CRYSTAL-GROWTH OF HIGH-SILICA ZSM-5 AT LOW-TEMPERATURE SYNTHESIS CONDITIONS

At the temperature of 90 degrees C and under atmospheric pressure, gro wth kinetics of high silica ZSM-5 was investigated through a long indu ction, nucleation and crystal growth periods. It was found the entire crystallization mechanism of ZSM-5 seems to be the combined process of the nucleation via solid-solid transformation, intergrowth among seed crystals and the normal growth in the reaction mixture. Nuclei were i nitially formed on the Si-rich surface of the amorphous intermediates, indicating that the reaction of TPA with Si species was prior to that with Al species. As the reaction time proceeded, various types of int ergrowth among the seed crystals were observed along with the crystals growing independently. The intergrowth seems to play a role for formi ng typical ZSM-5 crystal shapes. And then ZSM-5 crystals further grew in the reaction mixture, so that the bulk Si/Al-2 ratio of crystals ap proached that of the initial reaction mixture.