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An environmental catalyst derived from biological waste materials for green synthesis of biaryls via Suzuki coupling reactions
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 Baran a,∗ , Idris Sargin b , Murat Kaya b , Ayfer Mentes¸ a a b
Department of Chemistry, Faculty of Science and Letters, Aksaray University, 68100 Aksaray, Turkey 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
Article history: 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
a b s t r a c t 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. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Biaryl compounds have been utilized in various areas such as pharmacology, medicine, cosmetics due to their unique biological properties [1,2]. Synthesis of biaryl compounds has gained much recent attention and the palladium-catalyzed Suzuki coupling reaction has been efficiently used as a general method in the production of biaryls [2–4]. However, the Suzuki coupling reaction 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 synthesis 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].
∗ Corresponding author. E-mail address: talatbaran [email protected] (T. Baran). http://dx.doi.org/10.1016/j.molcata.2016.04.025 1381-1169/© 2016 Elsevier B.V. All rights reserved.
Chitosan is widely considered as an eco-friendly biopolymer due to its biodegradable, non-toxic and renewable nature. It is cheaply 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 reactive 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 coupling reaction. To further enhance its metal binding capacity; (1) chitosan is generally cross-linked with agents like glyoxal, glutaraldehyde and epichlorohydrin and also (2) chitosan composites are produced by incorporation of bio-based materials (e.g., cellulose, oil palm ash, cotton and alginate) exhibiting affinity for metal ions [10]. Ulva sp. is a ubiquitous fast growing macroalga [11]. Due to functional 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 demonstrated that it has high affinity for platinum group elements (i.e., rhodium, palladium and platinum) [13]. Palladium ions can coordinate with imine groups of glutaraldehyde 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
T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
in C-C coupling reactions. This study aimed (1) to synthesize a new green palladium catalyst for the Suzuki coupling reaction, (2) to characterise the chitosan-Ulva supported Pd(II) catalyst via analytical 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 without using any solvents and (4) to determine the reusability of the synthesized green catalyst.
217
Table 1 Optimization of base for Suzuki reaction. Base
Reaction yield (%)
NaOH KOH K2 CO3
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, deacetylation: 75–85%), glutaraldehyde (40%), CH3 COOH, Na2 PdCl4 , PdCl2 , phenyl boronic acid, aryl halides, NaOH, KOH, K2 CO3 , 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. Ulva powder (0.5 g) was added into 75 mL of chitosan solution (2% w:w, dissolved in 2% acetic acid solution) and stirred for 3 h. Chitosan-Ulva mixture was then dropped into alkaline water-methanol coagulation solution (water:methanol:NaOH; 100 mL:150 mL: 30 g) [14]. The beads were incubated in the coagulation solution for 16 h. Subsequently, the beads were removed by filtration and washed with water. In the cross-linking procedure, 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. 2.3. Synthesis of chitosan-Ulva supported Pd(II) catalyst Chitosan-Ulva composite bead (0.2 g) and Na2 PdCl4 (0.35 g) were added into 20 mL of water and stirred for 6 h at room temperature. 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. 2.4. Instrumentation FT-IR spectra of chitosan-Ulva composite bead and chitosanUlva 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).
Fig. 1. The effect of catalyst loading on Suzuki coupling reaction.
Fig. 2. The effect of reaction time on Suzuki coupling reaction.
3. Results and discussions 3.1. Catalytic performance of chitosan-Ulva supported Pd(II) catalyst 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 (K2 CO3 , 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 K2 CO3 gave the highest yield. Achieving high product yield with minimum amount of catalyst is desired for catalyst systems. Fig. 1 displays the relationship between the amount of the catalyst and the rate of product conversion. 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 Phenyl boronic acid (1.75 mmol), aryl halide (1.12 mmol), K2 CO3 (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.
3.1.3. Reusability test 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 reactivate 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. 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].
3.2.2. 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 modification of chitosan by cross-linking via NH2 groups disrupts hydrogen bonding and this lowers the thermal stability of chitosan [19]. In the
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 structure of chitosan following the coordination of Pd ions. Also, metal ions on chitosan can contribute to degradation of polymer chains as previously reported [20].
