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Coordination Chemistry Reviews 288 (2015) 118–143

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

j ourna l h omepage: www.elsev ier .com/ locate /ccr

eview

ilica-decorated magnetic nanocomposites for catalytic applications

anoj B. Gawandea, Yukti Mongab, Radek Zboril a, R.K. Sharmab,∗

Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University, Slechtitelu 11, 783 71lomouc, Czech RepublicDepartment of Chemistry, University of Delhi, Delhi 110007, India1

ontents

1. Introductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1191.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1191.2. Catalysis by nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1191.3. Nanocatalysts for sustainable organic protocols – a magnetic approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

2. Fabrication of metal-immobilized silica-coated magnetic nanocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1202.1. Fabrication of MNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1202.2. Surface modification of MNPs using silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1212.3. Surface functionalization of SMNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1232.4. Metal/metal complex immobilization of SMNPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

3. Applications of silica-coated magnetic nanocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1263.1. Carbon–carbon coupling reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1263.2. Acetylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1303.3. Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1303.4. Hydrogenation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1323.5. Olefin metathesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343.6. Asymmetric synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353.7. Photocatalytic activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373.8. Biocatalysis applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373.9. Miscellaneous applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

4. Conclusions and future scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

r t i c l e i n f o

rticle history:eceived 12 September 2014ccepted 2 January 2015vailable online 14 January 2015

eywords:ilica-coated magnetic nanocatalystsore–shellunctionalizationrganic transformations

a b s t r a c t

Stringent demands from society to address environmental issues and achieve an economically sustain-able environment have placed tremendous pressure on industries to replace homogeneous catalyst withrecoverable heterogeneous alternatives. Owing to recent advances in science and catalysis technology,these transformations have been revisited for the purpose of developing new nanocatalysts. In this review,we first discuss the background behind the huge interest in the important class of silica-coated magneticnanoparticles (SMNPs) as nanocatalysts. Next, we discuss the importance of the unique arrangementof silica over magnetic nanoparticles (MNPs). Particular attention is given to recent developments andadvances in spatial organization, functionalization and protection of these silica supported nanocatalysts.Synthetic protocols and applications of these nanomaterials for different organic transformations, such as

carbon–carbon coupling reactions, acetylation, oxidation, hydrogenation, olefin-metathesis, asymmetric synthesis, photo-catalytic and other green reactions, are briefly discussed, providing a basic outline of the work performed to date. Erecoverable, efficient and selecthe primary objective of these

∗ Corresponding author. Tel.: +420 585634385.E-mail addresses: manoj.gawande@upol.cz (M.B. Gawande), rksharmagreenchem@ho

1 Tel.: +91 011 27666250; fax: +91 011 27666250.

ttp://dx.doi.org/10.1016/j.ccr.2015.01.001010-8545/© 2015 Elsevier B.V. All rights reserved.

xploration of the potential contribution of these nanoparticles (NPs) astive catalysts in various industrially significant organic transformations is

methodologies.© 2015 Elsevier B.V. All rights reserved.

tmail.com (R.K. Sharma).

Chemistry Reviews 288 (2015) 118–143 119

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M.B. Gawande et al. / Coordination

. Introductions

.1. Background

In modern science and technology, there are always some newnnovations around the corner to investigate and explore [1,2].dvances in the area of catalysis and green sustainable technologiesequire long-term investment in research and development [3–5].he rising demand for a sustainable environment from society islacing tremendous pressure on chemists to transform catalyticechnologies from highly energy consumptive to energy neutralrocesses [6–8].

Despite the fact that heterogeneous catalysis is favoured forany applications, most catalytic processes employed for the man-

facture of bulk as well as fine chemicals are homogenous in nature.owever, homogeneous catalysts suffer from several drawbacks

elated to the prerequisites and handling of sensitive metal-ligandomplexes, deactivation or poisoning of the catalyst during reac-ion and difficulty in recovering and recycling expensive reagentshat impose hazardous impacts on the surrounding environment byroducing large amounts of waste materials [9–13]. Therefore, theajority of industrial-scale catalysts are desirably heterogeneous

o that they can be easily separated and tailored in accordanceith continuous and large-scale operations [14–17]. Yet, in contrast

o their homogeneous counterparts, they are much more difficulto develop practically, one reason is their complexity, which pre-ludes their analysis at a molecular level and development throughtructure–reactivity relationships. In addition, the scope of tradi-ional heterogeneous catalysts is rather limited owing to restrictedarticipation of the available active sites (only on the surface).o overcome this problem, several attempts have been made toombine different types of catalysts to obtain synergism, e.g. byombining a homogeneous catalyst with high selectivity and het-rogeneous catalyst with better industrial handling and potentialo use cleaner technology [18–23]. This can be achieved by effi-ient anchoring of the homogeneous metal complex (catalyst) onolid supports. This is known as “heterogenization of homogeneousatalytic systems” [24–26]. These third generation catalysts notnly preserve the activity and selectivity of homogeneous cata-ysts but also allow facile recovery and reuse of the catalyst akin toeterogeneous catalysts. However, during applications, problemsssociated with leaching of active catalytic molecules and partic-pation of sites on the surface of the solid support can decreasehe overall activity of these hybrid catalysts [27–29]. As a result,hemists have sought to find alternative catalytic systems thatffer comparable activity and selectivity but easier separation andeusability.

.2. Catalysis by nanoparticles

In the 21st century, various branches of scientific discipline werereated due to the expansion of science and technology. Many ofhese branches promise to offer further developments in the future.ne such popular branch that has provided solutions to many of thebovementioned problems is “Nanotechnology” [30–35].

Nanotechnology has become a strategic alternative for raising nation’s core competitiveness. It is also one of the areas wherendustries expect to realize leapfrogging development. Owing tohe great demand and advancement of chemical industries, Nano-echnology in turn, has given birth to a new technology revolutionnd created huge developments in catalysis as “nanocatalysis”. Dueo their nano-size, these catalysts have opened up new avenues

or producing a variety of products with extremely high activitynd selectivity, low energy consumption and long lifetime. Nano-etric catalysts also have the advantage of a large exposed surface

rea of the active component of the catalyst, thereby enhancing the

Fig. 1. Advantages of nanocatalysts.

contact between reactants and catalyst (Fig. 1). Their activity can bemanipulated by altering their sites and morphology, including size,shape composition electronic structure and thermal and chemicalstability [36,37].

1.3. Nanocatalysts for sustainable organic protocols – a magneticapproach

Green Chemistry is nowadays considered an attractive path-way because of its ability to improve chemical activities whilstavoiding the undesirable side effects of toxic and hazardous chem-icals. Nano-materials are suitable candidates for Green Chemistrycatalysis due to their high efficiency, great stability, durability andcost effectiveness [30–38]. Nanocatalysts also alleviate the needfor harsh reaction conditions and increase energy efficiency duetheir high catalytic activity, while their high selectivity reducesby-products and allows chemical reactions to be performed in aspecific manner with minimal consumption of substances [38–40].Although nanocatalysts have numerous advantages for practi-cal applications, their separation and regeneration is often notstraightforward. To overcome this problem various well-definednano-structured catalysts have been designed using the newsynthetic Nanotechnology. The introduction of magnetically recov-erable nanocatalysts has helped to meet important criteria for thedesign of many modern catalytic processes [34,41–51]. The use ofMNPs in catalysis not only solves the abovementioned problemsbut also addresses other Green Chemistry principles of “envi-ronmental remediation and development of alternative energysources” [28,44,52–57]. The unique advantages of magneticallyrecoverable nanocatalysts stem from their capability for easy andfast separation even in large sample volumes by using externalmagnetic forces with respect to the mother liquor without theneed for time-consuming centrifugation and filtration steps. Hence,the use of magnetically recoverable nanocatalysts is potentially a“double green dream”, which not only saves time but also avoidsproblems such as loss of catalyst, catalyst oxidation, and need foradditional solvent and subsequent generation of organic residues[58].

It is widely acknowledged that there is a growing need formore environmentally acceptable processes and catalysts in thechemical industry. This trend has become known as ‘Green Chem-istry’ or ‘Sustainable Technology’. The objective of this reviewis to present a broad overview of silica-decorated magneticallyrecoverable nanocatalysts. The main focus is to provide a brief

account of synthesis, characterization and catalytic applicationsof silica-based magnetic nanomaterial supported catalysts. Webegin with an introductory discussion on catalysis, including itsrecent advancement with Nanotechnology. Then, we discuss the

1 Chemistry Reviews 288 (2015) 118–143

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ole of MNPs as catalysts and the need for its encapsulationith silica. The review also covers all catalytic reactions catalyzed

y silica-decorated magnetically responsive based nanocatalysts.e believe that the in-depth discussion on versatile silica-based

ron-cored supported nanocatalysts and their applications woulde helpful to a broad community of scientists working in theelds of nanotechnologies, chemical engineering, organic chem-

stry, inorganic chemistry and materials chemistry and it is only matter of time before the great potential of silica-coated mag-etic nanocatalysts is realized and implemented on a far-reachingcale.

. Fabrication of metal-immobilized silica-coated magneticanocatalysts

The synthesis of a metal complex immobilized silica-coatedagnetic nanocatalyst was achieved using the steps depicted in

cheme 1 and Table 1.

.1. Fabrication of MNPs

The synthesis of magnetic iron oxide nanoparticles (NPs) up tillow has been carried out using physical methods (such as gas phaseeposition and electron beam lithography), wet chemical methodsnd biological methods.

Among the various methods available, chemical methodsnclude co-precipitation, thermal decomposition, microemulsionnd hydrothermal processes [99–101]. Owing to their straightfor-ard nature, such procedures for the preparation of iron oxideanocomposites have been widely used in basic research as well as

ndustrial applications [77]. One of the major advantages of usinghese protocols is that once the synthetic parameters are fixed, suchs reaction temperature, strength of the media, pH and Fe2+/Fe3+

atio, the quality of the resultant magnetic nanocomposites are fullyeproducible. However, with increasing societal and environmen-al concerns, these methods need to be replaced as most of themequire high temperature and rely on expensive and toxic reagents,hereby generating large amounts of hazardous by-products. Tovercome these problems, biological methods have been devel-ped, e.g. using micro-organisms for synthetic applications [85].owever, contamination with undesirable species and the need for

pecial conditions makes this route rather complicated. Thus, these of a “sustainable and green” approach for the synthesis of MNPsas attracted attention as an alternative to conventional methodsFig. 2).

With the rise in popularity of green sustainable technology andethodology, various benign routes for chemical synthesis have

een developed. Such methods are not only easy, economical andco-friendly but also safe to work with and handle. Recently, Wangt al. reported a facile and green method for synthesizing magneticanocomposites from rusty or scrap iron (Fe and Fe2O3) sources87]. In this method, scrap iron in de-ionized water was irradiatedor 30 min using a household microwave oven without the need forlkali, acid or high temperature. The MNPs formed were attachedo the surface of rusty iron. These NPs were separated from theurface by using ultrasonic waves and a magnet. The demonstratedrotocol was a simple and feasible way of preparation that offeredhe significant advantage of reclaiming and recycling non-pollutingaw materials such as scrap or waste iron.

Chin et al. have reported another simple and economicalethod. In this method, a relatively cheap and low toxicity

ron precursor, iron acetylacetonate Fe(acac)3, was used alongith environmentally benign and non-toxic polyethylene oxide

PEO) as the solvent and surfactant for the synthesis of MNPs88]. After optimizing various synthetic parameters, such as

Fig. 2. Green ways of synthesis for iron-oxide nanoparticles.

precursor concentrations, reaction durations and surfactants,Fe3O4 NPs of controllable size and shape were prepared. Lu et al.have demonstrated a facile and green synthetic approach forpreparing superparamagnetic Fe3O4 NPs using d-glucose as reduc-ing agent and gluconic acid (oxidative product of glucose) asstabilizer and dispersant [89]. Similarly, Demir et al. employedhydrothermal method for the synthesis of superparamagneticFe3O4 nanoparticles from maltose and its glucose derivative asthe reducing agent, and utilizing gluconic acid (the oxidativeproduct of glucose) as the stabilizer and dispering agent withiron-oxide precursor (FeCl3·6H2O) [114]. Demir also revealed othersugars such as galactose, mannose, and maltose for saccharide-assisted hydrothermal route using single iron precursor. Magneticcharacterization results revealed the obtained nanoparticles keepsuperparamagnetic nature [115]. The method was very simpleand could be carried out at mild temperature in aqueous solu-tion. Another sustainable approach was reported by Shahwan et al.[90] and Nadagouda et al. [91] using extracts of green tea leaves.The extract of tea contained an abundance of polyphenols. Suchpolyphenols are biodegradable, non-toxic and water soluble atroom temperature, unlike other polymers. In addition, they canform complexes with metal ions and cause their reduction. Thus,tea extract can be used as both a reducing and stabilizing agent forthe synthesis of NPs.

