Simple Process To Produce High-Yield Cellulose Nanocrystals Using

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Simple process to produce high-yield cellulose nanocrystals using recyclable citric/hydrochloric acids Hou-Yong Yu, Somia Yassin Hussain Abdalkarim, Heng Zhang, Chuang Wang, and Kam (Michael) Chiu Tam ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05526 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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ACS Sustainable Chemistry & Engineering

Simple process to produce high-yield cellulose nanocrystals using recyclable citric/hydrochloric acids

Houyong Yu

a*,b

, Somia Yassin Hussain Abdalkarim a Heng Zhanga, Chuang Wanga

and Kam Chiu Tamb*

a

The Key Laboratory of Advanced Textile Materials and Manufacturing Technology

of Ministry of Education, College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, China. E–mail addresses: [email protected]. b

Department of Chemical Engineering, Waterloo Institute for Nanotechnology,

University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada. E–mail addresses: [email protected].

E–mail addresses: E-mail: [email protected]; [email protected]. *Corresponding authors. Tel: +86-571-86843618; Fax: +86-571-86843619; Tel: 1-519-888-4567x38339; Fax: 1-519-888-4347 1 ACS Paragon Plus Environment

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

ABSTRACT: Cellulose nanocrystals (CNC) have great potentials in many applications, such as high-performance nanocomposites. However, there are many challenges in the industrial production of CNC, such as high cost of acid recovery, acid disposal, low yield, and poor thermal stability. In this study, a simple process to extract CNC via recyclable acid hydrolysis of microcrystalline cellulose (MCC) is presented. A high-yield (up to 87.8%) of carboxylated CNC was obtained using recyclable citric/hydrochloric acid mixtures compared to the 53.9% yield for sulfated CNC via recyclable H2SO4 hydrolysis. The mild acid mixtures could be readily recovered and recycled three times and showed a slight effect on the size of CNC, carboxyl content of Citrate CNC surface, zeta potential value, and thermal stability. Both charged Citrate CNC and sulfate CNC were excellent food Pickering emulsion stabilizers for soybean oil/water emulsion droplets, whose diameter decreased with increasing CNC contents. This work provides a simple and low-cost pathway to recover mineral or organic acids for the sustainable and green production of CNC with high yield and thermal stability while addressing the environmental issue of acid disposal in large-scale production of CNC. KEYWORDS: Recyclable acids hydrolysis, Cellulose nanocrystals, High yield, Food emulsion

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 INTRODUCTION Sustainable development that addresses the environmental challenges of many industrial processes is becoming more important in recent years. There is an increasing interest in the use of cellulose nanocrystals (CNC) extracted from wood pulp and biomass since they are derived from renewable sources. CNCs are attractive systems due to their excellent mechanical properties, ease of chemical modification, and stability in aqueous solution, making them attractive and promising nanomaterials for high-performance applications.1-3 Several types of CNC preparation process, such as acid and enzymatic hydrolysis, and mechanical treatment were used to extract CNC nanoparticles.4-7 Different types of acids, such as mineral acids, organic or organic peroxides have been utilized to extract CNC nanoparticles from various sources.8-10 Sulfuric, hydrochloric acids and their mixtures are the most common acids used for the hydrolysis of cellulose.2 The reaction conditions, such as starting material (nature of cellulose), acids type, and concentration of acids, temperature and time of hydrolysis will influence the properties of the CNC.11,12 With sulfuric acid, the extracted CNC displayed good suspension stability and dispersibility, and their yield can be improved up to 70% by simply controlling acid concentration. However, its thermal stability is also considerably low due to the negatively charged sulfate groups that promote degradation of cellulose at high temperature.5, 7, 13-15 the properties of CNC produced through sulfuric acids hydrolysis is provided in Scheme 1. However, the sulfate groups can be removed from CNCs by alkaline treatment at elevated temperatures, which is time-consuming especially in large-scale production.16 The hydrolysis of cellulose was also investigated using hydrochloric acids, and the resultant CNCs displayed high thermal stability. The yield is less than 20%, and the aqueous dispersion is less stable, and tends to flocculate due to the lower surface 3 ACS Paragon Plus Environment


charge density,10, 17-19 and this can be addressed by surface functionalization. Jia et al illustrated that CNC nanoparticles could be extracted via phosphoric acid hydrolysis of MCC, and the lower crystalline CNCs are good gelling material and emulsion stabilizer.20 More recently, our group extracted CNC nanoparticles using citric/hydrochloric acid mixtures at the hydrolysis time of 4 h, the rod-like CNCs have crystallinity of 91.6% with onset degradation temperature of 283.4 ℃, and possessed the highest carboxylic group content of 1.39 (mmol/g) with a zeta potential of up to -46.6 mV, compared to rod-like CNC prepared with other reaction time.21 Various extraction methods of CNCs via acid hydrolysis resulted in CNCs with high aspect ratio and yields. Although these methods are simple and easy to operate , but they suffer from major issues, such as corrosion, water consumption, and large acid content (1 kg of CNCs prepared via sulfuric acid hydrolysis required about 9 kg sulfuric acid), the summary of these studies is provided in (Scheme 1, as Previous work 1).22,23 In some instances, the mineral acids were used once, resulting in large amounts of acid wastes that contribute to the serious environmental challenge.14,24 Therefore, there is an urgent need to explore alternative ways to reuse mineral acid wastes after the acid hydrolysis of cellulose.25

