152 Cellulose – Fundamental Aspects

up to discussion [32-37].

amphiphilicity.

Water cannot be used to break the bonds in cellulose, yet both water-water, carbohydratewater and carbohydrate-carbohydrate hydrogen bond strengths are around 5kcal/mol [32]. Many forget to take this question into account when they reason about the dissolution of cellulose and its relative insolubility. It is obvious that hydrogen bonding is not the only relevant factor in the quest for a suitable solvent, yet the amphiphilic nature of cellulose is still widely overlooked. The last few years, a few eye-opening papers have taken this matter

**Figure 6.** Hydrogen bonds arrange cellulose chains in sheets, and the stacking that forms a three

A relatively new approach when trying to understand molecular interactions is computer simulations and modeling. Some very recently published papers describe this method, showing molecular origins of the recalcitrance of biomass in the terms of decrystallization and dissolution, and showing quantitatively the need for taking cellulose amphiphilicity into account [36-38]. For comparison it may be useful to study cyclodextrins. Typical cyclodextrins are oligosaccharides made up from six to eight 1→4 linked monosaccharides in a torus and are produced from starch but since starch as cellulose is built up from glucose, this may still serve as a model. Cyclodextrins are water soluble, but the arrangements of hydroxyl groups provide a pronounced

The inclusion or the interior of the sugar macro ring is if not fully hydrophobic at least significantly less hydrophilic than the outer part of the structure. As a result of this, cyclodextrins may serve as the host in inclusion complexes. Unusual hydrogen bonding patterns take place inside the cyclodextrin, due to the confined space and the hydrophobic nature of the torus, which was shown by Bezel et al. in a neutron scattering study [39]. This relates to and partly explains the concept of "high energy water" i.e. the water trapped in the cyclodextrin ring in a water solution which upon release by addition of a more hydrophobic substance contributes to a favorable enthalpy change. The driving force for inclusion of a hydrophobic substance in a cyclodextrin ring should be considered an "atypical" hydrophobic interaction, since it is not entropy driven [40]. An extensive overview of cyclodextrins as inclusion complexes was published by Clarke et al. and

explains many aspects of the amphiphilicity of these substances [41].

dimensional structure involves hydrophobic interactions.

Despite the challenges associated with the dissolution of cellulose, numerous options are available. Here the focus lies on direct solvents, i.e. solvents that do not form covalent bonds with the polymer but merely interact by physical means. This distinction between nonderivatizing and derivatizing solvents must be made in order to choose the correct solvent for a specific use. Solvents may further be classified as aqueous or non-aqueous, or as containing halogens or not or whatever else suits the user's purposes.

Solvents can be further classified based on their number of components. Both unicomponent, bicomponent and tricomponent solvent systems are known and including solvent mixtures the range is even broader. Since many non-derivatizing, or direct, solvents for cellulose includes water, as bulk solvent or as bound water in a salt or organic hydrate, it is not fully satisfactory to make a binary distinction between aqueous and non-aqueous solvents. Instead, solvents could be placed on a continuous scale that represents the amount of water in the system. However, herein the binary view is maintained.

#### **6.1. Aqueous media**

Being the most green of all chemicals, water is of course an attractive solvent and reaction media. In fact, pure water can actually be used in the field of cellulose technology. Water at around its critical point demonstrates some unique properties and can be an interesting media for various chemistry. Dissolution and hydrolysis of cellulose in subcritical and supercritical water was investigated by Sasaki et al. [42] but the dissolved cellulose in this case was probably shorter fragments and cellooligomers since severe depolymerization must take place before any dissolution can be expected as stated by more recent publications [43-44]. This means that however appealing, it is not yet possible to dissolve high DP cellulose in pure water. Water based solvent systems are plentiful though, and different additives can make water into a very efficient dissolution media. Some of the many alternatives available are listed below.

#### **6.2. Aqueous alkali**

Mercerization, the process of soaking cellulose in strong alkali to the extent that the crystal structure changes from cellulose I to cellulose II, is one of the most technically relevant processes in cellulose technology and is used to activate the hydroxyl groups of cellulose for further modification and/or dissolution. Cellulose is partly soluble in an aqueous solution of sodium hydroxide in a concentration of about 10 % w/w. The amount of cellulose that is soluble in NaOH/H2O depends on degree of polymerization and also mode of crystallinity [45]. Isogai investigated the solubility of cellulose from several sources but never succeeded in preparing cellulose solutions of higher concentrations than 5 % [46]. A method for complete dissolution of cellulose in lye solutions was also patented in 1994 [47]. For cellulose to dissolve in alkaline aqueous media, it needs to be cooled well below room temperature. Soube et al. completed the phase diagram for the ternary system cellulose/NaOH/H2O [48]. Taking the amphiphilic properties of cellulose into account, it is not surprising that it has been shown that cellulose in NaOH/H2O is in fact not completely dissolved but forms aggregates [49].

More recently, sodium hydroxide solutions with different additives have turned out to dissolve cellulose more efficiently than the binary NaOH/H2O system itself. Such additives are for example poly(ethylene glycol) (PEG) [50-51] and urea [52] and/or thiourea [53-55]. The fact that sodium hydroxide, PEG and urea are all environmentally friendly and pose low toxicity towards humans and animals makes these solvent systems interesting for large scale applications. However, mixed systems always pose high demands on recovery systems, and the amount of additives needed in these systems makes recovery and reuse quite necessary.

154 Cellulose – Fundamental Aspects

**6.1. Aqueous media**

**6.2. Aqueous alkali**

alternatives available are listed below.

dissolved but forms aggregates [49].

Solvents can be further classified based on their number of components. Both unicomponent, bicomponent and tricomponent solvent systems are known and including solvent mixtures the range is even broader. Since many non-derivatizing, or direct, solvents for cellulose includes water, as bulk solvent or as bound water in a salt or organic hydrate, it is not fully satisfactory to make a binary distinction between aqueous and non-aqueous solvents. Instead, solvents could be placed on a continuous scale that represents the amount

Being the most green of all chemicals, water is of course an attractive solvent and reaction media. In fact, pure water can actually be used in the field of cellulose technology. Water at around its critical point demonstrates some unique properties and can be an interesting media for various chemistry. Dissolution and hydrolysis of cellulose in subcritical and supercritical water was investigated by Sasaki et al. [42] but the dissolved cellulose in this case was probably shorter fragments and cellooligomers since severe depolymerization must take place before any dissolution can be expected as stated by more recent publications [43-44]. This means that however appealing, it is not yet possible to dissolve high DP cellulose in pure water. Water based solvent systems are plentiful though, and different additives can make water into a very efficient dissolution media. Some of the many

Mercerization, the process of soaking cellulose in strong alkali to the extent that the crystal structure changes from cellulose I to cellulose II, is one of the most technically relevant processes in cellulose technology and is used to activate the hydroxyl groups of cellulose for further modification and/or dissolution. Cellulose is partly soluble in an aqueous solution of sodium hydroxide in a concentration of about 10 % w/w. The amount of cellulose that is soluble in NaOH/H2O depends on degree of polymerization and also mode of crystallinity [45]. Isogai investigated the solubility of cellulose from several sources but never succeeded in preparing cellulose solutions of higher concentrations than 5 % [46]. A method for complete dissolution of cellulose in lye solutions was also patented in 1994 [47]. For cellulose to dissolve in alkaline aqueous media, it needs to be cooled well below room temperature. Soube et al. completed the phase diagram for the ternary system cellulose/NaOH/H2O [48]. Taking the amphiphilic properties of cellulose into account, it is not surprising that it has been shown that cellulose in NaOH/H2O is in fact not completely

More recently, sodium hydroxide solutions with different additives have turned out to dissolve cellulose more efficiently than the binary NaOH/H2O system itself. Such additives are for example poly(ethylene glycol) (PEG) [50-51] and urea [52] and/or thiourea [53-55]. The fact that sodium hydroxide, PEG and urea are all environmentally friendly and pose low toxicity towards humans and animals makes these solvent systems interesting for large

of water in the system. However, herein the binary view is maintained.

**Figure 8.** Additives in aqueous alkali solvents for cellulose. From left to right; poly(ethylene glycol), urea, thiourea.

Mechanical or chemical pretreatment is necessary for the dissolution to be efficient enough for industrial needs [56]. Several studies of cellulose in the aqueous NaOH/urea system using e.g. 13C NMR, 15N NMR, 1H NMR, FTIR, small angle neutron scattering and wide angle X-ray scattering suggest that the dissolution mechanism is based on the hydrates of NaOH that in low temperatures are able to form hydrogen bonds with the cellulose chain, while the urea molecules surrounds the cellulose/NaOH/H2O inclusion complex, screening it from other cellulose molecules and thereby prevent cellulose aggregation [57-59]. Being a hydrotrope, urea is expected to increase the solubility of a poorly water soluble solute in aqueous solutions. This is explained by the ability of hydrotropes to break water structures, i.e. the effect is explained by urea – water interactions, and/or a tendency of hydrotropes to interact with the solute itself by hydrophobic interactions. Interestingly, in the case of cellulose in aqueous NaOH/urea, no direct evidence of interaction between urea and cellulose was found. In the original NaOH/PEG/H2O publication by Yan and Gao [50] the dissolution mechanism was suggested to be similar to that in NaOH/urea/H2O. It was proposed that the solution is stabilized by polyethylene glycol chains, here acting as hydrogen bonding acceptors. It has also been suggested that the mechanism for dissolution in these types of solvent mixture relates to a charging up of the cellulose, i.e. turning it into a polyelectrolyte [32].

The solutions of cellulose in aqueous alkali with urea and/or thiourea are being thoroughly investigated for shaping purposes, i.e. regeneration of cellulose into fibers, membranes or similar. Recently, Yang et al. prepared high performance flexible films from different cellulose sources in a solvent of alkali and urea [60]. The films showed good gas barrier properties over the entire relative humidity range, and always one order of magnitude lower gas permeability than cellophane films which are prepared via the viscose process. Compared to conventional films from poly(ethylene) and poly(propylene), all cellulose films show increased water vapor permeability, but this is expected due to the intrinsic hydrophilicity of the polysaccharide structure. Regenerated cellulose fibers similar to viscose or Lyocell fiber have been wet spun from solutions of NaOH/urea [61] and NaOH/thiourea solutions [62-63].

The procedure for dissolving cellulose in aqueous alkali/urea-systems is based on a freezethaw method. The solvent is pre-cooled to below the freezing point of water before cellulose is added and kept cold until complete dissolution of the polymer [64]. Regeneration takes place by precipitation in e.g. dilute H2SO4, ethanol, butanol or acetone [60]. Lithium hydroxide and potassium hydroxide are expected to have a similar effect as sodium hydroxide. A strong impact of the choice of salt has been observed, and the dissolution power of the different systems is in the order LiOH/urea > NaOH/urea >> KOH/urea. Both LiOH and NaOH in combination with urea are able to rapidly dissolve cellulose, while KOH is not [64].

The possibility to perform homogeneous reaction in aqueous alkali/urea systems has not been neglected and it has been shown to be a stable media for etherification reactions. Carboxymethylation of cotton linters and microcrystalline cellulose was carried out in lithium hydroxide with urea and the degree of substitution could be controlled [65]. The product was water soluble CMC. Another important cellulose derivative, namely hydroxyethyl cellulose, was prepared from cotton linters by homogeneous reaction in sodium hydroxide with urea under mild conditions [66].

## **6.3. Inorganic metal complexes**

One interesting group of solvents is the one of inorganic metal complexes. The complexes are of transition metal ions and nitrous ligands. Some of the most common ones [67] are listed in Table 2.

As in many cases with solutions of transition metal complexes, most of the cellulose solvents in this group are strongly colored. This is true for example with Cuoxam, Nitren and Cuen which all are deep blue, while Pden only displays a weak yellow color. The color of the solvent or in some cases the cellulose – metal complex in itself may cause problems in analytical applications where light scattering or refractive index is used [68].

**Table 2.** Transition metal complexes with amines or NH3, their common names and chemical formula.

Saalwächter et al. compared several metal complexes and found that the coordinative binding metal complexes such as Cuoxam, Nitren and Cdtren were most efficient as cellulose solvents. The coordination takes place at the deprotonated olate anions at C2 and C3 at each AGU. Even though the dissolution appears to be to molecular level and via interaction with the C3 hydroxyl group, the cellulose chain stiffness appears to be considerable. The number of Kuhn segments, i.e. the number of efficient straight segments seconded by kinks, per polymer rarely exceeds 50 [68] which means that several monomers are assembled in rod like structures before an actual bend.

156 Cellulose – Fundamental Aspects

KOH is not [64].

listed in Table 2.

place by precipitation in e.g. dilute H2SO4, ethanol, butanol or acetone [60]. Lithium hydroxide and potassium hydroxide are expected to have a similar effect as sodium hydroxide. A strong impact of the choice of salt has been observed, and the dissolution power of the different systems is in the order LiOH/urea > NaOH/urea >> KOH/urea. Both LiOH and NaOH in combination with urea are able to rapidly dissolve cellulose, while

The possibility to perform homogeneous reaction in aqueous alkali/urea systems has not been neglected and it has been shown to be a stable media for etherification reactions. Carboxymethylation of cotton linters and microcrystalline cellulose was carried out in lithium hydroxide with urea and the degree of substitution could be controlled [65]. The product was water soluble CMC. Another important cellulose derivative, namely hydroxyethyl cellulose, was prepared from cotton linters by homogeneous reaction in

One interesting group of solvents is the one of inorganic metal complexes. The complexes are of transition metal ions and nitrous ligands. Some of the most common ones [67] are

As in many cases with solutions of transition metal complexes, most of the cellulose solvents in this group are strongly colored. This is true for example with Cuoxam, Nitren and Cuen which all are deep blue, while Pden only displays a weak yellow color. The color of the solvent or in some cases the cellulose – metal complex in itself may cause problems in

analytical applications where light scattering or refractive index is used [68].

Cadoxen [Cd(H2N(CH2)2NH2)3](OH)2 Cdtren [Cd(NH2CH2CH2)3N](OH)2 Cooxen [Co(H2N(CH2)2NH2)2](OH)2 Cupren [Cu(H2N(CH2)3NH2)2](OH)2

Cuen [Cu(H2N(CH2)2NH2)2](OH)2

Nioxen [Ni(H2N(CH2)2NH2)3](OH)2 Nitren [Ni(NH2CH2CH2)3N](OH)2 Pden [Pd(H2N(CH2)2NH2](OH)2 Zincoxen [Zn(H2N(CH2)2NH2)2](OH)2

**Table 2.** Transition metal complexes with amines or NH3, their common names and chemical formula.

Saalwächter et al. compared several metal complexes and found that the coordinative binding metal complexes such as Cuoxam, Nitren and Cdtren were most efficient as cellulose solvents. The coordination takes place at the deprotonated olate anions at C2 and C3 at each AGU. Even though the dissolution appears to be to molecular level and via

Cuam/Cuoxam/Schweitzer's reagent [Cu(NH3)4](OH)2

Nioxam [Ni(NH3)6](OH)2

sodium hydroxide with urea under mild conditions [66].

