Immobilized silver nanoparticles on silica gel as an efficient catalyst in nitroarene reduction

Article abstract of DOI:10.1016/j.cclet.2010.05.024

Nanoparticles have properties that can be fine-tuned by their size as well as shape. Hence, there is significant current interest in preparing nano-materials of small size dispersity and to arrange them in close-packed aggregates. This letter describes a way of synthesising silver nanoparticles and their protection to aggregate by silica gel. The combination of catalytic quantities of immobilized silver nanoparticles with reductive ability of NaBH₄ efficiently reduces aromatic nitroarenes to the corresponding amines in aqueous medium. Noteworthy is that highly chemoselective reactions were achieved in the presence of other functional groups such as halogen and carboxylic acid groups. The silver particles immobilized on silica gel are stable in the presence of oxygen for several months.

Full text of DOI:10.1016/j.cclet.2010.05.024

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 1015–1019
www.elsevier.com/locate/cclet
Immobilized silver nanoparticles on silica gel as an efficient
catalyst in nitroarene reduction
a
a
b
Ali Reza Kiasat a, , Roya Mirzajani , Fakhri Ataeian , Mehdi Fallah-Mehrjardi
*
a Chemistry Department, College of Science, Shahid Chamran University, Ahvaz 61357-4-3169, Iran
b Marine Chemistry Department, Khoramshahr Marine Science and Technology University, Khoramshahr 669, Iran
Received 18 November 2009
Abstract
Nanoparticles have properties that can be fine-tuned by their size as well as shape. Hence, there is significant current interest in
preparing nano-materials of small size dispersity and to arrange them in close-packed aggregates. This letter describes a way of
synthesising silver nanoparticles and their protection to aggregate by silica gel. The combination of catalytic quantities of
immobilized silver nanoparticles with reductive ability of NaBH4 efficiently reduces aromatic nitroarenes to the corresponding
amines in aqueous medium. Noteworthy is that highly chemoselective reactions were achieved in the presence of other functional
groups such as halogen and carboxylic acid groups. The silver particles immobilized on silica gel are stable in the presence of
oxygen for several months.
# 2010 Ali Reza Kiasat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Silver nanoparticle; Reduction; Aromatic nitro compounds; Aromatic amines; Sodium borohydride
The design and synthesis of materials in nanometer size is currently the subject of intense research because their
properties differ considerably from those of the corresponding bulk materials [1]. In particular silver nanoparticles are
particularly interesting due to their roles as substrates in studies of surface-enhanced Raman scattering [2], nonlinear
optical properties [3], and catalysis [4]. Avariety of preparation routes have been reported for the preparation of silver
nanoparticles; notable examples include, biological methods [5], photoreduction [6], reduction of silver salts by
different chemical reductants such as ascorbic acid [7], sodium citrate [8], hydrazine [9], sodium borohydride [10] or
some solvents (EtOH [11], DMF [12]). For protection of silver nanoparticles by silica gel various methods was
reported include photoreduction [13], voltammetry [14] and ultrasound [15].
Aromatic amines are important starting materials and intermediates for the manufacture of a great variety of
chemicals, such as dyestuffs, pharmaceutical products, agricultural chemicals, photographic chemicals, additives,
surfactants, textile auxiliaries, chelating agents and polymers. They are generally synthesized by the chemical reduction
of nitroarenes [16]. Although a variety of methods has been well documented [17] for this purpose, due to the stringent
environmental concerns constant efforts are being made to find safer alternative to the previously reported.
In this work, stable colloidal silver was prepared by reduction of silver nitrate with NaBH4. The silver nanoparticles
were characterized by transmission electron microscopy (TEM) and UV–vis spectra. For stabilization of nanoparticles
* Corresponding author.
E-mail address: akiasat@scu.ac.ir (A.R. Kiasat).
1001-8417/$ – see front matter # 2010 Ali Reza Kiasat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.05.024

