Influence of supercritical carbon dioxide treatment on the physicochemical properties of cellulose extracted from cassava pulp waste

J. of Supercritical Fluids 154 (2019) 104605

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Influence of supercritical carbon dioxide treatment on the physicochemical properties of cellulose extracted from cassava pulp waste Phawinee Nanta a , Kittiwut Kasemwong b,∗ , Wanwisa Skolpap a,d , Yusuke Shimoyama c a

Department of Chemical Engineering, Faculty of Engineering, Thammasat University, Pathumthani, 12120, Thailand NANOTEC Research Unit, National Nanotechnology Center, National Science and Technology Development Agency, 130 Thailand Science Park, Phaholyothin Rd., Khlong Luang, Pathumthani, 12120, Thailand c Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 S1-33, Ookayama, Meguroku, Tokyo, 152-8550, Japan d Center of Clinical Engineering, School of Engineering, Thammasat University, Pathumthani, 12120, Thailand b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• ScCO2 treatment increases cellulose decomposition temperatures, but reduces its fibril size. • Cellulose decomposition temperatures increase with elevating ScCO2 pressure from 8 MPa to 20 MPa. • Cellulose crystallinity changes as the ScCO2 treatment time extended to 120 min.

a r t i c l e

i n f o

Article history: Received 25 April 2019 Received in revised form 1 August 2019 Accepted 25 August 2019 Available online 27 August 2019 Keywords: Alternative technology Cellulose Renewable material Supercritical carbon dioxide

a b s t r a c t This study aims at investigating the influence of supercritical carbon dioxide (ScCO2 ) treatment on the physicochemical properties of cassava–based cellulose. The ScCO2 treatment was performed in a highpressure vessel by exposing the cellulose to ScCO2 without a co-solvent at variable pressure (P) ranging from 8 MPa to 20 MPa for 60 min. The system temperature (T) was set to 40 ◦ C, 60 ◦ C and 80 ◦ C. Subsequently, a field emission scanning electron microscopy analysis showed that the average fibril diameter of the cellulose was approximately two times less than that of the untreated cellulose. Differential scanning calorimetry and thermogravimetric analyses showed higher temperature shifting of the decomposition peak, which indicated improved thermal stability of the ScCO2 -treated celluloses. Additionally, a correlation test revealed that thermal stability of cellulose increased with the pressure of ScCO2 . © 2019 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. E-mail address: [email protected] (K. Kasemwong). https://doi.org/10.1016/j.supflu.2019.104605 0896-8446/© 2019 Elsevier B.V. All rights reserved.

Currently, plastic is primarily derived from petroleum resources that are depleting, limited, and unsustainable in the long term. The Klass model indicates that oil and natural gas will run out by 2042 [1]. Based on fossil fuel and consumption data, the CIA

