Ethanol dehydrogenative reactions catalyzed by copper supported on porous Al-Mg mixed oxides

Article abstract of DOI:10.1039/c8ra10076d

Mixed aluminum and magnesium oxides (AlMgO) prepared by means of an emulsion-mediated sol-gel method was impregnated with copper nitrate solution and used in the ethanol dehydrogenative reactions to produce acetaldehyde and ethyl acetate. The emulsified system allowed to obtain a macro-mesoporous support that resulted in an outstanding dispersion of copper. The porous catalyst was about 3 times more active than the non-porous counterpart, due to the formation on the support's surface of Cu⁰ together with the more active Cu⁺ species. In fact, the simultaneous presence of Cu⁺ and Cu⁰ were advantageous for the catalytic performance, as the turnover frequencies, were 122 and 166 h^-1 for the non-porous reference catalyst and for the porous one, respectively. Both catalysts deactivated due to copper particles sintering, however the porous one deactivated less, as a consequence of the better dispersion of the Cu species on the macro and mesoporous support. Acetaldehyde was the main product, however by increasing the contact time by 6.6 times, the conversion of ethanol on the non-porous catalyst reached about 90% with a selectivity to ethyl acetate of 20% by means of the coupling reaction of ethanol and acetaldehyde. The selectivity to ethyl acetate was favoured on an increased support/copper interface that is given by larger copper particles.

Full text of DOI:10.1039/c8ra10076d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Ethanol dehydrogenative reactions catalyzed by
copper supported on porous Al–Mg mixed oxides†
Cite this: RSC Adv., 2019, 9, 3294
a
Davi D. Petrolini,
Wellington H. Cassinelli,b Cristiane A. Pereira,c
Ernesto A. Urquieta-Gonzalez,c Celso V. Santillia and Leandro Martins
a
*
´
Mixed aluminum and magnesium oxides (AlMgO) prepared by means of an emulsion-mediated sol–gel
method was impregnated with copper nitrate solution and used in the ethanol dehydrogenative
reactions to produce acetaldehyde and ethyl acetate. The emulsified system allowed to obtain a macro–
mesoporous support that resulted in an outstanding dispersion of copper. The porous catalyst was about
3 times more active than the non-porous counterpart, due to the formation on the support's surface of
Cu0 together with the more active Cu+ species. In fact, the simultaneous presence of Cu+ and Cu0 were
advantageous for the catalytic performance, as the turnover frequencies, were 122 and 166 hꢀ1 for the
non-porous reference catalyst and for the porous one, respectively. Both catalysts deactivated due to
copper particles sintering, however the porous one deactivated less, as a consequence of the better
dispersion of the Cu species on the macro and mesoporous support. Acetaldehyde was the main
product, however by increasing the contact time by 6.6 times, the conversion of ethanol on the non-
porous catalyst reached about 90% with a selectivity to ethyl acetate of 20% by means of the coupling
reaction of ethanol and acetaldehyde. The selectivity to ethyl acetate was favoured on an increased
support/copper interface that is given by larger copper particles.
Received 7th December 2018
Accepted 10th January 2019
DOI: 10.1039/c8ra10076d
rsc.li/rsc-advances
Ethanol dehydrogenation stands out by the importance of
the products hydrogen and acetaldehyde, which can be used as
1. Introduction
an intermediates in the manufacture of acetic acid, acetic
anhydride, ethyl acetate, butyraldehyde, crotonaldehyde and n-
butanol.3,5–10 Though it has a lower hydrogen yield compared to
the ethanol reforming reactions, ethanol dehydrogenation is
carried out at lower temperatures. Even whether ethanol
reforming is operated at mild temperatures, it still generates
carbon monoxide, which causes loss of efficiency and early fuel
cell poisoning.8 Moreover, the coupling reaction to further give
ethyl acetate is a very attractive sustainable strategy,9,10 because
it enables the production of esters directly from alcohols.
Compared to the traditional esterication of ethanol with
carboxylic acids, the coupling is advantageous due to the
reduction of wastes and enhancement of process prot.
Some aspects are critical to guarantee high ethyl acetate yield
during the catalytic ethanol coupling reaction, such as the acid–
base property of the catalyst, which if not properly tuned, it can
lead to unwanted by-products. The porosity of the support is
another important property because it inuences the particle
size and the oxidation state of copper species, which are the
active ones for the mentioned reactions.11 A detailed kinetic and
mechanistic study on Cu/Cr2O3 showed the central role of
copper in the adsorption and dehydrogenation of ethanol,
which is due to its ability to preserve intact the C–C bonds and
facilitate the hydrogen transfer reactions.12 In addition, it was
emphasized the negative effect of the Brønsted acid sites of
The interest in biomass sources has mostly increased in the last
decades due to the intensication of the seek for biofuels and
renewable chemical products that might substitute fuels or
petroleum derived products,1,2 which are currently the most
important sources of the emission of greenhouse gases. Among
the several processes that are being used for producing biofuels,
ethanol from fermentation of biomass-derived sugars is the
oldest and most important process used in the biotechnology
industry. Beyond of its usage as fuel, ethanol is a versatile
building block for chemical industries because it can be cata-
lytically converted into several chemicals by means of dehy-
dration (to give ethylene), Guerbet coupling (n-butanol),
dehydrogenation (acetaldehyde and hydrogen) and by the
coupling reaction of acetaldehyde with ethanol (ethyl acetate).3,4
When the last two reactions occur in a sequence, the global
reaction process is called dehydrogenative coupling of ethanol,
giving hydrogen and ethyl acetate.
