Stabilizing nanoparticle catalysts in imidazolium-based ionic liquids: A comparative study

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Abstract

Room temperature imidazolium-based ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) have been used as effective liquid media for the synthesis of pure Au and bimetallic PdAu nanoparticles by direct synthesis and phase-transfer methods. The mode-of-stability, long-term stability, and long lifetimes of these ionic-liquid supported nanoparticle catalysts, all of which are important factors in determining the overall “greenness” of such materials, were investigated. Four different stabilizing systems in BMIMPF6 ILs were investigated: poly(vinylpyrrolidone) (PVP), 1-methylimidazole, 1-(2′-aminoethyl)-3-methylimidazolium hexafluorophosphate, and pure BMIMPF6 IL with the absence of a secondary stabilizer. The stability of pure Au nanoparticles synthesized by the above four stabilizers was studied using UV–vis spectroscopy and transmission electron microscopy (TEM). It was found that PVP-stabilized nanoparticles were the most stable to aggregation. The catalytic activity of the resulting PdAu nanoparticles was examined for the hydrogenation of 1,3-cyclooctadiene and 3-buten-1-ol across all of the systems to understand which stabilizer(s) are most optimal for nanoparticle catalyst synthesis and usage; particularly which systems have high catalytic activity and selectivity as well as long catalyst lifetimes. In agreement with Au nanoparticle stability results, PVP-stabilized PdAu nanoparticles were the most catalytically active due to improved nanoparticle stability, followed by nanoparticles stabilized by 1-methylimidazole, amine-functionalized IL, and the pure BMIMPF6 IL.

Graphical abstract

A comparative study of the stability and long-term catalytic activity of Au and bimetallic PdAu nanoparticles in imidazolium-based ionic liquids via four different stabilization methods.

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Introduction

Due to their unique physiochemical properties such as high polarity, excellent thermal stability and negligible vapor pressures, room temperature ionic liquids (ILs) have provided an opportunity for chemists to studying reactions in an unique reaction media [1], [2], [3]. In the past few years ILs have also proven to be excellent solvents for the immobilization and stabilization of metal nanoparticles, thus providing an excellent media for quasi-homogeneous catalysis [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. In particular, 1-butyl-3-methylimidazolium (BMIM)-based ILs have emerged as an effective media for the stabilization of nanoparticles [4], [5], [6], [7], [14], [17], [18], [19], [20], [21], [22]. Though there have been reports of the synthesis of gold and bimetallic nanoparticles in pure BMIM ILs without any additional stabilizers [17], [18], [19], a number of groups also reported the aggregation of gold nanoparticles in pure ILs [16], [23], [24]. To avoid these problems various secondary stabilizers such as poly(vinylpyrrolidone) (PVP) [25], [26], task-specific ILs [27], [28], [29], [30], [31], [32], [33], [34], and ionic liquid copolymers [35] have also been used for the synthesis of stable metal nanoparticles in ILs. In addition, our group has recently shown that low levels of 1-methylimidazole additives have dramatic effects on the stability of Au and bimetallic PdAu nanoparticles in imidazolium-based ionic liquids [36]. Thus while there have been a large number of different routes to synthesize and stabilize nanoparticle catalysts in ILs, to our knowledge there have been no comparative studies documenting the relative stability of nanoparticles using stabilizers in ILs or the effective relative activities and lifetimes of such nanoparticle/IL mixtures. This work is similar in nature to earlier pioneering work by El-Sayed and co-workers who showed that varying the type of stabilizer and the stabilizer/metal ratios had a profound effect on the activity and stability of Pt nanoparticles for Suzuki reactions in aqueous media, with the highest activities over short time periods often associated with particles which have the poorest long-term stabilities [37], [38], [39].

Deshmukh et al. were the first to show the formation of Pd nanoparticles in 1,3-di-n-butylimidazolium tetrafluoroborate ILs during Heck reactions [13]. Dupont and co-workers synthesized stable transition metal nanoparticles in pure imidazolium-based ILs without any secondary stabilizer, and the resulting nanoparticle catalysts were found to be active for a range of hydrogenation reactions [4], [5], [6], [7], [14], [20]. Gomez and co-workers synthesized stable Pd nanoparticles stabilized by pure ILs and showed that the resulting nanoparticles had higher catalytic activity for a variety of Suzuki C–C cross-coupling and sequential reactions [40], [41], [42]. Several groups have indicated that the stabilization of nanoparticles in ILs may be due to weak anion or cation interactions with nanoparticle surfaces [7], [8], [15], [16], while others have noted that impurities such as halides and water could also have significant effects on nanoparticle stability [11], [43]. Recently, we have shown that millimolar levels of 1-methylimidazole, a common starting material in imidazolium-based IL syntheses, can have dramatic effects on the stability of nanoparticles in ILs, which was a previously undocumented mode of stabilization [36]. In addition, we previously showed that highly stable Au, Pd and bimetallic PdAu nanoparticles can be synthesized by a simple phase-transfer method of PVP-stabilized nanoparticles from methanol to ILs, and that the resulting nanoparticles were catalytically active for a range of hydrogenation reactions [26].

