CO₂ assistant oxidative dehydrogenation of isobutane to isobutene catalyzed by ZnCaZr solid solution

CO₂-assisted oxidative dehydrogenation of isobutane to isobutene (CO₂-BDH) is an environmentally friendly low-carbon dehydrogenation process, which can effectively utilize greenhouse gas CO₂ while producing value-added product isobutylene. Besides, the soft oxidizing property of CO₂ can break the thermodynamic limitation of dehydrogenation reaction and avoid the problem of deep oxidation, which makes isobutylene highly selective. However, its industrialization is still challenged by the lack of green and efficient catalysts. In this work, xZn-CaZr solid solution catalysts was prepared by one-pot co-precipitation method and applied to CO₂-BDH reaction. The physicochemical properties of all catalysts were investigated by various means, and the structure-activity relationship and surface redox mechanism were described in combination with catalytic performance. The results show that xZn-CaZr catalysts formed solid solution structure with Zn species (6%−12%) existing in highly dispersion state, and the "confinement effect" given by mesoporous skeleton and strong metal-support interactions contributes to the stable distribution of nanoscale sites and generates more Zn-O-Zr interfaces. When excessive Zn species (16%) is added, the ZnO crystal will have obvious phase separation from the solid solution phase. The surface chemical states of different catalysts were analyzed by XPS, and it was found that the relative content of O_β increased first and then decreased with the increase of Zn content. In addition, the surface reduction characteristics of the catalyst indicated that the promotion of an appropriate amount of Zn species can improve the mobility of lattice oxygen, thus 0.4Zn-CaZr catalyst showing the highest relative content of O_β and the best oxygen conductivity. In the activity evaluation of different xZn-CaZr catalyst, 0.4Zn-CaZr catalyst shows the best catalytic dehydrogenation activity but its stability is poor, while 0.2Zn-CaZr catalyst has the best reaction stability. Moreover, the catalytic performance of ZnCaZr catalyst under different C₄H₁₀-CO₂ ratios was also investigated, which indicated that the higher CO₂ content in the feed gas was helpful to improve the catalytic stability and isobutene selectivity. Combined with the surface chemical state and carbon deposition information of the spent catalyst, it was found that the relative content of O_β on the surface of 0.4Zn-CaZr catalyst decreases obviously, but the carbon deposition rate was slow. On the contrary, the relative content of O_β for 0.2Zn-CaZr catalyst decreased less, but its carbon deposition rate was faster. We believe that the amount and mobility of lattice oxygen over xZn-CaZr catalysts were revealed as key factors in determining the catalytic performance. Notably, higher content and superior mobility of lattice oxygen can enhance the redox function of the solid solution catalyst itself and improve the activation performance of CO₂. The strong oxygen supply capacity can ensure the continuous MvK catalytic cycle on the Zn-O-Zr interface, and avoid the deep accumulation of inert carbon deposits while improving the dehydrogenation activity of isobutane and selectivity of isobutene. This study shed lights on the further design and development of green and efficient CO₂-BDH catalysts.