Cradle to grave: How green are ionic liquids?

Claims of green-ness often accompany papers featuring ionic liquids and these rest mainly on their ability to serve as non-volatile and non-flammable alternatives to traditional molecular solvents. As our understanding of this class of materials has grown, it is increasingly apparent that no single property other than melting point (inherent in the definition of this class of compounds) can be used to describe the entire class of compounds. One can no longer classify them unequivocally as green but one can definitely not dismiss ionic liquids as ungreen. This mini review takes a lifecycle analysis approach (from synthesis to disposal) to examining green as it pertains to ionic liquids. Recent progress in the field toward synthesis of greener ionic liquids and understanding their toxicity and biodegradability are addressed. The greenness of ionic liquids specifically with respect to their use in the energy domain as solvents for biomass dissolution, solar and fuel cells, and battery electrolytes is critically examined. The author further discusses, cost and performance, factors that are complementary to environmental benefits and cannot be ignored.