Successive vapour phase Guerbet condensation of ethanol and 1-butanol over Mg-Al oxide catalysts in a flow reactor

Article abstract of DOI:10.1016/j.apcata.2019.117265

The successive vapour phase condensation of ethanol and 1-butanol (via Guerbet reaction) in a flow reactor under atmospheric pressure was studied over catalytic Mg-Al oxide compositions. Wherein the vapour phase condensation of 1-butanol to 2-ethyl-1-hexanol in flow has been investigated for the first time. The acid/base capacity ratio, which is determined by the Mg/Al ratio, is an important characteristic for the activity and selectivity of Mg-Al oxide catalysts in the abovementioned processes. The carbon chain length of the reacting alcohols, an arrangement of surface active sites and other steric factors also have an impact on Guerbet condensation in the vapour phase. High productivity of Mg-Al oxide system to the Guerbet alcohols: 1-butanol – 25 g/(L_cat·h), 2-ethyl-1-hexanol – 19 g/(L_cat·h), has been achieved. The results have shown a prospect of successive conversion realization: 1) ethanol → 1-butanol; 2) 1-butanol → 2-ethyl-1-hexanol for the production of 2-ethyl-1-hexanol from ethanol.

Full text of DOI:10.1016/j.apcata.2019.117265

Journal Pre-proof
Successive vapour phase Guerbet condensation of ethanol and
1
-butanol over Mg-Al oxide catalysts in a flow reactor
Olga V. Larina, Karina V. Valihura, Pavlo I. Kyriienko, Nina V.
ˇ
Vlasenko, Dmytro Yu. Balakin, Ivan Khalakhan, Toma zˇ Cendak,
Sergiy O. Soloviev, Svitlana M. Orlyk
PII:
S0926-860X(19)30420-X
DOI:
https://doi.org/10.1016/j.apcata.2019.117265
APCATA 117265
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
27 June 2019
6 September 2019
19 September 2019
Please cite this article as: Larina OV, Valihura KV, Kyriienko PI, Vlasenko NV, Balakin DY,
ˇ
Khalakhan I, Cendak T, Soloviev SO, Orlyk SM, Successive vapour phase Guerbet
condensation of ethanol and 1-butanol over Mg-Al oxide catalysts in a flow reactor, Applied
Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117265
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Successive vapour phase Guerbet condensation of ethanol and 1-butanol over
Mg-Al oxide catalysts in a flow reactor
a*
a
a*
a
Olga V. Larina , Karina V. Valihura , Pavlo I. Kyriienko , Nina V. Vlasenko , Dmytro Yu.
b
c
d
a
a
Balakin , Ivan Khalakhan , Tomaž Čendak , Sergiy O. Soloviev and Svitlana M. Orlyk
aL.V. Pisarzhevsky Institute of Physical Chemistry of the NAS of Ukraine, 31 Prosp. Nauky,
3028 Kyiv, Ukraine.
0
bInstitute of Physics of the NAS of Ukraine, 46 Prosp. Nauky, 03028 Kyiv, Ukraine.
cCharles University, Faculty of Mathematics and Physics, Department of Surface and Plasma
Science, V Holešovičkách 2, 18000 Prague, Czech Republic.
dNational Institute of Chemistry, Department of Inorganic Chemistry and Technology,
Hajdrihova 19, 1001 Ljubljana, Slovenia.
*
Corresponding authors, e-mail: olga.larina@ukr.net; pavlo_kyriienko@ukr.net
Graphical abstract
1

Highlights


Vapour phase condensation of 1-butanol to 2-ethyl-1-hexanol over Mg-Al oxide solid
catalysts in flow under atmospheric pressure have been performed for the first time.
Experimental opportunities for production of 2-ethyl-1-hexanol from ethanol in a flow
reactor under atmospheric pressure by successive Guerbet condensation over Mg-Al
oxide catalysts has been shown.


The selectivity and yield of the main products in the process strong depends of
acid/base capacity ratio, which is determined by a Mg/Al ratio.
Optimal Mg/Al ratio and arrangement of surface active sites are different for the
reacting alcohols with different carbon chain length.
ABSTRACT:
The successive vapour phase condensation of ethanol and 1-butanol (via Guerbet
reaction) in a flow reactor under atmospheric pressure was studied over catalytic Mg-Al oxide
compositions. Wherein the vapour phase condensation of 1-butanol to 2-ethyl-1-hexanol in flow
has been investigated for the first time. The acid/base capacity ratio, which is determined by the
Mg/Al ratio, is an important characteristic for the activity and selectivity of Mg-Al oxide
catalysts in the abovementioned processes. The carbon chain length of the reacting alcohols, an
arrangement of surface active sites and other steric factors also have an impact on Guerbet
2

condensation in the vapour phase. High productivity of Mg-Al oxide system to the Guerbet
alcohols: 1-butanol – 25 g/(Lcat·h), 2-ethyl-1-hexanol – 19 g/(Lcat·h), has been achieved. The
results have shown a prospect of successive conversion realization: 1) ethanol → 1-butanol; 2) 1-
butanol → 2-ethyl-1-hexanol for the production of 2-ethyl-1-hexanol from ethanol.
KEYWORDS: Guerbet condensation, ethanol, 1-butanol, 2-ethyl-1-hexanol, flow reactor,
vapour phase, Mg-Al oxide system
1
. Introduction
Development of technologies for bio-ethanol upgrade into valuable chemicals and
fuels is an important issue in sustainable chemistry studies [1–3]. Ethanol is a flexible
building block for biorefinaries and can be used for the direct production of ethylene,
propylene, 1,3-butadiene and hydrocarbons, as well as for the production of oxygenated
molecules, such as ethyl acetate, acetic acid, 1-butanol, etc. Moreover, ethanol production
has been steadily increasing over the last decades [4].
2
-Ethyl-1-hexanol (2-EH) is the most widely applied compound among C –C11
6
group of higher alcohols. It is generally used in the production of plasticisers (dioctyl
terephthalate, phthalate, adipate), soaps, solvents, diesel additives and other special
chemicals [5]. The well-known industrial process of 2-EH production based on Guerbet
coupling reaction (Scheme 1) [6–8] involves exploitation of a batch reactor with
homogeneous-heterogeneous catalytic systems (liquid base + solid noble metal) [9–12].
This technology is associated with number of drawbacks: expensive separation,
3