Scheme 1. Comparison of the Citrate CNC extraction of this study with other conventional methods. 4 ACS Paragon Plus Environment

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Up to now, there are limited reported studies on the use of recyclable acids for the hydrolysis of cellulose.26 For example, Liu et al successfully recovered and recycled phosphoric acids to produce CNC with good yields without any loss of activity, and the resultant CNC showed higher thermal stability than that of CNC prepared by the partially sulfuric acids.27 Recently, Chen et al. successfully used recyclable solid organic acids at ambient pressure to prepare modified CNC and fibrils (CNF) with excellent thermal stability, large aspect ratio, and had good potential in the practical use for large-scale production. Their brief method which was used to recover the acids is provided as in (Scheme 1, as Previous work 2).28 Bian et al also reported a low-cost and sustainable production of wood-based nanomaterials, such as lignocellulosic nanofibrils (using mechanical fibrillation) and lignocellulosic crystalline nanofibrils (using dialysis) from wood pulp through recyclable acid hydrolysis of p-toluenesulfonic acid (p-TsOH) as acid hydrotrope for hydrolysis of cellulose. This study suggested that p-TsOH acid could rapidly and efficiently solubilize up to 85% of Birch-wood lignin at 80 ℃ after only 20 min. Their production process was cost-effective and favorable to the environment and demonstrated high-value usage of lignocelluloses from renewable resources.29 Previously, hydrochloric and organic acid modified CNCs were successfully investigated by using ramie fiber as starting materials, after removal of residual lignin and hemicelluloses by alkaline treatment, the ramie fibers were immersed in 0.4 M HCl overnight at ambient temperature, or in the high concentration of 80 wt% aqueous solutions of organic acid including (citric, malonic, and malic) to minimize the water consumption. This study is devolved green and solventless method to recycle recovered cellulose that is not converted to citrate modified CNCs. Besides the influence of reaction factors as concentration of hydrochloric acids and pKa value 5 ACS Paragon Plus Environment


of organic acids on the yield values and properties of modified CNCs were investigated. It was demonstrated that over subsequent reaction cycles increase the yield value of cellulose from 20 to 55%. Also, a yield value of citrate CNCs was increased from 5 to 20% by altering the morality of HCl solution from 0.025-0.1 M. Therefore, the improvement in the yield values could provide cost-efficient and environment-friendly production of ramie fibers-based nanomaterials.30 Hence, the recovering and recycling of mineral acids for the extraction of CNC require further research and development. In this study, CNC was synthesized from MCC with recycled citric/hydrochloric acid mixtures by optimizing the acid hydrolysis conditions. After separation of functionalized CNCs thorough the filtration processes, the residual acids could be easily recovered and mixed with 25% of the freshly-prepared acid mixture over a short time duration, and recycling experiments were examined for three times to investigate their effect in the yield values (See Scheme 2). Furthermore, the resultants CNC were compared to CNCs extracted with sulfuric acids. Besides, CNC is used as a sustainable Pickering emulsion stabilizer in the oil/water food system. It is well known that the mechanism of emulsion stabilization in Pickering emulsions differs from that of low molecular weight surfactants, such as amphiphilic polymers or surfactants, but they are stabilized using organic or inorganic particles.31 Recent studies have mainly focused on ways to improve the stability of Pickering emulsion using inorganic particles (e.g. silica) to prepare “dry” emulsions.32 Several studies have reported on enhancing the stability of emulsions using different types of cellulose nanocrystals or micro-fibrillated cellulose.33-35 Since the surface of CNC has a large number of hydroxyl groups, it could easily be modified with functional groups or polymer brushes for application in cosmetic, pharmaceutical, and functional food packaging’s in order to enhance their 6 ACS Paragon Plus Environment

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shelf life.36, 37 In this work, the CNC samples (Citrate CNCs) prepared using recycled acid treatment possesses similar characteristics that make them ideal candidates for such application. CNCs are ideal food emulsion stabilizers formulated as Pickering emulsions, where a relatively lower solid particle concentration is needed to control the droplet size and emulsion efficiency, compared to other types of particular stabilizers or surfactants.37,

38

Citrate CNCs also showed amphiphilic character as

other CNCs. As we know, hydroxyl and carboxyl groups in the amorphous cellulose chains gave hydrophilic character, while crystalline regions of Citrate CNCs rendered them hydrophobic character. Therefore, they could be readily adsorbed at the oil/water interface to form stable emulsions through the interactions of hydrophilic hydroxyl groups on the amorphous cellulose chains with water and the hydrophobic crystalline cellulose domains with oil.39, 40 We believe that the extraction of CNC by recycling acid mixtures results in a green and high-yield preparation method for large-scale production and broad application of cellulose nanomaterials.

