Advancements in Catalytic Depolymerization Technologies

Abstract

The increasing market demand and rising costs of raw materials have intensified interest in renewable and sustainable sources. As a result, the production of building-block chemicals from natural products or synthetic feedstocks has driven scientific research toward catalytic strategies for the depolymerization of these materials. Polymer chemistry offers significant opportunities for recycling, as polymer synthesis typically begins with monomeric units. Emerging non-destructive techniques now allow for the recovery of these original reagents. This review summarizes recent advances in catalytic methods for the depolymerization of polymers derived from both natural sources, such as cellulose and lignin, and synthetic sources, including conventional plastics. The review is structured in three main sections: catalytic depolymerization of cellulose, lignin, and plastics. Special emphasis is placed on recent studies that explore innovative methodologies. The raw materials obtained through these processes can be reintegrated into production cycles, contributing to the development of a fully circular economy.

Keywords: catalysis; circular economy; polymers; sustainable chemistry.

Scheme 1

Mechanism of depolymerization of cellulose [49].

Figure 1

Methodology for screening cellulose catalytic conversions [55].

Figure 2

Mechanocatalytic depolymerization of cellulose results in the formation of water-soluble oligomers [62].

Figure 3

A segment of macromolecular lignin structure with its H, G, and S moieties. The figure referred from [68].

Figure 4

Typical reaction mechanism of lignin under acidic conditions. (a) Self-condensation between lignin molecules; (b) reaction between lignin and phenol [78].

Scheme 2

Reproduced from Biswas et al., “Effects of solid base catalysts on depolymerization of alkali lignin for the production of phenolic monomer compounds”, Renewable Energy, Vol. 175, pp. 270–280, © 2021, with permission from Elsevier [87].

Scheme 3

Oxidative lignin depolymerization into aromatic compounds [95].

Figure 5

Schematic illustration of lignin degradation over the InₓS₃-C catalyst (a) and the degradation pathway of lignosulfonate (b). Reproduced from Li et al., Polymers, 2024, 16(17), 2388. Licensed under CC BY 4.0. https://doi.org/10.3390/polym16172388 [108].

Scheme 4

Depolymerization of lignin model compounds depolymerized by mediator–enzyme system [118].

Figure 6

A schematic representation of the reaction process starting from PET [124].

Figure 7

Recycling efficiency of (B) the Lewis acid catalyst [Fe(OTf)₃] and (A) the Brønsted acid catalyst (TfOH) [124].

Scheme 5

Adapted from Tanaka et al., ACS Materials Au, 2024, 4 (3), 335–345. © 2024 The Authors. Published by American Chemical Society under CC BY 4.0. Mechanism relative to the overall process divided into 3 reactions (A–C) [128].

Figure 8

A schematic representation of the double metal cyanide (DMC) complex catalyst [131].

Scheme 6

Ru pincer complex used to depolymerize low molecular weight polyamide [135].

Figure 9

Adapted from R. Coeck and D. E. De Vos, Chem. Commun., 2024, 60, 1444, under the terms of the Creative Commons Attribution-NonCommercial (CC BY-NC) licence (https://creativecommons.org/licenses/by-nc/4.0/, accessed on 6 June 2025). © The Royal Society of Chemistry [136].

Scheme 7

Reproduced from R. Coeck and D. E. De Vos, Chem. Commun., 2024, 60, 1444, under the terms of the Creative Commons Attribution-NonCommercial (CC BY-NC) licence (https://creativecommons.org/licenses/by-nc/4.0/). © The Royal Society of Chemistry [136].

Scheme 8

Chemoenzymatic polymerization/depolymerization of semiaromatic polyamides [138].

Scheme 9

Mn pincer complex used to depolymerize polyurethanes [146].

Figure 10

Catalytic depolymerization of three different polyurethanes (A–C) with the relative optimized reaction conditions and isolated yield.

Scheme 10

Lewis superacid bis(perchlorocatecholato)silane as a catalyst for C-O bond metathesis of polyethers [150].

Figure 11

Adapted from Ansmann et al., Angew. Chem. Int. Ed. 2023, 62, e202210132. © 2023 Wiley-VCH GmbH. Adapted with permission. Reaction scheme of the Lewis acid catalyzed selective degradation of 1,5-dimethoxypentane (A) and selective ring closing C-O bond metathesis of polyethylene glycol and polypropylene glycol (B) [150].

Scheme 11

Acetonitrile-adduct of a Lewis superacidic silane (Si(pinF)2·MeCN) as a catalyst for C-O bond metathesis of polyethers [151].