β@MFI Nanozeolite with Complete Shell for Improving Catalytic Cracking of Ethylcyclohexane

Zeolites with a core–shell structure can achieve a synergistic effect between core and shell zeolites to enhance the overall functionality of the zeolite. However, it is rather challenging to control the growth arrangement of shell zeolites due to lattice mismatch. In this study, a synthesis method was proposed for β@MFI nanozeolites via an in situ solid-phase synthesis. This strategy relies on the in situ crystallization of MFI zeolite on a silica shell, which is prepared using a surfactant-guided sol–gel approach to trap β nanozeolites. The silica shell alleviates the stress caused by the direct lattice mismatch between the two zeolites, while steam-assisted crystallization prevents nucleation in the liquid phase to preserve the shell. The ratio of particle sizes of the shell nanozeolite to the thickness of the shell was used to define core–shell nanozeolites with a complete shell with values >1. By studying the synthetic intermediates, the controlled in situ nucleation, growth, and fusion processes of shell zeolites on silica shells were demonstrated. Zeolites with a core–shell structure exhibit enhanced hydrothermal stability of the core zeolite and improved selectivity toward light olefins in catalytic cracking, indicating that the synthesized β@MFI nanozeolites can enhance the catalytic cracking reaction performance.