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Recent Strategies in Preparation of Cellulose Nanocrystals and Cellulose Nanofibrils Derived from Raw Cellulose Materials.

1. Introduction

Cellulose is the most abundant natural polymer and can be sustainably produced from various biomass. The global annual output of cellulose is approximately 75 to 100 billion tonnes [1]. The development and utilization of cellulose is of great significance to the sustainable development of human society, especially the energy scarcity in the future. Nanocelluloses (NCs) could be produced by the degradation of cellulose from various sources of biomass and have many unique properties, such as high specific surface area, high aspect ratio, high tensile strength and stiffness, low density, and low thermal expansion coefficient [2]. Recently, the application of NCs has attracted great attention [2-5]. NCs include cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), bacterial cellulose (BC), and electrospun cellulose nanofibers (ECNFs). CNCs and CNFs are more common than the other NCs. This is because CNCs and CNFs are gained by disintegration of raw cellulose materials into nanosize particles (top-down process), while BC and ECNFs are produced by a build-up of nanofibers from low molecular weight sugars by bacteria and electrospinning, respectively. This review will focus on CNCs and CNFs, while BC and ECNFs will not be covered. There is a significant difference between CNCs and CNFs in terms of dimension and crystallinity. CNCs are highly crystalline and usually have a length less than 500 nm, while CNFs are made up of amorphous and crystalline cellulose chains and have a length of up to several microns. Both of them have immense potential in the manufacture of renewable and biodegradable materials [6, 7]. They have been applied in the fields of biomedical engineering, food, sensor, packaging, optical and electronic devices, and so on [8-10]. Previous reviews have summarized their source, chemistry, and applications [2, 11-25]. The present review mainly focuses on the advances in preparation of CNCs and CNFs in order to display a clear development direction for the preparation of CNCs and CNFs.

2. Preparation of Cellulose Nanocrystals

CNCs have high crystallinity with diameter less than 100 nm and length less than 500 nm, which are formed though intra-and intermolecular interaction of cellulose macromolecules in a hydrogen bond way. CNCs are usually obtained through acid hydrolysis or enzymatic hydrolysis of cellulose pulp. The acid hydrolysis processes need to go through very harsh reaction conditions which usually require concentrated acid, while the enzymatic hydrolysis process requires really long time. These are mainly determined by the inherent stable structure of the materials that have been explained in a related review [5]. During the hydrolysis process, the amorphous regions of cellulose are more easily invested by acid compared to the crystalline regions, leading to first degradation of amorphous regions, while the crystalline regions are retained. Finally, whisker-like CNCs are obtained.

In retrospect, the first successful preparation of CNCs was in 1947 through hydrolyzing cellulose with hydrochloric acid and sulfuric acid by Nickerson and Habrle [26]. In 1951, Ranby prepared the stable CNCs colloidal suspensions though sulfuric acid hydrolysis of wood fiber [27]. In 1953, Mukherjee and Woods proved that the acicular particles obtained by sulfuric acid hydrolysis possessed nanometer size by X-ray technology and electron microscope and found that their crystal structure was the same as that of raw cellulose material [28]. Subsequently, Marchessault et al. found that the CNCs colloid suspension had birefringence [29]. After that, some new methods to prepare CNCs appeared gradually, such as enzymatic hydrolysis, oxidative degradation, ionic liquid, solid acid hydrolysis, organic acid hydrolysis, and subcritical hydrolysis, as shown Table 1. These will be described in the following sections.

2.1. Mineral Acid Hydrolysis. The mineral acid hydrolysis is the most common method for the preparation of CNCs. Various cellulose sources have been used for the preparation of CNCs by mineral acid hydrolysis [30-55], such as tomato peel [30], oil palm biomass [31], rice husk [33], and waste cotton cloth [34]. The typical mechanism is that the hydrogen ions from acid can easily invade the loose amorphous regions of cellulose to break the 1,4-[beta]-glycoside bonds, resulting in the hydrolysis of amorphous regions, while the crystalline region of cellulose could be retained in the process which is attributed to the inherent compact structure that prevented the permeation of the acid. Therefore, the relatively complete crystalline structure of CNCs can be obtained by the hydrolysis of mineral acid. The principle diagram of preparation is shown in Figure 1 [5].

The typically used mineral acids are sulfuric acid [30-33, 37, 38, 40-43, 56, 57], hydrochloric acid [58, 59], phosphoric acid [35, 60], hydrobromic acid [61], and their mixed acids [34, 36]. Sulfuric acid is the most typically used as it produces a negative surface charge on the particles which leads to more stable suspension. In general, the hydrolysis process needs the sulfuric acid concentration to be 60-65%, reaction temperature to be 40-50[degrees]C, and reaction time to be 30-60 min. However, the yield of CNCs is very low (less than 30 wt.%) due to the excessive degradation. Fan and Li optimized the sulfuric acid hydrolysis conditions with cotton pulp as raw material, and the yield of CNCs reached to 63.8% [62]. Chen et al. found that the yield of CNCs could be significantly improved by decreasing the concentration of sulfuric acid and prolonging reaction time. For example, the yield of CNCs could reach to 75.6% when 58 wt.% sulfuric acid was used at 56[degrees]C for 210 min [56]. CNCs from sulfuric acid hydrolysis have poor thermal stability due to the sulfate group which is a significant barrier for thermal process in composites [63]. The thermal stability of CNCs can be improved through neutralization with NaOH [58, 64, 65]. The morphology of CNCs can be also modified by controlling the reaction conditions. The spherical CNCs with average dimension of 35 nm could be obtained by using 63.5% [H.sub.2]S[O.sub.4] to hydrolyze microcrystal cellulose under the ultrasonic treatment combined with mechanical stirring for 3 h [66]. Azrina et al. also reported a similar condition for the preparation of spherical CNCs. This reaction was carried out with 64% (w/v) [H.sub.2]S[O.sub.4] solution at 45[degrees]C for 2 h in an ultrasound bath. In this reaction, the spherical CNCs with an average diameter of 30-40 nm were extracted from oil palm empty fruit bunch pulp [67].

Hydrochloric acid also was commonly used for the preparation of CNCs. The typical acid concentration, reaction temperature, and reaction time are 2.5N-6.0N, reflux temperature, and 2-4 h, respectively [58, 59]. Due to the lack of charge on the surface of CNCs, the CNCs prepared by hydrochloric acid are easily flocculation in water. But the thermal stability of CNCs by HCl is higher than that by [H.sub.2]S[O.sub.4] (see Figure 2). Yu et al. used hydrochloric acid to treat raw cellulose materials under hydrothermal conditions. The crystallinity of the resultant CNCs was 88.6% with high yield of 93.7% [58]. The maximum degradation temperature was 363.9[degrees]C which was determined by TGA analysis. Cheng et al. reported an inorganic chloride/hydrochloric acid system for extracting CNCs from microcrystalline cellulose (MCC) under hydrothermal conditions [68]. Four inorganic chlorides including Fe[Cl.sub.3]*6[H.sub.2]O, Cu[Cl.sub.2]*2[H.sub.2]O, Al[Cl.sub.3], and Mn[Cl.sub.2]*4[H.sub.2]O were investigated in this work. The research showed that the hydrolysis process was promoted significantly by inorganic chlorides especially ferric chloride via accelerating degradation of the amorphous region of cellulose.

Phosphoric acid, which belongs to middle strong acid (pKa = 2.12), could be used for the manufacture of CNCs with high thermal stability and stable suspension [69] (see Figure 2). Lu et al. reported a mechanochemical approach to manufacture bamboo cellulose nanocrystals (BCNCs) [70]. In this process, the bamboo cellulose pulp was chemically treated by phosphoric acid. Finally, short rod-like BCNCs with the length of 100-200 nm and the width of 15-30 nm were obtained. The crystalline structure transformed from cellulose I to cellulose II, and the crystallinity index declined from 66.44 to 59.62%, while the thermal stability became lower.

Hydrobromic acid (HBr) hydrolysis conditions assisted with ultrasonication to prepare CNCs were investigated by Sadeghifar et al. [61]. When the hydrolysis process was carried out at 2.5 M HBr under 100[degrees]C with sonication for 3 h, CNCs could be obtained at a yield of 60%. Moreover, Sucaldito and Camacho used HBr to hydrolyze fresh water green algae to achieve the highly crystalline CNCs (crystallinity index of 94.0%) [71]. The resultant CNCs have an average diameter of 20.0 ([+ or -] 4.4) nm, and the decomposition temperature reached 381.6[degrees]C.

Spherical CNCs could be also prepared by using a mixed acid system assisted with ultrasonication from MCC [64,72-74]. The mixed acid system is a mixture of sulfuric acid (98%, w/w), hydrochloride (37%, w/w), and [H.sub.2]O at a ratio of 3: 1: 6 (v/v) which was reported by Zhang et al. [72]. The process was carried out at 80[degrees]C by ultrasonicator heating for 8 h. The diameters of produced cellulose nanoparticles are in the range of 60-570 nm. The hydrolysis condition offered by Wang et al. was that the ratio of sulfuric acid (98%, w/w), hydrochloride (37%, w/w), and [H.sub.2]O was still 3:1:6 (v/v), while the ultrasonic treatment was at 50 Hz for 10 h [73]. The resultant spherical CNCs suspension with the dimension of 10-180 nm showed liquid crystalline property when the concentration was above 3.9%. A mixed acid system of hydrochloric acid and nitric acid (HCl/H[NO.sub.3]) could be used to prepare carboxylated cellulose nanocrystals (CCNCs) from MCC [75]. When the reaction was carried out under the mixed acid (HCl/H[NO.sub.3] = 7:3 (v/v)) concentration being 4M at 110[degrees]C for 3h, the resultant CCNCs were rod-like morphology (176 [+ or -] 6 nm length and 11 [+ or -] 2 nm width) and had excellent suspension stability and good thermal stability.

Overall, mineral acid hydrolysis method to produce CNCs is a simple method and has gone to pilot scale (quite a few around the world) and even demonstration plant (1 tonne/day at CelluForce, Canada). However, there are still some disadvantages in this method. First, the corrosion problem of the equipment is serious which leads to the insufferable cost of production. Furthermore, a large amount of waste acid and other pollutants will be produced in the process of acid hydrolysis, and it is very difficult to dispose and recover. So it is very necessary to develop green and low-cost CNCs production technologies [13].

2.2. Solid Acid Hydrolysis. Tang et al. reported a cation exchange resin hydrolysis method for the preparation of CNCs [76]. By hydrolyzing MCC with cation exchange resin (NKC-9), an optimum yield (50.04%) was achieved with a ratio of resin to MCC (w/w) being 10 and at 48[degrees]C for 189 min. The results showed that the diameter of CNCs was about 10-40 nm and the length was 100-400 nm. The crystallinity of resin-CNCs was higher than that of [H.sub.2]S[O.sub.4]-CNCs. In addition to the recoverable ability of cation exchange resin, the cation exchange resin-catalyzed hydrolysis is a promising approach for manufacturing of CNCs. Liu and coworkers reported the phosphotungstic acid (HPW) catalyzed-hydrolysis method with bleached hardwood pulp as raw material. CNCs with the diameter of 15-40 nm were given, which had high thermal stability and stable dispersion in aqueous phase [77]. The process is shown in Figure 3. The reaction was carried out with 75 wt.% phosphotungstic acid at 90[degrees]C for 30 h; the yield of CNCs was up to 60%. Phosphotungstic acid could be recovered through extraction with available ethyl ether. Because this method is a solid/liquid/solid three-phase reaction, the reaction efficiency is low and the reaction time is relatively long compared with the mineral acid hydrolysis method. Further research results showed that the reaction time was significantly shortened under the optimal conditions, and the obtained CNCs had a high crystallinity index (79.6%) with a 84% yield [78]. Torlopov et al. also reported a CNCs preparation process using phosphotungstic acid/acetic acid system to destruct cotton cellulose [79]. The resulting CNCs have high-crystalline structure and rod-like morphology.

Compared with the classic mineral acid hydrolysis method, solid acid hydrolysis method has mild condition and low corrosion to equipment, and the solid acid can be recycled. Moreover, the yield of CNCs is higher. However, the reaction efficiency is low and the reaction time is pretty long. Appropriate physical approach (such as microwave and ultrasound assisted) or suitable catalyst could be used to improve the reaction efficiency. At present, solid acid hydrolysis method to prepare CNCs is still in the stage of laboratory research.

2.3. Organic Acid Hydrolysis. Recently, organic acid hydrolysis was presented to produce NCs due to the fact that the organic acid is mild, recyclable, and environment-friendly and of low corrosiveness. However, in order to improve the efficiency of hydrolysis, higher temperature and longer reaction time are necessary because of the weak acidity of organic acid. Li and coworkers reported a two-step strategy to produce CNCs from bleached chemical pulp under mild conditions [80]. In the first stage, formic acid (FA) was used to hydrolyze the amorphous region of cellulose and release CNCs. In the second stage, the generated CNCs were further oxidized by TEMPO in order to increase the surface charge. The results showed that the CNCs modified by TEMPO have much higher crystalline and much more surface charge. In order to further increase the hydrolysis efficiency, Du et al. introduced the Fe[Cl.sub.3] to the FA system [63]. Figure 4 shows the process flow and the CNCs were obtained from bleached eucalyptus kraft pulp with a high yield (75%). The results showed that the crystallinity index of resultant CNCs could reach 75% when 0.015 M Fe[Cl.sub.3] was used. Moreover, up to 90% FA can be recovered by rotary evaporation, Fe[Cl.sub.3] could be recovered in the form of Fe(OH)3 precipitation generated by ammonia treatment, and the generated ammonium salt can be used as fertilizer. Thus, the Fe[Cl.sub.3]-catalyzed FA hydrolysis process was more sustainable than the traditional sulfuric acid hydrolysis process.

Then, dicarboxylic acid was used for CNCs production proposed by Chen et al. [91]. Similarly, CNCs usually were obtained at a low yield due to the low reactive activity of dicarboxylic acid. Thus, most of the hydrolyzed products are the insufficiently hydrolyzed fibers which can be used for preparing CNFs through subsequent mechanical disintegration. The advantages of using dicarboxylic acid for the preparation of NCs are that (1) carboxyl groups are introduced on the surface of CNCs and CNFs, as ester group could have been formed by the reaction of dicarboxylic acids with cellulose [92]; (2) solid dicarboxylic acid (e.g., oxalate and maleic acid) is of low corrosion to equipment and could be fully recovered by using crystallization technology; (3) the resultant CNCs and CNFs are thermally stable which is beneficial for the manufacture of composite. Li and coworkers used molten oxalate dihydrate to treat cellulose fibrils which is a free solvent for the preparation of CNCs [93]. In this process, the hydrolysis and esterification of cellulose could be carried out in one pot. CNCs could be gained in a high yield (80.6 wt.%). Xu and coworkers successfully prepared the CNCs with a mixed acid composed of hydrochloric acid and oxalate. The optimal condition is that the concentration of oxalate is 0.11 M with 10% HCl at 95[degrees]C for 4 h. Results showed that CNCs could be obtained with a high yield of 81.5% [94].

