Study on key step of 1,3-butadiene formation from ethanol on MgO/SiO2

Article abstract of DOI:10.1039/c4ra17070a

The structure and surface properties of the MgO/SiO₂ catalyst were studied by both experimental characterization and the simulation method. The adsorption properties of ethanol on different MgO/SiO₂ surfaces were researched by the density functional theory method and the result is that ethanol mainly adsorbed on MgO surface. The role of SiO₂ in increasing the MgO crystal defects in the MgO/SiO₂ has been obtained and the electronic properties of ethanol molecule before and after adsorption on the catalyst surfaces were compared. The initial step of ethanol dehydrogenation and the dehydration to 1,3-butadiene process, which is the dehydrogenation of ethanol to acetaldehyde reaction, on the flat sites, stepped sites and kinked sites was examined through the molecular simulation method. We investigated the preferable surface for the reaction of ethanol dissociation to an ethoxy group and the more active surface for the reaction of dehydrogenation of ethanol to acetaldehyde. The research results suggested that the stepped MgO surface is more active for the reaction of dehydrogenation of ethanol to acetaldehyde.

Full text of DOI:10.1039/c4ra17070a

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Study on key step of 1,3-butadiene formation from
ethanol on MgO/SiO2
Cite this: RSC Adv., 2015, 5, 25959
ab
Minhua Zhang,ab Meixiang Gao,ab Jianyue Chenab and Yingzhe Yu
*
The structure and surface properties of the MgO/SiO2 catalyst were studied by both experimental
characterization and the simulation method. The adsorption properties of ethanol on different MgO/SiO2
surfaces were researched by the density functional theory method and the result is that ethanol mainly
adsorbed on MgO surface. The role of SiO2 in increasing the MgO crystal defects in the MgO/SiO2 has
been obtained and the electronic properties of ethanol molecule before and after adsorption on the
catalyst surfaces were compared. The initial step of ethanol dehydrogenation and the dehydration to 1,3-
butadiene process, which is the dehydrogenation of ethanol to acetaldehyde reaction, on the flat sites,
stepped sites and kinked sites was examined through the molecular simulation method. We investigated
the preferable surface for the reaction of ethanol dissociation to an ethoxy group and the more active
surface for the reaction of dehydrogenation of ethanol to acetaldehyde. The research results suggested
that the stepped MgO surface is more active for the reaction of dehydrogenation of ethanol to
acetaldehyde.
Received 26th December 2014
Accepted 17th February 2015
DOI: 10.1039/c4ra17070a
www.rsc.org/advances
Makshina et al.12 also report that MgO/SiO2 is the most promising
catalyst in the reaction to transform ethanol to BD. However, the
1 Introduction
mechanism of the high MgO/SiO2 activity achieved and the roles
that SiO2 play in the catalysis of MgO/SiO2 are today open for
debate. The reaction mechanism suggested by Lebedev et al.,13
which is discussed in ref. 14, may be represented as follows:
1,3-Butadiene (BD) is one of the most essential basic chemicals
in the petroleum chemical industry. BD is a by-product of the
ethylene process typically isolated from the cracking of
naphtha.1 The recent trends in lightening of the feedstock of
steam crackers reduce the BD supply and may potentially lead
to a shortage in BD production in the coming years.2 An esti-
mated 9.3 million tons of BD were produced worldwide in 2005
(CMAI, 2006). The world capacity grew by 3.5% per year between
1997 and 2002.3 To accommodate this up-coming energy decit,
it is especially crucial to develop alternative technologies for BD
production from renewable resources such as bioethanol.
Recent research has shied toward bioethanol production from
non-food biomass feedstocks.4–6 Therefore, in the coming
number of years, BD production from bioethanol will be the
most promising, renewable and sustainable technology among
various on-purpose BD production processes.
