DOI: 10.1002/cssc.201100413

Highly Stable Noble-Metal Nanoparticles inTetraalkylphosphonium Ionic Liquids for in situ CatalysisAbhinandan Banerjee, Robin Theron, and Robert W. J. Scott*[a]

Introduction

Nanosized materials have attracted extensive interest in recentyears because of their unique structural and chemical proper-ties, which can often be modulated by changing parameterssuch as size, shape, and the surrounding environment of thenanomaterials.[1] Noble-metal nanoparticles (MNPs), in particu-lar, have shown impressive catalytic abilities for reactions ofboth scientific and industrial importance.[2] Although support-ed MNPs have traditionally been used as heterogeneous cata-lysts for enhanced reusability and facile product recovery,recent research has been focused on their use as “quasi-homo-geneous” catalysts.[3] Quasi-homogeneous nanocatalysis in-volves the synthesis and use of stable and catalytically activenanoparticles (NPs) in solution; this combines the benefits ofcatalyst modulation and recyclability without having to hetero-genize a homogeneous catalyst.[4] Although there is no dearthof either materials or methodologies for the synthesis of MNPsin solution, their lack of thermodynamic stability and conse-quent tendency to agglomerate and precipitate often preventtheir effective application as efficient catalytic systems.[5]

To date, the electrostatic and/or steric protection that stabi-lizes such NPs is introduced through the addition of an exter-nal stabilizer ; synthetic strategies, in which such stabilization isinherent in the reaction system, are few and far between.Stabilizers, such as poly(vinylpyrrolidone) and poly-N-donor li-gands, and molecular stabilizers, such as alkanethiolates, canstabilize MNPs; however, they can suffer from numerous disad-vantages: very efficient capping ligands often passivate thecatalytic activity of MNPs, and the protective ligand may notbe soluble in the medium of dispersion of MNPs.[6] Moreover,the concept of involving a secondary stabilizer in a catalystsystem just for the sake of protection may have undesirable ef-fects on the selectivity and the reactivity of the catalyzed reac-

tion. Herein, we show a process that involves the use of tet-raalkylphosphonium halide ionic-liquid (IL) solvents, whichallow for a robust and reusable catalytic MNP system in the ab-sence of organic solvents, secondary stabilizers, and excessivefunctionalization of the dispersion medium.

In the last decade, many groups have shown that ILs can beused as intriguing media for the stabilization of MNPs.[7] Immo-bilization of NPs by supporting them in an IL rather than on asurface provides catalytic centers in the liquid phase; therehave been many reports in the literature exploring the catalyticproperties of MNPs in imidazolium-based ILs since the pioneer-ing work of Dupont and co-workers.[8] Previously, well-definedPd NPs have been prepared in situ and used for C�C couplingreactions, such as Heck and Suzuki processes,[9] while we haveshown that bimetallic NPs can be transferred from water toimidazolium ILs for efficient nanocatalysis.[10] The reasonsbehind such efforts can be understood, if we consider the nu-merous attractive chemical properties of ILs, such as limitedvolatility and flammability, recyclability, wide ranges of solubili-ty and miscibility, and structural tunability (through both thecation and anion), although their green credentials are amatter of debate.[11] Despite these developments, optimal strat-egies for MNP stability in ILs are still being determined; thereis some evidence that MNP dispersions are inherently morestable in ILs because of the IL microstructure and/or weak in-teractions of the anions and/or cations of ILs with the NP sur-face, although we have previously shown that inherent impuri-

Gold and palladium nanoparticles were prepared by lithiumborohydride reduction of the metal salt precursors in tetraal-kylphosphonium halide ionic liquids in the absence of any or-ganic solvents or external nanoparticle stabilizers. These colloi-dal suspensions remained stable and showed no nanoparticleagglomeration over many months. A combination of electro-static interactions between the coordinatively unsaturatedmetal nanoparticle surface and the ionic-liquid anions, bol-stered by steric protection offered by the bulky alkylated phos-phonium cations, is likely to be the reason behind such stabili-zation. The halide anion strongly absorbs to the nanoparticle

surface, leading to exceptional nanoparticle stability in halideionic liquids; other tetraalkylphosphonium ionic liquids withnon-coordinating anions, such as tosylate and hexafluorophos-phate, show considerably lower affinities towards the stabiliza-tion of nanoparticles. Palladium nanoparticles stabilized in thetetraalkylphosphonium halide ionic liquid were stable, efficient,and recyclable catalysts for a variety of hydrogenation reac-tions at ambient pressures with sustained activity. Aerial oxida-tion of the metal nanoparticles occurred over time and wasreadily reversed by re-reduction of oxidized metal salts.

