NMR Study on the roles of Li+ in cellulose dissolution process - ACS

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NMR Study on the roles of Li+ in cellulose dissolution process Hailong Huang, Hao Ge, Jianhui Song, Yefeng Yao, Qun Chen, and Min Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04177 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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ACS Sustainable Chemistry & Engineering

NMR Study on the roles of Li+ in cellulose dissolution process Hailong Huang, Hao Ge, Jianhui Song, Yefeng Yao, Qun Chen, Min Xu* * Corresponding author: [email protected]

School of Physics and Materials Science & Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, No. 3663 North Zhongshan Road, Shanghai 200062, P R China ABSTRACT LiOH/urea aqueous solvent is a very efficient system to dissolve cellulose. However, there is still no mature theory to explain the roles of the Li+ ions to improve the dissolution ability of cellulose in the LiOH/urea solution. In this article, the existing forms of Li+ and the interactions of the components in LiOH/urea/cellulose system were investigated by solid-state NMR techniques. It was observed that as many as four types of Li+ existing in the LiOH/urea/cellulose system. Some direct evidences from the H-X(13C,6Li) 2D NMR experiments were provided to make clear the roles of Li+ ions in the dissolution of cellulose in LiOH/urea system and a 3D interaction model was proposed. The interactions among the components in the system were discussed on the molecular level. KEYWORDS: Cellulose, LiOH/urea solvent, Solid-state NMR, 2D LGHETCOR, Roles of Li+ INTRODUCTION Cellulose is the most promising natural material on the earth because of its abundant reserves, reproducibility and environmental friendliness. However, the strong and abundant intra- and intermolecular hydrogen bonds result in insolubility in water or several common organic solvents. Nowadays the viscose process is still the most widespread and dominated industrial process, which is very harmful to the

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environment and human health. In the past few decades, more environmental friendly and high-efficient solvent systems have been discovered including LiCl/DMAc1, N-Methylmorpholine-N-Oxide/water (NMMO/H2O)2, ionic liquids (ILs)3,4 and alkali/urea(thiourea) aqueous solutions5-7. The most interesting method is the alkali/urea(thiourea) aqueous solutions developed by Zhang’s research group. Alkali/urea(thiourea) aqueous solution was precooled to low temperature. Cellulose can be dissolved rapidly in the precooled NaOH/urea or LiOH/urea aqueous solvent system

(-5

to

-13.5

℃ ,

depends

on

different

composition)

on

vigorous stirring condition8,9. Many characterization methods have been applied to investigate the dissolution mechanism in alkali/urea system. Some valuable and exciting results were presented10. Combining WXRD, DLS and TEM experiment, Cai suggested an inclusion complexes (ICs) structure model which was the best recognized model to describe the dissolution process in NaOH(LiOH)/urea system up to now

11,12.

But

the roles and interaction of each component inside the ICs is still elusive. From the differential scanning calorimetry (DSC) method, Egal proposed that there was no direct interaction between urea and cellulose, but urea played a role of binding water to make cellulose-NaOH links more stable13. Liu clarified the interactions in the cellulose solution and a 3D structure of the cellulose chain in the cellulose/NaOH/urea aqueous solution through liquid NMR experiments14,15. It’s interesting that the ability of LiOH/urea aqueous solution to dissolve cellulose is the strongest in all alkali/urea solvents5. Xiong16 clarified the OH- anion played an important role in cellulose dissolution; while cations maintained the stability of the solution, which decided the dissolution ability of alkali solutions. Compared with Na+ and K+, the Li+ hydration ions stabilized cellulose. Wang17-19 found that the electrostatic force between OH- and cellulose dominated the inter-


