Selective oxidation of alcohols by supported gold ... · nanoparticles: recent advances Anuj S. Sharma, Harjinder Kaur* and Dipen Shah Industrially, oxidation of alcohols is very

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Selective oxidatio

ASaCG(PaopuHD

Gujarat University, Ahmedabad (Ion the DST Nanomission projectguidance of Dr Harjinder Kaur ainterest is in the nanoparticles and

Department of Chemistry, School of Science

E-mail: [email protected]; Fax: +91 79 2

Cite this: RSC Adv., 2016, 6, 28688

Received 4th December 2015Accepted 6th March 2016

DOI: 10.1039/c5ra25646a

www.rsc.org/advances

28688 | RSC Adv., 2016, 6, 28688–287

n of alcohols by supported goldnanoparticles: recent advances

Anuj S. Sharma, Harjinder Kaur* and Dipen Shah

Industrially, oxidation of alcohols is very important as they are starting materials for a variety of ketones,

aldehydes, acids, etc. required to produce plastics, detergents, paints, cosmetics, food additives and drug

intermediates. The potential of gold nanoparticles (AuNPs) in oxidation, in general, is immense owing to

its tendency to dissociate dioxygen. Supports play a vital role not only in the synthesis and stabilization of

nanoparticles, but can also look after the issues of sustainability by facilitating separation, recycling and

recovery of catalyst. In the oxidation of alcohols, supports also control the outcome of the reaction. This

review aims to systematically discuss the recent developments in the selective oxidation of alcohols

catalyzed by supported gold nanoparticles. We draw attention to the impressive progress that has taken

place in the oxidation of alcohols in the last thirteen years, leading to several green, sustainable and

highly efficient protocols. The effect of a support on the remarkable properties of gold nanoparticles in

terms of its catalytic activity, selectivity, recyclability, and stability is discussed at length. Mechanistic

aspects of these conversions are discussed in brief.

1. Introduction

Bulk gold typically exhibits a very poor catalytic activity as it isstable and inert in comparison to other transition metalspresent in the periodic table. Aer the discoveries that gold inthe nanoform is a catalytically active and effective green catalystfor the oxidation of CO and the hydrochlorination of ethylene atlow temperature by Haruta and Hutchings respectively, a lot of

nuj S. Sharma received his B.c. degree in Chemistry in 2011nd M. Sc. Degree in Organichemistry in 2013 from theujarat University, AhmedabadIndia). He completed his M.hil. in 2014 and submitteddissertation entitled “A Studyf oxidation reaction by sup-orted metal nanoparticles”nder the supervision of Drarjinder Kaur at Chemistryepartment, School of Science,ndia). Currently, he is workingand pursuing Ph. D. under thet same institute. His researchtheir catalytic activity.

s, Gujarat University, Ahmedabad, India.

6308545; Tel: +91 79 26300969

27

research has been devoted to its use in various oxidation reac-tions.1–7 Literature shows that catalysis by supported gold is oneof the fastest growing subjects in catalysis science today anda signicant portion of the work has been devoted to catalyticoxidation of organic compounds such as alcohols, polyols,carbonyl compounds, alkenes etc.8–11

Oxidation reactions using gold are of relevant interest as ithas been shown that oxygen gets chemically adsorbed onnanogold and thus, provides green processes based on the useof stable, selective and non-toxic heterogeneous catalysts withair or pure O2 as eco-friendly oxidant.12–19 The subject of

Harjinder Kaur completed post-graduation in chemistry fromKurukhetra University, Kuruk-shetra, Haryana, India and Ph.D. from Gujarat University,Ahmedabad, Gujarat, India. In2006, she joined chemistrydepartment as faculty aera brief stint at Nirma Institute ofTechnology, Ahmedabad. She isactively involved in the devel-opment of nanocatalyst fordeveloping green protocols. Her

research interests involve polymer–inorganic hybrid materials,supported nanoparticles, microwave assisted nanocatalysis inoxidation, hydrogenation and C–C coupling reactions.

This journal is © The Royal Society of Chemistry 2016

CrossMark:http://crossmark.crossref.org/dialog/?doi=10.1039/c5ra25646a&domain=pdf&date_stamp=2016-03-17

http://dx.doi.org/10.1039/c5ra25646a

http://pubs.rsc.org/en/journals/journal/RA

http://pubs.rsc.org/en/journals/journal/RA?issueid=RA006034

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. View Article Online

supported gold nanoparticle (AuNPs) catalysed oxidation hasbeen reviewed many times in the literature. The review reportedby Rossi and co-workers dealt with the entire work of selectiveoxidation conducted by supported gold.20 Aer that a criticalreview by Davis and co-workers illustrated the entire eld ofoxidation conducted by all kind of supported metal nano-particles.21 But, with burgeoning new reports on oxidation, it isimperative to focus on a particular group. Alcohols belong to animportant class of industrial chemicals that are highly usefulstarting materials for a variety of chemicals, intermediates,plastics, detergents, paints, cosmetics, food additives, phar-maceuticals etc.22 There is a huge potential for new catalyticsystems that can efficiently lead to conversion of alcohols tocorrespondingly aldehydes with remarkable selectivity, lowerenergy requirements and without using hazardous reagents.This is evident from the fact that 50% of the reports on AuNPscatalysed reactions are related to oxidation of alcohols. Themain objective of this review is to summarize the developmentin this eld.

2. Oxidation of alcohols

Selective oxidation of primary alcohols to aldehydes andrestricting its over oxidation to acids is an important issue.23–27

Various chromium reagents like chromic oxides,28 Jonesreagent,29 PCC (pyridium chlorochromate),30 PDC (pyridiumdichromate),31 activated DMSO (Swern oxidation),32 carbodi-mide (Ptzner–Moffatt oxidation),33 2-iodoxybenzoic acid,34

TPAP in presence of excess of NMO (Ley oxidation),35 TEMPO inthe presence of excess bleach (NaOCl) (oxoammonium-catalyzed oxidation),36 Sarett reagent,37 Collins reagent38 etc.have been investigated to accomplish this aim. These reagentsand salts are not only toxic but also produce voluminousamount of waste at the end of the reaction. They are also notsuitable for highly acid sensitive organic compounds. To over-come these drawbacks research has been directed towards thedevelopment of catalytic oxidation reactions. Many homoge-nous catalysts with well-dened active sites that provide rela-tively high activity and selectivity have been synthesized and

Dipen Shah was born in Ahme-dabad, India in 1987. Hereceived his B. Sc. degree(Chemistry-2007) and M. Sc.degree (Organic Chemistry-2009)from Gujarat University, Ahme-dabad (India). He received his M.Phil. (2011) and Ph. D. (2016)under the supervision of Dr Har-jinder Kaur from GujaratUniversity, Ahmedabad.Currently, he is working as anassistant professor in CCSIT-

Junagadh, Gujarat (India). His research interests are in the area ofadvanced nanomaterials, catalysis, Microwave assisted organicsynthesis and green chemistry.


reported e.g. Ru-ionic liquid@silica gel,39,40 Co(II), Ni(II), andCu(II) Schiff base complexes@zeolites,41 oxo vanadiumcomplexes,42 Cu–Py,43 Cu–Phen,43 Cu–Bipy,44 CuCl–TEMPO,45

Cu–diimine,46 and CuCl–DEBCO,47 Fe(II)–H2O2,48 CuBr,49 phos-phinite–ruthenium(II) complexes,50 etc. Peroxides and molec-ular oxygen were reported as oxidants. Major disadvantagesassociated with homogenous catalysts are their poor recycling,stability, and handling in industrial processes. On the otherhand, heterogeneous catalysts can easily overcome these prob-lems. Designing a highly active and selective heterogeneouscatalyst with the selectivity of homogeneous catalyst is a chal-lenge which has been overcome by the use of nanoparticles inthe recent times. Nanoparticles of many transition elementslike Ag, Pd, Ru, Au, Pt, Cu, etc., have been reported for theoxidation of aliphatic and aromatic primary alcohols.51–64

Among these, AuNPs have become favorites to researchers notonly due to their novelty but also due to several advantages.65

These benets involve the ability of gold to split elementaloxygen, in general nontoxic nature of gold and its excellentrecyclability. In this review, we present growing amount offundamental research dealing with selective oxidation of alco-hols with special reference to the role of support.

3. Type of support

Many natural and synthetic materials such as zeolites, metaloxides, activated carbon, carbon nanotubes (CNT), polymers,resins, proteins, rocks, biological materials, clays, ceramics,metal organic framework (MOFs) and membranes have beenused as support to stabilize metal nanoparticles. They have theability to control the particle growth and reduce the particleaggregation during reaction.66–69 One of the signicant featuresof gold catalysis is the dependence of its activity on the size andmorphology of nanoparticles and thus, supports play a crucialrole in the activity of catalyst (Fig. 1).70–74 Role of a support innano catalysis is manifold:

� They help in facile synthesis of nanoparticles and preventtheir agglomeration.

� They provide a framework for anchoring the catalyst thatfacilitates separation and recycling.

� They inuence the microenvironment of the reactants andenhance the reaction rates.

As far as the synthesis of supported nanoparticles is con-cerned, several approaches are currently available. Amongthem, three extremely useful methods are (1) impregnation, (2)deposition precipitation, and (3) immobilization. In theimpregnation method, the metal salts are dispersed in theexisting pores of support and then reduced. The main dis-advantage associated with this method is the difficulty in

Fig. 1 Schematic diagram of supported gold nanoparticles.

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Fig. 2 (A) Schematic representation of synthesis of MOFs, (B) attractive MOF applications [reprinted with permission from ref. 88. Copyright(2015) Royal Society of Chemistry].

Fig. 3 Schematic preparation of Au clusters@PCP or MOF-5 [reprin-ted with permission from ref. 90. Copyright (2008) John Wiley andSons].

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producing gold nanoparticles of required size unless thesupport has well dened porous structure. In depositionprecipitation method, the metal hydroxide or metal hydrateoxide is generally deposited on the surface of the support byincreasing the pH of the solution of metal salts in which thesupport material is added gradually. The obtained precipitatemay be nucleated by the surface functional groups, simulta-neously allowing the active phase to attach to the support orreduced aerwards. In immobilization method, gold nano-particles are rst synthesized and then dispersed on thesupport. This method is more commonly used for the prepa-ration of AuNPs on inorganic supports as compared to the othertwo methods.75–79 Literature survey revealed that supports playa crucial role in the performance of gold nanoparticle catalysedoxidation of alcohols. The same quantity of gold deposited onvarious supports can show different activity towards towardsame organic reaction and thus a wide variety of supports hasbeen studied. A summary is presented here.

Fig. 4 XPS spectrum in the Au 4f region of Au@UiO-66-NaBH4

[reprinted with permission from ref. 95. Copyright (2015) Royal Societyof Society].

3.1 Metal organic frameworks (MOFs)

Metal organic frameworks (MOFs) are crystalline porous solidsformed by either transition metal ions or clusters of metal ionthat occupy nodal positions in a crystalline framework.80–84 Themost attractive feature of MOFs is the ability to use minimumamount of metal ions and organic ligands to build maximumsurface area with predictable, controllable, tailorable, and post-modiable pores and cavities (Fig. 2). MOFs are a new class ofmaterials at the interface of heterogenous catalysts and supra-molecules. Excellent thermal stability of MOFs and narrow poresize make them excellent support for the stabilization of metalnanoparticles. The reviews reported by Zhang et al., Su et al. and

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Corma et al. provide the whole work on MOFs and their variousapplications.85–89

MOF-5, ZIF-8, ZIF-90, MIL-101, MCM-22 and UiO-66 are thecommon metal organic frameworks (MOF) that has been usedas support for the synthesis of AuNPs. Gold salt precursors suchas Me2Au(acac), HAuCl4 and ClAuCO has been used to prepareAu NPs with MOFs themselves acting as stabilizing materials asthey have inherent cavities suitable for the nucleation ofnanoparticles. Chemical vapour deposition (CVD),90 depositionreduction,90 in situ synthesis of MOF by solid grinding,90

impregnation (IP)91 and solution inltration92 are some of themethods used for the preparation of Au NPS@MOFs. In 2008,Haruta et al. reported Au clusters deposited onto porous MOF-5by solid grinding method (Fig. 3) without using any additionalstabilizing agent. However, the Au clusters though less than 2nm dimension were found to be bigger in size as compare topore size of MOF-5 and were mostly found on the outer surfacesof the support (Fig. 3).90 Microporous sodalite-like ZIF-8 (zeoliticimidazolate frameworks-8) and ZIF-90 (zeolitic imidazolateframeworks-90) with pore diameter of 1.2 nm and quite smallpore windows of 3. 5 A were used by Fischer et al. to impregnatepreformed AuNPs of size 1 to 5 nm in 2010. They reported betterstability of nanoparticles in ZIF-90 and owed it to the bettermatching of the nanoparticle size with the pore size of thesupport.91 In the same year, Li et al. and Tang et al. in



Fig. 5 (A) Schematic preparation of the Au@MOF material. (B) HRTEM images of Au@UiO-66 (NH2) [reproduced from ref. 92 with permissionfrom the Royal Society of Chemistry].

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a collaborative work reported Au@zeolite-type MOF (MIL-101)by using colloidal deposition (CD) method, impregnationmethod (IP) and deposition–precipitation (DP) method. MIL-101 has a crystalline framework with two kinds of quasi-spherical cages of 29 A and 34 A with a total surface area 4100� 200 m2 g�1. Polyvinyl pyrrolidone (PVP) was used as pro-tecting agent for the stabilization of gold nanoparticles in caseof colloidal deposition method. HR-TEM study revealed smallspherical gold nanoparticles of <3 nm size in the pores of MIL-101.93

A novel, innovative and highly efficient post functionaliza-tion method was developed by Gascon et al. for introducing

Fig. 6 Turnover frequencies for the oxidation of benzyl alcoholmeasured at 0.25 h [reprinted with permission from ref. 93. Copyright(2010) American Chemical Society].


additional metal functionalities in MOFs through oxamate aschelating agent and triethyl amine as reducing agent. Theamine functionalized MOF thus synthesized, was impregnatedwith AuNPs by sorption reduction method. From the TEManalysis, the author suggested that the stabilized gold nano-particles were highly dispersed with average size of z2.5 nm.However, in the absence of chelating agent bigger Au NPs wereobserved. Bimetallic gold and copper nanoparticles weresynthesized by Ziolek and co-workers on MCM-22. The synthe-sized bimetallic catalyst was characterized by using differenttechniques such as XRD, N2 adsorption/desorption, TEM, SEM,UV-visible, H2-TPR, FT-IR and ESR spectroscopy. The averagesize of Au NPs was found to be 21 nm. It was reported that thestability of gold on the MCM-22 surface was improved by theaddition of copper species.94

Au NPs supported on MOF UiO-66 were also reported byVoort and co-workers by sorption reduction method. Reductionof precursor with NaBH4 as well as H2 was tried and the averageAu NP size for the Au@UiO-66 by NaBH4 and Au@UiO-66-H2

was found to be 7 nm and 5 nm respectively. The stability andsize of stabilised AuNPs was determined by high-angle annulardark eld scanning transmission electron microscopy (HAADF-STEM) and XPS analysis. The XPS spectrum of Au(0)@UiO-66-NaBH4 showed the Au 4f doublet at Au 4f7/2 at 83.9 eV and Au4f5/2 at 87.5 eV (Fig. 4). Moreover, two other peaks which wererecognized as metallic Au and Au1+ species were observed at83.8 and 85.1 eV (Au 4f7/2).95 Modied UiO-66 (NH2)

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Scheme 1 Oxidation of various alcohols by Au NPs@MIL-101 catalyst.

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impregnated with gold nanoparticles by sorption–reductionmethod was investigated by Wang et al. The group reported thatin presence of amino functional group UIO-66 rapidly coordi-nated with HAuCl4 which behaved as precursor. This led tobetter distribution of HAuCl4 over the outer as well as innersurfaces of the MOF resulting in well distributed gold nano-particles of average size 2.8 nm (Fig. 5). XPS analysis, conrmedthat the AuNPs were in Au(0) state.92

In the catalytic oxidation of alcohols by AuNPs supported onMOFs, very high activity was reported but interestingly selec-tivity for product was different from different supports. E.g. inthe oxidation of benzyl alcohol by AuNPs@MOF-5 usingmolecular oxygen as oxidant, selectivity was for methyl benzoaterather than benzaldehyde. Interestingly, in the absence of base,selectivity for benzaldehyde increased to 31% though conver-sion was only 69%. Similar selectivity for methyl benzoate wasalso reported for Au NPs supported on ZIF-8 and ZIF-90.91 Incase of base free aerobic oxidation of benzyl alcohol usingAu@MIL-101 (CD/PVP) catalyst selectivity was for benzaldehydewith a TOF value of 258 h�1. High dispersion of AuNPs and theelectron donation effect of aryl rings present in the large cagesof the MIL-101 support were reported as the reason for its highcatalytic activity. Further, in elaborate studies using differentstabilizing agents, it was observed that activity was related to thesize of nanoparticles (Fig. 6). The versatility of catalyst wasinvestigated and the authors attempted oxidation of variousderivatives of benzyl alcohol and hetero aromatic alcohols(Scheme 1).93 Similar results were reported for the base freeaerobic oxidation of alcohols with AuNPs@UiO-66 (NH2) i.e.94% selectivity for benzaldehyde at 100 �C in DMF.92

Among all MOF supported AuNPs, best results were reportedby the AuNPs prepared by Vroot and co-workers using UiO-66 as

28692 | RSC Adv., 2016, 6, 28688–28727

support that gave 100% selectivity for benzaldehyde with 94%conversion of benzyl alcohol. Though, TON value reported was35 h�1 and TOF was only 7.5 h�1. This catalyst was highlyrecyclable, stable and TEM analysis of the reused catalyst didnot show any signicant change in the Au NP size (8 nm) aerthe second runs.95

Besides, benzyl alcohol oxidation some other alcohols havealso been oxidised using MOF supported gold nanoparticles.Haruta et al. investigated the catalytic activity of MOF-5 stabi-lized gold nanoparticles for the oxidation of 1-phenyl ethanol.Using molecular oxygen 79% yield of acetophenone was ob-tained in methanol in the presence of base at 80 �C in 23 h.90

Bimetallic Cu–Au on MCM-22 surface was reported for oxida-tion of methanol. It was concluded that presence of copperenhanced the mobility of oxygen promoting the conversion ofmethanol and hence, Au–Cu@MCM-22 showed higher catalyticactivity for oxidation of methanol at 523–623 K as compared togold alone.94

MOF and metal oxide mixed supports were investigated forstabilization of gold nanoparticles by Lu and co-workers.Different metal organic frameworks such as MOF-5,ZnO@MOF-5, and TiO2@MOF-5 were used to produceextremely small AuNPs of size 1–3 nm. The authors reportedgood reactivity, selectivity and recyclability for the benzylalcohol oxidation.96,97

3.2 Carbon materials

Carbon is a classical support that has been used since longwhereas, associated materials such as carbon nanotubes(CNTs), graphene are very popular and contemporary. Recently,nanostructured carbon materials with well-dened porositiesand high surface area has been reported that can be



Fig. 7 TEM images (A, B, D and E) and metal particle size distribution histogram (C and F) for the self-assembled Au (SH)-SC catalyst. (G) HRTEMof a representative gold nanoparticle in (D) [reprinted with permission from ref. 105. Copyright (2013) American Chemical Society].

