- Email: [email protected]
Facile rare-earth triflate-catalyzed esterification of cellulose by carboxylic anhydrides under solvent-free conditions
Polymer 184 (2019) 121916
Contents lists available at ScienceDirect
Polymer journal homepage: http://www.elsevier.com/locate/polymer
Short communication
Facile rare-earth triflate-catalyzed esterification of cellulose by carboxylic anhydrides under solvent-free conditions Suzuka Takeuchi, Akinori Takasu * Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Esterification Carbohydrate Cellulose Scandium triflate [Sc(OTf)3] Acetic anhydride
We report here the facile rare-earth triflate-catalyzed esterification of cellulose by carboxylic anhydrides, including acetic anhydride, at room temperature under solvent-free conditions. As a model, D-glucose was per aceylated using only equimolar amounts of acetic anhydrides against hydroxyl groups to give the expected 1,2,3,4,6-pentaacetyl α-D-glucopyranoside (crude yield 98%, 68% yield after recrystallization) under solvent-free condition. In the case of powdery microcrystalline cellulose, acetylation proceeded both in the absence of pyr idine and solvent to give peracetylated cellulose, even in the bulk. From 1H NMR measurements, the degree of substitution (DS) was 3.0 (100% acetylation) and we concluded that Sc(OTf)3-catalyzed solvent-free esterifica tion of cellulose had taken place. The number-average molecular weight (Mn), calculated using the 1H NMR intensity ratio based on the anomeric proton in the reducing terminus, was 1.7 � 104 (repeating unit of pyranose unit: 56).
1. Introduction Development of biomass-based functional materials is one of the most urgent subjects in the field of polymer chemistry from the view point of environmentally-benign technology [1]. Ten percent of the world’s pulp production is transformed into cellulose derivatives (4.4 million tons per year) [2]. Among chemical modifications of cellulose, the market for cellulose organic esters is the biggest (815 thousand tons) [3]. The cellulose esters of aliphatic C2 to C4 carboxylic acids, including cellulose acetate, are commercially used in in many practical industrial fields including coatings, films, textiles, and cigarette filter products [4]. However, the esterification process is still uneconomical because of the low reactivity of cellulose hydroxyl groups and a lack of a good solvent for cellulose [4]. Such esterifications, including acetylation, of carbohydrates are usually performed using an excess amount of carboxylic acid anhydride with pyridine as an acid trapper, ever since Behrend and Roth reported the acetylation of α- and β-glucoses in 1904 and 1907, respectively [5,6]. On the other hand, the powerful reagent, carboxylic acid chloride, is also used for the esterification, but its use suffers from the degradation of the cellulose backbone catalyzed by HCl released during the esterification [4]. In 1996, Ishihara et al. reported the scandium triflate [Sc(OTf)3]- catalyzed effective esterification of alcohols with some carboxylic
anhydrides, in which the esterification proceeded quantitatively even at 78 � C [7]. To the best of our knowledge, there are only a few reports dealing with esterifications, including the acetylation of monomeric sugars, that employs an excess amount of carboxylic anhydride (2–6 eq.) using indium triflate [8] and cerium triflate [9] as the catalysts. How ever, the facile esterification of cellulose by carboxylic anhydrides under solvent-free conditions is still a challenging subject, because there are no existing reports dealing with facile esterification of cellulose. In this study, we describe the Sc(OTf)3-catalyzed acetylation of powdery microcrystalline celluloses in the bulk at ambient temperature (Scheme 1). The degree of substitution and the structures were evaluated by 1H NMR and IR spectroscopies. 2. Experimental Materials. Cellulose (microcrystalline) was obtained from Sigma Aldrich and Sc(OTf)3 and propionic anhydride were purchased from Tokyo Kasei Co. (Tokyo, Japan). Acetic anhydride and D-glucose were obtained from Wako Pure Chemical Co., Inc. (Osaka, Japan) and any other solvents were used without further purification. Cellulose fiber (cotton) was provided by KURABO INDUSTRIES Ltd. (Osaka, Japan). Acetylation of Glucose. In a typical acetylation, powdered Dglucose (0.90 g, 5 mmol) and Sc(OTf)3 (24 mg, 0.05 mmol) are
* Corresponding author. E-mail address: [email protected] (A. Takasu). https://doi.org/10.1016/j.polymer.2019.121916 Received 2 July 2019; Received in revised form 24 September 2019; Accepted 17 October 2019 Available online 21 October 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
S. Takeuchi and A. Takasu
Polymer 184 (2019) 121916
Scheme 1. Schematic of solvent-free esterification including acetylation of cellulose using carbpxylic anhydrides (2 eq. relative to hydroxy groups) using Sc(OTf)3 as the catalyst (1 mol% relative to pyranose units) in the bulk.