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molecular interactions and the direct interactions existed between the cations and cellulose, and Li+ could penetrate closer to cellulose via ination of MD simulation, DSC and NMR diffusometry (PFG-SE NMR). Brendle20 provided the model of cellulose co-ordination in lithium-containing molten salt hydrate and explained the changes in the chemical shift of Li+ in the system These researches proved that the Li+ played an important role in the solution system. However, they could not explained how many kinds of Li+ were in the system and which kind of Li+ could form the complex with cellulose. Due to the fast ion movement rate and the dynamic equilibrium of cellulose dissolution, the direct evidence to indicate the interactions of lithium ions is still deficient. It could be difficult to distinguish clearly in liquid system21-23. The aim of this present work is to study the roles of Li+ in the dissolution of cellulose in LiOH/urea solvent system and the interactions between Li+ and other solvent components on the molecule level. All the solution samples were frozen within very short time by liquid nitrogen and then freeze-dried in order to maintain the same structure as the solutions. Due to the sensibility of 6Li to the coordination environment, solid-state NMR can be used as powerful tool for the investigation of the existence forms of Li+ ions24-26. In this paper, solid-state NMR experiments including

13C

heteronuclear

CP/MAS NMR, 6Li CP/MAS NMR, 6Li single-pulse NMR, 2D 1H-13C

and

1H-6Li

correlation NMR (LGHETCOR)27,28 were

performed. In addition, X-Ray Diffraction (WXRD) was adopted to help to confirm the ICs’ structure. Based on the results of these experiments, the roles of Li+ in the LiOH/urea/cellulose

system

and

a

possible

3D

interaction

model

of

LiOH/urea/cellulose system were proposed.


3


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EXPERIMENTS Materials Whatman CF-11 cellulose (Mn=8×104) used in all experiments was kindly provided by Hubei Chemical Fiber Co, Ltd. LiOH·H2O and urea were of analytical grade (Sinopharm Chemical Reagent Co., Ltd.) and used without further purification. Preparation of samples Cellulose (4g) was dissolved in the precooled (-13.5 ℃) 4.6wt% LiOH/ 15wt% urea/ 80.4wt% aqueous solution (100g) with stirring to obtain a transparent cellulose solution. Cellulose (4g) was swelled in 4.6wt% LiOH aqueous solution (100g) by the same method, the cellulose/LiOH solution was semitransparent and unstable compared with cellulose/LiOH/urea solution. The other prepared samples were 4.6wt% LiOH aqueous solution, 15wt% urea solution and 4.6wt% LiOH/ 15wt% urea aqueous solution. All the solutions were frozen rapidly by liquid nitrogen, and then were freeze-dried at -60 ℃ for 30 h. After freeze-drying, the solid samples were stored in a vacuum desiccator for the NMR Experiments. Solid-State NMR Experiments All of the

13C

and 6Li solid-state NMR experiments were performed on the

Bruker AVANCE III 600 WB spectrometer operating at 600.13, 150.96, and 88.36 MHz for 1H, 13C, and 6Li respectively at 25 ℃. A 4 mm triple resonance MAS probe was used for the experiments. The spin rate was 10 kHz in all the 13C and the 6Li NMR experiments. Solid-state NMR experiments including 13C CP/MAS NMR, 6Li CP/MAS NMR, 6Li single-pulse NMR, 2D heteronuclear 1H-13C and 1H-6Li


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correlation NMR (LGHETCOR) were performed. The 6Li and 13C chemical shifts were calibrated using LiCl aqueous solution (1 mol·L-1, δ= -1.06 ppm) and adamantane (δ= 38.56 ppm), respectively29. Figure 1 showed the pulse program for 2D 1H-X(6Li,13C) heteronuclear dipolar correlation

