Electron spin resonance of high-spin cobalt in microporous crystalline cobalt-containing aluminophosphates

Four highly crystalline, cobalt-containing microporous aluminophosphates (C oAPO-5, CoAPO-11, CoAPO-44, and CoAPO-46) have been investigated by using l iquid He X- and/or Q-band electron spin resonance (ESR) spectroscopy in ord er to investigate the coordination of high-spin cobalt before and after cal cination. The ESR spectra of the four zeolite structures are:characterized by an axial signal with an effective g(perpendicular to) approximate to 5.8 0-5.44 and g(parallel to) approximate to 2.00. Quantitative temperature dep endence measurements of this axial signal in the temperature range 4-30 K r eveal a Curie-Weiss behavior for both as-synthesized and calcined samples c onfirming (a) the m(s) = +/- 1/2 ground state of magnetically isolated high -spin cobalt and (b) a zero field splitting Delta > 0 cm(-1). Quantitation of the ESR signals indicated that most of the Co2+ is ESR active and that o nly about 30% of this Co2+ can be oxidized to the ESR-inactive Co3+ after c alcination. The spin Hamiltonian parameters of as-synthesized and calcined CoAPO-5 material, as determined by spectrum simulation and the microwave po wer saturation technique, support the presence of framework Co2+ in a flatt ened or elongated tetrahedron (D-2d).