Synthesis of Cu-MxOy/Al2O3 (M = Fe, Zn, W or Sb) catalysts for the conversion of glycerol to acetol: Effect of texture and acidity of the supports

Article abstract of DOI:10.1039/c5ra16166e

The purpose of this work is to study the relationships between the copper oxide structure, specific surface area, acidity of M_xO_y/Al₂O₃ (M = Fe, Zn, W and Sb) supports and subsequently the catalytic proprieties of the solids. The samples were characterized by XRD, N₂ adsorption/desorption isotherms and microcalorimetry of NH₃ adsorption. The XRD results (after and before reaction) of the Fe and Zn-containing solids present the formation of a copper-modified alumina structure (high dispersion of Cu species with a strong interaction with the support); however, it is not observed a copper-modified phase for the catalysts composed of W and Sb, since it was identified the isolated CuO phase (a lower dispersion of Cu with a poor interaction with the support). N₂ adsorption/desorption isotherms analysis shows that the copper-modified alumina samples are mesoporous materials and have a high surface area compared to the other catalysts, which did not presented a copper-modified phase. The catalytic tests ascribed that the presence of a copper-modified structure improves the activity, selectivity and mainly the stability for conversion of glycerol to acetol. W and Sb-based samples present low stability in glycerol transformation to acetol probably due to its acidity leading to formation of coke, blocking the active sites as a result of the low number of sites per area (low surface area) as well as the sintering of the copper phase. Thus, the results explain that the choice of metal elements in the solids composition affects its structural, textural, acidity proprieties and consequently the catalytic performance.

Full text of DOI:10.1039/c5ra16166e

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Page 1 of 22
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DOI: 10.1039/C5RA16166E
Synthesis of Cu-M
x y 2 3
O /Al O (M=Fe, Zn, W or Sb) catalysts for the conversion of
glycerol to acetol: Effect of texture and acidity of the supports.
c*
b
a
Tiago Pinheiro Braga , Nadine Essayem and Antoninho Valentini
a
Langmuir - Laboratório de Adsorção e Catálise, Departamento de Química Analítica e
Físico-Química, Universidade Federal do Ceará, Campus do Pici, CEP 60455-970,
Fortaleza CE, Brasil.
b
Institut de recherches sur la catalyse et l’environnement, 2 av. A. Einstein, 69626
Villeurbanne Cedex, France.
c
LABPEMOL-Laboratório de Peneiras Moleculares, Federal University of Rio Grande do
Norte, Campus do Pici, CEP 59078-970, Natal-RN, Brazil.
** Corresponding author. Tel: +33 (0) 472 445 399; fax: +33 (0) 472 445 399.
Eꢀmail addresses: tiagoquimicaufrn@gmail.com (Tiago P. Braga)
Abstract
The purpose of this work is to study the relationships between the copper oxide
structure, specific surface area, acidity of M O /Al O (M=Fe, Zn, W and Sb) supports and
x
y
2
3
subsequently the catalytic proprieties of the solids. The samples were characterized by XRD,
adsorption/desorption isotherms and microcalorimetry of NH adsorption. The XRD
N
2
3
results (after and before reaction) of the Fe and Znꢀcontaining solids present the formation of
a copperꢀmodified alumina structure (high dispersion of Cu species with a strong interaction
with the support); however, it is not observed a copperꢀmodified phase for the catalysts
composed of W and Sb, since it was identified the isolated CuO phase (a lower dispersion of
2
Cu with a poor interaction with the support). N adsorption/desorption isotherms analysis
shows that the copperꢀmodified alumina samples are mesoporous materials and have a high
surface area compared to the other catalysts, which did not presented a copperꢀmodified
phase. The catalytic tests ascribed that the presence of a copperꢀmodified structure improves
the activity, selectivity and mainly the stability for convhjmersion of glycerol to acetol. W and
Sbꢀbased samples present low stability in glycerol transformation to acetol probably due to its
acidity leading to formation of coke, blocking the active sites as a result of the low number of
sites per area (low surface area) as well as the sintering of the copper phase. Thus, the results
explain that the choice of metal elements in the solids composition affects its structural,
textural, acidity proprieties and consequently the catalytic performance.
Keywords: copper-modified; alumina; stability; glycerol; acetol.

