Three-stage TCF bleaching sequence

**Figure 6.** The brightness and viscosity profiles during three-stage totally chlorine-free bleaching of *Nypa fruticans* frond pulp at 20% AA dosage (O: oxygen; Psa: peroxymonosulfuric acid; Ep: alkaline **Figure 6.** The brightness and viscosity profiles during three-stage totally chlorine-free bleaching of *Nypa fruticans* frond pulp at 20% AA dosage (O: oxygen; Psa: peroxymonosulfuric acid; Ep: alkaline hydrogen peroxide). **Figure 6.** The brightness and viscosity profiles during three-stage totally chlorine-free bleaching of *Nypa fruticans* frond pulp at 20% AA dosage (O: oxygen; Psa: peroxymonosulfuric acid; Ep: alkaline hydrogen peroxide).

hydrogen peroxide).

as *E. globulus* pulp with 88.4% ISO and 6.0 cP [31].

*Molecules* **2022**, *27*, x FOR PEER REVIEW 9 of 17

**Figure 7.** Transformation of the *Nypa fruticans* unbleached pulp into pure cellulose during three-**Figure 7.** Transformation of the *Nypa fruticans* unbleached pulp into pure cellulose during threestage totally chlorine-free bleaching at 20% AA dosage (O: oxygen; Psa: peroxymonosulfuric acid; Ep: alkaline hydrogen peroxide). **Figure 7.** Transformation of the *Nypa fruticans* unbleached pulp into pure cellulose during three-stage totally chlorine-free bleaching at 20% AA dosage (O: oxygen; Psa: peroxymonosulfuric acid; Ep: alkaline hydrogen peroxide).

stage totally chlorine-free bleaching at 20% AA dosage (O: oxygen; Psa: peroxymonosulfuric acid; Ep: alkaline hydrogen peroxide). The α-cellulose and ash contents of the bleached pulp as pure cellulose in this study were 94.8 ± 2.8% and 0.2 ± 0.07%, respectively, as shown in Table 3. The final yield of the purified pulp was 28.0 ± 1.3% based on the material weight. The properties of pure cellulose include 92.2 ± 1.0% ISO brightness, 94.8 ± 2.8% α-cellulose, 7.9 ± 0.6 cP viscosity, which are acceptable levels for viscose rayon and cellulose derivatives. Regarding the ash content obtained in this study, namely 0.2 ± 0.07%, it was reported that dissolving pulps from The α-cellulose and ash contents of the bleached pulp as pure cellulose in this study were 94.8 ± 2.8% and 0.2 ± 0.07%, respectively, as shown in Table 3. The final yield of the purified pulp was 28.0 ± 1.3% based on the material weight. The properties of pure cellulose include 92.2 ± 1.0% ISO brightness, 94.8 ± 2.8% α-cellulose, 7.9 ± 0.6 cP viscosity, which are acceptable levels for viscose rayon and cellulose derivatives. Regarding the ash content obtained in this study, namely 0.2 ± 0.07%, it was reported that dissolving pulps from The α-cellulose and ash contents of the bleached pulp as pure cellulose in this study were 94.8 ± 2.8% and 0.2 ± 0.07%, respectively, as shown in Table 3. The final yield of the purified pulp was 28.0 ± 1.3% based on the material weight. The properties of pure cellulose include 92.2 ± 1.0% ISO brightness, 94.8 ± 2.8% α-cellulose, 7.9 ± 0.6 cP viscosity, which are acceptable levels for viscose rayon and cellulose derivatives. Regarding the ash content obtained in this study, namely 0.2 ± 0.07%, it was reported that dissolving pulps from non-woody materials with an ash content of approximately 0.7% still showed potential for the production of viscose rayon, carboxymethylcellulose, and other derivatives [60,61]. Therefore, pure cellulose was successfully produced from nipa palm fronds using prehydrolysis, soda-SAQ and TCF bleaching. The processes can further be utilized for the production of other useful products or materials.

