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 manganese-containing 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 extra-framework positions could be identified in the samples.