Direct Stepwise Oxidation of Methane to Methanol over Cu–SiO₂

Abstract

Cu supported on SiO2 can be used to directly convert methane to methanol in a stepwise process with no intrinsic need for a zeolite support. Effects of parameters such as the O2 activation temperature, activation time, CH4 reaction temperature, CH4 partial pressure (pCH4), and Cu wt % on methanol yield were investigated. Increasing the O2 activation temperature in the 200–800 °C range significantly improved the methanol yield, and when carried out at 800 °C, a methanol yield of 11.5 μmol/gcatalyst was obtained after reaction with methane at 200 °C for the sample with 2 wt % Cu. Yield per mole of Cu increased exponentially from 1.0 to 59.1 mmol with decreased Cu wt % from 30 to 1, respectively. The increase in the O2 activation time also strongly influenced the yield which corresponded to the increase in yield by a factor of >2 between 1 and 8 h. Increasing pCH4 from 0.05 to 1 atm resulted in a 5-fold increase in yield after activation at 450 °C; however, it resulted in at least 20% lower yields after activation at 800 °C showing that active sites of different nature were formed at different activation temperatures. The increase in yield with ramped O2 activation temperature correlated with the dehydration of the samples as evidenced by X-ray absorption near-edge spectroscopy (XANES) and via mass spectroscopy (MS) traces of H2O during the O2 activation step.