Efficient Cleavage of Strong Hydrogen Bonds in Cotton by Deep

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Efficient Cleavage of Strong Hydrogen Bonds in Cotton by Deep Eutectic Solvents and Facile Fabrication of Cellulose Nanocrystals in High Yields Yongzhuang Liu, Bingtuo Guo, Qinqin Xia, Juan Meng, Wenshuai Chen, Shou-Xin Liu, Qingwen Wang, Yixing Liu, Jian Li, and Haipeng Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b00954 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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ACS Sustainable Chemistry & Engineering

Efficient Cleavage of Strong Hydrogen Bonds in Cotton by Deep Eutectic Solvents and Facile Fabrication of Cellulose Nanocrystals in High Yields Yongzhuang Liu, Bingtuo Guo, Qinqin Xia, Juan Meng, Wenshuai Chen, Shouxin Liu, Qingwen Wang, Yixing Liu, Jian Li, Haipeng Yu* Key laboratory of Bio-based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin, 150040, P. R. China * Corresponding: Haipeng Yu, Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT

The content of cellulose in biomass is important for producing nanocellulose in high yield. Cotton fibers containing ultrahigh purity (~95%) cellulose are ideal feedstock for nanocellulose production. However, the presence of strong hydrogen bonding between the cellulose chains limits the use of cotton fibers for the production of nanocellulose in a facile and mild process. Here, efficient cleavage of the strong hydrogen bonds in cotton and ultrafast fabrication of cellulose nanocrystals (CNCs) with a high yield of 74.2% were first realized through 3-min microwave-assisted deep eutectic solvent pretreatment and a subsequent high-intensity ultrasonication process. The obtained CNCs had diameters of 3–25 nm and lengths ranged between 100 and 350 nm. The CNCs also displayed relative crystallinity of 82% and the thermal degradation temperature started from 320 °C. The study provides a green and efficient method for the mass production of cotton CNCs, and is expected to contribute to improving the refinery utilization of cotton feedstock. Keywords: cellulose nanocrystals, deep eutectic solvent, cotton, strong hydrogen bond, high yield

INTRODUCTION Cotton is an abundant, renewable and important resource in our daily lives.1 Cotton fibers (CFs) come from the outgrowths of epidermal cells during cell elongation and secondary cell wall synthesis. CFs contain the highest percentage (~95%) of cellulose among various biomass

resources.2

Cotton

cellulose

is

composed

of

linear

molecules

of

β-D-glucopyranosyl units that are associated laterally to form elementary fibrils and shows a high degree of polymerization (DP). The cellulose molecular chains are likely to bond tightly through strong hydrogen bonding.3,4 Noncellulosic components such as pectins and proteinaceous materials are located mainly in the outer waxy layer.1 It has also been reported that the primary cell wall and the covering of hemicellulose on the surfaces of cellulose is another critical factor that influences the disintegration of cellulose.5 Therefore, it is difficult to isolate cellulose from cotton using a chemical or mild mechanical process. Conventionally, CFs have been chemically hydrolyzed to produce pulp for paper


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making, microcrystalline cellulose (MCC) used as an excipient for tablets, a gentle filler in cosmetics and an additive to dietary food.6,7 CFs may also be enzymatically hydrolyzed to produce glucose, ethanol, or other chemicals.8 In recent years, the preparation of nanocellulose from CFs has been realized.3,9–16 The preparation of cellulose nanocrystals (CNCs) from CFs has mainly been implemented by using strong acid hydrolysis. However, the use of strong acids has drawbacks such as corrosion of the equipment and environmental pollution from waste liquors.2,9–15 A controlled microbial hydrolysis method has been used to produce CNCs from CFs, but it generally needed a long period.16 Until now, the preparation of cotton CNCs in a green and fast process together with a high yield remains a challenge. To increase the technical and economic feasibility of fabricating CNCs from cotton, it is very important to break the strong hydrogen bonds and obtain the cellulose easily. Deep eutectic solvents (DESs), which have both the characteristics of ionic liquids and organic solvents, are one of an emerging generation of green solvents.17 DESs have received extensive attention because they exhibit several advantages such as cheap and easy fabrication requiring no purification and insensitivity to water. The hydrogen bonds in a DES make it a good solvent for various materials including hydrogen-bond-rich polymers by competitively forming hydrogen bond interactions.18–21 Recently, Sirviö et al. reported that a choline chloride–urea based and acidic DES can be a hydrolytic media for lignocellulose pretreatment and the production of CNCs.22–24 Liu et al. reported that a microwave-assisted DES (MwDES) treatment was efficient for lignin and hemicellulose fractionation from wood and rapidly give high-purity cellulose, which was promising for CNCs production.25 DES has also been used for cellulose decrystallization and modification.26,27 Park et al. prepared antibacterial cotton fabrics by using DES as a solvent.28 To the best of our knowledge, the use of MwDES treatment for the fabrication of CNCs in high yields by cleaving the strong internal interactions in cotton has been rarely reported. In this paper, a choline chloride/oxalic acid dihydrate-based DES pretreatment and a high-intensity ultrasonication process were combined to realize the fabrication of CNCs from cotton in an environmentally friendly manner and with minimal steps. The



