Resin-trapped gold nanoparticles: An efficient catalyst for reduction of nitro compounds and Suzuki-Miyaura coupling

Article abstract of DOI:10.1016/j.molcata.2013.10.004

Gold in nanoparticle form shows good catalytic activity in contrast to bulk form and is finding applications in a variety of organic reactions. The present investigation describes direct deposition of gold nanoparticles onto commercially available resin by sorption reduction method. Uniformly dispersed nanoparticles of 3-8 nm dimensions were characterized by UV-visible spectroscopy, XRD, SEM and TEM, etc. The AuNPs were found to be remarkably stable and active catalysts for the selective reduction of nitro group under mild reaction conditions and microwave-assisted ligand-free Suzuki-Miyaura cross-coupling reaction between aryl halides and phenylboronic acid. Calculated rate constant (2.5 × 10^-2 s^-1) for the reduction of 4-nitrophenol is among the best reported in the literature. The versatility of both the protocols is demonstrated by taking a number of substrates.

Full text of DOI:10.1016/j.molcata.2013.10.004

Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Resin-trapped gold nanoparticles: An efficient catalyst for reduction
of nitro compounds and Suzuki-Miyaura coupling
Dipen Shah, Harjinder Kaur∗
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Gold in nanoparticle form shows good catalytic activity in contrast to bulk form and is finding applications
in a variety of organic reactions. The present investigation describes direct deposition of gold nanoparti-
cles onto commercially available resin by sorption reduction method. Uniformly dispersed nanoparticles
of 3–8 nm dimensions were characterized by UV–visible spectroscopy, XRD, SEM and TEM, etc. The AuNPs
were found to be remarkably stable and active catalysts for the selective reduction of nitro group under
mild reaction conditions and microwave-assisted ligand-free Suzuki-Miyaura cross-coupling reaction
between aryl halides and phenylboronic acid. Calculated rate constant (2.5 × 10−2 s−1) for the reduction
of 4-nitrophenol is among the best reported in the literature. The versatility of both the protocols is
demonstrated by taking a number of substrates.
Received 23 August 2013
Received in revised form 7 October 2013
Accepted 8 October 2013
Available online 17 October 2013
Keywords:
Resin impregnated AuNPs
Nitroarenes reduction
Suzuki-Miyaura reaction
Microwave heating
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
ever, the resultant catalysts often suffer from a significant loss
of catalytic activity during recycling because of weak interactions
Today, noble metals are being used as catalysts in the synthesis
of a variety of important organic compounds. An efficient, recy-
clable and cost-effective synthesis of noble metal nanocatalysts is of
great importance as it allows effective utilization of these expensive
metals. Two key factors that determine the catalytic performance
of such nanocatalysts are the particle size and the supports used
for the dispersion of the noble metals [1]. Small size of the parti-
cles enhances their surface area and hence the number of reactive
sites. On the other hand, choice of support plays a crucial role in
the accessibility to catalytic sites. In the last few decades, gold
nanoparticles (AuNPs) have been used in many areas of research
including electronics, biomedicines, optics, and catalysis due to
their strong optical, electrical, and chemical properties, as well as
their biocompatible nature. Catalytic activity has been extensively
reviewed in many reactions of both, industrial and environmental
importance [2]. To facilitate catalyst recovery, AuNPs are usually
dispersed onto solid matrices to prepare heterogeneous AuNP cat-
alysts. According to the literature [3–8], various materials have
been used as supporting matrices, including carbon nanotubes, sil-
ica, titania, ceria alumina, polymers, etc. Amongst them, alumina
is one of the most frequently used supports owing to its remark-
able properties such as high surface area, porous structure and
good mechanical strength [9,10]. The impregnation method is a
general strategy for the heterogenization of AuNPs [11,12]. How-
between the AuNPs and supporting matrices. To overcome these
disadvantages, much effort has been paid to the development of
new methodologies for the heterogenization of AuNPs. Yang et al.
