Mechanism of Brønsted-acid-promoted self-photosensitized [2+2] cycloaddition for synthesis of high-performance bio-spiral fuel

Photoinduced [2+2] cycloaddition of biomass-derived cycloolefin is a promising approach to synthesize high-energy bio-fuels, however, the conversion efficiency and selectivity are still low. Herein, we provide an acid-promoted photocycloaddition approach to synthesize a new kind of spiral fuel from biomass-derived cyclohexanone (CHOE) and camphene (CPE). Brønsted acids show higher catalytic activity than Lewis acids, and acetic acid (HOAc) possesses the best catalytic performance, with CHOE conversion up to 99.1%. Meanwhile, the HOAc-catalytic effect has been confirmed for [2+2] photocycloaddition of other biomass-derived ketenes and olefins. The catalytic mechanism and dynamics had been investigated, and showed that HOAc can bond with C=O groups of CHOE to form H-CHOE complex, which leads to higher light adsorption and longer triplet lifetime. Meanwhile, H-CHOE complex reduces the energy gap between CHOE LUMO and CPE HOMO, shortens the distance of ring-forming atoms, and then decreases the energy barrier (from 103.3 kcal/mol to 95.8 kcal/mol) of rate-limiting step. After hydrodeoxygenation, the targeted bio-spiral fuel shows high density of 0.992 g/cm3, high neat heat of combustion of 41.89 MJ/L, low kinetic viscosity of 5.69 mm2/s at 20°C, which is very promising to serve as high-performance aerospace fuel.