Rossi, l m (2014) magnetic nanomaterials in catalysis advanced

Volume 16 Number 6 June 2014 Pages 2889–3380

Green Chemistry Cutting-edge research for a greener sustainable future www.rsc.org/greenchem

ISSN 1463-9262

CRITICAL REVIEW Liane M. Rossi et al.

Magnetic nanomaterials in catalysis: advanced catalysts for magnetic separation and beyond

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Magnetic nanomaterials in catalysis: advanced catalysts for magnetic separation and beyond Liane M. Rossi,* Natalia J. S. Costa, Fernanda P. Silva and Robert Wojcieszak While magnetic separation techniques have long been in use, intensive research into superparamagnetic nanomaterials has accelerated the development of magnetically recoverable catalysts. Preparation techniques are currently undergoing rapid development and magnetic separation has been studied to facilitate the handling and recovery of enzyme, organo-, metal complex-, and nanoparticle-catalysts. In this article, we emphasize the preparation of support materials, because the choice of the correct support and the immobilization strategy are of primary importance in the development of high-quality magnetically recoverable catalysts. We summarize the representative methods for the synthesis of well-defined uncoated and coated magnetic nanomaterials. Recent scientific progress on the preparation of surface-modified magnetic nanomaterials and the most common synthetic approaches to attach or immobilize non-mag-

Received 29th January 2014, Accepted 2nd April 2014 DOI: 10.1039/c4gc00164h www.rsc.org/greenchem

netic catalytic active phases onto magnetic nanomaterials were discussed. Moreover, better control and understanding of the magnetic properties is now an essential tool not only in selecting the best preparation route for recoverable catalysts, but also for designing reactors (e.g., magnetic fluidized-bed reactors) and for developing magnetic field-driven technologies (e.g., changes in the catalytic output operating under an applied magnetic field).

Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, 05508-000 São Paulo, Brazil. E-mail: lrossi@iq.usp.br; Fax: +55 11 38155579; Tel: +55 11 30912181

Dr Liane M. Rossi received her BS degree in Chemical Engineering from the Federal University of Rio Grande do Sul – UFRGS (Brazil) in 1994 and her PhD in Chemistry from the Federal University of Santa Catarina – UFSC (Brazil) in 2001. After a two-year postdoctoral stay at UFRGS (Brazil) and a one-year postdoctoral stay at the University of New Orleans (USA), in 2004 she joined the Institute of Chemistry Liane M. Rossi at the University of São Paulo – USP (Brazil) where she has been an Associate Professor since 2010. Her research interests include novel approaches for the synthesis of supported metal nanoparticles with controlled size and morphology, the development of magnetically recoverable catalysts to facilitate catalyst recovery and recycling in liquid phase reactions, selective green reactions, including hydrogenations and oxidations, and biomass conversion into chemicals.

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Dr Natalia J. S. Costa received her BS degree in Chemistry (2008) and her PhD in Chemistry (2012) from the University of São Paulo – USP (Brazil). Currently, she is a postdoctoral researcher in Prof. Rossi’s group at the Institute of Chemistry – USP and participates in a collaboration project with LCC/CNRS-Toulouse (France). Her research interests include the development of ligand-assisted methods for the Natalia J. S. Costa preparation of supported metal nanoparticles, the preparation of supported nanocatalysts by decomposition of organometallic precursors and the preparation of nanocomposites such as metal nanoparticles embedded in inorganic oxides.

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1.

Critical Review

Introduction

Significant interest in catalytic routes for the synthesis of complex organic molecules has been driven by the economically competitive and ecologically aware market. Academic and industrial discoveries of efficient and selective catalysts for a wide variety of liquid and multiphase organic reactions have had an enormous influence on minimizing the costs of manufacturing and waste disposal. The practical application of catalysts in liquid phase reactions is hindered by both high costs and difficulties in catalyst separation and recycling. Many different approaches have been suggested to circumvent difficulties in catalyst separation, such as nanofiltration and new separation techniques based on liquid–liquid phase separation, including ionic liquids, fluorous phases, supercritical solvents, and polymeric supports.1 Each separation method has its own limitations of cost, efficiency, or generation of secondary waste.

Due to these issues, the so-called heterogenization of homogeneous catalysts on solid supports has received significant attention.2–5 Typical catalyst supports include polymers, carbon, silica, alumina, ceria, titania, and other metal oxides that can be separated by conventional separation techniques such as filtration and centrifugation. Superparamagnetic nanoparticles (NPs) have attracted attention as catalyst supports, because of their response to an applied magnetic field.

Magnetic separation has emerged as a robust, highly efficient and rapid catalyst separation tool with many advantages compared to catalyst isolation by the use of liquid–liquid extraction, chromatography, filtration or centrifugation. In fact, magnetic separation techniques have been used for decades in the mining and food processing industries to separate magnetic materials from non-magnetic materials on a wet or dry basis using eddy currents, electromagnets, and permanent

Fernanda P. Silva

Fernanda Parra da Silva received her BS degree in Chemistry (2008) and her MSc degree in Chemistry (2011) from the University of São Paulo – USP (Brazil). She is currently a PhD student in Prof. Rossi’s group at the Institute of Chemistry – USP. Her current research interests include the development of nonnoble metal nanoparticle catalysts for selective oxidation and hydrogenation reactions.

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magnets.6 Magnetic techniques are an inherent part of numerous material treatment operations and have undergone dramatic developments over the last 30 years.7 The first examples of the application of magnetic separation in the field of catalysis were based on the intrinsic magnetic and catalytic properties of Fe, Ni, Co, and iron oxides that can be directly collected and recovered by a remote magnetic field.8–12 Magnetic materials themselves can play a dual role in serving both as catalysts and magnetic carriers for catalysts. Numerous opportunities have arisen from the development of strategies to attach or immobilize non-magnetic catalytically active phases, such as metal complexes, metal or oxide nanoparticles, enzymes, or organocatalysts on superparamagnetic nanoparticles.13 An enormous number of magnetically recoverable (or retrievable) catalysts based on superparamagnetic supports can be found in recent reviews on this topic;14–19 however, we feel that more attention has been paid to the catalyst performance neglecting the fact that many of those catalysts were prepared with poorly characterized supports. We will avoid duplicating information and will focus our discussion on preparation methods and perspectives that take full advantage of the magnetic properties of these support materials in catalysis. We will summarize the most important aspects to be taken into account in choosing a magnetic material to be used as a magnetic support and will highlight representative methods for the synthesis of well-defined core– shell, anisotropic, and multicomponent magnetic nanostructures. We will present the recent scientific progress on the preparation of chemically modified magnetic nanomaterials for the design of magnetically recoverable catalysts. In addition, we will consider emerging opportunities for the design of reactors and catalysts operating under magnetic fields.

Dr Robert Wojcieszak received his Ph.D. in Physical Chemistry from the University of Lorraine in France in 2006. In 2008 he joined the IMCN at the Catholic University of Louvain in Belgium. In 2013 he was working on carbohydrate oxidation at the University of São Paulo in Brazil.

Later that year he joined the UCCS at the University of Lille 1 in France, and is currently a 1st class CNRS researcher in the Robert Wojcieszak UCCS-VALBIO group. He is involved in the development of new advanced catalysts for biomass transformation. His research is focused especially on the amination of bio-based alcohols and shape selectivity processes in heterogeneous catalysis.

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Critical Review

2. Magnetic separation Materials separation from product streams remains an issue for almost any manufacturing process. The nature of the material and the time and energy consumed usually determine which separation process will be chosen. Once the use of conventional separation methods like centrifugation or filtration became widespread, the use of magnetic separation was not always obvious, though if some part of a mixture is intrinsically magnetic, then magnetic separation is often the best choice. Magnetic separation generally offers high efficiency and specificity when compared with equivalent centrifugation or filtration methods.20–24 The use of magnetic separation occurs in a variety of sectors, with its use divided into two main categories: positive selection for the enrichment or concentration of target species (Fig. 1A) or negative selection for purification of feeds (Fig. 1B). In the first case, a magnetic carrier is introduced to selectively confer magnetic properties to non-magnetic target species that need to be separated from solution. An appropriate affinity ligand is coupled to the magnetic particles, which will thus exhibit high affinity towards the target species. The attachment or entrapment of the target species onto the surface of the magnetic material allows separation by magnetic decantation. In the second case, the nonmagnetic target species do not interact with magnetic carriers and remain in the supernatant while the magnetic material is removed from the solution by magnetic decantation.25,26 Examples of magnetic separation are the removal of tramp iron from a variety of feeds and the separation of ores of distinctly magnetic and nonmagnetic (or diamagnetic) particles.

In biotechnology, magnetic separators of relatively low field gradients are used in batch mode to concentrate surface-engineered magnetic beads from a suspension.27–30 Magnetic separation is possible in virtually any system, even when magnetic components are absent, through the employment of engineered magnetic materials as supports to attach the product of interest or the complementary component of a mixture.

While magnetic separation is, in principle, an excellent choice for many separation problems, its widespread appli-

Green Chemistry

cation has been limited by the complexity required in separator design and bead/nanoparticle technology. In the case of flow separation, the design of electromagnets is needed to create an external magnetic field. In batch separations, permanent magnets in a multipole configuration provide the relatively low gradients needed and their setup and use is easy and immediate.31,32 Most of the examples in the field of catalysis involve the engineering of magnetically retrievable nanomaterials in which the active catalyst phase is attached by covalent or electrostatic interactions. This kind of catalyst was in principle designed for application in batch reactors and represents a powerful tool in organic synthesis laboratories and a promising future for green alternatives in industry, such as magnetic fluidized-bed reactor technologies (see the section on magnetic reactors).

3. Choice of magnetic supports The appropriate choice of magnetic carriers can dramatically increase the efficiency of the separation process. The selection criteria should focus on the quality (morphology, size, and shape) and magnetic properties of the material. All substances are influenced to one degree or another by the presence of a magnetic field. The macroscopic magnetic properties of a material are a consequence of interactions between an external magnetic field and the magnetic dipole moments of the constituent atoms. The origin of the magnetic moments in atoms lies in two electronic movements related to the electron: the orbital movement of the electron around the nucleon, and the “spin” movement of the electron around its own axis. There is also a quantum exchange force that aligns ( parallel or antiparallel) the magnetic moments in neighbor atoms, thus each atom inside the material contributes individually for the total magnetic moment. The magnetic moments of the electrons with spin up are canceled by the magnetic moments with spin down, thus only the unpaired electrons are responsible for the magnetism in the material.33 If a material is inserted into a magnetic field H, the magnetic atomic moments in the material contribute to its magnetic response, also known as B, the intrinsic magnetic field of the material, which can be described by eqn (1): B ¼ μ0 ðH þ MÞ

ð1Þ

where µ0 is the magnetic permeability under vacuum, and M is the magnetization of the material.

Magnetization can be described as the vectorial measure of the magnetic dipole moment by volume unit in all the considered volume, and is represented by eqn (2): M ¼ m=V ¼ nμB =V

Fig. 1

Examples of the use of magnetic separation.

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ð2Þ

where m is the total magnetic moment, which corresponds to the number of atoms (n) that have unpaired electrons multiplied by the elemental magnetic moment µB (the Bohr magneton) divided by the volume V occupied by the material. The maximum possible magnetization, or saturation magnetization

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Critical Review

(Ms), represents the magnetization that results when all the magnetic dipoles are mutually aligned with the external field.

