A Surfactant‐Encapsulating Polyoxometalate Nanowire Assembly as a New Carrier for Nanoscale Noble‐Metal Catalysts

The loading of noble‐metal nanoparticles (NMNPs) onto various carriers to obtain stable and highly efficient catalysts is currently an important strategy in the development of noble metal (NM)‐based catalytic reactions and their applications. We herein report a nanowire supramolecular assembly constructed from the surfactant‐encapsulating polyoxometalates (SEPs) CTAB‐PW₁₂, which can act as new carriers for NMNPs. In this case, the Ag NPs are loaded onto the SEP nanowire assembly with a narrow size distribution from 5 to 20 nm in diameter; the average size is approximately 10 nm. The Ag NPs on the nanowire assemblies are well stabilized and the over agglomeration of Ag NPs is avoided owing to the existence of well‐arranged polyoxometalate (POM) units in the SEP assembly and the hydrophobic surfactant on the surface of the nanowire assembly. Furthermore, the loading amount of the Ag NPs can be adjusted by controlling the concentration of the AgNO₃ aqueous solution. The resultant Ag/CTAB‐PW₁₂ composite materials exhibit high activity and good stability for the catalytic reduction of 4‐nitrophenol (4‐NP) with NaBH₄ in isopropanol/H₂O solution. The NMNPs‐loaded SEP nanoassembly may represent a new composite catalyst system for application in NM‐based catalysis.     

Immobilized polyoxometalates onto mesoporous organically-modified silica aerogels as selective heterogeneous catalysts of anthracene oxidation

Immobilized molybdovanadophosphoric acids onto organically surface-modified silica aerogels were successfully prepared and investigated in heterogeneous catalysis of anthracene oxidation. The catalysts were obtained by supporting mono- and di-vanadium substituted molybdophosphoric acids on hybrid silica materials synthesized via the sol–gel process followed by surface amino-functionalization. The FTIR, DR UV–vis, and AA spectroscopy confirmed the loading and distribution of the polyoxometalate molecules on the surface of the aerogels. The nitrogen adsorption–desorption technique revealed a systematic decrease in the specific surface area and pore volume after the immobilization of the polyoxometalates. The application of the supported molecules as catalysts for anthracene oxidation showed 100% selectivity for 9,10-anthraquinone as opposed to the reactions conducted under homogeneous conditions. Moreover, at certain conditions, the catalytic activity of the supported polyoxometalates was greater than their corresponding free polyoxometalates with a clear effect of the surface chemical groups of the supporting silica aerogels. Additionally, the oxidant and solvent nature showed a crucial effect on the catalytic activity and selectivity of the immobilized polyoxometales. The heterogeneous catalysts were regenerated and reused over consecutive catalytic cycles reflecting a potential economic interest in these materials besides their high efficiency in heterogeneous catalysis.     

Nanoscale polyoxometalate‐based inorganic/organic hybrids

The latest advances in the area of polyoxometalate (POM)‐based inorganic/organic hybrid materials prepared by self‐assembly, covalent modification, and supramolecular interactions are presented. This Review is composed of five sections and documents the effect of organic cations on the formation of novel POMs, surfactant encapsulated POM‐based hybrids, polymeric POM/organic hybrid materials, POMs‐containing ionic crystals, and covalently functionalized POMs. In addition to their role in the charge‐balancing, of anionic POMs, the crucial role of organic cations in the formation and functionalization of POM‐based hybrid materials is discussed. DOI 10.1002/tcr.201100002     

复合仿生催化体系的构建与高效有氧氧化芳基烷烃

直接活化氧气氧化碳氢化合物是件挑战性的研究工作。根据生物酶很容易在温和条件下实现上述反应,以Keggin-型杂多酸[CuPW_(11)O_(39)]5-(简写为CuPW_(11))与金属-有机框架材料HKUST-1形成的复合材料Cu PW_(11)@HKUST-1为催化剂,以N-羟基邻苯二甲酰亚胺为助催化剂,构建模拟酶催化氧化反应体系。其中,CuPW_(11)@HKUST-1中的杂多酸作为氧化还原中心,N-羟基邻苯二甲酰亚胺作为电子供体。该复合模拟酶催化体系在催化活化氧气氧化芳基烷烃的反应中表现出了类似生物酶催化的性质,具有反应条件温和、转化率高、转化数高和选择性高等特点,其中产物产率与转化数分别高达99%和17700,为实现在温和条件下高效活化氧气氧化惰性有机物分子提供了一条切实可行的路线。     

Oxidation of Organosilanes with Nanoporous Copper as a Sustainable Non‐Noble‐Metal Catalyst

Although many noble‐metal catalysts have been used for the oxidation of organosilanes, there has been less success with non‐noble‐metal catalysts. Here, unsupported nanoporous copper (np‐Cu) is used to catalyze the oxidation of organosilanes under mild conditions. It is the first time that this reaction has been achieved with a heterogeneous copper catalyst with high activity and selectivity. Both water and alcohols are used as oxidants and the corresponding organosilanols and organosilyl ethers are obtained in high yield. The possible mechanism was obtained by kinetic studies. The catalyst could be reused at least five times without evident loss of activity. As a novel green catalyst np‐Cu should play a unique role in organic synthesis.     

