Synthesis of Quaternary Ammonium Room-Temperature Ionic Liquids and their Application in the Dissolution of Cellulose

: In this work, several kinds of quaternary ammonium-based room-temperature ionic liquids (QA RTILs) are synthesized by alkylation and ion-exchange reactions for the rapid dissolution of cellulose. The applications of cellulose materials have been limited due to their poor solubility in conventional organic solvents, because of a high degree of structural regularity and a large number of hydrogen bonds. The prepared ionic liquids were identiﬁed by nuclear magnetic resonance, elemental analysis, and liquid chromatography-mass spectrometry. The results indicated that N,N,N -triethylhexan-1-aminium acetate (N 6222 OAc), tetrahexylammonium acetate (N 6666 OAc), and N,N,N,N (cid:48) ,N (cid:48) ,N (cid:48) -hexaethyldecane-1,10-diaminium acetate (C 10 (N 222 OAc) 2 ) exhibited good cellulose-dissolution without any pretreatment. The regenerated cellulose ﬁlms with a low degree of crystallization of the cellulose II phase were also prepared easily in this process using N 6222 OAc due to its polar and small cation. These QA RTILs can be used as non-derivatizing solvents for cellulose and can also be easily recycled because of their thermostable and nonvolatile properties.


Introduction
Cellulose is the most abundant regenerative material in the world, and it has been widely used in many fields such as in medicine, food, packaging, and industrial materials due to its good physical properties [1]. It is a linear polysaccharide composed of d-glucose monomers linked by β-(1,4) glycosidic bonds. Its structure bears a high degree of regularity and exhibits a large number of hydrogen bonds; this renders cellulose insoluble in water and other common organic solvents, resulting in its limited application. Most of the methods available for cellulose processing are nongreen [2]; this necessitated the development of green solutions in order to dissolve cellulose. Recently, N-methylmorpholine oxide (NMMO) has been widely studied as the solvent used in the Lyocell process for glues, to regenerate cellulose fibers [3]. In comparison to the traditional method (Viscose process) [4], NMMO is a more environmentally friendly, non-derivatizing solvent. However, it still has some disadvantages, such as the requirement of high temperature to dissolve the cellulose, degradation of cellulose, side reactions of the solvent itself, which are caused without any antioxidant, and large investments in safety [5].
In contrast, room-temperature ionic liquids (RTILs) are excellent recyclable solvents, owing to their many advantages such as very low vapor pressure, flame retardancy, high thermal stability, and recyclability. Furthermore, RTILs can be easily modified by changing the structure of the cations and anions, which has broadened their range of application. Thus, imidazolium-based ionic liquids, such as 1-butyl-3-methylimidazolium chloride and 1-allyl-3-methylimidazolium chloride (AMIMCl) [1,6,7] have been widely studied to dissolve cellulose. However, the application of imidazolium-based ILs is still limited due to the complicated preparation process and expensive raw materials [8][9][10][11][12][13][14][15]. Herein, we present a facile and cost-effective method for the preparation of quaternary ammonium-based room-temperature ionic liquids (QA RTILs) using quaternary ammonium and acetate groups as the cation and anion, respectively. The prepared QA RTILs were identified by nuclear magnetic resonance (NMR), elemental analysis (EA), and liquid chromatography-mass spectrometry (LC/MS). The physical properties of QA RTILs, such as the melting point, the thermal decomposition temperature, and the viscosity, were also measured. The prepared QA RTILs exhibited excellent cellulose-dissolution ability and thus, they could be applied to prepare the cellulose films. The QA RTILs developed in this work are cheaper and less toxic than imidazolium-based ILs and are halogen-free. The structure and properties of the regenerated cellulose films were investigated in detail to compare the dissolution abilities of different solvents. In addition, the recovery of ILs was also examined to show the practicality of QA RTILs.

Synthesis of Ionic Liquids
Measured quantities of 1,10-dibromodecane (0.3 mol, limiting reactant) and triethylamine (1 mol, excess agent) were dissolved in 1 L of acetonitrile with vigorous stirring at 75 • C. After 5 h of reaction, the unreacted triethylamine and organic solvent were removed by vacuum evaporation (EYELA N-1100). N,N,N,N ,N ,N -hexaethyldecane-1,10-diaminium bromide (C 10 (N 222 Br) 2 , milky white powder) with over 98% purity was obtained after drying under vacuum at 80 • C for 3 h. Thereafter, equal moles of C 10 (N 222 Br) 2 (0.1 mol) and acetate acid (0.1 mol) were dissolved in 500 mL of water with vigorous stirring at room temperature. The viscous yellow ionic liquid (N,N,N,N ,N ,N -hexaethyldecane-1,10-diaminium acetate, C 10 (N 6222 OAc) 2 , coded as QA) with over 96% purity was obtained after the removal of hydrogen bromide and water by vacuum evaporation. The other two QA RTILs, N,N,N-triethylhexan-1-aminium acetate (N 6222 OAc, coded as NA) and tetrahexylammonium acetate (N 6666 OAc, coded as TA), were prepared by the same procedure with the exception that N,N,N-triethylhexan-1-aminium chloride and tetrahexylammonium bromide were used as their cation precursors. The analyzed data for QA are as follows: 1