spectroscopy

with

Lee-Goldburg

homonuclear

decoupling

(LGHETCOR). The protons evolve during t1 under suppression of the 1H homonuclear dipolar interactions with the FSLG30. LG-CP process was used to establish selective heteronuclear polarization transfer during a Cross-Polarization mixing time, while a ramped CP spin-lock pulse on the 13C or 6Li nuclei was applied to broaden the CP matching profile at high MAS rates (10k). The CP mixing time in the 2D correlation spectroscopy were 200 us and 1000 us in order to compare the short- and long-distance interaction between 1H and X(6Li,13C). WXRD experiments WXRD measurements were performed on Bruker D8 ADVANCE with Cu Ka(1.5406Å) radiation (40 kV, 40 mA). All the samples were placed on the same sample holder and scanned from 2θ=10°-45° at a speed of 10°min-1 at room temperature. RESULTS AND DISCUSSIONS Effect of Urea and LiOH from WXRD measures Figure 2 shows the 1D WXRD intensity of freeze-dried samples of pure LiOH, urea, LiOH/urea, LiOH/cellulose and LiOH/urea/cellulose solutions. The major peaks at 2θ of 22.4°, 24.7°, 29.4°, 31.8°, 35.6°, 37.2°, 40.6° and 41.8° are assigned to hexagonal crystals of urea, which can be found in freeze-dried samples of urea,


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LiOH/urea and LiOH/urea/cellulose solutions. The major peaks at 2θ of 20.4°, 32.6°and 35.8° are assigned to characteristic signals of LiOH, which can be obviously found in freeze-dried samples of LiOH and LiOH/cellulose solutions. Interestingly, in the freeze-dried sample of LiOH/urea, some new peaks at 2θ of 30.4° and 33.9° can be assigned to LiOH-urea complex probably. All the new peaks also can be found in the freeze-dried samples of LiOH/urea/cellulose.. Depending on the WXRD results and some other methods, Cai suggested that urea existed as inclusion complexes (ICs), whereas the complex associated with cellulose and LiOH was included in the ICs, leading to a urea shield for cellulose and LiOH12. It can make the cellulose dissolve in the solution thoroughly and keep stably. However, the ICs model only illustrates the role of urea in the dissolution system. It does not explain how the Li+ ions function in the ICs structure. So we try to use solid NMR to study the function of Li+ ions. The existing forms of Li+ in different systems As 6Li is very sensitive to the chemical environment, 6Li NMR analysis is a very useful tool for investigating the LiOH/urea/cellulose system. Figure 3 shows the 1H-6Li CP/MAS and 6Li quantitative single-pulse NMR spectra of the freezedried samples. In the 6Li single-pulse spectrum (Figure-3i) of LiOH, a main 6Li signal(δ=0.25 ppm) and a tiny 6Li signal(δ=-1.01 ppm) can be observed, while only the signal (δ=0.25 ppm) can be observed in the 1H-6Li CP/MAS spectrum(Figure3a) of LiOH. According to the 1H-6Li cross-polarization(CP) process, the signal (δ=0.25 ppm) is assigned to the Li+ ions closed to the 1H(OH-)，in this paper, we name it Li+-1 (Li+-OH-). The signal (δ=-1.01 ppm) is assigned to the Li+ ions away from 1H，in this paper, we name it Li+-4 (free Li+). From signal-pulse spectra above, the Li+-1 (Li+-OH-) signal and Li+-4 (free Li+) signal can also be observed in all the


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samples. Thus, it can be indicated that Li+-1 (Li+-OH-) and Li+-4 (free Li+) still exist in these solutions when the urea or cellulose was introduced into the LiOH solution. It is interesting that the amount of Li+-4 in LiOH/urea sample is much larger than pure LiOH sample. This is probably due to the interaction between urea and OH-, which makes more Li+ far away from OH-. Except the LiOH sample, more peaks can be observed in the other three samples, which mean more existing forms of Li+ existing in the mixed aqueous solution systems. The 6Li signal (δ= -0.23 ppm) can be named as Li+-2 and the 6Li signal (δ= -0.89 ppm) can be named as Li+-3. The two signals (Li+-2 and Li+-3) are due to the influence of urea or cellulose. The amounts of signals 1, 2, 3 and 4 for the quantitative experiments were shown in table 1. In order to make clear the attributions of these signals and the interactions of the components in the complex systems, more NMR experiments were performed. Intermolecular Interaction Revealed by 2D 1H-X(6Li,13C) correlation NMR experiments Advanced

