ESR Fine Structure of Manganese Ions in Zeolite A Detects Strong Variations of the Coordination Environment

The electron spin resonance spectra of Mn 2+ exchanged zeolite A have been investigated as a function of the monovalent co-cation (K +, Na +, Li +, Cs +, or NH 4 + ), Mn 2+ content, recording frequency, and temperature. Three new Mn 2+ species are observed with a well-resolved fine structure; this allows for the first time a direct quantitative determination of the zero-field splitting (ZFS) parameters in zeolites. In hydrated zeolites, three ESR active Mn 2+ species are observed, characterized by different values for the ZFS parameter D . Species I has D = 0.035 cm -1 . Species II is closer to a regular octahedron, with D = 0.010 cm -1 . Species III, with D = 0.14 cm -1, is in a strongly axially distorted coordination. Species I is dominant in MnKA, MnCsA, and MnNH 4 A, while II and III are found in MnNaA and MnLiA. In fully dehydrated zeolites, two species are observed. Species IV has a small hyperfine constant A and is present in dry NaA and KA. Species V is observed in dry LiA; it has axial symmetry with a large, temperature-dependent D . Species V probably represents Mn 2+ in a 3-fold coordination in a 6-ring. In partially hydrated zeolites, a tetrahedral species VI is observed. The spectroscopic data elucidate the location of manganese(II) ions in zeolite A, particularly at relatively low metal loadings.