[13] have used of an organic gel based on polymeric phloroglucinol
carboxylic acid and formaldehyde to support gold nanoparticles. Li
et al. [14] have synthesized stable AuNPs encapsulated in a silica
dendrimer organic–inorganic hybrid support as a recyclable cata-
lyst for the oxidation of alcohols. Ohno et al. employed high density
poly (methyl methacrylate) brushes to increase dispersion of gold
nanoparticles [15]. Miyamura et al. [16] have reported polystyrene
supported AuNPs as a catalyst for the aerobic oxidation of alcohols
to aldehydes and ketones under atmospheric conditions.
We have earlier synthesized palladium nanoparticles supported
on Amberlite XAD-4 and found that the resulting catalyst was
not only stable and recyclable but also showed catalytic activity
comparable to colloidal nanoparticles [17,18]. This prompted us
to study the catalytic behavior of other metals on this support.
In the present work, we have evaluated the catalytic activity of
resin-trapped AuNPs toward reduction of nitro compounds and
the Suzuki-Miyaura cross-coupling reaction of aryl halides with
arylboronic acid.
2. Experimental
2.1. Materials and instruments
∗
Corresponding author. Tel.: +91 79 26300969; fax: +91 79 26308545;
mobile: +91 9427628565.
All chemicals used were of the analytical grade or of the high-
est purity grade available. Gold chloride was purchased from
E-mail address: hk ss in@yahoo.com (H. Kaur).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.10.004

D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
71
Aldrich. Aryl halides and aryl nitro compounds were purchased
from Aldrich or Merck. Dichloromethane (DCM), diethyl ether
(Et2O), NaOH and NaBH4 were purchased from Finar Chemicals.
Amberlite XAD-4 (surface area 725 m2 g−1 mesh size 20–40) was
purchased from Aldrich. Water used in all experiments was puri-
fied by the Millipore-Q system. All glassware was thoroughly
cleaned with freshly prepared 3:1 HCl/HNO3 (aqua regia) prior to
use.
CEM benchmate microwave reactor was used for microwave
heating. UV–visible absorption spectra were acquired on a Jasco V-
570 UV–visible spectrophotometer. X-ray diffraction was recorded
on SEIFERT FPM, XRD 7 using Cu K␣ radiation (ꢀ = 1.5418) and filter
of nickel. Scanning Electron Microscopy (SEM) image of the bead
was taken on a Leo 1430 VP Electron Microscope after coating it
with palladium. High resolution transmission electron microscopy
(HR-TEM) pictures were taken using a Hitachi (H-7500) instrument.
The swollen resin beads were milled and a drop of alcoholic sus-
pension was placed onto a 200 mesh carbon coated copper grid. It
was then dried to evaporate the solvent and used for microscopy.
GC–MS measurements were carried on Perkin-Elmer USA Auto sys-
tem XL. 1H NMR spectra were recorded on a Bruker Advance II
400 NMR spectrometer. Inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES) measurements were carried out on
a HJY Ultima-2 instrument: power 1000 W, nebulizer flow 1.29,
nebulizer pressure 2.96, wave length 242.795 nm. LOD (limit of
detection) as determined by signal to noise ratio was found to be
0.25 mg/L.
Fig. 1. UV–visible spectrum for the resin-AuNPs, inset: picture of resin-AuNPs.
Beads taken with lateral illumination.
3. Results and discussion
3.1. Synthesis and characterization of resin supported AuNPs
Amberlite XAD-4 is a neutral, non-functional, hydrophobic,
macroporous, commercial and cross-linked polystyrene resin. The
resin is chemically and mechanically stable and there is no
interference with the reaction conditions. While the presence of
ligands/functional groups on a resin can control the size and sta-
bility of the nanoparticles, they have a negative influence from the
catalytic activity point of view i.e. reducing the interaction of cat-
alytic sites with the substrate. Hence, it seemed prudent to use the
resin as a support for nanoparticle synthesis. The resin beads turn
pink on impregnation of gold nanoparticles and since no other sta-
bilizing agent was used it is assumed that the nanoparticles are
trapped in the polymer network and stabilized solely by the steric
factor/electrostatic interaction of the benzene rings.
