Sustainable green catalysis by supported metal nanoparticles

The recent progress of sustainable green catalysis by supported metal nanoparticles is described. The template synthesis of metal nanoparticles in ordered porous materials is studied for the rational design of heterogeneous catalysts capable of high activity and selectivity. The application of these materials in green catalytic processes results in a unique activity and selectivity arising from the concerted effect of metal nanoparticles and supports. The high catalytic performances of Pt nanoparticles in mesoporous silica is reported. Supported metal catalysts have also been applied to biomass conversion by heterogeneous catalysis. Additionally, the degradation of cellulose by supported metal catalysts, in which bifunctional catalysis of acid and metal plays the key role for the hydrolysis and reduction of cellulose, is also reported. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 224–235; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900004     

介孔硅材料及其负载型催化剂去除挥发性有机物的最新进展

介孔硅材料由于具有大的比表面积,均一的孔结构和大的孔径,常被用于分离、吸附和催化等领域.本文综述了近年来国内外介孔硅材料及其负载型催化剂去除各类挥发性有机物(VOCs)的研究进展,主要包括烃类、甲醇、甲醛、丙酮、苯、甲苯、萘、乙酸乙酯等.讨论了介孔硅材料的结构对VOCs吸附过程的影响;介绍了不同催化剂消除各类VOCs的催化性能及反应机理,并重点评述了甲苯在不同催化剂上的研究进展.分析结果表明,介孔硅材料的表面环境、孔道结构以及宏观形貌是影响VOCs分子在介孔硅材料上吸附的主要因素;贵金属催化剂的应用需要提高其抗中毒性以及降低成本;过渡金属的研究应着重于研发高活性的负载型过渡金属复合氧化物催化剂.最后对国内外介孔硅材料及其负载型催化剂的发展进行了展望,今后催化剂的设计可以从“氧化硅载体”和“介孔孔道”两个方面展开,这将为设计合适的催化剂处理各类VOCs污染物提供一定参考.     

Cooperative catalysis by silica-supported organic functional groups

Hybrid inorganic-organic materials comprising organic functional groups tethered from silica surfaces are versatile, heterogeneous catalysts. Recent advances have led to the preparation of silica materials containing multiple, different functional groups that can show cooperative catalysis; that is, these functional groups can act together to provide catalytic activity and selectivity superior to what can be obtained from either monofunctional materials or homogeneous catalysts. This tutorial review discusses cooperative catalysis of silica-based catalytic materials, focusing on the cooperative action of acid-base, acid-thiol, amine-urea, and imidazole-alcohol-carboxylate groups. Particular attention is given to the effect of the spatial arrangement of these organic groups and recent developments in the spatial organization of multiple groups on the silica surface.     

Physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids

Experimental results on the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids are reported. The aerogels were prepared by hydrolysis and polycondensation of sodium silicate followed by subsequent washings, surface chemical modification and ambient pressure drying using 10 various acid catalysts consisting of strong and weak acids. The strength and concentration of acids have the major effect on the gelation of sol and hence the physico-chemical properties of the silica aerogels. Strong acids such as HCl, HNO₃ and H₂SO₄ resulted in shrunk (70–95%) aerogels whereas weak acids such as citric and tartaric acids resulted in less shrunk (34–50%) aerogels. The physical properties of silica aerogels were studied by measuring bulk density, volume shrinkage (%), porosity (%), pore volume, thermal conductivity, contact angle with water, Transmission Electron Microscopy (TEM), Atomic Absorption Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), Thermo Gravimetric-Differential Thermal (TG-DT) analyses and N₂ adsorption–desorption BET surface analyzer. The best quality silica aerogels in terms of low density (0.086 g/cm³), low volume shrinkage (34%), high porosity (95%), low thermal conductivity (0.09 W/m K) and hydrophobic (148°) were obtained for molar ratio of Na₂SiO₃:H₂O:citric acid:TMCS at 1:146.67:0.72:9.46 with 20 min gelation time. The resulting aerogels exhibited the thermal stability up to around 420 °C.     

