Hydrogenated intramolecular cyclization of diphenylmethane derivatives for synthesizing high-density biofuel

Multi-cyclic hydrocarbons from biomass are sustainable alternative for jet fuel. Here we report an unexpected hydrogenated intramolecular cyclization of diphenylmethane derivatives synthesized by alkylation of bio-derived compounds. With the presence of commonly used zeolite-Pd/C dual catalyst, conventional hydrodeoxygenation (HDO) occurs to produce dicyclohexylmethane. However, when only hydrogenation catalysts like Pd/C or Ni is used, novel intramolecular cyclization takes place to produce perhydrofluorene with the selectivity as high as 96.0%. The two pathways were illustrated by step-by-step controlled reactions in detail and a mechanism was proposed to explain the reason for perhydrofluorene formation. Moreover, this intramolecular cyclization is versatile for diphenylmethane with OCH3 or OH group at any position. The synthesized perhydrofluorene shows density of 0.96 g/mL, much higher than any biofuels reported and even higher than the widely used high-density fuel JP-10 derived from petroleum. It is believed that this biofuel can be excellent additive to improve the density of other jet fuels.