Highly selective 1-butanol obtained from ethanol catalyzed by mixed metal oxides: Reaction optimization and catalyst structure behavior

Article abstract of DOI:10.1016/j.mcat.2019.110516

Synthesis and characterization of a copper mixed metal oxide obtained from hydrotalcite precursor as well as catalytic runs for ethanol conversion to 1-butanol are described. Applying the surface response model, the reaction was optimized reaching an ethanol conversion of 79.6% into gaseous phase (69% of yield) and condensed phase (31% of yield) products using a batch reactor at 350 °C for 5 h. The main product of the condensed phase was 1-butanol with 25.4% yield and 32% selectivity, these results being among the higher ones reported for this reaction in the literature. Recycling catalyst experiments demonstrated that, for at least four cycles, the ethanol conversion remains almost constant but the 1-butanol yield and selectivity both decreased. XRD, EPR, surface area measurements and acidity/basicity experiments carried out after the first and fourth catalyst recycling cycles show major modifications in the initial mixed metal oxide structure and copper oxidation state indicating that the active catalytic species increases during the first catalytic run.

Full text of DOI:10.1016/j.mcat.2019.110516

MolecularCatalysis476(2019)110516
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Highly selective 1-butanol obtained from ethanol catalyzed by mixed metal
oxides: Reaction optimization and catalyst structure behavior
Marcos Rechi Siqueira⁎, Olavo Micali Perrone, Gustavo Metzker,
Daniela Correa de Oliveira Lisboa, João Cláudio Thoméo, Maurício Boscolo
São Paulo State University (UNESP), Institute of Biosciences, Humanities and Exact Sciences (IBILCE), São José do Rio Preto, SP, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Synthesis and characterization of a copper mixed metal oxide obtained from hydrotalcite precursor as well as
catalytic runs for ethanol conversion to 1-butanol are described. Applying the surface response model, the re-
action was optimized reaching an ethanol conversion of 79.6% into gaseous phase (69% of yield) and condensed
phase (31% of yield) products using a batch reactor at 350 °C for 5 h. The main product of the condensed phase
was 1-butanol with 25.4% yield and 32% selectivity, these results being among the higher ones reported for this
reaction in the literature. Recycling catalyst experiments demonstrated that, for at least four cycles, the ethanol
conversion remains almost constant but the 1-butanol yield and selectivity both decreased. XRD, EPR, surface
area measurements and acidity/basicity experiments carried out after the first and fourth catalyst recycling
cycles show major modifications in the initial mixed metal oxide structure and copper oxidation state indicating
that the active catalytic species increases during the first catalytic run.
Metal mixed oxides
Copper-modified hydrotalcite
Ethanol conversion
1-Butanol
1. Introduction
reaction pathways, although both start from ethanol. Hydrogen is ob-
tained by the ethanol reforming reaction, in which ethanol is fully
Ethanol, derived from sugar fermentation, is the biofuel produced in
largest volume from renewable sources [1]. Although it is mainly used
as fuel for combustion engines, ethanol has been studied as a possible
raw material in the chemical industry to produce hydrogen [2–5], bu-
tadiene [6,7], 1-butanol [8], acetaldehyde [9] and p-xylene [10] re-
presenting an alternative source of carbon-containing compounds in
place of petroleum.
converted into gaseous products [2,16] Other compounds, mostly 1-
butanol, are obtained from the Guerbet reaction [17]. This endothermic
process is the most accepted route for ethanol-based coupling reactions
to increase the number of carbons atoms in the chain consisting of at
least four steps: (i) ethanol dehydrogenation to acetaldehyde, (ii) the
aldol coupling reaction (iii) dehydration of the products formed by the
aldol reaction, and finally (iv) two successive hydrogenations yielding
1-butanol [8], as shown in Fig. 1. Furthermore, 1-butanol may restart
the cycle, generating longer carbon chain compounds. Although the
Guerbet reaction is a key contributor to the development of greener
industrial processes, ethanol conversion to higher molecular weight
products remains a challenge and has been studied less than the ethanol
reforming reaction.
The Guerbet reaction requires the utilization of a catalyst. Three
main classes of heterogeneous catalysts have been used for the Guerbet
reaction: hydroxyapatite [18] metal oxides [19], and magnesium-alu-
minum mixed metal oxides (MMO) obtained from hydrotalcite calci-
nation at high temperatures [20]. Hydrotalcites are layered double
hydroxides (LDH) composed of different Mg2+/Al3+ molar ratios being
Among these products, 1-butanol is desirable due to its possible use
as a fuel, as an industrial raw material (for the production of esters,
ethers and resins) and as a solvent [11]. Also, 1-butanol is noncorrosive,
is immiscible in water [8] and has an energy density of 36.6 kJ g−1
,
higher than ethanol (30 kJ g−1) and closer to gasoline (90%), making it
a possible alternative to high-octane rating fuels [12], an alternative to
biodiesel in diesel mixtures and an excellent substitute for ethanol in
gasoline blends [13]. Although this reaction makes a key contribution
for greener industrial processes, the ethanol conversion to higher mo-
lecular weight products remains a challenge and the process has been
studied less than the reforming reaction [14,15].
The above mentioned products can be obtained through two distinct
⁎ Corresponding author at: Universidade Estadual Paulista - UNESP, IBILCE, Postal address: Av. Cristóvão Colombo, 2265, São José do Rio Preto, São Paulo, 15054-
000 Brazil.
E-mail addresses: mrechi.siqueira@gmail.com (M. Rechi Siqueira), olavo_perrone@hotmail.com (O. Micali Perrone), metzkerg@gmail.com (G. Metzker),
dcolisboa@gmail.com (D.C. de Oliveira Lisboa), thomeo@ibilce.unesp.br (J.C. Thoméo), boscolo@ibilce.unesp.br (M. Boscolo).
https://doi.org/10.1016/j.mcat.2019.110516
Received 8 May 2019; Received in revised form 14 July 2019; Accepted 15 July 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
(reference material) and Cu10MMO used as catalysts.
2.2. Catalyst characterization
Powder X-ray diffraction (XRD) patterns of LDHs and MMOs were
obtained in a MiniFlex300 diffractometer (Rigaku, Tokyo, Japan), using
CuKα radiation (λ = 1.54 Å) operating at 40 kV and 30 mA at 2θ in the
range of 4 to 70 degrees. FTIR spectra were recorded from 4000 to
400 cm−1 on
a Spectrum Two spectrophotometer (PerkinElmer,
Waltham, USA) fitted with an attenuated total reflectance (ATR) device.