Cai et al. were the first to synthesize MNPs via soya bean sprout(SBS) templates under ambient temperature and normal atmo-sphere [92]. The reaction process was simple, eco-friendly andconvenient to handle. In principle, Fe3+ and Fe2+ ions in solutioncould be trapped on the cell wall of the SBSs by electrostatic attrac-tion and/or chelation. Subsequently, Fe3O4 NPs formed on the SBStemplates via an in situ co-precipitation process under alkalineconditions. The authors reported the presence of many reductivematerials in the SBS, e.g. vitamin C, which was believed to actas an antioxidant in the reaction. Hence, SBSs can be consideredan appropriate and available living bio-template for synthesizingFe3O4 NPs. As this method is clean, non-toxic and environmen-

tally benign, it represents an important advancement in the useof plants over toxic chemicals for the green synthesis of Fe3O4NPs. Ye et al. have reported another facile, solvent-free synthe-sis of highly crystalline and monodisperse Fe3O4 nano-crystallites

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 121

the p

a[ahnem5

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Scheme 1. Sequence of events in

t ambient temperature using a simple grinding method (Fig. 3)93]. The method utilized inexpensive and environmentally desir-ble starting materials, and did not require the use of toxic andigh boiling solvents. Pattanayak and Nayak [94] synthesized

ano-scaled zerovalent iron (nZVI) from Azadirachta indica (Neem)xtract under atmospheric conditions. The obtained iron NPs hadainly a zerovalent oxidation state and diameter within the range

0–100 nm.

able 1ethods used for the synthesis of silica-based magnetic nanocatalysts.

Entry Type of magneticnanoparticle

Type of method Approach

1 Magnetic nanoparticles

PhysicalGas phase d

Electron belithography

Chemical

Co-precipitMicroemulThermaldecomposiHydrotherm

Biological Biological mGreen methods Employs gr

methods

2 SMNPs Chemical

Stöber metMicroemulReverse miIn situ formAerosyl pyr

Arc-dischar

Forced hyd

3 Functionalized SMNPs Chemical Covalent gr

reparation of a nanocatalyst [59].

Conceivably, it is only a matter of time before the true potentialof these sustainable protocols for synthesis of MNPs are realizedand implemented on a far-reaching scale.

2.2. Surface modification of MNPs using silica

In recent years, magnetically recyclable catalysts have attracteda lot of attention [50–56]. However, in terms of stability, MNPs are

Main reagents/techniques used References

eposition Laser vaporization, thermalvaporization, arc discharge,plasma, vaporization and solarenergy-induced evaporation

[59–65]

am Electron beam lithograph [66–68]

ation Metal salts, base [69–71]sion Metal salts [72–74]

tionMetal salts, oleic acid [75–79]

al process Metal salts [80–84]ethods Uses microorganisms [85,86]

een Uses renewable resources [87–94]

hod TEOS, ethanol, [95–99]sion TEOS, oil phase, ethanol [99]croemulsion Oleic acid, surfactant, TEOS [100,101]ation TEOS, CTAB [102]olysis Silica precursor and iron

complex in gaseous phase[103–106]

ge method Micro-sized Fe and SiO2

powders[107,108]

rolysis Iron(III) acetylacetonate,sodium dodecylsulfate, TEOS

[109]

afting Silylating agent [110–113]

122 M.B. Gawande et al. / Coordination Chem

vnmgcialc(abohm[

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Kratschmer–Huffmann arc-discharge method [108]. This method

Fig. 3. Advantages of silica for MNPs coating.

ery sensitive to oxidation and agglomeration due to their magneticature and also exhibit high chemical reactivities as well as strongagnetic dipole interactions. When oxidized, thin oxide layers

enerally form on the surface of the particles, which dramaticallyhanges their properties. Natural agglomeration of these particlesnto larger clusters also restricts the use of such particles in variouspplications [116–118]. To overcome the abovementioned prob-ems, several encapsulation procedures have been proposed. Theoating strategies can be roughly divided into two major groups:1) coating with organic shells, including surfactant and polymers,nd (2) coating with inorganic components, including silica, car-on, precious metals or oxides, which can be created by gentlexidation of the outer shell of the NPs. Encapsulation of MNPsas been successfully employed by using carbon, silica, preciousetals, organic polymers, surfactants, organo-ligands and catalysts

117,119–127].Out of the various supports used for coatings and shells, silica

elivers key advantages (Fig. 3). Theoretically and experimen-ally, silica supports provide stability to MNPs by establishingquilibrium between the attractive and repulsive forces. Silicaoating alters the surface properties of MNPs from hydropho-ic to hydrophilic, which screens the magnetic dipolar attractionetween NPs [97,120,128–132]. Hence, silica coating allows theeneration of nanocatalysts that are inert, chemically and ther-ally stable under typical conditions where catalysts operate. Silica

upports provide not only stability and inertness but also facilitaterafting of specific ligands to the surface of MNPs owing to theresence of surface silanol groups [110,133,134,9,135–138].

Thus, silica-based magnetically recoverable nanocatalysts havehe following attractive features: (1) good dispersion of the activepecies (usually noble metal NPs) on the magnetic support, (2) suf-cient binding strength between the active species and magneticupport to enable recycling, (3) good chemical inertness of theagnetic support, (4) high saturation magnetization to facilitateagnetic separation, (5) soft ferromagnetism for redispersion, (6)

wide variety of silylating agents can be used, allowing pendantunctional groups in the inorganic framework, (7) attachment isasier on silica surfaces than on organic polymeric supports (which

ave a high number of cross-linking bonds), (8) low cost, readyvailability, mechanical robustness and straightforward synthesis,nd (9) a quasi-homogeneous state can be achieved (Fig. 4).

istry Reviews 288 (2015) 118–143

Thus, SMNPs offer powerful prospects for designing and synthe-sizing highly efficient and reusable magnetic nanocatalysts. Hence,by combining the unique properties of silica coatings and MNPs,a wide range of applications are opened up, not only just cataly-sis but also applications related to medicine, the environment andnanoscience [76,139–143].

Philipse et al. have argued that silica is the ultimate choicefor screening interactions between MNPs because of well-knownchemical silica-surface modification [95]. In a continuation of thiswork, different approaches have been explored to generate silicacoatings on superparamagnetic NPs. The well-known Stöber pro-cess [96] has often been used for silica coating MNPs based onhydrolysis and condensation of a sol-gel precursor, i.e. tetraethylorthosilicate (TEOS) in basic alcohol and water. The method directlycoats MNPs with amorphous silica because of the strong affinityof MNPs towards silica without the need for primer to promotethe deposition and adhesion of silica. Lu et al. investigated anothermodified sol-gel approach in which the thickness of silica coatingwas conveniently controlled in the range 2–100 nm by changingthe concentration of the sol-gel solution [97]. The ratio betweenthe concentration of iron oxide NPs and TEOS had to be optimizedin order to avoid homogeneous nucleation of silica in the mothersolution and thus formation of core-free silica spheres. Barnakovet al. synthesized and characterized the magnetic properties ofmagnetite/silica nanocomposites using a modified Stöber method[98]. The magnetic properties of silica/magnetite composite mate-rials can be easily manipulated by controlling the surface propertiesand silica coating thickness.

Another method for silica coating has been described by Tartajet al. based on a microemulsion approach [99], wherein micellesand inverse micelles were used to confine and control the sil-ica coating over MNPs. In this method, the precursor of silica, i.e.TEOS, readily dissolves in the external oil phase and interacts withwater within the micelle aggregates to produce hydrolyzed species(Si OH groups) that remain bound to the micelles due to theiramphiphilic character. The formation of particles then occurs notonly through polymerization of monomeric reactants into poly-meric reacting species but also through inter-droplet dynamicexchange. Yi et al. employed reverse microemulsion techniquescombined with template strategies for the synthesis of SiO2-coatedFe2O3 NPs with a controlled SiO2 shell thickness of 1.8–30 nm [100].This method was able to generate different nanoparticle architec-tures with tailored silica shell thickness and porosity.

A further method based on in situ formation of MNPs in thepores of pre-synthesized silica uses metal compounds as the sourceof magnetic phase [102]. A less widely used method of synthesis forSMNPs has been reported in several studies, in which SMNPs wereprepared by aerosyl pyrolysis of a precursor mixture composed ofsilicon alkoxides and metal compounds in a flame environment[103–106].

Another method of silica coating that is now widely used isthe arc-discharge method, which offers several unique advantages.Firstly, it employs micron-sized iron (Fe) and silica (SiO2) raw pow-ders of low cost and high purity. Secondly, stoichiometric levelsof oxygen supplied from the SiO2 raw powders favour the for-mation “core/shell” type nanocapsules. Zhang et al. synthesizedSiO2-coated Fe nanocapsules using an arc-discharge method [107].Their results showed that most of the as-prepared nanocapsuleswere spherical and core/shell structured with a silica coating ofaround 10–20 nm thickness. Fernández-Pacheco et al. investigatedthe preparation of encapsulated MNPs consisting of a metalliciron core and an amorphous silica shell by using a modified

provides a simple and inexpensive way to produce well-coatediron NPs. The obtained NPs exhibited a much stronger magneticresponse than any other composite material produced to date. The

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 123

Fig. 4. Advantages of SMNPs over simple homogeneous and heterogeneous catalysts.

ugh pr

mmft[ipoasam

TA

Scheme 2. Schematic representation of the mechanism of silica coating thro

ethod has also been used to investigate other types of inorganicatrices for encapsulating MNPs, but the rich chemistry and easy

unctionalization of silica on the outer surface makes the resul-ant NPs particularly promising for application as magnetic carriers132]. Ocana et al. have described another approach based on sil-ca and iron for preparing functionalized NPs (Scheme 2). In thisrotocol, silica-coated spheres were prepared over smooth layersf iron compounds via forced hydrolysis at 60–85 ◦C using iron(III)

cetylacetonate solutions containing silica cores in the presence ofodium dodecylsulfate (SDS) surfactant [109]. Table 2 represents

concise overview of different methods to obtain silica-coatedagnetic nanoparticles.

able 2 concise overview of the methods used for preparation of silica-coated nanoparticles inv

No Type of silica coating Size of the particles

1 Silica precipitationfrom an aqueousSodium silicatesolution

100 nm

2 Stöber process 0.20 �

3 A wet chemical process 2–100 nm

4 Modified Stöbermethod

30–40 nm

5 Reverse microemulsion 4.8–5.6 nm

6 Reverse microemulsionmethod

80 �m

7 Reverse micellemicroemulsion

64.5 nm/76.5

8 Aerosol pyrolysis ≤100 nm

9 Arc-discharge method 10–20 nm silica coating over 70–80

10 Arc-discharge method 10–30 nm

11 Forced hydrolysis 590 nm

ecipitation from Fe(acac)3 solutions in the absence or presence of SDS [109].

2.3. Surface functionalization of SMNPs

As already discussed, an important advantage of silica coatingover MNPs is the well-known surface chemistry and broad spec-trum of organosilanes available for modifying the silica surface withfunctional groups to give desired properties. Surface functional-ization also allows control over the interactions between the NPsand various chemical systems, which is essential for the effective

utilization of these materials in the field of catalysis [110]. Thus,to enhance the properties of SMNPs, the surface of silica-coatedmagnetic nanospheres have been modified with appropriate func-tionalizing agents to generate better dispersion of the catalyst in

olving the achieved size distribution.

