A MO-Theoretical Treatment of Toluene-Formaldehyde Reaction Catalyzed by Solid Acids

Abstract

The role of acid catalysts, such as platinum on silica-alumina, in the formation of xylenes rich in p-isomer from CH3C6H5 and HCHO in the presence of hydrogen was investigated using the extended Huckel molecular orbital theory. We made several findings. Firstly, the reactivity of CH3C6H5 increased remarkably on the basis of its chemisorption on the catalyst by means of the pseudo σ-orbital of CH3, while that of HCHO stayed almost constant, with or without chemisorption on the catalyst. Secondly, there was some evidence that p- or o-CH3C6H4CH2OH was a reaction intermediate. An assumption was made that CH3C6H4CH2OH resulted from the reaction between chemisorbed CH3C6H5 and physically adsorbed HCHO. Thirdly, the catalytic decomposition of the intermediate made available CH3C6H4CH2 radicals, which coupled with H radicals formed by the partial decomposition of HCHO and so provided the desired product, p-CH3C6H4CH3, with higher selectivity. Finally, detailed discussion was made with respect to the modes of the chemisorption of the reactants on the catalyst, together with the reaction mechanism, in accordance with our particular interests.