Determination of the Oxidation State of Antimony, Iron and Vanadium in Mixed Vanadium and Iron Antimonate Oxide Catalysts

Vanadium and mixed vanadium and iron antimonates with rutile-type structures have been studied by XANES at Sb L₁ edge, ⁵⁷Fe Mössbauer spectroscopy and ESR spectroscopy at 77 K. The results showed that both antimony and iron remained at their highest oxidation state, i.e. Sb^V and Fe^III, whereas vanadium was present as V^III and V^IV. Two types of V^IV species were distinguished corresponding to well-isolated vanadyl species in distorted octahedral coordination and vanadyl species in the same coordination but close to each other and in a dipole–dipole interaction. Both V^III and total V^IV concentrations decreased when the iron content increased, whereas isolated V^IV concentration increased first and then decreased, with a maximum for x = 0.2 in Fe_xV_1-xSbO₄. The observed variations in cationic composition are discussed in relation with the catalytic properties of the compounds in the ammoxidation of propane. Isolated V^IV-O moieties appeared to be the most active and selective catalytic sites.     

Silicate intercalated cobalt chromium–hydrotalcite (CoCr–HTSi): An environment-friendly recyclable catalyst for organic transformations

Silicate intercalated cobalt chromium hydrotalcite (CoCr–HTSi) was prepared for the first time by a simple sol–gel method. The resultant CoCr–HTSi yielded remarkable activity for allylic oxidation of cyclohexene and aromatic alcohols in the presence of environmentally-friendly conditions, viz., H₂O₂ as oxidant and methanol as oxidant. More importantly, the active species remain intact on the surface and no leaching occurred under the reaction conditions. The catalysts are recyclable even after four cycles, and have shown to be as truly heterogeneous in nature.     

Alkene oxidation catalyzed by transition metal substituted Keggin-type heteropolyanions

Alkene oxidations with various oxidants (tert-butyl hydroperoxide, iodosylbenzene and molecular oxygen in the presence of isobutyraldehyde (IBA)) catalyzed by transition metal monosubstituted heteropolyanions, PW₁₁MO ₃₉ ^n– (PW₁₁M; M=Co^II, Mn^II, Cu^II, Ti^IV, Ru^IV, V^V and Nb^V), have been studied. Orders of catalytic activity of PW₁₁M are different for the oxidants studied. Radical chain mechanisms are proposed fort-BuOOH and O₂/IBA. Preliminary coordination of the oxidant to PW₁₁M is not a necessary step of its homolytic activation. Epoxidation with PhIO requires its coordination to the catalyst and most likely includes the formation of active metal-oxo species.     

CUII(Sal-Ala)/MgAlLDH and CUII(Sal-Phen)/MgAlLDH as novel catalytic systems for cyclohexene oxidation by H2O2

We present here Cu^II(Sal-Ala)/MgAlLDH and Cu^II(Sal-Phen)/MgAlLDH complexes as novel catalytic systems for the cyclohexene oxidation by H₂O₂. The physical–chemical properties of the catalysts were investigated by XRD, FTIR, UV-Vis, XPS and TGA techniques. We present evidence that the properties of the Cu^II-based complexes and the specific properties of the LDHs matrix both beneficially contribute to establish the cyclohexene catalytic oxidation efficiency. Cu^II(Sal-Ala)/MgAlLDH showed the best catalytic activity with a cyclohexene conversion of 81% and 78% epoxide selectivity. The leaching effect of copper has not observed, and the CH epoxidation yields can be maintained during five catalytic cycles.     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Uptake of VIV ion of vanadocene dichloride by apo-transferrin investigated by ESR spectroscopy

The action mechanism of vanadocene dichloride, Cp₂VCl₂ (Cpη⁵-C₅H₅), has been investigated by interaction with apo-transferrin and albumin for its promising antitumor activities. Our ESR spectral studies clearly showed that V^IV ion is readily transported from vanadocene chloride to transferrin, and the V^IV ions are captured at the Fe(III) binding sites of transferrin as a VO²⁺ form. We also observed that some binuclear VO²⁺ complexes can efficiently transport the VO²⁺ ions to the apo-transferrin.     