 MATERIALS AND METHODS Materials. Commercial microcrystalline cellulose (MCC, about 10 µm of size) was provided by Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid (98%) and hydrochloric acid was supplied by Hangzhou Shuanglin Chemical Reagent Company. Citric acid and soybean oil were purchased from Hangzhou Gaojing Fine Chemical Industry Co., Ltd. and a local supermarket respectively.



Scheme 2. Schematic for the extraction of Citrate CNCs using recycled acid mixtures (inserts is the suspension stability of Citrate CNC suspension stability in H2O). CNC Preparation using recycled acid mixtures. The detailed process (Scheme 2) of extracting Citrate CNCs and Sulfate CNCs from the hydrolysis of MCC using citric/hydrochloric acid mixtures and sulfuric acid as described previously, where the CNCs produced under the C6H8O7/HCl ratio of 9:1 were more uniform and had relatively higher carboxyl contents, by optimizing the preparation conditions and different C6H8O7/HCl ratios.21 Briefly, Citrate CNC1 was prepared via the hydrolysis of MCC (2 g) dispersed in 100 mL acid mixture (90% citric acid/10% hydrochloric acid (v/v)) at 80℃ under continuous stirring (350 r/min) for 4 h, upon completion, the resultant suspension was rapidly cooled to room temperature. After that, the functionalized CNCs and recycled acid mixtures were separated by filtration processes and the CNC samples were washed for three times by successive centrifugations with deionized water-to-CNC ratio (liquid-to-solid ratio) (20:1 g/mL), and then the CNC suspension was dialyzed one day to remove the remaining acids with dialysis bag of (MW cutoff of 12000 Da) and the volume in the tube with deionized water-to-CNC suspension (liquid-to-liquid ratio) (7:3 v/v) against deionized water of 1000 ml for one day with six times change of water. The CNC suspension was concentrated and then freeze-dried. Meanwhile, the remaining acids with citrate 8 ACS Paragon Plus Environment

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were recovered after filtration process could be easily combined with 25% of the fresh acid mixture up to 100 mL to hydrolyze 2 g of new MCC to produced Citrate CNC2. Subsequently, Citrate CNC3 was prepared using twice recycled acid mixture and the residual byproducts of CNC. For comparison, 40 mL of 64 wt% sulfuric acid solution was employed to hydrolyze 2 g of MCC at 50 ℃ for 1 h under continuous stirring, and the CNCs obtained using the same recycled acid protocols are designated as Sulfate CNC1, Sulfate CNC2, and Sulfate CNC3 respectively.

 CHARACTERIZATION Field emission scanning electron microscopy (FE-SEM). The morphologies of Citrate CNCs and Sulfate CNCs were observed using a field emission scanning electron microscopy (FE-SEM, JSM-5610, JEOL, Japan) with an acceleration voltage of 2.0 kV. Around 200 fibers were used to determine the average geometrical dimensions of CNCs measured from FE-SEM images. Aqueous suspensions of approximately 0.01 wt% nanocrystals were dispersed in a beaker placed in a cold ultrasonic bath (model S7500, Branson) for 30−60 min, which was then deposited onto a silicon wafer. Besides, the morphologies of Citrate CNCs and Sulfate CNCs were conducted on Transmission electron microscopy (Philips CM10) at an acceleration voltage of 60 keV. Yield of cellulose nanocrystals. The CNC yield was calculated using equation 1: Yield (%) 

m2 100% m1

(1)

Where m1 is the mass of pure MCC (g), m2 is the mass of freeze-dried sample (g). Particle size and Zeta potential. The particle size and Zeta potential of Citrate CNCs and Sulfate CNCs were determined in a Nano ZS Malvern Zetasizer. 2.5 mg/mL aqueous suspensions of CNC were prepared diluted to 0.01 wt%, and the 9 ACS Paragon Plus Environment


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measurements were performed at 25℃ in triplicate. X-ray diffraction. X-ray diffraction (XRD, ARL X’TRA, Thermo Electron Corp) measurements were conducted to quantify the crystalline structure and crystallinity of the CNC using a monochromatic Cu Kα radiation at λ = 1.54056 Å (40 kV, 40 mA) in the range of 2θ = 5−60° with a scanning rate of 2° min−1. The crystallinity (XC) of all the samples was calculated according to the Segal equation 2) 41:

Xc 

I (200)  I ( amorphous) I ( 200)

100%

(2)

where I(200) is the overall intensity of the peak at 2θ = 22.7°, and I(amorphous) represents the intensity of the baseline at 2θ = 18°. Fourier transforms infrared spectroscopy (FTIR). Infrared spectra were recorded at room temperature on an FTIR spectrometer (Nicolet IS50, Thermo Electron Corp., USA). All freeze-dried samples were prepared in KBr pellets and analyzed using a spectral width ranging from 4000−400 cm−1 with a 2 cm−1 resolution and 32 scans. Thermogravimetric analysis (TGA). The thermal stability of the samples was measured using the Perkin Elmer PYRIS 1 thermogravimetric analysis (TGA). The samples (5–8 mg) were heated from room temperature to 600 ℃ at a rate of 10 ℃/min under a nitrogen flow rate of 30 mL min-1. Determination of surface group content and degree of substitution (DS). The sulfate contents of CNC were measured according to previously reported methods

21.

