Increasing shale gas production in the U.S. has incentivized the conversion of light alkanes derived from natural gas to olefins and aromatics. Reports in the literature have identified zinc-exchanged H-MFI zeolites (Zn/H-MFI) as active and selective catalysts for light alkane dehydrogenation and aromatization. However, the method of Zn/H-MFI preparation and the zinc loading of the catalysts affects the nature of Zn species formed, and hence the catalyst performance. In this study, Zn/H-MFI catalysts with varying zinc loadings (Zn/Al = 0.06 – 0.94) were prepared by solid-state ion exchange of ZnCl2. The effect of zinc loading on the zinc speciation and the catalyst activity and selectivity for propane dehydrogenation and cracking were investigated. The Zn/H-MFI catalysts were characterized using XRD, N2 physisorption, FTIR of adsorbed pyridine, 1H MAS NMR, 1H-27Al TRAPDOR NMR, UV-Vis DRS, and XAFS. The results suggest that as-prepared catalysts of all zinc loadings contain predominantly Lewis acidic [ZnCl]+ species, which are formed via reaction of ZnCl2 with the zeolite Brønsted acid protons. Additionally, catalysts with higher zinc loadings may contain ZnAl2O4 nanoparticles. Upon activation of the Zn/H-MFI catalysts in hydrogen, [ZnCl]+ sites are converted to [ZnH]+ sites. Under propane dehydrogenation reaction conditions, [ZnH]+ sites appear to react with trace amounts of water to form more active [ZnOH]+ sites. Kinetic studies of propane dehydrogenation and cracking suggest that both pathways proceed through a common surface intermediate and have similar apparent activation energies. Furthermore, [ZnOH]+ species formed at higher zinc loadings are more active for both propane dehydrogenation and cracking. Identifying how the zinc speciation and catalyst activity vary with zinc loading may help clarify how zinc-exchanged zeolite catalysts should be prepared to achieve high activity and selectivity for light alkane dehydrogenation and aromatization.

Propane dehydrogenation and cracking over zinc-exchanged H-MFI zeolites prepared by solid-state ion exchange of zinc chloride

ACS Fall 2021

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