Acidity of Group 5 Metal (V, Nb, Ta)-Substituted Keggin and Wells-Dawson Heteropolyacid (HPA) Catalysts and Their Application to Esterification of Acetic Acid with Ethanol

Abstract

Acidity of group 5 metal-substituted H4PW11M1O40 (M = V, Nb, Ta) Keggin and H7P2W17M1O62 (M = V, Nb, Ta) Wells-Dawson heteropolyacid (HPA) catalysts was measured by NH3-TPD experiment. Acidity of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts showed the same trend with respect to polyatom substitution in both series of HPA catalysts. Vapor-phase esterification of acetic acid with ethanol was carried out as a model reaction to correlate the acidity with the acid catalysis of the HPA catalysts. Yield for ethyl acetate (a product formed by acid catalysis of HPA) over H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts increased with increasing acidity of the HPA catalysts, regardless of the identity of HPA catalyst. Acidity of H4PW11M1O40 (M = V, Nb, Ta) and H7P2W17M1O62 (M = V, Nb, Ta) HPA catalysts could be utilized as a probe of acid catalysis for esterification of acetic acid with ethanol. Vapor-phase esterification of acetic acid with ethanol was carried out over H4PW11M1O40 (M = V, Nb, Ta) Keggin and H7P2W17M1O62 (M = V, Nb, Ta) Wells-Dawson heteropolyacid (HPA) catalysts. Yield for ethyl acetate (a major product formed by acid catalysis of HPA) increased with increasing acidity of the HPA catalysts, regardless of the identity of HPA catalyst.