Influence of Adding Molybdenum on Structure and Performance of FexOy/SBA-15 Catalysts in Selective Oxidation of Propene

Mixed iron and molybdenum oxide catalysts supported on nanostructured silica, SBA-15, were synthesized with various Mo/Fe atomic ratios ranging from 0.07/1.0 to 0.57/1.0. Structural characterization of as-prepared Mo_xO_y_Fe_xO_y/SBA-15 samples was performed by nitrogen physisorption, X-ray diffraction, and DR-UV-Vis spectroscopy. Adding molybdenum resulted in a pronounced dispersion effect on supported iron oxidic species. Increasing atomic ratio up to 0.21Mo/1.0Fe was accompanied by decreasing species sizes. Strong interactions between iron and molybdenum during the synthesis resulted in the formation of Fe−O−Mo structure units, possibly Fe₂(MoO₄)₃-like species. Reducibility of Mo_xO_y_Fe_xO_y/SBA-15 catalysts was investigated by temperature-programmed reduction experiments with hydrogen as reducing agent. The lower reducibility obtained when adding molybdenum was ascribed to both dispersion and electronic effect of molybdenum. Catalytic performance of Mo_xO_y_Fe_xO_y/SBA-15 samples was studied in selective gas-phase oxidation of propene with O₂ as oxidant. Adding molybdenum resulted in an increased acrolein selectivity and a decreased selectivity towards total oxidation products.     

溶剂热合成片状介孔CeO2及光致发光性能

本文以乙二醇为溶剂,Ce(NO3)3·6H2O为铈源,尿素为沉淀剂,采用了溶剂热法一步合成了二维片状结构的介孔CeO2,同时讨论了不同反应时间对晶型的影响。通过SEM、TEM、XRD、EDS和N2吸附-脱附等检测手段进行了表征,结果表明所合成CeO2是由大量较规整的片状结构单元组成,在片状结构上分布着均匀有序的介孔,产物具有立方萤石型结构,并考察了相应的光致发光性能。     

Main group supramolecular chemistry

Metal directed self-assembly has yielded a wide array of two- and three-dimensional structures with fascinating new chemical properties. These structures have typically been prepared utilizing transition metals as directing units, owing to the well-defined coordination preferences these metals exhibit. An area of growing research interest involves the preparation of structures containing main group elements as directing units. This tutorial review surveys the wide range of structure types available through this approach, specifically covering unique structure types accessible from the unusual coordination geometries often exhibited by the elements in Groups 12-17 of the periodic table. This review should be of interest to supramolecular and main group chemists, and researchers in the fields of crystal engineering, host-guest chemistry, and molecular recognition.     

Solvothermal Synthesis and Crystal Structures of Alkali Molybdates

Within the frame of systematic morphological studies concerning the solvothermal formation of nanoscale and microscale molybdenum oxides from the interaction of a molybdenum‐based precursor such as MoO₃⋅2 H₂O with ionic additives such as alkali and earth alkali halides, we studied – with the aim to elaborate preparative guidelines – the influence of the precursor structure and the alkali halide upon the crystal structure of the emerging alkali polymolybdates in terms of solvothermal fields and high‐throughput solvothermal techniques. The discussion of the resulting crystal structures revealed a structure‐directing potential of the alkali cations that was explored for the synthesis of new mixed alkali polymolybdates.     

Rb4Mo21Se24 containing Mo12 and Mo15 clusters

Rubidium molybdenum selenide, Rb₄Mo₂₁Se₂₄, crystallizes in the trigonal space group R. Its crystal structure consists of a mixture of Mo₁₂Se₁₄Se₆ and Mo₁₅Se₁₇Se₆ cluster units in a 1:2 ratio. Both units are interconnected through Mo—Se bonds. The Rb⁺ cations occupy large voids between the different cluster units.     

SYNTHESIS AND CRYSTAL STRUCTURE OF THE TRI-(μ2-METHOXY)-BRIDGED 'DOUBLE-CUBANE' CLUSTER COMPLEX (Et_4N)_3[Mo_2Fe_6S_8(OMe)_3(SPh)_6]

<正> The title compound was synthesized and crystal structure was solved. (Et4N)3[Mo2Fe6S8(OMe)3(SPh)6], Mr=1922.39, hexagonal, PGs/m, a = 17.750(3), c=15.880(2) A, V=4332.7(26) A3, Z-2, Dx=1.477 g/cm3, MoKα, λ= 0.71069 A, R = 0.059, Rw = 0.065, for 814 observed reflexions. The complex anion contains two Fe3MoS4 cubane-like units linked throug their molybdenum atoms by three ,μ2-methoxy-groups, and the overall structure of the anion has C3b symmetry.     

