Tandem pyrolysis catalytic reforming of plastic waste to high-yield monocyclic aromatics by one-step microwave catalysis

The recycling and processing of plastic waste to promote resource sustainability has emerged as a global concern. Selectivily upcycling polyolefins into valuable monocyclic aromatic hydrocarbons (MAHs) is a promising strategy to promote a circular economy but limited by low yield and selectivity. In this study, one-step microwave-triggered tandem pyrolysis coupling ZSM-5 catalytic reforming technology was explored, which is efficient for selectively transforming plastic into benzene, toluene, ethylbenzene, and xylene (BTEX) riched oil products. Pyrolysis and catalytic processes were achieved by mixing plastic and ZSM-5 with graphite, respectively, and placing them in the bottom and top layers of the reactor. Results indicate the mass ratio of plastic or ZSM-5 to graphite and microwave irradiation power would influence the selectivity and yield of MAHs. After careful investigation, the optimum reaction conditions are deemed to be 600 W, and the mass ratio of LDPE and ZSM-5 to graphite is 3:1 and 1:1, respectively, and delightful selectivity and yield of MAHs of 82.06% and 28.12 wt% are obtained under the above conditions. More importantly, the BTEX accounted for a significant amount in the MAHs with the selectivity and yield of 70.53% and 24.17 wt%, demonstrating a strong ability to produce high-value BTEX, which is attributed to the ZSM-5 providing a sufficient active site for reforming the pyrolysis vapors and promote the formation of BTEX by cracking, aromatization and Diels-Alder reactions. Additionally, gas products are rich in C2H4, C3H6 and C4H8 olefin monomers accounting for around 60% of the gas products, which also manifest high value. Besides, the ZSM-5 can be stabilized by exceeding three successive cycles, and this strategy is also suitable for the catalytic upcycling of PP and mixed plastics to produce MAHs selectively. Therefore, this one-step microwave-triggered tandem pyrolysis coupling ZSM-5 catalytic reforming technology provides a potential waste resource utilization method to convert plastic waste into high-value petrochemical products.