Incorporation of vanadium into the framework of hydroxyapatites: importance of the vanadium content and pH conditions during the precipitation step

Abstract

Even though vanadium-modified hydroxyapatite (V-HAp) samples are very promising systems for oxidative dehydrogenation of propane, the incorporation of vanadium into the hydroxyapatite framework was reported to be limited and to lead to over-stoichiometric compounds. Here, the synthesis of a Ca₁₀(PO₄)_6−x(VO₄)_x(OH)₂ stoichiometric solid solution using a co-precipitation method is monitored in the whole composition range (0 ≤ x ≤ 6) by controlling the pH of the precipitation medium, with continuous (the first series of samples) or periodic (the second series of samples) addition of NH₄OH during the precipitation step or during the maturation step, respectively. It is demonstrated that the changes in pH conditions result in materials of a substantial difference in terms of the final composition. From XRD patterns and Rietveld refinements, a solid solution V-HAp phase was found to be exclusively obtained for the first series of samples for x varying from 0 to 6. This also occurred in the second series of samples but only for x lower than 4. For 4 ≤ x ≤ 5.22, the materials were composed of a mixture of V-HAp and Ca₂V₂O₇, whereas for a x value of 6 only Ca₂V₂O₇ was formed. The predominance of polymeric V species in solution at a high vanadium concentration deduced from the diagram of speciation of vanadium accounts for the preferential formation of Ca₂V₂O₇ under these particular conditions. However, provided that a higher pH value was maintained, isolated VO₃(OH)²⁻ species are predominant, which accounts for the incorporation of isolated vanadates into the hydroxyapatite framework and for the well-controlled stoichiometry with Ca/(P + V) ratios found to be close to 1.67. Such a very good accommodation of vanadium in the hydroxyapatite framework is illustrated by the characterization of the local surrounding of phosphorus and vanadium species using ³¹P and ⁵¹V NMR, Raman and UV-vis spectroscopies.