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dissolution of the heterogeneous part in cotton and cleavage of the strong hydrogen bonds were investigated using DES by heating at 80−100 °C under 800 W microwave radiation for 3 minutes. The morphology of the pretreated CFs was observed through optical microscopy and scanning electron microscopy (SEM). Changes in the chemical composition were investigated by

13

C cross-polarization/magic angle spinning (13C

CP/MAS) nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. The obtained CNCs were characterized through transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analysis.

EXPERIMENTAL SECTION Materials. Raw CFs (Gossypium hirsutum L.) was purchased from Shihezi, Xinjiang, China and used without any pretreatment. Choline chloride and oxalic acid dihydrate were purchased

from

Aladdin

Reagent

Co.,

Ltd.

(Shanghai,

China).

Copper

(II)-ethylenediamine complex (1 M, molecular weight 121.63) for the DP test of cellulose was purchased from Guangfu Chemical Reagent Co., Ltd. (Tianjin, China). Sulfuric acid, sodium hydroxide, acetic acid and acetone were purchased from Kermel Chemical Reagent Co., Ltd. (Tianjin, China). All chemicals were of analytical purity. Preparation of DES. Typically, choline chloride and oxalic acid dihydrate were mixed at a molar ratio of 1:1 under reduced pressure at 80 °C for 1 h to give a clear viscous liquid. MwDES Pretreatment. CFs (0.25 g) were mixed with 10 g DES in a Teflon-lined polyetheretherketone tank. The tank was heated between 70 and 100 °C using a XH-800C Microwave Accelerated Reaction System (Xianghu Technology Co., Ltd., Beijing, China) under 800 W microwave radiation (Figure S1 in the Supporting Information (SI)). The program was set as listed in Table 1. After MwDES pretreatment, the undissolved matter was

separated and washed

with acetone

and

water.

For

convenience,

the

MwDES-pretreated CFs were named MwDES-70, MwDES-80, MwDES-90 and MwDES-100, where the suffix indicates the pretreatment temperature. The yield of the CFs was calculated as the weight percentage of oven-dried CFs to the oven-dried cotton feedstock.


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Ultrasonication Process. The pretreated CFs were first mixed with water at a concentration of 0.5 wt %. The suspensions were then processed using a JY99-IIDN ultrasonic generator (Scientz Biotechnology Co., Ltd, Ningbo, China) equipped with a 25 mm cylindrical titanium alloy probe. The ultrasonication was conducted at a frequency of 20 kHz with an output power 1200 W for 30 min, resulting in CNC suspensions. The obtained CNCs were named MwDES-70 CNCs, MwDES-80 CNCs, MwDES-90 CNCs and MwDES-100 CNCs. Characterization. The chemical compositions of the cotton and the resultant CNCs were determined as previously described (details are shown in the SI page S3-S4),25 and expressed as wt % content of cellulose, oligosaccharides and insoluble substances. Each sample was tested a minimum of five times and the average value was calculated. The morphology of the pretreated CFs was observed using a JSM-7500F SEM (JEOL, Tokyo, Japan) at 15 kV. Before observation, the samples were coated with platinum using a vacuum sputter coater. The DP of cellulose was measured with a 1835-4-0.80 Ubbelohde viscometer (Xinbiao Tengda Instruments, Beijing, China), using copper(II)ethylenediamine as solvent. The zeta potential (ZP) of the CNC suspensions was measured using dynamic light scattering with a zetasizer (Nano ZS, Malvern, UK). The charge densities of the CNCs were tested through conductometric titration with a DDS-11A conductivity meter (LEICI Instrument, Shanghai, China), and details are shown in the SI page S4. 13

