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.