多金属氧簇催化研究进展

多金属氧簇由于其组成和结构易于调控、具有酸性、氧化还原性、低毒性和低腐蚀性等优点,作为工业催化剂具有广阔的应用前景,是多酸化学领域的研究热点之一。本文综述了近5年来多金属氧簇在催化领域中研究的新进展,主要包括多金属氧簇的酸催化、氧化催化、双功能催化、加氢和活化二氧化碳合成碳酸酯等催化反应以及多金属氧簇的工业化应用等,并对未来发展趋势进行了展望。     

锰基氧化物与冷等离子体协同催化氧化VOCs研究进展

挥发性有机化合物(VOCs)对环境的严重污染和对人体的危害引起了人们的重视。冷等离子体与催化剂耦合形成的协同效应可显著提高VOCs低温转化速率,减少二次污染和降低能耗,具有较好的应用前景。协同催化效果主要取决于催化剂物化性能,可通过调控催化剂的组成、粒径和结构改善协同催化活性。锰基氧化物因其具有较高的储氧能力、稳定的晶体结构、较好的氧气活化性能和良好的抗中毒能力等优点而被广泛应用于冷等离子体协同催化净化VOCs的应用研究。通过金属离子掺杂调控锰基氧化物的离子价态、氧迁移率和氧气吸附量,是改善催化剂与等离子体协同催化性能主要方法。本文总结了近年来锰基氧化物与冷等离子体协同催化氧化VOCs的研究进展,主要包括氧化锰晶型、分散度和掺杂金属离子对协同催化氧化VOCs的活性影响趋势及反应机理。分析冷等离子体与锰基氧化物催化剂协同氧化VOCs中存在的问题并对其发展前景进行了展望。     

Expanding the Scope of Polyoxometalates as Artificial Proteases towards Hydrolysis of Insoluble Proteins

Despite the enormous importance of insoluble proteins in biological processes, their structural investigation remains a challenging task. The development of artificial enzyme-like catalysts would greatly facilitate the elucidation of their structure since currently used enzymes in proteomics largely lose activity in the presence of surfactants, which are necessary to solubilize insoluble proteins. In this study, the hydrolysis of a fully insoluble protein by polyoxometalate complexes as artificial proteases in surfactant solutions is reported for the first time. The hydrolysis of zein as a model protein was investigated in the presence of Zr(IV) and Hf(IV) substituted Keggin-type polyoxometalates (POMs), (Et₂NH₂)₁₀[M(α-PW₁₁O₃₉)₂] (M = Zr or Hf), and different concentrations of the anionic surfactant sodium dodecyl sulfate (SDS). Selective hydrolysis of the protein upon incubation with the catalyst was observed, and the results indicate that the hydrolytic selectivity and activity of the POM catalysts strongly depends on the concentration of surfactant. The molecular interactions between the POM catalyst and zein in the presence of SDS were explored using a combination of spectroscopic techniques which indicated competitive binding between POM and SDS towards the protein. Furthermore, the formation of micellar superstructures in ternary POM/surfactant/protein solutions has been confirmed by conductivity and Dynamic Light Scattering measurements.     

多金属氧酸盐催化的液相氧化反应*

介绍了多金属氧酸盐的结构和性能, 综述了这类化合物具有的多功能性和可调性,并对它们与环境温和、经济廉价的条件（溶剂,氧化剂等）的良好兼容性等特点进行了概述。多金属氧酸盐可以在原子/分子层次上进行催化剂的设计,在绿色催化和清洁生产等方面具有良好的应用前景。文本总结了多金属氧酸盐在液相清洁催化氧化方面的进展,主要针对以过氧化氢和分子氧为氧化剂的烯烃类化合物的环氧化反应、烷烃和芳香族化合物的氧化反应,并详细探讨了此类氧化反应的反应机理。同时也关注了多金属氧酸盐在仿生催化领域的应用。     

Polyoxometalate-based inorganic-organic hybrid film structure with reversible electroswitchable fluorescence property

