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 have been investigated, and show 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−1 to 95.8 kcal mol−1) of rate-limiting step. After hydrodeoxygenation, the targeted bio-spiral fuel shows high density of 0.992 g cm−3, high neat heat of combustion of 41.89 MJ L−1, low kinetic viscosity of 5.69 mm2 s−1 at 20 °C, which is very promising to serve as high-performance aerospace fuel.