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Functionalized chitosan polymerized with cyclodextrin decorated ionic liquid: Metal free and biocompatible catalyst for chemical transformations
International Journal of Biological Macromolecules 147 (2020) 399–407
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Functionalized chitosan polymerized with cyclodextrin decorated ionic liquid: Metal free and biocompatible catalyst for chemical transformations Samahe Sadjadi ⁎, Fatemeh Koohestani Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemicals Institute, PO Box 14975-112, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 14 September 2019 Received in revised form 8 January 2020 Accepted 8 January 2020 Available online 10 January 2020 Keywords: Heterogeneous catalyst Chitosan Cyclodextrin
a b s t r a c t A novel metal -free triple composite that benefits from the chemistry of chitosan, β-cyclodextrin and ionic liquid has been prepared through polymerization of vinyl functionalized chitosan and cyclodextrin decorated ionic liquid. The resulting compound, CS-CD-IL, was characterized and successfully applied as a heterogeneous catalyst for promoting Knoevenagel condensation reaction under mild reaction condition in aqueous media. It was believed that the presence of cyclodextrin in the structure of the catalyst could facilitate the reaction in aqueous media through transferring the hydrophobic substrate in the vicinity of the catalytic sites. Chitosan and ionic liquid, on the other hand, could activate the substrates via two pathways. The effects of the reaction variables as well as the recyclability of the catalyst were also investigated. The results confirmed high recyclability of the catalyst up to six reaction runs. Notably, the developed catalyst was versatile and could promote similar condensation reactions to furnish more complex chemicals such as Xanthane. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Among various catalyst supports, carbohydrates received considerable attention due to their biocompatibility, biodegradability and availability. In this context, various catalytic species were supported on the different carbohydrates such as chitosan, cyclodextrins, chitin, starch etc. [1–3]. On the other hand, carbohydrates were successfully utilized as hybrid/composite components to improve the feature of other catalyst supports. One of the mostly used carbohydrate in the field of catalysis is chitosan, CS. This natural biopolymer that can be achieved through deacetylation of chitin is available in large scale and contains both free amino and alcoholic functionalities in its backbone [4–6]. The presence of these functionalities allows further functionalization of CS with other functional groups such as ionic liquids (ILs). On the other hand, various metallic species can be immobilized on CS through electrostatic interactions and chelation mechanisms [7–9]. These features render CS a potential support for promoting heterogeneous catalysts [10,11]. ILs are organic salts composed of heteroatom-containing cations and small anion such as Cl− and Br−, tetrafluoroborate, ethyl sulfate, or hexafluorophosphate [12]. The cation in the structure of ILs is mostly N-containing heterocycles such as pyridinium and imidazolium. The
⁎ Corresponding author. E-mail address: [email protected] (S. Sadjadi).
https://doi.org/10.1016/j.ijbiomac.2020.01.089 0141-8130/© 2020 Elsevier B.V. All rights reserved.
features of ILs, including low toxicity, low melting point, high electric conductivity and tune-ability render them outstanding candidates for the catalytic purposes [13,14]. Cyclodextrins (CDs) that are cyclic oligosaccharides have also been extensively utilized for both homogeneous and heterogeneous catalysis. The main feature of CD is its cone-shape structure that provides a hydrophobic interior cavity with the capability of hosting various guest molecules with proper polarity and sizes. Considering the hydrophilic outer surface of CD, CD can serve as a molecular shuttle that is capable of transferring the hydrophobic molecules to the aqueous phase. This main feature along with the bio-compatibility, non-toxicity and availability of CD render it a convenient compound for designing catalysts and drug delivery systems [15–27]. Knoevenagel condensation reaction is one of the key reactions for the synthesis of α,β-unsaturated compounds that are important chemicals for the synthesis of more complicated chemicals [28,29]. This reaction is also useful for the synthesis of biologically active chemicals and therapeutic drugs such as nitrendipine and nifedipine. Interestingly, the use of this key reaction is also expanded to polymer science for the design of functional polymers. To date, various catalysts have been developed for promoting Knoevenagel condensation reaction [30–32]. Considering the wide range of use of this reaction, disclosing efficient protocols with use of heterogeneous and recyclable catalysts is of great importance. In the following of our study on carbohydrate based catalysts [33–35], herein we wish to report a novel metal-free catalyst based on
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combining the advantages of CD, CS and IL. To prepare the catalyst, CD was first tosylated and reacted with vinyl imidazole to furnish CD-IL, Fig. 1. Subsequently, CS was functionalized with 3- (trimethoxysilyl) propyl methacrylate and then polymerized with CD-IL. The resulting compound that benefited from the chemistry of CD and CS was then applied as a metal free and recyclable catalyst for promoting Knoevenagel condensation under mild reaction condition in aqueous media. The effects of the reaction variables such as, reaction time, temperature and catalyst loading were investigated to optimize the reaction condition and furnish the desired products in high yields. Then, the generality of this protocol was investigated for the condensation of various substrates. Moreover, the possible reaction mechanism and the possibility of generalizing this protocol to other condensation reactions were addressed. Finally, the recyclability of the catalyst was studied. 2. Experimental section 2.1. Materials The chemicals and solvents used for the preparation of the catalyst and examining its catalytic performance included, chitosan (CA), vinyl imidazole, 3- (trimethoxysilyl) propyl methacrylate, azobisisobutyronitrile (AIBN), p-toluenesulfonyl chloride (p-TsCl), acetone, β-cyclodextrin (CD), aldehydes, malononitrile, DMF, distilled water, all provided from Sigma-Aldrich. 2.2. Instruments Using various characterization techniques, the prepared catalyst was analyzed. The morphology of the catalyst was studied via recording its SEM images by a Tescan instrument with acceleration voltage of 20
kV. PERKIN-ELMER-Spectrum 65 apparatus was employed to record the FTIR spectrum of the catalyst. The thermal property of the catalyst was investigated by recording its thermogram via METTLER TOLEDO thermogravimetric analysis instrument. To carry out this analysis, the sample was heated under inert atmosphere with heating rate of 10 °C. min−1. To study the specific surface area of the catalyst, Belsorp Mini II instrument was applied. The preheating condition for this analysis was heating at 100 °C for 2 h. The XRD pattern of the catalyst was recorded by using Siemens, D5000. Cu Kα radiation from a sealed tube. 2.3. Catalyst preparation 2.3.1. CA functionalization: synthesis of CS-A Functionalization of CS was performed according to the previously reported method [36]. Briefly, CS (1.5 g) was dispersed in toluene (40 mL) and homogenized under ultrasonic irradiation of power 150 W for 15 min. Then, 3- (trimethoxysilyl) propyl methacrylate (1.5 mL) was added to the suspension and the resulting mixture was stirred overnight at 120 °C. At the end of the reaction, the precipitate was filtered off, washed with toluene several times and dried at 80 °C overnight. 2.3.2. Preparation of CD-OTs Tosylation of CD was carried out according to the previous reports [37]. Typically, to a solution of CD (15.86 mmol) in pyridine (200 mL), p-TsCl (7.9 mmol) was introduced. Then, the mixture was slowly stirred to dissolve p-TsCl. The resulting yellowish mixture was maintained at 0 °C for 2 days. Upon completion of the tosylation reaction, the mixture was diluted with distilled water (70 mL). To furnish the product, the solvent was evaporated and then cold water was added to the oily product.
