Synergy between Cu and Lewis Acidic Sites in Cu/Zn-FeOx Catalysts for the Selective Conversion of Poly(ethylene terephthalate) Waste to p-Xylene and Ethylene Glycol

Poly(ethylene terephthalate) (PET)-derived plastics are an important fraction of plastic waste. Selective hydrogenolysis of PET to p-xylene (xylene) and ethylene glycol (EG) could partially recover its value as a chemical feedstock. Herein, Cu/Zn-FeOx catalysts were prepared for the conversion of PET to xylene and EG in 1,4-dioxane at 180–200 °C. The results showed that Zn in Cu/Zn-FeOx created more Lewis acidic sites and enhanced the dispersion of metallic Cu species by developing a porous structure and promoting the reduction of CuO. Cu2.0/Zn1.0-FeOx was much more active than Cu/FeOx for the conversion of PET to xylene at 180 °C (yield: 93.7% vs 40.7%) but also for further hydrogenolysis of EG, diminishing EG in products. Cu1.0/Zn2.0-FeOx with a lower availability of Cu sites could also achieve a xylene yield of 98.6% and retain more EG (yield: 53.3%). A kinetic study showed that hydrogenolysis of intermediates like p-tolylmethanol but not hydrogenation was the rate-determining step. The lower activation energy for hydrogenolysis over Cu2.0/Zn1.0-FeOx than over Cu/FeOx (57.8 vs 65.8 kJ·mol–1) rendered the higher turnover frequency for xylene formation. In situ IR characterization showed that Lewis acidic sites induced by Zn addition were important for the adsorption/activation of O-containing functionalities (i.e., C–OH) of the reaction intermediates and consequently further hydrogenolysis.