The hydrothermal crystallization of zeolite MCM-22 was investigated at 423K in the Na2O-Al2O3-SiO2-H 2O-hexamethyleneimine (HMI) system under rotating and static synthesis conditions. Three different silica sources: silicic acid, silica gel, and Ultrasil precipitated silica were used. Under rotating conditions, all investigated silica sources yielded zeolite MCM-22. Silicic acid with the highest specific surface area (S=750m2g-1) had the shortest overall crystallization time followed by silica gel (S=500m2g-1) and Ultrasil (S=176m2g-1). Under static conditions only silicic acid resulted in zeolite MCM-22 and only with stirred aging or static aging. Reaction composition diagrams and crystallization kinetics for zeolite MCM-22 grown under both rotating and static (with 24-h static aging at 318K) conditions were determined at constant HMI/SiO2=0.35 and H2O/SiO2=19.5 ratios. Static syntheses resulted in a slightly narrower crystallization field than did rotating syntheses. Under both synthesis conditions, the mixtures with decreasing SiO2/Al2O3 (increasing Al content) resulted in a decreased induction period. However, decreasing SiO2/Al2O3 did not significantly influence the crystallization rate. Thus, the Al content of the synthesis mixture affected more the nucleation than the actual crystal growth. In mixtures with identical compositions, the final particle size was smaller and crystallization rate was higher in syntheses carried out under rotating synthesis conditions. These results suggest that silica dissolution was the rate-determining step in the crystallization of MCM-22, and that aging resulted in the formation of zeolite MCM-22 nuclei. Agitation prevented the excessive aggregation of the platelets into lamellar particles of MCM-22, but was not necessary for its formation.
- Crystal growth
- Crystallization kinetics
- Hydrothermal synthesis of MCM-22