Hierarchical Ga-MFI Catalysts for Propane Dehydrogenation

We report the synthesis, characterization, and enhanced propane dehydrogenation properties of hierarchical Ga-MFI zeolite catalysts synthesized by two different methods: (i) repetitive branching and (ii) utilization of a long chain alkyl SDA. Structural, compositional, and morphological characterizations confirm that the Ga-MFI catalyst materials have hierarchical structures including micropores and mesopores in a single particle and show that Si/Ga ratios comparable to bulk Ga-MFI catalysts can be obtained. Acid site analysis using NH3-TPD and pyridine adsorption followed by in situ IR spectroscopy allowed quantification of the number and types of acid sites present and show that the hierarchical catalysts have considerably higher Lewis acid site concentrations than the bulk catalysts. The hierarchical Ga-MFI catalysts show superior PDH performance (at 600 °C) compared to bulk Ga-MFI catalysts. Propane conversion rates are increased 2–6-fold and propylene selectivities are 10–100% higher. We also examine the effects of synthesis variations—such as addition of 3-mercaptopropyltrimethoxysilane during synthesis and H+ ion-exchange—and find that both these steps have a beneficial effect on PDH properties. Calculations suggest that PDH in both the bulk and the hierarchical Ga-MFI is not diffusion-limited. Therefore, the superior performance of hierarchical Ga-MFI is due to intrinsically higher activity and selectivity. Potential reasons for this behavior are outlined. The present findings showing enhancement of PDH in hierarchical Ga-MFI catalysts suggests that the utility of the hierarchical zeolites is not limited to diffusion-limited reactions involving large, bulky molecules and that they may be useful in more diverse applications involving gas-phase reactions with small molecules.