Nanoscale colloidal metals and alloys stabilized by solvents and surfactants Preparation and use as catalyst precursors

Solvent-stabilized organosols of the early transition metal series, e.g. Ti, Zr, Nb, and Mn, may be prepared by the reduction of the THF adducts or thioether solutions of the corresponding metal halides with K[BEt₃H]. Mono- and bimetallic organosols of Group 6–11 metals stabilized by tetraalkylammonium halides may be formed either by the reduction of the metal salts using NR₄ hydrotriorganoborates or conventional agents, e.g. H₂ or HCO₂H, after the pretreatment of the metal salts with NR₄X. The chemical reduction of transition metal salts in the presence of hydrophilic surfactants provides straightforward access to nanostructured mono- and bimetallic hydrosols. This synthesis can be performed even in water. Mono- and bimetallic nanoparticles stabilized by lipophilic or hydrophilic surfactants of the cationic, anionic or nonionic type serve as precursors for heterogeneous metal colloid catalysts effective for the hydrogenation and oxidation of organic substrates. Bimetallic precursors, e.g. Pt---Rh, have a synergic effect on the catalytic activity. A comparison of catalytic results and CO chemisorption experiments has revealed that the protecting surfactants still cover the nanoparticle surface after adsorption on supports, which markedly improves the lifetime of the catalysts. Chiral protecting agents may induce enantioselectivity in metal colloid catalysts.     

Preparation and catalytic properties of NR-stabilized palladium colloids

Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR] with the reducing agent [BEt₃H^?] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR₄X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl)₄Br to Pd(ac)₂ in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR-stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl)₄]₂PdCl₂Br₂ with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×10³ catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;10³ catalytic cycles.     

Sustainable Production of Chemicals via Hydrotreating of CO2 and Biomass Derived Molecules Using Heterogeneous Noble Metal Oxide Catalysts

The noble metals are widely used in heterogeneous catalysis and automobile industry. The limited natural sources and high cost of noble metals dictates improving the efficiency of modern industry. This review considers the applications of noble metal oxide as potential solutions to the sustainability issues, including biomass conversion, CO₂ capture and conversion, green fuel production, etc. Noble metal oxides with their different compositions (monometallic and bimetallic) and structures exhibit a wide range of properties in heterogeneous catalysis. Although platinum metals in an oxidized form may not be the most common choice in hydroprocesses; recently, there have been studies indicating that they were highly active and selective catalysts in hydrogenation and hydrogenolysis. This review outlines the most established noble metal oxide catalysts used in hydrogenation catalysis and shed the light on the relation of noble metal oxide species to catalyst selectivity based on state-of-the-art techniques. Finally, the perspectives on the application of noble metal oxide catalysts to produce value-added chemicals are discussed.     

Catalytic properties of transition metal salts immobilized on nanoporous silica polyamine composites II: hydrogenation

The transition metal compounds Pd(OAc)₂, RhCl₃·4H₂O and RuCl₃ · nH₂O were adsorbed onto the nanoporous silica polyamine composite (SPC) particles (150–250 µm), WP‐1 [poly(ethyleneimine) on amorphous silica], BP‐1 [poly(allylamine) on amorphous silica], WP‐2 (WP‐1 modified with chloroacetic acid) and BP‐2 (BP‐1 modified with chloroacetic acid). Inductively coupled plasma‐atomic emission spectrometry analysis of the dried samples after digestion indicated metal loadings of 0.4–1.2 mmol g^?1 except for RhCl₃·4H₂O on BP‐2 which showed a metal loading of only 0.1 mmol g^?1. The metal loaded composites were then screened as hydrogenation catalysts for the reduction of 1‐octene, 1‐decene, 1‐hexene and 1, 3‐cyclohexadiene at a hydrogen pressure of 5 atm in the temperature range of 50–90 °C. All 12 combinations of SPC and transition metal compound proved active for the reduction of the terminal olefins, but isomerization to internal alkenes was competitive in all cases. Under these conditions, selective hydrogenation of 1,3‐cyclohexadiene to cyclohexene was observed with some of the catalysts. Turnover frequencies were estimated for the hydrogenation reactions based on the metal loading and were in some cases comparable to more conventional heterogeneous hydrogenation catalysts. Examination of the catalysts before and after reaction with X‐ray photoelectron spectroscopy and transmission electron microscopy revealed that, in the cases of Pd(OAc)₂ on WP‐2, BP‐1 and BP‐2, conversion of the surface‐ligand bound metal ions to metal nano‐particles occurs. This was not the case for Pd(OAc)₂ on WP‐1 or for RuCl₃ · nH₂O and RhCl₃· 4H₂O on all four composites. The overall results are discussed in terms of differences in metal ion coordination modes for the composite transition‐metal combinations. Suggested ligand interactions are supported by solid state CPMAS ¹³C NMR analyses and by analogy with previous structural investigations of metal binding modes on these composite materials. Copyright © 2011 John Wiley & Sons, Ltd.     

