Facile extraction of thermally stable and dispersible cellulose

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Facile extraction of thermally stable and dispersible cellulose nanocrystals with high yield via a green and recyclable FeCl3-catalyzed deep eutectic solvent system Xianghao Yang, Hongxiang Xie, Haishun Du, Xinyu Zhang, Zhufan Zou, Yang Zou, Wei Liu, Hongyan Lan, Xinxing Zhang, and Chuanling Si ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00209 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Sustainable Chemistry & Engineering

Facile extraction of thermally stable and dispersible cellulose nanocrystals with high yield via a green and recyclable FeCl3-catalyzed deep eutectic solvent system Xianghao Yang†a, Hongxiang Xie†*a, Haishun Dub, Xinyu Zhangb, Zhufan Zoua, Yang Zoua, Wei Liua, Hongyan Lana, Xinxing Zhanga, and Chuanling Si*a,c a

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology,

Tianjin, 300457, China b

Department of Chemical Engineering, Auburn University, Auburn, AL 36849, USA

c

State Key Laboratory of Tree Genetics and Breeding, Northeast Forestry University, Harbin

150040, China † Both authors contributed equally. Corresponding authors *Chli. Si; *Hx. Xie. Tel.:(+86) 022-60601313. Fax: +86 22 60602510. E-mail addresses: [email protected] (Chli. Si); [email protected] (Hx. Xie).

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ABSTRACT: In the current work, a green and recyclable FeCl3-catalyzed deep eutectic solvent system (F-DES) was invented to fabricate cellulose nanocrystals (CNCs) with a high yield and excellent thermal stability. It was found that the optimum composition of the FeCl3-catalyzed deep eutectic solvent was composed of oxalic acid dihydrate (Oxd), choline chloride (ChCl) and FeCl3·6H2O in a mass ratio of 4:1:0.2 (corresponding to the molar ratio of 4.43:1:0.1). Results showed that CNCs with a diameter range of 5-20 nm and length of 50-300 nm could be isolated from bleached eucalyptus kraft pulp (BEKP) at a high yield (over 90% based on the cellulose content in BEKP) by a one-step F-DES treatment under mild conditions (80 °C, 6 h). The resultant CNCs showed a much higher thermal stability (onset thermal degradation temperature was over 310 °C) than the traditional sulfuric acid hydrolyzed ones, and also exhibited superior dispersion stability in water due to the introduction of carboxyl groups on the surface of CNCs by esterification. In addition, the separated F-DES could be directly reused to produce CNCs at least 3 times. Intriguingly, all the components of the reused F-DES could be separated by a simple separation process with few pollutants releasing into the environment. Therefore, the FDES process could be a green and economically feasible method for the preparation of thermally stable and dispersible CNCs.

KEYWORDS: Deep eutectic solvent (DES), Cellulose nanocrystals, FeCl3 catalysis, Organic acid hydrolysis, Oxalic acid, Choline chloride.

INTRODUCTION

Cellulose nanocrystals (CNCs), as renewable, biodegradable, non-toxic biomass nanomaterials, are ideal nanoscale building blocks in the composite formulation, and thus have attracted rapidly growing scientific and technological interests in recent years.1,2 Due to its


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unique physicochemical properties like low density, high specific surface area, outstanding mechanical properties and low thermal expansion,3,4 CNCs have been widely studied in the fields of biomaterials,5-7 packaging materials,8, 9 conductive materials,10, 11 sensors,12, 13 and various mechanically reinforced nanocomposites.14, 15 Generally, CNCs are mostly obtained via inorganic acid hydrolysis of raw cellulose materials.16, 17 During the hydrolysis process, the amorphous regions of cellulose are more easily hydrolyzed while the crystalline regions of cellulose are remained as CNCs, due to their inherent structure stability.13, 16, 18-20 However, the typical mineral acids like sulfuric acid,21 hydrochloric acid22 and phosphoric acid,23 usually cause some serious problems including equipment corrosion, severe environmental pollution and overdegradation of cellulose, etc.24,25 To overcome the above drawbacks, various new preparation methods have been developed in recent years, such as organic acid hydrolysis,26-28 ionic liquid treatment,1, 29-31 oxidation degradation,31-33 enzymatic hydrolysis34 and so on.17, 35 Among them, organic acid hydrolysis is a promising method due to the fact that organic acids are mild, recyclable, environment-friendly and low corrosive.36, 37 However, because of the weak acidity of organic acid, higher temperature and longer reaction time are needed to improve the hydrolysis efficiency.25 Ionic liquid also shows a great potential to prepare CNCs due to its reusability and the superiority for preparing functional CNCs.29-31, 38 However, the main obstacles for the mass production of CNCs using ionic liquid are the high cost and toxicity. It is noteworthy that deep eutectic solvents (DESs) share many characteristics and properties with ionic liquids, such as low melting points, low vapor pressure, and high thermal stability, which fully accord with the 12 principles of green chemistry and have emerged as a promising new generation of green solvent.39-41 DESs are generally formed by eutectic mixture of Lewis or Brønsted acids and bases, which include a