Cellulose Dissolution and Preparation of the Regenerated Cellulose Film
Measured quantities of microcrystalline cellulose (MCC) were added in 20 mL of QA RTILs at 50 • C with stirring. The dissolution process of MCC in the QA RTILs and other solutions was observed by polarized light microscopy. After the dissolution reaction for one h, the transparent cellulose solution with 2-12 wt % polymer concentration was coated on a glass plate using a spin coater. The coated solution was washed with running cold water to form a coagulated cellulose gel. The regenerated cellulose film was obtained after drying at 80 • C in a vacuum oven. The recovery of ILs was accomplished by evaporating the water from the precipitated liquid. The nomenclature of the regenerated cellulose film was as follows: C"X"-"Y", where X is the reaction solvent and Y is the cellulose content (wt %) in solvents. Reaction solvents were coded as W (deionized water), A (10 wt % acetic acid solution), and three ILs: NA, TA, and QA.

Analysis Instruments
NMR (Bruker Avance 500, Billerica, MA, USA), EA (Heraeus varioIII-NCH, Hanau, Germany), and LC/MS (Waters Micromass Q-Tof, Milford, MA, USA) were used to identify the chemical structure of the ionic liquids. The dissolution process of cellulose was observed by polarized light microscopy (PLM, Microtech DC-218w, HK, China). X-ray diffractometry (XRD, Bruker D2 Phaser, Billerica, MA, USA) and Fourier transform infrared (FTIR) spectroscopy (Bio-Rad Digilab FTS-3500, Hercules, CA, USA) were applied to analyze the degree of crystallinity and molecular structure of regenerated cellulose samples, respectively. The degree of polymerization (DP) and thermal stability of the cellulose materials were identified using an Ostwald viscometer in cupriethylenediamine hydroxide (CUEN) solution (DIN 54270, Deutsches Institut fur Normung E.V., Germany) and a thermogravimetric analyzer (TGA, NETZSCH TG 209 F1 Iris, Burlington, MA, USA), respectively. Scanning electron microscopy (SEM, JEOL JSM6500F, Tokyo, Japan) was used to observe the surface and the cross-sectional morphologies of the regenerated cellulose films.

The Physical Properties of the Prepared Ionic Liquids
The successful synthesis of three ionic liquids was thus established by the similarity between the estimated values and the obtained 1 H NMR, EA, and LC/MS data. As shown in Table 1, the viscosity of the ILs depends mainly on the intermolecular forces, so the viscosity increases with the increase in the carbon number of the cationic alkyl chain due to van der Waals forces. The reason for the lowest room-temperature viscosity of NA is its asymmetric structure, small molecular weight, and short side chain, which reduces the ion-ion forces and increases the ionic mobility. The experimental results showed that the viscosity of the ILs decreases with the increase in temperature due to the increase in the kinetic energy of the IL molecules. The viscosity of the ILs flow activation energy (E flow ) was calculated using the Arrhenius equation. As shown in Table 1, NA has the lowest E flow (8.1 kcal/mol). The E flow of all QA RTILs are lower than that of glycerol (12.3 kcal/mol) and imidazolium-based ILs (e.g., E flow of AlMeImCl is 14.4 kcal/mol), owing to the electrostatic forces found in ionic liquids [16]. This shows the convenience of operation using QA RTILs. Furthermore, the thermal decomposition temperature of the prepared QA RTILs was determined by TGA. The results indicated that the cation structure and alkyl chain length do not have a significant effect on the thermal stability of QA RTILs. The thermal stability of ionic liquids mainly depends on the substituents' electrical negativity and anionic nucleophilicity [17]. The thermal stability of QA RTILs can be improved by using fluoride-based anions, such as bis(trifluoromethylsulfonyl) amide and hexafluorophosphate (PF 6 ), but the cellulose dissolution ability of QA RTILs with fluoride-based anions is not good. For the purpose of comparison, we also prepared tetrahexylammonium hexafluorophosphate ILs (N 6666 PF 6 ). The thermal stability of N 6666 PF 6 (T d = 339 • C) is much better than that of TA (N 6666 OAc), but its melting temperature (134 • C) is also much higher than that of TA, thus resulting in its limited application. In our work, cellulose can be easily dissolved in these QA RTILs at mild temperatures; thus, high thermal stability is not very necessary for cellulose dissolution using these QA RTILs.

