Selective oxidation of methane to formaldehyde over Mo/ZrO 2 catalysts

Abstract

The catalytic performance of Mo/ZrO 2 catalysts in selective oxidation of methane to formaldehyde with molecular oxygen as oxidant has been investigated. The maximal yield of formaldehyde, ca. 4.0% (47.8% formaldehyde selectivity), was obtained on 12 wt.% Mo/ZrO 2 catalyst at 400 °C, 5.0 MPa, CH 4/O 2/N 2=10/1/3 and 12,000 ml g −1 catalyst h −1. The physicochemical properties of these catalysts, such as BET surface area, structure, particle size, reducibility, ion oxidation state and surface composition have been comparatively characterized by using BET, X-ray diffractometer (XRD), LR spectra (LRS), H 2-temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) technology. The results clearly show that some interaction between Mo and ZrO 2 occurred. Such interaction induced the changes of physicochemical properties, which in turn determined the catalytic performance. The effects were a function of the Mo-loading. Zr(MoO 4) 2 in Mo/ZrO 2 catalysts were closely related to the formation of formaldehyde. The methane conversion and formaldehyde selectivity has been also correlated to the density of molybdenum oxides. The higher the density of molybdenum oxide, the higher was the specific activity. The MoO species of Zr(MoO 4) 2 enable selective oxidation of methane to formaldehyde, while the many more lattice oxygen species and bulk MoO 3 accelerated the deep oxidation of product formaldehyde.