Room-Temperature Dissolution and Mechanistic Investigation of

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Room-temperature dissolution and mechanistic investigation of cellulose in a tetra-butylammonium acetate/dimethyl sulfoxide system Yao-Bing Huang, Ping-Ping Xin, Jia-Xin Li, Yue-Ying Shao, Chaobo Huang, and Hui Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01749 • Publication Date (Web): 04 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Sustainable Chemistry & Engineering

Room-temperature

dissolution

and

mechanistic

investigation of cellulose in a tetra-butylammonium acetate/dimethyl sulfoxide system Yao-Bing Huang,‡ Ping-ping Xin,‡ Jia-Xin Li,Yue-Ying Shao, Chao-Bo Huang and Hui Pan*

College of Chemical Engineering, Nanjing Forestry University, 159 Longpan Road, 210037, Nanjing, China. KEYWORDS: cellulose dissolution / tetrabutylammonium acetate / dimethyl sulfoxide / room temperature / dissolution mechanism ABSTRACT

The dissolution of cellulose in tetrabutylammonium acetate (TBAA) and dimethyl sulfoxide (DMSO) mixed solvent was studied at room temperature (approx. 25 oC). The ratio of TBAA in the mixed solvent system (WTBAA) was found to have great influence on the solubility of cellulose and the corresponding dissolution time. The mixed solvent of WTBAA=0.15 possessed the highest cellulose solubility and shortest dissolution time. Various cellulosic materials were well dissolved in the solvent with a maximum solubility up to 8.17 wt%. A mechanistic study regarding the interaction between the solvent system and the model compound cellobiose was conducted using, 1H NMR, 13C NMR, ATR-FTIR, conductivity and viscosity measurements. The

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results implied that TBAA existed at two different status in the mixed solvent as the ratio of TBAA varied (i.e. ion-split stage (WTBAA≤0.15) and ion-paired stage (WTBAA≥0.15)). WTBAA=0.15 was the turning point of these two stages and the mixed solvent displayed the best dissolving ability at this ratio. This finding suggests that a balance between the ion concentration and ion mobility is crucial to the dissolving ability of a mixed solvent. The solvation effect of the co-solvent DMSO helped to dissociate TBAA into free ions and facilitate the ion mobility. The hydroxyl protons of cellobiose were demonstrated to form strong hydrogen bonds with CH3COO-, which was the key to the dissolution of cellulose. Finally, the interaction between cellobiose and DMSO in the TBAA/DMSO/cellobiose system was investigated and was demonstrated as another important factor for the dissolution of cellulose by stabilizing the dissolved cellulose chain from further formation of inter- and intramolecular hydrogen bonds.

INTRODUCTION Cellulose is a plentiful and renewable resource in nature that has numerous applications traditionally in the textile and paper making industries and recently for bio-based chemicals and fuels.1,2 It is a polymer that is composed of a glucose unit linked by β-1, 4 glycosidic bonds.3 The abundant hydroxyl groups on the cellulose chain forms a strong, three-dimensional inter- and intramolecular hydrogen bonding network, which makes it insoluble in most organic solvents.4 Therefore, effective dissolution of cellulose a primary challenge for its further utilization. Traditional cellulose solvents such as carbon disulphide are not favoured due to toxicity and environmental pollution.5 An effective solvent systems for cellulose dissolution is of great importance for further advances in cellulose conversion.


