Isothermal conversion of methane to methanol over Cu-CHA using different oxidants

Methane oxidation to methanol in cyclic processes using CuO-zeolites has traditionally employed O₂ and N₂O as oxidants. This study explores the use of Cu-CHA zeolite, demonstrating that CO₂ can substitute O₂ in an isothermal reaction at 400 °C, analogous to previous findings using Cu-MAZ. The use of in situ UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and density functional theory (DFT) theoretical calculations identified the formation of a binuclear copper hydroxide complex, Z-[CuOH-HOCu]²⁺-Z, on the Cu-CHA. Initial treatment using CO₂ led to marginally superior catalytic activity, compared to the use of O₂ alone, indicating the stability of the Z-[CuOH-HOCu]²⁺-Z complex against self-reduction at 400 °C. In subsequent cycles, activation with O₂ facilitated the oxidation of adsorbed methanol, yielding water and reconstituting the active sites. Conversely, activation with CO₂ led to the partial desorption of methanol, precluding water production and subsequent catalyst regeneration. The findings suggested that both O₂ and CO₂ activations necessitated a post-reaction water extraction step, followed by thermal treatment to replenish the active sites. Importantly, the results indicated that CO₂ could be used as a viable alternative oxidant to O₂ in this catalytic process, potentially enhancing the sustainability of industrial methanol production.