Figure 6. PESs for MPV reduction catalyzed by MgO or ZnO.
The PESs of the MPV reduction by MgO or ZnO are shown in Fig. 6. As a first step of the MPV reduction, crotonaldehyde and ethanol are adsorbed on the catalyst. The energetics of Int14_Mg was computed to be -46.8 kcal/mol. Similar to the ethanol dehydrogenation, the proton from the OH group of ethanol is easily transferred to the catalyst where the barrier height was computed to be 1 kcal/mol. As noted, in the case of ZnO, ethanol is adsorbed in a dissociative manner. The PESs in Fig. 6 show that the energetics of Int15_Zn was -64.2 kcal/mol, which indicates the stronger basicity of ZnO. The second step of the MPV reduction is H atom abstraction of crotonaldehyde from the neighboring ethanol, whose TS corresponds to TS15_16_M(M =Mg , Zn ). The barrier height associated with this reaction were computed to be 22.9 and 15.5 kcal/mol for MgO and ZnO, respectively. The barrier was higher by 7.4 kcal/mol for MgO than for ZnO. The next step of the MPV reduction is a proton transfer to the O atom of crotonaldehyde as shown in Fig. 6. For MgO, the computed barrier height for this reaction was 8.1 kcal/mol. As a result of the proton transfer, Int7_Mg is formed, where its energetics lies above that of Int16_Mg , indicating the endothermic nature of the reaction. The energetics of Int16_Zn was computed to be -70.5 kcal/mol, remarkably larger than that of Int16_Mg . The proton transfer to the O atom requires an energy barrier of 24.2 kcal/mol in the case of ZnO as shown in Fig. 6. It is notable that the corresponding barrier height due to MgO is 8.1 kcal/mol, whose value is much lower than that due to ZnO. It is obvious that such a larger barrier by ZnO arises from the strong Lewis basicity of O in ZnO. This type of reaction is an example of a reaction where the strong Lewis basicity affects the catalytic performance in a negative manner. To gain information on structural aspects of Int16_M (M=Mg, Zn ) we show their molecular structure in Fig. 7. After this intermediate, the proton (labeled P in the figure) which was bonded to the catalyst, was transferred to the crotyl alcohol moiety viaT16-17_M (M=Mg or Zn). The distance between the proton and the neighboring crotyl alcohol is shown in Fig. 7 where the distance of MgO (1.86 Å) was shorter than that of ZnO (2.11 Å). The atomistic distance between the proton and the closest O atom in the catalyst was computed to be 0.99 and 0.97 Å for MgO and ZnO, respectively. All such results imply that transfer of proton from the catalyst should occur more easily on MgO.
Figure 7. Structure of Int16_Mg andInt16_Zn.
The energetics of Int17_Zn is endothermic by 23.7 kcal/mol with respect to Int16_Zn . After the formation ofInt17_Zn or Int17_Mg , desorption of crotyl alcohol and acetaldehyde occurs to regenerate the catalysts. The final state of the reaction lies 3.1 kcal/mol above the reference state.