Catalytic Pyrolysis of Poly(ethylene terephthalate) withMolybdenum Oxides for the Production of Olefins and TerephthalicAcid

A catalyst based on molybdenum oxides was prepared and used for the catalytic pyrolysis of PET (poly(ethyleneterephthalate)) in a H2atmosphere to facilitate the direct conversion of PET for producing olefins (ethylene and propylene) andTPA (terephthalic acid). At a reaction temperature of 400 degrees C, the yields of olefins and aromatic products were the highest, reaching10.10 and 52.16 wt %, respectively. Meanwhile, the carbon black could be entirely removed. The content of TPA in aromaticproducts was about 70%, and the content of benzoic acid reached 22.64%. Analyses using in situ gas chromatography (GC) andhigh-performance liquid chromatography (HPLC) were performed to clarify the possible reaction mechanism involved in thecatalytic pyrolysis of PET. Vinyl ester was found to be an important intermediate in the formation of olefins. The molybdenum oxidecatalyst promoted the cleavage of the C-O bond in the side chain of PET molecules to form TPA and monovinyl terephthalate. Bycatalytic hydrogenation, vinyl ester was easily attacked by H to drop offthe vinyl end to form olefins, simultaneously converting theester groups into a carboxyl group and further into a TPA. This is also the main path for the production of olefins. Meanwhile, thesource of olefins also involved the hydrogenation of the coordinated quaternary cycle transition state of vinyl esters and the Htransfer and cracking of monoethyl terephthalate. In addition, it was found that the presence of H2was the main reason for thereduction of carbon black products. This work is promising to promote the highly efficient utilization of waste PET and directionaladjustment of pyrolysis products.