purification, recovery and waste treatment sections and a requirement for noble metal
catalysts. Furthermore, water produced during Guerbet reaction leads to deactivation of
the liquid alkali catalyst, incomplete conversion of alcohols and formation of by-products
[
13].
Scheme 1, near here
Catalytic conversion of bio-ethanol (EtOH) into higher alcohols via C-C coupling
has attracted a scientific interest in recent years [1–4,14,15]. Dias et al. [16] have
developed a techno-economic analysis of prospect for 1-butanol (BuOH) production from
bio-ethanol by means of Guerbet condensation in a vapour phase plug flow reactor. The
authors have identified the main parameters (lower reactor investment costs, less rigorous
requirements for safety and shorter total operation period) allowing this process to be
more effective in comparison to the liquid phase synthesis. The conversion of EtOH to
BuOH in a flow over mixed oxide system is an already known process [17]. Patel et al.
[
18] have analysed techno-economic aspects of the catalytic production of 2-EH from
ethanol in a flow system in the presence of solid catalysts. According to their research the
process of producing 2-EH by means of staged condensation (1) EtOH → BuOH, 2)
BuOH → 2-EH) may be a cost-effective alternative to the traditional industrial method.
A self-condensation of n-butanal to 2-EH has been widely investigated. Hamilton
et al. [19] have reported the n-butanal aldol condensation and further selective
hydrogenation of 2-ethyl-2-hexanal to 2-EH in the presence of Pd/Na/SiO catalyst in a
2
fixed bed reactor with high selectivity of 2-EH (94.9% at the conversion of 42.4%). Li et
al. [20] have investigated the effect of reaction conditions on the one-pot sequential
synthesis of 2-EH catalysed by Ni/La−Al
2
3
O in a self-condensation reaction conducted at
453 K for 8 h, and a further hydrogenation reaction conducted at 453 K for 6 h under 4
4

MPa; n-butanal conversion attains 100 % with 2-EH selectivity of 67%. Liang et al. [21]
have reported the condensation of n-butanal to 2-EH over Ni/Ce−Al catalysts at 443
K, under 4 MPa and a reaction time of 8 h with full 100 % conversion of n-butanal and
6.9 % selectivity of 2-EH. However, an evaluation on reusability of Ni/Ce−Al O showed
that the recovered sample lost its catalytic activity for hydrogenation due to deactivation
of Ni species by the flaky boehmite γ-AlO(OH) formed from the hydration of γ-Al
2
O
3
6
2
2
3
O .
Patankar and Yadav [22] have performed the cascade engineered synthesis of 2-EH in a
one pot from n-butanal under solventless conditions with a trifunctional mixed metal
oxide catalyst containing 5 % Cu and MgO-Al
2
3
O (Mg/Al = 3) with 90 % selectivity. The
reaction rate constants have been calculated for this process and aldol condensation are
found to be a rate-controlling step.
There are few publications in the literature about one-pot direct synthesis of 2-EH
from BuOH in an autoclave in the presence of expensive palladium or iridium complexes
in combination with sodium butoxide as co-catalysts [23–26]. A vapour-phase
condensation flow system and the use of non-precious metals catalysts are more
responsive to current trends of sustainability processes. For realization of BuOH
conversion to 2-EH in a flow reactor in a gaseous phase, heterogeneous acid-base
catalysts are required.
Mg-Al mixed oxide catalysts derived from hydrotalcite (layered double
hydroxides) precursors have received a great attention [27–31] as a very promising
catalysts for the Guerbet condensation due to tuneable bifunctional acid–base properties,
high surface area, high quantity of defects, structural stability both at high temperatures
and in aqueous solutions, absence of toxic metals and relatively cheap cost. A number of
works [32,33] have reported the importance of Mg/Al ratio in the catalyst.
5

In the present work, a principal possibility of producing 2-EH from EtOH by
successive conversion: 1) EtOH → BuOH, 2) BuOH → 2-EH, in a flow reactor under
atmospheric pressure in the presence of Mg-Al oxide catalysts is presented. The effects of
Mg/Al ratio as well as temperature and liquid hourly space velocity on the catalytic
properties of Mg-Al oxide systems are discussed.
2
. Material and methods
2.1
Catalysts preparation
Mg-Al hydrotalcites with Mg/Al molar ratio of 1, 2, 3 and 4 were synthesized by
the co-precipitation method under conditions of high super saturation (pH 10–12). 1M
solutions of Mg(NO ·6H O and Al(NO ·9H O in various proportions were dropped
into 200 mL of a buffer solution containing 1.6 M NaOH and 0.1 M Na CO under
vigorous stirring at temperature of 358 K. The mixture was kept at 358 K for 24 hours
prior to a filtration step. The same method was used for MgO and Al synthesis. Then,
3
)
2
2
3
)
3
2
2
3
2
O
3
the precipitates were separated by hot filtration, washed several times with warm
deionized water to remove alkali metals and nitrate ions until neutral pH was reached, and
dried at 393 K for 6 h. The resulting hydrotalcites were calcined at 873 K for 5 hours to
obtain mixed oxides. This treatment temperature of Mg-Al hydrotalcites was chosen as
optimal according to the work [27].
2.2. Catalysts characterization
XRD patterns of the powder samples were recorded using D8 Advance (Bruker
AXS GmbH, Germany) diffractometer with monochromatized Cu-Kα radiation (nickel
filter, λ = 0.154 nm).
6