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Introduction
Copperꢀbased catalysts are well known as very active heterogeneous catalysts for various
1
2
reactions, such as steam reforming of methanol, CO oxidation and dehydrogenation
3
reactions. Particularly, copperꢀcontaining catalysts are being studied for the hydrogenolysis,
4
, 5, 6
dehydrogenation and dehydration of glycerol.
Glycerol is a major byproduct in the
biodiesel manufacturing process. The declining price of glycerol has sparked much interest in
7
, 8
transforming crude glycerol through catalytic processes. Specifically, the catalytic synthesis
of acetol from glycerol has received great attention, since it is the product of great commercial
value, which is used as reduced dyes, food industry (give aroma to foods), flavor compounds
in heated milk, skin tanning agent; it is also applied to produce chemicals such as propylene
glycol, propionaldehyde, acetone and furan derivatives (quite valuable chemical
9
ꢀ11
intermediate).
In addition, acetol is an interesting molecule from an astrophysical
12, 13
perspective.
There are many cases using the acidic catalysts in the chemical industries or for the
14, 15, 16
production of fine chemicals.
Alcohols dehydration is an important catalytic test to
1
7
identify acid in heterogeneous catalysts. Thus, a number of solid acid catalysts for the
dehydration of glycerol (polyol) have been reported including metal sulphates and
1
8, 19
phosphates, zeolites, metal oxides (Al
2
O
3
, TiO
2
, ZrO
2
) and heteropoly acids.
Furthermore,
Sato et al showed that copperꢀbased catalysts combined with an acidic support may promote
7
the dehydration of glycerol to form acetol with interesting catalytic properties.
In spite of this, these catalysts need to overcome various problems including harsh
reactions conditions, rapid catalyst deactivation due to coke deposition and the formation of
20, 21
unrequired byꢀproducts as well as the thermal catalytic sintering.
Therefore, the
development of a catalyst resistant to deactivation during the conversion of glycerol mainly in
gas phase is one of the most interesting research challenges.
The strong metal–support interactions (SMSI) between Cu phases and surface oxygen
22
defects are supposed to have a positive effect on catalyst proprieties. CuAl O and copperꢀ
2
4
modified alumina is formed by solid–solid interaction between CuO and Al
addition of small amounts of a certain foreign oxide such as ZnO and CeO to the system has
found to positively influence in the solid–solid interaction between CuO and Al O . These
2 3
O and the
2
2
3
23, 24
effects may also affect in the catalytic performance.
Recently, CuꢀZnOꢀbased catalysts
were found to be extremely efficient for catalytic hydrogenolysis of glycerol. Additionally,
high catalyst stability during the transformation of glycerol to propylene glycol was obtained

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2
5, 26
over CuꢀZnOꢀAl
alumina due to the interaction between CuO and Al
2 5
foreign oxide such as Fe , ZnO, WO and Sb O
2
O
3
based samples.
It is important to emphasize that the copperꢀmodified
and the effect of the addition a certain
on the copper oxide structure and on the
2
O
3
2
O
3
3
textural properties of the supports and consequently on the gasꢀphase conversion is rarely
reported in the literature, requiring further investigation.
Our objective is to investigate the effects of copper oxide structure as well as the
x y 2 3
texture and acidity of M O /Al O (M=Fe, Zn, W and Sb) supports on the gasꢀphase
conversion of glycerol to acetol.
Experimental section
Catalyst preparation
Polymeric precursor method
For 10FeAl catalyst, aluminum nitrate nonahydrate {Al(NO
nonahydrate {Fe(NO ·9H O}, citric acid monohydrate (CA){C
glycol (EG) {C H O } were used as starting chemicals. A CA/metal ratio of 2:1 (mol) was
3
)
3
2
·9H O}, iron nitrate
3
)
3
2
6
H
8
O
7
·H O}, and ethylene
2
2
6
2
used for all the samples. The metal amount is the sum of M (M=Fe, Zn, W or Sb) and Al. The
mass ratio of CA/EG was kept at 2:3. The sample was labeled XMAl, with X denoting the
M/Al weight ratio (10FeAl, 10ZnAl, 10SbAl and 10WAl).
For the synthesis process of the 10FeAl sample, where 0.07 is the Fe/Al weight ratio,
0.0075 and 0.11 mol of Fe and Al salts, respectively, were dissolved in distilled water at room
temperature. CA (0.2269 mol) was dissolved in ethanol at 50 ºC. Later the aqueous solution
was added to the CAꢀethanol solution and stirred during 60 min at 50 ºC. Subsequently, the
EG (0.3403 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 250 ºC for 1 h under air. The resulting precursor
composite was ground and treated at 500 ºC under air flow for 1 h. The same conditions were
used to prepare 10ZnAl, 10SbAl and 10WAl catalysts.
Wet impregnation method
Copperꢀbased catalysts were prepared by wet impregnation from 10FeAl, 10ZnAl,
10SbAl and 10WAl samples synthesized by polymeric precursor method (as described above)
with aqueous solutions using appropriate amounts of copper nitrate trihydrate
{
Cu(NO ) ·3H O} in a rotary evaporator. After impregnation, the samples were calcined at
3 2 2
500°C in muffle furnace for 1h (5°C/min). The sample was denominated XCuMAl, which X

