Conversion of ethanol to butadiene over mesoporous In2O3-promoted MgO-SiO2 catalysts

Article abstract of DOI:10.1016/j.mcat.2020.110984

Mesoporous MgO-SiO₂ mixed oxide catalysts were prepared for the conversion of ethanol to 1,3-butadiene. Mesoporosity was obtained by using SBA-15 material as support for magnesia or by applying one-pot synthesis method wherein magnesia precurso

Full text of DOI:10.1016/j.mcat.2020.110984

MolecularCatalysis491(2020)110984
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Conversion of ethanol to butadiene over mesoporous In2O3-promoted MgO-
SiO2 catalysts
Blanka Szabóa, Gyula Novodárszkia, Zoltán Maya, József Valyona, Jenő Hancsókb,
Róbert Barthosa,*
a Research Centre for Natural Sciences, Institute of Materials and Environmental Chemistry, Magyar tudósok körútja 2, Budapest 1117, Hungary
b Institute of Chemical and Process Engineering, University of Pannonia Veszprém H-8201, Hungary
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ethanol
Butadiene
MgO-SiO2 catalysts
Indium oxide
Acid-base properties
Mesoporous MgO-SiO2 mixed oxide catalysts were prepared for the conversion of ethanol to 1,3-butadiene.
Mesoporosity was obtained by using SBA-15 material as support for magnesia or by applying one-pot synthesis
method wherein magnesia precursor, such as, magnesium methoxide or ethoxide, was admixed to the synthesis
gel of the SBA-15 material. The SBA-15 support was wet-impregnated by Mg(NO3)2 solution or wet-kneaded
with Mg(OH)2 precipitate. The synthesized samples were mesoporous, however, the alkoxide in the synthesis gel
hindered the formation of regular SBA-15 structure. In wet-kneaded and one-pot-synthesized samples the pre-
sence of MgeOeSi bonds was confirmed by X-ray diffractometry. The acid-base properties of the preparations
were characterized by the room-temperature adsorption capacity for CO2 and NH3. Formation of MgeOeSi
bonds were shown to be responsible for the increased acidity/basicity of the samples. The best catalysts were the
wet-kneaded sample and the sample synthesized using methoxide as magnesium source. Over these catalysts the
butadiene yield reached 15 % at 400−425 °C. To enhance the ethanol-to-butadiene activity the mixed oxides
were promoted by In2O3. Additive In2O3 significantly improved dehydrogenation activity generating more
acetaldehyde, and suppressed dehydration activity giving less ethylene and diethyl ether. Butadiene yields above
40% were achieved.
1. Introduction
century. In the one-step process only, ethanol is introduced into a cat-
alytic reactor, whilst in two-step process first the ethanol is dehy-
1,3-butadiene (BD) is the most important conjugated diene. It is
used as monomer in the synthesis of poly-butadiene or as a building
block of different heteropolymers, such as, styrene-butadiene, nitrile-
butadiene rubber, styrene-butadiene latex and acrylonitrile-butadiene-
styrene resin. Synthesis of monomers, like hexamethylenediamine and
chloroprene, which are monomers of Nylon and Neoprene, respectively,
uses also BD as reactant [1]. Currently, BD is obtained mainly as by-
product of ethylene production by steam cracking naphtha. The feed-
stock lightening, such as, the use of shale gas for ethylene production
led to BD shortage and rise in BD prices [2]. An alternative way of
getting BD can be the catalytic coupling of ethanol to butadiene (ETB
reaction). Ethanol is one of the most abundant renewable raw mate-
rials. Its annual production rapidly grows which results in a price re-
duction [3]. The fundamentals of the ETB process were elaborated by
Lebedev [4] and Ostromisslensky [5], usually distinguished as one-step
and two-step process, respectively, in the first half of the previous
drogenated to acetaldehyde separately, then the produced acetaldehyde
is mixed up with ethanol and fed into the reactor. In condensation re-
action two acetaldehyde molecules can give crotonaldehyde, which is
then hydrogenated by another ethanol molecule to crotyl alcohol. Such
transfer hydrogenation is known as heterogeneous catalytic Meer-
wein–Ponndorf–Verley (MPV) reduction. In the final step crotyl alcohol
must to become dehydrated to get BD. The nice feature of the process is
that reducing agent ethanol is not lost as sacrifice but is converted to
acetaldehyde that is intermediate of the ETB process.
Among all the tested catalysts the MgO-SiO2 materials showed the
best activities in the ETB reaction [6–20]. Since ethanol dehydrogena-
tion to acetaldehyde is the rate determining step of the consecutive
reaction different metals/metal-oxides such as Zn, Zr, Cu, Au, and Ga
were added to the MgO-SiO2 catalyst to promote this step. It was
pointed out that the catalytic activity depends not only on the com-
position but also on the structure and morphology of the catalyst that
⁎ Corresponding author.
E-mail addresses: szabo.blanka@ttk.hu (B. Szabó), novodarszki.gyula@ttk.mta (G. Novodárszki), may.zoltan@ttk.hu (Z. May), valyon.jozsef@ttk.hu (J. Valyon),
hancsokj@almos.uni-pannon.hu (J. Hancsók), barthos.robert@ttk.hu (R. Barthos).
https://doi.org/10.1016/j.mcat.2020.110984
Received 7 December 2019; Received in revised form 21 April 2020; Accepted 27 April 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

B. Szabó, et al.
MolecularCatalysis491(2020)110984
determines the nature and amount of available active sites. Latter
properties can be directed by the method applied for catalyst pre-
paration. The methods reported for the preparation of MgO-SiO2 cata-
lyst were (i) mechanical mixing, (ii) co-precipitation, or (iii) wet
kneading of the components, and (iv) incipient wetness impregnation of
one component, usually the SiO2, with the precursor of the other one.