a
´
Instituto de Quımica, UNESP – Universidade Estadual Paulista, R. Prof. Francisco
Degni 55, 14800-900 Araraquara, SP, Brazil. E-mail: leandro.martins@unesp.br
b
˜
´
Instituto Federal de Sao Paulo – Campus Avare, Av. Prof. Celso Ferreira da Silva,
´
1333, Jardim Europa, 18707-150, Avare, SP, Brazil
c
˜
Centro de Pesquisas em Materiais Avançados e Energia – Universidade Federal de Sao
˜
Carlos, Rodovia Washington Luis, km 235, CEP 13565-905, Sao Carlos, SP, Brazil
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra10076d
3294 | RSC Adv., 2019, 9, 3294–3302
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Cr2O3 that prevent desorption of acetaldehyde and make this a reference support was obtained without the use of organic
a rate determinant step, which results in limited activity, pore structure directing agents (PSDAs), Pluronic 123 and n-
without considering the unwanted capacity of the Brønsted acid dodecane. The samples obtained without and using the PSDAs
sites to dehydrate ethanol.12 Therefore, the appropriate balance agents were named AlMgO and AlMgO–P, respectively, in which
of acid–base sites of the support is crucial for the dehydrogen- P stands for porous. The copper catalysts, with a nominal
ative coupling of alcohols.
content of 10 wt% of copper, were obtained by incipient wetness
The use of aluminum and magnesium mixed oxides derived impregnation of the calcined mixed oxides using an aqueous
from hydrotalcites as support for impregnation of copper solution of copper nitrate. Aer impregnation, the samples
ꢁ
species and forward application in the dehydrogenation of were calcined at 500 C for 3 h.
alcohols is motivating. Aer heat treatment, these mixed oxides
have various unique properties as providing an ideal balance of
acid and base sites13–17 and unique textural properties, which
2.2. Characterization of the supports and catalysts
have impact on metals dispersion and therefore on ethanol The porosity and macro and mesopore size distributions of the
conversion to acetaldehyde and ethyl acetate.3,5
calcined mixed AlMgO oxides and copper catalysts were
In a previous study,18–20 an emulsion-mediated method was assessed by mercury intrusion porosimetry in an AutoPore III
used to produce Al–Mg mixed oxides from hierarchically instrument from Micromeritics. The samples were degassed at
ordered hydrotalcite precursors that gave a high specic surface a pressure below 50 mPa before starting the measurements. The
area. It is expected that the textural properties of these materials Washburn equation28 was used to calculate the pore size by
may reduce metal agglomeration and increase the interparticle using mercury surface tension of 0.489 N mꢀ1 and contact angle
space, and then to have an incremental effect on the catalyst of 135ꢁ.
performance. In fact, in situ studies have shown that the cata-
The porosity of the samples was further characterized by
lytic reaction rates are signicantly optimized by changing the nitrogen adsorption–desorption isotherms at ꢀ196 ꢁC on an
metal particle size. Moreover, the particle size in copper based ASAP 2010 instrument from Micromeritics with relative pres-
catalysts affected the surface ratio of Cu0 to Cu+ and conse- sure ranging between 0.001 and 0.998. Prior to the analysis, the
quently the activity in reactions involving oxygenated interme- samples were vacuum-degassed at 200 ꢁC for 12 h under
diates.21–26 It is known that Cu0 supported catalysts are the most a pressure below to 10 mPa. Pore volumes, mesopore size
active for ethanol dehydrogenation, but further studies have distributions (BJH method) and BET surface area for P/P0 up to
shown that partially oxidized Cu+ directly inuence the activity 0.3 were determined from the isotherms.29
towards ethyl acetate.3,5,6,21,27
The microstructure of mixed oxides was analyzed by scan-
Herein, we examined how the textural properties and acid– ning electron microscopy (SEM), using a Philips XL 30 equip-
base properties of highly porous aluminum and magnesium ment. The samples were deposited on an aluminum sample
mixed oxides prepared from an emulsied synthesis system holder and sputtered with gold.
inuence the dispersion of copper species and their oxidation
X-ray diffraction proles of the copper catalysts were recor-
state and, consequently their catalytic performance on the ded using a Siemens D5000 diffractometer equipped with
dehydrogenative coupling of ethanol to produce acetaldehyde a CuKa radiation (selected by a curved graphite mono-
and ethyl acetate with an enhanced selectivity.
chromator). The protocol of the scanning steps was 2q range
from 3ꢁ to 80ꢁ, with step size of 0.02ꢁ, and counting time of 1 s.
Temperature programmed desorption of ammonia (NH3-
TPD) and of carbon dioxide (CO2-TPD) were performed sepa-
rately to assess the acid and base sites of the catalysts, respec-
2. Experimental
2.1. Synthesis of the supports and catalysts
The porous and the reference non-porous aluminum- tively. A very similar protocol was used for both analyses. ꢁFirstly,
magnesium mixed oxides, with molar fraction of Al/Mg of 0.5, 150 mg of catalyst was pre-treated by heating up to 500 C and
were obtained by sol–gel synthesis, according to a previous kept at this temperature for 1 h under a ow of 60 mL minꢀ1 of
method.18,19 The synthesis of the porous mixed oxides involved He. Aer that, the samples were cooled to 120 ꢁC for NH3
ꢁ
the use of aluminum tri-sec-butoxide (named as Al-But3), adsorption or to 50 C for CO2 adsorption. Then, a ow of 30
magnesium nitrate, n-dodecane as emulsication agent, a non- mL minꢀ1 of either NH3 (1% in He, v/v) during 0.5 h or pure CO2
ionic triblock copolymer as emulsion stabilizer (Pluronic P123) for 2 h was fed into the reactor. The physically adsorbed NH3 or
and ethanol as solvent. The molar composition was: 0.01 CO2 were purged for 1 h at the same temperature in which the
Pluronic : 1 Al-But3 : 1 Mg(NO3)2 : 15 ethanol. The preparation adsorption process occurred. Upon heating from the respective
of the sol involved the dispersion of Pluronic P123 in ethanol at adsorption temperature to 600 ꢁC, with a heating rate of
room temperature, followed by addition of Al-But3 and 10 ꢁC minꢀ1 and under a ow of 30 mL minꢀ1 of He, NH3 or CO2
magnesium nitrate. Subsequently, emulsication was achieved desorption was monitored by a thermal conductivity detector of
by adding 60 wt% of n-dodecane, under mechanical stirring. the Micromeritics Pulse Chemisorb 2705 equipment.
