Tungsten-based Catalysts Research Articles

Boosting tungsten -based catalyst activity for aerobic oxidative desulfurization of gas oil by cerium

This study is aimed at investigating the effect of tungsten-based catalysts for aerobic oxidative desulfurization (AODS) for gas oil. Two catalysts tungsten loaded on activated carbon (W/AC) and cerium-tungsten load on activated carbon (Ce–W/AC) were prepared, and they are characterized by Fourier transform infrared spectroscopy FTIR, X-ray diffraction (XRD), Scanning electronic microscopy (SEM), Energy dispersive spectroscopy EDX, and thermal behavior was examined by thermal graphometry analysis (TGA), the surface area was measure by Brunauer-Emmett-Teller (BET) method. The experiments were conducted in a lab-scale reactor. The primary investigation shows that Ce–W/AC was more active than W/AC. The second part examines the effect of reaction temperature, air flow rate, and catalyst dosage on sulfur removal efficiency by using Ce–W/AC, the ranges for the studied parameter are 150–200 °C, 20–60 mL/min, and 0.5–1 g for reaction temperature, air flow rate, and catalyst dosage respectively. The experiments were designed according to the Taguchi method. The experiments results show the sulfur removal efficiency ranged between 72.3 and 99.9. The findings demonstrate that increasing the reaction temperature leads to an increase in sulfur removal efficiency, while the increase in sulfur removal efficiency with air flow rate and catalyst dosage did not continue because it started decreasing after it reached a maximum value According to analysis variance ANOVA demonstrated that the effect of the studied variables ordered the following: reaction temperature air flow rate and finally catalyst dosage via their F-value where these values are 12.27, 4.09, and 0.16 for reaction temperature, air flow rate, and catalyst dosage respectively.

Unraveling the active states of WO3-based catalysts in the selective conversion of cellulose to glycols

Tungsten-based catalysts, including crystalline WO3 and H2WO4, have been reported to be efficient for the selective cleavage of C−C bonds of sugar intermediates involved in the conversion of cellulose to ethylene glycol and propylene glycol in combination with hydrogenation catalysts under H2 atmosphere. However, there is still not a consensus on the actual states of these catalysts during the reaction. Herein, we demonstrate that both WO3 and H2WO4 tended to undergo reduction to hydrogen tungsten bronze (HxWO3) species in the cellulose reaction, consisting of H0.23WO3 and H0.33WO3, which were then re-oxidized to WO3 upon exposure to ambient air after the reaction. Both WO3 and HxWO3 were insoluble in water and acted as the heterogeneous catalysts in the cellulose reaction, as further validated by the strong dependence of the catalytic activity of WO3 on its crystallite size and surface area. Such identification of the heterogeneous HxWO3 species, albeit exemplified here only from the in situ reduction of WO3 and H2WO4 in the cellulose reaction, unravels the active state of the tungsten-based catalysts under reductive reaction conditions.

Efficient catalytic conversion of biomass derived cellulose to short chain polyols

In efforts to replace petro-derived polyethylene terephthalate (PET) with a ‘green’ equivalent, technologies are being developed and scaled up for biobased production of the two components of PET viz. monoethylene glycol (MEG) and terephthalic acid. Herein, we report work on single-pot catalytic hydrogenolysis of cellulose isolated from lignocellulosic biomass into polyols with the major fraction being monoethylene glycol. A combination of silica-alumina-supported nickel (NSA) and bulk tungsten-based catalyst (HW) was investigated for selective conversion of cellulose to MEG. While raw rice straw showed negligible conversion, the two-step pretreated rice straw performed similar to pure cellulose. Ethylene glycol in up to 60 % yield was obtained over 0.15%NSA-0.2%HW within minutes at a hydrogen pressure lower than reported thus far for similar catalysts. Product buildup was seen to have no effect on the catalyst activity and a fed-batch approach could generate product stream in high concentrations with MEG as a single major product.

Molybdenum, Vanadium, and Tungsten-Based Catalysts for Sustainable (ep)Oxidation.

