Nanocellulose Mechanically Isolated from

Research Article pubs.acs.org/journal/ascecg

Nanocellulose Mechanically Isolated from Amorpha f ruticosa Linn. Xiao Zhuo,†,‡ Chang Liu,†,‡ Rutan Pan,†,‡ Xiaoying Dong,*,†,‡,§ and Yongfeng Li*,†,‡,§ †

Shandong Provincial University Key Laboratory of Silviculture, ‡Forestry College, and §Shandong Institute of Wood Science, Shandong Agricultural University, No. 61 Daizong Road, Taian 271018, China S Supporting Information *

ABSTRACT: Nanocellulose is gaining evident interest from researchers and engineers because of its renewability, biocompatibility, biodegradability, high mechanical strength, abundant hydroxyl groups for potential functionality, and extensive raw materials. Versatile sources are accordingly explored like harvested wood, annual plants, and agricultural residues. However, an abundant shrub plant, Amorpha f ruticosa Linn., has not yet been reported for isolating nanocellulose. We accordingly propose a green method with low energy consumption to extract nanocellulose from the vast shrub source via combined grinding and successive homogenization treatments. The derived nanocellulose possesses a fine structure with a diameter of ∼10 nm and an aspect ratio over 1000, high thermal stability with a maximum decomposition temperature of 337 °C, and similar composition with a hydroxyl group and a crystal I structure to that of natural cellulose. The demonstrated nanopaper presents visible light transmittance over 90% and haze below 15%, which further confirms the fine structure of the derived nanocellulose. Such a method could potentially broaden the major shrub plant with green, sustainable, up-scaled, and value-added applications in highend domains like electronics, biomedicine, aerospace, energy, environments, etc. KEYWORDS: Amorpha fruticosa Linn., Shrub plant, Nanocellulose, Green isolation, Grinding, High-pressure homogenization

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process).1−3,16−18 The main and versatile sources are harvested, renewable materials like wood, annual plants, and agricultural residues.19,20 Thus, vast investigations are concentrated on topdown isolation of nanocellulose from the abundant green resources including various species of wood, bamboo, cotton, soy hulls, hemp, sisal, branch-barks of mulberry, pineapple leaf, pea hull, coconut husk, banana rachis, sugar beet, rice straw, oat straw, wheat straw, corn straw, needles grass, and so on.1−3,17,18,21−34 The commonly employed top-down methods are mechanical treatments like grinding, blending, and homogenization or in combination with pretreatments like enzymatic hydrolysis, acid hydrolysis, and TEMPO-mediated oxidation.35−56 Among them, mechanical treatments are recognized as the most environmentally friendly and green isolation way due to the lack of utilization of chemical reagents.57 Homogenization and grinding as two kinds of wellknown mechanical treatments are accordingly explored extensively.1−3,17,18 The derived nanocelluloses can achieve diameters of several tens of nanometers with relatively narrow diameter distribution. Such a process essentially meets the requirement of sustainable development of our society through the employment of renewable resources and a green isolation method without utilization of chemical reagents. However, homogenization typically consumes huge amounts of energy,

INTRODUCTION Cellulose is the most abundant natural macromolecule which could be derived from plants, animals, and even marine organisms on earth.1 Nanocellulose is termed as a 1D nanomaterial with a diameter normally in nanoscale and a length typically in several hundreds of nanometers up to several micrometers, which is actually assembled from bundles of cellulose chains.2,3 It features three hydroxyl groups in each anhydroglucose unit, which renders hygroscopic character to nanocellulose, and accordingly results in relatively easier dispersion of the nanocellulose in water.4,5 The vast hydroxyl groups in the cellulose chain promote formation of intermolecular hydrogen bonding among polysaccharides, which facilitates the generation of the crystalline domain in nanocellulose.6,7 All these features endow nanocellulose with renewability, biocompatibility, biodegradation, high aspect ratio, excellent mechanical properties, superior thermal stability, and light transparency, which facilitates nanocellulose to be potentially applied as a reinforced nanomaterial for composite and coating and as a building block for packaging, separation film, biomedicine, and transparent nanopaper.8−12 Consequently, nanocellulose has received sharply increased interest from academia and industries and is becoming a hot research topic in the field of material science.13−15 Normally, nanocellulose can be derived either by disintegration of cellulose fibers from harvested sources, algae, and tunicate (top-down method) or by self-assembling of cellulose chains from bacteria or electrospinning equipment (bottom-up © 2017 American Chemical Society

