Acetone from ethanol employing ZnxZr1?xO2?y

Article abstract of DOI:10.1016/j.cattod.2016.09.011

The main purpose of this work is to contribute to the description of the acetone synthesis from ethanol employing Zn_xZr_1-xO_2-y based catalysts. The catalytic behavior of these solids was evaluated (isoconversion) in the acetone synthesis. The most active catalyst and m-ZrO₂ (used as a reference) were characterized by the following techniques: pyridine adsorption, TPD of NH₃, TPD of CO₂, TPD of ethanol followed by IR (DRIFTS)/MS, TPD of ethanol followed by XANES at the Zr K-edge and Zn K-edge and XRD in situ. The present study suggests that the main steps of the acetone generation from ethanol are the following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde. Both steps are related to strong basic and acid sites. Acetaldehyde reacts with the O of the solid solution generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate (strong basic sites) and generate acetone and CO₂. Water dissociates on the vacancies of the catalyst and reoxidizes the its surface, closing the catalytic cycle. All these steps might occur on Zn⁺² and on the species in its vicinity (XANES).

Full text of DOI:10.1016/j.cattod.2016.09.011

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Acetone from ethanol employing ZnxZr1−xO2−y
Leydi del R. Silva-Calpaa, Priscila C. Zonettib, Daniela C. de Oliveirac,
Roberto R. de Avilleza, Lucia G. Appelb,∗
a Departamento de Engenharia Química e de Materiais, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, rua Marquês de São Vicente 225, Gávea,
Rio de Janeiro, 22451-900, RJ, Brazil
b Divisão de Catálise e Processos Químicos, Instituto Nacional de Tecnologia, INT, Av. Venezuela 82/518, Rio de Janeiro, 21081-312, RJ, Brazil
c Laboratório Nacional de Luz Síncrotron, LNLS, rua Giuseppe Máximo Scolfaro 1000, Campinas, 13083-970, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 May 2016
Received in revised form 4 August 2016
Accepted 9 September 2016
Available online xxx
The main purpose of this work is to contribute to the description of the acetone synthesis from ethanol
employing ZnxZr1-xO2-y based catalysts. The catalytic behavior of these solids was evaluated (isocon-
version) in the acetone synthesis. The most active catalyst and m-ZrO2 (used as a reference) were
characterized by the following techniques: pyridine adsorption, TPD of NH3, TPD of CO2, TPD of ethanol
followed by IR (DRIFTS)/MS, TPD of ethanol followed by XANES at the Zr K-edge and Zn K-edge and XRD
in situ. The present study suggests that the main steps of the acetone generation from ethanol are the
following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde.
Both steps are related to strong basic and acid sites. Acetaldehyde reacts with the O of the solid solution
generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate
(strong basic sites) and generate acetone and CO2. Water dissociates on the vacancies of the catalyst
and reoxidizes the its surface, closing the catalytic cycle. All these steps might occur on Zn+2 and on the
species in its vicinity (XANES).
Keywords:
Acetone
Ethanol
Redox
Solid solution
Zinc
Zirconium
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Since the first decades of the last century, it is known that
acetone can be produced from ethanol [4]. This ketone is an inter-
Acetone,
a
water-white volatile and flammable liquid, is
mediate of the isobutene, isopropanol and propene production
from ethanol. Nevertheless, the steps of the acetone synthesis from
ethanol are not well established in the literature. In other words, the
reaction system related to the ethanol transformation into acetone
is a topic for discussion.
employed in the production of acetone cyanohydrins (ACH), which
is a precursor of methyl methacrylate (MMA). This compound is
employed in the production of polymers which have innumerous
applications. The second largest use of acetone is the manufacture
of bisphenol-A, which is used in the polycarbonate sector. Ace-
tone is also employed in the production of methyl isobutyl ketone,
isophorone and diacetone alcohol/hexylene glycol. Nowadays, the
cumene process is in charge of almost all the acetone production,
which is a co-product to phenol. Thus, the production of acetone
is dependent on the market demands for phenol. Moreover, the
cumene route, a three-step process, not only uses fossil feedstock
and shows low yields, but also is highly energy-consuming [1].
Ethanol is produced in huge amounts as a fuel from sugar-cane,
corn (1G ethanol) and agriculture residues (2G ethanol). Ethanol
is also considered a special platform molecule which is able to
generate many “drop-in” chemicals in one-step processes [2,3].
2CH3CH2OH + H2O → CH3COCH3 + CO2 + 4H2
Rodrigues et al. [5] studied the acetone synthesis employing a
physical mixture composed of Cu/ZnO/Al2O3 (CZA) and m-ZrO2.
They proposed the following steps for this synthesis: firstly, ethanol
adsorbs on both m-ZrO2 and CZA generating ethoxide species.
These species adsorbed on m-ZrO2 migrate to the CZA surface
and are dehydrogenated on Cuo to acetaldehyde. This aldehyde
migrates to m-ZrO2 and is oxidized to acetate by the superficial O of
the oxide. These carboxylates condense producing acetone, CO2 and
H2. Finally, H2O is dissociated on the Cuo (CZA) surface producing
H2 and oxidizing species which then migrate to m-ZrO2 recovering
its surface which was reduced by the carboxylates species synthe-
sis. Thus, the role of CZA is not only the generation of acetaldehyde,
but also the dissociation of H2O. This system shows the relevance
∗
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.2016.09.011
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L.d.R. Silva-Calpa, et al., Acetone from ethanol employing ZnxZr1−xO2−y, Catal. Today (2016),
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of the redox steps, i.e., the oxidation of acetaldehyde and the H2O
dissociation.
ried out at 400 ◦C. The catalyst mass was changed in order to reach
the isoconversion.
