Mechanistic Insights into the Zeolite-Catalyzed Isomerization and Disproportionation of m-Xylene

The mechanisms of m-xylene isomerization and disproportionation over 13 medium-pore zeolites and three large-pore ones are investigated. While H-TNU-10 and H-ZSM-57 with intersecting 10- and 8-ring channels were found to show considerably higher p/o ratios than H-ZSM-5, a commercial m-xylene isomerization catalyst, the GC-MS results from used zeolite catalysts demonstrate the intrazeolitic build-up of tri- and tetramethylated diphenylmethane species, whose existence during the m-xylene transformation over any acidic catalyst has not been experimentally verified until now. These dicyclic aromatic compounds were ascertained to serve as reaction intermediates of bimolecular m-xylene isomerization within the micropores not only of large-pore zeolites but also of medium-pore materials at temperatures lower than 523 K or so, once there are internal void spaces larger than 10-rings. Flushing experiments with used zeolites followed by GC-MS analyses strongly suggest that the high p-xylene selectivity found in some medium-pore zeolites is largely due to product shape selectivity rather than to transition state one. More importantly, the overall GC-MS results of our work demonstrate that transition state and product shape selectivities are experimentally distinguishable from each other.