Zwitterionic Surfactant stabilized palladium nanoparticles as catalysts in aromatic nitro compound reductions

Article abstract of DOI:10.5935/0103-5053.20150284

Palladium nanoparticles (NPs) stabilized by ImS3-14, a zwitterionic surfactant structurally related to ionic liquids, are revealed here to be good catalysts for the reduction of a large number of substituted aromatic nitro compounds. Our mass spectrometry results are consistent with the formation of amino products in a direct route, where the aromatic nitro compounds are initially reduced to nitroso compounds, which are then reduced to the hydroxylamine derivatives and finally to the anilines. Activation parameters showed that for most Pd catalysts reported in the literature, the mechanism seems to be similar, with lower enthalpy of activation (ΔH^?) being compensated by more negative entropy of activation (ΔS^?). As a result, the reaction is thermally compensated and the rate constants for most reactions rather similar. Furthermore, Pd NPs stabilized by ImS3-14 showed efficient catalytic activities for the reduction of aromatic nitro compounds, with high conversion and good selectivity even using very low loadings of metal.

Full text of DOI:10.5935/0103-5053.20150284

http://dx.doi.org/10.5935/0103-5053.20150284
J. Braz. Chem. Soc., Vol. 27, No. 2, 372-381, 2016.
Printed in Brazil - ©2016 Sociedade Brasileira de Química
0
103 - 5053 $6.00+0.00
Article
A
Zwitterionic Surfactant Stabilized Palladium Nanoparticles as Catalysts in
Aromatic Nitro Compound Reductions
Franciane D. Souza,* Haidi Fiedler and Faruk Nome*
Departamento de Química, Universidade Federal de Santa Catarina,
8
8040-900 Florianópolis-SC, Brazil
Palladium nanoparticles (NPs) stabilized by ImS3-14, a zwitterionic surfactant structurally
related to ionic liquids, are revealed here to be good catalysts for the reduction of a large number
of substituted aromatic nitro compounds. Our mass spectrometry results are consistent with the
formation of amino products in a direct route, where the aromatic nitro compounds are initially
reduced to nitroso compounds, which are then reduced to the hydroxylamine derivatives and finally
to the anilines. Activation parameters showed that for most Pd catalysts reported in the literature,
‡
the mechanism seems to be similar, with lower enthalpy of activation (ΔH ) being compensated
‡
by more negative entropy of activation (ΔS ). As a result, the reaction is thermally compensated
and the rate constants for most reactions rather similar. Furthermore, Pd NPs stabilized by ImS3-
1
4 showed efficient catalytic activities for the reduction of aromatic nitro compounds, with high
conversion and good selectivity even using very low loadings of metal.
Keywords: aromatic nitro reduction, mettalic nanoparticles, catalysis
1
1,14
Introduction
an imidazolium ring. Similar synthesis of palladium and
gold nanoparticles showing narrow size distribution used
zwitterionic surfactants of the type 3-(1-alkyl-3-imidazolio)
propane-sulfonate (ImS3-n, with n = 10, 12, 14 and 16,
Metallic nanoparticles (NPs) play a central role in
modern nanoscience because of their unique chemical
and physical properties, and several methods have been
15,16
Figure 1) in organic media.
1-6
developed for their synthesis. The most common methods
for their preparation involve the chemical reduction of
4
,7,8
metal salts and, due to the low stability in solution of
metallic nanoparticles, the synthesis is normally performed
Figure 1. Chemical structure of ImS3-n surfactants.
3
,4,7-10
in the presence of stabilizing agents.
Surfactants
are a versatile class of stabilizers, which show a broad
range of polar head groups and carbon chains and can
control size, shape and solubility of the nanoparticles.
The large proportion of their atoms on the surface
makes metallic nanoparticles targets of special interest for
11,12
3,8,17
9,18-21
12,22
application in catalysis.
properties and environments
The size,
shape, surface
9
5,9,23
Studies involving cationic, anionic and neutral surfactants
in nanoparticle synthesis are well known, but the use
of zwitterionic surfactants is relatively new. Early
work involving zwitterionic surfactants and metallic
of metal nanoparticles
can all have a significant effect on their catalytic activity.
The stabilizer plays an important role because it can reduce
the catalytic activity of the nanoparticle by hindering
substrate contact or by blocking product departure
13
nanoparticles by Reetz and Helbig describes the synthesis
of palladium nanoparticles (Pd NPs) using SB3-14 as a
stabilizer. Recently we described the preparation of Pd
NPs with narrow size distribution and high stability in
aqueous media using 3-(1-dodecyl-3-imidazolio)propane-
sulfonate (ImS3-12), a zwitterionic surfactant containing
2
4-26
from the surface.
Thus, there is a growing need for
the development of new methods for the synthesis of
well-dispersed nanoparticles and for stabilizers which do
not affect the catalytic activity.
The reduction of aromatic nitro compounds to anilines
is an important process because they are useful precursors
and intermediates in the preparation of dyes, polymers,
pharmaceutical substances, agricultural chemicals and
*
e-mail: franciane_dutra@hotmail.com, faruk.nome@ufsc.br

