Rice Straw Nanocelluloses: Process-Linked Structures, Properties

Downloaded by CITY UNIV OF HONG KONG on October 24, 2017 | http://pubs.acs.org Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1251.ch007

Chapter 7

Rice Straw Nanocelluloses: Process-Linked Structures, Properties, and Self-Assembling into Ultra-Fine Fibers Feng Jiang and You-Lo Hsieh* Fiber and Polymer Science, University of California-Davis, Davis, California 95616, United States *E-mail: [email protected].

Nanocelluloses isolated from agricultural biomass have attracted increasing attention as emerging renewable nanomaterials as well as ways to valorize these otherwise underutilized wastes from the food production system while alleviate the negative impact on our environment. Rice straw represents the largest quantity crop residue from the highest value crop in the world and contains ca. 40 % cellulose, similar to wood. Depending on the methods of isolation and derivation, a range of nanocelluloses with different surface chemistries and geometries have been facilely derived from rice straw via H2SO4 hydrolysis, TEMPO oxidation, and mechanical defibrillation, etc. These nanocelluloses, i.e., short and rigid cellulose nanocrystals (CNCs) as well as long and flexible cellulose nanofibrils (CNFs), exhibit significantly different crystalline and thermal characteristics and behaviors in liquids and from drying. These nanocelluloses self-assemble, under controlled freezing and freeze-drying, into tunable morphologies ranging from continuous nanofibers to sub-micron fibers, macroscopic films, to porous mass with intriguing amphiphilicity. This chapter focuses on self-assembling of rice straw nanocelluloses into fibrous structures. These diverse nanocelluloses and their self-assembled fibrous structures from a single agriculture biomass, rice straw, demonstrate the significance of processes in generating nanocelluloses and drying and how they offer precise control on creating the desired characteristics of

© 2017 American Chemical Society Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

nanocelluloses, nanoscale and micro-scale fibrous structures for potential applications.


Introduction As the most abundant natural polymer with an estimated 1.5x1012 ton annual production (1), cellulose is among the most desirable renewable substance to supply the ever increasing demands for materials. Cellulose synthesis in higher plants has been shown, from the freeze fracture of plasma membrane, to consist six globular terminal complexes in 25–30 nm wide symmetric rosettes, each synthesizes six parallel β-1,4-glucan chains that crystalize into a 36-chain microfibrils (2, 3). These microfibrils have nanoscale lateral dimensions ranging from as low as a few nanometers to several tens of nanometers (4–6). Owning to the tenacious intra- and inter-molecular associations via the abundant van der Waals and hydrogen bonds, cellulose microfibrils have been predicted to exhibit ultra-high tensile strength (7.5 GPa) and Young’s modulus (110–220 GPa), approaching those of carbon nanotubes (6–9). Depending on origins, cellulose in these microfibrils vary in molecular lengths or numbers of glucan units (300 to 10,000 degree of polymerization) (6), crystallinity (40–90 %) and crystallite dimensions (3–10 nm) as well as in either Iα or Iβ crystalline structure (6, 10). These crystalline nanocelluloses may be separated by overcoming the inter- and intra-fibril hydrogen bondings and van der Waals forces among semi-crystalline cellulose microfibrils by chemical, biological and mechanical means into smaller crystalline nanocelluloses (11–14). Earlier studies of nanocelluloses have focused primarily on those isolated from wood pulp (15–17), cotton fibers (18–20), and microcrystalline cellulose (21, 22) using various acids, oxidizing agents, enzymes and mechanical shear forces, etc. More recently, nanocelluloses from other sources, such as agricultural biomass, have become increasingly reported as a way to valorize the byproducts of food production system while alleviate the negative impact on the environment. However, the extent of effort and understanding on nanocelluloses from agricultural biomass to date is pale in comparison to those originated from wood. Generally, deriving nanocelluloses from agricultural biomass requires source-specific pretreatments to isolate cellulose due to their wide ranging structural and compositional differences. Source-linked structural differences have been evident as demonstrated by cross comparisons of nanocellulose characteristics derived by the same method, i.e., cellulose microfibrils from grinding of wood, rice straw and potato tuber (23) and cellulose nanocrystals from sulfuric acid hydrolysis of cotton, rice straw and grape skins (24). While process-linked structural differences in nanocelluloses from the same source have been commonly recognized among well reported on pulp and commercially available microcrystalline cellulose, those from agricultural biomass are scarce. Here, process-linked nanocelluloses from a single source, rice straw, known to date are presented and analyzed. Rice is the highest value crop and the third highest quantity crop after wheat and corn with annual global production of over 741 million tons in 2014 (25). Rice production, however, generates 134 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


