Silica-supported Vanadium Research Articles

Chemical looping methanol oxidation using supported vanadium phosphorous oxide carriers for formaldehyde production

A novel chemical looping scheme for selective oxidation of methanol has been reported, using silica-supported vanadium phosphorous oxide as the oxygen carrier.

Epoxidation of propane with oxygen and/or nitrous oxide over silica-supported vanadium oxide

Propane to propene oxide (PO) oxidation over V-containing mesoporous silica of SBA-3 structure has been studied using different oxidants (nitrous oxide, oxygen, and their mixture) in the temperature range 673–773 K. Electron spin resonance spectroscopy, ultraviolet–visible spectroscopy, and X-ray photoelectron spectroscopy (XPS), as well as X-ray diffraction, temperature-programmed reduction with hydrogen (H2 TPR), and low-temperature N2 adsorption/desorption, were applied for characterization of fresh and spent catalysts. XPS spectra and H2 TPR profiles revealed a significant reduction of V-species as a result of propane oxidation with N2O alone, which leads to a decrease in both propane conversion and the space–time yield (STY) of PO. The use of an N2O–oxygen mixture as an oxidant of propane allows the vanadium valence to be stabilized at a level similar to the initial sample, which results in stable activity with time on stream. Propane conversion of 40%, propylene selectivity of 45%, and propylene oxide selectivity of 11%, corresponding to a STY of propylene oxide of about 15 g kgcat-1h−1, have been obtained, which makes these results very promising compared with the data reported in the literature. Vanadium catalyst used with only oxygen results in stable propane conversion with high total oxidation and stable propene selectivity, although the STY of PO is 10 times lower. N2O applied as the only oxidant results in rapid catalyst deactivation, and after 2 h on stream, STY of PO is only 2.5 g kgcat-1h−1.

Single-Molecule Kinetics of Styrene Hydrogenation on Silica-Supported Vanadium: The Role of Disorder for Single-Atom Catalysts

A theoretical approach for the study of supported atom catalysis is developed based on recent advances in the study of single-molecule kinetics. This view is particularly useful in exhibiting the role of disorder in single-atom and single-site catalysts on amorphous supports. The distribution of passage times (or waiting times) through a complex catalytic network originating from a set of coupled active sites is described by a probability distribution function (PDF), f(t), that reflects the local environment of the reaction center. An efficient algorithm is developed based on the linear algebra of the Markov transition matrix that produces f(t) or its moments. The kinetics of the hydrogenation reaction of styrene on an organovanadium(III) catalyst supported on amorphous silica is studied. A kinetic model consisting of three intertwined catalytic cycles emanating from three chemically distinct active sites is proposed to describe the chemistry. Density functional theory (DFT) calculations are employed to determine the free energy barriers of the reactions, which are used to construct the rate coefficient matrix. The disorder induced by the amorphous support material is divided into a low-dimensional short-range component reflecting the covalent structures near the reaction center and a weaker long-range component modeling the bulk randomness. The results are computed and analyzed for a wide range of concentration values and disorder scenarios. The unusual structure in the f(t) PDF is found to occur for certain cases that reveal the contribution of multiple catalytic pathways acting in concert.

Ive Hermans

Ive Hermans

Comparing the Catalytic Activity of Silica-Supported Vanadium Oxides and the Polymer Nanofiber-Supported Oxidovanadium(IV) Complex toward Oxidation of Refractory Organosulfur Compounds in Hydrotreated Diesel

Silica-supported vanadium oxides (VxOy-silica 600 °C) and polymer nanofiber [2-(2′-hydroxy-5′-ethenylphenyl)imidazole (PIMv) and styrene (ST) copolymer]-supported oxidovanadium(IV) ([VIVO-p(PIMv-co-ST)]) were synthesized and used as catalysts for the oxidation of refractory organosulfur compounds in fuels in a continuous flow system. Conversion of dibenzothiophene (DBT) to dibenzothiophene sulfone (DBTO2) increased as the flow rate decreased, reaching 100% at flow rates of 0.1 and 0.2 mL/h for (VxOy-silica 600 °C) and [VIVO-p(PIMv-co-ST)], respectively. This was attributed to improved contact time between the catalyst and substrate, which allowed further oxidation to take place. However, the catalytic activity of VxOy-silica 600 °C dropped by 33% after the first oxidation cycle at a flow rate of 0.1 mL/h at 60 °C, unlike [VIVO-p(PIMv-co-ST)], which maintained its activity at 100% after three cycles. Optimized conditions were employed in the oxidation of a hydrotreated fuel sample (Sasol diesel 500) follow...

