Relay photo/thermal catalysis enables efficient cascade upgrading of sugars to lactic acid: Mechanism study and life cycle assessment

Chemocatalytic conversion of biomass-based saccharides to lactic acid (LA) is one of the prospective routes for biomass valorization, but generally suffers from harsh reaction conditions with relatively low selectivity toward C[sbnd]C bond cleavage. Herein, a novel ternary heterostructure g-C₃N₄/N-TiO₂/NiFe-LDH (NTCN/LDH) photocatalyst prepared by a simple wet-chemical method was demonstrated to be high-performance for cascade upgrading of various biomass sugars to LA (up to 99.0 % yield) in H₂O at 60 °C under visible-light irradiation for a short time (≥15 min). A relay photo/thermal catalytic mechanism was mainly responsible for the superior performance of NTCN/LDH in LA production, as elucidated by DFT calculations, in which NTCN/LDH provided suitable Lewis acid sites (i.e., adsorption sites) for thermocatalytic sugar isomerization (e.g., glucose-to-fructose conversion), while the constructed double type-II heterojunction could regulate the type and count of the formed reactive oxygen species (dominantly ·O₂⁻) to achieve precise photocatalytic C₃-C₄ bond breaking. The scale-up test could still maintain 92 % LA yield of the first-time experiment, indicating the eximious industrialization potential of the versatile NTCN/LDH catalyst. In addition, the life cycle assessment illustrated that the entire system has less impact on the environment than the current industrial LA production process.