Tailored and integrated production of functional cellulose nanocrystals

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Tailored and integrated production of functional cellulose nanocrystals and cellulose nanofibrils via sustainable formic acid hydrolysis: kinetic study and characterization Dong Lv, Haishun Du, Xinpeng Che, Meiyan Wu, Yuedong Zhang, Chao Liu, Shuangxi Nie, Xinyu Zhang, and Bin Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00714 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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ACS Sustainable Chemistry & Engineering

Tailored and integrated production of functional cellulose nanocrystals and cellulose nanofibrils via sustainable formic acid hydrolysis: kinetic study and characterization Dong Lv,†, ⊥ Haishun Du,†,§, ⊥ Xinpeng Che,†,‖ Meiyan Wu,† Yuedong Zhang,† Chao Liu,*,† Shuangxi Nie,‡ Xinyu Zhang,§ and Bin Li*,† † CAS

Key Laboratory of Biofuels, CAS Key Laboratory of Bio-based Material, Dalian National

Laboratory for Clean Energy, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong, 266101, China Mailing address: No. 189, Songling Road, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong, 266101, China ‡

Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Guangxi

University, Nanning, Guangxi, 530004, China Mailing address: No. 100, Daxue East Road, Guangxi University, Nanning, Guangxi, 530004, China § Department

of Chemical Engineering, Auburn University, Auburn, Alabama, 36849, USA

ACS Paragon Plus Environment

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Mailing address: 212 Ross Hall, Auburn, Department of Chemical Engineering, Auburn University, Auburn, Alabama, 36849, USA ‖

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao,

Shandong, 266042, China

Mailing address: No. 53 Zhengzhou Road, College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, Shandong, 266042, China

⊥

Both authors contributed equally.

Corresponding authors: *C. Liu: E-mail address: [email protected] (C. Liu) *B. Li. E-mail address: [email protected] (B. Li).

ABSTRACT: Cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) are of great interest to researchers due to their outstanding properties and wide application potentials. However, green and sustainable production of CNCs and CNFs is still challenging. In this work, the integrated and sustainable production of functional CNCs and CNFs was achieved by formic acids (FA) hydrolysis. Kinetic study for FA hydrolysis of cellulosic pulp was performed to investigate the hydrolysis mechanism. FA concentration of 80-98 wt.%, reaction temperature of 70-100 oC, and reaction duration up to 24 h were employed to capture the feature of the coexistence of a diversity of reaction products, i.e. CNCs, cellulose solid residue (CSR), cellulose formate (CF), xylose, glucose, and furfural. The separated CSR was further fibrillated


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to CNFs by homogenization. It was found that the yield, morphology, crystallinity, thermal stability, and degree of esterification of CNCs and CNFs were significantly affected by hydrolysis conditions (particularly for acid concentration). Detailed characterization indicated that the as-prepared CNCs exhibited high thermal stability (maximal weight loss temperature of 375 oC) and high crystallinity index of 79%. Both the resultant CNCs and CNFs showed good dispersibility in dimethylacetamide due to the introduction of ester groups on cellulose surface during FA hydrolysis. More interestingly, the regenerated CF was also a kind of functional CNFs with more ester groups. These ester groups would enable the CNCs/CNFs to be potentially used in polymeric materials due to the hydrophobic surface. Therefore, this study provided fundamental knowledge for the sustainable and integrated production of thermally stable and functional CNCs and CNFs with tailored characteristics.

KEYWORDS: Cellulose nanomaterials, Formic acid hydrolysis, Cellulose nanocrystals, Cellulose nanofibrils, Kinetic study.

INTRODUCTION Cellulose as a promising alternative material for petroleum-based materials is abundantly available in nature.1 Over the past few decades, the production of cellulose nanomaterials (CNM) from cellulosic materials has gained increasing attention, due to the appealing properties of these renewable nanomaterial with many potential applications, such as aerogel and hydrogel,2 reinforcement in nanocomposites,3 packaging materials,4 and biomedical materials.5 Based on the size, morphology and preparation techniques, two main kinds of CNM: cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) could be produced from lignocellulosic


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biomass by the top-down process.6 Among them, CNCs are rigid rod-like particles of 5-30 nm in diameter and several hundred nanometers in length, usually with a high crystalline structure.7 CNFs are flexible fiber-like with a diameter less than 100 nm and the length of 500 nm to several microns, which are composed of both disordered and crystalline domains.8 CNCs are mainly produced through strong mineral acid hydrolysis, such as sulfuric acid,9 hydrochloric acid,10 and phosphoric acid.11 Among them, sulfuric acid is the most commonly used acid for producing the sulfonated CNCs with good dispersibility in water.12 However, the sulfonated CNCs usually exhibit poor thermal stability due to the dehydration reaction induced by the residual sulfate groups, which prevents their further applications involved in thermal processing (e.g. injection molding, extrusion).13 The strong acid hydrolysis is indeed a simple and time-saving method for the preparation of CNCs, but the use of mineral acid usually inflicts large water usage, relatively low production yield, and disposal problem.14 Different from CNCs, CNFs are often produced through mechanical fibrillation such as high-pressure homogenization, microfluidization, grinding, and ultrasonication.15 However, mechanical fibrillation for CNFs production is very energy-intensive. Thus, pretreatment methods (e.g. enzymatic hydrolysis,16 TEMPO-mediated oxidation,17 carboxymethylation,18 and quaternization19) are proposed to reduce the energy consumption in the post mechanical deconstruction. Although the energy consumption could be remarkably reduced in this case, there are still many drawbacks in the existing pretreatment processes such as low processing efficiency, the use of hazardous reagent, and difficulty in chemical recovery.20 To improve the economics of CNM production, a suitable strategy was first proposed by Wang et al. to integrate the production of CNCs and CNFs to achieve near zero cellulose loss.21, 22

This strategy was based on the fact that a majority of cellulose was remained as the partially