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variety of anionic and cationic species.42-45 Among them, the oxalic acid/choline chloride (Ox/ChCl) system is a very promising DES due to its low freezing point,46 low-price and biodegradability.47 So far, the Ox/ChCl system has attracted rapidly growing interest in the fields of extraction of plant actives,48-50 CO2 absorbents,51 sustainable media organic reactions solvents,52 etc. Furthermore, due to its acidity and significant swelling ability for cellulose, the Ox/ChCl system has been widely utilized for conversion of biomass into value-added platform compounds and pre-treatment of lignocellulose in the field of biorefinery.48, 50, 52 Recently, Ox/ChCl system was investigated as an effective pre-treatment method for CNCs production. 53, 55

However, further mechanical disintegration (e.g. microfluidization or sonication) are necessary

for the preparation of CNCs. To the best of our knowledge, there is no report about directly extracting CNCs from cellulose raw materials using Ox/ChCl system. In the present work, we demonstrated a green and recyclable FeCl3-catalyzed deep eutectic solvent system (F-DES) to prepare CNCs from dry beached eucalyptus kraft pulp (BEKP), as shown in Figure 1. The F-DES system was formed by mixing oxalic acid dihydrate (Oxd), choline chloride (ChCl) and FeCl3·6H2O in a mass ratio of 4:1:0.2. It was found that the BEKP could be directly converted into CNCs at 80 °C for several hours with a high yield by using the F-DES treatment. The recovered F-DES could be directly reused to produce CNCs at least 3 times. Then, the components of the reused F-DES could be easily separated by a simple separation process. Results showed that more than 60% oxalic acid dihydrate and all of choline chloride could be regained which can be reused to prepare CNCs, the other oxalic acid could be recovered in the form of sodium oxalate. FeCl3 catalyst could be transferred to Fe(OH)3 precipitation, and the saccharides could be obtained via an isopropanol washing operation in the last step. The volatile solvents being used for separating, including ethanol and isopropanol,


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could be recovered by reduced pressure distillation. There were few pollutants releasing into the environment in the whole process. Therefore, a green and sustainable method was established for the preparation of CNCs in the present work.

Figure 1. Flow chart of preparation of CNCs by FeCl3-catalyzed deep eutectic solvent system. EXPERIMENTAL SECTION Materials. The BEKP was provided by Shandong Tai’an Bai Chun Paper Co., Ltd. The major chemical composition of BEKP was glucan (79.1±0.9%), xylan (15.2±0.5%), and lignin (0.1±0.1%). The BEKP was processed through a Wiley mill equipped with a 0.85 mm screen. The oxalic acid dihydrate (analytical grade) was purchased from Tianjin JingDong TianZheng Precision Chemical Reagent Factory. Choline chloride (analytical grade) was purchased from Meryer Chemical Technology Co., Ltd. FeCl3•6H2O (analytical grade) was purchased from Tianjin Fuchen Chemical Reagent Factory. And all other chemical reagents were of analytical