Fig. 3. Changes of degree of acetylation (%) as a function of concentration of Sc(OTf)3 catalyst (mol% to pyranose unit) (2.0 eq. acetic anhydride, 25 � C for 5h).
Fig. 1. 1H NMR spectrum of 1,2,3,4,6-pentaacetyl α-D-glucopyranoside (in CDCl3, 400 MHz).
Fig. 4. Photographs of cotton fiber before (left) and after (right) acetylation.
Scheme 2. Schematic of solvent-free acetylation of glucose powder using acetic anhydride (1 eq. to hydroxy groups) using Sc(OTf)3 as the catalyst (1mol % to pyranose units) in bulk.
Fig. 5. 1H NMR spectrum of cellulose acetate (upper) and cellulose acetate fiber (cotton) (bottom) (in CDCl3, 400 MHz).
Acetylation of Cellulose (microcrystalline). In a typical acetyla tion, microcrystalline cellulose (0.81 g, 5 mmol pyranose unit) and Sc (OTf)3 (24 mg, 0.05 mmol) were dissolved in acetic anhydride (2.8 mL, 30 mmol, 2 eq. to hydroxyl groups) in an eggplant flask. The solution was stirred at 25 � C for 5 h and the reaction mixture was poured into cold water to precipitate the product. The deposited material was dried under reduced pressure (yield 70%). For cellulose acetate 1 (Fig. 2). Acetylation of Cotton (cellulose fiber). In a typical acetylation, commercially available cotton (cellulose fiber) (0.81 g, 5 mmol pyranose
Fig. 2. 1H NMR spectrum of 2,3,6-triacetyl cellulose (in CDCl3, 400 MHz).
immersed in acetic anhydride (2.4 mL 25 mmol, 1 eq. to hydroxyl groups) in an eggplant flask. The suspension was immediately turned to be a solution and the solution was stirred at 25 � C for 5 h, and the re action mixture poured into cold water to cause precipitation. The deposited material (crude 98% yield) was purified by recrystallization from hot ethanol (see also Fig. 1) (see Scheme 2). 2
S. Takeuchi and A. Takasu
Polymer 184 (2019) 121916
unit), Sc(OTf)3 (24 mg, 0.05 mmol) were immersed in acetic anhydride (5.6 mL, 60 mmol, 4 eq. to hydroxyl groups) in an eggplant flask. The solution was stirred at 25 � C for 5 h and the reaction mixture was poured into cold water to cause precipitation. The deposited material was recovered quantitatively and dried under reduced pressure. For cellulose acetate fiber (cotton). 1H NMR (soluble part, 400 MHz, CDCl3, δ, ppm): 1.94, 2.01, 2.13 (3s, -OCH3, 9H), 3.51 (brs, H-5, 1H), 3.71 (brs, H-4, 1H), 4.06 (brs, H-6a, 1H), 4.34 (brs, H-6b, 1H), 4.41 (brs, H-1, 1H), 4.79 (brs, H-2, 1H), 5.07 (brs, H-3, 1H) (see also Fig. 5). IR (KBr disk, cm 1): 2943 – O(ester)], 1432 (δC-H), 1240 and 1032 [νC-O-C(ester)]. (νC-H), 1755 [νC– (see also Fig. S1). 3. Measurements 1
H NMR (400 MHz) spectra were acquired at 27 � C using a Bruker Analytik DPX400 spectrometer. The number average molecular weight (Mn) and the polydispersity index (Mw/Mn) of the polymers were determined by size exclusion chromatography (SEC) using a JASCO PU4185 pump system, a refractive index (RI) detector (JASCO RI-4035), and a Shodex GPC HK-404L column [eluent, chloroform (CHCl3); flow rate, 0.35 mL/min; temperature, 40 � C; Tosoh Corp.], and poly(styrene) standards. Fourier transform infrared spectroscopy (FT-IR) measure ments were conducted to monitor the acetylation. The measurements were also performed at room temperature using a FT/IR 430 spec trometer combined with an ATR attachment (JASCO Co.). Scanning electron microscopic (SEM) observation was performed using JEOL JSM-6010LA (Fig. S2).