Electroreduction of Diheptylviologen in a Self-Assembled Phospholipid Monolayer on Mercury and its Role as an Electron Transfer Mediator

The electroreduction mechanism of diheptylviologen (V²⁺) across a self-assembled dioleoylphosphatidylcholine (DOPC) monolayer supported on a mercury electrode has been investigated in aqueous 0.1 M KCl by the chronocoulometric technique. V²⁺ is reduced in two consecutive charge steps, 2V²⁺ + 3e → V⁺₂ and V⁺₂ + e → 2V⁰, involving 3/2 and 1/2 of a faraday per mole of reactant, respectively. Both charge steps are controlled by the rate of the elementary coupling step V⁺ + V⁰ → V⁺₂. V²⁺ acts as an electron transfer mediator from ferric and ferricyanide ions dissolved in the aqueous solution to the mercury surface across the DOPC monolayer, which is impermeable to these metal ions in the absence of V²⁺. The electron transfer mediation is more effective along the first charge step, 2V²⁺ + 3e → V⁺₂, and increases with the V²⁺ concentration in the lipid monolayer less than proportionally, thus pointing to the radical cation V⁺ as the most effective electron carrier from the mercury surface to the metal ions.     

Crystal Structures,Thermogravimetric and Magnetic Properties of Four Organodiamine Templated Vanadium Oxide Frameworks: Influences of Diaminoalkane Templates

Absract A series of four diaminoalkane templated vanadium oxide polymeric frameworks, [V₂^IVO₈V₂^VO₂](C₂H₁₀N₂), [V₂^IVO₈V₂^VO₂](C₃H₁₂N₂), [V₄^IVO₁₀V₂^VO₄](C₄H₁₄N₂) and [V₄^IVO₁₀V₂^VO₄](C₅H₁₆N₂), have been successfully prepared under hydrothermal conditions. The crystal structures are fully characterized revealing layered structures composed of common inorganic building units, namely {V^IVO₅} square pyramids and {V^VO₄} tetrahedra. The layer registries are different depending on the molecular structure of the diaminoalkanes, and can be accounted for by the organic-inorganic interface interactions. The analysis of hydrogen bonds indicates their important role in directing two- and three-dimensional structural architectures. The influences of different diaminoalkanes are also apparent in both thermogravimetric and complex magnetic behaviors, and are discussed in details.     

Reaction pathway of benzonitrile formation during toluene ammoxidation on vanadium phosphate catalysts

The reaction pathway of the ammoxidation of toluene on (VO)₂P₂O₇ used as catalyst and the interaction of potential intermediates with the pyrophosphate were studied by spectroscopic techniques (FTIR, EPR), temperature-programmed chemisorptions/ reactions (TPD, TPRS) and transient studies such as the temporal analysis of products (TAP) technique. NH₃ is chemisorbed on the catalyst surface, forming three different species, i.e., NH ₄ ⁺ ions located on BrØnsted sites, coordinatively bound NH₃ on Lewis sites and NH ₂ ^– groups, presumably P-NH₂. Toluene that is probably adsorbed on Lewis sites reacts in a first step to a benzyl radical. A subsequent partial oxidation by interaction of V^IV=O groups generates a V...O=CH-C₆H₅ surface structure. This benzaldehyde-like surface species reacts with adsorbed NH₃ according to a Langmuir-Hinshelwood mechanism. TAP experiments on ammonium-containing vanadium phosphates revealed that NH ₄ ⁺ ions could act as potential N-insertion species. No formation of benzylamine as well as the generation of V=NH surface groups as possible intermediates or N-insertion sites were observed.     

Study of SO2 oxidation over V2O5/activated carbon catalyst using in situ diffuse reflectance infrared Fourier transformation spectroscopy

The SO₂ oxidation over V₂O₅/AC catalyst was studied using an in situ diffuse reflectance infrared Fourier transformation spectroscopy technique at 120 °C. Results reveal that the surface oxygen groups could neither act as active sites for SO₂ oxidation nor supply the oxygen needed for V ^V ?V ^IV redox cycle. The vanadia species and gas phase oxygen are essential for SO₂ oxidation. During SO₂ oxidation over V₂O₅/AC, the surface hydroxyl groups involve in the formation of sulfate species. The role of water vapor in flue gas might be to supplement the hydroxyl groups consumed so that the SO₂ oxidation could continue.     

