Comparative studies on direct conversion of methane to methanol/formaldehyde over La–Co–O and ZrO2 supported molybdenum oxide catalysts

Abstract

The selective oxidation of methane with molecular oxygen over MoOx/La–Co–O and MoOx/ZrO2 catalysts to methanol/formaldehyde has been investigated in a specially designed high-pressure continuous-flow reactor. The properties of the catalysts, such as crystal phase, structure, reducibility, ion oxidation state, surface composition and the specific surface area have been characterized with the use of XRD, LRS, TPR, XPS and BET methods. MoOx/La–Co–O catalysts showed high selectivity to methanol formation while MoOx/ZrO2 revealed the property for the formation of formaldehyde in the selective oxidation of methane. 7 wt MoOx/La–Co–O catalyst gave 6.7 methanol yield (ca. 60 methanol selectivity) at 420 °C and 4.2 MPa. On the other hand, the maximal yield of formaldehyde ca. 4 (47.8 formaldehyde selectivity) was obtained over 12 wt MoOx/ZrO2 catalyst at 400 °C and 5.0 MPa. 7MoOx/La–Co–O catalyst showed higher modified H2-consumption than 12MoOx/ZrO2 catalyst. The reducibility and the O−/O2− ratio of the catalysts may play important roles on the catalytic performance. The proper reducibility and the O−/O2− ratio enhanced the production of methanol in selective oxidation of methane. [MoO4]2− species in MoOx/ZrO2 catalysts enable selective oxidation of methane to formaldehyde.