Preparation of Keggin and Preyssler heteropolyacid catalysts on amine-modified SBA-15 and their catalytic performances in esterification of n-butanol with acetic acid

The Keggin and Preyssler-structured tungstophosphoric acid catalysts anchored on the surface of modified SBA-15 (KNS and PNS) by grafting amine groups into system, and were characterized by infra-red spectroscopy, X-ray diffraction, N(2)-adsorption, Hammett indicator and UV-Vis spectroscopy. Their catalytic behaviors were investigated in the esterification of n-butanol with acetic acid. Characterization results for obtained catalysts verified the ordered hexagonal mesostructure of SBA-15 and preservation of the structures of heteropolyanions on amine-modified SBA-15. KNS and PNS catalysts with the 37 and 25% loadings of Keggin and Preyssler acids exhibited considerable conversions of n-butanol of 61.2 and 63.8% with both 100% selectivity for n-butyl acetate in the esterification reaction, respectively. Especially, tests of six-cycle reaction revealed good reusabilities of KNS and PNS due to their excellent water-resistance properties, and in contrast, the catalysts prepared by direct impregnation lost their high activities immediately at their second cycles. Active sites on the amine-modified SBA-15 retained the Keggin or Preyssler structure after reaction demonstrated by IR analysis. Also, reaction conditions such as the amount of catalyst, reaction time and temperature were explored.