Maleic acid was also used for the preparation of CNCs and the pretreatment of cellulose [95-98]. Wang and coworkers reported the preparation of CNCs using maleic acid to hydrolyze cellulose [95]. CNCs were obtained at a low yield, about 5%. The fibrous cellulosic solid residue could be converted to CNFs through the following mechanical disintegration. Bian et al. used concentrated maleic acid to hydrolyze unbleached mixed hardwood kraft pulp. The results were that the lignin-contain CNCs and CNFs were successfully prepared [97,98]. Bian and coworkers also reported a maleic acid catalyzed-hydrolysis reaction for the preparation of CNCs and CNFs. The bleached eucalyptus pulp and unbleached mixed hardwood kraft pulp were used as the raw materials, and typical operating conditions were acid concentrations of 50-70 wt.% at 100-120[degrees]C for 1-2 h. The results showed that CNCs with the length of approximately 239-336 nm were obtained, and CNFs could be gained by the further mechanical fibrillation of fibrous cellulosic solid residue [99]. Yu et al. reported an approach for extracting carboxylated CNCs by citric acid/hydrochloric hydrolysis of MCC [100]. In this work, rod-like CNCs with the length of 200-250 nm and the width of 15-20 nm were obtained. The CNCs extracted for 4 h had the highest carboxylic group content and high zeta potential value (up to -46.63 mV). By applying organic acid hydrolysis, some interesting functional groups could be introduced to the surface of CNCs through the ester bond between organic acid and CNCs in the hydrolysis process. For example, Boujemaoui et al. used 2-bromopropionic acid, 3-mercaptopropionic acid, 4-pentenoic acid, and 2-propynoic acid, respectively, to catalyze the hydrolysis of raw cellulose materials. Some active functional groups such as Br-, HS-, alkenyl, and alkynyl were grafted on the surface of CNCs correspondingly, which could be beneficial to further modify CNCs despite the very long reaction time (10 days) [101]. Overall, organic acid hydrolysis is more sustainable and environment-friendly than mineral acid hydrolysis for CNCs production. It is believed that the industrialization of CNCs by using organic acid hydrolysis will be realized in recent years.

2.4. Enzymatic Hydrolysis. Cellulase is a multicomponent enzyme system. Its active components include endoglucanases (EG), cellobiohydrolases (CBH), and [beta]-glucosidase (GB). The action mechanism of each component on cellulose is shown in Figure 5 [14, 81]. EG mainly acts on the amorphous region of cellulose to randomly hydrolyze the [beta]-1,4-glycosidic bond, so the long chain cellulose molecules are truncated into small molecular cellulose in the interior of cellulose; CBH mainly acts on the end of linear cellulose molecular to destroy the crystalline region of cellulose; GB is used to hydrolyze cellulose into glucose [81,102]. When CNCs are prepared by enzymatic hydrolysis, the destruction of the crystalline area of cellulose by CBH should be avoided as much as possible. Therefore, three components of cellulase should be separated from each other. EG is used to hydrolyze cellulose amorphous regions, and much more crystalline regions are retained in the process. Filson and coworkers successfully prepared CNCs using endoglucanase to hydrolyze recycled pulp [103]. They found that when the endoglucanase was increased to 420 EGU per 1 g pulp and the reaction temperature was at 50[degrees]C with microwave heating for 60 min, the CNCs with the diameter of 30-80 nm and the length of 100 nm-1.8 [micro]m could be obtained, the yield was up to 38.2%, the average zeta potential was -31.37 mV, and birefringence could be observed for the CNCs dispersion. Chen et al. prepared a spherical nanosized cellulose by the enzymatic hydrolysis of pulp fibers [104]. This research showed that enzymatic hydrolysis of pulp could give spherical CNCs with the diameter of ca. 30 nm, which had excellent monodispersity and uniformity. When the concentration of complex enzymes was 20 u/mL (cellulase : xylanase = 9:1), the yield of CNCs reached to 13.6%. However, when the single cellulase was used, even if the enzyme concentration reached up to 200 u/mL, only a mixture of strip and granular flocculation was obtained. Meyabadi et al. also reported the enzymatic hydrolysis method to produce the spherical CNCs with waste cotton as raw material combining with sonication [105]. The particle sizes of gained CNCs were less than 100 nm.

Overall, compared with the mineral acid hydrolysis, the enzymatic hydrolysis process for the preparation of CNCs is environment-friendly, while the experimental conditions are harsh, the yield of CNCs is low, and the reaction time is long. Enzymatic hydrolysis of raw cellulose materials is promising for the industrial production of CNCs, so further research of efficient enzymatic hydrolysis is imperative and urgent.

2.5. Oxidation Degradation. The hydroxyl groups on cellulose have high reactivity. They are easily oxidized by strong oxidants to aldehydes, ketones, and carboxyl groups so that their structures are destructed and the degree of polymerization is decreased. Therefore, some scientists have successfully prepared CNCs by use of the oxidation degradation method. Yang et al. adopted two-step oxidation method to prepare CNCs: softwood pulp was firstly oxidized by sodium periodate and then further oxidized by sodium chlorite, and finally two-step centrifugation was implemented to give CNCs (diameter/length, 13 nm/120-200 nm) and microfibrils (diameter/length, 120 nm/0.6-1.8 [micro]m) [106]. The resultant microfibrils could be further translated to CNFs by mechanical agitation. The separated CNCs with carboxyl groups on the surface had a high crystallinity (91%) and high charge density which could form a stable suspension in aqueous phase.

Ammonium persulfate (APS) also could be used to prepare carboxyl CNCs [57,107,108]. Hu et al.'s research results showed that the spherical shape particles with diameters of 20-70 nm were obtained by this method, and the crystallinity of CNCs was significantly improved after a series of chemical treatments [107]. The research results of Mascheroni and coworkers showed that CNCs produced through the APS treatment displayed higher charge densities, crystallinity, clarity of the solution and transparency of the coating compared to the CNCs produced by the [H.sub.2]S[O.sub.4] hydrolysis [108]. Leung and coworkers used the APS to oxidize the flax fibers to produce the surface-carboxylated CNCs [109]. The CNCs were needle-like with diameter of 3.8 nm and length of 150 nm, and the particle size distribution was uniform. In this method, the raw materials (such as hemp, flax, straw etc.) could be used directly for the preparation of CNCs without pretreatment process for the delignification. The reason is that APS has the delignin function in addition to oxidative degradation. Cheng and coworkers prepared the CNCs with 1 mol/L APS aqueous solution to oxidize lyocell fiber at 80[degrees]C for 16 h. The prepared CNCs were spherical with the diameter of 35 nm and a great deal of carboxyl groups were introduced on the surface of CNCs. The crystal type of resultant CNCs was cellulose II [110].

TEMPO was also used as oxidation to prepare carboxyl CNCs [111-113]. However, only subfraction of raw cellulose materials was oxidized to CNCs by using TEMPO; the rest of oxidized cellulose had much larger dimensions. It indicated that TEMPO was not as effective as APS on cellulose oxidation. Moreover, CNCs from TEMPO oxidation are highly carboxylated [114, 115], which provides the opportunity for functionalization [116,117].

Overall, compared with the mineral acid hydrolysis and enzymatic hydrolysis, the yield of CNCs was higher by the oxidation degradation method. However, in the process of reaction, a large number of oxidants were consumed, and the reaction time was longer. Also, a large amount of water and energy need to be consumed which leads to the fact that the cost of CNCs production was very high.

2.6. Ionic Liquid Method. Ionic liquid is a kind of organic salt solution with low melting point (less than 100[degrees]C) composed of organic cations and other anions. Compared to the traditional solvent, ionic liquid has advantages in terms of chemical stability, thermal stability, nonflammability, and low vapor pressure. It is also called "green solvent" [118]. In recent years, due to the unique properties of ionic liquid, it has been widely used to dissolve and separate lignocellulose and to prepare CNCs. Man and coworkers treated MCC with 1-butyl-3-methylimidazolium bisulfate ([BMIM]HS[O.sub.4]) to give CNCs with the diameter of 14-22 nm and the length of 50-300 nm. It was found that, compared with the original MCC, the CNCs remained cellulose I, while the thermal stability was significantly decreased [119]. Mao and coworkers treated MCC with [BMIM]HSO4 aqueous solution at 120[degrees]C for 24 h to give the CNCs at 48% yield [120]. Later, they hydrolyzed softwood, hardwood, and MCC by the optimizing two-stage method with [BMIM]HSO4 aqueous, and the yield of CNCs was 57.7%, 57%, and 75.6%, respectively, which was close to the theoretical value [121]. Tan et al. used [BMIM]HSO4 as both catalyst and solvent to prepare CNCs from MCC. Highly crystallized CNCs with diameter of 15-20 nm and length of 70-80 nm were obtained at 90[degrees]C [122]. Lazko and coworkers reported another method which is that cotton fibers were infiltrated firstly by [BMIM]Cl at 80[degrees] C, and 1-4 wt.% sulfuric acid was added subsequently. Then the reaction was kept for 2-16 h, and the CNCs were successfully prepared with the diameter of about 20 nm and the length of 150-350 nm. The analysis results showed that the CNCs prepared by [BMIM]Cl had less sulfonic group content on the surface than that of [BMIM]HS[O.sub.4], and their thermal stability was obviously increased [123]. Iskak et al. used [BMIM]Cl as ionic liquid to prepare CNCs [124]. In this work, they investigated the effects of the reaction time and temperature on the yield, size, and crystallinity index of CNCs. The results showed that CNCs with the particle size of 9 nm and the crystallinity index of 73% were obtained when the reaction time was 30 min. Meanwhile, when reaction temperature was 100[degrees]C, CNCs were obtained with a yield of 90% and crystallinity index of 76%. Abushammala and coworkers directly treated wood with 1-ethyl-3-methyl imidazole acetate ([EMIM][OAc]), and the surface acetylated CNCs were successfully prepared in a 44% yield. The type of resultant CNCs was cellulose I, and the crystallinity was 75%. The research showed that [EMIM][OAc] had three main functions in the process: (1) dissolving cellulose in situ when cotton fibers were infiltrated, (2) reducing intermolecular cohesion in wood by acetylation; (3) catalyzing the hydrolysis reaction of cellulose [125]. Miao and coworkers used tetrabutylammonium acetate/dimethylacetamide (TBAA/DMAc) system with acetic anhydride to treat hardwood pulp to gain the hydrophobic CNCs successfully. The reaction diagram is shown in Figure 6 [82]. The classical method of preparing hydrophobic CNCs included three steps: firstly, CNCs were prepared by sulfuric acid hydrolysis; secondly, CNCs were dispersed in an organic solvent by solvent exchange; finally, the CNCs were hydrophobically modified in organic solvent. Compared with traditional method, TBAA/DMAc and acetic anhydride system can be used to prepare hydrophobic CNCs in one pot. The process is simple, and the prepared CNCs with high thermal stability have acetyl group on the surface.

Overall, ionic liquid shows a great potential to prepare CNCs due to its reusability and the superiority for preparing functional CNCs. However, the main disadvantage is that ionic liquid is very expensive at present and has some toxicity. So it is still necessary for researchers to develop new, efficient, cheap, and safe ionic liquids.

2.7. Other Methods. Some new methods for the preparation of CNCs have been developed in recent years. Chen et al. used Cr[([NO.sub.3]).sub.3] to catalyze the hydrolysis of native cellulosic feedstock [126]. The CNCs could be obtained with high yield (83.6% [+ or -] 0.6%) when solid-liquid ratio was 1 : 30 and 0.8 M Cr[([NO.sub.3]).sub.3] was adopted at 80[degrees]C for 1.5 h. The produced CNCs showed a network-like long fibrous structure with the aspect ratio of 15.7. The crystallinity index was 86.5% [+ or -] 0.3% and the maximum thermal decomposition temperature was 344[degrees]C. The hydrolysis system of Cr[([NO.sub.3]).sub.3]- [H.sub.2]S[O.sub.4] was also investigated by Chen and coworkers [127]. In this work, the maximum yield and highest crystallinity of CNCs were obtained under hydrolysis conditions of 0.22 M Cr[([NO.sub.3]).sub.3], 0.80 M [H.sub.2]S[O.sub.4], and 82.2[degrees]C within 1 h. Other transition metals were examined as well for catalyzing the hydrolysis of raw cellulose materials to prepare CNCs, such as Fe(III), Al(III), Ni(II), Co(II), and Mn(II)[68,128,129], and they will not be elaborated in detail.

Novo and coworkers used subcritical water to hydrolyze cellulose for the preparation of CNCs [83, 130]. The subcritical water (120[degrees]C and 20.3 MPa for 60 min) possessed higher diffusion, activity, and ionization than common water. With that, cellulose materials could be hydrolyzed partly to provide CNCs at 21.9% yield [83]. The gained CNCs exhibited a high crystallinity index (79.0% determined by XRD), rod-like shape, and good thermal stability. The schematic diagram of the device is shown in Figure 7 [83]. During the experiment, the pressure inside the reactor could be decreased by opening the restrictor valve (due to the loss of a small volume of liquid) or be increased by injecting water with a precision pump. The cost of this method was compared with conventional one. The conventional production stage would produce CNCs with a cost of 1.54 $/kg, whereas the cost of preparing CNCs by the proposed subcritical treatment would be 0.02 $/kg. Later, Novo and coworkers systemically studied the effects of subcritical hydrolysis on the yield, crystallinity, hydraulic radius, and color of CNCs [130]. It was found that pressure played an important role in the yield of CNCs, and the temperature had a great influence on the color of CNCs. With the increase of temperature, the stability of CNCs aqueous suspensions was enhanced gradually but still far below the stability of CNCs suspension prepared by sulfuric acid hydrolysis. The main reason is that the surface charge density of CNCs prepared by this method is relatively low. The subcritical water method is very promising for industrial production of CNCs. Although the process requires a reactor that supports high pressures and high consumption of energy, it results in a greener method in terms of water and chemicals consumption reduction, lower effluent generation, and treatability of wastes.

Sirvio and coworkers reported that the deep eutectic solvent (DES) formed by choline chloride and oxalic acid could hydrolyze the wood fiber fine fibers into microfibers which were microfluidized sequentially to give the CNCs with the diameter of 9-17 nm and the length of 310-410 nm, which had high thermal stability and good dispersion stability in aqueous phase [131]. Compared with sulfuric acid hydrolysis, the preparation of CNCs with DES method has high yield (68-78%) and DES is recoverable, biodegradable, and environmentally friendly. However, the reaction degree is low, so it needs follow-up mechanical treatment to get the CNCs products. At present, the DES method to produce CNCs is in the laboratory study stage.

Nelson et al. reported that American Value Added Pulping technology (AVAP) developed by American Process Inc. (API) could produce NCs from different kinds of lignocellulosic materials. This technology provides a low-cost way to prepare NCs. It can prepare hydrophilic/hydrophobic CNCs and CNFs by controlling the content of lignin [84]. The process flow chart of nanoscale cellulose prepared by AVAP is shown in Figure 8. In this process, sulfur dioxide (S[O.sub.2]) and ethanol were used as pretreatment agents to remove lignin, hemicellulose, and the amorphous regions of cellulose from biomass. S[O.sub.2] was used as delignifying agent, and ethanol was used as a solvent which could dissolve resins and extractives, assist S[O.sub.2] penetrating easily into the wood, and reduce degradation of crystalline cellulose. The final morphology of NCs was modified by controlling temperature and the time of the pretreatment step. Nelson and Retsina used eucalyptus wood as raw material to produce the CNCs and CNFs using AVAP technology; the resultant CNCs with the diameter of 5-200 nm and the length of 50-500 nm had a high crystallinity of 93% [132]. In addition, AVAP method did not cause swelling and destruction to the crystalline region of cellulose. AVAP method can produce hydrophobicity and morphology adjustable CNCs, which greatly increases the potential application of CNCs in various fields. In April 2015, API started up a nanocellulose production line for production of the full range of AVAP nanocellulose products in Thomaston Georgia for sale under the BioPlus[TM] trade name.

