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Surface hydrophobicity is an important factor in controlling the catalytic activity of heterogeneous catalysts in various reactions, particularly liquid-phase reactions using water as the (co)solvent. In this study, the surface hydrophobicity of Fe-ZSM-5 was successfully controlled using a simple coating method in which furfuryl alcohol was used as the carbon precursor. Various techniques, such as N2 physisorption, temperature-programmed desorption of ammonia, and contact angle measurements of water droplets, were used to characterize the catalysts. Fe-ZSM-5 catalysts with different degrees of hydrophobicity were used for the aqueous-phase selective oxidation of methane with H2O2. The positive effect of the surface carbon coating on the catalytic performance was confirmed when the carbon content was not sufficiently high to block the pores. [ABSTRACT FROM AUTHOR]

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 dry reforming of methane (DRM) is a promising method for controlling greenhouse gas emissions by converting CO2 and CH4 into syngas, a mixture of CO and H2. Ni-based catalysts have been intensively investigated for their use in the DRM. However, they are limited by the formation of carbonaceous materials on their surfaces. In this review, we explore carbon-induced catalyst deactivation mechanisms and summarize the recent research progress in controlling and mitigating carbon deposition by developing coke-resistant Ni-based catalysts. This review emphasizes the significance of support, alloy, and catalyst structural strategies, and the importance of comprehending the interactions between catalyst components to achieve improved catalytic performance and stability. [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]

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]

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]

Fe-ZSM-5 is an active catalyst for the selective oxidation of methane with H 2 O 2 in liquid water. To prepare the active Fe-ZSM-5 catalyst, the effect of the Fe precursor on the catalytic performance was examined. Additionally, two preparation methods, wet impregnation (WI) and ion-exchange (IE), were also applied. The prepared catalysts were characterized using various techniques, such as nitrogen physisorption, X-ray diffraction, UV–Vis spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy after NO adsorption (NO-FTIR). Fe-ZSM-5 prepared from FeCl 2 appears to be the most active among the Fe-ZSM-5 catalysts prepared from FeCl 2 , FeSO 4 , Fe(CH 3 CO 2) 2 , FeCl 3 , Fe 2 (SO 4) 3 , Fe(C 5 H 7 O 2) 3 , and Fe(NO 3) 3 using the WI method. For Fe-ZSM-5 catalysts prepared using the IE method, divalent Fe precursors, such as FeSO 4 , FeCl 2 , and Fe(CH 3 CO 2) 2 , are better than trivalent Fe precursors, such as Fe 2 (SO 4) 3 , FeCl 3 , and Fe(NO 3) 3 , in terms of catalytic activity. The active catalyst has the dominant extra-framework Fe2+ species probed with UV–Vis spectroscopy and NO-FTIR. [Display omitted] • Partial oxidation of CH 4 with H 2 O 2 in water was examined over Fe-ZSM-5. • The effect of Fe precursor during the catalyst preparation was investigated. • The active sites were probed with FT-IR spectroscopy after NO adsorption. • Correlation was made between the activity and the peak intensity in NO-FTIR spectra. [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]

The oxidative coupling of methane (OCM) is operated at high temperatures and is a highly exothermic reaction; thus, hotspots form on the catalyst surface during reaction unless the produced heat is removed. It is crucial to control the heat formed because surface hotspots can degrade catalytic performance. Herein, we report the preparation of Mn2O3-Na2WO4/SiC catalysts using SiC, which has high thermal conductivity and good stability at high temperatures, and the catalyst was applied to the OCM. Two Mn2O3-Na2WO4/SiC catalysts were prepared by wet-impregnation on SiC supports having different particle sizes. For comparison, the Mn2O3-Na2WO4/SiO2 catalyst was also prepared by the same method. The catalysts were analyzed by nitrogen adsorption–desorption, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The transformation of SiC into α-cristobalite was observed for the Mn2O3-Na2WO4/SiC catalysts. Because SiC was completely converted into α-cristobalite for the nano-sized SiC-supported Mn2O3-Na2WO4 catalyst, the catalytic performance for the OCM reaction of Mn2O3-Na2WO4/n-SiC was similar to that of Mn2O3-Na2WO4/SiO2. However, only the surface layer of SiC was transformed into α-cristobalite for the micro-sized SiC (m-SiC) in Mn2O3-Na2WO4/m-SiC, resulting in a SiC@α-cristobalite core–shell structure. The Mn2O3-Na2WO4/m-SiC showed higher methane conversion and C2+ yield at 800 and 850 °C than Mn2O3-Na2WO4/SiO2. [ABSTRACT FROM AUTHOR]