This article gives an overview of the research activity of the LAC2 team at LCC developed at Castres in the field of sustainable chemistry with an emphasis on the collaboration with a research team from the University of Zagreb, Faculty of Science, Croatia. The work is situated within the context of sustainable chemistry for the development of catalytic processes. Those processes imply molecular complexes containing oxido-molybdenum, -vanadium, -tungsten or simple polyoxometalates (POMs) as catalysts for organic solvent-free epoxidation. The studies considered first the influence of the nature of complexes (and related ligands) on the reactivity (assessing mechanisms through DFT calculations) with model substrates. From those model processes, the work has been enlarged to the valorization of biomass resources. A part concerns the activity on vanadium chemistry and the final part concerns the use of POMs as catalysts, from molecular to grafted catalysts, (ep)oxidizing substrates from fossil and biomass resources.

Unveiling the mechanism for selective cleavage of C-C bonds in sugar reactions on tungsten trioxide–based catalysts

Conversion of naturally occurring sugars, the most abundant biomass resources on Earth, to fuels and chemicals provides a sustainable and carbon-neutral alternative to the current fossil resource-based processes. Tungsten-based catalysts (e.g., WO3) are efficient for selectively cleaving C-C bonds of sugars to C2,3 oxygenate intermediates (e.g., glycolaldehyde) that can serve as platform molecules with high viability and versatility in the synthesis of various chemicals. Such C-C bond cleavage follows a mechanism distinct from the classical retro-aldol condensation. Kinetic, isotope 13C-labeling, and spectroscopic studies and theoretical calculations, reveal that the reaction proceeds via a surface tridentate complex as the critical intermediate on WO3, formed by chelating both α- and β-hydroxyls of sugars, together with the carbonyl group, with two adjacent tungsten atoms (W-O-W) contributing to the β-C-C bond cleavage. This mechanism provides insights into sugar chemistry and enables the rational design of catalytic sites and reaction pathways toward the efficient utilization of sugar-based feedstocks.

Tungsten-Based Nanocatalysts: Research Progress and Future Prospects.

The high price of noble metal resources limits its commercial application and stimulates the potential for developing new catalysts that can replace noble metal catalysts. Tungsten-based catalysts have become the most important substitutes for noble metal catalysts because of their rich resources, friendly environment, rich valence and better adsorption enthalpy. However, some challenges still hinder the development of tungsten-based catalysts, such as limited catalytic activity, instability, difficult recovery, and so on. At present, the focus of tungsten-based catalyst research is to develop a satisfactory material with high catalytic performance, excellent stability and green environmental protection, mainly including tungsten atomic catalysts, tungsten metal nanocatalysts, tungsten-based compound nanocatalysts, and so on. In this work, we first present the research status of these tungsten-based catalysts with different sizes, existing forms, and chemical compositions, and further provide a basis for future perspectives on tungsten-based catalysts.

An inclusive perspective on the recent development of tungsten‐based catalysts for overall water‐splitting : A review

The problems of energy shortages, environmental pollution and the ever-increasing global population are creating serious concerns for mankind. For a sustainable future, it is essential to explore clean and renewable alternate energy sources. The green energy consumption model aims towards a future with a competent and sustainable hydrogen economy to fulfill the global energy demands. Water-splitting is a promising method for clean hydrogen production. A highly active noble metal-free catalyst is the main pre-requisite, to make the process energy-efficient and economical. An optimum catalyst lowers the energy required for the disassociation of water by driving down the overpotential of the process. Tungsten-based materials are an considered effective catalyst for water-splitting due to natural availability, environmental friendliness, high photostability, high electron mobility, large hole diffusion length and high catalytic activities. In recent years, a diverse range of tungsten-based materials has been developed as photo/electro catalysts for water-splitting. This review aims to present a comprehensive understanding and timely reference for the progress of tungsten-based catalysts in this field. This study begins with the fundamental principle of water splitting and the properties of tungsten and its hybrids then moves on to different techniques for producing binary/ternary nanocomposites for overall water splitting. A comprehensive overview of the recent developments in tungsten-based photo/electro catalysts for overall water splitting is then presented with the techniques to enhance the catalyst activity, such as morphological control, surface modification, heteroatom doping, and heterojunction construction. Finally, recommendations for the future are proposed.

