Nanofibers from Rice Straw

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

High-Yield Preparation of Micro/Nanofibers from Rice Straw Using Superextended Soda−Oxygen Cooking and High-Intensity Ultrasonication Lilong Zhang,†,§ Keli Chen,*,§ and Yulong Wu*,†,‡ Institute of Nuclear and New Energy Technology and ‡Beijing Engineering Research Center for Biofuels, Tsinghua University, Beijing 100084, China § Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China Downloaded via NOTTINGHAM TRENT UNIV on September 5, 2019 at 14:36:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Chemical purification through ultrasonic treatment as a method to prepare energy-saving and environmentally friendly nanofiber from natural lignocellulose has attracted wide attention in academic research. However, purifying cellulose through chemical pretreatment is complicated and inefficient, making it one of the bottlenecks that plague its industrial application. In this paper, the superextended soda−oxygen pulping process successfully purified rice straw cellulose fibers (total lignin 1.57%). Results showed that 82% of the hemicellulose can be removed with a high yield of 43.05% (w/w dry rice straw). In addition, micro/nanofibers can be isolated from the purified cellulose after the conventional ultrasonic process (2000 W, 2 h). The transmission electron microscope shows that the obtained micro/nanofibers were interconnected with fiber bundles, which formed entangled, weblike networks in the suspension. The thermogravimetric analysis results indicated that the degradation temperature of the micro/nanofibers can be increased to approximately 335 °C compared with that of 300 °C of the conventional pulp. After a simple vacuum filtration, a highly transparent film with a smooth surface was produced from micro/nanofibers. The production process has low cost, stable product quality, and high yield, and it is also easy to use in industrial production. KEYWORDS: rice straw, micro/nanofibers, soda−oxygen pulping, high-intensity ultrasonication

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INTRODUCTION

processes, which inevitably produce malodorous emissions due to the use of sulfur compounds, indispensably resulting in a complex wastewater treatment.7,8 Moreover, the high content of ash and silicon are accompanied by parenchyma cells. Therefore, black liquor (BL), which is derived from rice straw pulping, is difficult to be processed through the conventional recovery system. This dilemma is a the bottleneck for the development of rice straw pulping.9 Researchers continue to explore new pulping technologies to reduce pulping pollution and promote the resource utilization of rice straw through the industrial pulping process. Among these new technologies,

Rice straw is a valuable, renewable, and abundant biomass resource with a global yield of up to 2 billion tons annually. It can be used in many fields, such as feeding, composting, and fermentation.1−3 Hence, rice straw has been considered as a potential future energy and chemical resource for biorefinery. Separation of cellulose is considered as the basic step to establish an economical and sustainable lignocellulosic biorefinery that uses biomass resource such as rice straw.4,5 The conventional biorefining process, pulping, was deemed as the primary cellulose extraction process. It has many disadvantages, such as high energy consumption, high resource input, and high pollution discharge compared with the modern biomass conversion process.6 At present, the technologies commonly used worldwide for pulping are the Kraft and sulfite © XXXX American Chemical Society

Received: April 21, 2019 Revised: July 16, 2019 Published: August 19, 2019 A

DOI: 10.1021/acssuschemeng.9b02217 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Composition of Rice Straw % (w/w Dry Weight) ash

SiO2

1% NaOH extraction

9.47

13.06

49.46

pentosan nitric acid ethanol cellulose 18.92

34.81

benzene−ethanol extractives

holocellulose

kalson lignin

acid-soluble lignin

3.16

63.05

14.86

3.83

understanding the properties of the obtained pulps and its potential for extraction microfiber using high-intensity ultrasonication. This study focused not only on the effects of extended soda−oxygen pulping on the pulp properties but also its impact on subsequent ultrasonic treatment into micro/ nanofibers. The chemical component analysis and assembly spectroscopy techniques were used in this work to evaluate the pretreatment efficiency. Also, the ultrasonic process was evaluated by the properties of the resultant micro/nanofibers including its structure, morphology, crystallinity, and thermal degradation.23 Besides analyzing the breaking mode of the chemical linkages between lignins and the associated carbohydrates, the Fourier-transform infrared (FT-IR) and 2D-HSQC NMR techniques was also used to characterize the lignin derived from cooking waste water (black liquor).