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hydrolyzed cellulosic solid residue (CSR) after acid hydrolysis, which could be used to produce CNFs by subsequent mechanical fibrillation with a relatively lower energy input. Besides the above development towards the integrated production of CNM, some sustainable and environmentally friendly methods based on the replacement by recyclable chemicals have been invented to address the above drawbacks for the use of inorganic acids, such as solid acid (e.g. phosphotungstic acid) hydrolysis,23 organic acid (e.g. maleic acid and toluenesulfonic acid) hydrolysis,24, 25 and deep eutectic solvents treatment.26 Particularly, Zhu’s group demonstrated the tailored production of carboxylated CNCs and CNFs using recyclable di-carboxylic acids, which could achieve integrated production of thermally stable and carboxylated CNCs and CNFs with simple recovery of the acids.27, 28, 29 These promising findings showed great industrialization prospects for the production of CNM by green and sustainable processes.30, 31 It is noteworthy that preparation conditions can significantly affect the yield and property of the produced CNM. For example, Chen et al. demonstrated the tailored production of CNCs by controlling the concentration of sulfuric acid, which not only improved CNCs yields but also realized tailored production of CNCs with desired properties.32 Moreover, CNM with different properties determine different end applications. For instance, CNCs with high crystallinity and high thermal stability could be more suitable to be used as reinforcing nanofillers for nanocomposites.31 Therefore, it is of great importance to tailor the production of CNM with the desired properties for specific applications. Kinetic study as a mathematical tool is of great vital to optimize the production conditions of CNM towards large-scale process. For example, Wang et al. proposed a reaction kinetic model for CNCs production by sulfuric acid hydrolysis, and they demonstrated that acid concentration was the key factor to control the yield and property of CNCs.33 Xu et al. reported the kinetic


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study of CNCs production by oxalic acid hydrolysis with thoroughly study of acid concentration, addition of catalyst, and reaction time.34 Dong et al. developed a kinetic model of FA pretreatment of wheat straw to study the kinetics of delignification and polysaccharide solubilization during FA pretreatment.35 These fundamental works provided a great insight into the hydrolysis pathway of lignocellulose and controllable production of CNM. In our previous study, FA hydrolysis was demonstrated as an effective pretreatment for the preparation of CNM. It was found that the cellulose fibers were swelled and broken down during FA hydrolysis, which benefited to the subsequent fibrillation.20 Moreover, when FeCl3 was introduced into the reaction as the catalyst, CNCs could be more easily produced due to the enhanced hydrolysis efficiency,7 and the cellulosic solid residue (CSR) could be further fibrillated to CNFs with relatively low-intensity fibrillation.36 Thus, FeCl3-catalyzed FA hydrolysis could realize the integrated production of CNCs and CNFs. In addition, ester groups could be introduced on the surface of CNCs/CNFs due to the reaction of FA and cellulose, resulting in a better thermal stability and hydrophobic surface.36 Intriguingly, FA hydrolysis could be directly conducted for various lignocellulosic biomass (e.g. tobacco stalk,37 corn husk38) to produce lignin-containing CNFs, which could be used to fabricate cellulose nanopaper (CNP) with improved water-resistance, high toughness, and UV-blocking ability. Therefore, the FA hydrolysis is a clean and sustainable route for the production of thermally stable and functional CNM, and the product demonstrated great potential use in many fields. However, the kinetic and mechanism study for the integrated and tailored production of CNM based on FA hydrolysis has not been explored. In the present work, a kinetic study for the tailored and integrated production of CNCs and CNFs from cellulosic pulp via FA hydrolysis was conducted to better understand the mechanism


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of FA hydrolysis. Also, the morphology, chemical and crystalline structure, and thermal stability of the prepared CNCs and CNFs were thoroughly assessed. This fundamental study will be beneficial to the scale-up design for the production of CNM and the tuning of the properties of the resultant CNCs and CNFs for their specific end applications. EXPERIMENTAL SECTION Materials. Bleached eucalyptus kraft pulp (BEKP) was supplied by Mudanjiang Hengfeng Paper Co., Ltd., China. The glucan, xylan and lignin contents of BEKP were 79.5±0.3%, 15.5±0.2%, and less than 0.1%, respectively. Before FA hydrolysis, BEKP was disintegrated for 1 min with a kitchen blender, and then stored with a dryness of 94.5±0.3% at atmosphere temperature. FA (88 and 98 wt.%), anhydrous ferric chloride, and dimethylacetamide (DMAc) were purchased from Sinopharm Chemical Reagent Co., Ltd. FA hydrolysis. Reaction parameters of FA hydrolysis were controlled to capture all possible reactions. A relatively narrow range of acid concentration of 80-98 wt.%, reaction temperature of 70-100 oC, and a relatively large range of hydrolysis time (0-24 h) were employed. To facilitate the hydrolysis process, 8 wt.% of FeCl3 (based on the oven dried pulp) was added as catalyst, which was beneficial to the CNCs preparation with a high crystallinity index (CrI) according to our previous work.7 BEKP was directly treated with FA (1:30, w/v) under continuous mechanical agitation at 180 rpm. At regular intervals, time-dependent data were gained by sampling a certain amount of reaction mixture. After sampling, the sample was immediately centrifuged at 8000 rpm for 4 min to separate the precipitate and quench the reaction. The hydrolysate was collected for the analysis of sugars and sugar degradation products. The precipitate was washed with de-ionized water until neutrality. After that, the sediment was


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dispersed in DMAc and subsequently centrifuged at 5000 rpm for 4 min. The CNCs could be obtained in supernatant (because the obtained CNCs with a relatively high degree of substitution (DS) could be well dispersed in DMAc) and the remaining gel-like CSR was collected for further mechanical nanofibrillation to produce CNFs. High-pressure homogenization. The obtained CSR from the above FA hydrolysis was dispersed in DMAc with a concentration of 0.2 wt.%. Afterwards, the suspensions were passed through a high-pressure homogenizer (ATS Engineering Inc., China) for 3 times at 300 bar and then 7 times at 500 bar. After homogenization, the obtained CNFs were collected for further characterization. The integrated production of CNCs and CNFs via FeCl3-catalyzed FA hydrolysis is schematically shown in Fig. 1.

Figure 1. Schematic diagram of FeCl3-catalyzed FA hydrolysis for the integrated production of CNCs and CNFs Characterization. The chemical composition of BEKP was determined following the National Renewable Energy Laboratory procedure.39 The carbohydrate contents of the