Based on previous observation that the addition of manganese onto Ni/Al2O3 catalysts reduced the coke formation in the carbon dioxide reforming of methane, effects of the preparation method for Mn-promoted Ni/Al2O3 catalysts on the catalytic activity and stability were investigated. A coprecipitated catalyst, Ni–MnOx/MnAl2O4, showed higher coke resistance and more stable activity than an impregnated Ni/MnO/γ-Al2O3 catalyst and the well-known Ni0.03Mg0.97O catalyst at 923 K with a feed gas ratio CH4/CO2 of 1 without a diluent gas. In coprecipitated Ni–MnOx/MnAl2O4 catalysts, MnAl2O4, γ-Al2O3, and Ni(0) were observed as major crystalline phases with XRD and potassium was detected on the surface with EPMA. Comparison of H2 chemisorption and XRD suggested that the surface of large metallic nickel particles was partly blocked by manganese oxides, and the structure was confirmed by XAFS. For impregnated Ni/MnO/γ-Al2O3 catalysts, addition of potassium stabilized its catalytic activity further, to the level achieved by coprecipitated Ni–MnOx/MnAl2O4 catalysts. These results indicate that very stable Ni/Al2O3 catalysts for the carbon dioxide reforming of methane can be prepared by addition of both potassium and manganese as promoters. [Copyright &y& Elsevier]

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]

We report the barium zirconate supported NiO x catalysts (NiO x /BZO) for dry reforming of methane and feasibility test for their industrial application by using temperature-regulated chemical vapor deposition together with extrusion. Nickel oxide nanoparticles are well deposited and dispersed on BZO powders as well as structures in a gear form, showing high catalytic activity and extraordinary stability even at relatively lower temperature and higher space velocity. The NiO x /BZO catalyst are highly coke resistant for 50 h operation with almost negligible agglomeration of NiO x nanoparticles. Judging from XPS and high ion conducting properties of BZO supports, the –OH or –O species are expected to play an important role in promoting the self-decoking of surface carbon species to form CO and CO 2. [ABSTRACT FROM AUTHOR]

• CH 4 ex-situ decomposition supplied large amounts of H 2 than CH 4 or N 2 environment. • The 10Ni/5La 2 O 3 -5CeO 2 -Al 2 O 3 catalyst was used for CH 4 decomposition at 800 °C. • Yellow poplar vapors were upgraded over 1 wt% Ga/HZSM-5. • CH 4 ex-situ decomposition enhanced aromatics yield than CH 4 or N 2 environment. • CH 4 ex-situ decomposition can replace expensive reduction process of Ga/HZSM-5. The present study examined the effects of the pyrolysis environment on BTEX (benzene, toluene, ethylbenzene, and xylenes) production in the catalytic upgrading of yellow poplar pyrolysis vapors. Three different gas environments, N 2 , CH 4 , and pre-decomposed CH 4 stream (10 wt%-Ni/5 wt%-La 2 O 3 -5 wt% CeO 2 -Al 2 O 3), which is a mixture of H 2 (55.62%) and CH 4 , were studied using two types of zeolite catalysts, HZSM-5, and 1 wt% Ga/HZSM-5. The BTEX yields were enhanced linearly in the order N 2 < CH 4 < CH 4 ex-situ decomposition. The highest BTEX yield of 9.58 wt% was obtained under the CH 4 ex-situ decomposition environment over 1 wt% Ga/HZSM-5. The methane and hydrocarbons derived from biomass were activated on highly dispersed (GaO)+ sites and transformed smoothly to BTEX by aromatization on the BrØnsted acid sites of Ga/HZSM-5. The hydrogen produced from methane decomposition also assisted in aromatics production through the hydrodeoxygenation of methoxyphenols, guaiacols and catechols. [ABSTRACT FROM AUTHOR]

• Liquid-phase partial oxidation of methane with O 2 was achieved in the presence of H 2. • Pd supported on acid-functionalized porous polymer was used for in situ H 2 O 2 generation. • Fe-ZSM-5 was stable and active for methane oxidation with in situ H 2 O 2. • The combination of these two catalysts showed the TOF of 3.8 h−1 at 30 ℃. Aqueous-phase partial oxidation of methane over a combination of heterogeneous Fe-ZSM-5 and Pd catalysts was demonstrated in the presence of O 2 and H 2. Fe-ZSM-5 catalyst active for partial oxidation of methane with H 2 O 2 was prepared by an ion-exchange method. For comparison, homogeneous Fe species (FeSO 4) were also used as a catalyst for partial oxidation of methane. Several supported Pd catalysts for the direct synthesis of hydrogen peroxide from H 2 and O 2 were prepared using activated carbon (AC), AC treated with sulfuric acid (s-AC), as-synthesized hyper-crosslinked porous polymer (HCPP), HCPP with carboxylic group (c−HCPP), and HCPP with carboxylic and sulfonic groups (c-s−HCPP). Among them, the Pd catalyst supported on c-s−HCPP was effective for generating H 2 O 2 in situ , which enabled methane oxidation over Fe-ZSM-5 without any leaching. [ABSTRACT FROM AUTHOR]