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representing the percentage by weight of copper (5CuFeAl, 5CuZnAl, 5CuSbAl and
CuWAl).
Catalyst characterization
The crystal structure of the metal oxides was characterized by Xꢀray diffraction
analysis (XRD), with CuKα irradiation source (λ = 1.540 Å, 40 kV and 40 mA). Specific
surface area (BET) and pore volume of the catalysts were determined by N
5
2
adsorption/desorption isotherms, at liquid nitrogen temperature. The catalyst samples (80 mg)
were vacuum treated at 200 °C for 1 h.
3
The acid properties were measured by NH adsorption at 80° C, using a TianCalvet
calorimeter coupled with a volumetric equipment. The solids (0.1 g) were first vacuum treated
at 400 °C for 2 h. After the preꢀtreatment, 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 NH up to equilibrium and the differential enthalpy of adsorption was recorded
3
together with the amount of adsorbed ammonia.
Thermogravimetric analysis (TG) was carried out to estimate the amount of carbon
deposited, using a 5 ºC/min heating rate, under an air flow of 40 ml/min and 30 mg of sample.
Catalytic activity
Catalytic tests were carried out in a fixed bed microreactor, using 200 mg of sample.
2
All fresh catalysts were firstly reduced in H (25 mL/min) at 350°C for 1 h. After the
temperature of the catalyst bed had been maintained at 350°C in hydrogen flow for 1 h, an
aqueous solution of glycerol at 10 wt.% was fed through the reactor top by a microꢀpump at
3
ꢀ1
the feed rate of 0.03 cm .min , which corresponded to 0.001 mol of glycerol per minute, in
flow at 25 mL/min. The catalysts were tested at atmospheric pressure. The liquid products
H
2
were collected in an iceꢀwaterꢀsalt trap (ꢀ6 °C) in order to be analyzed every hour on a gas
chromatograph with a mass spectrometer and gfha gas chromatograph connected to a
hydrogen flame ionization detector (FIDꢀGC), using a capillary column purchased from
Varian, CP 9048 (length: 30 m, diameter: 0.32 mm, thickness of the stationary phase: 0.1
mm). Carbon balances were between 80 and 96 % in the catalytic experiments. Glycerol
conversion and acetol selectivity were calculated in the following way:
Amount of glycerol consumed mol
Cglycerol
=
x100
Amount of glycerol introduced to the reactor
Amount of obtained acetolmol
Sacetol
=
x100
Amount of glycerolconsumedmol
(2)

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Results and discussion
X-ray diffraction
The Xꢀray diffraction patterns are presented in Figure 1. The crystalline phases were
identified in comparison with ICDD files. The 10FeAl, 10ZnAl, 10SbAl and 10WAl samples,
without copper, are poorly crystalline material and present Xꢀray patterns of an amorphous
solid, Figure 1ꢀa. In spite of this, 10WAl catalyst show a broader peak related to WO phase
3
formation (JCPDS 04ꢀ013ꢀ0859). The XRDꢀamorphous diffractograms probably indicate that
the metal oxides structures are highly dispersed in the Al
features for catalytic supports.
2 3
O matrix, which are interesting
On the other hand, copperꢀcontaining solids prepared by wet impregnation method
presented peaks related to the formation of crystalline phases, Figure 1ꢀb. 5CuSbAl and
5
0
CuWAl samples points to the formation of a copper oxide phase (CuO, JCPDS 04ꢀ045ꢀ
937), besides the presence of a broader peak, concerning the formation of tungsten oxide
(WO
3
, JCPDS 04ꢀ013ꢀ0859) for 10WAl catalyst.
Contrarily, the 5CuFeAl and 5CuZnAl samples also synthesized by wet impregnation
route present broader peaks, which suggest the formation of a copperꢀmodified alumina phase
CuOꢀγꢀAl , JCPDS 00ꢀ034ꢀ0425ꢀFe and JCPDS 00ꢀ001ꢀ1153ꢀZn), indicating that copper
(
2 3
O
oxide is well dispersed in the support and probably interacting with its structure.
From these results one can notice that the choice of metal elements affect the structural
proprieties of the samples, whereas the Zn and Feꢀbased catalysts stimulate the formation of a
copperꢀmodified phase (highly dispersed copper oxide) compared to the Sb and Wꢀcontaining
solids, which showed the formation of single CuO (copper oxide with low dispersion and less
interaction with the support).
2 3
As mentioned above, the CuOꢀγꢀAl O (High dispersed copper oxide phase) is
probably formed as a result of the thermal solidꢀsolid reaction according to the following
reaction: CuO + Al O → CuOꢀγꢀAl O , which indicates that CuO diffused onto the surface
2
3
2
3
of defective spinelꢀtypeꢀγꢀAl
2 3 2 3
O . This indicate that CuOꢀA1 O interaction is promoted in the
samples composed of Fe and Zn, however, it is not possible in the catalyst containing Sb and
2
+
W. It is known that the addition of small amounts of other cations, such as Zn may influence
27ꢀ30
the chemical interaction between the CuO and Al
2
O
3
,
which may justify the results
observed in Figure 1. The close relationship of charge/radius ratio for Zn and Fe compared to
Cu or Al may justify this interaction for the Zn and Feꢀbased catalysts. However, the
relationship of charge/radius ratio for W and Sb retated to Cu or Al is not very close, which
may explain the difficulty of interaction between species. Previous papers demonstrate that

