D–π–A Triazine-Based Polymers Photoinduced Integrated Ring-Increasing Strategy of Biocyclohexene for Synthesizing High-Strained Cyclobutane Fuel

The [2 + 2] cycloaddition of cycloalkene is a promising strategy to synthesize a high-strained cyclobutane skeleton; however, the cycloaddition efficiency is still low. Herein, novel D–π–A triazine-based polymers are fabricated to boost the photooxidation/[2 + 2] cascade ring-increasing reaction of biocyclohexene for synthesizing high-energy-density cyclobutane fuel. TEPT-PT photocatalyst exhibits an unprecedented yield of 94.1% over the ring-increasing reaction, owing to the multicarbonyl groups endowed with excellent photooxidation ability and charge separation ability. Mechanistic studies reveal that singlet oxygen and superoxide radical synergistically intensify cyclohexene photooxidation to cyclohexenone, with singlet oxygen as the primary oxidant and superoxide radical as the secondary oxidant. The apparent activation energy of the [2 + 2] cycloaddition is 528.67 W/mol, significantly lower than that of the cyclohexene photooxidation step (2172.16 W/mol). After hydrodeoxygenation, the targeted cyclobutane biofuel is endowed with a high density of 0.912 g/mL and volumetric neat heat of combustion of 38.75 MJ/L, a significant increase of 16.9 and 16.1% compared with those of the RP-3 fuel, respectively. This study proposes a novel strategy for photocatalytic biomass upgrading to high-energy-density fuels using heterogeneous conjugated polymers.