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DMSO solution
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Characterization of lignocellulose aerogels fabricated using a LiCl/DMSO solution ⁎
Zhulan Liu, Jinxiu Wu, Jianyu Xia, Hongqi Dai, Yunfeng Cao , Zhiguo Wang
T
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Contact information: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, 159 Longpan Rd, Nanjing, 210037, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soybean stem Ethylenediamine LiCl/DMSO Lignocellulose aerogel Delignification
Lignocellulose aerogel has attracted great attentions in advanced renewable material research. Mostly, cellulose, hemicellulose or lignin should be separated tediously from the lignocellulose first then to fabricate the aerogel individually. Herein, lignocellulose mesoporous aerogels were prepared directly from lignocellulose/LiCl/DMSO solution. The soybean stem with different range of delignification were pretreated by ethylenediamine (EDA), then dissolved in LiCl/DMSO solvent followed by coagulating, solvent replacing and freeze drying sequentially. Rheology and compression tests were employed to measure the mechanical strength of the alcogel. Scanning electron microscopy (SEM), Brunauer-emmett-teller (BET), Thermo-gravimetric (TGA) and the gravimetric analysis were taken to characterize the morphology, porosity, thermal stability and swelling ability of the aerogels. Comparing these samples, the lignocellulose aerogel without delignification had the greatest swelling ability but a little lower porosity, mechanical strength and thermal stability. And as the delignification increased in a proper extent, which induce the lignin removal, the cellulose and hemicellulose dissociated certainly, the mechanical strength and thermal stability improved, a denser fibril network structure with lower pore diameter (DPore) and a higher SBET formed. While, exacerbate delignification could reduce the SBET and the uniformity of the pores in the aerogels remarkably. The performance of the aerogels could be controlled to some extent, by adjusting the degree of the delignification.
1. Introduction Aerogels are ultra-light solid materials with high porosities prepared by freeze-drying or critical point drying wet gel precursors which also called hydrogel or alcogel. During this process, the background liquid solvent is replaced by gas without the substantial collapse of the solid gel network due to rapid water sublimation (Bryning et al., 2007; Tan et al., 2001; Zhuo et al., 2016). Recently, these fragile networks, which have extremely low densities, large micropore volumes, and high inner specific surface areas (Bag et al., 2007), have been found to exhibit excellent mechanical properties (Kim et al., 2012), magnetic induction (Olsson et al., 2010), insulating and optical properties (Zhao et al., 2009), surface interface effects, dielectric properties (Lin et al., 2005), and quantum size effects (Hu et al., 2013). It has met extensive application in catalyst supports, films, separation, chemical analysis, sensing, energy technologies and energy absorption. Kistler first reported the synthesis of aerogels by supercritical drying silica, alumina, nickel tartrate, stannic oxide, tungstic oxide, gelatin, agar, nitrocellulose, cellulose and egg albumin (Kistler, 1931). It can be categorized as inorganic, organic or composite based on the starting
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materials used in the synthesis (Mu et al., 2016). Recently, natural polymer compounds have been widely used in materials applications. Not only is the production waste recycled to reduce environmental pollution, but the natural polymers also exhibit excellent performances (Nguyen et al., 2016; Rahbar Shamskar et al., 2016; Rotbaum et al., 2017; Zhang et al., 2017; Zhuo et al., 2016). In addition, these polymers can be used to address many materials safety problems. In particular, natural plant polysaccharides such as cellulose, are attracting much interest for using as raw materials in the preparation of high-quality aerogels (Labudek and Martiník, 2011; Liu et al., 2017; Wang et al., 2017a; Li et al., 2018). Aerogel preparation from cellulose can generally be divided into three steps (Chen et al., 2016; Liebner et al., 2008; Sescousse et al., 2011): cellulose dissolution, coagulation in anti-solvent, and the transformation of wet gels into aerogels (Sescousse et al., 2011). Because of the existence of lignin as well as the tightly packed molecular structure and strong intramolecular and intermolecular hydrogen bonding, it is difficult to dissolve lignocellulose in conventional solvents. Thus, in most reports of the renewable aerogel materials, cellulose, hemicellulose or lignin were isolated from lignocellulose by tedious and costly chemical treatment first, and then used individually
Corresponding author. E-mail addresses: [email protected] (Y. Cao), [email protected] (Z. Wang).
https://doi.org/10.1016/j.indcrop.2019.01.057 Received 11 February 2018; Received in revised form 27 December 2018; Accepted 26 January 2019 0926-6690/ © 2019 Published by Elsevier B.V.
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2.2. Methods
in the synthesis, which also lead to environmental pollution. Therefore, it is more desirable to prepare aerogels directly from lignocellulose instead of cellulose or any other single component. To prepare lignocellulose aerogels, the raw material must be dissolved in a solution first (Chen et al., 2016). The first lignocellulose aerogels were prepared by dissolving spruce wood in IL 1-butyl-3-methylimidazolium chloride, followed by coagulating the gel in aqueous ethanol and drying it with supercritical carbon dioxide (Aaltonen and Jauhiainen, 2009). There are some solvent systems that have been employed to dissolve lignocellulose, such as dimethyl sulfoxide/tetrabutylammonium fluoride (DMSO/TBAF), a dimethyl sulfoxide/imidazole (DMSO/NMI) binary system and imidazolium-based ionic liquids (ILs) (Kilpelainen et al., 2007; Lu and Ralph, 2003). However, the first two solvent systems require ball-milling the lignocellulose for a long time, and the dissolution in the IL was only achieved at a high temperature (above 110 °C). Wang et al observed that wood could be effectively dissolved in a LiCl/DMSO solvent system (Liu et al., 2015, 2016a; Wang et al., 2009). Chen et al prepared lignocellulose aerogels by dissolving ball-milled bagasse in LiCl/DMSO at a high temperature (110 °C), cyclically freezing and thawing the gel and then regenerating it with water (Chen et al., 2016). All these studies employed ball-milling and heating processes, both of which cause lignocellulose degradation and negatively impact the gelation. Even in the work of Mussana, the lignocellulose could be dissolved in ILS/DMSO co-solvent system at 80 °C, the solution should be cyclically frozen and thaw to get the aerogel (Muussana et al., 2018). Moreover, there are quite few information about the effect of the lignin on the preparation and properties of the aerogels. In our preliminary studies, ethylenediamine (EDA)-pretreated cellulose or lignocellulose could be completely dissolved in LiCl/DMSO to give a homogenous solution under moderate conditions (75 °C) (Liu et al., 2015; Wang et al., 2012). Because this method does not require ball-milling or high temperatures, the structural modification of lignin and cellulose degradation can be avoided. Additionally, the aerogel can be synthesized from cellulose/LiCl/DMSO solution by coagulating without any freezing-thawing process (Wang et al., 2012). Therefore, this method has great potential for the facile preparation of lignocellulose aerogels under reasonable conditions. In this study, lignocellulose aerogels with high porosity and good swelling properties were fabricated by dissolving EDA-pretreated soybean stem meal in LiCl/DMSO and then subjecting it to sequential coagulation, solvent replacement and freeze-drying processes. The morphology, thermal stability and swelling properties were characterized in detail to determine the effects of the lignin on the preparation and characterization of these aerogels.
