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 O-2 activation temperature, activation time, CH4 reaction temperature, CH4 partial pressure (pCH(4)), and Cu wt % on methanol yield were investigated. Increasing the O-2 activation temperature in the 200-800 degrees C range significantly improved the methanol yield, and when carried out at 800 degrees C, a methanol yield of 11.5 mu mol/g(ctalyst) was obtained after reaction with methane at 200 degrees 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 O-2 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 degrees C; however, it resulted in at least 20% lower yields after activation at 800 degrees C showing that active sites of different nature were formed at different activation temperatures. The increase in yield with ramped O-2 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 O-2 activation step.