Article pubs.acs.org/JAFC
Dissolution of Wood Pulp in Aqueous NaOH/Urea Solution via Dilute Acid Pretreatment Zhuqun Shi,* Quanling Yang,* Shigenori Kuga, and Yuji Matsumoto Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan S Supporting Information *
ABSTRACT: Wood pulps with certain amounts of lignin were successfully dissolved in aqueous NaOH/urea solution by subjecting them to the dilute acid pretreatment. After the acid hydrolysis, viscosity-average degree of polymerization (DPv) of the pulps decreased. The results revealed that both the DPv and lignin contents influenced the dissolved proportions of wood pulps. When they were not so high, the wood pulps could almost completely dissolve with dissolved proportions >90%. In particular, the acid-pretreated unbleached kraft pulp with DPv of about 500 and lignin content of 6.9% could dissolve in NaOH/urea solvent and achieve a maximum pulp concentration of 4 wt % in the obtained lignocellulose solution. Moreover, the acid-pretreated bleached thermomechanical pulp with a high lignin content of 14.2% also almost completely dissolved. The lignocellulose films prepared from these wood pulp/NaOH/urea solutions exhibited good transparency and bendability, thus maybe promising as new biobased materials. KEYWORDS: lignocellulose, dilute acid treatment, degree of polymerization, dissolved proportion
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INTRODUCTION The increasing resource demands and environmental problems of recent decades urgently require the utilization of renewable resources. Biomass, mainly composed of a wide variety of renewable agricultural feedstocks and wood, has attracted more and more attention due to its abundance, sustainability, and biodegradability.1 Wood is one of the most abundant biomasses on earth and is mainly composed of cellulose (40−50%), hemicellulose (20−30%), and lignin (15−25%).2 It has been widely used in the field of pulp and paper-making3 and recently has also been used for the production of nanocellulose materials,4−6 biofuel,7,8 and chemicals.9 Besides these applications, another promising application of wood pulp is to dissolve it using a solvent system to prepare lignocellulose solution and then regenerate it as lignocellulose materials.10−15 Moreover, direct dissolution of wood pulp could reduce the cost and pollution associated with dissolution of pulp after removal of all the hemicellulose and lignin of lignocellulose, leading to practical utilization of lignocellulose. However, wood pulp is difficult to dissolve in common solvents because of its complex structure.11 In recent years, a low-cost, nontoxic, and simple to handle solvent has been developed by Zhang’s group to dissolve cellulose and chitin, namely, NaOH/urea aqueous solvent at low temperature.16−22 They demonstrated that dissolution of cellulose or chitin at low temperature arises from a fast dynamic self-assembly process among solvent small molecules (NaOH, urea, and water) and the macromolecules (cellulose or chitin), leading to inclusion complex formation through hydrogen bonds, which is relatively stable at low temperature.17,21 If this solvent could also dissolve wood pulp including cellulose, hemicellulose, and lignin, the utilization of lignocellulose as a new material source would be facilitated and the application of NaOH/urea aqueous solvent would be widely expanded.23 In © XXXX American Chemical Society
our previous work, we showed that a small amount of hemicellulose and lignin restricted the dissolution of lignocellulose and that lignocellulose is hard to dissolve directly in NaOH/urea aqueous solvent.23 There should be some other factors, besides the interactions among cellulose, hemicellulose, and lignin in lignocellulose, contributing to the difficulty in dissolving lignocellulose. It was reported that the molecular weight of cellulose significantly affects its dissolution in the NaOH/urea system and celluloses with a viscosity-average molecular weight (Mη) lower than 1.2 × 105 could dissolve well in this system.24 Therefore, reducing the molecular weight (or degree of polymerization, DP) of lignocellulose by some pretreatment