1016
A.R. Kiasat et al. / Chinese Chemical Letters 21 (2010) 1015–1019
and prevention of their accumulation, chemical reduction of AgNO3 was done in the aqueous medium includes silica
gel. In continuation of our researches to develop green chemistry by using water as reaction medium [18], the
immobilized silver nanoparticles were used in reduction of nitroarenes to the corresponding aromatic amines with
NaBH4 in water under reflux conditions.
1. Experimental
Aqueous NaBH4 solution (30 mL, 8 mmol/L) was placed in an ice bath. After cooling, 1.5 g silica gel was added to
the solution. Then 10 mL AgNO3 solution (1 mmol/L) was added dropwise to this solution. The color of the solution
was turned yellow due to the formation of silver particles. After 30 min stirring, the solid was separated from solution
by centrifuge. The yellow precipitate was washed with water for several times and oven-dried at 80 8C. The
immobilized silver nanoparticle was stored in a dark colored bottle.
In a 50 mL round bottom flask, nitroaromatic compound (1 mmol), NaBH4 (4–6 mmol) and SiO2-Ag(nano) (1.5 g)
were added and completely mixed. Then water (5 mL) was added to the mixture and the resulting suspension was
stirred under reflux conditions for the time shown in Table 1. The progress of the reaction was monitored by thin layer
chromatography, TLC using Et2O/n-hexane (1/5) as eluent.
2. Results and discussion
Ag nanoparticles immobilized on silica gel was easily prepared by chemical reduction of AgNO3 by NaBH4 in
water and in the presence of silica gel. UV–vis spectroscopy is one of the most widely used techniques for structural
Table 1
Reduction of nitroarenes in the presence of silver nanoparticles immobilized on silica gel under reflux conditions in watera.
Entry
1
Substrate
NaBH4 (mmol)
Time (min)
Product
6
60
2
3
4
5
6
5
5
5
4
5
60
45
35
30
60
7
6
90
a
In all cases, conversion was 100%.

[(ig._1)TD$FIG]
A.R. Kiasat et al. / Chinese Chemical Letters 21 (2010) 1015–1019
1017
Fig. 1. UV–vis absorption spectrum of synthesized colloidal silver nanoparticles.
characterization of silver nanoparticles [19]. The absorption spectrum (Fig. 1) of the yellow silver colloids showed a
surface Plasmon absorption band with the maximum of 379 nm indicating the presence of spherical or roughly
spherical Ag nanoparticles, and this fact was confirmed by TEM images of Ag nanoparticles (Fig. 2a) and Ag
nanoparticles immobilized on silica gel (Fig. 2b).
In order to show the catalytic ability of this nanocatalyst in reduction reaction of nitroarenes, in a typical
experiment, nitrobenzene (1 mmol) and NaBH4 (6 mmol) were stirred under reflux conditions in water. TLC analysis
of the reaction mixture did not show completion of the reaction after 8 h and the reaction media was contaminated by
azobenzene. But by adding SiO2-Ag(nano) to the reaction media, the reaction was completed within 1 h and aniline was
[(ig._2)TD$FIG]
Fig. 2. TEM image of (a) synthesized colloidal silver nanoparticles, (b) the Ag nanoparticles immobilized on silica gel and (c) aggregated silver
nanoparticles.

1[(ig._3)TD$FIG] 018
A.R. Kiasat et al. / Chinese Chemical Letters 21 (2010) 1015–1019
Fig. 3. Scheme of the catalytic process occurring at silver nanoparticles supported on silica spheres. E: electrophile (AgNO2); N: nucleophile
(BH4ꢀ ions).
produced as only product in quantitative yield. The effect of silica gel was definitely confirmed by reaction of
nitrobenzene with NaBH4 under similar conditions, in the presence of Ag nanoparticle. TLC analysis of the reaction
mixture did not show completion of the reaction after 5 h. In addition the Ag nanoparticle was aggregated (Fig. 2c).
The catalytic role of silver nanoparticle can be explained by an electrochemical mechanism, where silver
nanoparticles-supported silica gel serve as an electron relay between oxidant and reductant, and electron transfer
occurs via the supported metal particles. In general, a nucleophile can donate electrons to metal particles and in
contrast, an electrophile would capture electrons form metal particles. As a general rule, the rate of a catalytic
reactions should be determined by the concentration of the catalyst and the difference in potential between the
reductant adsorbed metal particles and the oxidant used. In addition, the redox potential of a metal nanoparticle is
dependent on the size, and it increases with the increase of the size (Fig. 3) [20]. The hydroxyl functional groups of
silica gel interact with silver ions and improve the nanoparticles stability. The aggregation is prevented leading to
increase of catalytic active sites.
Reduction of different nitroarenes carrying activated and deactivated groups were carried out by use of NaBH4 in
the presence of silver nanoparticles immobilized on silica gel in water under reflux conditions (Table 1). The reduction
of nitroarenes took place smoothly and easily to afford the corresponding amines as exclusive products according to
TLC. It is worthy to note that the azoxy, azo and hydrazo compounds as the usual side products in reduction of
nitroarenes were not observed in this method. The scope and generality of this process is illustrated with several
examples and the results are summarized in Table 1.
In order to show the merit of our procedure for the reduction of nitroarenes, we have shown the advantages
of present procedure by comparing our results with those previously reported in the literature for the reduction
of nitrobenzene (Table 2). In conclusion, we have demonstrated an environmentally friendly route for reduction of
aromatic nitro compounds with NaBH4 in the presence of SiO2-Ag(Nano) in water that can be used for laboratory
synthesis and we feel that it may be a suitable addition to methodologies already present in the literature.
Table 2
Comparison of reduction of nitrobenzene with different methods.
Entry
Reaction conditions
Temp. (8C)
Time (h)
Yields (%)
Aniline
Azobenzene
1
2
3
4
5
6
7
8
NaBH4, DMSO [21]
85
2
8.5
4
22
50
74
26
–
NaBH4, Sulfolane [21]
100
96
Se, NaOAc, DMF [22]
100
100
100
100
100
100
Sm, 1,10-dioctyl-4,40-bipyridinium dibromide, MeOH [23]
NH4Cl, LiCl, NaBH4, Diglyme [24]
ClP(OEt)2, (iPr)2NEt, CHCl3, N2 [25]
ZrCl4, NaBH4, THF [26]
r.t.
3
–
162
50
5
–
12
6
–
Reflux
Reflux
–
SiO2-Ag(Nano), NaBH4, H2Oa
1
–
a
Present procedure.

