Nature of acid sites in SAPO5 molecular sieves. Part 1. - Effects of the concentration of incorporated silicon

Three SAPO5 samples of similar chemical composition, but with varying degrees of the concentration of silicon incorporated and hence varying concentrations of amorphous material, have been characterized by i.r. spectroscopy, temperature-programmed desorption and scanning electron microscopy. Several types of hydroxyl groups were observed and attributed to SiOH groups (3745 cm^-1), POH groups (3678 cm^-1) and SiOHAl groups (3626, 3605 and 3518 cm^-1). The relative intensities of the latter bands increased with increasing crystallinity of the material. The band at 3626 cm^-1 was attributed to SiOHAl groups pointing into the twelve-membered ring. Because the band at 3518 cm^-1 disappeared after the material was put in contact with ammonia, but not benzene, and because of its low wave-number, we conclude that the corresponding hydroxyl group is located in a six-membered ring. The shoulder at 3605 cm^-1 could not be assigned unequivocally. These three strong Brönsted-acid sites are caused by formal substitution of phosphorus by silicon in the ALPO5 lattice. Their acid strength is considerably lower than that of the Brönsted-acidic hydroxyl groups in high-silica zeolites and does not depend upon the concentration of silicon incorporated into the lattice or the amorphous phases present.