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Many solvent systems have been reported for cellulose dissolution including NMMO,6 LiCl/DMAc,7 and NaOH/urea.8 Recently, ionic liquids (ILs) are receiving more attentions as promising candidates for industrial dissolution of cellulose due to their excellent dissolving abilities and unique properties such as negligible vapour pressure, excellent solubility, thermal stability, and chemical inertness.9,10 In particular, imidazolium based ILs systems (e.g., 1-Allyl3-methylimidazolium

Chloride

([AMIM]Cl) ， 1-butyl-3-methylimidazolium

chloride

([BMIM]Cl) showed excellent capacities of dissolving cellulosic materials and biomass derived raw materials.11-14 These new solvent systems significantly stimulated the development of the dissolution of cellulose. Mechanistic investigations of the dissolution processes have been extensively studied. The ions of ILs and the co-solvent were identified as the main factors for cellulose dissolution. Generally, the anions of ILs were mainly responsible for the dissolution process by forming strong hydrogen bonding with the hydroxyl groups of cellulose and disrupting its inter- and intramolecular hydrogen bonding.15-17 For example, Remsing et al. 18 used NMR spectroscopy to study cellulose which was dissolved in 1-butyl-3-methylimidazolium chloride ([C4min]Cl), chloride ions were found to form stoichiometric hydrogen bonding with hydroxyl protons of cellulose, which was considered as the predominant factor for cellulose dissolution. Zhao et al. 16 investigated the specific roles of various anions in the cellulose dissolution abilities of imidazolium ILs with a fixed imidazolium cation. They found anions with high electronegativity and without the electron withdrawing group (i.e. Cl-, CH3COO− and (CH3O)2PO2−) were preferable hydrogen bond acceptors which could exhibit better dissolution performances. Apart from the anions, cations of ILs have also shown great influences on the dissolution of cellulose. Wang et al.19 investigated the dissolution of cellulose in 13 kinds of ILs with a fixed anion


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CH3COO- but varied cationic backbones. They found that a cationic backbone with acid protons without high electronegativity atoms (e.g. O, N) were better candidates for the dissolution of cellulose. Rabideau et al.20 reported that the dissolution of cellulose crystalline was initiated by the binding of anions to the external hydroxyl groups and then the cations separated the negatively charged moieties through their bulk volume. Other research has revealed that both anions and cations can form hydrogen bonds with cellulose.21It is interesting to note that the introduction of co-solvents (e.g. DMSO, DMAc) to ILs systems have also attracted much attentions due to their effective enhancement of cellulose dissolution. Andanson et al.

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employed the co-solvent DMSO to [C4C1IM][OAc] and greatly decreased the system’s viscosity and solvated ILs to form more free ions, which were considered as the key to better dissolution performances. More recently, quaternary ammonium ILs with an aprotic polar co-solvent were developed and showed remarkable abilities in the dissolution of cellulose.23-25 Compared with the aforementioned solvents, these systems are more efficient, lower energy-input and environmental friendly. Zhang et al.25 reported the pioneer work on the dissolution of cellulose in tetrabutylammonium acetate (TBAA) and DMSO mixed solvents at mild condition. However, a detailed study on the mechanistic of the dissolution process of this mixed solvent is still lacking. Herein, we reported the experimental and mechanistic studies of TBAA/DMSO mixed solvent for the dissolution of cellulose under mild conditions. Different ratios of the mixed solvent were prepared and used to study their dissolving properties, including the maximum dissolution quantity and dissolution time. The dissolution mechanism of cellulose in the mixed solvent was also investigated by using cellobiose as the model compound in the mixed solvent. The interactions between cellobiose, cation/anion of TBAA and DMSO were characterized by 1H