Fig. 8 (A) Preparation of AuNPs@thiol-Fru-d-NPS [reprinted withpermission from ref. 106. Copyright (2014) John Wiley and Sons].

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conveniently modied through different approaches to stabilizecatalyst.98 Prepared from mesoporous starch by controlledcarbonization carbon nanostructure has been reported for thepreparation of a wide range of supported metalnanoparticles.99–101

3.2.1 Activated carbon. Activated charcoal (AC) has beenimpregnated with gold nanoparticles by sol immobilization102

and wet impregnation103 methods. Generally, capping agentsare required to get uniform size of the nanoparticles.104 Zhangand co-workers aer supporting the nanoparticles on AC


removed the protecting PVP by washing. Specially synthesizedmesoporous carbon is another popular support for wetimpregnation of carbon nanoparticles.102 A novel one stepmethod to synthesize gold nanoparticles in mesoporouscarbon–silica support was reported by Wan et al. This materialwas synthesized by using a triblock copolymer Pluronic F127 asstructure directing agent, thiol group containing silane asa coordination agent, phenolic resin as a carbon source andHAuCl4 as gold precursor. TEM analysis, showed supportedAuNPs (average size 8.7 nm) inside highly stable mesostruc-tured carbon–silica material with hexagonally arranged pores(Fig. 7).105 In another novel approach, Shaabani and co-workersprepared a mesoporous carbon by hydrothermal method in theabsence of any kind of protecting agent and functionalized itwith thiol groups by treating with 3-thiol propyl triethoxy silane(Fig. 8). Au NPs were than impregnated by sorption reductionmethod using HAuCl4 as precursor and sodium borohydride asreducing agent.106

As far as the catalytic activity of gold nanoparticles towardsoxidation of alcohols is concerned, it was observed that thenanoparticles supported on mesoporous carbon were moreactive and stable compared to those stabilized on activatedcharcoal. Bimetallic nanoparticles107,108 were found to be moreactive than monometallic. But a detailed discussion of bime-tallic is beyond the scope of present review. AuNPs synthesised

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Scheme 2 Oxidation of some selected alcohols by Au NPs@thiol-Fru-d-NPs catalyst.

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in mesoporous carbon by Shaabani et al. showed excellentactivity in water toward benzyl alcohol oxidation using air asoxidant in the presence of a base (K2CO3). The best yield ofbenzaldehyde (>99%) was obtained aer 1 h. To evaluate theversatility of this catalyst, the authors investigated some otherderivatives of alcohols too (Scheme 2). Besides water, othersolvents such as acetonitrile, toluene, xylene, THF gave high yieldof benzaldehyde. They checked the activity of this catalyst usingvarious bases such as KOH, triethyl amine, and pyridine. Addi-tionally, the catalyst was reused six times without any signicantloss in its activity. Aer the sixth consecutive runs, the conver-sion of benzyl alcohol (100%) almost remained constant withslight decrease in selectivity of benzaldehyde (95%).106

The gold nanoparticles supported on mesoporous carbon–silica support synthesized by Wan et al. showed excellent cata-lytic performance in water at 90 �C and with oxygen as oxidant.The catalyst was stable, poison resistant and reusable with littleactivity loss due to metal leaching.105 Later on they removedsilica and resulting AuNPs were found to be even more activethan the earlier one. The catalyst displayed TOF value of 478 h�1

at 25 �C. Both these catalysts showed good conversion of benzylalcohol but selectivity towards benzaldehyde was poor and evenat very low temperatures benzoic acid was the major product.

3.2.2 CNT as support. Carbon nanotubes (CNTs) areanother carbon material that has been widely used as supportfor the metal nanoparticles.109,110 Their intrinsic propertiesconsisting of high surface areas, unique morphologies andhollow geometry make them extremely attractive as supports forheterogeneous catalysis.111–116 However, due to the inertness of

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the pristine CNTs, metal deposition requires activation. In2010, Prati and co-workers proposed a novel methodology forfunctionalising CNTs which included mild oxidation with nitricacid followed by amination by gaseous NH3. The material wasused to synthesize highly dispersed mono and bimetallic goldnanoparticles.117 Jiang and co-workers reported that introduc-tion of nitrogen groups on the surface of CNT increased thedispersion as well as stability of the metal nanoparticles owingto strong interaction between metal and nitrogen.118 Doris andco-workers in 2014 reported a layer by layer method to stabilizeAuNPs on coated CNT tubes. CNTs were functionalized by self-assembly of amphiphilic nitrilo triacetic-diyne lipid (DANTA)(Fig. 9). The DANTA organized itself as hemi micelles on thecarbon nanotube. Hence, nano ring type structures were formedwhere hydrophobic portion of DANTA was adsorbed on theCNTs by van der Waals type interactions and its hydrophilicanionic head was facing aqueous phase. This supported mate-rial displayed good activity for aerobic oxidation of alcohols atroom temperature in open ask (air).119 In another report by thisgroup, Au NPs of two different sizes were uniformly dispersedon these coated nanotubes and their activity for benzyl alcoholoxidation was evaluated. A clear effect of the size on the activityof catalyst was shown by these workers. They reported TOF of310 h�1 s-Au@CNT (size of Au NPs 3 nm) and it was 45 h�1 for l-Au@CNT (size of Au NPs 20 nm). The catalyst showed lowersintering rates and good recycling. However, poor selectivitytowards benzaldehyde was a major disadvantage.120

3.2.3 Graphene and graphene oxide. Graphene is a two-dimensional, single-atom thick sheet comprising of sp2-



Fig. 9 Synthesis of the Au NPs@CNT assembly [reprinted withpermission from ref. 119. Copyright (2013) Royal society of chemistry].

Fig. 11 XRD patterns of the Au/NG-x composites and NG (A). XRDpatterns of the Au/G-x composites and bare graphene (B) [reprintedwith permission from ref. 135. Copyright (2012) Royal Society ofChemistry].

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bonded carbon atoms arranged in honeycomb type structurewith high carrier mobility at room temperature (10 000 cm2 V�1

s�1), large surface area (2630 m2 g�1), excellent optical trans-parency (97.7%) and signicant thermal conductivity (3000–5000 Wm�1 K�1). Aer its discovery in 2004, it has made a crit-ical change in important elds such as material science andnanotechnology due to its mechanical, electronic, and thermalproperties. This interesting, and innovative material has alsoturned out to be a useful support for stabilization of metalnanoparticles.121–126 Graphene oxide (GO) which is the oxygen-ated form of graphene show very high structure defects ascompared to graphene.127–131 Both graphene and graphene oxidehave many potential applications but production, storage and

Fig. 10 Basic structural model of graphene and graphene oxide (GO) re


processing of these materials is a challenge. Commercially,graphene is synthesized by Hummers method which involvesan oxidation–reduction process, where the oxidation gives gra-phene oxide (GO) from the graphite and the reduction convertsgraphene oxide to graphene (Fig. 10). Though planer in shape,some chemical modication of graphene is necessary; other-wise irreversibly aggregation takes place via p–p stacking.132–134

Besides, chemical inertness of graphene affects the extent ofstabilization of AuNPs and achievement of narrow size distri-bution is difficult. Various modication of graphene such asnitrogen doped graphene,135 graphene graed with polydop-amine,136 graphene modied with octadecyamine, substitutedperylene attached graphene, graphene oxide (GO)137 reducedgraphene oxide (RGO) and supramolecular ionic liquid graedgraphene138 has been reported to overcome this problem andhighly stabilized gold nanoparticles (<5 nm) has been synthe-sized on them for catalytic applications.

In 2012, Wang et al. reported synthesis of AuNPs onnitrogen-doped graphene nanosheets (NG) supported catalyst.The introduction of nitrogen atom to the graphene oxide notonly modulated its electronic structure, but also changed thesurface properties of graphene oxide signicantly by intro-ducing pyrrolic and graphitic N species. This change wasevident in the XRD data as graphene showed a weak diffraction

draw from ref. 122.

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Fig. 13 Dispersity of GO (A), graphene (B), Au NPs@SIL-g-G (C) in H2O[reprinted with permission from ref. 138. Copyright (2013) RoyalSociety of Chemistry].

Fig. 14 Schematic preparing of Au@NiAl-LDH/RGO catalyst [reprintedwith permission from ref. 140. Copyright (2015) Royal Society ofChemistry].

Fig. 12 Schematic representation of layer-by-layer (LbL) assembled(Au/GO)n multilayer thin film for methanol oxidation [reprinted withpermission from ref. 139. Copyright (2012) John Wiley and Sons].

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line at 26.6� (d ¼ 0.34 nm) corresponding to the (002) reectionwhich disappeared on N doping. Two new peaks at 16.1� and22.9� were observed for N-doped graphene. Aer loading of goldusual peaks at 38.2�, 44.6�, 64.7� and 77.5� equivalent to the(111), (200), (220) and (311) crystal planes of metallic gold(Fig. 11) were reported. XPS results supported that pyrrolic andgraphitic N species doped into the graphene lattice playeda pivotal role in stabilizing gold nanoparticles by enhancingcharge transfer between the two. It was reported that thoughgraphene and NG have same morphology the presence ofnitrogen in NG led to the formation of smaller nanoparticles(average size 3.4 nm) as compared to graphene (30–100 nm)with better dispersion.135

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Graphene sheets have huge surface area. However, sepa-rating them and preventing their restacking is very important toutilize this unique property. A novel layer by layer (LbL)assembly of positively charged 4-dimethylaminopyridine(DMAP) coated AuNPs with negatively charged graphene oxide(GO) nanosheets (Fig. 12) was proposed by Kim et al. Dispersionof Au NPs was much higher on this bilayer assembly of 4-dimethylaminopyridine (DMAP) due to enhancement of surfacearea. The anodic current peaks were observed at 0.38 and 0.36 Vand a cathodic current peak at 0.11 and 0.17 V at 100 and 150 �Crespectively.139 Stability of AuNPs on graphene and theirdispersion was also enhanced by covalent functionalization ofgraphene with ionic liquid SIL (Fig. 13)138 and polydopamine.136

Li et al. synthesized a composite NiAl-LDH with RGO andimpregnated this with AuNPs by using sol-immobilizationmethod (Fig. 14). Raman spectroscopy and XPS data was usedto propose that NiAl-LDH/RGO composite had both defect sitesand oxygenic functional groups in RGO to control the direc-tional growth of Au nanoparticles thus leading to small particlesize of Au NPs (2.63 nm). The group was of the opinion thatpresence of Ni in LDH has a synergic effect that accelerated theelectrocatalytic activity for methanol oxidation.140

In 2015, Hutchings et al. reported bi-metallic Au–Pd NPssupported on graphene oxide sheets on graphene oxide sheetsseparated by the addition of TiO2. Signicant enhancement inthe activity of the catalyst was observed by simply hindering thestacking and agglomeration of graphene oxide sheets(Fig. 15).141

Excellent size control and dispensability of AuNPs on variousmodied graphene has led to excellent catalytic conversion ofbenzyl alcohol to benzaldehyde as reported by manyworkers.135,136 Au NPs@SIL-g-G prepared by Shaabani et al. hasbeen reported with remarkable selectivity towards benzalde-hyde (99%) with excellent conversion of benzyl alcohol (99%) in1 h. The catalyst also showed good recyclability and wassuccessfully used for the oxidation of a number of alcohols(Scheme 3).138



Fig. 15 (A) The systematicmethod used for preparing GO stabilized Au–Pd NPs on TiO2. (B) Representative BF-TEMmicrograph of the AuPd–9%GO/TiO2 sample. (Inset) Corresponding Au–Pd particle size distribution (PSD) (100 particles counted) [reprinted with permission from ref. 141.Copyright (2015) American Chemical Society].

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In 2013, Wang and co-workers used AuNPs and bimetallicAu–Pd@graphene for the methanol oxidation with 90.2%conversion and 100% selectivity for methyl formate witha TOF of 0.377 h�1 at 70 �C using molecular oxygen as oxidant.From the XPS reports, authors observed lowering in BE

Scheme 3 Oxidation of various alcohols by Au NPs@SIL-g-G supported


(binding energy) both in Au and Pd due to electron exchangebetween the two. Secondly, maximum shiing of �0.5 eV forAu 4f7/2 and �0.6 eV for the Pd 3d peak in Au2.0–Pd1.0/gra-phene was calculated (Fig. 16). This was attributed to con-ducting property of graphene which brought a strong

catalyst.

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Fig. 16 Au 4f (a) and Pd 3d (b) XPS spectra of the Au2.0/graphene (1),Pd1.0/graphene (2), Pd1.0/graphite (3), Au2.0–Pd1.0/CNTs (4), and Au2.0–Pd1.0/graphene (5) catalysts [reprinted with permission from ref. 142.Copyright (2013) Royal Society of Chemistry].

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synergism between Au and Pd which resulted in excellentcatalytic activity.142

AuNPs stabilized on graphene has also been used for elec-trocatalytic oxidation of ethanol. The catalytic activity was

Fig. 17 Oxidation of VA catalyzed by the Au/GQD composites. (a) UV-vtime at pH 8.5. The experiments were carried out with 15 mg of Au/GQDsVA was carried out with different catalysts under the same conditions fordifferent reaction temperatures for 1 h; (d) conversion rates of VA catalywith permission from ref. 144. Copyright (2015) Royal Society of Chemis

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studied in basic medium using cyclic voltammetry and chro-noamperometry techniques. The catalyst displayed excellentelectrocatalytic activity (13.16 mA cm�2 at a working potential of�0.12 V) and higher stability.143 Recently, Guo et al. has reportedhighly efficient oxidation of veratryl alcohol to veratryl aldehyde(VA) using AuNps supported on graphene quantum dots. TheAu@GQD was prepared through a facile one-pot reaction byusing GQDs, HAuCl4 and trisodium citrate as raw materials.From the TEM and HR-TEM study, the author suggested thatthe AuNPs are coated by GQDs thus making a core shell typestructure in Au/GQD. Importantly, the oxygen containinggroups on GQDs were reduced throughout during the reductionof Au precursor in presence of reducing agent. The Au@GQDcomposites showed higher dispersion and stability in aqueoussolution resulting in remarkable catalytic activity with excellentselectivity in acidic and alkaline conditions using H2O2 asoxidant (Fig. 17). It was shown that in the case of acidicconditions, singlet oxygen species generated by the reaction ofGQDs with H2O2 were the oxidising species whereas, in basicmedium, both GQDs and AuNPs contributed to the oxidation byproducing superoxide anion and traces of singlet oxygenspecies.144

3.3 Biopolymers and proteins

Biopolymers and biomass-related polymers are indeed attrac-tive candidates for use as supports for catalytic applicationsowing to their easy availability and low toxicity.145 At present,commonly used green stabilizing agents for nanoparticles are

is spectra of the oxidation products of veratryl alcohol versus reaction, 3 mM of H2O2 and 0.3 mM of VA at 50 �C; (b) the oxidation reaction of2 h; (c) conversion rates of VA catalyzed by the Au/GQDs at pH 8.5 withzed by the Au/GQDs at 50 �C for 2 h with varying pH values [reprintedtry].



Fig. 18 (A) Chitosan-stabilized (Au : Chit) gold nanoclusters. (B) The schematic representation of formation of Au : Chit. (a) Polycationic form ofchitosan, (b) formation of ion pairs with AuCl4

� and (c) Au : Chit formation. (b) (a) XPS spectra of core Au 4f, (b) N 1s and (c) O 1s. (d) FTIR spectra ofpure chitosan (i) and Au : Chit (ii) [reprinted with permission from ref. 157. Copyright (2011) Elsevier Publication].

Fig. 19 Schematic formation of Au@TiO2 heterodimer nanoparticlesredraw from the ref. 161.