The EPR measurements were performed using an EMX plus spectro-
meter (Bruker, Billerica, USA), operating at X-Band microwave fre-
quency (9.77 GHz). The samples were placed in a quartz capillary tube
(1 mm internal diameter) and the experiments were carried out at room
temperature. All thermal analysis measurements were carried out using
a TGA 4000 thermogravimetric balance– (PerkinElmer, Waltham, USA)
and the mass loss was measured from 100 °C to 650 °C (10 °C min−1) in
N2 flow (20 mL min−1) [20]. For the thermogravimetric temperature-
programmed reduction (TG-TPR) experiments, the sample was heated
to 650 °C in an ethanol saturated N2 atmosphere (20 mL min−1), and
the metal reduction temperature was determined by the mass change
resulting from the water lost due to the metal reduction (Eq.1)
Fig. 1. Proposed reaction mechanism for the conversion of ethanol into 1-bu-
tanol.
employed as catalyst precursors [21]. Mixed metal oxides derived from
LDH precursors have attracted attention because different metal ions
can be easily incorporated in the LDH precursor [22] (replacing Mg2+
or Al3+ ions) modifying the catalytic properties of these MMOs by
changing the acidity and basicity of the solid and the surface area re-
sulting in a possible fine-tuning of the selectivity and product yields of
the Guerbet reaction.
Δ
2 CH3CH2OH → 2CH3CHO + H2H2 + MO → M0 + H2O
(1)
Substitution of Mg2+ by Cu2+ ions on LDH increases the number of
acid sites in the derived MMO, improving the ethanol dehydrogenation
activity [23]. Also, copper demonstrated high hydrogenation activity
that is a key step in 1-butanol formation [17]. It has already been re-
ported that copper-doped metal oxides derived from hydrotalcites used
in the Guerbet reaction showed encouraging results [17,24,25] but
there is still a lack of information about how reaction parameters (time,
temperature and catalyst load) can influence the ethanol conversion
and the product formation. Also, the structural properties of catalysts
after recycling are not yet well explored.
Here, a copper-modified hydrotalcite (substituting 10 mol % of
Mg2+ by Cu2+, Cu10LDH) was synthesized as pre-catalyst and this
material was calcined at 450 °C to obtain the respective mixed metal
oxide (Cu10MMO) which was then used as the catalyst for the conver-
sion of ethanol to 1-butanol. The modified materials were characterized
by a series of spectroscopic techniques: Fourier Transform Infra-Red
(FTIR), X-Ray Diffraction (XRD), method of BET (Brunauer, Emmett
and Teller), Electron paramagnetic resonance (EPR) and
Thermogravimetric analysis (TGA). The surface response model was
applied aiming to maximize the ethanol conversion reaction to 1-bu-
tanol in batch autoclave reactors. Also, catalyst-recycling experiments
and catalyst characterization after the reaction cycles were conducted.
Mixed metal oxide basicity and acidity were determined by the CO2
or n-butylamine adsorption/desorption method in a TGA-4000 ther-
mogravimetric analyzer (PerkinElmer, Waltham, USA). Previously,
20 mg of samples were treated at 450 °C for 10 min under 20 mL min−1
flow of He. After that, 20 mL min−1 of CO2 or 20 mL min−1 of N2 sa-
turated with n-butylamine was added for 10 min for adsorption. Then,
the samples were heated from 35 to 900 °C at a rate of 10 °C min−1 and
the loss of mass caused by the removal of the absorbed CO2 or n-bu-
tylamine was monitored and used to determine the number and dis-
tribution of the basic and acid sites. The derivative mass loss was de-
convoluted into three desorption peaks to calculate the number of sites
classified as weak (W), medium (M), and strong (S).
Surface area was measured by the BET method (N2 adsorption)
using a Gemini VII surface area analyzer (Micromeritics, Norcross,
USA). Morphological characteristics of the solids were obtained by a
Leo 435VPi scanning electron microscope (Zeiss, Oberkochen,
Germany) (15 kV) and an Energy Dispersive Spectroscopy (EDS)
system, which was coupled to the SEM microscope with an acceleration
voltage of 3.7 keV and 5 iterations. Before taking the measurements, the
solids were prepared by gold sputtering on the surfaces
2.3. Reaction experimental design
2. Methods
The catalyst load (X1), reaction time (X2) and temperature (X3)
were selected as numerical variables, and the interactions between
them were studied using surface response methodology. The range
values correspond to 8 cubic points, 6 axial and 6 central cubic points,
resumed in Table 1. This design generated 40 experiments with 6 re-
plicates in central points for ethanol conversion (Table S1).
2.1. Synthesis
The copper-modified hydrotalcite (Cu10LDH, catalyst precursor)
was prepared by the co-precipitation method [26] using a 3:1 M ratio
previously studied (sum of divalent cations Mg2+ and Cu2+ = 3 and
trivalent cation Al3+ = 1). A solution of Na2CO3 (0.2 mol L−1) was
prepared and the pH adjusted to 10 using a NaOH solution (4 mol L−1).
A solution containing Mg(NO3)2.6H2O (2.55 mol L−1), Al(NO3)3.9H2O
(0.96 mol L−1) and Cu(NO3)2.3H2O (0.42 mol L−1), which substituted
10% of Mg2+, was slowly dripped into the carbonate solution under
controlled temperature (60 °C) and constant stirring. Once all the so-
lution had been added, the resulting slurry was aged for 12 h at 60 °C.
The solid was then filtered, washed with deionized water and dried at
100 °C for 12 h. The same procedure was followed to obtain a non-
modified hydrotalcite (LDH) used as the standard material for the
characterization analyses and reaction control. The resulting hydro-
talcites were calcined at 450 °C for 5 h giving rise to the MMO
The experiments were carried out in duplicate and the mean value
was adopted. The surface response model was used to study the effect of
Table 1
Independent variables and coded values used to optimize ethanol conversion.
Variables
Code units
Coded levels
−1.68
−1
0
1
1.68
Catalyst load (mg)
Reaction Time (h)
Reaction Temp. (°C)
X1
X2
X3
8
1
266
30
2
280
65
3.5
300
100
5
320
124
6
334
2

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
several factors that simultaneously influence the response (ethanol
conversion). As a consequence of the model, Eq. 2 was obtained to
evaluate the effectiveness of the model and the credibility of the ana-
lysis of variance.