C CP/MAS NMR analysis on the pretreated CFs and CNCs were carried out at room

temperature with a Bruker DRX-400 (Rheinstetten, Germany) spectrometer. Spectra were acquired with a 4 mm MAS probe using a combination of CP, MAS and high-power proton decoupling methods. A total of 800 scans were accumulated for each sample. The crystallinity index (CrI) of cellulose is calculated by dividing the area of the crystalline peak (integrating the peak from 87 to 93 ppm) by the total area assigned to the C4 peaks (integrating the region from 80 to 93 ppm). FTIR spectra were recorded using a Nicolet Magna 560 FTIR instrument (Thermo Fisher Scientific Inc., Waltham, USA) with a diamond attenuated total reflectance (ATR)



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attachment. The data were recorded over the range from 650 to 4000 cm−1 in absorbance mode with 32 scans per spectrum, with a resolution of 4 cm−1. TEM observation on the CNCs was performed using a JEM-2100 microscope (JEOL, Tokyo, Japan) at 200 kV. A droplet (5 µL) of diluted CNCs slurry was dropped on the carbon-coated electron microscopy grid, and was then negatively stained using 1 wt % phosphotungstic acid solution to enhance the contrast of the image. The dimensions of the CNCs were measured from the TEM images using a TDY-V5.2 image analysis system (Tianhong Precision Instruments, Beijing, China). The XRD patterns were obtained using a D/max2200 X-ray diffractometer (Rigaku, Tokyo, Japan) with Ni-filtered Cu Kα radiation (λ = 0.154 nm) at 40 kV and 30 mA. The diffraction data were collected from 2θ = 10–40° at a scanning rate of 4° min−1. The relative crystallinity index (CrI) was estimated using the Segal method.29 CrI(%) =

I 200 I am ×100 I 200

(1)

where I200 is the peak intensity of the (200) lattice diffraction at 2θ ≈ 22.6°, which represents both the crystalline and amorphous region material, and Iam is the diffraction intensity of amorphous fraction at 2θ ≈ 18°. The TG and DTG (a derivative of TG representing the corresponding rate of weight loss) analysis was performed using a Pyris6 TG analyzer (Perkin-Elmer, Waltham, USA) at a heating rate of 10 °C min−1 under a nitrogen atmosphere. The content of hydroxymethylfurfural (HMF) was quantitatively analyzed by ultraperformance liquid chromatography (UPLC, Waters, Milford, USA), with an Acquity BEH C18, 1.7 µm, 2.1 × 50 mm column and an ultraviolet detector at 280 nm. The mobile phase was acetonitrile/water (5/95 v/v) at a flow rate of 0.25 mL min−1.

RESULTS AND DISCUSSION Cellulose is the predominant component in cotton. In the raw cotton feedstock, the content of cellulose, oligosaccharides and insoluble substances was 94.2%, 1.6% and 0.5%, respectively (Table 2). The tight and ultrastrong inner hydrogen bond between the cellulose chains are difficult to access using conventional solvents and makes it hard to


