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.