3. Production of Cellulose Nanofibrils

Cellulose nanofiber [132], nanofibrillar cellulose [133, 134], and cellulose nanofibril [135] are the terms used for nanofibrillated cellulose [136]. The preparation of CNFs needs strong mechanical machining. However, according to the properties of raw materials and degree of processing, chemical pretreatments are carried out before mechanical machining [137]. A fact is that appropriate pretreatments of cellulosic fibers can increase the inner surface, alter crystallinity, and break hydrogen bonds of cellulose [138].

3.1. Mechanical Method. Mechanical approaches to prepare CNFs include homogenizing [139-144], grinding [145], twin-screw extrusion [146], cryocrushing [147], and high-intensity ultrasonication [148-151]. Among them, homogenizing and grinding are the most commonly used for the preparation of CNFs and the most effective for the scaling production of CNFs at present. At the same time, other methods have been also developed in order to find a more favorable solution for the industrial production of CNFs.

3.1.1. High Pressure Homogenization. Homogenizers and microfluidizers are typical equipment for the preparation of CNFs. They can disintegrate cellulose fibers into CNFs by high pressure homogenization. The process is that cellulose pulp is passed through a minute gap between an impacting and the homogenizing valve (see Figure 9 [14]) under high pressure and high velocity conditions, making the fibers sheared and to ensure cellulose fibrillation. Turbak et al. and Herrick et al. used high pressure homogenizer to treat 2 wt.% wood pulp for the first time in 1983 to produce CNFs with a diameter of less than 100nm [152, 153]. And then, homogenizers were widely used for the preparation of CNFs [139,154-158]. The main disadvantages include the following: (1) the high energy consumption for homogenization process could reach up to 70 MW h/t [159]; (2) the clogging of the system occurred frequently when using long fibers as raw materials [160]. However, when some pretreatment techniques (e.g., enzymatic, oxidation, and carboxylation) [87, 161, 162] were adopted, this energy consumption value was reduced significantly to 2 MW h/t, and the clogging problem could be solved as well [163].

3.1.2. Grinding Method. Ultrafine friction grinding is another common technique for the preparation of CNFs. The commonly used equipment is Supermasscolloider grinder (see Figure 9). Taniguchi and Okamura used Masuko's colloider grinder for the first time to grind 5-10 wt.% cellulose materials for 10 times and successfully prepared CNFs with diameter of 20-90 nm [164]. Since then, this method has attracted wide attention [165-170]. Gane et al. determined that adding kaolin or calcium carbonate is helpful for accelerating grinding in the grinding operation and will provide composite of CNFs and a mineral [171,172]. The working principle is that cellulose materials are sent to the grinding area consisting of a movable disc and a fixed disc, and the process of high-speed grinding and shearing was carried out to give CNFs lastly. Compared to the high pressure homogenization method, grinding method has the advantages of high efficiency, large capacity, low energy consumption, and being less prone to material clogging. However, because of the strong mechanical force in the grinding process, the fiber damage is more serious. As a result, the CNFs property is not well, such as low crystallinity and inferior thermal stability, which leads to the low physical strength of CNFs and the enhancement effect of material not as good as CNFs prepared by high pressure homogenization.

3.1.3. Ball Milling. Ball milling technique is also used for the production of CNFs. In this process, cellulose in a way of suspension can be disintegrated by the high energy collision between the balls (e.g., ceramic, metal, or zirconia) in a hollow cylindrical container while the container rotates. Zhang and coworkers studied the possibility of preparing CNFs from once-dried bleached softwood kraft pulp suspension by ball milling and investigated the effect of milling conditions [173]. The results showed that the size of milling ball had a decisive influence on the morphologies of produced nanocellulose (fibrous morphologies or particulates). In addition, ball-tocellulose mass ratio was very important as well as milling time for the controlling of fiber morphology. Alkali pretreatment was helpful for weakening hydrogen bonds and eliminating small particles. But the fibrous morphology was easily damaged in this process. When cerium-doped zirconia balls with diameter of 0.4-0.6 mm were adopted and without alkaline pretreatment, CNFs with diameter of 100 nm could be obtained within 1.5 h. Kekalainen et al. used ball milling technology to mill never-dried bleached hardwood kraft pulp with high solid contents ([greater than or equal to] 50%) to give CNFs with the diameter of 3.2 nm and the bundles of nanofibrils with the diameter of 10-150 nm [174].

3.1.4. Twin-Screw Extrusion Method. Twin-screw extrusion is another method for CNFs production. In the preparation process, cellulose pulp is fibrillated by two corotating, intermeshing screws mounted in a closed barrel. The advantage of twin-screw extrusion is that CNFs can be obtained with high solid contents (25-40 wt.%), which is very beneficial for transportation and storage [146]. The twin-screw extrusion process of the composites from thermoplastic starch and TEMPO-oxidized cellulose fibers was studied by Cobut and coworkers [175]. The CNFs with the diameter of ca. 30 nm were obtained during extrusion process. Hietala and coworkers studied this process for the bleached cellulose from softwood flour (particle size of 200-400 [micro]m) as raw materials [176]. The result was that CNFs were not gained; however, fibers with good dispersion in the starch matrix were given. Ho and coworkers studied this method for the preparation of CNFs from never-dried refined needle-leaf bleached kraft pulp [146], and the resultant CNFs were obtained at a solid content of 45 wt.%. Overall, twin-screw extrusion method is promising for the scale production of CNFs.

3.1.5. Cryocrushing Method. Cryocrushing method for the preparation of CNFs was studied by Dufresne and coworkers for the first time. The process is that water swollen cellulosic fibers were frozen firstly by liquid nitrogen and subsequently crushed by mortar grinding. Under mechanical impact, the pressure on the cell walls derived from ice crystals caused cell wall to break and release cell wall fragments [177]. Sain and coworkers studied the fibrillation of agricultural crops using cryocrushing method [156, 178, 179]. The results showed that the CNFs with the diameter of 50-100 nm and the crystallinity of 48% could be obtained, and the soybean stock nanofibers could be dispersed in acrylic oligomer emulsion as good as in water [156,180].

3.1.6. Blending Method. Uetani and Yano successfully prepared CNFs by using the high-speed blender to shear never-dried bleached softwood pulp [181]. The resultant CNFs had a uniform diameter of 15-20 nm when 0.7 wt.% cellulose pulp suspension was blended at 37,000 rpm. Jiang and Hsieh blended the cellulosic fibers suspension at 37,000 rpm and 97[degrees]C under different time conditions (up to 2h) to give the NCs with bimodal size distribution: (i) one was with the diameter of 2.7 nm and the length in range of 100-200 nm; (ii) the other was with the diameter of 8.5 nm and the length with few micrometers [182]. Nakagaito and coworkers improved the blender bottle, and the results showed that the time was halved for fibrillation process [183].

3.1.7. Aqueous Counter Collision. Aqueous counter collision (ACC) is another technology for CNFs preparation. Under high pressure, two jets of cellulose suspensions collide against each other, leading to wet pulverization of cellulose and liberation of CNFs. Kose et al. prepared CNFs from the previously homogenized bacterial cellulose aqueous suspension at a concentration of [less than or equal to] 0.4 wt.% using ACC method. The process was carried out under the ejecting pressure of 200 MPa for 80 passes [184]. The result was that CNFs with the diameter of ca. 30 nm were produced and cellulose Ia changed to cellulose I[beta] after ACC. But the crystallinity was still kept at >70%. Kondo and workers reported the preparation of CNFs from MCC by using ACC method [185]. The diameter of CNFs was ca. 15 nm and the length was ca. 700 nm. It is worth noting that in order to avoid clogging, the size of processed cellulosic material should be less than the nozzle diameter (ca. 150 [micro]m).

3.2. Pretreatments. The high energy consumption of CNFs by mechanical method is the mainly barrier to the scale production of CNFs, and cellulosic raw materials should be of small sizes for some equipment in order to avoid clogging. To solve these problems, a series of pretreatment methods have been developed, such as solvent-assisted pretreatment, enzyme hydrolysis, TEMPO oxidation, periodate oxidation, carboxymethylation, and cationization.

3.2.1. Solvent-Assisted Pretreatments. Traditionally, the cellulose pulp is pretreated by dilute base or dilute acid [158, 186-189]. For instance, bleached kraft bagasse pulp was pretreated with the aqueous solution of 5% NaOH at 50[degrees]C for 45 min so as to get rid of the residual hemicellulose and lignin and then was further ground to obtain the CNFs [158]. Degree of polymerization and cellulose content of the resultant CNFs were higher than the ones untreated or hydrolyzed with xylanase enzyme. Lee and coworkers used a low-concentration alkaline solution as pretreatment condition (2 wt.% NaOH solution below 0[degrees]C) to produce CNFs. The resulting CNFs with 90 nm diameter had relatively high thermal stability [188]. The maximum thermal decomposition temperature was 343[degrees]C, while it was 310[degrees]C for the CNFs prepared without the NaOH pretreatment. Since urea and ethylenediamine have the ability to loosen the hydrogen bonds and solubilize cellulose, they could be applied to pretreat cellulose [190]. Before further mechanical disintegration, a microemulsion (surfactant-oil-water systems) including urea or ethylenediamine was applied to treat kraft pulps. It was found that urea and ethylenediamine pretreatment could effectively accelerate the isolation of CNFs [190].

3.2.2. Organic Acid Hydrolysis Pretreatment. Recently, some organic acids, for example, formic acid and oxalate, were used to facilitate the cellulose. Du and coworkers used highly concentrated formic acid to hydrolyze bleached softwood pulp; then the formic esterified MCC passed through homogenizer without clogging to give CNFs [191]. The resultant CNFs showed high thermal stability (onset decomposition temperature ac. 291[degrees]C) and good dispersibility in DMSO, DMAc, and DMF, respectively, due to the introduction of surface formic ester groups. Bian et al. used p-toluenesulfonic acid (50-80 wt%) for the rapid and nearly complete dissolution of wood lignin at 80[degrees]C for 20 min (see Figure 10) [85]. In this process, the birch wood fiber fractions were hydrolyzed to lignocellulosic solid residue (LCSR) that was subsequently fibrillated into lignocellulosic nanofibrils (LCNFs). The dissolved lignin could be precipitated as lignin nanoparticles (LNPs) by diluting the concentration of p-TsOH to less than 10%, and the p-TsOH could be recovered by using commercially proven crystallization technology. The dicarboxylic acid was also used as the pretreatment of lignocellulose to prepare lignin containing cellulose nanocrystals (LCNCs) and LCNFs by Bian et al. [98]. The resultant LCNCs and LCNFs were less carboxylated compared to those lignin-free CNCs and CNFs. Therefore, they had lower charges. Overall, organic acid hydrolysis is an efficient pretreatment method, which is recycle and has low corrosion to equipment, and ester groups or carboxyl groups are grafted onto the surface of cellulose in the pretreatment process.

3.2.3. Enzyme Hydrolysis. The hydrolysis of cellulose can be catalyzed by some enzymes and the fibrillation can also be enhanced. The action mechanism of cellulase components to cellulose has been described in Section 2.4 and Figure 5. The enzyme pretreatment has a mild condition and a high selectivity and can make fibers produce holes, peeling, and fine fibrosis, which is conducive to the nanofibrillation of cellulose in subsequent mechanical processes and reduces the probability of blocking the homogeneous valve by cellulose slurry. The enzyme system could be multicomponent enzyme mixture [192-194] or the monocomponent [157, 170, 193, 195-198]. According to the reports, the monocomponent endoglucanase seems to give a better pretreatment effect [170,199,200].

3.2.4. TEMPO Oxidation. As we all know that the negatively charged groups (such as carboxyl or carboxymethyl) are introduced into cellulosic fibers and then the delamination of the nanofibrils can be improved, since there is electrostatic repulsion between cellulose nanofibrils with negative charge [201]. The process of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical oxidizing the primary alcohol groups of monosaccharides to carboxylic groups was reported by Davis and Flitsch in 1993 which has become one of the most extensive research topics in the area of oxidation of cellulose [202]. The typical oxidation system for cellulose is TEMPO/NaBr/NaClO [203], which has been extensively used for the pretreatment of raw cellulose materials before mechanical disintegration [22,189,204,205]. The mechanism of oxidation reaction is shown in Figure 11 [86]. Nitrosonium ions (+N=O) are generated in situ during the reaction of TEMPO radicals with the oxidants and they are used to oxidize the cellulose fibers. Consequently, the primary alcohol groups are first oxidized to aldehydes and further converted to carboxyl groups. At the same time, the depolymerization phenomenon of cellulose appears. There are two phenomena which may be caused by cellulose depolymerization: (i) [beta]-elimination could occur owing to C6 aldehyde groups presented in an alkaline environment and (ii) the cleavage of a hydroglucose unit took place owing to the existing of hydroxyl radicals [86, 206]. TEMPO/NaClO/NaCl[O.sub.2] system was also applied which overcame the depolymerization [207, 208]. The reaction was carried out in the neutral or weakly acidic environment and at 60[degrees]C for 72 h. The results demonstrated that most of original DP remained unchanged. TEMPO oxidation pretreatment remarkably reduced the energy consumption for the further mechanical disintegration of cellulose [86, 201, 203, 209-211]. Nelson et al. evaluated that the mechanical energy consumption after TEMPO oxidation was about 570 KWh/t, which was far lower than that of 70000 KWh/t without any pretreatment [84].

3.2.5. Periodate-Chlorite Oxidation. Periodate-chlorite is an effective oxidation system for transferring secondary alcohols of cellulose to carboxyl groups [212]. Liimatainen et al. used this method to oxidize cellulose to promote further mechanical decomposition of cellulose [87]. During the reaction, secondary alcohols of cellulose were firstly converted to aldehydes by sodium periodate, which were further oxidized to carboxyl groups by sodium chlorite (see Figure 12(a)). The CNFs with the diameter of 3-5 nm were obtained through sequential high pressure homogenization for up to 4 passes.

In this method, the glucopyranose ring was opened to provide 2,3-dicarboxylic acid cellulose (see Figure 12(b)) [87]. Moreover, the strength property of CNFs was not decreased by ring opening reaction [213-215].

3.2.6. Oxidative Sulfonation. Oxidative sulfonation of cellulose is another method for introducing negative charge groups onto the surface cellulose and can be used to enhance the fibrillation process [214, 216]. Liimatainen et al. treated bleached kraft hardwood pulp with periodate and bisulfite (see Figure 13), and CNFs with typical diameter of 10-60 nm were obtained by the following high pressure homogenization for 5 times. In this method, the used periodate could be efficiently recycled, so it is a potential green method for the pretreatment of cellulose [214].