Fig. 1. Schematic representation of synthesis of the catalyst.
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Upon addition of water, a white solid, CD-OTs, was obtained that was filtered and recrystallized from water. 2.3.3. CD decoration with Ionic liquid: synthesis of CD-IL The synthesis of CD-IL was carried out using the reported procedure with slight modification [38]. Initially, CD-OTs (3 g) in DMF was ultrasounded for 10 min. Then, a solution of vinyl imidazole (1.9 g in 5 mL DMF) was added under Ar flow and the resulting mixture was stirred at 90 °C overnight. Upon completion of the reaction, the mixture was cooled and acetone (20 mL) was added to furnish a white precipitate. The later was then filtrated and washed with acetone for several times and dried in oven at 60 °C overnight. 2.3.4. Polymerization of CD-IL and CS-A: synthesis of CS-CD-IL To form the CS-CD-IL, the mixture of CS-A (1 g) and CD-IL (1 g) in DMF (10 mL) was subjected to ultrasonic irradiation for 10 min. Subsequently, the mixture was stirred under Ar atmosphere at 70 °C and then a solution of AIBN (0.3 g in EtOH) was added. Subsequently, polymerization reaction was continued for 24 h. At the end of the reaction, the
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precipitate was filtered off, washed with DMF and ethyl acetate several times and dried in oven at 60 °C overnight. 2.4. Examining the catalytic activity 2.4.1. Knoevenagel condensation To perform Knoevenagel condensation, aldehyde (1 mmol), malononitrile (1.2 mmol) and catalyst (1 wt%) were mixed in H2O (3 mL) as solvent and stirred at for 1.5 h at room temperature. The progress of the reaction was monitored with TLC. Upon completion of the reaction, the mixture was cooled to ambient temperature and EtOH (20 mL) was added to the reaction mixture. Then, the catalyst was simply filtrated, washed with EtOH and dried at 100 °C in an oven for reusing. The yields of the reactions were calculated via GC. 2.4.2. Synthesis of Xanthane Typically, benzaldehyde (1 mmol), dimedone (2 mmol) and catalyst (0.03 g) were mixed in water (3 mL) and vigorously stirred at 50 °C for 2 h. The progress of the reaction was monitored via TLC. At the end of
Fig. 2. A: Thermogram of CS, CS-A, CD, CD-IL and CS-CD-IL. B: SEM images of CS-CD-IL.
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the reaction, the reaction mixture was diluted with EtOH (20 mL) and then, the catalyst was filtered. The synthesized Xanthane was purified through recrystallization from EtOH.
3. Result and discussion 3.1. Catalyst characterization To verify that CS was successfully functionalized, the thermograms of CS and CS-A were obtained and compared, Fig. 2A. The comparison of these two thermograms confirmed that the loading of 3(trimethoxysilyl) propyl methacrylate was ~ 4 wt%. The comparison of the thermograms of CD and CD-IL on the other hand showed lower thermal stability of CD-IL and an additional weight loss step. This observation indicates successful conjugation of IL. The thermogram of the catalyst showed that thermal stability of CSCD-IL was lower than that of CS. This value was higher compared to that of CD-IL. In the thermogram of the catalyst apart from the loss at ~150 °C that is due to the loss of water molecules, weight losses due to the degradation of CD-IL and CS can be observed.
In the following, the morphology of the catalyst was investigated by recording the SEM images of the catalyst, Fig. 2B. In the SEM images of the catalyst, small rods and spheres as well as large particles can be detected. Notably, the observed SEM images are distinguished from the SEM images of CD [39]. According to the literature [39], CS showed a plate-like particles. The small spherical aggregates and rods that can be seen on the surface of plate-like large particles can be assigned to the polymerized CD-IL on CS-A surface. Next, the XRD pattern of the catalyst was recorded, Fig. 3A. As shown, the catalyst showed a broad band at 2θ = 13–25°, indicating the amorphous form of CS-CD-IL. This observation is expectable as all components of the hybrid system are amorphous. The FTIR spectra of CS-A, CD-IL and the final catalyst are illustrated in Fig. 3B. The characteristic bands of CS-A included the bands at 3438 cm −1 (-OH), 2961 cm−1 (-CH2 ), 1652 cm −1 and 1078 (-Si-O) cm−1. The characteristic bands of CD-IL can be observed at 3405 cm−1 (-OH), 2927 cm−1 (-CH2) and 1665 cm−1 (-C=N). The FTIR spectrum of CS-CD-IL showed the characteristic bands of both CD-IL and CS. As the characteristic bands of CD-IL and CS-A mostly overlapped, FTIR spectroscopy cannot solely confirm the formation of the catalyst.
Fig. 3. A: The XRD pattern of CS-CD-IL and B: the FTIR spectra of CS-A, CD-IL and CS-CS-IL.
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The specific surface area of the catalyst was estimated to be b1 m2 g−1 (~ 0.7 m2 g−1). This value is less than the specific surface area of bare CS (4.5 m2 g−1). This observation can be attributed to the coverage of CS surface by CD-IL. 3.2. Catalytic activity of the catalyst To develop a metal free and bio-based catalyst, we designed CS-CDIL that benefits from the chemistry of carbohydrates and IL. In more detail, to take advantage of the capability of CD as phase transfer agent and IL, CD-IL was prepared and polymerized with CS-A. Through polymerization, CD-IL and the vinyl functionality of the CS-A tolerated radical polymerization to form CD-polyionic liquid (CD-PIL). The resulting catalyst was then applied as a metal-free catalyst for promoting Knoevenagel condensation reaction. First, the effects of CS-CD-IL amount, reaction solvent and temperature on the yield of the desired product were investigated. 3.2.1. Effect of catalyst amount To elucidate the effect of the catalyst amount on the yield of the reaction, Knoevenagel condensation reaction of benzaldehyde and
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malononitrile was selected as a model condensation reaction. Subsequently, the model reaction was performed in water as an environmentally benign solvent at ambient temperature by using various amounts of catalyst (o.5, 0.7, 1, 1.5 wt%). The yields of the desired product and the reaction time were measured for all reactions. The result, Fig. 4A, confirmed that increase of the catalyst loading from 0.5 to 1 wt% led to the increase of the yield of the product from 88 to 100%. Moreover, the increase of the catalyst amount from o.5 to 1 wt% accelerated the reaction and shortened the reaction time from 2.5 to 1.5 h. Notably, further increase of the catalyst loading did not affect the reaction time.