**6.3. Inorganic metal complexes**

Apart from complex coordination, metal complexes can interact with cellulose via pure Coloumb interactions. This is the case for Cuen [68], but not for Pden which forms square planar complexes with the AGUs of cellulose as with any other ligand [69-70].

Aqueous inorganic salts or metal complexes can also be used for dissolution and regeneration of cellulose. The Cupro process, using cuprammonium hydroxide (Cuam) as solvent, was invented already in the year 1890 and is still used today, although not in a huge scale [71]. The solvents have also proven viable reaction media. For example, completely homogeneous etherification of cellulose may be performed in Nitren to produce carboxymethyl cellulose in a one-phase-reaction with a regioselectivity and substitution pattern similar to the carboxymethyl cellulose obtained from commercial routes in NaOH slurries [72]. Unfortunately, some of the representatives of the inorganic metal complex based solvents initiate severe cellulose depolymerization in the presence of even traces of oxygen [73].

#### **6.4. Molten inorganic salt hydrates/concentrated inorganic salt solutions**

Molten inorganic salts have been pursued as cellulose solvents and reaction media for close to 100 years now and some are indeed able to dissolve cellulose without pretreatment and in reasonable concentrations. Both pure salt hydrates and mixtures of different salt hydrates as well as certain concentrated inorganic salt solutions may swell or dissolve cellulose [74]. The most commonly used solvents in this class are MgCl2\*6H2O, LiCl\*5H2O, LiClO4\*3H2O, ZnCl2\*4H2O, ZnCl2/H2O, LiSCN and Ca(SCN)2/H2O with or without adition of the sodium or potassium thiocyanate salt [75]. LiClO4\*3H2O is an extremely efficient solvent and cellulose in this particular salt gives clear solutions within a few minutes.

The dissolution process of cellulose in the aqueous Ca(SCN)2 solvent system was examined using IR spectroscopy by Hattori et al. and it seems to be based on complex formation and an addition compound between cellulose and the thiocyanate anion [76] but curiously enough, the crystallinity of regenerated cellulose product appears to depend on what kind of precipitation agent is used. In water, only cellulose II is recovered, but in methanol the product is mainly cellulose I. Upon precipitation in acetone the product is amorphous [77]. The interaction between cellulose and salt hydrates have also been examined using Raman spectroscopy [74], and of course different NMR spectroscopy techniques [78]. The variation in dissolution processes in different molten salt hydrates was again thoroughly investigated by e.g. 13C NMR spectroscopy and discussed by Leipner et al [79].

Homogeneous reactions to obtain cellulose derivatives may be carried out in molten inorganic salt hydrates. Heinze with coworkers reported successful carboxymethylation of cellulose in LiClO4\*3H2O with varying degree of substitution, and a statistical distribution of

substituents, showing that no part of the cellulose polymer was inaccessible to the reagents [72, 80]. Esterification in the form of acetylation in the molten inorganic salt LiClO4\*3H2O and the eutectic mixture of NaSCN/KSCN/LiSCN\*2H2O, as well as deacetylation reactions of cellulose triacetate in the concentrated salt solution of ZnCl2/H2O has been reported [81]. The acetylation reactions were performed with various acetylating agents such as vinyl acetate and acetic anhydride.

#### **6.5. Acidic solutions of cellulose**

It is sometimes claimed that mineral acids are able to dissolve cellulose. However, this must be put in context. Dilute acids swell cellulose, but dissolution can only be achieved using higher concentrations of acids and is expected to be associated with severe, if not complete, chain degradation over time if temperature is not kept very low. The concept of level-off degree of polymerization (LODP) was introduced as the chain length of cellulose after treatment in 2.5 N hydrochloric acid [82-83]. If the hydrolysis is allowed to continue the degree of polymerization will eventually reach the LODP which is thought to reflect the longitudinal size of the native cellulose crystals, since non-crystalline areas are preferably hydrolyzed over the crystallites.

Concentrated mineral acids such as hydrochloric acid, phosphoric acid or mixed acids are utilized as degradation media to prepare cellodextrins, short chained cellulose oligomers [84-87]. Phosphoric acids, with or without additives such as organic acids as a potent solvent for cellulose was patented already back in 1927 [88].

The transition from swelling to dissolution of cellulose in o-phosphoric acid, and its effect on the cellulose accessibility to enzymatic hydrolysis, was studied by Zhang et al. and dissolution took place without severe hydrolysis under the conditions used [89]. As often, water plays a crucial role in the dissolution behavior of cellulose in this solvent. Phosphoric acid is quite unique being triprotic and due to its ability to form dimers and even polymers. The composition of the acid is usually expressed in P2O5-concentration, which, at a concentration exceeding 74 % is anhydrous (superphosphoric acid). Thus, mixing different species of phosphoric acids may give a powerful cellulose solvent, claimed to rapidly dissolve up to 38 % w/w cellulose. This was shown recently and from these anisotropic solutions of cellulose with a DP of 700-800 in water free phosphoric acid, spinning dopes for production of textile fibers with only moderate chain degradation could be prepared. Fibers can be spun by air gap spinning in acetone and neutralized by Na2CO3. The resulting yarn showed extraordinary tenacity [90-91].

Carboxylic acids are not successfully used as direct dissolution media for cellulose. The acidity is low, and the acids are likely to react with the cellulose, resulting in cellulose derivatives that must be converted to pure cellulose in a second step. Acids in this group that may be used as solvents for cellulose include trifluoroacetic acid, dichloroacetic acid and formic acid with or without addition of sulfuric acid [92]. The dissolution is much faster if sulfuric acid is used as a catalyst. However, the polymers dissolved in these acids are not cellulose but the corresponding cellulose derivative, meaning these are in fact not direct solvents for cellulose but rather derivatizing solvents. Moreover, common for the cellulose derivatives produced via this route is unsurprisingly that they show hydrolytic instability.

#### **6.6. Non aqueous media**

158 Cellulose – Fundamental Aspects

acetate and acetic anhydride.

**6.5. Acidic solutions of cellulose**

hydrolyzed over the crystallites.

showed extraordinary tenacity [90-91].

for cellulose was patented already back in 1927 [88].

substituents, showing that no part of the cellulose polymer was inaccessible to the reagents [72, 80]. Esterification in the form of acetylation in the molten inorganic salt LiClO4\*3H2O and the eutectic mixture of NaSCN/KSCN/LiSCN\*2H2O, as well as deacetylation reactions of cellulose triacetate in the concentrated salt solution of ZnCl2/H2O has been reported [81]. The acetylation reactions were performed with various acetylating agents such as vinyl

It is sometimes claimed that mineral acids are able to dissolve cellulose. However, this must be put in context. Dilute acids swell cellulose, but dissolution can only be achieved using higher concentrations of acids and is expected to be associated with severe, if not complete, chain degradation over time if temperature is not kept very low. The concept of level-off degree of polymerization (LODP) was introduced as the chain length of cellulose after treatment in 2.5 N hydrochloric acid [82-83]. If the hydrolysis is allowed to continue the degree of polymerization will eventually reach the LODP which is thought to reflect the longitudinal size of the native cellulose crystals, since non-crystalline areas are preferably

Concentrated mineral acids such as hydrochloric acid, phosphoric acid or mixed acids are utilized as degradation media to prepare cellodextrins, short chained cellulose oligomers [84-87]. Phosphoric acids, with or without additives such as organic acids as a potent solvent

The transition from swelling to dissolution of cellulose in o-phosphoric acid, and its effect on the cellulose accessibility to enzymatic hydrolysis, was studied by Zhang et al. and dissolution took place without severe hydrolysis under the conditions used [89]. As often, water plays a crucial role in the dissolution behavior of cellulose in this solvent. Phosphoric acid is quite unique being triprotic and due to its ability to form dimers and even polymers. The composition of the acid is usually expressed in P2O5-concentration, which, at a concentration exceeding 74 % is anhydrous (superphosphoric acid). Thus, mixing different species of phosphoric acids may give a powerful cellulose solvent, claimed to rapidly dissolve up to 38 % w/w cellulose. This was shown recently and from these anisotropic solutions of cellulose with a DP of 700-800 in water free phosphoric acid, spinning dopes for production of textile fibers with only moderate chain degradation could be prepared. Fibers can be spun by air gap spinning in acetone and neutralized by Na2CO3. The resulting yarn

Carboxylic acids are not successfully used as direct dissolution media for cellulose. The acidity is low, and the acids are likely to react with the cellulose, resulting in cellulose derivatives that must be converted to pure cellulose in a second step. Acids in this group that may be used as solvents for cellulose include trifluoroacetic acid, dichloroacetic acid and formic acid with or without addition of sulfuric acid [92]. The dissolution is much faster if sulfuric acid is used as a catalyst. However, the polymers dissolved in these acids are not In 1934, Charles Graenacher reported the first non-aqueous direct solvent for cellulose in his patent where he applied N-alkylpyridinium salts not only as fairly efficient dissolution media but also for homogeneous reaction [93]. Since then a wide distribution of non aqueous media for cellulose treatment have developed, for uses both within industry and academia.

#### **6.7. DMAc/LiCl**

One of the most frequently used solvent systems for cellulose and other polysaccharides when it comes to homogenous modification and analytical work such as size exclusion chromatography is the dimethyl acetamide/lithium chloride mixture [94-96] first patented by McCormic in the early 80s [97]. The DMAc/LiCl solvent system is very efficient and will dissolve even high molecular weight cellulose like cotton linters or bacterial cellulose with negligible chain degradation at moderate temperatures. Besides that, major advantages are that the solutions are colorless and compatible with GPC columns.

**Figure 9.** Proposed interaction between solvent and polymer acting as the dissolution mechanism of the system cellulose in DMAc/LiCl, adapted from McCormick, Callais et al. 1985.

The DMAc/LiCl solvent system for cellulose seems to be very specific in its interaction. Neither DMAc with other lithium salts nor DMAc with other chloride salts seem to work in the same way and not even the similar solvent dimethyl formamide with addition of LiCl. The mechanism proposed by McCormick was that hydroxyl groups of cellulose interact with a lithium – DMAc macrocation via hydrogen bonding bridged by the chloride anion. The lithium cation interacts with the carbonyl oxygen via ion – dipole interaction [98]. For this interaction to take place, no cellulose bound water can be present. As a consequence of this, solvent exchange of the cellulose or distillation of the DMAc/LiCl/cellulose system is often necessary. This is a time and solvent demanding step and is one reason why this solvent system may not be preferred outside laboratory scale conditions. Moreover, the solvent itself needs to be dry, since it is easily disturbed by water impurities [99]. Despite early suggestions, the dissolution mechanism is disputed and still not completely understood. However, the DMAc molecule is not able to form hydrogen bonds to any large extent, and that should leave the inorganic ions of the mixtures highly active and free to interact with the polysaccharide instead [94].

This solvent system has been suggested for preparing regenerated cellulose. For example, cellulose beads for support in columns used for liquid chromatography have successfully been produced from DMAc/LiCl solutions and due to the efficiency of the solvent it was possible to optimize the bead performance by adjusting DP and concentration of cellulose [100]. DMAc/LiCl is also suitable for homogeneous derivatization of cellulose. As a "true" solvent where no covalent bonding between solvent and solute takes place, the cellulose is fully susceptible to reaction with other molecules [101-102].

In spite of its many uses, this solvent system experiences some problems. In size exclusion chromatography, unwanted effects have been noticed [103] and under thermal stress the cellulose solutions in DMAc/LiCl may fail to remain inert [104].

#### **6.8. DMSO/TBAF**

A new and very powerful solvent for cellulose consists of a mixture of dimethylsulfoxide (DMSO) and tetrabutylammonium fluoride (TBAF). It is an efficient solvent and can dissolve cellulose with a DP of up to 1200 within one hour at 60 °C [105]. So far, the DMSO/TBAF solvent system has been used mostly for analytical purposes e.g. NMR and GPC but also for homogenous reactions for chemical modification of cellulose, both in labscale only. Both etherifications and esterifications have been pursued with varying success [106-110].

**Figure 10.** Dimethylsulfoxide (left) and tetrabutylammonium fluoride (right), here represented as anhydrous.

TBAF is a very hygroscopic salt and water content in DMSO/TBAF/cellulose solutions may play a crucial role, depending on circumstances. Anhydrous TBAF is unstable [111-112] but TBAF with water content up to the trihydrate are excellent cellulose solvents, while higher water content is not tolerated. Other ammonium fluorides might be applicable as well [105].

#### **6.9. N-Methylmorpholine-N-oxide: NMMO**

Indisputable the most industrially successful of all non-derivatizing cellulose solvents is the N-methylmorpholine–N-oxide, commonly known as NMMO. Due to its ability to dissolve high concentrations of cellulose directly and without alteration of the chemical properties of the cellulose chain, as well as the possibility to recycle more than 99 % of the solvent after usage, this particular solvent has proven to be a viable commercial solvent system. Fibers produced from this solvent also show exceptional mechanical and tactile properties, making NMMO a serious competition to the old but incredibly successful solvent system NaOH/CS2 used in the viscose process. The NMMO solvent system has clear advantages in that it is non-derivatizing, since many process steps may be left out.

**Figure 11.** N-methylmorpholine-N-oxide, a cyclic, aliphatic tertiary amine oxide.

160 Cellulose – Fundamental Aspects

**6.8. DMSO/TBAF**

[106-110].

anhydrous.

interact with the polysaccharide instead [94].

fully susceptible to reaction with other molecules [101-102].

cellulose solutions in DMAc/LiCl may fail to remain inert [104].

