Homogeneous Transesterification of Sugar Cane Bagasse toward

Research Article pubs.acs.org/journal/ascecg

Homogeneous Transesterification of Sugar Cane Bagasse toward Sustainable Plastics Ming-Jie Chen,†,‡ Rui-Min Li,† Xue-Qin Zhang,‡ Jing Feng,† Jin Feng,† Chuan-Fu Liu,‡ and Qing-Shan Shi*,†

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†

State Key Laboratory of Applied Microbiology Southern China, Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, Guangdong Institute of Microbiology, Building of 58, No. 100 Central Xianlie Road Guangzhou 510070, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Homogeneous esterification of lignocellulosic agro-forestry wastes is an efficient approach to transform the biomass into value-added biobased materials. However, the application of fatty acid anhydrides or acyl chlorides as esterified reagents would yield equivalent acids which lead to significant decomposition of lignocellulosic biomass. The present study proposed an alternative chemical approach to prepare bioplastics from lignicellulosic biomass. Sugar cane bagasse esters were prepared by transesterification with vinyl esters using 1-ethyl-3-methylimidazolium acetate as duel solvent and activator. Solution-stated NMR studies suggested that the polysacchirides were highly reactive during the homogeneous transesterification. The chemical mechanism of the transesterification was demonstrated to be a carbene mechanism, which subsequently transferred into the intermediate of 2-alkanoyl-dialkylimidazolium. The as-prepared sugar cane bagasse esters were found to be solvable in common solvents. Bioplastic films were prepared by solution casting. Scanning electron microscopy and tensile testing studies showed that the sugar cane bagasse ester films were comparable to those of cellulose fatty esters. KEYWORDS: Biomass, Ionic liquid, Bioplastics, Biobased materials, Chemical modification

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INTRODUCTION Lignocellulose has been used as an engineering material for various applications since ancient times. However, it suffers from several disadvantages, including dimensional changes under changing atmospheric conditions and being susceptible to decay, incompatible to thermoplastics, and impossible to melt or dissolve.1 Regarding the fact that lignocelluloses are the most abundant and renewable resources on the earth, there are increasing interests in lignocellulose modification to enhance the properties of the material.2 Lignocellulose, such as sugar cane bagasse (SCB), consisting of cellulose, hemicellulose, and lignin, is a natural composite with complex physical structure compared to those of constructing materials, which makes it impossible to dissolve lignocellulose in traditional solvent systems.3 During the past four decades, the chemical modification of lignocellulose was performed in heterogeneous reaction media.4,5 The heterogeneous chemical modification of lignocellulose suffers from low efficiency;4 the reaction is mainly located within the highly lignified cell corner and the compound middle lamella, and it might not progress very deep into the cell wall.6 Cellulose © 2016 American Chemical Society

fractions remain unreactive, while lignin fractions achieve high degree of substitution (DS) upon the heterogeneous chemical modification.7 Ionic liquids known as green solvents have been developed for the biorefinery of woody biomass8,9 and biopolymers processing.10,11 They are addressed to show great potential for application on a commercial scale homogeneous chemical modification of cellulose, regarding their recyclability and high dissolution power.12 Homogeneous chemical modification of lignocellulose has been carried out using ionic liquids as reaction mediums.13 Highly substituted lignocellulose derivatives have been prepared using acid anhydride and acyl chloride as esterification reagents.13 However, the esterification between lignocellulose and acyl chlorides or anhydrides is a single-site reaction, yielding HCl or carboxylic acid as byproducts.1 The production of acids in the homogeneous esterification of lignocellulose would result in significant degradation of the Received: July 24, 2016 Revised: November 17, 2016 Published: November 19, 2016 360