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World Factbook states that oil and gas reserves will be depleted by 2052 and 2060, respectively [2]. Thus sustainable development initiatives have gained considerable attention. Renewable substances such as carbon dioxide, terpenes, vegetable oils, and carbohydrates are a possible substitute for fossil raw materials [3]. The Barbara group derived a new class of poly(esteralkenamer)s from the ferulic acid present in renewable lignocellulosic biomass that shows their thermostability over a temperature range of 283 ◦ C to 370 ◦ C [4]. Moreover, poly(propylene carbonate) has been synthesized from soybean oil at a yield of 72% and the number average molecular mass (Mn ) of 6498 g·mol−1 [5]. Recently, the Luchese group has successfully developed biodegradable films from corn and cassava starch that showed mechanical properties similar to low-density polyethylene film [6]. Cellulose is a biopolymer found in plant and animal cell walls and the most abundant renewable material that has the potential to replace fossil-based raw material for plastic production. Because of strong hydrogen bonding, both intra- and inter-chain, cellulose has a high melting point and glass transition temperature. The thermoplastic produced from cellulose was first developed by Alexander Parkes in 1850 and was patented as Parkesine in 1862, which is as a clothing waterproofer generated from cellulose treated with nitric acid. In 1865, John Wesley Hyatt modified Parkesine by adding camphor as a plasticizer and patented the modified Parkesine as celluloid in 1872 [7]. Celluloid was used in the film industry up until the 1950s when it was replaced by cellulose acetate safety film. Cellulose processability can be improved via typical reactions with various reagents, such as acetic acid, acetic anhydride, alkyl halide, and epoxy. However, these reactions are complicated owing to the accessibility limits of cellulose resulting from tight packing of the crystalline structure and its hydrogen bond network. A supercritical fluid, particularly carbon dioxide (ScCO2 ), an environmentally friendly and sustainable solvent, can be widely applied as an impregnation medium for cellulose modification [8–11] and a pretreatment for cellulose hydrolysis [12–15]. ScCO2 has been proposed as an alternative medium for acetylation of cellulose [16,17]. Matsunaga and coworkers developed an acetylation method for Sugi wood using ScCO2 . They found that their proposed ScCO2 method resulted in a higher percentage of acetylation compared to using liquid and vapor phase acetylation [16]. Nishino et al. found that the acetylation of plant cellulose fiber using in a ScCO2 medium requires less energy to cool the exothermic system compared to using methylene chloride or toluene as a medium [17]. ScCO2 is a promising, environmentally friendly solvent for effective cellulose processing technology; however, ScCO2 has been frequently applied with another cellulosic solvent, i. e., acetone [8], ethanol [9], water [12,18], dimethyl sulfoxide [19] and ionic liquids [20]. The effects of adding various co-solvents in ScCO2 treatments to changes in the physical and chemical properties of cellulosic materials have been studied extensively in numerous literature [18–20]. However, research into the effect of ScCO2 without a co-solvent is limited. In this work, the effect of processing conditions using ScCO2 without any cosolvents on modifying the structural and physicochemical properties of cassava–based cellulose was studied by varying temperatures (40 ◦ C, 60 ◦ C and 80 ◦ C) and pressures between 8 MPa and 20 MPa due to temperature and pressure dependence of diffusion characteristics of the penetrant ScCO2 .

2. Experiments 2.1. Materials Cassava pulp waste was obtained from the Chol Charoen Group, Thailand. Analytical grade sodium hydroxide (NaOH) and acetic

