Self-assembly kieselguhr-based ZSM-5 for catalytic pyrolysis of mask waste to produce high value-added aromatics

Abstract

The conversion of plastic waste to high value-added aromatics such as benzene, toluene, ethylbenzene, xylene (BTEX) through catalytic pyrolysis offers huge untapped opportunities for resource recovery and plastic pollution control. Economical equipment configuration, operation and maintenance, high safety factor, high efficiency, stability, low-cost catalyst synthesis design, high yield, high selectivity of products are the key to the practical application of catalytic pyrolysis. In present study, low-cost kieselguhr based ZSM-5 was synthesized by one-pot in situ hydrothermal method using kieselguhr as silicon-aluminum source. During the depolymerization-recrystallization process, the selective growth and self-assembly of crystals on the a, c axis formed large-grain of ZSM-5 with extended straight channels and maximized sinusoidal channels. Native transition and alkaline-earth metals were coordinated to the acidic sites of the silica-aluminum skeleton, achieving tunability of the catalyst acidity. Catalytic pyrolysis of mask waste at 550 °C in a fixed bed reactor achieved the highest pyrolysis oil yield (44.3 wt%) and BTEX selectivity (77.1 %) with p-xylene as the dominant xylene product (78.2 %). After 3 cycles of long-life experiments, the BTEX selectivity of the catalyst remained at 65.1 %, which is higher than that of commercial ZSM-5 (54.9 %), indicating the superior use stability. Molecular dynamics simulations showed that the excellent shape-selectivity of the catalyst is based on its unique pore structure, which facilitates the diffusion and transport of appropriately sized intermediates/products within the structure, resulting in products with advantageous distributions. This study proposes an efficient and feasible plastic closed-loop strategy for green upgrading of medical waste to high value-added chemicals.