2.2.1. Lignocellulose pretreatment with EDA Dried MCC or stem meal (L-0, L-0.5, L-1 and L-2) (10 g) was soaked in 200 ml of EDA and stirred continuously for overnight at room temperature. Then, the material was collected by filtration and freezedrying. These samples are denoted “Stem-EDA complex” or “MCC-EDA complex”. The residual EDA contents in the complexes were determined by titration of approximately 0.5 g samples in water with 0.1 mol/l hydrochloric acid, using methyl orange as the indicator, and the results are listed in Table 1. 2.2.2. Lignocellulose dissolution The Stem-EDA or MCC-EDA complex was suspended in 8% LiCl/ DMSO to a concentration of 5%. The mixture was stirred continuously at room temperature for 24 h and then at 60 °C for 2 h to obtain a homogeneous solution. 2.2.3. Lignocellulose aerogel preparation The homogeneous lignocellulose solution was centrifuged to degas first, then poured into a glass dish to form a 1 mm-thick layer and subsequently immersed in ethanol for gelation to get the alcogel. To remove LiCl, DMSO and residual EDA, the obtained alcogel was thoroughly washed with ethanol until no Cl− in the supernatant was detected by the silver nitrate solution. Then, the corresponding aerogel was obtained by solvent exchange with tert-butanol (TBA), followed by freeze-drying the alcogel above. The aerogels made from MCC, L-0, L0.5, L-1 and L-2 are referred to as S-MCC, S-0, S-0.5, S-1 and S-2, respectively. The entire preparation procedure is shown in Fig. 1. 2.2.4. SEM The macroscopic appearances of the lignocellulose aerogels were observed visually. The aerogel samples were frozen in liquid nitrogen and immediately fractured, then sprayed with a thin gold film on the surface. Their 3D cellulosic networks were characterized by scanning electron microscopy (SEM) (Quanta 200, FEI, USA). 2.2.5. Nitrogen adsorption measurements Nitrogen adsorption measurements were performed using an ASAP 2020 instrument (USA). The Brunauer-Emmett-Teller (BET) surface areas and pore sizes were obtained by measuring the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature. The BET specific surface areas were calculated from the isotherms. Before the measurements, the aerogel samples without any collapse were degassed at 70 °C for 24 h. 2.2.6. Rheology measurements Rheological test of the lignocellulose gels was conducted using an RS6000 Rheometer (HAAKER, German). A P20 TiL Platte and a P20 TiL cone plate can be used. The specimens were subjected to a strain sweep test over the range of 0-103 Pa at ambient temperature with 1 Hz of frequency. Storage modulus (G’) and loss modulus (G’’) were recorded for the lignocellulose alcogel containing abundant ethanol after solvent replacement but before freeze-drying. The samples were measured triplicate.
2. Materials and methods 2.1. Materials All chemicals were reagent grade. Deionized water was used in all the experiments. The microcrystalline cellulose (MCC) sample was obtained from Guoyao Chemicals Group (China). Soybean straw was collected from north Jiangsu, China. Air-dried soybean stem was first manually fractionated from soybean straw and then ground through 40to 80-mesh screens using a General Electric Wiley mill. The ground sample was subsequently extracted with a benzene-ethanol (2:1, v/v) mixture for 8 h to remove the extractives. The extractive-free sample was vacuum-dried at 45 °C for 24 h and used as the native lignocellulose source. Stem samples were delignified by adding NaClO2 and acetic acid to the stem meal suspension for 0.5, 1 or 2 h. The final lignin and polysaccharides contents were determined by the way of NREL/TP-51042618 (Sluiter et al., 2011), as listed in Table 1.
2.2.7. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the lignocellulose aerogels was performed on a TA Instruments Q500 thermogravimetric analyzer using approximately 5–10 mg samples. The experiments were conducted under nitrogen from 20 °C to 600 °C at a heating rate of 10 °C/ min. 2.2.8. Swelling properties A gravimetric method was employed to measure the swelling ratios of the aerogels in distilled water at different temperatures ranging from 294
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Table 1 Lignocellulose samples used to prepare the gels. Sample
Delignification time (h)
Lignin content (%)
Polysaccharide (%) Glucan
MCC L-0 L-0.5 L-1 L-2
– 0 0.5 1 2
– 23.24 21.33 18.91 14.24
± ± ± ±
0.03 0.11 0.05 0.09
– 37.39 44.12 45.46 48.68
± ± ± ±
Residual EDA content (%) Xylan
0.51 0.02 0.86 0.42
– 15.58 16.89 17.08 17.75
Others
± ± ± ±
0.75 0.55 0.26 0.2
– 3.79 4.01 4.50 4.62
± ± ± ±
Total
0.03 0.09 0.10 0.06
– 56.76 65.02 67.04 71.04
± ± ± ±
1.29 0.48 1.22 0.57
19.5 18.4 18.9 19.7 20.1
± ± ± ± ±
0.1 0.1 0.2 0.1 0.2
Note: Others in the polysaccharide include the Bhamnosan, Galactan and Araban. Total is the sum of Glucan, Xylan and Others.
staggered along the cellulose chain axis by half of a glucose ring. The fundamental reacting unit is a sheet of cellulose chains, but not individual cellulose chains (Loeb and Segal, 1955; Warwicker and Wright, 1967; Su et al., 1989; Wada et al., 2009). In addition, the lignin is an irregular polymer consisting of several monoaromatic monomers that form a 3D network, which makes lignocellulose essentially insoluble in common solvents. Therefore, finding an appropriate solvent is the major challenge in preparing lignocellulose gels. EDA is an efficient fiber-swelling agent. EDA solutions with concentrations greater than 60% can induce intramicellar swelling and lattice distention. When considerably higher EDA concentrations were used in previous studies, the [101] interplanar distance of the unit cell in cellulose I increased from 6.14 Å to 11.86 Å (Loeb and Segal, 1955). Consequently, the hydrogen bonds (C(6)eOH⋯OeC(3)) between the cellulose sheets are broken, and the hydroxyl groups (−OH) at C(6) and C(3) are exposed. As shown in Fig. 2, during EDA pretreatment, the abundant hydroxyl groups in cellulose accept hydrogen bonds from EDA amino groups to form a stable cellulose-EDA complex containing many hydrogen bonds (−OH⋯NH2−CH2−CH2−NH2⋯HO-), leading to the destruction of the hydrogen-bonding network between the microfibrils and other polysaccharides. EDA treatment is a simple method for enhancing the accessibility and chemical reactivity of crystalline cellulose. Then, in the subsequent dissolution procedure, LiCl and DMSO which are halogenated or polar, oxygen-containg nonaqueous solvents, compete with the amino groups to extract EDA from the cellulose, and generate new interactions in the lignocellulose (Sawada et al., 2013; Wada et al., 2008; Segal and Loeb, 1960). These interactions lead to the dissociation and dissolution of different lignocellulose
15 to 45 °C. The dried aerogel was immersed in distilled water at each temperature for a given amount of time. After removing the excess water on the surface with filter paper, the aerogel was weighed. The swelling ratio (SR) was defined as follows: SR = (Wt – Wd) × 100% / Wd
(1)
where Wt and Wd are the weights of the swollen aerogel at time t and the dry aerogel, respectively. The equilibrium swelling ratio (ESR) was defined to be the swelling ratio measured after the dried aerogel was immersed in distilled water for 24 h. 3. Results and discussion 3.1. Preparation and structure of the lignocellulose aerogels Lignocellulose or cellulose dissolution followed by coagulation in anti-solvent could result in the formation of swollen gels containing ambient liquid (Kistler, 1931). In this study, it was successful to form lignocellulose aerogel upon coagulating lignocellulose solution in antisolvent at 5% of lignocellulose concentration without freezing-thawing treatment, which is different from the aerogel preparation of Chen and Mussana (Chen et al., 2016; Muussana et al. (2018)). Lignocellulose are mainly constituted of cellulose, hemicellulose, and lignin in various proportions. According to the references, there are many intramolecular and intermolecular hydrogen bonds in cellulose. The cellulose chains form a sheet-like structure stabilized by intermolecular hydrogen bonds that are parallel to the pyranose rings. These sheets are
Fig. 1. Schematic representation of the lignocellulose aerogel preparation. 295
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Fig. 2. Mechanism of EDA pretreatment of cellulose.
in the solvent system but also has one or more active H atom to form a hydrogen bonding network with the lignocellulose solution (Loeb and Segal, 1955; Lv et al., 2011). Therefore, anhydrous ethanol, which is miscible in the LiCl/DMSO solution and also has a strong hydrogenbonding ability, was used as the anti-solvent for lignocellulose gelation in this study.
