Bio-Based Valorization of Lignin-Derived Phenolic Compounds: A Review

Abstract

Lignins are the most abundant biopolymers that consist of aromatic units. Lignins are obtained by fractionation of lignocellulose in the form of “technical lignins”. The depolymerization (conversion) of lignin and the treatment of depolymerized lignin are challenging processes due to the complexity and resistance of lignins. Progress toward mild work-up of lignins has been discussed in numerous reviews. The next step in the valorization of lignin is the conversion of lignin-based monomers, which are limited in number, into a wider range of bulk and fine chemicals. These reactions may need chemicals, catalysts, solvents, or energy from fossil resources. This is counterintuitive to green, sustainable chemistry. Therefore, in this review, we focus on biocatalyzed reactions of lignin monomers, e.g., vanillin, vanillic acid, syringaldehyde, guaiacols, (iso)eugenol, ferulic acid, p-coumaric acid, and alkylphenols. For each monomer, its production from lignin or lignocellulose is summarized, and, mainly, its biotransformations that provide useful chemicals are discussed. The technological maturity of these processes is characterized based on, e.g., scale, volumetric productivities, or isolated yields. The biocatalyzed reactions are compared with their chemically catalyzed counterparts if the latter are available.

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.…”

Garlic (Allium sativum) peel extracts and their potential as antioxidant and antimicrobial agents for food applications: influence of pretreatment and extraction solvent

“…In this study, the evaluation of two pretreatments of different natures (biological and thermal processing) applied to GP to facilitate the extraction of phenolic compounds, as well as the effect of different extraction solvents on the TPC, TFC, AC, and AA of GPE, was proposed. Thermal processing provides efficient lignin depolymerisation, high conversion yields, and a streamlined process with less complex equipment compared to other methods (Pandey & Kim, 2011; Chbani et al ., 2020; Zhou et al ., 2022), while biological treatments, such as enzymatic breakdown and microbial degradation, offer a more environmentally friendly approach and generally operate under mild conditions, including lower temperatures and pressures, reducing the energy requirements and the potential formation of harmful byproducts (Chio et al ., 2019; Zhao et al ., 2022; Martínková et al ., 2023).…”

From trash to cash: current strategies for bio-upcycling of recaptured monomeric building blocks from poly(ethylene terephthalate) (PET) waste

“…182 In addition, it is a non-environmentally friendly process and it still based on non-renewable sources. 183…”