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Wood Nanotechnology for Strong, Mesoporous, and Hydrophobic Biocomposites for Selective Separation of Oil/Water Mixtures Qiliang Fu, Farhan Ansari, Qi Zhou, and Lars A. Berglund ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00005 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Wood Nanotechnology for Strong, Mesoporous, and Hydrophobic Biocomposites for Selective Separation of Oil/Water Mixtures Qiliang Fu†, Farhan Ansari†, Qi Zhou*,†,‡, and Lars A. Berglund† †

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH

Royal Institute of Technology, SE-10044 Stockholm, Sweden ‡

School of Biotechnology, Royal Institute of Technology (KTH), AlbaNova University

Centre, 106 91 Stockholm, Sweden

KEYWORDS: delignification, wood modification, hierarchical, wetting, composite, mechanical

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ABSTRACT

Tremendous efforts have been dedicated to developing effective and eco-friendly approaches for separation of oil–water mixtures. Challenges remain in terms of complex processing, high material cost, low efficiency, and scale-up problems. Inspired by the tubular porosity and hierarchical organization of wood, a strong, mesoporous, and hydrophobic three-dimensional wood structure is created for selective oil/water separation. A delignified wood template with hydrophilic characteristics is obtained by removal of lignin. The delignified wood template is further functionalized by a reactive epoxy-amine system. This wood/epoxy biocomposite reveals hydrophobic/oleophilic functionality and shows oil absorption as high as 15 g per gram of wood/epoxy biocomposite. The wood/epoxy biocomposite has a compression yield strength and modulus up to 18 MPa and 263 MPa respectively at a solid volume fraction of only 12%. This is more than 20 times that of cellulose-based foams/aerogels reconstructed from cellulose nanofibrils. The favorable performance is ascribed to the natural hierarchical honeycomb structure of wood. Oil can be selectively absorbed not only from below but also from above the water surface. High oil/water absorption capacity of both types of wood structures (delignified template and polymer-modified biocomposite) allows for applications in oil/water separation.


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Templating is a common strategy in materials engineering to fabricate materials with welldesigned structure from macrometer to nanometer length scale.1 Porous templates have been artificially synthesized by using, for example, copolymers,2,3 emulsions,4 crystals,5,6 surfactant,7 colloids8,9 and supramolecular structures.10 These templates are used to produce functional materials for applications such as drug delivery, catalysis, photonics and electronic devices.11–14 Another approach is to use natural materials directly as templates, such as silica,15 zeolite,16 hair,17 pollen,18 spider silk,19 plant fibers20 and wood.21,22 Compared to the traditional approaches of materials design, biotemplating is a rapidly growing field, which uses the hierarchical structure of natural materials for tailoring of specific functions.23 Among the biotemplates, wood is abundant, low-cost, readily processed, environmentally benign, and can be conveniently engineered. Wood, as a hygroscopic and anisotropic template, inspires scientists and engineers to design porous materials utilizing its special structure and mechanical properties.24–26 It is a biological material with honeycomb-like cells, high stiffness/strength to weight ratio and high value for hierarchical order. In particular, it consists of parallel hollow tubes in which wood cell walls are layered with nanocomposite layers consisting of oriented cellulose microfibrils in a hydrated matrix of lignin and hemicellulose. By using native wood directly as a template, hybrid materials with anisotropic magnetic and fire-retardant properties have been developed by embedding magnetic particles or inserting calcium carbonate into wood cell walls27,28 or into lumen space29, respectively. The dimensional stability of native wood structures to moisture, and its durability against fungal decay can be improved by grafting polymers to the wood cell walls30,31 or by impregnation with polymers and/or inorganic nanoparticles.32,33 To increase the porosity of wood templates and facilitate the permeation of chemical moieties into the wood cell walls, matrix substances such as lignin and hemicellulose can be partially removed, generating 3 ACS Paragon Plus Environment