3.2.2. Effect of reaction time To study the effect of the reaction time on the yield of the reaction, the yields of the model product at short time intervals were measured. The diagram of the yield of the product versus reaction time, Fig. 4Bblue line, showed that upon passing the first 15 min, the yield of the product reached to 30%. Since then, the reaction proceeded slowly and only 10% increase of the yield of the reaction was observed per each 15 min. Upon further increase of the reaction time from 45 min to 90 min, the slope of the curve increased and the reaction completed after 90 min.
Fig. 4. A: The effect of the catalyst amount on the yield and reaction time of the model reaction and B: The effect of the reaction time on the yield of the model reaction in water at ambient temperature and 50 °C.
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Table 1 Study of the scope of the developed protocol for the Knoevenagel condensation reaction a.
Table 3 The comparison of the efficiency of the control catalysts with that of CS-CD-IL.
Entry
Aldehyde
Time (h:min)
Conversion (%)a
Yieldb (%)
Entry
Catalyst
Time (h:min)
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
Benzaldehyde 4-Chlorobenzaldehyde 4-Nitrobenzaldehyde 2- Nitrobenzaldehyde 4-Methoxybenzaldehyde 2- Methoxybenzaldehyde 4-Methyl benzaldehyde 4-Methylbenzaldehyde 3- Nitrobenzaldehyde Furfural Naphthaldehyde
1.5 1.5 1.5 1.5 2 2 2 2 1.5 2 2:30
100 100 100 90 100 98 95 95 95 90 73
100 100 100 90 100 98 95 95 95 90 73
1 2 3 4 5
CS CD IL CD-IL CS-CD-IL
2:10 2:10 1:30 1:30 1:30
88 90 92 97 100
a Reaction conditions: aldehyde (1 mmol), malononitrile (1.2 mmol), catalyst (1 wt%), H2O (2 mL). b Isolated yields.
3.2.3. Effect of reaction temperature It was investigated whether increase of temperature to 50 °C could accelerate the reaction. To this purpose, the model reaction was performed at 50 °C and the progress of the reaction was monitored at short time intervals, Fig. 4 B-red line. The results confirmed that at elevated temperature the reaction proceeded faster and after 15 min, 50% yield and conversion were observed. Then, the reaction proceeded slowly and only 10% increase of the reaction yield was observed per 15 min. After 1 h, 75% conversion and yield of the product were obtained. Then, the reaction lingered and full conversion was achieved after 1.5 h. This observation confirmed that although the reaction at higher temperature proceeded faster, after a determined time (for the model reaction 1 h), the reaction slowed down and both reactions at 25 and 50 °C completed after 1.5 h. Considering this result, the optimum reaction temperature was selected as 25 °C. 3.2.4. Generality of the developed protocol To study the scope of the reaction and elucidate whether the developed protocol can be generalized to other substrates, the Knoevenagel condensation reactions of various aldehydes with different functional groups were carried out, Table 1. As shown in Table 1, various substrates with different electronic features could undergo the reaction under CSCD-IL catalysis to furnish the corresponding products in high to excellent yields. It was postulated that CD in the structure of the catalyst could accelerate the reaction rate via forming inclusion complex with hydrophobic substrate (aromatic aldehyde). To validate this assumption, the yields of the reactions of two substrates with functional groups at the ortho and para positions were compared. In more detail, the yields of the reactions of 2- nitrobenzaldehyde and 2- methoxybenzaldehyde were compared with that of 4- nitrobenzaldehyde and 4- methoxybenzaldehyde (Table 1, entries 3 and 4 and entries 5 and 6). As shown, in both pairs, the yields of para -substituted substrates were higher than that of ortho substituted counterparts. This observation was attributed to the
more facile inclusion of para-substituted substrates into the cavity of CD. To further confirm the role of CD in the formation of the inclusion complex, Knoevenagel condensation reaction of a sterically demanding substrate, naphthaldehyde, was investigated (Table 1, entry 11). The result confirmed the lower yield and longer reaction time for this substrate. This observation can also confirm the role of CD as phase transferring agent. In the case of naphthaldehyde, the formation of the inclusion complex was less probable and consequently, longer reaction time was required. In the following, to elucidate the merit of CS-CD-IL catalyst for promoting the Knoevenagel condensation, the efficiency of CS-CD-IL for the model condensation reaction was compared with that of some catalysts reported in the literature. The results are summarized in Table 2. Considering the importance of Knoevenagel condensation reaction, this reaction was promoted using various kinds of catalysts, ranging from IL to metal organic frameworks (MOF) under different conditions. As can be seen in Table 2 (entry 5), CS was applied as a catalyst for this reaction. The precise comparison of the performance of CS and CS-CD-IL showed that although the yields of the desired product for both of the catalysts were comparative, the reaction temperature, reaction time and the required amount of the catalyst for CS catalysis were higher than those of CS-CD-IL. Moreover, the reaction under CS-CD-IL catalysis can proceed in water, while CS-catalyzed reaction was reported in EtOH. Knoevenagel condensation was also reported in the presence of metallic catalysts. As an example, PdNi@GO could lead to 95% yield of the product. Although the reaction condition was mild and the used amount of the catalyst was low, use of precious metal and graphene oxide as support rendered the procedure less attractive from the economic and environmental points of view. Use of metal-free bio-based catalysts, such as CS-CD-IL is more cost-effective and eco-friendly. As tabulated, use of ultrasonic irradiation (Table 2, entry 3) shortened the reaction time. However, the reaction temperature and the amount of the used catalyst were relatively higher than CS-CD-IL. This reaction was also reported under MOF catalysis. Although MOFs is an efficient catalysts for this reaction, the preparation of MOFs is relatively expensive and time consuming. ILs are another class of the catalysts that have been reported for this reaction. IL can be used both as a solvent and a catalyst. The results, Table 2, showed that bare IL is less efficient than CS-CD-IL. Moreover, in some cases, very long reaction times have been reported. Considering the results, it can be concluded that CS-CD-IL is an efficient catalyst for Knoevenagel condensation reaction.