**6.9. N-Methylmorpholine-N-oxide: NMMO**

extent, and that should leave the inorganic ions of the mixtures highly active and free to

This solvent system has been suggested for preparing regenerated cellulose. For example, cellulose beads for support in columns used for liquid chromatography have successfully been produced from DMAc/LiCl solutions and due to the efficiency of the solvent it was possible to optimize the bead performance by adjusting DP and concentration of cellulose [100]. DMAc/LiCl is also suitable for homogeneous derivatization of cellulose. As a "true" solvent where no covalent bonding between solvent and solute takes place, the cellulose is

In spite of its many uses, this solvent system experiences some problems. In size exclusion chromatography, unwanted effects have been noticed [103] and under thermal stress the

A new and very powerful solvent for cellulose consists of a mixture of dimethylsulfoxide (DMSO) and tetrabutylammonium fluoride (TBAF). It is an efficient solvent and can dissolve cellulose with a DP of up to 1200 within one hour at 60 °C [105]. So far, the DMSO/TBAF solvent system has been used mostly for analytical purposes e.g. NMR and GPC but also for homogenous reactions for chemical modification of cellulose, both in labscale only. Both etherifications and esterifications have been pursued with varying success

**Figure 10.** Dimethylsulfoxide (left) and tetrabutylammonium fluoride (right), here represented as

TBAF is a very hygroscopic salt and water content in DMSO/TBAF/cellulose solutions may play a crucial role, depending on circumstances. Anhydrous TBAF is unstable [111-112] but TBAF with water content up to the trihydrate are excellent cellulose solvents, while higher water content is not tolerated. Other ammonium fluorides might be applicable as well [105].

Indisputable the most industrially successful of all non-derivatizing cellulose solvents is the N-methylmorpholine–N-oxide, commonly known as NMMO. Due to its ability to dissolve high concentrations of cellulose directly and without alteration of the chemical properties of the cellulose chain, as well as the possibility to recycle more than 99 % of the solvent after usage, this particular solvent has proven to be a viable commercial solvent system. Fibers NMMO is completely soluble in water, and as a pure substance it is extremely hygroscopic. The high polarity of the N – O bond also results in a pronounced ability to form hydrogen bonds. There are two stable hydrates of NMMO, the monohydrate NMMO\*H2O and the 2.5 hydrate NMMO\*2.5H2O. As cellulose solvent the NMMO\*H2O is preferred, and cellulose is soluble all the way up to NMMO\*1.2H2O while for higher amounts of water precipitation of cellulose takes place. The NMMO oxygen is able to form two hydrogen bonds with nearby hydroxyl groups such in water or cellulose [113]. Hence, the monohydrate can dissolve cellulose while the 2.5-hydrate cannot. The competition of water and cellulose for these hydrogen bonds is the central feature of a whole industry based on dissolution and regeneration of cellulose in the NMMO system.

The procedure for dissolving cellulose in NMMO usually includes a first step where a suspension of cellulose in NMMO and a large excess of water is well mixed. The excess water provides low viscosity and thereby superior mixing. Surplus water is then removed by heat between 100 and 120 °C and reduced pressure until the point of complete cellulose dissolution is reached and the spinning dope is in its final form of approximately 14 % cellulose, 10 % water and 76 % NMMO.

The dissolution process of cellulose in NMMO at elevated temperatures is performed under strictly controlled conditions, since NMMO is thermally unstable. The N – O bond is energy rich and upon cleavage releases 222 kJ/mol. Furthermore, NMMO is an oxidant and is sensitive to all forms of catalytic impurities in the dope. NMMO was first considered as an unproblematic solvent without byproduct in the process of dissolving cellulose, but as the knowledge of the possible side reactions in the cellulose/NMMO have increased, the understanding of industrial challenges has improved [114-116]. Due to the reactivity of NMMO and the severe degradation of cellulose that may be the result of this, stabilizers must be added already in the beginning of the dissolution process. Such stabilizers are for example propyl gallate, which is the most known. This compound forms a strongly colored chromophore during usage, and other stabilizers are currently being investigated [117]. The degradation of NMMO and subsequent side reactions may include N-methylmorpholine, morpholine and formaldehyde. Some reactions are induced by transition metal ions such as iron and copper, why these must be strictly excluded from the process [118].

To be able to prepare cellulose solutions in NMMO without stabilizers and without chain degradation, temperature and/or time must be kept low and while this is not doable under commercial conditions, stabilizers are a crucial part of the process. To succeed without stabilizers, high shear must be applied throughout the dissolution and immediately subsequent spinning process [119]. This accelerates the dissolution process and results in a smooth dissolution without undissolved gel particles. Mechanical energy added to the system might have the role of facilitating breakage of cellulose – cellulose intermolecular bonds, enabling solvent molecules to coordinate to the polymer instead. In addition, the apparent viscosity decreases with shear stress due to cellulose alignment, as expected in non-Newtonian polymer solutions. Viscosity is of course also dependent on cellulose concentration and degree of polymerization of chosen substrate.

NMMO is now successfully applied as a non derivatizing cellulose solvent on industrial scale, and a huge amount of research is available on the subtle details on the interactions between polymers and solvent. For example, activation of cellulose seems to play a significant role in the aggregation behavior of cellulose in the NMMO monohydrate solution [120].

#### **6.10. Ionic liquids**

This broad class of solvents comprises low melting salts with an organic cation and an organic or inorganic anion. Several comprehensive overviews of the field have been published since 2002 when Swatloski found the neat imidazolium salt 1-butyl-3-methyl imidazolium chloride (BMIMCl) to dissolve cellulose [121]. As previously mentioned, Graenacher found already in 1934 that liquefied quaternary ammonium salts, alone or diluted in suitable solvents, could dissolve cellulose. He worked mostly with pyridinium chlorides, but at the time this did not attract the attention it might have deserved [93]. Ionic liquids (IL) represent chemicals simply defined as organic salts with a melting point below 100 °C and are not limited to being solvents. Room temperature ionic liquids (RTIL) are often considered the second generation of ionic liquids. The possibility to pair anions with cations yields an almost endless library of potential ionic liquids, and the possibilities to adjust chemical and physical properties of the resulting salts are immense. Since the late 1990s, the interest in ionic liquids has grown fast. Ionic liquids have now penetrated many areas of research and industrial applications of ionic liquids can be found in as wide spread areas as pharmaceutics, analytical chemistry, separation and extraction, materials science and as electrolytes in batteries. For cellulose applications the topic is an ever growing area of research. This relatively new solvent class has already shown great versatility in the field of cellulose technology, including dissolution for regeneration purposes [122-123], homogeneous derivatization [124], and biomass processing including wood component separation [125-127].

The ionic liquids that are able to dissolve cellulose include several classes of cations, and a multitude of anions. Some of the most common cations are imidazolium, pyridinium, ammonium and phosphonium derivatives, shown in Figure 12. The most popular cation used today is the imidazolium cation with different alkyl substituents. The effect of alkyl chain length on the cellulose dissolution ability was acknowledged already 2002 by Swatloski and has since then been observed by several groups [128-129]. Studies on the imidazolium cation itself, not taking into account its properties as a solvent, recently explained in detail the effect of cation symmetry and found effects on e.g. glass temperature, viscosity and ion mobility [130]. The asymmetrical cation provides the ionic liquid with lower viscosity, which indeed is an important feature for a solvent.

**Figure 12.** Technically relevant ionic liquid cations for cellulose processing.

162 Cellulose – Fundamental Aspects

[120].

**6.10. Ionic liquids**

separation [125-127].

To be able to prepare cellulose solutions in NMMO without stabilizers and without chain degradation, temperature and/or time must be kept low and while this is not doable under commercial conditions, stabilizers are a crucial part of the process. To succeed without stabilizers, high shear must be applied throughout the dissolution and immediately subsequent spinning process [119]. This accelerates the dissolution process and results in a smooth dissolution without undissolved gel particles. Mechanical energy added to the system might have the role of facilitating breakage of cellulose – cellulose intermolecular bonds, enabling solvent molecules to coordinate to the polymer instead. In addition, the apparent viscosity decreases with shear stress due to cellulose alignment, as expected in non-Newtonian polymer solutions. Viscosity is of course also dependent on cellulose

NMMO is now successfully applied as a non derivatizing cellulose solvent on industrial scale, and a huge amount of research is available on the subtle details on the interactions between polymers and solvent. For example, activation of cellulose seems to play a significant role in the aggregation behavior of cellulose in the NMMO monohydrate solution

This broad class of solvents comprises low melting salts with an organic cation and an organic or inorganic anion. Several comprehensive overviews of the field have been published since 2002 when Swatloski found the neat imidazolium salt 1-butyl-3-methyl imidazolium chloride (BMIMCl) to dissolve cellulose [121]. As previously mentioned, Graenacher found already in 1934 that liquefied quaternary ammonium salts, alone or diluted in suitable solvents, could dissolve cellulose. He worked mostly with pyridinium chlorides, but at the time this did not attract the attention it might have deserved [93]. Ionic liquids (IL) represent chemicals simply defined as organic salts with a melting point below 100 °C and are not limited to being solvents. Room temperature ionic liquids (RTIL) are often considered the second generation of ionic liquids. The possibility to pair anions with cations yields an almost endless library of potential ionic liquids, and the possibilities to adjust chemical and physical properties of the resulting salts are immense. Since the late 1990s, the interest in ionic liquids has grown fast. Ionic liquids have now penetrated many areas of research and industrial applications of ionic liquids can be found in as wide spread areas as pharmaceutics, analytical chemistry, separation and extraction, materials science and as electrolytes in batteries. For cellulose applications the topic is an ever growing area of research. This relatively new solvent class has already shown great versatility in the field of cellulose technology, including dissolution for regeneration purposes [122-123], homogeneous derivatization [124], and biomass processing including wood component

The ionic liquids that are able to dissolve cellulose include several classes of cations, and a multitude of anions. Some of the most common cations are imidazolium, pyridinium, ammonium and phosphonium derivatives, shown in Figure 12. The most popular cation

concentration and degree of polymerization of chosen substrate.

The dissolution mechanism of cellulose in ionic liquids has long been argued to be all about hydrogen bond interaction. Several studies have confirmed that the anion of the ionic liquid penetrates the cellulose structure and dissembles the native cellulose structure by competitive hydrogen bonding [131-133]. The anion acts as a hydrogen bond acceptor and the cation as a hydrogen bond donor.

In his original article Swatloski argued that hydrogen bond capability, and therefore the ability to dissolve cellulose, was directly related to the anion concentration in the close proximity of the polymer. This was said to be the reason for the fact that the butyl imidazolium salt but not the corresponding salts with higher alkyl chains could dissolve cellulose. Smaller cations simply allow for more anions to crowd around the cellulose chain [128]. The role of the cation in the dissolution mechanism is still disputed. Some simply leave the cation out of the discussion while others attributes the cation a more prominent role, e.g. as electron acceptor and hydrogen bond donor as according to Feng and Chen, c.f. Figure 12. In that case, both the cation and the anion need to be small enough to reach the hydroxyl groups of the cellulose, forming a electron donor – electron acceptor complex, break the polysaccharide – polysaccharide interactions and finally solubilize the polymer [134]. Proof of this is claimed to be the change in cellulose dissolution capability in imidazolium salts when the acidic proton on C2 is replaced by a methyl group [123].

Viscosity and other rheological properties are always important when working with polymer melts or solutions. One feature to examine is the Mark-Houwink parameters which relate the molecular weight of the polymer with the intrinsic viscosity of the solution according to the simple expression

$$\left[\eta\right] = KM^{\alpha} \tag{4}$$

where [η] is the intrinsic viscosity and M is the molecular weight if the polymer. K and α are the so called Mark-Houwink parameters. An indication of the polymer shape and indirectly a measure of the solvent quality can be acquired from these parameters. Cellulose solutions in ionic liquids have been studied by Gericke and co-workers who examined the Mark-Houwink parameters for cellulose/1-ethyl-3-methyl-imidazolium acetate (EMIMAc) solutions, and report α-values of 0.4 – 0.6 in the temperature range of 0 – 100 °C. A value of α around 0.8 indicates a "good" solvent and in general, α-values of 0.65 to ~1 have been reported for other cellulose solutions [135].

**Figure 13.** Interaction between cellulose and imidazolium type ionic liquid, adapted from Feng and co workers [134].

It has been shown numerous times that ionic liquids can be used as a reaction media for homogeneous derivatization as well as for dissolution of cellulose. Further, it has been proposed that due to their good dissolution properties, the use of ionic liquids can aid in control of degree of substitution in for example acetylation and tosylation reactions using various ionic liquids, reaction conditions and reactants [136-138]. Unlike in DMAc/LiCl solutions, no catalyst seems to be needed. The degree of substitution be controlled and a very wide range of DS can be achieved [136]. Silylations of cellulose in solutions of BMIMCl and EMIMAc using hexamethyldisilazane as silylating agent with high yields were also recently reported. Degree of substitution was controlled by reaction conditions and DS = 3 were achievable [139].

Ionic liquids as reaction media for homogeneous derivatization of cellulose turn out to be efficient enough to possibly compete with today's heterogeneous reaction routes. With optimization of reaction conditions the possibilities to control DS and possibly even substitution pattern opens up for production of new materials based on cellulose. One of the most important cellulose derivatives is still the cellulose acetate. Acetylation of cellulose can be performed using the ionic liquid not only as a solvent but as the actual reagent [140]. This was first discovered as an unexpected side reaction when in fact the goal was to react cellulose with acid chlorides, trityl chlorides and tosyl chlorides. The resulting polymer was acetylated, meaning that parts of the solvent, in this case the acetate anions of the ionic liquid EMIMAc, are consumed. In large scale applications this might turn out to be a serious problem, since recycling of the solvent requires that it does not degrade or react during the process. Not only will the recycling be more complicated due to side products, it will also be incomplete and in large scale application recycling of the solvent is necessary for both economic and environmental sustainability [141].

Mixed solvent systems provide further possibilities in cellulose processing. Adding a cosolvent might serve as a means to lower the viscosity of the solution and thereby facilitate fast dissolution rate and overall ease of handling. Mixtures of ionic liquids and organic solvents includes BMIMCl in 1,3-dimethyl-2-imidazolidinone (DMI) that, thanks to its low viscosity allows for efficient mixing and no agglomeration. This system is claimed to dissolve 10 % cellulose (Avicel) within a few minutes at 100 °C. This behavior is explained by the fact that only a fraction of IL in the proper molecular solvent may shift the solvatochromic Kamlet Taft parameters α, β and π\* to the point where cellulose is dissolved [142]. Remsing and coworkers studied the molecular interactions of BMIMCl in water and DMSO, respectively, and found that while water completely solvates the ions already at low concentrations high density clusters of ionic liquid were found even at concentrations as low as 10 %. The poor interaction between ions and DMSO thereby showed suggests that DMSO could be used as a rather inert co-solvent, leaving the ionic liquid intact to act as solvent [143].