Selectively chemo-catalytic hydrogenolysis of cellulose to EG and EtOH over porous SiO2 supported tungsten catalysts

Cellulosic ethanol produced from lignocellulose biomass can alleviate the shortage of conventional fossil energy supply and reduce global CO2 emissions. Wherein, hydrogenolysis of cellulose to ethanol is a new method for the synthesis of fuel ethanol, which could theoretically utilizes all carbon atoms in glucose in the direct retro-aldol condensation (RAC) reaction to produce ethanol, and can potentially break through the technical bottleneck of biological methods. Herein, we show that the benefits of the mesoporous structure of tungsten-based catalysts can be leveraged to influence the selective hydrogenolysis of cellulose into C2 products. Comparing the performance of different pore size SiO2 supported tungsten catalysts and detailed characterizations revealed that the mesoporous structure of supports can affect the morphology, crystal sizes, and surface chemistry of the catalysts, which presented a combined effect on the hydrogenolysis reaction. Whereby, 51.5 wt% ethylene glycol (EG) was obtained from the direct hydrogenolysis of cellulose over Ru-WOx/SiO2 (500 Ȧ) catalyst under 513 K, and 40.5 wt% ethanol (EtOH) was obtained from the direct hydrogenolysis of cellulose over Ir-WOx/SiO2 (500 Ȧ) catalyst under 553 K, respectively.

Regulating oxygen defects via atomically dispersed alumina on Pt/WOx catalyst for enhanced hydrogenolysis of glycerol to 1,3-propanediol

Tungsten-based catalyst has been widely investigated in the field of selective hydrogenolysis of secondary CO bond, such an important yet challenging strategy, in glycerol conversion, in which the product 1,3-propanediol (1,3-PDO) is of great value in polyester industry. Unfortunately, though it has been proved to be highly active, the identification of the intrinsic tungsten oxide active sites still remains unrevealed to date due to its complex microstructure. Herein, we incorporate atomically dispersed alumina as promoter in the context of the selective hydrogenation of glycerol and report a Pt/Al-WOx catalyst. This catalyst decidedly outperforms the unpromoted Pt/WOx, which elevates the 1,3-PDO yield to 2 times. Spectroscopy characterizations and chemisorption experiments have revealed that the high activity and selectivity of Pt/Al-WOx catalyst results from the more oxygen vacancies on WOx in-situ generation by the acceleration of atomically dispersed alumina in hydrogen atmosphere, which increases the selective adsorption of glycerol and the in-situ Brønsted acid sites for the selective activation of secondary CO bonds, thus largely augmenting the hydrogenolysis performance. This discovery not only provides the new strategy of defect engineering to enhance hydrogenolysis performance of secondary CO bond in biomass compounds, but reveals the unique role of the unsaturated coordination structure of WOx in chemoselective hydrogenolysis reactions.

Evaluation of Catalysts for the Metathesis of Ethene and 2-Butene to Propene

Different metathesis catalysts were evaluated regarding their activity for propene production from ethene and trans-butene feedstocks. Nickel, molybdenum, rhenium and tungsten, along with bimetallic nickel-rhenium systems were applied with commercial supports and self-synthesized MCM-41. For the latter support the Si/Al ratio was adjusted as an additional optimization parameter (Si/Al = 60). Attractive activities were observed using Re and NiRe based catalysts at moderate temperatures of 200–250 °C. In contrast, the tungsten-based catalysts were only active above 450 °C. Three catalysts, namely Re/AlMCM-41(60), NiRe/mix (1:1) and W/SiO2 offered propene selectivity’s exceeding 40% at attractive conversion rates. These catalysts were characterized by BET, powder XRD, NH3-TPD and TPR-TPO-TPR cycles. At specific reaction temperatures, reaction-regeneration cycles were performed, which revealed that for the Re and W catalysts the initial reactant conversions and propene selectivity can be recovered. In contrast, for the NiRe catalyst, a continuous, gradual and irreversible decrease of activity was observed. Even though the tungsten catalyst was operated at the highest temperature, no irreversible decrease in conversion and propene selectivity occurred. Therefore, this catalyst has potential as a promising candidate for the synthesis of propene.

Tungsten-Based Catalysts for Environmental Applications

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).