All cases the preparation was finished by calcination. Nevertheless,
there is no consensus about the best catalyst structure and method of
catalyst preparation. It is generally accepted that the ETB reaction de-
mands an appropriate balance of basicity and acidity. In MgO-SiO2
systems the role of magnesia component is to provide basic sites, while
acidic sites rise due to charge imbalance along Mg–O–Si bonds. The
number and strength of basic sites depends on the dispersity of the
magnesia, while the acidity of the mixed oxide is linked to the number
and nature of Mg–O–Si bonds. It was also shown that the BD yield
depends on the specific surface area [10], crystal size/shape [21,22]
and pore diameter of the MgO-SiO2 catalyst [23,24].
The mesoporous materials have several advantages compared to
microporous or non-porous materials. Mesopores foster mass and heat
transport and facilitate the high dispersion of the metal/metal oxide
promoters. Two major techniques were developed for the preparation
of basic catalyst that retain the favorable mesoporous structure: (i) the
two-step method, wherein the mesoporous framework is synthesized
first, followed by ion-exchange, impregnation, microwave irradiation or
anchoring of the basic guest, and (ii) the one-pot synthesis, where the
precursors of the basic guest and the mesoporous host are loaded to-
gether in a single reactor vessel.
samples were white and free of metal ions. It was also shown that both
anions and cations have an effect on the mesostructure of the SBA-15.
Wang et al. [34] found that the strength of effect follows the lyotropic
series NO3 < Cl− < CH3COO− of the anions. The MgO/mesoporous
−
silica preparations were used as adsorbents for CO2 [29,35] and cur-
cumin [28] or as catalyst in selective oxidation [27], transesterification
[30] and CO2 reforming of methane [36].
It is well known that SBA-15 material possesses hexagonally ar-
ranged, channel-like, parallel mesopores of about uniform diameter,
large specific surface area, and high silanol group concentration.
Catalytically active species can be stabilized in high dispersion within
the regular pore structure, providing very similar chemical environ-
ment for the active sites that is beneficial regarding the catalytic se-
lectivity. Thanks to its thick pore walls the SBA-15 material is known to
have excellent thermal, hydrothermal stability and outstanding acid
resistance [37]. Considering above benefits this work deals with MgO/
SBA-15 catalysts prepared by the methods of (i) impregnation, (ii) wet
kneading, and the (iii and iv) one-pot synthesis method using magne-
sium ethoxide or magnesium methoxide as Mg source. The MgO/SBA-
15 preparations were characterized by means of Inductively Coupled
Plasma - Optical Emission Spectrometry (ICP-OES), X-ray Diffraction
(XRD), N2 physisorption, and Transmission Electron Microscopy (TEM).
TPD curves of adsorbed carbon dioxide and ammonia were recorded to
learn more about the relative basicities and acidities of the samples. The
catalysts were tested in direct conversion of ethanol to butadiene. The
promoting effect of In2O3 on the ETB reaction is also discussed.
The imperceptible acidity and basicity limits the catalytic applica-
tion of ordered mesoporous silica. In order to exploit the benefits of its
mesoporosity, Chae et al. [21] used silicas, such as, SBA-15, KIT-6, and
MMS, as support for basic tantalum pentoxide and used the obtained
Ta2O5/silica catalyst for the transformation of ethanol/acetaldehyde
mixture to BD. The mesoporous catalysts were found to show better
coke tolerance, catalytic longevity, as well as, activity than catalysts
based on conventional silica. Klein and Palkovits [24] studied ZnO/
zeolite catalysts in the ETB reaction. Higher catalytic activity was at-
tained, if the zeolite particles contained not only micropores but also
mesopores. Fujita et al. [25] used Mg-incorporating MCM-41 meso-
porous silica materials as catalysts for the aldol condensation of 4-ni-
trobenzaldehyde and acetone. It was reported that the large surface
area and pore volume of the catalyst was favorable for the reaction.
Catalysts, containing both zinc and tantalum as active components on
different supports were compared by Pomalaza et al. [26] in the ETB
reaction. The mesoporous silica TUD-1 silica was found to be much
more advantageous support for magnesia than the dealuminated zeolite
Beta or the fumed silica.
To our knowledge mesoporous MgO-SiO2 materials have not been
examined yet in ETB reaction. Commonly, the mesoporous silica is
synthesized first and impregnated then by the precursor of MgO, such
as, Mg(NO3)2 [27] or Mg(CH3COO)2 [28]. Calcination of the prepara-
tion results in silica-supported MgO catalyst. Unfortunately, the catalyst
usually cannot fully retain the mesoporous structure of the support.
Zukal et al. [29] elaborated a modified method to avoid destruction of
silica mesopores. Magnesium acetate, impregnated on silica surface was
converted by oxalic acid to magnesium oxalate. The oxalate could be
converted to magnesia at relatively low temperature. According to an
alternative method magnesium salt and as-prepared micelle-templated
silica (MTS) were ground together at room temperature and calcined
then to remove the structure directing agent (SDA) of the MTS material
[30]. The basicity of the catalyst could be further enhanced by adding
alkali salt, such as KNO3 to the preparation [31]. In the so-called one-
pot method silica, magnesia source (magnesium nitrate or magnesium
acetate), and SDA are mixed to get a synthesis mixture [31–36]. In the
synthesis process silica is believed to incorporate magnesium ions in its
structure. However, Kónya et al. [33] demonstrated that during the
synthesis of SBA-15 in presence of different metal salts the fully washed
2. Experimental
2.1. Preparation of catalysts
Basic MgO/SBA-15 catalysts were prepared by impregnation and
wet kneading methods using SBA-15 material provided by Nanjing
XFNANO Materials Tech Co., Ltd, China, and a magnesia precursor.