Loading with which catalyst Reference

– [95]

– [96]– [97]– [98]

Palladium – SiO2/Fe3O4 [100]Rhodium- SiO2/Fe3O4 [101]

– [102]

– [103]nm iron nano core – [107]

– [108]– [109]

124 M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143

cid functionalized silica-coated magnetic nanoparticles [147].

sf

maocNtsasboigofi

fNamNasci(anlntnbisbboato

rmgT

palhr

Table 3Loading of Ir(III) ions onto magnetic supports [146].

Entry Support Metal loading [wt%]a

polyaminoamido (PAMAM) dendrons on silica-coated mag-netic nanoparticles [148]. After the dendronizing process, thesilica-coated magnetic nanoparticles showed more stability andsolubility in organic solvents (Scheme 4). The dendronized particles

Scheme 3. Schematic showing the synthesis of sulfuric a

olvents, and hence better performance in different chemical trans-ormations [111].

Surface functionalization has been mainly achieved by chemicalodification methods. The use of direct MNPs for function-

lization has become unpopular due to the use of high acidr strong oxidation that causes deterioration of these parti-les and thus loss of its magnetic activity. Hence, silica-coatedPs are preferred for functionalization. In this approach, func-

ionalized groups are introduced onto the surface of SMNPs byilanation using silane coupling agents. Various silane agents, e.g. 3-minopropyltriethyloxysilane (APTES), p-aminophenyl trimethyl-ilane (APTS) and mercaptopropyltriethoxysilane (MPTES), haveeen suggested as potential candidates for modifying the surfacef MNPs directly because they often enhance the compatibil-ty and provide a rather high density of surface functional endroups that facilitate further binding with other metal, polymersr biomolecules [112]. The hydroxyl groups present over the sur-ace of SMNPs help in latching on alkoxysilane groups, which cann turn allow binding of ligands or metal-ligand complexes.

Kralj et al. have established covalent grafting methodsor the functionalization of silica-coated superparamagneticPs by surface amino and/or carboxyl groups. To obtainmino-functionalized SMNPs, 3-(2-aminoethylamino) propyl-ethyldimethoxysilane (APMS) was grafted onto sileneous-coatedPs in an aqueous suspension [111]. The loading of the graftedmino groups was measured by conductometric titrations, whichhowed that the concentration of amino groups over the surfaceould be enhanced by increasing the amount of APMS in the graft-ng process. Carboxyl functionalization was attained in two ways:i) by a ring-opening linker elongation reaction between succinicnhydride and surface amines on amine-functionalized NPs in aon-aqueous medium, and (ii) by reacting APMS with SA, fol-

owed by grafting of the carboxyl-terminated reagent onto theanoparticle surface. Bumb et al. have reported a simple route forhe fabrication of amino- or thiol-functionalized superparamag-etic copolymer-silica nanospheres (NH2-SMCSNs/SH-SMCSNs)y embedding MNPs as a hydrophobic core surrounded by an

norganically cross-linked shell [122]. By combining uniquely auperparamagnetic core and silica cross-linked shell can not onlye the stability of nanocomposite spheres significantly enhancedut also the superparamagnetic characteristics of NH2-Si-MNPsr SH-Si-MNPs makes them promising candidates for biomedicalpplications. Importantly, NH2-Si-MNPs or SH-Si-MNPs can be fur-her used to facilitate the rapid transferral of NPs to a wide rangef end applications, such as catalysis and drug delivery.

Chang et al. investigated surface modification using a wideange of organic silanes to obtain various carboxyl ( COOH),ethyl phosphonate ( PO3), amino ( NH2) and phenyl ( Ph)

roups on the surface of silica encapsulated magnetic core NPs.he hydrophilic nature of the NPs modified with COOH andPO3 showed particular promise for loading water-soluble com-ounds, such as doxorubicin hydrochloride (DOX), via electrostatic

ttractions [113]. Functionalization with PO3 achieved a higheroading content compared to COOH modified NPs. On the otherand, phenyl ( Ph) loaded NPs displayed a controlled releaseate over a short period of time owing to weak hydrogen

1 Fe3O4@SiO2-NH2 1.42 Fe3O4@SiO2 0.17

a Determined by ICP-OES.

bonding interactions. Yang et al. employed N-heterocyclic car-bene (1,3-bis((4-triethoxysilyl-2,6-diisopropylphenyl)-imidazol-3-ium-trifluoro-methane sulfonate)) or a NHC moiety over aframework of Si@Fe3O4. The NPs were prepared with a sol–gel pro-cess using CTAB surfactant as template. After removing the CTAB,magnetic core–shell nanoporous organosilica microspheres withbuilt-in NHC moieties were obtained [144]. A novel way for the sta-bilization of gold NPs on silica-based magnetic supports involvesmodification with organosilanes. As Au NPs could not be stabi-lized on bare silica surfaces, they adopted an approach wherebythe Au NPs were firmly attached to silica-support surfaces previ-ously modified with amino groups [145]. Jacinto and his co-workersalso studied the interactions involved in metal immobilization onthe support and showed they were enhanced by functionalizing thesupport surface with amino groups. Functionalization of the silicasurfaces with amino groups not only improved the metal uptake butalso minimized metal leaching compared to non-modified silica, asshown in Table 3 [146].

Karimi prepared sulfuric acid functionalized silica-coated mag-netic nanoparticles (SSA-MNPs) by a simple one-step directreaction of chlorosulfonic acid with silica-coated magnetite (Fe3O4)MNPs [147]. The silica coated MNPs were prepared by modify-ing the surfaces of the magnetite nanoparticles with negativelycharged citrate groups, then coating with silica was achieved byreaction with tetraethylorthosilicate (Scheme 3). The preparedSSA-MNPs employed as efficient catalysts for the condensa-tion reaction of 6-amino-1,3-dimethyluracil with mono-, di-, ortrialdehydes to give the corresponding mono-, di- and tri[bis(6-aminopyrimidinyl)methanes] in good to excellent yields.

Abu-Reziq et al. demonstrated a method based on growing

Scheme 4. Polyaminoamido (PAMAM) dendrons based silica-coated magneticnanoparticles [148].

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 125

St

wra

ssotap

2

hgptiuihtaipWAcefbhm

cheme 5. Schematic showing the mechanism of silica coating through precipita-ion [149].

ere then phosphonated and complexed with [Rh(COD)Cl]2. Theesultant nanocomposites were then successfully applied in cat-lytic hydroformylation reaction.

Naeimi reported the preparation and application of Fe3O4@silicaulfonic acid as an efficient and reusable catalyst for one-potynthesis of 1-substituted 1H-tetrazoles from an amine, triethylrthoformate and sodium azide [149]. Good yields, short reactionimes, solvent-free conditions, non-toxicity and recyclability with

very easy operation are the most important advantages of theresented protocol (Scheme 5).

.4. Metal/metal complex immobilization of SMNPs

As already discussed, both conventional homogeneous andeterogeneous catalysts have a number of pros and cons, ran-ing from environmental and resource concerns (regarding theotential to recycle these materials), to the efficiency and effec-iveness of the actual catalytic species. Nowadays, the use of MNPss considered a sustainable approach. Various methods can besed to immobilize homogeneous catalytic entities onto magnet-

cally recoverable nano-supports to generate organic–inorganicybrid catalysts. These methods can be classified on the basis ofhe main interaction between the catalytically active componentnd support material. One of the most well-known approachesnvolves physisorption of the active component onto the sup-ort. In this case, the interaction is solely due to weak van deraals forces between the catalyst and the support [150,151].

nother very stable approach employs chemisorption of the activeonstituent onto the support. Covalent attachment will undoubt-dly produce stable catalysts and constitute generally the most

amous of catalysts that typically start from a highly active solu-le catalyst. Shylesh et al. synthesized an organic–inorganic hybrideterogeneous nanocatalyst by covalently latching oxodiperoxo-olybdenum (metal-ligand) complexes onto SMNPs [152]. The

HS(CH2)3Si(OMe)3

SH

SH

HS

HS

HSHS

NH

NH

H2N

H2N

H2NH2N

NH2 (CH

2 )2 HN(CH2 )3Si(OMe)3

SH-SiO2@

NH2-SiO2

Scheme 7. Pd(II) suppor

Scheme 6. Oxodiperoxomolybdenum complex tethered to SMNPs [152].

catalytic ability of the silica@magnetic core shell catalyst wascompared with that of MCM-41 and silica nanosphere supportedMo catalysts (with similar loading and simpler diamine ligandlike N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AAPS)) andperforms far better in comparison to the other sileneous heteroge-neous catalysts (Scheme 6).

A SiO2/Fe2O3 nanocomposite can be used as a magnetic catalystfor the hydrogenation of nitrobenzene. The silica surface was firstmodified with either amine or thiol derivatives [100]. Pd nanoclus-ters were then allowed to deposit on the derivatized SiO2/Fe2O3surface (Scheme 7). The dispersion and stability was higher in thecase of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS)compared to the thiol derivatized surface.

Park et al. have presented a novel innovative synthetic strat-egy for core–shell bimetallic NPs that are well defined, ligandfree and robustly fixed on the outermost surface of recyclablemagnetic silica microspheres (Scheme 8). The strategy includedseeding, coalescing the seeds to cores, and then growing shellsfrom the cores on aminopropyl-functionalized silica microspheresso that the cores and aminopropyl moieties were robustly embed-ded in the shell materials. Representative Au–Ag bimetallic NPsfixed on microspheres showed excellent catalytic performance thatremained consistent over repeated catalytic cycles [153]. Rahimiet al. described a novel synthesis of a new magnetic bromochromatehybrid nanomaterial, Fe3O4@SiO2@TEA@[CrO3Br], which was pre-pared by reacting triethylamine modified silica-coated magnetitenanoparticles with bromochromate anions [154]. The quantity ofchromium is approximately 0.38% confirms to the immobiliza-tion amount of [CrO3Br], which was equal to 0.007 mol/100 g.Though the author did not study the applications in this paperbut due to presence of CrO3Br as-prepared core–shell nanocata-

lyst, it was an important candidate for the oxidation of organicmatter, desulfurization of fuel and multicomponent reactions infuture.

SHSH

SH

SHSH

2NH2NH2NH2

NH2NH2

2

Fe2O3

@Fe2O3

Pd@SH-Si O2@Fe2O3

Pd@NH2- SiO2@Fe2O3

ted SMNPs [100].

126 M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143

3

estbhKt

3

ot

F

Table 4Comparison of silica-coated magnetic nanoparticles (SMNPs) with other heteroge-neous forms of silica gel (SG) and MSM-41 [158].

B(OH)2Br +Cs2CO 3

.

Entry Sample Yield (%)

1st run 4th run

1 Pd@SMNPs ≥99 94

Scheme 8. Core–shell bimetallic particles attached onto SMNPs [154].

. Applications of silica-coated magnetic nanocatalysts

There has been an increasing trend towards the use of SMNPs forfficient green chemical synthesis. In addition to facile separation,uch materials have a high selectivity, turn over number (TON) andurn over frequency (TOF). Magnetically recoverable materials haveeen used in a wide range of catalytic reactions, including oxidation,ydrogenation, C C coupling, asymmetric synthesis, hydration,noevenagel condensation, reduction, microwave-assisted reac-

ions in water and biocatalytic reactions (Fig. 5) [148–229].

.1. Carbon–carbon coupling reactions

Carbon–carbon bond formation is a key step in a wide rangef preparative organic syntheses [155–157]. These transforma-ions are simple, mild and versatile for creating a wide variety of

ig. 5. Applications of silica-coated magnetic nanoparticle supported catalysts.

2 Pd@MCM-41 88 783 Pd@SG 85 73

functional groups, often resulting in a high yield of stereospe-cific and regioselective products. For this reason, carbon–carboncoupling reactions are considered an extremely important toolin the field of catalysis. Recently, many silica-magnetic supportbased transition metal catalysts have been developed as excellentreagents for carbon–carbon bond formation in simple and complexstereo-controlled reactions [158].