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the copper oxide crystalline structure directly affects on the catalytic performance during the
3
1, 32
conversion of glycerol.
Therefore, in this paper, the transition phase was designated Cuꢀ
, whereas it is not observed
modified γꢀAl , which the copper is highly dispersed on γꢀAl O
2
O
3
2 3
defined XRD peaks concerning the crystalline phase of copper oxide.
Figure 1
The average crystallite diameter was estimated from the diffraction data using the
Scherrer formula, and the results are shown in Table 1. The average crystallites diameters of
the copperꢀmodified crystallites were found between 3.9 and 4.3 nm. Furthermore, the
average crystallite size for CuO phase was found between 4.8 and 5.7 nm. The reflections are
quite broad for the copperꢀmodified peaks, indicating the formation of fine nanostructure.
The Xꢀray diffraction may be used to indirectly measure (qualitatively) the
metal oxides dispersed throughout the base matrix from the diffractograms profile. Despite no
quantitative characterization of the copper dispersion/surface area has been performed, XRD
resultds is a good indication that the 5CuFeAl and 5CuZnAl samples synthesized by Pechini
method is a successful method for preparing nanometer-sized containing copperꢀmodified
alumina structure with high metal dispersion and strong interaction copper support compared
to 5CuWAl and 5CuSbAl solids, which probably will have very interesting catalytic
properties, since the catalytic activity usually has a direct relationship with the dispersion of
active sites.
Table 1
Specific surface area measurements
The nitrogen adsorption/desorption isotherms of the calcined sample are shown in
Figure 2 and subsequently the textural properties obtained are presented in Table 2.
Figure 2
Only the catalyst 10WAl exhibits a classic type I isotherm characteristic of
microporous materials according to the IUPAC classification. Contrarily, the other solids
(10FeAl, 10ZnAl, 10SbAl, 5CuFeAl, 5CuZnAl, 5CuSbAl and 5CuWAl) present a typical
type IV curve with H2 hysteresis loops, also characteristic of mesoporous materials. These
observations are confirmed by the pore diameter distribution.
Table 2 lists the surface area (SBET), total pore volume (Vp) and average pore diameter
(Dp) of solids. The 10FeAl, 10ZnAl, 10SbAl and 10WAl samples, without copper, presented

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2
a range of specific surface area between 330 and 244 m /g. Therefore, one can note that the
choice of metal elements (Fe, Zn, Sb or W) change the textural proprieties values of the
supports.
However, after the addition of copper by wet impregnation one can observed
changes in the surface area, total pore volume and average pore diameter data. For example,
the specific surface area for the 5CuZnAl materials compared to the 10ZnAl catalyst increases
2
from 244 to 409 m /g and the specific pore volume of the solid also increases from 0.20 to
3
0
.45 cm /g. On the other hand, the SBET for the 5CuWAl catalyst compared to the 10WAl
2
sample decrease from 294 to 32 m /g and the Vp of the solid also decrease from 0.16 to 0.04
3
cm /g.
Table 2
These textural effects in the porosity of the solids may be related to the information
evidenced in the XRD patterns (Figure 1). That is probably manifested by phase segregation,
chemical interactions and polymorphous transformations of the alumina support interacting
with the copper oxide. Interactions of the alumina support and copper oxide and formation of
copperꢀmodified structure, may explain the significant changes in the textural properties; this
phenomenon was observed in the Zn and Feꢀbased catalysts, however, it is not showed in the
Sb and Wꢀcontaining samples. Thus, the choice of metal elements affects clearly its structure
and texture, as observed in Figure 1 and 2 as well as the Table 1 and 2. The structural change
2
+
accompanied by the significant increase of surface area after impregnation of Cu and
recalcination using this synthetic route was previously explained for the catalysts containing
pure copper oxide and aluminum oxide due to the the redissolution and recrystallization of
transition alumina.
Microcalorimetry of NH adsorption
3
Calorimetric measurements are generally considered as reliable techniques to measure
the acidic strength of solid acids. Thus, the catalysts without and with copper were also
analyzed by calorimetry of NH
3
adsorption in order to determine the total acidity and the acid
strength distribution, Figure 3.
Figure 3

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Initially, the sample 10ZnAl, 10FeAl, 10SbAl and 10WAl, without copper, presented heat
of ammonia adsorption of 193, 161, 126 e 52 kJ/mol, respectively, Figure 3ꢀa. Therefore, the
first values confirm the following order of acid strength: 10ZnAl>10FeAl>10SbAl>10WAl
for the copperꢀfree solids. However, with greater coverage of ammonia, increasing the
3
number of moles of NH , it is noticed a progressive decrease in the heat of ammonia
adsorption, reaching a value of 36 and 8 kJ/mol for the samples 10ZnAl and 10WAl,
respectively. This profile characterizes the heterogeneity of the present acid sites, and that the
1
0ZnAl catalyst, despite higher acidity sites compared to other solids, these sites are present
in lower amount. Nevertheless, the sample 10WAl shows a profile, which indicates a solid
with a weak acidity and low amount of sites. Contrary, the supports 10FeAl and 10SbAl did
not show a rapid decrease of the heat of adsorption with the addition of the molecule probe
doses, indicating that these samples have higher amount of acid sites, with superior
homogeneity.
Figure 3ꢀc, relative to the samples without copper, which the number of moles of
ammonia adsorbed were normalized by the specific surface area, obtained from the N2
adsorption/desorption isotherms (Table 2). It is noted a profile similar to the Figure 3ꢀa
probably due to the fact that the surface area of these solids are very close.
On the other hand, after the addition of copper by wet impregnation, it was observed
differences in the microcalorimetry data, Figure 3ꢀb. In the first dose, the heat of adsorption
for the samples 5CuFeAl, 5CuWAl, 5CuSbAl and 5CuZnAl were 192, 140, 139 and 114
kJ/mol, respectively, indicating the following acid strength: 5CuFeAl>5CuWAl
≈
5CuSbAl>5CuZnAl. However, increasing doses of ammonia, it was observed an abrupt
drop of heat of NH adsorption with values of 27 and 25 kJ/mol, concerning the 5CuWAl and
3
5
CuSbAl catalysts, respectively. Therefore, it may be related to the elevated heterogeneity of
sites and the low surface area for these two solids (Table 2), providing lower number of acid
sites per gram of material. However, the 5CuFeAl and 5CuZnAl materials, otherwise shows
3
only a slight decrease in acidic sites with increased content of adsorbed NH and consequently
a better homogeneity (better distribution of the acids sites), as well as a higher amount of
adsorbed sites per gram of material. The high surface areas of these catalysts provide greater
number of acid sites.
Thus, the fact that 5CuWAl and 5CuSbAl samples indicate a rapid decrease in the
adsorption heat with the increase of the amount adsorbed may be correlated with the obtained
textural properties by N
2
adsorption/desorption isotherms, Figure 2 and Table 2. Among the
synthesized catalysts, Sb and Wꢀcontaining solids presented a low total pore volume (Table

