This study aims to investigate the degradation and stability of pulp treated in heterogeneous and homogeneous phases. The results showed that the homogeneous system 1-Allyl-3-Hexylimidazolium chloride (AHIMCl) ionic liquid exhibited special dissolubility for pulp samples, but showed lower thermal stability than the heterogeneous treatments by 20 wt% NaOH or 2-ethanediamine (EDA) solution. Compared with the 20 wt% NaOH solution, the 20 wt% EDA solution and AHIMCl treatments had special decrystallizing ability, and the 20 wt% EDA solution had lower reductions in the mean degree of polymerization of pulp after the treatments for 72 h at 5 oC. X-ray diffractogram (XRD) analysis showed that after the AHIMCl and 20 wt% EDA solution treatments, the 002 crystal plane size of the treated pulp samples (＜1 nm) became much lower than that of the raw pulp (5.09 nm). The diffracted intensity indicating 101 crystal planes nearly disappeared from the XRD curve of AHIMCl treated pulp samples. The X-ray photoelectron spectroscopy (XPS) analysis indicated that the significant reduction in C1s2 and O1s2 contents of the regenerated samples after the AHIMCl treatment implies that AHIMCl severely destroy the in crystalline and amorphous regions.
Degradation and stability of pulp treated in heterogeneous and homogeneous phases
Kunfeng Xia, Rendang Yang, Detao Liu,* Fei Yang, Bin Wang, and Long Li
This study aims to investigate the degradation and stability of pulp treated in heterogeneous and homogeneous phases. The results showed that the homogeneous system 1-Allyl-3-Hexylimidazolium chloride (AHIMCl) ionic liquid exhibited special dissolubility for pulp samples, but showed lower thermal stability than the heterogeneous treatments by 20 wt% NaOH or 2-ethanediamine (EDA) solution. Compared with the 20 wt% NaOH solution, the 20 wt% EDA solution and AHIMCl treatments had special decrystallizing ability, and the 20 wt% EDA solution had lower reductions in the mean degree of polymerization of pulp after the treatments for 72 h at 5 oC. X-ray diffractogram (XRD) analysis showed that after the AHIMCl and 20 wt% EDA solution treatments, the 002 crystal plane size of the treated pulp samples (＜1 nm) became much lower than that of the raw pulp (5.09 nm). The diffracted intensity indicating 101 crystal planes nearly disappeared from the XRD curve of AHIMCl treated pulp samples. The X-ray photoelectron spectroscopy (XPS) analysis indicated that the significant reduction in C1s2 and O1s2 contents of the regenerated samples after the AHIMCl treatment implies that AHIMCl severely destroy the hydrogen bonds in crystalline and amorphous regions.
Keywords: Degradation; Treatments; Pulp; Ionic liquid
Contact information: State Key Laboratory of Pulp and Paper Engineering. South China University of Technology, 510640, Guangzhou, P.R. China; *Corresponding author: email@example.com
Treatments for lignocelluloses have attracted great attention in recent years because of their potential to achieve new interesting properties (Dawson 1997; Farrell et al. 2006; Hahn-Hägerdal et al. 2006). The approaches for treatment including physicochemical methods, combined with biological or mechanical processes, have been universally used in the production of biodegradable plastics, paper, biofuels, and biocompatible composites (Cowling et al. 1976; Lynd et al. 1991; Sun et al. 2002; Pérez et al. 2002; Zhang et al. 2007; Liu et al. 2008; Pérez et al. 2010). For example, alkali liquors (e.g., NaOH and KOH) and amine-based compound solutions (e.g., ethanediamine and liquid ammonia) have been widely used for the treatments of lignocelluloses, making the application of bio-based resources in other industries possible (Ouajai et al. 2005; Orden et al. 2006; Pérez et al. 2010; Wang et al. 2010; Wan et al. 2011). In these processes, chemical and physical structures indicating the surface or/and interior of lignocelluloses are changed as a result of the conditions of treatment, and such changes have been used as the necessary base for the further applications.