The results show that only a small amount of pentosan, namely 1.0 ± 0.4%, remained in the purified pulp. Most of the pentosan in the materials can be isolated from the prehydrolysate for furfural production. From the sulfur-free soda cooking liquor, dissolved lignin can easily be precipitated based on a proposed method, such as the lignoboost process for raw

materials of biopolymers [62]. The sulfur-free and totally chlorine-free processes for cellulose purification from NPF can be developed in future into a promising biorefinery. in the purified pulp. Most of the pentosan in the materials can be isolated from the prehydrolysate for furfural production. From the sulfur-free soda cooking liquor, dissolved lig-

The results show that only a small amount of pentosan, namely 1.0 ± 0.4%, remained

non-woody materials with an ash content of approximately 0.7% still showed potential for the production of viscose rayon, carboxymethylcellulose, and other derivatives [60,61]. Therefore, pure cellulose was successfully produced from nipa palm fronds using prehydrolysis, soda-SAQ and TCF bleaching. The processes can further be utilized for the pro-


**Table 3.** Chemical composition of pure cellulose from *Nypa fruticans* fronds. nin can easily be precipitated based on a proposed method, such as the lignoboost process

**Table 3.** Chemical composition of pure cellulose from *Nypa fruticans* fronds.

Sulfur-free unbleached pulp 84.1 ± 2.7 3.3 ± 1.2 1.0 ± 0.5

**Components α-Cellulose (%) Pentosan (%) Ash (%)**  Materials 37.3 ± 2.1 24.0 ± 1.9 16.5 ± 2.2

*Molecules* **2022**, *27*, x FOR PEER REVIEW 10 of 17

duction of other useful products or materials.

#### **3. Materials and Methods** *3.1. Materials*

#### *3.1. Materials Nypa fruticans* fronds were collected from Muntai village, Bantan district of Bengkalis

*Nypa fruticans* fronds were collected from Muntai village, Bantan district of Bengkalis island, Indonesia. The fiber fragments with lengths of 0.5–1.0 cm were prepared after washing and sun-drying to approximately 90%. Figure 8a–c shows the nipa palm fronds and their chipped form. SAQ (1,4-dihydro-9,10-dihydroxyanthraxene sodium salt) was provided by Air Water Performance Chemical Inc., Kawasaki, Japan. Furthermore, Psa (H2SO5) was synthesized based on the method proposed by Kuwabara et al. [63] using 98% sulfuric acid (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and a 50% hydrogen peroxide aqueous solution (Mitsubishi Gas Chemical Company, Inc., Tokyo, Japan) at a molar ratio of 1:3 and 70 ◦C. island, Indonesia. The fiber fragments with lengths of 0.5–1.0 cm were prepared after washing and sun-drying to approximately 90%. Figure 8a–c shows the nipa palm fronds and their chipped form. SAQ (1,4-dihydro-9,10-dihydroxyanthraxene sodium salt) was provided by Air Water Performance Chemical Inc., Kawasaki, Japan. Furthermore, Psa (H2SO5) was synthesized based on the method proposed by Kuwabara et al. [63] using 98% sulfuric acid (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and a 50% hydrogen peroxide aqueous solution (Mitsubishi Gas Chemical Company, Inc., Tokyo, Japan) at a molar ratio of 1:3 and 70 °C.

**Figure 8.** Nipa palm fronds received (**a**); peeled (**b**), and; chipped (**c**).

#### **Figure 8.** Nipa palm fronds received (**a**); peeled (**b**), and; chipped (**c**). *3.2. Methods*

*3.2. Methods*  3.2.1. Prehydrolysis and Sulfur-Free Cooking

3.2.1. Prehydrolysis and Sulfur-Free Cooking The prehydrolysis and cooking of the NPF were carried out in a 350 mL stainlesssteel reactor (Taiatsu Techno Corporation, Tokyo, Japan), with a maximum temperature of 150 °C. The parameters used for the process include a distilled water to biomass ratio of 7 L/kg, a temperature of 150 °C, and a reaction time of 1–3 h. The wet solid residues The prehydrolysis and cooking of the NPF were carried out in a 350 mL stainlesssteel reactor (Taiatsu Techno Corporation, Tokyo, Japan), with a maximum temperature of 150 ◦C. The parameters used for the process include a distilled water to biomass ratio of 7 L/kg, a temperature of 150 ◦C, and a reaction time of 1–3 h. The wet solid residues without washing were subjected to soda-SAQ cooking using sodium hydroxide solution as fresh alkaline cooking liquor. The parameters used were a 0.1% SAQ dosage on the raw material weight, a 7 L/kg liquor to solid ratio, a temperature of 160 ◦C, 1–1.5 h cooking time, and active alkali (AA), namely Na2O, with dosages of 13, 17, 19, 20, and 25%. The residue obtained was processed into pulp using a disintegrator (FRANK-PTI GmbH), followed by screening, washing, and drying at 105 ◦C. Subsequently, the yield (%) of pulp was determined based on the raw materials' weight. The optimal prehydrolysis and cooking times were selected based on the yield and kappa number. The selected parameters, procedures, and analyses were carried out based on the method from previous studies for the production of dissolving pulp from non-wood materials with similar characteristics, such as oil palm empty fruit bunch [34]. Cooking with soda without SAQ as well as the kraft method using a mixture of sodium hydroxide and sodium sulfide (sulfidity of 30% as Na2O) were also carried out to compare the results from the selected soda-SAQ process.

