Abstract
Two distinct locations for tetrahedral aluminum in HZSM-5 have been identified, showing bond angles of 150°and 155°T-O-T, respectively. The former site is more abundant and has been indirectly associated with aluminum in isolated positions. Upon steaming its concentration decreases, following a first order rate law and leading to the formation of tetrahedrally coordinated extra-lattice aluminum as well as to invisible extra-lattice aluminum. The latter is speculated to be also tetrahedrally coordinated and kinetically linked to the visible portion of the extra-lattice aluminum. Both types of extra-lattice aluminum neutralize lattice charge and lead to a decrease of the concentration of bridging Si-OH-Al groups, which is initially more pronounced than the loss of lattice aluminum. With steaming duration the concentration of Bronsted acid sites stabilizes indicating that the extra-lattice aluminum atoms begin to form larger clusters at a rate equivalent to the rate of dealumination. The lattice aluminum with the more obtuse T-O-T angle is stable under the steaming conditions chosen. As its concentration is nearly equivalent to the concentration of aluminum sites sufficiently close to exchange Co 2+ ions, it is inferred that aluminum resisting dealumination constitutes these sites.
Original language | English |
---|---|
Pages (from-to) | 9-20 |
Number of pages | 12 |
Journal | Microporous and Mesoporous Materials |
Volume | 164 |
DOIs | |
State | Published - 1 Dec 2012 |
Keywords
- Al MAS NMR
- Site
- Tetrahedral extra framework aluminum
- Zeolite steaming