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\begin{document}

\title{Mechanistic Investigation on Ethanol to Butadiene Conversion Reaction
over Metal Oxide Clusters: A Computational Study}



\author[1]{Valeria Butera}%
\author[2]{Yusuke Tanabe}%
\author[2]{Yu Shinke}%
\author[1]{Tomohisa Miyazawa}%
\author[1]{Tadahiro Fujitani}%
\author[1]{Megumi Kayanuma}%
\author[1]{Yoong-Kee Choe}%
\affil[1]{Sangyo Gijutsu Sogo Kenkyujo Tsukuba}%
\affil[2]{Research Association of High-Throughput Design and Development for Advanced Functional Materials}%


\vspace{-1em}



  
  \date{May 16, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Density functional theory (DFT) calculations were conducted to
investigate mechanistic details of ethanol-to-butadiene conversion
reaction over MgO or ZnO catalyst. We evaluated the Lewis acidity and
basicity of MgO and ZnO and found that ZnO had the stronger Lewis
acidity and basicity compared with those of MgO. Potential energy
surfaces (PESs) of ethanol-to-butadiene conversion, which included
relevant transition states (TSs) and intermediates, were computed in
detail following the generally accepted mechanism reported in the
literature, where such mechanism included ethanol dehydrogenation, aldol
condensation, Meerwein-Pondorf-Verley (MPV) reduction and crotyl alcohol
dehydration. DFT results showed that ethanol dehydrogenation was the
rate limiting step of overall reaction when the reaction was catalyzed
by MgO. Also, DFT results showed that ethanol dehydrogenation occurred
more easily on ZnO compared with MgO where such a result correlated with
the stronger Lewis acidity of ZnO. In addition, we computed ethanol
dehydration which generates ethylene, one of the major undesired side
reaction products for butadiene formation. DFT results showed that ZnO
favored dehydrogenation over dehydration while MgO favored dehydration.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Butadiene is one of the most important bulk chemicals produced in the
petrochemical industry. For instance, it is used as a monomeric compound
for synthetic rubber. Currently, butadiene is mainly obtained from
petrochemical sources, as a byproduct of the naphtha steam cracking
process during ethylene production.\textsuperscript{{[}1{]}} Development
of a cheap and sustainable process for its production from biomass-based
resources would result in reduced reliance on oil resources. In this
context, the possible catalytic conversion of ethanol-to-butadiene has
attracted much interest. Obviously, the choice of catalysts has a strong
effect on the rate of butadiene formation, selectivity, and yield. As
such, many different catalysts have been explored in recent
years.\textsuperscript{{[}2-10{]}} Among them, the
MgO/SiO\textsubscript{2} catalyst has been widely
studied.\textsuperscript{{[}6{]}}

Baltrusaitis \emph{et al.} investigated the structure and reactivity of
MgO/SiO\textsubscript{2} focusing on the role of weak and strong basic
and acidic sites on the surface.\textsuperscript{{[}6{]}} Zhang \emph{et
al.} have studied the structural and surface properties of
MgO/SiO\textsubscript{2} by both experimental characterization and
simulation.\textsuperscript{{[}7{]}} Their X-ray analysis results showed
that MgO and SiO\textsubscript{2} maintain their native morphology, and
phases corresponding to MgSiO\textsubscript{4} were not observed. Based
on such observations, they concluded that the role of
SiO\textsubscript{2} in MgO/SiO\textsubscript{2} is to increase the
structural defects of MgO and, thereby, the catalytic activity.
Niiyama\emph{et al.} examined the correlation between acidity/basicity
and MgO/SiO\textsubscript{2} activity\textsuperscript{{[}11{]}} and
reported that butadiene formation includes dehydrogenation and
dehydration reactions, where the former takes place on basic sites and
the latter on acidic sites, suggesting the importance of both site types
for butadiene yield.