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NMR,

13

C NMR, ATR-FTIR, conductivity, and viscosity. The possible dissolution mechanism

was discussed based on the above results. EXPERIMENTAL Materials Cellulose filter papers (Whatman, ~1100DP), microcrystalline cellulose (MCC, SigmaAldrich, 20µm, ~250DP), α-cellulose (Aladdin, 25µm, ~180DP) and degreased cotton (Xuzhou health materials factory, ~4080DP) were obtained from commercial sources. All of them were dried overnight at 105 oC before further use. Tetramethylsilane (TMS, TCI, >90%), D-(+)Cellobiose (TCI, >98%), Tetrabutylammonium acetate (Sigma-Aldrich, 97%), Dimethyl sulfoxide (Energy Chemical 99.7%) and Deuterated dimethyl sulfoxide (DMSO-d6, SigmaAldrich, 99.97%) were used without further purification. Dissolution of cellulose In a typical cellulose dissolution experiment, a certain amount of cellulosic material (i.e. MCC, α-cellulose, filter paper, degreased cotton) was added to a 10 ml vial which contained TBAA/DMSO mixed solvent of different ratios. Then, the mixture was stirred in a water bath at 25 oC at an agitation speed of 600 rpm until complete dissolution. To determine the maximum solubility of cellulose in the mixed solvent, cellulosic material was gradually added to the mixed solvent until the mixture became optically clear as viewed using polarization microscopy. When the cellulose solution remained turbid after the mixture was allowed to stir for 2 additional hours, it was considered to have reached the saturation point of the solvent’s solubility. The solubility was then calculated from the amount of solvent and cellulosic material added. To determine the time of complete dissolution, the cellulose in TBAA/DMSO mixture was sampled and placed on


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a glass slide for the quick observation using polarization microscopy. When no cellulose crystal or particles were observed in the eyepiece (i.e., the image turned dark), the dissolution time was then determined. Additionally, 0.5 wt% MCC was dissolved in different ratios of TBAA/H2O mixed solvents at 25 oC for 48 h followed the same procedures as that of using TBAA/DMSO mixed solvent. Dissolution of cellobiose The dissolution procedure of cellobiose was similar to that of cellulose, a certain amount of cellobiose was added to a 10 ml vial which contained different amounts of the solvents. Then, the mixture was stirred in a water bath at 25 oC at an agitation speed of 600 rpm until complete dissolution. Sample preparation for characterization Sample series I and II were prepared by mixing different weights of TBAA in DMSO and DMSO-d6 (WTBAA=0.05, 0.10, 0.15, 0.20, 0.25), respectively. Sample series III and IV were prepared by dissolving different weights of cellobiose in DMSO (mass fraction of cellobiose in the mixture (Wcellobiose) were 0, 2.56, 4.60, 6.93, 8.79, 10.93, 14.59, and 17.67 wt%) and TBAA/DMSO-d6 (Wcellobiose=5, 15 wt%), respectively. Sample V and VI were prepared by dissolving different weights of cellobiose in TBAA/DMSO mixed solvent (Wcellobiose=5, 10, 15 wt%) and TBAA/DMSO (Wcellobiose=0, 5, 15, 25 wt%), respectively. Sample series VII were prepared by dissolving different weights of MCC in TBAA/DMSO mixed solvent (WTBAA=0.15, WMCC=1, 3, 5, 7 wt%). Sample VIII were prepared by dissolving different weights of MCC in TBAA/DMSO mixed solvent (WTBAA=0.15, WMCC=1, 5, 8 wt%). All the samples were prepared at 25 oC.


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Characterization ATR-FTIR characterization was conducted by using Thermo Nicolet 360 coupled with a Spectra Golden Gate MKII Single Reflection ATR accessory. The FTIR analysis was implemented between frequency range 4000-400 cm-1. Sample series I, III, V, VII and VIII were tested by ATR-FTIR (Figure S2, Figure 11, Figure 12, Figure S4, and Figure S5).. The 1H NMR spectra of sample series II (Figure 7) and 13C NMR spectra of sample series II, IV and VI (Figure 8, 10 and 9) were obtained by the nuclear magnetic resonance (NMR, Brucker Avance III 400 Hz). Each sample was transferred to a NMR vial with coaxial capillary inserts containing TMS for 1H NMR, 13C NMR. The viscosity of sample series I was determined by an Ubbelohde viscometer (dimmer 0.38mm) equipped with constant temperature water bath at 25 oC (Figure 4a). The conductivity of sample series I was measured at 25 oC on a DDS-11A conductometer (Shanghai Leici Chuangyi Instrument Factory, China). Data was collected in triplicate to ensure statistical validity (Figure 4b).

