MAGNETIC-RESONANCE STUDIES OF SAPO-44 AND MNAPSO-44

A variety of magnetic resonance experiments were performed to investigate the local environment of Mn in MnAPSO-44. MnAPSO-44 is a member of the aluminophosphate molecular sieves family and has a chabazite-like structure. Two samples with different Mn contents (Mn/(P + Al + Si) = 0.9 and 0.07 atom %) in their as-synthesized, calcined hydrated and dehydrated forms were studied. The P-31, Si-29, and Al-27 MAS NMR spectra are similar to those of the corresponding SAPO-44 samples showing only one type of TO4 tetrahedra. In the hydrated sample an Al-27 signal at -13 ppm, characteristic of octahedral Al, appears as well due to water coordination. EPR spectra were measured at X- and Q-band. The Mn(II) in the as-synthesized and calcined samples showed hyperfine splittings of 85 and 93 G, respectively, the latter being characteristic of octahedral environment. Dehydration at 400-degrees-C reduced the hyperfine constant to 65 G, indicating a change to tetrahedral coordination upon water removal. The nuclei in the immediate surrounding of the Mn were probed by the electron spin echo envelope modulation (ESEEM) technique. Both P-31 and Al-27 modulations were observed. The EPR and ESEEM results are interpreted in terms of Mn incorporation into tetrahedral framework sites for the sample with the low Mn content. The spatial distribution of the Mn throughout the sample was investigated by the ''2 + 1'' electron spin echo (ESE) experiment. It was found that only about 15% of the Mn(II) is homogeneously distributed and contributes to the echo signal.