Studies on formation of hollow silicalite-1 microcapsules

In the present work, the detailed formation process of hollow silicalite-1 microcapsules (HSMs) was investigated in the hydrothermal system of silicalite-1 pre-seeded mesoporous silica spheres (MSSs). The effects of the starting gel composition together with the hydrothermal synthesis conditions such as alkalinity, tetra-propylammonium cation (TPA+) content, and temperature on the features of microcapsules are comprehensively discussed, and a related mechanism is presented. It is found that the matching of core dissolution rate (Rcd) and silicalite-1 shell inter-growth rate (Rsi) is crucial for the formation of perfect HSMs. The proper addition of tetraethoxy-orthosilane (TEOS) as supplemental silica source could accelerate Rsi whereas excess TEOS will dramatically suppress the consumption of siliceous species from the MSS cores owing to the higher activity of siliceous species in TEOS for the zeolite crystallization. The excellent HSMs with dense inter-grown shell structure as well as perfect hollow center can be fabricated by controlling the relative rates of the dissolution of silica template core and the formation of zeolitic shell.