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

Abstract

International audienceCrystalline 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, 31P 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

Importance of the Nature of the Active Acid/Base Pairs of Hydroxyapatite Involved in the Catalytic Transformation of Ethanol to n‐Butanol Revealed by Operando DRIFTS

“…Similar trends were also obtained for HAp‐S and when characterizing these samples by means of the MBOH model reaction (SI 1 Figure S1a). Given that MBOH conversion on HAps was shown to be dependent on the OH − concentration and that the increase of the activation temperature favors dehydroxylation leading to the formation of O 2− species (Figure B), these results raise the question of the role of OH − versus O 2− in the catalytic transformation of ethanol to n ‐butanol. Ho et al.…”

“…Besides the bands associated to the OH groups from the channels, other low intensity ν OH bands are detected at higher wavenumbers (Figure C). The bands at 3678, 3672, 3660 and 3626 cm −1 are associated to terminating protonated phosphate groups located on the surface, whereas that at 3650 cm −1 originates from defective hydrogenophosphate groups present in the bulk ,. The slight modifications of the shape of the spectra in this region are significant of different relative intensities for the various contributions.…”

“…Hydroxyapatite samples were prepared according to the procedure described previously: the co‐precipitation of Ca(NO 3 ) 2 and (NH 4 ) 2 HPO 4 solutions beforehand adjusted at pH=10 was performed at 353 K under a N 2 flow to limit carbonation of the materials. As detailed elsewhere, depending on the pH control procedures, even with a nominal Ca/P ratio of 1.67 of the precursors in the solutions, hydroxyapatite samples can be obtained with different Ca/P ratios. The three HAp samples studied in this work have been obtained by periodic, continuous or without addition of NH 4 OH during the precipitation step, respectively.…”

“…The latter was shown to be influenced to a significant extent by the presence of structural defects, as illustrated by the more general formula of HAps: Ca 10‐x‐B (PO 4 ) 6‐x‐B (HPO 4 ) x (CO 3 ) A+B (OH) 2‐x‐2A‐B , in which A and B are related to carbonates located on hydroxyl and phosphate structural sites, respectively . The concentration of OH groups was thus found to be a more relevant descriptor than the Ca/P ratio to account for the basic reactivity of HAps . The discrimination between bulk and surface species remains complex and investigations aiming at characterizing the surface at a molecular level are needed to identify the nature of the surface sites likely involved in acid‐base catalysis.…”

“…Ca/P ratios lower than the stoichiometric ratio, i. e. 1.67, would favor acidity, while higher Ca/P ratios would rather promote basicity ,. Note, however, that the acid‐base properties of HAps do not always follow the above‐mentioned guidelines based on the Ca/P ratio, especially when comparing hydroxyapatite samples prepared under different conditions . It must be stressed that the Ca/P ratio does not only depend on the calcium content of the sample but also on its phosphorus content as phosphates, which itself strongly depends on the presence of carbonates.…”