Table 2 The comparison of the efficiency of Catalyst with previously reported catalysts for the Knoevenagel condensation. Entry
Catalyst
Catalyst amount
Solvent
Temperature (°C)
Time (h:min)
Yield (%)
Ref.
1 2a 3 4 5 6 7 8 9 10
CS-CD-IL [Zn2(TCA)(BIB)2,5]. (NO3) Caffein-SiO2@Fe3O4 PdNi@GO CS [H3N+–CH2-CH2−OH][CH3COO−] IL Amino Acid Amide based Ionic Liquid Activated Hf-UiO-66-N2H3 Glycine –
10 mg 0.003 mmol 50 mg 2 mg 25 mg 4 drop 30 mol% 20 mg – 2 mL
H2O Solvent free H2O (ultrasonic irradiation) H2O/EtOH EtOH Solvent free Solvent free EtOH [6-mim] PF6 2-hydroxyethylammonium formate
25 60 60 25 40 25 25 25 25 25
1:30 1:00 00:06 00:08 6:00 b1:00 00:30 4:00 22:00 5:00
100 N99 94 95 98 90.9 97 98 77 88
This work [40] [41] [42] [43] [44] [14] [45] [13] [46]
a
[Zn2(tricarboxytriphenyl amine)(1,3-bis(imidazol-1-ylmethyl)benzene)2,5].(NO3).
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Fig. 5. The plausible mechanism for Knoevenagel condensation.
Next, the contribution of each component of the catalyst to the catalysis was studied. To this purpose, the catalytic activities of some control catalysts, including CS, CD, IL and CD-IL for promoting the model Knoevenagel condensation reaction under optimum reaction condition were measured and compared with the catalytic activity of CS-CD-IL, Table 3. As shown in Table 3, the model reaction was first performed in the presence of CS. The result showed that CS could act as a catalyst. However, it catalyzed the reaction in longer reaction time and led to the lower yield of the desired product. In the following, the efficiency of CD was examined. It was found out that CD was catalytically active. Similar to CS, the model reaction in the presence of CD required longer reaction time compared to CD-CDIL. Moreover, lower yield of the product was furnished. The drawback of CD was its homogeneous nature that rendered its separation tedious. The study of the catalytic activity of bare IL also confirmed its lower activity compared to that of CS-CD-IL. These results
confirmed that although each component of the catalyst is catalytically active, use of them individually led to lower catalytic activity compared to the hybrid structure. This observation may be attributed to the synergistic effects among catalyst components. To further confirm this issue, another control catalyst, CD-IL, was prepared and its catalytic activity was compared with that of CD, IL and the catalyst. The result showed that the catalytic activity of CD-IL was higher than that of CD and IL, indicating the synergism between CD and IL. The lower catalytic activity of this control sample compared to that of CS-CD-IL proved that incorporation of CD-IL on CS could improve the catalytic activity. Finally, it was investigated whether this catalyst could promote similar condensation reaction to furnish more complicated chemicals such as Xanthane. To this purpose, a model Xanthane derivative from reaction of benzaldehyde and dimedone was synthesized under CS-CD-IL catalysis. Gratifyingly, the reaction under mild reaction condition
Fig. 6. The results of the recyclability test for the model reaction under optimum reaction condition.
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(using 0.03 g catalyst at 50 °C in aqueous media) led to the corresponding Xanthane derivative in 98% yield. This example indicated that CSCD-IL can be considered as a versatile catalyst that is capable of promoting various organic transformations.
Acknowledgements
3.3. Reaction mechanism
References
The plausible reaction mechanism is as follow: first, aldehyde is encapsulated by CD and transferred into the aqueous media in the vicinity of the catalytic sites. In the next step, both aldehyde and malononitrile will be activated. As the catalyst contained both CS and IL, the activation of the substrates can proceed by CS and IL, Fig. 5. In more detail, chitosan as a basic catalyst can activate malononitrile to form carbanions (pathway 1). Meanwhile, the amine group in the backbone of chitosan can produce an imine intermediate with aldehyde (pathway 1) [47]. On the other hand, the counter ion of IL can activate malononitrile to form an intermediate through pathway 2. Meantime, the imidazolium cations activate the carbonyl compound [48]. The reaction of the activated substrates, generated from each pathway can result in an intermediate that form the condensation product and free catalyst in the next step. 3.4. Recyclability of the catalyst In the next part, the recyclability of CS-CD-IL was investigated. At the end of the first run of the model Knoevenagel condensation, CSCD-IL was simply filtered and reused for the next reaction run. After each cycle, the yield of the condensation reaction was measured and compared with that of the fresh CS-CD-IL. Notably, all of the reactions were performed under the optimum reaction condition. The comparison of the yield of each reaction cycle, Fig. 6, confirmed that the catalyst could be successfully recycled for six reaction runs. Precisely, the catalyst preserved its catalytic activity for the second run of the reaction. Upon recycling for third to fifth reaction runs, slight loss of the catalytic activity was observed. Upon sixth reaction run, the loss of the catalytic activity increased and the yield of the product reached to 82%. 4. Conclusion Carbohydrate-based catalyst, CS-CD-IL, was prepared through functionalization of CS and formation of CD-IL followed by their radical polymerization. The catalytic activity of CS-CD-IL for promoting Knoevenagel condensation was investigated in aqueous media at ambient temperature. The results confirmed high catalytic activity of the catalyst and generality of the protocol. It was confirmed that all of the composite components could participate in the catalysis. In more detail, CD could form inclusion complex with the hydrophobic aldehydes and transfer them in the vicinity of the catalytic sites. On the other hand, IL and CS could activate both malononitrile and encapsulated aldehyde via two pathways. It is worth noting that the catalyst was recyclable for six reaction runs with slight loss of the catalytic activity. Moreover, it could successfully promote the synthesis of Xanthane derivative from reaction of benzaldehyde and dimedone. This result confirmed that CS-CD-IL could be considered as a versatile catalyst. CRediT authorship contribution statement Samahe Sadjadi: Conceptualization, Project administration, Writing - review & editing, Supervision, Methodology. Fatemeh Koohestani: Validation, Resources, Writing - original draft. Declaration of competing interest The authors declare no conflict of interest.
