纳米金催化一氧化碳氧化反应的理论研究

近年来,纳米金催化剂独特的催化性质,特别是其优异的低温催化氧化活性,引起了人们极大的研究热情.除低温选择氧化外,在精细化学品合成、大气污染物消除、氢能的转换和利用等领域也开发出了一系列有广泛应用前景的金催化反应.此外,体相金的化学惰性和纳米金的超高活性之间差异的“鸿沟”也引起了理论工作者浓厚兴趣,试图从原理上理解体相金和纳米金活性差异的根源. CO催化氧化是最具有代表性的研究金催化活性的化学反应,本文主要综述了近十多年来金催化 CO氧化反应理论计算方面的研究工作.一般认为, CO在纳米金表面的吸附是 CO氧化反应的初始步骤.密度泛函理论研究表明, CO在金表面的吸附强度主要与被吸附金原子的配位数有关：金配位数越低, CO的吸附能越强,部分研究结果表明两者之间存在近似的线性关系.我们研究发现, CO吸附强度也与被吸附金周围配位金原子的相对位置有关,其中位于正下方的配位金原子加强 CO吸附,而位于侧位的配位金原子则弱化 CO吸附,这显然削弱了 CO吸附与金配位数线性关系的可靠性.理论研究表明,在纯金表
　　面上 O2吸附强度一般很弱,只有在一些特殊结构的金团簇上才有较强的吸附,但在 Au/TiO2界面及 CeO2表面上 O2吸附较强.金表面原子氧的吸附和金的表面结构有关.我们发现,原子氧倾向于在金的表面形成一种线性的 O–Au–O结构以增加其稳定性.当金表面的氧覆盖度增大时,会形成一种金氧化物薄膜结构,其结构依赖于氧的化学势和金的表面结构.纳米金催化 CO氧化反应机理可能因体系、载体等的差异而不同.大部分理论计算结果表明,在纯金表面上 O2很难直接解离形成原子氧,因此反应机理可能是吸附的 CO先与 O2反应形成了一种 CO–O2中间体,然后解离形成 CO2.在 Au/TiO2和 Au/CeO2催化剂上 CO催化氧化机理争议很大,均有计算结果支持 LH机理和 M–vK机理.另外,根据实验上观察到了负载型纳米金能直接活化分子氧的结果,理论上也提出了分子氧先解离为原子氧再与 CO反应的氧解离机理.针对如何解离分子氧问题,人们分别提出了低配位金模型、正方形金结构模型、Ti5c模型及 Au/Ti5c模型等.我们也提出了一种独特的双直线 O–Au–O模型来理解 Au/TiO2或 Au/CeO2界面解离活化分子氧.理论计算结果表明,低配位的金,金和载体之间的电荷转移,以及金所表现出的强相对论效应对于纳米金的活性影响很大.需要特别指出的是,金的强相对论效应有助于理解金表面的 CO吸附与金配位的关系、金表面原子氧的吸附特性、金氧化物薄膜的结构和分子氧的活化等过程.我们认为,金的强相对论作用导致了体相金的化学惰性以及纳米金的活性,因此相对论效应的深入研究将有助于理解金催化 CO氧化反应机理,从而有助于深层次理解纳米金催化活性来源.     

Selective aqueous oxidation of alcohols catalyzed by copper (II) phthalocyanine nanoparticles

A new catalyst based on metallophthalocyanine nanoparticles has been synthesized and characterized by scanning electron microscopy (SEM). The aqueous oxidation of alcohols to the corresponding carbonyl compounds (aldehydes and ketones) has been studied using tetra-n-butyl-ammonium-peroxo-monosulfate (n-Bu₄NHSO₅) as an oxidant and a catalytic system consisting of copper (II) phthalocyanine nanoparticles in water. The highly selective oxidation gave excellent yields of related aldehydes or ketones without remarkable over-oxidation of the carboxylic acids. Organic co-solvents, surfactants, and co-catalysts were not used in this catalytic strategy. This strategy was green and time effective. The oxidant's by-product (TBAHSO₄) and catalyst can be efficiently recovered and reused several times without any significant change of catalytic activity.     