Oxidation catalysis with well-characterised vanadyl bis-bipyridine complexes encapsulated in NaY zeolite

Vanadyl exchanged faujasite (VO²⁺-NaY) allows bipyridine complexation, giving a heterogeneousship-in-a-bottle catalyst denoted as [VO(bpy)₂]²⁺-NaY. The [VO(bpy)₂]²⁺ complexes associated with the zeolite are characterised with FT-Raman, FT-IR, XPS, and diffuse reflectance spectroscopy (DRS), as well as with electron paramagnetic resonance (EPR). It is established that the cationic complexes are intrazeolitic and homogeneously distributed across the zeolite crystals and both the zeolite and the neutral bipyridine ligands stabilise V^IV. The catalytic oxidation of cyclohexane and cyclohexene with different peroxides or mono-oxygen atom donors in presence of several solvents is described. Good epoxide selectivity results from the complexation by bipyridine thus favoring the heterolytic over the homolytic decomposition pathway of V-peroxo-intermediates.     

In situ immobilization properties and mechanism of geopolymer microspheres after adsorbing Sr2+ and Cs+ (Sr/Cs@GPMs)

Herein, in situ immobilization properties and mechanism of the pre-prepared geopolymer microspheres after adsorption of Sr²⁺/Cs⁺ (Sr/Cs@GPMs) were studied. The Sr²⁺ and Cs⁺ were solidified via calcination and a GP slurry coating strategies. The conditions for leaching experiments were H₂O, 0.1 mol/L (NaCl, NaOH, and HCl). The calcination results showed that the pore volume of the two adsorbents gradually decreased with increasing temperature and the order of leaching rate of Sr²⁺/Cs⁺ in calcination of Sr/Cs@GPMs was: HCl > NaCl > NaOH > H₂O, while the leaching rate decreased with increasing temperature and met the national standards. The in situ immobilization mechanism revealed that the pores of the adsorbent disappeared after high-temperature calcination or ceramic reaction. The results of the GP slurry coating experiment showed that the leaching rates of Sr²⁺/Cs⁺ decreased with leaching cycles. The solidified slag-based GP with Sr/Cs@GPMs (12%) adsorbent in 0.1 mol/L HCl leaching environment had the 28-day leaching rate (R₂₈) and cumulative leaching fractions (P₂₈) for Sr²⁺ of 1.26 × 10^-3 cm/d and 0.057 cm, respectively, and 1.51 × 10^-3 cm/d and 0.127 cm for Cs⁺, respectively. These metrics met the requirements of the national standard, thus indicating that slag-based GP has value in treating radionuclides.     

Oxovanadium(IV) complexes based on S-alkyl-thiosemicarbazidato ligands. Synthesis,characterization, electrochemical,and antioxidant studies

Reaction of VOSO₄ with 2-hydroxy-napthaldeyde-S-R-thiosemicarbazones (R: methyl, ethyl, propyl or allyl) and salicyl aldehyde yielded five-coordinate oxovanadium(IV) complexes having a N¹,N⁴-diarylidene-S-R-thiosemicarbazidato structures. The compounds were characterized by elemental analysis, magnetic measurements, electronic, infrared, ¹H-NMR, and electron paramagnetic resonance (EPR) spectra. The X-band EPR signals were recorded from powder forms and also in solution. All the complexes have a single asymmetric line shape and theoretical fit studies prove the presence of axial symmetry around the paramagnetic vanadium ions. A computer simulation of the EPR spectrum of each complex was carried out to derive the related EPR parameters. Cyclic voltammograms of the complexes exhibited two metal-based reversible redox peaks around 500 and ?800?mV corresponding to one electron oxidation/reduction of V^IVO/V^VO and V^IVO/V^IIIO, respectively. The reductive response in the 50–350?mV region was assigned to ligand reduction. Antioxidant activities of the compounds were determined with CUPric Reducing Antioxidant Capacity, 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid, and 1,1-diphenyl-2-picrylhydrazyl assays. The test results indicated that the antioxidant capacity of the compounds increases with the carbon number of saturated hydrocarbon chain on sulfur atom.     

Direct incorporation of vanadium into three-dimensional KIT-6: 2. Reactivity test for styrene oxidation

The direct incorporation of vanadium into the three-dimensional (3-D) cubic Ia3d mesostructure designated as V-KIT-6 was prepared, and the material obtained therein showed a very high specific surface area of ∼1,000 m²/g with tunable pore diameters in a narrow distribution of sizes, ∼5.7 to 6.0 nm. The coordination and nature of the V sites in V-KIT-6 were characterized by ⁵¹V-spin-echo NMR analysis. It shows that after calcination, the V⁴⁺ species are totally oxidized to the V⁵⁺ state with 4- and 6-coordinated V-O environments in a highly dispersed state with much less crystalline V₂O₅ formation. The calcined V-KIT-6 materials showed excellent catalytic activity in the direct oxidation of styrene using tert-butyl hydroperoxide (TBHP) as an oxidant.     

