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

Abstract

Our study addresses a critical challenge in the development of thermally stable, selective, and efficient catalysts for lean methane (CH₄) combustion. This reaction plays a key role in environmental remediation by mitigating methane, a potent greenhouse gas emitted from industrial and natural sources, and supports clean energy applications. 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) was used to visualize 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 the MnFI sample. 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 h without deactivation or formation of H₂ or CO byproducts. Additionally, DFT modeling of key reaction intermediates revealed that the catalytic oxidation is initiated by 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.