Glycolysis of poly (ethylene terephthalate) using DBU-based ionic liquid catalysts

Abstract

This research investigated depolymerization of poly (ethylene terephthalate) (PET) via glycolysis using a series of ionic liquid (IL) catalysts. Organic acids were paired with 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU) to produce ionic liquids (DBU-ILs). The study explored the effect of structural variations in the organic acids on PET conversion and bis (2-hydroxyethyl) terephthalate (BHET) yield under optimized reaction conditions (specifically, EG:PET ratio of 5:1, temperature of 190 °C, reaction time of 120 min, and 5 mol% DBU in the catalyst relative to PET). The organic acids were selected to include variation in the (i) number of carboxylic acids available to complex with DBU and (ii) structure of the linker and pendant groups to adjust the lipophilicity of the catalyst. The catalytic activity of the DBU-IL increased with increasing lipophilicity of organic acid. Catalysts with organic acids with aromatic linkers (i.e. terephthalic acid) exhibited lower activities than aliphatic linkers at similar lipophilicity. Notably, a linear relationship was observed between the catalytic activity and lipophilicity for diacids with aliphatic linker of increasing chain length. The apparent rate constant for the DBU-azelaic acid was 32 % greater than for DBU-oxalic acid. Reusability studies showed that there was minimal degradation of the catalytic activity of the DBU-ILs over five reaction cycles.