The addition of a solution of ammonium hydroxide drop by
Proles of temperature programmed reduction with
drop under stirring led to the sol–gel transition. The as- hydrogen (H2-TPR) of the calcined copper catalysts were per-
ꢁ
synthesized samples were calcined at 500 C for 2 h under air formed using a Micromeritics Autochem 2920 equipment. A
in a conventional muffle oven. For sake of comparison, ow of 30 mL minꢀ1 of H2 (10% in He, v/v) was fed into the
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3294–3302 | 3295

View Article Online
RSC Advances
Paper
reactor and the temperature was raised from room temperature (10 ꢁC minꢀ1) under a ow of 100 mL mꢁinꢀ1 of H2 (5% in He, v/v).
to 500 C using a heating rate of 10 ꢁC minꢀ1. The hydrogen The samples were cooled down to 25 C under a He ow. The
ꢁ
consumption was monitored using a thermal conductivity reduced catalysts were exposed to a ow of 100 mL minꢀ1 of CO
detector. Using the same equipment, N2O chemisorption 2% in He, v/v during 30 min until saturation, and aerwards
experiments were performed to estimate the metallic surface purged with a ow of 100 mL minꢀ1 of He before the DRIFTS
area of the dispersed Cu0 species. The sample was previously spectra were collected. The temperature was then increased to
ꢀ1
reduced at 250 ꢁC for 30 min, and then cooled to 30 ꢁC and 500 C at a rate of 10 C min in He (100 mL minꢀ1). The CO
exposed to a ow of 30 mL minꢀ1 of N2O (1% in He, v/v) for adsorption spectra had two main signals centered at 2099 cmꢀ1
10 min. It is expected that the AlMgO support do not exhibit (corresponding to an assemblage of Cu0 + Cu+ species) and 2105–
a signicant interaction with N2O at this temperature. A second 2107 cmꢀ1 (corresponding to Cu+ species).21,31
H2-TPR was performed by increasing the temperature up to
ꢁ
ꢁ
ꢁ
400 C. The hydrogen consumption was used to calculate the
2.3. Catalytic activity
amount of surface oxidized copper aer N2O chemisorption. No
A microactivity reference system (PID Eng&Tech), operating in
bulk oxidation was observed for samples. The copper surface
continuous ow mode at atmospheric pressure, was used to
area (SCu) and particle isometric size (dCu) were estimated by
perform the dehydrogenative coupling of ethanol. The reactor
using eqn (1) and (2), respectively, and a correlation of 1.46 ꢂ
and main valves were inside a hot box kept heated at 180 ꢁC to
avoid condensation of liquid reactants and products. The
catalyst was reduced in situ at 250 ꢁC under a ow of 30
mL minꢀ1 of H2 (5% in N2, v/v). A xed bed reactor was used to
perform the reactions at 300 ꢁC for 6 h using 150 mg of
1019 Cu atoms per m2 and a stoichiometry of reduction of 2 Cu/
H2 were found.
SCu ¼ 6.4955 ꢂ 10ꢀ2 ꢂ C ꢂ D
(1)
ðVm=AmÞ
dCu ¼ 6
(2) powdered catalyst. The reactor was fed with ethanol (99 wt%) at
a ow rate of 0.1 mL minꢀ1, through a HPLC pump (Model 307,
Gilson) and a ow of 30 mL minꢀ1 of N2, as the carrier gas.
These conditions resulted in a weight hourly space velocity
(WHSV, stands for weight hourly space velocity and calculated
D
In eqn (1) and (2), C is the copper content in wt%, D is the
copper dispersion in %, Vm is the mean atomic density of
copper in g mLꢀ1 and Am is the atomic surface area in nm2 per
atom.30
as feed room temperature mass ow/catalyst mass) of 31.1 hꢀ1
.
The experiments at lower space velocity of 4.7 hꢀ1 were achieved
by using 240 mg of catalyst. The outlet products were analyzed
on line using a gas chromatograph (Model 2014, Shimadzu)
equipped with a ame ionization detector and a capillary Rtx-1
column of 30 m ꢂ 0.32 mm ID and a lm thickness of 1 mm.
In situ XANES measurements at the Cu K-edge (8979 eV) was
carried out at the D06A-DXAS beamline of the Brazilian
Synchrotron Light Laboratory (LNLS) at Campinas, Brazil. The
D06A-DXAS beamline was equipped with a focusing curved Si
(111) monochromator, operating in the Bragg mode for the
selection of the desired range of X-ray wavelengths (8900–9400
eV). The samples were prepared as self-supported pellets con-
taining 25 mg of catalyst mixed with boron nitride and placed
into a tubular quartz furnace (d ¼ 20 mm and X-ray path length
¼ 440 mm) sealed with Kapton refrigerated windows for the
transmission measurements. Temperature-resolved XANES
spectra at the Cu K-edge were acquired during TPR experiments
in which it was used a ow of 30 mL minꢀ1 of H2 (5% in He, v/v)
and a heating rate of 10 ꢁC minꢀ1 from room temperature up to
250 ꢁC. The calibration energy and normalization of XANES
spectra were performed using the Athena graphical interface
program. The information about the proportion of copper
species (Cu2+, Cu+ and Cu0) during the H2-TPR experiments for
Cu/AlMgO samples was achieved by the linear combination
tting (LCF) of known CuO, Cu2O and metallic Cu species.
