Green synthesis of a palladium-polyaniline nanocomposite for green Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1039/c5ra05007c

A palladium-polyaniline (Pd-PANI) nanocomposite was successfully synthesized using a one-pot green synthetic procedure in water by the reaction between aniline hydrochloride and potassium hexachloropalladate. Strong interaction between PANI and Pd was clearly evident in the UV-visible and Fourier transform infrared (FTIR) spectrum of the nanocomposite. Powder X-ray diffraction (XRD) patterns revealed the presence of Pd(0) in the nanocomposite with a fcc crystal structure. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) imaging showed that the Pd-PANI nanocomposite has spherical morphology with an average particle size of 175 ± 42 nm. High resolution TEM imaging and energy dispersive X-ray (EDX) spectroscopy studies revealed that very small nanoparticles of Pd (3.1 ± 0.9 nm) were found dispersed throughout the PANI matrix. X-ray photoelectron spectroscopy (XPS) revealed the presence of Pd(0) in the nanocomposite. The catalytic behavior of the Pd-PANI nanocomposite was studied for Suzuki-Miyaura coupling reactions in the presence of different bases in both organic and aqueous media. The results revealed that the Suzuki-Miyaura reaction proceeds much faster in water than in toluene. The excellent catalytic activity of the nanocomposite resulted in 86 and 91% yield in water and toluene respectively, when potassium carbonate was used as the base.

Full text of DOI:10.1039/c5ra05007c

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PAPER
Green synthesis of a palladium–polyaniline
nanocomposite for green Suzuki–Miyaura coupling
reactions†
Cite this: RSC Adv., 2015, 5, 33786
*
Sunil Dutt, Raj Kumar and Prem Felix Siril
A palladium–polyaniline (Pd–PANI) nanocomposite was successfully synthesized using a one-pot green
synthetic procedure in water by the reaction between aniline hydrochloride and potassium
hexachloropalladate. Strong interaction between PANI and Pd was clearly evident in the UV-visible and
Fourier transform infrared (FTIR) spectrum of the nanocomposite. Powder X-ray diffraction (XRD)
patterns revealed the presence of Pd(0) in the nanocomposite with a fcc crystal structure. Field emission
scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) imaging showed
that the Pd–PANI nanocomposite has spherical morphology with an average particle size of 175 ꢀ
42 nm. High resolution TEM imaging and energy dispersive X-ray (EDX) spectroscopy studies revealed
that very small nanoparticles of Pd (3.1 ꢀ 0.9 nm) were found dispersed throughout the PANI matrix.
X-ray photoelectron spectroscopy (XPS) revealed the presence of Pd(0) in the nanocomposite. The
catalytic behavior of the Pd–PANI nanocomposite was studied for Suzuki–Miyaura coupling reactions in
the presence of different bases in both organic and aqueous media. The results revealed that the
Suzuki–Miyaura reaction proceeds much faster in water than in toluene. The excellent catalytic activity
of the nanocomposite resulted in 86 and 91% yield in water and toluene respectively, when potassium
carbonate was used as the base.
Received 21st March 2015
Accepted 2nd April 2015
DOI: 10.1039/c5ra05007c
www.rsc.org/advances
Carbon–carbon coupling reactions such as Suzuki–Miyaura,
Heck, Sonogashira, Stille are very important to synthesize large
1. Introduction
number of pharmaceutical drugs, natural products, cosmetics,
polymers and liquid crystals.3 Pd is the catalyst of choice and is
being widely used as catalyst for Suzuki–Miyaura coupling
reactions.3 A plethora of literature is available on the develop-
ment of Pd catalysts for Suzuki–Miyaura coupling reactions.