Cellulose Dissolution
The process of 5 wt % cellulose dissolution in the NA ILs at 50 °C was observed using the polarizing microscope, as shown in Figure 1. The dissolution phenomena corresponding to QA and TA ILs were similar to those corresponding to NA, and the cellulose dissolution rate of these QA RTILs were in the order of NA > TA > QA. It was also found that the three QA RTILs could break cellulose intramolecular and intermolecular hydrogen bonds in a short time. The acetate anions in the QA RTILs exhibited strong hydrogen bonding capacity to destroy the cellulose structure. The conjunction of the spatial heterogeneity in QA RTILs with an amphiphilic cation triggered the aggregation of alkyl tails. The hydrophobic tail aggregation implies a trend to make other parts more hydrophilic and polar. Hence, an appropriate increase in alkoxy chains leads QA RTILs to be more powerful cellulose solvents, through increasing the spatial heterogeneity and the activity of acetate anions. [18,19] On the same molar basis, although QA has the longest alkyl chain and twice the acetate anion content of TA and NA, its dissolution ability for cellulose is the weakest among these QA RTILs. The amphiphilic properties of QA RTILs are decreased with the increase in alkyl number, resulting in an increase of the viscosity. NA with the polar cation and lower viscosity exhibits the best cellulose dissolution ability. The ILs with lower viscosity exhibit better ionic mobility, which plays an important role in the cellulose dissolution reaction. Figure 1 indicates that the cellulose main structure was destroyed in NA within 5 min, and many fine light spots were produced. The image became dark after 20 min of reaction due to the complete dissolution of cellulose. In the initial 5 min, the dissolution occurred very rapidly. Then, the dissolution rate decreased due to the crystalline structure in the residual cellulose fibrils (fine light spots labeled in Figure 1c) and the increase in ILs viscosity during the cellulose dissolution. The results show the good cellulose dissolution ability of NA, and the swelling of cellulose in NA was not observed, unlike in the case of the NMMO and AMIMCl systems. Cellulose dissolved well in the AMIMCl system at 60 °C, but cellulose swelling in AMIMCl was observed only at room temperature [7]. Moreover, we used water, dilute acetic acid, and dilute nitric acid in the dissolution test at 80 °C for 2 h, and only a two-phase liquid was formed due to the swelling of cellulose, and some cellulose was precipitated. These solutions are not easy to use as coating solutions for the preparation of regenerated cellulose films. Therefore, these solvents are not suitable as regenerating fiber film reagents.

Cellulose Dissolution
The process of 5 wt % cellulose dissolution in the NA ILs at 50 °C was observed using the polarizing microscope, as shown in Figure 1. The dissolution phenomena corresponding to QA and TA ILs were similar to those corresponding to NA, and the cellulose dissolution rate of these QA RTILs were in the order of NA > TA > QA. It was also found that the three QA RTILs could break cellulose intramolecular and intermolecular hydrogen bonds in a short time. The acetate anions in the QA RTILs exhibited strong hydrogen bonding capacity to destroy the cellulose structure. The conjunction of the spatial heterogeneity in QA RTILs with an amphiphilic cation triggered the aggregation of alkyl tails. The hydrophobic tail aggregation implies a trend to make other parts more hydrophilic and polar. Hence, an appropriate increase in alkoxy chains leads QA RTILs to be more powerful cellulose solvents, through increasing the spatial heterogeneity and the activity of acetate anions. [18,19] On the same molar basis, although QA has the longest alkyl chain and twice the acetate anion content of TA and NA, its dissolution ability for cellulose is the weakest among these QA RTILs. The amphiphilic properties of QA RTILs are decreased with the increase in alkyl number, resulting in an increase of the viscosity. NA with the polar cation and lower viscosity exhibits the best cellulose dissolution ability. The ILs with lower viscosity exhibit better ionic mobility, which plays an important role in the cellulose dissolution reaction. Figure 1 indicates that the cellulose main structure was destroyed in NA within 5 min, and many fine light spots were produced. The image became dark after 20 min of reaction due to the complete dissolution of cellulose. In the initial 5 min, the dissolution occurred very rapidly. Then, the dissolution rate decreased due to the crystalline structure in the residual cellulose fibrils (fine light spots labeled in Figure 1c) and the increase in ILs viscosity during the cellulose dissolution. The results show the good cellulose dissolution ability of NA, and the swelling of cellulose in NA was not observed, unlike in the case of the NMMO and AMIMCl systems. Cellulose dissolved well in the AMIMCl system at 60 °C, but cellulose swelling in AMIMCl was observed only at room temperature [7]. Moreover, we used water, dilute acetic acid, and dilute nitric acid in the dissolution test at 80 °C for 2 h, and only a two-phase liquid was formed due to the swelling of cellulose, and some cellulose was precipitated. These solutions are not easy to use as coating solutions for the preparation of regenerated cellulose films. Therefore, these solvents are not suitable as regenerating fiber film reagents.

