Comparative Assessment of Methods for Producing Cellulose I

Chapter 2

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Comparative Assessment of Methods for Producing Cellulose I Nanocrystals from Cellulosic Sources Jia Mao,1,2 Hatem Abushammala,1,2 Nicole Brown,3 and Marie-Pierre Laborie*,1,2 1Chair

of Forest Biomaterials, Faculty of Environment and Natural Resources, University of Freiburg, Werthmannstr. 6, 79085 Freiburg, Germany 2FIT - Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany 3Department of Agricultural and Biological Engineering, The Pennsylvania State University, 202 Forest Resources Building, University Park, Pennsylvania 16802, United States *E-mail: [email protected].

Current environmental concerns foster a strong interest in extracting polymers and building-blocks from lignocellulosic biomass. Among these, cellulose nanocrystals (CNCs) have raised both academic and industrial interest because of their advantageous properties. Alongside their high mechanical properties, low density, high surface area and biodegradability, CNCs also lend themselves to functionalization and self-assembly into interesting structures. As a result, they have shown great potential in a wide range of applications from the automotive industry, to packaging materials and tissue engineering. CNCs are mainly produced using a top-down hydrolytic approach from biomass. Production methods vary significantly, aiming at maximum CNC yield and quality, while also considering environmental impact and economics. This manuscript reviews the traditional and more recent routes for producing cellulose nanocrystals. An approach to compare effectiveness and environmental impact of CNC production

© 2017 American Chemical Society Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

from Brønsted acids is also proposed and implemented to compare the various production routes.


Introduction After plant fibers are sufficiently separated and degraded by chemical or mechanical means into smaller elementary constituents, small highly crystalline cellulosic fractions remain. Typically, this is accomplished by strong acid hydrolysis, and yields rod-like cellulose nanocrystals (CNCs) with lengths between 100-400 nm and widths of less than 10 nm (1–4). Although discovered much earlier, the utilization of nanocellulose as reinforcement for polymers has attracted the attention of materials scientists to the field intensively in the last 20 years (5). Since then, and owing to their diverse properties, research in CNCs has continued to escalate, both in terms of fundamental research, and industrial application. Bio-based and biocompatible, CNCs are also very rigid. They offer excellent mechanical properties with Young’s modulus of ~140 GPa and with a strength of 10 GPa (6). Depending on the isolation mechanism, the biomass feedstock selected and the treatment conditions, the surface charge density, density, electrical and optical properties of CNCs (7–9) can be in parts tailored. Further modulation of properties is possible by subsequent surface functionalization. As such, CNCs are foreseen as drivers of new technologies in the forest products and bio-based materials industries with emerging applications in regenerative medicine (10, 11), optics (12), composite films (13), hydrogels (14), coatings (15), pharmaceuticals and food packaging (16, 17) as well as in the automotive sector (18). Indeed, over the last 10 years, an increasing pool of research has emerged describing various production methods of CNCs as well as their potential applications. A growing number of academic articles, reviews and patents have been published (Figure 1). More than 7000 papers were found in academic databases, including ~800 patents on CNCs, with the highest citation being 148 times for a particular patent (19, 20). The topics mainly focus on fundamental studies, traditional production approaches, surface modification and functionalization, and the potential applications of CNCs in nanocomposites. The potential of CNCs has also triggered commercial scale production. Several commercial manufacturing facilities were recently built (Table 1). The first commercial manufacturer, CelluForce, was launched in Canada by FPInnovations in 2011. It is reported to produce around 1000 kilograms of CNCs per day. Nowadays, one can purchase cellulose nanocrystals with a wide range of properties and surface chemistry for a price ranging from 4-10 $/ kg to 1000-20000 $/kg (21–23). In commercial CNC production, the conventional concentrated sulfuric acid hydrolysis method is the most widely used approach.

20 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 1. Number of articles, reviews and patents on CNCs from a literature search (performed date: December, 2016; database: web of science, scifinder; keywords: cellulose nanocrystal/s, cellulose nanowhisker/s, nanocrystalline cellulose).


Table 1. Main Producers of CNCs on Industrial Scale

22


Company

Launch

Country

Source

Productivity (kg/day)

Method

Product

Ref.

CelluForce

2011

Canada

1000

Kraft pulp

Acid hydrolysis

CNCs

(24)

US Forest Products Laboratory

2012

US

10

Forest products

Acid hydrolysis

CNCs

(25)

Blue Goose Biorefineries

2013

Canada

10

Wood, Pulps, Recycled paper

H2O2 oxidization, alkaline extraction

CNCs

(21)

Melodea

2013

Israel

100

Cellulose-containing waste

n.a.

Carboxylated CNCs

(26)

Alberta Innovates Technology Futures (AITF)

2014

Canada

20

Wood, pulps

Acid hydrolysis

CNCs

(27)

American Process

2015

US

500

Forest and agricultural residue

n.a.

Hydrophobic CNCs

(28)

India Council for Ag. Research

2015

India

10

Cotton, agro-biomass, bagasse

n.a.

CNCs

(29)

Holmen AB

2016

Sweden

100

n.a.

n.a.

CNCs

(30)

Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Despite the large interest in CNCs, few reviews have comprehensively discussed the numerous approaches for producing CNC in terms of isolation efficiency and environmental impact. The objective of this review is to critically assess the numerous approaches reported to produce cellulose nanocrystals from cellulosic substrates. To that aim, the basic structure, biosynthesis and polymorphism of cellulose as it occurs in natural lignocellulosic resources is first reviewed. A systematic survey of the main routes and reactants utilized to produce CNCs from cellulosic sources follows. As Brønsted acid hydrolysis emerges as the most common route to produce CNCs from cellulose, together with a variety of hybrid methods based on acid hydrolysis, we then define simple parameters to quantify reaction efficiency and severity for any Brønsted-acid based hydrolysis process. This allows us in the last section to compare the various chemical routes surveyed in terms of CNC production efficiency and reaction severity, regardless of the raw material, reactant and reaction conditions employed. These parameters are proposed as tools for comparing and selecting a desirable reaction system based on specific constraints and priorities. Note that recent studies report the direct production of CNCs from much more complex lignocellulosic sources such as wood, using multiple steps or even single step (31–36). The resulting nanocrystals appear to have very distinct surface properties (35, 36). Also cellulose nanocrystals can be regenerated into anhydroglucose interlinked by β (1→4) glycosidic bonds. It is the major reinforcing polymer comprising plant cell walls, existing in the form of semicrystalline microfibrils. In nature, cellulose is generated from Terminal Enzyme Complexes (TCs), which are made of subunits arranged in a rosette (wood, plants) or a linear form (tunicate, algae, and so on). The glycosyl transferase catalytic sites in the TCs systematically add UDP-glucoses to the non-reducing ends of growing cellulose chains. Cellulose biosynthesis and the subsequent aggregation of single cellulose chains into bundles with disordered and crystalline domains are both highly complex phenomena, and not well understood. However, envisioning the resulting hierarchical structure is helpful for considering nanocrystal isolation methods. Despite recent debate on the crystalline nature of native cellulose elementary fibrils raised by Agarwal et al. (40), we consider the traditional organization of cellulose in wood as follows for this review: “Minisheets” comprised of a coplanar layer of cellulose chains represent the simplest level of association; within this layer, intermolecular hydrogen bonding bonds adjacent chains (Figure 2(a)). Several stacked minisheets constitute a crystallite 3-6 nm wide and 4-30 nm long (Figure 2(b)). This is our target—this is a cellulose nanocrystal. These crystallites do not exist discretely however, they are fractions within elementary fibrils, interspersed among disordered regions (Figure 2(c)). Stepping to a higher level, we see that these elementary fibrils assemble to form microfibrils (Figure 2(d)) (4, 41, 42). About 50% of woody biomass is composed of microfibrils. Each layer of the wood cell wall (the primary wall and the three secondary walls) is defined by aligned microfibrils deposited with a characteristic orientation (not shown). Importantly, this is not a chemically homogenous system. Between the microfibrils are hemicelluloses made of a variety of pentoses and hexoses, some acetylated, some bearing uronic acids, with branching and varying glycosidic 23 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


linkages. Also present is lignin, a phenolic polymer that confers rigidity and substantial recalcitrance to the cell wall.

Figure 2. Schematic diagram of forming one microfibril in wood (a) minisheet (b) cellulose crystallite (c) elementary fibril (d) microfibril (43). Even a simple rendering of biomass illustrates its complexity. Isolating the desired crystalline fractions is complex. Swelling the microfibrillar structure is required, allowing reagents to access the disordered fractions and degrade them. However, allowing too much access to reagents can ultimately lead to cellulose dissolution, or greatly reduced molecular weight. Both are undesirable. Dissolved cellulose re-crystallizes in a different arrangement (cellulose II, discussed below, which has greater stability but less strength). Similarly, native cellulose (cellulose I) with reduced molecular weight offers less strength. Optimizing reagent access to efficiently degrade disordered cellulosic fractions while recovering the native cellulose I crystalline regions at maximum length is thus a balancing act. In this review, we will later consider various chemical and physicochemical approaches to optimize cellulose nanocrystal recovery, and the relative severity of each approach. As mentioned above, cellulose crystals can exist in different forms, and the structure impacts the properties. Four structures are known: cellulose I, II, III and IV. Cellulose I is native, and has two distinct allomorphs: triclinic Iα and monoclinic Iβ (Figure 3(a)) (44, 45). These differ in their unit cell dimensions and crystallographic basis (Figure 3(b)). The ratio of Iα to Iβ in native celluloses varies depending upon the synthesizing plant or organism. For example, cellulose in bacteria and algae is enriched in Iα while in higher plants it consists mainly of Iβ. The parallel assembly of chains in cellulose I takes place through intramolecular bonding between C3 hydroxyl groups and the adjacent in-ring oxygen, C2 hydroxyl groups and the C6 hydroxymethyl groups, and also by the intermolecular hydrogen bonding between the C6 hydroxymethyl and C3 hydroxyl groups (Figure 4(a)). Unlike cellulose I, regenerated cellulose or cellulose II has an anti-parallel chain arrangement and a different hydrogen bonding pattern. Mainly the bonding between C3 and C6 transforms to C2 and C6 (Figure 4(b)) (46–48). Cellulose II is more stable than cellulose I; with sufficient chemical or thermal treatment, cellulose I converts to cellulose II irreversibly. However, cellulose II crystals are less desirable because they are associated with a lower Young modulus (49). Therefore, care must be taken with regard to the chemical and thermal approach to retain cellulose I crystals. With certain chemical treatments, cellulose III and cellulose IV crystals can also be produced, but these are beyond the scope of this review. 24 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 3. Schematic representation of single unit cell for triclinic cellulose Iα and monoclinic cellulose Iβ (a) (reproduced with permission from reference (50). Copyright 1991 American Chemical Society) and their crystallographic directions in the plane (b). (Reproduced from reference (51). Copyright 2003 Elsevier).

Figure 4. Inter and intra hydrogen-bonding in cellulose I (a) and cellulose II (b). (Reproduced from reference (48). Copyright 2006 Royal Society of Chemistry). To summarize, cellulose from biomass exists in a complex, hierarchical arrangement. We will next consider effective chemical treatments for isolating the crystalline fractions. Namely, these include acidic hydrolysis, alkaline degradation and oxidative degradation, but clearly, adequate access to the disordered regions must be afforded. The selective degradation of these disordered regions leaves the desired crystalline regions for harvesting. The structure of the crystals (I or II), as well as both the lateral dimensions and molecular weight, depend upon the isolation conditions and sources. Of the chemically viable strategies for CNC recovery (acidic hydrolysis, alkaline degradation, oxidative degradation), this review will focus on the main isolation route— Brønsted acid hydrolysis. Later we discuss how conditions can be optimized for obtaining desired cellulose I in high strength and in high yield (part 4). Also while some work is starting to show that CNCs can also be directly harvested from biomass, this review focuses on 25 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

“pure” cellulosic sources, such as microcrystalline cellulose (MCC), wood pulp and cotton.


