The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H<Subscript>2</Subscript>, CO,and CH<Subscript>4</Subscript>

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H₂, CO, and CH₄ was studied. It was found that the composite catalysts containing H _x MO₃ phases (M = W or Mo), which were formed by the reduction of MoO₃ and WO₃ oxides with hydrogen in the presence of deposited Pd, showed higher catalytic activity in the oxidation of small molecules (H₂, CO, and CH₄) with excess oxygen than the traditional Pd/Al₂O₃ deposited catalyst with the same content of the deposited metal. It was shown that the thermal stability of the H _x MO₃ phases was the limiting factor influencing the activity of these composite catalysts.     

Kinetics of molybdenum loss from iron molybdate catalysts under nitrogen atmosphere

The loss rate of molybdenum in Fe−Mo/SiO₂ catalyst has been measured. When the catalyst is composed of a Fe₂(MoO₄)₃ crystalline phase and monolayer dispersed MoO₃, the loss rate of MoO₃ varies with the content of MoO₃ in the catalyst. If a MoO₃ crystalline phase arises in the catalyst, the loss rate will be accelerated.     

A novel route to nanosized molybdenum boride and carbide and/or metallic molybdenum by thermo-synthesis method from MoO3, KBH4, and CCl4

Nanosized molybdenum boride and carbide were synthesized from MoO₃, KBH₄, and CCl₄ by thermo-synthesis method at lower temperature. The relative content of Mo, Mo₂C, and molybdenum boride in the product was decided by the molar ratio between MoO₃, KBH₄, and CCl₄. Increasing the molar ratio of CCl₄ to MoO₃ was favorable to the production of Mo₂C. Increasing the molar ratio of KBH₄ to MoO₃ was favorable to the production of molybdenum boride. By carefully adjusting the reaction conditions and annealing in Ar at 900°C, a single phase of MoB could be obtained.     

Comparative Investigation of Mo(CO)6 Adsorption on Clean and Oxidized Si(111) Surfaces

Mo(CO)₆ adsorption on the clean, oxygen-precovered and deeply oxidized Si(111) surfaces was comparatively investigated by high-resolution electron energy loss spectroscopy. The downward vibrational frequency shift of the C-O stretching mode in adsorbed Mo(CO)₆ illustrates that different interactions of adsorbed Mo(CO)₆ occur on clean Si(111) and SiO₂/Si(111) surfaces, weak on the former and strong on the latter. The strong interac-tion on SiO₂/Si(111) might lead to the partial dissociation of Mo(CO)₆, consequently the formation of molybdenum subcarbonyls. Therefore, employing Mo(CO)₆ as the precursor, metallic molybdenum could be successfully deposited on the SiO₂/Si(111) surface but not on the clean Si(111) surface. A portion of the deposited metallic molybdenum is transformed into the MoO₃ on the SiO₂/Si(111) surface upon heating, and the evolved MoO₃ finally desorbs from the substrate upon annealing at elevated temperatures.     

On the reactions of molybdenum and tungsten carbonyls with trimethyl- and triethyl-aluminium

Reactions of hexacarbonylmolybdenum, hexacarbonyltungsten and arene complexes of tricarbonylmolybdenum and tricarbonyltungsten with trimethyl- and triethyl-aluminium have been studied. It has been found, based on IR and NMR spectra, that trialkylaluminium does not form complexes with hexacarbonyls of molybdenum and tungsten. Arene (mesitylene, toluene and benzene) complexes of tricarbonylmolybdenum form 1 : 1 complexes with triethylaluminium, and arene complexes of tricarbonyltungsten form complexes with trimethyl- and triethyl-aluminium. Regardless of the molar ratios of reactants (arene)M(CO)₃/AlEt₃, only one of the three CO groups bonded to molybdenum or tungsten forms a complex with AlEt₃. Fast exchange between free and complexed trialkylaluminium and an exchange of trialkylaluminium between all three carbonyl groups have been observed in benzene, toluene and decalin solutions. In the ¹H NMR spectra of the products of the reactions of (mesitylene)Mo(CO)₃ with AIEt₃ and AIMe₃, signals at –9 to –14ppm (characteristic for molybdenum hydrides) were present. It confirmed an alkylation of molybdenum followed by β- or α-hydrogen elimination with the formation of the corresponding molybdenum hydrides, the actual catalyst of aromatic hydrocarbon hydrogenation.     