[a] A. Banerjee, R. Theron, Prof. R. W. J. ScottDepartment of Chemistry, University of Saskatchewan110 Science Place, Saskatoon SK S7N 5C9 (Canada)Fax: (+ 1) 306 966 4730E-mail : robert.scott@usask.ca

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ties in imidazolium ILs (such as unreacted 1-methylimidazole)and/or secondary stabilizers can dramatically affect the stabili-ty.[12] In addition, “task-specific” functionalized imidazolium andpyridinium ILs with thiol, alcohol, and nitrile moieties havebeen used for the synthesis of MNPs by many groups.[13]

To date, the majority of research concerned with MNP cata-lysts in ILs has involved imidazolium-based ILs; this can be par-tially attributed to the moderate viscosities of room-tempera-ture imidazolium ILs.[14] The room-temperature imidazolium ILshave weakly coordinating anions, such as BF4

� and PF6� , with

limited MNP stabilization capabilities. Moreover, the imidazoli-um cation can undergo deprotonation in the presence ofstrong bases to form carbenes,[15] and hexafluorophosphategroups can undergo anion hydrolysis to produce strongly toxicHF at moderate temperatures.[16] Another class of ILs is the tet-raalkylphosphonium family of ILs, which are considerably morestable in strongly basic media,[17] cost less, and are fairly chemi-cally inert (although Wittig substitution reactions on the phos-phonium center have been reported).[18] Several other groupshave previously shown that tetraalkylphosphonium ILs can beused successfully for MNP stabilization and catalysis. In particu-lar, Kalviri and Kerton reported a Suzuki coupling reaction cata-lyzed by Pd MNPs, which were formed in phosphonium ILs inthe absence of an external reductant, and it was postulatedthat the IL itself was acting as a reducing agent.[19] Yinghuaiet al. used the same IL to prepare Ru MNPs from an organome-tallic precursor with ethylene glycol as a reductant.[20] However,to the best of our knowledge, all of the tetraalkylphosphoniumIL systems studied to date have anions that either do not coor-dinate to metal surfaces or coordinate weakly to them. Wewere particularly intrigued by the P[6,6,6,14]X {P[6,6,6,14] = tri-hexyl(tetradecyl)phosphonium cation; X = Cl, Br} family ofroom-temperature ILs, because of their striking structural re-semblance to the quaternary ammonium surfactants whichwere already successfully used for electrostatic MNP stabiliza-tion.[21] We postulated that such P[6,6,6,14]X (X = Cl, Br) ILswould be particularly interesting to examine as efficient dual“solvent/stabilizers” for MNP synthesis.

Herein, we outline the synthesis of a catalytic system ofMNPs dispersed in several tetraalkylphosphonium ILs in the ab-sence of organic solvents and external stabilizers conventional-ly used for protection against MNP agglomeration. Au and PdMNPs synthesized directly in P[6,6,6,14]X (X=Cl, Br) ILs showedprolonged stability over several months, whereas MNPsformed in systems with weakly coordinating tosylate (OTs) andhexafluorophosphate anions showed poor stability towards ag-gregation. Pd MNPs dispersed in P[6,6,6,14]X (X=Cl, Br) ILswere recyclable catalysts for the hydrogenation of olefinic alco-hols, aromatic nitro compounds, 1,2-unsaturated carbonyls,and alkynes. The dependence of the catalytic activities on thenature of the IL used for dispersion of the MNPs further con-firms the important role played by the ILs in MNP stabilization,and sheds light on the exact mechanism of protection againstaggregation. In addition, we show that, although both the Auand Pd MNPs dispersed in P[6,6,6,14]X (X=Cl, Br) ILs are proneto oxidation in the presence of air, the MNPs can be easily re-

generated by re-reduction of the metal salts to reform MNPs,thus enhancing the recyclability of such MNP/IL composites.

Results and Discussion

Synthesis of MNPs in tetraalkylphosphonium ILs

Inherently stabilized Au and Pd MNPs were synthesized intetraalkylphosphonium ILs through in situ LiBH4 reduction,without the involvement of any organic solvents or secondarystabilizers. As these ILs are considerably viscous, cooling of theproduct led to highly viscous systems; however, the viscosityreduced drastically upon heating.

TEM images of Au MNPs synthesized in P[6,6,6,14]Cl andP[6,6,6,14]Br are shown in Figure 1 a and d, respectively. Theaverage sizes of the free Au MNPs synthesized in P[6,6,6,14]Cl

Figure 1. a) TEM image of Au MNPs prepared in P[6,6,6,14]Cl. b) TEM imageof Au MNPs prepared in P[4,4,4,1]OTs. c) Size distribution of Au MNPs inP[6,6,6,14]Cl. d) TEM image of Au MNPs prepared in P[6,6,6,14]Br. e) UV/Visspectra of 1.4 mm solutions of HAuCl4 (c), Au MNPs in P[6,6,6,14]Cl (b),and Au MNPs in P[6,6,6,14]Br (g) in cyclohexane.

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and P[6,6,6,14]Br determined from the TEM images were (4.1�0.7) and (5.4�0.9) nm, respectively. This is in agreement withthe UV/Vis spectra of the MNPs (Figure 1 e). The spectra of theAu MNPs in P[6,6,6,14]Cl show a broad plasmon band at l=

520 nm after reduction. For Au MNPs in P[6,6,6,14]Br, the plas-mon band is significantly shifted to higher wavelengths be-cause of the larger average particle size. Au MNPs synthesizedin tributyl(methyl)phosphonium tosylate (P[4,4,4,1]OTs) aggre-gated quickly (TEM micrograph shown in Figure 1 b) and pre-cipitated within a day.

Pd MNPs synthesized in P[6,6,6,14]Cl had a deep browncolor. A TEM image of Pd MNPs synthesized in P[6,6,6,14]Cl isshown in Figure 2 a; the average size of these particles was(2.3�0.6) nm. The UV/Vis spectrum, shown in Figure 2 c, showsan exponential rise in absorbance with decreasing wavelengthtypical of Pd MNPs in solution.

