Controlled Assembly of Lignocellulosic Biomass Components and

Research Article pubs.acs.org/journal/ascecg

Controlled Assembly of Lignocellulosic Biomass Components and Properties of Reformed Materials Jing Wang,† Ramiz Boy,‡ Ngoc A. Nguyen,‡ Jong K. Keum,§,∥ David A. Cullen,‡ Jihua Chen,∥ Mikhael Soliman,⊥ Kenneth C. Littrell,§ David Harper,† Laurene Tetard,⊥ Timothy G. Rials,† Amit K. Naskar,*,†,‡ and Nicole Labbé*,† †

Center for Renewable Carbon, University of Tennessee, 2506 Jacob Drive, Knoxville, Tennessee 37996, United States Materials Science and Technology Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States § Chemical and Engineering Materials Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States ∥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, United States ⊥ Nanoscience Technology Center, University of Central Florida, 12424 Research Parkway, Orlando, Florida 32826, United States ‡

S Supporting Information *

ABSTRACT: Reforming whole lignocellulosic biomass into value-added materials has yet to be achieved mainly due to the infusible nature of biomass and its recalcitrance to dissolve in common organic solvents. Recently, the solubility of biomass in ionic liquids (ILs) has been explored to develop alllignocellulosic materials; however, efficient dissolution and therefore production of value-added materials with desired mechanical properties remain a challenge. This article presents an approach to producing high-performance lignocellulosic films from hybrid poplar wood. An autohydrolysis step that removes ≤50% of the hemicellulose fraction is performed to enhance biomass solvation in 1-ethyl-3-methyl imidazolium acetate ([C2mim][OAc]). The resulting biomass−IL solution is then cast into free-standing films using different coagulating solvents, yet preserving the polymeric nature of the biomass constituents. Methanol coagulated films exhibit a cocontinuous 3Dnetwork structure with dispersed domains of less than 100 nm. The consolidated films with controllable morphology and structural order demonstrate tensile properties better than those of quasi-isotropic wood. The methods for producing these biomass derivatives have potential for fabricating novel green materials with superior performance from woody and grassy biomass. KEYWORDS: Regenerated whole biomass, Ionic liquid, Nanocomposite, Small-angle X-ray scattering, High performance lignocellulosic materials

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INTRODUCTION

physical and/or chemical treatment is required, including acid and alkaline pretreatment or other conversion process, to efficiently harness and utilize biomass constituents.5−8 The regeneration of “whole” biomass to manufacture materials often requires functionalization of biomass components. For example, Xie et al. employed acetic anhydride, acetyl chloride, and benzoyl chloride to modify wood dissolved in an ionic liquid.9 Ionic liquids are organic salts composed of cations and anions with melting points lower than 100 °C. Compared to common organic solvents, most ILs have high thermal stability and low vapor pressures near room temperature and are less hazardous

Lignocellulosic biomass, composed mostly of cellulose, hemicellulose, and lignin, is widely investigated as a renewable resource to generate fuels, chemicals, and products such as biodegradable plastics, textile fibers, membrane sensors, and other sustainable materials.1−3 These biobased materials are carbon neutral, potentially environmentally friendly, and usually produced from a purified stream of a specific polymeric constituent of lignocellulosic biomass. However, to this day, the utilization of biomass constituents to manufacture bulk materials consistently, with performance equivalent to synthetic analogues, remains challenging.4 Lignocellulosic biomass is not meltable and moldable like regular thermal plastic polymers and is insoluble in most commonly used organic solvents. Therefore, direct processing of biomass is difficult, and a © 2017 American Chemical Society

