On the Oxidative Valorization of Lignin to High‐Value Chemicals: A Critical Review of Opportunities and Challenges

Abstract

The efficient valorization of lignin is crucial if we are to replace current petroleum‐based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second‐generation biorefineries produce large quantities of low‐value technical lignin as a by‐product, which is often combusted on‐site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

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

“…, formic acid and oxalic acid) by ring-opening oxidation and benzoquinones and phenolic acids. 247–252 In comparison with the reductive mechanism, oxidative depolymerization results in an increase in functionalities. Recently, lignin was oxidatively depolymerized by electrochemical methods at low temperatures (0 °C–30 °C) for kinetic studies by in situ UV/VIS spectroscopy.…”

“…However, certain phenomena can affect the catalytic activity, stability and reusability of heterogeneous catalysts such as leaching, poisoning, coking and loss of surface area. 252 Currently, the stability and reusability of catalysts are not frequently reported in the literature but should be considered. The greenness of the oxidative depolymerization is evaluated by the CHEM21 metrics toolkit in Table 14.…”

“…Depolymerizing lignins with an oxidant (e.g., O 2 and H 2 O 2 ) acquires polyfunctional aromatic compounds with a higher oxygen content such as phenolic aldehydes and ketones (e.g., vanillin, syringaldehyde, acetovanillone, and acetosyringone) by side-chain oxidation, aliphatic carboxylic acids (e.g., formic acid and oxalic acid) by ring-opening oxidation and benzoquinones and phenolic acids. [247][248][249][250][251][252] In comparison with the reductive mechanism, oxidative depolymerization results in an increase in functionalities. Recently, lignin was oxidatively depolymerized by electrochemical methods at low temperatures (0 °C-30 °C) for kinetic studies by in situ UV/VIS spectroscopy.…”

Catalytic oxidative conversion of C/G-type lignin coexisting in Tung nutshells to aromatic aldehydes and acids

“…16,17 In the last decades, great efforts have been devoted to the oxidative cleavage of the C-O and C-C bonds in H/G/S-type lignins. 18,19 The pre-oxidation of C a alcohol to C a ketone is a common strategy to achieve the depolymerization of oxidized lignin. 20,21 It is generally recognized that the C a ketone structure is unfavorable for the formation of the benzylic cation, thus effectively restricting lignin condensation during depolymerization.…”

Lignin with Tunable and Predictable Yield and Molecular Properties

“…Lignin has been highlighted as an attractive substitute for petroleum-derived products and to mitigate global climate change. − Lignin is a complex natural macromolecule that composes around 10–30% of plants, and it contains chemically functional groups such as benzene rings and hydroxyl groups. , Owing to these molecular features, lignin can be directly employed in functional polymeric materials or as precursors to produce chemicals, fuels, and carbon materials. − A recent study that utilized lignin as a photocatalyst for the functionalization of C–H bonds has shown the potential multipurpose uses for lignin …”