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mechanically disintegrate it into CNCs. Generally, 0.5–30% acid, 1–10% alkali, and acidic sodium chlorite (NaClO2) loadings have been effective for the pretreatment of wood lignocellulosic feedstocks (Figure S2).30 However, when these processing methods (e.g., 5% H2SO4, 5% NaOH and NaClO2) were used to pretreat the cotton feedstock, there was no significant pretreatment effects; therefore, the pretreated CFs could not be wholly disintegrated into CNCs through the high-intensity ultrasonication process (Figure 1a–c and Figure S3). From the SEM observations of the CFs after acid and alkali pretreatments, there was little change in the morphology and surface of the CFs compared with those of untreated CFs, except that some pores clearly appeared among the microfibrils owing to the removal of the waxy layer and hemicellulose (Figure 2a,b and Figure S4). After NaClO2 pretreatment, some microfibrils parallel to the axis of the CFs appeared (Figure 2c), which might be owing to the exposure of the primary cell wall. After MwDES pretreatment at 80 °C for 3 min, some microfibrils with certain intersection angles were observed (Figure 2d). This revealed that the primary cell wall of cotton with low DP might be removed by MwDES pretreatment. When a subsequent ultrasonication process was performed on the MwDES-pretreated CFs, a homogeneous and stable suspension was obtained (Figure 1d). Therefore, the MwDES process was demonstrated as a fast and effective pretreatment method for ultrasonic disintegration of the CFs into CNCs. After the separation of the MwDES CFs, a 94.1% of DES could be recycled through a distillation process. From the 1H NMR analysis, the spectrum of the recycled DES was similar to that of the original DES (Figure S5), except that the peak of the oxalic acid was somewhat weaker, revealing the consumption of oxalic acid during the process. When the recycled DES was reused to pretreat the cotton, the results showed that the recycled DES remained useful and effective. The influence of the MwDES temperature on the yield of undissolved cellulose from cotton was investigated. Because the ultrasonication process does not have a significant effect on the degradation of cellulose, the yield of cellulose was theoretically equal to that of the ultrasonically disintegrated CNCs. When the cotton was pretreated by MwDES at 70 °C for 5 min, the yield of MwDES-70 CFs was 75.9% (Table 1). This was possibly because the hemicellulose and the heterogeneous cellulose in the primary cell wall were



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removed during the MwDES pretreatment. Optical microscope observations showed that the MwDES-70 CFs became somewhat shorter and presented an acid-swelling state as compared with untreated CFs (Figure 3a and Figure S6). Although the CNCs could be acquired in high yield through a pretreatment at 70 °C for 5 min, there were also some aggregates which had not been ultrasonically disintegrated into CNCs (Figure S7). When the pretreatment was performed at 80 °C for 3 min, the yield of MwDES-80 CFs slightly dropped to 74.2%, the fibers were shortened from several millimeters to approximately 100 µm and no more aggregates could be found (Figure 3b). As the temperature was increased to 90 °C and 100 °C, the yield of MwDES-90 CFs and MwDES-100 CFs remarkably declined to 62.4% and 57.8%, which might be because the hemicellulose and partial amorphous cellulose in the secondary cell wall was also degraded at higher temperatures, resulting in darkened suspension colors. The fibers were shorter and some fine particles could be observed in the MwDES-100 CFs (Figure 3c,d). The resultant CFs after pretreated at the temperatures 80–100°C could be well-nanofibrillated into CNCs by subsequent high-intensity ultrasonication processing. The DP and compositions of the MwDES CFs were studied and listed in Table 2. The DP values displayed a significant decrease from original 1600 to 149, 125 and 109 after MwDES pretreatments, which suggested that the molecular chains of cellulose had been broken at intervals in the amorphous region. The composition analysis showed that the insoluble materials remained stable during the pretreatment. Cellulose was still the predominant component (90%) in the MwDES CFs. The oligosaccharides content increased slightly with increasing temperatures. After pretreatments, the MwDES CFs suspensions (concentration: 0.5 wt %) were subjected to ultrasonic processing at 1200 W. The strong ultrasonic cavitation improved the disintegration of the CF fragments and generated CNCs. A comparison of the resultant CNCs suspensions is shown in Figure 3e–h. All the CNCs suspensions except that of MwDES-70 were stable and showed no obvious precipitates. This result suggested that the sizes of the CNCs were small enough to be well dispersed in aqueous suspensions. The surface change densities of the CNCs were determined by conductometric titration (Figure S8). The value of the charge density is 205.7 mmol kg–1. The result suggests that the


