(758f) Propane Dehydrogenation over Zn-Exchanged H-MFI Zeolites

Increasing shale gas production in the U.S. has incentivized the conversion of light alkanes derived from natural gas to olefins and aromatics. Lewis acidic metal cations exchanged into zeolites catalyze light alkane dehydrogenation and aromatization. Gallium-exchanged H-MFI zeolites (Ga/H-MFI) are highly active and selective catalysts for this purpose. However, Ga³⁺ cations catalyze undesired alkane cracking alongside dehydrogenation. Therefore, there is motivation to identify metals other than Ga that enhance the selectivity to dehydrogenation over cracking. This may be achieved using Zn/H-MFI. We have synthesized Zn/H-MFI via sublimation of ZnCl₂ into H-MFI, and have begun a detailed investigation of the structure of Zn sites prepared in this manner and of their role in light alkane cracking and dehydrogenation. Zn/H-MFI catalysts of varying Zn loading (Zn/Al = 0.06 – 0.94) were prepared by sublimation of ZnCl₂ into H-MFI. FTIR and FTIR of adsorbed pyridine show the near complete disappearance of bands attributed to Brønsted acid sites in Zn/H-MFI catalysts of all Zn loadings. Lewis acid sites are formed at low Zn loadings, and inactive ZnO species may form at higher loadings. ¹H-NMR and ¹H-²⁷Al TRAPDOR NMR show that Zn/H-MFI of all Zn loadings contain protons proximate to Al atoms, but their local chemical environment is altered. ²⁷Al-NMR shows that the local environment of Al atoms is altered as well. The kinetics of propane dehydrogenation and cracking were investigated over Zn/H-MFI catalysts of varying Zn loading. The product distribution is independent of propane partial pressure and temperature, suggesting that dehydrogenation and cracking proceed through a common surface intermediate and have similar apparent activation energies. The apparent activation energies for dehydrogenation and cracking are nearly independent of Zn loading, suggesting the active site identity does not vary with Zn loading.