On the Role of Vanadia Species for VOx／SiO2 in the Selective Oxidation of Methane

Various VOx/SiO2 catalysts were prepared by the methods of physical mixing, conventional wetness impregnation and ultrasonication-assistant impregnation. The catalysts were characterized by XRD, UV-Vis DRS, Raman, TPR, ESR and TPSR techniques and the nature of the vanadium species were correlated to their catalytic performance in the reaction of direct conversion of methane to formaldehyde. It is concluded that highly dispersed monomeric and low oligomeric vanadia species are formed on the sample prepared with both traditional wetness impregnation method and ultrasonication-assistant impregnation, whereas in the latter case, the amount of oligomeric vanadia species is much smaller. The V2O5 microcrystallines are the dominant species on the material prepared by physical mixing method. During the selective oxidation of methane, Vv species are reduced to V^IV paramagnetic species and both microcrystalline V205 species and oligomeric vanadia species are found to further disperse and transform into tetrahedral vanadia species. Based on the results of UV Raman spectroscopy and TPSR, C02 is suggested to be formed via two different routes, in which one is from the sequence reaction of CH4→ HCHO→ CO → CO2 over monomeric vanadia species, and the other is from the direct oxidation of methane to CO2 over oligomeric vanadia species. Oligomeric vanadia species is more active than monomeric vanadia species for methane activation.     

Isothermal Cyclic Conversion of Methane into Methanol over Copper‐Exchanged Zeolite at Low Temperature

Direct partial oxidation of methane into methanol is a cornerstone of catalysis. The stepped conversion of methane into methanol currently involves activation at high temperature and reaction with methane at decreased temperature, which limits applicability of the technique. The first implementation of copper‐containing zeolites in the production of methanol directly from methane is reported, using molecular oxygen under isothermal conditions at 200 °C. Copper‐exchanged zeolite is activated with oxygen, reacts with methane, and is subsequently extracted with steam in a repeated cyclic process. Methanol yield increases with methane pressure, enabling reactivity with less reactive oxidized copper species. It is possible to produce methanol over catalysts that were inactive in prior state of the art systems. Characterization of the activated catalyst at low temperature revealed that the active sites are small clusters of copper, and not necessarily di‐ or tricopper sites, indicating that catalysts can be designed with greater flexibility than formerly proposed.     

Effect of water on the catalytic oxidation of catechols

A dinuclear copper(II) complex derived from a new water-soluble pentadentate Schiff base backbone ligand has been prepared and characterized in solution and in the solid state. The complex has been found to accelerate the aerobic oxidation of 3,5-di- tert-butylcatechol (DTBC) into 3,5-di- tert-butylquinone (DTBQ) by 5 orders of magnitude, compared to the background reaction in aqueous methanol (k(cat)/k(non) = 160,000) at 30 degrees C. The transformation of the model substrate is considerably slower in pure methanol (k(cat)/k(non) = 60,000) under otherwise identical conditions. In-depth investigation of the catalytically active species revealed different structures for the copper(II) complex in methanol and in methanol/water mixtures.     

Pathways of Methane Transformation over Copper‐Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

The reaction of methane with copper‐exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic Cu^II species. Several types of surface‐bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono‐ and bidentate formates, Cu^I monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper‐exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.     

Mild, aqueous, aerobic, catalytic oxidation of methane to methanol and acetaldehyde catalyzed by a supported bipyrimidinylplatinum-polyoxometalate hybrid compound

We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center.     

Structure of copper sites in zeolites examined by Fourier and wavelet transform analysis of EXAFS

Copper-exchanged zeolites are a class of redox-active materials that find application in the selective catalytic reduction of exhaust gases of diesel vehicles and, more recently, the selective oxidation of methane to methanol. However, the structure of the active copper-oxo species present in zeolites under oxidative environments is still a subject of debate. Herein, we make a comprehensive study of copper species in copper-exchanged zeolites with MOR, MFI, BEA, and FAU frameworks and for different Si/Al ratios and copper loadings using X-ray absorption spectroscopy. Only obtaining high quality EXAFS data, collected at large k-values and measured under cryogenic conditions, in combination with wavelet transform analysis enables the discrimination between the copper-oxo species having different structures. The zeolite topology strongly affects the copper speciation, ranging from monomeric copper species to copper-oxo clusters, hosted in zeolites of different topologies. In contrast, the variation of the Si/Al ratio or copper loading in mordenite does not lead to significant differences in XAS spectra, suggesting that a change, if any, in the structure of copper species in these materials is not distinguishable by EXAFS.

The structure of copper-oxo species hosted in zeolites of various topology has been examined using wavelet and Fourier transform analysis of Cu K-edge EXAFS spectra.     