混合硝酸镍和醋酸镍制备的催化剂的特征和催化加氢抗硫性能

使用直接还原镍盐前体[Ni(NO_3)_2/γ-Al_2O_3, NiAc_2/γ-Al_2O_3或Ni (NO_3)_2-NiAc_2/γ-Al_2O_3]和镍氧化物前体的方法制备催化剂,研究了它们的 表面特征和甲苯加氢抗硫性能。还原镍盐得到的催化剂比还原其焙烧成的氧化物制 得的催化剂金属的还原和分散程度高。Ni(NO_3)_2-NiAc_2/γ-Al_2O_3分解得到的 氧化物前体的TPR在约415 ℃出现了较小数量的块状NiO的还原峰;而Ni(NO_3)_2- NiAc_2/γ-Al_2O_3的TPR中镍盐分解成氧化物时的耗氢量变小。用氢溢流的概念和 镍盐分解时的耗氢量可以将TPR的结果和金属的分散性关联起来。在镍盐前体催化 剂上甲苯的加氢具有较高的活性,而两种盐摩尔比为1：1时,盐前体催化剂反应活 性出现了一极大值,同时盐和氧化物前体催化剂都给出了抗硫性能的极大值。     

New supramolecular transition metal catalysts

The covalent connection of a catalytically active transition metal center with a water-soluble receptor (host molecule) generates a new type of supramolecular catalyst in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins, unusual degrees of substrate selectivity based on molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (water/organic) hydrogenation of nitro-aromatics also is a smooth process catalyzed by these supramolecular complexes. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system. Dendrimers having diphosphane moieties on the surface provide ligands for transition metals, the corresponding metal complexes (e.g., Pd) functioning as efficient catalysts which can be recycled due to their nanoscopic properties.     

Complex Reducing Agents (CRA's)—Versatile,Novel Ways of Using Sodium Hydride in Organic Synthesis

Why do we hardly use the simplest and, at the same time, inexpensive reducing agent sodium hydride in organic chemistry? To this question the answer is invariably: “It is too basic”. In this progress report we describe work we have performed aimed at controlling the basicity of NaH using sodium alcoholates and metal salts. The complex reducing agents (CRA's) developed (symbolized NaH-RONa-MX_n) allow organic halides, alkenes, alkynes and ketones to be reduced selectively. Highly regioselective 1,4- and 1,2-reductions of α,β-unsaturated ketones are easily performed using appropriate metal salts. Modified CRA's have proved to be excellent hydrosilylating reagents for carbonyl groups, non-pyrophoric heterogeneous hydrogenation catalysts, coupling reagents for aryl and vinyl halides, and reagents for the carbonylation of organic halides under very mild conditions. The study of these reactions opened up the field to phase-transfer-catalyzed photostimulated carbonylations as well as to S_RN1 reactions of metalates.–Thus, starting from the simple sodium hydride a large number of useful reagents have become accessible.     

Hybrid material from Zn[aminoacid]2 applied in the thio-Michael synthesis

Recently, methodologies that are in accordance with green chemistry principles have been garnering increasing attention. One of the most applied methods in this field is heterogeneous catalysis. In this context, many catalysts have been developed, and there is one remarkable class that has emerged: hybrid materials. Such heterogeneous catalysts are developed from organic and inorganic portions, especially from amino acids and metal salts, which are commonly found in the literature. Herein, we introduce Zn[Pro]₂ and Zn[Gly]₂ as heterogeneous catalysts in thio-Michael reactions via the implementation of two methods: via (1) a magnetic stirrer and (2) via an ultrasound device; the latter method resulted in minimally increased reaction yields in all cases.     