Metallo Salicylidenetriazol Complexes Encapsulated in Zeolite-Y: Synthesis, Physicochemical Properties and Catalytic Studies

Chromium(III), zinc(II) and nickel(II) complexes of thio-Schiff base derived from salicylaldehyde and 4-amino-2,4-dihydro-1,2,4-triazole-5-thione have been encapsulated in the nanopores of zeolite-Y by a flexible ligand method. The prepared encapsulated metal complexes have been characterized by surface analysis (XRD and N₂ adsorption/desorption), spectroscopic methods, chemical and thermal analyses. The various techniques of characterization used demonstrated that these complexes were effectively encapsulated in the zeolite supercages without structural modification or loss of crystallinity of the zeolite framework. The encapsulated complexes were screened as heterogeneous catalysts for various oxidation reactions such as phenol, cyclohexene and styrene oxidation, using H₂O₂ as an oxidant. Under the optimized conditions, these catalysts exhibited high to moderate activity. After a few cycles these catalysts were found to be stable and could be reused after recovering without detectable catalyst leaching or significant loss of activity.     

P-450 Type Activation of Dioxygen by Heterogenized Metal Porphyrins: Comparison with the Corresponding Homogeneous Systems

The protons of silanol groups on a silica or silica-alumina surface are replaced by metal(III) porphyrin cations, FeTPP⁺, MnTPP⁺, and CoTPP⁺ (TPP = tetraphenylporphyrin). The heterogenized iron(III) or manganese(III) porphyrin supported on silica thus prepared can activate dioxygen in the presence of excess amounts of NaBH₄ and cyclohexene, affording cyclohexanol and cyclohex-2-ene-1-ol in a 4:1 ratio, which is the same as the ratio observed in the known P-450 type oxidation of cyclohexene catalyzed by a homogeneous manganese porphyrin (MnTPPCl) in the presence of NaBH₄ used as an electron source. The heterogenized iron(III) porphyrin shows a reactivity comparable with that of the corresponding heterogenized manganese(III) porphyrin. On the other hand, substitution of the heterogenized iron(III) porphyrin by the corresponding homogeneous catalyst, FeTPPCl, results in drastic change in the product ratio; the ratio of cyclohexanol to cyclohex-2-ene-l-ol is reversed, suggesting that the oxidation of cyclohexene proceeds via radical chain autoxidation reactions. The catalytic activity for the P-450 type oxidation is diminished while FeTPPCl is converted to the μ-oxo-dimer (FeTPP)₂O. Cobalt(III) porphyrin in a homogeneous system also affords the autoxidation products. Such a mechanistic difference between the homogeneous and heterogeneous metal porphyrins in the oxidation of cyclohexene by dioxygen has been confirmed by kinetic studies in which the P-450 type oxidation and autoxidation obey different kinetics.     

A missing oxidation-state level in the family of polyoxo(azide)octadecavanadate(IV/V) clusters: Synthesis,structure and antitumoural properties of [VIV11VV7O44(N3)]10 − in a sodium containing-3D architecture

The missing V^IV₁₁V^V₇ oxidation-state member of the polyoxooctadecavanadate(IV/V) — azide family of inorganic host-guest systems has been incorporated in a Na^I-containing 3D architecture; the complex shows remarkable inhibition properties on human platelet aggregation and antitumoural activity against cancer cell lines.     

Synthesis, characterization of high Ti-containing Ti-MCM-41 catalysts and their activity evaluation in oxidation of cyclohexene and epoxidation of higher olefins

