Catalysis- Volume 28

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Catalysis

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A Specialist Periodical Report

Catalysis

Volume 28

A Review of Recent Literature

Editors

James J. Spivey,Louisiana State University, USAYi-Fan Han,East China University of Science and Technology,

Shanghai, ChinaK. M. Dooley,Louisiana State University, USA

Authors

Erfan Behravesh,bo Akademi University, Turku, FinlandYuxiang Chen,Carnegie Mellon University, Pittsburgh, PA, USAMarc-Olivier Coppens,University College London, UKWei-Lin Dai,Fudan University, Shanghai, ChinaJing Ding,Fudan University, Shanghai, ChinaAngelos M. Efstathiou,University of Cyprus, Nicosia, CyprusXiangchen Fang,Fushun Research Institute of Petroleum and

Petrochemicals and East China University of Science and Technology,Shanghai, China

Ruihua Gao,Fudan University, Shanghai, ChinaYun Hang Hu,Michigan Technological University, MI, USALeena Hupa,bo Akademi University, Turku, FinlandRongchao Jin,Carnegie Mellon University, Pittsburgh, PA, USAPhumelele E. Kleyi,Nelson Mandela Metropolitan University,

Port Elizabeth, South AfricaFrederic C. Meunier,CNRS, UniversiteLyon, FranceAlina Moscu,CNRS, UniversiteLyon, FranceDmitry Murzin,bo Akademi University, Turku, FinlandAdeniyi S. Ogunlaja,Nelson Mandela Metropolitan University,

Port Elizabeth, South AfricaChong Peng,Fushun Research Institute of Petroleum and Petrochemicals,

China and East China University of Science and Technology,Shanghai, China

Ayomi Sheamilka Perera,

University College London, UKTapio Salmi,bo Akademi University, Turku, FinlandYves Schuurman,CNRS, UniversiteLyon, FranceZenixole R. Tshentu,Nelson Mandela Metropolitan University,

Port Elizabeth, South AfricaRyan S. Walmsley,Rhodes University, Grahamstown, South Africa

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Wei Wei,Michigan Technological University, MI, USAXinli Yang,Fudan University, Shanghai, ChinaChenjie Zeng,Carnegie Mellon University, Pittsburgh, PA, USARonghui Zeng,Fushun Research Institute of Petroleum and

Petrochemicals, ChinaQuanjing Zhu,Fudan University, Shanghai, China

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ISBN: 978-1-78262-427-1PDF ISBN: 978-1-78262-685-5EPUB eISBN: 978-1-78262-805-7DOI: 10.1039/9781782626855ISSN: 0140-0568

A catalogue record for this book is available from the British Library

r The Royal Society of Chemistry 2016

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Preface

DOI: 10.1039/9781782626855-FP007

Chapter 1: Tungsten containing materials as heterogeneous catalysts for

green catalytic oxidation processThis review is given by Wei-Lin Dai, Jing Ding, Quanjing Zhu, RuihuaGao, Xinli Yang from Fudan university, Shanghai, China. It aims toprovide a comprehensive description of the recent advances in the fieldof tungsten-containing heterogeneous catalyst for green catalytic oxi-dation process. This review collects more than 90 literatures and consistsof three sections. The first part exhibits the advances in the pristinetungsten-based catalysts for the green catalytic oxidation process; thesecond one highlights various green catalytic oxidation reactions with

tungsten-based catalysts supported on different carriers; the last one il-lustrates the existing problems and outlook for the tungsten-basedcatalysts applied to the green catalytic oxidation reactions. The examplesdiscussed in this review highlight the need to design and synthesis oftungsten-based catalysts. Perhaps more importantly, they also are ofvalue for researchers in the area of heterogeneous catalysis to develophighly efficient green oxidation catalytic systems.

Chapter 2: Alumina ceramic foams as catalyst supportsIn the next review, Alumina ceramic foams as catalyst supports is re-viewed by Erfan Behravesh, Leena Hupa, Tapio Salmi, Dmitry Yu. Murzinfrom bo Akademi University, Finland. Ceramic foams have a wide rangeof potential applications in biomedicine, thermal insulation, filtration ofmolten metal alloys, absorption of environmental pollutants, catalystsupports, etc. Herein, three main methods of manufacturing ceramicfoams are introduced with the main emphasize on the replica technique.Furthermore, different techniques for improving structural properties ofceramic foams are reviewed. The focus of this review is on fabrication ofmacro-porous alumina foams with high interconnected porosity. In

addition, experimental data for manufacturing of ceramic foams viathereplica technique is represented along with literature surveys. Slurriesconsisted of alumina powder mixed in aqueous solutions of polyvinylalcohol (PVA) and magnesia and titania as sintering aids. The foams wereproduced by tuning different processing parameters to give propertiessuited for catalyst supports. These parameters included pore size of thepolyurethane (PU) foam used as a template, parameters in the PU foampretreatment, particle size of alumina powder in the slurry, slurry loadingand drying of the green alumina coated PU foam. Finally, the key factors

for optimizing ceramic foams in terms of mechanical strength andinterconnectivity are introduced together with an outlook for future ad-vances in ceramic foams as catalyst supports.

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Chapter 3: Recent advances in the synthesis and catalytic applications ofatomically precise gold nanoclustersThis review is contributed by Yuxiang Chen, Chenjie Zeng, and RongchaoJin from Carnegie Mellon University. This review summaries the recentadvances in the synthesis and catalytic application of atomically preciseAu

n(SR)

mnanoclusters. Structurally characterized nanoclusters can serve

as new model catalysts for obtaining atomic/molecular level insights intothe catalytic processes, including the precise size-dependent catalyticreactivity and how molecules are adsorbed and activated on the catalyticactive sites, as well as the structural sensitivity of the catalyst to the re-actions. While this area is still in its infancy, promising work has beenreported and demonstrated the catalytic power of atomically precisenanoclusters. Such reactions include catalytic oxidation, chemoselectivecatalytic hydrogenation, catalytic semihydrogenation, etc. In addition,precisely doped nanoclusters provide a unique opportunity to tune the

catalytic reactivity on a truly atom-by-atom basis. Overall, atomicallyprecise nanoclusters hold great promise in the discovery of uniquecatalytic processes as well as in advancing the fundamental under-standing of catalytic mechanisms at the atomic/molecular level.

Chapter 4: Research and Development of Hydrocracking Catalysts andtechnologyResearch and Development of Hydrocracking Catalysts and technology isreviewed by Chong Peng, Xiangchen Fang and Ronghui Zeng from bothFushun Research Institute of Petroleum and Petrochemicals, SINOPEC,China and East China University of Science and Technology, Shanghai,China. Hydrocracking (HCK), one of the main approaches to deep pro-cess heavy oil, is a catalytic conversion process where feedstock under-goes hydrogenation, S/N removal, molecular restructuring, cracking, andother reactions. It can process straight-run gasoline/diesel, vacuum gasoil, and other secondary processing fractions such as fluid catalyticcracking (FCC) diesel, FCC clarified oil, coker diesel, coker gas oil, anddeasphalted oil and produce various quality clean fuels such as liquefiedgas, gasoline, kerosene, jet fuel, diesel, and various quality petrochemical

materials such as light/heavy naphtha and tail oil. In this chapter, theresearch progress on commercial HCK technology and its relative cata-lysts are discussed. The typical technical characteristics and the repre-sentative processes from different corporations, such as Universal OilProducts, Albemarle, Criterion, Haldor Topsoe, and SINOPEC, are alsopresented. The development trend of HCK technology in the future isoutlined.

Chapter 5: Titano-silicates: Their history, evolution and scope of

applicationAyomi Perera and Marc-Olivier Coppens of University College Londonreview the rapidly evolving catalysis of titano-silicates, especially as ap-plied to selective oxidations. There have been significant advances inunderstanding how these materials function, and in the synthesis of newTi-silicate structures. A key application is in epoxide manufacture, and

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this application is reviewed here. But several potential green chemicalprocesses are under evaluation, and these are discussed and critiquedas well.

