MgO?SiO2 Catalysts for the Ethanol to Butadiene Reaction: The Effect of Lewis Acid Promoters

Article abstract of DOI:10.1002/cctc.202001007

MgO?SiO₂ samples, having the composition of natural talc (NT), were obtained by co-precipitation (CP) and wet kneading (WK) methods. The materials were used as catalysts of the ethanol-to-1,3-butadiene reaction. ZnO, Ga₂O₃ and In₂O₃ were tested as promoters. The catalyst WK gave the highest 1,3-Butadiene (BD) yield among the non-promoted catalysts because of the high specific surface area and strong basicity. Results suggested that over the neat WK catalyst the acetaldehyde coupling to crotonaldehyde was the rate-determining process step. Formation of crotyl alcohol intermediate was substantiated to proceed by the hydrogen transfer reaction between crotonaldehyde and ethanol. The crotyl alcohol intermediate becomes dehydrated to BD or, in a disproportionation side reaction, it forms crotonaldehyde and butanol. The promoter was found to increase the surface concentration of the reactant and reaction intermediates, thereby increases the rates of conversion and BD formation. The order of promoting efficiency was Zn>In>Ga.

Full text of DOI:10.1002/cctc.202001007

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: MgO-SiO2 Catalysts for the Ethanol to Butadiene Reaction: The
Effect of Lewis Acid Promoters
Authors: Róbert Barthos, Blanka Szabó, Gyula Novodárszki, Zoltán
Pászti, Attila Domján, József Valyon, and Jenő Hancsók
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1/2020

ChemCatChem
10.1002/cctc.202001007
FULL PAPER
MgO-SiO Catalysts for the Ethanol to Butadiene Reaction: The
2
Effect of Lewis Acid Promoters
[
a]
[a]
[a]
[b]
[a]
Blanka Szabó , Gyula Novodárszki , Zoltán Pászti , Attila Domján , József Valyon , Jenő
[c]
[a]
Hancsók Róbert Barthos*
[
a]
B. Szabó, Dr. G. Novodárszki, Dr. Z. Pászti, Prof. J. Valyon, Dr. R. Barthos
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences
Magyar tudósok körútja 2, Budapest 1117, Hungary
E-mail: barthos.robert@ttk.hu
[
[
b]
c]
Dr A. Domján
NMR Laboratory
Research Centre for Natural Sciences
Magyar tudósok körútja 2, Budapest 1117, Hungary
Prof. J. Hancsók
Institute of Chemical and Process Engineering
University of Pannonia
Egyetem utca 10, Veszprém 8201, Hungary
Supporting information for this article is given via a link at the end of the document.
[
11]
[15,29]
[5]
Abstract: MgO-SiO
NT), were obtained by co-precipitation (CP) and wet kneading (WK)
methods. The materials were used as catalysts of the ethanol-to-1,3-
butadiene reaction. ZnO, Ga and In were tested as promoters.
2
samples, having the composition of natural talc
Au , and Ga
Ta
, and metal combinations, like Zn-Hf , Zn-
[
7,30]
[13,23,26]
(
and Zn-Zr
have been reported to be active in the
ETB process.
O
2 3
O
2 3
The ETB process is a set of consecutive reactions. The
reaction goes through dehydrogenation of ethanol to
acetaldehyde, followed aldol coupling to crotonaldehyde that has
to become hydrogenated to crotyl alcohol. The alcohol is then
dehydrated to BD. According to the comparative assessment of
The catalyst WK gave the highest 1,3-Butadiene (BD) yield among
the non-promoted catalysts because of the high specific surface
area and strong basicity. Results suggested that over the neat WK
catalyst the acetaldehyde coupling to crotonaldehyde was the rate-
determining process step. Formation of crotyl alcohol intermediate
was substantiated to proceed by the hydrogen transfer reaction
between crotonaldehyde and ethanol. The crotyl alcohol
intermediate becomes dehydrated to BD or, in a disproportionation
side reaction, it forms crotonaldehyde and butanol. The promoter
was found to increase the surface concentration of the reactant and
reaction intermediates, thereby increases the rates of conversion
[
31]
Patel et al.
the BD production from bioethanol can be
beneficial in comparison to the petroleum-based BD production
process.
Magnesia-silica mixed oxide is the most frequently used
catalyst or catalyst support in the conversion of ethanol to BD.
There is a general agreement that the synthesis method and the
Mg to Si ratio determine the acid-base and redox properties and
and BD formation. The order of promoting efficiency was Zn> In> Ga. thereby, the hydrogenation-dehydrogenation and dehydration
activity of the catalyst. The acid-base pair sites are attributed to
the charge imbalance along the Mg-O-Si bonds. Pronounced
formation of such sites is expected if the homogeneity of the
oxide mixture is increased. Previous studies pointed out that
Introduction
subtle balance of acid-base and redox sites is necessary to
achieve high BD selectivity. The basic oxide ions of magnesia
Currently production of ethylene is based on the thermal
cracking of naphtha. In this process 1,3-Butadiene (BD), needed
for rubber manufacturing, is obtained as by-product. The
developments in ethylene production technologies, such as,
oxidative dehydrogenation of ethane led to BD shortage on the
market. There is a need for alternative technologies for BD
production. Recently the interest of both academic and industrial
research renewed in the production of BD from biomass-derived
[11]
initiate the dehydrogenation and aldol addition and condensation
[8,17,22]
reactions.
SiO
The roles of the silica component in the MgO-
2
mixed oxides are (i) to increase the amount of crystal
defects in the MgO structure, enhancing the dehydrogenation
[21]
activity, (ii) to increase the specific surface area (SSA) of the
catalyst, (iii) to generate hydrous magnesium silicates,
containing Mg-O-Si bonds showing weakly acidic character and
dehydration activity. The ETB selectivity could be enhanced by
[1,2]
feedstock.
reaction.
A possible process is the bioethanol-to-BD (ETB)
bringing all these properties in the harmony, which is the best
Two technologies were elaborated for the production of BD
[22]
regarding the reaction.
The MgO-SiO
2
catalysts are
or by co-
[3]
from ethanol. The Lebedev-process where the ethanol is
converted to BD over mixed oxide catalyst, like MgO-SiO or
, and the process of Ostromislensky where a mixture
[19–21,26]
synthesized either by wet kneading,
2
[14,20,22,23]
precipitation,
and, in case, by the incipient wetness
[4]
2 3
ZnO-Al O
impregnation of the obtained mixed oxide by a metal oxide
of ethanol and acetaldehyde is reacted over alumina or clay
catalysts. Recently, different oxides, mixed oxides, and zeolite
[11]
precursor compound.
The natural hydrated magnesium
silicate mineral (talc) can also serve as catalyst or catalyst
[
5–8]
[9]
supported catalysts, such as, SiO
2
,
SiO
2
-ZrO
2
,
zeolite
[25]
support.