Aerobic photocatalytic oxidation of activated benzylic and allylic alcohols to carbonyl compounds catalyzed by molecular iodine

An efficient, selective and environmentally benign photocatalytic system in acetonitrile has been developed for aerobic oxidation of activated benzylic and allylic alcohols into their corresponding aldehydes and ketones without the need for a transition metal in moderate to excellent yields with a catalytic amount of iodine. Very high inter- and intramolecular chemoselectivities are observed when benzylic OH groups are oxidized in the presence of aliphatic (nonbenzylic) hydroxyls.     

Sulfenamide catalyzed oxidation of alcohols to the corresponding carbonyl compounds with anhydrous chloramine-T

N-tert-Butylbenzenesulfenamide (1) catalyzed oxidation of various alcohols with stoichiometric amount of anhydrous chloramines-T (2) proceeded smoothly at room temperature to afford the corresponding carbonyl compounds in good yields.     

Efficient oxidation of alcohols to carbonyl compounds with molecular oxygen catalyzed by N-hydroxyphthalimide combined with a Co species

Highly efficient catalytic oxidation of alcohols with molecular oxygen by N-hydroxyphthalimide (NHPI) combined with a Co species was developed. The oxidation of 2-octanol in the presence of catalytic amounts of NHPI and Co(OAc)2 under atmospheric dioxygen in AcOEt at 70 degrees C gave 2-octanone in 93% yield. The oxidation was significantly enhanced by adding a small amount of benzoic acid to proceed smoothly even at room temperature. Primary alcohols were oxidized by NHPI in the absence of any metal catalyst to form the corresponding carboxylic acids in good yields. In the oxidation of terminal vic-diols such as 1,2-butanediol, carbon-carbon bond cleavage was induced to give one carbon less carboxylic acids such as propionic acid, while internal vic-diols were selectively oxidized to 1,2-diketones.     

Efficient selective oxidation of alcohols to aldehydes catalyzed by a morpholinone nitroxide

Efficient chemoselective oxidation of primary alcohols to the corresponding aldehydes is described. The transformation is promoted by a catalytic morpholinone nitroxide radical catalyst which can be easily synthesized. A broad range of substrates including aromatic and aliphatic primary alcohols are converted with excellent yields under mild conditions. The control experiments reveal that the morpholinone nitroxide can be used as a selective oxidant for primary alcohols in the presence of secondary alcohols.     

Partial oxidation of methane to syngas catalyzed by a nickel nanowire catalyst

A nickel nanowire catalyst was prepared by a hard template method,and characterized by transmission electron microscopy (TEM),N2 physical adsorption,X-ray photoelectron spectrometry (XPS),X-ray diffraction (XRD) and H2 temperature-programmed reduction (H2-TPR).The catalytic properties of the nanowire catalyst in the partial oxidation of methane to syngas were compared with a metallic Ni catalyst which was prepared with nickel sponge.The characterization results showed that the nickel nanowire catalyst had high specific surface area and there was more NiO phase in the nickel nanowire catalyst than in the metallic Ni catalyst.The reaction results showed that the nickel nanowire catalyst had high CH4 conversion and selectivities for H2 and CO under low space velocity.     

Oxidation of primary alcohols to aldehydes catalyzed by ruthenium compounds

The RuCl₃ and RuO₂·nH₂O catalyzed oxidation of alkanes, aromatic fatty acids, alcohols, citronellol, and hydroxycitronellol by NaOCl was studied in the diphase system CCl₄-aqueous NaOCl at pH 13–13.5. At 60–65°C, using 1–2 mole % of catalyst and a 1.5-fold molar excess of NaOCl, primary alkanols (hexanol-1, 2-ethylhexanol-1, decanol-1, hexadecanol-1) benzyl and 3-phenyl-propyl alcohols, and hydroxycitronellol are converted to the corresponding aldehydes with a selectivity of 70–90% and a yield of over 75%.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 115–121, January, 1991.     

Selective photocatalytic oxidation of alcohols by heteropolytungstates

A series of primary, secondary, saturated and unsaturated alcohols have been selectively oxidized photocatalytically to the corresponding aldehydes and ketones by a series of heteropolytungstates using near visible and UV light. Stoichiometrically, one photon adds two electrons onto the heteropolytungstates and produces one molecule of aldehyde or ketone. Using oxygen-saturated solutions, the photocatalytic cycle is obtained at the early stage of the photoredox reaction, with optimization of the quantum yield for the oxidized organic product being ∼ 0.1. After at least 20 photocatalytic cycles, when the production of the organic oxidation product was 20 times the amount of catalyst, there was no sign of decomposition of the catalyst.     

