Enhancement of Methanol Selectivity in the Products of Direct Selective Oxidation of Methane in CH4–O2–NO with Cu–ZnO/Al2O3

Enhancement of methanol selectivity in the products of the direct selective oxidation of methane with CH4–O2–NO in a gas-phase reaction was examined using a Cu–ZnO/Al2O3 catalyst. Three distinct reaction paths over the Cu–ZnO/Al2O3 catalyst were detected in the gas-phase selective oxidation of methane in CH4–O2–NO. The formation of CH3OH from CH2O–H2 and the water–gas shift reaction of CO–H2O progressed chiefly at around 250°C over Cu–ZnO/Al2O3 catalyst. The steam reforming reaction of CH3OH progressed over the same Cu–ZnO/Al2O3 catalyst at around 350°C and higher. Both CH3OH and CH2O were observed as C1-oxygenates at 550°C in the gas-phase selective oxidation of methane in CH4–O2–NO, but only CH3OH was observed as a C1-oxygenate in the presence of Cu–ZnO/Al2O3 catalyst in addition to the gas-phase selective oxidation of methane. The complete exhaustion of oxygen in the gas-phase selective oxidation of methane in CH4–O2–NO was a key to the effective use of Cu–ZnO/Al2O3 catalyst. Of the two reactions, CH3OH formation and water–gas shift over Cu–ZnO catalyst, the water–gas shift reaction progressed more over the catalyst with a higher surface area and with a lower surface Cu/Zn atomic ratio.