Gas-Phase conversion of glycerol to acetol: Influence of support acidity on the catalytic stability and copper surface properties on the activity

Article abstract of DOI:10.5935/0103-5053.20160134

Mesoporous mixed copper-aluminum oxides and copper-silicon oxides were synthesized with polymeric precursors route in order to evaluate the effect of the support acidity on the catalytic stability due to the carbon deposit and the copper surface characteristics on the catalytic activity for the gas-phase conversion of glycerol to acetol. The samples were characterized by different techniques such as inductively coupled plasma (ICP), thermogravimetry and differential thermal analyses (TGA-DTA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption/desorption isotherms, temperature-programmed reduction with H₂ (H₂-TPR) and microcalorimetry of NH₃ adsorption. The metallic copper surface was shown by XPS, which was observed an increase with the copper loading without marked changes between Si or Al support using the same copper content. The Cu-Al catalysts present acidic properties close to that of the pure alumina support while the Cu-Si solid is not acid, as expected. Reduced catalysts were evaluated in the reaction of glycerol conversion. The catalytic results showed a clear dependence of the glycerol conversion to acetol with the Cu metal surface and the initial catalytic properties did not depend on the support acidity, since the copper is the major active site. It was observed 95percent of acetol selectivity and 80percent of glycerol conversion for the best catalyst. However, the support acidity influenced the catalyst stability, since Cu-Al solid deactivated continuously by contrast to Cu-Si sample, which reached stability after 2 h of reaction. The higher acidity for the Al support leads to a greater carbon deposit compared to Si support, blocking the active sites and providing a rapid catalytic deactivation.

Full text of DOI:10.5935/0103-5053.20160134

http://dx.doi.org/10.5935/0103-5053.20160134
J. Braz. Chem. Soc., Vol. 27, No. 12, 2361-2371, 2016.
Printed in Brazil - ©2016 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
Gas-Phase Conversion of Glycerol to Acetol: Influence of Support Acidity on the
Catalytic Stability and Copper Surface Properties on the Activity
Tiago Pinheiro Braga,*,a Nadine Essayem,*,b Swamy Prakashb and Antoninho Valentinic
aLaboratório de Peneiras Moleculares (LABPEMOL), Instituto de Química, Universidade Federal
do Rio Grande do Norte, Lagoa Nova, 59072-970 Natal-RN, Brazil
bInstitut de Recherche sur la Catalyse et l’Environnement de Lyon, IRCELYON, CNRS, University of
Lyon 1, 2 avenue Albert Einstein, 69626 Villeurbanne, France
cLaboratório de Adsorção e Catálise (Langmuir), Departamento de Química Analítica e
Físico-Química, Universidade Federal do Ceará, Campus do Pici, 60455-970 Fortaleza-CE, Brazil
Mesoporous mixed copper-aluminum oxides and copper-silicon oxides were synthesized with
polymeric precursors route in order to evaluate the effect of the support acidity on the catalytic
stability due to the carbon deposit and the copper surface characteristics on the catalytic activity
for the gas-phase conversion of glycerol to acetol. The samples were characterized by different
techniques such as inductively coupled plasma (ICP), thermogravimetry and differential thermal
analyses (TGA-DTA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2
adsorption/desorption isotherms, temperature-programmed reduction with H2 (H2-TPR) and
microcalorimetry of NH3 adsorption. The metallic copper surface was shown by XPS, which was
observed an increase with the copper loading without marked changes between Si or Al support
using the same copper content. The Cu-Al catalysts present acidic properties close to that of
the pure alumina support while the Cu-Si solid is not acid, as expected. Reduced catalysts were
evaluated in the reaction of glycerol conversion. The catalytic results showed a clear dependence
of the glycerol conversion to acetol with the Cu metal surface and the initial catalytic properties did
not depend on the support acidity, since the copper is the major active site. It was observed 95%
of acetol selectivity and 80% of glycerol conversion for the best catalyst. However, the support
acidity influenced the catalyst stability, since Cu-Al solid deactivated continuously by contrast to
Cu-Si sample, which reached stability after 2 h of reaction. The higher acidity for the Al support
leads to a greater carbon deposit compared to Si support, blocking the active sites and providing
a rapid catalytic deactivation.