2D

heteronuclear

1H-X

(6Li,13C)

correlation

(HETCOR)

experiments with Lee-Goldburg homonuclear decoupling can provide high resolution in the hydrogen dimensionality and give a direct demonstration of the different components’ relationship. The spectrum in the 13C or 6Li dimensionality is the same as the 1D CP/MAS spectrum, and only the 1H signals which have the crosspolarization with the

13C

or 6Li can appear in the 1H dimensionality. Thus, the

advanced method can help us a lot in the study of cellulose dissolution process. Figure 4a and 4b are the 2D correlation spectra of urea(1H- 13C) and LiOH(1H- 6Li). We can find that there are only one set of cross-peaks (δH=-0.1 ppm, δLi=0.25 ppm) in Figure 4b observed, which is exactly assigned to Li+-1(Li+-OH-) and the unique


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hydrogen source (OH-). In the urea sample, there is also only one set of cross-peaks (δH=5.4 ppm, δC=162.8 ppm) appears, which mean there is only one kind of hydrogen. Figure 4c is the 2D 1H- 6Li correlation spectrum of LiOH/urea sample. It’s very interesting that there are three sets of cross-peaks can be clearly distinguished. The 6Li

peak at 0.25 ppm correlated with the proton dimension at about -0.1 ppm is

corresponding to the Li+-1(Li+-OH-) , which is same with the cross-peaks shown in Figure 4b (LiOH). It’s very meaningful that another two sets of cross-peaks were observed in the 2D spectrum of LiOH/urea sample compared with that of LiOH sample. Li+-2 resonance peak at -0.31ppm correlates with two kinds of hydrogen (δH=1.69 ppm and δH=7.98 ppm) in the case of adding urea. It’s obvious that the urea’s addition changed the hydrogen environment of partial Li+ ions a lot. As shown in 2D 1H- 13C correlation spectrum of LiOH/urea sample (Figure 4d) , the carbonyl of urea correlates with only one source of hydrogen which indicates that only one kind of amino groups (δH=5.4 ppm) exist in the LiOH/urea sample. Combined with the result, we can eliminate that the two kinds of hydrogen (δH=1.69 ppm and δH=7.98 ppm) appear in the hydrogen dimensionality of Figure 4c are not from the amino groups of urea directly, since the amino group is chemically bonded with the carbonyl group directly. Thus, the most probable explanation of the two kinds of hydrogen is therefore to be attributable to the OH- ions influenced by urea molecule. OH- ions play the role of bridge between Li+ and urea molecules as shown in the diagram of Figure 4. Depend on the previous research results, Liu15 and Song31 also put forward similar conclusions that OH- had some relationship with urea by liquid NMR and solid-state NMR methods. Figure 5 are the 2D 1H-13C correlation spectra of LiOH/urea, cellulose, LiOH/cellulose, LiOH/urea/cellulose samples. In the 2D

1H-13C

correlation


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spectrum of pure cellulose, there is a main 1H signal (δH=3.52 ppm) and a continued shoulder peak in the downfield region in the 1H dimensionality. Simultaneously, there are several characteristic peaks of cellulose in the 13C dimensionality as shown in Figure-5b. The regions of these hydrogen signals are all according with the results of liquid-state NMR which can afford a better resolution32,33. The 2D 1H-13C correlation spectrum of LiOH/cellulose (Figure 5c) is similar to that of cellulose (Figure 5b). But the

