J. Phys. Chem. C 2009, 113, 4721–4725
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Recyclable High Performance Palladium Heterogeneous Catalyst with Porous PVDF Binder from the Novel Gel Nanocomposite Route Sanjoy Samanta and Arun K. Nandi* Polymer Science Unit, Indian Association for the CultiVation of Science, JadaVpur, Kolkata-700032, India ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: January 22, 2009
A highly efficient, recyclable, size-tuneable Pd nanoparticle embedded porous heterogeneous catalyst is developed from poly(vinylidene fluoride) (PVDF)/ethylene carbonate (EC)/Pd gel nanocomposite. The dried gel nanocomposite has a fibrillar network structure and Pd nanoparticles of 3-6 nm diameters are dispersed on it. The oxidation of benzoin to benzil and benzyl alcohol to benzoic acid and reduction of nitrobenzene to aniline are studied with the above catalysts in water. The efficiency of the catalyst is dependent on nanoparticle size, and it is 2-2.5 times higher in porous state than that of its sintered (melt-cooled) state. At similar reaction conditions the present porous catalyst has 40 and 80 times higher efficiency than those of previously reported polymer supported Pd catalysts of similar nanoparticle size. The performance of present porous catalyst is 1.7-2.1 times higher than that of impregnated catalyst on PVDF film and it is 1.26-1.30 times more efficient than the same amount of Pd nanoparticle impregnated on activated (porous) carbon. The nanoparticle size remains unchanged even after three cycles of reaction. Possible reasons of the high performance of the present catalyst have been discussed from the multiporosity and site specificity of the binder polymer. The field of heterogeneous catalysis with metal nanoparticles (NPs) continues to grow in remarkable ways with increasing impact in various organic reactions over homogeneous catalysis.1-5 Heterogeneous catalysts are more powerful than homogeneous catalysts from the “industrial” points of view for its easy recovery, recyclablity, reducing product cost and raw materials wastage, easier isolation of product, etc. To fulfill the enhanced demands of the catalysts, polymeric binders are gaining importance for their NP stabilization power.3-5 Mainly inert, thermally stable and water insoluble but water dispersible polymers are very important to retain their heterogeneous identity during the reaction in water which is recently used as a medium for organic transformation6 due to its nontoxicity. There are some reports of polymer-Pd catalysts with hydrophilic and hydrophobic binders.3-5 The efficiency of such catalysts depends on its dispersion in solvents and on its site specificity, i.e, its capability to bring the reactant molecules on the catalyst surface. As the individual hydrophilic and hydrophobic binders can not often afford both heterogeneous nature of the catalyst and good dispersion in the medium some authors used cross-linked copolymer of hydrophobic (polystyrene) and hydrophilic (polyethylene glycol) moiety as binder for Pd nanoparticles.3a Palladium NPs immobilized on functionalized SBA-15 have recently been used to improve the catalytic efficiency using the channel structure of SBA-15.3b However, the methods of obtaining such catalysts are tedious and complicated. So an easy and alternative pathway to improve the catalytic efficiency is very essential to broaden the above field. We propose that if the polymeric binder is multiporous (combination of micro, meso, and macro pores)7a-c the catalytic activity is expected to be very high as the reactants would reach the catalyst surface through the pores for the transformations and the products would diffuse out without experiencing any * For correspondence E-mail:
[email protected]. Fax: (+91) 33 2473 2805.