Employing Sc/In2O3 or Y2O3-CeO2, Iwamoto [6] suggested dif-
ferent reactional steps for each catalyst for the acetone synthesis
from ethanol. On the one hand, employing Sc/In2O3, acetaldehyde is
oxidized by water or by the surface hydroxyl groups on this catalyst.
The acetic acid generated condenses producing acetone and CO2.
On the other hand, using Y2O3-CeO2, acetaldehyde is converted into
ethyl acetate (condensation) and then this ester is decomposed and
produces acetic acid and ethene. After that, acetone is generated by
the ketonization of acetic acid.
Sun et al. [7,8], suggested that acetone is an intermediate of
the isobutene generation from ethanol. They employed a ZnxZryOz
mixed oxide, which is very active, not only for the isobutene gen-
eration, but also for the acetone synthesis. The authors suggested
that acetone is generated by the dehydrogenation of ethanol and
aldol-condensation of acetaldehyde.
Recently, our group synthesized a superficial solid solution com-
posed of Zr and Zn on m-ZrO2, i.e., ZnxZr1−xO2−y/ZrO2, by placing
a Zn(NO3)2 solution in contact with m-ZrO2. After that, the sus-
pension was filtered, dried and calcined [9]. The XRD, XPS and
Raman spectroscopy analyses showed that ZnO was not synthe-
sized. Instead, Zn diffused into the first layers of the m-ZrO2 lattice.
The XRD and EPR results indicate the generation of O vacancies
when Zn is added to m-ZrO2. These results showed that Zn replaces
Zr in the m-ZrO2 lattice. This occurs due to the similar size of these
ions and also the different valences of these elements (Zr+4 and
Zn+2). It was verified that these O vacancies promote the O mobility,
which increases the reducibility of m-ZrO2 and the redox properties
of this oxide.
In order to identify the intermediates of the reaction the 0.7Zn
catalyst was tested at 400 ◦C using different masses of catalyst
(changing the residence time). Some catalytic tests were also car-
ried out employing this same catalyst (50 mg) using different
temperatures (350, 375, 400, 425, 450 and 475 ◦C). All of the others
parameters used in these tests are the same as described above.
The ethanol conversion was defined as the ratio of the moles of
ethanol consumed to the moles of ethanol introduced in the feed.
The definition of the selectivity to one specific compound is the
ratio of the number of carbon moles consumed to synthesize this
compound to the total number of carbon moles consumed.
The performances of m-ZrO2 and 0.7Zn catalysts in the water-
gas shift reaction (WGS) were evaluated measuring the CO
consumption rates under differential conditions (CO conversion
<10%) at 450 ◦C, 1 atm and H2O/CO = 1. Reagents and products were
analyzed by on-line gas chromatography.
2.3. Characterization
2.3.1. Chemical analyses
Chemical analyses of Zn and Zr were performed by an
inductively- coupled plasma – atomic emission spectrometer (ICP-
AES), Optima 300DV, Perkin Elmer Instruments, employing the
following conditions: plasma air, auxiliary air and Ar for the
nebulization flow rates of 15 L min−1, 0.2 L min−1, 0.60 L min−1
respectively and 1400 W of power.
,
2.3.2. Specific surface area
All in all, the main purpose of this work is to contribute to the
description of the acetone synthesis steps analyzing the role of the
redox, basic and acid properties of the catalysts when employing a
ZnxZr1−xO2−y system and m-ZrO2 as a reference.
The analyses were conducted employing
a Micrometrics
ASAP2010. The samples were pre-treated at 100 ◦C for 24 h, and
then submitted to an in situ treatment under vacuum at 150 ◦C for
2 h. The N2 adsorption occurred at −196 ◦C.
2. Experimental
2.3.3. Pyridine adsorption
The samples acid properties were evaluated by the adsorption
of pyridine followed by IR spectroscopy. The spectra were recorded
from thin (∼20 mg) self-supporting wafers using a Nicolet Magna
2000 FT-IR spectrophotometer. The samples were pretreated at
450 ◦C for 2 h under vacuum and exposed to high vacuum for 30 min
(10−7 Torr). Pyridine were adsorbed at 25 ◦C for 1 h at 2 Torr. Spec-
tra were collected after desorption at 150 ◦C for 30 min under high
vacuum. The absorption at 1445 cm−1 was employed for the calcu-
lation of the Lewis acid sites density. The FTIR spectra were divided
by the mass of the wafers in order to normalize the results.
2.1. Catalysts synthesis
A suspension composed of m-ZrO2 supplied by NORPRO (5 g)
and a 75 mL of aqueous solution of Zn(NO3)2·6H2O were heated at
70 ◦C under constant stirring for 4 h. After that, these solids were
filtered, dried and calcined at 450 ◦C for 12 h under synthetic air
flow. A sample of m-ZrO2 calcined at 450 ◦C for 12 h was used as a
reference. Three different concentrations (0.04, 0.08 and 0.12 M) of
the Zn precursor were employed in order to prepare three catalysts,
0.4Zn, 0.6Zn and 0.7Zn, respectively. The numbers 0.4, 0.6 or 0.7
refers to the wt.% of zinc in the catalysts.
