CO2-Assisted Direct Oxidation of Low-Concentration Methane to Liquid C1 Oxygenates over the Cu-ZSM-4 Catalyst

Abstract

Using CO2 as a mild oxidant to convert low-concentration methane (CH4) into high-value C1 liquid oxygenates offers a promising route for utilizing unconventional gas while reducing greenhouse gas emissions. However, achieving efficient CH4 activation and directional conversion remains challenging. In this study, copper-based zeolite catalysts (xCu-ZSM-4) prepared via an in situ ligand-assisted method were employed in a continuous fixed-bed reactor for atmospheric-pressure methane oxidation. Results showed that, with CH4 as the main reactant, co-feeding CO2 and H2O synergistically promotes methane direct oxidation. At a theoretical copper loading of 4 wt %, the total yield of liquid oxygenates under CO2 and H2O co-feeding conditions reached 54.1 μmol/gcat/h, markedly exceeding the yield of 35.6 μmol/gcat/h obtained with CO2 alone. Hence, this study systematically investigated the synergistic mechanism of CO2 and H2O co-feeding in promoting methane oxidation through diverse characterizations and density functional theory calculations. Specifically, spontaneously adsorbed CO2 is activated with a low energy barrier of 0.08 eV, cleaving one C=O bond; the dissociated oxygen atom binds to a copper site and readily combines with adjacent adsorbed hydrogen, thereby regenerating the [Cu2+-OH]+ active site. Meanwhile, H2O co-feeding is equally essential, lowering the methanol formation barrier to 0.27 eV (0.33 eV below the anhydrous system) and facilitating methanol desorption with a barrier of –0.19 eV. This ensures a continuous supply of active sites while promoting timely product desorption, thus avoiding deep oxidation and improving overall efficiency. These findings may offer valuable insights into designing efficient catalysts and optimizing reaction conditions for this field.