the highest quantities of residues, mostly as rice straw, owning to its higher residue to grain ratio of ca. 1–1.5 (26, 27). Optimal utilization of this vastly produced rice byproducts would have the highest potential and impact by creating economic returns and improving environment. Rice straw contains as high as 40% cellulose, similar to wood and with limited other use, making it ideal feedstock for nanocellulose production (28). Nanocelluloses from rice straw cellulose have been derived and systematically investigated using several established methods, including sulfuric acid hydrolysis (28–30), solid acid hydrolysis (31), 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) mediated oxidation (32), high-speed mechanical shearing (29), coupled TEMPO oxidation and blending (32) and aqueous counter collision treatment (33). These rice straw nanocellulose characteristics as affected by methods of isolation and derivation, their behaviors and relationships to the targeted applications are analyzed and reviewed to serve as an initial overview of how diverse nanocelluloses are optimally produced from a globally important and readily available feedstock. This current knowledge on rice straw nanocelluloses offers insight and guidance to similar effort on many other potential agricultural and other biomass sources.

Isolation of Rice Straw Nanocellulose Nanocelluloses from rice straw have been derived by means of acid hydrolysis (28, 29), oxidation (26, 32), mechanical defibrillation (23, 33, 34), and a combination of these methods (26, 32, 35). Similar to all lignocellulosic materials, cellulose in rice straw is embedded in the lignin and hemicellulose matrices, with protective outer silica layer that account for 13–15 % of rice straw (30, 35, 36). Cellulose in rice straw has been consistently isolated following a three-step procedure to sequentially remove wax, lignin, hemicellulose and silica, i.e., 1) Soxhlet extraction with 2:1 toluene/ethanol for 20 h to remove wax, pigment and oil, yield 94.4 %; 2) delignification with 1.4 % acidified NaClO2 at 70 °C for 5 h to remove lignin, yield 75.4 %; 3) bleaching with 5 % KOH solution at 90 °C for 2 h and then room temperature for 24 h to simultaneously remove hemicellulose and silica, yield 36.4 % of cellulose (28, 29, 35). A simplified isolation method that includes the same dewaxing step and two brief repeated alkaline treatments (4 % NaOH, 70 °C for 5 min), yielding 47.6 % cellulose rich solid (35), has also proven to be an effective alternative. The extracted rice straw cellulose appears as a whitish fibrous mass of mostly 5–10 μm wide and 50–300 μm long fibers as well as thin sheets of packed and highly oriented parallel microfibers (Figure 1). The cellulose fibers are highly crystalline with crystallinity of around 72 % (29), showing bright birefringence under polarized light microscope (Figure 1a and b).

135 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 1. Polarized light microscope (a,b) (a, Reprinted with permission from ref (33). Copyright 2016 American Chemical Society) and SEM (c,d) images of rice straw cellulose.

Acid Hydrolysis Although acid hydrolysis has been firstly implemented in isolating cellulose crystallites over 70 years ago (37, 38), its application in deriving cellulose nanocrystals (CNCs) from rice straw cellulose was only investigated and reported by our group since 2012 (24, 28–30). Rice straw cellulose isolated by the three-step process described previously was sulfuric acid hydrolyzed (64 wt%, 45 °C) to yield flat ribbon-like CNCs in decreasing lateral dimensions with longer hydrolysis time, i.e., 6.0 ± 2.1 nm thick, 30.7 ± 9.4 nm wide and 270 ± 106 nm long at 30 min and 5.1 ± 1.7 nm thick, 11.2 ± 3.6 nm wide and 117 ± 39 nm long at 45 min (28), where thickness was determined by atomic force microscopy (AFM) and the width and length by transmission electron microscopy (TEM). However, the yields are very low, at respective 6.4 and 4.8 % of rice straw cellulose. The CNC yield may be nearly tripled to 16.9 % by reducing the reaction time to 15 min, but their morphologies also become more heterogeneous and bimodally distributed as 2.1 ± 0.7 nm thick, 67 ± 17 nm long and 6.7 ± 1.8 nm thick, 166 ± 142 nm long, with the smaller CNCs gradually disappeared as the hydrolysis prolongs (29).



