Origin of Rate and Selectivity Trends and Exceptional Yield in Sulfur-Oxidative Propane Dehydrogenation Over Supported Vanadium Catalysts

Sulfur vapor (S2) is an effective alternative oxidant for propane dehydrogenation, (S)ODHP, with vanadium-based catalysts displaying promising performance and propylene selectivity significantly greater than with O2 as the oxidant (ODHP). However, a deeper mechanistic understanding of SODHP versus ODHP will be necessary for further catalytic performance. Here, we investigate these trends over vanadium catalysts supported by Al2O3, TiO2, ZrO2, and SiO2. First, support effects on V/MxOy-catalyzed SODHP are analyzed, revealing marked support effects on the catalyst reducibility and activity. The result is maximum propylene selectivity and yield of 96 and 41%, respectively, over a sulfided V/SiO2 catalyst, surpassing most metrics in the peer-reviewed ODHP literature. Second, propane overoxidation kinetic pathways for SODHP vs. ODHP are compared, revealing that, contrary to traditional oxide catalytic pathways, secondary oxidation of propylene is disfavored over sulfided surfaces. Density functional theory (DFT) analysis reveals that while the sulfided surfaces have propylene binding energies comparable to those of oxide surfaces, the former has less favorable thermodynamic barriers to deep propylene dehydrogenation pathways than the oxide surfaces. These results suggest more competitive propylene desorption vs. propylene overoxidation on sulfided surfaces leads to higher overall selectivity. Thus, SODHP is a promising catalytic alternative for propane oxidation that rivals the traditional O2-mediated approach.