Synthesis and characterization of acidic deep eutectic solvents based on p-Toluenesulfonic acid

Abstract

There is growing interest in identifying suitable, less hazardous deep eutectic solvent (DES) alternatives for conventional solvents and catalysts, particularly acid catalysts which can be volatile and highly corrosive. In this study, novel acidic DESs based on mixtures of either tetrabutylammonium hydrogensulfate (TBAHS) (DES1) or tetraphenylphosphonium bromide (TPPB) (DES2) with p-toluenesulfonic acid monohydrate (PTSAM) are prepared. The properties of these DESs were investigated using multiple analytical techniques, including density, viscosity, conductivity and diffusion measurements. DES1 formed a room-temperature liquid, whereas DES2 melted at 337 K at its eutectic point. Hammett acidity studies revealed that both the DESs show super acidity (H0DES1 = -2.91 and H0DES2 = -1.89) even when present in relatively dilute (40 mM) aqueous solutions. Conductivity measurements established that both DESs show good ionicity at elevated temperatures based on a Walden plot, although DES2 displayed reduced ionicity close to its melting point. Equilibrium structural arrangements for these DESs are proposed based on findings, with the formation of a complex hydrogen-bonded anion and isolated cation. These DESs are proposed as potential replacements for ionic liquids, and existing acid catalysts in acid catalyzed reactions.