The first step of the propylene generation from renewable raw material: Acetone from ethanol employing CeO2 doped by Ag

Article abstract of DOI:10.1016/j.cattod.2016.04.038

The catalytic behavior of CeO₂ and this oxide doped with Ag (AgCeO₂) were evaluated for the ethanol reaction to acetone in order to assess the mechanism proposed by Rodrigues et al. (2013). The oxides were characterized by BET, ICP-OES, TPD-NH₃, isopropanol dehydration/dehydrogenation model reaction, TPR-H₂, TPD-H₂O, WGS reaction and TPD-ethanol followed by DRIFTS-MS. The main effect of doping CeO₂ with Ag is the increase of the reducibility of this oxide which leads to a higher selectivity to acetone and a lower towards ethylene. The acetone synthesis reduces the catalyst. Ceria dissociates H₂O, which provides oxidant species for its own re-oxidation. Acetaldehyde is generated by the ODH reaction. The WGS reaction is a very interesting model for the redox reactions when H₂O is employed as an oxidant agent. The mechanism proposed by Rodrigues et al. (2013) describes the behavior of CeO₂ in the acetone synthesis from ethanol.

Full text of DOI:10.1016/j.cattod.2016.04.038

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Contents lists available at ScienceDirect
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The first step of the propylene generation from renewable raw
material: Acetone from ethanol employing CeO doped by Ag
2
∗
Adriana F.F. de Lima, Priscila C. Zonetti, Clarissa P. Rodrigues, Lucia G. Appel
Instituto Nacional de Tecnologia, Divisão de Catálise e Processos Químicos, Av. Venezuela 82/518, 20081-312, Rio de Janeiro, RJ, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic behavior of CeO2 and this oxide doped with Ag (AgCeO ) were evaluated for the ethanol
2
Received 7 December 2015
Received in revised form 29 February 2016
Accepted 29 April 2016
Available online xxx
reaction to acetone in order to assess the mechanism proposed by Rodrigues et al. (2013). The oxides were
characterized by BET, ICP-OES, TPD-NH3, isopropanol dehydration/dehydrogenation model reaction, TPR-
H2, TPD-H2O, WGS reaction and TPD-ethanol followed by DRIFTS-MS. The main effect of doping CeO2
with Ag is the increase of the reducibility of this oxide which leads to a higher selectivity to acetone
and a lower towards ethylene. The acetone synthesis reduces the catalyst. Ceria dissociates H2O, which
provides oxidant species for its own re-oxidation. Acetaldehyde is generated by the ODH reaction. The
WGS reaction is a very interesting model for the redox reactions when H2O is employed as an oxidant
agent. The mechanism proposed by Rodrigues et al. (2013) describes the behavior of CeO2 in the acetone
synthesis from ethanol.
Keywords:
Ethanol
Acetone
Silver
Ceria
Oxidation
© 2016 Elsevier B.V. All rights reserved.
1
. Introduction
and are easily carried out employing commercial catalysts [10,11].
However, there are few studies related to the synthesis of acetone
The commercial production of ethanol from cellulosic residues
straw and sugarcane bagasse) has recently started in the US and
from ethanol [3].
(
Brazil. This is a great opportunity for the generation of chemicals
from ethanol due to the synergies inherent in these facilities.
Ethanol is a very special platform molecule, which is able
to generate many different chemicals by the one-pot processes,
employing multifunctional catalysts [1–7]. These compounds are
the same molecules of the Petrochemical Industry. Thus, chem-
icals generated by fossil raw material can be, straightforwardly,
substituted by compounds synthesized by renewable feedstock, i.e.
ethanol.
Acetone is a very interesting example of a chemical synthesized
from ethanol. This cetone is an important solvent and also used in
the syntheses of drugs and polymers [3,8].
Some authors have proposed that propylene can be produced
from ethanol in three steps. Firstly, acetone is synthesized from
ethanol. Then, this cetone is hydrogenated to isopropyl alcohol, and
after that, this alcohol is dehydrated to propylene. This synthesis
would be carried out in three reactors [9]. Acetone hydrogenation
and alcohol dehydration reactions are described in the literature
2
CH CH OH + H O → CH COCH + CO + 4H
(1)
3
2
2
3
3
2
2
Studying the acetone synthesis from ethanol employing
Cu/ZnO/Al O + ZrO physical mixture, Rodrigues et al. proposed
the following mechanism: firstly, ethanol is dehydrogenated to
acetaldehyde on Cu/ZnO/Al O . This aldehyde migrates to ZrO
and then is oxidized to acetate species, which react (ketonization
reaction) generating acetone and CO . Finally, water is dissociated
on Cu/ZnO/Al O3 and the species generated migrate and reoxidize
2
3
2
2
3
2
2
2
ZrO2 [3].