The authors appreciate the partial support of Iran Polymer and Petrochemical Institute.
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Functionalized chitosan polymerized with cyclodextrin decorated ionic liquid: Metal free and biocompatible catalyst for chemical transformations Samahe Sadjadi ⁎, Fatemeh Koohestani Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemicals Institute, PO Box 14975-112, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 14 September 2019 Received in revised form 8 January 2020 Accepted 8 January 2020 Available online 10 January 2020 Keywords: Heterogeneous catalyst Chitosan Cyclodextrin
a b s t r a c t A novel metal -free triple composite that benefits from the chemistry of chitosan, β-cyclodextrin and ionic liquid has been prepared through polymerization of vinyl functionalized chitosan and cyclodextrin decorated ionic liquid. The resulting compound, CS-CD-IL, was characterized and successfully applied as a heterogeneous catalyst for promoting Knoevenagel condensation reaction under mild reaction condition in aqueous media. It was believed that the presence of cyclodextrin in the structure of the catalyst could facilitate the reaction in aqueous media through transferring the hydrophobic substrate in the vicinity of the catalytic sites. Chitosan and ionic liquid, on the other hand, could activate the substrates via two pathways. The effects of the reaction variables as well as the recyclability of the catalyst were also investigated. The results confirmed high recyclability of the catalyst up to six reaction runs. Notably, the developed catalyst was versatile and could promote similar condensation reactions to furnish more complex chemicals such as Xanthane. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Among various catalyst supports, carbohydrates received considerable attention due to their biocompatibility, biodegradability and availability. In this context, various catalytic species were supported on the different carbohydrates such as chitosan, cyclodextrins, chitin, starch etc. [1–3]. On the other hand, carbohydrates were successfully utilized as hybrid/composite components to improve the feature of other catalyst supports. One of the mostly used carbohydrate in the field of catalysis is chitosan, CS. This natural biopolymer that can be achieved through deacetylation of chitin is available in large scale and contains both free amino and alcoholic functionalities in its backbone [4–6]. The presence of these functionalities allows further functionalization of CS with other functional groups such as ionic liquids (ILs). On the other hand, various metallic species can be immobilized on CS through electrostatic interactions and chelation mechanisms [7–9]. These features render CS a potential support for promoting heterogeneous catalysts [10,11]. ILs are organic salts composed of heteroatom-containing cations and small anion such as Cl− and Br−, tetrafluoroborate, ethyl sulfate, or hexafluorophosphate [12]. The cation in the structure of ILs is mostly N-containing heterocycles such as pyridinium and imidazolium. The
⁎ Corresponding author. E-mail address: [email protected] (S. Sadjadi).
https://doi.org/10.1016/j.ijbiomac.2020.01.089 0141-8130/© 2020 Elsevier B.V. All rights reserved.
features of ILs, including low toxicity, low melting point, high electric conductivity and tune-ability render them outstanding candidates for the catalytic purposes [13,14]. Cyclodextrins (CDs) that are cyclic oligosaccharides have also been extensively utilized for both homogeneous and heterogeneous catalysis. The main feature of CD is its cone-shape structure that provides a hydrophobic interior cavity with the capability of hosting various guest molecules with proper polarity and sizes. Considering the hydrophilic outer surface of CD, CD can serve as a molecular shuttle that is capable of transferring the hydrophobic molecules to the aqueous phase. This main feature along with the bio-compatibility, non-toxicity and availability of CD render it a convenient compound for designing catalysts and drug delivery systems [15–27]. Knoevenagel condensation reaction is one of the key reactions for the synthesis of α,β-unsaturated compounds that are important chemicals for the synthesis of more complicated chemicals [28,29]. This reaction is also useful for the synthesis of biologically active chemicals and therapeutic drugs such as nitrendipine and nifedipine. Interestingly, the use of this key reaction is also expanded to polymer science for the design of functional polymers. To date, various catalysts have been developed for promoting Knoevenagel condensation reaction [30–32]. Considering the wide range of use of this reaction, disclosing efficient protocols with use of heterogeneous and recyclable catalysts is of great importance. In the following of our study on carbohydrate based catalysts [33–35], herein we wish to report a novel metal-free catalyst based on
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combining the advantages of CD, CS and IL. To prepare the catalyst, CD was first tosylated and reacted with vinyl imidazole to furnish CD-IL, Fig. 1. Subsequently, CS was functionalized with 3- (trimethoxysilyl) propyl methacrylate and then polymerized with CD-IL. The resulting compound that benefited from the chemistry of CD and CS was then applied as a metal free and recyclable catalyst for promoting Knoevenagel condensation under mild reaction condition in aqueous media. The effects of the reaction variables such as, reaction time, temperature and catalyst loading were investigated to optimize the reaction condition and furnish the desired products in high yields. Then, the generality of this protocol was investigated for the condensation of various substrates. Moreover, the possible reaction mechanism and the possibility of generalizing this protocol to other condensation reactions were addressed. Finally, the recyclability of the catalyst was studied. 2. Experimental section 2.1. Materials The chemicals and solvents used for the preparation of the catalyst and examining its catalytic performance included, chitosan (CA), vinyl imidazole, 3- (trimethoxysilyl) propyl methacrylate, azobisisobutyronitrile (AIBN), p-toluenesulfonyl chloride (p-TsCl), acetone, β-cyclodextrin (CD), aldehydes, malononitrile, DMF, distilled water, all provided from Sigma-Aldrich. 2.2. Instruments Using various characterization techniques, the prepared catalyst was analyzed. The morphology of the catalyst was studied via recording its SEM images by a Tescan instrument with acceleration voltage of 20
kV. PERKIN-ELMER-Spectrum 65 apparatus was employed to record the FTIR spectrum of the catalyst. The thermal property of the catalyst was investigated by recording its thermogram via METTLER TOLEDO thermogravimetric analysis instrument. To carry out this analysis, the sample was heated under inert atmosphere with heating rate of 10 °C. min−1. To study the specific surface area of the catalyst, Belsorp Mini II instrument was applied. The preheating condition for this analysis was heating at 100 °C for 2 h. The XRD pattern of the catalyst was recorded by using Siemens, D5000. Cu Kα radiation from a sealed tube. 2.3. Catalyst preparation 2.3.1. CA functionalization: synthesis of CS-A Functionalization of CS was performed according to the previously reported method [36]. Briefly, CS (1.5 g) was dispersed in toluene (40 mL) and homogenized under ultrasonic irradiation of power 150 W for 15 min. Then, 3- (trimethoxysilyl) propyl methacrylate (1.5 mL) was added to the suspension and the resulting mixture was stirred overnight at 120 °C. At the end of the reaction, the precipitate was filtered off, washed with toluene several times and dried at 80 °C overnight. 2.3.2. Preparation of CD-OTs Tosylation of CD was carried out according to the previous reports [37]. Typically, to a solution of CD (15.86 mmol) in pyridine (200 mL), p-TsCl (7.9 mmol) was introduced. Then, the mixture was slowly stirred to dissolve p-TsCl. The resulting yellowish mixture was maintained at 0 °C for 2 days. Upon completion of the tosylation reaction, the mixture was diluted with distilled water (70 mL). To furnish the product, the solvent was evaporated and then cold water was added to the oily product.
