Green heterogeneous Pd(II) catalyst produced from chitosan-cellulose micro beads for green synthesis of biaryls

Article abstract of DOI:10.1016/j.carbpol.2016.06.103

In green catalyst systems, both the catalyst and the technique should be environmentally safe. In this study we designed a green palladium(II) catalyst for microwave-assisted Suzuki [Formula presented] coupling reactions. The catalyst support was produced from biopolymers; chitosan and cellulose. The catalytic activity of the catalyst was tested on 16 substrates in solvent-free media and compared with those of commercial palladium salts. Reusability tests were done. The catalyst was also used in conventional reflux-heating system to demonstrate the efficiency of microwave heating method. We recorded high activity, selectivity and excellent TONs (6600) and TOFs (82500) just using a small catalyst loading (1.5?×?10^?3?mol%) in short reaction time (5?min). The catalyst exhibited a long lifetime (9 runs). The findings indicated that both green chitosan/cellulose-Pd(II) catalyst and the microwave heating are suitable for synthesis of biaryl compounds by using Suzuki [Formula presented] coupling reactions.

Full text of DOI:10.1016/j.carbpol.2016.06.103

Accepted Manuscript
Title: Green heterogeneous Pd(II) catalyst produced from
chitosan-cellulose micro beads for green synthesis of biaryls
Author: Talat Baran Idris Sargin Murat Kaya Ayfer Mentes¸
PII:
S0144-8617(16)30778-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.carbpol.2016.06.103
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22-6-2016
27-6-2016
Please cite this article as: Baran, Talat., Sargin, Idris., Kaya, Murat., &
Mentes¸, Ayfer., Green heterogeneous Pd(II) catalyst produced from chitosan-
cellulose micro beads for green synthesis of biaryls.Carbohydrate Polymers
http://dx.doi.org/10.1016/j.carbpol.2016.06.103
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Green heterogeneous Pd(II) catalyst produced from chitosan-cellulose micro beads for
green synthesis of biaryls
Talat Baran1*, Idris Sargin2, Murat Kaya2, Ayfer Menteş1
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1Department of Chemistry, Faculty of Science and Letters, Aksaray University, Aksaray,
68100 Aksaray, Turkey
2Department of Biotechnology and Molecular Biology, Faculty of Science and Letters,
Aksaray University, 68100 Aksaray, Turkey
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Corresponding author:
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Address: Department of Chemistry, Faculty of Science and Letters, Aksaray University,
Aksaray, Turkey, 68100
Tel/fax: +903822882142, E-mail address: talatbaran_66@hotmail.com
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Graphical abstract
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Highlights
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Green catalysis system consisting of biopolymer based heterogeneous Pd(II)-catalyst
High TONs (6600) and TOFs (82500) recorded for green chitosan/cellulose-Pd(II) catalyst
This green catalyst exhibited a better performance in Suzuki coupling reactions
Abstract
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In green catalyst systems, both the catalyst and the technique should be environmentally safe.
In this study we designed a green palladium(II) catalyst for microwave-assisted Suzuki C-C
coupling reactions. The catalyst support was produced from biopolymers; chitosan and
cellulose. The catalytic activity of the catalyst was tested on 16 substrates in solvent-free
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media and compared with those of commercial palladium salts. Reusability tests were done.
The catalyst was also used in conventional reflux-heating system to demonstrate the
efficiency of microwave heating method. We recorded high activity, selectivity and excellent
TONs (6600) and TOFs (82500) just using a small catalyst loading (1.5 x 10-3 mol %) in short
reaction time (5 min). The catalyst exhibited a long lifetime (9 runs). The findings indicated
that both green chitosan/cellulose-Pd(II) catalyst and the microwave heating are suitable for
synthesis of biaryl compounds by using Suzuki C-C coupling reactions.
