The importance of channel intersections in the catalytic performance of high silica stilbite

Abstract

Researchers at Chevron recently discovered a route for making high silica stilbite (STI), a zeolite with intersecting 10- and 8-ring channels. This zeolite, named SSZ-75, provides an interesting comparison to two other two-dimensional 10/8-ring zeolites, namely, ferrierite (FER) and ZSM-57 (MFS). The catalytic properties of these zeolites are characterized here using the Constraint Index test, adsorption measurements, and toluene disproportionation and its alkylation with isopropyl alcohol. Results are then compared to ZSM-5 (a well-characterized 3-dimensional 10-ring zeolite) and TON (a 1-dimensional 10-ring zeolite). While the micropore volumes for several of these five materials are similar, the Constraint Index, which considers the competitive cracking of n-hexane and 3-methylpentane, differs greatly. On the other hand, toluene conversions along with selectivity to p-xylene for MFI and STI are remarkably similar. This indicates that the reaction volume available at the intersections of the 10- and 8-ring channels in STI is comparable with that of the 10-ring channel intersections in MFI. However, in toluene alkylation with isopropyl alcohol, differences in intersection volume between MFI and STI are revealed. These results nicely demonstrate how the size of the pores and geometry of the channel intersections (rather than just the size of the individual pores) impact the catalytic properties of zeolites with multidimensional channel systems.