AuNPs exhibit strong surface plasmon resonance (SPR) absorp-
tion band that is absent in the spectrum of the bulk metal. This
band is dependent on the size, shape and aggregation of AuNPs.
Therefore, UV–visible spectroscopy is a useful tool to estimate
nanoparticle size, concentration, and aggregation level. In the
present case, the resin-AuNPs beads were milled with ethanol and
the solution centrifuged for 10 s. The supernatant solution was
analyzed by spectrophotometer against a blank prepared from sim-
ilarly treated resin beads. The SPR band maximum for AuNPs was
thus observed at 535 nm which is characteristic of gold nanoparti-
cles (Fig. 1). The red shift observed in the SPR band at 535 nm (Mie
theory predicts SPR at 510–515 nm for gold particles of 3–8 nm as
measured by TEM) can be due to stabilization of the nanoparticles
by the ꢁ electron cloud of the benzene ring [19].
The XRD pattern of the powdered resin beads is shown in Fig. 2.
The resin particles did not show any peak, only a broad hump was
observed, which indicated the amorphous nature of the polymer
matrix. When these beads were impregnated with gold nanopar-
ticles, characteristic peaks of gold at 2Â = 38.5 and 44.2 (JCPDS No.
04-0784) were observed, corroborating the immobilization of the
nanoparticles onto the resin beads. The bands were identified as
(1 1 1) and (2 0 0) reflections corresponding to the fcc lattice of gold
nanoparticles [20]. The broad nature of the bands was also indica-
tive of the nanosize of the particles.
SEM image of the resin beads did not show any particles or
agglomerations on the surface (Fig. 3). Thus, we inferred that all the
particles are trapped inside the matrix of the resin. TEM was used to
study the shape and size of nanoparticles. TEM pictures of AuNPs
taken after milling the beads (Fig. 4) showed spherical nanopar-
ticles embedded in polymer matrix. The size of the nanoparticles
varies between 3 and 8 nm. To determine the amount of gold loaded
2.2. Preparation of resin supported gold nanoparticles
(resin-AuNPs)
The resin supported gold nanoparticles were synthesized by a
method developed in our lab. Amberlite XAD-4 beads (5.0 g) were
washed repeatedly with hot water to remove salts, swollen in
ethanol and then equilibrated with 10 mL of 1 mmol solution of gold
chloride in ethanol. After 1 h, excess solution was drained and the
metal was reduced by passing cold aqueous NaBH4 (0.1 mol dm−3
)
solution. The resin particles were further washed with water to
remove excess reagent and stored in ethanol.
2.3. Reduction of nitro compound by resin-AuNPs
In a typical reduction protocol, 100 mg of catalyst was added
to 20 mL of methanol/water (1:1) solution containing 0.5 mmol of
nitro compound and 5 mmol of NaBH4. The mixture was vigor-
ously stirred at 40 ◦C. The reaction was monitored by Thin Layer
Chromatography (TLC) and reaction mixture was quenched by
extracting the organic derivatives with ethyl acetate. The solvent
was evaporated under vacuum to give crude product of corre-
sponding amine compound. Further purification was done through
column chromatography. The products were confirmed by their
melting points and 1H NMR spectroscopy.
2.4. Suzuki-Miyaura coupling catalyzed by resin-AuNPs
Into a 10 mL vial, phenylboronic acid (1.5 mmol), aryl iodide
(1.0 mmol), sodium hydroxide (3.0 mmol), ethanol (2.0 mL), water
(1.0 mL) and catalyst (200 mg wet resin) were taken and heated
in a CEM microwave (140 ◦C, 300 w) for 12 min. The reaction was
quenched by filtering the hot solution in 10 mL of cold water. The
resulting solution was extracted with Et2O/DCM (2 × 5 mL). The
combined extracts were dried over anhydrous MgSO4 and the sol-
vents were removed under vacuum. The crude products were then
recrystallized from appropriate solvent and characterized by 1H
NMR.

