ARTICLE pubs.acs.org/Langmuir
Imidazolium-Based Zwitterionic Surfactant: A New Amphiphilic Pd Nanoparticle Stabilizing Agent Bruno S. Souza,† Elder C. Leopoldino,† Daniel W. Tondo,† Jairton Dupont,‡ and Faruk Nome*,† † ‡
Departamento de Química, Universidade Federal de Santa Catarina, Florianopolis, Santa Catarina 88040-900, Brazil Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Porto Alegre RS, Brazil
bS Supporting Information ABSTRACT: Palladium nanoparticles (NPs) with an average size of 3.4 nm were prepared in water using imidazolium-based surfactant 3-(1-dodecyl-3-imidazolio)propanesulfonate (ImS3-12) as a stabilizer. The Pd NPs are highly dispersible in water and chloroform and were characterized by transmission electron microscopy, energydispersive X-ray spectroscopy, powder X-ray diffraction, and dynamic light scattering. The results indicate that in water the NP surface is covered with a double layer of ImS3-12 molecules. The NPs were effective in the aqueous biphasic hydrogenation of cyclohexene, with easy recycling and no loss of catalytic activity after four successive runs.
’ INTRODUCTION The synthesis of metallic NPs (NPs) in solution is a field that has advanced rapidly in the last 20 years.1 The main focus of synthetic methods is size and shape control, together with controlled composition of the metallic surface. Because the agglomeration of the NPs is the preferred thermodynamic path in solution, a wide range of stabilizers such as halides,2 polymers,3 dendrimers,4 phosphines,5 amines,6 thiols,6b,7 surfactants,8 and ionic liquids9 are added to prevent aggregation. In the field of catalysis, different stabilizers not only avoid the undesired coalescence but can act as active spectators that may alter the catalytic properties of the metal.5,10 Different stabilizers also allow differential dispersion of the zero-valent metal in a semiheterogeneous mode11 to match the desired use, as in biological6b or biphasic catalytic applications.10,12 In the latter case, catalysis by transition-metal NPs dispersed in water allows the development of synthesis procedures with low environmental impact, easy catalyst recycling, and separation from products.12b,d,13 However, most organic reactants are insoluble in water, and low reaction rates due to slow mass transfer from the organic to the aqueous phase are usually observed. Thus, the use of surfactant molecules that provide NP stabilization in water and assist the mass-transfer process are very attractive for biphasic catalytic applications.10,12a We recently described a new class of zwitterionic surfactants based on the imidazolium ring and have shown that, although formally neutral, they acquire anionic character in electrolyte solution.14 In the work here reported, we have prepared palladium NPs in water using one selected imidazolium-based zwitterionic surfactant as a stabilizer and show that ImS3-12 forms a double layer around the Pd surface. The Pd NPs were characterized both r 2011 American Chemical Society
in solution and in the solid phase, and after drying, they are easily redispersed in water or chloroform. The catalytic activity of the NPs dispersed in water was tested using the hydrogenation of cyclohexene, and the results indicate that ImS3-12 is an effective stabilizer that allows four successive recycles with no loss of catalytic activity.
’ EXPERIMENTAL SECTION Materials. Sodium hydride, imidazole, 1-bromododecane, 2-butyloctanol, cyclohexene, K2PdCl4, and NaBH4 were of analytical grade and were used without further purification. Dioxane was dried over 3 Å molecular sieves. The synthesis of dipolar surfactants ImS3-12 and ImS3-4/8 was based on the method used for the synthesis of ImS3-1414 starting from the N-alkylation of imidazole followed by reaction with 1, 3-propane sultone. The synthesis of ImS3-4/8 involves the preparation of alkyl bromide 2 starting from alcohol 1 (Scheme 1). Synthesis of 1-Dodecylimidazole. A solution of imidazole (16.3 g, 0.24 mol) in dry 1,4-dioxane (100 mL) was added to 150 mL of a suspension of oil-free sodium hydride (5.8 g, 0.24 mol) with stirring for 2 h at 90 °C. A solution of 1-bromododecane (29.9 g, 0.12 mol) in 1, 4-dioxane (100 mL) was then added dropwise to the reaction solution, and the mixture was stirred for 48 h at 90 °C. The solvent was removed on a rotary evaporator, giving a yellow residue that was dissolved in 200 mL of Received: September 6, 2011 Revised: November 21, 2011 Published: November 29, 2011 833
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Scheme 1. Synthesis of ImS3-4/8
prepared 0.30 mol 3 L 1 NaBH4 solution in water was quickly added. The aqueous solution became immediately black and was stirred for 24 h with no sign of precipitation for a period of 1 year. Preparation of Chloroform-Soluble Palladium NPs. The original nanoparticle solution was lyophilized, giving a black powder. This powder was dispersed in 50 mL of chloroform, the resulting black dispersion was filtered through a 0.2 μm PVDF membrane, and the insoluble inorganic salts were retained in the filter as a white solid. After chloroform removal in vacuum, a black hygroscopic material was obtained that was easily redispersible in water and chloroform.