environmentally friendly soda−oxygen pulping is expected to have high utility. To date, soda−oxygen pulping is considered the best option for alkaline pulping of rice straw till now. This kind of pulping method has many benefits according to environmental, economic, and energy consumption considerations.10,11 During the soda−oxygen pulping process, oxygen and alkali are applied instead of undesirable chemicals such as sulfides. Notably, the alkali recovery for BL as derived from the soda− oxygen pulp of rice straw shows a promising application in conventional recovery furnace after simple physical pretreatment.12 Our previous works also revealed that soda−oxygen pulp displayed good properties that can meet paper market’s economic demands.13 Hence, the fundamental theoretical basis for the industrial application of soda−oxygen cooking of rice straw has been successfully established. With the development of biorefinery technology, pulping is considered not only as a process of extracting cellulose process from the biomass but also an important pretreatment method for the preparation of cellulose-based product, such as microfibers and nanofibers. Ultrasound technology is an very effective method for the extraction of active ingredients of natural products like nanofiber preparation from lignocellulose.14 The nanofibers can be isolated from chemically pretreated wood cellulose by ultrasonic technique. To purify cellulose fibers, chemical pretreatment is an essential step to remove lignin and hemicellulose. However, the industrialization process was restricted by the chemical pretreatment step because of its low efficiency.15 To increase the technical and economic feasibility, isolating high-purity cellulose with less constraints is important. As a pretreatment method, soda−oxygen pulping has received attention in the pretreatment of straws for the extraction of cellulose material from the biomass.6,16,17 If this kind pretreatment method can successfully purify cellulose, then the technique to prepare micro/nanofibers from rice straw on an industrial scale using ultrasonic technique can be put into practice. Undoubtedly, the dosage of alkali dominates the soda− oxygen pulping process under stable oxygen condition. A desirable pulp with a high yield and viscosity can be obtained with the addition of suitable dosage of alkali during the pulping process.18 With sufficient oxygen, adding an excess alkali into the cooking process can prolong the soda−oxygen pulping process, resulting in the yield and viscosity losses.13,19,20 Therefore, a certain viscosity of the cellulose material should be produced during the pulping process depending on the alkali dosage. By extending the cooking process, suitable purified cellulose fibers can be produced for extracting micro/ nanofibers using ultrasonic technology. Hence, soda−oxygen pulping can purify cellulose that can meet the microfiber isolating needs. However, research studies on the preparation of micro/nanofibers in a soda−oxygen pulping system are yet to be carried out. Based on previous works on the soda−oxygen pulping of rice straw with conventional alkali charge (CASO),13,21,22 extended pulping process with high alkali dosage (HASO) was carried out in this paper. Emphasis of this work is on

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EXPERIMENTAL SECTION

Materials. Rice straw (LongGeng 31#) was harvested in JiaMusi, Heilongjiang province, China, in the summer of 2013. It was air-dried for over 24 h and then cut into length of 3−5 cm. The mean composition % (w/w) of the straw is shown in Table 1. Characterization of the materials was performed according to the Tappi test methods (Tappi, 2003−2004): 1% of NaOH solubles (Tappi T212 om-98), benzene−ethanol extractives (Tappi 204 cm97), α-cellulose (Tappi 203 om-93), and Kalson lignin contents (Tappi T 222 om-98). In addition, holocellulose was determined by treating the extracted rice straw with the NaClO2 solution (Browning, 1967). Cooking Trials. With the purpose of studying the difference in the resultant black liquors and between conventional alkali dosage and overdose pulps, two trials were individually performed at the same reactor and the detailed conditions were given as follows: maximum cooking temperature is 120 °C and the time of maximum temperature is 60 min, solid-to-liquor ratio is 1:5 (g/v), MgSO4 dosage is 0.5% (g/ g), and effective alkali (EA) content is 18% (w/dry straw) for the conventional dose and 36% (w/dry straw) for the overdose. One kilogram of rice straw was cooked in a 15 L stainless steel laboratory digester with temperature and pressure control. The endpoint of cooking was dependent on the pH of BL, which was down to pH 10 in accordance with our previous expectable pulping endpoints. The cooking process are shown in Figure S1. After cooking, each BL was separated from the pulp by centrifugation and washed with hot water until more than 95% of BL was removed from the pulp and stored in a refrigerator. Ultrasonic Fibrillation. According to the Yu’s research,22 two kind of pulps (CASO and HASO) were soaked in distilled water (1% by mass), and the obtained cellulose suspensions were then sonicated for 120 min using an ultrasonic processor (JY98-IIID, Ningbo Scientz Biotechnology Co., Ltd, China) at 20 kHz and an output power of 2000 W to isolate the fibers (UCASO and UHASO). The ultrasonic treatment was carried out in an ice bath; the ice was maintained through the entire ultrasonication process. Cellulose Composite Film Formation. The films were fabricated by the vacuum filtration of the dispersions of the pulps and micro/nanofibers samples through a vacuum filtration assembly and polycarbonate (PC) filtration membranes. More details are displayed in Figure S2 of the Supporting Information. Paper Property Analysis. The analysis of the typical composition, including folding endurance (TAPPI *T 511 om-13 Folding Endurance of Paper MIT Tester), bursting strength (TAPPI T 403 om-10 Bursting Strength of Paper), and tensile index (TAPPI T 404 wd-03 Tensile Breaking Strength and Elongation of Paper and B


