A multi-Technique Approach in Mechanistic and Kinetic Studies of The Oxidative Scission of Ketone over Supported Vanadium Oxides

Both lattice and gas phase oxygen play important roles in facilitating the oxidative cleavage of methyl ketones on vanadium oxide surfaces; however, their mechanistic contributions remain unclear. This study proposed a feasible approach to distinguish the contributions of each oxygen during the oxidative scission of ketones over the supported vanadium oxides. To this end, an online mass spectroscopic analysis of reaction products is employed during Temperature Programmed Surface Reactions (TPSR) and transient analysis of products. Specially, we examine trends in the oxidative scission of 3-methyl-2-butanone (3M2B) during TPSR experiments over γ-Al2O3 and VOX/γ-Al2O3 under anaerobic (He) and aerobic (15% O2/He) conditions, and we consider the approach to steady state in a packed bed reactor. We observe that the oxidative scission of methyl ketone can occur on reducible vanadium oxides and non-reducible γ-Al2O3, implying two parallel mechanisms exist – a combination of Mars-van Krevelen mechanism and Eley-Rideal mechanism. Under aerobic conditions, Eley-Rideal mechanism is more kinetically accessible. Moreover, the catalyst performance under steady-state conditions illustrates that the oxidative scission of methyl ketone forms acetone and acetic acid precursor undergo separate pathways with distinct intrinsic kinetics. When the material lacks the reducible lattice oxygen, acetate fragments keep binding on the surface and deactivate the material. Additionally, the combination of various active sites complicates the quantification of the accessible active sites under steady-state conditions and the comparison of reactivity among catalysts with surface structure, such as monomer, polymer, and bulk V2O5. Therefore, we applied Steady-state Isotopic Transient Kinetic Analysis (SSITKA) to obtain reaction kinetics data, such as concentration of various intermediates, surface residence time and surface coverage. Further, the microkinetic analysis combined with Raman and UV-Vis to elucidate the structure-property relationship for various loading of VOX/γ-Al2O3. The result indicates that the degree of oligomerization affects the reactivity. Monomer VO4 over low loading VOX/γ-Al2O3 (\u3c1 V nm-2) has the lowest reactivity and a higher degree of polymerization renders it higher reactivity. However, the reactivity contributed by the bulk of V2O5 is limited