Catalytic Pyrolysis of Poly(ethylene terephthalate) with Molybdenum Oxides for the Production of Olefins and Terephthalic Acid

A catalyst based on molybdenum oxides was prepared and used for the catalytic pyrolysis of PET (poly(ethylene terephthalate)) in a H2atmosphere to facilitate the direct conversion of PET for producing olefins (ethylene and propylene) and TPA (terephthalic acid). At a reaction temperature of 400 °C, the yields of olefins and aromatic products were the highest, reaching 10.10 and 52.16 wt %, respectively. Meanwhile, the carbon black could be entirely removed. The content of TPA in aromatic products was about 70%, and the content of benzoic acid reached 22.64%. Analyses using in situ gas chromatography (GC) and high-performance liquid chromatography (HPLC) were performed to clarify the possible reaction mechanism involved in the catalytic pyrolysis of PET. Vinyl ester was found to be an important intermediate in the formation of olefins. The molybdenum oxide catalyst promoted the cleavage of the C–O bond in the side chain of PET molecules to form TPA and monovinyl terephthalate. By catalytic hydrogenation, vinyl ester was easily attacked by H to drop off the vinyl end to form olefins, simultaneously converting the ester groups into a carboxyl group and further into a TPA. This is also the main path for the production of olefins. Meanwhile, the source of olefins also involved the hydrogenation of the coordinated quaternary cycle transition state of vinyl esters and the H transfer and cracking of monoethyl terephthalate. In addition, it was found that the presence of H2was the main reason for the reduction of carbon black products. This work is promising to promote the highly efficient utilization of waste PET and directional adjustment of pyrolysis products.