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

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, Mössbauer, 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 O2. 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.