Dynamics of redox behavior of nano-sized VOx species over Ti–Si-MCM-41 from time-resolved in situ UV/Vis analysis

Time-resolved in situ UV/Vis spectroscopy has been successfully applied for analyzing the reduction degree of highly dispersed VOx (0.4–1.1 V/nm2) species over Ti–Si-MCM-41 (Ti/Si = 0–1.5) materials in the oxidative dehydrogenation of propane (ODP) as well as for deriving the kinetic parameters of reduction (with C3H8) and reoxidation (with O2 and N2O) of these species. This quantitative kinetic study enabled to recognize at least two types of differently reducible VOx species in the samples with titanium. Three effects of titanium on redox properties of VOx species were identified: (i) the constant of reduction of oxidized VOx species by C3H8 increases, (ii) the constant of oxidation of reduced VOx species by O2 decreases; and (iii) the constant of oxidation of reduced VOx species by N2O increases. These effects are highly relevant for the ODP performance. The intrinsic ODP activity increases linearly with an increase in the constant of reduction of oxidized VOx species. Propene selectivity is determined by the ratio of the constants of reduction and reoxidation of VOx species; the higher the ratio, the higher the selectivity. This ratio can be tuned by the oxidizing agent (O2 vs. N2O) and by the Ti/Si ratio.