Current studies for treating lignocelluloses, such as alkali liquor treatments, have been mostly performed in a heterogeneous phase (Mannan 1993; Oh et al. 2005; Ouajai et al. 2005; Urreaga et al. 2007; Wan et al. 2011; Heinze et al. 2001). In most heterogeneous systems, however, these approaches cause various problems such as non-uniform reactions, lower efficiency, and heavy pollution. In this regard, homogeneous systems offer the potential not only to overcome these problems, but also to contribute innovation for treating lignocelluloses. For example, green ionic liquids (ILs) (e.g., 1-Butyl-3-Methylimidazolium Chloride (BMIMCl) and 1-Allyl-3-methylimidazolium Chloride (AMIMCl) ) have been recently developed, making it possible to treat celluloses in a homogeneous phase due to their perfect dissolution of lignocelluloses (Kantelinen et al. 1993; Fink et al. 2001; Ren et al. 2003; Cao et al. 2009; Lee et al. 2009). Treatments achieved in homogeneous phase may contribute to a complete and uniform change relative to both the surface and crystalline structure, an effect that is nearly impossible to achieve when employing heterogeneous phases. However, information on these distinctions in heterogeneous and homogeneous phases is rather scarce in the literature.
Pretreatments are widely regarded as an essential step in processing of lignocelluloses, rendering the material suitable for further processing in the production of biofuels, chemicals, and so on. This paper aims at providing a theoretical basis for pretreatments of lignocelluloses in the two different phases. In this work, Bleached Kraft Softwood Pulp (BKSP) was used as the lignocellulosic material. Heterogeneous NaOH and EDA solutions and homogeneous 1-Allyl-3-Hexylimidazolium Chloride (AHIMCl) treatments for lignocelluloses were investigated and compared. The effects of these treatments on the mean degree of polymerization, crystalline structure, and thermal stability of the pulp samples were investigated and compared by X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Thermogravimetric (TG), and Scanning Electron Microscope (SEM) analyses.
Bleached Kraft Softwood Pulp (Wood species, Pinus massoniana) was supplied by Guangdong Eagle Force Paper Co., Ltd (Guangdong, China). Copper (II)-ethylene-diamine complex (CED) was supplied by J&K Scientific Co., Ltd (Beijing, China). The ethylenediamine-to-copper ratio in the CED solution was 2.00±0.04 to 1, with copper molarity of 1.00±0.02 mole. Chemical agents including sodium hydroxide (NaOH) and EDA were purchased from Guangzhou Qianhui Bose Instrument Co., Ltd (Guangdong, China). 1-Allyl-3-Hexylimidazolium chloride ionic liquid (AHIMCl) (Chemical formula, C12H21N2Cl) was synthesized in accordance with the method of Liu et al. (2012): To N- Hexylimidazole in a glass-lined reactor, approximately 1:1.2 molar ratio of allyl chloride was added dropwise under argon gas atmosphere at room temperature. After completely adding the allyl chloride, the reaction mixture was stirred magnetically with reflux at 55 °C for about 10 h. After removing the residual allyl chloride under reduced pressure, the resulting liquid was repeatedly washed with an excess amount of ether to eliminate the residual N-hexylimidazole. The resulting IL solution was dried under vacuum at 80 °C for 72 h.
Pretreatment of the BKSP
The BKSP of Pinus massoniana species with weight-average length of 1 to 3 mm and weight-average width of 26 to 28 μm, was dispersed after being soaked in water for 24 h. Subsequently, the BKSP slurry was treated with 0 to 20 wt% aqueous NaOH and 20 wt% EDA solutions at 5 °C for 72 h. The pretreatment of the raw BKSP with AHIMCl was conducted as follows: In 5 mL of AHIMCl, approximately 8% of the raw BKSP was added and subsequently stirred by magnetic method at 100 °C until complete dissolution. After the pretreatments, the BKSP slurry was washed with deionized water until the pH of the washings was 7. After filtration, the treated samples were dried under vacuum at 40 °C for 24 h. The reason for placing the regenerated cellulose samples in water matter is that new hydrogen bonds between cellulose chains are restructured by water molecular after the cellulose solution is inserted into the water phase. In this process, anions and cations from ILs located between hydrogen bonds of cellulose chains are replaced by H+ and OH– from water molecular (Liu et al. 2012).
Mean degree of polymerizations analysis
The mean degree of polymerizations of the BKSP treated with aqueous NaOH and EDA solution with concentration from 1% to 20 wt% at 5 to 80 oC, were investigated by the viscosity method (GB, 1986).