Received: February 16, 2017 Revised: March 13, 2017 Published: March 15, 2017 4414

DOI: 10.1021/acssuschemeng.7b00478 ACS Sustainable Chem. Eng. 2017, 5, 4414−4420

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Flowchart of isolating nanocellulose from Amorpha f ruticosa and schematic illustration of the morphology at each step. (a) Schematic diagram of Amorpha f ruticosa; (b) 3D microstructure of stem wood of Amorpha f ruticosa which corresponds to (a); (c) digital photo of wood powders of Amorpha f ruticosa; (d) 3D microstructure of cell fiber corresponding to (b); (e) digital photo of aqueous suspension of the purified cellulose fibers ; (f) 3D microstructure of the purified cellulose fibers corresponding to (e); (g) digital photo of aqueous suspension of the derived nanocellulose; (h) 3D microstructure of nanocellulose corresponding to (g).

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whereas grinding normally results in a lower aspect ratio of nanocellulose due to the tremendous shearing force.1 Consequently, combined treatment of grinding and homogenization with optimum craft may be a desired way for isolating nanocellulose with lower energy consumption and fine nanocellulose, which has not yet been extensively explored. Despite the variety of explored sources, abundant shrub plants for isolating nanocelluloses are rarely reported. Amorpha f ruticosa Linn. is a species of deciduous shrub plants, which can tenaciously grow in harsh environments like rocks, slopes, and saline-alkali soil, etc.58,59 It can soften the soil and thus be known as an important plant for soil conservation especially in barren areas. Its stem can reach several meters in height (up to six meters) and several centimeters in diameter within one growth-year.58 Typically, it can be harvested many times once planted because of its fast-growing character.58 According to statistics, the plant can produce about two tons of stem wood per acre of land.60 The major stem sources are now commonly used as raw materials just for animal feed or handicrafts, resulting in inefficient utilization of the abundant resources.60 To the best of our knowledge, exploration on the Amorpha f ruticosa for isolating nanocellulose has not yet been reported. Therefore, this study intends to extract nanocellulose from the shrub plant via a combined mechanical treatment of grinding and successive homogenization processes. The morphological structure, composition, and the main properties of the derived nanocellulose are characterized and analyzed, which could provide a scientific basis for inexpensive, scalable, and high value-added utilization of such materials. Such exploration could potentially promote the green, healthy, and sustainable development of our society in an environmentally friendly mode.

EXPERIMENTAL SECTION

Raw Materials. Amorpha f ruticosa Linn. with cellulose content of ∼39.22%, hemicellulose content of ∼26.96%, and lignin content of ∼24.03%, obtained from a Taian suburb, was first ground into wood powders to pass through a 100-mesh sieve, then washed clean by DI water, and finally stored in a glass container after an oven-drying treatment. Toluene (>99%), anhydrous ethanol (>99.9%), potassium hydroxide (>95%), and glacial acetic acid (99.9%) were all purchased from Tianjin Kaitong Chemical Reagent Co., Ltd. in China. Sodium hypochlorite (∼10%) was provided by the Damao Chemical Reagent Factory in China. All the chemicals were directly used without further purification. Chemical Purification. 2.5 g wood powders were extracted by a mixed toluene-ethanol solution (volume ratio of 2:1) at 90 °C for 6 h. Then the wood powder was treated with sodium hypochlorite under acidic condition (glacial acetic acid) at 70 °C for 6 h to remove the lignin (the derived holocellulose contained cellulose of 78.28 wt % and hemicellulose of 21.72 wt %), followed by treatment with 2 wt % NaOH aqueous solution at 90 °C for 2 h to remove most of the hemicellulose. After that, the samples were again treated by sodium hypochlorite under acidic condition for 2 h, followed by final purification of 2 wt % NaOH solution at 90 °C for 2 h (the derived purified cellulose contained cellulose of 96.15 wt % and hemicellulose of 3.85 wt %). The whole process was performed with never-dried cellulose fibers to prevent the potential aggregation of cellulose chains. Mechanical Fibrillation. The purified cellulose fibers were dispersed in water to form a concentration of 0.3 wt %. The mixed suspension was then passed through a grinder (MKCA6-2J, Masuko Sangyo Co., Ltd., Kawaguchi, Japan) at 1500 rpm with a millstone gap of −5 for 5 min. Finally, the suspension was passed through a highpressure homogenizer (APV2000, SPX FLOW, Inc., Unna, Germany) under the first value pressure of 600 bar and the second value pressure of 100 bar for 20 min to obtain nanocellulose. The energy consumption for this mechanical treatment is calculated to be 16.4 MJ/kg, which is lower than the reported 97.2 MJ/kg of a single homogenizer treatment and 24.0 MJ/kg of a single microfluidizer treatment.61 4415


