An environmental catalyst derived from biological waste materials for green synthesis of biaryls via Suzuki coupling reactions

Article abstract of DOI:10.1016/j.molcata.2016.04.025

Synthesis of bio-macromolecular supported catalysts has gained much recent attention due to their greener nature. Among the biopolymers, chitosan is widely used as a support material due to its high affinity for metal ions. In this study, chitosan-Ulva sp. (green alga) composite microbeads were prepared as a support material for palladium catalyst. Ulva sp. particles were incorporated into chitosan matrix to enhance the interaction with palladium ions. The catalytic performance of chitosan-Ulva supported Pd(II) catalyst was investigated in the synthesis of biaryls via the Suzuki coupling reaction. All the experiments were conducted without using any solvent under the microwave irradiation, which is also considered as a green technique. This green catalyst exhibited high selectivity and efficiency in the reactions of phenyl boronic acid with different aryl halides in only 4 min at low temperature (50 °C). Excellent TON and TOF values were achieved for the catalyst; 4950 and 75000. In addition, the catalyst did not lose its activity even after 8 cycles. It showed high thermal stability (216.8 °C) and durability in presence of oxygen. This green catalyst has a potential to be used in pharmacology, medicine and cosmetics.

Full text of DOI:10.1016/j.molcata.2016.04.025

Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
An environmental catalyst derived from biological waste materials for
green synthesis of biaryls via Suzuki coupling reactions
Talat Barana,∗, Idris Sarginb, Murat Kayab, Ayfer Mentes¸ a
a Department of Chemistry, Faculty of Science and Letters, Aksaray University, 68100 Aksaray, Turkey
b Department of Biotechnology and Molecular Biology, Faculty of Science and Letters, Aksaray University, 68100 Aksaray, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of bio-macromolecular supported catalysts has gained much recent attention due to their
greener nature. Among the biopolymers, chitosan is widely used as a support material due to its high
affinity for metal ions. In this study, chitosan-Ulva sp. (green alga) composite microbeads were prepared
as a support material for palladium catalyst. Ulva sp. particles were incorporated into chitosan matrix to
enhance the interaction with palladium ions. The catalytic performance of chitosan-Ulva supported Pd(II)
catalyst was investigated in the synthesis of biaryls via the Suzuki coupling reaction. All the experiments
were conducted without using any solvent under the microwave irradiation, which is also considered as
a green technique. This green catalyst exhibited high selectivity and efficiency in the reactions of phenyl
boronic acid with different aryl halides in only 4 min at low temperature (50 ◦C). Excellent TON and TOF
values were achieved for the catalyst; 4950 and 75000. In addition, the catalyst did not lose its activity
even after 8 cycles. It showed high thermal stability (216.8 ◦C) and durability in presence of oxygen. This
green catalyst has a potential to be used in pharmacology, medicine and cosmetics.
Received 18 March 2016
Received in revised form 15 April 2016
Accepted 21 April 2016
Available online 22 April 2016
Keywords:
Chitosan
Algae
Eco-friendly
Palladium
Microwave
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Chitosan is widely considered as an eco-friendly biopolymer due
to its biodegradable, non-toxic and renewable nature. It is cheaply
Biaryl compounds have been utilized in various areas such as
pharmacology, medicine, cosmetics due to their unique biolog-
ical properties [1,2]. Synthesis of biaryl compounds has gained
much recent attention and the palladium-catalyzed Suzuki cou-
pling reaction has been efficiently used as a general method in the
production of biaryls [2–4]. However, the Suzuki coupling reac-
tion has serious disadvantages; (1) high reaction temperatures and
long reaction times, (2) non-environmental nature because of the
use of toxic organic solvents (3) labour-intensive nature and (4)
high operational costs [5]. To overcome these problems, microwave
irradiation technique has emerged as an efficient method in the
last decade [6]. This technique does not require harsh conditions
and enables high product yields in very short reaction times. The
most striking aspect of this technique is the elimination of organic
solvents by making it a more green-friendly method in the syn-
thesis of biaryl compounds. Due to their cleaner, biodegradable
and renewable properties, biopolymers have been widely used as
catalyst support material in Suzuki coupling reactions [7,8].