Nishiguchi et al. [12] showed that CeO2 is a promising and very
simple catalyst for the acetone synthesis from ethanol. Indeed, this
oxide shows strong basic sites and redox properties, and it can
also dissociate H O [13–15]. All these properties are very rele-
2
vant for the acetone synthesis. Moreover, some recent results (not
published yet) of our group found that doping CeO2 with Ag, the
selectivity to acetone increases meaningfully.
Recently, propene has been generated from ethanol in only one
step using CeO2 doped catalyst. The catalytic performance of this
oxide has proven that this synthesis is feasible. However, this cat-
alyst has to be improved in order to reach higher activities and
selectivities [16].
∗ Corresponding author.
E-mail addresses: lucia.appel@int.gov.br, lucia.appel@gmail.com (L.G. Appel).
http://dx.doi.org/10.1016/j.cattod.2016.04.038
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: A.F.F. de Lima, et al., The first step of the propylene generation from renewable raw material: Acetone
from ethanol employing CeO2 doped by Ag, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.038

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2
◦
shows a maximum at a temperature lower than 127 C, the ones
All in all, the aim of this work is to identify the role of Ag on
CeO2 in order to improve the description of the mechanism of the
acetone synthesis from ethanol.
◦
◦
between 127 C and 307 C, medium; and finally, the ones above
◦
307 C, strong basic sites.
The isopropanol dehydration/dehydrogenation model reactions
were employed in order to characterize the acid or basic properties
of the catalysts by comparing the acetone and propylene synthesis
rates [17]. The reactions were carried out using a fixed bed reactor
2
. Experimental
2.1. Catalysts preparation
◦
−1
at 150 C, 1 atm, 300 mg of catalyst and 50 mL min of N flow. Iso-
2
Two catalysts, CeO2 and CeO2 doped with Ag, were syn-
propanol vapors were generated by passing N2 through a saturator
at 10 C. The reagent and products were analyzed by on-line gas
chromatography.
◦
thesized by precipitation and co-precipitation methods using
(NH ) Ce(NO ) ] and AgNO as precursors. A strong base aqueous
[
4
2
3
4
3
solution was added dropwise to a (NH ) Ce(NO ) solution under
The reaction rates of the water-gas shift reaction (WGS) were
measured under differential conditions (CO conversion <10%) at
450 C, 1 atm, 100 mL min and H2O/CO = 1. Reagents and products
were analyzed by on-line gas chromatography.
4
2
3 4
continued stirring at room temperature in order to synthetize CeO2.
The same procedure was used for the CeO2 doped with Ag using a
◦
−1
−
1
solution containing Ce and Ag precursors (0.4 mol L ). The solids
obtained were filtered and washed with deionized water. Finally,
The TPD of ethanol followed by IR-MS (DRIFTS) spectroscopies
were carried out using a Nicolet iS50 FT-IR Spectrometer with
a MCT/B detector equipped with a diffuse reflectance assem-
bly chamber (Harrick) and ZnSe window. The IR spectra were
◦
◦
−1
they were calcined at 500 C for 1 h at 10 C min . The catalysts
were named as CeO2 and AgCeO2.
−
1
obtained using 4 cm
and 64 scans. The desorption was con-
2.2. Characterization
tinuously analyzed by on-line mass spectrometer (MS). The MS
◦
◦
The specific surface areas of the catalysts were obtained by N2
spectra were collected at every 50 C in the range of 50–450 C. The
main fragments related to the desorbed compounds, i.e., (m/z = 2
(H ), m/z = 18 (H O), m/z = 26 (ethylene), m/z = 29 (acetaldehyde),
adsorption employing a Micromeritics ASAP 2420. The samples
◦
were pre-treated at 100 C for 24 h, and then submitted to in situ
2
2
◦
treatment under vacuum at 120 C for 24 h. The N adsorption was
carried out at −196 C.
m/z = 31 (ethanol), m/z = 44 (CO ) and m/z = 43 (acetone)) were con-
2
2
◦
tinuously monitored by the MS. The intensities of these fragments
were mathematically treated in order to eliminate contributions
of more than one species. Initially, the catalysts were dried at
The Ag loading was determined using an inductively coupled
plasma optical emission spectrometer (ICP-OES), Horiba Scientific,
Ultima 2.