Heterogeneous Pd-catalysts are attractive than the traditional
homogeneous Pd-catalysts. This is due to the possibility of
catalyst recyclability and for avoiding toxicity imparted from
residual catalysts, especially in the synthesis of drug mole-
cules.4–6 Nano sized Pd-particles supported on a suitable
support are the most effective catalysts due to increase in
surface area to volume ratio, which consequently leads to better
interaction of the catalyst with the reactants.7
Another open challenge in the development of catalysts for
Suzuki–Miyaura coupling reactions is the need to develop
catalysts that are active in water as the reaction medium.8,9 Most
oen Suzuki–Miyaura coupling reactions are performed in
organic solvents. Recently, there is more focus on performing
the Suzuki–Miyaura coupling reactions in water as a green
solvent. A number of concerns such as cost, safety and health
hazards that are associated with the organic solvents can be
overcome by the use of water as a solvent. However, most
organic compounds are water insoluble and hence performing
reactions in water becomes difficult. A plethora of research is
A large number of scientic elds have benetted immensely
due to the advent of nanoscience and nanotechnology.1 One of
the many elds where nanotechnology was immediately applied
was heterogeneous catalysis. Catalysis is chiey a surface
phenomenon and reduction in particle size leads to highly
enhanced surface area and this in turn results in enhanced
catalytic activity. It is expected that nanocatalysts will provide a
breakthrough in cutting edge technologies. The development of
new catalysts, especially for different organic reactions is always
demanding. Cost effectiveness, enhancement in the product
yield and reduction in reaction time are the major drivers for the
development of novel catalysts. Need for developing ‘green’
synthetic processes has recently emerged as an additional driver,
mainly due to the increased awareness about environmental
concerns and legislations.2 In this context, nanotechnology and
green chemistry can come together for the development of such
catalysts and this is the central theme of the present paper.
School of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175005,
Himachal Pradesh, India. E-mail: prem@iitmandi.ac.in; Fax: +91-1905-300009; Tel:
+91-1905-300040
† Electronic supplementary information (ESI) available: Fig. S1, overall XPS
spectrum of the Pd–PANI nanocomposite, Fig. S2, NMR spectrum of biphenyl
and Fig. S3, NMR spectrum of octyl benzene. See DOI: 10.1039/c5ra05007c
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being done on developing methods for performing organic solution while maintaining the temperature at 3–5 ꢁC. The
reactions in water. The published literature until 2010 on reaction mixture was kept under ice cold condition for 12 hours
Suzuki–Miyaura coupling reactions in water have been reviewed for the completion of reaction. The product was collected by
by Polshettiwar et al.10 Evidently, most oen the Suzuki– centrifugation at 10 000 rpm for 30 min and repeatedly washed
Miyaura coupling reactions in water was performed by using with water. The puried product was dried in oven at 50 ꢁC for
homogeneous catalysts. Only a limited number of heteroge- 24 h and the dried sample was used for further characterization.
neous catalysts such as Pd supported on various silica supports
have been studied for Suzuki–Miyaura reactions in water.11–13
One notable common aspect of these catalysts is the presence of
2.3. Characterization techniques
The prepared Pd–PANI nanocomposite was characterized using
a number of techniques. UV-visible absorption spectrum of the
Pd–PANI nanocomposite in NMP was recorded using Shimadzu
UV-4250 double beam spectrophotometer. FTIR spectrum of the
Pd–PANI nanocomposite was recorded using Perkin Elmer
FT-IR spectrometer (Spectrum Two) in the frequency range from
4000 cmꢂ1 to 600 cmꢂ1 with a resolution of 4 cmꢂ1 and 8 scans.
A small amount of the Pd–PANI nanocomposite was mixed with
KBr powder and pelletized to record the FTIR spectrum. Blank
KBr pellet was used for background correction. XRD of the
Pd–PANI nanocomposite was recorded using Smart Lab X-ray
diffractometer (Rigaku, Japan) using Cu Ka radiation as X-ray
source (l ¼ 0.1542 nm, 40 mA, 45 kV) at room temperature.
