Isomerization and disproportionation of m-xylene

The conversion of m-xylene was performed on acid zeolites with ten- or twelve-membered rings, such as ZSM-5, ZSM-12, ZSM-48, OFF, MOR, FAU, Ω, β and L. All of these zeolites have distinctly different crystallographic structures with different shapes and dimensions of the intracrystalline cavities. This diversity of pore structures allowed to determine to what extent the pore shape and dimensions influence the selectivity of the reaction of m-xylene. The p/o selectivity in the isomerization of m-xylene is always high for ten-membered ring zeolites with crystals bigger than 1 μm. The moderate selectivity for p-xylene formation in some zeolites with twelve-membered rings is due to transition-state shape selectivity. The selectivity for disproportionation versus isomerization is very low for ten-membered ring zeolites and allows them to be distinguished easily from the twelve-membered ring zeolites, which show much more disproportionation. In the group of twelve-membered ring zeolites the disproportionation/isomerization selectivity does not correlate at all with the effective size of the intracrystalline cavities. The disproportionation/ isomerization selectivity is determined by the strong isolated acid sites which are present in cavities that allow the formation of at least one of the possible bimolecular transition-state complexes for disproportionation. The distribution of the trimethylbenzene isomers reflects important features of the shape of the pores, the presence and the position of larger spaces in lobes, cages or intersections adjacent to the twelve-membered ring channels of large pore zeolites.