Crystallization and morphology of zeolite MCM-22 influenced by various conditions in the static hydrothermal synthesis

MCM-22 zeolite was synthesized in various static hydrothermal systems with hexamethyleneimine (HMI) as the structure-directing agent. Synthesis parameters, such as silica source, alkaline source (Na+, K+ or NH4+), alkalinity of the reaction mixture, HMI/SiO2 ratio and seeding, were changed to investigate their effects on the crystallization and morphology of zeolite MCM-22. The produced samples were characterized by XRD, IR, SEM, N2 physisorption and ICP. Reaction conditions were revealed to influence the induction period and crystallization rate remarkably, and the sodium metasilicate as the silica source, the medium OH−/SiO2 molar ratio (0.15), the high HMI/SiO2 molar ratio and the addition of seeding could accelerate the crystallization. Moreover, by altering the reaction parameters, various morphologies of MCM-22 product with different particle sizes were achieved, such as spherical, rose-like, doughnut-like and platelet-like structures. The noteworthy observation was that with K+ as the alkaline source instead of Na+, a much shorter induction period and faster crystallization rate were obtained, resulting in a higher crystallinity and a smaller crystal size. K+ is more favorable to the formation of MCM-22 crystal than Na+ even at very low HMI/SiO2 ratios.