Received: May 24, 2017 Revised: July 15, 2017 Published: July 25, 2017 8044

DOI: 10.1021/acssuschemeng.7b01639 ACS Sustainable Chem. Eng. 2017, 5, 8044−8052

Research Article

ACS Sustainable Chemistry & Engineering and recyclable solvents.10 Hence, processes involving ILs are considered promising technology platforms for producing biochemicals and biomaterials.11 In 2002, Rogers and coworkers demonstrated that imidazolium-based ILs can dissolve cellulose by disrupting its hydrogen bonding network.12 Lindman et al. reported that cellulose is actually amphiphilic in nature, and a solvent that has both polar and nonpolar part can dissolve cellulose.13 Organic ionic liquids have both polar and nonpolar functional groups and thus help to dissolve lignocellulosic biomass.14 Since then, numerous studies have reported on biomass dissolution in various ILs above 100 °C. IL has been proposed to disrupt cellulose structures.15 When the temperature was higher than the glass transition temperature of lignin, the dissolution process was usually reported as more efficient.16 At present, biopolymer-based fibers and films have been produced from the blending of isolated cellulose, xylan, and lignin−IL solutions.17−19 However, only a few studies have explored the direct transformation of biomass−IL solutions into lignocellulosic biopolymer-based materials. These materials could be processed as fibers and films for textile and membrane sensor applications.20 In 2007, Fort et al. attempted to cast membranes directly from a wood/1-n-butyl3-methylimidazolium chloride ([C4mim]Cl) solution and concluded that hemicellulose and/or lignin were responsible for inhibiting the formation of hydrogels.21 Through an IL treatment at 110 °C for 16 h, Sun et al. were able to produce fibers from a bagasse−IL solution but not from a pine−IL solution unless the dissolution temperature was raised to higher temperature (175 °C) for a short time (30 min) to minimize polymer degradation.22 More recently, Abdulkhani et al. prepared composite films from a solution of ball-milled poplar wood in [C4mim]Cl cast in a Petri dish with water as the coagulant.23 However, this approach did not use most of the hemicellulose and lignin in the biomass and therefore could not directly convert whole biomass into a new value-added material. The authors concluded that lignin and hemicellulose acted as impurities which resulted in a material that was neither very strong nor tough ( water. Thus, in a water coagulation bath, both lignin and cellulose precipitate rapidly from the IL/biomass solution. Precipitation of cellulose and lignin is relatively slow but nearly simultaneous in DMAc/water mixture where IL, lignin, and cellulose have nearly similar interaction radii. In methanol, amorphous cellulosics precipitate first with very slow deposition of lignin from the solvent system while some lignin remains soluble in the coagulation bath. Thus, the fabricated IL/biomass film in methanol bath results in a low hydrophilic surface. The following discussion elucidates this hypothesis. The UV−vis data (Figure S2, Supporting Information) collected on the regenerated films corroborate the chemical composition data. The color of the films, varying from light to dark brown, and the UV−vis spectra support that each film has different light transmittance values in the 200−700 nm wavelength range. BF3 films absorb radiation close to 100% from UV to ∼450 nm, whereas BF2 absorbs only in UV up to ∼300 nm. Strong light absorbance is directly related to lignin content in the films; UV absorbance is proportional to the lignin fraction.36 The UV absorbance shift from BF2 to BF1 and BF3 is consistent with the increase in lignin content from

Table 1. Chemical Composition of Regenerated Biomass Filmsa

autohydrolyzed hybrid poplar BF1 (film from methanol bath) BF2 (film from DMAc/water bath) BF3 (film from water bath)

component

a

Chemical composition was calculated based on a dry basis of the sample. Carbohydrates are the sum of cellulose and hemicellulose. C/L is the ratio of carbohydrates over lignin content.

the highest lignin content (28.4%), and consequently the lowest carbohydrates to lignin (C/L) ratio among the three films. These variations in chemical composition could be explained as a direct consequence of differences in solubility of the biomass polymeric components in the coagulation baths. Moreover, carbohydrates hydrolysis could occur during the dissolution step, leading to monosaccharides, oligosaccharides, 8047


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ACS Sustainable Chemistry & Engineering

Figure 2. (a) X-ray diffraction patterns, (b) small angle neutron scattering, and (c) small-angle X-ray scattering of the methanol (BF1), DMAc/water (BF2), and water (BF3) coagulated biomass films.

