51V and 17O NMR studies of the mixed metal polyanions in aqueous V—Mo solutions

NaVO₃Na₂MoO₄ solutions acidified with HCl were studied at the atomic V/Mo ratios equal to 3 : 1, 1 : 1, 1 : 3, 1 : 6 and vanadium concentration [V] = 0.1, 0.04, 0.004 and 0.0004 M in the range pH 7-2. Their ⁵¹V NMR spectra (measured at H_ito = 7 T) were compared with those of VW solutions containing mixed metal complexes of known composition. The VMo₅O₁₉³⁻ (⁵¹V NMR chemical shift relative to VOCl₃, δ, −502 ppm), V₂Mo₄O₁₉⁴⁻ (δ −494), V₂Mo₄O₁₉³⁻ δ −507), V₉MoO₂₈⁵⁻ (δ −422, −492, −501, −512, −521.5) and HV₉MoO₂₈⁴⁻ polyanions (p.a.) have been found to be dominant mixed species in Na-V-Mo solutions. Along with them the V_itxMO_13-itxO^−itx−3₄₀ p.a.(x∼2–3) of the Keggin type (δ −496, −498, −516, −522) are supposed to be formed at pH < 4 in concentrated solutions ([V] > 0.01 M). The V₂Mo₆O₂₆⁶⁻ p.a., isolated at pH ∼ 5 as the sodium salt (solid state δ −482), seem to be present in concentrated Na-V-Mo solutions only as minor species. On dissolving the salt the V₂Mo₆O₂₆⁶⁻ p.a. mainly disproportionates into the complexes mentioned. From solutions containing mainly the V₉MoO₂₈⁵⁻ p.a. the sodium salt of V₁₀O₂₈⁶⁻ is crystallized. The V₉WO₂₈⁶⁻ p.a. are detected in VW solutions at V/W > 1. ¹⁷O and ⁹⁵Mo NMR spectra of some mixed complexes are described. The distribution diagrams for VMo and V-W solutions at [V] = 0.004 M and V/Mo(W) = 1:3, derived from their ⁵¹V NMR spectra, are given.     

Isothermal gravimetry and magnetic properties of CoOMoO3γAl2O3 catalysts

Isothermal gravimetry and magnetic susceptibility of MoO₃, MoAl₂O₃, CoAl₂O₃ and CoMoAl₂O₃ with/without Na⁺ ions have been studied in order to investigate the reducibility of the systems in H₂ H₂—hydrocarbons and H₂—hydro-carbon—thiophene. These studies have evidenced the formation of metallic cobalt during reduction of cobalt—moly catalysts containing Na⁺ ions in the Al₂O₃ support. This metallic cobalt accelerates the reduction of supported MoO₃. However, in the absence of sodium, cobalt exerts an inhibitory influence on the reduction of MoAl₂O₃. The inhibition is caused mainly due to retention of the water evolved during the process by well-dispersed Co²⁺ ions which are incapable of undergoing reduction. The presence of sulfur also kelps in suppressing the reduction to cobalt metal.     

Electronic structures and X-ray photoelectron spectra of MoO2 and Li2 MoO4

Electronic structures of MoO₂ (4d²) and molybdatc (4d^o) are calculated by the discrete-variational Xα method employing [Mo₂O₁₀^12? and [MoO₄]^2? clusters. The calculations indicate that the Mo—O bond is more covalent in the molybdatc than in MoO₂. Level structures for the valence band region arc in agreement with XPS spectra of MoO₂ and Li₂MoO₄.     

Some considerations about equations correlating valence and length of a chemical bond

It is confirmed that the Zachariasen equation is more accurate than the Brown-Shannon equation. Moreover, it is shown that, given the value of the parameter R₁ relative to the MoO bond in MoO₆ octahedra, the calculated valence of molybdenum in Mo(V)O₆ octahedra remains very close to 5 v.u. by varying the other bond parameter B in a wide range. For other oxidation states of molybdenum in MoO₆ octahedra, the difference between the calculated valence and the formal oxidation state shows different types of dependence on B. These behaviours could explain some errors in valence data of the literature. The parameters for the MoO bond in MoO₆ octahedra are recalculated as R₁=1.8788 Å and B=0.3046 Å for all molybdenum oxidation states from +3 to +6.     

