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

Abstract

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 CC 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.