Fig. 1. Schematic representation of synthesis of the catalyst.
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Upon addition of water, a white solid, CD-OTs, was obtained that was filtered and recrystallized from water. 2.3.3. CD decoration with Ionic liquid: synthesis of CD-IL The synthesis of CD-IL was carried out using the reported procedure with slight modification [38]. Initially, CD-OTs (3 g) in DMF was ultrasounded for 10 min. Then, a solution of vinyl imidazole (1.9 g in 5 mL DMF) was added under Ar flow and the resulting mixture was stirred at 90 °C overnight. Upon completion of the reaction, the mixture was cooled and acetone (20 mL) was added to furnish a white precipitate. The later was then filtrated and washed with acetone for several times and dried in oven at 60 °C overnight. 2.3.4. Polymerization of CD-IL and CS-A: synthesis of CS-CD-IL To form the CS-CD-IL, the mixture of CS-A (1 g) and CD-IL (1 g) in DMF (10 mL) was subjected to ultrasonic irradiation for 10 min. Subsequently, the mixture was stirred under Ar atmosphere at 70 °C and then a solution of AIBN (0.3 g in EtOH) was added. Subsequently, polymerization reaction was continued for 24 h. At the end of the reaction, the
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precipitate was filtered off, washed with DMF and ethyl acetate several times and dried in oven at 60 °C overnight. 2.4. Examining the catalytic activity 2.4.1. Knoevenagel condensation To perform Knoevenagel condensation, aldehyde (1 mmol), malononitrile (1.2 mmol) and catalyst (1 wt%) were mixed in H2O (3 mL) as solvent and stirred at for 1.5 h at room temperature. The progress of the reaction was monitored with TLC. Upon completion of the reaction, the mixture was cooled to ambient temperature and EtOH (20 mL) was added to the reaction mixture. Then, the catalyst was simply filtrated, washed with EtOH and dried at 100 °C in an oven for reusing. The yields of the reactions were calculated via GC. 2.4.2. Synthesis of Xanthane Typically, benzaldehyde (1 mmol), dimedone (2 mmol) and catalyst (0.03 g) were mixed in water (3 mL) and vigorously stirred at 50 °C for 2 h. The progress of the reaction was monitored via TLC. At the end of
Fig. 2. A: Thermogram of CS, CS-A, CD, CD-IL and CS-CD-IL. B: SEM images of CS-CD-IL.
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the reaction, the reaction mixture was diluted with EtOH (20 mL) and then, the catalyst was filtered. The synthesized Xanthane was purified through recrystallization from EtOH.
3. Result and discussion 3.1. Catalyst characterization To verify that CS was successfully functionalized, the thermograms of CS and CS-A were obtained and compared, Fig. 2A. The comparison of these two thermograms confirmed that the loading of 3(trimethoxysilyl) propyl methacrylate was ~ 4 wt%. The comparison of the thermograms of CD and CD-IL on the other hand showed lower thermal stability of CD-IL and an additional weight loss step. This observation indicates successful conjugation of IL. The thermogram of the catalyst showed that thermal stability of CSCD-IL was lower than that of CS. This value was higher compared to that of CD-IL. In the thermogram of the catalyst apart from the loss at ~150 °C that is due to the loss of water molecules, weight losses due to the degradation of CD-IL and CS can be observed.
In the following, the morphology of the catalyst was investigated by recording the SEM images of the catalyst, Fig. 2B. In the SEM images of the catalyst, small rods and spheres as well as large particles can be detected. Notably, the observed SEM images are distinguished from the SEM images of CD [39]. According to the literature [39], CS showed a plate-like particles. The small spherical aggregates and rods that can be seen on the surface of plate-like large particles can be assigned to the polymerized CD-IL on CS-A surface. Next, the XRD pattern of the catalyst was recorded, Fig. 3A. As shown, the catalyst showed a broad band at 2θ = 13–25°, indicating the amorphous form of CS-CD-IL. This observation is expectable as all components of the hybrid system are amorphous. The FTIR spectra of CS-A, CD-IL and the final catalyst are illustrated in Fig. 3B. The characteristic bands of CS-A included the bands at 3438 cm −1 (-OH), 2961 cm−1 (-CH2 ), 1652 cm −1 and 1078 (-Si-O) cm−1. The characteristic bands of CD-IL can be observed at 3405 cm−1 (-OH), 2927 cm−1 (-CH2) and 1665 cm−1 (-C=N). The FTIR spectrum of CS-CD-IL showed the characteristic bands of both CD-IL and CS. As the characteristic bands of CD-IL and CS-A mostly overlapped, FTIR spectroscopy cannot solely confirm the formation of the catalyst.
Fig. 3. A: The XRD pattern of CS-CD-IL and B: the FTIR spectra of CS-A, CD-IL and CS-CS-IL.
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The specific surface area of the catalyst was estimated to be b1 m2 g−1 (~ 0.7 m2 g−1). This value is less than the specific surface area of bare CS (4.5 m2 g−1). This observation can be attributed to the coverage of CS surface by CD-IL. 3.2. Catalytic activity of the catalyst To develop a metal free and bio-based catalyst, we designed CS-CDIL that benefits from the chemistry of carbohydrates and IL. In more detail, to take advantage of the capability of CD as phase transfer agent and IL, CD-IL was prepared and polymerized with CS-A. Through polymerization, CD-IL and the vinyl functionality of the CS-A tolerated radical polymerization to form CD-polyionic liquid (CD-PIL). The resulting catalyst was then applied as a metal-free catalyst for promoting Knoevenagel condensation reaction. First, the effects of CS-CD-IL amount, reaction solvent and temperature on the yield of the desired product were investigated. 3.2.1. Effect of catalyst amount To elucidate the effect of the catalyst amount on the yield of the reaction, Knoevenagel condensation reaction of benzaldehyde and
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malononitrile was selected as a model condensation reaction. Subsequently, the model reaction was performed in water as an environmentally benign solvent at ambient temperature by using various amounts of catalyst (o.5, 0.7, 1, 1.5 wt%). The yields of the desired product and the reaction time were measured for all reactions. The result, Fig. 4A, confirmed that increase of the catalyst loading from 0.5 to 1 wt% led to the increase of the yield of the product from 88 to 100%. Moreover, the increase of the catalyst amount from o.5 to 1 wt% accelerated the reaction and shortened the reaction time from 2.5 to 1.5 h. Notably, further increase of the catalyst loading did not affect the reaction time.