Fig. 20 Preparation of Au–Pd@TiO2 supported catalyst [reprintedwith permission from ref. 162. Copyright (2014) American ChemicalSociety].

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vitamins, citric acid, polyphenols, proteins, enzymes, poly-saccharides, carboxymethyl cellulose (CMC), and biodegradablepolymers like PLA, gelatin etc.146–155 Sorption reduction is themost common method used for preparing AuNPs supported onbiopolymers.

Chitosan is the second most abundant biopolymer in natureand widely studied for stabilization of nanoparticles.156 Sakur-aia et al. synthesized chitosan stabilized Au, Au–Pd and Au–Ptnanoparticles and to control the size of nanoparticles theyadopted a two step procedure. First ion pairs of AuCl4

� with


chitosan were prepared by dissolving gold precursor in a solu-tion of chitosan dispersed in acetic acid, followed by rapidreduction with cold NaBH4 solution. Nanoparticles with anaverage size of 2.3 nm with very narrow size distribution wereobtained. The interaction between gold and chitosan was

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Fig. 21 TEM images of bimetallic NPs (preparation conditions:temperature, 90 �C; time, 1 h; and stirring rate, 12 000 rpm) with insethistograms of particle size distributions: (a) Au(7) Pd(1), (b) Au(2) Pd(1),(c) Au(1) Pd(1), (d) Au(1) Pd (2), (e) Au(1) Pd (7) NPs [reprinted withpermission from ref. 162. Copyright (2014) American ChemicalSociety].

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depicted by XPS analysis. The group concluded that interactionbetween chitosan and AuNPs existed through the multi-dentateco-ordination of amine and hydroxyl groups which resulted inintense bands in the region of 650–500 cm�1 in the FTIRspectrum of the catalyst (Fig. 18).157 Deplanche and co-workersused palladized cell biomass in contact with H2 saturatedAu(III) solution to produce bimetallic nanoparticles.158

Wang and co-workers used an organic–inorganic hybridmaterial to prepare AuNPs of size. DNA/MMT hybrid supportwas synthesized and used to prepare fairly uniform nano-particles of less than 10 nm.159,160 This support gave amazingselectivity for ester in the oxidation of benzyl alcohol at 50 �Cusing CsCO3 as base and oxygen as oxidant. The versatility ofthe protocol was checked and the reaction with alcohols bearingelectron donating and withdrawing groups were carried outsmoothly. The reuse of the catalyst upto h cycle was estab-lished. With chitosan stabilized gold nanoparticles aldehydeand acid were obtained as major products.157

In 2009, Chen and co-workers reported an efficient and eco-friendly catalyst for the oxidation of methanol. They reported

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Janus type gold nanoparticles supported on TiO2. This materialwas synthesized by a sol–gel method using hydrophobic 1-hex-anethiolates and hydrophilic 2-(2-mercaptoethoxy) ethanol ascapping agents (Fig. 19). This synthesized catalyst was charac-terized by TEM, HR-TEM and UV visible spectroscopy tech-niques. From the TEM and HR-TEM analysis, it was shown thatmost of Au NPs have sizes between 2–6 nm. The materialexhibited excellent photocatalytic activity for the oxidation of0.14 mM methanol to formaldehyde. The group also suggestedthat the charge separation of photo generated electrons andholes at the Au@TiO2 support was assisted by their closeproximity to gold nanoparticles.161

In 2014, Li et al. reported a green bioreduction strategy forthe synthesis of gold nanoparticles and solvent free oxidationof benzyl alcohol. This group used Cacumen Platycladi extractto assemble bimetallic Au–Pd nanoparticles which were thenimmobilized onto TiO2 (Fig. 20). This catalyst was character-ized by TEM, X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS) and FT-IR (Fourier transform infraredspectroscopy) techniques. From the TEM analysis, author hassuggested that all stabilized Au–Pd NPs were spherical in shapeand highly dispersed with average size 8 � 1.1 nm (Fig. 21).This group achieved maximum conversion of benzyl alcohol(74.2%) with remarkable selectivity of benzaldehyde (95.8%) at90 �C with an oxygen ow rate of 90 mL min�1 under solventfree conditions. Excellent yield of benzaldehyde (98%) withremarkable TOF (589 h�1) was obtained by this group. Thecatalyst was highly recyclable and the authors reported that Au/Pd in molar ratio of 2 : 1 showed maximum catalytic activity.162

3.4 Synthetic polymers

Cross-linked polymers have been adopted as heterogeneoussupports for the stabilization of metal nanoparticles for the pastseveral decades.163 Their availability, ease of separation,enhanced stabilization of metal nanoparticles makes thempopular support especially for solution phase reactions. Manycommercially available linear as well as crosslinked polymerssuch as polyethyleneglycol (PEG),164 polystyrene (PS),165 poly-siloxane, polyisobutylene,166 polyvinyl pyridine,167 modiedpolystyrene,168,169 syndiotactic polystyrene-co-cis-1,4-poly-butadiene170 and dendrimers171 have been used for the synthesisof AuNPs and used to study oxidation of alcohols. Sorptionreduction is the most common method to support gold nano-particles on insoluble polymers whereas soluble polymers areutilized to synthesize nanoparticle dispersions in water andorganic solvents. The letter generally gives a better control overthe nanoparticles size. However, former has the advantage ofeasy recovery from the reaction mixture. Preformed AuNPs weresupported on polystyrene by dispersion method.165 Poly-vinylpyrrolidone (PVP),172,173 PANI174 are water soluble polymersthat have been most widely used to synthesize nanoparticles inaqueous solution with controlled particle size but their sepa-ration from reaction mixture is tedious.

Tsukuda et al. developed highly stable and durable goldnanoclusters (4 nm size) in water phase using star like thermosensitive polyvinyl vinyl ether (Fig. 22). These star like polymers



Fig. 22 Preparation and catalytic use of AuNPs [reprinted with permission from ref. 175. Copyright (2007) American Chemical Society].

Scheme 4 Oxidation of various alcohols by AuNPs@poly(EOEOVE) catalyst.

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 28688–28727 | 28701

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Fig. 23 The diagrams atop and below track the evolution of a single PS-b-P4VP micelle and an array of micelles. (i) Solution state self-assemblyof PS-b-P4VP into spherical micelle in toluene; (ii) spin coating micelles onto an electrode substrate; (iii) simultaneous loading of AuCl4

� andPtCl6

�2 ions into micellar array by immersion into a stoichiometrically tuned aqueous solution of the respective metal ions; (iv) reductive etchingof PS-b-P4VP template by Ar plasma etching [reprinted with permission from ref. 176. Copyright (2014) American Chemical Society].

Fig. 24 Preparation of PAMAM-supported Au NPs catalysts redraw from ref. 178.

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were synthesized by living polymerization of 2-2(ethoxy) ethoxyethyl vinyl ether. The resulting clusters effectively catalyzedaerobic oxidation of benzyl alcohol but with 99% selectivity forbenzoic acid. The authors studied the catalytic performance fordifferent alcohol derivatives (Scheme 4). The author suggestedthat the star-polymer-stabilized Au NPs are existing in hydro-phobic layers.175

Rider and co-workers synthesised very small and uniformbimetallic Pt–Au nanoparticles of 2–4 nm dimension usingpolystyrene-block-poly(4-vinylpyridine) block copolymer(Fig. 23) as template. The template was removed by etching aerdepositing the nanoparticles on electrode which were subse-quently used in methanol fuel cell. Self-assembly of copolymerformed micelles of 2–4 nm diameter that controlled the size ofnanoparticles. It was seen that the high density Pt–Au core shellbimetallic NPs were electrochemically more active for methanol

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oxidation as compared to Pt supported nanoparticles and weremore resistant to catalytic poisoning too.176

Dendrimers are macromolecules with tree-like architecturehaving a high degree of molecular uniformity, narrow molec-ular weight distribution, loosely packed interior pockets andhighly functionalized terminal surface. Such structural speci-city promises uniform formation of nanoparticles withpotential applications in catalysis. In 2012, Jiang et al. attachedPAMAM dendrimers to polystyrene in order to combine theadvantages of both heterogeneous and homogeneous catalysis.From the SEM and TEM analysis, the author has demonstratedthat the AuNPs were present both in the interior and on exteriorsurfaces of the polystyrene beads with sizes 5 to 10 nm. Theyield of benzaldehyde (99%) reported by this group under theoptimized conditions (Fig. 24).177,178



Fig. 25 Schematic preparation of Au NPs@MCBNs supported catalyst [reprinted with permission from ref. 179. Copyright (2015) AmericanChemical Society].

Fig. 26 Images for the recycling of the PVP-Au and 1-Au NPs[reprinted with permission from ref. 181. Copyright (2011) AmericanChemical Society].

Fig. 27 Polystyrene and its epoxide and alcohol derivatives used forstabilizing AuNCs.

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Recently in 2015, Zang and co-workers developed a new kindof polymer (MCBNs), a brush block terpolymer of [poly(p-chloromethylstyrene)-gra-poly(4-vinylpyridine)]-block-polystyrene[(PCMS-g-P4VP)-b-PS] and used it as a support for the stabili-zation and of AuNPs (Fig. 25). The immobilized AuNPs on thispolymer matrix showed nanoparticles of 1 nm dimension underTEM. This synthesized material exhibited remarkable activityfor the aerobic oxidation of 1-phenyl ethanol and it was shownthat acetophenone was the main product formed in the oxida-tion with 96% yield in 6 h.179


PVP stabilized nanoparticles have been studied by manyworkers and more than 90% conversion with excellent selec-tivity for benzaldehyde has been reported.180 Dyson and co-workers discovered that AuNPs stabilized by carboxylate modi-ed PVP can be separated from the reaction mixture by a newtool i.e. lowering the pH of the solution which caused the NPs toprecipitate. The AuNPs form pseudo homogenous aqueoussolutions at pH >2.5 and precipitate at pH <2.4. However,during oxidation of benzyl alcohol, this catalyst showed moreselectivity towards benzoic acid rather than benzaldehyde(Fig. 26).181 Tsukuda and co-workers stabilized AuNPs on thishydrophilic polymer (PVP) and investigated its catalytic activitytowards aerobic oxidation of benzyl alcohol in aqueousmedia atambient temperatures using molecular oxygen as oxidant.Kinetic measurements were performed to establish reactivity ofAuNPs of different sizes.182

Kobayashi et al. in 2007, developed a simple and efficientmethod for the synthesis of a highly controllable gold nano-clusters using AuClPPh3 complex as gold source and NaBH4 asreducing agent. They used modied polystyrene as support forthe stabilization of gold nanoclusters (AuNCs) (Fig. 27) of 1 nmdimension. This gold supported composite material showedexcellent catalytic activity for alcohol oxidation at roomtemperature using molecular oxygen at atmospheric pres-sure.168,169 Different solvents were studied by the authors andbest results were achieved in binary solvent like toluene/waterand benzotriuoride/water. The catalytic activity was also eval-uated for various derivatives of alcohols (Scheme 5). All of themgave good conversion (>75%) with >99% selectivity towardsaldehyde.168 In another report, the same group has used inte-grated carbon black (CB) in the polymer system (PI/CB-Au), toincrease the gold loading on polystyrene resin. The size ofAuNCs did not increase much, as the gold loading wasincreased from 5 wt% to 20 wt%.169

In the same year this group reported a novel gold-immobilized microchannel reactor for the efficient, green

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Scheme 5 Oxidation of selected alcohols by Au NPs@PI catalyst.

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oxidation of alcohols using molecular oxygen. They used a pol-ysiloxane-coated capillary column having 50 cm length and 250mm inner diameter which contained an inner lining made of50% phenyl and 50% n-cyanopropyl functionalities on siliconatoms in a lm of 0.25 mm thickness. The method for gold-immobilized on capillary column is shown in Fig. 28. The

Fig. 28 Immobilization of the gold catalyst. (a) Reduction of the cyanImmobilization of the gold catalyst redraw from the ref. 183.

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immobilized AuNPs in this column showed 99% conversion of1-phenyl ethanol to acetophenone in DCM using K2CO3 as base.The authors have reported that increasing the ow rate of theaqueous K2CO3 solution or oxygen gas did not affect theconversion. They also checked the activity of this catalyst foroxidation of different type of alcohols such as benzylic, aliphatic

o group to an amine. (b) Preparation of microencapsulated gold. (c)



Fig. 29 Schematic procedure for the synthesis of poly(o-phenylenediamine) submicrosphere supported gold nanoparticles redraw from the ref.186.

Fig. 30 Schematic preparation of PS/Au composite particles redraw from the ref. 165.

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and allylic alcohols. In all cases, high conversion with high yieldof product and recyclability was reported.183

Block copolymers are a type macromolecules that compriseof two separate polymer chains joined together. Such polymerswith segregation of hydrophilic/hydrophilic regions can providebetter stabilization and dispersion of the nanoparticles.184

Kawanami et al. derived a versatile, solution based techniquewith high degree of control over the size and spacing of nano-particles. AuNPs@amphiphilic block co-polymer (poly-(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide))(PEO-PPO-PEO) were synthesized and used for the oxidation ofalcohols using molecular oxygen as oxidant at 30 �C in thepresence of base (Na2CO3). Interestingly, the block copolymersacted both as reductants as well as stabilizer. The morphologyof stabilized AuNPs was spherical with size 4–12 nm. Theaverage particle size of Au NPs is 7.3 nm. The following grouphas suggested that AuNPs was highly stabilized mainly due tosteric factors. 99% conversion of benzyl alcohol was reportedbut the selectivity for benzaldehyde was only 8%. Benzoic acidand benzyl benzoate were produced as side products during thereaction due to over-oxidation of benzaldehyde.185

Highly active AuNPs@syndiotactic (polystyrene-co-cis-1,4-polybutadiene) multi-block copolymer were successfully usedfor the oxidation of primary and secondary alcohols by Grassiet al. Using molecular oxygen as oxidant. 1-Phenylethanol wasoxidised to acetophenone in high yields (96%) in 1 h, at 358 �C,whereas benzyl alcohol was quantitatively oxidised to benzal-dehyde with a selectivity of 96% in 6 h. Nanoporous crystallineform of syndiotactic polystyrene ensured facile and selectiveaccessibility for the substrates to the gold catalyst incarceratedin the polymer matrix.170


Gold nanoparticles of average size 5 nm were synthesised onpoly(o-phenylenediamine) submicrosphere of size 700 nm byimpregnating with precursor HAuCl4. No additional reducingagent was required and PoPD submicrospheres with abundantamino groups and p electrons in benzene rings acted asreducing and stabilising agents. The synthesised gold nano-particles showed good catalytic activity for oxidation of benzylalcohol to benzaldehyde in water with high yield and selectivity(Fig. 29). TEM images showed gold nanoparticles uniformlydeposited on the surfaces of PoPD submicrosphere186 Li et al. in2014 reported AuNPs@PS as highly active catalyst for benzylalcohol oxidation under atmospheric conditions using potas-sium carbonate as base in aqueous medium. This compositeshowed good recyclability and was reused several times withoutsignicant loss of activity. The PS microsphere was synthesizedusing conventional dispersion polymerization. TEM analysisshowed gold nanoparticle dispersed on the surface of PSmicrospheres. The average particle size of the Au NPs was about3.5 nm, 9.5 nm and 6.7 nm respectively. This catalyst was highlyselective for benzoic acid rather than benzaldehyde (Fig. 30).165

Polyaniline commonly known as PANI a conducting polymerwas used to synthesise mono and bimetallic gold nanoparticlesby Baiker and co-workers. PANI is a charge density donor and itspresence enhanced the activity of Au NPs. The size of the monoand bimetallic particles was controlled in a narrow range (2.4–3.7 nm) by using a colloidal preparation route. The authorsconcluded that if the content of Pd in bimetallic nanoparticlesis increased from 1 to 9 mol% signicant increase in the cata-lytic activity with 100% conversion of benzyl alcohol and 98%selectivity for benzaldehyde (98%) at 100 �C can be achieved. Aspolymer support polyaniline (PANI) was applied owing to its

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Fig. 31 Illustration of a layered double hydroxide, hydrotalcite, withhydroxide anions (red spheres) in the interstitial layer. The brucite-likelayers are shown in purple [reprinted with permission from ref. 195.Copyright (2014) Royal Society of Chemistry].

Fig. 33 Design of the Au NCs@LDH based catalyst redraw from theref. 201.