and abundantly produced by deacetylation of chitinous waste of
seafood industry processing shrimp, krill, crab and crayfish [9]. This
versatile biopolymer can be easily functionalised via pendant reac-
tive hydroxyl and amine groups on its backbone. Additionally, these
groups facilitate the binding of metal ions and therefore chitosan is
acknowledged as a supramolecular bio-ligand for the Suzuki cou-
pling reaction. To further enhance its metal binding capacity; (1)
chitosan is generally cross-linked with agents like glyoxal, glu-
taraldehyde and epichlorohydrin and also (2) chitosan composites
are produced by incorporation of bio-based materials (e.g., cellu-
lose, oil palm ash, cotton and alginate) exhibiting affinity for metal
ions [10].
Ulva sp. is a ubiquitous fast growing macroalga [11]. Due to func-
tional moieties (e.g., thiol, hydroxyl, carboxyl, amino and imidazole
groups) on the cell surface, biomass from Ulva sp. has been applied
in uptake of metal ions [12]. Especially, a recent study demon-
strated that it has high affinity for platinum group elements (i.e.,
rhodium, palladium and platinum) [13].
Palladium ions can coordinate with imine groups of glutaralde-
hyde cross-linked chitosan Schiff base. In coordination of palladium
ions with imine groups nitrogen atoms can function as donor atoms.
As mentioned, chitosan-Ulva sp. composite can bind palladium ions
efficiently and therefore they can exhibit desired catalytic activity
∗
Corresponding author.
E-mail address: talatbaran 66@hotmail.com (T. Baran).
http://dx.doi.org/10.1016/j.molcata.2016.04.025
1381-1169/© 2016 Elsevier B.V. All rights reserved.

T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
217
Table 1
in C-C coupling reactions. This study aimed (1) to synthesize a new
Optimization of base for Suzuki reaction.
green palladium catalyst for the Suzuki coupling reaction, (2) to
characterise the chitosan-Ulva supported Pd(II) catalyst via ana-
lytical tools (e.g., FT-IR, TGA, SEM-EDAX, XRD and ICP-OES), (3) to
investigate the performance of the catalyst in synthesis of biaryls
under mild conditions by microwave irradiation technique with-
out using any solvents and (4) to determine the reusability of the
synthesized green catalyst.
Base
Reaction yield (%)
NaOH
KOH
K2CO3
33
67
99
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid,
3.75 mmol base, 0.02 mol% chitosan-Ulva supported Pd(II) catalyst, 50 ◦C, 4 min
under microwave irradiation.
2. Experimental
2.1. Materials
Medium molecular weight Chitosan (448877-250G, deacetyla-
tion: 75–85%), glutaraldehyde (40%), CH3COOH, Na2PdCl4, PdCl2,
phenyl boronic acid, aryl halides, NaOH, KOH, K2CO3, MgSO4,
toluene and methanol were purchased from Sigma-Aldrich and
used as supplied. In experiments double distilled water was used.
2.2. Preparation of the catalyst support
In preparation of chitosan-Ulva beads, commercial medium
molecular weight chitosan (Sigma–Aldrich 448877-250G) was
used. Ulva biomass was collected in the banks of Uluırmak River
(Aksaray, Turkey, August 2015). After extensively washing with
water, Ulva samples were allowed to dry at room temperature.
Dried Ulva samples were ground and sieved with a 100 ␮ sieve and
then treated in NaOCl (2%) solution for 10 min at 40 ◦C.
Fig. 1. The effect of catalyst loading on Suzuki coupling reaction.