Ethanol Conversion = 551+ (0.464*X1) – (13.1*X2) – (3.86*X3) –
(0.00050*X1*X1)
-(1.125*X2*X2)+
(0.00664*X3*X3)
+
(0.0198*X1*X2) – (0.00105*X1*X3) + (0.0820*X2*X3)
(2)
This experimental design has been used to fit the second-order
model (Eq. 3) defined as follows to facilitate the optimal condition
calculations.
n
n
n
n
Y = b0 +
biXi +
biiXi2
+
bij Xi Xj
∑
∑
∑
∑
i=1
i=1
i=1
i=j
(3)
where: Y is the expected response, b0 the constant coefficient, bi is
linear coefficients, bii the quadratic coefficients, bij is an interaction
between coefficients and xi and xj are code of variables.
2.4. Catalytic assays and recycling
All catalytic reactions were performed in 10 mL stainless steel au-
toclave batch reactors [27]. For each catalytic run, 3 mL of anhydrous
ethanol and the needed amount of catalyst were loaded into the re-
actors. Anhydrous ethanol was used in the later tests because it presents
(on average) greater conversion of ethanol to condensable products.
The reactions were carried out in a muffle furnace (at 266, 280, 300,
320 and 334 °C) and for a time period of 1, 2, 3.5, 5 and 6 h as the
factorial design experiments predict. After a predetermined reaction
time, the reactor was cooled in water to quench the reaction. (Warning!
High pressure inside the reactor. Care must be taken). The reactors
were opened in a special apparatus to measure the amount of gas
generated during the reaction (described in details in SI). The gaseous
products were conducted into a graduated cylinder filled with water
and the volume displaced was correlated with the amount of gas gen-
erated [28].
For catalyst recycling tests, the catalyst was washed with hexane,
centrifuged and dried at 60 °C until the solvent had evaporated com-
pletely. To guarantee the same catalyst load in each cycle, the recycling
experiments started with four reactors using the reaction conditions
described before. After each run, the catalyst from all reactors was
collected, dried and redistributed for the next reaction cycle. Due to the
material lost in the course of these procedures, in the fourth reaction
cycle, the reaction was conducted with only three instead of four re-
actors simultaneously.
Fig. 2. XRD diffractograms for synthesized materials. (A) Non-modified hy-
drotalcite - LDH; (B) Copper-modified hydrotalcite (10 mol % of Cu2+) –
Cu10LDH. (C) Non-modified mixed metal oxide from hydrotalcite calcination at
450 °C - MMO; (D) Copper-modified mixed metal oxide (10 mol % of Cu2+
)
from hydrotalcite calcination at 450 °C – Cu10MMO. The number above each
peak is the Miller index.
carrier gas (1.0 mL min−1). The oven temperature ramp was as follows:
initial temperature of 50 °C for 5 min; heating up from 50 to 200 °C at
5 °C min−1 holding for 5 min. The injector was kept at 250 °C at a split
ratio of 150:1. The detector was kept at 250 °C.
The ethanol conversion and product selectivity were calculated
using Eqs. 4 and 5, where η is the number of moles.
η EtOH Initial − η EtOH unconverted
⎡
⎤
Conversion (%) =
Selectivity (%) =
*100
⎢
⎣
⎥
⎦
η EtOH Initial
(4)
(5)
η Desired Product
⎡
⎤
*100
2.5. Sample preparation, chromatographic analysis and calculations
EtOH Converted
⎣
⎦
The condensed phase products were separated from the catalyst by
centrifugation at 10 krpm for 10 min and filtrated in High performance
liquid chromatography (HPLC) syringe filters (0.22 μm X 25 mm).
The ethanol reaction products were identified and quantified using
a Clarus 680 gas chromatograph coupled with an MSD 600 T mass
spectrometer (PerkinElmer, Waltham USA) using electron impact io-
nization (70 eV) as ionization source. The analysis was performed in an
MS-5 column (30 m x 0.32 mm x 0.25 mm) (Agilent, Santa Clara, USA)
using helium as carrier gas (1.0 mL min−1). The injector temperature
was kept at 300 °C and the volume injected was 1 μL. Mass detection
was set in the 40–600 m/z range. The oven temperature ramp was as
follows: initial temperature of 50 °C for 6 min; heating up from 50 to
300 °C at 10 °C min−1 holding for 1 min. For both GC–MS and GC-FID,
samples were injected without dilution.
3. Results and discussion
3.1. Catalysts characterization
The XRD diffractograms of synthesized LDHs and MMOs are shown
in Fig. 2.
Table 2 shows the crystallographic data such as the distance
Table 2
Hydrotalcites and oxides lattice parameters.
a (Å)
c (Å)
D (Å)
I003/200
β(003)
Hydrotalcites (LDH)
LDH
Cu10LDH
Oxides (MMO)
MMO
Cu10MMO
3,06
3,05
17,34
16,97
72,40
93,95
2209
3158
1,14
0,87
The ethanol conversion was quantified using a Clarus 480 gas
chromatograph (PerkinElmer, Waltham, USA) coupled with a flame
ionization detector (FID). An Elite-5 (30 m x 0.25 mm x 0.25 mm)
chromatographic capillary column (PerkinElmer, Waltham, USA) was
used to perform the ethanol and product separation using N2 as the
420
419
–
–
447
405
1316
1217
–
–
3

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
between cations in the lamellar layer (a), the height of the unit cell (c)
crystallite size (D) and basal spacing (I003). Copper-modified hydro-
talcite (Cu10LDH) parameters are consistent with the ones found in the
literature [29,30]. However, the addition of copper causes an increase
in basal spacing (I003) and a reduction in peak width at half height
(β003) indicating decreased crystallinity [31]. The cell parameter (a) is
calculated by 2x d(110). There is no significant difference in this para-
meter comparing Cu10LDH and the reference material (LDH), sug-
gesting a successful Cu incorporation in the layers. Parameter (c) was
calculated by 3/2 [(d(003)+(2d(006))] and it can be seen that the in-
sertion of copper decreases this parameter. Parameter (D) is the particle
diameter and uses the mean of planes 003 and 006 [32]. As shown in
Table 2, the addition of copper results in an increase in particle size of
23% compared to the material without copper.
area, respectively [37]. The MMO and Cu10MMO oxides presented
three desorption peaks for acidity and basicity determination as fol-
lows: weak sites (between 80 and 180 °C), medium sites (usually be-
tween 180 and 380 °C) and strong sites (at temperatures around at
630 °C) [38,39]. The TPD profiles for MMO and Cu10MMO oxides are
shown in Supplementary Information Fig. S2
Table 3 shows the acid and basic site distribution of the MMOs.