DOI: 10.1021/acssuschemeng.6b01735 ACS Sustainable Chem. Eng. 2017, 5, 360−366

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ACS Sustainable Chemistry & Engineering lignocellulose fractions.4,14 A novel chemical pathway needs to be developed for the homogeneous chemical modification of lignocellulose to avoid fraction degradation. The homogeneous chemical modification of lignocellulose with cyclic anhydride is a tailored approach for designing flexible film materials from SCB.15 However, the processing results in materials bearing carboxyl groups make it hydrophilic,15 while hydrophobic materials from lignocellulose are under development. Acylation of lignocellulose with vinyl esters is an alternative esterification approach that does not liberate an acid as byproduct.16 Wood was successfully acetylated by the transesterification with vinyl acetate (VA) using potassium carbonate as catalyst.17−19 Relatively high weight percent gain (WPG) was obtained, as compared with those of the esterification between wood and acetic anhydride. 20 However, the previous researchers were limited to the heterogeneous transesterification using dimethylformamide (DMF) as reaction medium. In a recent study, homogeneous alkoxycarbonylation of cellulose was achieved in 1-ethyl-3-methylimidazolium acetate (EmimAc) by applying dialkycarbonates as reagents without any catalyst.21 Based on the novel synthesis approach, EmimAc was further described as dual solvent and activating reagent for the transesterification reaction of cellulose with isopropenyl acetate.22 An N-heterocyclic carbene reaction mechanism was suggested, though no detailed evidence and no precise chemical reaction path were provided.22 No hydrolysis of cellulose was demonstrated by gel permeation chromatography studies.21 However, the efforts are developed and limited on cellulose,23 which is obtained from lignocellulose with delignin (also known as pulping) processing, and subsequently with dehemicellulose processing. No such modification of lignocellulose is carried out, since the case of lignocellulose is more complex than that of cellulose.3 However, lignocellulose is much cheaper than cellulose. It would be of great interest to process lignocellulose into materials analogous to cellulose esters or synthetic plastics. According to previous studies, one would assume that EmimAc could be used as a duel solvent and activator for the homogeneous transesterification of lignocellulose with vinyl esters. Since vinyl esters have chemical structure analogous to isopropenyl acetate and dialkycarbonates, thermoplasticization of lignocellulose is expected, because of introduction of nonhydrogen-bonding alkanoyl side chains by homogeneous transesterification. In this study, we esterified SCB to different WPG in EmimAc with three different vinyl esters, namely vinyl acetate (VA), vinyl benzoate (VB), and vinyl laurate (VL). Fourier transform infrared spectoscopy (FT-IR) and solution state two-dimensional nuclear magnetic resonance (2D NMR) studies confirmed the transesterification between SCB and vinyl esters. The carbene mechanism was confirmed by NMR studies. The SCB esters were transformed into film materials to address potential application of the resulting materials as bioplastics.

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Cellulose (DP 900), VA, VB, and VL were purchased from SigmaAldrich (Shanghai, China). DMF, K2CO3, Na2CO3, CH3COOK, pyridine, and triethylamine were purchased from ALADDIN-E.COM (Shanghai, China). CHCl3 was purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Dissolution of SCB in EmimAc. SCB meal (0.5 g) was dispersed into EmimAc (19.5 g) at room temperature. The mixture was placed in a 110 °C oil-bath under N2 atmosphere for 5 h to guarantee complete dissolution. Homogeneous transesterification of SCB in EmimAc. Vinyl ester was added to the SCB/EmimAc solution. The amounts of the added vinyl esters are shown in the Supporting Information (Table S1−S3). The mixture was stirred at 70 °C under N2 atmosphere for 2 h. Isolation was carried out by precipitation of the product into 300 mL of isopropanol. The SCB esters were obtained after filtration, thoroughly washed with isopropanol (5 × 100 mL), and subsequently vacuum-dried at 50 °C for 24 h. Two repetitions of the transesterification were carried out. Transesterification performed in BmimCl (or AmimCl). SCB meal (0.5 g) was mixed with BmimCl or AmimCl (19.5 g) at room temperature. The mixture was placed in a 110 °C oil-bath under N2 atmosphere for 5 h for complete dissolution of SCB. After the dissolution, VA and catalyst, if any (K2CO3, Na2CO3, or CH3COOK), were added to the solution. The mixture was allowed to react under N2 atmosphere. The reaction factors, including amounts of VA, temperature, and reaction time, are listed in the Supporting Information (Table S4−S8). Isolation was carried out by precipitation of the product into 300 mL of isopropanol. The precipitant was collected, thoroughly washed with isopropanol (5 × 100 mL), and subsequently vacuum-dried at 50 °C for 24 h. Transesterification performed in VA. SCB meal (1 g) was mixed with VA (8 mL) at room temperature. EmimAc, if any, was added to the mixture. The reaction was carried out in an oil-bath for 2 h. The amount of EmimAc and the reaction temperatures are listed in the Supporting Information (Table S9). Isolation was carried out by filtration. The precipitant was thoroughly washed with isopropanol (7 × 100 mL), and subsequently vacuum-dried at 50 °C for 24 h. Two repetitions of the experiments were carried out. Organic bases activated transesterification in BmimCl. SCB meal (0.5 g) was dissolved in BmimCl (19.5 g) at 110 °C within 5 h under N2 atmosphere. Pyridine or triethylamine (8 mL) was added to the SCB solution in 70 °C. The solution was stirred for 30 min at 70 °C. Then, vinyl ester (8 mL) was added. The transesterification was performed at 70 °C in 2 h. Isolation was carried out by precipitation in 300 mL of isopropanol. The precipitant was collected by filtration, thoroughly washed with isopropanol (5 × 100 mL), and subsequently vacuum-dried at 50 °C for 24 h. Syntheses of cellulose esters in EmimAc. Cellulose (0.5 g) was dissolved in EmimAc (19.5 g) at 130 °C within 1 h. The cellulose solution was set in a 70 °C oil-bath. Vinyl ester (2 mL) was added. The reaction was performed at 70 °C within 2 h. Isolation was carried out by precipitation in 300 mL of isopropanol. The precipitant was collected by filtration, thoroughly washed with isopropanol (5 × 100 mL), and subsequently vacuum-dried at 50 °C for 24 h. Determination of WPG.4 WPG was applied to characterize the efficiency of the modification of SCB. WPG was calculated as