Cellulose Dissolution
The process of 5 wt % cellulose dissolution in the NA ILs at 50 °C was observed using the polarizing microscope, as shown in Figure 1. The dissolution phenomena corresponding to QA and TA ILs were similar to those corresponding to NA, and the cellulose dissolution rate of these QA RTILs were in the order of NA > TA > QA. It was also found that the three QA RTILs could break cellulose intramolecular and intermolecular hydrogen bonds in a short time. The acetate anions in the QA RTILs exhibited strong hydrogen bonding capacity to destroy the cellulose structure. The conjunction of the spatial heterogeneity in QA RTILs with an amphiphilic cation triggered the aggregation of alkyl tails. The hydrophobic tail aggregation implies a trend to make other parts more hydrophilic and polar. Hence, an appropriate increase in alkoxy chains leads QA RTILs to be more powerful cellulose solvents, through increasing the spatial heterogeneity and the activity of acetate anions. [18,19] On the same molar basis, although QA has the longest alkyl chain and twice the acetate anion content of TA and NA, its dissolution ability for cellulose is the weakest among these QA RTILs. The amphiphilic properties of QA RTILs are decreased with the increase in alkyl number, resulting in an increase of the viscosity. NA with the polar cation and lower viscosity exhibits the best cellulose dissolution ability. The ILs with lower viscosity exhibit better ionic mobility, which plays an important role in the cellulose dissolution reaction. Figure 1 indicates that the cellulose main structure was destroyed in NA within 5 min, and many fine light spots were produced. The image became dark after 20 min of reaction due to the complete dissolution of cellulose. In the initial 5 min, the dissolution occurred very rapidly. Then, the dissolution rate decreased due to the crystalline structure in the residual cellulose fibrils (fine light spots labeled in Figure 1c) and the increase in ILs viscosity during the cellulose dissolution. The results show the good cellulose dissolution ability of NA, and the swelling of cellulose in NA was not observed, unlike in the case of the NMMO and AMIMCl systems. Cellulose dissolved well in the AMIMCl system at 60 °C, but cellulose swelling in AMIMCl was observed only at room temperature [7]. Moreover, we used water, dilute acetic acid, and dilute nitric acid in the dissolution test at 80 °C for 2 h, and only a two-phase liquid was formed due to the swelling of cellulose, and some cellulose was precipitated. These solutions are not easy to use as coating solutions for the preparation of regenerated cellulose films. Therefore, these solvents are not suitable as regenerating fiber film reagents.

Cellulose Dissolution
The process of 5 wt % cellulose dissolution in the NA ILs at 50 • C was observed using the polarizing microscope, as shown in Figure 1. The dissolution phenomena corresponding to QA and TA ILs were similar to those corresponding to NA, and the cellulose dissolution rate of these QA RTILs were in the order of NA > TA > QA. It was also found that the three QA RTILs could break cellulose intramolecular and intermolecular hydrogen bonds in a short time. The acetate anions in the QA RTILs exhibited strong hydrogen bonding capacity to destroy the cellulose structure. The conjunction of the spatial heterogeneity in QA RTILs with an amphiphilic cation triggered the aggregation of alkyl tails. The hydrophobic tail aggregation implies a trend to make other parts more hydrophilic and polar. Hence, an appropriate increase in alkoxy chains leads QA RTILs to be more powerful cellulose solvents, through increasing the spatial heterogeneity and the activity of acetate anions. [18,19] On the same molar basis, although QA has the longest alkyl chain and twice the acetate anion content of TA and NA, its dissolution ability for cellulose is the weakest among these QA RTILs. The amphiphilic properties of QA RTILs are decreased with the increase in alkyl number, resulting in an increase of the viscosity. NA with the polar cation and lower viscosity exhibits the best cellulose dissolution ability. The ILs with lower viscosity exhibit better ionic mobility, which plays an important role in the cellulose dissolution reaction. Figure 1 indicates that the cellulose main structure was destroyed in NA within 5 min, and many fine light spots were produced. The image became dark after 20 min of reaction due to the complete dissolution of cellulose. In the initial 5 min, the dissolution occurred very rapidly. Then, the dissolution rate decreased due to the crystalline structure in the residual cellulose fibrils (fine light spots labeled in Figure 1c) and the increase in ILs viscosity during the cellulose dissolution. The results show the good cellulose dissolution ability of NA, and the swelling of cellulose in NA was not observed, unlike in the case of the NMMO and AMIMCl systems. Cellulose dissolved well in the AMIMCl system at 60 • C, but cellulose swelling in AMIMCl was observed only at room temperature [7]. Moreover, we used water, dilute acetic acid, and dilute nitric acid in the dissolution test at 80 • C for 2 h, and only a two-phase liquid was formed due to the swelling of cellulose, and some cellulose was precipitated. These solutions are not easy to use as coating solutions for the preparation of regenerated cellulose films. Therefore, these solvents are not suitable as regenerating fiber film reagents. Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 12

The Crystallinity of the Regenerated Cellulose Film
The cellulose structure contains both crystalline and non-crystalline regions. If the crystallinity of cellulose is increased, the hygroscopicity and the degree of swelling will be reduced, but the mechanical strength will be enhanced. It will be difficult for the reactive agent to enter the cellulose molecule because of the latter's stronger hydrogen bond. Therefore, the crystallinity of cellulose plays a significant role in its applications. After dissolving cellulose in QA RTILs, the solution was cast onto a glass plate and then coagulated in water. After removing and drying of the QA RTILs, a transparent cellulose film was obtained. The change in the crystallinity of cellulose was identified by X-ray diffractometry and Fourier transform infrared (FTIR) spectroscopy.