Regarding the reaction mechanism, because of its intrinsically complex
nature, several mechanisms have been
proposed.\textsuperscript{{[}12-19{]}} At present, the mechanism
described in Scheme 1 is generally accepted. This mechanism indicates
that two acetaldehyde molecules are formed as a consequence of
dehydrogenation of ethanol. Subsequently, an aldol condensation reaction
occurs leading to the formation of 3-hydroxybutanal. Dehydration of
3-hydroxybutanal produces crotonaldehyde, which is further hydrogenated
and dehydrated to form the final product, butadiene. Although many
researchers\textsuperscript{{[}20-28{]}} have subsequently adopted the
general features of this mechanism, different suggestions have been
proposed as to what constitutes the rate-determining step in the
reaction. Many undesired processes can also occur leading to the
production of different byproducts such as butanol, carbon dioxide and
ethylene.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Scheme 1. Reaction mechanism on the conversion of ethanol into
1,3-butadiene.

A number of computational studies on the reaction mechanism of
ethanol-to-butadiene conversion have been
reported.\textsuperscript{{[}7,25-28{]}} For examples,
Chieregato\emph{et al.} performed a computational modeling coupled with
an experimental study on a MgO cluster to investigate the reactivity of
ethanol.\textsuperscript{{[}24{]}} As a model for MgO nanocrystals,
(MgO)\textsubscript{10} or (MgO)\textsubscript{16} clusters were used.
They suggested that, based on their experimental and computational
results, crotyl alcohol and 3-buten-1-ol are the key intermediates and
precursors for butadiene formation. Zhang \emph{et al.} carried out a
periodic DFT study on the aldol condensation reaction on
MgO.\textsuperscript{{[}25{]}} According to their results, the barrier
height of the C-C coupling step in the aldol condensation reaction was
computed to be lower than the proton transfer step. Baltrusaitis\emph{et
al.} also used periodic DFT methods to explore the ethanol-to-butadiene
reaction mechanism.\textsuperscript{{[}26{]}}

To improve catalytic performance, a mixture of metal oxides has been
suggested.\textsuperscript{{[}3, 29{]}} The introduction of ZnO has been
reported to enhance the catalytic performance of
MgO/SiO\textsubscript{2}\textsuperscript{{[}4{]}}, which is of interest
for investigating details of the performance at the molecular level.
There have been several computational mechanistic studies on some metal
oxides such as
MgO,\textsuperscript{{[}25,26{]}}ZrO\textsubscript{2},\textsuperscript{{[}27{]}}Zr-SBA-15\textsuperscript{{[}28{]}},
but to our knowledge, no study has dealt with a detailed computational
investigation on full mechanistic aspects described in Scheme 1.
Moreover, the atomistic mechanism on the enhanced catalytic performance
by ZnO as a catalyst for the ethanol-to-butadiene conversion has still
been elusive. In designing new catalysts for ethanol-to-butadiene
conversion, inhibition of undesired side reactions is very
important;\textsuperscript{{[}1{]}} however, no study on the reaction
mechanisms of the side reactions at the molecular level has been
reported. Thus, in the present paper, we report results of our
computational investigation on each step of the complex reaction
conversion of ethanol to butadiene catalyzed by MgO or ZnO, to
understand how the different acidic and basic properties of catalysts
affect the reaction mechanism. In addition to the main reaction, we
consider also a side reaction leading to the formation of ethylene by
the dehydration of ethanol.

\textbf{Computational Details}

\emph{Catalyst modeling}

Recent studies suggest that mesoporous silica is a good catalyst support
for metal oxide catalysts. \textsuperscript{{[}30{]}} Obviously, in
mesoporous silica, the metal oxide catalyst exits as a nanocluster.
Thus, considering computational cost and accuracy, in the present study,
catalysts were modeled as ultrasmall nanoclusters. For MgO, we adopted
the structure reported in the literature.\textsuperscript{{[}31{]}} For
the ZnO cluster model, we replaced the Mg atoms of the
(MgO)\textsubscript{16} cluster by Zn atoms and performed geometry
optimization; the optimized structure is shown in Fig. 1 alongside that
of (MgO)\textsubscript{16}.

