A Study Of Liquid-Phase Catalyst Systems For Selective Oxidation Of Methane

Selective oxidation of methane to produce oxygenates remain one of the challenges in catalysis field. The reaction carried out in liquid phase with homogeneous catalysts becomes an important topic in this area due to its much gentle conditions over heterogeneous system. The conversion and selectivity varies in different solvent system which further enrich its connotation. In the present work, the selective oxidation of methane has been investigated in oleum with Pd/C catalysts and in acetonitrile with V-based catalysts. The main achievements are summarized as follows.(1) The Pd/C catalysts with different palladium loadings were prepared by impregnation using PdCl₂ as precursor. In all case, the major product in liquid phase was methyl sulfate with small mount of dimethyl sulfate. Both of them could be hydrolyzed into methanol. There was considerable amount of CO₂ in gas phase. Compared to palladium black, Pd/C catalyst showed better performance in terms of methane conversion and methanol yield. When the reaction was conducted with 5%Pd/C under the conditions of 4.0 MPa, 453 K and 4 h, a methane conversion of 23.6% with methanol selectivity of 69.5% was achieved. Characteristic studies by means of XRD, HRTEM, XPS, ICP-AES and CO adsorption showed that the active species might be Pd²⁺, which was generated in situ in the surface of active carbon. The Pd/C catalyst was recyclable, although its conversion and the selectivity showed gradual declines. The changes in catalytic performance during the catalyst reuse were mainly due to the leaching of palladium and the decrease in the palladium dispersion.(2) On the other hand, several V-based catalysts were employed for the selective oxidation of methane in acetonitrile in the presence of H₂O₂. The result showed that the VOSO₄ afforded the best performance. The activity was associated with several parameters, such as the amounts of catalyst and H₂O₂ and the reaction conditions. A methane conversion of 6.4% with the selectivity to formic acid at 51.6% was obtained over the VOSO₄ catalyst. The results of UV-Vis spectra and radical scavenger influence indicated that the reaction might be underwent a radical mechanism.