The sample was scanned in the 2q range of 10 to 90ꢁ at a
scanning rate of 2ꢁ minꢂ1 and a step size 0.02ꢁ. FESEM imaging
and elemental mapping of the Pd–PANI nanocomposite were
carried out using FEI Quanta FEG 450. TEM imaging and EDX of
the Pd–PANI nanocomposite was done using FEI Tecnai G2 200
S-Twin Model (200 kV) on carbon coated copper grids. XPS data
was collected using VG Microtech, model ESCA 3000 equipped
with ion gun (EX-05) for cleaning the surface. Atomic absorp-
tion spectroscopy (AAS) was carried out using LAB India AA
7000. The 1H spectra of the product of coupling reactions were
recorded using JEOL ECS-400 MHz NMR system in CDCl3.
an organic surfactant such as cetyl trimethylammonium
bromide (CTAB), polyethylene glycol (PEG) to facilitate the
mixing of the catalysts with water and to prevent aggregation.
Additionally, majority of these catalysts were made by tethering
Pd-containing organic compounds to the silica backbone.
Progressive deactivation of some of these catalysts was observed
due to the decomposition of the catalyst.14 Thus, only limited
attention was paid to develop Pd nanoparticles supported on a
suitable support as catalysts for performing Suzuki–Miyaura
reactions in water. Here we report the effective use of Pd–PANI
nanocomposite as a reusable catalyst for performing Suzuki–
Miyaura coupling reaction in water. Interestingly, it was also
found that the same catalyst can be used in organic solvents also.
PANI is a conducting polymer having very good environment
stability. PANI has been used as a catalyst support for a variety
of metals, metal ions and metal oxides including Pd. There are a
large number of methods reported for the synthesis of Pd or Pd
based catalysts.5 However, only very few reports are there on
green synthesis of Pd nanoparticles or their composites. Here
we report an one-pot green synthesis of Pd–PANI nano-
composite and its application as catalyst for Suzuki–Miyaura
coupling reaction. Synthesis of Pd–PANI catalyst was achieved
by using aniline hydrochloride as monomer and potassium
hexachloropalladate as oxidant in water as solvent. The
prepared nanocomposite was used as efficient catalyst in water
as well as toluene as reaction medium for Suzuki–Miyaura
coupling reactions with good yield.
2.4. Catalytic activity
2.4.1. Suzuki–Miyaura reaction in aqueous medium. The
catalytic activity of synthesized Pd–PANI nanocomposite was
tested for Suzuki–Miyaura coupling reaction using water as the
reaction medium (Scheme 1). The procedure for green Suzuki–
Miyaura coupling reaction described in the literature was
followed.15 Typically, to a 25 ml round bottom ask containing
2 ml water, 1.5 mmol phenylboronic acid, 1 mmol aryl halide,
3 mmol base and 0.01 mol% of Pd–PANI nanocomposite was
added. Reaction mixture was saturated with argon gas for
15 min and sealed with Teon rubber septum. The reaction was
carried out in argon atmosphere with stirring for 4 h at 90 C.
The product was extracted in ether and pure product was
collected using column chromatography.
2.4.2. Suzuki–Miyaura reaction in organic medium. The
catalytic activity of the prepared Pd–PANI nanocomposite was
2. Experimental section
2.1. Materials
Aniline hydrochloride, N-methyl pyrrolidone (NMP), potassium
hexachloropalladate (K2PdCl6), silica gel, bromo octane, bromo
benzene, iodo benzene, potassium carbonate (K2CO3), sodium
hydroxide (NaOH), triethylamine (Et3N), sodium phosphate
(NaPO4), phenyl boronic acid and toluene were used as received
from Sigma Aldrich. Hexane, ethyl acetate, diethyl ether and
thin layer chromatography (TLC) plates were purchased from
Merck. Ultra pure water (18.2 MU cm) from double stage water
purier (ELGA PURELAB Option-R7) was used to prepare all
solutions and in coupling reactions.
ꢁ
2.2. Preparation of the Pd–PANI nanocomposite
Freshly prepared aqueous solution of K2PdCl6 (20 ml, 50 mM)
was taken in a 50 ml beaker and was placed in an ice bath.
