Extraction of lignin from sugarcane bagasse by deep eutectic solvents (DESs)

The article aimed to research on the extraction of lignin from sugarcane bagasse by environmental method using deep eutectic solvents (DESs) containing choline chloride and formic acid. In the process, lignins dissolved in DESs were isolated from the solution by dilution with water and ethanol.

EXTRACTION OF LIGNIN FROM SUGARCANE BAGASSE BY

DEEP EUTECTIC SOLVENTS (DESs)

Bui Thi Thao Nguyen*, Nguyen Nhi Tru, Thai Ngoc Minh Hoang,

Nguyen Hoang Duong, Tran Manh Cuong

Faculty of Materials Technology, University of Technology - VNU HCM, 268 Ly Thuong Kiet,

Ward 14, District 10, Ho Chi Minh City, Viet Nam

*

Email: btnguyen@hcmut.edu.vn

Received: 15 July 2019; Accepted for publication: 30 September 2019

Abstract Lignin plays a crucial role as a structural material of plant cell walls and is also

known as a significant renewable resource that has the potential to be a raw material for

producing high value chemicals The article aimed to research on the extraction of lignin from

sugarcane bagasse by environmental method using deep eutectic solvents (DESs) containing

choline chloride and formic acid In the process, lignins dissolved in DESs were isolated from

the solution by dilution with water and ethanol The obtained lignin samples were characterized

with UV-Vis (Ultraviolet – visible spectroscopy), FT-IR (Fourier-transform infrared

spectroscopy), GPC (Gel permeation chromatography), and 1H-NMR (Proton nuclear magnetic

resonance) The initial results showed that the extracted lignin obtained adequately distinct

functional groups and the weight average molecular mass of the lignin was about 28265 g/mol

Keywords: lignin, extraction, sugarcane bagasse, deep eutectic solvents, DESs - soluble lignin

Classification numbers: 1.1.1, 2.3.1, 2.9.3.

1 INTRODUCTION

In recent years, due to environmental issues and depletion of non-renewable resources,

biodegradable materials gradually replaced traditional raw materials for applications in

packaging, medicine, textile industry and other areas The utilization of natural resources has

seen remarkable growth supported by modern technologies In particular, biodegradable

polymeric materials obtain significant interest and are used in diverse applications for

sustainable development

In this context, lignin - a biodegradable polymer - plays an important role as credible

alternative for industrial polymer and other derivatives Lignin is a part of plant cell walls,

linking cellulose and hemicellulose forming the structure of the plant tissue In terms of physical

characteristics, lignin is rigid, increasing the hardness of the cell wall Plants with robust

mechanical property contain high lignin content in their cell wall

Lignin is a complex cross-linked aromatic macromolecule which is polymerized from three

monolignol monomers, namely, p-coumaryl alcohol, guaiacyl alcohol, and syringyl alcohol

Therefore, the structural units of lignin are guaiacyl (G), syringyl (S) and p-hydroxyphenyl (H)

[1, 2] Depending on the plant species, lignin may consist almost entirely of one lignol or more

than one that is a mixture of two or three lignols The chemical linkages in lignin are C-O-C and

C-C linkages, which were named β-O-4, β-5,5-5, 5-O-4, β-1 and β-β, in which the ether linkages

are dominant (Fig.1) [3] The phenylpropane type alcohols units are connected by these linkages

to form lignin The hypothetical structural formula of softwood lignin was proposed by Alder in

1977 (Fig 1) [4]

Figure 1 (a) Adler’s hypothetical structural formula of softwood lignin [3], (b) some linkages of

lignin and (c) chemical structures of three phenylpropane-type alcohols [4]

Lignin attracts much interest from researchers due to its availability and multi-application

Lignin could be a by-product from the manufacturing process of industrial ethanol or be

extracted from biomass via the kraft process, the sulfite-pulping procedure, or

organic-solvent-based procedure The kraft and sulfite process use toxic chemicals and lignin

extracted by these processes is impure and has high level of contamination In an

organic-solvent-based procedure, biomass is treated in an organic solvent at high temperature,

leading to environmental friendliness However, this process is more expensive than the

previous procedures Recently, application of deep eutectic solvents (DESs) has emerged as a

new technique to extract lignin by using green solvents with dominant properties such as their

low cost, low toxicity, and biodegradability [5,6] DESs are formed by hydrogen bonding of two

chemicals acting as hydrogen-bond donors (HBD) or hydrogen-bond acceptors (HBA) [7]

Based on the property of the constituents, properties of DES can be adjusted according to

specific application Some types of DESs have been used in process of treating biomass by

selecting acid component of DESs DESs may be prepared by mixing inexpensive substances

such as formic acid, lactic acid, acetic acid (HBD) and choline chloride [8] Some previous

works used effectively the acid-based DESs to dissolve lignin from biomass, applied to lignin extraction procedure DESs have been used for the isolation of lignin from willow [5], wheat straw [9], and birch wood [10] The recent results have revealed that DESs based on choline chloride and organic acid were effective in extracting lignin from biomass [11] and the lignin obtained from the DESs based method has specific structural properties [12]

