nanocrystals. Herein, the cotton powder was introduced into microwave pressure
vessels and 1 M APS solution was added to fabricate CNCs. By varying the reaction
conditions with the microwave, high-quality CNCs with a defined hydrodynamic
diameter (150 nm) and zeta potential (À0.040 V) were produced in just 90 minutes.
The APS process has also been modified to produce CNFs. Filipova et al. (2018)
generated CNFs from bleached birch kraft pulp by resorting to combined APS
oxidation and mechanical treatment, based on a high shear laboratory mixer and
ultrasonication. The CNFs had a diameter of 20–300 nm, a crystallinity index of
74.3%, and a zeta potential À26.9 Æ 1.8 mV.
Ionic liquids (ILs) are liquid salt compounds that form both anionic and cationic
species at room temperature or near room temperature. They are considered a green
solvent due to several salient features, including very low or even negligible vapor
pressure, high thermal stability, and chemical stability, good solubility in polar and
nonpolar solvents, easy recovery, and nonflammability, among others (Usmani et al.
2020). Notably, their attractive applications in cellulose hydrolysis include homogenous catalysis, executed under mild conditions, with the possibility of re-extraction
of the solvent. Studies have been undertaken to facilitate NC generation from
microcrystalline cellulose (MCC), wood pulp, wax, etc., using ionic liquids as
solvent and catalyst, most commonly, 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO 4 ]), 1-ethyl-3-methylimidazolium acetate (EMIMAc), and
1-ethyl-3-methylimidazole chloride ([EMIM][Cl]) (Vinogradova and Chen 2016).
Tan et al. (2015) produced rod-shaped CNCs (diameter, 15 to 20 nm, and length,
70 to 80 nm) by dissolving MCC with [Bmim][HSO 4 ]. The CNCs were small in size
and high in crystallinity at the optimal reaction temperature of 90
C. Additionally,
mixed solvent systems were chosen by Miao et al. (2016) to prepare hydrophobic
CNCs from wood pulpboard. In their work, they used tetrabutylammonium acetatedimethylacetamide mixed solvent system in combination with acetic anhydride to
cleave the amorphous cellulose and to acetylate the hydroxyl groups. The method
has been shown to be very effective in generating nanocrystals, compatible with
polymer materials. Al Hakkak et al. (2019) attempted nonsolvent addition and
drying to produce rather rare spherical cellulose nanoparticles by dissolving MCC
with EMIMAc. The anhydrous environment assisted in preventing the aggregation
of the generated nanocellulose. Ionic liquids-assisted fabrication of NC can also be
combined with enzymatic or mechanical methods to yield a product with desired
properties. An important consideration in this aspect is the selection of enzymes,
where their activity and stability remain acceptable in the presence of ionic liquids.
In the presence of [Bmim][HSO 4 ], Zhao et al. (2017) resorted to enzymatic esterification for the production of cellulose nanocrystals. Phanthong et al. (2017) applied
ball milling in the presence of ionic liquid and produced cellulose nanocrystals
(Phanthong et al. 2017). Ball milling is considered a fast and a green way for milling
cellulose, which effectively affects the microscopic and macroscopic properties. As a
result, the combined approach achieved a yield of 93.1%, and multiple rounds of
reuse of the ILs were also possible. Prior to moving ahead, readers may peruse the
following table (Table 4.1) to have a contrasting account of the various methods used
for the preparation of NC.
72
E. M. Abda and R. Konwarh
vessels and 1 M APS solution was added to fabricate CNCs. By varying the reaction
conditions with the microwave, high-quality CNCs with a defined hydrodynamic
diameter (150 nm) and zeta potential (À0.040 V) were produced in just 90 minutes.
The APS process has also been modified to produce CNFs. Filipova et al. (2018)
generated CNFs from bleached birch kraft pulp by resorting to combined APS
oxidation and mechanical treatment, based on a high shear laboratory mixer and
ultrasonication. The CNFs had a diameter of 20–300 nm, a crystallinity index of
74.3%, and a zeta potential À26.9 Æ 1.8 mV.
Ionic liquids (ILs) are liquid salt compounds that form both anionic and cationic
species at room temperature or near room temperature. They are considered a green
solvent due to several salient features, including very low or even negligible vapor
pressure, high thermal stability, and chemical stability, good solubility in polar and
nonpolar solvents, easy recovery, and nonflammability, among others (Usmani et al.
2020). Notably, their attractive applications in cellulose hydrolysis include homogenous catalysis, executed under mild conditions, with the possibility of re-extraction
of the solvent. Studies have been undertaken to facilitate NC generation from
microcrystalline cellulose (MCC), wood pulp, wax, etc., using ionic liquids as
solvent and catalyst, most commonly, 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO 4 ]), 1-ethyl-3-methylimidazolium acetate (EMIMAc), and
1-ethyl-3-methylimidazole chloride ([EMIM][Cl]) (Vinogradova and Chen 2016).
Tan et al. (2015) produced rod-shaped CNCs (diameter, 15 to 20 nm, and length,
70 to 80 nm) by dissolving MCC with [Bmim][HSO 4 ]. The CNCs were small in size
and high in crystallinity at the optimal reaction temperature of 90
C. Additionally,
mixed solvent systems were chosen by Miao et al. (2016) to prepare hydrophobic
CNCs from wood pulpboard. In their work, they used tetrabutylammonium acetatedimethylacetamide mixed solvent system in combination with acetic anhydride to
cleave the amorphous cellulose and to acetylate the hydroxyl groups. The method
has been shown to be very effective in generating nanocrystals, compatible with
polymer materials. Al Hakkak et al. (2019) attempted nonsolvent addition and
drying to produce rather rare spherical cellulose nanoparticles by dissolving MCC
with EMIMAc. The anhydrous environment assisted in preventing the aggregation
of the generated nanocellulose. Ionic liquids-assisted fabrication of NC can also be
combined with enzymatic or mechanical methods to yield a product with desired
properties. An important consideration in this aspect is the selection of enzymes,
where their activity and stability remain acceptable in the presence of ionic liquids.
In the presence of [Bmim][HSO 4 ], Zhao et al. (2017) resorted to enzymatic esterification for the production of cellulose nanocrystals. Phanthong et al. (2017) applied
ball milling in the presence of ionic liquid and produced cellulose nanocrystals
(Phanthong et al. 2017). Ball milling is considered a fast and a green way for milling
cellulose, which effectively affects the microscopic and macroscopic properties. As a
result, the combined approach achieved a yield of 93.1%, and multiple rounds of
reuse of the ILs were also possible. Prior to moving ahead, readers may peruse the
following table (Table 4.1) to have a contrasting account of the various methods used
for the preparation of NC.
72
E. M. Abda and R. Konwarh