Catalysis Science & Technology最新文献

Direct dehydrogenation of methanol to formaldehyde and hydrogen is a “dream reaction” requiring catalysts, which are not only active in this highly endothermic reaction but also stable under harsh reaction conditions. Previous reports showed that materials with Zn₂SiO₄ exhibit a relatively high activity along with considerable long-time stability. However, neither detailed information on the physicochemical properties of such zinc silicates nor information on deactivation mechanisms was provided and discussed. In this study, the Zn : Si ratio has been varied to obtain different phases of zinc silicate and to investigate their specific activities in the methanol dehydrogenation reaction. Amorphous ZnO and SiO₂, as well as crystalline phases of zinc oxide and zinc silicate, viz. α-Zn₂SiO₄ and β-Zn₂SiO₄, were present in almost all materials in different concentrations. The β-Zn₂SiO₄ phase was found to be relatively unstable in methanol dehydrogenation similar to ZnO, which is readily reduced to metallic Zn. Since detailed material characterization was not reported in studies before, the catalytic role of different phases present in zinc silicate materials for the target reaction remained unclear. Some aspects of this role are addressed within this work with a focus on α-Zn₂SiO₄ and its potential as a catalyst for direct methanol dehydrogenation.

PtSn bimetallic catalysts are among the best-performing propane dehydrogenation (PDH) catalysts. However, understanding these catalysts remains limited due to the intricate nature of bimetallic systems and their dynamic structural evolution under reaction conditions. To address this challenge, we employ various in situ/operando techniques, including UV-vis, CO diffuse reflective infrared Fourier transform spectroscopy (CO-DRIFTS), near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and operando X-ray absorption spectroscopy (XAS), to elucidate the structural dynamics of PtSn/SiO₂ catalysts under reduction and working conditions. Our investigation reveals that the interactions between Pt, Sn, and SiO₂ support are strongly influenced by the synthesis procedures and the initial catalyst structure. Exposure to H₂ causes a reversible Sn–OH formation observed by modulation excitation spectroscopy (MES). A sequentially impregnated catalyst with a nominal Pt : Sn ratio of 1 : 3 and a co-impregnated catalyst with a ratio of 1 : 2 exhibit optimal performance for PDH. Despite distinct synthesis procedures and bulk structures, these two catalysts exhibit comparable surface properties and PDH performance, attributed to the dynamic migration of Sn species and formation of a Pt-rich metal surface under reductive atmospheres.

The creation and ethylene oligomerization function of silica-anchored group 4 metal (M = Ti, Zr, Hf) hydrides as a function of the anchor site structure are explored using density functional theory models. Oligomerization potential energy surfaces are sensitive both to the metal ion and to the structure of the anchoring site. Microkinetic models predict the oligomerization rate and degree of polymerization as a function of the site, temperature, and reactant pressures. Intrinsic catalytic activity is predicted to vary as Ti < Zr > Hf down the group, irrespective of site model, while absolute rates and product distributions are strongly sensitive to the site model, temperature, and hydrogen pressure. Catalysts based on these sites are plausible candidates for high-temperature ethylene oligomerization of interest for generating fuels from ethylene, but selectivity is expected to require careful control over the site structure and reaction conditions.

Hydroxylation of benzene directly into phenol without converting it into other compounds by using a metal complex-based oxidation catalyst is challenging because of the chemical stability of benzene. We demonstrated that a graphite-supported μ-nitrido-bridged iron phthalocyanine dimer, which acts as a potent iron–oxo-based molecular methane oxidation catalyst, can efficiently catalyze the direct benzene hydroxylation at 25 °C in an aqueous acetonitrile solution containing excess H₂O₂. It was confirmed that the catalytic benzene hydroxidation activity of the graphite-supported μ-nitrido-bridged iron phthalocyanine dimer was significantly higher than that of the silica gel-supported μ-nitrido-bridged iron phthalocyanine dimers.