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microwave pretreatment with DES led to the formation of negatively charged carboxyl moieties on the surfaces of CNCs. ZP values of the CNCs showed negative zeta potentials at approximately 20 mV. The result indicates that the CNCs were separated from one another and well dispersed in the suspensions. The ZP values were close to those that undergo a short acid hydrolysis process, but smaller than those that undergo a long acid hydrolysis time.31 The results suggested a DES-induced acidifying effect on the cellulose and therefore generated the electrostatic repulsive forces among the CNCs, which was conducive to a homogeneous dispersion of the CNCs (Figure S9). The morphology and size of the obtained CNCs was analyzed using TEM. Even though the same ultrasonication conditions were used, CNCs with different diameters were obtained. The TEM image of MwDES-70 CNCs showed massive CNCs with diameters of up to 25 nm, but some undisintegrated micrometer-sized short fragments also appeared (Figure S10). This might be caused by the insufficient reaction temperature during the MwDES pretreatment. Nanosized CNCs were clearly observed in the TEM images of MwDES-80, MwDES-90, and MwDES-100 CNCs (Figure 4). The MwDES-80 CNCs showed a diameter in the range of 3–25 nm and lengths in the range of 100–350 nm (Figure 4a and Figure S11). The diameter and length distribution of MwDES-90 CNCs were similar to those of MwDES-80 CNCs (Figure 4b). As the pretreatment temperature was increased to 100 °C, the diameters of the MwDES-100 CNCs decreased to less than 15 nm and the lengths became shorter (Figure 4c). The results reveal that CNCs could be obtained from pretreatments at different treating temperatures. However, at the treating temperature of 70 °C, the pretreated CFs were not wholly nanofibrillated to CNCs by subsequent high-intensity ultrasonication treatment, and some aggregates still existed. At the treating temperatures higher than 70 °C, well-nanofibrillated CNCs could be obtained. With the increase of the temperature, hemicellulose and partial amorphous cellulose in the CFs were degraded to some degree, resulting in darkened suspension colors. Meanwhile, the diameter and length of the resultant CNCs were also decreased. Therefore, from the view of high yield and energy conservation, the MwDES-80 pretreatment was an optimal choice. The structural elucidation of the MwDES-80 CFs and the resultant CNCs was



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performed using 13C CP/MAS NMR spectroscopy, as shown in Figure 5a. The noticeable signals between 60 and 120 ppm were assigned to cellulose. The signals at 174 and 21 ppm, which were attributed to the carbonyl carbons and methyl carbons in hemicellulose, were very weak.32 The typical cellulose carbon peaks C1–C6 remained unchanged after undergoing the MwDES pretreatment and the ultrasonication process. The FTIR spectra of MwDES-80 CFs and CNCs (Figure 5b) were further compared with raw cotton to reveal whether there were changes to the functional groups. The absorbance peaks at 1425, 1375, 1160, 1115, 1060 and 897 cm−1 are associated with the typical features of cellulose. The presence of the above-mentioned peaks was in agreement with the cellulose structure of cotton. In the FTIR spectrum of the MwDES-80 CFs, an absorption peak appeared at 1725 cm-1. This indicated the presence of C=O after the MwDES pretreatment, which may result from the carboxyl moieties on the surface or the exposure of hemiacetals after the degradation of amorphous cellulose. No apparent differences were observed in the spectrum of MwDES-80 CNCs, suggesting that the CNCs possessed the same chemical structures and functional groups as the MwDES CFs. The purity of the CNCs was determined by elemental analysis in Table S1. The results showed that the CHNS contents MwDES CFs and CNCs were close to those of CFs. In addition, the obtained CNCs were also soxhlet extracted with ethanol, and the extractives were characterized by GC/MS as shown in Figure S12. The GC/MS results did not show any extracted impurities. The remained solid after the ethanol extraction was 99.7% mass percentage of the initial CNCs. Both the results revealed that the CNCs were in high purity. XRD was used to investigate the crystal structure and relative crystallinity of the MwDES CFs and CNCs. As shown in Figure 6a, the raw cotton, MwDES CFs and CNCs all exhibited the typical diffraction peaks at approximately 2θ = 14.6°, 16.6° and 22.6°, which confirmed the crystal lattice type I of the native cotton cellulose. This result revealed that the MwDES pretreatment did not change the crystal structure of cellulose. The CrI, as a key factor in influencing the mechanical and thermal properties, was calculated according to the Segal’s method. The CrI of CFs was 64.88% (Figure 6b). After the MwDES pretreatment, the CrI values increased by 15–20% as compared with the original sample. The CrI values calculated by Segal’s method were also compared with