3.2.7. Carboxymethylation. Carboxymethylation is another method for the pretreatment of cellulose. The hydroxyl groups on cellulose could react with 1-chloroacetic acid under strong alkaline conditions to give the carboxymethylation products (see Figure 14(a)). Wagberg et al. reported the production of CNFs from carboxymethylated cellulose by high pressure homogenization [217]. The resultant CNFs had acylindrical cross section with the width of 5-15 nm and the length of ca. 1 [micro]m. Chen et al. reported the preparation of CNFs by high pressure homogenization of carboxymethylated cellulose raw fibers [88]. In this work, the carboxymethylation pretreatment included two steps: (1) basification of raw oven-dried cellulose fibers by using anhydrous NaOH/dimethyl sulfoxide; (2) further sodium acetoxylation of cellulose raw fibers by using sodium chloroacetate. Then the resultant MCC was extracted by deionized water which was beneficial to the collapse of the cell wall of cellulose fibers. Finally, the pretreated cellulose fiber suspension passed through homogenizer to give the CNFs. The resultant CNFs were used for manufacture of nanopaper which exhibited 90.5% optical transparency. Carboxymethylation pretreatment was also used by other researchers for preparing CNFs [218-222]. Overall, the carboxymethylation method is also a potential green pretreatment technology for the preparation of CNFs.

3.2.8. Cationization. The cationization of cellulose could be used for promoting the nanofibrillation of cellulose. In the process, the cations were introduced on the surface of cellulose by the quaternization of cellulose [223-225]. The fibrillation of cellulose was promoted by the electrostatic repulsion between the quaternary ammonium cations (see Figure 15 [89]). This method has been used for preparation of CNFs combination with following mechanical disintegration: (2,3-epoxypropyl) trimethylammonium chloride [89, 226, 227], (2-chloroethyl) trimethylammonium chloride [228], and (2-hydrazinyl-2-oxoethyl)-trimethylazanium chloride [162] were commonly used as cationization reagents. The process was carried out under alkaline environment, for example, NaOH/[H.sub.2]O/i-PrOH, NaOH/DMSO. Recent studies showed that, owing to significant antibacterial properties of cationic CNF film, and without quaternary ammonium leaking to the environment [89, 229], they could be applied into wound healing, food packaging, tissue, and so on.

3.2.9. Ionic Liquids. Ionic liquids could also be used for the pretreatment of cellulose. Li et al. successfully prepared CNFs using 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) to treat sugarcane bagasse combining with the following high pressure homogenization for the first time [230]. Ninomiya et al. also proposed an ionic liquid pretreatment method for the preparation of lignocellulose nanofibers (LCNFs) [231]. In this work, choline acetate as an ionic liquid was used to treat bagasse powder; then LCNFs were obtained through a subsequent mechanical nanofibrillation. However, ionic liquids are expensive, which will hinder the scaling application for the preparation of CNFs.

3.2.10. Deep Eutectic Solvent Pretreatments. Deep eutectic solvent (DES), which is composed of two or more compounds, has lower melt point than any component of its own. Due to its low cost, low toxicity, and recyclability, DES has been widely used in various fields [232]. Sirvio and workers proposed a bio-based DES consisting of urea and choline chloride (molar ratio of 2:1) which could be used to the pretreatment of cellulose pulp [233]. Li and coworkers used the urea-based DES as the pretreatment for the preparation of CNFs [234]. In this work, DES was composed of urea and ammonium thiocyanate or guanidine hydrochloride and was used to treat wood-derived cellulose fibers. The results showed that the cellulose fibers were well loosened and swelled by both DES systems. Then CNFs with the width of 13.0-19.3 nm were smoothly obtained through further mechanical nanofibrillation without clogging. A method for the preparation of anionic CNFs using DES as reaction medium was proposed by Sirvio and Visanko [90]. As is shown in Figure 16, the DES consisted of triethylmethylammonium chloride (TEMACl) and imidazole was liquid at room temperature. The ground wood pulp with a 27.4 wt% lignin was carboxylated by succinic anhydride during the DES pretreatment process under mild reaction conditions (2 h at 70[degrees]C). Then the anionic CNFs could be obtained through the followed microfluidization of carboxylated ground wood pulp. A microwave-assisted DES pretreatment method for fast CNCs production was proposed by Liu and coworkers [235]. It took only a little time (not more than 3 min) for the pretreatment of cotton, and then a high-intensity ultrasonication process was adopted. The resultant CNCs with the diameter of 3-25 nm and the length of 100-350 nm were obtained at a yield of 74.2%. Overall, DES pretreatment is efficient and environment-friendly and is a promising technology for the industrial production of NCs.

4. Conclusions

This review summarizes the preparation methods of CNCs and CNFs. For the preparation of CNCs, promising techniques were discussed besides the conventional mineral acid hydrolysis, such as organic acid hydrolysis, subcritical hydrolysis, AVAP method, and ionic liquid hydrolysis. Most of them have great possibility for further industrialization. However, there are still some problems that need to be solved. For example, the corrosion of equipment is serious and the postprocessing of waste acid is difficult for mineral acid hydrolysis, the preparation time is long and the technological requirements is rigorous for enzymatic hydrolysis, the reactivity is low for organic acid hydrolysis, there are a great deal of chemical consumption and wastewater treatment for oxidation degradation method, and the yield is low for subcritical water method. Therefore, in order to achieve the industrialization of various CNCs, a lot of research works need to be done. The appropriate catalysts should be developed to reduce the amount of acid or increase the efficiency of organic acid hydrolysis. The subcritical water method should be further improved to be suitable for the industrial production of CNCs. The DES method is promising and more attention should be paid to it. For the preparation of CNFs, various mechanical treatments are discussed, including high pressure homogenization, grinding method, ball milling, twin-screw extrusion, cryocrushing, blending, and aqueous counter collision. Among them, the twin-screw extrusion is the method with the most potential for the preparation of CNFs due to a high solid content of obtained CNFs. In order to reduce energy consumption of mechanical disintegration, the pretreatment of cellulose materials is necessary. Various pretreatment methods are summarized in this review. In this process, some special functional groups could be introduced to the surface of CNFs so that the physicochemical properties could be effectively adjusted.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.


This work was kindly supported by National Key Research and Development Program of China (Grant no. 2017YFB0307903), State Key Laboratory of Pulp and Paper Engineering (South China University of Technology) (201822), and the Foundation of Key Laboratory of Industrial Fermentation Microbiology of Ministry of Education and Tianjin Key Lab of Industrial Microbiology (Tianjin University of Science & Technology) (no. 2016IM003). Haishun Du acknowledges financial support from the China Scholarship Council.


[1] M. M. de Souza Lima and R. Borsali, "Rodlike cellulose microcrystals: structure, properties, and applications," Macromolecular Rapid Communications, vol. 25, no. 7, pp. 771-787, 2004.

[2] Y. Habibi, L. A. Lucia, and O. J. Rojas, "Cellulose nanocrystals: chemistry, self-assembly, and applications," Chemical Reviews, vol. 110, no. 6, pp. 3479-3500, 2010.

[3] J. F. Revol, L. Godbout, X.-M. Dong, D. G. Gray, H. Chanzy, and G. Maret, "Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation," Liquid Crystals, vol. 16, no. 1, pp. 127-134, 1994.

[4] X. M. Dong and D. G. Gray, "Induced circular dichroism of isotropic and magnetically-oriented chiral nematic suspensions of cellulose crystallites," Langmuir, vol. 13, no. 11, pp. 3029-3034, 1997.

[5] R. J. Moon, A. Martini, J. Nairn, J. Simonsen, and J. Young-blood, "Cellulose nanomaterials review: structure, properties and nanocomposites," Chemical Society Reviews, vol. 40, no. 7, pp. 3941-3994, 2011.

[6] J. A. Kelly, A. M. Shukaliak, C. C. Y. Cheung, K. E. Shopsowitz, W. Y. Hamad, and M. J. MacLachlan, "Responsive photonic hydrogels based on nanocrystalline cellulose," Angewandte Chemie International Edition, vol. 52, no. 34, pp. 8912-8916, 2013.

[7] M. Giese, L. K. Blusch, M. K. Khan, and M. J. MacLachlan, "Functional materials from cellulose-derived liquid-crystal templates," Angewandte Chemie International Edition, vol. 54, no. 10, pp. 2888-2910, 2015.

[8] J. Huang, H. Zhu, Y. Chen et al., "Highly transparent and flexible nanopaper transistors," ACS Nano, vol. 7, no. 3, pp. 2106-2113, 2013.

[9] J. A. Shatkin, T. H. Wegner, E. M. Bilek, and J. Cowie, "Market projections of cellulose nanomaterial-enabled products -Part 1: applications," Tappi Journal, vol. 13, no. 5, pp. 9-16, 2014.

[10] Y. H. Jung, T.-H. Chang, H. Zhang et al., "High-performance green flexible electronics based on biodegradable cellulose nanofibril paper," Nature Communications, vol. 6, article 7170, 2015.

[11] S. Eyley and W. Thielemans, "Surface modification of cellulose nanocrystals," Nanoscale, vol. 6, no. 14, pp. 7764-7779, 2014.

[12] C. Salas, T. Nypela, C. Rodriguez-Abreu, C. Carrillo, and O. J. Rojas, "Nanocellulose properties and applications in colloids and interfaces," Current Opinion in Colloid & Interface Science, vol. 19, no. 5, pp. 383-396, 2014.

[13] H. Zhu, W. Luo, P. N. Ciesielski et al., "Wood-derived materials for green electronics, biological devices, and energy applications," Chemical Reviews, vol. 116, no. 16, pp. 9305-9374, 2016.

[14] O. Nechyporchuk, M. N. Belgacem, and J. Bras, "Production of cellulose nanofibrils: a review of recent advances," in Industrial Crops and Products, vol. 93, pp. 2-25, 2016.

[15] H. P. S. Abdul Khalil, Y. Davoudpour, M. N. Islam et al., "Production and modification of nanofibrillated cellulose using various mechanical processes: a review," Carbohydrate Polymers, vol. 99, pp. 649-665, 2014.

[16] H. P. S. A. Khalil, A. H. Bhat, and A. F. I. Yusra, "Green composites from sustainable cellulose nanofibrils: a review," Carbohydrate Polymers, vol. 87, no. 2, pp. 963-979, 2012.

[17] M. T. Islam, M. M. Alam, A. Patrucco, A. Montarsolo, and M. Zoccola, "Preparation of nanocellulose: a review," AATCC Journal of Research, vol. 1, no. 5, pp. 17-23, 2014.

[18] N. Wang, E. Ding, and R. Cheng, "Surface modification of cellulose nanocrystals," Frontiers of Chemical Engineering in China, vol. 1, no. 3, pp. 228-232, 2007.

[19] N. Grishkewich, N. Mohammed, J. Tang, and K. C. Tam, "Recent advances in the application of cellulose nanocrystals," Current Opinion in Colloid and Interface Science, vol. 29, pp. 32-45, 2017.

[20] D. Trache, M. H. Hussin, M. K. M. Haafiz, and V. K. Thakur, "Recent progress in cellulose nanocrystals: Sources and production," Nanoscale, vol. 9, no. 5, pp. 1763-1786, 2017.

[21] S. Boufi, "6--Agricultural crop residue as a source for the production of cellulose nanofibrils," in ellulose-Reinforced Nanofibre Composites, pp. 129-152, Woodhead Publishing, 2017.

[22] H. Lee, J. Sundaram, and S. Mani, "Production of cellulose nanofibrils and their application to food: a review," in Nanotechnology: Food and Environmental Paradigm, R. Prasad, V Kumar, and M. Kumar, Eds., pp. 1-33, Springer, Singapore, 2017.

[23] B. L. Peng, N. Dhar, H. L. Liu, and K. C. Tam, "Chemistry and applications of nanocrystalline cellulose and its derivatives: a nanotechnology perspective," The Canadian Journal of Chemical Engineering, vol. 89, no. 5, pp. 1191-1206, 2011.

[24] W. Y. Hamad, Cellulose Nanocrystals: Properties, Production, and Applications, John Wiley and Sons Ltd Publishing, 2017.

[25] J. Aspler, J. Bouchard, W. Hamad, R. Berry, S. Beck, and X. Zou, "Review of nanocellulosic products and their application," in Biopolymer Nanocomposites: Processing, Properties, and Applications, A. Dufresne, S. Thomas, and P. L. Pothan, Eds., pp. 461-508, Wiley and Sons Publishing, 2013.

[26] R. F. Nickerson and J. A. Habrle, "Cellulose intercrystalline structure," Industrial & Engineering Chemistry, vol. 39, no. 11, pp. 1507-1512, 1947.

[27] B. G. Ranby, "Fibrous macromolecular systems. Cellulose and muscle. The colloidal properties of cellulose micelles," Discussions of the Faraday Society, vol. 11, pp. 158-164, 1951.

[28] S. M. Mukherjee and H. J. Woods, "X-ray and electron microscope studies of the degradation of cellulose by sulphuric acid," BBA--Biochimica et Biophysica Acta, vol. 10, no. C, pp. 499-511, 1953.

[29] R. H. Marchessault, F. F. Morehead, and N. M. Walter, "Liquid crystal systems from fibrillar polysaccharides," Nature, vol. 184, no. 4686, pp. 632-633, 1959.

[30] F. Jiang and Y.-L. Hsieh, "Cellulose nanocrystal isolation from tomato peels and assembled nanofibers," Carbohydrate Polymers, vol. 122, pp. 60-68, 2015.

[31] M. K. M. Haafiz, A. Hassan, Z. Zakaria, and I. M. Inuwa, "Isolation and characterization of cellulose nanowhiskers from oil palm biomass microcrystalline cellulose," Carbohydrate Polymers, vol. 103, no. 1, pp. 119-125, 2014.

[32] P. Lu and Y.-L. Hsieh, "Preparation and properties of cellulose nanocrystals: rods, spheres, and network," Carbohydrate Polymers, vol. 82, no. 2, pp. 329-336, 2010.

[33] N. Johar, I. Ahmad, and A. Dufresne, "Extraction, preparation and characterization of cellulose fibres and nanocrystals from rice husk," Industrial Crops and Products, vol. 37, no. 1, pp. 93-99, 2012.

[34] Z. Wang, Z. Yao, J. Zhou, and Y. Zhang, "Reuse of waste cotton cloth for the extraction of cellulose nanocrystals," Carbohydrate Polymers, vol. 157, pp. 945-952, 2017.

[35] Y. Tang, X. Shen, J. Zhang, D. Guo, F. Kong, and N. Zhang, "Extraction of cellulose nano-crystals from old corrugated container fiber using phosphoric acid and enzymatic hydrolysis followed by sonication," Carbohydrate Polymers, vol. 125, pp. 360-366, 2015.

[36] M. F. Rosa, E. S. Medeiros, J. A. Malmonge et al., "Cellulose nanowhiskers from coconut husk fibers: effect of preparation conditions on their thermal and morphological behavior," Carbohydrate Polymers, vol. 81, no. 1, pp. 83-92, 2010.

[37] W. P. Flauzino Neto, H. A. Silverio, N. O. Dantas, and D. Pasquini, "Extraction and characterization of cellulose nanocrystals from agro-industrial residue--Soy hulls," Industrial Crops and Products, vol. 42, no. 1, pp. 480-488, 2013.

