Striding the threshold of photocatalytic lignin-first biorefining via a bottom-up approach: from model compounds to realistic lignin

Abstract

Upgrading renewable lignocellulose into bulk chemicals and drop-in-fuels has been identified as a state-of-the-art paradigm to fulfill alternative energy demands. Lignin, as the largest precursor of alkyl aromatics, can be depolymerized effectively to maneuver task-specific phenols. Toward an ecofriendly lignin-first depolymerization, photocatalytic lignocellulose conversion, also termed photobiorefining, is of utmost importance to boost economic and technical feasibility of holistic biomass valorization. It is still an urgent but challenging task to rationally design light-driven lignin deconstruction with decent photocatalytic selectivity and durability for developing scalable photobiorefining. A bottom-up approach, beginning with representative model compounds as lignin intermediates, enables a chance to encompass well-defined photocatalytic mechanism/system for providing insights, feasibility, and guidelines before the technical or proto-lignin conversion. The emphasis of this review is mainly placed on photocatalytic lignin-first strategy that includes versatile photocatalyst types, transformation pathways, photoreactors, and reaction medium. The cleavage mechanisms of C–O (e.g., β-O-4′ and α-O-4′) and C–C (e.g., β-1′ and β-5′) linkages in native lignin and its models to form light aromatic hydrocarbons through homogeneous or heterogeneous photocatalysis are proposed. At the end, the remaining challenges for the selective control and practical spread of photocatalytic lignocellulose fractionation are elaborated, and burgeoning prospects to inspire opportunities on photocatalyst manipulation for efficient and sustainable light-mediated lignin conversion are also discussed. The present work sheds light on the structure–property–performance relationship in photocatalytic lignin valorization and forthcoming knacks to spark the bright pearl of high-performance photobiorefining.