AL-27 NMR, FT-IR AND ETHANOL-O-18 TPD CHARACTERIZATION OF FLUORIDED ALUMINA

A series of F/Al2O3 samples (wt% F = 0 to 20) has been studied using a combination of solid-state 27Al NMR, FT-IR, and ethanol-18O TPD techniques. Solid-state 27Al NMR is particularly sensitive to amorphous phases or small crystallites present on the catalyst surface, many of which cannot be detected by XRD. 27Al NMR shows the presence of three types of AlF3(H2O)n species (with n varying between 0 and 3) on fluorided alumina. FT-IR studies of ethanol adsorption show that fluoride blocks the sites required for dissociative chemisorption of ethanol. A similar analysis of adsorbed pyridine shows an increase in the number of Brønsted acid sites with the addition of up to 10 wt% fluoride. However, increasing the fluoride loading to 20 wt% decreases the number of Brønsted acid sites. The TPD of ethanol-18O from F/Al2O3 samples shows that at low levels fluoride serves to block Lewis acid sites, but at higher levels its predominant role is to increase the Brønsted acidity of the alumina surface. The pyridine adsorption and TPD experiments show that fluoride strengthens the remaining Lewis acid sites. © 1993 Academic Press, Inc.