Micelle-Hosted Palladium Nanoparticles Catalyze Citral Molecule

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Langmuir 2004, 20, 8537-8545

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Micelle-Hosted Palladium Nanoparticles Catalyze Citral Molecule Hydrogenation in Supercritical Carbon Dioxide Pascal Meric, Kai Man K. Yu, and Shik Chi Tsang* Surface and Catalysis Research Centre, School of Chemistry, University of Reading, Whiteknights, Reading, Berkshire RG6 6AD, U.K. Received February 20, 2004. In Final Form: May 29, 2004 A new approach of employing metal particles in micelles for the hydrogenation of organic molecules in the presence of fluorinated surfactant and water in supercritical carbon dioxide has very recently been introduced. This is allegedly to deliver many advantages for carrying out catalysis including the use of supercritical carbon dioxide (scCO2) as a greener solvent. Following this preliminary account, the present work aims to provide direct visual evidence on the formation of metal microemulsions and to investigate whether metal located in the soft micellar assemblies could affect reaction selectivity. Synthesis of Pd nanoparticles in perfluorohydrocarboxylate anionic micelles in scCO2 is therefore carried out in a stainless steel batch reactor at 40 °C and in a 150 bar CO2/H2 mixture. Homogeneous dispersion of the microemulsion containing Pd nanoparticles in scCO2 is observed through a sapphire window reactor at W0 ratios (molar water-to-surfactant ratios) ranging from 2 to 30. It is also evidenced that the use of micelle assemblies as new metal catalyst nanocarriers could indeed exert a great influence on product selectivity. The hydrogenation of a citral molecule that contains three reducible groups (aldehyde, double bonds at the 2,3-position and the 6,7-position) is studied. An unusually high selectivity toward citronellal (a high regioselectivity toward the reduction of the 2,3-unsaturation) is observed in supercritical carbon dioxide. On the other hand, when the catalysis is carried out in the conventional liquid or vapor phase over the same reaction time, total hydrogenation of the two double bonds is achieved. It is thought that the high kinetic reluctance for double bond hydrogenation of the citral molecule at the hydrophobic end (the 6,7position) is due to the unique micelle environment that is in close proximity to the metal surface in supercritical carbon dioxide that guides a head-on attack of the molecule toward the core metal particle.

Introduction The hydrogenation of organic compounds is a very important chemical process. Among all the hydrogenation reactions reported, the hydrogenation of R,β-unsaturated aldehydes is receiving the most current attention, as the hydrogenation of these compounds is of both industrial and fundamental importance.1,2 Within the R,β-unsaturated family, the hydrogenation of citral is an important reaction for its relevance to the perfumery industry.3,4 Citral can also be regarded as a model substrate for hydrogenation, as it possesses three different hydrogenation locations, namely, the isolated double bond at the hydrophobic end, the conjugated double bond at the hydrophilic end, and the carbonyl end group at the hydrophilic end. The possible reaction pathways of citral hydrogenation are presented in Scheme 1. It is noted from the scheme that a large variety of partially and fully hydrogenated products can be formed from the hydrogenation of this molecule under different conditions, but citronellal and citronellol are of the most useful products/ intermediates. As a result, it is desirable to obtain a high selectivity toward either one of these two products. In the past, considerable attention has been devoted toward studying the hydrogenation of this molecule under different conditions. This includes the hydrogenation carried out in the vapor phase5,6 and in the liquid phase with * Corresponding author. E-mail: [email protected]. (1) Gallezot, P.; Richard, D. Catal. Rev.sSci. Eng. 1998, 40, 81. (2) Claus, P. Top. Catal. 1998, 5, 51. (3) Salmi, T.; Maki-Arvela, P.; Toukoniitty, E.; Kalantar Neyestanaki, A.; Tiainen, L. P.; Lindfors, L. E.; Sjo¨holm, R.; Laine, E. Appl. Catal., A 2000, 196, 93. (4) De Simone, R. S.; Gradeff, P. S. U.S. Patent 4,029,709, 1977. (5) Sen, B.; Vannice, M. A. J. Catal. 1989, 115, 65. (6) Marinelli, T. B. L. W.; Nabuurs, S.; Ponec, V. J. Catal. 1995, 151, 431.