Compared to the reference material (MMO), the Cu10MMO oxide
showed an increase in both weak and strong basic sites, consequently
increasing the overall oxide basicity. However, regarding acid sites, the
copper led to an increase in weak sites and a slight decrease in medium
and strong acid sites in comparison with the MMO but the overall
acidity was higher than the reference material, following the same
tendency as that observed for the basic sites.
The crystallographic patterns of LDHs were transformed by calci-
nation as can be seen in Fig.1 (C and D). The XRD patterns of the MMO
and Cu10MMO show three peaks: (111), (200) and (220) characteristic
of periclase (MgO) (JCPDS 89–7746) and spinel (MgAl2O4) (JCPDS-
45946). After calcination, the peaks attributed to other oxides are not
seen, indicating the formation of amorphous material. Parameters (a)
and (D) were calculated using the plane (Miller index) (200) and can be
found in Table 2. The addition of copper decreases the crystallite size
(D), I200 and (a) indicating compaction and structural collapse but
without the formation of any crystalline regions.
The Cu10LDH catalyst thermogravimetric analysis shows two
thermal events (Supplementary Information Fig. S1). The first mass loss
(14.8%) occurs around 180 °C, and is correlated with the loss of ad-
sorbed and interlayer water, without structure collapse [33]. The
second mass loss (23.9%) occurs around 300 °C and is due to lamellar
dehydroxylation and decarbonation resulting in collapse of the struc-
ture [34].
The TPR analyses were performed using ethanol saturated N2 at-
mosphere as the reducing agent (Supplementary Information Fig. S1).
During the catalyst reduction, H2O is formed and consequently there is
a mass loss at temperatures higher than 100 °C. The reduction of the
copper containing catalyst shows at least two reduction peaks, the first
one at low temperatures is associated with Cu+2 reduction to Cu+1, the
second one is observed at higher temperatures and is associated to Cu+1
reduction to Cu0. [29–32]. The reduction starts at 550 °C and finishes at
585 °C, the maximum reduction temperatures associated with a mass
loss being 564 °C. Moreover, Karthikeyan and collaborators [14]
showed that high calcination temperatures decrease the number of
basic sites and keep the number of acid sites in the MMOs derived from
hydrotalcites almost constant. Since equilibrium between the number of
basic and acid sites is desirable, the calcination was carried out at
450 °C.
In the Cu10MMO material, there is a ratio of basic and acid sites (B/
A) equal to 1.55. The acid-base bifunctionality is necessary for the
Guerbet reaction and has already been confirmed in recent literature
[40]. This amphoteric property is important since ethanol dehy-
drogenation to acetaldehyde occurs at weak/medium basic sites, the
aldolization of aldehydes occurs at basic medium or strong sites and
finally, dehydration and hydrogenation occur on acid sites [19].
The LDHs infrared spectra (Supplementary Information, Fig. S3)
showed a wide and intense band around 3400 cm−1 attributed to vi-
brational stretching of OH groups (νOH) and water molecules present
on lamellae surfaces [41]. There is a discrete absorption at 1650 cm−1
attributed to water molecule vibration present in interlamellar layers
and the intense band at 1350 cm−1 indicates the carbonate ion in the
interlamellar region [42]. Bands in regions below 1000 cm−1 corre-
spond to particular vibrations between the metal and oxygen bonds
[33]. In general, no substantial differences were found in the IR spectra.
The SEM-EDS micrographs of MMO and Cu10MMO are depicted in
the Supplementary Information, Figs. S4 and S5. The rosette mor-
phology, typical for layered material, is not seen because this pattern is
destroyed by calcination, as observed by Coelho et al. [43]. The mi-
crographs of copper-modified materials showed a rounded shape pro-
file, suggesting the formation of agglomerates. Energy-dispersive
spectroscopy (EDS) provides information about the different elements
present in the outermost superficial layer of the material [30]. EDS
spectra, shown in Fig. S5, indicate the presence of Cu+2, Mg+2 and
Al+3 and the percentages of elements in different positions on the
sample are practically unchanged, meaning there is a good distribution
of the metallic elements.
Altogether, the techniques used show that the copper-modified LDH
and MMO were successfully synthesized and the introduction of copper
into the hydrotalcite structure mainly modifies the balance of basic and
acid sites in the catalyst but not the overall structure of the material.
Surface area, basicity, and acidity data for mixed metal oxides are
summarized in Table 3.
3.2. Reaction optimization and catalytic assays
The partial substitution of Mg2+ by Cu2+ (Cu10MMO) slightly de-
creases the surface area (178 to 171 m2 g−1) indicating that the Mg2+
substitution by Cu2+ did not deeply modify the material structure, as
already observed in the XRD diffractograms. The TPD technique pro-
vides the strength and number of acid and basic sites on the surface of
the catalyst determined by the desorption temperature and the peak
As a first attempt to optimize the ethanol conversion reaction, cat-
alytic runs were performed at a temperature of 280 °C, using 30 mg of
catalyst and 2 h of reaction, resulting in 20% ethanol conversion. In this
experiment, only 2% of the condensed phase was 1-butanol.
By using this preliminary result as a guide, the experimental
Table 3
Number of basic and acid sites of different strengths for MMO and Cu10MMO, derived from TPD−CO2 and TPD-n-butylamine.
a
b
Catalyst
SBET (m2 g−1
)
CO2 (mmol g−1
)
n-butylamine (mmol g−1
)
B/Ac
W
M
S
Total
W
M
S
Total
MMO
Cu10MMO
178
171
2
1
0.24
0.24
0.13
0.27
0.04
0.10
0.41
0.62
0.30
0.57
0.34
0.31
0.11
0.09
0.75
0.97
1.22
1.55
a
mmols of CO2 per gram of catalys.
b
c
mmols of n-butylamine per gram of catalyst.
basicity divided by acidity. dCu10MMO refers to copper-modified after recycling. W: weak; M: medium; S: strong.
4

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
Fig. 3. Response surface plot for the reaction variables investigated for the Guerbet reaction. (A) Correlation between catalyst load (X1) and reaction time (X2). (B)
Correlation between reaction time (X2) and temperature (X3).
planning was carried out aiming to optimize the ethanol conversion and
1-butanol yield by correlating three variables: catalyst load (X1), re-
action time (X2) and temperature (X3). The Supplementary Information
depicts the full table of response parameters (Table S1) and analysis of
variance (ANOVA) results for the model (Table S2).
and selectivities.