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Keywords: Suzuki coupling; glyoxal; Schiff base; microwave
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1. Introduction
Biaryl compounds are widely used in pharmacology, medicine and cosmetics (Cotugno,
Casiello, Nacci, Mastrorilli, Dell'Anna & Monopoli, 2014). So far various metal catalyst
systems have been tested in synthesis of biaryls through C-C Suzuki coupling reactions
(Cotugno, Casiello, Nacci, Mastrorilli, Dell'Anna & Monopoli, 2014; Heck, 1982). Many
solvents and ligands used in the syntheses of catalysts and biaryl compounds are harmful to
the environment (Das, Sarmah, Tairai & Bora, 2011). To make any catalyst system greener,
(1) the catalyst itself should have a green nature and (2) the process should not require the use
of harmful solvents.
Polysaccharides like chitosan and cellulose have been acknowledged as versatile support
materials for catalysts systems (Baruah, Das, Hazarika & Konwar, 2015; Calo et al., 2004;
Makhubela, Jardine & Smith, 2011). These biopolymers are easily available and considered as
low-cost. These polymers have metal binding hydroxyl and amine groups. These reactive
functional groups also enable different modifications like Schiff base condensation reactions
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with aldehydes and ketones (Baran, Menteş & Arslan, 2015). Schiff bases are easily produced
by forming imine groups. The resulting electron donor imine groups (-C=N) can provide
coordination sites for metal ions (Riham R. Mohamed & Fekry, 2011). Therefore, a cross-
linking agent like dialdhyde glyoxal can react with amine groups of chitosan by forming
Schiff base. Additionally, cross-linking procedure enhances the mechanical stability and
metal ion affinity of chitosan (Sargın, Kaya, Arslan, Baran & Ceter, 2015). These properties
are usually desired in designing of catalyst support materials.
In green catalyst systems the method to be followed should be environmentally safe, feasible
and less labour-intensive. The microwave irradiation technique is considered as
environmentally friendly and therefore it has become an important tool in Suzuki coupling
reactions in both homogeneous and heterogeneous catalyst systems (S. Varma, 1999; Singh,
Kaval, Tomar, Van der Eycken & Parmar, 2008). In microwave-assisted Suzuki coupling
reactions, heterogeneous catalysts are particularly preferred; (1) the catalysts exhibit higher
selectivity, (2) they produce less by-products, (3) they are easily recovered from the reaction
media, (4) they have higher reusability and (5) they requires less labour compared to
homogeneous catalysts (Hajipour, Karami & Pirisedigh, 2011).
In this study we aimed (1) to prepare a green chitosan-cellulose based catalyst support for
palladium ion and (2) to synthesize biaryl compounds through an environmentally safe
method, i.e., microwave irradiation technique, in a solvent free medium. We incorporated
cellulose particles into chitosan matrix to enhance its mechanical strength and metal ion
interaction. In characterisation of the catalyst we employed XRD, XPS, TGA, FT-IR, SEM-
EDAX, and ICP-OES analyses. The catalytic efficiency of the catalyst was tested in the
reactions of phenyl boronic acid with 16 different aryl halides. Microwave-assisted synthesis
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of biaryl compounds was accomplished in a solvent free medium in 5 min by using small
amount of catalyst (0.015 % mol) at low temperatures (about 50°C). We compared the
activity of the catalyst with those of commercial palladium compounds under the same
reaction conditions. We also tested the performance of our method against conventional
heating reflux system (90°C, 24 h).
2. Materials and methods
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2.1. Materials
Cellulose (22197-250G-F), chitosan (448877-250G, deacetylation: 75-85%), glyoxal (40%),
CH3COOH, PdCl2,NaCl, phenyl boronic acid, aryl halides, NaOH, KOH, Na2CO3, K2CO3,
Cs2CO3, toluene and ethanol were obtained from Sigma-Aldrich and used as supplied without
any purification.