X-Ray Diffraction Spectrum
The X-ray diffraction patterns of the cellulose films are shown in Figure 2. The diffraction peaks at 2θ = 22.6°, 16.4°, and 14.9° indicate that the crystal phase of the original cellulose (MCC) was cellulose type I. The crystal structure of cellulose was not significantly changed using water and acetic acid as the dissolution solvent, but cellulose type I was not observed in the CTA (cellulose dissolved by TA), CQA (cellulose dissolved by QA), and CNA (cellulose dissolved by NA) samples. The regenerated cellulose films, CTA, CQA, and CNA, exhibited the typical diffraction patterns of cellulose type II at 2θ = 21.2° and 20.3°. The results indicate the phase transformation from cellulose I to cellulose II after the dissolution and regeneration in QA RTILs. This phenomenon was also reported in another solvent system for cellulose. The crystallinity index (CrI) defined by Segal is applied to assess the degree of crystallization of the cellulose, as follows [20,21]: Where I200 is the peak intensity of the major crystallization peak (002) in the diffraction patterns, containing both the diffraction intensity of the crystalline and the amorphous regions and IAM is the intensity of the amorphous region. The diffraction peaks of I200 and IAM for cellulose type I are at 22.6° and 18°, respectively. The peaks at 2θ = 20.3° and 16.2° represent the crystalline and amorphous regions of the cellulose type II, respectively. According to the above equation, the degree of crystallization of the regenerated cellulose films was estimated, as shown in Table 2. The hydrothermal or acid treatments only slightly reduced the crystallinity of cellulose without changing its crystal phase. This result indicates that QA RTILs can rapidly break intermolecular and intramolecular hydrogen bonds and destroy the original crystalline structure. The crystallinity index of cellulose II is in the order of CNA-5 < CTA-5 < CQA-5, which is consistent with the viscosity of ILs. The kinetic energy of the dissolved cellulose molecules is lower in viscous ILs, so the recrystallization rate for cellulose II in QA is much higher than that in NA. The diffraction pattern of the CNA-5 film

The Crystallinity of the Regenerated Cellulose Film
The cellulose structure contains both crystalline and non-crystalline regions. If the crystallinity of cellulose is increased, the hygroscopicity and the degree of swelling will be reduced, but the mechanical strength will be enhanced. It will be difficult for the reactive agent to enter the cellulose molecule because of the latter's stronger hydrogen bond. Therefore, the crystallinity of cellulose plays a significant role in its applications. After dissolving cellulose in QA RTILs, the solution was cast onto a glass plate and then coagulated in water. After removing and drying of the QA RTILs, a transparent cellulose film was obtained. The change in the crystallinity of cellulose was identified by X-ray diffractometry and Fourier transform infrared (FTIR) spectroscopy.

X-Ray Diffraction Spectrum
The X-ray diffraction patterns of the cellulose films are shown in Figure 2. The diffraction peaks at 2θ = 22.6 • , 16.4 • , and 14.9 • indicate that the crystal phase of the original cellulose (MCC) was cellulose type I. The crystal structure of cellulose was not significantly changed using water and acetic acid as the dissolution solvent, but cellulose type I was not observed in the CTA (cellulose dissolved by TA), CQA (cellulose dissolved by QA), and CNA (cellulose dissolved by NA) samples. The regenerated cellulose films, CTA, CQA, and CNA, exhibited the typical diffraction patterns of cellulose type II at 2θ = 21.2 • and 20.3 • . The results indicate the phase transformation from cellulose I to cellulose II after the dissolution and regeneration in QA RTILs. This phenomenon was also reported in another solvent system for cellulose. The crystallinity index (Cr I ) defined by Segal is applied to assess the degree of crystallization of the cellulose, as follows [20,21]: where I 200 is the peak intensity of the major crystallization peak (002) in the diffraction patterns, containing both the diffraction intensity of the crystalline and the amorphous regions and I AM is the intensity of the amorphous region. The diffraction peaks of I 200 and I AM for cellulose type I are at 22.6 • and 18 • , respectively. The peaks at 2θ = 20.3 • and 16.2 • represent the crystalline and amorphous regions of the cellulose type II, respectively. According to the above equation, the degree of crystallization of the regenerated cellulose films was estimated, as shown in Table 2. The hydrothermal or acid treatments only slightly reduced the crystallinity of cellulose without changing its crystal phase. This result indicates that QA RTILs can rapidly break intermolecular and intramolecular hydrogen bonds and destroy the original crystalline structure. The crystallinity index of cellulose II is in the order of CNA-5 < CTA-5 < CQA-5, which is consistent with the viscosity of ILs. The kinetic energy of the dissolved cellulose molecules is lower in viscous ILs, so the recrystallization rate for cellulose II in QA is much higher than that in NA. The diffraction pattern of the CNA-5 film showed an amorphous structure. The effect of the cellulose content in NA on crystallinity was also investigated in this work. The type II crystallinity index increases with an increase in cellulose content. In the range of 2 to 10 wt % cellulose content, the degree of crystallinity for cellulose II increased slightly with the increase in cellulose content in ILs. As the cellulose content reached 12 wt %, the crystallinity index rapidly increased to 49%. The NA solution with 12 wt % cellulose became very viscous, and it reached a saturated or supersaturated state, leading to a rapid recrystallization reaction rate and hence could not be used as a proper coating solution. Therefore, the cellulose content in NA should be kept under 10 wt %. Moreover, 10 wt % cellulose can also be completely dissolved in NA at room temperature after two h of dissolution, but this regenerated cellulose film exhibits both cellulose I and II crystal structures (Cr I = 22%, Cr II = 15%). This indicates that moderate heating enhances the phase transformation of cellulose from type I to type II in the dissolution process.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 12 showed an amorphous structure. The effect of the cellulose content in NA on crystallinity was also investigated in this work. The type II crystallinity index increases with an increase in cellulose content. In the range of 2 to 10 wt % cellulose content, the degree of crystallinity for cellulose II increased slightly with the increase in cellulose content in ILs. As the cellulose content reached 12 wt %, the crystallinity index rapidly increased to 49%. The NA solution with 12 wt % cellulose became very viscous, and it reached a saturated or supersaturated state, leading to a rapid recrystallization reaction rate and hence could not be used as a proper coating solution. Therefore, the cellulose content in NA should be kept under 10 wt %. Moreover, 10 wt % cellulose can also be completely dissolved in NA at room temperature after two h of dissolution, but this regenerated cellulose film exhibits both cellulose I and II crystal structures (CrI = 22%, CrII = 15%). This indicates that moderate heating enhances the phase transformation of cellulose from type I to type II in the dissolution process.