The magnetic behavior of materials fits in two major groups. The materials that do not have permanent magnetization and exhibit a linear response to a moderate applied field, and the materials that have intrinsic magnetization and exhibit a nonlinear response to the applied field.34 The linear group comprehends materials with diamagnetic, paramagnetic or antiferromagnetic behavior. In these materials the magnetization M is proportional to the exciting field H. The proportionality constant is known as magnetic susceptibility (χ). The magnetic susceptibility is a dimensionless constant and is unique to each substance, having positive values for paramagnetic materials and negative values for diamagnetic materials.

The non-linear group is comprised of ferromagnetic and ferrimagnetic materials. These materials have intrinsic magnetization and exhibit high magnetic susceptibility.35 These materials are already magnetic even in the absence of a magnetic field.35,36 The magnetic susceptibility in ordered magnetic materials depends on the temperature and the applied field H, and this dependence could be detected by the shape of M × H magnetization curves at a given temperature. In nonliner media, an irreversibility in the magnetization process in the presence of an applied magnetic field results in hysteresis loops in M × H curves. This behavior is observed in large magnetic particles with a multi-domain structure formed by regions of uniform magnetization separated by domain walls.37 The hysteresis loops are related to the energy balance due to the domain wall motion between adjacent magnetic domains when the temperature and applied magnetic field are changed, provoking the alignment of the magnetic moments in the material. However, if the sample size is reduced, there is a critical volume below which the formation of multi-domains is not favorable and the particle remains as a single domain and their magnetic moments could be visualized as one large magnetic moment. Under these circumstances, when a magnetic field is applied, the energy is not dispersed by the motion of domain walls and, as a result, the material has a giant moment and is named a superparamagnet. These systems have no hysteresis and the data for different temperatures overlap into a universal M × H curve. The time required for spin reversal, the so-called relaxation time τ, depends on the energy barrier between the spin-up and spin-down states, also called easy axis orientation, and the temperature, and can be described by eqn (3):38 τ ¼ τ0 exp ðKV =kB TÞ

ð3Þ

The term K is the nanoparticle magnetic anisotropy energy density and V is its volume, therefore KV is the energy barrier associated with the magnetization moving from its initial easy axis direction, through a hard plane, to the other easy axis direction, and kBT is the thermal energy. With the decreasing particle size, kBT exceeds the energy barrier KV and the magnetization is easily flipped.39 If the particle magnetic moment reverses at times shorter than the experimental time scale, the

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system is called to be in a superparamagnetic regime, and if not, the system is called blocked. The temperature that separates these two states is called the blocking temperature (Tb) and depends on the anisotropy constant, the size of the particles, and the applied field. The blocking temperature can be calculated considering the time window of the measurement.40 In recent years, magnetic materials have been prepared as superparamagnetic nanoparticles using a variety of chemical methods for application in separation technologies. There are three ferromagnetic metals: Fe, Co, and Ni. These three metals have been prepared as monodispersed superparamagnetic nanoparticles by different methods of synthesis. The particle size can be controlled by changing the reaction parameters, such as temperature, time, concentration of the precursor, and nature of the surfactant, which play an important role in nucleation, growth, and stabilization of the nanoparticles.41 Many types of magnetic nanoparticles can be synthesized, including iron oxides (Fe2O3 and Fe3O4) and cobalt, manganese, nickel, and magnesium ferrites. The most commonly employed magnetic nanoparticles for different applications tend to be Fe3O4, MnFe2O4, and CoFe2O4 because they are easy to synthesize with size-monodisperse particles with high saturation magnetization. Table 1 shows the saturation magnetization of a range of metals, metal oxides, and ferrites.

The combination of high Ms and superparamagnetism makes such materials very attractive as magnetic carriers. In general, superparamagnetic nanoparticles are preferred because they are strongly attracted to an applied magnetic field due to the contribution of large magnetic moments within the individual particles and because they behave essentially as non-magnetic materials in the absence of an applied magnetic field. The removal of the applied magnetic field instantaneously reduces the overall magnetic moment back to zero and repeated cycles of separation and dispersion are possible because of the absence of “magnetic memory”. Additionally, materials ideal for magnetic separation techniques must be stable under reaction conditions (temperature, pressure, solvents, reagents, substrates, and products).

Magnetic nanoparticles can be either uncoated, such as those containing only molecular stabilizers, or coated with a layer of silica or carbon. Each type offers advantages and dis-

Table 1 Saturation magnetization (Ms) for various bulk ferrites and metallic Fe, Ni, Co42

Substance

Ms (emu g−1)

Fe Co Ni Fe3O4 γ-Fe2O3 MnFe2O3 CoFe2O4 NiFe2O4 CuFe2O4

222a, 218b 162.5a 57.5a 92b 76b 80b 80b 50b 25b

a

0 K. b 293 K

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Critical Review

advantages that depend on the application. Uncoated magnetic nanoparticles can be easily prepared and functionalized using strategies such as bi-functional ligands containing carboxylate, phosphonate, alkoxyorganosilane, etc. However, most of these materials easily oxidize in an air atmosphere, notably magnetite and zero-valence metals, and show low stability in acidic media, which may lead to changes in their magnetic properties. Coating the nanomaterial’s surface is a strategy to protect the magnetic content. A protective shell prevents the direct contact of the magnetic core with additional agents linked to the shell-material surface, thus avoiding unwanted interactions and oxidation. Furthermore, coating the magnetic cores with other oxides such as silica facilitates the immobilization of catalysts through a covalent approach and, in the case of mesoporous silica layers, increases the surface area.43 In general, coated magnetic nanomaterials are robust and thermally stable; however, the silica coating dissolves in basic media and polymers can be sensitive to reaction solvents, which may lead to premature catalyst decomposition. The coating of magnetic nanomaterials with a uniform layer of a second material is complex and is analyzed below.

3.1.

Uncoated magnetic nanoparticles

Superparamagnetic iron oxide NPs (SPIONs) are easier to synthesize and handle than zero-valence magnetic materials such as Fe, Co, and Ni; due to its high magnetization, magnetite (Fe3O4) is the most-exploited iron oxide nanomaterial for magnetic separation (Table 1).44 Fe3O4 is a ferrimagnetic mineral formed by the spinel structure containing the atoms of Fe(II) and Fe(III) occupying octahedral and tetrahedral sites of coordination when the oxide anions are arranged as a cubic closepacked lattice. The ferrimagnetism in magnetite is defined by the arrangement of the spins in Fe(II) that are antiparallel to the Fe(III) spins, while the interactions of the iron ions in different coordination sites results in incomplete cancellation of spin moments and a strong magnetization. The saturation magnetization values of magnetite nanoparticles are in general lower than the value for bulk magnetite, but the specific value depends on the synthetic procedure. Magnetic properties are also dependent on the size of the nanoparticle, i.e. changes in the crystal morphology.45 The most widely used method for synthesizing Fe3O4 NPs is aqueous coprecipitation from basic aqueous solutions of ferric and ferrous salts.46 The coprecipitation method is simple and eco-friendly, but results in polydispersed nanoparticles. In principle, the final material has acceptable magnetic properties to be used as a catalyst support. The widespread interest in obtaining magnetite nanoparticles for various technological and medical applications requires particles with uniform physical and chemical properties. Other synthetic options, such as thermal decomposition of Fe(III) complexes, are more labor-intensive and expensive than co-precipitation, but result in highly crystalline and monodispersed nanoparticles. Sun et al.47 and Hyeon et al.48–50 have contributed with several important methodologies to the synthesis of monodisperse iron oxide magnetic

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Green Chemistry

nanoparticles. This methodology has also been exploited to prepare magnetite nanoparticles for catalytic applications.51 Thermal decomposition methods are more suitable for obtaining nanomaterials with controlled morphology and size.

For example, dumbbell-like Au–Fe3O4 NPs are synthesized by thermal decomposition of iron-oleate in the presence of colloidal Au NPs,52 but the synthetic method does not fit well from the green perspective, as it consumes large volumes of high boiling-point solvents and surfactants. Magnetite nanoparticles can be further oxidized to maghemite or dissolved in an acidic medium. However, they are stable under basic conditions and can be used for the preparation of supported oxide/hydroxide catalysts, in contrast to common solid supports such as SiO2 and Al2O3. Iron hydroxide supported on magnetite nanoparticles is active for aldol reactions.51 Magnetite nanoparticles have intrinsic catalytic properties very wellknown in the Fenton reactions,53 but they also find application in other reactions, such as thiolysis of epoxides,54 coal liquefaction,55 and oxidation.56 Magnetite nanoparticles, when used as catalysts, should have uniform composition and size and a high surface area, for example as highly porous magnetite.57 When used as supports, magnetite nanoparticles can also affect the catalytic properties of the supported metal nanoparticle catalysts. For example, the heterostructures of Au and Fe3O4 have been suggested to be more active catalysts than their isolated components.58,59 The polarization effect at the Au–Fe3O4 interface also enhances the catalytic properties of Fe3O4. A cooperative effect was observed on the Pd–Fe3O4 catalyst in denitrification reaction. Sun et al.60 proposed a mechanism involving oxidation of the Fe(II) ions of magnetite to Fe(III) and reduction of nitrite to NH4+ that, in the presence of Pd, produces hydrogen-active species responsible for the subsequent reduction of Fe(III) to Fe(II). Kamonsatikul et al.61 demonstrated a synergistic effect in the oxidation of benzyl alcohol by Fe3O4 and γ-Fe2O3 NPs stabilized by ferrocene moieties. There is an increase in the conversion rate and selectivity of iron oxide nanoparticles stabilized by N-1-dodecylferrocenylmethylimine and N-1-dodecyl-ferrocenylmethylamine when compared to those stabilized by the common ligand dodecylamine. The proposed mechanism involves the adsorption of benzyl alcohol on the surface of nanoparticles and, in the second step, a Fenton-type reaction on both magnetic ironoxide nanoparticles and ferrocene moieties. These examples of the cooperative effect of the iron oxide nanoparticles suggest that magnetic separation is one of the advantages of using these materials as supports, and they can also contribute to the activity and selectivity of the catalysts.

Maghemite (γ-Fe2O3) has a lower saturation magnetization than magnetite (Table 1), but has the advantage of being more stable and resistant to acid environments and high temperatures.62 This material has been studied more because of its catalytic properties.63 Other ferrites (MFe2O4, M = Co, Mn, etc.) have also received attention in the field of catalysis as magnetically recoverable catalysts64 but not as much as magnetic supports. Superparamagnetic pure Fe(0) NPs can also serve as magnetically recoverable catalysts, but due to the easy oxi-

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dation of Fe(0) in air, the synthesis must be carried out under inert conditions. Better control of the final material can be achieved using organometallic precursors such as Fe(CO)565 or using strong reducing agents like alkyl Grignard or alkyl lithium reagents.66 Alloys containing iron, such as FePt, can also exhibit superparamagnetic properties for use as catalyst supports67 and are more stable than Fe(0). Both Fe and FePt, however, are more expensive and laborious to prepare than iron oxides. There is still room for the application of ferrites and superparamagnetic iron-based alloys as potential materials for magnetic separation and catalyst supports. Metallic Co and Ni also have noteworthy magnetic properties (Table 1). Due to their intrinsic magnetic properties, Co and Ni NPs could be used both as the catalyst phase and magnetic responsive material for separation by applying an external magnetic field. Several methods are available for the preparation of colloidal and supported Ni and Co NPs and their oxides, with different sizes and morphologies.68–71 However, the easy oxidation of metallic Ni and Co makes it necessary to stabilize these particles by coating them with strong coordinating ligands, inorganic oxides, or carbon. The material used in the magnetic nanoparticles’ coating can also act as a catalyst, for example TiO2-coated Ni NPs were active for photodegradation of organic pollutants.72 However, iron oxides are less toxic, cheaper, and easier to synthesize than Co and Ni and are thus preferred as catalyst supports. Nickel and cobalt oxide nanoparticles are less preferable as supports because they possess lower magnetization than their corresponding metals.