[
10]
[11–26]
Beta , and MgO-SiO
oxides, such as oxides of Zn
2
promoted with metals and/or metal
[
8,9,17,24,25,27]
[10,27,28]
[10,27]
, Ag
, Cu
,
1
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ChemCatChem
10.1002/cctc.202001007
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[
38]
Addition of a metal/metal-oxide promoter to the MgO-SiO
2
CO
2 2
-TPD and CO -FTIR studies of Menezes et al. suggested
significantly enhances the ethanol dehydrogenation and, as a
consequence, the aldol condensation becomes the rate-
determining step over the promoted catalysts. Angelici et al.
showed that copper additive increased the dehydrogenation
activity and weakened the acidity of the catalyst. Both effects
suppressed the undesired ethanol dehydration reaction to
ethylene or diethyl ether, hence higher BD yields could be
obtained.
that surface heterogeneity increases the concentration of basic
[20]
sites. Angelici et al.
showed that co-precipitated catalysts
[19]
contain higher amount of acid and strong-base surface sites
than those prepared by wet kneading. This finding and the
increased yield of ethylene and diethyl ether by-products in the
ETB reaction was explained by the better homogeneity of the
co-precipitated catalyst, i. e., by the formation of many Mg-O-Si
bonds.
Studies discuss the effect of the MgO/SiO
2
ratio on the BD
The relation of the method and conditions of catalyst
preparation, the structure and texture of the catalyst, and the
selectivity. At low magnesia contents ethylene and diethyl ether
are the main reaction products. By increasing the magnesia
content the BD selectivity gradually increases. Most often
volcano-type dependence was reported about the yield of BD as
function of the MgO content, The maximum yield was obtained
[
11,23,28]
ETB activity is not clearly known yet.
The present
systematic study intends to decrease the lack of knowledge on
this field. Two materials, having the same Mg to Si ratio as the
natural talc (NT), were prepared by co-precipitation and wet
kneading methods. The structure, texture, composition and acid-
base properties of the synthesized materials (CP and WK) and
the NT were thoroughly characterized and correlated with their
catalytic activity in the ETB reaction. The promoting effect of Zn,
Ga and In on the physico-chemical properties of the materials
and on their ETB activity was also investigated.
at MgO to SiO
.67).
2
molar ratio close to one (weight ratio
Another study reported that the natural talc,
having a MgO to SiO molar ratio of 0.5 showed good catalytic
performance in the ETB reaction. In contrast, Lewandowski et
[
11,12,22,24]
0
2
[
25]
[
26]
al. found the best activity at molar ratio 4.4 (weight ratio 3).
NMR studies of the acidity-structure relation led to adverse
[
14]
conclusions. Chung et al.
kneaded MgO-SiO catalysts, synthesized using different
magnesia sources. The BD selectivity was linked to hydrous
conducted NMR studies on wet-
2
Results and Discussion
2
9
magnesium silicates giving Si-NMR signals in the region of -85
to -98 ppm. In contrast, Zhu et al. attributed favourable activity
to amorphous magnesium silicate phases, giving Si signals at -
[
32]
Composition, texture and crystallinity
Compositional and textural characteristics of the three
7
0, -75 and -92 ppm.
MgO-SiO
Table 1. The Si to Mg weight ratio of the catalysts is close to the
.54 Si-to-Mg weight ratio in the chemical formula of natural talc
Mg Si 10(OH) ). Comparison of the bulk and surface
2
materials, used in the present study, are given in
The XPS technique can distinguish different chemical
surface environments. According to the literature, the Si and O
core electron binding energies are associated primarily with the
1
(
3
4
O
2
4
charge on the tetrahedral SiO building blocks, reflecting the
compositions indicate a slight surface enrichment of Mg for the
natural talc and for the co-precipitated material. This is not
surprising as the surface of most magnesium silicates tends to
[
33,34]
4-
linkage of these tetrahedral.
For example, if each SiO
4
unit
is connected to four identical units, there is no formal charge on
the tetrahedra. This structure is associated with high binding
energies of both the Si 2p (103.5-104.0 eV) and the O 1s
[36]
contain more Mg than the bulk.
The exception is the wet-
kneaded sample, which is somewhat Mg-deficient both at the
surface and in the bulk with respect to the natural talc. The SSA
of the NT sample is significantly lower than that of the
synthesized samples. According to the XRD and nitrogen
adsorption measurements summarized in Figures 1A and B,
respectively, this material is non-porous and is well crystallized.
Figure 1B shows that the CP and WK samples have micro and
mesoporosity.
[
35]
(
around 533 eV) core levels. If each tetrahedron shares three
oxygen ions with adjacent identical tetrahedral, forming an
infinite two-dimensional sheet, like in talc, then the repeating unit
4
-
is Si
result the Si and O binding energies are lower by about 0.5-0.6
eV. On the other extreme of scale, forsterite (Mg SiO ) contains,
tetrahedra, surrounded by Mg cations. The
charge on each SiO unit is -4 and the corresponding Si 2p and
O 1s binding energies are as low as 101.8 and 530.9 eV,
4
O10 and the formal charge on each tetrahedron is -1. As a
2
4
4
-
2+
independent SiO
4
4
Both samples show Type IV isotherm with H2 type
hysteresis loop and another hysteresis loop at p/p ˃0.9 due to
o
pores among agglomerated particles. The XRD patterns
[
36]
respectively.
The pronounced shift of the Si and O
photoelectron lines is accompanied by a similar but smaller shift
[
33]
of the Mg 2p line (from 50.7 eV in talc
forsterite).
to 50.2-50.4 eV in
Another study showed that incorporation of
magnesium into silica network lowers the Si binding energy into
[36]
Table 1. Characterization of the catalysts.
[
a]
[
32]
Catalyst
Si/Mg ratio
Surface
area
m g
Pore
Pore
diameter
nm
the range of 101.0 – 101.7 eV.
As result of Si incorporation in magnesia a new peak
appears in the Ultraviolet-Visible (UV-Vis) spectrum at 275 cm .
This spectral feature was brought in relation with the catalytic
[
b]
[c]
volume
2
-1
3
-1
cm g
-1
Natural talc
1.46 (1.40)
1.44 (1.33)
9.1
-
-
[21,37]
activity of the MgO-SiO
2
catalysts in the ETB reaction.
The
Co-
precipitaion
207.5
249.6
0.31
3.50
peak was attributed to the absorption of coordinatively
2
-
unsaturated O ions generated by the Si, built in the magnesia
structure.
Wet
1.61 (1.71)
0.76
6.40
The acid-base properties of MgO-SiO
2
catalysts are most
kneading
[
19,20,23]
[11,19,20]
often characterized by TPD
and FTIR
examinations
[
a] m/m, calculated from ICP-OES and XPS (in parentheses) analysis.
of species obtained from adsorption of basic or acidic molecules.
