No abstract

Pentacoordinated silicon units, SiO4/2F-, were found by solid-state NMR experiments in various as-made high-silica zeolites (Beta, SSZ-23, ITQ-3, ITQ-4, ZSM-12, Silicalite-1) that are prepared in the presence of fluoride ions as mineralizing agents. The SiO4/2F- units are part of the framework, being connected, through sharing of the four O atoms, with four neighboring tetracoordinated SiO4/2 units. These five-coordinate silicon units balance the charge of the cationic organic structure-directing agent. The 29Si chemical shift of these sites is between −140 and −150 ppm, and the 19F chemical shifts are between −56 and −78 ppm. The zeolites SSZ-23, ITQ-4, and Silicalite-1 show a dynamic motion of the fluoride ions at room temperature which is frozen out at a temperature of 130−140 K. In the case of fluoride motion, the 29Si chemical shifts are between −120 and −150 ppm, indicating an exchange between four- and five-coordinate silicon, which means that the fluoride ions exchange between different framework silicon sites. The rigid SiO4/2F- units show a characteristic parameter of the 19F chemical shift anisotropy:  the span value, Ω = δ11 − δ33, is between 80 and 87 ppm.

No abstract

Brønsted acid sites (BAS) in dry aluminosilicate zeolites, also known as bridging O H −H groups, may form hydrogen bonds with other framework oxygen atoms, O, in addition to BAS that are free of such interactions. These are referred to as perturbed and unperturbed BAS, respectively. However, hydrogen bonding may occur only for specific geometric orientations between two framework oxygen atoms, allowing an O H −H•••O interaction. To evaluate the possibility for such hydrogen bonding, we introduce an alignment angle κ that describes the angle between the Si−O H −Al bisector and the O H −O direction of a potential hydrogen-bonding donor−acceptor pair. A total of 27 zeolite framework topologies were analyzed, and the relevance of this approach was subsequently demonstrated for H-ZSM-5 with two different Si/Al ratios. Here, it could be unequivocally shown by advanced solid-state NMR methods that the debated 1 H NMR signal at ∼6 ppm arises due to the formation of hydrogen-bonded BAS.