back pressure provided the binder has good site specificity. Probably the micropores may not be very interesting because it is filling by nanoparticles. If the catalyst is embedded on polymers like partially polar β polymorphic poly(vinylidene fluoride) (PVDF) chain,8 it may impart some wetting property facilitating good dispersion of the catalyst in water and can afford the site specificity through hydrophobic interactions to the organic reactants. Recently, we have reported that multiporous polymers of high surface area and high pore volume can be obtained by drying thermoreversible polymer gels in a proper way.7a,b In this work, our aim is to embed palladium NPs on the porous surface of β PVDF, which is thermally stable up to 476 °C,8 less reactive with organic reagents due to high crystallinity,9 easily forms gels in different organic solvents,10 and acts as a stabilizer of metal NPs.8 Here, for the first time, we report a simple method for immobilizing palladium NPs with narrow size distribution on porous β-PVDF surface by gel nanocomposite (GNC) route using ethylene carbonate (EC) as a gelling medium.10b At the dissolution temperature of PVDF in EC, palladium acetate [Pd(OAc)2] produces Pd NPs by thermal decomposition.11 The catalytic activity of the dried porous gel nanocomposite is then evaluated in aqueous medium and is compared with those of previous workers.3a,b The effect of NP size on the catalytic activity as well as the effect of present multiporous binder on Pd catalytic activity compared with those of sintered catalyst (Pd-M), impregnated catalyst (Pd-I), and catalyst impregnated over porous carbon (Pd-C) is also tested. The formation of Pd(0) NPs in the gel samples has been established from the absence of the absorption peak at 393 nm of Pd(II) in the UV-vis spectrum (Figure 1).12,13 The TEM images (Figure 2a-c and Figure 1 in the Supporting Information) of dried gels also confirm the existence of Pd NPs on the porous surface of PVDF. The lattice spacing (0.226 nm) in the HRTEM image (Figure 2d) matched very well with the (111) lattice plane of palladium metal.2c The histograms in the inset of Figure 2a-c indicate that the size distribution of Pd NPs is
10.1021/jp810160r CCC: $40.75 2009 American Chemical Society Published on Web 02/20/2009
4722 J. Phys. Chem. C, Vol. 113, No. 11, 2009
Samanta and Nandi
Figure 3. FESEM pictures of (a) PVDF-EC gel (b) Pd 2 GNC. Figure 1. UV-vis spectra of Pd(OAc)2-EC solution and Pd 2 GNC (inset (a) pure gel and (b) Pd 2 GNC).
Figure 4. WAXS pattern of R-PVDF and those of dried gel and Pd 0.8 and Pd 2 nanocomposites.
Figure 2. TEM images of (a) Pd 0.2, (b) Pd 0.8, and (c) Pd 2 and (d) HRTEM image of Pd 2. Inset: Histogram of Pd nanoparticles in each micrograph.
very narrow. Probably, the specific interaction between >CdO groups of Pd(OAc)2 and >CF2 dipoles of PVDF14 is a reason for their homogeneous mixing. The Pd atoms, produced by thermal reduction11 produce Pd nanocrystals of uniform distribution by nucleation and growth mechanisms. On going from Pd 0.2 to Pd 2 (numbers represent the weight percent of Pd(OAc)2 with respect to PVDF used in the GNC preparation), larger NPs are produced. The size of NPs depends on nucleation and growth processes, where the later is again controlled by diffusion process.7a With increasing Pd concentration, high Pd(0) density produces bigger Pd nanocrystals. It is to be noted that Pd(OAc)2