Other groups have focused on the synthesis of task-specific ILs, which have specific functional groups attached to the imidazolium cations [30], [31], [44]. Such task-specific ILs have been shown to lead to enhanced nanoparticle stability and thus can be used to enhance catalytic activity of nanoparticles in ILs [27], [28], [29], [30], [32], [33], [34], [45]. For example, Kim et al. have documented the synthesis of gold and platinum nanoparticles using thiol-functionalized ILs which bind to the nanoparticle surface and act as stabilizers [28]. Similarly, nitrile-functionalized ionic liquids have been used as a stabilizer to prevent agglomeration of Pd nanoparticles for the Stille coupling reaction between iodobenzene and tributylphenyltin, and were found to prevent catalyst deactivation [33]. Recently, Niu and co-workers synthesized stable Au nanoparticles using a functionalized IL, 1-(3-aminopropyl)-3-methylimidazolium bromide, and the resulting nanoparticles showed enhanced electrocatalytic activity and high stability [32]. Intrigued by this work, we chose to use a similar amine-functionalized task-specific IL as a stabilizer/surfactant dissolved in BMIMPF6 for the synthesis of Au and bimetallic PdAu nanoparticles, and compare this stabilizer with other known methods for the stabilization of nanoparticles in ILs.

Given the numerous routes towards the synthesis of “stable” nanoparticles in ILs which exist in the literature, it can be very difficult to generalize as to which method(s) can lead to the optimal formation of stable, catalytically active nanoparticles. Thus, in order to compare a variety of stabilization methodologies, we synthesized pure Au and bimetallic PdAu nanoparticles in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) ILs by four different methods. Nanoparticles were synthesized in BMIMPF6 ILs by either direct synthesis or phase-transfer methods. In three different systems, PVP, 1-methylimidazole, and 1-(2′-aminoethyl)-3-methylimidazolium hexafluorophosphate (aemimPF6) were added as secondary stabilizers while in the fourth system nanoparticles were directly synthesized in BMIMPF6 ILs without any additional stabilizer. The long-term stability of pure Au nanoparticles in each of these systems was studied by UV–vis spectroscopy and transmission electron microscopy (TEM). Finally, in order to understand the effect that nanoparticle stability has on catalytic activity and catalyst lifetimes, bimetallic PdAu nanoparticles were synthesized by the four above-mentioned methods. Hydrogenation of two substrates, 1,3-cyclooctadiene and 3-buten-1-ol, was carried out to monitor their catalytic activity, and 1H NMR spectroscopy was used to measure the activity in terms of turnover number (mol product/mol catalysts).

Section snippets

Materials

1-Methylimidazole (99%) and 1-chlorobutane (99.5%) were purchased from Alfa and were distilled over KOH and P2O5, respectively, before use. Hexafluorophosphoric acid (ca. 65% solution in water), poly(vinylpyrrolidone) (M.W. 40,000), hydrogen tetrachloroaurate hydrate (99.9%), potassium tetrachloropalladate (99.99%), 3-buten-1-ol (98+%), and ethylenediamine (99%), and N-(2-bromoethyl)phthalimide (98+%) were purchased from Alfa and were used without further purification. 1,3-Cyclooctadiene (95%)

Results and discussion

Unraveling the possible modes of stabilization of nanoparticles in ionic liquids is a considerable challenge, particularly given the unique nature of the medium. In order to most effectively probe nanoparticle aggregation in solution, Au nanoparticles were chosen as the first system to investigate. The dipole–dipole interactions between plasmon bands of aggregated gold nanoparticles is manifested as a shift of the plasmon band wavelength, and thus a change in colour of the solution from red to

Nanoparticle catalysis

Having studied the stability of Au nanoparticles in BMIMPF6 by the four different stabilization strategies above, we wished to further investigate whether these findings would influence the catalyst lifetime of nanoparticles in BMIMPF6 as well. In order to do this, we chose to investigate bimetallic 3:1 PdAu nanoparticles as catalysts for the hydrogenation of 1,3-cyclooctadiene and 3-buten-1-ol. We have previously shown that PVP-stabilized PdAu bimetallic nanoparticles in BMIMPF6 are active

Conclusions

We have described four stabilization protocols for nanoparticle stabilization in BMIMPF6 ILs, and have shown that nanoparticle stability and thus catalytic activity of nanoparticles is dependent on the overall stability of the nanoparticles towards aggregation. The four different stabilization methods in BMIMPF6 which were used include the synthesis of nanoparticles in pure ILs, and the addition of secondary PVP, 1-methylimidazole, and aemimPF6 stabilizers. The activity and lifetimes of 3:1

Acknowledgements

The authors would like to acknowledge financial assistance from the National Sciences and Engineering Research Council of Canada (NSERC), and Prof. Ian Burgess for helpful discussions.

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