Catalytic behavior and kinetic features of FeOx/SBA-15 catalyst for selective oxidation of methane by oxygen

FeOx/SBA-15 catalysts with different iron contents were studied for the selective oxidation of methane by oxygen. The catalyst with an iron content of 0.05 wt% exhibited the highest single-pass formaldehyde yield (∼2% at 898 K). The selectivity to formaldehyde and the specific site rate for formaldehyde formation decreased with increasing iron content. The structure-performance correlation suggests that the isolated iron species account for the selective oxidation of methane to formaldehyde, whereas the FemOn clusters are less active and the crystalline Fe2O3 mainly catalyzes the complete oxidation of methane. Kinetic investigations over the 0.05 wt% FeOx/SBA-15 catalyst reveal that formaldehyde is the major primary product, and the consecutive oxidation of formaldehyde produces carbon monoxide as the main by-product. The reaction orders with respect to methane and oxygen were 1.0 and 0.20, respectively, and the activation energy was 102 kJ mol−1, which was significantly lower than those reported for other catalysts such as MoOx/SBA-15. Pulse reaction studies clarify that methane can react with the lattice oxygen but the products are only CO and CO2. Thus, the lattice oxygen cannot be responsible for formaldehyde formation. It is proposed that the activation of molecular oxygen on the reduced iron sites generated by methane molecules produces active oxygen species for the selective oxidation of methane.

FeOx/SBA-15 catalysts with low Fe contents can catalyze the selective oxidation of methane to formaldehyde efficiently. Isolated Fe species mainly account for the selective formation of HCHO. This report proposes that the FeII sites generated during the reaction function as the active centers for the activation of O2, forming active oxygen species for the conversion of CH4 to HCHO.Figure optionsDownload as PowerPoint slide