Cellulose dissolution in diallylimidazolium methoxyacetate + N-methylpyrrolidinone mixture

The utilization of cellulose in industrial applicat is of great significance to sustainable development of human society and reducing dependence on dwindling fossil resources. Nevertheless, this utilization of cellulose has actually been limited due to its insolubilization. Here, novel solvents consisting of diallylimidazolium methoxy acetate ([A2im][CH3OCH2COO]) and N-methylpyrrolidinone (NMP) were developed. The solubility of cellulose in [A2im][CH3OCH2COO]/NMP was determined, and the influence of [A2im][CH3OCH2COO]/NMP molar ratio on cellulose dissolution was systematically investigated. Meanwhile, we also presented the affecting factors of the cellulose material fabrication including preparation approach, [A2im][CH3OCH2COO] and cellulose solution concentration. Attractively, the [A2im][CH3OCH2COO]/NMP solvents display much powerful dissolution capacity for cellulose even at 25 °C (25.4 g 100 g−1). This is mainly ascribed to the combined factors: The hydrogen bond interactions of the H2, H4 and H6 in [A2im]+ and carboxyl O atom in [CH3OCH2COO]− with the hydroxyl H atom and O atom in cellulose; the dissociation of NMP towards [A2im][CH3OCH2COO]; the stabilization of NMP towards the dissolved cellulose chains. In addition, the thermostability and chemical structure of the regenerated cellulose from the solvents was also estimated.

This work aims to develop novel solvents which were expected to more efficiently dissolve cellulose than the solvents reported previously, and fabricate the cellulose material with varying morphologic structures using the solvents. Therefore, we here design novel solvents by combining [A 2 im][CH 3 OCH 2 COO] with NMP, and the solubilities of cellulose in the solvents were determined at 25 °C. Meanwhile, the influences of NMP/[A 2 im] [CH 3 OCH 2 COO] molar ratio on cellulose dissolution and the possible dissolution mechanism for cellulose were investigated. Additionally, the characterization of the regenerated cellulose from [A 2 im][CH 3 OCH 2 COO]/NMP solvent was completed to examine its chemical structure and thermostability.

Materials and Methods
Materials. Microcrystalline cellulose (MCC) was purchased from Sigma Aldrich Company, its viscosity-average degree of polymerization is 270. N-Allylimidazole (99%), allyl chloride (98%), ethoxyacetic acid (98%) and anion exchange resin (Ambersep 900-OH) were purchased from Alfa Aesar Company. N-Methylpyrrolidinone (NMP) (>99.9%) was purchased from Shanghai Aladdin biochemical technology Co., Ltd., and dried using 4 A molecular sieve before use. [ COO]/NMP(R NMP = 0.50)/cellulose mixture was stirred until cellulose was thoroughly solubilized. Then, additional cellulose was added. The procedure was repeated until cellulose could not be solubilized. The thorough dissolution of cellulose was monitored by a polarization microscope. If cellulose was thoroughly dissolved, the mixture was optically clear under the polarization microscope. Cellulose solubility at 25 °C was calculated based on the amount of solvent and cellulose added. The cellulose solubility data at 25 °C were shown in Table 1.
As an example, cellulose was dissolved in [A 2 im][CH 3 OCH 2 COO]/NMP(R NMP = 1) solvent to gain 1% of solution. The solution was poured in a Petri dish, taken off air bubble under vacuum for 30 min at ambient temperature. Then, the Petri dish containing the solution was immersed in distilled water to obtain cellulose hydrogel. The hydrogel was repeatedly washed with distilled water to remove [A 2 im][CH 3 OCH 2 COO] and NMP. The washed hydrogel was frozen for 8 h at −20 °C, and then freeze-dried in a FD-10 freeze-dryer (Henan Brother Equipment Co. Ltd., China) to obtain porous cellulose material. The material was named as PCM-1. PCM-3, PCM-5 and PCM-7 porous materials from 3%, 5% and 7% of cellulose solutions were prepared via a similar procedure to PCM-1, respectively. 13 c nMR and ftiR spectra measurements. Cellulose  COO]/NMP(R NMP = 2.43)/cellulose solution were performed on a spectrometer Nicolet iN10 spectrometer with Ge crystal ATR accessory at room temperature. Spectra were collected in high-resolution mode (4 cm −1 resolution and 64 scans) under an ATR 5% maximum pressure.  37 , where DMSO, DMF and DMA represent dimethyl sulfoxide, N,N-dimethylformamide and N,N-dimethylacetamide, respectively. This indicates that the cosolvent effects the cellulose dissolution, which is consistent with the results reported previously [32][33][34][35][36] . In addition, it was found that absorbent cotton (viscosity-average degree of polymerization of 1586) could be dissolved in [A 2 im] [CH 3 OCH 2 COO]/NMP(R NMP = 2.43) solvent at 50 °C while it was very difficult to dissolve in conventional solvents. It was also observed that 5.2% cellulose solution is very viscous and difficult to be stirred (see Fig. S1).  Fig. 1.

