Are deep eutectic solvents really green?: A life-cycle perspective

Abstract

Deep eutectic solvents (DESs) have generated great interest as promising green alternatives to replace conventional solvents due to their application-specific tunability, nonflamability, variable viscosity, low vapor pressure, and chemical (and thermal) stability. However, their greenness or sustainability remains unclear and requires rigorous quantification. This work uses life-cycle assessment (LCA) to evaluate the potential environmental impacts incurred from the production of a representative choline chloride (ChCl)/urea DES, reline. The environmental profile of reline is compared with that of common organic solvents (methanol, ethanol, dichloromethane (DCM), and ethyl acetate) on the basis of their utilization as a solvent for the oxidation of alcohol to the ketone; acetophenone. The results indicate that, in general, the DES imparts lower environmental impacts than DCM and ethyl acetate but has higher impacts than methanol and ethanol. Chemical constituent materials (urea, trimethylamine, hydrochloric acid, and ethylene oxide) required for the synthesis of reline DES contribute significantly to its life-cycle environmental impacts when compared to auxiliary processes. In addition to reline, the LCA of four other ChCl-based DESs is performed by substituting urea (hydrogen bond donor in the reline) with ethylene glycol, glycerol, citric acid, or glucose. ChCl/citric acid DES, the so-called natural deep eutectic solvent, imposes the highest environmental impacts among the studied ChCl-based DESs. This is partly due to the high water consumption and carbon dioxide emission during fermentation to synthesize the citric acid. Our study challenges the greennesses of DESs but more research with better data is required to corroborate our findings as some key inputs were modeled from a commercial patent.