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D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
3.2. Reduction of nitro compounds by resin-AuNPs
Reduction of nitro compounds is widely used in different indus-
tries like agrochemicals, dyes, pharmaceuticals, etc. Many reducing
agents have been utilized for their reduction, the most classic being
metallic reagents in the presence of an acid. However, this method-
ology is messy and environmentally hazardous [21] and therefore,
synthetic routes involving cleaner and cheaper alternatives are nec-
essary. One such methodology is to employ NaBH4 in water as the
hydride source in the presence of catalyst [22–24]. This reaction
is also used widely to evaluate catalytic activity of gold nanoparti-
cles. The versatility of resin-AuNPs catalyst in reducing different
nitro derivatives is presented in Table 1. The striking feature of
our catalyst is the quantitative reduction of halo nitro benzenes
and nitro amines (Table 1). In case of 4-nitrobenzaldehyde (entry
6) we found selective reduction of aldehyde over nitro group and
the major product was 4-nitrobenzyl alcohol. However, on increas-
ing the amount of sodium borohydride 4-aminobenzyl alcohol was
obtained as the sole product. In the presence of olefinic bond
(entry 7) surprisingly the double bond was reduced in preference
to the nitro group and major product was 2-nitrophenyl ethane
and not the expected phenyl ethenamine. Again on increasing the
concentration of sodium borohydride and time we could isolate 2-
phenyethylamine in quantitative yield. The method can be used as
a one step process for the synthesis of phenylethylamines.
Fig. 2. XRD pattern of the resin-AuNPs.
The kinetics of the catalytic reduction reaction was studied in
a standard quartz cuvette with 1 cm path length and 4.5 mL vol-
ume. 2.27 mL of water was mixed with 30 ␮L (10−2 M) 4-nitro
phenol solution, and finally 200 ␮L freshly prepared NaBH4 solu-
tion (10−1 M) was added. After mixing these solutions, 5 mg of
AuNPs were added in cuvette to study the reduction reaction.
UV–visible spectra were recorded at every 10 min interval in the
range of 200–500 nm for at least 60 min at 25 and 35 ◦C. The rate
constant was determined by measuring the change in absorbance
(Fig. 5) of a peak at 400 nm, the initially observed peak for the
nitrophenolate ion, as a function of time. The peak shows a grad-
ual decrease in intensity with time and a new peak appeared at
295 nm indicating the formation of 2-aminophenol. The reaction
shows a pseudo-first-order kinetics with respect to 4-nitrophenol
and the rate constants were calculated to be 1.7 × 10−2 s−1 and
2.55 × 10−2 s−1 at 25 ◦C and 35 ◦C respectively. The reduction was
found to occur only in the presence of our nanocatalyst and no
reduction occurred in the absence of AuNPs. Thus, indicating that
the catalytic reduction occurs at the surface of nanocatalyst. It was
also observed from Fig. 6 that the rate of the reaction increased with
the increase in the temperature. Table 2 shows a comparison of the
rate constants for the present method with those reported in the
Fig. 3. SEM image of resin-AuNPs.
onto the resin, 500 mg of resin was incinerated in a crucible for 4 h
at 550 ◦C. The residue was dissolved in 1 mL Aqua regia and diluted
to 10 mL with distilled water. The average concentration of gold (3
trials) as determined by ICP-AES was found to be 0.0032 mmol/g
(0.630 mg/g) of the resin.
Fig. 4. TEM image of resin-AuNPs at 100 and 200 nm resolution.

D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
73
Table 1
Resin-AuNPs catalyzed reduction of aromatic nitroarenes.