Parahydrogen-Induced Polarization Study of the Silica-Supported Vanadium Oxo Organometallic Catalyst.

Parahydrogen can be used in catalytic hydrogenations to achieve substantial enhancement of NMR signals of the reaction products and in some cases of the reaction reagents as well. The corresponding nuclear spin hyperpolarization technique, known as parahydrogen-induced polarization (PHIP), has been applied to boost the sensitivity of NMR spectroscopy and magnetic resonance imaging by several orders of magnitude. The catalyst properties are of paramount importance for PHIP because the addition of parahydrogen to a substrate must be pairwise. This requirement significantly narrows down the range of the applicable catalysts. Herein, we study an efficient silica-supported vanadium oxo organometallic complex (VCAT) in hydrogenation and dehydrogenation reactions in terms of efficient PHIP production. This is the first example of group 5 catalyst used to produce PHIP. Hydrogenations of propene and propyne with parahydrogen over VCAT demonstrated production of hyperpolarized propane and propene, respectively. The achieved NMR signal enhancements were 200–300-fold in the case of propane and 1300-fold in the case of propene. Propane dehydrogenation in the presence of parahydrogen produced no hyperpolarized propane, but instead the hyperpolarized side-product 1-butene was detected. Test experiments of other group 5 (Ta) and group 4 (Zr) catalysts showed a much lower efficiency in PHIP as compared to that of VCAT. The results prove the general conclusion that vanadium-based catalysts and other group 4 and group 5 catalysts can be used to produce PHIP. The hydrogenation/dehydrogenation processes, however, are accompanied by side reactions leading, for example, to C4, C2, and C1 side products. Some of the side products like 1-butene and 2-butene were shown to appear hyperpolarized, demonstrating that the reaction mechanism includes pairwise parahydrogen addition in these cases as well.

Operando Multiwavelength and Time-Resolved Raman Spectroscopy: Structural Dynamics of a Supported Vanadia Catalyst at Work

A novel operando spectroscopic approach combining multiwavelength and time-resolved Raman spectroscopy with gas-phase Fourier transform infrared (FTIR) spectroscopy is presented, supported by in situ UV–vis diffuse reflectance (DR) spectroscopy. The potential of this approach is demonstrated in a case study of the oxidative dehydrogenation (ODH) of ethanol over a silica-supported vanadia catalyst. The structural dynamics of the catalyst upon switching from oxidative to reactive conditions was extensively studied by Raman spectroscopy with different excitation wavelengths in the visible and UV, exploiting resonance effects in a targeted manner. Time-dependent correlation of Raman and IR spectra over several reaction cycles allows identification of active vanadia surface structures. Detailed Raman spectroscopic analysis reveals that the adsorption of ethoxy species onto dispersed VOx structures occurs via opening of both V–O–Si and V–O–V bonds. During reaction, large oligomeric VOx structures are decomposed...

Synthesis of 3-Substituted Indoles Using a Silica-Supported Vanadium Catalyst

Synthesis of 3-Substituted Indoles Using a Silica-Supported Vanadium Catalyst

Investigation of Silica-Supported Vanadium Oxide Catalysts by High-Field 51V Magic-Angle Spinning NMR

Supported V2O5/SiO2 catalysts were studied using solid-state 51V magic-angle spinning NMR at a sample spinning rate of 36 kHz and at a magnetic field of 19.975 T to provide a better understanding of the coordination of the vanadium oxide as a function of environmental conditions. Structural transformations of the supported vanadium oxide species between the catalyst in the dehydrated state and hydrated state under an ambient environment were revisited to examine the degree of oligomerization and the effect of water. The experimental results indicate the existence of a single dehydrated surface vanadium oxide species that resonates at −675 ppm and two vanadium oxide species under ambient conditions that resonate at −566 and −610 ppm. No detectable structural difference was found as a function of vanadium oxide loading on SiO2 (3% V2O5/SiO2 and 8% V2O5/SiO2). Quantum chemistry simulations of the 51V NMR chemical shifts on predicted surface structures were used as an aid in understanding potential surface va...

Potassium-modified silica-supported vanadium oxide catalysts applied for propene epoxidation

The influence of potassium ions on the structure and catalytic properties of silica-supported vanadium oxide catalysts was studied in the epoxidation of propene using N2O as an oxidant. The combination of different techniques (XRD, DR UV–vis, Raman, FTIR, and H2 TPR) for the characterization of potassium-doped V/SBA-15 catalysts reveals the formation of various types of vanadate species, including polyvanadates. Potassium changes the acid–base characteristics of the system and decreases the acidic character of surface vanadia. A decrease in the acidic character accounts for better selectivity in propene epoxidation; however, neutralization of active vanadium forms results in lower propene conversion. Textural, XRD and SEM/TEM, results indicate that the ordered hexagonal mesoporous structure with large pore diameters of the support is retained upon vanadium incorporation, although further modification with potassium ions results in a partial structure damage.

