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Hydrothermal Transformation of MFI Type Zeolites into Amorphous and Crystalline SiO2 Compounds

Postsynthesis acid or base treatment of zeolites ia a method frequently used for the change of silica/alumina ratio of the treated zeolite. Dissolution of silicalite-1 and ZSM-5 in 5 M NaOH solution at 60o C was investigated by different methods such as X-rax diffractometry, FTIR spectroscopy, scanning electron microscopy and measuring the silicon and aluminum concentrations in the liquid phase. Analysis of the experimental results showed that initial rate of dissolution is directly proportional to the surface area of the dissolving solid, but that the further course of the dissolution process considerably depends on the aluminum content in the dissolving solids, i.e. the rate of dissolution and equilibrium concentration of silicon in the liquid phase decrease with the increase of Al-content. Results of X-ray diffractometry showed that the dissolution of ZSM-5 was followed by its transformation into amorphous silica, and that silicalite-1 was rapidly transformed into a mixture of amorphous and crystalline SiO2 compounds.

Silicalite-1; ZSM-5; Hydrothermal transformation; Alkaline solutions; Amorphous SiO2; Crystalline SiO2

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