(745f) Kinetics Investigation of Ethanol Dehydration and Dehydrogenation over a Model Oxide Catalyst

Butadiene and butanol are common high-value chemicals that can be obtained via catalytic conversion of fermentation-derived ethanol. Such upgrading strategies, that combined chemical and biological catalysis, have the potential to be highly impactful. However, the reaction networks required for butadiene and butanol formation are still subject to debate. In this study, we propose a model that describes the first steps in these reactions as they occur over Mg-Si oxide catalysts. A 3wt % MgO/SBA-15 catalyst was chosen as a well-defined model system. We have probed the activity of the catalyst for the conversion of ethanol to ethylene and acetaldehyde at several ethanol partial pressures and reaction temperatures. The catalyst was found to deactivate according to two parallel, first-order processes. The acetaldehyde and ethylene synthesis rates, corrected for deactivation, were demonstrated to be in a saturation regime with respect to ethanol partial pressure in the range between 0.8-5.3 kPa. The apparent activation barrier for dehydration reaction was observed to be 67±8 kJ mol^-1 while that for the dehydrogenation reaction was only 19±8 kJ mol^-1. The low apparent barrier for dehydrogenation is consistent with strong adsorption of a reactive intermediate on the catalyst surface. There was a significant primary kinetic isotope effect for ethylene production when C₂D₅OD was employed, suggesting that C-H bond scission is the rate-controlling step during dehydration and that O-H bond cleavage is not kinetically significant. Conversely, there was no significant kinetic isotope effect observed for acetaldehyde production, signifying that the rate controlling step does not involve either C-H or O-H bond cleavage. Additionally, the acetaldehyde and ethylene synthesis rates were found to be inhibited by the presence of pyridine, indicating that the dehydration and dehydrogenation mechanisms both require acid sites. That there was no significant inhibition of the ethylene and acetaldehyde formation rates observed when 2,6-Di-tert-butylpyridine was employed indicates that these acid sites have Lewis acid character. Based on this data, we have proposed a mechanism describing the acetaldehyde formation wherein an ethanol molecule is first adsorbed on the surface, followed by O-H bond cleavage to form an ethoxide species. The adsorbed ethoxide subsequently undergoes C-H bond cleavage to produce acetaldehyde. The H atoms bound to the surface are combined and desorbed as H₂. The acetaldehyde desorption step is assumed to be kinetically significant step under these conditions. For ethylene formation, the adsorbed ethanol undergoes C-H and C-O bond cleavage to form ethylene, which desorbs from the surface. The H and OH fragments are combined and desorbed as H₂O. C-H bond cleavage is assumed to be the rate controlling step for dehydration, consistent with an E₂-type mechanism.