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Figure 2. Schematic illustration of the hierarchical structure of Amorpha f ruticosa wood and the corresponding images of the derived products at different scales. (a) Schematic diagram of 3D microstructure of the Amorpha f ruticosa wood; (b) SEM image of the Amorpha fruticosa wood at cross section; (c) schematic diagram of the cellulose fibers in the cell wall of Amorpha f ruticosa wood; (d) SEM image of the derived purified cellulose fibers corresponding to (c); (e) TEM image of a single purified cellulose fiber which corresponds to (c); (f) schematic diagram of nanocelluloses in the purified cellulose fibers; (g) SEM image of the derived nanocelluloses corresponding to (f); (h) TEM image of the derived nanocelluloses corresponding to (f). FTIR, XRD, and TG analyses, the wood sample was first ground into powder by a disintegrator and passed through a 100-mesh screen, followed by extraction with acetone for 24 h and toluene for 24 h, and then subsequently dried to a constant weight. Other samples were directly used for characterization after having been oven-dried at 105 °C for 24 h.

Preparation of the Transparent Nanopaper. To demonstrate the fine structure of the derived nanocellulose, the nanopaper from the nanocellulose was prepared. Briefly, 15 mL of nanocellulose suspension with 0.1 wt % concentration was passed through a filtrating equipment under vacuum suction conditions. The employed filter membrane is hydrophilic polytetrafluoroethylene with a mean pore size of 0.22 μm. Then, the wet membrane was set between two hydrophilic polytetrafluoroethylene membranes, followed by setting between two glasses for final oven drying at 80 °C for 12 h. The nanopaper was finally derived via the above procedure. Measurements. The nanocellulose was observed respectively with a Field Emission Scanning Electron Microscope (FE-SEM, JEM6610LV, JEOL USA Inc., Peabody, Massachusetts), a Transmission Electron Microscope (TEM, JEM-1400, JEOL USA Inc., Peabody, Massachusetts), and an Atomic Force Microscope (AFM, NaioAFM, Nanosurf AG, Liestal, Switzerland) in tapping mode. Before the SEM observation, the nanocellulose suspension is first freeze-dried at −55 °C, followed by gold-sputter-coating after being fixed on the conductive tape. For the TEM observation, the nanocellulose suspension is directly dropped on the copper screen, followed by negative staining by phosphotungstic acid, and finally dried at room temperature before examination. For the AFM observation, the nanocellulose suspension is first directly dropped on a silicon wafer and then dried at room temperature for further detection. Fourier Transform Infrared Spectroscopy (FTIR, Nicolet Magna 560, Thermo Nicolet Inc., USA) was used to analyze chemical components of the samples. An XRD instrument (D/max2200, Rigaku Corporation, Japan) was employed to measure the crystallinity of the derived nanocellulose. The test parameters included Cu butt, voltage of 40 kV, current of 30 mA, rotating speed of 4(o) /min, and step length of 0.02°. A thermogravimetric analyzer (TGA Q500, Waters, New Castle, DE) instrument was used for the TG/DTG analyses. 5−10 mg powder was used under continuous nitrogen flow, the heat rate was 10 °C/min, and the temperature range was from 35 to 450 °C. Before

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RESULTS AND DISCUSSION As far as we know, shrub plant wood is composed of cell fibers with a hollow structure (Figure 1a, b).62 Each cell fiber comprises a cell wall that is assembled from three main components, i.e., cellulose, hemicellulose, and lignin (Figure 1c, d). Typically, cellulose chains form nanofibrils with lateral dimensions of 3−25 nm that are ascribed to the strong intermolecular hydrogen bonds generated by the abundant hydroxyl groups. The nanocellulose is embedded in the matrix composed of lignin and hemicellulose. Consequently, we need to disintegrate the stem wood into nanocellulose upon the topdown way as shown in Figure 1. Stem wood is first ground into wood powders which has a cell lumen circled by a cell wall that contains the three major components (Figure 1a-d). Then, the wood powders are successively extracted to remove lignin and hemicellulose for purified cellulose fibers (Figure 1e, f). Finally, the aqueous suspension of the obtained purified cellulose fibers is passed through grinding and a homogenizer successively for mechanical isolation of nanocellulose (Figure 1g, h). The cellular structure of the Amorpha f ruticosa wood (Figure 1b, 2a) can be confirmed by the SEM observation that is shown in Figure 2b and Figure S1. After removing lignin and hemicellulose as a matrix in each cell fiber, we can obtain the 4416