Pathways of Methane Transformation over Copper-Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

The reaction of methane with copper-exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic Cu^II species. Several types of surface-bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono- and bidentate formates, Cu^I monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper-exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.     

醋酸溶液中Pd-CuPc/Y催化甲烷选择氧化制甲醇

以氯化铜、钼酸铵、苯酐、氯化铵、尿素和NaY分子筛为原料,采用苯酐-尿素法制备了酞菁铜/分子筛复合物CuPc/Y.采用等体积浸渍法将金属钯担载在CuPc/Y上制备了Pd-CuPc/Y催化剂,并在醋酸水溶液中考察了其催化甲烷选择氧化合成甲醇反应的性能,结果表明,催化性能与反应温度、溶剂中CH₃COOH与H₂O的混合比例、对苯醌用量、反应时间等因素有关,在0.5%Pd-0.5%CuPc/Y添加量0.5 g、CH₃COOH与H₂O体积比4∶1、对苯醌用量1 000 μmol、反应时间3 h、反应温度150 ℃的条件下,甲醇的最佳生成量为1 840 μmol.Pd-CuPc/Y催化剂可以多次循环使用,但由于催化剂流失和催化剂表面的钯粒子聚集的原因,循环使用后的催化剂催化活性有所下降.Pd-CuPc/Y在醋酸溶液中催化甲烷选择氧化合成甲醇是亲电取代反应和活性氧物种氧化共同作用的结果.     

Biosynthesis of methanol from methane by <Emphasis Type="Italic">Methylosinus trichosporium</Emphasis> OB3b

Methanol has recently attracted significant interest in the energetic field. Current technology for the conversion of methane to methanol is based on energy intensive endothermic steam reforming followed by catalytic conversion into methanol. The one-step method performed at very low temperatures (35°C) is methane oxidation to methanol via bacteria. The aim of this work was to examine the role of copper in the one-step methane oxidation to methanol by utilizing whole cells of Methylosinus trichosporium OB3b bacteria. From the results obtained it was found that copper concentration in the medium influences the rate of bacterial biomass growth or methanol production during the process of methane oxidation to methanol. The presented results indicate that the process of methane oxidation to methanol by Methylosinus trichosporium OB3b bacteria is most efficient when the mineral medium contains 1.0 × 10⁻⁶ mol dm⁻³ of copper. Under these conditions, a satisfactory growth of biomass was also achieved. Presented at the 35th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 26–30 May 2008.     

The application of FTIR in situ spectroscopy combined with methanol adsorption to the study of mesoporous sieve SBA-15 with cerium-zirconium oxides modified with gold and copper species

The molecular sieves SBA-15 with ceria, zirconia and mixed cerium-zirconium oxides were synthesized and used as supports for gold and copper species. The materials were characterised using a FTIR in situ spectroscopy combined with the adsorption of methanol and oxygen as probe molecules, which allowed the monitoring of the changes on the surface, in particular the creation of the intermediates species on the active sites of materials. The presence of interactions between gold and copper in bimetallic catalysts was considered during the reaction of methanol oxidation. The goal of this work was studied the changes of intermediates adsorbed on the surface of catalysts during the oxidation of methanol after treatment at selected temperature. The monometallic gold and the bimetallic copper–gold catalysts were tested in the reaction of methanol oxidation in the gas phase in situ.     

Electrochemical and spectroscopic studies of 2,3-dihydroxybenzoic acid,its oxidation products and their interaction with manganese(II), in dimethyl sulfoxide solutions

The electrochemical and spectroscopic behaviour of 2,3-dihydroxybenzoic acid (2,3-DHBA) and its oxidized forms have been studied in dimethyl sulfoxide solutions under aerobic and anaerobic conditions. The products resulting from the reaction with manganese(II) (in dimethyl sulfoxide) are also studied by cyclic voltammetry, u.v–vis., n.m.r. and e.s.r. spectroscopies. Under anaerobic conditions the anions of the ligand form stable complexes with manganese(II) and (III) of MnL₂ type, while in the presence of air the oxidized forms of the ligand react with manganese(II) to give mixed-valence species. The chemical stability of the semiquinone and its manganese complexes in addition to its photosensitivity is noteworthy. Calculations show that the electrogenerated manganese(III)–(2,3-DHB–semiquinone) system is stable, but redox-active and can undergo a two-electron exchange (per monomer). The dimeric (or oligomeric) species should be good candidates for water oxidation studies.     

