The Process of Mn(II) Incorporation into Aluminophosphate Zeotypes through High-Field ENDOR Spectroscopy and DFT Calculations

Abstract

High-field EPR and pulsed electron−nuclear double resonance (ENDOR) spectroscopies were used to investigate the formation of Mn−AlPO4-11, Mn−AlPO4-5, and Mn−SAPO-5. Samples recovered from reaction mixtures quenched at different times were subjected to EPR, ENDOR and X-ray diffraction (XRD) measurements, and the variations in the 31P and 1H hyperfine couplings, which are sensitive probes to the Mn−P interaction and the Mn(II) hydration, respectively, were followed. The intensity of the 1H ENDOR signal decreased with reaction time, showing that the amount of both water ligands and solvent water in the Mn(II) vicinity decreased. A relatively large isotropic 31P hyperfine coupling (Aiso(31P) ≈ 7 MHz), confirming the formation of Mn(II) framework sites, was found in all final products, whereas a smaller Aiso(31P), 4−5 MHz, was detected in samples quenched at early stages of the reaction. The latter was assigned to Mn(II) incorporated into a network of disordered aluminophosphate precursors. These precursors are formed prior to the detection of an XRD pattern, and are gradually transformed to the final three-dimensional crystalline structures. The changes in Aiso(31P) were attributed to transformations occurring both in the bonding topology and in the coordination sphere of Mn(II), where water ligands are gradually replaced by −O−P linkages. This interpretation was supported by the decrease in the intensity of the 1H ENDOR signals, and by a series of DFT cluster model optimizations on intermediates of the form [Mn(H2O)x(OP(OH)3)y]2+, where x + y = 6, 5 or 4, followed by calculations of hyperfine coupling constants. Although the theoretical hyperfine values were overestimated with respect to the experimental ones, a satisfactory correlation was found between the trends within the calculated Aiso(55Mn,31P), and the experimental trends observed during the molecular sieves formation.