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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]

Methane is an abundant resource and its direct conversion into value-added chemicals has been an attractive subject for its efficient utilization. This method can be more efficient than the present energy-intensive indirect conversion of methane via syngas, a mixture of CO and H2. Among the various approaches for direct methane conversion, the selective oxidation of methane into methane oxygenates (e.g., methanol and formaldehyde) is particularly promising because it can proceed at low temperatures. Nevertheless, due to low product yields this method is challenging. Compared with the liquid-phase partial oxidation of methane, which frequently demands for strong oxidizing agents in protic solvents, gas-phase selective methane oxidation has some merits, such as the possibility of using oxygen as an oxidant and the ease of scale-up owing to the use of heterogeneous catalysts. Herein, we summarize recent advances in the gas-phase partial oxidation of methane into methane oxygenates, focusing mainly on its conversion into formaldehyde and methanol. [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]

Abstract: We prepared Ru catalysts supported on various aluminum oxides with different crystalline phases, viz. α-Al2O3, κ-Al2O3, γ-Al2O3, η-Al2O3, δ-Al2O3 and θ-Al2O3, by the incipient wetness method and applied them to the selective CO oxidation in a hydrogen-rich stream. For comparison, the complete CO oxidation in the absence of H2 was also carried out over the same catalysts. Ru/α-Al2O3 showed the highest CO conversion among the tested catalysts, especially at low temperatures, irrespective of the presence of H2. For all of the catalysts, the catalytic activity for CO oxidation was much suppressed in the absence of H2 compared with that in the presence of H2. Several techniques: N2 physisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), CO chemisorption, temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of CO2 with mass spectroscopy and transmission electron microscopy (TEM) were employed to characterize the catalysts. The least amount of chemisorbed CO and CO2 was obtained at room temperature over Ru/α-Al2O3. The least amount of O2 was chemisorbed during TPO and the oxidized Ru species could be fully reduced in the presence of hydrogen at the lowest temperature over Ru/α-Al2O3 among the Ru/Al2O3 catalysts. Ru/α-Al2O3 can reduce a high inlet concentration of CO to less than 10ppm even in the presence of H2O and CO2 over a wide temperature range. [Copyright &y& Elsevier]

Abstract: We prepared various Ru catalysts supported on different supports such as yttria-stabilized zirconia (YSZ), ZrO2, TiO2, SiO2 and γ-Al2O3 with a wet impregnation method. We applied them to the selective CO removal in a hydrogen-rich stream via the preferential CO oxidation (PROX) and the selective CO methanation simultaneously. Among them, Ru/YSZ showed the highest CO conversion especially at low temperatures. Several measurements: the N2 physisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), the CO chemisorptions, the temperature-programmed oxidation (TPO), the temperature-programmed reduction (TPR), the temperature-programmed desorption (TPD) of CO2 with mass spectroscopy and the transmission electron microscopy (TEM), were conducted to characterize the catalysts. No linear correlation can be found between the amount of CO chemisorbed at 300K and the PROX activity. On the other hand, the facile activation of O2 appeared to be closely related to the high PROX activity, judging by the TPO experiment. In addition, the strong adsorption of CO2 suppressed the low-temperature PROX activity. Ru/YSZ can be easily oxidized and also reduced at low temperatures. It is found that Ru/YSZ uptakes only small amounts of CO2, which can be desorbed at low temperatures. Ru/YSZ can reduce the high inlet CO concentration to be less than 10ppm even in the presence of H2O and CO2. [Copyright &y& Elsevier]

Abstract: We conducted a comparative study for the preferential CO oxidation (PROX) over supported noble metal catalysts. The CO chemisorption in the absence and presence of H2, the O2 chemisorption, the temperature-programmed oxidation (TPO), the temperature-programmed reduction (TPR) after O2 chemisorption and the transmission electron microscopy (TEM) were conducted to characterize the catalysts. Commercial catalysts such as 1wt.% Pt/γ-Al2O3, 0.5wt.% Ru/γ-Al2O3, 0.5wt.% Rh/γ-Al2O3, 5wt.% Pt/γ-Al2O3, 5wt.% Ru/γ-Al2O3, and 5wt.% Rh/γ-Al2O3 were utilized. Among them, 5wt.% Ru/γ-Al2O3 showed the highest PROX activity. This catalyst can be considered to be promising because it can reduce the high inlet CO concentration to be less than 10ppm at low temperatures where the reverse water–gas shift reaction can be minimized. No detectable amount of chemisorbed O2 was measured at 373K over 1wt.% Pt/γ-Al2O3, 0.5wt.% Rh/γ-Al2O3 and 5wt.% Pt/γ-Al2O3. The moderate degree of CO and O2 chemisorption at reaction temperatures appeared to be essential for the optimum PROX activity. Although 0.5wt.% Ru/γ-Al2O3 and 5wt.% Ru/γ-Al2O3 have the similar particle size distribution based on TEM analysis, the latter catalyst with smaller amount of chemisorbed CO and O2 per Ru metal showed much better PROX activity. Based on TPO and TPR result, 0.5wt.% Ru/γ-Al2O3 can be oxidized at lower temperatures but can be reduced at higher temperatures compared with 5wt.% Ru/γ-Al2O3. Therefore, the easily reducible Ru species can give us the better PROX activity over a wide reaction temperature for Ru catalysts. [Copyright &y& Elsevier]