Keywords: copper catalysts, support acidity, catalytic stability, glycerol conversion, acetol
formation
Introduction
vegetable oil, can be considered as a renewable feedstock
to produce chemicals of economic interest, such as acetol,
propylene glycols or acrolein, which are usually made from
fossil resources.4-6
The conversion of renewable resources, such as plant
biomass, to produce fuels and chemicals, has become an
important research topic in catalysis.1,2 The utilization
of biomass as renewable resources is an issue to move
our economy towards a more sustainable future. The
produced amounts of renewable biomass fuels, such as
fatty acid methyl esters biodiesel fuel, are still increasing
year by year.3 Thus, glycerol, which is the byproduct of
the biodiesel production process by transesterification of
Glycerol conversion was extensively studied in liquid
phase to produce propylene glycols over metallic catalysts,
Ru, Pt, Ni, Cu.7,8 Gas phase studies are more recent and
have attracted increasing attention due to the possibility
to obtain, in these conditions, acrolein by removing two
water molecules from glycerol.9,10 Hydroxyacetone, also
named acetol, is another chemical obtained from glycerol
dehydration acquired by removing one water molecule.
This compound is also of interest since, via hydrogenation
step, 1,2-propanediol is obtained (Scheme 1). Acetol is
*e-mail: tiagoquimicaufrn@gmail.com,
nadine.essayem@ircelyon.univ-lyon1.fr

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Gas-Phase Conversion of Glycerol to Acetol
J. Braz. Chem. Soc.
reported to be formed in high selectivity when the reaction
is conducted in gas phase.
Two types of catalytic systems were described to be
efficient for the selective glycerol conversion to acetol
in gas phase: monofunctional acid or basic catalysts and
copper based materials.
Due to the toxicity of Cr, alternative Cu based catalysts
were investigated for this application. Cu/ZnO combined
with various supports, Al2O3, TiO2, ZrO2 were studied in
order to investigate any synergistic effects between metallic
copper and the oxide support on the gas phase glycerol
dehydration in H2 atmosphere.20 The authors proposed
different route to produce 1,2- or 1,3-propanediols, but
both routes involves a first acid catalyzed dehydration
step via protonation of the primary or secondary hydroxyl
group followed by a hydrogenation step into 1,2- or
1,3-propanediols, promoted by the metallic copper.
Under H2, Cu-H4SiW12O40/SiO2 was shown to directly
convert glycerol in 1,3-propanediol via proposed reaction
pathway, which involves a first dehydration step of glycerol
in 3-hydroxypropanal promoted by the acid sites and a
further hydrogenation step of 3-hydroxypropanal catalyzed
by the Cu sites.21 Interestingly, the authors showed that
in mild conditions (210 °C), H4SiW12O40/SiO2 catalyzed
glycerol dehydration in acrolein in high yield, while Cu
didn’t catalyze glycerol dehydration in 3-hydroxypropanal,
the first dehydration step which could further produce
acrolein via further dehydration step. One can remark
that the dehydration of glycerol to 3-hydroxypropanal
fits well with a Br∅nsted acid catalyzed step where the
secondary hydroxyl group of glycerol is protonated, leading
to a secondary intermediate carbenium ion, much more
stable than the primary one, which would be formed via
protonation of the primary hydroxyl groups (Scheme 2).
Copper based systems were also reported to be efficient
in N2 atmosphere. In particular, Sato et al.21 reported that
acetol was produced from glycerol in high selectivity over
Cr free supported Cu catalysts, but under N2 atmosphere
at moderate temperature, 250 °C. They found that the
acid-base properties of the supports affected the acetol
selectivity, while basic supports decreased glycerol
conversion and acetol selectivity, acid supports promoted
acetol selectivity. The authors concluded that the reaction of
At temperatures above 300 °C, acetol was obtained
from glycerol over monofunctional acid catalysts such as
ZrS, ZrW, ZrWSi, ZrWAl. A high yield in acetol of 74%
was achieved over ZrWAl at full glycerol conversion. The
authors proposed that weak acid sites were the actives
ones.11 Silica-aluminum catalysts, known for their acidic
properties, were also reported as efficient catalyst for acetol
formation at temperatures higher than 300 °C.12 In that
case, the authors have correlated the acetol selectivity to
the presence of strong Lewis acid sites as well as the basic
OH groups. Similarly, monofunctional basic catalysts were
also disclosed to catalyze acetol formation in high yield at
a reaction temperature as high as 360 °C.13
By contrast to acido-basic materials, copper based
catalysts were disclosed to dehydrate glycerol into acetol
in milder conditions, typically between 230-250 °C, at
atmospheric pressure. In most cases, the metallic copper
function is combined to acido-basic supports, and these
bifunctional catalytic systems are reported to be efficient
as well in N2 or H2 atmosphere. Copper chromite was
earlier identified as one of the most active catalysts for
converting glycerol to acetol.14-16 This is well exemplified
by the selective dehydration of glycerol over CuCrO
combined with the reactive distillation technology leading
to acetol yields higher than 90%.17,18 The authors had
already underlined that higher acetol yields were observed
under H2 atmosphere compared to N2. Moreover, copper
chromite was reported as an active system to directly
produce 1,2-propanediol in H2 atmosphere.19 The authors
proposed that the high acidity of CuCrO is responsible for
the first step of glycerol dehydration to acetol.
