Dissolution Mechanism of Cellulose in N,N-Dimethylacetamide

Jul 15, 2014 - National Research Center for Engineering Plastics, Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100...
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Dissolution Mechanism of Cellulose in N,N-Dimethylacetamide/ Lithium Chloride: Revisiting through Molecular Interactions Chao Zhang,†,‡ Ruigang Liu,*,† Junfeng Xiang,† Hongliang Kang,† Zhijing Liu,†,‡ and Yong Huang*,†,§ †

Sate Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Science, Beijing 100049, China § National Research Center for Engineering Plastics, Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Understanding the interactions between solvent molecules and cellulose at a molecular level is still not fully achieved in cellulose/N,N-dimethylacetamide (DMAc)/ LiCl system. In this paper, cellobiose was used as the model compound of cellulose to investigate the interactions in cellulose/DMAc/LiCl solution by using Fourier transform infrared spectroscopy (FTIR), 13C, 35Cl, and 7Li nuclear magnetic resonance (NMR) spectroscopy and conductivity measurements. It was found that when cellulose is dissolved in DMAc/LiCl cosolvent system, the hydroxyl protons of cellulose form strong hydrogen bonds with the Cl−, during which the intermolecular hydrogen bonding networks of cellulose is broken with simultaneous splitting of the Li+−Cl− ion pairs. Simultaneously, the Li+ cations are further solvated by free DMAc molecules, which accompany the hydrogen-bonded Cl− to meet electric balance. Thereafter, the cellulose chains are dispersed in molecular level in the solvent system to form homogeneous solution. This work clarifies the interactions in the cellulose/DMAc/LiCl solution at molecular level and the dissolution mechanism of cellulose in DMAc/LiCl, which is important for understanding the principle for selecting and designing new cellulose solvent systems.

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been widely used in the fields of cellulose analysis,16,17 shaping,18,19 and chemical modification.20−22 In the DMAc/LiCl system, LiCl was confirmed to remain in the Li+−Cl− ion pair form.23−25 Investigations on the cellulose/ DMAc/LiCl system have provided information about the interactions between the dissolved cellulose and DMAc/LiCl system. When cellulose was dissolved in DMAc/LiCl, the Cl− anions were observed to replace the OH···O hydrogen bonds between cellulose chains with the OH···Cl− hydrogen bonds.26 In the DMAc/LiCl solvent system, the Li+ cations associate tightly with the carbonyl oxygen of DMAc molecules to form a Li+(DMAc)x cationic complex, which has been investigated and elucidated by 13C NMR,27 infrared spectroscopy,28 X-ray crystallography,29 electrospray ionization mass spectrometry,30 and thermochemistry.31 Two typical dissolution mechanisms of cellulose in DMAc/LiCl have been proposed and both suggested the existance of Li+ cations in cellulose/DMAc/ LiCl solutions (Scheme 1). McCormick et al.26 hypothesized that the Li+(DMAc)x macro-cation complex associates with the Cl−, and there is no direct interaction between the Li+ and oxygen atoms in cellulose in cellulose/DMAc/LiCl solutions. Morgenstern et al.32 observed that the 7Li NMR chemical shift

INTRODUCTION Cellulose, which consists of β-(1 → 4)-linked glucose repeating units, is the most abundant renewable bioresource and the most promising alternative to petroleum-derived resources.1−7 The inter- and intramolecular hydrogen bond network in cellulose is strong,8,9 due to its chain stereoregularity and the abundant hydroxyl groups on cellulose chains. Therefore, cellulose cannot be melted or dissolved in common organic solvents, which limits the utilizations of cellulose. The key issue for successful dissolving cellulose is to break down the robust hydrogen bond network and hereafter to stabilize cellulose chains in the solution. Breakage of the hydrogen bond network of cellulose can be achieved either by derivation (e.g., in the viscose process) or dissolution of cellulose in direct solvents such as N-methylmorpholine-N-oxide (NMMO) monohydrate or ionic liquids (ILs).10−14 The traditional commercialized viscose process10 for producing regenerated cellulose products is a derivation approach, which is still the dominating method at present. However, the use of toxic CS2 and the byproducts in the viscose process lead to serious environmental pollution. Therefore, efforts on finding environmental friendly solvents for dissolving cellulose have attracted increasing attention. N,Ndimethylacetamide/lithium chloride (DMAc/LiCl) is a classical and direct solvent system for cellulose and plays a significant role in the history of cellulose science and technology.15 It has © 2014 American Chemical Society

