ReviewSustainable isolation of nanocellulose from cellulose and lignocellulosic feedstocks: Recent progress and perspectives
Introduction
As the oldest and most abundant polymer on earth, cellulose has played a significant and irreplaceable role in polymer history. Cotton, hemp, and bamboo fibers were the primary fiber sources in ancient China for clothing, building, and papermaking. Industrial application of cellulose and its derivatives started shortly after its first extraction in lab in 1838 by French chemist Anselme Payen (Payen, 1838). Cellulose nitrate was the first synthetic polymer synthesized by Pelouze in 1838 (Pelouze, 1838), followed by a more scientific approach by Christian Friedrich Schonbein in 1846 using sulfuric acid and nitric acid (Schonbein, 1846; Seymour & Kauffman, 1992). The first commercially practical synthetic fiber from nitrocellulose, with the intention to replace the expensive oriental silk, was synthesized by French chemist Comte de Chardonnay in 1884 (Kauffman, 1993). Regenerated cellulose was among the first in producing aerogel (Kistler, 1931, Kistler, 1932). However, the role of cellulose in modern life gradually shattered with the success of petroleum-based polymers in the 20th century.
Over the past two decades, cellulose has resurrected with renewed interest, owing to its natural abundance, renewability, biodegradability, and inherent biocompatibility. Such interest in cellulose was further boosted as advanced nanotechnology and characterization techniques were developed to isolate, modify, and characterize cellulose. Looking into its microscopic structure, the elementary fibril of natural cellulose consists of 30–40 glucan chains with cross section width of 3–5 nm (Moon et al., 2011). The glucan chains are highly orientated and packed in parallel through interchain hydrogen bonding and inter-sheet Van der Waals interactions to form a crystalline domain (Nishiyama et al., 2002). Along the elementary fibrils, some segments are distorted due to internal strains, forming tilted and twisted regions, deemed as amorphous region with highly accessible surface (Fig. 1) (D. Klemm et al., 2011; Rowland & Roberts, 1971). Some elementary fibrils form weak hydrogen bond with adjacent fibrils, as the surface glucan chains are slightly distorted from the highly ordered internal chains (Rowland & Roberts, 1971). With the presence of these accessible surfaces, either on the surface of the elementary fibrils or in the distorted region along the elementary fibrils, that are more susceptible to chemicals/enzymes and mechanical energy, it is possible to isolate the whole (or part of) elementary fibrils as nanocellulose.
As a type of green and sustainable nanomaterial, nanocellulose has exhibited great potential for applications in flexible electronics (Hoeng et al., 2016; D.W. Zhao, Zhu, et al., 2020; Li et al., 2021), sensors (Thomas et al., 2018; Q. Wang, Yao, et al., 2019; Ye et al., 2020), energy storage materials (W.S. Chen et al., 2018; Qin et al., 2020; Vilela et al., 2019), packaging (Norrrahim et al., 2021), food and pharmaceuticals (Kedzior et al., 2020; D. Klemm et al., 2018), biomedicine (Heise et al., 2021; N. Lin & Dufresne, 2014), and nanocomposite (Guan et al., 2020; Dufresne, 2019; Barhoum et al., 2018). The growing interest in utilizing nanocellulose has driven fervent research in the top-down isolation of nanocellulose, including cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). Though the terminology may vary from one study to another, CNCs typically refer to short, rigid, and rod-like nanocrystals with the distorted amorphous region completely degraded along the elementary fibrils (Fig. 1). CNFs commonly refer to long, flexible, and fibril-like nanofibrils. There are mainly two different kinds of nanofibrils, one is essentially made of individual elementary fibrils (with complete breaking of inter-fibril hydrogen bonding), while the other one contains bundles of elementary fibrils (with partial breaking of inter-fibril hydrogen bonding) (Fig. 1). The following text will not distinguish these two different CNFs.
