Polystyrene-supported imidazolium acidic ionic liquids: highly efficient catalysts for the synthesis of bisphenols

Abstract

A series of reusable polystyrene-supported imidazolium acidic ionic liquid catalysts were synthesized to replace industrial sulfuric acid for the synthesis of bisphenol compounds. These ionic liquids were covalently bonded to the support, reaching a high loading of 1.62–1.80 mmol g⁻¹, respectively. As expected, they achieved a catalytic effect comparable to that of sulfuric acid in the condensation reaction of 9-fluorenone with phenol. Under the optimum reaction parameters, a high yield and high selectivity of 99.9% and 99.7% were achieved for 9,9-bis(4-hydroxyphenyl)fluorene (BHPF). Moreover, this catalyst was also employed for synthesizing other bisphenol compounds and 99.0% conversion of the substrate was achieved. Based on the experimental results and density functional theory (DFT) calculations, a possible reaction mechanism was proposed, which indicated that 9-fluorenone and phenol were simultaneously activated through the synergistic catalysis of anions and cations in a polystyrene-supported ionic liquid catalyst. The recycling experiment results suggested that [PS-PsIm][CF₃SO₃] maintained a high catalytic activity in five reaction runs, making this a promising catalyst for the synthesis of bisphenol compounds.