Manganese-, cobalt-, and zinc-based mixed-oxide spinels as novel catalysts for the chemical recycling of poly(ethylene terephthalate) via glycolysis

Chemical recycling of post-consumer poly(ethylene terephthalate) (PET) into useful feedstock was carried out in the presence of novel mesoporous metal oxide spinel catalysts. ZnO (hexagonal), metal oxide spinels (Co3O4 and Mn3O4), and mixed metal oxide spinel (ZnMn2O4, CoMn2O4, and ZnCo2O4) catalysts were synthesised via the precipitation or co-precipitation method. The structural, textural, and acidity properties of the materials were examined using various characterisation tools, such as XRD, SEM/EDX, TEM, FT-IR, NH3-TPD, and BET surface area analyser. The depolymerisation of waste PET (mostly soft-drink bottles) to the monomer bis(2-hydroxyethyl) terephthalate (BHET) via glycolysis was performed using excess ethylene glycol (EG) in the presence of fabricated metal oxides as transesterification catalysts. The effect of different parameters, such as temperature, catalyst type, reaction time, EG/PET molar ratio, and catalyst/PET weight ratio, on the monomer yield were investigated. The results revealed that the catalyst that yielded the highest amount of BHET (92.2 mol%) under mild reaction conditions (260 °C and 5.0 atm) was zinc manganite tetragonal spinel (ZnMn2O4), which has tetrahedral Zn+2 ion and octahedral Mn+3 ion coordination with the spinel crystal structure. The high catalytic activity of this spinel may be attributed to its greater surface area, the presence of mild and strong acid sites, and its overall higher concentration of acid sites. Furthermore, the ZnMn2O4 structural properties were examined, and it was determined that the Zn+2/Mn+3 metal cations pair, their positioning in the crystal structure, and that the spinel geometry has a pronounced effect on the catalytic efficiency. The monomer, dimer, and oligomers were separated and characterised by thermal (DSC and TGA) and structural (NMR) analyses, which confirmed the purity and structure of the monomer and dimer. Based on the experimental observations, a reaction mechanism was also proposed. In conclusion, the present approach was an attempt to demonstrate a process consisting of a set of new catalysts with optimised process conditions for the maximum production of highly pure BHET.