Elucidating activity-property relationships of dual catalysts for poly(ethylene terephthalate) depolymerisation

Abstract

Polyethylene terephthalate (PET) is one of the most prominent single-use plastics. It can be readily circularised via chemical depolymerisation to monomer and subsequent repolymerisation to deliver virgin quality PET from waste. Chemical recycling-to-monomer (CRM) approaches rely on catalysts to enhance the activity of the depolymerisation process. Recently, dual-catalytic systems that combine inexpensive and readily available Lewis acids with simple organobases have displayed cooperative catalytic activity, among the highest reported for these processes; however, their mechanism of action, alongside the large number of possible combinations, makes identifying ideal catalyst species challenging. In this manuscript, we systematically explore the relationship between metal Lewis acids and organobases and their impact on PET glycolysis activity. These studies reveal a relationship between metal acetate electronegativity and the pKa of the organobase that can be used to predict the most active combination for a given metal salt or organobase. Furthermore, through preliminary DFT calculations, we demonstrate that the glycolysis likely follows a mechanism in which the base coordinates to the Lewis acid, acting as a single catalytic entity, and in which the acetate ligand assists with proton transfer.