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those by the solid state NMR method. The CrI of cotton and MwDES-80 CNCs were 60.68% and 75.87% by NMR method. The results are consistent with the values estimated using the Segal’s method. The increase in the relative CrI undoubtedly resulted from the dissolution of the heterogeneous primary cell wall and the removal of most of the oligosaccharides. The thermal stability of the MwDES CNCs was tested. TG and DTG curves are shown in Figure 6c,d. The initial decomposition temperature of raw cotton was about 300 °C, whereas those of the obtained CNCs slightly increased to approximately 320 °C. Similar results were also reflected in the temperature of the maximum degradation rate (Tmax). For example, the MwDES-80 CNCs had a higher Tmax of 354 °C compared with 335 °C of CFs. Taking into account all the above-mentioned results, to obtain CNCs in a high yield with good properties using less energy consumption, the MwDES pretreatment conditions of 80 °C for 3 min were considered optimal. A CNCs yield of 74.2% was obtained and 10.9 wt % HMF was simultaneously one-pot detected, suggesting that the transformation of glucose into HMF after isomerization and dehydration might have occurred (Figure S13, S14). The MwDES-80 CNCs obtained in this work were compared with the cotton CNCs reported in the literature, including the feedstock, preparation method, yield, diameter width, length, CrI, and Tmax of the CNCs. As listed in Table 3, strong sulfuric acid hydrolysis was the most commonly used method for the production of cotton CNCs.33–37 Although there were no substantial differences in the cotton feedstock content and the preparation method, the yields of CNCs from strong sulfuric acid hydrolysis were typically within 21.5–46.7%, which were far lower than that obtained using our method (74.2%). The other CNC preparation methods including TEMPO oxidation36, ionic liquid swelling combined with sulfuric acid hydrolysis37 and microbial hydrolysis,16 displayed lower efficiency and yield than the presented method. Overall, the morphology, dimensions, crystallinity and thermal stability of the MwDES-80 CNCs were comparable to the CNCs by conventional methods whereas the processing was greener and more efficient.



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A possible mechanism for the fabrication of CNCs from cotton using the presented method is illustrated in Figure 7. The choline chloride/oxalic acid dihydrate DES contains a high abundance of nonsymmetric chloride ions and carboxylic acid molecules, possessing the features of both ionic liquids and organic solvents. The strong hydrogen bond interactions in cotton may be weakened because of competing hydrogen bond formation between the chloride ions in DES and the hydroxyl groups in carbohydrates, consequently breaking the intramolecular hydrogen bond network. The oxalic acid solvent can dissolve the oligosaccharides and cellulose glucoses. Therefore, when an MwDES pretreatment is applied to cotton, the primary cell wall in cotton, most of the oligosaccharides and partial amorphous cellulose can be degraded and dissolved in DES, leaving some crystalline cellulose fragments. The linkages within the microfibers are weakened and the fragments are swollen to provide a higher accessibility. On this occasion, the high-intensity ultrasonication can play a disintegrating role in the separation of CNCs. The surface of the CNCs was slightly modified by the oxalic acid after the treatment. The carboxyl moieties on the surface of the CNCs can induce a well dispersion of the CNCs suspensions. Thus, the combination of MwDES and high-intensity ultrasonication is considered a substitute to strong acid hydrolysis for the fabrication of CNCs.

CONCLUSION In summary, this study demonstrated a facile, fast and green method to produce qualified CNCs with high yields from cotton. The choline chloride/oxalic acid dihydrate MwDES pretreatment was efficient for the cleavage of the strong hydrogen bonds in cotton and swelling of the undissolved fragments. The combination of MwDES pretreatment and high-intensity ultrasonication had a synergetic effect on disintegrating the fiber fragments into CNCs. The MwDES pretreatment under 800 W at 80 °C for 3 min and ultrasonication at 1200 W for 30 min was determined to be optimal, and the yield of CNCs was 74.2%. The CNCs showed uniform morphology with diameters in the range of 3–25 nm and lengths within 100–350 nm. They also showed high crystallinity (82%) and high thermal stability (>320 °C). Therefore, the presented method facilitates the green and fast processing of cotton while not affecting the yield and properties of the CNCs.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b009543. Experimental details for producing CNCs and related figures for additional characterizations (PDF).

AUTHOR INFORMATION Corresponding Author * H. Yu, E-mail: [email protected]. ORCID Haipeng Yu: 0000-0003-0634-7913 Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 31670583, 31622016), and the Natural Science Foundation of Heilongjiang Province of China (Grant No. JC2016002).

