A New Paradigm

Abstract

As an inedible component of biomass, lignin features rich functional groups that are desired for chemical syntheses. How to effectively depolymerize lignin without compromising the more valuable cellulose and hemicellulose has been a significant challenge. Existing biomass processing procedures either induce extensive condensation in lignin that greatly hinders its chemical utilization or focus on fully depolymerizing lignin to produce monomers that are difficult to separate for subsequent chemical synthesis. Here, we report a new approach to selective partial depolymerization, which produces oligomers that can be readily converted to chemically recyclable polymer networks. The process takes advantage of the high selectivity of photocatalytic activation of the β-O-4 bond in lignin by decatungstate catalysts (DT). In the photocatalytic system, DT works as both catalyst and oxidant, leading to two different reaction pathways, slow and steady bond cleavage pathway, and fast but limited oxidation pathway. The availability of exogenous electron mediators or external oxidants promotes cleavage or oxidation of this bond, respectively, enabling high degrees of control over the depolymerization and the stoichiometry of a key functional group, C═O, in the products. The resulting oligomers can then be readily utilized for the synthesis of polymer networks through reactions between C═O and branched −NH2 as a dynamic covalent cross-linker. Importantly, the resulting polymer network can be recycled to enable a circular economy of materials directly derived from biomass.