different solvents and cosolvents used.7,8 A large variety of catalysts of a different chemical nature have also been extensively investigated,9 which included the use of promoted and unpromoted metals/alloys,10-12 metal oxides,13,14 microporous supports,15 polymer fiber catalysts,16 and so forth. It has been shown that the selectivity of the reaction depends on some key parameters such as the type of metal and its particle size,17 the catalyst support,18-20 and the type of promoters/additives20-22. It is important to point out that the reaction selectivity and productivity toward desirable product(s) have been the (7) Mercadante, L.; Neri, G.; Milone, C.; Donato, A.; Galvagno, S. J. Mol. Catal. A: Chem. 1996, 105, 93. (8) Neri, G.; Mercadante, L.; Milone, C.; Pietropaolo, R.; Galvagno, S. J. Mol. Catal. A: Chem. 1996, 108, 41. (9) Gallezot, P.; Girior-Fendler, A.; Richard, D. In Catalysis of Organic Reactions; Pasco, W., Ed.; Dekker: New York, 1991; p 1. (10) Coq, B.; Figueras, F.; Geneste, P.; Moreau, C.; Moreau, P.; Warawdekar, M. G. J. Mol. Catal. 1991, 78, 211. (11) Richard, D.; Ockleford, J.; Girior-Fendler, A.; Gallezot, P. Catal. Lett. 1989, 3, 53. (12) Sordelli, L.; Psaro, R.; Vlaic, G.; Cepparo, A.; Recchia, S.; Dossi, C.; Fusi, A.; Zanoni, R. J. Catal. 1999, 182, 186. (13) Coq, B.; Kumbhar, P. S.; Moreau, C.; Moreau, P.; Warawdekar, M. G. J. Mol. Catal. 1993, 85, 215. (14) Hubaut, R.; Bonnelle, J. P.; Daage, M. J. Mol. Catal. 1989, 55, 170. (15) Gallezot, P.; Girior-Fendler, A.; Richard, D. Catal. Lett. 1990, 5, 169. (16) Aumo, J.; Lijla, J.; Maki-Arvela, P.; Salmi, T.; Sundell, M.; Vainio, H.; Murzin, D. Y. Catal. Lett. 2002, 84, 219. (17) Galvagno, S.; Milone, C.; Donato, A.; Neri, G.; Pietropaolo, R. Catal. Today 1993, 18, 349. (18) Singh, U. K.; Vannice, M. A. J. Catal. 2001, 199, 73. (19) Malathi, R.; Viswanath, R. P. Appl. Catal., A 2001, 208, 323. (20) Consonni, M.; Jokic, D.; Murzin, D. Y.; Touroude, R. J. Catal. 1999, 188, 165. (21) Poltarzewski, Z.; Galvagno, S.; Pietropaolo, R.; Staiti, P. J. Catal. 1986, 102, 190. (22) Aramendia, M. A.; Borau, V.; Jime´nez, C.; Marinas, J. M.; Porras, A.; Urbano, F. J. J. Catal. 1997, 172, 46.