Morphology of the samples was observed by means of Scanning Electron
Microscopy (SEM) using a Tescan MIRA 3 microscope operating at 30 keV electron
beam energy. Chemical composition mapping analysis was carried out using energy
dispersive X-ray spectroscopy (EDX) with a Bruker XFlash detector mounted directly
into the SEM.
27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments
were carried out on an Agilent Technologies VNMRS 600 MHz spectrometer (14.1 Tesla
2
7
5
1
1 mm bore Oxford superconducting magnet) operating at a Al Larmor frequency of
2
7
56.4 MHz, and using a 3.2 mm MAS probe head. The Al MAS NMR spectra were
recorded using a single pulse acquisition with small pulse angle (π/12), at a spinning
speed of 20 kHz and with a recycle delay of 4 s. All measurements were carried out at
room temperature employing AlCl
3
·6H O as a standard reference (0 ppm). All spectra
2
were normalized to the same area under the curves.
The acid-base characteristics of the samples were investigated on vacuum thermo-
gravimetric apparatus with quartz microbalance. The concentration of acidic and basic
sites was measured by the sorption of NH
3
and CO in a quasi-equilibrium regime (QE-
2
TD method of ammonia and carbon dioxide).[34] Prior to the adsorption measurements,
the samples were heated under vacuum (0.133 Pa) at 773 K to reach a constant weight.
The adsorption of NH
3
and CO was carried out at room temperature until no uptake was
2
observed, and the surplus was removed under vacuum. The weight was monitored during
stepwise increase of temperature and vacuum treatment. The total concentration of acidic
and basic sites was determined from the amount of NH
3
and CO adsorbed on the sample
2
surface at 323 K. The desorption temperature of the probe molecule was used as a
criterion for the strength of acidic/basic sites [35].
7

The nature of acidic sites of the samples was studied by Fourier transform infrared
FT-IR) spectroscopy of adsorbed pyridine. Before adsorption/desorption experiments,
(
-
2
the samples were pressed into thin wafers of ca. 10 mg·cm and placed inside the FT-IR
cell. The wafers were activated by calcination at 773 K for 1 h and then outgassed under
vacuum at 723 K (1 Pa) for 1 h. These wafers were exposed to gaseous pyridine at 423 K.
The spectra were recorded after desorption at defined temperature using Spectrum One
−
1
FT-IR spectrometer (Perkin Elmer, USA) with resolution of 1 cm , 12 scans. The
reported spectra were obtained after subtraction of the background spectra recorded after
calcination and prior to molecular probes adsorption.
The specific surface area of the samples was determined by a chromatographic method by
thermal desorption of nitrogen on a GKh-1 instrument, with a gas mixture containing 5
vol.% of N
.3. Catalytic activity measurement
Catalytic activity tests were carried out in a fixed-bed flow quartz reactor with
2
in helium.
2
inner diameter of 10 mm at 523–623 K and under atmospheric pressure. Catalyst samples
with grains of 1–2 mm were loaded into the reactor. Before catalytic tests, samples were
treated in a gas mixture of Ar and H
experiment temperature. Ethanol (94%, the rest – H
O) feed was introduced to the hot reactor via a syringe infusion pump for evaporation.
Reaction was carried out at fixed liquid hourly space velocity (LHSV). For ethanol
2
(50:50) at 773 K for 1 h and then cooled down to the
2
O) or 1-butanol (99.8%, the rest –
H
2
-1
−1
conversion, LHSV = 0.118; 0.235; 0.47 L∙Lcat ∙h (corresponding alcohol gas-hourly
−
1
space velocity (GHSV) = 460; 915; 1831 h ). For 1-butanol conversion, LHSV = 0.125;
-
1
−1
−1
0
.25; 0.5 L∙Lcat ∙h (corresponding GHSV = 306; 613; 1226 h ). The mixture of Ar and
-
1
H
2
with the ratio of 50:50 was used as a carrier gas with a flow of 10 mL·min . The
8

reactor effluent sampling for analysis was performed only after reaching of steady state of
the reaction (2 h after the beginning of catalytic experiment). The condensable liquid
products were collected into an ice-water cooled receiver with a sampling interval of 1 h.
The gas products after the cooled receiver were analysed with a sampling interval of 0.33
h. Catalytic indices are calculated as average values of multiple samples taken regularly
during a time on stream from 2 to 4 h. The reagents and reaction products were analysed on a
gas chromatograph (KristalLyuks 4000M, MetaChrome) equipped with a thermal conductivity
detector and a packed column (10 % NiSO
ionization detector and a capillary column (HP-FFAP, 50 m × 0.32 mm) for organic compounds.
Catalytic activity of the catalysts was characterized by conversion ( ), selectivity
4
on coal, 3 m × 3 mm) for CO and CO
2
, and a flame
X
to products (
S
), yield (
Y
) and productivity (P):
0
n
j
n
 n
i
i
X

 100%,
S

 100%,
0
i
j
0
i
n
(n  n
)
i
i
X
 S
Y
 LHSV    k
i
j
j
i
Y

,
P

,
j
100%
100%
j
where n0
is the initial amount of C moles of alcohol (EtOH or BuOH) in the feed;
in the stream of the
is the maximum possible amount of
n and
i
i
n
j
are the amount of C moles of the unreacted alcohol and product j
reaction products;
the main product (
ρ – liquid density of the alcohol, k
i
g
) that can be produced from 1 g of the alcohol.
S
  S
,
Y

 Y
,

g

g
where
S
,
Y
– sum of the selectivities and yields of resulting Guerbet alcohols g, namely:


