A. Philippou et Mw. Anderson, SOLID-STATE NMR INVESTIGATION OF THE ALKYLATION OF TOLUENE WITH METHANOL OVER BASIC ZEOLITE-X, Journal of the American Chemical Society, 116(13), 1994, pp. 5774-5783
The alkylation of toluene with methanol over different alkali-metal ca
tion 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 ter
ms 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 reson
ances corresponding to species not observed in the product stream appe
ar before and during the formation of products. All of these lines hav
e been assigned and indicate the presence of surface bound methoxy ion
s and surface formates. Formaldehyde is suggested to be the alkylating
agent for side-chain alkylation while methyl carbocations are respons
ible for ring alkylation. Over Na-X protonated metal methoxy species a
re observed and are the source of methyl carbocations. These surface c
omplexes 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 al
l three catalysts. it is found that in Cs-X the zeolite plays a crucia
l role gettering these highly reactive carbocations by binding them as
surface bound methoxy groups. This prevents ring alkylation occurring
on Cs-X.