(1) CH3CH2OH / CH3CHO + H2[
(2) 2CH3CHO / CH3CHOHCH2CHO
(3) CH3CHOHCH2CHO / CH3–CH]CH–CHO + H2O
(4) CH3–CH]CH–CHO + CH3CH2OH /
CH3CH]CHCH2OH + CH3CHO
(5) CH3CH]CHCH2OH / CH2]CH–CH]CH2 + H2O
In our previous work, we studied the reaction of MgO/SiO2-
and the mechanism of the catalytic conversion of ethanol to 1,3-
butadiene on magnesia–silica acidic–basic bifunctional cata-
lysts,15,16 which agrees with the research by Lebedev et al.13 The
research by Branda et al.17 shows that the catalytic activity of the
MgO surfaces is attributed to its crystal defects, but no one
accounts for the principle of ethanol conversion to BD on a
defective MgO structure.
Catalytic BD formation from ethanol is an industrially
proven route. From the 1920s to 1960s, ethanol was converted to
BD at 400–450 ꢀC using a one-step process or at 300–350 ꢀC
according to a two-step process using a Ta2O5/SiO2 catalyst.
Additionally, various catalysts have been proposed to perform
this reaction.7–11 Among all the catalysts that were suggested to
catalyze this reaction, MgO/SiO2 is the most widely studied, and
The aim of our present work is to apply a simulation method
to study the key steps of BD formation from ethanol on MgO
and SiO2, to analyze the roles that SiO2 play in the catalysis and
the different characters of the defective surface of MgO, which
aKey Laboratory for Green Chemical Technology of Ministry of Education, Tianjin
University R&D Center for Petrochemical Technology, Tianjin 300072, China.
E-mail: yzhyu@tju.edu.cn
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China
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makes contributions to the research of BD formation on MgO/ energy was calculated using the Rectied-Perdew–Burke–Ern-
SiO2 at the molecular level.
zerhof (RPBE) generalized gradient approximation (GGA).22 The
electron density and a self-consistent eld tolerance were
converged to within 2.0 ꢂ 10ꢁ5 eV and 1.0 ꢂ 10ꢁ5 Ha per atom,
respectively. The geometry optimizations were performed with
2 Experimental and computational
methods
ꢁ1
˚
DFT until the force on each atom was less than 0.004 Ha A
.
2.1 Preparation of the catalysts
3 Results and discussion
3.1 Characterizations of MgO, SiO2 and MgO/SiO2
The preparation method of wet-kneading was employed. The
MgO content of the MgO/SiO2 catalyst, the calcined temperature
and favorable reaction conditions have been studied. When the
MgO content was 80 wt%, the catalyst had the highest selectivity
as listed in Table 1.15,16
3.1.1 Crystallographic characterization. X-ray diffraction
patterns of the samples showed typical diffractograms of MgO/
SiO2, Fig. 1, with peaks at 36.6ꢀ, 42.8ꢀ, 62.3ꢀ and 78.6ꢀ, which
correspond to diffraction by the MgO planes (111), (200), (220)
and (222), respectively. The phase is unambiguously identied
according to the peaks at 20ꢀ to 30ꢀ, which corresponds to
amorphous SiO2. However, phases corresponding to MgSiO4
were not detected in the MgO/SiO2 catalyst. The X-ray analyses
show that MgO and SiO2 still maintain their native morphology
in the MgO/SiO2, and compared with MgO, sample MgO/SiO2
presents peaks wider and less intense.
2.2 Catalyst characterization
The solid structure in MgO, SiO2 and MgO/SiO2 samples were
determined by powder X-ray diffraction methods using a D/
max-2500 diffractometer and Ni-ltered CuKa radiation. The
UV reectance spectra of powdered samples were measured on
a U-3010 UV-Vis DRS spectrophotometer.