Degree of crystallinity analysis
The XRD method serves as the most widely used method to investigate the crystal structures of lignocelluloses (Park et al. 2010). The crystal structures of the BKSP treated with aqueous 20 wt% NaOH and EDA solutions at 5 oC, as well as AHIMCl, was analyzed using an XRD analyzer (Rigaku D/max-III X-ray diffractometer) set at 40 kV and 30 mA. Wide-angle x-ray intensities were collected for 2θ values ranging from 4° to 60°, with a step scanning rate of 8°/min and step increment of 0.04°.
The degree of crystallinity is calculated by the following equation (Segal et al. 1959),
where Xc indicates the crystallinity of BKSP sample; Iam is the intensity of amorphous regions of Cellulose I and Cellulose II at diffraction angle of 15.0°and 18.0°, respectively; and Ic is the intensity of crystal planes of Cellulose I and Cellulose II, respectively.
Crystallite size analysis
The crystallite size of BKSP sample was calculated by the Scherrer formula (Cao et al. 2002),
where K is the Scherrer constant (0.89), D is the apparent crystallite size (nm), λ is the wavelength of the X-ray (0.154056 nm), β is the full width at half maximum (rad), and θ is the diffraction angle.
Thermogravimetric analysis (TGA) was used to study the thermal stability of the BKSP treated with aqueous 20 wt% NaOH and EDA solutions, as well as AHIMCl. A TGA Q500 instrument was used to record weight loss within the range of room temper-ature to 500 oC, with a heating rate of 10 oC/min. The test samples for TGA were fibrous solids of about 25 mg. The flow rate of nitrogen source was 25 mL/min. The TGA pan was made of platinum.
X-ray photoelectron spectroscopy (XPS) analysis was conducted to study the effects of treatments on the main elements (e.g., C and O) on the BKSP. The surface of samples was observed using a Kratos AXis Ultra DLD (Kratos Company, Britain) at 7.9×10-10 vacuum degree by Mono (Al) source. Energy resolution was 0.48 eV, and the spatial resolution imaging was 3 µm. And the spatial resolution was 95 nm.
A scanning electron microscope (SEM) was used to examine the surface image of the raw BKSP samples and AHIMCl-treated BKSP samples. All the samples were fixed to a metal-base specimen holder using double-sided, sticky tape. The fixed samples were coated with gold, and they were then observed using a Philips XL-30 ESEM scanning electron microscope.
RESULTS AND DISUSSION
Effects of Treatments on the Mean Degree of Polymerization
The mercerized cellulose can be obtained by a treatment using a certain concentration of aqueous NaOH solution (Kolpak et al. 1978). In this process, aqueous NaOH solution swells and mercerizes the cellulose, changing the crystalline structures from cellulose I to cellulose II (Francis et al. 1978).
Figure 1 shows the effects of the different concentrations of aqueous NaOH solu-tion on the mean degree of polymerization of the BKSP. By increasing the concentration of aqueous NaOH solution from 1 wt% to 20 wt%, the mean degree of polymerization of the BKSP exhibited initially a decrease and subsequently an increasing tendency. When the raw BKSP was treated with lower concentrations of aqueous NaOH solution (≤5 wt%), the mean degree of polymerization slightly decreased because of cleavage of some of the glycosidic linkages in cellulose molecules from alkali degradation.
Fig. 1. Mean degree of polymerization as a result of the concentration and temperature of treatments of the raw BKSP by NaOH and EDA solutions. A = Mean degree of polymerization of the BKSP treated by 0 to 20 wt% NaOH solution at room temperature for 4 h; B = Mean degree of polymerization of the BKSP treated by 20 wt% EDA solution at room temperature for 72 h; C = Mean degree of polymerization of the BKSP treated by 20 wt% NaOH solution at 50 ºC for 4 h; D = Mean degree of polymerization of the BKSP treated by 20 wt% NaOH solution at room temperature for 72 h; E = Mean degree of polymerization of the BKSP treated by 20 wt% NaOH solution at 5 ºC for 72 h; F = Mean degree of polymerization of the BKSP treated by 20 wt% NaOH solution at 80 ºC for 4 h.