CH2Cl2 and washed three times with 50 mL of water. After the removal of CH2Cl2, the oily residue was distilled using a Kugelrohr apparatus under reduced pressure, giving 25.6 g of a colorless oil (yield 90%) that was characterized by 1H NMR (200 MHz) in CDCl3. δ (relative to TMS): 7.45 (s, 1H), 7.05 (s, 1H), 6.90 (s, 1H), 3.92 (t, 2H, J = 6.9 Hz), 1.76 (2H, m), 1.25 (18H, br s), 0.88 (t, 3H, J = 6.3 Hz).
Synthesis of 3-(1-Dodecyl-3-imidazolio)propanesulfonate (ImS3-12). A solution of 1,3-propanesultone (9.0 g, 0.074 mol) in acetone (80 mL) was slowly added in a round-bottomed flask to 1-dodecylimidazole (15.8 g, 0.067 mol) and acetone (80 mL) at 0 °C. The reaction mixture was then warmed to room temperature and stirred for 5 days. Filtration gave a white powder that was washed four times with fresh acetone, filtered, and dried under vacuum, giving 19.2 g (80.0%) of the zwitterionic ImS3-12 surfactant that was characterized by 1 H NMR (200 MHz) in CDCl3. δ (relative to TMS): 9.64 (s, 1H), 7.60 (s, 1H), 7.23 (s, 1H), 4.57 (t, 2H, J = 6.7 Hz), 4.25 (2H, t, J = 7.3 Hz), 2.85 (2H, t, J = 7.1 Hz), 2.39 (2H, quint, J = 7.1 Hz), 1.87 (2H, m), 1.25 (18 H, m), 0.88 (t, 3H, J = 6.2 Hz). Synthesis of 2-Butyl-octylbromide (2). Compound 2 was prepared by the adaptation of the method describes in ref 15. Samples of 11.16 g (0.06 mol) of 2-butyl-octanol (1) and 25 g (0.095 mol) of triphenylphosphine were dissolved in dichloromethane (40 mL) and cooled to 0 °C. N-Bromosuccinimide (16.2 g, 0.09 mol) was added, and the reaction mixture was stirred at room temperature for 12 h. The solvent was removed, and the residue was dispersed in hexane (a large amount of an orange solid did not dissolve). After solvent removal, the product was purified by column chromatography (silica gel) with hexane as the eluent, giving 14.5 g of 2 as a colorless oil (yield 95%) that was characterized by CG-MS (70 eV, Supporting Information). Synthesis of 1-(2-Butyloctyl)-imidazole (3). The synthesis and purification of 1-(2-butyloctyl)-imidazole (3) were performed as described for the synthesis of 1-dodecylimidazole using 2-butyl-octylbromide (0.05 mol) instead of 1-bromododecane. The reaction gave 11.2 g (yield 95%) of 1-(2-butyloctyl)-imidazole as a colorless oil that was characterized by GC-MS (70 eV, Supporting Information).