NP stability and regeneration

The P[6,6,6,14]Cl-stabilized MNPs remained unagglomeratedfor months under N2. This stability is exceptional compared tothat of MNPs stabilized in tetraalkylphosphonium ILs bearingpoorly coordinating anions, such as N(CN)2

� , OTs, and methyl-sulfate (SO4Me�), in which precipitation of the particles is ob-served within 12–24 h, or MNPs stabilized directly in imidazoli-um ILs, which tend to precipitate over several days or almostimmediately under hydrogenation conditions.[12] Under air atelevated temperatures, Au MNPs in the P[6,6,6,14]Cl IL show

very slow aerial oxidation, as the red Au0 system becomes col-orless and then pale yellow after several weeks at 90 8C; this islikely to be caused by oxidative etching of Au assisted byhalide absorption on the surface, leading to the formation ofAuI, AuIII, or a mixture of both.[22] The Pd MNPs in theP[6,6,6,14]Cl IL are completely oxidized over 48 h upon heatingin air at 90 8C. Oxidative etching of Pd in the presence of O2

and Cl� is well known: Kalviri and Kerton attribute their failureto grow Pd NPs in P[6,6,6,14]Cl in the absence of reductants tothis effect.[19] In both cases, the oxidized metal species couldbe readily re-reduced by dropwise addition of excess LiBH4.The UV/Vis spectra before and after regeneration and the TEMimages of the particles after re-reduction are shown inFigure 3. After oxidation and re-formation of the Pd and AuMNPs in P[6,6,6,14]Cl, TEM analysis revealed average particlesizes of (3.9�1.1) and (3.5�1.1) nm, respectively. The reasonsfor the moderate increase in Pd MNP size are still unknown.

Borohydride reduction is a well-documented method for thesynthesis of MNPs in conventional solvents, and its mechanismhas been thoroughly studied.[23] Reduction of dissolved metalsalts give metal atoms in the embryonic stage of nucleation,followed by irreversible ion/atom or atom/atom collisions thatproduce metal cluster “seeds”. It has been pointed out byDupont et al.[7b] that there are electrostatic forces at play be-tween the charged MNP surfaces and the three-dimensionalnetwork of cations and anions that constitute the ILs.

Dupont[7b] and others[24] proposed a two-phase system to ex-plain imidazolium IL stabilization of MNPs: a crystalline MNPand a semi-ordered IL phase. A similar model may also be ap-plied to the tetraalkylammonium phosphonium IL systems

Figure 2. a) TEM image of Pd MNPs prepared in P[6,6,6,14]Cl. b) Size distribu-tion of Pd MNPs prepared in P[6,6,6,14]Cl. c) UV/Vis spectra of 1.4 mm solu-tions of K2PdCl4 (c) and Pd0 in P[6,6,6,14]Cl (a) in cyclohexane.

Figure 3. TEM images of a) Pd MNPs and b) Au MNPs in P[6,6,6,14]Cl regen-erated after aerial oxidation followed by LiBH4 reduction. UV/Vis spectra ofthe re-reduction of c) Pd MNPs (c : Pd NPs, a : re-reduced Pd NPs, g :post-oxidation Pd) and d) Au MNPs in P[6,6,6,14]Cl (c : Au NPs, a : re-reduced Au NPs, g : post-oxidation Au).

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(Scheme 1), in which the halide anions bind strongly to themetal surface and the associated bulky tetraalkylphosphoniumcations offer steric protection. Significant work using tetraal-kylammonium halide systems, such as cetyltrimethylammoni-um bromide (CTAB) and tetraoctylammonium bromide (TOAB),have shown that halide absorption onto metal surfaces canlead to significant stability of the resulting particles.[25] Anothersignificant validation for this model is the direct dependenceof MNP stability on the anion composition of the IL: the stron-ger the coordinating ability of the anion (e.g. , Cl�>OTs�), thegreater the protection of the MNP surface, which leads to in-creased stability of the MNP in the IL.

Application of Pd MNPs in P[6,6,6,14]Cl IL for catalytichydrogenations

The catalytic behavior of Pd MNPs in P[6,6,6,14]Cl IL was evalu-ated by performing the hydrogenation of several organic com-pounds with unsaturated moieties present in their structures.Slightly elevated temperatures were necessary to reduce theviscosities of the ILs for attenuation of mass-transfer effects.Although the solubility of H2 in ILs is moderate, it has been ob-served that this solubility actually increases at higher tempera-tures, which is in contrast to other gases, such as CO2.[26] Semi-nal work by Brennecke et al. showed the dependence of gassolubility in an IL on the nature of the anion present.[27a] Earlier,Dyson et al. determined the Henry constant for H2 inP[6,6,6,14][PF3(C2F5)3] to be 70 MPa at 293 K, which indicatedthat H2 was more soluble (by an order of magnitude) in manyphosphonium ILs than the imidazolium or pyridinium ILs.[27b]

For the Pd MNP/P[6,6,6,14]Cl system, most of the hydroge-nations reached completion within a few hours. The progressof the reaction was followed by differential H2 pressure meas-urements; the details of which are given in the ExperimentalSection. The monometallic Pd MNPs in the P[6,6,6,14]Cl ILshowed high catalytic activity and considerable stability. It wasnoted earlier that the mechanism of stabilization of MNPs in-volves protective anchoring of the IL anions onto MNP sur-faces; the better the coordinating ability of the anion, the

greater the stabilization provided by it to the MNP. Similarly,one should expect a similar trend in catalytic activities. This isconfirmed by Figure 4, which shows the hydrogenation of 2-methyl-3-butene-2-ol at 75 8C by using 14.0 mm Pd MNPs in

different ILs. The relative turnover frequencies (TOFs) of this re-action over the first half hour can be seen in Figure 4 b; thelargest TOF of 10.2 min�1 was found for P[6,6,6,14]Cl comparedto only 1.8 min�1 for P[6,6,6,14]N(CN)2. Traces of precipitatewere observed in the reaction flask after completion of the re-action for MNPs in P[6,6,6,14]N(CN)2, P[4,4,4,1]OTs, andP[4,4,4,1]OSO3Me.