While the carboxyl group contents were determined by the electric conductivity titration method according to the previous reported method. 42 In brief, a dried sample around (0.02 g) was added into 0.01 M HCl (20 mL) with pH value in the range of 2.5−3.0 and the mixture was stirred to prepare a homogeneous suspension. Then, a 10 ACS Paragon Plus Environment

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0.01 M NaOH solution was added at a rate of 0.1 mL/min to make an alkaline suspension with up to pH = 11. The conductivity was recorded by a conduct meter (DDS-307) to determine the carboxylic acid contents according to the following eq 3. Besides the degree of substitution were calculated by equation 4: [COOH] =

(V2 - V1 )CNaOH w

(3)

0.5  COOH   10-3 DS  (1-176  0.5 COOH   10-3 )

 162

162(V2  V1 )C NaOH 2000 w  176(V2  V1 )C NaOH

(4)

Where (V2-V1) is the volume of NaOH (mL) required to deprotonate the carboxylic acids groups, CNaOH is the concentration of NaOH (M), and w is the weight of the CNCs samples (g). COOH is carboxyl group content of Citrate CNCs samples, and the values 162 and 176 correspond to the molecular weight of an anhydroglucose unit (AGU) and the molecular weight increase for the citrate CNC, respectively. The degree of substitution (DS) from FT-IR spectra. The degree of substitution (DS) for Citrate CNCs samples was determined from FTIR spectra.42 In details, the citrate CNCs with 1 mg/mL were transformed to their acid form (pH = 2) to shift the carboxyl absorption band by using the ultrasonic process. Then citrate CNCs samples were freeze-dried and analyzed as KBr pellets (1% cellulose in anhydrous KBr). The data was collected over 32 scans with resolutions of 4 cm-1. The degree of substitution values (DS) was calculated by the ratio of the intensity of the carbonyl peak [absorbance bands at 1735 cm-1 which were assigned to (C=O) in the acid form)] to that of the band near of 1060 cm-1 relating to the backbone structure of cellulose. The DS values of Citrate CNCs were calculated using following equation:

DS = 0.5(I1735 / I1060)

(5)



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Emulsion preparation and properties Emulsion preparation. Cellulose nanocrystals (concentrations of 0.1, 0.5, 1.0, 1.5, 2.0 and 5.0 wt%) were dispersed in acidic solution with 0.01 M hydrochloric acid (pH) to illustrate high ionic strength solution to prepare oil-in-water emulsions. The emulsions were prepared by dispersing soybean oil in a CNC aqueous dispersion (10% v/v of the whole emulsion), were prepared using an oil-to-water ratio of 1:9. The mixtures were then emulsified using an ultrasonic homogenizer for 10 min in an ice bath. Emulsion microscopy. The microstructure of the emulsions was observed by recording the images in an optical microscope (Nikon ECLIPSE TS100, Shanghai puch photoelectric technology co., LTD). The mean diameter of droplets was recorded from more than 150 droplets in several micrographs, and each sample was tested in triplicates. Emulsion physical stability. The stability of the emulsion was determined by an improvised centrifugation method.43,44 briefly, the emulsions placed in graduated tubes were centrifuged at 2200 g for 6 min at room temperature. The centrifugation generated an upper oil phase, a middle emulsion phase and an aqueous phase in the bottom. The initial volume of the emulsion and the volume of remaining emulsion (ER) after centrifugation were calculated based on equation 6:

ER 

V( remain emulsion volume ) V( initial emulsion volume )

 100%

(6)

Emulsion storage stability. Freshly prepared samples were transferred to 10 mL graduated tube. The stability of emulsion was visually checked after storage at 0, 14 and 28 days at room temperature. The volumes of the emulsion were calculated from the height measurement using a digital caliper. 12 ACS Paragon Plus Environment

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 RESULTS AND DISCUSSION

Figure 1. FE-SEM image of Citrate CNC1 (a), Citrate CNC2 (b), Citrate CNC3 (c), Sulfate CNC1 (d), Sulfate CNC2 (e), and Sulfate CNC3 (f). Morphological analysis. The FE-SEM images of Citrate CNC and Sulfate CNC prepared citric/hydrochloric acid mixtures and sulfuric acid hydrolysis is shown in Figure 1. The Citrate CNC was relatively well-dispersed and possessed a rod-like morphology (Figure 1a-c) with the length of 231.8-248.3 nm and diameter of 15.8-18.4 nm (Table 1). We observed that Citrate CNC2 and CNC3 possessed similar shape but larger diameter, compared to Citrate CNC1, indicating that there is a slight impact on the CNC morphology by using recycled acids for hydrolysis. The Sulfate CNCs possessed a similar rod-like morphology with a shorter length of 220.8-230.4 nm and diameter of 13.2-15.6 nm (Figure 1d-f), which was consistent with previously reported data.21,31 When using recycled acids, the dimension of the rods were larger, increasing from a diameter of 15.8 and length of 231.8 nm to 18.4 and 248.3 nm for Citrate CNC3, and from 13.2 and 220.8 nm to 15.6 and 230.4 nm for Sulfate CNC3, respectively. It should be noted that the acidity of sulfuric acid was stronger than the acid mixture, resulting in the degradation of the amorphous domains for MCC treated 13 ACS Paragon Plus Environment


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with sulfuric acid. Moreover, with an increasing number of times of recycled acid, the mixtures acids were diluted during the washings, resulting in a slight increase in the size of the CNCs. Moreover, TEM images in Figure 2 illustrate the length and diameter of various Citrate CNCs and Sulfate CNCs were about 200-250 nm and 10-20 nm, respectively.