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2.2. Instrumentation
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X-ray diffractograms were recorded on a Rigaku D max 2000 system (at 40 kV, 30 mA and
2θ with a scan angle: 5−50°). X-ray photoelectron spectrometry (XPS) of the catalyst was
recorded using a SPECS EA 300 ESCA System (Al monochromatic 50.2 W 200.0 μ 45.0°
187.85 eV). Thermal stability analysis was performed on an EXSTAR S11 7300 (nitrogen
atmosphere; heating range: 30-650°C). FT-IR spectra were recorded on a Perkin Elmer
Spectrum 100 FTIR spectrophotometer. The surface morphology examination was conducted
on a QUANTA-FEG 250 ESEM. The analysis of palladium and chloride ions on the catalyst
was done using an EDAX-Metek. Palladium ion content of the catalyst was analysed by using
Perkin Elmer Optima 2100 DV Inductively Coupled Plasma (ICP) Optical Emission
Spectrometer (OES). Analysis of biaryl compounds were done on GC-MS Agilent GC-7890
A- MS 5975. In the catalytic activity tests a domestic microwave oven was used.
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2.3. Synthesis of green chitosan/cellulose-Pd(II) catalyst
2.3.1. Preparation of the support material
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We designed glyoxal cross-linked-chitosan-cellulose composite microbeads as a support
material for palladium catalyst. The microbeads were prepared as follows: Cellulose particles
(500 mg) were dispersed in 50 mL of chitosan solution (2%, w:w). The resulting mixture was
dropped into solution of ethanol (60 mL), water (40 mL) and NaOH (12 g) solution to form
gelatinous microbeads. The microbeads were recovered by filtration, washed thoroughly with
distilled water, incubated in cross-linking solution containing ethanol (100 mL) and glyoxal
solution (0.5 mL) for at 70°C for 12 h. The cross-linked yellowish-brown composite
microbeads were filtrated out, rinsed with ethanol and water, allowed to dry at room
temperature.
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2.3.2.Pd(II) ion loading onto the support
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The support material (0.2 g) was suspended in water at 50°C for 1 h. Water solution of 0. 35 g
of Na2PdCl4 salt (20 mL) was added into the suspension and stirred for 6 h. Pd loaded
microbeads were separated by filtration, rinsed with water to remove free sodium, palladium
and chloride ions. Finally, green chitosan/cellulose-Pd(II) catalyst was oven-dried at 50°C.
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2.4. Catalytic activity tests of green chitosan/cellulose-Pd(II) catalyst
2.4.1. Determination of optimum test conditions
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Prior to catalytic activity tests for green chitosan/cellulose-Pd(II) catalyst, the conditions
(catalyst loading, reaction time and base system) were optimized. These parameters have
profound impact on the reaction yield and conversion rate of any catalyst system. To be
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industrially feasible, a catalyst has to possess high turnover number (TON) and turnover
frequency (TOF). In a catalyst system, low catalyst loading and short reaction time produce
high TON and TOF values. We carried out the optimisation tests on a model experiment. In
the experiment, we synthesized 4-methoxy biphenyl from phenyl boronic acid and 4-
bromoanisol under microwave irradiation. Catalyst loading experiments were done in a range
of 5x10−3‒2x10−2 mol % catalyst. Optimum catalyst loading was determined as 1.5x10−3 mol
% and the optimum reaction time was 5 min (Supplementary data). Also, the base used in a
catalyst system also greatly affects the reaction yield by playing a role in transmethylation
step. We tested 5 different base systems on the model reaction. K2CO3 gave the highest yield;
99% (Supplementary data).
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2.4.2. General procedure for microwave assisted synthesis of biaryls
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Upon determination of the optimum conditions, we employed the following procedure for the
synthesis of biaryls. Phenyl boronic acid (1.75 mmol), aryl halide (1.12 mmol), K2CO3 (3.75
mmol) and the catalyst (1.5x10−3 mol %) were put into a Schlenk tube and exposed to 400 W
microwave irradiation for 5 min at 50°C in solvent-free medium under oxygen atmosphere.
Biaryl products were extracted into organic phase using toluene-water extraction mixture (2:1
v:v). The organic phase containing the product was separated with a separatory funnel. To
remove water remaining in the organic phase, a drying agent (MgSO4) was added into the
organic phase. The anhydrous organic phase was subsequently evaporated to eliminate
toluene. Finally, the biaryl compounds were dissolved in acetone and shipped to GS-MS
analysis.
