Reaction Kinetics Analysis of Ethanol Dehydrogenation Catalyzed by MgO–SiO₂

Abstract

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−SiO 2 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−SiO 2 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−SiO 2 catalysts than with bulk MgO catalysts, which we attribute experimentally to an increase in the number of moderate-strength Mg 2+ − O 2− site pairs formed at the expense of strongly basic MgO sites.

Mechanistic investigation on ethanol‐to‐butadiene conversion reaction over metal oxide clusters

“…According to our DFT calculations, most energy demanding process in the ethanol‐to‐butadiene conversion catalyzed by MgO was ethanol dehydrogenation and this step was strongly influenced by the Lewis acidity of the catalyst, of which result is, to some extent, in line with a recent study on ethanol dehydrogenation catalyzed by MgOSiO 2 . [ 49 ] DFT calculation showed that ZnO is a better catalyst in this step, where such a performance arisen from its stronger Lewis acidity. Moreover, another important aspect of ZnO is that the catalyst favors ethanol dehydrogenation over the dehydration, whose feature was opposite to that of MgO.…”

First-principles-informed energy span and microkinetic analysis of ethanol catalytic conversion to 1,3-butadiene on MgO

“…The presence of strongly-bound intermediates has been observed experimentally under similar conditions, for example by Abdulrazzaq et al who also found that the apparent activation energy for dehydrogenation was lower than that for dehydration and suggested the low apparent barrier for dehydrogenation was linked to the abundance of a stable surface species. 62 Strongly-bound surface species were also observed with in situ spectroscopy by Taifan et al, with identified species including adsorbed ethanol/ethyoxy, acetaldehyde, aldol condensation and polymerization products, and C 4 species such as butanol and crotyl alcohol. 25,61 Hayashi et al reported significant quantities of oxygenates and longer hydrocarbons on MgO.…”

“…High acetaldehyde selectivity relative to ethylene -greater than 80 % at 723 K with ethanol partial pressure less than 10 kPa -has also been observed experimentally for a model MgO/SBA-15 catalyst. 62 The TOF for ethanol dehydration and Prins condensation (p156) was predicted by the ES analysis to be determined by the stability of state 6C (C 4 H 8 O) and the barriers to reach transition states 5B and 6B (depending on temperature). With the microkinetic model, we found surface coverage to be mostly C 4 H 8 O, with early presence of adsorbed ethanol as well as hydroxide (Fig.…”

Mechanistic Investigation on Ethanol to Butadiene Conversion Reaction over Metal Oxide Clusters: A Computational Study

“…According to our DFT calculations, most energy demanding process in the ethanol-to-butadiene conversion catalyzed by MgO was ethanol dehydrogenation and this step was strongly influenced by the Lewis acidity of the catalyst, of which result is, to some extent, in line with a recent study on ethanol dehydrogenation catalyzed by MgO-SiO 2 . [52] DFT calculation showed that ZnO is a better catalyst in this step, where such a performance arisen from its stronger Lewis acidity. Moreover, another important aspect of ZnO is that the catalyst favors ethanol dehydrogenation over the dehydration, whose feature was opposite to that of MgO.…”