Directly Converting Agricultural Straw into All-Biomass

Research Article pubs.acs.org/journal/ascecg

Directly Converting Agricultural Straw into All-Biomass Nanocomposite Films Reinforced with Additional in Situ-Retained Cellulose Nanocrystals Jinming Zhang,† Nan Luo,† Jiqiang Wan,†,‡ Guangmei Xia,† Jian Yu,† Jiasong He,† and Jun Zhang*,†,‡ †

CAS Key Laboratory of Engineering Plastics and CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: It is attractive and meaningful to effectively utilize agricultural straws for preparing high value-added materials. In this work, we employ corn husk as a model substance for agricultural straws. By using microcrystalline cellulose (MCC) as an adhesive and reinforcing phase, direct utilization of corn husk is achieved, and consequently, corn husk/MCC films are fabricated in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl). Corn husk is dissolved completely in AmimCl; then, MCC is added and partially dissolved by controlling the dissolution conditions. The undissolved nanocrystals from MCC are used as the reinforcing phase, and the dissolved MCC is used as the adhesive and part of the matrix. As a result, homogeneous, transparent, beige-colored corn husk/MCC nanocomposite films are obtained. The resultant nanocomposite films with the content of corn husk in a range of 50−71 wt % exhibit high tensile properties. The tensile strength and elastic modulus of nanocomposite films containing 50 wt % corn husk have reached 67 MPa and 4.4 GPa, respectively. Thus, this work provides a simple, economical, and effective method for converting sustainable biomass resources into valuable materials. KEYWORDS: Corn husk, Agricultural straw, All-biomass materials, Self-reinforced nanocomposite, Cellulose nanocrystals

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INTRODUCTION

Many researchers have made attempts to use agricultural straw as a raw material in recent decades. To date, there are several methods for utilizing agricultural straw. The first method is to convert agricultural straw to biofuels.5−11 Although there are extensive reports and significant progress, several technical and economical hurdles still need to be addressed before this technology can be widely utilized. The second method is to isolate cellulose or cellulose nanocrystals from agricultural straw.12−24 Then, the resultant cellulose is used to fabricate cellulose derivatives or cellulose nanocrystals for preparing functional nanocomposites. The complexity and low yield of the isolation process hinder its practical applications. The third method is to use agricultural straw or

Agricultural straw is an available, renewable, and tremendous bioresource and has been considered as a promising and sustainable future feedstock for energy and materials industries in the future. For example, as the most common agricultural straw, corn husk is estimated to generate approximately 70 million tons worldwide each year. However, most of it has not been effectively utilized and is just burnt in the fields, resulting in serious air pollution and high threat of fire. In fact, corn husk contains 38−50 wt % cellulose, 17−32 wt % hemicellulose, and 15−30 wt % lignin, which are conventional biomacromolecules for producing materials or chemicals.1−4 Therefore, it is attractive and important to achieve an effective, and even complete, utilization of agricultural straw via an environmentally friendly and feasible method, which will offer significant benefits to the economy and environment. © 2017 American Chemical Society

Received: February 15, 2017 Revised: April 5, 2017 Published: April 25, 2017 5127

DOI: 10.1021/acssuschemeng.7b00488 ACS Sustainable Chem. Eng. 2017, 5, 5127−5133

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Preparation Process of Corn Husk/MCC Nanocomposites

