Cobalt-exchanged hydroxyapatite catalysts: Magnetic studies, spectroscopic investigations, performance in 2-butanol and ethane oxidative dehydrogenations

A series of exchanged cobalt/calcium (Co2+/Ca2+) hydroxyapatite Ca10−xCox(PO4)6(OH)2 was synthesized and characterized by XRD, UV-visible-near-infrared (NIR) and IR spectroscopy, magnetic measurements (SQUID), and X-ray photoemission spectroscopy (XPS). The level of Co2+/Ca2+ exchange was limited to 1.35 wt% Co. After calcination in air at 550 °C, cobalt was still present as Co2+ and all samples were paramagnetic, showing that the apatite matrix impedes the oxidation of Co2+ and that the Co2+ ions are isolated, whatever the Co content. Magnetic measurements and UV-visible diffuse reflectance spectra show that the exchanged Co2+ ions are hosted by two types of sites (with octahedral and trigonal prismatic symmetries). XPS confirmed the surface cobalt enrichment and did not reveal Co3+ ions. Dehydrogenation of 2-butanol leads almost exclusively to the formation of butanone. As the Co content increases, the ketone yield passes through a maximum. In the oxidative dehydrogenation of ethane, the ethylene yield also reaches a maximum (22 mol%) for 0.96 wt% Co at 550 °C. These results are ascribed to (i) the partial compensation of the intrinsic dehydrogenating activity of cobalt by the decrease in basicity of apatite induced by the replacement of Ca2+ by Co2+, and (ii) the involvement of two types of sites.