The direct molecular oxygen partial oxidation of CH4 to dimethyl ether without methanol formation over a Pt/Y2O3 catalyst using an NO/NO2 oxygen atom shuttle

Abstract

The direct partial oxidation of CH4 to dimethyl ether (DME) on a Pt/Y2O3 catalyst was studied using a mixture of NO and O2 as the oxidant. The reaction was carried out in a fixed bed reactor at 0.1 MPa and 275–400 °C using 20% CH4, 1% NO, and 1% O2 in inert gas. No methanol was detected in the effluent and a contact time study demonstrated that DME was a primary product. The DME productivities were comparable to the oxygenate (methanol, formaldehyde) productivities obtained with stronger oxidants such as N2O, H2O2, and O3. The presence of Pt and the NO + O2 gas mixture was necessary for DME formation; without NO only CO2 was produced. During the methane partial oxidation reaction NO and NO2 were not reduced to N2, indicating that they worked as a shuttle to transfer oxygen from O2 to CH4. In situ Fourier transform infrared showed the formation of a bridged nitrate species on the Pt/Y2O3 catalyst which was associated with the reaction of CH4. A comprehensive study of this bridged nitrate species indicated they were formed on yttria sites close to Pt and were likely responsible for the formation of DME. The characterization of catalysts using X-ray diffraction showed that the Pt was highly dispersed and CO uptake measurements indicated a particle size of ~3 nm. Analysis by X-ray absorption fine structure measurements showed the presence of Pt oxide with Pt-O and Pt-Pt bonds.