Sustainability Assessment of Alternative Synthesis Routes to Aprotic Ionic Liquids: The Case of 1-Butyl-3-methylimidazolium Tetrafluoroborate for Fuel Desulfurization

Abstract

Advantages of ionic liquids (ILs) over volatile organic solvents in chemical processes include no or negligible evaporative losses and high tunability. However, the conventional production of aprotic ILs via metathesis can be unattractive (both economically and environmentally) because of its high complexity, while the performance of other synthesis routes remains unclear. Existing life-cycle assessments furthermore fail to combine the production and use phases of these solvents, leading to erroneous conclusion about their sustainability credentials. This paper compares a one-pot, halide-free production route to 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] against metathesis and two conventional fuel desulfurization solvents, namely, acetonitrile and dimethylformamide (DMF). Halide-free synthesis is predicted to reduce the cost and environmental impacts associated with the production of [BMIM][BF4] by 2–5-fold compared to metathesis. Upon including the use phase of the solvents in fuel desulfurization and accounting for the uncertainty in background data, halide-free [BMIM][BF4] consistently presents the lowest cost and environmental impacts, while DMF is the worst in class. As well as exemplifying the importance of synthesis routes of ILs on their sustainability, these results highlight the need to include the use phase of solvents for more comprehensive life-cycle assessments.