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grade and used as received from Meryer Chemical Technology Co., Ltd. without further purification. F-DES treatment process. The process for CNCs synthesis is displayed in Figure 1. In a typical experiment, 9 g ChCl, 36 g Oxd and 1.824 g FeCl3•6H2O (0.15mmol FeCl3•6H2O/g DES) were added into a 250 mL spherical flask successively, then the flask was heated up to 80 oC in an oil bath with stirring by agitator blade at 300 rpm. When the temperature reached up to 80 oC, 1 g oven dry BEKP was added into the F-DES system and then kept this temperature for 6 hours. Upon completion, the flask was cooled immediately to room temperature with tap water. Afterwards, the mixture was transferred into a beaker and then was diluted with 100 mL hot deionized water. The diluted mixture was centrifuged at 8000 rpm for 3 min to remove F-DES immediately. Subsequently, 1 mL of the separated supernatant was taken for the analyses of glucose and xylose by HPLC. The separated precipitates were washed for another three times with distilled water at 10000 rpm to remove remaining F-DES. After centrifugation, the precipitates were diluted with deionized water and the obtained suspensions were dialyzed against deionized water by a cellulose dialysis membrane (molecular weight cut-off is 14,000, DingGuo Co., Ltd.) until the conductivity of the suspensions approached that of deionized water. At last, the obtained CNCs suspensions (the consistency was about 1 %) were stored in a cold room (4 °C) for further treatments or tests. The yields of the obtained products were calculated based on the volume and solid content of the resultant CNCs suspensions. In addition, the F-DES can be recovered by rotary vacuum evaporation of the separated supernatant. Recovered F-DES can be reused directly for at least 3 times with raw pulp added only. Quantitative analysis of monomer sugars. The contents of glucose, xylose, arabinose, acetic acid, furfural, levulinic acid, HMF in supernatant from F-DES hydrolysis of BEKP were


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determined by HPLC (Agilent 1200, USA) with a differential refractive index detector and BIORAD Aminex HPX-87H (300 mm×7.8 mm) ion exclusion column. Using ultrapure water and sulfuric acid (5 mmol/L) as the mobile phase at a flow rate of 0.6 ml/min and a column temperature of 50 °C. Separation of reused F-DES. In a typical experiment, 11.2 g of the reused F-DES after 3 times was mixed with 13.8 g deionized water. Then, the mixture was heated up to 80 °C with a water bath until completely dissolved, and then was cooled to room temperature under nature environment. The precipitated crystals were filtered, washed by little water and dried sequentially to give 4.31 g oxalic acid dihydrate. The resultant filtrate was adjusted to alkaline (pH = 9-10) by NaOH so that iron complex and the residual oxalic acid were completely transformed into Fe(OH)3 precipitation and sodium oxalate, respectively. Then 175 mg Fe(OH)3 was obtained by filtrating and drying operation. The filtrate was dried by reduced pressure distillation. Next, the dried solid was washed with 100 mL ethanol to give 3.88 g sodium oxalate and the ethanol phase which contained choline chloric and saccharides was further dried by reduced pressure distillation. Residual solid mixture was washed by 100 mL hot isopropanol (60 °C) to give 290 mg insoluble saccharides. Finally, the isopropanol phase was dried by reduced pressure distillation to give 1.98 g choline chloride. Transmission electron microscope (TEM). The TEM images of CNCs samples were taken in a TEM microscope (JEOL JEM-F2100, Japan) with an accelerating voltage of 100 kV. Dilute CNCs suspensions with the concentration of 0.01 wt.% were ultrasonically treated (45 kHz and 180 W) for 10 min and subsequently deposited onto a carbon supported copper grid. After drying at room temperature, 20 μL uranyl acetate (1 wt.%) was dropped on the copper grid to dye the


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samples for 60 min in order to enhance the contrast of images.56 The size of the nanocellulose was manually measured with ImageJ (version FJJ1.5) as the tool.57 Scanning electron microscope (SEM). The sample was observed using a SEM (JEOL JSMIT300LV, Japan). The sample was coated with gold under vacuum before observation. Particle size distribution and zeta potential. The particle size distribution and zeta potential of CNCs were analyzed by a Zetasizer Nano ZS instrument (Malvern Instruments Ltd., UK) based on dynamic light scattering. All the sample suspensions were ultrasonically treated (45 kHz and 180 W) for 10 min before analysis, and the solid concentration of all samples was 0.15 wt%. For each sample, the measurements were conducted in triplicate and 10 runs were performed for each measurement. The average data was reported after repeated analyses. Conductometric titration. Conductometric titration of the CNCs samples with automatic potentiometric titrator (KEM AT510, Japan) was conducted as a quantitative method to determine the carboxyl group content. A small volume of a suspension containing 50 mg CNCs was added to 1 mmol/L NaCl solution (about 150 mL) and titrated using 2 mmol/L NaOH by adding approximately 0.2 mL in 30 seconds intervals.37 Fourier transform infrared spectroscopy (FTIR). FTIR spectra were obtained on a FTIR650 (Tianjin Gang Dong Sci. & Tech. Development Co., Ltd., China). The spectra were recorded at 25°C in the range of 400–4000 cm-1 for scans.58 X-ray diffraction (XRD) analysis. X-ray diffraction patterns of the BEKP and CNCs samples were characterized on an XRD-6100 X-ray diffractometer (Shimadzu, Japan), operating at 40 kV, 30 mA with Ni-filtered Cu Kα radiation at room temperature. The range of scatter in angle (2θ) was from 5° to 40° with a scan rate of 4 °/min. The crystallinity index (CrI) of each sample