would promote its dissolution. Acid hydrolysis of cellulose is usually used to reduce its DP by breaking the glucosidic linkages.25 The DP of cellulose decreased rapidly until it reached the so-called “leveling-off” or “limiting” degree of polymerization (LODP).26−29 It has been applied as a pretreatment to make cellulose molecules more accessible for chemicals or to produce glucose and biofuels.30−32 Traditional acid hydrolysis of wood for the production of fermentable sugars has been carried out at high acid concentration and low temperature or vice versa.33 Dissolution of cellulose by ethanol−hydrochloric acid pretreatment and 10−20 wt % sulfuric acid pretreatment has been reported.34,35 However, the effect of dilute sulfuric acid treatment on dissolution of lignocellulose in NaOH/urea solvent has never been investigated in detail until now. In this work, wood pulps were pretreated by dilute sulfuric acid (3 wt %) for different times and then subjected to Received: February 3, 2015 Revised: June 19, 2015 Accepted: June 23, 2015
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DOI: 10.1021/acs.jafc.5b01714 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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added into 5.00 mL of the cooled solution obtained above. The solution was neutralized by aqueous barium hydroxide solution to pH 5.5. After removal of the precipitate, about 20 mg of NaBH4 was added to the filtrate, and the reaction mixture was kept at room temperature for 24 h. Excess NaBH4 was degraded by acetic acid. After the mixture was evaporated to dryness, a small amount of methanol was added and dried. This was repeated several times, and then the mixture was heated in an oven at 105 °C for 15 min to ensure complete removal of the water. The dry residue was acetylated by 1 mL of acetic anhydride below 120 °C for 3 h and then analyzed by gas chromatography GC14b with FID (Shimadzu Co., Kyoto, Japan) using a column TC-17 (fused-silica capillary column, 30 m, 0.25 mm i.d, GL Science Inc., Tokyo, Japan) with a temperature program of 20 min at 220 °C, injection temperature of 220 °C, and detector temperature of 230 °C. X-ray Diffraction (XRD). The samples were dried in a vacuum oven with P2O5 at 40 °C for 48 h. XRD patterns of the samples were acquired in reflection mode using a RINT 2000 diffractometer (Rigaku, Tokyo, Japan) with monochromator-filtered Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA. Scans were obtained from 4 to 40° 2θ at a speed of 1° per minute. The crystallinity index (CI) was calculated from the height ratio between the intensity of the crystalline region and total region according to Segal’s method.38,39 Scanning Electron Microscopy (SEM). The surfaces of the wood pulps were coated with osmium using a Meiwafosis Neo osmium coater at 10 mA for 10 s and observed with a Hitachi S4800 fieldemission SEM at 2 kV. Optical Microscopy. The pulps were dispersed in the 7 wt % NaOH/12 wt % urea/81 wt % H2O solutions with pulp content of 1 wt %, and then the pulp dispersions were frozen and thawed. The images of the final dispersions (or solutions) were observed by using an optical microscope (BX-50 with a digital camera DP-20, Olympus, Japan). Viscosity-Average Degree of Polymerization (DPv) measurement. The samples (0.04 g) were dissolved in 0.5 M copper ethylenediamine (20 mL) for 30 min. The intrinsic viscosity of the solution was obtained by using a Cannon−Fenske capillary viscometer, and the value was converted to DPv values using the Mark−Houwink− Sakurada equation, [η] = 0.57 × DPv.40 Tensile Test. The tensile test of the lignocellulose film was performed using a Shimadzu EZ-TEST instrument equipped with a 500 N load cell. Rectangular strips 2 × 30 mm in size were cut from the lignocellulose film and tested with a span length of 10 mm at a rate of 1.0 mm min−1. The film was conditioned at 23 °C and 50% relative humidity for 2 days before the test.
dissolution in NaOH/urea aqueous solution. The effects of acid pretreatment time on DP, carbohydrate composition, kappa numbers, crystallinity indices, morphology, and dissolved proportions of lignocellulose were studied.