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nanoporosity in the wood cell walls.34 After this pretreatment step, resin impregnated wood based materials with high strength have been produced.35,36 In our recent work, transparent wood with 85% transmittance and 71% haze has been prepared through the impregnation of delignified wood templates with monomers and subsequent polymerization inside the cell wall and lumen pore space.37 As compared to conventional wood modification, wood based functional materials from nanoporous and bleached wood templates can be engineered for device applications, photonics, sensors, and many other application fields yet to be explored. The development of effective and eco-friendly approaches for separation of oil–water mixtures is important for treatment of industrial oil-containing waste water as well as for handling of marine oil spill accidents.38 Three-dimensional (3D) cellulosic materials, including aerogels and electrospun nanofibrous materials, have low cost, high oil/water separation efficiency, can be reused, and are promising for purification of oil-contaminated water.39–42 However, the preparation of cellulose aerogels involves a large number of processing steps including mechanical disintegration, chemical modification, sol-gel transformation, and careful drying. Surface modification is necessary to obtain superhydrophobic or underwater superoleophobic properties.41–49 Energy consuming methods such as freeze drying and freeze casting are often used in order to generate porous structures.50,51 Herein, we report the preparation of strong, mesoporous, and hydrophobic threedimensional wood structures for selective oil/water separation using native wood as the template. As shown in Figure 1, the highly porous delignified wood template with high hydrophilic/oleophobic performance is obtained by removal of lignin. In a second step, the delignified wood template is impregnated with bisphenol A diglycidyl ether (DGEBA) and jeffamine D-400 polyetheramine (PEA) in acetone. After curing, delignified wood/epoxy composites with hydrophobicity/oleophilicity and high oil absorption capacity (around 15 g/g) 4 ACS Paragon Plus Environment

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is successfully prepared. The chemical structure, morphology, mechanical properties, and selective wettability of the delignified wood template and delignified wood/epoxy biocomposite are characterized.

Figure 1. Schematic illustration of structural design of porous and functional wood materials for selective separation of oil-water mixtures. (a) native balsa wood, (b) delignified wood template, (c) delignified wood/epoxy biocomposite.

RESULTS AND DISCUSSION

Balsa wood (Ochroma Pyramidale) was used as the raw material. The macroscopic shape and microscopic morphology of balsa wood block is presented in Figure 2a. The native wood was yellowish before the extraction of lignin. Lignin was extracted by using 1 wt.% of sodium chlorite (NaClO2) in sodium acetate buffer solution at pH 4.6 and 80 °C. As shown in Figure 2b, the dry state delignified wood sample appears ivory white, suggesting that lignin with its chromophores was removed. The macroscopic shape and the microscopic honeycomb-like cell wall architecture of native wood were preserved after chemical extraction. At submicronscale, microscale pores were observed in the middle lamella and at cell wall corners due to the delignification (yellow dash in Figure 2b). In addition, the microstructure of balsa wood contains lumen channels and numerous pits in the fiber walls (Figure S1b, Supporting 5 ACS Paragon Plus Environment

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Information). After lignin removal, the pit membranes were open, and the coarse lumen channels at the scale of tens of micrometers were observed (Figure S1c, Supporting Information). The removal of lignin was also verified by the FTIR analysis. The absorbance peaks at 1520 and 1600 cm-1 in the native wood were ascribed to the aromatic skeletal vibrations and C=O stretching from lignin, respectively. None of the peaks remained in the delignified wood template (Figure S2, Supporting Information). The delignified wood template was then impregnated with 5 wt.% epoxy/amine/acetone solution, cured, and purified according to the method reported in our previous work.52 After modification, the microscale pores in the middle lamella and cell wall corners were infiltrated with epoxy (yellow dash in Figure 2c). In addition, the pit membranes were blocked and covered with a thin layer of epoxy, resulting in a smooth surface of lumen cell walls (Figure S1d, Supporting Information), due to the evaporation of acetone and formation of a thin epoxy coating of the inner surface of the lumen. The delignified wood/epoxy biocomposite was yellow due to infiltration of DGEBA-based epoxy in the microstructure.

Table 1. Composition, Brunauer–Emmet–Teller (BET) specific surface area (SSA), and porosity of balsa wood, delignified wood template, and delignified wood/epoxy biocomposite.

