Oxidative coupling of methane (OCM) is an attractive direct route for upgrading methane to valuable chemicals. In this study, temporal analysis of products (TAP) and steady-state experiments are conducted to understand the role of individual oxide phases and their combinations in supported Mn–Na2WO4/SiO2 catalysts for OCM. The results from TAP transient kinetic studies indicate that Mn plays an important role in promoting gas-phase oxygen activation, while NaO x /SiO2 and WO x /SiO2 are relatively inert toward gas-phase oxygen and methane activation. However, the supported catalyst combining Na and W in the form of Na2WO4 shows enhanced gas-phase oxygen activation, exhibiting a much lower oxygen activation energy (148 kJ/mol) and enhanced activity toward methane activation as compared to the individual supported oxide catalysts. The addition of Mn to Na2WO4/SiO2 further decreases the oxygen activation energy by 40 kJ/mol. Moreover, methane activation is also enhanced with CH3 as the main intermediate, but with increasing Mn content, more CH2 intermediates are observed. Different forms of oxygen (both dioxygen and atomic) are detected on the catalyst surface using isotopic pump/probe pulsing and their distribution is found to depend on the catalyst composition. An optimal Mn content in the Na2WO4/SiO2 catalyst system is needed to enhance the amount of dioxide surface species (e.g., superoxide 16O2 – or peroxide 16O2 2–) associated with Na2WO4, leading to high C2 selectivity for OCM. When the Mn content is too high, the larger MnO x domains are shown to contribute to the formation of higher concentrations of monoxide surface species that lead to nonselective OCM pathways. This insight from transient kinetic characterization using TAP combined with conventional steady-state studies provides a deeper understanding of the role of individual oxide phases and their combination on supported catalysts toward the formation of intermediate surface species and their impact on the OCM reaction mechanism. This knowledge is critical for designing superior catalyst formulations for OCM.
Irradiation of [Ru(bpy)2(bpSOp)](PF6)2 (where bpy is 2,2'-bipyridine and bpSOp is 1,3-bis(phenylsulfinyl)propane) results in the formation of two new isomers, namely the S,O- and O,O-bonded species. The crystal structure of the bis-thioether and bis-sulfoxide complexes are reported. NMR spectroscopy of the bis-thioether complex in solution is consistent with the molecular structure determined by diffraction methods. Further, NMR spectroscopy of the bis-sulfoxide complex reveals two conformers in solution, one that is consistent with the solid state structure and a second conformer showing distortion in the aliphatic portion of the chelate ring. Time-resolved visible absorption spectroscopy reveals isomerization time constants of 91 ps in dichloroethane (DCE) and 229 ps in propylene carbonate (PC). Aggregate isomerization quantum yields of 0.57 and 0.42 have been determined in DCE and in PC, respectively. The kinetics of the thermal reversion from the O,O- to S,O-bonded isomer are strongly solvent dependent, occurring with rates of 2.41 × 10(-3) and 4.39 × 10(-5) s(-1) in DCE, and 4.68 × 10(-4) and 9.79 × 10(-6) s(-1) in PC. The two kinetic components are assigned to the two isomers identified in solution.
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