Computational Modeling of the Nature and Role of Ga Species for Light Alkane Dehydrogenation Catalyzed by Ga/H-MFI

作者：Erum Mansoor, Martin Head‐Gordon, Alexis T. Bell · 发表于：ACS Catalysis · 年份：2018 · DOI：10.1021/acscatal.7b04295 · 被引用次数：120 · 研究领域：Zeolite Catalysis and Synthesis、Catalysis and Oxidation Reactions、Advanced Chemical Physics Studies

Ga-exchanged H-MFI zeolites are highly active for the dehydrogenation of light alkanes; however, both the nature of the active gallium species and the associated dehydrogenation mechanism have been difficult to establish. In this study, we examine the activity of Ga species in Ga/H-MFI by calculating the free energy landscapes on which all reactions occur. To this end, we use a hybrid quantum mechanics/molecular mechanics model for all electronic structure calculations. Quantum chemical calculations were carried out with a range-corrected functional and a good representation of dispersive interactions. The molecular mechanics part of our approach captures the long-range effects of Coulombic and dispersive interactions due to atoms in the extended framework. The rate-determining TS (RDTS) is identified by analysis of the free energy landscape for each mechanism, using the energetic span model. Our analysis reveals that, for reduced Ga/H-MFI, univalent and divalent gallium hydrides, [GaH 2 ] + and [GaH] 2+, respectively, are more active for ethane dehydrogenation in comparison to H + sites and Ga + sites. [GaH] 2+ sites consistently emerge as the most active sites for light alkane dehydrogenation, providing significant enthalpic stabilization to C–H cleavage TSs via alkyl and carbenium dehydrogenation routes. In contrast, carbenium-like C–H cleavage TSs occurring on Brønsted acid sites are enthalpically less favorable due to their limited electronic interactions with framework ...