1-butanol was more abundant than ethyl acetate. The minor pro-
ducts (sixty identified compounds) that contribute to 67% of the con-
densed phase are composed of esters (mostly ethyl and butyl butano-
ates), higher carbon chain alcohols (1-hexanol, 4-nonanol) and ketones
(3-hexanone, 2-heptanone). Table S3 in the Supplementary Material
summarizes the minor products identified by GC–MS.
The proposed reaction pathway for 1-butanol and ethyl acetate
formation is depicted in Fig. 5 [45]. As can be seen, there are two key
intermediates for the formation of the products, assuming the Guerbet
reaction: acetaldehyde and butyraldehyde. In the condensed phase
analysis, after 5 h of reaction, these two intermediates were not iden-
tified, suggesting that once formed on the catalyst surface, these species
are readily converted into ethyl acetate and/or 1-butanol.
The major selectivity presented for 1-butanol in the reaction may be
explained by two factors. Under the reaction conditions, reduction re-
actions are more feasible since, during ethanol dehydrogenation, metal-
hydride species are probably being formed on the catalyst surface. This
hypothesis is strongly supported by the possible high partial pressure of
hydrogen inside the batch reactor. From this perspective, butyr-
aldehyde reduction to 1-butanol is benefited instead of 1-ethoxyethanol
oxidation.
Analyzing the response surface graphics, Figs. 3A and B, generated
by the model, for the three-independent initial variables for ethanol
conversion, the reaction time variable (X2) is the most significant one,
followed by temperature (X3) and catalyst load (X1). Also, the inter-
actions between the variables were not significant as observed by the
Pareto diagram (Supplementary Information, Fig. S6). The response
surface graphic, (Fig. 3A) confirms the model prediction, since the
ethanol conversion is more sensitive to reaction time than to catalyst
load. In addition, correlating the reaction time (X2) and reaction tem-
perature (X3) variables and the latter is still the most important one for
the ethanol conversion yield as shown in the response surface plot
(Fig. 3B). As can be seen, at temperatures below 300 °C ethanol con-
version was lower than 25%. However, at temperatures above 300 °C,
conversion easily exceeds 30%, even for a shorter reaction time (3 h).
The highest ethanol conversion achieved was approximately 60% at
340 °C for a reaction time of 5 h and 100 mg of catalyst.
The surface response model can be described as functions of real
coded variables (X1, X2, and X3) as shown in Eq. 2. Fixing the max-
imum temperature at 350 °C the model shows that using a catalyst load
of 150 mg and 5 h of reaction, the expected ethanol conversion would
be 81%.
Applying the reaction parameters furnished by the model, the re-
action was carried out reaching an ethanol conversion of 79.6%, this
value being close to the model prediction. Since these conditions
proved to be the best ones, they were used as standard for studies
dealing with 1-butanol selectivity and catalyst recycling.
Regarding the composition of the products, they can be divided into
two phases-gaseous and condensed. The gaseous products correspond
to 69% of the total. These gas phase products were not identified and
quantified individually but the total amount of gases generated was
estimated by the displacement volume. As reported in the literature
[6–9] for similar reactions, they can be assumed to be a mixture with H2
[44], CO, CO2, CH4 and ethylene as their major constituents.
The condensed phase contributes with the other 31% of the reaction
products. The most representative products in the condensed phase are
1-butanol and ethyl acetate, both expected as Guerbet condensation
products.
The presence of Cu+2 in the catalyst plays an important role in the
1-butanol selectivity. The literature indicates that Cu+2 incorporated in
a heterogeneous catalyst enhances the hydrogenation activity, since the
hydride formations and its transfer over copper is favored. Also, the
copper incorporated in the hydrotalcite structure increased the number
of basic sites and acid sites. Thus, the increase in the acidity of the
catalyst should enhance the metal-hydride formation, enabling the
butyraldehyde hydrogenation to 1-butanol.
Recently, Barta and collaborators [17] converted ethanol to 1-bu-
tanol using copper and nickel (both metal ions in 10 mol % substitution
of Mg2+) modified mixed metal oxides derived from hydrotalcites,
reaching a 1-butanol yield of 21.7% and 72% of selectivity. However,
this catalyst showed an ethanol conversion of 47.6% at 310 °C. Re-
garding our study, Barta’s catalysts has similarities to the catalyst used
in this work, the obtained results follow the same tendency observed,
because the higher ethanol conversion and 1-butanol yield in the pre-
sent work (79.6 and 25.4%, respectively) is probably due to the higher
temperatures employed. However, the higher temperature is also
probably responsible for the lower selectivity (32%).
Fig. 4 summarizes the ethanol conversion, the portions corre-
sponding to the gaseous and condensed phases and the main products
of the latter (1-butanol and ethyl acetate) with corresponding yields
3.3. Catalyst recycling
Catalyst recycling is an important issue in the ethanol conversion
5

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
Fig. 4. Ethanol conversion, major product yields and selectivity
using Cu10MMO as catalyst.
when considering upscaling for industrial application. Also, taking into
account the environmental point of view, multiple catalyst recycling
diminishes the necessity of catalyst synthesis and, consequently, in-
creased raw material requirements.
Recycling experiments were conducted using the Cu10MMO catalyst
in four sequential cycles. Fig. 6 shows the ethanol conversion, 1-butanol
and ethyl acetate formation and product selectivity for the recycling
experiments.
As depicted in Fig. 6, after three recyclings the ethanol conversion
remains almost constant (ranging from 79 to 73%), demonstrating that
the catalyst is still active. However, product distribution changed, in-
dicating that 1-butanol production decreased (from 25.4 to 14%) while
the ethyl acetate production increased. Thus, the selectivity changed,
diminishing for 1-butanol (from 32 to 20%) and slight increasing for
ethyl acetate (from 10 to 12%).
changes are consequences of the reductive environment inside the re-
actor, due to the hydrogen generated during the Guerbet reaction. Since
it is well established that Cu2+ is easily reduced under a hydrogen at-
mosphere and at higher temperatures [45] the modifications seen in the
catalyst are expected. Also, the high pressure inside the reactor may
contribute to the material phase change observed.