Vol. 27, No. 2, 2016
Souza et al.
373
other products. Additionally, anilines appear in numerous
natural products and display a wide range of biological
basis of TEM image analysis of ca. 300 particles are shown
in Table 1. The increase in w results in an increase of the
0
2
,10,27-30
activities.
Moreover, nitro compounds are often
nanoparticle diameter, proving the influence of reverse
10,30,31
15
highly toxic and not readily biodegradable.
Reduction
micelle media in nanoparticle synthesis.
to anilines produces compounds which are much less toxic
and easily biodegradable. There is no shortage of methods
Table 1. Size and concentration of palladium in Pd NPs solutions
2
,27,29,32-36
to realize this reduction process,
but the poor
a
Diameter / nm
[Palladium] / ppm
40.7
selective reduction of the nitro to the amino-group in the
presence of other reducible functionalities can make this
reaction a challenging task.Additionally, the reduction of a
nitro group proceeds in a series of steps and hydroxylamine,
b
Pd NPs 1
4.2 ± 1.0
c
Pd NPs 2
5.8 ± 2.0
125.0
a
b
Measured by energy dispersive X-ray fluorescence; value taken from
c
2,27,33
reference 16; value taken from reference 15.
hydrazine and azoarene side products can be obtained.
Due to these disadvantages, the development of new
methods or the improvement of already established methods
remains the focus of extensive research efforts.
In the present study, we examine the efficiency of
Pd NPs prepared by chemical reduction in the presence
of a zwitterionic surfactant of the ImS3-n family, which
is structurally related to ionic liquids. The catalytic
activity of the Pd NPs was initially tested in the reduction
The concentration of palladium in the Pd NPs in solution
was determined by energy dispersive X-ray fluorescence
(EDXRF), a non-destructive technique which is ideal for
analyzing liquid or solid samples with limits of detection
4
0
in the ppm scale: the results are given in Table 1. Since
all the catalytic tests were performed in aqueous media,
chloroform removal and nanoparticle dispersion in aqueous
media was necessary before using the Pd NPs prepared in
reverse micelles. Dispersion of Pd NPs in pure water was
of 4-nitrophenol by NaBH , a reaction which can be
4
6
,37,38
conveniently followed by UV-Vis spectroscopy.
−
1
After optimization, the reduction reaction was examined
for a large number of aromatic nitro compounds with
different substituents. Finally, a catalytic mechanism for
the reduction reaction is proposed, based on the evidence
obtained applying electrospray ionization tandem mass
spectrometry [ESI-MS(/MS)] in negative ion mode to
monitor the course of reaction, which allowed us to detect
several intermediates.
not possible and a 0.08 mol L NaCl aqueous solution
was used to solubilize the nanoparticles in aqueous media.
The Pd NPs showed long-term stability, without signs of
precipitation or aggregation (Figure 2).
Results and Discussion
Preparation and quantification of palladium nanoparticles
Reverse micelles can be used as nanoreactors for the
synthesis of metallic nanoparticles due to the constrained
environment provided by this system. In fact, ImS3-14
was used to prepare palladium nanoparticles by mixing a
Figure 2. Cartoon model of the change in stabilization mechanism of Pd
NPs by ImS3-14 surfactant in organic and in aqueous media.
Reduction of 4-nitrophenol catalyzed by palladium
nanoparticles
−
1
reverse micelle solution containing 0.05 mol L ImS3-14
−
1
and an aliquot of 0.2 mol L K PdCl aqueous solution
2
4
−
1
in chloroform with a solution containing 0.05 mol L
ImS3-14 and an aliquot of 2.0 mol L ascorbic acid
as the reductant. The water to surfactant concentration
The catalytic activity of the different Pd NPs was
compared using the reduction of 4-nitrophenol (Nip) to
−
1
4-aminophenol (Amp), in the presence of NaBH (the
4
6
ratios, w , used for the preparation of Pd NPs were
[NaBH ] exceeds the Nip and catalyst concentrations).
0
4
15
39,40
w = 4.8 (Pd NPs 1) and w = 14.7 (Pd NPs 2).
The
The reaction can be conveniently monitored in the UV-Vis
region, following the disappearance of 4-nitrophenolate
ion at 400 nm, and the formation of a new absorption
0
0
Pd NPs obtained were characterized by transmission
electronic microscopy (TEM) and images showed that the
NPs are well dispersed and show no sign of aggregation (see
Figure S1 in the Supplementary Information (SI) section).
The mean diameters of the nanoparticles obtained on the
6
band around 300 nm, attributed to the formation of Amp.
Figure 3a shows typical spectral changes observed as a
function of time in the presence of Pd NPs 1 and Figure 3b