Fig. 6. FT-IR spectra of cotton (cellulose fiber) (above) acetylated cotton using Sc(OTf)3 catalyst (middle) and acetylated cotton using pyridine catalyst (bot tom) (in KBr disk).
the reactivity. 4.3. Acetylation of cotton (cellulose fiber) Finally, we tried the acetylation of cotton (cellulose fiber). Using 2 equivalents acetic anhydride, the acetylation proceeded even in a het erogeneous state and after acetylation for 5 h (at 25 � C) the acetylated fiber was obtained (Fig. 4). The acetylation occurred only on the fiber surface and the surface product was partially soluble in CDCl3 (soluble part: 5 wt%). The 1H NMR of the soluble part showed that acetylation had proceeded similarly to the microcrystalline cellulose (Fig. 5). In the IR spectrum, a new signal at 1755 cm 1 appeared, indicating the for mation of an ester linkage (Fig. 6, middle). Compared with a control experiment in which we acetylated cotton using pyridine as the catalyst (Fig. 6, bottom), this IR vibration signal with the Sc(OTf)3 present was stronger, supporting the idea that the triflate indeed catalyzed effective esterification of cotton fibers. In the ATR measurement, the relative intensity of the ester carbonyl vibration to hydroxyl stretching band became much stronger, which indicated that the acetylation occurred on the fiber surface (Fig. S1). In the SEM observations, the fiber structure remained even after acetylation (Fig. S2). These results showed that we could fabricate acetylated cotton even in the heterogeneous state.
4. Results and discussion 4.1. Acetylation of Glucose As a model reaction, powdered D-glucose was acetylated using equimolar acetic anhydride (relative to hydroxyl groups) using 1 mol % Sc(OTf)3 at 25 � C for 5 h (Scheme 2). As the reaction proceeded the mixture turned to a viscous syrup. The crude product (98% yield) ob tained after pouring the syrup into cold water was purified by recrys tallization from hot ethanol to give 1,2,3,4,6-pentaacetyl α-Dglucopyranoside (68% yield). The 1H NMR data indicated the expected structure, in which the α-anomer was diastereo-selectively obtained (Fig. 1). 4.2. Acetylation of cellulose (microcrystalline) Subsequently, acetylation of commercially available powder cellu lose was performed under similar conditions (Scheme 1). Using 2 equivalents acetic anhydride (to hydroxyl groups), the acetylation pro ceeded even in a heterogeneous state. After acetylation for 5 h (at 25 � C) homogeneous viscous materials were obtained. After reprecipitation into water, the degree of acetylation, i.e. the degree of substitution of the hydroxyl group (DP) estimated by 1H NMR in CDCl3 was 3.0, which means 100% of acetylation. The viscous solid was soluble in CHCl3 and partially soluble in tetrahydrofuran. The number-average molecular weight (Mn), calculated using a 1H NMR intensity ratio based on the anomeric doublet proton in the reducing terminus (α-anomer at 6.24 ppm, 3.4 Hz and β-anomer at 5.65 ppm, 7.6 Hz), was of 1.7 � 104 (from SEC measurement using CHCl3, Mn ¼ 1.72 � 104, Mw/Mn ¼ 2.26). In order to access the relationship between catalyst amount and degrees of substitution, we demonstrated the acetylations using different amounts of Sc(OTf)3. As shown in Fig. 3, when we used over 0.8 mol% of the catalyst, the acetylation seems to be complete. When we used 2eq. of propionic anhydride instead of acetic anhy dride for the esterification, the expected cellulose ester 2 was obtained (74% yield, see also Scheme 1). However, the degree of substitution (DS) is 1.7 even after an extended reaction time of 24h (Fig. S3). The results indicated that the alkyl chain length of the acid anhydrides influenced
