Catalytic and Mechanistic Insights of the Low‐Temperature Selective Oxidation of Methane over Cu‐Promoted Fe‐ZSM‐5

The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h^?1), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron‐ and copper‐containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h^?1 by using H₂O₂ as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by‐production of hydroxyl radicals. The catalytic system in question results in a low‐energy methane activation route, and allows selective C₁‐oxidation to proceed under intrinsically mild reaction conditions.     

Methyl chloride production from methane over lanthanum-based catalysts

The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl3), and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species by adsorbing dissociatively and forming OCl surface species, which can serve as an active site for methane activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine from -1 to +1 without any changes in the oxidation state of the underlying metal.     

Direct vapor phase propylene epoxidation over deposition-precipitation gold-titania catalysts in the Presence of H2/O2: Effects of support, neutralizing agent, and pretreatment

The effects of titanium connectivity, deposition solution neutralizing agent, and catalyst pretreatment were examined for a series of Au-on-titanium-containing supports for the direct gas-phase epoxidation of propylene using hydrogen and oxygen. The degree of titanium isolation was examined using pure titania, monolayer-titania on silica, submonolayer-titania on silica, and titanium silicalite-1 (TS-1) supports. Activity and selectivity were shown to increase as the degree of titanium isolation increased, with TS-1 and submonolayer-titania supports providing the best stability and yield. Isolation of the titanium was found to significantly reduce the cracking of propylene to ethanal and carbon dioxide. Sodium carbonate was found to be the best neutralizing agent for catalysts prepared using deposition-precipitation (DP). DP with ammonium hydroxide gave catalysts with reduced selectivity and activity. Titania-modified silica was found to produce better catalysts when the support was not calcined prior to gold deposition. Similarly, calcination was detrimental to catalysts prepared via deposition of a 2 nm gold colloid onto titania-modified supports even though the gold did not sinter. The beneficial effects of Ti site isolation and support acid/base control are best seen at higher temperatures, where only a few catalysts can maintain selectivity.