Kinetics of phosphotungstic acid-catalyzed condensation of levulinic acid with phenol to diphenolic acid: Temperature-controlled regioselectivity

Diphenolic acid (DPA) is a renewable compound to produce polycarbonates and epoxy resins. Diphenolic acid is produced by acid-catalyzed condensation of levulinic acid with phenol to form monophenolic acid. Subsequently, monophenolic acid undergoes condensation with phenol to form two DPA isomers, ortho-diphenolic acid (o,p′-DPA) and para-diphenolic acid (p,p′-DPA). Here we describe a kinetic analysis of solvent-free levulinic acid-phenol condensation catalyzed by phosphotungstic acid at temperatures between 70 and 140°C. The reaction appeared to be pseudo-first-order with respect to levulinic acid when the levulinic acid:phenol molar ratio was four or higher. We determined the kinetic parameters (reaction rate constants, pre-exponential factors, and activation energies) by fitting experimental results to simulated data. Although the activation energies of o,p′-DPA, and p,p′-DPA formation were higher than their corresponding reverse reactions, the kinetic analyses revealed that a steric effect controls the reaction products. Our findings demonstrated a simple temperature-controlled strategy to achieve a p,p′:o,p′ DPA molar ratio of 28.3 with 87% conversion and 98% selectivity to total DPA. This work provides a potential production route for p,p′-DPA from biomass.