2.3.4. TPD-CO2
The density of basic sites was determined by temperature pro-
grammed desorption of CO2 (TPD-CO2). The experiments were
carried out using a micro reactor system coupled to a QMS200
Balzers mass quadrupole spectrometer. The samples were treated
at 130 ◦C for 30 min under N2 flow (30 mL min−1) and reduced
under 10% H2/N2 flow (50 mL min−1) at 450 ◦C for 1 h. After that,
2.2. Catalytic tests
Catalytic tests were performed using a conventional system
with a fixed bed reactor at atmospheric pressure. The reagents
and products were analyzed on-line using a GC Agilent HP6890
equipped with two detectors (thermal conductivity detector and
flame ionization detector). The column employed was a Porapak-
Q/60 using He as the carrier gas. The samples were analyzed
every 23 min during 12 h on stream. The catalysts were previ-
ously dried at 130 ◦C with N2 (90 mL min−1) for 30 min. Then,
the catalysts were oxidized under 20% O2/He flow (40 mL min−1
)
at 450 ◦C for 1 h. The CO2 adsorption was conducted at room tem-
perature for 1 h (25 mL min−1). The desorption was performed by
heating (10 ◦C min−1) the sample from room temperature up to
450 ◦C under He flow (50 mL min−1). The fragment m/z = 44 was
continuously monitored by mass spectrometer. The TPD profiles
were decomposed employing Gaussian curves in order to quantify
the strength of the basic sites, as previously described by Carvalho
et al. [10] The basic weak sites were attributed to a curve, which
shows a maximum at temperature lower than 170 ◦C, medium sites
between 170 ◦C and 270 ◦C, and strong sites above 270 ◦C.
the samples were reduced under 10% H2/N2 flow (100 mL min−1
at 450 ◦C for 1 h. The gas stream composition and flow rate
were N2:H2O:C2H5OH = 91:8:1 mol% and 70 mL min−1
respec-
tively. Ethanol and H2O vapors were generated by passing N2
through two saturators, one at 4.9 ◦C and the other at 53.6 ◦C,
respectively. The catalytic tests at isoconversion (∼ 40%) were car-
)
,
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2.3.5. TPD-NH3
The density of acid sites of m-ZrO2 and 0.7Zn was deter-
mined by the temperature programmed desorption of ammonia
(TPD-NH3). The samples (600 mg) were dried at 130 ◦C for 30 min
under N2 flow (90 mL min−1). After that, they were reduced under
10% H2/N2 flow (100 mL min−1) at 450 ◦C (10 ◦C min−1) for 1 h,
purged and oxidized under synthetic air flow (90 mL min−1) at
450 ◦C for 30 min. The adsorption of 4% NH3/He was conducted at
100 ◦C for 30 min (30 mL min−1). The desorption was performed at
10 ◦C min−1, under He flow (30 mL min−1) from 100 ◦C to 450 ◦C.
The TPD profiles were decomposed in Gaussian curves in order to
quantify the weak, medium and strong acid sites [11]. The weak
strength sites of the samples are related to a curve which shows
a maximum at temperatures lower than 200 ◦C, the ones between
200 and 350 ◦C as medium; and finally, above 350 ◦C as strong ones.
2.3.6. TPD-ethanol followed by IR and mass spectroscopies
The temperature programmed desorption of ethanol (TPD-
ethanol) followed by infrared (DRIFTS) and mass (MS) spectro-
scopies were carried out using a Nicolet iS50 FT-IR Spectrometer
equipped with a MCT/B detector and a diffuse reflectance cham-
ber (Harrick) with ZnSe window. The samples were dried at 130 ◦C
for 30 min under He flow (20 mL min−1). Then, they were reduced
at 450 ◦C for 1 h under 22% H2/He (2 mL min−1) flow and purged
under He flow for 10 min. After that, the catalysts were oxidized
at 450 ◦C for 30 min under 21% O2/He flow (20 mL min−1). Pass-
ing He through an ethanol saturator maintained at 10 ◦C generated
vapor of ethanol. The adsorption of ethanol was conducted at 50 ◦C
for 1 h using an ethanol/He flow (10 mL min−1). Ethanol desorp-
tion was performed under He flow (40 mL min−1) at 20 ◦C min−1
from 50 ◦C to 450 ◦C. The compounds were continuously analyzed
by an on-line Dycor Mass Spectrometer, Ametek Process Instru-
ments. The DRIFTS spectra were obtained using 4 cm−1 and 64
scans. They were collected at each 50 ◦C in the range 50–450 ◦C. The
main fragments followed up during the analyses were: m/z = 2 (H2),
m/z = 18 (H2O), m/z = 26 (ethene), m/z = 29 (acetaldehyde), m/z = 31
(ethanol), m/z = 44(CO2) and m/z = 43 (acetone). The intensities of
these fragments were mathematically treated in order to eliminate
contributions of more than one species.
Fig. 1. Selectivities of m-ZrO2 and prepared samples at isoconversion
(∼40%). The flow rate, the gas mixture composition and temperature are
70 mL min−1, N2:H2O:C2H5OH = 91:8:1 mol% and 400 ◦C, respectively. The symbol
O.7Zn WW is related to the catalytic test carried out employing 0.7Zn at the same
conditions described above with adding H2O (N2:C2H5OH = 99:1 mol%).
for 1 h. The H2O vapors were generated by passing He (50 mL min−1
through a water saturator at 40 ◦C. The desorption was carried out
employing a He flow (50 mL min−1) at 10 ◦Cmin−1 from 25 to 450 ◦C.
The fragments, m/z = 2 (H2), m/z = 32 (O2) and m/z = 18 (H2O) were
continuously monitored during the analyses.