Moreover, the extraction process is also affected by other factors such as the mole ratio of DES constituents, the weight ratio between the DES and biomass, and the conditions and extraction method Generally, DESs melt at the eutectic temperature which is lower than the melting points of the individual components Therefore, acidic DES may be used at a temperature about 70 °C in most of cases [13] These DESs may be prepared by heating and stirring method and some conditions (heating time, temperature, stirring rate, etc.) could be altered according to the property of the compounds Generally, the components are put in a closed container and heated at about 70 °C under magnetic agitation until a homogeneous liquid

is obtained [14]

Sugarcane bagasse is an abundant residue from agricultural industry in Vietnam A large amount of bagasse is currently utilized wastefully or discharged into the environment as garbage Chemical composition of sugarcane bagasse is approximately 32–44 % cellulose, 27–32 % hemicellulose, 19–24 % lignin, 4–9 % other compounds, reported by different authors [15, 16] Therefore, extracting lignin from sugarcane bagasse is an effective solution using bagasse sources In the previous work [17], effect of functional groups in acid constituent of deep eutectic solvent for extraction of reactive lignin with the lactic acid and formic acid was investigated

In this paper, DESs containing choline chloride and formic acid were used to extract lignin from sugarcane bagasse The detailed characteristics of the obtained lignin samples are now revealed by several analytical methods, including UV-Vis (Ultraviolet-visible spectroscopy), FT-IR (Fourier-transform infrared spectroscopy), GPC (Gel permeation chromatography), and 1

H-NMR (Proton nuclear magnetic resonance)

2 MATERIALS AND METHODS 2.1 Preparation of bagasse powder

Sugarcane bagasse was obtained from local cane–juice shops (Ho Chi Minh, Vietnam) with sugarcane was grown in Tay Ninh province in the Southeast region of Vietnam After that, sugarcane bagasse was cut into 1-3 cm long (Fig 2a), and dried for 8 hours until the weight of bagasse was constant Next, the shorted sugarcane bagasse was grinded and sifted using 224 μm filtration to be turned into fine bagasse powder Water and ethanol-benzene were in turn used to purify sugarcane bagasse powder by Soxhlet equipment The obtained bagasse powder was used for the next experiment

2.2 Extraction of lignin from treated bagasse powder

DESs mixture including 16 g choline chloride and 25 g formic acid (1:5 molar ratio) was used to extract lignin from the treated bagasse powder The mixture was boiled at 70 oC, stirred at

200 rpm for about 3 hours to dissolve lignin of bagasse powder into DESs until homogeneous Subsequently, the bagasse-DESs mixture was vacuum-filtered to separate solid including

hemicellulose and cellulose from liquid containing DES-soluble lignin (DESL) Next, the DESL solution (Fig 2b) was mixed with ethanol After the indicated period of treatment, the DESL-ethanol was centrifuged at 5000 rpm in 15 min to precipitate lignin Then, lignin was washed with distilled water and dried before being characterized The obtained lignin was shown

in Figure 2c

Figure 2 Sugar bagasse (a), DESL (b), lignin (c)

2.3 Characterization

The lignin sample was characterized by FTIR spectra collected by Brucker Tensor 27 and GPC measured by Varian-PL-GPC 50 Plus Moreover, 1H-NMR spectra were recorded in deuterated dimethyl sulfoxide (DMSO-d6) on a Brucker Avance II 400 MHz NMR spectrometer and UV-vis spectrum was carried out by U-2910 UV-Vis double beam spectrophotometer

3 RESULTS AND DISCUSSION 3.1 FTIR-Spectra

According to Alder’s lignin formula (Fig 1), lignin possesses the following functional groups: methoxyl, hydroxyl, carboxyl, ketone and aldehyde groups [18] FTIR spectra of lignin were first revealed by Johnson in 1948[19] Through FTIR spectrum in Figure 3, the chemical structure of extracted lignin was reflected The spectrum showed up the broad band ranging from 3336-3380 cm-1 and the band centered about at 3362 cm-1, indicating hydroxyl groups in phenolic and aliphatic structures of lignin [20, 21] The band at 2944 cm-1 mainly arose from C-H stretching in aromatic methoxy groups and methylene groups The peak at 1707 cm-1 was originated from the conjugated carbonyl stretching, possibly referring the presence of hydroxylcinnamic ester in lignin sample Three strong peaks at 1601, 1513 and 1424 cm-1 corresponded to aromatic skeletal vibration of lignin The other peak was located at 1460 cm-1 indicated the asymmertric C-H deformation vibration in the aromatic ring [22, 23] The strong band intensity at 1328 cm-1 was associated with the structure of the syringyl lignin molecule breathing with C-O stretching and the peak at 834 cm-1 was related to the C-H out-of-plane bending vibrations of guaiacyl units of lignin [24, 25] The peak at 1272 cm-1 corresponded to guaiacyl ring breathing with C=O stretching and the peak at 1222 cm-1 referred to the vibration

