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 Brønsted 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 Co2+ ions, it is inferred that aluminum resisting dealumination constitutes these sites.
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