small pieces (2−3 cm long). Then, it was pulverized by a crusher and sieved through a sieve with 40 meshes per square inch. Microcrystalline cellulose (MCC, Vivapur 101) with a degree of polymerization (DP) of 220 was purchased from Beijing Fengli Jingqiu Commerce and Trade Co., Ltd. It was dried at 105 °C for 3 h under vacuum before use. The ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl) was synthesized in our laboratory by the method described in our previous work,57 and the water content in AmimCl was less than 0.3 wt % as measured by the Carl−Fischer method. Preparation of Corn Husk/MCC Films. The preparation procedure of regenerated corn husk/MCC films is shown in Scheme 1. A typical example is shown as follows. First, 1.0 g of corn husk powder was added to a round-bottom flask containing 24 g of AmimCl. The mixture of corn husk/AmimCl was mechanically stirred at 120 °C for 4 h to obtain corn husk/AmimCl solutions with mass fractions of corn husk in the range of 4 wt %. Then, 1.0 g of MCC and 24 g of fresh AmimCl were added to the corn husk/AmimCl solutions at 50 °C for 1−5 h. As the dissolution was being conducted, the mixtures of MCC, AmimCl, and corn husk/AmimCl were degassed at the same time by a vacuum pump. After that, an optically transparent “solution” was obtained and cast onto a glass plate to give a thickness of approximately 1.0 mm; then, by immediate coagulation in deionized water, a transparent corn husk/MCC nanocomposite hydrogel resulted. For residual ionic liquid in the regenerated samples to be removed, these hydrogels were further washed with distilled water at least three times until no Cl− ions were detectable by AgNO3 test. After drying in vacuum at 80 °C for 12 h, these corn husk/MCC nanocomposite films were kept in a desiccator prior to further characterization. Finally, after a simple rotary evaporation, AmimCl with high purity was recycled. Characterization. Wide-angle X-ray diffractograms (XRD) were recorded using an X-ray diffractometer (D/MAX-2500, Rigaku Denki, Japan). The X-ray radiation used was Cu Kα with a wavelength of 1.5406 Å generated at 40 kV and 200 mA. The radiation was irradiated perpendicular to the surface of the films. The natural corn husk and MCC were ground into powder, and the corn husk/MCC films were cut into strips 20 mm long and 15 mm wide for measurements. The solubility of corn husk and MCC in AmimCl was assessed using a Leica DMLP polarizing microscope (Leica Company, German). A droplet of solution was sandwiched between a clean glass slide and a coverslip for observing the dissolution. Fourier-transform infrared (FTIR) spectra were recorded using a PE-2000 spectrometer with the detector at a resolution of 4 cm−1, and 24 scans were acquired. Samples of corns husk and MCC were prepared by the KBr-disk method. The films were tested with the attenuated total reflection (ATR) method. Tensile testing was performed on an Instron 3365 with 5 kN load cell at a crosshead speed of 2 mm/min. The specimens were cut into rectangular-shaped strips 10 mm wide and 50 mm long. The average values and standard deviations were calculated from at least five samples. Transmission electron micrographs (TEM) were recorded using a JEOL JEM-2200FS transmission electron microscope with accelerating voltage of 200 kV. A thin droplet of solution was spread on copper grids and observed directly without staining.

its chars as low-cost adsorbents for removing various pollutants from water.25−31 During this process, a complete utilization of agricultural straw is realized, but these adsorbents generally require a treatment process to enhance their absorbing capability. The fourth method is to comminute agricultural straw and then to mix it with polymers for fabricating polymer composites.32−36 This is a simple, feasible, and economic method. However, the resultant products are low-grade, and polymers used in composites cannot be recycled and reused directly. Therefore, it is essential to find a simpler, more effective and economic method to obtain high value-added products for the efficient utilization of agricultural straw. Natural plants, such as wood, bamboo, reed, and straw, exhibit excellent mechanical properties because they have optimized hierarchical microstructures and natural cellulose nanocrystals (cellulose I) with high strength and regular arrangement along the longitudinal fiber axis.37−45 In theory, the elastic modulus and ultimate tensile strength of cellulose I crystallites can reach 138 and 17.8 GPa, respectively.46−48 After a completely dissolution and subsequent regeneration process, these optimized hierarchical microstructures and natural crystalline structures are broken and difficult to recover. Meanwhile, the presence of lignin and hemicellulose hinders the formation of structural materials. Fort et al. noted that, even if the raw material of lignocellulose was wood in which cellulose had a high degree of polymerization (DP), it is impossible to prepare structured cellulose hydrogels.58 In summary, it is difficult to directly obtain regenerated biomass-based materials with high mechanical strengths from lignocellulose. Recently, we found that, via selective dissolution of natural cellulose, cellulose nanocrystals were retained in situ; finally, strong and self-reinforced all-cellulose nanocomposite films were obtained.56 In this work, the same strategy was again employed. Corn husk was chosen as a model substance for agricultural straws. We fully take advantage of 1-allyl-3methylimidazolium chloride (AmimCl), i.e., its good solubility for lignocellulose49−53 and controllability of the dissolution process of lignocellulose. 54−56 Corn husk is dissolved completely in AmimCl; then, microcrystalline cellulose (MCC) is added and partially dissolved by controlling the dissolution conditions. As a result, those undissolved nanocrystals from MCC are used as the reinforcing phase, and the dissolved MCC is used as the adhesive; consequently, corn husk/MCC films are fabricated, and a direct utilization of corn husk is achieved.