Theoretical Si/Mg ratio is 1.54. [b] Gurvich method. [c] B.J.H. method.
2
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evidence the low crystallinity of the CP and WK samples. A
broad peak of the amorphous silica around 2Θ at about 23º can
be noticed on the diffractograms. Reflections of MgO having
periclase structure is present in the XRD pattern of the WK
sample (43.0º and 62.5º), whereas the CP sample shows broad
line centered at 35.3º and 61.0º indicating the presence of an
[
11]
amorphous magnesium silicate phase. These results suggest
2
+
that the CP sample is a more homogeneous mixture of Mg and
4
+
Si ions, while the WK sample consists of nano and microsize
domains of silica and magnesia probably linked at the contacting
surfaces of the two phases.
SEM-EDX studies
The morphology of the catalysts was studied by means of
SEM-EDX equipment. The elemental map of NT and CP
samples show that the Mg and Si are distributed
homogeneously. Some enrichment of Mg was observed on NT
2
sample. In contrast, the separate MgO and SiO phases are
clearly visible in the WK sample. The NT sample shows lamellar,
non-porous structure, while the CP and NT samples have
flower-like structure, rich in corners and edges. Due to their low
concentration (1 wt%) oxide dopants could not be identified in
the images, however, the results of quantitative analysis is
similar to the values determined by other methods. The images
and other details are shown in Figures S1-S2.
NMR studies
Due to the low level of crystallization XRD could not
provide adequate information about the bulk structure of the
1
29
1
samples. Therefore, H MAS NMR, Si CP MAS NMR, and H-
2
9
Si FSLG HETCOR NMR spectra were recorded to evaluate the
environment of the Si atoms and the possible formation of
magnesium silicate phases. Figure 2A shows that all the
hydroxyl groups have identical environment in the crystal
1
structure of the NT sample resulting in a single H MAS NMR
signal at 0.3 ppm. The relatively low chemical shift suggests that
the hydroxyl groups are isolated from each other. The CP
Figure 2. Solid state NMR spectra natural talc (NT), wet-kneaded (WK)
1
and co-precipitated (CP) samples. (A) H single pulse MAS NMR spectra,
29
1
1
29
(
B) Si{ H} cross-polarization MAS NMR spectra, and (C) H- Si FSLG
HETCOR representation of the results.
sample contains less hydrogen atoms than the NT sample. Two
different signals could be distinguished. The signal at 0.5 ppm
was assigned to "talc-like" and terminal hydroxyl groups. The
broader signal at 4.1 ppm belongs most probably to H-bonded
hydroxyl groups and strongly absorbed/structural water
molecules. The WK sample contains even less hydrogen atoms.
The signals are similar to those obtained for the CP sample, but
their intensity is much lower and the broad signal has a chemical
shift value of about 3.3 ppm indicating less pronounced H-
bonding.
2
9
The Si CP MAS NMR chemical shift depends on the
environment of the silicon atoms. The NT sample shows a single
well defined signal at -98 ppm (Figure 2B). Pure SiO
chemical shift around -110 ppm which decreases by replacing
the neighboring atoms of the SiO tetrahedra by H or Mg. In
2
has a
4
Figure 1. XRD patterns of (A) wet-kneaded (WK), co-precipitated (CP),
order to get spectra, which are easier to interpret direct
and natural talc (NT) samples and (B) N
2
adsorption isotherms of the
29
polarization Si spectra were recorded with relatively short
samples. Adsorption and desorption branches are indicated by full and
open symbols, respectively.
relaxation delay (5 sec) to filter out the signals of pure crystalline
or semi-crystalline SiO
Nevertheless, the spectrum of the WK sample remained
dominated by and signals of the SiO phase.
2 1
phases, having long T relaxation time.
Q
3
Q
4
2
3
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Table 2. XPS results for the natural talc (NT), the co-precipitated (CP) and the wet-kneaded (WK) materials.
a
Sample
Composition
at %
Mg: 16.7
Si: 20.3
O: 60.8
C 2.2
Binding energies, Auger kinetic energies, Auger parameters (eV)
b
c
d
Mg:Si
3.29:4
Mg 2p
Mg 1s
Mg KLL
Mg AP
Si 2p
Si KLL
Si AP
O 1s
C 1s
50.7
(2.1)
103.4
(2.1)
532.5
(2.4)
NT
1304.4
1179.7
2484.1
1608.5
1711.9
1711.6
1711.5
285.0
Mg: 15.2
Si: 17.4
O: 56.2
C: 9.6
5
0.2
102.4
(2.5)
531.7
(2.9)
285.0f
CP
3.49:4
2.70:4
1304.0
1304.3
1180.0
1179.9
2484.0
2484.2
1609.4
1608.7
(2.2)
288.6
e
Na: 1.7
Mg: 13.7
Si: 20.3
O: 59.6
C: 6.4
50.5
(2.8)
102.8
(2.6)
532.1
(3.0)
285.0
290.6
WK
g
[
[
a] Values in parentheses under the binding energy values indicate the full width at half maximum (FWHM, in eV) of the fitting Gaussian-Lorentzian product curve.
b] Mg Auger parameter: sum of the binding energy of the Mg 1s photoelectron peak and the kinetic energy of the Mg KL23 23 Auger peak. [c] Si Auger parameter:
23 Auger peak (excited by Bremstrahlung from the Al X-ray
L
sum of the binding energy of the Si 2p photoelectron peak and the kinetic energy of the Si KL23
L
anode). [d] the main component of the C 1s spectrum arising from hydrocarbons was used for charge compensation. [e] Na 1s binding energy: 1072.2 eV, Na KLL
+
Auger kinetic energy: 988.6 eV, Na Auger parameter: 2060.8 eV, all pointing to Na ions in a silicate and/or glass environment. [f] Small contribution, assigned to
carboxylic groups [e.g. NIST data base. [g] Minor contribution, assigned to carbonates.
Signal belonging to Si(OSi)
by the H- Si FSLG HETCOR spectrum (Figure 2C). No Q
3
OH (Q
3
) phase was clearly shown
indicates that they are significantly less interconnected than in
talc.
1
29
[33–36]
4
phase is detectable in the spectrum of CP sample (Figure 2B).
According to the correlation spectra (Figure 2C), signals
between -70 and -99 ppm originate from different magnesium
The Mg 2p, Si 2p, and O 1s binding energies of the WK
sample are intermediate between those of the NT and the CP
samples. The peaks are broader than either in the CP or the NT
samples, indicating that this material has the most
inhomogeneous structure. The XRD studies revealed the
presence of MgO crystallites, whereas the results of NMR
[14,32]
hydroxy silicate species.