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2.4.3. Synthesis of 4-methoxy-biphenyl with classical reflux-heating system
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To reveal the efficiency of the microwave irradiation, we synthesized 4-methoxy-biphenyl by
following the conventional reflux-heating system. Phenyl boronic acid (1.75 mmol), 4-
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bromoanisol (1.12 mmol), K2CO3 (3.75 mmol) and the catalyst (1.5x10−3 mol %) were added
into 6 mL of toluene in a Schlenk tube. The reaction mixture was stirred under reflux at 90°C
for 24 h under nitrogen atmosphere. Separation and analysis of 4-methoxy-biphenyl was
carried out by following the same procedure described in section of 2.4.2.
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2.4.4. Reusability test procedure for green chitosan/cellulose-Pd(II) catalyst
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Reusability tests were carried out on the model reaction under optimum conditions. At the end
of each run, green chitosan/cellulose-Pd(II) catalyst was recovered by filtration and rinsed
with hot water and ethanol to reactivate. The same reactivated catalyst was used in succeeding
experiments for 9 runs.
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3. Results and discussions
3.1. Characterisation of green chitosan/cellulose-Pd(II) catalyst
3.1.1. XRD and XPS analyses
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Chitosan has high crystalline structure due to its intra- and inter-molecular hydrogen bonds
(Wang, Du, Fan, Liu & Hu, 2005). Through modifications like Schiff base formation, cross-
linking or metal ion coordination to the groups of ‒NH2 and/or ‒OH, these strong hydrogen
bonds are lost by lowering the crystallinity of chitosan polymer (Antony, Theodore David,
Saravanan, Karuppasamy & Balakumar, 2013). As presented in Fig. 1, the crystallinity of
green chitosan/cellulose support was lower than that of pristine chitosan. The decrease in the
crystallinity index can be explained by the disruption of hydrogen bonding of chitosan in
cross-linking procedure. A number of free amino groups of chitosan were involved in the
formation of imine groups with ketone groups of glyoxal by dropping the crystallinity index
value. Also, green chitosan/cellulose-Pd(II) catalyst exhibited lower crystallinity than its
support; this can be attributed to the palladium coordination (Anan, Hassan, Saad, Butler &
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Mostafa, 2011). We calculated the crystallinity indices of pristine chitosan, the support and
the catalyst using the following equation; 82, 57 and 36% respectively (Zhang, Xue, Xue, Gao
& Zhang, 2005):
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Crystalline index (%) = [(I110− Iam)/I110] × 100
(1)
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where I110 denotes the maximum intensity at ~ 20° and Iam is the intensity of amorphous
diffraction at 16°.
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……………………………………………..Figure 1…………………………………..…..
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To confirm the presence of palladium ion on green chitosan/cellulose-Pd(II) catalyst, XPS
spectroscopy analysis was performed (please refer to supplementary data). In the XPS
spectrum, the peaks corresponding to palladium and chloride ions were observed,
demonstrating coordination of palladium ions to the support.
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3.1.2. Thermogravimetric analysis
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Chitosan exhibits high thermal stability due to its crystalline structure (Antony, Manickam,
Saravanan, Karuppasamy & Balakumar, 2013). The maximum thermal degradation of
chitosan was reported to be 302°C (Baran, Menteş & Arslan, 2015). We recorded higher
maximum thermal degradation of cross linked-chitosan-cellulose composite microbeads as
370°C (Fig. 2). It appears that cellulose particles together with cross-linking modification
contributed to the thermal stability of chitosan. However, a lower degradation temperature
was observed for the catalyst; 212°C. This can be ascribed to the catalytic activity of
palladium ion on the decomposition of the polymeric backbone of the support (Ziegler-
Borowska, Chełminiak & Kaczmarek, 2015). Additionally, the thermal decomposition of the
catalyst gave higher residual content than its support (Fig. 4), suggesting the presence of
palladium oxide in the residue (Demetgul, 2012). In a catalyst system, the catalyst is expected
to show high thermal stability (Baran, Açıksöz & Menteş, 2015). Green chitosan/cellulose-
Pd(II) catalyst exhibited a relatively high maximum degradation temperature; demonstrating
that the catalyst is suitable for catalyst systems running at elevated temperature.
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……………………………………………..Figure 2…………………………………..…..