2.3 Temperature-programmed surface reaction
3.1.2 Coordination characterization. The UV diffuse
reectance spectra of MgO/SiO2 and MgO are presented in
Fig. 2. The catalyst MgO/SiO2, in addition to an intense peak at
230 nm, shows a second peak at 275 nm. The MgO shows a peak
at 235 nm and a weak shoulder around 320 nm. Coluccia
et al.23,24 have observed peaks at 230 and 274 nm from a study of
several MgO samples, which they associated with four-
coordinate, and three-coordinate O2ꢁ ions, respectively. Thus,
we can make the conclusion that for the catalyst MgO/SiO2, the
addition of SiO2 would lead to structural modications in the
MgO such as an increase in the at, stepped and kinked defects
and in the coordinatively unsaturated O2ꢁ ions and Mg–O–Si
interactions, which provides a basis for establishing the model.
The temperature-programmed surface reaction experiments
were carried out in Auto II 2920 and products were analyzed by
an on-line mass spectrometer (Balzers Omnistar MS200). The
100 mg MgO or SiO2 (bought from Aladdin) was xed in a quartz
reactor. Before the experiment, the sample was pretreated in N2
at 500 ꢀC for 2 h at a heating rate of 20 ꢀC minꢁ1 from ambient
temperature to 500 ꢀC, and a N2 ow rate of 20 ml minꢁ1. Aer
the catalyst pretreated and the temperature drop to 400 ꢀC,
ethanol/He gas was injected to the catalyst surface through a
quantitative loop (1 ml). The MS was used to detect all possible
products: acetaldehyde, ethylene, and 1-butylene.
2.4 Computational methodology
3.2 The surface reaction of ethanol on MgO and SiO2
The calculations were performed by the density functional
theory program Dmol3 in Materials Studio of Accelrys, in which
the physical wave functions are expanded in the terms of
numerical basis sets.18,19 To maintain the balance between the
efficiency and computation accuracy, a DNP double numerical
basis set was used, and special points sampling integration
were employed, a 2 ꢂ 2 ꢂ 1 k-point grid was determined by the
Monkhorst–Pack method.20 The core electrons were treated with
DFT semi-core pseudopotentials.21 The exchange–correlation
The ion intensity of products against the reaction time on the
MgO and SiO2 surface, with ethanol pulse injection, are shown
Table
1 Influence of different MgO content on the catalytic
performance16
MgO
content Xethanol
%
SBD%
Sethylene%
Sdiethyl ether
%
Sacetic acid%
30%
40%
50%
70%
80%
90%
66.90
68.74
63.90
63.32
66.32
41.76
15.08 73.39
21.79 73.41
26.80 69.16
30.67 65.84
48.02 49.08
19.48 63.03
10.53
4.01
3.31
2.99
2.15
1.41
1.77
0.89
0.71
0.46
0.74
16.03
Fig. 1 XRD patterns for MgO/SiO2, MgO and SiO2 catalyst.
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the literature,2,8,12 the rst and key step is dehydrogenation of
ethanol to acetaldehyde. Thus, the study of the key step of the
formation of BD from ethanol at molecular level is a priority.
3.3 Models
As above, experimental research shows that the MgO and SiO2
still maintain their native morphology in MgO/SiO2 and pha-
ses corresponding to MgSiO4 are not detected. Thus, we model
MgO and SiO2, respectively. The partial structure of amor-
phous silica is similar to b-cristobalite; furthermore, the
physical properties such as the density, refractive index are
found to be close. Thus, we selected b-cristobalite with Fd3m
symmetries as the representative model of silica,25,26 as shown
˚
in Fig. 5. The lattice constant a ¼ b ¼ c ¼ 7.16 A agrees satis-
factorily with experimental measurements.27 The results of the
experiment show that the surface hydroxyls density of amor-
Fig. 2 UV diffuse reflectance spectra of MgO/SiO2 catalyst and MgO
catalyst samples.
