Noncorrosive ionic liquids composed of [HSO4] as esterification catalysts

Seven Brønsted acidic ionic liquids (BAILs) composed of [HSO4] were prepared, characterized, and used as catalysts of low dosage in various esterification reactions. Pyridinium hydrogensulfate ([Hpy][HSO4]), 2-methylpyridine hydrogensulfate ([Hmpy][HSO4]), and 1-methylimidazole hydrogensulfate ([Hmim][HSO4]) are found to be nearly noncorrosive, whereas N-(4-sulfonic acid)butyl triethylammonium hydrogensulfate ([BSEt3N][HSO4]) corrodes 316L stainless steel heavily. It is illustrated that the acidity and hydrophilicity can be tuned through the variation in cations to obtain BAILs of suitable catalytic activity. [Hpy][HSO4], as a typical noncorrosive BAIL, shows good catalytic activity in the esterification of n-butanol with acetic acid, primarily due to its suitable Brønsted acidity and highly hydrophilic nature to enable liquid–liquid biphasic catalysis, whereas [Hmpy][HSO4] and [Hmim][HSO4] require relatively large dosages to reach the equal level. The batch reaction distillation is further performed to investigate the esterification of acetic acid with n-butanol using [Hpy][HSO4] as catalyst of low dosage, and is found to be a promising strategy that causes the shift of chemical equilibrium further to the product side and leads up to the completion of the reaction. The noncorrosive BAILs are also tested to be catalysts of considerable activity for the esterification of other alcohols with acids in the absence of steric hindrance. [Hpy][HSO4] is recycled and reused seven times without significant losses of activity and quantity, showing the stability and recyclability of the noncorrosive BAILs. It is believed that the noncorrosive BAILs are prospective substitutes for corrosive mineral acids or solid resins in the catalytic distillation to overcome the corrosion of mineral acids or the difficult installation of solid catalysts in the column.