2.3.9. XRD (in situ)
X-ray diffraction (XRD) was performed using a D8 Advance
Bruker diffractometer equipped with an Anton Paar chamber, a
CuK␣ radiation source (1.5418 Å) and a Ni filter operated at 40 kV
and 40 mA. The angular range varied from 5◦ to 90◦, with incre-
ments of 0.05◦ and 2 s per step. The samples (m-ZrO2 and 0.7Zn)
were dried at 130 ◦C under N2 flow (15 mL min−1) for 30 min and
reduced under 2% H2/N2 flow (15 mL min−1) at 450 ◦C for 2 h. After
that the catalyst was purged under N2 flow down to room temper-
ature. The diffraction patterns were collected at room temperature
after drying and reduction. The diffraction patterns were analyzed
using the Rietveld method with fundamental parameters as imple-
mented in the software TOPAS Academic v. 4.1.11.
2.3.7. TPD-ethanol followed by XANES
X-ray Absorption Near Edge Spectroscopy (XANES) in situ were
carried out employing 0.7Zn at the Brazilian Synchrotron Light Lab-
oratory (LNLS) on XAFS2 beamline. The spectra were collected at
the Zn K-edge and Zr K-edge employing a double crystal Si 111
monochromator. The sample (100 mg) was dried at 130 ◦C under
He flow (95 mL min−1) for 30 min. Then, the catalyst was reduced
under 5% H2/He flow (100 mL min−1) at 450 ◦C for 1 h. After that, the
catalyst was purged under He flow and then oxidized at 450 ◦C for
30 min under 5% O2/He flow (50 mL min−1). The ethanol adsorp-
tion was carried out for 1 h at 30 ◦C using an ethanol/He flow
(10 mL min−1). The vapor of ethanol was generated passing He
through a saturator at 10 ◦C. The TPD of ethanol was performed
under He flow (38 mLmin−1) at 3 ◦C min−1 from 30 ◦C to 450 ◦C.
The spectra were collected in step-scan mode during the reduction,
oxidation, ethanol adsorption and the ethanol desorption steps.
The data processing and analysis was performed by the Demeter
software package [12].
3. Results and discussion
The Zn loading of the three prepared catalysts were analyzed
employing ICP-AES. The Zn concentrations of the 0.4Zn, 0.6Zn and
0.7Zn catalysts are 0.4, 0.6 and 0.7 wt.%, respectively. The specific
surface areas of both m-ZrO2 and Zn based catalysts were also mea-
sured. All the values obtained are in the proximity of 98 m2 g−1
.
3.1. Catalytic tests
The selectivities to acetone, ethene, acetaldehyde and CO2 at iso-
conversion of the Zn catalysts and m-ZrO2 samples are depicted in
Fig. 1. Methane and propene were also observed despite their very
low concentration. Each one of the four samples generates the same
compounds. Nevertheless, m-ZrO2 shows a much higher selectivity
to ethene than the others. This fact brings to light that Zn added to
m-ZrO2 change the catalytic behavior of this oxide. In spite of the
small increase of the acetaldehyde selectivity, which is observed
when the Zn concentration increases from 0.4 to 0.6 wt.%, it can be
2.3.8. TPD-H2O
The temperature programmed desorption of H2O was carried
out using a micro reactor system coupled to a mass spectrometer
QMS200 Balzers. The samples were dried at 130 ◦C for 30 min under
He flow (30 mL min−1). After that they were reduced at 450 ◦C for
1 h, 5% H2/N2 (50 mL min−1) and then purged with He for 30 min at
450 ◦C. The H2O adsorption was carried out at room temperature
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Fig. 3. Ethanol conversion and selectivities of 0.7Zn versus WHSV. The gas mixture
composition and temperature are N2:H2O:C2H5OH = 91:8:1 mol%, 400 ◦C, respec-
tively.
Fig. 2. Ethanol conversion and selectivities of 0.7Zn versus temperature. The
flow rate, the gas mixture composition and the weight of 0.7Zn are 70 mL min−1
N2:H2O:C2H5OH = 91:8:1 mol%, 50 mg, respectively.
,
and are in line with the results of Fig. 2 and the ones of Rodrigues
et al. [5].
inferred that 0.4Zn, 0.6Zn and 0.7Zn show similar selectivities to
ethylene, acetone, CO2 and acetaldehyde.
3.2. The acid and basic properties
The isoconversion tests of the prepared samples (0.4Zn, 0.6Zn
and 0.7Zn and m-ZrO2) were carried out employing the same exper-
imental conditions except for the catalysts mass. In order to reach
40% of conversion, 50, 40 and 33 mg of 0.4Zn, 0.6Zn and 0.7Zn, were
used, respectively. Thus, the lower the Zn concentration, the higher
the mass used in these tests. Indeed, the Zn/Zr atomic ratio obtained
by chemical analysis and the mass employed in these experiments
show a linear correlation (r2 = 0.995). This result reveals a direct
association between the catalysts behavior and their Zn content,
i.e., the higher the amount of zinc, the most active the catalyst is.
The selectivities data of these catalysts suggest that adding Zn to
m-ZrO2 there is an increase in the number of active sites without
changing significantly the catalytic properties of the catalysts.
The 0.7Zn was also tested at isoconversion (∼40%, 33 mg of cat-
alyst) without adding H2O to the feed (Fig. 1, 0.7Zn WW). The
temperature, ethanol concentration and flow rate values were the
same which were used in the tests described above. It can be noted
that the main products are ethene and acetaldehyde. The selectiv-
ity to acetone and CO2 is very low. This result shows that without
H2O the acetone synthesis does not occur.
As 0.7Zn showed the highest activity when it was compared with
the other catalysts, it was chosen to be analyzed by the techniques
described above. Besides that, m-ZrO2 is used as a reference.
