Journal of the American Chemical Society, Vol.121, No.14, 3368-3376, 1999
Five-coordinate silicon in high-silica zeolites
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-l) 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 Si-29 chemical 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 Silicalite-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 motion, the Si-29 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 F-19 chemical shift anisotropy: the span value, Omega = delta(11) - delta(33), is between 80 and 87 ppm.