VOLUME 19, NUMBER 5
MARCH 6, 2007
© Copyright 2007 by the American Chemical Society
Communications Selective Heterogeneous Catalytic Hydrogenation by Recyclable Poly(allylamine) Gel-Supported Palladium(0) Nanoparticles
Scheme 1. Hydrogenation Reactions
Yiying Hong and Ayusman Sen* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed October 23, 2006 ReVised Manuscript ReceiVed January 22, 2007
Achieving high substrate selectivity in catalytic reactions is an area of great interest.1-6 This has mainly been achieved by employing catalysts that exhibit either preferential substrate binding or are shape selective. Unfortunately, most of these systems are difficult to design and construct. In this context, hydrogels are simple to synthesize and can be vehicles for highly selective reactions. Catalysts can be supported inside hydrogels, and the porous structure allows ready diffusion of specific substrates to the catalyst center. For example, if Pd(0) nanoparticles were incorporated into a hydrogel and their aggregation was prevented,7-11 they * To whom correspondence should be addressed. E-mail:
[email protected].
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Mizuki, T.; Iwasawa, Y. Chem. Commun. 2006, 2833-2844. Somorjai, G. A. ACS Symp. Ser. 2005, 890, 210-219. Gates, B. C. Top. Organomet. Chem. 2005, 16, 211-231. Becker, J. J.; Gagne, M. R. Acc. Chem. Res. 2004, 37, 798-804. Gyula, T.; Istvan, P.; Arpad, M.; Istvan, H. Theochem. 2003, 666667, 69-77. Shape-selectiVe Catalysis: Chemicals Synthesis and Hydrocarbon Processing; Song, C., Garces, J. M., Sugi, Y., Eds.; ACS Symposium Series 738; American Chemical Society: Washington, DC, 2000. Iwasawa, T.; Tokunaga, M.; Obora, Y.; Tsuji, Y. J. Am. Chem. Soc. 2004, 126, 6554-6555. Tromp, M.; Sietsma, J. R. A.; van Bokhoven, J. A.; van Strijdonck, G. P. F.; van Haaren, R. J.; van der Eerden, A. M. J.; van Leeuwen, P. W. N. M.; Koningsberger, D. C. Chem. Commun. 2003, 128-129. Sato, F. Handbook of Organopalladium Chemistry for Organic Synthesis; John Wiley: Hoboken, NJ, 2002; Vol. 2, pp 2759-2765.
could be used to catalyze the preferential hydrogenation of hydrophilic substrates which would selectively enter the gel matrix. A SciFinder search using the keywords “gel” and “catalysis” revealed that preferential catalytic conversion of hydrophilic substrates by catalysts anchored within hydrogels has not been reported. In this paper we demonstrate (a) the synthesis and stabilization of metallic Pd(0) nanoparticles inside a hydrogel bearing coordinating amine groups and (b) the use of these nanoparticles for the highly selective hydrogenation of a variety of hydrophilic substrates (Scheme 1). The substrate test pairs differed by a single hydroxy group replacing a methyl counterpart. Cross-linked poly(allylamine) (cl-PAA) was used as the polymer matrix since it readily binds Pd(II) by coordination to the amine groups. Samples of cl-PPA with different degrees of cross-linking were prepared by radical-initiated aqueous polymerization of allylamine hydrochloride and 1,2(10) Blaser, H.-U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M. J. Mol. Catal. A: Chem. 2001, 173, 3-18. (11) Tsuji, J. Palladium Reagents and Catalysis; Wiley-VCH: New York, 1995.
10.1021/cm062526+ CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007
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Figure 1. Transmission electron microscopy of Na2PdCl4/cl-PAA (left) and recycled Pd(0)/cl-PAA recovered after typical hydrogenation reaction (right). Magnification: 15K. Scale bar: 500 nm.
Figure 2. Relative reactivity of 1-hexene (a) versus 5-hexen-1-ol (A) in the presence of 5% Pd(0)/C ([), Pd(0)/PA2 (2), and Pd(0)/PA4 (9).
bis(N-allylamino)ethane (see Supporting Information). Following synthesis, the material was neutralized, washed successively with water and acetone, and dried. Two different cl-PAA with varying degrees of crosslinking (equal to the ratio of the number of cross-linkers to the number of monomer units) were used: 1:21 (PA2) and 1:40 (PA4). Pd(0) loaded hydrogel (Pd(0)/cl-PAA) was easily prepared by exposing cl-PAA to Na2PdCl4 solution, washing with distilled water to remove the surface Pd(II) ions, and reducing the Pd(II) to Pd(0) by dihydrogen or by in situ reduction during the initial stage of substrate hydrogenation. Upon reduction the gel color changed from brown to black, and the transparent crystals of Na2PdCl4 were replaced by black Pd(0) nanoparticles (diameter: 21 ( 7 nm) homogeneously distributed inside the polymer matrix (Figure 1). Differential scanning calorimetry of the original cl-PAA gel and Pd/cl-PAA showed little difference in glass transition temperatures (see Supporting Information), indicating retention of the polymer network. Up to 58.20 wt % incorporation of Pd(0) was reached, as evidenced by thermogravimetric analysis (see Supporting Information). In order to probe selective olefin hydrogenation, water swollen Pd(0)/PA2 and Pd(0)/PA4 were used. Commercial 5%Pd(0) on carbon (Johnson Matthey) was used as the control catalyst. 5-Hexen-1-ol and 1-hexene were chosen as
Figure 3. Relative reactivity of p-nitrotoluene (a) versus p-nitrophenol (A) in the presence of Pd(0)/C ([) and Pd(0)/PA4 (9).