Standard Chemical Routes toward CNC Preparation Recall that three chemical mechanisms are possible for cleaving glycosidic bonds and therefore, in principle, producing CNCs from cellulosic sources: acidic hydrolysis, alkaline hydrolysis and oxidation. Biologically, enzymes also accomplish this, but they are beyond the scope of this review. Alkali hydrolysis has not been used to date to isolate CNCs from pure cellulosic sources although it is refered to in one report (52). In contrast, acidic hydrolysis is by far the most common route for CNC production, either as a pure chemical process or assisted with another process (these efforts will be reviewed in section 4). Of course, different types of acids exist. Compounds that readily dissociate to evolve a proton are known as Brønsted acids. This class of acids is most commonly used to obtain CNCs from cellulose. Lewis acids function differently; they have an empty orbital and thus can readily accept an electron pair. In fact, they are oxidizing agents. In this section, we thus begin our discussion with acidic CNC production routes with the common Brønsted acid hydrolysis and then review those processes relying on oxidative degradation. Essentially, this discussion surveys methods based on single reagents. Brønsted-Acid Catalyzed Hydrolysis Brønsted acids easily dissociate, producing hydronium ions in water (H3O+) (53–55). The generally accepted mechanism follows the pattern shown in Figure 5 (56–59). The hydronium ion acts as a catalyst, rapidly protonating the glycosidic oxygen. As the adjacent C-O bond cleaves, a cyclic carbonium ion forms at C1, which is then susceptible to the nucleophilic addition of H2O (I). The protonation might also take place at the ring oxygen, leading to breakage of ring to break (II). This final step regenerates the catalyst proton. Although Figure 5 illustrates the reaction on a disaccharide substrate, the typical case would be random cleavage within a disordered region of a cellulosic polymer chain, to forming two shorter chains. The first study reporting cellulose hydrolysis was published by Calvert in 1855 (60). A thorough study with strong mineral acids (sulfuric and hydrochloric acids) was conducted by Nickerson and Habrle in 1947 (61). They concluded that the disordered inter-crystalline regions were first attacked by acids. Two years later, Rånby first reported on stable aqueous colloidal suspensions of cellulose “micelles” obtained from cotton after hydrolysis with sulfuric acid (2.5 N) (62). Rod-like cellulose “micelles” with dimension of 7.3 x 46 nm (cellulose I) and 7.4 x 25.5 nm (cellulose II) were then revealed using electron microscopy in 1951 (63). Mukherjee reported crystallite thicknesses slightly increased following sulfuric acid hydrolysis using 2D X-ray diffraction in 1953 (64). Birefringent behaviour was first observed by Marchessault, who hydrolyzed cellulose powder and chitin with 2.5 M hydrochloric acid in 1959 (65). 26 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 5. Mechanism of acid-catalyzed hydrolysis of ß-1,4 glucan. Pathway I is initiated by the protonation of the glycosidic oxygen while Pathway II by the protonation of the ring oxygen. Pathway I is dominant over pathway II. Since the early studies, experimental conditions have been manipulated in attempts to optimize product yields, or achieve desired functional properties of the product. Such efforts vary liquid to solid ratio, acid strength, temperature, time, starting materials, and so on. After hydrolysis, the resulting cellulose suspension is continuously washed with deionized water and centrifuged till the suspension becomes turbid, indicating the presence of nanocrystals in appreciable concentration. Dialysis of the collected turbid suspension against deionized water is then performed to remove the residual free acid molecules until the pH of the suspension is stable (1, 66, 67). CNC aspect ratio, crystallinity index, thermal stability, surface properties as well as production yields are of a great importance for the use and application of CNCs. The major factors which influence these properties are the nature of the acid and the hydrolysis conditions, viz. acid concentration (C), liquid to solid ratio (L/S), reaction temperature (T) and time (t). As the source of the catalyst for the hydrolysis, the selection of the Brønsted acid has important impacts. Acid strength, viz. the ion dissociation capacity in water known as Ka, is the major factor governing hydrolytic conditions. A more convenient scale for acidic strength is the pKa, which is achieved by taking the negative logarithm of Ka. pKas range from -15 to 45, with the lower and more negative range of the spectrum being more effective acids, in other words, 27 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


those that dissociate more completely. Both inorganic and organic acids have been implemented in the production of CNCs are reviewed as follows: H3PW12O40. Phosphotungstic acid, H3PW12O40, a very strong acid has been recently used successfully for producing CNCs in high yield (60 w%) (68). The obtained CNCs also have high thermal stability because surface derivatization is scarce. Despite the high yield, drawbacks to this approach include complicated CNC collection after hydrolysis and the relatively long reaction time. H2SO4. Sulfuric acid is the most extensively reported acid for CNCs preparation. During the cleavage of the glycosidic bonds, sulfuric acid also functionalizes the surface of cellulose through esterification of the hydroxyl groups to form sulfated, and thus negatively charged nanocrystals. The anionic sulfate groups allow the formation of stable aqueous suspensions due to static repulsion between the anionically charged nanoparticles (62, 66). However, the sulfate groups decrease the thermal stability of CNCs produced. If protonated, they are themselves protic acids that can catalyze decomposition. If unprotonated, they can undergo elimination reactions that also decrease stability. The decomposition of sulfonated CNCs starts at 150 °C, much lower than the decomposition temperature (Td) of native cellulose at 250-300 °C. This early decomposition of sulfonated CNCs may also be impacted by the increased free volume around the areas in the highly sulfated regions, namely the surface and the chain-ends (66, 69). HCl / HBr. Both hydrochloric and hydrobromic acids have been reported as potent acids for nanocrystal production but with different hydrolysis parameters (Table 2). The obtained CNC suspensions are not functionalized by the halogen moieties. Thus, the resulting crystals are less charged, and tend to flocculate. These CNCs have good thermal stability (66, 70–73). A vapor HCl-mediated route was recently developed, affording higher accessibility of the acid, and yielding higher CNC production (74). H3PO4. Similar to the sulfuric acid method, phosphated-CNCs with native cellulose I structure have been prepared by phosphoric acid (75). They present a surface charge of ca. 10 mmol/kg, which is 10 times less than the surface charge for sulfated CNCs. Consequently, they exhibit better thermal stability (75). With the same acid, CNCs with cellulose II morphology have also been obtained as a result of the strong swelling effect of phosphoric acid (76). Subcritical water. The use of subcritical water to produce CNCs from cellulosic sources has been recently reported (77, 78). Subcritical water is pressurized water at temperatures between boiling and the critical temperature (374 °C). In this temperature region, hydrogen bonds within water are disrupted and thus water presents a lower equilibrium constant for ion dissociation (Kw) (79). Consequently, higher concentrations of ion species, such as H3O+, are obtained promoting a typical Brønsted acid catalyzed hydrolysis of cellulose. In studies of CNC production with subcritical water, a yield of ca. 5-25 w% of CNCs is reported depending on applied temperature (120-200 °C) and pressure (8.1-20.3 MPa) (78). Compared to the sulfuric acid-based method, the process viability is deemed favorable in terms of reduced water and chemical consumption, and effluent generation, although it requires significant energy (77). 28 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Table 2. Traditional Brønsted Acid-Mediated Hydrolysis Procedures for CNCs Production