According to Figure 4 b, the TOFs increase in the followingorder: P[6,6,6,14]N(CN)2<P[4,4,4,1]OTs<P[4,4,4,1]OSO3Me<P[6,6,6,14]PF6<P[6,6,6,14]Br ! P[6,6,6,14]Cl. The activities hereare substantial : we previously observed significantly lowerTOFs (approximately 4.5 min�1) for hydrogenations with polyvi-nylpyrrolidone (PVP)-stabilized Pd MNPs in imidazolium ILs.[10]

Trends can be deduced from this order, namely, shorteralkyl-chain-bearing phosphonium ILs are less efficient at pro-tecting against agglomeration than those with longer chains,

Scheme 1. Schematic representation of MNP stabilization by P[6,6,6,14]Cl IL.

Figure 4. a) Turnover number (TON) versus time plot for the hydrogenationof 2-methyl-3-butene-2-ol by using 14.0 mm Pd MNPs in various phosphoni-um ILs {^: P[6,6,6,14]Br, &: P[4,4,4,1]OTs, ~: P[6,6,6,14]PF6, � : P[6,6,6,14]Cl,*: P[4,4,4,1]OSO3Me, –: P[6,6,6,14]N(CN)2}. b) Comparison of the turnover fre-quency (TOF) per min of the reaction in different ILs.

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and the halides in general provide maximum stability to MNPs,leading to high catalytic activities. The first observation couldeasily be justified if, we consider the representation inScheme 1, in which a surrounding layer of tetraalkylphosphoni-um ions is shown to offer steric protection to the MNPs. Clear-ly, shorter alkyl chains would be less efficient at offering resist-ance to aggregation than longer ones. A number of other fac-tors make P[6,6,6,14]Cl the best IL for Pd MNP stabilization andcatalysis. Ferguson and Scovazzo found that the diffusivity ofmany gases at 30 8C was maximum in P[6,6,6,14]Cl[28] and ob-served that gas diffusivity in phosphonium-based ILs was sig-nificantly higher than diffusivity in imidazolium-based ILs ofequal viscosity. P[6,6,6,14][NTf2] with a viscosity of 200 cP(1 P = 0.1 Pa s) has the same gas diffusivity as an imidazolium-based IL with a viscosity of 20 cP. They attributed this to thesmall size of Cl� , which prevents it from competing with diffus-ing gas molecules to fill the same microvoids within the IL.However, of greater importance is the idea of MNP stabilizationby anionic species of the IL. Among all of the ILs studied, theanion with the largest surface charge density is Cl� . In otherwords, the catalytic activities of MNPs dispersed in ILs dependoverwhelmingly on the stability of the MNPs in those ILs. AsCl� is capable of offering excellent electrostatic protection tothe MNP surface, it enhances the catalytic abilities of the MNPsdispersed in Cl-bearing ILs. A similar effect is observed for Br� ,but to a smaller extent, which may be explained by the largersize and smaller surface charge density of Br� . All other anionsshow comparatively smaller catalytic efficiencies and for OTs�

and OSO3Me� ILs, precipitation of Pd MNPs was observedwithin several days. The only IL that shows a slight deviationfrom this trend is P[6,6,6,14]PF6, the large, poorly coordinatinganion of which should lead to poor MNP stabilization; howev-er, the comparatively higher stability of Pd MNPs in this IL canbe explained by the moderate Cl� content of the IL of200 ppm, even after repeated washings (P[6,6,6,14]PF6 was syn-thesized from P[6,6,6,14]Cl by using ion exchange).

The selectivity of a catalyzed reaction is also profoundly in-fluenced by the surrounding environment of the catalyst. ThePd MNPs in P[6,6,6,14]Cl catalyzed the hydrogenation of vari-ous organic molecules with unsaturated moieties (such as ole-finic and acetylinic multiple bonds) in their structures. Accord-ing to Table 1, yields were almost quantitative, except for sty-rene, for which polystyrene was also obtained. The aromaticmoiety remained unaffected in all relevant reactions. The PdMNPs in P[6,6,6,14]Cl showed very high thermal stability to rel-atively elevated temperatures (up to approximately 140 8C);thus, a relatively high reaction temperature was chosen forseveral of the hydrogenations. Of the various organic mole-cules hydrogenated, the catalytic activity was found to be thelowest for the hydrogenation of trans-cinnamaldehyde; thiscould be because of the presence of highly conjugated unsa-turation, along with steric effects of the bulky benzene ring at-tached to the double bond. The selectivity towards partial hy-drogenation of 1,3-cyclooctadiene to generate cyclooctene(rather than the completely saturated cyclooctane) was investi-gated; this was found to be a function of time and tempera-ture of the reaction. Reaction for 4.5 h at 80 8C not only re-

