Water-soluble gold nanoparticles: Recyclable catalysts for the reduction of aromatic nitro compounds in water

Article abstract of DOI:10.1039/d0ra02131h

A structure/catalytic activity study of water-soluble gold nanoparticles, stabilized by zwitterionic ligands derived from imidazolium salts, in the reduction of aromatic nitro compounds in pure water at different temperature, as well as their recyclabilit

Full text of DOI:10.1039/d0ra02131h

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Water-soluble gold nanoparticles: recyclable
catalysts for the reduction of aromatic nitro
compounds in water
Cite this: RSC Adv., 2020, 10, 15065
ab
ab
Gustavo A. Monti,ab N. Mariano Correa,
R. Darıo Falcone,
´
c
ab
*
*
Gustavo F. Silbestri
and Fernando Moyano
A structure/catalytic activity study of water-soluble gold nanoparticles, stabilized by zwitterionic ligands
derived from imidazolium salts, in the reduction of aromatic nitro compounds in pure water at different
temperature, as well as their recyclability, was performed. Our studies indicate that the nanoparticles
synthesized by an easy, fast and reproducible process, need a short characteristic induction time to
restructure the surfaces and make them active. The differences observed in the catalytic activity of the
nanoparticles, determined by using the typical Langmuir–Hinshelwood model, are strongly based on the
degree of coverage and spatial arrangement of the imidazolium salts on them. Finally, we demonstrate
that gold nanoparticles stabilized by non-traditional ligands can be an excellent choice for nitro
compound degradation.
Received 6th March 2020
Accepted 2nd April 2020
DOI: 10.1039/d0ra02131h
rsc.li/rsc-advances
In the eld of catalysis, one of the fasted growing areas in
nanoscience, different stabilizers not only avoid the undesired
Introduction
aggregation and coalescence but can act as active spectators
that may attend the catalytic properties of the metal.13,14 At the
same time, the stabilization of M–NPs by the coordinating
capping ligands, results in a change of the surface properties
and the surface accessibility of the nanoparticles particularly
with regard to applications in catalysis.15 Catalysis by transition-
metal NPs dispersal in water allows the development of
synthesis procedures with low environmental impact, easy
catalyst recycling, and separation from products.16,17
It is known that, due to their high surface energy, M–NPs
tend to aggregate. To avoid it, ligands such as thiols, amines,
disulfures, thioethers, phosphines, N-heterocyclic carbenes
(NHCs) or imidazolium salts are uses.3 An alternative way for
stabilizing metal nanoparticles can be the use of ionic liquids
(ILs); the well-established and probably most studied ILs
In recent years, many books and reviews about the application
of gold in homogeneous catalysis have appeared. Normally,
Au1+ or Au3+ organic complexes are used as catalysts.1,2
However, recently, Wang et al.3 conducted a detailed review of
the synthetic methods and functional properties of various
nanostructured materials of noble metals (especially Au and
Ag). Also, they present and discuss the applications of nano-
materials for chemical systems and biosensors. The interest in
metallic nanoparticles (M–NPs) has increased due to their
numerous applications such as in medicine,4,5 optics,6,7 elec-
tronics,8 luminescence9 and especially because of their poten-
tial use in catalysis.10–12
It is well known that, when the size of a system is reduced to
the nanometer scale, matter exhibits some specic properties
that may be signicantly different from the physical properties
of the massive material. These properties depend on several
factors, such as the shape, size and functionalization of the
surface of the NPs and, they can be adapted if you have control
over the morphology and size of the system, which can be
achieved by manipulating the synthesis conditions of the NPs.
contain
an
imidazolium
cation
(e.g.
