| Abstract: | Dealuminated Y-type zeolites were prepared by SiCl4- and hydrothermal treatment. Using these techniques, the modified zeolites had framework aluminum contents ranging from ca. 1Al/unit cell (u.c.) to ca. 38 Al u.c. The catalytic activity of these materials was determined using hexane cracking as a test for strong acidity. The dealuminated materials exhibited an enhanced activity relative to their nondealuminated precursors. The starting H-Y zeolite with 55 Al/u.c. had an activity much lower than would be expected based on the total number of Bronsted sites. For dealuminated zeolites, the activity increased linearly with framework Al content for zeolites with framework aluminum contents between 0 and 30 Al/u.c. This linear relationship implied that the turn-over-frequency (TOF) was constant for the framework sites. These results qualitatively agree with a model for strong acidity which is based on isolated Al atoms being responsible for the strong acidity. In a subsequent study Na+-poisoning was examined to determine the effect of sodium addition on the catalytic activities of these materials. These materials were found to have drastically reduced activities upon sodium addition, with each sodium atom effectively neutralizing the catalytic activity previously thought to be due to five framework Bronsted acid sites. These results are interpreted to mean that isolated framework Al atoms are a necessary but not sufficient condition for strong acidity, and only ca. 1/5 of the framework Al atoms are associated with this strong acidity. Extra framework AI is believed to impart, through inductive effects, strong acidity to these framework Bronsted sites. An infrared study of the hydroxyl region revealed bands at 3675 and 3595-3610 cm^-1(± 2 cm^-1) that are attributed to protons associated with two different types of extra framework aluminum oxide or oxyhydroxide. A band at 3695 cm^-1 is attributed to Na+ interacting with water which is hydrogen-bonded to a framework oxygen atom. |
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