(560ds) Mechanistic Understanding of the Role of Ga in the Dehydrogenation of Ethane on Ga/Al2O3 Catalyst

Owing to abundant shale gas resources, catalytic production of light alkenes from shale gas is an alternative to the conventional steam cracking process. Ga-modified Al₂O₃ has been found to be a potential catalyst for dehydrogenation of light alkanes to alkenes with selectivity of 85 % or higher.^1-2 However, the role of Ga is not mechanistically known especially since the Al₂O₃ support is also known to be active for alkane dehydrogenation.³

In this work, we use density-functional theory calculations and first-principles microkinetic modelling to investigate the mechanism of ethane dehydrogenation on Ga-modified γ-Al₂O₃ (110) surfaces and elucidate the role of Ga sites. We consider γ-Al₂O₃ (110) surfaces with Ga atoms either as surface adatoms (Ga/Al₂O₃) or as same-valence dopants (GaAl₂O₃). The Ga/Al₂O₃ surface is constructed by condensation of Ga(OH)₃ with surface hydroxyls of γ-Al₂O₃ (110). The doped GaAl₂O₃ surface is constructed in standard manner, viz. by replacing the undercoordinated and tetrahedral Al sites on the surface of Al₂O₃ with Ga.

The dehydrogenation of ethane entails sequential activation of two C-H bonds and H₂ recombination. We investigate these elementary steps on numerous sites of the two Ga-modified alumina (110) surfaces and compare with the energetics of the reaction on pure alumina (110). The first C-H bond activation invariably entails proton transfer to a basic O atom of the surface while engaging a vicinal Lewis acidic metal atom to stabilize the excess negative charge on the resulting carbanion (C₂H₅^-). Of the possible channels for the second C-H bond activation, proton transfer to a second O atom is energetically unfavorable as it would entail recombination of two protons. Instead, energetically more favorable pathways are those that involve either hydride formation on a Lewis metal center followed by proton-hydride recombination, or direct hydride-proton recombination without mediation by a Lewis acid site.

We find that on both Ga and Al sites, the transition states in the stepwise hydride pathway are lower in energy than in the direct hydride-proton recombination pathway. Reaction path analysis obtained from microkinetic model shows a higher reaction flux for the stepwise hydride-proton recombination pathway over the direct hydride-proton recombination pathway. Furthermore, although the Al sites are more Lewis acidic than the Ga sites, the Al-O-Ga bridges are more basic that the Al-O-Al ones. As a result, the Al-O-Ga sites stabilize surface intermediates more strongly. O atoms bridging the Ga and Al exhibit higher Lewis basicity than those bridging two Al atoms, resulting in significantly lower energy intermediates on Ga-O site pairs of Ga/Al₂O₃ when compared to the Al-O site pairs of Al₂O₃. Interestingly, Ga-O site pair stabilizes the intermediates more than the transition states. This observed stabilization of the intermediates due to the addition of Ga can potentially influence the reaction rates by changing the surface coverages of the reaction intermediates. The stable intermediates on Ga can also potentially generate new catalytic cycles thereby unravelling new active sites.

References:

1. Xu, B., Zheng, B., Hua, W., Yue, Y., and Gao, Z. “Support Effect in Dehydrogenation of Propane in the Presence of CO2 over Supported Gallium Oxide Catalysts” Journal of Catalysis 239, no. 2 (2006):470–477.

2. Chen, M., Xu, J., Su, F. Z., Liu, Y. M., Cao, Y., He, H. Y., and Fan, K. N. “Dehydrogenation of Propane over Spinel-Type Gallia-Alumina Solid Solution Catalysts” Journal of Catalysis 256, no. 2 (2008):293–300.

3. Rodemerck, U., Kondratenko, E. V, Otroshchenko, T., and Linke, D. “Unexpectedly High Activity of Bare Alumina for Non-Oxidative Isobutane Dehydrogenation” (2016): 12222–12225.