Recent Advances in Heterogeneous Photocatalysis for Lignin Valorisation

Lignin, one of the most abundant renewable aromatic biopolymers on earth, represents a promising feedstock for producing high-value chemicals capable of replacing fossil-derived resources, yet its structural complexity poses significant barriers to efficient valorization. In recent years, photocatalytic transformation has emerged as an attractive strategy to overcome these limitations, employing heterogeneous catalysts to harness solar energy for the selective cleavage and functionalization of lignin under mild and sustainable conditions. This review provides a comprehensive overview of recent progress in heterogeneous photocatalysts designed for lignin degradation, emphasizing how material composition, morphological features, surface properties, and band-gap engineering influence catalytic efficiency and selectivity. Key reaction pathways and mechanistic insights are discussed to elucidate the roles of photo-generated charge-carriers, reactive oxygen species, and catalyst–lignin interactions in driving depolymerization and upgrading processes. Furthermore, we analyze current challenges—including low reaction selectivity, catalyst deactivation, and limited scalability—and highlight emerging strategies aimed at improving catalyst stability, enhancing visible-light utilization, and promoting targeted product formation. By critically examining these advancements and limitations, this review outlines future opportunities for the development of efficient, robust, and economically viable photocatalytic systems to enable the sustainable and large-scale valorization of lignin.