the precipitate changed to greyish-black. The Pd@MCs that formed
were centrifuged (4000 rpm, 2 min), washed with de-ionized water,
dried at room temperature and stored for further studies.
proves that the activity of the Pd nanoparticles is retained in
the MCs.
After the reaction, the Pd@MC catalyst was separated from
the reaction mixture by centrifugation (4000 rpm, 2 min). It
should be noted that in the case of Pd–PLL, catalyst recovery
was not possible at the same centrifugal speed. In Fig. 3(b)–(c),
the optical and TEM images of the used catalysts reveal the
intact hollow structure of the MCs. The reusability of the
separated catalyst was checked in the hydrogenation of
nitrobenzene to aniline, and it was found to be as active as
in the 1st cycle.w The activity was consistently similar, even up
to the 5th cycle. An AAS analysis of the MCs after the 5th
cycle showed a Pd content of 4.28 wt%, indicating no appar-
ent reduction in metal content. Since the MCs’ structures
remain mostly unaltered, it is believed that the active Pd sites
also remain unchanged, and hence the catalytic activity is
not affected.
y Pd@MCs as a catalyst for hydrogenation reactions:To demonstrate
the catalytic activity of the Pd@MCs, the hydrogenation of nitro-
arenes to anilines was used as a model reaction. In a typical reaction,
2 mmol of nitroarene was dissolved in 5 mL of ethanol with 0.005 g of
catalyst under 1 atmosphere of hydrogen. The reaction progress was
monitored by thin layer chromatography (TLC) and gas chromato-
graphy (GC). After the reaction, the hollow silica Pd@MCs were
separated from the reaction mixture by centrifugation (4000 rpm,
2 min). Thereafter, the catalyst was washed with ethyl acetate and used
in the next reaction.
1 J. Tsuji, Palladium Reagents and Catalysts, Wiley, Chichester,
1995; J.-L. Malleron, J.-C. Fiaud and J.-Y. Legros, Handbook of
Palladium-Catalyzed Organic Reactions, Academic Press, London,
2000.
2 G. Marck, A. Villiger and R. Buchecker, Tetrahedron Lett., 1994,
35, 3277; C. R. LeBlond, A. T. Andrews, Y. Sun and J. R. Sowa,
Jr, Org. Lett., 2001, 3, 1555; B. M. Choudary, S. Madhi, N. S.
Chowdari, M. L. Kantam and B. Sreedhar, J. Am. Chem. Soc.,
2002, 124, 14127.
The combination of nanoparticles, polyamines and porous
MC structure probably give rise to synergistic effects that
enhance the activity and durability of the catalyst. Thus, the
above unique approach of encapsulating nanoparticles in a
MC structure may find wide application in other transition
metal-catalyzed reactions. Moreover, this method can also
be modified to create different pore sizes or architectures in
the MC, depending upon the size of the silica particles and
the way they are assembled, thereby leading to shape selec-
tive catalysis.
3 R. Raja, T. Khimyak, J. M. Thomas, S. Herman and B. F. G.
Johnson, Angew. Chem., Int. Ed., 2001, 40, 4638.
4 R. Akiyama and S. Kobayashi, Angew. Chem., Int. Ed., 2001,
40, 3469; A. M. Jansson, M. Groti, K. M. Halkes and M.
Meldal, Org. Lett., 2002, 4, 27; C. Ramarao, S. V. Ley, S. C.
Smith, I. M. Shirley and N. DeAlmeida, Chem. Commun.,
2002, 1132; C. K. Y. Lee, A. B. Holmes, S. V. Ley, I. F.
McConvey, B. Al-Duri, G. A. Leeke, R. C. D. Santos and J.
P. K. Seville, Chem. Commun., 2005, 2175; K. Okamoto, R.
Akiyama, H. Yoshida, T. Yoshida and S. Kobayashi, J. Am.
Chem. Soc., 2005, 127, 2125; H. Oyamada, R. Akiyama, H.
Hagio, T. Naito and S. Kobayashi, Chem. Commun., 2006,
4297.
5 R. M. Crooks, M. Zhao, L. Sun, V. Chechik and L. K. Yeung,
Acc. Chem. Res., 2001, 34, 181; R. Gopidas, J. K. Whitesell and M.