◦
◦
130 C for 30 min under N2 flow and reduced at 450 C for 1 h
−
1
The TPD of H O was carried out using a micro reactor system
under 10% H /N2 flow (100 mL min ). After that, the samples
were oxidized at 450 C for 30 min under 20% O /He flow. Ethanol
vapors were generated by passing He through a saturator at 10 C.
2
2
◦
coupled to a mass spectrometer QMS200 Balzers. The samples were
2
◦
−1
◦
dried at 130 C for 30 min under He flow (30 mL min ). After that,
◦
◦
−1
they were reduced to 450 C (10 C min ) for 1 h and then purged
with He for 1 h at 450 C. The H O adsorption was conducted at
The ethanol adsorption was conducted employing an ethanol/He
◦
−1 ◦
flow (20 mL min ) for 1 h at 50 C. The desorption was performed
2
− ◦ −1 ◦
1
room temperature for 1 h. The H O vapors were generated by pass-
employing a He flow (40 mL min ) at 20 C min from 25 C to
2
−1
◦
−
◦ ◦
450 C and remaining at 450 C for 30 min.
ing He (50 mL min ) through a saturator at 40 C. The desorption
was carried out employing a He flow (50 mL min ) at 10 C min
from 25 to 450 C. The fragments, m/z = 2 (H ) and m/z = 18 (H O)
1
◦
−1
◦
2
2
were continuously monitored during the analyses.
2.3. Catalytic tests
Before using the following techniques: TPR, TPD of NH , TPD
3
of CO , isopropanol model reaction and finally, the water gas shift
The catalytic tests were performed using a conventional sys-
2
◦
reaction, the samples were firstly dried at 130 C under N or He
tem with a fixed bed reactor at 1 atm. Ethanol and H O vapors
2
2
−
−1
1
flow (30 mL min ) for 30 min. After that, they were reduced at
50 C (10 C min ) for 1 h. Next, they were purged and oxidized
were generated by passing N2 through two saturators, one
◦
◦
◦
◦
4
at 4.9 C and the other at 53.6 C, respectively. A mixture of
−1
◦
−1
with synthetic air (30 mL min ) at 450 C for 30 min.
The TPR was carried out using 150 mg of the catalyst, 10% H /N
flow (30 mL min ) and 10 C min . The amount of H2 consumed
during the reduction was measured by a thermal conductivity
detector (TCD). Reduction profiles were normalized using the mass
of the samples and the H2 signal intensity.
N :H O:C H OH = 91:8:1 mol% was fed at 50 mL min . The cata-
2
2
2
5
◦
2
2
lysts were dried at 130 C for 30 min under N flow and reduced at
2
−1
◦
−1
◦ −1
450 C for 1 h under 10% H /N flow (100 mL min ).
2 2
The reaction rates were measured under differential conditions
◦
◦
(conversion <10%) at 250 C and 400 C. The isoconversion catalytic
◦
−1
tests (30% conversion) were performed at 400 C and 50 mL min
.
The acid sites densities of the prepared samples were deter-
The products were analyzed on-line by gas chromatography using
GC Agilent 6890 equipped with two detectors (thermal conductiv-
ity and flame ionization), methanator and a Porapak-Q/60 ft column
using He as the carrier gas. The samples were analyzed every 27 min
during 14 h on stream.
The products formation rates were obtained multiplying the
ethanol consumption rates by the molar selectivities. The ethanol
conversion was defined as the ratio of the moles of ethanol con-
sumed to the moles of ethanol introduced in the feed. The definition
of the selectivity to one specific compound is the ratio of the num-
ber of carbon moles consumed to synthesize this compound to the
total number of carbon moles consumed. Considering the acetone
synthesis reaction (see Section 1), 75% is the highest selectivity to
acetone that can be obtained. The carbon balance was higher than
95% for all tests. The deactivation phenomenon was not observed
during the 14 h of time on stream.