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
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coordination of Pd(II) ions with the ligand. These observations are in line with existing literature reports [22,23]. 3.2.4. EDAX spectrum 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 presented in Fig. 6. The peaks of palladium and chloride ions in the spectrum demonstrated the formation of Pd(II) catalyst. The elemental 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 content of O-carboxymethyl chitosan Schiff bases-supported catalysts ([OCMCS-3a]·2H2 O and [OCMCS-4a]·2H2 O) as 13.02% and 15.63% [8]. It appeared that chitosan-Ulva composite served as a better ligand for palladium ions. 3.2.5. Crystallinity Previous studies have established that chitosan polymer exhibits high crystallinity due to inter-and intra-molecular hydrogen 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]. 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 aldehyde 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 chitosanUlva composite beads. 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) formation 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 compared 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
Table 2 Effect of chitosan-Ulva supported Pd(II) catalyst on Suzuki C-C reactions.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
X
Y
Reaction yield (%)
TON
TOF
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 4-CH3 4-OCH3 3-NO2 4-OCH3 3-NO2 4-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
Reaction conditions: 1.12 mmol aryl halide, 1.87 mmol phenyl boronic acid, 3.75 mmol K2 CO3 , 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)
Table 3 Effect of different Pd catalysts on C-C coupling reaction. Catalyst
Reaction yield (%)
PdCl2 Na2 PdCl4 Chitosan-Ulva supported Pd(II) catalyst
39 52 99
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 substitution 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). 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 commercially available palladium salts (PdCl2 and Na2 PdCl4 ) were also carried out under optimized conditions. The catalytic productivity of chitosan-Ulva supported Pd(II) catalyst was compared to those of PdCl2 and Na2 PdCl4 in the model reaction. The results were summarised in Table 3. Considering the product yields (4methoxybiphenyl), chitosan-Ulva supported Pd(II) catalyst showed a far better catalytic performance than those of commercial Pd catalysts PdCl2 and Na2 PdCl4 . 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 catalysts retained their activities after fifth run [1]. In another study
T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
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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]. 4. Conclusions 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 ecofriendly, cost-effective and simple method, in the test reactions we did not use any solvent and achieved high selectivity and product 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 chitosanalgal-based metal catalysts for different catalyst systems such as Heck, Ullmann and catalysed-decomposition reactions. 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. References [1] B.C.E. Makhubela, A. Jardine, G.S. Smith, Appl. Catal. A-Gen. 393 (2011) 231–241.
[2] P. Cotugno, M. Casiello, A. Nacci, P. Mastrorilli, M.M. Dell’Anna, A. Monopoli, J. Organomet. Chem. 752 (2014) 1–5. [3] J. Sun, Y. Fu, G. He, X. Sun, X. Wang, Appl. Catal. B-Environ. 165 (2015) 661–667. [4] D. Zhang, T. Cheng, G. Liu, Appl. Catal. B-Environ. 174-175 (2015) 344–349. [5] P. Das, C. Sarmah, A. Tairai, U. Bora, Appl. Organomet. Chem. 25 (2011) 283–288. [6] B.K. Singh, N. Kaval, S. Tomar, E. Van der Eycken, V.S. Parmar, Org. Process Res. Dev. 12 (2008) 468–474. [7] V. Calo, A. Nacci, A. Monopoli, A. Fornaro, L. Sabbatini, N. Cioffi, N. Ditaranto, Organometallics 23 (2004) 5154–5158. [8] T. Baran, E. Ac¸ıksöz, A. Mentes¸, J. Mol. Catal. A: Chem. 407 (2015) 47–52. [9] N. Yan, X. Chen, Nature 524 (2015) 155–157. [10] W.W. Ngah, L. Teong, M. Hanafiah, Carbohyd. Polym. 