

Q.2 Explain Greenhouse effects and its impacts.

Q.3 Give brief note on types of greenhouses.

Q.4 Discuss the use of solar energy for space heating, water heating and production of electricity.

Q.5 Discuss the use of indirect solar energy for generation of electrical power.

Q.6 Write short notes on the following:

1. Solar greenhouse
2. Solar production of electricity

Q.7 Explain the solar desalination process and solar cooking process.

Q.8 Write informative note on wind power with its merits and limitations.

Q.9 Define green chemistry and explain 12 principles of green chemistry.

Q.10 Explain any two greener methods with examples.

Q.11 Give a brief note on greenhouse concept.

Q.12 What is the importance of green synthesis?