Entry
Substrate
Product
Time (min) at 40 ◦
C
aYield (%)
NO2
NH2
1
20
85
NO2
NH2
OH
OH
2
20
82 (85)b
NO2
NH2
NH2
NH2
NH2
NH2
3
4
25
25
95
90
NO2
NH2
NO2
NH2
Br
Br
5
25
90
NO2
NO2
CH2OH
CHO
6
7
25
30
85
80
Fig. 5. UV–visible spectra for the reduction of 4-nitrophenol measured at 10 min
intervals (a) at 25 ◦C (b) at 35 ◦C.
H2
C
H
C
HC
NO2
H2C
NO2
Aromatic nitroarenes (0.5 mmol); NaBH4 (5 mmol); Au catalyst (100 mg resin);
20 mL of MeOH/H2O (1:1); 40 ◦C 20 min.
a
Isolated yield.
% Conversion.
b
literature. The activation energy of the reaction at different temper-
atures was calculated from the Arrhenius equation. The activation
energy calculated for the p-nitrophenol (4-NP) to p-aminophenol
(4-AP) test reaction is 29.8 2 kJ per mole. The apparent activa-
tion energies for the same reaction carried out with different gold
nanoparticles (various shapes, different supports) range from 30 to
58 kJ per mole.
Fig. 6. Plot of ln(At/A0) versus time for the reduction of 4-nitrophenol.
Table 2
Systematic literature survey of rate constant at room temperature for the reduction of 4-nitrophenol.
Entry
Sample
Carrier system
k (s−1
)
Ref.
1
2
3
4
5
6
Resin-AuNPs
AuNPs
Au-resin
AgNPs/GR-G3.0 PAMAM
Au/PMMA
PEI capped AuNPs
Non-ionic resin
2.5 × 10−2
1.78–13.2 × 10−3
0.16 × 10−3
21.7 × 10−3
7.9 × 10−3
This work
[25]
[26]
[27]
[28]
PAMAM dendrimer
Ion-exchange resin
PAMAM dendrimer
Poly(methyl methacrylate)
Polyelectrolyte
5.2 × 10−2
[29]

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D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
Fig. 7. Recycling of the catalyst for the reduction of 4-nitrophenol.
Recyclability of the resin-AuNPs catalyst was also investigated.
Fig. 9. Effect of base on the yield of standard Suzuki-Miyaura coupling reaction.
The resin beads were recovered by simple filtration and were
washed with hot water/ethanol to remove any sorbed products.
The resin-AuNPs were reused without obvious loss of their cat-
alytic activity, up to four cycles. Recyclability of the resin is shown
in Fig. 7.
reaction. The reaction did not proceed to completion when either
Na2CO3 or K2CO3 were used as base (Fig. 9).
The effect of catalyst concentration was carried out by vary-
ing the amount of catalyst while keeping the other parameters
constant. It was observed that 200 mg of resin was sufficient to com-
plete the reaction (Fig. 10). The TON and TOF (h−1) were calculated
to be 1666 and 8330, respectively.
3.3. Suzuki-Miyaura reaction catalyzed by resin-AuNPs
Suzuki reaction of aryl halides with phenylboronic acid, which
is predominately catalyzed by palladium (Pd) catalysts, is a power-
ful and convenient synthetic method for the generation of biaryl in
organic chemistry. There are only a few reports of AuNPs catalyzed
Suzuki reaction. Han et al. [30] first reported the activity of PATP
polymer-stabilized gold nanoparticles for Suzuki-Miyaura cross-
coupling reaction of aryl halides with arylboronic acids in water. Li
et al. [31] reported AuNPs supported on graphene as active cata-
lyst for the Suzuki reaction of aryl halides with arylboronic acid in
water under aerobic conditions. We explored the catalytic activity
of resin-AuNPs for the Suzuki reaction. The reaction was success-
fully carried out in methanol/water under aerobic conditions in a
microwave. It was also observed that water was required for the
reaction as no reaction took place when pure ethanol was used.
