Catalytic activities of various moderately strong solid acids and their correlation with surface polarity parameters

The specific rate constant k' of the surface-mediated hydride-transfer reaction of 1.4-cyclohexadiene with triphenylmethylium is strongly dependent on the nature of the solid acid catalyst used. The catalysis of this hydride-transfer reaction by 30-moderately strong solid acid catalysts, e.g., silicas, aluminas, aluminosilicates, and titanium dioxide particles, has been studied. Generation of the triphenylmethylium when chlorotriphenylmethane is chemisorbed to the solid acid catalyst was used for the kinetic measurement. The individual, pseudo-first-order rate constant, k, of the hydride-transfer reaction increases linearly with the amount of solid acid catalyst used. The specific rate constant, k', based on the surface area of the solid acid, was used to quantifiy the catalytic activity of the solid acids, k' increases in the order silicas < aluminosilicates < titanium dioxides congruent to aluminas. Ln k' can be be correlated with the corresponding surface polarity parameters of the solid acid, e.g., Reichardt's E-T(30), Gutmann's acceptor number AN, and Kamlet-Taft's parameters a (hydrogen-bond-donating acidity) and pi* (dipolarity/polarizability). It is shown that the catalytic activity of a moderately strong solid acid catalyst can be characterized by Gutmann's acceptor number AN or the Kamlet-Taft a value. The different classes of solid acids, e.g., silicas, aluminas, and aluminosilicates, respectively, give different dependencies of k' as a function of their solid acid acidity parameters.