3.2.2. Effect of reaction time To study the effect of the reaction time on the yield of the reaction, the yields of the model product at short time intervals were measured. The diagram of the yield of the product versus reaction time, Fig. 4Bblue line, showed that upon passing the first 15 min, the yield of the product reached to 30%. Since then, the reaction proceeded slowly and only 10% increase of the yield of the reaction was observed per each 15 min. Upon further increase of the reaction time from 45 min to 90 min, the slope of the curve increased and the reaction completed after 90 min.
Fig. 4. A: The effect of the catalyst amount on the yield and reaction time of the model reaction and B: The effect of the reaction time on the yield of the model reaction in water at ambient temperature and 50 °C.
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Table 1 Study of the scope of the developed protocol for the Knoevenagel condensation reaction a.
Table 3 The comparison of the efficiency of the control catalysts with that of CS-CD-IL.
Entry
Aldehyde
Time (h:min)
Conversion (%)a
Yieldb (%)
Entry
Catalyst
Time (h:min)
Yield (%)
1 2 3 4 5 6 7 8 9 10 11
Benzaldehyde 4-Chlorobenzaldehyde 4-Nitrobenzaldehyde 2- Nitrobenzaldehyde 4-Methoxybenzaldehyde 2- Methoxybenzaldehyde 4-Methyl benzaldehyde 4-Methylbenzaldehyde 3- Nitrobenzaldehyde Furfural Naphthaldehyde
1.5 1.5 1.5 1.5 2 2 2 2 1.5 2 2:30
100 100 100 90 100 98 95 95 95 90 73
100 100 100 90 100 98 95 95 95 90 73
1 2 3 4 5
CS CD IL CD-IL CS-CD-IL
2:10 2:10 1:30 1:30 1:30
88 90 92 97 100
a Reaction conditions: aldehyde (1 mmol), malononitrile (1.2 mmol), catalyst (1 wt%), H2O (2 mL). b Isolated yields.
3.2.3. Effect of reaction temperature It was investigated whether increase of temperature to 50 °C could accelerate the reaction. To this purpose, the model reaction was performed at 50 °C and the progress of the reaction was monitored at short time intervals, Fig. 4 B-red line. The results confirmed that at elevated temperature the reaction proceeded faster and after 15 min, 50% yield and conversion were observed. Then, the reaction proceeded slowly and only 10% increase of the reaction yield was observed per 15 min. After 1 h, 75% conversion and yield of the product were obtained. Then, the reaction lingered and full conversion was achieved after 1.5 h. This observation confirmed that although the reaction at higher temperature proceeded faster, after a determined time (for the model reaction 1 h), the reaction slowed down and both reactions at 25 and 50 °C completed after 1.5 h. Considering this result, the optimum reaction temperature was selected as 25 °C. 3.2.4. Generality of the developed protocol To study the scope of the reaction and elucidate whether the developed protocol can be generalized to other substrates, the Knoevenagel condensation reactions of various aldehydes with different functional groups were carried out, Table 1. As shown in Table 1, various substrates with different electronic features could undergo the reaction under CSCD-IL catalysis to furnish the corresponding products in high to excellent yields. It was postulated that CD in the structure of the catalyst could accelerate the reaction rate via forming inclusion complex with hydrophobic substrate (aromatic aldehyde). To validate this assumption, the yields of the reactions of two substrates with functional groups at the ortho and para positions were compared. In more detail, the yields of the reactions of 2- nitrobenzaldehyde and 2- methoxybenzaldehyde were compared with that of 4- nitrobenzaldehyde and 4- methoxybenzaldehyde (Table 1, entries 3 and 4 and entries 5 and 6). As shown, in both pairs, the yields of para -substituted substrates were higher than that of ortho substituted counterparts. This observation was attributed to the
more facile inclusion of para-substituted substrates into the cavity of CD. To further confirm the role of CD in the formation of the inclusion complex, Knoevenagel condensation reaction of a sterically demanding substrate, naphthaldehyde, was investigated (Table 1, entry 11). The result confirmed the lower yield and longer reaction time for this substrate. This observation can also confirm the role of CD as phase transferring agent. In the case of naphthaldehyde, the formation of the inclusion complex was less probable and consequently, longer reaction time was required. In the following, to elucidate the merit of CS-CD-IL catalyst for promoting the Knoevenagel condensation, the efficiency of CS-CD-IL for the model condensation reaction was compared with that of some catalysts reported in the literature. The results are summarized in Table 2. Considering the importance of Knoevenagel condensation reaction, this reaction was promoted using various kinds of catalysts, ranging from IL to metal organic frameworks (MOF) under different conditions. As can be seen in Table 2 (entry 5), CS was applied as a catalyst for this reaction. The precise comparison of the performance of CS and CS-CD-IL showed that although the yields of the desired product for both of the catalysts were comparative, the reaction temperature, reaction time and the required amount of the catalyst for CS catalysis were higher than those of CS-CD-IL. Moreover, the reaction under CS-CD-IL catalysis can proceed in water, while CS-catalyzed reaction was reported in EtOH. Knoevenagel condensation was also reported in the presence of metallic catalysts. As an example, PdNi@GO could lead to 95% yield of the product. Although the reaction condition was mild and the used amount of the catalyst was low, use of precious metal and graphene oxide as support rendered the procedure less attractive from the economic and environmental points of view. Use of metal-free bio-based catalysts, such as CS-CD-IL is more cost-effective and eco-friendly. As tabulated, use of ultrasonic irradiation (Table 2, entry 3) shortened the reaction time. However, the reaction temperature and the amount of the used catalyst were relatively higher than CS-CD-IL. This reaction was also reported under MOF catalysis. Although MOFs is an efficient catalysts for this reaction, the preparation of MOFs is relatively expensive and time consuming. ILs are another class of the catalysts that have been reported for this reaction. IL can be used both as a solvent and a catalyst. The results, Table 2, showed that bare IL is less efficient than CS-CD-IL. Moreover, in some cases, very long reaction times have been reported. Considering the results, it can be concluded that CS-CD-IL is an efficient catalyst for Knoevenagel condensation reaction.
