The Reduction-Coupled Oxo Activation (ROA) Mechanism Responsible for the Catalytic Selective Activation and Functionalization of n-Butane to Maleic Anhydride by Vanadium Phosphate Oxide

We report here the results of density functional theory quantum mechanical (QM) studies of the detailed chemical mechanism underlying the n-butane selective oxidation to form maleic anhydride (MA) on vanadyl pyrophosphate [(VO)₂P₂O₇] and vanadyl phosphate [VOPO₄] surfaces. This QM-derived mechanism differs substantially from previous suggestions but is in excellent agreement with key experimental observations. We find that the O(1)=P bond of the oxidized X1 phase of the VOPO₄ surface is the active site for initiating the VPO chemistry, by extracting the H from the n-butane C–H bond. This contrasts sharply with previous suggestions, all of which involved the V=O bonds. The ability of O(1)=P to cleave alkane C–H bonds arises from a new unique mechanism that decouples the proton transfer and electron transfer components of this H atom transfer reaction. We find that the juxtaposition of a highly reducible V⁺⁵ next to the P=O bond but coupled via a bridging oxygen dramatically enhances the activity of the P=O bond to extract the proton from an alkane, while simultaneously transferring the electron to the V to form V⁺⁴. This Reduction-Coupled Oxo Activation (ROA) mechanism had not been known prior to these QM studies, but we believe that it may lead to a new strategy in designing selective catalysts for alkane activation and functionalization, and indeed it may be responsible for the selective oxidation by a number of known mixed metal oxide catalysts. To demonstrate the viability of this new ROA mechanism, we examine step by step the full sequence of reactions from n-butane to MA via two independent pathways. We that find that every step is plausible, with a highest reaction barrier of 21.7 kcal/mol.     

正丁烷异构化的研究进展

正丁烷异构化产物异丁烷是烷基化反应的主要原料和合成MTBE、ETBE等汽油添加剂的重要前躯体。从烷烃异构化反应机理、影响因素、工艺进展、催化剂研究进展等几个方面,论述了烷烃异构化催化剂的发展概况。催化剂研究新进展主要包括：贵金属改性、非贵金属改性、Mo（W）体系、硫酸锆、杂多酸等,并展望了新型催化剂的应用前景。     

Influence of Milling Media on the Physicochemicals and Catalytic Properties of Mechanochemical Treated Vanadium Phosphate Catalysts

Abstract  

VOHPO₄·0.5H₂O synthesized via the alcohol reduction of VOPO₄·2H₂O was mechanochemical treated for 30 min in three different media, i.e. cyclohexane, ethanol and air. XRD results revealed that their structure became less crystalline compared to the unmilled material. SEM showed that the particles for the milled materials become smaller and unique features were observed in the different type of media used. The reactivity of the oxygen species linked to V⁵⁺ and V⁴⁺ were also affected by the milling process. The selectivity to maleic anhydride from n-butane oxidation were observed to increase in line with the increase in the oxygen species associated with V⁵⁺ and the presence of isolated V⁵⁺ phase. A correlation was observed between the crystallite size of the pyrophosphate phase at (020) plane with the maleic anhydride selectivity.     

Ionic Liquid-Assisted Synthesis of Vanadium Phosphate Catalysts from Phosphorous Acid for Selective Oxidation Reactions

Catalysis Letters - Vanadium oxyphosphate (VPO) with a high surface area was synthesized in a mixed solvent of ionic liquid [Bmim]Br and water, using phosphorous acid to reduce V5+ to V4+ and as a...     

Cracking of n-Butane Over Alkaline Earth-Containing HZSM-5 Catalysts

A series of catalysts, NiSO₄/SiO₂–Al₂O₃, for ethylene dimerization were prepared by the impregnation method using aqueous solution of nickel sulfate. Although SiO₂–Al₂O₃ without NiSO₄ was inactive as catalyst for ethylene dimerization, the SiO₂–Al₂O₃ with NiSO₄ exhibited high catalytic activity even at room temperature. The high catalytic activity of NiSO₄/SiO₂–Al₂O₃ was closely correlated with the increase of acidity and acid strength due to the addition of NiSO₄. The sample having 15 wt% of NiSO₄ and calcined at 500 °C for 1.5 h exhibited maxima for catalytic activity and acidity. In view of IR results of CO adsorbed on NiSO₄/SiO₂–Al₂O₃, it is concluded that the active sites responsible for ethylene dimerization consist of a low-valent nickel, Ni⁺, and an acid.     

