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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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<p>Lignin is a complex polyaromatic macromolecule and a potential source of various sustainable materials and feedstock chemicals. To this end, researchers have made some considerable efforts in the high-value applications of lignin, even though there is a limited success so far. This is mainly because the exact structure of native lignin is still virtually unknown due to its highly heterogeneous nature. Besides, technical lignin undergoes unintended structural modifications during the chemical pulping and extraction processes making its final structure even more complicated. Therefore, understanding the lignin structure and its macromolecular characteristics is essential for its proper utilization. In this study, two technical lignins, such as indulin AT and alkali-treated lignin, were investigated for thermal and structural characterization. Various thermal behaviors were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Indulin AT was found to be thermally more stable compared to alkali lignin. Structural characterization was performed using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and proton nuclear magnetic resonance spectroscopy (¹H NMR). Cupric oxide oxidation was utilized to selectively degrade the lignin into its monomers (H/G/S-moieties), which were identified with GC-MS. The results suggested that the selected lignins are mainly composed of G-type monomers. The detailed characterization studies also revealed some minor structural differences between the two lignins due to their respective delignification process. Indulin AT underwent higher structural modifications due to the harsher delignification process and hinted to show more recalcitrance toward depolymerization than alkali lignin.</p>

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.