(298a) Copper Speciation in Cu-Mordenite Catalysts for the Oxidation of Methane to Methanol
Methane from abundant natural gas reserves represents a valuable potential feedstock for the chemical industry, but the requirement of liquefying natural gas for storage and transport renders much of the gas present in remote deposits economically inaccessible. Copper-exchanged zeolite catalysts are capable of chemically converting CH4 to higher-value liquid CH3OH with high selectivity, but low yields have prevented industrial application and hampered active site characterization. Utilizing the H+ form of the zeolite instead of the Na+ form has been shown to increase CH3OH productivity, which should facilitate identification of the Cu active site, but the active site structure in Cu-H-MOR has not yet been unambiguously defined. Under conditions of high CH4 pressure, the C1 product yield during cyclic operation of the Cu-H-MOR catalyst was 0.42 mol (mol Cu)-1. Linear combination fitting of the Cu K-edge X-ray absorption spectrum of the Cu-H-MOR catalyst showed that 83% of the Cu was auto-reduced in He, which is assumed to be the redox-active fraction of Cu for CH3OH formation. Normalizing the product yield to the redox-active fraction of Cu gave a reaction stoichiometry of 0.50 mol (mol Cu)-1, consistent with the presence of dicopper active sites. The O2-activated dicopper sites were further characterized by EXAFS, UV-vis and resonance-Raman spectroscopy.