Dissolution of high-silica zeolites in alkaline solutions .2. Dissolution of 'activated' silicalite-1 and ZSM-5 with different aluminum content

Dissolution of organic-free ('activated') high-silica pentasils (silicalite-l and ZSM-5 with different Al-content) in 5 M NaOH solution at 60 degrees C was investigated by different methods such as X-ray diffractometry, Fourier transform infrared spectroscopy (FTIR), scanning-electron microscopy (SEM) and measuring the silicon and aluminum concentration in the liquid phase. Analysis of the experimental results showed that the process of dissolution of the 'activated' high-silica pentasils is controlled by two essential reactions; a forward one caused by breaking of the Si-O-Si and/or Si-O-Al bonds owing to the action of OH- ions from solution, and a backward one caused by the reaction between soluble species from the liquid phase or by their reaction on/with the surface of the dissolving solid. The rate of the forward reaction decreased with an increase of the Al-content in the ZSM-5 crystals, whereas the backward reaction resulted in the formation of amorphous SiO2 (ZSM-5) and/or different crystalline modifications of SiO2 or even different hydrates of sodium silicate (silicalite-l). A kinetic equation of the dissolution process was derived on the basis of the proposed mechanism of dissolution.