REFERENCE 1. Wakelyn, P. J.; Bertoniere, N. R.; French, A. D.; Thibodeaux, D. P.; Triplett, B. A.; Rousselle, M. A.; Gamble, G. R. Cotton Fiber Chemistry and Technology. CRC Press, 2006.



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Thermostable

Cellulose

Nanocrystals

via

Two-step

Swelling-Hydrolysis Extraction. Cellulose 2014, 21, 4195–4207.


Ionic

Liquid


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Figure 1 1200W ultrasonic treatment on the pretreated CFs (concentration: 0.5% w/w) for 30 min. The CFs were pretreated by: (a) 5% H2SO4 at 90 °C for 2 h, (b) 5% NaOH at 90 °C for 2 h, pH=11, (c) NaClO2 pulping process and (d) MwDES at 80 °C for 3 min.


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Figure 2 SEM images of the pretreated CFs by (a) 5% H2SO4 at 90 °C for 2 h, (b) 5% NaOH at 90 °C for 2 h, pH=11, (c) NaClO2 pulping process, and (d) MwDES at 80 °C for 3 min.



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Figure 3 Optical photographs and polarized optical microscope images of the MwDES pretreated CFs and the corresponding CNCs suspensions: (a, e) MwDES-70; (b, f) MwDES-80; (c, g) MwDES-90 and (d, h) MwDES-100.


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Figure 4 TEM images, diameter and length distribution histograms of (a) MwDES-80 CNCs, (b) MwDES-90 CNCs and (c) MwDES-100 CNCs.



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Figure 5 (a) 13C CP/MAS NMR spectra and (b) FTIR spectra of the CFs, MwDES-80 CFs and MwDES-80 CNCs.


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Figure 6 (a) XRD curves, (b) relative crystallinity, (c) TG curves and (d) DTG curves of CFs and the CNCs.



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Figure 7 Schematic diagram of fabrication of CNCs by through MwDES pretreatment and a subsequent high-intensity ultrasonication process.


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Table 1 The yield of MwDES pretreated CFs at different conditions Temperature /°C

Power /W

Time /min

Yield /%

MwDES-70 CFs

70

800

5

75.9

MwDES-80 CFs

80

800

3

74.2

MwDES-90 CFs

90

800

3

62.4

MwDES-100 CFs

100

800

3

57.8



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Table 2 Composition and DP analysis of the CFs and MwDES pretreated CFs Cellulose /% Oligosaccharides /% Insolubles /%

DP

CFs

94.2

1.6

0.5

1600

MwDES-80 CFs

90.3

5.4

0.5

149

MwDES-90 CFs

89.8

6.0

0.4

125

MwDES-100 CFs

88.4

7.6

0.5

109


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Table 3 Comparison of the CNCs prepared in this work and those in previous literatures Feedstock

Method

Temp. Time Yield Diameter Length /nm /°C /h /% /nm

MwDES Cotton fiber hydrolysis, 80 0.5 74 ultrasonication SA(65%) Cotton linter 45–72 0.5 – hydrolysis SA(64%) Cotton pulp 50 5 62.1 hydrolysis SA(64%) Cotton fiber hydrolysis, 45 1 43.5 ultrasonication SA(63.9%) Cotton fiber 50 0.75 41.7 hydrolysis SA(64%) Cotton fabric 45 1 30–35 hydrolysis SA(65%) Cotton fiber hydrolysis, 45 1.5 52–65 ultrasonication SA(60%) Cotton linter 45 1 – hydrolysis SA(64%) 40 2 12.8 Cotton linter hydrolysis SA(63.5%) Cotton MCC hydrolysis, 44 3 21.5 ultrasonication Mixed acid 55 7 46.7 Cotton cloth hydrolysis TEMPO Cotton linter 30 4 45.7 oxidation Ionic liquid swelling and Cotton fiber 80 3–18 15 SA(1-4%) hydrolysis Microbial Cotton MCC 25 120 22 hydrolysis DES Wood pulps 60–100 2–6 66–88 hydrolysis

CrI /%

Tmax Ref. /°C

3–25

100–350

82

354

This work

6–70

25–320

–

–

9

–

248

85

–

10

–

197

–

–

11

–

Efficient Cleavage of Strong Hydrogen Bonds in Cotton by Deep

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