(Zn)MgAl Hydrotalcite-Based Composite Oxide Nanostructures for Propane Dehydrogenation

Different amounts of Zn contained MgAl-layered double hydrotalcites topologically transformed into mixed oxide (xZn-O) nanomaterials through calcination at a high temperature. The degree of polymerization and reducibility of ZnO_x species have been systematically investigated, as they are closely linked to the activity of propane dehydrogenation (PDH). For the xZn-O (x < 25%) catalysts, highly dispersed Zn species dominate owning to the strong lattice confinement effect of MgAl_xO_y supports. These catalysts exhibit a high turnover frequency (TOF) value, 93% propylene selectivity, and a slow increase in C₃H₈ conversion due to their strong resistance to over reduction. With an increase in Zn content, surface-supported ZnO nanoparticles and bulk ZnO crystal species emerge. Based on the results of characterization and calculation, as the degree of polymerization of the ZnO_x species increases, they become more easily reduced, and the activation energy for C₃H₆ formation decreases, suggesting that the coordinatively unsaturated ZnO_x species nearby oxygen vacancies are the active sites for PDH. However, these also lead to a decrease in C₃H₆ selectivity and an increase in coke selectivity (deactivation constant). Therefore, the appropriate degree of polymerization and reducibility of ZnO_x species is critical for achieving efficient catalytic PDH.