As the concentration of aqueous NaOH solution was further increased to 20 wt%, the mean degree of polymerization of the BKSP increased. Moreover, the mean degree of polymerization of the BKSP samples treated with 15 wt% and 20 wt% aqueous NaOH solutions was greater than that of the raw BKSP. Small polyose derivatives resulting from the aqueous NaOH solution with higher concentrations are probably reaggregated in the process of water for washing at 5 oC. This contributes to an increase of viscosity of the cellulose solution in CED solvent. Viscosity of cellulose solution is used to estimate the mean degree of polymerizations of cellulose samples. With the treatment of 20 wt% NaOH solution, however, the mean degree of polymerization of the BKSP samples decreased significantly with the increase in treating temperature or time (See Fig. 1). Instead of polymerization, severe alkaline degradation occurred at high temperatures (e.g., 80 °C). Compared with the 20 wt% NaOH solution treatment, the 20 wt% EDA solution treatment had lesser effect on the mean degree of polymerization of the BKSP.
Crystalline Structure of the Treated BKSP
Treatments may influence not only the mean degree of polymerization, but also the crystal structures of the BKSP samples. In the present study, the aqueous NaOH and EDA solutions exhibited chemical effects on the cellulose structure partly because of the cleavage of cellulose chains by alkaline reaction. However, a previous study reported that the ILs investigated displayed mainly physical effects on cellulose structure by breaking the hydrogen bonds between cellulose chains (Pinkert et al. 2009). Figure 2 illustrates the effects of the aforementioned treatments on the crystal structures of the BKSP samples.
After the aqueous NaOH and AHIMCl solution treatments, the crystal form of cellulose I of the raw BKSP had been transformed to cellulose II. The distinct diffracted intensity of the above BKSP samples at 2θ values was less intense. Moreover, the intensity shifted to a lower 2θ value of 21.20° after the 20 wt% aqueous NaOH solution treatment at 5 oC. A more distinct decrease in diffracted intensity was observed after the AHIMCl treatment; the weak diffracted intensity of the BKSP samples treated with AHIMCl at 2θ values shifted to a lower 2θ value of 21.72°. Under this condition, the diffracted intensity indicating 101 crystal plane nearly disappeared (as shown in Fig. 2). The crystalline structure of the BKSP was damaged by ILs (Miyafuji et al. 2009).
As shown in Fig. 3, the aforementioned treatments damaged the crystal structure of cellulose. The degree of crystallization of raw BKSP decreased from 63.93% to 61.35% and 52.96% after the 20% aqueous NaOH and the AHIMCl treatments, respectively. However, after the 20% aqueous EDA solution treatment the degree of crystallization of raw BKSP decreased to approximately 40.5%. This result suggests that EDA has a special decrystallization effect on lignocelluloses (Nada et al. 1990).
The 002 crystal plane size of the BKSP samples varied almost in the same manner during the aforementioned treatments. After the AHIMCl and EDA solution treatments, the 002 crystal plane size of the BKSP samples (1 nm) became lower than that of the raw BKSP (5.09 nm). This result shows that, unlike the aqueous NaOH treatment, the EDA solution and AHIMCl treatments can severely damage the crystalline structure of lignocelluloses.
Fig. 2. XRD patterns of the raw BKSP (a), 20 wt% NaOH-treated BKSP samples (b), 20 wt% EDA-treated BKSP samples (c) for 72 h at 5 ºC, and AHIMCl-treated BKSP samples (d)
Fig. 3. 002 crystal plane size and degree of crystallinity of the BKSP samples (a) resulting from 20 wt% NaOH (b) and EDA (c) treatments for 72 h at 5 ºC, and AHIMCl (d) treatments.
Electron Spectroscopy for Chemical Analysis
X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate in detail the EDA solution and AHIMCl-treated BKSP samples. As shown in Fig. 4, the main elements (i.e., C and O) were observed on the surface of the BKSP samples after the treatments. In addition, the N and Cl elements were observed on the surface of the BKSP samples treated with AHIMCl. These observations indicate that a small amount of AHIMCl residue was retained on the surface of the BKSP despite abundant washings by water.
Fig. 4. XPS wide scan spectra of the raw BKSP and the BKSP samples treated by AHIMCl and 20 wt% EDA, respectively.