Aqueous solution of aniline hydrochloride (20 ml, 0.1 M) was
prepared separately and then added drop wise to the Pd salt Scheme 1 Suzuki–Miyaura coupling reaction in water.
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tested for Suzuki–Miyaura coupling reaction in organic medium
also (Scheme 2). Typically, to a 50 ml round bottom ask con-
taining 5 ml toluene, 1 mmol of bromo octane, 1.5 mmol phenyl
boronic acid, 3 mmol of a base and 0.036 mol% of the Pd–PANI
nanocomposite was added. The reaction mixture was then
reuxed in an oil bath under magnetic stirring at 90 ꢁC for 9 h.
The product was extracted with ethyl acetate and the pure
product was collected using column chromatography. Similar
procedure was followed for the Suzuki–Miyaura reaction
between bromo benzene and phenylboronic acid (Reaction (2)
in Scheme 2).
2.5. Effect of base
Effect of change in the base that was used for the coupling
reaction was studied systematically. Five different bases namely
NaOH, K2CO3, Na2CO3, Na2PO4 and (C2H5)3N were used for all
the three Suzuki–Miyaura reactions that are presented in
Schemes 1 and 2 to study effect of base on the yield of the
reaction.
Fig. 1 UV-visible absorption spectrum of Pd–PANI nanocomposite.
absorption bands that are peaking at 425 nm and 600 nm
correspond to p–p* transition within benzenoid units and
3. Results and discussion
transition from benzenoid to quinoid of PANI respectively.18
A
The reaction between K2PdCl6 and aniline hydrochloride was
instantaneous, even in ice-cold conditions, as we observed
change in color of the solution of the latter immediately on the
addition of the former. The colorless solution of aniline
hydrochloride changed initially to brown and then a green
colored precipitate was formed. The color of the prepared
Pd–PANI nanocomposite was green aer extraction and puri-
cation. This indicates that the PANI in the nanocomposite was
in its emeraldine salt form. Oxidation of aniline takes place
simultaneously with the reduction of palladium ions on mixing
leading to the formation of the Pd–PANI nanocomposite. The
Pd salt K2PdCl6 was reported as oxidant for the synthesis of
PANI using aniline monomer in organic solvents.16 The present
synthesis proceeds in aqueous medium in a single step and
hence is a ‘green’ procedure.
blue shi of the p–p* transition band (benzenoid to quinoid
units) was observed in the nanocomposite when compared to
pure PANI. This indicates that there is interaction between
palladium nanoparticles and nitrogen atom of the PANI.16
Recorded FTIR spectrum of the prepared Pd–PANI nano-
composite is shown in Fig. 2. The most important characteristic
vibrational bands of Pd–PANI nanocomposite were observed at
3454 cmꢂ1 (N–H stretching), 1571 cmꢂ1 (quinoid ring stretch-
ing), 1492 cmꢂ1 (benzenoid ring stretching), 1305 cmꢂ1
(secondary C–N stretching), 1119 cmꢂ1 (vibration band of
dopant anion) and 803 cmꢂ1 (para disubstituted benzene
rings).19 Also the shiing of N]Q]N stretching (Q ¼ quinoid
unit) and N–B–N stretching (B ¼ benzenoid unit) towards lower
frequencies are due to the interaction between PANI and Pd.20
3.1. Characterization
The UV-visible absorption spectrum of the prepared Pd–PANI
nanocomposite is shown in Fig. 1. The Pd–PANI nanocomposite
showed three characteristic absorption peaks at wavelength of
290 nm, 425 nm and 600 nm. The peak at 290 nm is due to
plasmon absorption of Pd nanoparticles.17 The two broad
Scheme 2 Suzuki–Miyaura coupling reaction in toluene.
33788 | RSC Adv., 2015, 5, 33786–33791
Fig. 2 FTIR spectrum of Pd–PANI nanocomposite.