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was calculated using the Segal method showed in Eq (1) where I200 is the maximum peak intensity at the overall intensity of the peak at 2θ =22-23°, and Iam is the minimum intensity which represents the intensity of the baseline at 2θ =18-19°, where θ is the corresponding Bragg angle.59 CrI (%) = (I200-Iam)/I200

Eq(1)

Thermal gravimetric (TG) analyses. TGA was carried on a thermogravimetric analyzer (TA instruments T650, USA). In this work, samples were first dried at 50 °C for 4 h prior to testing. Mass of each sample was 3-4 mg and the carrier gas was nitrogen at a flow rate of 50 mL/min. Each sample was heated from 20 °C to 600 °C at 10 °C/min to record the TGA and differential thermogravimetric analysis (DTG) curves.60 RESULTS AND DISCUSSION Preparation of CNCs from the dry BEKP Sirviö et al.53 reported that CNCs could be prepared via a two-step process including the pretreatment of pulp by a 1:1 molar ratio of Oxd/ChCl and sequential mechanical disintegration. It was found that only microfibers could be obtained after pre-treatment, indicating the weak hydrolysis ability of Oxd/ChCl system. Therefore, we initially tried to increase the proportion of Oxd to enhance the hydrolysis activity. However, when the Oxd/ChCl system in a 4:1 mass ratio was used to treat the BEKP at 80 °C for 7 h (Entry 2, Table 1), only microcrystalline celluloses with a particle size of 5-10 μm were obtained (showed in Figure S1). To further increase the hydrolysis activity, it is necessary to introduce a suitable catalyst into this system. It was reported that FeCl3 could be an ideal catalyst for the hydrolysis of cellulose61 and has been applied for the preparation of CNCs by formic acid hydrolysis.19,27 Thus, we tried to introduce FeCl3 to Oxd/ChCl


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system in the current study. As expected, it was found that CNCs with a particle size of 710±143 nm and a crystallinity index (CrI) of 80.5% were obtained in a 76% yield by introducing a little amount FeCl3·6H2O (0.1 mmol FeCl3·6H2O per gram DES) to the reaction (Entry 3, Table 1). Moreover, when the amount of FeCl3·6H2O was further increased to 0.15 mmol/g, the particle size of the resultant CNCs remarkably decreased to 270±92 nm, indicating excellent catalytic effect of FeCl3·6H2O for the CNCs production. As shown in Figure 2, TEM and AFM images showed that the resultant CNCs had whisker morphology with the width of 5-20 nm and the length of 50-300 nm. Table 1. Preparation conditions and the properties of resultant CNCsa. Entry

FeCl3·6H2O (mmol/g DES)

a

Rmb

Time

Yield

Average size

Zeta potential

COOH

(h)

(%)

(nm)c

(mV)

(mmol/g CNCs)

1d

－

－

－

－

－

－

－

2

－

4:1

7

86

5152±3328

－

－

3

0.1

4:1

6

76

710±143

-32.3±0.5

0.09±0.02

4

0.15

4:1

6

73

270±92

-32.9±0.7

0.18±0.03

5

0.2

4:1

6

72

268±75

-35.4±0.9

0.19±0.06

6

0.25

4:1

6

72

262±75

-37.5±0.2

0.20±0.05

7

0.3

4:1

6

71

258±54

-37.8±0.6

0.20±0.01

8

0.15

4:1

7

67

239±49

-38.6±0.9

0.21±0.02

9

0.15

4:1

5

75

502±185

-34.8±0.3

0.13±0.07

10

0.15

4:1

4

80

890±380

-32.6±0.8

0.09±0.03

11

0.15

4:1

3

81

1948±269

-30.2±0.1

0.05±0.01

12

0.15

3:1

6

72

917±270

－

－

13

0.15

2:1

6

73

2195±658

－

－

14

0.15

1:1

6

88

5726±3856

－

－

All of above reactions were carried out at 80°C; b Rm represents the mass ratio of Oxd/ChCl; c

Average size represents hydrodynamic diameter based on dynamic light scattering (DLS), roughly representing the length, but not the width of CNCs samples; d Entry 1 showed the data of raw pulp.