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Chemical Composition of Pulps CASO HASO

yielda

viscosity (mL/g)

Klason ligninb

acid-soluble ligninb

α-celluloseb

pentosansb

ashb

SiO2

45.32 43.05

830 355

4.77 1.13

0.52 0.44

73.27 78.76

11.96 2.17

8.70 16.83

7.81 5.97

c

a

Percent yield of the components % (w/w dry straw); b% (w/w pulp); c(w/w ash). Fourier-Transform Infrared (FT-IR) Spectroscopy Analysis. The FT-IR spectra of the lignin samples were determined by Tensor 27 (Bruker company, Germany) in the range of 4000−400 cm−1 with a KBr disk containing about 1% finely ground. HSQC Analysis. Two-dimensional NMR spectroscopy spectra of the lignin samples were recorded by a AVANCE 800 MHz spectrometer (Bruker company, Germany) equipped with a cryogenically cooled z-gradient triple resonance probe at 25 °C. The 60 mg of acetylated lignin samples was dissolved in 0.5 mL of CDCl3, and the central solvent peaks (δH/δC, 7.26/77.23) were assigned as internal reference.25 The spectral widths of 1H and 13C dimensions were 5000 and 13 200 Hz, respectively. The 1H and 13C dimensions both collected 1024 points with a recycle delay of 1 s. The number of transients was 64, and 256 times increments were recorded in the 13C dimension.

Paperboard) were carried out with the referenced standard methods. The analyses were conducted with three repetitions, and the relative standard deviation was below 5%. Scanning Electron Microscopy (SEM). Imaging was performed with a scanning electron microscope (VEGA3-SBH, TESCAN). The samples were air-dried prior to imaging and mounted on aluminum stubs using a conductive carbon tape. The stubs were then sputtercoated with approximately 10 nm of gold sputter coater (SBC-12, KYKY Technology Co., Ltd, Beijing, China). The imaging was performed with a beam accelerating voltage of 10 kV. Transmission Electron Microscope (TEM). Drops of dilute cellulose microfiber suspensions were deposited into glow-discharged carbon-coated TEM grids. The excess liquid was absorbed by a piece of filter paper. After the specimen had been completely dried, it was observed using a Jeol Jem 2100f electron microscope operated at 30 kV. Crystallinity Measurements. To examine and compare the crystalline structure of both untreated and alkaline-treated samples, Xray diffraction (XRD) was performed on an Empyrean diffractometer (PANalytical B.V., Almelo, Holland) with a Cu Kα radiation of wavelength (λ) (Kα1) 0.15406 nm generated at 40 kV and 40 mA. The samples were dried in a vacuum oven with P2O5 at 35 °C for 48 h. The scans were obtained from the 2θ values of 10−90° at a scanning speed of 1°/min. The crystallinity (CI) of the cellulose samples was calculated according to the peak deconvolution method with the ratio between the area of the crystalline contribution and total area shown in eq 1

