Solar Lignin Reforming with Tunable Selectivity Using a Hybrid Photocatalyst in Aqueous Solution.

Abstract

The valorization of lignin provides an important means to access sustainable base chemicals, but current approaches are hampered by the lack of selective and scalable systems in aqueous solution. Here, we report a ZnIn2S4 (ZIS)-based photocatalytic system that directs photogenerated electrons to cocatalysts to control product selectivity by using concentrated solar irradiation. By introducing a phosphonated molecular Ni cocatalyst for hydrogen evolution (NiP), the conversion of the lignin model compound 2-phenoxy-1-phenylethanol shifts from phenol and acetophenone on bare ZIS to H2 and 2-phenoxy-1-phenylethanone on the ZIS|NiP system (5 sun). The latter achieves a turnover number (TONH2) of up to 120 molH2 molNiP-1. Integration of a phosphonated Ni bis(terpyridine) cocatalyst (NitpyP) in a CO2-saturated aqueous solution redirects photogenerated electrons toward syngas (H2/CO) production (TONsyngas = 48), whereas immobilization of formate dehydrogenase (FDH) produces formate (TONformate = 2200). The hybrid photocatalyst could also convert polymeric lignin, albeit with lower activity (e.g., TONH2 = 18 for organosolv lignin). Two days of outdoor testing under real sunlight confirms the system's robustness under fluctuating environmental conditions, with solar heating (to ∼60 °C) enhancing performance and selectivity. Our findings establish photocatalyst tuning as a strategy to control product selectivity in solar lignin reforming, which can be more widely applied in other photocatalytic reactions in the future.