molecules in the LiCl/DMSO solution. A homogeneous lignocellulose solution in 8% LiCl/DMSO was obtained by lignocellulose dissolution into independent fiber chains. These chains were then coagulated by adding an anti-solvent which is soluble in DMSO and contains active H atoms that are likely to form hydrogen bond with the lignocellulose. Because the amount of the antisolvent added to the system is significantly larger than that of LiCl/ DMSO, the lignocellulose gels can be thoroughly washed. If the antisolvent does not have active H atoms (anti-solvent A), it only dilutes the lignocellulose/LiCl/DMSO solution and cannot form a 3D network structure with the lignocellulose. However, if the anti-solvent has active H atoms (anti-solvent B), its ability to form hydrogen bonds to the lignocellulose controls the gel formation. If anti-solvent B has a relatively weak hydrogen-bonding ability, many of the hydrogen bonds between and within the lignocellulose chains reform, resulting in lignocellulose precipitation from the solution system. In contrast, if antisolvent B exhibits a strong hydrogen-bonding ability, it dynamically competes with the lignocellulose chains to form hydrogen bond with other lignocellulose chains and solution. Consequently, the highly compact, complex entangled network of the lignocellulose gel shown in Fig. 3 can be produced by hydrogen bonding between the solution and lignocellulose, and between the lignocellulose chains. In conclusion, the anti-solvent used for lignocellulose gelation should not only be miscible
3.2. Morphology of the lignocellulose aerogels observed by SEM MCC and soybean stem with different lignin contents were pretreated by EDA (L-MCC, L-0, L-0.5, L-1 and L-2) and dissolved in 8% LiCl/DMSO to obtain homogenous solutions. After gelation, the resulted alcogel was thoroughly washed by immersion in ethanol, then transferred to TBA and subsequently freeze-dried to get the corresponding aerogels (S-MCC, S-0, S-0.5, S-1 and S-2). During this process, the shape and bulk network structure of the alcogels were preserved well. As shown in Fig. 4, the surfaces of the lignocellulose aerogels with different lignin contents are slightly collapsed. In contrast, the fractured cross sections reveal unique bulk cellulose network structures with many pores. The differences in the surface external and bulk internal structures are probably due to the variations in the gelation conditions at these two positions. During the gelling process, the liquid composition at the surface external changed dramatically, whereas it changed
Fig. 3. Schematic illustration of the nanostructural reorganization in the lignocellulose/LiCl/DMSO solution. (a) Lignocellulose/LiCl/DMSO suspension, (b) homogenous lignocellulose solution, (c) lignocellulose alcogel, (d) lignocellulose aerogel. 296
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Fig. 4. SEM images of the lignocellulose aerogels.
53.27 m2/g which is much smaller than that of S-MCC. These results might be attributed to the differences in the homogeneity and uniformity of the pore dimensions of these two aerogels. After 0.5 h or 1 h of delignification, the Dpore value decreased to nearly 16 nm, and the SBET value increased to 61.21 m2/g for S-0.5 and 69.14 m2/g for S-1, respectively. When the delignification time was increased to 2 h, the resulting lignocellulose aerogel S-2 has a larger Dpore value but a smaller SBET value than S-1, which is consistent with the morphological analysis. In the undelignified lignocellulose, the lignin, hemicellulose and cellulose are interconnected in a complex structure and cannot be dissociated into independent cellulose or hemicellulose chains to generate a network structure. Therefore, the complex bulk structure of S-0 exhibits a film-like or floccular morphology, and S-0 thus has a relatively large DPore value and small SBET value. After 0.5 h or 1 h of delignification, some of the lignin was removed, enabling the hemicellulose and cellulose molecular chains in S-0.5 and S-1 to unravel and connect to each other by hydrogen bonding to form 3D fibril networks with higher SBET and smaller DPore values. However, when the delignification time was 2 h, the lignin, hemicellulose and cellulose were separated to a greater degree, resulting in the formation of more and larger pores. Thus, the DPore value of the lignocellulose aerogel S-2 is larger, whereas the SBET value is smaller. All the SBET values in this study are much higher than those of lignocellulose aerogels (2–7.5 m2/ g) prepared by cyclically freezing and thawing a wood solution in an ionic liquid and then drying with supercritical carbon dioxide (Li et al., 2011; Lu et al., 2012), which is regarded as the most effective drying method for preparing homogenous cellulose aerogels (Hoepfner et al., 2008).
gradually in the bulk internal, and lead to slow coagulation and relaxation rearrangement which help to give the network structure (Wang et al., 2012). In addition, during freeze-drying, the liquid on the surface was sublimed much faster than that inside the gel, resulting in different surface and bulk structures. The images in Fig. 4 show that the bulk microstructure of the lignocellulose aerogels is significantly affected by the delignification degree. The lignocellulose aerogel S-0 made from undelignified soybean stem L-0 has a complex bulk structure that appears film-like or floccular instead of network-like, and micropores are distributed throughout the film. This result is attributed to the complex interwoven structure of the cellulose, hemicellulose and lignin in L-0, which prevents them from separating into independent cellulose or hemicellulose chains to generate a network structure. However, although S-0.5 made from L-0.5 still exhibits some film-like or floccular morphological features, the bulk lignocellulose aerogel has a network structure with more pores. When the delignification time is increased to 1 h, the internal of S-1 has a unique cellulose network structure, as shown by the fractured cross section. In addition, the pores in S-1, which originate from the interconnected long and fairly straight fibrils, have a considerably more uniform distribution than those of S-0 and S-0.5. It is thus concluded that as the delignification time increased, the lignocellulose dissociated into single fibril chains, then connected to each other by reforming the hydrogen bonds to form 3D networks with large interstitial spaces. When the delignification time was increased to 2 h, the resulting lignocellulose aerogel S-2 has a distinct fibril network structure; However, the pore distribution is not as uniform as those of S-0.5 and S-1.
3.3. BET porosity measurements of the lignocellulose aerogels 3.4. Mechanical strength of the lignocellulose gels The pore sizes and volumes of the series of aerogels were measured to furtherly characterize the mesoporous structures. Nitrogen adsorption experiments at liquid nitrogen temperature provide information on the structure of porous materials. The N2 adsorption-desorption isotherms of the lignocellulose aerogels are shown in Fig. 5. The adsorption isotherms exhibit the inverse-S shape of a type IV adsorption isotherm with the adsorption-desorption hysteresis loop, meaning the average pore diameter is larger than 10 nm. The BET and BJH methods were used to determine the pore size distribution. The results show that proper delignification leads to a decrease in the pore size and increase in the specific surface area (SBET) and the pore density of the lignocellulose aerogels. As shown in Table 2, the average pore diameter Dpore of the lignocellulose aerogel S-0 made from L-0 is 20.281 nm which is very close to that of S-MCC made from L-MCC. However, the SBET of S-0 is
The mechanical strength of the lignocellulose gels is important for their application. From the optical image in Fig. 6, the lignocellulose alcogel S-2 could bear the weight of 20 g without obvious deformation. For the viscoelastic properties, the storage modulus G’ represents stored energy in materials during elastic deformation, which reflects the elasticity property or the solid-like behaviour of the materials; the loss modulus G’’ represents the lost energy due to viscous deformation, which reflects viscosity property or the liquid-like behaviour (Mushi et al., 2016). In other words, storage modulus G’ exhibits the stiffness of materials (Liu et al., 2016b). Moreover, the cross point of G’ and G’’ is the critical stress value indicating that the gel can bear the shear stress before irreversible deformation. As shown in Fig. 6, the G’ values are much higher than G’’ for all alcogel over the whole stress range, implying elastic gel characteristics. It also can be observed that the 297
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Fig. 5. N2 adsorption-desorption isotherms of the lignocellulose aerogels. (a) S-0, (b) S-0.5, (c) S-1, (d) S-2, (e) S-MCC.
cross point of G’ and G’’ with the sequence of S-MCC > S-2 > S0.5 > S-0.