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RESULTS AND DISCUSSION Pulp Characterization. With increase in initial alkali loading, the pulping time would be prolonged under stable oxygen pressure condition. As shown in Figure S1, the cooking time can extend to 10 h, which was three times that of the conventional pulping process. The synergistic effect between oxygen and alkali on pulp could lead to a decrease in viscosity and a deep delignification. The degree of degradation of the pulp during the cooking process can be revealed by the viscosity of the pulp. Compared with the CASO pulp (CASOP) (830 mL/g), the viscosity of the HASO pulp (HASOP) was only half of the former. The delignification degree could be represented by the Kalson lignin remaining in the pulp. In contrast to the residual Kalson lignin of CASO (4.77%), the pulp of HASO just contains 1.13% Kalson lignin, which also indicated that the extended pulping process has deeper delignification. It means that the extended soda− oxygen pulping largely increased the delignification degree but significantly decreased the viscosity of the pulp. The results were also in agreement with the conclusion of the previous work.26 Elisabet reported that owing to high alkalinity (EA = 50%), a prominent acceleration in the delignification rate was obtained while the viscosity of the pulp largely decreased. During such conditions of a long cooking time and high concentration of alkali pulp, polysaccharides including cellulose and hemicellulose are degraded significantly and lead to considerable decrease of pulp yield. However, some interesting results may challenge the theoretical understanding about high-alkali pulping. As shown in Table 2, the yield of pulp from HASO (43.05%) just decreased lightly, which is about 2.17% lower than that of conventional pulping. The αcellulose retained in the pulp showed different result compared with pulp yield. HASO has ca. 5% higher α-cellulose content than that of CASO. Therefore, this indicated that HASO pulping has selectivity on lignin removal even pulp viscosity decreased considerably. The synergistic effect between alkali and oxygen was suitable for the extraction of cellulose because oxidatively active components like •OH and −OOH are more aggressive toward lignin, whereas cellulose was hardly decomposed under mild conditions.27,28 Ultimately, the α-

2θ 2

CI = 1 −

∫ Iam d2θ A am = 1 − 22θθ21 A simple ∫2θ1 Isample d2θ

(1)

where Aam = area under the amorphous curve (deg); Asimple = area under the sample intensity curve (deg); 2θ1 = 13.5°; and 2θ2 = 49.5°. Thermogravimetric Analysis (TGA). Thermogravimetric (TG) analysis was performed to compare the degradation characteristics of the cellulose fibers and the microfiber obtained from using highintensity ultrasonication process. The TGA of the samples was performed using a simultaneous thermal analyzer (NETZSCH STA449C) with a heating rate of 10 K/min under nitrogen (flow rate of 50 cm3/min) in the temperature range from 20 to 500 °C. Isolation of Lignin from Black Liquors. The obtained BLs were mixed well with a RW 20 digital overhead type mechanical stirrer (IKA, Germany). SiO2 was first removed by acid precipitation with 12% (w/w) sulfuric acid when the pH of BLs reached 8.5 and centrifuged at 8000 rpm for 20 min. With more than 12% (w/w) sulfuric acid added into BLs, the pH of the supernatant was adjusted to 2.0 and then the supernatant was centrifuged at the same condition as in the last step to isolate lignin. The isolated lignin was washed with distilled water and then freeze-dried as crude lignin. Characterization of Lignin. The crude lignin was purified and etherified according to the literature24 before the characterization of lignin. Molecular Weight Analysis. Gel permeation chromatography (GPC) is a method for deriving molecular weights. In this paper, it was used to determine the molecular weights of the lignin. The GPC comprises Waters 1525 binary high-performance liquid chromatography pump, a Waters 717 plus autosampler, a Waters 2414 refractive index detector, and a Breeze (V3.3) GPC workstation (Waters). The purified and etherified lignin samples were (3 mg) was dissolved in 1 mL of tetrahydrofuran. The eluent flow was 1 mL/min, and the column temperature was maintained at 35 °C during the analysis. C


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ACS Sustainable Chemistry & Engineering

