关键词: 五配位铝位点, 钒基催化剂, 丙烷脱氢, VO_x聚合度

Propane dehydrogenation is an important route for propylene production. Among various non-noble metal catalytic systems, VO_x-based catalysts have attracted considerable attention owing to their excellent dehydrogenation activity, high selectivity, and relatively low cost. Nevertheless, the practical application of VO_x catalysts is still limited by rapid deactivation caused by coke deposition. The coke formation rate is linearly correlated with the polymerization degree of surface vanadium species. In this work, a series of VO_x/Al₂O₃ catalysts with different VO_x polymerization degrees were constructed by regulating the content of penta-coordinated aluminum sites (Al^V) in Al₂O₃ supports. The Al₂O₃ supports were synthesized via urea-assisted hydrothermal crystallization of pseudo-boehmite followed by calcination, with the Al^V content [5.52% (w)~19.60% (w)] tuned by adjusting the synthesis conditions. The effect of Al^V sites on the structural evolution of VO_x species and propane dehydrogenation performance was systematically studied. The obtained samples were characterized by ²⁷Al MAS NMR, XRD, UV-vis, Raman, H₂-temperature-programmed reduction (TPR) and NH₃-temperature-programmed desorption (TPD), and the catalytic performance was evaluated in a fixed-bed reactor at 550 ℃. Coke deposition and propylene adsorption-desorption behaviors were further investigated by thermogravimetric analysis, C₃H₈-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and C₃H₆-DRIFTS. Results showed that, with increasing Al^V content in the Al₂O₃ support, the surface vanadium species gradually transformed from highly polymerized VO_x species to low-polymerized VO_x species and further toward isolated species. As the Al^V content increased from 5.52% (w) to 19.60% (w), the apparent band gap increased almost linearly from 2.44 to 2.84 eV. Meanwhile, highly polymerized VO_x decreased from 21.20% to be below the detection limit, and low-polymerized VO_x content reached a maximum of 84.74%. This indicated that Al^V sites played an important role in tuning the dispersion state and polymerization degree of vanadium oxide species on alumina. Correlation analysis revealed that the low-polymerized VO_x species was positively correlated with propylene selectivity and negatively correlated with the deactivation rate constant. When low-polymerized VO_x exceeded 80%, propylene selectivity surpassed 98.4% and the deactivation rate constant fell below 0.075 h^-1. This fraction can thus serve as a quantitative descriptor for catalyst design. Among the prepared catalysts, the VO_x/Al₂O₃ catalyst containing 11.26% (w) of Al^V possessed the highest percentage of low-polymerized VO_x species. It exhibited the best performance in propane dehydrogenation, with a propane conversion of 15.0%, a propylene selectivity of 98.5%, and a propylene space-time yield of 2.8 mmol·g_cat^-1·h^-1. Its stability was also significantly higher than that of the catalyst supported on commercial alumina. This study confirmed that Al^V sites promote the formation of low-polymerized VO_x species, which acted as the dominant active species for propane dehydrogenation. Mechanistic studies demonstrated that low-polymerized VO_x possesses moderate surface acidity and facilitates propylene desorption. The enrichment of low-polymerized VO_x species enhanced the propylene formation rate while suppressing deep dehydrogenation and coke formation, and ultimately improving catalyst stability. This study provided a feasible strategy for optimizing VO_x/Al₂O₃ catalysts, achieving by regulating coordination environment of Al species in Al₂O₃ supports. It also offered guidance for designing efficient and stable non-noble metal catalysts for propane dehydrogenation.

Key words: penta-coordinated aluminum sites, VO_x-based catalysts, propane dehydrogenation, VO_x polymerization degree