Jean-Marc Krafft

Insight into the structure and localization of the titania overlayer in TiO2-coated SBA-15 materials

Titania-coated SBA-15 materials (Ti-SBA-n with n = 1–4) were synthesized by successive grafting of titanium isopropoxide in iso-propanol. The investigation of the nature and localization of the TiO2 overlayer allowed us to conclude that there was formation of very small (UV-visible, EXAFS, XANES), ca. 2 nm or smaller (XRD and TEM) TiO2 domains that cover only partially the silica surface (CO adsorption). These nano TiO2 domains are located on or close to the mesopore surface (EELS, XRD and N2-sorption) rather than in the micropores as previously reported.

Influence of acid–base properties of Mg-based catalysts on transesterification: role of magnesium silicate hydrate formation

The transesterification reaction assisted through heterogeneous basic catalysis was thoroughly studied because of its importance in transforming biomass, as for biodiesel production or lactone opening. As catalysts with the strongest basic properties are not always the most efficient ones, a series of magnesium-based materials, exhibiting a large range of acido–basic properties, was investigated. Moreover, in order to compare gas and liquid phases operating conditions, a model reaction (transesterification of ethyl acetate with methanol) was chosen. It appears that gas phase transesterification (at 393 K) requires strong basic sites, whereas magnesium silicate, exhibiting moderate basicity together with acidic properties, is a very reactive catalyst in the liquid phase (at 333 K) depending on its preparation method. The set of experimental data (XRD, XPS, DRIFTS, MEB, 29Si and 25Mg NMR) demonstrated that a magnesium silicate hydrate structure (MSH) is formed at the surface of the most active silicates. It is thus concluded that different mechanisms operate under gas and liquid conditions, and that among the magnesium silicate materials, the MSH phase exhibits specific acido–basic properties beneficial to this kind of reaction.

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.

Role of Water on the Activity of Magnesium Silicate for Transesterification Reactions

This study aims to reveal how water adsorbed on to a solid can influence its catalytic properties for transesterification in liquid phase. A commercial magnesium silicate was subjected to a range of thermal pretreatments and used for the transesterification of ethyl acetate with methanol. Conversion of ethyl acetate decreased with increasing pretreatment temperature, in direct relation to the release of the water content of the magnesium silicate. Thermogravimetric analysis, diffuse reflectance infrared Fourier transform (DRIFT), and 1H NMR spectroscopies revealed that physically adsorbed water had little influence on the reactivity. The water incorporated within the catalysts, however, which desorbs at higher temperatures, played a key role on the conversion. Calorimetry, in situ DRIFT spectroscopy, and 1H NMR characterization indicate that two kinds of active site exist. These are created from the water coordinated to magnesium located on the edge of the clay‐like particles or in the defects present in the silicate layer, respectively. Their role could be to stabilize methanol deprotonated by basic Mg‐OH groups, activate the ester, or help the departure of the alkoxyl moiety.