SYNTHESIS OF AL,SI-MFI-TYPE ZEOLITES IN THE PRESENCE OF F- ANIONS - STRUCTURAL AND PHYSICOCHEMICAL CHARACTERISTICS

MFI-type zeolites prepared in the presence of Al and Pr4N+ in fluoride media at pH 5-10 were characterized with various techniques (SEM; XRD; chemical analysis; F-19, Al-27, and Si-29 MAS n.m.r. n-hexane adsorption) in their as-synthesized, calcined, and postsynthesis-treated forms. The purity and the crystallinity of the materials are comparable with those synthesized in OH- medium. Although the overall Si/Al ratio can be as low as 12, a part of Al does not display its normal tetrahedral coordination in the framework. This type of aluminum, whose amount increases during removal of the template by calcination, can be revealed by F-19 MAS n.m.r. after a treatment of the calcined samples with Pr2NH2F solutions. It is also responsible for the modification of the adsorption kinetics of n-hexane.