Sheet-like silicalite-1 single crystals with embedded macropores displaying superior catalytic performance

Abstract

Unique sheet-like Silicalite-1 single crystals containing intracrystalline macropores (HMS-Silicalite-1) were prepared by urea-assisted, dry-gel synthesis, which uses mesoporous silica spheres (MSSs) as both the silica precursor and the macropore template. Urea was found to inhibit crystal growth in the b-axis direction and, therefore, play a pivotal role in the formation of the sheet-like crystals. Zeolite growth occurred via “dissolution–crystallization” transformation of MSSs to produce macro/microporous Silicalite-1 crystals. Systematic studies revealed that the urea/SiO₂ and H₂O/SiO₂ ratios affect the morphology of the resulting Silicalite-1 zeolites and that the thickness of HMS-Silicalite-1 platelets can be tuned between 190 nm and 400 nm. It was also found that the H₂O/SiO₂ and urea/SiO₂ ratios can be modulated to match the “inhibition” and “dissolution–crystallization” processes. HMS-Silicalite-1 was assessed as a catalyst for the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam and found to exhibit significantly greater resistance to deactivation than conventional Silicalite-1 and hierarchical macro/microporous Silicalite-1. The superior performance of HMS-Silicalite-1 is attributed to the presence of crystals of Silicalite-1 that are thin in the direction of the b-axis and contain significant macropores (150 to 400 nm in size). The results of this study offer a novel, yet simple, method for simultaneous control of the zeolite crystal aspect ratio and macropore content to produce catalysts that are highly efficient for diffusion-limited reactions involving large reactant molecules.