Preparation method effect on the properties of Ziegler-type hydrogenation catalysts based on bis(acetylacetonato)cobalt

The turnover frequency and turnover number of Ziegler-type systems based on Co(acac)₂ · nH₂O (n = 0, 0.5, and 2) and AlEt₃ or AlEt₂(OEt) are reported in relation to the Al: Co molar ratio and to the concentrations of the initial components. Proton donor compounds have an effect on the catalytic characteristics of the systems. ESR data are presented for the interaction of Co(acac)₂ with organometallic compounds (AlEt₃, AlEt₂(OEt), Li(n-Bu), and (C₆H₅CH₂)MgCl) in the presence of various arenes. Seven preparation methods and a formation scheme are suggested for catalytic species that are active in styrene hydrogenation and are based on Co(acac)₂ combined with AlEt₃ or AlEt₂(OEt) and.     

Chromium,molybdenum and ruthenium complexes of chloranilic acid

Chloranilic acid (H₂CA) reacts with Cr(CO)₆ and CrCl₃ to give the tris derivatives Cr(H₂CA)₃ and Cr(HCA)₃. Spectroscopic measurements on the two complexes reveal that the ligand–metal bond is of the semiquinone type. Susceptibility determinations showed effective magnetic moments of 1.82 and 1.07_B. Examination of Cr(H₂CA)₃ by cyclic voltammetry revealed two redox reactions due to tautomeric interconversion through electron transfer. Boiling Mo(CO)₆ with H₂CA in air gave MoO₃(HCA) in which the molybdenum atom is in the +5 oxidation state. The i.r. spectrum of the MoO₃(HCA) displayed bands due to terminal M=O bonds. The cluster compound Ru₃(CO)₁₂ reacted with H₂CA to give Ru₃(CO)₁₀(-H)(HCA), the i.r. spectrum of which revealed that the ligand is bound to the metal as a catechol moiety. The spectroscopic and magnetic studies of the molybdenum and ruthenium complexes confirmed the proposed structures.     

Synthesis of mono- and binuclear dioxomolybdenum(VI) complexes derived from N(4)-substituted thiosemicarbazones: X-ray crystal structures of [(MoO2L)2], [MoO2Lpy] and [MoO2Lpy]

Four molybdenum(VI) thiosemicarbazonato complexes have been synthesized and characterized. The dinuclear complexes [(MoO₂L¹)₂] (1) and [(MoO₂L²)₂] (3) have been prepared by the reaction of [MoO₂(acac)₂] with 2-hydroxyacetophenone N(4)-cyclohexyl (H₂L¹) and N(4)-phenyl (H₂L²) thiosemicarbazones in alcoholic medium. Mononuclear dioxomolybdenum(VI) complexes of the type [MoO₂L¹py] (2) and [MoO₂L²py] (4) have been prepared by the reaction of 1 or 3 with pyridine (py) in alcoholic medium. In all the complexes, molybdenum is coordinated by two terminal oxo-oxygen atoms, (O_t), oxygen, nitrogen and sulfur atoms from the principal ligand and by an oxygen atom from the second unit in 1, and by a nitrogen atom from pyridine in complexes 2 and 4. All complexes have been spectroscopically characterized. The molecular structures of complexes 1, 2 and 4 have been determined by the single crystal X-ray diffraction method.     

Reducibility of Mo species in Pt/MoO3 and PtMo/Al2O3 catalysis

Recucibility of Mo species in Pt/MoO₃ and PtMo/Al₂O₃ has been investigated by temperature-programmed reduction (TPR), temperature-programmed desorption of hydrogen (H₂-TPD) and temperature programmed electronic conductivity (TPEC) techniques. In Pt/MoO₃ at H₂ atmosphere, it was found by TPEC and TPR that, a slight amount of Pt could activate the transfer of the species and H atoms between H₂ and MoO₃, and thus accelerate the reduction of MoO₃. In PtMo/Al₂O₃, TPR and H₂-TPD revealed that the reduction of surface Mo species could also be facilitated by Pt. Two kinds of hydrogen molybdenum species were proposed on the surface of the catalyst after prereduction.     

Reduction of sulfoxides catalyzed by oxo-complexes

This work reports the catalytic activity of the oxo-complexes HReO₄, MoO₂(acac)₂, WO₂Cl₂, and VO(acac)₂ in the reduction of sulfoxides with PhSiH₃ or HBcat. The results obtained showed that the catalyst systems PhSiH₃/HReO₄ (5 mol %) and HBcat/HReO₄ (5 mol %) are highly efficient for the deoxygenation of sulfoxides. The complex MoO₂(acac)₂ was also efficient, but the reactions required more time and heating. Finally, the complexes WO₂Cl₂ and VO(acac)₂ showed a moderate activity.     

