Insights into the Speciation of Cu in the Cu-H-Mordenite Catalyst for the Oxidation of Methane to Methanol

The proton form of Cu-exchanged mordenite (Cu-H-MOR) was prepared via ion-exchange, and the nature of the active Cu species in the cyclic oxidation of CH4 to CH3OH was investigated by high-pressure reactivity testing, X-ray absorption spectroscopy (XAS), and H-2 temperature-programmed reduction (TPR). Increasing the CH4 pressure from 1 to 35 bar and the reaction time from 4 to 20 h increased the product yield from 0.30 to 0.42 mol (mol Cu)(-1), suggesting that at lower pressures and shorter reaction times, the CH4 activation reaction is not complete and the active site for CH3OH formation likely contains fewer than three Cu atoms. Linear combination fitting of the Cu K-edge X-ray absorption near edge spectra showed that 83% of the Cu in freshly prepared Cu-H-MOR can be autoreduced in He at 723 K. Analysis of the extended X-ray absorption fine structure of Cu-H-MOR after activation in O-2 at 723 K resulted in an oxygen coordination number of 2.9. The product yield normalized by the redox-active Cu fraction was 0.50. All of these findings are consistent with a dicopper active site. The same fraction of nonreducible Cu was observed by autoreduction in He and TPR in H-2, suggesting that redox-inert Cu is inactive toward CH4 oxidation.