Increasing para-Xylene Selectivity during Polyolefins to Aromatics via an Olefin Intermediate Tandem Catalysis Process

An emerging alternative for petroleum-based aromatics, catalytic cracking of waste plastics to produce benzene, toluene, ethylbenzene and xylene (BTEX), particularly high-valued para-xylene (PX), provides a sustainable pathway for plastics recycling. This study proposed an olefin-intermediate tandem catalysis process to increase the selectivity of PX during polyolefins-to-aromatics conversion. First, the short b-axis HZSM-5 (sHZSM-5) with a high external surface and nano straight channels along the b-axis was synthesized for cracking of polyolefins into short-chain olefins. Compared with microsized zeolite (mHZSM-5), sHZSM-5 and P-sHZSM-5 increased the yields of C₂-C₄ olefins to 55.2% and 67.7 wt % from 46.3 wt %, respectively. Subsequently, the produced olefins underwent selective aromatization over 3Si/Zn/P-ZSM-5 with the weak external surface acid sites to increase the selectivity of PX in X. Experimental results showed that the use of 3Si/Zn/P-ZSM-5 with SiO₂ coating obtained a higher PX selectivity of up to 73.4% with a relatively lower BTEX yield of 28.5 wt %. By contrast, Zn/P-ZSM-5 without SiO₂ coating obtained a lower PX selectivity of 27.2% and a higher BTEX yield of 42.6 wt % due to its strong external surface acid sites. The olefin-intermediate tandem catalysis process filled the gap in the research of catalytic cracking of plastics for PX production.