An Effective Hybrid Heterogeneous Catalyst to Desulfurize Diesel: Peroxotungstate@Metal-Organic Framework

Design and Synthesis of Amphiphilic Catalyst [C16mim]5VW12O40Br and Its Application in Deep Desulfurization with Superior Cyclability at Room Temperature

Achieving long-term stable deep desulfurization at room temperature and recovering high value-added sulfone products is a challenge at present. Herein, a series of catalysts [C_nmim]₅VW₁₂O₄₀Br (C_nVW₁₂, 1-alkyl-3-methylimidazolium bromide tungstovanadate, n = 4, 8, 16) were presented for the room temperature catalytic oxidation of dibenzothiophene (DBT) and its derivatives. Factors affecting the reaction process, such as the amount of catalyst, oxidant, and temperature, were systematically discussed. C₁₆VW₁₂ showed higher catalytic performance, and 100% conversion and selectivity could be achieved in 50 min with only 10 mg. The mechanism study showed that the hydroxyl radical was the active radical in the reaction. Benefiting from the "polarity strategy", the sulfone product accumulated after 23 cycles in a C₁₆VW₁₂ system, and the yield and purity were about 84% and 100%, respectively.

Immobilization of Peroxo-Heteropoly Compound and Palladium on Hydroxyapatite for the Epoxidation of Propylene by Molecular Oxygen in Methanol

Peroxo-heteropoly compound PO₄[W(O)(O₂)₂] was synthesized on calcium-deficient hydroxyapatite using a reaction of surface [HPO₄]^2- groups on hydroxyapatite with a Na₂[W₂O₃(O₂)₄] aqueous solution. The vibration of [HPO₄]^2- at 875 cm^-1 became very weak, and the vibration of the peroxo-oxygen bond [O-O]^2- at 845 cm^-1 appeared in the FT-IR spectrum of the solid product, indicating that PO₄[W(O)(O₂)₂] was formed on the surface of hydroxyapatite. The formed solid sample was further reacted with PdCl₂(PhCN)₂ in an acetone solution to fix PdCl₂ between the O sites on the hydroxyapatite. Elemental analyses proved that the resultant solid contained 1.2 wt.% Pd, implying that PdCl₂ molecules were immobilized on the surface of hydroxyapatite. The hydroxyapatite-based hybrid compound containing Pd and PO₄[W(O)(O₂)₂] was used as a heterogeneous catalyst in a methanol solvent for propylene epoxidation by molecular oxygen in an autoclave batch reaction system. A propylene conversion of 53.4% and a selectivity for propylene oxide of 88.7% were obtained over the solid catalyst after reaction at 363 K for 8 h. The novel catalyst could be reused by a simple centrifugal separation, and the yield of propylene oxide did not decrease after the reaction for five runs. By prolonging the reaction time to 13 h, the highest yield of propylene oxide at 363 K over the solid catalyst was obtained as 53.8%, which was almost the same as that of the homogeneous catalyst containing PdCl₂(PhCN)₂ and [(C₆H₁₃)₄N]₂{HPO₄[W(O)(O₂)₂]₂} for the propylene epoxidation. Methanol was used as a solvent as well as a reducing agent in the propylene epoxidation by molecular oxygen. Small particles of Pd metal were formed on the surface of the hybrid solid catalyst during the reaction, and acted as active species to achieve the catalytic turnover of PO₄[W(O)(O₂)₂] in the propylene epoxidation by molecular oxygen in methanol.

