Production of lactic acid esters catalyzed by heteropoly acid supported over ion-exchange resins

Abstract

The heterogeneous liquid-phase esterification reaction of lactic acid with ethyl alcohol accompanied with lactoyllactic acid hydrolysis over heteropoly acid supported on ion exchange resin catalysts was investigated at 343 K with ethanol to lactic acid molar ratio of 1∶1. The catalysts with 5–20% of tungstophosphoric (H₃PW₁₂O₄₀·xH₂O) and molybdophosphoric (H₃PMo₁₂O₄₀·xH₂O) acid type of heteropoly acids over Lewatit^® S100 showed higher activities than the resin itself. DRIFTS and XRD analysis indicated the formation of amorphous heteropoly acid layer, which is strongly bonded to the resin surface with hydrate structure. Reaction experiments showed molybdophosphoric acid has slightly higher activity than the tungstophosphoric acid, which yields slower deactivation. The reaction model was established by considering the hydrolysis reaction of lactoyllactic acid under the reaction conditions and the esterification step. Reaction rate constants for homogeneous self catalyzed, ion exchange resin catalyzed, and kinetic contribution of heteropoly acid loading were calculated. Kinetic analysis of the reaction data revealed that the active proton sites do not increase proportionally with the amount of heteropoly acids over the resin surface and proton efficiency of the catalysts decreases with increasing loading. It was observed that molybdophosphoric acid loaded S100 catalysts have higher proton efficiency than the tungstophosphoric acid loaded counterpart.