Organic Transformations on Metal Nanoparticles: Controlling Activity

Invited Feature Article pubs.acs.org/Langmuir

Organic Transformations on Metal Nanoparticles: Controlling Activity, Stability, and Recyclability by Support and Solvent Interactions Inge Geukens and Dirk E. De Vos* Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium

ABSTRACT: The different mechanisms by which the support and the solvent can influence the catalytic properties of a metal nanoparticle (NP) are reviewed. The use of a support not only significantly facilitates the recycling of NPs but also has many additional advantages varying from enhanced stabilization of the NP dispersion to the alteration of the electronic properties of the metal, shape selectivity effects, and even active participation in the reaction mechanism. The correct choice of solvent, on the other hand, can drastically influence properties such as the morphology of the particles and, in the case of alloys, determine the composition of the NPs. Judicious solvent selection also enhances recyclability and stability, and in some cases, the solvent plays a cocatalytic role. Despite the many beneficial effects of combining metal NPs with the correct support or solvent, many processes are not well understood. Further research should be conducted on elucidating the general mechanisms behind the support−NP or solvent−NP interactions.

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INTRODUCTION Nanoparticles (NPs) are an intriguing class of materials that can be used as catalysts in organic reactions, in environmental cleanup, and in energy conversions.1 They are often described as semiheterogeneous catalysts. On one hand, they exhibit a high activity comparable to that of homogeneous catalysts because of the high surface area of the particles. On the other hand, they are more readily recycled than purely homogeneous catalysts, giving them a more heterogeneous character.2 Immobilizing nanoparticles on a support remains one of the most applied methods for improving their recyclability, and even simple devices such as Teflon-coated stirring bars can be a vector for NP recycling.3 Frequently, some activity is lost after immobilizing the particles compared to the free particles, for instance, because diffusion to the supported particles is slowed down by the surrounding matrix. However, there are also systems for which activity, stability, and even selectivity can be enhanced through the contact of the particles with the support. In the first part of this Feature Article, such beneficial effects will be highlighted using specific examples, focusing on the application of metal NPs in organic reactions. Another factor that can decisively influence the catalytic properties of NPs is © XXXX American Chemical Society

the solvent in which the particles are synthesized and/or in which the catalytic reaction is conducted. In contrast to often studied aspects such as the synthesis precursors, reducing agents,4 and stabilizers used,5 the influence of the solvent has not been studied in the same systematic fashion. In the second part, this subject will be reviewed not only regarding properties such as stability and activity but also regarding the enhanced recyclability that can be achieved using the correct solvent. Control of Stability, Activity, and Selectivity through Support−Nanoparticle Interactions. Ideally, a support promotes the catalytic activity of NPs by stabilizing their high dispersion toward aggregation, which would reduce the number of active surface atoms. The first method to do this is through pore confinement: being trapped inside the pores of the support, the particles are sterically inhibited to aggregate, often leading to more stable dispersions than in the case of free NPs. Zeolites are often used for this purpose (Figure 1a),6 but other materials can also be used, such as metal organic frameworks Received: November 21, 2012 Revised: January 15, 2013

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dx.doi.org/10.1021/la304639z | Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