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EXPERIMENTAL SECTION

Materials. Corn husk was obtained from the Anhui province of China. It was washed by water, dried in the sun, and then cut into 5128


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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration for directly fabricating agricultural straw into all-biomass nanocomposite films reinforced with additional in situretained cellulose nanocrystals.

Table 1. Mechanical Properties of Corn Husk/MCC Nanocomposite Films Prepared under Different Conditions sample

content of corn husk in nanocomposites/wt %

dissolution time of MCC/h

S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9

33 50 60 67 71 50 50 50 50

4 4 4 4 4 1 2 3 5

tensile strength/MPa 36 67 53 59 54 22 49 65 65

Scanning electron micrographs (SEM) were observed using a JEOL JSM-6700F scanning electron microscope at an accelerating voltage of 10 kV. The cross sections of frozen-and-fractured films were coated with platinum before observation.

± ± ± ± ± ± ± ± ±

4 5 3 5 4 2 3 4 2

tensile modulus/GPa 4.1 4.4 3.7 3.5 4.2 1.9 3.8 3.7 4.2

± ± ± ± ± ± ± ± ±

0.2 0.5 0.5 0.4 0.5 0.1 0.3 0.3 0.4

elongation at break/% 4.1 2.7 4.0 4.0 1.6 1.8 3.8 4.0 4.5

± ± ± ± ± ± ± ± ±

0.4 0.3 0.4 0.2 0.2 0.4 0.3 0.4 0.5

concentration of the corn husk solution was increased to 10 wt %, it was still difficult to maintain a stable shape of the corn husk hydrogels. Fort et al. had found a similar phenomenon in wood/ionic liquid solution and suggested the presence of lignin and/or hemicellulose to prevent the formation of structured cellulose hydrogels.58 Moreover, after a dissolution-regeneration process, the natural crystalline structure of cellulose (cellulose I) was broken once and difficultly reconstructed again. Thus, it was difficult to directly obtain regenerated cornhusk hydrogels with highly stable shape. Cellulose nanocrystals (cellulose I), which exist in all natural plants, have excellent mechanical properties and often act as a reinforcing agent for fabricating polymer nanocomposites. During the dissolution process, natural cellulose nanocrystals can be retained by controlling the dissolution conditions of plants. On the basis of this phenomenon, self-reinforced allcellulose nanocomposite films were successfully prepared in our previous work.56 In this work, we took this strategy again. Microcrystalline cellulose (MCC) was added to the corn husk/ AmimCl solutions; then, a low dissolution temperature (50 °C) was used to make sure that cellulose nanocrystals in MCC were retained (Figure S2). The undissolved nanocrystals from MCC are used as the reinforcing phase (Figure 3), and the dissolved MCC is used as the adhesive and part of the matrix. As a result, homogeneous, transparent, and beige-colored corn husk/MCC nanocomposite hydrogels and films were obtained with high shape stability, as shown in Figure 1. The beige color of corn husk/MCC nanocomposite hydrogels and films originates from the presence of lignin and chromogenic extract in the corn husk, which had not been bleached before use. This phenomenon indicated that complete utilization of corn husk was achieved.

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RESULTS AND DISCUSSION The dissolution of lignocellulose strongly depends on five factors as follows: (a) particle size of raw material, (b) water content of raw material, (c) temperature of dissolution, (d) structure of ionic liquids, and (e) water content of ionic liquids. Zavrel et al. pointed out that AmimCl was the most effective ionic liquid for dissolving wood chips by a high-throughput screening test.49 Thus, in our work, AmimCl was chosen as the solvent for dissolving corn husk. The corn husk was pulverized, sieved, and dried carefully before dissolution, which is described in detail in the Experimental Section. Then, after 4 h of dissolution at 120 °C, the cellulose in the corn husks was found to dissolve completely in AmimCl (Figure S1), and a relatively transparent 4 wt % corn husk/AmimCl solution was obtained. Generally, it was considered that all of the components in the corn husk were dissolved in AmimCl. When the corn husk/ AmimCl solutions with higher concentrations were being prepared, a higher temperature or longer dissolution time was necessary (Figure S1). In AmimCl, the highest concentration of 10 wt % corn husk could be obtained as the dissolution time was prolonged. The resultant corn husk/AmimCl solutions were immersed in water to remove ionic liquids and exchange the liquid phase to obtain hydrogels. However, the corn husk hydrogels were so fragile that only a mild touch could definitely destroy it to pieces. We tried many times to strengthen the shape stability of the corn husk hydrogels but all of the efforts failed. Even if the 5129


