INORGANIC AND NANO-METAL CHEMISTRY
7
Table 3. Comparison of the catalytic activity of MPEG-Ag nanocomposite with other catalystsa.
Entry
Catalyst system
Conditions
Ref
1
2
3
4
5
Ag NPs graphite grafted with hyperbranched poly(amidoamine) dendrimer
Synthesized Ag-Polyaniline Using Ionizing Radiation
Ag/Al2O3 Nanocatalyst
Core-shell Ag@Ni magnetic nanocatalyst
MPEG-Ag nanocomposite
2/5 mg catalyst, 5mM of NaBH4 (1ml), water, rt, 25 min
20 mg catalyst, 5mM NaBH4 (1ml), 3 min
5/8 mg catalyst, 5mM of NaBH4 (1ml), water, rt , 22 min
75 mg catalyst, isopropylalkohol, 12 min
18 mg catalyst, 5mM of NaBH4 (1ml), water, rt, 50 min
[5]
[31]
[5]
[4]
[This work]
aReaction conditions: 1 ml of 4-nitrophenol (5 ꢃ 10ꢀ1 mM),
–
narrower range of particle sizes from 4-9 nm with a max- compound and BH4 The catalytic behavior of the synthe-
imum close to 8 nm. The TEM image of MPEG-Ag nano-
composite displayed Ag NPs with a particle size of around
25 nm on MPEG. The absence of small NPs of MPEG in
composite could be attributed to the coverage of these nano-
particles by Ag NPs due to the high interaction between
these nanoparticles.
sized MPEG-Ag nanocomposite was compared with some of
the reported catalytic systems (Table 3).
The reusability of the MPEG-Ag nanocomposite for cata-
lytic reduction of 4-NP in the presence of NaBH4 was dis-
played in the inset of Figure 6A. It was noticed that the
catalytic activity of the nanocomposite has no indicative
decrease after five consecutive cycles.
The reduction of 4-nitrophenol carried out in water solv-
ent at room temperature (25 ꢂC) as a model reaction to
study the catalytic activity of MPEG-Ag nanocomposite
using NaBH4-catalyst with different amounts (0.009, 0.018,
0.036 and 0.054 g) was added to the reaction mixture (Table
1). 0.018 g was selected as the best result. To monitor the
reduction process of nitro-compounds, UV–Vis absorption
spectra were recorded with time. In the absence of MPEG-
Ag nanocomposite, the corresponding product was not
obtained. The reaction was carried out in the water media
as a safe and green solvent and other solvents were not
tested. With optimized amounts, MPEG-Ag nanocomposite
catalyzed the reduction of the various nitro-aromatic com-
pounds in water at 25 ꢂC, and desired amino derivatives
were obtained in high yields (Table 2). The yield of reaction
was calculated from UV-Vis curves using the following
equation:
Conclusion
In summary, the loading of Ag NPs on modified PEG with
a
silane ligand (3, 30-bis-(3-triethoxysilylpropyl)-2, 20-
dithioxo [5, 50] bithiazolidinylidene-4, 40-dione) was carried
out in the presence of Et3N. First, PEG was modified with
the silane ligand in the presence of Et3N in toluene, and
then the MPEG containing spherical nanoparticles was used
as a substrate for the synthesizing of Ag NPs. The structure
of MPEG-Ag nanocomposite was confirmed by UV-Vis,
XRD, FT-IR, SEM, EDX, and TEM analysis. Obtained
MPEG-Ag nanocomposite was used as the catalyst for the
reduction of the nitro-aromatic compounds in water at
25 ꢂC for 50 min, and corresponding amino compounds
were produced in high yield. Also, recoverability of the cata-
lyst was investigated, and show excellent stability and
reactivity for five times without any change in catalytic
activity. Further studies on the loaded of other Nobel nano-
particles on our modified PEG and electrocatalysis behavior
are underway.
A0 ꢀ A
% ¼
ꢃ 100
A0
where A0 and A are the absorbance value of the initial con-
centration and absorbance value after the com-
plete reduction.
The reduction process of 4-NP was investigated by UV-
Vis spectroscopy. As shown in Figure 6, before the addition
of NaBH4, the absorption spectrum was observed at 320 nm
due to the characteristic peak of NP. After the addition of
NaBH4, the absorption peak shifted to 400 nm, which corre-
sponds to the generation of 4-nitrophenolate ions. In the
absence of a catalyst, this peak remained unchanged until
24 h. With the addition of MPEG-Ag catalyst to the reaction
mixture, yellow color of the mixture changed to the color-
less, the peak height at 400 nm decreased, and a new peak
was appeared around 300 nm (Figure 6A). These changes
attribute to the reduction of 4-nitrophenol to 4-aminophe-
nol.[6] The reduction process of 4-NB in the presence of
MPEG-Ag catalyst was also shown in Figure 6B. The reduc-
tion process of 4-NB was completed after 30 min that it is
lower than the reduction time of 4-NP which, this could be
related to the presence of hydroxyl groups in 4-NP.
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In the proposed reaction mechanism, MPEG-Ag nano-
composite plays a significant role in the transfer of electrons
from reducing ions to the nitro groups. The surface of
nanocomposite provides adsorption sites for nitro aromatic