Preparation of monocyclic aromatic hydrocarbons from chlorine-containing mixed waste plastics via tandem catalysis coupled with hydrothermal pretreatment

Pyrolytic conversion of chlorine-containing mixed plastic (CMP) wastes into monocyclic aromatic hydrocarbons (MAHs) suffered the chlorinated pollutants and low product yield, making it difficult for continuous industrial operation. Herein, a novel method of tandem catalytic pyrolysis of CMP coupled with hydrothermal pretreatment (HY) for the clean production of high-yield MAHs was investigated. The pyrolytic kinetics of the feedstocks obtained after HY was analyzed, and the synergistic mechanism of HY and tandem catalysis over carbon-based catalyst (Fe/Ni/Mo@N/C) and zeolite (HZSM-5) was established. The results showed that the homogenization effect of HY on CMP was the key to improving its catalytic activity. High pretreatment temperature effectively weakened the branch-chain bond of the polyolefin molecules and dramatically reduced the pyrolytic activation energy of CMP. Additionally, the β-scission and alkylation reactions in the catalytic process were significantly promoted by HY, and subsequently improved the yields of toluene and xylene in HY-tandem catalysis, the selectivity of MAHs and benzene, toluene, ethylbenzene, and xylene (BTEX) reached 84.09% and 71.29%, respectively. The increased yield of BTEX favoured by the synergistic effect of pretreatment and catalysis was promoted to 351.97 mg/g in HY-tandem catalysis, whose yield was 9.52 times that of thermal pyrolysis. After the catalytic reaction of 360 min and transforming 18 times of the mixed plastic wastes, the coke amount of the microporous catalyst was still lower than 4 wt%. This work provides a potentially transformative solution to the challenge of upcycling unsorted plastic waste.