Synthesis,features and thermal decomposition of hexachlorostannates and hexachloroplatinates of tris(2,2′-dipyridyl)metal(II) and tris(1,10-phenanthroline)metal(II) (metal=Ru,Fe, Ni)

Several complex salts of general formula [M^II(dipy)₃]M^IVCl₆ or [M^II(phen)₃]M^IVCl₆ (whereM ^II=Ru, Fe, Ni andM ^IV=Sn, Pt) were synthetized and subjected to thermal analyses. Heating of these derivatives leads to the release of organic fragments and chlorine, which are often involved in oxidation processes. The residues comprise metal oxides or pure metals (e.g. Pt). Differences in the structures and features of the ligand molecules, revealed on the basis of quantum-chemistry calculations, account qualitatively for the differences in behaviour and stability of the complex compounds studied.This work was financed by the Polish State Committee for Scientific Research (KBN) under grant 2 0679 91 01 (contract no. 1156/2/91).     

The versatility of metal halide-quaternary ammonium salt catalysts for organic processes. From homogeneous to glass-encapsulated ion pairs

Ion pairs generated from transition metal halides and quaternary onium salts are versatile catalysts for many organic processes. Under phase transfer conditions, RhCl₃- and/or PtCl₄-Aliquat 336^® catalyze (a) double bond migration in allylic compounds, (b) disproportionation of cyclic 1,3-dienes, (c) selective transfer reduction of alkenes, alkynes, α,β-carbonyl compounds, and aroyl chlorides by polymethylhydrosiloxane, (d) hydrogenation of double, triple, and aromatic C-C bonds at room temperature, (e) cyclooligomerization of mono-, di-, and triacetylenes, and (f) addition of water, sulfur, and carbon monoxide to alkynes. In processes (a)-(c) the metal catalyst can be recovered in the aqueous phase by treatment of the reaction mixture with lipophilic anions. Two alternative methods for the recovery of the ion pair catalysts have been investigated. One is based on catalyst encapsulation in sol-gel matrices and the other employs polystyrene-supported ion pairs.     

Chemical looping of metal nitride catalysts: low-pressure ammonia synthesis for energy storage

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates. Although the reaction is thermodynamically favourable at ambient temperature and pressure, the catalytic synthesis of ammonia (NH₃), a fertilizer and chemical fuel, from N₂ and H₂ requires some of the most extreme conditions of the chemical industry. We demonstrate how ammonia can be produced at ambient pressure from air, water, and concentrated sunlight as renewable source of process heat via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into ammonia. Separating ammonia synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. This is rationalized via electronic structure calculations with the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. The predicted trends are confirmed experimentally with evolution of 56.3, 80.7, and 128 μmol NH₃ per mol metal per min at 1 bar and above 550 °C via reduction of Mn₆N_2.58 to Mn₄N and hydrogenation of Ca₃N₂ and Sr₂N to Ca₂NH and SrH₂, respectively.     

过渡金属氧化物掺杂对铜锰氧化物催化CO氧化性能的影响

以乙酸铜和乙酸锰为铜锰前驱体,以NH₄HCO₃为沉淀剂,相应金属硝酸盐为掺杂剂,采用共沉淀法制备了不同过渡金属氧化物掺杂的铜锰氧化物催化剂.?采用N₂物理吸附、X射线衍射,氢气-程序升温还原和原位红外漫反射光谱等方法对催化剂进行了表征,考察了系列催化剂上CO反应性能.?结果表明,掺杂过渡金属氧化物可以调变催化剂对CO的吸附能力,进而影响催化剂性能.     

Metal/zirconia catalysts for the synthesis of methanol: characterization by vibrational spectroscopy

The activation of carbon dioxide by catalytic hydrogenation has been studied as a route for methanol synthesis. Metal/zirconia catalysts suitable for this reaction have been prepared by (i) activation of amorphous metal alloys [1] or (ii) coprecipitation of amorphous zirconia and metal oxides [2]. Vibrational spectroscopy has been used to obtain information on the catalytic reaction mechanism, by the in situ identification of adsorbed species and intermediates under reaction conditions.The reverse water-gas shift reaction, producing CO from CO₂ and hydrogen, plays a crucial role in the reaction mechanism. This reduction is shown to proceed via surface formate, adsorbed close to the metal/zirconia interface. Over Pd/ZrO₂ and Ni/ZrO₂, formate is reduced to methane without further observable intermediates. Pivotal intermediates on the route to methanol, as observed on Cu/ZrO₂ catalysts, are -bound formaldehyde and surface methylate. Addition of silver as a promoter can result in enhanced selectivities and productivities for methanol formation. The synergy between the two metals becomes evident from the spectroscopic measurements; the most prominent feature of the silver-promoted catalysts is a high concentration of surface formaldehyde, which is either preferentially formed or stabilized by the silver component.     