◦
mined by the TPD of NH . This molecule was adsorbed at 100 C
3
−
1
for 30 min under 4%NH /He flow (60 mL min ). Desorption was
3
−1
◦
−1
◦
conducted at 10 C min , 50 mL min of He flow until 450 C. The
TPD profiles were decomposed in lorentzians curves in order to
quantify the weak, medium and strong acid sites. The acid sites
related to a curve which shows its maximum value at tempera-
tures below 200 C are assigned as weak, those between 200 and
50 C as medium; and finally, above 350 C as strong ones.
The basic sites densities of the prepared samples were deter-
◦
◦
◦
3
mined by the TPD of CO . The CO adsorption was conducted at
2
2
−1
room temperature for 1 h (20 mL min ). The TPD-CO2 measure-
ments were carried at 20 C min , under He flow (50 mL min
up to 450 C. The TPD profiles were decomposed in Gaussian curves
in order to quantify the weak, medium and strong basic sites. The
basic sites which are assigned as weak are related to a curve which
◦
−1
−1
)
◦
Please cite this article in press as: A.F.F. de Lima, et al., The first step of the propylene generation from renewable raw material: Acetone
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3
. Results and discussion
Table 1 exhibits the surface areas, Ag loading, acid and basic
sites densities, the isopropanol consumption and also acetone gen-
eration rates. On the one hand, it can be observed an increase of
weak and strong sites densities when Ag is added to CeO . On the
2
other hand, a small decrease of the acid sites densities (weak and
medium) is observed when CeO2 is doped with this metal. These
solids when compared with acid oxides, for instance, ␥-alumina,
they show much lower strength and densities than the ones of ␥-
alumina [18,19]. It can be also verified that the surface areas do not
change when Ag is added to the support.
The isopropanol dehydration/dehydrogenation model reactions
were employed in order to evaluate the acidity and basicity of the
samples. When Ag is added to CeO , the rates of acetone generation
2
and isopropanol consumption show a small increase suggesting
that there is a small rise in the density of the basic sites. Moreover,
propene was not observed. The isopropanol model reaction results
observed when Ag is added to CeO2 compared with the ones of the
TPD-CO2 show the same tendency with different intensities. This
might be related to the different nature of these measurements.
The TPD of CO , TPD of NH and the isopropanol model reactions
2
3
results exhibit, as well described in the literature, the basic charac-
ter of these samples and their low acidity. It is worth stressing that
CeO2 only shows Lewis acid and basic sites [20].
Using Raman spectroscopy and XRD experiments (not shown),
it was not possible to detect changes in the crystalline structure of
CeO when Ag is added to this oxide. In addition, the XPS measure-
2
ments were not able to provide information related to the oxidation
state and superficial concentration of Ag. This occurs due to the low
loading of this metal in the catalyst sample.
Fig. 1 shows the TPR profiles of both samples. The reduction
degree of CeO and AgCeO are 49% and 44%, respectively. As it can
2
2
be observed, CeO shows two peaks. The one observed at low tem-
2
◦
perature (312 C) is related to the elimination of the surface capping
oxygen of CeO2 and the other one (545 C) to the reduction of the
Fig. 1. TPR profiles of the prepared catalysts.
◦
bulk of the oxide particles [14,21]. The doped catalyst shows the
same peaks of CeO2. The one related to the catalyst surface reduc-
H O on CeO2 surface generated hydroxyls species which can
undergo two competing reactions: the associative desorp-
2
◦
tion is shifted to 155 C. The H2 consumption values related to the
tion of OH(a) releasing H O and creating oxygen vacancies
2
−
2
peaks at low temperature are very similar (∼4.7 mmol m ). More-
(
OH(a) + OH(a) → H O + Olattice + Ovacancy) and the other one which
2
(g)
over, these values are close to the ones proposed by Perrichon et al.
generates H (OH + OH(a) → H
+ 2Olattice). The authors demon-
2
(a)
2(g)
[
21] for the reduction of the superficial species of CeO . Differently
2
strated that the presence of O vacancies in CeO2 (reduced CeO2)
promote the H2 formation. In other words, O vacancies on CeO2
surfaces control the reactivity of the surface hydroxyls species.