Shylesh et al. synthesized an organic–inorganic hybrid hetero-geneous nanocatalytic system by grafting palladium dichloride totriphenylphosphine-immobilized trimethoxysilyl-functionalizedSi-MNPs (Pd@Si-MNP)[158] for use in the Suzuki–Miyaura cross-coupling reaction. The activity and stability of the synthesizedmagnetically recoverable nanocatalyst were compared with thoseof other popular heterogeneous solid supports, i.e. silica gel(SG) and mesoporous MCM-41, as shown in Table 4. For this,the palladium(II)-phosphine precursor was heterogenized oncommercially available silica gel [159] as well as on othersileneous mesoporous MCM-41 supports [160]. Notably, the activ-ity and stability of the three catalysts followed the order ofPd@SMP > Pd@MCM-41 > Pd@SG. The high catalytic activity ofPd@SMP is likely due to its unique quasi-homogeneous proper-ties, namely, its nanoscale particle size and location of the activesites on the surface of the dense and almost non-porous silicashell, which prevents limiting effects of diffusion. More than 98%of the catalyst could be recovered simply by fixing a magnet nearto the reaction vessel. Another Fe3O4/SiO2 magnetite nanoparticlesupported-biguanide palladium acetate complex has been reportedby Beygzadeh et al. [161] The catalyst was efficiently used forSuzuki-cross coupling of aryl halides with arylboronic acids inaqueous media. It was also easily separated by applying an externalmagnet without the need for expensive ultracentrifugation and wasreused several times with no loss of catalytic activity. The recov-ered catalyst was also studied by transmission electron microscopy(TEM), which confirmed the preservation of the core–shell struc-ture and good dispersion of the produced palladium NPs [161].

Yang et al. have successfully synthesized silica-magneticcore–shell functionalized with a bulky N-heterocyclic carbene (N,N-bis(2,6-diisopropylphenyl)imidazol-2-ylidene [162]. The ligandshowed good coordination capability for metallation withPd(acac)2 (acac = acetylacetonate). The strategy of using metal-ligand binding to a silica-iron nano-support provides higher loadingin comparison with direct binding of metals onto a magnetic silicasupport. The reported Pd-NHC loaded sileneous magnetic materialwas active towards the Suzuki–Miyaura couplings of challengingaryl chlorides under relatively mild conditions (Scheme 9). The cat-alytic activity of these functionalized NPs was higher than that ofmesoporous silica-based catalysts and commercially available Pd/Ccatalyst.

Yoon et al. have described a scheme in which a magnetite/silicananoparticle-supported N-heterocyclic carbene nickel catalyst

(Mag-Si@NHC-Ni) was synthesized from imidazolium with N-picolyl moieties. The resulting nanocomposite was used as anefficient catalyst in the C S coupling of various aryl halides withthiols. Although the scope of the catalytic activity was tested for a

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 127

MSN-I

weha

eNfeefTrcasoufoosNaoBpat

odoNrdwe

mi(

Scheme 9. Catalytic activity of Pd(II)@

ide range of substrates, the reaction yield was not greatly influ-nced by the electronic properties of the substituent on the arylalides or thiol substrates. The catalyst was also easily recoverednd recycled with almost consistent activity [163].

Similarly, Nador et al. presented a versatile magnetically recov-rable catalyst consisting of copper NPs on silica-coated maghemitePs. The catalyst was prepared under mild conditions by employing

ast reduction of anhydrous CuCl2 with lithium sand in the pres-nce of catalytic amounts of DTBB (4,4′-di-tert-butylbiphenyl) aslectron carrier. The catalyst performance was evaluated for dif-erent coupling and cycloaddition reactions of terminal alkynes.he reported copper-based catalyst was very efficient and easilyeusable in Glaser alkyne dimerization reactions in THF, multi-omponent Huisgen 1,3-dipolar cycloaddition reactions in waternd the three-component synthesis of propargylamines underolvent-free conditions [164]. Sheykhan et al. immobilized a seriesf Brønsted acids (HA) onto superparamagnetic Fe2O3@SiO2 andsed them as catalysts for the synthesis of biologically importantormamidines. Excellent yields were obtained with all catalystsver a short reaction time. Among the range of Brønsted acidsn silica-magnetic supports tested as catalysts, immobilized HBF4hows the best catalytic properties [165]. SuperparamagneticPs modified by sulfuric acid groups (�-Fe2O3@SiO2-OSO3H)re straightforward and green catalysts for the rapid synthesisf aminoimidazopyridine skeletons via the Ugi-like Groebke-lackburn-Bienaymé three-component reaction (Scheme 10). Theroducts were prepared under solvent-free conditions without anydditives and the catalyst was recovered and reused for five reac-ion cycles [166].

A new protocol [167] for the one-pot multicomponent synthesisf diazepine derivatives at moderate temperature uses a 1,2-iamine, a linear or cyclic ketone and an isocyanide in the presencef a catalytic amount of silica-supported iron oxide (Fe3O4/SiO2)Ps (Scheme 11). This new and efficient method for the prepa-

ation of synthetically, biologically and pharmaceutically relevantiazepine derivatives included some important features, such easyorkup procedure, reusability of catalyst, high atom economy,

xcellent yields and mild reaction conditions [167].

Claesson et al. employed a stable dispersion to synthesize

onodisperse magnetizable colloidal silica particles functional-zed with a homogeneous catalyst, i.e. PCP–pincer Pd-complexScheme 12) [168]. The catalytic activity of the prepared

PR for Suzuki Miyaura reactions [162].

nanocatalyst was demonstrated towards C C bond formation reac-tions in a cross-aldol reaction.

Mondal et al. synthesized a magnetic NP conjugated meso-porous nanocatalyst (Fe3O4@mesoporous SBA-15) with a highsurface area by chemical conjugation of MNPs with functional-ized mesoporous SBA-15 [169]. Functionalized mesoporous SBA-15containing surface carboxyl and amino groups was synthesizedvia the thiol-ene click reaction between cysteine hydrochlorideand vinyl functionalized SBA-15. The catalytic activity of therobust, safe and magnetically recoverable Fe3O4@mesoporous SBA-15 nanocatalyst was evaluated in the Biginelli reaction undermild conditions for the synthesis of a diverse range of 3,4-dihydropyrimidin-2(1H)-ones (Scheme 13).

Rostamizadeh et al. directly synthesized a (�-Fe2O3)-MCM-41-SO3H catalyst by the reaction between chlorosulfonic acid andsilica-coated NPs (�-Fe2O3)-MCM-41 and used it as a magnet-ically recyclable catalyst for the efficient one-pot synthesis ofN-aryl-2-amino-1,6-naphthyridine derivatives [170]. The catalystwith 10 wt% of loaded iron oxide NPs could be recovered fromthe reaction mixture by application of an external magnet andreused without significant decrease in activity even after 5 runs.The prepared catalyst exhibited better activities than other com-mercially available sulfonic acid catalysts. Zhang et al. synthesizeda catalyst by anchoring 3-sulfobutyl-1-(3-propyltriethoxysilane)imidazolium hydrogen sulfate onto the surface of silica-coatedFe3O4 NPs (Scheme 14) [171]. Owing to the combination of nano-support features and flexible imidazolium linkers, it acts as a“quasi-homogeneous” catalyst to effectively catalyze the one-potsynthesis of benzoxanthenes by the three-component condensa-tion of dimedone with aldehyde and 2-naphthol under solvent-freeconditions. More importantly, the catalyst could be easily recov-ered and reused up to six times without significant loss of catalyticactivity.

Rafiee and Eavani synthesized a magnetically recoverablecatalyst by immobilization of H3PW12O40 on the surface ofsilica-encapsulated Fe2O3 NPs (Scheme 15) [172]. The acidity ofthe catalyst was measured by potentiometric titration with n-butylamine. The catalyst showed an excellent distribution of acidic

sites, suggesting that the catalyst possessed a higher number ofsurface active sites compared to its homogeneous analogues. Thecatalytic activity was probed towards one-pot three-componentMannich-type reactions of aldehydes, amines and ketones in water

128 M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143

Scheme 10. Catalytic activity of a �-Fe2O3@SiO2-OSO3H nanocatalyst for Groebke-Blackburn-Bienaymé three-component reactions [166].

Scheme 11. Catalytic activity of a Fe3O4@SiO2 nanocatalyst for multi-component reactions [167].

Scheme 12. Catalytic activity of a PCP–pincer Pd-complex immobilized silica-coated nanocatalyst for cross-aldol condensation reactions [168].

Scheme 13. Catalytic activity of a cysteine hydrochloride immobilized silica-coated nanocatalyst for Biginelli reactions [169].

Scheme 14. Catalytic activity of a 3-sulfobutyl-1-(3-propyltriethoxysilane) immobilized silica-coated nanocatalyst for the synthesis of benzoxanthenes [171].

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 129

d silica-coated nanocatalyst for Mannich-type reactions [172].

aiwcer

facetmtt(21roeftnba4etiu(aeta

reusability over multiple cycles without appreciable loss in cat-

S[

Scheme 15. Catalytic activity of a H3PW12O40 immobilize

t room temperature. Excellent conversions were obtained, show-ng that the catalyst had strong and sufficient acidic sites, which

ere responsible for its catalytic performance. After the reaction,atalyst/product separation was easily achieved by applying anxternal magnetic field and more than 95% of the catalyst wasecovered.

Nemati and Saeedirad prepared a MNP supported silica sul-uric acid catalyst for the synthesis of pyrimido[4,5-b]quinolinesnd indeno fused pyrido[2,3-d]pyrimidines in water under mildonditions [173]. The corresponding products were obtained inxcellent yields. The efficiency of the catalyst remained unal-ered even after being reused for three cycles. These advantages

ake this protocol attractive for large-scale synthesis. Ros-amizadeh et al. prepared a new (�-Fe2O3)-MCM-41-SO3H directlyhrough reaction of chlorosulfonic acid with silica-coated NPs�-Fe2O3)-MCM-41 and employed a one-pot synthesis for N-aryl--amino-1,6-naphthyridine derivatives. The catalyst contained0 wt% of loaded iron oxide NPs and could be recovered from theeaction mixture by applying an external magnet and reused with-ut significant decrease in activity even after 5 runs. The catalystxhibited better activities than other commercially available sul-onic acid catalysts [170]. The same research group also reportedhe synthesis and application of an amino acid based magneticanocatalyst (�-Fe2O3-MCM-41@l-prolinium nitrate). The immo-ilization of l-prolinium nitrate (environmentally friendly aminocid-based ionic liquid) inside the mesochannels of �-Fe2O3-MCM-1 generated a new solid nanocatalyst that could be used as anfficient heterogeneous catalyst for the one-pot oxidative cycliza-ion direct synthesis of quinazolin-4(3H)-one derivatives fromsatoic anhydride, aldehyde or alkyl halide and primary aminesnder mild reaction conditions without the need for oxidantScheme 16). Moreover, in these reactions, only the hybrid cat-lyst catalyzed the reactions, whereas no activity was shown by

ither �-Fe2O3-MCM-41 or l-prolinium nitrate alone. The charac-eristic features of the hybrid catalyst were attributed to its acidicnd oxidative behaviour [174].

cheme 16. Catalytic activity of �-Fe2O3-MCM-41@l-prolinium nitrate as a nanocatalys174].

Scheme 17. Catalytic activity of a silica supported Zr (IV) nanocatalyst forFriedel–Crafts, Knoevenagel and Pechmann condensation reactions [175].

