Metal-free and mild photo-thermal synergism in ionic liquids for lignin Cα–Cβ bond cleavage to provide aldehydes

Abstract

Direct cleavage of the C–C bond in lignin linkages is a promising route to afford value-added aromatics, which, however, usually involves metal-based catalysts and harsh conditions. Here, a photo-thermal strategy is reported to deconstruct the C_α–C_β bond in lignin β-O-4 and β-1 interlinkages in the ionic liquid [BMim][NTf₂]. It was found that the synergism of UV light and heating could trigger the Norrish type I reaction by inducing the cleavage of the C_α–C_β bond and afford aromatic aldehydes through a free radical route in modest yields. Meanwhile, [BMim][NTf₂] could interact with lignin moieties and stabilize the intermediates, which significantly contribute to the C_α–OH protonation and accelerate lignin interunit linkage breakage. In this unique route, the C_α–C_β bond could be cleaved at 50 °C under UV light irradiation without the presence of a metal. Further comparison with photochemical or thermochemical alone strategies demonstrates that the photo-thermal route offers a significant improvement in the C_α–C_β bond cleavage reactivity. Alkali lignin could also be degraded into aromatic monomers in [BMim][NTf₂] using the protocol. Given the necessity of biomass upgradation, this work may provide a green and unique technology to boost the inert C–C bond cleavage upon photo-thermal synergism under metal-free and mild conditions that may underpin future advances in photo-thermal transformation and renewable energy production.