Nov, 2020 - By Tejaswini Kasture
Green chemistry is the design of chemical products and processes that reduce the use or generation of hazardous substances. Green chemistry applies across the life cycle of a chemical product, including its design, manufacture, use, and ultimate disposal. Green chemistry is also known as sustainable chemistry. Green chemistry aims to do the following:
- prevent pollution at the molecular level
- is a philosophy that applies to all areas of chemistry, not a single discipline of chemistry
- apply innovative scientific solutions to real-world environmental problems
- result in source reduction and prevent pollution
- reduce the negative impacts of chemical products and processes on human health and the environment
- eliminate hazard from existing products and processes
- design chemical products and processes to reduce their intrinsic hazards
Importance and Benefits of Green Chemistry
The fact that the chemical industry must not adversely affect the environment for future generations has been the driving force behind the development of green chemistry. This is not a separate branch of chemistry, but an approach that permeates every stage of process development.
The aspiration can be summed up in one word: sustainability. Sustainable development meets the needs of the present without compromising the ability of future generations to meet their own needs. The problems it aims to address are:
- The depletion of finite oil, gas and mineral resources
- The production of waste, some of it harmful to living organisms
- Reagents and processes that present a risk to human health and the environment
- Products, which, when disposed of, do not degrade easily.
- Cleaner air: Lesser release of hazardous chemicals in the air, leading to less damage to lungs
- Cleaner water: Lesser release of hazardous chemical wastes in water leading to cleaner drinking and recreational water
- Increased safety for workers in the chemical industry: Less use of toxic materials which leads to less need for personal protective equipment and lower potential for accidents (e.g., fires or explosions)
- Safer consumer products of all types: New, safer products will become available for purchase; some products (e.g., drugs) will be made with less waste; some products (i.e., pesticides, cleaning products) will be replacements for hazardous products
- Safer food: Elimination of persistent toxic chemicals that can enter the food chain; safer pesticides that are toxic only to specific pests and degrade rapidly after use
- Less exposure to such toxic chemicals as endocrine disruptors
- Many chemicals end up in the environment through intentional release (e.g., pesticides), through unintended releases (including emissions during manufacturing), or through disposal. Green chemicals either degrade to innocuous products or are recovered for further use.
- Plants and animals suffer less from toxic chemicals in the environment
- Lower potential for global warming, ozone depletion, and smog formation
- Less chemical disruption of ecosystems
- Less use of landfills, especially hazardous waste landfills
Economy and business:
- Higher yield for chemical reactions, consuming smaller amounts to obtain the same amount of product
- Fewer synthetic steps, often allowing faster manufacture of products, increased plant capacity, and conservation of energy and water
- Reduced waste, eliminating costly remediation, hazardous waste disposal, and end-of-the-pipe treatments
- Allow replacement of a purchased feedstock by a waste product
- Better performance so that less product is needed to achieve the same function
- Reduced use of petroleum products, slowing their depletion and avoiding their hazards and price fluctuations
- Reduced manufacturing plant size or footprint through increased throughput
- Improved competitiveness of chemical manufacturers and their customers
- Increased consumer sales by earning and displaying a safer-product label (e.g., Safer Choice labeling)
Principles of Green Chemistry
- 1. Prevent waste: Design chemical syntheses to prevent waste. Leave no waste to treat or clean up.
- Maximize atom economy: Design syntheses so that the final product contains the maximum proportion of the starting materials. Waste few or no atoms.
- Design a less hazardous chemical syntheses: Design syntheses to use and generate substances with little or no toxicity to either humans or the environment.
- Design safer chemicals and products: Design chemical products that are fully effective yet have little or no toxicity
- Use safer solvents and reaction conditions: Avoid using solvents, separation agents, or other auxiliary chemicals. If you must use these chemicals, use safer ones.
- Increase energy efficiency: Run chemical reactions at room temperature and pressure whenever possible
- Use renewable feedstock: Use starting materials (also known as feedstock) that are renewable rather than depletable. The source of renewable feedstock is often agricultural products or the wastes of other processes; the source of depletable feedstock is often fossil fuels (petroleum, natural gas, or coal) or mining operations.
- Avoid chemical derivatives: Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.
- Use catalysts instead of stoichiometric reagents: Minimize waste by using catalytic reactions. Catalysts are effective in small amounts and can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and carry out a reaction only once.
- Design chemicals and products to degrade after use: Design chemical products to break down to innocuous substances after use so that they do not accumulate in the environment.
- Analyze in real time to prevent pollution: Include in-process, real-time monitoring and control during syntheses to minimize or eliminate the formation of byproducts.
- Minimize the potential for accidents: Design chemicals and their physical forms (solid, liquid, or gas) to minimize the potential for chemical accidents including explosions, fires, and releases to the environment.
Green Chemistry Examples
A. Innovative propylene oxide process
Companies DOW and BASF jointly developed a technology of conversion of hydrogen peroxide into propylene oxide (HPPO) that has significant "green" advantages over competing technologies:
- It uses hydrogen peroxide and propylene as raw materials, producing only propylene oxide and water
- It reduces waste water by 70-80%
- It uses 35% less energy
- Its capital cost is 25% less
- It avoids the need for co-product infrastructure and markets
B. Advanced amine technology for carbon capture
Alstom-DOW pilot plant captures CO2 from new or existing industrial facilities with an improved sustainability profile:
- Pilot plant in West Virginia is designed to capture 1,800 tons CO2 per year
- Advanced Amine process leads the industry in carbon capture efficiency
- Capture rate ~90% with 99.5% purity of CO2
- Process significantly reduces parasitic energy requirements
C. Metathesis catalysis for making high-performing, green specialty chemicals at advantageous costs
Elevance employs Nobel-prize-winning catalyst technology to break down natural oils and recombine the fragments into novel, high performance green chemicals. These chemicals combine the benefits of both petrochemicals and bio-based chemicals. Elevance produces specialty chemicals for many uses, e.g., concentrated cold-water detergents that provide better cleaning with reduced energy costs.
