Extraction of Cellulose Acetate from Cajuput (Melaleuca leucadendron) Twigs and Sugarcane (Saccharum officinarum) Bagasse by Environmentally Friendly Approach

This study was carried out to investigate the extraction of cellulose acetate (CA) from cajuput (Melaleuca leucadendron) twigs and sugarcane (Saccharum officinarum) bagasse using an environmentally friendly method. At first, cellulose was extracted from cajuput twigs (CT) and sugarcane bagasse (SB) through prehydrolysis followed by soda (NaOH) pulping and elemental chlorine-free (ECF) bleaching. Later, the extracted cellulose was acetylated using iodine (I) as a catalyst. The obtained CA was characterized by Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), scanning electron microscope (SEM), and X-ray diffraction. FTIR and NMR analysis proved the replacement of free OH (hydroxyl) groups by acetyl groups. The degree of substitution (DS) showed the acetylation capability of cellulose extracted from CT and SB as well. The diameter of CA and its crystallinity index (CrI) were measured by SEM and X-ray diffraction, respectively. The cellulose content was 85.4 and 89.5% for CT and SB, respectively after the pulping and bleaching. The diameter of CA extracted from CT was approximately 10 μm and it was approximately 20 to 30 μm for SB. The CrI of the CA extracted from SB and CT was 75.6 and 60.2, respectively. Furthermore, the thermal gravimetric analysis showed that CA extracted from CT and SB was thermal resistant. Therefore, CT and SB will be potential alternative resources for CA production using the mentioned method.


Introduction
The Research on the extraction of value-added biomaterials from biomass is one of the top most interesting areas for scientists [1][2][3][4]. Cellulose, the most abundant component of biomass, has been known as the essential material for the production of textile fibers, cellulose derivatives, pharmaceuticals substances, food additives, etc. Cellulose derivative, namely, cellulose acetate (CA) or an ester of cellulose, is the most commercially used and widely applied in the industrial sectors. CA is an interesting and very useful material due to its large spectrum of utilities. The main applications of CA are in the field of food packaging, textile industries, gas separator membrane, ultrafiltration membrane, coating, and bioplastics [5][6][7][8][9][10][11][12]. CA has also been applied in the military and defence area, such as for less-prone low vulnerability ammunition (LOVA) systems. CA has been reported as promising binders for LOVA gun propellant [13,14].
Besides thermochemical and hydrolytic routes to transform biomass into useful fuels and chemicals, catalytic conversion by using chemicals is industrially attractive because of its fastest conversion rate and controllable selectivity. CA is produced by introducing the acetyl group to cellulose. Generally, acetic anhydride is used as an acetylating agent by using H 2 SO 4 as a catalyst [15]. However, this strong acid is considered less environmentally friendly among other catalysts because of its toxicity properties. In another study, NaHSO 4 has been used as a catalyst to reduce H 2 SO 4 and acetylation reaction temperature [16]. Meanwhile, Wu et al. have reported synthesis CA in the absence of any catalysts by using ionic liquids at room temperature [17]. Moreover, the use of iodine has been proposed for the esterification of corn starch and rice husk (RH) [18][19][20]. On the other hand, the preparation of high-purity cellulose is an important step for CA production. Pretreatment is one of the key factors to separate cellulose from other components especially lignin and hemicellulose before the acetylation process. The biological pretreatment of biomass has been reported previously [21,22]. Meanwhile, other methods, such as mechanical, thermal, and chemical pretreatments have been studied to purify cellulose [23][24][25][26][27][28]. Sodium hydroxide and elemental chlorine-free (ECF) bleaching agents are the most commonly used chemicals in the pulp and paper industries [29,30].
Wood pulps are the most traditional source of cellulose for CA production [31]. However, the deforestation issue causes a shortage of wood supply. Therefore, alternative resources such as agricultural waste have been considered as a potential sources of cellulose. In Indonesia, there are abundant biomass resources that are specially generated from agro-industrial sectors. Cajuput twigs (CT) and sugarcane bagasse (SB) are the potential biomass resources among others. Indonesia produces 600 to 650 tons of cajuput oil every year. After distillation, twigs, and leaves are the leftovers. The calculated biomass potency from cajuput oil distillation mills is approximately 32,500-65,000 ton/year [32,33]. Additionally, the Indonesian Ministry of Agriculture reported that sugar production is approximately 2.5 million tons in 2019 from 0.5 million hectares of the planted area [34]. SB potency was approximately 10 million tons and was continuously increasing in 2019. It can be used as a potential resource for the biorefinery process. Therefore, CT and SB can be potential resources for the synthesis of CA.
However, there is no recorded single study on CA synthesis from CT. Meanwhile, Candido et al. have extracted CA from SB using acid (H 2 SO 4 ) treatment and H 2 SO 4 as a catalyst during acetylation [35]. In addition, a single study had not been reported on the preparation of CA from CT and SB using eco-friendly combinative pretreatment methods. We have selected iodine as the catalyst in the presence of acetic anhydride. Prehydrolysis and Iodine as a catalyst for acetylation during CA extraction have not been studied until now. Therefore, the application of an environmentally friendly approach for CA production from CA and SB is contemporary considering environmental issues. That is why this study has been conducted to characterize the CA obtained from CT and SB through prehydrolysis treatment followed by soda (NaOH) pulping, ECF bleaching, and acetylation using iodine as a catalyst. Furthermore, the extracted CA has been compared with commercial CA.