5. Conclusions We have developed a new facile method for the esterification of cellulose materials. The esterification occurred even at room tempera ture and a quite high degree of acetylation was attained even in a bulk condition at room temperature. In the best of our knowledge, this is the first report dealing with facile esterification in solvent-free (bulk) con dition. This procedure could be applied to cellulose fiber (cotton) and we could sucessfully fabricated acetylated cotton fibers. These fundamental results provide a new route to synthesizing a series of cellulose de rivatives using carboxylic anhydrides without the use of pyridine. Author contributions The manuscript was written through contributions of all authors./All authors have given approval to the final version of the manuscript./ zThese authors contributed equally. (match statement to author names with a symbol)
3
S. Takeuchi and A. Takasu
Polymer 184 (2019) 121916
Acknowledgements [3]
This work was funded by the Ministry of Education, Science, and Culture of Japan (Grant-in-Aid for Development Scientific Research, no. 22750104, 15K04872, and 18K19112).
[4] [5]
Appendix A. Supplementary data
[6]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121916.
[7]
References
[8]
[1] D.K. Schneiderman, M.A. Hillmyer, 50th anniversary perspective: there is a great future in sustainable polymers, Macromolecules 50 (10) (2007) 3733–3749. [2] A. Reveley, Review of cellulose derivatives and their industrial applications, in: J. F. Kennedy, G.O. Phillips, D.J. Wedlock, P.A. Williams (Eds.), Cellulose and its
[9]
4
Derivatives: Chemistry, Biochemistry and Applications, Ellis Norwood, New York, 1985, pp. 211–225. J. Engelhardt, Sources, industrial derivatives and commercial application of cellulose, Carbohydr Eur 12 (1995) 5–14. C. Vaca-Garcia, S. Thiebaud, M.E. Borredon, G. Gozzelino, Cellulose esterification with fatty acids and acetic anhydride in lithium chloride N,N-dimethylacetamide medium, J. Am. Oil Chem. Soc. 75 (2) (1998) 315–319. R. Behrend, P. Roth, Ueber die Birotation der Glucose, Justus Liebigs Ann. Chem. 331 (3) (1904) 359–382. R. Behrend, Ueber glucose, sowie deren Phenylhydrazone und oxime, Justus Liebigs Ann. Chem. 353 (1) (1907) 106–122. K. Ishihara, M. Kubota, H. Kurihara, H. Yamamoto, Scandium trifluoromethanesulfonate as an extremely active Lewis acid catalyst in acylation of alcohols with acid anhydrides and mixed anhydrides, J. Org. Chem. 61 (14) (1996) 4560–4567. N.P. Bizier, S.R. Atkins, L.C. Helland, S.F. Colvin, J.R. Twitchell, M.J. Cloninger, Indium triflate catalyzed peracetylation of carbohydrates, Carbohydr. Res. 343 (10–11) (2008) 1814–1818. G. Bartoli, R. Dalpozzo, A. De Nino, L. Maiuolo, M. Nardi, A. Procopio, A. Tagarelli, Per-O-acetylation of sugars catalyzed by Ce(OTf)3, Green Chem. 6 (4) (2004) 191–192.