EXPERIMENTAL SECTION

WPG = (m1 − m0)/m0 × 100%

Materials. SCB was provided by Guangxi Gui-Tang Group Co., Ltd. (Guigang, China). It was washed with hot water (90 °C) five times to remove any water-soluble residues and then ground into 20to 40-mesh particles and dewaxed by toluene/ethanol (2:1 v/v) for 10 h. The dewaxed SCB was oven-dried at 50 °C for 24 h and subjected to planetary ball-milled treatment for 4 h. EmimAc, 1-butyl-3-methylimidazolium chloride (BmimCl), and 1allyl-3-methylimidazolium chloride (AmimCl) were supplied by Shanghai Cheng Jie Chemical Co. Ltd. (Shanghai, China).

(1)

where m0 is the mass of the added SCB and m1 is the mass of modified SCB. Determination of the DS.24 About 0.2 g of SCB ester sample was stirred for 30 min in 10 mL of aqueous ethanol (70%). NaOH aqueous solution (0.5N, 5 mL) was added, and the mixture was stirred for 48 h at 60 °C. The unreacted NaOH was back-titrated with 0.1 N aqueous HCl. Two repetitions of the titration were carried out. The DS was calculated as 361


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ACS Sustainable Chemistry & Engineering (5 × 0.5 − 0.1Ve) − (5 × 0.5 − 0.1VC) 0.2 − [(5 × 0.5 − 0.1Ve) − (5 × 0.5 − 0.1VC)] × m × 10−3 (2) where Ve and VC are the volumes (mL) of HCl solution added to the SCB ester sample and to the SCB sample (control), respectively. m is the molar mass of the attached RCO acyl residue. Bioplastic film prepared from SCB esters. For SCB acetates and benzoates, a weight sample (0.6 g) was dissolved in 10 mL of DMF. The SCB ester solution was cast onto a polytetrafluoroethene mold and dried in a cabinet oven with air circulation at 50 °C for 12 h to obtain the film. For SCB laurates, a sample of 0.6 g was dissolved in 20 mL of CHCl3. The solution was cast onto a polytetrafluoroethene mold and dried at room temperature for 12 h. The obtained films were subsequently oven-dried at 50 °C for 12 h. Characterization. FT-IR spectra were recorded on an FT-IR spectrophotometer (Bruker) using a KBr disc containing 1% finely ground samples. Thirty-two scans were collected from 4000 to 400 cm−1 at a resolution of 4 cm−1. The NMR spectra were recorded on a Bruker Advance III 600 spectrometer (Germany). The 1H NMR and heteronuclear singular quantum correlation (HSQC) analyses were performed at room temperature (298 K) with 16 scans and a receiver gain of 187. Differential scanning calorimetry (DSC) analysis was performed on a TA Q500 analyzer (TA Instruments, USA) in nitrogen atmosphere. Sample weight: 4−5 mg. Heating rate: 10 °C/min. To perform scanning electron microscopy (SEM) analysis, the films were fixed on a metal stub using carbon tape and coated with gold. LEO 1530VP (LEO, Germany) with an accelerating voltage of 10 kV was used to obtain the secondary electron images The thermogravimetric analysis (TGA) was performed on a Q500 theromgravimetric analyzer (TA Instruments, USA) in nitrogen atmosphere. Sample weight: 9−10 mg. Heating rate: 10 °C/min. Tensile testing was performed with an Instron Universal testing machine 5565 fitted with a 100 N load cell. The test was according to “ASTM D882-12”. The grips length was set to 30 mm. Test specimens were cut to 15 mm width × 70 mm length. The rate of grip separation was set to 1 mm/min. The contact angle measurements were performed with a Dataphysics OCA40 Micro (Germany). A small piece, 1 × 1 cm, of film was placed on a microscope slide glass for the measurement. A water droplet of 3 μL was maintained on the film. Measurements were repeated 5 times. The measurements were carried out at 23 °C and a humidity of 50%.

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Figure 1. FT-IR spectra of SCB and SCB acetate (WPG, 22.3%).