164 Cellulose – Fundamental Aspects

workers [134].

were achievable [139].

economic and environmental sustainability [141].

reported for other cellulose solutions [135].

a measure of the solvent quality can be acquired from these parameters. Cellulose solutions in ionic liquids have been studied by Gericke and co-workers who examined the Mark-Houwink parameters for cellulose/1-ethyl-3-methyl-imidazolium acetate (EMIMAc) solutions, and report α-values of 0.4 – 0.6 in the temperature range of 0 – 100 °C. A value of α around 0.8 indicates a "good" solvent and in general, α-values of 0.65 to ~1 have been

**Figure 13.** Interaction between cellulose and imidazolium type ionic liquid, adapted from Feng and co

It has been shown numerous times that ionic liquids can be used as a reaction media for homogeneous derivatization as well as for dissolution of cellulose. Further, it has been proposed that due to their good dissolution properties, the use of ionic liquids can aid in control of degree of substitution in for example acetylation and tosylation reactions using various ionic liquids, reaction conditions and reactants [136-138]. Unlike in DMAc/LiCl solutions, no catalyst seems to be needed. The degree of substitution be controlled and a very wide range of DS can be achieved [136]. Silylations of cellulose in solutions of BMIMCl and EMIMAc using hexamethyldisilazane as silylating agent with high yields were also recently reported. Degree of substitution was controlled by reaction conditions and DS = 3

Ionic liquids as reaction media for homogeneous derivatization of cellulose turn out to be efficient enough to possibly compete with today's heterogeneous reaction routes. With optimization of reaction conditions the possibilities to control DS and possibly even substitution pattern opens up for production of new materials based on cellulose. One of the most important cellulose derivatives is still the cellulose acetate. Acetylation of cellulose can be performed using the ionic liquid not only as a solvent but as the actual reagent [140]. This was first discovered as an unexpected side reaction when in fact the goal was to react cellulose with acid chlorides, trityl chlorides and tosyl chlorides. The resulting polymer was acetylated, meaning that parts of the solvent, in this case the acetate anions of the ionic liquid EMIMAc, are consumed. In large scale applications this might turn out to be a serious problem, since recycling of the solvent requires that it does not degrade or react during the process. Not only will the recycling be more complicated due to side products, it will also be incomplete and in large scale application recycling of the solvent is necessary for both Recently an interest in so called switchable or distillable ionic liquids has risen. One important reason for this is the need to recycle and also purify the ionic liquid after use. By reintroducing molecular traits of the ionic liquid, i.e. turning it into a molecular solvent (mixture), it may be possible to distill the components and thereby achieve high purities. Thermal instability of the solvent may actually have a profound impact on the choice of recycling techniques. The possibility of irreversible decomposition of the ionic liquid must be taken into account, and there are several routes by which the e.g. the imidazolium cation can decompose [144]. This concept may contradict the often assumed statements that ionic liquids in general are both non-volatile and thermally stable. Imidazolium based ionic liquids are known to decompose under temperatures exceeding 200 °C and reduced pressure. EMIMCl shows fragmentation into 1-methylimidazole, 1-ethylimidazole, chloromethane and chloroethane [145]. The fragments obtained by thermolysis can be further distilled and separated for later use as starting materials in a reaction to regenerate the original ionic liquid [146]. Problems arise when the thermal decomposition occurs unexpected or via unknown routes. Recent investigations have shown that common ionic liquids such as EMIMCl and BMIMCl start to degrade already at temperatures around 120 °C [144]. The free acids resulting from the decomposition may cause problems in carbohydrate processing, since depolymerization can be expected.

In 2009, BASF patented a method for distillation of ionic liquids such as the cellulose solvent EMIMAc. For a successful distillation, the ionic liquid must be converted into volatile compounds, which upon condensation again form the wanted salt. The results were good, but not outstanding, with a yield of > 90 % and a purity of > 95 % at 170 °C [147]. The failure to reach better results is most likely due to side reactions where unwanted decomposition occurs.

Recently, a new class of distillable tetramethylguanidine based ionic liquids was found to dissolve cellulose. In this case, the ionic liquid will dissociate and form the volatile corresponding acid and base pair [29]. Decreased temperature will again form the ionic liquid, according to the equilibrium in Figure 14. In this case, the distillation can be very successful and the reported yield and purity both exceeded 99 %.

**Figure 14.** Distillation of a tetramethylguanidine based ionic liquid.

## **7. Cellulose solutions**

Due to the strong tendency to self-aggregation in cellulose and the challenge of finding a proper solvent, most industrial cellulose solutions are actually not true molecular solutions. Knowledge of the character of the solution system is of importance for optimization of industrial systems as well as in academia, e.g. for characterization of the polymer using size exclusion chromatography. Comparisons between the cellulose structures in different solvents may be performed using e.g. small angle X-ray scattering (SAXS), small angle light scattering (SLS), dynamic light scattering (DLS) and depolarized dynamic light scattering (DDLS) [148].

Several different models of partially substituted cellulose in solution have been proposed. By light scattering experiments it is possible to approximate the aggregation number, and also the shape of the cellulose aggregates, if present. Models suggest star-branched macromolecules or fringed micelles [149-150]. However, these models were based on experiments using cellulose derivatives and even low degree of substitution may significantly change the behavior of the polymer in a given solvent.

For high concentration cellulose solutions very different behavior can be observed. Lyotropic mesophases or liquid crystalline phases in solutions of NMMO/cellulose are thought to be possible if the concentration of cellulose exceeds 20 % [151-153]. Comparable solutions can also be formed in an ammonia/ammonium thiocyanate solvent. The nature of the solution can be controlled to selectively produce lyotropic cellulose mesophases having either nematic or cholesteric character. High strength, high modulus fibers can be produced from a solution of cellulose in the nematic phase. Films can be prepared from mesophase solutions of cellulose that are mainly in the cholesteric phase. Material properties vary significant with cellulose concentration [154]. Similar solutions for fiber spinning applications have been prepared and solution anisotropy have been observed using inorganic acids [155] and ionic liquids [121, 133, 156] at cellulose concentrations over 10 %. For DMAc/LiCl solutions of cellulose, the polymer concentrations needs to exceed 15 % [98].

The ordered structure can be induced not only in the liquid state but of course also in the gel like state. Nematic ordered films were prepared from molecularly dispersed DMAc/LiCl solutions by stretching of a water swollen gel phase attained by slow coagulation in water vapor [157]. Still, the mechanisms of liquid crystalline phase transition and gel−sol transition are not yet fully understood and much research is left to do. The question must be raised whether to actually define liquid crystalline phases as true solutions when obvious anisotropy is present.

## **8. Conclusions**

166 Cellulose – Fundamental Aspects

**7. Cellulose solutions**

anisotropy is present.

**Figure 14.** Distillation of a tetramethylguanidine based ionic liquid.

significantly change the behavior of the polymer in a given solvent.

Due to the strong tendency to self-aggregation in cellulose and the challenge of finding a proper solvent, most industrial cellulose solutions are actually not true molecular solutions. Knowledge of the character of the solution system is of importance for optimization of industrial systems as well as in academia, e.g. for characterization of the polymer using size exclusion chromatography. Comparisons between the cellulose structures in different solvents may be performed using e.g. small angle X-ray scattering (SAXS), small angle light scattering (SLS), dynamic light scattering (DLS) and depolarized dynamic light scattering (DDLS) [148].

Several different models of partially substituted cellulose in solution have been proposed. By light scattering experiments it is possible to approximate the aggregation number, and also the shape of the cellulose aggregates, if present. Models suggest star-branched macromolecules or fringed micelles [149-150]. However, these models were based on experiments using cellulose derivatives and even low degree of substitution may

For high concentration cellulose solutions very different behavior can be observed. Lyotropic mesophases or liquid crystalline phases in solutions of NMMO/cellulose are thought to be possible if the concentration of cellulose exceeds 20 % [151-153]. Comparable solutions can also be formed in an ammonia/ammonium thiocyanate solvent. The nature of the solution can be controlled to selectively produce lyotropic cellulose mesophases having either nematic or cholesteric character. High strength, high modulus fibers can be produced from a solution of cellulose in the nematic phase. Films can be prepared from mesophase solutions of cellulose that are mainly in the cholesteric phase. Material properties vary significant with cellulose concentration [154]. Similar solutions for fiber spinning applications have been prepared and solution anisotropy have been observed using inorganic acids [155] and ionic liquids [121, 133, 156] at cellulose concentrations over 10 %. For DMAc/LiCl solutions of cellulose, the polymer concentrations needs to exceed 15 % [98].

The ordered structure can be induced not only in the liquid state but of course also in the gel like state. Nematic ordered films were prepared from molecularly dispersed DMAc/LiCl solutions by stretching of a water swollen gel phase attained by slow coagulation in water vapor [157]. Still, the mechanisms of liquid crystalline phase transition and gel−sol transition are not yet fully understood and much research is left to do. The question must be raised whether to actually define liquid crystalline phases as true solutions when obvious The development and optimization of efficient cellulose solvents have had tremendous impact on cellulose processing for well over a hundred years, and will continue to be important as the interest in cellulose itself as a basis for new materials and other applications is increasing. The most important question when considering the dissolution of cellulose is what the purpose of keeping the cellulose in solution will be. For homogeneous reactions in small scale the requirements on the solvents may be completely different from those in a large scale industrial facility for dissolution and shaping of cellulose where the purpose of regeneration is not only to stabilize the physical structure but also regain the original chemical composition of the polymer. Degree of polymerization, source and purity of the cellulose as well as required concentration in the solution may also have a profound effect on the choice of solvent.

A wide variety of suitable solvents for cellulose are already available, both aqueous and non-aqueous. Despite this, feverish activity goes on in both industry and academia to optimize existing solvent systems and develop new ones. Ideally, such a new cellulose solvent would of course be efficient and inert towards the polymer itself. It would also be 100 % recyclable and non-hazardous to environment and man. To meet future requirements on "green chemistry" and sustainability, it would be produced from non-hazardous starting materials and upon final degradation give non-hazardous, biodegradable products only. Proper life cycle assessment could be conducted to see if any new solvent meets these demands.

New materials produced from cellulose raw material are claimed to fall under the category of sustainable development because the cellulose is a polymer that replenishes itself in plants by air, water and solar energy. However, the chemistry we apply to modify and shape the cellulose to whatever form we desire must also be considered. Solvents are no exception, especially not since they are often used in great excess meaning large quantities. Truly sustainable polymer based materials are only achieved by using a sustainable polymer in combination with a sustainable chemistry to finalize the product. The quest for the cellulose solvent of the future goes on.

### **Author details**

Carina Olsson *Corresponding Author Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden*

Gunnar Westman *Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden*

## **Acknowledgement**

The authors would like to acknowledge Prof. Herbert Sixta and Dr. Merima Hasani for valuable discussions and critical reading of the manuscript. Many thanks also to Södra Cell for financial support.

#### **9. References**

[14] Gardner, K.H. and Blackwell, J. (1974). The Structure of Native Cellulose. Biopolymers, 13(10): 1975-2001.

168 Cellulose – Fundamental Aspects

**Acknowledgement**

for financial support.

**9. References**

The authors would like to acknowledge Prof. Herbert Sixta and Dr. Merima Hasani for valuable discussions and critical reading of the manuscript. Many thanks also to Södra Cell

[1] Cross, C.F., Bevan, E.T. and Beadle, C. (1893). Thiokohlensäureester der Cellulose.

[2] Loth, F., Schaaf, E., Fink, H.P., Kunze, J. and Gensrich, H.J. Verfahren zur Herstellung von Cellulosecarbamat in Einem Inerten Organischen, mit Wasser Nicht Mischbaren

[3] Rentto, M.E. (2011). Avilon Develops Environmental Friendly Cleantch Innovation. Available: http://www.cisionwire.com/neo-industrial-plc-g/r/avilon-develops-

[6] Bovey, F.A. and Winslow, F.H. (1979). Macromolecules. An Introduction to Polymer

[7] Zugenmaier, P. (2008). History of Cellulose Research. In: Crystalline Cellulose and

[8] Chu, S.S.C. and Jeffrey, G. (1968). The Refinement of the Crystal Structures of -D-

[9] Ferrier, W.G. (1963). The Crystal and Molecular Structure of -D-glucose. Acta

[10] Nehls, I., Wagenknecht, W., Philipp, B. and Stscherbina, D. (1994). Characterization of Cellulose and Cellulose Derivatives in Solution by High Resolution 13C-NMR

[11] Nishiyama, Y., Langan, P. and Chanzy, H. (2002). Crystal Structure and Hydrogen-Bonding System in Cellulose Iβ from Synchrotron X-ray and Neutron Fiber Diffraction.

[12] Goring, D.A.I. and Timell, T.E. (1962). Molecular Weight of Native Celluloses. Tappi,

[13] Nishikawa, S. and Ono, S. (1913). Transmission of X-rays Through Fibrous, Lamellar and Granular Substances. Proceedings of the Tokyo Mathematico-Physical Society, 7:

environmental-friendly-cleantech-innovation,e200454. Accessed 2012 April 13. [4] Manjkow, J., Papanu, J.S., Hess, D.W., Soane, D.S. and Bell, A.T. (1987). Influence of Processing and Molecular Parameters in the Dissolution Rate of Poly-(Methyl Methacrylate Thin Films. Journal of the Electrochemical Society, 134(8): 2003-2007. [5] Miller-Chou, B.A. and Koenig, J.L. (2003). A Review of Polymer Dissolution. Progress in

Berichte der deutschen chemischen Gesellschaft, 26(1): 1090-1097.

Lösungsmittel. Germany patent (2004).

Polymer Science, 28(8): 1223-1270.

Crystallographica, 16: 1023-1031.

45(6): 454-460.

131-138.

Science. (New York: Academic press, inc.).

Cellulose Derivatives. Berlin: Springer Verlag. pp. 7-51.

Spectroscopy. Progress in Polymer Science, 19(1): 29-78.

Journal of the American Chemical Society, 124(31): 9074-9082.

glucose and Cellobiose. Acta Crystallographica, 24: 830-838.

[46] Isogai, A. and Atalla, R.H. (1998). Dissolution of Cellulose in Aqueous NaOH Solutions. Cellulose, 5(4): 309-319.