Infrared Spectroscopy
The crystallinity and change in the molecular structure of cellulose were analyzed by infrared spectroscopy in the range from 800 to 1500 cm −1 [22,23]. As shown in Figure 3, the functional groups of the regenerated cellulose film (CNA-12) are very similar to those of the original cellulose. The major characteristic peaks of cellulose are located in the range between 1465 and 1565 cm −1 . There was no new functional group appearing in this range, indicating that the dissolution and regeneration of cellulose in QA RTILs are physical reactions. However, a significant and sharp peak representing CNA-12 was found at 898 cm −1 ; this peak is derived from the β-(1,4) glycosidic bond that corresponds

Infrared Spectroscopy
The crystallinity and change in the molecular structure of cellulose were analyzed by infrared spectroscopy in the range from 800 to 1500 cm −1 [22,23]. As shown in Figure 3, the functional groups of the regenerated cellulose film (CNA-12) are very similar to those of the original cellulose. The major characteristic peaks of cellulose are located in the range between 1465 and 1565 cm −1 . There was no new functional group appearing in this range, indicating that the dissolution and regeneration of cellulose in QA RTILs are physical reactions. However, a significant and sharp peak representing CNA-12 was found at 898 cm −1 ; this peak is derived from the β-(1,4) glycosidic bond that corresponds to the C-H deformation with ring vibration contribution and O-H bending. This peak indicates that the cellulose was converted into cellulose II [22]. As compared to the original cellulose, the CH 2 bending vibration bonding of CNA-12 shifted from 1430 to 1424 cm −1 with lower peak intensity. This reduction in CH 2 bending vibration intensity is due to the weakening of the C 3 and C 6 hydrogen bonds during the cellulose transformation from type I to type II. Furthermore, a peak at 1112 cm −1 in the original cellulose curve disappeared in the CNA-12 curve. This was probably due to the difference of C-O stretching vibration caused by the cellulose ring skeleton and the weak hydrogen bond of cellulose II [23]. The IR (Infrared radiation) absorption peaks at 1432 and 898 cm −1 are very different in the original cellulose and regenerated sample, so the intensity ratio of A 1432 /A 898 can be used as the crystallinity index (or named Lateral Order Index (LOI)). Nelson further claimed another index to represent the degree of crystallization of the regenerated cellulose: Total Crystallinity Index (TCI, A 1372 /A 2904 ) [22]. Table 3 depicts that the LOI value of CNA-10 decreased from 2.525 to 0.534, and its TCI value also dropped from 0.118 to 0.022. As can be seen, the lower the cellulose content in QA RTILs, the smaller the values of LOI and TCI are. The results of the FTIR were consistent with their XRD patterns. As the cellulose structure is destroyed in the dissolution process, some defective areas will be produced to enhance the surface reactivity of cellulose in the applications, such as in the preparation of the polymer, paint, fiber, and membranes. In addition, the difference in the peak intensity at 1635-1645 cm −1 , indicating the O-H stretching vibration of the adsorbed water, was also observed. The absorption intensity of CNA-12 was significantly higher than that of the original cellulose. The reason for this phenomenon is that the amorphous structure of cellulose increased after dissolution in ILs, and then, the number of free hydroxyl bonds increased. Water molecules can easily penetrate the amorphous region to form temporary hydrogen bonds with free hydroxyls, thus enhancing its hygroscopicity.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 12 to the C-H deformation with ring vibration contribution and O-H bending. This peak indicates that the cellulose was converted into cellulose II [22]. As compared to the original cellulose, the CH2 bending vibration bonding of CNA-12 shifted from 1430 to 1424 cm −1 with lower peak intensity. This reduction in CH2 bending vibration intensity is due to the weakening of the C3 and C6 hydrogen bonds during the cellulose transformation from type I to type II. Furthermore, a peak at 1112 cm −1 in the original cellulose curve disappeared in the CNA-12 curve. This was probably due to the difference of C-O stretching vibration caused by the cellulose ring skeleton and the weak hydrogen bond of cellulose II [23]. The IR (Infrared radiation) absorption peaks at 1432 and 898 cm −1 are very different in the original cellulose and regenerated sample, so the intensity ratio of A1432/A898 can be used as the crystallinity index (or named Lateral Order Index (LOI)). Nelson further claimed another index to represent the degree of crystallization of the regenerated cellulose: Total Crystallinity Index (TCI, A1372/A2904) [22]. Table 3 depicts that the LOI value of CNA-10 decreased from 2.525 to 0.534, and its TCI value also dropped from 0.118 to 0.022. As can be seen, the lower the cellulose content in QA RTILs, the smaller the values of LOI and TCI are. The results of the FTIR were consistent with their XRD patterns. As the cellulose structure is destroyed in the dissolution process, some defective areas will be produced to enhance the surface reactivity of cellulose in the applications, such as in the preparation of the polymer, paint, fiber, and membranes. In addition, the difference in the peak intensity at 1635-1645 cm −1 , indicating the O-H stretching vibration of the adsorbed water, was also observed. The absorption intensity of CNA-12 was significantly higher than that of the original cellulose. The reason for this phenomenon is that the amorphous structure of cellulose increased after dissolution in ILs, and then, the number of free hydroxyl bonds increased. Water molecules can easily penetrate the amorphous region to form temporary hydrogen bonds with free hydroxyls, thus enhancing its hygroscopicity.