Diffuse reectance infrared Fourier transform spectroscopy
(DRIFTS) of CO adsorption on copper catalysts were recorded
using a Thermo Nicolet iS50 FTIR spectrophotometer equipped
with a MCT detector and a Harrick diffuse reectance infrared
Fourier transform spectroscopy cell with a CaF2 window. The
spectra were collected on a basis of 40 scans and a resolution of
4 cmꢀ1 using 150 mg of sample. Initially, the samples were pre-
3. Results and discussion
3.1. Textural characteristics of the porous supports and
chemical speciation of the dispersed copper sites
The effect of PSDAs in the textural properties of the supports
and, consequently, on those of the copper catalysts was studied
by means of mercury intrusion porosimetry, nitrogen phys-
isorption and scanning electron microscopy (Fig. 1 and Table
1). The mixed oxide, AlMgO–P, synthesized in the presence of
PSDAs presented considerably higher pore volume and BET
area than the reference non-porous support as a consequence of
the voids le aer removal of the emulsier agent (n-dodecane).
A previous study elucidated that the porosity is created by the
stacking of layers of the hydrotalcites precursors randomly
positioned around n-dodecane droplets.18,19 The macropores
arose from the droplets of n-dodecane, while the mesopores
were due to the slit-like arrangement of layers of the precursors.
Although the hydrotalcite structure no longer exist aer calci-
nation of the samples, the porous characteristic is preserved
and it can be useful for the dispersion of active metal sites and
for improved accessibility of chemicals. Aer copper nitrate
impregnation and calcination, Cu/AlMgO–P sample showed
a great decrease in the pore volume and surface area. It can be
related to the partial blockage of pores, as well as to a collapse of
treated at 500 C for 30 min (10 C minꢀ1) under a ow of He
ꢁ
ꢁ
of 100 mL minꢀ1, and then, cooled down and reduced at 250 ꢁC
3296 | RSC Adv., 2019, 9, 3294–3302
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 1b shows the X-ray diffraction patterns of the copper
catalysts. The reference sample exhibited mainly diffraction
peaks at 43.4ꢁ (200), 64.3ꢁ (220) and at 32.5ꢁ (110), 35.4ꢁ (002),
38.6ꢁ (111), 48.5ꢁ (202), 53.6ꢁ (021) corresponding to MgO and
CuO structures, respectively. On the other hand, the porous
support exhibited only diffraction peaks corresponding to MgO,
as an indication of higher dispersion of copper oxides. Possibly,
due to capillarity effects during drying of impregnated samples,
copper solution ows into the pore voids of the porous support,
allowing a high dispersion of copper species. According to
earlier studies performed with copper supported on g-Al2O3,32,33
the formation of a monolayer coverage and, consequently,
saturation of the surface occurs at a loading in-between 4 and
5 wt% of copper per 100 m2 gꢀ1 of the support. The specic area
of the reference sample Cu/AlMgO is just above this limit
(10 wt% of copper for 166 m2 gꢀ1) and surface bulk copper oxide
particles are prone to be formed aer the saturation of the
surface by a copper monolayer. Therefore, most probably, the
reference sample contains heterogenous distribution of copper
species, constituted by well-dispersed monolayered particles
and segregated ones.
Further evidence of the improved textural properties of the
porous support was provided by nitrogen physisorption
measurements (Fig. 1c) and BJH pore size distribution curves
(Fig. 1d). The hysteresis between the adsorption and desorption
branches for Cu/AlMgO–P catalyst is typical of the existence of
mesopores. The smooth slopes of the isotherms, together with
the non-parallel behavior of the adsorption and desorption
curves, indicated a distinct H3 hysteresis cycle for pores with
a broad distribution resulting from the aggregation of plate-like
particles, particularly for the catalyst Cu/AlMgO–P with an
average mesopore diameter of 3.3 nm. The preparation method
provided a catalyst with a BET area greater than the reference
(Table 1).
Fig. 1 (a) Mercury intrusion porosimetry (opened and filled symbols
represent the supports and the copper containing catalysts, respec-
tively); (b) X-ray diffraction; (c) nitrogen physisorption (opened and
filled symbols represent the desorption and adsorption branches,
respectively); (d) BJH pore size distribution of the desorption branches
for Cu/AlMgO samples; and scanning electron microscopy images of:
(e) AlMgO and (f) AlMgO–P supports.
the pore walls during the process of impregnation and drying.