170 Cellulose – Fundamental Aspects

[32] Lindman, B., Karlström, G. and Stigsson, L. (2010). On the Mechanism of Dissolution of

[33] Lindman, B. (2010). Amphiphilic Biopolymers. In: M. Fanun (Ed.), Colloids in

[34] Biermann, O., Hädicke, E., Koltzenburg, S. and Müller-Plathe, F. (2001). Hydrophilicity and Lipophilicity of Cellulose Crystal Surfaces. Angewandte Chemie International

[35] Bergenstråhle, M., Mazeau, K. and Berglund, L.A. (2008). Molecular Modeling of Interfaces Between Cellulose Crystals and Surrounding Molecules: Effects of

Caprolactone Surface Grafting. European Polymer Journal, 44(11): 3662-3669. [36] Cho, H.M., Gross, A.S. and Chu, J.-W. (2011). Dissecting Force Interactions in Cellulose Deconstruction Reveals the Required Solvent Versatility for Overcoming Biomass

Recalcitrance. Journal of the American Chemical Society, 133(35): 14033-14041. [37] Gross, A.S. and Chu, J.-W. (2010). On the Molecular Origins of Biomass Recalcitrance: The Interaction Network and Solvation Structures of Cellulose Microfibrils. The Journal

[38] Beckham, G.T., Matthews, J.F., Peters, B., Bomble, Y.J., Himmel, M.E. and Crowley, M.F. (2011). Molecular-Level Origins of Biomass Recalcitrance: Decrystallization Free Energies for Four Common Cellulose Polymorphs. The Journal of Physical Chemistry B,

[39] Betzel, C., Saenger, W., Hingerty, B.E. and Brown, G.M. (1984). Topography of Cyclodextrin Inclusion Complexes, Part 20. Circular and Flip-Flop Hydrogen Bonding in Beta-Cyclodextrin Undecahydrate: a Neutron Diffraction Study. Journal of the

[40] Griffiths, D.W. and Bender, M.L. (1973). Cycloamyloses as Catalysts. Advances in

[41] Clarke, R.J., Coates, J.H. and Lincoln, S.F. (1988). Inclusion Complexes of the Cyclomalto-Oligosaccharides (Cyclodextrins). In: R. S. Tipson and H. Derek (Eds.), Advances in Carbohydrate Chemistry and Biochemistry. Academic Press. pp. 205-249. [42] Sasaki, M., Fang, Z., Fukushima, Y., Adschiri, T. and Arai, K. (2000). Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water. Industrial & Engineering

[43] Sasaki, M., Adschiri, T. and Arai, K. (2004). Kinetics of Cellulose Conversion at 25 MPa

[44] Yu, Y. and Wu, H. (2009). Characteristics and Precipitation of Glucose Oligomers in the Fresh Liquid Products Obtained from the Hydrolysis of Cellulose in Hot-Compressed

[45] Kamide, K., Okajima, K., Matsui, T. and Kowsaka, K. (1984). Study on the Solubility of Cellulose in Aqueous Alkali Solution by Deuteration IR and 13C NMR. Polymer Journal,

Water. Industrial & Engineering Chemistry Research, 48(23): 10682-10690.

in Sub- and Supercritical Water. AIChE Journal, 50(1): 192-202.

Cellulose. Journal of Molecular Liquids, 156(1): 76-81.

of Physical Chemistry B, 114(42): 13333-13341.

American Chemical Society, 106(24): 7545-7557.

Chemistry Research, 39(8): 2883-2890.

Biotechnology. CRC Press. pp. 1-7.

Edition, 40(20): 3822-3825.

115(14): 4118-4127.

Catalysis, 23: 209-61.

16(12): 857-866.

[78] Brendler, E., Fischer, S. and Leipner, H. (2001). 7Li NMR as Probe for Solvent–Cellulose Interactions in Cellulose Dissolution. Cellulose, 8(4): 283-288.

172 Cellulose – Fundamental Aspects

6(1): 84-89.

4(5): 835-844.

Diego: Academic press.

Journal, 30(1): 43-48.

American Chemical Society. pp. 91-101.

Ltd.

Macromolecular Bioscience, 4(12): 1105-1112.

NaOH/Thiourea Aqueous Solutions. Polymer, 47(8): 2839-2848.

functionalization. Progress in Polymer Science, 26(9): 1689-1762.

Coordinating Solvent. Macromolecules, 30(24): 7638-7640.

Cellulose in Unconventional Media. Cellulose, 6(2): 153-165.

Molten Salts as Solvents for Cellulose. Cellulose, 10(3): 227-236.

Solution. Macromolecular Symposia, 294(2): 125-132.

Macromolecules, 33(11): 4094-4107.

[62] Ruan, D., Zhang, L., Zhou, J., Jin, H. and Chen, H. (2004). Structure and Properties of Novel Fibers Spun from Cellulose in NaOH/Thiourea Aqueous Solution.

[63] Chen, X., Burger, C., Fang, D., Ruan, D., Zhang, L., Hsiao, B.S. and Chu, B. (2006). X-ray Studies of Regenerated Cellulose Fibers Wet Spun From Cotton Linter Pulp in

[64] Cai, J. and Zhang, L. (2005). Rapid Dissolution of Cellulose in LiOH/Urea and

[65] Qi, H., Liebert, T., Meister, F., Zhang, L. and Heinze, T. (2010). Homogenous Carboxymethylation of Cellulose in the New Alkaline Solvent LiOH/Urea Aqueous

[66] Zhou, J., Qin, Y., Liu, S. and Zhang, L. (2006). Homogenous Synthesis of Hydroxyethylcellulose in NaOH/Urea Aqueous Solution. Macromolecular Bioscience,

[67] Heinze, T. and Liebert, T. (2001). Unconventional methods in cellulose

[68] Saalwächter, K., Burchard, W., Klüfers, P., Kettenbach, G., Mayer, P., Klemm, D. and Dugarmaa, S. (2000). Cellulose Solutions in Water Containing Metal Complexes.

[69] Ahlrichs, R., Ballauff, M., Eichkorn, K., Hanemann, O., Kettenbach, G. and Klüfers, P. (1998). Aqueous Ethylenediamine Dihydroxo Palladium(II): A Coordinating Agent for Low- and High-Molecular Weight Carbohydrates. Chemistry – A European Journal,

[70] Hanemann, O. and Ballauff, M. (1997). Chain Conformation of Cellulose in a

[71] Woodings, C. (2001). Regenerated Cellulose Fibres. Cambridge: Woodhead publishing

[72] Heinze, T., Liebert, T., Klüfers, P. and Meister, F. (1999). Carboxymethylation of

[73] Sjöström, E. (1993). Wood Chemistry: Fundamentals and Applications (2nd ed.). San

[74] Fischer, S., Leipner, H., Thümmler, K., Brendler, E. and Peters, J. (2003). Inorganic

[75] Fischer, S. and Thümmler, K. (2010). Molten Inorganic Salts as Reaction Medium for Cellulose. In: Cellulose Solvents: For Analysis, Shaping and Chemical Modification.

[76] Hattori, M., Koga, T., Shimaya, Y. and Saito, M. (1998). Aqueous Calcium Thiocyanate Solution as a Cellulose Solvent. Structure and Interactions with Cellulose. Polymer

[77] Hattori, M., Shimaya, Y. and Saito, M. (1998). Structural Changes in Wood Pulp Treated by 55 wt% Aqueous Calcium Thiocyanate Solution. Polymer Journal, 30(1): 37-42.

NaOH/Urea Aqueous Solutions. Macromolecular Bioscience, 5(6): 539-548.

Sulfoxide/Tetrabutylammonium Fluoride. Macromolecular Rapid Communications, 25(9): 916-920.

[111] Sharma, R.K. and Fry, J.L. (1983). Instability of Anhydrous Tetra-Normal-Alkylammonium Fluorides. Journal of Organic Chemistry, 48(12): 2112-2114.

174 Cellulose – Fundamental Aspects

2394-2401.

Research. Science, 212(4496): 749-753.

Stability. Cellulose, 9(1): 41-53.

Chemistry and Physics, 201(8): 882-889.

and Catalyst. Carbohydrate Research, 327(3): 345-352.

Modification. American Chemical Society. pp. 103-118.

Macromolecular Chemistry and Physics, 201(6): 627-631.

Ions and Water. Biomacromolecules, 10(9): 2401-2407.

and Microcrystalline Celluloses. Cellulose, 14(4): 283-293.

Fluoride Trihydrate. Macromolecular Bioscience, 4(11): 1008-1013.

[97] McCormick, C.L. Novel Cellulose Solutions. (1981).

[95] Austin, P.R., Brine, C.J., Castle, J.E. and Zikakis, J.P. (1981). Chitin: New Facets of

[96] McCormick, C.L. and Callais, P.A. (1987). Derivatization of Cellulose in Lithium Chloride and N-N-Dimethylacetamide Solutions. Polymer, 28(13): 2317-2323.

[98] McCormick, C.L., Callais, P.A. and Hutchinson, B.H. (1985). Solution Studies of Cellulose in Lithium Chloride and N,N-Dimethylacetamide. Macromolecules, 18(12):

[99] Potthast, A., Rosenau, T., Buchner, R., Röder, T., Ebner, G., Bruglachner, H., Sixta, H. and Kosma, P. (2002). The Cellulose Solvent System N,N-Dimethylacetamide/Lithium Chloride Revisited: the Effect of Water on Physicochemical Properties and Chemical

[100] Oliveira, W.D. and Glasser, W.G. (1996). Hydrogels from Polysaccharides. I. Cellulose Beads for Chromatographic Support. Journal of Applied Polymer Science, 60(1): 63-73. [101] El Seoud, O.A., Marson, G.A., Ciacco, G.T. and Frollini, E. (2000). An Efficient, One-Pot Acylation of Cellulose under Homogeneous Reaction Conditions. Macromolecular

[102] Tosh, B., Saikia, C.N. and Dass, N.N. (2000). Homogeneous Esterification of Cellulose in the Lithium Chloride-N,N-Dimethylacetamide Solvent System: Effect of Temperature

[103] Bikova, T. and Treimanis, A. (2002). Problems of the MMD Analysis of Cellulose by

[104] Potthast, A., Rosenau, T., Sixta, H. and Kosma, P. (2002). Degradation of Cellulosic

[105] Heinze, T. and Köhler, S. (2010). Dimethyl Sulfoxide and Ammonium Fluorides Novel Cellulose Solvents. In: Cellulose Solvents: For Analysis, Shaping and Chemical

[106] Heinze, T., Dicke, R., Koschella, A., Kull, A.H., Klohr, E.-A. and Koch, W. (2000). Effective Preparation of Cellulose Derivatives in a New Simple Cellulose Solvent.

[107] Östlund, Å., Lundberg, D., Nordstierna, L., Holmberg, K. and Nydén, M. (2009). Dissolution and Gelation of Cellulose in TBAF/DMSO Solutions: The Roles of Fluoride

[108] Chen, Y., Stipanovic, A., Winter, W., Wilson, D. and Kim, Y.-J. (2007). Effect of Digestion by Pure Cellulases on Crystallinity and Average Chain Length for Bacterial

[109] Ass, B.A.P., Frollini, E. and Heinze, T. (2004). Studies on the Homogeneous Acetylation of Cellulose in the Novel Solvent Dimethyl Sulfoxide/Tetrabutylammonium

[110] Hussain, M.A., Liebert, T. and Heinze, T. (2004). Acylation of Cellulose with N,N′- Carbonyldiimidazole-Activated Acids in the Novel Solvent Dimethyl

SEC using DMA/LiCl: A Review. Carbohydrate Polymers, 48(1): 23-28.

Materials by Heating in DMAc/LiCl. Tetrahedron Letters, 43(43): 7757-7759.

[138] Cao, Y., JinWub, Zhangb, J., Li, H., Zhanga, Y. and Heb, J. (2009). Room Temperature Ionic Liquids (RTILs): A New and Versatile Platform for Cellulose Processing and Derivatization. Chemical Enginering Journal, 147: 13-21.

176 Cellulose – Fundamental Aspects

4975.

424.

(12): 1271-1273.

1941-1947.

Chemistry, 55(22): 9142-9148.

of Physical Chemistry B, 115(20): 6572-6584.

Cellulose. Macromolecules, 38(20): 8272-8277.

Biomacromolecules, 10(5): 1188-1194.

[125] Sun, N., Rahman, M., Qin, Y., Maxim, M.L., Rodriguez, H. and Rogers, R.D. (2009). Complete Dissolution and Partial Delignification of Wood in the Ionic Liquid 1-ethyl-3-

[126] Ilkka Kilpeläinen, H.X., Alistair King, Mari Granstrom, Sami Heikkinen, Dimitris S. Argyropoulus. (2007). Dissolution of Wood in Ionic Liquids. Agricultural and Food

[127] Fort, D.A., Remsing, R.C., Swatloski, R.P., Moyna, P. and Rogers, G.M.R.D. (2007). Can Ionic Liquids Dissolve Wood? Processing and Analysis of Lignocellulosic Materials

[128] Swatloski, R.P., Spear, S.K., Holbrey, J.D. and Rogers, R.D. (2002). Dissolution of Cellulose with Ionic Liquids. Journal of the American Chemical Society, 124(18): 4974-

[129] Vitz, J., Erdmenger, T., Haensch, C. and Schubert, U.S. (2009). Extended Dissolution Studies of Cellulose in Imidazolium Based ionic Liquids. Green Chemistry, 11(3): 417-

[130] Zheng, W., Mohammed, A., Hines, L.G., Xiao, D., Martinez, O.J., Bartsch, R.A., Simon, S.L., Russina, O., Triolo, A. and Quitevis, E.L. (2011). Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids. The Journal

[131] Remsing, R.C., Swatloski, R.P., Rogers, R.D. and Moyna, G. (2006). Mechanism of Cellulose Dissolution in the Ionic Liquid 1-N-Butyl-3-Methylimidazolium Chloride: a 13C and 35/37Cl NMR Relaxation Study on Model systems. Chemical Communications

[132] Zhang, J., Zhang, H., Wu, J., Zhang, J., He, J. and Xiang, J. (2010). NMR Spectroscopic Studies of Cellobiose Solvation in EmimAc Aimed to Understand the Dissolution Mechanism of Cellulose in Ionic Liquids. Physical Chemistry Chemical Physics, 12(8):

[133] Zhang, H., Wu, J., Zhang, J. and He, J. (2005). 1-Allyl-3-Methylimidazolium Chloride Room Temperature Ionic Liquid: A New and Powerful Nonderivatizing Solvent for

[134] Feng, L. and Chen, Z.-l. (2008). Research Progress on Dissolution and Functional Modification of Cellulose in Ionic Liquids. Journal of Molecular Liquids, 142(1-3): 1-5. [135] Gericke, M., Schlufter, K., Liebert, T., Heinze, T. and Budtova, T. (2009). Rheological Properties of Cellulose/Ionic Liquid Solutions: From Dilute to Concentrated States.

[136] Wu, J., Zhang, J., Zhang, H., He, J., Ren, Q. and Guo, M. (2004). Homogeneous Acetylation of Cellulose in a New Ionic Liquid. Biomacromolecules, 5(2): 266-268. [137] Granström, M., Kavakka, J., King, A., Majoinen, J., Mäkelä, V., Helaja, J., Hietala, S., Virtanen, T., Maunu, S.-L., Argyropoulos, D.S. and Kilpeläinen, I. (2008). Tosylation and Acylation of Cellulose in 1-Allyl-3-Methylimidazolium Chloride. Cellulose, 15: 481-488.

with 1-N-Butyl-3-Methylimidazolium Chloride. Green Chemistry, 9: 63-69.