Single-Site VOx Moieties Generated on Silica by Surface Organometallic Chemistry: A Way To Enhance the Catalytic Activity in the Oxidative Dehydrogenation of Propane

We report here an accurate surface organometallic chemistry (SOMC) approach to propane oxidative dehydrogenation (ODH) using a μ2-oxo-bridged, bimetallic [V2O4(acac)2] (1) (acac = acetylacetonate anion) complex as a precursor. The identity and the nuclearity of the product of grafting and of the subsequent oxidative treatment have been systematically studied by means of FT-IR, Raman, solid-state (SS) NMR, UV–vis DRS, EPR and EXAFS spectroscopies. We show that the grafting of 1 on the silica surface under a rigorous SOMC protocol and the subsequent oxidative thermal treatment lead exclusively to well-defined and isolated monovanadate species. The resulting material has been tested for the oxidative dehydrogenation of propane in a moderate temperature range (400–525 °C) and compared with that of silica-supported vanadium catalysts prepared by the standard impregnation technique. The experimental results show that the catalytic activity in propane ODH is strongly upgraded by the degree of isolation of the VO...

Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts

We investigate the chemical reactions involved during the synthesis of supported vanadium oxide catalysts using chemical grafting of vanadium oxytriisopropoxide (VO(OiPr)3) to thermally pretreated silica under solvent-free conditions. VO(OiPr)3 is found to react with both site-isolated silanol (Si−OH) groups and strained siloxane (≡Si−O−Si≡) bridges at the silica surface. Solid-state 51V and 13C MAS NMR confirms the formation of two slightly different vanadium species associated with the two anchoring mechanisms. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ Raman spectroscopy, and thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) were used to study the subsequent calcination, revealing the formation of a transient V—OH intermediate upon the release of propene, followed by the formation of isolated VO4 surface species upon elimination of water. X-ray absorption spectroscopy (XAS) and 51V MAS NMR of the calcined material confirm the conversion of the two original vanadium sites to a species with a single isotropic shift, confirming the formation of isolated, tetrahedral VO4 sites.

Propane-to-propene oxide oxidation on silica-supported vanadium catalysts with N2O as an oxidant

Propane-to-propene oxide oxidation was carried out at 673 and 703K and for different contact times on vanadium catalyst supported on mesoporous silica of SBA-3 structure in the presence of N2O as an oxidant. For comparison, direct propene epoxidation was also investigated. Vanadium catalysts for propane oxidation undergo deactivation with time on stream. It was noted that the higher the reaction temperature the faster the deactivation. The very small amount of coke deposit in the spent catalyst would indicate that the vanadium reduction is responsible for catalyst deactivation. FT-IR spectroscopy was used to study the adsorbed surface species involved in the selective propane oxidation. Considering the alteration of selectivity toward oxygen-bearing products with propane/propene conversion influenced by reaction temperature and contact time, some tentative reaction paths for the selective oxidation of propane have been proposed.

A unified view on heterogeneous and homogeneous catalysts through a combination of spectroscopy and quantum chemistry.

Identifying catalytically active structures or intermediates in homogeneous and heterogeneous catalysis is a formidable challenge. However, obtaining experimentally verified insight into the active species in heterogeneous catalysis is a tremendously challenging problem. Many highly advanced spectroscopic and microscopic methods have been developed to probe surfaces. In this discussion we employ a combination of spectroscopic methods to study two closely related systems from the heterogeneous (the silica-supported vanadium oxide VOx/SBA-15) and homogeneous (the complex K[VO(O2)Hheida]) domains. Spectroscopic measurements were conducted strictly in parallel for both systems and consisted of oxygen K-edge and vanadium L-edge X-ray absorption measurements in conjunction with resonance Raman spectroscopy. It is shown that the full information content of the spectra can be developed through advanced quantum chemical calculations that directly address the sought after structure-spectra relationships. To this end we employ the recently developed restricted open shell configuration interaction theory together with the time-dependent theory of electronic spectroscopy to calculate XAS and rR spectra respectively. The results of the study demonstrate that: (a) a combination of several spectroscopic techniques is of paramount importance in identifying signature structural motifs and (b) quantum chemistry is an extremely powerful guide in cross connecting theory and experiment as well as the homogeneous and heterogeneous catalysis fields. It is emphasized that the calculation of spectroscopic observables provides an excellent way for the critical experimental validation of theoretical results.