3.2.

Coated magnetic nanomaterials

3.2.1. Silica coating. Magnetic nanomaterials coated with silica have received a great deal of attention in the last few decades, especially for applications in catalysis and biomedicine. Silica coating has several advantages, including high stability in aqueous media and easy surface functionalization.

From a practical point of view, it is highly desirable that every iron oxide nanoparticle is coated with a homogeneous silica shell. The formation of core-free silica particles must be avoided, because they cannot be removed by the applied magnetic field during the magnetic separation process. Inequality in the core number or silica shell thickness also results in an irregular response to the applied magnetic field and compromises the efficiency of separation.

Sol–gel processes. Sol–gel processing has been widely adopted for coating magnetic nanomaterials with silica because it is a simple and surfactant-free procedure. However, acquisition of a true core–shell structure is questionable. The sol–gel method relies on the use of the well-known Stöber process,73 in which silica spheres are formed by the hydrolysis and condensation of a silicon alkoxide precursor in a basic alcohol–water mixture. Ohmori and Matijević demonstrated that a sol–gel Stöber method can be used to coat hematite particles with uniform layers of silica by hydrolysis of tetraethylorthosilicate (TEOS) in water, ammonia, and 2-propanol mixtures.74 The main limitation of the sol–gel method is the lack of stability of colloidal magnetic nanoparticles in the

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Critical Review

alcohol–water mixture; they typically precipitate before the deposition of silica. Philipse et al.75 prevented this problem of flocculation by modifying the surface of magnetite nanoparticles with sodium silicate before coating them with silica using the sol–gel method. The silica coating was carried out by adding TEOS to a mixture of ethanol and ammonia containing the surface-modified magnetite nanoparticles. Lu et al.76 showed that a commercially available ferrofluid can be directly coated with silica shells through TEOS hydrolysis using a sol– gel protocol. The silica shell and core–shell structure was confirmed by transmission electron microscopy (TEM) analysis.

However, the researchers provided no information about the surface properties of the magnetic cores, nor did they explain why the cores do not flocculate in the mixture of 2-propanol and water. Deng et al.77 performed a systematic study on the formation of silica-coated magnetite particles via the sol–gel approach, where the magnetite nanoparticles were prepared by co-precipitation and stabilized with citrate. They studied the influence of reaction parameters – including the type of alcohol, the volume ratio of alcohol to water, and the amount of aqueous ammonia and TEOS – on the formation of silicacoated magnetite nanoparticles. Their conclusion was that silica-coated magnetite particles with uniform dispersion and typical core–shell structure can be prepared in a solution with a 4 : 1 volume ratio of ethanol to water. Increasing the amount of ammonia improved the morphology of silica-coated magnetite nanoparticles, but magnetite-free silica particles were also formed by the rapid hydrolysis of TEOS. An increase in the amount of TEOS leads to a gradual increase in the thickness of the silica coating and the capability of producing larger silicacoated magnetite particles with a more regular shape.

Microemulsion methods. Microemulsion methods have been used for coating nanomaterials with silica.78 Well-controlled core–shell morphologies are obtained using these methods, which rely on the hydrolysis and condensation of TEOS around either pre-synthesized or co-precipitated magnetic NPs dispersed in the surfactant assemblies. At the microscopic level, a microemulsion consists of nanometer-sized droplets that can be used as confined reaction media. The microemulsion method can be regarded as a confined sol–gel method where the spherical-shaped silica is formed by the ammonia base-catalyzed hydrolysis of TEOS. In theory, the size of the nano-sized droplets and the thickness of silica shells can be regulated by changing the microemulsion formulation, such as the selection of a surfactant. In practice, however, it proved to be very difficult to control the final size of the particles.

Magnetic nanoparticles (typically Fe, γ-Fe2O3, or Fe3O4) can be prepared in situ. Tartaj and Serna79 prepared monodisperse silica spheres containing superparamagnetic α-Fe NPs in a reverse microemulsion formed by an aqueous solution of FeCl2, the nonionic surfactant polyoxyethylene(5)nonylphenyl ether (Igepal CO-520), and heptane. The addition of TEOS and ammonia induced the reduction of FeCl2, producing Fe NPs and the hydrolysis of TEOS. After thermal annealing, a protective layer of Fe2SiO4 was formed, which provided high stability to the iron cores. Analogously, iron oxide nanoparticles can be

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Critical Review

prepared in situ by a co-precipitation method in a microemulsion.80 Lee et al.81 reported the preparation of magnetic nanoparticles in situ by adding hydrazine to a reverse microemulsion (sodium dodecyl–benzenesulfonate–xylene– water) containing the metal salt precursors ([Fe3+]/[Fe2+] = 2) (Fig. 2). The silica layer is formed simply by adding TEOS during the formation of the magnetite nanoparticles. The amount of TEOS injected at the final step determines the thickness of the silica shell. However, the in situ preparation of nanoparticles often results in poor-quality magnetic cores with large size dispersion, low crystallinity and often a mixture of several iron oxide phases.

Magnetic nanoparticles can be prepared prior to silica shell formation and introduced as a component of the microemulsion method. Vestal and Zhang82 used a reverse microemulsion with Igepal CO-520 to coat pre-formed high-quality ferrite nanoparticles (MFe2O4; M = Co, Mn, Fe, Ni, and Mg, etc.) with silica. Yi et al.83 used a similar reverse microemulsion with Igepal CO-520 to synthesize silica-coated magnetic Fe2O3 NPs and multifunctional particles containing CdSe quantum dots and magnetic nanoparticles.84 The researchers studied the influence of the amount of Igepal CO-520, ammonia, the number of core nanoparticles, and TEOS on the coating quality and observed that the shell thickness increased with an increase in the TEOS content and aging time and a decrease in the number of core nanoparticles and surfactant content.85 A suitable ratio of Igepal CO-520 to ammonia, which determines the number and size of the aqueous domain, is crucial for a one-to-one matching of the number of core nanoparticles with that of aqueous domains, in order to avoid the formation of undesirable core-free silica particles.

Cannas et al.86 showed that the microemulsion method with

Fig. 2

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Igepal CO-520 can be used to provide a uniform coat of highquality magnetic nanoparticles prepared through thermal decomposition of metal acetylacetonates in the presence of a mixture of oleic acid and oleylamine with a silica shell. Both the ferrite core nanoparticles (8 nm) and the silica-coated core–shell particles (shell thickness ∼12–13 nm) have a narrow particle size distribution. More recently, Ding et al.87 revisited the reverse microemulsion method with the surfactant Igepal CO-520 to extend the preparation of core–shell nanoparticles to Fe3O4 NPs of different sizes. They studied the regulations that produced uniform core–shell nanoparticles with a single core (without core-free silica particles) and with different shell thicknesses using differently sized-Fe3O4 NPs (Fig. 3). The authors showed that the silica coating parameters suitable for Fe3O4 NPs of a given size are not always applicable to those of other sizes. Moreover, a match of the number of Fe3O4 NPs with the aqueous domain number is essential for a successful synthesis. The authors showed that a small aqueous domain (determined by the ammonia to Igepal CO-520 ratio) was suitable for coating ultrathin silica shells, while the large aqueous domain was required to coat thicker shells. Although increasing the content of both ammonia and TEOS can increase the silica shell thickness, a good match between these two components is essential to avoid the formation of core-free silica particles.

The mechanism of silica coating in a reverse microemulsion system has recently been discussed. Selvan et al.,88 Darbandi et al.,89 and Koole et al.,90 when studying the coating of quantum dots with silica, proposed a ligand exchange mechanism to explain the coating process. Vogt et al.91 examined the coating of oleic acid-capped iron oxide nanoparticles with silica, and suggested that the ligand exchange of oleic

Schematic diagram of the one-pot synthesis of silica-coated magnetite NPs.

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Fig. 3 TEM images of Fe3O4 NPs with sizes of (a) 3–5 nm, (b) 8.8 nm, (c) 10.1 nm, (d) 12.2 nm, (e) 13.5 nm, and (f ) 19.1 nm. The insets show the corresponding silica coating. Scale bar = 20 nm. Reprinted with permission from ref. 87.

acid and partially-hydrolyzed TEOS explains the phase transfer of the magnetic nanoparticles with hydrophobic ligands from the oil phase to the water phase that is essential for the formation of a uniform core–shell morphology. The mechanism of the silica coating was also investigated by Ding et al.87 using Fourier transform infrared (FTIR) analysis and is schematically illustrated in Fig. 4. First, Igepal CO-520 forms micelles in a cyclohexane solution, because of its hydrophilic groups. When

Fig. 4

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Fe3O4 NPs are added to the solution, a ligand exchange between oleic acid and Igepal CO-520 occurs. Ammonia is added and fills the remaining Igepal CO-520 micelles. The micelle size is enlarged and forms a reverse microemulsion system. The addition of TEOS to the microemulsion system causes its hydrolysis at the oil–water interface and the subsequent ligand exchange with Igepal CO-520 chemically absorbed on the Fe3O4 NPs surface. This process transfers the Fe3O4 NPs to the water phase and the hydrolyzed TEOS undergoes a condensation process and forms the silica shells. The high quality silica-coated magnetite material prepared by the microemulsion method with Igepal CO-520 has been used as a support for many types of catalysts. Rossi et al.92 reported a modified formulation for the microemulsion process and were able to scale up the synthesis to prepare up to 6–7 grams of uniformly sized silica-coated magnetite. The magnetic nanocomposite exhibits a saturation magnetization of 9 emu g−1 (total mass of material) at 300 K, which corresponds to 69 emu g−1 of Fe3O4. The reverse micellar microemulsion method is very efficient, reproducible, and easy to scale up, while maintaining a well-defined core–shell morphology. Nevertheless, it generates a considerable amount of chemical waste, primarily because of the amount of solvent and surfactant used. In order to become economically and environmentally sound, Rossi et al. developed a methodology that recovered all the microemulsion components, including the surfactant, to be recycled for a new synthesis.

Magnetic NPs with mesoporous silica coating (such as Fe3O4@mSiO2) have received attention because they combine high surface area and magnetic separability.85,93–97 The main strategy involves the simultaneous sol–gel polymerization of tetraethoxysilane (TEOS) and n-octadecyltrimethoxysilane (C18TMS) that, after calcination to remove the organic groups of C18TMS, generates the mesoporous silica shell (Fig. 5).

3.2.2. Carbon coating. Recently, carbon-protected magnetic nanoparticles have come in for greater scrutiny due to

Proposed mechanism for coating of pre-formed magnetic nanoparticles with silica in a microemulsion.