Fig. 2 depicts that the H generation and H O desorption occur at
from the metals of the Pt group, the dissociation of H2 on clean
Ag is highly activated. However, according to Mohammad et al.
[
22], O species on Ag are able to dissociate H . Thus, considering
2
2
2
the pretreatment employed for the TPR measurements, it can be
proposed that the remaining O on Ag might dissociate H2 promot-
low temperatures for both reduced catalysts. These spectra exhibit
that both reactions described above might occur simultaneously.
ing the reduction of CeO . However, another possibility can also
2
Indeed, the H O and H2 spectra are very similar for these cata-
2
be raised; Ag atoms and/or Ag clusters can act as donor species,
becoming oxidized, donating the valence electrons to the oxide and
lysts. Therefore, it can be suggested that Ag seems not to change
the amount of the CeO2 vacancies suggesting that this metal is not
replacing Ce in the lattice of CeO2 [25].
3
+
promoting the Ce ions generation, thus, reducing CeO2 [23].
Therefore, the TPR analyses show that CeO redox properties are
2
Ceria as support of different metals is frequently employed in
the WGS reaction. Zonetti et al. [26] observed that CeO2 and other
mixed oxides can promote the RWGS without any metal. The Redox
mechanism [27], which seems to prevail in these cases, can be
described according to the following steps: initially, CO is oxidized
to CO by the O of the CeO lattice. Then, this oxide is reduced gen-
promoted by Ag as it decreases the reduction temperature of the
CeO2 surface capping oxygen.
Fig. 2 depicts the spectra of TPD-H O on CeO2 and AgCeO2.
2
The interaction of CeO2 with H O is well described in the
2
literature. Chen et al. [24] showed that the adsorption of
2
2
Table 1
Specific surface area (A), Ag loading, acetone generation (-racetone) and isopropanol consumption (-risop) rates, densities of acid sites (A), weak (Aw) and medium (Am) and
densities of basic sites (B), weak (Bw), medium (Bm) and strong (Bs).
A (m2
g
−1
)
Ag (%wt)
-risop
−1
␮mol gcat min
racetone
A (␮mol g
−1
)
−1
B (␮mol g )
Amostra
−
1
(
)
Aw
Am
Bw
Bm
Bs
CeO2
AgCeO2
75
76
–
0.02
236
285
236
285
38
31
89
69
24
37
16
17
12
17
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Fig. 2. Profiles of TPD-H2O on CeO2 and AgCeO2.
erating O vacancies. After that, H O dissociates, re-oxidizing the
oxide and producing H2.
multiplying the ethanol consumption rate by the molar selectivity.
Therefore, adding Ag to CeO2 the selectivity to acetone increases.
The WGS reaction and the acetone synthesis from ethanol show
two similar steps. The first one is the oxidation of the C O bond,
which in the case of the WGS reaction generates CO2 and, in the
acetone synthesis, is related to the oxidation of acetaldehyde to
acetate species.
2
Table 2 shows that the AgCeO2 WGS rate is 60% higher than
the one of CeO . Considering the Redox mechanism, this result can
2
be associated with the higher reducibility of AgCeO , since both
2
catalysts seem to show the same catalytic behavior for the H O
2
dissociation (see Fig. 2). Moreover, the Redox mechanism proposes
that the catalyst is reduced by CO. Thus, according to Preda and
Pacchioni [23], it can be suggested that the role of Ag is associated
According with the Redox mechanism these oxidations reduce
the catalysts and the H O decomposition re-oxidizes them. As
2
with the promotion of the reduction of Ce4+ to Ce3+
.
discussed previously, only the first step is affected by Ag. Conse-
The ethanol consumption rates and the acetaldehyde, CO2 and
quently, the higher rate of acetone generation of AgCeO compared
2
acetone generation rates are shown on Table 2. Adding Ag to CeO ,
with CeO2 can be associated with the higher reducibility of this
oxide promoted by Ag. Moreover, the ethanol consumption rates
depicted on Table 2 show that the redox properties are not associ-
ated with the rate limiting step of the acetone synthesis.
As Table 1 depicts Ag promotes a raise of the number of basic
sites. One can propose that the condensation of acetates might be
favored by adding Ag to CeO2.