Synthesis and crystal chemistry of NH3(MoO3)3

NH₃(MoO₃)₃ crystallizes with hexagonal symmetry, space group $\text{P6}{}_3\text{m}$, lattice constants a = 10.568 Å, c = 3.726 Å, and Z = 2. The crystal structure has been determined by Patterson synthesis and refined assuming isotropic temperature factors to a final conventional R value of 0.085. The structure shows a three-dimensional arrangement built up of double chains of distorted MoO₆ octahedra, parallel to the [001] direction. The octahedral double chains are linked among each other through common oxygen atoms. In addition to the shared oxygen atoms, each molybdenum is coordinated to one terminal oxygen. MoO distances range from 1.645 to 2.378 Å and OMoO angles from 74.3 to 114.3°. These results are consistent with the fact that molybdenum in high-valence states shows octahedral coordination with terminal oxygens.     

How Strain Affects the Reactivity of Surface Metal Oxide Catalysts

Highly dispersed molybdenum oxide supported on mesoporous silica SBA‐15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2–2.5 Mo atoms nm^?2). X‐ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO₄ units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature‐programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O‐K‐edge at high Mo loadings are explained by distorted MoO₄ complexes. Limited availability of anchor silanol groups at high loadings forces the MoO₄ groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.     

Preparation and properties of rare-earth-molybdenum(V) oxides Ln3MoO7

Phase equilibrium studies in Ln₂O₃MoO₂MoO₃ systems indicate, for a Ln₂O₃:Mo ratio of 3:2, the existence of a new molybdenum (V)-rare-earth oxide Ln₃MoO₇. Ternary oxides have been prepared for Ln ≡ La-Ho and Y. For Ln ≡ La-Eu, Ln₃MoO₇ compounds form at 1473 K under oxygen partial pressures ranging from 10^−7.3 to 10^−11.6 atm. For Ln ≡ Gd-Ho and Y compounds form above 1573 K and at 1473 K the stability range is about 10⁻⁹ to about 10⁻¹⁰ atm. Lattice parameters deduced from X-ray diffraction patterns are reported. For the large Ln cations, lanthanum to europium, all reflections could be indexed in an orthorhombic C-centred cell isotypic to Nd₃NbO₇. For Ln ≡ Gd-Ho and Y, strong f.c.c.-type reflections show that the structure is defect fluorite. Stability ranges in terms of oxygen partial pressure and crystal chemical properties are discussed with respect to the rare-earth elements. Magnetic susceptibility measurements of La₃MoO₇ confirm the oxidation state + 5 for molybdenum.     

The crystal structure of Ba2V2O7

Ba₂V₂O₇ is triclinic with a = 13.571(3), b = 7.320(2), c = 7.306(2) Å, α = 90.09(1), β = 99.48(1), β = 99.48(1), γ = 87.32(1)°, V = 7.15.1 ų, Z = 4, and space group $\text{P}\text{1}$. The crystal structure was solved by Patterson and Fourier methods and refined by full-matrix least-squares analysis to a R_w of 0.034 (R = 0.034) using 2484 reflections measured on a Syntex $\text{P}\text{1}$ automatic four-circle diffractometer. The structure has two unique divanadate groups that are repeated by the b and c lattice translations to form sheets of divanadate groups parallel to (100). These sheets are linked by four unique Ba atoms that lie between these sheets. Ba(1) and Ba(3) are coordinated by eight oxygens arranged in a distorted biaugmented triangular prism and a distorted cubic antiprism, respectively. Ba(2) is coordinated by 10 oxygens arranged in a distorted gyroelongated square dipyramid and Ba(4) is coordinated by nine oxygens arranged in a distorted triaugmented triangular prism. These coordination numbers are substantiated by a bond strength analysis of the structure, and the variation in 〈BaO〉 distances is compatible with the assigned cation and anion coordination numbers. Both divanadate groups are in the eclipsed configuraton with 〈VO(br)〉 bond lengths of 1.821(4) and 1.824(4) Å and VO(br)V angles of 125.6(3) and 123.7(3)°, respectively. Examination of the divanadate groups in a series of structures allows certain generalizations to be made. Longer 〈VO(br)〉 bond lengths are generally associated with smaller VO(br)V angles. When VO(br)V < 140°, the divanadate group is generally in an eclipsed configuration; when VO(br)V > 140°, the divanadate group is generally in a staggered configuration. Nontetrahedral cations with large coordination numbers require more oxygens with which to bond, and hence O(br) is more likely to be three coordinate, with the divanadate group in the eclipsed configuration. In the eclipsed configuration, decrease in VO(br)V promotes bonding between O(br) and nontetrahedral cations, and hence smaller nontetrahedral cations are generally associated with smaller VO(br)V angles.     

