In situ FT-IR microscopic investigationof metal substituted AIPO4-5 single crystals

First direct spectroscopic evidence for the existence of Brønsted acid sites in AlPO4-5 (AFI) single crystal molecular sieves substituted with divalent metal cations is reported. Ammonia and pyridine were used as probe molecules. Compared with unsubstituted AlPO4-5 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 (¿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.