Dealumination of HZSM-5 via steam-treatment

Two distinct locations for tetrahedral aluminum in HZSM-5 have been identified, showing bond angles of 150 degrees and 155 degrees 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 Co2+ ions, it is inferred that aluminum resisting dealumination constitutes these sites. (C) 2012 Elsevier Inc. All rights reserved.