Ulva powder (0.5 g) was added into 75 mL of chitosan solu-
tion (2% w:w, dissolved in 2% acetic acid solution) and stirred
for 3 h. Chitosan-Ulva mixture was then dropped into alka-
line water-methanol coagulation solution (water:methanol:NaOH;
100 mL:150 mL: 30 g) [14]. The beads were incubated in the coag-
ulation solution for 16 h. Subsequently, the beads were removed
by filtration and washed with water. In the cross-linking proce-
dure, the beads were transferred into crosslinking solution (45 mL
methanol and 0.4 mL of glutaraldehyde solution of 25%) and heated
under reflux in a fume hood to arrest any glutaraldehyde vapour.
Finally, glutaraldehyde cross-linked beads were rinsed with water
and dried at room temperature.
Fig. 2. The effect of reaction time on Suzuki coupling reaction.
2.3. Synthesis of chitosan-Ulva supported Pd(II) catalyst
Chitosan-Ulva composite bead (0.2 g) and Na2PdCl4 (0.35 g)
were added into 20 mL of water and stirred for 6 h at room temper-
ature. Then, yellow-brownish beads were recovered by filtration
and washed with water to remove any uncomplexed ions. Finally,
the chitosan-Ulva supported Pd(II) catalyst was oven-dried at 50 ◦C.
3. Results and discussions
3.1. Catalytic performance of chitosan-Ulva supported Pd(II)
catalyst
2.4. Instrumentation
3.1.1. Determination of optimum conditions
To determine the optimum experimental parameters, the C-C
coupling reaction of 4-bromoanisole with phenyl boronic acid was
carried out on a model reaction. All the parameters i.e., base system
(K2CO3, NaOH and KOH), catalyst amount (0.005–0.025% mol), and
reaction time (1–5 min) were studied under microwave irradiation.
In C-C coupling reactions, base system plays a key role during the
transmetallation. Therefore, as presented in Table 1 three different
base systems were tested and K2CO3 gave the highest yield.
Achieving high product yield with minimum amount of cata-
lyst is desired for catalyst systems. Fig. 1 displays the relationship
between the amount of the catalyst and the rate of product conver-
sion.
FT-IR spectra of chitosan-Ulva composite bead and chitosan-
Ulva supported Pd(II) catalyst were recorded on a Perkin Elmer
Spectrum 100 FTIR spectrophotometer. Thermal stability of the
beads was investigated on an EXSTAR S11 7300 under nitrogen
atmosphere in a range of 30–650 ◦C. Examination of the surface
morphology of the beads was carried out on QUANTA-FEG 250
ESEM. The presence of Pd ions on the catalyst was detected using
EDAX-Metek. The XRD patterns of the beads were obtained on a
Rigaku D max 2000 system at 40 kV, 30 mA and 2␪ with a scan angle
from 5◦ to 50◦ using Cuk␣1 radiation (␭ = 1.54059 Å). Palladium
ion content of the catalyst was determined by using PerkinElmer
Optima 2100 DV Inductively Coupled Plasma (ICP) Optical Emission
Spectrometer (OES).
The test experiments with the model reaction demonstrated
that optimum reaction time was 4 min (Fig. 2).

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T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
Fig. 3. FT-IR spectra of (a) chitosan, (b) chitosan-Ulva composite bead and (c) chitosan-Ulva supported Pd(II) catalyst.
Fig. 4. TG/DTG diagrams of (a) chitosan, (b) chitosan-Ulva composite bead and (c) chitosan-Ulva supported Pd(II) catalyst.
3.1.2. General procedure for the Suzuki coupling reaction
3.1.3. Reusability test
Phenyl boronic acid (1.75 mmol), aryl halide (1.12 mmol), K2CO3
(3.75 mmol) and the catalyst (0.02%) were mixed without using
any solvent and subsequently the mixture was exposed to the
microwave irradiation (at 400 W) at 50 ◦C for 4 min. After the
reaction was terminated, the organic phase was extracted from
the mixture into toluene-water (4:2 v:v). To ensure the complete
removal of water from the organic phase, MgSO4 was added into
the organic phase. The chemical composition of the synthesized
biaryls in the organic phase were analysed by using GC–MS Agilent
GC-7890 A- MS 5975.