acid (CH3 COOH) were purchased from Carlo Erba Reagents, France. Technical grade (80%) sodium chloride (NaClO2 ) was obtained from Sigma-Aldrich, USA., and carbon dioxide (99.5%) was purchased from Praxair, Thailand. 2.2. Preparation of cellulose powder Cellulose was extracted from de-starched cassava pulp (DCP) via alkaline treatment and bleaching processes. The alkaline treatment involved reacting DCP 4% (w/v) with NaOH solution (4% (w/v)). The reaction was conducted in an autoclave (TOMY SX-700, Japan) at 120 ◦ C for 1 h. The residue was collected and washed with tap water until neutral. Then, the bleaching process was performed by adding 1% (w/v) of NaClO2 and 4% (w/v) of CH3 COOH. The mixture was placed in a 120 ◦ C water bath for 24 h. The resulting white pulp was collected and washed several times with tap water. The collected pulp was resuspended in water and homogenized at 8000 rpm for 5 min (ULTRA-TURRAX T25 Digital High-Speed Homogenizer, IKA, USA) and dried using a pilot-scale Niro MOBILE MINOR model 2000 Spray Dryer. 2.3. ScCO2 treatment The experiments were performed in batch supercritical extraction equipment, as shown in Figs. 1 and S1. Cellulose powder was loaded into the sample holder of a high-pressure stainless steel vessel, as shown in Fig. S1(a). Carbon dioxide from the gas cylinder flowed to the vessel through the flow instrument (Siemens, Denmark), heat exchanger, and a high-pressure pump (model P2004, Thar Instrument Inc., USA). Then, the vessel was heated. The pressure and temperature in the vessel were controlled in the range of 8 MPa to 20 MPa and 40 ◦ C to 80 ◦ C, respectively. The treatment time was set at 60 min. The treated cellulose was removed after cooling and depressurizing the system to atmospheric pressure. 2.4. Characterization 2.4.1. Surface morphological analysis The surface morphology of the cellulose sample was analyzed using a field emission scanning electron microscopy (FE-SEM) technique on the FEI Versa 3D model (Thermo Fisher Scientific Inc., USA) operating at 5 kV. The samples were sputter coated with a 7 nm layer of silver using a Leica EM ACE600 model high vacuum sputter coater (Leica Microsystems Ltd., Singapore). The fiber diameters of the sample were determined using ImageJ Java - based processing software. 2.4.2. Specific surface area and pore volume analyses The total specific surface area and pore volume of the cellulose before and after ScCO2 treatment were determined using a Brunauer-Emmett-Teller (BET) technique. To remove adsorbed water and vapors, prior to the measurement, the cellulose sample was dried in vacuum at 100 ◦ C by using BELPREP-vacII (MicrotracBEL Corp., Japan). Then, nitrogen adsorption-desorption was measured using a BELSOR-max model (BEL MicrotracBEL Corp., Japan) surface area analyzer. 2.4.3. Crystallinity analysis Several methods can be applied to determine cellulose crystallinity, e.g., X-ray diffraction (XRD), Fourier transform spectroscopy (FTIR), and solid-state 13 C nuclear magnetic resonance (NMR) [21]. XRD of all samples was performed on a Bruker D8 Advance Diffractometer (Massachusetts, USA) using CuK˛ radiation with a coupled mode between 4◦ and 30◦ with a step size at 0.05◦ and a 2 s count time. To calculate the degree of crystallinity (Cr) and crystallinity index (C.I.) of the sample, Herman’s equation

P. Nanta, K. Kasemwong, W. Skolpap et al. / J. of Supercritical Fluids 154 (2019) 104605

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Fig. 1. Schematic of supercritical carbon dioxide treatment set up. (a) CO2 cylinder, (b) flow instrument, (c) heat exchanger 1, (d) high-pressure pump, (e) heat exchanger 2, (f) a high-pressure vessel, (g) de-pressurized valve, (h) CO2 solvent trap, (i) product line.

(amorphous subtraction and peak deconvolution method) [22] and Segal’s equation (peak height method) [23] were applied. These equations are expressed as follows, respectively. Cr = Acrystalline /Atotal

(1)

C.I. = (I002 − IAM )/I002 = 1 − (IAM /I002 )

(2)

Here, Cr is a degree of crystallinity,Acrystallin e is the area under the crystalline peaks, Atotal is the total area of the spectra, C.I. is crystallinity index, IAM is the intensity of amorphous diffraction and I002 is the maximum intensity of the (200) lattice diffraction. Origin software was employed to integrate the area under the amorphous and crystalline peaks using Gaussian model fitting and to determine the intensity of each lattice diffraction. FTIR has been used to characterize the crystallinity of cellulose by measuring the relative peak height or area [24–26]. FTIR is considered one of the most uncomplicated methods; however, it only provides relative values. The treated cellulose sample and KBr powder were weighed in a cellulose mass fraction of 1% in KBr and then ground to form a KBr pellet. To prepare the KBr pellet, an adequate amount of fine powder of the cellulose/KBr mixture was placed to cover the bottom of a stainless steel pellet die. FTIR spectra of all samples were measured using a Nicolet 6700 model FTIR microscope (Thermo Fisher Scientific Inc., USA) in the range 4000 cm−1 to 400 cm−1 in transmission mode. The C.I. was determined by calculating relative peak intensities of the FTIR spectra at the position between 1372 cm−1 and 2900 cm−1 as expressed in Eq. (3) and those of the FTIR spectra at the position between 1430 cm−1 , and 893 cm−1 as expressed in Eq. (4).