1
-butanol, 2-ethyl-1-butanol, 1-hexanol, 2-ethyl-1-hexanol, and 1-octanol.
9

3
. Results and Discussion
3
.1
Structural and textural characteristics
Powder X-ray patterns of as-prepared catalysts are shown in Figure 1. The patterns of
synthesized precursors exhibit a series of sharp reflections for the (0 0 3), (0 0 6), (1 1 0), (1 1 3)
planes and more broad peaks for the (0 0 9), (0 1 5), (0 1 8) planes, characteristic of hydrotalcite
crystalline structures, which consists of layered double hydroxides with brucite-like layers.[36]
After sample calcination at 873 K the patterns exhibit weak broad peaks at approximately 37°,
4
3°, 63°, which corresponds to diffraction of (1 1 1), (2 0 0), (2 2 0) planes and can be assigned
to MgO phase (periclase) [37]. The wide peaks indicate a low crystallinity and/or small crystal
sizes of the identified phases. If a separate Al-containing phase such as γ-Al is present, its
2
O
3
crystallinity is too weak to be identified by XRD. It can indicate a very high dispersion of Al
in the MgO structures with formation of Mg-Al mixed oxide periclase-like phase [38]. The
2
O
3
formation of crystal MgAl
2
4
O phase is not obvious, since the main lines of spinel species can be
hidden under the major MgO phase reflections if small amounts are present in the samples.
Figure 1, near here
Figure 2, near here
SEM images have been recorded to investigate the morphology of calcined
samples with different Mg/Al ratios. The micrographs of the Mg-Al oxide catalysts with
Mg/Al molar ratios in the range from 1 to 4 (Figure 2) show a well-developed layered
structure [39,40]. One can distinguish that Al content in the samples influences the
particle size and morphology. Mg-Al (4:1) sample with the highest crystallinity possesses
a well-developed platelet structure typical for layered materials. It is formed by
1
0

aggregates consisting of thin plate-like crystals 150−200 nm in size. Mg-Al (3:1) and Mg-
Al (2:1) samples are formed by the same aggregates. They are composed of plate-like
crystals which are smaller in size (⁓ 100 nm) and significantly thinner than those in Mg-
Al (4:1). Besides, the micrographs of Mg-Al (3:1) and Mg-Al (2:1) allow assuming the
presence of an amorphous constituent in these samples. The images of Mg-Al (1:1) depict
more amorphous aggregates; plate-like crystals are less noticeable. The uniform
distribution of magnesium and aluminium on the surface of the samples with the ratio of
Mg/Al = 1, 2 and 3 (EDX elemental mapping images in Figure S1, ESI) indicates the
homogeneity of the prepared Mg-Al oxide system. For Mg-Al (4:1) sample, a segregation
of Al to the surface can be observed.
Figure 3, near here
27Al MAS NMR of the samples after treatment at 873 K has been performed in
order to identify the nature of alumina species (Figure 3A). It can be noticed that two
3
+
signals are observed on the spectra: one related to Al cations tetrahedrally coordinated
3
+
to oxygen (Altetra, δ = 68–83 ppm), and the other one can be assigned to Al cations
octahedrally coordinated to oxygen (Alocta, δ = 11–18 ppm). The signal of Alocta species
2
7
consists of at least two components. Hence, the Al MAS NMR spectra have been fitted
using a Czjzek model [41] implemented in DMFit software. This model is often used
[
42,43] to simulate MAS experiments for quadrupolar nuclei in glassy, amorphous and
disordered materials and describes the structural disorder around the probe nucleus at two
or three atomic positions scale which results from a variation of the quadrupolar
interaction (variation of the electronic field gradient).
Table 1, near here
1
1

The fitting results are summarized in Figure 3B and Table 1. If only three
resonances are introduced in the model, the spectrum cannot be correctly fitted. Thus,
fourth line located at approximately 69 ppm has been introduced. Lines located at 16–18
and 79–83 ppm may be assigned to octahedrally and tetrahedrally coordinated aluminium
atoms involved in a highly dispersed Mg-containing phase (not obviously detected by
XRD), and the lines at ⁓ 11 and 69 ppm – to γ-alumina, as a segregated phase. It should
be mentioned that only Mg-Al oxide samples with high and low amount of alumina
contains such segregated γ-alumina phase.
Thus, raw synthesized precursors of Mg-Al oxide compositions constitute
hydrotalcite crystalline structures, which consists of layered double hydroxides with
brucite-like layers. After calcination at 873 K a very high dispersion of γ-Al
structures with formation of Mg-Al mixed oxide periclase-like phase has been observed.
The formation of MgAl is not obvious, but the presence of Mg-O-Al bonds in the
samples under study is confirmed by NMR.
2
O
3
in MgO
2
O
4
3
.2
Acid-base properties
The acid-base characteristics (the concentration and strength of acidic and basic sites) of
the Mg-Al oxide catalysts have been determined using QE-TD of ammonia and carbon dioxide.
The nature of acidic sites has been studied by FT-IR spectroscopy of adsorbed pyridine.
Figure 4, near here
Figure 4 shows distribution profiles of acidic and basic sites on desorption
temperature of NH
3
and CO , which characterizes a strength of acidic/basic sites [35].
2
Table 2 represents the data on acid-base capacity and surface density of the sites.
1
2

