Principles of Green Chemistry
Paul Anastas and John C. Warner defined green chemistry as the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture, and applications of chemical products.*
The 12 principles of green chemistry are:
- It is better to prevent waste than to treat or clean up waste after it is formed.
- Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
- Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
- Chemical products should be designed to preserve efficacy of function while reducing toxicity.
- The use of auxiliary substances (e.g. solvents and separation agents) should be made unnecessary wherever possible and innocuous when used.
- Energy requirements should be recognized for their environmental and economic impacts and should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.
- A raw material or feedstock should be renewable rather than depleting wherever technically and economically practicable.
- Unnecessary derivatization (blocking group, protection and deprotection, temporary modification of physical or chemical processes) should be avoided whenever possible.
- Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
- Chemical products should be designed so that at the end of their function they do not persist in the environment and break down into innocuous degradation products.
- Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
- Substances and the form of a substance used in a chemical process should be chosen to minimize potential for chemical accidents, including releases, explosions, and fires.
These 12 principles fall into 4 broad groups:
- reduce energy use
- reduce waste
- reduce hazards
- reduce resource use and utilize renewable resources
In practice, making a chemical process comply with the principles of green chemistry may mean:
- redesigning production methods to use different starting materials
- using different reaction conditions (eg, different solvents or catalysts)
- using production methods with fewer steps
* Anastas, P.T., Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press, Oxford, 1998
Reducing Energy Use
- Inexpensive, toxic 1,2-ethanediol (ethylene glycol) is the main constituent of the antifreeze used in automobiles. Professor Suppes developed a system for converting waste 1,2,3-propanetriol (glycerin or glycerol) from biodiesel or soap production to 1,2-propanediol (propylene glycol) using a copper chromite catalyst which lowers the temperature of the conversion. The polypropene (polypropylene) produced in this way could be cheap enough to replace ethylene glycol in antifreeze.
- Sertraline is the active ingredient in Zoloft, a pharmaceutical drug used in the treatment of depression. The commercial process first used by Pfizer to produce sertraline used 979 L of solvents (ethanol, ethyl acetate, THF, hexane and toluene) per kg of product. The newer microwave-mediated synthesis uses only 256 L of solvents per kg of product, and, uses only 3 solvents in total; ethanol, ethyl acetate and methanol.
- Sildenafil citrate is the active ingredient in Viagra. In the first commercial synthesis of sildenafil citrate by Pfizer, 31 L of solvents per kg of product were used. The newer microwave-mediated synthesis requires only 10 L of solvents per kg of product for the production of sildenafil citrate.
- Flammable and carcinogenic solvents such as tetrachloromethane (carbon tetrachloride) and perchloroethylene have been used as dry cleaning solvents. Supercritical carbon dioxide, a fluid with physical properties between those of liquid and gaseous carbon dioxide, can be used as a safer alternative to dissolve grease.
- Ozone-depleting CFCs or flammable hydrocarbons had been used as a blowing agent in the production of polystyrene foam. Dow Chemical discovered that supercritical carbon dioxide, which is neither ozone-depleting nor flammable, could be used as an effective blowing agent, and, because the carbon dioxide they use is sourced from other industries where it as a waste product, its impact on the human-induced greenhouse effect is negligible.
- Many of the insecticides we have used to control pests in crops have had deleterious effects on the health of humans and other aminals. The CSIRO in Australia is developing a new class of insecticides which are molecules that interact with a specific insect's hormones, causing that type of insect to moult prematurely and die, but do not effect other plant or animal species.
- Toxic tributyltin oxide, TBTO, has been used on ships as an antifoulant, that is, to reduce the growth of plants and animals on ship hulls, but it persists in the environment for a long time. 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one is the active ingredient in a newer alternative, Sea-Nine, a product which is toxic to the animals that grow on ship hulls but does not accumulate in marine animals like shellfish.
Reducing Resource Use and Utilizing Renewable Resources
- Traditionally, the polymers used in making textiles, cutlery and food packaging, have been produced from non-renewable petroleum-based materials. Lactic acid, which is a renewable resource produced by fermenting corn, can be used to produce a polymer known as polylactic acid (PLA). Polylactic acid (PLA) can be recycled or composted.
- Many materials used as building materials are made from non-renewable resources such as iron in steel. A biodegradable composite material made out of renewable resources and with the tensile strength of steel has been developed by scientists in the USA using flax yarn embedded in soy protein polymer resin.