2.4.4. Thermal properties analysis The thermal properties of the cellulose samples were characterized using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. The TGA measurement was conducted using the TGA2 model (Mettler Toledo, USA). Here, the actual amount (approximately 10 mg) of the dried cellulose sample was kept in a 70 ␮L alumina crucible. The sample was then heated from 30 ◦ C to 600 ◦ C at a heating rate of 10 ◦ C·min−1 under N2 flow. The DSC measurement was performed using the DSC823e model (Mettler Toledo, USA) at a heating rate of 10 ◦ C·min−1 (range 25 ◦ C to 600 ◦ C) under N2 atmosphere at a flow rate of 10 mL·min−1 . The dried sample was weighed in a standard 40 ␮L aluminum crucible and then twice the deionized water weight was added to each sample. The closed-lid crucibles were then homogenized in an ultrasonic bath for 15 min and kept in a refrigerator for 24 h prior to measurement. The change in the thermal behavior of the cellulose after treatment was evaluated based on measurements of the peak temperature (Tp ) and enthalpy change (H). The Hterm was evaluated using the fundamental thermodynamic relation, as expressed in Eq. (5):



H =

Cp dT

Where H is the change of enthalpy between two states (J·g−1 ), and CP is the specific heat capacity (J·◦ C−1 g−1 ) calculated using Eq. (6). Cp =

Q˙ m

(6)

Here, Q is heat flux (J·s−1 ),m is mass (g), and  is the heating rate

C.I. = I1372 /I2900

(3)

(◦ C·s−1 ).

C.I. = I1430 /I893

(4)

2.5. Data analysis

Here, I1372 , I2900 , I1430 and I893 are the intensity of the FTIR spectra at 1372 cm−1 , 2900 cm−1 , 1430 cm−1 , and 893 cm−1 , respectively. Solid state 13 C NMR spectroscopy was used to evaluate the crystallinity of untreated and treated cellulose samples by employing Newman’s method [27]. The NMR measurement was performed on a Bruker Ascend 400 WB spectrometer (USA). To estimate the degree of crystallinity, the crystalline peak area that integrated peaks between 87 ppm and 93 ppm was normalized to the total area assigned to C4 peaks integrating between 80 ppm and 93 ppm.

(5)

Analysis of variance (one-way ANOVA) was employed to determine statistically significant differences among the untreated and supercritical treated (ScCO2 -treated) celluloses, with a confidence interval of 95% (p-value < 0.05). Tukey’s multiple comparison test was then performed by comparing differences between the means of each factor level. Pearson’s and Spearman’s rho correlation tests were performed to investigate the strength and direction of the relation among treatment conditions (T and P) and the properties of the treated cellulose. The relation was evaluated using the correlation coefficient

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Fig. 2. FE-SEM image of untreated cellulose.

(r), which takes a value between −1 and +1 (−1 ≤ r ≤ 1). Here, positive and negative values suggest direct and inverse correlations, respectively (0.00 to 0.19, very weak; 0.20 to 0.39, weak; 0.40 to 0.59, moderate; 0.60 to 0.79, strong; 0.80 to 1.00, very strong). 3. Results and discussion 3.1. Surface morphology of ScCO2 -treated cellulose The high-resolution organization of the cellulose fibril was observed using FE-SEM. Figs. 2 and 3 show FE-SEM images of the untreated and ScCO2 -treated cellulose samples, respectively. As shown in Fig. 2, the fibrils of the untreated cellulose formed a strong reciprocal pack and compact aggregates that differed from the treated cellulose (Fig. 3). This is consistent with the results of Zheng [18], which reported disruption of the cellulosic structure