Methylimidazolium Acetate. Green Chemistry, 11(5): 646-655.

## **Rapid Dissolution of Cellulose in Ionic Liquid with Different Methods**

Wu Lan, Chuan-Fu Liu, Feng-Xia Yue and Run-Cang Sun

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/52517

## **1. Introduction**

178 Cellulose – Fundamental Aspects

Polymer, 40(23): 6443-6450.

Therefrom. (1996).

Unites States of America patent (1988).

[153] Kim, S.O., Shin, W.J., Cho, H., Kim, B.C. and Chung, I.J. (1999). Rheological Investigation on the Anisotropic Phase of Cellulose-MMNO/H2O Solution System.

[154] Cuculo, e.a. Anisotropic Cellulose Solutions, Fibers, and Films Formed Therefrom.

[155] Boerstoel, H.N.K.B., M.; Westerink, J.B. Cellulose Solutions and Products Made

[156] Song, H., Niu, Y., Wang, Z. and Zhang, J. (2011). Liquid Crystalline Phase and Gel−Sol Transitions for Concentrated Microcrystalline Cellulose (MCC)/1-Ethyl-3- Methylimidazolium Acetate (EMIMAc) Solutions. Biomacromolecules, 12(4): 1087-1096. [157] Tetsuo Kondo, E.T., and R. Malcolm Br. (2001). "Nematic Ordered Cellulose": A

Concept of Glucan Chain Association. Biomcaromolecules, 2(4): 1324-1330.

Cellulose, the most widespread natural polymers, can be used as a green source for fabricating biodegradable and biocompatible materials with attractive properties by chemical modification or mixing with other components (Potthast et al., 2006). Effective utilization of cellulose not only mitigates the fossil resources crisis but also protects the environment of the Earth. However, cellulose is extraordinarily difficult to be dissolved due to its considerable hydrogen bonds and partially crystalline structure.

Ionic liquids (ILs), also known as room temperature ionic liquids (RTILs), are organic salts comprised completely of ions. They are regarded as novel green solvents and have attracted tremendous attentions in analysis chemistry, electric chemistry, catalysis chemistry and organic synthesis (Forsyth et al., 2002; Turner et al., 2003) because of their unique physicochemical properties, such as chemical and thermal stability, non-flammability and immeasurably low vapor pressure (Feng & Chen, 2008; Cao et al., 2009). Furthermore, it is feasible to modify the physicochemical properties such as viscosity, melting point, polarity and solubility of the ILs through changing the structure of cations and anions (Hermanutz et al., 2008). Recently, ILs were applied in the field of biomass research, such as dissolution and the following homogeneous modification of cellulose (Forsyth et al., 2002; Cao et al., 2010). Swatloski and his group (Swatloski et al., 2002) reported the pioneer study on the dissolving capability of a series of ILs containing 1-butyl-3-methylimidazolium ([C4mim]+) as cation for cellulose. Among this kind of ILs, 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) exhibited the best solubility, which could dissolve up to 10 wt % of pulp cellulose with a degree of polymerization of (DP) of 1000 at 100 oC. Soon after, 1-allyl-3-methylimidazolium chloride ([Amim]Cl) was synthesized and found to be a powerful solvent of cellulose (Zhang et al., 2005). Cotton cellulose could be dissolved in [Amim]Cl within 120 min at 110 oC. Heinze and his co-workers (Heinze et al., 2005) tested the solubility of [C4mim]Cl for cellulose and the results showed that [C4mim]Cl could dissolved cotton linter (DP=1198) with the concentration of 10 wt%, though a relatively long time (12 h) was required.

© 2013 Liu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Liu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In view of long treatment duration, some pretreatment or auxilary methods should be introduced to reduce the dissolution time of cellulose in ILs. Ultrasonic and microwave are the most popular activation methods or "unconventional" physical agents in the concept of "Green Chemistry" (Kardos & Luche, 2001). Ultrasonic treatment has been well established in the separation of plant materials, particularly for extraction of low molecular weight substances (Tang et al., 2005; Gadhe et al., 2006). The mechanical and chemical effects of ultrasound are believed to accelerate the extraction of organic compounds from plant materials due to disruption of cell walls and to enhance mass transfer in cell walls. Microwave heating, as an alternative to conventional heating technique, has been successfully applied in organic synthesis and carbohydrate degradation (Satge et al., 2002; Ma et al., 2009; Ramier et al., 2012) due to its thermal effects and non-thermal effects, raised from the heating rate, acceleration of ions and molecules collision, and rapid rotation of dipoles. In addition, microwave energy can easily penetrate to particle inside hence all particles can be heated simultaneously, which resolve the heat transfer resistance problem and make it more uniform and efficient than conventional heating methods (Hou et al., 2008). Consequently, it is reasonable to assume that cellulose could be rapidly dissolved in ILs using ultrasonic and microwave as pretreatment or auxilary methods to active cellulose structure. Actually Mikkola et al (Mikkola et al., 2007) and Swatloski et al (Swatloski et al., 2002) have already mentioned that ultrasonic and microwave irradiation can significantly accelerate dissolution of cellulose in ILs. Besides ultrasonic and microwave, ethanol treatment and mechanical milling were also believed to enhance cellulose dissolution rate in ILs. Mechanical milling was one of the most common mechanical pretreatment methods which can cut down the particle size and reduce molecular weight. Previous studies (Evtuguin et al., 2001; Fujimoto et al., 2005) have already applied mechanical milling to break lignocellulose structure in order to dissolve lignin from biomass matrix with organic solvents. Ethanol treatment was a new kind of pretreatment method to increase cellulose dissolution rate in our lab because it was considered to be capable of preventing cornification of cellulose fiber.

In this article, different pretreatment methods were applied to accelerate dissolution of cellulose in ILs, including mechanical milling, ethanol treatment, ultrasonic irradiation and microwave irradiation. Moreover, the effects of different pretreatment conditions on dissolution rate were also investigated, for example, the ethanol treatment time, mechanical milling time, ultrasonic and microwave irradiation power and duration. In addition, ultrasonic and microwave irradiation were also applied during the dissolution process of cellulose samples. We also investigated the influence of various reaction conditions on acceleration of cellulose dissolution and physicochemical properties of regenerated cellulose, such as degree of polymerization (DP) and crystallinity.

### **2. Experimental**

#### **2.1. Materials**

Sugarcane bagasse, obtained from a local sugar factory (Guangzhou, China), was first dried in sunlight and then cut into small pieces. The cut SCB was ground and screened to prepare 40-60 mesh size particles. The ground SCB was dried again in a cabinet oven with air circulation for 16 h at 50 oC.

1-Butyl-3-methylimidazolium chloride ([C4mim]Cl) was provided by the Chemer Chemical Co., Ltd., Hangzhou, China, and used as received. All of other chemicals used were of analytical grade and obtained from Guangzhou Chemical Reagent Factory, China.

#### **2.2. Isolation of cellulose from bagasse**

180 Cellulose – Fundamental Aspects

cornification of cellulose fiber.

**2. Experimental**

**2.1. Materials**

In view of long treatment duration, some pretreatment or auxilary methods should be introduced to reduce the dissolution time of cellulose in ILs. Ultrasonic and microwave are the most popular activation methods or "unconventional" physical agents in the concept of "Green Chemistry" (Kardos & Luche, 2001). Ultrasonic treatment has been well established in the separation of plant materials, particularly for extraction of low molecular weight substances (Tang et al., 2005; Gadhe et al., 2006). The mechanical and chemical effects of ultrasound are believed to accelerate the extraction of organic compounds from plant materials due to disruption of cell walls and to enhance mass transfer in cell walls. Microwave heating, as an alternative to conventional heating technique, has been successfully applied in organic synthesis and carbohydrate degradation (Satge et al., 2002; Ma et al., 2009; Ramier et al., 2012) due to its thermal effects and non-thermal effects, raised from the heating rate, acceleration of ions and molecules collision, and rapid rotation of dipoles. In addition, microwave energy can easily penetrate to particle inside hence all particles can be heated simultaneously, which resolve the heat transfer resistance problem and make it more uniform and efficient than conventional heating methods (Hou et al., 2008). Consequently, it is reasonable to assume that cellulose could be rapidly dissolved in ILs using ultrasonic and microwave as pretreatment or auxilary methods to active cellulose structure. Actually Mikkola et al (Mikkola et al., 2007) and Swatloski et al (Swatloski et al., 2002) have already mentioned that ultrasonic and microwave irradiation can significantly accelerate dissolution of cellulose in ILs. Besides ultrasonic and microwave, ethanol treatment and mechanical milling were also believed to enhance cellulose dissolution rate in ILs. Mechanical milling was one of the most common mechanical pretreatment methods which can cut down the particle size and reduce molecular weight. Previous studies (Evtuguin et al., 2001; Fujimoto et al., 2005) have already applied mechanical milling to break lignocellulose structure in order to dissolve lignin from biomass matrix with organic solvents. Ethanol treatment was a new kind of pretreatment method to increase cellulose dissolution rate in our lab because it was considered to be capable of preventing

In this article, different pretreatment methods were applied to accelerate dissolution of cellulose in ILs, including mechanical milling, ethanol treatment, ultrasonic irradiation and microwave irradiation. Moreover, the effects of different pretreatment conditions on dissolution rate were also investigated, for example, the ethanol treatment time, mechanical milling time, ultrasonic and microwave irradiation power and duration. In addition, ultrasonic and microwave irradiation were also applied during the dissolution process of cellulose samples. We also investigated the influence of various reaction conditions on acceleration of cellulose dissolution and physicochemical properties of regenerated

Sugarcane bagasse, obtained from a local sugar factory (Guangzhou, China), was first dried in sunlight and then cut into small pieces. The cut SCB was ground and screened to prepare

cellulose, such as degree of polymerization (DP) and crystallinity.

The dried SCB powder was first dewaxed with toluene-ethanol (2:1 v/v) in a Soxhlet apparatus for 6 h. The dewaxed SCB was treated with distilled water (800 mL) for 2 h at 80 oC. Further the insoluble residue was delignified with sodium chlorite at pH 3.8-4.0, adjusted by 10% acetic acid, at 75 oC for 2 h. After filtration, the residue was sequentially washed with distilled water and 95% ethanol, and then dried in a cabinet oven at 50 oC overnight. Finally the holocellulose obtained was extracted with 10% KOH (600 mL) at 20 oC for 10 h to release the hemicelluloses. The residue was collected by filtration and washed thoroughly with distilled water until the filtrate was neutral, then dried in an oven at 50 oC for 16 h.

## **2.3. Pretreatment of cellulose**

#### *2.3.1. Mechanical ball milling*

Dry cellulose samples were subjected to ball milling for 6, 12, 18, and 24 h, respectively, on PM 0.4L planet ball mill machine (PM 0.4L Nanjing Chishun Science & Technology Co., Ltd, China). Ball milled samples were dried in an oven at 50 oC for 6 h.

#### *2.3.2. Ethanol treatment*

Residue collected from alkali treatment of holocellulose was washed with distilled water and 95% ethanol and then dispersed in 95% ethanol and subjected to mechanical agitation for 4, 6, 8, and 10 h, respectively, to make cellulose adsorb sufficient ethanol. Then the sample was filtrated by Buchner funnel and dried in cabinet oven with air circulation for 8 h at 55 oC

#### *2.3.3. Ultrasonic irradiation*

10 g of cellulose sample was suspended in 95% ethanol solvent with mechanical agitation. It was irradiated with ultrasound at the power of 30W, 50W, and 70W for 10, 20, 30 and 40 min, respectively, at room temperature (25 oC). Ultrasonic treated samples were filtrated and dried in oven for 8 h at 55 oC.

#### *2.3.4. Microwave irradiation*

10 g of cellulose sample was suspended in 95% ethanol solvent with mechanical agitation and then subjected to microwave oven (XH-100B, Beijing Xianghu Technology Development Co., Ltd., China) for 200, 400, 600 W microwave irradiation with 20, 30, 40 min at room temperature and 50 oC, respectively. Microwave treated samples were filtrated and dried in oven for 8 h at 55 oC.

## **2.4. Dissolution and regeneration of cellulose**

## *2.4.1. Dissolution of cellulose without auxilary method*

Cellulose or pretreated cellulose sample was added into a flask containing [C4mim]Cl with a weight ratio of 2% (cellulose to [C4mim]Cl). The mixture of cellulose/[C4mim]Cl was stirred continuously in an oil bath at 110 oC with N2 atmosphere to form clear solution.

## *2.4.2. Dissolution of cellulose with ultrasonic assisted*

2% dried cellulose sample with ethanol pretreatment was added into a flask with [C4mim]Cl at 110 oC. N2 was purged continuously and the mixture was stirred. Then the mixture was irradiated at 110 oC for 5, 10, 15, and 20 min, respectively, with ultrasound provided with a horn at sonic power of 20, 30, 40, 50, 60 and 75 W, respectively. After ultrasound irradiation, the mixture was stirred at 110 oC until cellulose was completely dissolved.

#### *2.4.3. Dissolution of cellulose with microwave irradiation*

Molten [C4mim]Cl was mixed with cellulose sample with ethanol pretreatment (2 wt%) in a three-neck flask. The flask was positioned in the microwave reactor (XH-100B) and heated by the microwave irradiation of 300W, 400W, 500W, 600W, and 700W, respectively, to a specific temperature of 85 oC, 90 oC, 95 oC, 100 oC, 105 oC, 110 oC, and 115 oC, respectively, and stirred until a clear cellulose/IL solution was formed. The temperature was real-time monitored by a sensor directly contacted with the reaction medium. When a required temperature was reached it was maintained by impulse microwave irradiation at low heating power. The accuracy of temperature was ±0.2 oC for measurement and ±1oC for controlment.

#### *2.4.4. Regeneration of cellulose*

After complete dissolution, the cellulose/IL mixture was slowly poured into 250 mL ethanol with vigorous agitation. The precipitate was collected by filtration and washed thoroughly with ethanol to eliminate [C4mim]Cl, and then freezen-dried for 24 h. The yield was determined from the regenerated cellulose on the basis of initial freezen-dried measurements. To reduce errors and confirm the results, all experiments were performed at least in duplicate, and the yield represents the average value.

#### **2.5. Characterization of the native, pretreated and regenerated cellulose**

#### *2.5.1. Degree of polymerization*

The cupriethylene-diamine method (British standard) was used for determining the limiting viscosity number [η] and DP of native, pretreated, and regenerated cellulose. The viscosity DP was calculated from the following equation, DP0.90 = 1.65 [η]. Molecular weight (Mw) of cellulose was then calculated from DP by multiplied by 162, the Mw of an AGU.

