Highly selective olefin epoxidation with aqueous H2O2 over surface-modified TaSBA15 prepared via the TMP method

Trialkylsiloxy-modified Ta(v) centers on mesoporous silica exhibit excellent selectivity for epoxide formation (>98% after 2 h) in the oxidation of cyclohexene using aqueous H2O2 as the oxidant; the modified catalysts exhibit an increased lifetime, retaining high selectivity after 6 h of reaction (>95% epoxide).     

A new layered metal-organic framework as a promising heterogeneous catalyst for olefin epoxidation reactions

A new layered MOF material [Co(Hoba)(2)·2H(2)O] (1) (H(2)oba = 4,4'-oxybis(benzoic acid)) has been synthesized and used as a highly recyclable heterogeneous catalyst for olefin epoxidation reactions. Both high conversion (96%) and high selectivity of epoxide products (96%) are achieved.     

Fluorinated alcohols enable olefin epoxidation by H2O2: template catalysis

Experimental observations show that direct olefin epoxidation by H(2)O(2), which is extremely sluggish otherwise, occurs in fluorinated alcohol (R(f)OH) solutions under mild conditions requiring no additional catalysts. Theoretical calculations of ethene and propene epoxidation by H(2)O(2) in the gas phase and in the presence of methanol and of two fluorinated alcohols, presented in this paper, show that the fluoro alcohol itself acts as a catalyst for the reaction by providing a template that stabilizes specifically the transition state (TS) of the reaction. Thus, much like an enzyme, the fluoro alcohol provides a complementary charge template that leads to the reduction of the barrier by 5-8 kcal mol(-)(1). Additionally, the fluoro alcohol template keeps the departing OH and hydroxyalkenyl moieties in close proximity and, by polarizing them, facilitates the hydrogen migration from the latter to form water and the epoxide product. The reduced activation energy and structural confinement of the TS over the fluoro alcohol template render the epoxidation reaction observable under mild synthetic conditions.     

gamma-1,2-H2SiV2W10O40] immobilized on surface-modified SiO2 as a heterogeneous catalyst for liquid-phase oxidation with H2O2

An organic-inorganic hybrid support has been synthesized by covalently anchoring an N-octyldihydroimidazolium cation fragment onto SiO2 (denoted as 1-SiO2). This modified support was characterized by solid-state 13C, 29Si, and 31P NMR spectroscopy, IR spectroscopy, and elemental analysis. The results showed that the structure of the dihydroimidazolium skeleton is preserved on the surface of SiO2. The modified support can act as a good anion exchanger, which allows the catalytically active polyoxometalate anion [gamma-1,2-H2SiV2W10O40]4- (I) to be immobilized onto the support by a stoichiometric anion exchange (denoted as I/1-SiO2). The structure of anion I is preserved after the anion exchange, as confirmed by IR and 51V NMR spectroscopy. The catalytic performance for the oxidation of olefins and sulfides, with hydrogen peroxide (only one equivalent with respect to substrate) as the sole oxidant, was investigated with I/1-SiO2. This supported catalyst shows a high stereospecificity, diastereoselectivity, regioselectivity, and a high efficiency of hydrogen peroxide utilization for the oxidation of various olefins and sulfides without any loss of the intrinsic catalytic nature of the corresponding homogeneous analogue of I (i.e., the tetra-n-butylammonium salt of I, TBA-I), although the rates decreased to about half that with TBA-I. The oxidation can be stopped immediately by removal of the solid catalyst, and vanadium and tungsten species can hardly be found in the filtrate after removal of the catalyst. These results rule out any contribution to the observed catalysis from vanadium and tungsten species that leach into the reaction solution, which means that the observed catalysis is truly heterogeneous in nature. In addition, the catalyst is reusable for both epoxidation and sulfoxidation without any loss of catalytic performance.     

A novel route for in-situ H2O2 generation from selective reduction of O2 by hydrazine using heterogeneous Pd catalyst in an aqueous medium

Hydrogen peroxide in high yields can be generated with high efficiency at mild conditions (25 degrees C and atmospheric pressure) with the formation of only environment-friendly by-products (N2 and H2O) by a reduction of O2 by hydrazine from its hydrate/salt with its complete conversion in a short reaction period (相似文献   

Kinetics of the oxygenation of unsaturated organics with singlet oxygen generated from H2O2 by a heterogeneous molybdenum catalyst