In Situ IR Spectroscopic Studies on Molybdenum Nitride Catalysts: Active Sites and Surface Reactions

Recent IR spectroscopic studies on the surface properties of fresh Mo₂N/-Al₂O₃ catalyst are presented in this paper. The surface sites of fresh Mo₂N/-Al₂O₃, both Mo⁺ (0<<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm^-1, due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo₂N/-Al₂O₃ behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo₂N/-Al₂O₃ catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo₂N/-Al₂O₃ catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites.     

Crystal structure, vibrational properties and luminescence of NaMg3Al(MoO4)5 crystal doped with Cr ions

Crystals of NaMg₃Al(MoO₄)₅ doped with 0.5% Cr³⁺ ions have been synthesized and characterized by a single-crystal X-ray structure analysis and IR, Raman, electron absorption and luminescence spectroscopic studies. It has been shown that NaMg₃Al(MoO₄)₅ crystallizes in the structure, with a=6.8744(8) Å, b=6.9342(7) Å, c=17.605(2) Å, α=87.788(8)°, β=87.727(9)°, γ=78.501(9)°, Z=2. The characteristic feature of the structure is its enormously large thermal displacement parameter for sodium, even at 105 K. The IR and Raman spectra indicate significant interactions between the MoO₄²⁻ ions in the structure. The electron absorption, excitation and luminescence studies have shown that there are at least two different sites of incorporated Cr³⁺ ions in the NaMg₃Al(MoO₄)₅ crystal structure. They differ themselves by strength of crystalline field. One of them is characterized by Cr³⁺ in low ligand field and ⁴T₂→⁴A₂ emission whereas the second is characterized by higher strength of the crystal field and dominant ²E→⁴A₂ emission. Temperature-dependent studies show that the compound does not exhibit any phase transition.     

Pyrolysis-controlled synthesis and magnetic properties of sol–gel electrospun nickel cobaltite nanostructures

Poly(chloropropyl-methyl)silsesquioxanes (PCMSQ) were prepared using the base catalyzed sol–gel processing on methyltrimethoxysilane (MTMS) and 3-chloropropyltriethoxysilane (CPTES) with 5:5, 6:4, and 7:3 molar ratios in methanol and water. The PCMSQ with 6:4 molar ratio of MTMS:CPTES, which has the maximum yield, according to the elemental analysis, was chosen and some chlorine atoms of the chloropropyl groups were changed to different amines by refluxing it with ethylenediamine (en), diethylenetriamine (dien), ortho-phenylenediamine (opda), and 2-imidazolidinethion (imt). The amine grafted PCMSQ were then used to support MoO₂(acac)₂ complex and dien grafted PCMSQ with higher metal content was applied to the epoxidation of cis-cyclooctene with TBHP. The product yields were studied by gas liquid chromatography and the catalytic procedure was optimized for the parameters involved such as the solvent and oxidant. The catalytic activity of this catalyst also was investigated toward epoxidation of some other alkenes. It was also applied to check its reuse ability.

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In this research, micro-spherical poly-organo-silsesquioxane was prepared using the base catalyzed sol–gel processing of methyltrimethoxysilane and 3-chloropropyltriethoxysilane with different molar ratios. The sample which has the maximum yield, was used to graft MoO₂(acac)₂ and for the successful anchoring, some chlorine atoms of the chloropropyl side groups were changed to amines. Then the prepared catalyst with higher molybdenum content was used for the epoxidation of cis-cyclooctene and some other alkenes with TBHP.

     

Preparation and Characterization of Molybdenum Oxide Thin Films by Sol-Gel Process

This paper presents a new sol-gel process to prepare molybdenum oxide thin films. A molybdenum acetylacetonate sol was prepared by employing the system CH₃COCH₂COCH₃/MoO₃/C₆H₅CH₃/HOCH₂CH₂OCH₃. A molybdenum acetylacetonate gel was prepared by addition of aqueous NH₃. Thermal gravimetry (TG) and differential thermal analyses (DTA) of the gel suggested that crystallization of MoO₃ occurs in a 140 K temperature range around 508°C. MoO₃ films were prepared on fused silica, Si (111) and Al₂O₃ (012) substrates by annealing spin coating films of the sol in oxygen environment at 508°C. X-ray diffraction (XRD) showed that all films crystallize in -MoO₃ structure, and crystallites on fused silica substrate are arbitrarily oriented while those on Si (111) and Al₂O₃(012) substrates oriented in the (010) direction. SEM investigations showed that MoO₃ grains of all films are randomly distributed, with a longitudinal dimension of about 1–5 m and the film thickness is about 1 m.     