3.1.3. FT-IR spectra analysis
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Through condensation of ‒NH2 groups of chitosan with ketone groups of glyoxal imine
groups (C=N) are formed. The stretching of imine groups of the support was observed at 1636
cm−1 (Fig. 3). We did not observe any band stretching that can be attributed to carbonyl
groups of glyoxal (at around 1750 cm−1), confirming the involvement of all ketone groups of
glyoxal in the formation of Schiff base. In case of the catalyst, the vibration of bands for
imine groups appeared at lower wavenumber; 1621 cm−1. This can be ascribed to the
coordination of palladium ions to imine groups of the ligand. These findings are in the line
with literature reports (Demetgul, 2012).
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……………………………………………..Figure 3…………………………………..…..
3.1.4. SEM-EDAX and ICP-OES studies
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SEM images clearly showed that both the support and the catalyst microbeads had nearly
spherical formation (Fig. 4a,c). At higher magnifications (Fig. 4b,d), the support displayed a
fibrous and porous surface. Following the metal ion coordination, the catalyst surface was
changed into rougher morphology and fibrous structure disappeared. EDAX spectrum of the
catalyst also confirmed the presence of palladium ions on the surface (Fig. 5). We determined
palladium ion content of the catalyst using EDAX and ICP-OES and we observed closer
values; 20.75% and 21.25%.
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3.2. Application of green chitosan/cellulose-Pd(II) catalyst
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Catalysts which are capable of displaying activity on different substrates are considered to be
suitable for industrial processes. Catalysts exhibiting high TONs and TOFs are classified as
high turnover catalyst (HTC) (Cui, Zhang & Li, 2005; Farina, 2004), and such catalysts are
preferred in industrial applications rather than laboratory scale studies. Particularly biaryl
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compounds which are used in pharmacology, cosmetics and medicine are synthesized via
palladium-catalysed Suzuki cross-coupling reactions (Farina, 2004). We tested the catalytic
activity of green chitosan/cellulose-Pd(II) catalyst on 16 substrates in C‒C coupling reactions
under microwave irradiation without using any solvents. The catalyst displayed a high
catalytic performance in synthesis of biaryls (Table 1) in very short reaction time. Using low
catalyst loading, we achieved high TON and TOF values; 6600 and 82500. Also, chemical
identification of the biaryls was carried out by using GC-MS and no by-products were
detected in the spectra (please refer to supplementary data), indicating the selectivity of the
catalyst. These findings demonstrated that green chitosan/cellulose-Pd(II) catalyst has high
activity on various substrates and it has good selectivity in C‒C coupling reactions by
producing high TONs and TOFs. Additionally, the green microwave heating technique proved
to be effective in the reactions. These properties are usually desired in application of a catalyst
system in broad scale-up applications.
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Table 1. Effect of green chitosan/cellulose-Pd(II) catalyst on Suzuki C‒C reactions
Entry
Boronic acid
Substrate
Product
H3CO
Yield (%)
TON
5933
TOF
74162
1
89
2
78
5200
65000
3
4
99
85
6600
5667
82500
70837
5
73
4867
60837
6
7
8
84
55
56
5600
3667
3733
70000
45837
46662
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73
76
4867
5067
60837
63337
10
11
12
22
38
1467
2533
18337
31662
13
14
63
25
4200
1667
52500
20837
OCH3
CI
15
16
10
9
667
600
8350
7500
NO2
CI
17a
28
1867
23337
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18b
19c
20*
31
47
46
2067
3133
3067
25837
39162
38337
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Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K2CO3, 0.015 % green chitosan/cellulose-Pd(II) catalyst, 50 °C, 5 min. under
MW
a: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K2CO3, 0.015 % PdCl2, 50 °C, 5 min. under MW
b: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K2CO3, 0.015 % PdCl2(CH3CN)2, 50 °C, 5 min. under MW
c: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol K2CO3, 0.015 % Na2PdCl4 50 °C, 5 min. under MW
*: Conventional heating method: 1.12 mmol aryl halide, 1.87 mmol phenyl boronic acid, 3.75 mmol K2CO3, 0.015 % mol catalyst, 90 °C, 24 h, 5 mL toluene
TON: (turnover number, yield of product/ per mol of Pd).