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supernatant were determined using high-performance anion-exchange chromatography with pulsed amperometric detection (Dionex ICS-3000, USA). Identical chromatography system and conditions were used according to our previous work.7 Degradation products of sugars in the supernatant were analyzed by High Performance Liquid Chromatography (HPLC, Waters 2489, USA) with ultraviolet detector (284 nm) and SunFire C18 (4.6 mm × 250 mm) chromatographic column. Ethanol/water (volume ratio of 1:4) was used as mobile phase with flow rate of 1 mL/min at 35 oC. The yields of CNCs, CSR and cellulose formate (CF) were obtained using gravimetric method.40 The morphologies of CSR samples were observed by a scanning electron microscope (SEM, Hitachi S-4800, Japan) at 3.0 or 5.0 kV. A transmission electron microscope (TEM, Hitachi H-7600, Japan) was used to observe the morphologies of CNM. Fourier transform infrared (FTIR) analysis was conducted by a Thermo Nicolet FTIR spectrometer (Nicolet 6700, USA) in the wavenumber range of 400-4000 cm−1 with a resolution of 4 cm−1. The X-ray diffraction (XRD) patterns were obtained by an X-ray diffractometer (Bruker Discover D8, Germany) with Ni-filtered Cu Kα radiation at 40 kV and 40 mA. Scattering angle (2θ) ranged from 5 to 40 ° and the scanning rate of 4 °/min were employed. The CrI of each sample was calculated according to the Segal method.41 The thermal stability was studied through a thermogravimetric analyzer (TA Q600, USA). The temperature elevated from 25 to 600 oC at a heating rate of 10 oC/min under the protection of nitrogen (25 mL/min). The dispersibility tests of CNM suspensions were carried out by centrifugation and solvent exchange with DMAc and water, respectively. Samples were treated by a VWR ultrasonic cleaner (45 kHz and 180 W) for 10 min and the pictures were taken immediately after ultrasonic treatment and after standing for 48 h, respectively.


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Kinetic modelling. Cellulose, hemicellulose, and lignin are the three main components of pulp fibers, but the trace level of lignin (< 0.1%) in BEKP was neglected. Cellulose consists of two fractions, namely, crystalline region and disordered region. The hydrolysis rates of these two regions are varied greatly. As reported, the disordered region of cellulose had a 2-30 times faster hydrolysis rate compared to the crystalline region, because the crystalline region with more regular structure showed bigger resistance to acid attack.42 Along with the hydrolysis of disordered region, the rod-like CNCs could be obtained because of the release of crystalline region. The hydrolysis of cellulose and hemicellulose will inevitably result in the generation of monosaccharides, which also might be further degraded into downstream products like 5hydroxylmethfurfural (HMF) and furfural, respectively. However, according to our previous work,20 HMF was undetectable in the supernatant of FA hydrolysis due to the weak acidity of FA (pKa = 3.77) compared to mineral acids like H2SO4 (pKa = -3.0). It was also reported that FeCl3 had little catalytic activity on dehydration of glucose to HMF.43 Therefore, it was assumed that there was no further degradation product of glucose in this study. In addition, as reported, the de-polymerization of oligomers to monosaccharides was much faster than the formation of oligomers from glycan.44 Thus, the concentration of oligomers was very low, which could be neglected. Also, FA could swell and react with cellulose to form CF without using catalyst,45 and the resultant CF could be further degraded to glucose.20 In addition, soluble oligo-cellulose formate (or glucose formate) may be generated as the degradation product of CF. Considering its small amount and difficulty in quantization, the oligo-cellulose formate (or glucose formate) was neglected in this work. Based on the analysis above, it could be proposed that the FA hydrolysis of BEKP could involve four pathways, which are schematically represented in Fig. 2.


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Figure 2 Schematic of the proposed reactions for FA hydrolysis of BEKP for the production of CNCs and CSR. The first and principal pathway was the production of CNCs, which was attributed to the depolymerization of disordered cellulose, as represented by a rate constant of k1. Part of the produced CNCs could be further degraded to glucose under severe hydrolysis conditions (represented by a rate constant of k2). The second pathway was the direct hydrolysis of cellulose to release glucose with a rate constant of k3. The third pathway was the formation of CF and its further degradation which could be represented by the rate constants of k4 and k5, respectively. The last pathway was the degradation of hemicellulose to xylose, as represented by a rate constant of k6. Unlike glucose, xylose could be further degraded to furfural with a rate constant of k7. After FA hydrolysis, the remaining gel-like CSR was collected to go through further homogenization to produce CNFs. First order reactions were used to develop a mathematic model for the discussed scheme, which was normally used to model the hydrolysis of lignocellulose.46 (1) Production of CNCs


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The CNCs production relates to the first three pathways in the scheme (Fig. 2), which all eventually contributed to the generation of glucose. It is easy to understand that there is a fraction of cellulose which is susceptible to be hydrolyzed under a certain hydrolysis condition.35 Here, a parameter 𝜀 (0 ≤ 𝜀 ≤ 1) was defined to describe this hydrolysable fraction of cellulose, which could also be called as “hydrolysis degree”. With the degradation of cellulose, CNCs and CF are produced and then both could be partially degraded to release glucose. Using first order kinetics, the hydrolysis of cellulose for CNCs production could be expressed by the following differential equations:

-

dCcel dt

dCCNCs dt dCCF dt dCglu dt

=

=

(k1 + 𝑘3 + k4)Ccelt

=

k1Ccelt - k2CCNCs

(1)

(2)

k4Ccelt - k5CCF

(3)

= 1.111(k2CCNCs + k3Ccelt +

k5CCF)

(4)

Where Ccel is cellulose concentration remaining in the hydrolysate, g/L; Ccelt is the concentration of hydrolysable cellulose containing in unhydrolyzed solid at a certain time, g/L; CCNCs is the concentration of CNCs, g/L; CCF is the concentration of CF, g/L; Cglu is the concentration of glucose, g/L; the coefficient 1.111 is for the molecular weight conversion of glucan to glucose. The cellulose remaining in the reactive system in the terms of 𝜀 is: Ccelt = Ccel - Ccel0 (1 -

ε)

(5)

Where Ccel0 is the initial cellulose concentration, g/L.


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Integrate the above differential equations using the initial conditions of Ccel = Ccel0, CCNCs = 0, CCF = 0 and Cglu = 0 at t = 0, the following equations were obtained. Ccel =

CCNCs =

k1Ccel0ε k1 + k3 + k4 - k2

k1 + k3 + k4 - k5

𝐶Glu =

+

ε (1 - exp( - (k1 + k3 + k4)t)) ]

k4Ccel0ε

CCF =

-

Ccel0 [1 -

(exp( - k2t) - exp( - (k1 + k3 + k4)t))

1.111k3Ccel0ε

(7)

(exp( - k5t) - exp( - (k1 + k3 + k4)t))

1.111k1k2Ccel0ε k1 + k3 +

(6)

1 - exp( - k2t)

k4 - k2

(

k2

+

(8)

exp( - (k1 + k3 + k4)t) - 1 k1 + k3 +

)

k4

exp( - (k1 + k3 + k4)t) - 1

1.111k4k5Ccel0ε k1 + k3 + k4 -

k1 +

(

k3 +

k4

1 - exp( - k5t)

k5

k5

+

exp( - (k1 + k3 + k4)t) - 1 k1 +

k3 +

k4

)

(9)

