Extraction of Lignin from Sawdust (Chlorophora excelsa) ^†

Abstract

Sawdust is an abundant source of lignocellulosic biomass, presenting a sustainable alternative to fossil fuels for producing aromatics, fuels, and chemicals. Lignin, a crucial component, can be depolymerized into valuable aromatics or used for polymer synthesis due to its multiple hydroxyl groups. This study focuses on extracting lignin from Chlorophora excelsa sawdust via organosolv technology. The characterization of sawdust revealed 41.15% cellulose, 28.63% hemicellulose, and 26.13% lignin. The extraction process involved treating sawdust at varying temperatures (100–200 °C) with an ethanol–water solution and sulfuric acid. The optimal yield of 49.81% lignin occurred at 160 °C, confirming the chemical properties and composition of the extracted lignin.

1. Introduction

In recent years, the increasing consumption of fossil fuels has raised serious environmental concerns, driving a growing interest in alternative renewable resources. Biomass is emerging as a promising feedstock for the production of fuels, aromatics, chemicals, and energy. The relevance of specific biomass types in a region depends on factors such as availability, technical feasibility, economic viability, and environmental impact. In Nigeria, waste wood, landscape maintenance wood, and forestry biomass hold considerable potential [1].

Lignocellulosic biomass—primarily composed of lignin, hemicellulose, and cellulose, with minor amounts of ash and inorganic salts—is widely recognized as a sustainable resource. Lignin, in particular, is the leading renewable source of aromatic compounds and is considered a viable alternative to petroleum-based products [2].

Structurally, lignin is a three-dimensional amorphous polymer composed of ether and carbon–carbon bonds, formed from three main phenylpropane monomers, namely sinapyl (syringyl), p-coumaryl (hydroxyphenyl), and coniferyl (guaiacyl) alcohols. These units are linked by various bonds, including β-β, 4-O-5, β-O-4, α-O-4, β-5, 5-5, and β-1 [3].

Lignin’s high hydroxyl group functionality enables its use in diverse polymer applications. However, its heterogeneity increases during extraction, influencing its downstream applications. The rigidity of its aromatic backbone also makes it suitable for material development [4]. The characteristics and yield of lignin vary depending on the biomass source and the extraction method used, which in turn affects depolymerization outcomes. Therefore, the selection of appropriate pretreatment techniques is crucial to optimize yield and tailor lignin properties [5].

Lignin extraction methods from lignocellulosic biomass are typically categorized into four types, including physical (e.g., size reduction), chemical (e.g., alkaline, acidic, oxidative), solvent-based (e.g., organosolv, phosphoric acid, ionic liquids), and biological (e.g., enzymatic or fungal) processes [6]. Key processing parameters—such as temperature, pressure, pH, and solvent type—significantly affect the chemical structure and bonding within lignin [7].

Organosolv lignin is soluble in alkaline conditions but precipitates under mildly acidic conditions, enabling effective depolymerization. However, the organosolv method has notable drawbacks, including challenges in solvent recovery, risks posed by flammable solvents, high operational pressures, and the formation of undesired by-products due to side reactions like ethoxylation, aryl ether bond cleavage, and condensation [8].

This study investigates the extraction of lignin from Chlorophora excelsa sawdust using a mixture of ethanol, sulfuric acid, and water at various temperatures (100–200 °C). The goal is to disrupt lignin’s structure to facilitate dissolution in the solvent in order to extract the lignin from the sawdust and determine its yield and characterization using FT-IR, CNSNa, density, melting point, and ash and moisture content analyses. Elemental analysis was conducted using the Walkley–Black method, a flame photometer, atomic absorption spectrophotometry, and the wet digestion method to determine the chemical composition of carbon, sodium, sulfur, and nitrogen. Acidic organosolv conditions promote the cleavage of C-O-C ether bonds between lignin and hemicellulose, accelerating the extraction process. Lignin has the potential for the production of aromatics, fuels, and chemicals due to it aromatic nature and abundant availability [9]. Limited research has been conducted on Chlorophora excelsa sawdust; however, Watkins et al. [10] reported extracting lignin from alfalfa fiber sawdust at 105 °C, yielding 34% using the organosolv method with formic and acetic acid as catalysts.

