Mechanistic studies of the side-chain alkylation of toluene with methanol on basic zeolites Y by multi-nuclear NMR spectroscopy

Abstract

The side-chain alkylation of toluene with methanol on cesium-exchanged zeolites Y, with and without subsequent impregnation with cesium hydroxide, was investigated by 133Cs and 13C MAS NMR spectroscopy. In the first step, the catalysts under study were characterized by different spectroscopic techniques showing a high dispersion of the oxidic guest compound on the zeolitic host and an influence of the guest compound on the intrinsic base strength of the host framework. Adsorption of toluene led to 133Cs MAS NMR signals pointing to a strong interaction of the toluene molecules with the cesium cations. 13C MAS NMR experiments on the zeolites Y loaded with pure methanol and toluene/methanol mixtures revealed a rapid decomposition of formaldehyde formed by dehydrogenation of methanol. Interestingly, on the impregnated zeolite Y, formaldehyde decomposition was inhibited by toluene. Parallel to the conversion of methanol to formaldehyde, surface-bound formate species were formed. Since these species were stable at high reaction temperatures, as well, they may be responsible for catalyst deactivation.