Scheme 1. Conversion of glycerol to acetol or acrolein.

Vol. 27, No. 12, 2016
Braga et al.
2363
Scheme 2. Mechanism for the glycerol dehydration using acid catalysts.
glycerol conversion over Cu/Al2O3 is not only catalyzed by
Cu, but the acid support should assist the metallic function.
Thus, they first proposed that acetol formation over Cu
based catalysts might proceed via a homolytic dissociation
of the primary C−OH bond to produce acetol. Recently,
the mechanism proposed by Sato et al.21 was supported by
the results obtained by Bienholz et al.22 over Cu/SiO2 in
gas phase at 250 °C, in H2 atmosphere. Indeed, the authors
observed a linear relationship between the copper surface
and the TOF, indicating that copper is primarily responsible
for the glycerol conversion to acetol and the acidic, basic
or neutral supports have a secondary role.21,22
From this brief literature overview, it is clear that Cu
based catalysts are efficient catalytic systems to promote
selective glycerol conversion to acetol formation in gas
phase, but it also appears that there is no agreement on the
nature of the active sites nor on the prevailing mechanism
and on the catalysts deactivation.
In other respects, in order to improve the performances
of oxides catalysts, involving copper supported ones,
intense researches are currently carried out in order to
develop new methodologies for material synthesis with a
focus on enhanced surface area, mesoporosity and narrow
pore distribution.23-25 Among them, aluminum and silicon
oxide compositions are of great interest due to strong
potential as catalyst supports.15
The polymeric precursor method derived from the
Pechini method is one of this methodology, allowing the
synthesis of mesoporous oxides.16 Accordingly, due to the
potential of this synthesis method,17,26 in this paper, we
present interesting results on the transformation of glycerol
to 1-hydroxyacetone over mesoporous copper-silica or
copper-aluminium systems obtained by the polymeric
precursor route and it was studied the effect of the support
acidity (Al or Si) on the catalytic stability due to the
deposited coke and the influence of copper surface aspects
on the catalytic activity.
Experimental
Catalyst preparation
This method is based on the chelation of metallic
cations (Cu2+, Si4+ and Al3+) by citric acid (CA) in a water
solution followed by a polyesterification of the remaining
carboxylic acid function with ethylene glycol. Aluminum
nitrate nonohydrate {Al(NO3)3·9H2O}, copper nitrate
trihydrate {Cu(NO3)2·3H2O}, tetraethylorthosilicate
(TEOS), citric acid monohydrate (CA) {C6H8O7·H2O},
and ethylene glycol (EG) {C2H6O2} were used as starting
chemicals. A CA/metal ratio of 2:1 (mol) was used for all
the samples. The metal amount is the sum of Cu, Al or Si.
The mass ratio of CA/EG was kept at 2:3. The different
samples were labeled as xCuSi or xCuAl, with x denoting
the Cu molar percentage (100 × Cu/Al + Cu or 100 × Cu/
Si + Cu ). The sample containing only aluminum oxide
was denominated Al.
For the synthesis process of the 3CuAl sample, where
3 is the mol% of Cu/(Cu + Al), 0.0038 and 0.110 mol of
Cu and Al salts, respectively, were dissolved in distilled
water at room temperature. CA (0.2312 mol) was dissolved
in ethanol at 50 ºC. Then the aqueous solution was added
to the CA-ethanol solution and stirred during 60 min at
50 ºC. Subsequently, the EG (0.3497 mol) was added, and
the mixture was stirred for 4 h at 100 ºC until it became a
viscous resin. The resin was treated at 300 ºC for 1 h under
air. The resulting precursor composite was ground and
treated at 500 ºC under air flow for 2 h. Similar conditions
were used to prepare the other samples 1CuAl, 7CuAl,
8CuSi and Al.

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J. Braz. Chem. Soc.
Catalysts characterizations
thermal conductivity detector was used to follow the H2
consumption.
The solids were analyzed by inductively coupled
plasma-atomic emission spectrometry (ICP-AES) after
dissolution by acid attack. The analyzed Cu contents by
ICP are the expected ones (Table 1).