Figure 2. TEM image of Citrate CNC1 (a), Citrate CNC3 (b), Sulfate CNC1 (c) and Sulfate CNC3 (d). Table 1. Size, Zeta potential, and group content of all Citrate CNCh and Sulfate CNC samples. Length

Diameter

Zeta potential

Group content

(nm)

(nm)

(mV)

(mmol/g)

Citrate CNC1

231.8± 23.2

15.8 ± 3.0

-45.6 ± 2.1

1.40 ± 0.15

Citrate CNC2

236.2 ±21.4

17.9 ± 4.1

-43.5 ± 2.0

1.36 ± 0.20

Citrate CNC3

248.3 ±16.1

18.4 ± 3.9

-40.5 ± 1.6

1.28 ± 0.18

Sulfate CNC1

220.8±26.6

13.2 ± 2.7

-22.5 ± 1.0

0.35 ± 0.08

Sulfate CNC2

226.8±21.4

14.3 ± 1.9

-21.1 ± 1.2

0.34 ± 0.06

Sulfate CNC3

230.4±23.5

15.6 ± 2.3

-20.3 ± 1.0

0.34 ± 0.07


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Suspension Stability. The samples of Citrate CNCs were dispersed in water for 48 h by the sonication and the images of the Citrate CNC after each cycle were provided as shown in Figure 3. It is found that no noticeable precipitations of CNCs were observed at the bottom of bottles after sonication (2 h, and 24 h). However, after 48 h at 20 ℃, flocculation of citrate CNCs have observed as well as a phase where the suspension was maintained. It can be concluded that the high carboxylic content had a significant effect on the improvement of suspension stability.

Figure 3. Images for Citrate CNC suspension stability in H2O Yield. The issue of low yield in CNC preparation is a critical factor that should be addressed in the commercial production of CNC. The yield of CNC samples prepared via the hydrolysis of two kinds of recycled acid mixtures is shown in Figure 4a.

Figure 4. Yield of Citrate CNC and Sulfate CNC (a), Comparison of CNC yield in literatures and this study: Microbial hydrolysis method (microbial hydrolysis of MCC),28

mechanical

treatment

method

(mechanical

treatment

(ultrasonication-centrifugation) of MCC,45 Sulfuric acid hydrolysis method (sulfuric 15 ACS Paragon Plus Environment


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acid hydrolysis of MCC),46 HCl hydrolysis method (HCl hydrolysis of MCC),47 subcritical hydrolysis method (subcritical hydrolysis of MCC),48 phosphoric acid hydrolysis method (phosphoric acid hydrolysis of MCC).49 Clearly, yields of 53.9-54.8% were found for three kinds of Sulfate CNC prepared by recycled H2SO4 hydrolysis, while the Citrate CNCs samples possessed a higher yield up to 86.5-87.8%, indicating that efficient removal of amorphous regions, besides the short reaction time could promote high yields. In addition, with recycled acid mixtures, the number of times of acid recycling showed the insignificant effect on the yield values for CNC samples, for example, the Citrate CNC3 possessed a slightly lower yield of 86.5% compared to 87.8% for Citrate CNC1. Figure 4b summarizes the yield of CNCs prepared from MCC as starting materials via different methods, acids, and conditions. It is obvious that a yield of up to 86% could be achieved with recycled acid mixtures over 4 h, which was obviously higher than 22% for

microbial

hydrolysis

of

MCC,

20%

for

mechanical

treatment

(ultraconservative-centrifugation) of MCC,45 30% for sulfuric acid hydrolysis of MCC.46 21.9% for subcritical hydrolysis of MCC,48 Compared to all these hydrolysis methods, our Citrate CNC prepared by hydrolysis with recycled acid mixtures exhibited high yields, which was in agreement with several studies performed yield value of 85% for CNC obtained by HCl hydrolysis of MCC,47 and 87% for phosphoric acid hydrolysis of MCC.49


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Figure 5. X-ray diffraction patterns of Citrate CNC and Sulfate CNC. X-ray diffraction (XRD). It is well known that the crystallinity of CNC is an important factor in controlling the rigidity and thermal stability.50 Therefore, the change of the crystalline structure and crystallinity was evaluated from X-ray diffraction spectra. In Figure 5a, all the CNC samples exhibited similar characteristic cellulose I pattern, similar to that of MCC, such as the peaks at 2θ=14.9°, 16.5°, 20.5°, 22.7° and 34.5° assigning to (1ī0), (110), (012), (200) and (004) reflection planes, respectively.20 It indicated that the typical cellulose I crystal lattice of all the CNC samples was unchanged after hydrolysis. Compared with MCC, very weak intensity of (004) plane appeared for CNCs samples, while the stronger intensity of (004) plane was evident for the Citrate CNC samples. At the same time, the Citrate CNC samples showed similar crystallinity values around 83.1% for (Citrate CNC1), 82.6% (Citrate CNC2), and 82.4% (Citrate CNC3), which were higher than 76.3-77.4% for Sulfate CNCs samples and 70.5% for the starting materials (MCC). This further confirmed that the higher crystallinity of Citrate CNC samples could be achieved, compared with that of Sulfate CNC. Additionally, sulfuric acid was indeed more aggressive to the MCCs than the acid mixture, besides the amorphous regions, and some partial damage in the crystalline regions was hydrolyzed, demonstrating a lower crystallinity. However, recycled acid mixtures had a slight impact on the crystallinity of CNCs, 17 ACS Paragon Plus Environment


