(154h) Catalytic Conversion of Waste PET into Bhet over Mn-Substituted MgAl2O4 Spinel As a Reusable and Regenerable Heterogeneous Catalyst

Catalytic glycolysis with ethylene glycol (EG) is a promising chemical recycling method that produces monomer-level PET with lower CO₂ emissions than other methods and various catalysts have been investigated. In the glycolysis reaction, metal oxides as heterogeneous catalysts have several advantages for separation, preparation cost, and reusability with regeneration by rinsing or thermal treatment. However, the use of a high concentration of EG to achieve high selectivity to BHET monomer in the PET glycolysis facilitates the deposition of heavy residue on the catalyst and dissolution of metal oxide into the solvent at the high reaction temperature, which leads to permanent loss of catalytic activity and difficulty of product purification by the metal contaminants. Therefore, it is indispensable to develop not only a highly stable catalyst in the reaction condition but also a recyclable and regenerable catalyst.

Spinel oxides are one of the representative materials that have excellent thermal and chemical stability and the acidic and basic properties of the materials can be tuned by partial substitution with various metal cations. Taking into account the characteristics of spinel oxide, Mn-substituted MgAl₂O₄ spinel oxides (Mn_xMg_1-xAl₂O₄, x= 0-1.0) were successfully synthesized by co-precipitation and applied to PET glycolysis as the robust catalyst. Mn²⁺ could substitute the Mg²⁺ in the MgAl₂O₄ spinel structure up to 25% without the formation of free Mn oxide which was confirmed by the increase of lattice parameter (a) from 0.8063 nm to 0.8096 nm. At higher Mn contents (x ≥ 0.3), isolated Mn₂O₃ was detected and the lattice parameters were constant at 0.8103 nm. The PET glycolysis was conducted at 190 °C for 3 h in reflux conditions. The conversion of PET over the Mn_xMg_1-xAl₂O₄ catalyst is monotonically increased from 37.1% (x=0) to 100% (x = 0.25) and the selectivity to BHET was achieved to 95.9%. The concentration of oxygen defects generating both moderate acidic and basic sites in the MgAl₂O₄ spinel increased with the increase of substituted Mn content which corresponded to the enhancement of the catalytic results. The recyclability and regenerability of Mn-substituted MgAl₂O₄ were also conducted.