Search

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 selective oxidation of methane into methane oxygenates such as CH3OH and HCOOH with O2 is a promising technology for direct methane conversion. This study successfully demonstrates the partial oxidation of methane over Pd−Fe/ZSM‐5 catalysts in water, where H2O2 was generated in situ from H2 and O2. A low Pd content is essential for preventing agglomeration of Pd nanoparticles, and intimate contact between Pd and Fe species is beneficial for achieving high yields of methane oxygenates. This one‐body catalyst showed better catalytic performance than that of previously reported ones, especially at low temperatures. 0.28 %Pd–0.56 %Fe/ZSM‐5 exhibited a stable catalytic performance in a continuous flow reactor. [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]

The selective oxidation of methane into methane oxygenates such as CH3OH and HCOOH with O2 is a promising technology for direct methane conversion. This study successfully demonstrates the partial oxidation of methane over Pd−Fe/ZSM‐5 catalysts in water, where H2O2 was generated in situ from H2 and O2. A low Pd content is essential for preventing agglomeration of Pd nanoparticles, and intimate contact between Pd and Fe species is beneficial for achieving high yields of methane oxygenates. This one‐body catalyst showed better catalytic performance than that of previously reported ones, especially at low temperatures. 0.28 %Pd–0.56 %Fe/ZSM‐5 exhibited a stable catalytic performance in a continuous flow reactor. [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]

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]

[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]

• 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]