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/350143/articles/475062-mechanistic-investigation-on-ethanol-to-butadiene-conversion-reaction-over-metal-oxide-clusters-a-computational-study}

\textbf{Figure 1.} (MgO)\textsubscript{16} and
(ZnO)\textsubscript{16}cluster models. (Color description Mg: yellow; O:
red; Zn: cyan)

As shown in Fig. 1, the optimized structure of the
(ZnO)\textsubscript{16} cluster exhibits a feature similar to those of
(MgO)\textsubscript{16}. Vibrational frequency calculation confirms that
the structure of (ZnO)\textsubscript{16} (Fig. 1) corresponds to the
potential energy minimum. To check the possibility of other minima
structures of (ZnO)\textsubscript{16}, we further applied one of the
global optimization algorithms \textsuperscript{{[}32{]}} implemented in
the atomic simulation environment (ASE)
program.\textsuperscript{{[}33,34{]}} The result of the global
optimization indicates that the structure shown in Fig. 1 is a global
minima.

\emph{Details of DFT calculations}

Geometry optimizations and frequency calculations for all reactants,
intermediates, products, and transition states (TSs), were performed
using DFT, as implemented in the Gaussian 16 quantum chemistry
package.\textsuperscript{{[}35{]}} We selected the B3LYP
functional,\textsuperscript{{[}36,37,38,39{]}} which combines Becke's
1988 exchange functional with the correlation functional by Lee, Yang,
and Parr.

As for the basis sets, the full-electron basis set 6-311G** for O, H, C,
Mg and Zn atoms was chosen. After geometry optimization calculations,
vibrational frequency calculations were conducted to check the nature of
stationary points on potential energy surfaces (minima or TS). Intrinsic
reaction coordinate (IRC) calculations \textsuperscript{{[}40{]}} were
performed to confirm the obtained reactant and product connected through
the TS.

\textbf{Results and Discussion}

Because of its complexity, the overall reaction mechanism was divided
into 4 different parts leading to the formation of 1) acetaldehyde; 2)
crotonaldehyde; 3) crotyl alcohol; and 4) butadiene. Each part will be
discussed separately in this section.

\emph{Acidity and basicity of clusters}

The catalytic behavior of metal oxides is related to the fact that their
surface has both Lewis acidic (metal atoms) and basic sites (oxygen
atoms).\textsuperscript{{[}41{]}} In experiments, the acidity or
basicity strength of the surface is typically examined using probe
molecules such as CO\textsubscript{2} (adsorbed on a basic site), or
NH\textsubscript{3} (adsorbed on an acidic
site).\textsuperscript{{[}42{]}} Thus, to understand the cluster's
intrinsic basic or acidic nature, we first investigated the adsorption
energy of CO\textsubscript{2} or NH\textsubscript{3} on the metal oxide
clusters where the adsorption energy (\emph{E} \textsubscript{ads}) is
defined as follows:

\emph{E} \textsubscript{ads} = \emph{E} \textsubscript{ads+c} --\emph{E}
\textsubscript{c} -- \emph{E} \textsubscript{adsorbate}

\emph{E} \textsubscript{abs+c} indicates the energy of the cluster
adsorbed by the adsorbate molecule, \emph{E} \textsubscript{c} indicates
the energy of the bare cluster and \emph{E}
\textsubscript{adsourbate}indicates the energy of adsorbate molecule.