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high conducting and redox properties and hydrophobic naturein most of the polar as well as non-polar solvent.174

3.5 LDH

Clays are another cheap and green material which is frequentlyused for supporting metal nanoparticles. Among the manyclasses of clays LDH (layered double hydroxide) is the mostcommonly usedmaterial. It is anionic clay, which belongs to thefamily of brucite Mg(OH)2 like layered inorganic materials(Fig. 31). It has a number of advantages viz. high versatility incomposition, high adsorption capacity, tunable basicityetc.187–197 Basicity of the clay has synergistic effect on the cata-lytic activity of the gold nanoparticles. The ion exchange abilityof the clay helps in facile sorption of the precursor. Theprecursor is generally loaded onto the catalyst by co-precipita-tion196 deposition precipitation197 sol immobilization198

methods followed by reduction. Besides concentration of theprecursor, size of the nanoparticle was also controlled by thenature of reducing agent. The stronger the reducing agentsmaller is the size of nanoparticles. Kaneda and co-workersreported that the size of Au NPs decreased more in thefollowing order hydrazine > molecular hydrogen > potassiumborohydride. In almost all the cases gold nanoparticles of lessthan 10 nm were achieved easily.196

In 2011, Zheng and co-workers synthesized a eco-friendly,magnetic separable novel core shell Au NPs@Fe3O4@MgAl-LDH. The authors rst synthesized coated negatively chargedFe2O3 magnetic nanospheres with MgAl-LDH. Fe2O3 nano-particles were of average size 450 nm on which LDH shells of 20nm thickness were grown. Au NPs with an average size of 7 nmwere deposited on this composite material by deposition–precipitation method (Fig. 32). The catalyst was easily separated

Fig. 32 The synthetic strategy of an Fe3O4@MgAl-LDH@Au catalyst red

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by magnet and recycled upto ve consecutive runs without anydrop in yield.199

Li and co-worker synthesized gold nanoparticles on a seriesof transition metal modied M-HT (M ¼ Cr3+, Mn2+, Fe3+, Co2+,Ni2+, Cu2+, Zn2+). They rst prepared the HT precursor(MgAl2(OH)12CO3) by homogeneous precipitation method usingurea hydrolysis. Later on, HT with different metal ions wasproduced by calcination–reconstruction process. AuNPs of size2.7–3.9 nm were deposited by homogeneous precipitationmethod and used for a base free protocol for the oxidation ofvarious substituted benzyl alcohol.200 In 2014, Zhang and co-workers supported glutathione capped gold sol in water onM3Al-layered double hydroxide (M ¼ Mg, Ni, Co). The gluta-thione capped gold nanoparticles of average size 1.5 � 0.6 wereprepared and then different amount of nanoparticles weredispersed in deionized water along with Mg3Al-LDH. Thedispersion aer centrifugation was calcinated under N2 toremove the capping agent. Highly dispersed gold nanoparticleson LDH were thus obtained (Fig. 33).201

Ni–Al layered LDH is another clay material which has beenexplored as support by many workers.202,203 The basic and redoxproperties were adjusted by adjusting the ratio of Ni : Al toachieve the best synergistic effects between AuNPs and thesupport. The activity of the obtained Au catalysts was found tobe structurally associated with the arrangements of NiO6 andAlO6 octahedrons present on the surface of hydroxide layers.Deposition precipitation method was used to prepare AuNPs of4.5 nm dimension. XPS studies indicated enhanced interactionbetween AuNPs and support which was indicated as the reasonfor better activity of catalyst.

A number of studies for benzyl alcohol oxidation using LDHsupported gold nanoparticles have carried out and reported attemperatures ranging from 40 �C up to 120 �C with very highTON and TOF using dioxygen as oxidant but in organic solvents.E.g. Kaneda and co-workers reported a turnover number (TON)and turnover frequency (TOF) of 200 000 and 8300 h�1 respec-tively in the oxidation of 1-phenylethanol. Even, less reactivecyclohexanol derivatives were easily oxidized by the AuNPs@HT

raw from the ref. 199.



Scheme 6 Oxidation of various alcohols by Au@HT supported catalyst.

Fig. 34 Oxidation of benzyl alcohol to corresponding to benzaldehyde by Au@Cr-HT support [reprinted with permission from ref. 204.Copyright (2014) Elsevier Publication].

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catalyst with excellent yield in basic medium (Na2CO3).196 Wangand co-workers reported best conversion of benzyl alcohol(>99%) with excellent selectivity towards benzaldehyde (99%) in9 h, in p-xylene at 120 �C using Au NPs@HT as catalyst. Toevaluate the activity of this catalyst, authors also oxidizedvarious substituted alcohols, heterocyclic alcohols, andaliphatic alcohols (Scheme 6) with high conversion and selec-tivity. They observed a unique effect of Au loading on the cata-lytic activity.197

Among the several transition metal doped clays synthe-sized by Li and co-workers, the best yield of benzaldehydewas achieved in toluene under atmospheric O2 at 373 K in 0.5h in case of Cr3+ containing Au NPs@Cr-HT support. Theactivity of the various catalysts was found to decrease in thefollowing order Au NPs@HT < Au NPs@Cu-HT < Au NPs@Zn-HT < Au NPs@Mn-HT < Au NPs@Co-HT < Au NPs@Fe-HT <


Au NPs@Ni-HT < Au@Cr-HT. It was shown that the synergybetween metallic AuNPs and hydrotalcite support whichinvolved both the basic sites of the surface of the hydrotalciteand the Cr cations was responsible for the increased activity.Highest TOF of 930 with 99% selectivity for benzaldehyde wasreported. Versatility of the protocol for various substitutedbenzyl alcohol as well as 1-phenyl ethanol was also re-ported.200 Hensen et al. also reported that the interactionbetween gold and Cr content of HT support has synergisticeffect on the oxidation of alcohol. As the content of Cr wasincreased on the HT support, it resulted in an increase in thesurface area and basicity of the support. The 1830 h�1 (TOF)was reported by this group and from the kinetic isotopeeffect, it was concluded that the cleavage of O–H bond wasaccelerated by the presence of Cr ions (Fig. 34). The size ofAuNPs also decreased on increasing the Cr content of the

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Fig. 35 Synthesis procedure of sub-nanometer-sized Au clusterswithin SBA-15 using Au11:TPP as a precursor [reprinted with permissionfrom ref. 227. Copyright (2009) American Chemical Society].

Fig. 37 (A) Synthesis of magnetite–silica–gold nanocomposite. (B)Vial A shows the typical nanocomposite solution before and vial Bshows the nanocomposite after being subjected to a bar magnet fora period of 10 min [reprinted with permission from ref. 230. Copyright(2015) American Chemical Society].

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support. From the UV-Raman, X-ray absorption and photo-electron spectroscopic studies, the group revealed that thestrong Au–Cr synergy is directly related to a Cr3+ 4 Cr6+

redox cycle at the Au/Cr-HT interface, where O2 activationtakes place by electron transfer.204 Recently, Zhang and co-workers reported highest TOF (46 500 h�1) for the conver-sion of 1-phenyethanol with stabilized AuNPs on LDH thatalso gave acetophenone in 99% yield in toluene under basefree conditions in 0.5 h. The activity of this catalyst fordifferent alcohol derivatives was also investigated.201

3.6 Zeolites, silica

Zeolites and silicas are the most widely used industrial supportsowing to their high thermal stability, variable dimensions of thepores, high ability of absorption, porosity, highly crystalline natureand shape selectivity. Zeolites have an extremely large internalsurface area in relation to their external area and an orderedporous structure which make them suitable for supportingnanoparticles. Mesoporous silica with uniform pore sizes orchannels (2–10 nm range) is commercially available in well-known

Fig. 36 Schematic preparation of Au@(S)-(�)-2-pyrrolidinone-5-carbox

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phases likeMCM-41, MCM-48, SBA-15 andMCM-50.205,206 They arehighly ordered structures of low cost and their commercial avail-ability makes them highly suitable supports for industrial appli-cations.207–209 Moreover, conned void spaces in zeolites restrictthe growth of various nanoclusters and lead to an increase in theircatalytic performance. Heterogenization of gold nanoparticles onthese supports has been reported by deposition reduction, depo-sition precipitation and impregnation methods.210–226

Tsukuda and co-workers impregnated Au clusters (z1 nm)within the mesoporous silica (SBA-15, MCF, HMS) using

ylic supported nanocatalyst redraw from the ref. 228.



Scheme 7 Subsequent reactions of benzaldehyde during oxidation of benzyl alcohol redraw from ref. 231.

Scheme 8 (a) Synthesis of nanoporous ionic organic networks (PIONs), NMP: N-methyl-2-pyrrolidinone; (b) controllable route to Au@PIONhybrid materials [reprinted with permission from ref. 234. Copyright (2015) American Chemical Society].

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triphenylphosphine-protected Au11 (Au11:TPP) sol as precursor(Fig. 35). Aer impregnation organic protecting layer wasremoved by calcination.227 Wang et al. modied the surface ofSBA-15 with (S)-(�)-2-pyrrolidinone-5-carboxylic acid in order tocontrol the size of nanoparticles (Fig. 36). The average sizes ofAu nanoparticles in Au@SBA-15-Py and Au@SBA-15 are esti-mated to be 2.1 nm and 2.2 nm.228

A novel ceria mixed silica was reported by Gong et al. tosynthesize gold nanoparticles. An amino functionalized SBA-15was synthesized and impregnated with AuNps by depositionreduction method. Aerwards, Au@SBA-15 materials wasimpregnated with different concentrations of cerium nitrate


solution and sonicated to prepare Au–xCeO2@SBA-15. Thelatter was calcinated to obtain the nal Au–[email protected] of CeO2 not only enhanced the stability of theAuNPs but also increased the activity of the catalyst. From theXPS report, authors observed that the Au species existed as Au0

in Au@SBA-15 whereas, on addition of CeO2 to Au@SBA-15, theAu species existed in several oxidation states viz. Au0, Au+ andAu3+. This was reported as the reason for large enhancement inthe selectivity of catalyst for benzaldehyde from the meagre 2–18% aer inclusion of CeO2.229 Another mixed support was re-ported by Gole et al. 2015. The Fe3O4–SiO2–AuNPs was preparedby co-precipitation of iron salts, reduction of gold chloride, and

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Scheme 9 Oxidation of various cyclohexanol with Au@PION-1 catalyst.

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silica formation in single step (Fig. 37). D-Glucose was added asstabilizing agent for AuNPs. Besides higher stability of thenanoparticles, the catalyst was easily separable owing to itsmagnetic nature.230

In their extensive research, Hutchings et al. studied theoxidation of benzyl alcohol using various zeolite supports andcompared them. They found that selectivities for acidicsupports were lower than those for non-acidic supports. Thegroup also concluded that the selectivity towards the benzal-dehyde decreased with corresponding increase in side productssuch as dibenzyl ether and benzyl benzoate. They proposed thatthe latter is likely to be formed by the oxidation of the hemi-acetal intermediate generated by the combination of benzylalcohol and benzaldehyde (Scheme 7). Further, the addition ofPd into the catalyst improved the activity without loss of theselectivity for benzaldehyde.231 In another investigation by thesame group, gold nanoparticles synthesized on hierarchicaltitanium silicates (TS-1), were reported as better catalystscompared to other commercial silicas. They concluded that thepresence of secondary porosities in this material playeda crucial role in the catalyst activity as they enhanced the metaldispersion and improved the accessibility to the active sites.232

One again bimetallic Au–Pd was found to be more active thangold alone towards oxidation of benzyl alcohol.

In the case with silica modied with 2-pyrrolidinone-5-carboxylic acid228 the authors reported more than 90% conver-sions of benzyl alcohol. However, the catalyst showed selectivityfor benzoic acid rather than benzaldehyde in water medium.

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The deposition of nano ionic liquid layer on a porousmaterial and using it to support for catalyst conceptually createsa new fundamental research area that solves certain problemsassociated with homogenous catalysis.233 The nanoporous ionicorganic networks (PIONs) with excellent ionic density weresynthesized by substitution nucleophilic bimolecular reaction(Scheme 8) on silica by Dai et al. in 2015. This mixed supportwas then impregnated with gold nanoparticles of size 1–2 nm.The authors studied the oxidation of a number of secondaryalcohols at 60 to 140 �C in toluene with quantitative conversionwith >99% selectivity for corresponding ketone. Versatility ofboth the catalyst was reported by studying a number of exam-ples (Scheme 9).234

3.7 Metal oxides

In general, metal oxides offer high thermal and chemicalstabilities combined with a well-developed porous structurehaving high surface areas, meeting the requirements for mostapplications. They can also be easily prepared and furtherfunctionalised, adding value to their use as supports.Silica,235–238 alumina,239–241 titania,242–245 ceria,246 and zirconia247

are the most commonly employed supports. Depending on thechemical reactivity of the support, metal oxides can be classiedas inert (SiO2) or reactive (CeO2). Reactive oxides have beenknown to participate in the reaction. Lately, super-paramagnetic oxides (e.g. Fe3O4, Co3O4) have also becomeimportant materials for the immobilisation of metal nano-particles owing to their facile separation capabilities. Sol-



Table 1 Comparative data for oxidation of benzyl alcohol by various metal oxide supported Au NPsa

Entry Catalyst Con. of BA (%) Sel. of BZ (%) T (�C) Time (h) Ref.

1 Au@SiO2 (CP) 1.9 100 100 3 2482 Au@CeO2 (IP) 3.4 100 100 3 2483 Au@TiO2 (IP) 0.65 100 100 3 2484 Au@Fe2O3 (IP) 7.1 87.6 100 3 2485 2.5% Au@TiO2 (IP) 96.7 15.3 100 8 2496 Au@TiO2 P25

b (WIP) 3.3 55.7 80 4 2507 [email protected]

b (WIP) 19.9 88.4 80 4 2508 [email protected]–CH3I





b (WIP) 2.4 4.8 80 4 25013 1% Au@MgAl-MMO (SI) 90 92.5 140 4 25114 0.65% Au@MgAl-MMO (SI) 28 95 140 4 25115 1% Au@MnO2 (SI) 5.6 100 120 96 25216 1% Au@MnO2 (WSI) 5.5 100 120 96 25217 1% Au@MnO2 (SI) 8.6 99.0 120 6 25218 1% Au@MnO2 (WSI) 8.1 100 120 6 25219 2.5% Au@TiO2 (IP)** 76.6 50.9 100 5 25320 2.5% Au@TiO2 (IP)** 46.2 93.8 100 5 25321 2.5% Au@TiO2 (IP)** 3.8 >99.9 100 5 25322 2.5% Au@ZnO (IP)** 38.1 81.5 100 5 25323 2.5% Au@SiO2 (IP)** 33.6 59.3 100 5 25324 2.5% Au@Co3O4 (IP)** 65.8 55.7 100 5 25325 2.5% Au@MgO (IP)** 47.4 58.0 100 5 25326 1% Au@TiO2 (IP) 16.0 100 127 3 25427 1% Au@Fe2O3 (FSP) 5.5 100 127 3 25428 Au@MnO2–R (HDP) 40.7 40.1 120 5 25529 Au@MnO2–C (HDP) 13.6 13.3 120 5 25530 Au@Fe2O3 (HDP) 9.8 99.3 120 5 25531 Au@TiO2 (HDP) 30.5 99.6 120 5 25532 Au@CeO2

b (IP) 24 99 100 3 25633 Au@TiO2

b (IP) 25 99 100 3 25634 1% Au@TiO2 (IP) 0.99 100 130 24 25735 Au@TiO2* (DP) 63.1 79.2 96 2 25836 Au@CoOx*(DP) 34.0 85.2 96 2 25837 Au@NiO* (DP) 37.5 82.5 96 2 25838 Au@CuO*(DP) 39.0 75.5 96 2 25839 Au@ZnO*(DP) 45.0 71.2 96 2 25840 Au@ZrO2*(DP) 59.5 81.5 96 2 25841 Au@La2O3*(DP) 55.2 70.1 96 2 25842 Au@Sm2O3*(DP) 50.4 73.2 96 2 25843 Au@Eu2O3*(DP) 45.5 75.3 96 2 25844 Au@Yb2O3*(DP) 47.5 80.6 96 2 25845 Au@CeO2

b (DP) 16.72 >99 90 3 25946 Au@Mn–CeO2

b (DP) 43.22 >99 90 3 25947 Au@TiO2 (HDP) 85 >99 320 2 26048 Au@MgOc (DP)** 99 92 110 2 26149 Au@MgFe2O4 (SI) 35 59 100 4 26250 Au@MgFe2O4 (SI)** 62 36 100 4 26251 Au@MgFe2O4 (SI)** 32 66 100 4 262

a O2 as oxidant, CP¼ co-precipitation, IP¼ impregnation, FSP¼ Flame Spray Pyrolysis, HD¼ hydrothermal precipitation, SI¼ sol-immobilisation,DP ¼ deposition precipitation, WIP ¼ wet-impregnation, WSI ¼ Wet Sol-Immobilisation. b ¼Toluene using as solvent. c ¼Methanol using assolvent, Con. (%) ¼ conversion of benzyl alcohol, Sel. (%) ¼ selectivity of benzaldehyde, * ¼ TBHP as oxidant, ** ¼ base added.

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immobilization, deposition–precipitation, co-precipitation,hydrothermal deposition precipitation and impregnation arethe common methods used for the preparation of supportedgold nanoparticles on metal oxides support. A summary of thesupports used and the methods used for AuNPs impregnationare tabulated in Table 1.


It has been observed that metallic gold species (Au0) arepresent of inert or non-reducible supports such as alumina.263

Whereas, gold in three different states (Au0, Au1+, Au3+) havebeen observed on the surface of reactive oxides such as CeO2,264

U3O8 (ref. 263) etc. Another support in which gold is reported indifferent oxidation states is b-MnO2. Cao et al. synthesized Au

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Fig. 38 (A) TEM image of pure MnO2 nanorods. (B) HRTEM image and electron diffraction pattern (inset) of MnO2 nanorod. In the HRTEM image,the spacing between the lattice fringes is ca. 0.32 nm, which corresponds well with the (110) crystal plane of b-MnO2 (pyrolusite). (C) TEM imageof Au/MnO2–R catalyst (D) HRTEM image of Au/MnO2–R catalyst. (E) TEM image of Au/MnO2–C (F) size distributions of Au nanoparticles ob-tained for Au/MnO2–R and Au/MnO2–C [reprinted with permission from ref. 255. Copyright (2008) American Chemical Society].

