Development of Diallylimidazolium Methoxyacetate/DMSO (DMF/DMA) Solvents for Improving Cellulose Dissolution and Fabricating Porous Material

Cellulose is the most abundant natural biopolymer, with unique properties such as biodegradability, biocompability, nontoxicity, and so on. However, its extensive application has actually been hindered, because of its insolubility in water and most solvents. Herein, highly efficient cellulose solvents were developed by coupling diallylimidazolium methoxyacetate ([A2im][CH3OCH2COO]) with polar aprotic solvents dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Attractively, these solvents showed extraordinarily powerful dissolution performance for cellulose (e.g., 26.1 g·100g−1) in [A2im][CH3OCH2COO]/DMSO(RDMSO = 1.01 solvent even at 25 °C), which is much more advantageous over previously reported solvents. To our knowledge, such powerful cellulose solvents have not been reported before. The cellulose dissolution mechanism is proposed to be of three combined factors: (1) The hydrogen bond interactions of the H2, H4 and H6 in [A2im]+ and the carboxyl O atom in [CH3OCH2COO]−, along with the hydroxyl H atom and O atom in cellulose, are main driving force for cellulose dissolution; (2) the dissociation of [A2im][CH3OCH2COO] by DMF increases the anion and cation concentrations and thus promotes cellulose dissolution; (3) at the same time, DMF also stabilizes the dissolved cellulose chains. Meanwhile, the porous cellulose material with a varying morphologic structure could be facially fabricated by modulating the cellulose solution concentration. Additionally, the dissolution of cellulose in the solvents is only a physical process, and the regenerated cellulose from the solvents retains sufficient thermostability and a chemical structure similar to the original cellulose. Thus, this work will provide great possibility for developing cellulose-based products at ambient temperatures or under no extra heating/freezing conditions.


Introduction
Fossil-based products have created our brilliant and splendid civilization for the development of human society, but in the meantime they have also brought some harm to the ecological environment and to human health. Therefore, the utilization of ecofriendly and nontoxic products has widely been recognized in modern society. Among the promising alternatives to fossil-based products, cellulose, the most abundant biopolymer resource on earth, has attracted increasing attentions due to its fascinating (DMA) (99.5%) were purchased from Tianjin kemio chemical reagent Co., Ltd. (Tianjin, China). DMSO, DMF and DMA were dried with a 4A molecular sieve before use. [A 2 im][CH 3 OCH 2 COO] was synthesized and purified by using a similar procedure described in the literature [25].

Cellulose Dissolution
In a typical dissolution experiment, cellulose was added to a 25 mL colorimetric tube which contained 2.0 g of [A 2 im][CH 3 OCH 2 COO]/DMF, and the tube was sealed with parafilm. The tube was then immersed in an oil bath (DF-101S, Gongyi Yingyu Instrument Factory), and the bath temperature was controlled to be 25 ± 0.5 • C. After the cellulose was completely dissolved, the solution became completely clear, and no cellulose particle was observed under the polarization microscope (Nanjing Jiangnan Novel Optics Co. Ltd., Nanjing, China). Then, additional cellulose was added. When the cellulose solution became saturated, judged by the fact that cellulose could not be dissolved further, and cellulose particles could be observed under the polarization microscope, the addition of cellulose stopped. The cellulose solubility (expressed by gram per 100g of solvent) at 25 • C was calculated from the amount of the solvent and cellulose added.

Preparation of Porous Cellulose Materials
1%, 3%, 5% and 7% cellulose solutions were obtained by dissolving cellulose in [A 2 im][CH 3 OCH 2 COO]/DMSO(R DMSO = 1) solvent, respectively. The cellulose solutions were transferred into a Petri dish, and then coagulated in distilled water to obtain gels, followed by washing repeatedly with distilled water to ensure that the [A 2 im][CH 3 OCH 2 COO]/DMSO solvent had been washed out. The washed gels were frozen for 8 h at −20 • C, and then freeze-dried using an FD-10 freeze-dryer (Henan Brother Equipment Co. Ltd., China). The cold trap temperature was below −45 • C and the vacuum pressure was below 0.1 MPa during the freeze-drying process. The porous materials prepared from these 1%, 3%, 5% and 7% cellulose solutions were denoted as DMSO- The porous materials were characterized using scanning electron microscopy (SEM). The SEM images of the freeze-dried materials were frozen in liquid nitrogen, immediately snapped. The fracture surfaces of the films were sputtered with gold, and then photographed.