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Figure 2. (A) FTIR spectra and (B) XRD curves of MCC, corn husk, and corn husk/MCC nanocomposite films. The dissolution times of MCC were 1, 2, 3, and 5 h for samples S-6−9, respectively.

Figure 3. TEM micrographs of corn husk/MCC/AmimCl solutions of samples (A) S-7 and (B) S-8. The dissolution time of MCC for S-7 is 2 h and for S-8 is 3 h.

processing window for fabricating corn husk/MCC films with optimal tensile strength. In summary, the optimal preparation conditions for fabricating corn husk/MCC nanocomposite films were as follows. The MCC of 0.4 g and AmimCl of 9.6 g were added to 4 wt % corn husk/AmimCl solution of 10.0 g. Then, the mixture was mechanically stirred at 50 °C for 4 h. The maximum tensile stress and elastic modulus of the corresponding corn husk/MCC nanocomposite films (sample S-2) were 67 MPa and 4.4 GPa, respectively. These mechanical properties were similar to those of pure cellulose films,21 which were prepared using cellulose extracted from corn husks as the feedstock. However, the extraction process included multiple steps, such as acid−base and decolorization treatments; thus, it is tedious and causes environmental pollution. In contrast, the dissolution-regeneration method in ionic liquid used in this work was simple, economical, and environmentally friendly, by which corn husk-based films with high mechanical strength were obtained. More importantly, the resultant all-biomass nanocomposite films are fully biodegradable and have an enough mechanical strength that they could be used as packaging tapes and wrapping papers to replace conventional petroleum polymer films in some fields. More characterization measurements will help to understand the processing, structure, and properties relationship of these corn husk/cellulose films. In FTIR spectra of corn husk/MCC films (Figure 2A and Figure S4), there are characteristic peaks of lignin (1248 cm−1, C−O stretching; 1510 cm−1, aromatic ring stretching) and hemicellulose (1740 cm−1, CO stretching),21,62,63 indicating that all of the main components

More importantly, the corn husk/MCC nanocomposite films exhibited high mechanical strength due to the cellulose nanocrystal as the reinforcing phase and the dissolved MCC as the adhesive. The tensile properties of corn husk/MCC films obtained under various preparation conditions are listed in Table 1 and Figure S3. As the content of corn husk in corn husk/MCC films increased from 33 to 50 wt %, the tensile strength increased significantly. However, as the corn husk content increased further, the tensile strength of corn husk/ MCC films decreased slightly due to the lack of sufficient MCC to bind various components together. Even so, when the content of corn husk exceeded 33 wt %, the tensile strength of corn husk/MCC films was higher than 50 MPa, indicating that the reinforcing role of cellulose nanocrystals remained strong enough. The tensile elongation of the obtained corn husk/ MCC films was ∼2−5%, which is similar to that of pure cellulose films prepared with ionic liquids59 and other solvent systems.60 Because cellulose a has rigid backbone structure, the pure cellulose film is often brittle. However, the toughness of cellulose film can be effectively improved by adding plasticizers, such as glycerol, into the washing bath during the preparation process.61 In addition, as the dissolution time of MCC in AmimCl increased, the tensile strength of corn husk/MCC films initially increased and then reached a maximum after 3−5 h of dissolution of MCC. This phenomenon designated that, after a dissolution process for a certain amount of time, the distribution, size, and content of cellulose nanocrystals in MCC/corn husk/AmimCl solutions reached an optimum, as shown in Figure 3. This result also means that there is a wide 5130


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Figure 4. SEM images of corn husk/MCC nanocomposite films of samples (A) S-1, (B) S-3, (C) S-5, (D) S-6, (E) S-8, and (F) S-9.