suggesting a similar hydrolysis efficiency of the recycled acids.

Figure 6. FTIR spectra (a), carboxyl and sulfate group contents (b) of Citrate CNC and Sulfate CNC. FT-IR Characterization. Figure 6 shows the FT-IR spectra of MCC, Citrate CNC, and sulfate CNC. All characteristic peaks of MCC were observed in the spectra of CNC samples, including the O-H stretching vibration around 3445 cm-1, C-H stretching vibration at 2898 cm-1, the H-C-H and O-C-H in-plane bending vibrations at 1429 cm-1, and the C-H deformation vibration at 1376 cm-1. This result indicating that the cellulose structure of CNC samples was preserved after the hydrolysis of MCC. In addition, a new carbonyl peak at 1735 cm-1 was observed, revealing the existence of carboxyl groups (-COOH) on the surface of Citrate CNC, which were generated by the esterification reaction between hydroxyl groups of cellulose and carboxyl groups of citric acid.21 Moreover, the carboxyl group contents of Citrate CNC2 (1.36 ± 0.20mmol/g) and Citrate CNC3 (1.28 ± 0.18 mmol/g) possessed a slightly lower carboxyl content of 1.40 ± 0.15 mmol/g for Citrate CNC1. This result indicated that the recycled acids with more recycling times showed a marginal effect on the carboxyl content of the citrates CNC surface, which was supported by the zeta potential results. This can be explained by during the CNC preparation, hydrochloride acid was indeed as volatile acid and caused the changes of Citric acid/HCl ratio, but 18 ACS Paragon Plus Environment

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the hydronium ions (H3O+) from HCl as catalysts was used to catalyze the esterification of hydroxyl groups on the exposed MCC chains with carboxyl groups of Citric acid (C6H8O7). Therefore, the morphology and structure size of Citrate CNCs were almost retained (see Figure 2) by using recycled acids with a freshly same acid mixture. For CNCs, a new S=O stretching peak at 1205 cm-1 appeared, indicating the existence of S=O stretching from sulfate groups in the Sulfate CNCs through the esterification of hydroxyl groups during the sulfuric acid hydrolysis of MCC.51 The sulfate contents of CNC were measured according to previously reported methods.21 The results indicated the recycled times did not have any effect in the sulfate groups content of (0.34-0.35 mmol/g) but yielded a slight reduction in zeta potential values as in Table 1. From Table 2, it is obvious that the degree of substitution (DS) exhibited a maximum value of 0.129 occurred for the Citrate CNC1. Besides DS Value was slightly decreased from 0.129 as recycling times increased to 0.116 for Citrate CNC3, this result was consistent with zeta potential measurement (Table 1). For more investigation, the DS values also calculated from FT-IR spectra.42 It is found that the DS values of citrate CNC1, CNC2, CNC3 were 0.125, 0.117, and 0.106 respectively (Table 2). Therefore, this result was in agreement with DS values calculated by the conductometric method. More significantly, Table 2 shows an evaluation study on the degree of substitution for Citrate CNCs in this work to compare with other DS values for Citrate CNC prepared from ramie fiber.30 Compared to Citrate CNC prepared from ramie fiber, our Citrate CNCs showed lower DS values, this result could be due to



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lower carboxyl group content (1.40 ± 0.15 mmol/g) compared to their content from ramie fiber (1.884 ± 0.124 mmol/g). Besides, ramie fibers have lower crystallinity than microcrystalline cellulose, and thus more hydroxyl groups in the amorphous region were more easily modified by carboxylation. Therefore, the citrate degree of substitution for Citrate CNCs obtained from our work (MCC as starting materials) was smaller compared with Citrate CNCs from ramie fiber. Table 2. Carboxyl group content and summary of a study investigating various determination methods of the degree of substitution (DS) for Citrate CNC COOH

Conductometric

FTIR

NMR

mmol/g

DS

DS

DS

Malonate CNCs

1.108±0.094

0.14

‒

0.16

Spinella et al.30

Malate CNCs

1.617±0.17

0.19

‒

0.18

Spinella et al. 30

Citrate CNCs

1.884±0.124

0.23

‒

0.22

Spinella et al. 30

Citrate CNC1

1.40± 0.15

0.129

0.125

–

This work

Citrate CNC2

1.34± 0.20

0.123

0.117

‒

This work

Citrate CNC3

1.28± 0.18

0.116

0.106

‒

This work


References

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Figure 7. TGA (a), DTG (b) of Citrate CNCs, Sulfate CNCs, and the Tmax of Citrate CNCs and Sulfate CNCs in this study and CNC from H2SO4 hydrolysis of MCC*,52 Citric acid/HCl hydrolysis of ramie fibers *,30 ammonium persulfate (APS)-oxidized viscose fibers,53 APS hydrolysis of lyocell fibers* (c).54 *Estimated value. Table 3. Thermal stability parameters of Citrate CNC and Sulfate CNCs Citrate