Thermal decomposition of bivalent transition metal malonates in various atmospheres

The thermal decomposition of the malonates of bivalent transition metals (Mn, Fe, Co, Ni, Cu and Zn) was investigated by mainly TG-DTA, X-ray diffraction analysis and evolved gas analysis in atmospheres of N₂, CO₂ and O₂ and in the air. It was shown that CO₂ has an inhibiting effect on the decomposition whereas O₂ and air have the accelerating effects on the basis of N₂. The decomposition of the salts investigated can be classified into three groups from solid decomposition products: Mn and Zn malonates gave the metal oxides including 1–1.5 moles of elementary carbon, while Cu and Ni malonates gave the metals with 1–1.5 moles of the carbon. Fe and Co malonates in the last group gave once the metal oxides with 1-0.5 moles of the carbon and the oxides produced were subsequently reduced to the metals by the carbon. A possible reaction mechanism for the malonates was discussed and compared with those of the corresponding oxalates and succinates.     

Polymeric Heterogeneous Catalysts of Transition‐Metal Oxides: Surface Characterization,Physicomechanical Properties,and Catalytic Activity

We investigate the physicomechanical properties of polymeric heterogeneous catalysts of transition‐metal oxides, specifically, the specific surface area, elongation at break, breaking strength, specific electrical resistance, and volume resistivity. Digital microscopy, Fourier‐transform infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, and energy‐dispersive analysis are used to study the surfaces of the catalysts. The experimental results show that polymeric heterogeneous catalysts of transition‐metal oxides exhibit high stability and can maintain their catalytic activity under extreme reaction conditions for long‐term use. The oxidation mechanism of sulfur‐containing compounds in the presence of polymeric heterogeneous catalysts of transition‐metal oxides is confirmed. Microstructural characterization of the catalysts is performed by using X‐ray computed tomography. The activity of various catalysts in the oxidation of sulfur‐containing compounds is determined. We demonstrate the potential application of polymeric heterogeneous catalysts of transition‐metal oxides in industrial wastewater treatment.     

Ultrasound-assisted rapid reduction of nitroaromatics to anilines using gallium metal

Abstract

The reduction of nitroaromatic compounds to anilines is widely used throughout organic synthesis. Typical methods of performing this transformation utilize hydrogenation over a pyrophoric catalyst or a finely divided reducing metal, which often affords heterogeneous mixtures that are difficult to purify. Herein, we report for the first time the use of gallium metal as a reducing agent in organic synthesis. The reaction proceeds under aerobic conditions and affords homogeneous mixtures for a convenient workup. Using this method, twelve anilines were obtained in 33% to quantitative yields with short reaction times of 10-60?minutes.     