Total concentration of acidic sites in parent MgO is relatively small, and accounts
for 0.15 mmol NH /g. The acidity spectrum of this sample includes only super weak and
weak acidic sites; a limit temperature of NH desorption is 473 K. Introduction of
3
3
aluminium cations into MgO leads to an increase in total acidity of the Mg-Al oxide
system due to the formation of a large number of medium and strong acidic sites. As a
result, a limit temperature of ammonia desorption for Mg-Al oxide samples reaches
6
73 K. In general, when aluminium oxide is present in the system, total acidity of the
samples passes through a maximum corresponding to Mg-Al (3:1) sample (С
.80 mmol NH /g). This sample is characterized by the highest concentration of strong
acidic sites (0.24 mmol NH /g). For parent Al , total concentration of acidic sites
/g.
Σ
=
0
3
3
2
O
3
accounts for 0.94 mmol NH
Table 2, near here
3
Since Mg-Al oxide samples have different specific surface areas, a density of
acidic sites on the surfaces become an important characteristic. The obtained results
(Table 2) indicate that despite an inversely proportional dependence of specific surface
area on Mg/Al ratio, a change in this ratio from 4 to 1 leads to a dome-like increase in a
density of acidic sites on the surface with a maximum for Mg-Al (3:1) sample. As in the
2
case of acid capacity, a maximum acid density (6.73 µmol NH
3
/m ) corresponds to Mg-
Al (3:1) sample.
Regarding basic characteristics of Mg-Al oxide system, it has been found that the
introduction of aluminium cations to MgO leads to a non-monotonic dependence of total
3
+
base capacity on Mg/Al ratio. The addition of a small amount of Al (Mg-Al (4:1)
sample) results in a decrease in the number of basic sites as compared with those for
parent MgO. Reducing of Mg/Al ratio to 3 causes disappearance of strong basic sites, as
1
3

result, limit temperature of CO desorption for Mg-Al (3:1) sample declines to 623 K.
2
Further increase in the aluminium content contributes to disappearance of the basic sites
with Tdes. = 523-623 K and formation of stronger basic sites. As a result, limit temperature
of CO
time, medium basic sites reappear on the surface of Mg-Al (1:1) sample. For parent
Al , limit temperature of CO desorption does not exceed 673 K.
Total base capacity of the samples with the ratios of Mg/Al = 1, 2 and 3 has a close
order (0.21–0.3 mmol CO /g), but a density of surface basic sites for the various samples
2
desorption for Mg-Al (2:1) and Mg-Al (1:1) samples rises to 723 K. At the same
2
O
3
2
2
differs significantly. Total base density for Mg-Al oxide system varies from 2.11 to
2
1
.34 µmol CO
2
/m in a non-monotonic sequence:
Mg-Al (3:1) > Mg-Al (1:1) > Mg-Al (4:1) > Mg-Al (2:1).
As mentioned in the Introduction part, for efficient Guerbet process a catalyst
should have different types of sites. Table 2 shows acid/base capacity ratio (ABCR) for
all investigated samples. This indicator for parent MgO has the lowest value (0.8). The
addition of aluminium cations to MgO causes an increase in ABCR to 3.5. Further
decrease in Mg-Al ratio leads to a decrease in acid/base capacity ratio. In general, for Mg-
Al oxide compositions, when Mg/Al ratio changes from 1 to 4, a directly proportional
dependence of ABCR on Mg/Al ratio is observed. For parent Al
.9.
2
3
O , ABSR accounts for
3
Figure 5, near here
FT-IR spectra of pyridine adsorbed on Mg-Al oxide system, MgO and Al
2
3
O are
–
1
depicted in Figure 5. The bands at 1598 and 1446 cm correspond to pyridine
coordinatively bonded to weak Lewis acidic sites (LAS), as proved by their disappearance
after desorption of pyridine at 473 K. In the case of magnesium oxide, the intensity of the
1
4

band at 473 K is low, which indicates a low content of LAS. The introduction of
aluminium leads to an increase in the intensity of this band and, accordingly, the amount
of this type of acidic sites, which confirms the previously obtained results of QE-TD.
Significant changes in the transition from the sample with a lower aluminium content to
the sample with its high content is not observed. The bands characteristic of pyridine
adsorption on Bronsted acid sites are not detected in the spectra of all samples.
Thus, acidic and basic Lewis sites are present on the surfaces of all investigated
samples; Bronsted sites are not identified. The introduction of aluminium cations into
MgO leads to increase in the concentration of acidic sites, the highest content of such
sites is reached for Mg-Al (3:1) sample. An increase in the concentration of basic sites is
also observed in the samples with large amounts of aluminium (ratios of Mg/Al = 1, 2 and
3). The acid-base capacity ratio for Mg-Al oxide systems changes in inverse proportion to
the amount of aluminium introduced into the compositions.
Summarizing the results of the acid-base characteristics studied and NMR, XRD
and SEM data, the following types of acid-base Lewis pairs are assumed to be present on
the surface of Mg-Al oxide systems: Mg-O-Mg (basic character) due to the presence of
MgO, Al-O-Al (acidic character) due to the presence of Al
2
3
O , and acid-base Lewis pairs
of the Al-O-Mg type formed in the contact zone of magnesia and alumina phases during
the calcination of hydrotalcites.
3
.3
Catalytic properties
The main results of catalytic tests of Mg-Al oxide system (as well as parent MgO and
) for vapour phase condensation of EtOH to BuOH in a flow reactor at 573 K are
Al
2
O
3
summarized in Table 3. In the series of Mg-Al oxide catalysts (including MgO and Al
2
3
O ) EtOH
conversion rises with an increase in the content of aluminium oxide in the samples. Addition of
1
5

aluminium oxide to magnesia lead to a slightly lower BuOH selectivity in comparison with
parent MgO. However, with further increase in Al content the selectivity for the target
product is increased passing through the maximum for Mg-Al (2:1) sample. For this catalyst
2
O
3
1
7.5 % BuOH yield is achieved (selectivity of 65.2 % at EtOH conversion of 26.9 %).
Scheme 2, near here
EtOH-to-BuOH conversion involves a complex sequence of reactions. The main
steps of the Guerbet coupling are dehydrogenation of EtOH to acetaldehyde (AA), aldol
self-condensation reaction to croton aldehyde (CA), hydrogenation of the condensation
product to give BuOH (Scheme 1). Together with general path of the Guerbet
condensation a number of side reactions also take place during the process: dehydration
of EtOH to ethylene (E) and diethyl ether (DEE), disproportionation of AA to ethyl
acetate (EA) by Tishchenko reaction, and others (Scheme 2).
Table 3, near here
For Mg-Al oxide catalysts under study, AA is observed in the product stream as
primary product of EtOH dehydrogenation (Table 3). At the same time, aldol
condensation products (i.e. CA) are generally not detected which can be attributed to the
fact that this reaction occurs rapidly after initiation. DEE is the main EtOH dehydration
product. Its selectivity rises with an increase in the content of aluminium oxide in the
catalysts. The parent Al
0.0%). Small fractions of E, acetone (Ac), EA and other side products are also identified
in the product stream.
2
O
3
is especially selective for DEE formation (selectivity of
8
It is noteworthy that resulting BuOH reacts with itself and with initial reagent
EtOH) to form higher Guerbet alcohols. As a result, four product options exist: three
(
1
6

