Synergistic Catalysts of Lewis Acid and Brønsted Acid for Highly Efficient Preparation of Bisphenol Compounds

Bisphenol compounds play a pivotal role as essential diol monomers in the synthesis of numerous engineering plastics. However, conventional methods for their preparation often involve the use of concentrated inorganic acids, leading to significant environmental pollution and equipment corrosion. Herein, we developed a strong inorganic acid-free synergistic catalytic system consisting of a Lewis–Brønsted acid catalyst composed of ZnCl2 and 3-mercaptopropionic acid. This system enabled the highly efficient preparation of bisphenol compounds through the Friedel–Crafts alkylation reaction of ketones with phenols or phenoxyethanol. The highest selectivity of 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene can reach 96.4% at 110 °C for 2 h, which was higher than that of the traditional sulfuric acid method with a selectivity of 85.0%. A mechanistic study revealed that the carbonyl group of the ketone was first activated by the coordination ability of Zn2+ from the Lewis acid, which facilitated the subsequent protonation process initiated by the acidic site of Brønsted acid.