(2) Hydrolysis of xylan to xylose Similar with the hydrolysis of cellulose, part of hemicellulose could be more easily hydrolyzed compared to other part of xylan. Herein, a coefficient η (0 ≤ η ≤ 1) was also introduced as the hydrolysis degree of xylan to distinguish these two fractions. The hydrolysis of xylan can be described as:

-

d𝐶Xn dt

dCxyl dt

=

=

k6𝐶Xnt

1.136k6𝐶Xnt - k7𝐶xyl

(10)

(11)


13


dCfur dt

= k7Cxyl

Page 14 of 45

(12)

Where CXnt is the concentration of hydrolysable xylan, g/L; CXn is the concentration of the remaining xylan, g/L; Cxyl is the xylose concentration in the reaction system, g/L; Cfur is the concentration of furfural, g/L; 1.136 is the coefficient for the molecular weight conversion of xylan to xylose. Likewise, CXn could also be expressed in the terms of η:

CXnt = CXn - CXn0(1 - η)

(13)

Where CXn0 is the initial concentration of xylan in BEKP, g/L. Introducing the initial conditions of CXn = CXn0, Cxyl = 0 and Cfur = 0 at t = 0, the integral form of the above equations were obtained:

CXn = CXn0[1 - η(1 - exp( - k6t))]

Cxyl =

Cfur =

1.136k6𝐶Xn0η k6 -

k7

(exp( - k7t) - exp( - k6t))

1.136k6CXn0η k6 - k7

(14)

(1 - exp( - k7t)) -

(15)

1.136k7CXn0η k6 -

k7

(1 - exp( - k6t))

(16)

(3) The yields of CNCs, CSR and CF The yields of CNCs, CSR, and CF were calculated based on the initial cellulose mass concentration of BEKP which was expressed as Cpulp, eq. (17) was employed to calculate the yield of CNCs:

YCNCs =

CCNCs Cpulp

×

100%

(17)


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By the use of eq. (7), the yield of CNCs is transformed to: k1Ccel0ε

YCNCs =

Cpulp(k1 + k3 + k4 -

(exp( - k2t) - exp( - (k1 + k3 + k4)t)) × 100%

k2)

(18)

Similarly, the yield of CNFs can be deduced from the concentration of CSR in the reaction system, which is the sum of un-hydrolyzed cellulose and xylan. The time course concentration of CSR (by introducing eq. (6) and (14)) is: Ccel + CXn = Ccel0[1 - ε(1 - exp( - (k1 + k3 + k4)t))] + - k6t))] (19)

CCSR =

CXn0[1 - η(1 - exp(

Thus, the yield of CNFs can be calculated as:

YCNFs

=

CCSR

=

CCel0[1 - ε(1 - exp( - (k1 + k3 + k4)t))] +

Cpulp × 100%

CXn0[1 - η(1 - exp( - k6t))]

Cpulp

(20)

And CF yield could be given by:

YCF =

CCF

×

Cpulp

100%

(21)

Use eq. (8), the time courses yield of CF is obtained, YCF =

k4Ccel0ε

Cpulp(k1 + k3 + k4 (22)

k5)

(exp( - k5t) - exp( - (k1 + k3 + k4)t)) × 100%

(4) Maximum yield of the desired product


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As can be seen from eq. (18), it’s easy to discover that the yield of CNCs could reach a maximum value for a given hydrolysis condition, which means that there is an optimal time for CNCs production. Take the derivative of eq. (18), the optimal time is gained as:

to =

ln((k1 +

k3 + k4)/k2))

k1 + k3 + k4 -

(23)

k2

It is worth to notice that the optimal time exists only when k2 is not equal to zero. Then the corresponding maximum yield of CNCs is obtained: k2

YCNCs max =

k1Ccel0ε Cpulp(k1 +

(

k3 + k4)

k1 + k3 + k2

)

k4 k2 - k1 - k3 - k4

(24)

RESULTS AND DISCUSSION To simplify discussion, labels (xx, yy, zz) were used to describe hydrolysis conditions of acid concentration (CFA, wt.%), temperature (T, oC), and hydrolysis time (t, h), respectively, and the total yield of CNCs plus CNFs was marked as YCNM. Kinetic rate constants obtained by fitting the experimental yields of CNCs, CSR, CF, glucose, and xylose to the model are listed in Table S1. CNCs production.


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Figure 3. Kinetic model fitted CNCs yields at different hydrolysis conditions in comparison with experimental results (CFA = 80 and 84 wt.% (a), 88 wt.% (b), 92 wt.% (c), 98 wt.% (d)). (xx, yy) in the legend represents CFA and reaction temperature, respectively. Comparison of time-dependent experimental data with the model-fitted values for CNCs yields is displayed in Fig. 3, which shows good agreement, and CFA played a dominate role in CNCs production compared to reaction temperature and time. For instance, only 8.3% of the maximum CNCs yield was obtained under CFA of 80 wt.% at 90 oC for 24 h, while the value could reach to 30.2% with the CFA of 98 wt.% at 90 oC for 6 h. In addition, temperature affected CNCs yields as well, especially with a lower CFA. For example, the maximum yields of CNCs increased from 8.2 to 19.9% when temperature elevated from 70 to 80 oC under the CFA of 88 wt.%. Certainly, hydrolysis time also had clear impact on CNCs production under the fixed CFA and hydrolysis temperature (Fig. 3). For most of the employed hydrolysis conditions, the maximum CNCs yields could be achieved within 24 h. However, for the case with a lower CFA of 80 and 84 wt.% (Fig. 3a), the maximum


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CNCs yields were achieved after 24 h hydrolysis even at a relatively high temperature, indicating the insufficient hydrolysis of pulp fibers under these mild conditions. Therefore, this relatively low CFA was not suitable for CNCs production with a high yield. Yet, there was a palpable decrease of CNCs yields under severe hydrolysis conditions after the point of maximum CNCs yields. For example, under the high CFA (98 wt.%) and high temperature (100 ºC), the CNCs yields decreased remarkably with the increase of hydrolysis time (after 4 h). This phenomenon could be ascribed to the further degradation of CNCs into oligomeric and monomeric sugars. Therefore, the production of CNCs depended on the competing reactions between the hydrolysis of BEKP to CNCs (k1) and the degradation of CNCs to glucose (k2). Thus, to get a relatively high yield of CNCs, the hydrolysis conditions should be well controlled to avoid the insufficient hydrolysis and the excess de-polymerization of cellulose to glucose. XRD analysis of CNCs obtained by various hydrolysis conditions could also indicate the degree of hydrolysis process. As shown in Fig. S1, all samples showed positions of diffraction peaks at 2θ = 15.1, 16.5, and 22.6°, corresponding to the (1-10), (110), and (200) crystallographic planes of cellulose Iβ, respectively.47 Moreover, the CrI of CNCs rapidly increased from 65.6 (BEKP) to 71.0% after hydrolysis with CFA of 88 wt.% at 90 oC for 1 h, and reached a maximum value of 79.1% after hydrolysis for 6 h. This result could be ascribed to the fast hydrolysis of hemicellulose and disordered regions of cellulose at the early stage of FA hydrolysis.

