Topological model for the compared acidity of SAPOs and SiAl zeolites

For both SiAl zeolites and SAPOs, the protons belong to bridging hydroxyls in the Si-OH-AL species. To make a comparison possible, the acid sites are described by the environment of Si atoms in such a moiety. It is shown that in SiAl zeolites and in SAPOs, the occupation by Al, Si, or P of the first and second shell of tetrahedra around a central Si atom governs the acid strength through the electronegativity of atoms and the connectivity of tetrahedras. In SAPOs, the strength of acidity increases first, as the first layer contains more Si, and, second, as the second layer is richer in P. Since Si-O-P bonds are not likely, the addition of Si above an upper limit depending on the structure type tends to generate siliceous islands. This increases the acid strength by forming Si (nAl) species, with n < 4, at the border of the islands. The number of species and the value of n depend on the size of the islands, itself directly related to the Si content and to the connectivity of tetrahedra, i.e., to the topology of the structure. The formation of pure Si islands decreases the expected number of protons per Si. A correlation is described in both SiAl zeolites and SAPOs between the ranking of acid strengths and the chemical shift of 29Si in MAS n.m.r. when the type and number of first and second neighbors (Al, Si, or P) is changing.