The reusability tests were carried on the model reaction. At
the end of each run, the catalyst was recovered from the reaction
medium and then washed with water and hot methanol to reac-
tivate the catalyst. The reactivated catalyst was used in the model
coupling reaction for 8 times.
3.2. Characterisation of chitosan-Ulva supported Pd(II) catalyst
3.2.1. Chemical composition
FT-IR spectra of chitosan, chitosan-Ulva composite bead and
chitosan-Ulva supported Pd(II) catalyst are displayed in Fig. 3. As

T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
219
Fig. 5. SEM images of chitosan-Ulva composite bead (a–c) and chitosan-Ulva supported Pd(II) catalyst (d-f).
Fig. 6. EDAX spectrum of chitosan-Ulva supported Pd(II) catalyst.
seen in the spectrum of chitosan-Ulva composite bead, 1730 cm−1
carbonyl groups of glutaraldehyde and 1590 cm−1 (NH2 bending of
chitosan) did not appear in the spectrum (Fig. 3b). On the other
hand, a band was observed at 1647 cm−1 and this band can be
attributed to the stretching of imine groups of cross-linked chitosan
bead [15]. The observation of Schiff base formation in the structure
demonstrated that cross-linking of chitosan was achieved.
present study, DTGmax of chitosan-Ulva composite bead was found
to be 270.7 ◦C. In addition, the coordination of Pd ions reduced the
DTGmax value; 216.8 ◦C. Decrease in DTGmax value of chitosan-Ulva
supported Pd(II) catalyst can be ascribed to lower crystalline struc-
ture of chitosan following the coordination of Pd ions. Also, metal
ions on chitosan can contribute to degradation of polymer chains
as previously reported [20].
In the spectrum of chitosan-Ulva supported Pd(II) catalyst, imine
groups stretching was shifted to 1630 cm−1. This shift (17 cm−1
)
can account for coordination of Pd ions to nitrogen atoms of imine
groups [16,17].
3.2.3. Surface morphology
Chitosan polymer usually has a smoother surface, but after a
chemical modification such as cross-linking, Schiff base formation,
carboxymethylation and complexation with metal ions, its surface
exhibits distinct characteristics [21]. The surface becomes rough,
layered or even some cracks can be observed. As seen in Fig. 5,
the surface of chitosan-Ulva composite beads became rougher and
layered during the cross-linking procedure, demonstrating Schiff
base formation. On the other hand, the surface of chitosan-Ulva
supported Pd(II) catalyst exhibited a relatively smoother surface.
The roughness and the piles of stacks were cleared off after the
3.2.2. Thermal stability
Thermograms of chitosan, chitosan-Ulva composite bead and
chitosan-Ulva supported Pd(II) catalyst was recorded (Fig. 4). Due
to the strong hydrogen bonding of its strands, chitosan exhibits
high thermal stability [16,18]. The maximum thermal degradation
of chitosan (DTGmax) was recorded at 302 ◦C. Chemical modifica-
tion of chitosan by cross-linking via NH2 groups disrupts hydrogen
bonding and this lowers the thermal stability of chitosan [19]. In the

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T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
Table 2
coordination of Pd(II) ions with the ligand. These observations are
in line with existing literature reports [22,23].
Effect of chitosan-Ulva supported Pd(II) catalyst on Suzuki C-C reactions.