Samples

Lignina (%) 24.9 ± 1.4 2.9 ± 0.3

Hemicelluloseb (%) 24.4 ± 1.1 16.4 ± 1.4

Celluloseb (%) 50.7 ± 2.6 80.7 ± 2.0

Epoxy resin (%) -

BET SSA (m2 g-1) 1.3 ± 0.2 21.0 ± 1.1

Porosityc (%) 88.9 ± 1.6 93.3 ± 1.0

Balsa wood Delignified wood Delignified 2.3 ± 0.2 13.1 ± 1.2 64.3 ± 1.5 20.3 ± 3.0 1.4 ± 0.2 88.3 ± 1.7 wood/epoxy a Lignin content was acid-insoluble lignin in wood determined according to TAPPI T222 om-83. b Hemicellulose and cellulose content in wood was determined by sugar analysis. c Porosity was determined from density of wood sample using equation (1), Supporting Information.


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Figure 2. The hierarchical structure demonstrated by photographs of the wood samples (10×15×15 mm3) and cross-sectional field emission scanning electron microscopy (FE-SEM) images of the cell walls for (a) balsa wood, (b) delignified wood template, and (c) delignified wood/epoxy biocomposite. (d) Pore volume distributions and (e) N2 adsorption/desorption isotherms of balsa wood, delignified wood template and delignified wood/epoxy biocomposite. 7 ACS Paragon Plus Environment

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High magnification images reveal the nanoscale structure of the cell wall (Figure 2). As long as lignin is embedded in the cell walls, the S2 layer of balsa wood appears nonporous (Figure 2a). After partial removal of lignin and hemicellulose, nanoscale pores (white arrows in Figure 2b) were observed in the S2 layer, leading to a ca. 5% increase in porosity. The total porosity of the delignified wood specimen is as high as 93.3% (Table 1). The lignin content decreased from 24.9% for native balsa wood to less than 2.9% for the delignified wood template. There was an 8% reduction of hemicellulose after delignification. The relative cellulose content increased from 51 to 81% for delignified wood template (Table1). The skeleton of the wood cell wall was well preserved, suggesting successful delignification without defibrillation of cellulose microfibrils. The pore size and fibril diameter of the delignified wood template were affected by the drying process. As shown in the ultrahigh resolution SEM images of delignified wood template (Figure S3, Supporting Information), nanofibril bundles in the fiber walls were exposed after freeze drying, whereas much smaller and more uniform pores and discrete cellulose microfibrils were observed after super-critical drying. This is because the liquid carbon dioxide undergoes gas-phase transition without surface tension effects during the depressurization process. The cell wall of delignified wood/epoxy biocomposite exhibited a nonporous cell wall structure (Figure 2c) due to the infiltration of epoxy in the nanopores in the cell walls. This is also in line with the similar porosity (ca. 88%) measured for native balsa wood and delignified wood/epoxy biocomposite. The epoxy weight content in the delignified wood/epoxy biocomposite was 20.3% (Table 1). Note that the volume fraction of solid is only 11.7%. The pore size distribution of the wood structures were examined by Brunauer–Emmet– Teller (BET) measurements through physisorption of nitrogen (N2). A maximum specific surface area (SSA) value of 21 m2 g-1 was obtained after the removal of lignin, whereas SSA values of 1.3 m2 g-1 and 1.4 m2 g-1 were measured for the native balsa wood and the 8 ACS Paragon Plus Environment

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delignified wood/epoxy biocomposite, respectively. Estimated nanoscale pore volume distributions versus pore sizes in the range of 2–90 nm for the wood structures are compared in Figure 2d. Interestingly, nanoscale pore size in the range of 2–20 nm is the largest pore volume fraction for the delignified wood template. Data are estimated from nitrogen desorption isotherms following the Barrett-Joyner-Halenda (BJH) calculation model. This is similar to data previously reported for delignified spruce wood, which possess pores with sizes in the range of 2–14 nm.53,54 The N2 adsorption/desorption capabilities of delignified wood template is much higher than those for balsa and delignified wood/epoxy biocomposites (Figure 2e), due to the high SSA of nanoscale pores in the cell walls. Similar trends of pore volume distribution and N2 absorption/desorption isotherms are observed for balsa and delignified wood/epoxy biocomposites. Based on the BET and SEM results, a cellulose-based template with hierarchical pores and high SSA was obtained by lignin removal. Epoxy was then successfully infiltrated in the cell wall of the delignified wood template.

