Catalysis by Heteropoly Compounds .XXII. Reactions of Esters and Esterification Catalyzed by Heteropolyacids in a Homogeneous Liquid-Phase Effects of the Central Atom of Heteropolyanions Having Tungsten as the Addenda Atom

Abstract

Decomposition of isobutyl propionate (IBP) (Reaction (1)), ester exchange of IBP with acetic acid (Reaction (2)), ester exchange of IBP with n-propyl alcohol (Reaction (3)), and esterification of propionic acid with isobutyl alcohol (Reaction (4)) have been studied in a homogeneous liquid phase using heteropolyacids H m XW 12O 40 ( X = P, Si, Ge, B, and Co) having the Keggin structure and H 6P 2W 18O 62, having the Dawson structure. The results clearly indicated that the catalytic behavior of heteropolyacids remarkably depended on the kind of reaction system, namely, the basicity of reactants. For Reaction (1), the catalytic activities of the heteropolyacids were 60-100 times higher than those of H 2SO 4 and p-toluenesulfonic acid. Among the heteropolyacids, the activity was in the order H 3PW 12O 40 ≥ H 4SiW 12O 40 ∼ H 4GeW 12O 40 > H 5BW 12O 40 > H 6CoW 12O 40. This order suggests that the activity follows the order of the acid strength of the solutions of heteropolyacids, which increases with the decrease in the negative charge of the polyanion. H 6P 2W 18O 62 showed an activity between those of H 5BW 12O 40 and H 6CoW 12O 40. Water molecules in the system retarded Reaction (I). For Reaction (2), the activity order was similar to that for Reaction (I) in the absence of water. However, contrary to Reaction (1), the addition of water greatly accelerated Reaction (2). It was presumed that, in the presence of water, Reaction (2) proceeds via the hydrolysis of IBP to propionic acid and isobutyl alcohol, followed by the esterification of isobutyl alcohol with acetic acid. On the other hand, the difference in the activity between the heteropolyacids and H 2SO 4 was not significant for Reactions (3) and (4), for which the effective acid strength of the catalysts may level off as a consequence of the leveling effect of the reactant alcohols.