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Nanopowders of aluminum-substituted (0−20 mol %) hydroxyapatite (HA) with the average size of 40−60 nm were synthesized by the precipitation method from nitrate solutions. A series of samples were studied by various analytical tools to elucidate the peculiarities of Al introduction. Electron paramagnetic resonance and pulsed electron-nuclear double resonance data demonstrate that incorporation of Al resulted in a decrease in the concentration of impurity carbonate anions and lead to an increase in the number of protons in the distant environment of the impurity nitrogen species. Density functional theory calculations show that the Al 3+ incorporation is accompanied by the local positional rearrangement and the distortion of anion channel geometry. An in vitro test conducted on MG-63 cells demonstrates the cytocompatibility and magnification of the surface matrix characteristics with Al doping.

Mesoporous iron-doped hydroxyapatite (HA) powders with a surface area up to 141 m 2 /g were synthesized and characterized by a variety of analytical and computational approaches, including X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF), atomic emission spectrometry with inductively coupled plasma (AES-ICP), Fourier transform infrared absorption (FTIR), nitrogen adsorption−desorption (BET), scanning and transmission electron microscopy (SEM and TEM), electron paramagnetic resonance (EPR), and density functional theory (DFT). Based on the data of TEM with mapping, the homogeneous distribution of Fe was evidenced. Fe 3+ ions were detected by EPR, and according to DFT, Fe 3+ occupied the Ca(2) position. The second part of the manuscript was dedicated to evaluating the catalytic properties of the developed HA powders for oxidative desulfurization, which is a promising alternative to hydrotreating for fuel purification. For this purpose, the molybdenum was impregnated on the HA, and iron-HA powders and the influence of its amount and the iron content were investigated. The optimal process parameters such as rotation speed, amount of H 2 O 2 , reaction time, temperature, and quantity of the catalyst were established and for the first time, to the authors' best knowledge, complete conversion of dibenzothiophene in the presence of an HA-based catalyst was achieved due to a combination of active sites of iron cations and molybdate anions.

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