Stabilization of ZnO Catalysts for Polyesters Hydrolytic Depolymerization by Incorporation into ZrO2 Lattice: A Polylactic Acid Case Study

Abstract

Depolymerization is an effective strategy to achieve circularity in polyesters management. However, most of current technologies require organic solvents, homogeneous catalysts, or harsh reaction conditions, while generating considerable amounts of undesired products. Hydrolysis over heterogeneous ZnO catalyst using neat water has shown to be a sustainable method for selective depolymerization, although limited by the interaction of ZnO with the nascent carboxylic acids monomers produced, which leads to catalyst dissolution and deactivation, hence to poor catalyst reusability. Herein we demonstrated that the use of ZnO–ZrO₂ mixed oxide catalysts is a successful strategy to avoid Zn leaching in solution, while maintaining catalyst activity. The hydrolytic depolymerization of polylactic acid over ZnO–ZrO₂ mixed oxides at 130 °C is investigated as reference reaction, showing that catalysts with up to 10% wt Zn content resulted in complete conversion and 100% selectivity to lactic acid, with negligible Zn leaching over repeated catalyst reuses. The catalysts are characterized by a combination of solid-state techniques, suggesting that ZnO stabilization occurs upon incorporation into the lattice of an inert ZrO₂ phase.