Steering K/C dual sites to orient Lewis acid/oxygen species for photo-upgrading of biomass sugars into lactic acid

Photocatalytic selective synthesis of lactic acid (LA) from biomass sugars with a single heterogeneous catalyst is promising but challenging due to the multiple reaction steps involved. Herein, a K-doped C-rich red polymerized carbon nitride (RPCN) photocatalyst with uniform K/C dual sites was constructed by a molten salt template method, which was highly efficient for cascade isomerization dehydration of glucose to LA with > 90% selectivity under visible light and gentle conditions. Control experiments and theoretical calculations expounded that the introduced K/C dual sites could improve the light capture ability and photogenerated charge separation efficiency, while the K species provided sufficient Lewis acid sites (adsorption sites) for the isomerization of glucose to fructose. Meanwhile, the introduced C sites that substitute N atoms could promote electrons to be captured by adsorbed oxygen for selective generation of superoxide radicals, which was highly efficient for the scission of the C3–C4 bond in fructose, exclusively furnishing LA. Importantly, the RPCN photocatalyst was also suitable for the photocatalytic upgrading of various biomass saccharides into LA with high yields of 81.3%–95.3% and could be recycled for five consecutive cycles. The tailored construction of dual sites by localization of space charge lightens an avenue for multi-step conversion of biomass with pronounced selectivity. Graphical abstract