Inspired by the increasing demand for synthetic rubber the investigation of the ethanol to butadiene (ETB) reaction was launched at the beginning of the last century. Due to the rapid development of the petrochemical industry this technology soon became completely uninterested. Even today 99.9 % of the butadiene (BD) is produced from fossil carbon sources. Nowadays, this production process suffers from high and increasing cost. The ethanol-to-butadiene (ETB) process, where the ethanol is made by fermentation of non-food, biobased carbohydrates became again an environment benign, timely alternative. Magnesia-silica mixed oxides are the most frequently used catalysts of the ETB reaction because of their relatively high activity and BD selectivity. Present thesis work concerns the ETB reaction over MgO-SiO2 mixed oxide catalysts. The research aimed to understand the reaction mechanism using catalysts of systematically varied structure and properties. The catalyst structure was controlled by using new preparation methods. The results, described below, can contribute to the development of efficient and green ETB technology.
Based on the experimental results a likely reaction pathway was substantiated. Accordingly, the reaction is initiated by ethanol oxidation to acetaldehyde, continued by direct ethanol-acetaldehyde coupling to get crotyl alcohol, and finished by crotyl alcohol dehydration to BD.
It was proved that addition of transition (Zn, Ga) or post- transition (In) metal oxide to a magnesia-silica preparation can significantly increase the BD yield, which effect was interpreted by the accelerated ethanol dehydrogenation step that introduces the consecutive reaction sequence leading to BD. The different metal oxide additives enhanced the acidity and basicity of the magnesia-silica catalyst to about the same level however, the activity of the catalysts was different. Relation was found between the ETB activity of the catalyst and the chemical hardness of the metal ions in the modifying oxide. I showed that the magnesia-silica mixed oxide catalysts, made of high-specific-surface-area (high-SSA) MgO gave significantly higher BD yields than the similar catalyst preparations made from low-SSA MgO. The relative activities of these catalysts could not be changed by metal oxide modification, indicating that the catalysts retained most of the properties of the modified magnesia-silica mixed oxides.
The highest BD yield was obtained over MgO-SiO2 catalysts, having high-SSA, mesoporous MgO component. These catalysts have high amount of basic MgO on the surface facilitating the ethanol-acetaldehyde coupling to get crotyl alcohol, whereas the catalyst surface has adequate, balanced acidity to initiate the crotyl alcohol dehydration to BD.