different cross-coupling products formed from the different alcohols – 2-ethyl-1-butanol
2-EB), 1-hexanol (HeOH), 1-octanol (OcOH), and a self-coupling product – 2-EH. They
(
are detected in the reaction products with comparatively high yield. Butanal and butenes
are also founded in the resulting mixture as primary products of BuOH dehydrogenation
and dehydration. The composition with a ratio of Mg/Al = 4 is the most selective (24.9%)
one for butenes. Therefore, Mg-Al-oxide catalysts are active not only for EtOH-to-BuOH
conversion, but also for higher alcohol condensation in vapour phase.
Figure 6, near here
Overall Mg/Al ratio dependences of EtOH conversion, sum of the selectivities and
yields of the main reaction products – BuOH and higher Guerbet alcohols – are
summarized in Figure 6. Additionally, productivities of Mg-Al oxide system in EtOH-to-
BuOH and EtOH-to-Guerbet alcohols processes are shown. According to the obtained
data, selectivities and yields of BuOH and higher Guerbet alcohols have a similar trend.
The best result is achieved over Mg-Al (2:1) sample providing productivity of 25
g/(Lcat·h) for BuOH and 33 g/(Lcat·h) – for total amount of Guerbet alcohols (BuOH +
heavier alcohols).
A summary of catalytic properties of Mg-Al oxide system (with parent MgO and Al
2
O
3
for comparison) in the process of vapour phase condensation of BuOH to 2-EH in a flow
reactor at 573 K is shown in Table 4.
Table 4, near here
The dependence of catalytic properties on the ratio of Mg/Al in Mg-Al oxide composition
is quite similar to the process of EtOH condensation to BuOH. With an increase in the
content of aluminium oxide in the samples BuOH conversion also rises.
1
7

Selectivity of 2-EH is decreased initially with the addition of aluminium oxide as
compared to parent magnesium oxide, then rises with further increase of Al content in
2
O
3
the samples passing through the maximum for Mg-Al (1:1) and then again declines for
parent alumina. 2-EH yield of 10.8% (selectivity of 57.3% at BuOH conversion of 18.8%)
is achieved using a composition with a ratio of Mg/Al = 1 providing productivity of
1
9 g/(Lcat·h). The selectivity of BuOH dehydration products – butene and dibutyl ether
(
DBE) – rises with an increase in the content of Al in the compositions. The parent
2
O
3
alumina is especially selective for butene and dibutyl ether formation (selectivity of 21.2
and 69.9%). Butanal is observed in the product stream as a primary product of BuOH
dehydrogenation.
Figure 7, near here
Vapour phase condensation of EtOH to BuOH over the sample with the best
obtained catalytic results in this process at 573 K has been also performed at different
reaction temperatures (523 and 623 K). Temperature dependences of EtOH conversion,
sum of the selectivities, yields and productivity to BuOH and other Guerbet alcohols for
Mg-Al (2:1) composition are shown in Figure 7. EtOH conversion over the tested catalyst
grows up with temperature. On the contrary, the selectivities of both BuOH and other
Guerbet alcohols tend to decrease due to more active formation of side products of
dehydration (E, butenes, DEE) and etherification (Ac, EA) (Table S1). The highest BuOH
yield of 22.5% and productivity of 32 g/(Lcat·h) are observed over Mg-Al (2:1)
composition at 623 K. For total amount of Guerbet alcohols these indices are 30.1% and
4
2 g/(Lcat·h).
Figure 8, near here
1
8

Figure 8 depicts temperature dependences of BuOH conversion, 2-EH selectivity,
yield, and productivity of Mg-Al (1:1) and Mg-Al (2:1) compositions in vapour phase
condensation of BuOH to 2-EH. BuOH conversion grows up with increasing temperature,
but 2-EH selectivity is drastically decreased causing the decline of 2-EH yield. The best
indices of 2-EH yield over investigated catalysts at 523 K are close: 12.7-13.2%.
In both EtOH-to-BuOH and BuOH-to-2-EH processes, the formation of dehydration
(E/butenes, DEE/DBE) and dehydrogenation (AA/butanal) products is largely increased with
temperature rising and vice versa (Tables S1, S2, S3).
According to thermodynamic calculations [13,44], the selectivity to BuOH (for EtOH-to-
BuOH conversion) and 2-EH (for BuOH-to-2-EH one) depends on the thermodynamic features
of these Guerbet condensation processes. The equilibrium yield of C-C condensation products
for both processes is decreased with increasing temperature. However, for BuOH conversion
such decrease is more substantial than for EtOH conversion. Therefore, BuOH-to-2-EH process
is expediently carried out at lower temperature than EtOH-to-BuOH one.
Figure 9, near here
The experiments of vapour phase condensation of EtOH to BuOH and BuOH to 2-
EH over the samples with the best performance have been also conducted at various
LHSV at 573 K. The main catalytic indices depending on LHSV in EtOH-to-BuOH
process over Mg-Al (2:1) composition are shown in Figure 9. EtOH conversion and sum
of the yields of resulting Guerbet alcohols declines with rising of EtOH load, but total
selectivity of Guerbet alcohols (BuOH + higher alcohols) remains at almost the same
level (86.5-88.6%). Thereby, high productivity of 0.049 kg/(Lcat·h) for BuOH and 0.060
kg/(Lcat·h) for total amount of Guerbet alcohols (BuOH + heavier alcohols) is achieved at
1
9