Oxidovanadium(V) Anchored to Silanol-Functionalized Polyoxotungstates: Molecular Models for Single-Site Silica-Supported Vanadium Catalysts

The metalation of two different types of silanol-decorated polyoxotungstates, [XW9O34–x(tBuSiOH)3]3– (X = P, x = 0; X = Sb, x = 1) and [PW10O36(tBuSiOH)2]3–, by Cl3VO or (iPrO)3VO has been achieved. The characterization of the resulting oxidovanadium(V) complexes [XW9O34–x(tBuSiO)3VO]3– (X = P, 3; X = Sb, 3′) and [PW10O36(tBuSiO)2VO(iPrO)]3– (4) has been fully detailed, including X-ray analysis. These compounds are present in monomeric forms and therefore represent original molecular models for tris-grafted and bis-grafted isolated (V═O)3+ species dispersed onto silica. Their ability as precatalysts for the epoxidation of cyclic olefins and allylic alcohols with tert-butyl hydroperoxide (TBHP) has been studied. The results suggested that a confined tris-grafted species, such as 3 or 3′, does not act as an efficient catalyst whereas a more labile bis-grafted species, such as 4, does. To gain a better understanding, we have assessed their suitability for ligand exchange with alcohols and TBHP and performed ...

Normal mode analysis of silica-supported vanadium oxide catalysts: Comparison of theory with experiment

The vibrational structure of silica-supported vanadium oxide species has been studied by normal mode analysis using polyhedral oligomeric silsesquioxanes (POSSs) to describe the silica support. The analysis reveals that the vanadium oxide-related vibrational bands are characterized by significant contributions of several force constants. Their discussion in terms of single bond-unit designators is therefore not adequate. The consideration of the silica support is shown to be of importance for the vibrational spectrum. The theoretical results are fully consistent with experimental data for a silica-supported vanadium oxide catalyst with a vanadium coverage of 0.7atoms/nm2.

Promoting effect of carbon dioxide on the dehydrogenation of ethylbenzene over silica-supported vanadium catalysts

Dehydrogenation of ethylbenzene (EB) to styrene (ST) in the presence of carbon dioxide (CO2) was carried out over silica-supported vanadium catalysts (VOx/SiO2) to investigate the role of CO2 played in this reaction coupling process. A prominent promoting effect of CO2 on EB dehydrogenation is observed; over VOx/SiO2 with a vanadium loading of 0.8mmol/g-SiO2, ST yield at 550°C in CO2 is 2.05 times higher than that in an inert atmosphere of nitrogen and the catalyst also deactivates much more slowly in CO2. CO2 as a soft oxidant can effectively keep/regain high valence vanadium species that are highly active for EB dehydrogenation, which is then conducive to enhancing EB conversion and suppressing catalyst deactivation. Both carbonaceous deposition and deep reduction of the active vanadium species contribute to the catalyst deactivation; however, CO2 is only effective on alleviating the catalyst deactivation by protecting the high valance vanadium species from deep reduction, but is invalid in suppressing coke formation.

Silica-supported vanadium oxide nanoparticles as efficient heterogeneous catalyst for hydrolysis of sucrose

The reaction kinetics of the hydrolysis of sucrose by solid catalysts was investigated using polarimetry. In this research work the V2O5/SiO2 was selected as appropriate catalyst. This catalyst was prepared by impregnation method. At the optimizing conditions the activation parameters have been evaluated using the Arrhenius and Eyring plots. The structure and particle size of catalyst has been studied with the use of XRD.

Oxo/imido heterometathesis of N-sulfinylamines and carbonyl compounds catalyzed by silica-supported vanadium oxochloride

A series of silica-supported vanadium oxo complexes has been prepared via water-free grafting VOCl 3 onto the silica surface. The obtained materials have been characterized with Raman, diffuse reflectance FTIR (DRIFT) and UV–vis, 51V solid-state NMR and X-ray photoelectron spectroscopies, elemental analysis and N 2 physisorption. The samples with a low vanadium loading (1–5%) have been found to be comprised primarily of the isolated tetrahedral d 0 units ( SiO)V( O)Cl 2, whereas the materials with a higher vanadium content are contaminated with a significant amount of d 1 vanadium species. The silica-supported vanadium complexes act as heterogeneous catalysts for oxo/imido heterometathesis between N-sulfinylamines and carbonyl compounds affording imines and SO 2. Grafting VOCl 3 onto silica leads to a dramatic enhancement of its catalytic activity. A novel water-free express method for preparation of imines of wide range of aldehydes and some ketones has been developed. Noteworthy, this is the first example of transition metal mediated heterometathetical imidation of ketones.

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