#### *2.5.2. Fourier transform infrared*

182 Cellulose – Fundamental Aspects

oven for 8 h at 55 oC.

**2.4. Dissolution and regeneration of cellulose**

*2.4.1. Dissolution of cellulose without auxilary method*

*2.4.2. Dissolution of cellulose with ultrasonic assisted*

*2.4.3. Dissolution of cellulose with microwave irradiation*

least in duplicate, and the yield represents the average value.

*2.4.4. Regeneration of cellulose*

*2.5.1. Degree of polymerization*

Co., Ltd., China) for 200, 400, 600 W microwave irradiation with 20, 30, 40 min at room temperature and 50 oC, respectively. Microwave treated samples were filtrated and dried in

Cellulose or pretreated cellulose sample was added into a flask containing [C4mim]Cl with a weight ratio of 2% (cellulose to [C4mim]Cl). The mixture of cellulose/[C4mim]Cl was stirred

2% dried cellulose sample with ethanol pretreatment was added into a flask with [C4mim]Cl at 110 oC. N2 was purged continuously and the mixture was stirred. Then the mixture was irradiated at 110 oC for 5, 10, 15, and 20 min, respectively, with ultrasound provided with a horn at sonic power of 20, 30, 40, 50, 60 and 75 W, respectively. After ultrasound irradiation,

Molten [C4mim]Cl was mixed with cellulose sample with ethanol pretreatment (2 wt%) in a three-neck flask. The flask was positioned in the microwave reactor (XH-100B) and heated by the microwave irradiation of 300W, 400W, 500W, 600W, and 700W, respectively, to a specific temperature of 85 oC, 90 oC, 95 oC, 100 oC, 105 oC, 110 oC, and 115 oC, respectively, and stirred until a clear cellulose/IL solution was formed. The temperature was real-time monitored by a sensor directly contacted with the reaction medium. When a required temperature was reached it was maintained by impulse microwave irradiation at low heating power. The

After complete dissolution, the cellulose/IL mixture was slowly poured into 250 mL ethanol with vigorous agitation. The precipitate was collected by filtration and washed thoroughly with ethanol to eliminate [C4mim]Cl, and then freezen-dried for 24 h. The yield was determined from the regenerated cellulose on the basis of initial freezen-dried measurements. To reduce errors and confirm the results, all experiments were performed at

The cupriethylene-diamine method (British standard) was used for determining the limiting viscosity number [η] and DP of native, pretreated, and regenerated cellulose. The viscosity

continuously in an oil bath at 110 oC with N2 atmosphere to form clear solution.

the mixture was stirred at 110 oC until cellulose was completely dissolved.

accuracy of temperature was ±0.2 oC for measurement and ±1oC for controlment.

**2.5. Characterization of the native, pretreated and regenerated cellulose**

The FT-IR spectra of the native and regenerated cellulose were obtained on an FT-IR spectrophotometer (Nicolet 510) using a KBr disc containing 1% finely ground samples. Thirty-two scans were taken for each sample in the range 4000~400 cm−1 with a resolution of 2 cm−1 in the transmittance mode.

## *2.5.3. Wide angle X-ray diffraction*

The XRD spectra were recorded at room temperature with a Rigaku D/MAX-IIIA X-ray diffractometer (Japan) in the scattering angle range from 5o to 45o with 8o/min scanning speed and a 2θ step interval of 0.02o. Cellulose samples were cut into small pieces and laid on the glass sample holder, analyzed under plateau conditions. Ni-filtered Cu Kα1 radiation (λ=0.154 nm) generated at a operating voltage of 40 kV and current of 30 mA was utilized.

#### *2.5.4. Solid-state CP/MAS 13C Nuclear Magnetic Resonance*

The solid-state CP/MAS 13C NMR spectra were measured on a Bruker DRX-400 spectrometer with 5mm MAS BBO probe at the frequency of 100 MHz employing both Cross Polarization and Magic Angle Spinning and each experiment was recorded at ambient temperature. Acquisition time was 0.034 s, the delay time 2 s, and the proton 90o pulse time 4.85 μs. Each spectrum was obtained with an accumulation of 5000 scans.

## *2.5.5. Thermogravimetric analysis*

Thermal analysis of the native and regenerated cellulose was performed on thermogravimetric analysis (TGA) on a thermal analyzer (SDT Q500, TA Instrument). The sample weighed between 7 and 11 mg and the scans were run from room temperature to 600 oC at a rate of 10 oC/min under nitrogen flow.

## **3. Results and discussion**

## **3.1. Dissolution and regeneration of cellulose with different pretreatment methods**

It took quite long to completely dissolve cellulose in ILs because of the high crystallinity and tremendous hydrogen bonding in cellulose. When cellulose dissolved in ILs its crystalline structure was totally destroyed thus no bright eyeshot, which represented the crystalline structure of cellulose, could be found under polarizing microscope. Consequently, in this study we employed polarizing microscope to monitor the dissolution process and defined that dissolution of cellulose was completed when polarizing microscope was in black eyeshot. Ethanol treatment, mechanical milling, ultrasonic irradiation and microwave irradiation were applied to pretreat cellulose for enhancement of cellulose dissolution in ILs, and the results are showed in Table 1.

**Table 1.** Dissolution of cellulose in [C4mim]Cl pretreated with different methods.

Yield of regenerated cellulose was approximately between 94%~118%. On one hand, cellulose would probably absorb inorganic salt, such as impurity of IL, during dissolution process. In this case yield would increase or even beyond 100%. On the other hand, cellulose was also likely to be degraded and produced oligosaccharide or even monosaccharide which could hardly be recovered. In addition, washing step and filtration process also contributed to the losses. According to the yield of regenerated cellulose, it was reasonable to deduce that cellulose did not degraded significantly during dissolution process in ILs at 110 oC. However, yields of mechanical milling samples were relatively lower, because mechanical milling was able to degrade cellulose molecular and break the macromolecules into small fragments. In the following dissolution and regeneration process cellulose molecules were further destroyed and fragments with relatively lower molecular weights were produced which cannot be precipitated and recovered. The same results were also discovered for samples pretreated with relatively high power of microwave.

184 Cellulose – Fundamental Aspects

<sup>15</sup>Microwave

<sup>16</sup>Microwave

<sup>17</sup>Microwave

<sup>18</sup>Microwave

<sup>19</sup>Microwave

and the results are showed in Table 1.

irradiation were applied to pretreat cellulose for enhancement of cellulose dissolution in ILs,

1 - - - - 11.1 h 1.18 2 Mechanical milling - 6 h 25 oC 8.9 h 0.95 3 Mechanical milling - 12 h 25 oC 7.6 h 0.94 4 Mechanical milling - 18 h 25 oC 6.8 h 0.95 5 Mechanical milling - 24 h 25 oC 6.4 h 0.94 6 Ethanol treated - 4 h 25 oC 4.5 h 1.04 7 Ethanol treated - 6 h 25 oC 3.8 h 1.06 8 Ethanol treated - 8 h 25 oC 3.1 h 1.05 9 Ethanol treated - 10 h 25 oC 2.6 h 1.06 10 Ultrasonic irradiation 30 W 20 min 25 oC 2.2 h 1.05 11 Ultrasonic irradiation 30 W 30 min 25 oC 1.5 h 1.06 12 Ultrasonic irradiation 30 W 40 min 25 oC 0.9 h 1.04 13 Ultrasonic irradiation 75 W 20 min 25 oC 1.4 h 1.02 14 Ultrasonic irradiation 75 W 40 min 25 oC 0.8 h 1.01

Time Yield Pretreatment Power Duration Temperature

irradiation 200 W 30 min 25 oC 2.6 h 1.06

irradiation 400 W 30 min 25 oC 2.4 h 1.02

irradiation 600 W 30 min 25 oC 1.6 h 1.04

irradiation 200 W 20 min 50 oC 1.3 h 1.03

irradiation 600 W 20 min 50 oC 0.9 h 0.98

Yield of regenerated cellulose was approximately between 94%~118%. On one hand, cellulose would probably absorb inorganic salt, such as impurity of IL, during dissolution process. In this case yield would increase or even beyond 100%. On the other hand, cellulose was also likely to be degraded and produced oligosaccharide or even monosaccharide which could hardly be recovered. In addition, washing step and filtration process also contributed to the losses. According to the yield of regenerated cellulose, it was reasonable to deduce that cellulose did not degraded significantly during dissolution process in ILs at 110 oC. However, yields of mechanical milling samples were relatively lower, because mechanical milling was able to degrade cellulose molecular and break the macromolecules into small fragments. In the following dissolution and regeneration process cellulose molecules were further destroyed and fragments with relatively lower molecular weights

**Table 1.** Dissolution of cellulose in [C4mim]Cl pretreated with different methods.

No. Pretreatment Condition Dissolution

**Figure 1.** Degree of polymerization of pretreated and regenerated cellulose. Sample No. is in accordance with Table 1. Left bar is DP of pretreated sample, right bar is DP of regenerated sample.

[C4mim]Cl was one of the most widely studied ILs for cellulose dissolution, but it took 11.1 h to totally dissolve unpretreated cellulose sample at 110 oC, which significantly limited the large-scale utilization of ILs in biomass. Commercial application would be promising if the dissolution of cellulose was improved. As shown in Table 1, dissolution time decreased from 11.1 h to about 6.4~8.9 h when cellulose were subjected to mechanical milling for 6, 12, 18, and 24 h (samples 2~5), indicating the great effect of particle size on cellulose dissolution. However, the fine cellulose sample after mechanical milling would be easily agglomerated during mixing cellulose in [C4mim]Cl because of its high viscosity. Outside of agglomerates were swollen cellulose macromolecules and inside was dry sample. This kind of particles would seriously prolong the dissolution time and make it difficult to operate during dissolution process. Ethanol pretreated cellulose (samples 6~9) could be dissolved in [C4mim]Cl at 110 oC within 2.6~4.5 h. During ethanol pretreatment, free water and bound water in cellulose could be replaced by ethanol, which was able to evaporate more quickly during drying, therefore inhibiting high moisture sample become cornification. Cellulose samples after ethanol pretreatment were loosen and porous, which enabled it to open the structure and enlarge superficial area, facilitating penetration of ILs into cellulose. So ethanol pretreatment was one of the effective and feasible methods to improve cellulose dissolution. In addition, dissolution time would be further reduced when ethanol pretreated samples were subjected to ultrasonic and microwave irradiation. It needed only 2.2 h at 110 oC in [C4mim]Cl to entirely dissolve 2% cellulose pretreated with 30 W ultrasound for 20 min. When the power increased to 75 W, the sample dissolution time was reduced to 0.9 h. Further prolonging irradiation time to 40 min also decreased dissolution time to 0.8 h. The cellulose samples pretreated with microwave irradiation exhibited the similar results. It should be noted that cellulose irradiated with high microwave power at high temperature was more likely to be degraded.

Fig. 1 shows the DP of pretreated and regenerated cellulose. DP of cellulose samples after ethanol treatment, mechanical milling, ultrasonic irradiation and microwave irradiation were obviously lower than the DP of native cellulose. For ultrasonic and microwave pretreatment, enhancement of power and irradiation time further decreased the DP of cellulose, even though the decrement was marginal. For example, DP was reduced from 1083 to 994 when ultrasonic power rose from 30 W to 75 W. In addition, mechanical milling for 12 h significantly brings down DP from 1263 to 378, demonstrating the serious degradation of cellulose during mechanical milling. Moreover, DP of all the samples declined slightly after dissolution in [C4mim]Cl at 110 oC and regeneration, indicating that cellulose molecules were broken into relatively low molecular fragments.

#### **3.2. Dissolution and regeneration of cellulose assisted with ultrasonic**

To further study the influence of ultrasonic irradiation on cellulose dissolution in ILs, we tried to apply ultrasonic irradiation directly during the dissolution process as not a pretreatment but an auxilary method. Results showed that it was more effective to utilize ultrasonic in dissolution process than in pretreatment, as listed in Table 2. Cellulose could be dissolved in [C4mim]Cl within 1 h when irradiated by 30W ultrasonic for 20 min. With the same irradiation power and duration, it required 2.2 h to entirely dissolve sample when the ultrasonic was employed as pretreatment method. Enhancement of cellulose dissolution with ultrasonic was probably because ultrasound irradiation provided a greater penetration of ionic liquid into cellulose and improved mass transfer. When the dissolving system was irradiated with ultrasound, the extreme condition was probably obtained. It was said that transient temperature of at least 5000 K and pressure up to 1200 bar could be achieved, accompanied by vigorously physical agitation and shock (Kardos & Luche, 2001; Adewuyi, 2001). All of the abovementioned effects provided excellent conditions for cellulose dissolution in IL. This beneficial effect of ultrasound techniques has also been reported in isolation of cell wall polymers (Aliyu & Hepher, 2000). Cellulose dissolution time decreased from 147 (sample 24) to 120 (sample 20), 110 (sample 25) and 100 min (sample 26) with the improvement of ultrasonic power from 20 to 30, 40 and 50 W. However, further improvement of ultrasonic power (samples 26–28) did not result in the decrement in cellulose dissolution time. It seems that increase of power within small range was not as efficient as extension of duration. Compared with sample 3 obtained with one-step ultrasound irradiation, sample 11 with two-step irradiation with 5 min interval needed the same time to totally dissolve sample as well, indicating that the same ultrasound energy obtained from same power and total irradiation duration did not result in different effect on cellulose dissolution.

was more likely to be degraded.

cellulose dissolution.

samples were subjected to ultrasonic and microwave irradiation. It needed only 2.2 h at 110 oC in [C4mim]Cl to entirely dissolve 2% cellulose pretreated with 30 W ultrasound for 20 min. When the power increased to 75 W, the sample dissolution time was reduced to 0.9 h. Further prolonging irradiation time to 40 min also decreased dissolution time to 0.8 h. The cellulose samples pretreated with microwave irradiation exhibited the similar results. It should be noted that cellulose irradiated with high microwave power at high temperature

Fig. 1 shows the DP of pretreated and regenerated cellulose. DP of cellulose samples after ethanol treatment, mechanical milling, ultrasonic irradiation and microwave irradiation were obviously lower than the DP of native cellulose. For ultrasonic and microwave pretreatment, enhancement of power and irradiation time further decreased the DP of cellulose, even though the decrement was marginal. For example, DP was reduced from 1083 to 994 when ultrasonic power rose from 30 W to 75 W. In addition, mechanical milling for 12 h significantly brings down DP from 1263 to 378, demonstrating the serious degradation of cellulose during mechanical milling. Moreover, DP of all the samples declined slightly after dissolution in [C4mim]Cl at 110 oC and regeneration, indicating that

cellulose molecules were broken into relatively low molecular fragments.