Contents lists available at ScienceDirect
Polymer journal homepage: http://www.elsevier.com/locate/polymer
Short communication
Facile rare-earth triflate-catalyzed esterification of cellulose by carboxylic anhydrides under solvent-free conditions Suzuka Takeuchi, Akinori Takasu * Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Esterification Carbohydrate Cellulose Scandium triflate [Sc(OTf)3] Acetic anhydride
We report here the facile rare-earth triflate-catalyzed esterification of cellulose by carboxylic anhydrides, including acetic anhydride, at room temperature under solvent-free conditions. As a model, D-glucose was per aceylated using only equimolar amounts of acetic anhydrides against hydroxyl groups to give the expected 1,2,3,4,6-pentaacetyl α-D-glucopyranoside (crude yield 98%, 68% yield after recrystallization) under solvent-free condition. In the case of powdery microcrystalline cellulose, acetylation proceeded both in the absence of pyr idine and solvent to give peracetylated cellulose, even in the bulk. From 1H NMR measurements, the degree of substitution (DS) was 3.0 (100% acetylation) and we concluded that Sc(OTf)3-catalyzed solvent-free esterifica tion of cellulose had taken place. The number-average molecular weight (Mn), calculated using the 1H NMR intensity ratio based on the anomeric proton in the reducing terminus, was 1.7 � 104 (repeating unit of pyranose unit: 56).
1. Introduction Development of biomass-based functional materials is one of the most urgent subjects in the field of polymer chemistry from the view point of environmentally-benign technology [1]. Ten percent of the world’s pulp production is transformed into cellulose derivatives (4.4 million tons per year) [2]. Among chemical modifications of cellulose, the market for cellulose organic esters is the biggest (815 thousand tons) [3]. The cellulose esters of aliphatic C2 to C4 carboxylic acids, including cellulose acetate, are commercially used in in many practical industrial fields including coatings, films, textiles, and cigarette filter products [4]. However, the esterification process is still uneconomical because of the low reactivity of cellulose hydroxyl groups and a lack of a good solvent for cellulose [4]. Such esterifications, including acetylation, of carbohydrates are usually performed using an excess amount of carboxylic acid anhydride with pyridine as an acid trapper, ever since Behrend and Roth reported the acetylation of α- and β-glucoses in 1904 and 1907, respectively [5,6]. On the other hand, the powerful reagent, carboxylic acid chloride, is also used for the esterification, but its use suffers from the degradation of the cellulose backbone catalyzed by HCl released during the esterification [4]. In 1996, Ishihara et al. reported the scandium triflate [Sc(OTf)3]- catalyzed effective esterification of alcohols with some carboxylic
anhydrides, in which the esterification proceeded quantitatively even at 78 � C [7]. To the best of our knowledge, there are only a few reports dealing with esterifications, including the acetylation of monomeric sugars, that employs an excess amount of carboxylic anhydride (2–6 eq.) using indium triflate [8] and cerium triflate [9] as the catalysts. How ever, the facile esterification of cellulose by carboxylic anhydrides under solvent-free conditions is still a challenging subject, because there are no existing reports dealing with facile esterification of cellulose. In this study, we describe the Sc(OTf)3-catalyzed acetylation of powdery microcrystalline celluloses in the bulk at ambient temperature (Scheme 1). The degree of substitution and the structures were evaluated by 1H NMR and IR spectroscopies. 2. Experimental Materials. Cellulose (microcrystalline) was obtained from Sigma Aldrich and Sc(OTf)3 and propionic anhydride were purchased from Tokyo Kasei Co. (Tokyo, Japan). Acetic anhydride and D-glucose were obtained from Wako Pure Chemical Co., Inc. (Osaka, Japan) and any other solvents were used without further purification. Cellulose fiber (cotton) was provided by KURABO INDUSTRIES Ltd. (Osaka, Japan). Acetylation of Glucose. In a typical acetylation, powdered Dglucose (0.90 g, 5 mmol) and Sc(OTf)3 (24 mg, 0.05 mmol) are
* Corresponding author. E-mail address: [email protected] (A. Takasu). https://doi.org/10.1016/j.polymer.2019.121916 Received 2 July 2019; Received in revised form 24 September 2019; Accepted 17 October 2019 Available online 21 October 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
S. Takeuchi and A. Takasu
Polymer 184 (2019) 121916
Scheme 1. Schematic of solvent-free esterification including acetylation of cellulose using carbpxylic anhydrides (2 eq. relative to hydroxy groups) using Sc(OTf)3 as the catalyst (1 mol% relative to pyranose units) in the bulk.
