Microwave synthesis of MnAPO/MnAPSO-44 and MnAPO-5 - Stability of Mn2+ ions on framework positions

Aluminophosphate molecular sieves of structure 44 and structure 5 with incorporation of manganese (up to 11 mass% MnO) were prepared by microwave synthesis. The MnAPO samples were characterized in comparison to the corresponding SAPO samples by X-ray diffraction, light microscopy, infrared spectroscopy, thermal analysis, ESR spectra, nitrogen adsorption and calorimetric measurements of the heat of NH3 adsorption. Manganese replaces aluminium on tetrahedrally coordinated framework positions. The ESR measurements reveal a cluster-like arrangement of Mn2+ ions (Mn2+-O-P-O-Mn2+). The as-synthesized samples are colourless. After calcination in air under static conditions a blue/deep blue colour is observed which is associated with the full adsorption capacity. The adsorption of polar molecules, as water and ammonia, causes marked structure destruction, and the colour changes from blue to pink or to grey shades. P-OH groups and Lewis acid sites are detected by infrared spectroscopy.