Table 1 shows the results of a drying experiment of sample
AlMgO–P that was put in contact with water and rotaevaporated
in order to follow the impact of sample drying. The pores
shrank from 3.7 to 1.7 cm3 gꢀ1 only due to capillary action of
evaporating water. Considering that for the sample containing
copper Cu/AlMgO–P the pore volume was much smaller, i.e. 0.8
cm3 gꢀ1, both pore blockage and capillary forces were possibly
responsible for that decrease. The same behavior was observed
by Cassinelli et al. that studied porous Cu/Al2O3.6
The metallic copper dispersion was assessed by nitrous oxide
chemisorption followed by temperature programed reduction
with hydrogen (Fig. 1S†). Firstly, the catalysts were reduced
ꢁ
under a hydrogen stream at 250 C for complete reduction of
copper, followed by exposure to dilute nitrous oxide ow that
caused the oxidation of the surface copper species forming an
oxygen layer around the metallic particles. The stoichiometry of
the reduction of the surface copper oxides allowed to estimate
the relative contribution of surface copper species. Table 2
Table 1 Pore volumes determined by mercury intrusion porosimetry, BET specific surface area and acid and base sites quantification by
adsorption of NH3 and CO2, respectively
Pore volume (cm3 gꢀ1
)
Acid sites
Base sites
Sample
Meso
Macro
Total
BET area (m2 gꢀ1
)
Total NH3 (mmol gꢀ1
)
Total CO2 (mmol gꢀ1
)
AlMgO
AlMgO–P
Cu/AlMgO
Cu/AlMgO–P
AlMgO–P (H2O)a
0.1
1.8
0.2
0.4
0.6
0.3
1.9
0.3
0.4
0.9
0.4
3.7
0.5
0.8
1.7
166
260
120
206
—
0.78
0.43
0.29
0.36
—
0.45
0.43
0.40
0.37
—
a
AlMgO–P sample put in water and evaporated, simulating copper wet impregnation.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3294–3302 | 3297

View Article Online
RSC Advances
Paper
Cu+ intermediate during the reduction process. On the other
hand, the porous samples submitted to the reduction treatment
exhibited a noticeable shoulder in the ascending margin of the
spectrum. The LCF of the XANES spectra in Fig. 2 shows that in
fact the copper on the catalyst Cu/AlMgO reduces from Cu2+ to
Cu0 with a minor formation of the Cu+ intermediate. For the Cu/
AlMgO–P catalyst, although the reduction of Cu2+ to Cu+ started
earlier, at approximately 20 min under H2 stream, Cu+ reached
a portion close to 25% that can be associated to a kinetically
controlled reduction of Cu+ to Cu0, possibly due to the mono-
layer dispersion of copper species on the AlMgO–P support.
Since, Cu+ was previously considered the most active species to
promote the activation of ethanol,5,6 its formation is highly
desirable for the ethanol dehydrogenation reactions.
FTIR spectra of CO adsorbed on reduced copper catalysts in
the high frequency (HF) region of 2160–2020 cmꢀ1 and in the
low frequency (LF) region of 1850–1150 cmꢀ1 are shown in Fig. 3
and 3S,† respectively. It has been reported35 that CO adsorption
on copper surfaces show IR bands in the HF spectral region
associated with linear or bridged CO species interacting with
Table 2 Copper dispersion, metallic area and particle size for copper
impregnated on AlMgO supports, before (B) and after (A) ethanꢀo1l
dehydrogenation reaction carried out at 300 ꢁC and WHSV of 31.1 h
Cu dispersion
(%)
Metallic area
Particle size
(nm)
Sample
(m2 gꢀ1
)
B
A
Cu/AlMgO
Cu/AlMgO–P
Cu/AlMgO
50
85
42
50
29
55
27
34
2.1
1.2
2.5
2.1
Cu/AlMgO–P
illustrates the dispersion degrees, Cu surface area and Cu
average particle size. As expected, the Cu/AlMgO–P catalyst had
a superior dispersion of copper of 85% in comparison to the
50% of the reference sample, thus conrming the positive effect
of the emulsion mediated synthesis in producing a porous
support for dispersing copper. The dispersion degree and the
average Cu particle size follow the same pattern as Cu surface
area, i.e. for Cu/AlMgO–P high dispersion leads to a low particle
size (1.2 nm). It is worthy to note that compared with appro-
priate literature data obtained over copper-based mesoporous
materials, Cu/AlMgO–P catalyst shows very high dispersion and
surface area for copper. For example, at similar copper loading
of 10 wt%, copper supported on all-silica mesoporous SBA-15
material had only 37% of copper dispersion.34
Additional information about copper species distribution on
supports was provided by XANES spectra (Fig. 2S†) during
reduction of copper by heating from 25 to 400 ꢁC under a H2/He
atmosphere. For the reference catalyst, the CuO reduction
started at approximately 25 min aer heating (plateau of 220 ꢁC
in Fig. 2) promoting only a minor change in the spectra of
Fig. 2S,† related to the direct transformation of Cu2+ to Cu0,
without the clear identication of a shoulder in the ascending
margin around 9002 eV,5,6,21,27 a ngerprint of the presence of
Fig. 3 FTIR spectra of CO desorption on reduced: (a) Cu/AlMgO; (b)
Fig. 2 Copper species along reduction with H2 up to 250 ꢁC for the Cu/AlMgO–P catalysts in the region of 2160–2020 cmꢀ1; (c) relative
Cu/AlMgO and Cu/AlMgO–P catalysts, obtained from XANES intensity of bands displayed in (a) and (b) vs. desorption temperature of
measurements at Cu K-edge (spectra are in Fig. 2S†).
CO.
3298 | RSC Adv., 2019, 9, 3294–3302
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
copper species, such as, Cu2O or Cu0 sites. However, herein, explanation was based on the reduction of hydroxyl groups in
based on the XANES results of copper supported samples, CuO the surface of the AlMgO–P sample due to the interaction with
specie is highly improbable, and the three strong overlapping the hydrophobic emulsion during the synthesis that leads to
bands with maximum at 2107, 2106 and 2103 cmꢀ1 of Cu/ a support with a considerably low acidity and basicity. In
AlMgO–P catalyst are probably related to chemisorption of CO comparison to other supports found in the literature (see data
over Cu+ and/or Cu0 species (Fig. 3S†). According to Hadjiivanov collection in Table 1S†), hydrotalcites derived oxides are among
et al.,36 linear Cu0–CO carbonyls may be adsorbed at the same those with lower acidity/basicity. By supporting copper, the
frequency, as Cu+–CO, when copper is highly dispersed on the catalyst had a pronounced decrease in acidity due to the selec-
surface. These copper species are better distinguished via tive combination of the Lewis alkaline sites from copper oxides
thermal stability, once the Cu0–CO surface carbonyls are easily with acidic sites on AlMgO–P. A milder decrease in basicity was
wrecked by increasing temperature, whereas the Cu+ cations also observed, which may be related to binding of Cu to certain
produce more stable surface carbonyls, as a result of the base sites as well. However, these events were fairly benecial to
strongest s-bond.31,35 Fig. 3c shows the relative intensity of avoid the formation of undesired by products, specially ethene
chemisorbed CO bands in the spectra of Fig. 3a and b, and diethyl ether that can be formed on those sites.