Chapter 6: Nanofiber-supported metal-based catalystsA group headed by Zenixole Tshentu from Nelson Mandela Metropolitan

and Rhodes Universities have reviewed nanofiber-supported metal cata-lysts. The review covers most of the conventional catalytic transitionmetal/metal oxides supported on different types of electrospun nanofi-bers. Catalytic metal ion complexes supported on electrospun nanofi-bers,via coordination to the desired functional groups of polymer chains,have also been discussed. While the use of electrospun nanofibers ascatalyst support is still at its infancy stage, several application studieshave shown that the use of nanofiber-based catalytic materials exhibitedgood catalytic activity as a result of the increased surface area-to-volume

ratio. There is discussion of catalyst reusability and challenges associatedwith the use of electrospun nanofibers in catalysis.

Chapter 7: Elucidation of Mechanistic and Kinetic Aspects of Water-GasShift Reaction on Supported Pt and Au Catalysts via Transient IsotopicTechniquesAngelos Efstathiou of the University of Cyprus comprehensively sum-marizes recent work from the low-temperature watergas shift catalysisliterature. He places special emphasis on steady-state isotopic transientkinetic analysis and other transient isotopic techniques to probe mech-anism and determine important kinetic parameters for supported Pt andAu catalysts on reducible and non-reducible metal oxides. These resultsare extended and put into context by comparing to predictions of recentcomputational (DFT) studies.

Chapter 8: Recent progresses on the use of supported bimetallic catalystsfor the preferential oxidation of CO (PROX)Alina Moscu, Yves Schuurman, and Frederic Meunier (Institut deRecherches sur la Catalyse et lEnvironnement de Lyon) report on current

progress and prospects on PROX, an essential step in fuel processing ofhydrocarbons to produce CO-free dihydrogen. This can be converted intoclean energy, particularly in PEM fuel cells. The CO just downstream ofWGS must be reduced to low levels, often to ppm concentrations. Im-provements must be addressed at low temperatures. Recent studies haveshown progress in bimetallic compounds compared to monometalliccounterparts. The improved activity of alloys and bimetallic appear to beto geometric or electronic effects. The review here pays particular atten-tion to in situIR-based studies realized over Pt-based formulations, since

CO is both a reactant and a molecular probe enabling the determinationof the state of metals under reaction conditions. Reaction results atconditions at the working catalyst are essential. In situ and operandoconditions enable can be used to probe the true active phases, becausealloy segregation can readily occur even due to minor modification of theexperimental conditions.

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Chapter 9: 3D MoS2/Graphene Hybrid Layer Materials as Counter Elec-trodes for Dye-Sensitized Solar CellsWei Wei and Yun Hang Hu (Michigan Tech) discuss the development ofdye-sensitized solar cells (DSSCs), which have attracted considerable at-tention as an alternative to conventional silicon based solar cells becauseof their low cost, low energy consumption, simple fabrication process,

and high power conversion efficiency. Typically, DSSCs are composed of aphotoelectrode (a transparent conducting subtract with a dye coated TiO2film), an electrolyte, and a counter electrode (CE). The synthesis andDSSC counter electrode applications of graphene sheets and MoS2 ma-terials are briefly reviewed. Furthermore, in this chapter, they report anew method to synthesize 3D MoS2/graphene hybrid layer materials ascounter electrode catalysts for DSSCs.

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CONTENTS

Cover

Image provided courtesy ofcomputational science companyAccelrys (www.accelrys.com). Anelectron density isosurface mappedwith the electrostatic potential for anorganometallic molecule.This shows the charge distributionacross the surface of the moleculewith the red area showing thepositive charge associated with thecentral metal atom. Research carriedout using Accelrys MaterialsStudioss.

Preface vii

Tungsten containing materials as heterogeneous catalysts forgreen catalytic oxidation process

1

Wei-Lin Dai, Jing Ding, Quanjing Zhu, Ruihua Gao andXinli Yang

1 Introduction 1

2 Pristine W-based catalyst for green catalytic oxidation 2

3 W-based catalyst supported on different carriers for

green catalytic oxidation

6

4 Conclusion and outlook 23

Acknowledgements 24

References 24

Alumina ceramic foams as catalyst supports 28

Erfan Behravesh, Leena Hupa, Tapio Salmi andDmitry Yu. Murzin

1 Introduction 28

2 Experimental 32

3 Results and discussion 35

4 Conclusions and outlook 46

Acknowledgements 47

References 47

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Recent advances in the synthesis and catalytic applicationsof atomically precise gold nanoclusters

51

Yuxiang Chen, Chenjie Zeng and Rongchao Jin

1 Introduction 51

2 Synthesis, structure, and properties of gold nanoclusters 53

3 Catalytic properties of Aun(SR)mnanoclusters 654 Summary 82

Acknowledgements 83

References 83

Research and development of hydrocracking catalysts andtechnology

86

Chong Peng, Xiangchen Fang and Ronghui Zeng

1 Introduction 86

2 History 87

3 Hydrocracking catalyst 88

4 Hydrocracking processes 94

5 Kinetic models of hydrocracking 110

6 Consideration for hydrocracking technology development 116

References 117

Titano-silicates: highlights on development, evolution andapplication in oxidative catalysis

119

Ayomi Sheamilka Perera and Marc-Olivier Coppens

1 Introduction 119

2 Synthesis and characterisation of titano-silicates 122

3 Application of titano-silicates as oxidative catalysts 124

4 Outlook: is the future bright for titano-silicate catalysts? 134

5 Summary 137Acknowledgements 138

References 138

Nanofiber-supported metal-based catalysts 144

Adeniyi S. Ogunlaja, Phumelele E. Kleyi, Ryan S. Walmsley andZenixole R. Tshentu

1 Introduction 144

2 Nanofiber-supported metal/metal oxide catalysts 146

3 Nanofiber-supported metal complexes 156

4 Challenges associated with electrospun nanofibers 165

5 Conclusions 169

References 169

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Elucidation of mechanistic and kinetic aspects of watergasshift reaction on supported Pt and Au catalysts viatransientisotopic techniques

175

Angelos M. Efstathiou

1 Introduction 175

2 Watergas shift reaction mechanisms 1803 SSITKAoperandomethodology 187

4 Application of SSITKA and other transient isotopic

techniques towards elucidation of WGS reaction

mechanisms

206

5 Conclusions 230

Acknowledgements 231

References 231

Recent progresses on the use of supported bimetallic catalystsfor the preferential oxidation of CO (PROX)

237

Alina Moscu, Yves Schuurman and Frederic C. Meunier

1 Benefits and challenges associated with the use of

multi-metallic materials as catalysts and electrodes

237

2 Hydrogen as energy carrier in fuel cell applications 238

3 Main features of the preferential CO oxidation (PROX) 240

4 Platinum-tin-based catalysts 2515 Surface analyses byin situ infrared spectroscopy 254

6 Conclusions 262

References 262

3D MoS2/Graphene hybrid layer materials as counter electrodesfor dye-sensitized solar cells

268

Wei Wei and Yun Hang Hu

1 Introduction 268

2 Preparation of DSSCs 270

3 Characterization of DSSCs 272

4 Conclusions 278

Acknowledgements 278

References 278

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Tungsten containing materials asheterogeneous catalysts for greencatalytic oxidation process

Wei-Lin Dai,* Jing Ding, Quanjing Zhu, Ruihua Gao andXinli YangDOI: 10.1039/9781782626855-00001

This chapter provides a comprehensive description of the recent advances in the field

of tungsten-containing heterogeneous catalysts for green catalytic oxidation processes.

