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.

Identifying zeolite catalysts that can simultaneously optimize p-xylene selectivity and feed utilization is critical to toluene alkylation with methanol (TAM). Here, we show that zeolite MCM-22 (MWW) has an exceptional catalyst lifetime in the TAM reaction at high operating pressure, conversion, and selectivity. We systematically probe the catalytic behavior of active sites in distinct topological features of MCM-22, revealing that high p-xylene yield and catalyst stability are predominantly attributed to sinusoidal channels and supercages, respectively. Using a combination of catalyst design and testing, density functional theory, and molecular dynamics simulations, we propose a spatiotemporal coke coupling phenomenon to explain a multistage p-xylene selectivity profile wherein the formation of light coke in supercages initiates the deactivation of unselective external surface sites. Our findings indicate that the specific nature of coke is critical to catalyst performance. Moreover, they provide unprecedented insight into the synchronous roles of distinct topological features giving rise to the exceptional stability and selectivity of MCM-22 in the TAM reaction.

The synthesis of zeolite UZM-8 with disordered MWW monolayers in the presence of diethyldimethylammonium hydroxide as the sole structuredirecting agent is described. Under the synthesis conditions studied here, crystallization of pure UZM-8 was possible only from synthesis mixtures with a narrow range (15-25) of Si/Al ratios. The physicochemical properties of H-UZM-8 and its catalytic properties for the isopropylation of benzene to form cumene are compared with those obtained from the other members of MWW family of zeolites, i.e., H-MCM-22, H-ITQ-2, and H-MCM-56 with similar Si/Al ratios.

The direct conversion of ethene to propene (ETP) is a potentially important route for the selective production of the latter olefin. Here we report that after some time on stream, H-UZM-35, an MSE-type large-pore zeolite, shows much better propene yield than H-SSZ-13, the best catalyst for the ETP reaction thus far. The key to this improvement is the presence of large cylindrical cages in H-UZM-35 that allows the easy formation of isopropylnaphthalene-based reaction centers for ETP catalysis, while being relatively resistant to coke formation. Mild dealumination was found to further mitigate catalyst deactivation.