of the C-O-C linkages in ethers and esters or phenolic hydroxyl Next, the peaksand1170 cm-1

associated with p-hydroxyphenyl structures of lignin breathing with C=O stretching in ester

groups Furthermore, the band at 1124 cm-1 was associated with the C-H in-plane deformations vibration in syringyl aromatic ring types and 1036 cm-1 was originated from aromatic C-H

in-plane deformation and C-O bending vibration in primary alcohols, guaiacyl type [25] This FTIR spectra of the product was similar to the spectra of lignin in some previous works [20-25]

Figure 3 FTIR spectra of lignin extracted from sugarcane bagasse.

3.2 1 H-NMR Spectra

Figure 4 1H-NMR spectra of lignin extracted from sugarcane bagasse

In 1H-NMR spectra of lignin extracted from sugarcane bagasse (Fig 4), the two signals at 2.50 and 3.33 ppm were associated with DMSO-d6 and HDO, respectively The signal at 7.42 ppm arose from the aromatic protons in positions 2 and 6 in structures containing a Cα=O group or

HCα=Cβ structure, revealing the presence of p-courmarate-type structure and hydroxylcinnamic

acid in lignin [26] The two shifts in the range of 7.30-6.70 and 6.70 -6.20 ppm exhibited for

aromatic protons in guaiacyl and syringyl units, respectively [27] The existence of the H-β and H-α

in β-O-4 linkage were reflected by the signals at 4.37 and 4.90 ppm, respectively [28, 29] The peak located at 3.70 ppm referred to the resonance of the methoxyl and side-chains protons in β-β, β-1

linkages and aliphatic hydroxyl groups Moreover, the chemical shifts around 3.10 ppm were possibly originated from the protons of anhydroxylose units [27] which may be exist in lignin sample The 1H-NMR spectra was consistent with the results reported by other authors [30]

3.3 UV-Vis Spectra

UV-Vis spectroscopy was used to further study the structural components of lignin Figure 5 showed UV-Vis absorption spectrum of lignin sample The spectrum presented absorbance maximum between 260 nm and 350 nm The absorption peak at 270 nm indicated the important component of lignin which was the polylignol copolymerized from syringyl, guaiacyl,

and p-coumaryl units [31-33] The absorption of 324 nm region confirmed the presence of

hydroxylcinnamic acid esters which were ferulic acid ester and p-coumaric acid ester [33] The value of A324/A270 was about 0.62, proving the existence of low content of hydroxylcinnamic acid esters in lignin sample [32-33] The max peak located at 270 nm, confirming the presence of high

amount of lignin molecular units, including syringyl, guaiacyl, and p-coumaryl in the lignin

sample

Figure 5 UV-Vis spectra of lignin extracted from sugarcane bagasse

3.4 GPC Results

Table 1 The GPC results of average molecular weight and polydispersity index from lignin.

From GPC results (Tab 1), the values of number average molecular weight (Mn) and weight average molecular weight (Mw) of lignin molecular were 17197 g/mol and 28265 g/ mol, respectively Furthermore, the polydispersity index of lignin (PD = Mw/Mn) which revealed the molecular weight distribution of the lignin polymer was about 1.6, being seen in Table 1 The

lignin had low PD with the value of PD being lower than 2 This result demonstrated that the lignin molecular had narrow molecular weight distribution Moreover, these data confirmed that lignin molecular which was extracted from sugarcane bagasse remained a polymer molecular, not being broken by extraction method

4 CONCLUSIONS

Comparing with numerous methods which have been used to extract lignin from biomass such as kraft process, sulfite-pulping, and organic-solvent-based procedure, using DESs to extract lignin from sugarcane bagasse has been the method of using low toxic and biodegradable solvents Extracting lignin from sugarcane bagasse by DESs is a green and efficient method using bagasse sources which are enormous agricultural by-products DESs which were used for the extraction process of this work included choline chloride and formic acid The isolated product was studied for characterization and chemical structure FTIR, 1H-NMR, and UV-vis spectra confirmed that the product had adequate functional groups of lignin, such as methoxyl, hydroxyl, carboxyl and ketone groups, containing structural components of lignin such as syringyl, guaiacyl, and p-hydroxyphenyl units The results from this study proved that lignin was extracted successfully by DESs and the lignin molecular has the weight average molecular mass

of 28265 g/mol and low polydispersity index which are dominant properties of the research

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