Recently, Sharma and co-workers investigated an efficientmagnetically retrievable zirconium (IV) based nanocatalyst[175]. The reported catalyst was synthesized via covalentgrafting of 3-hydroxy-2-methyl-1,4-naphthoquinone onto amine-functionalized SMNPs followed by complexation with ZrOCl2(Zr-HMNQ@Si-MNP). The ability of the resultant organic–inorganichybrid nanocatalyst was successfully evaluated for a range of indus-trially important organic transformations, such as Friedel–Crafts,Knoevenagel and Pechmann condensation reactions (Scheme 17).The magnetically recoverable nano-hybrid catalyst displayed a highTON and chemo-selectivity for the desired product, as well as good

alytic activity.The same authors reported a magnetically recoverable, efficient

and selective copper-based nanocatalyst fabricated via covalent

t for the one-pot oxidative cyclization straight synthesis of quinazolin-4(3H)-one

130 M.B. Gawande et al. / Coordination Chem

St

gnAwatacte

3

dhastAurac

3

ticn

Ss

cheme 18. Catalytic activity of a silica supported Cu (II) nanocatalyst for the syn-hesis of secondary amines [176].

rafting of 2-acetylthiophene onto a silica-coated magneticano-support followed by metallation with copper acetate (Cu-cTp@Si-MNP) [176]. The catalytic efficacy of the nanocatalystas evaluated for the synthesis of industrially important alkylated

mines (Scheme 18). The catalytic nanocomposite offered highurnover frequency and selectivity for secondary amines undererobic conditions. Furthermore, the heterogeneous nature of theatalyst allowed easy magnetic recovery and regeneration, makinghe reported protocol highly beneficial for addressing industrial andnvironmental concerns.

.2. Acetylation

A silica-coated magnetic nanoparticle-supported 4-N,N-imethylaminopyridine (DMAP) analogue was used as a robusteterogeneous nucleophilic catalyst of unprecedented activitynd recyclability (Scheme 19) [177]. The catalyst was easilyeparated from the products by exposure of the reaction vesselo an external magnet and decantation of the reaction solution.fter washing with THF to remove residual product and dryingnder high vacuum, the remaining catalyst could be recovered andeused. The catalyst was compatible with the hindered acylatinggent isobutyric anhydride. No catalyst degradation (physical orhemical) was discernible after 30 consecutive catalyst cycles.

.3. Oxidation

Oxidation reactions are of vital importance to chemical indus-

ries because of their potential for generating a wide variety ofmportant compounds used in the pharmaceutical and fine chemi-al industries [178,179]. Traditionally, oxidation has been achievedon-catalytically by using stoichiometric quantities of oxidant in

OOO

Si N

OHO

O

4-N,N-dimethylaminopy ridin eimmobilized silica-coa tednanocatal yst

N

Ac2O, NEt3, CH2Cl2

Yiel d = 98-91 %

cheme 19. Catalytic activity of a 4-N,N-dimethylaminopyridine immobilizedilica-coated nanocatalyst for acetylation reactions [177].

istry Reviews 288 (2015) 118–143

the presence of mineral acids, which generates huge amountsof toxic waste. To alleviate these environmental issues, effortshave been made by scientists to develop oxidation reactions thatembrace the principles of Green Chem. [180,181]. In light of theseconcerns, the use of silica based MNPs as a solid support for hetero-geneously catalyzed oxidation reactions are highly effective. Jacintoet al. developed a magnetically separable silica-based Ru catalystthat was highly active for the oxidation of aryl and alkyl alco-hols to aldehydes under 3 bar of molecular oxygen at 100 ◦C [182].The oxidation of non-activated alcohols (such as octanol) or diolsto diketones proceeded smoothly with a high rate of conversionand selectivity (no carboxylic acid formed). The catalyst synthe-sized using an amino-functionalized support was highly stable anddevoid of metal leaching during the reaction. The magnetic prop-erties of the nanocatalyst enabled its separation from the reactionmedia. The same authors also prepared a gold-based catalyst [182].In this case, the methodology used to achieve metal reduction,either by heating under an air atmosphere or by using hydrogen,had a crucial effect on the particle dispersion and distribution onthe magnetic support, which in turn significantly influenced thecatalytic activity. The Au catalyst was highly active for the aero-bic oxidation of alcohols and was efficiently recovered by magneticseparation with negligible Au leaching.

Tang et al. investigated TiO2-functionalized silica-based MNPsfor diverse oxidation reactions [183]. Different amounts of tetra-butyl ortho titanate (TBOT) were used to cover the outer shellof Si-MNPs. As the TBOT was expected to diffuse into the silicalayer, the authors discussed substitution of Si sites by Ti. Amongall the catalysts investigated, materials with 18.9% TiO2 showedthe best catalytic performance for the epoxidation of stilbene inthe presence of tBuOOH. Further increasing the titanium concen-tration in the outer shell decreased the catalytic activity, which wasattributed to the formation of unreactive Ti O Ti units instead ofcatalytically active Ti O Si units.

Mori et al. have described a two-step process involving a ligandexchange step with (3-mercaptopropyl) triethoxysilane and a sol-gel process using TEOS and TBOT for the preparation of TiO2/SiO2coated MNPs [184]. Characterization of the resulting catalyst con-firmed the presence of tetrahedrally co-ordinated titanium in theexternal layer of the nanocomposite. This catalyst was employedfor four consecutive reactions with consistent activity (TON = 48)for the epoxidation of cyclooctene using 30% H2O2 as oxidant.Rayati and Abdolalian derived a Schiff base ligand from 5-bromo-2-hydroxybenzaldehyde and 2,2′-dimethyl-propylenediamine (H2L)and used it to synthesize the corresponding dioxomolybdenum(VI)complex (Mo(O)2L), which was characterized by spectroscopicmethods [185]. The adsorption of Mo(O)2L on the surface of silica-coated magnetite NPs via hydrogen bonding led to the formation of(Fe2O3)–MCM-41–Mo(O)2L, which was subsequently investigatedas a heterogeneous catalyst for the efficient and highly selectiveoxidation of a wide range of olefins with hydrogen peroxide andtert-butyl hydroperoxide in ethanol. Under reflux conditions, theoxidation of cyclooctene in the presence of tert-butyl hydroperox-ide or hydrogen peroxide led to formation of the respective epoxideas the sole product.

A ruthenium hydroxide supported on Si-MNPs was employedas an efficient heterogeneous catalyst for the liquid-phase oxida-tion of a wide range of alcohols using molecular oxygen as the soleoxidant in the absence of co-catalysts or additives [186]. The cat-alyst was fabricated by loading an amino modified support withruthenium (III) ions from an aqueous solution of ruthenium (III)chloride followed by treatment with sodium hydroxide to form

ruthenium hydroxide species. Several carbonylic mono-terpenoidsimportant for the fine chemical and pharmaceutical industries wereobtained in good to excellent yields starting from biomass-basedmonoterpenic alcohols, such as isoborneol, perillyl alcohol, carveol

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 131

ca-coa

aeSecTatohsatwelsclrbaSptwstdim

c

Scheme 20. Catalytic activity of a H3PW12O40 immobilized sili

nd citronellol. The catalyst showed no metal leaching and wasasily recovered by application of an external magnet and re-used.ilva and his coworkers reported another novel magnetically recov-rable cobalt oxide (CoO) catalyst synthesized by deposition ofatalytically active CoO NPs on silica-coated magnetite NPs [187].he catalyst was highly efficient towards cyclohexene oxidationnd selective towards the allylic oxidation product. In contrasto other cobalt and iron species, the catalytic activity of cobaltxide (CoO) was much higher for the allylic oxidation of cyclo-exene. Escamilla-Perea et al. examined the effect of silica SBA-15ubstrate modification with variable amounts of Fe2O3 (5, 10, 15nd 20 wt%) on the catalytic response of supported gold catalystsowards CO oxidation at 20 ◦C [188]. The catalytic activity increasesith increasing Fe2O3 loading, although the response was not lin-

ar. The highest catalytic activity was observed for the catalystoaded with 15 wt% Fe2O3. With the aid of XRD and XPS analy-es, this behaviour was explained in terms of the largest Fe2O3luster dispersion on the surface of the SBA-15 substrate and itsarge stability during the on-stream reaction. An easy syntheticoute for preparing a nanocatalyst comprising H3PW12O40 immo-ilized on surface-modified Fe3O4 magnetite NPs and successfullypplied it for the oxidation of dibenzothiophene, is illustrated incheme 20 [189]. This magnetic catalyst showed high catalyticerformance towards the oxidation of dibenzothiophene in ace-onitrile in the presence of hydrogen peroxide and high conversionsere obtained. This catalyst was easily separated from the reaction

olution by applying an external magnetic field and recycled severalimes. Oliveira and co-workers also performed chemoselective oxi-ation of alcohols, a reaction of great interest for the fine chemical

ndustry, using gold NPs adhered to silica-based magnetic supportsodified with organosilanes [145].Masteri-Farahani and Tayyebi synthesized a highly effi-

ient catalyst by covalent binding of a Schiff base ligand

Scheme 21. Catalytic activity of a MoO2salpr immobilized silica

ted nanocatalyst for the oxidation of dibenzothiophene [189].

(N,N′-bis(3-salicylidenaminopropyl)amine) (salpr) onto the surfaceof silica-encapsulated magnetite nanoparticles (Si-MNPs) followedby complexation with MoO2(acac)2 [190]. The catalytic activity ofthe prepared MoO2salpr/SI-MNPs nanocatalyst was examined inthe epoxidation of olefins with tert-butyl hydroperoxide (TBHP)and cumene hydroperoxide (CHP) (Scheme 21). Ucoski et al.have reported the immobilization of different cationic and neu-tral metalloporphyrins (MPs) on silica-coated Fe3O4 NPs using abasic hydrolytic sol–gel process [191]. Strong electrostatic interac-tions between the hydroxylated surface of the silica and cationicMPs favoured immobilization of the charged MPs. In contrast, neu-tral MPs probably established weak bonds with the silica and wereeasily leached from the support. The catalytic activity of the immo-bilized MPs towards the oxidation of cyclooctene, cyclohexeneand cyclohexane was investigated using iodosylbenzene as oxygendonor. The immobilized MPs were used at least for five succes-sive cyclooctene oxidation reactions – the yields did not decrease,confirming that the catalyst could be reused [191].

Hamadi and co-workers have reported phosphotungstic acid(PTA) supported on imidazole-functionalized silica-coated cobaltferrite NPs. The immobilized phosphotungstic acid was an efficientheterogeneous catalyst for the synthesis of 1-aminophosphonatesunder solvent-free conditions at room temperature. The FTIR spec-trum of the recovered catalyst showed no change even after its usefor five times, which indicates that no significant leaching of thePTA species from the support occurred on using and reusing thecatalyst [192].

Sharma and Monga synthesized a highly competent zinc-based nanocatalyst by covalent implanting of 2-acetylpyridine over

amine-functionalized silica@magnetite NPs, followed by metalla-tion with zinc acetate (Zn-AcPy@Si-MNPs) (Scheme 22) [193]. Theresultant nanocomposite was highly active and efficient for oxida-tion of various aromatic amines to obtain azoxyarenes. The high

-coated nanocatalyst for the epoxidation of olefins [190].

132 M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143

NH2N+

O-

N

R

R R

Aromatic amine Azo xya ren e

Zn- Acteylp yridine@ Si-MN Ps

30% H2O2, ACN, 80 oC

Yield = 81-99%

Sn

ttrp[

as(ptttmsToats

Mtswaasrpd

ecowpe(acs

cheme 22. Catalytic activity of a zinc-acetylpyridine immobilized silica-coatedanocatalyst for the oxidation of aromatic amines [193].

urnover number (TON), mild reaction conditions and high selec-ivity for azoxyarenes with sustained catalytic activity makes theeported protocol useful, highly compliant and economical in com-arison to the other non-magnetic heterogeneous catalytic systems193].

Chen et al. developed a new magnetically separable nanocat-lyst by covalent binding of a Schiff base ligand, N,N-bis(3-alicylidenaminopropyl)amine (salpr), on the surface of SMNPsFe3O4/SiO2) followed by complexation with Cu(OAc)2. Therepared Fe3O4/SiO2/Cu(II)salpr catalyst exhibited high activityowards the selective oxidation of various alkyl aromatics withert-butyl hydroperoxide as oxidant [194]. Recently, Fang et al. syn-hesized a novel catalyst based on thiol-functionalized silica-coated

agnetic mesoporous nanocrystals as support and Au NPs as activeites via a facile and environment-friendly approach (Scheme 23).he synthesized catalyst exhibited high catalytic activity towardsxidations of cyclohexene and styrene by molecular oxygen attmospheric pressure. The high activity was mainly attributed tohe strong interactions between the Au NPs and thiol groups on theurface of the support [195].