13, 48

However, when the hydrolysis time was longer than 6 h, part of the crystalline

regions of CNCs started to degrade. Therefore, the CrI of CNCs decreased to 72.8 and 66.9% after 12 and 24 h, respectively. In addition, elevated temperatures could obtain the CNCs with a relatively higher CrI. For instance, with the CFA of 88 wt.% and hydrolysis time of 12 h, the CrI values of CNCs produced at 70, 90, and 100 oC were 69.2, 72.8, and 73.4%, respectively.


18

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Furthermore, CFA could significantly affect the CrI of CNCs as well. The CrI of CNCs decreased along with the increasing of CFA. For example, after hydrolysis for 12 h at 90 oC, the CrI of CNCs with CFA of 80, 88, and 98 wt.% were 73.2, 72.8, and 62.5 %, respectively. Semi-quantitative analyses of the lengths for different CNCs samples obtained from different hydrolysis conditions were estimated by dynamic light scattering (DLS), as displayed in Fig. S2. CNCs showed relatively narrow particle size distribution. The average lengths of the CNCs samples (from a to h) were 345, 270, 192, 289, 276, 235, 168, and 124 nm, respectively. Samples a, b, and c showed the CNCs length largely decreased with the increase of CFA. Samples b, d, e, and f showed the shorter CNCs length with the elevated reaction temperatures. Furthermore, based on the mean sizes of the samples b, g, and h, it was found that the length of CNCs reduced with the prolonged the reaction time, which was due to the fact that FA could further degrade the resultant CNCs to glucose. The particle size of the prepared CNCs was comparable with the CNCs produced by other organic acid (e.g. oxalic acid).27


19


Page 20 of 45

Figure 4. TEM images of CNCs obtained under the hydrolysis conditions of (80, 90, 2) (a), (88, 90, 2) (b), (98, 90, 2) (c), (88, 70, 2) (d), (88, 80, 2) (e), (88, 100, 2) (f), (88, 90, 12) (g) and (88, 90, 24) (h) and the length distribution of CNCs from sample (88, 90, 12) (i). TEM analysis was conducted to observe the fine morphologies of the prepared CNCs. As shown in Fig. 4a, under mild hydrolysis conditions, crystal aggregates and long CNCs could be obviously observed due to the incomplete fibrillation and the less-charged surface of the produced CNCs. Increasing hydrolysis severity (like using higher CFA (Fig. 4b and 4c compared to Fig. 4a), higher reaction temperature (Fig. 4f compared to Fig. 4b) or prolonged reaction time (Fig. 4g and 4h compared to Fig. 4b)) could improve crystal separation with more uniform and shorter CNCs particle sizes, which was in line with the average particle sizes shown in Fig. S2. Consequently, relatively homogenous CNCs samples could be obtained by tuning the reaction


20

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conditions. As can be seen in Fig. 4h, very short CNCs lengths (with the average length of 104 nm) were obtained when the reaction time was prolonged to 24 h. As shown in Fig. 4i, the length of CNCs prepared under the condition of (88, 90, 12) was in the range of 50-293 nm (based on measurements of 100 individual CNCs with clearly identifiable ends), and the average length (141 nm) of the CNCs was basically consistent with the results (168 nm) from DLS analysis (Fig. S2). The diameter distribution (Fig.S3) of the prepared CNCs under the condition of (88, 90, 12) presented that the diameter range and the average diameter were 5-21 and 11 nm, respectively. Production of CSR and CNFs.

Figure 5. Kinetic model fitted CSR yields at different hydrolysis conditions in comparison with experimental results (CFA = 80 and 84 wt.% (a), 88 wt.% (b), 92 wt.% (c), 98 wt.% (d)). (xx, yy) in the legend represents CFA and reaction temperature, respectively. As shown in Fig. 5, a relatively large range of CSR yields from 100% to near zero could be obtained under the hydrolysis conditions investigated. CSR yields rapidly decreased to approximately 3.5% after hydrolysis for 6 h under CFA of 98 wt.%, demonstrating the fast


21


Page 22 of 45

hydrolysis rate of pulp fibers under strong hydrolysis conditions. Since the resultant CSR was further used to prepare CNFs by homogenization, a relatively high yield was desired. Therefore, high CFA like 98 wt.% or high hydrolysis temperatures like 100 oC were not suitable to achieve a relatively high YCNM. SEM analysis of BEKP and CSR were conducted to well illustrate morphological changes along with hydrolysis proceeding. Fig. S4 exhibited that the compact structure of BEKP was broken up during FA hydrolysis, and fiber fines and fragments could be distinctly observed. Also, the morphology of the fiber surface became rough, and the internal microfibril bundles could be seen (Fig. S4b). This result was because of the fact that FA could infiltrate into the interior region of pulp fibers, swell the fibers, and hydrolyze hemicellulose and disordered domains of cellulose.40 When mild hydrolysis conditions like (80, 90, 2) were employed (Fig. S4b), the length of CSR samples changed slightly compared to BEKP, which might cause a high energy consumption and machine blockages during the post-homogenization. Also, the CNFs produced under this mild hydrolysis condition were heavily entangled with long lengths and big diameters (Fig. 6b). Therefore, even though high CNFs yields could be obtained under mild hydrolysis conditions, these experimental conditions were not suitable for efficient and high-quality of CNFs production. Properly increasing CFA and reaction temperature or prolonging reaction time could result in more fiber fines and fragments. The swelling and fragmentation of cellulose fibers could largely facilitate the following homogenization process with lower energy input.20 TEM images of the prepared CNFs are shown in Fig. 6. With relatively mild hydrolysis conditions (Fig. 6a, 6d, 6e), large microfibril bundles could be clearly observed due to the incomplete fibrillation, while with the increase of hydrolysis severity, relatively homogenous CNFs samples with shorter CNFs lengths and smaller diameters could be obtained (Fig. 6c, 6f


22

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and 6h). As shown in Fig. 6i, the diameter of the CNFs sample in Fig. 6g were 8-30 nm (based on the measurement of 100 individual CNFs with clearly identifiable ends).