The effect of various parameters such as time, catalyst con-
centration and effect of base were studied taking the coupling of
4-iodoanisole and phenylboronic acid as the standard reaction. The
time course of the reaction was studied by quenching the reaction
at different time intervals and the reaction mixture was analyzed
by GC–MS. It was observed that after 12 min (Fig. 8) the peak for 4-
iodoanisole disappeared completely and 4-methoxybiphenyl was
identified as the sole coupling product by its mass spectrum. The
effect of base on the reaction was also studied and it was observed
that three moles of NaOH were required for the completion of the
We also carried out Suzuki cross-coupling with other substi-
tuted iodobenzenes containing both electron withdrawing and
electron releasing groups. The results are summarized in Table 3. In
all the cases very high conversion of iodobenzenes was observed in
short reaction times. Bromobenzene and chlorobenzene were also
used for coupling with phenylboronic acid (Table 3, entries 8 and
9). Their reactivities were however, found to be lower than that
of iodobenzene. This decrease may be attributed to the differences
in the strengths of C I bond, C Br bond and C Cl bond. The recy-
clability of the catalyst was also investigated and the results are
shown in Fig. 11. The catalysts was recovered by simple filtration
and washed with alcohol/water and reused. The slight yield loss
was observed after five cycles.
3.4. Leaching studies of the catalyst
Leaching can decrease the efficiency and service life of a catalyst.
In order to study the leaching of catalyst during the reaction, hot
filtration test was performed in both the reactions. In case of nitro
group reduction the resin beads were filtered off from the reaction
mixture after 5 min and the filtrate was monitored for continued
activity by UV–visible spectroscopy. The results revealed that after
the removal of the resin particles, the reaction stopped (Fig. 12),
indicating the absence of active gold species in the reaction mixture.
Fig. 8. Time course of Suzuki-Miyaura reaction between phenylboronic acid and
Fig. 10. Effect of catalyst concentration on the standard Suzuki-Miyaura coupling
4-iodoanisole.
reaction.

D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
75
Table 3
Resin-AuNPs catalyzed Suzuki-Miyaura coupling.
Entry
Aryl halide
Product
Time (min)
aYield (%)
I
1
12
80
I
OCH3
COCH3
CHO
OCH3
I
2
12
85
COCH3
I
3
12
85
CHO
I
4
12
75
NO2
NO2
I
5
12
80
CH3
CH3
I
6
15
80
Br
Br
Br
7
8
12
20
75
45
Cl
9
20
Trace
Aryl halide (1.0 mmol); phenylboronic acid (1.5 mmol); Au catalyst (200 mg resin); NaOH (3.0 mmol); EtOH (2 mL); H2O (1 mL), (microwave conditions, 140 ◦C, 12 min).
a
Isolated yield.

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D. Shah, H. Kaur / Journal of Molecular Catalysis A: Chemical 381 (2014) 70–76
AuNPs was studied for the reduction of nitro group and the Suzuki-
Miyaura cross coupling reaction. The rate constant and the apparent
activation energy calculated for the reduction of 4-nitrophenol is
comparable to the best reported in the literature. The catalyst can
also be used for one step synthesis of phenylethylamines. The cat-
alyst can be utilized for Suzuki-Miyaura cross coupling reaction
of various aryl halides. However, the activity is less compared to
similarly prepared palladium nanoparticles [16]. It is also notewor-
thy that the catalyst showed excellent recyclability for both the
reactions.
Acknowledgements
Authors are grateful to the UGC, New Delhi for the BSR fel-
lowship. SAIF, Panjab University, Chandigarh, for the NMR spectra,
Sicart, Vallabh Vidhya Nagar, Gujarat, for GC–MS and TEM. CSM-
CRI, Bhavnagar, for SEM pictures. Dr. Sareen PRL, Ahmedabad for
ICP-AES studies, O2H Ahmedabad for ESI Mass.
Fig. 11. Recycling of the catalyst for the standard Suzuki Miyaura coupling reaction.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.
2013.10.004.
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4. Conclusion
In summary, we report that Amberlite XAD-4 can be used to syn-
thesize highly stable supported AuNPs. The catalytic activity of the