Citrate

Citrate

Sulfate

Sulfate

Sulfate

CNC1

CNC2

CNC3

CNC1

CNC2

CNC3

MCC

aT (oC) 0

298.8

283.4

288.5

292.1

228.2

228.4

227.9

aT (oC) 5%

309.6

295.4

298.4

303.9

237.0

237.4

238.8

337.2

347.0

357.6

245.4

245.6

245.3

aT

max

(oC)

aT 0

357.5

, T5%, Tmax were calculated from TGA curves Thermal stability. Generally, some nanofillers in food emulsions required

high-temperature treatment prior to use, thus the thermal stability of CNC nanofillers should be evaluated for food emulsion system.55 The TGA and differential thermal gravimetric (DTG) curves of CNC samples are shown in Figure 7, and the corresponding thermal parameters are summarized in Table 3. All the Citrate CNC samples displayed similar degradation temperature as MCC, whereas the Sulfate CNC samples yielded a lower degradation temperature with a two-step degradation peak 21 ACS Paragon Plus Environment


(Figure 7b). These results indicated that Sulfate CNCs had poorer thermal stability than Citrate CNCs. Sulfuric acid hydrolysis yielded surface sulfated groups that lowered the degradation temperature of CNCs as the removal of sulfate groups in the sulfated anhydroglucose units required less energy. This enhanced the decomposition or depolymerization of the cellulose by direct catalytic degradation process.18,56,57 It was obvious that as times of recycled acid treatment increased, the degradation temperature was increased significantly for both types of Citrate and Sulfate CNCs. It is well-known that the thermal stability of CNC depended on the size, crystallinity, surface functional groups and remaining acids.58 For the similar size and crystallinity, more unstable carboxyl groups/sulfated groups on the surface of CNC samples resulted in a lower degradation temperature. Especially, the T0 values of Sulfate CNC1, Sulfate CNC2, and Sulfate CNC3 were 228.2, 228.4 and 227.9 ℃, while Citrate CNC samples showed higher T0 values of 283.4-292.1oC. Citrate CNC3 yielded the best thermal stability among all the CNC samples, the T0, T5%, and Tmax values were increased by 64.2, 65.1 and 112.3 ℃, compared to Sulfate CNC3. Particularly, the Tmax value of Citrate CNC3 increased by 20.4 ℃, compared to Citrate CNC1. More importantly, for the Tmax values of reported values (H2SO4 hydrolysis MCC,52 APS-oxidized viscose fibers,53 APS hydrolysis of lyocell fibers54), higher thermal stability was observed for CNC samples prepared via the hydrolysis of recycled acids was in agreement with the previous reported study of citric acid/HCl hydrolysis of ramie fibers30. Emulsion preparation and characteristics. The diameter of the emulsion droplets in different CNC concentrations displayed similar morphology with the size of between 2-20 µm (Figure 8). At 1.0% CNC, as shown in Figure8a-f, all the emulsion droplets with different recycled acid hydrolyzed CNCs possessed similar 22 ACS Paragon Plus Environment

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size ranging from 3.12-3.21 μm, indicating that there was no observable effect of CNC prepared using recycled acid treatment times on the droplet size of food emulsions. When the content of Citrate CNCs or Sulfate CNCs was further increased, the droplets size decreased from 17.8 to 1.01 μm when the Citrate CNC content was increased from 0.1 to 5.0 wt%. It suggests that the surface of the oil droplets was stabilized and covered by the Citrate CNC or Sulfate CNC nanoparticles, leading to the reduction of droplets size.



Figure 8. Optical microscopy images of emulsions prepared with 1.0 wt% of Citrate CNC1 (a), Citrate CNC2 (b), Citrate CNC3 (c), with 1.0 wt% of Sulfate CNC1(d), Sulfate CNC2(e), Sulfate CNC3(f), and prepared with Citrate CNC3 of 0.1 wt% (g), 0.5 wt% (h), 1.5 wt% (i), 2.0 wt% (j), 5.0 wt% (k), Sulfate CNC3 of 0.1 wt% (l), 0.5 wt% (m), 1.5% (n), 2.0 wt% (o), and 5.0 wt% (p). Mean diameters of emulsion droplets were represented for each image. With increasing Citrate CNC3 contents (as a model sample), a gradual reduction in the mean diameter of emulsion droplets was observed (Figure 8g-k), these results were ascribed to the large interfacial area (amphiphilic character) by increasing the Citrate CNC3 contents.40 At low Citrate CNC3 contents could cause the breakup of the emulsion due to the insufficient interfacial area, which can result in emulsion instability at low emulsifier content. A high Citrate CNC3 content with more carboxyl amounts would induce strong repulsion between the droplets that minimized the frequency of droplet collision yielding a smaller droplet size.38 These results signify that further aqueous phase or oil separation can be restricted by a higher content of Citrate CNC3, leading to increasing emulsion volume with good stability. Similar behavior was found for Pickering emulsions prepared with the Sulfate CNC prepared using sulfuric acids (Figure 8l-p). The oil droplet size decreased from 17.0 μm with low content of 0.1 wt% Sulfate CNCs to 1.05 μm as the concentration of Sulfate CNC increased to 5.0 wt% at a fixed emulsion content. This result indicates that high contents Sulfate CNC yielded emulsions with smaller droplet size. 24 ACS Paragon Plus Environment

