High Field³¹P ENDOR of MnAlPO₄-20:  Direct Evidence for Framework Substitution

Abstract

The incorporation of Mn(II) into framework sites in the aluminophosphate zeotype AlPO 4 -20, an analog of sodalite, has been investigated using pulsed electron nuclear double resonance (ENDOR) spectroscopy at 95 GHz. The field sweep echo-detected EPR spectrum showed the presence of a single Mn(II) site with a 55 Mn hyperfine coupling of 8.7 mT. ENDOR spectra, recorded using the Mims and Davies sequences, consist of an 27 Al signal at the Larmor frequency and a 31 P doublet corresponding to a hyperfine coupling of 8 MHz. The symmetry of the doublet about the 31 P Larmor frequency indicates that it originates from the M S = ± 1 / 2 manifolds and that the interaction is primarily isotropic. The relatively large 31 P hyperfine interaction and the weak interaction with 27 Al provide unique and direct evidence for Mn(II) substitution of framework Al. X-band electron spin echo envelope modulation (ESEEM) measurements showed only signals at the 1 H, 14 N, 27 Al, and 31 P Larmor frequencies. The first two are due to weak dipolar interaction with the template molecules, while the others reflect interactions with the framework.