Molecular Understanding of the Bulk Composition of Crystalline Nonstoichiometric Hydroxyapatites: Application to the Rationalization of Structure-Reactivity Relationships

Crystalline hydroxyapatite samples (HAps) have been prepared by using the co-precipitation method under various pH conditions, leading to nonstoichiometric solids (1.65 < Ca/P < 1.77). The aim of this study was to rationalize the sensitivity of the catalytic activity of HAps to their bulk compositions going from the macroscopic level expressed by the Ca/P ratio to the molecular level properties of the bulk. From DRIFT, P-31 NMR and Raman characterizations, hydroxyapatites were obtained with a range of structural defects compared with the ideal stoichiometric compound. If the amount of HPO42- and B-type carbonates directly impacts the Ca/P ratio, it is not the case for A-type carbonates. All these defects, and especially the A-type carbonates, participate in the modulation of OH content inside the channels. Irrespective of the Ca/P values, the OH concentration appears to be perfectly related to the surface basic reactivity measured through 2-methyl-3-butyn-2-ol (MBOH) conversion. Thus, except for the similar carbonate content (in cases of low Ca/P values), the Ca/P ratio is not sufficient to predict the catalytic behavior of all HAps synthesized under various conditions: in the case of variable carbonate content monitored under different pH conditions, a larger range of Ca/P ratio can be obtained including over-stoichiometric HAps samples (Ca/P > 1.67), and the bulk OH concentration becomes a much better descriptor than the Ca/P ratio to account for the basic reactivity.