A.R. Kiasat et al. / Chinese Chemical Letters 21 (2010) 1015–1019
1019
Acknowledgment
Partial supports for this work by Shahid Chamran University Research Council are gratefully acknowledged.
References
` ` `
[1] O. Cıntora-Gonzalez, C. Estournes, M. Richard-Plouet, et al. Mater. Sci. Eng. 15 (2001) 179.
[2] W.J. Pileth, J. Phys. Chem. 86 (1982) 3461.
[3] F. Hache, D. Richard, C. Flytzanis, J. Opt. Soc. Am. B3 (1986) 1647.
[4] A. Henglein, J. Lillie, J. Phys. Chem. 85 (1981) 1246.
[5] N. Vigneshwaran, N.M. Ashtaputre, P.V. Varadarajan, et al. Mater. Lett. 61 (2007) 1413.
[6] H.H. Huang, X.P. Ni, G.L. Loy, et al. Langmuir 12 (1996) 909.
[7] D. Wang, C. Song, Z. Hu, et al. Mater. Lett. 59 (2005) 1760.
[8] H. Jia, J. Zeng, W. Song, et al. Thin Solid Films 496 (2006) 281.
[9] Y. Zhu, M. Ji, H. Zheng, et al. Mater. Lett. 58 (2004) 1121.
[10] L. Limsavarn, V. Sritaveesinsub, S.T. Dubas, Mater. Lett. 61 (2007) 3048.
[11] A. Pal, S. Shah, S. Devi, Mater. Chem. Phys. 114 (2009) 530.
[12] Y. Gao, P. Jiang, L. Song, et al. J. Cryst. Growth 289 (2006) 376.
[13] V. Hornebecq, M. Antonietti, T. Cardinal, et al. Chem. Mater. 15 (2003) 1993.
[14] A.M. Bond, W. Miao, T.D. Smith, et al. Anal. Chim. Acta 396 (1999) 203.
[15] C.K. Huang, C.Y. Chen, J.L. Han, et al. J. Nanopart. Res. 12 (2009) 199.
[16] M. Lauwiner, P. Rys, J. Wissmann, Appl. Catal. A: Gen. 172 (1998) 141.
[17] (a) F. Fache, E. Schulz, M.L. Tommasino, et al. Chem. Rev. 100 (2000) 2159;
(b) I. Kazmierski, J.M. Paris, C. Gosmini, et al. Tetrahedron Lett. 44 (2003) 6417;
`
(c) S. Ung, A. Falguieres, A. Guy, C. Ferroud, Tetrahedron Lett. 46 (2005) 5913;
(d) Y. Moglie, C. Vitale, G. Radivoy, Tetrahedron Lett. 49 (2008) 1828.
[18] (a) A.R. Kiasat, R. Badri, S. Sayyahi, Chin. Chem. Lett. 19 (2008) 1301;
(b) A.R. Kiasat, M. Fallah-Mehrjardi, Catal. Commun. 9 (2008) 1497;
(c) A.R. Kiasat, R. Mirzajani, H. Shalbaf, et al. Chin. Chem. Lett. 20 (2009) 1025.
´
´
[19] G.R. Gattorno, D. Dıaz, L. Rendon, et al. J. Phys. Chem. B 106 (2002) 2482.
[20] Z.J. Jiang, C.Y. Liu, L.W. Sun, J. Phys. Chem. B 109 (2005) 1730.
[21] R.O. Hutchins, D.W. Lamson, L. Rua, et al. J. Org. Chem. 36 (1971) 803.
[22] X. Liu, S. Lu, J. Mol. Catal. A: Chem. 212 (2004) 127.
[23] C. Yu, B. Liu, L. Hu, J. Org. Chem. 66 (2001) 919.
[24] C. Yang, C.U. Pittman Jr., Synth. Commun. 28 (1998) 2027.
[25] A.S. Kende, J.S. Mendoza, Tetrahedron Lett. 32 (1991) 1699.
[26] K.P. Chary, S.R. Ram, D.S. Iyengar, Synlett (2000) 683.