normalized by their maximum intensity. Carbonyl species
adsorbed on Cu/AlMgO catalyst are totally absent at 150 ꢁC,
while for the Cu/AlMgO–P catalyst complete desorption of CO
3.2. Dehydrogenative reactions of ethanol to acetaldehyde
and ethyl acetate
occurred at 255 ꢁC, that may be related to the higher amount of
surface Cu+. The LF region in Fig. 4S† presented bands at 1635,
1547, 1375, 1343 and 1220 cmꢀ1, referring to carbonate species
interacting with basic surface sites as OH groups and O2ꢀ
centers or even acid–base pairs (due Mg2+–O2ꢀ–Al3+), which are
very similar for both samples, as expected by the TPD-CO2,
which showed minor differences for samples Cu/AlMgO and
Cu/AlMgO–P (Table 1 and Fig. 5S†). The bands located at 1635
and 1220 cmꢀ1 are attributed to bridged bidentate carbonates
The copper impregnated catalysts were investigated in the
ꢁ
dehydrogenation of ethanol (Scheme 1) at 300 C under space
velocities of 31.1 hꢀ1 (Fig. 4) and 4.7 hꢀ1 (Fig. 5), and under
a period of 6 h to evaluate the activity and deactivation. Catalytic
tests were also performed using the Cu-free supports (Fig. 6S†)
and the ethanol conversion was negligible, being below 1%. On
the other hand, the copper catalysts, had a high activity in
selectively converting ethanol into acetaldehyde (Fig. 4),
reaching conversion values of 28 and 61% for Cu/AlMgO and
Cu/AlMgO–P samples, respectively. Ethyl acetate, ethene,
diethyl ether, n-butanal, crotonaldehyde, methyl ethyl ketone,
acetone and butanol were minor products summing less than
2%. Thus, the porous catalyst was about 3 times more active,
because of the higher dispersion of copper and creation of
a singular environment that led to the formation of Cu+ species
on the catalyst surface. In fact, the presence of partially reduced
copper Cu+ and Cu0 are advantageous for the catalytic perfor-
mance, as the initial turnover frequencies, considering the
surface copper, were 122 and 166 hꢀ1 for the reference and for
the porous catalyst, respectively.
(b-HCO3ꢀ), whereas the bands 1547 and 1375 cmꢀ1 and
2ꢀ
1343 cmꢀ1 are assigned to carbonate monodentate (m-CO3
)
2ꢀ
and bidentate (b-CO3 ), respectively.13 Typically, monodentate
carbonate formation requires low-coordination oxygen anions
(strong base sites), provided by MgO.13
The acid and base properties of the copper catalysts were
determined from the temperature programmed desorption
(TPD) of NH3 or CO2, respectively. Fig. 5S† shows the TPD
proles of NH3 and CO2 and the integration of the peaks are
given in Table 1. The deconvolution by means of a Gaussian
function was used to discriminate the desorption peaks for
weak, medium and strong sites (Fig. 5S†), which reect the
different binding acid–base sites available on the surface. The
porous support (AlMgO–P) presented lower acidity than the
reference (AlMgO), as reported in the previous study.19 The
The catalysts were not completely steady for a time-on-
stream (TOS) of 6 h. The notable deactivation of the Cu/
AlMgO reference sample was related to the harsh copper
Scheme 1 Dehydrogenative reactions of ethanol to give acetaldehyde and ethyl acetate as main products. The dashed path corresponds to the
Tishchenko mechanism that might take place on the support/copper interface.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3294–3302 | 3299

View Article Online
RSC Advances
Paper
Fig. 4 (a) Conversion of ethanol at 300 ꢁC and WHSV of 31.1 hꢀ1; (b) selectivity over Cu/AlMgO–P; (c) Cu/AlMgO catalysts. Ethyl acetate, ethene,
diethyl ether, butanal, crotonaldehyde, methyl ethyl ketone, acetone and butanol were less than 2%.
dispersion and to the sintering of copper particles that were (Table 2S†). The result can be explained based on the Tish-
loosely supported on the non-porous sample. The porous chenko mechanism,7,37–40 which consists of four main steps
catalyst, in contrast, deactivated less, because of the physical depicted in Scheme 1 and that involve the participation of the
barrier that well dispersed copper particles placed on the support/copper interface. Firstly, it occurs the formation an
surface of macro and mesopores of the support had to sinter. alkoxide (CH3CH2Oꢀ) adsorbed on AlMgO base sites (step 1)
Both catalysts suffered some extent of deactivation along 6 h at that reacts with the aldehyde formed on the copper phase (step
300 C (21.1% for Cu/AlMgO against 8.2% for Cu/AlMgO–P) as 2). At this step, the presence of Cu+ in combination with Cu0
ꢁ
a consequence of a slow copper sintering, as revealed by aids to promote the formation of acetaldehyde (which is more
chemisorption experiments depicted in Table 2, however, its evident in the porous catalyst). Then, the alkoxide reacts with
effect was more pronounced on the non-porous reference the aldehyde to form a hemiacetal (step 3). Finally, the hemi-
catalyst.