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2
), hence, the ammonia accessibility towards the acid sites may be limited for these two
materials. This fact is confirmed in Figure 3ꢀd, in which the number of moles of NH was
normalized by surface area values shown in Table 2. It is noticed that the sample 5CuSbAl,
3
5
5
CuFeAl and 5CuZnAl presented a similar amount and strength of acid sites. The catalyst
CuWAl confirms a minor amount of acidic sites. Thus, if the reaction which the catalysts is
dependent on acid sites (as the glycerol conversion), the surface area influence the number of
adsorption sites and will certainly influence on the catalytic performance.
Glycerol dehydration is a classic acid catalyzed reaction. The amount, strength and
type of acid sites (Brønsted or Lewis) strongly affect the catalytic performance of solid acid
19, 33
catalysts.
It is well known that the product selectivity from glycerol dehydration depends
on which hydroxyl group is firstly removed, terminal or secondary hydroxyl. Acetol is the
product when the terminal hydroxyl is initially removed from glycerol. On the other hand, 3ꢀ
hydroxypropanal is formed as an intermediate, when the secondary hydroxyl group is
33, 34
abstracted first, and posteriorly is converted to acrolein during the second dehydration.
Furthermore, the type of acid sites (Bronsted and/or Lewis) is well known to affect the
glycerol dehydration mechanism. The Bronsted acid sites favor the removal of middle
hydroxyl groups to form acrolein, and Lewis acid sites tend to remove the terminal hydroxyl
33, 34
groups toward acetol.
Thus, it is expected that the synthesized catalysts have higher
selectivity to acetol, since unhydrated metal oxides (MꢀO) solids preferably have Lewis sites.
Catalytic activity
The conversion of glycerol and the acetol selectivity as a function of time for the four
different catalysts (10FeAl, 10ZnAl, 10SbAl and 10WAl), without copper, is shown in Figure
4
.
Figure 4
The results explain that the samples 10ZnAl, 10SbAl and 10WAl are almost inactive
after 300 min of reaction, indicating that only the acid properties probably play a secondary
role during the transformation of glycerol, since the pure supports showed no interesting
activities in reaction. On the other hand, 10FeAl catalyst presents higher activity and stability
compared to the others three solids. Additionally, Figure 4ꢀb presents that the solid 10ZnAl,

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1
0FeAl and 10SbAl area higher selectivity for the acetol formation. Contrarily, 10WAl
sample, especially during the first 60 minutes, presented a large amount of acrolein.
2
It is reported in literature that the promotional effect of H is related to the formation
δ+
δ−
n
of H (WO
3
)
Brønsted acid sites through the local reduction of polytungstate domains,
without the removal of lattice oxygen atoms. These sites catalyze dehydration reactions much
33,
more effectively than acidic OH groups in W–O–W species under a reducing environment.
3
5, 36
The presence of Bronsted sites for the Wꢀbased solids may justify the observed
selectivity to acrolein for the catalysts 10WAl and 5CuWAl. The acrolein was observed only
for these two samples.
On the other hand, all the four different Cuꢀcontaining catalysts (5CuFeAl, 5CuZnAl,
5
CuSbAl and 5CuWAl) are very active during the first sixty minutes of reaction, Figure 5ꢀa.
These results propose that copper species combined with an acid site provides an active
species for the conversion of glycerol to acetol formation. However, the differences in
catalytic stability can be observed after 300 min of time on stream. 5CuSbAl and 5CuWAl
samples present a rapid deactivation, while the 5CuFeAl and 5CuZnAl solids show an
excellent performance for the conversion of glycerol to acetol (high stability).
Figure 5
The profiles of acetol selectivity are presented in Figure 5ꢀb. 5CuFeAl and 5CuZnAl
catalysts display similar acetol selectivity, near 90%, after 4 h of time on stream. Conversely,
5
CuWAl sample demonstrate lower acetol selectivity probably due to the tungsten ability to
form bronsted acid sites, hence, leading to the formation of unrequired products such as
acrolein.
Sato et al. studied the vapour phase glycerol dehydration/dehydrogenation to acetol,
on copper based catalysts, which the dehydrogenation was catalyzed by Cu through the
formation of Cu alkoxide species. Glycerol may be dehydrated through the dehydrogenation–
dehydration–hydrogenation sequence of 1,3ꢀpropanediol. In analogy to the dehydration of
1
,3ꢀpropanediol, glycerol may be converted to 2ꢀhydroxypropanal, which is readily
tautomerized to acetol. It was mentioned that the presence of an acid support may promote the
7
, 34
dehydrationvia elimination of OH anion, optimizing the conversion of glycerol to acetol.
Thus, copperꢀbased catalysts combined with an acid support are very selective to acetol
according to the mechanism proposed in the literature, in agreement with the results presented
in Figure 5 and 6.