The results of the optimized structures of the metal oxide clusters
adsorbed by the probe molecule are presented in Fig. 2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure. 2} . Optimized structures of probe molecules ((a)
CO\textsubscript{2}, (b)NH\textsubscript{3}) adsorbed on MgO or ZnO
clusters. (Color description Mg: green; O: red; Zn: cyan; C: grey, H:
white, N: blue)

For MgO, the distance between the C atom of CO\textsubscript{2} and one
of the O atoms of the MgO cluster was computed to be 2.51 \selectlanguage{ngerman}Å and\emph{E}
\textsubscript{ads} was computed to be -7.8 kcal/mol where the (-) sign
indicates net stabilization upon the complex formation. For ZnO, the
figure indicates that CO\textsubscript{2} interacts more strongly with
the metal oxide, which is manifested by the short distance (1.37 Å)
between the C atom of CO\textsubscript{2} and one of the O atoms of the
ZnO cluster. The resulting \emph{E} \textsubscript{ads} was computed to
be -36.8 kcal/mol, which indicates that CO\textsubscript{2}is chemically
adsorbed. Moreover, it is seen in the figure that the distances between
the Zn atom and the two O atoms of CO\textsubscript{2}are around 2.1 Å,
implying that, in addition to the C atom, the two O atoms interact with
the cluster. Such a structure is similar to the tridentate configuration
of chemically absorbed CO\textsubscript{2} on a Zn (0001)
surface.\textsuperscript{{[}43{]}} Also, the above trend is in
accordance with a result of a microcalorimetric study by Auroux \emph{et
al.} , where they reported that the order of basicity of pure metal
oxide surfaces is ZnO \textgreater{} MgO.\textsuperscript{{[}44{]}} In
Fig. 2 (b), the optimized structure of metal oxides with
NH\textsubscript{3} is presented. As shown, the distance between the
Lewis acidic site (the metal atom) of the cluster with the N atom of
NH\textsubscript{3} was computed to be 2.21 and 2.00 Å for MgO and ZnO
clusters, respectively. A corresponding \emph{E} \textsubscript{ads}
value for ZnO was computed to be -27.3 kcal/mol, which is, again, larger
than that of MgO (-21.5 kcal/mol). Thus, DFT result indicates that the
Lewis acidity of ZnO is stronger than that of MgO.

\emph{Ethanol adsorption}

A very first part of the overall mechanism is the conversion of ethanol
into acetaldehyde, which starts with the adsorption of ethanol on the
cluster. Ethanol adsorption on the MgO surface was investigated by
Branda \emph{et al} .\textsuperscript{{[}45{]}} using an embedded
cluster model approach. The authors considered four different absorption
sites: terrace, edge, O-apical, and Mg-apical; their positions on the
MgO (100) surface are described in Fig. 3. According to their results,
the absorption energy becomes stronger in the following order: terrace
\textless{} edge \textless{} O-apical \textless{} Mg-apical.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 3.} Four different sites of the MgO (100) surface.

Branda \emph{et al.} \textsuperscript{{[}45{]}} found that dissociative
adsorption occurs at the sites with defects (edge and apical) resulting
in the consequent H transfer from the OH group in ethanol to the O atom
of the MgO surface. However, when ethanol is adsorbed on the surface
without defects (terrace) two bonds are formed: 1) a hydrogen bond
between the H of the ethanol molecule and an O of the surface; and 2) an
interaction between a Mg atom of the surface and the O of ethanol. In
the present study, we also considered terrace and O- and Mg-apical
ethanol adsorption. In agreement with the result of Branda \emph{et al.}
,\textsuperscript{{[}45{]}} we found that nondissociative adsorption of
ethanol occurs on the terrace site. However, the adsorption of ethanol
molecule on the apical sites is dissociative only in the O-apical case
while no ethanol dissociation occurs when ethanol is adsorbed on the
Mg-apical site. The computed absorption energies (\emph{E}
\textsubscript{ads}) are -21.0, -29.0 and -37.9 kcal/mol for the
terrace, Mg-apical, and O-apical sites, respectively.