becomes reduced to Pd(0) at 180 °C, which is lower than the usual temperature required in the bulk state (200-260 °C).15 This may be due to the easier degradation of the complex of EC with Pd(OAc)211b,16 (Figure 2 in the Supporting Information). From the atomic absorption spectra, it is evident that transformation of Pd(OAc)2 into Pd(0) is about 95% complete (Table 1). The FESEM pictures indicate fibrillar network structure in dried PVDF gel and in the dried PVDF-Pd gel nanocomposites (Figure 3a,b and Figure 3 in the Supporting Information).7 It is likely to be responsible for the enhancement of catalytic efficiency because of large surface area of the pores obtained from drying gels. The presence of NPs has no effect on gel morphology, probably due to the very small size (3-6 nm) of the particles. The solvent subtracted FT-IR spectra (Figure 4a,b in the Supporting Information) indicate the presence of IR absorbance peaks at 840, 508, and 475 cm-1 which remain
TABLE 1: Atomic-Absorption Spectroscopy Data for Pd Concentration in the Pd 0.5 and Pd 2 Catalysts before and after Reaction of Entry 3
catalyst
theoretical amount of Pd(0) per gram of nanocomposite (mmol)
experimental amount of Pd(0) per gram of nanocomposite before reaction (mmol)
Pd 2
89.0 × 10-3
85 × 10-3
Pd 0.5
22.3 × 10-3
20.4 × 10-3
experimental amount of Pd(0) per gram of nanocomposite after reaction (mmol) 84.1 × 10-3 (after 3rd cycle) 20.0 × 10-3 (after 1st cycle)
Recyclable Palladium Heterogeneous Catalyst
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TABLE 2: Characteristics of Pd 2 Catalysed Oxidation and Reduction Reactions in Aqueous Medium at Atmospheric Pressure and Its Comparison with Literature Data
unchanged in the dried gels and in the gel nanocomposites. This indicates the formation of piezoelectric β polymorph of PVDF in both pure gel10b and in the dried gel nanocomposites. The conclusion is also supported from WAXS pattern of dried gels where the single diffraction maxima at 2θ ) 20.8° correspond to that of β-polymorph of PVDF (Figure 4). Due to the partial polar nature of the β-PVDF chain,8b the catalyst remains dispersed in water. The thermal stability of GNCs has increased to some extent, more than that of pure PVDF as the onset degradation temperature increases from 461 to 467 °C in the composites (Figure 5 in the Supporting Information).
Figure 5. Recyclability chart of reactions in (a) entry 1 and (b) entry 3 of Table 2 in different cycles. (c) Yield (%) of reactions in entries 1, 2, and 3 using different porous [Pd 2 and Pd-C (Pd impregnated on activated carbon)] and nonporous [Pd-I (Pd impregnated on PVDF film) and Pd-M (melt cooled Pd-2)] Pd catalysts in the first cycle.
With the PVDF fibrillar network-supported Pd nanoparticles in hand, we now concentrate our attempts on oxidation and reduction reactions. It will be worthy to carry out these reactions in aqueous medium using the above catalysts instead of the traditional methods in which the undesirable organic solvents are used. Two oxidations and one reduction reactions (Table 2) are carried out to evaluate the catalytic activity of GNCs. Both the oxidation of benzoin to benzil and benzyl alcohol to benzoic acid (Table 2, entries 1 and 2) are done in aqueous medium by refluxing 3 h and 2.5 h, respectively, under atmospheric pressure in presence of oxygen. The reduction of nitrobenzene to aniline (Table 2, entry 3) is carried out at room temperature in presence of hydrogen under stirring for 2.5 h. The products are separated by column chromatography. Notably, we have obtained very high yield for the above reactions i.e.
Figure 6. Conversion vs time plot of three reactions (Table 2) using Pd 2 and Pd 0.5 catalysts at the same reaction conditions.
