MAS n.m.r. and e.s.r. studies of MnAlPO5

Abstract

27Al and 31P magic angle spinning ( MAS) n.m.r. and e.s.r. measurements have been performed on MnAlPO5 in order to obtain direct evidence for Mn +2 incorporation into tetrahedral sites in the framework. Two samples differing in their Mn content have been studied, and their results were compared with those obtained from impregnated Mn-AlPO5 and exchanged Mn-SAPO5 where the Mn +2 does not occupy framework sites. The MAS n.m.r. spectrum showed a large anisotropy, manifested in numerous side bands due to the paramagnetic shifts induced by dipolar interactions between the unpaired electrons of the Mn +2 and the 31P and 27Al nuclei in its vicinity. On dehydration, a considerable linewidth narrowing has been observed, attributed to a decrease in the electronic relaxation time induced by increase in Mn +2 spin-exchange interaction. Calculated 31P MAS n.m.r spectra, where the paramagnetic shift anisotropy was obtained from X-ray diffraction data of AlPO5 and taking into account the Mn content obtained from the chemical analysis, showed qualitative agreement with the experimental spectra. Comparison of the simulated and experimental spectra indicates that, indeed, part of the Mn +2 cations occupy framework sites; it does, however, also suggest that extraframework Mn +2 cations are present as well. These conclusions are further supported by e.s.r. measurements. The e.s.r. spectrum of MnAlPO5 shows resolved hyperfine lines with splitting characteristics of octahedral Mn +2, supporting the existence of extraframework Mn +2. On dehydration, the spectrum of MnAlPO5 coalesce into a single absorption line. This coalescence is attributed to increase in the spin-exchange interaction due to migration of probably extraframework Mn +2 toward the framework.