Solvent Influence on the Catalytic Activity and Surface Polarity of Inorganic Solid Acids

The surface-mediated hydride-transfer reaction of 1,4-cyclohexadiene with triphenylmethylium induced by two silicas, an alumina, and an aluminosilicate as solid acid catalyst, respectively, has been kinetically studied as a function of the polarity of the surrounding solvent. The specific rate constants k ‘ have been determined in 10 different solvents. Generally, k ‘ decreases with increasing polarity of the solvent. Kamlet−Taft's α (hydrogen-bond acidity) and π* (dipolarity/polarizability) parameters of the solid acid/solvent interface have been determined for alumina and aluminosilicate in various solvents. Fe(phen) 2 (CN) 2 [ cis -dicyanobis(1,10-phenanthroline)iron(II), ( 1 )] and Michler's ketone [4,4‘-bis( N, N -dimethylamino)benzophenone, ( 2 )] were used as solvatochromic surface polarity indicators. The UV/vis spectra of the two surface polarity indicators 1 and 2 adsorbed on solid acid catalysts from the solvents were measured by the reflection mode and the UV/vis absorption maxima were used to calculate the α and π* values of the catalysts. α of the solid acid catalyst/solvent interface decreases with increasing β term (hydrogen-bond accepting) ability of the solvent, whereas the π* term of the solvent marginally modifies the interfacial polarity. k ‘ increases for each individual solvent with increasing β of the catalyst solvent interface in the order series silica < alumina < aluminosilicate. It can be shown that α of the solid acid catalyst/solvent interface decreases with increasing β term (hydrogen-bond accepting) ability of the solvent, whereas the π* term of the solvent marginally modifies the interfacial polarity.