Pressurized ex-situ catalytic co-pyrolysis of polyethylene and lignin: Efficient BTEX production and process mechanism analysis

Ex-situ catalytic co-pyrolysis of plastic and biomass to aromatics has been extensively investigated, simultaneously, aromatization have been reported to be promoted under pressure, but available research did not discuss the combined effect of catalytic co-pyrolysis and pressure on aromatics production. In this work, ex-situ catalytic co-pyrolysis of polyethylene and lignin over HZSM-5 under a series of pressure (0.1–0.8 MPa) in a Py-GC/MS system was investigated. We show that pressurized operation during co-pyrolysis of polyethylene and lignin leads to a large increase in BTEX relative yield while maintaining catalyst stability. A very high BTEX relative yield of 70.94% can be achieved at temperature of 650 °C, pressure of 0.5 MPa, polyethylene to lignin ratio of 1:1, and catalyst to raw material ratio of 4:1. Ratio of two raw materials significantly affects efficiency of hydrogen radicals-mass transfer by regulation of hydrogen radical emission and reception. A peak value of BTEX relative yield of 70.27% was obtained at a polyethylene to lignin ratio of 4:3. Among BTEX, benzene and toluene are more prone to alkylate and polycondensate into highly branched and polycyclic aromatics, due to a lower steric hindrance. Hydrogen radicals-mass transfer has a significant effect on BTEX production, and mass transfer efficiency can be regulated by appropriate pressure and raw materials ratios. Meanwhile, pressurized operation has a dual positive effect on coke inhibition via promoting hydrogen radicals-mass transfer and water release, such that catalyst stability and BTEX production are promoted. This study offers new insight into efficient production of BTEX.