The ethanol conversion and selectivities versus the reaction
temperature for 0.7Zn are shown in Fig. 2. Increasing the tem-
perature the ethanol conversion increases and the selectivity to
acetaldehyde decreases (from 375 ◦C), whereas the one to acetone
(until 425 ◦C) and CO2 increase. The selectivity to acetone shows a
maximum at 425 ◦C. The selectivity to isobutene rises above 425 ◦C.
According to Sun et al. [7] this last compound is generated by the
condensation of three molecules of acetone.
Table 1 exhibits the acid and basic sites densities of the catalysts.
On the one hand, when NH3 is used as probe molecule, it can be
observed that adding Zn to m-ZrO2, the density of the strong acid
sites decreases considerably, whereas the one of the medium and
weak sites increases. On the other hand, the density of the strong
basic sites increases, whereas the one of the medium and weak sites
decreases (TPD-CO2).
Only Lewis acid sites were observed when pyridine is employed
as a probe of acid sites [13]. In this case, the acid sites density of m-
ZrO2 is around sevenfold higher than the one of the Zn catalyst. This
result is similar to the ratio of the strong acid sites densities of m-
ZrO2 to 0.7Zn measured by NH3. It indicates that the data generated
by pyridine are mainly related to the strong acid sites. This might
occur due to the basic strength of these probe molecules and also
the experimental conditions employed.
It is worth stressing that the changes in the acid and basic prop-
erties of m-ZrO2 introduced by Zn might contribute to modify the
catalytic behavior of this oxide.
Acetaldehyde can be synthesized by dehydrogenation of ethanol
[14]. The first step of this reaction occurs on an acid-strong base
pair site which breaks the O H bond of ethanol and generates an
ethoxide species. After that the ␣-hydrogen of the ethoxide group is
abstracted by another strong basic site producing acetaldehyde and
H2 [14]. Table 1 exhibits that m-ZrO2 and 0.7Zn show strong basic
sites and also acid sites. Thus, these oxides are able to dehydro-
genate ethanol. Acetaldehyde can also be produced by the oxidative
dehydrogenation of ethanol (ODH). This reaction might also con-
tribute to the synthesis of this aldehyde. The density of strong
basic sites and redox properties are very important for this syn-
thesis, being the former more relevant [15]. Moreover, according
to Rodrigues et al. [5] strong basic sites are also very relevant for
the acetate condensation which generates acetone and CO2.
Some experiments were also carried out increasing the space
velocity of the reaction (Fig. 3). It was observed that as the residence
time decreases, the conversion of ethanol and the selectivity to ace-
tone and CO2 decrease, whereas the selectivity to acetaldehyde
increases. These results indicate that ethanol generates acetalde-
hyde and then this aldehyde synthesizes acetone, CO2 and H2. These
data highlight that acetaldehyde is an intermediate of this reaction
3.3. TPD – ethanol followed by IR spectroscopy
The m-ZrO2 and 0.7Zn were analyzed employing the TPD of
ethanol followed by IR and mass spectroscopies. Figs. 4 and 5
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Table 1
Density of weak (W), medium (M), strong (S) and total (T) basic sites and acid sites.
Sample
Basic sites density (␮mol CO2 g−1
)
Acid sites density (␮mol NH3 g−1
)
Acid sites density (ua g−1, pyridine)
T
W + M
S
T
W + M
S
T
m-ZrO2
0.7Zn
191
147
46
69
237
216
18
28
25
5
43
33
182
26
m-ZrO2. Both figures also show a band around 1159 cm−1 and a
tiny absorption at 1076 cm−1 at low temperature. These bands can
be associated with v(CO) vibrations of monodentate and bidentate
ethoxide species, respectively. As the temperature increases the
high intensity band shifts from ∼1159 cm−1 to ∼1144 cm−1 (see
Figs. 4, 5). This indicates that the adsorption sites of the ethox-
ide species are changing. Other vibrations, 1447 and 1381 cm−1
,
related to the ethoxide species can be assigned to ␦as(CH3) and
␦s(CH3), respectively. On the one hand, the intensities of the ethox-
ide species bands decrease as the temperature increases. On the
other hand, increasing the temperature, the intensity of the bands
around 1587, 1442, 1420 cm−1, assigned to ␯ (OCO), ␦as(CH ), ␯
as
s
3
(OCO) of the acetate species, respectively, increase as well [16],
being this effect more intense in the case of 0.7Zn. Thus, it can be
proposed that ethoxide species are dehydrogenated to acetalde-
hyde (Fig. 3), which is then oxidized to acetate species. These
species condensate promoted by strong basic sites and generate
acetone and CO2 [5,17]. Moreover, the bands around 1554 and
1356 cm−1 can the associated with bidentate carbonates [16]. Thus,
the acetate species condensate or oxidize producing CO2, which
react with the catalysts producing carbonates.
The TPD of ethanol employed He as the carrier gas. Therefore,
according to the Mars and Van Krevelen mechanism, the O of the
m-ZrO2 or 0.7Zn lattice is the oxidant agent of the generation of
acetates and carbonates species during these experiments. Thus,
the O consumption of the lattice changes the catalysts surface.
Binet et al., [18] showed that the v(CO) vibrations of the monoden-
tate ethoxide species adsorbed on CeO2 shift to lower frequencies
when oxygen is eliminated from the surface. Therefore, the shifts
observed in the case of the v(CO) vibrations of the monodentate
ethoxide species (∼1159 cm−1 to ∼1144 cm−1) can be associated
with removal of the superficial O of the m-ZrO2 and 0.7Zn surfaces.