Fig. 3. Changes of degree of acetylation (%) as a function of concentration of Sc(OTf)3 catalyst (mol% to pyranose unit) (2.0 eq. acetic anhydride, 25 � C for 5h).
Fig. 1. 1H NMR spectrum of 1,2,3,4,6-pentaacetyl α-D-glucopyranoside (in CDCl3, 400 MHz).
Fig. 4. Photographs of cotton fiber before (left) and after (right) acetylation.
Scheme 2. Schematic of solvent-free acetylation of glucose powder using acetic anhydride (1 eq. to hydroxy groups) using Sc(OTf)3 as the catalyst (1mol % to pyranose units) in bulk.
Fig. 5. 1H NMR spectrum of cellulose acetate (upper) and cellulose acetate fiber (cotton) (bottom) (in CDCl3, 400 MHz).
Acetylation of Cellulose (microcrystalline). In a typical acetyla tion, microcrystalline cellulose (0.81 g, 5 mmol pyranose unit) and Sc (OTf)3 (24 mg, 0.05 mmol) were dissolved in acetic anhydride (2.8 mL, 30 mmol, 2 eq. to hydroxyl groups) in an eggplant flask. The solution was stirred at 25 � C for 5 h and the reaction mixture was poured into cold water to precipitate the product. The deposited material was dried under reduced pressure (yield 70%). For cellulose acetate 1 (Fig. 2). Acetylation of Cotton (cellulose fiber). In a typical acetylation, commercially available cotton (cellulose fiber) (0.81 g, 5 mmol pyranose
Fig. 2. 1H NMR spectrum of 2,3,6-triacetyl cellulose (in CDCl3, 400 MHz).
immersed in acetic anhydride (2.4 mL 25 mmol, 1 eq. to hydroxyl groups) in an eggplant flask. The suspension was immediately turned to be a solution and the solution was stirred at 25 � C for 5 h, and the re action mixture poured into cold water to cause precipitation. The deposited material (crude 98% yield) was purified by recrystallization from hot ethanol (see also Fig. 1) (see Scheme 2). 2
S. Takeuchi and A. Takasu
Polymer 184 (2019) 121916
unit), Sc(OTf)3 (24 mg, 0.05 mmol) were immersed in acetic anhydride (5.6 mL, 60 mmol, 4 eq. to hydroxyl groups) in an eggplant flask. The solution was stirred at 25 � C for 5 h and the reaction mixture was poured into cold water to cause precipitation. The deposited material was recovered quantitatively and dried under reduced pressure. For cellulose acetate fiber (cotton). 1H NMR (soluble part, 400 MHz, CDCl3, δ, ppm): 1.94, 2.01, 2.13 (3s, -OCH3, 9H), 3.51 (brs, H-5, 1H), 3.71 (brs, H-4, 1H), 4.06 (brs, H-6a, 1H), 4.34 (brs, H-6b, 1H), 4.41 (brs, H-1, 1H), 4.79 (brs, H-2, 1H), 5.07 (brs, H-3, 1H) (see also Fig. 5). IR (KBr disk, cm 1): 2943 – O(ester)], 1432 (δC-H), 1240 and 1032 [νC-O-C(ester)]. (νC-H), 1755 [νC– (see also Fig. S1). 3. Measurements 1
H NMR (400 MHz) spectra were acquired at 27 � C using a Bruker Analytik DPX400 spectrometer. The number average molecular weight (Mn) and the polydispersity index (Mw/Mn) of the polymers were determined by size exclusion chromatography (SEC) using a JASCO PU4185 pump system, a refractive index (RI) detector (JASCO RI-4035), and a Shodex GPC HK-404L column [eluent, chloroform (CHCl3); flow rate, 0.35 mL/min; temperature, 40 � C; Tosoh Corp.], and poly(styrene) standards. Fourier transform infrared spectroscopy (FT-IR) measure ments were conducted to monitor the acetylation. The measurements were also performed at room temperature using a FT/IR 430 spec trometer combined with an ATR attachment (JASCO Co.). Scanning electron microscopic (SEM) observation was performed using JEOL JSM-6010LA (Fig. S2).