The pyrolysis step was monitored by thermogravimetric
analysis (TGA) and differential thermal analysis (DTA),
using a 5 ºC min-1 heating rate, under air flow of 40 mL min-1
and 30 mg of sample. The crystal structure of the metal
oxides was characterized by X-ray diffraction analysis
(XRD), with Cu Kα irradiation source (λ = 1.540Å, 40 kV
and 40 mA).
Catalytic activity
Catalytic tests were carried out in a fixed bed
microreactor, using 200 mg of catalyst. All fresh catalysts
were firstly reduced in H2 (25 mL min-1) at 250 °C for 1 h.
Then, an aqueous solution of glycerol at 10 wt.% was
pumped at the feed rate of 1.8 cm3 h-1, then vaporized and
fed through the reactor in a down flow mode together with
a gas (hydrogen) flow rate of 25 mL min-1. The glycerol
feed corresponded to 0.06 mol of glycerol per hour. The
liquid products were collected in an ice-water-salt trap
(−6 °C) in order to be analyzed, every hour by the gas
chromatography-mass spectrometry (GC-MS). A gas
chromatography-flame ionization detector (GC-FID) was
connected to the gas effluent. The capillary column was
purchased from Varian, CP 9048 (length: 30 m, diameter:
0.32 mm, thickness of the stationary phase: 0.1 mm).
Carbon balances were between 77 and 88% in the catalytic
experiments. Glycerol conversion and acetol selectivity
were calculated in the following way:
Catalysts specific surface area (BET) and pore volume
were determined by N2 adsorption/desorption isotherms, at
liquid nitrogen temperature. The catalyst samples (80 mg)
were vacuum pretreated at 200 °C for 1 h.
The acid properties were measured by calorimetry of
NH3 adsorption at 80 °C, using a Tian-Calvet calorimeter
coupled with a volumetric equipment. The reduced solids
(0.1 g), placed in the calorimetry cell, were first vacuum
treated at 400 °C for 2 h. After the pre-treatment, the cell
was then introduced in the calorimeter at 80 °C and the
experiment was performed. The oxide samples were then
contacted with small doses of NH3 up to equilibrium and the
differential enthalpy of adsorption was recorded together
with the amount of adsorbed ammonia.
(1)
(2)
X-Ray photoelectron spectroscopy (XPS) analysis
were performed on the calcined samples and after in situ
reduction at 250 °C, in equipment connected to the XPS
spectrometer. The XPS analyses were carried out on a
Kratos Ultra DLD spectrometer using monochromated
Al Kα radiation (10 mA, 15 kV). All spectra were taken
in the hybrid (combined electrostatic and magnetic lens)
mode.All spectra were referenced to the C1s line at binding
energy 284.6 eV, characteristic of ever-present adventitious
carbon (C−C and C−H). This peak position was obtained
after Shirley background subtraction and decomposition
of the C1s peak envelope using a Gaussian-Lorentzian
(70-30%) curve fit. All other photoelectron peaks were
background-subtracted and curve-fit in the same manner.
Quantification was carried out with the VISION software
supplied. The relative sensitivity factors (RSF) applied
here are inherent to this software and incorporate Wagner
photoelectron cross-sections and analyser transmission
correction.
Results and Discussion
Thermal analysis
Figure 1 shows the TGA-DTA curves of the precursor
powder that were obtained after heating the synthesized
polymeric resin at 300 °C for 1 hour in air. The first weight
loss (100-150 ºC) is ascribed to the release of physisorbed
water and remaining ethylene glycol (organic precursor).
This event corresponds to an endothermic peak in the DTA
curve. The main weight loss is observed from 300 to 500 °C
for the Cu-Al samples. For the Cu free sample,Al, or 8CuSi
the weight losses was prolonged at higher temperature, 550
and 600 °C, respectively. These important weight losses are
related to the decomposition of the residual organic material
that corresponds to a breakaway of the polymeric chains
formed by the polyesterification reaction as well as to the
decomposition of nitrates wastes.27,28 This process occurs
with an important exothermic phenomenon, as pointed out
in the DTA curves.
The temperature of reduction for the CuO based
catalysts was obtained by temperature-programmed
reduction (H2-TPR) analysis from 25 to 930 °C in a
quartz reactor using a 8% H2/N2 mixture (25 mL min-1) at
a heating rate of 10 °C min-1 and 20 mg of the catalyst. A
From the results of thermal analyses of Cu-Al samples,
one can note a shift to lower temperatures of the complete

Vol. 27, No. 12, 2016
Braga et al.