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MATERIALS AND METHODS
Materials. Softwood unbleached kraft pulp (UKP) with a kappa number of about 28, another softwood unbleached kraft pulp (UKP′) with a higher lignin content, and a softwood thermomechanical pulp (TMP) were used as the wood pulp samples, which were kindly provided by Oji Paper Co., Ltd., Japan. NaClO2 was purchased from Sigma-Aldrich Co. Other reagents and solvents were of laboratory grade and were used as received from Wako Pure Chemicals, Tokyo, Japan. Chlorite Delignification. The UKP or TMP with a weight of 5 g was delignified by a mixture of 1 g of NaClO2, 0.3 mL of acetic acid, and 160 g of water four times. The mixture was kept at room temperature. It took 40 min each time. The mixture was occasionally shaken by hand. Softwood bleached kraft pulp (BKP) with low lignin content and bleached TMP (BTMP) were finally obtained after filtration. Dilute Acid Hydrolysis. Three grams of dried original pulp (UKP, UKP′, and TMP) or bleached pulp (BKP and BTMP) was dispersed in 100 g of 3% H2SO4 aqueous solution. The mixture was heated by water bath at 80 °C for different times (0.5, 1, 2, 3, or 4 h). After the acid hydrolysis, the pulps were separated by filtration and kept without drying. The pulp samples were coded as UKP-hydrolysis time (UKP0.5, UKP-1, UKP-2, UKP-3, or UKP-4), UKP′-hydrolysis time (UKP′4 or UKP′-8), TMP-hydrolysis time (TMP-4 or TMP-8), BKPhydrolysis time (BKP-0.5, BKP-1, BKP-2, BKP-3, or BKP-4), and BTMP-hydrolysis time (BTMP-4), respectively. The pulps without acid treatment were coded UKP-0, BKP-0, UKP′-0, TMP-0, and BTMP-0, respectively. Dissolution. The original pulp or acid-treated pulp was dispersed in a 7 wt % NaOH/12 wt % urea/81 wt % H2O solution with pulp content of 1 wt %, and then the pulp dispersion was frozen and thawed. The resulting mixture was centrifuged at 10000 rpm for 10 min at 5 °C and then filtered.16 The remaining undissolved fractions were washed by 7 wt % NaOH/12 wt % urea/81 wt % H2O three times and further washed by water until they were neutral. Then they were dried in a vacuum oven with P2O5 at 40 °C for 48 h and weighed. The dissolved proportion of the pulp was calculated according to the equation m − m1 dissolved proportion = 0 × 100% m0 (1)
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RESULTS AND DISCUSSION Effect of Acid Treatment on Characteristics of Pulps. Figure 1 shows the yields of the wood pulps after acid
where m0 is the dry mass of the pulp before dissolution and m1 is the dry mass of the residual undissolved pulp. The dissolved proportion measurements were repeated more than three times, and their average was obtained. Film Preparation. UKP′-4 was dissolved in NaOH/urea solvent to obtain a lignocellulose solution with 4 wt % lignocellulose content. After centrifugation, the solution was cast on a glass plate and soaked in 5 wt % aqueous H2SO4 for 5 min, following the previously reported method.19 The regenerated lignocellulose gel was washed thoroughly with water and air-dried at 23 °C and 50% relative humidity for more than 1 day to obtain the lignocellulose film. Lignin Content Determination. The Klason method and kappa number method were used to determine the lignin content of the samples.36 Sugar Analysis. The carbohydrate compositions of the samples were determined by the alditol−acetate method.37 Sulfuric acid (72 wt %, 1 mL) was added to 100 mg samples, kept at room temperature for 4 h, and then diluted with water to adjust the concentration of sulfuric acid at 4 wt %. Hydrolysis of the samples in 4 wt % sulfuric acid was performed in an autoclave at 120 °C for 1 h. After cooling to room temperature, the hydrolyzed solution was filtered by glass filter, and the filtrate was adjusted to 100 mL with water. As the internal standard, 1.00 mL of myo-inositol aqueous solution (1.00 mg/mL) was
Figure 1. Effects of acid hydrolysis time on the yields of the wood pulps. Yield is based on the weight of the samples before acid hydrolysis. B
DOI: 10.1021/acs.jafc.5b01714 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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of UKP and BKP were almost unchanged for the different acid hydrolysis times, and in this case their influence on the dissolution of wood pulp in NaOH/urea system could be excluded. Figure 3 shows the XRD patterns of the wood pulps before and after acid hydrolysis, and the effects of acid hydrolysis time on the crystallinity indices (CI) of the wood pulps are shown in Figure S1. Compared with the original wood pulp, the crystallinity indices of the acid hydrolysis pulps increased. The crystallinity indices of UKP increased from 66 to 77% and that of BKP increased from 73 to 79%, with the increase in acid hydrolysis time from 0 to 4 h. Moreover, the increase in the crystallinity index of UKP after 1 h of acid hydrolysis