Preparation of Raw Materials
CT and SB were sourced from the cajuput oil mill and sugarcane mill, respectively, Yogyakarta, Indonesia. At first, CT and SB were washed and sun-dried to obtain a moisture content of around 10%. Dried CT and SB were then cut into 0.5-1.0 cm in length. At last, CT and SB samples were airdried for 3 days at 20 ± 2 °C to obtain a moisture content of approximately 8-10%. For analytical purposes, CT and SB samples were ground to 40-80 mesh by a grinder. Standard CA with an average Mn ~ 30,000 by GPC was obtained from Aldrich chemistry, and commercial pure cellulose C6228 was collected from Merck. Laboratory grade 100% acetic acid (glacial) (CH 3 COOH), iodine (I), 95-97% sulfuric acid (H 2 SO 4 ), 99% sodium hydroxide (NaOH), 99% sodium thiosulfate pentahydrate (Na 2 S 2 O 3 · 5H 2 O), and 99% sodium hydrogen sulfate monohydrate (NaHSO 4 · H 2 O) reagents were supplied by Merck but 98% acetic anhydride ((CH 3 CO) 2 O) reagent was delivered by ajax chemicals.

Prehydrolysis
Prehydrolysis of CT and SB were done in a stainless-steel reactor having 255 mm in height and 76 mm in diameter at 150 ± 2 ℃ for 2 h. Distilled water was used for prehydrolysis and the solid-to-liquid ratio was 1:12 for both types of materials. The solid residue was filtered for pulping [36].

Pulping
Soda pulping of CT and SB were conducted using the same reactor as used in prehydrolysis. In pulping process, the solid to liquor ratio was 1:7, and 10% (w/w%) of NaOH was used. The pulping temperature and time were 150 ℃ and 2 h, respectively. The pressure was about 3-4 atmospheric pressure (atm) during the pulping process. Black liquor was removed, and the pulps were washed properly to remove all NaOH.

Bleaching
The chemical ECF bleaching for CT and SB pulps was carried out separately in two stages in the Erlenmeyer flask. In the first stage, pulps were mixed with 0.5% (w/v) NaClO 2 and 0.5 ml CH 3 COOH. The bleaching conditions were solid to liquor ratio 1:12, pH 2-3, temperature 80 °C, and time 90 min. After the first stage, the pulps were filtrated and washed with distilled water until the pH reached a value of 7. In the last stage, the pulps were mixed with 10% (v/v) H 2 O 2 for 90 min at 70 °C under the solid to liquor ratio of 1:12. At the end of this stage, pulps were filtered and washed until the pH value was 7.

Cellulose Acetate Synthesis
The synthesis of CA from CT and SB cellulose was done separately in a 250 ml Erlenmeyer flask. For CA synthesis, 1 g of each type of cellulose was mixed with 50 ml acetic anhydride and 1.5 g iodine, and the mixture was stirred at 100 °C for 5 h in the oil bath. Subsequently, sodium thiosulfate pentahydrate was added to the mixture and stirred until the color changed. Then, the mixture temperature was lowered to room temperature (25 °C) and 150 ml ethanol was added and stirred for 60 min. After that, the solid residue was filtrated and washed with 75% (v/v) ethanol and distilled water until pH 7. At the end of the process, the residue was dissolved in the dichloromethane for 60 min at room temperature, and the solution was then filtrated to obtain CA. CA was formed as a white solid layer in the flask after evaporating the filtrate.

