New insights into the transformation of ethanol to butadiene over MgO-SiO2 catalysts, prepared by means of the sol-gel technique, have been gained via characterization, catalytic tests, and in situ infrared diffuse reflectance spectroscopy. Catalysts with low Si-content, i.e., with a Mg/Si atomic ratio in the range between 9 and 15, gave superior butadiene yields, because of the proper combination of strong basic sites, required for ethanol activation, and a moderate number of medium-strength acid sites, needed for the dehydration of intermediately formed alkenols to butadiene. A model of the medium-strength, Lewis-type acid sites, consisting of Mg2+ cations with neighbouring Si4+ atoms, has been proposed; these sites are converted into Bronsted sites in the presence of water, a co-product of the multistep transformation of ethanol into C-4 molecules.
