Studies On The Selective Oxidation Of Methane Over Copper-and Iron-containing Heterogeneous Catalysts

This dissertation focuses on the studies of the selective oxidation of methane to formaldehyde over Cu/SBA-15 and Fe/SBA-15 catalysts. The nature of the active sites has been elucidated through the correlation of catalytic behaviors and characterizations of these catalysts. Kinetic measurements and pulse studies have been exploited to study the reaction mechanisms over these two kinds of catalysts.The comparison of the catalytic performances of SBA-15-supported various transition metal species (the mole ratio of transition metal to silicon is 1/13200) showed that HCHO yield and turnover frequency (TOF) for HCHO formation over the Cu/SBA-15 and Fe/SBA-15 catalysts were higher than other transition metal-containing catalysts. The optimization for the Cu/SBA-15 catalysts with different copper contents revealed that the best catalytic performance was obtained over the sample with a Cu content of 0.008 wt%. The TOF for HCHO formation could reach to 5.6 s^-1 over this catalyst, which was remarkably higher than those reported to date. For the series of Fe/SBA-15 catalysts with different Fe contents, the highest HCHO yield (1.9%) was gained over the sample with an Fe content of 0.05 wt%. As the Cu and Fe contents were increased to 1.0 wt% and 4.6 wt%, respectively, CuO and Fe₂O₃ crystallites were observed from XRD, and these crystallites could catalyzed the deep oxidation to COx. This was different from the result observed over the MoOx/SBA-15 catalyst, where the MoO3 clusters enhanced the HCHO formation. We conclude that the highly dispersed copper and iron sites, especially the isolated CuII and FeIII sites, are responsible for the selective oxidation of CH4 to HCHO.The reaction of the lattice oxygen with CH4 pulses over the 0.008 wt% Cu/SBA-15 and 0.05 wt% Fe/SBA-15 suggested that the lattice oxygen was not responsible for HCHO formation. The pulse reaction using (CH₄ + O₂) pulses over Cu/SBA-15 could produce HCHO, and we found that both CH4 conversion and HCHO selectivity increased significantly with the pulse numbers at the initial stage. It was of interest that the pulse number to reach the constant CH4 conversion and HCHO selectivity (induction period) decreases with decreasing the P(O₂)/P(CH₄) in the pulse, or with introducing a small amount of H2 into the (CH₄ + O₂) pulse. The same phenomenon was also observed over the 0.05 wt% Fe/SBA-15 catalyst if the P(O₂)/P(CH₄) in the pulse was sufficiently low. EPR characterizations confirmed that the Cu^II and Fe^III in the catalysts could be reduced by CH₄. EPR studies also demonstrated that a part of CuII and FeIII sites underwent reductions during the flow reactions and the pulse reactions. On the basis of the results described above, we speculate that the reduced Cu or Fe sites (i.e. Cu^I or Fe^II) may participate in the activation of molecular oxygen, forming an active oxygen species for the conversion of CH4 to HCHO.