Raw Materials
Chemical Analyses Chemical analysis of CT and SB materials was performed. Acid-insoluble lignin was examined using T 222 om-11. Meanwhile, acid-soluble lignin was determined according to the method of UV-vis spectrometric using wavelength 205 nm (TAPPI um 250). Extractive was measured as per the standard T 204 om-88. The sugar contents of raw material were analyzed by high-performance liquid chromatography waters e2695 after acid hydrolysis of extractive free samples at 120 °C for 1 h using 72% sulfuric acid followed by 4% sulfuric acid. The analysis was done after filtering and diluting the samples [37].

Extracted Cellulose
Chemical Analyses The chemical analysis for the extracted cellulose from CT and SB was done following the similar procedure described above for raw materials. However, the samples were used directly without any extractive extraction.

Cellulose Acetate
Functional Groups Functional groups of extracted CA from CT and SB were studied by using Fourier transmission infrared (FTIR) spectroscopy IR Prestige 21 Shimadzu. The samples were blended with KBr followed by compressing this mixture in the sample disk [16]. The used transmission wavenumber was in the range of 4000-400 cm −1 . To compare the acetylation performance of these studied materials, functional groups of commercial CA and cellulose were studied by FTIR. For the furthermore confirmation of acetylation, the 1 H nuclear magnetic resonance (NMR) was performed for CA of CT and SB [38]. Twenty mg of the sample were dissolved in 0.5 mL of dimethylsulfoxide (DMSO). The NMR spectra were recorded on a Jeol 500 MHz instrument, and chemical shifts were reported in parts per million from tetramethylsilane.

Degree of Substitution
The degree of substitution (DS) was determined following the saponification reaction according to Zhou et al. (2016) [39]. For the determination of DS, 1 g of each type of CA sample was transferred to 250 ml Erlenmeyer flask and 40 ml of 75% (v/v) ethanol was then added to the flask. The mixture was heated at 60 °C for 30 min. Then, 50 ml of 0.5 N NaOH was added to the mixture and heated at 60 °C for 30 min. The flask was tightly closed and left to stand at 25 °C for 72 h. After that, the excess NaOH was titrated with 0.5 N HCl using the indicator of phenolphthalein until the disappearance of pink color. After the NaOH titration, about 1 ml of HCl was added to the mixture overnight. In the next, back titration was conducted by 0.5 N NaOH with the indicator of phenolphthalein until the appearance of faint pink color after vigorously shaking. The same procedure was applied for the blank sample. The percentage of acetyl (AC) and DS were calculated following Eq. (1) and Eq. (2), respectively. (1) where V 1 and V 2 are required volume of NaOH for titration of the sample and the blank, respectively, V3 and V4 are required volume of HCl for titration of the sample and the blank, respectively, normality of NaOH and HCl is the concentration of NaOH and HCl, respectively, and W represents the weight of the sample.

Crystallinity
The crystallinity of extracted CA from CT and SB was performed by X-ray diffraction (XRD). A analytical X-ray diffractometer with Cu Kα 1.5405 nm radiation at 400 kV and 300 mA was used to investigate the X-ray diffraction spectra. This analysis was also done for commercial CA and cellulose to compare the crystallinity of extracted CA of this study. The crystallinity index (CrI) was calculated using the following equation: where I max is the highest peak intensity of the crystalline fraction and Imin is the lowest intensity peak of the amorphous region.

Thermal Property
Thermal Gravimetric Analysis (TGA) was used to analyze its thermal property [16]. The heating rate was 10 •C/ min maintaining the temperature range of 30 -650 •C. A TA-Instruments SDT Q600 controlled the heating rate and temperature.

Morphological Properties
The surfaces of the extracted CA from CT and SB were analyzed by using the scanning electron microscope (SU-3500 from Hitachi, Tokyo, Japan) with the magnification of 500 × to 2500 × magnification. The used voltage for imaging was 5 kV.