Table 2. Mechanical properties of native balsa wood, delignified wood template and delignified wood/epoxy biocomposite at low (50%) and high (100%) relative humidity (RH).

Sample

Balsa wood Delignified wood Delignified wood/epoxy

Density [kg m-3]

25 °C, 50% RH E [MPa] σy [MPa]

25 °C, 100% RH E [MPa] σy [MPa]

174 ± 17 123 ± 16 196 ± 7

216.6 ± 28.0 184.9 ± 12.0 263.0 ± 13.0

78.0 ± 7.0 76.0 ± 4.0 160.5 ± 15.0

17.1 ± 0.7 13.2 ± 1.4 18.1 ± 1.0


6.3 ± 0.3 4.9 ± 0.7 9.9 ± 1.0

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Figure 3. Compressive stress-strain curves of native balsa wood, delignified wood template, delignified wood/epoxy biocomposite at (a) 50% and (b) 100% relative humidity (RH). (c) Young’s modulus and (d) yield strength as function of density for the wood structures prepared in this work as compared to cellulose-based foams or aerogels from literature 58-70. The compressive stress–strain curves of the wood structures in longitudinal direction at 50% and 100% relative humidity (RH) are presented in Figure 3a and 3b, respectively. The compressive mechanical properties are summarized in Table 2. At RH of 50%, all wood structures showed initial linear elastic deformation at low strain, followed by yielding and non-linear plastic response between 10–15% of strain. Subsequently, the cell walls collapsed 10 ACS Paragon Plus Environment

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in the plateau region between 15–80% of strain, followed by cell wall densification between 80–95% of strain as a result of the elimination of air voids. This is in agreement with the mechanical behavior of native balsa wood with a density of 265 kg m–3 in axial direction reported by Borrega and Gibson.55 It is worth noting that the stress-strain curves in compression of wood specimens with a higher density in axial direction have a different shape.56 The Young’s modulus (E) and yield strength (σy) of delignified wood template were 184.9 ± 12.0 MPa and 13.2 ± 1.4 MPa, respectively, lower than for balsa and the delignified wood/epoxy biocomposite. This is mainly attributed to defects generated by lignin removal. There are micro-scale cavities in the cell wall corners and middle lamella (Figure 2b), as well as nano-scale pores in the cell walls. The low density (123 ± 16 kg m-3) and lack of lignin matrix in delignified wood cell wall also contributed to low compression strength. The E and σy values of the delignified wood/epoxy biocomposite were 263.0 ± 13.0 MPa and 18.1 ± 1.0 MPa, which are 21.0% and 5.8% higher than those for the native balsa wood, respectively. At RH of 100%, the transition from elastic deformation to a plateau after the yielding point was much smoother for all wood structures as compared to that at RH of 50%. The E and σy values of balsa at 100% RH were as low as 78.0 ± 7.0 MPa and 6.3 ± 0.3 MPa. These values are only ca. 36% of those at 50% RH. The moisture content of native balsa wood at 100% RH was 27.8%, which is a substantial difference to the 7% at 50% RH (Figure S4, Supporting Information). Adsorbed moisture acts as a plasticizer for the cell wall. The E and σy values of delignified wood template were also significantly decreased at 100% RH (Table 2). Interestingly, the E and σy values of the delignified wood/epoxy biocomposite were as high as 160.5 ± 15.0 MPa and 9.9 ± 1.0 MPa, respectively, due to the reduced moisture sensitivity at 100% RH. The nano-scale pores inside the cell walls of delignified wood were replaced by epoxy (Figure 2c and Figure S1d, Supporting Information). The exposed surface hydroxyl


