Efficient separation of cellulose from Bamboo by organic alkali

doi:10.21203/rs.3.rs-1848602/v1

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Efficient separation of cellulose from Bamboo by organic alkali

https://doi.org/10.21203/rs.3.rs-1848602/v1

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This study focuses on the effective separation of cellulose from bamboo through a two-step process. Several organic alkalies 2-pyrrolidinone, ethylurea, dibutylamine, N-methylformamide and tetramethyl guanidine were used to remove lignin and isolate cellulose from massive bamboo. The results showed that tetramethyl guanidine can effectively remove lignin and hemicellulose while retaining almost all the cellulose in the residual solid. The removal of lignin and hemicellulose achieved 86.0% and 84.0% after heating for 3h at 150°C, respectively. Subsequently, the final remove of lignin and hemicellulose increased to 91.5% and 93.8%, respectively, after a simple alkaline H₂O₂ bleach treatment. Interestingly, the loss of cellulose was very small after two-step treatments, and 96.9% of the component was still retained. The crystalline increased from 61.3–68.4% in XRD graphs due to the remove of lignin and hemicellulose. The SEM images indicated that the diameter of cellulose bundles decreased from 80–100µm to about 50µm after organic alkali treatment, and then the fiber bundle was completely separated into a single long fiber with a diameter of about 10 µm after H₂O₂ bleaching. The FT-IR spectra confirmed the high selective removal of lignin and hemicellulose. Two-dimensional ¹H-¹³C NMR (2D HSQC) spectra were analyzed to investigate the lignin structure, and found that only the signals of –OCH₃, C_γ-H_γ in β-O-4' (A_γ) and β-β' (C_γ) structures and C₅-H₅ in guaiacyl (G₅) did not disappear after two-step treatment.

cellulose

tetramethyl guanidine

organic alkalis

lignin

bamboo

With the consumption of fossil resources and the resulting environmental problems, biomass is considered to be an alternative kind of environment-friendly natural resources (Li et al. 2015). As one of the main components of biomass, cellulose could be widely found from plant, some species of algae and biofilms secreted by bacteria (Moon et al. 2011). Since its sustainable potential and unique multidimensional structure, it is not only used in the paper industry, but also is attractive in oilfield application, filler in rubber, nanogenerators, catalyst support, nanocomposite films and textile industry (Damay et al. 2019; Li et al. 2015; Liu et al. 2021). Bamboo is widely distributed in America, Asia and Africa. And due to its fast growth speed and short cycle, bamboo is regarded as an important renewable biomass, which is widely used in papermaking industry (Silva et al. 2020; Muhammad et al. 2019). Compared with wood, bamboo has higher cellulose content, which is more conducive to the development and utilization of lignocellulosic biomass industry (Rusch et al. 2021).