Similar results regarding XRD and catalyst surface area modification
during the Guerbet reaction were recently reported by Ford and co-
workers [46]. Their results were obtained with 20% Cu2+ substitution
in the catalyst instead of the 10% used in this work and under different
reaction conditions using Cl− as additive for the reactions. However,
since the results reported in the literature and herein follow the same
tendency, it is indicative that the mixed metal oxide used as a catalyst
for the Guerbet reaction behaves similarly, even under slightly different
reaction conditions.
Trying to address the catalyst surface behavior during the catalytic
runs, XRD experiments were performed. Fig. 7 shows the diffractograms
for the Cu10MMO; (A) after the reaction, (B) after the first catalytic
cycle and (C) after the fourth cycle.
The diffractogram (B) shows that the Cu10MMO underwent major
modifications, indicating the formation of three different oxidation
states ranging from +2 to 0, as indicated in the Fig.7. Furthermore, the
peak shape changed significantly in diffractograms (B) and (C) in-
dicating that, after the reaction, the size of the crystallites increased.
This new material proved to be stable since it was still active after at
least four reaction cycles.
4. Conclusion
Ethanol conversion to 1-butanol was achieved with a 25.4% yield
and 32% selectivity in the condensed phase employing a copper mixed
metal oxide as the catalyst after 5 h of reaction at 350 °C, achieving
ethanol conversion of 79.6%. The surface response model was applied
to obtain the best reaction conditions based on the reaction time,
temperature and catalyst load. This showed that reaction time is the
most important variable for the reaction. The catalyst maintained the
ethanol conversion and main condensed phase products over four re-
cycling cycles, clearly indicating the robustness of the synthesized
material. Although the ethanol conversion over a number of recycling
cycles was almost constant, the 1-butanol yield and selectivity changed,
probably due to structural modifications on the catalyst surface. Due to
the reductive nature of the products generated by the Guerbet reaction,
the MMO suffered structural modifications and copper oxidation state
changes, as shown by XRD and EPR, indicating that the catalyst is a
mixture of Cu2+, Cu+ and Cu0 species generated in situduring the first
reaction cycle.
To confirm the changes in the copper oxidation during the catalytic
reactions, EPR experiments were performed. Fig. 8 shows the spectra of
the MMO (A) before and (B) after the first catalytic cycle.
The spectrum of Fig.8(A) indicates that MMO is composed of Cu2+
ions in an octahedral coordination arrangement, showing three distinct
g values (g1 = 2.55, g2 = 2.15 and g3 = 2.05). After the first catalytic
run, as shown in Fig. 8(B), the Cu2+ signal almost disappeared, with
only a low-intensity signal with a g value of 2.05. This observation
indicates that the Cu2+ ions in the Cu10MMO structure were reduced to
Cu+ and/or Cu0 species, due to the hydrogen production during the
ethanol reactions. However, there is still a small amount of Cu2+ in the
Cu10MMO structure, shown by the presence of the low-intensity signal
seen. The change in the copper oxidation state seen in the EPR ex-
periments corroborates the XRD observations.
Acknowledgments
The authors would like to thank the São Paulo Research Foundation
- FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for
their financial support (Grant nos. 2015/11588-4; 2016/15575-7;
2017/13230-5; 2013/23886-4) and to the National Council for the
The surface area and XRD data are coherent since the increase in the
crystallites in the material leads to a decrease in the surface area (from
178
1–98
2). Figs. 7 and 8 show the real nature of the catalytic
Improvement of Higher Education
-
CAPES (Coordenação de
species. It is clear that the fresh Cu10MMO underwent copper oxidation
and structure modifications, giving rise to a new oxide material during
the first catalytic run and this material was still catalytically active over
at least four cycles. The copper oxidation state and catalyst structural
Aperfeiçoamento de Pessoal de Nível Superior) for their National Post
Doctorate Program - PNPD (Programa Nacional de Pós Doutorado) post-
doctoral fellowship grant no.1760327) to G.M. The authors also are
indebted to Professor Peter C. Ford, University of California Santa
Fig. 5. Proposed reaction pathway for ethanol conversion into 1-
butanol and ethyl acetate based on the Guerbet reaction.
6

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
Fig. 6. Ethanol conversion and 1-butanol and ethyl acetate yields
and selectivity for recycling experiments. Bars color code:
Conversion; Selectivity; 1-butanol; ethyl acetate.
[4] K.D. Dewoolkar, P.D. Vaidya, Tailored hydrotalcite-based hybrid materials for hy-
drogen production via sorption-enhanced steam reforming of ethanol, Int. J.
Hydrogen Energy 41 (2016) 6094–6106, https://doi.org/10.1016/j.ijhydene.2015.
10.034.
[5] A.D. Shejale, G.D. Yadav, Cu promoted Ni-Co/hydrotalcite catalyst for improved
hydrogen production in comparison with several modified Ni-based catalysts via
steam reforming of ethanol, Int. J. Hydrogen Energy 42 (2017) 11321–11332,
https://doi.org/10.1016/j.ijhydene.2017.03.052.
[6] V.L. Sushkevich, I.I. Ivanova, Mechanistic study of ethanol conversion into buta-
diene over silver promoted zirconia catalysts, Appl. Catal. B Environ. 215 (2017)
36–49, https://doi.org/10.1016/j.apcatb.2017.05.060.
[7] L.H. Chagas, C.R.V. Matheus, P.C. Zonetti, L.G. Appel, Butadiene from ethanol
employing doped t -ZrO 2, Mol. Catal. 458 (2018) 272–279, https://doi.org/10.
1016/j.mcat.2018.01.018.
[8] S. Chakraborty, P.E. Piszel, C.E. Hayes, R.T. Baker, W.D. Jones, Highly selective
formation of n-Butanol from ethanol through the guerbet process: a tandem cata-
lytic approach, J. Am. Chem. Soc. 137 (2015) 14264–14267, https://doi.org/10.
1021/jacs.5b10257.
[9] K.K. Ramasamy, M. Gray, H. Job, C. Smith, Y. Wang, Tunable catalytic properties of
bi-functional mixed oxides in ethanol conversion to high value compounds, Catal.
Today 269 (2016) 82–87, https://doi.org/10.1016/j.cattod.2015.11.045.
[10] J. Pang, M. Zheng, R. Sun, A. Wang, X. Wang, T. Zhang, Synthesis of ethylene glycol
and terephthalic acid from biomass for producing PET, Green Chem. 18 (2016)
342–359, https://doi.org/10.1039/C5GC01771H.
Fig. 7. XRD diffractograms comparison of fresh Cu10MMO (A), after first re-
action cycle (B) and after fourth reaction cycle (C).