XPS examinations
The bonding environment of Si, Mg, and O on the surface
of the mixed oxide catalysts was explored by XPS. Apart from
these components, XPS identified carbon in all samples, which
is attributed to the adventitious hydrocarbon contamination. The
2 3
showed the formation of hydrated SiO (Q ) in the material.
Nevertheless, the broad and smooth character of the XPS peaks
did not permit their reliable resolution into component peaks.
The Si Auger parameter at 1711.5 eV is clearly below the
narrow range around 1712 eV, usually reported for silicates or
CP sample contained
a small amount of sodium; the
concentration of all other possible contaminants remained below
the detection limit of XPS.
crystalline SiO
gel-like silica, i.e., strongly hydrated SiO
2
. Latter finding substantiates the presence of a
[
33]
2
component.
It is
The Mg 2p, Si 2p, O 1s, and C 1s spectra are presented
as Supporting Information (Figure S3). All photoelectron peaks
were adequately modeled by a single peak of symmetric shape
plausible that the bulk-sensitive XRD and NMR and the surface-
sensitive XPS measurements reveal different features of this
sample. While the bulk-sensitive techniques indicate the
(
except the C 1s peak of the CP and WK samples). The binding
presence of MgO- and SiO
interpreted as indication of formation of interfacial Si-Mg mixed
oxide between the SiO and MgO domains and on the surface of
the particles. It wasshown before that in systems prepare by
MgO deposition onto clean or oxidized Si surface or by Mg
2
-like regions, the XPS data can be
energies of the Mg, Si, and O core levels for the natural talc
sample showed very good agreement with the data found in the
literature for talc materials.
The main results of the XPS measurements are summarized in
Table 2. For the first sight it appears that the binding energy of
the Mg 2p electrons are rather similar in the different samples. It
2
[33–35,39,40]
(Cf. Supporting Information).
deposition onto SiO
silicate layer was formed during annealing.
mixed Si-Mg oxide phase is more favored by thermodynamics
than the coexistence of separate MgO and SiO phases. Such
2
surface, an interfacial disordered Mg-
[
42,43]
Obviously, a
[
34,35,41]
is not surprising as, according to literature data,
the Mg
binding energy, the Auger kinetic energy, and the Auger
parameter values seem to overlap for many oxygen-containing
magnesium compounds, such as, MgO and Mg-silicates.
However, the width of the Mg 2p, Si 2p and O 1s peaks,
measured for the synthetic preparations, is broader than the
width of the corresponding peaks of natural talc, suggesting that
the chemical environment of these components on the surface is
more inhomogeneous then on the surface of natural talc. This
finding is in accordance with the disordered nature of thematerial
demonstrated also by both the XRD and NMR examinations.
With respect to those in talc the Mg 2p, Si 2p, and O 1s binding
energies were significantly lower for the CP sample. The Si 2p
and the O 1s peaks shifted by approximately 1 eV, whereas the
shift of the Mg 2p line was smaller. These results suggest
2
intermixing could be initiated by the 500 °C calcination of the WK
material. Under these circumstances the surface of the WK
sample should contain not only SiO and MgO domains but also
2
certain amounts of disordered Mg-silicates, which is in line with
the high extent of inhomogeneity, suggested by the XPS results.
A particular feature of the WK sample is the rather strong high
binding energy C 1s peak around 290.6 eV, which appears in
addition to the common hydrocarbon-related carbon peak. The
intensity of this peak amounts to about 25 % of the total C signal
intensity (Figure S3D). The high binding energy indicates the
[
36,41]
presence of highly oxidized, carbonate-like carbon species.
It may be noted that while such carbonate contributions are
rarely observed on silica surfaces, they readily form on MgO
[
44]
increased negative charge on the SiO
4
tetrahedral units, which
exposed to CO
2
-containing ambient.
The appearance of the
carbonate signal for the WK sample and its lack for the NT and
4
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Table 3. Results of CO
2
and NH
3
Temperature Programmed Desorption (TPD) measurements. Natural talc (NT), Co-precipitated sample (CP),Wet-kneaded
sample (WK)
Catalyst
Support
1 % Ga
O
2 3
1 % In
2
O
3
1 % ZnO
C
7.3
19.2
34.8
[
a,b]
[a,c]
acidic
[a,b]
basic
[a,c]
acidic
[a,b]
[d]
[a,c]
acidic
[a,b]
basic
[a,c]
C
acidic
15.6
693.7
879.5
C
basic
C
C
C
C
basic
C
[
d]
[d]
[d]
NT
CP
WK
7.7
10.5
94.5
17.1
412.0
461.1
n.m.
22.7
52.2
n.m.
394.0
910.2
n.m.
27.3
45.5
n.m.
735.1
933.2
[
°
a] Concentrations in µmol/g units. [b] Adsorption of CO
C min to 500 °C and held at this temperature for 1h. [c] Same procedure by using 13 kPa NH . [d] Non measured.
2
at 13 kPa and room temperature, flushed for 15 min, evacuated, then ramped up in He flow at a rate of 10
-
1
3
CP samples confirm that the former indeed contains MgO-like
surface regions.
the base strength of the adsorbing surface sites. The integrated
band area gives an approximate measure of the amount of
adsorption sites. Figure 3A shows that the WK sample contains
the highest number of basic sites, whereas the NT sample of low
surface area has only small amount of similar sites. This is in
TPD and FTIR characterization of the acid-base properties
There is agreement in the literature that the acid-base
properties of catalysts play a key role in the ETB reaction. For
characterization of acid-base properties we should know the
number, strength, and strength distribution of the acid and base
sites. The most commonly used methods characterize is the
adsorption of acidic or basic probe molecules on the catalyst
surface. Usually some spectral change of the adsorbed probe is
determined, or the strength of the adsorption is related to the
thermal stability of the adsorbed species. The use of probe
molecules raises several issues: (i) the acidity/basicity of the
probe molecule delimits the sites that can be characterized, (ii)
the adsorption interaction changes the acid-base properties of
the catalyst, (iii) shape and size selective adsorption inhibitions
may occur, (iv) the stoichiometry of the adsorption is not always
2
accordance with the results of the CO -TPD study. Unlike the
spectra of the NT and CP samples the FTIR spectrum of the WK
sample exhibits two CD component peaks, suggesting the
presence of two kinds of adsorption sites having different base
-1
strengths (Figure 3A). The broad band at 2234 cm indicates
the presence of sites having stronger basicity than the sorption
sites of the other samples. The adsorption of CO
can be responsible for the pronounced tailing of the CO
2
on these sites
-TPD
2
peak at its high-temperature side (Figure S4C). Figure 3A shows
that addition of metal oxide significantly decreased the relative
and absolute amounts of strong base adsorption sites. While the
total peak area increased only slightly, the area ratio of the
-1
peaks at about 2230 and 2260 cm changed significantly. The
ratio that was 4.94 for the WK sample dropped to 0.42, 0.30,
and 0.21 for the In/WK, Zn/WK and Ga/WK samples,
respectively. It is clear that the additive decreased the basicity of
the WK catalysts.
