Enhanced methane combustion over monoclinic single-site Mn-containing nanosized MFI zeolite catalyst

Overview & Synopsis

Our study addresses a critical challenge in the development of thermally stable, selective, and efficient catalysts for lean methane (CH4) combustion. This reaction is important for environmental remediation, particularly in mitigating methane, a potent greenhouse gas from both industrial and natural sources, as well as for applications in clean energy technologies. Incorporating transition metals into nanosized zeolite frameworks offers synergistic benefits, including enhanced thermal stability and active Lewis acid sites. In this study, manganese (Mn) was directly incorporated into a pure silica MFI zeolite (MnFI) to generate active sites for catalytic lean methane (CH4) combustion. The monoclinic P21/n space group and the reduced silanol site density observed for the MnFI sample indicate the successful incorporation of Mn into the zeolite framework. High-angle annular-dark field scanning transmission electron microscopy (HAADF-STEM) visualized the isolated Mn atoms within the zeolite framework. The synergy between the Mn sites and the monoclinic structure enhanced the thermal stability and introduced active Lewis acid sites in sample MnFI. Catalytically, the MnFI zeolite outperformed a reference Mn-containing sample prepared by incipient wetness impregnation, achieving higher CH4 conversion under lean conditions. Notably, MnFI maintained a stable CH4 conversion of 65.1% at 590 °C over 120 hours without deactivation or formation of H2 or CO byproducts. Additionally, DFT modeling of key reaction intermediates revealed that the catalytic oxidation initiates with the reductive dissociative adsorption of CH4 at zeolite oxygen centers, followed by the sequential adsorption of the first and second O2 molecules at the Mn sites. The overall process involves the reduction of Mn4+ to Mn2+ and subsequent reoxidation.