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Fig. 5 TEM images of magnetic nanoparticles with mesoporous silica shell. Reprinted with permission from ref. 96.

their many advantages over polymer and silica coatings.40 Carbon materials have much higher chemical and thermal stability and are impermeable to most chemicals, completely shielding the inner magnetic core. The well-developed graphitic carbon layers provide an effective barrier against oxidation and acid erosion. Moreover, carbon coating is more often applied to nanoparticles in the metallic state, and thus the nanomaterials have a higher magnetic moment than the corresponding oxides. A variety of carbon-coated iron-based binary alloys (FeCu, FeCo, and FeNi) were prepared by the sequential spraying, chemical precipitation, and controlled pyrolysis at elevated temperatures.13 Carbon-coated magnetite, however, is more difficult to obtain due to the facile reduction of Fe3O4 to Fe0 and carbide under typical reaction conditions.

The few examples described in the literature are based on the precipitation of iron oxide nanoparticles followed by stabilization with tensoactive molecules. The stabilized nanoparticles are then mixed with different organic carbon precursors such as glucose, polyethylene glycol, or citric acid, and treated hydrothermically at temperatures of 200–600 °C. Tristão et al.98 prepared magnetic carbon-coated Fe3O4 particles in a single step, combining the reduction of Fe2O3 with a chemical vapour deposition process using methane. Fe2O3 was directly reduced with methane at temperatures between 600 and 900 °C to produce mainly Fe3O4 particles coated with up to 4 wt% of amorphous carbon. The as-prepared catalysts were active in hydrodechlorination reaction. Nikitenko et al.99 reported a sonochemical procedure that leads to air-stable carbon-coated Co NPs. Unfortunately, the obtained particles were polydisperse and not very uniform. Another method described by Johnson et al.100 produced carbon-coated magnetic Fe and Fe3C NPs by a simple pyrolysis of iron stearate at high temperatures under an argon atmosphere. The

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as-obtained carbon-coated magnetic nanoparticles were stable at up to 400 °C under air. This direct one-step salt conversion process has a lot of advantages and might be scaled up.

However, the nanoparticles produced by this method show a broad size distribution, with a diameter ranging from 20 to 200 nm and the cores are covered with 20 to 80 graphene layers. Almost the same method was used by Lu et al.101 to prepare highly stable carbon-coated Co NPs with a size of about 11 nm. In the procedure, the Co NPs were first coated with furfuryl alcohol and then carbonized to carbon during pyrolysis. The obtained carbon layer protected the Co NPs against air oxidation and erosion by strong acids and bases.

Moreover, the stability of the carbon layer depends on the carbon precursor. Indeed, if cetyltrimethylammonium bromide (CTAB) was used as the carbon source, the carbon coating was not perfect and the cobalt core was leached with an acid. Similar carbon-encapsulated Co NPs were prepared through the pyrolysis of a metallic Co NP composite (ca. 8–10 nm) and poly(styrene-b-4-vinylphenoxyphthalonitrile).102 In this case, the obtained carbon-coated Co NPs were highly stable. The long-term study showed that their high saturation magnetizations (ca. 95–100 emu g−1) were maintained for at least one year under ambient conditions. Grass et al.103–106 reported on a continuous process for the synthesis of carbon-coated Co NPs by a reducing flame-spray pyrolysis. A substantial amount of carbon-coated ferromagnetic nanoparticles (>30 g h−1) with a thin graphene layer (1 nm) was easily prepared. The intrinsically pyrophoric metal cores exhibit very high thermal and chemical stability and moreover, the carbon layer deposited on the cobalt core has no detrimental effect on the magnetization.

3.2.3. Polymer coating. More and more attention is being paid to organic polymer materials as coatings for magnetic nanoparticles. Polymeric coating materials can be classified into synthetic and natural.107 Polymers based on poly(ethylene-co-vinyl acetate), poly(vinyl pyrrolidone) (PVP), poly(lacticco-glycolic acid) (PLGA), poly(ethylene glycol) (PEG), and poly(vinyl alcohol) (PVA) are typical examples of synthetic polymeric systems. Natural polymer systems include gelatin, dextran, chitosan, pullulan, starch, etc.108,109 In general, the methods used for the coating of magnetic nanoparticles with surfactants or polymers differ from those used for inorganic oxides.40 In this case the polymers or surfactants can be chemically anchored or physically adsorbed on magnetic nanoparticles to form either a single or double layer.110,111 This adsorption creates repulsive (mainly as steric repulsion) forces that stabilize the magnetic particles and avoid precipitation.

Polymers containing functional groups, such as carboxylic acids, phosphates, and sulfates, are suitable for coating iron oxide-based magnetic materials. The most commonly used polymers are poly( pyrrole), poly(aniline), poly(alkylcyanoacrylates), poly(methylidene malonate), and polyesters, such as poly(lactic acid), poly(glycolic acid), poly(e-caprolactone), and their copolymers.112–114 The most important applications of polymer-coated magnetic particles are magnetic field-directed drug targeting and as contrast agents in magnetic resonance

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imaging. Both applications are possible due to the biocompatibility of these materials. However, polymer-coated magnetic particles also find use in the field of catalysis.115 Pourjavadi et al.116 prepared a functionalized poly(ionic liquid) coated magnetic nanoparticle (Fe3O4@PIL) catalyst by polymerization of functionalized vinylimidazolium in the presence of surface modified magnetic nanoparticles. The polymer layers coating the surface of the magnetic nanoparticles confer good thermal stability and recyclability. The poly(ionic liquid)-coated magnetic nanoparticles form a novel class of acidic heterogeneous catalysts which are particularly suitable for the practice of organic synthesis in an environmentally friendly manner. The catalyst was used for the protection of aldehydes by conversion to 1,1-diacetate under solvent free and room temperature conditions. Also, the catalyst shows good activity for the deprotection reaction of acylals.

Chu et al.117 reported the synthesis of polymer-coated magnetite nanoparticles by a single inverse microemulsion. The first step is the preparation of magnetic nanoparticles in a microemulsion using a sodium bis(2-ethylhexylsulfosuccinate) surfactant and toluene as the organic phase. After polymerization, the particles were recovered by precipitation in an excess of an acetone–methanol mixture at a 9 : 1 ratio. The asobtained particles have superparamagnetic properties and a narrow size distribution centred around 80 nm. Xu et al.118 reported that single iron oxide nanoparticles (ca. 10 nm) can be embedded in polystyrene spheres through emulsion polymerization to give stable superparamagnetic photonic crystals. Vestal et al.119 have used this method for coating MnFe2O4 NPs with polystyrene. Another method used for polymer coating is oxidative polymerization; for example, polyaniline in the presence of the oxidant ammonium peroxodisulfate was used to prepare polydispersed (20–30 nm) and core–shell particles.120 Flesch et al.121 successfully anchored methacrylate polymerizable groups to the surface of maghemite nanoparticles using methacryloxypropyltrimethoxysilane (MPS). This strategy results in hybrid particles with a diameter of about 100 nm containing up to 8 g of polymer per gram of maghemite. Lee et al.122 prepared core–shell Ni/NiO NPs stabilised with imidazole. The addition of excess ethanol followed by centrifugation enabled the isolation of nanoparticles in the form of a black powder.

Some critical aspects regarding polymeric coatings may affect the performance of a magnetic nanoparticle system. The length or molecular weight of the polymer and the manner in which the polymer is anchored or attached also influence the properties of the particles.123 Stevens et al.124 prepared core– shell iron oxide-polymer nanocrystals for immobilizing Pd catalysts via the emulsion polymerization approach. Highly crystalline and monodisperse γ-Fe2O3 nanocrystals (11 nm), coated with a layer of oleate, were encapsulated inside the interior cores of micelles due to the hydrophobic alkyl chains of the oleate molecules. Different polymers such as styrene, 4-vinylbenzene chloride (VBC), and 1,4-divinylbenzene (DVB) were used and trapped into the micellar cores. Moreover, 2,2′-azobis(2-methylpropionitrile) (AIBN) was used as a free radical

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initiator. The as-prepared Pd–NHC catalysts were stable and demonstrated high catalytic activity in the Suzuki cross-coupling reactions.

3.2.4. Metal hydroxide and oxide coating. Inorganic oxides other than silica, such as titania, alumina and ceria, have widespread use as catalyst supports and may find a wide range of useful applications when associated with magnetic nanoparticles. The methodologies used include the direct coating of magnetic nanomaterials and the post-coating of silicacoated magnetic nanomaterials. Álvarez et al.125 reported a sol–gel method to coat magnetite with titania, with the best results obtained when a layer of silica was added before coating with titania. Ye et al.126 prepared peapod-like nanostructures through the controlled deposition of titania over silica-coated magnetite by adjusting the amount of the titania precursor, tetrabutyltitanate (TnBT). He et al.127 reported the direct coating of magnetite with titania by a homogeneous precipitation method with urea and Ti(SO4)2. They were able to control the rate of hydrolysis and condensation of the TiO2 precursor using urea, which slowly releases ammonia and controls the generation of OH−, and consequently covers the magnetite nanoparticles homogeneously. Highly-ordered TiO2coated Fe2O3 core–shell arrays have been fabricated by a stepwise, seed-assisted, hydrothermal approach.128 Vejpravova et al.129 based their preparation on precipitation of a positively-charged TiO2 layer on negatively-charged γ-Fe2O3 NPs modified by citric acid.129 They investigated the phase composition and its relation with the particle size and magnetic properties. The sample dried at 200 °C consisted of maghemite, anatase, and a rutile phase in an approximately 1 : 1 : 1 ratio.

When the samples were annealed at up to 700 °C, the hematite phase was formed and its content and particle size gradually increased with the increasing annealing temperature. A facile process was reported to prepare zirconia-coated hematite particles in an aqueous solution.130 An amorphous ZrO2 shell of about 2–3 nm was homogeneously deposited on the hematite surface.

Mi et al.131 reported the preparation of magnesium–aluminum layered double hydroxide-coated magnetite nanoparticles as supports for gold nanocatalysts. The magnetic support was prepared via dropwise addition of magnesium and aluminum salts, and alkaline solutions in a suspension containing magnetite prepared using the solvothermal method. The resulting support, with a honeycomb-like morphology, was used to stabilize Au NPs prepared by the deposition–precipitation method.

3.2.5. Metal coating Core–shell particles. Pure iron, cobalt, and nickel nanoparticles are chemically unstable in air and easily oxidized, which limits their utility. The coating of such magnetic particles with a noble metal layer protects the particles from oxidation. Deposition of precious metals on magnetic nanoparticles can occur through different reactions such as microemulsion,132 precipitation133,134 or redox transmetalation.135 The synthesis of platinum-coated cobalt particles by refluxing Co NP colloids (ca. 6 nm) and [Pt(hfac)2] (hfac = hexafluoroacetylacetonate) in a nonane solution containing dodecyl isocyanide as a stabilizer was reported by Park et al.135 The obtained particles were

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air-stable and redispersable in typical organic solvents. The reaction byproduct was separated and analyzed as [Co(hfac)2], indicating that the formation of the core–shell structure was driven by redox transmetalation reactions between Co0 and Pt2+. Platinum forms a shell around the cobalt core and the shell surface is stabilized by dodecyl isocyanide-capping molecules. Magnetic studies of the Co–Pt core–shell nanoparticles indicate that they retain most magnetic properties of the pure Co core. This catalyst has the advantage of using platinum atoms economically, because only the outer atoms are accessible for the reagents, and the magnetic cobalt core plays a critical role in the separation and recycling of the catalyst. The Co– Pt NPs exhibited excellent catalytic activity for the hydrogenation of a number of unsaturated organic molecules under mild conditions, and magnetic separation and recycling ability. The catalyst was also highly selective in the reduction of aldehydes to the corresponding alcohols upon hydrogenation in the presence of a ketone. In the case of benzaldehyde and cyclohexanone hydrogenation, only benzaldehyde was converted to benzyl alcohol. These noteworthy results demonstrated the efficacy of Co–Pt NPs as a bifunctional nanoplatform system for catalytic applications, and also demonstrated their magnetic separation and recycling capabilities.136 The redox transmetalation reaction technique was successfully used for the preparation of other magnetic core–shell nanostructures, such as Co–Au, Co–Pd, Co–Pt, and Co–Cu.137 Gold-coated magnetite nanoparticles have recently become the focus of research efforts that exploit the novel nanoarchitecture of these materials as platforms for magnetically controlled biomedical, analytical, and catalytic applications.