Fig. 6. FT-IR spectra of cotton (cellulose fiber) (above) acetylated cotton using Sc(OTf)3 catalyst (middle) and acetylated cotton using pyridine catalyst (bot tom) (in KBr disk).
the reactivity. 4.3. Acetylation of cotton (cellulose fiber) Finally, we tried the acetylation of cotton (cellulose fiber). Using 2 equivalents acetic anhydride, the acetylation proceeded even in a het erogeneous state and after acetylation for 5 h (at 25 � C) the acetylated fiber was obtained (Fig. 4). The acetylation occurred only on the fiber surface and the surface product was partially soluble in CDCl3 (soluble part: 5 wt%). The 1H NMR of the soluble part showed that acetylation had proceeded similarly to the microcrystalline cellulose (Fig. 5). In the IR spectrum, a new signal at 1755 cm 1 appeared, indicating the for mation of an ester linkage (Fig. 6, middle). Compared with a control experiment in which we acetylated cotton using pyridine as the catalyst (Fig. 6, bottom), this IR vibration signal with the Sc(OTf)3 present was stronger, supporting the idea that the triflate indeed catalyzed effective esterification of cotton fibers. In the ATR measurement, the relative intensity of the ester carbonyl vibration to hydroxyl stretching band became much stronger, which indicated that the acetylation occurred on the fiber surface (Fig. S1). In the SEM observations, the fiber structure remained even after acetylation (Fig. S2). These results showed that we could fabricate acetylated cotton even in the heterogeneous state.
4. Results and discussion 4.1. Acetylation of Glucose As a model reaction, powdered D-glucose was acetylated using equimolar acetic anhydride (relative to hydroxyl groups) using 1 mol % Sc(OTf)3 at 25 � C for 5 h (Scheme 2). As the reaction proceeded the mixture turned to a viscous syrup. The crude product (98% yield) ob tained after pouring the syrup into cold water was purified by recrys tallization from hot ethanol to give 1,2,3,4,6-pentaacetyl α-Dglucopyranoside (68% yield). The 1H NMR data indicated the expected structure, in which the α-anomer was diastereo-selectively obtained (Fig. 1). 4.2. Acetylation of cellulose (microcrystalline) Subsequently, acetylation of commercially available powder cellu lose was performed under similar conditions (Scheme 1). Using 2 equivalents acetic anhydride (to hydroxyl groups), the acetylation pro ceeded even in a heterogeneous state. After acetylation for 5 h (at 25 � C) homogeneous viscous materials were obtained. After reprecipitation into water, the degree of acetylation, i.e. the degree of substitution of the hydroxyl group (DP) estimated by 1H NMR in CDCl3 was 3.0, which means 100% of acetylation. The viscous solid was soluble in CHCl3 and partially soluble in tetrahydrofuran. The number-average molecular weight (Mn), calculated using a 1H NMR intensity ratio based on the anomeric doublet proton in the reducing terminus (α-anomer at 6.24 ppm, 3.4 Hz and β-anomer at 5.65 ppm, 7.6 Hz), was of 1.7 � 104 (from SEC measurement using CHCl3, Mn ¼ 1.72 � 104, Mw/Mn ¼ 2.26). In order to access the relationship between catalyst amount and degrees of substitution, we demonstrated the acetylations using different amounts of Sc(OTf)3. As shown in Fig. 3, when we used over 0.8 mol% of the catalyst, the acetylation seems to be complete. When we used 2eq. of propionic anhydride instead of acetic anhy dride for the esterification, the expected cellulose ester 2 was obtained (74% yield, see also Scheme 1). However, the degree of substitution (DS) is 1.7 even after an extended reaction time of 24h (Fig. S3). The results indicated that the alkyl chain length of the acid anhydrides influenced