A series of titanium rich isomorphous substituted Ti MCM-41 and HMS materials have been synthesized with different Si/Ti ratios. The highest amount of Ti incorporated in synthesis gel is in TiMCM-41 (Si/Ti = 10). Whereas for TiHMS catalysts, Ti is incorporated up to Si/Ti = 50 successfully without forming any extra framework TiO₂. Cyclohexene epoxidation reaction with dry tert-butyl hydroperoxide (TBHP) as an oxidant has been studied to evaluate the catalytic properties of Ti substituted mesoporous catalysts. The effect of molar ratio of substrate:oxidant in this reaction has been studied and high conversion, high selectivity were achieved at 2: 1 molar ratio with TiMCM-41 (Si/Ti =25). Dry TBHP (in dichloromethane) and chloroform were found as good oxidant and solvent system for this reaction. Pure siliceous mesoporous silica and low `Ti' substituted mesoporous silicas were found to be efficient catalysts for oxidation of cyclohexene. An interesting variation of the selectivity from allylic oxidation to epoxidation during oxidation of cyclohexene was observed with an increase in the Ti amount in the mesoporous framework. The allylic oxidation of cyclohexene has been carried out using molecular oxygen as an oxidant and in the presence of a small amount of TBHP as initiator. Siliceous HMS materials gave better conversion compared to MCM-41 type of materials and other conventional silicas like silica gel, fumed silica etc. in allylic oxidation of cyclohexene. Epoxidation of higher cyclic olefins like cyclooctene, cyclododecene, cis-cyclododecene and linear olefins 1-Heptene, cis-2-hexene, 1-undecene was carried out over TiMCM-41 (Si/Ti = 25). Ti substituted mesoporous catalysts were characterized by elemental analysis, XRD, FTIR, UVDRS, ²⁹SiMASNMR, BET surface area and pore size distribution techniques.     

Surface analytical approach to TiCl3-based Ziegler-Natta catalysts combined with microstructure analysis of polymer

In the field of Ziegler-Natta catalysis (ZNC), most of the previous works have been focused on the characterization of either catalyst surface or polymer microstructure but the combined studies of these two aspects are still limited. A clearer understanding concerning the isospecific nature of active sites on the donor-free TiCl₃ - based catalyst has been presented in this work in terms of both surface characteristics of the catalyst and microstructure of polypropylene (PP) produced. Ti- and Al-species existing in the catalyst surface have been identified by high resolution X-ray photoelectron spectroscopic (XPS) and solid-state magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR) techniques, respectively. Moreover, grinding effect on catalyst performance has been carried out to investigate the nature of active sites by particle size distribution (PSD) method. Microstructures of PPs produced using the surface characterized catalysts have been investigated by temperature rising elution fractionation (TREF), ¹³C NMR and GPC methods. It can be demonstrated that the surface structure of catalyst is dominantly affected by the type of Al-alkyl cocatalyst used as well as the preparation method of catalyst whilst it precisely determine the nature of isospecificity of active sites and its distribution. Two different types of surface Ti⁺³-species have been identified on the catalyst surface depending on the different vacant coordination states of surface titanium species. On the other hand, ²⁷Al MAS NMR experiments have shown the presence of surface aluminum in four (Al^IV), five (Al^V) and six (Al^VI) coordination number of multiple environments. The states of surface Al-species are sensitive to coordination number and symmetry of the local environment around the aluminum nuclei. Higher isospecificity of active sites is seen to be related to the higher fulfilled coordinated states of surface Ti- and Al-species.     

Sc3+-doping effects on porous Li3V2(PO4)3/C cathode with superior rate performance and cyclic stability

An enhanced sol-gel combustion method was used to synthesize different porous Sc³⁺-doped Li₃V_2-xSc_x(PO₄)₃/C (x = 0.00, 0.05, 0.10 and 0.15) compounds. The substitution of Sc³⁺ into the V³⁺ sites of Li₃V_2-xSc_x(PO₄)₃/C expands the lattice volume along with the enlargement of Li⁺ diffusion channel, which is beneficial for Li⁺ transportation and ionic conductivity improvement. Besides, the Sc³⁺ doping content exhibits a great impact on the morphology of Li₃V_2-xSc_x(PO₄)₃/C composite. The pristine Li₃V₂(PO₄)₃/C are constituted of porous particles and nanorods, and the ratio of nanorods to particles can be controlled by adjusting the amount of Sc³⁺ doping since the ratio of nanorods to particles decreases with increasing Sc³⁺ doping content. When Sc³⁺ doping content increases to a certain level (x = 0.15, Li₃V_1.85Sc_0.15(PO₄)₃/C), the nanorods are hardly seen. Li₃V_1.90Sc_0.10(PO₄)₃/C with higher tapped density, better reversibility, smaller resistance and larger Li⁺ diffusion coefficient demonstrates outstanding rate performance and cyclic stability, together with high specific discharge capacities of 130.2 and 92.9 mAh g⁻¹ at 0.5 and 20 C, respectively. Furthermore, a superior specific discharge capacity of 85.8 mAh g⁻¹ was retained at 20 C following 1000 cycles. Overall, a novel approach for the preparation of high-performance Li₃V_2-xSc_x(PO₄)₃/C cathodes with different morphologies for lithium-ion batteries is provided.