General Procedure for the Hydrogenation of Cyclohexene and Recycle Tests. In a Fischer Porter glass flask, 200 μL of a
nanoparticle dispersion (0.54 μmol Pd) was dispersed in 1.8 mL of H2O containing 10 mg of ImS3-4/8. This dispersion was maintained under 2 bars of H2 pressure for 1 h. After this period, 1 mL of cyclohexene (9.87 mmol) was added, giving a cyclohexene to Pd ratio of ca. 18 300. The biphasic mixture was kept at 35 °C, and the reaction system was pressurized to 2 bars with hydrogen gas, followed by pressure release through an auxiliary bleed valve. This procedure was repeated five times to ensure the absence of other gases in the reaction vessel. The system was pressurized again at 2 bars (gauge pressure), and the heterogeneous mixture was vigorously stirred with a magnetic bar at 1500 rpm. During the hydrogenation, the pressure was maintained constant and hydrogen gas was provided from a tank attached to a pressure transducer interfaced through a Novus converter to a PC. After the hydrogenation was complete, the pressure was released and the aqueous phase, containing the Pd NPs, was allowed to separate from the organic phase by standing. The organic phase was removed, the Fischer Porter flask was recharged with 1 mL of cyclohexene, and the hydrogenation reaction was repeated. An identical rate was obtained using 2 mL of cyclohexene. In all reactions, the rate of cyclohexene hydrogenation calculated from the hydrogen consumption was within 5% error of that calculated by GCMS analysis. The hydrogenation of cyclohexene with Pd/C 9.8% was conducted in a similar way using 20 mg of catalyst (18.41 μmol Pd). Reactions that followed at agitation speeds higher than 1000 rpm gave the same rate of hydrogen consumption. Thus, under our hydrogenation conditions (1500 rpm), the kinetics is probably controlled by the chemical reaction and not by the dissolution of hydrogen gas. XRD and TEM Analyses. The black solid obtained after NaCl removal was analyzed in a Siemens D500 diffractometer using a graphite crystal as a monochromator and a Cu Kα radiation source. TEM analysis was performed in a JEOL JEM-1011 transmission electron microscope operating at 100 kV at LCME/UFSC (Florianopolis, Brazil). HR-TEM analysis was performed in a JEOL JEM-3010 transmission electron microscope operating at 300 kV equipped with a Noram Voyager EDS probe at the LME/LNLS (Campinas, Brazil). The sample for TEM was prepared by the deposition of the chloroform Pd/ImS3-12 dispersion in a carbon-coated copper grid. DLS Measurements. Dynamic light scattering measurements were performed at 25 °C using a Brookhaven Instrument (90Plus/BI-MAS) with a laser operating at 657 nm and a scattering angle of 90°. The prepared Pd NP dispersion in water, containing 2.7 mM Pd, 9.09 mM
Synthesis of 3-(1-(2-Butyloctyl)-3-imidazolio)propanesulfonate (ImS3-4/8). The synthesis and purification of ImS3-4/8 were performed as described for the synthesis of ImS3-12 using 1-(2-butyloctyl)imidazole (0.04 mol) instead of 1-dodecylimidazole. The reaction gave 10.0 g (yield 70%) of the zwitterionic ImS3-4/8 as a white powder that was characterized by 1H NMR (200 MHz) in CDCl3. δ (relative to TMS): 9.58 (s, 1H), 7.69 (s, 1H), 7.15 (s, 1H), 4.59 (t, 2H, J = 6.8 Hz), 4.14 (2H, d, J = 7.1 Hz), 2.85 (2H, t, J = 6.8 Hz), 2.40 (2H, quint, J = 7.2 Hz), 1.82 (1H, m), 1.25 (16 H, m), 0.88 (t, 6H, J = 6.1 Hz).