was more than that of BKP; this was probably due to there being more amorphous components present in UKP-0 than in BKP-0, such as lignin, hemicellulose, and amorphous cellulose. SEM images of the wood pulps before and after acid hydrolysis are shown in Figure 4. UKP-0 and BKP-0 show relatively smooth and flat surfaces, and many random microfibril bundles were present on the surface of UKP-0, probably owing to the S1 layer of the wood fiber. However, after acid hydrolysis, these random microfibril bundles disappeared and some oriented fiber bundles appeared, forming a rough surface. This change in morphology of wood fiber was probably due to the S1 layer of the wood fiber having been removed by acid hydrolysis and the secondary wall of the wood fiber appearing.34 Moreover, compared with UKP, the change in morphology of BKP after acid hydrolysis was more obvious. The surface of BKP became very rough, and some microfibril bundles were stripped from the surface of the wood fiber after acid hydrolysis. This may have been because lignin exerted a preventative influence on the acid hydrolysis of wood fiber, and the lower lignin content of BKP compared with UKP led to a greater change in morphology after the acid hydrolysis. Dissolution of Acid-Treated Unbleached Kraft Pulp and Bleached Kraft Pulp. Figure 5 shows the photos of different UKP/NaOH/urea solutions and BKP/NaOH/urea solutions after freezing and thawing. The UKP-0/NaOH/urea solution and BKP-0/NaOH/urea solution were not transparent. However, after 2 h of acid pretreatment, UKP and BKP gave transparent NaOH/urea solution, suggesting that most of the UKP-2 and BKP-2 was dissolved in the NaOH/urea solution. The effect of acid pretreatments on the dissolution of wood pulps was also studied by optical microscopy. The relevant optical microscope images of different UKP/NaOH/urea solutions after freezing and thawing are shown in Figure 6. In the case of the UKP-0/NaOH/urea dispersion, most of the wood fibers kept their original morphology, although some of the wood fibers were swollen. However, even after 0.5 h of acid hydrolysis, the morphology of UKP-0.5 changed obviously. Most pulps were dissolved or became highly swollen fibers, fragments, balloons, or flat rings in the insoluble fractions. Moreover, with increasing acid hydrolysis time, the insoluble fractions were reduced. UKP-3/NaOH/urea shows a clear solution image, indicating that after 3 h of acid hydrolysis, the wood pulp almost completely dissolved in the NaOH/urea aqueous solution. Figure 7 shows the dissolved proportions and DPv of wood pulps pretreated by dilute sulfuric acid. After 4 h of acid hydrolysis, DPv of UKP and BKP decreased from 1150 to 550 and from 1300 to 520, respectively, whereas the dissolved proportions of UKP and BKP increased from 20.7 to 94.7% and
hydrolysis based on the weight of the wood pulps before acid hydrolysis. The yields decreased only a little after acid hydrolysis. Even after 4 h of acid hydrolysis, the yield of softwood BKP was still high (97.3%) and slightly higher than 91.7% of softwood UKP. This was probably because BKP contains less acid soluble components, such as lignin and hemicellulose, after delignification compared with UKP. Especially, the yields of the wood pulps with different acidtreated times (0.5−4 h) were almost the same. As will be seen later, the amounts of the main components (cellulose, hemicellulose, and lignin) of the wood pulps barely changed under the mild dilute acid hydrolysis condition, which is the reason for the high yields of UKP and BKP after acid hydrolysis. The kappa numbers of the wood pulps are shown in Figure 2. After acid hydrolysis, the kappa numbers of UKP and BKP
Figure 2. Effects of acid hydrolysis time on the kappa numbers of the wood pulps.
slightly decreased and kept a similar level with different acidtreated times (0.5−4 h). Table 1 shows the carbohydrate Table 1. Carbohydrate Compositions of the Wood Pulps before and after Pretreatment by Sulfuric Acid Hydrolysis sample
glucose (%)
xylose (%)
mannose (%)
galactose (%)
UKP-0 UKP-0.5 UKP-1 UKP-2 UKP-3 UKP-4
71.1 78.3 83.1 84.7 81.1 84.1
6.5 6.5 7.3 7.2 7.2 6.9
5.0 6.8 5.4 5.7 3.8 5.4
0.8 0.8 0.2 0.6 0.5 0.4
BKP-0 BKP-0.5 BKP-1 BKP-2 BKP-3 BKP-4
82.1 83.3 84.8 85.4 85.9 86.9
7.4 7.4 7.5 7.2 6.8 7.2
5.6 5.6 5.5 6.0 5.5 5.5
0.8 0.5 0.5 0.6 0.5 0.5
compositions of the wood pulps before and after dilute sulfuric acid treatment. The results indicated that the hemicellulose contents of the pulps barely changed under the different acid treatment times (0.5−4 h). The results of carbohydrate compositions together with the kappa numbers (Figure 2) and yields of the pulps (Figure 1) confirmed that the amounts of the main components (cellulose, hemicellulose, and lignin) C
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Figure 3. XRD patterns of the wood pulps before and after acid hydrolysis.