Table 2 Porous properties of the lignocellulose aerogels. Samples
Specific surface area (SBET) (m2/g)
Pore diameter (DPore) (nm)
S-0 S-0.5 S-1 S-2 S-MCC
53.27 61.21 69.14 64.29 61.04
20.281 16.709 16.301 17.813 20.199
± ± ± ± ±
1.70 2.01 2.13 1.89 2.43
± ± ± ± ±
3.5. Thermogravimetric analysis of the lignocellulose aerogels
0.854 0.048 0.031 0.10 0.52
The thermal stability of the lignocellulose aerogel was examined by thermogravimetry analysis (TGA) in a nitrogen environment. As shown in Fig. 7, the lignocellulose delignification affects the thermal stability of the lignocellulose aerogels. In all the TGA curves, small weight losses are observed below 100 °C, due to the apparent evaporation of adsorbed water from the lignocellulose aerogels. The onset decomposition temperatures of S-0 and S-0.5 which made from the raw stem (L-0) and the stem delignified for 0.5 h (L-0.5), respectively, are approximately 200 °C. While, the onset decomposition temperatures of S-1 and S-2 are increased to 225 °C around. The maximum decomposition temperature of S-0 and S-0.5 are almost 287 °C, but that of S-1 and S-2 are rise up to about 315 °C which is a little bit lower than that of MCC (328 °C). Additionally, the char yields of S-0 and S-0.5 at 600 °C are 28.66% and 23.10%, respectively. But for S-1 and S-2, their char yields are reduced to approximately 15%. All these results show that lignocellulose delignification improves the thermal stability and decreases the char yield of the lignocellulose aerogels. Combining the data in Table 1, in the aerogels S-0 and S-0.5 made from the raw stem L-0 and the slightly delignified stem L-0.5, the lignin and carbohydrate including cellulose
Fig. 6. Mechanical strength of the lignocellulose alcogels.
delignification of the stem could affect the mechanical strength of these gels. For S-0 which made from L-0 without any delignification, it has the lowest mechanical strength with about 110 Pa of G’ and 13 Pa of G’’. After 0.5 h of delignification, some of the lignin was removed, then the strength of S-0.5 is relatively improved to approximate 250 Pa of G’. If the delignification time was prolonged to 2 h, more lignin was removed but the cellulose and hemicellulose were remained well, which lead to the mechanical strength of the alcogel S-2 raised to about 1250 Pa obviously. However, the mechanical strength of these three lignocellulose gels are all much lower than that of S-MCC which made form MCC without any lignin. As a result, the existence of the lignin in the stem with slight or even without delignification would decrease the mechanical strength of the lignocellulose gels by weakening the interactions between cellulose or hemicellulose chains during the formation of the gels. It is also can be proved by the critical stress value at the
Fig. 7. TGA curves of the lignocellulose aerogels. 298
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lignin content decreased to 14.24%, but the total polyssacrides content increased obviously to 71.04% including 48.68% of glucan, 17.75% of xylan and 4.62% of others. These data proved that the lignin was furtherly removed during the 2 h of delignification, but the cellulose and hemicellulose were remained well and their abundant of hydrophilic groups such as –OH were exposed, which is benefit for improving the swelling property of the S-2. As a result, the SR of S-2 was certainly raised to 960% which was higher than that of S-1 but still lower than S0. The ESR of the lignocellulose aerogels at different temperatures are listed in Table 3, which also shows the relationship between the stem lignin content and ESR. S-0 clearly has the highest ESR, followed by S-2, S-MCC, S-0.5 and S-1 sequentially. These results are consistent with the SR results at 15 °C. Moreover, as the temperature raised, the hydrogen bonding between the aerogel and water weaken, and consequently the aerogel network structure shrinked. Thus, the ESR decreased. Additionally, it is concluded that the ESR decreases with the delignification degree increment in a proper range. However, excessive delignification results in a certain elevation of the ESR, which is attributed to the lignin removal and the exposure of the hydrophilic groups in cellulose and hemicellulose, but it is still lower than that of S-0 which made from L-0 without any delignification.
Fig. 8. Swelling ratios of the lignocellulose aerogels at 15 °C. Table 3 ESR of the lignocellulose aerogels at different temperatures. Aerogel samples ESR at different temperatures (%)
15 °C 20 °C 25°C 30°C 35°C 40°C 45 °C
S-0
S-0.5
S-1
S-2
S-MCC
2550.00 2286.57 2000.00 1870.73 1739.58 1543.24 1369.70
1157.27 1128.85 1127.78 1077.14 1028.33 926.42 677.27
935.71 876.92 771.43 680.37 666.67 645.79 551.10
2126.79 1547.67 1395.89 1290.70 1156.96 1121.43 966.67
1502.70 1624.24 1040.14 1144.29 990.36 989.52 867.41
4. Conclusions Highly porous lignocellulose aerogels were synthesized by dissolving EDA-pretreated soybean stem meal in LiCl/DMSO and sequentially coagulating, solvent replacing and freeze-drying. This series of lignocellulose aerogels exhibited great water adsorption and swelling properties. Appropriate delignification facilitates the formation of a continuous 3D fibril network with desired SBET and Dpore. As the delignification increased in a certain range, the lignin content of the raw soybean stem decreased and the polysaccharide content increased relatively, then resulted in the improvement of the mechanical strength of the alcogel and thermal stability of the aerogel, as well as the formation of the denser fibril network structure with lower Dpore and higher SBET which cause the decline of the swelling ability. But the excessive delignification for 2 h, had negative impact on the 3D network formation and the pore uniformity of the lignocellulose aerogels. However, because of the further lignin removal and the hydrophilic groups in the remained cellulose and hemicellulose exposure, the swelling ability of S-2 was some higher than that of S-1. But this data was still lower than that of S-0. It is possible to adjust the delignification to control the structure and swelling ability of the lignocellulose aerogel.
and hemicellulose, were well preserved, and the amorphous cellulose regions were also relatively unchanged, then result in their poor thermal stability. As the delignification time increased to 1 or 2 h, some of the lignin was removed and its content was notably decreased to 18.91% and 14.24%, respectively. While the relative contents of the cellulose and hemicellulose which could be presented by the polysaccharide relatively content, were increased greatly. All these alternations were help to improve the thermal stability. Besides, the amorphous cellulose regions were also somewhat degraded during delignification (Wang et al., 2017b; Muussana et al., 2018; Li et al., 2015). However, the highly ordered crystalline regions, in which the cellulose molecules are firmly bound together, were preserved well, leading to a significant improvement in the aerogel thermal stability. In addition, because the cellulose and hemicellulose have much lower C:H elemental ratio than the lignin, the aerogel char yield decreased with delignification time extending (Muussana et al., 2018; Li et al., 2015). This result is supported by the fact that S-MCC which does not have any lignin, has the lowest char yield.
Acknowledgements This work was supported by the financial support from the National Natural Science Foundation of China (No. 31600473, 31670591), the Natural Science Foundation of Jiangsu Province (BK20160928), and PAPD of Jiangsu Higher Education Institutions.
