*Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents DOI: http://dx.doi.org/10.5772/intechopen.89878*

Deep eutectic solvents (DESs) are a new class of green organic solvents; they are in the continuity of molten salt and ionic liquid solvent, but they are less toxic and easier to use. A deep eutectic solvent is composed of at least two compounds, a Lewis or Brönsted acid and a base. According to its constituents, a DES can be classed in one of the four existing categories listed in **Table 3**.

The association of these compounds with a specific ratio forms an eutectic mixture with a melting temperature far below than of its constituents.

In the case of type III, it is accepted that the self-association occurs via hydrogen bonding interactions between the hydrogen bond donor and the hydrogen bond


*ChCl, choline chloride; OAD, oxalic acid dihydrate; t, toluenesulfonic; AH, aminoguanidine hydrochloride; GH, guanidine hydrochloride; APA, anhydrous phosphoric acid.*

#### **Table 4.**

*Overview of the different DES pretreatments tested for cellulose nanocrystals obtention.*

—with specific morphology and surface properties. A recent review of Roman et al. [44] explored CNC toxicity. Results of this study correlate with previous results of Lin et al. [39] and Kovacs et al. [45], demonstrating that CNC are not toxic by ingestion or dermal contact and for aquatic organisms. However, pulmonary and cytotoxicity are less ideal since their toxicity depends on CNC properties and form (especially if they are in powder form, since they are more volatile). In any case, toxicity of CNC is low, especially when they are in wet-state or in composites, films, or coatings, for example, not constraining development of new CNC-based

**Market Applications Exploited CNC properties References**

Coatings for flexible packaging Morphology

Filtration Mesoporous films and membranes High specific surface area

High mechanical properties Filmogenic properties Morphology

Rheological properties

High mechanical properties

Electrical insulating Piezoelectric properties Surface area

Hydrophilicity

Low toxicity Colloidal stability High mechanical properties Surface reactivity

Strength Large surface area

Emulsion interfacial stabilization

Morphology

[30, 31]

[32, 33]

[34–36]

[37]

[38, 39]

[40]

[43]

[41, 42]

Composites/films Nanocomposites

Electronics/sensors Flexible electronics

Energy Supercapacitors

E-paper Piezoelectric sensors

Biomedical Biocomposites for bone/tooth replacement Drug delivery Protein immobilization Wound dressings Biosensors

Flexible batteries

Cosmetics Hydrogels and foams Colloidal stability

Security Security papers and inks Iridescent properties

*Main market applications of CNC and corresponding properties and literature references.*

Coatings/paints/ adhesives

Flexible packaging Optical films

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

**3. Deep eutectic solvent for cellulose nanocrystals preparation**

chemicals and are difficult to industrialize.

As described in the first part of this chapter, the traditional methods used to obtain cellulose nanocrystal consist of strong acid hydrolysis, enzymatic hydrolysis, or oxidation reactions. These treatments allow to hydrolyzing the amorphous regions in cellulose chains, and they are often followed by mechanical or ultrasonic treatment to homogenize the suspension. However, these methods use toxic

**Type Component 1 Component 2** I Quaternary ammonium salt Metal chloride II Quaternary ammonium salt Metal chloride hydrate III Quaternary ammonium salt Hydrogen bond donor IV Metal chloride hydrate Hydrogen bond donor

products.

**Table 2.**

**Table 3.**

**86**

*The four DES families.*

acceptor. The strong hydrogen bonds between the different compounds prevent the crystallization of each product and decrease the melting point of the mixture below room temperature [46]. Type III DESs are easy to obtain by simply mixing the compounds with the right ratio at a temperature higher than the melting point during 1 h. The mixture obtained is homogenous and transparent and has a low vapor pressure. However, the main drawback of deep eutectic solvents is their price. Nevertheless, some studies show that it is possible to recycle them between

three and five times depending on the DES components and the usage [47]. Moreover, a subclass of DESs, named natural deep eutectic solvents (NADES), is formulated using bio-based compounds; these solvents are environmentally friendly and

*Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents*

DESs and NADES can be helpful in organic chemistry; indeed they can replace some toxic organic solvents; Zdanowicz summarized all their possible application domains for polysaccharide processing [48]. Among all these applications, one of them consists of the use of type III DESs as an acidic hydrolytic solvent for cellulose

This principle has been studied for the first time by Sirviö et al. [49]; they manage to extract individual nanocrystals using a choline chloride: oxalic acid dihydrate treatment from dissolving pulp. Different DESs had been prepared in different ratios and studied to hydrolyze the amorphous part of cellulose fibers from different sources: an overview of these treatments is summarized