Oxidative cleavage of cycloalkenes using hydrogen peroxide and a tungsten-based catalyst: towards a complete mechanistic investigation

The identification of intermediates and by-products issuing from the oxidative cleavage of cycloolefins allows proposing of a reaction mechanism.

Differences in acid and catalytic properties of W incorporated spherical SiO2 and 1%Al-doped SiO2 in propene metathesis

Propene self-metathesis to ethylene and 2-butene was investigated over tungsten-based catalysts (9 wt.% W) supported on silica and 1%Al-doped SiO2, prepared by different processes. On pure SiO2, incorporating WOx improved tungsten dispersion and formation of active tetrahedral WOx species, compared to the impregnated ones. However, negative effect was found when tungsten was incorporated in the 1%Al-doped SiO2 as the catalysts contained more crystalline WOx, lower tetrahedral species, and lower Lewis acid sites, compared to the W-imp 1Al2O3/SiO2 prepared by impregnation. Incorporating both W and Al in the SiO2 by sol-gel method may lead to suppression of bridge hydroxyl Si(OH)Al group, as a consequence more crystalline WOx was formed. Nevertheless, the abundance of Bronsted and Lewis acid was found to play important role in the propene metathesis and the secondary metathesis of 1-butene reactions than surface silanol and the presence of tetrahedral tungsten oxide species on the 1%Al-doped SiO2 supported catalysts.

Epoxidation of microalgal biomass-derived squalene with hydrogen peroxide using solid heterogeneous tungsten-based catalyst

Squalene structurally resembles petroleum resources, and is efficiently produced by Aurantiochytrium mangrovei, a microalga, and could be used effectively as an alternative oil resource. An efficient heterogeneous catalyst system for the epoxidation of squalene was developed, comprising the heterogeneous catalyst [N(C6H13)4]3[PW4O8(O2)8] for the successful epoxidation of squalene with H2O2 in t-BuOH. The addition of the tetraalkylammonium salt [N(C6H13)4]Cl, and H2O2 as the oxidant allowed quantitative epoxidation of squalene.

Crystal orientation-dependent activity of tungsten-based catalysts for selective catalytic reduction of NOx with NH3

Hexagonal WO3 crystals with [1 1 0] and [0 0 1] orientations (denoted as H-110 & H-001) were synthesized and impregnated with 5 wt% V2O5 for NH3-SCR catalysis. H-110 with dominant (0 0 1) exposed facet possesses 72% NOx conversion at 400 °C while H-001 with dominant (1 0 0) exposed facet only achieves 17%. The maximal NOx conversion over V/H-110 and V/H-001 catalysts was enhanced to 96% and 47%, respectively. The catalysts were characterized by H2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, X-ray photoelectron spectroscopy (XPS), Raman, ultraviolet-visible (UV-vis) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy. The more coordinative unsaturated W6+ atoms on (0 0 1) facet generated more acid sites and NH3 was more strongly adsorbed on H-110 surface. The better match relationship between vanadia and WO3 (0 0 1) exposed facets led to the formation of more polymeric vanadates on V/H-110 catalyst. The higher proportioned V4+ and chemisorbed oxygen on V/H-110 catalyst also facilitated NH3-SCR reaction.

Precise determination of Tafel slopes by DEMS. Hydrogen evolution on tungsten-based catalysts in alkaline solution

Abstract High-purity hydrogen generation for primary energy production is one of the main goals of the renewable energy consumption model. In this regard, water electrolyzers (WE) are projected to fulfil this requirement since they can be coupled to renewable sources, which provide the necessary energy to generate the water splitting reaction. Nowadays, no-noble materials are usually employed as low-cost catalysts to overcome the hydrogen evolution reaction (HER). However, kinetics and mechanistic studies of the HER from faradaic currents is becoming more challenging due to the overlap of by-side reactions such as catalyst oxides reduction. Herein, for the first time ionic currents from differential electrochemical mass spectrometry (DEMS) are employed to overcome this issue. Thus, the m/z = 2 signal is followed during the hydrogen evolution on tungsten-based catalysts in basic medium and accurate Tafel slopes are achieved and the rate determining step (RDS), as well as the reaction mechanism can be easily attained.

