Homogeneous Transesterification of Sugar Cane Bagasse toward

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Homogeneous Transesterification of Sugarcane Bagasse Toward Sustainable Plastics Ming-Jie Chen, Rui-Min Li, Xue-Qin Zhang, Jing Feng, Jin Feng, Chuan-Fu Liu, and Qing-Shan Shi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01735 • Publication Date (Web): 19 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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ACS Sustainable Chemistry & Engineering

Homogeneous Transesterification of Sugarcane Bagasse Toward Sustainable Plastics

Ming-Jie Chen†‡, Rui-Min Li†, Xue-Qin Zhang‡, Jing Feng†, Jin Feng†, Chuan-Fu Liu‡ and Qing-Shan Shi*†

†

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.

*

Corresponding author: Room 406, Building of 58, No. 100 Central Xianlie Road,

Guangzhou 510070, China. Tel.: +86 20 87137652. E-mail: [email protected].

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Abstract Homogeneous esterification of lignocellulosic agro-forestry wastes is an efficient approach to transform the biomass into value-added bio-based materials. However, the application of fatty acid anhydrides or acyl chlorides as esterified reagents would yield equivalent acids which lead to significantly decomposition of lignocellulosic biomass. The present study proposed an alternative chemical approach to prepare bio-plastics from lignicellulosic biomass. Sugarcane 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

reactive

intermediate

of

2-alkanoyl-dialkylimidazolium. The as-prepared sugarcane bagasse esters were found to be solvable in common solvents. Bio-plastic films were prepared by solution casting. Scanning electron microscope and tensile testing studies showed that the sugarcane bagasse ester films were comparable to those of cellulose fatty esters. Keywords: Biomass; Ionic liquid; Bio-plastics; Bio-based materials; Chemical modification


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Introduction Lignocellulose has been used as engineering material for various applications since the ancient times. However, it suffers from several disadvantages, including dimensional changes under changing atmospheric conditions, susceptible to decay, incompatible to thermoplastics, impossible to be melted or dissolved.1 Regarding 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 sugarcane bagasse (SCB), consisting of cellulose, hemicellulose and lignin, is a nature composite with complex physical structure compared to those of constructing materials, which make it impossible to dissolve lignocellulose with traditional solvent systems.3 During the past four decades, the chemical modification of lignocellulose was performed in heterogeneous reaction mediums.4,5 The heterogeneous chemical modification of lignocellulose suffers from low efficiency,4 the reaction is mainly located within highly lignified cell corner and the compound middle lamella, and might not progress very deep into the cell wall.6 Cellulose fractions remain unreactive, while lignin fractions achieve at high 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 processing10,11. 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 3 ACS Paragon Plus Environment


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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 the HCl or carboxylic acid as by-products.1 The production of acids in the homogeneous esterification of lignocellulose would result in the significant degradation of the lignocellulose fractions.4,14 Novel chemical pathway is in need 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 flims materials from SCB.15 However, the processing results in materials bearing carboxyl groups making it hydrophilic,15 while hydrophobic materails from lignocellulose is under developed. Acylation of lignocellulose with vinyl esters is an alternative esterification approach that does not liberate an acid as by-product.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

researches

were

limited

to

the

heterogeneous

transesterification using dimethylformamide (DMF) as reaction mediums. 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 4 ACS Paragon Plus Environment

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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 detail 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 cellulose23 which is obtained from lignocellulose with de-lignin (also known as pulping) processing, and subsequently with de-hemicellulose processing. None of such modification of lignocellulose is carried out, since the case of lignocellulose is more complex than that of cellulose3. However, lignocellulose is much cheaper than cellulose. It would be of great interest to processing lignocellulose into materials analog 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 non-hydrogen 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 5 ACS Paragon Plus Environment


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studies. The SCB esters were transformed into film materials to address potential application of the resulted materials as bio-plastics. Experimental Materials SCB was provided by Guangxi Gui-Tang Group Co., Ltd. (Guigang, China). It was washed with hot water (90 oC) five times to remove any water-soluble residues, then ground into 20- to 40-mesh particles and dewaxed by toluene/ethanol (2:1 v/v) for 10 h. The dewaxed SCB was oven-dried at 50 oC for 24 h and subjected to planetary ball-milled treatment for 4 h. EmimAc,

1-butyl-3-methylimidazolium

chloride

(BmimCl)

and

1-allyl-3-methylimidazolium chloride (AmimCl) were supplied by Shanghai Cheng Jie Chemical Co. Ltd. (Shanghai, China). Cellulose (DP 900), VA, VB and VL were purchased from Sigma-Aldrich (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 oC oil-bath under N2 atmosphere for 5 h to guarantee complete dissolution. 6 ACS Paragon Plus Environment

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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 oC under N2 atmosphere for 2 h. Isolation was carried out by precipitation of the product into 300 mL isopropanol. The SCB esters were obtained after filtration, thoroughly washed with isopropanol (5×100 mL), and subsequently vacuum dried at 50 oC 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 oC 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 isopropanol. The precipitant was collected, thoroughly washed with isopropanol (5×100 mL), and subsequently vacuum dried at 50 oC 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. Amount of EmimAc, reaction temperature are listed in supporting information (Table S9). Isolation was carried out by filtration. The precipitant was thoroughly washed with 7 ACS Paragon Plus Environment


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isopropanol (7×100 mL), and subsequently vacuum dried at 50 oC 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 oC within 5 h under N2 atmosphere. Pyridine or triethylamine (8 mL) was added to the SCB solution in 70 oC. The solution was stirred for 30 min at 70 oC. Then, vinyl ester (8 mL) was added. The transesterification was performed at 70 oC in 2 h. Isolation was carried out by precipitation in 300 mL isopropanol. The precipitant was collected by filtration and thoroughly washed with isopropanol (5×100 mL), subsequently vacuum dried at 50 oC for 24 h. Syntheses of cellulose esters in EmimAc Cellulose (0.5 g) was dissolved in EmimAc (19.5 g) at 130 oC within 1 h. The cellulose solution was set at 70 oC oil-bath. Vinyl ester (2 mL) was added. The reaction was performed at 70 oC within 2 h. Isolation was carried out by precipitation in 300 ml isopropanol. The precipitant was collected by filtration, thoroughly washed with isopropanol (5×100 mL), and subsequently vacuum dried at 50 oC for 24 h. Determination of WPG4 WPG was applied to characterize the efficiency of the modification of SCB. WPG was calculated as: WPG = (m1 - m0)/m0 × 100% where, m0 is the mass of the added SCB, m1 is the mass of modified SCB. Determination of the DS24 8 ACS Paragon Plus Environment

(1)

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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 oC. The unreacted NaOH was back-titrated with 0.1N aqueous HCl. Two repetition of the titration were carried out. The DS was calculated as: ሺହ×଴.ହି଴.ଵ௏೐ ሻିሺହ×଴.ହି଴.ଵ௏಴ ሻ ଴.ଶିሾሺହ×଴.ହି଴.ଵ௏೐ ሻିሺହ×଴.ହି଴.ଵ௏಴ ሻሿ×୫×ଵ଴షయ

(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. Bio-plastic film prepared from SCB esters For SCB acetates and benzoates, a weight sample (0.6 g) was dissolved in 10 mL DMF. The SCB ester solution was cast onto a polytetrafluoroethene mould and dried in a cabinet oven with air circulation at 50 oC for 12 h to obtain the film. For SCB laurates, sample of 0.6 g was dissolved in 20 mL CHCl3. The solution was cast onto a polytetrafluoroethene mould and dried at room temperature for 12 h. The obtained films were subsequently oven-dried at 50 oC 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 receiver gain 9 ACS Paragon Plus Environment


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of 187. Differential scanning calorimetry (DSC) analysis was performed on a TA Q500 analyser (TA Instruments, USA) in nitrogen atmosphere. Sample weight: 4−5 mg. Heating rate: 10 oC/min. To perform scanning electron microscope (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 analyser (TA Instruments, USA) in nitrogen atmosphere. Sample weight: 9−10 mg. Heating rate: 10 °C/min. Tensile testing was performed with Instron Universal testing machine 5565 fitted with a 100 N load cell. The test was according to “ASTM D882 - 12”. Grips length was set 30 mm. Test specimens were cut to 15 mm width × 70 mm length. Rate of grip separation was set 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. Water droplet of 3 uL was maintained on the film. Measurements were repeated 5 times. The measurements were carried out at 23 oC and humidity of 50%. Results and discussion Synthesis of SCB esters by transesterification 10 ACS Paragon Plus Environment

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

Acetylated SCB were 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 oC. The acetylated SCB was confirmed by FT-IR spectra (Figure 1), band assignment was according to references25,26. The band intensities of 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 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 the necessity 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’) and non-substituted polysaccharides (C2 and C3) were observed, suggesting 11 ACS Paragon Plus Environment


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the partially substitution of SCB cellulose and hemicellulose fractions by the acetyl group. The DS of polysaccharides at C2 and C3 position 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 homogenous transesterification of lignocellulose. It should be noted that cellulose is known to be hardly reactivity to acetylating agents in the heterogeneous esterification of lignocellulose.7

Acetylated SCB with WPG of -3.4%

C3’

C2’ C2,3,5

C1

Acetylated SCB with WPG of 22.3%

C2’ C2,3,5

C3’

C1

Figure 2. HSQC spectra of acetylated SCB.


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

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 relationship to the temperature. Raising the temperature from 70 oC to 110 oC, 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 B3LYP/6-311++Gሺ2d, pሻ level with GAMESS29,30 according to the carbene mechanism (Figure 4) using methnol as a model reagent. The activation energy decreased from 42.0 kcal to 36.1 kcal using EmimAc as activator (Figure S1). To take the 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 of 11.3% to 60.6% and SCB laurates with WPG in range of 3.9% to 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. Linear relationship between 13 ACS Paragon Plus Environment


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DS and WPG was observed, suggesting that it was satisfied to characterize the efficiency the esterification of SCB with WPG. Thus, the following discussion will base 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 oC to 130 oC and using K2CO3, Na2CO3, or CH3COOK as catalyst (Table S4-S8, Figure S4). It should be noted that the potassium salts are good catalyst for the transesterification of wood using DMF as reaction mediums.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 DMF19. 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. 14 ACS Paragon Plus Environment

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To further address the activator effect of EmimAc, the transesterification was heterogeneous performed directly in VA (Table S9). However, no reaction was observed by mixing SCB with VA directly at both 70 oC and 110 oC. 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 oC. Further increacse the amount of EMIMAc to 100%wt, SCB acetates with WPG of 6.7%, and 24.3% were obtained at 70 oC and 110 oC, 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) at 110 oC oil bath for 2 h. Then EmimAc was recovered by filtration, thoroughly washing with VA and subsequently 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.

Figure 4. Possible reaction mechanism of SCB with VA in EmimAc. 15 ACS Paragon Plus Environment


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The major difference between EmimAc and BmimCl (or AmimCl) is the counter anion. The acetate anion (CH3COO-) shows very strong nucleophilicity and basicity, while the Cl- does not. Transesterification of vinyl ester with SCB in EmimAc may achieve 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 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 studies (Table S10). The resulted 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 the ‘Path A’ (Figure 4). Upon mixing VA and EmimAc at molar ratio of 1:1, the mixture transformed into solid after 24 h (Figure S8), which suggested the chemical reaction between VA and EmimAc. It is well known that carbene is presented in EmimAc, due to interaction 16 ACS Paragon Plus Environment

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between acetate anion and the acidic C2-H2 of 1-ethyl-3-methylimidazolium (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.

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

The mixture of VA and EmimAc was applied to 1H NMR studies (Figure 5). The chemical signal at 10.06 ppm, 7.90 ppm and 7.81 ppm corresponding to H2, H4 and H5 originating from 1-ethyl-3-methylimidazolium, shifted to 9.74 ppm, 7.85 ppm 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

1-ethyl-2-acetyl-3-methylimidazolium cation were observed, suggesting the chemical reaction between 1-ethyl-3-methylimidazolium and VA at the C2 position (Figure 5). Increase the molar ratio of VA to 2:1, chemical signals for H4 and H5 were 17 ACS Paragon Plus Environment


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observed only for 1-ethyl-2-acetyl-3-methylimidazolium, suggesting the totally conversion

of

1-ethyl-3-methylimidazolium

1-ethyl-2-acetyl-3-methylimidazolium.

The

carbene

and

carbene

mechanism

was

into further

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 evidences 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 pKa value of 10.72 is a much stronger base than pyridine with pKa value of 5.17.37 It is suggested that 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 employing methyl esters as the transesterification reagent, BmimCl as reaction medium, and 1,5,7-triazabicyclo[4.4.0]dec-5-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. 18 ACS Paragon Plus Environment

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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. 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 oC for SCB benzoate with WPG of 34.3%-60.5%, and that at 158-166 oC 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 alkanoyl side chain introduced, the more flexible SCB ester did, owing to the decrease in the Tg.

Figure 6. UV-Vis curves of SCB ester films.

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 advantages of the dissoluble SCB esters, 19 ACS Paragon Plus Environment


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bio-plastic films were prepared by solution casting. The SCB esters films were semitransparent with yellow colour. The result is different from that of cellulose ester film which is transparent without any colours21. The difference is attributed to the present of lignin (well-known chromophores) in SCB. The films showed low visible light transmittance with strong ultraviolet light blocking ability (Figure 6). To give light on 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 thermal degradation on set 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 groups. In a general remark, both the increase in WPG and chain length of the fatty acyl groups would decrease the moisture content of SCB ester. The thermal degradation on set temperature of SCB esters decreased with the increasing WPG. However the chain length of the substituted fatty acyl groups have no effect on the thermal degradation on set 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, long acyl chain (the SCB laurate ones) 20 ACS Paragon Plus Environment

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showed 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 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 suffer from heavy energy and chemical consumption. At last, contact angle studies (Figure S15 an Table S14) demonstrated the SCB esters were hydrophobic materials. The contact angles with water of SCB ester films passed through 87o to 103o, 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 87o to 106o.39 The increment of both the fatty chain length and WPG values would increase the hydrophobicity of the materials. 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 bio-plastic films were prepared by solution casting. The SCB ester films showed comparable performance to those of cellulose derivatives. The 21 ACS Paragon Plus Environment


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study presented an alternative approach to transform lignocellulosic biomass into bio-plastics without pretreatment of lignocellulose. Supporting Information 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 are included. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements 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). 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. 22 ACS Paragon Plus Environment

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TABLE OF CONTENTS (TOC) GRAPHIC

Homogeneous Transesterification of Sugarcane Bagasse Toward Sustainable Plastics

Ming-Jie Chen†‡, Rui-Min Li†, Xue-Qin Zhang‡, Jing Feng†, Jin Feng†, Chuan-Fu Liu‡ and Qing-Shan Shi*†

Sugarcane bagasse was transformed into hydrophobic bio-plastics by homogenous transesterification using 1-ethyl-3-methylimidazolium acetate as dual solvent and activator.


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Homogeneous Transesterification of Sugar Cane Bagasse toward

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