Spectroscopic evidence for the presence of internal silanols in highly siliceous ZSM-5

Preparación y caracterización de la zeolita MCM-22 y de su precursor laminar

“…Este tipo de bandas asociadas a silanoles internos pueden verse afectadas por la localización de éstos en la estructura. Así, teniendo en cuenta que en esta zeolita existen supercavidades delimitadas por anillos de 12 MR y canales de 10 MR, y considerando que este tipo de bandas aparecen en la zeolita Y a ~ 3700 cm -1 y en la ZSM-5 centradas en ~ 3500 cm -1 [16][17][18] , la banda que aparece a mayor frecuencia de vibración podría corresponder a los silanoles en defectos producidos en las supercavidades, mientras que los que aparecen a ~ 3500 cm -1 se asignarían a defectos en los canales de 10 MR similares a los de ZSM-5. Al disminuir el contenido en aluminio de las muestras la cantidad de silanoles externos decrece, aumentando la de silanoles internos.…”

Hypothetical Structures of Magadiite and Sodium Octosilicate and Structural Relationships Between the Layered Alkali Metal Silicates and the Mordenite- and Pentasil-Group Zeolites

“…Considering that the number of defect sites in ZSM-5 decreases as the AI content of the solids increases (Woolery et al, 1986), the zeolitic network is probably stabilized if A1 is present during the formation of the zeolite. Gatti et al (1986) found that magadiite or ZSM-5-type structures are synthesized together from mixtures having 5iO2/A1203 > 100, and magadiite is the preferred product if A1 is almost totally absent.…”

“…Recent IR and NMR evidence has suggested the presence of intemal Si-OH groups in samples of highly siliceous ZSM-5 (Woolery et al, 1986;Boxhorn et al, 1984;Brtmner, 1987). If zeolite-like domains exist in magadiite as postulated above, the presence of internal silanol groups in siliceous zeolites may possibly be tak- The a-and b-axis projections of the idealized structure of ZSM-5 (Kokotailo et al, 1978) and the c-axis projection of mordenite (Meier, 1974) are shown in Figure 7.…”

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

“…For H/ZSM5-17 two intense bands were observed at 3590 and 3740 cm −1 . They are assigned respectively to Brønsted acid groups (associated with framework aluminum (Si(OH)Al)) and to terminal silanol groups (located inside the zeolite channels and on the external surface) [46][47][48], respectively. A small shoulder detected at 3650 cm −1 is attributed to hydroxyl groups of extra-framework aluminum species [49].…”