chemically introduced functional groups include epoxy, hydroxyl, carbonyl, and carboxyl functionalities, which provide anchor points for the NPs and, for example, allowed to immobilize Ru NPs to use in hydrogenation reactions.13 Finally, poly(vinylpyrrolidone) (PVP)-stabilized Au−Pd NPs showed a higher turnover frequency for the oxidation of alcohols to carbonyl compounds when deposited on basic aluminate spinel supports: the PVP stabilizers have a high affinity for the basic sites of the support, leading to an increased dispersion of the Au−Pd NPs whereas the basic sites participate in a parallel manner in the oxidation reaction (vide infra).14 Besides enhanced stabilization, the interaction between the NPs and the anchor points can often lead to electronic modifications in the NPs, which can alter their catalytic activity (vide infra). The support can also actively participate in the reaction mechanism either by cocatalyzing an intermediate step or by suppressing an (unwanted) side reaction. An example is the use of Au NPs immobilized in the pores of Cr-MIL-101, a metal− organic framework consisting of Cr ions linked by terephthalate, for cyclohexane oxidation using molecular oxygen. The coordinatively unsaturated Cr3+ sites of the MOF increase the rate of the catalytic reaction because they cocatalyze the decomposition of cyclohexyl hydroperoxide, an intermediate formed during the reaction, to the desired cyclohexanone/ cyclohexanol products.15 Another example of the synergetic action between the active sites of a support and the nanoparticle is that of Ag/Al2O3, which was shown by Shimizu et al. to be a useful catalyst for the oxidant-free dehydrogenation of alcohols. The basic sites of alumina first activate the alcohol to form an alkoxide, which is then transformed into a carbonyl compound by the Ag NPs, forming Ag−H species. In a final step, acid sites on alumina promote the desorption of H2, regenerating the Ag surface and the basic sites.16 Another example in which basic sites initiate the reaction comes from work by Prati et al., who chemically modified carbon nanofibers (CNFs) by treating them first with HNO3 and then with ammonia at different temperatures to introduce basic properties into the material. At lower temperatures (473 K), amide-like groups are formed that can be transformed into pyridine-like groups at higher temperatures (673−873 K), increasing the hydrophobicity of the surface. At 873 K, even stronger oxygen basic sites were observed. They found that by using these supports to immobilize Au NPs and applying them for the oxidation of glycerol, a higher turnover frequency was observed

Figure 1. (a) Encapsulation of metal nanoparticles inside the pores of a NaA zeolite: pore confinement enhances the stabilization of the particles (reprinted with permission from ref 6. Copyright 2010, American Chemical Society). (b) Formation of Ce−O bonds with the NPs leading to enhanced stabilization when ceria is used as the support.10

(MOFs),7 carbon nanotubes (CNTs),8 and even membranes in which the NPs can be encapsulated.9 An advantage is that the size of the NPs can be tuned by the size of the pore to which they are confined. A second method to enhance the stabilization of the NPs is by creating anchor points for which the particles show a high affinity, thereby decreasing their tendency to migrate over the surface and create larger particles. These anchor points can be defect sites in the support, but they can also be introduced into the support by chemical modification.10 The stabilization of Cu NPs used for the oxidation of butane can, for example, be significantly improved by introducing elements such as Ce, Zr, La, and Cs into the Al2O3 support material. In all cases, the dispersion of Cu was better when the additives were present because of the interaction of Cu with the sites created by the additives.11 In the case of ceria (CeO2), research has shown that strong Ce− O−M bonds are formed (M = metal), explaining the high stability of NPs on this support (Figure 1b).10 Another example is the N doping of carbon supports to immobilize Pt and Pt alloys for the electrooxidation of methanol and the electroreduction of oxygen, which results in smaller particle sizes and enhanced dispersion on the support.12 Thermally reduced graphite oxides are another class of carbon materials that have only recently been rediscovered as supports for NPs. The

Scheme 1. Reaction Scheme of the Racemization of (S)-(−)-1-Phenylethylamine Using Pd as a Catalysta

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The basic support used to immobilize the NPs suppresses the formation of bis(1-phenylethyl)amine, a condensation product of the amine, which can further react to form ethylbenzene and the amine. B


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Scheme 2. Dissociative Adsorption of O2 on Ceria21

Figure 2. Possible mechanism for the activation of molecular oxygen by molecular hydrogen at the Au−O−Ti interface in the epoxidation of propylene.