unequivocal, etc.. In the present study TPD patterns of NH
3
and
CO , as well as, FTIR spectra of adsorbed CDCl and Py were
2
3
determined and evaluated to characterize the acid-base
properties of the catalysts. Results derived from the TPD curves
shown in Figures S4 and S5 are summarized in Table 3. The
Vibrations of the aromatic ring of adsorbed Py molecules
can be used to distinguish Py, coordinatively bonded to Lewis
smallest amounts of adsorbed CO
the NT sample, which can be explained by the very low surface
area of the mineral. The WK sample has higher CO uptake than
the CP sample, whereas their NH uptakes is similar. The higher
CO uptake suggests that the WK sample is more basic than the
2 3
and NH were measured for
+
acid sites or to Brønsted acid sites in protonated form, (PyH )
+
2
(Figure 3B). The band, diagnostic for PyH should appear at
−
1
3
about 1545 cm . The MgO-SiO
2
catalysts did not have Brønsted
2
acid sites, strong enough to protonate Py. The infrared
CP sample. This can correspond to the presence of MgO
domains in the former sample, as suggested by the XPS data.
The dopant ZnO had virtually no effect on the acid-base
properties of the NT sample. The corresponding properties of
the In
2
O
3
and Ga
2
O
3
doped NT sample were not tested. It is
uptake of the
noteworthy that the additives approached the CO
2
catalysts, i.e., the basicity of the CP and WK catalysts
approached to each other. The addition of metal oxides
significantly increased the ammonia uptake, i.e., acidity of the
catalysts. The Ga/CP sample was an exception. In this series of
measurements it could be observed that the addition of metal
oxide dopant brought the ammonia uptake of the samples
almost to the same level (Table 3).
The order of basicity of the neat MgO-SiO
supported by the CDCl adsorption measurements shown in
Figure 3A. The presented spectra do not show bands at 2139
2
samples is also
3
-1
[20]
and 2086 cm , which Angelici et al. assigned to CDCl
3
, bound
to strong basic sites. The spectra in Figure 3A are more similar
[
45]
Figure 3. FTIR spectra of adsorbed (A) CDCl
were pre-treated in vacuum at 450 °C for 1 h. The spectra were recorded
at room temperature (A) in the presence of CDCl at about 933 Pa
pressure, and (B) after adsorption of pyridine at 666 Pa pressure at 200 °C
and evacuation at the same temperature for 30 min.
3
and (B) pyridine. The pellets
to those reported by Paukshtis et al. in their pioneering work.
-1
The CD frequency of free CDCl
Interaction of CDCl molecules and basic surface sites shifts the
CD stretching band to lower wavenumbers.
displacement reflects the strength of interaction and, thereby,
3
molecules is at 2260 cm .
3
3
[
20,45]

The band
5
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ChemCatChem
10.1002/cctc.202001007
FULL PAPER
adsorption band of pyridine bound to Lewis acid sites is around
-1
1
445 cm . Comparing the spectra of Py bound to neat MgO-
SiO samples after identical sample treatments, it can be
2
concluded that the NT sample does not bind any Py under the
experimental conditions used, whereas the WK sample can bind
and retain slightly more Py than the CP sample (Figure S4A).
Figure 3B shows that addition of only 1 wt.% dopant significantly
increases the Py uptake of the WK sample and also increases
its Py bonding strength. Based on the thermal stability of the Py
-1
species giving the band at 1445 cm , the following order of
acidity can be established: In/WK >Zn/WK > Ga/WK (Figure 3B).
The spectra of the entire series of experiments and the
integrated absorbance values of selected peaks are summarized
in Figure S6 and Table S2.
ETB activity and selectivity
The initiation step of the ETB process is the
dehydrogenation of ethanol to acetaldehyde. It is generally
believed that the hydrogen formed during the dehydrogenation
of ethanol hydrogenates the crotonaldehyde, formed from the
acetaldehyde by aldol coupling, to get the crotyl alcohol
intermediate of BD formation. Our results (vide infra) supports
the idea that the formation of the unsaturated alcohol takes
place predominantly by hydrogen transfer between ethanol and
crotonaldehyde according to the Meerwein – Ponndorf – Verley
Figure 5. Conversion of ethanol over (A) natural talc, (B) co-precipitated,
and (C) wet-kneaded MgO-SiO
2
catalysts as function of reaction
-1 -1
temperature. The WHSV was 0.5 gethanol•gcat
h
at a total gas flow rate of
3
-1
3
0 cm min . The partial pressure of ethanol was about 15 kPa in He. The
conversion curve is labelled by letter C. Selectivity curves are given for
butadiene (BD), acetaldehyde (AA), ethylene (EE), diethyl ether (DEE),
butanol (BOL) and butanes (BUE).
quite similar. The NT catalyst, having much lower surface than
latter catalysts, was much less active. Over each of the catalysts
the main reaction products were diethyl ether at lower
temperatures, and ethylene at higher temperatures. This
temperature dependence is understandable regarding that the
bimolecular dehydration of ethanol is exothermic, whereas the
monomolecular reaction is endothermic. The best performing
ETB catalyst was the WK sample having BD selectivity around
30 % in almost the whole examined temperature region. The
data in Tables 3, S1, and S2 show that this catalyst has the
strongest basic properties and that the number and strength of
Lewis acid sites on it are also higher than those of the other two
samples. The favourable catalytic activity of the WK sample is
most probably due to cooperation of relatively strong basic and
acidic sites on its surface.
(
MPV) mechanism to give acetaldehyde side product. Regarding
the ETB reaction the MPV process is favourable compared to
the direct hydrogenation by H because it is highly selective in
2
the reduction of the carbonyl group and lives the double bound
untouched. Moreover, the process generates acetaldehyde from
the reducing reactant ethanol, which aldehyde can then
participate then in the aldol coupling reaction step of the ETB
process. The heterogeneous MPV process can take place either
[
46]
[8,47–49]
[48]
on Lewis acidic or basic sites.
that over MgO-SiO catalysts the basic sites are responsible for
the MPV activity of the ethanol. Ordomsky et al. compared the
activity of ZnO /SiO and MgO/SiO catalysts in acetaldehyde
condensation and found that over ZnO containing catalyst the
initial acetaldehyde conversion is somewhat higher. Over MgO-
SiO catalysts the aldol condensation step is considered to take
place on an acid-base pair sites by concerted mechanism.