Benign and economic silica-coated magnetically recoverablen-porphyrin catalytic system reported for oxidation of indus-

rially and biologically important substrates including olefins,aturated hydrocarbons, alcohols and organosulfur compounds inater using tert-butyl hydroperoxide [196]. The activity, selectivity

nd scope of the reaction were explored with a variety of substratesnd reactions proceeded effectively and smoothly in the absence ofurfactants, organic co-solvents (Scheme 24). The separation andecycling of nanocatalyst as well as isolation of water-insolubleroducts were simple, effective and economical in this clean oxi-ation method.

Sun and co-workers fabricated organic–inorganic hybrid het-rogeneous nanocatalysts via covalent anchoring of cobalt(II) oropper(II) acetylacetonate complexes ([Co(acac)2] or [Cu(acac)2])nto core–shell structured Fe3O4@SiO2 previously functionalizedith 3-aminopropyltriethoxysilane (APTES)[197]. The catalyticerformance of the prepared nanocatalysts was evaluated in thepoxidation of styrene using eco-friendly air as the oxygen source

Scheme 25). Both of the nanocomposites Fe3O4@SiO2–NH2–Cond Fe3O4@SiO2–NH2–Cu presented in Table 5 in excellent styreneonversion and good epoxide selectivity in contrast to the corre-ponding homogeneous counterparts.

Scheme 23. Catalytic activity of gold NPs on a thiol-functionalized magnetic m

Scheme 24. Catalytic activity of silica-coated magnetically recoverable Mn-porphyrin catalytic system for the oxidation of different arenes [196].

3.4. Hydrogenation reactions

Hydrogenation is one of the most widely used organic transfor-mations in organic synthesis because of its immense applicationsin different industries [198–200]. Most current hydrogenationcatalysts are based on homogeneous rhodium (Rh), iridium (Ir),ruthenium (Ru), nickel (Ni) and palladium (Pd) catalysts [201–205].Due to the obstacles encountered in recovering these metals, recentresearch has focused on ways to use these metals for hetero-geneous hydrogenation catalysts. In 2008, Jacinto et al. reportedthe first magnetically recoverable Rh(0) nanoparticle-supportedcatalyst for the hydrogenation of cyclohexene (180,000 mol/molRh) and benzene (11,550 mol/mol Rh) under mild conditions.A thiol-modified silica-based magnetically retrievable palladiumnanocatalyst [207] was examined towards the hydrogenation ofcyclohexene under solvent-free conditions. The reported turn-over-frequency was 11500 h−1, which is very high and equivalentto commercially available Pd/C catalysts. Notably, the catalyst could

be recovered and reused 20 times without loss of activity. Theimmobilization of Rh and Ru NPs on amine-functionalized SMNPswas also investigated for the same reactions [77,142]. The hydro-genation of cyclohexene in the presence of Ru/NH2@Si-MNPs gave

esoporous silica sphere catalyst for the aerobic oxidation of olefins [195].

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 133

OOO

TEOS

Sol-ge l

APTES

Dry toluene ,reflux

OOO

Si (CH2)3 NH2

O

OM

O

O

Dry toluene,reflux, 12h

Si (CH2)3 N OM

O O

O

M = Cu, Co

90.8% co nv.: 63. 7% select ivity

Air, 80 oC

86.7% co nv.: 51. 4% select ivity

Fe3O4 Fe3O4@SiO2Fe3O4@SiO2NH2

Fe3O4@SiO2NH2M

Catalyst

Fe3O4@SiO2NH2Co

Fe3O4@SiO2NH2Cu

Scheme 25. Catalytic activity of magnetically recoverable heterogeneous nanocatalysts Fe3O4@SiO2–NH2–M (M = Cu or Co) for the oxidation of olefins [197].

(HO)3Si(CH2)3NH2

-3H2O

TEOS

NH2

NH2

H2NNH2

NH2NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

H2N

H2N

NH2

H2N

H2N

H2N

H2N

H2N

H2N

H2NFe3O4 Si-Fe3O4

NH2-Si- Fe3O4

O

OCu

O

O

CHCl3, Reflux

NH2

NH2

NCu

O

NH2

Cu(II)-Acac@N H2-Si- Fe3O4

Magnetically responsiv ecatalyst

NO2

NH2

NH2

NH2NaBH4, H2O

Yield = 92-100 %

Scheme 26. Cu(acac)2@Am-Si-MNPs catalyzed reduction of nitro compounds [209].

134 M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143

Table 5Aerobic epoxidation of styrene over various catalysts [197].

No. Catalyst Styrene conversion Epoxide selectivity

1 Fe3O4@SiO2–NH2–Co 90.8 63.786.7

mRfsntmotlutpo6laVn(Thm

ebfTeaase

irFgcsrpt

a

Sp

electrostatic attachment also enabled ready reloading of the cat-

2 Fe3O4@SiO2–NH2–Cu

ore than 99% conversion under mild reaction conditions, whereash-NH2@Si-MNPs showed 18 times higher activity in terms of TOF

or a similar reaction. The same authors also reported surfactant-tabilized magnetite-silica supported rhodium nanocatalysts. Theanocomposite catalyst Fe3O4@SiO2-Rh(0) NPs was highly activeowards the solventless hydrogenation of model olefins and aro-

atic substrates under mild conditions, with turnover frequenciesf up to 143,000 h−1 [101]. An Ir(0) catalyst prepared on a func-ionalized magnetic-silica support showed a high degree of metaleaching into the liquid products. The Ir(0) catalyst was able to besed to hydrogenate various alkene derivatives under mild condi-ions and its catalytic performance was comparable to that of otherlatinum group metal catalysts prepared by the same group (withrder Rh > Ir > Pd > Pt@Ru; cyclohexene hydrogenation at 75 ◦C and

atm H2) and superior to many other Ir(0) catalysts reported in theiterature [146]. The catalytic performance in the hydrogenation oflkenes was compared with that of Rh and Pt catalysts. Baig andarma performed a one-pot synthesis of ruthenium NPs on mag-etic silica, which involved in situ generation of magnetic silicaFe3O4@SiO2) and ruthenium nanoparticle immobilization [208].he transfer hydrogenation of carbonyl compounds occurred inigh yield and with excellent selectivity using the catalyst undericrowave irradiation conditions.Recently, Sharma et al. reported the fabrication of a highly

fficient magnetically separable nanocatalyst by covalent immo-ilization of a copper(II) acetylacetonate complex onto amine-unctionalized silica@magnetite NPs(Cu(acac)2@Si-MNP) [209].he catalytic performance of the synthesized nanocomposites wasxamined for the degradation of various organic nitro-analogues inqueous medium at room temperature using sodium borohydrides a source of hydrogen (Scheme 26). The reported nanocatalyticystem was able to selectively reduce the nitro group even in pres-nce of other sensitive functional groups.

A one-step synthesis of magnetite silica-supported rutheniumnvolves in situ generation of magnetic-silica (Fe3O4@SiO2) anduthenium nanoparticle immobilization [210]. The applicatione3O4@SiO2/Ru demonstrated for the catalytic transfer hydro-enation reaction of carbonyl compounds. The hydrogenation ofarbonyl compounds occurs in high yield along with excellentelectivity under the microwave irradiation (Scheme 27). All theeactions with different substrates were completed within a timeeriod of 30–45 min using microwave in contrast to several hours

aken in conventional heating experiments.

Li et al. fabricated a novel catalyst using layer-by-layerssembly of polyelectrolyte–gold nanoparticle multilayer films on

cheme 27. Fe3O4@SiO2/Ru catalyzed hydrogenation reaction of carbonyl com-ounds [210].

51.4

Fe3O4@silica core–shell microspheres [211]. Their protocol pro-vided a convenient method for designing a bi-functional hybridcomposite combining both catalytic and magnetic performance.Au NPs were effectively immobilized in the polyelectrolyte layerwithout blocking the catalytic sites. The obtained hybrid magneticmicrospheres exhibited high catalytic performance in both organicand inorganic reduction reactions. The use of magnetic supportfor the immobilization of Au NPs provided a facile, clean, fast andefficient means of separation of the catalyst at the end of the reac-tion cycle. Similarly, Shin et al. demonstrated a facile fabrication ofsilver-deposited silanized magnetite over SMNP (Fe3O4/SiO2@Ag)beads (Scheme 28) [212]. The prepared Fe3O4/SiO2@Ag particleswere employed as a solid-phase catalyst for the reduction of 4-nitrophenol (4-NP) in the presence of sodium borohydride. Thereduction of other nitrophenols, such as 2-nitrophenol (2-NP) and3-nitrophenol (3-NP), was also tested using the Fe3O4/SiO2@Ag NPsas catalysts and their rate of reduction follows the order 4-NP > 2-NP > 3-NP. The Fe3O4/SiO2@Ag particles could be separated fromthe product using an external magnet and can be recycled a numberof times after the quantitative reduction of nitrophenols.

Chi et al. have developed a straightforward method forgenerating a core–shell structured Fe3O4@SiO2–Ag magneticnanocomposite by an in situ wet chemistry route with the aid ofpolyvinylpyrrolidone as both reductant and stabilizer. This methodcan effectively prevent Ag NPs from aggregating on the silicasurface, yielding highly dispersed and small-sized Ag NPs. The as-prepared nanocomposite comprised a central magnetite core with astrong response to external fields, an interlayer of SiO2 and numer-ous highly dispersed Ag NPs with a narrow size distribution [213].

3.5. Olefin metathesis

With the advancement of efficient nanocatalysts, the olefinmetathesis reaction has emerged as a powerful tool for the forma-tion of C C bonds [214–216]. Byrenes et al. managed to producea nanocatalyst in one step from a commercially available secondgeneration Grubbs catalyst by immobilization of the homoge-neous catalyst onto magnetically separable silica-coated nanosizediron oxide particles (Scheme 29) [217]. The resultant ruthenium(II) alkylidene catalyst provided pseudo-homogeneous reactiv-ity coupled with an in-built facile recovery option. The use of

alyst and reuse of the functionalized MPs. Magnetic retrieval ofthe immobilized catalyst simplified product isolation and catalystrecycling.

Scheme 28. Catalytic activity of Fe3O4@SiO2 for the reduction of nitrophenols [212].

M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143 135

Scheme 29. Catalytic activity of a ruthenium alkylidene immobilized silica-coated nanocatalyst for the metathesis of olefins [217].

Scheme 30. Cu(II)-azabis(oxazoline)-complex immobilized on silica-covered magnetic nanocomposites for asymmetric benzoylations [221].

oated

3

oH

Sp

Scheme 31. Catalytic activity of a chiral rhodium immobilized silica-c

.6. Asymmetric synthesis

Asymmetric catalysis is one of the most powerful meth-ds for producing enantiomerically pure compounds [218–220].owever, despite huge efforts devoted to this subject, the

cheme 32. Catalytic applications of Fe3O4@SiO2 for the degradation of organic dyeollutants [228].

nanocatalyst for the hydrogenolysis of bicyclo[2.2.2.]oct-7-enes [225].

contribution of asymmetric catalysis to the overall produc-tion of chiral chemicals has been so far lower than originallyexpected considering the isolation problems of homogeneouscatalysis. As chiral ligands/catalysts are relatively expensive,hydrogenation of the chiral catalyst is desirable to aid cat-alyst recovery. Thus the use of SMPs provides a lower costmethodology offering recycling of the catalyst. Recently, severalresearch groups have reported that various asymmetric catalysts onsilica-based magnetically recoverable support show high enantio-selectivity and activity towards various chemical transformations[221–224]. Schätz et al. prepared Cu(II)-azabis(oxazoline)-compleximmobilized on silica covered magnetic nanocomposites with awell-defined globular structure bearing azide end groups [221].The resulting catalysts were tested in asymmetric benzoyla-tions of hydrobenzoin (±)16, and rivalled or exceeded the bestresults obtained previously for either supported or non-supportedcatalysts with respect to selectivity, yield and recyclability(Scheme 30).