Abstract: The catalytic stability of MoO3/SiO2 and TiO2/SiO2, which have been reported to be active for transesterification between dimethyl carbonate and phenol, was examined in a batch reactor for the same reaction. X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and the temperature programmed oxidation (TPO) were conducted to characterize the catalysts. The gradual decrease in the activity was observed over both catalysts with increasing the number of a batch reaction. The strong adsorption of reactants was observed from the temperature programmed oxidation experiment. The deactivated MoO3/SiO2 can be regenerated after oxidation in air at 823K. However, the deactivated TiO2/SiO2 can be only partially regenerated even after oxidation in air at 973K. Some of the Mo species in MoO3/SiO2 were leached out during a reaction. This dissolved homogeneous Mo species appeared to take part in this reaction. [Copyright &y& Elsevier]

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]

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]

Graphical abstract Highlights • A new core@shell catalyst is developed for partial methane oxidation. • SiO 2 @V 2 O 5 @Al 2 O 3 nanostructure is an efficient and thermally stable catalyst. • The thickness of Al 2 O 3 shells over SiO 2 @V 2 O 5 cores can be controlled. • SiO 2 @V 2 O 5 @Al 2 O 3 -(50) nanostructures feature the best catalytic activity at 600 °C. • A highly disperse vanadium species is formed. Abstract The stable tetrahedral geometry and high C H bond dissociation energy of methane complicate its direct catalytic conversion; for example, the selective oxidation of methane to formaldehyde, which avoids the production of carbon dioxide by full oxidation and is therefore important for the versatile utilization of natural gas, is still viewed as challenging. Here, we utilize hydrothermal synthesis followed by atomic layer deposition (ALD) to prepare an efficient and thermally stable catalyst based on novel SiO 2 @V 2 O 5 @Al 2 O 3 core@shell nanostructures, showing that the thickness of Al 2 O 3 shells over SiO 2 @V 2 O 5 cores can be tuned by controlling the number of ALD cycles. Catalytic methane oxidation experiments performed in a flow reactor at 600 °C demonstrate that SiO 2 @V 2 O 5 @Al 2 O 3 nanostructures obtained after 50 ALD cycles exhibit the best catalytic activity (methane conversion = 22.2%; formaldehyde selectivity = 57.8%) and outperform all previously reported vanadium-based catalysts at 600 °C. The prepared catalysts are subjected to in-depth characterization, which reveals that their Al 2 O 3 shell provides new surfaces for the generation of highly disperse T d monomeric species with a V O Al bond by promoting interactions between Al 2 O 3 and V 2 O 5 nanoparticles during ALD. Moreover, the surface Al 2 O 3 shell is found not only to protect V 2 O 5 nanoparticles against sintering at 600 °C, but also to anchor the produced T d monomeric vanadium species responsible for the high catalytic performance. [ABSTRACT FROM AUTHOR]

Abstract: We propose an activation method to control Ru particle size in supported Ru catalysts, viz. commercial Ru/Al2O3 and home-made Ru/SiO2, in order to increase the catalytic activity for the preferential CO oxidation (PROX). Ru particle size is controlled by adjusting pre-treatment conditions and it affects the catalytic activity for the PROX over supported Ru catalysts. Several measurements: inductively coupled plasma-atomic emission spectroscopy (ICP-AES), bright-field transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), CO chemisorption, and O2 chemisorption were conducted to characterize the catalysts. The co-presence of H2 and O2 is essential for controlling the Ru particle size accurately. The PROX activity especially at low temperatures increases with increasing particle size of Ru, which seems to be closely related to the adsorption behavior of O2 on Ru surface. [Copyright &y& Elsevier]