An in situ13C MAS NMR study of toluene alkylation with methanol over H-ZSM-11

This chapter aims to clarify the mechanism of toluene alkylation with methanol using in situ 13C M.AS NMR technique. The attention is focused on the following main aspects: 1) Identification of the reaction pathways operating under the NMR/batch conditions; 2) identification of the reaction intermediates, which may play the role of alkylating agents; 3) identification of the primary and secondary reaction products. The identification of the main reaction products and intermediates, and the quantification of the NMR results provide new information concerning the nature of the methylating agents, the primary products of alkylation and the whole reaction network. During the initial stages of the reaction, methanol partially converts to dimethyl ether (DME).The reaction then proceeds via two parallel reaction pathways. The first pathway, which is the major one, includes toluene methylation with DME or MeOH to yield o- and p-xylenes in the statistical ratio of two to one. The primarily formed xylenes undergo isomerization and further alkylation to tri and tetramethylbenzenes. The observation of diphenylmethane-like species as reaction intermediates suggests transalkylation of alkylaromatics. The second reaction pathway includes toluene alkylation with the fragments formed upon methanol conversion to hydrocarbons and thus, leads to various alkyltoluenes. The latter, in turn, undergo dealkylation, fragmentation and transalkylation to yield mainly xylenes, ethyltoluenes and polymethylbenzenes.