Comparative Evaluation of Mn(II) Framework Substitution in MnAPSO-34 and Mn-impregnated SAPO-34 Molecular Sieves Studied by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopy

Abstract

MnAPSO-34 and Mn-impregnated SAPO-34(Mn-SAPO-34) sample were prepared with various manganese contents and studied by electron spin resonance(ESR) and electron spin echo modulation(ESEM). Electron spin echo modulation analysis of 0.07mol % Mn(relative to p) in MnAPSO-34 with adsorbed D2O shows two deuteriums at 0.26 nm and two at 0.36 nm from Mn. This suggests that two waters hydrate an MnO4 configuration with a D-O bond orientation for the waters as expect for a negatively charged site at low manganese content (0.1 mol%), the ESR spectra of MnAPSO-34 and MnH-SAPO-34 exhibit the same parameters(g  2.01 and A  89 G), but the spectra obtained from MnAPSO-34 samples are better resolved. The decomposition temperature of as-synthesized MnAPSO-34 were in the range of 200 – 600 oC of the morpholine which is 12 oC higher than that in as-synthesized MnH-SAPO-34. Infrared spectra showed that the position of a band at 3450 cm-1 shifted about 15 cm-1 toward higher energy in MnAPSO-34 versus MnH-SAPO-34. The modulation depth of the two-pulse ESE of MnAPSO-34 with adsorbed D2O is deeper than that of MnH-SAPO-34 with adsorbed D2O. Three-pulse ESEM of MnAPSO-34 and MnH-SAPO-34 with adsorbed deuterium oxide shows that the local environments of manganese in the hydrated samples are different, suggesting that Mn(II) is framework substituted in MnAPSO-34 since it obviously occupies an extraframework position in MnH-SAPO-34.