Over the past few years, researches have made considerable efforts to separate cellulose from biomass. Since hemicellulose was easily to hydrolyze, the key problem of isolating cellulose from biomass is how to effectively degrade and remove lignin (Zhou et al. 2017). So far, the methods of removing lignin and isolating cellulose from lignocellulosic biomass include acidic aqueous solution, alkaline aqueous solution, ionic liquid, deep eutectic solvents and organic solvents (Wang et al. 2021; Lin et al. 2021; Guo et al. 2018; Ponnusamy et al. 2019). Chemical pulping was a typical chemical process of lignocellulosic delignification for the production of cellulose. Acid sulfite/sulfate and alkaline sulfate cooking were the main technical methods of cellulose separation in paper pulping (Aro and Fatehi. 2017; Wanrosli et al. 2007). Although the removal of lignin was effective, the subsequent treatment of water pollution was quite troublesome. In order to solve this problem, organic solvent method has attracted extensive attention due to the advantages of the high solubility of organic solvent to lignin and the recyclability (Zhang et al. 2016). Zhong et al. (2021) used 85.0 % aqueous formic acid as solvent, and obtained 38.9% yield of cellulose from corn stover at 130 ℃ for 30 min. Ionic liquids (ILs) were being widely used in biomass isolation ascribing to its high thermal stability and nearly non-volatility (Wang et al. 2021; Liu et al. 2012; Halder et al. 2019). Financie et al. (2016) used a hydrophilic ILs [EMIM][DEP] to treat oil palm frond biomassfollowed by enzymatic delignification, and the content of lignin and hemicellulose decreased from 24.0% to 8.5%, 20% to 12.1%, respectively. [Bmim][OAc] was applied to corn stover at 110 °C, and removed the lignin of 31.47% and xylan of 43.95% (Zhang et al. 2015). Recently, deep eutectic solvents (DESs) with physical and chemical properties similar to ionic liquids have also been used to dissolve lignin and separate cellulose from biomass (Tan et al. 2020). Bamboo residue was treated in a deep eutectic solvent composed of choline chloride and lactic acid in a molar ratio of 1:1 at 130 °C for 90 min and the removal yields of xylan and lignin were 56% and 26%, respectively (Wu et al. 2021). Likewise, Wang reported a two-step treatment process of moso bamboo powder and obtain cellulose with a purity of 92.7% and 80.1% of delignification after hydrothermal treatment at 200 °C for 10 min, followed by lactic acid-choline chloride (10:1) DESs treatment at 140°C for 6h (Wang et al. 2021). Despite considerable efforts, the separation of cellulose from biomass currently encounters low cellulose purity and delignification, high loss of cellulose, high cost of recovery of solvents and environmental pollution (Wang et al. 2021).

Herein, in order to solve the mentioned problems in the process of separating cellulose from biomass, a two-step process was proposed as illustrated in Fig. 1 In the first step, a two-component organic alkali-water system was used for treatment of block Sinocalamus Affinis at 150°C. This process can effectively dissolve lignin and hemicellulose into the mixed solvents and the solvents could be recovered and reused by vacuum distillation, and the lignin and hemicellulose solids obtained by distillation can be further processed into high value-added products. This closed-loop treatment process produced no pollution. To obtain higher purity cellulose, a simple alkaline H₂O₂ bleaching treatment was used. Finally, a purity of 94% bamboo cellulose was obtained with only 3.1% cellulose loss.

Materials

1,1,3,3-tetramethyl guanidine (TMG), ethyl alcohol, methanol, acetonitrile, acetone, anhydrous formic acid, sodium silicate, magnesium sulfate, sodium hydroxide, 2-pyrrolidinone, ethylurea (Urea), dibutylamine, N,N-dimethylformamide (DMF), N-methylformamide (NMF) was purchased from Kelong reactant company. All the above reagents were purchased as analytical grade and were used as received. Sinocalamus Affinis (SA) was obtained from Dazhou, Sichuan province in China.

Organic alkali treatment

The dried SA was cut into small pieces with dimensions of about 1.5-2 cm (length), 3–5 mm (width) and 3–4 mm (thickness) to be used for the following experiments. A typical organic alkali treatment processes was carried out as follows: 1g bamboo pieces and 15ml solution containing TMG, DMF and water were put into a 25ml capacity autoclave and heated at 150℃ for 3h. After cooling to room temperature, the solid residue was separated by filtration and then washed with water for 3 times, and dried to obtain light yellow crude cellulose. Hemicellulose and lignin were dissolved in brown black filtrate. The filtrate was distilled under reduced pressure to remove the solvent. After adding water, the brown precipitate was lignin, and hemicellulose was dissolved in the upper aqueous solution. After filtration and drying, brown powdered lignin was obtained.

Alkaline bleaching

The crude cellulose solid was subjected to an alkaline bleaching treatment. The crude cellulose solid was put into a solution containing 3wt% sodium hydroxide, 6wt% hydrogen peroxide, 2wt% sodium silicate and 0.3wt% magnesium sulfate and heated at 60 ℃ for 5 hours. Then filter to obtain white fluffy cellulose.