Synthesis of transparent silica aerogels using tetraalkylammonium fluoride catalysts

Tetraalkylammonium fluoride salts have been employed as catalysts for the synthesis of silica aerogels by a two-step, sol–gel method. Aerogel materials were characterized by N₂ physisorption and SEM. The effect of the type of catalyst on the optical transparency of obtained aerogels has been examined. It has been found that such compounds allow the synthesis of silica aerogels with the highest optical transmittance ever reported for such materials. The optimal catalysts are tetrabutyl and tetraoctyl ammonium fluoride, with which aerogels with transparency as high as 96% and extinction coefficient as low as 3.5 m⁻¹ can be prepared.     

Recent developments in use of heteropolyacids, their salts and polyoxometalates in organic synthesis

Heteropolyacids, their salts and polyoxometalates have been used as catalysts in wide range of organic reactions such as multi components, oxidation, reductions, electrochemical and photochemical reactions. There have been tremendous and continued efforts to modify the catalytic performance of heteropoly acids such as supporting them on MCM, silica gel, etc. In this review, we have attempted to bring some of new applications of heteropolyacids in organic reactions and recent approaches on modifying them, into focus.     

Tailoring the catalytic performance of sol-gel-encapsulated tetra-n-propylammonium perruthenate (TPAP) in aerobic oxidation of alcohols

Sol-gel nanohybrid silica particles organically modified and doped with the ruthenium species tetra-n-propylammonium perruthenate (TPAP) are highly efficient catalysts for the selective oxidation of alcohols to carbonyl groups with O(2) at low pressure in toluene. The materials are easily prepared by a one-step sol-gel process, and their catalytic performance can be optimised by tailoring the conditions of their synthesis by hydrolytic co-polycondensation of tetramethoxysilane (TMOS) and alkyltrimethoxysilanes R-Si(OMe)(3) in the presence of TPAP. Eventually, heterogeneous catalysts considerably more active than the unsupported perruthenate were obtained, while also being leach-proof and recyclable. The correlation between the materials' activity, surface polarity and textural properties suggests valuable information on the chemical behaviour of sol-gel catalytic materials in oxidation catalysis; this is of interest in view of the importance of efficient solid catalysts for the selective oxidation of alcohols with O(2).     

Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (-273 to >1000 °C) and pressure (UHV to ?100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman-IR, Raman-UV-vis, Raman-EPR) and the operando Raman spectroscopy methodology (e.g., Raman-MS and Raman-GC) is allowing for the establishment of direct structure-activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).     

Catalytic Chemistry of Supported Heteropolyacids and Their Applications as Solid Acids to Industrial Processes

This article reviews recent research and development of supported heteropolyacid (HPA) catalysts; focusing on the acidic and catalytic properties. First the basic knowledge of solid HPAs is provided briefly to facilitate understanding of heterogeneous catalysis of HPAs. Secondly, the structure as well as the physical and chemical properties of supported HPA catalysts is described. Especially the layer structure of HPA dispersed on the surface of SiO₂ and the changes in the surface acidity with the loading level of HPA are discussed. For this purpose, temperature programmed desorption of benzonitrile devised by Okuhara and Kamiya’s group is useful. This method is capable to assess the surface acidity of the supported HPA, which controls the catalytic activity for the reactions proceeding by surface-type catalysis. Then, two new industrial processes developed by Showa Denko K. K. are described; (i) production of ethyl acetate from ethylene and acetic acid and (ii) oxidation of ethylene to acetic acid. Supported HPA catalysts are utilized for both processes, which were recently much improved by finely controlling the catalysts and reaction conditions.     

