Silica-supported Vanadium Oxide Research Articles

Mesoporous sol-gel silica supported vanadium oxide as effective catalysts in oxidative dehydrogenation of propane to propylene.

Vanadium oxide incorporated mesoporous silica (V-m-SiO2) were designed and synthesized using a surfactant-modified sol-gel method. Detailed characterization shows that monomeric [VO4] sites containing one terminal V[double bond, length as m-dash]O bond and three V-O-support bonds are dominated until atomic ratio of vanadium to silicon approaches to 5%. It is also confirmed that such V-m-SiO2 catalyst contains high proportion of vanadium oxide species interacting strongly with silica. Compared to vanadium oxide supported mesoporous silica (V/m-SiO2) prepared using a traditional impregnation method, present V-m-SiO2 catalyst exhibits more superior ability to catalyze oxidative dehydrogenation of propane to propylene. By correlation with structural data, superior catalytic performance of present V-m-SiO2 catalyst can be reasonably attributed, in part, to its favorable geometric and electronic properties rendered by the unique preparation method.

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.

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...

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.

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.

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.

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.

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.

Reply to Comment on “Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts”

Reply to Comment on “Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts”

Hydrogen‐Atom Abstraction from Methane by Stoichiometric Vanadium–Silicon Heteronuclear Oxide Cluster Cations

Vanadium-silicon heteronuclear oxide cluster cations were prepared by laser ablation of a V/Si mixed sample in an O(2) background. Reactions of the heteronuclear oxide cations with methane in a fast-flow reactor were studied with a time-of-flight (TOF) mass spectrometer to detect the cluster distribution before and after the reactions. Hydrogen abstraction reactions were identified over stoichiometric cluster cations [(V(2)O(5))(n)(SiO(2))(m)](+) (n=1, m=1-4; n=2, m=1), and the estimated first-order rate constants for the reactions were close to that of the homonuclear oxide cluster V(4)O(10) (+) with methane. Density functional calculations were performed to study the structural, bonding, electronic, and reactivity properties of these stoichiometric oxide clusters. Terminal-oxygen-centered radicals (O(t)*) were found in all of the stable isomers. These O(t)* radicals are active sites of the clusters in reaction with CH(4). The O(t)* radicals in [V(2)O(5)(SiO(2))(1-4)](+) clusters are bonded with Si rather than V atoms. All the hydrogen abstraction reactions are favorable both thermodynamically and kinetically. This work reveals the unique properties of metal/nonmetal heteronuclear oxide clusters, and may provide new insights into CH(4) activation on silica-supported vanadium oxide catalysts.

Alkali Metal Ion-Modified Vanadium Mononuclear Complex for Photocatalytic Mineralization of Organic Compounds

Modification of silica-supported vanadium oxide (VS) photocatalysts with alkali metal ions enhanced the photocatalytic activity of the vanadium(V) mononuclear complex for oxidative decomposition of gaseous organic compounds into carbon dioxide. Action spectrum analysis revealed that the onset wavelength for cesium-ion-modified VS was 420 nm, which was ca. 20 nm longer than that with a titania photocatalyst, P25.

ChemInform Abstract: Structure and Reactivity of Silica-Supported Vanadium Oxide

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Catalyst Structure-Performance Relationship Identified by High-Throughput Operando Method: New Insight for Silica-Supported Vanadium Oxide for Methanol Oxidation

The reaction mechanism of methanol oxidation catalyzed by vanadium oxides on a silica support (V2O5/SiO2) was investigated in a high-throughput operando reactor coupled with a Fourier transform-infrared (FT-IR) imaging system for rapid product analysis and six parallel, in situ Raman spectroscopy probes for catalyst characterization. Up to six V2O5/SiO2 catalysts with different vanadium loadings (i.e., from 0 to 7%) were simultaneously monitored under identical experimental conditions. The specific Raman bands of the different catalysts in the six parallel reaction channels are quantitatively determined in this work. Under steady-state reaction conditions, the Raman intensities of C–H stretch in Si–O–CH3 and V–O–CH3 were extensively studied at different reaction temperatures and different vanadium loadings. For the first time, we observed enhanced Si–O–CH3 formation on V2O5/SiO2 catalysts with low vanadium loadings. We attribute this phenomenon to surface cluster edge activation. Careful comparison of the in situ Raman intensity of V–O–CH3 on V2O5/SiO2 catalysts revealed different methoxy formation mechanisms in different reaction temperature regimes.

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V═O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quantitat...