RESULTS AND DISCUSSION Dissolution of cellulose in the TBAA-DMSO mixed solvent Different weight ratios of TBAA/DMSO mixed solvent were prepared to investigate their maximum solubility of cellulose at 25 oC (The weight ratio of TBAA in the mixed solvent system was denoted as WTBAA). Typical images of the dissolution of MCC in the mixed solvent were recorded (Figure S1). In the images, clear cellulose particles can be seen at the beginning of the dissolution process, then they disappeared with time. Figure 1 shows the maximum solubility


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of MCC at different ratios of TBAA/DMSO. As WTBAA increased from 0.05 to 0.15, the solubility of MCC increased from 1.27 wt% to 8.17 wt%, which indicated that the amount of TBAA is the determining factor for the dissolution behaviour of the mixed solvent. Maximum cellulose solubility appeared at WTBAA=0.15. Further increase of TBAA ratio to 0.20 did not significantly affect the MCC solubility. However, a dramatic decrease in the MCC solubility was observed when the TBAA ratio increased to 0.25, and only 1.76 wt% MCC was completely dissolved in this mixed solvent. The dissolution time of MCC in the mixed solvent was also investigated and the results are presented in Figure 2. When 4 wt% MCC was added to the TBAA/DMSO mixed solvent (WTBAA=0.05), only partial dissolution was observed even with extended dissolution time. Much of the MCC were deliberated from aggregates to small fascicles and the mixture became a turbid solution. When WTBAA increased to 0.10, MCC was well dissolved in 7 min. The fastest dissolution was observed when WTBAA was 0.15 and only 2 min was used for the complete dissolution of MMC, which was consistent with the same ratio of TBAA that obtained the maximum dissolution quantity. Further increasing the WTBAA resulted in longer dissolution time. Noteworthy, 4 wt% MCC cannot be completely dissolved in the WTBAA=0.25 mixed solvent. This result indicated that the ratio of TBAA in the mixed solvent had a great influence on the dissolution of cellulose. And WTBAA=0.15 of TBAA in the mixed solvent was the best option for the dissolution of cellulose under the specific reaction condition. Cellulosic materials with different DP had different dissolution behaviours in the mixed solvent (Figure 3). Four cellulosic raw materials (1 wt%) were dissolved in the WTBAA=0.15 mixed solvent. MCC, α-cellulose, and filter paper were completely dissolved within 1.5 min, 1.7min, and 3.5 min, respectively. The dissolution time increased as the DP of the materials


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increased. And it took 13 min to dissolve degreased cotton due to its higher molecular weight and larger size. The above experiment further demonstrated that the mixed solvent had remarkable ability in dissolving different types of cellulosic materials. The viscosity and conductivity of the mixed solvent at different TBAA ratios were also measured. As shown in Figure 4a, the viscosity of the mixed solvent gradually increased as the ratio of TBAA increased at the entire range of TBAA ratios investigated. However, the conductivity of the mixed solvent showed different trends as the ratio of TBAA increased, which could be divided into two stages (Figure 4b). Figure 5 shows a brief diagram of these two stages/status of TBAA ions during the solvation by DMSO. For the first stage, the conductivity increased with an increase of the ratio of TBAA, which implied that within a certain TBAA to DMSO ratio (i.e. < WTBAA=0.15), TBAA molecules were partially split into free ions and the percentage of free ions split from TBAA (degree of dissociation) almost proportioned to the ratio of TBAA. At the second stage, the conductivity increased more slowly as the ratio of TBAA increased from 0.15 to 0.25. The result suggested that the dissociation degree of TBAA decreased at this stage and more TBAA existed in the form of ion-pairs with less free ions at higher TBAA ratios. In short, TBAA was solvated by DMSO and formed a relatively constant percentage of free ions at lower TBAA ratios (< WTBAA=0.15). However, the percentage of free ions did not increase as the TBAA ratio continuously increased because more TBAA existed as ion-pairs (> WTBAA=0.15). This preliminary finding was strongly related to the different dissolution behaviour of cellulose in the mixed solvent at different ratios (discussed later).26 Previous studies have reported that the addition of a co-solvent to the imidazolium based ILs can effectively reduce the viscosity of the mixed solvent, which can then facilitate the mobility of ions and increase their dissolution ability.2,27 Based on the above results, it can be speculated that