X
Y
Reaction yield (%)
TON
TOF
3.2.4. EDAX spectrum
1
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
Br
Br
I
I
I
I
CI
CI
CI
2-OCH3
3-OCH3
4-OCH3
4-CH3
3-NH2
3-NO2
4-NO2
2-CH3
74
87
99
52
73
86
88
46
47
88
72
68
57
15
3700
4350
4950
2600
3650
4300
4400
2300
2350
4400
3600
3400
2850
750
56061
65909
75000
39394
55303
65152
66667
34848
35607
66667
54545
51515
43182
11364
To validate the coordination of Pd ions with the binding groups
on the surface of chitosan-Ulva composite beads, EDAX spectrum
of chitosan-Ulva supported Pd(II) catalyst was recorded and pre-
sented in Fig. 6. The peaks of palladium and chloride ions in the
spectrum demonstrated the formation of Pd(II) catalyst. The ele-
mental composition (%) of the catalyst was recorded from the
EDAX data as follows; C: 30.92, N: 9.91, O: 29.6, Cl: 9.32 and Pd:
20.25%. Palladium ion content of the catalyst was also determined
by ICP-OES as 19.36%. This high palladium content was desirable
for the catalyst. In our previous study we recorded palladium con-
tent of O-carboxymethyl chitosan Schiff bases-supported catalysts
([OCMCS-3a]·2H2O and [OCMCS-4a]·2H2O) as 13.02% and 15.63%
[8]. It appeared that chitosan-Ulva composite served as a better
ligand for palladium ions.
9
4-CH3
10
11
12
13
14
4-OCH3
3-NO2
4-OCH3
3-NO2
4-CH3
Reaction conditions: 1.12 mmol aryl halide, 1.87 mmol phenyl boronic acid,
3.75 mmol K2CO3, 0.02 mol% chitosan-Ulva supported Pd(II) catalyst, 50 ◦C, 4 min
under microwave irradiation.
TON: (turnover number, yield of product/per mol of Pd).
TOF: (turn over frequency, TON/time of reaction)
3.2.5. Crystallinity
Previous studies have established that chitosan polymer
exhibits high crystallinity due to inter-and intra-molecular hydro-
gen bonds [24]. Deformation of these strong hydrogen bonds via
cross-linking, Schiff base formation or metal coordination results
in a decrease in the crystallinity index of chitosan [21,25]. Analysis
of XRD patterns of chitosan-Ulva composite bead revealed that the
beads had lower crystalline structure than chitosan (Fig. 7). The
following equation gave the crystallinity indices of chitosan-Ulva
composite bead and the catalyst; 62% and 6%, respectively. Also, in
our previous study we calculated the crystallinity index of chitosan
as 82% [16].
Table 3
Effect of different Pd catalysts on C-C coupling reaction.
Catalyst
Reaction yield (%)
PdCl2
Na2PdCl4
39
52
99
Chitosan-Ulva supported Pd(II) catalyst
Reaction conditions: 1.12 mmol 4-bromoanisole, 1.87 mmol phenyl boronic acid,
3.75 mmol base, 0.02 mol% catalyst, 50 ◦C, 4 min under microwave irradiation.
are generally attributed to the ionic radii and bond dissociation
energies of halides [1,8]. In addition, the positions of substitu-
tion groups on phenyl rings, i.e., para-, meta- and ortho-, greatly
affected the reaction yields in all the C-C coupling reactions with
all aryl halides. The reaction yields followed the order of para-
> meta- >ortho-substituent. As presented in Table 2, in the presence
of electron withdrawing groups, the reactions gave higher yields
than in the presence of electron donating groups. High TON and
TOF values are important properties for the catalyst systems for
industrial applications [28]. In this study excellent TON (4950) and
TOF (as high as 75000) were achieved for chitosan-Ulva supported
Pd(II) catalyst (Table 2, entry 3).
Crystallineindex(%) = [(I110 − Iam)/I110] × 100
(1)
where I110 denotes the maximum intensity at ∼20◦ and Iam is the
intensity of amorphous diffraction at 16◦.
This lower crystallinity likely stemmed from the cross-linking
treatment in which amino groups on chitosan reacted with alde-
hyde ends of the cross-linking agent by forming imine groups.