Figure 6. TEM images of CNFs obtained under the hydrolysis conditions of (80, 90, 2) (a), (88, 90, 2) (b), (98, 90, 2) (c), (88, 70, 2) (d), (88, 80, 2) (e), (88, 100, 2) (f), (88, 90, 12) (g) and (88, 90, 24) (h) and the width distribution of CNFs from sample (88, 90, 12) (i). Formation of CF. As shown in Fig. 7, the profiles of CF yields displayed significant increase from lower than 2% to over 40% along with the increase of CFA from 80 to 98 wt.%, indicating the crucial role of CFA for CF formation. Similar to CNCs, CF also experienced a de-polymerization process to glucose, which was conspicuous under severe hydrolysis conditions, especially at a high CFA. It was worth to mention that CF yields showed rapid increase under severe hydrolysis conditions. For instance, CF with a high yield over 40% could be achieved within 5 h hydrolysis under CFA of 98


23


Page 24 of 45

wt.% at 100 oC, indicating the fast esterification of cellulose under high FA concentration. Although the experiments were not exhaustive enough, the results showed that the CF yields dramatically increased when CFA was higher than 92 wt.%.

Figure 7. Kinetic model fitted CF yields at different hydrolysis conditions in comparison with experimental results (CFA = 80 and 84 wt.% (a), 88 wt.% (b), 92 wt.% (c), 98 wt.% (d)). (xx, yy) in the legend represents CFA and reaction temperature, respectively. FTIR spectra are shown in Fig. S5. Compared with BEKP, a new band was formed at 1720 cm-1 in the spectrum of CNCs, CSR, CNFs, and CF. The new band could be ascribed to the C=O stretching of ester group,45 and the intensities of this new band for the samples were in the order of CF > CNCs > CSR = CNFs, which represented the degree of esterification of each substrate. The degree of esterification of CNCs was relatively bigger than that of CSR, probably due to more exposed hydroxyl groups on the surface of CNCs could react with FA. CSR and CNFs had similar degree of esterification, because CNFs was produced by CSR via physically mechanical process without any chemical reactions. Increasing CFA, bigger absorption peaks of C=O


24

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stretching could be observed for both CNCs and CNFs (e.g. sample of (98, 90, 12)), indicating the bigger degree of esterification of CNCs and CNFs under higher CFA. Therefore, to some extent, the hydrolysis conditions (particularly for CFA) could be controlled to tailor the integrated production of CNCs and CNFs with the desired degree of esterification and morphologies. Degree of esterification could be estimated by the degree of substitution (DS) of the samples determined using a back-titration method.20 The results are given in Fig. S6, which is consistent with FTIR analysis, and the DS of the obtained CNCs and CNFs were all less than 0.5. It was worth mentioning that CF with an obviously higher DS (1.42) was soluble in FA. Previous work reported that CF with the DS no less than 1.3 was soluble in FA.45 In addition, CF could be fast regenerated from FA hydrolysate by adding de-ionized water (Fig. S7), and the regenerated CF was floated on the surface of water, indicating its low density. To further illustrate the morphology of the regenerated CF, TEM images were taken for the sample under hydrolysis condition of (88, 90, 6). Interestingly, the regenerated CF had similar morphology like CNFs (Fig. S8a), with the diameter range and the average diameter of 7-27 and 16.1 nm, respectively (Fig. S8b). Compared to CNCs and CNFs, CF with a higher DS might be more beneficial for the compatibility with polymeric matrix materials (e.g. polylactic acid). Generation of glucose and xylose. The hydrolysis of glucan to glucose showed a linearly increasing tendency with the duration of hydrolysis (Fig. 8), and 5-HMF was not detected in hydrolysate, which was due to the weak acidity of FA.49 There were three origins of glucose: direct de-polymerization of BEKP, the degradation of CNCs, and the degradation of CF. At the initial phase of hydrolysis, the glucose mainly came from straight hydrolysis of BEKP. However, the de-polymerization of CNCs and CF would produce more and more glucose as the reaction proceeded, especially under high CFA


25


Page 26 of 45

and high reaction temperatures. As displayed in Fig. 8, glucose concentration had a pronounced increase with the increase of CFA, which was in keeping with the change of CSR yields.

Figure 8. Kinetic model fitted glucose concentrations under different hydrolysis conditions in comparison with experimental results (CFA = 80 and 84 wt.% (a), 88 wt.% (b), 92 wt.% (c), 98 wt.% (d)). (xx, yy) in the legend represents CFA and reaction temperature, respectively.


26

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Figure 9. Kinetic model fitted xylose concentrations at different hydrolysis conditions in comparison with experimental results (CFA = 80 and 84 wt.% (a), 88 wt.% (b), 92 wt.% (c), 98 wt.% (d)). (xx, yy) in the legend represents CFA and reaction temperature, respectively. Different from the hydrolysis of glucan, xylan showed relatively fast hydrolysis rate, especially at the beginning of FA hydrolysis. This was because hemicellulose could be more easily to be hydrolyzed than cellulose for FA hydrolysis.43 Thus, xylose concentration reached a peak in the first 5 h in the range of hydrolysis conditions. Over 70% xylose was released for all the reaction conditions investigated. Yet, Fig. 9 shows that a significant drop could be seen in the profiles of xylan hydrolysis. For instance, xylose concentration declined from 5 to 1.8 g/L in the time range of 3 to 24 h under CFA of 98 wt.% at 100 oC, indicating that there was the further degradation product of xylose which was confirmed to be furfural. By and large, furfural concentration was lower compared with other products. In spite of the small amount, furfural could significantly affect the color of hydrolysate from transparent to near black (Fig. S9), which was due to the generation of chromophoric groups by side reactions like condensation and coking reactions.50 Maximum yields of CNCs.

