95Mo and 17O NMR studies of aqueous molybdate solutions

Aqueous molybdate solutions with molybdenum concentrations of [Mo]=1, 0.4, and 0.2 M have been studied by NMR at pH 7-1 and in 0.3-6 M HClO4. The 95Mo NMR spectrum of isopolyanion (IPA) Mo7O24(6-) (I) at pH=5 consists of a signal at 210 ppm and two overlapping peaks at 32 and approximately 15 ppm with the intensity ratio approximately 1:4:2, and that of beta-Mo8O26(4-) (II) consists of two signals at approximately 100 and 10 ppm with the intensity ratio approximately 1:3. A broad 95Mo NMR line at around 0 ppm was observed in the pH range of IPA Mo36O112(8-) (III), and a signal of cationic oxospecies including MoO2(2+) (IV) was observed from -62 to -69 ppm. Two protonation sites of IPA I have been identified from 17O NMR spectra, which suggests binding of up to two protons. The distribution diagram, derived from the 95Mo NMR spectra, is given for [Mo]=0.4 M. The 95Mo NMR signals shift to lower frequencies with increasing number and strength of the Mo-O terminal bonds.     

Polyoxomolybdate promoted hydrolysis of a DNA-model phosphoester studied by NMR and EXAFS spectroscopy

Hydrolysis of (p-nitrophenyl)phosphate (NPP), a commonly used phosphatase model substrate, was examined in molybdate solutions by means of (1)H, (31)P, and (95)Mo NMR spectroscopy and Mo K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. At 50 °C and pD 5.1 the cleavage of the phosphoester bond in NPP proceeds with a rate constant of 2.73 × 10(-5) s(-1) representing an acceleration of nearly 3 orders of magnitude as compared to the hydrolysis measured in the absence of molybdate. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed in solutions where [Mo(7)O(24)](6-) is the major species in solution. Mixing of NPP and [Mo(7)O(24)](6-) resulted in formation of these two intermediate complexes that were detected by (31)P NMR spectroscopy. Complex A was characterized by a (31)P NMR resonance at -4.27 ppm and complex B was characterized by a (31)P NMR resonance at -7.42 ppm. On the basis of the previous results from diffusion ordered NMR spectroscopy, performed with the hydrolytically inactive substrate phenylphosphonate (PhP), the structure of these two complexes was deduced to be (NPP)(2)Mo(5)O(21)(4-) (complex A) and (NPP)(2)Mo(12)O(36)(H(2)O)(6)(4-) (complex B). The pH studies point out that both complexes are hydrolytically active and lead to the hydrolysis of phosphoester bond in NPP. The NMR spectra did not show evidence of any paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), and indicating that the cleavage of the phosphomonoester bond is purely hydrolytic. The Mo K-edge XANES region also did not show any sign of Mo(VI) to Mo(V) reduction during the hydrolytic reaction. (95)Mo NMR and Mo K-edge EXAFS spectra measured during different stages of the hydrolytic reaction showed a gradual disappearance of [Mo(7)O(24)](6-) during the hydrolytic reaction and appearance of [P(2)Mo(5)O(23)](6-), which was the final complex observed at the end of hydrolytic reaction.     

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM-5 Zeolite Identified by 2D 1H−95Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy

Methane dehydroaromatization (MDA) over Mo-modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D ⁹⁵Mo NMR and 2D ¹H-⁹⁵Mo heteronuclear correlation solid-state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM-5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoO_xC_y species, with the more carbidic one (MoO_xC_y-II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM-5 pores are related to the MDA activity.     

Structural studies of NaPO3-MoO3 glasses by solid-state nuclear magnetic resonance and Raman spectroscopy

Vitreous samples were prepared in the (100 - x)% NaPO(3)-x% MoO(3) (0 相似文献   

A high-resolution MAS NMR study on the potential catalysts Mo/HBeta for olefin metathesis: The interaction of Mo species with HBeta zeolite