29


Acid

*

Acid Strength (pKa)

Surface charge

Hydrolysis conditions t (h)

T (°C)

[Acid]

Yield (w%)

Td (°C)

L/S (v/w)

Ref.

H3PW12O40

-13*

75 w%

90

30

80

n.a

350

60

(68)

HBr

-9

2.5 M

100

3

50

n.a

n.a

70

(70)

HCl

-8

4-6 M

80

3-4

30

0

270-350

10-20

(8, 69, 72)

H2SO4

-3

60-65 w%

45-55

1-2

8.75-10

0.1-1% (sulfate)

150-250

20-30

(1, 80)

H3PO4

2.1

85 w%/10.7 M

100

1.5-2

5-10

10.8 mmol/kg (phosphate)

300-350

76-80

(75, 76)

Hammett acidity function (H0) for phosphotungstic acid



Oxidative Degradation Oxidation has been repeatedly and successfully utilized for CNC production. Of course, oxidation can be understood in two ways: as a loss of electrons to an electron acceptor (Lewis acid), or as a net increase in the incorporation or contribution of oxygen containing groups, as is the case in many of the chemistries used for CNC production. Methods commonly used include the TEMPO/NaClO/ NaBr oxidation (81–83), which has been demonstrated on cotton (84), jute (85), oil palm (86), and most recently, was modified to examine a bromide-free route (87). Ammonium persulfate has also been used to produce CNCs from MCC and wood pulp (88, 89). It has low long-term toxicity, high water solubility and low cost, thus it is favored over its sodium and potassium counterparts (89, 90). Sodium metaperiodate has also been used on wood pulp to produce amphiphilic CNCs via oxidation and reductive amination with three types of butylamine isomers (91). Depending on the extent of butylamino-functionalization, 50 w% of the CNCs were produced with tunable hydrophilicity/ hydrophobicity.

Optimization and Comparative Assessment of CNC Production Methods The general efficacy of CNC production methods is linked to the complexity of cellulose hydrolysis, which can be summarized in the following factors: 1) Limited accessibility into cellulose fibers. 2) Kinetics of hydrolysis. 3) Competing mechanisms of swelling, dissolution and hydrolysis. Native cellulose fibers are difficult to access for common liquids due to the rigid hydrogen bonding network. Breakage of both inter- and intra-molecular hydrogen bonding while maintaining the crystalline integrity of cellulose is necessary for effective hydrolysis, eventually leading to CNC production. Competing kinetics are driven by the characteristics of the cellulose (92–94), the solvent properties (95) and the experimental conditions (96). For effective recovery of native cellulose I nanocrystals, hydrolysis must occur under heterogeneous conditions, yet disordered regions must be highly swollen and accessible to reactants. The consideration of this complexity has led to optimization studies with standard Brønsted acids in the first place but also to hybrid approaches combining several reagents and/or physico-mechanical processes. The vast array of hybrid approaches, as reviewed next, highlight the need to quantitatively compare the efficiency and the chemical footprint of CNC production. Such tools are also proposed in this section and implemented on the entire paletteof the most common single reagents and hybrid methods. Optimization of Single Brønsted Acids Processes Hydrolysis conditions have been studied within a certain range for the different single Brønsted acid methods (Table 2). Sulfuric acid concentrations 30 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


between 60 w% and 65 w% are most commonly used. Concentrations over 65 w% induce complete dissolution of cellulose and reduce accordingly the yield (97), while lower concentrations lead to production of cellulose-II nanocrystals as a result of regeneration (37). Overall temperature and time are optimum in the range of 40-50 °C and 1-2 h with above-noted sulfuric acid concentration; 80 °C and 3-4 h are optimum with hydrochloric acid; 100 °C and 3 h with hydrobromic acid; 100 °C and 1.5-2 h for the phosphoric acid method (Table 2). The obtained yield using the H2SO4-mediated method is around 20-30 w% from pulp, which is much lower than the theoretical yield considering that cellulose crystallinity index ranges from 50 to 60%. Several optimization studies have been conducted with the assistance of design of experiments (DOE). Response surface methodology has been applied to improve CNC yield for the first time by Bondeson, et al. who obtained 30 w% yield from MCC (1). The optimum conditions consisted of using C=63.5 w%, T=44°C, t=2.2h and L/S=10.2 mL/g., A higher yield of 64.1 w% was achieved from beaten cotton pulp using sulfuric acid hydrolysis conditions 65 w%, 50 °C, 5 h and 30 mL/g (98). Wang, et al. (99) and Loelovich (97) also studied the hydrolysis conditions and optimized the reaction to a maximum yield of 59.7 w% and 70-75 w% for wood pulp and MCC, respectively. In Ioelovich’s study, this high yield was achieved by an additional mechanical treatment. Indeed, hybrid processes have emerged in the last five years as a strategy to either increase fiber accessibility or enhance effective hydrolysis. Efforts at Decreasing Process Harshness and Improving Efficiency A large array of approaches (Table 3) has been considered with a view to i) decreasing the process severity and or/ ii) enhancing the yield/reaction efficiency. These methods are surveyed next.

