MgO/SiO2 prepared by wet-kneading as a catalyst for ethanol conversion to 1,3-butadiene: Prins condensation as the predominant mechanism

MgO/SiO2 catalysts prepared by wet-kneading with various compositions were applied to directly convert ethanol to 1,3-butadiene. The MgO/SiO2 molar ratio significantly affected the structural, textural, and surface properties of the catalysts, leading to different activities toward ethanol conversion. At low MgO loading (51 mol %), magnesium silicate is formed on the amorphous silica surface, resulting in a higher activity for ethanol dehydration to ethylene. Increasing the MgO loading, crystalline magnesium oxide is formed in addition to the magnesium silicate, producing strong basic sites capable of increasing the ethanol conversion to 1,3-butadiene. Interestingly, for the mixed oxides prepared herein, a linear correlation was found between the forward reaction rate for 1,3-butadiene formation as a function of the reaction rate for acetaldehyde and ethylene. This behavior suggests a predominance of the Prins condensation mechanism. Therefore, the paired arrangement and the strength of the acid-basic conjugate sites of these catalysts must be determinant for the dehydrogenation and dehydration of ethanol and the condensation of the respective intermediates acetaldehyde and ethylene to form 1,3-butadiene.