higher for BF1, suggesting a lower fraction of (11̅0) cellulose crystals planes aligned in the same plane with the film surface. Moreover, both peaks slightly shift to lower angles (red dashed arrows), indicating an increase in d-spacing, the distance between the order planes. The presence of the very broad and shallow diffraction peak (Figure 2a) near 35−37° in the starting hemicellulose extracted sample (AHP) and the DMAc/waterand water-based films (BF3 and BF2) originates from the organization of cellulose chains into microfibers. Interestingly, this peak does not appear in the methanol-based films (BF1) where coagulation occurs very slowly. These differences in cellulose chain orientation could be explained by the coagulation process and the interaction between solvent systems and biomass components discussed earlier with the Hansen solubility parameters. The highest binary solubility radius values, Rij, of water−lignin and water− amorphous cellulose systems (Table 2) indicate that water is a poor solvent for both lignin and amorphous cellulose (which exists mainly in amorphous state in IL solution). Therefore, these two components rapidly precipitated in the water coagulation bath, resulting in strong interactions with hydroxyl groups of the glucopyranose ring and inducing a hydrophilic surface on the obtained film. The hydrophilicity of these films was verified by strong adhesion forces between the PFM− cantilever tip and the film surfaces (Figure S3, Supporting Information). The water films exhibit the highest measured force, corroborating the Hansen solubility data and the above discussion. The PFM data suggest that the surface of the BF2 and BF3 films coagulated in water and DMAc/water is hydrophilic. We anticipate that the high polarity of water and DMAc/water solvents (Table 2) promotes strong hydrogen bonding interactions with hydroxyl groups of cellulose molecules at their interfaces. It was revealed that the hydroxyl

11.2 to 18.4 and 28.4%, respectively (Table 1). On the other hand, cellulose has a very wide transmittance range, from ∼200 to 800 nm.37 The increasing trend of light transmittance from BF3 to BF1 and BF2 is due to cellulose content increasing from 58.5 to 68.8 and 82.5%, respectively. The transparency of BF1 and BF2 in the visible suggests potential use of these films for applications such as coatings and food packaging, while the absorbance of lignin-rich materials such as in the water coagulated films (BF3) could be of interest as UV protective materials or chemicals.38 Morphologies of the Biomass Films. The exposure of hybrid poplar biomass to [C2mim][OAc] disrupts cellulose crystallinity and modifies the biomass structure and morphology, leading to complete dissolution. After coagulation in a solvent bath, the cellulose component regains some of its molecular alignment, and the different cellulose crystal structure directs unique film morphologies. The rates of IL− coagulating solvent exchange and the solvent polarity significantly contribute to this structural formation. X-ray diffraction (XRD) was used to determine the effect of coagulating solvent properties on the crystal structure formation and cellulose alignment directions of our regenerated films. The XRD diffraction pattern of the starting hybrid poplar material (AHP) reveals three distinct peaks at around 35, 22, and 16°. The regenerated films display two clear peaks at 20 and 12° that correspond to the order in planes (110) and (11̅0), respectively (Figure 2a). The relative position of these diffraction peaks is indicative of the conversion of cellulose crystal type I to cellulose crystal type II after dissolution and regeneration.39 For both BF2 and BF3, the (110̅ ) reflection is more intense than the (110) reflection, suggesting that the (11̅0) crystal planes are predominantly parallel to the surface of these two films.40 In contrast, the intensity of the (110) is 8048


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ACS Sustainable Chemistry & Engineering groups of cellulose crystalline molecules are organized on the surface of (11̅0) crystal planes.40,41 Therefore, the directional interactions of hydrogen bonds between the solvents and hydroxyl groups of cellulose molecules induced the alignment of the (11̅0) crystal planes parallel to the water and DMAc/ water film surfaces. This argument was demonstrated by the measured XRD data discussed in the previous section (Figure 2a). On the other hand, methanol, with the highest solubility of lignin (Rij = 11.8 MPa0.5), should cause a less hydrophilic surface in the as-prepared films. The PFM data (Figure S3, Supporting Information) confirm this, in which the normalized adhesion force measured from the BF1 (methanol) regenerated film is much lower (from 3 to 4 times) than the force obtained from the BF2 and BF3 films. We anticipate that the high population of hydrophilic surfaces containing both amorphous cellulose and (11̅0) cellulose crystals from BF3 and BF2 promotes moisture absorption into the films, causing a decrease in those films’ mechanical properties, as discussed in the next section. To better understand the effect of solvents on the new materials formed, the morphology of the films at different scattering vector (Q) ranges was investigated by SANS and SAXS. BF3 exhibits a shoulder in the SANS curve within the Qrange of 0.001−0.01 Å−1, whereas BF1 and BF2 show powerlaw behavior in the measured Q-range without any shoulder or peak (Figure 2b). The clearly discernible scattering shoulder of BF3 indicates the presence of nanoscale domains (lignin) demonstrated by the differences in neutron scattering density from the matrix. The SANS curves of BF1 and BF2 show power-law behaviors with a fractal dimension D ∼ 3.2, indicative of a 3D-network structure. This 3D-network structure may be formed through the percolation of lignin aggregates and cellulose fibrils. The disappearance of scattering peaks could also imply that either the films possess no nanoscale domain or, if they exist, the nanoscale domains have no neutron scattering density contrast with the surrounding matrix. The average radius of gyration, ⟨Rg⟩, of the lignin domains in BF3 obtained from Guinier−Porod model fit42 to the SANS curve is ∼46 nm. As seen in Figure 2c, the SAXS curve of BF1 has a discernible scattering shoulder in the Qrange of 0.01−0.1 Å-1. The Guinier−Porod model fit to the SAXS curve of BF1 reveals the existence of nanoscale domains D with D being with ⟨Rg⟩ = ∼10 nm. Because R g =