RESULTS AND DISCUSSION

Synthesis of SCB esters by transesterification. Acetylated SCB was obtained using EmimAc as solvent and VA as transesterification reagent without any catalysts. After 2 h of reaction, WPG of 22.3% was achieved at 70 °C. The acetylated SCB was confirmed by FT-IR spectra (Figure 1); band assignment was according to refs 25 and 26. The band intensities 1740 cm−1, attributed to the CO stretching vibration, 1378 cm−1, attributed to the CH3 bending vibration, and 1240 cm−1, attributed to C−O stretching vibration, associated with the ester group, increased due to the introduction of the acetylate groups. The band at 3428 cm−1, attributed to the O−H stretching vibration, shifted to 3478 cm−1 and decreased in intensity, due to the substitution of the O−H groups by the acetylate groups. It should be noted that catalyst is suggested to be necessary for the transesterification of woody biomass with VA.27,28 It would be interesting to reduce the catalyst use in the view of reaction handling. The chemical reaction of VA with SCB was further confirmed by liquid-state 2D NMR (HSQC, Figure 2). Both chemical signals of substituted polysaccharides (C2′ and C3′)

Figure 2. HSQC spectra of acetylated SCB.

and nonsubstituted polysaccharides (C2 and C3) were observed, suggesting the partial substitution of SCB cellulose and hemicellulose fractions by the acetyl group. The DS of polysaccharides at the C2 and C3 positions was semiquantitatively studied by spectral integration of HSQC. The integral intensity of substituted polysaccharides (C2′ and C3′) increased with the increasing WPG, suggesting the highly chemical reactivity of cellulose and hemicellulose during the homogeneous transesterification of lignocellulose. It should be noted that cellulose is known to be hardly reactive to acetylating agents in the heterogeneous esterification of lignocellulose.7 The DS of the acetylated SCB could be controlled by stoichiometric methods. Acetylated SCB with WPG varying from −3.4% to 22.3% was prepared by adjusting the VA dosage of 0.25 to 4 mL/g (Table S1). However, The WPG showed no 362


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ACS Sustainable Chemistry & Engineering

Figure 3. (a) Plotting of WPG to VA dosage; (b) plotting of DS to WPG.

potassium salts are good catalysts for the transesterification of wood using DMF as reaction medium.19 The inactivation of the salts in BmimCl and AmimCl may be attributed to the fact that both the potassium salt and VA are insoluble in the ionic liquids, while they are dissolvable in DMF.19 On the other hand, vinyl ester, K2CO3, and CH3COOK are dissolvable in EmimAc. The transesterification was further carried out in EmimAc with VA as reagent and K2CO3/CH3COOK (5 wt %, mass ratio to SCB) as catalyst. SCB acetates with WPG of 25.8% and DS of 14.3 mmol/g, and WPG of 21.8% and DS of 14.0 mmol/g were obtained using K2CO3 and CH3COOK as catalyst, respectively. The addition of K2CO3 or CH3COOK to EmimAc resulted in the increment of DS of SCB acetates noticeably. The activation of K2CO3 and CH3COOK in EMIMAc for the transesterification suggested that the solvent effect of EmimAc is crucial for the transesterification of SCB. To further address the activator effect of EmimAc, the transesterification was heterogeneously performed directly in VA (Table S9). However, no reaction was observed by mixing SCB with VA directly at both 70 and 110 °C. Upon the addition of 5 wt % (mass ratio of SCB) EmimAc to the mixture of SCB and VA, SCB acetate with WPG of 2.3% was obtained at 110 °C. On further increasing of the amount of EMIMAc to 100 wt %, SCB acetates with WPG of 6.7%, and 24.3% were obtained at 70 and 110 °C, respectively. The acetylation of SCB was confirmed by FT-IR studies (Figure S5). The bands corresponding to the acetyl groups increased in intensity with the increasing WPG. To elucidate whether EmimAc got consumed during reaction, the transesterification was carried out by mixing SCB (1 g), EmimAc(1 g), and VA (8 mL) in a 110 °C oil bath for 2 h. Then EmimAc was recovered by filtration, thoroughly washing with VA and subsequent evaporation of VA. The result showed that EmimAc was recovered 100%. 1H NMR studies showed that no chemical changes occurred to EmimAc (Figure S6). Those results showed that EmimAc acted as an activator for the transesterification. The major difference between EmimAc and BmimCl (or AmimCl) is the counteranion. The acetate anion (CH3COO−) shows very strong nucleophilicity and basicity, while the Cl− does not. Transesterification of vinyl ester with SCB in EmimAc may be achieved via two different reaction pathways according to the nucleophilicity and the basicity of CH3COO−, respectively (Figure 4).31−33 According to the anhydride mechanism (Path A), an anhydride is formed as intermediate between the acetate anion and VA. If VB or VL is applied as the reagent instead of VA, a mixed anhydride of acetic benzoic anhydride or acetic laurel anhydride would form as the intermediate, and a final

relationship to the temperature. Raising the temperature from 70 to 110 °C, the WPG did not increase (Figure 3). The result suggested that there was relatively low reaction activation energy between SCB and VA using EmimAc as reaction medium. The relatively low activation energy was confirmed by quantum chemistry simulation at the B3LYP/6-311++G(2d, p) level with GAMESS29,30 according to the carbene mechanism (Figure 4) using methnol as a model reagent. The activation

