Second-Generation <i>meta</i>-Phenolsulfonic Acid–Formaldehyde Resin as a Catalyst for Continuous-Flow Esterification

A second-generation <i>m</i>-phenolsulfonic acid–formaldehyde resin (PAFR II) catalyst was prepared by condensation polymerization of sodium <i>m</i>-phenolsulfonate and paraformaldehyde in an aqueous H₂SO₄ solution. This reusable, robust acid resin catalyst was improved in both catalytic activity and stability, maintaining the characteristics of the previous generation catalyst (<i>p</i>-phenolsulfonic acid–formaldehyde resin). PAFR II was applied in the batchwise and continuous-flow direct esterification without water removal and provided higher product yields in continuous-flow esterification than any other commercial ion-exchanged acid catalyst tested.