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Figure 2. TEM images of CNCs prepared by the F-DES system (a-c corresponding to the samples from Entry 4, 7, and 8 in Table 1 respectively). AFM images (d and e) of the CNCs samples produced under conditions of Entry 4. A series of experiments were further carried out to optimize the reaction conditions for the production of CNCs. When the concentration of FeCl3·6H2O was increased from 0.15 mmol/g to 0.3 mmol/g, there is no obvious change for the yield and particle size (Entry 4-7, Table 1). This result was also confirmed by the TEM characterization (see Figure 2a, 2b) that the particle sizes of CNCs were almost the same with each other. Therefore, a reasonable dosage of FeCl3·6H2O at 0.15 mmol/g was used in this system. Moreover, when the reaction time was prolonged from 3 h to 7 h, the results showed that not only the yield, but also the particle size and CrI of the resultant CNCs were significantly affected (Entry 4, Entry 8-11 in Table 1). With the increase of reaction time, the size of CNCs reduced at first and then tended to be a constant value (200-300 nm based on DLS), and the CrI of CNCs exhibited a trend of first increase and then decrease. It may be the reason that the hydrolysis rate in amorphous regions is much faster than that in crystalline


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regions13, so the particle size of CNCs decreased and the initial CrI increased with the reaction time increasing in the early stage. While during the later stage, the hydrolysis reaction mainly occurred in the crystalline regions due to the depletion of amorphous region, therefore, the particle size tended to be constant owe to the slow hydrolysis rate on crystalline regions, and the CrI began to decrease slightly. Thus, 6 h could be the optimal reaction time. Finally, the effect of the mass ratio of Oxd/ChCl (Rm) was investigated (Entry 4, Entry 12-14, Table 1). When Rm changed from 4:1 to 1:1, the particle size significantly increased from 270±92 nm to 5726±3856 nm, which indicated an obvious decrease of reactivity. The results suggested that high proportion of Oxd is necessary for the preparation of CNCs in this reaction system. It may be due to the reason that the esterification of cellulose with oxalic acid and the acidity were both enhanced by increasing the concentration of Oxd, which could accelerate the intermolecular hydrogen bond destruction and hydrolysis of cellulose. Overall, an ideal F-DES for the one-step preparation of CNCs was the composition of Oxd/ChCl/FeCl3·6H2O in a mass ratio of 4:1:0.2 at 80 ºC for 6 h, corresponding to Entry 4 in Table 1. To further clarify the degradation process of cellulose during F-DES treatment, small molecular degradation products were analysed by HPLC, as shown in Figure 3a. In the reaction process, glucose and xylose were generated in a yield range of 0.58-9.0% and 10.55-14.07% respectively based on the dry BEKP while without any further degradation products like 5hydroxymethylfurfural (HMF), furfural or levulinic acid. Therefore, the results suggested that the F-DES system was very mild for the preparation of CNCs with a high yield. For example, the yield of CNCs under optimal reaction condition (Oxd/ChCl/FeCl3·6H2O in a mass ratio of 4:1:0.2 at 80 °C for 6 h) is 73% based on raw BEKP and over 90% based on the cellulose content in BEKP, which is much higher than the yield (around 30%) of CNCs produced by traditional


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sulfuric acid hydrolysis.16 Moreover, when the reaction time was longer than 4 h, the yield of glucose continued to increase while the yield of xylose tended to be constant, which further indicated the hydrolysis of cellulose mainly occurred in the crystalline regions in the later stage.

