DISSOLUTION OF HIGH-SILICA ZEOLITES IN ALKALINE-SOLUTIONS .1. DISSOLUTION OF SILICALITE-1 AND ZSM-5 WITH DIFFERENT ALUMINUM CONTENT

Dissolution of high-silica pentasils (silicalite-1, 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 (FTIR) spectroscopy, scanning-electron microscopy (SEM), measuring of specific surface area, determination of particle size distribution and measuring the silicon and aluminum concentration in the liquid phase. Analysis of the experimental results showed that the initial rate of dissolution is directly proportional to the surface area of the dissolving solids and that the rate constant is a linear function of the silica-to-alumina ratio of the dissolving solids. The influence of other factors, such as concentration of silicate anions and tetrapropylammonium (TPA(+)) ions in the liquid phase, preferential leaching of silicon, selective attack of OH- ions on the crystal surface, etc., on the course of dissolution and properties of the dissolving solids are discussed.