1-ethyl-3-
methylimidazolium or 1-butyl-3-methylimidazolium) and
weakly-coordinating anions such as tetrauoroborate [BF4]ꢀ,
hexauorophosphate [PF6]ꢀ and triuoromethylsulfonate(tri-
ate) [OTf]ꢀ or bis(triuoromethylsulfonyl)imide(triimide)
[NTf2]ꢀ.18 NHCs have been used to synthesize Au-
and Ag–
19,20
NPs21 as a model synthesis for the formation of either
conglomerates or 3D networks. Glorius and co-worked
described the synthesis and catalytic activity of Au- and Pd–
NPs stabilized with NHCs bearing sulfonate and carboxylate
aInstituto para el desarrollo agroindustrial y de la salud, IDAS, (CONICET – UNRC),
Argentina
b
´
´
groups.22 Also, de Jesus and Chaudret have prepared, by thermal
Departamento de Quımica, Universidad Nacional de Rıo Cuarto, Agencia Postal # 3.,
´
´
Rıo Cuarto, C.P. X5804BYA, Argentina. E-mail: fmoyano@exa.unrc.edu.ar
decomposition, Pt–NPs using sulfonated NHC as stabilized
ligands.23 On the other hand, Nome et al. have prepared Pd–
NPs, water-soluble, using imidazolium-based surfactant 3-(1-
c
´
´
Instituto de Quımica del Sur (INQUISUR), Departamento de Quımica, Universidad
´
Nacional del Sur (UNS), CONICET, Av. Alem 1253, B8000CPB, Bahıa Blanca,
Argentina. E-mail: gsilbestri@uns.edu.ar
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dodecyl-3-imidazole)propanesulfonate. The NPs were effective intermediates and/or product decomposition. In all cases, we
and recycling in the aqueous biphasic hydrogenation of cyclo- observed that the reaction was completed aer 30 min.
ꢀ
ꢁ
hexene24 and, recently we have reported the synthesis, electro-
static stabilization and characterization of Au–NPs in aqueous
medium using sulfonated-imidazolium salts as stabilized.25,26
However, these stabilizer systems may impede the activity of the
M–NPs for a given reaction. At this point, a model reaction that
is well known and, without the generation of by products, would
be of great help.27 Pal28 and Esumi29 were the rst to identify the
catalytic reduction of p-nitrophenol,30 is perhaps the most used
reaction, to test the catalytic activity of M–NPs in aqueous
solution.
ADNB
A0DNB
Fig. 2 shows a graph of ln
versus time (t) for typical
reaction between DNB and Hz, catalyzed by Au–NPs. As it can be
observed, the graph denotes two proles. In the rst part (red
dots), it can be seen that there is an induction time (t0), in which
no reductions take places. In the second part (black dots), the
reaction follows a rst-order rate law. This behavior was also
observed in the catalytic reduction of different substrates with
metal nano-catalysts.32,33 For example, during the catalytic
reduction of p-nitrophenol in the presence of different catalysts,
an induction period has been observed by many groups which
can be as long as several minutes.32,33 This period is usually
ascribed to the diffusion time required for p-nitrophenol to be
adsorbed onto de catalyst's surface before the reaction could
start.12,34 On the other hand, the induction time can be assigned
to a surface restructuring necessary to render the M–NPs as an
active catalyst35 or, also it may be the time necessary to remove
surface oxides.36 All data published so far by different groups
show that the reduction of nitrophenol proceeds on the surface
of the M–NPs and quantitative analysis of this surface reaction
can be performed.31,35 Since our data indicate that the reduc-
tions begin aer t0, we determine all kapp values in the presence
of different Au–NPs as shown in Fig. 2.
Continuing with our research on water-soluble gold–NPs,
herein we report the structure/catalytic activity study of Au–NPs
stabilized with different imidazolium salts (1–3, Scheme 1) in
the reduction of aromatic nitro compounds in aqueous medium
as well as their recyclaꢁbility. Kinetic data have been obtained
ꢁ
between 25 C and 45 C by monitoring the concentrations of
1,4-dinitrobenzene (DNB) by UV-vis spectroscopy and, the
kinetic were modeled in terms of the Langmuir–Hinshelwood
model.31
Results and discussion
Following eqn (6), the k values can be determined by varying
kapp with DNB concentrations. Fig. 3 shows the experimental
data of kapp at different DNB concentrations in the presence of
different Au–NPs (1–3). In all case, the proles show an increase
of kapp values when DNB concentration increases, which is
typically due to a Langmuir–Hinshelwood mechanism.31 These
proles clearly indicate that the catalysis process involving Au–
NPs is a supercial process, where the reagents compete for the
free regions on the Au–NPs surface, so that, when DNB
concentrations increase, more DNB molecules are adsorbed on
the catalyst and the kapp values increases.
