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Due to the increased and excessive consumption of fossil fuels, sustainable alternative energy sources are badly needed to replace fossil fuels. The conversion of biomass into energy and value-added chemicals is one of the most promising potential pathways to solve this problem. Millions of tons of lignin, one of the major components of biomass, are produced annually as a byproduct of various industries, where it is treated as a low-value material. However, since it has an aromatic polymer nature, lignin is a proven source for different value-added products. Studies suggest that the selective cleavage of a specific bond of the complex lignin structure is one of the major challenges of converting lignin to a targeted product. In this study, eight different lignin depolymerization methods, both traditional and green, are reviewed. Acid and base catalytic depolymerization methods are straightforward, but due to their low selectivity and comparatively severe reaction conditions, they are expensive and not eco-friendly. Pyrolysis-based depolymerization comes with similar problems but has a higher conversion. In contrast, greener approaches, such as oxidative, microwave-assisted, super/sub-critical fluids (SCF), ionic liquid (IL), and deep eutectic solvent (DES)-based depolymerization techniques, have shown higher efficiency in terms of converting the lignin into phenolic compounds even under milder reaction conditions. SCF, IL, and DES-based approaches will likely become more popular in the future for their greener nature. Overall, depolymerization of lignin with greener technologies could make this process more economically viable and sustainable.

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Lignocellulosic biomass is a renewable resource that contains three major constituents: cellulose, hemicellulose, and lignin. Lignin is a potential source of aromatic phenols. The extraction and subsequent depolymerization of lignin was studied using pine sawdust and pistachio shells. Lignin extraction used 70:30 methyl isobutyl ketone:ethanol followed by 0.1M H2SO4. The extraction yield of lignin was 15.78 ± 3.38% from pistachio shells and 18.86 ± 1.52% from pine sawdust. The extracted lignin was characterized using Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and 1H-NMR spectroscopy. The extracted lignin was depolymerized using subcritical water and a Ni-Graphene catalyst at 240 °C for 10 min. The depolymerization products were identified as phenolic monomers, such as phenol, guaiacol, vanillin, syringol, guaiacylpropane, syringaldehyde, coniferaldehyde, synapyl alcohol, and synapyl aldehyde, using GC-MS.

The catalytic depolymerization of alkali lignin into phenolic monomers was studied using subcritical water. In this study, subcritical water was used as the greener solvent with heterogeneous catalysts. The goal of this study was to screen for the best catalyst for the depolymerization, to optimize the reaction conditions, and to increase the yield of the phenolic monomers. The depolymerization reactions were performed at 200 and 240 °C for 5, 10, and 15 min, using subcritical water as the solvent with different catalysts. The treatment of the lignin sample with Ni-Graphene catalyst in subcritical water at 240 °C for 10 min resulted in the highest total yield of phenolic monomers, which was 41.16 ± 0.27 mg/g of alkali lignin. The catalysts also resulted the highest yield for each of the phenolic monomers guaiacol (G), vanillin (G), and homovanillic acid (G) compared to other catalysts studied. The optimized method proved to be an excellent approach to depolymerize alkali lignin.