Catalytic activity and process variables optimization during microwave synthesis of methyl caprylate over various multi-SO3H functionalized ionic liquids

Abstract

The catalytic performances of a series of multi-SO3H functionalized ionic liquid catalysts during synthesis of methyl caprylate under microwave irradiation condition were investigated. The physicochemical properties of these Brønsted acidic ionic liquids were characterized by a variety of different analytical and spectroscopic techniques such as TGA, NMR, FT-IR, and UV–vis. Among various ionic liquids examined, [Tshx][HSO4]4 (i.e., N,N′,N″,N‴-tetrapropane sulfonic hexamethylenetetramine tetrahydrosulfate) showed the highest catalytic activity and superior recyclability. Process variables during esterification of caprylic acid and methanol, viz. amount of catalyst, methanol/n-caprylic acid molar ratio, microwave temperature, microwave power and time were explored and optimized by the response surface methodology (RSM). Accordingly, an optimum methyl caprylate yield of 96.2% was achieved over [Tshx][HSO4]4 catalyst under the optimized conditions: catalyst amount 0.3 mmol, methanol/n-caprylic acid molar ratio 7:1, temperature 80 °C, microwave power 400 W and reaction time 35 min.