Mechanism of Si/Al ratio tuning in Cu-ZSM-5-catalyzed highly efficient methane-to-methanol conversion

The Si/Al ratio of Cu-ZSM-5 zeolites is systematically investigated to regulate the catalyst structure and performance in the selective oxidation of methane to methanol under mild conditions. A series of Cu-ZSM-5-X（CZX） catalysts with varying Si/Al ratios are prepared via wet impregnation. Their physicochemical properties are characterized in detail using techniques such as X-Ray Diffraction （XRD）, N₂ physisorption, X-Ray Photoelectron Spectroscopy （XPS）, and Electron Paramagnetic Resonance （EPR）. Results reveal that the Si/Al ratio governs the distribution of copper valence states. Among the series, CZ50 （measured Si/Al = 44.58） possesses the highest proportion of active Cu⁺ sites, with copper being highly dispersed on the zeolite support. Under reaction conditions of 70℃, 3 MPa CH₄, and 0.5 M H₂O₂ as the oxidant, CZ50 exhibits optimal catalytic performance, achieving a methanol productivity of 46.46 mmol·g_cat⁻¹·h⁻¹ with 86.22% selectivity. This performance significantly surpasses that of catalysts with other Si/Al ratios. Mechanistic studies indicate that Cu⁺ sites efficiently activate H₂O₂ to generate ·OH radicals, which subsequently attack methane to form the key ·CH₃ intermediate. In situ IR spectroscopy combined with radical-quenching experiments confirms that the distinct electronic structure of CZ50 effectively stabilizes this methyl intermediate, thereby inhibiting deep oxidation to CO₂. Through precise tuning of the zeolite Si/Al ratio, the valence state of copper active centers is modulated to favor the formation of highly efficient and stable Cu⁺ sites. This strategy achieves highly productive and selective conversion of methane to methanol under mild conditions. The optimized CZ50 catalyst demonstrates methanol yields that exceed most reported systems, representing a key advance toward the practical realization of direct methane conversion and providing a novel design principle and a solid theoretical foundation for developing efficient, stable non-precious-metal catalysts for methane valorization.