Figure 3. AFM topography (a−f) and TEM images (g−i) of air-dried biomass films coagulated in methanol (BF1) (a, d, and g), DMAc/ water (BF2) (b, e, and h), and water (BF3) (c, f, and i).

BF1 (Figure 3g), although the lignin interpenetrated into the matrix and formed a cocontinuous 3D-network with the cellulose fibril crystals as revealed by the SANS data. BF2, with the lowest lignin content, does not exhibit such domains, but a two-phase morphology is observed (Figure 3h). The domains (dark regions) possess very large features over 300 nm that are beyond SANS and SAXS detection limits. Overall, these AFM and TEM observations are consistent with the neutron and Xray scattering results. Mechanical Properties of Biomass Films. As shown in Figure 4a, the dynamic mechanical storage moduli of all the films slightly decrease with increasing temperature due to film softening as the glass transition is approached. BF1 has the highest storage modulus (E′) from 25 to 250 °C, while at low temperatures BF3 has relatively higher storage modulus than BF2. Above 80 °C, BF2 shows higher E′ than BF3, reflecting the reinforcement effect of higher cellulose content in the films. Overall, the IL regenerated biomass films are very rigid, and the storage moduli drop is less than 1 order of magnitude in the investigated temperature range. At 250 °C, the E′ values remain above 1 GPa. BF2 and BF3 exhibit an obvious transition at about 175 °C (Figure 4b), corresponding to the Tg of lignin. BF1 shows a transition at approximately 185 °C. The high Tg of BF1 is possibly due to the cocontinuity of the lignin phase percolated with the cellulose crystals and the elimination of low molecular weight lignin fractions during film regeneration in methanol. Strong interactions between lignin phase and cellulose can inhibit macromolecular chain movements, causing an increase in Tg. Tensile stress−strain analysis of the films, presented in Figure 5 and Table 3, highlights significant differences in the mechanical properties of our biobased materials. Of the three films, BF1 has the highest Young’s modulus (5.9 GPa) and tensile strength (70 MPa), indicating that these films possess a more optimal C/L ratio, and a more homogeneous morphology. Compared to that of biopolymer films reported in the literature, BF1 exhibits a significantly higher modulus and tensile strength.23,38 For example, Chen et al. prepared biomass films with 2.3 GPa Young’s modulus and 38 MPa strength.38

2 3/5

diameter,42 the diameters of lignin domains of BF1 and BF3 are ca. 26 and 120 nm, respectively. The SANS and SAXS curves of BF2, exhibiting power-law behavior, may indicate the absence of nanoscale domains. The combined SANS and SAXS analyses are consistent with the TEM results (Figure 3). AFM was conducted to characterize the texture of the film surface (Figures 3a−f) and to expand on the information provided by neutron and X-ray scattering. The comparison of the surface morphology below 3 μm scans (Figure 3a−c) reveals finer globular structures in BF1 and BF2 at the smaller scale, although the shape of the features is significantly different (Figures 3d and e). BF2 presents the roughest surface of the three (Rq ∼ 22 nm measured on Figure 3e). The fast dissolution of IL into BF2 (DMAc/water), Rij = 15.4 MPa0.5, significantly contributed to the formed surface. A representative TEM image of BF3 shows distinct darker features that we attribute to the phase separation of aggregated lignin domains (50−100 nm) dispersed in the carbohydrates matrix (Figure 3i). Similar domains with much smaller sizes are observed for 8049


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Figure 4. DMA data for biomass films coagulated in methanol (BF1), DMAc/water (BF2), and water (BF3): (a) log storage modulus as a function of temperature and (b) tan δ as a function of temperature.

