Influence of Adsorbed Molecules on the Configuration of Framework Aluminum Atoms in Acidic Zeolite-β. A ²⁷Al MAS NMR Study

Abstract

The formation of octahedral framework aluminum sites in acidic zeolite-depends on the adsorption of water. This could be shown by 27 Al MAS NMR spectroscopy under controlled atmosphere conditions. The 27 Al MAS NMR spectrum of the hydrated zeolite shows two lines corresponding to tetrahedral and octahedral Al sites, respectively. These NMR lines disappear on dehydrated samples. The adsorption of methanol or pyridine as probe molecules on the dehydrated zeolite samples restores only the tetrahedral (framework) aluminum sites in the 27 Al MAS NMR spectrum due to the relaxation of the distorted zeolite lattice by the adsorbed molecules. The acid-catalyzed reaction (470 K) of adsorbed methanol molecules on zeolite-, which supplies water, brings back the 27 Al MAS NMR signal caused by the octahedral framework aluminum sites.

Three-Coordinate Aluminum in Zeolites Observed with In situ X-ray Absorption Near-Edge Spectroscopy at the Al K-Edge: Flexibility of Aluminum Coordinations in Zeolites

“…The formation of octahedral aluminum occurs already at room temperature. 31,32 The presence of this octahedrally coordinated species has been well documented for zeolite H-Beta, [42][43][44][45][46] but it is also reported for other zeolites. 31,32,47 It is generally assumed that this octahedral species is attached to the framework and that partial hydrolysis of the framework took place.…”

Zeolite Beta: The Relationship between Calcination Procedure, Aluminum Configuration, and Lewis Acidity

“…Several studies point to the flexibility of the coordination sphere of the aluminum atoms in zeolite Beta (3)(4)(5)(6)(7)(8). In addition to tetrahedrally coordinated framework aluminum, octahedrally coordinated framework aluminum has been proposed, consisting of an aluminum connected with four bonds to the framework, one water molecule, and a hydronium ion (3)(4)(5)(6).…”

“…In addition to tetrahedrally coordinated framework aluminum, octahedrally coordinated framework aluminum has been proposed, consisting of an aluminum connected with four bonds to the framework, one water molecule, and a hydronium ion (3)(4)(5)(6). The amount of these types of aluminum was shown to depend on the pretreatment of zeolite Beta and on the water content (3,6).…”

“…In addition to tetrahedrally coordinated framework aluminum, octahedrally coordinated framework aluminum has been proposed, consisting of an aluminum connected with four bonds to the framework, one water molecule, and a hydronium ion (3)(4)(5)(6). The amount of these types of aluminum was shown to depend on the pretreatment of zeolite Beta and on the water content (3,6). FTIR and 29 Si and 27 Al MAS NMR were applied to study the changes induced by various calcination conditions (shallow vs deep-bed (4)), temperature treatments and interactions with molecules like ammonia, pyridine, benzene, n-hexane, and water (7,9).…”

“…Instead of the poorly defined, inhomogeneous deep-bed calcination procedure, all additional treatments were performed using a well-defined shallow calcination bed while varying the atmosphere above the zeolite sample. Hot spots and autosteaming (6), intrinsically related to deep-bed calcination procedures under an oxidative atmosphere, will be prevented in this way, thus permitting a more reproducible calcination method.…”