13C

signals splitting in the spectrum of cellulose almost

disappears in the spectra of the LiOH/cellulose and LiOH/urea/cellulose samples, indicating an average ‘‘g-t’’ conformation of cellulose II due to being dispersed thoroughly by LiOH or urea[34]. From the 2D spectrum of freeze-dried sample of LiOH/urea/cellulose system(Figure 5d), two remarkable sets of cross-peaks are obtained. The carbon peak at 162.8 ppm (carbonyl of urea) correlates with the proton dimension at about 5.21 ppm (protons of urea). The carbon peaks between 50 ppm and 110 ppm (carbon of cellulose) yield correlate with protons around 3.52 ppm (protons of cellulose). We also did the similar experiment with a short contact time (200 us, Figure 5e) in order to see whether there are some peaks lose or not, but no difference was observed. If there is a direct interaction between cellulose and urea, the cross peaks between carbons of cellulose and hydrogens of urea or the cross peaks between carbons of urea and hydrogens of cellulose will be observed through cross-polarization process. In fact, they don’t appear in the 1H-13C 2D spectrum. It reveals that the direct distance between urea and cellulose is longer than 0.5 nm which is the longest distance of cross-polarization process35. The result is a powerful and direct evidence to prove the conclusion that there is no direct interaction between cellulose and urea proposed before13,21. In the 2D 1H-6Li correlation spectrum of LiOH/cellulose(Figure 6b), two 6Li(Li+-1,Li+-3)

peaks can be observed in the 6Li dimensionality, which is same with


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the 1D 1H-6Li CP/MAS spectrum shown in Figure 3c. According to the results and analysis before, the set of cross-peaks(δH=-0.1 ppm, δLi=0.25 ppm) can be assigned to Li+-OH- which means the existence of free Li+-OH- in LiOH/cellulose system. In 1D 1H-6Li spectrum of LiOH/cellulose (Figure 3c), the assignation of Li+-3 characteristic peak (δLi=-0.89 ppm) wonder us a lot. The new Li+-3 characteristic peaks must have some direct connection with the cellulose dissolution process in LiOH systerm. In Figure 6b, we can find that the Li+-3 peak correlates with the hydrogen peak around 3.52 ppm which can be attributed to the hydrogen of cellulose (shown in Figure 6d). It indicates that a part of Li+ ions combined with cellulose chains directly in LiOH/cellulose system[16-19]. It’s the first time to obtain a direct result to prove the relationship between Li+ and cellulose on the molecule level. We can speculate that after OH- ions plays a role to split the intermolecular hydrogen bond between cellulose chains5, a part of Li+ ions combines with the oxygen groups of cellulose36,37, and the rest of Li+ exist in the system as Li+-OH- and free Li+ existing forms. The process is the main reason that the alkali aqueous solution can partially dissolve cellulose. It was known the dissolution ability of LiOH/urea aqueous solution is the best among these several kinds of alkali/urea solutions (LiOH, NaOH, KOH). The difference of dissolution ability is probably due to the coordination ability between alkali metal ions (Li+, Na+, K+) and cellulose[16-19]. The stronger complexation action leads stronger dissolution ability. Four sets of cross-peaks can be observed in the 2D spectrum of freeze-dried LiOH/urea/cellulose sample (Figure 6c). All these cross-peaks can be observed in the LiOH/urea and LiOH/cellulose samples mentioned before. Combined the 1D 6Li quantitative single-pulse NMR spectrum (Figure 3iv), four kinds of existing forms of Li+ are present in the LiOH/urea/cellulose system. Some Li+(Li+-3) ions combine with cellulose which mainly occupy the space close to cellulose chain through the