The interfacial interaction of Mo species with the HBeta zeolite was studied by multinuclear MAS NMR, XRD and N₂ adsorption. As proved by the quantitative ²⁷Al MAS NMR, this interaction is so strong as to dealuminate the framework of HBeta, and leads to a new peak appearing at −14 ppm, which indicates the formation of crystalline Al₂(MoO₄)₃. This can also be detected by XRD measurements when the Mo loading is as high as 9.0 wt.%. The corresponding quantitative ²⁹Si and ¹H MAS NMR spectra show that the amount of silanols and Brønsted acidic sites decrease obviously with increasing Mo loading. This also reveals an interaction between Mo species and HBeta support through an oxygen bridge resulting from condensation with the hydroxyls on the support. At higher Mo loadings, the interaction is so strong that it results in an extraction of aluminum from the zeolite framework, and subsequently appearance of Al₂(MoO₄)₃ and loss of Brønsted acidic sites. These can be correlated to the low catalytic activity of Mo/HBeta in metathesis of ethylene and 2-butylene to propylene.     

Quadridentate bridging EO(4)(2-) (E = S, Mo, W) ligands and their role as electronic bridges

Three compounds containing two quadruply bonded Mo(2)(DAniF)(3) (DAniF = N,N'-di-p-anisylformamidinate) units linked by tetrahedral EO(4)(2-) anions (E = S, Mo, W) have been prepared and characterized by crystallography and NMR. The linkers in these [Mo(2)(DAniF)(3)](2)(mu-EO(4)) compounds hold the Mo(2) units in an approximately perpendicular orientation and mediate strong electrochemical communication between them. Each of the three compounds shows two quasireversible (mu-SO(4)) or fully reversible (mu-MoO(4), mu-WO(4)) features in its cyclic voltammogram corresponding to successive oxidation of each of its Mo(2) units. The DeltaE(1/2) values are the largest thus far measured for Mo(2)-X-Mo(2) bridged complexes and may be sufficiently large to permit isolation of the singly oxidized species.     

Probing crystal structures and transformation reactions of ammonium molybdates by 14N MAS NMR spectroscopy

The unique high-resolution feature offered by 14N magic-angle spinning (MAS) NMR spectroscopy of ammonium ions has been used to characterize the crystal structures of various ammonium molybdates by their 14N quadrupole coupling parameters, i.e., CQ, the quadrupole coupling constant, and etaQ, the asymmetry parameter. Two polymorphs of diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction (XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two sets of characteristic 14N CQ and etaQ values have been obtained. Similarly, the well-characterized ammonium polymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24.4H2O, and (NH4)6Mo8O27.4H2O also give rise to distinct and characteristic 14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24.4H2O spectrum requires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (nu(r) = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity. These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessment of the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experiments performed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 and mP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.     

Methane dehydrogenation and aromatization in the absence of oxygen on Mo/HZSM-5: A study on the interaction between Mo species and HZSM-5 by using Al and Si MAS NMR

By using a high-resolution solid state nuclear magnetic resonance spectrometer with ²⁷Al and ²⁹Si probes, the interaction between Mo species and HZSM-5 of frsol|Mo/HZSM-5 catalysts has been studied. The results show that there is a strong interaction between Mo species and HZSM-5 zeolite. The framework aluminum in the zeolite can be easily extracted by the introduction of Mo species. The extractability of framework aluminum by Mo species increases with increasing Mo loading and the calcination temperature. The extraction process leads to the formation of non-framework Al at first and then a new crystalline phase of Al₂(MoO₄)₃. The dealumination of the catalyst having a Mo loading of 15% and had been calcined at 973 K is so severe that all the aluminum in the framework are extracted and no framework Al could be detected by ²⁷Al MAS NMR. The catalyst, therefore, lost its catalytic activity for methane dehydrogenation and aromatization in the absence of oxygen. The Si/Al ratio measured from ²⁹Si MAS NMR further confirms the dealumination process observed by ²⁷Al MAS NMR. The MAS NMR results give us an evidence that Al₂(MoO₄)₃ crystallites are much less active for the reaction.     

Determination of the Mo surface environment of Mo/TiO2 catalysts by EXAFS,XANES and PCA

The local structure of oxidic Mo/TiO₂ catalysts (0.5 to 13.5 wt.% Mo) has been studied using EXAFS and XANES. Both EXAFS and XANES results suggest that the Mo surface phase is octahedrally coordinated for all Mo loadings. The EXAFS results were also examined using principal component analysis (PCA) to determine the number of Mo species present on the Mo/TiO₂ catalysts. Results from PCA of the Mo EXAFS spectra suggested the presence of three Mo species: two surface species and bulk MoO₃.     

