Phosphotungstic acid immobilized on sulphonic-acid-functionalized SBA-15 as a stable catalyst for the esterification of cyclohexene with formic acid

Abstract

Abstract Phosphotungstic acid (HPW) was immobilized on a sulphonic-acid-functionalized SBA-15 mesoporous molecular sieve through a one-step hydrothermal synthesis method, using non-ionic surfactant pluronic P123 as a structure-directing agent in acidic media, tetraethoxysilane (TEOS) as a silicon source and 3-trimethoxysilyl-1-propanethiol (MPTMS) with hydrogen peroxide (H2O2) oxidized to a sulfonic acid group. The obtained composite material HPW-SBA-15-SO3H has a high specific surface area, large pore volume and increased strong acidity. The effect of raw-material ratio on the catalyst was investigated. Catalysts that were prepared under optimal conditions of a molar ratio of (MPTMS)/ (MPTMS + TEOS) of 1:10, a molar ratio of (H2O2)/(MPTMS) of 18:1 and a mass ratio of (HPW)/(TEOS) of 1:10 exhibited an excellent catalytic activity in the esterification reaction of cyclohexene with formic acid. The cyclohexene conversion exceeded 86.7% and the cyclohexyl formate selectivity was 98.1% after 6 h at 353 K with a mole ratio of cyclohexene to formic acid of 1:3 and a catalyst dosage of 5.7% (w/w). The composite catalyst had a higher catalytic stability than the sulfonic-acid-functionalized SBA-15 and SBA-15-immobilized HPW individually. The cyclohexene conversion reached 72.0% even after reused four times. Fourier transform infrared and temperature-programmed desorption of ammonia revealed that a strong interaction existed between the sulfonic-acid group and the HPW, which prevented acid-active component of HPW and sulfonic-acid group loss from the carrier.