This chapter contains three sections. The first exhibits the advances in the pristine

tungsten-based catalysts for the green catalytic oxidation process. The second highlights

various green catalytic oxidation reactions with tungsten-based catalysts supported on

different carriers. The third illustrates the existing problems and outlook for the tungsten-

based catalysts applied in the green catalytic oxidation reactions. All these contributionsprovide a proper guide and overview to tungsten-based catalysts for the sake of the better

development of highly efficient green oxidation catalytic systems.

1 Introduction

Green Chemistry is now a central issue in both academia and industrydue to global contamination and other environmental and health risks ofchemicals. Oxidation is a core technology for converting petroleum-based

materials to useful chemicals of a higher oxidation state. However, thetraditional methods of oxidation always require the use of stoichiometriclevels of oxidant such as chromates, permanganates and periodates,

which could result in complex heat management needs and by-productsthat are harmful to the environment.1 As the increasing emerge ofthe environmental issue, searching for a green synthesis method ofthe catalysts and further for an environmentally friendly process of thecatalytic oxidation are urgently needed to meet the challenging environ-mental demands and industrialization requirements. Nowadays, a cleanand environmentally friendly process using environmental benignoxidants such as air,2 oxygen3 or hydrogen peroxide4 are preferred.

In recent years, there has been a growing interest in tungsten-basedcatalyst due to their properties and wide potential applications. This kindof materials exhibits unique acidity that improves the activity andselectivity, chemical and thermal stability and environmental protection,thus being regarded as a kind of promising catalyst owing to its versatileapplication, including the metathesis and isomerization of alkenes,5 se-lective oxidation of unsaturated compounds,6 hydrodesulfurization andhydrocracking of heavy fractions in petroleum chemistry,7,8 and de-

hydrogenation of alcohols.9 Moreover, the tungsten-based catalyst can beapplied to the photocatalytic oxidation reaction, especially tungsten

Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and

Innovative Material, Fudan University, Shanghai 200433, P. R. China.

E-mail: [email protected]

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trioxide. Tungsten trioxide, with a band gap between 2.4 and 2.8 eV, is avisible light response catalyst with stability in acidic reaction conditions.10

The deeply positive level of the valence band (VB) makes it suitable forachieving the efficient oxidative decomposition of the organic compoundsunder solar or visible irradiation. In addition, tungsten-based catalysts alsohave been widely used in the field of electrocatalysis. For instance, tung-

sten trioxide, dependent on its own special electrochemical properties, canbe used as an anode catalyst for hydrogen oxidation in proton exchangemembrane fuel cells (PEMFCs), promoting the development of PEMFCs.

In this chapter, we aim to provide a comprehensive description ofthe recent advances in the field of tungsten-containing heterogeneouscatalyst for green catalytic oxidation process. Though significantadvances have been achieved in the development of the tungsten-containing heterogeneous catalyst in green catalytic oxidation process,there is rarely relevant review focusing on the aspect. Hence, this is our

main driving force to contribute to the fundamental exemplificationsin this field. This review collects more than 90 literatures and consistsof three sections. The first part exhibits the advances in the pristinetungsten-based catalysts for the green catalytic oxidation process; thesecond one highlights various green catalytic oxidation reactions withtungsten-based catalysts supported on different carriers; the last oneillustrates the existing problems and outlook for the tungsten-basedcatalysts applied to the green catalytic oxidation reactions. Herein, wesincerely wish to give colleagues a proper guide and overview to tungsten-based catalysts for the sake of the better development of highly efficientgreen oxidation catalytic systems.

2 Pristine W-based catalyst for green catalytic oxidation

Pristine tungsten-based catalyst mainly includes tungsten trioxide,tungstic acid, tungsten carbide, etc. Recently, a great deal of researchbased on pristine tungsten-based catalysts was focused on various greencatalytic oxidation reactions, which exhibited excellent catalytic per-formance and chemical stability.

2.1 Tungsten trioxide catalyst for green catalytic oxidationTungsten trioxide (WO3) is an important rare earth oxide and can be usedin the field of gas sensors,11 catalysis,12 electrochemistry13 and photo-catalysis14 because of its unique gasochromic, acidity, electrochromicand photochromic properties. Tungsten trioxide, a very efficient catalystfor various acid catalyzed reactions due to its solid acidic and redoxproperties, is applied to some oxidation reactions, such as the selectiveoxidation of alcohols or aldehydes and the epoxidation of alkenes.

Suet al.synthesized hexagonal single crystal WO3nanorods with the aidof p-aminobenzoic acid through a facile hydrothermal method.15 Thecatalyst showed good performance for oxidation of cyclohexene to adipicacid compared to commercial WO3. Meanwhile, WO3, with a band gapbetween 2.4 and 2.8 eV, is a visible light response catalyst with stabilityunder acidic conditions, which makes it a suitable choice for the

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photocatalytic oxidation of organic pollutants under solar irradiation.The possible photocatalytic oxidation process using WO3 as a photo-catalyst can be expressed by the following set of equations.

WO3 hv-eh1 (1)

H2Oh1-H2O

1-HOH1 (2)

O2 e

-O2 (3)

H2O-HOH1 (4)

HOh1-HO (5)

O2H1-HOO (6)

O2H1-HO2

(7)

HO2 HO2-H2O2

1

2O2 (8)

H2O2H1 e-HOH2O (9)

When photocatalyst WO3 is irradiated with light energy above 2.7 eV,electron (e) and hole (h1) pairs are generated (eqn (1)). Then, thephotogenerated holes can react with water hydroxide ions to producehydroxyl radicals (eqn (2) and (4)). Meanwhile, the reduction of dissolvedoxygen by the photogenerated electrons results in the generation ofsuperoxide anion radicals. Meanwhile, radicals can be initiated, gener-ating a variety of charged neutral and ionic species (eqn (59)).

Villaet al.reported that mesoporous WO3was prepared by replicatingtechnique using ordered mesoporous silica KIT-6 as the template, whosesurface area reached up to 151 m2 g1.16 The photocatalytic oxidation ofCH4 into CH3OH over this catalyst using electron scavengers and H2O2

was studied and the photocatalytic activity of WO3 toward methanolproduction could be enhanced by a factor of 2.5 and 1.7 when addingFe31 and Cu21, respectively. Meanwhile, the photocatalytic activity issignificantly affected by the particle morphology (including shape andsize) of WO3. Hameed et al. further claimed that the photocatalytic

activity for the degradation of phenols could be enhanced by controllingthe morphology of the photocatalyst (disc-shaped WO3).

17 The possibleroute of degradation/mineralization of phenolic substrates was furtherinvestigated in Fig. 1. WO3 NPs are also shown to possess excellentphotocatalytic degradation of pharmaceutical compounds, like Lidocainefrom water.18 The study suggests that photocatalytic property of WO3NPs

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can be taken for large-scale application in pharmaceutical industries forthe efficient removal of pharmaceutical stuff from effluents.

2.2 Tungstic acid catalyst for green catalytic oxidationTungstic acid, as a well-known and widely used solid acid catalyst,has received a considerable attention due to its non-toxicity, costeffectiveness, ease of handling and good selectivity and activity for a

wide variety of reactions. Our group firstly reported a green procedure for

O-heterocyclization of 1,5-cyclooctadiene (COD) by catalytic oxidationwith aqueous H2O2.

19 The effect of the amount of the tungstic acid waswell studied as well as the molar ratio of COD and H2O2and the volumeratio of the solvent to reactant. The green catalytic oxidation processhad a lot of advantages compared with the other traditional chemicalmethods and the tungstic acid catalyst could also be easily recovered.Meanwhile, our group also developed a new economic and green route tosynthesize phthalide from 1,2-benzenedimethanol using aqueoushydrogen peroxide as the oxidant and tungstic acid as the catalyst under

organic solvent-free conditions.

20

The desired product with high purityand good yield was obtained. The tungstic acid catalyst could be easilyfiltrated after reaction and reused for more than 6 times. The above twogreen catalytic oxidation processes are firstly reported and can open up anew prospect for the application of tungsten-based catalysts.