Approaches for Decreasing the Severity of the Process The reaction acidity can be easily reduced by mixing acids, masking the acid in a complex and/or assisting its efficacy with a physico-mechanical treatment.

Acid Mixtures In general, CNC properties, such as surface hydrophilicity/hydrophobicity and charge, thermal stability and so on, can be tailored with selection of acid mixtures (100, 101). Less acidic environments can be also generated by mixing two strong acids with lower concentration or by combining one strong and one weak acid. Mixtures of H2SO4 and HCl have been commonly employed with ultrasonication to prepare sphere-like CNCs (102–104). Both native and regenerated spherical cellulose have thereby been produced (103, 104). H2SO4 or HCl have also been mixed with weaker acids, such as acetic acid or butyric acid. With this acid mixture, higher CNCs yield of 85 w% with rod-like cellulose I morphology could be obtained from cellulose pulp (105). Formic acid (98 w%) 31 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

was also used, generating ca. 70 w% of nanoparticles after 6 hours of reaction at 95 °C, and only 0.5 hour when adding 0.7 w% of HCl (106). A novel method utilizing a maleic anhydride/H2SO4 mixture was recently designed by first adding the mixture to the pulp and then ball milling it for 1h before heating. Around 60 w% of cellulose I nanocrystals were thereby obtained from the pulp (107). Lower yields of 22-35 w% were obtained from cotton with HCl/ CH3CH2CH2COOH (108). Acid mixtures therefore appear as very potent avenues to reduce acidity and at the same time increase yield.


Thermo-Physico-Mechanical Treatments Ultrasonication. Exposing a material to ultrasound generates cavitation bubbles by nucleation, growth and collapse. This creates powerful shock waves in the liquid phase, which attack and loosen the surface and the disorganized region of the cellulose. As a result, chemical reagents have greater access (105, 109, 110). By ultrasonicating pulp prior to acid hydrolysis, CNC yield could be increased from 15 w% to 33 w% in comparison to non-pretreated pulp (111). However, ultrasonication was found to alter the shape of the crystals from a rod-like geometry to a more spherical shape. High Pressure Homogenization. Homogenization is a common mechanical method to produce microfibrillated cellulose. It creates strong shear forces, which “peel” the cellulose fibers. However, it can also be helpful to loosen the structure of fibers without changing their crystalline structure. Pan et al, reported that much milder acidic conditions (20 w%, 2 h, 20 °C) were sufficient for the preparation of high crystallinity index (CrI of 89.7 %) CNCs from MCC with the assistance of homogenization (112). Microwave Heating. Unlike conventional heating, microwaves are a form of electromagnetic radiation at specific frequencies, typically targeting water molecules, causing them to vibrate. These vibrations generate heat. Thus, this approach can be more efficient than convective heating approaches, since it is not limited by a time-dependent heat diffusion process (113). Utilizing microwave heating, Kos et al reported a yield of ca. 38 w% of rather thick (29 x 360 nm) CNCs, was prepared from MCC with H2SO4 in only 10 min (114). A yield of 38 w% was achieved with enzyme-catalyzed hydrolysis under microwave heating compared to a lower yield of 29 w% when conventionally obtained (115). A yield of 78 w% of CNCs could also be obtained from filter paper by combining both microwave heating and ultrasonication during sulfuric acid hydrolysis (116). Ionizing Radiation. This approach uses radiolysis to break chemical bonds. Kortokov et al. applied this technique as a pre-treatment under controlled conditions, so that cellulose crystalline structure could be maintained while the disordered region was degraded (117). Around 60 w% of nanocrystals with cellulose I morphology could thereby be obtained from radiolysis-pretreated MCC using a low concentration of H2SO4 (4M) and H2O2 oxidation (117, 118). Autoclaving. The highest CNC yields ever reported have been achieved by Yu, et al. (119), who used an hydrothermal treatment generated with autoclave during acid hydrolysis with 6M HCl at 110 °C for 3 h; this procedure yielded 32 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

ca. 80 w% of CNCs from wood pulp, and ca. 90 w% from MCC. However, this process is energy-intensive.


Masked Strong Brønsted Acids Strong acids can be bound into complexes such that their acidic function is first activated when demasked. This is the case in ionic liquids (ILs) containing a strong acid as anion such as hydrogen sulfate, which upon addition of water might dissociate and release the acid. For example 1-Butyl-3-methyl imidazolium hydrogen sulfate ([Bmim]HSO4) has been successfully utilized for CNC production (120–122). First used by Man, et al. in its pure form, it produced relatively thick nanoscale cellulose of 14-22 nm from MCC at 70-90°C for 1 h (120). Building on this first success, Mao et al. evidenced the favorable role of water to efficiently produce CNCs (121). Further, by decoupling the swelling step from the hydrolysis step, whereby hydrolysis was triggered by addition of water after a fixed period of swelling in the pure ionic liquid, near theoretical yields of ca. 80 w% and ca 60 w% could be achieved for MCC and wood pulps, respectively (123). In principle, ILs paired with any strong acidic anions can catalyze cellulose hydrolysis, given the presence of catalytic quanitites of water which are inherently present in cellulose (124). With this strategy, a series of heteropolyacid (HPA) ionic liquids such as [C4H6N2(CH2)3SO3H]3-nHnPW12O40 (n=1,2,3) has also been successfully used to catalyze the hydrolysis of cellulose (125). Hydrophobic CNCs were prepared using a IL-based cosolvent system containing tetrabutylammonium acetate (TBBA) and dimethylacetamide (DMAc) (126). The concentrations of TBA+, CH3-, COO– were gradually diluted by DMAc, decreasing the interactions between CH3COO- and the cellulose, such that only the disordered regions were dissolved. Meanwhile, heterogeneous surface modification by acetic anhydride produced acetylated CNCs. The production of CNC from subcritical water as recently reported by Bras et al. might also be akin to a “masked” acid strategy since bringing water to its subcritical state causes an increase in hydronium ions concentration (77, 78).

