Aluminum, zeolite dealumination, ammonium hexafluorosilicate

Evidence that something more than a proper Alf distribution is needed to create strong acidity first came from the experiments of Beyerlein et al. (7). Using the acid-catalyzed conversion of isobutane as a measure of strong acidity, they found that a dealuminated zeolite prepared by treatment with ammonium hexafluorosilicate (AHF) exhibited much less carbonium ion activity than might be expected, based on the number of Alf atoms. This treatment leaves very little extraframework Al in the zeolite. When the sample was mildly steamed, the activity became considerably greater. The authors concluded that the enhanced acidity was a result of a synergism between the framework Bronsted sites and the Lewis sites associated with extraframework aluminum. 

Chemical dealumination involves reaction of the zeolite framework with any one of a variety of reagents(2). In this work, zeolites were reacted with ammonium hexafluorosilicate in aqueous solution(9-12) to prepare dealuminated products. Aluminum was extracted from the zeolite framework and removed from the crystal as a soluble fluoroaluminate complex the resulting lattice vacancies are believed to be filled by silicon in solution. Composition profiles of chemically dealuminated zeolites (AFS) are homogeneous and indicate the entire crystal is accessible for dealumination(13). Sorption data indicate that AFS zeolites do not possess a secondary pore system although pore blockage may occur due to occlusion of fluoroaluminate species(13). 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

Min investigated the reaction mechanism of this dealumination approach.[24] Breck and Skeels had found that, in the ammonium hexafluorosilicate solution, the extraframework silicon atoms could substitute for the aluminum atoms in the crystal lattice of the zeolite framework, but the mechanism of the whole reaction process was not clear. Through 29Si,27Al-MAS-NMR and IR characterization techniques, Min and coworkers investigated the removal of A1 atoms from the zeolite framework, the chemical environment of the extraframework silicon atoms, and the species of the silicon that can be inserted into the Al-removal framework vacancies during the reaction process, and finally they proposed the reaction mechanism [Equations (6.18)-(6.25)] as follows. 

A more in-depth study on FAU-structiu e zeolites with high siUca content by Beyerlein et al. [108] addresses the influence of extra-framework species (EFAL) on the catalytic activity for the isobutane conversion. Overall, the cracking activity increased linearly with the concentration of framework aluminum sites for all catalysts. However, FAU samples dealuminated with ammonium hexafluorosilicate (AHF) showed a lower activity compared to conventionally hydrothermally treated catalysts. Steaming of the chemically dealuminated catalysts led to an increased activity in cracking isobutane compared to the parent FAU. This is interpreted as an indication that extraframework aluminum species, typically acting as Lewis add sites, are interacting with Bronsted acid sites to form new and stronger addic sites. 

Early in the 1980s, Breck and Skeels developed a new method for the dealumination of medium- and large-pore zeolites. It was first described in a patent [180] assigned to the Union Carbide Corp. (application filed in 1981) and then presented at the 6th International Zeolite Conference in 1984 [181]. Their fundamental idea was to treat a zeolite slurried in water with an aqueous solution of an agent which extracts aluminum from the framework, provides ligands for the formation of a thermodynamically strongly favored, soluble aluminum complex and serves as an extraneous source of silicon atoms filling up the framework vacancies formed upon extraction of aluminum. Breck and Skeels realized that only soluble hexafluorosilicate salts, especially the ammonium and lithium salts, meet the requirements of such a process. The overall process of this dealumination process can be described by Eq. (6). 

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