Niiyama et al.
found
2
[50]
2
2
2
2
2
[
16]
Figures 5A and B show that especially at low temperatures
the acetaldehyde was the main reaction product over the NT
and CP catalysts, poorly performing in the ETB reaction. The
Nevertheless, the catalyst of the one-step Lebedev-process
must have also hydrogenation-dehydrogenation activity (Figure
high acetaldehyde selectivity is associated with a low yield of C
4
4).
products. This suggests that over these two catalysts the
aldehyde coupling catalytic function is weak. Besides generating
BD, the WK catalyst also initiates formation of butanol and
butenes in significant amounts, confirming that this catalyst is
highly active in aldol coupling and dehydration reactions. This
activity can be attributed to the strong basicity of this catalyst
Obviously, the MPV reaction gives less acetaldehyde than
needed to maintain the reaction. Thus, the dehydrogenation
activity of the catalyst has not only to initiate the reaction, but
also has to maintain high enough concentration of acetaldehyde
intermediate. In this respect the use of ethanol-acetaldehyde
mixture as reactant makes the dehydrogenation function of the
catalyst of secondary importance.
(
Table 3).
In accordance with other studies the presumed
Figure 5 shows the catalytic performance of neat MgO-
intermediates of BD formation, such as, 3-hydroxybutanal,
crotonaldehyde, and crotyl alcohol are absent or appear only in
negligible amounts even at very low space times (Figures 5 - 7).
The transformation of crotyl alcohol to BD is facile over each
catalyst explaining the absence of crotonaldehyde and crotyl
alcohol in the product mixtures. The high activity of the WK
catalyst in converting crotonaldehyde to crotyl alcohol
2
SiO preparations in the ETB transformation. As function of
temperature the conversion over the CP and WK catalysts was
intermediate and C
4
products makes the WK preparation to the
most active ETB catalyst.
Figure 4. Formation of crotyl alcohol in the ETB process.
6
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ChemCatChem
10.1002/cctc.202001007
FULL PAPER
Figure 6. Effect of 1 wt.% (A) Ga
2
O
3
, (B) In
2
O
3
, and (C) ZnO promoter on
Figure 8. Catalytic conversion of crotyl alcohol over (A) NT, (B) CP, and
-
1
-1
-1 -1
the conversion of ethanol to butadiene. The WHSV was 0.5 gethanol·
g
cat
h
cat
(C) WK catalysts. The WHSV was 0.125 galcohol·g h at a total gas flow
3 -1
3
-1
at a total gas flow rate of 30 cm min . The partial pressure of ethanol was
about 15 kPa in He. See the Fig.5. for the legends.
rate of 30 cm min . The partial pressure of crotyl alcohol was about 4 kPa
in He. See the Fig.5. for the legends. CA=crotonaldehyde and
BAL=butanal.
Interestingly, butanol was a side product that appeared in
clearly detectable amount (Figures 5-7) according to Scalbert et
hydrogenation of crotyl alcohol
evolved in aromatization side process. GC-MS measurements
showed traces of C aromatics, such as, xylenes and styrene,
were formed in low-temperature reactions. The results, obtained
studying the crotonaldehyde conversion, did not support this
consuming the hydrogen
[
51]
al.
the crotyl alcohol can be hydrogenated to butanol or
dehydrated to BD depending on the reaction conditions and the
properties of the used catalyst.
8
In order to better understand the formation and
notion. In the reaction over neat MgO-SiO
2
preparations
transformation processes of C
reaction of crotyl alcohol over unpromoted MgO-SiO
Figure 8). Full conversion was reached at around 325 °C. Over
each catalyst the main reaction products were BD, formed by
crotyl alcohol dehydration, and butanol. Interestingly,
crotonaldehyde was also obtained. The highest amount of
crotonaldehyde was obtained over the NT catalyst probably
because this catalyst was hardly active in its further conversion.
It cannot be excluded that butanol formation could happen by
4
intermediates, we studied the
formation of polyolefins and aromatics poisoned the catalysts
very quickly. Addition of hydrogen to the reactant did not stop
rapid poisoning. It was observed that butanol and
crotonaldehyde were obtained together with similar selectivites.
Results seem to support that disproportionation of crotyl alcohol
2
catalysts
(
(
Figure 9) could have been responsible for the butanol formation
during the ETB process and not a direct crotonaldehyde
hydrogenation.
Figure 6 shows that the conversion of ethanol and the
selectivity of BD formation were significantly increased by all
three applied metal oxide additives. Because the WK catalyst
showed the best activity in the ETB reaction the promoting effect
of Ga-, In-, and Zn-oxide was investigated using this material as
support. In general, the In and Zn promoters suppressed
dehydration reactions and increased both ethanol conversion
and BD selectivity (Figure 6). The Ga changed only the
conversion, but the product selectivities were very similar to
those obtained using the non-promoted WK sample. The
positive effect of In dopant is significant at temperatures 250-
3
2
50 °C. The Zn/WK performs about 50 % BD selectivity in the
50-375 °C temperature range. At higher temperatures the full
analysis was difficult because a number of different C
trienes, and aromatics were formed.
5 8
-C dienes,
The addition of Zn to NT remarkably suppressed the
formation of ethylene and diethyl ether. The selectivity of BD
formation was not affected by this dopant. However, the ethanol
conversion to acetaldehyde was significantly higher than over
Figure 7. Effect of 1 wt.% ZnO additive on the activity and selectivity of (A)
NT and (B) CP samples in the ETB reaction. The WHSV was
-
cat
1
-1
3
-1
0.5 gethanol·
g
h at a total gas flow rate of 30 cm min , The partial
pressure of ethanol was about 15 kPa in He. Section C shows the effect of
WHSV on the product yield and conversion over 1 wt.% ZnO/WK sample
at 300 °C, and ethanol partial pressure of about 15 kPa. The total flow rate
3
-1
was varied in the 5-120 cm min range. See the Fig.5. for the legends.
Figure 9. Disproportionation reaction of crotyl alcohol.
7
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ChemCatChem
10.1002/cctc.202001007
FULL PAPER
the bare support, suggesting that dehydrogenation was
promoted but the formed acetaldehyde hardly took part in the
aldol coupling reaction. Addition of ZnO to the CP sample
resulted in a remarkably increase of ethanol conversion and BD
selectivity, while the selectivity for the dehydration products
remained close to or below 10 %. This result suggests that the
rate controlling step of ETB reaction over the CP catalyst could
have been the step of ethanol dehydrogenation to acetaldehyde.
Unlike to the Zn/NT catalyst, the aldol coupling activity of the
Zn/CP catalyst was sufficient to bring the reaction further and
get BD. Figure 7C shows the effect of space time on the
conversion of ethanol and the yield of the products over the 1 %
ZnO/WK catalyst. The yield of acetaldehyde passed through
maximum indicating that it was a primary product that was
further converted in secondary aldol condensation reaction.
There is evidence in the literature that ZnO is able to bind
[
52]
heterolytically dissociated hydrogen on its surface. However,
our attempt to detect activated hydrogen on any of our catalysts
was unsuccessful. It has also been shown (vide ultra) that
molecular hydrogen is unable to hydrogenate crotonaldehyde
under the reaction conditions used. This is strong evidence that
the crotyl alcohol intermediate of the ETB reaction could have
been formed from crotonaldehyde by the MPV mechanism over
our catalysts.