Tungsten-based catalysts for lignin depolymerization: the role of tungsten species in C–O bond cleavage

Tungsten-based catalysts with designed tungsten species are synthesized and the role of each species in hydrocracking of both lignin model compounds and real lignin is deeply studied.

Adsorption properties of the tetragonal P4/nmm WO3 (100) surface toward molecules involved in the hydration of ethylene

Industrial demand for ethanol has stimulated research on improved catalysts for ethylene hydration. WO3 and other tungsten-based catalysts have been used in direct hydration of ethylene to ethanol, showing high conversion rate and selectivity toward this reaction. By using first-principles calculations based on spin polarized density functional theory, including also dispersion forces, this paper presents the adsorption properties of molecules involved in the ethylene hydration on the tetragonal P4/nmm WO3 (100) surface. The inclusion of dispersion forces, improved our lattice parameters prediction with respect to previous theoretical studies. For the WO3 tetragonal crystal, the (100) surface contains five chemically distinct atoms: a five-fold coordinate W5c, three different types of two-fold binding oxygen O2c, and a single-coordinated oxygen O1c. The energetically favored adsorption sites of water, ethylene, and ethanol (including also hydrogen) are determined here for the first time on this surface. The surface presents high reactivity towards water and ethylene, the single-fold coordinated oxygens stabilize the adsorbed molecules and after adsorption the tungsten atoms take a W6c role. The obtained configurations of the reactants suggest a Rideal–Eley mechanism of ethylene over adsorbed water. These results reveal the interactions between the selected tetragonal WO3 surface and the molecules involved in the hydration of ethylene, which ultimately determine the reaction mechanism for ethanol production.

Oxidative Cleavage of Fatty Acid Derivatives for Monomer Synthesis

Oxidative cleavage of fatty acids and fatty acid derivatives is a practical way to obtain bifunctional molecules that can be used in polycondensation reactions. Diacids, hydroxyacids, and amino acids can then be used to produce polyesters or polyamides and also a large range of other products, such as lubricants and plasticizers. Ozonolysis has long been the sole industrial process for oxidative cleavage, but recently, routes using hydrogen peroxide as a clean oxidant have regained interest. Hydrogen peroxide is easier to use, but the kinetics of the catalyzed reactions are still slow. Although several catalytic systems have been described in the literature, tungsten-based catalysts are still the preferred choices. Different catalysts can trigger different mechanisms, such as a radical mechanism instead of a catalytic reaction. In addition, some side products and co-products often disregarded in the literature, such as shorted cleavage products, indicate the presence of side reactions that affect the quality of the final products. The oxidative cleavages in continuous and batch processes have significant differences, which are discussed with an illustration of our understanding of the process used by Matrica S.p.A.

Initiation of heterogeneous Schrock-type Mo and W oxide metathesis catalysts: A quantum thermochemical study

Three mechanistic pathways were explored theoretically to study the electronic features and thermodynamics behind the initiation of silica-grafted molybdenum and tungsten oxide metathesis pre-catalysts in light olefins (ethylene and propylene) to form Schrock-type carbene structures. At 298 K and 1 atm, the investigated activation process was found to be exothermic for all but one of the carbene species, where the pathway involving a secondary carbene center and liberating acetone as a byproduct was more favorable (by about 10 kcal/mol) than the process involving a primary carbene center and evolving propionaldehyde. The most exothermic carbene to form (−20.9 kcal/mol) had its both propylidene and propoxy groups bonded through their secondary carbon atoms. A competing pathway suggested for an ethylene feedstock and tungsten-based catalyst, which involved the production of acetaldehyde was also viable. The generation of carbene species from the W-based pre-catalyst was less exothermic than the Mo-based pre-catalyst, however. The computational data with the activation pathways at hand stressed the benefits of initiation at low temperatures, the presence of adjacent trapping heterogeneities for the acetone or propanal by-products, and the beneficial role of the presence of an isopropoxy ligand. The localized-orbital locator maps indicated the polar covalent characteristics of the metal–carbene bonds, which were polarized towards the C atom. The analysis of the density of states indicated the destabilization of the highest occupied molecular orbitals (HOMOs) of the pre-catalysts upon carbene generation, where the alkylidene fragment and transition metal both contributed to the HOMO of the carbene complex.