CNCs, since its first isolation in 1947 by treating cotton with 2.5 N sulfuric acid (Nickerson & Habrle, 1947), has been isolated using various types of mineral acids, including sulfuric acid (Dong et al., 1998), hydrochloric acid (Araki et al., 1998; Battista & Smith, 1962), hydrochloric acid vapor (Kontturi et al., 2016), and phosphoric acid (Espinosa et al., 2013). CNF was firstly isolated from wood using homogenizer in 1983 by Herrick et al. (1983) and Turbak et al. (1983), followed by the development of other high-shear treatments, including microfluidizer (Zimmermann et al., 2004), grinder (S. Iwamoto et al., 2007), aqueous counter collision (F. Jiang et al., 2016; Kose et al., 2011), high-speed blender (F. Jiang & Hsieh, 2013), and ultrasonication (W.S. Chen et al., 2011). However, high shear treatment alone cannot completely break the inter-fibril hydrogen bonds to liberate individual elementary fibrils. To overcome this difficulty, chemical or biological pretreatments can be introduced prior to high shear operation to yield CNFs. These pretreatment methods include the use of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation (T. Saito et al., 2006; T. Saito & Isogai, 2004), periodate-chlorite oxidation (Liimatainen et al., 2012), carboxymethylation (Aulin et al., 2010), and enzymatic degradation (X.S. Han et al., 2020; Han et al., 2021; Henriksson et al., 2007). All these conventional technologies for nanocellulose isolation have been well and extensively reviewed by many researchers, covering CNCs (Brinchi et al., 2013; Habibi et al., 2010; Mishra et al., 2019; Trache et al., 2017; Vanderfleet & Cranston, 2020), CNFs (Isogai et al., 2011; Nechyporchuk et al., 2016), and MFCs (Khalil et al., 2014; Osong et al., 2016). For instance, Vanderfleet and Cranston examined various factors including the sources of biomass, pretreatment methods, and isolation routes and their impact on the key characteristics of the isolated nanocellulose, such as surface chemistry and charge density, aspect ratio, colloidal stability, thermal stability, and crystallinity (Vanderfleet & Cranston, 2020). In this review, these conventional isolation methods will not be elaborated further.
Despite their capability in producing nanocellulose, the commercialization of conventional nanocellulose isolation approaches is challenged by the associated economic and environmental concerns. Implementing sulfuric acid, a most commonly-used mineral acid, not only incurs the generated CNC with lowered yield and reduced thermal stability, but also leads to costly recovery of the mineral acid and other valuable hydrolysates (Filson & Dawson-Andoh, 2009; F. Jiang et al., 2010; J. Wang et al., 2021). Alternatively, the use of hydrochloric acid alone, another common mineral acid, results in a limited CNC yield of <20% (Corrêa et al., 2010; Rosa et al., 2010), unless cation exchange resin (L.R. Tang et al., 2011) or energy-intensive hydrothermal treatment (H. Yu et al., 2013) is applied simultaneously. For CNF production, the application of high shear treatment alone is hindered by the high energy intensity and the low uniformity of the product (F. Jiang & Hsieh, 2013; Y. Jiang et al., 2018; C. Wang, Yuan, et al., 2020). On the other hand, chemical pretreatments involve the handling and consumption of toxic and/or costly reagents such as TEMPO (T. Saito et al., 2006; T. Saito & Isogai, 2004) and periodate (Liimatainen et al., 2012), or employ the intensive use of organic solvents for carboxymethylation (Aulin et al., 2010), making these techniques less attractive for potential industrial operators. Though enzymatic degradation appears to be a promising method for sustainable nanocellulose production (X.S. Han et al., 2020; Han et al., 2021; Henriksson et al., 2007), the current protocol barely has a chance to be commercialized given the limitations, predominantly including the extremely long reaction time, the use of pricy reagent (enzyme) and its poor recyclability.
Over the past few years, isolating nanocellulose in a more sustainable and efficient way has drawn extensive interest. Particular interest has been focusing on certain pretreatment techniques that employ the use of organic acids/anhydride, solid acid, Lewis acid, ionic liquid, and deep eutectic solvent (DES). All these techniques share one or more “green” perspectives including less corrosion to reactors, higher nanocellulose yield, easier chemical recyclability, lower energy consumption, reduced treatment and purification steps, as well as improved processing robustness towards various substrates. As more attention is being paid to the overall sustainability of nanocellulose isolation, this article provides a critical review of these sustainable nanocellulose isolation methods. The terminology of nanocellulose used in this paper aligns with Fig. 1, and therefore may be different from the source papers. Key pretreatment conditions of selected representative works with the characteristics of the isolated nanocellulose are summarized in Table 1.