Chemoselective C-H oxidation of alcohols to carbonyl compounds with iodosobenzene catalyzed by (salen)chromium complex

Primary and secondary alcohols with benzylically and allylically activated C-H bonds are chemoselectively oxidized to the corresponding carbonyl compounds by the (salen)Cr(III) complex I as the catalyst and iodosobenzene as the oxygen source; the oxidizing species is the Cr(V) oxo complex. Allylic alcohols with fully substituted double bonds give appreciable amounts of epoxides besides the C-H oxidation products enones, while saturated alcohols are less readily oxidized.     

Guerbet reaction of primary alcohols leading to beta-alkylated dimer alcohols catalyzed by iridium complexes

[IrCl(cod)]2 and [Cp*IrCl2]2 complexes catalyzed efficiently the Guerbet reaction of primary alcohols to beta-alkylated dimer alcohols in good yields. For instance, the reaction of 1-butanol in the presence of [Cp*IrCl2]2 (1 mol %), t-BuOK (40 mol %), and 1,7-octadiene (10 mol %) produced 2-ethyl-1-hexanol in 93% yield. Various primary alcohols undergo the Guerbet reaction under the influence of Ir complexes to give the corresponding dimer alcohols in good yields. This method provides an alternative direct route to beta-alkylated primary alcohols which are prepared by aldol condensation of aldehydes followed by hydrogenation.     

Selective and regiospecific oxidation of dithiaalkanes in a gold(III) catalyzed phase-transfer process

The oxidation of dithiaalkanes has been carried on in a gold(III) catalyzed phase-transfer process. Using a higher concentration of catalyst and oxidant (procedure $\text{A}$) the bis(benzylthio)alkanes Bz-S-(CH₂)-S-Bz (Bz = benzyl) were ·Oxidized to the corresponding monosulfoxide when n = 1 and to disulfoxide [bis(benzylsulfinyl)alkanes] when n = 2, 3, 4, 5, and 8. In more diluite conditions (procedure $\text{B}$) high yields of monosulfoxides were obtained also in the case of n = ·2 and 3 while the selectivity decreased for n = 4, 5, and 8. The oxidation of asymmetric dithiaalkanes, R-S-(CH₂)₃-S-R' (RR' = BzPh, BzCy, CyPh; Ph = phenyl, Cy = cyclohexyl), under experimental conditions $\text{B}$ was regiospecific leading either to the formation of only one type of monosulfoxide (RR' = BzPh) or to the formation of both types of monosulfoxides but in different yield (RR' = BzCy and CyPh); in all cases the alkylic and less hindered sulfur atom was oxidized in preference.     

Selective aerobic oxidation of hydroxy compounds catalyzed by photoactivated ruthenium-salen complexes (selective catalytic aerobic oxidation)

Selective oxidation of alcohols to the corresponding carbonyl compounds is one of the most fundamental reactions in organic synthesis. Traditional methods for this transformation generally rely on stoichiometric amount of oxidants represented by Cr(VI) or DMSO reagents, though their synthetic utility is encumbered by unpleasant waste materials. From ecological and atom-economic viewpoints, catalytic aerobic oxidation is much more advantageous because molecular oxygen is ubiquitous and the byproduct is basically non-toxic water or hydrogen peroxide. On the other hand, phenol derivatives undergo oxidative coupling, forming C-C or C-O bond, through radical intermediates coupled with an electron-transfer process. Molecular oxygen is also well known to serve as electron acceptor in this reaction. Thus, a variety of transition metal complexes have so far been examined for aerobic oxidations of alcohols and phenols, and high catalytic activities have been achieved in some cases. However, stereo- and chemo-selective aerobic oxidations are still limited in number and are of current interest. Presented in this paper is our recent studies on catalytic aerobic oxidations with photoactivated nitrosyl ruthenium-salen complexes, including asymmetric oxidation of secondary alcohols to ketones (kinetic resolution), enantioselective oxidative coupling of 2-naphthols to binaphthols and oxygen-radical bicyclization of 2,2'-dihydroxystilbene, chemoselective oxidation of primary alcohols to aldehydes and diols to lactols, and asymmetric desymmetrization of meso-diols to lactols.     

