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

Abstract

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 ZnCl 2 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 ZnAl 2 O 4 /ZnAl 2 O 4−x Cl 2x nanoclusters. Postsynthetic treatment in He and subsequently in 2.5% H 2 in He at 773 K removes Cl and adsorbed HCl, resulting in the formation of [ZnH] + cations. Studies of C 3 H 8 dehydrogenation and cracking suggest that in the absence of cofed H 2 , [ZnH] + cations are transformed to bridging Zn 2+ cations, which exhibit higher C 3 H 8 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, C 3 H 8 partial pressure, and temperature. The turnover frequency for propane dehydrogenation and cracking increases with Zn loading, which we propose is due to localization of Zn 2+ 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 Zn 2+ is independent of C 3 H 8 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 C 3 H 8 dehydrogenation.

Atom-by-Atom Synthesis of Multiatom-Supported Catalytic Clusters by Liquid-Phase Atomic Layer Deposition

“…Comparing Al/Zn and Zn 2 /Al catalysts, the pre-exponential factor increases, which can simply be explained by the previously discussed increased number of active sites present in Zn 2 /Al. From the Arrhenius plots slopes, an apparent activation energy of 145 ± 1 kJ mol –1 for all zinc-containing catalysts was calculated, which falls in the expected range from the literature (121–159 kJ mol –1 ). , …”

“…From the Arrhenius plots slopes, an apparent activation energy of 145 ± 1 kJ mol −1 for all zinc-containing catalysts was calculated, which falls in the expected range from the literature (121−159 kJ mol −1 ). 73,74 The increased Zn mobility might also favor the displacement of the zinc atom off of the silica surface, leading to irreversible deactivation. Therefore, placing an aluminum next to a zinc atom and adding an anchor on the other side could limit displacement and increase stability.…”

Silicalite-1 Stabilizes Zn-Hydride Species for Efficient Propane Dehydrogenation

“…The erosion of [ZnH] + species upon D 2 treatment is related to the decreased PDH activity under C 3 H 8 –D 2 reactant, further revealing the unique role of [ZnH] + species for propane activation. Indeed, the [ZnH] + species were widely reported in aluminosilicate zeolites, but their activity for alkane activation is debated. − The present work not only reveals that the [ZnH] + could be stabilized by silicalite zeolites but also highlights its unique role in propane activation. More importantly, it is found that the [ZnH] + formation could be regulated by the amount of [ZnOH] + in as-prepared catalysts for improving PDH performance.…”

Ga+-Chabazite Zeolite: A Highly Selective Catalyst for Nonoxidative Propane Dehydrogenation

“…However, the cost and fast deactivation of Pt and the environmental impact associated with Cr have stimulated the exploration of alternative catalysts . Ga-, − V-, − Zr-, − Zn-, − Co-, − and Fe-based materials − and nanocarbon catalysts, , among others, have been investigated for this purpose. In particular, oxide supports (silica or zeolites) − containing single metal sites (Ga + , Zn 2+ , Co 2+ , Fe 2+ , etc.)…”