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P. S. Rathore et al.
proceeded faster within 90 min and the kmax shifted from
381 to 307 nm. Whereas, in the absence of NiNPs, the peak
at 381 nm corresponding to PNA did not disappear even
after 24 h of reflux.
under high H2 pressure (0.1–7 MPa) for 2–4 h [29]. This
opens up new possibilities for eco-friendly synthesis of
aromatic amines.
Furthermore, experiments were performed using a dif-
ferent mole ratio of NaBH4 in standard reaction. On using
NaBH4 less than 0.005 mol, reaction yield was less than
10 % even after 24 h of reflux (Table 1, entry 4). When we
increased moles of NaBH4 from 0.005 to 0.007, the yield
increased to 31 5 % (Table 1, entry 5), whereas using
0.0132 mol of NaBH4 resulted into a remarkable yield
(95 2 %) at 35 °C within 180 min (Table 1, entry 6).
However, lowering the amount of NaBH4 caused an
increase in the reaction time and hence an adequate
quantity of NaBH4 was used in optimum reaction condi-
tion. Good to high yields were obtained at 80 °C temper-
ature and the reaction was completed within 15 min. This
shows that as the reaction temperature increases, reaction
time decreases (Table 1, entries 8, 9).
3.2 Decolourization of Azo Dyes
Degradation of dyes, especially azo dyes which contribute
to about 70 % of all used dyes, is difficult due to their
complex structure and synthetic nature [30]. The most
commonly used method is adsorption which causes the
high operational cost because regeneration is generally not
so effective [31]. The use of noble metal NPs like Pd [32],
Au [33], Ni [34] and Ag [35] in dye decolourization or
reduction has been reported. This is the first report on the
MW assisted decolourization reaction of azo dye by NiN-
Ps. As a representative case, the catalytic activity of NiNPs
was first investigated in the decolourization of Orange II
(OR-II) in MW, a kind of azo dye with –N=N– bonds. UV–
Vis spectra in Fig. 2 shows that before the addition of
NiNPs, there was the maximum absorption band centered
at 483 nm, which could be assigned to the conjugated
system formed by the –N=N– bonds of OR-II. The colour
of the dye is attributed to this maximum absorption band,
which could be used to monitor the decolourization reac-
tion. Ascorbic acid alone could not induce complete dec-
olourization of the dye in MW. But when irradiated in
presence of NiNPs ascorbic acid led to complete deco-
lourization of the dye. The absorption band at 483 nm
decreased and disappeared, which could be ascribed to the
loss of conjugation of –N=N– bonds due to electron
transfer through surface of NiNPs (Scheme 3). As dis-
cussed by Pande and co-workers [36], when the size of
bulk metals goes to the nanoscale, electron transfer is
efficient, in addition to large surface area. Thus the electron
transfer between the dye and ascorbic acid occurs before
rapid diffusion pulls them apart [37].
Variation in quantity of NPs (Table 1, entries 10–14)
under optimum reaction conditions showed that even
10 mg was sufficient for catalyzing the reaction at RT but
took 300 min for maximum conversion (Table 1, entry 10).
On the other hand with 40 mg the reaction time decreased
to 180 min (Table 1, entry 13). Beyond 50 mg the reaction
time and yield did not change (Table 1, entry 14).
Under the optimized conditions, we investigated the
scope of converting various aromatic nitro compounds to
aromatic amines by using NiNPs, NaBH4 and water
(Scheme 2). NiNPs was applied for all substrates and they
almost gave same yield in similar reaction time (Table 2).
The results were satisfactory for various aromatic nitro
substrates with different functional groups (Table 2). Fur-
ther, this catalytic system works efficiently in aqueous
system and therefore no hazardous solvent was required.
This reaction has been investigated in the presence of other
protic solvents such as methanol, ethanol, and ethylene
glycol; however, none of them gave satisfactory results
(data not given). In an aqueous system NaBH4 gives
sodium metaborate (NaBO2) [29] which is the only waste
generated during the reaction. However sodium metaborate
can be recycled using magnesium hydride (MgH2) or
magnesium silicide (Mg2Si) by annealing (350–750 °C)
Although the role of starch is only to cap the NiNPs and
prevent its oxidation [26] its role as adsorbent needs to be
assessed in the present case since starch is highly active
towards adsorption of dye molecules [38]. To rule out the
possibility of adsorption, a control experiment was per-
formed with only starch. No significant decrease in the
absorbance was observed indicating that the decolouriza-
tion is not simply due to dye adsorption onto the support.
Also under MW condition adsorption will not occur due to
the high kinetic energy of dye molecules.
Similarly, in the study of decolourization of other azo
dyes with AA, it was noted that the absorption peak at
corresponding wavelength (kmax) (Table 3) of all azo dyes
significantly decreases indicating that NiNPs catalyzes the
decolourization of azo dyes.
The experiment was also successful with a mixture of
dyes. In all experiment decolourization occurred within
Scheme 2 General scheme for reduction of various nitro aromatics
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