The Role of VO Sites on the Oxidation of Methane to Formaldehyde over V/SiO 2

Abstract

V-incorporated mesoporous KIT-6 silica materials with different V content (xV-KIT-6) were used for partial oxidation of methane (POM) with dioxygen under atmospheric pressure. Turnover frequency of CH4 (TOFCH4) is basically indistinguishable for highly dispersed VOx species under (sub-)monolayer coverage on SiO2. However, the polymerization degree of VOx species significant affect HCHO selectivity. At 625 °C, 3V-KIT-6 catalyst exhibited best catalytic performance of POM: methane conversion was 8.1%, formaldehyde selectivity was 26.1% and selectivity of the incomplete oxidation products was 90.5%. Compared with 3V-KIT-6 catalyst, the supported 1.1V/KIT-6 catalyst with same vanadium content and dispersity exhibit poor high-temperature TOFHCHO and low-temperature HCHO selectivity, suggesting that the supported and incorporated VOx species possess different coordination environments, such as VO and VOSi bond length and bond angle. To further investigate their microstructure effect on catalytic performance, in situ and operando UV Raman spectroscopy was used to investigate their different redox properties of VOx species. Under the pure CH4 atmosphere, their VOV or VOSi lattice oxygen are more reducible than VO lattice oxygen, suggesting the former are active sites. Supported VOx species are more easily reduced by CH4 than the incorporated ones, the VO lattice oxygen of 3V-KIT-6 catalyst cannot be completely consumed by CH4 after 1h, at least 30% VO bonds maintained. When 5% O2 was cofed, the VO bonds on 3V-KIT-6 and 1.1V/KIT-6 recovered quickly, indicating that the partially reduced VOx species are oxidized by O2 faster than their reduction by CH4. Thus, the activation of CH4 on VOx species should be the rate-determining step. Some unsaturated VOx species are present on 1.1V/KIT-6 easily, those may generate unselective electrophilic oxygen species for overoxidation.