- Significant energy savings
- Reduction of greenhouse gas emissions by 50% (compared to petrochemical technologies)
D. An efficient biocatalytic process to manufacture simvastatin
Simvastatin, a drug for treating high cholesterol, is manufactured from a natural product. The traditional multistep synthesis was wasteful and used large amounts of hazardous reagents. Professor Y. Tang (UCLA) conceived a synthesis using an engineered enzyme and a practical low-cost feedstock. Codexis optimized both the enzyme and the chemical process.
- Great reduction of hazard
- Less amount of waste
- Cost-effective approach
- Better meets needs of customers
E. Enzymes save energy and wood fiber for manufacturing high-quality paper and paperboard
Traditionally, making strong paper requires costly wood pulp, energy-intensive treatment, or chemical additives but that may overcome. Buckman’s Maximyze enzymes modify the cellulose in wood to increase the number of fibrils that bind the wood fibers to each other, thus making paper with improved strength and quality − without additional chemicals or energy. Buckman's process also allows papermaking with less wood fiber and higher percentages of recycled paper, enabling a single plant to save $1 million per year.
F. Gas-expanded liquids for sustainable catalysis
Gas-expanded liquid (GXL) is a substance generated by dissolving a compressible gas (for example, CO2 or a light olefin) in a regular liquid substance at mild pressures (tens of bar). When CO2 is used as an expansion gas, this process produces CO2-expanded liquid (CXL). An attractive feature of GXLs is that combines the advantages of compressed gases and of traditional solvents. GXLs retain the beneficial attributes of the conventional solvent (polarity, catalyst/reactant solubility) but provide higher miscibility of permanent gases (O2, H2, CO, etc.), as compared to organic solvents at ambient conditions. GXLs also results in enhanced transport rates compared to regular liquid solvents. The enhanced gas solubility in GXLs have been exploited to alleviate gas starvation (often encountered in homogeneous catalysis with conventional solvents). Environmental advantages of GXL include:
- replacement of harmful organic solvents with environmentally benign CO2;
- reduced flammability due to CO2presence in the vapor phase;
- and lower process pressures (tens of bar) compared to supercritical CO2(hundreds of bar) which is linked to energy savings.
i. Cleaning products
The discovery of a catalytic chemical process (metathesis) earned the Nobel Prize in Chemistry in 2005. Using much less energy, it can reduce greenhouse gas emissions for many key processes. The process is stable at normal temperatures and pressures, can be used in combination with greener solvents, and is likely to produce less hazardous waste. Furthermore, progress in the development of this process led to the Presidential Green Chemistry Challenge Award in 2012, when metathesis was demonstrated to break down natural oils and recombine the fragments into high-performance chemicals. Metathesis has implications for the manufacture of, among others, detergents.
ii. Computer chips
Many chemicals, and resources such as water and energy are needed to manufacture computer chips. A 2003 study estimated a ratio of 630:1 in terms of chemicals and fossil fuels required to make one computer chip – i.e. it takes 630 times the weight of the chip in source materials to make one chip (in comparison, the ratio for manufacturing a vehicle is 2:1).
Advances in Green Chemistry are as follows:
- It is a new process that uses supercritical carbon dioxide in one of the steps of chip preparation, significantly reducing the chemicals, energy and water needed in the production process.
- Innovation using chicken feathers to make computer chips. The protein, keratin, in the feathers is used to make a fibre form that is both light and tough enough to withstand mechanical and thermal stresses. The result is a feather-based printed circuit boardthat works at twice the speed of traditional circuit boards. While this technology is still in the works for commercial purposes, the research has led to other uses of feathers as source material, including for biofuel.
Pharmaceutical research, besides investigating new medical solutions, also focuses on ways to reduce harmful side-effects and processes that produce less toxic waste. Some Green Chemistry successes include:
- New biocatalysts – using an enzymatic process originally developed for the treatment of type 2 diabetes but holding promise for other drugs as well – which reduce waste, improve yield and safety, and eliminate the necessity for a metal catalyst.
- A new synthesis using an engineered enzyme and low-cost feedstock for a well-known high cholesterol medicine was optimized to be more cost-effective, as well as to greatly reduce hazard and waste.
iv. Biodegradable plastics
Many companies are developing plastics made from renewable, biodegradable sources. Some of the key developments include:
- New food containers made from a method where microorganisms convert corn starch into a resin that is just as strong as the rigid petroleum-based plastic used for containers such as water bottles and yogurt pots.
- Fully biodegradable bags made of a compostable polyester film with cassava starch and calcium carbonate. The bags are tear-resistant, puncture-resistant, waterproof, printable and elastic; as well as able to disintegrate into water, CO2, and biomass in industrial composting systems.
Oil-based paints containing synthetic resin made from dicarboxylic acid (known as alkyd paints), emit organic compounds. These volatile compounds evaporate from the paint as it dries and have environmental impacts. Improvements in this area include:
- Replacing fossil-fuel-derived paint resins and solvents with a mixture of soya oil and sugar cuts hazardous volatiles by half. These bio-based oils are used to replace petroleum-based solvents, creating safer paints with less toxic waste.
- Water-based acrylic alkyd paints with low volatile organic compounds that can be made from recycled soda bottle plastic, acrylics, and soybean oil. In 2010, enough of these paints were manufactured to eliminate more than 362 874 kg of volatile organic compounds.
The ultimate aim of green chemistry is to entirely cut down the stream of chemicals pouring into the environment and save the human health.