2
28
˚
phous silica is 4.9 OH/100 A , while that of b-cristobalite
2
˚
(111) is 4.55 OH/100 A . The supercell for the (111) surface
include a (2 ꢂ 2) surface unit cell and 4 atomic layers with a
in Fig. 3 and 4. The products are acetaldehyde, ethylene and 1-
butene on the MgO surface, while only ethylene is formed from
ethanol on the SiO2 surface. Further, we can speculate that the
effect of ethanol on MgO: ethylene is formed from ethanol
through dehydration; ethanol is dehydrogenated to acetalde-
hyde; acetaldehyde underwent an aldol-condensation with
reduction and dehydration forming 1-butene. Similarly, the
process of ethanol on SiO2 is dehydration of ethanol to ethylene.
Based on the process of formation of BD from ethanol listed in
˚
vacuum region of 15 A, and maintain the other two layers of
the slab xed in the geometry.
Avoiding unphysical dipole–dipole interactions between two
consecutive slabs, we adopted an asymmetry supercell in which
the atoms of the uppermost layer were satised by OH and the
bottom Si atoms were unsatised.
The alkalinity of metal oxide mainly lies in O2ꢁ; however, the
catalytic activity of MgO surfaces is attributed to their crystal
defects.17 Just as the results of coordination characterization,
Fig. 3 The ion intensity of products versus reaction time on the MgO surface.
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Fig. 4 The ion intensity of products versus reaction time on the SiO2 surface.
three-coordinated Mg2+ adsorption site at the kink corner (Mg3c
and O3c).
In our work, we used the geometry of periclase. The surface
energy of MgO (100) is the lowest of; MgO (100), MgO (110),
MgO (111)/Mg and MgO (111) by calculations, which agrees with
Refson et al.29 Jose A. Rodriguez's work has shown that slabs of
3–4 layers provide a very good representation of the MgO (100)
surface.30 Taking the three types of the above defects into
consideration, the model employed in this paper is presented in
Fig. 6. The geometry optimization of bulk MgO provides us a
˚
typical rock-salt structure with a0 ¼ 4.28 A, which is close to the
31
˚
experiment measurements (4.22 A).
Fig. 5 Crystal models for the SiO2 surface (a). The model for b-cris-
tobalite; (b). The model for b-cristobalite (111) surface.
3.4 Surface adsorption
3.4.1 The adsorption of ethanol on SiO2. Taking several
adsorption sites and geometry of ethanol on the SiO2 surface
into consideration, the calculation results show that ethanol is
adsorbed on a certain site of b-cristobalite (111) and the most
stable bonding congurations are shown in Fig. 7. The driving
force of ethanol adsorption on the b-cristobalite (111) is that
the addition of SiO2 increases the defects of MgO. Thus, the
study of reaction species adsorbed on the MgO defects is of
great signicance. There are three coordination sites of the
MgO surface: ve-coordinated Mg2+ ion at a terrace (Mg5c and
O5c), four-coordinated Mg2+ ion at a step site (Mg4c and O4c),
Fig. 6 Optimized geometry diagrams of different MgO surfaces.
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Dissociative adsorption also appears when ethanol is
adsorbed on the kink sites of MgO, and the adsorption energy is
˚
3.39 eV. The bond length of O–H in ethanol is 4.630 A, and the
fractured H forms an OH with O3c on the kink surface of MgO
˚
(bond length ¼ 0.967 A). The distance between O in ethanol and
˚
˚
˚
Mg5c, Mg3c, Mg4c is 2.403 A, 1.982 A, 2.131 A, respectively.
Bond length of O–H forming on kink surface of MgO is the
shortest, which illustrates that the interaction between ethanol
and the kink surface of MgO is the strongest.
In summary, the above results reveal that ethanol is prefer-
ably adsorbed on the defective sites of MgO. However, the
adsorption of ethanol on MgO or SiO2 is competitive adsorp-
tion, it will produce the by-product of ethylene when ethanol is
adsorbed on the SiO2.24
3.4.3 Analysis of density of states. In order to deeply
comprehend the interaction between ethanol and SiO2 or MgO,
we also studied density-of-states (DOS) of some relative atoms.