acetal dehydrogenates on copper phase (step 4). According to
Ethanol conversion was also examined at a WHSV of 4.7 hꢀ1 the results of Scotti et al. that studied Cu/ZrO2 catalysts,40 we can
(Fig. 5), i.e. by decreasing the space velocity and consequently predict herein that the steps 1 and 4 are promoted by AlMgO/Cu
increasing the contact time in 6.6 times. Ethanol conversion interface and that improved hydrogen spillover takes place,
and selectivity to all products were noticeably improved: the thus allowing the interface to work ttingly. It seems that the
conversion increased by a factor of 3.5 and 1.4, for the reference interface plays a major role in the overall catalyst performance,
and for the porous catalyst, respectively. The main product, because the superior capacity of the porous catalyst to dehy-
ethyl acetate, was mostly produced on the reference non-porous drogenate ethanol did not allow to obtain higher selectivity to
catalyst (35 against 20% on the porous one). That behavior ethyl acetate. Lastly, the cooperative effect of acid and base sites
reveals that the coupling reaction of ethanol and acetaldehyde of suitable strength of the Al–Mg oxides was important to give
is favored on larger copper particles.37 The differences in high selectivity to ethyl acetate, as reported elsewhere.7,40
selectivity to ethyl acetate could rely on the support/copper
In order to conrm that the catalyst deactivation was due to
interface, which differs on copper particles of different sizes copper sintering and not to coke deposition, aer running
Fig. 5 (a) Conversion of ethanol at 300 ꢁC and WHSV of 4.7 hꢀ1; (b) selectivity over Cu/AlMgO–P; (c) Cu/AlMgO catalysts, before and after
regeneration by calcination at 500 ꢁC and reduction at 250 ꢁC. Diethyl ether, butanal, crotonaldehyde, methyl ethyl ketone, acetone and butanol
were less than 2%.
3300 | RSC Adv., 2019, 9, 3294–3302
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
ethanol dehydrogenation for 6 h the catalysts were calcined at images and Prof. J. M. C. Bueno for the CO/DRIFTS adsorption
500 ꢁC followed by reduction at 250 ꢁC in H2. Aer regeneration, experiments.
the catalytic activity was not restored at all and the copper
particle size increased (Fig. 5 and Table 2). The increase in
particle size between 2–2.5 nm appears to favor to increase the
References
selectivity of ethyl acetate to about 40% and decrease acetal-
dehyde to about 55%. Also, the regeneration process leads to an
increase of sintering of the copper particles of the catalysts
which increased ethyl acetate selectivity up to about 8%. The
sintering process is more evident in the non-porous Cu/AlMgO
(Fig. 5c). Finally, aer the thermal treatment at 500 ꢁC and
sintering of copper, the surface of the support led to the
occurrence of the dehydration on ethanol to ethene, with
a selectivity of about 5%.
1 E. S. Shuba and D. Kie, Renewable Sustainable Energy Rev.,
2018, 81, 743–755.
2 H. H. Mardhiah, H. C. Ong, H. H. Masjuki, S. Lim and
H. V. Lee, Renewable Sustainable Energy Rev., 2017, 67,
1225–1236.
3 A. G. Sato, D. P. Volanti, D. M. Meira, S. Damyanova,
E. Longo and J. M. C. Bueno, J. Catal., 2013, 307, 1–17.
4 A. G. Sato, D. P. Volanti, I. C. De Freitas, E. Longo, J. Maria
and C. Bueno, Catal. Commun., 2012, 26, 122–126.
5 W. H. Cassinelli, L. Martins, M. Magnani, S. H. Pulcinelli,
V. Briois and C. V. Santilli, RSC Adv., 2016, 6, 20453–20457.
6 W. H. Cassinelli, L. Martins, A. R. Passos, S. H. Pulcinelli,
A. Rochet, V. Briois and C. V. Santilli, ChemCatChem, 2015,
7, 1668–1677.
7 F. Zaccheria, N. Scotti and N. Ravasio, ChemCatChem, 2018,
10, 1–11.
8 D. A. Morgenstern and J. P. Fornango, Energy Fuels, 2005, 19,
1708–1716.
9 S. K. Rout, S. Guin, K. K. Ghara, A. Banerjee and B. K. Patel,
Org. Lett., 2012, 14, 3982–3985.
10 J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, J. Am.
Chem. Soc., 2005, 127, 10840–10841.
4. Conclusions
Hydrotalcite derived mixed oxides of Al and Mg were success-
fully prepared by an emulsion mediated method and impreg-
nated with copper nitrate. The combination of texture
characterization techniques, such as mercury porosimetry and
nitrogen physisorption, disclosed the production of a more
porous material with a higher BET area. The acidity and basicity
of the catalysts decreased aer impregnation with copper, due
to metal coating on these sites. The porous catalyst exhibited
more available metallic sites and greater dispersion on the
surface of the material, as demonstrated by H2-TPR together
with N2O chemisorption.
11 H. Miura, K. Nakahara, T. Kitajima and T. Shishido, ACS
Omega, 2017, 2, 6167–6173.
From the LCF of XANES spectra and FTIR-CO measure-
ments, the presence of a larger amount of Cu+ intermediate was
observed, which was advantageous for the conversion of
ethanol to acetaldehyde, as the turnover frequencies, were 122
and 166 hꢀ1 for the non-porous reference and for the porous
catalyst, respectively. Both catalysts deactivated due to copper
sintering, however the porous catalyst deactivated less, because
of the difficulties that the physical barrier related with a higher
surface area generated by the presence of macro and meso-
pores, which allowed to well disperse the copper particles and
consequently minimizing their sintering. By increasing the
contact time by 6.6 times, the formation of ethyl acetate was
mostly favored on the non-porous catalyst, because its forma-
tion is favored on the support/copper interface given by larger
copper particles.