Figure 3. (a) The yield of glucose and xylose based on dry BEKP, (m, n) represents the sample from the condition of (m mmol/g FeCl3·6H2O, reacting for n h). (b) FTIR spectra of samples from Entry 1, 3, 4, 7, 8, 11 in Table 1. (c) XRD patterns of samples and the corresponding CrI (d). To investigate the change of surface chemistry during F-DES treatment, FTIR spectra of the original BEKP and the as-prepared CNCs were collected. As shown in Figure 3b, a new band at 1737 cm-1 could be found in the FTIR spectrum of CNCs although it was very weak, which indicated the existence of C=O. It proved that the esterification between oxalic acid and BEKP occurred during the hydrolysis reaction. In addition, other bands (e.g. the bands at 3420, 2900, 1640, 1430, 1163, 1113, and 897 cm-1) in the spectra of CNCs and original BEKP were parallel,


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indicating the cellulose Iβ structure remains unchanged after F-DES treatment. As shown in Figure 3c and Figure S2, XRD patterns of the obtained CNCs showed diffraction peaks at 2θ = 15.1°, 16.5°, 22.6°and 34.6°, corresponding to the (1-10), (110), (200) and (004) crystallographic planes of characteristic diffraction peaks of cellulose Iβ, which was consistent with FTIR analysis. In addition, conductometric titration analysis demonstrated that the carboxyl content on the surface of CNCs was inverse correlated with the particle size of CNCs (Entry 3-11, Table 1). This was due to the fact that more carboxyl groups could be held on the surface of CNCs due to the increase of specific surface area with the particle size reducing. Also, the values of zeta potential for the obtained CNCs were below -32 mV owing to the introduction of negative charges, which were sufficient to promote stable water suspension. In addition, Figure 4 displayed the characteristic flow birefringence phenomena of CNCs in aqueous solution when observed between two crossed linear polarizing films, indicating the excellent dispersibility of CNCs in water.

Figure 4. Photograph of the CNCs suspension (left, Entry 4) and the corresponding photograph showing the typical flow birefringence phenomena (right) observed under polarized light.


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Figure 5. TG and DTG curves of the samples from Entry 1, 3, 4, 7, 8, 11 in Table 1 at a heating rate of 10 °C/min. Figure 5 shows the thermal gravimetric (TG) and derivative thermogravimetric (DTG) curves of the original BEKP and as-prepared CNCs. The start decomposition temperature (Tstart), onset decomposition temperature (Ton) and maximum decomposition temperature (Tmax) were summarized in Table S2. As shown in Figure 5 and Table S2, the thermogravimetric analysis showed that the Tstart of the as-prepared CNCs were slightly reduced with the increase of reaction time and all lower than that of original BEKP. It was probably due to the increased amount of carboxyl groups on the surface of CNCs, which could lead to the formation of unstable anhydroglucuronate units and decomposed at the early stage.32 Moreover, the Tmax of the asprepared CNCs were all slightly higher than that of BEKP except that of CNCs from Entry 8 in Table 1. It may be due to the removal of the amorphous regions of cellulose, as well as the increase of crystallinity, while the destruction of some crystalline areas by excessive hydrolysis led to the reduction of the Tmax of CNCs from Entry 8 in Table 1. The Ton of all the resultant CNCs were over 310 oC, indicating that the obtained CNCs showed much higher thermal stability than the traditional sulfuric acid hydrolyzed ones.19 It was noteworthy to mention that


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thermal stability of the resultant CNCs was much higher than that of the ones produced by the two-step process.53 This may be due to more amorphous parts of cellulose was hydrolyzed during F-EDS treatment. Reuse of F-DES It is very necessary and valuable to realize the reuse of F-DES, which could make the preparation cost of the CNCs sufficiently reduced and protect the environment. In this work, the separated F-DES after recovery was directly reused to prepare CNCs for three times. The results showed that CNCs with narrow size distribution could be still obtained in a high yield (> 70%), as shown in Table 2. Compared with the initial reaction, the hydrolysis activity of F-DES decreased gradually with the increase of reuse times, which could be seen from the increase of the CNCs size (Table 2) This was probably because the increscent degradation products in supernatant (see Figure 6a) led to low hydrolysis efficiency. Table 2. The reutilization of F-DES for the preparation of CNCs. Recycle number

Yield

Average size

(%)

(nm)

1

73±3

567±216

2

75±9

602±259

3

75±7

649±275

As shown in Figure 6a, the contents of glucose and xylose in the reused F-DES increased with the increasing reusing number. Moreover, the colors of the separated F-DES deepened with the increasing reuse times, as shown in Figure 6b. After reusing for 3 times, the content of glucose and xylose reached up to 5.25 g/kg and 15.5 g/kg, respectively, but still no excessive degradation products like HMF, furfural, levulinic acid were found, which further proved the reaction conditions were mild.