5. Conclusions We have developed a new facile method for the esterification of cellulose materials. The esterification occurred even at room tempera ture and a quite high degree of acetylation was attained even in a bulk condition at room temperature. In the best of our knowledge, this is the first report dealing with facile esterification in solvent-free (bulk) con dition. This procedure could be applied to cellulose fiber (cotton) and we could sucessfully fabricated acetylated cotton fibers. These fundamental results provide a new route to synthesizing a series of cellulose de rivatives using carboxylic anhydrides without the use of pyridine. Author contributions The manuscript was written through contributions of all authors./All authors have given approval to the final version of the manuscript./ zThese authors contributed equally. (match statement to author names with a symbol)
3
S. Takeuchi and A. Takasu
Polymer 184 (2019) 121916
Acknowledgements [3]
This work was funded by the Ministry of Education, Science, and Culture of Japan (Grant-in-Aid for Development Scientific Research, no. 22750104, 15K04872, and 18K19112).
[4] [5]
Appendix A. Supplementary data
[6]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121916.
[7]
References
[8]
[1] D.K. Schneiderman, M.A. Hillmyer, 50th anniversary perspective: there is a great future in sustainable polymers, Macromolecules 50 (10) (2007) 3733–3749. [2] A. Reveley, Review of cellulose derivatives and their industrial applications, in: J. F. Kennedy, G.O. Phillips, D.J. Wedlock, P.A. Williams (Eds.), Cellulose and its
[9]
4
Derivatives: Chemistry, Biochemistry and Applications, Ellis Norwood, New York, 1985, pp. 211–225. J. Engelhardt, Sources, industrial derivatives and commercial application of cellulose, Carbohydr Eur 12 (1995) 5–14. C. Vaca-Garcia, S. Thiebaud, M.E. Borredon, G. Gozzelino, Cellulose esterification with fatty acids and acetic anhydride in lithium chloride N,N-dimethylacetamide medium, J. Am. Oil Chem. Soc. 75 (2) (1998) 315–319. R. Behrend, P. Roth, Ueber die Birotation der Glucose, Justus Liebigs Ann. Chem. 331 (3) (1904) 359–382. R. Behrend, Ueber glucose, sowie deren Phenylhydrazone und oxime, Justus Liebigs Ann. Chem. 353 (1) (1907) 106–122. K. Ishihara, M. Kubota, H. Kurihara, H. Yamamoto, Scandium trifluoromethanesulfonate as an extremely active Lewis acid catalyst in acylation of alcohols with acid anhydrides and mixed anhydrides, J. Org. Chem. 61 (14) (1996) 4560–4567. N.P. Bizier, S.R. Atkins, L.C. Helland, S.F. Colvin, J.R. Twitchell, M.J. Cloninger, Indium triflate catalyzed peracetylation of carbohydrates, Carbohydr. Res. 343 (10–11) (2008) 1814–1818. G. Bartoli, R. Dalpozzo, A. De Nino, L. Maiuolo, M. Nardi, A. Procopio, A. Tagarelli, Per-O-acetylation of sugars catalyzed by Ce(OTf)3, Green Chem. 6 (4) (2004) 191–192.