Synthesis of 9-fluorenone by the micro-nanobubble-enhanced ozone oxidation of fluorene

This article proposes a greener and more efficient method for preparing 9-fluorenone from fluorene via ozone oxidation and the use of micro-nanobubble technology. The effectiveness of ozone micro-nanobubble mass transfer in different polar organic solvents was first studied. Equilibrium concentrations of ozone were higher in alcoholic solvents than in solvents such as ethyl acetate, acetone, dichloromethane, chloroform, and 1,2-dichloroethane, which showed similar equilibrium concentrations of ozone. The variation in hydrogen bonding or dipole–dipole interactions between ozone and solvent molecules caused this change in solubility. Using ethyl acetate as the ideal solvent, with an ozone: fluorene molar ratio of 1.6:1 and reaction duration of 90 min, optimal conditions for ozone oxidation resulted in a 66% yield of 9-fluorenone. We also postulated the chemical mechanism involved in fluorine oxidation by ozone. The presence of hydroxyl radicals during the reaction was confirmed via electron paramagnetic resonance spectra. Indirect ozone oxidation predominated in particular reactions. No requirement of a catalyst and ease of this method makes it well suited for the large-scale manufacturing of 9-fluorenone.