Lineare oligophosphaalkane: XVIII. Addition Ph-funktioneller methylenbisphosphane an die MoMo-Dreifachbindun in (η5-C5H5)2Mo2(CO)4

By means of the addition of the PH-functional methylenebisphosphanes R¹R²-PCH₂PR³H (PCP) to the MoMo triple bond in (η⁵-C₅H₅)₂Mo₂(CO)₄(MoMo) the complexes (η⁵-C₅H₅)₂Mo₂(CO)₄(PCP) containing a five-membered ring system Mo₂P₂C are obtained. Starting with unsymmetrically substituted methylenebisphosphanes R′₂PCH₂PRH only one isomer is formed, while the disecondary derivatives RHPCH₂PHR (as the diastereomeric mixture) gave two isomers of (η⁵-C₅H₅)₂Mo₂(CO)₄(PCP) (A₂ and AB) as indicated by the ³¹P{¹H} and ¹³C{¹H} NMR spectra.X-ray structural analysis of the derivative of the racemate of t-BuHPCH₂PH(t-Bu) space group C2/c, monoclinic, a 18.034(2), b 14.909(1), c 11.106(1) Å, α 90, β 99.788(8), γ 90°) reveals a puckered Mo₂P₂C five-membered ring system (dihedral angle PMoMo′P′ 54.4(2)°) with square-pyramidal coordination geometry at the Mo atoms. Two of the CO ligands (C(6)O(1) and C(6′)O(1′)) are almost coplanar with the molybdenum atoms, while the terminal CO groups (C(7)O(2) and C(7′)O(2′)) are about orthogonal (dihedral angle C(7)MoMo′C(7′) 88.4(3), MoMo′ 3.2109(4), MoP 2.4567(8), PC(8) 1.834(3), PH(P) 1.37(3) Å).     

The molybdenum-molybdenum triple bond—XVII. Syntheses,spectroscopic and structural characterizations of Mo4F4(O—t-Bu)8 and Mo2F2(O—t-Bu)4(PMe3)2 (MoMo)

Hydrocarbon solutions of Mo₂(O—t-Bu)₆ and PF₃ (2 equiv) yield Mo₄F₄(O—t-Bu)₈, I, and PF₂(O—t-Bu). Compound I contains a bisphenoid of molybdenum atoms with two short MoMo distances, 2.26 Å, and four long MoMo distances, 3.75 Å, corresponding to localized MoMo triple bonding and non-bonding distances, respectively. The tetranuclear compound may be viewed as a dimer, [Mo₂(μ₂-F)₂(O-t-Bu)₄]₂, and addition of PMe₃ to hydrocarbon solutions of I yields Mo₂F₂(O—t-Bu)₄(PMe₃)₂, II, which contains an unbridged MoMo triple bond of distance 2.27 Å. Each molybdenum atom is coordinated to two oxygen atoms, one fluorine atom and the phosphorus atom of the PMe₃ ligand in a roughly square planar manner. The overall central Mo₂O₄F₂P₂ skeleton has C₂ symmetry and NMR studies (¹H, ¹⁹F and ³¹P) are consistent with the maintenance of this type of structure in solution. Infrared and electronic absorption spectral data are reported. These are the first compounds containing fluorine ligands attached to the (MoMo)⁶⁺ unit.     

Use of overtone bands to monitor the state of the catalyst active phases during infrared studies of adsorption and catalytic reactions

Overtone bands are observable in the i.r. spectra of mixed and supported oxide catalysts, and may be informative on the state of the active phases and their interactions with absorbates. In the cases of supported V₂O₅/TiO₂, MoO₃/Al₂O₃ and WO₃/Al₂O₃ the behaviour of such bands indicates that single uncoupled MO bonds are exposed on the surface. In the case of bulk VPO catalysts they are informative on the presence of oxidized phases.     

Polarized IR and Raman spectra of kdy(moo4)2 single crystals,and the 92MO-100MO isotope effect

Polarized IR and Raman spectra below 4000 cm^?1 for single crystals of KDy(MoO₄)₂ are presented and discussed in relation to molecular and crystal structures. The spectroscopic data indicate additional intermolecular interactions due to pair coupling of the molybdate tetrahedra. This effect, combined with the multilayer crystal structure, is found to restrict the vibrational degrees of freedom, disturbing the selection rules for vibrational transitions. The spectra of first and higher-order transitions are described on the basis of ⁹²Mo/¹⁰⁰Mo isotopic-exchange effects and Y/Dy and K/Na substitutions.     