\emph{Ethanol dehydrogenation}

As described in Scheme 1, the first part of ethanol-to-butadiene
conversion reaction is the dehydrogenation reaction of ethanol to form
acetaldehyde. Fig. 4 shows PESs of the dehydrogenation reaction
catalyzed by MgO or ZnO, also including the schematic representations of
all the intermediates and TSs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{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 \textbf{Int1\_Mg} followed
by the transfer of a H atom of the OH group to the MgO cluster that
leads to \textbf{Int2\_Mg} whose details are shown in Fig. 4. As a
result of \textbf{TS1-2\_Mg} , ethanol is now converted to ethoxide
(CH\textsubscript{3}CH\textsubscript{2}O\textsuperscript{-}). The
computed energy barrier for \textbf{TS1-2\_Mg} is 1.7 kcal/mol implying
that this hydrogen atom transfer is virtually barrierless.
Once\textbf{Int2\_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 H\textsubscript{2}
molecule,\textbf{Int3\_Mg} via \textbf{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
H\textsubscript{2} molecule and acetaldehyde from
\textbf{Int3\_Mg}regenerated 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 \textbf{Int1\_Zn} , the energy of which is computed to
be exothermic by -34.2 kcal/mol. The energetics of \textbf{Int1\_Zn} is
larger than that of \textbf{Int2\_MgO} , whose result correlates with
the result of NH\textsubscript{3} 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 \selectlanguage{greek}α-\selectlanguage{english}H atom transfer
from ethoxide to the cluster. In the case of ZnO, our attempt to search
for a TS that corresponds to \textbf{TS2-3\_Mg} , a reaction that the
adsorbed H atom on the cluster abstracts the \selectlanguage{greek}α-\selectlanguage{english}hydrogen atom of
ethoxide, was unsuccessful. According to our computation, ethanol
dehydrogenation catalyzed by ZnO occurs via \textbf{TS2-3\_Zn} (Fig. 4).
Before the TS occurrence, another intermediate, \textbf{Int2\_Zn} is
formed, where in case of \textbf{Int2\_Zn} , the \selectlanguage{greek}α-\selectlanguage{english}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
at\textbf{TS2-3\_Zn} , where one is transferred to Zn and another to O.
Such a TS is also found in ethanol dehydrogenation catalyzed by
Al\textsubscript{2}O\textsubscript{3}.\textsuperscript{{[}46{]}} Since
the Lewis acidity of Al in Al\textsubscript{2}O\textsubscript{3} is very
strong,\textsuperscript{{[}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 \textbf{TS2-3\_Zn} is 24.5 kcal/mol
above \textbf{Int2\_Zn} and 31.4 kcal/mol above \textbf{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. \textbf{Int3\_Zn} corresponds to an intermediate where
acetaldehyde, the final product of ethanol dehydrogenation, is adsorbed
on the protonic H atom bonded to ZnO. \textbf{Int3\_Zn} is followed
by\textbf{Int4\_Zn} , which is generated upon desorption of
acetaldehyde. It is seen in Fig. 4 that the energy barrier
(\textbf{TS3-4\_Zn} , 19.1 kcal/mol) is required upon H\textsubscript{2}
molecule formation.\textbf{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.

\emph{Aldol condensation}

The next step of ethanol-to-butadiene conversion is the aldol
condensation of two acetaldehyde molecules to form 3-hydroxybutanal,
followed by dehydration to produce crotonaldehyde. PESs for aldol
condensation, obtained from DFT calculations, were shown in Fig. 5.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{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
of\textbf{Int6\_Mg} and \textbf{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
(\textbf{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 form\textbf{Int8\_M (M} =\textbf{Mg} or \textbf{Zn)} via
\textbf{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 \textbf{Int8\_M}(Fig. 5) and the resulting intermediate now had
four C atoms, which constitutes the basic skeleton of butadiene. After
the formation of\textbf{Int8\_M} , the proton on the metal oxide is
transferred back to the O atom via \textbf{TS8-9\_M} to form
3-hydroxybutanal (\textbf{Int9\_M} ). The barrier for this step was very
low (Fig. 5). We note that the energetics of \textbf{Int6\_M} and
\textbf{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 \textbf{Int6\_M} from \textbf{Int9\_M} is not
sufficient.

