Surface Interaction of Fluorine with Silica- and Alumina-Based Materials

6 Surface interaction of fluorine with silica- and alumina-based materials. 

Delmotte et al. (1990) used F NMR to study crystalline microporous alumino- and gallosilicates synthesised in a fluoride-containing medium. The spectra contained a resonance at - 129 ppm, assigned to Si-F species. The spectra of samples prior to calcination showed a peak at — 155 ppm, as was also found in a calcined gallium-containing sample. A signal observed in the as-synthesised materials at — 64 to — 75 ppm was attributed to ion-paired free fluoride. 

Adsorption of gas molecules can be used to characterise the active sites in zeolites. Xenon may be used for this purpose and monitored by Xe NMR (see Section 9.3.7), but adsorption of CF4, monitored by F NMR, has been suggested as an alternative (Yang et al. 2001). Such measurements place strong constraints on the modelling of the molecular level adsorption. 

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