c nMR and ftiR analysis of cellulose dissolution mechanism.
It is clear that, after the addition of cellulose to [A 2 im][CH 3 OCH 2 COO]/NMP(R NMP = 2) solvent, the signals of the C2 and C4 atoms in imidazolium ring markedly moves upfield (a marked decrease of chemical shift). This indicates that in [A 2 im][CH 3 OCH 2 COO]/NMP(R NMP = 2)/cellulose(8%) solution, as the results of the interaction between the acidic H2, H4 protons and the hydroxyl oxygen atoms in cellulose through hydrogen bonding, the electron cloud density of the C2 and C4 atoms increased, leading to the upfield of their chemical shifts. Moreover, the hydrogen bond interaction between H2 proton and the hydroxyl oxygen in cellulose are stronger than that between H4 proton and the hydroxyl oxygen in cellulose. The carboxyl C10 atom demonstrates the signal considerably moved downfield (chemical shift increases significantly). This suggests that there's strong hydrogen bond formed between the carboxyl oxygen atom in [CH 3 OCH 2 COO] − and the hydroxyl proton of cellulose. The electron cloud density of C10 atom thus decreases to move its chemical shift downfield. Meanwhile, strong interaction between the hydroxyl oxygen in cellulose and H6 atom leads to the upfield movement of the signal of C6 atom on allyl chain. Moreover, the electron cloud density redistribution may cause the upfield shift of C9 atom and downfield shift of C7 atom. In addition, O8 interacts with hydroxyl hydrogen atom through the hydrogen bond to increase the chemical shift of C8 atom. Nevertheless, the chemical shift of C5 atom remains still. Based (2019)     Morphology and structure of the porous cellulose material. SEM images of the fracture surfaces of the porous cellulose materials PCM-1, PCM-3, PCM-5, and PCM-7 are shown in Fig. 3. PCM-1 has a fluffy and porous structure which is composed of randomly oriented cellulose sheets, with the sheets being twisted and broken. Different from PCM-1, PCM-3, PCM-5, and PCM-7 exhibit long channel structures which were composed of adjacent sheets. This is an indication that the concentration of cellulose solution significantly affects the morphology of the cellulose material.
It is interesting to find that the morphologic structures of the cellulose materials prepared from 3-5% cellulose solution are quite similar to that reported by Xu et al. in which the porous cellulose material was prepared by   As a comparison, we also prepared a cellulose film, and its SEM images are shown in Fig. 4. As shown in Fig. 4, the fracture surface of the film exhibits a homogeneous and dense structure. The morphology of the cellulose film is quite similar to those reported in the literatures 42,43 . Figure 5 shows the XRD patterns of the original cellulose and regenerated cellulose film. It is indicated in Fig. 5 that the original cellulose displays the typical diffraction peaks of cellulose I at 2θ = 15.2°, 16.4°, 22.5°, 34.6°4 4 . In the meantime, the typical diffraction patterns of cellulose II are observed at 2θ = 12.5°, 20.3° and 21.2° for the regenerated cellulose exhibits 45 . This is indicative of a conversion from cellulose I to cellulose II. Figure 6 shows the FTIR spectra of the original and the regenerated cellulose. The FTIR spectra of the regenerated cellulose from [A 2 im][CH 3     COO]/NMP solvent in the dissolution and regeneration processes. XRD studies confirm the conversion from cellulose I to cellulose II after the original cellulose is dissolved and regenerated in [A 2 im][CH 3 OCH 2 COO]/NMP solvent. Therefore, this work provides a viable strategy for the practical application in cellulose processing/conversion even at as low as 25 °C.