To form crotonaldehyde, dehydration of 3-hydroxybutanal should occur as
the next step. \textbf{TS9-10\_M} in Fig. 5 corresponds to C-H bond
cleavage occurring at the \selectlanguage{greek}α-\selectlanguage{english}C of 3-hydroxybutanal. The resulting
intermediate is \textbf{Int10\_M} . In the case of MgO, we could
locate\textbf{Int10'\_Mg} where the hydroxy group is not bonded to the
catalyst, followed by the formation of \textbf{Int10\_Mg} . The barrier
height of \textbf{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 \textbf{Int10\_M} , a
reaction proceeds to form crotonaldehyde via \textbf{TS10-11\_M} , which
corresponds to the TS of the OH group transfer to the catalyst, followed
by the formation of \textbf{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. \textbf{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 \textbf{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
of\textbf{Int12\_Zn} is lower than that of \textbf{Int13\_Zn} implying
that H\textsubscript{2}O is easily dissociated into H and OH on ZnO. The
aldol condensation followed by the dehydration is endothermic by 3.6
kcal/mol.

\emph{Crotonaldehyde reduction}

Reduction of crotonaldehyde to crotyl alcohol can be performed by H
atoms from ethanol in the Meerwein-Pondorf-Verley (MPV) reduction,
where, in the reduction process, ethanol acts as a donor of H atoms to
crotonaldehyde.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{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 \textbf{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 \textbf{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 \textbf{TS15\_16\_M}(\textbf{M}
=\textbf{Mg} , \textbf{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, \textbf{Int7\_Mg} is formed, where its energetics lies above
that of \textbf{Int16\_Mg} , indicating the endothermic nature of the
reaction. The energetics of \textbf{Int16\_Zn} was computed to be -70.5
kcal/mol, remarkably larger than that of \textbf{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 \textbf{Int16\_M} (\textbf{M=Mg, Zn} ) we show
their molecular structure in Fig. 7. After this intermediate, the proton
(labeled \textbf{P} in the figure) which was bonded to the catalyst, was
transferred to the crotyl alcohol moiety via\textbf{T16-17\_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 \selectlanguage{ngerman}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.

\textbf{Figure 7}. Structure of \textbf{Int16\_Mg}
and\textbf{Int16\_Zn}.

The energetics of \textbf{Int17\_Zn} is endothermic by 23.7 kcal/mol
with respect to \textbf{Int16\_Zn} . After the formation
of\textbf{Int17\_Zn} or \textbf{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.