3.6. Swelling properties of the lignocellulose aerogels In Fig. 8, when the lignocellulose porous aerogels were immersed in the water at 15 °C, they quickly absorbed large amounts of water. Their weight increased continuously in the first 15 min and then remained unchanged at longer immersion times. After 15 min, the swelling ratio (SR) of S-0 reached to 1495% which is the highest than that of the others. For S-0.5 and S-1, the SR were reduced to 1003% and 590%, respectively. Considering the results of the SEM and BET, the delignification degree significantly affects the morphology and the porous properties of the lignocellulose aerogels, which accordingly affect the swelling properties. Due to the undissociated lignocellulose films or flocs in S-0 which was made from the undelignified raw stem L-0, there are many macropores inside that could absorb large amounts of water quickly. In contrast, during 0.5 h or 1 h of delignification, some of the lignin was removed from the lignocellulose. The cellulose and hemicellulose dissociated and became entangled to form a denser fibril network, which result in a smaller pore diameter and consequently lower SR. However, if the delignification was performed for 2 h, the
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Characterization of lignocellulose aerogels fabricated using a LiCl/DMSO solution ⁎
Zhulan Liu, Jinxiu Wu, Jianyu Xia, Hongqi Dai, Yunfeng Cao , Zhiguo Wang
T
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Contact information: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, 159 Longpan Rd, Nanjing, 210037, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soybean stem Ethylenediamine LiCl/DMSO Lignocellulose aerogel Delignification
Lignocellulose aerogel has attracted great attentions in advanced renewable material research. Mostly, cellulose, hemicellulose or lignin should be separated tediously from the lignocellulose first then to fabricate the aerogel individually. Herein, lignocellulose mesoporous aerogels were prepared directly from lignocellulose/LiCl/DMSO solution. The soybean stem with different range of delignification were pretreated by ethylenediamine (EDA), then dissolved in LiCl/DMSO solvent followed by coagulating, solvent replacing and freeze drying sequentially. Rheology and compression tests were employed to measure the mechanical strength of the alcogel. Scanning electron microscopy (SEM), Brunauer-emmett-teller (BET), Thermo-gravimetric (TGA) and the gravimetric analysis were taken to characterize the morphology, porosity, thermal stability and swelling ability of the aerogels. Comparing these samples, the lignocellulose aerogel without delignification had the greatest swelling ability but a little lower porosity, mechanical strength and thermal stability. And as the delignification increased in a proper extent, which induce the lignin removal, the cellulose and hemicellulose dissociated certainly, the mechanical strength and thermal stability improved, a denser fibril network structure with lower pore diameter (DPore) and a higher SBET formed. While, exacerbate delignification could reduce the SBET and the uniformity of the pores in the aerogels remarkably. The performance of the aerogels could be controlled to some extent, by adjusting the degree of the delignification.
1. Introduction Aerogels are ultra-light solid materials with high porosities prepared by freeze-drying or critical point drying wet gel precursors which also called hydrogel or alcogel. During this process, the background liquid solvent is replaced by gas without the substantial collapse of the solid gel network due to rapid water sublimation (Bryning et al., 2007; Tan et al., 2001; Zhuo et al., 2016). Recently, these fragile networks, which have extremely low densities, large micropore volumes, and high inner specific surface areas (Bag et al., 2007), have been found to exhibit excellent mechanical properties (Kim et al., 2012), magnetic induction (Olsson et al., 2010), insulating and optical properties (Zhao et al., 2009), surface interface effects, dielectric properties (Lin et al., 2005), and quantum size effects (Hu et al., 2013). It has met extensive application in catalyst supports, films, separation, chemical analysis, sensing, energy technologies and energy absorption. Kistler first reported the synthesis of aerogels by supercritical drying silica, alumina, nickel tartrate, stannic oxide, tungstic oxide, gelatin, agar, nitrocellulose, cellulose and egg albumin (Kistler, 1931). It can be categorized as inorganic, organic or composite based on the starting
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materials used in the synthesis (Mu et al., 2016). Recently, natural polymer compounds have been widely used in materials applications. Not only is the production waste recycled to reduce environmental pollution, but the natural polymers also exhibit excellent performances (Nguyen et al., 2016; Rahbar Shamskar et al., 2016; Rotbaum et al., 2017; Zhang et al., 2017; Zhuo et al., 2016). In addition, these polymers can be used to address many materials safety problems. In particular, natural plant polysaccharides such as cellulose, are attracting much interest for using as raw materials in the preparation of high-quality aerogels (Labudek and Martiník, 2011; Liu et al., 2017; Wang et al., 2017a; Li et al., 2018). Aerogel preparation from cellulose can generally be divided into three steps (Chen et al., 2016; Liebner et al., 2008; Sescousse et al., 2011): cellulose dissolution, coagulation in anti-solvent, and the transformation of wet gels into aerogels (Sescousse et al., 2011). Because of the existence of lignin as well as the tightly packed molecular structure and strong intramolecular and intermolecular hydrogen bonding, it is difficult to dissolve lignocellulose in conventional solvents. Thus, in most reports of the renewable aerogel materials, cellulose, hemicellulose or lignin were isolated from lignocellulose by tedious and costly chemical treatment first, and then used individually
Corresponding author. E-mail addresses: [email protected] (Y. Cao), [email protected] (Z. Wang).
https://doi.org/10.1016/j.indcrop.2019.01.057 Received 11 February 2018; Received in revised form 27 December 2018; Accepted 26 January 2019 0926-6690/ © 2019 Published by Elsevier B.V.
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2.2. Methods
in the synthesis, which also lead to environmental pollution. Therefore, it is more desirable to prepare aerogels directly from lignocellulose instead of cellulose or any other single component. To prepare lignocellulose aerogels, the raw material must be dissolved in a solution first (Chen et al., 2016). The first lignocellulose aerogels were prepared by dissolving spruce wood in IL 1-butyl-3-methylimidazolium chloride, followed by coagulating the gel in aqueous ethanol and drying it with supercritical carbon dioxide (Aaltonen and Jauhiainen, 2009). There are some solvent systems that have been employed to dissolve lignocellulose, such as dimethyl sulfoxide/tetrabutylammonium fluoride (DMSO/TBAF), a dimethyl sulfoxide/imidazole (DMSO/NMI) binary system and imidazolium-based ionic liquids (ILs) (Kilpelainen et al., 2007; Lu and Ralph, 2003). However, the first two solvent systems require ball-milling the lignocellulose for a long time, and the dissolution in the IL was only achieved at a high temperature (above 110 °C). Wang et al observed that wood could be effectively dissolved in a LiCl/DMSO solvent system (Liu et al., 2015, 2016a; Wang et al., 2009). Chen et al prepared lignocellulose aerogels by dissolving ball-milled bagasse in LiCl/DMSO at a high temperature (110 °C), cyclically freezing and thawing the gel and then regenerating it with water (Chen et al., 2016). All these studies employed ball-milling and heating processes, both of which cause lignocellulose degradation and negatively impact the gelation. Even in the work of Mussana, the lignocellulose could be dissolved in ILS/DMSO co-solvent system at 80 °C, the solution should be cyclically frozen and thaw to get the aerogel (Muussana et al., 2018). Moreover, there are quite few information about the effect of the lignin on the preparation and properties of the aerogels. In our preliminary studies, ethylenediamine (EDA)-pretreated cellulose or lignocellulose could be completely dissolved in LiCl/DMSO to give a homogenous solution under moderate conditions (75 °C) (Liu et al., 2015; Wang et al., 2012). Because this method does not require ball-milling or high temperatures, the structural modification of lignin and cellulose degradation can be avoided. Additionally, the aerogel can be synthesized from cellulose/LiCl/DMSO solution by coagulating without any freezing-thawing process (Wang et al., 2012). Therefore, this method has great potential for the facile preparation of lignocellulose aerogels under reasonable conditions. In this study, lignocellulose aerogels with high porosity and good swelling properties were fabricated by dissolving EDA-pretreated soybean stem meal in LiCl/DMSO and then subjecting it to sequential coagulation, solvent replacement and freeze-drying processes. The morphology, thermal stability and swelling properties were characterized in detail to determine the effects of the lignin on the preparation and characterization of these aerogels.
