Isomerization and formation of xylenes over ZSM-22 and ZSM-23 zeolites

The shape selective catalytic properties of H-ZSM-22 and H-ZSM-23 in the isomerization of meta-xylene and methylation of toluene are compared with those of H-ZSM-5. Their activities in both these reactions decrease in the order ZSM-5 > ZSM-23 > ZSM-22. Their para selectivities, however, follow the reverse order. Even though all three zeolites contain pores constituted by 10 membered rings, differences in the shape of the pores as well as the presence of pore intersections (in ZSM-5) lead to significant differences in their shape selective catalytic properties. While loss of xylenes in the xylene isomerization process occurs due to transalkylation reactions over ZSM-5 zeolites, dealkylation is the main cause over ZSM-22. ZSM-23 occupies an intermediate position. ZSM-22 and ZSM-23 are shown to have superior shape selectivity vis-à-vis ZSM-5 even at high equivalent conversion levels