largest (0.47 L/(Lcat·h)) EtOH load. It should be noted that individual BuOH selectivity is
increased with the LHSV rising, conversely to selectivity of higher Guerbet alcohols,
which is decreased with EtOH load (Table S4). So, at higher LHSV butanol is less
involved in subsequent condensation reactions with the formation of heavier alcohols.
Figure 10, near here
The dependences of catalytic properties of Mg-Al (1:1) composition in BuOH-to-
2
-EH process on LHSV are depicted in Figure 10. EtOH conversion and 2-EH yield are
decreased with rising of BuOH load. The selectivity of 2-EH is decreased too due to
incomplete conversion of butanal and formation of side dehydration products (butenes,
DBE) (Table S5). The highest productivity of Mg-Al (1:1) to 2-EH – 19 g/(Lcat·h) – is
achieved at LHSV of 0.25 L/(Lcat·h).
3
.4
General discussion
A number of works [17,32,33,45] are related to investigation of the mechanism of ethanol
to 1-butanol conversion, however, only one theoretical paper [13] can be found about the
mechanism of the 1-butanol to 2-ethyl-1-hexanol process. Let's analyse the similarities in
reaction paths of these two different processes over the same series of catalysts.
Di Cosimo et al. [33] have shown that condensation of acetaldehyde is the slowest
step of Guerbet reaction on mixed Mg-Al oxides. Usually the catalytic performance of
Mg-Al oxide compositions in individual aldol condensation reactions is correlated with
their acid-base properties, more particularly, the highest conversion is observed over
compositions with the highest density of basic sites [40]. However the Guerbet reaction is
a complex process with more complicated dependences. According to previous analysis
of the reaction mechanism, it can be suggested that for efficient conversion of EtOH or
2
0

BuOH the catalysts should have different types of sites, meaning acidic and basic ones are
equally important. The basic sites allow the Mg-Al oxide compositions to exhibit activity
in key steps of the process – dehydrogenation and aldolization. Apart from that, Lewis
acidic sites act like adsorption sites for intermediates. Hence, low catalytic activity of
parent MgO in both EtOH-to-BuOH and BuOH-to-2-EH conversion processes (Tables 3
2
−
and 4) can be explained by the presence of predominantly isolated O basic sites which
hinder formation of the alkoxide intermediates by alcohol dissociative adsorption [32]. It
is in agreement with QE-TD results (Figure 4, Table 2), where parent MgO exhibits the
highest surface oxygen concentration and the highest base density. At the same time, the
3
+
2−
presence of a high density of Al -O pairs on a Al
2
3
O surface, in particular Lewis acidic
sites, can be responsible for dehydration of ethanol to ethylene (BuOH to butenes), and
the coupling with dehydration to diethyl ether (dibutyl ether) [32].
There is an interesting tendency when Mg-Al oxide samples with a high and low
amount of alumina catalyse the side reactions of dehydration more effectively than the
samples with middle Al content (Tables 3 and 4). Seemingly, in the catalysts with a
medium amount of alumina, Al-O-Mg species (Lewis acid-base pairs) located at the
surface of a brucite-like mixed oxide in an oxygen environment do not favour dehydration
of EtOH to ethylene and diethyl ether. Therein, Al-O-Al species like in parent Al
the active sites of the dehydration [46]. This assumption is confirmed by NMR data
Figure 3, Table 1): only Mg-Al oxide samples with a high and low amount of alumina
2
3
O are
(
contains a segregated γ-alumina phase. Moreover, for a Mg-Al (4:1) sample, the
segregation of alumina to the surface is observed by EDX mapping (Figure S1).
Figure 11, near here
2
1

Aldol self-condensation of aldehyde involves formation of carbanion species on adjacent
acidic and strong basic sites, where acidic sites adsorb aldehyde molecules and basic sites
abstract the proton from this aldehyde, generating a carbanion intermediate. After that, carbanion
induces nucleophilic attack toward other adsorbed aldehydes, thus forming a new C–C bond and
yielding an aldol product [29,33]. Such a mechanism explains the reacting of resulting butanol
with itself or the initial ethanol to form 1-hexanol, 2-ethyl-1-butanol, 2-ethyl-1-hexanol, and 1-
octanol.
It should be mentioned that the investigated Mg-Al oxide samples are more
selective for cross-coupling (EtOH + BuOH) products – HeOH and 2-EB, than for co-
condensation (BuOH + BuOH) products – 2-EH and OcOH (Table 3). It is in agreement
with the classic dependence of a reaction rate on reagents concentration: since the
concentration of ethanol/acetaldehyde is higher, the quantity of resulting HeOH is larger.
However, a stearic or electronic effect also impacts on the relative rates of carbanion
formation [17,47].
As mentioned above, the acid/base capacity ratio (ABCR) for the samples under
study changes with the composition from the lowest value (0.8) for parent MgO to the
highest (3.9) – for parent Al
2
3
O , intermediate ABCR values correspond to Mg-Al oxide
catalysts (Table 2). Therein, there are dome-like dependences of the productivity of Mg-
Al oxide compositions in the EtOH → BuOH and BuOH → 2-EH processes on acid/base
capacity ratio (Figure 11). An analysis of these dependencies suggests that the catalysts
with a medium value of ABCR are more active and selective towards the target alcohol
product (BuOH or 2-EH). The optimum acid/base capacity ratio varies from 3 to 2 with
the increase in a carbon chain length of the reacting alcohol (from C
2
to C , i.e. from
4
2
2