Neatu and co-workers reported the synthesis of magnetite col-loids modified with cinchonidine embedded in silica and usedthem for the hydrogenolysis of bicycle [2.2.2] oct-7-enes [222].The effect of various modifications was also evaluated but most

136 M.B. Gawande et al. / Coordination Chemistry Reviews 288 (2015) 118–143

O

OH

OH

OH

OHO

O

OH

OH

OH

OH O

OH

OH

OH

OH

OH +O

OH

OH

OH

OH

HO

Sucrose Glucose Fructose

Fe3O4@SiO2@SO3H

O

OH

OH

OH

OH

O

OH

OH

OH

OH Fe3O4@SiO2@SO3H O

OH

OH

OH

OH

OH2

Glucos eGalactos e

Conversion = 93%

Conversio n = 88 %

O

Scheme 33. Catalytic activity of a Fe3O4@SiO2@SO3H nanocatalyst for the synthesis of biofuels [233].

Fe (II)+

Fe(III)

NH4OH TEOSNH4OH

Fe3O4Si-Fe3O4

HPW

HPW-Si-Fe3O4

OCH2OH CH2OH

OHOH

OOHO

OOO

EMFFructose

EtOHH+

54.8% 83.6%

talyst

hetfbiitwe

Scheme 34. Catalytic activity of a Fe3O4-Si@HPW nanoca

ave no influence on the catalytic properties of the materials. How-ver, the nature of substituents on the substrate severely influencedhe activity. In particular, small substituents, such as hydrogen,avoured interactions with the catalytically active sites, whereasulky substituents, such as phenyl groups, caused a drastic decrease

n activity. In a similar strategy, cinchondine modified Pt was

mmobilized on silica MNPs for the enantioselective hydrogena-ion of �-ketoesters and ���-trifluroacetophenone. The catalystas recycled up to eight times with only negligible decrease of

nantioselectivity (1st run 57% e.e. vs. 8th run 57% e.e.).

Scheme 35. Catalytic activity of an ionic liquid immobilized silic

for the etherification of 5-hydroxymethylfurfural [234].

Sun et al. synthesized a magnetically recoverable chiral rhodiumcatalyst that exhibited excellent catalytic activity (up to 99% e.e.)and enantioselectivity (up to 97% e.e.) towards the asymmetrictransfer hydrogenation of aromatic ketones in aqueous medium.The catalyst was easily recovered by application of a small magnetand reused up to ten times without affecting its enantioselec-

tivity, demonstrating good potential for industrial applications(Scheme 31) [223].

A new magnetic mesocellular mesoporous silica support fea-turing a 3D open-pore structure, was developed to achieve highly

a-coated nanocatalyst for the epoxidation of olefins [235].

Chem

earptatsrwy

3

Saat2gicatFi(cwatfactohoFtaefdpca[FopsotsPF

(cdtbri

The comparison of different aforementioned silica-magnetic

M.B. Gawande et al. / Coordination

fficient, filtration-free recycling of chiral ligands for catalyticsymmetric dihydroxylation [224]. The 3D open pore structure waseported to enhance the reaction kinetics compared to other meso-orous materials. Reactions using chiral ligands immobilized onhis magnetic silica system exhibited almost the same reactivitynd enantioselectivity as those obtained in the homogeneous reac-ion. Even in the case of trans-stilbene and in contrast to otherilica supports available, the enantioselectivity obtained with theeported catalyst did not decrease with repeated use and remainedithin a 1% margin of the initial run (>99.5–98.5%) with >95%

ields.

.7. Photocatalytic activity

Gad-Allah et al. prepared magnetically separable TiO2/iO2/Fe3O4 nanocomposites with different core (Fe3O4) diametersnd silica contents by a sol-gel technique [225]. The photocatalyticctivity of the prepared samples was investigated towards the pho-odegradation of methyl orange. The obtained results showed that5–45 mm core diameter gave the maximum activity because itives rise to an optimal surface area and light transmittance. Vary-ng the silica content had a significant effect on the activity of theomposite. Using more than 10 wt% of silica reduced the catalystctivity because of the increase in particle diameter and reduc-ion of surface area. Yuan et al. successfully synthesized core–shelle3O4@SiO2@meso-TiO2 via a facile two-step sol–gel method, aim-ng to remove unwanted organic compounds from aqueous mediaScheme 33). The prepared nanomaterial comprised a magneticore, SiO2 interlayer and mesoporous TiO2 outer shell [226] andas synthesized using cetyl trimethylammonium bromide (CTAB)

s a pore-forming agent and TiOSO4 as the titanium source. Whenhe molar ratio of TiOSO4/CTAB = 8:1, a mesoporous structure wasormed within TiO2 shell. Because of the large BET surface area cre-ted by the mesoporous TiO2 shell, the Fe3O4@SiO2@meso-TiO2omposite exhibited high photocatalytic activity for the degrada-ion of rhodamine B in aqueous suspension, which was higher thanbtained with Fe3O4@SiO2@solid-TiO2. Meanwhile, Aliyan et al.ave described the preparation of a nanocatalyst via modificationf the surface of meso-structured silica (SBA-15) by immobilizinge3O4 (Scheme 32). The catalytic activity of Fe3O4@SBA-15 towardshe photodegradation of malachite green (MG) was explored in

photo-catalytic reactor using a UV lamp as light source. Theffect of various experimental parameters on the degradation per-ormance of the process was evaluated by examining catalystosage, initial dye concentration and pH of the dye solution in theresence of Fe3O4@SBA-15 as photo-catalyst. The reported photo-atalyst exhibited a significantly high catalytic stability and thectivity loss was negligible even after five MG degradation cycles227]. Huang et al. successfully synthesized polyaniline-modifiede3O4/SiO2/TiO2 composite microspheres by sol–gel reactionsn Fe3O4 microspheres followed by the chemical oxidativeolymerization of aniline [228]. The synthesized multilayer-tructured composites were evaluated for the photodegradationf methylene blue under visible light. The effect of varyinghe polyaniline (PANI) amount on the photocatalytic activityhowed that Fe3O4/SiO2/TiO2 composites with about 2.4–4.1 wt%ANI gave higher photocatalytic efficiency than that of simplee3O4/SiO2/TiO2.

Sahoo et al. conducted an adsorption study of methyl orangeMO) on monodispersed magnetic mesoporous manganese ferriteomposites of about 200 nm size [229]. The observed degra-ation was attributed to adsorption of the dye molecule into

he porous structure of catalyst, followed by decolourizationy the central magnetic MnFe3O4 NPs. The MO degradationate was high at low pH (pH 2.0) and decreased with increas-ng pH, while the process of decolourization of MO was most

istry Reviews 288 (2015) 118–143 137

favourable at acidic pH. The degradation efficiency of MO wasenhanced by sonication and photolysis under sunlight. The authorsconcluded that they had designed a catalyst which showed adsorp-tion, degradation, recovery and reusability simultaneously in onesystem.

3.8. Biocatalysis applications

Several researchers have investigated new heterogeneous waysto alleviate the problems of fossil fuel depletion and global warming[230–232]. Cellulose decomposition is one of the most importantbiorefinery processes for the production of biofuels and bioplas-tics. The use of MNPs as solid catalysts offers a powerful approachfor carrying out hydrolysis at the interface between the solid sur-face and macromolecular cellulose (Scheme 33). The nanoparticleacid catalysts are easily dispersed in aqueous solution, resultingin facile interaction with cellulose, and hence high yield. Takagakiet al. designed magnetically recoverable silica NPs functionalizedwith sulfonic acid groups and found they served as a highly activesolid acid catalyst for the hydrolysis of disaccharides (sugar andcellubiose) and polysaccharides with facile magnetic separation[233].

Zhang and co-workers prepared magnetically recoverable cat-alyst by decorating phosphotungstic acid (HPW) on silica-coatedFe3O4 nanoparticles (Fe3O4@SiO2-HPW) [234]. The magneticnanoparticle (Fe3O4@SiO2) supported HPW catalyst (Fe3O4@SiO2-HPW) was prepared by simple co-precipitation followed bysol-gel approach. The outer shell of silica provides suitablesites for surface functionalization with HPW, which showed anexcellent catalytic activity for the synthesis of EMF from HMFand fructose (Scheme 34). Under optimal reaction conditions,EMF was obtained in a high yield of 83.6% by the etherifica-tion of 5-hydroxymethylfurfural. EMF could be also synthesizeddirectly from fructose in a yield of 54.8% via a one-pot reactionstrategy.

3.9. Miscellaneous applications

A copper nanomaterial supported on magnetic SiO2/CoFe2O4NPs can serve as a cost-effective nanocatalyst for the hydrolysisof ammonia-borane (NH3BH3) [235]. Nowadays, ammonia-boraneis considered one of the most promising solid hydrogen carri-ers because of its high gravimetric hydrogen storage capacityand low molecular weight. The reported catalytic system wasreproducibly prepared by wet-impregnation of Cu(II) ions onSiO2/CoFe2O4, which was followed by in situ reduction of theCu(II) ions on the surface of the magnetic support during thehydrolysis of NH3BH3 (Scheme 35). This protocol provided a veryhigh turnover frequency (2400 h−1) at room temperature, whichis not only higher than all known non-noble metal catalystsbut also more advanced than the majority of noble metal basedhomogeneous and heterogeneous catalysts employed for the samereaction.

Ma et al. prepared a new magnetic nanoparticle-supported anti-mony catalyst and evaluated it as a recoverable catalyst for theClauson-Kaas reaction [236]. The reaction proceeded efficiently inaqueous medium to give the corresponding N-substituted pyrrolesin high yield. The immobilized catalyst was easily recovered bymagnetic separation and recycled up to six times without signif-icant loss of its catalytic activity.

core–shell catalysts is explained in Table 6. As explained in above-mentioned section, these nanocatalysts are used not only fordifferent applications but these also proved themselves as impor-tant candidate for future industrial catalyst.

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118–143Table 6Various silica-coated magnetic core–shell catalysts used for different chemical transformations [158–236].

No Catalyst Type of reaction Reaction Catalytic performance (solvent,reaction temp., time, yield)

Recyclability Leaching test Reference

1 Pd@Si-MNPC C couplingreaction

Suzuki–Miyaura Dioxane, 80 ◦C, 15 h, 99–73% Recyclable upto eight times Negligible [158]2 Fe3O4/SiO2 magnetite

nanoparticlesupported-biguanidepalladium acetate complex

Suzuki H2O/Ethanol, 80 ◦C, 1–9 h,98–55%

At least eight runs Not given [161]

3 Pd-NHC@SMP Suzuki–Miyaura Isopropyl alcohol, 80 ◦C, 8 h,99–59%

Six times Not given [162]

4 Mag-Si@NHC-Ni C S coupling Cs2CO3, DMF, 100 ◦C, 92–62% Three times Detected after 4th run [163]5 Copper NPs on silica-coated

maghemite NPsCoupling and cycloaddition reactions THF, 100 ◦C, 1–20 h, 95–40% Three times Negligible [164]

6 Brønsted acids(HA)@Fe2O3@SiO2

C C couplingreaction

Synthesis of biologicallyimportant formamidines

Solvent-free, RT, 100–80% 2–5 runs Not given [165]

7 �-Fe2O3@SiO2-OSO3H Ugi-likeGroebke-Blackburn-Bienayméthree-component reaction

Solvent-free, 35 ◦C, 1 h, 94–85% Not mentioned Not given [166]

8 Fe3O4/SiO2 Multicomponent synthesis ofdiazepine derivatives

Ethanol, R.T., 3–6 h, 98–85% Not mentioned Not given [167]

9 PCP–pincerPd-complex@Si-MNPs

Cross-aldol reaction iPr2EtN (10 mol%), CH2Cl2, R.T.,40% (TOF = 16 h−1)

Reusable Not given [168]

10 Fe3O4@mesoporous SBA-15@cysteine hydrochloride

Biginelli reaction Ethanol, 363 K, 5–12 h, 85–10% Seven times within the withinexperimental error limit

[169]

11 (�-Fe2O3)-MCM-41-SO3H One-pot synthesis of N-aryl-2-amino-1,6-naphthyridinederivatives