Preparation and Characterization of Regenerated Cellulose Film
As an example, cellulose was dissolved in [A 2 im][CH 3 OCH 2 COO]/DMSO(R DMSO = 0.5) solvent to obtain a cellulose solution with 5.0 g·100g −1 of solubility. The cellulose solution was cast onto a glass plate to give a thickness of about 0.3 mm, then we took off air bubbles in a vacuum oven, and then immediately coagulated in the water to obtain a transparent regenerated cellulose film.
The regenerated cellulose film was washed with distilled water to ensure that the [A 2 im][CH 3 OCH 2 COO]/DMSO solvent had been washed out and dried at 60 • C in a vacuum oven. The dried cellulose film was named, as were denoted, as DMSO-CF. DMF-CF from [A 2 im][CH 3 OCH 2 COO]/ DMF(R DMF = 0.5) solvent and DMA-CF from [A 2 im][CH 3 OCH 2 COO]/DMA(R DMA = 0.5) solvent were prepared using a similar procedure to DMSO-CF, respectively. The regenerated cellulose film was employed for the measurements of XRD, FTIR spectroscopy and TGA.
FTIR spectra were recorded on a Necolet Nexus spectrometer with KBr pellets. A total of 16 scans were taken for each sample at a resolution of 2 cm −1 . The XRD patterns were collected on a BrukerD8Advance diffraction spectrometer with Cu-Ka radiation (λ= 1.54 Å) over the range 3-60 degrees (2θ) at a scan speed of 2 degrees (2θ) per minute. Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 449 C thermal analyzer using alumina crucibles. The measurements were carried out under flowing N 2 at a heating rate of 10 • C·min −1 .

13 C NMR Spectra Measurements
Measurements of 13 Figure 1 for easy understanding.  Figure 1 for easy understanding. 223

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As an example, [A2im][CH3OCH2COO]/DMSO was employed to understand the possible 230 dissolution mechanism of cellulose. It can be seen from Figure 1 and Table 2  As an example, [A 2 im][CH 3 OCH 2 COO]/DMSO was employed to understand the possible dissolution mechanism of cellulose. It can be seen from Figure 1 and Table 2 that the addition of cellulose to the [A 2 im][CH 3 OCH 2 COO]/DMSO(R DMSO = 2) solvent leads to a marked upfield shift for the C2 atom and a weak upfield shift for the C4 atom (a decrease of chemical shift). This indicates that in the [A 2 im][CH 3 OCH 2 COO]/DMSO(R DMSO = 2)/cellulose (8%) solution, the acidic H2 proton strongly interacts with the hydroxyl oxygen in cellulose by hydrogen bond formation, which leads to the increase of the electron cloud density of C2 atom, thus its chemical shift moves upfield. Similarly, the acidic H4 proton weakly interacts with hydroxyl oxygen. The signal of the carboxyl C10 atom considerably moves downfield (a considerable increase of chemical shift). This suggests that the carboxyl oxygen atom in [CH 3 OCH 2 COO] − forms a strong hydrogen bond with the hydroxyl proton of cellulose, resulting in the decrease of the electron cloud density of the C10 atom, thus its chemical shift moves downfield. It is also found that the signal of C6 atom on the allyl chain also moves upfield, implying that the hydroxyl oxygen in cellulose strongly interacts with the H6 atom. The observable upfield shift of the C9 atom and downfield shift of the C7 atom may be due to the redistribution of the electron cloud density. Additionally, the increased chemical shift for the C8 atom is mainly ascribed to the hydrogen bond interaction between O8 and the hydroxyl hydrogen atom. However, little change has been observed for the chemical shift of the C5 atom. The above results indicate that the main driving force of the dissolution of cellulose in the [A 2 im][CH 3 OCH 2 COO]/DMSO(R DMSO = 2) solvent primarily results from the interactions of the H2, H4 and H6 protons in cation with the hydroxyl oxygen in cellulose as well as the carboxyl oxygen atom in [CH 3 OCH 2 COO] − with the hydroxyl hydrogen in cellulose, which is consistent with the results aforementioned. At the same time, [A 2 im][CH 3 OCH 2 COO]/DMF(DMA) solvents display similar dissolution mechanisms for cellulose (see Table 2).
It is also found from Figures S1-S6    This is mainly due to the hydrogen bond interactions of the H2 atom in [A 2 im] + with the hydroxyl oxygen in cellulose and the carboxyl oxygen atom in [CH 3 OCH 2 COO] − with the hydroxyl hydrogen in cellulose [40]. In the meanwhile, [A 2 im][CH 3 OCH 2 COO]/DMF(DMA) solvents also display similar changing trends (see Figures S7 and S8).