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Fig. 5 High resolution TEM image of the Pd–PANI nanocomposite.
the elemental map of Pd as shown in Fig. 6b and c it is evident
that Pd nanoparticles are present throughout the spherical
particles of Pd–PANI nanocomposite. From a comparison of the
elemental map of Pd and N, it can be seen that the Pd nano-
particles predominates the surface as the concentration of Pd
was higher than N.
EDX spectroscopic analysis of the sample deposited on TEM
grid was also done and a typical spectrum is shown in Fig. 7.
The Pd element is clearly seen in EDX spectrum. Presence of Cl
was also detected in EDX. This is due to the emeraldine salt
form of PANI. AAS studies revealed that the nanocomposite
contained 9% (by wt) of Pd.
Fig. 3 XRD pattern of Pd–PANI nanocomposite.
The XRD pattern of the prepared Pd–PANI nanocomposite is
shown in Fig. 3. The broad peak centered at 25ꢁ having a
shoulder at 20ꢁ is due to the crystalline domains of PANI.21,22
There were a number of other distinct peaks observed in the
XRD pattern corresponding to Pd in the Pd–PANI nano-
composite. Peaks at 2q 40ꢁ, 46ꢁ, 68ꢁ, 82ꢁand 86ꢁ are due to the
(111), (200), (220), (113) and (222) reection planes of metallic
Pd with fcc lattice structure.23 These peaks also matched well
with the standard values (JCPDS le no. 87-0638) and with the
reported data for Pd.24
Pd–PANI nanocomposite was further characterized by XPS as
shown in Fig. 8 and S1 ESI.† The metallic state of Pd(0) in
nanocomposite was conrmed by the XPS peaks at 339.4 eV
The morphology of the prepared Pd–PANI nanocomposite was
studied using FE-SEM and TEM imaging and the corresponding
images are shown in Fig. 4. It is evident from the FE-SEM image
that the Pd–PANI nanocomposite has spherical morphology with
average particle size of 175 ꢀ 42 nm. The same morphology and
particle size was conrmed from TEM images also. Surface of the
Pd–PANI nanocomposite appeared rough in the FESEM images.
It shows that the Pd nanoparticles are present on the surface of
the nanocomposite. A high resolution TEM image is shown in
Fig. 5. From Fig. 5, it is clear that Pd is present in the form of
small spherical nanoparticles in the PANI matrix. The particle
size of the Pd nanoparticles was 3.0 ꢀ 0.9 nm.
3
3
(Pd(0) d5/2) and 344.7 eV (Pd(0) d3/2) respectively. Evidently,
there was signicant shi in the binding energy values and this
must be due to interaction of Pd(0) with PANI.25–27 Peak at
binding energies of 200.3 eV, 285.2 eV, 401.5 eV, 535.7 eV
corresponds to Cl 2p, C 1s, N 1s and O 1s respectively in the core
level XPS spectrum (Fig. S1, ESI†). The core level spectrum also
matched with the reported data.28
The elemental mapping of Pd and N was done using energy
dispersive X-ray (EDX) spectroscope attached to the FESEM and
the corresponding elemental map is presented in Fig. 6. From
Fig. 6 (a) FESEM image of Pd–PANI nanocomposite. Elemental map
Fig. 4 FESEM and TEM image of Pd–PANI nanocomposite.
of: (b) Pd and N together, (c) Pd alone and (d) N alone.
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Table 1 Study of effect of base on the yield of the Suzuki–Miyaura
coupling reaction in water and toluene as reaction medium
% Yield of reaction
Scheme
NaOH
K2CO3
Na2CO3
Na2PO4
(C2H5)3N
1
84
82
85
86
91
88
80
76
79
77
74
63
73
58
61
2 (R1)
2 (R2)
reaction was relatively slow in toluene compared to water. In
water, the coupling reaction between bromobenzene and
phenyl boronic acid completed in 4 h, whereas in toluene it took
9 h to complete the same reaction. The coupling reaction
between octylbromide and phenylbornic acid also proceeded
with good yield in toluene. The formation of the products
(biphenyl and octyl benzene) was conrmed by using NMR
Fig. 7 EDX spectrum of the Pd–PANI nanocomposite.
