Modulating the crystal phase of Zr-based solid acid catalysts to boost the synthesis of 9,9-bis(4-hydroxyphenyl)fluorene

Abstract

The crystal phase of Zr-based solid acid catalysts was modulated by a two-step precipitation method for strongly bonding with sulfate groups. The catalytic performance of these catalysts was subsequently evaluated for condensation of 9-fluorenone with phenol. The results revealed that the catalytic activity of the catalysts was positively correlated with the acidity of the catalysts. Specifically, the SZr@Zr-2 catalyst exhibited the best catalytic performance with a 9-fluorenone conversion of 99.92% and 9,9-bis(4-hydroxyphenyl)fluorene (BHPF) selectivity of 99.86% under the optimized reaction conditions of 110 °C, 3 h and phenol to 9-fluorenone mole ratio of 6. It was demonstrated that the Zr(OH)₄@Zr(OH)₄-2 substrate prepared by two-step precipitation inherited rich Zr(OH)₄ species, which could be easily bonded with more sulfate groups. After calcination, these species were subsequently transformed into tetragonal ZrO₂ species induced by sufficient interaction with sulfate groups. The coordination between sulfate groups and tetragonal ZrO₂ enhanced the acidity of the SZr@Zr-2 catalyst and then boosted the condensation of 9-fluorenone with phenol for BHPF synthesis.