Moreover, two catalysts were synthesized by the so-called one-pot
method using magnesium ethoxide and methoxide as Mg source.
The impregnation was carried out by adding 200 mL 0.5 M solution
of Mg(NO3)2·6H2O (Sigma-Aldrich, 99%) to 10.00 g of SBA-15, dried at
120 °C for 2 h. The water was removed under continuous stirring at
70 °C. The sample was dried then at 120 °C overnight and calcined in air
at 500 °C for 5 h. The sample was designated as IMP.
For the wet kneading procedure Mg(OH)2 precipitate was made. To
a sodium hydroxide solution Mg(NO3)2·6H2O solution was added at pH
of 12. During formation of precipitate the pH was maintained by slow
addition of 2 M NaOH. The precipitate was aged in the mother-lye for
24 h, washed then to remove sodium and dried at 120 °C overnight. The
wet kneading was carried out by stirring of 2.15 g Mg(OH)2 and 5.00 g
of dried SBA-15 in 100 mL of distilled water at room temperature for
5 h. The suspension was centrifuged, the solid was dried at 120 °C
overnight and calcined in air at 500 °C for 5 h. The sample was desig-
nated as WK.
The one-pot synthesis with magnesium ethoxide was performed as
follows: 16 g template for SBA-15 synthesis (Pluronic P-123, Aldrich)
was dissolved in 600 mL 0.03 M HCl solution by stirring at room tem-
perature for 2 h. A second mixture was made by mixing 37 mL TEOS
(Tetraethyl orthosilicate, Aldrich, 98%) and 8.20 g magnesium ethoxide
(Aldrich, 98%) in 250 mL in methanol. Thereafter, the second mixture
was dropwise added to the vigorously stirred Pluronic P-123 solution at
room temperature. After stirring for further 24 h at 40 °C the mixture
was transferred into a Teflon-lined autoclave and treated for a 24 h at
100 °C. The solid sample was washed with distilled water until the su-
pernatant was free of chloride. The product was dried overnight at
120 °C, and the template was removed by calcination in air. The sample
was calcined by heating up at a rate of 1 °C min−1 to 500 °C and keeping
at this temperature overnight. The sample was designated as OPET.
For the one-pot synthesis with magnesium methoxide first a pre-
2

B. Szabó, et al.
MolecularCatalysis491(2020)110984
hydrolized TEOS solution was made using the same amounts of
Pluronic P-123 template, HCl solution and TEOS as used for the
synthesis with magnesium ethoxide (vide ultra). To this stirred solution
155 mL magnesium methoxide solution (7−8 wt.% in methanol, Alfa
Aesar) was added dropwise. The gelation of the obtained sol started
immediately. The viscous gel was stirred for further 30 min and after
was transferred into a Teflon lined autoclave. The procedure was fin-
ished as described above for the OPET sample. The sample was desig-
nated as OPMET.
Catalysts, promoted by In2O3 were obtained by impregnating the
mixed oxide catalysts using an about 33 mM/dm3 solution of In
(NO3)3·xH2O (Alfa Aesar 99.99% metal purity) in an amount needed to
get catalysts having In2O3 content of about 2.0 wt.%, 5.0 wt.%, and
10.0 wt.%. In order to get appropriate metal concentration for the ali-
quots the water content of the nitrate salt was determined by thermo-
gravimetry. The number before the abbreviation of the catalysts name
denotes the In2O3 content of the sample in weight percent, e.g., 5IMP
means 5 wt.% In2O3 on the surface of the catalyst, made by the im-
pregnation method.
flushed through the catalyst bed with a helium flow. The effect of
weight hourly space velocity (WHSW) on the conversion was measured
by 14.7 vol.% ethanol/He feed in the flow rate range of 5–120 cm3
min−1 at 350 °C temperature. The effect of reaction temperature was
examined
at
the
total
flow
rate
of
30 mL min−1
(WHSV = 0.5 gethanol gcat−1 h−1). All gas lines of the reactor system
were kept at 120 °C temperature to avoid the condensation of ethanol
and reaction products. The reaction products were analyzed by on-line
Shimadzu GC-2010 gas chromatograph equipped with two FID detec-
tors. A Chrompack PLOT Fused Silica column with Al2O3/KCl sta-
tionary phase (50 m long, 0.32 mm diameter) was used for the analysis
of hydrocarbon products and
a HP-PLOT-U column (30 m long,
0.32 mm diameter) was used for the analysis of oxygenates. The cali-
bration of the GC was carried out for the reactant and each product
separately. The conversion of ethanol was calculated from the ethanol
concentration of the feed and the reactor effluent. The selectivity cal-
culation was based on the carbon atom content of the products. The
carbon balance accuracies were usually better than 95%.
3. Results
2.2. Characterization
XRD patterns of the MgO-SiO2 samples are shown on Fig. 1. Each
sample exhibit a broad diffraction line at around 2θ = 23°, which can
be attributed to amorphous silica.
On the XRD patterns of one-pot-synthesized samples two additional
lines can be observed at 2θ = 35.14° and 60.62° (Fig. 1a, OPET,
OPMET). According to the literature these lines indicate MgeOeSi
bonds and are interpreted as magnesia-silica hydrates [20] or chrysotile
[38]. However, on the diffractograms the (00l) indexed lines are
lacking and the observed broad lines can be attributed with great un-
certainty to talc structure disordered in direction c.
The composition of the samples was determined by inductively
coupled plasma optical emission spectrometry (ICP OES) with axial
plasma observation and capability for simultaneous and multi-element
determination (Spectro Genesis).