Preparation of Palladium NPs Stabilized with ImS3-12 (Pd/ImS3-12). The Pd NPs were obtained by the chemical reduction method using NaBH4 as the reductant and K2PdCl4 as the metal precursor, following a procedure similar to that used by Vasylyev et al.10 A solution containing 1.75 mmol of ImS3-12 (627.44 mg), 14 mmol of NaCl (818.3 mg), and 0.52 mmol of K2PdCl4 (171.4 mg) in 175 mL of doubly distilled water was magnetically stirred in a 500 mL round-bottomed flask in air. Under vigorous stirring, 17.5 mL of a freshly 834
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visualization of the Pd NPs. By freeze-drying the thus-prepared NP water dispersion, a black solid was obtained. This material was characterized by XRD: sharp signals corresponding to the diffraction planes of NaCl were observed, but the reflections due to the presence of Pd nanocrystals were very weak. Thus, to characterize the Pd NPs properly by TEM and XRD, we used the chloroform-soluble Pd NPs prepared as described above. After chloroform evaporation, a black hygroscopic powder was obtained that was directly characterized by XRD and TEM. Figure 1A shows the TEM picture of the Pd/ImS3-12 NPs redispersed in chloroform. As can be seen, the NPs are well dispersed in the grid and show no sign of aggregation. The HRTEM image shown in Figure 1B indicates that the NPs are roughly spherical in shape, with a mean diameter of 3.4 ( 1.2 nm (Figure 1C) on the basis of TEM image analysis of ∼300 particles. Figure 1D shows a large nanoparticle oriented along the (200) axis, and the inset shows the fast Fourier transformation indicating an interplanar spacing of 0.19 nm, in agreement with the interplanar spacing of 0.1915 nm for the (200) plane in 4 nm palladium NPs as described by Lamber et al.18 Figure 2A shows the EDS spectrum of the NaCl-free Pd NPs where the characteristic O and S due to the ImS3-12 coating and the Pd peaks are observed. The absence of Cl peaks allows us to rule out stabilization provided by halides that were present in the synthesis procedure. The XRD spectrum of Pd/ImS3-12 is shown in Figure 2B, and four diffractions peaks were assigned as corresponding to the Pd(0) (111), (200), (220), and (311) diffraction planes.19 By means of the so-called Scherrer formula20 and assuming round particles (K = 0.9),21 we calculate an average particle size of about 3.5 nm, in excellent agreement with the TEM data. As described above, after removal of the excess salt, the Pd NPs could be easily redispersed in both chloroform and deionized water. Because ImS3-12 is not soluble in deionized water, this is strong evidence that the ImS3-12 ionic groups are strongly associated with the Pd nanoparticle surface, lowering the interactions between the imidazolium and sulfonate groups of neighboring molecules.17 Therefore, it is reasonable to assume that at the interface both cationic and anionic moieties are near the nanoparticle whereas alkyl side chains are directed away it. Thus, the colloidal stabilization mechanism is probably not a pure electrostatic double layer but a template effect due to the heterogeneous structure (polar and apolar domains)22 of the zwitterionic surfactant-like imidazolium ionic liquid. The Pd NPs stabilized by ImS3-12 exhibit distinctive behavior, being dispersible in electrolyte solution, deionized water, and chloroform, whereas alkanothiolate-, alkylphosphine-, and alkylamine-protected NPs do not usually exhibit this amphiphilic behavior.23 Water-dispersible Pd NPs stabilized by other dipolar ionic surfactants, such as sulfobetaine 124 and, more recently, phosphine 2,25 have been reported (Scheme 2), although it is suggested that Pd NPs stabilized by 1 are not stable when the [surfactant] content falls below the critical micelle concentration.26 It is worth noting that, although frequently waterdispersible NPs are unstable in electrolyte solution,12c,25,27 the use of ImS3-12 as a protective agent prevents particle aggregation in electrolyte solution. Under our experimental conditions, ImS3-12 forms aqueous micelles that readily incorporate [PdCl4]2 into the Stern layer of the zwitterionic micelle as previously shown for the binding of a series of anions to the ImS3-14 surfactant.14 Figure 3 shows UV vis spectral changes of a solution containing 1 mM K2PdCl4
Figure 1. Transmission electron microscopy micrographs at (A) 100 and (B) 300 kV of the Pd/ImS3-12 NPs. (C) Size distribution of the prepared NPs. (D) HR-TEM image of a single nanoparticle oriented along the (200) axis. The inset shows the FFT image where is possible to see the spots corresponding to an interplanar spacing of 0.19 nm. ImS3-12, and 72 mM NaCl, was diluted 10-fold and carefully filtered twice through a 0.20 μm PVDF membrane before the measurements. The reported particle size corresponds to the number distribution determined by the non-negative least-squares algorithm implemented in the instrument, which considers the difference between the refractive index of the sample and the solvent, the temperature, and the scattering angle. Because the Pd nanoparticles absorb light at 657 nm, it is necessary to take into account the use of the real (n) and imaginary (k) refractive indexes of Pd, which were taken from the Handbook of Optical Constants of Solids.16 Experiments carried out using different particle concentrations afforded similar results.