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the dissolution of cellulose in a TBAA/DMSO system is a synergic result from both TBAA and the co-solvent DMSO. And the balance between the concentration of free ions and the mobility of the mixed solvent is crucial for the dissolving ability of the TBAA/DMSO system. Mechanism of cellulose dissolution in TBAA/DMSO mixed solvent The dissolution mechanism of cellulose in the TBAA/DMSO mixed solvent was investigated by using cellobiose as the model compound, which has been widely studied in many previously reported work. The interactions between cellobiose and the mixed solvent were carefully studied and discussed. The structures of TBAA, DMSO, and cellobiose are shown in Figure 6 and each carbon atoms in the structures were labelled as well. Solvation of TBAA by DMSO Different ratios of TBAA in TBAA/DMSO-d6 mixed solvent were prepared and characterized by 1H NMR and

13

C NMR to investigate the interaction between TBAA and

DMSO in the absence of cellobiose. The results are shown in Figure 7. Two obvious changes were observed in the 1H NMR spectrum. Firstly, the peak of Ca-H (3.3ppm) of the TBAA cation moved consistently shielded when WTBAA decreased from 0.25 to 0.05. The shielded shift of Ca-H as the amount of DMSO increased in the TBAA/DMSO mixture was caused by the increase of the electron density around the H atom, which was most likely due to the formation of an H-bond between the Ca-H and the O atom of DMSO. The other notable change in the 1H NMR spectrum belongs to the Ce-H (1.6ppm) of the TBAA anion, which is more complicated than that of the CaH. It firstly moved deshielded as the ratio of DMSO increased, and then shielded with further increase of the ratio of DMSO. And WTBAA of 0.15 is the turning point of the changes. This specific changes can be well explained by the possible two different existing forms (Figure 4b)


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of TBAA in the mixed solvent. According to the conductivity experiments, we referred WTBAA ranging from 0.25 to 0.15 as the “ion-paired” stage and WTBAA from 0.15 to 0.05 as the “ionsplit” stage, indicating the status of the cation and anion of TBAA at different TBAA/DMSO ratios. At the “ion-paired” stage, TBAA in DMSO existed mainly as ion-pairs. At the same time, hydrogen bonds between cations and anions also existed as reported by several previous studies.28,29 When the amount of DMSO gradually increased in the mixed solvent, more hydrogen atoms from DMSO could interact with the carboxylate anions of the TBAA and form hydrogen bonds with the anion, which would then reduce the electron density on the carboxylate anions and simultaneously deshielded the Ce-H by the electron-withdrawing inductive effect from the carboxylate anion.30 When DMSO was continually added to the mixed solvent at the “ion-split” stage, more TBAA ion-pairs were split into free ions by the solvation effect of DMSO, resulting in the disruption of the strong interaction between tetrabutylammonium cation and acetate anion (Ca-H…-OOCCH3) of the TBAA ion pair. Consequently, the electron cloud moved back to oxygen and increased the electron density of carboxylate anions. 31-34 Therefore, the electron density on Ce-H would increase as a consequence of the electron-donating effect of the carboxyl functional groups. The above interactions between these ions or ion-pairs were further confirmed by the

