Efficient esterification reaction of palmitic acid catalyzed by WO3-x/mesoporous silica

Abstract

Different solid catalysts were prepared by impregnation of tungsten species on commercial mesoporous silica. The resulting materials were characterized in order to elucidate their composition, properties and acidity. Moreover, the catalytic results were carefully studied, in an attempt to reach insights about the catalytic active species involved in the investigated reactions. The obtained solids were nominated Q15W-X, where X = 1 means an impregnation using ammonium paratungstate (APT), X = 2 sodium tungstate (Na2WO4) and X = 3 tungstic acid dihydrate (W-2) as tungsten source. They were evaluated as catalysts towards the esterification of palmitic acid by methanol, the average conversion was found to be between 80 and 90% and the catalysts were reused in four more reaction cycles, keeping similar conversion results as in the first use. The spectroscopic analyses suggested that the formation of polytungstate species over the silica with different tungsten species, mainly W5+ and W6+ is probably influence the esterification under mild conditions. Such species are not present in the solid resulting from the impregnation of W-2 on glass beads (GB) and the solid WO3 obtained by the calcination of W-2 (WO3-C), which presents poor catalytic activity and are mostly constituted by monoclinic WO3.