Chitosan-Ulva supported Pd(II) catalyst showed more amorphous
nature than chitosan and chitosan-Ulva composite bead. As
previously reported, amorphous nature of chitosan composites
facilitates diffusion of ions or molecules into chitosan matrices [26].
As noted in previous Section 3.2.4, high palladium content of the
catalyst could be attributed to the amorphous nature of chitosan-
Ulva composite beads.
3.3.1. Comparison of the performance of the catalyst with
commercial Pd catalysts
To determine the feasibility of chitosan-Ulva supported Pd(II)
catalyst for C-C coupling reactions, experiments with commer-
cially available palladium salts (PdCl2 and Na2PdCl4) were also
carried out under optimized conditions. The catalytic productiv-
ity of chitosan-Ulva supported Pd(II) catalyst was compared to
those of PdCl2 and Na2PdCl4 in the model reaction. The results
were summarised in Table 3. Considering the product yields (4-
methoxybiphenyl), chitosan-Ulva supported Pd(II) catalyst showed
a far better catalytic performance than those of commercial Pd
catalysts PdCl2 and Na2PdCl4.
3.3. Catalytic performance of the catalyst in C-C coupling
reactions
Firstly, to prove the catalytic activity of chitosan-Ulva supported
Pd(II) catalyst, the control experiments were conducted with
catalyst-free model reactions under optimum conditions (Please,
refer to the Section 3.1.1). No product (4-methoxy biphenyl) for-
mation was observed in the catalyst-free media. Secondly, the
catalytic performance of chitosan-Ulva supported Pd(II) beads was
tested in 14 different C-C coupling reactions. In these experiments
aryl bromides, aryl iodides and aryl chlorides were reacted with
phenyl boronic acid. GC–MS spectra of the synthesized biaryls
were recorded (Please, refer to Supplementary data Fig. S1–S16).
The corresponding conversion yields, turnover number (TON) and
turnover number frequency (TOF) values are listed in Table 2.
Noticeably higher reaction productivity was observed for aryl
bromides. Aryl iodides also produced higher conversion yields com-
pared to aryl chlorides. Due to their poor activity, the coupling
reactions with aryl chlorides gave the lowest product yields [27].
These differences observed in the reaction yields of biaryl halides
3.3.2. Reusability of chitosan-Ulva supported Pd(II) catalyst
Used chitosan-Ulva supported Pd(II) catalyst was reactivated
according to the procedure mentioned in Section 3.1.3. Reusability
of the Pd(II) catalyst was investigated on the specified model
reaction under optimised reaction conditions. The catalyst showed
a better performance up to 8 runs, and then a slight decrease was
observed in its activity (Fig. 8). In earlier study, Pd nano-sized
particles supported-chitosan and 6-deoxy-6-amino chitosan cat-
alysts retained their activities after fifth run [1]. In another study

T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
221
Fig. 7. XRD patterns of (a) chitosan-Ulva composite bead and (b) chitosan-Ulva supported Pd(II) catalyst.
Fig. 8. Reusability of chitosan-Ulva supported Pd(II) catalyst.
chitosan-supported Pd-nanoparticles in ionic liquids were used up
to six runs [2].
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In this study a green palladium(II) biocomposite catalyst was
synthesized by using two different biomaterials (chitosan and Ulva
sp.) to enhance palladium ion binding capacity of the support
material. The catalytic productivity of chitosan-Ulva supported
Pd(II) catalyst was tested in the synthesis of biaryl compounds
by employing microwave irradiation technique. With this eco-
friendly, cost-effective and simple method, in the test reactions we
did not use any solvent and achieved high selectivity and prod-
uct yields under mild conditions in only 4 min under microwave
irradiation. This study demonstrated that chitosan matrix that has
been reinforced with a biomaterial having affinity for palladium
ions makes a perfect support for the palladium catalyst. Further
studies should explore the catalytic activity of different chitosan-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.04.
025.
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