In recent years, the disposal of untreated lignin waste from lignocellulosic biomass emerging from many industries, such as paper pulp, biorefineries and agro-sectors has gathered a lot of attention, as it not only poses environmental problems due to its non-degradability but also can be utilised as a starting material in the manufacturing of lignin-based commercial products. A monolayer of hemicellulose covers the cellulose in the lignocellulose structure, which then embeds it in the central composition of lignin and hemicellulose, creating a complex network [1]. Lignin, a naturally stable phenolic compound, is the second most abundant natural polymer; it accounts for 10–25% of lignocellulosic biomass. Being water-insoluble, it serves as the bonding agent that links cellulose and hemicellulose [2]. The side branches of the complex polymer structure are made up of methoxy groups, phenolic hydroxyl groups, and a few aldehyde groups at the end. It is present in the plant cell wall and is used to bind cells and provide durability to plant matter. Enzymatic polymerization of three phenols—coniferyl alcohol, p-coumaryl alcohol, and sinapyl alcohol—results in many functional groups and linkages. Phenylpropane units in lignin form characteristic linkages, including β-β, β-O-4, and β-5 linkages. The three phenylpropanoid types of lignin are syringyl (S), p-hydroxyphenyl (H), and guaiacyl (G) [3].
Coconuts contain abundant amounts of lignin (37–42%, w/w). Although there are differences depending on the species, location, and degree of maturation of the nuts, coir mainly stands out from other plant fibres due to its high lignin content [4]. Deep Eutectic Solvents (DES) and Organosolv processes are environmentally friendly solvent treatments for lignin extraction. specialised mixtures of two or three components in precise molar ratios, characterised by lower freezing points than their constituents along with significantly lowered melting points (> 50°C) compared to ideal mixtures. Organosolv, on the other hand, employs lower temperatures (< 150°C) to maintain lignin in its original state. When alcohols, especially ethanol (EtOH), are utilized, they typically yield high-quality lignin with remarkable preservation of the substantial β-aryl ether composition while containing minimal carbohydrates, ash, and other impurities not derived from lignin. These specialized mixtures have lower freezing and melting points and offer biocompatibility, reduced toxicity, enhanced biodegradability, and cost-effectiveness [5, 6].
The primary objective of this research work is to study the effectiveness of green solvent treatments like Organosolv using the catalyst and DES, a combination of choline chloride (ChCl) and several hydrogen bond donors employed for lignin extraction from coconut coir. The structural changes that occurred during Organosolv and DES treatment were analysed using Fourier transform infrared (FT-IR) and lignin was characterised using 1H Nuclear magnetic resonance (NMR) spectroscopy. This study aims to extract lignin in its native form using green solvents that can be recovered and reused for further experiments. This extracted lignin can be utilized in various industrial and commercial applications.