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Figure 3. Characterization of chemical components of Amorpha f ruticosa wood at each isolation step. (a) Schematic illustration to morphology evolution of chemical components of the Amorpha fruticosa wood during the nanocellulose-isolating process; (b) FTIR spectra of the chemical components at different isolation steps; (c) XRD curves of the chemical components at different isolation steps; (d) TG curves of the chemical components at different isolation steps; (e) the onset and maximum decomposition temperatures of the chemical components at different isolation steps which correspond to (d).

wood at 1580 and 1500 cm−1, assigned to the skeleton vibration of CC at an aromatic ring of lignin, almost disappeared in holocellulose, indicating removal of lignin after natural wood exposing to NaClO 2 /H + conditions.23,24 The peak of holocellulose at 1737 cm−1, assigned to the stretching vibration of CO, disappeared in the purified cellulose fiber, confirming removal of most of the hemicellulose after KOH treatment on holocellulose.36,37The nanocellulose presents almost similar FTIR spectra to that of the purified cellulose fiber, suggesting the composition of the purified cellulose fibers remained unchanged after grinding and homogenization treatments. In other words, the derived nanocellulose still maintains the abundant hydroxyl groups. The FTIR spectra of Figure 3b prove the evolution of the compositions as described in Figure 3a. Figure 3c shows that all the XRD curves present similar diffraction peaks at 2θ = 16.2° and 22.3°, which correspond to the (101) plane and (002) plane, respectively. That means cellulose retains the crystal I morphology during the whole process. According to the Segal method, the crystallinity of natural wood, holocellulose, purified cellulose fiber, and the nanocellulose is calculated to be 54.63%, 64.01%, 79.87%, and 54.67%, respectively.36,37,46−48 The purified cellulose fiber has the highest crystallinity due to the gradual removal of amorphous lignin and hemicellulose. A similar result on

purified cellulose fiber (Figure 2c) that is described in the SEM image (Figure 2d). The purified cellulose fiber has an average diameter of ∼10 um, which is similar to its original cell fiber in the stem wood (Figure 2b). It means that the nanocelluloses still entangle each other to maintain the fiber hollow form which is proved by the TEM observation (Figure 2e). Figure 2e clearly shows that the nanocellulose loosely entangled within the purified cellulose fiber has a mean diameter of about 10 nm and a length longer than 1 μm. Theoretically, removal of the matrix could loosen the intermolecular interactions of nanocelluloses. Thus, a combined mechanical treatment is employed to fibrillate the nanocellulose (Figure 2f). The isolated nanocellulose possesses 1D morphology with a diameter in nanoscale and a length in micrometers (Figure 2g). The mean diameter and length of each nanocellulose approach ∼10 nm and over 10 μm, leading to an aspect ratio beyond 1000 (Figure 2h, Figure S2). The diameter statistics indicate that the diameter sizes present a normal distribution with over 87% diameters in the range of 8−15 nm (Figure 2h, inset), suggesting the feasibility of the combined grinding and homogenization treatments. During the whole disintegration process, the chemical components of lignin and hemicellulose are gradually removed, accompanied by morphology evolution of cellulose fibers as shown in Figure 3a. Figure 3b describes the peaks of natural 4417


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Figure 4. Demonstration of the derived nanocellulose as a building block for the transparent nanopaper. (a) Schematic illustration of preparation of the transparent nanopaper from the derived nanocellulose; (b) AFM image of the surface of the derived film; (c) the diameter distribution of nanocelluloses in the surface of the film which corresponds to (b); (d) transmittance and haze curves of the nanopaper and the digital photo of the nanopaper (inset); (e) SEM image of the surface of the transparent nanopaper; (f) SEM image of the cross section of the transparent nanopaper.