In 2016, Sirviö tried three different DESs with choline chloride as hydrogen bond acceptor and oxalic acid (anhydrous or dihydrate), p-toluenesulfonic acid monohydrate, and levulinic acid as hydrogen bond donors [49]. Only DESs composed of choline chloride/oxalic acid dihydrate (ChCl:OAD) with a 1:1 molar ratio are allowed to obtain, after mechanical disintegration and CNC suspension. Different batches were prepared using different times and temperatures (**Figure 8(a–d)**). Cellulose nanocrystals obtained after 2 h of treatment at 120°C had the highest aspect ratio with a mean length of 353 16 nm and diameter of 9.9 0.7 nm (**Figure 8d**). The study showed that the final width of CNCs depends on the

Ling et al. studied the effect of ChCl:OAD treatment on cellulose nanocrystal structure [51]; three molar ratios were chosen 1:1, 1:2, and 1:3, under two temperatures 80 and 100°C. Cellulose nanocrystals suspensions were obtained in every case (**Figure 8(e–g)**). Lower crystallinity and lamellar structures were observed for CNCs with lower acid content, and hydrogen bonds were more broken with higher acid ratio (ChCl:OAD = 1:3) during the DES treatment. Moreover, these CNCs

In order to decrease the treatment temperature and increase the CNC yield, Yang et al. proposed to add a catalyst (FeCl3) during the DES treatment [53]. They found out that the optimum conditions for the treatment were 80°C and 6 h using a deep eutectic solvent composed of choline chloride, oxalic acid dihydrate, and FeCl36H2O with a molar ratio of 1:4.43:0.1. Cellulose nanocrystals with a length between 50 and 300 nm and a diameter range of 5–20 nm were isolated from

Using ChCl:OAD (1:1) DES, Laitinen et al., in 2017, were able to obtain a CNC suspension after 30 min of pretreatment at 100°C and a microfluidizer treatment. The cellulose nanocrystals obtained had a low charged content and could be used as

In 2017, interesting work was published by Liu et al. [52]. They reported an ultrafast fabrication of CNCs using the DES ChCl:OAD (1:1) assisted by microwave pretreatment. In only 3 min, the CNCs obtained had diameters between 100 and 25 nm and lengths between 100 and 350 nm (**Figure 8(k–m)**). The yield was 74.2%,

Another DES composed of choline chloride and citric acid with a molar ratio of 2:1 allowed Ibrahim et al., in 2018, to hydrolyze lignocellulosic materials and to

Some deep eutectic solvents can dissolve cellulose; it is the case for the DESs composed of guanidine hydrochloride and anhydrous phosphoric acid (1:2). This

obtained were better dispersed and had a higher aspect ratio.

eucalyptus kraft pulp with a yield of 90% (**Figure 8(h–i)**).

effective oil–water Pickering stabilizers (**Figure 8(j)**) [50].

and the nanocrystals' crystallinity was higher than 82%.

obtain cellulose nanocrystals [55].

**89**

their price can be lower.

*DOI: http://dx.doi.org/10.5772/intechopen.89878*

nanocrystal obtention.

pretreatment temperature [57].

in **Table 4**.

#### **Figure 8.**

*Transmission electron micrographs of CNCs using DES pretreatment: (a) ChCl:OAD 1:1, 2 h100°C [49]; (b) ChCl:OAD 1:1, 4 h100°C [49]; (c) ChCl:OAD 1:1, 6 h100°C [49]; (d) ChCl:OAD 1:1, 2 h120°C [49]; (e) ChCl:OAD 1:1, 1 h100°C [51]; (f) ChCl:OAD 1:2, 1 h100°C [51]; (g) ChCl:OAD 1:3, 1 h100°C [51]; (h) ChCl:OAD 1:4 + cat:FeCl3.6H2O (0.15 mmol/gDES), 6 h–80°C [53]; (i) ChCl:OAD 1:4 + cat:FeCl3.6H2O (0.3 mmol/gDES), 6 h80°C [53]; (j) ChCl:OAD 1:1, 30 min100°C [50]; (k) microwave-assisted ChCl:OAD 1:1, 3 min80°C [52]; (l) microwave-assisted ChCl:OAD 1:1, 3 min90°C [52]; (m) microwave-assisted ChCl:OAD 1:1, 3 min100°C [52]; (n) guanidine hydrochloride:anhydrous phosphoric acid 1:2, 24 h room temperature [56]; (o) aminoguanidine hydrochloride:glycerol 1:2, 10 min70° C [54]; (p) aminoguanidine hydrochloride:glycerol 1:2, 10 min 70°C [54].*

### *Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents DOI: http://dx.doi.org/10.5772/intechopen.89878*

three and five times depending on the DES components and the usage [47]. Moreover, a subclass of DESs, named natural deep eutectic solvents (NADES), is formulated using bio-based compounds; these solvents are environmentally friendly and their price can be lower.

