Preparation of MCM-22/ZSM-5 core–shell structured zeolite for catalytic cracking of polyethylene to selectively produce paraxylene

Abstract

The core–shell structured MCM-22/ZSM-5 zeolite (MZCS50) was synthesized through a combination of hydrothermal crystallization (to form the MCM-22 core) and steam-assisted crystallization (to form the ZSM-5 shell). The MCM-22 core exhibited lamellar cake-like shape with a diameter of 3 ∼ 4 µm, while the ZSM-5 shell consisted of aggregated nanoparticles. MZCS50 possessed a high specific surface area, a micropore-mesopore coexisting structure, and was Brønsted acid-dominant material. MZCS50 was used as a catalyst for the cracking of low-density polyethylene (LDPE) to produce BTEX (benzene, toluene, ethylbenzene and xylenes). The results showed that 550 °C was the optimal temperature for LDPE cracking. The maximum liquid yield achieved was 23.4 %, with selectivities to BTEX, xylenes and p-xylene of 79.8 %, 43.2 %, and 21.3 %, respectively. Repeated cracking experiments demonstrated that MZCS50 exhibited good stability and superior catalytic performance in converting toluene to xylenes and further transforming xylene isomers into p-xylene. These results indicate that the core–shell structured MZCS50 is a promising catalyst for converting plastic wastes into valuable p-xylene.