For contrast, just the O, H in ethanol and Mg, and O in MgO
were selected to calculate DOS. Fig. 9 displays calculated DOS
plots for the systems. For ethanol, the O 2p levels are located at
3 to 7 eV and states that it contains a mixture of Mg, while for O
in MgO surface and H of OH in ethanol the s character appear
between ꢁ5 and ꢁ9 eV. Furthermore, the overlay area gradually
increases and the peak of a strong hybrid appears. This illus-
trates the interaction between H of OH in ethanol and O on the
surface of the catalyst gradually increases from SiO2, terrace,
step to kink.
Fig.
7 Stable geometric configurations of CH3CH2OH molecule
adsorbed on the b-cristobalite (111) surface.
hydrogen bonds are formed between the Si–OH and OH in
ethanol. However, the adsorption energy is 0.58 eV, and the
adsorption behavior is a weak adsorption.
3.4.2 The adsorption of ethanol on MgO. The stable
geometric congurations of ethanol adsorbed on the terrace,
step and kink surfaces of MgO are shown in Fig. 8. First, we
discuss the stabilities and geometrical parameters of ethanol
adsorbed on the terrace sites of MgO, C–C slopes on the surface
and the two bonds formed between the ethanol molecule and
catalyst surface: H of OH in ethanol with O on the MgO surface
3.5 Surface reaction
˚
(band length ¼ 1.776 A); O in ethanol with Mg on the MgO
3.5.1 The dehydrogenation of ethanol on at MgO surface.
The adsorption of ethanol on at MgO has been introduced as
part of the adsorption of ethanol on MgO. The initial, transi-
tion, and nal states of the dehydrogenation of ethanol on the
at MgO surface are shown in Fig. 10. The reaction starts by the
H atom moving towards the catalyst surface and the H atom
adsorbs at the fcc site (at the TS). The O–H bond distance is
˚
surface (band length ¼ 2.352 A). The adsorption energy is 0.58
eV, which is slightly larger than ethanol on SiO2.
Dissociative adsorption appears when ethanol is adsorbed
on the step sites of MgO, and the adsorption energy is 1.92 eV.
The bond length of O–H in ethanol is 2.001 A, which indicates
the cleavage of O–H. The fractured H forms an OH on the at
surface of MgO, and the bond length is 0.985 A. The O–Mg bond
˚
˚
˚
˚
elongated to 3.024 A (at the TS) from the initial 0.982 A. The
reaction barrier is 1.63 eV and the reaction energy is ꢁ3.44 eV,
indicating a strong exothermic reaction.
˚
is formed between the Mg5c (bond length ¼ 1.977 A), Mg4c
˚
(bond length ¼ 2.221 A) and ethoxy group.
Fig. 8 Stable geometric configurations of the CH3CH2OH molecule adsorbed on different MgO surfaces.
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Fig. 11 Stable geometric configuration of the CH3CH2OH molecule
adsorbed on stepped MgO surface.
3.5.2 The dehydrogenation of ethanol on stepped MgO
surface. Dissociative adsorption appeared when ethanol is
adsorbed on the stepped sites of MgO, as shown in Fig. 11, and
the adsorption energy is 1.92 eV. The bond length of O–H in
˚
ethanol is 2.001 A, which indicates the cleavage of O–H bond.
Branda et al.17 investigated the adsorption of ethanol on
different defective sites of MgO, and drew the conclusion that
dissociative adsorption appeared when ethanol was adsorbed
on the low coordination of the MgO surface, whereas there was
poor adsorption for at MgO surface, and our research studies
agree with theirs'.
3.5.3 The dehydrogenation of ethanol on kinked MgO
surface. Fig. 12 shows that dissociative adsorption also appears
when ethanol is adsorbed on the kink sites of MgO, and the
adsorption energy is 3.39 eV. The fractured H forms an OH with
Fig. 9 DOS profiles of the CH3CH2OH molecule before and after
adsorption on SiO2 and three different MgO surfaces.
