Copper-catalyzed selective oxidation of methane by oxygen:: Studies on catalytic behavior and functioning mechanism of CuOx/SBA-15

While copper is the active center of particulate methane monooxygenase in methanotrophic bacteria, there are few studies to utilize synthetic copper catalysts for the selective oxidation of methane by oxygen. In this work, we have found that the copper ions attached on mesoporous silica SBA-15 with high dispersion can catalyze the selective oxidation of methane to formaldehyde by oxygen efficiently. The catalyst with a copper content of 0.008 wt % (Si/Cu = 13200) exhibits the best catalytic performance, and the specific site rate for formaldehyde formation can reach 5.6 mol (mol Cu)(-1) s(-1), significantly higher than those reported to date for other catalysts. We have elucidated that the oxidation of methane produces formaldehyde as a major primary product together with a small amount of carbon dioxide, while carbon monoxide is formed mainly via the consecutive oxidation of formaldehyde over our copper-based catalyst. Pulse reaction studies have indicated that methane molecules can react with the lattice oxygen of the catalyst, producing carbon oxides, and Cull in the catalyst is reduced at the same time. Detailed pulse reaction studies combined with EPR characterizations suggest that the reduced copper (probably Cut) sites generated by methane molecules during the reaction account for the activation of molecular oxygen, forming active oxygen species for the selective oxidation of methane to formaldehyde.