Reaction Kinetics Analysis of Ethanol Dehydrogenation Catalyzed by MgO-SiO2

The Mg-catalyzed dehydrogenation of ethanol to yield acetaldehyde is an important step in the Lebedev reaction. In this work, we prepared a model MgO-SiO2 catalyst by impregnation of MgO onto an SBA-15 support and used this material to study the reaction kinetics of ethanol dehydrogenation to acetaldehyde. The rates of acetaldehyde and ethylene production were measured for ethanol partial pressures ranging from 0.92 to 5.25 kPa. Both rates are fractional order at 723 K, decreasing to nearly zero-order at 648 K. Consistent with the literature for MgO-SiO2 Lebedev catalysts, both basic sites and Lewis acidic sites were observed on this catalyst. The rates of both acetaldehyde and ethylene were inhibited by pyridine but not by 2,6-ditertbutylpyridine, suggesting that both reactions involve not only basic but also Lewis acidic sites. To elucidate the origin of this cooperativity, a microkinetic model was constructed using a recently published mechanism for the Lebedev reaction catalyzed by MgO. The model was fit to our data using four fitting parameters. The fitting suggests that adsorbed ethanol and hydrogen atoms have a weaker bond with mixed-oxide MgO-SiO2 catalysts than with bulk MgO catalysts, which we attribute experimentally to an increase in the number of moderate-strength Mg2+O2- site pairs formed at the expense of strongly basic MgO sites.