2.2.1. Lignocellulose pretreatment with EDA Dried MCC or stem meal (L-0, L-0.5, L-1 and L-2) (10 g) was soaked in 200 ml of EDA and stirred continuously for overnight at room temperature. Then, the material was collected by filtration and freezedrying. These samples are denoted “Stem-EDA complex” or “MCC-EDA complex”. The residual EDA contents in the complexes were determined by titration of approximately 0.5 g samples in water with 0.1 mol/l hydrochloric acid, using methyl orange as the indicator, and the results are listed in Table 1. 2.2.2. Lignocellulose dissolution The Stem-EDA or MCC-EDA complex was suspended in 8% LiCl/ DMSO to a concentration of 5%. The mixture was stirred continuously at room temperature for 24 h and then at 60 °C for 2 h to obtain a homogeneous solution. 2.2.3. Lignocellulose aerogel preparation The homogeneous lignocellulose solution was centrifuged to degas first, then poured into a glass dish to form a 1 mm-thick layer and subsequently immersed in ethanol for gelation to get the alcogel. To remove LiCl, DMSO and residual EDA, the obtained alcogel was thoroughly washed with ethanol until no Cl− in the supernatant was detected by the silver nitrate solution. Then, the corresponding aerogel was obtained by solvent exchange with tert-butanol (TBA), followed by freeze-drying the alcogel above. The aerogels made from MCC, L-0, L0.5, L-1 and L-2 are referred to as S-MCC, S-0, S-0.5, S-1 and S-2, respectively. The entire preparation procedure is shown in Fig. 1. 2.2.4. SEM The macroscopic appearances of the lignocellulose aerogels were observed visually. The aerogel samples were frozen in liquid nitrogen and immediately fractured, then sprayed with a thin gold film on the surface. Their 3D cellulosic networks were characterized by scanning electron microscopy (SEM) (Quanta 200, FEI, USA). 2.2.5. Nitrogen adsorption measurements Nitrogen adsorption measurements were performed using an ASAP 2020 instrument (USA). The Brunauer-Emmett-Teller (BET) surface areas and pore sizes were obtained by measuring the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature. The BET specific surface areas were calculated from the isotherms. Before the measurements, the aerogel samples without any collapse were degassed at 70 °C for 24 h. 2.2.6. Rheology measurements Rheological test of the lignocellulose gels was conducted using an RS6000 Rheometer (HAAKER, German). A P20 TiL Platte and a P20 TiL cone plate can be used. The specimens were subjected to a strain sweep test over the range of 0-103 Pa at ambient temperature with 1 Hz of frequency. Storage modulus (G’) and loss modulus (G’’) were recorded for the lignocellulose alcogel containing abundant ethanol after solvent replacement but before freeze-drying. The samples were measured triplicate.
2. Materials and methods 2.1. Materials All chemicals were reagent grade. Deionized water was used in all the experiments. The microcrystalline cellulose (MCC) sample was obtained from Guoyao Chemicals Group (China). Soybean straw was collected from north Jiangsu, China. Air-dried soybean stem was first manually fractionated from soybean straw and then ground through 40to 80-mesh screens using a General Electric Wiley mill. The ground sample was subsequently extracted with a benzene-ethanol (2:1, v/v) mixture for 8 h to remove the extractives. The extractive-free sample was vacuum-dried at 45 °C for 24 h and used as the native lignocellulose source. Stem samples were delignified by adding NaClO2 and acetic acid to the stem meal suspension for 0.5, 1 or 2 h. The final lignin and polysaccharides contents were determined by the way of NREL/TP-51042618 (Sluiter et al., 2011), as listed in Table 1.
2.2.7. Thermogravimetric analysis Thermogravimetric analysis (TGA) of the lignocellulose aerogels was performed on a TA Instruments Q500 thermogravimetric analyzer using approximately 5–10 mg samples. The experiments were conducted under nitrogen from 20 °C to 600 °C at a heating rate of 10 °C/ min. 2.2.8. Swelling properties A gravimetric method was employed to measure the swelling ratios of the aerogels in distilled water at different temperatures ranging from 294
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Table 1 Lignocellulose samples used to prepare the gels. Sample
Delignification time (h)
Lignin content (%)
Polysaccharide (%) Glucan
MCC L-0 L-0.5 L-1 L-2
– 0 0.5 1 2
– 23.24 21.33 18.91 14.24
± ± ± ±
0.03 0.11 0.05 0.09
– 37.39 44.12 45.46 48.68
± ± ± ±
Residual EDA content (%) Xylan
0.51 0.02 0.86 0.42
– 15.58 16.89 17.08 17.75
Others
± ± ± ±
0.75 0.55 0.26 0.2
– 3.79 4.01 4.50 4.62
± ± ± ±
Total
0.03 0.09 0.10 0.06
– 56.76 65.02 67.04 71.04
± ± ± ±
1.29 0.48 1.22 0.57
19.5 18.4 18.9 19.7 20.1
± ± ± ± ±
0.1 0.1 0.2 0.1 0.2
Note: Others in the polysaccharide include the Bhamnosan, Galactan and Araban. Total is the sum of Glucan, Xylan and Others.
staggered along the cellulose chain axis by half of a glucose ring. The fundamental reacting unit is a sheet of cellulose chains, but not individual cellulose chains (Loeb and Segal, 1955; Warwicker and Wright, 1967; Su et al., 1989; Wada et al., 2009). In addition, the lignin is an irregular polymer consisting of several monoaromatic monomers that form a 3D network, which makes lignocellulose essentially insoluble in common solvents. Therefore, finding an appropriate solvent is the major challenge in preparing lignocellulose gels. EDA is an efficient fiber-swelling agent. EDA solutions with concentrations greater than 60% can induce intramicellar swelling and lattice distention. When considerably higher EDA concentrations were used in previous studies, the [101] interplanar distance of the unit cell in cellulose I increased from 6.14 Å to 11.86 Å (Loeb and Segal, 1955). Consequently, the hydrogen bonds (C(6)eOH⋯OeC(3)) between the cellulose sheets are broken, and the hydroxyl groups (−OH) at C(6) and C(3) are exposed. As shown in Fig. 2, during EDA pretreatment, the abundant hydroxyl groups in cellulose accept hydrogen bonds from EDA amino groups to form a stable cellulose-EDA complex containing many hydrogen bonds (−OH⋯NH2−CH2−CH2−NH2⋯HO-), leading to the destruction of the hydrogen-bonding network between the microfibrils and other polysaccharides. EDA treatment is a simple method for enhancing the accessibility and chemical reactivity of crystalline cellulose. Then, in the subsequent dissolution procedure, LiCl and DMSO which are halogenated or polar, oxygen-containg nonaqueous solvents, compete with the amino groups to extract EDA from the cellulose, and generate new interactions in the lignocellulose (Sawada et al., 2013; Wada et al., 2008; Segal and Loeb, 1960). These interactions lead to the dissociation and dissolution of different lignocellulose
15 to 45 °C. The dried aerogel was immersed in distilled water at each temperature for a given amount of time. After removing the excess water on the surface with filter paper, the aerogel was weighed. The swelling ratio (SR) was defined as follows: SR = (Wt – Wd) × 100% / Wd
(1)
where Wt and Wd are the weights of the swollen aerogel at time t and the dry aerogel, respectively. The equilibrium swelling ratio (ESR) was defined to be the swelling ratio measured after the dried aerogel was immersed in distilled water for 24 h. 3. Results and discussion 3.1. Preparation and structure of the lignocellulose aerogels Lignocellulose or cellulose dissolution followed by coagulation in anti-solvent could result in the formation of swollen gels containing ambient liquid (Kistler, 1931). In this study, it was successful to form lignocellulose aerogel upon coagulating lignocellulose solution in antisolvent at 5% of lignocellulose concentration without freezing-thawing treatment, which is different from the aerogel preparation of Chen and Mussana (Chen et al., 2016; Muussana et al. (2018)). Lignocellulose are mainly constituted of cellulose, hemicellulose, and lignin in various proportions. According to the references, there are many intramolecular and intermolecular hydrogen bonds in cellulose. The cellulose chains form a sheet-like structure stabilized by intermolecular hydrogen bonds that are parallel to the pyranose rings. These sheets are
Fig. 1. Schematic representation of the lignocellulose aerogel preparation. 295
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Fig. 2. Mechanism of EDA pretreatment of cellulose.
in the solvent system but also has one or more active H atom to form a hydrogen bonding network with the lignocellulose solution (Loeb and Segal, 1955; Lv et al., 2011). Therefore, anhydrous ethanol, which is miscible in the LiCl/DMSO solution and also has a strong hydrogenbonding ability, was used as the anti-solvent for lignocellulose gelation in this study.
