Xylene isomerization and aromatic alkylation in zeolites NU-87, SSZ-33, β, and ZSM-5: molecular dynamics and catalytic studies

The unique pore topology of zeolite NU-87, with 10-membered ring (MR) channels intersected by perpendicular 12-MR cavities, can offer new opportunities for alkylaromatics isomerization, as well as for aromatics alkylation. The catalytic performance has been studied for meta-xylene isomerization and disproportionation, as well as toluene alkylation with methanol, ethanol, propanol, and isopropanol. Product distribution was interpreted on the basis of pore topology and compared to zeolites with 12-MR (β), 10- and 12-MR (SSZ-33), and 10-MR (ZSM-5) zeolites. The presence of cavities allows more space for the location of bulky intermediates and/or products, and also traps the molecules, allowing consecutive reactions that lead to thermodynamic equilibrium. Channels, on the other hand, allow diffusion without trapping, if their free diameter is large enough. 10-MR channels in NU-87 are smaller than in ZSM-5 and this limits diffusivity. Furthermore, trimethylbenzenes formed will tend to equilibrate, but only the smaller isomer, i.e., 1,2,4-trimethylbenzene, fits in the NU-87 cavity and will have a small chance to diffuse. Catalytic experiments as well as atomistic molecular dynamics are used to interpret the behavior of NU-87 and to explain the results in comparison with other structures.