Maximizing light olefins and aromatics as high value base chemicals via single step catalytic conversion of plastic waste

Abstract

Chemical recycling of plastic waste via thermochemical processes is essential to move to a carbo-circular economy, reducing our dependence of fossil resources. However, recovering monomers from polyolefins in one or multiple steps is challenging due to their chemical inertness. In the present work, a tandem micro-pyrolyzer coupled to comprehensive two-dimensional gas chromatography and FID/ToF-MS detectors was utilized to study the performance of industrial formulations of steam-treated FCC catalysts and HZSM-5 additives for the in-line catalytic upgrading of polyolefin pyrolysis products towards light olefins and aromatics in two steps. When upgrading pyrolysis vapors from LDPE over the steam-treated catalysts at 600 °C, CH4 yields did not exceed 0.5 wt% due to their low acidity. The FCC catalyst formulations obtained higher yields of C5-C11 aliphatics (up to 42 wt%) and were more active in converting C12 + products than the HZSM-5-containing additives. The highest C2-C4 olefin selectivity of 53 wt% was obtained using a bare steam-treated HZSM-5 additive. With this catalyst, at higher catalyst loading and temperature (700 °C), the light olefin yield reached 69 wt% (19% ethylene, 22% propylene, 10% 1,3-butadiene, and 18% other C4 olefins). Importantly, similar yields of light olefins with even higher propylene yields of 31 wt% were obtained when processing real post-consumer mixed polyolefin waste. An adjusted loading of unsteamed catalyst (to obtain a similar level of initial conversion of C12 + products) showed higher coking propensity and deactivated more rapidly than its steam-treated version. The research results show great potential for the pyrolysis of mixed polyolefin waste followed by the direct in-line upgrading of the pyrolysis vapors to produce high value base chemicals such as C2-C4 olefins, aromatics, and naphtha-range aliphatics at high selectivity while limiting formation of coke, CH4, and H2.