A series of Pd/Al₂O₃ catalysts with metal weight loadings of 1.0 wt%, 2.5 wt%, and 5.0 wt% were synthesised by chemical vapour impregnation (CVI) and used for the total oxidation of propane. All the catalysts were highly active for propane total oxidation. Extensive characterisation showed essentially identical catalyst structural and chemical characteristics, with consistent nanoparticle size, dispersion, and metal oxidation state regardless of metal loading. The major difference between catalysts was the number of surface palladium sites which scaled directly with metal loading. Turnover frequency calculations showed that the intrinsic activity of each catalyst is the same, with conversion scaling with the number of active sites. The number of active sites was normalised experimentally with catalyst performance proving to be identical regardless of weight loading. This study shows that CVI is a technique that can produce active catalysts with high levels of control and consistency of active metal nanoparticles as a function of loading. The same level of control over dispersion and activity was not achieved when catalysts were prepared by conventional aqueous impregnation. The fundamental understanding of CVI is important for the design of highly active catalysts, which is exemplified for propane total oxidation, but has wider significance for other applications of supported metal catalysts.

Sodium alginate is a biomass-derived polysaccharide. A novel hydrogel (SA–H₃PO₄) was prepared by taking advantage of the hydrogen bonding interactions between sodium alginate and phosphoric acid. Phosphoric acid molecules are highly dispersed in the hydrogel. A phosphorus-doped carbon material (PC-700) with a high content of P was derived through the calcination of the SA–H₃PO₄ xerogel under N₂ at 700 °C. PC-700 was used for the catalytic oxidation of furfural to maleic acid with the involvement of H₂O₂. The reaction route was investigated and 9 representative reactions for the reaction network were determined. The reaction rate constants and apparent activation energies of the reactions were obtained. Using the kinetic parameters, the kinetic models can simulate the concentration profiles in good agreement with the experimental data. Based on ample experimental results, the C₃PO species in PC-700 were identified as the active sites. The production of maleic acid from furfural over PC-700 is mainly achieved through the oxidation of furfural to 5-hydroxyfuran-2(5H)-one (HFONE) and the direct oxidation of furfural to maleic acid via β-formylacrylic acid (β-FA) formation. PC-700 has shown excellent catalytic activity for the production of maleic acid from furfural in the presence of H₂O₂. For the oxidation of 1000 mM furfural, a maleic acid yield of 74.8% and furfural conversion of 100% were obtained at 80 °C within 5 h.

Ionic liquid-mediated synthesis of Bi₄O₅Br₂ and BiOBr was carried out in non-polar solvents (glycerol, ethylene glycol) and a polar solvent (0.1 M mannitol). The effect of elongation of alkyl side chains (C4mim⁺, C8mim⁺, and C16mim⁺) of imidazolium ionic liquids, which act as a source of bromide and template, on the morphological, optical, and photocatalytic properties of materials was investigated. The crystallite size, morphology, particle size, energy bandgap, and exposure of (110), (001), and (102) facets were effectively tuned by selecting the proper ionic liquid–solvent system. The self-assembly of ILs and their role in forming Bi-based crystallites in non-polar and polar solvents differed. The most effective 5-fluorouracil was photooxidized over the samples prepared in C4mim⁺ – 0.1 M mannitol solution, while the best Cr(vi) photoreduction occurred with the sample formed in C4mim⁺ – glycerol. Molecular dynamics simulation correlated the length of alkyl side chains of imidazolium ILs with an increase in the number of “free” –OH groups of the solvent, which interacted with BiOBr nuclei during synthesis, fine-tuning its photocatalytic activity.

In order to meet the climate goals of the Paris Agreement and limit the potentially catastrophic consequences of climate change, we must move away from the use of fossil feedstocks for the production of chemicals and fuels. The conversion of synthesis gas (a mixture of hydrogen, carbon monoxide and/or carbon dioxide) can contribute to this. Several reactions allow to convert synthesis gas to oxygenates (such as methanol), olefins or waxes. In a consecutive step, these products can be further converted into chemicals, such as dimethyl ether, short olefins, or aromatics. Alternatively, fuels like gasoline, diesel, or kerosene can be produced. These two different steps can be combined using bifunctional catalysis for direct conversion of synthesis gas to chemicals and fuels. The synergistic effects of combining two different catalysts are discussed in terms of activity and selectivity and compared to processes based on consecutive reaction with single conversion steps. We found that bifunctional catalysis can be a strong tool for the highly selective production of dimethyl ether and gasoline with high octane numbers. In terms of selectivity bifunctional catalysis for short olefins or aromatics struggles to compete with processes consisting of single catalytic conversion steps.