Core–Shell Structured Zeolite Catalysts with Minimal Defects for Improvement of Shape Selectivity

Abstract

An aluminum-containing zeolite (shell) was coated with aluminum-free zeolite (core) by crystal overgrowth to form zeolites with a core–shell structure (core–shell zeolites). Fluoride ion as a mineralizer in both the core zeolite synthesis and the crystal overgrowth of the core zeolite with the shell zeolite was necessary to form the core–shell zeolite with minimal defects, leading to complete passivation of acid sites on the external surface. When the core–shell zeolite with MFI structure was used as a catalyst for toluene methylation with methanol, the selectivity for p-xylene reached 95.8%. This indicated that there were no acid sites on the external surface. The core–shell zeolite with TON structure was pressed to break needle-like particles for the formation of new acid sites on the pore mouths of smaller broken particles. The broken core–shell zeolite showed high catalytic activity and selectivity for skeletal isomerization of n-tetradecane because the acid sites on the side surface of the needle-like particles, which mainly catalyzed the cracking of alkanes, were passivated. The core–shell zeolites with minimal defects can be used for other applications such as precursors of hollow zeolites with minimal defects.