Repeated use of supported H3PW12O40 catalysts in the liquid phase esterification of acetic acid with butanol

Abstract

Tungstophosphoric acid (HPA) catalysts supported over SiO2, ZrO2 and carbon were prepared and characterized. Its repeated use in the liquid phase reaction of esterification of acetic acid with butanol was particularly studied. The results revealed that silica- and zirconia-supported HPA had an intrinsic activity per unit protonic acid site greater than that of an Amberlyst W35 resin. Its repeated use was, however, impeded because of the dissolution of HPA in the reaction medium. Carbon-supported HPA catalysts had a slightly lower activity but they displayed a sustained conversion level upon reuse in subsequent catalytic tests. Inspection of the solvent media after the reaction in a UV-visible spectrophotometer revealed that both butanol and water leached HPA from the silica- and zirconia-supported catalysts and that the effect was minimal in the case of the carbon catalysts. It was rationalized that [PW12O40]3- anions are strongly adsorbed at pH 1-2 on the carbon support due to the electrostatic interaction with the carbon surface protons of zero-point-of-charge (ZPC) equal to 6.