by supercritical carbon dioxide. Moreover, Peciulyte et al. reported that cellulose fibril aggregates are primarily characterized by a high proportion of the fibril surface area, crystallinity, and porosity [28]. In this study, the surface area and pore volume (BET) were analyzed to verify the fibril dissociation phenomenon. After the ScCO2 treatment, the cellulose surface area was increased from 0.75 m2 g−1 to 12.10 m2 g−1 (approximately 16 times) as the pore volume of the cellulose was expanded. Fig. 3 shows the surface morphology of the ScCO2 -treated cellulose. The results demonstrate a more fibrillated network structure of small fibrils compared to the untreated cellulose. The fibrils of the cellulose treated at low pressure and temperature formed a dense alignment (Figs. 3(a) and (b)), while smooth and web-like network structures were more pronounced at higher temperature and pressure. The average fibril diameters (ADs) of the untreated and ScCO2 -treated cellulose are listed in Table 1. The reciprocal pack characteristic of the untreated cellulose results in a large diameter of the cellulose fibrils. After ScCO2 treatment, the AD of the cellulose was reduced to 20 nm to 37 nm due to dissociation of the fibrils. The AD of untreated cellulose is significantly different from the celluloses after ScCO2 treatment under 15 MPa and 20 MPa. However, the AD of untreated cellulose insignificantly differs from the celluloses after ScCO2 treatment under 8 MPa. 3.2. Crystalline property of ScCO2 -treated cellulose The crystalline properties of the cellulose before and after ScCO2 treatment were evaluated according to the Cr and C.I. Cr is the fractional amount of crystalline in the polymer sample, and the C.I. is an empirical measure of the relative amount of crystalline

Fig. 3. FE-SEM images of treated cellulose samples under ScCO2 treatment for 60 min at (a) 8 MPa, 40 ◦ C, (b) 15 MPa, 40 ◦ C, (c) 20 MPa, 40 ◦ C, (d) 8 MPa, 60 ◦ C, (e) 15 MPa, 60 ◦ C, (f) 20 MPa, 60 ◦ C, (g) 8 MPa, 80 ◦ C, (h) 15 MPa, 80 ◦ C and (i) 20 MPa, 80 ◦ C.

Processing condition P (MPa)

T (◦ C)

Untreated 8 8 8 15 15 15 20 20 20

40 60 80 40 60 80 40 60 80

Average diameter (nm) 50.93 ± 25.41b 29.03 ± 8.94ab 29.08 ± 11.87ab 37.09 ± 35.98ab 20.43 ± 5.99a 19.16 ± 6.57a 20.23 ± 6.69a 23.36 ± 7.83a 26.38 ± 10.31a 32.55 ± 16.00ab

TGA results

Crystallinity

DSC results

Cr1 (%)

C.I.2

C.I.3

To (◦ C)

Td (◦ C)

Loss4 (%)

To (◦ C)

Tp (◦ C)

H (J·g−1 )

49.66 ± 1.46ab 50.17 ± 2.41ab 53.93 ± 1.05b 50.49 ± 3.52ab 50.60 ± 2.16ab 51.25 ± 3.02ab 49.00 ± 2.15ab 46.96 ± 1.93a 48.02 ± 0.82ab 52.52 ± 0.13ab

0.79 ± 0.00a 0.79 ± 0.05a 0.82 ± 0.05a 0.78 ± 0.04a 0.79 ± 0.05a 0.80 ± 0.06a 0.79 ± 0.08a 0.78 ± 0.00a 0.79 ± 0.04a 0.82 ± 0.01a

0.79 ± 0.11a 0.72 ± 0.00a 0.75 ± 0.08a 0.72 ± 0.02a 0.73 ± 0.04a 0.71 ± 0.07a 0.68 ± 0.05a 0.77 ± 0.03a 0.73 ± 0.02a 0.80 ± 0.02a

310.73 ± 0.78 ab 310.02 ± 0.03 a 310.73 ± 0.20 ab 311.45 ± 0.14 bcd 311.99 ± 0.47 cd 311.30 ± 0.50 bcd 310.50 ± 0.14 ab 311.15 ± 0.01 bc 312.34 ± 0.03 de 313.07 ± 0.46 e

339.18 ± 0.11 a 341.51 ± 0.12 b 342.68 ± 0.24 c 343.51 ± 0.19 d 344.18 ± 0.44 d 343.01 ± 0.08 e 341.84 ± 0.50 cd 343.34 ± 0.01 b 344.84 ± 0.34 f 345.01 ± 0.49 f

