Propane Dehydrogenation and Cracking over Zn/H-MFI Prepared by Solid-State Ion Exchange of ZnCl2

Light alkanes in shale gas are an attractive source of carbon for the production of alkenes and aromatics compared to petroleum-derived naphtha. Zinc-exchanged zeolite H-MFI (Zn/H-MFI) is active and selective for light alkane dehydrogenation and dehydroaromatization. In this study, Zn/H-MFI with varying Zn/Al ratios was prepared via solid-state ion exchange (SSIE) of ZnCl2 and characterized by various methods. As-prepared Zn/H-MFI with Zn/Al ≤ 0.52 contains isolated [ZnCl]+ and [ZnCl(HCl)]+ species; Zn/H-MFI with higher Zn loadings also contains ZnAl2O4/ZnAl2O4–xCl2x nanoclusters. Postsynthetic treatment in He and subsequently in 2.5% H2 in He at 773 K removes Cl and adsorbed HCl, resulting in the formation of [ZnH]+ cations. Studies of C3H8 dehydrogenation and cracking suggest that in the absence of cofed H2, [ZnH]+ cations are transformed to bridging Zn2+ cations, which exhibit higher C3H8 dehydrogenation activity and selectivity relative to [ZnH]+ cations. The kinetics of dehydrogenation and cracking over Zn/H-MFI were investigated as a function of Zn loading, C3H8 partial pressure, and temperature. The turnover frequency for propane dehydrogenation and cracking increases with Zn loading, which we propose is due to localization of Zn2+ cations either at increasingly distant pairs of Al atoms or at the β-site in the MFI framework. The selectivity to dehydrogenation over cracking over Zn2+ is independent of C3H8 partial pressure and temperature, consistent with dehydrogenation and cracking pathways that proceed via a common surface intermediate and have similar enthalpies of activation. The product distribution is thus determined by the entropy of activation for each pathway, which is less negative in the case of C3H8 dehydrogenation.