Introduction
Green chemistry can be defined as the formation of chemical products to reduce and eliminate the use of hazardous compounds. Green chemistry uses innovative research for the development of alternative green and sustainable technologies. Green chemistry is evolving branch of science that is trying to reduce and eliminate the impact of hazardous chemicals on the environment and human health. (rsc.org) First research on the green chemistry occurred in the 1990s. Today green chemistry is applied to all industry sectors. The green chemistry design is trying to achieve the sustainability.
Discussion
There are twelve principles designed to develop alternative green and sustainable technologies, to reduce hazardous compounds.
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Principle 1: Waste prevention
Principle one states that the waste should be prevented, instead of treating and cleaning up the waste at the end of production. Planning ahead and reducing the waste during every step of the production stage, reduces the overall amount of waste. In the industry, a measure of waste is known as E-factor. E-factor can be defined as the weight of the waste coproducing to the waste of the final product. Waste prevention is also achieved by following mass intensity process which is expressed as the ratio of the weight of all materials (water, solvents, reagents, raw materials, etc.) used to the weight of the API (active pharmaceutical ingredient).
Before 12 principles were introduced, during the manufacturing of the product there would be over 100 kilograms of waste produced. However, if the first principle of green chemistry is followed, the amount of waste can be reduced up to 10 times. (American Chemical Society. 2019)
Principle 2: Atom economy
Atom economy states that waste should be reduced by maximizing the number of atoms of all reagents that are used to formulate the final product. For a reaction the efficiency is measured by calculating the percentage yield. It is desirable to have 100% percentage yield, but very often that is not the case. The percent atom economy is calculated by the ratio of the weight of desired products divided by the total of the weight of reactants used. Eg.(American Chemical Society. 2019)
For the reaction above the percentage atom economy is 50%, this means that only half of atoms of the reagents are used to give a final product. Chemists then have to work on maximising the usage of atoms of reagents. The percentage atom economy should be as high as possible, too achieve minimal waste.
3. Less hazardous chemical synthesis
Principle 3 works on designing the safest route to synthesize the product. Synthetic reaction should use and produce substances that have little or no toxicity to human health and the environment. Very often it is hard for scientists to adhere to this principle because most of the chemical compounds are toxic and they have to be used for a chemical reaction to be successful. Replacing toxic substance is difficult because it often does not result in the high percentage yield, when using different compounds that is less toxic. Until new synthetic route is designed, previous toxic material must be used. Reactive compounds are toxic compounds. Reactive compounds lead to reactions that are kinetically and thermodynamically favourable. (American Chemical Society. 2019)
4. Designing safer chemicals
Principle 4 looks into designing chemicals products that carry out their medical, industrial or any other role. It is important that the product has minimal toxicity to humans and the environment. To design safer chemicals, chemists have to understand how the substances work in the body and the environment. Sometimes the toxicity of the product is unavoidable, but the alternative route of synthesis must be looked for. Adhering to principle 4, may be very challenging for the chemists trying to design the synthesis route. It is difficult to minimize toxicity and maintaining the efficacy and the function of the product. Highly reactive substances are often used in the synthesis of the product. However, they react with biological targets in humans and environment, and result in undesirable effect. American Chemical Society. (2019).
5. Safer solvents and auxiliaries
Solvents are crucial parts of the synthetic reaction. Without the solvent reactions wouldn’t proceed. Solvents are responsible for mass and energy transfer during the reactions. It is difficult to replace toxic solvent, if the reaction cannot proceed without it. Most solvents are toxic, volatile, flammable and explosive. Solvents greatly contribute to the toxicity of the compound. During the chemical operation, 50-80% of the mass in the batch is from the solvents. During the synthesis, solvents are heated, cooled, distilled, pumped, filtered, etc. American Chemical Society. (2019).