Degree of Polymerization of the Regenerated Cellulose Film
The degree of polymerization (DP) of cellulose has a crucial effect on its mechanical properties. The dissolution time, temperature, cellulose concentration in the solvent, and type of solvent have obvious effects on the DP of the regenerated cellulose materials. The effects of solvents and cellulose concentration on the DP are shown in Table 3. Three interesting phenomena were observed: First, the DP of cellulose could be reduced by its reaction with acetic acid. However, the cellulose dissolution ability of acetic acid was not sufficient for preparing a smooth film. Second, in the CNA-X samples, the DP and TCI both increased with an increase in the cellulose content. The more viscous NA ILs could retard the degradation of regenerated cellulose and increase the recrystallization rate for type II cellulose. Thus, the change in DP is the same as the changing trend of crystallization. For example, the DP of CNA-12 is kept at 87% DP of the original cellulose even if the crystal phase of CNA-12 is completely transformed from cellulose I to cellulose II. This shows that the regenerated cellulose prepared from the concentrated cellulose ILs exhibits good mechanical properties. Third, in comparison with CNA-5, CTA-5, and CQA-5, CQA-5 has the highest degree of crystallinity, while its DP is least among the three ILs. However, the TCI values of CQA-5 and CNA-12 are similar, but the DP of CQA-5 is much less than that of CNA-12. The reason for this inconsistency could be the different cation structures of ILs. QA has a very long carbon chain ((CH 2 ) 10 ), and both ends of this chain are bonded with tertiary ethyl ammonium acetate (N(C 2 H 5 ) 3 AC). The recrystallization rate of CQA-5 is relatively rapid because QA exhibits several acetic anions and has a high viscosity. Further, as an IL molecule penetrates into the cellulose polymers, the retention time of the ILs in the cellulose polymers is proportional to the IL viscosity. Thus, QA can destroy more glycosidic or hydrogen bonds, resulting in the severe degradation of the cellulose polymer and the small DP of the CQA sample. In addition, the CNA-5 solution exhibited low crystallinity and moderate viscosity and DP; therefore, it is quite a good coating solution for the preparation of thin films on most substrates.

Thermal Stability of the Regenerated Cellulose Films
TGA curves for the regenerated cellulose films are shown in Figure 4. Arapid decomposition in a narrow temperature range from 285 to 310 • C was observed for the original cellulose sample. The regenerated samples exhibited lower onset temperatures for decomposition (from 165 to 175 • C), but gave higher char yields (nonvolatile carbonaceous material) on pyrolysis, as indicated by the high residual masses after the decomposition step. This result illustrates that many hydrogen bonds of cellulose are destroyed after dissolution in ILs, so the ratio of the lose amorphous structure was increased, causing a reduction in the thermal stability of the regenerated cellulose. The regenerated cellulose contains two types of anhydroglucose with different backbone structures: β-cellulose and γ-cellulose [24][25][26]. The onset temperatures of these two structures are lower than that of the α-cellulose (the original natural cellulose), but the two structures exhibit larger residual masses after thermal treatment; e.g., CQA-5 has nearly four times the residual weight of MCC. This thus indicates that the regenerated cellulose materials can be applied in the preparation of porous carbon nanofibers, activated carbon, and biomaterials.

Morphology of the Regenerated Cellulose Film
The 3000× magnification SEM images of the surface of the original and regenerated cellulose films are shown in Figure 5. As can be seen, rough morphology and fiber texture are observed on the MCC and CA-5 surfaces. After regeneration of the ILs, the material changed significantly, displaying a smooth surface. The nonporous structure of the CNA samples shows the good dissolution ability of NA. The smoother regenerated films are obtained by using the ILs with better dissolution ability. The cross-sectional SEM images show uniformity from the interior to the surface, indicating the dense texture of the prepared film. The fibrous network of CQA-5 is more obvious than those of CNA-5 and CTA-5, due to the high degree of crystallization in the case of CQA-5. In addition, the fibrous texture on CNA samples increased with the cellulose content.