Syntheses and spectroscopic and structural characterization of molybdenum(VI) citrato monomeric raceme and dimer, K4[MoO3(cit)].2H2O and K4[(MoO2)2O(Hcit)2].4H2O

Investigation of the aqueous coordination chemistry for citrate and molybdenum(VI) resulted in the isolation of molybdenum(VI) citrato monomeric raceme and dimer K4[MoO3(cit)].2H2O (1) and K4[(MoO2)2O(Hcit)2].4H2O (2) (H4cit = citric acid). Complex 1 can serve as the first structurally characterized monomeric citrato molybdate and may represent an early mobilized precursor in the biosynthesis of FeMo-co (FeMo-cofactor). The two complexes have been characterized by elemental analyses and IR and NMR spectroscopies. The IR and NMR spectra are consistent with a monomeric species or a monooxo-bridged dinuclear structure, as revealed by a single crystal X-ray diffraction study. Compound 1 is monoclinic space group P2(1)/c with a = 7.225(1) A, b = 9.151(2) A, c = 22.727(2) A, beta = 94.93(1) degrees, V = 1497.1(7) A3, and Z = 4. Full-matrix least-squares refinement resulted in residuals of R = 0.027 and Rw = 0.032. The molybdenum atom forms an octahedral coordination with three oxo groups and one tridentate citrate, in which the latter is coordinated through the alkoxy and vicinal carboxyl and much more weakly by one of the two terminal groups [2.411(3) A]. Compound 2 is triclinic space group P1 with a = 8.2728(8) A, b = 8.9514(8) A, c = 10.0605(9) A, alpha = 101.673(8) degrees, beta = 100.672(7) degrees, gamma = 112.938(7) degrees, V = 642.5(3) A3, and Z = 1. Full-matrix least-squares refinement resulted in residuals of R = 0.033 and Rw = 0.039. The complex anion contains a linear (O2Mo)O(MoO2) core with the bridging oxo group lying at the center of inversion symmetry (Mo-Ob-Mo, 180 degrees). Each citrate ligand is three-coordinated to one molybdenum atom through the deprotonated hydroxy, alpha-carboxyl, and one beta-carboxyl group, making each metal atom six-coordinate.     

Conformational dimorphism of 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane: solvent-induced crystallization of a metastable polymorph containing boat-shaped molecules

Two crystalline modifications of 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane have been experimentally obtained as phase pure products and studied by single-crystal X-ray diffraction. The six-membered heterocycles adopt a chair conformation in the alpha-phase; this polymorph is accessible by crystallisation from solution and from the melt. In contrast, the beta-form is built up from boat-shaped molecules; it can exclusively be crystallised from n-hexane. At the molecular level, formation energies of the 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane conformers have been compared by using molecular mechanics, semiempirical and ab-initio quantum mechanical calculations. Possible reasons for the selective formation of the alpha- or beta-phase in specific solvents have been considered. Formation of the metastable phase is suggested to occur via a hypothetical intermediate of composition [(GeCl2CH2)3].0.5C6H14. For such an in-silico solvate, a crystal structure of favourable lattice energy, closely related to the experimentally observed beta-modification, has been found through global energy minimisation. Elimination of the n-hexane molecules from this computer-generated solid and subsequent simulated annealing resulted in a crystal structure that corresponds to the experimentally observed beta-phase within the limits of the force field calculations. This scenario implies solvent directed crystallisation of a metastable polymorphic molecular crystal.     

Olefin epoxidations in the ionic liquid [C4mim][PF6] catalysed by oxodiperoxomolybdenum species in situ generated from molybdenum trioxide and urea–hydrogen peroxide: The synthesis and molecular structure of [Mo(O)(O2)2(4-MepyO)2]·H2O

Commercially available molybdenum(VI) compounds, including molybdenum trioxide, were successfully employed as catalyst precursors in the epoxidation of olefins with urea–hydrogen peroxide adduct (UHP) in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [C₄mim][PF₆]. After oxidation, the corresponding epoxides were isolated by extraction with diethyl ether. Additionally the ionic liquid–catalyst mixture was recycled and reused in further catalytic cycles. The catalytic species is assumed to be an oxodiperoxomolybdenum species which forms in situ. A representative complex of this type was thus isolated and characterised. Reaction of excess 4-methylpyridine-1-oxide (4-MepyO) with MoO₃ dissolved in aqueous hydrogen peroxide afforded [Mo(O)(O₂)₂(4-MepyO)₂]·H₂O (1) as yellow crystals. Compound 1, an active epoxidation catalyst, was subsequently characterised and its structure determined by X-ray crystallography.     