4724 J. Phys. Chem. C, Vol. 113, No. 11, 2009 92%, 86%, and 98% for entries 1, 2, and 3, respectively. The organic substrates diffuse onto the dispersed polymer matrix due to strong hydrophobic force forming a dense reaction center at the porous surface, where the Pd NPs come in contact with the substrates transforming them into products quickly with high yield. Blank measurements were also carried out for the catalytic activity in the pure dried gel; however, we did not observe any progress of reactions for all the three entries with the pure multiporous PVDF. Therefore, Pd nanoparticles are serving as a catalyst for the above reactions.17 After the first cycle, the catalyst is separated by filtration and washed by water, finally by dichloromethane to refresh the catalyst and is reused for second and third cycles for the same reaction to examine the recyclablity. The yields of cycles 1, 2, and 3 are almost same (Figure 5 and Figure 6 in the Supporting Information). During the reactions the Pd NPs do not diffuse out into the aqueous phase, as confirmed from the atomic absorption spectroscopic data of Pd concentration before and after reactions (Table 1). The invariance of NP size after 3 cycles of reaction (entry 2) is evidenced from TEM micrograph of Pd 2 (Figure 7 in the Supporting Information). The average NP size is 5.98 ( 0.44 which is the same with that of before the starting of reaction (6.0 ( 0.3). So it is apparent from Table 2 that Pd-PVDF GNCs behave as a good recyclable heterogeneous catalyst. To understand the effect of multiporous morphology of the dried gel, the above catalysts were melted at 210 °C for 1 h and then cooled to 30 °C. The substance does not show any porous morphology (Figure 3b in the Supporting Information). In Figure 5c the yields produced under same reaction conditions are compared, and it is evident that the yield is 2-2.5 times higher in the Pd 2 catalyst than that in the melt-cooled (sintered) catalyst. To further test the effect of porosity, we have impregnated same amount of Pd nanoparticles on the surface of equal weight PVDF films as in Pd 2 catalyst by evaporating the methanol solution of Pd(OAc)2 and then fixing the nanoparticles by raising the film temperature to 150 °C. The reactions were then performed under similar conditions of Table 2 and the reactions are found to be slower, e.g., 43, 47, and 59% yields are obtained for entries 1, 2, and 3, respectively (cf. Figure 5c). So the porous Pd 2 catalyst is more efficient than the impregnated catalyst by 2.1, 1.8, and 1.7 times for entries 1, 2, and 3, respectively. In order to compare the catalytic efficiency of Pd 2 with normal porous binder e.g, activated carbon, the same amount of Pd(0) was impregnated on the equal mass of activated carbon (Pd-C) (as the PVDF content in Pd 2) via dispersion and sonication (60W, Eyela, AVIOC) in ethanol medium. The dried catalyst was used to perform the reactions of Table 2 under the same reaction conditions, and the yields are 71, 68, and 77% for entries 1, 2, and 3, respectively (cf. Figure 5c). Hence multiporous Pd 2 catalyst is about 1.3 times more efficient than that of Pd-C catalyst. The multiporosity of the present catalyst not only enables to expose larger surface area of Pd nanoparticles for catalytic activity but also facilitates easy diffuse out of the products without developing any back pressure. The effect of NP size on catalytic activity is also tested for the above reactions using Pd 0.5 [diameter (D) ) 3.4 ( 0.4 nm] and Pd 2 [D ) 6.0 ( 0.3 nm] with same amount of Pd in the reaction. In Figure 6 the conversion vs time is compared for Pd 0.5 and Pd 2 catalysts, and it is evident that reactions with Pd 0.5 are faster than that of Pd 2 catalyst. A comparison of reactions of entry 1 and 2 with those of other workers (Table 2) at similar reaction conditions indicates that the catalytic
