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.

Ruthenium-catalyzed dehydrogenation of ammonia boranes.

The dehydrogenation of ammonia borane (AB) and methylammonia borane (MeAB) is shown to be catalyzed by several Ru-amido complexes. Up to 1 equiv of H2 (1.0 system wt %) is released from AB by as little as 0.03 mol % Ru within 5 min, and up to 2 equiv of H2 (3.0 system wt %) are released from MeAB with 0.5 mol % Ru in under 10 min at room temperature, the first equivalent emerging within 10 s. Also, a mixture of AB/MeAB yields up to 3.6 system wt % H2 within 1 h with 0.1 mol % Ru. Computational studies were performed to elucidate the mechanism of dehydrogenation of AB. Finally, it was shown that alkylamine-boranes can serve as a source of H2 in the Ru-catalyzed reduction of ketones and imines.