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Figure 9. Effects of Citrate CNC3 (a), Sulfate CNC3 (b) contents of (0.1, 0.5, 1.0, 1.5, 2.0 and 5.0 wt%) and Citrate CNC3 content of 5.0 wt% with 10 mg/mL NaCl in emulsion (c) on the stability of oil/water emulsions at 0 day and after 14 and 28 days of storage. Emulsion stability. The previous study by Irina et al. showed that it was not possible to make stable emulsions with CNC that possessed surface charge density greater than 0.03 (e/nm2).59 Therefore, by adding electrolytes such as NaCl to screen the electrostatic forces, stable emulsions were might be obtained in our current study. In order to simulate actual food emulsion conditions, the effect of Citrate CNC3 or Sulfate CNC3 concentration on the storage stability was evaluated by visual inspection (Figure 9). After preparation, the emulsion droplets displayed good stability, where the creaming effect decreased with increasing CNC content. After 14 days, different degrees of phase separation was evident for all the emulsions, and this phenomenon was reduced with increasing CNC contents. The results indicated that the more aqueous phase will be trapped by higher CNC contents, leading to larger 25 ACS Paragon Plus Environment


emulsion volume with good stability. Beyond 28 days, the volume of the emulsion remained mostly unchanged, suggesting that CNC-stabilized oil/water emulsions possessed good storage stability due to the carboxyl or sulfate groups on the CNC samples. For more investigation, 10 mg/mL NaCl salt was added in emulsion with Citrate CNC3 content of 5.0 wt% in order to improve the stability of the emulsion (Figure 9c). It is understandable that stable emulsions were obtained with a high content of Citrate CNC 3 (5.0 wt%), more Citrate CNC3 could enhance the viscosity of the emulsion to result in the separation of oil and aqueous phase. The mean droplet diameter of Citrate CNC3 and Sulfate CNC3 oil/water emulsions in different CNC contents at 0, 14 and 28 days of storage are shown in Figure 10. The diameter of emulsion droplets with low and high concentration remained unchanged over a long duration of 28 days, indicating that the CNC-stabilized emulsion prepared using the CNC prepared using fresh and recycled acid mixtures could be used for preparing food emulsions.

Figure 10. Mean droplet diameter of Citrate CNC3 (a) and Sulfate CNC3 (b) oil/water emulsions prepared using different CNC contents at 0 day and after 14 and 28 days of storage.

 CONCLUSIONS In this study, a green and high-yield (up to 87.8%) production of sulfated and 26 ACS Paragon Plus Environment

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carboxylated CNCs using recyclable H2SO4 and citric/hydrochloric acid mixtures was reported. With citric/hydrochloric acids, carboxylated Citrate CNCs were produced where the carboxyl groups of the citric acid were grafted on the CNC surface. The combination of recycled and fresh citric/hydrochloric mixtures used in the hydrolysis process showed slight influence in the chemical structure, crystallinity and thermal property of Citrate CNC and Sulfate CNC. Importantly, Citrate CNC-3 possessed the best thermal stability, where the T0, T5%, and Tmax values were increased by 64.2, 65.1 and 112.3℃, compared to Sulfate CNC3. Surface functionalization through carboxylation yielding high thermal stability of CNC offers a green and scalable process to produce renewable CNC. Thus, the present study has significant practical importance to offer promising economic opportunities with the little environmental impact that reduces contamination from acid wastes. The CNCs produced from this process have potential application in the preparation of Pickering emulsion in food systems.

 AUTHOR INFORMATION Corresponding Author *Hou-Yong Yu (H.Y. Yu); Tel.: 86 571 86843618; E–mail addresses: [email protected]. *Kam Chiu Tam; Tel: 1-519-888-4567-38339; Fax: 1-519-888-4347. E–mail addresses: [email protected]. Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENTS 27 ACS Paragon Plus Environment


The project was funded by Candidates of Young and Middle Aged Academic Leader of Zhejiang Province, “521” Talent Project of Zhejiang Sci-Tech University, and the Young Elite Scientists Sponsorship Program by CAST. The research funding from CelluForce and FP Innovations facilitated research on CNC. K.C.T. wishes to acknowledge funding from CFI and NSERC.



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Graphical Abstract

High-yield extraction method of CNC by recycling mixed acid hydrolysis as ideal choice for large-scale production of CNC nanomaterials.


Simple Process To Produce High-Yield Cellulose Nanocrystals Using

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