A heterogeneous catalyst containing MoO42- exchanged on layered double hydroxides (Mo-LDHs) is used to produce 1O2 from H2O2, and with this dark 1O2, unsaturated hydrocarbons are oxidized in allylic peroxides. The oxidation kinetics are studied in detail and are compared with the kinetics of oxidation by 1O2, formed from H2O2 by a homogeneous catalyst. A model is proposed for the heterogeneously catalyzed 1O2 generation and peroxide formation. The model divides the reaction suspension in two compartments: (1) the intralamellar and intragranular zones of the LDH catalyst; (2) the bulk solution. The 2-compartment model correctly predicts the oxidant efficiency and peroxide yield for a series of olefin peroxidation reactions. 1O2 is generated at a high rate by the heterogeneous catalyst, but somewhat more 1O2 is lost by quenching with the heterogeneous catalyst than using the homogeneous catalyst. Quenching occurs mainly as a result of collision with the LDH hydroxyl surface, as is evidenced by using LDH supports containing strong 1O2 deactivators such as Ni2+. A total of 15 organic substrates were peroxidized on a preparative scale using the best Mo-LDH catalyst under optimal conditions.     

Synthesis, structural elucidation, and application of a pyrazolylpyridine-molybdenum oxide composite as a heterogeneous catalyst for olefin epoxidation

The reaction of [MoO(2)Cl(2)(pypzEA)] (1) (pypzEA = ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-yl]acetate) with water in a Teflon-lined stainless steel autoclave (100 °C) or in an open reflux system leads to the isolation of the molybdenum oxide/pyrazolylpyridine composite material [Mo(2)O(6)(HpypzA)] (2; HpypzA = [3-(pyridinium-2-yl)-1H-pyrazol-1-yl]acetate). The solid state structure of 2 was solved through single crystal and powder X-ray diffraction analyses in conjunction with information derived from FT-IR and (13)C CP MAS NMR spectroscopies and elemental analyses. In the asymmetric unit of 2, two crystallographically distinct Mo(6+) centers are bridged by a syn,syn-carboxylate group of HpypzA. The periodic repetition of these units along the a axis of the unit cell leads to the formation of a one-dimensional composite polymer, (∞)(1)[Mo(2)O(6)(HpypzA)]. The outstretched pyrazolylpyridine groups of adjacent polymers interdigitate to form a zipper-like motif, generating strong onset π-π contacts between adjacent rings of coordinated HpypzA molecules. The composite oxide 2 is a stable heterogeneous catalyst for liquid-phase olefin epoxidation.     

Kinetics of olefin copolymerization with heterogeneous Ziegler-Natta catalysts

The kinetics of the ethylene/1-hexene copolymerization reaction with a Ti-based Ziegler-Natta catalyst has been studied. Kinetic analysis established the existence of several populations of active centers in the catalyst. The centers differ in two aspects: their ability to incorporate α-olefin units into copolymer chains (i.e., their reactivity ratios) and the average molecular weights of the polymer chains they produce. The centers of different populations are formed at different rates and have different kinetic stabilities. As a consequence, both the molecular weight distributions of the copolymers and their compositional distributions are relatively broad and change with in time. Two kinds of catalyst poisons were found. The poisons of the first type, arylalkoxysilanes, preferentially deactivate the centers which have the highest ability to copolymerize α-olefins with ethylene. These poisons decrease the average α-olefin content in the copolymers and the fraction of their olefin-rich components. The poisons of the second type, conjugated dienes, preferentially deactivate the centers which have the lowest ability to copolymerize α-olefins with ethylene. These poisons significantly increase the content of the olefin-rich components in the copolymers.     

Spin surface crossing in chromium-mediated olefin epoxidation with O(2)

[CpCr(mu-Cl)Cl](2) reacted with dioxygen (O(2)) to produce CpCr(O)Cl(2) (1), which has been structurally characterized. Although 1 oxidized PPh(3) and 1,4-cyclohexadiene catalytically, it did not epoxidize olefins. DFT calculations have been performed on the system to characterize the potential energy surface for the epoxidation of ethylene and, in particular, the consequences of the crossing from the doublet surface of the starting materials to the quartet surface of the product (i.e. a chromium(III) epoxide adduct). These calculations suggested that "spin-blocking" was not a significant problem and that the reaction of CpCr(O)Cl(2) (3) with ethylene should have a lower activation barrier. On the basis of this computational prediction, 3 was prepared; it was found to epoxidize olefins stoichiometrically.     