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The other products selectivity (1,2ꢀpropanediol, acrolein, acetaldehyde, allyl alcohol,
acetone and condensation products) is presented in Table 3. For example, the sample 5CuWAl
presented the 1,2ꢀpropanediol and others products selectivity of 31.2, and 34.6%, respectively.
Table 3
The gradual catalyst deactivation for the 5CuSbAl and 5CuWAl solids, observed in
Figure 5ꢀa, can be attributed to high sintering of copper (low metal support interaction) due
the formation of single CuO phase (low metallic dispersion), Figure 1 and 7. It is very well
known that copper species are quite favorable to suffer sintering during the catalytic process
due to the drastic reaction conditions for the transformation of glycerol in gas phase. The
acidity of the WO /Al O and Sb O /Al O supports (Figure 3ꢀb e 3ꢀd), and consequently, the
3
2
3
2
5
2
3
formation of undesired byꢀproducts causing the carbon deposit may also favor the catalytic
deactivation (covering of the sites and blockage of pore). The low dispersion of copper
species (wellꢀdefined peaks observed in the XRD patterns) e and the low number of sites per
gram (low surface area, Table 2) for the 5CuSbAl and 5CuWAl samples favor the
rapid deactivation by carbon deposit (blocking active sites), justifying the sharp drop in
glycerol conversion after 60 min.
On the contrary, the best stability for 5CuFeAl and 5CuZnAl catalysts can be ascribed
the presence of a copperꢀmodified phase (high metallic dispersion, strong metal support
interaction and higher surface area), which presents greater resistance to suffer sintering (high
structural stability), as observed in XRD results, Figure 1 and 7. The carbon deposit due to its
acidity (Figure 3ꢀb e 3ꢀd) is also occurring for these two catalysts, however, the greater
number of sites per area (Table 2) and the largest dispersion of the copper sites (Figure 1)
slows the total coverage of active sites and blockage of pore by carbon and the catalyst
deactivation, favoring its high conversion and stability (Figure 5 and 6).
From these results presented one can note that the structural, textural and acidity
proprieties and accordingly the catalytic performance, depend on the choice of metal elements
employed (Fe, Zn, Sb and W) due to the fact that the Fe and Znꢀbased samples presented the
copperꢀmodified structure, greater number of acid sites per area and higher surface area,
subsequently, better conversion and high stability during the glycerol to acetol related to W
and Sbꢀbased solids.
It is important to highlight that experiments were performed in the presence of
hydrogen in order to promote the hydrogenation of acetol to 1,2ꢀpropanediol, however, under

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the applied reaction conditions the reaction rate in the hydrogenation is much lower compared
to the reaction rate of the dehydration of glycerol, justifying the higher selectivity to acetol
related to the 1,2ꢀpropanediol (Table 3 and Figure 4b). The low selectivity to 1,2ꢀpropanediol
compared to acetol is due to the fact that under atmospheric pressure and elevated temperature
its production is disadvantaged, since the hydrogenation of acetol is an exothermic reaction.
3
4, 37
The remarkable activity and stability for the 5CuFeAl catalyst make the study of recycling
particularly interesting. The four catalytic tests are presented in Figure 6. For recycling
experiments, the sample used in a first run was reduced again in H (25 mL/min) at 350°C for
2
1
h after each run and reused in the conversion of glycerol to acetol at 250°C for 6h, as
described for the fresh catalysts.
These results clearly indicated that for the 5CuFeAl sample no significant decrease in the
catalytic activity and acetol selectivity were observed after the second recycling test.
However, the conversion of glycerol achieved over 63 % after the fourth recycling
experiment, but the acetol selectivity is similar to the first run. Thus, the results ascribed that
the catalytic activity and the acetol selectivity did not decrease dramatically during the
recycling experiments, considering that no special treatment (reoxidation of deposited coke)
was employed after every reuse. Thus, the recycling experiments also confirm the high
resistance to deactivation for the 5CuFeAl solid, as previously mentioned, is due to high
metallic dispersion, strong metal support interaction and high surface area, according to the
2
XRD and N physisorption results.
Figure 6
In order to obtain information concerning the reasons of the decrease in the conversion
of glycerol, principally for the samples 5CuSbAl and 5CuWAl, the TG analysis was carried
out for all the Cuꢀbased catalysts, after the catalytic test. The TG results for all the samples
present a carbon elimination (burning) in the temperature range of 300 and 600ºC; it shows
that the carbon deposit is more pronounced for the 5CuZnAl sample, as it is illustrated in
Table 3. Furthermore, a relationship between the amount of acid sites from calorimetric
measurements (Figure 4ꢀb) and the carbon deposited is observed, indicating that a higher
acidity leads to a superior coke deposition.
Additionally, the spent Cuꢀbased solids were also analyzed by XRD in order to obtain
information relating to the thermal sintering (structural stability) and the metalꢀsupport
interaction. The diffractograms are presented in Figure 7 and the average crystallite sizes were
estimated using the Scherrer’s formula, Table 4.