83 (2011) 1446–1456. [11] M. Guidone, C.S. Thornber, Harmful Algae 24 (2013) 1–9. [12] K. Masakorala, A. Turner, M.T. Brown, Environ. Pollut. 156 (2008) 897–904. [13] A. Turner, M.S. Lewis, L. Shams, M.T. Brown, Marine Chem. 105 (2007) 271–280. [14] A. Pal, S. Pan, S. Saha, Chem. Eng. J. 217 (2013) 426–434. [15] M.M. Beppu, R.S. Vieira, C.G. Aimoli, C.C. Santana, J. Membr. Sci. 301 (2007) 126–130. [16] T. Baran, A. Mentes¸, H. Arslan, Int. J. Biol. Macromol. 72 (2015) 94–103. [17] C. Demetgul, M. Karakaplan, S. Serin, Synth. Monomers Polym. 11 (2008) 565–579. [18] F.A.A. Tirkistani, Degrad. Stab. 60 (1998) 67–70. [19] R. Antony, S.T. David, K. Saravanan, K. Karuppasamy, S. Balakumar, Spectrochim. Acta A. 103 (2013) 423–430. [20] M. Ziegler-Borowska, D. Chełminiak, H. Kaczmarek, J. Therm. Anal. Calorim. 119 (2015) 499–506. [21] R. Antony, S.T.D. Manickam, K. Saravanan, K. Karuppasamy, S. Balakumar, J. Mol. Struct. 1050 (2013) 53–60. [22] C. Demetgul, Carbohyd. Polym. 89 (2012) 354–361. [23] T. Baran, A. Mentes¸, Int. J. Biol. Macromol. 79 (2015) 542–554. [24] N.A. Anan, S.M. Hassan, E.M. Saad, I.S. Butler, S.I. Mostafa, Carbohyd. Res. 346 (2011) 775–793. [25] W. Li-xia, W. Zi-wei, W. Guo-song, L. Xiao-dong, R. Jian-guo, Polym. Adv. Technol. 21 (2010) 244–249. [26] E. Guibal, Sep. Purif. Technol. 38 (2004) 43–74. [27] A.F. Littke, C.Y. Dai, G.C. Fu, J. Am. Chem. Soc. 122 (2000) 4020–4028. [28] J.T. Singleton, Tetrahedron 59 (2003) 1837–1857.
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 Baran a,∗ , Idris Sargin b , Murat Kaya b , Ayfer Mentes¸ a a b
Department of Chemistry, Faculty of Science and Letters, Aksaray University, 68100 Aksaray, Turkey 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
Article history: 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
a b s t r a c t 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. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Biaryl compounds have been utilized in various areas such as pharmacology, medicine, cosmetics due to their unique biological properties [1,2]. Synthesis of biaryl compounds has gained much recent attention and the palladium-catalyzed Suzuki coupling reaction has been efficiently used as a general method in the production of biaryls [2–4]. However, the Suzuki coupling reaction 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 synthesis 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].
∗ Corresponding author. E-mail address: talatbaran [email protected] (T. Baran). http://dx.doi.org/10.1016/j.molcata.2016.04.025 1381-1169/© 2016 Elsevier B.V. All rights reserved.
Chitosan is widely considered as an eco-friendly biopolymer due to its biodegradable, non-toxic and renewable nature. It is cheaply 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 reactive 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 coupling reaction. To further enhance its metal binding capacity; (1) chitosan is generally cross-linked with agents like glyoxal, glutaraldehyde and epichlorohydrin and also (2) chitosan composites are produced by incorporation of bio-based materials (e.g., cellulose, oil palm ash, cotton and alginate) exhibiting affinity for metal ions [10]. Ulva sp. is a ubiquitous fast growing macroalga [11]. Due to functional 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 demonstrated that it has high affinity for platinum group elements (i.e., rhodium, palladium and platinum) [13]. Palladium ions can coordinate with imine groups of glutaraldehyde 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
T. Baran et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 216–221
in C-C coupling reactions. This study aimed (1) to synthesize a new green palladium catalyst for the Suzuki coupling reaction, (2) to characterise the chitosan-Ulva supported Pd(II) catalyst via analytical 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 without using any solvents and (4) to determine the reusability of the synthesized green catalyst.