13

C

NMR analysis (Figure 8 and Table S1). The chemical shifts of Ca, Cb, Cc and Cd on the cations of TBAA all move shielded as the DMSO ratio increased (WTBAA from 0.25 to 0.05), which indicated the formation of hydrogen bond between the cation hydrogen and the oxygen of DMSO, as illustrated in the 1H NMR (Figure 7) 33,35 The chemical shift of Cf followed the same trend as that of the Ce-H, which could also be explained by the same interaction between TBAA and DMSO at the “ion-paired” and “ion-split” stages as the amount of DMSO increased in the


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mixed solvent system. The chemical shift of Ce atoms was in the opposite direction to that of Cf atoms. Given the adjacent position of Ce to Cf, the opposite shift of Ce is most likely due to the electron redistribution effect.36 The existence of the two stages of the TBAA cation and anion was further demonstrated by the ATR-FTIR analysis of the TBAA-DMSO mixed solvent (Figure S2). A gradual red shift of peak 1376cm-1, which is assigned to the asymmetric stretching vibration of the carboxyl group, with the increase of DMSO content in the mixed solvent was observed at the ion-paired stage (WTBAA from 0.25 to 0.15). This red shift of carboxyl group could be caused by the gradually reduced electron density of its oxygen, indicating the increasing interaction between the oxygen as the electron donor and the hydrogen from DMSO as the electron accepter.37 This peak was reversely blue shifted at the “ion-split” stage, suggesting the increase of the electron density of the oxygen, which was caused by the splitting of the cation and anion of TBAA from ion-pair to free ions as discussed previously. Interaction between cellobiose and TBAA in cellobiose/TBAA/DSMO-d6 mixed system Generally, cellobiose has a higher solubility than cellulose in the mixed solvent. Therefore, several different amount of cellobiose were dissolved in the TBAA/DMSO-d6 (WTBAA=0.15) mixed solvent to evaluate the roles of cations and anions of TBAA during the dissolving process. The chemical shifts of the carbons of TBAA at different loading of cellobiose were compared with pure mixed solvent without cellobiose. And the difference of the chemical shifts (Δδ) are presented in Figure 9. Ce and Cf displayed the most significant changes in the chemical shift among all carbons on TBAA. As the amount of cellobiose increased, Cf moved deshielded while Ce moved shielded. It has been reported that carboxylate anion can easily form strong hydrogen bonding with hydrogen from the hydroxyl groups on the cellulose chain by disrupting the


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internal hydrogen bond network of the hydroxyl group themselves.38 This strong interaction led to the dissolution of cellulose in the mixed solvent. Therefore, the deshieded shift of Cf should be attributed to the formation of the hydrogen bond between the oxygen on the carboxylate anion of TBAA and the hydrogen on the hydroxyl group of cellobiose, which consequently decreased the electron density of Cf. The shielded shift of Ce is likely due to the electron redistribution effect as its adjacent position to Cf.

19,38

In addition, the slight change in the chemical shift of Ca~Cd

carbons at even high loading of cellobiose indicated that the interaction between the cation of TBAA and cellobiose were very weak. The interaction between cellulose and TBAA/DMSO systems were also characterized. Different amounts of MCC were dissolved in the WTBAA=0.15 mixed solvent and subjected to ATR-FTIR characterization. As shown in Figure S4, the peaks at 1583cm-1 and 1376cm-1 were assigned to the symmetric and asymmetric stretching vibration of the carboxyl group, respectively. With an increase in MCC loading, these two peaks became weaker. This variation suggested that more hydrogen bonds between oxygen on CH3COO- and MCC were formed, which therefore reduced the vibration of the carboxylate groups on TBAA. This result is in agreement with that of the interaction between TBAA and cellobiose. The chemical shifts of the carbons on cellobiose at two concentrations in the TBAA/DMSO solvent system were also compared. And the differences of the chemical shift of all carbons in cellobiose are presented in Figure 10 and Table S2. It was found that each carbon at one pyranoid ring of cellobiose shifted to the same direction as their counterparts at the other ring (e.g., C1 and C7, C5 and C11, etc.), indicating they had similar interactions with the solvent system during the dissolution process. For one pyranoid ring, the chemical shift at C1 and C5 moved deshielded while C2 and C3 moved shielded at lower cellobiose concentration. The


