Figure 5 . PESs for the formation of crotonaldehyde from two
acetaldehyde molecules catalyzed by MgO or ZnO.
Acetaldehyde molecules formed in the previous step adsorbed on the metal
oxide whose energetics are shown in Fig. 5 along with the PESs of the
reaction. Similar to the adsorption of ethanol, the adsorption energy of
acetaldehyde was larger for ZnO than for MgO (see energetics ofInt6_Mg and Int6_Zn ). This result also reflects the
strong Lewis acidic nature of the Zn atom in ZnO. The next step of aldol
condensation is carbanion formation, which results from proton transfer
from the methyl group of acetaldehyde to the metal oxide cluster, where
the relevant TS is schematically described in Fig. 5 (TS6-7_Mg,
TS6-7_Zn ). The barrier heights of the proton transfer were computed to
be 9.8 kcal/mol and 4.3 kcal/mol for MgO and ZnO, respectively. The
smaller barrier height of ZnO implies the strong Lewis basic nature of
the O atom in ZnO. The resulting carbanion can now attack the C atom of
the carbonyl group of the neighboring acetaldehyde to formInt8_M (M =Mg or Zn) via TS7-8_M .
The associated barrier heights for this step were computed to be 6.0 and
11.8 kcal/mol for MgO and ZnO, respectively. The calculated atomistic
charge of the carbanion was -0.79 e for MgO and -0.74 e for ZnO, which
indicated the carbanion on MgO was more anionic. The lower barrier
height of the C-C bond formation step due to MgO catalyst thus was
contributed from the larger anionic nature of carbanion on MgO. The C-C
bond formation reaction in aldol condensation forms Int8_M(Fig. 5) and the resulting intermediate now had four C atoms, which
constitutes the basic skeleton of butadiene. After the formation ofInt8_M , the proton on the metal oxide is transferred back to
the O atom via TS8-9_M to form 3-hydroxybutanal
(Int9_M ). The barrier for this step was very low (Fig. 5). We
note that the energetics of Int6_M and Int9_M , which
is the final product of aldol condensation, are virtually identical,
i.e., less than 2 kcal/mol, regardless of the catalysts. This result
implies that the thermodynamic driving force that leads to the formation
of Int6_M from Int9_M is not sufficient.
To form crotonaldehyde, dehydration of 3-hydroxybutanal should occur as
the next step. TS9-10_M in Fig. 5 corresponds to C-H bond
cleavage occurring at the α-C of 3-hydroxybutanal. The resulting
intermediate is Int10_M . In the case of MgO, we could locateInt10’_Mg where the hydroxy group is not bonded to the
catalyst, followed by the formation of Int10_Mg . The barrier
height of TS9-10_M differs by the involved catalysts as shown
in Fig. 5. In the case of MgO, the barrier height was computed to be
20.9 kcal/mol, whereas that of ZnO is 8.9. Thus, ZnO is a more effective
catalyst for this step. After the formation of Int10_M , a
reaction proceeds to form crotonaldehyde via TS10-11_M , which
corresponds to the TS of the OH group transfer to the catalyst, followed
by the formation of Int11_M . The barrier height of this
elementary reaction by MgO was computed to be 8.4 kcal/mol and that of
ZnO, 8.4 kcal/mol. Int11_M is an intermediate where
crotonaldehyde, proton and the OH group are adsorbed on the catalyst
where the latter two groups are chemically adsorbed. Desorption of
crotonaldehyde leads to the formation of Int12_M as described
in Fig. 5. After desorption, water-molecule formation on the catalyst
and water desorption where the processes finally regenerate the
catalysts. The PESs in Fig. 5 show that the energetics ofInt12_Zn is lower than that of Int13_Zn implying
that H2O is easily dissociated into H and OH on ZnO. The
aldol condensation followed by the dehydration is endothermic by 3.6
kcal/mol.