Received: June 17, 2014 Revised: July 15, 2014 Published: July 15, 2014 9507

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V were prepared by dissolving different weights of cellobiose, DCOA, and EC in DMAc/LiCl (mole ratio of 1:0.088), respectively. Sample VI was prepared by dissolving EC in DMAc with the molar ratio of cellobiose units of EC (COEC) to DMAc of 0.016:1. Sample VII was prepared by dissolving EC in DMAc/LiCl, where the molar ratio of COEC:LiCl:DMAc is 0.016:0.176:1. Sample series I−VII were prepared at 60 °C and then cooled to room temperature after dissolution. Sample series VIII and IX were prepared by dissolving different weights of cellobiose in Mili-Q water or DMSO, respectively, at room temperature. ATR-FTIR Experiments. ATR-FTIR experiments were carried out on a Bruker Tensor 27 FTIR spectrometer equipped with a Specac Golden Gate ATR attachment. The ATR attachment contains a single-bounce ZnSe crystal. All spectra were collected at a 2 cm−1 resolution with 128 scans over a window from 1000 to 4000 cm−1. NMR Experiments. The 13C NMR spectra of sample series I, II, VI, and VII were acquired on a Bruker AV 600 spectrometer with 128 scans for 13C NMR measurements at room temperature. All samples were placed in a 5 mm NMR tube with a coaxial capillary insert containing CDCl3 to provide a field-frequency lock and NMR external standard. The 35Cl NMR spectra of sample series III and IV were acquired on a Bruker AV III 500 spectrometer with 128 scans for 35Cl NMR measurements at room temperature. All samples were placed in 5 mm NMR tube with a coaxial capillary insert containing 1 M KCl in D2O to provide a field-frequency lock and NMR external standard. The 7Li NMR spectra of sample series IV and V were acquired on a Bruker AV 600 spectrometer with 128 scans for 7 Li NMR measurements at room temperature. All samples were placed in a 5 mm NMR tube with a coaxial capillary insert containing 2.6 M LiCl in D2O to provide a field-frequency lock and NMR external standard. Conductivity Measurements. The conductivities of sample series II, VIII, and IX were measured on a DDS-307A Conductivity Meter (Shanghai INESA Scientific Instrument Co., Ltd., China). Each data point was measured after equilibrating the samples at 25 °C for 1 h.

Scheme 1. Dissolution Mechanism of Cellulose in DMAc/ LiCl Proposed by (a) McCormick et al.26 and (b) Morgenstern et al.32

is a strong function of cellulose concentration, which confirmed that there are close interactions between the Li+ cations and cellulose chains. By these results, they suggested that one DMAc molecule in the inner coordination sphere of Li+ is replaced by one cellulose hydroxyl group when dissolving cellulose in DMAc/LiCl.32 All-atom molecular dynamics simulations also indicated that the preferential interaction of Li+ with glucan chains in DMAc drives cellulose dissolution.33 However, previous work mainly focused on the distinctions of ions before and after dissolving cellulose in DMAc/LiCl, few work paid attention to the variation of DMAc molecules in this system. Elucidation of molecular interactions and the disruption of the robust hydrogen bonds network of cellulose in DMAc/LiCl system can reveal the mechanism of cellulose dissolution in this system. Furthermore, the resulting knowledge can contribute to the development of new cellulose solvents and pretreatment technologies for biomass conversion. Cellobiose, the repeating unit of cellulose, has been extensively used as the model compound of cellulose to study the interactions between cellulose and small molecules. 34−37 In this work, the interactions in the system were revisited by investigating the variation of DMAc molecules by using attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR), 13 C, 35 Cl, and 7 Li NMR spectroscopies and conductivity. It is found that when cellobiose is dissolved in DMAc/LiCl, hydrogen bonds are formed between the Cl− anions and the hydroxyl protons of cellobiose and the Li+ cations, solvated by DMAc molecules in DMAc/LiCl, are further solvated by additional DMAc molecules, by which the Li+−Cl− ion pairs are split. This work clarifies the dissolution mechanism of cellulose in DMAc/LiCl.

