Insight into the dissolution–crystallization strategy towards macro/meso/microporous Silicalite-1 zeolites and their performance in the Beckmann rearrangement of cyclohexanone oxime

Abstract

Hierarchical macro/meso/microporous Silicalite-1 single crystals were successfully fabricated based on an in situ dissolution–crystallization strategy without the assistance of secondary templates and the corresponding fabrication mechanism was systematically investigated in this study. Mesoporous silica sphere (MSS) precursors could provide a quasi “space-occupying” effect to construct intracrystalline macropores while functioning as silica sources. Various factors including the amount of tetrapropylammonium hydroxide, crystallization temperature and initial water content in synthesis mixtures were investigated considering their distinct effects on the dissolution and crystallization processes. It has been found that the intracrystalline macropores could be only formed via precise controlling of the velocity difference between the outside-in dissolution of MSS and the crystallization of Silicalite-1 zeolite. Based on the recognition of the fabrication mechanism, further modified Silicalite-1 with much higher porosity (meso-/macropore volume was increased from 0.15 to 0.26 cm³ g⁻¹) was synthesized successfully. Owing to the introduction of macropores and mesopores, the concentration and accessibility of active hydroxyl groups as well as the diffusion properties over hierarchical Silicalite-1 were both significantly enhanced and improved. And therefore, the hierarchical samples showed higher catalytic activities and superior catalyst lifetimes in the Beckmann rearrangement of cyclohexanone oxime compared to conventional microporous Silicalite-1. The present study provides a new understanding of the evolution mechanism of amorphous precursors to hierarchical zeolites in a secondary-template-free system, which is of great significance for the development of direct fabrication methods towards hierarchical zeolites and the Beckmann rearrangement reaction.