Figure 4. Relative reactivity of 4-methylbenzyl alcohol (a) versus 4-hydroxybenzyl alcohol (A) in the presence of Pd(0)/C ([), fresh Pd(0)/ PA4 (9), and Pd(0)/PA4 in the 10th recycle (2).
the two alkenes, the former being the more hydrophilic of the two. The reaction was carried out by adding a 1:1 molar ratio of the two alkenes and the swollen Pd/gel conjugate to toluene (total alkene/toluene/gel ) 1:5:0.5, v/v) and exposing
Communications
the mixture to dihydrogen (10 psi) at room temperature. Under these conditions only approximately 3% of 5-hexen1-ol was absorbed into the gel. The progress of the hydrogenation was followed by gas chromatographic analysis of the toluene fraction. The preferential hydrogenation of 5-hexen-1-ol can be clearly seen by plotting the conversion of 1-hexene against 5-hexen-1-ol (Figure 2). The selectivity increased in the order of Pd(0)/C catalyst < Pd(0)/PA2 < Pd(0)/PA4. With Pd(0)/PA4, when 90.3% of the 5-hexen-1-ol had converted, the conversion of 1-hexene was only 5.7%. In contrast, Pd/C showed very little selectivity. It is not clear at this point why the more cross-linked PAA (PA2) shows a lower selectivity. Possibly, the lower incorporation of water into the more cross-linked material makes it less hydrophilic and therefore less selective toward 5-hexen-1-ol. Consistent with this hypothesis is the lower swelling ratio in water for the more cross-linked gel (ratio of the diameter of swollen to original gel particle: PA2, 2; PA4, 3.2). Pd(0)/PA4 was subsequently used in two other test reactions. Reduction of nitroarenes to anilines is a synthetically important transformation.12-15 While Pd(0)/C showed higher conversion of p-nitrophenol over p-nitrotoluene, essentially 100% selectivity was observed with wet Pd(0)/PA4 in chloroform (Figure 3). Deoxygenation of alcohols is another useful reaction in organic synthesis. The most common method involves radical chemistry15 and introduces radical initiators such as azobisisobutyronitrile (AIBN) and peroxides.16 The drawbacks include demanding control of the reactions and the difficulty in scale-up.16 We found that in the chloroform medium, Pd(0)/PA4, gave 100% conversion and essentially 100% selectivity for the more hydrophilic of the test pair (Figure 4). (12) Larock, R. C. ComprehensiVe Organic Transformations: A Guide to Functional Group Preparations; Wiley-VCH: New York, 1999. (13) Sandler, S. R.; Karo, W. Organic Functional Group Preparation; Academic Press: New York, 1968. (14) Kabalka, G. W.; Verma, R. S. Compr. Org. Synth. 1991, 8, 363379. (15) Hudlicky, M. Reductions in Organic Chemistry; American Chemical Society: Washington, DC, 1996. (16) Park, H. S.; Lee, H. Y.; Kim, Y. H. Org. Lett. 2005, 7 (15), 31873190.
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Note that high selectivity was observed even though both starting substrates have a hydroxyl substituent. The retention of the product in the gel was ∼0%, relative to p-xylene (standard). In all the reactions, the product solution was easily separated from the Pd(0)/gel conjugate simply by decanting. Furthermore, the catalyst system can be recycled at least 10 times without significant loss of activity or selectivity (see Supporting Information). As shown in Figure 4, the selectivity for a reaction using Pd(0)/PA4 in the 10th cycle (2) is essentially identical to that obtained using fresh Pd(0)/PA4 (9). Similar observations were made for the other test pairs. There was no observable leaching of the supported Pd(0) species;10,17 following hydrogenation and removal of the gel, the addition of dihydrogen to the toluene solution of the substrates did not result in further hydrogenation. In conclusion, we have demonstrated the facile synthesis of Pd(0) incorporated hydrogel with a homogeneous distribution of Pd(0) nanoparticles within the polymer network. The water-swollen Pd(0)/gel was used to selectively hydrogenate/ deoxygenate hydrophilic compounds. Very high selectivities were observed in three test reactions: hydrogenation of olefins and nitroarenes and deoxygenation of alcohols. The selectivity was lower with a more cross-linked hydrogel. The catalyst system is easy to recycle without loss of selectivity and activity. This procedure for the selective hydrogenation of hydrophilic compounds in the presence of hydrophobic counterparts is superior to those presently known in terms of both simplicity and the ease of separation and recycling of the catalyst. Acknowledgment. We thank the Department of Energy, Office of Basic Energy Sciences, for funding. Supporting Information Available: Experimental details and other relevant supporting data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM062526+ (17) Ciriminna, R.; Pagliaro, M. Curr. Org. Chem. 2004, 8, 1851-1862.