10.1021/la049549s CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004

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Scheme 1. Reaction Pathways of Citral Hydrogenation

chief criteria for determining the reaction conditions (temperature, pressure, solvent, additive, etc.) and choice of catalysts. However, during the past decade, the progressive expansion of green chemistry, linked to the necessary substitution of environmentally friendly processes for polluting processes, led to a rapid exploration of alternative chemistry systems. Thus, many existing chemical processes have recently been reassessed/reconsidered with respect to their environmental impacts on society. Attention has been particularly paid to the use of supercritical carbon dioxide (scCO2) as a new but clean solvent medium to replace conventional organic solvent(s). Its ability to dissolve gases such as dihydrogen or dioxygen facilitates its use for hydrogenation and oxidation reactions, which could also lead to an elimination of interphase mass transfer limitations that are commonly encountered during catalysis in the liquid phase. As a result, environmental benefits could be substantiated with an enhancement in reaction rate/productivity.23 Thus, scCO2 has recently received considerable attention for catalytic reactions such as hydroformylation,24,25 oxidation,26-30 and hydrogenation.31-42 In addition, the use of (23) Baiker, A. Chem. Rev. 1999, 99, 453. (24) Klinger, R. J.; Rathke, J. W. J. Am. Chem. Soc. 1994, 116, 4772. (25) Schmid, L.; Schneider, M. S.; Engel, D.; Baiker, A. Catal. Lett. 2003, 88, 105. (26) Steele, A. M.; Zhu, J.; Tsang, S. C. Catal. Lett. 2000, 73, 9. (27) Zhu, J.; Robertson, A.; Tsang, S. C. Chem. Commun. 2002, 18, 2044. (28) Jenzer, G.; Mallat, T.; Baiker, A. Catal. Lett. 2001, 73, 5. (29) Jenzer, G.; Schneider, M. S.; Wandeler, R.; Mallat, T.; Baiker, A. J. Catal. 2001, 199, 141. (30) Jenzer, G.; Sueur, D.; Mallat, T.; Baiker, A. Chem. Commun. 2000, 2247. (31) Minder, B.; Mallat, T.; Pickel, K. H.; Steiner, K.; Baiker, A. Catal. Lett. 1995, 34, 1. (32) Hitzler, M. G.; Poliakoff, M. Chem. Commun. 1997, 17, 1667. (33) Pillai, U.-R.; Sahle-Demessie, E. Chem. Commun. 2002, 5, 422. (34) Minder, B.; Mallat, T.; Pickel, K. H.; Steiner, K.; Baiker, A. Catal. Lett. 1995, 34, 1. (35) Zhao, F.; Ikushima, Y.; Chatterjee, M.; Shirai, M.; Arai, M. Green Chem. 2003, 5, 76. (36) Zhao, F.; Ikushima, Y.; Shirai, M.; Ebina, T.; Arai, M. J. Mol. Catal. A: Chem. 2002, 180, 259. (37) Yeung, L. K.; Lee, C. T.; Johnston, K. P.; Crooks, R. M. Chem. Commun. 2001, 21, 2290.

molecular assemblies such as micelles in catalysis has long been receiving much attention, as it mimics enzymes, the biological catalysts, in organic synthesis.43 Micellar catalysts contain microscopic solvent droplets carrying catalytic species, which are stabilized by self-organized surfactant molecules in an immiscible solvent. The formation of water-in-CO2 microemulsions first originated from the pioneering work of Johnston et al.,44,45 who discovered that polyperfluoroammonium surfactant ions could form a stable aqueous microemulsion in scCO2 but most of the conventional surfactants failed to do so. It is believed that fluorosurfactant molecules possessing both a hydrophilic headgroup and carbon dioxide-philic C-F moieties provide a layer of weakly attractive self-assemblies, which covers the highly cohesive aqueous core and stabilizes the micelle system in scCO2. Thus, micelles in supercritical carbon dioxide as a catalytic reaction medium may offer a new scenario in this area. We have recently reported that Co(II) species in micelles in supercritical carbon dioxide/oxygen mixtures can act as catalytically active species for toluene oxidation.27 Very recently, a new but elegant work on the synthesis of metal nanoparticles in micelles in the water-in-scCO2 microemulsions and their use as a new hydrogenation catalyst for simple substrates has also been described.41,42 Here, we present our study on the hydrogenation of citral using micelle-hosted Pd in supercritical carbon dioxide/H2 following this area. Since the use of micellar (38) Bertucco, A.; Canu, P.; Devetta, L.; Zwahlen, A. G. Ind. Eng. Chem. Res. 1997, 36, 2626. (39) Adams, D. J.; Chen, W.; Hope, E. G.; Lange, S.; Stuart, A. M.; West, A.; Xiao, J. Green Chem. 2003, 5, 118. (40) Chaterjee, M.; Ikushima, Y.; Zhao, F. Y. Catal. Lett. 2002, 82, 141. (41) Ohde, H.; Wai, C. M.; Kim, H.; Kim, J.; Ohde, M. J. Am. Chem. Soc. 2002, 124, 4540. (42) Meric, P.; Yu, K. M. K.; Tsang, S. C. Catal. Lett. 2004, 95, 39. (43) Tascioglu, S. Tetrahedron 1996, 5, 11113. (44) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (45) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399.