Catalytic and Mechanistic Insights of the Low‐Temperature Selective Oxidation of Methane over Cu‐Promoted Fe‐ZSM‐5

The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h^?1), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron‐ and copper‐containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h^?1 by using H₂O₂ as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by‐production of hydroxyl radicals. The catalytic system in question results in a low‐energy methane activation route, and allows selective C₁‐oxidation to proceed under intrinsically mild reaction conditions.     

Investigating the innate selectivity issues of methane to methanol: consideration of an aqueous environment

The higher reactivity of the methanol product over the methane reactant for the direct oxidation of methane to methanol is explored. C–H activation, C–O coupling, and C–OH coupling are investigated as key steps in the selective oxidation of methane using DFT. These elementary steps are initially considered in the gas phase for a variety of fcc (111) pristine metal surfaces. Methanol is found to be consistently more reactive for both C–H activation and subsequent oxidation steps. With an aqueous environment being understood experimentally to have a profound effect on the selectivity of this process, these steps are also considered in the aqueous phase by ab initio molecular dynamics calculations. The water solvent is modelled explicity, with each water molecule given the same level of theory as the metal surface and surface species. Free energy profiles for these steps are generated by umbrella sampling. It is found that an aqueous environment has a considerable effect on the kinetics of the elementary steps yet has little effect on the methane/methanol selectivity-conversion limit. Despite this, we find that the aqueous phase promotes the C–OH pathway for methanol formation, which could enhance the selectivity for methanol formation over that of other oxygenates.

Consideration of the selectivity of methane to methanol in the aqueous phase with AIMD.     

Conversion of methane to methanol at the mononuclear and dinuclear copper sites of particulate methane monooxygenase (pMMO): a DFT and QM/MM study

Methane hydroxylation at the mononuclear and dinuclear copper sites of pMMO is discussed using quantum mechanical and QM/MM calculations. Possible mechanisms are proposed with respect to the formation of reactive copper-oxo and how they activate methane. Dioxygen is incorporated into the Cu(I) species to give a Cu(II)-superoxo species, followed by an H-atom transfer from a tyrosine residue near the monocopper active site. A resultant Cu(II)-hydroperoxo species is next transformed into a Cu(III)-oxo species and a water molecule by the abstraction of an H-atom from another tyrosine residue. This process is accessible in energy under physiological conditions. Dioxygen is also incorporated into the dicopper site to form a (mu-eta(2):eta(2)-peroxo)dicopper species, which is then transformed into a bis(mu-oxo)dicopper species. The formation of this species is more favorable in energy than that of the monocopper-oxo species. The reactivity of the Cu(III)-oxo species is sufficient for the conversion of methane to methanol if it is formed in the protein environment. Since the sigma orbital localized in the Cu-O bond region is singly occupied in the triplet state, this orbital plays a role in the homolytic cleavage of a C-H bond of methane. The reactivity of the bis(mu-oxo)dicopper species is also sufficient for the conversion of methane to methanol. The mixed-valent bis(mu-oxo)Cu(II)Cu(III) species is reactive to methane because the amplitude of the sigma singly occupied MO localized on the bridging oxo moieties plays an essential role in C-H activation.     

The local environment of Cu+ in Cu-Y zeolite and its relationship to the synthesis of dimethyl carbonate

Cu-exchanged Y zeolite was investigated in order to determine the location of the copper cations relative to the zeolite framework and to determine which Cu cations are active for the oxidative carbonylation of methanol to dimethyl carbonate (DMC). Cu-Y zeolite was prepared by vapor-phase exchange of H-Y with CuCl. The oxidation state, local coordination, and bond distances of Al and Cu were determined using Al K-edge and Cu K-edge X-ray absorption spectroscopy (XAS). Complimentary information was obtained by H2 temperature-programmed reduction and by in-situ infrared spectroscopy. Cu-Y has a Cu/Al ratio of unity and very little occluded CuCl. The average Al-O and Al-Cu bond distances are 1.67 angstroms and 2.79 angstroms, respectively, and the average Cu-O and Cu-Si(Al) bond distances are 1.99 angstroms and 3.13 angstroms, respectively. All of the Cu exchanged is present as Cu+ in sites I', II, and III'. Cu-Y is active for the oxidative carbonylation of methanol, and at low reactant contact time produces DMC as the primary product. With increasing reactant contact time, DMC formation decreases in preference to the formation of dimethoxy methane (DMM) and methylformate (MF). The formation of DMM and MF is attributed to the hydrogenation of DMC and the hydrogenolysis of DMM, respectively. Observation of the catalyst under reaction conditions reveals that most of the copper cations remain as Cu+, but some oxidation of Cu+ to Cu2+ does occur. It is also concluded that only those copper cations present in site II and III' positions are accessible to the reactants, and hence are catalytically active. The dominant adsorbed species on the surface are methoxy groups, and adsorbed CO is present as a minority species. The relationship of these observations to the kinetics of DMC synthesis is discussed.     

