Understanding the\nRole of the Surface Acidity of MFI\nZeolites during LDPE Cracking: Decomposition Temperature and Product\nDistribution

The utilization of zeolites in the catalytic cracking\nof plastics\nhas garnered attention as a promising recycling method. Zeolitic micropores\nare uniform and exhibit shape selectivity, but their sizes are very\nsmall compared to those of polymer molecules. Consequently, the reactions\noccurring on the external surfaces and near the pore mouths of zeolites\nare crucial for polymer cracking. However, the role of zeolitic surfaces\nin polymer cracking has not been clarified. In this study, we controlled\nthe external surfaces of zeolites (particle size, external surface\nareas, and location of the surface acid sites). These catalysts were\nemployed in the cracking of low-density polyethylene (LDPE), and the\neffects of their surface properties on the reactions were investigated.\nThe external surface areas of ZSM-5 zeolites were controlled by changing\ntheir particle sizes and desilication conditions. Although the LDPE\ncracking temperatures strongly depended on the zeolitic surface area,\nthe temperature was plateaued when the external surface area exceeded\napproximately 90 m<sup>2</sup>/g. To investigate the role of surface\nacidity further, we prepared core–shell-type ZSM-5/silicalite-1\nzeolites with various shell thicknesses. The silicalite-1 coating\nsignificantly reduced the LDPE cracking activity of ZSM-5 zeolites,\nand the cracking temperature increased with the increase in silicalite-1\nshell thickness. However, the gaseous product distributions were shifted\ntoward lower hydrocarbons by the increase in inert shell thickness,\nindicating that the initial cracking and excessive reactions on the\nexternal acid sites led to the production of large C5 and C6+ products.\nThis study revealed that the initial cracking reactions occur at the\ninner acid sites near the pore mouth as well as the external surface\nand that the smaller reactants diffuse into deep active sites. These\nfindings are anticipated to offer valuable insights into the development\nof zeolite catalysts suitable for the catalytic cracking of polymers.