duced the extent of conversion to approximately 90 %, butalso gave more cyclooctene (78 %, as opposed to 61 % for a re-action for 8 h at 110 8C). Similar observations were recorded fortrans-cinnamaldehyde and 3-hexyn-1-ol. For the former, reac-tion for 5 h at 90 8C produced 79 % 3-phenylpropanal and 21 %of the phenyl alcohol; the extent of conversion was also foundto drop to approximately 85 %. Another possible hydrogena-tion product, cinnamyl alcohol, was not detected at all. This isin accordance with previous results in other systems.[10] In thehydrogenation of 3-hexyn-1-ol, hexenols and hexanol wereformed as the only products ; a reaction for 6 h at 60 8C gaverespective yields of 18 and 82 %. When the reaction time wasreduced to 2.5 h and the temperature to 55 8C, the hexenolyield increased to 41 %, whereas 59 % of hexanol was pro-duced. Combined evidence, therefore, indicates that the hy-drogenations proceed by the accepted Horiuti–Polanyi mecha-nism, with one unsaturated moiety reduced at a time.[1] Expo-sure to H2 for smaller amounts of time and/or at a lower tem-perature favors the partially hydrogenated product.

The catalytic efficiency of the regenerated MNPs in an IL isshown in Figure 5, which shows the recyclability of this systemfor the hydrogenation of 2-methyl-3-butene-2-ol. For the recy-clability evaluation, the same system (Pd MNPs in P[6,6,6,14]Cl)was used repeatedly for the reduction of 2-methyl-3-butene-2-ol at 75 8C. For each cycle, the catalytic activity was obtainedby performing differential pressure measurements of H2 insidethe sealed reaction flask; the products and unreacted substrate

Table 1. Data for the hydrogenation of unsaturated species using H2 inthe presence of 14 mm Pd NP/P[6,6,6,14]Cl.[a]

Reactant Product(s) Yield[b]

[%]T[8C]

t[h]

>99 75 2

90 70 2.5

52[c] 100 2

>99 (1: 61, 2 : 38)90 (1: 78, 2 : 22)

11080

84.5

PhNO2 PhNH2 95 115 6

>99 (3 : 18, 4 : 82)92 (3 : 41, 4 : 59)

6055

62.5

>99 (5 : 8, 6 : 92)85 (5 : 21, 6 : 79)

12090

85

[a] The substrate (1–2 mL) was added to the 14 mM Pd NP/P[6,6,6,6,14]Clcatalyst system (10 mL) and heated at the given temperature for thegiven length of time under an atmosphere of H2 at ambient pressures.[b] Yields calculated from the areas of the signals in the 1H NMR spectraof the pure products and/or residual reactants recovered from the reac-tion mixture after the completion of the reaction by using vacuum strip-ping at high temperatures. [c] Extensive formation of polystyrene gran-ules that precipitated can explain the low yield.

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were removed by vacuum stripping, and the catalyst systemwas used for the next cycle. After removal of volatile com-pounds from the catalytic mixture, there was very little loss inactivity for the hydrogenation of the substituted allyl alcohol(<5 % after 5 cycles), which confirmed the re-usability/recycla-bility of the catalysts. The physical appearance of the systemcan be seen in Figure 5b; there are no significant differencesafter five catalytic cycles or after the oxidation/reductionprocedure.

However, moderate particle-size growth was observed uponuse of the Pd MNPs as hydrogenation catalysts, as shown inFigure 6 b; after five cycles, the average size of the particlesgrew to (5.5�1.9) nm. We are uncertain as to why this particlesize growth occurs, but no MNP precipitation was observed inthe system after multiple cycles. This growth could be reversedby Pd oxidation in air at elevated temperatures followed by re-reduction of the Pd salts (Figure 5 b).

Conclusions

We have synthesized Au and Pd NPs in a variety of tetraalkyl-phosphonium ILs by in situ reduction without the use of any

organic solvents. These MNPs were found to have stabilitiesranging from days to months, depending upon the composi-tion of the IL. The Pd NPs showed excellent activities for thehydrogenation of a range of substrates (cyclohexene, 2-methyl-3-butene-2-ol, nitrobenzene, 1,3-cyclooctadiene, trans-cinnamaldehyde, and 3-hexyn-1-ol) when using H2 at ambientpressures. Unreacted substrates (if any) and products wereeasily removed from the catalytic mixture by applying avacuum treatment. The catalytic dispersions of Pd MNPs inP[6,6,6,14]Cl were stable for months, highly recyclable, andcould be readily regenerated after aerial oxidation.

Experimental Section

Materials

All chemicals, except for the ones listed below, were purchasedfrom Sigma Aldrich and used as received. HAuCl4 and K2PdCl4

(both 99.9 %, metals basis) were obtained from Alfa Aesar andwere stored under vacuum and flushed with N2 after every use.Trihexylphosphine (98 %), as well as commercial samples of theionic liquids (ILs), were generously donated by Cytec Industries.Commercial samples of ILs were dried under vacuum at 70 8C for10–12 h with stirring before use. Deuterated solvents were pur-chased from Cambridge Isotope Laboratories. 18 MW cm Milli-Qwater (Millipore, Bedford, MA) was used.