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RESULTS AND DISCUSSION The interactions in the cellulose/DMAc/LiCl system were investigated according to the following order. First, the solvation of lithium cations was investigated by 13C NMR and FTIR, which confirmed the strong association of the Li+ cations with DMAc molecules. Second, the hydrogen bond between the hydroxyl protons in cellobiose and the Cl− anions was characterized by 35Cl NMR. It can be concluded that all the hydroxyl protons of cellobiose formed hydrogen bonds with the Cl− anions rather than with DMAc molecules in the DMAc/ LiCl system in the case of an excess amount of LiCl. The solvation of the Li+ cations in cellobiose/DMAc/LiCl system was then investigated by 7Li NMR, 13C NMR, and FTIR. When the hydroxyl protons of cellobiose form strong hydrogen bonds with Cl−, the Li+−Cl− ion pairs split and the Li+ cations get further solvated by DMAc. Finally, the splitting of the Li+−Cl− ion pairs was confirmed by using conductivity measurements. Solvation of Lithium Cations by DMAc. The 13C chemical shift of the carbonyl carbon (Table 1) in DMAc continuously moves from 170.10 ppm for pure DMAc to 171.55 ppm for the solution of molar ratio of LiCl:DMAc equaling to 0.197:1. Precipitate appears in the solution with the

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EXPERIMENTAL SECTION Materials. N,N-Dimethylacetamide (DMAc, AR) and dimethyl sulfoxide (DMSO, AR) were purchased from Sinopharm Chemical Regent Beijing and dried over 4 Å molecular sieves before use. Anhydrous lithium chloride (LiCl, 99%), D-(+)-cellobiose (98+%), and D-cellobiose octaacetate (DCOA) were purchased from Alfa Aesar and dried in vacuum at 60 °C before use. Ethyl cellulose (EC, 48.0−49.5 wt % of ethoxyl group, the degree of substitution is 2.54) was purchased from Fluka and also dried in the vacuum at 60 °C before used. Sample Preparation. Sample series I were prepared by mixing different amounts of LiCl into DMAc. Sample series II were prepared by dissolving a different weight of cellobiose in DMAc/LiCl (mole ratio of 1:0.176). Sample series III, IV, and 9508

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Table 1. 13C Chemical Shift of the Carbonyl Group at Different Molar Ratio of LiCl to DMAc in DMAc/LiCl System [LiCl ]:[DMAc]

δ 13C (−CO)

0:1 0.022 0.044 0.066 0.088 0.109 0.132 0.154 0.176 0.197 0.219

170.10 170.34 170.55 170.71 170.99 171.03 171.17 171.32 171.46 171.55 171.54

molar ratio of LiCl:DMAc exceeding 0.197:1 when the solution is cooled down to room temperature from 60 °C. So the 13C chemical shift of the carbonyl carbon shows almost no change when the molar ratio of LiCl:DMAc equals 0.219:1 (Table 1). The bounding of the Li+ cations to carbonyl oxygen of DMAc leads to the decrease in electron density on the carbonyl carbon, which causes the deshielding of the carbonyl carbon and results in the downfield shift of the chemical shift of the carbonyl carbon. Figure 1 shows the stretch vibration of the carbonyl group in DMAc molecules in the DMAc/LiCl solutions with different content of LiCl. The results indicate that the absorbance band of the carbonyl groups gradually red shifts with the increase in the content of LiCl in the DMAc/ LiCl solution, until the molar ratio of LiCl:DMAc is above 0.154:1. The red shift of the stretch vibration of the carbonyl group results from the interaction between Li+ and the carbonyl oxygen of DMAc. When the molar ratio of LiCl:DMAc is higher than 0.154:1, most of the DMAc molecules associate with the Li+ cations. As the concentration of lithium further increases, the residual free DMAc molecules continuously bind to the Li+ cations. However, the number of the residual free DMAc molecules is too small to cause further red-shift of the carbonyl stretch vibration when they bind to the Li+ cations. Both FTIR and 13C NMR results confirm the strong association of the Li+ cations with DMAc molecules, which is similar to the literature results.27−31 Hydrogen Bonds between Hydroxyl Protons in Cellobiose and Cl−. Cellulose is generally needed to be activated by DMAc before the dissolution in DMAc/LiCl, which inevitably amalgamates DMAc into cellulose and makes it hard for us to ascertain the accurate quantity of cellulose in the sample preparation for the investigations. Therefore, cellobiose, the repeating unit of cellulose, was used as the model compound of cellulose. Figure 2 shows the 35Cl NMR spectra of cellobiose and D-cellobiose octaacetate (DCOA) solutions in DMAc/LiCl. In the cellobiose/DMAc/LiCl solution, the peak of 35Cl becomes wider and wider with the increase of the molar ratio of hydroxyl groups on cellobiose to LiCl ([−OH]:[LiCl]) and totally disappears at the [−OH]: [LiCl] around 1 (Figure 2a). It is known that the 35Cl nucleus possesses an electric quadrupole moment, which interacts with the electric field gradient in the nucleus. The peak width of the 35 Cl NMR spectra is mainly affected by the quadrupolar interaction. When hydroxyl protons of cellobiose form hydrogen bonds with the Cl− anions, the electron cloud of the Cl− is polarized and the magnitude of the electronic field