有序多孔过渡金属氧化物及其负载贵金属催化剂对挥发性有机物氧化的催化性能

大部分的挥发性有机物(VOCs)污染环境,危害人身健康.目前,我国虽然已开展了治理 VOCs污染的工作,但还缺乏有效的、拥有自主知识产权的 VOCs治理技术,因此研发新型高效 VOCs处理技术迫在眉睫.催化氧化法是公认的最有效消除 VOCs的途径之一,而高性能催化剂的研发是实现该过程的关键.近年来,人们围绕消除 VOCs的高效且价廉的催化剂的研发开展了卓有成效的工作,许多过渡金属氧化物、混合或复合金属氧化物及其负载贵金属催化剂均被认为是有效的催化氧化材料.与体相材料相比,多孔材料具有发达的孔道结构和高的比表面积,一方面有利于反应物的扩散、吸附和脱附,因而具有更高的催化活性和选择性;另一方面有利于活性组分(如贵金属等)在多孔材料表面的高分散,抑制活性组分的烧结,因而具有更好的催化稳定性.本文简述了近年来多孔金属氧化物在环境污染物消除领域的研究进展,阐述了以有序介孔或大孔过渡金属氧化物、钙钛矿型氧化物和负载贵金属催化剂的制备及其对典型 VOCs(如苯系物、醇类、醛类及酮类等)氧化的催化性能,重点介绍了四类催化材料,包括有序介孔过渡金属氧化物或复合氧化物(Co3O4, MnO2, Fe2O3, Cr2O3和 LaFeO3等)催化剂,有序介孔金属氧化物负载贵金属(Au/Co3O4, Au/MnO2和 Pd/Co3O4等)催化剂,三维有序大孔过渡金属氧化物或复合氧化物(Fe2O3, LaMnO3, La0.6Sr0.4MnO3和 La2CuO4等)催化剂,以及三维有序大孔金属氧化物负载贵金属(Au/Co3O4, Au/LaCoO3, Au/La0.6Sr0.4MnO3和 AuPd/Co3O4等)催化剂的制备及其物化性质与对苯、甲苯、二甲苯、乙醇、丙酮、甲醛、甲烷或氯甲烷等 VOCs氧化的催化性能之间的相关性.借助二氧化硅或聚甲基丙烯酸甲酯微球等硬模板,采用纳米浇铸法可制备出二维或三维的有序单一或多级孔道结构的金属氧化物.研究表明,多孔金属氧化物的催化性能远优于其体相甚至纳米催化剂的.有序多孔材料的优异催化性能与其拥有大的比表面积、高的吸附氧物种浓度、优良的低温还原性、独特的孔道结构、活性组分的高分散以及贵金属与氧化物载体之间的强相互作用等有关.探明影响催化剂活性的因素有利于从原子水平上认识催化过程,为新型高效催化剂的设计与制备奠定基础.本文还指出了此类研究中存在的一些问题,例如利用硬模板法制备多孔材料的缺点是目标催化剂的收率低,硬模板浪费严重,大规模制备多孔催化剂势必增加制备成本,这些问题有待于妥善解决.与此同时,还展望了 VOCs消除技术的未来发展趋势,采用多种技术联用的方法有望最大程度地提高 VOCs的消除效率.     

Thermogravimetric study on the reduction process of unsupported and carbon-supported Fe,Mo and Fe−Mo catalysts for fischer-tropsch synthesis

The catalytic performance of unsupported and carbon-supported Fe, Mo and Fe?Mo catalysts for Fischer-Tropsch synthesis is greatly influenced by the final reduction states of the catalysts. In this investigation, the reduction process of the catalysts by H₂ was studied by using TG-DTG. The reduction process depends not only on the reducibility of metals but also on the nature of the support. Methanation of the support occurred as soon as the supported metals were completely reduced for the carbon-supported catalysts. For these, the reduction temperature should by carefully selected so that the metal oxides are reduced as completely as possible, whilst the methanation of the support must be avoided to obtain optimum reduced catalysts.     

Oxidative Mechanochemistry: Direct,Room‐Temperature,Solvent‐Free Conversion of Palladium and Gold Metals into Soluble Salts and Coordination Complexes

Noble metals are valued, critical elements whose chemical activation or recycling is challenging, and traditionally requires high temperatures, strong acids or bases, or aggressive complexation agents. By using elementary palladium and gold, demonstrated here is the use of mechanochemistry for noble‐metal activation and recycling by mild, clean, solvent‐free, and room‐temperature chemistry. The process leads to direct, efficient, one‐pot conversion of the metals, including spent catalysts, into either simple water‐soluble salts or metal–organic catalysts.     

Comparative Study of Reduced and Sulfide Catalysts Based on Transition Metals in Conversion of CO and H<Subscript>2</Subscript>

Metallic (reduced) and sulfide heterogeneous catalysts based on Co and Mo and modified with alkali metals (K and Na) are comparatively studied in the synthesis of hydrocarbons and alcohols from syngas (CO + H₂). A significant amount of hydrocarbons is formed, when syngas is converted on cobalt and molybdenum metal catalysts. Modification of these catalysts with an alkali metal leads to suppression of the hydrogenation reaction and a decrease in the hydrocarbon yield; the yield of alcohol is low. A significantly higher amount of alcohol is obtained on molybdenum disulfide modified with potassium and cobalt in comparison with metal systems. The differences in catalytic activity are probably due to the structure of the active phase and different mechanisms of the reaction.