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chemical shift at C4 and C6 did not change significantly at different cellobiose concentrations. The deshielded shift at C1 and C5 could be due to more hydrogen bonds between the oxygen on the pyranoid ring and the hydrogen on the cation of TBAA at lower cellobiose concentration (i.e., higher TBAA amount in the system), which would subsequently decrease the electron densities at C1 and C5 adjacent to the oxygen. At the same time, more hydrogen bonds also formed between the hydroxyl hydrogen at C2 and C3 the oxygen on TBAA anion, therefore, increasing the electron densities at C2 and C3 and shifting them to upfield. Interaction between cellobiose and DMSO in the cellobiose/TBAA/DSMO mixtures The interaction between cellobiose and DMSO in the mixed solvent was studied by dissolving different amounts of cellobiose in pure DMSO and in a TBAA/DMSO mixed solvent. Figure 11 shows the ATR-FTIR spectrum of cellobiose in pure DMSO at different cellobiose concentrations. It can be seen that the C-S-C vibration at 953 cm-1 almost remained unchanged as the concentration of cellobiose increased, while the intensity of peak 1042cm-1, which was assigned to S=O stretching vibration, gradually decreased with the increase of cellobiose. Given the above fact, it can be deduced that, instead of being affected directly by the concentration of cellobiose, the decrease in the intensity of S=O stretching vibration should be attributed to the interactions between the S=O group and the hydroxyl group in cellobiose, which results in weakening the vibration of S=O and consequently reducing the peak intensity. 38 A similar trend was observed in the spectrum of cellobiose dissolved in the TBAA/DMSO mixed solvent at different concentrations (Figure 12). This result suggested the existence of interactions between cellobiose and DMSO in the mixed solvent, albeit in the presence of TBAA. In addition, we also investigated the interaction between cellulose and DMSO. As shown in Figure S5, the intensity of the S=O vibration peak at 1042 cm-1 gradually decreased with the increasing loading of MCC,


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which was consistent with that of cellobiose. However, the peak intensity did not change as much as that observed in the cellobiose/TBAA/DMSO system. This could be due to the fact that only part of the hydroxyl groups on cellulose can interact with DMSO, even at the complete dissolution of cellulose in the mixed solvent because dissolved cellulose exists mainly in the form of small aggregates and many hydroxyl groups were still in the interior of the aggregates and thus cannot be accessed by DMSO 32 Blank experiments were carried out by using a TBAA/H2O mixed solvent (WTBAA =0.05, 0.10, 0.15, 0.20, 0.25) for cellulose dissolution. MCC was added to each mixed solvent (0.5 wt%) and stirred at room temperature for 48h. The solutions remained turbid during the dissolution process. It can be seen from polarized microscopy that cellulose was poorly dissolved as shown in Figure S3. It has been reported that the addition of a protic polar solvent (e.g. water) to the ILs would preferentially solvated anions rather than cations, which would hinder the interaction of anions with cellulose, and thereby decrease the cellulose solubility.29 Therefore, we can deduced that DMSO, on the one hand, decreased the viscosity of ILs and promoted the dissociation of ion-paired ILs into free ions. On the other hand, it can interact with hydroxyl groups, possibly forming hydrogen bonds, on cellulose and thereby stabilize the dissolved cellulose chain from further reforming inter- and intramolecular hydrogen bonding.

