UC Berkeley

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, [GaH2]+ 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 O atoms and H+ sites. Activation enthalpy barriers for dehydrogenation determined using the energetic span analysis are in good agreement with those measured experimentally. Accounting for entropy of activation reveals that constrained TSs become less favorable in free energy with increasing chain length. We also find that an increase in enthalpic favorability of the alkyl mechanism is observed with increasing chain length for the TS responsible for the second C-H cleavage step leading to alkene formation.