Concluding Remarks
In the present paper, we explored the complex reaction mechanism of ethanol-to-butadiene conversion reaction on two metal oxides, MgO and ZnO using DFT. PESs of all elementary reactions associated with the conversion reaction were computed including relevant TSs and intermediates. We found that ethanol dehydrogenation, which was the first step of the conversion reaction, was very energy demanding compared with other steps when MgO was used as a catalyst. DFT calculations showed that ZnO was a better catalyst for most of elementary reactions involved in the conversion reaction. Especially, its better ability as an ethanol dehydrogenation catalyst was worth mentioning, where it resulted from the strong Lewis acidic character of ZnO. Moreover, the performance of ZnO as an aldol condensation catalyst was very good since the highest barrier of aldol condensation catalyzed by ZnO was 11.8 kcal/mol. In aldol condensation, the Lewis acidic and basic sites of catalyst participated in the reaction, so that ZnO shows a better performance as a catalyst in comparison with MgO. For the MPV reduction, our calculation showed that MgO was a better catalyst for this step. Butadiene formation through crotyl alcohol dehydration did not depend much on the catalysts considered in the present study while this step was predicted to be the rate determining step of ZnO catalyzed reaction. DFT calculations predicted ethanol dehydration, one of the undesired side reactions, occurred more easily on ZnO than on MgO at Lewis acid/base sites of the catalyst. Ethanol dehydration on Brøntsed base site was not likely to occur since the barriers were higher than those of Lewis acid/base site. Although ZnO’s barrier height for ethylene formation was lower than that of MgO, the barrier height of ethylene formation was higher than that of acetaldehyde formation in the case of ZnO. Thus, one can suggest that the use of ZnO should enhance selectivity for acetaldehyde formation over ethylene formation, which also affects yield of butadiene formation. In view of Lewis acidity or basicity, the use of metal oxide catalyst having strong Lewis acidity or basicity can lower several elementary reactions in the conversion reaction. However, as demonstrated, such a catalyst is likely to increase the production of undesired side products. Thus, the optimal strength (not too strong or not too weak) of Lewis acidity and basicity for optimal performance of the catalyst could be suggested, which requires more extensive screening of catalysts.