Analytical methods and instruments

The contents of cellulose, lignin and hemicellulose of raw bamboo and the treated bamboo were calculated according to the method provided in the literatures (Qi et al. 2006; Hu et al. 2014). The crystallinity of sample was calculated according to the formula of relative crystallinity of cellulose (Ren et al. 2021; Li et al. 2009; Chen et al. 2011).

Scanning electron microscopy (SEM, JSM-7500F, JEOL, Japan); Fourier transform infrared spectra (FT-IR, Nicolet NEXUS 670, American); Nuclear magnetic resonance (NMR, AM-400, Bruker, Switzerland) of samples were detected at 300K using DMSO-d₆ as the solvent; X-ray diffraction (XRD, Shimazu XRD-6100, Japan) of samples were grinded and laminated at room temperature and then made into a 2θ strength curve with a scan range of 5° to 40°; Planetary ball mill (YXQM-8L).

Milled Wood Lignin (MWL) was obtained according to Bjorkman method. And the procedures are as follows: 10g of the bamboo powder was extracted with toluene-ethanol (2:1, v/v) for 24h. Then, the samples were dried at 70℃ for 3h. The dried samples were milled for 8h at a speed of 250 rpm using a planetary ball mill (15 balls of 2cm diameter and 30 balls of 1cm diameter). The ball-milled sample was extracted in 96% dioxane for 24h under 60℃ with reflux condensation (solid-to-liquid ratio was 1:10 g/mL). The mixture was centrifuged and filtered, the residue was washed with dioxane for three times and the supernatant was transferred together. The collected supernatants were combined at reduced pressure and then dried in a vacuum oven at 70°C for 3h to obtain MWL.

Effects of different organic alkalies on chemical composition

One way to separate cellulose from lignocellulose is that lignin and hemicellulose were sheared from cellulose and can be dissolved in a solvent, while cellulose exists in solid form. Acid or alkali can cut off the linking group between cellulose and lignin or hemicellulose, thereby stripping and freeing lignin and hemicellulose. However, acids may also hydrolyze cellulose into soluble sugars, resulting in a large loss of cellulose yield. In this work, we use the strategy of combining alkaline organic solvent and water to separate cellulose from sinocalamus affinis. Several organic alkalies 2-pyrrolidone, N-methylformylanilide (NMF), N-ethylurea (Urea), dibutylamine and 1, 1, 3, 3-tetramethyl guanidine (TMG) were selected to investigate the effects on three components in bamboo. The small pieces of bamboo were put into organic alkali-water solvent and heated for 3 hours at 150°C, and the residual solids were analyzed after filtering and washing with water. The contents of cellulose, lignin and hemicellulose in raw bamboo and the residual solids were listed in Table 1

Table1 The contents of cellulose, lignin and hemicellulose in raw bamboo and the residual solids treated by various organic alkali systems at 150 °C for 3h.

Sample	Content (%)				Removal (%)
Sample	cellulose	hemicelluose		lignin	cellulose	hemicellulose	lignin
Raw bamboo (SA)^a	48.5	18.9	29.4		-	-	-
2-Pyrrolidone treated^a	47.6	11.2	28.8		4.9	42.6	5.4
NMF treated^a	51.0	10.2	26.7		6.4	52.1	19.1
Urea treated^a	57.9	9.4	25.3		3.1	59.7	27.9
Dibutylamine treated^b	54.9	9.0	26.6		6.3	61.0	25.2
TMG treated^b	78.1	4.1	11.7		1.9	86.9	75.7

^a1g SA, 1mol organic alkali, 10ml H₂O; ^b1g SA, 1mol organic alkali, 5ml H₂O

From the Table 1, it can be found that the contents of cellulose, hemicellulose and lignin in raw Sinocalamus Affinis were 48.5%, 18.9% and 29.4%, respectively. The presence of nitrogen-containing organic bases made the aqueous solution alkaline, which degrading lignin and hemicellulose. Under the conditions of this study, the organic alkalis 2-pyrrolidone, NMF, Urea, and dibutylamine caused obvious effects on the removal of hemicellulose, and the removal ratio ranged from 42.6% to 61.0%. However, the removal of lignin by these systems was weak, and the removal ranged from 5.4% to 27.9%. Obviously, the strongest organic alkali TMG exhibited the best efficiency, and the removal of lignin and hemicellulose reached 75.7% and 86.9%, respectively, at 150°C for 3 hours. Fortunately, the loss of cellulose was only 1.9% during this process.