’ RESULTS AND DISCUSSION Palladium NPs stabilized by zwitterionic surfactant ImS3-12 were prepared by the chemical reduction method in water using excess NaBH4. As previously reported for tetradecyl zwitterionic surfactant ImS3-14,14 ImS3-12 is insoluble in pure water and becomes soluble in electrolyte solution. In the solid state, ionic interactions and strong hydrogen bonding occurring between the sulfonate group of one surfactant and the imidazolium group of another17 are strong enough to preclude dissolution. In aqueous electrolyte solution, these interactions are reduced and imidazolium-based surfactant ImS3-12 is readily soluble. Thus, the synthesis of the Pd NPs stabilized by the zwitterionic surfactant was performed in aqueous 80 mM NaCl solution. The optimum ratio between surfactant ImS3-12 and the Pd2+ salt was found to be 3:1 with no sign of precipitation even after 1 year under ambient conditions. When the ImS3-12 to Pd2+ ratio was reduced to 2:1, the formation of Pd black was observed at the bottom of the reaction flask. Because of the large amount of NaCl present in the Pd/ImS3-12 water dispersion, attempts to characterize the NP water dispersion directly by TEM afforded poor images, and NaCl microcrystals were observed to obstruct the 835
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Figure 2. (A) EDS spectrum of the Pd/ImS3-12 NPs. The arrow indicates the position of the chlorine signal that is clearly absent in the sample. The Cu peak arises from the grid, and the Si peak arises from X-ray emission from the EDS detector. (B) XRD spectrum of Pd/ImS3-12 NPs showing the characteristic reflections of the (111), (200), (220), and (311) planes. The XRD spectrum of the palladium bulk is also shown for comparison.
Scheme 2. Dipolar Stabilizers Employed in Water-Soluble NP Preparation
caused by increasing the ImS3-12 concentration. As can be seen, the addition of ImS3-12 leads to an increase in the molar absorptivity of Pd2+ at 344 nm, and this effect occurs only after the cmc of ImS3-12 has been reached. For comparison, Figure 3 also shows the variation of UV vis absorbance at different ImS312 concentrations in the absence of Pd2+. These results clearly show that the [PdCl4]2 complex is readily incorporated into ImS3-12 micelles whereas no evident association occurs between [PdCl4]2 and ImS3-12 monomers (i.e., before micelle formation). Upon addition of NaBH4, the Pd2+ salt is reduced to metallic Pd, which spontaneously aggregates on the ImS3-12 micelle surface. Because a single micelle carries on average about 60 monomers, it is expected to incorporate ca. 20 [PdCl4]2 anions, which is clearly not enough to allow for the formation of the detected Pd nanoparticles (average diameter 3.4 nm), and several micelles must participate in the process. In water, the most common mechanism of stabilization provided by surfactant molecules is the formation of a double layer around the NPs, preventing particle coalescence.28 The adhesion of the surfactant headgroup is fundamental to colloidal stability, and the formation of the double layer around the nanoparticles occurs in a single spontaneous process, with both layers growing simultaneously. Therefore, the proposed mechanism of formation and stabilization of Pd NPs stabilized by ImS3-12 is shown in Scheme 3 and avoids the formation of a monolayer, which would be unstable in aqueous solutions, with the hydrophobic tail in the direction of the bulk aqueous phase. Although the EDS spectrum indicates the presence of sulfur atoms, it is difficult to visualize the double-layer packing on the surface of Pd NPs directly from TEM micrographs (Figure 1), which in most cases provides reliable information regarding the metal core.29 Information about the presence of the surfactant is implicit in the EDS spectrum, and somewhat more detailed information can be inferred from the TGA experiments (see below).
Figure 3. Absorbance at 344 nm as a function of [ImS3-12] in the (b) absence and (9) presence of 1 mM K2PdCl4. For both experiments, [NaCl] = 80 mM and 25 °C.