Figure 2. AFM height images (5 μm ×5 μm) and thickness distribution of nanocellulose derived from rice straw; a, d). CNC (29, 30); b,e). HoloCNC (30) (a and b. Reproduced with permission from ref (30). Copyright 2015 American Chemical Society. d. Reproduced with permission from ref (29). Copyright 2013 Elsevier); c,f). CNF from solid acid hydrolysis (31) (Reproduced with permission from ref (31). Copyright 2015 American Chemical Society); g,j) CNF from TEMPO oxidation (32) (Reproduced with permission from ref (32). Copyright 2013 Royal Society of Chemistry); h,k) HoloCNF from TEMPO oxidation (26) (Reprinted with permission from ref (26). Copyright 2015 American Chemical Society); i,l) CNF from ACC treatment (33). (l. Reprinted with permission from ref (33). Copyright 2016 American Chemical Society). Sulfuric acid hydrolysis (64 wt%, 45 °C and 45 min) of the less purified holocellulose isolated by the first two steps, i.e., eliminating the third alkaline leaching (30), yielded 11.6 % holoCNCs from holocellulose, nearly 2.5 times of CNCs from the three-steps purified rice straw cellulose. These holoCNCs had similar dimensions (4.1 ± 1.6 nm thick, 6.4 ± 1.9 nm wide, and 113 ± 70 nm long) as CNCs (4.7 ± 1.3 nm thick, 6.4 ± 1.2 nm wide, and 143 ± 31 nm long) as well as 137 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


ribbon-like short crystallites (Figure 2 and 3 a, b, d, e). Besides the most commonly reported sulfuric acid hydrolysis, rice straw cellulose was also hydrolyzed using a solid acid catalyst synthesized by lignin-based activated carbon fibers to a 8.1 % yield of cellulose nanofibrils (2.1 ± 1.0 nm thick, 3.1 ± 0.7 nm wide and up to 1 μm long) (Figure 2 and 3 c and f) with 69 % yield of glucose in three cycles (31). The much longer and flexible cellulose nanofibrils obtained from solid acid hydrolysis as compared to the short ribbon-like rigid cellulose nanocrystals from liquid mineral acid hydrolysis is attributed to its weaker acid characteristics and action by surface hydrolysis rather than by diffusion of the small but strong mineral acids. In fact, these cellulose nanofibrils from solid acid hydrolysis are similar in dimension as those from TEMPO mediated oxidation described later, but differ in type and extent of surface charges. In all cases for sulfuric acid hydrolysis of rice straw cellulose, the yields are less than 17 % of rice straw cellulose (or 9 % of the rice straw). The CNC yields from rice straw is similar to those from softwood pulp (17.2 % at 65 % H2SO4, 45 °C, 30 min) (39), but inferior to microcrystalline cellulose (30 % at 63.5 wt% H2SO4, 44 °C, 130 min) (22), possibly due to lesser crystalline structure of rice straw. Cellulose is, however, much more easily liberated from rice straw, negative or low cost crop residues, thus offering CNCs as significant high value-added coproducts. Furthermore, while nanocellulose yield was low in the case of solid acid hydrolysis, the process produces CNFs with dimensions that rival most CNFs and, most intriguingly, the overall value-added conversion was potentially 100% due to the exclusive co-production of glucose and the reusable solid acid catalysts. TEMPO-Mediated Oxidation TEMPO mediated oxidation is a relative mild reaction that selectively oxidizes the primary hydroxy groups on crystalline surfaces and inter-fibril cellulose segments to carboxylate groups, without breaking the cellulose chains at low oxidant dosage (40). This is in contrast to the strong acid hydrolysis that causes chain scission of cellulose in the amorphous regions and removes the hydrolyzed sugars and fragments, leaving the acid-resistant and highly crystalline domains. We have shown that TEMPO oxidation (5 mmol/g NaClO/cellulose) alone can convert 19.7 % of rice straw cellulose (or 7.2 % of rice straw) to 1.7 nm wide and hundreds nm to micrometer long cellulose nanofibrils (CNFs) (29). TEMPO oxidation (5 mmol/g NaClO/cellulose) followed by mechanical blending (37, 000 rpm, 30 min) maximized the CNF yield to 96.8 % (or 35.2 % of rice straw) as well as generated finer and more uniform CNFs (1.5 ± 0.5 nm thick, 2.1 ± 0.4 nm wide, and up to 1 μm long) (Figure 2 and 3 g, j) (32). The same coupled TEMPO oxidation and blending of rice straw holocellulose converted 92.7 % of holocellulose into hocellulose nanofibrils (HCNFs) or 33.7 % of rice straw, slightly lower than that of CNFs (26). HCNFs are similarly thin and long, but slightly wider (1.4 ± 0.6 nm thick, 2.9 ± 0.8 nm wide and up to several micrometers long) than CNF from more purified rice straw cellulose (Figure 2 and 3 h, k). HCNFs also appeared to be more heterogeneous than the CNFs and contained some spherical nanoparticles that were thought to be silica. Neither HCNF yield nor quality is higher than CNF from the same coupled TEMPO 138 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


