Resolving Mn framework sites in large cage aluminophosphate zeotypes by high field EPR and ENDOR spectroscopy

Abstract

The structural features of Mn(II) incorporated into two large cage zeotypes, Mn-UCSB-10Mg and Mn-UCSB-6Mg, were explored by combining multifrequency CW-EPR with W-band ENDOR spectroscopy. As-synthesized samples, dried both at room temperature and 150 °C, were examined and the results were compared with the reference samples Mn-AlPO₄-20 and Mn-AlPO₄-5. High-frequency CW EPR experiments (at W- and G- bands) resolved two main types of Mn(II) framework sites with significantly different ⁵⁵Mn hyperfine couplings and slightly different g values. These results were further corroborated by ⁵⁵Mn ENDOR spectra, which enabled a more accurate determination of the two hyperfine coupling values and revealed the presence of a third species. ENDOR experiments carried out at magnetic fields away from the central, |−1/2,m_I〉 → |+1/2,m_I〉 EPR transitions, established a negative sign for A_iso(⁵⁵Mn). By comparison with as-synthesized samples that were mildly dehydrated the various species were assigned to framework sites with different degrees of water coordination. While one species is similar to the distorted (pseudo) tetrahedral sites found in the reference Mn-AlPO₄-20,5 samples, the other two experience interaction with weakly bound water ligands. The transformation between the three types upon dehydration and rehydration is reversible. In an attempt to improve the spectral resolution, W-band EPR and ENDOR measurements were carried out on single-crystals of Mn-UCSB-10Mg (typical size of ∼0.02 mm³). Similar to the polycrystalline sample, two main A_iso(⁵⁵Mn) components were resolved in the EPR spectra, their relative populations, however, differed from that of the polycrystalline material. This difference is attributed to variations in the water content originating from a crystal size effect. Surprisingly, the single crystal spectra did not show better resolution, and moreover, they did not exhibit significant orientation dependence in most of the experiments. These findings are ascribed to the presence of several chemically distinguishable sites combined with the multiplicity of symmetry related tetrahedral sites in one zeotype unit cell. Such a situation leads to an effective ‘powder like’ spectrum due to the small anisotropy of the magnetic interactions involved.