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Correlating Acid Properties and Catalytic Function: A First-Principles Analysis of Alcohol Dehydration Pathways on Polyoxometalates

Density functional theory calculations and reactivity data were used to examine the mechanism of alcohol dehydration on Keggin-type polyoxometalate (POM) catalysts and the influence of the POM composition and the degree of substitution of the alcohol on kinetically relevant elimination steps. Dehydration was found to proceed through E1 pathways in which the alcohol C—O bond is cleaved heterolytically via a carbenium-ion transition state. Dehydration rates were found to depend on the elimination rate constant and the equilibrium constant for the formation of unreactive alcohol dimers. E2-type elimination transition states, involving concerted C—H and C—O bond cleavage, were not found. The extent of substitution at the α-carbon on the alcohol was found to lead to marked effects on elimination barriers, because substitution increases the proton affinity of the alcohol and the stability of the carbenium-ion transition state. Changes in the central and addenda atoms of the POM cluster and the presence of n-don...