toward the metal−support interface because of the decreased negative charge of the oxygen atoms in the support. This lowers the electron density on the Pt surface, leading to less strongly adsorbed molecules, and results in faster hydrogenation. Also, the selectivity is affected by the acidity because the Lewis acid sites on the support form docking points for the CO bonds of the reactant. Hence, the CO bond is more likely to be hydrogenated than the CC bond, while the latter is favored on most catalysts for hydrogenation.23 Supports are often modified to impart electronic changes to the metal nanoparticle. An example is the use of different chemically modified CNFs to influence the selectivity of Pd NPs for the hydrogenation of acetylene. Via the oxidative treatment of CNFs, O functional groups were introduced onto the carbon surface, making it more acidic. The selectivity for ethylene is slightly higher on the acidic supports, which was again explained by the lower electron density of the CNF, which influences the properties of Pd and increases the adsorption strength of acetylene compared to the adsorption of ethylene. This reduces the chance that ethylene overreduction occurs. Additionally, because of the changed adsorption of acetylene, oligomer and coke formation on the surface were suppressed.24 Supports such as CNTs display particular electronic properties that can influence the properties of the metal NPs located on them.8 The activity of Cu NPs deposited on the inside and outside of CNTs, for instance, was compared for the hydrogenation of methyl acetate to methanol and ethanol. This showed a higher activity for the particles deposited inside the tube because the reducibility of the catalyst in the presence of hydrogen and CO is markedly better on the inside of the tube than outside.25 Similar effects were discovered earlier by Chen and co-workers for iron nanoparticles deposited on the inner walls of CNTs for the Fischer−Tropsch reaction.26 The examples above illustrate the complex interactions that can take place between carbon supports and NPs. The different interactions that can take place between oxide supports and metal NPs and their influence on the electronic properties of the metal have been described in a recent review.27 New species can also be formed at the interface between the nanoparticle and the support, which display higher catalytic activities than the NPs themselves. This phenomenon is often invoked to explain the enhanced activity of Au NPs on reducible oxide supports, such as TiO2, for the selective

for the materials treated at high temperature because the basic sites help to promote the formation of an alcoholate from glycerol on Au and the subsequent C−H cleavage to form glyceraldehyde. 17 A final example is the Pd-catalyzed racemization of chiral benzylic amines. In this case, the selectivity can be drastically enhanced by immobilizing Pd on basic supports such as BaSO4 because they suppress the formation of amine condensation products, which can in secondary reactions form ethylbenzene (Scheme 1), both processes being unwanted.18 Another way in which the support can initiate the reaction is by activating one of the reactants through dissociative adsorption, which creates more reactive species in close proximity to the metal NPs and facilitates their further reaction. An example is given by Shekhar et al. for the water−gas shift reaction using Au NPs on Al2O3 and TiO2. They found that the support participates in activating H2O and that the support that binds H2O more strongly, in this case, TiO2, exhibits the highest activity.19 Ceria is also often used for this purpose because it can undergo fast reduction/oxidation cycles (CeO2 ↔ Ce2O3), allowing oxygen to adsorb on the surface in the form of superoxide and peroxide species, which eventually leads to the complete incorporation of molecular oxygen into the ceria lattice (Scheme 2). This property, together with the high lability of the lattice oxygen, makes ceria an excellent support to use in oxidation reactions in the presence of precious metal NPs.20,21 Finally, carbon supports have been shown to activate hydrogen peroxide by homolytically forming hydroxyl and hydroperoxyl radicals on their surface, enhancing the activity of Au NPs on these supports for the degradation of organic compounds.22 Metal−support interactions can affect several NP properties, but a very important effect is the alteration of the electronic properties of the metal through the contact of the NPs with the support. This process can change the adsorption affinity of the metal for the reactants and/or products, which in turn can affect its catalytic activity. Handjani et al. examined the influence of the surface acidity of mesoporous silicates SBA15 and Al-SBA-15 and of Al2O3 on the catalytic activity of Pt NPs with similar dispersions and sizes with respect to the hydrogenation of cinnamaldehyde. They found that the rate of hydrogenation is enhanced by stronger acidity. On the acidic support, the electron density of the Pt nanoparticle is shifted C