2.3 Tungsten carbide catalyst for green catalytic oxidationTungsten carbide (WC), as a green catalyst for electron-transfer reactions,has been extensively investigated in various applications, such as

hydrogenolysis and isomerization reactions, fuel cells, hydrogen evo-lution and catalytic oxidation, because it exhibits catalytic propertiessimilar to those of noble metals as well as the chemical and thermalstability. WC has been reported to possess three different crystallinephases (b-W2C, a-WC and b-WC1x) depending on the reaction con-ditions and the synthetic route. New methods of thermo-programmed

Fig. 1 The possible route of degradation/mineralization of phenolic substrates. Reprintedwith M. Aslam et al., Morphology controlled bulk synthesis of disc-shaped WO3 powderand evaluation of its photocatalytic activity for the degradation of phenols, J. Hazard.Mater., 276, 120128. Copyright (2014), with permission from Elsevier.17

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elaboration of carbide synthesis have led to significant improvement inthe catalytic oxidation application, due to WC with relatively high specificsurface areas. Meanwhile, tungsten carbide also has a well knowncatalytic action on the process of oxidation of hydrogen in acidic media

where it has a particular chemical stability and it is more resistant topoisoning agents than noble metals. Although the catalytic efficiency of

WC is much lower than that of noble metals, a considerable interest hasbeen taken in the field of fuel cells for the development of inexpensivesystems. More recently, WC has been taken into consideration as acatalyst that could convert H2and O2to water without the drawbacks ofnoble metals which can contaminate the active materials and enhancedischarge and sulphation. Many investigations have been carried out toobtain highly active tungsten carbide and determine its electrochemicalbehavior. Penazzi et al. studied electrochemical oxidation of hydrogenusing WC as catalyst.21 The final results suggested that the rate of

chemical oxidation of hydrogen was proportional to PH2only. Meanwhile,the effect of WC on electrochemical oxidation of hydrogen increasedwith the applied potential and was proportional to the specific BETarea of the catalyst. Wanget al.demonstrated for the first time that theintrinsic catalytic activity of WC nanorods (WC NRs) towards typicalperoxidase substrate, such as 3,30,5,50-tetramethyl-benzidine (TMB)and o-phenylenediamine (OPD) in the presence of hydrogen peroxide(H2O2).

22 Compared to natural enzyme HRP, WC NRs exhibited superiorcatalytic activity and good reutilization. The schematic illustration ofoxidation reaction of TMB and OPD for the intrinsic peroxidase-likeactivity of WC NRs was further studied, as demonstrated in Fig. 2.

Although homogeneous tungsten-based catalysts exhibit superiorcatalytic performance in the field of traditional catalytic oxidation,photocatalytic oxidation and electrochemical oxidation, they still restrictfurther applications in industry due to the difficulty in separation

Fig. 2 The schematic illustration of oxidation reaction of TMB and OPD for the intrinsicperoxidase-like activity of WC NRs. Reprinted from N. Li et al., Novel tungsten carbidenanorods: an intrinsic peroxidase mimetic with high activity and stability in aqueous andorganic solvents, Biosens. Bioelectron., 54, 521527. Copyright (2014), with permissionfrom Elsevier.22

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and reuse. Hence, it is urgent to design new kinds of tungsten-basedheterogeneous catalysts.

3 W-based catalyst supported on different carriers forgreen catalytic oxidation

Tungsten-based materials, as an outstanding catalyst, have been appliedto many green catalytic oxidation reactions, such as conventional greenoxidation including selective oxidation of unsaturated compounds, se-lective oxidation of saturated hydrocarbons,etc.; photocatalytic oxidationfor degradation of organic pollutants and unsaturated compounds, etc.;electrocatalytic oxidation, for instance, methanol electrochemicaloxidation, etc. However, the homogeneous catalytic system restricts itsfurther application in industry for the difficult separation and reuse ofthe homogeneous tungsten-based catalyst. One of the most promising

ways to accomplish this aim is to design the W-containing heterogeneouscatalysts. Hence, many heterogeneous tungsten-based catalysts sup-ported on different carriers (such as metal oxide, siliceous mesoporousmolecular sieves and carbon materials) have been designed for greencatalytic oxidation that show good performance in the target reaction.

3.1 W-based catalyst supported on metal oxidesThe metal oxide, dependent on its simple preparation, strong surfaceacidic sites, high thermal stability and good catalytic activities, has beenconsidered as a kind of useful material in the catalytic oxidation field.23

Tin dioxide (SnO2), by virtue of its unique chemical and mechanicalstabilities as well as two specific characteristics: variation in valence stateand oxygen vacancies defects,24 has been widely used as one of themost important smart and functional materials for technological andindustrial applications, such as transparent conductive electrodes, anodematerials for lithium-ion batteries, solar energy conversion, electro-chemical devices, antistatic coatings and catalysis.2531 So far, there havebeen several studies on the WO3/SnO2 composites applied to the green

catalytic oxidation reactions. Our group reported the effect of calcinationtemperature of the support and the catalyst of WO3/SnO2on the catalyticoxidation of 1,2-benzenedimethanol by H2O2as shown in Scheme 1.

32 Aseries of WO3/SnO2 composite catalysts for the catalytic oxidation wereprepared by co-precipitation-impregnation method and characterized

with various techniques. The characterization results showed that thecalcination temperature of the catalyst and the support was essential tothe structural evolution of the WO3/SnO2. The studies of the catalyticbehavior of WO3/SnO2catalyst in the selective oxidation of 1,2-benzene-

dimethanol to phthalide using aqueous H2O2 as the oxidant suggestedthat the optimized calcination temperature of the support and thecatalyst was 1023 and 823 K, respectively, which was mainly attributed tothe high dispersion of tungsten species and few W61 ions enteredinto SnO2 lattice. The latter would decrease inevitably the amount ofsurface active tungsten species. Meanwhile, Kamataet al.demonstrated a

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spheres, the strong interaction, and the medium-strong acidity of thecatalyst were all considered to contribute to the superior catalyticbehavior of the WO3/TiO2catalyst in the selective oxidation of CPE to GA.The optimal tungsten content was 20 wt%, and the GA yield over thiscatalyst reaches 69%.

In addition, WO3/TiO2is also a popular photocatalyst for degradation

of organic pollutants that has already been well studied by severalresearch groups. Zhanget al. developed a hierarchically nanostructured

WO3/TiO2 photocatalyst via the subsequent hydrothermal treatment ofelectrospun TiO2nanofibers in the presence of tungstic acid.

38As shownin Fig. 4, the WO3 seed layer on the TiO2 nanofibers provided growthsites, facilitating the nucleation and growth in the solution, thusTiO2nanofibers and WO3nanorods were closely combined together. The

WO3/TiO2demonstrated enhanced visible light absorption and increasedphotocatalytic degradation of organic pollutants, due to the migration of

electronhole pairs between TiO2and WO3, thus increasing the lifetimeof the charge carriers, large surface areas and light utilization andthe unobstructed migration of electrons to the surface. The energy bandstructure and electronhole pair separation in the hierarchicallystructured TiO2/WO3 nanofibers were described in Fig. 5. Wang et al.investigated a low temperature peptization process to construct a

Fig. 3 SEM and TEM images of the mesoporous titania microspheres (a and b) and the20 wt% WO3/TiO2 catalyst (c and d). Reprinted from X.-L. Yang et al., Synthesis of novel

coreshell structured WO3/TiO2spheroids and its application in the catalytic oxidation ofcyclopentene to glutaraldehyde by aqueous H2O2, J. Catal., 438450. Copyright (2005),with permission from Elsevier.37

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uniform composite structure containing amorphous WO3 and anataseTiO2.

39 Visible-light photocatalytic activity of the as-prepared WO3/TiO2composite was induced by UV pre-irradiation. The whole consecutiveprocedure could be defined as a consecutive photocatalytic processas shown in Fig. 6, which was beneficial to the efficient use of solarenergy.

