Chemical recycling of plastic waste to monomers: Effect of catalyst contact time, acidity and pore size on olefin recovery in ex-situ catalytic pyrolysis of polyolefin waste

The catalytic upgrading of post-consumer mixed polyolefinic waste pyrolysis vapors has been studied over five different solid acid catalysts to understand the effect of vapor-catalyst contact time and catalyst properties on the recovery of light olefins (C2-C4). HZSM-5 with Si/Al ratio of 11 and 27 produced the highest light olefins yields (83.4 wt% and 72.1 wt%, respectively) at a very short contact time of ∼5 ms while extending the contact time (≥23 ms) decreased the olefin yields and resulted in higher yields of aromatics (∼30.0 wt% and ∼40.0 wt%, respectively). Short contact time (5 ms) was not sufficient for HZSM-22 to crack pyrolysis vapors to C2-C4 olefins (8.0 wt%) but increasing contact time improved C2-C4 yields to 53.5 wt% without increased production of aromatics, which may be attributed to its small pore size (shape selectivity). SAPO-34, SAPO-11, and Al-MCM-41 were not selective to light olefins at any studied contact time.