Figure 1. (a) FTIR spectra and (b) peak position of carbonyl stretch vibration band as a function of the molar ratio of LiCl:DMAc.

gradient becomes larger compared that of the free ions.35,38 As a result, the spin−spin nuclear relaxation rate increases and the peak becomes wider and wider. Because of a rapid exchange between the free and the hydrogen-bonded Cl− anions, the peak width of the observed 35Cl signal is the weighted sum of that corresponding to the free Cl− anions and that interacting with cellobiose.35,38 In order to confirm the interactions between Cl− and hydroxyl groups of cellobiose in the system, the 35Cl NMR spectra of DCOA/DMAc/LiCl solutions with the different content of DCOA were also investigated and the results are shown in Figure 2b. DCOA is a derivative of cellobiose, in which all the hydroxyl groups are substituted by acetate groups. The results indicate that the content of DCOA has no effect on the 35Cl NMR spectra, which is because there is no hydroxyl group in DCOA and the status of the Cl− anions keep unchanged with the increase in DCOA content in the DCOA/DMAc/LiCl solutions. The above results confirmed that the Cl− ions can only associate with the hydroxyl protons of cellobiose through the hydrogen bonds in cellobiose/ DMAc/LiCl solutions. The stoichiometric characteristics of the hydroxyl groups with LiCl have been investigated in other sugar/DMAc/LiCl 9509

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Figure 2. 35Cl NMR spectra of the solution of (a) cellobiose and (b) cellobiose octaacetate dissolving in DMAc/LiCl. The molar ratio of LiCl:DMAc is constant at 0.088:1. The ratios in (a) are the molar ratio of hydroxyl groups of cellobiose to LiCl. The different concentrations connected by dashed lines are at the same mass fraction of the solute to DMAc weight.

Figure 3. FTIR spectra of the stretch vibration of carbonyl group of DMAc at different molar ratios of hydroxyl group of cellobiose:LiCl in the DMAc/LiCl system with the molar ratio of LiCl:DMAc is 0.176:1. The concentration of LiCl is chosen to exclude the influence of a large amount of free DMAc molecules (the DMAc molecules that are not associated with Li+ ion).

