Synthesis of MFI-type zeolites under atmospheric pressure

Silicalite and highly silicious ZSM-5 were synthesized using two reaction m ixtures with different crystal growth environments, a dispersed low density mixture and a separated high density mixture, at 93+/-3 degreesC under the atmospheric pressure. Nucleation behavior and the transformation process o f two mixtures were investigated utilizing various analytical techniques su ch as XRD, FT-IR, TGA, SEM, and pH measurement. During the induction period , the same intermediate phase was observed in both mixtures. The presence o f this phase was found to be dependent on the degree of dilution of the rea ction mixture. After the induction period, a sharp increase in both the deg ree of crystallization and the pH of the reaction mixture was detected. Thi s indicates that the pH change in the reaction mixture is closely related t o the crystallization process. From these observations, a crystallization m echanism is proposed on the basis of the appearance of stable silicate spec ies and the role of OH- ions during the induction period. According to this mechanism, MFI-type zeolite grows by condensation reaction among the stabl e silicate species formed during the induction period.