X-ray diffraction patterns were recorded at ambient conditions by
Philips PW 1810/3710 diffractometer applying monochromatized CuKα
(λ = 0.15418 nm) radiation (40 kV, 35 mA) and proportional counter.
Data were collected between 3° and 75° 2θ, in 0.02° steps for 0.5 s in
each step. The low-angle XRD measurements were executed by Philips
X’Pert MPD system using 2θ−ω scan method between 0.6–3.0 2θ, in
0.002 steps for 10 s in each step.
Nitrogen physisorption measurements were carried out at 77 K
using Thermo Scientific Surfer automatic, volumetric adsorption ana-
lyzer. Before adsorption measurement, samples were outgassed under
vacuum for 2 h at 523 K.
Sample morphology was studied by an FEI Tecnai G2 20 X Twin
transmission electron microscope (TEM) at a 200 kV accelerating vol-
tage. Samples were drop-cast from distilled water suspension onto a
copper mounted holey carbon film.
Temperature-programmed desorption (TPD) measurements were
carried out by CO2 and NH3 using a flow-through microreactor (I.D.
4 mm) made of quartz. About 100 mg of catalyst sample (particle size:
0.315–0.65 mm) was placed into the microreactor and was pre-treated
in a 30 cm3 min−1 flow of O2 at 500 °C for 1 h, then flushed by N2
(30 cm3 min−1) at the same temperature for 15 min. The pre-treated
sample was evacuated at 500 °C, cooled to room temperature and
contacted with NH3 or CO2 at 13 kPa pressure. After 15 min of contact
the physiosorbed molecules were removed by evacuation. Then the
reactor temperature was ramped up in He flow (20 cm3 min−1) at a rate
of 10 °C min−1 to 500 °C and held at this temperature for 1 h, while the
effluent gas was passed through a dry ice/acetone trap and a thermal
conductivity detector (TCD). Data were collected and processed by
computer. Calculation of the adsorbed amount of NH3 or CO2 is based
on the peak areas. The TCD was calibrated by passing known amounts
of NH3 or CO2 through the detector.
On the XRD pattern of the IMP sample no characteristic lines can be
observed. This suggests that the dispersion of MgO over the SBA-15
support is high enough to be XRD amorphous.
In contrast, the XRD lines of MgO appear on the diffractogram of the
calcined WK sample at 2θ = 42.92° and 62.04°. The line, also appearing
at 2θ = 35.52°, and the shoulder at 2θ = 60.62° proves that during the
wet kneading and calcination process MgeOeSi bonds were formed. In
the XRD patterns of indium doped samples no lines of indium oxide
appeared proving the absence of crystalline indium oxide (only the
pattern of 10OPMET is shown in Fig. 1a). Only the IMP and WK samples
give reflections characteristic for the hexagonally ordered two-dimen-
sional pore system of SBA-15 materials (Fig. 1b).
Fig. 2a shows the N2 adsorption-desorption isotherms of the neat
basic catalyst preparations. According to the IUPAC classification [39]
the isotherms of parent SBA-15 and IMP samples are of Type IV, those
of the OPET and OMET samples are of Type II, while the isotherm of the
WK sample is the combination of the Type II and IV isotherms. Hys-
teresis loops can be observed on each isotherm indicating the presence
of mesoporous system wherein capillary condensation occurs. Only the
isotherm of neat SBA-15 material shows adsorption and desorption
branches characteristic for H1 type hysteresis loops stemming from
fairly regular array of mesopores having narrow distribution of sizes. In
case of IMP and WK samples the hysteresis loop is somewhat distorted
compared to that of neat SBA-15 material, indicating that the in-
troduction of magnesia mechanically and/or chemically shattered the
pore system. The isotherm of WK sample presents open hysteresis loop
even at relative pressure close to one. This may indicate large meso-
pores among agglomerated MgO nanoparticles. The hysteresis loop on
the isotherm of the OPET sample resembles to H3 type loop which is
characteristic for the morphology of aggregated plate like particles
[40]. The OPMET sample exhibits H2 type hysteresis loop. Such loop is
usually found on the isotherm of inorganic oxide gels having pore
system made up of interconnected network of pores of different sizes
and shapes [40]. The N2 adsorption-desorption isotherms of the in-
dium-oxide-promoted samples are shown on Fig. 2b. The H2 type
2.3. Catalytic activity
Catalytic reactions were carried out at atmospheric pressure in a
fixed-bed, continuous flow glass tube (∅ = 10 mm) microreactor. Prior
to reaction the catalysts were activated in oxygen flow (20 mL min−1
)
for 1 h at 450 °C. In the reaction 1 g of catalyst (particle size
0.315−0.65 mm) was used. By a Gilson 307 HPLC Piston Pump re-
actant ethanol was fed in an evaporating zone kept at 120 °C and
3

B. Szabó, et al.
MolecularCatalysis491(2020)110984
Fig. 1. XRD patterns of MgO-SiO2 samples in
a) wide and b) small-angle 2θ region. Samples
were prepared by impregnation of SBA-15
material (IMP), wet kneading of SBA-15 ma-
terial and Mg(OH)2 precipitate (WK), and by
modified micelle-templated SBA-15 synthesis
method using magnesium ethoxide (OPET), or
magnesium methoxide (OPMET) as magne-
sium source. The In-promoted sample, con-
taining 10 wt.% In2O3 (10OPMET) was made
from the OPMET sample by wet impregnation.
hysteresis loops suggest that the more or less ordered SBA-15-like ma-
terials became restructured during the In-doping procedure. The data
obtained from ICP-OES analysis and N2 adsorption measurements are
summarized in Table 1. The silica to magnesia mass ratio of the sam-
ples, except that of the OPET sample, was near to one. The SSA of the
one-pot synthesized samples was higher than that of the IMP and WK
samples. Relative to the specific pore volume of parent SBA-15 material
magnesia introduction reduced the pore volume of WK and IMP sam-
ples, indicating that impregnation partially clogged the pores. At the
same time the diameter of the most frequent pores remained practically
unchanged suggesting that part of the channels remained intact. The In-
modification of IMP and WK samples led to increased specific surface
area (SSA, Table 1). In contrast, the SSA and pore volume of the OPET
and OPMET samples decreased upon introduction of In2O3 (Table 1).