oxidation and blending process as in comparing HCNC to CNC from H2SO4 hydrolysis. This is attributed to the mild TEMPO reaction conditions compared to sulfuric acid hydrolysis and the presence of impurities in holocellulose that consume the oxidizing reagent.

Figure 3. TEM images and width distribution of nanocellulose derived from rice straw; a, d). CNC (30) (a. Reprinted with permission from ref (30). Copyright 2015 American Chemical Society); b,e). HoloCNC (30); c,f). CNF from solid acid hydrolysis (31) (f. Reproduced with permission from ref (31). Copyright 2015 American Chemical Society); g,j) CNF from TEMPO oxidation (32) (j. Reproduced from ref (32) with permission from ref (32). Copyright 2013 Royal Society of Chemistry); h,k) HoloCNF from TEMPO oxidation (26) (Reprinted with permission from ref (26). Copyright 2015 American Chemical Society); i,l) CNF from ACC treatment (33). (l. Reprinted with permission from ref (33). Copyright 2016 American Chemical Society).



Mechanical Defibrillation Chemical approaches of deriving nanocelluloses inevitably change their surface chemistries, e.g., replacing the hydroxyls with sulfate groups on CNCs and the C6 primary hydroxyls with carboxylate groups on CNFs. Therefore, to produce nanocelluloses without affecting surface chemistry, the strong interfibrillar hydrogen bondings and possibly, the weaker van der Waals interaction must be broken by non-chemical means. Mechanical grinding of rice straw cellulose at 1,500 rpm was reported to generate 12–35 nm wide microfibrils while yield was not reported (23). Combined ultrasonication (1000 W, 5 min) and high-pressure homogenization (20 min) has been reported to produce several tens of nm wide large bundles of CNFs, but without a yield value (34). We have employed high speed blending (37, 000 rpm for up to 2 h) to collect 12 % CNFs from rice straw cellulose (4.4 % of original rice straw), in bimodally distributed short (2.7 ± 1.2 nm wide and 100–200 nm long) and long (8.5 ± 4.1 nm wide and several micrometers long) CNFs (29). A more intensive and scalable aqueous counter collision (ACC) method that utilizes high-speed collision of two aqueous jets of cellulose suspensions documented on microbial cellulose (41) and microcrystalline cellulose (42) was applied (180 MPa, 30 passes, 15 kWh/kg energy consumption) to fully defibrillate rice straw cellulose into CNFs of varying sizes (33). By differential centrifugation with increasing centrifugal forces, ACC defibrillated rice straw nanocellulose could be separated into four fractions by the significantly different sedimentation coefficient differentiated by their widths, i.e., 6.9 % in 80–200 nm, 14.4 % in 20–80 nm, 20.3 % in 5–20 nm, and 58.4 % in less than 5 nm lateral dimensions. Statistical analysis of the narrowest group, i.e., the

Rice Straw Nanocelluloses: Process-Linked Structures, Properties

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