The reaction between DNB and Hz was explored in the absence
of Au–NPs and under this experimental condition no catalytic
activity was observed (absorption spectra for 30 min remains
unchanged). However, in the presence of catalysts, we observe
changes in the spectra indicating that the reduction reaction
takes place. Fig. 1 shows typical absorption spectra for the
reduction reaction at different times in water, catalyzed by
different Au–NPs (1–3) at 25 ꢁC. It can be seen an increase in the
absorbance value at lmax ¼ 380 nm with time, corresponding to
the product reactions absorbance (p-NA) and, a decrease in the
absorbance value at lmax ¼ 284 nm corresponding to the reac-
tive (DNB) consumption. Also, it can be observed a clear iso-
sbestic point at lmax ¼ 304 nm. This evidences the lack of
The adjustments obtained with the mechanism are consis-
tent with Wunder et al.37 and k values were obtained by
Scheme 1 Au–NPs surfaces with different sulfonated-imidazolium salts.
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Fig. 1 Representative absorption spectra at different time for the reductions reactions of DNB catalyzed by different Au–NPs: (a) 1, (b) 2 and, (c)
3. [DNB] ¼ 5 ꢂ 10ꢀ5 M, [Hz] ¼ 0.1 M and, [Au–NPs] ¼ 1 ꢂ 10ꢀ8 M.
arrangement and the degree of coverage are responsible for that
the 3 show greater efficient catalytic in the model reaction at
ꢁ
25 C.
Dependence of the reaction rate with temperature
To explain thermodynamically the k values obtained for Au–
NPs, we select those Au–NPs that have a similar size (2 and 3,
see Table 2) and; we evaluate the behavior of catalysts against
the temperature in the reaction. Fig. 4 shows the kapp values as
a function of the concentrations DNB at different temperatures
Fig. 2 Typical time trace of the absorption of the DNB at lmax
¼
for 2 and 3, respectively.
284 nm: ( ) induction time, t0; ( ) section from which kapp is taken.
[DNB] ¼ 5 ꢂ 10ꢀ5M, [Hz] ¼ 0.1 M and [Au–NPs] ¼ 1 ꢂ 10ꢀ8 M.
As expected, the kapp values increase with temperature
according to the Langmuir–Hinshelwood mechanism, which
was used to determine the values of k at different temperatures
as described above. Thus, it is possible to express the depen-
dence k with temperature in terms of the Arrhenius equation.35
adjusting the experimental data to eqn (6) and, assuming n ¼
0.6 for the adsorption of DNB, while m ¼ 1 for the adsorption
of Hz. These values were calculated previously37 ꢁ following
Gaussian distributions. The k values obtained at 25 C for 1, 2
and 3 are 1.03 ꢂ 10ꢀ3 mol m2 sꢀ1, 4.07 ꢂ 10ꢀ4 mol m2 sꢀ1 and
3.10 ꢂ 10ꢀ3 mol m2 sꢀ1, respectively. An interesting analysis
arises when comparing the k of Au–NPs of similar shape and
size (2 and 3) with the degree of coverage and spatial arrange-
ment.38 For 2, there are more molecules of imidazolium salts
that cover the Au–NPs and it is due to a better compaction
around them.26 This makes it difficult to approach the Au–NP
surface of the reagents. Nevertheless 3 have a different
arrangement around Au–NPs and then it is less compacted to 2;
the reagents can easily access the surface of Au–NPs therefore,
a higher value k is observed. In other words, both the spatial
Ea
RT
k ¼ k0eꢀ
(1)
It is an equation that ts experimental data in most of the
situations, where k0 is the frequency factor of the surface reac-
tion and, Ea the true activation energy which we measure
in kJ molꢀ1. Taking the natural logarithm of eqn (1), the
Arrhenius equation can be rearranged as:
Ea
ln k ¼ ln k0 ꢀ
(2)
RT
Fig. 5 shows the dependence k with temperature in terms of
the Arrhenius linear equation to different Au–NPs (2 and 3).
Fig. 3 Dependence of kapp with the DNB concentration at T ¼ 25 ꢁC for each Au–NPs: (a) 1, (b) 2 and (c) 3. The red lines adapt to the Langmuir–
Hinshelwood model according to eqn (6). The k value was obtained considering the total surface area calculate of the catalyst for each NPs, see
Table 2.