Zirconia (ZrO2)-supported tungsten oxide catalysts have been exten-sively studied in recent years because of their ability to catalyze a wide

range of reactions such as selective reduction of NOx to N2, oligo-merization ofoC20alkanes to gasoline, diesel and lubricants (C301), andisomerization of alkanes.40,41 However, the oxidation properties of thezirconia-supported tungsten oxides were rarely reported. Our groupfirstly reported that a series of tungsten oxide supported on commercialZrO2 that was synthesized via a traditional impregnation method were

Fig. 4 Schematic illustration showing the formation of the hierarchically structured TiO2/WO3 nanofibers. Reprinted from ref. 38 with permission from The Royal Society of

Chemistry.

Fig. 5 Schematic diagram showing the energy band structure and electronhole pairseparation in the hierarchically structured TiO2/WO3 nanofibers. Reprinted from ref. 38,with permission from The Royal Society of Chemistry.

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applied to the oxidative lactonization of 1,2-benzenedimethanol to

phthalide with H2O2, as shown in Fig. 7.42 The tungsten precursor andthe calcination temperature were crucial to the dispersion and the natureof the tungsten species on ZrO2and the excellent catalytic performanceof the catalyst prepared by calcination at 823 K after using tungsticacidoxalic acid complex as the tungsten precursor was attributed tothe presence of polymeric WO6 units. Galano et al. experimentallyand theoretically explored the dependence between catalytic activity of

WOxZrO2 system in the oxidation of dibenzothiophene (DBT) and itsrelationship with local acidity.43 The structural requirements indicated

that the oxidative efficiency (per W-atom) increased as the WOxsurfacedensity become larger, up to 7 W nm2. These results strongly suggestedthat n-meric domains and/or WO3xnanoparticles (NPs) anchored on thesurface were more reactive than monomeric and three-dimensionalstructures and WOx domains of intermediate size provided a bettercompromise between surface acidity and catalytic efficiency. The

Fig. 6 Schematic illustration of the consecutive photocatalytic process. Reprinted fromref. 39 with permission from The Royal Society of Chemistry.

Fig. 7 Schematic illustration of the selective oxidation of 1,2-benzenedimethanol.Reprinted from Q. J. Zhu et al., Effect of tungsten precursor on the high activity of theWO3/ZrO2catalyst in the oxidative lactonization of 1,2-benzenedimethanol,Appl. Catal. A:Gen., 435, 141147. Copyright (2012), with permission from Elsevier.42

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theoretical analysis revealed that the combined presence of Lewis andBronsted sites energetically favored the formation of peroxometallatecomplexes and that OOH addition reached its maximum for the min-imum (non-zero) number of Lewis sites and maximum number ofBronsted sites.

Aluminum oxides, especially g-Al2O3, have been widely employed as

supports for catalytic oxidation reactions because of their high surfacearea, thermal and chemical stability and low cost. As previous literaturereported, aluminum oxides (Al2O3)-supported tungsten based catalystshave been widely applied to the green catalytic oxidation process. Lu et al.developed a novel oxidation method of alcohols to the correspondingaldehydes and ketones with high efficiency under molecular oxygenin the presence of the 1,1-diphenyl-2-picrylhydrazyl (DPPH) catalystand the tungsten oxide/alumina (WO3/Al2O3) co-catalyst.

44 Variousaromatic, alicyclic, and aliphatic alcohols could be converted to their

corresponding carbonyl compounds in excellent yields. Suzuki et al.reported an efficient and selective aerobic oxidative transformation ofprimary amines to oximes proceeded with high efficiency under mildconditions in the presence of the DPPH catalyst and WO3/Al2O3 co-catalyst.45 Various alicyclic and aliphatic amines could be converted totheir corresponding oximes in excellent yields. In the DPPHWO3/Al2O3system, DPPH acted as an electron transfer mediator, and an alkyl-hydroperoxide intermediate was transformed into an oxime by the WO3/

Al2O3 co-catalyst. Meanwhile, the reaction could be rationalized by as-suming the mechanism, as depicted in Scheme 2.

Vanadium oxide (V2O5) is an attractive material due to its good cata-lytic, electrical and optical properties. The good catalytic activity is theresult of easy reduction and oxidation between the multiple oxidationstates of vanadium in V2O5.

46 Meanwhile, mesoporous vanadium oxidesolid, by virtue of thermal and mechanical stability, plays a key role assupport in the catalytic oxidation.47 Makgwaneet al.studied an efficientroom temperature oxidation process of cyclohexane to cyclohexanone (K)and cyclohexanol (A) by highly active nanostructured WO3/V2O5 com-posite as the catalyst.48 The catalyst exhibited high catalytic activity with

up to 90% conversion and excellent recyclability and stability. The cata-lytic results appeared to suggest a possible existence of a strong inter-action effect between the two combined metal oxides that enhanced theirperformance when compared to individual oxides. The enhanced per-formance of WO3/V2O5 was partly attributed to the redox and possiblestructural modifications due to the strong metal-support interaction. Thepossible mechanism route of cyclohexane oxidation over WO3/V2O5catalyst was shown in Scheme 3.

The metal oxides as the support of W-based catalyst have performed

satisfactorily in green catalytic oxidation processes. Nevertheless, thereare still many problems in the practical applications of these reportedcatalysts, including the needs of expensive raw materials of the metaloxides and the poor recycling efficiency of the catalyst. In order to workout the drawbacks mentioned above, siliceous mesoporous supportshave been widely investigated.

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3.2 W-based catalyst supported on siliceous mesoporous molecularsieves

Silicon materials with unique physicochemical properties have receivedtremendous attention in many fields, such as solar energy conversion,optoelectronics, biological sensors and supports.49 Porous silicon, withthe intrinsic properties of silicon and the unique features of a porousstructure, has been considered as one of the most attractive materials foruse in catalysis,50 luminescence,51 supercapacitors52 and batteries.53

Until now, many efforts have been made in the preparation of poroussilicon. A variety of mesoporous silicon materials have been extensivelystudied, such as mesoporous MCM-41, MCM-48, HMS, MCF, SBA-15

and so on.Since the first invention of MCM-41 in 1992 by the Mobile researchers,

MCM-41 material has attracted much interest because of their potentialapplications in many fields of science and engineering, such asadsorption, separation, and catalysis involving bulky molecules.54 Inparticular, its remarkable textural properties, such as high surface area

Scheme 2 Proposed mechanism for DPPHWO3/Al2O3 catalyzed aerobic oxidation ofprimary amines. Reprinted with permission from K. Suzuki, T. Watanabe and S. I.Murahashi, J. Org. Chem., 2013, 78, 2301. Copyrights (2013), American ChemicalSociety.45

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and large pore volume, make it very suitable for application as catalystsupports. Li et al. utilized the hydrolysis of tetraethylortho silicate andammonium tungstate in the presence of cetylpyridinium bromide astemplate in strongly acidic medium to synthesize a novel tungsten-containing MCM-41 mesoporous molecular sieve.55 The tungsten-containing MCM-41 mesoporous molecular sieve was applied to thecyclohexene conversion, which strongly suggested that the isolated Wsites incorporated in the framework of W-MCM-41 afforded the mesos-tructure with high catalytic activity with respect to the crystalline WO3inpromoting the hydroxylation of cyclohexene using dilute H2O2 asterminal oxidant. Our group reported the W-MCM-41 catalyst for theselective oxidation of CPE to GA with aqueous hydrogen peroxide.56 It was

found that tungsten species stably existed in the silica-based matrix ofMCM-41 up to a Si/W molar ratio of 40. The W-MCM-41 catalyst exhibitedthe highest activity and selectivity in the selective oxidation of CPE to GA.The convenient separation of the W-MCM-41 catalyst from the reactionproducts mixture and its longer lifetime made it more feasible than thecorresponding homogenous catalysts when applied in industrial use.