Micelle-Hosted Palladium Nanoparticles

catalysts in scCO2 is a very new research direction, issues such as comparative reaction activity or catalyst lifetime/ recyclability in scCO2, which will require detailed and extensive investigations and comparisons, will not be presented here. However, two fundamental issues are particularly focused on in this paper, namely, to carry out a direct visualization of microemulsions containing metal nanoparticles in scCO2 through our sapphire window reactor and to investigate reaction parameters affecting product selectivity in hydrogenation, both of which are important with regard to the future developments of this new type of catalysis. In particular, the direct visualization experiment is useful for revealing the working state of catalysis and for ascertaining whether a true homogeneous microemulsion is established (gaining insight into where catalysis has taken place). Since it is well-known that blending additives could change the phase properties of the mixture but there is a limited predictability of the theoretical calculations of the phase behaviors, a direct visualization and postcatalysis characterization of microemulsions in scCO2 through a sapphire window reactor is reported. The influence of several key parameters such as the medium, the molar water-to-surfactant ratio (the W0 ratio), the concentration of the micelle, and the nature of the surfactant countercation on product distribution are also described. Experimental Section Analytical grade citral, decane, perfluorotetradecanoic acid, and ammonia solution in methanol were purchased from Aldrich. Cyclohexane and methanol were supplied by Fischer Scientific. All of these compounds were used without further purification. Synthesis of the Fluorinated Ammonium Surfactant. Perfluorotetradecanoic acid [CF3(CF2)12COOH] was dissolved in methanol under stirring at a temperature of 40 °C. We reported earlier the synthesis, purification, and characterization of Co(II) perfluorotetradecanoate salt.27 Using a similar methodology, a 5-fold excess of 2.0 M ammonia solution in methanol was added to the acid solution; the mixture was then kept at 40 °C under vigorous stirring for 24 h. The generated CF3(CF2)12COO-NH4+ was collected as a dried powder by evaporating off the solvent and excess ammonia at 60 °C overnight. A slightly yellow colored solid was obtained. The purity of the synthesized material was confirmed by elemental analysis, and also, the formation of ammonia salt was monitored by Fourier transform infrared (FTIR). A clear shift in wavenumber at the CdO stretching absorption region was observed when the ammonium salt was formed due to the modification in conjugation (from 1768 to 1683 cm-1). To ensure no acid was left over in the ammonium salt, the absence of an acid absorption peak was checked. The fluorosurfactant anion salts with countercations including Li+, Na+, and K+ were prepared in a similar way by adding excess LiCO3, NaCO3, and KCO3 powder, respectively, to the methanol solution (acid) with the only extra step of filtering off the excess carbonate powder before the evaporation. For each modified surfactant, a shift of the CdO stretch absorption peak was observed after the modification. Catalytic Tests. The hydrogenation reaction of citral was carried out in a 300 mL stainless steel reactor (Parr Instrument, model number 4561 reactor) equipped with a heating jacket, an outlet for gas with a needle valve, and an overhead stirrer. Typically, palladium nitrate (0.25 mmol), citral (0.5 mL), decane (internal standard, 0.1 mL), perfluorotetradecanoate ammonium surfactant (0.25 mmol), and the appropriate amount of water required to saturate the scCO2 under the reaction conditions (i.e., 0.4768 mL for the 300 mL reactor, 140 bar scCO2, 40 °C) plus the extra amount of water required to give a W0 (molar water-to-surfactant) ratio of 30 were placed in the reactor. Dried CO2 was then pumped into the autoclave reactor using a booster pump to reach the desired pressure (140 bar) at 40 °C. Stirring was achieved by means of an overhead magnetic stirrer, the motor of which was set at 3/4 max speed (∼550 rpm). The mixture was then stirred overnight in order to create the microemulsion