Synthesis of the tetraalkylphosponium ILs

The synthesis of P[6,6,6,14]Cl and the generation of nanosized Auand Pd clusters in the IL were performed by using standardSchlenk techniques. For a typical synthesis of P[6,6,6,14]Cl, trihexyl-phosphine was added to a slight excess of 1-chlorotetradecaneunder N2 at 145 8C, stirred overnight, and vacuum stripped to

Figure 5. a) Recyclability of Pd MNPs prepared in P[6,6,6,14]Cl for hydroge-nation of 2-methyl-3-butene-2-ol. The final column shows the TOF per minafter complete Pd oxidation at 90 8C in air over two days followed by re-re-duction of Pd with LiBH4. b) From left to right: 14 mm Pd NPs in P[6,6,6,14]Clafter 5 catalytic cycles, after oxidation, and finally, after re-reduction withLiBH4.

Figure 6. a) TEM image of Pd MNPs in P[6,6,6,14]Cl regenerated after aerialoxidation followed by LiBH4 reduction; size distributions are given in c).b) TEM image of Pd MNPs in P[6,6,6,14]Cl after five cycles of catalytic hydro-genations; size distributions are given in d).

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remove volatile impurities.[29] A transparent, pale-yellow viscous ILwas generated in 90 % yield. Chloride contents of the ILs preparedby metathesis were checked by using an AgNO3 test. 1H and31P NMR spectra of the as-prepared samples matched with thosereported in the literature.[27] The commercial sample of P[6,6,6,14]Clsupplied by Cytec was of comparable purity. P[6,6,6,14]Br was syn-thesized by using an identical procedure and was a deep yellowviscous liquid obtained in 93 % yield; it was also indistinguishablefrom the dried Cytec samples in terms of purity. P[6,6,6,14]PF6 wassynthesized from P[6,6,6,14]Cl by using an additional ion-exchangestep, in which HPF6 was added dropwise to ice-cold P[6,6,6,14]Clunder N2 and stirred overnight under ambient temperatures. Thewaxy white solid generated was washed repeatedly with deionizedwater until the washings failed to generate a precipitate upon re-action with a large excess of a 0.05 m AgNO3 solution. It was thendried under vacuum, giving a yield of 95 %. P[4,4,4,1]OTs,P[4,4,4,1]OSO3Me, and P[6,6,6,14]N(CN)2 ILs were donated by Cytecand were moderately heated under vacuum to remove volatile im-purities prior to use.

Synthesis of Pd and Au MNPs in ILs

For the synthesis of Pd MNPs, K2PdCl4 (45 mg; 0.14 mmol on thebasis of Pd content) was added under N2 to a sample of the IL(10 mL) at 80 8C (all of the ILs studied were liquids at this tempera-ture) and vigorously stirred to give a 14 mm solution. The solutionwas cooled to 60 8C, and a stoichiometric excess of LiBH4 reagent(1.5 mL, 2.0 m in THF) was injected dropwise over a period of5 min. Rapid effervescence followed, and the entire solution turnedbrown, indicating NP formation. After the addition of LiBH4, volatileimpurities were removed by vacuum stripping the system at 80 8C.The Pd MNP solution thus obtained was stored under N2 incapped vials until use. For the synthesis of Au MNPs, an identicalsolvent-free procedure was followed, except the reactions wereperformed in air. HAuCl4 (20 mg, equivalent to 0.05 mmol of Au)was dissolved in P[6,6,6,14]X (10 mL; X = Cl, Br) at 80 8C to give agolden yellow solution, which turned violet and then wine-redupon the dropwise addition of 2.0 m LiBH4 reagent (1.5 mL) aftercooling the solution to 60 8C.

General procedure for hydrogenation reactions

Hydrogenation reactions were performed in a Schlenk flask, withthe stem connected to an H2 gas source and the sealed necklinked to a differential pressure gauge [Model 407910, Extech In-struments with a resolution of 100 Pa and an accuracy of 5 % at(23�5) 8C]. The H2 supply was stopped, and the stopcock wasclosed before the reactant was injected into the reaction vessel.The progress of the reaction was monitored by performing time-dependent measurements of the H2 pressure inside the sealedflask. The temperatures to which the reaction mixtures wereheated and the reaction times of the systems studied are given inTable 1. The accuracy of the pressure data was verified by vacuumstripping the product and leftover substrate from the reaction mix-ture and by using 1H NMR spectroscopy to determine the ratio be-tween the product(s) and the unreacted substrate (if any) from theareas of the signals. In all cases, no significant difference wasfound between the two sets of data. The catalytic systems wereused repeatedly for recyclability studies. Parameters such as turn-over number (TON) and effective turnover frequency (TOF,molH2

molmetal�1 min�1) could be determined from H2 pressure data.

TOFs from NMR spectroscopy were also determined from the slopeof linear plots of TON as a function of time and were consistently

within 5 % of values obtained by using differential pressure meas-urements. Yields and product distributions were studied by usingNMR spectroscopy.