Figure 1. Pathway of the isolation of the micro/nanofibers from the rice straw by superextended soda−oxygen cooking and ultrasonication. Step 1: cellulose was purified by extended soda−oxygen pulping. SEM images of A(1) conventional pulp and A(2) purified cellulose after extended soda− oxygen pulping. Step 2: micro/nanofibers separation process. After ultrasonic treatment, bundled fibers in the conventional pulp hardly separated into a signal microcellulose, whereas the entangled, weblike networks of the signal microcellulose were obtained in the HASO pulp as shown in images B(1) and B(2). During the experiment, paper and film were directly moved from the supporting substrate after filtration. Top images C(1) and D(1) pertain to the production of the conventional pulp. High-transmittance and smooth-surface films were produced by micro/nanofibers as illustrated in C(2) and D(2).

nanofibers could be found on the flattened surface of the HASO pulp as shown in the bottom portions of Figure 1. This morphology was a result of the removal of lignin from the straw. Lignin is an adhesive that can glue the cellulose and hemicellulose together. The decrease in the lignin content not only loosened and shrank the fibers but also dispersed the adjacent microfibers. After ultrasonic pretreatment, the original cellulose tubular structure was completely destroyed, which roughened the surface of the cellulose. These microsized fibers were reportedly composed of strong hydrogen-bonded nanofibers. Individualized nanofiber bundles with a width of 10−30 nm can be observed on the surface of the microsized cellulose fibers (Figure 2B(2)). However, the samples still maintained

cellulose became the predominant component and accounted for 78.37% of the remaining material. Peeling reaction, termination reaction, and alkaline hydrolysis as the main degradation reactions of polysaccharides at the pulping condition should be considered.29 During the conventional pulping process, peeling reaction, as the primary losses of polysaccharides during the conventional pulping process, is initially start at the reducing end and unstable side groups or unordered structure. However, during the process of soda−oxygen pulping, the reducing ends of carbohydrate and lignin can be oxidized by active oxygen groups. It suggested that the peeling reaction seems unlikely to continue.30 Besides, under such highly alkaline condition in HASO pulping, termination reactions are favored by abundant hydroxide ion concentration and the structure of polysaccharides is stabilized. A large amount of acidic degradation products could be formed, which can neutralize the alkali and precipitate SiO2 on the fiber.31 The above-mentioned reactions favored the yield of pulp. On the other hand, alkaline hydrolysis, which was adverse to viscosity, can result in the depolymerization of cellulose and thus decrease the fiber strength property especially under HASO pulping condition.32,33 •OH, obtained from the pulping process, could be maintained at a high level when the hydroxide ion concentration OH− is sufficient. Therefore, the shearing action of •OH could be strengthen during the HASO pulping process. Also, the extended pulping process could continuously remove lignin from the residual straw without a large polysaccharides loss. Morphology of the Chemically Purified Cellulose Fibers and Micro/Nanofibers. The microstructures of the rice straw, soda−oxygen pulp, and ultrasonically treated pulp were observed by SEM. Compared with the wrinkled surface of the CASO pulp, the venation of the stem vascular bundles distinctly flattened and destroyed after HASO pulping. This appearance was largely a result of the extended pulping process. The resulting loosened tissue collapsed and shrank and the adjacent fibrous bands were exposed because of the removal of lignin. Furthermore, the original cylindrical fibrous bands were deformed, bended, and even exploded after an extended soda−oxygen pulping. As the result of structural changes, the original tubular structure of cellulose became flattened. In terms of the pulp cellulose surface, certain micro/

Figure 2. SEM images of A(1) CASOP, B(1) HASOP, and pulps after ultrasonic pretreatment: A(2) UCASO and B(2) UHASO.

the initial cell shape (Figure 2A(1)). That is, a tangle of fiber bundle structures existed in the CASO pulp. The cellulose fibers were separated into individual microsized fibers from the HASO pulp. The fibers can be dispersed after ultrasonic treatment. Therefore, the surface of the paper produced by fibers after ultrasonic treatment appeared smooth without any D