12 S. W. Colley, J. Tabatabaei, K. C. Waugh and M. A. Wood, J.
Catal., 2005, 236, 21–33.
13 V. K. Diez, C. R. Apesteguia and J. I. Di Cosimo, J. Catal.,
2003, 215, 220–233.
´
14 P. J. Luggren, C. R. Apesteguıa and J. I. Di Cosimo, Fuel,
2016, 177, 28–38.
´
15 P. J. Luggren, C. R. Apesteguıa and J. I. Di Cosimo, Top.
Catal., 2016, 59, 196–206.
´
16 J. J. Bravo-Suarez, B. Subramaniam and R. V. Chaudhari,
Appl. Catal., A, 2013, 455, 234–246.
17 J. T. Kozlowski and R. J. Davis, ACS Catal., 2013, 3, 1588–
1600.
´
18 D. D. Petrolini, E. A. Urquieta-Gonzalez, S. H. Pulcinelli,
C. V. Santilli and Martins, Microporous Mesoporous Mater.,
2017, 240, 149–158.
´
19 D. D. Petrolini, A. V. da Silva Neto, E. A. Urquieta-Gonzalez,
Conflicts of interest
S. H. Pulcinelli, C. V. Santilli and L. Martins, RSC Adv., 2018,
8, 6039–6046.
20 D. D. Petrolini, S. H. Pulcinelli, C. V. Santilli and L. Martins,
J. Sol-Gel Sci. Technol., 2014, 71, 9–15.
There are no conicts to declare.
21 P. C. P. Caldas, J. M. R. Gallo, A. Lopez-Castillo, D. Zanchet
and J. M. C. Bueno, ACS Catal., 2017, 7, 1–28.
Acknowledgements
The authors gratefully acknowledge the nancial support 22 R. A. Hoyt, M. M. Montemore, E. C. H. Sykes and E. Kaxiras,
˜
provided by the Sao Paulo State Research Foundation (FAPESP,
grant numbers #2013/01328-0 and #2015/05321-5) and CNPq 23 J. Quesada, L. Faba, E. Dıaz and S. Ordonez, ChemCatChem,
(grant number #306326/2017-5). The authors also acknowledge 2018, 10, 3583–3592.
the XAFS1 beamline of the Brazilian Synchrotron Light Labo- 24 M. Ohira, H. Liu, D. He, Y. Hirata, M. Sano, T. Suzuki and
J. Phys. Chem. C, 2018, 122, 21952–21962.
´
ratory (LNLS) for the XAS measurements, LCE-UFSCar for SEM
T. Miyake, J. Jpn. Pet. Inst., 2018, 61, 205–212.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 3294–3302 | 3301

View Article Online
RSC Advances
Paper
25 Z. Lu, D. Gao, H. Yin, A. Wang and S. Liu, Ind. Eng. Chem. 32 B. R. Strohmeier, D. E. Leyden, R. S. Field and
Res., 2015, 31, 301–308. D. M. Hercules, J. Catal., 1985, 94, 514–530.
26 E. V. Shelepova, L. Y. Ilina and A. A. Vedyagin, React. Kinet., 33 R. Friedman, J. Catal., 1978, 55, 10–28.
Mech. Catal., 2017, 122, 385–398.
27 I. Ro, Y. Liu, M. R. Ball, D. H. K. Jackson, J. P. Chada,
34 C. S. Chen, C. C. Chen, C. T. Chen and H. M. Kao, Chem.
Commun., 2011, 47, 2288–2290.
C. Sener, T. F. Kuech, R. J. Madon, G. W. Huber and 35 M. Neurock, Z. Tao, A. Chemburkar, D. D. Hibbitts and
J. A. Dumesic, ACS Catal., 2016, 6, 7040–7050. E. Iglesia, Faraday Discuss., 2017, 197, 59–86.
28 E. W. Washburn, Proc. Natl. Acad. Sci. U. S. A., 1921, 7, 115– 36 K. I. Hadjiivanov and G. N. Vayssilov, Adv. Catal., 2002, 47,
116. 307–511.
29 S. J. Gregg, K. S. W. Sing and H. W. Salzberg, J. Electrochem. 37 J. Sun and Y. Wang, ACS Catal., 2014, 4, 1078–1090.
Soc., 1967, 114, 279C.
30 H. Masuda, M. Suzuki, K. Ehara, M. Fuji, T. Fukui, H. Suzuki,
38 M. Nielsen, H. Junge, A. Kammer and M. Beller, Angew.
Chem., Int. Ed., 2012, 51, 5711–5713.
J. Tatami, K. Hayashi and K. Toda, Nanoparticle Technology 39 P. C. Zonetti, J. Celnik, S. Letichevsky, A. B. Gaspar and
Handbook, 2008, pp. 3–48. L. G. Appel, J. Mol. Catal. A: Chem., 2011, 334, 29–34.
31 W. W. Wang, W. Z. Yu, P. P. Du, H. Xu, Z. Jin, R. Si, C. Ma, 40 N. Scotti, F. Zaccheria, C. Evangelisti, R. Psaro and
S. Shi, C. J. Jia and C. H. Yan, ACS Catal., 2017, 7, 1313–1329.
N. Ravasio, Catal. Sci. Technol., 2017, 7, 1386–1393.
3302 | RSC Adv., 2019, 9, 3294–3302
This journal is © The Royal Society of Chemistry 2019