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NPs@b-MnO2 by homogenous deposition precipitation usingurea as the precipitation agent and commercial MnO2 or MnO2

nanorods as support. From the XPS studies, the authorproposed that both reduced and oxidized Au species on theMnO2 nanorods support before and aer the reaction. Signi-cantly enhanced catalytic activity was observed for gold catalystsupported on MnO2 nanorods, as compared with that oncommercial MnO2 powders. Secondly, the enhanced catalyticactivity of the Au/MnO2–R catalyst was attributed to the bene-cial presence of higher amount of oxidized gold species and

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surface oxygen vacancies resulting from the strong interactionbetween Au and the more reactive surface of MnO2 nanorods.TEM images indicated that the MnO2 nanorods have welldened rod like morphology with diameters of 40–100 nm andlength ranging between 2.5 and 4.0 mm. Further, HRTEMstudies showed that MnO2 nanorods are crystalline in natureshowing d spacing characteristic of 110 plane of b-MnO2. Thestructure feature of MnO2 nanorods were not altered ondepositing gold. Au NPs were highly dispersed on the MnO2

surface, and the size distribution ranged from 2.0 to 10.0 nm



Fig. 39 Schematic preparation of magnetically recoverable Au NPs redraw from the ref. 266.

Fig. 40 TEM images of catalyst 3 (a) and catalyst 4 (b). Histogram showing particle size distribution and lognormal fitting of the Au NPs of catalyst3 (c) and the Au NPs of catalyst 4 (d) [reprinted with permission from ref. 266. Copyright (2010) Royal Society of Chemistry].

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with the average size of 4.4 and 5.0 nm for Au/MnO2–R and Au/MnO2–C (Fig. 38). In addition, the HR-TEM image of the Au/MnO2–R catalyst also showed that the gold NPs on the surface ofMnO2 nanorods are mainly semispherical in shape.255

Another, especially noteworthy synthesis is of magneticallyseparable nanoparticles by Rossi et al. in 2009. First oleic acidcoated Fe3O4 NPs and the core shells Fe3O4@SiO2 compositewere prepared as per literature265 and then Au3+ was


immobilization on the surfaces of core shell silica coatedmagnetite NPs. Aer that the metal was reduced by twodifferent methods viz. thermal reduction in air and by hydrogenreduction at mild temperature. This synthesized material wasmodied with amino groups by treatment with 3-amino pro-pyltriethoxysilane (APTES) in dry toluene under N2 atmosphereto give amino functionalized support (Fe3O4@SiO2/NH2)(Fig. 39). Aer that the material was washed with toluene and

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Scheme 10 Oxidation of geraniol catalyzed by Au@CeO2 catalyst.

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dried at 100 �C. From the TEM analysis, the group reported thatthe support composed of magnetic cores (z10 nm) which werespherically coated with silica spheres (z40 nm). The averagesize of stabilized AuNPs was found to be 5.9 nm (Fig. 40). Theauthors investigated the activity of this magnetic recoverablematerial for benzyl alcohol oxidation. The best conversion(84.3%) with good yield of benzaldehyde (95%) in the presenceof base (K2CO3) was reported.266

Huchings and co-workers reported very selective, mild, basefree and solvent free methodology for the aerobic oxidation ofsome important aromatic alcohols such as geraniol, benzylalcohol and some aliphatic alcohols. They synthesizedAu@CeO2 using co-precipitation, deposition precipitation andimpregnation methods and achieved 100% selectivity for theoxidation of benzyl alcohol and octan-1-ol with TOF 150 h�1 and26 h�1 by using this catalyst. For the comparative study, theauthor also tried various other oxides such as SiO2 and TiO2 butAu@CeO2 was highly selective as well as active. The catalyst was

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used to explore oxidation of geraniol with excellent conversion.Various products identied during the oxidation of geraniolwere linalol, b-pinene, limonene, g-terpinene, nerol and tracesof geranic acid. The selectivity of this catalyst towards citral waspoor (Scheme 10).248

In 2007, Chaudhary et al. reported solvent free protocol forthe synthesis of chlorine free benzaldehyde. They synthesizedU3O8 supported gold nanoparticles by four different methodsviz. impregnation (IP), deposition precipitation (DP), homoge-nous deposition precipitation (HDP) and co-precipitation (CP).The catalyst prepared by HDP method showed much betteractivity compared to others. Though, different solvents such astoluene, p-xylene, DMF and DMSO were tried best yield wasobtained in solvent free conditions. General applicability of theprotocol was checked using various substituted benzyl alcohols(Scheme 11).267 Secondly, the author indicated that the stabi-lized gold atoms were in three different states (Au0, Au1+, Au3+).Whereas, only metallic gold species (Au0) was observed on the



Scheme 11 Oxidation of various alcohols by Au@U3O8 catalyst.

Fig. 41 Oxidation of allylic alcohols under solvent free conditions inthe presence of gold and palladium catalysts redraw from the ref. 268.

Scheme 12 (A) The proposed intermediates for the formation of thecorresponding lactone by oxidation of 1,2-benzenedimethanol. (B)The proposed intermediate for the formation of benzaldehyde byoxidation of meso-hydrobenzoine.

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surface of nano gold deposited on non-reducible metal oxidesuch as alumina.263

Corma et al. suggested that the combination of small crystalsize gold (2–5 nm) and nano crystalline ceria (5 nm) as a highlyactive, selective and recyclable catalyst for the solvent and basefree oxidation of alcohols to aldehyde and ketones with highTON and TOF using oxygen at atmospheric pressure. XPSanalysis revealed stabilized AuNPs having surface atoms inAu3+, Au+ and Au0 state. The Au NPs formed on the Au@CeO2

catalyst interact with the nano-metric ceria surface which


stabilizes the positive oxidation states of gold by creating Ce3+

and oxygen decient sites in the ceria. The TOF value 250 000h�1 was reported by this same group.264 Corma et al. and Garciaet al. have suggested that AuNPs supported on nano-crystallineceria were more active and highly chemo selective for theaerobic oxidation of allylic alcohols (Fig. 41).268 They were of theopinion that the lattice oxygen vacancies, which are moreabundant in nanoparticulated ceria favoured interaction andphysisorption of molecular oxygen. Benzylic alcohols eitherprimary or secondary were converted to the corresponding

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Scheme 13 General oxidation of alcohols (A) primary alcohol (B) secondary alcohol (C) tertiary alcohol.

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aldehydes or ketones in quantitative yields. The presence ofsubstituents on the aromatic ring such as NO2–, Cl or –OCH3

inuenced the reaction rate but did not affect the selectivity ofthe product. The Au@npCeO2 catalyst was also tested for theaerobic oxidation of vicinal-diols. Oxidation of 1,2-benzenedimethanol led to benzofuranone in quantitative yields.However, hydro-benzoin underwent oxidative C]C cleavage toform benzaldehyde as the main product (Scheme 12).269

In 2009, Cao et al. reported base free protocol for alcoholoxidation using hydrogen peroxide as oxidant. The groupsynthesized various metal oxides supported catalyst but AuNPs@TiO2 gave the best conversion (99%) with excellent yield(98%). It was shown that small AuNPs can substantially facilitatethe crucial H2O2 decomposition. The synergetic interactionbetween AuNPs and TiO2 support was responsible to achievinghigh activity in the H2O2 mediated oxidation of alcohols. TEMand XPS results showed essentially no changes in the meandiameters of the Au NPs and in the metallic state of Au.253,270

Collaborative work done by Li et al. and Hensen et al. showedthat gold nanoparticles supported on basic hydrozincite andbismuth carbonate support displayed good activity in solvent free

Scheme 14 The proposed mechanism for the aerobic oxidation ofalcohols redraw from the ref. 264.

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system. The basicity of the support played a crucial role inenhancing the activity of this catalyst. The group checked varioussupports such as series of metal (Zn, Bi, Ce, La, and Zr) carbonatefor the stabilization of gold nanoparticles. Among all, bismuthcarbonate and hydrozincite showed excellent activity. AuNPs wereloaded on this support by decomposition precipitation ofaurochloric acid. Maximum benzaldehyde yield of 54% was ach-ieved in case of hydrozincite based catalyst. This yield wasconsiderably higher than that of Au@TiO2 and Au@CeO2. Stuckyet al. also explored low cost promoters such as metal carbonates,acetates or borate as support for AuNPs and used them for aerobicoxidation of alcohols under solvent free conditions at 100 �C. Bychoosing appropriate promoters, product selectivity and alcoholconversion has been simultaneously improved.271

4. Mechanism of alcohol oxidation

Outcome of alcohol oxidation is highly dependent on the type ofcarbon atom to which hydroxyl group is attached. Primaryalcohols are easily oxidized to aldehydes but over oxidationleads to formation of carboxylic acids. According to the mech-anism, primary alcohols on oxidation form aldehydes, whichmay get transformed to carboxylic acid on further oxidationthrough a hydrated aldehyde stage. Secondary alcohols arenormally oxidized to ketones while tertiary alcohols are mostlyunaffected by oxidation (Scheme 13).23–27

Controlling oxidation to aldehyde is the most importantgoal. Several attempts have been made to nd out the reactionpathway of alcohol oxidation over supported gold catalyst usingmolecular oxygen. The adsorption and dissociation of molec-ular oxygen on extended gold nanoparticles surfaces is ener-getically unfavourable. There is growing consensus that it is lowcoordinated gold atoms that can actively adsorb oxygen. Thusoxygen dissociation depends on the morphology of gold nano-particles more than size, Au(100) facet being most active. Thereis also evidence of support playing a key role in this.272 As far asoxidation of alcohols is concerned, according to the commonmechanism reported by Corma et al.,264 Chechik et al.273 andNishimura et al.,274 the oxidation of alcohol generally involvesthree main steps. In the rst step, alcohol molecules are



Fig. 42 The mechanism for dehydrogenation of allyl alcohol to generate allyloxide on the O/Au(111) surface via (a) an oxygen adatom and (b)a hydroxyl species [reprinted with permission from ref. 276. Copyright (2014) American Chemical Society].

Fig. 44 Schematic preparation of bimetallic Au–Cu@TiO2-NB[reprinted with permission from ref. 283. Copyright (2014) RoyalSociety of Chemistry].

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adsorbed on the surface of gold nanoparticles. In the secondstep, b-hydride elimination take place to generate an activecarbonyl species and a metal hydride (Au–H). While in the thirdstep, the metal–hydride (Au–H) is oxidized by elemental oxygento regenerate the gold surface (Scheme 14). Nishimura et al. alsoreported that the rate of catalytic reaction decreased on additionof little quantity of radical scavengers such as BHT and TEMPO.They believed that the scavengers decreased the rate of thereaction by combining with the radical intermediatespecies.274,275 Chechik et al. explored the mechanism by usingelectron paramagnetic resonance (EPR) spectroscopy and spintrapping. With the help of isotope labelling, they proved thatthe hydrogen in the spin adduct originates from the cleavage ofthe C–H bond in the alcohol molecule. The formation of thehydrogen spin adducts most likely resulted from the abstractionof hydrogen from the Au surface.

Mullins et al.,276 Yang et al.277 and Li et al.278 investigated thereactivity of oxygen and hydroxyl species on the surface of gold

Fig. 43 Dehydrogenation of methanol to formaldehyde via OOH*. (a) TTransfer of b-H to OOH* to form formaldehyde and hydrogen peroxideChemical Society].


during the partial oxidation of allyl alcohol. The reactionmechanism for allyl alcohol oxidation was inuenced by therelative proportions of reactive species on the gold surface(Fig. 42). Activation barrier calculated by DFT studies indicatedthat the hydroxyl hydrogen of allyl alcohol is readily abstractedby adsorbed oxygen or hydroxyl species on the gold surface to

ransfer of a-H to OOH* to form methoxy and hydrogen peroxide. (b)[reprinted with permission from ref. 278. Copyright (2013) American

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Scheme 15 Oxidation of different substituted alcohol with Au@zeolite catalyst.

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generate a surface-bound allyl oxide intermediate. This inter-mediate on a-dehydrogenation generated acrolein via interac-tion with an oxygen surface hydroxyl species.276

Li et al., also explained the oxidation of methanol. Accordingto them molecular oxygen can be activated via a hydroperoxyl(OOH) intermediate produced by abstracting a hydrogen atomfrom co-adsorbed methanol or water (Fig. 43). The formed OOHintermediate either directly reacts with methanol to produceformaldehyde or dissociates into adsorbed atomic oxygen andhydroxyl.277,278

5. Photocatalytic oxidation

Photocatalytic organic transformation, a new reaction way tocarry out organic reactions at room temperature and ambientpressure are becoming popular. Its biggest advantage is theproduct selectivity as compared to thermal reactions whereunstable intermediates produce byproducts. Plasmonic metalssuch as gold, with exceptional absorption of visible light havethe potential to photo-catalyse organic reactions.279–281 In 2011,Kominami developed eco-friendly protocol for the oxidation ofbenzyl alcohol using Au@cerium oxide as catalyst. Exception-ally high selectivity (99%) with 99% conversion was reported in20 h. Versatility of the protocol was also established. This reportinspired a number of other research groups to explore thephotocatalytic properties of supported gold catalyst.282 Likewise,Liu et al. reported that Au–Cu/TiO2-NB supported materialpossessed excellent catalytic activity. A new photo-deposition

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technique by galvanic replacement was developed to synthe-size this catalyst (Fig. 44). The authors reported 93% conversionwith 98% selectivity for benzaldehyde at 240 �C in 8 h when Au/Cu in the ratio of 1 : 2.3 was used.283

Ke et al. reported a novel green tool for the oxidation ofbenzyl alcohol to benzaldehyde with high selectivity (99%)under visible-light irradiation at ambient temperature. Thisgroup synthesized Au@zeolites (Y) catalyst by using reductionmethod and checked its catalytic activity for the oxidation ofvarious substituted benzyl alcohols (Scheme 15). A correlationof surface plasmon resonance (SPR) effect of gold nanoparticles,the adsorption capability of zeolite supports and the molecularpolarities of aromatic alcohols, on photocatalytic activity wasdemonstrated.284 In a similar study by Zhu et al. AuNPs@zeolitewere shown to have a strong ability to absorb visible and ul-traviolet light. The catalyst prepared by impregnation methodsshowed 23% conversion of benzyl alcohol with 100% selectivityfor benzaldehyde with molecular oxygen in the presence of baseat ambient temperature. The authors concluded that presenceof base (NaOH) signicantly increased the photocatalyticactivity of the supported gold nanoparticles.285

In 2014, Liu et al. reported a novel green and highly efficientmethodology for the photocatalytic oxidation of alcohol usingAu@hydrotalcite in the absence of base under visible light. Thiscatalyst was prepared by a sequential deposition reductionmethod. Benzyl alcohol conversion of 57.41% with 93.2%selectivity for benzaldehyde under visible light in 24 h wasachieved by this group. The catalyst showed good recyclability



Fig. 45 (A) Effect of light intensity on the photooxidation of benzyl alcohol. (B) Effect of the range of light wavelengths on the photooxidation ofbenzyl alcohol [reprinted with permission from ref. 286. Copyright (2014) Elsevier Publication].

Fig. 46 The mechanism for the aerobic oxidation of benzyl alcoholover the Au–Pd/TiO2 NB nanostructures by using visible light irradia-tion [reprinted with permission from ref. 275. Copyright (2015) RoyalSociety of Chemistry].

Fig. 47 Dependence of TOF on the mean Au nanoparticle size for theAu/HT-catalyzed oxidant-free dehydrogenation of benzyl alcohol.Reaction conditions: catalyst 0.10–0.20 g; benzyl alcohol 1–2 mmol;solvent (p-xylene) 5 mL; air flow rate 3 mL min�1; T ¼ 393 K [reprintedwith permission from ref. 194. Copyright (2011) John Wiley and Sons].

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and was easily recovered by centrifugation from the reactionmixture and aer washing with deionized water and ethanol itcould be reused for the next batch. The authors investigated theeffect of solvents on reaction rates and studied solvents liketoluene, mesitylene, benzotriuoride and 1,4 dioxane. Thepolar solvents did not give good results. More than 40%conversion of benzyl alcohol with excellent selectivity was re-ported in non-polar solvents. However, best conversion(72.93%) was found in benzotriuoride. Effect of metal loadingand wavelength of light was studied and it was reported that theconversion of benzyl alcohol increased as the wavelengthincreased mainly due to the strong absorption of gold nano-particles at 515–535 nm (Fig. 45).286

In a collaborative work, Xu et al. and Liu et al. proposeda mechanism for the photocatalytic oxidation of benzylalcohol275 using Au–Pd bimetallic nanoparticles supported onTiO2 nanobelts (Fig. 46). The proposed photo catalytic mecha-nism is similar to the aerobic dehydrogenation. Interestingly, itwas shown that the presence of PdNPs play more important rolefor the cleavage of O–H and C–H bonds. Secondly, it was foundthat the electronic structure of Pd strongly impacts oxygenactivation, and electrons transfer ability. In the rst step, the


activated Au sites transferred electrons to both Pd sites and TiO2

nanobelts. These electrons, which are present in the Pd sitesand the heterostructured nanobelts can ll the unoccupiedorbitals of oxygen molecules. Thus, transient anion O–O�

species are formed which can cleave the O–H bond of thealcohol to form an alkoxide intermediate. While in the secondstep, the intermediate formed undergoes a hydride transferfrom C–H to the positively charged Au to form benzaldehydeand Au–H species. In the third step, the cleavage of the Au–Hspecies take places and the active sites of the catalyst arerecovered.

