Abstract:

This work demonstrates an improved (NH₄)₂SiF₆ treatment to enhance the hydrothermal stability of mesoporous SBA-15 zeolite. In this treatment, Al³⁺ ions are incorporated into SBA-15 zeolite first, then it is treated with 5% (NH₄)₂SiF₆ solution according to 1% SiO₂ of SBA-15 and finally washed with HCl (2 mol/L) to remove the pre-incorporated Al³⁺ ions. The obtained SBA-15 exhibits higher hydrothermal stability than that without pre-incorporated Al³⁺. Compared with the latter, the sample maintains a better ordered mesostructure and a larger surface area (271 m²/g) after hydrothermal treatment at 800 ℃ for 12 h in 100% steam. The results show that incorporating Al³⁺ ions into SBA-15 zeolite before (NH₄)₂SiF₆ treatment obviously promotes the stabilization effect of (NH₄)₂SiF₆ treatment. The mechanism suggests that the incorporated Al³⁺ ions can effectively capture F^- ions that have been released from (NH₄)₂SiF₆, and thus reduce their etching into the SBA-15 zeolite framework. This ensures that the positive factors of (NH₄)₂SiF₆ treatment, such as silicon insertion and surface hydrophobization by F^- ions, play effective roles in the improvement of the hydrothermal stability of SBA-15 zeolite. This promoting effect of the Al³⁺ ions is closely related to the method that is used to introduce the Al³⁺ and the SBA-15 zeolite processing temperature.

Key words: SBA-15 zeolite, Mesoporous material, Hydrothermal stability, Ammonium fluorosilicate, Al³⁺ ion incorporation, Si insertion