a Only the highest barrier for each reaction is
shown.b Includes dehydration of 3-hydroxybutanal.c 31.4 kcal/mol, if the barrier is calculated
from Int1_Zn. The barrier height regarding the
H2 molecule formation was not considered.
In the case of ZnO, the computation showed that ethanol dehydrogenation
occurs more easily in comparison MgO where such a result is in line with
an experimental study which reports that the introduction of Zn into
catalysts such as MgO2/SiO2 or talc
enhances the catalytic activity for ethanol
dehydrogenation.[48] ZnO appears to be a very good
catalyst for aldol condensation and the following dehydration, the
barrier of which was about half that of MgO. On the other hand, ZnO’s
performance as a catalyst for the MPV reduction is slightly worse than
that of MgO. Both oxides show virtually identical barrier height for
crotyl alcohol dehydration.Side reactionsOne of the major undesired side reactions of ethanol-to-butadiene
conversion is the formation of ethylene via ethanol
dehydration.[1] Indeed, metal oxides have been
used for ethanol dehydration catalysts.[49,50] One
study reported that the Lewis acidity of metal oxides affects the
catalytic performance of the dehydration.[50]Dehydration of alcohol occurs via either an E1 or E2-type elimination
reaction; according to a previous DFT study on the dehydration reaction
over Al2O3, an E2 reaction is
favored.[49]PESs for the dehydration of ethanol occurred via the E2 mechanism for
ZnO and the E1 mechanism for MgO are shown in Fig. 9. While MgO follows
the E1 mechanism, the energetics of the intermediate
(INT2_Mg_Dehydration) is virtually identical to that ofTS1_Mg_Dehydration, implying that the reaction mechanism is
similar to that of the E2 mechanism. The computed barrier height was
40.5 and 38.9 kcal/mol for ZnO and MgO, respectively. Also, the
energetics of TS2_Mg_Dehydration was 41.9 kcal/mol above that
of INT1_Mg_Dehydration. This result indicated that the
dehydration occurs more easily using ZnO as a catalyst. As the TS
cartoon (TS_Zn_Dehydration) in Fig. 9 shows, the dehydration
of ethanol via the E2 mechanism included the C-O bond and the C-H bond
cleavage, where the former occurs at the Lewis acidic site and the
latter at the Lewis basic site. Obviously, the strong Lewis acidity and
basicity of ZnO cause the side reaction to occur more easily in
comparison with MgO.Figure 9. PESs for ethylene production via dehydration of
ethanol catalyzed by MgO or ZnO. (M=Mg, Zn)
In addition, the dehydration, e.g., the 3-hydroxybutanal dehydration
(Fig. 5) results in the attachment of the OH group on the cluster (seeInt12_Zn or Int12_Mg in Fig. 5) where the OH groups
can act as Brøntsed base sites. Thus, we further investigated the
dehydration of ethanol on Brøntsed acid/base sites.Figure
10. PESs for ethylene production via dehydration of ethanol on Brøntsed
base site. (M=Mg, Zn)
Fig. 10 shows that PESs for dehydration of ethanol considering that
water was already adsorbed in a dissociative manner as Hhwafffwathe OH
group and H on catalyst. INT1_Brønsted is a complex on which
the OH group and H are chemically adsorbed on the catalysts; ethanol is
also adsorbed on the catalyst. The OH group of ethanol interacts with
the metal atom of the metal oxide catalyst. The computed barrier heights
of the dehydration were 39.5 and 43.3 kcal/mol for ZnO and MgO,
respectively, which are higher than the dehydration at the Lewis
acid/base sites. Moreover, as shown in Fig. 10, the energetics of the
product (INT2_Brønsted) is higher than that of reactant
(INT1_Brønsted), which is in contrast to the dehydration at
the Lewis acid/base site. As such, the dehydration that involves the
Brøntsed base site is not preferred compared with the dehydration on
Lewis acid/base sites. Compared with the ethanol dehydrogenation (see
the PESs Fig. 4), ZnO is a better catalyst for dehydrogenation whereas
MgO favors dehydration over dehydrogenation because the barrier for the
dehydration is lower than that for dehydrogenation.Effects of Lewis acidity and basicityDetails of the PESs of each of the elementary reactions of
ethanol-to-butadiene conversion have been discussed up to this point.
