Hélène Lauron-Pernot

Efficient base catalysts are required for the conversion of oxygenated molecules, such as those derived from biomass. In this context, zinc silicates are promising candidates, as they combine a moderate Lewis base with a cation of moderate Lewis acidity. In this work, we show that co‐precipitation yields novel bulk materials whose composition can be tuned by controlling the synthesis pH. Their specific surface areas were determined by N ₂ physisorption, and XRD revealed ill‐crystallized silicate phases. Their acid–base properties, probed by the conversion of 2‐methyl‐3‐butyn‐2‐ol (MBOH), are strongly correlated with the Zn/Si ratio: the reactivity of MBOH through the basic pathway increases with the synthesis pH, and hence with the Zn/Si molar ratio. Infrared spectroscopy of adsorbed pyridine and CO shows that both the amount of silanol groups and the strength of the Lewis acid sites associated with zinc species are strongly affected by the synthesis conditions. Finally, the role of residual K ⁺ cations, whose content varies in parallel with the Zn/Si ratio, is discussed, and a tentative description of the acid–base pair sites is proposed.

Synthesis process tunes the acid–base properties of magnesium silicates leading to high selectivity in valuable pentenoates.

Magnesium silicates combining acidic and basic surface properties are known to be interesting as heterogeneous catalysts. Nevertheless, their catalytic performances are highly dependent on the synthesis method used. In this study, a series of magnesium silicates was synthesized for the first time using a coprecipitation method with a micromixer. It is first shown that changes in synthesis/precipitation pH led to magnesium silicates with different Mg/Si ratios: the higher the synthesis pH, the higher the Mg/Si ratio. Moreover, prepared silicates with a final Mg/Si ratio greater than 0.7, thus prepared at high pH, exhibit negligible specific surface area, whereas relatively high values (>180 m²/g) have been obtained for lower Mg/Si ratios. A set of experimental characterization data obtained by N₂ physisorption, SEM, XRD, TGA-DTA as well as Raman and ²⁹Si NMR spectroscopies are presented and discussed. They show the existence of two distinct families with a similar Magnesium Silicate Hydrate (MSH) phase, but they reveal different aggregation states and textural properties. Finally, the surface acid–base reactivity of the co-precipitated magnesium silicates was determined using the model reaction of 2-methylbut-3-yn-2-ol (MBOH) conversion. The results obtained suggest that it is possible to prepare silicates with a wide range of surface acid–base properties, from purely basic solids to those with both acidic and basic properties, by adjusting the final Mg/Si ratio via the control of the synthesis parameters.

Choosing acido-basic properties of magnesium silicates to tune the transesterification mechanism in the liquid phase.