6. Effect of particle size

It is understood that as the size of the nanoparticles decreases,its surface area to volume ratio increases. The free surface areaof the nanoparticles is highly important because that increasesthe contact between reactant molecules and catalytic sites.Highly controllable size, shape and morphology make catalystmore successful and selective in many organic transformations.The effect of size on catalytic activity of AuNPs is even more

RSC Adv., 2016, 6, 28688–28727 | 28719


Fig. 49 Stability and recyclability of supported Au NPs to the othercatalyst.

Fig. 50 Catalytic performance of Au NPs@RGO catalysts during therecycling experiment [reprinted with permission from ref. 137. Copy-right (2013) Elsevier Publication].

Fig. 48 1-Phenylethanol conversion and H2O2 decomposition versusgold particle dimension; oxidation of 1-phenylethanol : 10 mmol 1-phenylethanol, Au/TiO2 (Mintek), 10 mL H2O, 90 �C, 5% H2O2 addeddropwise, substrate : H2O2 : Au ¼ 100 : 110 : 1; H2O2 decomposition:1.5 wt% H2O2 aqueous solution 10 mL, Au/TiO2 (Mintek) 80 mg, 25 �Cfor 80min in open air. The rate of decomposition was calculated basedon the amount of H2O2 remaining, which was measured by the titra-tion of 0.1 M Ce(SO4)2 with 1.5 w/v% ferroin solution as an indicator[reprinted with permission from ref. 270. Copyright (2009) RoyalSociety of Chemistry].

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pronounced as it is inactive in bulk form. The catalytic activityof gold catalyst decreases dramatically with increase in size. Inmost of the reports, the authors have concluded that very smallAu NPs (<5 nm) with low loading are very active as well asselective for the alcohol oxidation as compare to bigger size (>5nm) Au NPs.287–289

The effect of size on the activity of catalyst was studied indetail by Fang et al. They proposed that AuNPs supported on HTexhibited higher conversion of benzyl alcohol (94%) when thegold loading was 0.06 wt% (size 4 nm). However, the conversiondecreased as the gold loading was increased up to 0.26 wt%which led to increase in particle size (13.5 nm). In anotherreport, they presented a linear relationship between the size ofAuNPs and the TOF values. Similarly, the work done by Zhanget al. and Wang et al. indicated that the catalytic activity of thecatalyst in terms of TOF value was depended on the size ofAuNPs (Fig. 47) for the oxidation of benzyl alcohol. As theaverage diameter of Au NPs increased from 4 nm to 12 nm, theTOF decreased from 800 h�1 to 200 h�1.194

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According to Corma et al. and Garcia et al. activity of AuNPscorrelates linearly with the total number of external gold atomsand with the surface coverage of the support. They reported thatduring the oxidation of cinnamyl alcohol, TOF value decreasedby increasing the size of AuNPs. However, the selectivity towardscinnamaldehyde remains unchanged that indicated that theselectivity is independent of gold particle size. They studiedAuNPs having sizes 5–25 nm with cubic shape modelled in fcccrystal lattice and best TOF value was reported for AuNPs of size5 nm.269 Similar results were obtained by Cao et al. Theyproposed that the use of smaller Au nanoparticles gave higheractivity. Interestingly, the AuNPs having size less than 5 nmsubstantially facilitate the H2O2 decomposition (99% conver-sion of 1-phenyl ethanol in H2O2). The conversion decreasedwith increasing in size (Fig. 48).270 Similar results were reportedby many others too and it can be concluded that activity of goldis directly related to size.

7. Recyclability

In the past few decades, the supported AuNPs have beendeveloped as a sustainable replacement to conventional homoand heterogeneous catalysts as they combine the ease of sepa-ration with high catalytic activity. Which are the fundamentalrequirements for an ideal catalyst or green catalyst. The biggestadvantage associated with gold catalyst is its higher stabilityand remarkable recyclability as compared to supported transi-tion metal nanoparticles (Fig. 49). In solution phase reactionspolymeric supported nanoparticles can be separated and recy-cled more easily compared to zeolites and oxides which areavailable in powder form. Similarly, nano supports are also noteasy to handle. To overcome these problems, severalapproaches have been developed, such as the immobilization ofAu NPs on a magnetic solid support, which rapidly separatesnanoparticles from the reaction mixture.

Other important issues that are directly associated withrecycling are leaching of the metallic species from



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nanoparticles and agglomeration of nanoparticles. Both leadto decrease in catalytic activity. Among the various supportsreported in literature, polymeric supports such as macroporous divinylbenzene cross-linked material have shownexcellent recyclability.290–292 Nanoparticles attached by cova-lent bonding to the support are generally sufficiently robust tosurvive the harsh reaction conditions; this binding andadsorption process minimizes catalyst leaching and allows thecatalyst to be reused several times. It has been also observedthat the catalyst in which nanoparticles are embedded in thematrix are more stable compared to those where they arepresent on the surface.

In 2008, Kobayashi et al. have investigated highly recyclablepolymer-incarcerated carbon-stabilized gold nanoclusters (PI/CB-Au) for the oxidation of secondary alcohols. The catalystwas easily recovered by simple ltration method and reusedwithout signicant loss of its catalytic activity. No leaching wasobserved in solution. The inclusion of carbon black (CB) to thecomposition of the catalyst enhances the stability of goldnanoclusters. The catalyst could be recovered and reused up tove times without any loss of its activity. Aer ve runs, the yieldfor acetophenone was >99%.169 Li et al. reported that AuNPs@PS composite with excellent recyclability and stabilitytowards benzyl alcohol oxidation. Aer the reaction, the catalystwere easily recovered by centrifugation method and used for thenext run. The activity of this composite was maintained up tove runs with same yield.165

Rossi et al.266 and Zhang et al.199 have reported magneticseparable catalyst to facilitate its from the reactionmixture. But,the conversion decreased and reached 35% aer the fourth runthough selectivity was unchanged. ICP-AES studies indicatedhardly any leaching. Slight change was observed in the size ofAuNPs aer two consecutive runs. In case of Au NPs@HTcatalyst, the size of AuNPs increased up to 0.1 nm. However, sizeof Au NPs was increased up to 0.5 nm for the Au NPs@g-Al2O3

catalysts and the Au NPs were coagulated in case of AuNPs@MgO catalyst.198

Huang et al. and Ma et al. investigated the reusability of AuNPs@RGO catalysts for the oxidation of benzyl alcohol (Fig. 50).Aer the reaction, the catalyst was recovered by simple ltra-tion. The ltrate material was dried in vacuum at roomtemperature for 12 h. Aer complete drying, it was ready to usefor the next batch. Aer ve runs, the conversion of benzylalcohol (BA) was decreased steadily from 65% to 40% while theselectivity towards benzaldehyde remained more than 90%. Theactivity of the catalyst was decreased mainly due to the massloss during the ltration method.137

To conclude supported nanoparticles can be separated usingsimple ltration, centrifugation and magnetic separation.Recycling upto ve cycles is common. Leaching is either absentor negligible. Small increase in size of gold nanoparticles hasbeen reported in some cases.

8. Conclusion and outlook

Heterogenization of nanoparticles using supports is great stepin bridging the gap between homogenous and heterogeneous


catalysis. Alcohols are an important raw materials for manychemical industries and development of green and sustainablemethods for their oxidation is crucial for them. The usage ofgold nanoparticles is advantageous in this respect as they showbetter recycling compared to other transition metals. Especiallynoteworthy achievements of AuNPs in alcohol oxidation arebase free, solvent free reactions using molecular oxygen asoxidant. However, in case of oxidation alcohols designingcatalyst that can control the outcome of a reaction in terms ofactivity and selectivity is the biggest challenge. Activity of goldnanoparticles is largely dependent on the size and greaterunderstanding of nano particle synthesis in recent time hasallowed a much better precision in particle size, shape,composition etc. Thus, great improvements in activity of thecatalysts is reported. In oxidation of secondary alcohols usingAuNPs very high TOFs of the order of 104 or more are easilyachieved. But, in oxidation of primary alcohols besides activity,restricting the outcome of reaction to aldehyde stage is rathertricky as enhancement of catalytic activity leads to acids ratherthan aldehydes. This is where the nature of support has playeda key role i.e. optimisation of activity with selectivity.

Many supports starting from conventional C, SiO2, zeolites,polymers to more advanced CNTs, MOF, graphenes, nanooxides have been explored to achieve the target of high activitywith total selectivity for aldehydes. On one hand this goal hasbeen achieved by enhancing hydrophobicity in polymersupports on the other enhanced interaction of oxides supportssuch as CeO2 and TiO2 has yielded wonderful results. Stillothers like zeolites have enhanced photochemical oxidations.MOFs are very useful in controlling size of nanoparticles andwith many of them oxidative esterication was exclusively ach-ieved. The present review has collected many examples from theliterature that clearly prove the intrinsic activity of supportedAuNPs for some of the important alcohol oxidations. Oxidationof allyl alcohols to useful 3-hydroxypropionic acid is one ofthem. Though, some scientists have focused on stabilisation ofvery small sized clusters (<1 nm) with good dispersion onsupports that has not led to selectivity as far as oxidation ofprimary alcohols is concerned. Recyclability upto ve cycles wasshown by many catalyst and leaching is negligible from most ofthe supports probably because of heterogeneous nature ofcatalysis. With judicious choice of support combining activity,selectivity, accessibility of catalytic sites (low catalyst conc.) andease in handling AuNPs are certainly promising in the eld ofalcohol oxidation. As far as industrialisation is concernedchallenges in terms of scalability, economy and recyclability ofthe catalyst still exist.

Acknowledgements

Authors are grateful to Nanomission DST Project for thenancial support to carry out this work. The authors arethankful to the Department of Chemistry, Gujarat University,Ahmedabad, for providing the necessary facilities. INFLIBNETGandhinagar, are acknowledged for providing the e-sourcefacilities.

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References

1 G. J. Hutchings and M. Haruta, Appl. Catal., A, 2005, 291, 2.2 M. Haruta, Chem. Rec., 2003, 3, 75.3 M. Haruta, Catal. Today, 1997, 36, 153.4 M. Haruta, Gold Bull., 2004, 37, 27.5 M. Hurata, T. Kobayashi, H. Sano and N. Yamada, Chem.Lett., 1987, 2, 405.

6 G. J. Hutchings, J. Catal., 1985, 96, 292.7 S. J. Freakley, Q. He, C. J. Kiely and G. J. Hutchings, Catal.Lett., 2015, 145, 71.

8 L. Prati and G. Martra, Gold Bull., 1999, 32, 96.9 M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114,1827.

10 M. Stratakis and H. Garcia, Chem. Rev., 2012, 112, 4469.11 J. K. Mobley and M. Crocker, RSC Adv., 2015, 5, 65780.12 J. A. Rodriguez, G. Liu, T. Jirsak, J. Hrbek, Z. Chang,

J. Dvorak and A. Maiti, J. Am. Chem. Soc., 2002, 124, 5242.13 A. Taketoshi and M. Haruta, Chem. Lett., 2014, 43, 380.14 G. J. Hutchings, M. Brust and H. Schmidbaur, Chem. Soc.

Rev., 2008, 37, 1759.15 C. D. Pina, E. Falletta and M. Rossi, Chem. Soc. Rev., 2012,

41, 350.16 D. T. Thompson, Top. Catal., 2006, 38, 231.17 G. J. Hutchings, Gold Bull., 2004, 37, 1.18 X. Liu, L. He, Y. M. Liu and Y. Cao, Acc. Chem. Res., 2014, 47,

793.19 M. Haruta, J. Nanopart. Res., 2003, 5, 3.20 C. D. Pina, E. Falletta, L. Prati andM. Rossi, Chem. Soc. Rev.,

2008, 37, 2077.21 S. E. Davis, M. S. Ide and R. J. Davis, Green Chem., 2013, 15,

17.22 M. Haruta, Nature, 2005, 437, 1098.23 J. March, Advanced Organic Chemistry: Reactions;

Mechanism, and Structure, Wiley, New York, 3rd edn, 1985,ISBN 0-471-85472-7.

24 H. C. Brown and C. P. Garg, J. Am. Chem. Soc., 1961, 83,2952.

25 E. J. Corey and G. Schmidt, Tetrahedron Lett., 1979, 20, 399.26 M. Hunsen, Tetrahedron Lett., 2005, 46, 1651.27 J. Herscovici and K. Antonakis, J. Chem. Soc., Chem.

Commun., 1980, 561.28 J. C. Collins, W. W. Hess and F. J. Frank, Tetrahedron Lett.,

1968, 9, 3363.29 C. Wiles, P. Watts and S. J. Haswell, Tetrahedron Lett., 2006,

47, 5261.30 E. J. Corey and J. W. Suggs, Tetrahedron Lett., 1975, 16, 2647.31 S. Czernecki, C. Georgoulis, C. L. Stevens and

K. Vijayakumara, Tetrahedron Lett., 1985, 26, 1699.32 A. J. Mancuso, S. L. Huang and D. Swern, J. Org. Chem.,

1978, 43, 2480.33 K. E. Ptzner and J. G. Moffatt, J. Am. Chem. Soc., 1965, 87,

5661.34 D. B. Dess and J. C. Martin, J. Am. Chem. Soc., 1991, 113,

7277.

28722 | RSC Adv., 2016, 6, 28688–28727

35 S. V. Ley, J. N. William, P. Griffith and S. P. Marsden,Synthesis, 1994, 639.

36 P. L. Anelli, C. Biffi, F. Montanari and S. Quici, J. Org. Chem.,1987, 52, 2559.

37 E. J. Corey and G. Schmidt, Tetrahedron Lett., 1979, 20, 399.38 J. C. Collins, W. W. Hess and F. J. Frank, Tetrahedron Lett.,

1968, 9, 3363.39 R. Ciriminna, P. Hesemann, J. J. Moreau, M. Carraro,

S. Campestrini and M. Pagliaro, Chem.–Eur. J., 2006, 12,5220.

40 Y. Xie, Z. Zhang, S. Hu, J. Song, W. Li and B. Han, GreenChem., 2008, 10, 278.

41 A. Mobinikhaledi, M. Zendehdel and P. Safari, React. Kinet.,Mech. Catal., 2013, 110, 497.

42 K. Marui, Y. Higashiura, S. Kodama, S. Hashidate,A. Nomoto, S. Yano, M. Ueshima and A. Ogawa,Tetrahedron, 2014, 70, 2431.

43 C. Jallabert and H. Riviere, Tetrahedron Lett., 1977, 14, 1215.44 X. Liu, A. Qiu and D. T. Sawyer, J. Am. Chem. Soc., 1993, 115,

3239.45 M. F. Semmelhack, C. R. Schmid, D. A. Cortes and

C. S. Chou, J. Am. Chem. Soc., 1984, 106, 3374.46 B. Betzemeier, M. Cavazzini, S. Quici and P. Knochel,

Tetrahedron Lett., 2000, 41, 4343.47 S. Mannam, S. Alamsetti, S. K. Alamsetti and G. Sekar, Adv.

Synth. Catal., 2007, 349, 2253.48 C. Walling and S. Kato, J. Am. Chem. Soc., 1971, 93, 4275.49 B. Betzemeiera, M. Cavazzinib, S. Quicib and P. Knochela,

Tetrahedron Lett., 2000, 41, 4343.50 E. Duliere, M. Devillers and J. M. Brynaert, Organometallics,

2003, 22, 804.51 M. J. Beier, T. W. Ransen and J. D. Grunwaldt, J. Catal.,

2009, 266, 320.52 N. Dimitratos, A. Villa, D. Wang, F. Porta, D. Su and L. Prati,

J. Catal., 2006, 244, 113.53 K. Mori, T. Hara, T. Mizugaki, K. Ebitani and K. Kaneda, J.

Am. Chem. Soc., 2004, 126, 10657.54 T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki,

K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2008,47, 138.

55 L. F. Liotla, A. M. Venezia, G. Deganello, A. Longo,A. Martorana, Z. Schay and L. Guczi, Catal. Today, 2001,66, 271.

56 J. Chen, Q. Zhang, Y. Wang and H. Wan, Adv. Synth. Catal.,2008, 350, 453.

57 N. Zotova, K. Hellgardt, G. H. Kelsall, A. S. Jessiman andK. K. Hii, Green Chem., 2010, 12, 2157.

58 Y. Uozumi and R. Nakao, Angew. Chem., Int. Ed., 2003, 42,194.

59 X. Yang, X. Wang and J. Qui, Appl. Catal., A, 2010, 382, 131.60 Y. Shiraishi, D. Tsukamoto, Y. Sugano, A. Shiro,

S. Ichikawa, S. Tanaka and T. Hirai, ACS Catal., 2012, 2,1984.

61 X. Jiang, H. Liu, H. Liang, G. Jiang, J. Huang, Y. Hong,D. Huang, Q. Li and D. Sun, Ind. Eng. Chem. Res., 2014,53, 19128.



Review RSC Advances

Publ

ishe

d on

08

Mar

ch 2

016.

Dow

nloa

ded

by S

.V. N

atio

nal I

nstit

ute

of T

echn

olog

y on

19/

03/2

016

06:2

1:35


62 C. M. A. Parlett, P. Keshwalla, S. G. Wainwright,D. W. Bruce, N. S. Hondow, K. Wilson and A. F. Lee, ACSCatal., 2013, 3, 2122.