2
◦
the ethanol consumption rate at 250 C does not change and only
◦
acetaldehyde is observed as a product. At 400 C the ethanol con-
sumption rate of these samples is also almost the same. On the
one hand, the acetone synthesis rate of AgCeO2 is twofold higher
than the one of CeO . On the other hand, the rate of acetaldehyde
2
generation of AgCeO2 is twofold lower. According to the reaction
stoichiometry, the CO2 generation rates are similar to the ones of
The catalytic behavior of both prepared catalysts at isoconver-
sion (30%) is depicted on Fig. 3. The main products are acetone,
acetone. The generation of H and a small amount of ethylene were
2
also observed. As these data depict Ag promotes the oxidation of
acetaldehyde to acetone. The acetone generation rate is obtained
acetaldehyde, ethylene and CO . Acetone and CO2 are obtained
2
in equimolar amounts (see Eq. (1)). At these experimental condi-
tions, the dehydration of ethanol occurs. Traces of other products,
e.g. methane, CO, propylene and butene were also observed. These
results show that doping CeO2 with Ag there is an increase of the
Table 2
selectivity to acetone and CO and a decrease of the ethylene selec-
2
−
1
−1
CO consumption rate of WGS (-rCO, ␮mol gcat min ), rates of ethanol consumption
tivity. The sample doped with Ag shows the higher selectivity to
acetone again.
− −1 ◦ ◦
− −1 ◦ ◦ −1 −1
1
(
-rEtOH, ␮mol gcat min ) at 400 C and 250 C and rate of acetaldehyde generation
1
(
4
␮mol gcat min ) at 400 C and 250 C, acetone generation (␮mol gcat min ) at
00 C and CO2 generation (␮mol gcat min ) at 400 C.
◦
−1
−1
◦
The TPD of ethanol over oxidized catalysts was analyzed by
infrared and mass spectroscopy, simultaneously. Fig. 4 and 5 depict
the IR spectra of ethanol adsorption and desorption on CeO2 and
◦
Catalysts -rCO 450
C
-rethanol racetaldehyde -rethanol racetone racetaldehyde rCO2
◦
◦
C
250
C
400
−
1
−1
AgCeO in the 1300–800 cm and 1800–1200 cm range, respec-
2
CeO2
AgCeO2
2.6
4.2
20
20
20
20
270
290
50
105
140
73
63
92
tively.
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Fig. 3. Selectivities to acetone, acetaldehyde, ethylene and CO2 of CeO2 and AgCeO2
at isoconversion (30%). The gas mixture composition, flow rate, temperature, the
−
1
◦
catalyst weight are N2:H2O:C2H5OH = 91:8:1 mol%, 50 mL min , 400 C and 50 mg,
respectively.
−
1
Figs. 4A and 5A show absorptions at 1119/1098 cm
and
062/1054 cm which can be assigned to monodentate and biden-
tate ethoxide species (v(CO)), respectively [28,29]. The vibrations
−1
1
−1
at 1119 and 1054 cm can be attributed to ethoxide adsorbed on
Ce , whereas the bands at 1098 and 1062 cm to these species
4+
−1
3
+
on Ce [30]. These results show the redox behavior of these
catalysts during the TPD of ethanol. The intensity of the absorp-
tions related to the ethoxide species decrease as the temperature
increases, being the lower energy vibrations the most stable (1098
and 1054 cm ). At 400 C both absorptions disappear suggesting
that these oxides might show the same ethoxide consumption rate.
According to Li et al., bands at 905 cm and 885 cm (Figs. 4A and
A) can also be attributed to monodentate and bidentate ethoxide
species (ꢀs(CCO)), respectively. Moreover, vibrations at 1476 (Figs.
−1
◦
−
1
−1
5
−
1
4
, 5B, arrows) and 1380 cm (Figs. 4B and 5B) can also be assigned
to ␦as (CH , CH ) and ␦s (CH , CH ) of the ethoxide species, respec-
3
2
3
2
◦
tively [27]. These ethoxide vibrations also disappear at 400 C.
At low temperature (50 C, adsorption), CeO shows vibrations
at 1571, 1421 and 1441 cm which can be assigned to ꢀas(OCO),
◦
2
−1
ꢀ
s(OCO) and ␦as(CH ) of acetate species (Fig. 4B), respectively.
