(289e) Tuning Zeolite-Supported Metal Catalysts By Varying Heteroatom Distribution and Composition: Characterization By Catalysis and Probe Molecule Adsorption Spectroscopy

The influence of zeolite heteroatom distribution (e.g. paired vs. isolated sites) and composition (e.g. Al, Fe, Ga) on the electron density, catalytic activity and stability of supported metal cations is examined. A variety of extra-framework cation probes of Al sites were characterized by probe molecule adsorption with solid-state FTIR spectroscopy. The influence of distribution and composition is also probed by ethane dehydrogenation or ethylene dimerization catalysis. Zeolite heteroatom distributions and compositions influence catalytic performance for solid acid catalyzed reactions including 1-hexene cracking, methanol-to-olefins, and propene oligomerization. With the increasing relevance of metal-exchanged zeolites for new applications in catalysis, developing new characterization protocols and synthesis-structure-property relationships for transition metal-exchanged zeolites is critical.

The stabilization of active metal sites during ethane dehydrogenation was influenced by availability of zeolite framework Al sites; in the absence of available Al sites, Cr sites quickly deactivated. It was also found that greater Cr/Al molar ratios yielded a greater abundance of electron-rich Cr²⁺ species after reduction. This trend correlated with enhanced ethane dehydrogenation activity but decreased stability in time-on-stream studies. Both high activity and enhanced stability were achieved with Cr/Ca-ZSM-5. Increasing in electron density by varying the zeolite heteroatom composition in the order: [Fe] > [Ga] > [Al] was observed via CO adsorption on supported Ni cations. C₂H₄ dimerization activity increased with increasing electron density of the Ni cation. Despite similarities in reported acid site strength, the solid acid sites on [Fe]-Beta in this work had significantly lower activity than those on [Ga]-Beta for the skeletal isomerization of linear butenes as well as C₂H₄ dimerization. Taken together, these results demonstrate the ability of heteroatom distribution and composition to further tune adsorption and catalysis by zeolite-supported metals.