Effects of different proportion of TMG, DMF and water

The solubility of solvent to lignin has an important influence on delignification. Previous studies by Raikwar et al (2021) and Xue et al (2016) have confirmed that DMF was an excellent solvent for the dissolution of lignin. In this study, DMF was selected as cosolvent to increase the solubility of lignin.

The effects of various volume proportions of TMG, DMF and H₂O on the contents of cellulose, lignin and hemicellulose in residual solid were investigated after heating for 3h at 150°C. The results were illustrated in Fig. 2. As can be seen from the figure, when only water and TMG or DMF existed, the removal of hemicellulose and lignin was far inferior to those of the three solvents coexistence. When the ratio of TMG:DMF:H₂O arrived 7:3:5, the removal of hemicellulose and lignin were the highest, reaching 84.0% and 86.0%, and cellulose retention yield can reach 97.8%. In conclusion, 7:3:5 was the best ratio under this treatment condition in this study.

Alkaline bleaching treatment

After the bamboo was treated with high temperature organic alkali, the remaining solid still contains 7.3% lignin and 5.4% hemicellulose, respectively. In order to obtain high-purity cellulose, an alkaline H₂O₂ bleaching treatment was employed to remove the residual lignin. The remaining solid was put into a solution containing 3wt% sodium hydroxide, 6wt% hydrogen peroxide, 2wt% sodium silicate and 0.3wt% magnesium sulfate and heated at 60 ℃ for 5 hours. Then filter to obtain white fluffy cellulose with a purity of 94% and brightness (ISO) of 66.5%. As a result, after this two-step treatment, lignin and hemicellulose in bamboo were removed by 91.5% and 93.8%, respectively, and the loss of cellulose was only 3.1%.

Microscopic investigation of samples

The SEM images of bamboo powder, cellulose fibers after organic alkali and alkaline bleaching were showed in Fig. 3. The SEM images intuitively showed the microscopic morphology of fibers of raw bamboo and treated cellulose. It can be seen from Fig. 3a and Fig. 3b that in bamboo raw powder, about 15–20 individual fibers form fiber bundles with a diameter of about 80–100µm, and the periphery was closely connected with hemicellulose and lignin. After high-temperature organic alkali treatment, the connected lignin and hemicellulose reduced, and the diameter of the fiber bundle was reduced to about 50µm (Fig. 3c), indicating that this treatment process can dissociate the fiber bundles. After the alkali bleaching treatment, no lignin and hemicellulose were found around the fibers, and the fiber bundles were completely separated into individual fibers with a diameter of about 10µm (Fig. 3d), this indicating that the treatment can completely dissociate the fiber bundles to obtain long bamboo fiber.

FT-IR analysis

FT-IR spectra analysis was performed to investigate the varieties of chemical functional groups after these treatments. Figure 5 showed the FT-IR spectra of bamboo powder, and residual solids treated after organic alkali and after alkaline bleaching. In Fig. 5, the characteristic peaks of lignin appeared at 1729, 1634, 1453, 1422, 1240 and 833 cm^− 1. The peak at 2944 cm^− 1 belonged to C-H vibration of -CH₂ in lignin. The peaks at 1602, 1508 and 1422 cm^− 1 were attributed to the skeleton vibration of benzene ring, and the peak at 1729 cm^− 1 was due to C = O stretching. The peak at 1370 cm^− 1 represents the phenolic hydroxyl groups. After organic alkali treatment, it was obviously seen that the characteristic peaks of hemicellulose and lignin disappeared, leaving the absorption peaks of cellulose, whose characteristic peak was at 1314, 1160, 896 cm^− 1 (Wei et al. 2016). Especially, the absorption peak at 1240 cm^− 1, which belongs to the C-O-C stretching vibration of the aromatic ether, disappeared. The absorption peak near 1314 cm^− 1, which was attributed to the in-plane bending of hydroxyl group, was obviously strengthen, this indicated a high selective removal of lignin and hemicellulose through organic alkali treating process, which was consistent to the chemical titration analysis above. After alkaline bleaching, the disappearance of the peak at 1631cm^− 1 and the appearance of the peak at 1640cm^− 1 may be due to the formation of ketone oxidation of hydroxyl group of lignin in the presence of hydrogen peroxide.

