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Methane is an abundant and relatively clean fossil fuel resource; therefore, its utilization as a chemical feedstock has a major impact on the chemical industry. However, its inert nature makes direct conversion into value-added products difficult under mild conditions. Compared to the gas-phase selective oxidation of methane, there have been several recent advances in the liquid-phase conversion of methane. This review categorizes the reports on the liquid-phase selective oxidation of methane according to the solvent and oxidant used. The advantages and disadvantages of each approach are discussed. High yields of methyl bisulfate as a methanol precursor can be achieved using SO3 in sulfuric acid; however, more attention should be paid to the separation process and overall economic analysis. However, the aqueous-phase selective oxidation of methane with in situ generated H2O2 is quite promising from an environmental point of view, provided that an economical reducing agent can be used. Based on the current state-of-the-art on this topic, directions for future research are proposed. [ABSTRACT FROM AUTHOR]

The direct oxidation of methane to methanol is considered challenging due to the intrinsically low reactivity of the C–H bond of methane and the formation of a large number of unstable intermediates (methanol, formaldehyde, and formic acid) relative to the yield of methane. However, promising advances have recently been reported in this area based on the use of electrochemical systems that differ from traditional thermal catalysis. In this review, the recent advances in direct and indirect electrochemical methane conversion with homogeneous catalysts are reviewed and discussed, especially under low-temperature conditions. Finally, the limitations of the current electrochemical methane conversion technology and future research directions are discussed. [ABSTRACT FROM AUTHOR]

In this study, a Cu-promoted Fe/ZSM-5 catalyst was examined to reveal the effects of Cu species in selective oxidation of methane into methane oxygenates using H2O2 in water. Cu/ZSM-5, Cu-Fe/ZSM-5, and Fe/ZSM-5 catalysts were prepared using wet impregnation, solid-state ion exchange, and ion-exchange methods. Various techniques, including nitrogen physisorption, temperature-programmed reduction with H2, UV-vis spectroscopy, and FT-IR spectroscopy after NO adsorption, were utilized to characterize the catalysts. The promotional effect of Cu on the Cu-Fe/ZSM-5 catalyst in terms of methanol selectivity was confirmed. The preparation method has a considerable influence on the catalyst performance, and the ion-exchange method is the most effective. However, leaching of the Cu species was observed during this reaction, which can affect the quantification of formic acid by 1H-NMR. The homogeneous Cu species increase hydrogen peroxide decomposition and CO2 selectivity, which is undesirable for this reaction. [ABSTRACT FROM AUTHOR]

The effect of halide ion on the catalytic activity for the aqueous-phase partial oxidation of methane with H 2 O 2 generated in situ over Pd/C was examined. Various halide ions, including F-, Cl-, Br-, and I- were compared. Among them, Br- and I- are effective for the synthesis of H 2 O 2 from H 2 and O 2 , resulting in the synthesis of methane oxygenates from methane over Fe/ZSM-5. They show different product yields as a function of their concentrations. The highest total product yields were obtained with [KBr] = 5 mM and [KI] = 0.05 mM, respectively. Compared with Br-, the highest product yield was obtained with a much lower concentration of I-, guaranteeing that no detectable leaching of metal from Fe/ZSM-5 and Pd/C was found. The total productivity was 4.64 mmol/g cat. /h over Fe/ZSM-5 at 50 ℃ with H 2 O 2 generated in situ over Pd/C from H 2 and O 2 in the presence of 0.05 mM of KI. [Display omitted] • Partial oxidation of CH 4 was examined over Fe/ZSM-5 with in situ generated H 2 O 2. • The effect of halide anion was investigated over Pd/C. • Br- and I- are effective for the synthesis of methane oxygenates from methane. • The high productivity can be achieved in the presence of KI without metal leaching. [ABSTRACT FROM AUTHOR]

Direct conversion of methane into value‐added chemicals is a challenging but worthwhile subject. In this communication, the direct conversion of methane into methane oxygenates was achieved in an aqueous solution at room temperature using iron salts and Pd/C as catalysts and hydrogen peroxide as an oxidant. The hydrogen peroxide can be directly added or generated in situ from hydrogen and oxygen. The Pd/C catalyst greatly enhanced the reaction rate together with the iron salts. The effect of some parameters such as reaction temperature, reaction time, pH, acid type, and catalyst amount on the yields of the methane oxygenates was also investigated. When hydrogen peroxide was directly added, the turnover frequency (TOF), defined as the moles of methane oxygenates per moles of Fe per unit time, at 293 K was 29 h−1 at pH=2.3. The TOF at 293 K was 42 h−1 at pH=1.3 with in situ generated hydrogen peroxide from hydrogen and oxygen. [ABSTRACT FROM AUTHOR]

