Modulating structural oxygen/crystallinity enables ambient cascade photo-upgrading of biomass saccharides to lactic acid

Abstract

Photocatalytic transformation of biomass into biofuels and value-added chemicals is of great significance for carbon neutrality. Metal-free carbon nitride has extensive applications but with almost no absorption and utilization of near-infrared light, accounting for 50% of sunlight. Here, a molten salt-assisted in-plane “stitching” and interlayer “cutting” protocol is developed for constructing a highly crystalline carbon nitride catalyst containing structural oxygen (HC-CN). HC-CN is highly efficient for the photothermal cascade transformation of biomass-derived glucose into lactic acid (LA) with an unprecedented yield (94.3%) at 25°C under full-spectrum light irradiation within 50 min, which is also applicable to quantitatively photo-upgrading various saccharides. Theoretical calculations expound that the light-induced glucose-to-catalyst charge transfer can activate the C _β –H bond to promote the rate-determining step of intramolecular hydrogen shift in glucose-to-fructose isomerization. Meanwhile, the introduced structural oxygen in HC-CN can not only facilitate the local electric field formation to achieve rapid charge transport/separation and regulate selective •O₂ ⁻ generation for oriented C3–C4 bond cleavage of fructose but also narrow the energy band gap to broaden the light absorption range of HC-CN, contributing to enhanced LA production without exogenous heating. Moreover, HC-CN is highly recyclable and exhibits negligible environmental burden and low energy consumption, as disclosed by the life cycle assessment. Tailored construction of full-spectrum light adsorption and versatile reaction sites provides a reference for implementing multi-step biomass and organic conversion processes under mild conditions.