12 Principles of Green Chemistry and Green Solvents

In 1998, in a book entitled Green Chemistry: Theory and Practice, Paul Anastas and John Warner put forth 12 principles of green chemistry. Currently, the American Chemical Society(ACS) includes them on their web site, linking each principle to a practical example.

  1. Prevention It is better to prevent waste than to treat or clean up waste after it has been created.

  2. Atom Economy Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.

  3. 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.

  4. Designing Safer Chemicals Chemical products should be designed to affect their desired function while minimizing their toxicity.

  5. 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.

  6. 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.

  7. Use of Renewable Feedstocks A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

  8. 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.

  9. Catalysis Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

  10. 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.

  11. 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.

  12. 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.

It’s reassuring to see a foundation for green chemistry laid out. But at the same time, we can’t help wonder how widespread the adherence to those principles actually is. One research center that is committed to making these 12 principles as feasible as possible is the Green Chemistry Centre of Excellence at the University of York in England. (The ACS site does not link any of the principles to them perhaps because the ACS showcases only industrial and academic applications from within the United States.) With regard to principle 5, the University of York investigates the use of both supercritical and liquid carbon dioxide as a solvent.

Recall CO2‘s phase diagram. Below its triple point(518 kPa at −56.6 °C) sublimation is possible—the familiar behavior of dry ice. Above that point, CO2 liquid is possible as the molecule goes through its three states with increasing temperature. But above its critical point of 304.25 K  7.39 MPa or about 7400 kPa, it forms a supercritical fluid.CO2phase
Supercritical CO(sCO2) which exists at high pressures, has interesting hybrid properties: it acts like a gas in having high diffusion rates but also resembles a liquid in density and dissolving power, both of which are variable with slight changes in pressure.

sCO2 is non-polar, but unlike many traditional non-polar solvents it is not toxic, leaves no traces (principle 10) and can be recovered. It can extract waxes, natural products and liquid crystals.BioSolvents Also greener than conventional solvents are biological ones such as glycerin, ethanol, limonene, 2-methyl-tetrahydrofuran and the degreaser ethyl lactate. These can be generated from food waste, revealing how compost is not the only useful product that can be derived from what most people perceive as trash.

When either sCO2 or biological solvents are used as medium for reactions, they are less likely to “poison” catalysts.   When catalysts accumulate impurities or get “poisoned” , they can no longer be used to speed up reactions. So by keeping catalysts “cleaner” with the use of green solvents, they perform syntheses with less waste. There are exceptions: Ziegler catalysts are poisoned by CO2 through the formation of CO. A more important issue is that the high pressures required to generate CO2 drive up the cost. Returning to advantages, some green-solvent-assistant syntheses are also more selective when only one chiral component is desired. In fact many enzymatic reactions can operate in supercritical fluids. Such properties of the solvents help us respect green chem principle 6.

Professor James Clark, the director of U of York’s Green Chem Centre, which is the largest of about 9 green centers in Britain, is connected with India’s IGCW (Industrial Green Chemistry World) whose core objective is

to drive industry implementation of green chemistry and engineering-based technologies to sustainably address priority and pressing environmental challenges of our chemical industry.

From http://www.york.ac.uk/chemistry/staff/academic/a-c/jclark/
From http://www.york.ac.uk/chemistry/staff/academic/a-c/jclark/

Although about 85% of petroleum is used for fuel, the 15% whose distillates are used for petrochemicals, plastics, lubricants, asphalt and other products accounts for about half of the industry’s profits. Renewable sources of energy have to replace petroleum as a fuel, but we also need a sustainable source of materials for organic compounds. For this reason, the center is hoping that biorefineries will soon find their way into large industry. If we again refer to the green principles, a biorefinery uses atom economy and produces less hazardous chemical syntheses; it designs safer and more biodegradable compounds, using better solvents and makes use of renewable feedstocks.

One thought on “12 Principles of Green Chemistry and Green Solvents

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  1. Very interesting. First principle reminds me of the precautionary principle. It is sad to see that so many people are not aware of this kind of thinking, however one calls it. I remind a frustrating conversation I had many years ago with a hardcore neoliberal economist who was convinced that polluting was just another variable one considers in calculations of feasibility of a business, pollute first – clean after (if law mandates, only) kind of thinking. I only hope this kind of corporate irresponsibility changes with time. Another excellent and rich post by you, thanks!

    Liked by 1 person

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