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groups of cellulose were also modified covalently by epoxy.57 Effects of reduced moisture sensitivity due to epoxy modification have been reported previously by Ansari et al.52 Notably, the specific modulus (1.2–1.5 MPa kg-1 m3) and specific yield strength (0.095– 0.107 MPa kg-1 m3) of native balsa wood, delignified wood and wood/epoxy biocomposite in the present study are much higher than the cellulose-based 3D foams and aerogels reported in literature (Figure 3c, 3d and Table S1, Supporting Information). The compressive mechanical properties of cellulose-based three dimensional structured foam and aerogel materials with densities in the range of 7–340 kg m-3, are in the following ranges: E = 0.006–21 MPa, and σy = 0.003–1.8 MPa.58–70 Native wood with highly oriented cellulose microfibrils has a natural honeycomb-like structure with much higher compressive strength than for man-made foam architectures using nanocellulose. The cellulose microfibrils in the wood structures are well oriented in the original fiber direction as shown in Figure S3, Supporting Information. The water and oil adsorption behavior of the delignified wood template and delignified wood/epoxy biocomposites are presented in Figure 4. Data are in the form of contact angle, capillary water absorption, and oil/water absorption capacity. When a drop of water was placed on the surface of the longitudinal cross-section, the water droplet was immediately absorbed into the samples of native wood and delignified wood template (Figure 4a and 4b). For delignified wood/epoxy biocomposite (Figure 4c), the initial contact angle was rather high (140°) and decreased slightly to 125° over a period of 3 min (Figure S5, Supporting Information). This indicates that the delignified wood/epoxy biocomposite is not only hydrophobic, but also has much lower rate of water absorption. In contrast, the highly porous delignified wood template is hydrophilic.


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Figure 4. Water and oil absorption of balsa, delignified wood template and delignified wood/epoxy biocomposite. Photographs of a water (dyed with Congo Red) droplet on the surface of (a) balsa, (b) delignified wood template and (c) delignified wood/epoxy biocomposite surface at 0s, 1s, 2s, and 3min. (d) Capillary absorption (transport) of water (dyed with Congo Red) in balsa, delignified wood template, and delignified wood/epoxy biocomposite in longitudinal direction. Demonstration of underwater absorption of methylene chloride (dyed with Oil Red O) using (e) balsa wood; (f) delignified wood template, and (g) delignified wood/epoxy biocomposite. The absorption of a droplet of silicone oil (dyed with Oil Red O) for the delignified wood/epoxy biocomposite in (h) longitudinal and (i) transverse


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directions. (j) Water absorption capacity of the wood structures. (k) Absorption capacities of different type of oils for delignified wood/epoxy biocomposite. The water or oil absorption/adsorption capacity is critical in selective oil/water separation.71 The capillary absorption of water in longitudinal direction is shown in Figure 4d and Video S1, Supporting Information. Interestingly, the transport distance of water for the delignified wood/epoxy biocomposite is only 2 mm at 70s, while the transport distance of water in original balsa wood and delignified wood template samples are 14 mm and 20 mm, respectively. The transport distance depends on the parameters in the capillary suction equation (3), Supporting Information. With the water surface tension (γ), local acceleration (g) and the liquid density (ρ) being constant for all the samples, the height of absorption (i.e. transport distance) is dependent on the contact angle of water (cosθ) and the radius of the pores (r). The contact angles of water for balsa wood and delignified wood template are close to 0° because of capillary absorption. On the other hand, the pore size in cell walls for delignified wood template is much larger (Figure 2b and 2d, nano- and microscale) than that of balsa and delignified wood/epoxy biocomposite. In contrast, the delignified wood/epoxy biocomposite has not only larger pores (microscale lumens, Figure 2c) but also a large contact angle (Figure 4c and Figure S5, Supporting Information). According to the capillary suction equation (3), Supporting Information, delignified wood template displays the highest capillary suction, while delignified wood/epoxy biocomposite shows the smallest water transport distance in longitudinal direction. The dramatic water absorption of delignified wood template in longitudinal direction also suggests that the removal of hydrophobic lignin is beneficial for efficient water absorption. In the transverse direction (Figure S6 and Video S2, Supporting Information), the delignified wood/epoxy biocomposite also shows distinct hydrophobic