Characterization of Raw Materials and Extracted Cellulose
The chemical composition of CT, SB, and extracted cellulose from CT and SB has been presented in Table 1. The pressure build up during the pulping process is about 3-4 atm. The cellulose and hemicellulose content of SB material were higher than those of CT material. Conversely, the total lignin and water extractive contents of CT were higher than those of SB. Furthermore, after the delignification, the cellulose content of CT increased by 85.4%, a little lower than that of SB that is 89.5%. Delignification depends on the type and lignin content of raw materials [40]. Therefore, it was observed that the delignification of SB was easier than that of CT since the lignin content of CT was higher than that of SB. Meanwhile, regarding acidinsoluble lignin, the decreasing percentage of SB (75.9%) was higher than that of CT (48.3%). After the prehydrolysis and pulping, the remaining lignin in the pulps was removed via the ECF method using NaClO 2 and H 2 O 2 . Hypochlorite preferentially destroys certain groups of lignin. Meanwhile, peroxides have been known to be effective bleaching agents and can improve brightness without significant yield loss [41]. On the other hand, prehydrolysis has a great contribution to remove hemicellulose along with lignin [29]. Therefore, the cellulose content of CT and SB was increased after prehydrolysis followed by soda pulping and ECF bleaching. However, the remaining lignin content and hemicellulose content was higher for CT, which may need to consider by controlling prehydrolysis and pulping conditions along with bleaching conditions. Furthermore, the purity of cellulose of CT and SB was comparable to findings in previous studies. Cellulose content of date palm frond [42], oil palm empty fruit bunch (EFB) [43], rice straw [43] and SB [35] were 65.0 to 78.0, 72.4, 76.1 and 85.8%, respectively. Therefore, the extracted cellulose from SB for this study was higher than date palm fronds and rice straw, while it was very close to SB observed in previous studies. On the other hand, purity of cellulose was in the range of date palm frond and lower than that of EFB and SB findings from other studies. This might be due to higher lignin content of CT raw material (35.20%) compared to date palm frond (25.0%) [42], EFB (28.1%) [43], rice straw (11.5) [43] and SB (24.0%) [35]. Interestingly, the delignification and CT (85.4%) and SB (89.5%) for this study were close to SB (92%) obtained through acid treatment by other researchers. Thus, the prehydrolysis technique may reduce the chemical consumption leading to lowering the negative impact on the environment. Figure 1 represents the IR spectra of CA of CT and SB, commercial cellulose, and commercial CA. It was presented that there was neither stretching nor vibration of carbonyl ester at approximately 1730 cm −1 for cellulose. Meanwhile, for commercial CA as well as CA of CT and SB, the peaks of carbonyl ester (C = O) were detected at this bandwidth. Previous studies reported that the peak height of approximately 1728-1743 cm −1 is an indication of C = O stretching of carbonyl ester band that is specific for acetyl groups [18,44]. Therefore, the CA was extracted from CT and SB successfully.

Cellulose acetate Characterization
During the acetylation, polar hydroxyl groups (-OH) in the cellulose were substituted by acetyl groups (-CH 3 COO). In this study, the method of acetylation of CT and SB was different from the conventional CA production method, in which the acetylation of cellulose comprises activation, acetylation, and hydration steps. In this work, acetate anhydride and iodine were directly mixed with cellulose. Reportedly, iodine can form a complex with glucose polymers such as cellulose, and this reaction may help the solubility of cellulose in the acetic anhydride [45]. Biswas et al. (2009) have also shown that the acetylation and CA increase as a function of iodine concentration [45]. The ratio of the iodine concentration to the pulp weight in this study was 1.5:1.
In addition, spectral studies of the proton by NMR ( 1 H NMR) had also shown the proof of acetylation of CT and SB (Fig. 2). The anhydroglucose unit (AGU) protons were detected between δ = 3.0 and δ = 5.2. These results were similar with previous study [46]. Meanwhile, signals of the acetyl groups were located between δ = 1.5 and δ = 2.5. In the spectra of CA for CT, two dominants chemical shifts at δ 2.071 and δ 2.040 refer to acetyl-CH 3 protons. On the other hand, only one acetyl-CH 3 chemical shift was observed at δ 2.029 for the spectra of CA of SB. These results confirm that CA was formed from each material. Moreover, CA extracted from CT may contain di-acetate because it has two chemical shifts. On the other hand, CA extracted from SB may be dominated by mono-acetate and contained a small portion of di-acetate. Table 2 presents the DS and yield of the product. Theoretically, the DS for CA is 1, 2, and 3, for mono-acetate, di-acetate, and tri-acetate, respectively, which is corresponded to the hydroxyl group that could be acetylated. In this study, DS for CA of CT and CA of SB were 1.90 and 1.78, respectively. Both types of CA are close to DS 2 that is usually categorized as diacetate and could be the mix of mono-acetate and di-acetate. Das et al. have reported that the DS will be influenced by reaction conditions such as reactants concentration, time, and temperature [18]. In the reaction, carbonyl carbon of acetic anhydride is activated  by iodine; subsequently, the oxygen from the hydroxyl group of cellulose attacks this carbonyl carbon resulting in sp3 hybridization [18,19]. The yield of CA represents the weight of the product compared with that of initial cellulose ( Table 2). It showed that yields were higher for both CT and SB acetylated celluloses. Compared with a previous study by Das et al., the DS for CA extracted from CT and SB was lower than that of rice husk (RH). However, the yield of RH acetylated cellulose was lower than that of CT and SB. The temperature of reaction for CT and SB was 100 °C that may result in a too fast acetylation rate when compared with the reaction temperature of 80 °C for RH, and the CA nucleus cannot be formed further. Based on the DS value, extracted CA from CT and SB contain not only CA but also remaining cellulose that cannot be converted completely. In others previous studies, DS of CA extracted from date palm frond [42], EFB [43], rice straw [43], SB [35] and bamboo [47] were 1.02 to 3.01, 0.84, 0.21, 2.52 and 2.48 to 2.84, respectively. The obtained results of DS for CT and SB were higher and lower than those of previous studies. Another previous study observed DS of CA obtained from SB 2.52. In this case, the authors used combination method of acid (H 2 SO 4 ) hydrolysis followed by soda (NaOH) pulping, and H 2 O 2 bleaching for cellulose extraction. In acetylation process, the same authors used H 2 SO 4 as catalyst to extract CA [35].