217
Table 1 Optimization of base for Suzuki reaction. Base
Reaction yield (%)
NaOH KOH K2 CO3
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, deacetylation: 75–85%), glutaraldehyde (40%), CH3 COOH, Na2 PdCl4 , PdCl2 , phenyl boronic acid, aryl halides, NaOH, KOH, K2 CO3 , 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. Ulva powder (0.5 g) was added into 75 mL of chitosan solution (2% w:w, dissolved in 2% acetic acid solution) and stirred for 3 h. Chitosan-Ulva mixture was then dropped into alkaline water-methanol coagulation solution (water:methanol:NaOH; 100 mL:150 mL: 30 g) [14]. The beads were incubated in the coagulation solution for 16 h. Subsequently, the beads were removed by filtration and washed with water. In the cross-linking procedure, 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. 2.3. Synthesis of chitosan-Ulva supported Pd(II) catalyst Chitosan-Ulva composite bead (0.2 g) and Na2 PdCl4 (0.35 g) were added into 20 mL of water and stirred for 6 h at room temperature. 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. 2.4. Instrumentation FT-IR spectra of chitosan-Ulva composite bead and chitosanUlva 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).
Fig. 1. The effect of catalyst loading on Suzuki coupling reaction.
Fig. 2. The effect of reaction time on Suzuki coupling reaction.
3. Results and discussions 3.1. Catalytic performance of chitosan-Ulva supported Pd(II) catalyst 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 (K2 CO3 , 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 K2 CO3 gave the highest yield. Achieving high product yield with minimum amount of catalyst is desired for catalyst systems. Fig. 1 displays the relationship between the amount of the catalyst and the rate of product conversion. The test experiments with the model reaction demonstrated that optimum reaction time was 4 min (Fig. 2).
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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 Phenyl boronic acid (1.75 mmol), aryl halide (1.12 mmol), K2 CO3 (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.
3.1.3. Reusability test 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 reactivate 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
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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. 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].
3.2.2. 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 modification of chitosan by cross-linking via NH2 groups disrupts hydrogen bonding and this lowers the thermal stability of chitosan [19]. In the
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 structure of chitosan following the coordination of Pd ions. Also, metal ions on chitosan can contribute to degradation of polymer chains as previously reported [20].
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
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coordination of Pd(II) ions with the ligand. These observations are in line with existing literature reports [22,23]. 3.2.4. EDAX spectrum 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 presented in Fig. 6. The peaks of palladium and chloride ions in the spectrum demonstrated the formation of Pd(II) catalyst. The elemental 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 content of O-carboxymethyl chitosan Schiff bases-supported catalysts ([OCMCS-3a]·2H2 O and [OCMCS-4a]·2H2 O) as 13.02% and 15.63% [8]. It appeared that chitosan-Ulva composite served as a better ligand for palladium ions. 3.2.5. Crystallinity Previous studies have established that chitosan polymer exhibits high crystallinity due to inter-and intra-molecular hydrogen 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]. 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 aldehyde 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 chitosanUlva composite beads. 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) formation 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 compared 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
Table 2 Effect of chitosan-Ulva supported Pd(II) catalyst on Suzuki C-C reactions.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
X
Y
Reaction yield (%)
TON
TOF
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 4-CH3 4-OCH3 3-NO2 4-OCH3 3-NO2 4-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
Reaction conditions: 1.12 mmol aryl halide, 1.87 mmol phenyl boronic acid, 3.75 mmol K2 CO3 , 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)
Table 3 Effect of different Pd catalysts on C-C coupling reaction. Catalyst
Reaction yield (%)
PdCl2 Na2 PdCl4 Chitosan-Ulva supported Pd(II) catalyst
39 52 99
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 substitution 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). 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 commercially available palladium salts (PdCl2 and Na2 PdCl4 ) were also carried out under optimized conditions. The catalytic productivity of chitosan-Ulva supported Pd(II) catalyst was compared to those of PdCl2 and Na2 PdCl4 in the model reaction. The results were summarised in Table 3. Considering the product yields (4methoxybiphenyl), chitosan-Ulva supported Pd(II) catalyst showed a far better catalytic performance than those of commercial Pd catalysts PdCl2 and Na2 PdCl4 . 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 catalysts retained their activities after fifth run [1]. In another study
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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]. 4. Conclusions 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 ecofriendly, cost-effective and simple method, in the test reactions we did not use any solvent and achieved high selectivity and product 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 chitosanalgal-based metal catalysts for different catalyst systems such as Heck, Ullmann and catalysed-decomposition reactions. 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. References [1] B.C.E. Makhubela, A. Jardine, G.S. Smith, Appl. Catal. A-Gen. 393 (2011) 231–241.
