Organocatalytic Glycolysis of Polyethylene Terephthalate and Product Separation by Membrane Filtration

In recent years, researchers have developed metal-free ionic organocatalysts for PET depolymerisation that have exhibited good catalytic activity, thermal stability and low environmental toxicity. This work aims to study the scalability, reaction kinetics and product separation of the chemical recycling reaction as a step towards developing a scalable process. An ionic organocatalyst comprising 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) and para-toluene sulfonic acid (pTSA) was selected as a potential glycolysis catalyst. The process was successfully scaled by a factor of 40, with complete conversion of PET achieved in 72 minutes (10g PET, 25 mol% catalyst, 180 ^oC, 20 equiv. ethylene glycol) in a batch reactor. The glycolysis kinetics were found to obey a modified shrinking core model, in which PET ester bonds are cleaved at the surface of the PET particles and the activation energy was experimentally calculated to be 61-66 kJ mol^-1. The kinetic model was adapted to account for product inhibition, where hydrolysis of bis-hydroxyethyl terephthalate (BHET) by extraneous water led to the formation of carboxylate moieties which were found to deactivate the catalyst and inhibit the glycolysis reaction. Furthermore, DBU:pTSA can be separated from ethylene glycol by nanofiltration using a commercial TS40 membrane at elevated temperatures between 80 and 120 °C. A rejection of 93% can be achieved at 2.24 MPa independent of temperature, while the highest flux of 17.7 kg m^-2 s^-1 was obtained at 120 °C. The presence of BHET only slightly reduced the catalyst rejection, but the flux dropped to 7.4 kg m^-2 s^-1. As BHET only resulted in a rejection of 41%, nanofiltration offers the possibility of separating both catalyst and BHET at elevated temperatures.