Polyoxometalates-based heterogeneous catalysts in acid catalysis

Acid catalysis, one of the most important industrial processes, suffers from the toxicity, corrosion and recyclability problems of conventional homogeneous acid catalysts. Thus, the development of green heterogeneous acid catalysts becomes the focus of fundamental research and industrial catalysis. As a class of discrete anionic metal-oxygen clusters with tunable structure at the molecular and atomic scales, polyoxometalates(POMs) benefit from their super strong Br?nsted acidity, high proton mobility,and thermal stability. POMs-based heterogeneous catalysts have been used as the potential green alternatives to conventional homogeneous acid catalysts. In this review, we summarize recent progress on the design strategies of the POMs-based heterogeneous catalysts and their catalytic properties in acid-catalyzed reactions, where they are combined with functionalized cations, modified through covalent interactions, supported onto the non-precious metal support, and introduced into the framework of porous polymers. The design, functional strategies and catalytic performance of these POMs-based heterogeneous catalysts in specific acid-catalyzed reactions are emphasized.     

Immobilized metalloporphyrins on 3-aminopropyl-functionalized silica support as heterogeneous catalysts for selective oxidation of primary and secondary alcohols

Abstract  

Iron(III), manganese(III), and cobalt(II) complexes of meso-tetrakis(p-chlorophenyl)porphyrin (Fe(TClPP)X, Mn(TClPP)X, and Co(TClPP)X, X = Cl or OAc) were immobilized onto 3-aminopropyl-functionalized silica (SF-3-APTS). SF-3-APTS acts as both axial base and support for immobilization of these metalloporphyrins. The obtained heterogeneous catalysts were characterized by Fourier transform infrared (FT-IR), UV–Vis, and inductively coupled plasma (ICP) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and thermogravimetric analysis (TGA) techniques. Their catalytic activity as biomimetic catalysts was investigated for the selective oxidation of primary and secondary benzylic alcohols to the corresponding carbonyl compounds with t-butylhydroperoxide as oxidant. SF-3-APTS–Fe(TClPP)Cl demonstrated higher catalytic activity than SF-3-APTS–Mn(TClPP)Cl and SF-3-APTS–Co(TClPP)OAc. The presence of electron-withdrawing substituents on benzylic alcohols enhances the rate of catalytic oxidation. SF-3-APTS–Fe(TClPP)Cl could be reused at least four times without significant loss of its catalytic activity.     

Effect of the nature of the support for copper-cerium oxide catalysts on selective oxidation of CO in hydrogen-rich mixtures

We have studied the catalytic properties of copper-cerium oxide catalysts, supported on zirconium, aluminum, titanium, and manganese oxides, in the reaction of selective oxidation of CO in hydrogen-rich mixtures. We have shown that the high activity and selectivity of catalysts supported on zirconium and aluminum oxides is connected with the presence of (in addition to divalent copper) higher amounts of copper in the (+1) oxidation state in the catalysts. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 2, pp. 119–124, March–April, 2006.     

Potential Utilization of Metal–Organic Frameworks in Heterogeneous Catalysis: A Case Study of Hydrogen‐Bond Donating and Single‐Site Catalysis

Crystalline solid materials are platforms for the development of effective catalysts and have shown vast benefits at the frontiers between homogeneous and heterogeneous catalysts. Typically, these crystalline solid catalysts outperformed their homogeneous analogs due to their high stability, selectivity, better catalytic activity, reusability and recyclability in catalysis applications. This point of view, comprising significant features of a new class of porous crystalline materials termed as metal‐organic frameworks (MOFs) engendered the attractive pathway to synthesize functionalized heterogeneous MOF catalysts. The present review includes the recent research progress in developing both hydrogen‐bond donating (HBD) MOF catalysts and MOF‐supported single‐site catalysts (MSSCs). The first part deals with the novel designs of urea‐, thiourea‐ and squaramide‐containing MOF catalysts and study of their crucial role in HBD catalysis. In the second part, we discuss the important classification of MSSCs with existing examples and their use in desired catalytic reactions. In addition, we describe the relative catalytic efficiency of these MSSCs with their homogeneous and similarly reported analogs. The precise knowledge of discussed heterogeneous MOF catalysts in this review may open the door for new research advances in the field of MOF catalysis.     