However, it was found that directly coating magnetic particles with gold is very difficult, because of the dissimilar nature of the two surfaces.40,138 Ban et al.139 have synthesized goldcoated iron nanoparticles with approximately 11 nm core size and a gold shell of about 2.5 nm thickness. The coating was achieved by a partial replacement reaction in a polar aprotic solvent. The solution of FeCl3 dissolved in 1-methyl-2-pyrrolidinone (NMPO) was added to an NMPO solution containing sodium and naphthalene under stirring at room temperature.

The Fe3+ ions were reduced by sodium to form the metallic cores. After removal of sodium chloride by centrifugation, and the addition of 4-benzylpyridine as a capping agent at elevated temperatures, the iron nanoparticles were coated with gold by the addition of dehydrated HAuCl4 dissolved in NMPO. Goldcoated iron nanoparticles have also been prepared by a reverse microemulsion method. The inverse micelles were formed with cetyltrimethylammonium bromide (CTAB) as a surfactant, 1-butanol as a co-surfactant, and octane as the continuous oil phase.140 Zhang et al.141 reported a new method for the preparation of gold-coated iron nanoparticles by the combination of wet chemistry and laser irradiation. Pre-synthesized iron nanoparticles and gold powder were irradiated using a laser in a liquid medium. The iron cores (18 nm) were covered with a gold shell of approximately 3 nm in diameter.

Dumbbell-like particles. Dumbbell-like particles are formed by two or more different nanoparticle components that are in

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intimate contact but do not form core–shell nanostructures.

The promising features of dumbbell-like particles are connected with their tunable and controllable asymmetric structure, which allows the presence of nanoparticles with different properties in a single system. The nanoscale junctions present in these structures facilitate electron transfer across the interface, changing the local electronic structures and thus inducing new physical and chemical properties that are not present in the single-component nanoparticles.142 Bifunctional composite Au–Fe3O4 NPs were prepared via the decomposition of Fe(CO)5 over the surface of pre-synthesized Au NPs, followed by oxidation in air (Fig. 6).52 The particle structure was formed through epitaxial growth of iron oxide on the Au seed. Similarly, dumbbell-like Pt–Fe3O4 NPs were prepared by epitaxial growth of Fe onto Pt nanoparticles followed

by Fe oxidation.143 The Pt–Fe3O4 dumbbell-like particles show a 20-fold increase in catalytic activity toward oxygen reduction reaction compared with isolated Pt NPs. The synthesis approach to Au–Fe3O4 NPs using Au NPs as seeds has also been generalized to the synthesis of other dumbbell-like particles with the noble metals Ag, Pt, and Pd or alloys with the other component being iron or cobalt oxides.143,144 Ag–Fe3O4 NPs can be prepared by controlled nucleation of Ag on pre-synthesized Fe3O4 NPs.145–147 An organic solution containing Fe3O4 NPs was mixed with an aqueous solution of AgNO3 and agitated by ultrasonication. The Fe3O4 size was controlled by reductive decomposition of iron acetylacetonate, Fe(acac)3, while the size of the Ag NPs was regulated by controlling the amount of AgNO3 added to the reaction mixture147 Au–FePt

Fig. 6 TEM and STEM images of the dumbbell-like Au–Fe3O4 NPs: (A) TEM image of the 3–14 nm Au–Fe3O4 NPs; (B) TEM image of the 8–14 nm Au–Fe3O4 NPs; (C) HAADF-STEM image of the 8–9 nm Au– Fe3O4 NPs; and (D) HRTEM image of one 8–12 nm Au–Fe3O4 NP. Reprinted with permission from ref. 52.

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dumbbell-like particles have been synthesized by catalytic growth of Au on an FePt NP surface.148,149

4. Direct deposition of catalysts on magnetic nanomaterial surfaces The deposition or precipitation of a catalyst phase directly on magnetite or other magnetic nanoparticles can, in principle, follow a similar methodology to the preparation of supported heterogeneous catalysts on other inorganic oxides, such as SiO2, Al2O3, TiO2, CeO2, or carbon materials. The magnetic nanoparticles can either be prepared in situ by co-precipitation of the mixture of iron chlorides and other metal salts (catalyst metal precursors) in the presence of stabilizers150–154 or presynthesized by co-precipitation and then impregnated with metal salts (catalyst metal precursors) followed by metal reduction,55,155–157 or used for deposition of oxides.158 Commercially available magnetite powders have been used as the catalyst for deposition of metal oxide catalysts employed in many different organic transformations.159–169 In general, direct deposition methods do not provide any control of the morphology and size of the deposited metallic or oxide phases. Metal leaching can be an issue for such supported catalysts, but this concern has not been clearly elucidated in those studies. Another approach that has received attention is the sol-immobilization method, which consists of the impregnation of the support with pre-synthesized colloidal nanoparticles.56,170 Excellent control of the nanoparticle morphology can be achieved during the synthesis step and their size can be regulated by the choice of stabilizing agents and synthesis conditions, independent of the support used. The low affinity of the metal nanoparticles to the support can be a problem.

Some strategies are available to improve the metal–support interaction, such as the example reported by Jiang et al.51,171 The authors prepared 1,6-hexanediamine-stabilized magnetite nanoparticles by a thermal decomposition method, which resulted in free amines on the iron oxide surface that improved the impregnation of a pre-formed bimetallic colloidal catalyst.

Rossi et al.172 modified the surface of iron oxide nanoparticles using 3-mercaptopropionic acid (MPA) or a series of alkoxyorganosilanes173 for the immobilization of Pd catalysts. The modification of the surface of magnetic nanomaterials with organic molecules (ligands) has been used as a strategy to improve the impregnation of metal NPs, or metal ions and complexes, for the preparation of magnetically recoverable catalysts, and this topic will be detailed later in this review.

5. Functionalization of magnetic supports for the design of advanced catalysts Tailoring the surface properties of magnetic nanoparticles, often accomplished by coating them within a shell of a pre-

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Critical Review

ferred material or by direct surface modification with desired functional groups, can be performed by three different approaches: (a) the non-covalent adsorption of bifunctional molecules, surfactants or polymers; (b) coating with other oxides, carbon, polymers, or metallic layers producing core–shell or heterodimer dumbbell-like nanostructures; and (c) the covalent approach to forming a relatively stable linker between hydroxyl groups on the nanoparticle surface and anchoring agents such as carboxylic acid, phosphonic acid, and dopamine derivatives.174,175 The functionalization of the surface of magnetic nanomaterials is an important step in the design of supported catalysts,174,176 because it allows the immobilization of a wide range of molecular catalysts, biocatalysts, organocatalysts, and metal nanoparticles.92,172,177 In the ensuing sections, we will highlight the synthesis strategies generally used to functionalize magnetic nanomaterial surfaces, especially magnetite, carbon-coated magnetite, and silicacoated magnetite, by the covalent approach, as summarized in Fig. 7. The well-known functionalization strategies for carbon and silica can be transposed to carbon- and silica-coated magnetic nanoparticles. Here, only selected examples of metal complexes, metal nanoparticles, and organocatalysts immobilized on magnetic nanomaterials prepared using the most suitable functionalization approaches will be discussed. For more examples, please refer to other recently published review papers.14,16,17 5.1.

Functionalization strategies for ferrites

5.1.1. Carboxylate. Carboxylates are known to attach to the metal oxide surface by strong bidentate metal–carboxylate bonds. Bifunctional molecules containing a carboxylic acid group and another functional group (R) can be used to functionalize the surfaces of magnetic nanoparticles such as magnetite and other ferrites (Fig. 7A(1)). The carboxylic acid group will bind to the nanomaterial surface (most probably as a carboxylate) and the other terminal group (R) will face outside resulting in functionalized surfaces. The high stability of the carboxylate–oxide linkage holds the key for the preparation of organic ferrofluids based on oleic acid-stabilized magnetic nanoparticles. The carboxylate group binds to the iron oxide surface and the hydrophobic tail provides the steric hindrance that stabilizes the nanoparticles in solution. Fourier transform infrared studies revealed that oleic acid was chemisorbed onto the Fe3O4 NPs as a carboxylate.178 The wavenumber separation between the νas (COO−) and νs (COO−) IR bands (1639–1541 = 98 cm−1) was ascribed to the chelating bidentate, where the interaction between the COO− group and the Fe atom was covalent. The formation of chemical bonds between the iron oxide substrate and the oxygen atoms of the carboxylic acid was also supported by X-ray photoelectron spectroscopy (XPS) results.179 The surface of iron oxide nanoparticles was functionalized with –SH terminal groups using 3-mercaptopropionic acid for the immobilization of a Pd catalyst.180 Functional acids containing multiple double bonds such as linolenic (LLA) and linoleic (LEA) acids or pyridine moieties such as 6-methylpyridine-2-carboxylic acid (MPCA), isonicotinic acid

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Fig. 7 Different approaches to functionalize magnetic nanoparticles (MNPs): (A) iron oxide MNPs; (B) silica-coated MNPs and (C) carbon-coated MNPs.

(INA), 3-hydroxypicolinic acid (HPA), and 6-(1-piperidinyl)pyridine-3-carboxylic acid (PPCA) were used in the functionalization of magnetic nanoparticles. Both carbon–carbon double bonds and pyridine terminal groups on the surface of the magnetic support can be reacted with palladium complexes to form catalytically active species on the exterior of magnetic nanoparticles.181 The complex [Rh(cod)(η6-benzoic acid)]BF4 could be directly immobilized on the surface of ferrite nanoparticles through the carboxylic acid approach.182 The supported catalyst showed excellent catalytic activity and regioselectivity in hydroformylation reactions, which is comparable to that of its homogeneous counterpart. [Rh(cod)(μ-S(CH2)10CO2H)]2 could also be directly immobilized on the surface of ferrite nanoparticles through the carboxylic acid approach.183 5.1.2. Dopamine. The linkage of dopamine (2-(3,4-dihydroxyphenyl)ethylamine) on the surface of magnetic nanoparticles is based on the chelation of the hydroxyl groups of dopamine with the under-coordinated surface iron atoms (Fig. 7A(2)). It produces stronger covalent bonding than oleic acid on the surface of magnetite nanoparticles and contains amine groups that allow the direct immobilization of catalysts or the post-functionalization for the preparation of more complex ligands and catalysts. The amine groups of the dopamine molecules form a positively charged shell, which effectively prevents aggregation of the magnetic nanoparticles.