~（95）Mo NMR谱及其在钼化学研究中的应用

对~（９５）ＭｏＮＭＲ的特点、提供的信息、发展历史以及研究现状作了较为全面的介绍。结合本实验室研究工作和部分文献报道对其在钼化学研究中的应用进行了较为详细的论述，同时还指出影响~（９５）ＭｏＮＭＲ的因素并进行了简单分类和讨论。     

Transition from a nonbonding to a bonding interaction in a tetranuclear [Mo2]2(mu-OR)4 cluster

Tetranuclear Mo4 clusters with two quadruply bonded Mo2(4+) units, [Mo2(cis-DAniF)2] (DAniF = N,N'-di-p-anisylformamidinate), linked by alkoxides (OCH3 for 1 and OC2H5 for 4) have been prepared. The nonbonding separation between the midpoints of the quadruply bonded units, ca. 3.24 A, is the shortest among compounds having two linked Mo2(4+) units. Electrochemical measurements show two redox waves for each compound with large DeltaE(1/2) values (554 and 587 mV for 1 and 4, respectively) that correspond to K(C) values on the order of 10(9). The large electronic communication is attributed to the short separation between dinuclear units that favor direct delta-to-delta orbital interactions between the two dimetal centers. Compound 1 was chemically oxidized using stoichiometric amounts of ferrocenium salts to a one-electron oxidation product 2 (in which the counteranion is PF6-) and a two-electron oxidation product 3 (which contains two BF4- anions). Upon oxidation there are significant decreases in the distance between the two [Mo2] units to 3.100 A and then to 2.945 A. The mixed-valence species 2 shows two broad absorption bands at 5900 and 7900 cm(-1) in the NIR region which are assigned to the HOMO-1 --> SOMO and HOMO-2 --> SOMO transitions. Compound 3 is fluxional in solution, as shown by variable-temperature 1H NMR spectra. The sharp signals in the NMR spectrum at -50 degrees C and the lack of an EPR signal suggest that this species is diamagnetic and that a four-center, two-electron bond is formed in the cyclometallic Mo4 cluster. To a first-order approximation, an average bond order of 0.25 is assigned to the bonding interaction between the two Mo atoms along the long edges of the rectangle defined by the four Mo atoms.     

Syntheses and characterization of oxygen/sulfur-bridged incomplete cubane-type clusters, [Mo3S4Tp3]+ and [Mo3OS3Tp3]+, and a mixed-metal cubane-type cluster, [Mo3FeS4ClTp3]. X-ray structures of [Mo3S4Tp3]Cl, [Mo3OS3Tp3]PF6, and [Mo3FeS4ClTp3

The reaction of [Mo3S4(H2O)9]4+ (1) with hydrotris(pyrazolyl)borate (Tp) ligands produced [Mo3S4Tp3]Cl x 4 H2O ([3]Cl x 4 H2O) in an excellent yield. An X-ray structure analysis of [3]Cl x 4 H2O revealed that each molybdenum atom bonded to the Tp ligand. We report four salts of 3, [3]Cl x 4 H2O, [3]tof x 2 H2O, [3]PF6 x H2O, and [3]BF4 x 2 H2O in this paper. The solubility and stability of the chloride salt in organic solvents differ completely from those of the other salts. We have also prepared a new compound, [Mo3OS3Tp3]PF6 x H2O ([4]PF6 x H2O), via the reaction of [Mo3OS3(H2O)9]4+ (2) with KTp in the presence of NH4PF6. All the molybdenum atoms bonded to Tp ligand. 1H NMR signals corresponding to nine protons bonded to three pyrazole rings in one Tp were observed in a spectrum (at 253 K) of [3]BF4 x 2 H2O. It shows that cluster 3 has a 3-fold rotation axis in CD2Cl2 solution. Twenty-one 1H NMR signals corresponding to twenty-seven protons bonded to nine pyrazole rings in three Tp ligands were observed in a spectrum (at 233 K) of [4]PF6 x H2O; obviously, 4 has no 3-fold rotation axis, in contrast to 3. The short CH...mu3S distance caused large upfield chemical shifts in the 1H NMR spectra of 3 and 4. The reaction of 3 with metallic iron in CH2Cl2 produced [Mo3FeS4XTp3] (X = Cl (5), Br (6)). X-ray structure analysis of 5 has revealed the existence of a cubane-type core Mo3FeS4. Complex 3 functions as a metal-complex ligand for preparing a novel mixed-metal complex even in nonaqueous solvents. The cyclic voltammogram of 5 shows two reversible one-electron couples (E(1/2) = -1.40 and 0.52 V vs SCE) and two irreversible one-electron oxidation processes (E(pc) = 1.54 and 1.66 V vs SCE).     