molecules in the LiCl/DMSO solution. A homogeneous lignocellulose solution in 8% LiCl/DMSO was obtained by lignocellulose dissolution into independent fiber chains. These chains were then coagulated by adding an anti-solvent which is soluble in DMSO and contains active H atoms that are likely to form hydrogen bond with the lignocellulose. Because the amount of the antisolvent added to the system is significantly larger than that of LiCl/ DMSO, the lignocellulose gels can be thoroughly washed. If the antisolvent does not have active H atoms (anti-solvent A), it only dilutes the lignocellulose/LiCl/DMSO solution and cannot form a 3D network structure with the lignocellulose. However, if the anti-solvent has active H atoms (anti-solvent B), its ability to form hydrogen bonds to the lignocellulose controls the gel formation. If anti-solvent B has a relatively weak hydrogen-bonding ability, many of the hydrogen bonds between and within the lignocellulose chains reform, resulting in lignocellulose precipitation from the solution system. In contrast, if antisolvent B exhibits a strong hydrogen-bonding ability, it dynamically competes with the lignocellulose chains to form hydrogen bond with other lignocellulose chains and solution. Consequently, the highly compact, complex entangled network of the lignocellulose gel shown in Fig. 3 can be produced by hydrogen bonding between the solution and lignocellulose, and between the lignocellulose chains. In conclusion, the anti-solvent used for lignocellulose gelation should not only be miscible
3.2. Morphology of the lignocellulose aerogels observed by SEM MCC and soybean stem with different lignin contents were pretreated by EDA (L-MCC, L-0, L-0.5, L-1 and L-2) and dissolved in 8% LiCl/DMSO to obtain homogenous solutions. After gelation, the resulted alcogel was thoroughly washed by immersion in ethanol, then transferred to TBA and subsequently freeze-dried to get the corresponding aerogels (S-MCC, S-0, S-0.5, S-1 and S-2). During this process, the shape and bulk network structure of the alcogels were preserved well. As shown in Fig. 4, the surfaces of the lignocellulose aerogels with different lignin contents are slightly collapsed. In contrast, the fractured cross sections reveal unique bulk cellulose network structures with many pores. The differences in the surface external and bulk internal structures are probably due to the variations in the gelation conditions at these two positions. During the gelling process, the liquid composition at the surface external changed dramatically, whereas it changed
Fig. 3. Schematic illustration of the nanostructural reorganization in the lignocellulose/LiCl/DMSO solution. (a) Lignocellulose/LiCl/DMSO suspension, (b) homogenous lignocellulose solution, (c) lignocellulose alcogel, (d) lignocellulose aerogel. 296
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Fig. 4. SEM images of the lignocellulose aerogels.
53.27 m2/g which is much smaller than that of S-MCC. These results might be attributed to the differences in the homogeneity and uniformity of the pore dimensions of these two aerogels. After 0.5 h or 1 h of delignification, the Dpore value decreased to nearly 16 nm, and the SBET value increased to 61.21 m2/g for S-0.5 and 69.14 m2/g for S-1, respectively. When the delignification time was increased to 2 h, the resulting lignocellulose aerogel S-2 has a larger Dpore value but a smaller SBET value than S-1, which is consistent with the morphological analysis. In the undelignified lignocellulose, the lignin, hemicellulose and cellulose are interconnected in a complex structure and cannot be dissociated into independent cellulose or hemicellulose chains to generate a network structure. Therefore, the complex bulk structure of S-0 exhibits a film-like or floccular morphology, and S-0 thus has a relatively large DPore value and small SBET value. After 0.5 h or 1 h of delignification, some of the lignin was removed, enabling the hemicellulose and cellulose molecular chains in S-0.5 and S-1 to unravel and connect to each other by hydrogen bonding to form 3D fibril networks with higher SBET and smaller DPore values. However, when the delignification time was 2 h, the lignin, hemicellulose and cellulose were separated to a greater degree, resulting in the formation of more and larger pores. Thus, the DPore value of the lignocellulose aerogel S-2 is larger, whereas the SBET value is smaller. All the SBET values in this study are much higher than those of lignocellulose aerogels (2–7.5 m2/ g) prepared by cyclically freezing and thawing a wood solution in an ionic liquid and then drying with supercritical carbon dioxide (Li et al., 2011; Lu et al., 2012), which is regarded as the most effective drying method for preparing homogenous cellulose aerogels (Hoepfner et al., 2008).
gradually in the bulk internal, and lead to slow coagulation and relaxation rearrangement which help to give the network structure (Wang et al., 2012). In addition, during freeze-drying, the liquid on the surface was sublimed much faster than that inside the gel, resulting in different surface and bulk structures. The images in Fig. 4 show that the bulk microstructure of the lignocellulose aerogels is significantly affected by the delignification degree. The lignocellulose aerogel S-0 made from undelignified soybean stem L-0 has a complex bulk structure that appears film-like or floccular instead of network-like, and micropores are distributed throughout the film. This result is attributed to the complex interwoven structure of the cellulose, hemicellulose and lignin in L-0, which prevents them from separating into independent cellulose or hemicellulose chains to generate a network structure. However, although S-0.5 made from L-0.5 still exhibits some film-like or floccular morphological features, the bulk lignocellulose aerogel has a network structure with more pores. When the delignification time is increased to 1 h, the internal of S-1 has a unique cellulose network structure, as shown by the fractured cross section. In addition, the pores in S-1, which originate from the interconnected long and fairly straight fibrils, have a considerably more uniform distribution than those of S-0 and S-0.5. It is thus concluded that as the delignification time increased, the lignocellulose dissociated into single fibril chains, then connected to each other by reforming the hydrogen bonds to form 3D networks with large interstitial spaces. When the delignification time was increased to 2 h, the resulting lignocellulose aerogel S-2 has a distinct fibril network structure; However, the pore distribution is not as uniform as those of S-0.5 and S-1.
3.3. BET porosity measurements of the lignocellulose aerogels 3.4. Mechanical strength of the lignocellulose gels The pore sizes and volumes of the series of aerogels were measured to furtherly characterize the mesoporous structures. Nitrogen adsorption experiments at liquid nitrogen temperature provide information on the structure of porous materials. The N2 adsorption-desorption isotherms of the lignocellulose aerogels are shown in Fig. 5. The adsorption isotherms exhibit the inverse-S shape of a type IV adsorption isotherm with the adsorption-desorption hysteresis loop, meaning the average pore diameter is larger than 10 nm. The BET and BJH methods were used to determine the pore size distribution. The results show that proper delignification leads to a decrease in the pore size and increase in the specific surface area (SBET) and the pore density of the lignocellulose aerogels. As shown in Table 2, the average pore diameter Dpore of the lignocellulose aerogel S-0 made from L-0 is 20.281 nm which is very close to that of S-MCC made from L-MCC. However, the SBET of S-0 is
The mechanical strength of the lignocellulose gels is important for their application. From the optical image in Fig. 6, the lignocellulose alcogel S-2 could bear the weight of 20 g without obvious deformation. For the viscoelastic properties, the storage modulus G’ represents stored energy in materials during elastic deformation, which reflects the elasticity property or the solid-like behaviour of the materials; the loss modulus G’’ represents the lost energy due to viscous deformation, which reflects viscosity property or the liquid-like behaviour (Mushi et al., 2016). In other words, storage modulus G’ exhibits the stiffness of materials (Liu et al., 2016b). Moreover, the cross point of G’ and G’’ is the critical stress value indicating that the gel can bear the shear stress before irreversible deformation. As shown in Fig. 6, the G’ values are much higher than G’’ for all alcogel over the whole stress range, implying elastic gel characteristics. It also can be observed that the 297
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Fig. 5. N2 adsorption-desorption isotherms of the lignocellulose aerogels. (a) S-0, (b) S-0.5, (c) S-1, (d) S-2, (e) S-MCC.
cross point of G’ and G’’ with the sequence of S-MCC > S-2 > S0.5 > S-0.