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Figure 7
The diffraction pattern presented in Figure 7 for the 5CuSbAl and 5CuWAl samples
2+
suggest the Cu reduction during the reaction process. The results of the Sb and Wꢀbased
catalysts point to the cuprous oxide formation (Cu O, JCPDS 04ꢀ003ꢀ6433) and metallic
2
copper phase (Cu, JCPDS 01ꢀ071ꢀ4611), indicating that the oxidation states of copper are 0
and +1 after reaction.
Great attention was given to the transformation of the catalytic phase during reaction,
6
, 7
and the oxidation state of copper during the conversion of glycerol.
It was already
mentioned that the presence of metallic copper may favor the conversion of glycerol to acetol.
3
8
On the other hand, the XRD patterns for the 5CuFeAl and 5CuZnAl catalysts ascribed
that the copperꢀmodified alumina structure (JCPDS 00ꢀ034ꢀ0425ꢀFe and JCPDS 00ꢀ001ꢀ
153ꢀZn) was maintained after transformation of glycerol, whereas that the Fe and Znꢀ
containing diffractograms after the reaction (Figure 7) are very similar to the fresh catalyst
Figure 1), which may justify the best catalytic stability, Figure 5ꢀa. Thus, the copperꢀsupport
interaction for the 5CuFeAl and 5CuZnAl samples was stronger compared 5CuSbAl and
1
(
5
CuWAl to solid, justifying its better stability.
The average crystallite diameter using the Scherrer formula for the spent catalysts is
illustrated in Table 4. The average crystallites diameters of the metallic Cu particles were
found between 28.0 and 42.0 nm for the 5CuSbAl and 5CuWAl samples. Contrarily, the
copperꢀmodified phase for the 5CuFeAl and 5CuZnAl catalysts show high sinteringꢀresistance
ability (strong metalꢀsupport interaction) due to the particle sizes after and before reaction are
very similar, as can observed in Table 1 and 4.
Table 4
Conclusions
The XRD, N adsorption isotherms and microcalorimetry of NH adsorption results
2
3
showed that the choice of metal elements affect its structure, texture, acidity and subsequently
the catalytic performance. Interactions of the copper oxide and aluminum oxide and formation
of copperꢀmodified alumina phase occurred in the Fe and Znꢀcontaining solids, however, it is
not observed in the Sb and Wꢀbased samples. The solids composed of Cu/Zn/Al and Cu/Fe/Al
showed the best performance for conversion of glycerol to acetol. The catalysts with a copperꢀ

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modified structure (Fe and Znꢀbased solids) achieved higher catalytic stability compared to
the samples containing only isolated copper oxide (W and Sbꢀcontaining solids). It was found
that sintering of Cu was more pronounced in the Sb and Wꢀbased samples related to Fe and
Znꢀcontaining solids.
Acknowledgments
The authors acknowledge the “Universidade Federal do Ceará” (UFC) and Institut de
recherches sur la catalyse et l’environnement (IRCE) in LyonꢀFrance.
Notes and references
1
2
3
4
5
6
7
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3

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Tables
Table 1. Average crystallite sizes of the Cuꢀcontaining solids after calcination process.
Samples crystallite sizes (nm)
CuO
ꢀ
2 3
CuOꢀγꢀAl O
5
CuFeAl
3.9
ꢀ
5
CuSbAl
29.8
26.3
ꢀ
5
CuWAl
ꢀ
5
CuZnAl
4.3
Table 2. Textural properties of the samples.
2
3
Samples
0FeAl
SBET (m /g)
330
Vp (cm /g)
Dp (nm)
3.0
1
0.30
0.20
0.16
0.16
0.30
0.45
0.03
0.04
10ZnAl
10SbAl
244
4.1
245
4.2
1
0WAl
CuFeAl
CuZnAl
CuSbAl
CuWAl
294
2.6
5
270
4.1
5
409
3.9
5
18
5.3
5
32
4.1