Table 1. The calculated maximum CNCs yields, the corresponding CSR yields and optimal hydrolysis time compared with the experimental results. CFA (wt.%) 80 80 84 84 88 88 88 88 92 92

T (oC) 90 100 90 100 70 80 90 100 70 80

topt (h) 35.3 43.5 30.5 27.7 86.9 56.7 15.4 12.6 22.8 19.3

texp (h) 24 24 24 24 24 24 14 12 22 18

YCNCs max (%) eq.(24) exp γ 8.2 8.3 16.0 16.5 15.9 17.4 18.1 18.5 8.2 8.4 19.9 20.1 24.2 25.4 26.6 26.7 15.2 16.3 24.3 23.9

YCSR (%) eq.(20) exp 65.2 64.4 59.1 56.1 52.1 49.8 45.9 44.0 68.7 68.2 47.5 45.7 26.9 30.2 25.8 25.6 47.0 46.2 30.9 33.5

YCNM (%) eq exp 73.4 72.7 75.1 72.6 68.0 67.2 64.0 62.5 76.9 76.6 67.4 65.8 51.1 55.6 52.4 52.3 62.2 62.5 55.2 57.4


27


92 92 98 98 98 98

90 100 70 80 90 100

12.5 10.2 16.2 11.1 6.0 3.9

12 8 14 10 6 4

26.9 29.1 13.3 16.7 27.3 26.8

28.4 29.0 14.3 18.7 30.2 29.0

19.5 17.6 23.6 11.0 4.5 5.7

Page 28 of 45

23.2 26.7 26.8 15.3 6.9 6.1

46.4 46.7 36.9 27.7 31.8 32.5

51.6 55.7 41.1 34.0 37.1 35.1

As discussed above, the achievable maximum CNCs yields and the corresponding hydrolysis time could be obtained by eq. (23) and (24), respectively. The calculated maximum CNCs yields, the corresponding CSR yields, and the optimal hydrolysis time, compared with the experimental results are all listed in Table 1, which presents that a good agreement was obtained between the experimental data and the calculated value, and YCNM decreased with the increasing of hydrolysis severity. CNCs yields were significantly affected by CFA, and 20% more CNCs yields could be achieved when CFA was higher than 84 wt.% due to the sufficient cellulose de-polymerization. However, when CFA was over 92 wt.%, cellulose was more likely transformed to CF, resulting in a low yield of CNM (below 50%). Therefore, CFA of 84-92 wt.% was more suitable to obtain a relatively high yield of YCNM. Also, maximum CNCs yields could be achieved within 14 h for most of the employed hydrolysis conditions. Thus, a relatively short hydrolysis time should be used to avoid the degradation of CNCs. Based on the above analysis, CFA of 84-92 wt.%, reaction temperature of 80-100 oC, and hydrolysis time of 4-14 h were more suitable to achieve high YCNM (without the consideration on the regeneration of CF). Sensitivity analysis. To assess the reliability of regression-obtained kinetic constants, sensitive analysis was conducted. Here, an acid impact factor p was introduced in expanded Arrhenius equation to correlate rate constants k with reaction temperature (T) and CFA, k = k0exp( -

Ea

)CpFA

RT

(25)


28

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Where k0 is the pre-exponential factor, Ea is the activation energy, and p is the reaction order regarding to CFA. Eq. (25) can also be expressed as: lnk =

lnk0 -

Ea RT

+

plnCFA

(26)

Thus, k0, Ea and p can be obtained by plotting lnk with CFA and 1/T. The results are listed in Table S2. Then the rate constants could be expressed using the following eq. 59583 8.1658 )𝐶FA 𝑅𝑇

k1 =

0.0019 ×

k3 =

4.944 × 107 ×

k4 =

1.3207 × 10 ―21 × exp( -

k6 =

2.7554 × 10 ―4 ×

exp( -

exp( -

(27a)

72015 0.6249 )𝐶FA 𝑅𝑇

exp( -

(27b)

49440 19.4356 )𝐶FA 𝑅𝑇

(27c)

24865 5.2455 )𝐶FA 𝑅𝑇

(27d)

Note that the relationships for k2, k5, and k7 were not established due to the lack of enough data. Fig. S10 shows the comparison between the values of k2, k5, k7 predicted by eq. (27) and the ones obtained by fitting the experimental data. A good agreement was achieved and only a few data showed large difference, which indicated a relatively good reliability of the fitting. Also, the Ea of cellulose degradation to glucose was higher than that of hemicellulose degradation to xylose (Table S2), which confirmed the easier de-polymerization of hemicellulose. The degrees of hydrolysable cellulose and xylan are functions of hydrolysis temperature and CFA. Thus, a temperature and CFA -related severity based on the combined severity factor concept was built (e.q. 28). 𝑅 = CpFAR0 = exp( -

T - 𝑇𝑟 S )CFA 14.75

(28)


29


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Where T is the temperature, oC; Tr is a reference temperature which was chosen as 25 oC; the exponent s represents the acid dependence of FA hydrolysis. The relationship between ε or η and R could be proposed as, ε(or η) = 1 ―

𝐴

(29)

R𝑡

Where A is a constant; t is the exponents related to the impacts of CFA and temperature. The equation could also be expressed as, ln

1 = ―ln𝐴 + 𝑠lnCFA + t lnR0 1 ― ε(𝑜𝑟 η)

(30)

Thus, A, s, and t can be obtained by plotting ln[1/(1-ε(or η))] with CFA and R0, as listed in Table S2. Then, ε and η could be expressed as, ε=1―

η=1―

3.376 × 1020

(31a)

R00.8669 C14.5289 FA 3.834 × 105

(31b)

R00.1958 C4.3341 FA

The degrees of hydrolyzable cellulose and xylan predicted using eq. (31a) and (31b) were compared with the ones determined by fitting the experimental data, and excellent agreement was obtained (Fig. S10), indicating the good validity of the combined severity factor for the derivation of eq. (31a) and (31b). Properties of the prepared CNCs and CNFs.


30

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Figure 10. XRD pattern (a) and CrI (b) of BEKP, CNCs, CSR, CNFs and CF under hydrolysis conditions of (88, 90, 6). As shown in Fig. 10, the CrI of the samples could be sorted in an ascending order of CNFs < CF < BEKP < CSR < CNCs. Compared to the CrI of 65.6% for BEKP, the CrI of CNCs was 79.1%, and about 21% increase of CrI was mainly due to the removal of hemicellulose and disordered regions of cellulose during FA hydrolysis.13 The CrI of CSR was slightly decreased compared to CNCs because of the insufficient hydrolysis of the disordered region. Significant decrease about 16% of CrI for CNFs was observed compared to that of BEKP, which was due to the harsh damages in the crystal region of cellulose by high shear force during homogenization.51 Fig. 10 also presents that the diffraction peaks of CF at 2θ = 14.9, 16.5, and 22.6° were decreased, while its diffraction peaks at 2θ = 20° was enhanced, which revealed that the crystal form of the CF had changed to cellulose II from cellulose I.45 This transformation could be ascribed to the rapid destroy of intermolecular hydrogen bonds and the original crystalline forms.52 In addition, the CF had a CrI of 61.0%, which was higher than that of the CNFs


31


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obtained in this work (54.9%), and the CNFs (< 50%) produced by traditional TEMPO oxidation method.53 This result was probably due to the different hydrogen bonds of cellulose II with cellulose I. More and stronger hydrogen bonds could be formed by both parallel and antiparallel packing polarities of cellulose II during recrystallization in regeneration.37, 54