2365
Figure 2. X-Ray diffraction profile of the samples after the calcination
at 500 °C.
formula. The results indicate the comparable sizes of
crystalline CuO phase with average crystallites sizes of
24.6, 23.5 and 26.1 nm for 3CuAl, 7CuAl and 8CuSi,
respectively. Furthermore, one can note that the Cu loading
increase does not result in larger CuO crystallites, which
do not seem to depend on the nature of the support, Si or
Al oxides. This probably accounts for the synthesis method
which prevents ionic interactions between the mineral
precursors caused by the chelating properties of citric acid
as regards to the Cu cations. Moreover, the relative high-
temperature calcination, 500 °C, as regards to the sintering
ability of copper might also contribute to the close Cu
crystallites sizes obtained for the different systems.
Figure 1. Thermal analysis: TGA profile and the respective DTA.
carbon elimination. This suggests that copper might
catalyze the elimination of the organic precursors. However,
this seems to be true only for the CuAl system, since over
CuSi, the carbon release was observed up to 600 °C.
This shift in the temperature events can influence
the texture and morphology of the samples. Employing
organic compounds (CA and EG) in the preparation of the
catalysts, which are removed as volatile materials during
the calcination steps, probably results in cavities formations
and simultaneous formation of the oxides structure. The
textural properties of the materials (specific surface areas,
pores diameter and pores volume) and the structural
features of the final oxides are most closely related to the
calcination process.
Specific surface area results
The nitrogen adsorption/desorption isotherms are
presented in Figure 3. The samples Al, 1CuAl, 3CuAl
and 7CuAl showed similar isotherms characteristic
of mesoporous materials (type II according to IUPAC
classification and H3 hysteresis loop). This is usually
assigned to slit-shaped pores due to aggregates of
particles. On the other hand, the sample 8CuSi presents
isotherms type IV, characteristic of mesoporous catalyst
(H2 hysteresis loop). Type H2 loop indicates that the pore
structure is complex and tend to be made of interconnected
networks of pores with different sizes and shapes.29 These
differences are confirmed by the pores diameter distribution
(Barrett-Joyner-Halenda (BJH) method).
X-Ray diffraction
The X-ray diffraction patterns are presented in Figure 2.
It is observed that the samples Al and 1CuAl are poorly
crystalline materials and present X-ray patterns of an
amorphous solid. Nevertheless, the 2θ degree value of the
broader peaks (18.6, 20.4, 40.6 and 53.3 degrees) and their
relative intensities suggest the β-Al(OH)3 phase formation
(JCPDS 00-008-0096) for the 3CuAl and Al solids. On the
other hand, the samples 3CuAl, 7CuAl and 8CuSi display the
formation of a single Cu phase (CuO, JCPDS 04-006-2679).
The CuO crystallites sizes of the samples were
calculated from the XRD patterns using the Debye-Scherrer
Table 1 lists the specific surface area (SBET), total
pore volume (Vp) and average pores diameter (Dp) of
solids. The increase of the copper loading in the Al oxides
solids provides samples with improved textural features.

2366
Gas-Phase Conversion of Glycerol to Acetol
J. Braz. Chem. Soc.
might have occurred. These differences were also observed
in the TGA-DTA results (Figure 1).
XPS analysis
Quantitative XPS analysis of the calcined samples
show that the superficial atomic concentration of copper
increased from 3CuAl to 7CuAl, logically. Note that the
Cu coverage of 7CuAl and 8CuSi are very close. Moreover,
the results indicate that the Cu coverage was not changed
significantly after the reduction step at 250 °C (Table 2).
These results, combined with the isotherms data, indicate
that despite the sample 8CuSi have greater surface area
related to the solid 7CuAl the Cu surface coverage is very
similar, probably indicating that the catalytic performance
for these two catalysts will be very close for reactions in
which copper is the major active site.
For 8CuSi, the Cu2p3/2 photoelectron spectrum present a
main peak observed at 932.9 eV, and satellite lines at higher
binding energy values, characteristic of CuII oxydation state
(Figure 4). The binding energy of the main Cu2p3/2 peak is
in good agreement with the presence of CuO, as observed
in XRD results (Figure 2).
Figure 3. N2 adsorption/desorption isotherms of the catalysts.