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characteristic as compared to balsa and the delignified wood template, indicating low water absorption due to the hydrophobic epoxy infiltration. Selective oil adsorption of the three wood structures were demonstrated by a droplet of methylene chloride (dyed with Oil Red O) located at the bottom of the water container (Figure 4e-g and Video S3, Supporting Information). The oil droplet (methylene chloride) could not be absorbed by native balsa and the delignified wood template. In contrast, water was instantaneously absorbed into these two hydrophilic wood structures (8s), showing the hydrophilic nature. The physical structure of the material is obviously important for oil/water separation. Nanoporosity is generated in the fiber walls of delignified wood template (Figure 2b), which facilitates water absorption by capillary action and the delignified wood template is thus hydrophilic. In addition, the microscale voids, in cell wall corners and middle lamella (Figure 2c), lumen channels and pits (Figure S1, Supporting Information), also contribute to the hydrophilic sorption in native balsa and the delignified wood template. Interestingly, the delignified wood/epoxy biocomposite immediately absorbed the oil droplet under water (Figure 4g). The removal of n-hexane (dyed with oil red O) from the surface of water by the delignified wood/epoxy biocomposite was also observed (Figure S7 and Video S4, Supporting Information). Thus, the delignified wood/epoxy biocomposite selectively absorbs oil under water, and also on the water surface. In addition, it exhibits directional oil absorption feature since the pores in the wood biocomposite are highly directional. As shown in Figure 4h, when a droplet of silicone oil was placed on the surface perpendicular to the longitudinal direction, it was quickly adsorbed into the wood structure owing to pore (lumen) openings are directly exposed to the oil droplet. Interestingly, little oil absorption was observed when the droplet was placed on the transverse surface exposing to cell walls coated with epoxy and the oil was evenly spread on the surface due to the hydrophobicity of the biocomposite (Figure 4i). 15 ACS Paragon Plus Environment

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As shown in Figure 4j, the water absorption capacity was significantly decreased from 28 g/g for the delignified wood template to 0.3 g/g for the delignified wood/epoxy biocomposite as calculated from equation (4), Supporting Information. However, the absorption capacities of different oils and organic solvents were around 6-20 g/g for delignified wood/epoxy biocomposite, as shown in Figure 4k and Figure S8, Supporting Information, respectively. Native balsa wood contains numerous pits and pit membranes on the fiber walls, connecting adjacent fiber walls/tracheids/ray cells/vessels for water absorption/transportation (Figure S1b, Supporting Information). After delignification, the pit membranes were open due to the removal of lignin from the membranes (Figure S1c, Supporting Information), increasing water conductance. In addition, the extraction of hydrophobic lignin leads to exposure of the hydrophilic cellulose microfibrils in the fiber walls (Figure S3, Supporting Information). For the delignified wood/ epoxy biocomposite, the interface between lumen and cell wall was coated a thin layer of hydrophobic epoxy polymer. Thus, the pits on the fiber walls were blocked with hydrophobic polymer (Figure 2c and Figure S1d, Supporting Information). In conventional hydrophobic wood biocomposites, the hydrophobic polymers not only infiltrate the free space in the cell walls but also the lumen space so that water transport pathways are completely blocked. In the present work, the polymer is only infiltrated inside the cell wall. The delignified wood/epoxy biocomposite is thus highly porous and hydrophobic, which is different from traditional wood modification methods.

CONCLUSIONS

In summary, we have created wood-based structures for oil/water separation. The starting template was a mesoporous delignified wood nanostructure, with nanoscale pores and microscale lumen channels. The highly porous delignified wood template showed hydrophilicity, and water was selectively absorbed through spontaneous wetting and capillary 16 ACS Paragon Plus Environment

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action. A functional delignified wood/epoxy biocomposite with hydrophobicity and enhanced oleophilicity was then prepared by the impreganation of the delignified wood template with epoxy. The hygromechanical stability was improved due to the hydrophobic nature and epoxy cell wall impregnation. Oil can be selectively absorbed not only under water but also from the water surface using the delignified wood/epoxy biocomposite. High oil/water absorption capacity of delignified wood structures with and without epoxy allows for applications in oil/water separation. More importantly, both wood structures exhibit favorable mechanical properties as compared to widely studied reconstructed nanocellulose foam and aerogel materials. This is ascribed to the natural hierarchical structure of wood, which includes highly oriented cellulose microfibrils, and wood cell walls with anisotropy. From the present study, we anticipate that composite materials can be produced by coupling material engineering with chemistry for the processing of hierarchical structures scaled from nano-, micro- to macroscales. Functionalities can be further tailored by chemical modification, thus providing expected properties that allow applications in structural materials, photonics, sensors, and optical devices.