DESs and NADES can be helpful in organic chemistry; indeed they can replace some toxic organic solvents; Zdanowicz summarized all their possible application domains for polysaccharide processing [48]. Among all these applications, one of them consists of the use of type III DESs as an acidic hydrolytic solvent for cellulose nanocrystal obtention.

This principle has been studied for the first time by Sirviö et al. [49]; they manage to extract individual nanocrystals using a choline chloride: oxalic acid dihydrate treatment from dissolving pulp. Different DESs had been prepared in different ratios and studied to hydrolyze the amorphous part of cellulose fibers from different sources: an overview of these treatments is summarized in **Table 4**.

In 2016, Sirviö tried three different DESs with choline chloride as hydrogen bond acceptor and oxalic acid (anhydrous or dihydrate), p-toluenesulfonic acid monohydrate, and levulinic acid as hydrogen bond donors [49]. Only DESs composed of choline chloride/oxalic acid dihydrate (ChCl:OAD) with a 1:1 molar ratio are allowed to obtain, after mechanical disintegration and CNC suspension. Different batches were prepared using different times and temperatures (**Figure 8(a–d)**). Cellulose nanocrystals obtained after 2 h of treatment at 120°C had the highest aspect ratio with a mean length of 353 16 nm and diameter of 9.9 0.7 nm (**Figure 8d**). The study showed that the final width of CNCs depends on the pretreatment temperature [57].

Ling et al. studied the effect of ChCl:OAD treatment on cellulose nanocrystal structure [51]; three molar ratios were chosen 1:1, 1:2, and 1:3, under two temperatures 80 and 100°C. Cellulose nanocrystals suspensions were obtained in every case (**Figure 8(e–g)**). Lower crystallinity and lamellar structures were observed for CNCs with lower acid content, and hydrogen bonds were more broken with higher acid ratio (ChCl:OAD = 1:3) during the DES treatment. Moreover, these CNCs obtained were better dispersed and had a higher aspect ratio.

In order to decrease the treatment temperature and increase the CNC yield, Yang et al. proposed to add a catalyst (FeCl3) during the DES treatment [53]. They found out that the optimum conditions for the treatment were 80°C and 6 h using a deep eutectic solvent composed of choline chloride, oxalic acid dihydrate, and FeCl36H2O with a molar ratio of 1:4.43:0.1. Cellulose nanocrystals with a length between 50 and 300 nm and a diameter range of 5–20 nm were isolated from eucalyptus kraft pulp with a yield of 90% (**Figure 8(h–i)**).

Using ChCl:OAD (1:1) DES, Laitinen et al., in 2017, were able to obtain a CNC suspension after 30 min of pretreatment at 100°C and a microfluidizer treatment. The cellulose nanocrystals obtained had a low charged content and could be used as effective oil–water Pickering stabilizers (**Figure 8(j)**) [50].

In 2017, interesting work was published by Liu et al. [52]. They reported an ultrafast fabrication of CNCs using the DES ChCl:OAD (1:1) assisted by microwave pretreatment. In only 3 min, the CNCs obtained had diameters between 100 and 25 nm and lengths between 100 and 350 nm (**Figure 8(k–m)**). The yield was 74.2%, and the nanocrystals' crystallinity was higher than 82%.

Another DES composed of choline chloride and citric acid with a molar ratio of 2:1 allowed Ibrahim et al., in 2018, to hydrolyze lignocellulosic materials and to obtain cellulose nanocrystals [55].

Some deep eutectic solvents can dissolve cellulose; it is the case for the DESs composed of guanidine hydrochloride and anhydrous phosphoric acid (1:2). This

acceptor. The strong hydrogen bonds between the different compounds prevent the crystallization of each product and decrease the melting point of the mixture below room temperature [46]. Type III DESs are easy to obtain by simply mixing the compounds with the right ratio at a temperature higher than the melting point during 1 h. The mixture obtained is homogenous and transparent and has a low vapor pressure. However, the main drawback of deep eutectic solvents is their price. Nevertheless, some studies show that it is possible to recycle them between

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

*Transmission electron micrographs of CNCs using DES pretreatment: (a) ChCl:OAD 1:1, 2 h100°C [49]; (b) ChCl:OAD 1:1, 4 h100°C [49]; (c) ChCl:OAD 1:1, 6 h100°C [49]; (d) ChCl:OAD 1:1, 2 h120°C [49]; (e) ChCl:OAD 1:1, 1 h100°C [51]; (f) ChCl:OAD 1:2, 1 h100°C [51]; (g) ChCl:OAD 1:3, 1 h100°C [51]; (h) ChCl:OAD 1:4 + cat:FeCl3.6H2O (0.15 mmol/gDES), 6 h–80°C [53]; (i) ChCl:OAD 1:4 + cat:FeCl3.6H2O (0.3 mmol/gDES), 6 h80°C [53]; (j) ChCl:OAD 1:1, 30 min100°C [50]; (k) microwave-assisted ChCl:OAD 1:1, 3 min80°C [52]; (l) microwave-assisted ChCl:OAD 1:1, 3 min90°C [52]; (m) microwave-assisted ChCl:OAD 1:1, 3 min100°C [52]; (n) guanidine hydrochloride:anhydrous phosphoric acid 1:2, 24 h room temperature [56]; (o) aminoguanidine hydrochloride:glycerol 1:2, 10 min70°*

