DRIFTS study of surface of γ-alumina and its dehydroxylation

gamma-Alumina surfaces pretreated at different temperatures (25 to 450 degrees C) and their rehydration after pyridine adsorption were studied using the conventional pyridine adsorption/DRIFTS technique. The results show that hydration at ambient temperature cannot convert all of the surface-coordinatived unsaturated Al sites to six-coordination, and weak and medium strong Lewis acid sites can still be observed for the fully hydrated gamma-alumina. Dehydration at above 200 degrees C starts to generate strong, Lewis acid sites. The number of weak, medium strong, and strong Lewis acid sites changes with dehydration temperature and all types of Lewis acid sites have adjacent OH groups. A good correlation between the number of Lewis acid sites and the intensity of perturbed OH groups is observed. Rehydration of the pyridine-adsorbed 450 degrees C pre-dehydrated alumina leads to removal of the pyridine adsorbed on the strong and medium strong Lewis acid sites and recovery of the surface of the original alumina. Water cannot replace the pyridine adsorbed on the weak Lewis acid sites.