Understanding the Role of the Surface Acidity of MFI Zeolites during LDPE Cracking: Decomposition Temperature and Product Distribution

The utilization of zeolites in the catalytic cracking of plastics has garnered attention as a promising recycling method. Zeolitic micropores are uniform and exhibit shape selectivity, but their sizes are very small compared to those of polymer molecules. Consequently, the reactions occurring on the external surfaces and near the pore mouths of zeolites are crucial for polymer cracking. However, the role of zeolitic surfaces in polymer cracking has not been clarified. In this study, we controlled the external surfaces of zeolites (particle size, external surface areas, and location of the surface acid sites). These catalysts were employed in the cracking of low-density polyethylene (LDPE), and the effects of their surface properties on the reactions were investigated. The external surface areas of ZSM-5 zeolites were controlled by changing their particle sizes and desilication conditions. Although the LDPE cracking temperatures strongly depended on the zeolitic surface area, the temperature was plateaued when the external surface area exceeded approximately 90 m²/g. To investigate the role of surface acidity further, we prepared core–shell-type ZSM-5/silicalite-1 zeolites with various shell thicknesses. The silicalite-1 coating significantly reduced the LDPE cracking activity of ZSM-5 zeolites, and the cracking temperature increased with the increase in silicalite-1 shell thickness. However, the gaseous product distributions were shifted toward lower hydrocarbons by the increase in inert shell thickness, indicating that the initial cracking and excessive reactions on the external acid sites led to the production of large C5 and C6+ products. This study revealed that the initial cracking reactions occur at the inner acid sites near the pore mouth as well as the external surface and that the smaller reactants diffuse into deep active sites. These findings are anticipated to offer valuable insights into the development of zeolite catalysts suitable for the catalytic cracking of polymers.