Figure 8. PESs for the 1,3-butadiene production catalyzed by MgO or ZnO. Dehydration of crotyl alcohol consists of two elementary reactions. PESs for the dehydration is shown in Fig. 8. The first one is the transfer of a H atom from the terminal methyl group of crotyl alcohol to the O atom of the catalyst as shown in Fig. 8. The barrier heights of this step were computed to be 39.3 and 38.3 kcal/mol for MgO and ZnO, respectively. The next step of dehydration is the C-O bond cleavage leading to the OH group transfer to the Lewis acidic site of the catalyst, which corresponds to TS19-20_M(M=Mg, Zn). MgO is more efficient in this step (5.0 kcal/mol) than ZnO (16.1 kcal/mol). Int20_M(M=Mg, Zn) represents the intermediate where butadiene is adsorbed on top of the catalyst. Desorption of butadiene leads to the formation of Int21_M (M=Mg, Zn). The OH and H which were adsorbed on the catalyst were desorbed leading to the regeneration of the catalysts. The dehydration is endothermic by 12.0 kcal/mol.  Summary of main reactionTable 1 lists the highest barriers heights of each step. The highest barrier for ethanol-to-butadiene conversion by MgO occurs at the ethanol dehydrogenation step. This result reflects the lower acidity of MgO. The barrier heights of other steps were lower than that of ethanol dehydrogenation, which implies that an admixture of catalysts having a higher ethanol dehydrogenation activity with MgO should improve the overall performance of ethanol-to-butadiene conversion.Table 1. Summary of highest barrier heights (kcal/mol)a