**3.2. Dissolution and regeneration of cellulose assisted with ultrasonic**

To further study the influence of ultrasonic irradiation on cellulose dissolution in ILs, we tried to apply ultrasonic irradiation directly during the dissolution process as not a pretreatment but an auxilary method. Results showed that it was more effective to utilize ultrasonic in dissolution process than in pretreatment, as listed in Table 2. Cellulose could be dissolved in [C4mim]Cl within 1 h when irradiated by 30W ultrasonic for 20 min. With the same irradiation power and duration, it required 2.2 h to entirely dissolve sample when the ultrasonic was employed as pretreatment method. Enhancement of cellulose dissolution with ultrasonic was probably because ultrasound irradiation provided a greater penetration of ionic liquid into cellulose and improved mass transfer. When the dissolving system was irradiated with ultrasound, the extreme condition was probably obtained. It was said that transient temperature of at least 5000 K and pressure up to 1200 bar could be achieved, accompanied by vigorously physical agitation and shock (Kardos & Luche, 2001; Adewuyi, 2001). All of the abovementioned effects provided excellent conditions for cellulose dissolution in IL. This beneficial effect of ultrasound techniques has also been reported in isolation of cell wall polymers (Aliyu & Hepher, 2000). Cellulose dissolution time decreased from 147 (sample 24) to 120 (sample 20), 110 (sample 25) and 100 min (sample 26) with the improvement of ultrasonic power from 20 to 30, 40 and 50 W. However, further improvement of ultrasonic power (samples 26–28) did not result in the decrement in cellulose dissolution time. It seems that increase of power within small range was not as efficient as extension of duration. Compared with sample 3 obtained with one-step ultrasound irradiation, sample 11 with two-step irradiation with 5 min interval needed the same time to totally dissolve sample as well, indicating that the same ultrasound energy obtained from same power and total irradiation duration did not result in different effect on a Samples 20-28 were obtained with one-step ultrasound irradiation, while sample 29 was irradiated for 5 min, stirred for 5 min in interval, and then irradiated for another 5 min.

**Table 2.** Dissolution and regeneration of cellulose in [C4mim]Cl assisted with ultrasound irradiation

38 400 115 8.5 -

DP of native and regenerated cellulose is also listed in Table 2. After dissolution and regeneration, DP decreased from 1263 for native cellulose to 1103 for regenerated cellulose sample obtained without ultrasound irradiation. Ultrasound irradiation for 5, 10, 15, and 20 min led to a slight reduction of DP to 799, 800, 744 and 682, respectively. Therefore, direct utilization of ultrasonic in cellulose dissolution degraded cellulose molecular more significantly than using it as pretreatment method. However, the increase of ultrasound power from 20 to 40, 50, and 75 W resulted in an improvement of DP from 946 to 962, 1022, and 1019. The reason for this improvement is not clear and perhaps due to the reassociation and polymerisation of the radicals formed at higher ultrasound power.

#### **3.3. Dissolution and regeneration of cellulose assisted with microwave**

Microwave was also applied as a heating method instead of pretreatment to improve cellulose dissolution. Table 3 shows the results of cellulose dissolution and regeneration

**Table 3.** Dissolution and regeneration of cellulose in [C4mim]Cl assisted with microwave irradiation

under microwave heating with different power and temperature. Obviously, cellulose dissolved in [C4mim]Cl far more quickly with microwave heating than with traditional heating, like in oil bath. Cellulose could be dissolved in [C4mim]Cl within 50 min at 90 oC under 300 W microwave heating. Dissolution rate would further increase with the increment of microwave power and reaction temperature. Heating with 400W microwave at 115 oC, ethanol treated sample was completely dissolved in only 8.5 min. It should be noted that, however, the yield of regenerated cellulose were about 86~94%, lower than that heating in oil bath. The low yield suggested that degradation of cellulose was more severe under microwave heating than in oil bath.

#### **3.4. FT-IR spectra**

FI-IR characterization was employed to measure the changes of the structure of regenerated cellulose samples obtained after pretreatment and dissolution in [C4mim]Cl assisted with or without ultrasonic and microwave irradiation. Fig. 2 illustrates FT-IR spectra of native cellulose (spectrum 1) and regenerated cellulose sample 17 (spectrum 2, pretreated with 600 W microwave irradiation for 20 min at 25 oC). In these two spectra, the absorbances at 3416, 2916, 1638, 1417, 1323, 1161, 1046, and 895 cm-1 are associated with native cellulose (Oh et al., 2005; Liu et al., 2006). Bands in the two spectra were rather similar and no new band was displayed in spectrum of regenerated cellulose, indicating that the structure of both samples are similar and [C4mim]Cl is direct solvent for cellulose. However, there were still slightly differences in the bands in the spectra of samples dissolved with ultrasonic irradiation and microwave heating (Figures not showed). The absorption from O-H stretching moved to higher wavenumbers from 3407 cm-1 to 3423 cm-1 after regeneration, indicating the broken hydrogen bonds in cellulose. In addition, the peak for CH2 bending shifted to a lower frequency from 1431 cm-1 to 1419 cm-1, which was a hint for the splitting of hydrogen bonds in C6-OH (Zhang et al., 2005).

The value of N\_O'KI index, proposed by Nelson and O'connor, represents the relative changes of cellulose crystallinity. It can be calculated using the formula N\_O'KI =a1372/a2900 (Liu & Sun, 2010), in which a1372 and a2900 are the absorbance intensities of the bands at 1372 and 2900 cm-1 for O–H bending and C–H stretching, respectively, in FT-IR spectra of cellulose. In present study, N\_O'KI of regenerated cellulose was calculated from FT-IR by the ratio of two absorbances at 2900 cm-1 and 1372 cm-1, and the results are listed in Table 2. As seen in Table 2, N\_O'KI of regenerated cellulose from ultrasonic dissolution sample increased from 0.930 to 0.963 and 1.084 when the irradiation time increase from 0 to 5 and 10 min. However, it was reduced to 0.975, 0.966 as prolonging the irradiation duration to 15 and 20 min. The N\_O'KI of regenerated cellulose changed irregularly with the enhancement of ultrasonic power, and the reason need to be further studied. On the other hand, N\_O'KI of regenerated cellulose from microwave heating samples rose with the enhancement of heating power and increase of temperature. It was probably because that cellulose samples with lower molecular fragments were more likely to recrystallize during regeneration. In addition, N\_O'KI of the regenerated cellulose from mechanical milling samples dissolved in different temperature (110, 120, 130, and 140 oC) were also measured. Results demonstrated that the crystallinity increase with the enhancement of temperature, similar to that of microwave heating samples.

188 Cellulose – Fundamental Aspects

**3.4. FT-IR spectra**

microwave heating than in oil bath.

in C6-OH (Zhang et al., 2005).

under microwave heating with different power and temperature. Obviously, cellulose dissolved in [C4mim]Cl far more quickly with microwave heating than with traditional heating, like in oil bath. Cellulose could be dissolved in [C4mim]Cl within 50 min at 90 oC under 300 W microwave heating. Dissolution rate would further increase with the increment of microwave power and reaction temperature. Heating with 400W microwave at 115 oC, ethanol treated sample was completely dissolved in only 8.5 min. It should be noted that, however, the yield of regenerated cellulose were about 86~94%, lower than that heating in oil bath. The low yield suggested that degradation of cellulose was more severe under

FI-IR characterization was employed to measure the changes of the structure of regenerated cellulose samples obtained after pretreatment and dissolution in [C4mim]Cl assisted with or without ultrasonic and microwave irradiation. Fig. 2 illustrates FT-IR spectra of native cellulose (spectrum 1) and regenerated cellulose sample 17 (spectrum 2, pretreated with 600 W microwave irradiation for 20 min at 25 oC). In these two spectra, the absorbances at 3416, 2916, 1638, 1417, 1323, 1161, 1046, and 895 cm-1 are associated with native cellulose (Oh et al., 2005; Liu et al., 2006). Bands in the two spectra were rather similar and no new band was displayed in spectrum of regenerated cellulose, indicating that the structure of both samples are similar and [C4mim]Cl is direct solvent for cellulose. However, there were still slightly differences in the bands in the spectra of samples dissolved with ultrasonic irradiation and microwave heating (Figures not showed). The absorption from O-H stretching moved to higher wavenumbers from 3407 cm-1 to 3423 cm-1 after regeneration, indicating the broken hydrogen bonds in cellulose. In addition, the peak for CH2 bending shifted to a lower frequency from 1431 cm-1 to 1419 cm-1, which was a hint for the splitting of hydrogen bonds

The value of N\_O'KI index, proposed by Nelson and O'connor, represents the relative changes of cellulose crystallinity. It can be calculated using the formula N\_O'KI =a1372/a2900 (Liu & Sun, 2010), in which a1372 and a2900 are the absorbance intensities of the bands at 1372 and 2900 cm-1 for O–H bending and C–H stretching, respectively, in FT-IR spectra of cellulose. In present study, N\_O'KI of regenerated cellulose was calculated from FT-IR by the ratio of two absorbances at 2900 cm-1 and 1372 cm-1, and the results are listed in Table 2. As seen in Table 2, N\_O'KI of regenerated cellulose from ultrasonic dissolution sample increased from 0.930 to 0.963 and 1.084 when the irradiation time increase from 0 to 5 and 10 min. However, it was reduced to 0.975, 0.966 as prolonging the irradiation duration to 15 and 20 min. The N\_O'KI of regenerated cellulose changed irregularly with the enhancement of ultrasonic power, and the reason need to be further studied. On the other hand, N\_O'KI of regenerated cellulose from microwave heating samples rose with the enhancement of heating power and increase of temperature. It was probably because that cellulose samples with lower molecular fragments were more likely to recrystallize during regeneration. In addition, N\_O'KI of the regenerated cellulose from mechanical milling samples dissolved in different temperature (110, 120, 130, and 140 oC) were also measured. Results demonstrated

**Figure 2.** FT-IR spectra of native cellulose (spectrum 1) and regenerated cellulose sample 17 (spectrum 2).

**Table 4.** Relative changes of crystallinity of regenerated cellulose dissolved with ultrasonic and microwave assistance, calculated from FT-IR spectra.

**Table 5.** Relative changes of crystallinity of regenerated samples from dissolution of mechanical milling cellulose at different temperature.

**Figure 3.** Solid state CP/MAS 13C-NMR spectra of native cellulose (spectrum a) and regenerated cellulose sample 23 (spectrum b) and sample 38 (spectrum c).

#### **3.5. Solid-state CP/MAS 13C NMR spectra**

CP/MAS 13C NMR spectroscopy is one of the most effective methods to characterize lignocellulosic materials directly and is able to provide detailed information on solid samples. Fig. 3 shows the CP/MAS 13C-NMR spectra of native cellulose (spectrum a) and regenerated cellulose sample 23 (spectrum b) and sample 38 (spectrum c). The region between 50 and 105 ppm is characterized for the carbons of carbohydrate. The signals from cellulose at 105.7 (C-1), 89.6 and 84.7 (C-4 of crystal cellulose and amorphous cellulose, respectively), 75.8 and 73.5 (C-2, C-3 and C-5), 65.8 and 63.8 (C-6 of crystal cellulose and amorphous cellulose, respectively) are all observed (Focher et al., 2001; Maunu, 2002; Bardet et al., 2002; Liu et al., 2007a; Liu et al., 2009). However, the spectra also exhibited differences at C-4 and C-6 signals. In spectrum b (sample 23), signals for C-4 and C-6 of crystal cellulose disappeared, indicating that the regenerated sample from ultrasonic assisted dissolution cellulose contained mainly amorphous structure. Nonetheless, signal specific of C-4 of amorphous cellulose was degenerated and that of crystal cellulose was reappeared in spectrum c (sample 38). The two signals contributing to C-6 of crystal and amorphous cellulose were merged into one. This spectrum hinted that cellulose macromolecular formed crystalline structure in the regeneration process after dissolve in [C4mim]Cl with relative high power of microwave heating.

**Figure 4.** Wide-angle X-ray diffraction curves of native (spectrum 1) and regenerated cellulose sample 20 (spectrum 2), 31 (spectrum 3), and 34 (spectrum4).

#### **3.6. Wide-angle X-ray diffraction**

190 Cellulose – Fundamental Aspects

cellulose at different temperature.

Temperture/oC a2900 (C-H) a1372 (O-H) N\_O'KI 110 0.263 0.243 1.082 120 0.362 0.327 1.107 130 0.363 0.313 1.160 140 0.468 0.396 1.182

**Table 5.** Relative changes of crystallinity of regenerated samples from dissolution of mechanical milling

**Figure 3.** Solid state CP/MAS 13C-NMR spectra of native cellulose (spectrum a) and regenerated

CP/MAS 13C NMR spectroscopy is one of the most effective methods to characterize lignocellulosic materials directly and is able to provide detailed information on solid samples. Fig. 3 shows the CP/MAS 13C-NMR spectra of native cellulose (spectrum a) and regenerated cellulose sample 23 (spectrum b) and sample 38 (spectrum c). The region between 50 and 105 ppm is characterized for the carbons of carbohydrate. The signals from cellulose at 105.7 (C-1), 89.6 and 84.7 (C-4 of crystal cellulose and amorphous cellulose, respectively), 75.8 and 73.5 (C-2, C-3 and C-5), 65.8 and 63.8 (C-6 of crystal cellulose and amorphous cellulose, respectively) are all observed (Focher et al., 2001; Maunu, 2002; Bardet et al., 2002; Liu et al., 2007a; Liu et al., 2009). However, the spectra also exhibited differences at C-4 and C-6 signals. In spectrum b (sample 23), signals for C-4 and C-6 of crystal cellulose disappeared, indicating that the regenerated sample from ultrasonic assisted dissolution

cellulose sample 23 (spectrum b) and sample 38 (spectrum c).

**3.5. Solid-state CP/MAS 13C NMR spectra**

Wide-angle X-ray diffraction is the one of the most valuable technique to measure the crystalline structure of material. In this article wide-angle X-ray diffraction was used to characterize the influence of different pretreatment methods, ultrasonic irradiation and microwave heating on crystalline structure of regenerated cellulose. Fig. 4 exhibits the wideangle X-ray diffractogram of native (spectrum 1) and regenerated cellulose sample 20 (spectrum 2), 31 (spectrum 3), and 34 (spectrum4). Spectrum 1 gave strong crystalline peak at 22.3° for (002) crystal plane and at 16.1° merged from peaks of (110) and (11�0) crystal