Figure 4. PESs for the formation of acetaldehyde from ethanol
catalyzed by MgO or ZnO.
Among possible adsorption sites, we first considered a terrace site. A
first step of the formation in this case is Int1_Mg followed
by the transfer of a H atom of the OH group to the MgO cluster that
leads to Int2_Mg whose details are shown in Fig. 4. As a
result of TS1-2_Mg , ethanol is now converted to ethoxide
(CH3CH2O-). The
computed energy barrier for TS1-2_Mg is 1.7 kcal/mol implying
that this hydrogen atom transfer is virtually barrierless. OnceInt2_Mg is produced, the reaction can proceed via two
different routes: 1) direct interaction of the H atom transferred to the
MgO cluster, with one H of the methyl group of ethanol and consequent
formation of acetaldehyde and an H2 molecule,Int3_Mg via TS2-3_Mg ; and 2) a stepwise mechanism in
which the H atom of the methyl group is first transferred to the
cluster, which leads to the formation of acetaldehyde. Both mechanisms
have been explored revealing that the former is the only possible one
when the MgO catalyst is used, even though a very high energy barrier
corresponding to 43.3 kcal/mol is computed. Desorption of
H2 molecule and acetaldehyde from Int3_Mgregenerated the catalyst.
When the Mg-apical case is considered, no TS related to the first H
transfer has been found, which indicates that ethanol dehydrogenation is
not likely to occur at the Mg-apical site of the cluster. Considering
the O-apical case, adsorption of ethanol occurs in a dissociative
manner. Our attempts to locate a TS that leads to the formation of
acetaldehyde at the O-apical site were also unsuccessful.
When the same reaction occurred on the ZnO cluster, the dissociative
adsorption of ethanol takes place also on the terrace site, leading to
the formation of Int1_Zn , the energy of which is computed to
be exothermic by -34.2 kcal/mol. The energetics of Int1_Zn is
larger than that of Int2_MgO , whose result correlates with the
result of NH3 adsorption on the clusters described
above, stressing the strong Lewis acidic nature of Zn in the ZnO
cluster. The next step of the dehydrogenation is the α-H atom transfer
from ethoxide to the cluster. In the case of ZnO, our attempt to search
for a TS that corresponds to TS2-3_Mg , a reaction that the
adsorbed H atom on the cluster abstracts the α-hydrogen atom of
ethoxide, was unsuccessful. According to our computation, ethanol
dehydrogenation catalyzed by ZnO occurs via TS2-3_Zn (Fig. 4).
Before the TS occurrence, another intermediate, Int2_Zn is
formed, where in case of Int2_Zn , the α-H is pointing to the
Zn atom and the O-H bond of ethanol is re-formed. As described in Fig.
4, two H atoms are simultaneously transferred to ZnO atTS2-3_Zn , where one is transferred to Zn and another to O.
Such a TS is also found in ethanol dehydrogenation catalyzed by
Al2O3.[46] Since
the Lewis acidity of Al in Al2O3 is very
strong,[47] it is implied that the occurrence of
the simultaneous H atom transfer, which is not observed in MgO-catalyzed
ethanol dehydrogenation, is associated with the strong Lewis acidity of
the metal oxides. The energetics of TS2-3_Zn is 24.5 kcal/mol
above Int2_Zn and 31.4 kcal/mol above Int1_Zn . Such
values are lower than that of the ethanol dehydrogenation barrier
catalyzed by MgO to some extent. As such, our computational results
indicate that acetaldehyde production occurs more easily on the ZnO
cluster. Int3_Zn corresponds to an intermediate where
acetaldehyde, the final product of ethanol dehydrogenation, is adsorbed
on the protonic H atom bonded to ZnO. Int3_Zn is followed byInt4_Zn , which is generated upon desorption of acetaldehyde.
It is seen in Fig. 4 that the energy barrier (TS3-4_Zn , 19.1
kcal/mol) is required upon H2 molecule formation.Int5_Zn can be described as a hydrogen molecule adsorbed on
ZnO, where its desorption re-generates the catalyst. Overall ethanol
dehydrogenation is endothermic by 20.1 kcal/mol.