Novel Keggin-type H4PVMo11O40-based ionic liquid catalysts for n-caprylic acid esterification

Abstract

A series of new Keggin-type heteropolyacid (HPA)-based ionic liquid catalysts were synthesized from molybdovanadophosphoric acid (H4PVMo11O40) and propane sulfonated ionic liquid (MIM-PS). The physicochemical properties of these inorganic–organic composite catalysts were characterized by various techniques such as FT-IR, TGA, high-resolution 1H and 13C NMR, and solid-state 31P MAS NMR. Their catalytic performances were evaluated for esterification of n-caprylic acid to methyl caprylate, among them, the [MIM-PSH]2.0H2.0PVMo11O40 catalyst revealed an optimal reaction activity. Moreover, an experimental design was exploited via response surface methodology (RSM) on the basis of the Box–Behnken design (BBD). Accordingly, a maximum methyl caprylate yield of 95.6% was obtained over the [MIM-PSH]2.0H2.0PVMo11O40 catalyst under the optimal conditions: catalyst to n-caprylic acid molar ratio 0.52 mol%, methanol/acid molar ratio of 7, reaction time 3.0 h, and temperature 363 K. These results were in conformity with those predicted by a mathematical model. A desirable durability may also be inferred for the reported catalyst; no considerable change in catalyst performance was observed during six successive cyclic operations. The improved acidic strength and lower molecular transport resistance of the catalysts were accounted for the superior catalytic performance and durability observed.