Degree of Substitution
The obtained yield of CA for CT and SB were 89.3 and 99.3%, respectively in this study. Barkalow et al. achieved a 75.0 -80.0% yield of CA extracted from mechanical pulp [48]. Again, the CA yield from RH was 66.0% [18]. The CA yield for both types of materials (CT and SB) was higher than in previous studies.

Thermal Property
From Fig. 3, the thermogravimetry analysis (TGA) of CA of CT and SB and commercial CA can be observed. There was a slight mass loss of approximately 5% up to 200 °C. It was less than the finding of other researchers such as Filho et al., 2008;Candido et al., 2017 [35, 44]. This loss is corresponding to the loss of volatile compounds and H 2 O bound to the hydrophilic (OH) groups of CA chains [49]. In the next stage, mass loss was 70-80% at 200-380 °C. It is the result of the pyrolytic decomposition of the CA polymer chain followed by deacetylation along with the decomposition of lignin and hemicellulose chain [49]. The main thermal decomposition occurred at 380-600 °C. It is due to the carbonization of CA [49]. A similar result was also obtained by other researchers where CA underwent major degradation between 330 °C and 450 °C [50]. The thermal degradation of CA obtained from CT and SB was more or less similar to commercial CA at the first stage but it was higher than commercial CA in the second stage. The thermal degradation of CA obtained from SB and commercial CA showed a similar amount of loss in the third stage while it was lower for CA obtained from CT (Fig. 3).

Crystallinity
In Fig. 4, the sharp high-intensity peak at 2Ɵ = 22° describes the crystalline nature of cellulose, and the CrI of cellulose was 86.6. The commercial CA did not present any intensity peak that confirmed CA was amorphous and this result was similar with previous study [16]. After the acetylation, the CrI of the CA extracted from SB and CT was 75.6 and 60.2, respectively. Meanwhile, there was no peak detected at 2Ɵ = 22° for commercial CA. It can be assumed that the crystallinity of cellulose decreased during the acetylation process. Moreover, the CrI of CA extracted from SB was higher than those of CA extracted from CT.

Morphological Properties
The morphologies of CA obtained from CT and SB were compared with those of cellulose. Figure 5 shows that the fiber diameter of acetylated cellulose of CT and SB was decreased compared with commercial cellulose. The diameter of commercial cellulose ranged from 26 to 29.9 μm. Meanwhile, the diameter of CA extracted from CT and SB was approximately 10 μm and it was 20 to 30 μm for CA extracted from SB. Therefore, the acetylation process influences the structure of cellulose fibers.

Conclusions
Cellulose acetate (CA) was successfully synthesized from sugarcane bagasse (SB) and cajuput twigs (CT) through an environmentally friendly approach. The prehydrolysis followed by soda pulping and acetylation using iodine as catalyst showed suitability to extract CA from CT and SB. The synthesized CA from CT and SB was confirmed by its FTIR and 1 H NMR spectra. Thermal property, crystallinity index, and morphological properties of CA extracted from CT and SB showed better performance compared to commercial CA. The degree of substitution (DS) showed the suitability of acetylation of extracted cellulose from CT and SB as well. DS of CA for CT and SB was in the range of values observed in previous studies. On the other hand, the CA yield of CT and SB was higher than in previous studies. Therefore, the use of prehydrolysis and iodine as catalysis instead of H 2 SO 4 for the production of CA can reduce environmental pollution. Further study is needed to apply this approach to extract CA from other raw materials.