A study of the conversion of ethanol to 1,3-butadiene: effects of chemical and structural heterogeneity on the activity of MgO–SiO2 mixed oxide catalysts

Abstract

The ethanol-to-butadiene (ETB) transformation proceeds through consecutive reactions, involving hydrogenation/dehydrogenation, C–C coupling, and dehydration. Uniform active sites are needed to attain high catalytic selectivity. It is a challenge to generate a catalyst containing three kinds of co-operating active sites in high homogeneity. Lacking dehydration activity, basic MgO is active in converting ethanol mainly to acetaldehyde and butanol, whereas the main products obtained over SiO₂ catalysts are dehydration products ethylene and diethyl ether. 1,3-Butadiene could be obtained over MgO–SiO₂ mixed oxide catalysts, having acidic and basic sites of strength and concentration favoring all three reactions. Silica was either precipitated over the surface of MgO, or wet-kneaded with MgO to get mixed oxide catalysts. More active ETB catalysts were obtained if the MgO component has a higher specific surface area. XRD, EDS, XPS and acidity/basicity examinations showed that Mg atoms got incorporated into the silica phase, generating new Lewis acid surface sites. An amorphous MgO–SiO₂ mixed oxide preparation, having the highest surface Mg/Si ratio and atomic homogeneity, had the highest activity and 1,3-butadiene selectivity. The catalyst was obtained by hydrolyzation/condensation/precipitation of an Si,Mg–alkoxide solution within carbon mesopores and burning out the carbon/precipitate material. The catalytic ETB mechanism is discussed.