The role of MPV reaction in the synthesis of propene from ethanol through the acetone route

Article abstract of DOI:10.1016/j.catcom.2020.106096

Physical mixtures of AgCeO₂ and ZrO₂/SiO₂ were employed in the ethanol conversion to propene in the presence of water through the acetone route. The oxides were characterized by XRD, N₂ physical adsorption, isop

Full text of DOI:10.1016/j.catcom.2020.106096

Catalysis Communications 145 (2020) 106096
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
The role of MPV reaction in the synthesis of propene from ethanol through
the acetone route
⁎
C.R.V. Matheus, E. Falabella S. Aguiar
Chemistry School, Technology Center, UFRJ, Av. Athos da Silveira Ramos, 149 - Cidade Universitária, 21941-972 Rio de Janeiro, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Physical mixtures of AgCeO
2
and ZrO
2
/SiO
2
were employed in the ethanol conversion to propene in the presence
Heterogeneous catalysis
Ethanol
of water through the acetone route. The oxides were characterized by XRD, N physical adsorption, isopropanol
2
dehydration/ dehydrogenation and ethanol to acetone MPV model reactions. The mixtures were tested under
Propene
different WHSVs and temperatures. This study presents new perspectives for the propene synthesis from ethanol.
MPV reduction and dehydration are both important for the overall reaction. Understanding how to balance these
reactions is an important step towards the synthesis of new catalysts able to produce propene from ethanol, with
high selectivity to propene.
Green chemistry
One pot
1
. Introduction
This work intends to present a catalytic system capable of con-
ducting the transformation of ethanol into propene with low amounts of
Propene is an important molecule which is attracting attention as its
ethene and to discuss the effects of each model reaction on the per-
formance of the whole reaction system. A comparison between this and
the previous mixture [7] will be made in order to understand how these
reactions are important for the system. For that reason, the following
model reactions have been used to determine the activity/selectivity of
the catalytic mixtures deployed: isopropanol dehydration/ dehy-
drogenation and MPV reduction. The first produces propene (dehy-
dration) or acetone (dehydrogenation) whereas the second, acet-
aldehyde and isopropanol.
demand grows while the world looks for sustainable processes [1,2].
Although some routes are under investigation for green propene [2–5],
the acetone route [1,6] seems to be the cleanest among them all. It may
be represented by Eq. (a)-(f). Some catalytic systems have already been
proposed for this route: Sc/In
2
O
3
[2], Y
2
[7].
O
3
_CeO
2
[3] and a physical
mixture comprising AgCeO + t-ZrO
2
2
2CH3CH2OH
CH3CHCH2 + CO2 + 3H2
(a)
1
2
CH3CH2OH + O2
CH3CHO + H2 O
(b)
(c)
(d)
(e)
2. Experimental
2
CH3CHO + H2 O
CH3COCH3 + CO2 + H2
2.1. Catalysts
CH3CHOHCH3
CH3CHCH2 + H2 O
The CeO
2
catalyst was synthesized by precipitating solubilized
CH3CH2OH
CH2 CH2 + H2 O
[
(NH Ce(NO
4
)
2
3
) ] (Sigma-Aldrich) with a strong base (NaOH – Sigma-
6
CH3CH2OH + CH3COCH3
CH3CHOHCH3 + CH3CHO
(f)
Aldrich) in aqueous solution, added dropwise, with continuous stirring
at room temperature. The solid obtained was filtered and washed with
distilled water until neutral pH. Eventually, it was calcined at 500 °C for
In previous works [7], we have assessed some steps of this reaction,
presenting thermodynamic calculations and experimental results. We
have been able to show that reactions from a to f represent well the
propene production from ethanol by employing a physical mixture
−1
1
h at 10 °Cmin . The resultant was submitted to wetness impregna-
tion using an AgNO (Sigma-Aldrich) solution. The solid with 0,02% w/
3
w Ag was dried overnight at 100 °C, calcined at 500 °C for 4 h under
comprising AgCeO
2
and t-ZrO . Important characteristics of the cata-
2
−1
flow of synthetic air at 3 °Cmin , milled and passed through a sieve
with a 90 μm mesh.
lytic mixtures for the reaction were revealed by using model reactions
such as the isopropanol dehydration/ dehydrogenation and the Meer-
wein-Pondorf-Verley hydrogen transfer from ethanol to acetone.
Silica (Saint-Gobain NorPro) was also submitted to wetness
⁎
Corresponding author.
E-mail address: efalabella@eq.ufrj.br (E.F.S. Aguiar).
https://doi.org/10.1016/j.catcom.2020.106096
Received 24 May 2020; Received in revised form 20 June 2020; Accepted 22 June 2020
Available online 24 June 2020
1566-7367/ © 2020 Elsevier B.V. All rights reserved.