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ACS Sustainable Chemistry & Engineering significant irregularity. This result suggests that the connection between the micro/nanofibers is no longer tight after the extended soda−oxygen pulping process. That is, micro/ nanofibers contained in the HASO pulp can be separated more easily than those in the CASO pulp through ultrasonic treatment. Therefore, it can offer a new method to individualize microfiber from rice straw after soda−oxygen pretreatment using high-intensity ultrasonication. If the soda−oxygen pulping process can further remove the lignin and hemicellulose from the pulp, then homogeneous nanofibers may be obtained. To reveal the structures of cellulose bundles extracted from pulps by ultrasonic method, the samples were further investigated by TEM observation. The dimension of the UCASO pulp has not reached the micron size. Hence, only the UHASO pulp was investigated. Figure 3 displays the TEM images of the isolated fibers obtained from the HASO pulp after a 2 h high-intensity ultrasonication treatment at an output power of 1000 W.

on the surface of the cellulose displayed ribbonlike structures comprising 2 nm wide parallel-aligned cellulose nanofibers. A structure with small uniform length ranging from 100 to 200 nm and nanofiber bundles was obtained from the HASO pulp after the ultrasonic treatment. Although this phenomenon only occurred on the surface of the cellulose, it indicates that the hydrogen bonding between the nanofibers can be broken after the HASO pulping process. The bundles were interconnected with other nanofibers and bundles, which formed entangled, weblike networks on the surface. X-ray Diffraction (XRD) Analysis of the Pulp before and after Ultrasonic Treatment. Cellulose crystallinity in individualized fibers as the key factor determining their mechanical and thermal properties is of interest. The XRD graph (Figure 4) evidently shows that all fibers exhibited sharp peaks around 2θ = 16−22.6°, which represent a typical cellulose I form. This indicates that the crystalline form of cellulose was not changed during chemical and ultrasonic treatments. However, with more lignin and hemicellulose in the amorphous regions being dissolved from the straw, the crystallinity has a different rate of increase. Figure 4 shows a significant increase in crystallinity from 51.29% for the original wood fibers to 59.34% for the CASO pulp. The extended soda−oxygen pulping process has a deeper delignification, which has been discussed in the section “Pulp Characterization”, and the crystallinity can further increase to 63.67% in the HASO pulp. These results agree with the previous works that show an increase in crystallinity after chemical treatment.34,35 Ultrasonication can lead to the realignment of the cellulose molecules.36 After the ultrasonic treatment, the crystallinity increased by 2% and a weak peak was formed in 33°. It implied that the ultrasonic treatment had an effect on the crystal regions of the cellulose. However, with more lignin and carbohydrate dissolved from the pulp, the crystallinity slightly increased while the extent of decrease was reduced, which means that after extended soda−oxygen pulping, the crystallinity region of the HASO pulp became more stable than that of the CASO pulp. Thermostability Analysis of the Pulp before and after Ultrasonic Treatment. Thermal properties of biomaterial is important for their applicability in biocomposite processing. Figure 5 displays the TG curves of the rice straw fibers, pulp fibers, and micro/nanofibers obtained after ultrasonic treatments. Without exception, a small weight loss in the range of 25−100 °C due to the release of water and the low-molecular-

Figure 3. TEM micrographs of the HASO pulp after ultrasonic treatment.

As illustrated in Figure 4A, large aggregates consisting of wirelike cellulose fibers with microscale widths were observed for the HASO pulp after ultrasonic pretreatment. A number of branches of small bundles or partly individualized nanofibers were attached to the aggregates as well. Several portions of the fiber bundles retained a structure that is similar to that of the CASO pulp before the ultrasonic treatment; that is, tight and orderly nanofibers were arranged in a row. Other nanofibers can be separated out from the main body of the cellulose, However, these fibers continued to bond, which satisfactorily agreed with the SEM images. The exposed nanofiber bundles

Figure 4. XRD peaks and crystallinity increase of the original pulps and pulps after ultrasonic treatment. E


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Figure 5. TG curves of the original untreated rice straw, original pulps, and pulps after ultrasonic treatment.