Fig. 5. XPS spectra of C1s of the raw BKSP (A), EDA-treated BKSP (B), and AHIMCl-treated BKSP (C) samples
Fig. 6. XPS spectra of O1s of the raw BKSP (A), EDA-treated (B), and AHIMCl-treated BKSP samples (C) samples
Peak deconvolution was applied to investigate the spectra of C1s and O1s peaks to determine the changes in the chemical elements and bonds information after the treatments. The peaks resulted in three carbon (C1s1, C1s2, and C1s3) and two oxygen (O1s1 and O1s2) components. A previous study reported that the binding energy of C1s1, C1s2, and C1s3 is 285.0 eV, 286.8 eV, and 288.0 eV, respectively, in the cellulosic solid phase (Gelius et al. 1970). C1s1 indicates the carbon element linked to hydrogen or to carbon (-C-H or -C-C). C1s2 shows carbon element linked to a single oxygen element (-C-O-), whereas C1s3 bonds to two non-carbonyl oxygen atoms [-C (O)-O-] or a single carbonyl oxygen atom (-C=O) (Dorris et al. 1978a,b). Remarkable differences in the relative contents of C1s1, C1s2, and C1s3 components were observed after the treatments (as shown in Fig. 5). As shown in Table 1, the binding energy of C1s1, C1s2, and C1s3 was 284.56 eV, 286.38 eV, and 288.03 eV, respectively. These results are consistent with those reported in the literature (Dawson, 1997; Hua et al. 1993a; Hua et al. 1993b; Jaić et al. 1996; Wistara et al. 1999). The XPS spectrum of the BKSP samples treated with EDA solution and AHIMCl was dominated by the C1s1 peak (C-C and C-H bonds) with a relatively low peak width of 1.50 eV.
Previous studies suggested that C1s1 mainly implies the content of lignin and extractives, because carbohydrates give rise only to C1s2 and C1s3 peaks (Ahmed et al. 1987; Kamdem et al. 1991). The relative C1s1, C1s2, and C1s3 contents of the raw BKSP samples were 7.13 %, 51.84 %, and 41.03 %, respectively (as shown in Table 1). The data imply that the BKSP contains only marginal lignin and mostly cellulose and hemicellulose (Dorris et al. 1978a; Hua et al. 1993a).
Table 1. C1s XPS Data for the BKSP Samples
Compared with the raw BKSP, the BKSP samples treated with EDA had more C1s2 and less C1s1 content, whereas that treated with AHIMCl had less C1s1 and C1s2 contents but more C1s3content (as shown in Table 1). The decrease in the relative C1s1 content shows that the carbohydrate on the surface of the BKSP increased after the EDA solution and AHIMCl treatments. This contribution of C1s2 to C-O bonding indicates that there were changes in the hydrogen bonds in the cellulose region. Thus, the AHIMCl treatment can severely destroy the abundant hydrogen bonds in cellulose regions. This conclusion can be explained by the fact that the cation [AHIM+] and anion [Cl–] from AHIMCl attack the hydrogen bonding in crystalline and amorphous regions (Pinkert et al. 2009). However, the C1s2 of the BKSP samples treated with EDA solution tended to be higher, which might have resulted from the reconstruction of hydrogen bonding.
The C-O bonding in lignocelluloses mainly consists of C-OH-O and free C-OH, which can be expressed by O1s1 and O1s2 XPS peaks, respectively (Hua et al. 1993b). Therefore, the relative O1s1 content mainly reflects the changes in hydrogen bonds between oxygen elements and hydroxyl groups, whereas O1s2 exists only in hydroxyl groups (Ahmed et al. 1987a,b). The O1s1 and O1s2 XPS peaks of the above treating samples are also shown in Fig. 6. The O1s1 and O1s2 binding energies of the raw BKSP were 532.36 and 533.98 eV, respectively (see Table 2).
Table 2. O1s XPS Data of the BKSP Samples
These results agree well with those previously reported (Dorris et al. 1978a,b; Ahmed et al. 1978a,b). After the AHIMCl treatment, the O1s1 of the BKSP samples decreased by 4.66%. This result indicates that the hydrogen bonds in the BKSP samples were destroyed by AHIMCl, an effect that was also observed in the C1s XPS peak discussed above. The high O1s1content observed is probably the result of the reconstruction of hydrogen bonding during washing of the BKSP/AHIMCl liquid with water. After the EDA solution treatment, the O1s1content slightly increased.