NMR analysis

In order to investigate the effect of two treatment processes on lignin, the chemical structure information of both Milled Wood Lignin (MWL) and H₂O₂ bleaching treated lignin (BTL) was analyzed by two-dimensional ¹³C-¹H NMR (2D HSQC). 2D HSQC NMR spectra of the lignin showed the predominance of lignin signals together with the remaining sugars. The side chain region (δ_C/δ_H 110 − 50/5.5–2.5 ppm) and aromatic region (δ_C/δ_H 150 − 100/8.5–5.5 ppm) of ¹³C-¹H 2D HSQC NMR spectra of Milled Wood Lignin (MWL) and H₂O₂ bleaching treated lignin (BTL) were illustrated in Fig. 6 The main substructures of two kinds of lignin, including different units linked by ether and C-C bonds, and carbohydrates were depicted in Fig. 7, and their corresponding signal assigned in the HSQC spectra were listed in Table 2.

As shown in Fig. 6, the side-chain regions (δ_C/δ_H 110 − 50/5.5–2.5 ppm) of NMR spectra of MWL and BTL both showed prominent signals corresponding to methoxyls (δ_C/δ_H 56.18/3.74 and 56.22/3.72 ppm). As seen in Fig. 6b, the spectra of MWL showed abundant chemical structure information, as observed in the previous reports (Wen et al. 2013; Hu et al. 2014). The C_α-H_α correlations in β-O-4' substructures were observed at δ_C/δ_H 72.82/4.92 and 71.55/4.72 ppm for structures linked to S and G lignin units.

The C_β-H_β correlations were found at δ_C/δ_H 81.82/4.02 ppm for β-O-4' S units. Likewise, The C_γ-H_γ and C₆-H₆ correlations in β-O-4' appeared at δ_C/δ_H 60.71/3.51 and 128.36/7.32 ppm, respectively. Signals for phenylcoumaran substructures (B) were also detected in the spectrum of MWL, with their C_α-H_α, C_γ-H_γ correlations at δ_C/δ_H 89.16/5.30 and 68.02/4.15 ppm. A strong signal at δ_C/δ_H 72.70/3.51 ppm was corresponded to the resinols structures (C) with its C_γ-H_γ. Whereas, in the BTL spectrum (Fig. 6d), the signals of chemical groups were obviously different and there were new signals (δ_C/δ_H 100.45/5.11 and 72.01/3.60 ppm) appeared in BTL except for methoxyl and C_γ-H_γ still existed in alkyl-aryl ethers (A) and resinols (C), which may be caused by the residues of xylose and hexose in lignin due to hydrolysis of cellulose and hemicellulose.

In the aromatic region (δ_C/δ_H 150 − 100/8.5–5.5 ppm), the main cross-signals in MWL including syringyl units (S), guaiacyl units (G), p-hydroxyphenyl units (H), which S-lignin showed a prominent signal for the C_2,6-H_2,6 corralations in oxidized syringyl units (S') with a C_α ketone were observed at δ_C/δ_H 106.79/7.22 ppm. The G lignin units showed different correlations for C₂-H₂ (δ_C/δ_H 111.27/6.97 ppm), C₅-H₅ and C₆-H₆ (δ_C/δ_H 116.29/6.98 and 116.51/6.57 ppm).