65.25 ± 1.67a 67.46 ± 1.23 ab 67.48 ± 0.96 ab 67.76 ± 1.68 ab 69.10 ± 0.56 b 67.30 ± 0.20 ab 67.38 ± 0.30 ab 68.87 ± 1.66 b 68.75 ± 0.63 ab 68.11 ± 1.95 ab

321.08 ± 0.62 a 322.89 ± 1.09 a 340.52 ± 0.66 b 344.18 ± 0.58 d 341.63 ± 1.36 bc 340.13 ± 0.63 b 342.76 ± 0.45 cd 355.07 ± 0.01 e 342.86 ± 0.23 cd 343.02 ± 0.12 cd

341.68 ± 0.00 a 347.83 ± 0.01 b 362.39 ± 0.01 d 362.44 ± 0.16 d 362.45 ± 0.01 d 361.37 ± 0.01 c 361.64 ± 0.01 c 363.23 ± 0.32 e 363.27 ± 0.01 e 363.25 ± 0.01 e

53.52 ± 6.80 ab 63.59 ± 1.09 b 82.87 ± 4.28 c 62.58 ± 2.55 ab 60.84 ± 7.90 ab 45.15 ± 2.60 a 51.08 ± 3.01 ab 101.23 ± 14.33 d 68.3 ± 1.45 bc 54.21 ± 3.01 ab

1 Percentage of crystallinity estimated using Herman’s equation, 2 Crystallinity index estimated using Segal’s equation, 3 Crystallinity index estimated using FTIR technique, 4 % Loss of decomposed cellulose, Data represent as means ± standard deviation (SD), a–f Mean values with different superscript letters in the same column are significantly different (p < 0.05), Increasing mean values are represented in ascending alphabetical order.

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Table 1 Average fibril diameter, crystallinity, and thermal behavior of cassava–based cellulose before and after 60-min ScCO2 treatment.

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Fig. 4. (a) XRD and (b) FTIR spectra of untreated and ScCO2 -treated celluloses under 40 ◦ C, 60 min of treatment time and various pressures. at 8 MPa, ; ScCO2 treatment at 15 MPa, ; ScCO2 treatment at 20 MPa.

material in cellulose (crystal and amorphous). The Cr and C.I., common quantitative indicators of crystallinity, are defined by various techniques such as XRD, FTIR, and 13 C NMR. XRD and FTIR spectra of untreated and ScCO2 -treated celluloses at 40 ◦ C and 60 min of treatment time are shown in Fig. 4. Amongst cellulose samples after the ScCO2 treatment, the difference between the individual standard XRD and FTIR spectra pattern and the observed patterns were not noticed. The peak separation method was used to evaluate the Cr from XRD spectra. Note that the Cr values calculated using Eq. (1) are shown in Table 1. The Cr value of the untreated cellulose was 49.66%, and those of the ScCO2 -treated celluloses were between 46.96% and 53.93%. The C.I. was evaluated according to

; control,

; ScCO2 treatment

the peak height technique using Eq. (2) and the FTIR method using Eqs. (3) and (4). The C.I. value of the untreated cellulose calculated by the peak height and FTIR methods were 0.79, and those of ScCO2 treated celluloses calculated by both techniques were in the range of 0.68 and 0.82. The estimated Cr and C.I. values using various equations were shown in Fig. S2. As shown, the Cr and C.I. values of the ScCO2 treated celluloses are indistinguishable from those of the untreated cellulose, which suggests that ScCO2 alone may be insufficient to destroy the amorphous region of the cellulose. The statistical result also supports that the crystalline properties (Cr and C.I.) of cellulose (Table 1) were not altered significantly by ScCO2 treatment.