Au／TS-1@Mesosilica双功能催化材料的制备及其催化丙烯直接环氧化性能

设计制备了一种新型微孔介孔复合核壳结构钛硅分子筛TS-1@Mesosilica（TS-l@Ms）,核为MFI结构钛硅分子筛TS-1,壳层为以非离子表面活性剂P123为模板剂组装形成的介孔氧化硅．壳层氧化硅具有三维蠕虫状孔道结构,有利于微孔和介孔部分的连通及反应物和产物的扩散．通过沉积沉淀法将金纳米粒子负载在壳层介孔孔道,和TS-1中的钛活性中心协同,形成适合于C3H6和H2、O2直接气相环氧化制备环氧丙烷（PO）的双功能催化材料．实验结果表明,Au／TS-1@MS在空速8000mLg-h、温度473K条件下连续反应132h,活性和选择性没有明显下降,丙烯转化率保持在3．7％左右,PO选择性87％以上．     

Kinetics of MTO-catalyzed olefin epoxidation in ambient temperature ionic liquids: UV/Vis and 2H NMR study

The kinetics of oxygen-atom transfer from the peroxo complexes of methyltrioxorhenium (MTO) to alkenes in ionic liquids have been investigated. Noncatalytic conversions of alkenes to epoxide were monitored by UV/Vis at 360 nm, where the monoperoxorhenium (mpRe) and diperoxorhenium (dpRe) complexes absorb. Water- and peroxide-free dpRe was prepared in situ by the reaction of MTO and urea hydrogen peroxide (UHP) in dry THF. The observed biexponential time profiles in conjunction with kinetic modeling allow the assignment of the fast step to the reaction of olefin with dpRe (k4) and the slow step to the analogous reaction with mpRe (k3). In most of the tudied ionic liquids, k4 approximately 5 x k3. 2H NMR experiments conducted with [D3]dpRe under non-steady-state conditions confirm the speciation of the catalytic system in ionic liquids and assert the validity of the UV/Vis kinetics. Deuteriated alkenes were used to study the catalytic epoxidation and dihydroxylation of alkenes by 2H NMR spectroscopy. The values of k4 for alpha-methylstyrene in several ionic liquids exceed what is observed in acetonitrile by an order of magnitude. While the rate of olefin epoxidation is unaffected by the nature of the ionic liquid cation, a discernible kinetic effect is observed with coordinating anions such as nitrate.     

Iron-substituted SBA-15 microparticles: a peroxidase-like catalyst for H2O2 detection

In this communication, we demonstrate our recent finding that iron-substituted SBA-15 (Fe-SBA-15) microparticles possess intrinsic peroxidase-like activity and can catalyze the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) by H(2)O(2) to develop a blue color in aqueous solution, leading to a simple approach towards colorimetric detection of H(2)O(2) with a linear detection range from 0.4 μM to 15 μM (r = 0.997) and a detection limit of 0.2 μM.     

Liquid-phase oxidation of alkylaromatics by a H-atom transfer mechanism with a new heterogeneous CoSBA-15 catalyst

A new pseudo-tetrahedral Co(II) complex has been grafted onto the surface of SBA-15 and successfully utilized for the catalytic oxidization of alkylaromatic substrates with tert-butyl hydroperoxide (TBHP)via an H-atom transfer mechanism.     

AlCl3.6H2O as a catalyst for simple and efficient synthesis of gem-dihydroperoxides from ketones and aldehydes using aqueous H2O2

AlCl₃.6H₂O was explored as an efficient catalyst for the synthesis of ingem-dihydroperoxides (DHPs) from ketones and aldehydes. The reactions took place within a short period of time using (30%) aqueous H₂O₂ as a ??green?? oxidant in acetonitrile under neutral conditions at room temperature to afford the products in high yields.     

Highly selective olefin epoxidation with manganese triazacyclononane complexes: Impact of ligand substitution

Manganese complexes of 1,4,7-triazacyclononane with different substituents catalyze the selective epoxidation of a large number of olefins to epoxides with H₂O₂. The activities of complexes with methyl (L₁), 2-hydroxybutyl (L₂) and acetato (L₃) substituents are compared. The effects of solvent and temperature on the epoxide yield are very different for the three complexes. It is proposed that these differences are related to the binding of the pendant arms in Mn---L₂ and Mn---L₃ complexes. In general, acetone or methanol are preferred solvents. Variations of stereoretention are also observed: with Mn---L₁ in acetone, isomer scrambling occurs, while with Mn---L₁ in methanol, the epoxidation is almost stereospecific. UV-visible and electron spin resonance spectroscopy are used to characterize the state of manganese under oxidizing conditions.