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( Figure 4a). After the addition H2O to cellulose solution, the cellulose molecules closely stack along

Thermostability and Chemical Structure of the Regenerated Cellulose
TGA curves for the original cellulose and the regenerated cellulose film are shown in Figure 5.  43]. This indicates that the transformation from cellulose I to cellulose II occurred via the dissolving-freezing-thawing procedure. Additionally, a glass substrate was used to cement/fix each regenerated cellulose film to facilitate the XRD measurements of the regenerated cellulose samples.  FTIR spectra of the original cellulose and the regenerated cellulose films DMSO-RCF, DMF-RCF and DMA-RCF are shown in Figure 7. Clearly, the FTIR spectra of the three regenerated cellulose films are quite similar, and no new peaks are observed in the regenerated cellulose film sample. This suggests that no chemical reaction occurs between the cellulose and the [A 2 im][CH 3 OCH 2 COO]/DMSO(DMF, DMA) solvents during the dissolution and regeneration processes of the cellulose. The absorption bands at 1423 cm −1 in the samples DMSO-RCF, DMF-RCF and DMA-RCF are assigned to the CH 2 scissoring vibration. These bands were weakened and shifted to a lower wavenumber compared to the peak at 1431 cm −1 for the original cellulose, suggesting the destruction of an intra-molecular hydrogen bond involving O6 [44][45][46]. The new shoulders at 990 cm −1 in the samples DMSO-RCF, DMF-RCF and DMA-RCF could be assigned to the C-O stretching vibration in the amorphous region [47]. The O-H vibrations in the samples DMSO-RCF, DMF-RCF and DMA-RCF shift to a higher wavenumber (3419 cm −1 ), which is a hint for the breaking of hydrogen bonds to some extent [48,49]. The absorption bands in the range of 1164-1061 cm −1 belong to the C-O-C stretching of the original cellulose [50]. The presence of such bands in the absorption of the samples DMSO-RCF, DMF-RCF and DMA-RCF suggests that the macromolecular structure of cellulose is not descontructed after the regeneration of the cellulose.

Conclusions
Novel and efficient cellulose solvents were developed by coupling [A 2 im][CH 3 OCH 2 COO] with DMSO(DMF, DMA). Attractively, these solvents showed extraordinarily powerful dissolution performances for cellulose. For example, as high as 26.1 g·100g −1 of cellulose solubility was obtained in the [A 2 im][CH 3 OCH 2 COO]/DMSO (R DMSO = 1.01) solvent, even at 25 • C. The cellulose dissolution mechanism is suggested to be that: 1) The cellulose dissolution mainly results from the interactions of the H2, H4 and H6 in [A 2 im] + , as well as carboxyl O atom in [CH 3  COO] and the cellulose solution concentration, and has nothing to do with DMSO(DMF, DMA). It was also found that the dissolution of cellulose in the solvents is only a physical process, and the regenerated cellulose from the solvents retains sufficient thermostability and similar chemical structure to the original cellulose. Therefore, this study provides mild and efficient cellulose solvent systems, which has potential applications in developing cellulose-based products, even at ambient temperatures.

Conflicts of Interest:
The authors declare no conflict of interest.