Figure 10. Effect of the ethanol conversion on the distribution of the
reaction products over doped CP samples, doped by 1 wt.% Zn, In, or Ga-
oxide doped. The reaction temperature was 350 °C, the WHSV was varied
-1 -1
cat
in the 0.2 - 1.0 galcohol·g h range. The partial pressure of ethanol in He
was about 8 kPa. Over ZnO/CP the low conversion levels were achieved
by diluting of 0.50 g catalyst with 0.50 g inert SiC.
In our opinion, the positive effect of the metal oxide
dopants on the ETB reaction cannot be directly related to the
induced changes in the acid-base properties of the catalysts and
[
17]
al. made an attempt to determine the activation energy of the
ethanol conversion and the formation of the main products over
2
+
to the enhanced dehydrogenation activity. It is obvious that Zn ,
3
+
3+
4+
In and Ga cations are stronger Lewis acids than Si and
2 2
MgO-SiO and Zn/MgO-SiO catalysts. The reported Arrhenius
2
+
Mg cations. Stronger Lewis acid sites necessarily generate
stronger Lewis base site
plots suggest that all the reactions were handled as processes
of zero order kinetics. In this case only the apparent rate
constants (kapp) are equal with the initial rate of the reaction that
can be experimentally determined. Anyhow, the obtained kapp
includes the intrinsic k values of all the processes that contribute
2
-
O
anions. The adsorption
measurements clearly demonstrated that the additive oxides
significantly enhanced both the acidity and basicity of the
catalysts. Higher ethanol conversions and higher product
formation rates over doped samples can be explained by the
increased surface concentration of the reactant and the reaction
intermediates.
to forming and consuming the surface intermediate of
a
transformation and determines its concentration. Obviously, no
strong mechanistic conclusions can be established on the
values of the apparent activation energies (Eapp), derived from
such Arrheniius plots. Getting deeper knowledge of complex
reaction networks like the ETB reaction requires theoretical and
microkinetic analysis.
The increasing aldehyde concentration as function of
space time (Figure 7C) must be paralleled by an increasing
surface concentration of the aldehyde activated for aldol
[
53]
coupling, most probably in the form of surface enolate.
The rate of aldol coupling increases with increasing space
time but it remains the rate-controlling process step (Figure 7C).
The steady-state product composition varies as the
concentration of the surface species changes with the
conversion. Figure 10 shows the product distribution over CP
supported metal oxide catalysts at conversion levels of 10 to
The metal additive promotes the ethanol dehydrogenation
activity. If this activity change is not paralleled by increased aldol
condensation activity acetaldehyde intermediate accumulate on
the surface and appears in the product mixture in higher
concentration. This phenomenon was demonstrated by the
[
30]
study of Pomalaza et al.
The metal oxide additives, used in
5
0 %. The results show that higher C
4
and BD selectivities were
present study, had the positive effect on the BD yields in the
order Zn> In> Ga.
achieved in all cases at higher conversions, i.e., at longer space
is times.
The outstanding promoting effect of ZnO is thought to be
related to its electronic structure. According to the HSAB (Hard
and Soft Acids and Bases) theory, on the scale of soft and hard
In general the applied metal additives were found to
increase the activity and modify selectivity (Figure 10). The
dopant increased not only the number active sites but also the
rate constants of the different reactions. As a consequence the
concentration of surface intermediates can increase and the
relative amounts of the surface intermediates of different
products can change. The composition of the product mixture
changes accordingly. We needed each rate and adsorption
equilibrium constant (k and K) and the temperature dependence
of these constants to give full description of the complex network
2
+
ions, Zn belongs to the borderline group, while the other two
4
+
2+
cations, as well as, the Si and Mg ions, belong to the group of
[54,55]
[39]
hard Lewis acids.
Hayashi et al.
based on experimental
and computational results suggested that chemical hardness is
one of the chemical descriptors, which permit to predict the
2
effect of dopant on the ETB activity of MgO–SiO catalysts. The
2
+
softness of Zn ion seems to be favourable regarding the BD
selectivity of the catalysts at lower temperatures, whereas at
a
and give the activation energy (E ) of each reaction. Wang et
8
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ChemCatChem
10.1002/cctc.202001007
FULL PAPER
higher temperatures it promotes formation of polyolefins and
aromatics.
Two gels were made and mixed to get catalyst WK. To get the first gel a
solution was made by dissolving 51.28 g Mg(NO ) in 200 mL ion-free
3 2
water than the pH of the solution was adjusted to 12.0 by adding to it 0.5
M NaOH solution dropwise. A suspension was obtained that was aged
overnight at room temperature. It was then centrifuged to separate the
precipitate and washed with deionized water until the pH of the
supernatant reached 7.0. The washed gel was dried at 120 °C overnight.
The second gel was prepared adding dropwise 41.6 g TEOS to 440 mL
It is also interesting, that the ZnO additive suppresses
ethanol dehydration, nevertheless dehydration selectivity
increases with increasing conversion. With increasing
2 3 2 3
conversion the dehydration activity of the In O and Ga O
doped catalysts changes in the opposite direction. Obviously the
distribution of the ethanol conversion between dehydration and
dehydrogenation depends both on the catalyst properties and
the level of conversion.
of 1.5 M NH
was washed and dried at 120 °C overnight. In 100 mL deionized water
.8 g of the first dried gel (Mg(OH) ) and 8.0 g of the second dried gel
SiO ) were mixed and stirred at room temperature for 5 h (wet kneading).
3
solution under vigorous stirring at 70 °C. The obtained gel
5
2
(
2
The solid was then separated, dried at 120 °C overnight, and calcined in
air at 500 °C for 5 h. The Si to Mg weight ratio of the prepared samples
was adjusted to about 1.54 that is similar to that of the natural talc. . The
Conclusion
impregnation of the MgO-SiO
2
preparations were executed by 33 mmol
O (Fluka ≥99 %, In(NO ·xH O (Sigma
·xH O (Sigma Aldrich ≥99.9 %) in the
-
3
Natural talc and two MgO-SiO
2
catalysts, having the same Mg/Si
dm solutions of Zn(NO
3
)
2
·6H
2
3
)
3
2
ratio as the natural talc, but synthesized in different ways, such
as, co-precipitation and wet kneading, were tested in the
Aldrich ≥99.99 %, or Ga(NO
3
)
3
2
amount to obtain catalysts containing about 1 wt.% of metal oxides.
ethanol-to-butadiene (ETB) reaction. The effect of ZnO, In
2 3
O
The ICP-OES measurements with a simultaneous plasma emission
spectrometer (Spectro Genesis) and axial plasma observation were used
to determine the elemental composition of the samples.