\emph{Crotyl alcohol dehydration}

The final part of the overall mechanism is the dehydration of crotyl
alcohol to ultimately produce butadiene.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\textbf{Figure 8}. PESs for the 1,3-butadiene production catalyzed by
MgO or ZnO. Dehydration of crotyl alcohol consists of two elementary
reactions. PESs for the dehydration is shown in Fig. 8. The first one is
the transfer of a H atom from the terminal methyl group of crotyl
alcohol to the O atom of the catalyst as shown in Fig. 8. The barrier
heights of this step were computed to be 39.3 and 38.3 kcal/mol for MgO
and ZnO, respectively. The next step of dehydration is the C-O bond
cleavage leading to the OH group transfer to the Lewis acidic site of
the catalyst, which corresponds to
\textbf{TS19-20\_M}(\textbf{M}=\textbf{Mg}, \textbf{Zn}). MgO is more
efficient in this step (5.0 kcal/mol) than ZnO (16.1 kcal/mol).
\textbf{Int20\_M}(\textbf{M}=\textbf{Mg}, \textbf{Zn}) represents the
intermediate where butadiene is adsorbed on top of the catalyst.
Desorption of butadiene leads to the formation of \textbf{Int21\_M
(M=Mg, Zn).}\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}The OH and H which were adsorbed on the catalyst were
desorbed leading to the regeneration of the catalysts. The dehydration
is endothermic by 12.0 kcal/mol. \begin{CJK}{UTF8}{gbsn}\textbf{　}\end{CJK}\selectlanguage{english}\emph{Summary of main
reaction}Table 1 lists the highest barriers heights of each step. The
highest barrier for ethanol-to-butadiene conversion by MgO occurs at the
ethanol dehydrogenation step. This result reflects the lower acidity of
MgO. The barrier heights of other steps were lower than that of ethanol
dehydrogenation, which implies that an admixture of catalysts having a
higher ethanol dehydrogenation activity with MgO should improve the
overall performance of ethanol-to-butadiene conversion.\textbf{Table 1.}
Summary of highest barrier heights (kcal/mol)\emph{\textsuperscript{a}}\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& Type of reactions & Type of reactions & Type of reactions & Type of
reactions\tabularnewline
\midrule
\endhead
& Ethanol dehydrogenation & Aldol condensation
\emph{\textsuperscript{b}} & MPV reduction & Crotyl alcohol
dehydration\tabularnewline
MgO & 43.3 & 20.9 & 22.9 & 39.3\tabularnewline
ZnO & 24.5\emph{\textsuperscript{c}} & 11.8 & 24.2 & 38.3\tabularnewline
\bottomrule
\end{longtable}

\emph{\textsuperscript{a}} Only the highest barrier for each reaction is
shown.\emph{\textsuperscript{b}} Includes dehydration of
3-hydroxybutanal.\textsuperscript{\emph{c}} 31.4 kcal/mol, if the
barrier is calculated from \textbf{Int1\_Zn}. The barrier height
regarding the H\textsubscript{2} 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 MgO\textsubscript{2}/SiO\textsubscript{2} or talc
enhances the catalytic activity for ethanol
dehydrogenation.\textsuperscript{{[}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.\emph{Side reactions}One of the major
undesired side reactions of ethanol-to-butadiene conversion is the
formation of ethylene via ethanol dehydration.\textsuperscript{{[}1{]}}
Indeed, metal oxides have been used for ethanol dehydration
catalysts.\textsuperscript{{[}49,50{]}} One study reported that the
Lewis acidity of metal oxides affects the catalytic performance of the
dehydration.\textsuperscript{{[}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
Al\textsubscript{2}O\textsubscript{3}, an E2 reaction is
favored.\textsuperscript{{[}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 (\textbf{INT2\_Mg\_Dehydration}) is virtually identical
to that of\textbf{TS1\_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 \textbf{TS2\_Mg\_Dehydration} was 41.9 kcal/mol above
that of \textbf{INT1\_Mg\_Dehydration}. This result indicated that the
dehydration occurs more easily using ZnO as a catalyst. As the TS
cartoon (\textbf{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.\textbf{Figure 9}. PESs for ethylene production via
dehydration of ethanol catalyzed by MgO or ZnO.\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}(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 (see\textbf{Int12\_Zn}
or \textbf{Int12\_Mg} in Fig. 5) where the OH groups can act as Br\selectlanguage{ngerman}øntsed
base sites. Thus, we further investigated the dehydration of ethanol on
Brøntsed acid/base sites.\textbf{Figure 10}. PESs for ethylene
production via dehydration of ethanol on Brøntsed base site.\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}(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. \selectlanguage{ngerman}\textbf{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 (\textbf{INT2\_Brønsted}) is higher than that of reactant
(\textbf{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.\emph{Effects of
Lewis acidity and basicity}Details 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 \selectlanguage{greek}α-\selectlanguage{english}H transfer from ethanol. The C-H bond
cleavage is also found in other reactions during ethanol-to-butadiene
conversion. For example, \textbf{TS9-10\_M} (M=Mg or Zn) shown in Fig. 5
and\textbf{TS18-19\_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 is\textbf{TS19-20\_M} (M=Mg or Zn).
Associated\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}barrier\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}heights\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}are\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}lower\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}for\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}MgO-catalyzed\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}reactions\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}than\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}for
ZnO\begin{CJK}{UTF8}{gbsn}　\end{CJK}\selectlanguage{english}catalyzed reactions of \textbf{TS19-20\_M}.\textbf{Figure 11}.
Structure of \textbf{Int19\_Mg} and\textbf{Int19\_Zn}.

