Location of Mn(II ) ions in manganese aluminophosphate molecular sieves: a comparative study of MnAPO-11 and MnAPO-41

Abstract

Manganese-incorporated aluminophosphate molecular sieves, MnAPO-11, MnAPO-41 and MnSAPO-41, were prepared by hydrothermal methods. The samples were characterized by the techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric/differential thermal analysis (TGA/DTA), magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy (31P and 27Al ) and electron spin resonance (ESR) spectroscopy. Thermal analysis data reveal that complete oxidation of the template takes place at higher temperatures in the manganese-containing samples than in the AlPO4s. The greater difficulty in template removal from manganesecontaining samples is attributed to the presence of protonated template ions compensating the negative charge created by manganese present in the framework. ESR studies reveal that a larger fraction of the manganese in AlPO4-11 is present in the framework than in AlPO4-41. At least four different Mn2+ species in both framework and extraframework positions could be identified in the samples.