Synthesis of acidic-basic Y zeolite from kaolin: catalytic and mechanism study in the cracking of polyethylene

Cracking of plastic waste by the acidic FAU-type zeolites faces challenges of low production of light olefins, a high amount of coke formation, and expensive process. Basic zeolites might be a better alternative; however, their preparation is difficult and the related cracking mechanism is unknown. We addressed these difficulties by direct synthesis of a hierarchical acidic-basic FAU-type zeolite (HY-BS) using silica extracted from a low-grade kaolin containing alkaline (earth) metals and investigated the impact of basicity in the linear low density polyethylene (LLDPE) cracking mechanism and products distribution. Compared to an acidic HY zeolite, the HY-BS presented comparable density of Brønsted acidity but a drastically higher density of the basicity. The HY-BS had comparable activity to the reference HY zeolite, but it produced twice light olefins (30.5 vs. 16.2%), triple lighter aromatics (41.0 vs. 13.8%), and less coke in LLDPE cracking. The thermodynamic investigation of thermal and zeolitic cracking of LLDPE was conducted based on the TGA results, along with kinetic analysis using four model-free techniques and a differential method. According to kinetic and thermodynamic parameters and product distribution, the HY-BS exhibited a dual cracking pathway that first followed a carbocation mechanism before switching to a radical mechanism as the reaction progressed, accounting for the reason behind improved light olefins selectivity. Consequently, direct promotion of the basicity of the zeolites synthesized from impure kaolin was effective to overcome difficulties facing plastic waste cracking.