A mini review on photocatalytic lignin conversion into monomeric aromatic compounds†

Abstract

To meet the growing demand for chemicals and energy while minimizing environmental impact, it is necessary to employ sustainable resources instead of fossil fuel feedstock and develop green processes to upgrade the chemical industry. Lignin, as the only abundant natural aromatic structured biomass, has been identified as a promising feedstock for producing monomeric aromatics through the depolymerisation process. However, the robust structure of lignin, coupled with the limitations of conventional catalytic methods, presents significant challenges for converting lignin into aromatic monomers. Recently, photocatalysis has emerged as a remarkable platform for versatile lignin transformation and has achieved some significant progress. The unique photogenerated reactive species as well as the mild reaction conditions enable the selective cleavage of targeted interunit bonds in lignin while preserving the aromatic structures and functional groups for further processing. In this review, we aimed to provide an overview of the state-of-the-art advances in key photocatalytic processes applied for the conversion of lignin using various photocatalysts. This review focuses on the latest investigations into lignin conversion mechanisms, emphasizing selective C–O and C–C bond cleavage. Additionally, we discuss catalyst modification strategies and key factors influencing the lignin conversion process based on significant recent publications. Furthermore, we elucidate future perspectives and challenges for the photocatalytic conversion of lignin into valuable products. Hopefully, this mini review will inspire future work on the photocatalytic lignin conversion process to achieve lignin valorisation and a sustainable supply of aromatic chemicals.