solutions, such as methyl-β-D-glucopyranoside39 and N-acetylD-glucosamine,40 by using 1H NMR. In both cases, the 1H NMR signals of the hydroxyl protons of the sugars were strongly shifted to low field until [−OH]:[LiCl] equals one.39,40 The solvation of glucose and cellobiose by the ionic liquid 1-n-butyl-3-methylimidazolium chloride has also been found involving hydrogen bonding between the carbohydrate hydroxyl protons and the ionic liquid chloride ions in a 1:1 stoichiometry.35 The association constant between the hydroxyl protons and the Cl− anions is quite large and almost all the hydroxyl groups of saccharide form strong hydrogen bonds with Cl− anions in the DMAc/LiCl system when the LiCl is in excess. In the present work, 35Cl NMR results confirm the stoichiometry of [−OH]:[LiCl] in cellobiose/DMAc/LiCl solution. One hydroxyl proton only can form one hydrogen bond with one Cl− anion as the hydrogen bond donor.41 In addition, cellobiose cannot be dissolved in pure DMAc. Therefore, it is reasonable to conclude that all the hydroxyl protons of cellobiose formed hydrogen bonds with the Cl− anions rather than with DMAc molecules in the DMAc/LiCl system in the case of excess amount of LiCl. Solvation of Li+ Cations by DMAc in Cellobiose/ DMAc/LiCl System. A different weight ratio of cellobiose was dissolved in DMAc/LiCl, in which the molar ratio of LiCl:DMAc was 0.176:1. The influence of cellobiose on the DMAc molecules was investigated by ATR-FTIR and 13C NMR. Figure 3 shows the FTIR spectra of cellobiose/DMAc/ LiCl solutions with different contents of cellobiose. The results indicate that the band of stretch vibration of carbonyl group slightly shifts to the red region with the increase in molar ratio of [−OH]:[LiCl]. Table 2 lists the 13C chemical shift of the carbonyl groups of DMAc in cellobiose/DMAc/LiCl solutions with different content of cellobiose. It can be seen that the 13C chemical shift of the carbonyl group moves to downfield with the increase of [−OH]:[LiCl]. Two different interactions are proposed to relate these shifts, namely a direct and an indirect interaction. The direct interaction means the dissolved cellobiose interacts with the DMAc molecules directly, while the indirect interaction represents that the dissolved cellobiose first affects the dissolved ions in the DMAc/LiCl cosolvent

Table 2. Chemical Shift of DMAc Carbonyl Carbon Atoms in 13C NMR Spectra at Different Molar Ratio of Hydroxyl Group to LiCl in Cellulose/DMAc/LiCl Systema

a

[−OH ]:[LiCl]

δ 13C (−CO)

0:1 0.50:1 0.99:1 1.48:1 1.98:1

171.46 171.47 171.53 171.55 171.58

The molar ratio of LiCl:DMAc is constant at 0.176:1.

system and then influences the DMAc molecules due to the interaction between the dissolved ions and DMAc molecules. As discussed above, the hydroxyl protons of cellobiose form strong hydrogen bonds with the Cl− anions rather than with the carbonyl groups of DMAc molecules. When [−OH]:[LiCl] is less than one, all the hydroxyl protons form strong hydrogen bonds with the Cl− anions and cannot interact with the DMAc molecules. Therefore, the red shift of the stretch vibration of carbonyl groups in FTIR spectra and the downfield shift of carbonyl carbon in 13C NMR spectra could not be attributed to the influence of hydroxyl protons. However, when the molar ratio of [−OH]:[LiCl] is higher than one, hydrogen bonds could be formed between the free hydroxyl groups of cellulose and the carbonyl groups of DMAc molecules, which can also cause the red shift of the carbonyl stretch vibration and the downfield shift of the 13C chemical shift of the carbonyl carbon. The other structure units of cellulose except for the hydroxyl protons may interact with DMAc molecules. In order to clarify the origin of these shifts, ethyl cellulose (EC) with the degree of substitution of ethyl groups of 2.54 was dissolved in pure DMAc and DMAc/LiCl cosolvent system. Figure 4 shows the FTIR spectra of DMAc, EC/DMAc, DMAc/LiCl, and EC/ DMAc/LiCl. The results indicate that the stretch vibration 9510

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dissolved in DMAc/LiCl at different concentrations. These shifts could be attributed to the further solvation of Li+ cations by DMAc molecules after dissolving cellobiose in the system. The chemical shift of Li+ nucleus in 7Li NMR spectra as a function of molar ratio of molar cellobiose to LiCl (ncellobiose:nLiCl) in the cellobiose/DMAc/LiCl solutions is shown in Figure 5a. The molar ratio of DMAc:LiCl was fixed

Figure 4. Stretch vibration of carbonyl group in FTIR spectra of pure DMAc, solution with the molar ratio of COEC:DMAc being 0.016:1, solution with the molar ratio of LiCl:DMAc being 0.176:1, and solution with the molar ratio of COEC:LiCl:DMAc being 0.016:0.176:1.

