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 degreesC, 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. (C) 2004 Elsevier Inc. All rights reserved.