To investigate its origin, we present the molecular structure
of\textbf{Int19\_Mg} and \textbf{Int19\_Zn} , whose details are shown in
Fig. 11 along with some structural parameters. As mentioned above, these
intermediates are followed by \textbf{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
for\textbf{Int19\_Mg} than for \textbf{Int19\_Zn} (2.22 \selectlanguage{ngerman}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
Zn\textsubscript{1}-Zn\textsubscript{2} is longer than that (3.76 Å) of
Mg\textsubscript{1}-Mg\textsubscript{2.} Such a result indicates that,
structurally, the Mg\textsubscript{1}-Mg\textsubscript{2} 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 Zn\textsubscript{1}-Zn\textsubscript{2} 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-SiO\textsubscript{2}.\textsuperscript{{[}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.

\textbf{Concluding Remarks}

In the present paper, we explored the complex reaction mechanism of
ethanol-to-butadiene conversion reaction on two metal oxides, MgO and
ZnO using DFT. PESs of all elementary reactions associated with the
conversion reaction were computed including relevant TSs and
intermediates. We found that ethanol dehydrogenation, which was the
first step of the conversion reaction, was very energy demanding
compared with other steps when MgO was used as a catalyst. DFT
calculations showed that ZnO was a better catalyst for most of
elementary reactions involved in the conversion reaction. Especially,
its better ability as an ethanol dehydrogenation catalyst was worth
mentioning, where it resulted from the strong Lewis acidic character of
ZnO. Moreover, the performance of ZnO as an aldol condensation catalyst
was very good since the highest barrier of aldol condensation catalyzed
by ZnO was 11.8 kcal/mol. In aldol condensation, the Lewis acidic and
basic sites of catalyst participated in the reaction, so that ZnO shows
a better performance as a catalyst in comparison with MgO. For the MPV
reduction, our calculation showed that MgO was a better catalyst for
this step. Butadiene formation through crotyl alcohol dehydration did
not depend much on the catalysts considered in the present study while
this step was predicted to be the rate determining step of ZnO catalyzed
reaction. DFT calculations predicted ethanol dehydration, one of the
undesired side reactions, occurred more easily on ZnO than on MgO at
Lewis acid/base sites of the catalyst. Ethanol dehydration on Brøntsed
base site was not likely to occur since the barriers were higher than
those of Lewis acid/base site. Although ZnO's barrier height for
ethylene formation was lower than that of MgO, the barrier height of
ethylene formation was higher than that of acetaldehyde formation in the
case of ZnO. Thus, one can suggest that the use of ZnO should enhance
selectivity for acetaldehyde formation over ethylene formation, which
also affects yield of butadiene formation. In view of Lewis acidity or
basicity, the use of metal oxide catalyst having strong Lewis acidity or
basicity can lower several elementary reactions in the conversion
reaction. However, as demonstrated, such a catalyst is likely to
increase the production of undesired side products. Thus, the optimal
strength (not too strong or not too weak) of Lewis acidity and basicity
for optimal performance of the catalyst could be suggested, which
requires more extensive screening of catalysts.

\textbf{Acknowledgement}

This study has been supported by a project JPNP16010, commissioned by
the New Energy and Industrial Technology Development Organization
(NEDO).

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