A novel inorganic-organic hybrid film structure based on polyoxometalate and conventional organic dye has been fabricated, whose fluorescence can be reversibly switched using the electrochromic component to activate or suppress the related fluorescence quenching mechanism upon applying reduction or oxidation potentials of polyoxometalates.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]?n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

Suzuki coupling reaction catalyzed heterogeneously by Pd(salen)/polyoxometalate compound: another example for synergistic effect of organic/inorganic hybrid

A hybrid compound consisting of palladium(salen) [salen = N,N′‐bis(salicylidene)ethylenediamine] complex covalently linked to a lacunary Keggin‐type polyoxometalate, K₈[SiW₁₁O₃₉](POM), was synthesized and characterized by FT‐IR, elemental analysis, inductively coupled plasma and diffuse reflectance UV–visible spectroscopic methods. The hybrid, [Pd(salen)–POM], was investigated in the Suzuki cross‐coupling in EtOH/H₂O under mild reaction conditions. In comparison to the corresponding organic and inorganic moiety, the hybrid has shown greatly improved catalytic activity, and much higher yields toward coupling products were obtained with a low catalyst loading for various aryl halides, including unreactive and sterically hindered ones. The catalyst also exhibited prominent recyclable performance and no obvious loss of activity was observed after six consecutive runs. Copyright © 2013 John Wiley & Sons, Ltd.     

Molecular oxygen and oxidation catalysis by phosphovanadomolybdates

The history of aerobic catalytic oxidation mediated by a subclass of polyoxometalates, the phosphovanadomolybdates of the Keggin structure, [PV(x)Mo(12-x)O40](3+x)-, is described. In the earlier research it was shown that phosphovanadomolybdates catalyze oxydehydrogenation reactions through an electron-transfer oxidation of a substrate by the polyoxometalate that is then reoxidized by oxygen. These aerobic oxidations are selective and synthetically useful in various transformations, notably diene aromatization, phenol dimerization and alcohol oxidation. Oxygen transfer from the polyoxometalate to arenes and alkylarenes was also discussed as a homogeneous analog of a Mars-van Krevelen oxidation. "Second generation" catalysts include binary complexes of the polyoxometalate and a organometallic compound useful, for example, for methane oxidation and nanoparticles stabilized by polyoxometalates effective for aerobic alkene epoxidation.     

Nanoscaled Copper Metal–Organic Framework (MOF) Based on Carboxylate Ligands as an Efficient Heterogeneous Catalyst for Aerobic Epoxidation of Olefins and Oxidation of Benzylic and Allylic Alcohols

Aerobic epoxidation of olefins at a mild reaction temperature has been carried out by using nanomorphology of [Cu₃(BTC)₂] (BTC=1,3,5‐benzenetricarboxylate) as a high‐performance catalyst through a simple synthetic strategy. An aromatic carboxylate ligand was employed to furnish a heterogeneous copper catalyst and also serves as the ligand for enhanced catalytic activities in the catalytic reaction. The utilization of a copper metal–organic framework catalyst was further extended to the aerobic oxidation of aromatic alcohols. The shape and size selectivity of the catalyst in olefin epoxidation and alcohol oxidation was investigated. Furthermore, the as‐synthesized copper catalyst can be easily recovered and reused several times without leaching of active species or significant loss of activity.     

Applications of inorganic‐organic hybrid architectures based on polyoxometalates in catalyzed and photocatalyzed chemical transformations

The scope of this article is to reveal the fruitful combination of POM species with metal coordination complexes, leading to the construction of several efficient multifunctional catalysts. In this review, we try to underscore various catalytic and photocatalytic reactions catalyzed by POM‐based inorganic‐organic hybrid. Notably, it has been well established that depending on the type of the reaction, the activity and selectivity of these hybrid catalyst can be drastically improved by the rational and correct choice of the organic metal complex and POM anion providing a marriage of convenience.     

Preparation,characterization and catalytic activity studies of organoruthenium‐supported polyoxotungstates on SBA‐15

Organoruthenium‐supported polyoxometalates [(RuC₆H₆)XW₉O₃₄]^7? (XWRu; X = As, P) were selected as samples to study their catalytic activities towards the solvent‐free oxidation of n‐hexadecane. First of all, the XWRu were deposited on 3‐aminopropyltriethoxysilane‐modified SBA‐15 to prepare solid catalysts, which were characterized using powder X‐ray diffraction, nitrogen adsorption measurements, Fourier transform infrared reflectance spectroscopy and X‐ray photoelectron spectroscopy. Subsequently, their catalytic performances and stabilities were assessed through the oxidation of n‐hexadecane using air as the oxygen source without any additives and solvents, and the influences of the loading amount, catalyst amount, reaction time and reaction temperature on the catalytic activities were investigated. The results indicated the influence of the central heteroatoms X of XWRu on the catalytic activities. Copyright © 2014 John Wiley & Sons, Ltd.     