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hydrogenation of carbonyl compounds and is referred to as SMSI (strong metal−support interaction).28 The idea is that metal cations with lower valency or oxygen vacancies are formed on the support next to the metal site; these new sites can interact with the carbonyl group through its oxygen atom and hence activate it for hydrogenation.29,30 Au on TiO2 is also used to produce propylene oxide from propylene in the presence of a H2/O2 mixture. The contact area between Ti and Au seems to provide the active sites for the reaction: when the number of Au−Ti interactions using different synthesis methods is increased, more active and selective catalysts are obtained, whereas Au and TiO2 alone provide little to no activity, indicating a synergetic action between Au and Ti.31 This observation seems to support the reaction mechanism of Haruta and co-workers, who suggested that the active oxygen species are produced at the interface between Ti and Au by the reductive activation of O2 by H2 via a hydroperoxide mechanism (Figure 2).32 A final example is the catalytic behavior of Pd NPs for the hydrogenation of campholenic aldehyde on several metal oxides. Using NiO and Co3O4, a special effect was observed: metallic Ni and Co were formed at the interface, resulting in the formation of Pd−Ni or Pd−Co species at the interface. The synergetic effect increased the overall activity of the Pd nanoparticles.30 It is also possible that active sites become more accessible when the particles are immobilized on a support, compared to the free nanoparticles as a stabilized sol. This was observed for preformed Au−Pd nanoparticles that were stabilized by PVP and subsequently immobilized inside a porous polyimide membrane by diffusion-induced phase inversion. Comparing the activity of these membrane-occluded nanoparticles with that of the free nanoparticles for the oxidation of aliphatic, allylic, and benzylic alcohols to carbonyl compounds showed an enhancement of the activity for the membrane. The authors ascribed this to the partial removal of the PVP molecules from the nanoparticles, making the catalytic sites more accessible. Another advantage of working with NPs in membranes is that the open structure of the membrane solves diffusion problems during the reaction.33 Besides the enhanced stabilization of NPs through pore confinement, the internal pore architecture and functionality of a support can have additional effects on the catalytic outcome of a reaction. For example, the reactants can locally accumulate inside the support, increasing the overall reaction rate on the NPs located inside the support, although this can also have adverse effects on the selectivity and long-term activity of the NPs. Materials in which this is often encountered are CNTs.8 Enhanced activity was observed for Cu NPs located inside CNTs for the hydrogenation of methyl acetate because hydrogen can accumulate inside the tube.25 Another important concept for which zeolite materials are often applied is shape selectivity. To illustrate this, three different supportsa KL zeolite, graphite, and ZrO2were used to study the influence of the support on the selectivity of Ru NPs for the hydrogenation of citral, which has both CC and CO bonds that can be hydrogenated (Figure 3). The selectivity for the hydrogenation of CO is much higher on the zeolite than on the other two supports, especially at high conversions. Easily accessible active sites inside the zeolite predominantly hydrogenate the CC bonds but are also very prone to poisoning by decarbonylation and oligomerization products and become inactive at high conversions. The more confined particles are far less sensitive to this poisoning effect because citral can adsorb

Figure 3. At high conversions, the selectivity for unsaturated alcohols can be enhanced when KL zeolites are used as support for Ru NPs during the hydrogenation of citral as a result of geometrical restrictions.

only with the CO group (Figure 3), leading to good selectivities for CO hydrogenation at high conversions.34 Another example is the immobilization of Ru NPs in different types of small-pore zeolites for use in the competitive hydrogenation of 1-hexene and 2,4,4-trimethyl-1-pentene. The materials in which the Ru NPs were completely located inside the pores of the zeolite showed enhanced selectivity for the hydrogenation of 1-hexene because the bulky 2,4,4trimethyl-1-pentene has difficulty diffusing through the narrow channels of the zeolite.35 Finally, the support can influence the composition and structure of alloys, which in turn can influence their catalytic properties. This was shown for Pt−Cu alloys on CeO2 and Fe 2 O 3 supports. On the latter support, less Cu was incorporated into the Pt lattice because of the stronger interaction of Cu with the support than with Pt. In ceria, this phenomenon was not observed, and more active alloy structures were formed for the oxidation of CO.36 In a similar way, different catalytic sites are formed in Pt−Ni−Co alloys when immobilized on TiO2, SiO2, or carbon supports, which again influences their activity in the oxidation of CO (Figure 4).37 Solvent Choice as a Tool to Enhance the Stability, Activity, and Recyclability of Nanoparticles. Just like the support, solvents can influence the catalytic properties of nanoparticles in several ways, either during the synthesis of the NPs or during a catalytic reaction. Like the supports, solvents can influence the stability of the particles. This was shown for