3
The CeO2 spectra show that as the temperature increases the
relative intensity of the acetate vibrations changes exhibiting that
other species emerge. As well known, [28] acetate species adsorbed
on oxides with redox properties can be oxidized to carbonate
species. Moreover, carbonates can be produced by the interaction
of these catalysts with CO2 generated from the acetone synthesis,
(Table 2, Fig. 3). According to the literature [31,32], many differ-
ent species, i.e., monodentate, polydentate, symmetric, polymeric
carbonates and hydrogenocarbonate can be generated by the inter-
action between CO2 and CeO2.
Yee et al. [28] observed that acetate species are transformed into
carbonate species during TPD on an oxidized CeO . They assigned
2
−
1
the vibrations at 1568 and 1341 cm
to bidentate carbonates
−
1
and the ones at 1428 cm
the most intense. Moreover, bands at 1438, 1538 and 1345 cm
to symmetric carbonates, which are
Fig. 4. In situ DRIFTS spectra of adsorption and desorption of ethanol on CeO2: A)
−
1
−1
−1
.
1300–800 cm and B) 1800–1200 cm
were observed for CeO previously reduced. The first vibration was
2
associated with symmetric carbonate species and the others to
bidentate carbonates. The most intense band, also in this case, is
the symmetric carbonate one.
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Analyzing Fig. 4B and considering that the CeO2 might be par-
tially reduced during the TPD of ethanol, it can be suggested that
◦
at temperatures higher than 300 C, the main species on the CeO2
surface are the acetate, symmetric and bidentate carbonates. Con-
sidering the shape of these absorptions at these conditions, it
cannot be ruled out that other types of carbonate species might
also occur on CeO2.
The TPD of ethanol on AgCeO at low temperature (adsorption at
2
◦
−1
5
0 C) also shows vibrations at 1557 and 1427 cm , which can be
assigned to ꢀas(OCO) and ꢀs(OCO) of acetate species, respectively
Fig. 5B). It can be verified that in the case of AgCeO2 the relative
intensity of the acetate species absorptions does not change. At
(
◦
−1
temperatures higher than 300 C, a small band at 1341 cm can
be observed which is related to bidentate carbonate species on the
catalyst surface. Indeed, the same carbonate species observed for
CeO2 might be on the AgCeO2 surface, admitting a lower concen-
tration of symmetric carbonate. These results show the effect of
Ag on the CeO2 surface. In this case, the presence of other types of
carbonates species cannot also be ruled out.
These spectra depict that the oxygen of the lattice of both cata-
lysts is able to oxidize ethoxide species to acetates species reducing
partially the surface of CeO and AgCeO2 (see ethoxide species
2
3
+
4+
adsorbed on Ce and Ce , Figs. 4A and 5A) at very low tempera-
ture.
Fig. 6A and B depict the MS spectra of the TPD of ethanol over
CeO2 and AgCeO , respectively. Both figures show the spectra of
2
the same compounds which are the following: H O, H , ethanol,
2
2
acetone, CO , acetaldehyde and ethylene. The acetone spectra of
2
◦
CeO show two peaks (Fig. 6A): one at 300 C and another around
2
◦
◦
◦
4
50 C. Fig. 6B depicts the same peaks at 234 C and 450 C. These
results are in line with the data on Table 2 which are related to the
higher activity of the doped catalyst in the acetone synthesis.
The same pattern of the acetone spectra (see the lines) is
observed for the ethanol, H O, CO , and acetaldehyde (only at
2
2
high temperatures) spectra for both catalysts. Therefore, it can be
inferred that these compounds might be intermediates or products
of the same reaction. As the H signal occurs at temperatures higher
2
than the first peak of acetone, it can be proposed that acetaldehyde
is not generated by dehydrogenation but by the oxidative dehydro-
genation (ODH) of ethanol. After that, this aldehyde is oxidized to
acetate species (Figs. 4 and 5), which condense producing acetone
and CO . The ethylene and H O spectra show that the dehydration
2
2
of ethanol also occurs during the TPD. Thus, the ethoxide species
Figs. 4A and 5A) can be oxidized to acetaldehyde or dehydrated
(
to ethylene. It is worth stressing that H O is not only produced
2
by the dehydration reaction but also by the ODH of ethanol. Both
the H2 spectra indicate that the dissociation of water also occurs
during the TPD experiments. The oxidant species produced by the
H O decomposition regenerate the catalysts after the oxidations to
2
acetaldehyde and carboxylate species. The “re-oxidation” of these
catalysts is related to the recovery not only of the redox properties
of the catalysts but also the basicity of the samples. The basic sites
are associated with the ketonization and dehydration reactions.
