The use of auxiliary substances (e.g. solvents, separation agents) should be made unnecessary wherever possible and innocuous when used

What is an auxiliary substance? It depends on whom you ask. The average synthetic chemist, when reading this principle (Fig. 1), would think of all of the auxiliary substances required for a chemical synthesis and subsequent product purification: solvents, drying agents, column packing agents, etc. Another kind of chemist, such as a formulation chemist, might think of an entirely different set of substances: surfactants, preservatives, colours, stabilizers, viscosifiers, and all of the other substances that are not absolutely required for the function of the product but improve the product’s performance, ease of use, and appearance. The principle may have originally been written from the perspective of a synthetic chemist, but the principle is equally valid from the perspectives of all kinds of chemists and chemical engineers.

Fig. 1 The 12 Principles of Green Chemistry¹

From a synthetic chemist’s perspective, the word “solvent” stands out as perhaps the most important word in the principle. Solvents are the largest mass input in a fine chemical synthesis and the largest component of waste produced. The life cycle impacts of fine chemical syntheses can easily be dominated by the impacts due to solvent rather than impacts due to reagents or catalysts, simply because the mass of solvent used is so much greater than the mass of reagents or catalysts. Fortunately, there has been extensive academic and industrial research on how to reduce the environmental impact of solvent use,² generally by two strategies: identifying ways to reduce solvent use and identifying greener solvents.

Methods for reducing solvent use include solventless synthesis and using one solvent for more than one step. The humorous theorem called Murphy's Law of Solvents³ stipulates that “The best solvent for any process step is bad for the next step”. Thus, at the end of each reaction or separation step, the solvent that was used for that step must be removed and replaced with a solvent that meets the needs of the next step. This results in excessive solvent usage. This problem can be solved by using either (a) a compromise solvent, meaning a solvent that performs adequately for two or more steps in a row, or (b) a switchable or tunable solvent, meaning a solvent that meets the needs of the first step and then can have its properties switched or tuned to meet the needs of the second step.

The second strategy, identifying greener solvents, has attracted far more attention. Many papers have compared common conventional solvents for their impact on the environment, health, and safety (EHS) and suggested that chemists and engineers use only those that are found to be among the best by those criteria. While those papers have been useful contributions, they generally consider neither the environmental impact of the manufacture of the solvent nor the effect of the solvent on reaction performance and ease of post-reaction separation. Including those factors makes the selection of the greenest solvent a far more difficult task; one that can only be achieved by life cycle analysis applied to a specific application. Very few such papers have been published so far, but let us all hope that more and more of them will appear in the near future.

While the solvent may be the most obvious “auxiliary substance” to a chemist, there are many other substances that should also be considered. Drying agents, chromatographic column packings, and separation agents (e.g. saline solution, acid/base wash, activated carbon) are other examples of auxiliary substances used during or after a reaction. The search for greener substitutes for such materials has not been nearly as active or high profile as the search for greener solvents; that is a great opportunity for future growth of the field.

From the perspective of a formulations chemist, the expression “auxiliary substance” refers to an entirely different set of materials: the additives in formulations that may not be crucial to the functioning of the product but make the product better for the user. Green chemistry efforts towards reducing the impact of such auxiliary substances⁴ are not as well represented in the academic literature as work on greener reaction media. Research into the identification of greener surfactants^5,6 has been widely published, but work on greener viscosifiers and stabilizers has been stronger in industry than academia. Drying agents, which also represent a great opportunity for future green chemistry research, are useful in chemical synthesis, consumer products, and in other industries; for molecular sieves the major usage is in insulating glass while the biggest application of silica gel is in packaging.⁷

Even greener solvents, which have been so intensively studied as media for chemical syntheses, have been largely ignored in the literature for other applications, despite the fact that the majority of solvent usage is not as reaction media. For example, methylene chloride, a solvent that certainly needs to be replaced, is primarily used for formulations such as paint strippers, adhesives, and aerosols.⁸ Even looking only at industrial usage, the majority (60%) of emissions of chlorinated solvents in the US come from industries other than the chemical industry, according to Toxics Release Inventory data.⁹

If we truly want green chemistry to bring about the replacement of hazardous auxiliary substances with greener alternatives, then we have to broaden the application of principle 5 to include all uses of these substances. The principle should be interpreted to include different applications (not only chemical synthesis), different industries, different products, and as a result, a very wide range of auxiliary substances. Whatever the process or product, auxiliary substances should be avoided where possible and innocuous when used.

Notes and references

1. P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, 1998.
2. F. M. Kerton and R. Marriott, Alternative Solvents for Green Chemistry, RSC Publishing, Cambridge, UK, 2nd edn, 2013.
3. P. G. Jessop, S. Trakhtenberg and J. Warner, in Innovations in Industrial and Engineering Chemistry. A Century of Achievements and Prospects for the New Millennium, ed. W. H. Flank, M. A. Abraham and M. A. Matthews, ACS Symposium Series, Washington, 2008, pp. 401–426.
4. P. G. Jessop, F. Ahmadpour, M. A. Buczynski, T. J. Burns, N. B. Green II, R. Korwin, D. Long, S. K. Massad, J. B. Manley, N. Omidbakhsh, R. Pearl, S. Pereira, R. A. Predale, P. G. Sliva, H. VanderBilt, S. Weller and M. H. Wolf, Green Chem., 2015, 17, 2664–2678.
5. Surfactants from Renewable Resources, ed. I. J. Mikael Kjellin, Wiley, Chichester, UK, 2010.
6. P. Foley, A. Kermanshahipour, E. S. Beach and J. B. Zimmerman, Chem. Soc. Rev., 2011, 41, 1499–1518.
7. A. P. Cohen, “Desiccants”, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-Interscience, 2003.
8. M. T. Holbrook, “Methylene Chloride”, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-Interscience, 2003.
9. TRI Explorer, United States Environmental Protection Agency, https://iaspub.epa.gov/triexplorer/tri_release.chemical.

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