Table 1 Catalytic performance of 1–Rh and 2–Rh NPs in arene hydrogenation
Substrate
Phenol
Stab.
Solvent
Water
Con. (%)
100
Sel. (%)
1
Cyclohexanol
9.8
Cyclohexanone
0.1
9
—
Anisole
Styrene
1
1
Water
Water
100
100
Ethylbenzene
4
Ethylcyclohexane
55.7
4.2
Ethyl-benzene
n-Propyl-benzene
Toluene
1
1
2
2
Water
Water
[bmim][BF ]
4
99
69
95
97
Ethylcyclohexane
Propylcyclohexane
Methylcyclohexane
Methylcyclohexane
Toluene
2 4
[C OHmim][BF ]
Reaction conditions: reactant (0.5 mmol), 1 Â 10À3 mmol of Rh NPs (Rh : stabilizer = 1 : 20) in 1 ml water or IL, H
(20 atm), 60 1C, 2 h.
2
Dr V. Laporte and S. Katsyuba are thanked for helpful
discussions on the XPS data and IR spectra, respectively.
Notes and references
1
2
A. Roucoux, J. Schulz and H. Patin, Chem. Rev., 2002, 102, 3757.
For examples see: (a) C. Chiappe, D. Pieraccini, D. Zhao, Z. Fei
and P. J. Dyson, Adv. Synth. Catal., 2006, 348, 68; (b) I. Favier,
M. Gomez, G. Muller, D. Picurelli, A. Nowicki, A. Roucoux and
´
J. Bou, J. Appl. Polym. Sci., 2007, 105, 2772; (c) D. D. Ankola,
K. Ravi, F. Chiellini and R. Solaro, Macromolecules, 2009, 42,
7
388; (d) G. V. Ramesh, S. Porel and T. P. Radhakrishnan, Chem.
Soc. Rev., 2009, 38, 2646; (e) J. Dupont and J. D. Scholten, Chem.
Soc. Rev., 2010, 39, 1780.
D. Zhao, M. Wu, Y. Kou and E. Min, Catal. Today, 2002, 74, 157.
Fig. 4 Zeta potential of PVP–Rh, 1–Rh and PAA–Rh NPs at
pH = 7.
3
1
4
4 (a) Y. Li and M. A. El-Sayed, J. Phys. Chem. B, 2001, 105, 8938;
b) A. Roucoux, J. Schulz and H. Patin, Adv. Synth. Catal., 2003, 345,
22; (c) C. A. Stowell and B. A. Korgel, Nano Lett., 2005, 5, 1203.
5 N. Yan, X.-C. Xiao and Y. Kou, Coord. Chem. Rev., 2010, 254, 1179.
Electrostatic stabilization was assessed by zeta potential
measurements (Fig. 4), and as expected, the Rh NPs stabilized
by PVP have a low overall charge, indicated by a zeta potential
of À20 mV. The negative charge might originate from absorbed
(
2
6
D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed., 2005,
4, 7852.
M. Bencini, E. Ranucci, P. Ferruti, C. Oldani, E. Licandro and
4
3
anions such as chloride from the NP precursor (RhCl ) or
7
electron donation from PVP, as demonstrated in the case
S. Maiorana, Macromolecules, 2005, 38, 8211.
1
5
of a Au NP–PVP system. Compared to PVP–Rh, a highly
negative value (À64 mV) is observed for 1–Rh, indicative of an
electrical double-layer around the NPs, and resulting in a
Columbic repulsion between them. In general, NPs with zeta
potentials higher than Æ60 mV are regarded as being highly
8 (a) N. Yan, J. Zhang, Y. Tong, S. Yao, C. Xiao, Z. Li and Y. Kou,
Chem. Commun., 2009, 29, 4423; (b) N. Yan, J. Zhang, Y. Yuan,
G. Chen, P. J. Dyson, Z. Li and Y. Kou, Chem. Commun., 2010,
4
6, 1631.
9 C. Zhao, W. Gan, X. Fan, Z. Cai, P. J. Dyson and Y. Kou,
J. Catal., 2008, 254, 244.
10 Y. Wang, J. Ren, K. Deng, L. Gui and Y. Tang, Chem. Mater.,
16
stable, and NPs stabilized by carboxylate-containing polymers
2
000, 12, 1622.
1
7
tend to have zeta potentials ranging from À40 to À60 mV.
For comparison, polyacrylic acid (PAA) coated Rh NPs were
prepared in water by NaBH reduction (PAA–Rh) and their
1
1 S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation
for Organic Synthesis, John Wiley & Sons, 2001, p. 427.
1
2 (a) J. Schulz, A. Roucoux and H. Patin, Chem.–Eur. J., 2000, 6, 618;
4
(
b) A. Roucoux, J. Schulz and H. Patin, Adv. Synth. Catal., 2003, 345,
22; (c) P. Migowski and J. Dupont, Chem.–Eur. J., 2006, 13, 32;
(d) C. Zhao, H. Wang, N. Yan, C. Xiao, X. Mu, P. J. Dyson and
Y. Kou, J. Catal., 2007, 250, 33; (e) B. Leger, A. Denicourt-Nowicki,
A. Roucoux and H. Olivier-Bourbigou, Adv. Synth. Catal., 2008, 350,
53; (f) B. Leger, A. Denicourt-Nowicki, H. Olivier-Bourbigou and
A. Roucoux, Inorg. Chem., 2008, 47, 9090.
13 K. W. Wulser and M. A. Langell, Catal. Lett., 1992, 15, 39.
zeta potential was found to be À49 mV, in agreement with the
2
1
6
literature values. The reason for the more negative zeta
potential of 1–Rh compared to PAA–Rh is not clear, but is
likely to be due to combined electron donation from both the
amide and the carboxylate group.
´
1
´
In summary, we have shown that Rh NPs coated by 1 exhibit
superior thermal and catalytic stability to PVP coated NPs. The
enhanced stability may be attributed to the presence of
the carboxylate group that may provide weakly coordinate to
the NP surface and an electrical double-layer that helps prevent
aggregation. Combined these additional interactions result in
NPs that can be recycled and reused without loss of activity.
N. Yan thanks the EU for Marie Curie International
Incoming Fellowship (Number: 252125-TCPBRCBDP).
Prof. J. A. Hubbell and Dr A. Vandervlies are acknowledged
for assistance in zeta potential measurement. Prof. H. A. Klok
and Sanhao Ji are thanked for carrying out the GPC analysis.
1
4 R. G. Finke, in Metal Nanoparticles: Synthesis, Characterization
and Applications, ed. D. L. Feldheim and C. A. Foss, Jr., Marcel
Dekker, New York, 2002, ch. 2, p. 17–54.
1
1
5 H. Tsunoyama, N. Ichikuni, H. Sakurai and T. Tsukuda, J. Am.
Chem. Soc., 2009, 131, 7086.
6 ‘‘Zeta Potential of Colloids in Water and Waste Water’’, ASTM
Standard D 4187, American Society for Testing and Materials,
1985.
17 For examples see: (a) S. Chen and K. Kimura, Langmuir, 1999, 15,
1075; (b) K. Yoosaf, B. I. Ipe, C. H. Suresh and K. G. Thomas,
J. Phys. Chem. C, 2007, 111, 12839; (c) J. Hang, L. Shi, X. Feng
and L. Xiao, Powder Technol., 2009, 192, 166; (d) K. Mori,
A. Kumami, M. Tomonari and H. Yamashita, J. Phys. Chem. C,
2009, 113, 16850.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2529–2531 2531