3.2. Catalytic activity
1
spectroscopy and the typical H NMR spectrum is reported in
The catalytic activity of the Pd–PANI nanocomposite was
studied in both the aqueous and organic reaction medium and
the results are discussed in the following section.
Fig. S2 and S3 ESI† and the results are summarized below.
Octyl benzene. 1H NMR (500 MHz, CDCl3) d 7.60–7.58 (m, 2H),
d 7.45–7.42 (m, 2H), d 7.36–7.33 (m, 1H), d 3.40 (t, J ¼ 6.9 Hz,
2H), d 1.88–1.82 (m, 2H), d 1.43–1.40 (m, 2H), d 1.30–1.25 (m,
8H), 0.88 (t, J ¼ 7.0 Hz, 3H).
3.2.1. Suzuki–Miyaura coupling reaction. The Pd–PANI
nanocomposite showed very good catalytic activity for Suzuki–
Miyaura reaction in water and the reaction was almost complete
in 4 h. Water and diethyl ether was added to the reaction
mixture aer completion of the reaction. The oil and aqueous
phases were separated. To the oil phase excess amount of
sodium sulphate was added and diethyl ether was evaporated
on rotary evaporator under reduced pressure. Column chro-
matography of crude product was done on silica gel column
using hexane and ethyl acetate mixture (95 : 5) to collect the
pure product and yield was calculated. The formation of the
product was conrmed by using NMR spectroscopy and the
The Suzuki–Miyaura reaction is usually performed in pres-
ence of a base and it is known that the nature of the base affects
the completion of reaction.29 Hence, the effect of changing the
base on the yield of the product was studied using ve different
bases. The yield of the reaction with each base in different
Suzuki–Miyaura reaction in both water and toluene is presented
in Table 1.
It can be seen from the table that the yield of the reaction
varies signicantly with the change in base. Yield for the Suzuki–
Miyaura reaction in aqueous medium varied from 86% to 73%
with the change in base. In the organic medium the yield
changed from 91% to 58% and 88% to 61% respectively in
Reaction (1) and Reaction (2). Higher yield was obtained when
the base used was K2CO3, irrespective of the reaction medium
and reactants. So, it was concluded that K2CO3 is the best base
among all for the prepared Pd–PANI catalyst in the Suzuki–
Miyaura coupling reactions that were studied. Raque et al., also
reported K2CO3 as the best base for Suzuki–Miyaura reaction
with Pd–PANI as catalyst.29 However, NaOH and Na2CO3 also
gave good yields. Easy separation and reusability of the catalyst
is the major attraction for heterogeneous catalysts such as Pd–
PANI. Recyclability of the catalysts was not tested due to the
small scale of reaction with very low concentration of catalysts.
1
1
typical H NMR spectrum is reported in Fig. S2 ESI.† The H
NMR data is summarized below.
1
Biphenyl. H NMR (400 MHz, CDCl3) d 7.60–7.57 (m, 4H), d
7.45–7.41 (m, 4H), d 7.36–7.32 (m, 2H).
The Pd–PANI nanocomposite showed good catalytic activity
in Suzuki–Miyaura reactions in toluene medium also. The
4. Conclusions
In summary, a simple single step and green method for the
synthesis of Pd–PANI nanocomposite has been developed. The
nanocomposite has a spherical morphology and contains very
small Pd nanoparticles dispersed in the PANI matrix. Strong
interaction between the Pd nanoparticles and the PANI was
clearly evident. The prepared Pd–PANI nanocomposite showed
very good catalytic activity for Suzuki–Miyaura coupling
Fig. 8 XPS spectrum of the Pd core level.
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˜
´
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Acknowledgements
We are thankful to AMRC, IIT Mandi for laboratory and the
characterization facilities. Financial support from DST, New
Delhi, India is also acknowledged hereby. We are also thankful
to CMSE, NIT Hamirpur (H.P.), India for providing FE-SEM
imaging facilities.
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