Pure ImS3-12 surfactant is soluble in chloroform or electrolyte solution but is not soluble in deionized water. The amphiphilic behavior or ImS3-12 also makes Pd NPs dispersible in either polar or apolar solvents. Because the surfactant interacts with the nanoparticle surface via the ionic group(s), the alkyl chain must point outward from the nanoparticle surface. NMR experiments in CDCl3 (Figure S1 and Table S1) show that there is a strong interaction between the Pd NPs and the surfactant headgroup. Changes in 1H chemical shifts of ImS3-12 show that the resonances corresponding to the imidazolium hydrogens and those of CH2 in the tether are those affected significantly by the Pd NPs. The large, broad main-chain resonance at δ = 1.25 and the ω terminal methyl group resonance (0.88 ppm) are not affected. As a consequence of the preferential orientation of the surfactant, the main-chain methylenes and the ω methyl group are outside the zone affected by the Pd NPs. These results are strongly indicative that in this particular case the metallic surface is more hydrophilic in nature, favoring binding of the surfactant headgroup. Thus, most probably, in chloroform the outer layer of the Pd NPs is detached from the external layer of the Pd NPs and the 836
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Scheme 3. Aqueous Formation and Stabilization of Pd Nanoparticles by a Double Layer of ImS3-12
Figure 4. (A) Size distribution of Pd NPs as determined by DLS in 7.2 mM NaCl solution. (B) Cartoon depicting the stabilization of the Pd nanoparticle by a double layer of ImS3-12.
ImS3-12 surfactant is dissolved in the bulk solvent either as a monomer or reverse micelles. As a result, the Pd NPs remain stabilized by a monolayer coating in the organic solvent. In fact, after solvent removal from a clear Pd/ImS3-12 NP dispersion in chloroform, a black solid is obtained and again this material is easily dispersed in water. Thus, the double-layer coating has a dynamic nature, and in water the surfactant double layer in the NP surface is essential to avoiding particle agglomeration. To address the double-layer formation, we investigated the Pd/ImS3-12 NPs by dynamic light scattering (DLS), and Figure 4A shows the distribution of diameters obtained for a dispersion in 7.2 mM NaCl solution. As can be seen, DLS yields an average diameter of about 16 nm, which is considerably larger than the 3.4 nm measured by TEM and XRD. Because the size of one ImS3-12, estimated using ab initio methods (Figure S2 in the ESI), is about 2.1 nm, the DLS results are in good agreement with the model depicted in Figure 4B, where a double layer of ImS3-12 covers the metal surface, allowing stabilization in water. Aqueous solutions of 0.05 M ImS3-12 surfactant in the presence of added salts (NaCl or NaClO4) show small spherical micelles with a hydrodynamic radius of 2.4 nm. A common technique employed for the characterization of bilayer structures is thermogravimetric analysis (TGA), and the decomposition curves for pure ImS3-12 and chloroform-soluble
Pd/ImS3-12 NPs (see above) are shown in Figure 5. The curve obtained for the Pd NPs (Figure 5A) shows a complex decomposition mechanism with three weight losses (i.e., 300 380 °C, 55%; 380 460 °C, 20%; and 580 900 °C, 10%). In contrast, the thermogravimetric profile of pure ImS3-12 (Figure 5B) shows two weight losses (310 410 °C, 95%; and 410 700 °C, 5%). The complex weight loss profile shown in Figure 5A is characteristic of bilayer-coated NPs and similar to those reported for water-dispersible nanostructures stabilized by surfactants.28 Although a thermogravimetric profile for Pd NPs alone without ImS3-12 would be useful in discussing the coverage of ImS3-12, in the absence of surfactant we cannot isolate any type of Pd NPs; therefore, this particular control experiment cannot be done. For Pd/ImS3-12 NPs, the first weight loss occurring from 300 to 380 °C is probably related to the desorption of the secondary (outer) layer of ImS3-12 and uncoordinated surfactant present as micelles or in monomeric form (see above). The observed weight loss occurring above 380 °C is most probably related to the decomposition of the inner (ImS3-12 coordinated to the Pd nanoparticle) layer. In fact, our results are very similar to those obtained by Roucoux et al. for rhodium nanoparticles stabilized by a bilayer of cationic surfactants.12d The catalytic activity of the Pd NPs was probed using the hydrogenation of cyclohexene, which was carried out using 1 mL 837
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Figure 5. Thermogravimetric profiles of (A) the Pd NPs stabilized by ImS3-12 and (B) pure ImS3-12. Also shown is the derivative of the weight loss per minute (DTGA).