Figure 4. Possible reaction mechanism of SCB with VA in EmimAc.

energy decreased from 42.0 to 36.1 kcal using EmimAc as activator (Figure S1). To take advantage of EmimAc as reaction medium for the transesterification, SCB benzoate and SCB laurate (Table S2−S3) were prepared using VB and VL as reagents, respectively. SCB benzoates with WPG in range 11.3%−60.6% and SCB laurates with WPG in range 3.9%− 84.3% were obtained. The chemical structure of the SCB esters was confirmed by FT-IR studies (Figure S2−S3). The DS of acylated SCB was further studied by alkaline hydrolysis and titration. A linear relationship between DS and WPG was observed, suggesting that it was satisfactory to characterize the efficiency of the esterification of SCB with WPG. Thus, the following discussion will be based on the result of WPG. Mechanism of the transesterification. To address the success of EmimAc as a dual solvent and activator for the transesterification, the transesterification of SCB with VA was performed in 1-butyl-3-methylimidazolium chloride (BmimCl) and 1-allyl-3-methylimidazolium chloride (AmimCl). No reactions were achieved in the BmimCl and the AmimCl, even though changing the reaction temperature from 70 to 130 °C and using K2CO3, Na2CO3, or CH3COOK as catalyst (Table S4−S8, Figure S4). It should be noted that the 363


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ACS Sustainable Chemistry & Engineering product of SCB acetate would be achieved.32 However, as we performed the transesterification of SCB with VB or VL, corresponding SCB benzoate and SCB laurate, but not SCB acetate, was achieved. To further confirm the reaction mechanism, the reactions of cellulose with vinyl esters were studied (Table S10). The resulting cellulose derivatives were characterized by FT-IR and solution state 1H NMR (Figure S7). Cellulose acetate, cellulose benzoate, and cellulose laurate were obtained, corresponding to the usage of VA, VB, and VL as transesterification reagent, respectively. These results suggested that the transesterification between SCB with vinyl esters was unlikely to take place according “Path A” (Figure 4). Upon mixing VA and EmimAc at molar ratio 1:1, the mixture transformed into solid after 24 h (Figure S8), which suggested the chemical reaction between VA and EmimAc. It is wellknown that carbene is presented in EmimAc, due to interaction between acetate anion and the acidic C2−H2 of 1-ethyl-3methylimidazolium (Figure 4).34−36 The carbene mechanism (Figure 4, Path B) was confirmed by liquid-state NMR studies of the mixture of EmimAc and VA. The mixture of VA and EmimAc was applied to 1H NMR studies (Figure 5). The chemical signal at 10.06, 7.90, and 7.81

confirmed by HSQC study on the mixture of VA and EmimAc (Figure S9). The signal for C2−H2 disappeared due to the full substitution of H2 of 1-ethyl-3-methylimidazolium by the acetyl groups. These results gave further evidence to the carbene mechanism. According to the carbene mechanism, the addition of a base into the mixture of SCB solution in BmimCl and vinyl esters should result in the acylated SCB. No reaction was observed using pyridine as the base, while the transesterification was observed using triethylamine as the base (Table S11, Figure S10). Triethylamine with a pKa value of 10.72 is a much stronger base than pyridine with a pKa value of 5.17.37 It is suggested that a very strong base is necessary to activate the transesterification between SCB and vinyl esters. It is reported that cellulose esters with DS up to 0.69 are achieved by employing methyl esters as the transesterification reagent, BmimCl as reaction medium, and 1,5,7-triazabicyclo[4.4.0]dec5-ene as activator.38 However, the WPG of acylated SCB was low (Table S11) using BmimCl as reaction medium and triethylamine as activator, due to the fact that vinyl esters are insoluble in BmimCl (Figure S11). The results addressed the significance of EmimAc used as dual solvent and activator for the transesterification of lignocellulose with vinyl esters. Properties of the acylated SCB. Internal plasticizing effects of the transesterification on SCB were expected owing to the broken down hydrogen-bonding network associated with the substitution of hydroxyl groups by flexible alkanoyl side chains. The glass transition temperature (Tg) of the SCB esters was studied to understand the internal plasticization and flexibility of the materials. The SCB acetates and SCB showed no Tg by DSC studies (Figre S12). The Tg was observed at 191−199 °C for SCB benzoate with WPG of 34.3%−60.5%, and that at 158−166 °C for SCB laurate. Long chain acyl groups and high WPGs tended to result in SCB esters with low Tg. The results suggested that the longer the alkanoyl side chain introduced, the more flexible the SCB ester, owing to the decrease in the Tg. Noticeably, the acylated SCB were dissolvable in traditional solvents, such as DMF or CHCl3 (Table S12). It is impossible to dissolve native SCB or SCB regenerated from ionic liquids, which remains a major challenge in efforts to transform the biomass into advanced materials. Taking advantage of the dissoluble SCB esters, Bioplastic films were prepared by solution casting. The SCB ester films were semitransparent, with a yellow color. The result is different from that of the cellulose ester film, which is transparent without any color.21 The difference is attributed to the presence of lignin (wellknown chromophores) in SCB. The films showed low visible light transmittance with strong ultraviolet light blocking ability (Figure 6). To give light to the potential application of the SCB ester films, SEM, TGA, tensile testing, and contact angle measurements were performed to understand the structural factors and performance of the films. SEM studies (Figure S13) showed that the SCB ester films displayed a relatively smooth and uniform morphology, similar to those of cellulose carbonate.21 TGA studies (Figure S14) showed that the homogeneous transesterification would result in the decrease of moisture content and initial thermal degradation temperature of SCB. In a typical case, the moisture content decreased from 11.5% (for the SCB sample) to 1.84% (for the SCB laurate with WPG of 84.3%) due to the introduction of hydrophobic fatty acyl