Dehydrogenative Cracking of n-Butane over Modified HZSM-5 Catalysts

Dehydrogenative cracking reaction of n-butane was studied using HZSM-5 catalyst modified with various metal oxides. Alkaline earth (magnesium), transition metal (cobalt) and rare earth (lanthanum) elements are used for the modification. The selectivity of the products was studied at low conversion (20%). Methane, ethane, ethylene, propylene, butenes and butadiene were the main products. With the use of the cobalt- or magnesium-containing HZSM-5, dehydrogenative cracking was observed and the selectivity of ethylene was much larger than that of ethane. On the other hand, the selectivity of ethylene and ethane were almost the same in the reaction using the lanthanum-containing HZSM-5. It is considered that the cobalt- and magnesium-loaded sites on HZSM-5 played an important role in the dehydrogenative cracking.     

Synthesis of Vanadium Phosphate Catalysts by Hydrothermal Method for Selective Oxidation of n-butane to Maleic Anhydride

Two vanadium phosphate catalysts (VPH1 and VPH2) prepared via hydrothermal method are described and discussed. Both catalysts exhibited only highly crystalline pyrophosphate phase. SEM showed that the morphologies of these catalysts are in plate-like shape and not in the normal rosette-type clusters. Temperature-programmed reduction in H₂ resulted two reduction peaks at high temperature in the range of 600–1100 K. The second reduction peak appeared at 1074 K occurred as a sharp peak indicated that the oxygen species originated from V⁴⁺ phase are having difficulty to be removed and their nature are less reactive compared to other methods of preparation. Modified VPH2 gave better catalytic performance for n-butane oxidation to maleic anhydride contributed by a higher BET surface area, high mobility and reactivity of the lattice oxygen associated to the V⁴⁺ which involved in the hydrocarbon’s activation. A slight increased of the V⁵⁺ phase also enhanced the activity of the VPH2 catalyst.     

Aromatization of n-Butane Over Supported Mo2C Catalysts

Similarly to the case of methane, ethane and propane, Mo₂C deposited on ZSM-5 significantly enhanced the aromatization of n-butane observed on ZSM-5 (SiO₂/Al₂O₃ ratio of 80) alone. The catalytic performance of Mo₂C/ZSM-5 sensitively depended on its preparation and pretreatment. The selectivity of aromatics measured for pure ZSM-5 increased from 11-13% to 28-34% at the conversion level of 60-65%. The formation of aromatics was also observed over Mo₂C/SiO₂.     

Active Sites Characterization in Mixed Vanadium and Iron Antimonate Oxide Catalysts for Propane Ammoxidation

Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained show that in VSbO₄ mixed oxide catalyst systems, substitution of vanadium by small quantities of iron has a strongly positive effect on the activity of the catalysts. With such a system, the selectivity to acrylonitrile decreases while the selectivity to propene increases. At high levels of vanadium substitution by iron, the activity was low but the selectivity towards acrylonitrile was enhanced. The characterization of the catalysts carried out using several techniques, such as X-ray diffraction, Mössbauer spectroscopy, X-ray absorption, and electron microscopy, has shown that a continuous solid solution was formed between VSbO₄ and FeSbO₄. At low iron loading, Fe³⁺ substituted for V⁴⁺ in the cationic-deficient structure V⁴⁺_0.64V³⁺_0.28Sb⁵⁺_0.92♢_0.16O₄, leading to a structure with both cationic and anionic vacancies, Fe³⁺_xV³⁺_0.33V⁴⁺_0.41−xSb⁵⁺_0.92♢_0.18O_4−x, and generating very active sites for alkane activation. At high iron loading, only V³⁺ was present in the bulk of the solids and only Fe³⁺ contributed to the isolation of the vanadium sites, which are selective to acrylonitrile.     

Dehydrogenation and Isomerization of n-Butane or Isobutane Over Cr Catalysts Supported on Zeolites

Dehydrogenation and isomerization of n-butane or isobutane into isobutene over Cr catalysts supported on zeolites were investigated. We found that Cr catalyst supported on H-SSZ-35-type zeolite, having one-dimensional cage-type channel structure, was very effective for this reaction and the yield of isobutene was 5.44% from n-butane and 9.57% from isobutane. In this reaction, it is suggested that dehydrogenation of butanes is accelerated by Cr₂O₃ loading, and solid acidity of the zeolite support favors isomerization.     

