THE DEVELOPMENT OF STRONG ACIDITY IN HEXAFLUOROSILICATE-MODIFIED Y-TYPE ZEOLITES

Catalysts have been prepared by treating a parent Na,NH4-Y(54) zeolite containing 54 Alf/u.c. with ammonium hexafluorosilicate (AHFS). The resulting zeolites, containing between 39 and 26 Al/u.c., were crystalline, but were relatively inactive for the cracking of n-hexane at 400°C. After steaming they became active, and their activity was greater than that of a parent zeolite which had been steamed to the same framework aluminum (Alf) content. To achieve maximum activity by steaming the amount of A1 removed from the AHFS-treated zeolite was equivalent to ca. one Al atom per β cage. The enhanced activity of the steamed AHFS-treated material is believed to result from the presence of isolated Al in the framework and a cationic form of extraframework Al in the β cages. It is apparent from the 29Si NMR spectrum that the resonance of Si atoms connected to three Alf atoms is not present in the AHFS-treated samples. The absence of this 29Si resonance is consistent with an increase in isolated Alf atoms. All of the zeolites contained an octahedral form of extraframework Al which has no affect on acidity or activity. The most active zeolites are characterized by infrared bands at 3602 and 3525 cm-1, which are assigned to hydroxyl groups in the large and small cages, respectively. The zeolites prepared with AHFS retained less than 0.3 wt% fluorine. It is unlikely that this fluorine is responsible for the enhanced acidity of the more active catalysts. Impregnation of the less active catalysts with NH4F increased the activity; however, this phenomenon results from dealumination rather than from the electronic effect of fluorine. © 1992.