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formation of coordination between oxygen groups of cellulose and Li+ as shown in the diagram of Figure 6. Partial Li+ in the form of Li+-2 (Li+-OH--urea) wrap around the cellulose chain without direct interaction with cellulose. Certainly, some Li+-1 (Li+-OH-) and Li+-4 (free Li+) still exist in the LiOH/urea/cellulose system as substitution. Actually, nearly Li+-1 and free Li+-4 can be observed in all samples’ spectra. Maybe the existence of Li+-1 and free Li+-4 ensures a relatively strong dissolution ability of LiOH/urea system by “buffer function” when the content of LiOH changes a little. These several Li+ existing forms on the molecule level consist with the WXRD results which suggest a complex condition in LiOH/urea/cellulose system. In brief, according to the above analysis, a 3D model on the molecular level is proposed in Figure 7. In the stable LiOH/urea/cellulose system, all these four forms of Li+ exist and play important roles respectively. Some Li+ ions coordinate with oxygen groups of cellulose forming Li+-3 to destroy the inter-hydrogen bonding. Some Li+ ions are in the form of Li+-2 (Li+-OH--urea) to prevent the self-aggregation of cellulose chains. Thanks to Li+-2 (Li+-OH--urea), LiOH/urea/cellulose system is much more stable and homogenous than LiOH/cellulose system. Also some Li+-OH(Li+-1) and free Li+(Li+-4) exist in the system as shown in the model. Conclusion: In this study, combined WXRD and several powerful Solid-State NMR approaches (1D 6Li NMR , 1H-13C and 1H-6Li 2D LGHETCOR NMR ), we distinguished Li+ ions existing forms and clarified their roles in the dissolution of cellulose in the LiOH/urea system. Some Li+ ions coordinate with cellulose chain, and quite amount of Li+ ions in the form of Li+-OH--urea surround the Li+-cellulose chains which prevent the self-aggregation of cellulose chains. The coordination


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ability of alkali metal ions (Li+, Na+, K+) with cellulose probably determines the ability to dissolve cellulose. A possible 3D model on the molecular level has been proposed according to the analysis. All these results demonstrate that solid-state high resolution NMR technology can be a powerful and unique tool to investigate the mechanism of cellulose dissolution process in alkali/urea system and help to promote the applications of the potential cellulose solvent system. Acknowledgements: This work is sponsored by the National Natural Science Foundation of China (21875068).


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Aqueous

Solutions.

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DOI:

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(23) Wernersson, E.; Stenqvist, B.; Lund, M. The Mechanism of Cellulose Solubilization by Urea Studied by Molecular Simulation. Cellulose 2015, 22 (2), 991-1001, DOI 10.1007/s10570-015-0548-8. (24) Wan, C.; Hu, M. Y.; Borodin, O.; Qian, J.; Qin, Z.; Zhang, J.; Hu, J. Z. Natural Abundance 17O, 6Li NMR and Molecular Modeling Studies of the Solvation Structures of Lithium Bis(Fluorosulfonyl)imide/1,2-Dimethoxyethane Liquid Electrolytes. J Power Sources 2016, 307, 231-243, DOI: 10.1016/j.jpowsour.2015.12.120. (25) Brendler, E.; Fischer, S.; Leipner, H. 7Li NMR as Probe for Solvent–Cellulose Interactions in Cellulose Dissolution. Cellulose 2001, 8 (4), 283-288, DOI: 10.1023/A:1015120107514. (26) Witt, E.; Chandran, C. V.; Heitjans, P. Slow Ion Exchange in Crystalline Li2SO4 ⋅ H2O: A 6Li 2D EXSY NMR Investigation. Solid State Ionics 2017, 304, 60-64, DOI: 10.1016/j.ssi.2017.03.016. (27) Vogt, F. G.; Clawson, J. S.; Strohmeier, M.; Edwards, A. J; Pham, T. N.; Watson, SA, Solid-State NMR Analysis of Organic Cocrystals and Complexes. Cryst Growth Des 2009, 9 (2), 921-937, DOI: 10.1021/cg8007014. (28) Woźniak-Braszak, A.; Jurga, K.; Baranowski, M. The Lipari-Szabo Model-Free Analysis as a Method for Study of Molecular Motion in Solid State Heteronuclear Systems Using NMR Off-Resonance. Appl Magn Reson 2016, 47, 567-574, DOI 10.1007/s00723-016-0785-5. (29) Xu, Z.; Stebbins, J. F. 6Li Nuclear Magnetic Resonance Chemical Shifts, Coordination Number and Relaxation in Crystalline and Glassy Silicates. Solid State Nucl Mag 1995, 5 (1), 103-112, DOI: 10.1016/0926-2040(95)00026m. (30) Van Rossum, B. J.; Förster, H.; de Groot, H. J. M. High-Field and High-Speed CP-MAS13C NMR Heteronuclear Dipolar-Correlation Spectroscopy of Solids with Frequency-Switched Lee–Goldburg Homonuclear Decoupling. J Magn Reson 1997, 124 (2), 516-519, DOI: 10.1006/jmre.1996.1089. (31) Song, J.; Ge, H.; Xu, M.; Chen, Q.; Zhang, L. Study On the Interaction Between Urea and Cellulose by Combining Solid-State 13C CP/MAS NMR and Extended Hückel Charges. Cellulose 2014, 21 (6), 4019-4027, DOI 10.1007/s10570-014-0461-6. (32) Yang, X.; Müller, K. The Guest Ordering and Dynamics in Urea Inclusion Compounds Studied by Solid-State 1H

