In situ FT-IR microscopic investigation of metal substituted AlPO₄-5 single crystals

First direct spectroscopic evidence for the existence of Bronsted 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 hand 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 (V(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.