Lindqvist@Nanoporous MOF-Based Catalyst for Effective Desulfurization of Fuels

An effective and sustainable oxidative desulfurization process for treating a multicomponent model fuel was successfully developed using as a heterogeneous catalyst a composite material containing as an active center the europium Lindqvist [Eu(W₅O₁₈)₂]^9- (abbreviated as EuW₁₀) encapsulated into the nanoporous ZIF-8 (zeolitic imidazolate framework) support. The EuW₁₀@ZIF-8 composite was obtained through an impregnation procedure, and its successful preparation was confirmed by various characterization techniques (FT-IR, XRD, SEM/EDS, ICP-OES). The catalytic activity of the composite and the isolated EuW₁₀ was evaluated in the desulfurization of a multicomponent model fuel containing dibenzothiophene derivatives (DBT, 4-MDBT and 4,6-DMDBT) with a total sulfur concentration of 1500 ppm. Oxidative desulfurization was performed using an ionic liquid as extraction solvent and aqueous hydrogen peroxide as oxidant. The catalytic results showed a remarkable desulfurization performance, with 99.5 and 94.7% sulfur removal in the first 180 min, for the homogeneous active center EuW₁₀ and the heterogeneous EuW₁₀@ZIF-8 catalysts, respectively. Furthermore, the stability of the nanocomposite catalyst was investigated by reusing and recycling processes. A superior retention of catalyst activity in consecutive desulfurization cycles was observed in the recycling studies when compared with the reusing experiments. Nevertheless, the nanostructure of ZIF-8 incorporating the active POM (polyoxometalate) was shown to be highly suitable for guaranteeing the absence of POM leaching, although structural modification was found for ZIF-8 after catalytic use that did not influenced catalytic performance.

Ionic Liquid-Based Polyoxometalate Incorporated at ZIF-8: A Sustainable Catalyst to Combine Desulfurization and Denitrogenation Processes

An effective and sustainable process capable of simultaneously execute desulfurization and denitrogenation of fuels is in fact an actual necessity in the refinery industry. The key to achieve this goal is the parallel oxidation of sulfur and nitrogen compounds present in fuels, which is only achieved by an active and recovered catalyst. A novel heterogeneous catalyst was successfully prepared by the encapsulation of an imidazolium-based polyoxometalate (POM) into a ZIF-8 framework ([BMIM]PMo₁₂@ZIF-8). This composite material revealed exceptional catalytic efficiency to concurrently proceed with the oxidative desulfurization and denitrogenation of a multicomponent model fuel containing various sulfur and nitrogen compounds. A complete removal of all these compounds was achieved after only one hour and the catalyst system was able to be reused for ten consecutive cycles without loss of efficiency. In fact, an ionic liquid POM was incorporated in the ZIF-8 for the first time, and this composite compound was originally applied as a catalyst for simultaneous oxidative desulfurization and denitrogenation processes.

Assembling Ultrafine SnO2 Nanoparticles on MIL-101(Cr) Octahedrons for Efficient Fuel Photocatalytic Denitrification

Effectively reducing the concentration of nitrogen-containing compounds (NCCs) remains a significant but challenging task in environmental restoration. In this work, a novel step-scheme (S-scheme) SnO2@MCr heterojunction was successfully fabricated via a facile hydrothermal method. At this heterojunction, MIL-101(Cr) octahedrons are decorated with highly dispersed SnO2 quantum dots (QDs, approximate size 3 nm). The QDs are evenly wrapped around the MIL-101(Cr), forming an intriguing zero-dimensional/three-dimensional (0D/3D) S-scheme heterostructure. Under simulated sunlight irradiation (280 nm < λ < 980 nm), SnO2@MCr demonstrated superior photoactivity toward the denitrification of pyridine, a typical NCC. The adsorption capacity and adsorption site of SnO2@MCr were also investigated. Tests using 20%SnO2@MCr exhibited much higher activity than that of pure SnO2 and MIL-101(Cr); the reduction ratio of Cr(VI) is rapidly increased to 95% after sunlight irradiation for 4 h. The improvement in the photocatalytic activity is attributed to (i) the high dispersion of SnO2 QDs, (ii) the binding of the rich adsorption sites with pyridine molecules, and (iii) the formation of the S-scheme heterojunction between SnO2 and MIL-101(Cr). Finally, the photocatalytic mechanism of pyridine was elucidated, and the possible intermediate products and degradation pathways were discussed.