vacuum suction conditions. The employed filter membrane is hydrophilic polytetrafluoroethylene with a mean pore size of 0.22 μm. AFM observation shows that the nanocellulose uniformly distributed within the derived film after vacuum filtration and forms an entangled network (Figure 4b, Figure S3a). Each single nanocellulose on the surface of the film presents a fine structure with a uniform diameter of ∼10 nm (Figure 4c, Figure S3b), which is consistent with the results of TEM observation. The nanopaper derived from the above oven-dried film presents high visible light transmittance over 90% from a wavelength of 400 nm up to 800 nm and low haze of 8% up to 15% from a wavelength of 400 to 800 nm (Figure 4d). Such high transmittance and low haze benefit the nanopaper like glass that light can penetrate directly through the nanopaper without large light scattering. Consequently, the green leaf behind the nanopaper is clearly visible (Figure 4d, inset). SEM observation further shows that the surface of the nanopaper is quite smooth, and the comprised nanocelluloses still maintain a fine structure with nanoscale diameters (Figure 4e), which leads to light transmission without obvious light scattering.45 The cross section shows a layered structure with nanometer thickness and densely packed layers without obvious gaps or holes between them (Figure 4f). The above results should be ascribed to layer-by-layer self-assembly of nanocellulose with strong hydrogen bonding generated by the abundant hydroxyl groups. Consequently, we could reasonably believe that the employed mechanical treatment facilitates the successful isolation of nanocellulose from Amorpha f ruticosa wood, whose fine structure leads to the nanopaper with high transmittance and low haze.

holocellulose should also be ascribed to the removal of amorphous lignin. However, the crystallinity of nanocellulose decreases from 64.01% to 54.67%, which should be ascribed to the decreasing alignment of the cellulose chains. That is to say, the high-intensity shearing force and impact force generated by grinding and homogenization treatments break the hydrogen bonds between the cellulose chains to some extent, leading to the crystallinity decreased.45 As hemicellulose has the lowest thermal decomposition temperature among the three major components, the natural wood presents the lowest thermal stability in comparison with the purified cellulose fibers and nanocellulose (Figure 3d). The lowest onset and maximum decomposition temperature of natural wood in Figure 3e corresponds to the above results as shown in Figure 3d. The purified cellulose typically has a small amount of hemicellulose, leading to slightly lower thermal stability than that of nanocellulose (Figure 3d, e). However, the difference of the thermal stability between the purified cellulose fibers and nanocellulose is quite small (Figure 3d), such that both of them present nearly identical onset and maximum thermal decomposition temperatures (Figure 3e). Overall, the nanocellulose obtained after a series of treatments on natural wood presents a similar composition, crystal form, and thermal stability to those of cellulose fibers in natural wood. To demonstrate the fine structure of the derived nanocellulose, we prepared the nanopaper with the nanocellulose as a building block via a suction filtration method (Figure 4a). Fifteen mL of nanocellulose suspension with 0.1 wt % concentration was passed through filtrating equipment under 4418


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CONCLUSIONS This study successfully isolates nanocellulose from Amorpha f ruticosa wood by grinding and successive homogenization treatments. The as-prepared nanocellulose presents an average diameter of ∼10 nm and a length over 10 μm, leading to an aspect ratio higher than 1000. The fine nanocellulose still retains abundant hydroxyl groups and a crystal structure of natural cellulose I and presents a maximum decomposition temperature of 337 °C, remarkably higher than that of natural wood. The demonstrated nanopaper shows light transmittance beyond 90% and haze below 15%, indicating the fine structure of the building block with uniform diameter in nanoscale. Such green isolation of nanocellulose from the Amorpha f ruticosa wood could potentially realize value-added applications of the abundant shrub resources, for example in areas of microfluidic chips, flexible electronics, reinforced composites, and food packaging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00478. Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (X.D.). ORCID

Yongfeng Li: 0000-0002-6394-0222 Author Contributions

X.Z. and C.L contributed equally to this work. X.Z., X.D., and Y.L. designed the experiment. C.L. and X.Z. performed the whole experiment and drew the figures. R.P. and X.D. carried out measurements of the light transmittance of nanopaper. X.D. and Y.L. wrote the paper. All of the authors commented on the final manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China, Grant. No. 31300479, and Key Special Foundation for the National Key Research and Development Program of China, Grant. No. 2016YFD0600704, and Project of the Shandong Province Higher Educational Science and Technology Program, Grant. No. J15LC13.

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