Figure 4. The support can determine the composition and structure of an immobilized nanoalloy, which in turn can influence the catalytic properties of the alloy.37 D


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the hydrogenation of α,β-unsaturated carbonyl compounds to allylic alcohols by PVP-stabilized Au NPs. By replacing the alcohol solvents, which are normally used, with an amide such as N,N-dimethylacetamide or N,N-dimethylformamide (DMF), the activity of the Au NPs increased. The influence of the solvent was twofold: (1) the Au metal precursor was more readily soluble in the amide solvent and (2) the PVP molecules interacted more favorably with the amide molecules, which resulted in better unfolding of their chains and hence a better stabilization of the nanoparticles (Figure 5). As a plus,

acetalization, an unwanted side reaction that often occurs in alcohol solvents, did not proceed in the amide solvents, which also increased the overall selectivity of the reaction.38 The solvent can also eliminate the necessity of adding stabilizers to the solution. An example is the stabilization of Pd NPs formed in situ during the Heck reaction by introducing Pd(II) acetate into polar solvents such as propylene carbonate and N-methyl2-pyrrolidone. No external phosphine ligands or polymers needed to be added to the reaction mixture to stabilize the particles because the solvent provided for this, though the exact mechanism behind this stabilization was not elucidated.39,40 Ionic liquids (ILs) are a special class of solvents that can strongly influence the stability of nanoparticles. ILs can take over the stabilizing function of ligands and can thus serve as both a solvent and a stabilizer at the same time. Replacing an organic solvent with an ionic liquid can significantly enhance the stability of the particles and hence also influence their activity.41,42 This was demonstrated for aromatic amination reactions using Ni nanoparticles. In toluene, low conversions (4%) were observed because the Ni NPs that were formed during the coupling reaction were unstable and became inactive. When the ionic liquid tetraoctylphosphonium bromide was used, the conversion increased to 53% just by the increased stabilization of the NPs.43 Depending on which type of ionic liquid is used, different degrees of stabilization can be observed. Luska and Moores found, for example, that Ru NPs were more stabilized in phosphonium-based ionic liquids than in imidazolium-based ionic liquids for the hydrogenation of

Figure 5. Schematic representation of how switching from alcohol solvents to amide solvents can enhance the stabilization of PVP/Au NPs through the chain unfolding of the stabilizing polymer.38

Figure 6. Depending on the solvent and metal precursor, different morphologies can be observed when synthesizing Rh NPs in ethylene glycolderived solvents using PVP as a stabilizer, ranging from icosahedra and cubes to triangular plates. TFA is trifluoroacetate; the scale bar is 20 nm (adapted with permission from ref 53; copyright 2011, American Chemical Society). E


