Photocatalytic controlled oxidation reaction of methane

Abstract

The CH4 partial oxidation into value-added chemicals by solar means has been discussed as alternative for emission abatement, a fundamental topic for sustainable production with lower global warming gas emissions in future. So far, various semiconductor photocatalysts have been developed. However, the understanding of the main factors influencing photocatalysts' activities on controlled oxidation of methane to methanol is still an issue, especially due to methane overoxidation to CO2. Thus, we propose that the semiconductor must have a valence band (VB) favorable to produce hydroxyl radicals (•OH) and a conduction band (CB) not advantageous to superoxide radical (O2•-). Furthermore, the concentration of the hydroxyl radicals is fundamental and should be fine-tuned for selectively oxidize methane. O2 also is the key oxidant for process control, since O2 may scavenging methyl radicals (•CH3) that further react with •OH to form methanol. Our results showed that the required band edge positions for photocatalysts (e.g., Bi2O3) seems correct to obtain significant amounts of desirable chemical products, taking as main products methanol (3700 μmol g-1) and acetic acid acetic acid (~2036 μmol g-1 h-1) from pure CH4 at room temperature and atmospheric pressure. Moreover, longer hydrocarbons (e.g., ethanol and acetone) could be produced depending on the reaction condition. ESR experiments proved the formation of •CH3 and •OH, and isotope labeling experiment with 13CH4 as the reactant also was conducted, confirming that CH3OH comes from CH4 photooxidation. Another sustainable route to control the CH4 oxidation driven by chloride intermediates in solution using Bismuth-based semiconductors excited in visible light was investigated. BiOCl, a perovskite layered material, exhibited promising photocatalytic performance for methane conversion to methanol (1300 µmol g-1), acetic acid (435 µmol g-1), and ethanol (57 µmol g-1) without foreign radicals to control the reaction. Our findings are significant to new level of understanding in methane’s efficient partial photooxidation, which opens a way to control competitive reactions by appropriate band edge positions in photocatalysts to greater selectivity and avoid its overoxidation to CO2.