The impregnation of the OPMET sample with indium salt resulted in a
further decrease of the pore volume and the most frequent pore size.
The pores, formed between aggregated MgO-SiO2 nanoparticles made
the mesopore structure of the OPET sample bimodal. The pore volumes
and pore size data of Table 1 are relevant for this bimodal structure.
Textural properties of the samples were characterized by means of
TEM images (Fig. 3). The obtained results are in harmony with the
conclusions drawn above from XRD and N2 physisorption results. The
partially damaged SBA-15 structure can be observed on the images of
the IMP and WK samples, whereas images of the OPMET and OPET
samples are similar to those usually obtained for inorganic oxides/
mixed oxides. On the images of the In-doped samples no regular
channel system is discernable. In this respect the 5IMP sample seems to
be an exception. No In2O3 crystallites show up on any image of the In-
containing samples.
Results of ETB reaction over MgO-SiO2 preparations are shown in
Fig. 2. N2 adsorption-desorption isotherms of the samples at −196 °C. Adsorption and desorption branches are indicated by full and open symbols, respectively. For
the meaning of sample designations see the legend of Fig. 1.
4

B. Szabó, et al.
MolecularCatalysis491(2020)110984
Table 1
The ETB activity of OPMET catalysts containing 2 wt.%, 5 wt.% and
10 wt.% In2O3 was compared. Over the catalyst, containing 5% In2O3
the EE selectivity was only 20% even at the highest applied reaction
temperature and the BD selectivities remained above about 45% on all
the applied reaction temperatures (Fig. 6b). At lower and higher In
contents the EE selectivity was higher and lower, respectively. It has to
be also noticed that not only the BD but also the AA yields are higher
than that over the In-free catalysts. The AA selectivity changes parallel
with the In content of the catalysts. Its increase occurs on the expense of
butadiene yields. The stability test over 5OPMET catalyst at 375 °C
shows that after 10 h time on stream the conversion falls from the initial
52% to 39%. After 25 h it stabilizes at 30%. We observed continuous
fall and increase of BD and AA selectivities, respectively (Fig. 6d). In
view of formation rates the highest values were achieved over WK and
5OPMET samples at 425 °C, by reaching the values of 0.9 and
1.7 mmolBD*gcat−* 1 h−1, respectively. These values are in the same
range as it is reported by Da Ros et al. [19] for ZrZn-MgO/SiO2 catalysts
(0.6–3 mmolBD*gcat−* 1 h−1) and a somewhat lower than earned by Se-
kiguchi et al. [41] for the zinc containing talc catalysts
(8–19 mmolBD*gcat−* 1 h−1).
Characterization of the catalysts.
a
a
Sample ID
SiO2
MgOa
In2O3
SSAb
PVc
PDd
wt%
100
m2/g
494
191
356
594
486
334
326
327
400
402
411
cm3/g
1.20
0.51
0.90
1.38
0.48
0.26
0.31
0.31
0.39
0.50
0.85
nm
SBA-15
IMP
WK
–
–
–
–
–
8.09
7.89
8.05
37.81
3.72
3.62
3.56
3.58
3.47
3.73
25.70
57.81
51.22
70.93
48.96
47.72
49.02
44.16
54.41
48.84
65.91
42.19
48.78
29,07
51.04
50.31
46.17
46.04
39.71
46.52
27.03
OPET
OPMET
2OPMET
5OPMET
10OPMET
5IMP
–
1.97
4.81
9.80
5.88
4.65
7.01
5WK
5OPET
a
Elemental composition determined by ICP-OES measurements.
Specific Surface Area.
Pore volume calculated by Gurvich method.
b
c
d
The most frequent pore diameter calculated from desorption branch by
BJH method.
For better understanding the role of MgO and In2O3 the catalytic
activity of neat MgO and In2O3/SBA-15 was compared. Fig. 7a shows
that in studied temperature range BOL (∼30%), AA, BD and CA (∼
20%−20%) were formed with almost constant selectivity at low con-
version level (∼0.5%–10%) over MgO catalyst. It is also worth noting
that only EE was identified among the dehydration products with a
selectivity of 2%–10%, whereas over pure MgO-SiO2 catalysts (cf.
Fig. 4) DEE and EE were obtained with a selectivity of 70%–90% in the
temperature range applied. AA was formed over the In2O3/SBA-15
catalyst (Fig. 7b) with high selectivity (∼95%), whereas DEE, EE and
ethyl acetate could be detected as other products. It can also be seen in
the figure that ethanol conversion was significantly higher over In2O3/
SBA-15 catalyst than over neat MgO, however, the conversion over the
In2O3/SBA-15 catalyst was lower in the whole temperature range than
over any of the MgO-SiO2 catalysts (Fig. 4).
Fig. 4. Similar conversion curves were obtained for all four catalysts.
The main reaction products were formed in ethanol dehydration reac-
tions. The exothermic reaction of diethyl ether (DEE) formation domi-
nated at lower temperatures, whereas the endothermic ethylene (EE)
formation reaction prevailed at higher temperatures. The highest BD
selectivity was achieved using the WK catalyst. It was about 20%–30%
in the applied 250−425 °C temperature range. Over other catalysts, not
regarding the selectivity at low conversion, the BD selectivity was
around 10%. Small amounts of acetaldehyde (AA) and butene isomers
(BUE, 1%–5% selectivity) were also identified in the product mixture.
