Investigating the Greenness of Ionic Liquids

Objective

After researching ionic liquids and developing a framework for classifying them into “greenness” categories, select two particular ionic liquids from different categories and subject them to a comparative sustainability assessment.

Background

Green chemistry engages with sustainability issues at the most elemental level. The Environmental Protection Agency describes green chemistry as “the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances,” and notes that it applies throughout the lifecycle of a chemical product, from design and manufacture through use and reuse/recycling.

An ionic liquid is an ionic salt with a melting point below 100°C (as opposed to an ionic salt like sodium chloride, with a melting point of 800°C). The use of ionic liquids in research in the last fifteen years has expanded rapidly, and they are now being used industrially as well. Ionic liquids can be used in various applications including chemical reaction solvents, absorption fluids for gas separations, heat transfer solvents, biological media, capacitor media, and electrolytes in batteries and other electrochemical systems. Recently there has been much hope (and some hype?) to the effect that ionic liquids can be greener substitutes for volatile organic compounds (VOCs). VOCs, the most common organic solvents, are toxic, flammable, and major sources of environmental pollution. Ionic liquids, with their relatively low flammability and volatility, are seen as potentially capable of avoiding many of the containment, waste disposal, and safety requirements of VOCs—with attendant financial savings as well. However, not all ionic liquids are inherently green, and some are extremely toxic, especially in aquatic environments.  Much remains to be learned about the toxicity, biological activity, and biodegradability of ionic liquids.

Suggested Approaches

(i)  Conduct a thorough literature search that aims for a solid and updated understanding of ionic liquids, with emphasis on issues related to their comparative environmental benefits.

(ii)  On the basis of this research, develop a defensible system for classifying ionic liquids into potential “degrees-of-greenness” groups.

(iii)  Select two ionic liquids, from different greenness groups. Perform a comparative sustainability analysis of these two ionic liquids, using life cycle assessment or another applicable sustainability quantification system. The analysis will require a host of threshold decisions as to the particular context under examination, and the lifecycle boundary conditions that will be applied. The analysis will certainly cover the manufacturing stage, and potentially use phases as well.