8CuSi possesses a significant higher specific surface
area (603 m2 g-1) and total pore volume (0.6 cm3 g-1), in
comparison with the other catalysts (Al, 1CuAl, 3CuAl
and 7CuAl) which have specific surface areas lower than
100 m2 g-1, however, the materials are mesoporous in all
cases, with pore size of 40.0 Å. Thus, it appears that the
nature of the component used as support influences greatly
the textural properties, probably caused by the type of
inorganic precursor. Indeed, the Al and Si precursors are
distinct, Al nitrate and tetraethylorthosilicate (TEOS),
respectively. Thus, starting from TEOS in the case of
silica might, most likely, prevents the formation chelated
Si specie with citric acid and thereafter a different process
For 3CuAl and 7CuAl, the Cu2p3/2 photoelectron
spectrum shows a main component at 932.9 eV with a
shoulder at higher binding energy, 935 eV, which represents
ca. 30% of the total peak area. One can notice that for
3CuAl and 7CuAl, the relative intensity of the satellite
peaks are higher compared to 8CuSi (Figure 4).
Table 1. Chemical analyses, textural properties and amount of carboneous deposits on the spent catalysts
Sample
Al
Cu / wt.%
−
SBET / (m2 g-1)
Vp / (cm3 g-1)
0.03
Dp / nm
15.3
8.5
C / wt.%
4.2
7
1CuAl
3CuAl
7CuAl
8CuSi
0.66
32.0
41.4
99.2
603.6
0.04
6.5
3.16
0.06
10.6
8.2
5.3
6.75
0.06
20.6
7.0
7.80
0.6
4.0
SBET: Specific surface area ;Vp: total pore volume ; Dp: average pores diameter; C: Carbon content obtained from TGA analysis of spent catalysts recovered
after 5 h of recation in standard conditions.
Table 2. XPS analyses of calcined and reduced samples: binding energies of Cu2p3/2, Cu superficial atomic concentrations
ElCu2p3/2 (eV)
CuII
Cu superficial atomic
concentration / atom%
Catalyst
CuII
CuI/0
3CuAl calcined
934.8
932.7
main component
−
0.66
3CuAl reduced
7CuAl calcined
−
−
932.1
0.51
0.80
934.8
932.8
−
main component
7CuAl reduced
8CuSi calcined
8CuSi reduced
−
−
−
−
933.5
−
931.9
−
932.7
0.90
0.88
0.87

Vol. 27, No. 12, 2016
Braga et al.
2367
surface.After in situ reduction at 250 °C, the satellite peaks
disappeared logically and the Cu2p3/2 photoelectron spectra
present a single finer peak at ca. 932 eV for all samples,
characteristic of the reduced species CuI and/or Cu0
(Figure 5). Unfortunately, we failed to discriminate between
the formation of metallic copper, Cu0 or CuI species based
on the kinetic energy of the auger electrons L3M4.5M4.5,
because the signals were too broad, which precludes any
reliable measure. Thus, temperature programmed reduction
analysis is indispensable to confirm the copper oxidation
state after the reduction step at 250 °C.
Figure 4. Cu2p3/2 photoelectron spectra for calcined samples, 3CuAl,
7CuAl and 8CuSi.
One can recall that it was previously reported that the
intensity of the satellite bands (Isat) compared to that of the
main peak (Imp), was correlated with the Cu2+ symmetry.
Thus, for CuO where Cu2+ is in a square planar symmetry,
an Isat/Imp value close to 0.5 was reported when Cu2+ is in
a spinel structure; in a tetrahedral environment, a higher
relative intensity of the satellite bands was measured up to
0.8.30,31 Thus, for 7CuAl, the presence of the shoulder at
935 eV, a higher binding energy value than that tabulated
for CuO and already reported for the spinel CuCr2O4
together with the presence of more intense satellite bands,
strongly support the partial insertion of Cu in a spinel
phase such as CuAl2O4 which would exist, at least, on the
Figure 5. Cu2p3/2 photoelectron spectra for reduced samples, 3CuAl,
7CuAl and 8CuSi.
Temperature programmed reduction (TPR)
H2-TPR profiles of different samples presented a
single band of hydrogen consumption (Figure 6). The
TPR results showed the influence of the support (Al or
Si oxide) on the reducibility of copper oxide, which is
evidenced for low amounts of CuO. The solid 1CuAl
with the slightest amount of copper oxide has a higher

2368
Gas-Phase Conversion of Glycerol to Acetol
J. Braz. Chem. Soc.
reduction temperature compared to the catalyst 7CuAl,
probably due to the greater interaction between copper
oxide and the Al support.
reaction conditions.36-41 Xiao et al.33 proposed that the
metal copper was mainly responsible for the activation
of hydrogen, while Cu+ was for dehydration due to the
presence of Lewis acid sites. The dehydration also takes
place at metallic copper site, since glycerol may also be
dehydrated to acetol. Thus, according to the H2-TPR and
XPS results, which indicate the presence of Cu0 and Cu+
in the applied reaction conditions, it is expected that the
solids present interesting catalytic properties in the glycerol
conversion to acetol and 1,2-propanediol.