Template-free sol–gel synthesis of high surface area mesoporous silica based catalysts for esterification of di-carboxylic acids

High surface area mesoporous silica based catalysts have been prepared by a simple hydrolysis/sol–gel process without using any organic template and hydrothermal treatment. A controlled hydrolysis of ethyl silicate-40, an industrial bulk chemical, as a silica precursor, resulted in the formation of very high surface area (719 m²/g) mesoporous (pore size 67 Å and pore volume 1.19 cc/g) silica. The formation of mesoporous silica has been correlated with the polymeric nature of the ethyl silicate-40 silica precursor which on hydrolysis and further condensation forms long chain silica species which hinders the formation of a close condensed structure thus creating larger pores resulting in the formation of high surface mesoporous silica. Ethyl silicate-40 was used further for preparing a solid acid catalyst by supporting molybdenum oxide nanoparticles on mesoporous silica by a simple hydrolysis sol–gel synthesis procedure. The catalysts showed very high acidity as determined by NH₃-TPD with the presence of Lewis as well as Brønsted acidity. These catalysts showed very high catalytic activity for esterification; a typical acid catalyzed organic transformation of various mono- and di-carboxylic acids with a range of alcohols. The in situ formed silicomolybdic acid heteropoly-anion species during the catalytic reactions were found to be catalytically active species for these reactions. Ethyl silicate-40, an industrial bulk silica precursor, has shown a good potential for its use as a silica precursor for the preparation of mesoporous silica based heterogeneous catalysts on a larger scale at a lower cost.     

Role of support-nanoalloy interactions in the atomic-scale structural and chemical ordering for tuning catalytic sites

The understanding of the atomic-scale structural and chemical ordering in supported nanosized alloy particles is fundamental for achieving active catalysts by design. This report shows how such knowledge can be obtained by a combination of techniques including X-ray photoelectron spectroscopy and synchrotron radiation based X-ray fine structure absorption spectroscopy and high-energy X-ray diffraction coupled to atomic pair distribution function analysis, and how the support-nanoalloy interaction influences the catalytic activity of ternary nanoalloy (platinum-nickel-cobalt) particles on three different supports: carbon, silica, and titania. The reaction of carbon monoxide with oxygen is employed as a probe to the catalytic activity. The thermochemical processing of this ternary composition, in combination with the different support materials, is demonstrated to be capable of fine-tuning the catalytic activity and stability. The support-nanoalloy interaction is shown to influence structural and chemical ordering in the nanoparticles, leading to support-tunable active sites on the nanoalloys for oxygen activation in the catalytic oxidation of carbon monoxide. A nickel/cobalt-tuned catalytic site on the surface of nanoalloy is revealed for oxygen activation, which differs from the traditional oxygen-activation sites known for oxide-supported noble metal catalysts. The discovery of such support-nanoalloy interaction-enabled oxygen-activation sites introduces a very promising strategy for designing active catalysts in heterogeneous catalysis.     

Preparation and Catalytic Properties of Polymer Supported Dendritic Metal Complex

Polymer supported materials are extensively used as oxidizing agent, reducing agent catalysts, photosensitizers ion exchange resins and agriculturally and pharmacologically active agents1. The application of polymer metal complexes has been widely investigated2. The polymer supported complex undergoes swelling in a suitable solvent medium and provides enough surface area in carrying out electron transfer reactions, which clearly emphasizes the influence of a polymer network in heterogeneous catalysis.In the present, we have succeeded in the grafting of "dendrimer-like" hyperbranched polymer onto the surface of chloromethyl polystyrene reactions.All the catalysts show promising catalytic activities for the oxidation of iso-propylbenzene in the mild reaction condition, in each case, hypnone 1, 2-phenyl-2-propanol 2 were obtained as the major products     

Synthesis of MWCNTs doped sodium silicate based aerogels by ambient pressure drying

