Effect of aluminum on the formation of zeolite MCM-22 and kenyaite

The synthesis of zeolite MCM-22 and kenyaite under static hydrothermal cond itions using hexamethyleneimide (HMI) as a directing agent was investigated . Kenyaite was the favored product instead of the purr silica analogue of M CM-22 for a pure silica synthesis mixture. With increasing SiO2/Al2O3 ratio , the crystallized products varied from purr MCM-22 to a mixture of MCM-22 and kenyaite, and finally pure layered silicate kenyaite. The fraction of M CM22 crystallites increases, whilst that of kenyaite decreases with the alu minum content. All the products occlude HMI molecules, and the amount of HM I increases with the aluminum content. MCM-22 and kenyaite have different l ayer structures, but the Si atoms in the HMI-included kenyaite have similar NMR chemical shifts as those in MCM-22. Aluminum atoms, which are present as tetrahedral framework species, are most possibly incorporated into the f ramework of MCM-22 only. The presence of aluminum seems to be critical for the formation of MCM-22 zeolite. (C) 2001 Elsevier Science B.V. All rights reserved.