Figure 5. Variation of 7Li chemical shift in DMAc/LiCl (with the molar ratio of DMAc:LiCl being 1:0.088) as a function of the molar ratio of (a) cellobiose/LiCl, (b) GREC/LiCl, and (c)DCOA/LiCl.

band of DMAc carbonyl groups at 1635 cm−1 blue shifts slightly to 1636 cm−1 by dissolving ethyl cellulose in DMAc. The molar ratio cellobiose unit of EC (COEC) to DMAc is 0.016:1. In the DMAc/LiCl cosolvent system with the molar ratio of LiCl:DMAc of 0.176:1, the stretch vibration band of DMAc carbonyl groups shift to 1625 cm−1 (Figure 4), which confirms the interaction between DMAc and LiCl. The addition of EC in DMAc/LiCl, with the molar ratio of COEC: LiCl:DMAc of 0.016:0.176:1, has no effect on the stretch vibration of DMAc carbonyl groups (Figure 4). The 13C chemical shift of DMAc carbonyl groups is listed in Table 3.

at 1:0.088 in all solutions. The results show that the 7Li chemical shift moved to upfield with the increasing cellobiose concentration. Morgenstern et al.32 also observed that the chemical shift of 7Li decreased with the increasing cellulose concentration in the cellulose/DMAc/LiCl system, but they assigned the chemical shift to the contribution of the interaction between the Li+ cations and the oxygen atoms of cellulose. For the purpose to clarify the interactions that involved the Li+ in the system, the chemical shift of 7Li by dissolving EC and D-cellobiose octaacetate (DCOA) in DMAc/LiCl with the molar ratio of DMAc:LiCl of 1:0.088 were also investigated. It was found that the changes of the chemical shift of the 7Li NMR signal with the increase in the concentration of EC and DCOA in the DMAc/LiCl system is far less than that in the cellobiose/DMAc/LiCl solutions (Figure 5, panels b and c). In the DOCA/DMAc/LiCl solutions, no obvious change can be observed with the increase in the DOCA concentration. The minor upfield shift of the 7Li NMR signal with the increase in DOCA concentration is attributed to the change of the bulk magnetic susceptibility of the solution and weak dispersion interaction between Dcellobiose octaacetate and Li+ cations.42 As for EC/DMAc/LiCl solutions, except for the change of the bulk magnetic susceptibility of the solution and weak dispersion interaction, the remaining hydroxyl groups (about 0.92 hydroxyl group per cellobiose unit) can still cause the upfield shift in the 7Li NMR signal. Therefore, it can be concluded that the upfield shift of the 7Li NMR signal is strongly correlated with the hydroxyl protons of cellobiose. In consideration of all the above results, the mechanisms of the variation of DMAc and Li+ cations can be depicted as follows. When cellulose is dissolved in the DMAc/LiCl cosolvent system, hydrogen bonds can be formed between the hydroxyl protons and the Cl− anions, the Li+−Cl− ion pairs,23−25 are thereafter split and the Li+ cations are further solvated by DMAc, which led to the red shift of carbonyl stretch vibration and upfield shift of the 7Li NMR signal and downfield shift of the carbonyl carbon 13C NMR signal.

Table 3. Chemical Shift of DMAc Carbonyl Carbon Atoms in 13C NMR Spectra under Different Conditions sample

δ 13C (−CO) (ppm)

DMAc DMAc/LiCla EC/DMAcb cellobiose/DMA/LiClc EC/DMAc/LiCld

170.10 171.46 170.05 171.53 171.44

a

nDMAc:nLiCl = 1:0.176. bnCOEC:nDMAc = 0.016:1. cncellobiose:nDMAc:nLiCl = 0.022:1:0.176. dnCOEC:nDMAc:nLiCl = 0.016:1:0.176.

The results show that the 13C chemical shift of the carbonyl carbon moves upfield after dissolving EC in pure DMAc or in DMAc/LiCl with the molar ratio of LiCl:DMAc of 0.176:1. The above results confirmed that the red shift of the stretch vibration band (Figure 3) and the downfield shift of 13C chemical shift of DMAc carbonyl groups (Table 2) cannot be ascribed to the direct interaction between the dissolved cellobiose and the DMAc molecules. As mentioned above, the association of Li+ cations with the DMAc molecules led to the red shift of the stretch vibration band of the carbonyl group in the FTIR spectra and the downfield shift of carbonyl carbon of DMAc molecules in 13C NMR spectra. The same trend arose when cellobiose was 9511