[38] N. A. Rosli, I. Ahmad, and I. Abdullah, "Isolation and characterization of cellulose nanocrystals from agave angustifolia fibre," Bioresources, vol. 8, no. 2, pp. 1893-1908, 2013.

[39] E. Espino, M. Cakir, S. Domenek, A. D. Romen-Gutierrez, N. Belgacem, and J. Bras, "Isolation and characterization of cellulose nanocrystals from industrial by-products of Agave tequilana and barley," Industrial Crops and Products, vol. 62, pp. 552-559, 2014.

[40] F. Kallel, F. Bettaieb, R. Khiari, A. Garcia, J. Bras, and S. E. Chaabouni, "Isolation and structural characterization of cellulose nanocrystals extracted from garlic straw residues," Industrial Crops and Products, vol. 87, pp. 287-296, 2016.

[41] A. Mandal and D. Chakrabarty, "Isolation of nanocellulose from waste sugarcane bagasse (SCB) and its characterization," Carbohydrate Polymers, vol. 86, no. 3, pp. 1291-1299, 2011.

[42] P. Lu and Y.-L. Hsieh, "Cellulose isolation and core-shell nanostructures of cellulose nanocrystals from chardonnay grape skins," Carbohydrate Polymers, vol. 87, no. 4, pp. 2546-2553, 2012.

[43] H. A. Silverio, W. P. Flauzino Neto, N. O. Dantas, and D. Pasquini, "Extraction and characterization of cellulose nanocrystals from corncob for application as reinforcing agent in nanocomposites," Industrial Crops and Products, vol. 44, pp. 427-436, 2013.

[44] P. Lu and Y.-L. Hsieh, "Preparation and characterization of cellulose nanocrystals from rice straw," Carbohydrate Polymers, vol. 87, no. 1, pp. 564-573, 2012.

[45] R. M. Sheltami, I. Abdullah, I. Ahmad, A. Dufresne, and H. Kargarzadeh, "Extraction of cellulose nanocrystals from mengkuang leaves (Pandanus tectorius)," Carbohydrate Polymers, vol. 88, no. 2, pp. 772-779, 2012.

[46] S. Thambiraj and D. Ravi Shankaran, "Preparation and physicochemical characterization of cellulose nanocrystals from industrial waste cotton," Applied Surface Science, vol. 412, pp. 405-416, 2017.

[47] R. Rampazzo, D. Alkan, S. Gazzotti, M. A. Ortenzi, G. Piva, and L. Piergiovanni, "Cellulose Nanocrystals from Lignocellulosic Raw Materials, for Oxygen Barrier Coatings on Food Packaging Films," Packaging Technology and Science, vol. 30, no. 10, pp. 645-661, 2017.

[48] W. Lei, C. Fang, X. Zhou et al., "Cellulose nanocrystals obtained from office waste paper and their potential application in PET packing materials," Carbohydrate Polymers, vol. 181, pp. 376-385, 2018.

[49] T. Taflick, L. A. Schwendler, S. M. L. Rosa, C. I. D. Bica, and S. M. B. Nachtigall, "Cellulose nanocrystals from acacia bark-Influence of solvent extraction," International Journal of Biological Macromolecules, vol. 101, pp. 553-561, 2017.

[50] A. Moradbak, P. M. Tahir, A. Z. Mohamed, M. M. Abdi, R. L. Razalli, and R. Halis, "Isolation of cellulose nanocrystals from Gigantochloa scortechinii ASAM pulp," European Journal of Wood and Wood Products, pp. 1-7, 2017.

[51] F. V. Ferreira, M. Mariano, S. C. Rabelo, R. F. Gouveia, and L. M. F. Lona, "Isolation and surface modification of cellulose nanocrystals from sugarcane bagasse waste: from a micro- to a nano-scale view," Applied Surface Science, vol. 436, pp. 1113-1122, 2018.

[52] H. Lu, Y. Gui, L. Zheng, and X. Liu, "Morphological, crystalline, thermal and physicochemical properties of cellulose nanocrystals obtained from sweet potato residue," Food Research International, vol. 50, no. 1, pp. 121-128, 2013.

[53] F. B. de Oliveira, J. Bras, M. T. B. Pimenta, A. A. da Silva Curvelo, and M. N. Belgacem, "Production of cellulose nanocrystals from sugarcane bagasse fibers and pith," Industrial Crops and Products, vol. 93, pp. 48-57, 2016.

[54] S. Bano and Y. S. Negi, "Studies on cellulose nanocrystals isolated from groundnut shells," Carbohydrate Polymers, vol. 157, pp. 1041-1049, 2017.

[55] M. Mariano, R. Cercena, and V Soldi, "Thermal characterization of cellulose nanocrystals isolated from sisal fibers using acid hydrolysis," Industrial Crops and Products, vol. 94, pp. 454-462, 2016.

[56] L. Chen, Q. Wang, K. Hirth, C. Baez, U. P. Agarwal, and J. Y. Zhu, "Tailoring the yield and characteristics of wood cellulose nanocrystals (CNC) using concentrated acid hydrolysis," Cellulose, vol. 22, no. 3, pp. 1753-1762, 2015.

[57] N. Kasiri and M. Fathi, "Production of cellulose nanocrystals from pistachio shells and their application for stabilizing Pickering emulsions," International Journal of Biological Macromolecules, vol. 106, pp. 1023-1031, 2018.

[58] H. Yu, Z. Qin, B. Liang, N. Liu, Z. Zhou, and L. Chen, "Facile extraction of thermally stable cellulose nanocrystals with a high yield of 93% through hydrochloric acid hydrolysis under hydrothermal conditions," Journal of Materials Chemistry A, vol. 1, no. 12, pp. 3938-3944, 2013.

[59] J. Araki, M. Wada, S. Kuga, and T. Okano, "Flow properties of microcrystalline cellulose suspension prepared by acid treatment of native cellulose," Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 142, no. 1, pp. 75-82, 1998.

[60] S. Li, C. Li, C. Li et al., "Fabrication of nano-crystalline cellulose with phosphoric acid and its full application in a modified polyurethane foam," Polymer Degradation and Stability, vol. 98, no. 9, pp. 1940-1944, 2013.

[61] H. Sadeghifar, I. Filpponen, S. P. Clarke, D. F. Brougham, and D. S. Argyropoulos, "Production of cellulose nanocrystals using hydrobromic acid and click reactions on their surface," Journal of Materials Science, vol. 46, no. 22, pp. 7344-7355, 2011.

[62] J.-S. Fan and Y.-H. Li, "Maximizing the yield of nanocrystalline cellulose from cotton pulp fiber," Carbohydrate Polymers, vol. 88, no. 4, pp. 1184-1188, 2012.

[63] H. Du, C. Liu, X. Mu et al., "Preparation and characterization of thermally stable cellulose nanocrystals via a sustainable approach of Fe[Cl.sub.3]-catalyzed formic acid hydrolysis," Cellulose, vol. 23, no. 4, pp. 2389-2407, 2016.

[64] N. Wang, E. Ding, and R. Cheng, "Thermal degradation behaviors of spherical cellulose nanocrystals with sulfate groups," Polymer Journal, vol. 48, no. 12, pp. 3486-3493, 2007.

[65] H. Kargarzadeh, I. Ahmad, I. Abdullah, A. Dufresne, S. Y. Zainudin, and R. M. Sheltami, "Effects of hydrolysis conditions on the morphology, crystallinity, and thermal stability of cellulose nanocrystals extracted from kenaf bast fibers," Cellulose, vol. 19, no. 3, pp. 855-866, 2012.

[66] R. Xiong, X. Zhang, D. Tian, Z. Zhou, and C. Lu, "Comparing microcrystalline with spherical nanocrystalline cellulose from waste cotton fabrics," Cellulose, vol. 19, no. 4, pp. 1189-1198, 2012.

[67] Z. A. Z. Azrina, M. D. H. Beg, M. Y. Rosli, R. Ramli, N. Junadi, and A. K. M. M. Alam, "Spherical nanocrystalline cellulose (NCC) from oil palm empty fruit bunch pulp via ultrasound assisted hydrolysis," Carbohydrate Polymers, vol. 162, pp. 115-120, 2017.

[68] M. Cheng, Z. Qin, Y. Chen, S. Hu, Z. Ren, and M. Zhu, "Efficient extraction of cellulose nanocrystals through hydrochloric acid hydrolysis catalyzed by inorganic chlorides under hydrothermal conditions," ACS Sustainable Chemistry & Engineering, vol. 5, no. 6, pp. 4656-4664, 2017.

[69] S. Camarero Espinosa, T. Kuhnt, E. J. Foster, and C. Weder, "Isolation of thermally stable cellulose nanocrystals by phosphoric acid hydrolysis," Biomacromolecules, vol. 14, no. 4, pp. 1223-1230, 2013.

[70] Q. Lu, W. Lin, L. Tang, S. Wang, X. Chen, and B. Huang, "A mechanochemical approach to manufacturing bamboo cellulose nanocrystals," Journal of Materials Science, vol. 50, no. 2, pp. 611-619, 2014.

[71] M. R. Sucaldito and D. H. Camacho, "Characteristics of unique HBr-hydrolyzed cellulose nanocrystals from freshwater green algae (Cladophora rupestris) and its reinforcement in starch-based film," Carbohydrate Polymers, vol. 169, pp. 315-323, 2017.

[72] J. Zhang, T. J. Elder, Y. Pu, and A. J. Ragauskas, "Facile synthesis of spherical cellulose nanoparticles," Carbohydrate Polymers, vol. 69, no. 3, pp. 607-611, 2007.

[73] N. Wang, E. Ding, and R. Cheng, "Preparation and liquid crystalline properties of spherical cellulose nanocrystals," Langmuir, vol. 24, no. 1, pp. 5-8, 2008.

[74] F. Niu, M. Li, Q. Huang et al., "The characteristic and dispersion stability of nanocellulose produced by mixed acid hydrolysis and ultrasonic assistance," Carbohydrate Polymers, vol. 165, pp. 197-204, 2017.

[75] M. Cheng, Z. Qin, Y. Chen, J. Liu, and Z. Ren, "Facile one-step extraction and oxidative carboxylation of cellulose nanocrystals through hydrothermal reaction by using mixed inorganic acids," Cellulose, vol. 24, no. 8, pp. 3243-3254, 2017.

[76] L.-R. Tang, B. Huang, W. Ou, X.-R. Chen, and Y.-D. Chen, "Manufacture of cellulose nanocrystals by cation exchange resin-catalyzed hydrolysis of cellulose," Bioresource Technology, vol. 102, no. 23, pp. 10973-10977, 2011.

[77] Y. Liu, H. Wang, G. Yu, Q. Yu, B. Li, and X. Mu, "A novel approach for the preparation of nanocrystalline cellulose by using phosphotungstic acid," Carbohydrate Polymers, vol. 110, pp. 415-422, 2014.

[78] Q. Lu, Z. Cai, F. Lin, L. Tang, S. Wang, and B. Huang, "Extraction of cellulose nanocrystals with a high yield of 88% by simultaneous mechanochemical activation and phosphotungstic acid hydrolysis," ACS Sustainable Chemistry & Engineering, vol. 4, no. 4, pp. 2165-2172, 2016.

[79] M. A. Torlopov, E. V. Udoratina, I. S. Martakov, and P. A. Sitnikov, "Cellulose nanocrystals prepared in H3PW12O40-acetic acid system," Cellulose, vol. 24, no. 5, pp. 2153-2162, 2017.

[80] B. Li, W. Xu, D. Kronlund et al., "Cellulose nanocrystals prepared via formic acid hydrolysis followed by TEMPO-mediated oxidation," Carbohydrate Polymers, vol. 133, pp. 605-612, 2015.

[81] L. R. Lynd, P. J. Weimer, W. H. Van Zyl, and I. S. Pretorius, "Microbial cellulose utilization: fundamentals and biotechnology," Microbiology and Molecular Biology Reviews, vol. 66, no. 3, pp. 506-577, 2002.

[82] J. Miao, Y. Yu, Z. Jiang, and L. Zhang, "One-pot preparation of hydrophobic cellulose nanocrystals in an ionic liquid," Cellulose, vol. 23, no. 2, pp. 1209-1219, 2016.

[83] L. P. Novo, J. Bras, A. Garcia, N. Belgacem, and A. A. S. Curvelo, "Subcritical water: a method for green production of cellulose nanocrystals," ACS Sustainable Chemistry & Engineering, vol. 3, no. 11, pp. 2839-2846, 2015.

[84] K. Nelson, T. Retsina, M. Iakovlev et al., "American process: production of low cost nanocellulose for renewable, advanced materials applications," in Materials Research for Manufacturing: An Industrial Perspective of Turning Materials into New Products, L. D. Madsen and E. B. Svedberg, Eds., vol. 224, pp. 267-302, Springer International Publishing: Cham, 2016.

[85] H. Bian, L. Chen, R. Gleisner, H. Dai, and J. Y. Zhu, "Producing wood-based nanomaterials by rapid fractionation of wood at 80[degrees]C using a recyclable acid hydrotrope," Green Chemistry, vol. 19, no. 14, pp. 3370-3379, 2017.

[86] A. Isogai, T. Saito, and H. Fukuzumi, "TEMPO-oxidized cellulose nanofibers," Nanoscale, vol. 3, no. 1, pp. 71-85, 2011.

[87] H. Liimatainen, M. Visanko, J. A. Sirvioo, O. E. O. Hormi, and J. Niinimaki, "Enhancement of the nanofibrillation of wood cellulose through sequential periodate-chlorite oxidation," Biomacromolecules, vol. 13, no. 5, pp. 1592-1597, 2012.

[88] J. Chen, M. Akin, L. Yang et al., "Transparent electrode and magnetic permalloy made from novel nanopaper," ACS Applied Materials & Interfaces, vol. 8, no. 40, pp. 27081-27090, 2016.

[89] A. Chaker and S. Boufi, "Cationic nanofibrillar cellulose with high antibacterial properties," Carbohydrate Polymers, vol. 131, pp. 224-232, 2015.

[90] J. A. Sirvio and M. Visanko, "Anionic wood nanofibers produced from unbleached mechanical pulp by highly efficient chemical modification," Journal of Materials Chemistry A, vol. 5, no. 41, pp. 21828-21835, 2017.

[91] L. Chen, J. Y. Zhu, C. Baez, P. Kitin, and T. Elder, "Highly thermal-stable and functional cellulose nanocrystals and nanofibrils produced using fully recyclable organic acids," Green Chemistry, vol. 18, no. 13, pp. 3835-3843, 2016.

[92] T. C. Allen and J. A. Cuculo, "Cellulose derivatives containing carboxylic acid groups," Journal of Polymer Science: Macromolecular Reviews, vol. 7, no. 1, pp. 189-262, 1973.

[93] D. Li, J. Henschen, and M. Ek, "Esterification and hydrolysis of cellulose using oxalic acid dihydrate in a solvent-free reaction suitable for preparation of surface-functionalised cellulose nanocrystals with high yield," Green Chemistry, vol. 19, no. 23, pp. 5564-5567, 2017.

[94] W. Xu, H. Grenman, J. Liu et al., "Mild oxalic-acid-catalyzed hydrolysis as a novel approach to prepare cellulose nanocrystals," ChemNanoMat, vol. 3, no. 2, pp. 109-119, 2017.