Figure 5. Formation of 2-alkanoyl-dialkylimidazolium evidenced by 1 H NMR studies.

ppm corresponding to H2, H4, and H5 originating from 1-ethyl3-methylimidazolium, shifted to 9.74, 7.85, and 7.76 ppm, respectively, upon the addition of 0.5 mol/mol VA to EmimAc. Chemical signals for H4 and H5 originating from carbene and 1ethyl-2-acetyl-3-methylimidazolium cation were observed, suggesting the chemical reaction between 1-ethyl-3-methylimidazolium and VA at the C2 position (Figure 5). Upon increase of the molar ratio of VA to 2:1, the chemical signals for H4 and H5 were observed only for 1-ethyl-2-acetyl-3methylimidazolium, suggesting the totally conversion of 1ethyl-3-methylimidazolium and carbene into 1-ethyl-2-acetyl-3methylimidazolium. The carbene mechanism was further 364


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01735. Tables of the synthetic condition; FT-IR of the modified samples; NMR of the modified samples;, TGA, DSC, SEM, and contact angle of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

Figure 6. UV−vis curves of SCB ester films.

*Tel.: +86 20 87137652. E-mail: [email protected].. ORCID

Qing-Shan Shi: 0000-0003-4228-3080

groups. As a general remark, both the increase in WPG and chain length of the fatty acyl groups would decrease the moisture content of the SCB ester. The thermal degradation of the set temperature of SCB esters decreased with the increasing WPG. However, the chain length of the substituted fatty acyl groups has no effect on the initial thermal degradation temperature. Tensile testing studies (Table S13) showed that the acetylation and benzoylation had no significant effect on the mechanical property of the films as compared with those prepared directly from SCB. However, the long acyl chain (the SCB laurate ones) showed an internal plasticized effect on the films, as the elongation at break of the film increased noticeably from around 4% (for the SCB acetate films) to 59.23% (for the SCB laurate film with WPG of 84.3%). Interestingly, the SCB ester films showed comparable mechanical properties to those of cellulose fatty esters.39 It would be an advantage to prepare materials with performance comparable to that of cellulose directly from the whole lignocellulosic biomass, since the isolation of cellulose40 from lignocellulosic biomass always suffers from heavy energy and chemical consumption. At last, contact angle studies (Figure S15 and Table S14) demonstrated the SCB esters were hydrophobic materials. The contact angles with water of SCB ester films passed through 87° to 103°, depending on both the fatty chain length and the WPG values. These results are comparable to those of cellulose fatty acid ester (DS of 1.7−3.0) films which pass through 87° to 106°.39 The increment of both the fatty chain length and WPG values would increase the hydrophobicity of the materials.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (31600477), State Key Laboratory of Pulp and Paper Engineering (201602), and the Scientific and Technological Project of Guangdong Province (No. 2013B091500080, 2015A010105019, and 2016A010103020).

