Electron Spin Resonance of High-Spin Cobalt in Microporous Crystalline Cobalt-Containing Aluminophosphates

Four highly crystalline, cobalt-containing microporous aluminophosphates (CoAPO-5, CoAPO-11, CoAPO-44, and CoAPO-46) have been investigated by using liquid He X- and/or Q-band electron spin resonance (ESR) spectroscopy in order to investigate the coordination of high-spin cobalt before and after calcination. The ESR spectra of the four zeolite structures are characterized by an axial signal with an effective g_⊥ ≈ 5.80-5.44 and g∥ ≈ 2.00. Quantitative temperature dependence measurements of this axial signal in the temperature range 4-30 K reveal a Curie-Weiss behavior for both as-synthesized and calcined samples confirming (a) the m_s = ± 1/2 ground state of magnetically isolated high-spin cobalt and (b) a zero field splitting Δ > 0 cm^-1. Quantitation of the ESR signals indicated that most of the Co²⁺ is ESR active and that only about 30% of this Co²⁺ can be oxidized to the ESR-inactive Co³⁺ after calcination. The spin Hamiltonian parameters of as-synthesized and calcined CoAPO-5 material, as determined by spectrum simulation and the microwave power saturation technique, support the presence of framework Co²⁺ in a flattened or elongated tetrahedron (D_2d).