#### 3.2.2. Totally Chlorine-Free Bleaching

A modified and simple three-stage sequence of TCF bleaching, including oxygen (O), Psa, and alkaline hydrogen peroxide (Ep) treatments and procedures, was carried out for the soda-SAQ pulp based on the multi-stage sequence proposed by a previous study [34]. This TCF sequence has been suggested as a solution for the pulp and paper industry to prevent pollution during bleaching [32,34]. The conditions of each stage are presented in Table 4. Depending on the TCF bleaching stage, a pulp consistency (PC) of 10 or 30% was prepared by carefully mixing the pulps with distilled water. The particles of wet pulp were mixed with NaOH in a polyethylene bag by hand, providing limited homogeneity of the treatment specifically to the samples with 30% PC; however, successful results for bleaching were obtained in this study. The PC value was then determined with the TAPPI method T 240-om 93 [64]. Regarding the oxygen delignification step, a portion of oxygen was added to the reaction mixture at the beginning of the process until the pressure was reached, followed by the reaction at the determined temperature and time (T-t). We did not use a high-share mixer for medium-consistency oxygen bleaching. Meanwhile, at the Psa stage, a target amount of Psa and a small amount of NaOH aqueous solution for pH adjustment were added to the pulp suspension in a polyethylene bag at the determined T-t. With respect to the E<sup>p</sup> stage, target amounts of H2O<sup>2</sup> and NaOH were also added to the pulp suspension in a polyethylene bag and at the determined T-t. These two later steps of bleaching at lower consistency (10%) were carried out for easier comparison with past results in the literature (Appendix A). The brightness and viscosity after each stage were obtained using methods explained in the next section.

**Table 4.** Conditions of three-stage totally chlorine-free bleaching for *Nypa fruticans* frond soda pulp.


#### 3.2.3. Chemical Analysis of Materials and Pulp

For raw materials, the contents of holocellulose, alpha (α)-cellulose, pentosan, acidinsoluble lignin (Klason lignin), extractives, and ash were determined using the TAPPI methods, namely T 249 om-00, T 203 cm-99, T 223 cm-10, T 222 om-06, T 204 cm-07, and T 211 om-07, respectively [65–70]. Meanwhile, the acid-soluble lignin was determined with UV-vis spectrophotometry at 205 nm using a gram extinction coefficient of 110 L/g cm [TAPPI method: T 250 um-62]. The methods T236 om-99, T452 om-08, and T230 om-04 were used to determine the kappa number, brightness, and viscosity, respectively [71–74]. The pulp viscosity was also converted into the degree of polymerization or *DP* based on the equation proposed by Shi et al. [75] (Equation (1)):

$$DP^{0.905} = 0.75V \tag{1}$$

where *V* represents the pulp viscosity (mPa·s or cP) after the process.

#### 3.2.4. Statistical Analysis

In this study, at least two independent experiments and two measurements for each were carried out for each condition/method. The average values ± standard deviations were calculated and presented. Furthermore, a *t*-test with significance at *p* < 0.05 was used to compare any two pulp yield or kappa number values with different processing conditions/methods (Systat v13.2.01, Systat Statsoft Inc., San Jose, CA, USA), as described by Hart and Sharp [76]. It was also used to compare any two brightness or viscosity values in the TCF bleaching experiment.

#### **4. Conclusions**

*Nypa fruticans* frond waste is a promising raw material for producing pure cellulose and its derivatives, as well as biofuels and chemicals, due to a reasonable amount of αcellulose and a relatively low lignin content, namely 37.3% and 18.3%, respectively. The selected prehydrolysis sulfur-free soda process with soluble anthraquinone (SAQ) catalyst has more advantages compared to the kraft and soda processes due to the decrease in the kappa number by ≥3.6 points at a 19–20% active alkali or AA dosage. Soda-SAQ also exhibited higher pulp yields of 6% at the same dosage. The optimal conditions for the soda-SAQ process include 3 hours of prehydrolysis at 150 ◦C and cooking for 1.5 h at 160 ◦C with a 20% AA concentration. Prehydrolysis sulfur-free soda cooking with the SAQ catalyst followed by three-stage totally chlorine-free bleaching with oxygen, peroxymonosulfuric acid, and alkaline hydrogen peroxide stages can produce pure cellulose as a dissolving pulp from NPF waste. The final properties of the product obtained include 92.2% ISO brightness, 94.8% α-cellulose, 7.9 cP viscosity, and 0.2% ash content. The results show the benefits and details of the prehydrolysis sulfur-free soda process with SAQ to produce pure cellulose that is acceptable for viscose rayon and cellulose derivatives. Further studies can be conducted to investigate and develop its potential for biorefinery because 99% of the pentosan was removed in the prehyrolysate and cooking liquor. The production of the viscose rayon and cellulose derivatives, such as cellulose nitrate/acetate from the obtained prehydrolysis soda-SAQ pulps, can also be explored.