3
74
Zwitterionic Surfactant Stabilized Palladium Nanoparticles as Catalysts in Aromatic Nitro Compound Reductions
J. Braz. Chem. Soc.
−
1
−1
Figure 3. (a) Variation in UV-Vis absorption spectra of reduction of 4-nitrophenol (0.066 mmol L ) with NaBH (9.71 mmol L ) catalyzed by palladium
4
−1
−1
nanoparticles (Pd NPs 1-0.0167 mmol L ) ([NaCl] = 0.033 mmol L ). (b) Time-dependence of the absorption of 4-nitrophenolate ions at 400 nm
[4-nitrophenol] = 0.066 mmol L ; [NaBH ] = 9.71 mmol L ; [Pd NPs 1] = 0.0167 mmol L , [NaCl] = 0.033 mmol L ).All reactions were carried at 25 °C.
−1
−1
−1
−1
(
4
shows the time dependence of the reduction of Nip at
before falling (Figure S3 in SI section), with the maximum
indicative of the presence of 4-hydroxylaminophenol,
which is expected to show absorbance near 300 nm,
before being further reduced toAmp. The observed profile
indicates that reduction occurs in several steps, where the
intermediates leave the nanoparticle surface after reduction
then return to the surface for subsequent steps of the
reduction to 4-aminophenol. According to Figure 4, the
reaction proceeds in three steps: the very fast reduction of
Nip to 4-nitrosophenol and then to 4-hydroxylaminophenol,
which is reduced to the aniline in the final, slowest step.
In order to confirm the UV-visible analysis, we performed
some mass spectrometry experiments.
6
,37,38
4
00 nm.
In the absence of the Pd NPs 1 catalyst, the
absorbance at 400 nm remains unaltered. Similar spectral
changes were obtained for all the Pd NPs systems studied.
In these reactions, after the addition of NaBH there
4
6
,37
is a short induction time, common to heterogeneous
catalysts and corresponding to the time required for
catalyst activation. This induction time has been attributed
to such factors as the diffusion-controlled adsorption of
substrates on the nanoparticle surface; the presence of
−
dissolved oxygen in water reacting faster with BH than
4
the reaction of borohydride with Nip, poisoning of the
nanoparticle surface by a metal oxide layer and the slow
6,37,38,41,42
surface restructuring of the nanoparticles.
The absence of a well-defined isosbestic point in
Figure 3a indicates that Nip is not converted to Amp in a
single step and, as can be seen in Figure S2 (SI section),
the changes in the UV spectra are complex. In fact,
following the absorbance changes in 296 nm, which is
the wavelength corresponding to the formation of Amp,
an initial increase in absorbance reaches a maximum
Mass spectrometric study of the Nip reduction
−
The reduction of Nip by BH catalyzed by palladium
4
nanoparticles was confirmed performing ESI-MS
experiments, with the equipment operating in negative
ion mode. Figure 5 shows the ESI(−)-MS spectrum of an
aliquot taken after the addition of NaBH solution to the
4
Figure 4. Cartoon model describing the reduction of 4-nitrophenol and intermediates on the nanoparticle surface.

Vol. 27, No. 2, 2016
Souza et al.
375
solution containing Nip and palladium nanoparticles. The
reduction was carried out with lower Pd NP concentration
to slow the reactions and allow the detection of the reaction
intermediates. Several peaks were observed, consistent
with the products and intermediates of the reaction
according to Figure 4. However, no signal corresponding
to 4-nitrosophenolate was detected, indicating that this
particular intermediate is consumed very fast.
aromatic nitro compounds are initially reduced to nitroso
compounds, which are then reduced to the hydroxylamine
derivatives and finally to the anilines.
Kinetic study of the Nip reduction
As the concentration of borohydride remains essentially
constant during the reaction, the pseudo first order analysis
could be used to evaluate the catalytic rate. The apparent
pseudo first order rate constants (k ) were determined from
app
the slope of the linear correlation of ln(A/A ) with time t,
0
shown in Figure 3b and the values of kapp as a function of the
concentrations of 4-nitrophenol and sodium borohydride
are shown in Figure 6. As shown in Figure 6a, an increase
in the concentration of Nip leads to a decrease in the value
of the experimental pseudo first order rate constant (kapp)
and the effect is observed with both Pd NPs, being slightly
more pronounced in the presence of the smaller Pd NPs 1.
As expected, the effect is related to the bigger surface
area of these nanoparticles, when compared with the same
concentration of Pd NPs 2. It is interesting to note that the
decrease in kapp corresponds to the plateau region in a typical
plot of initial rate versus [Nip] in terms of the Langmuir-
Hinshelwood model, indicating that diffusion and binding
of the substrates to the surface of the catalyst followed by
reaction on the catalyst surface are rate limiting (see inset
in Figure 6a). Conversely, as can be seen in Figure 6b, the
Figure 5. ESI-MS spectrum in negative ion mode of an aliquot
taken after 5 min of reaction in the presence of Pd NPs. The reaction
−
1
−1
conditions were [Nip] = 0.093 mmol L ; [NaBH ] = 9.71 mmol L ;
[
4
−1
−1
NaCl] = 0.033 mmol L ; [Pd] = 0.0019 mmol L and aliquots were
diluted ten times and injected in the mass spectrometer.
The presence of the intermediates is strong evidence that
after each step of the reduction, the organic intermediates
leave the surface of the nanoparticle, allowing both recovery
of the surface and detection of the intermediates by ESI-MS.
In fact, mass spectrometric analysis using ESI-MS in the
negative mode monitors the reaction collecting snapshots of
its anionic composition. Reagent, intermediates and products
present as anions are expected to be transferred directly from
the reaction solution to the gas phase and then detected by
ESI-MS. The peak at m/z 138.0 refers to the deprotonated
increase in the concentration of NaBH leads to an increase
4
in kapp and the diameter of nanoparticle has no effect on the
rate profile, indicating that recovery of the catalytic surface,
6
,37
via reduction with NaBH play a fundamental role.
4
The explanation for the characteristic dependence of k
app
−
on the concentration of Nip and BH is that both reactants
4
compete for free places on the nanoparticle surface and the
reaction takes place only between adsorbed species on the
surface. If most places are occupied by a single species, the
reaction rate will be decreased. This is observed at higher
6,37,38
4-nitrophenoxide ion and the m/z 107.9 fragment indicates
concentrations of Nip.
the formation of 4-aminophenolate, the final product of
reaction. Furthermore, in this spectrum, we detected the
deprotonated 4-hydroxyaminophenol ion of m/z 123.9, an
intermediate of the reaction and this peak displaying a lower
intensity when compared to the 4-nitrophenolate signal. The
ESI(−)-MS spectrum for Pd NPs 2 and ESI(+)-MS and ESI-
MS/MS spectra for both systems are shown in Figures S4 to
S8 in the SI section. ESI(+)-MS and the cleavage observed in
the ESI-MS/MS spectra (see SI section) showed several ions
consistent with the presence of Nip (peak of m/z = 138.0),
Amp (peak of m/z = 107.9) and 4-hydroxyaminophenol (peak
of m/z = 123.9). Clearly, our results are consistent with the
formation of amino products in a direct route, where the
The kapp is found to be proportional to the total surface
area available in the metal nanoparticle and associated
with zero-order reaction dependence with respect to
−
41
BH . Thus, the rate of reduction of Nip can be defined
4
6,37,41
by equation 1:
dc
Nip
= �k c = �k Sc
(1)
app Nip
1
Nip
dt
where kapp is the apparent rate constant, k is the rate
1
constant normalized to S, the total surface area of metal
nanoparticles normalized to the unit volume of the reaction
system. Figure 7 shows the plots of kapp as a function of
the surface area S for palladium nanoparticles. The rate