[11] S. Bepari, S. Basu, N.C. Pradhan, A.K. Dalai, Steam reforming of ethanol over
cerium-promoted Ni-Mg-Al hydrotalcite catalysts, Catal. Today 291 (2017) 47–57,
https://doi.org/10.1016/j.cattod.2017.01.027.
[12] V.C.S. Palla, D. Shee, S.K. Maity, Conversion of n-butanol to gasoline range hy-
drocarbons, butylenes and aromatics, Appl. Catal. A Gen. 526 (2016) 28–36,
https://doi.org/10.1016/j.apcata.2016.07.026.
[13] W.R. da S. Trindade, R.G. dos Santos, Review on the characteristics of butanol, its
production and use as fuel in internal combustion engines, Renewable Sustainable
Energy Rev. 69 (2017) 642–651, https://doi.org/10.1016/j.rser.2016.11.213.
[14] K.K. Ramasamy, M. Gray, H. Job, D. Santosa, X.S. Li, A. Devaraj, A. Karkamkar,
Y. Wang, Role of calcination temperature on the hydrotalcite derived MgO-Al2O3 in
converting ethanol to butanol, Top. Catal. 59 (2016) 46–54, https://doi.org/10.
1007/s11244-015-0504-8.
[15] J.M. Hidalgo, Z. Tišler, D. Kubička, K. Raabova, R. Bulanek, (V)/Hydrotalcite, (V)/
Al2O3, (V)/TiO2 and (V)/SBA-15 catalysts for the partial oxidation of ethanol to
acetaldehyde, J. Mol. Catal. A Chem. 420 (2016) 178–189, https://doi.org/10.
1016/j.molcata.2016.04.024.
[16] N. AlO, S.J. Han, Y. Bang, J.H. Song, J. Yoo, S. Park, K.H. Kang, Hydrogen pro-
duction by steam reforming of ethanol over, Catal. Today 41 (2015) 2554–2563,
https://doi.org/10.1016/j.cattod.2015.07.041.
[17] Z. Sun, A.C. Vasconcelos, G. Bottari, M.C.A. Stuart, G. Bonura, C. Cannilla,
F. Frusteri, K. Barta, Efficient catalytic conversion of ethanol to 1-butanol via the
guerbet reaction over copper- and nickel-doped porous, ACS Sustain. Chem. Eng. 5
(2017) 1738–1746, https://doi.org/10.1021/acssuschemeng.6b02494.
[18] A. Fihri, C. Len, R.S. Varma, A. Solhy, Hydroxyapatite: a review of syntheses,
structure and applications in heterogeneous catalysis, Coord. Chem. Rev. 347
(2017) 48–76, https://doi.org/10.1016/j.ccr.2017.06.009.
[19] G. Tesquet, J. Faye, F. Hosoglu, A. Mamede, F. Dumeignil, M. Capron, Applied
Catalysis A : general Ethanol reactivity over La 1 + x FeO 3 + ı perovskites, Appl.
Catal. A Gen. J. 511 (2016) 141–148.
[20] M. León, E. Díaz, S. Ordóñez, Ethanol catalytic condensation over Mg-Al mixed
oxides derived from hydrotalcites, Catal. Today 164 (2011) 436–442, https://doi.
org/10.1016/j.cattod.2010.10.003.
[21] G.S. Macala, T.D. Matson, C.L. Johnson, R.S. Lewis, A.V. Iretskii, P.C. Ford,
Hydrogen transfer from supercritical methanol over a solid base catalyst: A model
for lignin depolymerization, ChemSusChem 2 (2009) 215–217, https://doi.org/10.
1002/cssc.200900033.
[22] M.S. Maru, S. Ram, R.S. Shukla, N.H. Khan, Ruthenium-hydrotalcite (Ru-HT) as an
effective heterogeneous catalyst for the selective hydrogenation of CO 2 to formic
acid, Mol. Catal. 446 (2018) 23–30, https://doi.org/10.1016/j.mcat.2017.12.005.
[23] M. Jabłońska, K. Nothdurft, M. Nocuń, V. Girman, R. Palkovits, Redox-performance
correlations in Ag–Cu–Mg–Al, Ce–Cu–Mg–Al, and Ga–Cu–Mg–Al hydrotalcite
Fig. 8. EPR spectra of (A) fresh and (B) after the first catalytic run.
Barbara (UCSB) for the scientific inspiration and for reading the final
version of the manuscript and Professor Daniel R. Cardoso (IQSC-USP)
for the use of the EPR spectrometer (São Paulo Research Foundation
grant number 92/03515-3).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110516.
References
[1] C. Angelici, B.M. Weckhuysen, P.C.A. Bruijnincx, Chemocatalytic conversion of
ethanol into butadiene and other bulk chemicals, ChemSusChem 6 (2013)
1595–1614, https://doi.org/10.1002/cssc.201300214.
[2] G. Busca, U. Costantino, T. Montanari, G. Ramis, C. Resini, M. Sisani, Nickel versus
cobalt catalysts for hydrogen production by ethanol steam reforming: Ni-Co-Zn-Al
catalysts from hydrotalcite-like precursors, Int. J. Hydrogen Energy 35 (2010)
5356–5366, https://doi.org/10.1016/j.ijhydene.2010.02.124.
[3] S. De, J. Zhang, R. Luque, N. Yan, Ni-based bimetallic heterogeneous catalysts for
energy and environmental applications, Energy Environ. Sci. 9 (2016) 3314–3347,
https://doi.org/10.1039/c6ee02002j.
7

M. Rechi Siqueira, et al.
MolecularCatalysis476(2019)110516
and Fe(III) stable photocatalysts for Orange II degradation, Appl. Clay Sci. 132–133
(2016) 641–649, https://doi.org/10.1016/j.clay.2016.08.016.
derived mixed metal oxides, Appl. Catal. B Environ. 207 (2017) 385–396, https://
doi.org/10.1016/j.apcatb.2017.01.079.
[24] I.C. Marcu, D. Tichit, F. Fajula, N. Tanchoux, Catalytic valorization of bioethanol
over Cu-Mg-Al mixed oxide catalysts, Catal. Today 147 (2009) 231–238, https://
doi.org/10.1016/j.cattod.2009.04.004.
[25] J.J. Bravo-suárez, B. Subramaniam, R.V. Chaudhari, Applied Catalysis A : general
Vapor-phase methanol and ethanol coupling reactions on CuMgAl mixed metal
oxides, Applied Catal. A, Gen. 455 (2013) 234–246, https://doi.org/10.1016/j.
apcata.2013.01.025.