Untersuchungen zum ternären System V/Mo/O

Investigation on the Ternary System V/Mo/O During chemical transport reactions mixtures of pseudo-binary line of intersection V₂O₅/MoO₃ are separated into two phases, the V₂O₅-(α) phase, which MoO₃-content depends on the oxygen partial pressure during the deposition and the MoO₃ phase which contains no more than 1% (n/n) V₂O₅. Ternary compounds do not exist on the pseudo-binary line. V₉Mo₆O₄₀ is formed by the reaction of V₂O₅ and MoO₃ (3:2) under exclusion of oxygen. The compound may be chemically transported under the own oxygen coexistence pressure. It was shown by total pressure measurements of V₂O₅/MoO₃ starting mixtures that the insertion of MoO₃ in the α-phase and the formation of the V₉Mo₆O₄₀ phase respectively is connected with elimination of oxygen and the reduction of V^V to V^IV in equivalence of the quantity of incorporated MoO₃.     

Nano‐MoO3‐modified MCM‐22 for methane dehydroaromatization

The work reported was aimed at a simple method to improve the catalytic activity of Mo/HMCM‐22 in methane aromatization. The catalysts were characterized using X‐ray diffraction, scanning electron microscopy, N₂ adsorption–desorption, NH₃ temperature‐programmed desorption, infrared spectra of pyridine adsorption, X‐ray photoelectron spectroscopy and thermogravimetric analysis. Physicochemical measurements indicated that Mo species with smaller size in HMCM‐22 would sublimate more easily and form Mo species at the atomic/molecular level and then interact well with the internal Brønsted acid sites to form Mo–O–Al active species. Catalytic results confirmed that nano‐MoO₃‐modified HMCM‐22 showed higher methane conversion and aromatics yield (13.1 versus 8.9%) than commercial MoO₃‐modified HMCM‐22 (11.0 versus 7.5%). In addition, nano‐MoO₃‐modified HMCM‐22 showed better durability compared with commercial MoO₃‐modified MCM‐22. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization and Physical Properties of PbO-As2O3 Glasses Containing Molybdenum Ions

PbO-As₂O₃ glasses containing different concentrations of MoO₃ ranging from 0 to 1 mol% (in steps of 0.2) were prepared. The samples were characterized by X-ray diffraction, differential thermal analysis and scanning electron microscopy. A number of studies, viz., optical absorption, magnetic susceptibilities, ESR spectra, IR spectra, elastic properties (Young's modulus E, shear modulus G and microhardness H) and dielectric properties (constant ε, loss tan δ, a.c. conductivity σ_ac over a range of frequency and temperature and breakdown strength), have been carried out on these glasses. Optical absorption, ESR and magnetic susceptibility measurements suggest that when MoO₃ concentration is greater than 0.4 mol% in the glass matrix, molybdenum ions exist in Mo⁵⁺ state with Mo(V)O⁻₃ complexes that act as modifiers in addition to Mo⁶⁺ state with MoO₄ and MoO₆ structural groups. The studies on elastic and dielectric properties indicate that the mechanical and insulating strengths of the glass are considerably high when the content of MoO₃ is about 0.4 mol% in the glass matrix.     

The characteristic IR spectra of heterothiometallic cluster compounds

This article summarizes and presents the obtainable characteristic IR spectra of the existing heterothiometallic cluster compounds containing the [MXS₃]²⁻ (X=O, S; M=V, Mo, W, Re) moiety. The MS stretching vibration modes are classified into four categories including ν(MS_t), ν(Mμ₂-S), ν(Mμ₃-S) and ν(Mμ₄-S) according to the different conjunction ways between the transition metal and sulfur atoms. The structures of the heterothiometallic cluster compounds could be inferred from their characteristic IR spectra, the core structure's symmetry of the heterothiometallic clusters and the M/M′ ratio.     

95Mo NMR spectral studies on seven-coordinate molybdenum(II) isocyanide complexes

