Poisoning effect of polyvinyl chloride on the catalytic pyrolysis of mixed plastics over zeolites

Nowadays, the chemical recycling is applied for only 1% of total waste plastics, largely due to contaminants in plastic waste and difficulty in product control. As the major contaminant, polyvinyl chloride (PVC) often forms corrosive hydrogen chloride (HCl) during the chemical recycling, which may cause severe catalyst deactivation and equipment damage. However, the investigation on catalytic pyrolysis (the major route for plastics chemical recycling) of the PVC containing mixed plastics has been rarely reported. Here, catalytic co-pyrolysis of PVC and polyethylene (PE) was studied over an aromatization catalyst, Pt/ZSM-5, since the basic building block aromatics are desired products from plastics chemical recycling. The poisoning effect of PVC vapor on the catalyst stability was explored by collective efforts of thorough product analysis and catalyst characterization. It was found that the HCl evolving from PVC has an autocatalytic effect that promotes the scission of dehydrochlorinated PVC, resulting in the high yield of benzene and acetylene from PVC. On the other hand, the presence of PVC suppressed the aromatics formation from PE, largely due to the poisoning effect of PVC-derived HCl on the Pt/ZSM-5. The deactivation is irreversible as evidenced by the decreased zeolite crystallinity and the loss of strong acid sites that are key to the aromatization, possibly due to the removal of framework Al upon the interaction with HCl. The modification with octadecylphosphonic acid only slightly alleviated the PVC poisoning effect. The insights on the PVC poisoning of zeolite catalysts provided in this work may guide the process design of chemical recycling of PVC containing waste plastics.