3
76
Zwitterionic Surfactant Stabilized Palladium Nanoparticles as Catalysts in Aromatic Nitro Compound Reductions
J. Braz. Chem. Soc.
−
−1
−1
Figure 6. (a) Dependence of the apparent rate constant k on the concentration of Nip at [BH ] = 9.71 mmol L ; [NaCl] = 0.033 mmol L ;
app
4
−1
[
Pd] = 0.0167 mmol L . The insert shows the dependence of initial rate on the concentration of Nip; (b) dependence of the apparent rate constant k on
app
−
−1
−1
−1
2
−1
the concentration of BH at [Nip] = 0.093 mmol L ; [NaCl] = 0.033 mmol L ; [Pd] = 0.0167 mmol L . () Pd NPs 1 (S = 0.21 m L ); () Pd NPs 2
4
2
−1
(
S = 0.14 m L ). All reactions were carried at 25 °C.
−
1
−2
constant k obtained for Pd NPs 1 = 0.142 s m L and for
1
−1
−2
Pd NPs 2 = 0.299 s m L. The Pd NPs 2 showed a higher
catalytic activity than Pd NPs 1, indicating the influence of
nanoparticle size on the catalytic activity.
Thermodynamic parameters
Thermodynamic parameters measured for the reduction
reactions of Nip in the presence of Pd NPs 1 and Pd NPs 2,
are summarized in Table 2 and were calculated using
standard equations described in the experimental section
and values of the apparent rate constant (k ) at different
app
temperatures given in the SI section (Figure S9 in the SI
section).
Figure 7. Apparent rate constant kapp as a function of the surface area
normalized to the unit volume of the reaction system S for palladium
Table 2 compares activation parameters for the Nip
reduction reaction in the presence of Pd NPs with several
other systems. For both nanoparticles and for other systems
−1
-
−1
nanoparticles ([4-nitrophenol] = 0.093 mmol L ; [BH ] = 9.71 mmol L ;
4
−1
[NaCl] = 0.033 mmol L , at 25 °C). () Pd NPs 1; () Pd NPs 2.
‡
reported in the literature (Table 2), negative values of ΔS
43
were obtained, probably due to a more ordered structure in
the transition state related to the adsorption of Nip onto the
surface of palladium nanoparticles. In general, the reaction
seems to be thermally compensated and, as can be seen in
Table 2. Thermodynamic parameters for the Nip reduction catalyzed by palladium
‡
−1
‡
−1
−1
‡
−1
−1
Catalyst
ΔH / (kJ mol )
ΔS / (J mol K )
−127.4
ΔG / (kJ mol )
E / (kJ mol )
Reference
a
Pd NPs 1
Pd NPs 2
G4-OH(Pd )
45.94
50.66
43.8
40.6
16.4
29
83.50
83.97
90.9
89.3
88.2
88
48.5
53.2
50.9
46.9
17.9
30.6
31.4
This work
−111.8
This work
−158.0
43
43
43
44
44
4
0
G5-OH(Pd )
−163.6
80
G6-OH(Pd160
)
−240.8
Pd DENs
−198
13
Pd DENs
27
−204
88
55
‡
‡
‡
ΔH : enthalpy of activation; ΔS : entropy of activation; ΔG : Gibbs energy of activation; E : energy of activation.
a