Liang et al.184 reported a facile one-step approach to fabricate dopamine-coated magnetic nanoparticles. The dopamine was added to the mixture of ferric chloride and sodium sulfite and the magnetic nanoparticles were formed in situ after adding NH4OH, resulting in dopamine-coated iron oxide nanoparticles. The dopamine molecule was used to immobilize 2–3 nm gold nanoparticles onto magnetite nanoparticles. The dopamine was strongly coordinated to iron cations on the Fe3O4 surface and the amino groups of dopamine were easily bonded to the surface of gold nanoparticles. Magnetic nanomaterials functionalized with dopamine and containing –NH2 terminal groups were used without further modification for the immo-

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bilization of metal catalysts, including Ni,185 Pd,185–189 and RuOH.190 Mazur et al.175 presented an easy and versatile one-step surface functionalization strategy allowing simultaneous immobilization of various functionalities onto magnetic nanoparticles (Fig. 8). They modified magnetic nanoparticles simultaneously with dopamine anchors bearing azide, maleimide, and amine terminal groups by simple immersion of freshly prepared iron oxide nanoparticles in an anhydrous acetonitrile solution containing the respective dopamine derivatives in stoichiometric amounts. Xu et al.191 used the dopamine approach to provide further functionality to magnetic nanoparticles using nitrilotriacetic acid (NTA). It was suggested that bidentate enediol ligands such as dopamine convert the under-coordinated Fe surface sites back to a bulk-like lattice structure with an octahedral geometry for oxygen-coordinated iron, which explains the tight binding of dopamine to iron oxide. Many different post-functionalization strategies for the anchored dopamine result in supported organocatalysts.

Gleeson et al.192 prepared a magnetic nanoparticle-supported chiral 4-dimethylaminopyridine (DMAP) catalyst, which was highly active in enantioselective acylation of alcohols and can be recycled to an unprecedented extent (>30 times). The same authors193 pointed out that this methodology cannot be generalized, because a marked drop in catalyst activity and selectivity can occur after immobilization. A significant level of background catalysis by the nanoparticles in the absence of the organocatalyst was considered the main reason for the overall poor stereoselectivity of the immobilized catalyst.

Some authors have claimed that dopamine functionalization can improve the magnetic properties of Fe3O4. Nagesha et al.194 synthesized Fe3O4 NPs by the thermal decomposition method with oleic acid as the surfactant and then performed a dopamine ligand exchange. Both oleic acid and dopamine were covalently bonded to the surface via a chelating bidentate interaction with the iron species. A surprising and significant increase in the remanence, saturation magnetization, and blocking temperature of the particles was found after dopa-

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Fig. 8

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One-step surface functionalization strategy allowing simultaneous incorporation of various functionalities onto magnetic nanoparticles.

mine functionalization. TEM and X ray diffraction (XRD) studies revealed no change in the particle size or structure.

The results were correlated with an increase in the magnetic size of the nanoparticle core induced by the dopamine ligand exchange process. These effects were tentatively attributed to surface bonding effects that alter the canted magnetic state of the Fe3O4 NPs. A detailed examination of the dopamine-based surface modification of iron oxide NPs was reported by Zhang et al.195 They used dopamine and L-dopa (3,4-dihydroxy-Lphenylalanine) as two surface modifiers for iron oxide nanoparticles. The dopamine and L-dopa were stable after being immobilized onto the surface of iron oxide NPs and the iron oxide nanoparticles dispersed very well in water after surface modification. The magnetic properties showed that the blocking temperature of the dopamine- or L-dopa-decorated iron oxide nanoparticles changed very little over 20 days, confirming the long-term stability of these surface-modified nanoparticles.

5.1.3. Glutathione. Glutathione (GSH) is a linear tripeptide synthesized from three amino acids L-glutamate, L-cysteine, and glycine. These functional groups offer the possibility of being coupled and further cross-linked to form a polymerized structure. The thiol group of cysteine is suggested as the active ligand group that binds covalently to the nanoparticle surface (Fig. 7A(3)).196 Ma et al.197 reported a facile one-step approach to prepare glutathione-modified magnetite NPs by co-precipitation of ferric and ferrous salts in an alkaline solution in the presence of glutathione. After stirring the solution of ferric and ferrous salts in an alkaline medium for 30 min, the temperature was raised to 85 °C and GSH was added to the mixture.

The product was retrieved using a magnet, washed 5 times with deionized water, and finally dispersed in water. Baig et al.17 prepared a magnetically recoverable organocatalyst by coating magnetite particles with glutathione. The recyclable catalyst showed excellent activity for the Paal–Knorr reaction of a variety of amines and, crucially, the entire process was carried out in an aqueous medium, without using organic solvents in the reaction; it was also recyclable. The reaction was

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also carried out simply by removing the product layer and adding fresh benzylamine and tetrahydro-2,5-dimethoxyfuran, and similar results were obtained.198 5.1.4. Phosphonate. The linkage of phosphonate precursors (molecules containing P–OH, P–OR, or P–O-groups) on the surface of magnetic nanoparticles is based on the chelation of the hydroxyl groups of the phosphonate group with the surface iron atoms through the formation of M–O–P covalent bonds (Fig. 7A(4)). The attachment of the phosphonate precursor can occur by mono-, bi-, or tridentate pathways.199 Palladium nanoparticles were immobilized onto a magnetic nanomaterial using a phosphonate linkage.200 Vaquer et al.201 used a phosphonate-functionalized terpyridine to prepare molecular Ru(III) complexes that were bonded to magnetite nanoparticles through a phosphonate linkage. The magnetiteanchored Ru(III) complexes were excellent stereoselective catalysts for the epoxidation of olefins (cis-olefins generate only the corresponding cis-epoxides). Hu et al.202 investigated the immobilization of an asymmetric Ru catalyst onto magnetite nanoparticles by two different approaches (Fig. 9). In the first method the Ru(II) complex was reacted directly with magnetite nanoparticles by ultra-sonication in anhydrous methanol and an argon atmosphere. The molecular Ru(II) catalyst contains (R,R) 1,2-diphenylethylenediamine (DPEN) and 4,4′-bisphosphonic acid-substituted 2,2′-bis(diphenylphosphino)-1,1′binaphthyl (BINAP). This last ligand was responsible for the linkage of the complex to the magnetic nanoparticle surface.

The resulting catalyst was highly active for the asymmetric hydrogenation of aromatic ketones with 99% conversion and enantioselectivity of 98.1% at 0.1 mol% of Ru(II). The catalyst was magnetically recovered and reused with success. In the second method, the Ru catalyst was built on the magnetite nanoparticle surface by grafting a DPEN derivative onto magnetic nanoparticles, followed by complexation of a BINAP derivative. In this method, however, the catalyst did not show good activity in asymmetric hydrogenation of aromatic ketones, likely because of the oxidation of the Ru(II) intermedi-

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Fig. 9

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Immobilization of an asymmetric Ru catalyst on magnetite nanoparticles using different approaches.

ate. A similar post-grafting process was used to prepare supported organocatalysts containing a proline ligand. The magnetic nanoparticles were functionalized with 3-azidopropyl phosphonic acid followed by an azide–alkyne cycloaddition to obtain the proline catalyst.203 5.2. Functionalization strategy using alkoxyorganosilanes: ferrites and silica-coated ferrites The most deeply studied technique for the functionalization of the magnetic nanomaterial surface employs a condensation reaction using alkoxyorganosilanes, (OR′)3Si(CH2)3-R. The hydrolysis process between the alkoxide terminal groups present in the monosilane and the external silanol groups from the matrix of silica or the external hydroxyl groups from iron oxides allows the covalent functionalization of the magnetic nanomaterial surface with a variety of functionalized organic molecules. The modification of magnetic nanomaterials via alkoxyorganosilane chemistry can be performed directly on the ferrite nanoparticle surface or after coating with silica, as represented in Fig. 7A(5) and 7B. In both cases, the linkage of the alkoxyorganosilane occurs via the M–O–Si bond, due to the presence of reactive –OH groups on the magnetite and silica-coated magnetite surfaces.

The functionalization of solid supports via covalent bonding has become the most-employed method of heterogenization of homogeneous catalysts and many review articles have covered this topic.5,204 The simplest way to functionalize magnetic supports by this method is to take advantage of commercially available alkoxyorganosilanes. Wang et al.205 functionalized magnetite nanoparticles with 3-aminopropyl(triethoxysilane) and used the terminal amino-groups for the immobilization of the Pd catalyst. However, the Pd NPs could

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not be easily distinguished from the iron oxide support and the poorly characterized catalyst was active but not stable for recycling. Yi et al.206 reported the preparation of a well-characterized catalyst with highly dispersed Pd NPs on silica-coated magnetic nanoparticles functionalized with 3-aminopropyl(triethoxysilane) and 3-mercaptopropyl(trimethoxysilane). The high stability of the nanocatalysts supported on silica-coated magnetic nanoparticles made this support more promising compared to the uncoated magnetic nanomaterials from the practical point of view. Silica-coated magnetic nanoparticles functionalized with alkoxyorganosilanes were extensively explored for the immobilization of metal nanoparticle catalysts.

The simple functionalization of the silica-coated magnetic nanoparticles with amino-groups using 3-aminopropyl(triethoxysilane) improved the metal loading and dispersion of supported metal nanoparticles (for example, Au,207,208 Rh,209 Pt,210 CoO,211 Ru,212 Ir,213 and AuPd214 NPs). Oliveira et al.215 reported that the impregnation of Au(III) ions is very limited on silica surfaces, but could be improved by the functionalization of the silica surfaces with amino-groups. It was shown by X-ray absorption near-edge structure (XANES) analysis that there is a strong metal–support interaction (formation of Au(σ+) species) on amino-functionalized surfaces, which was not observed for metal ions adsorption on the surfaces of non-functionalized supports.

Very promising results were obtained by changing the surface terminal groups using different alkoxyorganosilanes.

Rossi et al.216 reported the preparation of Pd NPs by an impregnation–reduction process using an amine and an ethylenediamine-functionalized silica-coated magnetic support.

The average size of the Pd NPs formed on the amine and the ethylenediamine-functionalized support was about 6 and 1 nm, respectively, and the catalytic activity in a model hydro-

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genation reaction was higher in the case of 6 nm nanoparticles. The ligand grafted onto the support surface strongly affected the size of the nanoparticles and its catalytic activity.

Silva et al.173 also investigated the effect of different ligands grafted directly onto iron oxide surfaces. Amino, ethylenediamino, and thiol groups were used to immobilize Pd NPs onto magnetite. The catalytic activity of those catalysts in the hydrogenation of cyclohexene was significantly different, with higher activity for the amino-modified supports, and the selectivity in the semi-hydrogenation of alkynes was also strongly affected by the ligands. The Pd catalyst support on thiol-modified magnetite presented the highest selectivity among the three groups. More sophisticated ligands, such as phosphine,217 terpyridine,218 and polyamidoamine dendrimers,171 were explored for the immobilization of Pd nanoparticles onto silica-coated magnetic supports.

Critical Review

There are a large number of examples of supported catalysts (organocatalysts and transition metal complexes) immobilized onto superparamagnetic supports via alkoxyorganosilane chemistry. We will summarize the main strategies used for the immobilization process in both uncoated and silica-coated magnetic supports. Commercially available alkoxyorganosilanes containing chloropropyl, aminopropyl, azidopropyl, and mercaptopropyl terminal functionalities are used as starting materials and can be modified further for the preparation of more complex organic ligands, creating different types of new chemical bonds (Fig. 10 and 11). The transformation of the alkoxyorganosilane precursors into the desired functionality can be performed before or after grafting the alkoxyorganosilanes onto the support surface.

