Crystal structures of dehydrated H chabazite pretreated at 320.degree.C, and at 600.degree.C after steaming

Abstract

Publication costs assisted by the Katholieke Universiteit LeuvenTwo crystal structures of dehydrated NH4-exchanged chabazite crystals were studied by use of single-crystal X-ray diffraction methods after dehydration at 320 "C, and at 600 "C following a steaming period. No significant differences are observed between the two structures; there is no preference of the protons for a single oxygen species nor any indication of A1 removal from the framework after steaming. A strong correlation between T-0-T bond angles and T-0 distances in adjacent tetrahedra shows an identical slope with that of dehydrated H mordenite. The framework oxygens, all pointing either to the hexagonal prisms or to the centers of the framework eight-rings, prohibit the free vibration of OH bonds in the large cavity. This explains the 30 cm-' decrease of the HF bands as compared to the value predicted from the framework composition.

Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

“…To our best knowledge, only Bordiga et al assigned this band to “extra lattice or partially extra lattice Al–OH vibrations”. Many researchers associate the appearance of both the ∼3680 cm –1 H-SAPO-34 and ∼3660 cm –1 H-CHA FTIR bands with sample hydrolysis, showing that the intensity of these bands substantially increases in the presence of H 2 O. − ,, It has been also reported that the acidic OH groups of both molecular sieves tend to connect at least 1 molecule, but often 3 or 4 H 2 O molecules, via hydrogen bonds. ,, However, our measurements were carried out on samples calcined at 500 °C or higher temperatures in air, and in addition they were also in situ evacuated at ∼10 –5 Torr at 500 °C before both the TR and the DRIFT measurements, which were carried out at room temperature afterward. This pretreatment removes all molecular water from the zeolites as one could confirm from the lack of any measurable 1640 cm –1 FTIR band, associated with the well-known bending vibration of molecular water.…”

“…They do not show up in the computer models of these molecular sieves; i.e., they seem not to be associated with the structural Brønsted acidic sites. In the course of our literature survey we have found that those researchers who use DRIFT technique − generally find these bands more intense than those who use TR technique. − In case of the H-SAPO-34 most researchers assign the band near ∼3680 cm –1 to surface P–OH vibration, following Peri’s early empirical assignment from the TR spectra of an amorphous AlPO 4 . Yet, his spectra also showed the parallel appearance surface Al–OH vibrations near 3800 cm –1 , which are absent from our and many other authors’ H-SAPO-34 spectra (Figure a).…”

Oxydative Deammonisierung von NH4-Zeolithen

“…Es ist allgemein bekannt, dab H-Zeolithe unter hydrothermalen Bedingungen gewisse strukturelle Verfinderungen erleiden, indem tetraedrisch koordiniertes Aluminium aus dem Anionengitter des Zeoliths austritt und kationische Gitterpositionen einnimmt [12,13]. Zumindest beim H-Chabasit ftihrt jedoch die Einwirkung von Wasserdampf nicht zum Austritt von AI aus dem Gitter [11,14], aber trotzdem zu wesentlichen Ver~inderungen im Verhalten gegentiber thermischen und hydrothermalen Einwirkungen [11 ]. Da bei der oxydativen Deammonisierung Wasser als eines der Reaktionsprodukte entsteht, stellt sich die Frage, ob im Verlauf dieser Reaktion auch die unter hydrothermalen Bedingungen verlaufenden, zu StrukturS.nderungen fiihrenden Prozesse vor sich gehen.…”

Structure Refinement of the Partially Dehydrated Natural Chabazite Ca_l.57Na_0.49Al_3.39Si_8.55O₂₄·11.53H₂O

“…consensus on the positions of water molecules and exchangeable cations in chabazites had not been obtained, although considerable efforts had been devoted to the determinations of their positions [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Structure Refinement of the Partially Dehydrated Natural Chabazite Ca 157 Na 0 .4,Al 33 ,J)i 85 5024•11.53H20…”