Vol. 27, No. 2, 2016
Souza et al.
377
Figure 8, lower activation enthalpies lead to more negative
entropies of activation (which is equivalent to higher
values of the Arrhenius A term). As a result of the thermal
compensation, although the change in enthalpy of activation
is about 30 kJ mol , the free energies of activation do not
reflect directly the change (Figure 8) and there is a much
smaller difference in rate constant (and free energies of
activation) than expected in terms of the enthalpy changes.
Table 3. Reduction of mono substituted aromatic nitro compounds
catalyzed by palladium nanoparticles
a
−
1
Substituent
X
Pd NPs 1
Conversion / % Conversion / %
Pd NPs 2
TON
100
H
2
3
4
2
4
4
-OH
-OH
-OH
-NH2
-NH2
100
100
100
83
-CH OH
2
b
3
-NH2
83
a
All the reactions were carried out nitro using (0.05 mmol) of the aromatic
nitro compound, 1 mol% of Pd NPs, 11 equivalents of NaBH and 9.9 mL
4
b
of THF/H O (1:2.3); TON (ratio of moles of desired product formed to
2
moles of catalyst) calculated using the amount product formed in 30 min.
Table 4. Reduction of halo-substituted nitro aryl compounds catalyzed
a
by palladium nanoparticles
Figure 8. Enthalpy () and free energy () of activation as a function
of the change in entropy of activation, showing a typical thermal
compensation reaction for the reduction using different palladium systems
as catalysts.
Substituents
X,Y
Pd NPs 1
Selectivity / %
Pd NPs 2
Selectivity / %
b
b
2
4
2
3
3
3
-F (Y=H)
-Br (Y=H)
-Cl-5-OH
-Cl-4-NH2
-Br-4-OH
-Cl-4-OH
100
69
100
54
Reduction of aromatic nitro compounds
100
100
97
100
100
100
100
To evaluate the scope and limitations of the Pd NPs,
several substituted aromatic nitro compounds were tested
to verify the reduction to anilines. Initially, the reduction
of mono substituted aromatic nitro compounds by NaBH4
in a fixed time of 30 min were tested (Table 3) and the
conversions obtained were, in all cases 100%, except for
100
2
3
4
-I (Y=H)
-I (Y=H)
-I (Y=H)
b
b
−
−
3-nitroaniline with Pd NPs 2 which was 83%. These results
a
All the reactions were carried out using (0.05 mmol) of the aromatic nitro
indicate that the position of the substituents had little or no
effect on the reactivity.
compound, 1 mol% of Pd NPs, 11 equivalents of NaBH and 9.9 mL of
4
b
THF/H
O (1:2.3); for iodo-nitrobenzene, the selectivity values are not
2
shown because a complex mixture products was obtained.
In a second set of experiments, the reduction of halo-
substituted aromatic nitro compounds with one or two
substituents were evaluated (Table 4). All the reactions
showed 100% conversion of reagents to products. However,
in some cases, other products were formed besides the
expected aniline derivative. For 4-bromonitrobenzene,
dehalogenation and the formation of azo products
was observed. In the case of 2-bromo-4-nitrophenol,
debrominated product was found in the reduction using
Pd NPs 1 as catalyst. Reactions of the iodonitrobenzenes
produced a complex mixture of dehalogenated and azo
products and was not analyzed in detail. Although Pd
catalysts are known to show a high tendency to catalyze
products were obtained only with iodo derivatives and, in a
minor proportion, with the bromo substituted nitro arenes.
Finally, the reduction of substituted aromatic nitro
compounds containing groups which can be reduced
by NaBH was evaluated (Table 5). The reduction of
4
nitrobenzaldehydes showed that, besides the reduction of
the nitro group, the aldehyde function was also reduced
with formation of the corresponding aminobenzyl alcohol.
In the reaction of 2-nitrobenzaldehyde, a small amount of
N-phenylformamide was detected as side product. Finally,
the reaction of 4-nitroacetophenone produced a mixture of
4-aminoacetophenone and 4-aminostyrene.
4
5-48
halogen displacement;
in this work, dehalogenated

3
78
Zwitterionic Surfactant Stabilized Palladium Nanoparticles as Catalysts in Aromatic Nitro Compound Reductions
J. Braz. Chem. Soc.
Table 5. Reduction of nitro substituted aryl compounds with other
reducible group catalyzed by palladium nanoparticles
allowed us to understand that the reaction proceeds
via nitroso and hydroxylamine derivatives, and that the
intermediate molecules leave the nanoparticle surface,
with the reaction proceeding in several steps where
adsorption and desorption are observed in each step of
the reaction.Activation parameters show that mechanisms
seem to be similar for most Pd catalysts reported in
the literature, with lower activation enthalpies being
compensated by more negative entropies of activation;
as a result, the reaction is thermally compensated and
the rate constants for most reactions are rather similar.
a
Reagent
substituent
Product
substituent
Pd NPs 1
selectivity / % selectivity / %
Pd NPs 2
b
b
4
2
3
-COCH3
-CHO
-CHO
-CHO
4-COCH3
43
92
40
87
2-CH OH
2
3-CH OH
100
100
100
100
2
4
4-CH OH
2
a
All the reactions were carried out using (0.05 mmol) of the aromatic nitro
compound, 1 mol% of Pd NPs, 11 equivalents of NaBH and 9.9 mL of
4
Experimental
THF/H O (1:2.3).
2
Materials
Other milder reducing agents were tested, but
hydroquinone and formic acid led to precipitation of the
Pd NPs and ammonium formate was unable to reduce the
nitro group after 20 h in these conditions. Using hydrogen
gas, precipitation of Pd NPs 3 was observed, but Pd NPs 2
showed good conversions and selectivity for the reduction
of a variety of nitroaromatic compounds (Table 6).
The ImS3-14 surfactants were synthesized as described
4
9
previously. K PdCl with a purity grade of 98% was
2
4
purchased from Aldrich, Brazil. The chloroform used was
distilled before use. Milli-Q water with a resistivity higher
than 18.2 MΩ cm was used in all experiments. All other
chemicals were of analytical grade and were used without
any purification.
a
Table 6. Reduction of aromatic nitro compounds using Pd nanoparticles
Figure 9. Chemical structure of ImS3-14 surfactant.
Reagent
substituent
Product
substituent
d
b
Conversion / %
TON
Preparation of palladium nanoparticles in reverse micelles
The palladium nanoparticles were synthesized at
H
H
4
4
4
4
-OH
4-OH
4-NH2
-NH2
different water to surfactant concentration ratios (w ),
0
1
00
100
15,40
-CH OH
4-CH OH
using the methodology previously described.
In fact, a
2
2
water to surfactant concentration ratio of 4.8 was used for
-COCH3
-Br
4-COCH3
4-H
the preparation of Pd NPs 1. A higher ratio w = 14.7 was
0
4
needed for the preparation of Pd NPs 2.
a
All the reactions were carried out using (0.05 mmol) of the nitro aryl
compounds, 1 mol% of Pd NPs, 4 bar of H and 1.9 mL of THF/H O
1:2.3). TON: ratio of moles of desired product formed to moles of catalyst.
The palladium nanoparticles obtained were characterized
by transmission electronic microscopy (TEM). The
samples for TEM analysis were prepared by deposition
of the nanoparticle dispersion in a carbon-coated copper
grid and analyzed in a JEOL JEM-2100 transmission
electron microscope operating at 200 kV at LCME/UFSC,
Florianópolis, Brazil.
2
2
(
Conclusions
Pd NPs stabilized by ImS3-14 showed efficient
catalytic activities for the reduction of nitroarenes
in the presence of NaBH , with high conversion and
4
good selectivity. Substituents had little or no effect
on the reactivity or selectivity and azo products were
observed in significant amounts only in the reactions of
Nanoparticles redissolution in water
The chloroform was removed through rotary evaporation
and the black powder obtained was dissolved using a
iodo derivatives. The reduction with H showed good
2
−
1
conversion and selectivity with no sign of hydroxylamine
products. The reduction of 4-nitrophenol was examined
by UV-visible and mass spectroscopic techniques and
0.08 mol L NaCl aqueous solution. The solution was
stirred until complete dissolution of the Pd NPs.