Langmuir, Vol. 20, No. 20, 2004 8539 system. Then, the pressure was topped up to 150 bar by adding 10 bar H2 and the reaction was followed for 4 h. At the end of the reaction, the autoclave was allowed to cool to room temperature and the reaction mixture was then vented via double liquid traps containing cyclohexane in order to remove soluble species in the carbon dioxide. Thereafter, the reactor was opened and the remaining residue was extracted with another portion of the solvent. The resulting solutions were combined, and the solid containing metal particles was separated and collected from the solution by filtration. The filtrate was dried over sodium sulfate and then filtered before evaporating off the cyclohexane. The leftover product was dissolved in another clean cyclohexane solvent and injected into both gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) instruments for product identification. The Hewlett-Packard 5890 gas chromatograph was equipped with a flame ionization detector and a capillary column (30 m × 0.22 mm ID BPX5 0.25). Conversion and product selectivity were determined on the basis of all detected products with reference to the internal standard. For the experiments carried out in the liquid and vapor phases, the same experimental setup, the same experimental/workup procedures and an identical quantity of citral, decane (internal standard), water, fluorosurfactant, catalyst, and hydrogen, was used. The only difference is that the scCO2 was replaced by 40 mL of cyclohexane (liquid phase experiment) and 10 bar hydrogen (vapor phase experiment), respectively. Visualization of the Microemulsion through a Sapphire Window Reactor. Experiments were carried out in an ∼30 mL stainless steel Parr reactor equipped with two high-pressure transparent sapphire windows with a window separation of ∼4.5 cm. Appropriate amounts of reactants, palladium nitrate, surfactant, and water were introduced into the reactor in order to create the same conditions (40 °C and 150 bar overall pressure) as those in the 300 mL reactor where catalysis was performed. The only other difference between the two reactors was the stirring means, where the small reactor was stirred by using a magnetic stirrer at the bottom of the reactor. UV-vis characterization of the ammonium perfluorotetradecanoate in methanol was first carried out in a standard 1 cm length UV curette cell, showing an absorption maximum at λ ) 266 nm,  ) 8.733 × 104 dm3 mol-1 cm-1, the absorption of which was assigned to be the electronic excitation of the carboxylate group. On the basis of this value, the quantity of the ion pairs and the time required to form a homogeneous microemulsion in scCO2 in the sapphire window reactor were estimated. Catalyst Characterization. Transmission electron microscopy (TEM) analysis was used to characterize particle size, structure, morphology, and composition through direct imaging, electron diffraction, and elemental analysis of selected areas. A Philips CM20 microscope operating at 200 kV equipped with an Oxford Instrument EDS 6767 energy dispersive X-ray (EDX) analyzer was used. High-resolution images were observed with a JEOL JEM-2010 transmission electron microscope equipped with a high-resolution pole piece (1.5 Å point-to-point resolution) operating at 200 kV. The sample was gently grinded, suspended in 2-propanol, and placed on a carbon-coated copper grid followed by the solvent evaporation. Electron micrographs and EDX analyses of selected areas of the sample were also taken. X-ray powder diffraction (XRD) analysis of the samples was performed with a Simen Instrument X-ray diffractometer using Ni-filtered copper with a radiation of KR1 ) 1.540 56 Å. Phase identification of our samples was carried out by comparing the collected spectra with the published files from the International Centre for Diffraction Data (JCPDS-1996). The average size of a metal particle was calculated on the basis of peak broadening using the Scherrer equation. Instrumental peak broadening was also taken into account.