[95] R. Wang, L. Chen, J. Y. Zhu, and R. Yang, "Tailored and integrated production of carboxylated cellulose nanocrystals (CNC) with nanofibrils (CNF) through maleic acid hydrolysis," ChemNanoMat, vol. 3, no. 5, pp. 328-335, 2017.

[96] F. Yeganeh, R. Behrooz, and M. Rahimi, "The effect of sulfuric acid and maleic acid on characteristics of nano-cellulose produced from waste office paper," International Journal of Nano Dimension, vol. 8, no. 3, pp. 206-215, 2017.

[97] H. Bian, L. Chen, H. Dai, and J. Y. Zhu, "Effect of fiber drying on properties of lignin containing cellulose nanocrystals and nanofibrils produced through maleic acid hydrolysis," Cellulose, vol. 24, no. 10, pp. 4205-4216, 2017.

[98] H. Bian, L. Chen, H. Dai, and J. Y. Zhu, "Integrated production of lignin containing cellulose nanocrystals (LCNC) and nanofibrils (LCNF) using an easily recyclable di-carboxylic acid," Carbohydrate Polymers, vol. 167, pp. 167-176, 2017.

[99] H. Bian, L. Chen, R. Wang, and J. Y. Zhu, "Green and low-cost production of thermally stable and carboxylated cellulose nanocrystals and nanofibrils using highly recyclable dicarboxylic acids," Journal of Visualized Experiments, vol. 2017, no. 119, Article ID e55079, pp. 1-7, 2017.

[100] H.-Y. Yu, D.-Z. Zhang, F.-F. Lu, and J. Yao, "New approach for single-step extraction of carboxylated cellulose nanocrystals for their use as adsorbents and flocculants," ACS Sustainable Chemistry & Engineering, vol. 4, no. 5, pp. 2632-2643, 2016.

[101] A. Boujemaoui, S. Mongkhontreerat, E. Malmstrom, and A. Carlmark, "Preparation and characterization of functionalized cellulose nanocrystals," Carbohydrate Polymers, vol. 115, pp. 457-464, 2015.

[102] S. Zhou and L. O. Ingram, "Synergistic hydrolysis of carboxymethyl cellulose and acid-swollen cellulose by two endoglucanases (CelZ and CelY) from Erwinia chrysanthemi," Journal of Bacteriology, vol. 182, no. 20, pp. 5676-5682, 2000.

[103] P. B. Filson, B. E. Dawson-Andoh, and D. Schwegler-Berry, "Enzymatic-mediated production of cellulose nanocrystals from recycled pulp," Green Chemistry, vol. 11, no. 11, pp. 1808-1814, 2009.

[104] X. Chen, X. Deng, W. Shen, and M. Jia, "Preparation and characterization of the spherical nanosized cellulose by the enzymatic hydrolysis of pulp fibers," Carbohydrate Polymers, vol. 181, pp. 879-884, 2018.

[105] T. F. Meyabadi, F. Dadashian, G. Mir Mohamad Sadeghi, and H. Ebrahimi Zanjani Asl, "Spherical cellulose nanoparticles preparation from waste cotton using a green method," Powder Technology, vol. 261, pp. 232-240, 2014.

[106] H. Yang, M. N. Alam, and T. G. M. van de Ven, "Highly charged nanocrystalline cellulose and dicarboxylated cellulose from periodate and chlorite oxidized cellulose fibers," Cellulose, vol. 20, no. 4, pp. 1865-1875, 2013.

[107] Y. Hu, L. Tang, Q. Lu, S. Wang, X. Chen, and B. Huang, "Preparation of cellulose nanocrystals and carboxylated cellulose nanocrystals from borer powder of bamboo," Cellulose, vol. 21, no. 3, pp. 1611-1618, 2014.

[108] E. Mascheroni, R. Rampazzo, M. A. Ortenzi, G. Piva, S. Bonetti, and L. Piergiovanni, "Comparison of cellulose nanocrystals obtained by sulfuric acid hydrolysis and ammonium persulfate, to be used as coating on flexible food-packaging materials," Cellulose, vol. 23, no. 1, pp. 779-793, 2016.

[109] A. C. W. Leung, S. Hrapovic, E. Lam et al., "Characteristics and properties of carboxylated cellulose nanocrystals prepared from a novel one-step procedure," Small, vol. 7, no. 3, pp. 302-305, 2011.

[110] M. Cheng, Z. Qin, Y. Liu et al., "Efficient extraction of carboxylated spherical cellulose nanocrystals with narrow distribution through hydrolysis of lyocell fibers by using ammonium persulfate as an oxidant," Journal of Materials Chemistry A, vol. 2, no. 1, pp. 251-258, 2014.

[111] S. Montanari, M. Roumani, L. Heux, and M. R. Vignon, "Topochemistry of carboxylated cellulose nanocrystals resulting from TEMPO-mediated oxidation," Macromolecules, vol. 38, no. 5, pp. 1665-1671, 2005.

[112] M. Hirota, N. Tamura, T. Saito, and A. Isogai, "Water dispersion of cellulose II nanocrystals prepared by TEMPO-mediated oxidation of mercerized cellulose at pH 4.8," Cellulose, vol. 17, no. 2, pp. 279-288, 2010.

[113] J. Peyre, T. Poakkonen, M. Reza, and E. Kontturi, "Simultaneous preparation of cellulose nanocrystals and micron-sized porous colloidal particles of cellulose by TEMPO-mediated oxidation," Green Chemistry, vol. 17, no. 2, pp. 808-811, 2015.

[114] M. Hirota, K. Furihata, T. Saito, T. Kawada, and A. Isogai, "Glucose/glucuronic acid alternating co-polysaccharides prepared from TEMPO-oxidized native celluloses by surface peeling," Angewandte Chemie International Edition, vol. 49, no. 42, pp. 7670-7672, 2010.

[115] Y. Okita, T. Saito, and A. Isogai, "Entire surface oxidation of various cellulose microfibrils by TEMPO-mediated oxidation," Biomacromolecules, vol. 11, no. 6, pp. 1696-1700, 2010.

[116] S. Ifuku, M. Tsuji, M. Morimoto, H. Saimoto, and H. Yano, "Synthesis of silver nanoparticles templated by TEMPO-mediated oxidized bacterial cellulose nanofibers," Biomacromolecules, vol. 10, no. 9, pp. 2714-2717, 2009.

[117] A. P. Mangalam, J. Simonsen, and A. S. Benight, "Cellulose/DNA hybrid nanomaterials," Biomacromolecules, vol. 10, no. 3, pp. 497-504, 2009.

[118] M. J. Earle and K. R. Seddon, "Ionic liquids. Green solvents for the future," Pure and Applied Chemistry, vol. 72, no. 7, pp. 1391-1398, 2000.

[119] Z. Man, N. Muhammad, A. Sarwono, M. A. Bustam, M. V. Kumar, and S. Rafiq, "Preparation of Cellulose Nanocrystals Using an Ionic Liquid," Journal of Polymers and the Environment, vol. 19, no. 3, pp. 726-731, 2011.

[120] J. Mao, A. Osorio-Madrazo, and M.-P. Laborie, "Preparation of cellulose I nanowhiskers with a mildly acidic aqueous ionic liquid: reaction efficiency and whiskers attributes," Cellulose, vol. 20, no. 4, pp. 1829-1840, 2013.

[121] J. Mao, B. Heck, G. Reiter, and M.-P. Laborie, "Cellulose nanocrystals' production in near theoretical yields by 1-butyl-3- methylimidazolium hydrogen sulfate ([Bmim]HS[O.sub.4])--Mediated hydrolysis," Carbohydrate Polymers, vol. 117, pp. 443-451, 2015.

[122] X. Y. Tan, S. B. Abd Hamid, and C. W. Lai, "Preparation of high crystallinity cellulose nanocrystals (CNCs) by ionic liquid solvolysis," Biomass & Bioenergy, vol. 81, pp. 584-591, 2015.

[123] J. Lazko, T. Senachal, N. Landercy, L. Dangreau, J.-M. Raquez, and P. Dubois, "Well defined thermostable cellulose nanocrystals via two-step ionic liquid swelling-hydrolysis extraction," Cellulose, vol. 21, no. 6, pp. 4195-4207, 2014.

[124] N. A. M. Iskak, N. M. Julkapli, and S. B. A. Hamid, "Understanding the effect of synthesis parameters on the catalytic ionic liquid hydrolysis process of cellulose nanocrystals," Cellulose, vol. 24, no. 6, pp. 2469-2481, 2017.

[125] H. Abushammala, I. Krossing, and M.-P. Laborie, "Ionic liquid-mediated technology to produce cellulose nanocrystals directly from wood," Carbohydrate Polymers, vol. 134, pp. 609-616, 2015.

[126] Y. W. Chen, T. H. Tan, H. V. Lee, and S. B. A. Hamid, "Easy fabrication of highly thermal-stable cellulose nanocrystals using Cr[([NO.sub.3]).sub.3] catalytic hydrolysis system: a feasibility study from macroto nano-dimensions," Materials, vol. 10, no. 1, article 42, 2017.

[127] Y. W. Chen, H. V. Lee, and S. B. A. Hamid, "A response surface methodology study: effects of trivalent [Cr.sup.3+] metal ion-catalyzed hydrolysis on nanocellulose crystallinity and yield," Bioresources, vol. 11, no. 2, pp. 4645-4662, 2016.

[128] Y. W. Chen, H. V. Lee, and S. B. A. Hamid, "Preparation of nanostructured cellulose via Cr(III)- and Mn(II)-transition metal salt catalyzed acid hydrolysis approach," Bioresources, vol. 11, no. 3, pp. 7224-7241, 2016.

[129] Y. W. Chen, H. V. Lee, and S. B. Abd Hamid, "Preparation and characterization of cellulose crystallites via Fe(III)-, Co(II)- and Ni(II)-assisted dilute sulfuric acid catalyzed hydrolysis process," Journal of Nano Research, vol. 41, pp. 96-109, 2016.

[130] L. P. Novo, J. Bras, A. Garcia, N. Belgacem, and A. A. da Silva Curvelo, "A study of the production of cellulose nanocrystals through subcritical water hydrolysis," Industrial Crops and Products, vol. 93, pp. 88-95, 2016.

[131] J. A. Sirvia, M. Visanko, and H. Liimatainen, "Acidic deep eutectic solvents as hydrolytic media for cellulose nanocrystal production," Biomacromolecules, vol. 17, no. 9, pp. 3025-3032, 2016.

[132] K. Nelson and T. Retsina, "Innovative nanocellulose process breaks the cost barrier," TAPPI Journal, vol. 13, no. 5, pp. 19-23, 2014.

[133] S. Ahola, M. Osterberg, and J. Laine, "Cellulose nanofibrils-Adsorption with poly(amideamine) epichlorohydrin studied by QCM-D and application as a paper strength additive," Cellulose, vol. 15, no. 2, pp. 303-314, 2008.

[134] P. Stenstad, M. Andresen, B. S. Tanem, and P. Stenius, "Chemical surface modifications of microfibrillated cellulose," Cellulose, vol. 15, no. 1, pp. 35-45, 2008.

[135] M. Henriksson, L. A. Berglund, P. Isaksson, T. Lindstrom, and T. Nishino, "Cellulose nanopaper structures of high toughness," Biomacromolecules, vol. 9, no. 6, pp. 1579-1585, 2008.

[136] L. Brinchi, F. Cotana, E. Fortunati, and J. M. Kenny, "Production of nanocrystalline cellulose from lignocellulosic biomass: technology and applications," Carbohydrate Polymers, vol. 94, no. 1, pp. 154-169, 2013.

[137] V. S. Chauhan and S. K. Chakrabarti, "Use of nanotechnology for high performance cellulosic and papermaking products," Cellulose Chemistry and Technology, vol. 46, no. 5-6, pp. 389-400, 2012.

[138] M. Szczesna-Antczak, J. Kazimierczak, and T. Antczak, "Nanotechnology- methods of manufacturing cellulose nanofibers," Fiber and Textile in Eastern Europe, vol. 20, no. 91, pp. 8-12, 2012.

[139] A. Dufresne, D. Dupeyre, and M. R. Vignon, "Cellulose microfibrils from potato tuber cells: processing and characterization of starch-cellulose microfibril composites," Journal of Applied Polymer Science, vol. 76, no. 14, pp. 2080-2092, 2000.

[140] M. E. Malainine, M. Mahrouz, and A. Dufresne, "Thermoplastic nanocomposites based on cellulose microfibrils from Opuntia ficus-indica parenchyma cell," Composites Science and Technology, vol. 65, no. 10, pp. 1520-1526, 2005.

[141] I. Sira and D. Plackett, "Microfibrillated cellulose and new nanocomposite materials: a review," Cellulose, vol. 17, no. 3, pp. 459-494, 2010.

[142] R. Zuluaga, J.-L. Putaux, A. Restrepo, I. Mondragon, and P. Ganan, "Cellulose microfibrils from banana farming residues: isolation and characterization," Cellulose, vol. 14, no. 6, pp. 585-592, 2007.

[143] A. Ferrer, I. Filpponen, A. Rodriguez, J. Laine, and O. J. Rojas, "Valorization of residual Empty Palm Fruit Bunch Fibers (EPFBF) by microfluidization: production of nanofibrillated cellulose and EPFBF nanopaper," Bioresource Technology, vol. 125, pp. 249-255, 2012.

[144] S.-Y. Lee, S.-J. Chun, I.-A. Kang, and J.-Y. Park, "Preparation of cellulose nanofibrils by high-pressure homogenizer and cellulose-based composite films," Journal of Industrial and Engineering Chemistry, vol. 15, no. 1, pp. 50-55, 2009.

[145] S. Panthapulakkal and M. Sain, "Preparation and characterization of cellulose nanofibril films from wood fibre and their thermoplastic polycarbonate composites," International Journal of Polymer Science, vol. 2012, Article ID 381342, pp. 1-6, 2012.

[146] T. T. T. Ho, K. Abe, T. Zimmermann, and H. Yano, "Nanofibrillation of pulp fibers by twin-screw extrusion," Cellulose, vol. 22, no. 1, pp. 421-433, 2015.

[147] A. Chakraborty, M. Sain, and M. Kortschot, "Cellulose microfibrils: a novel method of preparation using high shear refining and cryocrushing," Holzforschung, vol. 59, no. 1, pp. 102-107, 2005.

[148] A. N. Frone, D. M. Panaitescu, D. Donescu et al., "Preparation and characterization of PVA composites with cellulose nanofibers obtained by ultrasonication," Bioresources, vol. 6, no. 1, pp. 487-512, 2011.

[149] R. K. Johnson, A. Zink-Sharp, S. H. Renneckar, and W. G. Glasser, "A new bio-based nanocomposite: Fibrillated TEMPO-oxidized celluloses in hydroxypropylcellulose matrix," Cellulose, vol. 16, no. 2, pp. 227-238, 2009.

[150] E. H. Qua, P. R. Hornsby, H. S. S. Sharma, G. Lyons, and R. D. McCall, "Preparation and characterization of Poly(vinyl alcohol) nanocomposites made from cellulose nanofibers," Journal of Applied Polymer Science, vol. 113, no. 4, pp. 2238-2247, 2009.

