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

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.

Toluene disproportionation was studied at 500 °C for STI, MFS, and MFI having a silica-to-alumina ratio of ∼50. STI, the framework of which consists of intersecting 10- and 8-ring channels, shows similar xylene-to-benzene selectivity as MFI (a well-characterized 3-dimensional 10-ring structure) and is noticeably different from MFS, another 2-dimensional framework with intersecting 10- and 8-ring channels. These results highlight the relatively large internal volume of STI, a physical characteristic that can be directly linked to its synthesis and the subunits, which make-up its structure.Figure optionsDownload high-quality image (36 K)Download as PowerPoint slideHighlights
► Zeolites, STI, MFS, FER, MFI, and TON at SAR of ∼50 differ in catalytic performance.
► Variations are due to differences in internal reaction space at pore intersections.
► Multiple adsorption and catalytic tests were performed to fully characterize STI.
► The anomalous CI of 3 for STI is explained by larger volume at its pore intersections.
► Toluene reactions provide further evidence of larger intersection volume in STI.