Aerobic oxidation of organic compounds catalyzed by vanadium compounds

Aerobic oxidation of α-hydroxy ketones catalyzed by dichloroethoxyoxovanadium in ethanol causes a carbon–carbon bond cleavage that produces diesters or diketones. This reaction is highly chemoselective, and disecondary glycols do not react at all. However, ditertiary glycols effectively react with dichloroethoxyoxovanadium or trichlorooxovanadium to provide the corresponding ketones. Aerobic oxidation of α-hydroxy ketones catalyzed by dichloroethoxyoxovanadium or trichlorooxovanadium in aprotic solvents almost quantitatively affords the corresponding α-diketones. The reaction of tertiary cyclopropanol compounds with vanadyl acetylacetonate under an oxygen atmosphere causes fragmentation of the cyclopropane moiety to produce β-hydroxy ketones and β-diketones. For the 6-substituted bicyclo[4.1.0]heptanol derivatives, the endoperoxides are also obtained together with β-hydroxy ketones. Conversely, 2-ethoxycarbonylcyclopropyl silyl ethers produce γ-oxocarboxylate derivatives given the same reaction conditions. Monothioacetals are easily deprotected into carbonyls using a catalytic amount of trichlorooxovanadium in 2,2,2-trifluoroethanol under an oxygen atmosphere. Thiols are converted into the corresponding disulfides by the aerobic oxidation catalyzed by trichlorooxovanadium in the presence of molecular sieves 3A. Polymer-supported vanadium compounds are synthesized by the reaction of vanadium oxytrichloride with polymers bearing hydroxyl moieties. The catalyst prepared from TentaGel S OH was highly active and reusable for the aerobic oxidations.     

Tandem oxidation of allylic and benzylic alcohols to esters catalyzed by N-heterocyclic carbenes

N-heterocyclic carbenes catalyze the oxidation of allylic, propargylic, and benzylic alcohols to esters with manganese(IV) oxide in excellent yields. A variety of ester derivatives can be synthesized, including protected carboxylates. This one-pot tandem oxidation represents the first organocatalytic oxidation of alcohols to esters. Saturated esters can also be accessed from aldehydes using this method. Through the utilization of a chiral catalyst, the acyl-heteroazolium intermediate becomes a chiral acylating agent, which can desymmetrize meso-1,2-diols. [reaction: see text].     

Generation of alkyl hypochlorites in oxidation of alcohols with carbon tetrachloride catalyzed by vanadium and manganese compounds

Primary alcohols and diols with various structures were subjected to transformations into esters, aldehydes, ketones, and lactones under the action of carbon tetrachloride in the presence of manganese compounds (MnCl₂, MnO₂, Mn(OAc)₂, Mn(acac)₃) and vanadium compounds (VCl₅, V₂O₅, VO(acac)₂) as catalysts. These transformation proceeded with the involvement of alkyl hypochlorites, which were generated in the course of oxidation of alcohols with carbon tetrachloride catalyzed by manganese or vanadium compounds. The optimum molar ratios between the catalyst and reagents were determined, and the reaction conditions for the highly selective synthesis of esters, aldehydes, ketones, and lactones from alcohols were found.     

Aerobic oxidation of benzyl alcohols by MoVI compounds

Selective and controlled aerobic oxidation of activated benzyl alcohols to the corresponding aldehydes is achieved in refluxing CH₃CN using catalytic amounts of MoO₂Cl₂(L)₂ where L is DMSO, DMF or THF. The catalysis reactions are possible under open air in the absence of any other external co‐oxidants. However, bubbling of oxygen to the reaction mixture is useful in making the catalysis reaction sustained. Both activated and deactivated varieties of α‐substituted benzyl alcohols (secondary alcohols) give ketones in the same reaction conditions. The inexpensive catalyst is selective towards activated primary benzyl alcohols and also, being mild, stops the oxidation at the aldehyde stage, making it synthetically useful. Copyright © 2006 John Wiley & Sons, Ltd.