63 B. Karimi, M. Khorasani, H. Vali, C. Vargas and R. Luque,ACS Catal., 2015, 5, 4189.

64 Y. Shiraishi, H. Sakamoto, Y. Sugano, S. Ichikawa andT. Hirai, ACS Nano, 2013, 7, 9287.

65 J. Y. Lee, H. Lee and H. S. Kim,Mater. Sci. Forum, 2005, 475–479, 2463.

66 D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., 2005, 117,8062.

67 A. S. Sharma, D. Shah and H. Kaur, RSC Adv., 2015, 5, 42935.68 X. Pan and X. Bao, Chem. Commun., 2008, 6271.69 R. M. Deshpande, V. V. Buwa, C. V. Rode, R. V. Chaudhary

and P. V. Mills, Catal. Commun., 2002, 3, 269.70 M. Boudart, Adv. Catal., 1969, 20, 153.71 D. Barkhuizen, I. Mabaso, E. Viljoen, C. Welker, M. Claeys,

E. V. Steen and J. C. Q. Fletcher, Pure Appl. Chem., 2006, 78,1759.

72 M. Valden, X. Lai and D. W. Goodman, Synergism, 1998,281, 1647.

73 S. Sakurai and M. Haruta, Catal. Today, 1996, 29, 361.74 J. F. Jia, K. Harak, J. N. Kondo, K. Domen and K. Tamaru, J.

Phys. Chem., 2000, 104, 11153.75 Liquid Phase Oxidation viaHeterogenous, Catalysis Organic

Synthesis and Industrial Applications, ed. M. G. Clerici and O.A. Kholdeeva, John Wiley & Sons Publications, 2013.

76 R. J. Chimentao, I. Kirm, F. Medina, X. Rodrıguez,Y. Cesteros, P. Salagre, J. E. Sueiras and J. L. G. Fierro,Appl. Surf. Sci., 2005, 252, 793.

77 L. Peng, J. Zhang, S. Yang, B. Han, X. Sang, C. Liu, X. Ma andG. Yang, Chem. Commun., 2015, 51, 4398.

78 N. S. Patil, B. S. Uphade, P. Jana, S. K. Bharagava andV. R. Choudhary, J. Catal., 2004, 223, 236.

79 J. Liu, F. Wang, S. Qi, Z. Gu and G. Wu, New J. Chem., 2013,37, 769.

80 J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C. Y. Su,Chem. Soc. Rev., 2014, 43, 6011.

81 A. Dhakshinamoorthy and H. Garcia, Chem. Soc. Rev., 2012,41, 5262.

82 S. R. Batten, B. F. Hoskins and R. Robson, J. Am. Chem. Soc.,1995, 117, 5385.

83 S. Kitagawa, S. Kawata, Y. Nozaka and M. Munakata, J.Chem. Soc., Dalton Trans., 1993, 1399.

84 N. Ahmad, A. H. Chughtai, H. A. Younus and F. Verpoort,Coord. Chem. Rev., 2014, 280, 1.

85 O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser,C. E. Wilmer, A. A. Sarjeant, R. Q. Snurr, S. T. Nguyn,A. O. Z. R. Yazaydın and J. T. Hupp, J. Am. Chem. Soc.,2012, 134, 15016.

86 H. Deng, S. Grunder, K. E. Cordova, C. Valente,H. Furukawa, M. Hmadeh, F. Gandara, A. C. Whalley,Z. Liu and S. Asahina, Science, 2012, 336, 1018.

87 H. Furukawa, Y. B. Go, N. Ko, Y. K. Park, F. J. U. Romo,J. Kim, M. O'Keeffe and O. M. Yaghi, Inorg. Chem., 2011,50, 9147.


88 N. Ahmad, H. A. Younus, A. H. Chughtai and F. Verpoort,Chem. Soc. Rev., 2015, 44, 6804.

89 A. Corma, H. Garcıa and F. X. L. Xamena, Chem. Rev., 2010,110, 4606.

90 T. Ishida, M. Nagaoka, T. Akita and M. Haruta, Chem.–Eur.J., 2008, 14, 8456.

91 D. Esken, S. Turner, O. I. Lebedev, G. V. Tendeloo andR. A. Fischer, Chem. Mater., 2010, 22, 6393.

92 Y. Luan, Y. Qi, H. Gao, N. Zheng and G. Wang, J. Mater.Chem. A, 2014, 2, 20588.

93 H. Liu, Y. Liu, Y. Li, Z. Tang and H. Jiang, J. Phys. Chem. C,2010, 114, 13362.

94 P. Kaminski, I. Sobczak, P. Decyk, M. Ziolek, W. J. Roth,B. Campo and M. Daturi, J. Phys. Chem. C, 2013, 117, 2147.

95 K. Leus, P. Concepcion, M. Vandichel, M. Meledina,A. Grirrane, D. Esquivel, S. Turner, D. Poelman,M. Waroquier, V. van Speybroeck, G. van Tendeloo,H. Garcıa and P. van Der Voort, RSC Adv., 2015, 5, 22334.

96 M. Muller, S. Turner, O. I. Lebedev, Y. Wang, G. vanTendeloo and R. A. Fischer, Eur. J. Inorg. Chem., 2011, 1876.

97 J. Zhu, P. C. Wang and M. Lu, Appl. Catal., A, 2014, 477, 125.98 D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato, Chem.

Rev., 2006, 106, 1105.99 R. Luque, V. Budarin, J. H. Clark and D. J. Macquarrie, Appl.

Catal., B, 2008, 82, 157.100 V. Budarin, J. H. Clark, J. J. E. Hardy, R. Luque,

K. Milkowski, S. J. Tavener and A. J. Wilson, Angew.Chem., 2006, 118, 3866.

101 V. Budarin, J. H. Clark, F. E. I. Deswarte, J. J. E. Hardy,A. J. Hunt and F. M. Kerton, Chem. Commun., 2005, 2903.

102 Y. Hao, G. P. Hao, D. C. Guo, C. Z. Guo, W. C. Li, M. R. Liand A. H. Lu, ChemCatChem, 2012, 4, 1595.

103 Q. He, P. J. Miedziak, L. Kesavan, N. Dimitratos, M. Sankar,J. A. L. Sanchez, M. M. Forde, J. K. Edwards, D. W. Knight,S. H. Taylor, C. J. Kiely and G. J. Hutchings, FaradayDiscuss., 2013, 162, 365.

104 S. Yan, S. Zhang, Y. Lin and G. Liu, J. Phys. Chem. C, 2011,115, 6986.

105 S. Wang, Q. Zhao, H. Wei, J. Q. Wang, M. Cho, H. S. Cho,O. Terasaki and Y. Wan, J. Am. Chem. Soc., 2013, 135, 11849.

106 M. Mahyari, A. Shaabani, M. Behbahani and A. Bagheri,Appl. Organomet. Chem., 2014, 28, 576.

107 A. Villa, N. Janjic, P. Spontoni, D. Wang, D. S. Su andL. Prati, Appl. Catal., A, 2009, 364, 221.

108 L. Prati, F. Porta, D. Wang and A. Villa, Catal. Sci. Technol.,2011, 1, 1624.

109 C. G. Wildgoose, C. E. Banks and R. G. Compton, Small,2006, 2, 182.

110 C. Wang, S. Guo, X. Pan, W. Chen and X. Bao, J. Mater.Chem., 2008, 18, 5782.

111 K. Okamoto, R. Akiyama, H. Yoshida, T. Yoshida andS. Kobayashi, J. Am. Chem. Soc., 2005, 127, 2125.

112 Z. Liu, X. Lin, J. Y. Lee, W. Zhang, M. Han and L. M. Gan,Langmuir, 2002, 18, 4054.

113 Y. Xing, J. Phys. Chem. B, 2004, 108, 19255.

RSC Adv., 2016, 6, 28688–28727 | 28723


RSC Advances Review

Publ

ishe

d on

08

Mar

ch 2

016.

Dow

nloa

ded

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.V. N

atio

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nstit

ute

of T

echn

olog

y on

19/

03/2

016

06:2

1:35


114 H. S. Kim, H. Lee, K. S. Han, J. H. Kim, M. S. Song,M. S. Park, J. Y. Lee and J. K. Kang, J. Phys. Chem. B, 2005,109, 8983.

115 J. Y. Lee, H. Lee and H. S. Kim,Mater. Sci. Forum, 2005, 475–479, 2463.

116 J. Kong, M. G. Chapline and H. Dai, Adv. Mater., 2001, 13,1384.

117 A. Villa, D. Wang, P. Spontoni, R. Arrigo, D. Su and L. Prati,Catal. Today, 2010, 157, 89.

118 K. Jiang, A. Eitan, L. S. Schadler, P. M. Ajayan andR. W. Siegel, Nano Lett., 2003, 3, 275.

119 R. Kumar, E. Gravel, A. Hagege, H. Li, D. V. Jawale,a D. Verma, I. N. N. Namboothiri and E. Doris, Nanoscale,2013, 5, 6491.

120 D. V. Jawale, E. Gravel, V. Geertsen, H. Li, N. Shah,R. Kumar, J. John, I. N. N. Namboothiri and E. Doris,Tetrahedron, 2014, 70, 6140.

121 M. Liu, R. Zhang and W. Chen, Chem. Rev., 2014, 114, 5117.122 D. Chen, H. Feng and J. Li, Chem. Rev., 2012, 112, 6027.123 G. Brumel, Nature, 2009, 458, 390.124 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang,

M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos andA. A. Firsov, Nature, 2005, 438, 197.

125 A. A. Balandin, S. Ghosh, W. Z. Bao, I. Calizo,D. Teweldebrhan, F. Miao and C. N. Lau, Nano Lett., 2008,8, 902.

126 Y. B. Zhang, Y. W. Tan, H. L. Stormer and P. Kim, Nature,2005, 438, 201.

127 R. S. Ruoff, Nat. Nanotechnol., 2008, 3, 10.128 A. Lerf, H. Y. He, M. Forster and J. Klinowski, J. Phys. Chem.

B, 1998, 102, 4477.129 T. Szaboo, O. Berkesi, P. Forgoo, K. Josepovits, Y. Sanakis,

D. Petridis and I. Dekaany, Chem. Mater., 2006, 18, 2740.130 F. Kim, L. J. Cote and J. X. Huang, Adv. Mater., 2010, 22,

1954.131 G. Eda and M. Chhowalla, Adv. Mater., 2010, 22, 2392.132 J. Liu, J. Tang and J. J. Gooding, J. Mater. Chem., 2012, 22,

12435.133 H. Chen, M. B. Muller, K. J. Gilmore, G. G. Wallace and

D. Li, Adv. Mater., 2008, 20, 3557.134 W. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson,

J. J. Gooding and F. Braet, Angew. Chem., Int. Ed., 2010,49, 2114.

135 X. Xie, J. Long, J. Xu, L. Chen, Y. Wang, Z. Zhang andX. Wang, RSC Adv., 2012, 2, 12438.

136 F. Ren, C. Zhai, M. Zhu, C. Wang, H. Wang, D. Bin, J. Guo,P. Yang and Y. Du, Electrochim. Acta, 2015, 153, 175.

137 X. Yu, Y. Huo, J. Yang, S. Chang, Y. Ma andW. Huang, Appl.Surf. Sci., 2013, 280, 450.

138 M. Mahyari, A. Shaabani and Y. Bide, RSC Adv., 2013, 3,22509.

139 Y. Choi, M. Gu, J. Park, H. K. Song and B. S. Kim, Adv.Energy Mater., 2012, 2, 1510.

140 M. Y. Miao, J. T. Feng, Q. Jin, Y. F. He, Y. N. Liu, Y. Y. Du,N. Zhang and D. Q. Li, RSC Adv., 2015, 5, 36066.

28724 | RSC Adv., 2016, 6, 28688–28727

141 J. Wang, S. A. Kondrat, Y. Wang, G. L. Brett, C. Giles,J. K. Bartley, L. Lu, Q. Liu, C. J. Kiely and G. J. Hutchings,ACS Catal., 2015, 5, 3575.

142 R. Wang, Z. Wu, C. Chen, Z. Qin, H. Zhu, G. Wang,H. Wang, C. Wu, W. Dong, W. Fana and J. Wang, Chem.Commun., 2013, 49, 8250.

143 Q. Wang, X. Cui, W. Guan, X. Zhang, C. Liu, T. Xue,H. Wang and W. Zheng, Microchim. Acta, 2014, 181, 373.

144 X. Wu, S. Guo and J. Zhang, Chem. Commun., 2015, 51,6318.

145 V. Budarin, J. H. Clark, R. Luque, D. J. Macquarrie andR. J. White, Green Chem., 2008, 10, 382.

146 G. Glaspell, H. M. A. Hassan, A. Elzatahry, V. Abdalsayedand M. S. Shall, Top. Catal., 2008, 47, 22.

147 F. He and D. Zhao, Environ. Sci. Technol., 2005, 39, 3314.148 K. R. Reddy, N. S. Kumar, P. S. Reddy, B. Sreedhar and

M. L. Kantam, J. Mol. Catal. A: Chem., 2006, 252, 12.149 N. Kotelnikova, U. Vainio, K. Pirkkalainen and R. Serimaa,

Macromol. Symp., 2007, 254, 74.150 A. Corma, P. Concepcion, I. Dominguez, V. Fornes and

M. J. Sabater, J. Catal., 2007, 251, 39.151 B. W. Zhu, T. T. Lim and J. Feng, Chemosphere, 2006, 65,

1137.152 M. J. Laudenslager, J. D. Schiffman and C. L. Schauer,

Biomacromolecules, 2008, 9, 2682.153 Y. Hong and A. Sen, Chem. Mater., 2007, 19, 961.154 G. Schmid, V. Maihack, F. Lantermann and S. Peschel, J.

Chem. Soc., Dalton Trans., 1996, 589.155 J. Virkutyte and R. S. Varma, Chem. Sci., 2011, 2, 837.156 M. N. V. Ravi Kumar, R. A. A. Muzzarelli, C. Muzzarelli,

H. Sashiwa and A. J. Domb, Chem. Rev., 2004, 104, 6017.157 A. Murugadossa and H. Sakuraia, J. Mol. Catal. A: Chem.,

2011, 341, 1.158 K. Deplanche, I. P. Mikheenko, J. A. Bennett, M. Merroun,

H. Mounzer, J. Wood and L. E. Macaskie, Top. Catal.,2011, 54, 1110.

159 L. Tang, X. Guo, Y. Li, S. Zhang, Z. Zha and Z. Wang, Chem.Commun., 2013, 49, 5213.

160 Y. Hong, X. Jing, J. Huang, D. Sun, T. O. Wubah, F. Yang,M. Du and Q. Li, ACS Sustainable Chem. Eng., 2014, 2, 1752.

161 S. Pradhan, D. Ghosh and S. Chen, ACS Appl. Mater.Interfaces, 2009, 1, 2060.

162 Y. Hong, X. Jing, J. Huang, D. Sun, T. O. Wubah, F. Yang,M. Du and Q. Li, ACS Sustainable Chem. Eng., 2014, 2, 1752.

163 N. Panziera, P. Pertici, L. Barazzone, A. W. Caporusso,G. Vitulli, P. Salvadori, S. Borsacchi, M. Geppi,C. A. Veracini, G. Martra and L. Bertinetti, J. Catal., 2007,246, 351.

164 H. E Ling, W. U. T. Bin, H. U Baoji, C. Yan, W. U. S. Xiang,J. Tao and H. B. Xing, Sci. China: Chem., 2010, 53, 1592.

165 Y. Li, Y. Gao, C. Yang, S. Sha, J. Hao and Y. Wu, RSC Adv.,2014, 4, 24769.

166 A. M. Caporusso, P. Innocenti, L. A. Aronica, G. Vitulli,R. Gallina, A. Biffis, M. Zec-ca and B. Corain, J. Catal.,2005, 234, 1.

167 J. C. Garcia-Martinez, R. Lezutekong and R. M. Crooks, J.Am. Chem. Soc., 2005, 127, 5097.



Review RSC Advances

Publ

ishe

d on

08

Mar

ch 2

016.

Dow

nloa

ded

by S

.V. N

atio

nal I

nstit

ute

of T

echn

olog

y on

19/

03/2

016

06:2

1:35


168 H. Miyamura, R. Matsubara, Y. Miyazaki and S. Kobayashi,Angew. Chem., Int. Ed., 2007, 46, 4151.

169 C. Lucchesi, T. Inasaki, H. Miyamura, R. Matsubara andS. Kobayashi, Adv. Synth. Catal., 2008, 350, 1996.

170 A. Buonerba, C. Cuomo, S. O. Sanchez, P. Canton andA. Grassi, Chem.–Eur. J., 2012, 18, 709.

171 W. L. Wei, H. Y. Zhu, C. L. Zhao, M. Y. Huang andY. Y. Jiang, React. Funct. Polym., 2004, 59, 33.

172 H. Tsunoyama, H. Sakurai, Y. Negishi and T. Tsukuda, J.Am. Chem. Soc., 2005, 127, 9374.

173 S. P. Flores, D. Wang, D. A. Wheeler, R. Newhouse,J. K. Hensel, A. Schwartzberg, L. Wang, J. Zhu,M. B. Flores and J. Z. Zhang, J. Mater. Chem., 2011, 21, 2344.