Volatility and solubility of solvents greatly contributes to the pollution of air, water and land. Instead of solvents, chemists are trying to use safer substances such as water, ionic liquid and supercritical liquids. (pubs.rsc.org)
When choosing the replacement solvent, water is often used. Many reactions can be run in water. Water is safe and its waste doesn’t pose hazard to the environment and human health. The issue with water is that organic substances are not soluble in water and reaction cannot proceed. (pubs.rsc.org)
Supercritical liquids are also used to replace toxic solvents. Supercritical liquids are constantly heated and compressed over their critical point. Supercritical liquids are derived from water, carbon dioxide, methanol, etc. The most common supercritical liquid used is carbon dioxide derivative. It is safe to use and easy to handle. (pubs.rsc.org)
6. Design of energy efficiency
In the chemical reaction, energy is used heating, cooling, pumping, separation, etc. Favourable reactants are the one that can be used in the reaction at the room temperature and pressure. Energy that is not used in the reaction is waste. Alternative uses of energy are now used as an energy source. Solar power, wind and hydro power are often used. (pubs.rsc.org)
7. Use of renewable feedstocks
Most of the product are derived from petroleum and natural gas. Petroleum and natural gases are not renewable and can be depleted. This principle is looking at the usage of natural renewable sources that can be used as fuel and materials needed for chemical reactions. Some of the materials used are cellulose, starch, glycerine, etc. (pubs.rsc.org)
8. Reduce derivatives
One of the most important aspects of green chemistry is to reduce the usage of derivative and protecting groups. (Compound Interest. (2019) In the synthesis of the product, protecting groups are used to prevent alteration of the part of the compound, while transformation is occurring on the other part of the compound’s structure. Enzymes are used to replace the usage of derivative and protecting groups. Enzymes are specific and react with a specific substrate while leaving the rest on the molecule unchanged. (pubs.rsc.org)
9. Catalysis
Using catalysts in the reaction increases the atom efficiency and reduces the waste in the chemical reaction. Catalysts are used to speed up the rate of chemical reaction without being used up in the reaction. Catalysts can be recycled and used up in the reaction repeatedly. Catalysts reduce the energy input and contribute to better product selectivity. (Pubs.rsc.org, 2019)
10. Design for biodegradation
Chemical products should be non-toxic and able to carry out their function. However, this is often not the case as most chemicals are toxic. When toxic products break down, they cause pollution to the environment and pose the hazard to human health. Organic compounds are pollutant and they don’t break down and persist in the environment. When possible, these chemicals should be replaced “greener” compounds such as water, biodegradation and UV light. (Compound Interest. (2019)
11. Real time analysis for pollution prevention
Green analytical chemistry is trying to demonstrate how much waste chemicals are producing. Every reaction has to be carefully monitored to prevent any negative outcome.
Analytical chemistry generates a lot of waste. Analysing chemical reactions usually required extraction, separation, purification, etc. of the chemicals. A lot of analysis requires the use of solvents. As solvents are toxic, alternative substances are used. Supercritical fluids are often used to replace solvents. They are non-toxic and don’t cause pollution. (Pubs.rsc.org, 2019)
12. Safer chemistry for accident prevention
Chemicals are toxic, volatile, explosive, etc. and as such they pose risk to health. Professionals dealing with chemicals are exposed to hazards. Managing hazards, reduces the risks. If possible, all hazards should be eliminated from the chemical processes. This is not always possible; therefore, risks must be minimised. (Compound Interest. (2019)
Today, green chemistry is applied everywhere. Green chemistry has positive impact on the environment. It reduces the number of toxic elements being released in the environment. Most organic compounds are pollutants that do not break down and as such penetrate into the environment. Green buildings use solar design, natural ventilation and “green friendly” materials.
Wind generators are another example how green chemistry is used to produce the energy from a renewable source. Green buildings use innovative technology to reduce the pollution of the environment. (Aircconline.com. 2019).
One of the main applications of green chemistry in the pharmaceutical industry if the use of green solvents. During pharmaceutical production solvents are used in large volume. Solvents are toxic and volatile compounds and they generate a lot of waste that causes pollution of the environment. In the pharmaceutical industry, solvents account for over 80% of the mass used. Water and glycerol are “green solvents” that are readily used replace solvents. Super critical carbon dioxide has very low toxicity and is used for chemical extraction. Super critical carbon dioxide must be kept over its super critical temperature and pressure.
Green chemistry is greatly applied to the usage of biomass and biomass conversion. Cellulose and lignin conversion are widely used in pharmaceutical industry. 20% of Earth’s biosphere is lignin. Lignin can mainly be used to replace the use of oils in the production. (Chembioagro.springeropen.com. 2019)
Cellulose and hemicellulose account for 50-80% of wood biomass. They can be hydrolysed into simple monomeric sugars that have low molecular weight. Using a catalyst cellulose and hemicellulose can be converted into sugars and oligomers. (Chembioagro.springeropen.com. 2019) A lot of natural and renewable sources of energy can be found in the nature. Renewable sources of energy should be used over fossil fuels. (Taylor & Francis. 2019) Lignocellulosic biomass is composed of cellulose, lignin and hemicelluloses. Pre-treatment of lignocellulosic biomass is very expensive. At the end stage lignin and cellulosic biomass is obtained. They can then be used as source of energy. (Taylor & Francis. 2019)
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