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hydrogenation of trans- and cis-stilbene. The authors found that the olefinic double bond of stilbene is preferentially adsorbed on the Pt(100) facet, which prevails in nanocubes; this enhances the hydrogenation rate for these particles.56 For more extensive information on the influence of the size and shape of NPs on catalysis, two recent reviews are available.57,58 In ionic liquids, different morphologies can also be observed,46 which can again influence catalysis. This was shown for nickel nanoparticles synthesized in the ionic liquid tetrabutylammonium bromide, which were used for the racemization of chiral amines. When synthesized in toluene, spherical nanoparticles were obtained that were unstable and precipitated during the reaction, while in the ionic liquid, stable cubic particles were obtained. Despite the fact that larger nanoparticles were synthesized in the ionic liquid, the activity of the particles for the racemization of amines did not decrease significantly and even higher selectivities were observed, which was attributed to the shape of the particles via a mechanism similar to that previously mentioned for Pd NPs in hydrogenation reactions.59 Specific solvents can also be chosen to enhance the recyclability of the NPs. Poly(ethylene glycol) (PEG), for example, is often used for this purpose because it is immiscible with common organic solvents such as diethyl ether and nhexane. The latter facilitates the separation of organic products after reaction. For instance, Pd nanoparticles were synthesized in PEGs of various molecular weights (400−4000 g·mol−1) for the hydrogenation of olefins. In the case of cyclohexene and 1hexene, spontaneous phase separation occurred after cooling the reaction mixture to room temperature, after which the products could be simply decanted from the NPs containing PEG phase. When the olefins contained different functional groups, they could be extracted from the PEG phase by diethyl ether. Up to 10 reaction cycles could be performed using this system without any significant loss in activity. As a plus, PEG also has the ability to reduce the metal precursors intrinsically during synthesis, making the addition of external reducing agents redundant.60 Other solvents that can be used to recycle NPs through extraction or via biphasic systems include aqueous solvents,61−64 fluorous solvents,65−67 and ionic liquids.43,68−70 For the latter, not only multiphasic systems or extraction can be used but other methods are also available. By exploiting the negligible vapor pressure of ionic liquids, for example, certain products can be removed by simply distilling them from the IL phase.71 Furthermore, the ionic liquid can be immobilized on a solid support, forming supported ionic liquid phases (SILPs) that can strongly facilitate the recycling of the NPs.72,73 A final way in which solvents can be used to recycle NPs is to decrease (or increase) the affinity between the stabilizing layers around the particles. When adding a solvent for which the stabilizers show little affinity, the particles tend to cluster together and precipitation can occur, allowing the facile removal of the particles from the reaction medium after reaction. Solvents often used for this purpose are ethanol and methanol,74 but other solvents such as supercritical CO2 and squalane can also be used.75,76 After precipitation, the particles can be redispersed in the original solvent and can begin a new reaction cycle. An example is the addition of methanol to phosphine-dendrimerstabilized Pd NPs that were used for the Suzuki coupling. This method allowed efficient recycling.77 Finally, the solvent obviously can also actively participate in the reaction. The tunability of ionic liquids is a great advantage in this respect because specific functions can be introduced that

cyclohexene and that the type of anion also significantly influenced the stabilizing properties of the IL.44 Many research groups have attempted to find correlations between the type of ionic liquid and the stability and activity of a metal nanoparticle for a certain reaction. Although some correlations were found, mostly for imidazolium-based ionic liquids, no clear rules have been established to find the most stable/active ionic liquid− nanoparticle combination because they depend on both the cation and anion of the ionic liquid, on the synthesis method of the NPs, the metal used, and the reaction type for which the NPs are being used.45−47 Therefore, trial and error methods are usually applied. However, once an active IL−NP system is found, further improvements can be made by modifying the ionic liquid and introducing chemical functions that can enhance their stabilization ability. These task-specific or functionalized ILs have already been applied several times to enhance the long-term stability of metal nanoparticles. Examples are nitrile-functionalized pyridinium ionic liquids that were used to stabilize Pd NPs during Stille and Suzuki reactions. The catalytic activities of the Pd NPs in the unfunctionalized ILs and the nitrile-functionalized ILs were comparable, but after repeated recycling, significant loss of activity was observed in the unfunctionalized IL, whereas no change in activity was observed in the functionalized IL.48 For more extensive information on the use of metal NPs in functionalized ILs for catalysis, a recent review is available.49 Besides their influence on stability, solvents can have an effect on the eventual shape, size, and, in the case of alloys, composition of the nanoparticles. Several binary mixtures of water with different organic solvents were tested to synthesize Pt/C and PtxNi/C electrocatalysts for borohydride synthesis: the solvent composition influenced not only the average size of the particles but also the composition of the Pt−Ni alloy. The solvent can influence the synthesis process in several ways. The support−solvent interaction, for example, influences not only the wetting of the surface but also the process of adsorption of the metal precursor, whereas the viscosity of the solvent influences the diffusion of the precursor to and from the nanoparticle. Furthermore, different metal−solvate complexes can be formed that can display different redox potentials. All of these factors can determine the final size and composition of the nanoparticles on the support.50 Synthesizing FexPt100 − x NPs, which are mainly used as electrocatalysts,51 in toluene, oleylamine and 1,2-dichlorobenzene resulted not only in different sizes and alloy compositions but also in different morphologies depending on which solvent was used. This phenomenon was ascribed to the stabilizing properties of the solvent that can influence the surface energy of the different crystal facets in the nanoparticle.52 A final example is the use of various ethylene glycol-derived solvents for the synthesis of Rh NPs, which find applications in oxidation, reduction, hydroformylation, and cross-coupling reactions. Using PVP as a stabilizing agent, different nanoparticle morphologies were observed (Figure 6). Although the metal salt precursor has the most important influence on the morphology, the solvent helped to fine- tune the particles’ uniformity.53 In general, the eventual morphology of the nanoparticles is determined by the interplay between the solvent and the nature and concentration of the metal precursor and added stabilizer.54,55 An example of the influence of the morphology on the activity of nanoparticles is given by Cao et al., who synthesized Pt nanocrystals as cubes, tetrahedra, cuboctahedra, and nanowires to check the influence of the shape on the F