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Fig. 4 Dependence of kapp with the DNB concentration at different temperature: ( ) 25 ꢁC, ( ) 35 ꢁC and, ( ) 45 ꢁC for 2 and 3.
Fig. 5 Arrhenius analysis of the k obtained at different temperatures using two Au–NPs: 2 and 3.
The activation energies values found for the reaction cata- spherical polyelectrolyte brushes that ensure their stability
lyzed by 2 and 3 are 34 kJ molꢀ1 and 13 kJ molꢀ1, respectively. against aggregation; they found a value to 19 kJ molꢀ1. However,
Our results, shows that the lowest value of the activation energy to determine the activity of M–NPs, the activation energy is
determined corresponds with highest the value k. Furthermore, a difficult parameter to interpret, so k is used as a comparison
a similar value of the activation energies in metallic nano- criterion. Table 1 shows the kinetic parameters obtained in this
particles was assigned to spontaneous surface reconstruction in work and those reported in literature.
the absence of a substrate. In this work during the catalytic
If we compare the k obtained for the reduction reaction
reduction of p-nitrophenol, the t0 found for each kinetics and catalyzed by our Au–NPs (1–3) with other catalytic systems, we
the activation energies determined, may indicate that there is can note that they are in an intermediate region. The values
restructuring on surfaces of Au–NPs to use as an active catalyst. found for k indicate that they are less active in relation to Au–
It is necessary remark that the value of activation energies NPs stabilized with CTAB. However, 1–3 are two orders of
found to different nanocatalysts39 varies in the range from 10 to magnitude more active than other systems based on supported
60 kJ molꢀ1. Even more, these ndings are similar to those Au–NPs. On the other hand, 1–3 have a catalytic activity about
Ballauff et al.37 were the Au–NPs are immobilized on cationic a thousand times higher than the Ag–NPs. We can note that
only Pd–NPs have a considerably higher k than the rest of the
catalytic systems. It is important to mention that, a variety of
parameters have been reported in the literature in order to
compare the activity between different catalytic systems. Most
Table 1 Kinetic parameters in the reduction of DNBa
M–NPs
Size (nm)
k (mol m2 sꢀ1
)
Reference
of these studies simply make use of the pseudo-rst order rate
constant (kobs or kapp), without taking in consideration the
catalyst loading. However, the determined value of TON (the
number of moles of substrate/the number of moles of catalyst)
for Au–NPs is around to 15 000.
Finally, it is important to mention that the synthesized
nanoparticles not only showed an interesting catalytic effect in
water even at room temperature, they can also be recovered and
reused. Fig. 6 shows that aer the rst catalytic cycle there are
1
2
3
9.0
15.0
15.0
6.5
3.0
4.0
13.41 ꢂ 10ꢀ3
2.79 ꢂ 10ꢀ3
46.83 ꢂ 10ꢀ3
50.00 ꢂ 10ꢀ2
1.00 ꢂ 10ꢀ4
1.51 ꢂ 102
This work
This work
This work
40
31
41
42
Au–NPsb
Au–NPsc
Pd–NPsd
Ag–NPse
2.5
1.00 ꢂ 10ꢀ4
a
b
c
d
25 ꢁC. Stabilized with CTAB. Supported on polyelectrolytes. On
the surface of Fe3O4@dextrano. e Stabilized with exopolysaccharide.
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appropriate amount of this solution -to obtain a given concen-
tration (1 ꢂ 10ꢀ4 M) of the metal precursor-was transferred to
a volumetric ask, and the acetonitrile was evaporated by
bubbling dry N2; then, 1 ꢂ 10ꢀ4 M aqueous solution of
sulfonated-imidazolium salt was added. Hydrazine was dis-
solved in the same aqueous solution. Hydrazine (Hz) concen-
tration was 1 ꢂ 10ꢀ3 M when mixed (without stirring) with the
metal precursor solution. The time course of the reaction was 3
seconds. Then, when the Au–NPs (1–3) were formed, the
suspensions turned pink along the time.25,26
Fig. 6 Conversion rate based on the number of cycles in which 2
(black bars) and 3 (red bars) were catalytically active.