Catalytic Combustion of Methane over High Copper-Loading ZSM-5 Catalysts

Two series of Cu/ZSM-5 catalysts,loading from 5 to 20 wt% CuO,were prepared by the deposition-precipitation and impregnation methods,respectively.The catalysts prepared by the impreg- nation method showed better catalytic performances than those prepared by the deposition-precipitation method and the increase of copper loading favored methane conversion.20Cu(I)/ZSM-5 had the highest activity with T_(90%)of 746 K,and for 20Cu(D)/ZSM-5,T_(90%)was as high as 804 K.The characteriza- tion of X-ray diffraction(XRD),temperature-programmed reduction(TPR),temperature-programmed desorption(TPD),and X-ray photoelectron spectroscopy(XPS)revealed that the dispersion of cop- per species could be improved by using the deposition-precipitation method instead of the impregnation method,but the fraction of surface CuO,corresponding to active sites for methane oxidation,was larger on 20Cu(I)/ZSM-5 than 20Cu(D)/ZSM-5.The results of Pyridine-Fourier transform infrared spectrum (Py-FT-IR)showed that a majority of Lewis acidity and a minority of Brφnsted acidity were present on Cu/ZSM-5 catalysts.20Cu(I)/ZSM-5 presented more Lewis acid sites.The number of Lewis acid sites changed significantly with preadsorption of oxygen.Adsorption of methane and oxygen on acid sites was observed.The properties of Cu/ZSM-5 catalysts were correlated with the activity for methane oxidation.     

铜（II）化合物作用下2-綦胺的氧化偶合反应

2-萘胺（1）在甲醇中与铜胺络合物[x(CuCl_2):x(苄胺或乙醇胺）=2：1]室温 下反应得到产率为74%的1,1'-联-2-萘胺（2）。在醋酸铜、硝酸铜或高氯酸铜的催 化下,1在甲醇中被氢气氧化为2-氨基-1,4-萘醌-4-2'-萘亚胺（3）,产率86%。 用X-ray单晶衍射证实3的乙酰化产物结构为2-乙酰氨基-1,4-萘醌-E-4-2'-萘亚胺 （4）。循环伏安研究表明,铜（II）化合物的氧化反应选择性与其还原峰电位有 关。电喷雾质谱的分析表明1,2-萘醌-2-亚胺（6）是生成3的中间体。     

Methane Over-Oxidation by Extra-Framework Copper-Oxo Active Sites of Copper-Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

Copper-exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over-oxidation products like CO and CO₂. However, mechanistic pathways for methane over-oxidation by copper-oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over-oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over-oxidation by [Cu₃O₃]²⁺ active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ-oxo atom of the active site. This μ-(O−CH₃) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper-monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper-monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step must be stabilized away from copper-oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper-oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane-to-methanol conversion.     

Intrinsic bioremediation of gas condensate hydrocarbons

Condensate liquids have been found to contaminate soil and ground water at two gas production sites in the Denver Basin operated by Amoco Production Co. These sites have been closely monitored since July 1993 to determine whether intrinsic aerobic or anaerobic bioremediation of hydrocarbons occurs at a sufficient rate and to an adequate end point to support a no-intervention decision. Ground water monitoring, soil gas analysis, and analysis of soil cores suggest that bioremediation is occurring at these sites by multiple pathways, including aerobic oxidation, sulfate reduction, and methanogenesis. Results of over two years of monitoring of ground water and soil chemistry at these sites are presented to support this conclusion.

     

Rifampicin inhibits alpha-synuclein fibrillation and disaggregates fibrils

The aggregation of alpha-synuclein in dopaminergic neurons of the substantia nigra is a critical step in the pathogenesis of Parkinson's disease. We show that the antibiotic rifampicin inhibited alpha-synuclein fibrillation and disaggregated existing fibrils in a concentration-dependent manner. Size-exclusion chromatography data indicated that rifampicin stabilized alpha-synuclein as both a monomer and soluble oligomers comprised of partially folded alpha-synuclein. Experiments using aged samples of rifampicin indicated that the most active species in inhibiting fibrillation and disaggregating fibrils is an oxidation product of rifampicin, which was confirmed in experiments under anaerobic conditions. These results indicate that rifampicin-mediated inhibition of alpha-synuclein fibrillation and disaggregation of fibrils involves preferential stabilization of monomeric and soluble oligomeric forms, and that rifampicin potentially may have therapeutic application for Parkinson's disease.