The chemical treatments probably contribute to the changes in the thermal stabilities of the BKSP samples. Figure 7 shows the TGA curves of the raw BKSP and the BKSP samples treated with aqueous NaOH and EDA solutions, as well as with AHIMCl. From room temperature to 150 oC, the weight losses of the BKSP samples after aqueous 20 wt% NaOH, EDA solutions, and the AHIMCl treatments were 3.01%, 3.67%, and 11.63%, respectively. The weight losses of the BKSP samples after AHIMCl treatment were higher than those of the other samples.
As shown in Fig. 7, the greatest decomposition of the AHIMCl-treated BKSP samples occurred at 255.1 oC (Td), which was lower than that of EDA- (345.9 oC) and NaOH-treated (346.1 oC) BKSP samples. The combined weight loss of the AHIMCl-, EDA-, and NaOH-treated BKSP samples at 150 oC to Td was 41.42%, 55.77%, and 57.13%, respectively (as shown in Fig. 7).
Based on the similar combined weight loss and highest decomposition tempera-ture of the BKSP samples, commercial EDA and NaOH solutions did not have a signify-cant effect on thermal stabilities of the BKSP samples. This finding indicates that nearly all the damage in the crystalline structure and the structural rearrangements of cellulose chains results from the dissolution of the BKSP in AHIMCl (Muhammad et al. 2010).
Fig. 7. Temperature dependencies of weight loss for the BKSP samples by the treatments of aqueous NaOH and EDA solutions, as well as with AHIMCl
Fig. 8. SEM image of raw BKSP (A) and the regenerated samples treated by AHIMCl (B) (×200)
The results showed that the BKSP samples after the treatments by 20 wt% NaOH and EDA solutions still retained nearly the same morphology in heterogeneous phases. No single fiber from the BKSP samples was observed, however, after the treatments with AHIMCl, due to the dissolution in a homogeneous phase. As shown in the SEM images, the regenerated samples treated with AHIMCl exhibited an unordered rearrangement of cellulose chains that was in contrast to the raw BKSP samples, which consisted of glossy arrangements of the BKSP fibers (See Fig. 8).
- Increasing the treating temperature or time decreased the mean degree of polymerize-tion of the BKSP in the 20 wt% aqueous NaOH solution.
- An increased mean degree of polymerization of the BKSP was observed when increasing the concentration of aqueous NaOH solution from 5 wt% to 20 wt%.
- 20 wt% EDA solution treatment had a negligible effect on the mean degree of polymerization of the BKSP, but it had a great effect on the degree of crystallization and the 002 crystal plane sizes of the BKSP samples.
- After the AHIMCl treatment, the degree of crystallization and 002 crystal plane sizes of the BKSP samples decreased significantly, and resulted in poor thermal stability.
- Compared with the raw BKSP, the EDA-treated BKSP had more C1s2 and less C1s1 content, whereas the AHIMCl-treated BKSP samples had less C1s1 and C1s2 content. The reduction in C1s2 and O1s2 contents of the AHIMCl-treated BKSP samples indicates that AHIMCl can destroy hydrogen bonds.
The authors acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 31000285), the Fundamental Research Funds for the Central Universities, South China University of Technology, PR China (No. 2011ZM0102), and State Key Lab of Subtropical Building Science, South China University of Technology, PR China (No. 2010KB21).
Ahmed, A., Adnot, A., Grandmaison, J. L., Kaliaguine, S., and Doucet, J. (1987a). “ESCA analysis of cellulosic materials,” Cell. Chem. Technol. 21(5), 483-492.
Ahmed, A., Adnot, A., and Kaliaguine, S. (1987b). “ESCA study of the solid residues of supercritical extraction of Populus tremulóides in methanol,” J. Appl. Polym. Sci. 34(1), 359-375.
Cao, Y., and Tan, H. M. (2002). “Effects of cellulase on the modification of cellulose,” Carbohyd. Res. 337(14), 1291-1296.
Cao, Y., Wu, J., Zhang, J., Li, H. Q., Zhang, Y., and He, J. S. (2009). “Room temperature ionic liquids (RTILs): A new and versatile platform for cellulose processing and derivatization,” Chem. Eng. J. 147(1), 13-21.
Cowling, E. B., and Kirk, T. K. (1976). “Properties of cellulose and lignocellulosic materials as substrates for enzymatic conversion processes,” Biotechnology Bioengineer Symposia; United States; 6, 95-123.