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REFERENCES

(1) Hill, C. A. S. Wood Modification: Chemical, Thermal and Other Processes; John Wiley & Sons Ltd.: Chichester, England, 2006. (2) Vaidya, A. A.; Gaugler, M.; Smith, D. A. Green route to modification of wood waste, cellulose and hemicellulose using reactive extrusion. Carbohydr. Polym. 2016, 136, 1238−1250. (3) Hon, D. N.-S.; Shiraishi, N.: Wood and Cellulosic chemistry; Marcel Dekker, Inc: New York, 2001. (4) Xie, H.; King, A.; Kilpelainen, I.; Granstrom, M.; Argyropoulos, D. S. Thorough chemical modification of wood-based lignocellulosic materials in ionic liquids. Biomacromolecules 2007, 8, 3740−3748. (5) Rowell, R. M. Chemical modification of wood: A short review. Wood Mater. Sci. Eng. 2006, 1, 29−33. (6) Keplinger, T.; Cabane, E.; Chanana, M.; Hass, P.; Merk, V.; Gierlinger, N.; Burgert, I. A versatile strategy for grafting polymers to wood cell walls. Acta Biomater. 2015, 11, 256−263. (7) Rowell, R. M.; Simonson, R.; Hess, S.; Plackett, D. V.; Cronshaw, D.; Dunningham, E. Acetyl distribution in acetylated whole wood and reactivity of isolated wood cell-wall components to acetic-anhydride. Wood Fiber Sci. 1994, 26, 11−18. (8) George, A.; Brandt, A.; Tran, K.; Zahari, S. M. S. N. S.; KleinMarcuschamer, D.; Sun, N.; Sathitsuksanoh, N.; Shi, J.; Stavila, V.; Parthasarathi, R.; Singh, S.; Holmes, B. M.; Welton, T.; Simmons, B. A.; Hallett, J. P. Design of low-cost ionic liquids for lignocellulosic biomass pretreatment. Green Chem. 2015, 17, 1728−1734. (9) Badgujar, K. C.; Bhanage, B. M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. 2015, 178, 2−18. (10) Cevasco, G.; Chiappe, C. Are ionic liquids a proper solution to current environmental challenges? Green Chem. 2014, 16, 2375−2385. (11) Livi, S.; Duchet-Rumeau, J.; Gérard, J.-F.; Pham, T. N. Polymers and ionic liquids: A successful wedding. Macromol. Chem. Phys. 2015, 216, 359−368. (12) Gericke, M.; Fardim, P.; Heinze, T. Ionic Liquids  Promising but Challenging Solvents for Homogeneous Derivatization of Cellulose. Molecules 2012, 17.745810.3390/molecules17067458 (13) Miyafuji, H. Application of ionic liquids for effective use of woody biomass. J. Wood Sci. 2015, 61, 343−350. (14) da Costa Lopes, A. M.; Bogel-Łukasik, R. Acidic ionic liquids as sustainable approach of cellulose and lignocellulosic biomass

CONCLUSION

SCB esters were successfully synthesized by homogeneous transesterification using EmimAc as a duel solvent and activator. The DS of SCB could be adjusted by stoichiometric methods. The transesterification of lignocellulose with vinyl ester in EmimAc was demonstrated to be a carbene mechanism. The formation of 2-alkanoyl-1,3-dialkylimizolium cation as reactive intermediate between EmimAc and vinyl esters was confirmed by NMR studies. SCB esters were dissolvable in common solvents, and bioplastic films were prepared by solution casting. The SCB ester films showed comparable performance to those of cellulose derivatives. The study presented an alternative approach to transform lignocellulosic biomass into bioplastics without pretreatment of lignocellulose. 365


