Incorporation of Transition Metal Ions into MeAPO/MeAPSO Molecular Sieves

Abstract

The synthetic incorporation of transition metal ions into aluminophosphate and silicoaluminophosphate molecular sieve frameworks is studied by electron spin−echo modulation (ESEM) spectroscopy. Based mainly on bulk chemical composition of the product, it has been suggested that divalent and trivalent metal ions such as Mn(II), Co(II), Cr(III), and Fe(III) substitute for framework aluminum sites. On the basis of 31P ESEM results, we originally suggested a framework phosphorus site for the incorporation of metal ions such as Ni(II) and Cr(III) in NiAPSO-5 and CrAPSO-5 molecular sieves. This site was later supported for other metal ions such as Ti(IV), V(IV), and Cu(II). In this work, we have extended such studies to Mn(II) and Fe(III) to better understand the metal substitution sites in AlPO4 frameworks. Irrespective of the particular metal ion, the 31P ESEM spectra of MeAPO/MeAPSO molecular sieves can be interpreted best on the basis of metal incorporation at a framework phosphorus site. Strong 27Al modulations are also observed for these materials, which supports the 31P ESEM result that incorporated metal ions are located nearer to aluminum than phosphorus.