Methane has been reported to be directly converted into value-added products through various methods. Among them, photoelectrochemical (PEC) methane conversion is considered an eco-friendly method because it utilizes solar light and is able to control the selectivity to different products by means of application of an external bias. Recently, some PEC methane conversion systems have been reported, but their performance efficiencies are relatively lower than those of other existing thermal, photocatalytic, and electrochemical systems. The detailed mechanism of methane activation is not clear at this stage. In this review, various catalytic materials and their roles in the reaction pathways are summarized and discussed. Furthermore, promising semiconductor materials, co-catalysts, and oxidants have also been proposed. Finally, direct and indirect pathways in the design of the PEC methane conversion system have been discussed. [ABSTRACT FROM AUTHOR]

Copper- and vanadium-catalyzed oxidation of methane into methyl trifluoroacetate and formic acid with in situ generated H2O2 from H2 and O2 over Pd/C was investigated in trifluoroacetic acid (TFA) and/or trifluoroacetic anhydride (TFAA) as a solvent. Metallic palladium was responsible for the in situ generation of hydrogen peroxide from dihydrogen and dioxygen. Divalent copper hydroperoxide and monoperoxomonovanadate, VO(O2)+, seemed to be related to C&z.sbnd;H bond activation. The radical chain reaction did not occur in either copper- or vanadium-catalyzed methane oxidation. However, methyl radical appeared to be produced in vanadium system but not in copper system. [Copyright &y& Elsevier]

Liquid-phase selective oxidation of methane into methane oxygenates, including methanol and formic acid, with molecular oxygen was investigated using Fe-zeolites and Pd/activated carbon in the presence of molecular hydrogen as a reducing agent. Various Fe-zeolites such as Fe-ZSM-5, Fe-mordenite, Fe-β, Fe-Y, and Fe-ferrierite were prepared by ion-exchange and compared for this reaction. Among them, Fe-ZSM-5 was selected for further study because this catalyst showed high activity in the selective oxidation of methane with relatively less leaching. Further, the effect of reaction temperature, pH, and the amount of catalyst was examined, and detailed investigations revealed that the leached Fe species, which were facilitated in the presence of acid, were mainly responsible for methane oxidation under the given reaction conditions. [ABSTRACT FROM AUTHOR]

Front Cover: Aqueous-Phase Selective Oxidation of Methane with Oxygen over Iron Salts and Pd/C in the Presence of Hydrogen (ChemCatChem 17/2019) Shows the catalyst system composed of homogeneous iron salt and heterogeneous Pd/C which are responsible for aqueous-phase methane oxidation with oxygen in the presence of hydrogen even at low temperatures near 0 °C. When hydrogen peroxide was directly added, the turnover frequency (TOF), defined as the moles of methane oxygenates per moles of Fe per unit time, at 293 K was 29 h SP -1 sp at pH = 2.3. [Extracted from the article]

[Display omitted] • Aqueous-phase partial oxidation of CH 4 with H 2 O 2 was examined over Fe-ZSM-5. • The effect of the catalyst preparation method for Fe-ZSM-5 was investigated. • Ion-exchange method is the most effective for the catalytic activity. • Isolated or oligonuclear extra-framework Fe species are proved to be most active. • The optimum Fe content exists to maximize the total product yield per consumed H 2 O 2. The effect of the catalyst preparation method on the catalytic activity of Fe-ZSM-5 for the partial oxidation of methane with H 2 O 2 in an aqueous phase was examined. UV–vis spectroscopy and FT-IR spectroscopy after NO adsorption were used to characterize the prepared catalysts. Various preparation methods, including wet impregnation, solid-state ion-exchange, chemical vapor impregnation, hydrothermal synthesis, and aqueous phase ion-exchange were compared. Among them, the aqueous phase ion-exchange method demonstrated the best results in producing isolated or oligonuclear extra-framework Fe species, which showed the highest catalytic activity. On the other hand, bulk iron oxides did not decompose H 2 O 2 nor did they activate methane. The total product yield and the amount of H 2 O 2 consumed increased with increasing Fe content in the case of the Fe-ZSM-5 catalyst prepared using an ion-exchange method. However, the fraction of less active Fe species, such as iron oxide clusters and larger iron oxide aggregates on the external crystal surface, increased with the Fe content, resulting in a decrease in the total product yield per Fe content and the amount of H 2 O 2 consumed per Fe content. [ABSTRACT FROM AUTHOR]