Development of thermodynamic activity coefficients to describe the catalytic performance of supported polyoxometalate catalysts

Abstract

Turnover frequencies for methanol dehydration and oxidation with supported polyoxometalate catalysts exhibit a strong dependence on the extent of proton exchange with other cations. Harned’s rule, which is typically used to describe the thermodynamic activity of acid and salt solutions, was used to model thermodynamic activity coefficients for cation-exchanged, supported polyoxometalates. The resulting activity coefficients described the dehydration and oxidation turnover frequencies of supported, cation-exchanged phosphomolybdate catalysts remarkably well. This approach permits turnover frequencies for gas-solid contact catalysis to be defined in terms of the thermodynamic activity rather than the concentration of active sites, in this case the polyoxometalate protons. The success of a single-parameter activity coefficient model to describe the thermodynamic activity and the catalytic activity of surface acid sites suggests that cation exchange alters the solvation environment of these sites.