[151] E. H. Qua, P. R. Hornsby, H. S. S. Sharma, and G. Lyons, "Preparation and characterisation of cellulose nanofibres," Journal of Materials Science, vol. 46, no. 18, pp. 6029-6045, 2011.

[152] A. F. Turbak, F. W. Snyder, and K. R. Sandberg, "Microfibrillated cellulose, a new cellulose product: properties, uses, and commercial potential," in Proceedings of the Journal of Applied Polymer Science. Applied Polymer Symposium, vol. 37, pp. 815-827, ITT Rayonier Inc., Shelton, WA, USA, 1983.

[153] F. W. Herrick, R. L. Casebier, J. K. Hamilton, and K. R. Sandberg, "Microfibrillated cellulose: morphology and accessibility," in Proceedings of the Journal of Applied Polymer Science. Applied Polymer Symposium, vol. 37, pp. 797-813, Shelton, WA, 1983.

[154] S. Iwamoto, A. N. Nakagaito, H. Yano, and M. Nogi, "Optically transparent composites reinforced with plant fiber-based nanofibers," Applied Physics A: Materials Science & Processing, vol. 81, no. 6, pp. 1109-1112, 2005.

[155] A. N. Nakagaito and H. Yano, "The effect of morphological changes from pulp fiber towards nano-scale fibrillated cellulose on the mechanical properties of high-strength plant fiber based composites," Applied Physics A: Materials Science & Processing, vol. 78, no. 4, pp. 547-552, 2004.

[156] B. Wang and M. Sain, "Isolation of nanofibers from soybean source and their reinforcing capability on synthetic polymers," Composites Science and Technology, vol. 67, no. 11-12, pp. 2521-2527, 2007.

[157] M. Henriksson and L. A. Berglund, "Structure and properties of cellulose nanocomposite films containing melamine formaldehyde," Journal of Applied Polymer Science, vol. 106, no. 4, pp. 2817-2824, 2007.

[158] M. L. Hassan, E. A. Hassan, and K. N. Oksman, "Effect of pretreatment of bagasse fibers on the properties of chitosan/microfibrillated cellulose nanocomposites," Journal of Materials Science, vol. 46, no. 6, pp. 1732-1740, 2011.

[159] O. Eriksen, K. Syverud, and 0. Gregersen, "The use of microfibrillated cellulose produced from kraft pulp as strength enhancer in TMP paper," Nordic Pulp & Paper Research Journal, vol. 23, no. 3, pp. 299-304, 2008.

[160] K. L. Spence, R. A. Venditti, Y. Habibi, O. J. Rojas, and J. J. Pawlak, "The effect of chemical composition on microfibrillar cellulose films from wood pulps: mechanical processing and physical properties," Bioresource Technology, vol. 101, no. 15, pp. 5961-5968, 2010.

[161] I. Besbes, M. R. Vilar, and S. Boufi, "Nanofibrillated cellulose from Alfa, Eucalyptus and Pine fibres: preparation, characteristics and reinforcing potential," Carbohydrate Polymers, vol. 86, no. 3, pp. 1198-1206, 2011.

[162] H. Liimatainen, T. Suopajorvi, J. Sirvio, O. Hormi, and J. Niinimaoki, "Fabrication of cationic cellulosic nanofibrils through aqueous quaternization pretreatment and their use in colloid aggregation," Carbohydrate Polymers, vol. 103, no. 1, pp. 187-192, 2014.

[163] M. Ankerfors, C. Aulin, and T. Lindstrom, "Nanocellulose research and developments at Innventia," in Proceedings of the TAPPI International Conference on Nanotechnology for Renewable Materials, pp. 600-634, USA, June 2011.

[164] T. Taniguchi and K. Okamura, "New films produced from microfibrillated natural fibres," Polymer International, vol. 47, no. 3, pp. 291-294, 1998.

[165] S. Iwamoto, A. N. Nakagaito, and H. Yano, "Nano-fibrillation of pulp fibers for the processing of transparent nanocomposites," Applied Physics A: Materials Science & Processing, vol. 89, no. 2, pp. 461-466, 2007.

[166] K. L. Spence, R. A. Venditti, O. J. Rojas, Y. Habibi, and J. J. Pawlak, "A comparative study of energy consumption and physical properties of microfibrillated cellulose produced by different processing methods," Cellulose, vol. 18, no. 4, pp. 1097-1111, 2011.

[167] M. Bulota, K. Kreitsmann, M. Hughes, and J. Paltakari, "Acetylated microfibrillated cellulose as a toughening agent in poly(lactic acid)," Journal of Applied Polymer Science, vol. 126, no. 1, pp. E448-E457, 2012.

[168] M. Jonoobi, A. P. Mathew, and K. Oksman, "Producinglow-cost cellulose nanofiber from sludge as new source of raw materials," Industrial Crops and Products, vol. 40, no. S1, pp. 232-238, 2012.

[169] Q. Q. Wang, J. Y. Zhu, R. Gleisner, T. A. Kuster, U. Baxa, and S. E. McNeil, "Morphological development of cellulose fibrils of a bleached eucalyptus pulp by mechanical fibrillation," Cellulose, vol. 19, no. 5, pp. 1631-1643, 2012.

[170] O. Nechyporchuk, F. Pignon, and M. N. Belgacem, "Morphological properties of nanofibrillated cellulose produced using wet grinding as an ultimate fibrillation process," Journal of Materials Science, vol. 50, no. 2, pp. 531-541, 2014.

[171] P. A. C. Gane, J. Schoelkopf, and D. Gantenbein, "Process for the production of nano-fibrillar cellulose suspensions," US Patent, vol. WO2010112519, no. A1, 2010.

[172] J. C. Husband, P. Svending, D. R. Skuse et al., "Paper filler composition," US Patient, vol. WO2010131016, no. A2, 2010.

[173] L. Zhang, T. Tsuzuki, and X. Wang, "Preparation of cellulose nanofiber from softwood pulp by ball milling," Cellulose, vol. 22, no. 3, pp. 1729-1741, 2015.

[174] K. Kekalainen, H. Liimatainen, F. Biale, and J. Niinimaki, "Nanofibrillation of TEMPO-oxidized bleached hardwood kraft cellulose at high solids content," Holzforschung, vol. 69, no. 9, pp. 1077-1088, 2015.

[175] A. Cobut, H. Sehaqui, and L. A. Berglund, "Cellulose nanocomposites by melt compounding of TEMPO-Treated wood fibers in thermoplastic starch matrix," Bioresources, vol. 9, no. 2, pp. 3276-3289, 2014.

[176] M. Hietala, P. Rollo, K. Kekoloinen, and K. Oksman, "Extrusion processing of green biocomposites: Compounding, fibrillation efficiency, and fiber dispersion," Journal of Applied Polymer Science, vol. 131, no. 6, Article ID 39981, 2014.

[177] A. Dufresne, J.-Y. Cavaille, and M. R. Vignon, "Mechanical behavior of sheets prepared from sugar beet cellulose microfibrils," Journal of Applied Polymer Science, vol. 64, no. 6, pp. 1185-1194, 1997.

[178] A. Bhatnagar and M. Sain, "Processing of cellulose nanofiber-reinforced composites," Journal of Reinforced Plastics and Composites, vol. 24, no. 12, pp. 1259-1268, 2005.

[179] A. Alemdar and M. Sain, "Isolation and characterization of nanofibers from agricultural residues--wheat straw and soy hulls," Bioresource Technology, vol. 99, no. 6, pp. 1664-1671, 2008.

[180] B. Wang, M. Sain, and K. Oksman, "Study of structural morphology of hemp fiber from the micro to the nanoscale," Applied Composite Materials, vol. 14, no. 2, pp. 89-103, 2007.

[181] K. Uetani and H. Yano, "Nanofibrillation of wood pulp using a high-speed blender," Biomacromolecules, vol. 12, no. 2, pp. 348-353, 2011.

[182] F. Jiang and Y.-L. Hsieh, "Chemically and mechanically isolated nanocellulose and their self-assembled structures," Carbohydrate Polymers, vol. 95, no. 1, pp. 32-40, 2013.

[183] A. N. Nakagaito, K. Ikenaga, and H. Takagi, "Cellulose nanofiber extraction from grass by a modified kitchen blender," Modern Physics Letters B, vol. 29, no. 6-7, Article ID 1540039, pp. 1540039-1540044, 2015.

[184] R. Kose, I. Mitani, W. Kasai, and T. Kondo, ""Nanocellulose" as a single nanofiber prepared from pellicle secreted by gluconacetobacter xylinus using aqueous counter collision," Biomacromolecules, vol. 12, no. 3, pp. 716-720, 2011.

[185] T. Kondo, R. Kose, H. Naito, and W. Kasai, "Aqueous counter collision using paired water jets as a novel means of preparing bio-nanofibers," Carbohydrate Polymers, vol. 112, pp. 284-290, 2014.

[186] A. de Campos, A. C. Correa, D. Cannella et al., "Obtaining nanofibers from curauaa and sugarcane bagasse fibers using enzymatic hydrolysis followed by sonication," Cellulose, vol. 20, no. 3, pp. 1491-1500, 2013.

[187] A. N. Nakagaito, K. Ikenaga, and H. Takagi, "Cellulose nanofiber extraction from grass by a modified kitchen blender," Modern Physics Letters B, vol. 29, no. 6-7, Article ID 1540039, 2015.

[188] H. Lee, J. Sundaram, L. Zhu, Y. Zhao, and S. Mani, "Improved thermal stability of cellulose nanofibrils using low-concentration alkaline pretreatment," Carbohydrate Polymers, vol. 181, pp. 506-513, 2018.

[189] Y. Feng, T. Cheng, W. Yang et al., "Characteristics and environmentally friendly extraction of cellulose nanofibrils from sugarcane bagasse," Industrial Crops and Products, vol. 111, pp. 285-291, 2018.

[190] C. A. Carrillo, J. Laine, and O. J. Rojas, "Microemulsion systems for fiber deconstruction into cellulose nanofibrils," ACS Applied Materials & Interfaces, vol. 6, no. 24, pp. 22622-22627, 2014.

[191] H. Du, C. Liu, Y. Zhang, G. Yu, C. Si, and B. Li, "Preparation and characterization of functional cellulose nanofibrils via formic acid hydrolysis pretreatment and the followed high-pressure homogenization," Industrial Crops and Products, vol. 94, pp. 736-745, 2016.

[192] G. Siqueira, S. Tapin-Lingua, J. Bras, D. da Silva Perez, and A. Dufresne, "Morphological investigation of nanoparticles obtained from combined mechanical shearing, and enzymatic and acid hydrolysis of sisal fibers," Cellulose, vol. 17, no. 6, pp. 1147-1158, 2010.

[193] Y. Qing, R. Sabo, J. Y. Zhu, U. Agarwal, Z. Cai, and Y. Wu, "A comparative study of cellulose nanofibrils disintegrated via multiple processing approaches," Carbohydrate Polymers, vol. 97, no. 1, pp. 226-234, 2013.

[194] Y. Chen, Y. He, D. Fan, Y. Han, G. Li, and S. Wang, "An efficient method for cellulose nanofibrils length shearing via environmentally friendly mixed cellulase pretreatment," Journal of Nanomaterials, vol. 2017, Article ID 1591504, 12 pages, 2017.

[195] M. Henriksson, G. Henriksson, L. A. Berglund, and T. Lindstram, "An environmentally friendly method for enzymeassisted preparation of microfibrillated cellulose (MFC) nanofibers," European Polymer Journal, vol. 43, no. 8, pp. 3434-3441, 2007.

[196] M. Paakko, M. Ankerfors, H. Kosonen et al., "Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels," Biomacromolecules, vol. 8, no. 6, pp. 1934-1941, 2007.

[197] S. Nie, K. Zhang, X. Lin et al., "Enzymatic pretreatment for the improvement of dispersion and film properties of cellulose nanofibrils," Carbohydrate Polymers, vol. 181, pp. 1136-1142, 2018.

[198] I. B. Tabar, X. Zhang, J. P. Youngblood, and N. S. Mosier, "Production of cellulose nanofibers using phenolic enhanced surface oxidation," Carbohydrate Polymers, vol. 174, pp. 120-127, 2017.

[199] L. Rosgaard, S. Pedersen, J. Langston, D. Akerhielm, J. R. Cherry, and A. S. Meyer, "Evaluation of minimal Trichoderma reesei cellulase mixtures on differently pretreated Barley straw substrates," Biotechnology Progress, vol. 23, no. 6, pp. 1270-1276, 2007.

[200] X. Turon, O. J. Rojas, and R. S. Deinhammer, "Enzymatic kinetics of cellulose hydrolysis: A QCM-D study," Langmuir, vol. 24, no. 8, pp. 3880-3887, 2008.

[201] M. Baackstroam, S. Bolivar, and J. Paltakari, "Effect of ionic form on fibrillation and the development of the fibre network strength during the refining of the kraft pulps," O Papel, vol. 73, no. 7, pp. 57-65, 2012.

[202] N. J. Davis and S. L. Flitsch, "Selective oxidation of monosaccharide derivatives to uronic acids," Tetrahedron Letters, vol. 34, no. 7, pp. 1181-1184, 1993.

[203] T. Saito, Y. Nishiyama, J. L. Putaux, M. Vignon, and A. Isogai, "Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose," Biomacromolecules, vol. 7, no. 6, pp. 1687-1691, 2006.

[204] I. Besbes, S. Alila, and S. Boufi, "Nanofibrillated cellulose from TEMPO-oxidized eucalyptus fibres: effect of the carboxyl content," Carbohydrate Polymers, vol. 84, no. 3, pp. 975-983, 2011.

[205] J. Zhang, H. Song, L. Lin, J. Zhuang, C. Pang, and S. Liu, "Microfibrillated cellulose from bamboo pulp and its properties," Biomass & Bioenergy, vol. 39, pp. 78-83, 2012.

[206] S. Coseri, G. Biliuta, B. C. Simionescu, K. Stana-Kleinschek, V. Ribitsch, and V. Harabagiu, "Oxidized cellulose--Survey of the most recent achievements," Carbohydrate Polymers, vol. 93, no. 1, pp. 207-215, 2013.

[207] M. Zhao, J. Li, E. Mano et al., "Oxidation of primary alcohols to carboxylic acids with sodium chlorite catalyzed by TEMPO and bleach," The Journal of Organic Chemistry, vol. 64, no. 7, pp. 2564-2566, 1999.

[208] T. Saito, M. Hirota, N. Tamura et al., "Individualization of nanosized plant cellulose fibrils by direct surface carboxylation using TEMPO catalyst under neutral conditions," Biomacromolecules, vol. 10, no. 7, pp. 1992-1996, 2009.

[209] S. P. Mishra, A.-S. Manent, B. Chabot, and C. Daneault, "The use of sodium chlorite in post-oxidation of TEMPO-oxidized pulp: Effect on pulp characteristics and nanocellulose yield," Journal of Wood Chemistry and Technology, vol. 32, no. 2, pp. 137-148, 2012.

[210] R. Shinoda, T. Saito, Y. Okita, and A. Isogai, "Relationship between length and degree of polymerization of TEMPO-oxidized cellulose nanofibrils," Biomacromolecules, vol. 13, no. 3, pp. 842-849, 2012.