Solvent-free, 120 ◦C,110–185 min, 98–84%

Five runs Not found (checked by FTIR) [170]

12 3-Sulfobutyl-1-(3-propyltriethoxysilane)imidazolium hydrogensulfate@Si-MNPs

One-pot synthesis ofbenzoxanthenes by thethree-componentcondensation of dimedonewith aldehyde and 2-naphthol

Neat, 30–60 min, 90 ◦C, 94–84% Six cycles Negligible [171]

13 H3PW12O40@Si-MNPs Three-componentMannich-type reactions

H2O (5 mL), RT,15–180 min,98–71%

Five times Negligible [172]

14 (�-Fe2O3)-MCM-41-SO3H Synthesis ofpyrimido[4,5-b]quinolines andindeno fusedpyrido[2,3-d]pyrimidines

H2O, 70 ◦C, 25–40 min, 95–81% Three times Not Given [173]

15 �-Fe2O3-MCM-41@l-proliniumnitrate

One-pot oxidative cyclizationdirect synthesis ofquinazolin-4(3H)-onederivative

Solvent free, 100 ◦C, 10–40 min,97–80%

Three cycles No leaching (Checked by XED) [174]

16 Zr-HMNQ@Si-MNP Friedel–Crafts, Knoevenageland Pechmann condensationreactions

Friedel–Crafts: Solvent-free, RT,30 min, 99–94%Knoevenagel: Solventfree, RT,20 min, 99–93%Pechmann: Solventfree, 110 ◦C,20 min, 99–96%

Six cycles Not detected(checked byICP-MS)

[175]

17 Cu-AcTp@Si-MNP N-Alkylation of amines Solventless, 10 h, KOH, 100 ◦C,air

Nine cycles Not detected [176]

18 DMAP@Si-MNPs Acetylation NEt3, DCM, RT, 16 h, 98–91% 30 times Not given [177]19 Silica-based Ru catalyst

Oxidation

Oxidation of aryl and alkylalcohols to aldehydes

Toluene (1 mL), 3 h, 100 ◦C,3 atm O2, 99–82%

Recyclable Negligible (ICP-MS detected) [182]

20 TiO2-functionalizedsilica-based MNPs

Epoxidation TBHP, ACN:DMF, 24 h, 15% Not given Not given [183]

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21 TiO2/SiO2 coated MNPs Epoxidation H2O2, 70 ◦C, 24 h, 99% Four cycles Not given [184]22 Fe2O3)–MCM-41–Mo(O)2L Epoxidation Ethanol, H2O2 (20 mmol), 8 h,

100–63%Six times Not detected [185]

23 Ruthenium hydroxidesupported on Si-MNPs

Epoxidation Toluene, 80–120 ◦C, 5–10 h,98–11%

Recyclable Not detected [186]

24 CoO NPs on silica-coatedmagnetite

Epoxidation Cyclohexene: cyclohexane,75 ◦C, PO2 = 3 atm, 6 h, 96.5%

Recyclable Not detected [187]

25 Au-SBA-15-MNPs CO Oxidation Molar flow rate of the CO inthis gas mixture was5.95 × 10−7 mol/s, 8 h

Not given Stable [188]

26 H3PW12O40@Si-MNP Oxidation of dibenzothiophene ACN, H2O2, 30 ◦C, 7 h 4–5 cycles Detected after 4 cycles [189]27 Mo-Salpr-Si-MNP Epoxidation of olefins TBHP: CHCl3, 6 h, 99.9–56%

CHP: CHCl3, 6 h, 95–45%Recyclable Not detected (checked by VSM) [190]

28 Metalloporphyrins (MPs) onsilica-coated Fe3O4 NPs

Oxidation of cyclooctene,cyclohexene and cyclohexane

DCM:ACN (1:1), under argon,1 h

5 cycles Not detected [191]

29 Phosphotungstic acid (PTA)supported onimidazole-functionalizedsilica-coated cobalt ferrite NPs

�-Aminophosphonatessynthesis

Neat, RT, 15 min., 97–68% 5 cycles 0.92% leaching detected in thefirst run, but negligible in thenext runs

[192]

30 Zn-AcPy@Si-MNPs Oxidation of amines H2O2, ACN, 80 ◦C, 1 h, 99–81% 5 cycles Negligible (detected by AAS) [193]31 Fe3O4/SiO2/Cu(II)salpr Oxidation of various alkyl

aromatics80% TBHP, Solvent free, 80 ◦C,12 h, 95–82%

8 cycles Not given [194]

32 Au NPs on thiol-functionalizedsilica-coated magneticnanosupport

Oxidations of cyclohexene andstyrene

TBHP, toluene, 373 K, 8 h,87–35%

5 cycles Not given [195]

33 Mn-porphyrin-Si-MNPs Olefins, saturated hydrocarbon,alcohols and organosulfurcompounds

TBHP, H2O, Recyclable Negligible [196]

34 Fe3O4@SiO2–NH2–Co andFe3O4@SiO2–NH2–Cu

Epoxidation Air, 80 ◦C,Fe3O4@SiO2–NH2–Co: 90.8%conv.: 63.7% selectivityFe3O4@SiO2–NH2–Cu: 86.7%conv.: 51.4% selectivity

4 cycles Negligible [197]

35 Rh(0) nanoparticle-supportedcatalyst

Hydrogenation

Hydrogenation of cyclohexene H2 (6 atm.), 75 ◦C, 99% conv. 20 cycles Negligible [206]

36 Pd- thiol-modified silica-basedmagnetically

Hydrogenation of cyclohexene H2 (10 atm.), 75 ◦C,:Catalyst/substrateratio = 1/1000, 99%

5 cycles Less than 0.01 ppm [208]

37 Ru0-NH2@Si-MNPs Hydrogenation of cyclohexene 10 bar H2, room temperature,2–6 h, 100–1.5%

Observed starting the fourthrun, although the catalystremains active for up to 7cycles

Detected for 7 cycles [111]

38 Fe3O4@SiO2-Rh(0) NPs Hydrogenation of modelolefins and aromatic substrates

Under microwave irradiation inwater 1–2.5 h, 100 ◦C, 99–87%

Recycles Not detected [208]

39 Cu(acac)2@Am-Si-MNPs Reduction of nitro compounds NaBH4, H2O, 5–60 min., RT,100–92%

9 cycles Not detected [209]

40 Fe3O4@SiO2/Ru Hydrogenation reaction ofcarbonyl compounds

KOH (0.2 mmol), inisopropanol under MWIrradiation, 100 ◦C, 30–45 min.,88–65%

3 cycles Not detected [210]

41 polyelectrolyte–gold@Fe3O4–silica core–shellmicrospheres

Hydrogenation of nitrophenols NaBH4, 25 ◦C 5 cycles Negligible [211]

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Table 6 (Continued )

No Catalyst Type of reaction Reaction Catalytic performance (solvent,reaction temp., time, yield)

Recyclability Leaching test Reference

42 silver-deposited silanizedmagnetite nanoparticles

Reduction of nitrophenols NaBH4, RT, 20 cycles Not detected [212]

43 Fe3O4@SiO2–Ag magnetic Reduction of nitrophenols NaBH4, RT, 8 min. 15 cycles Not detected [213]

44 Ruthenium alkylidene Olefin metathesis reaction DCM/toluene, 2–3 h, RT, 95% 5 cycles Negligible [217]45 Cu(II)-azabis(oxazoline)-

complexAsymmetric

Asymmetric benzoylations ofhydrobenzoin

DIPEA, 0 ◦C, 3 h, CH2Cl2 Not given Not given [221]

46 Magnetite colloids modifiedwith cinchonidine embeddedin silica

Hydrogenolysis of bicycle[2.2.2] oct-7-enes

Ethylacetate, autoclave at40 bar, 24 h, 298–353 K

Recyclable Not given [222]

47 Magnetically recoverable chiralrhodium catalyst

Asymmetric transferhydrogenation of aromaticketones

HCO2Na, Bu4NBr, 40 ◦C, 1 h,97.2–80.3% (97.7–84% ee)

Ten cycles Not given [223]

48 Mesocellular mesoporous silicasupport

Asymmetric dihydroxylation K3Fe(CN), K2CO3, t-BuOH–H2O(v/v¼1:1, 3 mL), 99.5–95.5%

Eight cycles Not given [224]

49 TiO2/SiO2/Fe3O4

Photocatalyticacitivity

Photodegradation of methylorange

Batch mode, 100 W highpressure mercury lamp, therole of silica coating wasobserved, increase in the ratewas observed with inhance insilica content but decline wasobserved after 14% of silicacoating

Not given Not given [225]

50 Fe3O4@SiO2@meso-TiO2 Degradation of rhodamine B Photoreactor with quartzjacket and (50 W) highpressure mercury lamp withmain emission wavelength313 nm and an average lightintensity of 2.85 mW cm−1,30 ◦C, is efficiency comparableto that of the well-knowncommercial photocatalystDegussa P25.

Ten cycles Not given [226]

51 Fe3O4@SBA-15 Photodegradation of malachitegreen

Photocatalytic reactor using UVlamp as a light source, pH 5.2

5 cycles Not given [227]

52 Polyaniline-modifiedFe3O4/SiO2/TiO2

Oxidative polymerization ofaniline

Photocatalytic reactor using UVlamp as a light source, 30 min.

Not given Not given [228]

53 Magnetic mesoporousmanganese ferrite composites

Methyl orange H2O2, mili-Q water, 30 min. 5 cycles Not given [229]

54 Sulfonic acid over SiO2/Fe3O4 Biocatalysisapplications

Hydrolysis of disaccharides Sucrose hydrolysis: water(3.0 mL), 373 K, 20 min, 93%Cellobiose hydrolysis:, H2O(3.0 mL), 393 K, 18 h, 88%

Three times Not given [233]

55 Phosphotungstic acid (HPW)supported on silica-coatedFe3O4 nanoparticles

The etherification of5-hydroxymethylfurfural

Ethanol 5 times Not given [234]

56 SiO2/CoFe2O4 NPs Miscellaneousapplications

Hydrolysis of ammonia-borane(NH3BH3)

NH3BH3, N2 gas, RT, 10 cycles Negligible [235]

57 Magneticnanoparticle-supportedantimony catalyst

Clauson–Kaas reaction H2O, reflux, 100 ◦C,40–120 min, 96–55%

6 cycles Very low (0.3 ppm) [236]

Chem

4

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Ntpspc

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M.B. Gawande et al. / Coordination

. Conclusions and future scope

In this review, we have provided an overview of the rationalesign of silica-coated magnetically recoverable nanocatalysts forarious organic transformations and other important applications.e have also discussed the importance of the inimitable arrange-ent of SMNPs. The functionalization and modification of SMNPs,hich introduces additional functionality to the NPs, has led to

hem gaining increasing interest for a wide range of applications.n this review, particular attention has been devoted to recentevelopments in the spatial organization, synthesis and protectionf these silica-supported nanocatalysts. The sustainable synthe-is of MNPs using less toxic and readily available precursors, asell as environmentally benign solvents or supports, under ambi-

nt conditions was explained. Various applications of these NPsn some challenging reactions, such as oxidation, carbon–carbonoupling reactions, olefin metathesis and photo- and bio-catalysis,ere discussed, which covered not only classical approaches for

uch reactions but also included methods with alternative renew-ble energy sources, such as microwave, flow reactors sonicationtc. Although remarkable progress has been made with these cat-lysts, further work is needed to discover other new and advancedpplications for these catalysts, such as for Green Chem., and envi-onmentally sustainable protocols.

The take-home message from our brief article is that newanotechnology is breathing new life into the design and prepara-

ion of well-defined heterogeneous catalysts with highly desirableroperties. By combining these new self-assembly syntheses withurface-science and theoretical mechanistic studies, it should proveossible to develop a new generation of highly stable and selectiveatalysts in the future.

cknowledgements

Y. Monga thanks the DST (Department of Science and Tech-ology), New Delhi, India, for awarding an Inspire Fellowship.he authors gratefully acknowledge the support by the projectO1305 of the Ministry of Education, Youth and Sports of the Czechepublic.

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