ESR fine structure of manganese ions in zeolite A detects strong variations of the coordination environment

The electron spin resonance spectra of Mn²⁺ exchanged zeolite A have been investigated as a function of the monovalent co-cation (K⁺, Na⁺, Li⁺, Cs⁺, or NH₄⁺), Mn²⁺ content, recording frequency, and temperature. Three new Mn²⁺ 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 Mn²⁺ 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 MnNH₄A, 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 Mn²⁺ 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.