2. Methodology of Extraction of Lignin from Sawdust (Chlorophora excelsa)

Sawdust from Chlorophora excelsa was sourced in a powder form from Tudun Wada, Kaduna State, Nigeria. The powder sawdust was sieved to achieve a particle mesh size of 0.5 μm. A proximate analysis of the pretreated sawdust was performed to determine the percentages of hemicellulose, cellulose, and lignin contents. The neutral detergent fiber (NDF) and acid detergent fiber (ADF) of 0.5 g of a prepared sawdust sample were analyzed. The residue from the ADF analysis was used to determine the content of the lignin in the sawdust. The percentage of hemicellulose was obtained by subtracting the percentage of ADF from the percentage of NDF, while that of cellulose was calculated by subtracting the percentage of lignin from the percentage of ADF [11].

The process of producing lignin from sawdust (Chlorophora excelsa) was performed by mixing an ethanol–water (60/40 W/W) solution at a sawdust-to-liquid ratio of 1:10 (W/W) at 100 °C, 120 °C, 140 °C, 160 °C, 180 °C, and 200 °C for 1 h 30 min in the presence of 20% tetraoxosulphate (VI) acid. The reaction mixture was then allowed to cool to room temperature, after which acidified water was added to obtain a pH of 2 to precipitate the lignin. Filtration was performed to recover the lignin, which was then washed and dried at 50 °C for 24 h [6,10,11,12]. A digital temperature control and pressure gauge were used to measure the temperature and pressure of the extraction.

3. Results and Discussion

The lignin produced from sawdust was obtained using a solvent system comprising water, ethanol, and tetraoxosulphate (VI) acid at pH of 2. This approach aimed to degrade the lignin substrate in solution and facilitate its recovery through subsequent precipitation and washing. The organosolv process, conducted under acidic conditions, cleaves the ether bonds between lignin and hemicellulose, thereby improving the extraction efficiency. Acid precipitation is widely adopted for lignin extraction from biomass due to its ability to induce complete precipitation within a pH range of 2–4 [9,13].

3.1. Organosolv Lignin Extracted from Sawdust (Chlorophora excelsa)

Lignin was produced from sawdust; the percentage yield of the produced organosolv lignin was determined gravimetrically, and the result is presented in

Table 1. Percentage yield of the produced organosolv lignin at different temperatures.

S/N	Description	Lignin Yield (%)
1	Treated at 100 °C	15.17 ± 0.28
2	Treated at 120 °C	27.41 ± 0.12
3	Treated at 140 °C	28.70 ± 0.32
4	Treated at 160 °C	49.81 ± 0.56
5	Treated at 180 °C	29.90 ± 0.33
6	Treated at 200 °C	24.68 ± 0.19

. The table shows the yield of the lignin obtained at different temperatures. The raw sawdust gave 26.13% lignin, 28.63% hemicellulose, and 41.15% cellulose. The treated organosolv lignin at 160 °C gave the highest lignin yield of 49.81%, surpassing the 34% yield reported by Watkins et al. [10] for alfalfa fiber sawdust at 105 °C.

Similarly, Kim et al. [13] reported a 11.3% lignin yield from black industrial waste sawdust at 120 °C and pH 2, which is notably lower than the yield obtained in this study at 160 °C. The carbonization of cellulose into a lignin-like substance in the presence of acid at a high temperature increases the lignin content in the solid residue precipitate [14,15]. The composition of lignin in biomass varies depending on the age, environmental factors, source, and chemical process used in the treatment [13]. The biomass used in this study was obtained from sawdust (Chlorophora excelsa), whereas Kim et al. [13] and Watkins et al. [10] utilized black industrial waste sawdust and alfalfa fiber sawdust, respectively.

3.2. Fourier-Transform Infrared Spectroscopy (FT-IR)

The chemical structures of organosolv lignin extracted through the organosolv process from sawdust at 100 °C, 120 °C, 140 °C, 160 °C, 180 °C, and 200 °C were analyzed using FT-IR, as shown in Figure 1. Organosolv lignin produced at 160 °C exhibits peaks at 3753.4 cm⁻¹, 3652.8 cm⁻¹, and 3332.2 cm⁻¹, which could be assigned to a wide absorption band that represents the hydroxyl group in aliphatic and aromatic phenolic OH groups, as shown in Figure 1, respectively.

