Iron-calcium-hydroxyapatite catalysts: Iron speciation and comparative performances in butan-2-ol conversion and propane oxidative dehydrogenation

Iron/calcium-hydroxyapatite catalysts containing different amounts of iron - Fe(x)/CaHAp - were prepared by cation exchange in aqueous media and characterized by XRD, IR, UV-vis, Mossbauer. XPS spectroscopies and temperature programmed reduction (TPR). The iron was found in the samples exclusively as isolated Fe3+ species distributed in CaHAp between distorted octahedral and lower coordination sites. These catalysts were tested in butan-2-ol conversion with and without oxygen in the reaction mixture. The increase of iron content enhanced the acid properties of the catalysts and the butan-2-ol dehydration (butenes yield around 55%) in the presence of O-2. In propane oxidative dehydrogenation (ODH), they exhibit good propene selectivity (35-90%) but with limited conversion. The maximum propene yield (6.2%) is obtained at low Fe content whereas the dehydrogenation of butan-2-ol into methyl ethyl ketone (MEK) in the presence of dioxygen is much higher on pure CaHAp than on Fe(x)/CaHAp. The sites generated at low Fe content were identified as Fe3+-O-Ca2+ species. Those sites, contrarily to Fe3+-O-Fe3+ which appear at high iron loadings, provide a reasonable performance in propane ODH, most likely because they maintain the needed basicity for the hydrogen abstraction from the propane. This confirms the essential role played by the acid-base properties in the dehydrogenation processes. (C) 2010 Elsevier B.V. All rights reserved.