The ⁹⁵Mo NMR spectra of a series of seven-coordinate molybdenum(II) isocyanide complexes of the types [Mo(CNR)_7-nL_n](PF₆)₂ (R = CH₃, CHMe₂, CMe₃, C₆H₁₁, CH₂Ph; L = py, bpy, Me₂bpy, phen, dppe, P-n-Bu₃; n = 0,1,2) [Mo(CNC-Me₃)₆X]PF₆ (X = Cl, Br, I) and [{Mo(CNCMe₃)₄(NN)}₂(μ-CN)](PF₆)₃ (NN = bpy, Me₂bpy, phen) have been studied. The ⁹⁵Mo chemical shift range for this group of complexes is about 1100 ppm. An increase in the size of the R group attached to the isocyanide ligand generally tends to shield the ⁹⁵Mo nucleus. Replacement of the isocyanide ligand with a phosphorus ligand also increases the shielding, whereas the replacement of isocyanide with a heterocyclic nitrogen donor leads to deshielding by 800–900 ppm. This group of complexes shows a normal halogen dependence, i.e. replacement of Cl^? by Br^? and I^? increases the shielding of the ⁹⁵Mo nucleus. The cyano-bridged cations [{Mo(CNCMe₃)₄(NN)}₂(μ-CN)]³⁺ (NN = bpy, Me₂bpy, or phen) show two ⁹⁵Mo NMR signals, one for the molybdenum coordinated to the carbon of the bridging CN and one for the N-coordinated molybdenum. Comparison of the chemical shifts and linewidths of the cyano-bridged species with those of the corresponding mononuclear molybdenum(II) complexes [Mo(CNCMe₃)₅(NN)](PF₆)₂ leads to the assignment of the more deshielded signal to the N-coordinated molybdenum. The ¹⁴N and ³¹P NMR spectra for these complexes have also been measured, as have the ¹³C NMR spectra of the pairs of complexes [Mo(CNCMe₃)₅(NN)](PF₆)₂ and [{Mo(CNCMe₃)₄(NN)}₂(μ-CN)](PF₆)₃ (NN = bpy or phen). The ¹⁸³W NMR spectra for [W(CNR)₅(bpy)](PF₆)₂ (R = CMe₃ and CH₂Ph), show that the δ(¹⁸³W)/δ(⁹⁵Mo) chemical shift ratios for isocyanide complexes are different from the ratio found for M⁰ and M^VI.     

β-环糊精诱导的三氧化钼水溶性的增加

报道了由β-环糊精诱导的三氧化钼(MoO₃)水溶性增加及其与β-环糊精浓度的相关性, 认为MoO₃水溶性的增加是由于β-环糊精的加入导致了MoO₃在水中的沉淀-溶解平衡向溶解方向移动. 几个独立的实验证明了由β-环糊精和钼酸根离子(MoO₄^2-)组成的灰蓝色产物的形成. 结果表明: (1) X射线光电子能谱显示, 与MoO₃相比, 灰蓝色产物中钼的3d_5/2和3d_3/2电子轨道的结合能有明显的降低; (2) 在灰蓝色产物中出现了213 nm处的强吸收带(Mo＝O)和775 nm处的宽吸收峰(Mo―O); (3) 由于MoO₄^2-的影响, β-环糊精的质子有很明显的低场漂移. 总之, 我们的工作不仅明确地揭示了β-环糊精和MoO₄^2-之间的分子-离子相互作用, 同时揭示了这种相互作用在改善MoO₃物理性质上的潜在应用价值.     

Derivative analysis of Raman spectra of liquid water in the OH (D) stretching region

Second derivative analysis of Raman spectra of H₂O, D₂O and HOD in liquid phase at room temperature for parallel and perpendicular polarized modes in the OH and OD stretching regions is reported. Five components obtained at approximately 3215, 3375, 3455, 3535 and 3640 cm⁻¹ for the second derivative plots of Raman spectra of liquid water are explained as due to the presence of three types of associated water species with (i) both OH bonds involved in moderately strong hydrogen bonds (SS), (ii) both OH bonds involved in weak hydrogen bonds (WW), and (iii) one OH bond involved in strong and one in weak hydrogen bonds (SW) respectively. The derivative plots obtained for Raman spectra of D₂O and HOD also contain features expected to be present on the basis of this model.     

EPR of copper ions in Pb<Subscript>2</Subscript>MoO<Subscript>5</Subscript> crystals

The article presents the results of EPR studies of Pb₂MoO₅ crystals containing a copper impurity. Based on the analysis of angular dependences of the EPR spectra, it is found that copper ions incorporate into the structure of the Pb₂MoO₅ crystal in the Cu²⁺ state and occupy the molybdenum site with the formation of a linear extended Cu(II)–V(O)–Pb(IV) defect along the а axis of the crystal. An oxygen vacancy appears in the structure of the defect to compensate the charge and the lead ion acquires the Pb⁴⁺ charge state. According to the structure of this center, one magnetically non-equivalent position with the direction of main values of А and g of A(Cu)_zz and g_zz tensors parallel to the а axis is observed in the EPR spectra. Moreover, the EPR spectra exhibit an addition hyperfine structure from one lead atom on which the unpaired electron density is 0.061%. The obtained data on the structure of the defect formed when the copper impurity incorporates into the Pb₂MoO₅ crystal provided the assumption that the observed light scattering when the light beam is directed perpendicular to the а axis may be due to the cooperative effect of the presence of di- and tetravalent ions substituting for molybdenum in the linear configuration of Pb–O–Mo bonds.