Now we discuss the barrier heights of the elementary reactions in
conjunction with the Lewis acidity/basicity of the metal oxide. One of
the most notable features affected by the Lewis acidity/basicity is
ethanol dehydrogenation because, as noted, it depends strongly on the
metal’s Lewis acidity. As the result of DFT calculations discussed in
the previous section indicate, the Mg atom in MgO does not have
sufficient Lewis acidity to facilitate the C-H bond cleavage
corresponding to the α-H transfer from ethanol. The C-H bond cleavage
is also found in other reactions during ethanol-to-butadiene conversion.
For example, TS9-10_M (M=Mg or Zn) shown in Fig. 5 andTS18-19_M (M=Mg or Zn) (Fig. 8) also correspond to the TS of
the C-H bond cleavage and the latter occurs on the Lewis basic site. For
this C-H bond cleavage, the performance of ZnO as a catalyst is better
than that of MgO, which correlates with the stronger Lewis basicity of
the O atoms of ZnO.
By
contrast, the transfer of the OH group, which includes the C-O bond
cleavage, appears to occur more easily on MgO. An example isTS19-20_M (M=Mg or Zn).
Associated barrier heights are lower for MgO-catalyzed reactions than for
ZnO catalyzed reactions of TS19-20_M.Figure 11. Structure of Int19_Mg andInt19_Zn.
To investigate its origin, we present the molecular structure ofInt19_Mg and Int19_Zn , whose details are shown in
Fig. 11 along with some structural parameters. As mentioned above, these
intermediates are followed by TS19-20_M (M=Mg or Zn), which
corresponds to the TS of the C-O bond cleavage leading to butadiene
formation. As shown in Fig.11, the distance between the terminal carbon
atom and the closest metal (Mg or Zn) atom was computed to be longer forInt19_Mg than for Int19_Zn (2.22 Å for MgO, 2.03Å
for ZnO). This result also reflects the strong Lewis acidity of the Zn
atom in the ZnO cluster. Also, the distance between the oxygen atom of
the OH group and the neighboring metal atom was predicted to be 1.90 Å
and 2.18 Å for MgO and ZnO, respectively. As such, the shorter O-Mg
distance implies that the transfer of the OH group to the catalyst
occurs more easily on MgO than ZnO. Moreover, DFT calculations predicted
that the bond distance (4.08 Å) of
Zn1-Zn2 is longer than that (3.76 Å) of
Mg1-Mg2. Such a result indicates that,
structurally, the Mg1-Mg2 distance is
similar to the molecular size of crotonaldehyde. Thus, the terminal C
atom and the O atom of the OH group of crotonaldehyde can be
simultaneously bonded to the catalyst in the case of MgO. In contrast,
the Zn1-Zn2 distance is longer than the
molecular size of crotonaldehyde, which hinders the simultaneous bonding
of the OH group and the terminal carbon of crotonaldehyde to the
catalyst. Considering the above results, it could be reasonable to
conclude that the lower OH transfer barrier height due to MgO arises
from the structural mismatch between the adsorbate and the catalyst.
The ethanol-to-butadiene conversion reaction is very complex, being
composed of series of elementary reactions. Depending on the nature of
each elementary reaction, the Lewis acidity or basicity of metal oxide
catalyst affects the performance of the catalyst in a positive or
negative manner, as discussed above. According to our DFT calculations,
most energy demanding process in the ethanol-to-butadiene conversion
catalyzed by MgO was ethanol dehydrogenation and this step was strongly
influenced by the Lewis acidity of the catalyst, of which result is, to
some extent, in line with a recent study on ethanol dehydrogenation
catalyzed by MgO-SiO2.[52] DFT
calculation showed that ZnO is a better catalyst in this step, where
such a performance arisen from its stronger Lewis acidity. Moreover,
another important aspect of ZnO is that the catalyst favors ethanol
dehydrogenation over the dehydration, whose feature was opposite to that
of MgO.