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Figure 7. Hydrogenolysis reaction of aromatic ketones using Pd/C catalysts in acid-functionalized ionic liquids. The acid function can catalyze an intermediate reaction step, leading to increased alkylbenzene yields in the presence of the ionic liquid.79

making it difficult to find the clear mechanisms behind the enhanced activities observed in the literature. Hence, a lot of trial and error is encountered in the literature. Only after finding a good combination have efforts been made to explain the observed interactions. More consistent and broad experimental research should therefore be conducted to explore this fascinating research field further.

can contribute to the reaction, such as acidic or basic functions, allowing the solvent to play a cocatalytic role in the reaction. Wang et al., for instance, used ethanolamine-functionalized ammonium-based ionic liquids as a reaction medium for Pd NPs to use in the Heck reaction. The ionic liquid not only stabilizes the NPs but also acts as a base in the reaction cycle, so no external bases need to be added during the reaction. After a series of reactions, though, the IL would need to be recovered again because a stoichiometric amount of HX salt is formed during the Heck reaction, which reacts with the IL.78 Another example is the use of acid-functionalized ionic liquids for the hydrogenolysis of aromatic ketones to produce alkylbenzenes using Pd/C as catalyst. Increased yields were observed in the presence of betainium-based ionic liquids because they can catalyze an intermediate reaction step, as shown in Figure 7.79

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+32) 16-321998. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.G. acknowledges the Research Foundation Flanders (FWO) for financial support as a research assistant. This research was also funded by the Industrieel Onderzoeksfonds KU Leuven (project IKP/10/005) and by the SBO-project MAPIL (IWT Flanders). Our efforts in organic catalysis are equally supported by KU Leuven (Metusalem grant CASAS) and the Belgian Science Policy (IAP 7/05 Functional Supramolecular Systems).

CONCLUDING REMARKS The environment of a nanoparticle can significantly influence its catalytic properties, whether it is through contact with a support or through the action of the surrounding solvent. The interaction between a support and NPs can be quite complex and affect the catalytic outcome in numerous ways. Enhanced stabilization of the particles through pore confinement and the creation of anchor points to which NPs can be attached is only one aspect. The support can also actively participate during the reaction by catalyzing intermediate reactions, suppressing side reactions, and/or enriching reactants around the active sites. Furthermore, strong interactions between the support and the metal NPs not only can alter the electronic properties of the NPs but also can result in new species at the interface with enhanced catalytic activity. Pores and channels inside a support can affect the diffusion phenomena and profoundly alter the activity and selectivity of the particles. Additionally, working with alloys adds more complexity because the support can also control the structure of alloys and alter their active sites. As for supports, the interaction between NPs and a solvent can be quite complex, influencing the stability, size, and shape of nanoparticles and, in the case of alloys, even affecting the composition of the particles. Applying the right solvent can facilitate the recycling of the NPs, and synergetic actions can take place between the NPs and a task-specific solvent. Ionic liquids are especially useful for these applications because they can be modified to enhance the stability, recyclability, and activity of the NPs, making them excellent solvents to use in combination with NPs. Looking over these examples, it is obvious that many factors can influence the final properties of a nanoparticle, often

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REFERENCES

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