Phase formation in the systems Ag2MoO4-MO-MoO3 (M=Ca, Sr, Ba, Pb, Cd, Ni, Co, Mn) and crystal structures of Ag2M2(MoO4)3 (M=Co, Mn)

Phase equilibria in the systems Ag₂MoO₄-MMoO₄ (M=Ca, Sr, Ba, Pb, Ni, Co, Mn) and subsolidus phase relations in the systems Ag₂MoO₄-MO-MoO₃ (M=Ca, Pb, Cd, Mn, Co, Ni) were investigated using XRD and thermal analysis. The systems Ag₂MoO₄-MMoO₄ (M=Ca, Sr, Ba, Pb, Ni) belong to the simple eutectic type whereas in the systems Ag₂MoO₄-MMoO₄ (M=Co, Mn) incongruently melting Ag₂M₂(MoO₄)₃ (M=Co, Mn) were formed. In the ternary oxide systems studied no other compounds were found. Low-temperature LT-Ag₂Mn₂(MoO₄)₃ reversibly converts into the high-temperature form of a similar structure at 450-500°C. The single crystals of Ag₂Co₂(MoO₄)₃ and LT-Ag₂Mn₂(MoO₄)₃ were grown and their structures determined (space group , Z=2; lattice parameters are a=6.989(1) Å, b=8.738(2) Å, c=10.295(2) Å, α=107.67(2)°, β=105.28(2)°, γ=103.87(2)° and a=7.093(1) Å, b=8.878(2) Å, c=10.415(2) Å, α=106.86(2)°, β=105.84(2)°, γ=103.77(2)°, respectively) and refined to R(F)=0.0313 and 0.0368, respectively. The both compounds are isotypical to Ag₂Zn₂(MoO₄)₃ and contain mixed frameworks of MoO₄ tetrahedra and pairs of M²⁺O₆ octahedra sharing common edges. The Ag⁺ ions are disordered and located in the voids forming infinite channels running along the a direction. The peculiarities of the silver disorder in the structures of Ag₂M₂(MoO₄)₃ (M=Zn, Mg, Co, Mn) are discussed as well as their relations with analogous sodium-containing compounds of the structural family of Na₂Mg₅(MoO₄)₆. The phase transitions in Ag₂M₂(MoO₄)₃ (M=Mg, Mn) of distortive or order-disorder type are suggested to have superionic character.     

Effect of Mo loading on transesterification activities of MoO3/γ-Al2O3 catalysts prepared by conventional and slurry impregnation methods

Summary The effect of Mo loadings and preparation methods, slurry and conventional impregnation, on the performances of alumina-supported MoO₃ catalysts in transesterification of dimethyl oxalate (DMO) with phenol was investigated. Slurry prepared MoO₃/-Al₂O₃ catalyst exhibited higher activity and dispersion capacity than conventional one. Slurry MoO₃/water was used instead of an ammonium heptamolybdate solution. Highly dispersed amorphous Mo catalysts were obtained, closely related to the catalytic activities without calcination, waste solutions, and calcining nitrogenous gases.     

Combinatorial topology of uranyl molybdate sheets: syntheses and crystal structures of (C6H14N2)3[(UO2)5(MoO4)8](H2O)4 and (C2H10N2)[(UO2)(MoO4)2]