Amine alkylation is among the most common reactions to obtain functionalized nanoparticles via organic modification

Fig. 10

Examples of functionalization of magnetic nanoparticles using 3-aminopropyl(triethoxysilane).

Fig. 11

Examples of functionalization of magnetic nanoparticles using alkoxyorganosilanes.

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of alkoxyorganosilanes, and can be applied to organosilanes containing aminopropyl and chloropropyl functionalities. For example, a magnetically recoverable acid catalyst was obtained by the reaction of an aminopropyl-functionalized magnetic support with chlorosulfonic acid. This approach was used to prepare sulfamic acid-functionalized magnetite NPs that were active as acid catalysts for synthesis of α-amino nitriles219 and synthesis of quinolines.220,221 Recently, Koukabi et al.222 showed that functionalization with sulfonic acid can take place by the direct reaction of chlorosulfonic acid with the hydroxyl groups on the magnetite nanoparticle surface.

The preparation of more complex ligands has been achieved via the formation of imine, amine, and amide bonds using 3-aminopropyl(triethoxysilane) or aminopropyl-functionalized magnetic supports as the starting material (Fig. 10).

The reaction of 3-aminopropyl(triethoxysilane) and an aldehyde-containing ligand resulted in the covalent immobilization of the ligand through the formation of an imine bond. A series of supported Schiff base complexes were prepared by reacting 3-aminopropyl(triethoxysilane) and salicylaldehyde and were used as the catalyst for the synthesis of 1,1-diacetates from aldehydes.223 Shylesh et al.224 immobilized oxodiperoxomolybdenum complexes by the reaction of aminopropyl groups grafted onto silica-coated magnetic nanoparticles and an ester group present in a modified 2-(1-alkyl-3-pyrazolyl)pyridine ligand. Ding et al.225 immobilized a copper(I) complex after grafting the N,N,N″,N″-tetraethyldiethylenetriamine ligand by the acylation of aminopropyl-functionalized magnetite nanoparticles with acryloyl chloride (i.e., the Schotten– Baumann reaction), followed by the reaction of the terminal CvC bonds with N,N,N″,N″-tetraethyldiethylenetriamine.

Using a similar strategy, Hirakawa et al.226 reported the immobilization of the [CpRu(η3-C3H5)(2-pyridinecarboxylato)]PF6 complex on a magnetic support to be used as a catalyst for deallylation reaction. The first step for preparing the supported Ru complex involved the formation of a carboxylic acid chloride by the reaction of 2-allyl hydrogenpyridine-2,4-dicarboxylate and thionyl chloride. Then, the direct reaction of 5-(triethoxysilyl)pentyl-1-amine functionalized the silica-coated magnetic support with the carboxylic acid chloride-modified ligand resulting in the amide bond linkage. The easy acylation of amines with anhydride is another way to promote the catalyst linkage via amide bond formation. This method was explored by Xu et al.227 as one of the steps to prepare silicacoated magnetite nanoparticles modified with 1-benzyl-1,4dihydronicotinamide. An amide bond was formed by the linkage of nicotinic anhydride with the amino groups present on the support surface.

The immobilization of ionic liquids is also promising for the preparation of magnetically recoverable catalysts since the ionic liquid can create an environment that promotes high affinity of the catalyst and the substrate,228,229 and the ionic liquid itself can act as a catalyst.230 The immobilization of ionic liquids on magnetic supports can occur via reaction of an alkoxysilane containing chloropropyl group as a starting material and a nitrogenated compound. The reaction of a

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3-chloropropyl(triethoxysilane) or chloropropyl-functionalized magnetic support with imidazol derivatives is the most common method. This reaction occurs easily with a simple mixture of the reagents in an inert atmosphere with or without an organic solvent.231,232 Ionic liquid-modified magnetic nanoparticles were used to support Pt NPs as magnetically recoverable selective hydrogenation catalysts.232 Supported quaternary ammonium salts are used for the same purpose and can be prepared similarly, via reaction of a ternary amine and a chloropropyl-modified surface.233,234 The immobilization of different complexes follows a similar strategy. Saeedi et al.235 prepared a supported manganese(III) tetrapyridylporphyrin complex by the reaction of chloropropyl-functionalized silicacoated magnetite NPs with an aromatic nitrogen present in the porphyrin ligand. Garcia-Garrido et al.236 prepared silicacoated magnetite NPs functionalized with a ruthenium-arene1,3,5-triaza-7-phosphatricyclo catalyst by the reaction of 3-iodopropyl(trimethoxysilane) and the 1,3,5-triaza-7-phosphatricyclo ligand. A halide can also be present in the ligand, in which case the reaction proceeds between the ligand and the aminofunctionalized magnetic support, as reported by Zeng et al.237 for the preparation of a supported Cu(I)-bis(oxazolinyl)pyridine ( pybox) complex. The aminopropyl-functionalized silica-coated magnetite NPs were mixed with a 4-bromo-substituted phenylpybox ligand, but in this case the reaction occurred via CuBrBINOL-catalyzed N-arylation.238 The functionalization of magnetic nanomaterials has also been achieved using 3-azidopropyl(trimethoxysilane) as the starting material. Riente et al.239,240 reported the use of a copper(I)-catalyzed alkyne–azide cycloaddition (CuAAC) reaction for the preparation of an immobilized first generation MacMillan imidazolidin-4-one catalyst225 and (S)-α,α-diphenylprolinol trimethylsilyl ether catalyst,226 starting from magnetic nanomaterials functionalized with 3-azidopropyl(trimethoxysilane). The resulting heterogenized organocatalysts were successfully explored for the Friedel Crafts alkylation of N-substituted pyrroles with R,β-unsaturated aldehydes225 and the Michael addition of propanal to b-arylsubstituted nitroolefin reactions respectively.226 Both catalysts could be easily reused after magnetic separation.

Mercaptopropyl-alkoxysilane can also be modified to promote the linkage of homogeneous catalysts onto magnetic supports. The reaction of the thiol groups and a non-aromatic CvC bond via Michael addition was successfully applied to attach (S)-diphenylprolinol trimethylsilyl ether241 and copper(II)-poly(N-vinylimidazole) catalysts.242 Other sophisticated approaches were also explored in the literature, such as the urethane linkage using 3-isocyanatopropyl(triethoxysilane)243 and polymerization using a 3-methacryloxypropyltrimethoxysilane precursor.244 Selected examples using the strategies discussed above are shown in Fig. 12.

5.3.

Functionalization strategy using dendrimers

Dendrimers are a relatively novel class of polymers with a welldefined three-dimensional structure buildup of functional groups, symmetry perfection, nanosize, and internal cavities

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Fig. 12

Critical Review

Selected examples of functionalized magnetic nanomaterials.

that are suitable for the immobilization of metal complexes,245 metal nanoparticles,246,247 and enzymes248 onto magnetic nanomaterials. Dendrimer-coated magnetic nanoparticles (DcMNPs) are synthesized through different cycles or generations by adding branched monomers that react with the functional groups of the iron oxide core. In general, aminomodified magnetic nanoparticles prepared by the co-precipitation method (whether coated with silica or not) and functionalized with 3-aminopropyl(trimethoxysilane) were used as the initiators. Polyamidoamine (PAMAM) dendrons were constructed on the particles containing amino groups by the Michael-type addition of methyl acrylate to produce the amino propionate ester and the subsequent amidation of the resulting ester groups with ethylenediamine. This process was repeated until the desired number of generations was achieved, so as to introduce a dense outer amine shell through a cascade type generation. Uzun et al.248 immobilized glucose oxidase onto the amine-rich surface by a standard bioconjugation procedure using the well-known cross-linking reagent glutaraldehyde (Fig. 13). Abu-Reziq et al.245 phosphonated PAMAM-dendronized silica-coated magnetic nanoparticles by reaction of the terminal amino groups with diphenylphosphinomethanol prepared in situ from diphenylphosphine with paraformaldehyde. The phosphonated magnetic nanomaterial was used for the immobilization of [Rh(COD)Cl]2 and applied in hydroformylation reactions.

Chou and Lien249 also used PAMAM dendrimer-coated magnetic nanoparticles as regenerable adsorbents capable of effectively removing Zn(II) from aqueous solutions. Niu and Crooks250 reported recent developments related to the catalytic properties of dendrimer-encapsulated metal nanoparticles (DEMNs). Particular emphasis was placed on the specific func-

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tions and properties of these materials that are a direct consequence of their dendritic architecture. Dendrimers are particularly attractive hosts for catalytically active metal nanoparticles for the following five reasons: (1) bearing fairly uniform composition and structure, the dendrimer templates themselves yield well-defined nanoparticle replicas; (2) the nanoparticles are stabilized by encapsulation within the dendrimer, and therefore do not agglomerate during catalytic reactions; (3) the nanoparticles are retained within the dendrimer primarily by steric effects and thus a substantial fraction of their surface is unpassivated and available to participate in catalytic reactions; (4) the dendrimer branches can be used as selective gates to control the access of small molecules (substrates) to the encapsulated (catalytic) nanoparticles; (5) the dendrimer periphery can be tailored to control the solubility of the hybrid nanocomposite and used as a handle to facilitate linking to surfaces and other polymers.250 5.4. Functionalization strategy for carbon-coated nanomaterials The layers of carbon formed on the surface of magnetic nanoparticles are chemically similar to the graphite layers on multiwalled carbon nanotubes, and the well-developed functionalization protocols for such nanomaterials could be applied. The most promising strategy for the functionalization of carbon-coated surfaces is the use of diazonium chemistry, which yields chloro-, nitro-, amino-103 and azido-functionalized251 magnetic materials (Fig. 14).

The functional groups described above can be further modified to add new ligands on the carbon-coated surfaces.

Schätz et al.251 used a copper(I)-catalyzed alkyne–azide cycloaddition reaction to graft a propargylated norbornene deriva-

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Green Chemistry

Fig. 13

Modification of magnetic nanoparticles with PAMAM dendrimers for invertase immobilization.

Fig. 14

Strategy for the functionalization of carbon-coated surfaces using diazonium chemistry.

tive onto azide-functionalized carbon-coated Co NPs and by a subsequent ring-opening metathesis polymerization with triphenylphosphine-functionalized norbornenes, they prepared a ROMPgel covalently grafted onto the magnetic nanomaterials.

The material was used as a support for the immobilization of palladium catalysts for a Suzuki–Miyaura cross-coupling reaction. The surfaces of magnetic carbon coated-Co NPs can also be functionalized with complex molecules utilizing the versatile copper-catalyzed alkyne–azide cycloaddition.252 The exceptional stability of the carbon-coated Co NPs enables the nanopowder to tolerate several successive reaction cycles

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without any significant loss in catalyst activity. Furthermore, their excellent magnetic properties permitted the rapid separation and quantitative recycling of Co NPs by simple magnetic decantation. The recovered nanoparticles can be reused subsequently without any further purification.

