Mechanistic Investigation on Ethanol to Butadiene Conversion Reaction
over Metal Oxide Clusters: A Computational Study
Abstract
Density functional theory (DFT) calculations were conducted to
investigate mechanistic details of ethanol-to-butadiene conversion
reaction over MgO or ZnO catalyst. We evaluated the Lewis acidity and
basicity of MgO and ZnO and found that ZnO had the stronger Lewis
acidity and basicity compared with those of MgO. Potential energy
surfaces (PESs) of ethanol-to-butadiene conversion, which included
relevant transition states (TSs) and intermediates, were computed in
detail following the generally accepted mechanism reported in the
literature, where such mechanism included ethanol dehydrogenation, aldol
condensation, Meerwein-Pondorf-Verley (MPV) reduction and crotyl alcohol
dehydration. DFT results showed that ethanol dehydrogenation was the
rate limiting step of overall reaction when the reaction was catalyzed
by MgO. Also, DFT results showed that ethanol dehydrogenation occurred
more easily on ZnO compared with MgO where such a result correlated with
the stronger Lewis acidity of ZnO. In addition, we computed ethanol
dehydration which generates ethylene, one of the major undesired side
reaction products for butadiene formation. DFT results showed that ZnO
favored dehydrogenation over dehydration while MgO favored dehydration.