**2. Cellulose nanocrystals from classical acid hydrolysis**

#### **2.1 From cellulose to cellulose nanocrystals**

#### *2.1.1 Structure of cellulose*

Cellulose can be extracted from a large variety of sources, kike wood (the main source), seed fibers (cotton), bast fibers (flax, jute, ramie), some animal species (tunicates), fungi, and fruits, with different cellulose contents [1]. With more than 10<sup>11</sup> tons of cellulose produced each year [2], with less than 5% is extracted for applications, cellulose is the most abundant polymer on our planet [3]. Historically, cellulose was discovered after being extracted with nitric acid from several plants by the French researcher Anselme Payen in 1838, who characterized the residual compound with chemical formula C6H10O5 [3, 4]. In 1939, the name "cellulose" was for the first time introduced in the scientific community. After almost 200 years of cellulose extraction, modification, and industrial use, this sustainable and biodegradable polymer is currently used for several applications, from paper and cardboard to biomedical, building, textile, cosmetics, pharmacy, and composites [5, 6]. Indeed, intrinsic properties of cellulose fibers—abundancy, renewability, and availability—as well its fibrillary structure or mechanical properties (strength, flexibility) make them materials of choice for such applications. Indeed, in their natural form, cellulose fibers are included in hemicellulose- and lignin-based matrix like a natural composite and acts as the primary compound of plant cell walls by providing high mechanical properties and maintaining their structure.

polymerization (DP) is expressed as a function of the AGU unit number and depends on the cellulose source and isolation process (e.g., DP between 300 and 1700 for wood pulp and 800–1000 for cotton) [3]. The numerous hydroxyl groups—three per AGU—induce possible functionalization of cellulose as well as intra- and intermolecular hydrogen bonds in and between cellulose chains. These

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

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

*(a) Chemical structure of cellulose chain and (b) representation of some hydrogen bonds between two cellulose*

interactions form stabilized and flexible cellulose filaments: the cellulose

Moreover, cellulosic chains are rearranged into different regions: the ordered crystalline and the disordered amorphous ones. Indeed, a cellulose chain can be represented as a crystalline wire connected by amorphous areas (see **Figure 3(b)**). It explains the aggregation of cellulose chains and thus their arrangement into microfibrils. The latter are assembled in bundles, themselves assembled in cellulose fibers, with a semi-crystalline structure. Cellulose crystals present four polymorphs: cellulose I, II, III, and IV. Cellulose I is the most abundant form in nature and is present under cellulose Iα and Iβ forms whose ratio depends on the source and affect cellulose properties [3, 4]. Crystallinity of cellulose varies according to the source and is in the large range of 40–80% [3], leading to highly cohesive fibers. Looking more precisely at the level of the microfibrils, they are composed of elementary fibrils, with a diameter around 5 nm. This general structure provides

microfibrils (see **Figure 3(a)**).

**Figure 2.**

*chains.*

**79**

Looking more precisely at cellulose structure, it is a linear homopolymer of β-Dglucopyranose (C6H12O6) units. These anhydro-D-glucose units (AGU) are linked by β-(1–4)-glycosidic linkage, a covalent bonding between equatorial OH group in C4 and C1 carbon atom of the next unit. Every unit is twisted at 180°C with respect to its surrounding environment and is in chair conformation, with the three hydroxyl groups in equatorial position. Cellobiose (C12H22O11)—the combination of two anhydroglucose units (AGU)—is the repeating unit of cellulose [2, 3]. Cellulose general formula is represented in **Figure 2(a)**.

End groups of cellulose polymer are chemically different: one nonreducing end and one reducing bearing aldehyde group. Note that cellulose degree of

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

#### **Figure 2.**

**2. Cellulose nanocrystals from classical acid hydrolysis**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

ing high mechanical properties and maintaining their structure.

general formula is represented in **Figure 2(a)**.

**78**

Cellulose can be extracted from a large variety of sources, kike wood (the main source), seed fibers (cotton), bast fibers (flax, jute, ramie), some animal species (tunicates), fungi, and fruits, with different cellulose contents [1]. With more than 10<sup>11</sup> tons of cellulose produced each year [2], with less than 5% is extracted for applications, cellulose is the most abundant polymer on our planet [3]. Historically, cellulose was discovered after being extracted with nitric acid from several plants by the French researcher Anselme Payen in 1838, who characterized the residual compound with chemical formula C6H10O5 [3, 4]. In 1939, the name "cellulose" was for the first time introduced in the scientific community. After almost 200 years of cellulose extraction, modification, and industrial use, this sustainable and biodegradable polymer is currently used for several applications, from paper and cardboard to biomedical, building, textile, cosmetics, pharmacy, and composites [5, 6]. Indeed, intrinsic properties of cellulose fibers—abundancy, renewability, and availability—as well its fibrillary structure or mechanical properties (strength, flexibility) make them materials of choice for such applications. Indeed, in their natural form, cellulose fibers are included in hemicellulose- and lignin-based matrix like a natural composite and acts as the primary compound of plant cell walls by provid-

