Organic-free synthesis of ZSM-5 with narrow crystal size distribution using two-step temperature process

Abstract

The MFI type of highly silica zeolite, ZSM-5 with well-defined crystal morphology was successfully synthesized in the absence of organic template using two-step temperature process. In this process, the crystallization process was conducted at two different temperatures by separating the nucleation and crystallization stage; that is, the nucleation stage at higher temperature (190 °C) to accelerate the nucleation followed by the crystallization stage at lower temperature (150 or 165 °C) so as to control the crystal size and size distribution. The effect of nucleation rate on the final crystal size and size distribution adopting higher temperature of the first stage for nucleation than that of the second stage for crystallization was investigated. The results show that two-step temperature process could control the crystal size and size distribution of ZSM-5. The result also shows that the time duration at higher temperature for the acceleration of nucleation strongly affects the crystal size and size distribution. It implies that rapid nucleation at higher temperature forms large number of nuclei followed by slow crystal growth probably due to the lack of energy at lower temperature. It is very critical, therefore that the optimum time duration at higher temperature should be chosen to accelerate the nucleation within initial stage.