Results and Discussion Microemulsion Formation. Initial experiments were first carried out to determine the “solubility” of the fluorinated surfactant. The sapphire windows allow visualization of almost the entire vessel content such that any phase segregation can be easily noticed. It was found that properly dried ammonium perfluorotetradecanoate

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Figure 2. Microemulsion visualization through sapphire windows: (a) catalyst slurry at 25 °C and 67 bar (liquid CO2); (b) most of the slurry disappeared and reached a transparent state at 40 °C and 140 bar CO2; (c) after addition of 10 bar H2, a homogeneously brown “solution” observed.

Figure 1. Concentration of ammonium perfluorotetradecanoate vs the time required to form an microemulsion based on UV-vis analysis through a sapphire window reactor. Conditions: 30 mL sapphire window reactor, 0.32 mmol of ammonium perfluorotetradecanoate (excess), 217 µL of water (excess, 45 µL of water is actually required to saturate the solvent), 140 bar CO2, 40 °C.

under the dry environment remained almost insoluble in supercritical carbon dioxide. On the other hand, it could be “dissolved” under “wet” conditions. This fact agrees well with the previous observations that water is required to saturate the supercritical carbon dioxide before a microemulsion appears.27 Figure 1 shows clearly that the ammonium perfluorotetradecanoate dissolves into wet supercritical carbon dioxide (forming a totally transparent microemulsion which is consistent with previous observations of microemulsion formation). To estimate the quantity of surfactant in the wet supercritical phase, it is assumed that a value of  ) 8.733 × 104 dm3 mol-1 cm-1 (derived from the methanol solution) is similar in the case of scCO2 (otherwise unavailable). A molar concentration of surfactant of 1.963 × 10-3 M (equivalent to 0.0589 mmol or 0.043 g) was detected in the wet supercritical carbon dioxide in the small sapphire window reactor by UV-vis analysis (a very slight chemical shift to 267 nm in scCO2). Also, it is noted that under constant stirring, it still took a rather long period of time (900 min) to establish the stable “microemulsion” condition at the region of the sapphire windows (∼3.15 cm from the bottom of the reactor) in this small reactor using magnetic stirring at the bottom. This may not be ideal from the catalysis viewpoint, since it is a long activation period. However, when the preparation was carried out in a large reactor using overhead stirring, a much shorter time was required (90-120 min). Thus, more effective stirring would be required to establish the microemulsion state in a shorter period of time. In addition, time is required for the fluorinated species to undergo self-assembly with the water/scCO2 in order to override the strong electrostatic interactions within the ionic solid precursor. The stability of water-in-scCO2 microemulsion formation at different W0 ratios (we fixed the amount of surfactant to 0.0589 mmol but varied the amount of water) has been investigated using the sapphire window reactor. Figure 2 presents a direct visualization of the fluid through the sapphire windows under different conditions. As can be seen from these pictures, separate phases of liquid CO2 and an insoluble surfactant/Pd nitrate/water mixture were clearly visible below the supercritical temperature (room temperature, Figure 2a). Upon raising the temperature to 40 °C at 140 bar, a single phase transparent fluid was

Figure 3. A typical XRD pattern of micelle-hosted Pd showing reflection peaks due to Pd and the selected Pd(111) angular broadenings from 35 to 45° indicating an average particle size of 4.01 nm taking the instrumental broadening into account.