Vol. 27, No. 2, 2016
Souza et al.
379
−
1
Palladium quantification
where kapp is the apparent rate constant (in s ), T is
the temperature (in K), R is the universal gas constant
−
1
−1
The quantification of palladium in the solutions was
performed using energy dispersive X-ray fluorescence
(8.314472 J mol K ), k is the Boltzmann constant
B
(1.3806503 × 10 m kg s K ), h is Planck’s constant
(6.6261764 × 10 J s), ΔH is the enthalpy of activation
and ΔS corresponds to the entropy of activation. From the
−
23
2
−2
−1
−
34
‡
(
EDXRF). The EDXRF measurements were performed
‡
using a S2 Ranger (Bruker, Germany) with temperature
controlled to 23 ± 1 °C. Samples of 5 mL were placed in
an X-Cell (with a diameter of 40 mm) and covered with a
‡
‡
‡
values of ΔH and ΔS , the Gibbs energy of activation (ΔG )
was calculated using equation 3:
5
μm thick polypropylene film, special for XRF analyses.
‡
‡
‡
The S2 Ranger measurements were carried out using a
Pd X-ray tube, operated with Cu filter and 50 kV and
ΔG = ΔH – TΔS
(3)
250 μm. An automatic sampler and EQUA ALL software
Mass spectrometric study of the Nip reduction
were used for instrument control, data collection, and data
analysis (Bruker, Gemany). Before analysis, instrument
calibration and a stability check were performed. For the
The mass spectrometer system consisted of a hybrid
triplequadrupole/linear ion trap mass spectrometer QTrap
3200 (Applied Biosystems/MDS Sciex, Concord, Canada)
coupled to a Harvard Pump 11 Plus (Harvard Apparatus,
Holliston, MA) for sample infusion. The experiments were
performed using the Turbo VTM source (electrospray-
ESI) in positive and negative ion mode. The capillary
needle was maintained at 5500 V, and the declustering
potential (DP) was set to 35 V. Synthetic air was used as
nebulizer gas (GS1) at a pressure of 15 psi, and nitrogen
was used at 10 psi as Curtain GasTM in the interface as
well as collision gas (CAD GasTM) at 6 arbitrary units
in the LINAC collision cell. The sample solution was
40
nanoparticle synthesized in w 14.7 it was necessary to
0
dilute the solution.
Kinetic studies of 4-nitrophenol reduction
The kinetic measurements were performed in a 3 mL
quartz cell using a HP 8453 spectrophotometer operating
in the range of 190-820 nm coupled to a thermostatic bath.
First, were added an aliquot of 4-nitrophenol, an aliquot
of palladium nanoparticle, 50 µL of 2.0 mol L−1 NaCl
solution (to force the solubilization of nanoparticles) and
an aliquot of water to complete 2 mL of solution. After
cell thermostatting at 25 °C, the reactions were started
−
1
diluted to 1.3 mg L of Nip in water solution and infused
−
1
at a rate of 10 μL min using the built-in infusion pump.
Then, electrospray mass and tandem mass experiments
(MS2) were performed. The starting solution contained
by the addition of 1 mL of NaBH solution, freshly
4
prepared. Reactions were followed at 400 nm, referring to
−
1
−1
4-nitrophenolate.
0.093 mmol L of 4-nitrophenol, 9.71 mmol L NaBH ,
4
−
1
−1
In this study, the 4-nitrophenol concentration was first
0.033 mmol L NaCl and 0.0019 mmol L Pd and the
aliquots were taken after the addition of NaBH4.
−
1
varied from 0.006 to 0.12 mmol L , keeping NaBH and
4
−
1
nanoparticle concentrations at 9.71 and 0.0167 mmol L ,
respectively. Then the NaBH concentration was varied
General procedure for the reduction of aromatic nitro
compounds with NaBH4
4
−1
from 0.83 to 15.8 mmol L , keeping the 4-nitrophenol and
−1
nanoparticles concentrations at 0.093 and 0.0167 mmol L ,
respectively. Finally, the nanoparticle concentration
A mixture of 3 mL of 0.015 mol L− aromatic nitro
1
−
1
was varied from 0.0013 to 0.04 mmol L , keeping the
compound dissolved in tetrahydrofuran (THF), an aliquot
−
1
4
9
-nitrophenol and NaBH concentrations at 0.093 and
.71 mmol L , respectively.
of palladium nanoparticles, 300 µL of 2.0 mol L NaCl
4
−1
solution and an aliquot of water were added into a reaction
−
1
For the calculation of the thermodynamic activation
flask, totalizing 9 mL. Then, 900 µL of 0.5 mol L NaBH
4
parameters, rate constant (k ) were determined at different
solution freshly prepared was added to the above solution
app
temperatures. The concentration of reactants and catalyst
under continuous stirring. The final ratio H O:THF was
2
−
1
−1
−
were 0.093 mmol L 4-nitrophenol, 9.71 mmol L BH ,
2.3:1. The mixture was stirred at room temperature for
30 min. After this time, diethyl ether was added and
the organic phase was separated and analyzed by gas
chromatography (GC/MS) (chromatograph Agilent
Technologies model 7820A coupled to mass spectrometer
Agilent Technologies model 5975, operating with a
column HP5-ms).
4
−1
−1
0
.033 mmol L NaCl and 0.0167 mmol L Pd. The data
were treated using the Eyring equation (equation 2):
�
�





k T
h 
��
��
B
k
=
exp
exp �
(2)
app







R
RT





3
80
Zwitterionic Surfactant Stabilized Palladium Nanoparticles as Catalysts in Aromatic Nitro Compound Reductions
J. Braz. Chem. Soc.
General procedure for the reduction of aromatic nitro
compounds with H2
10. Nemanashi, M.; Meijboom, R.; J. Colloid Interface Sci. 2013,
389, 260.
1
1. Drinkel, E.; Souza, F. D.; Fiedler, H. D.; Nome, F.; Curr. Opin.
Colloid Interface Sci. 2013, 18, 26.
In a Fischer-Porter glass flask, 0.9 mL of 0.015 mol L−1
aromatic nitro compound dissolved in THF, an aliquot of
palladium and 90 µL of 2.0 mol L−1 NaCl solution was
12. An, K.; Somorjai, G. A.; ChemCatChem 2012, 4, 1512.
13. Reetz, M. T.; Helbig, W.; J. Am. Chem. Soc. 1994, 116, 7401.
14. Souza, B. S.; Leopoldino, E. C.; Tondo, D. W.; Dupont, J.;
Nome, F.; Langmuir 2012, 28, 833.
dispersed in 1.9 mL of H O. The reaction system was
2
pressurized to 4 bars with hydrogen gas, followed by
pressure release through an auxiliary bleed valve. This
procedure was repeated three times to ensure the absence
of other gases in the reaction vessel. The system was
pressurized again at 4 bars, the mixture was vigorously
stirred with a magnetic bar at 1500 rpm and the pressure was
maintained constant. After 4 h, the pressure was released
and diethyl ether was added to glass flask. The organic
phase was separated and analyzed by GC/MS.
15. Souza, F. D.; Souza, B. S.; Tondo, D. W.; Leopoldino, E. C.;
Fiedler, H. D.; Nome, F.; Langmuir 2015, 31, 3587.
16. Fernandes, S. C.; de Souza, F. D.; de Souza, B. S.; Nome, F.;
Vieira, I. C.; Sens. Actuators, B 2012, 173, 483.
17. Hwang, C. G.; Sang-Ho, K.; Hoon, O. J.; Kim, M. R.; Choi,
S. H.; J. Ind. Eng. Chem. 2008, 14, 864.
18. Bhattacharjee, S.; Dotzauer, D. M.; Bruening, M. L.; J. Am.
Chem. Soc. 2009, 131, 3601.
1
9. Campbell, P. S.; Santini, C. C.; Bayard, F.; Chauvin, Y.;
Supplementary Information
Colliere, V.; Podgorsek, A.; Gomes, M. F. C.; Sa, J.; J. Catal.
2
010, 275, 99.
Supplementary data (TEM microscopy of Pd NPs,
UV-Vis absorption spectra, ESI-MS and ESI-MS/MS
spectrum and thermodynamics plots for reduction
aromatic nitro compound) are available free of charge at
http://jbcs.sbq.org.br as PDF file.
20. Zhu, Y. J.; Zaera, F.; Catal. Sci. Technol. 2014, 4, 3390.
21. Miller, L.; Winter, G.; Baur, B.; Witulla, B.; Solbach, C.;
Reske, S.; Linden, M.; Nanoscale 2014, 6, 4928.
22. Cuenya, B. R.; Acc. Chem. Res. 2013, 46, 1682.
23. Sadeghmoghaddam, E.; Gu, H. M.; Shon, Y. S.; ACS Catal.
2
012, 2, 1838.
Acknowledgements
24. Zhong, R. Y.; Yan, X. H.; Gao, Z. K.; Zhang, R. J.; Xu, B. Q.;
Catal. Sci. Technol. 2013, 3, 3013.
We are grateful to INCT-Catálise, PRONEX, FAPESC,
CNPq and CAPES for their support of this work and
to LCME/UFSC for technical support during electron
microscopy work.
25. Li, Y.; El-Sayed, M. A.; J. Phys. Chem. B 2001, 105, 8938.
26. Ciganda, R.; Li, N.; Deraedt, C.; Gatard, S.; Zhao, P. X.;
Salmon, L.; Hernandez, R.; Ruiz, J.; Astruc, D.; Chem.
Commun. 2014, 50, 10126.
2
7. Dey, R.; Mukherjee, N.; Ahammed, S.; Ranu, B. C.; Chem.
Commun. 2012, 48, 7982.
References
2
8. Gkizis, P. L.; Stratakis, M.; Lykakis, I. N.; Catal. Commun.
2013, 36, 48.
1
2
3
4
. Jeong, C.; Park, J.; Bae, J. W.; Suh, Y. W.; Catal. Commun.
014, 54, 1.
. Kim, E.; Jeong, H. S.; Kim, B. M.; Catal. Commun. 2014, 45,
5.
. Goesmann, H.; Feldmann, C.; Angew. Chem., Int. Ed. 2010, 49,
362.
. Pachon, L. D.; Rothenberg, G.; Appl. Organomet. Chem. 2008,
2, 288.
2
29. Corma, A.; Serna, P.; Science 2006, 313, 332.
30. Chen, L. F.; Hu, J. C.; Qi, Z. W.; Fang, Y. J.; Richards, R.; Ind.
Eng. Chem. Res. 2011, 50, 13642.
2
31. Agrawal, A.; Tratnyek, P. G.; Environ. Sci. Technol. 1996, 30,
153.
1
32. Mirza-Aghayan, M.; Boukherroub, R.; Rahimifard, M.;
Bolourtchian, M.; Appl. Organomet. Chem. 2010, 24, 477.
33. Corma, A.; Concepcion, P.; Serna, P.; Angew. Chem., Int. Ed.
2007, 46, 7266.
2
5
6
. Kralik, M.; Biffis, A.; J. Mol. Catal. A: Chem. 2001, 177, 113.
. Herves, P.; Perez-Lorenzo, M.; Liz-Marzan, L. M.; Dzubiella, J.;
Lu, Y.; Ballauff, M.; Chem. Soc. Rev. 2012, 41, 5577.
34. Pehlivan, L.; Metay, E.; Laval, S.; Dayoub, W.; Demonchaux, P.;
Mignani, G.; Lemaire, M.; Tetrahedron Lett. 2010, 51, 1939.
35. Pogorelic, I.; Filipan-Litvic, M.; Merkas, S.; Ljubic, G.;
Cepanec, I.; Litvic, M.; J. Mol. Catal. A: Chem. 2007, 274,
202.
7
8
9
. Astruc, D.; Lu, F.;Aranzaes, J. R.; Angew. Chem., Int. Ed. 2005,
4
4, 7852.
. Roucoux, A.; Schulz, J.; Patin, H.; Chem. Rev. 2002, 102,
757.
. Yan, N.; Xiao, C. X.; Kou, Y.; Coord. Chem. Rev. 2010, 254,
179.
3
st
36. Hudlicky, M.; Reductions in Organic Chemistry; 1 ed.; Ellis
1
Horwood Limited: Chichester, 1984.