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Solid state 13 C NMR was applied to confirm a narrow change in the Cr of the cellulose after treatment. Fig. S3 shows the 13 C NMR spectrum of the untreated cellulose. Here, the focusing signals for determination of Cr were the C-4 carbons of the cellulose located between 80 ppm and 93 ppm. Notably, signals between 87 ppm and 93 ppm correspond to a crystalline form, whereas signals between 80 ppm and 87 ppm correspond to an amorphous form. The result reveals that the Cr of the untreated cellulose was 42.4%, and that of the ScCO2 -treated cellulose changed slightly to 43.1% after the treatment under 20 MPa and 80 ◦ C for 60 min. This is consistent with the Cr results obtained by XRD analysis. An additional experiment was performed to study the influence of exposure time of ScCO2 (60 min and 120 min) on the degree of crystallinity of the cellulose for some treatments. The multiple comparison analysis results for the ScCO2 -treated celluloses at different operating times are illustrated in Table 2. As compared to the Cr value of 50% for untreated cellulose, those values were varied between 48% and 51% after 60-min ScCO2 treatment of cellulose, whereas those values increased up to 53% after 120-min ScCO2 treatment. Increasing operating time seemed to improve cellulose crystallinity; however, statistically relevant changes were not observed. Only at constant ScCO2 pressure of 20 MPa, a significant increase in cellulose crystallinity was observed on increasing the ScCO2 treatment time from 60 min to 120 min. It may be hypothesized that higher ScCO2 exposure pressure and time are required to change the crystallinity of cellulose. This hypothesis is supported by the study of Tsutsumi et al. about the effects of ScCO2 treatment on the crystallinity behavior of poly(L-lactide), whose thermal properties are similar to cellulose. After treating poly(L-lactide) with ScCO2 for 3 h at 40 ◦ C its crystallinity was increased at elevated pressure [29]. 3.3. Thermal property of ScCO2 -treated cellulose TGA and DSC were employed to analyze the thermal properties of the ScCO2 -treated cellulose samples. The results are summarized in Table 1. Estimation of the thermal stability of the cellulose after treatment was studied via TGA based on the initial decomposition temperature (To ), decomposition temperature (Td ), and mass change of the cellulose portion (% Loss). TGA and Derivative thermogravimetry (DTG) thermograms are shown in Fig. 5. The significant mass loss for each sample appeared in the temperature range 300 ◦ C to 350 ◦ C due to decomposition of the cellulose sam-

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Table 2 Crystallinity of cassava–based cellulose before and after ScCO2 treatment for 60 min and 120 min. Processing condition

Cr1 (%)

P (MPa)

T (◦ C)

t (min)

Untreated 8 8 15 20 8 8 15 20

40 80 80 60 40 80 80 60

60 60 60 60 120 120 120 120

50 ± 1.5ab 50 ± 2.4ab 51 ± 3.5ab 49 ± 2.2ab 48 ± 0.8a 52 ± 2.6b 52 ± 0.5ab 52 ± 2.0b 53 ± 3.2b

1 Percentage of crystallinity estimated using Herman’s equation, Data represent as means ± standard deviation (SD), a–b Values with different superscript letters in the same column are significantly different (p < 0.05), Increasing mean values are represented in ascending alphabetical order.

ple. Here, as a result of the biochar decomposition [30,31], a slight mass loss was observed between 350 ◦ C and 600 ◦ C. The untreated cellulose sample started to decompose at 310.73 ◦ C and reached decomposition temperature (Td ) at 339.18 ◦ C. The cellulose after ScCO2 treatment reached Td at approximately 342 ◦ C to 345 ◦ C. As shown in Table 1, ScCO2 treatment of celluloses caused changes in their Td values. Higher ScCO2 pressure contributed to the increase of Td values of celluloses. Consequently, the increasing trend of Td indicates increased thermal stability of the cellulose after ScCO2 immersion. This result is consistent with the result obtained by Li et al. [32]. They found higher crystallinity and thermal stability of cellulose after treatment with a steam explosion (SE). It could be said that the ScCO2 treatment functions in the same manner as the SE. In addition, increasing trend of % Loss was observed after the ScCO2 treatment. During TGA measurement, the decomposed mass of ScCO2 -treated cellulose increased from 65.25% to 67% after the ScCO2 treatment under 8 MPa and 15 MPa and increased to 68% after the treatment under 20 MPa. This can be attributed to purity enhancement of the cellulose. Fig. 5 clearly shows decreasing biochar composition (350 ◦ C to 600 ◦ C) of the ScCO2 -treated cellulose in the sample that was consequently affected by increased percentage of cellulose. In other words, ScCO2 treatment has the capability of biochar extraction, which was also reported in Manjare et al. [33]. The literature review stated that supercritical fluids

