extraction, including high-pressure homogenization (HPH) or microfluidization,
grinding, ball milling, steam explosion, ultrasonication, cryocrushing, high-speed
blending, and so on (Xie et al. 2018). The mechanical approaches normally produce
long fiber networks with a typical diameter of less than 100 nm and lengths of 0.1 to
2 mm. The final step is usually concerned with the recuperation of NC and neutralization of toxic chemicals generated thereof, especially for acid treatments, and
thereby incurring additional costs. Other drawbacks, often cited with the mechanical
methods, include the requirement of bulky equipment, clogging, and the energyintensive nature of the process. As a result, these difficulties limit large-scale
commercialization of NC and often lead to the search for a greener way of
nanocellulose extraction. It should also be clear that some of the chemical or
mechanical methods have been developed primarily for cellulose extraction used
in the food, pharmaceutical, and biofuels industries, as evinced by research on NC in
the past two decades. For these and other methods, the readers are suggested to refer
to recently published excellent reviews (Xie et al. 2018; Phanthong et al. 2018). In
the context of “going green,” it is also pertinent to mention that research has been
directed toward exploring a number of enzymes (Michelin et al. 2020) for the
production of NC (Fig. 4.2).
Enzymatic hydrolysis of lignocellulose with cellulase releases the cellulose in
combination with pre-treatment methods that are used to delignify the feedstock
(Karim et al. 2017; Ribeiro et al. 2019; Squinca et al. 2020). However, large-scale
production of NC suffers from practical snags due to processing costs, scale-up
problems, and a lack of efficient enzymes. Moving ahead, as exemplary approaches,
we have presented our discussion on two NC-synthesis approaches, based on
ammonium persulfate (APS) and ionic liquids in the subsequent section.
Sulfuric acid hydrolysis of lignocellulosic feedstock has a long application
history in the delignification and production of cellulose. In fact, other acids,
including hydrochloric acid, phosphoric acid, hydrobromic acid, and formic acid,
have also been used, but the agglomeration of the resulting product tends to limit
their use (Yu et al. 2013). The use of sulfuric acid also generates sulfonated NC with
a stable dispersion but with poor thermal stability, which constrains their use in
composite materials. Additionally, the corrosion of equipment and the large amount
of toxic chemicals generated by the process have raised environmental concerns,
therefore, requiring a toxicity neutralization step that further increases the cost of the
manufacturing process (Leung et al. 2011). Recently, the use of ammonium
persulfate (APS) as an oxidant has been viewed as an eco-friendly method of
producing CNC (Zhang et al. 2016). The method had attracted considerable attention
due to its low long-term toxicity, high water solubility, low cost, and lesser use of
harsh extractive agents. In addition, directly carboxylated cellulose nanocrystals are
produced from lignocellulose by concomitant removal of lignin, hemicellulose, and
other plant contents (Zhang et al. 2016; Oun and Rhim 2017, 2018; Liu et al. 2020).
The free radicals generated in the oxidation process are responsible for removing the
noncellulosic components as well as breaking down the amorphous region, thereby
assisting the extraction of CNCs (Mascheroni et al. 2016). The resulting carboxylated CNCs are amenable for additional surface modification and tailoring with
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E. M. Abda and R. Konwarh
grinding, ball milling, steam explosion, ultrasonication, cryocrushing, high-speed
blending, and so on (Xie et al. 2018). The mechanical approaches normally produce
long fiber networks with a typical diameter of less than 100 nm and lengths of 0.1 to
2 mm. The final step is usually concerned with the recuperation of NC and neutralization of toxic chemicals generated thereof, especially for acid treatments, and
thereby incurring additional costs. Other drawbacks, often cited with the mechanical
methods, include the requirement of bulky equipment, clogging, and the energyintensive nature of the process. As a result, these difficulties limit large-scale
commercialization of NC and often lead to the search for a greener way of
nanocellulose extraction. It should also be clear that some of the chemical or
mechanical methods have been developed primarily for cellulose extraction used
in the food, pharmaceutical, and biofuels industries, as evinced by research on NC in
the past two decades. For these and other methods, the readers are suggested to refer
to recently published excellent reviews (Xie et al. 2018; Phanthong et al. 2018). In
the context of “going green,” it is also pertinent to mention that research has been
directed toward exploring a number of enzymes (Michelin et al. 2020) for the
production of NC (Fig. 4.2).
Enzymatic hydrolysis of lignocellulose with cellulase releases the cellulose in
combination with pre-treatment methods that are used to delignify the feedstock
(Karim et al. 2017; Ribeiro et al. 2019; Squinca et al. 2020). However, large-scale
production of NC suffers from practical snags due to processing costs, scale-up
problems, and a lack of efficient enzymes. Moving ahead, as exemplary approaches,
we have presented our discussion on two NC-synthesis approaches, based on
ammonium persulfate (APS) and ionic liquids in the subsequent section.
Sulfuric acid hydrolysis of lignocellulosic feedstock has a long application
history in the delignification and production of cellulose. In fact, other acids,
including hydrochloric acid, phosphoric acid, hydrobromic acid, and formic acid,
have also been used, but the agglomeration of the resulting product tends to limit
their use (Yu et al. 2013). The use of sulfuric acid also generates sulfonated NC with
a stable dispersion but with poor thermal stability, which constrains their use in
composite materials. Additionally, the corrosion of equipment and the large amount
of toxic chemicals generated by the process have raised environmental concerns,
therefore, requiring a toxicity neutralization step that further increases the cost of the
manufacturing process (Leung et al. 2011). Recently, the use of ammonium
persulfate (APS) as an oxidant has been viewed as an eco-friendly method of
producing CNC (Zhang et al. 2016). The method had attracted considerable attention
due to its low long-term toxicity, high water solubility, low cost, and lesser use of
harsh extractive agents. In addition, directly carboxylated cellulose nanocrystals are
produced from lignocellulose by concomitant removal of lignin, hemicellulose, and
other plant contents (Zhang et al. 2016; Oun and Rhim 2017, 2018; Liu et al. 2020).
The free radicals generated in the oxidation process are responsible for removing the
noncellulosic components as well as breaking down the amorphous region, thereby
assisting the extraction of CNCs (Mascheroni et al. 2016). The resulting carboxylated CNCs are amenable for additional surface modification and tailoring with
70
E. M. Abda and R. Konwarh