Mesoporogen-free synthesis of hierarchical HZSM-5 for LDPE catalytic cracking

Abstract

Herein, a mesoporogen-free and steam-assisted route is proposed to synthesize highly crystalline hierarchical HZSM-5 using Stöber silica spheres as a precursor. The effect of crystallization time on the morphology and structural characteristics of the prepared zeolites was extensively investigated. It was found that the crystallinity of zeolites increases gradually in the first 20 h and then decreases with the extension of the crystallization time. A typical sample with the crystallization time of 20 h named as HZSM-5-20h exhibits the highest crystallinity (100%) and the largest mesopore volume (0.13 cm³ g⁻¹). Further, a nucleation–aggregation–growth mechanism based on the evolution of “water pools” of silica spheres has been proposed to elucidate the structural transformation of here reported hierarchical HZSM-5. When used for the catalytic cracking of low density polyethylene (LDPE), HZSM-5-20h greatly reduces the decomposition temperature from 448 to 397 °C compared with microporous ZSM-5. Such improved performance can be attributed to its large mesopore volume and good crystallinity, and consequently improved accessibility of mesopore surface active sites that could not be used originally.