[26] R. Dębek, K. Zubek, M. Motak, M.E. Galvez, P. Da Costa, T. Grzybek, Ni-Al hy-
drotalcite-like material as the catalyst precursors for the dry reforming of methane
at low temperature, Comptes Rendus Chim. 18 (2015) 1205–1210, https://doi.org/
10.1016/j.crci.2015.04.005.
[27] C.M. Bernt, H. Manesewan, M. Chui, M. Boscolo, P.C. Ford, Temperature tuning the
catalytic reactivity of Cu-Doped porous metal oxides with lignin models, ACS
Sustain. Chem. Eng. 6 (2018) 2510–2516, https://doi.org/10.1021/acssuschemeng.
7b03969.
[28] M. Chui, G. Metzker, C.M. Bernt, A.T. Tran, A.C.B. Burtoloso, P.C. Ford, Probing the
lignin disassembly pathways with modified catalysts based on Cu-Doped porous
metal oxides, ACS Sustain. Chem. Eng. 5 (2017) 3158–3169, https://doi.org/10.
1021/acssuschemeng.6b02954.
[29] S.J. Han, Y. Bang, J.H. Song, J. Yoo, S. Park, K.H. Kang, I.K. Song, Hydrogen pro-
duction by steam reforming of ethanol over dual-templated Ni-Al2O3 catalyst,
Catal. Today 265 (2016) 103–110, https://doi.org/10.1016/j.cattod.2015.07.041.
[30] N.T. Thao, L. Thi, K. Huyen, Catalytic oxidation of styrene over Cu-doped hydro-
talcites, Chem. Eng. J. 279 (2015) 840–850, https://doi.org/10.1016/j.cej.2015.05.
090.
[31] Z.Q. Zhang, H.Y. Zeng, X.J. Liu, S. Xu, C.R. Chen, J.Z. Du, Modification of MgAl
hydrotalcite by ammonium sulfate for enhancement of lead adsorption, J. Taiwan
Inst. Chem. Eng. 60 (2016) 361–368, https://doi.org/10.1016/j.jtice.2015.10.034.
[32] P. Xiong, X. Ji, X. Zhao, W. Lv, T. Liu, W. Lu, Materials design and control synthesis
of the layered double hydroxide with the desired basal spacing, Chemometr. Intell.
Lab. Syst. 144 (2015) 11–16, https://doi.org/10.1016/j.chemolab.2015.03.005.
[33] J. Nowicki, J. Lach, M. Organek, E. Sabura, Transesterification of rapeseed oil to
biodiesel over Zr-dopped MgAl hydrotalcites, Appl. Catal. A Gen. 524 (2016)
17–24, https://doi.org/10.1016/j.apcata.2016.05.015.
[37] O. Micali, F. Lobefaro, M. Aresta, F. Nocito, M. Boscolo, A. Dibenedetto, Butanol
synthesis from ethanol over CuMgAl mixed oxides modi fi ed with palladium (II)
and indium (III), Fuel Process. Technol. 177 (2018) 353–357, https://doi.org/10.
1016/j.fuproc.2018.05.006.
[38] J.H. Earley, R.A. Bourne, M.J. Watson, M. Poliakoff, Continuous catalytic upgrading
of ethanol to n -butanol and &C 4 products over Cu/CeO catalysts in supercritical
2
CO 2, Green Chem. 17 (2015) 3018–3025, https://doi.org/10.1039/C4GC00219A.
[39] A.D. Patel, S. Telalović, J.H. Bitter, E. Worrell, M.K. Patel, Analysis of sustainability
metrics and application to the catalytic production of higher alcohols from ethanol,
Catal. Today 239 (2015) 56–79, https://doi.org/10.1016/j.cattod.2014.03.070.
[40] D. Gabriëls, W.Y. Hernández, B. Sels, P. Van Der Voort, A. Verberckmoes, Review of
catalytic systems and thermodynamics for the Guerbet condensation reaction and
challenges for biomass valorization, Catal. Sci. Technol. 5 (2015) 3876–3902,
https://doi.org/10.1039/C5CY00359H.
[41] A. Fahami, G.W. Beall, Mechanosynthesis and characterization of Hydrotalcite like
Mg-Al-SO4-LDH, Mater. Lett. 165 (2016) 192–195, https://doi.org/10.1016/j.
matlet.2015.11.132.
[42] L.M. Rodriguez-Chiang, J. Llorca, O.P. Dahl, Effect of Fe-Zn-Mg-Al hydrotalcites on
the methane potential of synthetic sulfate-containing wastewater, J. Water Process
Eng. 10 (2016) 120–127, https://doi.org/10.1016/j.jwpe.2016.03.001.
[43] A. Coelho, O.M. Perrone, E. Gomes, R. Da-Silva, J.C. Thoméo, M. Boscolo, Mixed
metal oxides from sucrose and cornstarch templated hydrotalcite-like LDHs as
catalysts for ethyl biodiesel synthesis, Appl. Catal. A Gen. 532 (2017) 32–39,
https://doi.org/10.1016/j.apcata.2016.12.012.
[44] C.H. Campos, G. Pecchi, J. Luis, G. Fierro, P. Osorio-vargas, Enhanced bimetallic
Rh-Ni supported catalysts on alumina doped with mixed lanthanum-cerium oxides
for ethanol steam reforming, Mol. Catal. 469 (2019) 87–97, https://doi.org/10.
1016/j.mcat.2019.03.007.
[45] M.M. Günter, T. Ressler, R.E. Jentoft, B. Bems, Redox behavior of copper oxide/zinc
oxide catalysts in the steam reforming of methanol studied by in situ X-ray dif-
fraction and absorption spectroscopy, J. Catal. 203 (2001) 133–149, https://doi.
org/10.1006/jcat.2001.3322.
[46] J.A. Barrett, Z.R. Jones, C. Stickelmaier, N. Schopp, P.C. Ford, A Pinch of Salt
Improves n-Butanol Selectivity in the Guerbet Condensation of Ethanol Over Cu-
Doped Mg/Al Oxides, (2018), https://doi.org/10.1021/acssuschemeng.8b03589.
[34] L.S. Liji, R. Mehedi, M. Malmivirta, P. Paturi, M. Lastusaari, M.M. Dîrtu, Y. Garcia,
P. Fardim, Heteronuclear nanoparticles supported hydrotalcites containing Ni(II)
8