MCM-48 with a 3D cubic mesostructure consists of two inter-penetrating continuous networks of chiral channels. These enantiomericpairs of porous channels are separated by an inorganic wall that followed

exactly the gyroid (G-surface) infinite periodic minimal surface (IPMS).This unique 3D channel network is thought to provide a highly openedporous host that provides easy and direct access for guest species, thusfacilitating inclusion or diffusion throughout the pore channels withoutpore blockage.5759 Hence, the structural peculiarities of the MCM-48make it a potentially very interesting and promising catalyst/catalyst

Scheme 3 Plausible mechanism route to cyclohexane liquid-phase oxidation productsformation. Reprinted from P. R. Makgwaneet al., Efficient room temperature oxidation ofcyclohexane over highly active hetero-mixed WO3/V2O5oxide catalyst, Catal. Commun.,54, 118123. Copyrights (2014), with permission from Elsevier.48

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support, adsorbent, sensor and an excellent inorganic template for thesynthesis of nanostructures.60 Zhenget al. reported a series of MCM-48supported 12-tungstophosporic acid mesoporous materials that weresynthesizedviaa wet impregnation method. The characterization resultsindicated that the mesoporous phase of MCM-48 supports remainedalmost unchanged upon the HPW loading, while the long-range order

decreased noticeably. Furthermore, HPW/MCM-48 was proved to be anefficient catalyst for the green synthesis of benzoic acid with aqueoushydrogen peroxide as oxidant and the 35 wt% loading of HPW proved tobe optimal under the system.61 Meanwhile, our group firstly developed anovel W-containing MCM-48 catalyst under hydrothermal conditionsviapH adjustment using tetraethoxysilane (TEOS) as Si source, Na2WO4as Wsource and cetyltrimethylammonium bromide (CTAB) as the structure-directing template.62 The as-synthesized material showed the typicalstructure of MCM-48. WO3species were highly dispersed into the lattice

of the bulk and might be imbedded separately, which could be served asthe active centers for the selective oxidation of CPE to GA. The FT-IR-pyridine adsorption confirmed the presence of strong Brnsted acid sitesand Lewis acid sites upon incorporating tungsten oxide species into theMCM-48 materials, which were beneficial to the catalytic performance.The optimal tungsten content was 20 wt% and the GA yield over thiscatalyst exceeded 66%, higher than those over commercial and xerogelsilica supported WO3 catalyst or WO3/TiO2-SiO2 catalyst, suggesting itspromising potential use in industry.

Siliceous mesocellular foams (MCFs) with well-defined ultra largemesopores and hydrothermally robust frameworks were first describedthrough an oil-in-water microemulsion method by Winkel et al.63,64 MCFmaterials have gained many benefits from such a facilitated synthesismethod such as well defined pore and wall structure, thick walls, andhighly hydrothermal stability, which give MCFs unique advantages ascatalyst supports. Cao et al. developed high-quality mesoporous W-MCFmaterials featuring a well-defined three dimensional (3D) mesoporosityand ultra large mesopores with different Si-to-W ratios via a directhydrothermal method.65A high tungsten content up to 20 wt.% could be

well incorporated into the framework of the MCF material. The W-dopedMCF materials appeared to be suitable as catalysts in the selectiveoxidation of 1,3-butanediol to 4-hydroxy-2-butanone in the hydrogenperoxide system, as shown in Scheme 4. The catalyst exhibited a highactivity and an extremely high stability as a function of the test and reusedue to the presence of three dimensional mesocellular networks withultralarge mesopores which favors the diffusion of reactants andproducts. Our group also studied WO3-containing mesocellular silicafoam catalysts.66 These catalysts were synthesized viaan in situmethod

by using TEOS and sodium tungstate as precursors via a traditionalimpregnation method. Both catalysts exhibited excellent performance forthe target reaction (O-heterocyclization). The ultra large mesopores of thecatalysts were helpful for the transport of the large raw material andproducts during the reaction. Meanwhile, the recycling experimentresults indicated that the in situ method-derived catalyst showed much

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better stability than the impregnation one, suggesting that the tungstenoxide on the catalyst synthesized by the impregnation method was moreeasily aggregated after the reaction than the one on the catalyst syn-thesized by the in situ method. Whats more, our group presented thatthe ammonium acetate-treated WO3-MCF (AMA-treated WO3-MCF) cata-lysts exhibited good performance and retained the special structure ofthe supports under special pre-treatment conditions.67 The recyclingexperiment demonstrated the excellent stability of the AMA-treated

WO3-MCF catalyst. Clearly, the tungsten clusters on the surface were

single-site {WO4} species bound strongly to silica through WOSicovalent bonds, as illustrated in Scheme 5.

Hexagonal mesoporous silica (HMS), commonly synthesized by theassembly pathway of hydrogen-bonding interactions between neutralprimary alkylamine and neutral inorganic precursors at room tem-perature,68 has high surface area and large and uniform pore size.

Scheme 4 The oxidation process of 1,3-butanediol by H2O2. Reprinted from Y. Su et al.,Tungsten-containing MCF silica as active and recyclable catalysts for liquid-phaseoxidation of 1,3-butanediol to 4-hydroxyl-2-butanone, Appl. Catal. A: Gen., 315,91100. Copyrights (2006), with permission from Elsevier.65

Scheme 5 Incorporating catalytic oxotungsten tetrahedra centers into the framework ofMCF. Reprinted from R. H. Gao et al., High-activity, single-site mesoporous WO3-MCFmaterials for the catalytic epoxidation of cycloocta-1,5-diene with aqueous hydrogenperoxide,J. Catal., 256, 259267. Copyrights (2008), with permission from Elsevier.67

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These features extend its applications in the field of catalysis, molecularsieving, and supports. Moreover, HMS possesses a much thicker frame-

work wall, smaller domain size with short channels, and larger textualmesoporosity.6971 These properties are distinguishable from those ofMCM-41, and provide better transport channels for reactants to accessthe active centers and better diffusion channels for products to move out

than their MCM-41 analogs. Our group studied the tungsten-containingHMS via dodecylamine as template at room temperature.72 The as-prepared material was very active as a catalyst for the selective oxidationof CPE to GA with environmentally benign hydrogen peroxide as theoxidant. Complete conversion of CPE and very high yield of GA (76%)

were obtained over the W-HMS catalyst with a Si/W molar ratio at 30.Furthermore, almost no tungsten species were leached into the reactionsolution, enabling the catalyst to be employed for many reactioncycles without dramatic deactivation. Meanwhile, our group reported a

novel and green route for the selective oxidation of CPE oxide to GA byusing aqueous H2O2 as the oxidant and WS2@hexagonal mesoporoussilica (WS2@HMS) material as the catalyst, which displayed a very largesurface area, high efficiency, excellent selectivity and outstandingreusability.73

SBA-15, a type of ordered mesoporous material achieved by using atriblock copolymer as template under strongly acidic conditions, is apromising candidate in catalysis since it possesses a high surface area(6001000 m2 g1) and uniform tubular channels with tunable porediameters in the range of 530 nm.74 Our group firstly demonstrated theuse of W-doped SBA-15 catalyst prepared by a novel in situ synthesismethod as highly efficient catalyst for the direct production of GA viaselective oxidation of CPE by using non-aqueous hydrogen peroxide asthe green oxidant.75 Obviously, the morphology and structure still re-mained unchanged after WOx species doping. The heterogeneous

W-doped SBA-15 catalyst exhibited an excellent activity and selectivity forthe selective transformation of CPE to GA and the presence of a highsurface concentration of WOx species dispersed on well orderedhexagonal pore walls of SBA-15 support was essential to the superior

performance of the catalyst for the selective oxidation of CPE. Jia et al.developed mesoporous SBA-15 materials modified with oxodiperoxotungsten complexesviaa post-grafting route as efficient catalysts for theepoxidation of olefins with hydrogen peroxide.76 The preparation ofhybrid mesoporous SBA-15 materials was presented in Scheme 6.Compared with the catalytic properties of hybrid SBA-15 materialscontaining different ligands, the catalyst bearing pyrazolylpyridine ligandexhibited relatively high recoverability, stability and very high efficiencyof H2O2 utilization under optimized conditions. The catalytic activity

could be further improved by using solvent mixtures of CH3CN andCH3COOH at a temperature as low as 35 1C. Furthermore, reasons forimproving catalytic performance of the hybrid material were discussedand this phenomenon could be attributed to a suitable coordinationinteraction between the chelate ligand and the WO(O2)2fragment and thestructure and surface properties of SBA-15 support.