Molybdenum magnetic shielding and quadrupolar tensors for a series of molybdate salts: a solid-state 95Mo NMR study

A series of molybdate, MoO4(2-), salts have been studied using solid-state 95Mo NMR spectroscopy at applied magnetic field strengths of 11.75, 17.63 and 21.14 T. In contrast to previous investigations, the principal components of the Mo shielding and EFG tensors have been obtained, as well as their relative orientations. At the fields employed, the anisotropic Mo shielding and quadrupolar interactions make significant contributions to the observed 95Mo central transition NMR lineshapes. Based on available structural data, the extent of distortion of the MoO4(2-) anion from T(d) symmetry is reflected in the observed 95Mo nuclear quadrupolar coupling constants for the molybdate salts with divalent cations (i.e., Ca2+, Sr2+, Cd2+, Ba2+, Pb2+), but no correlation is found for molybdate salts containing the monovalent alkali metal (Li+, K+, Rb+, Cs+) cations.     

A solid-state 95Mo NMR and computational investigation of dodecahedral and square antiprismatic octacyanomolybdate(IV) anions: is the point-charge approximation an accurate probe of local symmetry?

Solid-state 95Mo NMR spectroscopy is shown to be an efficient and effective tool for analyzing the diamagnetic octacyanomolybdate(IV) anions, Mo(CN)(8)4-, of approximate dodecahedral, D(2d), and square antiprismatic, D(4d), symmetry. The sensitivity of the Mo magnetic shielding (sigma) and electric field gradient (EFG) tensors to small changes in the local structure of these anions allows the approximate D(2d) and D(4d) Mo(CN)(8)4- anions to be readily distinguished. The use of high applied magnetic fields, 11.75, 17.63 and 21.1 T, amplifies the overall sensitivity of the NMR experiment and enables more accurate characterization of the Mo sigma and EFG tensors. Although the magnitudes of the Mo sigma and EFG interactions are comparable for the D(2d) and D(4d) Mo(CN)(8)4- anions, the relative values and orientations of the principal components of the Mo sigma and EFG tensors give rise to 95Mo NMR line shapes that are significantly different at the fields utilized here. Quantum chemical calculations of the Mo sigma and EFG tensors, using zeroth-order regular approximation density functional theory (ZORA DFT) and restricted Hartree-Fock (RHF) methods, have also been carried out and are in good agreement with experiment. The most significant and surprising result from the DFT and RHF calculations is a significant EFG at Mo for an isolated Mo(CN)(8)4- anion possessing an ideal square antiprismatic structure; this is contrary to the point-charge approximation, PCA, which predicts a zero EFG at Mo for this structure.     

Methane dehydro-aromatization over Mo/HZSM-5 catalysts in the absence of oxygen: Effect of steam-treatment on catalyst stability

The effect of steam-treatment to HZSM-5 zeolite and Mo/HZSM-5 with a steaming time range of 0.5-1 h on the catalytic performance of methane dehydro-aromatization (MDA) over Mo/HZSM-5 catalyst prepared with impregnation has been studied in detail in combination with the characterization of 1H MAS NMR technique. Both the deactivation rate constant (kd) and the Brnsted acid sites per unit cell were calculated to quantitatively evaluate the stability of Mo/HZSM-5 catalysts treated with steam at 813 K before and after impregnation of molybdenum species, and the corresponding variation of their Brnsted acid sites. The results reveal that a V-shape relationship between kd and the number of B1 acid sites per unit cell is presented on Mo/HZSM-5 catalyst under the tested steam-treatment and reaction conditions.     