Moreover, as well known, DRIFTS measurements are not quantita-
tive. However, it can be observed that the intensity of the acetate
species vibrations of 0.7Zn is much higher than the ones of m-ZrO2.
The insertion of Zn in the m-ZrO2 lattice promotes its redox proper-
ties rendering 0.7Zn more active for the oxidation of the oxygenate
species (see Introduction). It is worth stressing that ethene is not
observed in the m-ZrO2 DRIFTS spectra due to the low intensity of
its absorption band.
Fig. 4. TPD spectra of ethanol over m-ZrO2 (in situ DRIFTS): (a) 150 ◦C, (b) 250 ◦C, (c)
300 ◦C, (d) 350 ◦C, (e) 400 ◦C, and (f) 450 ◦C, respectively.
3.4. TPD – ethanol followed by mass spectroscopy
The spectra of ethanol, acetone, acetaldehyde, ethene, H2 and
CO2 of the TPD of ethanol followed by mass spectrometry (MS) are
depicted in Fig. 6.
Both catalysts show spectra which exhibit similar shapes except
for the one of acetaldehyde. In spite of the different catalytic behav-
ior of 0.7Zn and m-ZrO2 (Fig. 1), these results suggest that the
phenomena that occur on these catalysts during the TPD of ethanol
are almost the same. The first step of the acetone synthesis is
the generation of acetaldehyde [5] (Fig. 3). The spectra of this
aldehyde for both catalysts depict that, at low temperature (tem-
perature < ∼280 ◦C), it is entirely consumed generating acetone and
CO2. It is worth stressing that at this temperature there is ethanol
adsorbed on the catalysts. Therefore, the generation of acetalde-
Fig. 5. TPD spectra of ethanol over 0.7Zn (in situ DRIFTS): (a) 150 ◦C, (b) 250 ◦C, (c)
300 ◦C, (d) 350 ◦C, (e) 400 ◦C, and (f) 450 ◦C, respectively.
exhibit the DRIFTS spectra of 0.7Zn and m-ZrO2, respectively. These
figures show bands at 1256 cm−1. These peaks can be assigned
to ␦as(OH) of ethanol. As the temperature increases, the inten-
sity of these bands decrease. This effect is more relevant in the
case of 0.7Zn due to the fact that this catalyst is more active than
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Table 2
6
Amount of H2 produced during the TPD-H2O and CO consumption rate of WGS at
450 ◦C (-rCO).
samples
−1
H2 ␮molgcat
−rCO ␮molgcat−1min−1
m-ZrO2
0.7Zn
0.68
1.33
20
75
[20,21]. When H2O is adsorbed on the O vacancies it generates two
hydroxyl species, which react producing H2 and two O on the sur-
face of CeO2, thus, oxidizing its surface. Previous work of our group
showed that 0.7Zn and m-ZrO2 are also able to dissociate H2O, being
the 0.7Zn catalyst the most efficient one due to its higher amount
of vacancies [9]. The TPD of water was carried out employing these
samples, 0.7Zn and m-ZrO2. Two broad peaks of H2 were observed
with their maxima around 100 and 160 ◦C for 0.7Zn and m-ZrO2,
respectively. Table 2 depicts that 0.7Zn generates much more H2
than m-ZrO2. Therefore, the H2 spectra of the TPD-ethanol of both
catalysts can be associated with the dissociation of H2O. More-
over, the H2 peak intensity of 0.7Zn is much higher than the one
of m-ZrO2, which is in line with Table 2 results. Thus, after the first
peaks of acetone and CO2, the H2O dissociation reoxidizes the cat-
alysts and the reaction occurs again (CO2 and acetone peaks). Fig. 1
exhibits that without H2O there is a very low selectivity to ace-
tone synthesis. This happens because without the reoxidation of
the catalyst, the catalytic cycle does not close and the reaction can-
not occur. Both samples also show that the generation of H2 occurs
before the ethene desorption (see the dots). This occurs because the
dehydrogenation of acetaldehyde also generates H2 [14]. The dif-
ferent shapes of the acetaldehyde spectra might be associated with
the different consumption rates of this aldehyde as an intermediate
species. The acetaldehyde MS signal of m-ZrO2 emerges at 345 ◦C,
between the two peaks of CO2 or acetone peaks. At this tempera-
ture the surface of the catalyst is reduced. As a consequence, the
acetaldehyde oxidation rate is very low and this aldehyde is des-
orbed. In the case of the 0.7Zn spectrum, a shoulder can be observed
at around this same temperature. However, the maximum of the
acetaldehyde spectrum is at 450 ◦C, which is the same temperature
of the CO2 and acetone desorption peaks (high temperature). At
the temperature of 450 ◦C acetaldehyde might be also synthesized
by the oxidative dehydrogenation reaction. This result suggests
that at high temperature the acetaldehyde generation is not the
slower step of the acetone synthesis, but rather the oxidation of
acetaldehyde.
Fig. 6. TPD spectra of ethanol over m-ZrO2 and 0.7Zn (analyzed by MS).
hyde seems to be the slower step of the acetone synthesis at these
low temperatures for both catalysts.
Some catalytic tests were carried out employing m-ZrO2 and
0.7Zn at the same experimental conditions used in the isoconver-
sion tests (Fig. 1), except for the high values of the residence times
and low temperatures (400 mg, 40 mL min−1, 250 ◦C). Both cata-
lysts generate almost only acetaldehyde at very low conversions,
which are much smaller than the equilibrium conversions (thermo-
dynamic data of the dehydrogenation of ethanol [19]. These results
suggest that the acetaldehyde synthesis is a slow step under these
conditions (at low temperature).