Catalytic experiments in water
The reactions were followed measuring the increase at the
maximum absorption band of the p-nitroaniline (p-NA) at lmax
¼ 380 nm (3 ¼ 10 427 Mꢀ1 cmꢀ1) at different temperature, T1 ¼
25.0 ꢃ 0.1 ꢁC, T2 ¼ 35.0 ꢃ 0.1 ꢁC and, T3 ¼ 45.0 ꢃ 0.1 ꢁC.
Absorbance values at different times, were recorded in a Hew-
lett–Packard, UV-visible 8453 spectrophotometer equipped with
a thermostated cell (3 mL of volume and 1 cm path length).
The UV-visible spectroscopic analysis (Scheme 2) shows that
the reduction of DNB catalyzed by Au–NPs in water, produces p-
NA in quantitative yields.
signicant losses in the conversion. Two possible explanations
for this observation are: (i) the loss of Au–NP in the catalyst
recovery process in each cycle or, (ii) since the catalytic mech-
anism proposed is a process on the surface of Au–NPs, where
the reagents compete for the free regions, the active sites can be
occupied or modied aer the reaction. Further work is in
development in our laboratories focusing on the recovery
process in each cycle of these water-soluble Au–NPs.
To start a kinetic run, 0.1 M solution of hydrazine in water
was prepared in presence of Au–NPs, [Au–NPs] ¼ 1 ꢂ 10ꢀ8 M. In
a thermostated cell that contained the hydrazine and Au–NPs
solutions, the reduction reactions were initiated by addition of
different mL of the stock solution of DNB in order to have 3 mL
of solution with the desired [DNB] concentrations. DNB
Experimental section
General
Tetrachloroauric acid (HAuCl4, Sigma-Aldrich) as the precursor
and hydrazine monohydrate (N2H4$H2O, Sigma-Aldrich) as the
reducing agent, both for the synthesis of Au–NPs were employed
as received. The substrate 1,4-dinitrobenzene (DNB) from
Sigma-Aldrich was employed as received. Ultrapure water was
obtained from Labconco equipment model 90901-01. Imidazo-
lium salts [1,3-bis(2,6-diisopropyl-4-sodiumsulfonatophenyl)
imidazolium, 1-mesityl-3-(3-sulfonatopropyl)imidazolium and
1-(3-sulfonatopropyl)imidazolium] were prepared and charac-
terized according to reported procedures.43,44
concentration in water between 1.5 ꢂ 10ꢀ5 M to 1.5 ꢂ 10ꢀ4
M
was varied.
Recover of Au–NPs
Aer the reaction is concluded, the Au–NPs are removed from
the reactions medium by centrifugations for 25 min at 13.000
RPM and washed with pure water. Then, Au–NPs are suspended
with 3 mL of water and sonicated for 5 min. The surface plas-
mon resonance (SPR) of Au–NPs was checked by UV-visible. In
Synthesis of water-soluble Au–NPs
A stock solution of metal precursor ([HAuCl4] ¼ 0.06 M) was addition, the complete removal of the substrate (DNB) was
prepared in acetonitrile (Sintorgan HPLC quality). Following, an conrmed by UV spectroscopy. Finally, to start other catalytic
Scheme 2 Reduction of DNB using Au–NPs (1–3) as a catalyst and representative absorption spectra of the reactive (black line) and product (red
line) in water.
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Table 2 Catalyst parameters of the different Au–NPs used
reaction cycle, the desired amount of substrate is added from
the stock solution.
1
2
3
Analytical concentrationa (M)
Sizeb (nm)
1 ꢂ 10ꢀ8
1 ꢂ 10ꢀ8
1 ꢂ 10ꢀ8
Langmuir–Hinshelwood (LH) model
9
15
13
To quantify the catalytic reduction of 1,4-dinitrobenzene reac-
tion, the well-known equations of Langmuir–Hinshelwood were
applied.31 It is an idealized model of bimolecular surface-
mediated reactions and, involves certain considerable limita-
tions in its application to real systems; nevertheless, it deserves
some attention because this model is the basis for studying
many reactions. The model assumes a uniform solid surface
with no interaction between adsorbed species and, that the
Morphology
Spherical
Spherical
67
Spherical
48
Degree of surface coverage of 59
the Au–NPsc (%)
Total surface area of the
3.03 ꢂ 10ꢀ4 8.43 ꢂ 10ꢀ4 6.33 ꢂ 10ꢀ4
catalyst (m2)
a
Molar concentration of the Au–NPs solutions were calculated by
dividing the total number of gold atoms over the average number of
b
c
product of reaction is not absorbed on surface. In the LH model, gold atoms per Au–NP.35 Diameters determined by TEM. Obtained
by TGA analysis.