Furthermore, it is probable that under the employed
reaction conditions the acetol selectivity is larger compared
to the 1,2-propanediol selectivity, since the reaction rate in
the hydrogenation is much lower compared to the reaction
rate of the dehydration of glycerol. Under atmospheric
pressure and elevated temperature the 1,2-propanediol
production is disadvantaged, since the hydrogenation of
acetol is an exothermic reaction.33-35,40,41
Figure 6. Temperature programmed reduction curves of the catalysts.
Furthermore, as expected, the intensity of Cu2+
reduction peaks increase with the increase of copper
amount, confirming that the formed peak is incontestably
related to the reduction of CuO (majority) and copper spinel
(minority), as seen in the X-ray diffractograms (Figure 2)
and XPS results (Figures 4 and 5).
On the other hand, the solid 8CuSi shows the formation
of a shoulder around 220 °C and an intense band at 255 °C.
The shoulder at 220 °C may indicate the presence of Cu+,
while the second one may be due to the reduction of Cu2+
species to Cu0; again in agreement with the XRD and XPS
results. H2-TPR results confirmed the presence of CuI and
Cu0 after pre-treatment in hydrogen atmosphere at 250 °C
before the reaction, considering that the copper oxide was
not completely reduced at this temperature.
However, based on published work32 and considering
that there was the formation of copper oxide nanoparticles
as observed in the X-rays results, one cannot rule out the
possibility that the observed shoulder is related to highly
dispersed CuO species (more easily reducible) and the
second peak is concerning the reduction of CuO bulk (more
difficult to reduce). Additionally, it is known that a copper
oxide particle located within the pores of the support shows
a harder reduction process compared to a particle positioned
on the external surface. Thus, this may also be linked with
the textural properties (Figure 3).
Catalytic activity
The catalytic performances of the samples are depicted
in Figure 7a (conversion of glycerol) and Figure 7b (acetol
selectivity). The samples with the highest copper loading
are active and highly selective, with acetol selectivity
higher than 90% (7CuAl and 8CuSi) and an initial glycerol
conversion higher than 85%. Note that if these two catalysts
present different BET surface area, the reduced samples
present similar Cu surface as determined by XPS analysis
(Figure 4 and Table 2). This similar copper surface may
be correlated with the same initial glycerol conversion
measured on 7CuAl and 8CuSi. However, the catalytic
activities of these two catalysts show distinct evolutions
with time on stream. The 8CuSi sample presents a short
initial deactivation, however, its catalytic activity is
stabilized after 2 h with glycerol conversion and acetol
selectivity of 66 and 96%, respectively. Other products
It is important to report that it is given a great attention
for the change of the catalytic phase and the oxidation
state of copper during the glycerol conversion.33-35 It was
already mentioned that the presence of Cu0/CuI and the
synergetic effect of Cu0 and Cu+ may favor the conversion
of glycerol to acetol and 1,2-propanediol under specific
Figure 7. Catalytic performance as a function of time for the catalysts:
(a) conversion of glycerol; (b) acetol selectivity.

Vol. 27, No. 12, 2016
Braga et al.
2369
are observed in low amount, such as 1,2-propanediol,
propanoic acid and small amounts of acetic acid.
Among the Al-Cu series, 7CuAl exhibits the best
catalytic properties (Figure 7). In particular, one can notice
that the initial activity measured on 7CuAl and 3CuAl
are closely correlated with the Cu surface measured by
XPS on the reduced samples. On the other hand, pure
aluminum oxide is almost inactive and less selective for
hydroxyacetone. Its behavior is comparable to 1CuAl, with
less than 1 wt.% of Cu.
It is important to highlight that when the reactions are
performed under N2 atmosphere, all the samples exhibited
very poor activities.
All these results strongly suggest that reduced copper
species are the active species for the selective conversion
of glycerol to acetol under the reaction condition, while the
Al and Si supports would not play a major role in acetol
selective formation in agreement with previous reports from
Bienholz et al.22 However, copper metal sites combined
with an appropriate support can play a major role in the
activity, selectivity and stability of samples.21
In spite of the equivalent initial glycerol conversion
for 7CuAl (86%) and 8CuSi (90%) well correlated to
their copper surface (XPS results), 7CuAl solid shows a
continuous deactivation during the course of the reaction
while 8CuSi is rapidly stabilized. 3CuAl shows also a
deactivation, but less pronounced than 7CuAl (Figure 7).