Approaches for Increasing Efficiency of the Process With appropriate chemical pretreatments, cellulose structure can be swollen and opened up offering more accessibility for acid-attack. To that aim, swelling treatments and TEMPO-mediated oxidations have been combined with acid hydrolysis (Table 3).

Swelling Treatment NaOH and DMSO are good swelling agents for cellulose (127, 128). In these solvent systems, homogeneous swelling can occur without dissolution, depending 33 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


on conditions. Due to its high dissolving capacity, high concentrations of NaOH could alternatively induce cellulose mercerization, changing its crystalline structure into cellulose II. Consequently, CNCs produced from NaOH-pretreated cellulose turn into spherical grains with cellulose II structure. On the other hand, DMSO-pretreatment yields CNCs with rod-like shape and native microstructure (129). CNC yields of 32.4 w% have been achieved when applying swelling pretreatment to cotton fibers compared to 14 w% without the pretreatment (129). Careful control of the swelling parameters is necessary to avoid damage to the crystalline regions of cellulose and maintain its original structure. Some ILs with powerful dissolving capacity for cellulose can also be tailored to function as swelling agents. 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) was used as swelling agent during CNC production (38, 130). A binary IL system containing [Bmim]Cl and 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([SBMIM]HSO4) was also used to produce CNCs from cotton fibers by first swelling at room temperature and then initiating hydrolysis by water addition at 80 °C. The production however led to morphologically heterogeneous nanoparticles containing both celluloses I and II (122, 130). Occurrence of cellulose regeneration is due to the strong dissolving capacity of [Bmim]Cl (38, 39).

TEMPO-Mediated Oxidation TEMPO (2,2,6,6-Tetramethylpiperidinyloxy) combined with NaBr and NaOCl is a stable radical system. It was first applied as an oxidizer for cellulose, whereby NaOCl acted as a selective oxidant on the primary C6 alcohol group; catalytic amounts of NaBr and TEMPO were used at pH of 10-11 (131). The more negatively charged carboxyl groups induced strong repulsion between cellulose fibers and thereby eased fibrillation. When using TEMPO/NaBr/NaOCl-mediated pretreatment before acid hydrolysis, yields increased to 37- 56 w% compared to ca. 10-20 w% from cotton fibers with solely sulfuric acid (84, 102). However, the crystallinity index decreased to 50-60 %. It can also be applied for CNC production in non-acidic environments as noted previously; however, the pH responsiveness of these nanocrystals will be quite different given the oxidation at C6 and resulting anionic charge. While there clearly is an infinite array of combinations, routes and conditions for CNC production, predominantly utilizing some sort of Brønsted acid, no quantitative parameter has been proposed to date to comprehensively evaluate and compare these different routes in terms of efficiency and chemical harshness. In the next section, we propose three parameters which start to fulfill this agenda.


Table 3. Hybrid Brønsted Acid-mediated Approaches for CNC Production

35


Source

Chemical treatment

Mechanical treatment

Yield (w%)

CNCs Morphology

Ref.

**Size

*Shape

CrI (%)

(nm) MCC

H2SO4 (98 w%)/ HCl (37 w%) /H2O mixture (3:1:6)

Ultrasonication, 50 Hz,10h

n.a.

S-I

x(20-90)

n.a.

(103)

MCC, pulp

H2SO4 (10 w%)/H2O2 (1 w%) mixture, 85-98 °C, 2h

Radiolysis

60

R-I

(25-120)x(100-900)

71

(117, 118)

MCC

[Bmim]HSO4, 70-90 °C, 1h

n.a.

n.a.

R-I

(14-22)x(50-300)

82-91

(120)

MCC, cotton, pulp

(NH4)2S2O8, 60 °C, 16h

n.a.

65, 81, 36

R-I

(5-7)x(121-128)

83, 91, 81

(90)

MCC

cation exchange resin(NKC-9), 50 °C,3h

Ultrasonication

50.04

R-I

(10-40)x(100-400)

84

(132)

MCC

HCl (6M)

Autoclaving, 110°C, 3h

93.7

R-I

(16±2)x(262±14)

76

(119)

MCC

[Bmim]Cl, 125 °C, 5h

Homogenization

30-40

R-II

12x112

50

(38)

MCC

H2SO4,50%,70 °C,10min

Microwave heating

82

R-I

(125)x(1310)

83

(114)

MCC, softwood and hardwood kraft pulps

[Bmim]HSO4, 100-130 °C, 13h

n.a.

80, 60

R-I

(5-7)x(120-300)

77-82

(123)

Continued on next page.


Table 3. (Continued). Hybrid Brønsted Acid-mediated Approaches for CNC Production

36


Source

Chemical treatment


Yield (w%)

CNCs Morphology

Ref.

**Size

*Shape

CrI (%)

(nm) MCC

Subcritical H2O, 120-200 °C, 8.1-20.3 Mpa, 60 min

Ultrasonication

5-25

R-I

55x242

79

(77)

MCC

TEMPO/NaClO/NaBr, 30 °C, 2 h

Ultrasonication

93

R-I

6x122

72

(86)

Wood pulp

CH3COOH,16h, RT; H2SO4, 80 °C, 3-7h

Ultrasonication, 68-75°C,0-6h

85

R-I

x(10-100)

80

(105)

Wood pulp

maleic anhydride/H2SO4/pulp mixture, 90 °C, 1h

Milling, 500 rpm, 0.5-2h; U

61.1

R-I

(20-100)x(200-960)

80

(107)

Wood pulp

NaIO4/LiCl, 75 °C, 3 h

Homogenization

45-50

R-I

4x150

50-60

(91)

Wood pulp

TBAA/DMAcsolventmixture, acetic anhydrid, 65 °C, 2 h

Ultrasonication

n.a.

