Abstract
First direct spectroscopic evidence for the existence of Brønsted acid sites in AlPO 4-5 (AFI) single crystal molecular sieves substituted with divalent metal cations is reported. Ammonia and pyridine were used as probe molecules. Compared with unsubstituted AlPO 4-5 single crystals, the metal substituted samples exhibit well-developed bands of isolated hydroxyl groups and a broad absorption band indicating the existence of hydrogen bonded OH groups. Upon adsorption of pyridine on the metal substituted samples the formation of a new, relatively free vibrating hydroxyl group ( ν oh = 3690–3694 cm −1 was observed, giving evidence for a strong Lewis type interaction leading to the cleavage of a metal-oxygen bond close or at the position of the substituting metal cation.
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