˚
O3c on the kink surface of MgO (bond length ¼ 0.967 A). The
distance between O in ethanol and Mg5c, Mg3c, Mg4c is 2.403
˚
˚
˚
A, 1.982 A, 2.131 A, respectively.
Fig. 10 Potential energy curve for the formation of the ethoxy group
from ethanol dehydrogenation on flat MgO surface.
Fig. 12 Stable geometric configuration of the CH3CH2OH molecule
adsorbed on kinked MgO surface.
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Fig. 13 Potential energy curve for the ethanol dehydrogenation to ethoxy group on stepped and kinked MgO surface.
From the above analysis, we can conclude that the reaction
of ethanol to ethoxy group on three surfaces show activity
4 Conclusions
increasing from at < step < kink, which agrees with the PDOS
analysis. Branda et al. also draw the conclusion that the
adsorption of ethanol on the at surface is weak and ethanol
shows dissociative adsorption on low coordination.17,32
3.5.4 The dehydrogenation of ethoxy group on stepped and
kinked MgO surface. The above results show that adsorption
of ethanol on at MgO surface is poor, and the activation
energy of ethanol dehydrogenation to the ethoxy group is
higher, which indicates the dehydrogenation of ethanol to
ethoxy group reacts on the defective sites of the MgO surface.
Thus, it would be interesting to study the dehydrogenation of
the ethoxy group on the step and kink defects of the MgO
catalysts and make comparisons with the perfect surfaces
studied here.
Through the catalysts characteristics and analysis, we have
established models of MgO and SiO2 surface structures, espe-
cially the three defective structures in MgO. The role of SiO2 in
MgO/SiO2 has been recognised, showing that the structural
defects of MgO were increased by adding SiO2 in the catalyst,
which is in favor for the reaction of dehydrogenation of ethanol
to acetaldehyde. It provides theoretical guidance for the
improvement of catalyst performance in order to enhance the
conversion of ethanol to 1,3-butadiene.
The reaction of ethanol dissociation to an ethoxy group was
preferable on defective MgO surfaces. The order of adsorption
energy of ethanol on three different MgO surfaces is terrace site
< step site < kink site. The activation energy for the reaction of
ethanol dissociation to an ethoxy group on the at MgO surface
is 1.63 eV, which indicates it is more difficult to react on the at
MgO surface.
The stepped MgO surface is more active for the reaction of
dehydrogenation of ethanol to acetaldehyde. The adsorption
energy of the ethoxy group on stepped and kinked MgO surface
is 1.89 eV and 3.16 eV, respectively. However, the activation
energy of dehydrogenation of the ethoxy group to acetaldehyde
on stepped MgO surface is 1.30 eV, which is lower than that on
the kinked MgO surface (1.42 eV).
Fig. 13(a) shows the potential energy curve for the ethanol
dehydrogenation to the ethoxy group on stepped MgO
˚
surface. The C–H bond is elongated to 2.084 A at the TS from
˚
the initial length of 1.113 A, while it is further elongated to
˚
2.977 A in the nal state. The distance of H in the ethoxy
group to O in stepped MgO surface decreases gradually, until
˚
nally reaching 0.997 A. The reaction barrier is 1.30 eV, and
the reaction energy is ꢁ1.19 eV, indicating an exothermic
reaction.
Fig. 13(b) shows the potential energy curve for the ethanol
dehydrogenation to the ethoxy group on kinked MgO surface.
Acknowledgements
˚
The C–H bond is elongated to 1.753 A at the TS from the
˚
initial length of 1.105 A, while it is further elongated to 1.854
˚
The authors thank the key laboratory for green chemical tech-
nology of ministry of education, for technical assistance.
A in the nal state. The distance of H to O, in the ethoxy group
on the stepped MgO surface, decreases gradually and nally
˚
reaches 0.990 A. The reaction barrier is 1.42 eV and the
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