MATERIALS AND METHODS

Materials. Balsa wood (Ochroma Pyramidale) was purchased from Wentzels Co. Ltd, Sweden. The wood sample was cut into two different sizes, 20 × 5 × 5 mm3 and 10 × 15 × 15 mm3 (longitudinal × radial × tangential). Sodium chlorite (NaClO2) and tert-butanol were purchased from Sigma–Aldrich, Germany. Ethanol and acetone were purchased from VWR, Sweden. Bisphenol A Diglycidyl Ether (abbreviated in DGEBA) was bought from TCI chemicals, and Jeffamine D-400 polyetheramine (PEA) was provided by Huntsman, USA. Preparation of delignified wood template. The balsa samples were chemically extracted with 1 wt.% of NaClO2 in sodium acetate buffer solution at pH 4.6 and 80 °C for 6 h or 18 h 17 ACS Paragon Plus Environment

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depending on the sample size. The NaClO2/sodium acetate buffer solution must be changed every 6h. After delignification, the delignified wood samples were rinsing with deionized water completely. The delignified samples were frozen at -20 °C for 5h before they were subjected to sublimation/freeze drying for 24 h. Preparation of delignified wood/epoxy biocomposite. The porous delignified wood template was impregnated with 5 wt.% of epoxy/acetone solution (the ratio of DGEBA:PEA was 65:35, w/w) according to the method reported previously.52 The epoxy solution was impregnated into the wood template by using 0.3 bar vacuum for 30 min. These impregnated samples were polymerized by increasing temperature step by step in an oven, 30 °C, 60 °C and 90 °C for 3h respectively, followed by curing at 120 °C for 12h. The cured biocomposite samples were thoroughly washed by acetone twice to remove extra epoxy. Characterizations. The wood samples were fractured in liquid nitrogen, and the crosssections were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan). Klason lignin content was determined by acid hydrolysis with 72 wt.% sulfuric acid at 121 °C according to TAPPI T222 om-83. Sugar analysis was performed on a Dionex ICS-3000 High Performance Anion Exchange Chromatography (HPAEC). Brunauer–Emmet– Teller (BET) measurements were carried out on the equipment of Micromeritics ASAP 2020 through N2 physisorption at relative pressure between 0.05 and 0.3. The compression test, in axial direction, was carried out using an Instron 5966 equipped with a 10 kN load cell at 25 °C and 50% and 100% relative humidity. Water contact angles were measured using a CAM200 contact angle meter (KSV instruments Ltd, Helsinki, Finland). Wettability and absorption capability of water, oils and organic solvents are reported in Supporting Information.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. FE-SEM images of the lumen channels, pores and microfibrils structure in wood, FTIR spectra of native and delignified wood template, Summary of mechanical data from current work and previous published results, Photographs of water contact angle and oil absorption, Video records of water and oil absorption. (file type, PDF, AVI) AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed, Tel: +46 8 790 96 25. E-mail: [email protected] ACKNOWLEDGMENT The Wallenberg Wood Science Center (WWSC) is acknowledged for financial support for this work. The authors also thank Ms. V. Guccini for kind help with BET measurement. Q. Fu is grateful to China Scholarship Council (CSC) for supporting PhD study. REFERENCES (1)

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BRIEFS The highly porous delignified wood template with high hydrophilic performance is obtained by the removal of lignin from native balsa wood. The delignified wood template is further impregnated with epoxy. The delignified wood/epoxy biocomposite with hydrophobicity/oleophilicity and high oil absorption capacity is successfully prepared. Both wood structures exhibit favorable mechanical properties. SYNOPSIS


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Wood Nanotechnology for Strong, Mesoporous ... - ACS Publications

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