Both the acetone and CO2 spectra of the 0.7Zn catalyst show two
peaks (Fig. 6). The same occurs for the acetone and CO2 spectra of
the m-ZrO2 catalyst. The first peaks of the CO2 and acetone spectra
are almost at the same temperature for both catalysts. However,
when the ratio of the intensities of CO2 to the acetone peaks for
the 0.7Zn catalyst (low temperature peaks) is compared with the
same ratio for the m-ZrO2 catalyst, it can be observed that the for-
mer is much higher. The same result can be verified for the bands of
CO2 and acetone at high temperature. It can be proposed that the
0.7Zn catalyst promotes the total oxidation of the acetates (CO2
high intensity peaks), which is in line with its improved redox
properties (see Introduction, [9]).
Oxygen was not admitted in the system during the TPD of
ethanol. Thus, it can be suggested that the O of the 0.7Zn and m-
ZrO2 catalysts surfaces are consumed by the synthesis of acetone
and total oxidation. The ethoxide species shifts exhibited by the
DRIFTS spectra (see Figs. 4 and 5) are related to the O elimination
from the catalysts. Moreover, after the acetone and CO2 desorption
first peaks, as the surface of the catalysts are reduced, the ketone
synthesis is almost extinguished.
Zirconia synthesizes a larger amount of ethene at a slightly lower
temperature than 0.7Zn (see MS spectra). This result is in line with
the catalytic results (Fig. 1). According to Di Cosimo [14], ethene
can be synthesized by the E2 mechanism. It is related to the simul-
taneous abstraction of OH and ␤-H by a pair of acid and basic Lewis
sites, respectively. It is known that the higher the strength of the
acid sites, the higher the rate to the ethylene synthesis. Both m-ZrO2
and 0.7Zn show strong acid sites (Table 1). However, the one with
Zn exhibits a much lower density of these sites. Thus, the catalytic
behavior related to the ethylene synthesis of these two catalysts
(Fig. 1) is associated with their density of strong acid sites depicted
on Table 1.
3.5. The WGS reaction as a model reaction
The redox [22] and associative (carboxyl [23] and formate [24])
mechanisms have been proposed for the water gas shift (WGS)
reaction employing Pt or other metals supported on reducible
oxides. Zonetti et al. [25] verified that some oxides (without metal)
promote the reverse water gas shift reaction (RWGS) and follow
the redox mechanism. Considering the principle of the microscopic
reversibility, this might also occur in the case of the WGS reaction.
Thus, the WGS reaction on oxides can be described as following:
initially, CO is oxidized to CO2 by the O of the oxide lattice; then,
this catalyst is reduced generating O vacancies; finally, H2O disso-
ciates on this vacancies, re-oxidizing the oxide and producing H2.
The slower step of this mechanism is the reduction of the catalyst.
Indeed, the oxidation step of acetaldehyde to acetate species is very
similar to the WGS reaction (CO + H2O → CO2 + H2). Both are related
to the oxidation of C O species using H2O as an oxidant agent.
Thus, it can be proposed that the WGS reaction can be employed as
a model reaction for probing the redox properties of the catalysts
employed in the acetone synthesis.
The dehydration of ethanol generates ethene and H2O. The H2O
dissociation on CeO2 (reduced) is well described in the literature
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Fig. 7. TPD spectra of ethanol over 0.7Zn (XANES at Zn K-edge): (a) initial sample,
(b) sample at the end of the reduction, (c) sample at the end of the oxidation, (d)
sample at the end of ethanol adsorption, (e) sample at the end of ethanol desorption,
(f) ZnO reference and (g) Zn foil reference; [inset] figure shows the relative intensity
on the white line region.
Table 2 exhibits the CO consumption rates of m-ZrO2 and 0.7Zn
in the WGS reaction. The 0.7Zn catalyst shows a higher consump-
tion rate of CO. It is almost fourfold higher than m-ZrO2. It oxidizes
CO faster because it shows higher reducibility [25]. Considering the
similarity of the CO oxidation with the acetaldehyde oxidation, it
can be suggested that 0.7Zn is more active for this acetaldehyde oxi-
dation. This result is in line with the TPD-ethanol that shows that
0.7Zn is very active for acetaldehyde selective and total oxidation.
Fig. 8. XANES spectra at Zr K-edge collected at the beginning (a) and at the end (b)
of the TPD of ethanol.
0.7Zn is reduced during the pretreatment (Fig. 7b), a decrease in
the white line intensity when compared with Fig. 7a is observed.
According to Waychunas et al. [28], this change is associated with
the protonation of Zn (Zn-O → Zn-OH). Recently, Schimming et al.
[29] proved that the CeO2 vacancies are able to dissociated H2. Thus,
it might be suggested that H2 can also be dissociated on the 0.7Zn
catalyst vacancies generating hydroxyl species on the remaining
O of the 0.7Zn surface. The catalyst reoxidation (Fig. 7c) slightly
decreases the protonation of its surface. The spectra depicted on
Fig. 7d and e were collected after the ethanol adsorption and des-
orption (TPD of ethanol), respectively. Both spectra show a decrease
of the white line intensity which can be associated with the adsorp-
tion of oxygenate species on Zn+2 [30,31].
3.6. TPD – ethanol followed by XANES
Fig. 7 shows the experimental XANES at Zn K-edge of 0.7Zn dur-
ing the TPD of ethanol (after normalization procedure). The spectra
a, b, c, d, e, f and g are related to (a) initial sample, (b) sample at
the end of the reduction, (c) sample at the end of the oxidation, (d)
sample at the end of the ethanol adsorption, (e) sample at the end
of the ethanol desorption, (f) ZnO reference and (g) Zn foil refer-
ence. The [inset] figure shows the relative intensities of the spectra
in the white line region.