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Scheme 6 Schematic representation for the preparation of hybrid mesoporous SBA-15 materials. Reprinted modified with oxodiperoxo tungsten complexes as efficient catalyss for the epoxidation of olefins with hydroCopyrights (2009), with permission from Elsevier.76

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In this section, several heterogeneous silica-supported WO3 catalystshave been designed that show good performance to the target greenoxidation reaction. However, due to the complexity of the preparationprocedures, the need for expensive raw materials and the difficulties oflarge-scale production, the synthesis of new support materials is stillunder development. During the past few years, novel carriers such as

sustainable carbon materials (including polymeric graphitic carbonnitride, graphene, ordered mesoporous carbons) have attracted con-siderable attention. So far, there are no reports on the application ofsilica-supported WO3 catalysts to the photocatalytic or electrochemicalapplications.

3.3 W-based catalyst supported on sustainable carbon materialsCarbon material remains one of the most attractive and well-studied

material systems in the scientific community due to its amazing varietyand versatility in combination with low cost, availability, and wide ran-ging properties. The physical chemical, optical and electronic propertiesof carbon materials vary according to its allotropic form and also greatlydepend on its structure, morphology and surface composition. Highsurface area carbon materials have been extensively used for sorption,sensing, photovoltaic, catalysis and storage applications.7781

Polymeric graphitic carbon nitride (g-C3N4), an appealing and potentialmaterial, has recently received considerable attention. It has been widelyused in photocatalysis, fuel cells and gas storage, by virtue of its variousphysicochemical properties, such as excellent thermal and chemicalstability, nontoxic, electrical conductivity, energy storage and gasadsorption. Simultaneously, g-C3N4 is a p-conjugated polymer semi-conductor with a layered structure formed by tri-s-triazine constructionunit and an optical band gap of 2.7 eV, which has shown some photo-catalytic activity under visible light. Nevertheless, its specific surface areais small, and the recombination of photo-generated electron-hole pairs ishigh, which restrict the further improvement of the catalytic activity.82

WO3is also a semiconductor material with an optical band gap of 2.7 eV,

and it exhibits photocatalytic activity under visible light.83 Since theoptical band gap of WO3is almost the same as that of g-C3N4, WO3andg-C3N4 are simultaneously excited. Therefore, it is expected that thecomposite structure may improve the photocatalytic activity of g-C3N4by increasing the number of photogenerated electronhole pairs.84

WO3/g-C3N4 composites have been attracting much attention in recentyears, especially in the field of photocatalysis. Li et al. reported a novelWO3/g-C3N4 composite photocatalyst via a calcination method.

85 Thehighest MB (methylene blue) and 4-CP (p-chlorophenol) degradation

efficiency for the WO3/g-C3N4 (9.7%) composite respectively reached upto 97% and 43% under visible light irradiation, while pure g-C3N4 onlyinduced 81% degradation of MB within 3 h and 3% degradation of 4-CP

within 6 h. A possible mechanism of the visible light activity of aWO3/g-C3N4 catalyst was proposed and illustrated in Fig. 8. Katsumataet al.prepared g-C3N4/WO3composites by a physical mixing method and

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Fig. 9 Possible reaction mechanism of cycloalkene oxides over WO3/g-C3N4 compos-ites. Reprinted from J. Ding, Carbon nitride nanosheets decorated with WO3 nanorods:Ultrasonic-assisted facile synthesis and catalytic application in the green menufacture ofdialdehydes, Appl. Catal. B: Environ., 165, 511518. Copyrights (2015), with permissionfrom Elsevier.87

Scheme 7 Possible degradation mechanism of CH3CHO over a g-C3N4/WO3compositephoto catalyst under visible light irradiation. Reprinted from K. Katsumata, Preparation ofgraphitic carbon nitride (g-C3N4)/WO3 composties and enhanced visible-light-drivenphotodegeneration of acetaldehyde gas, J. Hazard. Mater., 260, 475482. Copyrights(2013), with permission from Elsevier.86

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WO3RGO (WO3-reduced graphene oxide) nanocomposite by the directhydrothermal growth of WO3 nanoplates on FTO substrates andsubsequent in situ photo reduction to deposit RGO layers on the WO3nanoplate surface.92 Photo anodes made of the WO3RGO nanocom-posites showed an enhanced photocurrent of 2.0 mA cm2 at a bias of1.23 Vvs. RHE, which was higher than that of the pristine WO3 nano-

plates and among the best values reported for the hydrothermallysynthesized WO3-based photoanodes. This improved photocurrent wasattributed to both the reduced charge recombination in ultrathin WO3nanoplates and enhanced charge transfer at the electrode/electrolyteinterface. Devadoss et al. presented the fabrication of grapheneWO3membranes as a potential alternative photoanode in photoelectron-chemical glucose sensing applications and the possibility of fortifying itsphotocatalytic activity using plasmonic gold nanoparticles.93 The photo-electron chemical tests illustrated that the performance of AuNPs

supported on grapheneWO3 membrane was superior to other systemswithout backbone conducting channel. The mechanism of glucose oxi-dation at the grapheneWO3Au hybrid membrane modified with glu-cose oxidase (GOD) enzyme was depicted in Fig. 10.

Ordered mesoporous carbons (OMCs), by virtue of their appealingstructural characteristics, such as periodic and uniform mesopores and

Fig. 10 Schematic representation of (a) grapheneWO3Au triplet junction for glucosesensing, (b) energy levels at grapheneWO3Au photoelectrode under light illuminationand glucose oxidation mechanism. Reprinted from A. Devadoss, P. Sudhagar, S. Das, S. Y.Lee, C. Terashima, K. Nakata, A. Fujishima, W. Choi, Y. S. Kang and U. Paik,ACS Appl. Mater.Interfaces, 2014, 6, 4864. Copyright (2014), with permission from American ChemicalSociety.93

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large surface area, have been extensively studied in various applications,such as electrochemistry, photocatalysis, catalysis and so on. Meanwhile,mesoporous carbons with 3D pore structures can also promote the masstransport of both reactants and by-products. He et al.developed a simpleself-assembly route to prepare ordered mesoporous C-WO3 nanocom-posites with large surface area and excellent corrosion resistance.94 Theabove detailed preparation process for OMC/WC was schematically de-

scribed in Fig. 11. Tungsten oxide grew in the form of rods as a result ofthe confinement effect of the porous structure, improving graphitizationdegree and hydrophilicity. Meanwhile, the ordered mesoporous CWO3nanocomposites, as electrocatalyst carriers for PEMFC, displayed su-perior electrocatalytic activities compared with pure ordered mesoporouscarriers. Wanget al. firstly presented a facile soft-template synthesis oforder 2D hexagonal mesoporous tungsten carbide (OMC/WC) compositenanomaterials with a surface area of 538 m2 g1.95 The Pt nanoparticlessupported on OMC/WC revealed a better performance than that of the

commercial PtRu@C catalyst for methanol electro-oxidation.

4 Conclusion and outlook

In this contribution, some recent promising examples among tungstencontaining materials, which have already been extensively applied in the

Fig. 11 Schematic illustration of the formation process of highly ordered mesoporousC-WO3 films. Reprinted from ref. 94, with permission from The Royal Society ofChemistry.

