Transalkylation of trimethylbenzenes

Catalytic isomerizations of ethylbenzenes and xylenes over zeolites are commercial processes and have been used as test reactions of acid catalysts. Corma and Sastre26 have recently suggested that xylenes can form via transalkylation of trimethylbenzene which is believed to be an intermediate in the isomerization of p-xylene. A general scheme as that shown in Eq. 626 was proposed on the basis of kinetic and mass spectrometric data. The reactant p-xylene was believed to produce m-xylene as a primary product but also rearranges in the pores of ultrastable faujasite zeolites to form o-xylene which appears as a primaiy product. In addition, trimethylbenzenes were formed along with toluene. 

Further isomerization of m-xylene as well as transalkylation of trimethylbenzene and toluene to form m-xylene can occur. Evidence for the bimolecular transalkylation mechanism was provided by observation of a peak at m/e 109 in the mass spectra for CD3 substitution of toluene. These data rule out unimolecular 1,2-methyl shifts as the sole means of formation of xylenes. The higher the A1 content of the ultrastable faujasite the greater the extent of bimolecular transalkylation. These observations have significant implications for unimolecular kinetic models that have been proposed as well as reported activation energies and turnover frequencies. 

P-lO - Transalkylation of trimethylbenzene with toluene over large pore zeolites... 

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