*C [54]; (p) aminoguanidine hydrochloride:glycerol 1:2, 10 min 70°C [54].*

**Figure 8.**

**88**

solvent was studied by Sirviö to treat dissolving pulp during 24 h and then regenerate it into cellulose nanoparticles [56] (**Figure 8n**)).

**References**

642-17370-7\_2

1066-1076

3358-3393

1519-1542

2012. 460 p

3941-3994

**91**

1996;**4**:173-207

[1] Varshney VK, Naithani S. Chemical functionalization of cellulose derived from nonconventional sources. In: Kalia S, Kaith BS, Kaur I, éditeurs. Cellulose Fibers: Bio- and Nano-Polymer Composites: Green Chemistry and Technology. Berlin, Heidelberg: Springer, Berlin, Heidelberg; 2011. p. 43–60. DOI: 10.1007/978-3-

*DOI: http://dx.doi.org/10.5772/intechopen.89878*

[9] Ranby BG. Fibrous macromolecular systems. Cellulose and muscle. The colloidal properties of cellulose micelles. Discussions of the Faraday Society.

[10] Ranby BG, Ribi E. Über den feinbau der zellulose. Experientia. 1950;**6**:12-14

[11] CelluForce [Internet]. CelluForce. 2016. Available at: https://www.celluf

International Activities on Cellulose

[13] Cranston E. Overview of Canada. Chiba, Japan: TAPPI Nano Division;

[14] Lima MM de S, Borsali R. Rodlike cellulose microcrystals: Structure, properties, and applications. Macromolecular Rapid

Communications. 2004;**25**(7):771-787

[15] Habibi Y, Lucia LA, Rojas OJ. Cellulose nanocrystals: Chemistry, selfassembly, and applications. Chemical Reviews. 2010;**110**(6):3479-3500

[16] Camarero Espinosa S, Kuhnt T, Foster EJ, Weder C. Isolation of thermally stable cellulose nanocrystals

[17] Chen L, Wang Q, Hirth K, Baez C, Agarwal UP, Zhu JY. Tailoring the yield and characteristics of wood cellulose nanocrystals (CNC) using concentrated acid hydrolysis. Cellulose. 2015;**22**(3):

[18] Viet D, Beck-Candanedo S, Gray DG. Dispersion of cellulose nanocrystals in polar organic solvents. Cellulose.

by phosphoric acid hydrolysis. Biomacromolecules. 2013;**14**(4):

1223-1230

1753-1762

2007;**14**:109-113

orce.com/en/technology/

Nanomaterials. 2017

2019

[12] ISO/TC 6/TG 1-Cellulose Nanomaterials. Summary of

1951;**11**:158-164

*Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents*

[2] Suhas GVK, Carrott PJM, Singh R, Chaudhary M, Kushwaha S. Cellulose: A

[3] Klemm D, Heublein B, Fink H-P, Bohn A. Cellulose: Fascinating biopolymer and sustainable raw material. Angewandte Chemie International Edition. 2005;**44**(22):

[4] O'Sullivan AC. Cellulose: The structure slowly unravels. Cellulose.

[5] Habibi Y. Key advances in the chemical modification of nanocelluloses. Chemical Society Reviews. 2014;**43**(5):

[6] Dufresne A. Nanocellulose: From Nature to High Performance Tailored

Materials. Berlin: de Gruyter;

[7] Capron I, Rojas OJ, Bordes R.

[8] Moon RJ, Martini A, Nairn J, Simonsen J, Youngblood J. Cellulose nanomaterials review: Structure, properties and nanocomposites. Chemical Society Reviews. 2011;**40**(7):

Science. 2017;**29**:83-95

Behavior of nanocelluloses at interfaces. Current Opinion in Colloid & Interface

review as natural, modified and activated carbon adsorbent. Bioresource Technology. 2016;**216**:

In addition to the hydrolyzation of cellulose amorphous part, some DESs can simultaneously chemically modify the CNC surface. For example, Li et al. obtained cationic nanocrystals using aminoguanidine hydrochloride and glycerol mixture with a molar ratio of 1:2 (**Figure 8(o–p)**) [54].
