Role of Magnesium Silicates in Wet-Kneaded Silica–Magnesia Catalysts for the Lebedev Ethanol-to-Butadiene Process

Wet-kneading is a technique commonly used for the synthesis of SiO₂–MgO catalysts for the Lebedev ethanol-to-butadiene process, with catalyst performance known to depend heavily on the preparation parameters used in this method. Here, the large influence of Mg precursor and MgO content on morphology, chemical structure (as determined by TEM­(-EDX), FT-IR, XRD and solid-state ¹H–²⁹Si cross-polarized MAS NMR), and on catalyst performance is demonstrated. The Mg precursor used is found to influence the extent of magnesium silicate formation during wet-kneading, as estimated from TEM and FT-IR, which, in turn, was found to correlate with catalyst performance. Accordingly, the catalyst synthesized from a nanosized Mg­(OH)₂ precursor (SiO₂–MgO (III)_nano), showing the highest degree of chemical contact between the SiO₂ and MgO components, gave the highest butadiene yield. Variation of the Mg/Si ratio in a series of SiO₂–MgO (III)_nano materials showed a volcano-type dependence of the butadiene yield on MgO content. ¹H–²⁹Si CP-MAS NMR studies allowed for the identification of the type and an estimation of the amount of magnesium silicates formed during wet-kneading. Here, we argue that the structural characteristics of the hydrous magnesium silicates, lizardite and talc, formed during catalyst preparation, together with the ratio of the magnesium silicates to MgO, determine the overall acid/base properties of the SiO₂–MgO (III)_nano catalyst materials and as a result, catalyst performance.