Crotyl alcohol and crotonaldehyde was present only in traces.
The addition of 5 wt.% In2O3 to the MgO-SiO2 catalysts remarkably
enhanced ethanol conversion and improved BD selectivity (Fig. 5). The
activity improvement is more pronounced below about 350 °C reaction
temperature than at higher temperatures. The higher BD selectivities
and yields are paralleled with reduced yields of DEE and EE. The
conversion to EE rapidly increases on the expense of conversion to BD
as we increase the reaction temperature. The In2O3 in the catalysts also
increases the selectivities for AA, BUE, and butanol (BOL), relative to
the corresponding selectivities of the In-free catalysts.
The acidity and basicity of the catalysts were characterized by their
room-temperature adsorption capacity for NH3 and CO2, respectively
(Table 2). Fig. 8 shows the acidity and basicity of the neat and In2O3-
modified catalysts and their activity in the ETB reaction at the same
conversion. Identical conversions were achieved over the different
Fig. 3. TEM images of the samples. For the meaning of the sample designations see the legend of Fig. 1.
5

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MolecularCatalysis491(2020)110984
Fig. 4. Catalytic conversion of ethanol/He mixture at atmospheric pressure as function of reaction temperature over MgO-SiO2 catalysts using flow-through mi-
h−1 and about 15 kPa, respectively. The conversion curve is labelled by letter C.
−1
croreactor. The WHSV and the partial pressure of ethanol were 0.5 gethanol·gcat
Selectivity curves are given for butadiene (BD), diethyl ether (DEE), ethylene (EE), acetaldehyde (AA), and butenes (BUE). For the meaning of sample designations
see the legend of Fig. 1.
catalysts by selecting the suitable WHSV of ethanol. The WHSV was set
by adjusting the feed-rate of the 14.7 vol.% ethanol/He reactant mix-
ture at 350 °C temperature. From the neat MgO-SiO2 preparations at
25% conversion level the WK catalyst showed the highest BD selectivity
(Fig. 8a). The other In-free samples possess similar or higher con-
centration of acid sites, as indicated by the NH3 adsorption capacity,
and lower concentration of basic sites, suggested by the adsorption
capacity for CO2. Low concentration of basic sites combined with high
concentration of acidic sites results in a high dehydration activity, thus
higher ethylene-plus-diethyl-ether selectivities in the ETB reaction on
the expense of BD formation.
Addition of 5 wt.% In2O3 to the MgO-SiO2 preparations brings to the
Fig. 5. Catalytic conversion of ethanol/He mixture over In-modified MgO-SiO2 catalysts. For reaction conditions, catalyst and curve identifications see the legends of
Fig. 1 and 4. In the designation 5OPET the prefix number 5 indicates that the catalyst contains 5 wt.% In2O3.
6

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MolecularCatalysis491(2020)110984
Fig. 6. Activity and selectivity of In2O3-modified MgO-SiO2 catalysts in the ETB reaction between 250 and 425 °C temperatures. The catalyst was modified by a) 2 wt.
% (2OPMET), b) 5 wt.% (5OPMET), and c) 10 wt.% (10OPMET) In2O3. Section d) shows the stability of the 5OPMET catalyst at 350 °C. The ethanol WHSV and partial
h−1 and about 15 kPa, respectively.
−1
pressure were 0.5 gethanol gcat
Fig. 7. Ethanol conversion activity and selectivity of a) MgO and b) 5 wt.% In2O3/SBA-15 catalysts. The WHSV and the partial pressure of ethanol were
−1
0.5 gethanol gcat
h−1 and about 15 kPa, respectively.
7

B. Szabó, et al.
MolecularCatalysis491(2020)110984
MgO content by using this method is around 30 wt% [32].
Table 2
Results of CO2 and NH3 Temperature Programmed Desorption (TPD) mea-
surements.
We carried out one-pot sol-gel synthesis using TEOS as silica source
and magnesium alkoxide as magnesium source. The difficulty of
synthesis lies in the different hydrolysis rates of the TEOS and magne-
sium alkoxide. Moreover, the magnesium ethoxide is not soluble in
TEOS, so it must be dissolved in methanol before adding to the synth-
esis gel. The XRD patterns in Fig. 1 show that the faster hydrolysis of
magnesium alkoxide did not result in the formation of separate MgO
phase. Crystalline magnesium silicates could be observed besides
amorphous silica. Another advantage of the sol-gel synthesis besides of
good homogeneity of the mixed oxide is that it is possible to achieve
high SSA. The SSA of OPMET and OPET samples are 486 and 594 m2/g,
respectively (Table 1). These values are close to those reported for one-
pot preparations obtained using inorganic salts as magnesium source
(400−600 m2/g). On the basis of nitrogen adsorption isotherms and
TEM images it can be concluded that our OPMET and OPET samples are
mesoporous but does not have SBA-15 structure. It may be supposed
that the SDA promoted the formation of mesopores, but the diversity of
the chemical properties of TEOS and magnesium alkoxide prevent ar-
rangement to a regular array having narrow pore size distribution that
is characteristic of the SBA-15 material. The impregnation and wet
kneading steps have only partially damaged the original SBA-15
structure and the SSA of the samples remain considerably high (191 and
356 m2/g for IMP and WK samples, respectively) despite the con-
siderably high MgO content of the samples (∼50 wt.%). Impregnation
by In(NO3)3 solution caused structural rearrangement of the OPET and
OPMET preparations. The IMP and WK preparations suffered total loss
of their mesoporous structure.