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field of green catalytic oxidation, have been presented. Tungsten-basedcatalyst, as a promising material for green catalytic oxidation process,such as selective oxidation of unsaturated compounds, photocatalyticoxidation of organic pollutants and electrochemical catalytic oxidation inthe field of proton exchange membrane fuel cell, exhibits excellentcatalytic activity, chemical and thermal stability and efficient separation

ability from the solvent mixture. However, considerable efforts are stillrequired to solve the following issues.

(i) More efficient, stable and cheap catalysts should be exploited tofulfill the catalytic oxidation processes. Extensive studies have beenconducted to investigate metal oxides-supported WO3and found that thesynthesis of these materials required expensive raw materials, similar tothe silica-supported WO3 catalysts. Meanwhile, the complexity of thepreparation procedures and the difficulties of large-scale productions ofsilica-supported WO3 catalysts have limited their wide applications.

As novel supports, porous carbon materials will be an alternative andinteresting candidate in the applications of catalytic oxidation process,especially in the conventional green catalytic oxidation processes.

(ii) The application scope limits of some tungsten containingmaterials are still far from being reached. For example, few literatureshave been reported on tungsten-based catalyst supported on sustainablecarbon materials in conventional catalytic oxidation including selectiveoxidation of unsaturated organic substrates.

(iii) In the presence of hydrogen peroxide oxidant, the leaching of

active tungsten species and deactivation of the W-based catalyst in theoxidation process are big challenges for industrial application. Till now,no commercial tungsten-based heterogeneous catalysts are found, thusresearch on tungsten-based material with high activity and stability andits industrial application may be the hot point in this area.

In summary, it is reasonable to believe that new catalysts (besidescarbon materials) and/or green catalytic processes based on tungsten-based material will continue to be explored in the near future.

Acknowledgements

This work was financially supported by the Major State Basic ResourceDevelopment Program Grant No. 2012CB224804) and NNSFC (Project20973042, 21173052, 21373054).

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regardless the preparation method.2 It should be noted that the micro-structural features are influenced by the processing route for manu-facturing of the porous material.

Cellular ceramics are divided into foams, honeycombs, connectedrods, connected fibers, hollow spheres and bio-template structures.3

According to the unrivaled properties of ceramic foams like high per-

meability, high external surface, high temperature stability and goodthermal shock resistance, they have a wide range of applications. Theapplications of cellular ceramics include filtration of molten metals andhot gases, thermal and acoustic insulation, light-weight structural com-ponents, support for fuel cells, electrodes, sensors, bioreactors, radiantburners as well as scaffolds for bone replacement.47 In addition, ceramicfoams have broad applications in different catalytic processes as catalystsupports. They have crucial benefits including high yield and selectivity,improved temperature control and heat management. Ceramic foams

have shown several advantages over packed beds of catalyst particleswhich have made them an interesting field of study for different re-actions. The main advantages of ceramic foams are (1) the ability tomatch the shape and size of the reactor for easier loading of long andnarrow tubes; (2) reduced pressure drop relative to packed beds, savingenergy costs; (3) higher effectiveness factor because of higher externalsurfaces; and (4) enhanced heat transfer, avoiding hot spots and allowingbetter reactor stability for highly exothermic reactions.8 In addition tothese advantages, many other benefits of structured catalyst supports toovercome technical problems are reviewed by Moulijn et al.9

Pressure drop and heat transfer are the main transport properties ofthe foams. Many efforts have been made aiming to study the pressuredrop in ceramic foams such as investigation of the effect of differentparameters on pressure drop.10,11 Basically, pressure drop in ceramicfoams depends on the fraction of the pore volumes between the struts.Richardsonet al. however, showed that changes in texture of the surfacee.g., by washcoating the surface, influence the pressure drop in ceramicfoams implying the pressure drop in foams also depends on the physicalcharacteristics of the surface.12

Enhanced convection in ceramic foams due to turbulence in tortuouspores and radiation between the struts leads to a better heat transferin foams compared to fixed bed reactors. For instance, Peng andRichardson developed a suitable radial heat transfer correlation for onedimensional reactor model.13 The reactor contained 30 PPI a-Al2O3cer-amic foam. The authors compared the heat transfer between equivalentceramic foam and particle beds. It was shown that the ratio of heat transfercoefficient to particles increased. The increase is even higher at higherReynolds numbers. In addition, they also found that washcoating of the

foams with 30 PPI of pore density increases the heat transfer coefficient.There are many heat transfer limited operations including bothendothermic and exothermic reactions. Twigg and Richardson compileda list of these reactions including partial oxidations, alkylations, oxy-chlorination, hydrogenation and dehydrogenations, for which ceramicfoams as catalyst supports could be beneficial (Table 1).14

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FischerTropsch synthesis is one example of complex and exothermicreactions which can be successfully done by using foams. Brown et al.compared Co/g-Al2O3foam catalyst with powders and pellets.

15A highereffectiveness factor and reaction rate were achieved for the catalystfoams. Other catalytic applications of ceramic foams have been reportedfor exothermic reactions such as steam reforming, especially methanesteam reforming and ethylene epoxidation using Rh/g-Al2O3 and Ag/g-Al2O3for which a higher reaction rate and turnover frequency comparedto catalysts in powder form were achieved.13,15,16

Apart from applications dealing with transport properties, most ap-plications of foam catalysts are related to chemistry that occurs on thesurface of the struts. These applications include: selective CO oxi-dation,17 solar methane reforming with CO2,

18 methanol to olefins,19

catalytic combustion20 and oxidative dehydrogenation for which desir-able intermediates are produced in consecutive reactions.21

Ceramic foams with high porosity are produced using one of the threemain processing routes: replica technique, direct foaming and sacrificialtemplate. The manufacturing methods differ in terms of processing

features and final properties of the foams. Majority of ceramic foams isproduced by the replica technique. The replica method was reported bySchwartswalder and Somers in 196322 and since then it has been utilizedin several studies devoted to the preparation of ceramic foams. Thismethod consists of slurry preparation, pretreatment of the polyurethane(PU) foam in order to adhere more slurry to the surface, impregnation of

Table 1 Classification of industrial catalytic processes for which ceramic

foams could be beneficial.14 Reproduced from M. V. Twigg and J. T.

Richardson, Theory and Applications of Ceramic Foam Crystals. Chem.

Eng. Res. Des.,80. Copyright (2002) with permission from Elsevier.

Type Example

Exothermic nature

Partial oxidation Ethylene to ethylene oxidePartial oxidation o-Xylene to phthalic anhydridePartial oxidation Propene to acrylic acidPartial oxidation Butane to maelic anhydridePartial oxidation Methanol to formaldehyde

Alkylation Benzene to ethyl benzeneAlkylation Diethylbenzene or cumeneOxidative rearrangement Water gas shift Oxychlorination Acetic acid to vinyl acetateOxidation Ethylene to ethylene dichlorideHydrogenation Methanol synthesis

Hydrogenation Methanation of CO/CO2Hydrogenation FischerTropsch synthesisEndothermic nature

Dehydrogenation Ethylbenzene to styreneDehydrogenation Cyclohexane to benzeneOxidation Cyclohexane to cyclohexanoneDehydrogenation Butanol to methyl ethyl ketoneSteam reforming Natural gas to synthesis gasSteam reforming Naphtha to synthesis gas

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PU foam by dipping or even spraying with ceramic slurry23 drying of thefoam and finally sintering of the ceramic particles. In addition, duringsintering the polymer template is burnt out by applying a high tem-perature to get the final material with the desired properties.3

There are different templates which can be used as cellular structure inreplica technique. These templates can be either synthetic including

polymeric24

and carbonic foams25

or even natural ones such as coral26

orwood.27

Replica technique is a well-established method in order to producecellular ceramics. With this technique open porosity up to 90% with cellsizes ranging from a few hundred micrometers to several millimeters isachievable. This method is simple and flexible because it is applicablefor any ceramic material which can be dispersed into a suspension.Moreover, this method is cost effective without toxic emissions

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