Cis di-oxomolybdenum(VI) complexes with a tridentate ONO donor ligand; synthesis,X-ray crystal structure,spectroscopic properties and oxotransfer chemistry

The molybdenum complexes of Schiff base ligands viz. [MoO₂LH₂O] where L¹ = tris(hydroxymethyl)(salicylide-neamino)methane, L² = tris(hydroxymethyl)(2,3-dihydroxybenzylideneamino)methane and L³ = tris(hydroxymethyl)(2,3,4-trihydroxybenzylideneamino)methane have been synthesized and characterized by spectroscopic and electrochemical techniques. The X-ray crystal structure of the complex [MoO₂L¹H₂O] reveals a distorted octahedral geometry with one ligand and a water molecule coordinated to the MoO₂ center. No previous complex of this type contains a coordinated water molecule. The complex undergoes an oxotransfer in the presence of Bu₃P to form a -oxobridged molybdenum(V,V) dimer. This rules out Mo—S coordination as a prerequisite for oxotransfer in such molybdenum(VI) complexes.     

Mo/Rh carboxylate: heterometallic compound built of homometallic paddlewheel units

Mixed-metal molybdenum(II)/rhodium(II) tetra(trifluoroacetate) of the composition [(MoRh)(O2CCF3)4] has been obtained from the gas-phase reaction between volatile carboxylates, [Mo2(O2CCF3)4] and [Rh2(O2CCF3)4]. This is an interesting system for which a single-crystal X-ray investigation fails to provide an unambiguous evidence of whether the product consists of the initial homometallic or newly formed heterometallic paddlewheel units. In the solid-state structure both metal atoms occupy the same crystallographic position, while the M-M and M-O distances are averaged with respect to the parent homometallic compounds. Nevertheless, the results of mass-spectrometric and magnetic measurements clearly indicate that the title bimetallic carboxylate contains a statistical mixture of homometallic dimolybdenum and dirhodium units. The product can be considered as a result of cocrystallization of isomorphous paddlewheel molecules.     

Studies on the reduction of the Fe2O3MoO3 system and its interaction with synthesis gas (CO+H2)

The structural changes of iron—molybdenum mixed oxide systems during calcination and reduction were studied. The oven-dried precipitated mass contains excess molybdenum as polymolybdic ions, which is transformed into Fe₂(MoO₄)₃ and MoO₃ on heat-treatment of the sample above 400°C. The reduction of Fe₂(MoO₄)₃ proceeds through the formation of FeMoO₄ and FeMoO₃. On complete reduction, it gives a mixed crystal of iron and molybdenum. MoO₃ is also simultaneously reduced to elemental molybdenum through the formation of MoO₂ as an intermediate oxide.The interaction of the reduced mass with synthesis gas indicates that the iron—molybdenum mixed crystal is active for the hydrogenation of CO molecules. This mixed lattice is also stable towards the carburization process under synthesis gas.     

Mo2B4O9—Connecting Borate and Metal-Cluster Chemistry

We report on the first thoroughly characterized molybdenum borate, which was synthesized in a high-pressure/high-temperature experiment at 12.3 GPa/1300 °C using a Walker-type multianvil apparatus. Mo₂B₄O₉ incorporates tetrahedral molybdenum clusters into an anionic borate crystal structure—a structural motif that has never been observed before in the wide field of borate crystal chemistry. The six bonding molecular orbitals of the [Mo₄] tetrahedron are completely filled with 12 electrons, which are fully delocalized over the four molybdenum atoms. This finding is in agreement with the results of the magnetic measurements, which confirmed the diamagnetic character of Mo₂B₄O₉. The two four-coordinated boron sites can be differentiated in the ¹¹B MAS-NMR spectrum because of the strongly different degrees of local distortions. Experimentally obtained IR and Raman bands were assigned to vibrational modes based on DFT calculations.     

Mn(approximately)2.4Mo6O9: first example of empty twin chains of edge-sharing M6 octahedra in transition metal cluster chemistry

The novel ternary reduced molybdenum oxide Mn(approximately)(2.4)Mo(6)O(9) has been synthesized by solid-state reaction at 1400 degrees C for 96 h in sealed molybdenum crucibles. Electron diffraction studies showed that Mn(approximately)(2.4)Mo(6)O(9) presents a complex crystal structure with a 3d incommensurate modulation. The average crystal structure was determined on a single-crystal by X-ray diffraction in the orthorhombic space group Pnma with the following lattice parameters: a = 16.4824(2) A, b = 2.8273(2) A, c = 17.3283(2) A, Z = 4. The Mo network consists of empty twin chains of trans-edge-sharing octahedra that occur for the first time in a solid-state compound. The Mo-Mo distances within the chains range from 2.62 to 2.92 A, and the Mo-O distances from 1.99 to 2.17 A as usually observed in the reduced molybdenum oxides. Single-crystal resistivity measurements show that Mn(approximately)(2.4)Mo(6)O(9) is metallic between 4.2 and 300 K. The magnetic susceptibility data indicate paramagnetic behavior due to the Mn(2+) moment at high temperatures with a weak ferromagnetic behavior below 80 K.     