Besides, the signals corresponding to C_2,6-H_2,6 appeared at δ_C/δ_H 130.55/7.18 ppm was belong to H lignin (Jose etal. 2012; Wen etal. 2013). However, In the BTL spectrum (Fig. 6c), there was only C₅-H₅ of G lignin appeared at δ_C/δ_H 115.88/6.86 ppm in BTL, this indicates that lignin has been seriously degraded after two-step treatment. Other signals in the aromatic regions showed signals corresponding to p-coumarates (PCA), ferulate (FA) and p-hydroxybenzoate (PB) substructures. The cross signals of PCA and FA corresponding to the C_α-H_α and C_β-H_β at δ_C/δ_H 144.83/7.52 and 114.73/6.27 ppm, respectively. In addition, the signals corresponding to the C_2,6-H_2,6, C_3,5-H_3,5 and C_γ-H_γ of p-coumarates (PCA) were also observed at δ_C/δ_H 130.58/7.51, 116.09/6.79 and 65.90/4.33 ppm. The C_2,6-H_2,6 correlations of PB appeared at δ_C/δ_H 131.80/7.79 ppm. The signals for the C_β-H_β correlations of substructures J were found at δ_C/δ_H 127.23/6.76 ppm. Furthermore, the signals δ_C/δ_H 100.63/4.89, 95.26/4.64 and 97.32/4.29 ppm were observed as lignin carbohydrate complex. Other signals from β-D-xylopyranoside units (X) were also observed, which its C₂-H₂, C₃-H₃, and C₄-H₄ correlations were at δ_C/δ_H 70.79/3.05, 71.59/3.14 and 77.65/3.31 ppm (Santos etal. 2015; Yuan etal. 2011).

In this study, a simple and effective method for separating cellulose from block Sinocalamus Affinis was proposed. The solvent can be recovered by distillation to obtain water-insoluble solid lignin. This process was environment friendly and could produce almost no environmental pollutants.

The organic strong alkali TMG has the best removal effect on lignin and hemicellulose, and the addition of DMF and water improves the delignification effect. The diameter of cellulose bundles reduced from 80–100µm to about 50µm after organic alkali treatment, and then the fiber bundle was completely separated into a single fiber with a diameter of about 10µm after H₂O₂ bleaching treatment. NMR spectra were compared by MWL in detail, which showed that bamboo lignin was seriously degraded after two-step treatment. And the final remove of lignin and hemicellulose arrived 91.5% and 93.8%, respectively and a purity of 94% bamboo cellulose was obtained with only 3.1% cellulose loss.

The method proposed in this study can not only improve the paper pulping process, but also provide an important technology for the preparation of dissolving pulp in the textile industry due to the effective separation of long fibers.

Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.

Availability of data and materials All data generated during this study are included in this published article.

Competing interests The authors have no relevant financial to disclose.

Funding The National Natural Science Foundation of China (No. 92056116).

Authors' contributions Li-Yu Chen: Investigation, Experiments, Data collection and Analysis, Writing—original draft; Xiang-Guang Meng: Conceptualization, Methodology, Supervision, Resources, Writing—review&Editing; Wen-Wang Yu: Experiments; Yu-Lian Zhang: Partial Data collection; Zi-Yu Gan: Partial Data Analysis; Cheng Yang: Funding, Writing—review.

Acknowledgements This study was funded by the National Natural Science Foundation of China (No. 92056116) and financial support from the fundamental research funds for the central universities of China.

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No competing interests reported.

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Editorial decision: Major revision
09 Aug, 2022
Editor assigned by journal
09 Aug, 2022
Submission checks completed at journal
07 Aug, 2022
First submitted to journal
11 Jul, 2022

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Efficient separation of cellulose from Bamboo by organic alkali

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Version 1

Abstract

Figures

Introduction

Experimental

Materials

Organic alkali treatment

Alkaline bleaching

Analytical methods and instruments

Results And Discussion

Effects of different organic alkalies on chemical composition

Effects of different proportion of TMG, DMF and water

Alkaline bleaching treatment

Microscopic investigation of samples

FT-IR analysis

NMR analysis

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1