S4. This finding indicates that the micro/nanofibers produced during the ultrasonication process could combine the surrounding fibers to form a compact structure. The macroscopical machine property of the films can also increase as follows. This kind structure could increase the shearing force between the microfibers. Also, the macroscopical machine property of the films also increased as follows. Among them, the folding endurance of the microfiber film increased 22 times, while that of the conventional pulp increased two times. Particularly, through the comprehensive comparison of mechanical properties, the UHASO paper showed a higher mechanical property after the ultrasonic treatment than before the treatment. All these factors make it a suitable candidate for application in many material devices. Therefore, the micro/ nanofibers with high tensile strength, good transparency, and high thermal stability have great potential application in the fields requiring high quality of packages, biosensors, and portable electron devices. Structural Elucidation of Lignin Derived from BL. To further elucidate the structural transformation of lignocellulose and the breakdown mode of the chemical bonds between lignin and associated carbohydrates, the FT-IR and 2D-HSQC NMR techniques were used to characterize the samples. The lignin fractions of the lignocellulose subjected to ethanol pretreatment at 180 °C were selected as representatives. Figure S5 reveals that the lignin composition of CASO has two obvious differences with HASO in the FT-IR spectrum. On the one hand, CASO has a sharp peak at 1740 cm−1, which is attributed to carboxylic ether. Furthermore, it has a weak signal at 1640 cm−1, which is assigned to uronic acids. However, HASO has an opposite signal intensity at the abovementioned signal’s positions. Therefore, after prolonged cooking process, uronic acids are converted into carboxylic acid. On the other hand, CASO compositions have obvious strong sharp peaks at 890 and 790 cm−1, which were assigned to the C−C stretching vibration in a ring structure of sugar and lignin. That is, the extended synergistic effect can break down the linkage of the lignin−carbohydrate complex (LCC). As shown in the HSQC spectrum of the CASO lignin in Figure 6, the side-chain regions (δC/δH 50−110/2.5−6.0 ppm) and aromatic regions (δC/δH 100−135/5.5−8.5 ppm) provided the main signals. The main cross-signals in the 2DHSQC spectra were assigned according to the literature38−40 and are listed in Table S2. The main different region appeared at the carbohydrate regions (δC/δH 91−105/3.9−5.4 ppm), which are mainly assigned to the LCC structures.41 Signals

weight compounds like small-molecule acid was found. Compared with original fibers, the pulps’ peaks showed a higher decomposition temperature at 335 °C. Especially for the HASO pulp, its peak appeared in 360 °C. The higher temperature of the thermal decomposition of the HASO pulp is related to the partial removal of the hemicellulose and lignin. Besides, the higher crystallinity of the cellulose could improve its thermostability.37 The micro/nanofibers obtained after ultrasonic treatments exhibited a degradation behavior similar to that of the HASO pulp. The decomposition temperature of the CASO pulp after ultrasonic treatment starting at approximately 335 °C implied that the process had an effect on the thermal decomposition of the cellulose. This result is also consistent with XRD analyses, indicating that the ultrasonic process could slightly increase the crystallinity. Also, more thermostable cellulose could be gained after the ultrasonic treatment. Structure and Properties of Paper and Film Properties. Given the significant decrease in viscosity, the strength of the HASO paper largely decreased compared with that of the CASO paper. Table 3 shows that the biggest decline was Table 3. Physical Properties of Paper and Film

CASO HASO UCASO UHASO

folding endurance (n)

tensile breaking strength (N·m/g)

burst strength (kPa·m2/g)

78 2 127 45

23.78 8.3 58.7 17.8

178 34 265 69

folding endurance, which decreased by 95%, whereas the tensile breaking strength decreased by 84%. The paper strength indexes suggested that the HASO pulp cannot be used for any kind of paper-making process. Although this property appeared to be disadvantageous in the paper-making process, the HASO pulp may have another advantage in the extraction of micro/ nanofibers. Additional lignin can be extracted from the straw with deep delignification. In addition, additional micron fibers may be separated without the bond of lignin. As shown in Figure S3, the micro/nanofibers obtained from the cellulose connected with one another formed a weblike structure after ultrasonic treatment. This kind structure can increase the shearing force between microfibers. After the paper-making process, the films made by pulp after ultrasonic pretreatment achieved a smooth outer surface shown in Figure F


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Figure 6. HSQC NMR spectra of the lignin obtained from BLs derived from CASO (A) and HASO (B). The main lignin substructures are (A) βO-4′ substructures, (B) phenylcoumarane substructures formed by β-5/α-O-5 linkages, (C) resinol substructures formed by β−β′/α-O-γ′/γ-O-α′ linkages, (G) guaiacyl units, (S) syringyl units, and (S′) oxidized syringyl units bearing a carbonyl group at Cα.