Dawson, R. S. W. (1997). “Determination of surface changes in Pinus radiata fibre on chemical treatment by X-ray photo-electron spectroscopy (XPS),” European Journal of Wood and Wood Products 55(1), 57-58.
Dorris, G. M., and Gray, D. G. (1978a). “The surface analysis of paper and wood fibers by ESCA I,” Cell. Chem. Technol. 12, 9-23.
Dorris, G. M, and Gray, D. G. (1978b). “The surface analysis of paper and wood fibers by ESCA II. Surface composition of mechanical pulps,” Cell. Chem. Technol. 12, 721-734.
Farrell, A. E., Plevin, R. J., Turner, B. T., Jones, A. D., O’Hare, M., and Kammen, D. (2006). “Ethanol can contribute to energy and environmental goals,” Science 27(311), 506-508.
Fink, H. P., Weigel, P., Purz, H. J., and Ganster, J. (2001). “Structure formation of regenerated cellulose materials from NMMO-solutions,” Prog. Polym. Sci. 26(9), 1473-1524.
Francis, J., Kolpak, M. W., and John, B. (1978). “Mercerization of cellulose: 1. Determination of the structure of mercerized cotton,” Polymer 19(2), 123-131.
GB5888-86, 1986. “Testing method of degree of polymerization of ramie cellulose,” Standardization Administration of the People’s Republic of China.
Hahn-Hägerdal, B., Galbe, M., Gorwa-Grauslund, M. F., Lidén, G., and Zacchi, G. (2006). “Bio-ethanol – The fuel of tomorrow from the residues of today,” Trends Biotechnol. 24(12), 549-566.
Heinze, T., and Liebert, T. (2001). “Unconventional methods in cellulose functionalization,” Prog. Polym. Sci. 26(9), 1689-1762.
Hua, X., Kaliaguine, S., Kokta, B.V., and Adnot, A. (1993a). “Surface analysis of explosion pulps by ESCA. Part 1. Carbon (1 s) spectra and oxygen-to-carbon ratios,” Wood Sci. Technol. 27(6), 449-459.
Hua, X., Kaliaguine, S., Kokta, B. V., and Adnot, A. (1993b). “Surface analysis of explosion pulps by ESCA Part 2. Oxygen (1 s) and sulfur (2p) spectra,” Wood Sci. Technol. 28(1), 1-8.
Jaić, M., Živanović, R., Stevanović-Janežić, T., and Dekansld, A. (1996). “Comparison of surface properties of beech- and oakwood as determined by ESCA method,” European Journal of Wood and Wood Products 54(1), 37-41.
Kamdem, D. P., Riedl, B., Adnot, A., and Kaliaguine, S. (1991). “ESCA spectroscopy of poly (methylmethacrylate) grafted onto wood fibers,” J. Appl. Polym. Sci. 43(10), 1901-1912.
Kantelinen, A., Hortling, B., Ranual, M., and Viikari, L. (1993). “Effects of fungal and enzymatic treatments on isolated lignins and on pulp bleachability,” Holzforchung 47(1), 29-35.
Kolpak, F. J., and Blackwell, J. (1978). “Mercerization of cellulose. 2. The morphology of mercerized cotton cellulose,” Polymer 19(2),132-135.
Lee, S. H., Doherty, T. V., Linhardt, R. J., and Dordick, J. S. (2009). “Ionic liquid-mediated selective extraction of lignin from wood leading to enhanced enzymatic cellulose hydrolysis,” Biotechnol. Bioeng. 102(5), 1368-1376.
Liu, D. T., Li, J., Yang, R. D., Mo, L. H., Huang, L. H., Chen, Q. F., and Chen, K. F. (2008). “Preparation and characteristics of molded biodegradable cellulose fibers/MPU-20 biocomposites (CFMCs) by steam injection technology,” Carbohyd. Polym. 74(2), 290-300.
Liu, D. T., Xia, K. F., Cai, W. H., Yang, R. D., Wang, L. Q., and Wang, Q. (2012). “Investigations about dissolution of cellulose in the 1-allyl-3-alkylimidazolium chloride ionic liquids,”Carbohyd. Polym. 87(2), 1058-1064.
Lynd, L. R., Cushman, J. H., Nichols, R. J., and Wyman, C. E. (1991). “Fuel ethanol from cellulosic biomass,” Science 251 (4999), 1318-1323.