Catalyst
Support
5% In2O3
a
b
a
b
Cbasic
Cacidic
Cbasic
Cacidic
μmol/g (μmol/m2)
3.16 (0.017)
64.25 (0.180)
25.44 (0.043)
19.14 (0.039)
μmol/g (μmol/m2)
26.51 (0.066)
47.42 (0.118)
36.74 (0.113)
42.09 (0.102)
IMP
WK
OPET
OPMET
343.60 (1.796)
429.94 (1.207)
948.19 (1.598)
523.24 (1.077)
1070.17 (2.676)
922.66 (2.297)
934.66 (2.271)
905.96 (2.776)
a
Adsorption of CO2 at 13 kPa and room temperature, flushed for 15 min,
evacuated, then ramped up in He flow at a rate of 10 °C min−1 to 500 °C and
held at this temperature for 1 h.
b
Same procedure by using 13 kPa NH3.
same level NH3 and CO2 adsorption capacity of the samples (Fig. 8b and
c). Sample 5IMP is an exception. The somewhat higher acidity and
lower basicity of the 5IMP sample led to lower BD and higher ethylene-
plus-diethyl-ether selectivities. It can be also noticed that the best BD
selectivities were achieved over the In2O3 promoted one-pot synthe-
sized samples. As the In2O3 promotes the dehydrogenation of ethanol,
the AA selectivities are also higher over the promoted samples. This is
more significant at lower conversion level (i.e., at lower space time),
when the lower concentrations favor less AA consuming side reactions.
Nevertheless, the enhanced AA formation facilitates the aldol addition
step and, thereby, increases the yield of butenes and C4-oxygenates.
4. Discussion
According to the generally accepted mechanism, the ETB reaction
takes place in the following consecutive reaction steps with the in-
volvement of specific catalytic sites. [42] In the first and second reac-
tion steps ethanol undergoes dehydrogenation over basic sites to form
AA, which undergoes then through aldol addition reaction also over
basic sites. Taifan et al. [43] studied the ETB reaction over wet-kneaded
MgO-SiO2 catalyst by in-situ DRIFTS and DFT studies, and concluded
that the conversion of ethanol to AA takes place through the surface
ethoxy groups formed on MgO, followed by hydrogen loss from its
alpha carbon. The results shown in Fig. 7a support this theory, as the
reaction gives AA as well as C4 products.
Among the non-doped MgO-SiO2 catalysts the WK sample showed
the highest butadiene selectivity. The XRD pattern of the sample
(Fig. 1) shows that wet kneading-generated MgO and SiO2 islands are
linked by MgeOeSi bonds. In the other samples the magnesium is well
dispersed in the SiO2 framework and is a part of MgeOeSi linkages.
The presence of separated MgO phase increases the CO2 adsorption
capacity of the catalysts and the basicity of the sample, whereas the
The heterogeneity/homogeneity of MgO/SiO2 systems has a deci-
sive role on the butadiene yields in the ETB reaction. There is a con-
tradiction in the literature about the incorporation of magnesium from
its salt (nitrate, acetate) into the silica framework of the SBA-15 during
one-pot synthesis. In publications which claim that the incorporation of
the magnesium is possible the washing step was omitted [32,36]. It was
also observed that the synthesis product contains chloride ions in non-
negligible amount that can come from the hydrochloric acid component
of the synthesis mixture [32]. The obtained MgO/SBA-15 material
shows the same features as the pure silica SBA-15, i.e., sharp XRD re-
flection at low-angle and hexagonally ordered cylindrical mesopores,
appearing on the TEM images. Wu et al. [31] claim that by using one-
pot synthesis method the surface of the SBA-15 pores becomes deco-
rated by magnesium species. This species is believed to get converted to
smooth MgOx layer on the pore walls during calcination. Formation of
some MgeOeSi bonds is also possible in reaction of the magnesium
species and the silanol groups of SBA-15 material. The upper limit of
Fig. 8. Bar and line chart of product selectivities and acid-base properties, respectively. Selectivities of a) neat, and b), c) 5 wt.% In2O3/MgO-SiO2 catalysts are shown
at conversion level a) 25%, b) 30%, and c) 65%. The identical conversions with the different catalysts were achieved by adjusting the ethanol WHSV. The lines and
the values on the secondary axis give the room-temperature NH3 and CO2 adsorption capacities of the catalysts determined by TPD measurements.
8

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MolecularCatalysis491(2020)110984
MgeOeSi bonds are responsible for the enhanced acidity and dehy-
dration activity of the sample (Table 2 and Fig. 8a).
sites and the surface concentration of the reactant and intermediates.
Comparing the NH3 adsorption data collected in Table 2 and the
measured conversion values, it can be seen that catalysts with higher
NH3 uptake convert ethanol with higher conversion. Results suggest
that the selectivities of the different MgO-SiO2 catalysts, promoted by
the same amount of In2O3, is determined by the properties of the sup-
port. However, an exception to this rule is the OPMET catalyst that
suggests that results can be improved by applying innovative synthesis
methods.
As it is discussed in the Introduction Section addition of transition
metals/metal oxides to the MgO-SiO2 catalysts enhances conversion as
well as selectivity for BD by promoting dehydrogenation of ethanol to
AA. As it can be clearly seen in Fig. 7b, In2O3 promotes the dehy-
drogenation of ethanol to AA with high efficiency. It is also worth to
note that, although In2O3 is a strong Lewis acid (increased the ammonia
uptake of the samples by two to three times, see Table 2), no significant
amounts of dehydration products were formed in its presence. The re-
sults presented on Fig. 5 and 6 fully support the promoting effect of
In2O3 additive. It also can be concluded from the results that by steering
reaction toward the dehydrogenation pathway the dehydration step is
suppressed and besides the selectivity of BD the selectivity of other C4
products (mainly BUE and BOL) also increases to some extent. This
phenomenon was investigated in details on the best performing OPMET
catalyst (Fig. 6). The results show that by increasing the added amount
of indium oxide, the selectivity of products formed by dehydration was
gradually reduced. However, it was also observed that upon increasing
the indium oxide content over 5 wt.% the BD selectivity did not show
further increase but instead the acetaldehyde selectivity was growing.