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CONCLUSIONS An IL system that used hybrid poplar wood as the raw material was investigated as a mean to directly regenerate biomass films with high mechanical performance. Biomass films free of defects and wrinkles were generated from the autohydrolyzed hybrid poplar/[C2mim][OAc] solution by use of methanol, DMAc/water, or water as coagulant bath. The physicochemical properties of the regenerated films were found to be dependent on the coagulant characteristics, primarily defined by their solubility parameters. Methanol-coagulated biomass films were found to have a structure more homogeneous than those coagulated in DMAc/water and water, mostly due to the slower diffusion of the IL during coagulation. DMAc/water-coagulated films had the least lignin content, and water-coagulated films had the largest discrete lignin domains (∼120 nm) as measured by scattering tools. In addition, methanol-coagulated films possessed the ideal ratio of carbohydrates to lignin and a cocontinuous network, giving them the best mechanical properties. Our approach has potential for the regeneration of whole biomass materials which would contribute to the green manufacturing of engineered bioproducts with improved carbon sequestration and ecological benefits.

Figure 5. Representative stress−strain curves of biomass films coagulated in methanol (BF1), DMAc/water (BF2), and water (BF3).

Table 3. Tensile Testing Results of Biomass Films Coagulated in Methanol (BF1), DMAc/Water (BF2), and Water (BF3)

a

mini

thickness (mm)

Young’s modulus (GPa)

BF1 BF2 BF3

0.025 (0.001)a 0.029 (0.002) 0.033 (0.002)

5.9 (0.3) 3.9 (0.7) 4.5 (0.9)

limitation at break (%)

tensile strength (MPa)

1.6 (0.4) 1.4 (0.3) 0.6 (0.1)

70 (5) 37 (6) 22 (7)

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Standard deviation calculated from six individual measurements.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01639. Storage modulus (G′) and loss modulus (G″) of hybrid poplar/[C2mim][OAc] solution, UV−vis transmittance of biomass films, PFM curves illustrating the hydrophobicity of biomass films, SEM images of cross sections of biomass films, and thermogravimetric data of biomass films (PDF)

Interestingly, our regenerated biomass films are stronger than their corresponding wood counterparts. For example, an average parallel-to-grain tensile strength of 51 MPa was reported for a typical poplar wood,43 whereas the modulus of elasticity of hybrid poplar wood varies from 3.0 GPa (perpendicular to grain) to 7.0 GPa (parallel to grain).44 Stress−strain curves showed that BF1 and BF2 films have elongation greater than that of BF3 (Table 3 and Figure 5). The lower tensile strength of BF2 and BF3 is likely due to the heterogeneity of the network structure, which has been demonstrated to negatively affect tensile strength.45,46 In addition, the greater elongation of BF1 and BF2 is likely due to their high cellulose content, 68.8 and 82.5%, respectively, because the high molecular weight of this polymer constituent has been reported to substantially improve the plasticity of films.23 Scanning electron microscopy images of cross sections of dry biomass films are shown in Figure S4 (Supporting Information).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Phone: 865-946-1126. *E-mail: [email protected]. ORCID

David Harper: 0000-0003-2783-5406 Nicole Labbé: 0000-0002-2117-4259 Notes

The authors declare no competing financial interest. 8050


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ACKNOWLEDGMENTS This work is dedicated to the friendship and memory of Dr. Luc Moens, an ionic liquid expert who made this research paper possible through his help and support over the years. This project was funded by the Southeastern Sun Grant Initiative and the University of Tennessee Office of Research. R.B., N.A.N, and A.K.N. acknowledge support from BioEnergy Technologies Office of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. TEM (J.C., D.A.C., and R.B.), SAXS (J.K.K.), and SANS (K.C.L. and J.K.K.) experiments were conducted at the Center for Nanophase Materials Sciences (CNMS) and GP-SANS beamline, High Flux Isotope Reactor (HFIR); both are a DOE Office of Science User Facility. J.W. and R.B. contributed equally to this work.

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