*Noncumulative evolution of the number of publications and patents dealing with CNC (source: SciFinder, April 2019—Descriptors, cellulose nanocrystal, cellulose nanorod, rodlike cellulose, cellulose nanowire, cellulose crystallite, cellulose nanoparticle, cellulose whiskers, nanocrystalline cellulose—Language, English).*

Looking more precisely at cellulose structure, it is a linear homopolymer of β-Dglucopyranose (C6H12O6) units. These anhydro-D-glucose units (AGU) are linked by β-(1–4)-glycosidic linkage, a covalent bonding between equatorial OH group in C4 and C1 carbon atom of the next unit. Every unit is twisted at 180°C with respect

to its surrounding environment and is in chair conformation, with the three

and one reducing bearing aldehyde group. Note that cellulose degree of

hydroxyl groups in equatorial position. Cellobiose (C12H22O11)—the combination of two anhydroglucose units (AGU)—is the repeating unit of cellulose [2, 3]. Cellulose

End groups of cellulose polymer are chemically different: one nonreducing end

**2.1 From cellulose to cellulose nanocrystals**

*2.1.1 Structure of cellulose*

**Figure 1.**

*(a) Chemical structure of cellulose chain and (b) representation of some hydrogen bonds between two cellulose chains.*

polymerization (DP) is expressed as a function of the AGU unit number and depends on the cellulose source and isolation process (e.g., DP between 300 and 1700 for wood pulp and 800–1000 for cotton) [3]. The numerous hydroxyl groups—three per AGU—induce possible functionalization of cellulose as well as intra- and intermolecular hydrogen bonds in and between cellulose chains. These interactions form stabilized and flexible cellulose filaments: the cellulose microfibrils (see **Figure 3(a)**).

Moreover, cellulosic chains are rearranged into different regions: the ordered crystalline and the disordered amorphous ones. Indeed, a cellulose chain can be represented as a crystalline wire connected by amorphous areas (see **Figure 3(b)**). It explains the aggregation of cellulose chains and thus their arrangement into microfibrils. The latter are assembled in bundles, themselves assembled in cellulose fibers, with a semi-crystalline structure. Cellulose crystals present four polymorphs: cellulose I, II, III, and IV. Cellulose I is the most abundant form in nature and is present under cellulose Iα and Iβ forms whose ratio depends on the source and affect cellulose properties [3, 4]. Crystallinity of cellulose varies according to the source and is in the large range of 40–80% [3], leading to highly cohesive fibers. Looking more precisely at the level of the microfibrils, they are composed of elementary fibrils, with a diameter around 5 nm. This general structure provides

**Figure 3.**

*Schematization of a simplified (a) composition of cellulose fiber (extracted and adapted from [7]) and (b) arrangements of crystalline and amorphous domains in cellulose chains (extracted from [8]).*

visualization of the different scales inside the cellulose fiber. In addition to being environmentally relevant, cellulose fibers present interesting mechanical properties, ability for further surface modification, low toxicity, low cost, and other properties making them outstanding materials for a lot of traditional as well as innovative applications.

CNC. Concerning their industrialization, around 10 CNC producers can be recorded, with annual production up to 400 tons/year. These productions are significantly lower than those of CNF, but requirement of more chemicals and difficult industrial production steps (washing, dialysis, and sonication) can easily explain this difference. **Table 1** shows the non-exhaustive list of CNC producers

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

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

CelluForce Inc. USA 400 America Process Inc. (GranBio) USA 200 Alberta Pacific Forest Industries Inc. Canada 180 (expected in 2021)

Anomera Inc. Canada 11 Forest Products Laboratory (FPL) USA 5 University of Maine USA 4 Blue Goose Biorefineries Inc. Canada 4 Cellulose Lab Canada 4 Advanced Cellulosic Material Inc. Canada 1 FPInnovations Canada 0.5 InnoTech Alberta Canada 0.3 Embrapa/National Nanotechnology Laboratory for Agriculture Brazil Pilot Melodea Israel Pilot

*and (c) wood cellulose nanocrystals (CNC) (extracted from [8]).*

*TEM images of (a) microfibrillated cellulose (MFC), (b) TEMPO-oxidized nanofibrillated cellulose (NFC),*

**Company Country Annual production capacity (tons)**

*Main CNC producers and their location and annual production capacity (data extracted from Refs. [12, 13]).*

**Figure 4.**

**Table 1.**

**81**
