Sustainability of green solvents – review and perspective

Abstract

Solvents define pivotal properties for chemical processing and chemical reactions, and can be as game-changing as catalysts. A solvent can be the key to a good chemical process, rather than being just an asset and a reaction space. It determines the solubility, meaning the concentration at which reactants can be processed, determines the stability of excited states, and thus guides the potential-energy curves of activation. As a net outcome, solvents rule over productivity and economic/environmental benefit. Expanding the capabilities of conventional solvents, a new class of so-called master solvents has been proposed and investigated in the last two decades, which have also been termed “green” or “designer” solvents. They comprise first of all ionic liquids and supercritical carbon dioxide, yet also deep eutectic, thermomorphic and fluorous solvents. There is a vast literature on and experience with these solvents and their applications. However, their sustainability has been reported mostly by considering only one class of master solvent and hardly benchmarking several classes. Thus, a comparative, complete, and holistic view of their sustainability is missing. This perspective review aims to fill this gap and focuses on a holistic overview of sustainability assessments of master solvents. It encompasses green chemistry principles, life cycle assessments, and modern circularity evaluations based on the “10R framework”; standing for 10 principles starting with “R” such as recycling, reuse, repair, rethink, remanufacture, and so on. This review will show what “green” actually denotes and means for each class of master solvent when it comes to a ‘weighting’ by a certified sustainability assessment.