Synergy between Cu and Lewis Acidic Sites in Cu/Zn-FeO _x 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 degrees 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. Cu-2.0/Zn-1.0-FeOx was much more active than Cu/FeOx for the conversion of PET to xylene at 180 degrees C (yield: 93.7% vs 40.7%) but also for further hydrogenolysis of EG, diminishing EG in products. Cu-1.0/Zn-2.0-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 Cu-2.0/Zn-1.0-FeOx than over Cu/FeOx (57.8 vs 65.8 kJmol(-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.