The stability study of the 5OPMET catalysts (Fig. 6d) also showed that
during the 30 h time on stream the activity of the catalyst and the BD
selectivity decreased, while the acetaldehyde selectivity increased. The
selectivity of the other products remained virtually unchanged.
Angelici et al. [7] stated that “Studies on these dehydrogenation
promoters typically report limited characterization data on both the
acid–base and structural properties, making the explanations for the
observed beneficial or detrimental catalytic effects often somewhat
speculative.” In present study the structures of the catalysts were
characterized by XRD and TEM measurements, while the acid-basic
properties of the catalysts were characterized by their NH3 and CO2
adsorption capacity. Results presented on Fig. 8 suggested that there is
a correlation between acid-base properties and catalytic activity of the
samples. However, in a complex reaction network of ETB reaction, it is
not only the acidity/basicity that affects the selectivity of the formation
of different products. Fig. 8a shows that the selectivity of BD (and of
other C4 products) is the highest over the WK sample on which the CO2
adsorption capacity is the highest and the amount of adsorbed NH3 is
almost the same or lower than on the other samples. The lower basicity
and higher acidity of samples results in a low BD and high dehydration
(i.e. ethylene and diethyl ether formation) activity. Fig. 2 shows that
the structure of the samples changed significantly upon promotion by
indium oxide. The hysteresis loops on the N2 adsorption-desorption
isotherms suggest that the pore structure of the samples became very
similar after the impregnation and their structure is most similar to
inorganic oxide gels with pore system made up of interconnected net-
work of pores of different sizes and shapes. It can be also noticed that
over the best performing catalysts (5WK, 5OPET, 5OPMET) the NH3
and CO2 adsorption capacities are very similar, while the 5IMP sample
possess higher acidity and lower basicity what resulted in a lower BD
and higher dehydration activity.
5. Conclusions
Four methods were applied to prepare MgO-SiO2 catalysts for con-
version of ethanol to butadiene. Besides common impregnation and wet
kneading methods the one-pot synthesis method was employed adding
magnesium methoxide or ethoxide as magnesium source to the synth-
esis gel of SBA-15 material. The XRD results showed that all the
methods, except the impregnation method resulted in formation of
MgeOeSi linkages. Separate MgO phase was detected in the wet-
kneaded sample only. All the preparations are mesoporous with rather
high specific surface area (190−590 m2/g). The magnesium alkoxides
in the gel of micelle-templated synthesis hindered the formation of SBA-
15 structure. Over neat MgO-SiO2 catalysts 10%–30% butadiene se-
lectivities were achieved. The main reaction products were diethyl
ether at lower and ethylene at higher temperatures. The addition of
5 wt.% In2O3 to the MgO-SiO2 preparations significantly increased the
butadiene selectivity. For instance, the In-promoted magnesium-con-
taining silica catalyst, obtained by micelle-templated synthesis method
showed 45%–60% butadiene selectivity in the temperature range of
250−425 °C. The ethylene/diethyl ether selectivity could be sup-
pressed below 10% by increasing the indium oxide loading to 10 wt.%.
However, the selectivity for butadiene was also reduced because the
rate of acetaldehyde coupling was not high enough to consume all the
acetaldehyde formed at an increased rate. At 25% conversion the best
performing neat MgO-SiO2 preparation was the wet-kneaded sample.
This sample has the highest basicity and about the same or lower
acidity than the rest of the catalyst preparations. Introduction of In2O3
changes both the structure and the acid-base properties of the neat
MgO-SiO2 samples. The pore structure of the samples becomes quite
similar. The samples of similar acid-base properties show similar cata-
lytic activity. The catalyst having relatively high CO2 and low NH3
adsorption capacity showed high butadiene selectivity. The highest
dehydration activity and lowest butadiene selectivity was observed
using the catalyst made from MgO-impregnated SBA-15 silica by in-
dium oxide introduction. This catalyst was the most acidic and the less
basic.
CRediT authorship contribution statement
József Valyon: Supervision. Jenő Hancsók: Supervision.
Declaration of Competing Interest
As it can be seen from the data in Table 2 and Fig. 5 and 6, after the
addition of In2O3, the acidity of the samples (i.e., the NH3 uptake) as
well as the conversion and butadiene selectivity also increased sig-
nificantly for all catalysts. It is worth considering whether these positive
changes can be explained by the increased “acidity”? Independent
measurements showed well the role of the catalyst components in the
reaction. Dehydrogenation of ethanol and aldol condensation take place
over MgO, whereas In2O3 significantly increases the acetaldehyde
concentration in the system, while MgeOeSi bonds promote the de-
hydration step. A good catalyst for butadiene selectivity is formed when
these three functions work together. However, it is questionable whe-
ther the appropriate ratio of the three functions can be characterized by
acidity/basicity measurements only? In heterogeneous catalytic reac-
tions, the rate of the reaction is determined also by the number of active
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
The authors acknowledge the financial support of the project of the
Economic Development and Innovation Operative Program of Hungary,
GINOP-2.3.2-15-2016-00053: Development of liquid fuels having high
hydrogen content in the molecule (contribution to sustainable mobi-
lity). The Project is supported by the European Union. Thanks is due to
the support provided by the European Union and the State of Hungary,
9

B. Szabó, et al.
MolecularCatalysis491(2020)110984
co-financed by the European Regional Development Fund in the fra-
mework of the project No. VEKOP-2.3.2-16-2017-00013.
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