and Ga dopants was also investigated. Examination of the
2
O
3
neat mixed oxide catalysts showed that the catalytic activity of
natural talc was low. This can be related to its non-porous
structure and low specific surface area. The highest BD yields
were found over the wet-kneaded catalyst. Since the acidity of
the samples was similar but their basicity was much different,
the highest ETB activity was attributed to the highest basicity of
the catalyst prepared by wet kneading. It was shown that the
catalyst prepared by wet kneading is inhomogeneous on the
nano and microscale. It contains silica and strong base
magnesia domains bound by Si-O-Mg bonds. Doping by metal
oxide increased the acidity of the synthetic catalysts to about the
same level. The basicity and the activity of the preparations
were modified to different extents. The increased catalytic
activity was explained by the increased concentration of the
reactant/intermediate on the catalysts surface, while the
differences in product distributions were interpreted by the
chemical difference of the metal oxides. The oxide dopants
enhanced the BD yields in the order Zn> In> Ga. Investigation of
X-ray diffraction patterns were recorded by a Philips PW 1810/3710
diffractometer using monochromatized CuKα (λ=0.15418 nm) radiation
(40 kV, 35 mA) and proportional counter. The patterns, recorded at
ambient temperature, were collected between 3° and 75° 2Θ range in
0.02° steps, in each step for 0.5 s.
The nitrogen adsorption/desorption isotherms of the samples were
recorded at -196 °C using Thermo Scienctific Surfer automatic,
volumetric adsorption analyser. Prior the measurements the samples
were activated under vacuum at 250 °C for 120 minutes.
A computer controlled flow-through TPD system, equipped with a U-tube
(I.D. 4 mm) quartz reactor and thermal conductivity detector (TCD), was
used to determine NH and CO uptakes under conditions permitting
3
2
comparison of the results. (For experimental details cf. the legends of
Figures. S4 and S5.)
the reaction of
C
4
intermediates suggested that the
Solid state Magic Angle Spinning Nuclear Magnetic Resonance (MAS
NMR) spectra of samples were recorded by Varian System spectrometer,
crotonaldehyde hydrogenation step in the reaction chain, giving
crotyl alcohol took place according to the MPV reduction
mechanism. Parallel to crotyl alcohol dehydration to BD the
disproportionation of the alcohol to butanol and crotonaldehyde
also proceeded.
1
operating at 400 MHz for the H frequency, with a Chemagnetics 4.0 mm
29
narrow-bore double resonance T3 probe. To get Si Cross Polarization
Magic Angle Spinning (CP MAS) spectra 8000 transients were recorded
using SPINAL-64 decoupling with 1 ms contact time, and 20 sec recycle
delay. Polydimethylsiloxane (PDMS) was used as external chemical shift
1
29
reference (0.21 ppm for the H and 0 ppm for the Si scale) The 90°
2
9
pulse lengths were 5.4 μs for Si and 2.9 μs for the proton channel. For
1
29
Experimental Section
obtaining the H- Si FSLG HETCOR (Frequency-Switched Lee-
Goldburg Heteronuclear Correlation) spectra 384 transients were
recorded with 64 increments in the t
The FSLG scaling factor was 0.53. The measuring temperature was
8 °C, the spinning rate of the rotor was 12 kHz in all measurements.
1
dimension with 10 sec recycle delay.
Natural talc (Tital VT) was obtained from Sibelco Specialty Minerals
Europe, The Netherlands. This material contains Fe-, Mg- and Ca-oxide
impurities in low amount (<0.25 wt.%).
1
X-ray photoelectron spectra were recorded by an EA 125 electron
spectrometer (OMICRON Nanotechnology GmbH, Germany). Non-
monochromatic AlK radiation (1486.6 eV) was used as excitation source.

Two solutions were made to synthesize the CP preparation: in 400 mL
methanol 52.7 g Mg(NO
3
)
2
(Sigma-Aldrich, 99 %), and in 100 mL
methanol 55.5 g tetraethyl orthosilicate TEOS (> 98 % Fluka AG) was
dissolved. The mixture of the two solutions was dropwise added to 1 M
NaOH solution under vigorous stirring. The pH of the mixture was
maintained at 10.5±0.2 by continuously adding to it the needed amount
of 1 M NaOH solution. The obtained suspension was aged for 1 week at
room temperature then it was repeatedly centrifuged and washed with
deionized water. The pH of the last supernatant reached the value of 7.0.
The separated gel was dried at 120 °C overnight and calcined at 500 °C
for 5 h.
Calibration of the energy scale of the instrument was checked according
to the ISO 15472 standard. The estimated accuracy of the reported
binding energy values is 0.2 eV. Detailed spectra used for quantitative
analysis and study of the chemical environment of the sample
components were recorded in the Constant Analyser Energy mode (pass
energy: 30 eV, resolution: around 1 eV. Powder samples were pressed
into pellets before mounting on standard Omicron sample plates. As the
samples were insulating materials, considerable charging was observed
during the measurements. Dealing with silicate materials, generally the C
9
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ChemCatChem
10.1002/cctc.202001007
FULL PAPER
[
40]
1
s line is used for compensation of the charging effect. Accordingly, in
[
[
3]
4]
S. V. Lebedev, J. Gen. Chim. 1933, 3, 698–708.
J. Ostromisslenski, J. Russ. Phys. Chem. Soc. 1915, 47, 1472–
506.
the present study the lowest binding energy component of the C 1s
spectrum, arising from hydrocarbons at 285.0 eV, was selected as
reference point. With this calibration reasonable binding energy values
were obtained for both Si 2p electrons around 103 eV and Mg 2p
electrons around 50.5 eV. The measured spectra were processed by
fitting Gaussian-Lorentzian product peaks to the data with the CasaXPS
1
[
5]
T. De Baerdemaeker, M. Feyen, U. Müller, B. Yilmaz, F.-S. Xiao, W.
Zhang, T. Yokoi, X. Bao, H. Gies, D. E. De Vos, ACS Catal. 2015, 5,
3393–3397.
[
6]
software package after removal of Shirley or linear background.
Quantitative evaluation of the XPS data was performed by using the XPS
[
[
6]
7]
N. Fairley, “CasaXPS software package,” can be found under
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FULL PAPER
Entry for the Table of Contents
Structure and activity of MgO-SiO
Favorable wet-kneaded preparation contains MgO and SiO
was reduced to crotyl alcohol by MPV H-transfer reaction and disproportionation of crotyl aldehyde generated butanol by-product.
2
catalysts having the composition of natural talc were studied in ethanol-to-butadiene reaction.
2
nano and microdomains. It was shown that crotonaldehyde intermediate
2 2 3 2 3
ZnO was better Lewis-acid promoter of wet-kneaded MgO-SiO than In O or Ga O . The differences in catalytic activity were
interpreted by the chemical nature of dopants.
1
2
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