Fig. 5. (a) TGA and (b) DTG thermograms of untreated and ScCO2 -treated celluloses under 40 ◦ C, 60 min of treatment time and various pressures. treatment at 8 MPa, ; ScCO2 treatment at 15 MPa, ; ScCO2 treatment at 20 MPa.

; control,

; ScCO2

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can be used to extract and purify various natural extracts, such as hydrocarbons in coal [34]. The DSC measurements of peak temperature (Tp ) and enthalpy change (H) demonstrate the thermal behavior of the cellulose after treatment. As shown in Table 1, the enthalpies of all samples were positive, which means that heat release occurred during cellulose decomposition. The decomposed products of cellulose originating from the cleavage of ether were released and then reacted with each other to form solid char in the vapor phase [35], which resulted in heat being released to the surrounding environment. The maximum temperature of the exothermic peaks (Tp ) was between 330 ◦ C to 360 ◦ C. The Tp and H values obtained for the untreated cellulose sample were 341.68 ◦ C and 53.52 J·g−1 , respectively while those for ScCO2 -treated cellulose were located between 348 ◦ C to 364 ◦ C and between the 51 J·g−1 to 83 J·g−1 regions, respectively. Note that the improved thermal stability of the cellulose under ScCO2 treatment may have been due to slight elevation of the Tp and H values. This is consistent with the TG analysis results described in the previous paragraph and the DSC results for the cellulose after biofield energy treatment [36,37]. The Tp and H values of cellulose were shifted to higher temperature after the ScCO2 treatment. However, the only significant shifts of Tp values were observed on increasing pressure of ScCO2 treatment. The influences of pressure (P) and temperature (T) of ScCO2 treatment on the cellulose properties were evaluated via a correlation test. The results are shown in Table S1. The results reveal that only P affects Tp at a significance level of 0.05. Note that P also affects To , Td and % Loss at a significance level of 0.10. The Pearson and Spearman correlation coefficients listed in Table S1 demonstrate a strong positive relation of P with Tp , To , Td , and % Loss responses with corresponding r values of 0.74, 0.65, 0.63, and 0.60, respectively. Thus, increasing the pressure of the ScCO2 treatment increases the Tp , To , Td , and % Loss characteristics of the cellulose.

4. Conclusions In this study, cassava–based cellulose samples were treated by ScCO2 exposure for 60 min under pressure and temperature of 8 MPa to 20 MPa and 40 ◦ C to 80 ◦ C, respectively. After ScCO2 treatment, the surface area and pore volume of the cellulose were enhanced by 16% owing to the dissociation of cellulose fibrils. The AD was reduced by a factor of two, while the thermal properties, including Tp , Td , and %Loss of the cellulose increased, which resulted in a higher thermal stability of the treated cellulose. Extending ScCO2 treatment time at 20 MPa, 60 ◦ C from 60 min to 120 min caused the significantly increased Cr value of cellulose by 5%. Acquired results demonstrated that increasing ScCO2 pressure favored cellulose thermal stability improvement.

Acknowledgments This work was financially supported by the Royal Golden Jubilee (RGJ) Ph.D. program Thailand Research Fund (TRF) [grant number PHD/0265/2553]. We also acknowledge European Union EP7 Commissioners under the project title of Gains from Losses of Root and Tuber Crops: Gratitude (http://www.fp7-gratitude.eu) and Ministry of Science and Technology.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.supflu.2019. 104605.

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