Characterization

UV/Vis spectra were obtained by using a Varian Cary 50 Bio UV/Visible spectrophotometer with a scan range of l= 200–800 nmand an optical path length of 1.0 cm. 1H and 31P NMR spectra wereobtained by using a Bruker 500 MHz Avance NMR spectrometer;chemical shifts were referenced to the residual protons of the deu-terated solvent. TEM analyses of the Au and Pd MNPs in differentILs, both before and after catalytic cycles, were conducted byusing a Philips 410 microscope operating at 100 kV. The sampleswere prepared by ultrasonication of a 1 % solution of the MNP/ILsolution in CHCl3 followed by dropwise addition onto a carbon-coated copper TEM grid (Electron Microscopy Sciences, Hatfield,PA). To determine particle diameters, a minimum of 100 particlesfrom each sample from several TEM images were manually mea-sured by using the ImageJ program.[30]

Acknowledgements

We would like to thank the NSERC and the University of Sas-katchewan for funding, Dr. Pia Wennek for her help and support,and Tesfalidet Balcha for help with the TEM imaging and picturesof the samples. A.B. would like to acknowledge scholarships fromthe University of Saskatchewan and VWR. We would also like toexpress our gratitude towards Al Robertson of Cytec for donationof the phosphonium ILs as well as the phosphine precursors.

Keywords: catalysis · green chemistry · hydrogenation · ionicliquids · nanoparticles

[1] a) H. Bçnnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, B.Korall, Angew. Chem. 1991, 103, 1344 – 1346; b) D. Astruc, Nanoparticlesand Catalysis (Ed. : D. Astruc), Wiley-VCH, Weinheim, 2008 ; c) G. Schmid,Chem. Rev. 1992, 92, 1709 – 1727; d) A. T. Bell, Science 2003, 299, 1688 –1691; e) E. Roduner, Chem. Soc. Rev. 2006, 35, 583 – 592; f) T. Zeng, W.-W.Chen, C. M. Cirtiu, A. Moores, G. Song, C.-J. Li, Green Chem. 2010, 12,570 – 573.

[2] a) H. Bçnnemann, G. Braun, W. Brijoux, R. Brinkmann, A. S. Tilling, K. See-vogel, K. Siepen, J. Organomet. Chem. 1996, 520, 143 – 162; b) C. N. R.Rao, G. U. Kulkarni, P. J. Thomas, P. P. Edwards, Chem. Soc. Rev. 2000, 29,27 – 35; c) N. Toshima, T. Yonezawa, New J. Chem. 1998, 22, 1179 – 1201;d) C. M. Cirtiu, A. F. Dunlop-Bri�re, A. Moores, Green Chem. 2011, 13,288 – 291.

[3] a) A. Biffis, S. Cunial, P. Spontoni, L. Prati, J. Catal. 2007, 251, 1 – 6;b) C. Rangheard, C. d. J. Fernandez, P.-H. Phua, J. Hoorn, L. Lefort, J. G.De Vries, Dalton Trans. 2010, 39, 8464 – 8471; c) S. M. Zhang, C. L. Zhang,J. W. Zhang, Z. J. Zhang, H. X. Dang, Z. S. Wu, W. M. Liu, Acta Phys. Chim.Sin. 2004, 20, 554 – 556.

[4] a) A. Safavi, S. Zeinali, Colloids Surf. A 2010, 362, 121 – 126; b) G. S. Fon-seca, J. B. Domingos, F. Nome, J. Dupont, J. Mol. Catal. A : Chem. 2006,248, 10 – 16; c) Y. S. Salprima, R. N. Dhital, H. Sakurai, Tetrahedron Lett.2011, 52, 2633 – 2637.

[5] a) H.-C. Schwarzer, W. Peukert, AIChE J. 2004, 50, 3234 – 3247; b) M. Man-teghian, S. Ghader, Chem. Eng. Technol. 2009, 32, 835 – 839.

[6] a) K. S. Nagabhushana, H. Bçnnemann, Nanotechnol. Catal. , Vol. 1 (Eds. :B. Zhou, S. Hermans, G. A. Somorjai), Springer, New York, 2004, pp. 51 –72; b) N. Yan, Y. Yuan, P. J. Dyson, Chem. Commun. 2011, 47, 2529 –2531; c) V. Cal�, A. Nacci, A. Monopoli, A. Detomaso, P. Iliade, Organo-metallics 2003, 22, 4193 – 4197; d) X. Mu, J. Meng, Z. Li, Y. Kou, J. Am.

Gre

enC

hem

istr

yb

yN

ano

cata

lysi

s

ChemSusChem 2012, 5, 109 – 116 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 115

Nanoparticles in Tetraalkylphosphonium Ionic Liquids

Chem. Soc. 2005, 127, 9694 – 9695; e) B. L�ger, A. Denicourt-Nowicki, H.Olivier-Bourbigou, A. Roucoux, ChemSusChem 2008, 1, 984 – 987.

[7] a) D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. 2005, 117, 8062 – 7872;b) J. Dupont, D. de Oliviera Silva, Nanoparticles and Catalysis (Ed. : D.Astruc), Wiley-VCH, Weinheim, 2008, Chapter 6, p. 195; c) P. Migowski, J.Dupont, Chem. Eur. J. 2007, 13, 32 – 39; d) J. Huang, T. Jiang, H. X. Gao,B. X. Han, Z. M. Liu, W. Z. Wu, Y. H. Chang, G. Y. Zhao, Angew. Chem.2004, 116, 1421 – 1423; Angew. Chem. Int. Ed. 2004, 43, 1397 – 1399.

[8] a) P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. DeSouza, J. Dupont, Poly-hedron 1996, 15, 1217 – 1219; b) J. Dupont, G. S. Fonseca, A. P. Umpierre,P. F. P. Fichtner, S. R. Teixeira, J. Am. Chem. Soc. 2002, 124, 4228 – 4229;c) G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, S. R. Teixeira, J. Dupont,Chem. Eur. J. 2003, 9, 3263 – 3269.