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Figure 1

Figure 1. Pulse program for 2D 1H-X LGHETCOR spectroscopy


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Figure 2

Figure 2. WXRD pattern of freeze-dried samples of (a)LiOH/urea/cellulose, (b)LiOH/cellulose, (c)LiOH/urea, (d)urea and (e)LiOH solutions


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Figure 3

Figure 3. Left: 1H-6Li CP/MAS spectra of freeze-dried samples of (a) pure LiOH, (b) LiOH/urea, (c) LiOH/cellulose and (d) LiOH/urea/cellulose, the CP mixing time is 1ms. Right: 6Li quantitative single-pulse spectra with 1H decoupling during signal acquisition of freeze-dried samples of (i) LiOH, (ii) LiOH/urea, (iii) LiOH/cellulose and (iv) LiOH/urea/cellulose, the delay time is 7000s.


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Figure 4

Figure 4. 2D LGHETCOR spectra of (a) pure urea(1H-13C), (b)LiOH(1H-6Li), (c) LiOH/urea sample (1H-6Li) and (d) LiOH/urea sample (1H-13C), the CP mixing time is 1ms.


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Figure 5

Figure 5. 1H-13C 2D LGHETCOR spectra of (a) LiOH/urea, (b)cellulose, (c) LiOH/cellulose, (d) LiOH/urea/cellulose sample (mixing time 1ms) and (e) LiOH/urea/cellulose sample (mixing time 200 us).


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Figure 6

Figure 6. 2D LGHETCOR spectra of freeze-dried samples of (a) LiOH/urea (1H6Li),

(b) LiOH/cellulose (1H-6Li), (c) LiOH/urea/cellulose (1H-6Li) and (d) cellulose (1H-13C), the CP mixing time is 1ms.


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Figure 7

Li +-2 O

O

H2N

OH -

C

NH2

N H2

Li+-4

OH-

C

N H2

H2N

=

Li +-2

=

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C

NH2

O

Li+-4 Li +-2

H Li+ O N C

Li+-3

Li+-1 Li+-3

Figure 7. The molecular model of LiOH/urea/cellulose solution system.


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Table 1.

Page 24 of 25

The contents of different forms of Li+ in the LiOH/urea/cellulose system from the 6Li quantitative single-pulse NMR spectrum

Sample LiOH LiOH/urea LiOH/cellulose LiOH/urea/cellulose

Li+-1 (%) 91.15 22.35 76.79 37.40

Li+-2 (%)

Li+-3 (%)

54.85 35.50

18.39 20.98

Li+-4 (%) 8.85 22.80 4.82 6.12


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Table of Content

Through solid-state NMR techniques, four existing forms of Li+ ions were observed; the roles of the Li+ ions and interaction were investigated to expound the mechanism of cellulose dissolution process in LiOH/urea/cellulose system.


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NMR Study on the roles of Li+ in cellulose dissolution process - ACS

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