CONCLUSIONS The dissolution of cellulose in a TBAA/DMSO mixed solvent was investigated. 15 wt% TBAA (WTBAA=0.15) in the mixed solvent was found to exhibit the best performance in the dissolution of cellulose, with a maximum of 8.17 w% MCC solubility. Different type of


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cellulosic materials with different DP were well dissolved in the mixed solvent. The viscosity and conductivity of the mixed solvent showed different trends as the ratio of TBAA in the mixed solvent varied, indicating the two different conditions of TBAA (i.e., “ion-aired” and “ionsplit”). WTBAA=0.15 was the turning point of the two conditions and the mixed solvent displayed the best dissolving ability at this ratio. This finding suggests that a balance between the ion concentration and ion mobility is crucial to the dissolving ability of a mixed solvent. The interaction between carboxylate anions of TBAA and the cellulose model compound cellobiose, forming strong hydrogen bonds between (anions) O…H-O (cellobiose), were found to be the key factor for the dissolving behaviour of cellulose in the mixed solvent. Meanwhile, DMSO not only reduced the viscosity of the mixed solvent but also interacted with cellobiose to stabilize the dissolved cellobiose molecule from reforming inter- and intra- hydrogen bonds. AUTHOR INFORMATION Corresponding Author Hui Pan (Email: [email protected]), Tel:(86) 25-85427233 Author Contributions ‡ These authors equally contributed to the work. All authors have given approval to the final version of the manuscript. Funding Sources The authors are grateful for the financial support by the forestry industry research special funds for public welfare projects (201504602) and the Jiangsu Specially-Appointed Professor program of the State Minister of Education of Jiangsu Province.


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Figure 1. Effect of WTBAA on the solubility of MCC.

Figure 2. Effect of WTBAA on the dissolution time of 4 wt% MCC.


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Figure 3. Dissolution time of different cellulosic materials (1 wt%) in the mixed solvent (WTBAA=0.15).


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Figure 4. Viscosity (a) and electrical conductivity (b) measurement of the TBAA/DMSO mixed solvent of WTBAA from 0.05 to 0.25.

Figure 5. Simulative ion-paired and ion-split stages of TBAA in DMSO.


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Figure 6. Structures of TBAA, DMSO and cellobiose

Figure 7. The 1H NMR spectra of TBAA in TBAA/DMSO-d6 solution.


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Figure 8. The 13C NMR chemical shifts of TBAA in TBAA/DMSO-d6 solution.


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Figure 9. The changes of

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C NMR chemical shifts of TBAA at different concentrations of

cellobiose in the TBAA/DMSO mixed solvent. Condition: WTBAA=0.15, 25oC. ∆δ5=δ5-δ0, ∆δ15=δ15-δ0, ∆δ25=δ25-δ0 where δ0, δ5, δ15 and δ25 were the chemical shifts of C atoms on TBAA at 0 wt%, 5 wt%, 15 wt% and 25wt% concentrations of cellobiose in TBAA/DMSO-d6 mixed solvent, respectively.


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Figure 10. The changes of

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C NMR chemical shifts of cellobiose in TBAA/DMSO-d6

solvent. Conditions: WTBAA=0.15. ∆δ=δ5-δ15, where δ5 and δ15 were the chemical shifts of each C atoms on cellobiose at 5 wt% and 15 wt% concentrations in TBAA/DMSO-d6, respectively.


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Figure 11. ATR-FTIR spectra of S=O stretching vibration of DMSO at different loading of cellobiose in the pure DMSO system.


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Figure 12. ATR-FTIR spectra of S=O stretching vibration of DMSO at different loading of cellobiose in TBAA/DMSO/cellobiose system (WTBAA=0.15).


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For Table of Contents Use Only

Room-temperature dissolution and mechanistic investigation of cellulose in a tetrabutylammonium acetate/dimethyl sulfoxide system

Yao-Bing Huang,‡ Ping-ping Xin,‡ An-Feng Liu, Xin-Cheng Zhou and Hui Pan *

Dissolution of cellulose were carried out in the tetrabutylammonium acetate/dimethyl sulfoxide mixed solvent at room temperature and the dissolving mechanism was also investigated.


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Room-Temperature Dissolution and Mechanistic Investigation of

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