Table 2 The comparison of the efficiency of Catalyst with previously reported catalysts for the Knoevenagel condensation. Entry
Catalyst
Catalyst amount
Solvent
Temperature (°C)
Time (h:min)
Yield (%)
Ref.
1 2a 3 4 5 6 7 8 9 10
CS-CD-IL [Zn2(TCA)(BIB)2,5]. (NO3) Caffein-SiO2@Fe3O4 PdNi@GO CS [H3N+–CH2-CH2−OH][CH3COO−] IL Amino Acid Amide based Ionic Liquid Activated Hf-UiO-66-N2H3 Glycine –
10 mg 0.003 mmol 50 mg 2 mg 25 mg 4 drop 30 mol% 20 mg – 2 mL
H2O Solvent free H2O (ultrasonic irradiation) H2O/EtOH EtOH Solvent free Solvent free EtOH [6-mim] PF6 2-hydroxyethylammonium formate
25 60 60 25 40 25 25 25 25 25
1:30 1:00 00:06 00:08 6:00 b1:00 00:30 4:00 22:00 5:00
100 N99 94 95 98 90.9 97 98 77 88
This work [40] [41] [42] [43] [44] [14] [45] [13] [46]
a
[Zn2(tricarboxytriphenyl amine)(1,3-bis(imidazol-1-ylmethyl)benzene)2,5].(NO3).
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Fig. 5. The plausible mechanism for Knoevenagel condensation.
Next, the contribution of each component of the catalyst to the catalysis was studied. To this purpose, the catalytic activities of some control catalysts, including CS, CD, IL and CD-IL for promoting the model Knoevenagel condensation reaction under optimum reaction condition were measured and compared with the catalytic activity of CS-CD-IL, Table 3. As shown in Table 3, the model reaction was first performed in the presence of CS. The result showed that CS could act as a catalyst. However, it catalyzed the reaction in longer reaction time and led to the lower yield of the desired product. In the following, the efficiency of CD was examined. It was found out that CD was catalytically active. Similar to CS, the model reaction in the presence of CD required longer reaction time compared to CD-CDIL. Moreover, lower yield of the product was furnished. The drawback of CD was its homogeneous nature that rendered its separation tedious. The study of the catalytic activity of bare IL also confirmed its lower activity compared to that of CS-CD-IL. These results
confirmed that although each component of the catalyst is catalytically active, use of them individually led to lower catalytic activity compared to the hybrid structure. This observation may be attributed to the synergistic effects among catalyst components. To further confirm this issue, another control catalyst, CD-IL, was prepared and its catalytic activity was compared with that of CD, IL and the catalyst. The result showed that the catalytic activity of CD-IL was higher than that of CD and IL, indicating the synergism between CD and IL. The lower catalytic activity of this control sample compared to that of CS-CD-IL proved that incorporation of CD-IL on CS could improve the catalytic activity. Finally, it was investigated whether this catalyst could promote similar condensation reaction to furnish more complicated chemicals such as Xanthane. To this purpose, a model Xanthane derivative from reaction of benzaldehyde and dimedone was synthesized under CS-CD-IL catalysis. Gratifyingly, the reaction under mild reaction condition
Fig. 6. The results of the recyclability test for the model reaction under optimum reaction condition.
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(using 0.03 g catalyst at 50 °C in aqueous media) led to the corresponding Xanthane derivative in 98% yield. This example indicated that CSCD-IL can be considered as a versatile catalyst that is capable of promoting various organic transformations.
Acknowledgements
3.3. Reaction mechanism
References
The plausible reaction mechanism is as follow: first, aldehyde is encapsulated by CD and transferred into the aqueous media in the vicinity of the catalytic sites. In the next step, both aldehyde and malononitrile will be activated. As the catalyst contained both CS and IL, the activation of the substrates can proceed by CS and IL, Fig. 5. In more detail, chitosan as a basic catalyst can activate malononitrile to form carbanions (pathway 1). Meanwhile, the amine group in the backbone of chitosan can produce an imine intermediate with aldehyde (pathway 1) [47]. On the other hand, the counter ion of IL can activate malononitrile to form an intermediate through pathway 2. Meantime, the imidazolium cations activate the carbonyl compound [48]. The reaction of the activated substrates, generated from each pathway can result in an intermediate that form the condensation product and free catalyst in the next step. 3.4. Recyclability of the catalyst In the next part, the recyclability of CS-CD-IL was investigated. At the end of the first run of the model Knoevenagel condensation, CSCD-IL was simply filtered and reused for the next reaction run. After each cycle, the yield of the condensation reaction was measured and compared with that of the fresh CS-CD-IL. Notably, all of the reactions were performed under the optimum reaction condition. The comparison of the yield of each reaction cycle, Fig. 6, confirmed that the catalyst could be successfully recycled for six reaction runs. Precisely, the catalyst preserved its catalytic activity for the second run of the reaction. Upon recycling for third to fifth reaction runs, slight loss of the catalytic activity was observed. Upon sixth reaction run, the loss of the catalytic activity increased and the yield of the product reached to 82%. 4. Conclusion Carbohydrate-based catalyst, CS-CD-IL, was prepared through functionalization of CS and formation of CD-IL followed by their radical polymerization. The catalytic activity of CS-CD-IL for promoting Knoevenagel condensation was investigated in aqueous media at ambient temperature. The results confirmed high catalytic activity of the catalyst and generality of the protocol. It was confirmed that all of the composite components could participate in the catalysis. In more detail, CD could form inclusion complex with the hydrophobic aldehydes and transfer them in the vicinity of the catalytic sites. On the other hand, IL and CS could activate both malononitrile and encapsulated aldehyde via two pathways. It is worth noting that the catalyst was recyclable for six reaction runs with slight loss of the catalytic activity. Moreover, it could successfully promote the synthesis of Xanthane derivative from reaction of benzaldehyde and dimedone. This result confirmed that CS-CD-IL could be considered as a versatile catalyst. CRediT authorship contribution statement Samahe Sadjadi: Conceptualization, Project administration, Writing - review & editing, Supervision, Methodology. Fatemeh Koohestani: Validation, Resources, Writing - original draft. Declaration of competing interest The authors declare no conflict of interest.
The authors appreciate the partial support of Iran Polymer and Petrochemical Institute.
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