Catalyst-free lignin photothermal valorization via light-promoted aromatic C–H cleavage

Abstract

Chemical conversion offers a promising way to valorize native lignin, which serves as a sustainable alternative to petroleum-derived carbon and hydrogen sources. However, the selective transformation of lignin using conventional thermochemical approaches remains challenging because it is difficult to control the site for C–H/C/O bond activation and cleavage. Although catalysts can significantly improve reaction selectivity, they are prone to gradual deactivation. Herein, we introduce a light-driven selective valorization of lignin to hydrogen and carbon under catalyst-free conditions. Broad-spectrum sunlight photons are absorbed by lignin's conjugated chromophores and then trigger aromatic C–H bond activation and dissociation, assisted by an excited-state conformation transition of lignin's multi-aromatic building blocks. This results in the production of solid carbon and hydrogen-rich gas (63.3%) at a yield of up to 190 mL H₂ per g_Lignin, 112 times that of conventional thermochemical routes. The solar thermal approach potentially offers a sustainable solution to lignin valorization.