MECHANISTIC STUDIES OF THE SIDE-CHAIN ALKYLATION OF TOLUENE WITH METHANOL ON BASIC ZEOLITES-Y BY MULTINUCLEAR NMR-SPECTROSCOPY

The side-chain alkylation of toluene with methanol on cesium-exchanged zeolites Y, with and without subsequent impregnation with cesium hydr oxide, was investigated by Cs-133 and C-13 MAS NMR spectroscopy. In th e first step, the catalysts under study were characterized by differen t spectroscopic techniques showing a high dispersion of the oxidic gue st compound on the zeolitic host and an influence of the guest compoun d on the intrinsic base strength of the host framework. Adsorption of toluene led to Cs-133 MAS NMR signals pointing to a strong interaction of the toluene molecules with the cesium cations. C-13 MAS NMR experi ments on the zeolites Y loaded with pure methanol and toluene/methanol mixtures revealed a rapid decomposition of formaldehyde formed by deh ydrogenation 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 temperat ures, as well, they may be responsible for catalyst deactivation. (C) 1998 Elsevier Science B.V. All rights reserved.