Vol. 27, No. 2, 2016
Souza et al.
381
3
3
3
4
7. Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M.; J. Phys.
45. Leger, B.; Nowicki, A.; Roucoux, A.; Rolland, J. P.; J. Mol.
Catal. A: Chem. 2007, 266, 221.
Chem. C 2010, 114, 8814.
8. Wunder, S.; Lu,Y.;Albrecht, M.; Ballauff, M.; ACS Catal. 2011,
46. Lu, Y. M.; Zhu, H. Z.; Li, W. G.; Hu, B.; Yu, S. H.; J. Mater.
Chem. A 2013, 1, 3783.
1
, 908.
9. Zapp, E.; Souza, F. D.; Souza, B. S.; Nome, F.; Neves,A.;Vieira,
I. C.; Analyst 2013, 138, 509.
47. Chen, J.; Zhang, Y.; Yang, L.; Zhang, X.; Liu, J.; Li, L.;
Zhang, H.; Tetrahedron 2007, 63, 4266.
0. Fiedler, H. D.; Drinkel, E. E.; Orzechovicz, B.; Leopoldino,
E. C.; Souza, F. D.; Almerindo, G. I.; Perdona, C.; Nomet, F.;
Anal. Chem. 2013, 85, 10142.
48. Viciu, M. S.; Grasa, G.A.; Nolan, S. P.; Organometallics 2001,
20, 3607.
49. Tondo, D. W.; Leopoldino, E. C.; Souza, B. S.; Micke, G. A.;
Costa,A. C.; Fiedler, H. D.; Bunton, C.A.; Nome, F.; Langmuir
2010, 26, 15754.
4
4
1. Signori, A. M.; Santos, K. D.; Eising, R.; Albuquerque, B. L.;
Giacomelli, F. C.; Domingos, J. B.; Langmuir 2010, 26, 17772.
2. Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T.; Langmuir 2010,
2
6, 2885.
Submitted: September 17, 2015
4
3. Noh, J. H.; Meijboom, R.; Appl. Surf. Sci. 2014, 320, 400.
4. Bingwa, N.; Meijboom, R.; J. Phys. Chem. C 2014, 118, 19849.
Published online: October 29, 2015
4