TOF: (turn over frequency, TON/time of reaction)
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3.3. Comparison with commercial palladium catalysts
The catalytic performance of green chitosan/cellulose-Pd(II) catalyst was compared to those
of commercially available palladium salts (PdCl2, PdCl2(CH3CN)2 and Na2PdCl4) in the
synthesis of 4-methoxy biphenyl under the optimum conditions. Among the commercial salts,
Na2PdCl4 gave the highest product yield; 47% (please, refer to entries 3, 17, 18 and 19 in
Table 1). However, the reaction catalyzed by green chitosan/cellulose-Pd(II) catalyst
produced a much higher product yield; 99%. This high conversion rate suggests that
chitosan/cellulose microbeads were suitable support for palladium(II) catalyst.
3.4. Comparison of microwave radiation with heating-reflux system in coupling reaction
To reveal the efficiency of the green microwave irradiation method we followed in this study,
we also synthesized 4-methoxy biphenyl using green chitosan/cellulose-Pd(II) catalyst in the
heating-reflux system (24 h, 90°C in toluene). A low product yield was recorded for the
system; 46% (entry 20 in Table 1). On the other hand, we recorded a higher reaction yield
(99%) for the green method (entry 3 in Table 1) (5 min, 50°C in solvent-free media). These
findings indicated that microwave irradiation method can be used in synthesis of biaryl
compounds.
3.5. Reusability of green chitosan/cellulose-Pd(II) catalyst
Catalysts with long lifetime are highly desired in catalyst systems employed in industrial
operations. Reusability tests of green chitosan/cellulose-Pd(II) catalyst were conducted on the
model reaction of 4-bromoanisol with phenyl boronic acid under optimum conditions. The
catalyst worked efficiently up to 9 cycles. We recorded still high TONs (2867) and TOFs
(35837) even after 9 runs (Table 2). Considering its long lifetime and high TON and TOF
values, green chitosan/cellulose-Pd(II) catalyst can be used in industrial applications.
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Table 2. Reusability of green chitosan/cellulose-Pd(II) catalyst
*Suzuki C-C reaction
The number of cycle
Yield (%)
TON
6600
6133
6067
5333
4600
3867
3867
3533
2867
TOF
82500
76662
75837
66662
57500
48337
48337
44162
35837
1 st
2 nd
3 rd
4 th
5 th
6 th
7 th
8 th
9 th
99
92
91
80
69
58
58
53
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Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid, 3.75 mmol base, 0.015 mol % green
chitosan/cellulose-Pd(II) catalyst, 50 °C, 5 min under MW
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4. Conclusions
This study indicated that a biopolymer based catalyst can be designed and used effectively in
the synthesis of biaryls via Suzuki C-C coupling reactions by microwave heating. Chitosan-
cellulose composite microbeads proved to be an excellent supramolecular ligand for
palladium ion. Microwave irradiation showed a better performance over conventional heating-
reflux method, and the products were successfully obtained in solvent-free media at low
temperatures. Further, in this study green chitosan/cellulose-Pd(II) catalyst exhibited excellent
properties such as high selectivity and activity, high TONs and TOFs, long lifetime, thermal
stability and reusability. This environmentally safe catalyst can be transferred into up-scale
industrial operations.
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Figure Captions
Fig.1 XRD diagram of a) chitosan b) cross linked-chitosan-cellulose composite micro beads,
c) green chitosan/cellulose-Pd(II) catalyst
Fig.2 TG/DTG spectra of a) cross linked-chitosan-cellulose composite micro beads, b) green
chitosan/cellulose-Pd(II) catalyst
Fig. 3 FT-IR spectra of a) cross linked-chitosan-cellulose composite micro beads, b) green
chitosan/cellulose-Pd(II) catalyst
Fig. 4 SEM images of cross linked-chitosan-cellulose composite micro beads (a,b), green
chitosan/cellulose-Pd(II) catalyst (c,d)
Fig. 5 EDAX spectrum of green chitosan/cellulose-Pd(II) catalyst
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