Section snippets
Organic acids
CNCs production mediated by strong mineral acid (such as H2SO4) are challenged by several key drawbacks including massive production of contaminated effluent, high corrosion hazards to the facility, and limited yield (Bondeson et al., 2006; L.H. Chen, Zhu, et al., 2016b). In contrast, organic acid may serve as a more sustainable alternative, due to its weaker acidity, non- or reduced corrosivity, and easier recycling nature. Particularly, the relatively lower boiling temperature of most organic
Organic acid anhydrides
Acetic anhydride has been primarily used in synthesizing cellulose acetate following either heterogenous or homogeneous reactions, as discussed in Section 2.1.1 of this article. Recently, acetylated nanocellulose (both CNCs and CNFs) has been isolated following the same pyridine catalyzed acetic anhydride esterification reaction (Jonoobi et al., 2010; Sofla et al., 2019) (Fig. 4a). This processing medium can help swell cellulose fibers, leading to a DS value of 1.16. The acetylated kenaf fibers
Solid acid
In order to counteract the negative effect of mineral acid hydrolysis, such as the strong acidity that leads to reactor corrosion and excessive hydrolysis, as well as the difficulty in the recovery and reuse of chemicals, solid acid has been employed in isolating nanocellulose, including acidic cation exchange resin (Ahmed-Haras et al., 2020; L.R. Tang et al., 2011), lignin based solid acid (Hu et al., 2015; Zhu et al., 2020), and bio-char based solid acid (Y.F. Zhao, Lei, et al., 2020).
Other soluble Brønsted and Lewis acid
The major obstacle with regard to solid acid catalyzed hydrolysis is the low contact area in the heterogeneous reaction medium, which limits cellulose hydrolysis efficiency. To address this issue, soluble heteropoly acids such as phosphotungstic acid and transition metal-based Lewis acid have been used to isolate nanocellulose. Phosphotungstic acid (H3PW12O40, HPW) contains abundant Brønsted acid sites that can break the glycosidic bonds in cellulose and enables CNC isolation at high
Ionic liquid
Ionic liquid (IL), composed of an organic cation and an inorganic or organic anion, features its appearance as a liquid at room temperature or near room temperature (Sankhla et al., 2021). IL has recently drawn wide attention in providing a sustainable alternative to volatile organic solvents for industrial processing, due to its special characteristics of low flammability, non-volatility, non-explosiveness, possible high thermal stability, and ease of recycling (Q. Zhang et al., 2011). Many
Deep eutectic solvent
Deep eutectic solvents (DESs) are a type of solvents typically comprising two compounds, a hydrogen bond donor and a hydrogen bond acceptor (Le Gars et al., 2019). The correlation between these two compounds at a particular ratio leads to a eutectic mixture with the melting point much lower than any of its constituents. Depending on whether the nanocellulose surface group is modified, DESs systems for nanocellulose isolation can be classified into two types: non-derivatization and
Conclusion and perspectives
In conclusion, this article provides a critical review on recent progress in sustainable nanocellulose isolation technologies, including hydrolysis and/or derivatization using organic acid and anhydride, solid acid, recyclable Brønsted and Lewis acid, ionic liquid, and deep eutectic solvent. Compared to the conventional approaches that incur numerous economic and environmental concerns, these novel methods demonstrated clear advantages covering one or more of the following aspects: reduced
CRediT authorship contribution statement
Jungang Jiang: Conceptualization, Writing – original draft, Writing – review & editing. Yeling Zhu: Conceptualization, Writing – original draft, Writing – review & editing. Feng Jiang: Conceptualization, Resources, Supervision, Writing – original draft, Writing – review & editing.
Acknowledgment
This research was undertaken, in part, thanks to funding from the Canada Research Chairs program (231928). We gratefully acknowledge the financial support of the Province of British Columbia through the Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNRORD).
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