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.     

Solid state crystalline and liquid crystalline structure of semifluorinated 1-bromoalkane compounds

A series of semifluorinated 1-bromoalkane (SFBA) mesogens have been synthesized and characterized to better understand their solid state crystalline and liquid crystalline structures. In the solid state, the local conformation of the fluorocarbon segments becomes disordered once the fluorocarbon chain reaches a length above eight CF units. This is evident from the pronounced decrease of molar melting enthalpy. An increasing amount of helix and helix reverse conformations and increasingly disordered packing can also be observed with each addition of a fluorocarbon segment. X-ray diffraction peaks in the small angle region can be indexed by a tilted, two dimensional layered (herring bone) structure. The crystal structure is similar to a type of plastic crystal in which the amphiphilic character is clear, because the two segments of fluorocarbon and hydrocarbon are almost immiscible. Heating of F(CF2)12(CH2)10Br leads to a transition from plastic crystal to smectic B, as revealed by time-resolved XRD and FTIR analysis. At this solid-to-liquid transition temperature, conformational analysis confirmed an onset of the CH2 gauche conformation within the hexagonal lattice, most likely due to changes occurring in the hydrocarbon segment, and a sudden increase of helix defects along the fluorocarbon segment. The disordered helix rigid-rod structure of the fluorocarbon segment and its poor compatibility with the hydrocarbon segment play an important role in the crystalline solid and liquid crystalline structures.     

Interlayer-type Crystal Structure of N-（1-Adamantyl） urea

1 INTRODUCTION In the context of supramolecular chemistry, mo- lecules are joined together by intermolecular interac- tions to form a supramolecule whose physical pro- perties largely depend on the orientation and packing of molecules in the crystal structures[1]. The adaman- tane is a kind of cage alkane with high symmetry and stable framework. Its derivatives are extensively applied in the fields of medicine, macromolecular materials, aviation and so on due to the unique structures and …     

Structural modulation of molybdenyl iodate architectures by alkali metal cations in AMoO3(IO3) (A = K, Rb, Cs): a facile route to new polar materials with large SHG responses

Three new molybdenyl iodates, KMoO3(IO3) (1), RbMoO3(IO3) (2), and CsMoO3(IO3) (3), have been prepared through the hydrothermal reactions of MoO3 with AIO4 (A = K, Rb, or Cs) at 180 C. These compounds are isolated as nearly colorless, air-stable crystals. Single-crystal X-ray diffraction experiments reveal that 1 possesses a corrugated layered structure constructed from molybdenum oxide chains that are bridged by iodate anions. The puckering of the layers is caused by the alignment of bent molybdenyl (MoO2(2+)) groups along one side of the molybdenum oxide chains. The K+ cations separate these layers from one another and serve to balance charge. In contrast, compounds 2 and 3, which are isostructural, form three-dimensional structures with small cavities filled with Rb+ or Cs+ cations. The differences between the structures of 1 and those of 2 and 3 are due to rotation of the molybdenyl units as translation occurs down the molybdenum oxide chains in order to accommodate the increased size of the Rb+ and Cs+ cations. This rotation allows for the iodate anions to bridge the molybdenum oxide chains in an additional dimension, creating a three-dimensional network structure. Furthermore, while 1 crystallizes in a centrosymmetric space group, 2 and 3 crystallize in polar space groups. Second-harmonic generation measurements on 2 and 3 show large responses of 400x alpha-quartz. Differential scanning calorimetry measurements demonstrate that 2 and 3 are thermally stable to 494 and 486 C, respectively. UV-vis diffuse reflectance spectra of these compounds show a high degree of transparency from 1 to 3 eV and a band gap of 3.1 eV.     

应用晶体结构数据进行化学反应途径研究 Ⅰ. 五配位钼化合物Mo(L)5的动态立体化学

The principle of structure correlation is applied to 28 crystal structures which contain 44 different five-coordinate molybdenum fragments. The data derived map the reaction pathway for a bimolecular ligand substitution reaction at tetrahedral molybdenum centers. The dihedral-angle method from Muetterties and Guggenberger is employed in order to describe the interconversion between the trigonal bipyramidal (TBP) and the square pyramidal (SQP) conformations via the Berry pseudorotation mechanism in the system of Mo(L)_5 complexes. This structure correlation method provides a convincing mapping of the Berry pathway in this paper.     

The Transition Metal-Nitrogen Multiple Bond

Numerous nitrido complexes of transition metals show very short metal-nitrogen bond lengths, suggesting M?N-triple bonds. At present, compounds of this type are being intensively investigated. In particular the molybdenum complexes are considered as model substances for the study of at least an intermediate step of N₂-assimilation. This article contains a review of the structure and bonding, as well as syntheses and reactions of these complexes.