High resolution NMR unraveling Cu substitution of Mg in hydrotalcites–ethanol reactivity

Abstract

Hydrotalcite-type Mg6−xCuxAl2(OH)16CO3·4H2O solids (with x=0, 1, 3, 5) were synthesized as precursors of mixed oxides obtained by subsequent calcination. Two families of samples (synthesized by coprecipitation with and without sonication) were thoroughly characterized by XRD, TG–TDA, H2-TPR, NMR, SEM and N2-physisorption. Their reactivity towards ethanol was further assessed. Sonication enabled considerably reducing the preparation duration. MgO and Al2O3 were formed upon calcination of the hydrotalcites under air at 400°C, while copper-enriched phases (e.g. CuO, spinel like structure…) were formed from the solids with higher Cu contents. Magnesium atoms substitution by copper atoms in the hydrotalcite structure was evidenced for the first time in this kind of materials using high-resolution solid state NMR. Indeed, a very unusual but remarkable “Fermi contact effect” was responsible for specific features observed on the NMR spectra. Further, partial substitution of magnesium by copper led to an increase in specific surface area, with the formation of particles of a smaller size. Copper addition considerably enhanced the dehydrogenation properties of the solids at low temperature (∼250°C), with a higher reactivity towards ethanol, and higher selectivities to acetaldehyde.