Axial base‐controlled catalytic activity,oxidative stability and product selectivity of water‐insoluble manganese and iron porphyrins for oxidation of styrenes in water under green conditions

A series of water‐insoluble iron(III) and manganese(III) porphyrins, FeT(2‐CH₃)PPCl, FeT(4‐OCH₃)PPCl, FeT(2‐Cl)PPCl, FeTPPCl, MnT(2‐CH₃)PPOAc, MnT(4‐OCH₃)PPOAc, MnT(2‐Cl)PPOAc and MnTPPOAc, in the presence of imidazole (ImH), F^?, Cl^?, Br^? and acetate were used as catalysts for the aqueous‐phase heterogeneous oxidation of styrenes to the corresponding epoxides and aldehydes with sodium periodate. Also, the effect of various reaction parameters such as reaction time, molar ratio of catalyst to axial base, type of axial base, molar ratio of olefin to oxidant and nature of metal centre on the activity and oxidative stability of the catalysts and the product selectivity was investigated. Higher catalytic activities were found for the iron complexes. Interestingly, the selectivity towards the formation of epoxide and aldehyde (or acetophenone) was significantly influenced by the type of axial base. Furthermore, Br^? and ImH were found to be the most efficient co‐catalysts for the oxidation of olefins performed in the presence of the manganese and iron porphyrins, respectively. The optimized molar ratio of catalyst to axial base was different for various axial bases. Also, the order of co‐catalyst activity of the axial bases obtained in aqueous medium was different from that reported for organic solvents. The use of a convenient axial base under optimum reaction catalyst to co‐catalyst molar ratio in the presence of the manganese porphyrin gave the oxidative products with a conversion of ca 100% in a reaction time of less than 3 h. However, the catalytic activity of the iron porphyrins could not be effectively improved by increasing the catalyst to co‐catalyst molar ratio.     

Soft Matter Approaches for Enhancing the Catalytic Capabilities of Polyoxometalate Clusters

In this minireview, we discuss the recent efforts on expanding the catalytic capabilities of polyoxometalates (POM) through emulsion catalysis approaches with novel catalytic-active POM–organic hybrid clusters as emulsifiers. The hybrid emulsifiers include surfactant encapsulated POM complexes, molecular POMs–organic hybrids, and POM-based solid nanoparticles. With such novel approaches the catalytic efficiency of the POMs can be significantly improved by enhancing the compatibility of the POMs with organic media, providing catalytic interface for biphasic reactions, as well as easier preparation, and better recyclability. Particularly, a simple, green chemistry method to prepare metal nanoparticle materials with POMs as both reducing and capping agents in aqueous is reviewed.     

Highly Efficient Supramolecular Catalysis by Endowing the Reaction Intermediate with Adaptive Reactivity

A new strategy of highly efficient supramolecular catalysis is developed by endowing the reaction intermediate with adaptive reactivity. The supramolecular catalyst, prepared by host–guest complexation between 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO) and cucurbit[7]uril (CB[7]), was used for biphasic oxidation of alcohols. Cationic TEMPO⁺, the key intermediate, was stabilized by the electrostatic effect of CB[7] in aqueous phase, thus promoting the formation of TEMPO⁺ and inhibiting side reactions. Moreover, through the migration into the organic phase, TEMPO⁺ was separated from CB[7] and recovered the high reactivity to drive a fast oxidation of substrates. The adaptive reactivity of TEMPO⁺ induced an integral optimization of the catalytic cycle and greatly improved the conversion of the reaction. This work highlights the unique advantages of dynamic noncovalent interactions on modulating the activity of reaction intermediates, which may open new horizons for supramolecular catalysis.     

Recent Advances in Supramolecular Gels and Catalysis

Over the past two decades, supramolecular gels have attracted significant attention from scientists in diverse research fields and have been extensively developed. This review mainly focuses on the significant achievements in supramolecular gels and catalysis. First, by incorporating diverse catalytic sites and active organic functional groups into gelator molecules, supramolecular gels have been considered as a novel matrix for catalysis. In addition, these rationally designed supramolecular gels also provide a variety of templates to access metal nanocomposites, which may function as catalysts and exhibit high activity in diverse catalytic transformations. Finally, as a new kind of biomaterial, supramolecular gels formed in situ by self‐assembly triggered by catalytic transformations are also covered herein.