A peak at 1107.0 cm⁻¹, present in the lignin samples extracted at 100 °C, 120 °C, 140 °C, 160 °C, and 200 °C, suggests the presence of guaiacyl and syringyl units within the lignin structure. The peak observed at 2918.5 cm⁻¹ across all temperatures corresponds to the presence of methyl and methylene (-CH₂) groups. The peak at 1028.7 cm⁻¹, which is consistent across all lignin samples, corresponds to aromatic C-H stretching. Bands at 1103.3 cm⁻¹, 1312.0 cm⁻¹, 1215.1 cm⁻¹, 1129.4 cm⁻¹, and 1155.5 cm⁻¹, observed in all samples, are associated with ether (C-O-C) linkage stretching and carbonyl (C=O) groups. The band at 1595.3 cm⁻¹ corresponds to aromatic skeletal vibrations in the lignin samples.

While the lignin samples from different temperatures exhibit similar types of functional groups, they differ in the intensities of their spectral bands. Specifically, the aromatic skeletal vibration bands (1400–1600 cm⁻¹) are more pronounced in the lignin extracted at 160 °C, and it also displays multiple hydroxyl functional groups (3200–3400 cm⁻¹). Generally, the lignin produced at various temperatures contains aromatic chains linked to alkanes, alkyl groups, hydroxyl groups, aldehydes, methoxyl groups, and some ether bonds. These observations align with trends reported in similar studies, such as those by Watkins et al. [10], on the formic and acetic acid pulping of lignin from alfalfa fiber sawdust, and Kim et al. [13], on lignin extracted from black industrial waste sawdust using the organosolv process.

3.3. Elemental Analysis and Physical Properties of Produced Organosolv Lignin

The elemental analysis and physical properties of the organosolv lignin extracted from sawdust are summarized in

Table 2. Properties of organosolv lignin extracted from sawdust.

Properties	Composition (%)
Carbon	60.00 ± 1.99
Sodium	0.02 ± 0.002
Nitrogen	0.40 ± 0.03
Sulfur	0.88 ± 0.01
Ash (inorganic)	0.95 ± 0.02
Moisture content	1.88 ± 0.01
Bulk density (g/cm3)	0.264 ± 0.01

. During the purification process, sulfuric acid effectively affected the concentration of the inorganic contaminants of the extracted lignin, resulting in an ash content of 0.95%. This outcome is consistent with findings by Naron et al. [11], who reported ash content ranging from 0.95% to 4.5 wt%. The organosolv lignin demonstrated a high carbon content of 60.00%, attributable to the low inorganic particle concentration of 0.95%. This result corroborates the findings of Domínguez-robles et al. [16] and Graglia [17], who observed that lignin generally exhibits a high carbon content.

The organosolv lignin exhibits low sulfur and nitrogen contents of 0.88% and 0.40%, respectively. These values align with the findings of Graglia [17], who reported that lignin extracted via the organosolv process typically contains sulfur levels between 0.4% and 0.9% and nitrogen levels between 0.1% and 0.4%, indicating a high degree of purity. Additionally, the organosolv lignin shows a minimal sodium impurity of 0.02%, reflecting substantial hydrocarbon content. Graglia [17] documented a maximum sulfur content of 5% in lignin samples, suggesting that elemental analysis is a reliable method for assessing the purity of lignin. The moisture content of the organosolv lignin was 1.88%. The bulk density of the produced lignin was 0.264 g/cm³ at a melting point of 155 °C, which is less than the one produced by Biopiva [18,19] (0.45–0.60 g/cm³) at a melting point of 200 °C. This is probably because bulk density decreases with an increase in temperature.

4. Conclusions

The extraction of organosolv lignin from sawdust (Chlorophora excelsa) was successfully investigated through the organosolv process under acidic conditions and using ethanol/water as a solvent at different temperatures (100–200 °C). The sawdust used consisted of 41.15% cellulose, 28.63% hemicellulose, and 26.13% lignin. The highest yield of organosolv lignin obtained from sawdust was 49.81% at 160 °C. The yield of the lignin was obtained with a negligible amount of sulfur via acidification under mild reaction conditions. The elemental analysis for the organosolv lignin indicated carbon (60.00%), sodium (0.02%), nitrogen (0.40%), and sulfur (0.88%). The high amount of carbon and low quantity of sodium and sulfur indicated that the produced organosolv lignin is good for aromatization. The significance of this work is that different industries can easily identify organosolv lignin extracted from sawdust (Chlorophora excelsa) as a source of raw material for valorization due to its high percentage of carbon. It was also observed that the FTIR analysis of the lignin produced at 160 °C has multiple OH functional groups, which is evidence that more lignin was produced at 160 °C.