Origin of active sites on silica–magnesia catalysts and control of reactive environment in the one-step ethanol-to-butadiene process

Wet-kneaded silica–magnesia is a benchmark catalyst for the one-step ethanol-to-butadiene Lebedev process. Magnesium silicates, formed during wet kneading, have been proposed as the active sites for butadiene formation, and their properties are mainly explained in terms of the ratio of acid and base sites. However, their mechanism of formation and reactivity have not yet been fully established. Here we show that magnesium silicates are formed by the dissolution of Si and Mg subunits from their precursors, initiated by the alkaline pH of the wet-kneading medium, followed by cross-deposition on the precursor surfaces. Using two individual model systems (Mg/SiO₂ and Si/MgO), we demonstrate that the location of the magnesium silicates (that is, Mg on SiO₂ or Si on MgO) governs not only their chemical nature, but also the configuration of adsorbed ethanol and resulting selectivity. By using an NMR approach together with probe molecules, we demonstrate that acid and basic sites in close atomic proximity (~5 Å) promote butadiene formation. [Figure not available: see fulltext.].