Figure 6. (A) Effect of addition of ImS3-12 on the electronic absorption spectrum of Pd2+ in the presence of (9) 2 and (•) 5 mM ImS3-4/8. For both experiments, [K2PdCl4] = 1 mM, [NaCl] = 80 mM, and T = 25 °C. The insets are expanded views of the cmc regions. (B) Cartoon depicting the doublelayer packing with ImS3-12 and ImS3-4/8 around a Pd nanoparticle.
of cyclohexene and 200 μL of Pd/ImS3-12 dispersed in 1.8 mL of water, rendering a biphasic system. The initial cyclohexene to Pd ratio was 18 300, and the reaction was conducted at a constant hydrogen pressure of 2 bars at 35 °C. Hydrogenation conducted using the as-prepared Pd NPs (i.e., those stabilized with 3 equiv of ImS3-12) led to formation of Pd black and a drastic fall in catalytic activity during the first catalytic cycle. To increase the nanoparticle stability and avoid aggregation, another 10 equiv of ImS3-12 was added at the beginning of the reaction. Under these conditions, there is no precipitation of Pd black, though cyclohexene is hydrogenated slowly. At the end of the reaction (48 h), an emulsion had formed and the aqueous and organic phases could not be separated on standing. We repeated the hydrogenation reaction by adding 10 mg of ImS3-4/8 as a costabilizer to a fresh dispersion of 200 μL of Pd/ImS3-12 in 1.8 mL of water. At the end of cyclohexene hydrogenation, no sign of Pd aggregation was observed and the organic products could be conveniently separated from the catalyst. It should be noted that surface tension measurements
show that ImS3-4/8 does not form micelles even at 0.01 M (Figure S3), which is its maximum solubility in water at 25 °C. The addition of ImS3-4/8 to 1 mM K2PdCl4 was examined by following UV vis changes (Figure S4), with the results showing no interaction between the organic additive and the tetrachlopalladate dianion. However, when we follow the incorporation of the [PdCl4]2 complex into ImS3-12 micelles in the presence of different amounts of ImS3-4/8 (Figure 6A), we can clearly see that the cmc is markedly reduced from 1.2 mM in the absence of ImS3-4/8 (Figure 3) to 0.8 and 0.3 mM in the presence of 2 and 5 mM ImS3-4/8, respectively. Ab initio calculations of the structure ImS3-4/8 will not help us to understand the stabilization mechanism, and the important effects, shown in Figure 6, reflected in the decrease in the cmc of the ImS3-12 surfactant caused by the addition of ImS3-4/8 suggest that the zwitterionic additive most probably favors micellar packing in aqueous solutions and will be expected to be incorporated into the double layer around the Pd NPs, thus increasing the colloid stabilization (Figure 6B). 838
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zwitterionic imidazolium-based surfactants is very promising for biphasic NP catalysis because it provides both NP stabilization and good mass-transfer rates for apolar compounds. Indeed, the zwitterionic imidazolium-based material combines the structural organization of the ionic liquids with the polar and polar domains of concentrated surfactant media and thus opens a new opportunity for the generation of tailor-made amphiphilic colloidal stabilizing agents for catalysis and separation processes.
’ ASSOCIATED CONTENT
bS
Figure 7. Hydrogenation of 1 mL of cyclohexene using 0.54 μmol Pd NPs stabilized with ImS3-12 and ImS3-4/8. Reaction conditions: 2 bars of H2, 35 °C, 10 mg of ImS3-4/8, and 1500 rpm. Also shown is the biphasic system containing cyclohexane and the Pd NPs in the upper and lower phases, respectively.
Figure 7 illustrates the kinetics of conversion of cyclohexene to cyclohexane using Pd NPs. As can be seen, no induction period is observed and from 0 to 75% conversion the reaction is zeroth order in cyclohexene. The calculated TOF is almost the same for all reactions, and a mean value of 1000 h 1 was calculated in the linear region of the graph (see linear fittings). For comparison, the hydrogenation of cyclohexene using conventional Pd/C under the same reaction conditions gave a TOF of 430 h 1. Figure 7 also shows a picture of the biphasic mixture at the end of the fourth hydrogenation run after 10 min of standing, which shows the excellent phase separation. After the fourth catalytic cycle, the aqueous phase (showing no sign of aggregation) was analyzed by TEM, and the results show that the nanoparticles containing both ImS3-12 and ImS3-4/8 are slightly smaller and are less polydisperse than the nanoparticles containing only ImS3-12 and prepared as described in Figure 1 (Supporting Information, Figures S5 and S6). It is important to notice that after four catalytic cycles the TOF number remains essentially constant and, accordingly, significant migration of Pd from the aqueous to the organic phase can be neglected and any eventual leakage does not affect the catalytic activity.