EtOH to BuOH) during Guerbet coupling. The similar dependencies of the indices of the
Guerbet process on the ratio of acidic/basic sites has been also reported for
hydroxyapatites [48,49].
It is also noteworthy that the total density of acidic sites is not a critical parameter
for EtOH-to-BuOH conversion. For Mg-Al (3:1) and Mg-Al (1:1) samples, total acid
density values differ twice, but BuOH yields are fairly close. Hence, more important
parameter is the nature of formed acid-base pairs and their availability.
For efficient realization of the BuOH-to-2-EH process, the presence of a strong
basic sites is required; 2-EH selectivity is a higher for the samples with a higher content
of strong basic sites (Mg-Al (1:1) and Mg-Al (2:1) samples, Table 2 and 4). However,
over MgO and Mg-Al (4:1) the selectivity towards 2-EH is significantly lower than over
the samples with a high aluminum content. This may be caused by a keto-enol
tautomerism of the formed intermediate product or an inhibition of
Meerwein−Ponndorf−Verley reduction of aldol condensation product over acid-base pairs
formed by MgO. It leads to transformation of the initial alcohol into undesirable side C-C
products. Over parent alumina, BuOH-to-2-EH conversion also proceeds with low
selectivity. Thus, a catalyst with acidic pairs including both magnesium and aluminum
cations is preferably required for BuOH-to-2-EH process.
Consequently, the acid/base capacity ratio is an important characteristic of activity
and selectivity of Mg-Al oxide catalysts in the process of vapour phase condensation of
ethanol to 1-butanol and 1-butanol to 2-ethyl-1-hexanol in flow. The BuOH-to-2-EH
process is a quite similar to EtOH-to-BuOH conversion, but all dependences are not
replicated fully. Thus, not only acid-base properties of the catalyst, but also the carbon
2
3

chain length of the reacting alcohols, an arrangement of surface active sites and other
steric factors have an impact on the Guerbet condensation in vapour phase.
Conclusions
In this study the research results of the vapour phase condensation of BuOH to 2-
EH over solid catalysts in a flow reactor under atmospheric pressure are presented for the
first time. Conversion of EtOH and BuOH, and the selectivity towards resulting Guerbet
alcohols essentially depend on the acid-base characteristics of the Mg-Al oxide system,
which are determined by the Mg/Al ratio. The presence of acid-base Lewis pairs of the
Al-O-Mg type, formed in the contact zone of magnesia and alumina phases during the
calcination of hydrotalcites, is required for transformation of the initial alcohols into the
target products (1-butanol and 2-ethyl-1-hexanol). The different optimal ratio of
acidic/basic sites on the catalyst surface for conversion of EtOH to BuOH and BuOH to
2
-EH may be the cause of the largest yield of BuOH (17.5% with selectivity 65.2 %)
being achieved in the presence of a composition with a ratio of Mg/Al = 2, and the largest
yield of 2-EH (10.8 % with selectivity 57.3 %) being obtained over a composition with
the Mg/Al = 1 ratio. Additional influence on Guerbet condensation in a vapour phase has
the carbon chain length of the reacting alcohols and an arrangement of active sites on the
surface. The potential opportunity of the 2-ethyl-1-hexanol production from ethanol using
the successive Guerbet condensation EtOH → BuOH and BuOH → 2-EH in a flow
reactor under atmospheric pressure over Mg-Al oxide catalytic systems has been shown.
Acknowledgements
2
4

This work was financially supported by programs of National Academy of Sciences of
Ukraine KPKVK 6541230 “Support for the development of priority areas of scientific research”.
The authors, in particular O.V. Larina, K.V. Valihura, D.Yu. Balakin, I. Khalakhan and
T. Čendak acknowledge CERIC-ERIC Consortium for access to experimental facilities at
Nuclear Magnetic Resonance Spectrometer “Magic” at Slovenian NMR Centre (Proposal
number: 20182046) and Field Emission Scanning Electron Microscope at Charles University in
Prague (Proposal number: 20182124), and financial support.
2
5

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Captions for Figures and Scheme
Figure 1: XRD of the prepared samples before (a) and after treatment at 873 K (b): 1 –
MgO, 2 – Mg-Al (4:1), 3 – Mg-Al (3:1), 4 – Mg-Al (2:1), 5 – Mg-Al (1:1); 6 – Al
2
3
O .
Figure 2: SEM images of the samples after a treatment at 873 K: A – Mg-Al (4:1), B –
Mg-Al (3:1), C – Mg-Al (2:1), D – Mg-Al (1:1).
Figure 3: 27Al MAS NMR spectra of the samples after a treatment at 873 K (A) and the
corresponding fit calculating using DMFit software (B): 1 – Mg-Al (4:1), 2 – Mg-Al
(
3:1), 3 – Mg-Al (2:1), 4 – Mg-Al (1:1).
Figure 4: Differential QE-TD-curves of ammonia (A) and carbon dioxide (B) for the samples: 1
MgO, 2 – Mg-Al (4:1), 3 – Mg-Al (3:1), 4 – Mg-Al (2:1), 5 – Mg-Al (1:1), 6 – Al
Figure 5: FTIR spectra of adsorbed pyridine over the samples after desorption at 423 K
dash lines) and 473 K (solid lines): 1 – MgO, 2 – Mg-Al (4:1), 3 – Mg-Al (3:1), 4 – Mg-
Al (2:1), 5 – Mg-Al (1:1); 6 – Al
–
2
3
O .
(
2
3
O .
Figure 6: Mg/Al ratio dependences of EtOH conversion, sum of the selectivities and
yields of resulting Guerbet alcohols, and productivity of Mg-Al oxide compositions in
EtOH-to-BuOH and EtOH-to-Guerbet alcohols processes (T = 573 K, LHSV = 0.235
L/(Lcat·h)).
Figure 7: Temperature dependences of EtOH conversion, sum of the selectivities and
yields of resulting Guerbet alcohols, and productivity of Mg-Al (2:1) composition in
EtOH-to-BuOH and EtOH-to-Guerbet alcohols processes (LHSV = 0.235 L/(Lcat·h)).
2
9