The room temperature reactions of triisobutylgallium (GaⁱBu₃) with the surfaces of either γ-alumina or fumed silica lead to supported Ga sites of different nuclearities. Mononuclear Ga sites are formed on γ-alumina while dinuclear Ga sites are formed on silica. Both materials catalyze propane dehydrogenation at 550 °C. GaⁱBu₃/γ-Al₂O₃exhibits significantly higher activity, however, the nature of the Ga species present at high temperature and under reaction conditions has yet to be identified. Using operando X-ray absorption spectroscopy (XAS) combined with online gas chromatography-mass spectrometry (GC-MS), we monitored the evolution at the Ga K-edge as well as the C₁ – C₄ hydrocarbons in the gas phase. In particular, we observed transitions of the initial grafted states as the materials were heated from room temperature to 550 °C under He. The X-ray absorption near edge structure (XANES) and GC-MS analysis reveals that isobutyl ligands are gradually replaced by oxygen-donor ligands derived from both oxide surfaces. Density Functional Theory (DFT) simulations allowed us to interpret the impact of ligand variations on the Ga K-edge XANES features, including white line intensity and edge position. On alumina, the mononuclear Ga(III) sites are tricoordinate at 550 °C in He. GaⁱBu₃/SiO₂ undergoes a complex, multi-stage structural transformation, resulting in a mixture of Ga(I) and mononuclear Ga(III) sites at 550 °C. Upon switching the gas flow to propane (20 vol% in He), the XANES indicates that the Ga(I) sites in GaⁱBu₃/SiO₂ slowly revert to mononuclear Ga(III), while the corresponding sites in GaⁱBu₃/γ-Al₂O₃ remained almost unchanged. These findings demonstrate a correlation between Ga structure and catalytic activity, contributing to our understanding of the structure-activity relationships in Ga-based catalysts.

Fernando Vila

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