We will report recent investigations on the propane and ethane dehydrogenation activity and speciation of extra-framework Ga cations over aluminosilicate zeolites ZSM-5 (MFI), chabazite (CHA) and SUZ-4 (SZR). In ZSM-5 zeolites using a combination of quantitative FTIR spectroscopy of adsorbed pyridine, kinetic studies of ethane and propane dehydrogenation over zeolites with a range of Si/Al and Ga/Al ratios, and quantitative pulse-titration reactions, we have been able to differentiate two types of Ga species: divalent Ga species balancing paired framework Al atoms, (Ga2O2)2+, and isolated Ga+ sites. Isolated Ga+ is redox-active, while (Ga2O2)2+ species are not; Ga+ catalyzes propane and ethane dehydrogenation faster and more selectively than H+ Bronsted acid sites, but (Ga2O2)2+ catalyzes the dehydrogenation reaction at rates as high as 18 times the rates of Ga+. Gallium incorporation into CHA-zeolites, in contrast, forms only isolated Ga+ sites, while the structure-property relations of Ga-SUZ-4 are under investigation. We will compare these results to parallel studies carried out over Zn- and In-exchanged zeolite to establish a framework to understand the differences in reactivity of these various catalysts.

Gallium speciation in aluminosilicate zeolites and catalytic light-alkane dehydrogenation

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