Ionic Liquid Catalysts for Poly(ethylene terephthalate) Glycolysis: Use of Structure Activity Relationships to Combine Activity with Biodegradability

The chemical recycling of poly(ethylene terephthalate) (PET) plastic by catalytic glycolysis is an enabling technology for the circular economy that is attracting burgeoning academic and commercial interest. Ionic liquids are emerging as a versatile catalyst class for this transformation, yet general strategies for how both high activity and biodegradability can be incorporated into catalyst design have not yet emerged. Beginning with an active literature catalyst incorporating a phosphonium cation of concern from a biodegradability standpoint, a structure activity relationship study involving 33 systematically varied ionic liquid catalysts was undertaken, which highlighted (inter alia) the contribution of cation lipophilicity to activity and identified the hydrocinnamate and benzoate counteranions as highly serviceable. This allowed the design of a superior, high-activity catalyst, which remained of biodegradative concern. Subsequently, the structure-activity relationships and general principles uncovered in this study informed a biodegradability/activity-guided approach to catalyst design, leading to the development of three highly active catalysts that were either known to be readily biodegradable or comprised biodegradable anions and cations. All three significantly outperformed a benchmark cholinium ion-based glycolysis catalyst at low catalyst loadings of 1 mol %.