An equilibrium analysis of the aqueous H+-MoO4(2-)-(HP)O(3)2- and H+-MoO4(2-)-(HP)O(3)2--HPO4(2-) systems

The speciation in the phosphitomolybdate system, H+-MoO4(2-)-(HP)O(3)2-, has been determined from combined potentiometric and 31P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5<-log[H+]<6.2, 40.0相似文献   

Synthesis and Crystal Structure of a Mo-S Cluster Compound Coordinated by Isonicotinate

A cluster compound, Mo3S4(DTP)3(IN)(Py)·EtOH I (DTP = diethyl dithiophospha- te, IN = isonicotinate) has been rationally synthesized and characterized by single-crystal X-ray diffraction. The crystal belongs to the triclinic system, space group P1 with a = 13.6345(2), b = 13.7805(2), c = 14.5400(1) , α = 107.078(1), β = 113.380(1), γ = 96.310(1)o, C25H45Mo3- N2O9P3S10, Mr = 1218.96, V = 2315.75(5) 3, Z = 2, Dc = 1.748 g/cm3, μ = 1.399, F(000) = 1224, R = 0.0425 and wR = 0.1144 for 6335 observed reflections (I > 2σ(I)). 31P NMR spectrum has been measured with the resonance 110.050 and 108.783 (2:1).     

Synthesis and Structure of Dinuclear Molybdenum Carbonyl Thiolate Compound [Et4N ] 2 [Mo2 (CO) 8 (SC6H4-OCH3-p) 2]

Reaction of Mo(CO)6 with p-H3CO-C6H4SNa and Et4NCl · H2O in CH3CN afforded a dinuclear molybdenum(0) compound [Et4N]2[Mo2 (CO)8 (SC6H4-OCH3-p)2] (1). The crystal structure was determined by X-ray diffraction. The crystallographic data: C38H54Mo2N2O10S2, Mr = 954.87, triclinic, P-1, a = 11.348 (7), b =11.616(5), c=10.065(7) A, a=113.86(4), β=111.39(5), γ=91.92(5)°, V=1104.0(1) A3, Z=1, Dc=1.44 g/cm3, F(000)=492, μ=7.0cm-1, Final R=0. 046 and Rw=0. 049 for 2657 reflections with I＞3. Oσ(I). The X-ray structure analysis revealed that the Mo2S2 core of 1 is planar. The geometry around each Mo atom is a distorted octahedron, the two octahedrons form an edge-sharing bioctahedron. The bond. In addition, the 95Mo NMR chemical shift of 1 is discussed.     

傅里哀变换红外发射光谱法研究金属氧化物催化剂 I. 实验建立及氧化钼的还原-氧化研究

本文报导了傅里哀变换红外发射光谱法研究多相氧化物催化剂的实验装置的建立及实验条件的考察。设计加工了一个温度范围宽, 可在各种气氛和真空条件下原位研究氧化物催化剂的红外发射池。以MoO_3为例, 考察了样品厚度, 测试温度对发射光谱的影响。当样品量小于1 mm·cm~(-2)时, 随样品量增加, 谱峰强度线性增加, 但峰形不发生变化。当样品量高于1 mg·cm~(-2)时, 随样品量增加, 谱图严重畸变。随样品温度升高,谱峰强度近似线性增加。并用该方法研究了MoO_3在还原和再氧化过程中表面氧物种的变化。结果表明桥式MO—O—Mo物种比端基Mo=O物种较易还原, 再氧化时, Mo=O物种比Mo—O—Mo物种优先形成。     

负载型钼基催化剂上甲烷,乙烷无氧芳构化反应研究

研究了不同载体钼基催化剂上甲烷，乙烷的无氧芳构化反应。在所采用的载体中，ＨＺＳＭ－５具有最佳性能，对甲烷的芳构化反应，Ｍｏ／ＨＺＳＭ－５催化剂表现出较高的活性和芳烃选择性；而Ｍｏ／Ａｌ２Ｏ３或Ｍｏ／ＳｉＯ２催化剂则相对较差。对于乙烷的反应，钼物种的存在更有利于甲烷或乙烯的生成，芳烃选择性相对较低。钼物种较强的断键能力可能是使甲烷Ｃ－Ｈ键活化的原因。