Selective hydrodeoxygenation of polyethylene terephthalate plastic wastes into 1,4-dimethylcyclohexane by Ir-ReOx/SiO2 + HZSM-5 catalytic system

Abstract

In order to mitigate the environmental impact of polyethylene terephthalate (PET) plastic pollution and enhance waste resource utilization, we developed a novel Ir-ReO_x/SiO₂ + HZSM-5 catalytic system for the hydrodeoxygenation (HDO) of PET plastic wastes into cycloalkanes, particularly 1,4-dimethylcyclohexane (DMCH). Under mild conditions (190 °C, 3 MPa H₂) with a certain amount of water in cyclopentane solvent, the highest yield of DMCH reached 95.8%, contributing to an overall yield of cycloalkanes at 98.4% within a reaction time of 4 h from HDO of virgin PET. The HDO of PET into DMCH involved initial hydrogenation of PET aromatic ring, then depolymerized by direct hydrogenolysis of acyl C–O bond in the ester group and subsequent cascade HDO of alcoholic C–O bond to saturated ester and alcohol intermediates, and further HDO of these oxygenates converting into desired DMCH product. The Ir-ReO_x/SiO₂ catalyst exhibits fully reduced metallic Ir nanoparticles along with partially reduced ReO_x species that are highly dispersed on both SiO₂ surface and Ir nanoparticle surfaces. The oxophilic nature of ReO_x species facilitates the activation of C–O bonds, the acidic HZSM-5 zeolite for the promotion of dehydration reaction, and the protic property of H₂O for the enhancement of hydrogenation PET contributed to highly HDO activity of this system. Furthermore, the catalytic system is applicable for the HDO of diverse real-world PET plastic wastes including Coca-Cola™ bottles, green Sprite bottles and white and red discarded fiber cloths, generating DMCH with a yield of up to 95.9% and cycloalkanes with a yield of up to 99.5%. This innovative process presents a new avenue for efficient hydrodeoxygenation conversion of PET into easily separable DMCH which serves as a crucial building block for the production of terephthalic acid to realize the chemical recycling of PET plastic wastes towards a green circular economy.