MULTINUCLEAR MAS NMR AND IR SPECTROSCOPIC STUDY OF SILICON INCORPORATION INTO SAPO-5, SAPO-31, AND SAPO-34 MOLECULAR-SIEVES

Al-27, P-31, Si-29, and H-1 MAS n.m.r. as well as diffuse reflectance i.r. spectroscopy were used to investigate the isomorphous substitution of silicon into the framework of aluminophosphate molecular sieves with different pore sizes. Si-29 MAS n.m.r. was found to be the only direct method to study this type of silicon incorporation. In all SAPOs investigated, the incorporation of silicon generates two different kinds of Bronsted acidic sites irrespective of the number of nonequivalent T sites in the framework. The results of both the i.r. spectroscopy and the H-1 MAS n.m.r. spectroscopy in the presence of sorbed molecules corroborate the commonly accepted assignment of these two different species to undisturbed hydroxyls and hydroxyls interacting with further framework oxygen, respectively.