Recent Advances in Lignin Depolymerization Techniques: A Comparative Overview of Traditional and Greener Approaches

Abstract

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.

Capacity of Aqueous Solutions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Acetate to Partially Depolymerize Lignin at Ambient Temperature and Pressure

“…A number of lignin-derived compounds were identified in the post-treatment aqueous phases, with the full list and corresponding yields (interpreted as mass of compound per unit mass of the original Indulin AT) being given in Table S1 in the Supporting Information. Apart from the polyphenolic compound epicatechin, the two (monophenolic) compounds obtained in a higher yield are vanillin and guaiacol, in line with what has been typically reported in the depolymerization of kraft lignins. ,− These two substances are likely produced by the cleavage of β-O-4 bonds in the lignin polymeric structure by the action of the IL, , and further analysis will specifically focus on them. From a mechanistic perspective, and according to a suggestion from the literature, the cleavage of these β-O-4 bonds may take place heterolytically via a six-membered transition state, with participation of both cation and anion of the IL.…”

Formaldehyde-Free and Phenol-Free Non-toxic Phenolic Resins with High Thermostability

“…As an alternative to phenol, “simple”, low toxicity, and available phenolic compounds were targeted. Methoxy and methyl phenolic compounds were selected on the one hand from a methodological point of view to compare them to their hydroxylated equivalents, and on the other hand because the depolymerization compounds of lignin display such types of groups. − We chose to classify (Table ) the phenolic compounds from the number of activating groups with the mesomeric donor effect (+M). The functionality (number of reactive sites) is also given, i.e., the number of unsubstituted aromatic carbons in the ortho or para position of a hydroxyl group.…”

Sustainable lignin modifications and processing methods: green chemistry as the way forward

“…97 In [Ch]Cl, the chloride ions act as a bridge to form hydrogen bonds between the free OH groups of lignin and methanol in the case of AL depolymerization. 80 Also, ILs and DESs contain cations and anions, which help to weaken or break the bonds (see Fig. 6) and interlinkages among the monomers of lignin.…”

“…In contrast, DESs are easier to formulate, less toxic and environmentally friendly, making them more favorable and safer chemicals, which can act as both the solvent and catalyst. 80 Bohre et al provided a brief overview of SILCs for sustainable biomass valorization to high-value chemicals and fuels. In addition, SILCs have attracted increasing attention due to their ability to overcome the drawbacks associated with conventional ILs.…”