Table 2 Porous properties of the lignocellulose aerogels. Samples
Specific surface area (SBET) (m2/g)
Pore diameter (DPore) (nm)
S-0 S-0.5 S-1 S-2 S-MCC
53.27 61.21 69.14 64.29 61.04
20.281 16.709 16.301 17.813 20.199
± ± ± ± ±
1.70 2.01 2.13 1.89 2.43
± ± ± ± ±
3.5. Thermogravimetric analysis of the lignocellulose aerogels
0.854 0.048 0.031 0.10 0.52
The thermal stability of the lignocellulose aerogel was examined by thermogravimetry analysis (TGA) in a nitrogen environment. As shown in Fig. 7, the lignocellulose delignification affects the thermal stability of the lignocellulose aerogels. In all the TGA curves, small weight losses are observed below 100 °C, due to the apparent evaporation of adsorbed water from the lignocellulose aerogels. The onset decomposition temperatures of S-0 and S-0.5 which made from the raw stem (L-0) and the stem delignified for 0.5 h (L-0.5), respectively, are approximately 200 °C. While, the onset decomposition temperatures of S-1 and S-2 are increased to 225 °C around. The maximum decomposition temperature of S-0 and S-0.5 are almost 287 °C, but that of S-1 and S-2 are rise up to about 315 °C which is a little bit lower than that of MCC (328 °C). Additionally, the char yields of S-0 and S-0.5 at 600 °C are 28.66% and 23.10%, respectively. But for S-1 and S-2, their char yields are reduced to approximately 15%. All these results show that lignocellulose delignification improves the thermal stability and decreases the char yield of the lignocellulose aerogels. Combining the data in Table 1, in the aerogels S-0 and S-0.5 made from the raw stem L-0 and the slightly delignified stem L-0.5, the lignin and carbohydrate including cellulose
Fig. 6. Mechanical strength of the lignocellulose alcogels.
delignification of the stem could affect the mechanical strength of these gels. For S-0 which made from L-0 without any delignification, it has the lowest mechanical strength with about 110 Pa of G’ and 13 Pa of G’’. After 0.5 h of delignification, some of the lignin was removed, then the strength of S-0.5 is relatively improved to approximate 250 Pa of G’. If the delignification time was prolonged to 2 h, more lignin was removed but the cellulose and hemicellulose were remained well, which lead to the mechanical strength of the alcogel S-2 raised to about 1250 Pa obviously. However, the mechanical strength of these three lignocellulose gels are all much lower than that of S-MCC which made form MCC without any lignin. As a result, the existence of the lignin in the stem with slight or even without delignification would decrease the mechanical strength of the lignocellulose gels by weakening the interactions between cellulose or hemicellulose chains during the formation of the gels. It is also can be proved by the critical stress value at the
Fig. 7. TGA curves of the lignocellulose aerogels. 298
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lignin content decreased to 14.24%, but the total polyssacrides content increased obviously to 71.04% including 48.68% of glucan, 17.75% of xylan and 4.62% of others. These data proved that the lignin was furtherly removed during the 2 h of delignification, but the cellulose and hemicellulose were remained well and their abundant of hydrophilic groups such as –OH were exposed, which is benefit for improving the swelling property of the S-2. As a result, the SR of S-2 was certainly raised to 960% which was higher than that of S-1 but still lower than S0. The ESR of the lignocellulose aerogels at different temperatures are listed in Table 3, which also shows the relationship between the stem lignin content and ESR. S-0 clearly has the highest ESR, followed by S-2, S-MCC, S-0.5 and S-1 sequentially. These results are consistent with the SR results at 15 °C. Moreover, as the temperature raised, the hydrogen bonding between the aerogel and water weaken, and consequently the aerogel network structure shrinked. Thus, the ESR decreased. Additionally, it is concluded that the ESR decreases with the delignification degree increment in a proper range. However, excessive delignification results in a certain elevation of the ESR, which is attributed to the lignin removal and the exposure of the hydrophilic groups in cellulose and hemicellulose, but it is still lower than that of S-0 which made from L-0 without any delignification.
Fig. 8. Swelling ratios of the lignocellulose aerogels at 15 °C. Table 3 ESR of the lignocellulose aerogels at different temperatures. Aerogel samples ESR at different temperatures (%)
15 °C 20 °C 25°C 30°C 35°C 40°C 45 °C
S-0
S-0.5
S-1
S-2
S-MCC
2550.00 2286.57 2000.00 1870.73 1739.58 1543.24 1369.70
1157.27 1128.85 1127.78 1077.14 1028.33 926.42 677.27
935.71 876.92 771.43 680.37 666.67 645.79 551.10
2126.79 1547.67 1395.89 1290.70 1156.96 1121.43 966.67
1502.70 1624.24 1040.14 1144.29 990.36 989.52 867.41
4. Conclusions Highly porous lignocellulose aerogels were synthesized by dissolving EDA-pretreated soybean stem meal in LiCl/DMSO and sequentially coagulating, solvent replacing and freeze-drying. This series of lignocellulose aerogels exhibited great water adsorption and swelling properties. Appropriate delignification facilitates the formation of a continuous 3D fibril network with desired SBET and Dpore. As the delignification increased in a certain range, the lignin content of the raw soybean stem decreased and the polysaccharide content increased relatively, then resulted in the improvement of the mechanical strength of the alcogel and thermal stability of the aerogel, as well as the formation of the denser fibril network structure with lower Dpore and higher SBET which cause the decline of the swelling ability. But the excessive delignification for 2 h, had negative impact on the 3D network formation and the pore uniformity of the lignocellulose aerogels. However, because of the further lignin removal and the hydrophilic groups in the remained cellulose and hemicellulose exposure, the swelling ability of S-2 was some higher than that of S-1. But this data was still lower than that of S-0. It is possible to adjust the delignification to control the structure and swelling ability of the lignocellulose aerogel.
and hemicellulose, were well preserved, and the amorphous cellulose regions were also relatively unchanged, then result in their poor thermal stability. As the delignification time increased to 1 or 2 h, some of the lignin was removed and its content was notably decreased to 18.91% and 14.24%, respectively. While the relative contents of the cellulose and hemicellulose which could be presented by the polysaccharide relatively content, were increased greatly. All these alternations were help to improve the thermal stability. Besides, the amorphous cellulose regions were also somewhat degraded during delignification (Wang et al., 2017b; Muussana et al., 2018; Li et al., 2015). However, the highly ordered crystalline regions, in which the cellulose molecules are firmly bound together, were preserved well, leading to a significant improvement in the aerogel thermal stability. In addition, because the cellulose and hemicellulose have much lower C:H elemental ratio than the lignin, the aerogel char yield decreased with delignification time extending (Muussana et al., 2018; Li et al., 2015). This result is supported by the fact that S-MCC which does not have any lignin, has the lowest char yield.
Acknowledgements This work was supported by the financial support from the National Natural Science Foundation of China (No. 31600473, 31670591), the Natural Science Foundation of Jiangsu Province (BK20160928), and PAPD of Jiangsu Higher Education Institutions.
3.6. Swelling properties of the lignocellulose aerogels In Fig. 8, when the lignocellulose porous aerogels were immersed in the water at 15 °C, they quickly absorbed large amounts of water. Their weight increased continuously in the first 15 min and then remained unchanged at longer immersion times. After 15 min, the swelling ratio (SR) of S-0 reached to 1495% which is the highest than that of the others. For S-0.5 and S-1, the SR were reduced to 1003% and 590%, respectively. Considering the results of the SEM and BET, the delignification degree significantly affects the morphology and the porous properties of the lignocellulose aerogels, which accordingly affect the swelling properties. Due to the undissociated lignocellulose films or flocs in S-0 which was made from the undelignified raw stem L-0, there are many macropores inside that could absorb large amounts of water quickly. In contrast, during 0.5 h or 1 h of delignification, some of the lignin was removed from the lignocellulose. The cellulose and hemicellulose dissociated and became entangled to form a denser fibril network, which result in a smaller pore diameter and consequently lower SR. However, if the delignification was performed for 2 h, the
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