Morphology of the Regenerated Cellulose Film
The 3000× magnification SEM images of the surface of the original and regenerated cellulose films are shown in Figure 5. As can be seen, rough morphology and fiber texture are observed on the MCC and CA-5 surfaces. After regeneration of the ILs, the material changed significantly, displaying a smooth surface. The nonporous structure of the CNA samples shows the good dissolution ability of NA. The smoother regenerated films are obtained by using the ILs with better dissolution ability. The cross-sectional SEM images show uniformity from the interior to the surface, indicating the dense texture of the prepared film. The fibrous network of CQA-5 is more obvious than those of CNA-5 and CTA-5, due to the high degree of crystallization in the case of CQA-5. In addition, the fibrous texture on CNA samples increased with the cellulose content.

Morphology of the Regenerated Cellulose Film
The 3000× magnification SEM images of the surface of the original and regenerated cellulose films are shown in Figure 5. As can be seen, rough morphology and fiber texture are observed on the MCC and CA-5 surfaces. After regeneration of the ILs, the material changed significantly, displaying a smooth surface. The nonporous structure of the CNA samples shows the good dissolution ability of NA. The smoother regenerated films are obtained by using the ILs with better dissolution ability. The cross-sectional SEM images show uniformity from the interior to the surface, indicating the dense texture of the prepared film. The fibrous network of CQA-5 is more obvious than those of CNA-5 and CTA-5, due to the high degree of crystallization in the case of CQA-5. In addition, the fibrous texture on CNA samples increased with the cellulose content.

Recycling of the Ionic Liquids
After the regeneration of cellulose, the residual NA ILs in the coagulation bath were recovered by distilling water using a vacuum rotary evaporator. The purity and structure of the recovered NA are the same as those of the prepared NA, as confirmed by 1 H NMR from 0.5 to 4 ppm (as shown in Figure 6). A recovery ratio of 81% for NA was obtained in this work, with a major loss resulting from the NA ILs adhering on the walls of the distillation bottle. Thus, the recovery rate will be reasonably enhanced in a large-scale production process. Moreover, the recovered NA was used to prepare the regeneration of cellulose films again, and the sample was coded as R-CNA-5. As shown in Figure 7, no significant difference in the XRD pattern between CNA-5 and R-CNA-5 was observed. The results show the stability of QA RTILs in the cellulose dissolution process. It is apparent that this advantage will promote the industrial application of QA RTILs.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 12 After the regeneration of cellulose, the residual NA ILs in the coagulation bath were recovered by distilling water using a vacuum rotary evaporator. The purity and structure of the recovered NA are the same as those of the prepared NA, as confirmed by 1 H NMR from 0.5 to 4 ppm (as shown in Figure 6). A recovery ratio of 81% for NA was obtained in this work, with a major loss resulting from the NA ILs adhering on the walls of the distillation bottle. Thus, the recovery rate will be reasonably enhanced in a large-scale production process. Moreover, the recovered NA was used to prepare the regeneration of cellulose films again, and the sample was coded as R-CNA-5. As shown in Figure 7, no significant difference in the XRD pattern between CNA-5 and R-CNA-5 was observed. The results show the stability of QA RTILs in the cellulose dissolution process. It is apparent that this advantage will promote the industrial application of QA RTILs.   After the regeneration of cellulose, the residual NA ILs in the coagulation bath were recovered by distilling water using a vacuum rotary evaporator. The purity and structure of the recovered NA are the same as those of the prepared NA, as confirmed by 1 H NMR from 0.5 to 4 ppm (as shown in Figure 6). A recovery ratio of 81% for NA was obtained in this work, with a major loss resulting from the NA ILs adhering on the walls of the distillation bottle. Thus, the recovery rate will be reasonably enhanced in a large-scale production process. Moreover, the recovered NA was used to prepare the regeneration of cellulose films again, and the sample was coded as R-CNA-5. As shown in Figure 7, no significant difference in the XRD pattern between CNA-5 and R-CNA-5 was observed. The results show the stability of QA RTILs in the cellulose dissolution process. It is apparent that this advantage will promote the industrial application of QA RTILs.

Conclusions
The results show that the novel acetate-based ionic liquids are effective, non-derivatizing solvents for cellulose. The untreated microcrystalline cellulose was rapidly dissolved in the ionic liquids, and its crystal phase was transformed to cellulose II completely under mild heating conditions. The cellulose film regenerated by coagulation with water exhibited good mechanical properties and a smooth texture. Among these ionic liquids, NA with the small polar cation was found to be the best cellulose-dissolving reagent, and it could be easily recycled. These ionic liquids can be considered to be recyclable solvents that can replace traditional dissolving solvents (DMAc/LiCl) and reduce the toxic waste generated in the present viscose fiber synthesis process. In addition, the preparation steps of these ionic liquids developed in this study are simple enough for mass production.