then observed for W0 ratios ranging from 2 to 20 (Figure 2b). It is noted that this temperature was well above the theoretical critical temperature for pure CO2 (31.1 °C), although modification of the critical temperature of the overall fluid by the presence of substrates was not known. The observation of a transparent single phase is consistent with the water-in-scCO2 microemulsion state already reported in the literature.27 Figure 2c presents the color change of the solution upon the addition of H2. The microemulsion appeared to be stable and homogeneous (single phase), and the color of the fluid became gradually intensified to a brown “solution”. Similar observations were made using a W0 ratio of 30. To an extreme, when the W0 ratio was set at 100-200, a clear distinction of phases (water droplets and solid powder were segregated near the bottom of the reactor in a clear scCO2 phase) was evidenced despite the fact that the reactor had been left for stirring for more than 24 h. Thus, a large quantity of liquid water at these extremely high W0 ratios was unable to enter into the microemulsion state but remained as physical large droplets dispersing in the solvent. As a result, most of the Pd species would therefore partition into these water puddles, depriving the Pd from the microemulsion state. It is unfortunate that it proved difficult to quantitatively derive the distribution of Pd in micelles and in water puddles at different W0 ratios (a large error by the UV-vis determination at such low concentrations combined with the spectral contamination due to the physical dispersion of large non-microemulsion water droplets in the medium). The sizes of the micelle-hosted Pd under a stable microemulsion regime (W0 ratio ) 30) have been carefully determined by XRD (on the basis of the Debye-Scherrer equation and by taking the instrumental broadening into account) and TEM. As can be seen from Figure 3 (XRD), the average size of the Pd nanoparticle obtained under supercritical conditions is ∼4.01 nm. Figure 4 shows

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Figure 4. (A) A typical TEM image of micelle-hosted Pd nanoparticles in scCO2. (B) The size histogram of the micelle-hosted Pd showing the core metal with an average size of 3.65 ( 0.85 nm (standard deviation).

Figure 5. High-resolution TEM (HRTEM) image of a typical micelle-hosted Pd particle (scale bar, 3.5 nm) and the corresponding model of a cuboctahedral Pd cluster.

indeed an extremely uniform metal size of ∼3.65 ( 0.85 nm with a sharp size distribution by the TEM characterization. Careful examination of a typical micelle-hosted Pd nanoparticle by TEM under a high magnification clearly suggests that many of them show indeed a defined cubooctahedron shape, as shown in Figure 5. As a comparison, much more poorly defined particles with a broader size distribution were obtained under the same conditions where the Pd was not in the microemulsion state (W0 ratio ) 100). Occasionally, even larger particle clusters were observed in this sample (Figure 6). It is also noted that a similar result was obtained in the sample pretreated with hydrogen before the application of scCO2 (see Table 1). Influence of the Experimental Conditions. The hydrogenation reaction of citral was carried out under different conditions, such as in the vapor phase, in the liquid phase (cyclohexane), and in the supercritical carbon dioxide phase. The results are presented in Table 1. A complete conversion of citral (>99%) was quickly achieved right away after the addition of H2 (83%.

Figure 6. (A) A typical TEM image of Pd nanoparticles in scCO2 at W0 ) 100. (B) The size histogram of the corresponding Pd showing a metal particle size of 8.22 ( 4.08 nm (standard deviation) with a multimodal size distribution.

A small degree of hydrogenation at the terminal carbonyl group to corresponding unsaturated alcohols [citronellol (CIOL) and traces of 3,7-dimethyloct-2-enol and geraniol], saturated alcohols [3,7-dimethyloctanol (DMOL)], and cyclic products (isopulegol) was also observed (Scheme 1).

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Table 1. Influence of Experimental Conditions on Citral Hydrogenationa product distribution (%) solvent

working state W0 ratio

vapor cyclohexane solution CO2 ME (H2 after) CO2 ME (H2 after) in buffer CO2 H2 before ME

CIAL

DHAL

30 30

1 19 68 67

82 66 12 7

30

30

54

a

Conditions: 300 mL reactor, 0.5 mL of citral (all consumed), 0.25 mmol of Pd nitrate, 0.25 mmol of F-surfactant, 140 bar CO2 or 40 mL of cyclohexane, 10 bar H2, 40 °C, 4 h. Buffered solution was used instead of deionized water, which was prepared using a saturated aqueous solution of potassium dihydrogen phosphate (pH 7.5).

