Five-coordinate silicon in high-silica zeolites

Pentacoordinated silicon units, SiO4/2F-, were found by solid-state NMR exp eriments in various as-made high-silica zeolites (Beta, SSZ-23, ITQ-3, ITQ- 4, ZSM-12, Silicalite-l) that are prepared in the presence of fluoride ions as mineralizing agents. The SiO4/2F- units are part of the framework, bein g connected, through sharing of the four O atoms, with four neighboring tet racoordinated SiO4/2 units, These five-coordinate silicon units balance the charge of the cationic organic structure-directing agent. The Si-29 chemic al shift of these sites is between -140 and -150 ppm, and the F-19 chemical shifts are between -56 and -78 ppm. The zeolites SSZ-23, ITQ-3, and Silica lite-l 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 motio n, the Si-29 chemical shifts are between -120 and -150 ppm, indicating an e xchange between four- and five-coordinate silicon, which means that the flu oride ions exchange between different framework silicon sites. The rigid Si O4/2F- units show a characteristic parameter of the F-19 chemical shift ani sotropy: the span value, Omega = delta(11) - delta(33), is between 80 and 8 7 ppm.