Effects of tetrahedral molybdenum oxide species and MoOx domains on the selective oxidation of dimethyl ether under mild conditions

Abstract

A new preparation method for MoO₃–SnO₂ catalysts precipitated by HNO₃ was developed to selectively synthesize industrially useful chemicals formaldehyde and methyl formate via oxidation of environmentally friendly feedstock dimethyl ether. By adjusting the structure of MoO₃–SnO₂, the selectivity to formaldehyde and methyl formate can reach as high as 95.0% and 82.4%, respectively, under different conditions. The conclusion has been drawn, based on the experimental results, that the formation of tetrahedral molybdenum oxide species and SnMoO₄ species favors the reaction of dimethyl ether to formaldehyde, and that the formation of MoO_x domains favors the direct oxidation reaction of dimethyl ether to formaldehyde. The XRD and Raman results suggest that the formation of tetrahedral molybdenum oxide species has a positive correlation with the high selectivity to formaldehyde and indicates that the formation of the MoO_x domains favors the high selectivity to methyl formate. The XPS results of the MoO₃–SnO₂ catalysts with different SnO₂ contents demonstrate that formaldehyde is not readily desorbed from Mo1Sn2 and Mo1Sn3 because the electron-poor Mo cations of the domains supported on the surface of the catalysts strengthen their affinity for binding electron-donating dimethyl ether-derived intermediates and formaldehyde, which favors the subsequent reaction of formaldehyde to methyl formate. These interesting findings reveal that tetrahedral molybdenum oxide species and MoO_x domains play an important role in the selective oxidation of dimethyl ether to formaldehyde and methyl formate, and give insight into the rational design of the MoO₃–SnO₂ catalyst structure for dimethyl ether oxidation in future studies.