R-I

(20-30)x(300-500)

51%

(126)

Reject pulp

Enzymolysis, 50 °C, 45 min

Microwave heating

29-38.2

R-I

(30-80)x(100-1800)

n.a.

(115)

Cellulose fibers

NaOH and DMSO, H2SO4 (36M)/ HCl (12.1M)/ H2O mixture ( 3:1:6),80°C, 8h

Ultrasonication

62-76

S-II

x85

82

(104)

Paper

H2SO4,50%,70 °C,1.5h

Microwave heating

80

R-I

n.a.

78

(116)

Cotton

HCl (0.027M) /CH3COOH (17.5M), HCl (0.027M) /CH3CH2CH2COOH (10.9M) mixtures, 105 °C, 0-25h

Blending, 20-40 min

15-35, 22

R-I

(29-45)x(170-280) (34-36)x(208-226)

n.a.

(108)


37


Source

Chemical treatment


Yield (w%)

CNCs Morphology

Ref.

**Size

*Shape

CrI (%)

(nm)

*

Cotton

ZnCl2 , 80-100°C, 1-4h

Homogenization,8000 rpm; Ultrasonication,5080°C,1-5h

n.a.

n.a.

n.a.

n.a.

(133)

Cotton

TEMPO-NaBr-NaClO, 25°C, 0-14h

Ultrasonication

35

R-I

(5-10)x(100-400)

83-85

(84)

Cotton

DMSO/NaOH, Cellulase

Ultrasonication

32

R-I, S-II

(10-40)x(70-280), 6x20

78

(129)

Cotton

[Bmim]Cl, [SBMIM]HSO4, 80°C, 80min

n.a.

30

R-I/-II

20x300

95

(122)

S-I, S-II: sphere-like cellulose I, II; R-I, R-II: rod-like cellulose I, II.

**

Size:thickness x length


Proposal for Quantification of Process Harshness and Efficiency With the aim to provide for quantitative tools to enable comparison of production efficiency and environmental footprint in protic Brønsted-acid methods (123), three parameters can be defined as follows:


-

-

-

Molar ratio of protons to anhydroglucose ([H+]/[AGU]): [H+]/[AGU] can be used to evaluate the actual acidity per glycosidic linkage of the reaction mixture used for CNC production when protic acids are used, as most commonly done. Further, it provides a direct measure of the efficiency of the actual hydrolysis reaction mechanism under given conditions. Combined severity factor (CSF): a combined severity factor (CSF) is a common tool in chemical engineering to quantify the severity of a thermochemical process performed at various pH. Likewise, it can be used as an indicator of the reaction severity of a CNC production method. When a reaction takes place in an acidic environment, it is defined from the reaction temperature (T), a reference temperature (Tref) most often set at 100°C , time (t) and the initial pH value of the reaction (134):

Relative crystalline region recovery (CR): since the ultimate goal of nanocrystal preparation is to obtain 100% of the crystalline fraction of the cellulose fibers in the form of CNCs with native microstructure, relative crystalline region recovery (CR) offers a tool to estimate the crystalline fraction recovered during CNC production. CR is not only related to yield, but also to the crystallinity index of both the raw material and the obtained CNCs , and as such provides a good normalization of process efficiency (123).

where MCNCs, CrICNCs and Mo, CrIo are the mass, the crystallinity index of the CNCs and the raw material, respectively, and Yw is the gravimetric yield of CNCs. All three parameters can give insight on different aspects of the process. [H+]/ [AGU] molar ratio is not only focusing on the acidity; the actual reaction efficiency can be also easily estimated. CSF is a more holistic parameter giving an average value for harshness of the conditions. CR can assist with comparing actual yields of crystal recovery, combining both CNC mass yield and quality.



Critical Comparison of All Brønsted Acid-Mediated Processes Compiling the available data from the literature related to Brønsted acidmediated routes (Table 4), [H+]/[AGU], CSF and CR were estimated as indicators of reaction acidity, process harshness and efficiency (Figure 6(a), (b) and (c)). The following discussion will mainly categorize all methods into two groups: 1) Single Brønsted acid-mediated methods 2) Hybrid Brønsted acid-mediated methods. The molar ratio of [H+]/[AGU] depends on the concentration of acid and its ratio to the starting material. The higher the [H+]/[AGU], the more acidic and thereby corrosive the environment. Figure 6(a) shows two distinct regions regarding [H+]/[AGU] values for traditional strong acidic methods and the recent modified acidic methods, which have ranges of 20-70 and 0.1-10, respectively. CR has a similar trend as yield: broader distributions and lower averaged values are obtained using traditional acidic methods ranging from 15 % to 85 %, while in general more crystalline regions can be recovered with the hybrid methods, i.e. the recent routes under less acidic and corrosive environments are more efficient for CNC production. However, there is still some space for higher crystalline region recovery. It is interesting to note that traditional hydrolytic methods using other strong acids are quite different from the sulfuric acid method (Table 2): less protons are needed in the HCl- and HBr-hydrolysis, while the H3PO4-based hydrolysis requires higher amounts of protons. This relates to the different pKa of the acids indicating their hydrolytical strength in solution. HCl and HBr as strong acids have a pKa of -8 and -9, respectively, and thereby dissociate very well, while H3PO4 as a weak acid has a higher pKa of 2.1 and thus a high concentration of H3PO4 is needed for hydrolysis. Phosphotungstic acid as a heteropoly acid has both strong catalyzing and oxidizing capacity and thus requires the lowest amount of protons ([H+]/[AGU]=9.97) among the strong acids used for CNC production. In general, most CSF values for traditional acid hydrolysis range from 0.5 to 2.5, while hybrid methods vary vastly in severity. However, higher CR around 60-80 w% are also achieved with hybrid methods compared to traditional methods (Figure 6(b)). Note that the highest yield (93.7 w%) of cellulose I nanocrystals was obtained with HCl hydrolysis under intensive hydrothermal treatment (119), yet obviously it is one of the harshest with a CSF value of 3.33. When comparing CSF and acidity, although the conventional single acid methods generally have lower CSF, more acid is involved in CNC production in lower CR; however, it would be desirable to reduce the consumed energy (relating to T and t) for the hybrid acidic methods, which have higher CSF even at much lower acidities (Figure 6(c)). Either more concentrated acid solutions with higher [H+]/[AGU] or higher T and t inputs leading to higher CSF are needed for CNC production. Overall, the extensive work to develop more environmentally friendly technologies with high efficiency has met with large success. Near theoretical yields from the most common cellulosic sources (MCC or pulp) have been already obtained, yet the processes are energy intensive.