[211] B. Wu, B. Geng, Y. Chen, H. Liu, G. Li, and Q. Wu, "Preparation and characteristics of TEMPO-oxidized cellulose nanofibrils from bamboo pulp and their oxygen-barrier application in PLA films," Frontiers of Chemical Science and Engineering, vol. 11, no. 4, pp. 554-563, 2017.

[212] U.-J. Kim and S. Kuga, "Ion-exchange chromatography by dicarboxyl cellulose gel," Journal of Chromatography A, vol. 919, no. 1, pp. 29-37, 2001.

[213] H. Liimatainen, N. Ezekiel, R. Sliz et al., "High-strength nanocellulose-talc hybrid barrier films," ACS Applied Materials & Interfaces, vol. 5, no. 24, pp. 13412-13418, 2013.

[214] H. Liimatainen, M. Visanko, J. Sirvio, O. Hormi, and J. Niinimaki, "Sulfonated cellulose nanofibrils obtained from wood pulp through regioselective oxidative bisulfite pretreatment," Cellulose, vol. 20, no. 2, pp. 741-749, 2013.

[215] P. A. Larsson, L. A. Berglund, and L. Wagberg, "Highly ductile fibres and sheets by core-shell structuring of the cellulose nanofibrils," Cellulose, vol. 21, no. 1, pp. 323-333, 2014.

[216] J. A. Sirvio, A. Kolehmainen, M. Visanko, H. Liimatainen, J. Niinimaki, and O. E. O. Hormi, "Strong, self-standing oxygen barrier films from nanocelluloses modified with regioselective oxidative treatments," ACS Applied Materials & Interfaces, vol. 6, no. 16, pp. 14384-14390, 2014.

[217] L. Wagberg, G. Decher, M. Norgren, T. Lindstrom, M. Ankerfors, and K. Axnas, "The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes," Langmuir, vol. 24, no. 3, pp. 784-795, 2008.

[218] C. Aulin, S. Ahola, P. Josefsson et al., "Nanoscale cellulose films with different crystallinities and mesostructures--their surface properties and interaction with water," Langmuir, vol. 25, no. 13, pp. 7675-7685, 2009.

[219] C. Aulin, J. Netrval, L. Wagberg, and T. Lindstrom, "Aerogels from nanofibrillated cellulose with tunable oleophobicity," Soft Matter, vol. 6, no. 14, pp. 3298-3305, 2010.

[220] C. Eyholzer, A. Borges De Couraca, F. Duc et al., "Biocomposite hydrogels with carboxymethylated, nanofibrillated cellulose powder for replacement of the nucleus pulposus," Biomacromolecules, vol. 12, no. 5, pp. 1419-1427, 2011.

[221] I. Siro, D. Plackett, M. Hedenqvist, M. Ankerfors, and T. Lindstrom, "Highly transparent films from carboxymethylated microfibrillated cellulose: the effect of multiple homogenization steps on key properties," Journal of Applied Polymer Science, vol. 119, no. 5, pp. 2652-2660, 2011.

[222] C. Eyholzer, N. Bordeanu, F. Lopez-Suevos, D. Rentsch, T. Zimmermann, and K. Oksman, "Preparation and characterization of water-redispersible nanofibrillated cellulose in powder form," Cellulose, vol. 17, no. 1, pp. 19-30, 2010.

[223] X. Cai, B. Riedl, and A. Ait-Kadi, "Effect of surface-grafted ionic groups on the performance of cellulose-fiber-reinforced thermoplastic composites," Journal of Polymer Science Part B: Polymer Physics, vol. 41, no. 17, pp. 2022-2032, 2003.

[224] A. P. Abbott, T. J. Bell, S. Handa, and B. Stoddart, "Cationic functionalisation of cellulose using a choline based ionic liquid analogue," Green Chemistry, vol. 8, no. 9, pp. 784-786, 2006.

[225] Y. Song, Y. Sun, X. Zhang, J. Zhou, and L. Zhang, "Homogeneous quaternization of cellulose in NaOH/Urea aqueous solutions as gene carriers," Biomacromolecules, vol. 9, no. 8, pp. 2259-2264, 2008.

[226] C. Aulin, E. Johansson, L. Wagberg, and T. Lindstrom, "Self-organized films from cellulose i nanofibrils using the layer-by-layer technique," Biomacromolecules, vol. 11, no. 4, pp. 872-882, 2010.

[227] A. Olszewska, P. Eronen, L.-S. Johansson et al., "The behaviour of cationic NanoFibrillar Cellulose in aqueous media," Cellulose, vol. 18, no. 5, pp. 1213-1226, 2011.

[228] T. T. T. Ho, T. Zimmermann, R. Hauert, and W. Caseri, "Preparation and characterization of cationic nanofibrillated cellulose from etherification and high-shear disintegration processes," Cellulose, vol. 18, no. 6, pp. 1391-1406, 2011.

[229] S. Saini, C. Y. Falco, M. N. Belgacem, and J. Bras, "Surface cationized cellulose nanofibrils for the production of contact active antimicrobial surfaces," Carbohydrate Polymers, vol. 135, pp. 239-247, 2016.

[230] J. Li, X. Wei, Q. Wang et al., "Homogeneous isolation of nanocellulose from sugarcane bagasse by high pressure homogenization," Carbohydrate Polymers, vol. 90, no. 4, pp. 1609-1613, 2012.

[231] K. Ninomiya, M. Abe, T. Tsukegi et al., "Lignocellulose nanofibers prepared by ionic liquid pretreatment and subsequent mechanical nanofibrillation of bagasse powder: Application to esterified bagasse/polypropylene composites," Carbohydrate Polymers, vol. 182, pp. 8-14, 2018.

[232] E. L. Smith, A. P. Abbott, and K. S. Ryder, "Deep eutectic solvents (DESs) and their applications," Chemical Reviews, vol. 114, no. 21, pp. 11060-11082, 2014.

[233] J. A. Sirvio, M. Visanko, and H. Liimatainen, "Deep eutectic solvent system based on choline chloride-urea as a pre-treatment for nanofibrillation of wood cellulose," Green Chemistry, vol. 17, no. 6, pp. 3401-3406, 2015.

[234] P. Li, J. A. Sirvio, A. Haapala, and H. Liimatainen, "Cellulose nanofibrils from nonderivatizing urea-based deep eutectic solvent pretreatments," ACS Applied Materials & Interfaces, vol. 9, no. 3, pp. 2846-2855, 2017.

[235] Y. Liu, B. Guo, Q. Xia et al., "Efficient cleavage of strong hydrogen bonds in cotton by deep eutectic solvents and facile fabrication of cellulose nanocrystals in high yields," ACS Sustainable Chemistry & Engineering, vol. 5, no. 9, pp. 7623-7631, 2017.

Hongxiang Xie, (1,2,3) Haishun Du, (4) Xianghao Yang, (1) and Chuanling Si (1,2,3)

(1) Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China

(2) State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou 510640, China

(3) Key Laboratory of Industrial Fermentation Microbiology of Ministry of Education and Tianjin Key Lab of Industrial Microbiology, Tianjin University of Science and Technology, Tianjin 300457, China

(4) Department of Chemical Engineering, Auburn University, Auburn, AL 36849, USA

Correspondence should be addressed to Chuanling Si;

Received 20 January 2018; Accepted 21 February 2018; Published 17 April 2018

Academic Editor: Tongqi Yuan

Caption: Figure 1: Schematic diagram of CNCs prepared by acid hydrolysis: (a) idealized cellulose microfibril showing one of the suggested configurations of the crystalline and amorphous regions and (b) CNCs after acid hydrolysis dissolved the disordered regions. Adapted from [5].

Caption: Figure 2: Schematic diagram of thermal stability contrast of CNCs from sulfuric acid hydrolysis, hydrochloric acid hydrolysis, or phosphoric acid hydrolysis [69].

Caption: Figure 3: The overall procedure for the manufacture of CNCs by using HPW (HPW: phosphotungstic acid, [H.sub.3]P[W.sub.12][O.sub.40]) [77].

Caption: Figure 4: Overall flow-process diagram of the preparation of CNCs by Fe[Cl.sub.3]/FA hydrolysis [63].

Caption: Figure 5: Schematic diagram of different cellulase impacts on cellulose, adapted from [81].

Caption: Figure 6: One-pot preparation of hydrophobic CNCs in TBAA/DMAc with acetic hydride (upper part) and the more typical route (lower part) [82].

Caption: Figure 7: Scheme of subcritical water batch setup [83].

Caption: Figure 8: A simplified process flow diagram of the AVAP NCs process [84].

Caption: Figure 9: The most applied mechanical treatment processes used in the fabrication of CNFs and their schematic diagram: (a) homogenizer, (b) microfluidizer, and (c) grinding. The photographs were adapted from [14],, and

Caption: Figure 10: Schematic diagram of the preparation of LCNFs and LNPs by using p-TsOH pretreatment [85].

Caption: Figure 11: Regioselective oxidation of C6 primary hydroxyls of cellulose to C6 carboxylate groups by TEMPO/NaBr/NaClO system in water at pH 10-11 [86].

Caption: Figure 12: Schematic diagrams of cellulose carboxylation via periodate- chlorite: (a) chemical formula for cellulose carboxylation via periodate-chlorite oxidation and (b) preparation of CNFs by periodate-chlorite oxidative system [87].

Caption: Figure 13: Schematic diagram of cellulose sulfonation via periodate- bisulfite treatment.

Caption: Figure 14: Schematic diagrams of (a) chemical formula for carboxymethylation of cellulose by using chloroacetic acid and (b) carboxymethylation pretreatment of cellulose for the preparation of CNFs and the application of resultant CNFs as nanopaper and flexible electrode [88].

Caption: Figure 15: Schematic diagram of cellulose cationization using (2,3- epoxypropyl) trimethylammonium chloride [89].

Caption: Figure 16: Schematic diagram of DES pretreatment for the preparation of anionic CNFs [90].
Table 1: Introduction of various preparation methods of CNCs
including major chemical reagents, typical conditions, yield,
thermal stability, and some other advantages/disadvantages.

                 Major chemical             conditions
Method               reagents                [ref.] (a)

Mineral             [H.sub.2]             64wt.% [H.sub.2]
acid               S[O.sub.4],              S[O.sub.4],
hydrolysis        HCl, [H.sub.3]          at 45[degrees]C
                 P[O.sub.4], HBr          for 30 min [30].

                                             3 h [58].

Solid                Cation                   75 wt.%
acid              exchange resin          Phosphotungstic
hydrolysis          (NKC-9),                 acid, at
                 Phosphotungstic         90[degrees]C for
                       acid                  30h [77].

Organic           Formic acid,         0.015M Fe[Cl.sub.3],
acid               oxalic acid,                88wt.%
hydrolysis         Maleic acid,          HC[O.sub.2]H, at
                p-toluenesulfonic          95[degrees]C
                       acid                 for 6h [63].

                                           Melt oxalate
                                           dihydrate, at
                                       110[degrees]C for 2h,
                                          then, sonicated
                                            for 18 min
                                             [91, 93].

Enzymatic           Cellulase             20 u/mL complex
hydrolysis                             enzymes (cellulase :
                                         xylanase = 9 : 1),
                                        at 50[degrees]C for
                                             5 h [104].

Oxidation      [(N[H.sub.4]).sub.2]           1mol/L
degradation        S[O.sub.5],          [(N[H.sub.4]).sub.2]
                  NaI[O.sub.4],           S[O.sub.5], at
                  NaCl[O.sub.2]          80[degrees]C for
                                            16 h [110].

Ionic          [BMIM]HS[O.sub.4],       [BMIM]HS[O.sub.4],
liquid              [BMIM]Cl,            at 70-90[degrees]C
                   [EMIM][OAc],            for 1 h [119].

Transition      Cr(III), Fe(III),       Solid-liquid ratio
metal           Al(III), Ni(II),        was 1 : 30 and 0.8M
catalysis         Co(II), Mn(II)      Cr[([NO.sub.3]).sub.3]
                                          was adopted at
                                       80[degrees]C for 1.5h

Subcritical             --               [H.sub.2]O, 120 C
water                                       and 20.3MPa
                                         for 60 min [130].

DES             Choline chloride/     Choline chloride/Oxalic
                   Oxalic acid           acid dihydrate =
                                       1 : 1 (Molar ratios),
                                      at 100[degrees]C for 6h,
                                          then Mechanical
                                        disintegration with
                                       microfluidizer [131].

AVAP             Sulfur dioxide/          S[O.sub.2]/EtOH/
                     Ethanol                [H.sub.2]O,

              Yield (b)    [T.sub.max] (c)
Method            (%)        ([degrees]C)

Mineral          15.7           253.2
                  80            347.2

Solid             60             350

Organic           75             355
                 80.6            360

Enzymatic        13.6             --

Oxidation         35            330.9

Ionic             --             260

Transition       83.6            344

Subcritical      21.9            345

DES               78             300

AVAP              24              --

Method                 disadvantages              Ref. (d)

Mineral        It is difficult to treat waste     [35-75]
acid             acid. [H.sub.2]S[O.sub.4]
hydrolysis        has a serious corrosion
                 problem to equipment. The
                    CNCs have excellent
                   suspension stability.
                 It is difficult to treat
               waste acid. HCl has a serious
                    corrosion problem to
                  equipment. The CNCs have
                 bad suspension stability.

Solid            Phosphotungstic acid could       [76-79]
acid                be recovered through
hydrolysis      extraction with ethyl ether
                  and is low corrosion to

Organic             Formic acid could be        [63, 80, 91-
acid                recovered by reduced            101]
hydrolysis      pressure distillation and is
                      low corrosion to
                 Oxalate dihydrate could be
                recovered by crystallization
                  and is low corrosion to

Enzymatic              This method is          [81, 102-105]
hydrolysis       environment-friendly, but
                   the cost is very high.

Oxidation         Carboxylation CNCs could       [106-117]
degradation         be produced. A large
                  number of oxidants were
                  consumed in the process.

Ionic              Ionic liquid could be       [82, 118-125]
liquid          reused. It has advantage to
                 preparing functional CNCs.
                  However, ionic liquid is
                 very expensive at present.

Transition       Transition metal could be     [68, 126-129]
metal               used to enhance the
catalysis        hydrolysis reactivity and
                 reduce the amount of acid.

Subcritical            The method is             [83, 130]
water             environment-friendly. The
                 cost of preparing CNCs was
                 estimated to be 0.02 $/kg.
                     The CNCs have bad
                   suspension stability.

DES                 DES is recoverable,            [131]
                     biodegradable, and
                 environmentally friendly.
                   However, the reaction
                 degree is low, so it needs
                    follow-up mechanical
                 treatment to get the CNCs

AVAP                 AVAP method could           [84, 132]
                   produce hydrophobicity
                 and morphology adjustable
                   CNCs. The method is low
                 cost and has been used for
                  industrial production of

(a) [ref.]: the reference of typical conditions; (b) Yield: the
details come from the reference of corresponding typical
condition; (c) [T.sub.max]: the max decomposition temperature,
the details come from the reference of corresponding typical
condition; (d) Ref.: the reference of corresponding method of
preparing CNCs.
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Author:Xie, Hongxiang; Du, Haishun; Yang, Xianghao; Si, Chuanling
Publication:International Journal of Polymer Science
Date:Jan 1, 2018
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