[9] a) V. Cal�, A. Nacci, A. Monopoli, A. Fornaro, L. Sabbatini, N. Cioffi, N. Di-taranto, Organometallics 2004, 23, 5154 – 5158; b) M. Moreno-MaÇas, R.Pleixats, Acc. Chem. Res. 2003, 36, 638 – 643; c) J. G. de Vries, DaltonTrans. 2006, 421 – 429.

[10] a) P. Dash, N. A. Dehm, R. W. J. Scott, J. Mol. Catal. A: Chem. 2008, 286,114 – 119; b) P. Dash, S. Miller, R. W. J. Scott, J. Mol. Catal. A : Chem. 2010,329, 86 – 95.

[11] a) M. Freemantle, An Introduction to Ionic Liquids, RSC Publishing, Cam-bridge, 2010 ; b) F. M. Kerton, Alternative Solvents for Green Chemistry,RSC Publishing, Cambridge, 2009 ; c) T. Welton, Green Chem. 2011, 13,225; d) J. P. Hallett, T. Welton, Chem. Rev. 2011, 111, 3508 – 3576.

[12] P. Dash, R. W. J. Scott, Chem. Commun. 2009, 812 – 814.[13] a) L. Luo, N. Yu, R. Tan, Y. Jin, D. Yin, D. Yin, Catal. Lett. 2009, 130, 489 –

495; b) A. Stojanovic, D. Kogelnig, L. Fischer, S. Hann, M. Galanski, M.Groessl, R. Krachler, B. K. Keppler, Aust. J. Chem. 2010, 63, 511 – 524;c) K. S. Kim, D. Demberelnyamba, H. Lee, Langmuir 2004, 20, 556 – 560;d) C. Vilain, F. Goettmann, A. Moores, P. Le Floch, C. Sanchez, J. Mater.Chem. 2007, 17, 3509 – 3514.

[14] L. G. S�nchez, J. R. Espel, F. Onink, G. W. Meindersma, A. B. de Haan, J.Chem. Eng. Data 2009, 54, 2803 – 2812.

[15] L. S. Ott, M. L. Cline, M. Deetlefs, K. R. Seddon, R. G. Finke, J. Am. Chem.Soc. 2005, 127, 5758 – 5759.

[16] M. G. Freire, C. M. S. S. Neves, I. M. Marrucho, J. A. P. Coutinho, A. M. Fer-nandes, J. Phys. Chem. A 2010, 114, 3744 – 3749.

[17] L. Ford, F. Atefi, R. D. Singer, P. J. Scammells, Eur. J. Org. Chem. 2011,942 – 950.

[18] M.-C. Tseng, H.-C. Kan, Y.-H. Chu, Tetrahedron Lett. 2007, 48, 9085 – 9089.[19] H. A. Kalviri, F. M. Kerton, Green Chem. 2011, 13, 681 – 686.[20] Z. Yinghuai, E. Widjaja, S. L. P. Sia, W. Zhan, K. Carpenter, J. A. Maguire,

N. S. Hosmane, M. F. Hawthorne, J. Am. Chem. Soc. 2007, 129, 6507 –6512.

[21] E. Guyonnet Bil�, R. Sassine, A. Denicourt-Nowicki, F. Launay, A. Rou-coux, Dalton Trans. 2011, 40, 6524 – 6531.

[22] M. Dasog, R. W. J. Scott, Langmuir 2007, 23, 3381 – 3387.[23] H. Bçnnemann, K. S. Nagabhushana, Surface and Nanomolecular Cataly-

sis (Ed. : R. Richards), Taylor and Francis, London 2006, Chapter 3, p. 63 –73.

[24] a) C. Hardacre, J. D. Holbrey, S. E. J. McMath, D. T. Bowron, A. K. Soper, J.Chem. Phys. 2003, 118, 273 – 278; b) P. J. Dyson, J. S. McIndoe, D. B.Zhao, Chem. Commun. 2003, 508 – 509.

[25] a) O. R. Miranda, N. R. Dollahon, T. S. Ahmadi, Cryst. Growth Des. 2006, 6,2747 – 2753; b) F. X. Chen, G. Q. Xu, T. S. A. Hor, Mater. Lett. 2003, 57,3282 – 3286.

[26] J. Kumełan, A. P.-S. Kamps, D. Tuma, G. Maurer, J. Chem. Eng. Data 2006,51, 1364 – 1367.

[27] a) J. L. Anthony, J. L. Anderson, E. J. Maginn, J. F. Brennecke, J. Phys.Chem. B 2005, 109, 6366 – 6374; b) P. J. Dyson, G. Laurenczy, C. A. Ohlin,J. Vallance, T. Welton, Chem. Commun. 2003, 2418 – 2419.

[28] L. Ferguson, P. Scovazzo, Ind. Eng. Chem. Res. 2007, 46, 1369 – 1374.[29] C. J. Bradaric, A. Downard, C. Kennedy, A. J. Robertson, Y. H. Zhou, Green

Chem. 2003, 5, 143 – 152. .[30] G. H. Woehrle, J. E. Hutchison, S. Ozkar, R. G. Finke, Turk. J. Chem. 2006,

30, 1 – 13.

Received: November 7, 2011Published online on December 15, 2011

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Ch

emistry

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ocatalysis

116 www.chemsuschem.org � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2012, 5, 109 – 116

R. W. J. Scott et al.