Methane Oxidation by Aqueous Osmium Tetroxide and Sodium Periodate: Inhibition of Methanol Oxidation by Methane

AgII -Mediated Electrocatalytic Ambient CH4 Functionalization Inspired by HSAB Theory

Although most class (b) transition metals have been studied in regard to CH₄ activation, divalent silver (Ag^II ), possibly owing to its reactive nature, is the only class (b) high-valent transition metal center that is not yet reported to exhibit reactivities towards CH₄ activation. We now report that electrochemically generated Ag^II metalloradical readily functionalizes CH₄ into methyl bisulfate (CH₃ OSO₃ H) at ambient conditions in 98 % H₂ SO₄ . Mechanistic investigation experimentally unveils a low activation energy of 13.1 kcal mol^-1 , a high pseudo-first-order rate constant of CH₄ activation up to 2.8×10³  h^-1 at room temperature and a CH₄ pressure of 85 psi, and two competing reaction pathways preferable towards CH₄ activation over solvent oxidation. Reaction kinetic data suggest a Faradaic efficiency exceeding 99 % beyond 180 psi CH₄ at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.

Kinetic UV-Vis Spectroscopic and DFT Mechanistic Study of the Redox Reaction of [OsVIIIO4(OH)n]n- (n = 1, 2) and Methanol in a Basic Aqueous Matrix

This combined experimental and computational study builds on our previous studies to elucidate the reaction mechanism of methanol oxidation by Os^VIII oxido/hydroxido species (in basic aqueous media) while accounting for the simultaneous formation of Os^VII species via a comproportionation reaction between Os^VIII and Os^VI. UV-Vis spectroscopy kinetic analyses with either CH₃OH or the deuterated analogue CD₃OH as a reducing agent revealed that transfer of α-carbon-hydrogen of methanol is the partial rate-limiting step. The resulting relatively large KIE value of approximately 11.82 is a combination of primary and secondary isotope effects. The Eyring plots for the oxidation of these isotopologues of methanol under the same reaction conditions are parallel to each other and hence have the same activation enthalpy [Δ^⧧H° = 14.4 ± 1.2 kcal mol^-1 (CH₃OH) and 14.5 ± 1.3 kcal mol^-1 (CD₃OH)] but lowered activation entropy (Δ^⧧S°) from -12.5 ± 4.1 cal mol^-1 K^-1 (CH₃OH) to -17.1 ± 4.4 cal mol^-1 K^-1 (CD₃OH). DFT computational studies at the PBE-D3 level with QZ4P (Os) and pVQZ (O and H) basis sets provide clear evidence to support the data and interpretations derived from the experimental kinetic work. Comparative DFT mechanistic investigations in a simulated aqueous phase (COSMO) indicate that methanol and Os^VIII first associate to form a noncovalent adduct bound together by intermolecular H-bonding interactions. This is followed by spin-forbidden α-carbon-hydrogen transfer (not O-H transfer) from methanol to Os^VIII by means of HAT, which is found to be the partial rate-limiting step. Without the organic and inorganic fragments dissociating from each other during the entire stepwise redox reaction (in order to avoid formation of highly energetically unfavorable monomer species), the HAT step is followed by PT and then ET before the final product monomers formaldehyde and Os^VI dissociate from each other. DFT-calculated Δ^⧧H° is within 5 kcal mol^-1 of the experimentally obtained value, while the DFT Δ^⧧S° is three times larger than that found from the experiment.

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 H(2)O(2) 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 C(1)-oxidation to proceed under intrinsically mild reaction conditions.