Hence, one can think that the gradual catalyst deactivation
is probably caused by the acidity of the aluminum based
support, which leads to higher carbon deposition.
Calorimetric measurements are generally considered as
a reliable technique to measure the acidic strength of solid
acids in addition to the quantification of the number of acid
sites when isotherms of the acid probe molecules adsorption
are recorded in parallel. Therefore, in order to confirm that
the pronounced deactivation of 7CuAl compared to 8CuSi
is due to the higher acidity of the Al support compared to
the Si support, the reduced samples Al, 7CuAl and 8CuSi
were analyzed by calorimetry of NH3 adsorption (Figure 8).
The calorimetry results presented in Figures 8a and 8b
confirm that the samples containing aluminium oxide (Al and
7CuAl) show superior acidity compared to the silica-based
oxide (8CuSi). If the three solids present a few amount of
strong acid sites with a differential heat of NH3 adsorption
higher than 160 kJ mol-1, the specific amounts of acid sites
of intermediate strength, with a differential heat of NH3
adsorption higher than 110 kJ mol-1 are significantly higher
for the Al based solids. The calorimetric curve of 8CuSi
which shows a rapid decrease of the differential heat of NH3
adsorption give evidences of the poor acidity of the silica
support, as expected. On the other hand, it was observed
Figure 8. Differential heat of NH3 adsorption as a function of NH3
coverage: (a) micromol of ammonia adsorbed per gram of the catalyst;
(b) micromol of ammonia adsorbed per meter square of the catalyst.
for the catalysts based on Al, a larger number of acid sites,
but the surface is heterogeneous in term of acid strength,
according to the progressive decrease of the differential heat
of NH3 adsorption with the NH3 coverage. Furthermore, it is
observed that 7CuAl and the Cu freeAl solid present similar
calorimetric curves when the amount of absorbed NH3 is
expressed per m2. This indicates that reduced Cu species
do not provide additional acid sites nor modify the intrinsic
acidic features of the support in a significant way.
To further consolidate the cause of deactivation during
the reaction, the spent catalysts were analyzed by TGA and
the results are presented in Table 1. The results indicate
the marked higher carbon amount determined on 7CuAl
compared to 8CuSi, 20.6 against 7.0 wt.%. Therefore, this
result confirms the origin of the deactivation. The acidity
of theAl based catalyst favors the formation of carboneous
deposits on the surface due to its acidity, which may
indirectly hinder the reduced copper role in the glycerol
dehydration due to the covering of active sites by carbon.
Thus, it is essential to control the acid-metallic properties in
order to obtain good results of catalytic activity, selectivity
and especially stability.
Conclusions
In this paper, mesoporous mixed copper-aluminum and
copper-silicium oxides were synthesized by the polymeric

2370
Gas-Phase Conversion of Glycerol to Acetol
J. Braz. Chem. Soc.
precursor method and tested as catalysts in the gas-phase
conversion of glycerol to acetol in H2 atmosphere, in mild
temperature conditions, 250 °C and under atmospheric
pressure. The selective glycerol conversion to acetol
depends directly on the copper surface as determined
by XPS analysis performed on in situ reduced solids.
The nature of the support component, Si or Al, does not
influence the initial catalytic performances, using the same
copper content with a similar copper surface, but affect the
stability of the catalytic systems. While Cu supported on
the silica support exhibits a good catalytic stability after
2 h of reaction, the Cu-Al systems exhibit continuous
deactivations which were correlated to the acidity of theAl
support as clearly shown by calorimetry of NH3 adsorption.
The greater acidity for the acidAl support provides a higher
carbon deposit compared to Si neutral support, justifying
the sharp deactivation for the 7CuAl solid due to covering
of the active sites by coke.
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The results of XPS and H2-TPR analysis together with
the results of calorimetry support the mechanism in which
the active species for glycerol dehydration to acetol is a
reduced Cu species CuI and/or Cu0. The acid sites present
on the Cu based catalyst do not participate effectively in
the dehydration step for the experimental conditions used,
playing a secondary role, but they have a negative role on
the catalyst stability due to the excessive carbon deposit.
Therefore, the proposed mechanism should most likely
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already proposed,21,22 is in good agreement with the
reaction product, acetol, which can hardly be formed via
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The authors acknowledge the Universidade Federal do
Ceará (UFC) and Institut de Recherches sur la Catalyse et
l’Environnement de Lyon (IRCELYON) in Lyon-France.
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