On the Environment of the Active Sites in Phosphate Modified Silica–Zirconia Acid Catalysts

The acid sites of a series of phosphated silica–zirconia catalysts have been quantified by the combined use of FTIR and TGA and the data related to the activity of the samples for the isomerisation of but-1-ene. The catalyst series show a linear correlation indicative of constant TOF when the number of Brønsted acid sites is plotted against catalytic activity. However, this relationship only holds when the number of Lewis acid sites exceeds that of Brønsted acid sites suggesting that an optimal arrangement may involve both types of site. The TOF is less when the Lewis/Brønsted ratio falls below 1, a scenario that dominates at high Brønsted acid site densities. When this site arrangement prevails, a higher than expected cis/trans ratio is found in the isomerisation selectivity. These differences in the environment of the active Brønsted acid site may also be deduced from differences in the molar absorption coefficients of adsorbed basic probe molecules.     

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H₂-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO₄ · 1/2 H₂O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H₂-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V⁴⁺–O^? pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.     

在VPO催化剂上丁烷选择氧化的质谱研究

The selective oxidation of n-butane to maleic anhydride(MA) on a vanadium-phosphorus oxide (VPO) catalyst was studied using on-line gas-chromatography combined with mass spectrometry(GC-MS) and transient response technique.The reaction intermediates,butene and furan,were found in the reaction effuent under near industrial feed condition (3% butane 15% O2),while dihydrofuran was detected at high butane concentration (12% butane,5%O2).Some intermediates of MA decomposition were also identified.Detection of these intermediates shows that the vanadium phosphorus oxides are able to dehydrogenate butane to butene,and butene further to form MA.Based on these observations,a modified scheme of reaction network is proposed.The transient experiments show that butane in the gas phase may directly react with oxygen both on the surface and from the metal oxide lattice,without a proceeding adsorption step.Gas phase oxygen can be adsorbed and transformed to surface lattice oxygen but it can not participate in selective oxidation.Adsorbed oxygen leads to deep oxidation,while lattice oxygen leads to selective oxidation.     

磷钒酸盐红色发光粉的制备及性能研究

采用燃烧法合成了一种近紫外激发的Ca3((P,V)O4)2:Eu3+发光材料,用X射线衍射谱、荧光光谱对样品进行了表征。结果表明,荧光粉基质Ca3((P,V)O4)2具有畸变的Ca3(VO4)2的结构,VO34-取代了部分的PO34-,但Ca2+的半径与Eu3+接近,因此Eu3+容易进入晶格,表现出良好的发光性能。荧光光谱分析发现荧光粉可被376 nm紫外光激发,主发射峰值位于620 nm(Eu3+离子的5D0→7F2跃迁)的红光,同时详细研究了Eu3+离子掺杂浓度、引发温度及尿素用量对荧光粉发光性能的影响。     

Mechanical Activation of Tetracalcium Phosphate

It was found that prolonged high-energy ball-milling of Hilgenstokite (tetracalcium phosphate, TTCP) resulted in a decrease in both particle and crystallite size, leading to a mechanical activation of the compound. This mechanically activated material demonstrated a high reactivity such that, in contrast to highly crystalline TTCP, a setting reaction with water to nanocrystalline hydroxyapatite (HA) and Ca(OH)₂ could be achieved at 37°C. However, crystalline TTCP is practically unreactive at physiologic temperatures because of the formation of a thin HA layer on the particle surface preventing further reaction.     

Magnetic Behavior and Microstructure of Vanadium Phosphate Glasses

The magnetic properties and microstructures of the vanadium phosphate glass system over the composition range 60 to 90 mol% V₂O₅ were investigated to study magnetic ordering in the glass and the effect of microstructure on its magnetic properties. Direct antiferromagnetic coupling between V⁴⁺ ions in the glassy matrix exists, and a transition temperature near - 70°C was observed. As-cast glasses with high V₂O₅ concentrations separated into two glassy phases; this separation increased the ESR line width as a result of inhomogeneity broadening. The separation, which concentrated the vanadium ions in a vanadium-rich phase, caused a hysteresis in the plot of ESR line intensity vs temperature at the transition temperature. Reduction of the vanadium ions by dextrose added to the melt enhanced phase separation and resulted in weak antiferromagnetic transitions at +70° and -120°C, the Neel temperatures of VO₂ and V₂O₃, respectively.     

磷酸铁锂和磷酸钒锂复合正极材料研究进展

自锂离子电池被发现以来,对锂离子电池的正极材料的研究也趋于白热化.继LiCoO2之后兴起了一系列可以作为锂离子电池正极的材料.由于稳定性好,安全性能高,绿色环保而且资源比较丰富,LiFePO4和Li3V2(PO4)3成为锂离子正极材料的候选材料之一,也是现在的研究重点.此文章主要对LiFePO4和Li3V2(PO4)3的结构特点,二者复合材料的复合机制以及电化学性能进行论述,并对LiFePO4和Li3V2(PO4)3复合锂离子电池正极材料的应用及发展方向进行说明.