Two new mixed organic-inorganic uranyl molybdates, (C₆H₁₄N₂)₃[(UO₂)₅(MoO₄)₈](H₂O)₄ (1) and (C₂H₁₀N₂)[(UO₂)(MoO₄)₂] (2), have been obtained by hydrothermal methods. The structure of 1 [triclinic, , Z=1, a=11.8557(9), b=11.8702(9), c=12.6746(9) Å, α=96.734(2)°, β=91.107(2)°, γ=110.193(2)°, V=1659.1(2) Å] has been solved by direct methods and refined on the basis of F² for all unique reflections to R1=0.058, which was calculated for the 5642 unique observed reflections (|F_o|?4σ_F). The structure contains topologically novel sheets of uranyl square bipyramids, uranyl pentagonal bipyramids, and MoO₄ tetrahedra, with composition [(UO₂)₅(MoO₄)₈]⁶⁻, that are parallel to (−101). H₂O groups and 1,4-diazabicyclo [2.2.2]-octane (DABCO) molecules are located in the interlayer, where they provide linkage of the sheets. The structure of 2 [triclinic, , Z=2, a=8.4004(4), b=11.2600(5), c=13.1239(6) Å, α=86.112(1)°, β=86.434(1)°, γ=76.544(1)°, V=1203.14(10) Å] has been solved by direct methods and refined on the basis of F² for all unique reflections to R1=0.043, which was calculated for 5491 unique observed reflections (|F_o|?4σ_F). The structure contains topologically novel sheets of uranyl pentagonal bipyramids and MoO₄ tetrahedra, with composition [(UO₂)(MoO₄)₂]²⁻, that are parallel to (110). Ethylenediamine molecules are located in the interlayer, where they provide linkage of the sheets. All known topologies of uranyl molybdate sheets of corner-sharing U and Mo polyhedra can be described by their nodal representations (representations as graphs in which U and Mo polyhedra are given as black and white vertices, respectively). Each topology can be derived from a simple black-and-white graph of six-connected black vertices and three-connected white vertices by deleting some of its segments and white vertices.     

Rhodium(I) complexes with 1′-(diphenylphosphino)ferrocenecarboxylic acid as active and recyclable catalysts for 1-hexene hydroformylation

The rhodium complex trans-[Rh(CO)(Hdpf-κP)(dpf-κ²O,P)] (1), (Hdpf = 1′-(diphenylphosphino)ferrocenecarboxylic acid) was used as an efficient and recyclable catalyst for 1-hexene hydroformylation producing ca. 80% of aldehydes at 10 atm CO/H₂ and 80 °C. After the reaction, unchanged complex 1 was separated from the reaction mixture and used again three times with the same catalytic activity. The effect of modifying ligands, phosphines and phosphites, on the reactivity of 1 was investigated. The active catalytic systems containing 1 or trans-[Rh(CO)(L)(dpf-κ²O,P)] (2) were formed in situ from acetylacetonato rhodium(I) precursors [Rh(CO)₂(acac)] (3) or [RhL(CO)(acac)] (4) and Hdpf or Medpf (L = phosphine, Medpf = methyl ester of Hdpf).     

A novel Mo(V) oligophosphate built up of di- and triphosphate groups: Cs(MoO)4(P2O7)2(P3O10)

A Mo(V) oligophosphate, built up of di and triphosphate groups, Cs(MoO)₄(P₂O₇)₂(P₃O₁₀) has been synthesized for the first time. This compound crystallizes in the triclinic P−1 space group with , , , α=94.534(6)°, β=102.520(6)°, γ=103.663(4)°. This original structure can be described by the association of MoO₆ octahedra, MoP₂O₁₁ units built up of one P₂O₇ group sharing two apices with the same MoO₆ octahedron, and triphosphates groups P₃O₁₀. The resulting tridimensional framework forms large S-shaped tunnels running along c where the Cs⁺ cations are located.     

Crystal growth and electrical properties of lithium,rubidium, and cesium molybdenum oxide bronzes

Crystals of Li_0.33 MoO₃ (blue), Rb_0.23MoO₃ (blue) and Cs_0.31MoO₃ (red) were grown by electrolysis from MoO₃M₂MoO₄ melts (M =alkali metal) with composition 70–77 mole% MoO₃. Melts richer in M₂MoO₄ produced MoO₂ only. Correlation is made between bronze formation and the coordination of Mo in the melt and in the equilibrium solid phase M₂Mo₄O₁₃. Li_0.33MoO₃ and Cs_0.31MoO₃ are semiconductors with high-temperature-range activation energies 0.16 and 0.12 eV. Rb_0.23MoO₃ has an electrical behavior similar to that of blue K_xMoO₃ with a semiconductor-metal transition at (170 ± 5) K. ESR spectra observed in Li_0.33MoO₃ and Rb_0.23MoO₃ single crystals at 4.2 K show extensive delocalization of the 4d¹ electron associated with Mo(V) centers. Attempts to grow molybdenum bronzes containing Ca or Y were unsuccessful.