Green chemistry is the design of chemical products and processes in order to minimize their impact on the environment.
This concept has emerged in the early 1990s and has gained much importance since its birth.
This article presents briefly the twelve principles of green chemistry, and then details some important aspects of green chemistry.
The twelve principles of green chemistry
Twelve principles have been invented to explain how to put green chemistry into practice:
- Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.
- Atom economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
- Less hazardous chemical syntheses: Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
- Designing safer chemicals: Chemical products should be designed to affect their desired function while minimizing their toxicity.
- Safer solvents and auxiliaries: The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.
- Design for energy efficiency: Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.
- Use of renewable feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
- Reduce derivatives: Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
- Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
- Design for degradation: Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.
- Real-time analysis for pollution prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.
- Inherently safer chemistry for accident prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires.
The atom economy is the maximization of the incorporation of the reagents into the product during a chemical reaction.
An ideal synthesis incorporates the totality of the atoms of the reagents into the product, and does not generate any waste.
During the hydrogenation of an alkene, both atoms of the dihydrogen are added to the alkene and thus incorporated into the product. This synthesis is perfect from an atom economy point of view.
The use of a protecting group during a synthesis is not atom-economical at all because it must be removed by the end of the synthesis, thus generating waste.
To quantify the atom-economy of a synthesis, the E Factor has been invented: E = (mass of waste / mass of product). A green reaction should have the lowest possible E factor, ideally close to 0.
Green reaction media
The classical organic solvents exhibit several problems. First, thy are often flammable or very flammable, which causes safety problems. They are volatile, and emit Volatile Organic Compounds (VOCs) in the atmosphere, which are a problem for health and for the greenhouse effect. Some of these solvents are toxic and/or carcinogenic. They represent a very large part of the chemicals used by the chemical industry, so improvements in the area of solvents are necessary to make chemistry greener. The main alternatives to the traditional solvents will be presented here.
Solventless reactions are reactions performed without using a solvent. The absence of a solvent is the greenest reaction medium. They can have advantages such as an increase in the reaction rate, an easier purification and a lower price. Unfortunately, highly exothermic reactions cannot be carried out without a solvent, because they need it to help dissipating the heat of reaction.
Supercritical carbon dioxide, i.e. carbon dioxide heated above 304 K and compressed above 74 MPa, has an behaviour that is intermediate between the behaviours of gases and liquids. It can be used to perform extractions or chemical reactions. It is very advantageous because it is not toxic and can be removed extremely easily at the end of the process by lowering pressure.
Water can also be used as a solvent. Even if a majority of organic compounds are not soluble in water, it can be used as a reaction medium. Water is very cheap, very safe because uninflammabale, non toxic, and can lead to interesting results in terms of selectivity and reaction rate.
Ionic liquids have gained much interest recently as green reaction media. Ionic liquids are salts which are liquid at room temperature. They are very little volatile, and they are not flammable. They often have good solubility properties for both ionic and and nonionic compounds.
If none of these green reaction media suits to the reaction that must be developed, problematic organic solvents can still be replaced by less dangerous and greener organic solvents.
Problematic organic solvents include chloroform, dichloromethane, diethyl ether, benzene, dimethylformamide, etc. Greener organic solvents include cyclopentyl methyl ether, toluene, methyltetrahydrofuran, N-methylpyrrolidone, etc.
Nowadays, the majority of organic chemicals are derived from petroleum, but petroleum reserves are limited and not renewable. Chemistry will need, one day or another, to find renewable sources of raw materials. A renewable feedstock for chemistry is biomass, the materials produced by living beings (mainly plants).
Some raw materials that can be obtained from biomass include starch, cellulose, lignin and vegetable oils. They can be converted to a variety of chemicals including methanol, ethanol, lactic acid, glycerol, furfural an many others.
The use of renewable feedstocks for chemistry is a rather young area of development, and much more efforts will be needed before we are able to fully replace petroleum.
The use of comestible biomass as a renewable feedstock for chemistry poses one ethical problem: should we use it as a source of renewable chemicals of for alimentation? One solution would be to use mainly non-comestible biomass such as products derived from wood or agricultural waste.
A catalyst is a chemical added in small quantity to a chemical mixture in order to facilitate a reaction. It works by lowering the energy needed to perform the reaction.
Using a catalyst allows to perform a reaction at a lower temperature and to make it faster. It often increases the selectivity of the reaction, thus producing less waste, increasing the yield, and facilitating the purification. An easy purification, a low quantity of waste and a higher yield make the use of catalysts very economically interesting.
Catalysts are already widely used industrially, but many challenges remain in the design of green catalysts. Catalysts containing a metal should use a non-toxic, cheap and widely available metal. Their catalytic activity and lifetime should be the highest possible. They should be easily recyclable, which means they should be easily separated from the reaction mixture.
A green chemical should have a safe production process. Efforts must be made to reduce the risk of accident and to reduce the harm that can be caused in case of accident during the production. This can be achieved by avoiding the use of flammable, explosive and toxic reagents, reaction media and intermediates.
During their use, chemicals should be safe. They should neither cause health problems for their users and the other people, nor damage the environment by their use.
Finally, after they have been used, green chemicals should be degradable in a safe and non-polluting way, and ideally biodegradable, which means degradable in the environment.