11-Molybdo-1-vanadophosphoricacid H4PMo11VO40 supported on ammonia-modified silica as highly active and selective catalyst for oxidation of methacrolein

An efficient photocatalyst based on H5PMo10V2O40/UiO-66-NH2 for direct hydroxylation of benzene to phenol by H2O2

To realize the direct hydroxylation of benzene to phenol by hydrogen peroxide, an efficient photoactive catalyst system was prepared by the recombination of H₅PMo₁₀V₂O₄₀ and UiO-66-NH₂. The heterpolyacid was uniformly distributed on the UiO-66-NH₂, and the combination was stable. The composite could effectively photocatalyze the direct hydroxylation of benzene to phenol by H₂O₂ in the mixture solution of acetonitrile and acetic acid. The yield and selectivity were 14.08% and 98.8% under the optimum condition, respectively. The performance of the catalyst still maintained well after 5 catalytic cycles. Hence, the investigated catalyst system might be applied in the field of hydroxylation of benzene to phenol.

Elucidating the role of H2O in promoting the formation of methacrylic acid during the oxidation of methacrolein over heteropolyacid compounds

The involvement of water in the selective oxidation of MAL to MAA over a pure Keggin-type H₃PMoO₁₂O₄₀ catalyst was investigated using an in situ DRIFTS reactor coupled with a mass spectrometer for the first time to elucidate the reaction pathway associated with water. Comparing the spectra and activity data using D₂O instead of H₂O during transient switching experiments has allowed us to evaluate the possible active sites where D₂O is activated. It has been found that, during the cycling switches of D₂O in and out of the MAL + O₂ gas feed at 320 °C, the formation of MAA-OD product is increased and decreased when D₂O is added and removed, respectively. This suggests that the deuterium from D₂O is involved in the production of gas phase MAA-OD. In addition, the in situ DRIFTS-MS results obtained from the isotopic switches between D₂O and H₂O reveal changes in the characteristic infrared bands of the Keggin unit between 1200 and 600 cm^-1. It is found that the isotopic exchange possibly occurs on the bridging oxygen of Mo-O-Mo unit, where water is activated for the formation of MAA. Based on the in situ DRIFTS-MS analysis from the transient switching experiments, the reaction mechanism associated with the effect of water on the selective oxidation of MAL to MAA over Keggin-type H₃PMoO₁₂O₄₀ catalyst is proposed.

Biodiesel Production from Low-Quality Oils Using Heterogeneous Cesium Salts of Vanadium-Substituted Polyoxometalate Acid Catalyst

This research aims at developing an efficient and reusable catalyst to improve biodiesel production processes. To achieve this, a vanadium-substituted polyoxometalate (POM) acid, namely H6PV3MoW8O40, was firstly prepared, and then the heterogenzation of the homogeneous Keggin-type heteropoly acids was performed by the partial proton substitution by monovalent large cesium cations with the formation of solid Cs2H4PV3MoW8O40 catalysts. Several techniques, such as X-ray diffractometer, Fourier transform infrared, coupled plasma–atomic emission spectrometry, Diffuse reflectance ultraviolet–visible spectrum, thermal gravimetric analysis and N2 adsorption–desorption techniques, were employed to characterize the as-prepared solid catalyst. The solid acid catalyst had the capacity to catalyze both the transesterification of soybean oil and esterification of free fatty acids (FFAs) simultaneously, providing an efficient production process for the production of biodiesel from low-quality oils. Under the operational conditions of a methanol/oil molar ratio of 30:1, a catalyst dosage of 5 wt.%, a reaction temperature of 140 °C, and a reaction duration of 8 h, an oil conversion of 92.2% was attained with the total FFA transformation to biodiesel. Furthermore, the catalyst could be reutilized for several cycles with no significant drop in its activity, thus having great potential for application with a bright perspective in the production of biodiesel, especially from low-quality oil feedstocks.

Comparison in thermal stability and catalytic performance of H4PMo11VO40 heteropolyacid supported on mesoporous and macroporous silica materials

Ordered mesoporous silica, SBA-15 and MCM-41, and three-dimensionally ordered macroporous SiO2 were used as the supports of H4PMo11VO40 heteropolyacid for methacrolein oxidation. The dispersion and structural evolutions of the heteropolyacid along with thermal treatment were investigated. It was found that the heteropolyacid entered the one-dimensional mesoporous channels of SBA-15 and MCM-41, and the crystallization and growth were limited, leading to high dispersion of the heteropolyacid. However, the thermal stability was decreased under high dispersion. The migration of the heteropolyacid was observed to the end of the one-dimensional channels of SBA-15 and the outer surface of MCM-41 with calcination, accompanied by the decomposition of the heteropolyacid and the formation of MoO3. In comparison, the crystallization and growth of heteropolyacid were not limited in the open macropores of three-dimensionally ordered macroporous SiO2. Dispersed particles on the surface of the macropores with size of about 5 nm exhibited a higher thermal stability. The decomposition of the heteropolyacid in the SBA-15 and MCM-41 supported catalysts resulted in the loss of strong acid sites, causing low selectivity to methacrylic acid in methacrolein oxidation. High thermal stability with high exposure of the active sites in the three-dimensionally ordered macroporous SiO2 supported catalyst contributed to the enhancement in the catalytic performance.

Selective oxidation of methacrolein to methacrylic acid over H4PMo11VO40/C3N4-SBA-15

Molybdovanadylphosphoric acid (HPMV) was supported on a carbon nitride-modified SBA-15 (CN-SBA-15) molecular sieve to enhance its catalytic performance for oxidation of methacrolein (MAL) to methacrylic acid (MAA). HPMV/CN-SBA exhibited increased catalytic activity (20%) and five times greater MAA selectivity (98.9%) compared to bulk HPMV. HPMV supported on CN-SBA-15 exhibited much better catalytic performance as compared to that on other supports, such as KIT-6, HY zeolite, TiO2, Al2O3, SiO2, CNTs, and NH3-modified CNTs. The supported HPMV was well characterized by FT-IR, XRD, SEM, N2 physical desorption, TG-DTA, NH3-TPD, CO2-TPD, XPS, and solid-state NMR. The CN minimized the interaction between the silica support and HPMV. HPMV was successfully separated from SBA-15, which was restricted by CN to increase stability and prevent interaction between the catalysts and support that would lead to decomposition of the catalysts during calcination and reaction. HPMV reacted with amino groups on the CN, which improved MAA selectivity and enhanced the thermal stability of the supported heteropoly acid (HPA) catalysts. This work identifies a new approach to preparing highly efficient and stable supported HPA catalysts for oxidation reactions.

Low-temperature catalytic oxidative coupling of methane in an electric field over a Ce-W-O catalyst system

We examined oxidative coupling of methane (OCM) over various Ce–W–O catalysts at 423 K in an electric field. Ce₂(WO₄)₃/CeO₂ catalyst showed high OCM activity. In a periodic operation test over Ce₂(WO₄)₃/CeO₂ catalyst, C₂ selectivity exceeded 60% during three redox cycles. However, Ce₂(WO₄)₃/CeO₂ catalyst without the electric field showed low activity, even at 1073 K: CH₄ Conv., 6.0%; C₂ Sel., 2.1%. A synergetic effect between the Ce₂(WO₄)₃ structure and electric field created the reactive oxygen species for selective oxidation of methane. Results of XAFS, in-situ Raman and periodic operation tests demonstrated that OCM occurred as the lattice oxygen in Ce₂(WO₄)₃ (short W–O bonds in distorted WO₄ unit) was consumed. The consumed oxygen was reproduced by a redox mechanism in the electric field.