SOLID-STATE NMR INVESTIGATION OF THE ALKYLATION OF TOLUENE WITH METHANOL OVER BASIC ZEOLITE-X

The alkylation of toluene with methanol over different alkali-metal cation forms of zeolite X has been studied by C-13 solid-state NMR with magic-angle spinning (MAS NMR). The catalysts used were Na-X, K-X, and Cs-X. The results indicate that the fate of the methanol, both in terms of surface complexation and decomposition, plays an important role in this alkylation process. The alkylation reaction starts at ca. 340 degrees C with complete conversion at 450 degrees C. A number of resonances corresponding to species not observed in the product stream appear before and during the formation of products. All of these lines have been assigned and indicate the presence of surface bound methoxy ions and surface formates. Formaldehyde is suggested to be the alkylating agent for side-chain alkylation while methyl carbocations are responsible for ring alkylation. Over Na-X protonated metal methoxy species are observed and are the source of methyl carbocations. These surface complexes are found to a lesser extent in K-X and are not formed in Cs-X. Over Cs-X the interaction of methanol with the basic sites results in the formation of formaldehyde. In addition, methyl carbocations can be formed during the formation of the surface formates observed in all three catalysts. it is found that in Cs-X the zeolite plays a crucial role gettering these highly reactive carbocations by binding them as surface bound methoxy groups. This prevents ring alkylation occurring on Cs-X.