Figure 4. SEM images of the wood pulps before and after acid hydrolysis.
influencing the dissolved proportion of the wood pulp in this case is the molecular weight of the wood pulp, and the effect of lignin content could be neglected. However, it does not mean that the presence of lignin does not affect the dissolution of the pulps into this solvent. After acid hydrolysis, the lignin molecule must be cut into smaller fragments even though they are not removed from the pulp.41 This fragmentation of lignin must affect the interaction between lignin and polysaccharide. The results of dissolved proportions of the wood pulps were in good agreement with the results of their appearances (Figure 5) and optical microscope images (Figure 6).
from 20.5 to 96.3%, respectively. It was indicated that the wood pulps with lower DP had higher dissolved proportions in the NaOH/urea solvent system, which was similar to the dissolution of cotton linters cellulose in this solvent system.24 In fact, even though the acid hydrolysis time was only 2 h, the dissolved proportions of UKP-2 and BKP-2 had already increased to about 90%, which was much better than those of the untreated UKP-0 and BKP-0. Moreover, the dilute acidtreated wood pulps still kept a relatively high DPv of about 600. Furthermore, this figure also shows that under the same acid hydrolysis conditions, the dissolved proportions of UKP and BKP were similar. This indicated that the main factor D
DOI: 10.1021/acs.jafc.5b01714 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 5. Photographs of different UKP/NaOH/urea solutions and BKP/NaOH/urea solutions after freeze and thaw. Figure 7. Effects of acid hydrolysis time on the dissolved proportions and viscosity-average degrees of polymerization (DPv) of the wood pulps.
To further confirm that wood pulp with low molecular weight was easily dissolved in NaOH/urea solvent, regenerated wood pulps were prepared by regeneration of once-dissolved pulp with ethanol. DPv values of the original wood pulp samples and the regenerated wood pulp samples are shown in Figure 8. BKP-3 with a low DPv of 530 almost completely dissolved in NaOH/urea solvent (Figure 7). After dissolution and regeneration, DPv of the regenerated BKP-3 was about 400, not much lower than that of the original BKP-3. This result indicated that the dissolution of wood pulp in NaOH/urea solvent followed by regeneration resulted in only a slight degradation of cellulose. Furthermore, UKP-1 and BKP-1 could not completely dissolve in the NaOH/urea solvent. The DPv of the regenerated UKP-1 was about 410, which was much lower than that of the original UKP-1 (700). The DPv of the regenerated BKP-1 was about 390, also much lower than that of the original BKP-1 (730). This indicates again that the low molecular weight fraction of pulp was easy to dissolve in the NaOH/urea solvent, which is consistent with the results of the dissolved proportions shown in Figure 7. Dissolution of Wood Pulps with High Lignin Contents. To study whether the acid-treated pulp with high lignin content (>3%) could dissolve in NaOH/urea solvent or not, the different acid hydrolysis pulps with relatively high lignin contents were dissolved, and their relevant dissolved proportions are shown in Table 2. UKP′ had a lignin content of 6.1%. After acid hydrolysis for 4 h, it could dissolve well in NaOH/urea solvent, and its dissolved proportion increased from 20 to 93%. The dissolved proportion of the original TMP was very low (about 10%), probably due to its high lignin content (about 29%). After acid hydrolysis for 4 h, the dissolved proportion of TMP increased to 33%, although the dissolved proportion was still very low. Even after acid hydrolysis for 8 h, TMP still could not completely dissolve and the dissolved proportion was 90%. For instance, the UKP′-4 pulp with DPv of about 500 and lignin content of 6.9% could dissolve in NaOH/urea solvent and achieve a maximum pulp concentration of 4 wt % in the obtained lignocellulose solution. Moreover, lignocellulose materials could be prepared from these wood pulp/NaOH/urea solutions. Figure 10 shows appearances of the UKP′-4 pulp, the relevant lignocellulose film, and the stress−strain curve of the lignocellulose film. The film with yellow color exhibited good transparency and bendability. It possessed a tensile strength of 82 MPa, Young’s modulus of 3.7 GPa, and elongation at break of 10.3%. These lignocellulose films are promising as new biobased materials and the applications of lignocellulose would be widely expanded. This may promote the creation of new material and chemical streams from nonfood agriculture to high-tech industry. Meanwhile, the enhanced utilization of these wood
ACKNOWLEDGMENTS We acknowledge Prof. Akira Isogai of The University of Tokyo for supply of the viscometer and optical microscopy for this work and support by the China Scholarship Council (CSC) for ZS.
The authors declare no competing financial interest.
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DOI: 10.1021/acs.jafc.5b01714 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
