Self-photosensitized cycloaddition induced synthesis of a high-density fuel with ultra-low freezing point using bulk bio-benzaldehyde and furans

Photocatalytic cycloaddition of biomass cycloolefins is a promising strategy for synthesizing high-density fuels; however, the current feedstocks are limited to cyclic ketenes that offer lower yields. Herein, we propose a universal self-photosensitized [2 + 2] cycloaddition approach to achieve ultra-low freezing point biofuels using biomass-derived bulk aromatic aldehydes, furans and olefins, which cannot be accomplished via thermal catalytic conversions. The triplet self-photosensitized mechanism was thoroughly demonstrated using a combination of monochromatic light excitation, triplet quenching, phosphorescence quenching, Stern–Volmer kinetic analysis and DFT calculations. Under room temperature, catalyst-free and solvent-free conditions, the optimal benzaldehyde conversion and cycloadduct selectivity reached 92.6% and 98.0%, respectively. This self-photosensitized [2 + 2] cycloaddition strategy could be expanded to various biomass-derived aromatic aldehydes, furans and olefins. In particular, the synthesized biofuel possessed a density of 0.805 g mL−1, an ultra-low freezing point of lower than −85 °C and an outstanding cryogenic kinematic viscosity of 28.7 mm2 s−1 at −60 °C, indicating its potential as a high-density and low-freezing-point fuel component and a low-freezing-point aerospace fuel additive. This work developed a versatile and eco-friendly route for photocatalytic biomass valorization to high-quality biofuels.