Carbon-coated surfaces feature novel strategies in noncovalent functionalization through the π–π interaction with aromatic compounds. A palladium complex noncovalently attached to cobalt-graphene core–shell nanobeads via pyrene tags for the hydroxycarbonylation of aryl halides in water (Fig. 15) was prepared by Wittmann et al.106 The dissociation

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Fig. 15

Critical Review

Carbon-coated Co NPs with pyrene tags for a thermally-triggered catch-and-release system.

of the immobilized compounds in polar solvents is prevented due to their very strong attraction to the graphene layer.

However, desorption of the aromatic anchors was thermally triggered to allow the catalyst to become homogeneous during the course of the reaction at 100 °C. In this catch-and-release strategy, the pyrene moieties could be reabsorbed on the carbon surface by cooling the solution back to ambient temperature.

6. Reactor design for catalysts immobilized on magnetic supports In heterogeneous catalysis, which is preferred in industrial applications, it is necessary to understand the contact between the phases involved and to describe the transport of reactants, intermediates, and products both within each phase and between phases.253 Thus, reaction engineering was developed as a discipline by introducing the scientific principles that quantify the interactions between chemical kinetics and the transport of momentum, heat, and mass. The resulting multiscale methodology provides the means for comparison of different reactor types253 and the evaluation of their performance in terms of productivity, yield, selectivity, and possible separation of products from catalysts.

High-gradient magnetic separation (HGMS) is a well-known method used to separate magnetic materials from a non-magnetic liquid medium. This process has traditionally been applied in kaolin clay benefaction, the removal of iron particles from process streams in steel, and in power plants.254 HGMS has been applied to more complex separations through the use of functionalized magnetic particles to remove environmental contaminants selectively,254 and has also received recent attention for separating magnetically recoverable supported catalysts.14,16,17 An HGMS system generally consists of a column packed with a bed of magnetically susceptible wires placed inside an electromagnet. When a magnetic field is applied across the column, the wires dehomogenize the mag-

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netic field in the column, producing large field gradients around the wires that attract magnetic particles to their surfaces and trap them there.24 The industrial application of HGMS to separate suspensions of magnetically recoverable catalysts still offers great potential for the development of green processes. Magnetic separation allows highly efficient catalyst recovery and product isolation in a minimum number of process steps. It should be considered a green separation technology for the following reasons: (1) clean, fast and selective separation of magnetic components from non-magnetic components; (2) low energy consumption; (3) no solvent consumption in the separation step; (4) low solvent consumption in the washing step; (5) catalyst separation under a controlled atmosphere avoids catalyst exposure to air and consequent decomposition or oxidation; (6) avoidance of the catalyst mass loss that may occur during transfer processes (i.e., from one container to another in conventional separation processes).

The magnetic properties of the materials, however, open up opportunities for applications other than their easy separation from products. One elegant application that takes advantage of the magnetic properties of such materials is the fluidization of catalyst particles in a gas–liquid stream in the presence of a spatially uniform and time-invariant applied magnetic field oriented axially relative to the fluidizing fluid flow.255 This is the operating principle of magnetically stabilized bed (MSB) reactors (Fig. 16).256 They integrate the advantages of a fixed bed with those of the conventional fluidized bed reactors in order to intensify the reaction process, providing efficient interphase mass transfer properties, low pressure drop, high productivity, absence of particle clogging, minimal loss of solid particles, and magnetically driven mobility of tiny particles to enable convenient catalyst loading and unloading.

MSB reactors for immobilized enzyme reactions have been reported,257 notably the recent application of the immobilized lipase for the continuous production of biodiesel.258 Fermentation with immobilized cells on magnetic particles has also employed the MSB technology.259 Hydrogenations,260–262 dehalogenation,263 olefin oligomerization,264 CO methana-

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Fig. 16

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Schematic diagram of the magnetically stabilized bed (MSB) reactor and experimental apparatus.

tion,265,266 CO2 reforming of methane,267 and desulfurization268 are all examples of processes that have been tested in MSB reactors with advantages compared to fluidized bed reactors without a magnetic field.

Peng et al.264 evaluated Ni–Al2O3–Fe3O4 catalysts in light fluid catalytic cracking (FCC) gasoline olefin oligomerization and found that the MSB reactor allowed a significant intensification of the light FCC gasoline olefin oligomerization process by enhancing both interphase mass transfer and operational flexibility. The same group also studied a novel magnetic support based on alumina-coated NiFe2O4 spinel ferrite. A Pd catalyst prepared with this support was tested for acetylene hydrogenation reaction in a MSB reactor. The results showed that the magnetic Pd–Al2O3 catalyst incorporated into the MSB reactor significantly enhanced the productivity and achieved a C2H2 conversion close to 100% and a C2H4 selectivity of 84%.262 Reiser and Stark tested azabis(oxazoline)copper complexes supported on silica-coated magnetite and carbon-coated cobalt in a modified MSB reactor.252 Magnetic fluidization of the supported catalyst in a microreactor was suggested to improve fluid dynamics, avoiding clogging of nanofiltration membranes, which would inevitably provoke a flow collapse.

After comparing the two solids, they suggested that cobalt particles were better retained than magnetite, showing only negligible nanoparticle leaching (<1% after 60 h). The carboncoated cobalt nanoparticles (Co/C) tagged with azabis(oxazoline)-copper(II) complexes were tested for the kinetic resolution of racemic 1,2-diphenylethane-1,2-diol via asymmetric monobenzoylation under batch conditions and in a continuous flow-type reactor.269 The extremely high ferromagnetism of the cobalt cores not only facilitates the recycling of the catalyst via magnetic decantation in the batch reactions but also enables a novel continuous flow-reactor design, as shown in Fig. 17. The Co/C particles were retained at a moderate flow rate (0.2 mL

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Fig. 17 Co/C-supported catalyst in a jointed glass column contained by an external magnetic field (left) and agitated in the rotating magnetic field (right). Reprinted with permission from ref. 269.

min−1), showing only negligible catalyst leaching. Misuk et al.270 combined the magnetic and catalytic behaviour of imidazol-based magnetic ionic liquids. The magnetic fixation of a magnetic ionic liquid catalyst in a micro/meso-sized channel was used to form a liquid fixed-bed (LFB) reactor. Recently, Cheng et al.271 employed a magnetic sulphonated poly(styrenedivinylbenzene) resin catalyst in an MSB reactor to enhance the etherification of FCC light gasoline. They demonstrated that the catalytic performance of the magnetic acid resin catalyst in the magnetic reactor is substantially enhanced when compared to its performance in a conventional fixed-bed reactor under otherwise identical operation conditions. The catalyst that was immobilized onto a magnetic support has the potential to be loaded and unloaded continuously in the magnetic reactor, which will greatly simplify the current complex industrial etherification process. As stated by the authors, the “magnetic catalyst-on-magnetic reactor” strategy can find wide applications in many reactions of significance in both the energy and environmental sectors, especially in cracking, dehydrogenation, and reforming that are plagued by fast catalyst deactivation. In this context, this strategy envisages significant

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changes in the economic and environmental impacts of chemical production processes, and will greatly stimulate the design of new synthetic approaches for magnetic catalytic materials. It is evident that the preparation of materials exhibiting strong magnetic properties is a prerequisite for employment in an MSB reactor.

The first commercial application of a magnetically stabilized bed reactor debuted in 2003 in a caprolactam hydrorefining unit using an amorphous Ni-based hydrogenation catalyst.261 There are now five industrial MSB reactor units operating with a 2 Mt per year total production capacity, and six industrial MSB reactor units are under construction.256 This is a very promising technology that can be applied for any kind of solid–gas and solid–liquid reaction in continuous flow, with the catalyst retained magnetically in the reactor. However, the industrial application of MSB is still restricted by a lack of appropriate catalysts immobilized onto magnetic supports.

7. Catalytic activity affected by the presence of a magnetic field The most widespread application of magnetic nanoparticles as catalysts or catalyst supports is the simple use of their magnetic properties to separate the catalyst from reaction products by applying an external magnetic field;15,16 however, there has been less discussion of possible changes in the catalytic output by means of an external magnetic field applied during reaction. This is the case when the magnetic material is the catalytic active phase;272,273 very recently, it has been suggested that a non-magnetic catalyst phase supported on a magnetic core can change the adsorption geometry and consequently the catalytic output in the presence of an applied magnetic field. Sá et al.274 reported the behavior of Pt nanoparticles supported on carbon-coated Co NPs in the catalytic oxidation of CO in the presence of an applied magnetic field. They showed that the magnetic core is able to inductively alter the electronic structure of the non-magnetic Pt active site. Changes in the electronic structure of Pt caused by a localized magnetic field result in changes in the adsorption geometry of CO. The resulting field could induce changes in the electronic structure of surface Pt sites because the generated magnetic moment in the particles is about four orders of magnitude larger than the dipolar coupling between two atomic spins. Cobalt plasmon excitation with an external magnetic field results in an enhancement of electric field at the surface, which can induce changes in the Pt electronic structure via either energy transfer or enhanced emission. This electronic change makes the CO adsorption geometry change from atop to bridged, and consequently the catalytic output. This observation opens the possibility of catalytic control by means of an external magnetic field, even if the active site is not magnetic as is true for the vast majority of catalytically active metals.

Another way to take advantage of the magnetic properties of the magnetic catalysts or supports is inductive heating.

Magnetic nanoparticles have the capacity of heating up when

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Critical Review

exposed to an alternating magnetic field. This property has been used in magnetic hyperthermia therapy to kill tumor cells, but can also find applications in the field of catalysis.

Two examples show that the reactivity of gel-immobilized enzymes can be controlled by magnetic heating.275,276

8.

Conclusions

Synthetic strategies for magnetic materials with controlled size, composition, and structure are currently undergoing rapid development and open enormous possibilities for the preparation of magnetic catalysts (or non-magnetic catalysts immobilized onto magnetic supports). The development of functionalization strategies for uncoated and coated magnetic nanomaterials allows the combination of an enormous number of possibilities for the immobilization of catalytically active species (organocatalysts, metal complexes, metal nanoparticles, enzymes, etc.). The most common applications of catalysts immobilized onto magnetic supports still rely on catalyst separation and recovery facilitated by magnetic separation in batch reactions. Magnetic separation is an environmentally friendly alternative for the separation and recovery of catalysts, since it minimizes the use of solvents and auxiliary materials, reduces the operation time, minimizes the catalyst loss by preventing mass loss and oxidation, and energy. The unique combination of superparamagnetic nanoparticles and catalytic active species presents the opportunity to solve a range of catalyst recovery problems to which no other filtration technique is easily applicable. However, the successful application of the magnetically stabilized bed reactor is a very elegant example on how to take advantage of the magnetic properties of catalysts, though the application of the magnetic field fluidization technology is still limited to few examples. This particular technology has the potential for enormous growth, though the industrial application of magnetically stabilized bed reactors is still restricted by the lack of appropriate magnetic catalysts (or catalysts immobilized onto magnetic supports) with high magnetization and uniform magnetic properties for the whole sample. It is clear that additional benefits can be derived from the magnetism of these advanced catalysts. Even more narrowly focused examples include magnetic inductive heating and possible changes in the catalytic output by means of an external magnetic field applied during reaction. The possibility of catalytic control by means of an external magnetic field, even if the active site is not magnetic, which is the case for the vast majority of catalytically active metals, is an open research field with immense possibilities for further development in the field of supported catalysts and goes far beyond the obvious application for magnetic separation.

Acknowledgements The authors acknowledge support from the Brazilian funding agencies Fundação de Amparo à Pesquisa do Estado de São

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Paulo (FAPESP), Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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