[2] P. Cotugno, M. Casiello, A. Nacci, P. Mastrorilli, M.M. Dell’Anna, A. Monopoli, J. Organomet. Chem. 752 (2014) 1–5. [3] J. Sun, Y. Fu, G. He, X. Sun, X. Wang, Appl. Catal. B-Environ. 165 (2015) 661–667. [4] D. Zhang, T. Cheng, G. Liu, Appl. Catal. B-Environ. 174-175 (2015) 344–349. [5] P. Das, C. Sarmah, A. Tairai, U. Bora, Appl. Organomet. Chem. 25 (2011) 283–288. [6] B.K. Singh, N. Kaval, S. Tomar, E. Van der Eycken, V.S. Parmar, Org. Process Res. Dev. 12 (2008) 468–474. [7] V. Calo, A. Nacci, A. Monopoli, A. Fornaro, L. Sabbatini, N. Cioffi, N. Ditaranto, Organometallics 23 (2004) 5154–5158. [8] T. Baran, E. Ac¸ıksöz, A. Mentes¸, J. Mol. Catal. A: Chem. 407 (2015) 47–52. [9] N. Yan, X. Chen, Nature 524 (2015) 155–157. [10] W.W. Ngah, L. Teong, M. Hanafiah, Carbohyd. Polym. 83 (2011) 1446–1456. [11] M. Guidone, C.S. Thornber, Harmful Algae 24 (2013) 1–9. [12] K. Masakorala, A. Turner, M.T. Brown, Environ. Pollut. 156 (2008) 897–904. [13] A. Turner, M.S. Lewis, L. Shams, M.T. Brown, Marine Chem. 105 (2007) 271–280. [14] A. Pal, S. Pan, S. Saha, Chem. Eng. J. 217 (2013) 426–434. [15] M.M. Beppu, R.S. Vieira, C.G. Aimoli, C.C. Santana, J. Membr. Sci. 301 (2007) 126–130. [16] T. Baran, A. Mentes¸, H. Arslan, Int. J. Biol. Macromol. 72 (2015) 94–103. [17] C. Demetgul, M. Karakaplan, S. Serin, Synth. Monomers Polym. 11 (2008) 565–579. [18] F.A.A. Tirkistani, Degrad. Stab. 60 (1998) 67–70. [19] R. Antony, S.T. David, K. Saravanan, K. Karuppasamy, S. Balakumar, Spectrochim. Acta A. 103 (2013) 423–430. [20] M. Ziegler-Borowska, D. Chełminiak, H. Kaczmarek, J. Therm. Anal. Calorim. 119 (2015) 499–506. [21] R. Antony, S.T.D. Manickam, K. Saravanan, K. Karuppasamy, S. Balakumar, J. Mol. Struct. 1050 (2013) 53–60. [22] C. Demetgul, Carbohyd. Polym. 89 (2012) 354–361. [23] T. Baran, A. Mentes¸, Int. J. Biol. Macromol. 79 (2015) 542–554. [24] N.A. Anan, S.M. Hassan, E.M. Saad, I.S. Butler, S.I. Mostafa, Carbohyd. Res. 346 (2011) 775–793. [25] W. Li-xia, W. Zi-wei, W. Guo-song, L. Xiao-dong, R. Jian-guo, Polym. Adv. Technol. 21 (2010) 244–249. [26] E. Guibal, Sep. Purif. Technol. 38 (2004) 43–74. [27] A.F. Littke, C.Y. Dai, G.C. Fu, J. Am. Chem. Soc. 122 (2000) 4020–4028. [28] J.T. Singleton, Tetrahedron 59 (2003) 1837–1857.