Samanta and Nandi efficiency of the present porous catalyst is 117 and 47 times higher than those of cross-linked polystyrene-poly(ethylene glycol)-Pd catalyst3a and SBA-15 stabilized Pd nanoparticle catalyst with channel structure.3b The catalytic efficiency of the porous catalyst has been compared from the ratio of Pd (mmol/g of nanocomposite) used to obtain approximately same yield at the same reaction conditions. Thus the catalytic efficiency of Pd 2 is (10/0.085)) 117 and (4/0.085)) 47 times higher than that of Uozumi et al.3a and Karimi et al.,3b respectively. The catalytic efficiency depends on available surface area, hence on nanoparticle size. The average size of Pd 2, ref 3a, b are 6, 7, and 9 nm, respectively, giving the ratio of total surface area for the same amount of Pd as 0.17, 0.21, and 0.25 for ref 3a, b and Pd 2, respectively. So the reaction rate should be proportional in similar fashion. Normalizing the effect of the NP surface area with that of ours, the present Pd 2 catalyst is 80 and 40 times more efficient than that of Uozumi et al.3a and Karimi et al.,3b respectively. The polar β polymorphic PVDF and multiporosity of the binder are the causes of such increased catalytic efficiency. So, a cheap, recyclable, high performance Pd nanoparticle embedded porous heterogeneous catalyst is developed by PVDF/ Pd(OAC)2/ethylene carbonate gel nanocomposite route, and it may be a novel way of preparing high performance heterogeneous catalysts, in general. Supporting Information Available: Experimental details, TEM and FESEM images, Pd-complex structure, FTIR, TGA, IR, and NMR characterization of reaction products. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Liz-Marzan, L. M.; Kamat, P. V. Nanoscale materials; Kluwer Academic Publisher: Boston, 2003. (b) Chen, Y. W. J. Nanopart. Res. 2006, 8, 223. (c) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (2) (a) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249. (b) Wilson, O. M.; Knecht, M. R.; Garcia-Martinez, J. C.; Croocks, R. M. J. Am. Chem. Soc. 2006, 128, 4510. (c) Piao, Y.; Jang, Y.; Shokouhimehr, M.; Lee, I. S.; Hyeon, T. Small 2007, 3, 255. (d) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (3) (a) Uozumi, Y.; Nakao, R. Angew. Chem., Int. Ed. 2003, 42, 194. (b) Karimi, B.; Abedi, S.; Clark, J. H.; Budarin, V. Angew. Chem., Int. Ed. 2006, 45, 4776. (c) Brink, G.-J. t.; Arends, I. W. C. E.; Sheldon, R. A. Science 2000, 287, 1636. (d) Kakiuchi, N.; Maeda, Y.; Nishimura, T.; Uemura, S. J. Org. Chem. 2001, 66, 6620. (4) (a) Kamiya, I.; Ogawa, A. Tetrahedron Lett. 2002, 43, 1701. (b) Four, P.; Guibe, F. J. Org. Chem. 1981, 46, 4439. (c) Helminen, J.; Paatero, E.; Hotanen, U. Org. Process Res. DeV. 2006, 10, 51. (d) Guino´, M.; Hii, K. K(M). Tetrahedron Lett. 2005, 46, 7363. (e) Mirza-Aghayan, M.; Boukherroub, R.; Bolourtchian, M.; Hosseini, M. Tetrahedron Lett. 2003, 44, 4579. (5) (a) Ohde, H.; Wai, C. M.; Kim, H.; Kim, J.; Ohde, M. J. Am. Chem. Soc. 2002, 124, 4540. (b) Borszeky, K.; Mallat, T.; Baiker, A. Catal. Lett. 1999, 59, 95. (c) Nishio, R.; Sugiura, M.; Kobayashi, S. Org. Biomol. Chem. 2006, 4, 992. (6) (a) Li, C. J.; Chan, T.-H. Organic Reactions in Aqueous Media; Wiley-VCH: New York, 1997. (b) Lindstrom, U. M. Chem. ReV. 2002, 102, 2751. (7) (a) Dasgupta, D.; Nandi, A. K. Macromolecules 2005, 38, 6504. (b) Dasgupta, D.; Nandi, A. K. Macromolecules 2007, 40, 2008. (c) Malik, S.; Roizard, D.; Guenet, J. M. Macromolecules 2006, 39, 5957. (8) (a) Manna, S.; Batabyal, S. K.; Nandi, A. K. J. Phys. Chem. B 2006, 110, 12318. (b) Manna, S.; Nandi, A. K. J. Phys. Chem. C 2007, 111, 14670. (9) Nandi, A. K.; Mandelkern, L. J. Polym. Sci., Part B: 1991, 29, 1287. (10) (a) Dikshit, A. K.; Nandi, A. K. Macromolecules 2000, 33, 2616. (b) Dasgupta, D.; Manna, S.; Garai, A.; Dawn, A.; Rochas, C.; Guenet, J. M.; Nandi, A. K. Macromolecules 2008, 41, 779. (c) Mal, S.; Nandi, A. K. Polymer 1998, 39, 6301. (11) (a) Aymonier, C.; Bortzmeyer, D.; Thomann, R.; Mulhaupt, R. Chem. Mater. 2003, 15, 4874. (b) Ho, P.-F.; Chi, K.-M. Nanotechnology 2004, 15, 1059.
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