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Table 3. Catalytic selectivity observed for others products and amount of coke for the spent
catalysts.
Samples
Selectivity (%)*
**
1
,2ꢀpropanediol
Others products
C (wt%)
3.4
5
CuFeAl
8.9
10.0
31.2
4.6
4.6
5
CuSbAl
13.8
34.6
10.1
2.0
5
CuWAl
1.1
5
CuZnAl
7.9
*
Values for reaction period of 60 min. C: Carbon content in catalysts after reaction from TGA
**
analysis. Other products: acrolein, acetaldehyde, allyl alcohol, acetone and condensation
products.
Table 4. Average crystallite sizes of the Cuꢀbased solids for the spent catalysts.
Samples
crystallite sizes (nm)
Cu
Cu
2
O
2 3
CuOꢀγꢀAl O
5
CuFeAl
ꢀ
ꢀ
4.9
ꢀ
5
CuSbAl
10.0
18.2
ꢀ
41.8
28.0
ꢀ
5
CuWAl
ꢀ
5
CuZnAl
4.5
Figures
Figure 1
o
o
o
(
(
(
o) CuO-γ−Al O3
o
2
WO3
*
-
*) CuO-γ−Al O3
*
2
-
-
b
a
-) CuO
*
*
-
*
-
-
10FeAl
5CuFeAl
5
CuSbAl
CuWAl
1
0ZnAl
1
0SbAl
5
10WAl
5CuZnAl
5
10 15 20 25 30 35 40 45 50 55 60 65 70
15 20 25 30 35 40 45 50 55 60 65 70 75
2θ (°)
2θ(°)

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Figure 2
2
1
1
00
50
00
2
1
1
00
50
00
5CuFeAl
1
0FeAl
0ZnAl
a
b
5
0
5
0
1
20
300
9
6
3
0
0
0
200 5CuZnAl
100
20
1
1
00
1
5
5CuSbAl
10SbAl
8
6
0
0
10
5
3
0
9
6
3
0
20 5CuWAl
0
10WAl
10
0
0
0
0,0 0,2 0,4 0,6 0,8 1,0
,0 0,2 0,4 0,6 0,8 1,0
Relative pressure (P/P0)
Relative pressure (P/P0)

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Figure 3
2
1
160
1
1
1
8
6
4
00
80
2
1
1
00
60
20
1
1
1
1
0FeAl
0SbAl
0WAl
0ZnAl
a
b
5
CuFeAl
5CuSbAl
5
5
CuWAl
CuZnAl
40
20
00
0
0
0
8
0
0
0
4
20
0
50 100 150 200 250 300
n NH adsorbed (micromol/g)
0
70 140 210 280 350 420
n NH adsorbed (micromol/g)
3
3
2
1
160
00
80
2
1
1
00
c
1
1
1
1
0FeAl
0SbAl
0WAl
0ZnAl
d
60
20
140
120
100
5CuFeAl
5CuSbAl
5CuWAl
5CuZnAl
8
4
0
0
0
8
6
4
0
0
0
20
0
.0 0.2 0.4 0.6 0.8 1.0
2
0.0 0.5 1.0 1.5 2.0 2.5
2
n NH adsorbed (micromol/m )
n NH adsorbed (micromol/m )
3
3

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Figure 4
1
00
a
1
1
1
1
0FeAl
0ZnAl
0SbAl
0WAl
8
6
4
2
0
0
0
0
0
1
1
1
20
10
00
acetol selectivity
1,2-propanediol selectivity
propanoic acid selectivity
b
Others (acrolein, acetaldehyde,
allyl alcohol, acetone and condensation products)
9
8
7
6
5
0
0
0
0
0
40
3
2
1
0
0
0
0
1
00
200
time (min)
300
1
0FeAl
10ZnAl
10SbAl
10WAl
Catalyts
Figure 5
00
1
00
90
1
a
b
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
8
7
6
0
0
0
5CuFeAl
5
5
5
CuSbAl
CuWAl
CuZnAl
50
4
3
2
1
0
0
0
0
0
0
100
200
300
0
100
200
300
time (min)
time (min)

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Figure 6
00
5CuFeAl
Conversion of glycerol
Acetol selectivity
1
90
80
70
60
50
40
30
20
10
0
1° day (360min) 2° day (360min) 3° day (360min) 4° day (360min)
Catalyst reusability
Figure 7
.
-o
(
(
(
o) CuO-γ−Al O3
2
o
.
o
*) CuO-γ−Al O4
*
2
*
*
.
*
-) Cu O et (
.) Cu
*
-
2
*
-
-
5
CuFeAl
5
CuSbAl
5
CuWAl
5
CuZnAl
1
5 20 25 30 35 40 45 50 55 60 65 70 75
2θ (°)

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Figure captions
Figure 1. Xꢀray diffraction profile of the samples, after the calcination. a) samples without
copper b) Cuꢀbased catalysts.
Figure 2.
2
N adsorption/desorption isotherms of the catalysts; a) samples without copper b)
Cuꢀbased solids.
3
Figure 3. Differential heat of NH adsorption as a function of ammonia coverage. (a and c):
copper free samples; (b and d): copperꢀbased solids. (a and b): normalized by the mass of
the catalyst; (c and d): normalized by specific surface area from N physisorption results.
2
Figure 4. Catalytic performance of the samples without copper; (a) conversion of glycerol,
(b) product selectivity.
Figure 5. Catalytic tests of the Cuꢀcontaining catalysts: (a) conversion of glycerol, (b) acetol
selectivity.
Figure 6. Catalyst reusability for the sample 5CuFeAl.
Figure 7. Xꢀray diffraction diffractograms of the solids after the catalytic tests.