Efficient glycolysis of recycling poly(ethylene terephthalate) via combination of organocatalyst and metal salt

The recycling of discarded polyethylene terephthalate (PET) is an effective way of catering to global sustainable economic development strategies. It is well known that catalysts play a crucial role in the degradation of PET, based on the fact that most of the currently available catalysts have some disadvantages, including complex preparation process, severe reaction conditions and insufficient reaction efficiency. Herein, we report a combination of metal catalyst and organoazacyclic 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) as cooperating catalysts were used to depolymerize waste polyethylene terephthalate into bis(2-hydroxyethyl) terephthalate (BHET) directly. Screening of several metal salts under optimal conditions showed that zinc acetate ((Zn(OAc)2) was the most effective co-catalyst. The waste PET could be completely depolymerized at 180 °C for 77 min with the weight ratio of EG/PET was 3:1 and the molar ratio of Zn(OAc)2/DBU was 1:2, and the content of BHET in the recrystallized product reached 78.2%. Kinetic studies showed that the activation energy of depolymerization of PET was 115.48 kJ/mol. It was speculated that the hydrogen bond formed by Zn(OAc)2/DBU and EG exhibited excellent catalytic effect in the glycolysis process. Therefore, it is hopeful that this simple and mild composite catalyst can provide industrial ideas for recycling other waste polyesters.