Characterization of Supported Vanadium Oxide Species on Silica: A Periodic DFT Investigation

The geometry, energetic, and spectroscopic properties of molecular structures of silica-supported vanadium oxide catalysts are studied using periodic density functional calculations. Isolated vanadia units deposed on amorphous silica are modeled at low coverage, 0.44 atoms nm−2. The models are built following the grafting process through the reaction of a vanadium precursor with surface silanols: OV(OH)3 + (Si−OH)n → OV(OH)3−n(O−Si)n + nH2O (with n = 1−3). The most stable grafted structures involve one vanadyl group together with n(V−O−Si) bonds. The predominance of the vanadate groups is analyzed as a function of hydration by means of atomistic thermodynamics. At dehydrated conditions, the trigrafted pyramidal OV(O−Si)3 species are predominant, whereas partial hydration stabilizes digrafted OV(OH)(O−Si)2 and monografted OV(OH)2(O−Si) species. The harmonic vibrational spectra for selected models are compared to recent experimental infrared and Raman data, for representative bands, and vibrational modes. Hydration effects are discussed in terms of thermodynamic stability and vibrational spectra. The results obtained in this study show that the pyramidal OV(O−Si)3, digrafted OV(OH)(O−Si)2, and monografted OV(OH)2(O−Si) models can exist at a support surface, a trend in agreement with recent experimental findings.

SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE WITH AMMONIA OVER SILICA-SUPPORTED VANADIUM OXIDE CATALYST

The selective catalytic reduction (SCR) of nitric oxide with excess ammonia in the presence of oxygen on silica-supported vanadium oxide catalyst was studied in a packed-bed reactor, and a mathematical model was proposed for the processes occurring in the reactor. Experimental data were presented for evaluation of the accuracy of the proposed model. Good agreement was observed between the measured and calculated values of the conversion in the outlet of the reactor. Once the validity of the proposed model was verified, it was used to examine the effects of different parameters such as feed temperature, inlet feed composition, and gas hourly space velocity (GHSV) on the conversion of NO over V2O5/SiO2 catalyst for practical application. The results for the employed catalyst showed that high NO conversion occurred at temperatures of 280°–300°C, GHSV less than 2000 h−1 (STP), and O2 concentration greater than 10% v/v. These results clearly demonstrate the high potential for this catalyst to be applied commercially for the control of NOx emissions from flue gases of different sources.

Resonance Raman Spectroscopy of Discrete Silica-Supported Vanadium Oxide

Vanadium oxide deposited as discrete oxovanadium groups, [(−O)3V═O], in transparent silica xerogels were investigated by resonance Raman spectroscopy. Spectra were collected at 351 and 257 nm excitation into two distinct absorption bands of the oxovanadium site. Three new bands associated with vibrations of the vanadium oxide site were observed at 496, 568, and 720 cm−1. From these additional modes and the previously known vibrations at 1064, 1033, and 923 cm−1 an empirical force field was determined from which a normal-mode analysis of the primary stretching vibrations of the vanadium oxo group was carried out. This analysis indicates that for most of the observed bands the interfacial Si−O−V stretches are the primary component, and in fact, only the weak band at 923 cm−1 was dominated by the terminal V═O stretch. Shifts in the band positions with 18O isotopic enrichment are in general agreement with the normal-mode analysis, moreover, the enrichment indicates that the bridging groups are generally quite...

Ice-Assisted Preparation of Silica-Supported Vanadium Oxide Particles

Vanadium oxide particles were prepared by physical vapor deposition of vanadium in oxygen ambient onto ice-precovered well-ordered silica thin films. Morphology, electronic structure, and vibrational properties of the vanadia deposits were studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and infrared reflection absorption spectroscopy. The results show that the ice behaves as an oxidative agent, that favors vanadium oxidation up to V4+, and as a buffer layer that precludes strong interaction of the V adatoms with the silica film. At room temperature, upon desorption of the unreacted water, nanosized particles of vanadia hydroxide-like gel, containing V−OH and, to a lesser extent, vanadyl (VO) species, are formed. Vacuum annealing at 550 K leads to the total dehydration and partial reduction of the particles to V2O3, which expose the V-terminated surface. Subsequent reoxidation in 10-6 mbar of O2 irreversibly transforms the surface to the VO ter...

Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1–16 wt.% V has been systematically investigated by infrared, Raman, UV–vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V–OH groups, characterized by a band at 3660 cm −1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H 2O. Further hydration resulted in the appearance of a V–O–V vibration band located around 700 cm −1 pointing to the formation of di- or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a V⋯V contribution was detected for the hydrated species. The IR, Raman and UV–vis data suggested a pyramidal anchoring of the dehydrated VO x species, whereas, the EXAFS data pointed to the presence of single V–O–Si bonded VO x species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. Combining the results of this study with data from our previous studies [D.E. Keller, F.M.F. de Groot, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 109 (2005) 10223; D.E. Keller, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 110 (2006) 14313] as well as literature led to a reaction scheme in which a monomeric VO x species anchored by three Si–O–V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VO x species anchored by one Si–O–V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V–O–H and Si–O–H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure ( i.e., Si–O–V O(OH) 2) could be present under catalytic conditions where H 2O is a reaction product ( e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si–O) 3–V O) and the umbrella (Si–O–V O(OH) 2) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions.