Thermal stability of sugarcane bagasse derivatives bearing carboxyl groups synthesized in ionic liquid

Chen, M. J., Feng, J., and Shi, Q. S. (2016). "Thermal stability of sugarcane bagasse derivatives bearing carboxyl groups synthesized in ionic liquid," BioRes. 11(3), 6254-6266.

Abstract

To illuminate changes in the thermal stability of lignocellulosic biomass by homogeneous chemical modification in ionic liquids, sugarcane bagasse derivatives bearing carboxyl groups were prepared in ionic liquids. Fourier transform infrared (FT-IR) spectroscopy and solid-state nuclear magnetic resonance (NMR) confirmed the chemical structure of the derivatives. Sugarcane bagasse derivatives with degree of substituted OH as high as 9.93 mmol/g were achieved. The homogeneous esterification was demonstrated to be a more efficient approach than heterogeneous ones. Based on thermogravimetric analysis, the onset degradation temperature of sugarcane bagasse decreased dramatically to 185 °C, 160 °C and 140 °C, using succinic anhydride, maleic anhydride, and phthalic anhydride as reagent, respectively. A first-order degradation kinetic model was applied to obtain the degradation activation energies of sugarcane bagasse. The results showed that homogeneous chemical modification significantly decreased the thermal stability of sugarcane bagasse by reducing the onset degradation temperature and degradation activation energies.

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Thermal Stability of Sugarcane Bagasse Derivatives bearing Carboxyl Groups Synthesized in Ionic Liquid

Ming-Jie Chen, Jin Feng, and Qing-Shan Shi *

Keywords: Ionic liquids; Sugarcane bagasse; Thermal stability; Cyclic anhydride; Esterification

Contact information: State Key Laboratory of Applied Microbiology Southern China, Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, Guangdong Institute of Microbiology, Guangzhou 510070, China; *Corresponding author: shiqingshan@hotmail.com

INTRODUCTION

Growing concerns about the longevity and stability of fossil resources and their associated greenhouse gas emissions have driven the development of chemicals and materials from agricultural or woody biomass (Xia et al. 2016). Sugarcane bagasse from sugar industrial residues is one of the most abundant sources of biomass and consists of lignin, cellulose, and hemicellulose. There has been increasing interest in converting this biomass into novel materials without the decomposition of its components (Chen et al. 2015). However, the materials prepared from native sugarcane bagasse generally suffer from low performance (Chen and Shi 2015).

Chemical derivatization can be an effective way to attach functional groups to materials derived from sugarcane bagasse. This can be done via the substitution of the hydroxyl groups in the lignin, cellulose, and hemicellulose fractions. The esterification of sugarcane bagasse with cyclic anhydrides is a conventional method to prepare derivatives containing carboxyl groups (Gurgel et al. 2008; Chen et al. 2013). Thermoplasticization of the esterified sugarcane bagasse with cyclic anhydrides was demonstrated, and very compact self-adhesion esterified sugarcane bagasse composites was formed under hot-pressing processing (Hassan et al. 2000). Regarding the thermoplastic processing of a material, thermal stability of the softened material during thermo-mechanical processing is an essential aspect, which can positively influence the molding technology (Cui et al. 2013).

The thermal properties of esterified sugarcane bagasse are dependent on the reaction methods. The thermal stability of bagasse fibers is decreased by the solvent-free esterification with succinic anhydride, which is immobilized on the fiber by acetone solution impregnation (Hassan et al. 2000). The thermal stability of modified Posidonia, which is prepared with maleic and phthalic anhydrides as reagents and tripropylamine as a catalyst under ultrasonic pretreatment, is reported to be higher than that of raw Posidonia (Chadlia 2010).

Opposite results for the thermal stability of modified lignocellulose suggest that such a method would be in demand to perform thermal stability analysis while a novel chemical approach is developed for the preparation of cyclic-anhydride-modified lignocellulose.

The preparation of carboxyl functional sugarcane bagasse is conventionally performed in a heterogeneous medium because it is impossible to dissolve the biomass in traditional solvents. As a result of the heterogeneous reaction, the chemical modification of sugarcane bagasse always suffers from low efficiency. Only a small part of the hemicellulose takes part in esterification, while cellulose fractions remain hardly reactive to the anhydrides during the heterogeneous reaction because of the low accessibility of cellulose and hemicellulose in the heterogeneous medium, which is associated with the difficulty of chemical diffusion into the cell wall (Rowell et al. 1994b).

Ionic liquids with strong hydrogen bonding donor-acceptor ability allow the complete dissolution of woody biomass (Li et al. 2015); they have been used for the homogeneous chemical modification of biomass to achieve a high degree of substitution (Zhang et al. 2016). However, limited information on thermal stability is available for the as-prepared lignocellulosic derivatives.

Yuan et al. (2011; 2010) report that the homogeneous esterification of wood with acyl chloride in ionic liquids decreases its thermal stability by the destruction of the crystalline structure associated with a high degree of substitution. Xie et al. (2007) report that the acetylation of spruce with acetic anhydride has no effect on its thermal stability, and benzoylation increases the thermal stability of spruce when ionic liquid is used as the reaction medium. In summary, there is no general agreement regarding how the homogenous esterification using ionic liquids as reaction medium affects the thermal stability of lignocellulose.

1-Allyl-3-methylimidazolium chloride (AmimCl) represents one of the most widely used ionic liquids for the homogeneous chemical modification of natural macromolecules, due to its low melting point (52 °C) and powerful solubility for macromolecules. Starch sulfates with degree of substitution 1.37 have been reported using AmimCl as solvent at 40 °C within 75 min (Kärkkäinen et al. 2016). 6-O-trityl cellulose with degree of substitution 1.0 has been synthesized in AmimCl with triphenylmethy chloride as regents (Lv et al. 2015).

To evaluate the thermal stability of sugarcane bagasse derivatives bearing carboxyl groups prepared in ionic liquid medium, thermogravimetric analysis (TGA) of the esterified sugarcane bagasse were performed. AmimCl was selected as the solvent for the homogenous esterification of sugarcane bagasse with cyclic anhydrides. The as-prepared sugarcane bagasse esters were confirmed by FT-IR and solid-state NMR studies.

EXPERIMENTAL

Materials

Sugarcane bagasse was obtained from Guangxi Gui-Tang Group Co., Ltd. (Guigang, China). It was washed with hot water (90 °C) five times and then air-dried to remove any water-soluble residues. The washed sugarcane bagasse was ground into 20- to 40-mesh particles and dewaxed by toluene/ethanol (2:1 v/v) in a Soxhlet extractor for 10 h. The dewaxed sugarcane bagasse was oven-dried at 50 °C for 24 h and subjected to planetary ball-milled treatment for 4 h. Sugarcane bagasse is composed of 19.3% lignin, 44.7% glucose, and 29.7% xylose according to the standard NREL methods (Sluiter et al. 2008).

AmimCl was supplied by Shanghai Cheng-Jie Chemical Co., Ltd. (Shanghai, China). Dimethyl sulphoxide (DMSO), succinic anhydride, maleic anhydride, phthalic anhydride, and isopropanol were purchased from Aladdin Industrial, Inc. (Shanghai, China).

Methods

Dissolution of sugarcane bagasse in AmimCl

Sugarcane bagasse meal (1 g) was dispersed into 5 mL of DMSO at room temperature, and 10 g of AmimCl was added to the mixture. The sugarcane bagasse/DMSO/AmimCl mixture was placed in a 110 °C oil-bath under a N₂ atmosphere for 12 h to guarantee complete dissolution.

Preparation of control sample

Upon the complete dissolution of sugarcane bagasse, the solution was transferred to a 90 °C oil-bath and stirred under N₂ atmosphere for 90 min. The as-prepared sugarcane bagasse solution was allowed to cool to room temperature and then poured into 300 mL of isopropanol. The precipitate was collected, thoroughly washed with isopropanol (5 × 100 mL), and oven-dried at 50 °C for 24 h to obtain the control sample. The control sample was composed of 16.8% lignin, 47.5% glucose, and 28.8% xylose according to the standard NREL methods (Sluiter et al. 2008).

Esterification of sugarcane bagasse

The sugarcane bagasse/DMSO/AmimCl solution was set in a 90 °C oil-bath. Cyclic anhydride (succinic anhydride, maleic anhydride, or phthalic anhydride) was added. The amounts of the added anhydride are shown in Table 1. The resulting solution was stirred under N₂ atmosphere for 90 min. The esterified sugarcane bagasse was separated by pouring the resulting solution into 300 mL of isopropanol. The precipitate was collected, thoroughly washed with isopropanol (5 × 100 mL), and oven-dried at 50 °C for 24 h to obtain the sugarcane bagasse derivatives. The weight present gain (WPG) of the sugarcane bagasse derivatives was calculated according to Eq. 1,

WPG = 100% × (m₁− m₀)/m₀ (1)

where m₀ and m₁ are the oven-dry weights of the original sugarcane bagasse (1 g) and the sugarcane bagasse derivatives, respectively.

The degree of substituted OH was calculated according to Yuan et al. (2010),

Substituted OH = 1000 × (m₁− m₀)/(m₀·M_anhydride) (mmol/g) (2)

where M_anhydride is the molar mass of the applied anhydride.

Characterization

Fourier transform infrared spectroscopy (FT-IR) results were recorded using a Bruker Tensor 27 FT-IR spectrophotometer (Ettlingen, Germany). The samples (1 mg) were dispersed in 100 mg of KBr. The mixed powder was pressed into a disk at 10 MPa, which was immediately analyzed. Thirty-two scans were collected per spectrum at a resolution of 4 cm⁻¹.

Solid-state ¹³C NMR spectra were collected using a Bruker AV-III 400 M spectrometer with the cross polarization/magic angle spinning (CP/MAS) technique (Lan et al. 2011).

The TGA/derivative thermogravimetry (DTG) curves were collected using a TA SDT Q500 thermal analyzer (TA Instruments, New Castle, DE, USA). The samples (~10 mg) were heated from 40 °C to 600 °C at a heating rate of 10 °C/min under a N₂ atmosphere (flow rate of 25 mL/min).

The decomposition kinetics were modeled according to the Broido equation (Broido 1969; Khalil et al. 2011),

where T is temperature (K), R is the gas constant (8.314 J/(mol K)), E is the activation energy (kJ/mol), Z is the frequency factor, β is the heating rate, T_m is the temperature at the maximum degradation rate, and α is the fraction of decomposition at temperature T, calculated according to Eq. 4,

where m_T, m₄₀, and m₆₀₀ are the values of residue mass at temperatures T, 40 °C, and 600 °C, respectively.

RESULTS AND DISCUSSION

Preparation of the Sugarcane Bagasse Derivatives

The esterification of sugarcane bagasse with cyclic anhydride is an efficient way to prepare functional materials from the biomass. In previous studies, ionic liquids were used as excellent solvents for the homogeneous modification of sugarcane bagasse with maleic anhydride, and the WPG was controlled by stoichiometric methods (Chen et al. 2013). The carboxylic groups would further react with the O-H groups, resulting in diesters when the temperature reached 100 °C (Matsuda 1987). To ensure the production of carboxylic groups by the esterification between sugarcane bagasse and cyclic anhydride, the temperature was set to 90 °C. In the present study, sugarcane bagasse derivatives with WPG varying from 1.1% to 184% were synthesized by adjusting the anhydride dosage from 10 mmol/g to 50 mmol/g (Table 1). Sugarcane bagasse succinate with WPG of 98.1% was obtained within 90 min at 90 ^oC. It should be noted that it takes 200 h to achieve lignocellulosic succinate with WPG of 90% for a heterogeneous reaction at the boiling point of xylene (approximate 140 ^oC) (Rowell et al. 1994a). Sugarcane bagasse phthalate with WPG no more than 40% is achieved using pyridine as dual catalyst and heterogeneous reaction medium with 75 min, though under the assistant of ultrasound irradiation (Liu et al. 2006). It is suggested that AmimCl is an advanced homogeneous reaction medium to prepare sugarcane bagasse derivatives with high WPG.

Table 1. Preparation of the Sugarcane Bagasse Derivatives

According to data shown in Table 1, substitution of OH groups depended on the type of anhydride. When the amount of anhydride was 10 to 30 mmol/g, the degree of substituted OH decreased in order of Succinic anhydride > Phthalic anhydride > Maleic anhydride. It is expected that succinic anhydride is the most reactive to sugarcane bagasse, as both maleic anhydride and phthalic anhydride are conjugated molecules analogous to succinic anhydride. Previous studies have demonstrated the higher reactivity of succinic anhydride than maleic anhydride (Vaidya et al. 2016). Thus, it is reasonable that sugarcane bagasse succinate had the highest substituted OH. However, the degree of substituted OH decreased in another order of Maleic anhydride > Phthalic anhydride ≈ Succinic anhydride, when the amount of anhydride was 50 mmol/g.

The reaction reached chemical equilibrium with the increasing anhydride amount; thus, a similar degree of substituted OH was observed for sugarcane bagasse succinate and phthalate at anhydride amount of 50 mmol/g. As to the situation of maleic anhydride, self-polymerization may take place as a side reaction, thus resulting in sugarcane bagasse maleate with the lowest degree of substituted OH at anhydride amount of 10 to 30 mmol/g. With further increase of the maleic anhydride amount to 50 mmol/g, some of the poly(maleic anhydride) was attached onto sugarcane bagasse, resulting in a sharply increasing WPG.

Chemical Confirmation of the Sugarcane Bagasse Derivatives

The chemical structure of the sugarcane bagasse derivatives was confirmed by FT-IR studies (Fig. 1). The bands at 1735 cm⁻¹ and 1164 cm^-1 were attributed to the C=O stretching vibration and C-O stretching vibration, respectively. These bands increased in absorption intensity after esterification, suggesting the introduction of ester groups and the successful attachment of the anhydrides onto the sugarcane bagasse. The band at 834 cm^-1 assigned to alkyl C-H deformation increased sharply in intensity for sugarcane bagasse succinate. New bands at 1290 cm^-1 and 740 cm^-1 assigned to aromatic carboxylic C-O stretching and aromatic C-H deformation, respectively, were observed for the sugarcane bagasse phthalates. The results indicated the successful esterification of sugarcane bagasse and cyclic anhydrides.

Fig. 1. FT-IR spectra of the sugarcane bagasse derivatives

Solid-state ¹³C NMR studies provided further evidence for sugarcane bagasse derivatives (Fig. 2). The signal at 172 ppm corresponded to the ester groups (−COO), and it increased in signal intensity after modification. The signal shifted to 168 ppm for the phthalic anhydride-modified sugarcane bagasse (Fig. 2, PA5) because of the conjugation of C=O and the phenyl ring. For the sugarcane bagasse phthalate (Fig. 2, PA5), a strong chemical signal at 130 ppm was observed that represented the introduction of a phenyl ring. For the sugarcane bagasse succinate (Fig. 2, SA5), a strong chemical signal corresponding to -CH₂– was observed at 29.3 ppm. The NMR results suggested that the esterification between sugarcane bagasse and cyclic anhydride occurred.

Fig. 2. Solid-state ¹³C NMR spectra of the sugarcane bagasse derivatives

Thermal Stability of the Modified Sugarcane Bagasse

Figure 3 shows the TGA/DTG curves of the sugarcane bagasse derivatives. The onset degradation temperature of sugarcane bagasse (Fig. 3a) was 230 °C. The thermal degradation of sugarcane bagasse could be divided into three stages. During the first stage, a weight loss of 23.8% was observed in the temperature range of 230 °C to 285 °C. For the second stage, a weight loss of 31.5% was observed in the temperature range of 285 °C to 345 °C. For the third stage, a weight loss of 10.6% was observed in the temperature range of 345 °C to 460 °C. The three stages are generally related to the degradation of hemicellulose, cellulose, and lignin, respectively.

The onset degradation temperature of sugarcane bagasse succinates was 185 °C, showing no relationship to the WPG. The onset degradation temperature decreased gradually to 160 °C with increasing WPG for the sugarcane bagasse maleate, while the sugarcane bagasse phthalates showed the lowest onset degradation temperature at 140 °C (PA2 through PA5). These results suggested that the chemical modification dramatically decreased the thermal stability of sugarcane bagasse. The decrease in the thermal stability of sugarcane bagasse derivatives was attributed to the decomposition of sugarcane bagasse fractions during chemical modification, as the introduction of an acid into the biomass/ionic liquid solution leads to the hydrolysis of polysaccharides (Li et al. 2008).

Unlike native sugarcane bagasse, the thermal degradation of sugarcane bagasse derivatives did not correspond to the lignin, cellulose, and hemicellulose fractions. The sugarcane bagasse succinate showed three degradation stages. However, a weight loss of 53% to 60% was observed at a temperature of 285 °C. The sugarcane bagasse maleates and phthalates showed four stage degradation models, and the weight loss values at 285 °C were 45% to 47% and 52% to 69%, respectively. These results showed that the sugarcane bagasse derivatives had noticeably lower thermal stabilities than the native sugarcane bagasse. Notably, all samples presented a peak at approximately 325 °C for the DTG curves. This peak generally corresponds to the rapid degradation of cellulose (Zhang et al. 2014), and it decreased in height with increasing WPG because of the reaction of cellulose with anhydrides.

The sugarcane bagasse derivatives prepared in AmimCl showed lower thermal stability than those prepared under heterogeneous medium in previous reports (Bodirlau et al. 2008; Chadlia 2010; Khalil et al. 2011). The difference may be attributed to the degradation and decrystallization of sugarcane bagasse fractions during the homogeneous esterification. Similar results have been observed for cellulose and hemicellulose (Liu et al. 2007; Peng et al. 2010). The onset degradation temperature of xylan-rich hemicellulose decreases from 200 °C to 185 °C associated with the decreasing of molecular weight from to 42169 g/mol to 29010 g/mol by homogeneous esterification with maleic anhydride in ionic liquid (Peng et al. 2010). The onset degradation temperature of cellulose decreases from 253 °C to 208 °C associated with the total transformation of crystalline structure into amorphous forms by homogenous esterification with phthalic anhydride in ionic liquid (Liu et al. 2007).

Fig. 3. TGA/DTG curves of the (a) sugarcane bagasse succinates, (b) sugarcane bagasse maleates, and (c) sugarcane bagasse phthalates

Thermal Degradation Kinetics Studies

For the thermal degradation kinetic studies, a first-order degradation reaction was assumed, and the kinetic parameters for various degradation stages were determined according to Eq. 3 (Gao et al. 2006). The activation energies (E) determined from the slopes of the plots of ln(lnα) vs. 1/T according to Broido’s equation for various stages of the samples are presented in Tables 2 and 3. According to the TGA/DTG analysis, a three-stages degradation model was applied for sugarcane bagasse and sugarcane bagasse succinate to calculate the degradation activation energy. In case of sugarcane bagasse maleates and phthaltes, a four-stages degradation model was applied.

Table 2. Activation Energies of Sugarcane Bagasse and Succinates

Table 3. Activation Energies of Sugarcane Bagasse Maleates and Phthalates

For sugarcane bagasse, the activation energy of the first stage of degradation (E₁) was 59.0 kJ/mol, which was close to that of the second stage (E₂; 55.7 kJ·mol⁻¹), and the activation energy of the third stage (E₃) was 17.4 kJ/mol. After modification with succinic anhydride, the activation energies of E₁, E₂, and E₃ decreased. The value of E₂ was noticeably less than that of E₁ for sugarcane bagasse succinates. E₁ showed no relationship to the WPG, while E₂ tended to decrease with increasing WPG. E₃ was 8.3 kJ/mol for the SA2 through SA5 samples, which was much lower than in the control sample.

For sugarcane bagasse modified with maleic anhydride and phthalic anhydride, there were four stages in the degradation model. In general, the first thermal degradation stage of sugarcane bagasse maleates and phthalates showed the highest activation energy (E₁), while the fourth thermal degradation stage showed the lowest activation energy (E₄), which was similar to the trend observed for sugarcane bagasse succinates. The values of E₁, E₂, and E₃ for sugarcane bagasse derivatives were much lower than those of the Control sample. The E₁ and E₄ of sugarcane bagasse maleates and sugarcane bagasse phthalates showed no relationship to the WPG, while the samples with higher WPG tended to have lower E₂ and E₃ values.

The cumulative activation energies (E_T) of sugarcane bagasse derivatives were lower than that of the control sample, with the exception of MA1 and PA1. The high activation energies of MA1 and PA1 may have resulted from the low WPG and the four-stage degradation model. The E_T of the sugarcane bagasse derivatives decreased with increasing WPG. The low activation energies suggested that homogeneous chemical modification dramatically decreased the thermal stability of sugarcane bagasse.

Kinetics studies suggested that homogeneous esterification would result in the decrease of degradation activation energies of sugarcane bagasse. Similar results have been reported for cellulose-graft-poly(L-lactic acid) homogeneous prepared using AmimCl as medium (Dai et al. 2014). The activation energy of cellulose-graft-poly(L-lactic acid) decreases with the increasing poly(L-lactic acid) content (Dai et al. 2014). However, opposite results have been reported for those prepared in heterogeneous media. Khalil et al. (2011) report that the activation energy of Acacia mangium increases from 70 kJ/mol to 94 kJ/mol after heterogeneous esterification with succinic anhydride. It is suggested that homogeneous esterification applying ionic liquids as reaction medium would result in a lowering of thermal stability of lignocellulose, while heterogeneous esterification would not. The difference is attributed to the degradation and decrystallization along with homogeneous esterification (Poletto et al. 2012).

CONCLUSIONS

Homogeneous esterification using ionic liquids as solvents was demonstrated to be a more efficient approach than heterogeneous esterification for the modification of lignocellulose.
TGA studies suggested the thermal stability of sugarcane bagasse was dramatically decreased, with a reduction in the onset degradation temperature and degradation activation energies upon homogeneous chemical modification in ionic liquid.
The sugarcane bagasse modified with maleic anhydride and phthalic anhydride conformed to four-step thermal degradation models, while the unmodified sugarcane bagasse and those modified with succinic anhydrides followed three-step thermal degradation models.
Lignocellulose derivatives prepared homogeneous ionic liquids showed lower thermal stability than those prepared in heterogeneous media.

ACKNOWLEDGMENTS

This work was financially supported by the Scientific and Technological Project of Guangdong Province (Nos. 2013B091500080, 2013B050800023, and 2015A010105019).

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Article submitted: December 16, 2015; Peer review completed: March 30, 2016; Revised version received and accepted: May 27, 2016; Published: June 2, 2016.

DOI: 10.15376/biores.11.3.6254-6266