Figure 6. Estimated relative crystalline recovery (CR) as a function of (a) [H+] /[AGU] (b) combined severity factor (CSF) and (c) CSF as a function of [H+] /[AGU] regarding CNCs production methods: traditional Brønsted acid methods (solid square); hybrid Brønsted acid-based method (solid circle) (all data can be found in Table 4.)


Table 4. Summary of CNCs Yields with Different Acid Hydrolysis Methods and the Estimated CSF, [H+]/[AGU] and CR

41


Sources

Acid

CSF

Yield (%)

[H+]/[AGU] (mol/mol)

CR (%)

Ref.

Traditional Brønsted acid-mediated methods MCC

H2SO4

73

1.47

25.7

n.a.

(97)

MCC

H2SO4

40

1.79

33.2

41.5

(111)

MCC

H2SO4

30

1.77

32.8

n.a.

(1)

Bleached Eucalyptus Kraft pulp

H2SO4

61,65

1.42, 1.12

29.6

79.6, 84.8

(135)

Kraft eucalyptus pulp

H2SO4

60

1.66

24.9

n.a.

(99)

Bleached hardwood pulp

H3PW12O40

60

2.85

10.0

78.5

(68)

Bleached softwood Kraft pulp

H2SO4

38

2.05

28.4

58.6

(136) (75)

Cotton

H3PO4

76-80

3.46

52.0

n.a.

Cotton

HBr

70

2.65

20.3

79.6

(70)

97.3

n.a.

(98)

28.4

n.a.

(80) (73)

Cotton Cotton

H2SO4 H2SO4

64 44

2.31 1.46

Cotton

HCl

10-20

2.39

19.4

n.a.

Pineapple leaf

H2SO4

65

1.42

59.7

74.1

(137)

Sugarcane bagasse

H2SO4

58,50

1.17, 1.57

66.4

66.8, 46.4

(138)

Corncob

H2SO4

50

1.42

44.6

57.1

(139)

Continued on next page.


Table 4. (Continued). Summary of CNCs Yields with Different Acid Hydrolysis Methods and the Estimated CSF, [H+]/[AGU] and CR

42


Sources

Acid

CSF

Yield (%)


CR (%)

Ref.

Kenaf bast fibers

H2SO4

41

1.77

32.8

n.a.

(140)

Phormium tenax fibers

H2SO4

35

1.16

32.5

n.a.

(141)

Mengkuang leaves

H2SO4

28

1.30

59.3

n.a.

(142)

Soy hulls

H2SO4

20

1.01

32.5

21.9

(143)

Banana plants

H2SO4

23

0.58

72.6

n.a.

(144)

Syngonanthus nitens

H2SO4

14

1.62

66.4

15.8

(145)

Improved Brønsted acid-mediated methods MCC

HCl

94

3.33

58.2

101.2

(119)

MCC

(NH4)2S2O8

83

0.32

0.5

70.0

(90)

MCC

H2SO4

82

1.38

46.0

84.0

(114)

MCC

[Bmim]HSO4

78

2.13

0.3

78

(123)

MCC

H2SO4

50

n.a.

32.5

56.1

(132)

MCC

[Bmim]HSO4

48

2.38

0.2

61

(121)

MCC, recycled pulp

Maleic acid

10

2.49

67.0

10.4

(115)

MCC

[Bmim]HSO4

n.a.

n.a.

0.3

n.a.

(120)

Softwood bleached kraft pulp

HCl

75

1.67

4.9

99.0

(146)

Wood pulp

H2SO4+maleic acid

61

2.31

7.8

76.4

(107)


Acid

CSF

Yield (%)


CR (%)

Ref.

Cellulose pulp, cotton, beet pulp, MCC

H2SO4

60

1.75, 2.71

1.4, 2.7

77.1

(118)

Wood pulp

[Bmim]HSO4

58

3.01

2.4

76

(123)

Filter paper

H2SO4

78

2.22

64.4

81.0

(116)

Cotton

CH3COOH

85

1.63

3.0

n.a.

(105)

Cotton

HCl+CH3COOH

15-35, 22

0.61

9.0

n.a.

(108)

123.2

n.a.

(104)

pure cellulose fibers

HCl+ H2SO4

74

3.45

43


Sources



Conclusions The principles of green chemistry as outlined by Anastas (147, 148) advocate among others for a minimization of energy consumption, atom economy, and maximization of reaction yields. All these principles direct researchers to develop “greener” processes. Balance between input (raw materials, energy cost) and output (product yield, waste) always needs to be considered. CNCs are traditionally prepared with strong Brønsted acids (mainly H2SO4, HCl) at low temperature within short periods of time. A series of hybrid processes have further been designed, which afford CNCs in higher yields and using more mildly acidic environments, albeit often with higher energy costs. Depending on the constrains of a particular supply chain and producer, one or another process might be desirable and process selection might be guided with the quantitative parameters viz. crystalline recovery, combined severity factors and [H+]/[AGU] ratio. Although these parameters and this review exclusively relate to CNC production from pure cellulosic source, it is clear that recent advances in obtaining CNCs with one reactant directly from a complex lignocellulosic source such as wood (35, 36) circumvents the need to pulp wood in the first place and should be advantageous from the environmental standpoint.

Acknowledgments The authors would like to thank Prof. Chip Frazier and Prof. Kevin Edgar, from the department of Sustainable Biomaterials at Virginia Tech, USA, for their insightful input on this manuscript.

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Comparative Assessment of Methods for Producing Cellulose I

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