**Author Contributions:** Conceptualization, E. and H.O.; methodology, E., S. and D.A.; software, E. and S.; validation, E. and S.; formal analysis, E., A.A. and H.O.; investigation and supervision, S. and D.A.; resources, E., A.A. and H.O.; data curation, E., S. and D.A.; writing—original draft preparation, E.; writing—review and editing, E., A.A. and H.O.; project administration, E. and D.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by the Directorate General of Education, Culture, Research, and Technology, Indonesia, through the World Class Research (WCR) Scheme with Project Agreement number 1395/UN19.5.1.3/PT.01.03/2021.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are grateful to Retno Dwi Astutik and Donny Tusary, students at the Chemical Engineering Department, Faculty of Engineering, for their valuable contributions in the preparation and investigation of this study within the framework of the WCR project.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A**

#### **High Consistency of Oxygen Bleaching**


**References** 

and petiole. *Elixir Appl. Chem.* **2012**, *45*, 7664–7668.

8. Make pulp into small, bulky and fluffy particles (diameter 5–10 mm). 9. Put pulp in a polypropylene plastic bottle and put it in a reactor (Figure A2). 10. Charge oxygen gas into the reactor under 0.35–0.50 MPa (3.5–5.0 bars) at room temperature for

8. Make pulp into small, bulky and fluffy particles (diameter 5–10 mm).

*Molecules* **2022**, *27*, x FOR PEER REVIEW 13 of 17

1. Measure the water content of the air-dried pulp sample.

**High Consistency of Oxygen Bleaching** 

6. If the NaOH is 2N (2 mol/L), then add 1.25 ml.

polyethylene plastic bag

30% of pulp consistency.

with hands (Figure A1).

distilled water.

**Appendix A** 


2. Measure the pulp sample with 10.0 g of oven-dried weight, and then tear it, and put them in a

3. Prepare 20 mL (or A: 10 mL for MgSO4·7H2O2 and B: 10 mL for NaOH) of distilled water for

5. If the pulp sample was 10.0 g, then add 2.5 mL of 1 N (1 mol/L) NaOH (0.1 g of NaOH) into the

7. Drop A: 10 mL of Mg solution containing 100 mg of MgSO4·7H2O2 to the pulp sample, and mix the pulp sample in a polyethylene bag with hands (Figure A1). Drop B: 12.5 mL or 11.25 mL of the NaOH solution to the pulp sample, and mix the pulp sample in a polyethylene bag

4. Prepare the targeted NaOH and Mg dosage. Example 1.0% and 0.1% respectively.


**Figure A1.** Put pulp in polyethylene plastic bag and make pulp into small particles (diameter 5–10 **Figure A1.** Put pulp in polyethylene plastic bag and make pulp into small particles (diameter 5–10 mm).

(**a**) (**b**) **Figure A2.** (**a**) Put pulp in a polypropylene plastic bottle, and (**b**) then put the plastic bottle into the reactor. 443. **Figure A2.** (**a**) Put pulp in a polypropylene plastic bottle, and (**b**) then put the plastic bottle into the reactor.

(**a**) (**b**)

1. Akpakpan, A.E.; Akpabio, U.D.; Obot, I.B. Evaluation of physicochemical properties and soda pulping of *Nypa fruticans* frond

3. Harun, N.Y.; Saeed, A.A.H.; Ramachandran, V.A/L.A. Abundant nipa palm waste as bio-pellet fuel. *Mater. Today*. **2021**, *42*, 436–

2. CABI. *Nypa fruticans*. Available online: https://www.cabi.org/isc/datasheet/36772 (accessed on 3 July 2022).

**Figure A3.** Charge oxygen gas under 0.35–0.50 MPa at room temperature.

(**a**) (**b**)

**Figure A2.** (**a**) Put pulp in a polypropylene plastic bottle, and (**b**) then put the plastic bottle into the

reactor.

**Figure A3.** Charge oxygen gas under 0.35–0.50 MPa at room temperature. **Figure A3.** Charge oxygen gas under 0.35–0.50 MPa at room temperature.

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