Improving hydrothermal stability of acid sites in MFI type aluminosilicate zeolite (ZSM-5) by coating MFI type all silica zeolite (silicalite-1) shell layer

Zeolites have been applied in various chemical processes due to their multi-functionality. In practical use of zeolites, their hydrothermal stability is one of the important factors. We focused on the coating with hydrophobic silicalite-1, which is all silica MFI zeolite. In this work, we investigated the hydrothermal stability of ZSM-5 (MFI type aluminosilicate zeolite) coated with silicalite-1 (all silica MFI zeolite) by the combination of steam treatment and NH3 temperature programed desorption (NH3-TPD). The peak area at higher temperature (h-peak area) in NH3-TPD profiles for core-shell MFI zeolite (ZSM-5/s-1) was unchanged even after steam treatment for 8 h in contrast to uncoated ZSM-5. These results strongly indicate that extremely high Si/Al ratio on the external surface coated with silicalite-1 is highly effective for improving the hydrothermal stability; the hydrophobicity of silicalite-1 probably prevents the penetration of steam into Al sites of ZSM-5. This work opens up a new function of the silicalite-1 layer on ZSM-5 other than improvement of shape selectivity.