Photocatalytic depolymerization of lignin via oxidizing cleavage of Cα-Cβ bonds in micellar aqueous media

Abstract

Photocatalytic depolymerization of lignin to prepare high-value chemicals is a promising way to promote the valuable utilization of lignin. However, the complexity and stubbornness of lignin structure seriously decrease the photocatalytic efficiency and selectivity. Herein, the micellar aqueous media (SDS-8/HCl) consisting of sodium lauryl sulfonate and hydrochloric acid was successfully prepared. Photocatalyst TiO₂ and SDS-8/HCl system can effectively depolymerize the typical β-1 lignin models and ethanol organosolv lignin to value-added chemicals by oxidizing cleavage of lignin C_α-C_β bonds. The addition of hydrochloric acid solution (1 mol/L) improves the selectivity of photocatalytic breaking of lignin C_α-C_β bonds. Chlorine ions are oxidized to chlorine radicals by photogenerated holes and hydroxyl radicals, dramatically increasing the photocatalytic efficiency. Electron paramagnetic resonance technique and Gas chromatography-mass spectrometry were used to demonstrate the presence of chlorine radicals. Under optimal conditions, the conversion of substrate Dpol is 98.4 %, and the obtained products are mainly benzaldehyde and benzoic acid. Isotope labeling experiments show that water is also involved in photocatalytic reactions and the oxygen needed to form the product benzaldehyde comes from water. Single-electron transfer processes are possible photocatalytic mechanisms that differ from the previous reports. Importantly, water and chlorine ions were found to be involved in photocatalytic reactions for the first time and promote the cleavage of lignin C_α-C_β bonds. This work provides new ideas for photocatalytic cleavage of lignin C_α-C_β bonds in heterogeneous photocatalytic systems using micellar aqueous media.

Keywords: Chlorine radical; Lignin C(α)-C(β) bonds; Micellar aqueous media; Photocatalysis; TiO(2).