【Others 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

      The efficient utilization and conversion of light alkanes is significantly important for realizing a green and low-carbon society. As an important industrial and model reaction, non-oxidative propane dehydrogenation (PDH), acting as an alternative and on-purpose propene production technology to traditional oil-based technologies (steam or catalytic cracking of different oil fractions), has received increasing attention due to the advantages of abundant light alkane resource, higher propene selectivity, and easy separation of reaction products. The growing demand for propene has also driven the rapid development of the PDH process. In the aspect of fundamental research, PDH has been recognized as a platform reaction for studying the C–H bond activation over various types of catalysts including noble metal, and metal oxide-based materials.

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 Ga₂O₃ with 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 experimental and theoretical studies.

      ● The presence of hydroxyl groups on the Ga₂O₃ surface 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–O_v 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.