high-Mw molecule fraction into low-molecular-weight lignin groups. In comparison with CASO, HASO has lower Mw and Mn and a narrower molecular weight distribution. Since each lignin unit has C9 formula,42 TASO has an average of nine lignin units, whereas CASO has 11 lignin units counted as Mn. Hence, the small lignin groups were gained from the extended soda−oxygen pulping process. Considering the previous statement about the HSQC spectrum, the synergistic effect between oxygen and alkali could break down the LCC bonds of the aryl ethers and phenyl glycosides. More free lignin fractions underwent the scission of C−C and C−O−C bonds to generate specific phenolic compounds with small Mw.

were not observed in the HASO lignin, implying that LCC bonds were cleaved during the extended soda−oxygen pulping process. In the side-chain regions, except for the strong methoxy group signals (δC/δH 56.1/3.77 ppm), except for the strong methoxyl group signals (δC/δH 56.1/3.77 ppm). However, the signals of β−β′ were weak, whereas those of β-5′ could not even be detected in the HASO lignin. Furthermore, a semiquantitative method was used to identify the relative abundance of the main structures as presented in Table S3. In the aromatic regions, the syringyl/ guaiacyl (S/G) ratio of the lignin samples was calculated based on the percentage of C2,6−H2,6 correlations from the S units and C2−H2 plus C6−H6 correlations from the G units. Most of the linkages were β-O-4′ with a relative percentage of 95.3%. With the prolonging of the pulping process, the relative percentage of the β-O-4′ linkages decreased to 89.2%, whereas that of other linkages increased.34 The lignin fractions were apparently degraded with prolonged soda−oxygen process. These parts of the lignin groups might have degrade into lowmolecular-weight fragments. At the same time, the signal intensities of S2/6 (δC/δH 103.8/6.64 ppm) and G5 (δC/δH 115.4/6.81 ppm) were decreased, which means that the extended oxidation may selectively degrade the G- and S-type lignins. Table 4 shows the weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity (Mw/Mn) of CASO and HASO through GPC analysis. Prolonged oxidation reaction of oxygen could degrade the

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CONCLUSIONS This study demonstrated the high delignification and fractionation efficiency of lignocellulose biomass by the extended soda−oxygen cooking process. The extended synergistic effect between soda and oxygen had selectivity for lignin removal and relatively decreased pulp viscosity. This green and efficient biorefining process can prove that chemicalpurified cellulose fibers can be separated into micron-sized ones after ultrasonic treatments. A better thermal stability of this type of microfiber compared with that of the original straw fiber suggested that the microfiber can be potentially applied to wide biomaterial fields. This kind sample method for extracting micro/nanofibers confirmed that the soda−oxygen pulp is a potential and effective method for use in lignocellulose biorefineries.

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Table 4. Weight-Average (Mw) and Number-Average (Mn) Molecular Weights and Polydispersity (Mw/Mn) of the AcidInsoluble Lignin

Mw (g/mol) Mn (g/mol) Mw/Mn

CASO

HASO

4025 1886 2.13

2518 1516 1.66

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02217. Heating rate and cooking process (Figure S1); cellulose composite film forming process (Figure S2); SEM images of pulps before and after ultraphonic pretreatG


Research Article


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ment (Figure S3); SEM images of the films (Figure S4); FT-IR spectra of lignins samples (Figure S5); main functional groups of lignin and carbohydrate chemical assignment of FT-IR and 13C NMR spectra (Tables S1 and S2); percentages of the substructures in the lignin samples on the basis of contour integration of the HSQC spectra (Table S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.C.). *E-mail: [email protected]. Tel: (+86) 10-89796163. Fax: (+86) 10-69791464 (Y.W.). ORCID

Keli Chen: 0000-0001-9287-7632 Yulong Wu: 0000-0003-0212-6689 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (Nos. 21276119, 21838006, and 21776159) and the National Key R&D Program of China (No. 2018YFC1902101).

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Nanofibers from Rice Straw

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