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Figure 6. (a) The Contents of xylose and glucose in recovered F-DES with different reusing times. (b) Images of supernatant with different colors, the reusing time of supernatant was 0-3 from left to right. Finally, the separation of the reused F-DES was investigated in this work. As shown in Figure 7, oxalic acid dihydrate could be obtained at a 60% recovered rate by recrystallization process. Then, the filtrate was adjusted to alkaline (pH = 9-10) by NaOH so that iron complex and the residual oxalic acid were completely transformed into Fe(OH)3 precipitation and sodium oxalate, respectively. Then Fe(OH)3 was filtrated out and the filtrate was dried by reduced pressure distillation. Next, the dried solid was washed with ethanol to give purified sodium oxalate and the ethanol phase which contained choline chloric and saccharides was further dried by reduced pressure distillation. In the last step, the residual solid mixture was washed by hot isopropanol to dissolve choline chloride but leave saccharides. The isopropanol could be recycled as well as ethanol in the separation process. Thus, each component of the reused F-DES was separated reliably through this simple separation process. The recovered oxalic acid dihydrate and choline


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chloride can be used to prepare CNCs for next cycle, Fe(OH)3 and sodium oxalate are both important industrial chemicals. Saccharides can be used as biological fermentation raw materials. Therefore, all the chemicals can be recovered, and there are few pollutants releasing into the environment during the whole process, indicting a green and sustainable route for the effective preparation of functional CNCs.

Figure 7. The Flow chart of separation of recovered F-DES.

CONCLUSION In this study, a facile and sustainable method by one-step treatment of F-DES was established to prepare thermally stable and dispersible CNCs from BEKP. The optimal F-DES was composed by Oxd/ChCl/FeCl3·6H2O in a 4:1:0.2 mass ratio which was proved to have the best swelling ability and strongest hydrolysis activity towards BEKP. The optimal reaction condition for the CNCs production was at 80 °C for 6 hours using the optimal F-DES system. Under this condition only glucose and xylose were obtained without any further degraded products (e.g. HMF, furfural), indicating the present method was much milder than the traditional inorganic acid hydrolysis. The whisker-like CNCs could be produced with a high yield (>90% based on the cellulose content in BEKP), high crystallinity (over 80%), excellent thermal stability (Tmax over 355 °C) and great disperse stability in water. Moreover, the separated F-DES could be directly


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reused to prepare CNCs for 3 times. The reused F-DES could be further separated by a simple process which could completely recover all the used chemicals, as well as the hydrolyzed saccharides. Thus, the present study provides a green and sustainable approach for the effective preparation of thermally stable and dispersible CNCs.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxx. SEM image of the sample from Entry 2 in Table 1 (Figure S1), XRD patterns of BEKP and the obtained samples at different conditions (Figure S2), CrI data of the BEKP and CNCs samples (Table S1), thermal degradation data of the BEKP and CNCs samples (Table S2) (PDF) AUTHOR INFORMATION Corresponding Author *Tel.: (+86) 022-60601313. Fax: +86 22 60602510. E-mails: [email protected] (Si, Chli.), [email protected] (Xie, Hx.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We gratefully acknowledge the financial support from the State Key Laboratory of Tree Genetics and Breeding (K2017101) and Postdoctoral Science Foundation of China (2018M631749). In addition, H. Du acknowledges the financial support from the China Scholarship Council (201708120052). ABBREVIATIONS F-DES, FeCl3-catalyzed deep eutectic solvent system DES, deep eutectic solvent CNC, cellulose nanocrystals BEKP, beached eucalyptus kraft pulp Oxd, oxalic acid dihydrate ChCl, choline chloride

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For Table of Contents Use Only

A brief (~20 words) synopsis This paper demonstrated that thermally stable and dispersible cellulose nanocrystals was extracted from cellulose pulp by a mild and sustainable FeCl3-catalyzed deep eutectic solvent system.


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Facile extraction of thermally stable and dispersible cellulose

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