adhesion between matrix and reinforcing phase have a significant influence on the mechanical performance of materials. Because the reinforcing phase and reinforced matrix had a similar chemical structure, the cross-section of corn husk/ MCC films was uniform, compact, and relatively smooth, and there was no aggregate (Figure 4). Meanwhile, no piece of cellulose nanocrystals was found in these SEM images due to the nanoscale dimensions of the nanocrystals. These phenomena indicate the good dispersion of nanocrystals in the matrix.

in corn husk were utilized effectively. In addition, comparing FTIR spectra of corn husk/MCC films with those of MCC and corn husk, no new peaks appeared. Obviously, it was a physical dissolution process of corn husk in AmimCl. In the XRD profile of MCC (Figure 2B), there is a strong crystalline peak at 22.8° for (200) crystal plane, a small peak at 34.6° for (004) crystal plane, and a broad peak at 15.8° overlapped by 15.1° (11̅0) crystal plane and 16.8° (110) crystal plane, indicating a typical cellulose I crystalline structure. With the increased dissolution time of MCC, the characteristic peaks of cellulose I, 15.1° (1−10), 16.8° (110), and 22° (200), become progressively indistinct and are covered by a broad diffraction peak in the range of 2θ = 15−30° (Figure 2B and Figure S5), overlapped mainly by two peaks of cellulose II at 20.1° (110) and 21.9° (020) and the peak of amorphous cellulose at 17.3°.56 These phenomena confirmed that MCC was being dissolved as the time prolonged, and the main allomorph of the resultant cellulose nanocomposites were cellulose II and amorphous. Considering the tiny size and relatively low content of remaining cellulose I crystals, this phenomenon is operative. Therefore, in the regenerated corn husk/MCC nanocomposites, there existed a small amount of undissolved cellulose I crystals. The existence of undissolved cellulose nanocrystals was directly confirmed by TEM observations as well. TEM images revealed that there were many nanocrystals in relatively transparent corn husk/MCC/AmimCl “solutions”, as shown in Figure 3. The width and length of nanocrystals are 5−50 and 50−100 nm, respectively, which are consistent with the dimensions of elementary fibrils and microfibrils of cellulose I crystal.54 With the increase of dissolution time, the aspect ratio and content of cellulose nanocrystals decrease, and the distribution of cellulose nanocrystals became more uniform than in the initial stage. Because of the weak contrast between cellulose nanocrystals and the background, it was difficult to obtain clearer TEM micrographs of corn husk/MCC/AmimCl “solutions”. On the basis of the TEM observation, it could be predicted that, with the increase of the dissolution time, the mechanical performance of cellulose nanocomposites would first increase, reach a maximum at medium dissolution time, and then decrease, which is reflected in the mechanical test in Table 1. It is common that the mechanical properties strongly depend on the microstructure of materials. The compact extent and the

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CONCLUSIONS

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ASSOCIATED CONTENT

A direct utilization of corn husk was achieved by using ionic liquid AmimCl as the solvent and MCC as the adhesive and reinforcing phase. Corn husk was dissolved completely in AmimCl at 120 °C for 4 h. The highest concentration of 10 wt % corn husk/AmimCl solution could be obtained as the dissolution time was prolonged. Then, MCC was added and partially dissolved by controlling the dissolution conditions. The undissolved nanocrystals from MCC were used as the reinforcing phase, and the dissolved MCC was used as the adhesive and matrix. Eventually, homogeneous, transparent, and beige-colored corn husk/MCC nanocomposite films were obtained with high mechanical properties. Because all the reinforcing phase, adhesive phase, and reinforced matrix were composed of cellulose, the resultant corn husk/MCC films showed excellent interfacial compatibility and were fully biocompatible and biodegradable, combined with being lightweight and high strength. The optimum tensile strength and elastic modulus of nanocomposite films with 50 wt % corn husk reached 67 MPa and 4.4 GPa, respectively. Thus, this work provided an easy, economical, and effective method to accomplish direct utilization of agricultural straws and fabricate high value-added materials.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00488. POM images of corn husk/AmimCl and corn husk/ MCC/AmimCl solutions, stress−strain curves of corn husk/MCC films, FTIR spectra, and XRD curves (PDF) 5131


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jinming Zhang: 0000-0003-3404-4506 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Nos. 51425307, 51573196, and 21374126) and the Program of Taishan Industry Leading Talents (Shandong Province).

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Directly Converting Agricultural Straw into All-Biomass

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