174 S. Marx and A. Baiker, J. Phys. Chem. C, 2009, 113, 6191.175 S. Kanaoka, N. Yagi, Y. Fukuyama, S. Aoshima,

H. Tsunoyama, T. Tsukuda and H. Sakurai, J. Am. Chem.Soc., 2007, 129, 12060.

176 K. Mikkelsen, B. Cassidy, N. Hofstetter, L. Bergquist,A. Taylor and D. A. Rider, Chem. Mater., 2014, 26, 6928.

177 V. S. Myers, M. G. Weir, E. V. Carino, D. F. Yancey, S. Pandeand R. M. Crooks, Chem. Sci., 2011, 2, 1632.

178 J. Zheng, S. Lin, X. Zhu, B. Jiang, Z. Yang and Z. Pan, Chem.Commun., 2012, 48, 6235.

179 P. Shi, C. Gao, X. He, P. Sun andW. Zhang,Macromolecules,2015, 48, 1380.

180 N. K. Chaki, H. Tsunoyama, Y. Negishi, H. Sakurai andT. Tsukuda, J. Phys. Chem. C, 2007, 111, 4885.

181 Y. Yuan, N. Yan and P. J. Dyson, Inorg. Chem., 2011, 50,11069.

182 H. Tsunoyama, N. Ichikumi, H. Sakurai and T. Tsukuda, J.Am. Chem. Soc., 2009, 131, 7086.

183 N. Wang, T. Matsumoto, M. Ueno, H. Miyamura andS. Kobayashi, Angew. Chem., Int. Ed., 2009, 48, 4744.

184 Y. Or, B. Samanta and V. M. Rotello, Chem. Soc. Rev., 2008,37, 1814.

185 X. Wang, H. Kawanami, N. M. Islam, M. Chattergee,T. Yokoyama and Y. Ikushima, Chem. Commun., 2008, 4442.

186 J. Han, Y. Liu, L. Li and R. Guo, Langmuir, 2009, 25, 11054.187 F. Cavani, F. Triru and A. Vaccari, Catal. Today, 1991, 11,

173.188 P. J. Sideris, U. G. Nielsen, Z. Gan and C. P. Grey, Science,

2008, 321, 113.189 D. G. Evans and R. C. T. Slade, Struct. Bonding, 2006, 119, 1.190 L. Desigaux, M. B. Belkacem, P. Richard, J. Cellier, P. Leone,

L. Cario, F. Leroux, C. T. Gueho and B. Pitard, Nano Lett.,2006, 6, 199.

191 M. Fogg, A. L. Rohl, G. M. Parkinson and D. O. Hare, Chem.Mater., 1999, 11, 1194.

192 S. Nishimura, A. Takagaki and K. Ebitani, Green Chem.,2013, 15, 2026.

193 J. C. A. A. Roelofs, D. J. Lensveld, A. J. van Dillen and K. P. deJong, J. Catal., 2001, 203, 184.

194 W. Fang, J. Chen, Q. Zhang, W. Deng and Y. Wang, Chem.–Eur. J., 2011, 17, 1247.

195 M. R. Sturgeon, M. H. O'Brien, P. N. Ciesielski, R. Katahira,J. S. Kruger, S. C. Chmely, J. Hamlin, K. Lawrence,


G. B. Hunsinger, T. D. Foust, R. M. Baldwin, M. J. Biddyand G. T. Beckham, Green Chem., 2014, 16, 824.

196 T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa andK. Kaneda, Adv. Synth. Catal., 2009, 351, 1890.

197 W. Fang, Q. Zhang, J. Chen, W. Deng and Y. Wang, Chem.Commun., 2010, 46, 1547.

198 C. Xu, Z. Wang, X. Huangfu and H.Wang, RSC Adv., 2014, 4,27337.

199 F. Mi, X. Chen, Y. Ma, S. Yin, F. Yuan and H. Zhang, Chem.Commun., 2011, 47, 12804.

200 P. Liu, Y. Guan, R. A. V. Santen, C. Li and E. J. M. Hensen,Chem. Commun., 2011, 47, 11540.

201 L. Li, L. Dou and H. Zhang, Nanoscale, 2014, 6, 3753.202 J. Zhao, G. Yu, K. Xin, L. Li, T. Fu, Y. Cui, H. Liu, N. Xue,

L. Peng and W. Ding, Appl. Catal., A, 2014, 482, 294.203 J. Wang, X. Lang, Z. Bao, M. Jia, J. Wang, X. Guo and J. Zhao,

ChemCatChem, 2014, 6, 1737.204 P. Liu, V. Degirmenci and E. J. M. Hensen, J. Catal., 2014,

313, 80.205 J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz,

C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olsonand E. W. Sheppard, J. Am. Chem. Soc., 1992, 114, 10834.

206 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartulli andJ. S. Beck, Nature, 1992, 359, 710.

207 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson,B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.

208 C. Li and Y. Liu, Bridging Heterogenous and HomogenousCatalysis, Concepts, Stratergies and Applications, Wiley-VCH, 2014.

209 L. B. Sun, X. Q. Liu and H. C. Zhou, Chem. Soc. Rev., 2015,44, 5092.

210 M. N. Garcia, M. Martis, D. L. Castello, D. C. Amoros,K. Mori and H. Yamanshita, Catal. Sci. Technol., 2015, 5,364.

211 A. M. G. Pedrosa, M. J. B. Souza, D. M. A. Melo andA. S. Araujo, Mater. Res. Bull., 2006, 41, 1105.

212 L. Wang, J. Zhang, X. Yi, A. Zheng, F. Deng, C. Chen, Y. Ji,F. Liu, X. Meng and F. S. Xiao, ACS Catal., 2015, 5, 2727.

213 J. Jae, G. A. Tompsett, A. J. Foster, K. D. Hammond,S. M. Auerbach, R. F. Lobo and G. W. Huber, J. Catal.,2011, 279, 257.

214 J. Kang, K. Cheng, L. Zhang, Q. Zhang, J. Ding, W. Hua,Y. Lou, Q. Zhai and Y. Wang, Angew. Chem., Int. Ed., 2011,50, 5200.

215 M. V. Patil, M. K. Yadav and R. V. Jasra, J. Mol. Catal. A:Chem., 2007, 277, 72.

216 R. I. Kureshy, S. Singh, N. H. Khan, S. H. R. Abdi, E. Sureshand R. V. Jasra, J. Mol. Catal. A: Chem., 2007, 264, 162.

217 A. K. Shah, N. H. Khan, G. Sethia, S. Saravanan,R. I. Kureshy, S. H. R. Abdi and H. C. Bajaj, Appl. Catal.,A, 2012, 419–420, 22.

218 K. Komura, Y. Taninaka, Y. Ohtaki and Y. Sugi, Appl. Catal.,A, 2010, 388, 211.

219 M. Zahmakıran, Y. Tonbul and S. Ozkar, J. Am. Chem. Soc.,2010, 132, 6541.

220 J. Hajek, N. Kumar, P. Maki-Arvela, T. Salmi andD. Y. Murzin, J. Mol. Catal. A: Chem., 2004, 217, 145.

RSC Adv., 2016, 6, 28688–28727 | 28725


RSC Advances Review

Publ

ishe

d on

08

Mar

ch 2

016.

Dow

nloa

ded

by S

.V. N

atio

nal I

nstit

ute

of T

echn

olog

y on

19/

03/2

016

06:2

1:35


221 Z. Konya, V. F. Puntes, I. Kiricsi, J. Zhu, J. W. Ager, M. K. Ko,H. Frei, P. Alivisatos and G. A. Somorjai, Chem. Mater.,2003, 15, 1242.

222 X. Y. Zhang, D. X. Liu, D. D. Xu, S. Asahina, K. A. Cychosz,K. V. Agrawal, A. Wahedi, A. Bhan, A. Hashimi, O. Terasaki,M. Thommes and M. Tsapatsis, Science, 2012, 336, 1684.

223 Y. Y. Sun and R. Prins, Angew. Chem., Int. Ed., 2008, 47,8478.

224 G. T. Neumann and J. C. Hicks, ACS Catal., 2012, 2, 642.225 W. C. Yoo, S. Kumar, R. L. Penn, M. Tsapatsis and A. Stein,

J. Am. Chem. Soc., 2009, 131, 12377.226 J. Li, A. Corma and J. Yu, Chem. Soc. Rev., 2015, 44, 7112.227 Y. Liu, H. Tsunoyama, T. Akita and T. Tsukuda, J. Phys.

Chem. C, 2009, 113, 13457.228 L. Wang, X. Meng, B. Wang, W. Chi and F. S. Xiao, Chem.

Commun., 2010, 46, 5003.229 T. Wang, X. Yuan, S. Li, L. Zeng and J. Gong, Nanoscale,

2015, 7, 7593.230 M. Kokate, S. Dapurkar, K. Garadkar and A. Gole, J. Phys.

Chem. C, 2015, 119, 14214.231 G. Li, D. I. Enache, J. Edwards, A. F. Carley, D. W. Knight

and G. J. Hutchings, Catal. Lett., 2006, 110, 7.232 I. Moreno, N. F. Dummer, J. K. Edwards, M. Alhumaimess,

M. Sankar, R. Sanz, P. Pizarro, D. P. Serrano andG. J. Hutchings, Catal. Sci. Technol., 2013, 3, 2425.

233 A. Riisager, R. Fehrmann, H. Haumann, B. S. K. Gorle andP. Wasserscheid, Ind. Eng. Chem. Res., 2005, 44, 9853.

234 P. Zhang, Z. Qiao, X. Jiang, G. M. Veith and S. Dai, NanoLett., 2015, 15, 823.

235 A. Barau, V. Budarin, A. Caragheorgheopol, R. Luque,D. J. Macquarrie, A. Prelle, V. S. Teodorescu andM. Zaharescu, Catal. Lett., 2008, 124, 204.

236 A. Sandoval, A. Gomez-Cortes, R. Zanella, G. Diaz andJ. M. Saniger, J. Mol. Catal. A: Chem., 2007, 278, 200.

237 R. B. Bedford, U. G Singh, R. I. Walton, R. T. Williams andS. A. Davis, Chem. Mater., 2005, 17, 701.

238 S. Senkan, M. Kahn, S. Duan, A. Ly and C. Ledholm, Catal.Today, 2006, 117, 291.

239 A. Gniewek, J. J. Ziolkowski, A. M. Trzeciak, M. Zawadzki,H. Grabowska and J. Wzryszcz, J. Catal., 2008, 254, 121.

240 D. Barkhuizen, I. Mabaso, E. Viljoen, C. Welker, M. Claeys,E. V. Steen and J. C. Q. Fletcher, Pure Appl. Chem., 2006, 78,1759.

241 N. Perkas, Z. Zhong, J. Grinblat and A. Gedanken, Catal.Lett., 2008, 120, 19.

242 M. Haruta, Chem. Rec., 2003, 3, 75.243 T. Ishida and M. Haruta, Angew. Chem., 2007, 119, 7288.244 M. A Aramendia, J. C. Colmenares, A. Marinas,

J. M. Marinas, J. M. Moreno, J. A. Navio and F. J. Urbano,Catal. Today, 2007, 128, 235.

245 A. Abad, A. Corma and H. Garcia, Chem.–Eur. J., 2008, 14,212.

246 J. M. Moreno, M. A. Aramendia, A. Marinas, J. M. Marinasand F. J. Urbano, Appl. Catal., B, 2007, 76, 34.

247 M. J. Jacinto, P. K. Kiyohara, S. H. Masunaga, R. F. Jardimand L. M. Rossi, J. Mol. Catal. A: Chem., 2008, 338, 52.

28726 | RSC Adv., 2016, 6, 28688–28727

248 D. I. Enache, D. W. Knight and G. J. Hutchings, Catal. Lett.,2005, 103, 43.

249 D. I. Enache, J. K. Edwards, P. Landon, B. S. Espriu,A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely,D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362.

250 J. Sa, S. F. R. Taylor, H. Daly, A. Goguet, R. Tiruvalam, Q. He,C. J. Kiely, G. J. Hutchings and C. Hardacre, ACS Catal.,2012, 2, 552.

251 J. Feng, C. Ma, P. J. Miedziak, J. K. Edwards, G. L. Brett,D. Li, Y. Du, D. J. Mor-gan and G. J. Hutchings, DaltonTrans., 2013, 42, 14498.

252 M. Alhumaimess, Z. Lin, Q. He, L. Lu, N. Dimitratos,N. F. Dummer, M. Conte, S. H. Taylor, J. K. Bartley,C. J. Kiely and G. J. Hutchings, Chem.–Eur. J., 2014, 20, 1701.

253 N. Zheng and G. D. Stucky, Chem. Commun., 2007, 3862.254 B. Kimmerle, J. D. Grunwaldt and A. Baiker, Top. Catal.,

2007, 44, 1.255 L. C. Wang, Y. M. Liu, M. Chen, Y. Cao, H. Y. He and

K. N. Fan, J. Phys. Chem. C, 2008, 112, 6981.256 P. Haider, B. Kimmerle, F. Krumeich, W. Kleist,

J. D. Grunwaldt and A. Baiker, Catal. Lett., 2008, 125, 169.257 P. Fristrup, L. B. Johansen and C. H. Christensen, Catal.

Lett., 2008, 120, 184.258 V. R. Choudhary and D. K. Dumbre, Ind. Eng. Chem. Res.,

2009, 48, 9471.259 S. Mandal, C. Santra, K. K. Bando, O. O. Jamesa, S. Maity,

D. Mehta and V. Chowdhury, J. Mol. Catal. A: Chem.,2013, 378, 47.

260 A. Kumar, V. P. Kumar, B. P. Kumar, V. Vishwanathan andV. R. Choudhary, Catal. Lett., 2014, 6, 1285.

261 Z. Wang, C. Xu and H. Wang, Catal. Lett., 2014, 144, 1919.262 E. M. de Moura, M. A. S. Garcia, R. V. Goncalves,

P. K. Kiyohara, R. F. Jardim and L. M. Rossi, RSC Adv.,2015, 5, 15035.

263 V. R. Choudhary and D. K. Dumbre, Top. Catal., 2009, 52,1677.

264 A. Abad, P. Concepcion, A. Corma and H. Garcia, Angew.Chem., 2005, 44, 4066.

265 M. J. Jacinto, O. H. C. F. Santos, R. F. Jardim, R. Landersand L. M. Rossi, Appl. Catal., A, 2009, 360, 177.

266 R. L. Oliveira, P. K. Kiyohar and L. M. Rossi, Green Chem.,2010, 12, 144.

267 V. R. Choudhary, R. Jha and P. Jana, Green Chem., 2007, 9,267.

268 A. Abad, C. Almela, A. Corma and H. Garcia, Chem.Commun., 2006, 3178.

269 A. Abad, A. Corma and H. Garcia, Chem.–Eur. J., 2008, 14,212.

270 J. Ni, W. J. Yu, L. He, H. Sun, Y. Cao, H. Y. He and K. N. Fan,Green Chem., 2009, 11, 756.

271 J. Yang, Y. Guan, T. Verhoeven, R. V. Santen, C. Li andE. J. M. Hensen, Green Chem., 2009, 11, 322.

272 M. Boronat and A. Corma, Dalton Trans., 2010, 39, 8538.273 M. Conte, H. Miyamura, S. Kobayashi and V. Chechik, J.

Am. Chem. Soc., 2009, 131, 7189.274 S. Nishimura, Y. Yakita, M. Katayama, K. Higashimine and

K. Ebitani, Catal. Sci. Technol., 2013, 3, 351.



Review RSC Advances

Publ

ishe

d on

08

Mar

ch 2

016.

Dow

nloa

ded

by S

.V. N

atio

nal I

nstit

ute

of T

echn

olog

y on

19/

03/2

016

06:2

1:35


275 T. Jiang, C. Jia, L. Zhang, S. He, Y. Sang, H. Li, Y. Li, X. Xuand H. Liu, Nanoscale, 2015, 7, 209.

276 G. M. Muller, L. Zhang, E. J. Evans, T. Van, G. Henkelmanand C. B. Mullins, J. Am. Chem. Soc., 2014, 136, 6489.

277 L. Wang, C. He, W. Zhang, Z. Li and J. Yang, J. Phys. Chem.C, 2014, 118, 17511.

278 C. R. Chang, X. F. Yang, B. Long and J. Li, ACS Catal., 2013,3, 1693.

279 C. Y. Yang, Z. Wang, T. Q. Lin, X. M. Xie and M. H. Jiang, J.Am. Chem. Soc., 2013, 135, 17831.

280 X. B. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331,746.

281 Z. Wang, C. Y. Yang, T. Q. Lin, X. M. Xie and M. H. Jiang,Energy Environ. Sci., 2013, 6, 3007.

282 A. Tanaka, K. Hashimoto and H. Kominami, Chem.Commun., 2011, 47, 10446.

283 Q. Jia, D. Zhao, B. Tang, N. Zhao, H. Li, Y. Sang, N. Bao,X. Zhang, X. Xu and H. Liu, J. Mater. Chem. A, 2014, 2,16292.


284 X. Zhang, X. Ke and H. Zhu, Chem.–Eur. J., 2012, 18, 8048.285 H. Zhu, X. Chen, Z. Zheng, X. Ke, E. Jaatinen, J. Zhao,

C. Guo, T. Xie and D. Wang, Chem. Commun., 2009, 7524.286 J. Yu, J. Li, H. Wei, J. Zheng, H. Su and X. Wang, J. Mol.

Catal. A: Chem., 2014, 395, 128.287 X. Xie, Y. Li, Z. Q. Liu andM. Haruta, Nature, 2008, 458, 746.288 T. Ishida and M. Haruta, Angew. Chem., Int. Ed., 2007, 46,

7154.289 Nanoparticles and Catalysis, ed. D. Astruc, Wiley-VCH Verlag

GmbH, Weinheim, 2008.290 Nanocatalysis: Synthesis and applications, ed. V.

Polshettiwar and T. Asefa, John Wiley & Sons Publication,2013.

291 Metal Nanoparticles for catalysis Advances and Applications,ed. F. Tao, The Royal Society of Chemistry, 2014.

292 A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed.,2006, 47, 7896.

RSC Adv., 2016, 6, 28688–28727 | 28727


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