Such a hydrogenation of the two CdC bonds preferably over the aldehyde end group by Pd under all conditions (the liquid, vapor, and supercritical phases) is not surprising, as Pd metal is well-known to favorably catalyze the hydrogenation of double bonds but is sluggish to hydrogenate the carbonyl group.18 As can be seen from Table 1, it is interesting to find that the product distribution is greatly affected by the conditions used. A high selectivity toward the fully hydrogenated aldehyde DHAL was observed in the vapor phase (82%), but the selectivity decreased when the reaction was carried out in the liquid phase (66%). It is even more interesting to note that when the reaction took place in the presence of micelle-hosted Pd in supercritical carbon dioxide the partially hydrogenated aldehyde CIAL was the main product (68%) under the same prolonged period of reaction time. It is well documented that, for conventional metal catalysts, the subsequent hydrogenation of the unactivated double bond of the citral is very difficult to inhibit. Hence, a high selectivity to saturated aldehyde is normally obtained over Pd, even for a short contact time.46 It is expected that a strongly bound partially hydrogenated species on a metal surface facilitates the subsequent attack of the remaining double bond by surface hydrogen (particularly the sideways attack of the molecule assessing both the double bonds by the metal surface). As a result, a number of ways for modifying noble metals (i.e., blending Fe) to guide the aldehyde molecule for a head-on attack to the metal surface have been studied.46 In the present study, the surprisingly high CIAL selectivity we obtained when the reaction was carried out over the unpromoted Pd under supercritical carbon dioxide suggests the prominent effect of the microemulsion on the product distribution of the reaction. It is known that the pH inside microemulsion droplets in scCO2 is typically 3, as determined with fluorescence47 and absorbance48 probes. To gain further insight into whether the unusual selectivity is due to the low pH or due to this particular micellar environment, the catalytic reaction was carried out using a saturated phosphate buffer solution at a pH of 7.5 rather than the deionized water (can stabilize the pH under the reaction conditions). It is interesting to find that almost identical results in terms of product distribution and citral conversion were obtained. This experiment rules out the possibility that the low pH accounts for the product selectivity. To further illustrate that our results are indeed related to the presence of the micelle host, a (46) Augustine, R. L. Heterogeneous Catalysis for the Synthetic Chemist; Marcel Dekker Inc.: New York, 1996. (47) Niemeyer, E. D.; Bright, F. V. J. Phys. Chem. B 1998, 102, 1474. (48) Holmes, J. D.; Steytler, D. C.; Rees, G. D.; Robinson, B. H. Langmuir 1998, 14, 6371.

Figure 7. Profile curves of product selectivity vs W0 ratio (b, dihydrocitronellal; 9, citronellal; 0, 3,7-dimethyloctanol; O, citronellol). Conditions: 300 mL reactor, 0.5 mL of citral (all consumed), 0.25 mmol of Pd nitrate, 0.25 mmol of F-surfactant, 140 bar CO2, 10 bar H2, 40 °C, 4 h.

new experimental procedure done by adding H2 immediately prior to the addition of CO2 was carried out. A switch of the product distribution was observed as 54% selectivity toward DHAL, somewhat comparable to the selectivity of this product in cyclohexane, was detected (Table 1). We believe that the significant switch of the product distribution must relate to whether the microemulsion was formed before catalysis or not, since the UV-vis experiments indicated that a relatively long time was required to establish a stable microemulsion state from the surfactant salt we used. Thus, when hydrogen was added prior to the formation of the microemulsion, the fast hydrogenation reaction was well completed (