Mannan, K. M. (1993). “X-ray diffraction study of jute fibers treated with NaOH and liquid anhydrous ammonia,” Polymer 34(12), 2485-2487.
Miyafuji, H., Miyata, K., Saka, S., Ueda, F., and Mori, M. (2009). “Reaction behavior of wood in an ionic liquid, 1-ethyl-3-methylimidazolium chloride,” J. Wood Sci. 55, 215-219.
Muhammad, N., Man, Z., Khalil, M. A.B., and Maitra, S. (2010). “Studies on the thermal degradation behavior of ionic liquid regenerated cellulose,” Waste and Biomass Valorization 1(3), 315-321.
Nada, A. M. A., Shabaka, A. A., Yousef, M. A., and Abd-El-Nour, K. N. (1990). “Infrared spectroscopic and dielectric studies of swollen cellulose,” J. App. Poly. 40(5-6), 731-739.
Orden, M. U., and Urreaga, J. M. (2006). “Photooxidation of cellulose treated with amino compounds,” Polym. Degrad. Stabil. 91(9), 2053-2060.
Ouajai, S., and Shanks, R. A. (2005). “Composition, structure and thermal degradation of hemp cellulose after chemical treatments,” Polym. Degrad. Stabil. 89(2), 327-335.
Oh, S. Y., Yoo, D. I., Shin, Y., Kim, H. C., Kim, H. Y., Chung, Y. S., Park, W. H., and Youk, J. H. (2005). “Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy,” Carbohyd. Res. 340(15), 2376-2391.
Park, S., Himmel, M. E., Baker, J. O., and Johnson, D. K. (2010). “Cellulose crystallinity index: Measurement techniques and their impact on interpreting cellulase performance,” Biotechnol. Biofuels 3, 1-10.
Pérez, J., Muñoz-Dorado, J., Rubia, T., and Martínez, J. (2002). “Biodegradation and biological treatments of cellulose, hemicellulose and lignin: An overview,” Int. Microbiol. 5(2), 53-63.
Pérez, S., and Samain, D. (2010). “Structure and engineering of celluloses,” Adv. Carbohyd. Chem. Bi. 64, 25-116.
Pinkert, A., Kenneth, N., Marsh, S. P., and Mark, P. S. (2009). “Ionic Liquids and their Interaction with Cellulose,” Chem. Rev. 109(12), 6712–6728.
Ren, Q., Wu, J., Zhang, J., He, J. S., and Guo, M. L. (2003). “Synthesis of 1-allyl, 3-methylimidazolium-based room temperature ionic liquid and preliminary study of its dissolving cellulose,” Acta Polym. Sin. 3, 448-451.
Segal, L., Creely, J. J., Martin, A. E., and Conrad, C. M. (1959). “An empirical method for estimating the degree of crystallinity of native cellulose using the X ray diffreactometer,” Text Res. J. 29(10), 786-794.
Sun, Y., and Cheng, J. Y. (2002). “Hydrolysis of lignocellulosic materials for ethanol production: A review,” Bioresource Technol. 83(1), 1-11.
Urreaga, J. M., and Orden, M. U. “Modification of cellulose with amino compounds: A fluorescence study,” Carbohyd. Polym. 69(1), 14-19.
Wan, C. X., Zhou, Y. G., and Li, Y. B. (2011). “Liquid hot water and alkaline pretreatment of soybean straw for improving cellulose digestibility,” Bioresource Technol. 102(10), 6254-6259.
Wang, Z. Y., Keshwani, D. R., Redding, A. P., and Cheng, Y. J. (2010). “Sodium hydroxide pretreatment and enzymatic hydrolysis of coastal Bermuda grass,” Bioresource Technol. 101(10), 3583-3585.
Wistara, N., Zhang, X. J., and Young, R. A. (1999). “Properties and treatments of pulps from recycled paper. Part II. Surface properties and crystallinity of fibers and fines,” Cellulose 6(4), 325-348.
Zhang, W., Liang, M., and Lu, C. H. (2007). “Morphological and structural development of hardwood cellulose during mechanochemical pretreatment in solid state through pan-milling,” Cellulose 14(5), 447-56．
Article submitted: January 4, 2012; Peer review completed: February 26, 2012; Revised version received: March 20, 2012; Accepted: March 23, 2012; Published: March 26, 2012.