Effect of Ga₂O₃ content on the activity of Al₂O₃-supported catalysts for the CO₂-assisted oxidative dehydrogenation of propane

Propylene production through the CO₂-assisted oxidative dehydrogenation of propane is considered an effective route for addressing the ever-increasing demand for propylene and simultaneously utilizing CO₂. In this study, a series of alumina-supported gallium oxide catalysts with variable Ga₂O₃ loadings was synthesized, characterized, and evaluated with respect to their activity for the oxidative dehydrogenation of propane with CO₂. Surface basicity was measured through CO₂-TPD experiments using mass spectrometry (MS) and in situ FTIR spectroscopy techniques, while surface acidity was determined by employing potentiometric titration and pyridine adsorption/desorption experiments. XRD, BET, and SEM-EDS techniques were also applied for the determination of the catalysts’ physicochemical and morphological properties. The results showed that surface basicity was maximized for the sample containing 20 wt.% Ga₂O₃, whereas surface acidity monotonically increased with an increasing Ga₂O₃ loading. Catalytic activity was found to be strongly influenced by the Ga₂O₃ concentration and optimized for the 30%Ga₂O₃-Al₂O₃ catalyst, which was characterized by moderate surface acidity and basicity. This catalyst was not only able to enhance propane's conversion into propylene, which reached 59% at ~600 ^oC with a corresponding propylene yield of 39%, but also to limit the undesired reactions of propane hydrogenolysis and propane/propylene decomposition, which were responsible for the formation of C₂H₄, CH₄, C₂H₆, and coke. Time-on-stream stability tests showed that the 30%Ga₂O₃–Al₂O₃ catalyst exhibited very good stability at 550 °C for 12 h, where byproduct formation and carbon deposition were limited, whereas it was gradually deactivated with the time on stream when the reaction occurred at elevated temperatures (>600 °C). Mechanistic studies conducted using in situ FTIR and transient-MS techniques indicated that the reaction proceeded through a two-step oxidative route, with the participation of CO₂ in the abstraction of H₂, originated by propane dehydrogenation, through the RWGS reaction, shifting the thermodynamic equilibrium towards propylene generation.