(670g) The Unique Impact of Organic Structure-Directing Agents on Aluminum Speciation in Zeolite SSZ-39 Catalysts

The catalytic versatility of zeolite is attributed in part to the diverse speciation of aluminum active sites. Due to their outstanding properties and shape selectivity, zeolites are used in numerous industrial applications for processes such as ion exchange, absorbance, and catalysis. The performance of zeolite catalysts is closely related to the coordination of aluminum in either framework or extra-framework locations, which create Brønsted and Lewis acids, respectively. Recent studies have shown that “framework–associated” aluminum species, which are not fully coordinated within the zeolite framework, can undergo reversible transformation from tetrahedral or octahedral coordination as a function of reaction conditions. Understanding the nature of the aluminum species, as well as developing novel synthesis methods to tailor their structure, is critical to developing advanced catalysts for a broad range of industrial applications.

In this presentation, we will discuss the synthesis and characterization of a small pore zeolite, SSZ-39 (AEI), which was prepared using two different organic structure-directing agents (OSDAs) with subtle differences in chemical composition. Time-resolved analysis of various stages during crystallization reveal unique intermediates with high defect densities depending on OSDA selection wherein transformations in crystal morphology at later stages of growth remove defects. Our findings reveal that a unique crystallization pathway using one of the OSDAs results in intermediate structures that impact local speciation of Al sites. Differences in local structure result in distinct performance of each catalyst in cracking reactions used as benchmarks to evaluate SSZ-39. Our findings reveal that zeolite AEI catalysts exhibit unusually high conversion and light olefin selectivity, and a stability that surpasses commercial ZSM-5. Zeolite structure is assessed using a combination of synergistic state-of-the-art characterization techniques to better understand the role of OSDAs on aluminum speciation and to develop robust structure-performance relationships in zeolite catalysis.