Jun Kubo

Influence of Preparation Factors on Ca/P Ratio and Surface Basicity of Hydroxyapatite Catalyst

We aimed at precision control of the basicity of hydroxyapatite (Ca10(PO4)6(OH)2; HAP) and paid attention to four key factors in the synthesizing process—the solutions Ca/P ratio, pH, temperature and stirring time—and compared their influence on the Ca/P ratio and basicity of HAP. Among them, solution pH had the greatest influence on Ca/P ratio within the bulk of the catalyst. This was followed in order of influence by the Ca/P ratio of the solution (the ratio of raw materials), solution temperature and stirring time. In addition, we found that samples of HAP with the same Ca/P ratio had almost the same basic site density, even if they were synthesized using different combinations of preparation factors. This indicates that the basic site density of HAP prepared by the precipitation method depends only on the Ca/P ratio within the bulk of the catalyst.

Control of calcium accessibility over hydroxyapatite by post-precipitation steps: influence on the catalytic reactivity toward alcohols

Hydroxyapatites are increasingly used as heterogeneous catalysts since they present atypical behaviours for many acid base reactions. The aim of this study was to discuss the possible involvement of Ca2+ Lewis and/or PO-H Brønsted acid sites belonging to the hydroxyapatite system in the conversion of 2-methylbut-3-yn-1-ol, a model molecule that is known to account for the acid base properties, and of ethanol into n-butanol. A series of hydroxyapatite samples with similar bulk properties was prepared from a lone precipitation batch, but by varying the conditions of the washing and drying steps. Although the surface depth probed by XPS exhibited similar average composition, ISS analysis revealed a gradient of calcium concentration in the first surface layers. In fact, the different conditions of drying and washing resulted in a modulation of the relative amount of Ca2+ and PO-H accessible on the top surface, as revealed by the adsorption of the CO molecule monitored by FTIR. The conversion in the two alcohol molecules is linearly dependent on the nature of the acid base pairs involved: when accessible on the top surfaces, due to their stronger acidity, the Ca2+ Lewis acid sites are preferentially involved, but they are less efficient than PO-H, as illustrated by the linear decrease of the conversion levels with the increasing relative amount of accessible Ca2+ cations. It is thus concluded that PO-H sites enhance the performances of the catalysts for the two reactions, and that washing and drying conditions allowing us to decrease the calcium accessibility at the benefit of PO-H should be favoured.