Abstract
High silica ZSM-5 zeolites were synthesized by using microporous silica derived from kaolin and were characterized by XRD, FTIR, SEM and N2 adsorption. The microporous silicas were prepared by calcining kaolinite at 600°C and selectively leaching with H2S04 solution for different times. Microporous silicas with Si02/ Al203 ratios of 70 and 710 were used as the starting materials. These powders were then converted into ZSM-5 by hydrothermal treatment at 150° in the presence of the templating agent tetrapropylammonium hydroxide. ZSM-5 zeolite formation was observed as early as 3h in the both samples. The growth rates of the crystals were found to depend on the Si02/Al203 ratios of the starting materials. A high Si02/Al203 ratio resulted in more uniform and well-crystallized ZSM-5 zeolite. The change of crystal morphology was examined as a function of hydrothermal treatment using XRD and SEM, showing that the preferential growth of (020) plane resulted in thick platy ZSM-5 crystals in the high silica batch. The specfiic surface areas obtained were 300-340 m2/g and the pore size distributions showed a considerable change due to differences in the size and morphology of the ZSM-5 crystals.