the apparent kinetic rate constant, kapp, is strictly proportional
to the total surface of nanoparticles (S) and the concentration
of Hz. The, this can be dened through (eqn (3)).31,37
dcDNB
the size, the morphologic and, the degree of surface coverage of
the Au–NPs incorporated into the reaction medium. These
ꢀ
¼ kScDNBcHZ ¼ kappcDNB
(3)
dt
where cDNB and cHZ is the concentrations of DNB and Hz, values have been determined in our research group and due
respectively at time t and, S is the total surface area of Au–NPs reported26 (summarized in Table 2).
per unit volume of the solution. As the diffusion of the reagents
and the adsorption–desorption steps are fast, the rate of the
bimolecular reaction on the surface is proportional to the
Conclusion
product of the fractions of the surface covered by the two
reacting species so, we can rewrite the eqn (3) as:
Three new water-soluble Au–NP (1–3) synthesized in our labo-
ratory, proved to be an excellent active catalysts for the reduc-
tion of 1,4-dinitrobenzene at room temperature in water. In all
cases, there is an induction time, and the activation energy
found is suitable for restructuring at the surface of Au–NP,
being consistent with other metal nanoparticles used for this
reaction. Not least, all the nanoparticles were reusable in water
for 3 cycles, using a quick, easy, economical and non-polluting
method.
dcDNB
¼ ꢀkS qDNBqHz
(4)
dt
where qDNB and qHz are fractions of surface sites occupied by
DNB and Hz, respectively. The k is the bimolecular rate constant
and can be obtained from the kinetics analysis. The reactants,
DNB and Hz, follow the Langmuir isotherm:37
n
ðKDNBcDNB
Þ
qDNB
¼
(4a)
n
m
1 þ ðKDNBcDNBÞ þ ðKHzcHz
Þ
Conflicts of interest
m
ðKHzcHz
Þ
There are no conicts to declare.
qHz
¼
(4b)
n
m
1 þ ðKDNBcDNBÞ þ ðKHzcHz
Þ
Here, KDNB and KHz are the adsorption coefficients of DNB
and Hz, respectively, and cDNB and cHz symbolize the respective
Acknowledgements
concentrations in solution. The exponent n and m are related to Financial support from the Consejo Nacional de Inves-
´
´
the heterogeneity of the solid surface of the NPs. In others tigaciones Cientıcas y Tecnicas (PIP CONICET 112-2015-
´
words, DNB and Hz may occupy adsorption sites on the surface 0100283), Universidad Nacional de Rıo Cuarto, Universidad
of different energies.31,37 Taking into account eqn (3) and (4a), Nacional del Sur, Agencia Nacional de Promocion Cientıca y
´
´
´
Tecnica (PICT-2015-2151 and PICT-2015-0585, PICT 2018-0507),
eqn (5) and (6) can be obtained:
´
and Ministerio de Ciencia y Tecnologıa (PID 2018, Gobierno de
n
m
dcDNB
dt
kSðKDNBcDNBÞ ðKHzcHz
Þ
ꢀ
¼
¼ kappcDNB
(5)
(6)
´
la provincia de Cordoba) is gratefully acknowledged. N. M. C., R.
D. F., F. M. and, G. F. S. hold a research position at CONICET.
G. M. thanks from CONICET for a research doctoral fellowship.
2
m
n
ð1 þ ðKDNBcDNBÞ þ ðKHzcHzÞ Þ
m
n
nꢀ1
kSKDNB cDNB ðKHzcHz
ð1 þ ðKDNBcNBÞ þ ðKHzcHzÞ Þ
Þ
kapp
¼
2
m
n
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