Scheme 1. Reaction mechanism on the conversion of ethanol into 1,3-butadiene.

A number of computational studies on the reaction mechanism of ethanol-to-butadiene conversion have been reported.^[7,25-28] For examples, Chieregatoet al. performed a computational modeling coupled with an experimental study on a MgO cluster to investigate the reactivity of ethanol.^[24] As a model for MgO nanocrystals, (MgO)₁₀ or (MgO)₁₆ clusters were used. They suggested that, based on their experimental and computational results, crotyl alcohol and 3-buten-1-ol are the key intermediates and precursors for butadiene formation. Zhang et al. carried out a periodic DFT study on the aldol condensation reaction on MgO.^[25] According to their results, the barrier height of the C-C coupling step in the aldol condensation reaction was computed to be lower than the proton transfer step. Baltrusaitiset al. also used periodic DFT methods to explore the ethanol-to-butadiene reaction mechanism.^[26]

To improve catalytic performance, a mixture of metal oxides has been suggested.^{[3, 29]} The introduction of ZnO has been reported to enhance the catalytic performance of MgO/SiO₂^[4], which is of interest for investigating details of the performance at the molecular level. There have been several computational mechanistic studies on some metal oxides such as MgO,^[25,26]ZrO₂,^[27]Zr-SBA-15^[28], but to our knowledge, no study has dealt with a detailed computational investigation on full mechanistic aspects described in Scheme 1. Moreover, the atomistic mechanism on the enhanced catalytic performance by ZnO as a catalyst for the ethanol-to-butadiene conversion has still been elusive. In designing new catalysts for ethanol-to-butadiene conversion, inhibition of undesired side reactions is very important;^[1] however, no study on the reaction mechanisms of the side reactions at the molecular level has been reported. Thus, in the present paper, we report results of our computational investigation on each step of the complex reaction conversion of ethanol to butadiene catalyzed by MgO or ZnO, to understand how the different acidic and basic properties of catalysts affect the reaction mechanism. In addition to the main reaction, we consider also a side reaction leading to the formation of ethylene by the dehydration of ethanol.