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cosolvent system can be depicted as that shown in Figure 7. Li+ and Cl− form ion pairs in DMAc/LiCl system.23−25 When cellulose is dissolved in the DMAc/LiCl cosolvent system, the hydroxyl protons of cellulose will form strong hydrogen bonds with the Cl−, during which the intermolecular hydrogenbonding networks of cellulose is broken with simultaneously splitting the Li+−Cl− ion pairs. Then, the Li+ cations get further solvated by free DMAc molecules, and the further solvated Li+ cations accompany the hydrogen-bonded Cl− to meet the electric balance. Thereafter, cellulose chains were dispersed in the molecular level in the solvent system to form homogeneous solution. It should be noted that the results in this paper cannot provide the information on the time scale of the different steps of the dissolution of cellulose in DMAc/LiCl. Such work may be possibly carried out in the future, depending on the new experimental techniques and methods that can be introduced in this field.

The depiction was confirmed by the conductivity measurements of the DMAc/LiCl solution with dissolution of cellobiose (Figure 6). The results show that the conductivity

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CONCLUSIONS Cellobiose was used as a model compound of cellulose to study the interactions in the cellulose/DMAc/LiCl system. ATRFTIR, 13C, 35Cl, and 7Li NMR and conductivity results confirmed that the Li+−Cl− ion pairs in the DMAc/LiCl system are split when cellobiose is dissolved in the DMAc/LiCl cosolvent system and results in the increase in the conductivity of cellobiose/DMAc/LiCl solutions. The Cl− anions form hydrogen bonds with the hydroxyl protons of cellobiose, and there is no obvious interaction between the hydroxyl protons of cellobise and the carbonyl groups in DMAc molecules. At the same time, the Li+ cations, solvated by DMAc, are further solvated by the additional DMAc molecules, causing the red shift of the stretch vibration band of the carbonyl group of DMAc in FTIR spectra and the downfield of the 13C chemical shift of the carbonyl carbon in DMAc molecules. On the basis of the experimental results, the dissolution mechanism of cellulose in DMAc/LiCl is depicted as follows. The hydroxyl protons of cellulose can form strong hydrogen bonds with the Cl− anions, during which the intermolecular hydrogen bonding networks of cellulose is broken with simultaneous splitting of the Li+−Cl− ion pairs. Then, the Li+ cations get further solvated by free DMAc molecules and accompany the hydrogen-bonded Cl− anions to meet the electric balance. Thereafter, cellulose chains can disperse in molecular level in the solvent system to form homogeneous solution. This work clarifies the nature of the dissolution of cellulose in the DMAc/LiCl cosolvent, which is important for understanding the principle for selecting and designing new cellulose solvent systems.

Figure 6. Conductivity variation of the solutions with the increase of cellobiose concentration, respectively. (The conductivity variation is relative to the solution without cellulose, respectively.)

of cellobiose/DMAc/LiCl solutions increases with the increasing cellobiose concentration. Generally, the increase in the conductivity could be attributed to two reasons. One is the direct effect by the dissolving cellobiose. The other is that the dissolution of cellobiose can affect the ions in the system and lead the increase in ionic conductivity. For the clarification of the above-mentioned reasons for cellobiose/DMAc/LiCl solutions, the conductivity of the cellobiose/H2O and cellobiose/DMSO solutions were also measured. The results show that there is almost no change in the conductivity of the cellobiose/H2O and cellobiose/DMSO solutions with the increase in the cellobiose concentration. Generally, the ionic conductivity of the solution is determined by the product of concentration and the mobility of free ions.43 When cellobiose is dissolved in the DMAc/LiCl system, the mobility of free ions is generally decreased.37 Therefore, the increase in the conductivity of the cellobiose/DMAc/LiCl solutions with the increase in cellobiose concentration must be attributed to the increase in the dissociation constant of both Li+−Cl− ion pairs and the concentration of free ions. With consideration of the above results and discussion, the mechanism for the dissolution of cellulose in the DMAc/LiCl

Figure 7. Schematic of the interaction among Li+ cation, Cl− anion, and DMAc when cellulose dissolves into the DMAc/LiCl system. 9512

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ASSOCIATED CONTENT

S Supporting Information *

The complete author list of refs 8 and 13. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel/Fax: +86-10-82618573. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from National Natural Science Foundation of China (Grants 21274154 and 51373191) is gratefully acknowledged.

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