A. Fox, Nano Lett., 2003, 3, 1757; C. Ornelas, L. Salmon, J. R.
Aranzaes and D. Astruc, Chem. Commun., 2007, 4946.
6 Z. Hou, N. Theyssen, A. Brinkmann and W. Leitner, Angew.
Chem., Int. Ed., 2005, 44, 1346; S. Pathak, M. T. Greci, R. C.
Kwong, K. Mercado, G. K. S. Prakash, G. A. Olah and M. E.
Thompson, Chem. Mater., 2000, 2, 1985; J. Y. Shin, B. S. Lee, Y.
Jung, S. J. Kim and S. Lee, Chem. Commun., 2007, 5238.
7 S. Kidambi, J. Dai, J. Li and M. L. Bruening, J. Am. Chem. Soc.,
2004, 126, 2658.
8 Y. Li and M. A. El-Sayed, J. Phys. Chem. B, 2001, 105, 8938.
9 R. K. Rana, V. S. Murthy, J. Yu and M. S. Wong, Adv. Mater.,
2005, 17, 1145; V. S. Murthy, R. K. Rana and M. S. Wong,
J. Phys. Chem. B, 2006, 110, 25619; J. Yu, V. S. Murthy, R. K.
Rana and M. S. Wong, Chem. Commun., 2006, 1097; M. S. Wong,
J. N. Cha, K. S. Choi, T. J. Deming and G. D. Stuky, Nano Lett.,
2002, 2, 583.
10 C. D. Wagner, W. M. Riggs, L. E. Davis and J. F. Moulder, in
Handbook of X-Ray Photoelectron Spectroscopy, ed. G. E. Muilenberg,
Perkin-Elmer Corporation, Eden Prairie, MN, USA, 1978.
11 M. Rossi, F. P. Silva, L. L. R. Vono, P. K. Kiyohara, E. L. Duarte,
R. Itri, R. Landers and G. Machado, Green Chem., 2007, 9, 379; J.
He, I. Ichinose, T. Kunitake, A. Nakao, Y. Shiraishi and N.
Toshima, J. Am. Chem. Soc., 2003, 125, 11034.
This work was supported by the European Community
Sixth Framework Program through a STREP grant to the
SELECTNANO Consortium, contract no. 516922.03/25/
2005. The authors thank Dr S. V. Manorama and Dr Shashi
Singh for TEM analysis and Dr Rajiv Trivedi for GC analysis.
Notes and references
z Synthesis of Pd@MCs: To encapsulate Pd inside the MCs, PLL was
used as the structure-directing agent. In a typical process, 1.3 mL of
PLL (2 mg mLꢀ1, 150 kDa) was gently vortex-mixed for 10 s with
7.8 mL of trisodium citrate solution (5.36 mM), keeping the ratio, R,
of the total negative charge on the citrate to the total positive charge
on the PLL at 10 : 1. The slightly turbid solution was aged for 30 min
and then vortex-mixed with 7.8 mL of silica sol (Snowtex-O, particle
diameter 13 ꢂ 3 nm; 20 wt% SiO2) for 20 s to form the MCs. The
cloudy suspension was allowed to age for 2 h, then centrifuged
(4000 rpm, 2 min) to remove the unreacted reagents. The colourless
precipitate was redispersed in 2.5 mL of trisodium citrate (5.36 mM)
and aged for another 12 h. To this suspension was added 1.25 mL of
sodium tetrachloropalladate (0.01 M), and this was stirred for 30 min
to allow the binding of [PdCl4]2ꢀ with the positively charged poly-
amines present inside the MC. The solution was then centrifuged to
remove the unbound [PdCl4]2ꢀ ions, and the yellow precipitate was
redispersed in 2 mL of trisodium citrate (5.36 mM). To reduce the
entrapped Pd ions, 100 mL of sodium borohydride (0.01 M) was added
and the vessel kept closed for 30 min, during which time the colour of
12 D. K. Yi, S. S. Lee and J. Y. Ying, Chem. Mater., 2006, 18, 2459.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4165–4167 | 4167