The XANES spectra at the Zr K-edge of 0.7Zn were also collected
during the TPD of ethanol on the 0.7Zn catalyst (Fig. 8). These spec-
tra show features that can be assigned to the monoclinic m-ZrO2
[31]. The XANES spectra at the Zr K-edge of 0.7Zn catalyst did not
change after ethanol adsorption and desorption procedures.
On the one hand, the XANES at Zn K-edge of 0.7Zn spectra show
changes in the intensity of the white line which are associated with
the interaction of some species with Zn during or after the reduc-
tion step and TPD of ethanol. On the other hand, XANES at Zr K-edge
spectra of the same catalyst submitted to these same experimental
procedures do not change. This result shows that the TPD −ethanol
occurs on Zn+2 and its vicinity and also that Zr+4 species show very
low activity. Indeed, Fig. 1 shows that a small amount of Zn is able to
drop the selectivity of ethylene from 68% to 12%. This behavior can
be associated not only with changes in the acidity of the catalyst
promoted by Zn, but also to the increase of redox properties which
promote the acetone synthesis. This result shows that the active
sites of 0.7Zn are Zn+2 and its vicinity. Indeed, the catalytic test (iso-
conversion tests) are in line with the XANES results which shows
that adding Zn to m-ZrO2 increases in number of active sites with-
out changing significantly the catalytic properties of the catalysts.
Moreover, XANES at Zr K-edge and XANES at Zn K-edge spectra not
only confirm the solid solution formation, but also exhibit that it
The position of the absorption edge in the XANES spectra reveals
the oxidation state of Zn. The Zn◦ atoms (Fig. 7g) exhibits the edge
at 9659 eV, whereas the one of Zn2+ (ZnO, Fig. 7f) at 9662 eV, which
is ascribed to a 1s → 4p electronic transition of Zn2+ ions [26,27].
The Zn K-edge position of 0.7Zn is observed at ∼9662.4 eV at
the TPD of ethanol. No energy shifts were verified during the TPD.
The value of the absorption edge is equivalent to the one of ZnO
(zincite), indicating that Zn oxidation state in this catalyst is Zn2+
.
Zinc in the zincite (Zn2+ ions) is in a tetrahedral geometry. However,
the spectrum shape of 0.7Zn (Fig. 7a–e) cannot be designated as ZnO
(see Fig. 7f). Thus, it can be suggested that Zn2+ is a component of
a solid solution or it is in the hydrozincite phase, which shows a
mixture of tetrahedral and octahedral sites [28]. Indeed, this result
is in line with our previous study which proposed that: firstly, Zn+2
is in a solid solution in the superficial layers of m-ZrO2 (see Intro-
duction) and secondly that there is no ZnO in the 0.7Zn sample [9].
The TPD of ethanol on 0.7Zn spectra indicated that the superficial O
of 0.7Zn is employed to oxidize ethanol to acetone or more specifi-
cally acetaldehyde to acetate species and CO2. However, the XANES
spectra show that Zn+2 is not reduced.
The spectra depicted in Fig. 7a–e are very similar. However,
some changes can be observed on the white line intensity. When
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is stable during the TPD of ethanol. Some spectra collected during
the acetone synthesis (reaction) confirm the stability of the solid
solution.
4. Conclusion
The present study suggests that the main steps of the acetone
generation from ethanol are the following: firstly, ethoxide species
are generated and, then, they are dehydrogenated to acetaldehyde.
Both steps are related to strong basic and acid sites. According to
the Mars and Van Krevelen mechanism, acetaldehyde reacts with
the O of the solid solution surface generating acetate species and
vacancies on the catalyst surface. These carboxylate species con-
densate (strong basic sites) and generate acetone and CO2. Water
dissociates on the vacancies of the catalyst and reoxidizes the cata-
lyst surface, closing the catalytic cycle. All these steps might occur
on Zn+2 and on the species in its vicinity.
3.7. The XRD “in situ”
The catalysts 0.7Zn and m-ZrO2 both oxidized and reduced in situ
were analyzed by XRD (see Table S1, Supporting information). All
the four samples show m-ZrO2 and small amount of t-ZrO2. The
ZnO was not observed. The addition of Zn to m-ZrO2 significantly
changes the lattice parameters of this oxide, which can be better
observed considering the cell volume values. The Rietveld refine-
ment is able to model the substitution of Zr by Zn atoms. It was
also observed that there is an increase of the oxygen vacancies
concentration when Zn is added to m-ZrO2. Considering the very
small amount of tetragonal phase in the samples, it was not possi-
ble to infer if Zn substitutes Zr in this phase. Table S1 showed that
reduction treatments at temperatures lower than 450 ◦C did not
change the crystallographic characteristics of the studied samples.
This result is in line which the XANES ones.
Acknowledgments
The authors are indebted to Eng. Johnatan Celnik and
the LACAT/INT staff for their assistance in the experimental
work. Brazilian Synchrotron Light Laboratory (LNLS) is gratefully
acknowledged for the use of XAFS2 beamline installation (Project
no. XAFS1-17747). The financial support of FAPERJ, CNPq and CAPES
(Brazil) is also acknowledged.
Appendix A. Supplementary data
3.8. Final comments
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.09.
011.
On the one hand the XANES and XRD results show that the
reduction of 0.7Zn by H2 or acetaldehyde does not significantly
change the monoclinic structure. On the other hand, it is possi-
ble to infer considering the TPD-ethanol results that superficial O
of 0.7Zn is consumed by the redox process.
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