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ACS Sustainable Chemistry & Engineering conversion without additional catalysts. ChemSusChem 2015, 8, 947− 965. (15) Chen, M.-J.; Shi, Q.-S. Transforming sugarcane bagasse into bioplastics via homogeneous modification with phthalic anhydride in ionic liquid. ACS Sustainable Chem. Eng. 2015, 3, 2510−2515. (16) Chen, J.; Xu, J.; Wang, K.; Cao, X.; Sun, R. Cellulose acetate fibers prepared from different raw materials with rapid synthesis method. Carbohydr. Polym. 2016, 137, 685−692. (17) Ö zmen, N.; Ç etin, N. S.; Tingaut, P.; Sèbe, G. A new route for the functionalisation of wood through transesterification reactions. Eur. Polym. J. 2006, 42, 1617−1624. (18) Jebrane, M.; Sèbe, G. A novel simple route to wood acetylation by transesterification with vinyl acetate. Holzforschung 2007, 61, 143− 147. (19) Jebrane, M.; Sèbe, G. A new process for the esterification of wood by reaction with vinyl esters. Carbohydr. Polym. 2008, 72, 657− 663. (20) Ç etin, N. S.; Ö zmen, N.; Birinci, E. Acetylation of wood with various catalysts. J. Wood Chem. Technol. 2011, 31, 142−153. (21) Labafzadeh, S. R.; Helminen, K. J.; Kilpeläinen, I.; King, A. W. T. Synthesis of cellulose methylcarbonate in ionic liquids using dimethylcarbonate. ChemSusChem 2015, 8, 77−81. (22) Kakuchi, R.; Yamaguchi, M.; Endo, T.; Shibata, Y.; Ninomiya, K.; Ikai, T.; Maeda, K.; Takahashi, K. Efficient and rapid direct transesterification reactions of cellulose with isopropenyl acetate in ionic liquids. RSC Adv. 2015, 5, 72071−72074. (23) Hinner, L. P.; Wissner, J. L.; Beurer, A.; Nebel, B. A.; Hauer, B. Homogeneous vinyl ester-based synthesis of different cellulose derivatives in 1-ethyl-3-methyl-imidazolium acetate. Green Chem. 2016, 18, 6099−6107. (24) Peydecastaing, J.; Vaca-Garcia, C.; Borredon, E. Accurate determination of the degree of substitution of long chain cellulose esters. Cellulose 2009, 16, 289−297. (25) Santoni, I.; Callone, E.; Sandak, A.; Sandak, J.; Dirè, S. Solid state NMR and IR characterization of wood polymer structure in relation to tree provenance. Carbohydr. Polym. 2015, 117, 710−721. (26) Traoré, M.; Kaal, J.; Martínez Cortizas, A. Application of FTIR spectroscopy to the characterization of archeological wood. Spectrochim. Acta, Part A 2016, 153, 63−70. (27) Jebrane, M.; Pichavant, F.; Sèbe, G. A comparative study on the acetylation of wood by reaction with vinyl acetate and acetic anhydride. Carbohydr. Polym. 2011, 83, 339−345. (28) Cetin, N. S.; Ozmen, N. Acetylation of wood components and fourier transform infra-red spectroscopy studies. Afr. J. Biotechnol. 2011, 10, 3091−3096. (29) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General atomic and molecular electronic structure system. J. Comput. Chem. 1993, 14, 1347−1363. (30) Gordon, M. S.; Schmidt, M. W. Chapter 41 - Advances in electronic structure theory: GAMESS a decade later. In Theory and Applications of Computational Chemistry; Frenking, G., Kim, K. S., Scuseria, G. E., Eds.; Elsevier: Amsterdam, 2005; pp 1167−1189. (31) Clough, M. T.; Geyer, K.; Hunt, P. A.; Son, S.; Vagt, U.; Welton, T. Ionic liquids: not always innocent solvents for cellulose. Green Chem. 2015, 17, 231−243. (32) Köhler, S.; Liebert, T.; Schöbitz, M.; Schaller, J.; Meister, F.; Gü n ther, W.; Heinze, T. Interactions of ionic liquids with polysaccharides 1. Unexpected acetylation of cellulose with 1-ethyl3-methylimidazolium acetate. Macromol. Rapid Commun. 2007, 28, 2311−2317. (33) Ebner, G.; Schiehser, S.; Potthast, A.; Rosenau, T. Side reaction of cellulose with common 1-alkyl-3-methylimidazolium-based ionic liquids. Tetrahedron Lett. 2008, 49, 7322−7324. (34) Gurau, G.; Rodríguez, H.; Kelley, S. P.; Janiczek, P.; Kalb, R. S.; Rogers, R. D. Demonstration of chemisorption of carbon dioxide in 1,3-dialkylimidazolium acetate ionic liquids. Angew. Chem., Int. Ed. 2011, 50, 12024−12026.

(35) Wei, X.; Han, Z.; Zhang, D. Theoretical study on the mechanism of the side reaction of 1-butyl-3-methylimidazolium cation with D-glucose. Carbohydr. Res. 2013, 374, 40−44. (36) Aggarwal, V. K.; Emme, I.; Mereu, A. Unexpected side reactions of imidazolium-based ionic liquids in the base-catalysed Baylis-Hillman reaction. Chem. Commun. 2002, 1612−1613. (37) Speight, J. G. Lange’s Handbook of Chemistry, 16th ed.; McGrawHill: New York, 2005. (38) Schenzel, A.; Hufendiek, A.; Barner-Kowollik, C.; Meier, M. A. R. Catalytic transesterification of cellulose in ionic liquids: sustainable access to cellulose esters. Green Chem. 2014, 16, 3266−3271. (39) Crépy, L.; Chaveriat, L.; Banoub, J.; Martin, P.; Joly, N. Synthesis of Cellulose Fatty Esters as PlasticsInfluence of the Degree of Substitution and the Fatty Chain Length on Mechanical Properties. ChemSusChem 2009, 2, 165−170. (40) Radotić, K.; Mićić, M.: Methods for Extraction and Purification of Lignin and Cellulose from Plant Tissues. In Sample Preparation Techniques for Soil, Plant, and Animal Samples; Micic, M., Ed.; Springer: New York, NY, 2016; pp 365−376.

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Homogeneous Transesterification of Sugar Cane Bagasse toward

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