Figure 11. TG (a) and DTG (b) curves of original BEKP, CNCs, CSR, CNFs and CF under hydrolysis condition of (88, 90, 6) The thermal stability of BEKP, CNCs, CSR, CNFs, and CF were studied. The TG and derivative thermogravimetric (DTG) curves of the samples are shown in Fig. 11, and the thermal degradation onset temperature (Ton) and maximal weight loss temperature (Tmax) for all samples are listed in Table 2. Compared to BEKP, an obvious increase of the Ton and Tmax of CNCs was found, indicating the improvement of thermal stability of CNCs, which probably due to two reasons, (1) amorphous hemicellulose and non-crystalline regions of cellulose were hydrolyzed, and (2) many thermally unstable hydroxyl groups were replaced by the thermally stable ester groups.7 This good thermal stability endowed the prepared CNCs with a distinct advantage for thermal processing at high temperatures. Both Ton and Tmax of CNCs were higher than that of CSR due to the more thorough removal of disordered materials of CNCs during FA hydrolysis.


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After homogenization, the obtained CNFs had a lower Ton and Tmax than CSR, because of the damages in crystalline region during homogenization, as verified by XRD (Fig. 10). However, the thermal stability of the obtained CNFs was still better than that of CNFs produced by TEMPO-mediated oxidation.53 In addition, the regenerated CF showed different decomposition behavior, and the degradation of CF occurred within a relatively wide temperature range of 310.3-369.4 oC. The Ton and Tmax of CF were higher than the representative values for CNFs produced by sulfuric acid hydrolysis or traditional TEMPO oxidation method.53,55 The relatively high thermal stability of CF was due to the presence of the thermally stable ester groups and the relatively higher CrI (Fig. 10b). Table 2. The Ton and Tmax of BEKP, CNCs, CSR, CNFs and CF Sample Ton (oC)

BEKP 325

CNCs 350

CSR 347

CNFs 301

CF 310

Tmax (oC)

373

397

382

347

369

The dispersibility of the prepared CNCs and CNFs in water and DMAc are illustrated in Fig. 12. After ultrasonic treatment, both the CNCs and CNFs samples could be well dispersed in water and DMAc, respectively. CNCs and CNFs could remain well dispersion in DMAc even after 48 h standing. The well dispersibility of CNCs and CNFs in DMAc was due to the formation of surface ester groups, which largely increased their compatibility in organic solvents.20 However, the CNCs with low surface charge was not stable in water7 and obvious flocculation could be observed after 48 h standing.


33


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Figure 12. The stability of CNCs and CNFs in water and DMAc (a) and the typical flow birefringence phenomena of CNCs in DMAc (b). The characteristic flow birefringence phenomena of CNCs and CNFs in water and DMAc were observed between two crossed linear polarized films. Only CNCs suspension in DMAc could exhibit the clear retained flow birefringence. This phenomenon was because the randomly oriented CNCs could self-assemble into chiral nematic ordered structures in DMAc.7

CONCLUSION In this study, FA hydrolysis were used as a sustainable and feasible process for the tailored and integrated production of CNCs and CNFs. Reaction kinetics of different preparation parameters of acid concentration, reaction temperature and reaction time were thoroughly studied, and the hydrolysis mechanism of cellulose pulp by FA hydrolysis could be simplified as four main phenomenological parallel reactions: de-polymerization of cellulose to CNCs, transformation of cellulose to CF by esterification, direct hydrolysis of disordered regions of cellulose to glucose, and hydrolysis of hemicellulose to xylose. Under more severe conditions, CNCs and CF could be further hydrolyzed to glucose, and xylose could be further degraded to furfural. Acid concentration played a dominate role in the production of CNCs. Hydrolysis conditions of acid concentration of 84-92 wt.%, reaction temperature of 80-100 oC and hydrolysis time of 4-14 h


34

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were more suitable to achieve high YCNM (CNCs + CNFs). More interestingly, the regenerated CF was a new kind of CNFs with a high DS and the crystal form of cellulose II. Characterization results showed that both the obtained CNCs and CNFs exhibited high thermal stability, and good dispersibility in DMAc due to the formation of ester groups on the surface of CNM. Moreover, CNCs with higher crystallinity displayed the special optical property of characteristic flow briefringence phenomena in DMAc. Therefore, the tailored and integrated production of functional CNCs and CNFs could be achieved by the clean and sustainable FA hydrolysis process, and the prepared CNM with tunable properties may have large potential applications (e.g. reinforcing agent for polymeric matrix materials, water resistant and functional CNP).

ASSOCIATED CONTENT Supporting Information. Kinetic constants obtained by fitting of experimental data (Table S1), XRD pattern and CrI of samples (Figure S1), particle size distribution of CNCs (Figure S2), SEM images of BEKP and CSR (Figure S4), FTIR spectra of samples (Figure S5), DS of samples (Figure S6), Quick regeneration of CF from hydrolysate (Figure S7), TEM images of regenerated CF (Figure S8), Color changes of hydrolysate (Figure S9), Comparisons of kinetic constants and degree of hydrolysable cellulose and xylan between those obtained from fitting experimental data and predictions for correlations (Figure S10), kinetic constants for calculation of rate constant and hydrolysis degrees of cellulose and hemicellulose (Table S2). The following files are available free of charge. Supplementary tables and figures (PDF)


35


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Liu, C.); * Tel.: +86-532-80662725, Fax: +86-532-80662724, E-mail: [email protected] (Li, B.). Author Contributions The manuscript was written through contributions of all authors. The first two authors contributed equally and all authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.31870568, and No. 31700509), the Primary Research and Development Plan of Shandong Province (No. 2016CYJS07A02), the “Transformational Technologies for Clean Energy and Demonstration”, Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA21060201), as well as the Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (No. KF201702, and No. KF201709). In addition, H. Du acknowledges the financial support from the China Scholarship Council (No. 201708120052). ABBREVIATIONS BEKP, bleached eucalyptus kraft pulp FA, formic acid DMAc, dimethylacetamide


36

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CSR, cellulose solid residue CNCs cellulose nanocrystals CNFs, cellulose nanofibrils CF, cellulose formate DS, degree of substitution CNP, cellulose nanopaper

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For Table of Contents Use Only

A brief (~20 words) synopsis The tailored and integrated production of functional CNCs and CNFs were achieved by sustainable FA hydrolysis, and the hydrolysis mechanism of cellulose pulp was investigated.


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Tailored and integrated production of functional cellulose nanocrystals

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