【Domestic Papers】Hydroxyl-facilitated efficient propane dehydrogenation over bare Ga₂O₃ via altering reaction pathway

      Researchers from the Beijing Institute of Technology have published a dissertation titled "Hydroxyl-facilitated efficient propane dehydrogenation over bare Ga₂O₃ via altering reaction pathway" in Journal of Catalysis.

Background

      With the increasing demand for green and low-carbon development, the efficient utilization of light alkanes has become an important research topic. Non-oxidative propane dehydrogenation (PDH) is regarded as a key alternative to conventional oil-based routes for propene production due to abundant feedstock resources, high propene selectivity, and facile product separation, and it has already been industrially implemented. However, currently used industrial catalysts largely rely on noble metals or chromium-based systems, which suffer from high cost or environmental concerns. This has motivated intensive efforts to develop low-cost and environmentally benign catalysts. Among metal-oxide catalysts, GaO_x-based materials have attracted considerable attention because of their favorable dehydrogenation activity and environmental friendliness. Nevertheless, most previous studies focus on supported or doped Ga₂O₃ systems, while the intrinsic catalytic behavior, reaction pathways, and structure–activity relationships of bare Ga₂O₃ remain insufficiently understood. Therefore, a systematic investigation of the structural and surface properties of bare Ga₂O₃ and their roles in C–H bond activation and PDH reaction mechanisms is essential, providing fundamental insights for the rational design of efficient and sustainable PDH catalysts.

Abstract

      Designing suitable catalysts for catalytic processes involving alkanes and efficiently activating C−H bond in light alkanes are both theoretically and practically significant. The present study shows that bare Ga₂O₃ without any supported species or dopants can effectively catalyze the PDH reaction. A clear structure–activity relationship based on crystallite size, acid density, and hydroxyl group density is established, the smaller the crystallite size, the higher the activity. Ga₂O₃ with a smaller crystallite size owns a higher concentration of hydroxyl species and oxygen vacancies. Comprehensive experimental investigations and DFT calculations indicate that the presence of oxygen vacancy in Ga₂O₃ decreases the apparent activation energy of PDH compared with the pristine Ga₂O₃ surface. Importantly, the existence of hydroxyl groups on the Ga₂O₃ surface can significantly improve the ability to activate C−H bond by altering the PDH reaction from a non-oxidative to the oxidative pathway at the initial stage. It is expected that these findings offer fundamental insights into regulating the physicochemical properties of metal oxides for efficient C−H bond activation and hydrogenation reactions.

Highlights

      ● A series of bare Ga₂O₃with different crystallite size were synthesized by different methods.

      ● The smaller the crystallite size of Ga₂O₃, the higher the PDH activity.

      ● The structure–activity relationship and reaction mechanism were investigated by complementary characterization techniques and DFT calculations.

      ● The presence of hydroxyl groups on the Ga₂O₃surface can significantly improve the ability to activate C−H bond.

Conclusion

      In summary, this study investigated the structure–activity relationship of bare Ga₂O₃ for PDH reaction by combining complementary characterization techniques with DFT calculations. It is found that the crystallite size has strong effects on the microstructure, acidic, and surface properties of Ga₂O₃. The smaller the size of crystallite, the higher the concentration of acid sites and hydroxyl species, and therefore the higher the rate of propene formation in PDH. The presence of hydroxyl species on the Ga₂O₃ surface alters the reaction pathway of PDH from a non-oxidative pathway to the oxidative pathway at the initial stage concerning the first C−H bond activation, significantly decreasing the reaction barrier of C−H bond activation. In addition, Ga₂O₃ with smaller crystallite size owns a high concentration of oxygen vacancy forming Ga₂O₃- Ov sites, which shows a lower apparent activation barrier than that of Ga₂O₃ and Ga₂O₃-H surfaces. Moreover, Ga₂O₃ with different crystallite sizes exhibits very high propene selectivity (>97%) and the selectivity of cracking products is low over a wide range of propane conversions. It is believed that the present study contributes to deeply understanding the C−H bond in light alkanes and promoting the development of Ga₂O₃- based materials in PDH reaction.