Therefore, the re-oxidation of the catalysts promotes a new cycle of
acetone and ethylene syntheses which occur at high temperatures
◦
(
see peaks at 450 C).
◦
Adding the acetaldehyde and acetone synthesis rates at 400 C,
which are depicted on Table 2, the values obtained are similar.
Moreover, the rates of the acetaldehyde synthesis are the same at
◦
2
00 C. Taking into account that acetone is generated from acetalde-
hyde it can be suggested that Ag does not change the ODH rates
under these conditions (conversion lower than 10%).
Fig. 5. In situ DRIFTS spectra of adsorption and desorption ethanol on AgCeO2: A)
−1
−1
.
1300–800 cm and B) 1800–1200 cm
The hydrogen transference from the alkoxide species to the
superficial O of CeO has been identified as the first step of the ODH
2
reaction and the rate determining step of this reaction as well. After
that H O is generated, desorbed and an O vacancy emerges. These
2
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Fig. 6. TPD of ethanol on CeO2 (A) and AgCeO2 (B).
two steps are associated with the redox properties [33]. The results
obtained (Table 2) show that Ag does not change the CeO2 ODH
activity. Thus, they indicate that the ethoxide synthesis is a slow
step at low conversion.
lower. This might occur due to the competition between the dehy-
dration and the ODH reactions for their mutual intermediate i.e.
ethoxide species under these conditions. As Ag promotes the redox
properties of CeO2, the ethoxide consumption of the ODH reaction
might be faster than the one of the dehydration reaction.
According to Di Cosimo et al. [34], when oxides which show
only Lewis sites are employed, ethylene can be synthesized from
ethanol by two different mechanisms. The first one (E1cB) needs
both strong basic sites and acid sites. The first step is the ethanol
adsorption followed by the O H bond rupture on an acid-strong
base pair site generating ethoxide species. After that, a proton is
abstracted leading to a carbanion, which then generates ethylene.
The second mechanism is the E2 elimination, which uses Lewis acid
and basic sites of balanced strength. The acid sites are in charge of
the OH abstraction. They should be stronger than the ones which
promote the E1cB mechanism.
The mechanism proposed by Rodrigues et al. [3] appears to
describe most of the steps of the CeO and AgCeO reaction systems.
2
2
Ceria is able to generate acetaldehyde, oxidize this aldehyde to
acetate species and condensate these carboxylates to acetone. This
oxide also dissociates H O, which provides oxidant species for its
2
own (CeO ) re-oxidation. The increase of the density of strong basic
2
sites caused by Ag might promote the condensation step. However,
the main role of Ag is to promote the oxidation of acetaldehyde to
acetate species inhibiting the dehydration reaction and increasing
the selectivity to acetone.
Taking the acid and basic properties of the prepared samples
(
strong basic sites and weak acid sites) into account it can be sug-
4
. Conclusions
gested that the most likely mechanism of ethanol dehydration is
the first one mentioned (E1cB).
At low conversion the generation of ethylene is very small
for both catalysts. However, at a higher conversion (Fig. 3, con-
version = 30%), it can be observed that CeO2 shows much higher
In short, the main effect of doping CeO2 with Ag is the increase
of the reducibility of this oxide which leads to a higher selectivity
to acetone and a lower towards ethylene. Ceria dissociates H O,
which provides oxidant species for its (CeO ) own re-oxidation.
Acetaldehyde is generated by the ODH reaction. The WGS reac-
tion is a very interesting model for the redox reactions when H O
2
2
selectivity to ethylene than AgCeO . Indeed, adding Ag to CeO2 the
basicity increases, whereas the selectivity to ethylene is threefold
2
2
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is employed as an oxidant agent. The mechanism proposed by
Rodrigues et al. describes the behavior of CeO2 in the acetone syn-
thesis from ethanol.
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Please cite this article in press as: A.F.F. de Lima, et al., The first step of the propylene generation from renewable raw material: Acetone
from ethanol employing CeO2 doped by Ag, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.04.038