1
H NMR of the Pd/ImS3-12 NPs, UV vis spectral changes, Cartesian coordinates of ImS312, surface tension measurements, TEM picture of the Pd nanoparticles after the fourth catalytic run, and characterization of all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful to INCT-Catalise, PRONEX, FAPESC, CNPq, and CAPES for their support of this work and to LME/LNLS and LCME/UFSC for technical support during electron microscopy work. We also thank Prof. Paulo A. Suarez for his valuable comments on the thermogravimetric analysis. ’ REFERENCES (1) (a) Astruc, D. Nanoparticles and Catalysis; Wiley-VCH: Weinheim, Germany, 2008. (b) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001.(c) Rothenberg, G. Catalysis: Concepts and Green Applications; Wiley-VCH: Weinheim, Germany, 2008. (2) Ott, L. S.; Cline, M. L.; Finke, R. G. J. Nanosci. Nanotechnol. 2007, 7, 2400–2410. (3) Hirai, H.; Yakura, N.; Seta, Y.; Hodoshima, S. React. Funct. Polym. 1998, 37, 121–131. (4) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 8572–8580. (5) Tamura, M.; Fujihara, H. J. Am. Chem. Soc. 2003, 125, 15742–15743. (6) (a) Rao, C. R. K.; Lakshminarayanan, V.; Trivedi, D. C. Mater. Lett. 2006, 60, 3165–3169. (b) Manea, F.; Bindoli, C.; Polizzi, S.; Lay, L.; Scrimin, P. Langmuir 2008, 24, 4120–4124. (7) Shen, C. M.; Su, Y. K.; Yang, H. T.; Yang, T. Z.; Gao, H. J. Chem. Phys. Lett. 2003, 373, 39–45. (8) (a) Bonnemann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Tilling, A. S.; Seevogel, K.; Siepen, K. J. Organomet. Chem. 1996, 520, 143–162. (b) Wu, S. H.; Chen, D. H. Chem. Lett. 2004, 33, 406–407. (c) Yang, C. C.; Wan, C. C.; Wang, Y. Y. J. Colloid Interface Sci. 2004, 279, 433–439. (9) (a) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316–5317. (b) Redel, E.; Thomann, R.; Janiak, C. Inorg. Chem. 2008, 47, 14–16. (c) Dupont, J.; Scholten, J. D. Chem. Soc. Rev. 2010, 39, 1780–1804. (d) Gutel, T.; Santini, C. C.; Philippot, K.; Padua, A.; Pelzer, K.; Chaudret, B.; Chauvin, Y.; Basset, J. M. J. Mater. Chem. 2009, 19, 3624–3631. (10) Vasylyev, M. V.; Maayan, G.; Hovav, Y.; Haimov, A.; Neumann, R. Org. Lett. 2006, 8, 5445–5448. (11) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852–7872.
’ CONCLUSIONS The present work shows that very stable NPs can be prepared in water by a simple method using a zwitterionic imidazoliumbased surfactant as a stabilizer. The NPs have been characterized in the solid phase and in aqueous solution, and the results indicate that stabilization is provided by the formation of a double layer of ImS3-12 molecules around the Pd surface. However, the colloidal stabilization mechanism is probably not a pure electrostatic double layer but is a template effect due to the heterogeneous structure (polar and nonpolar domains) of ImS3-12. The NPs were used for the biphasic hydrogenation of cyclohexene. The use of 3 equiv of ImS3-12 per Pd atom proved not to be enough to provide NP stability to the catalytic application. When the NPs were further stabilized with additional ImS3-4/8, good conversion to cyclohexane was obtained and the catalyst could be recovered four times without a loss of catalytic activity by simple phase separation. The class of 839
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