The successful incorporation of multiwalled carbon nanotubes (MWCNTs) into silica aerogels prepared by sol–gel method is reported herein. Pure silica aerogels prepared using sodium silicate precursor by ambient pressure drying are so fragile that they cannot be used easily. MWCNTs were used as reinforcements to improve the mechanical properties of silica aerogels. Results show that inserting small amounts of MWCNTs in the gels causes enhanced dimensional stability of silica aerogels. The silica aerogels were prepared by doping MWCNTs in silica matrix before gelation. The influence of MWCNTs on some microstructural aspects of silica matrix has been studied using nitrogen adsorption–desorption isotherms. From SEM study it is confirmed that the silica particles get capped on the surface of MWCNTs suggesting an enhanced toughness. Further, FTIR, Raman, EDAX, thermal conductivity and hydrophobicity studies of these doped aerogels were carried out. By addition of MWCNTs, silica aerogels were formed with 706 m²/g BET and 1,200 m²/g Langmuir surface areas and 149^o contact angle. Low density (0.052 g/cc) and low thermal conductivity (0.067 W/m K) MWCNTs doped silica aerogels were obtained for the molar ratio of Na₂SiO₃::H₂O::MWCNTs::citric acid::TMCS at 1::146.67::2.5 × 10⁻³::0.54::9.46 respectively with improved mechanical strength.     

氧化锰八面体分子筛(OMS-2)在有机分子清洁合成中的催化氧化应用

氧化锰八面体分子筛具有优异的氧化性、离子交换性和导电性等性能,被广泛应用于环保、半导体、有机合成等诸多领域。由于体相存在混合价态Mnn+和丰富的表面缺陷空位,使该分子筛同O2或H2O2等绿色氧化剂之间容易发生快速电子转移,活化绿色氧化剂,近年来作为非均相催化剂和功能性载体应用于有机分子的清洁合成中,表现出优异的催化活性、反应选择性和结构稳定性。本文综述了近年来OMS-2催化剂在有机分子清洁氧化合成中,及作为具有电子转移介质性能的载体材料的研究进展,并对未来发展提出了展望。     

Effect of supporting surface layers on catalytic activities of gold nanoparticles in CO oxidation

The surfaces of fumed silica materials were modified with a surface sol-gel process for catalysis applications. This surface-modification approach allows not only a monolayer growth of TiO(2) or Al(2)O(3) but also a stepwise double-layer growth of TiO(2)/TiO(2), Al(2)O(3)/Al(2)O(3), TiO(2)/Al(2)O(3), or Al(2)O(3)/TiO(2) on the surfaces of the silica materials with a monolayer precision. XRD analyses revealed that the coated monolayers and double layers of TiO(2) and Al(2)O(3) were amorphous. Gold nanoparticles were successfully deposited on the above six surface-modified silica materials via a deposition-precipitation method. The catalytic activities of these six gold catalysts for CO oxidation are highly dependent on the structures of their surface monolayers or double layers. The gold catalyst supported on the silica material functionalized with a TiO(2) monolayer (Au/TiO(2)) is the most active in both as-synthesized and oxidized forms, while the gold catalyst supported on the silica material functionalized with an Al(2)O(3)/TiO(2) double layer (Au/Al(2)O(3)/TiO(2)/SiO(2)) is the most active in the reduced form among the six catalysts. Surprisingly, the gold catalyst supported on the silica material functionalized with a TiO(2)/Al(2)O(3) double layer (Au/TiO(2)/Al(2)O(3)/SiO(2)) has much less activity than Au/Al(2)O(3)/TiO(2)/SiO(2) under all various treatments, underscoring the sensitivity of the catalytic activity to the structure of the supporting surfaces.     

甲苯类化合物的选择性氧化

综述了以O2为清洁氧源甲苯类化合物的气相和液相选择性氧化。甲苯类化合物的气相选择性氧化中主要介绍了金属氧化物、分子筛和负载型催化剂并对影响催化剂活性和选择性的因素进行了分析;甲苯类化合物的液相选择性氧化中重点介绍了MC(mid-century)催化体系及其反应机理方面的研究进展,并特别介绍了仿生催化在甲苯类化合物催化氧化中的应用。对各催化氧化体系的应用前景进行了展望。