C.R.V. Matheus and E.F.S. Aguiar
Catalysis Communications 145 (2020) 106096
impregnation of a zirconyl nitrate (Sigma-Aldrich) solution in order to
obtain catalysts with 2.5%, 5.0% and 7.5% w/w of ZrO . The resultant
2
was dried overnight at 100 °C and calcined at 500 °C for 2 h at 10
−
1
−1
°
Cmin
under air flow (30 mLmin ).
2.2. Characterization
The BET specific surface areas (SBET) of the catalysts were obtained
by N physisorption at −196 °C employing a Micromeritics ASAP 2010.
2
The samples were first heated at 100 °C for 12 h and then treated in situ
under vacuum at 350 °C for 4 h.
X-ray diffraction (XRD) was performed using a Bruker D8 Advance
instrument equipped with a LynxEye detector, Ni filter and CuKα
(
1.5418 Å) radiation. Data were collected at room temperature, from
1
0° to 90°, with a step size of 0.02° and 1 s per step.
In order to test the catalysts performance for intermediate steps, two
model reactions were carried out: the isopropanol dehydrogenation/
dehydration reaction and the hydrogenation of acetone by ethanol
(
MPV reduction). The tests were performed in a fixed-bed reactor and
Fig. 1. XRD diffractograms of zirconia on silica in different mass contents
(
2.5%, 5% and 7.5% ZrO2) and AgCeO .
2
monitored online by gas chromatography with flame ionization (FID)
and thermal conductivity (TCD) detectors. Both experiments were
conducted under differential conditions at 200 °C and atmospheric
Table 1
BET area and MPV and isopropanol model reactions results.
pressure. The reagents vapors were generated by passing N through
2
saturators with isopropanol (for its dehydrogenation/ dehydration) or
ethanol and acetone (for the MPV reduction).
[
a]
[b]
[c]
[d]
ra[e]
Catalyst
S
BET
Ri
Ra
Ri/Ra
rp
rp/ra
Before performing the isopropanol and the MPV model reactions and
2.5ZrO
2
/ SiO
2
2
205
207
206
3.31
3.73
4.29
2.93
3.32
3.51
1.13
1.12
1.22
0.03
0.05
0.06
0.05
0.07
0.02
0.64
0.78
4.11
5
ZrO
2
/ SiO
2
the catalytic tests, the samples were firstly dried at 150 °C under N
2
flow (30
−1
−1
7,5ZrO
2
/ SiO
mLmin ) for 30 min. After that, they were reduced at 500 °C (10 °Cmin
)
for 1 h under 10% H
2
/N
2
flow, than purged with N and oxidized with
2
[
a] BET area. [b] Ri isopropanol generation rates. [c] Ra acetaldehyde gen-
−
1
synthetic air (90 mLmin ) at 500 °C for 30 min. Exceptions were the MPV
model reaction, where catalysts were reduced and oxidized at 400 °C and
the catalytic tests, with reduction at 400 °C and no following oxidation.
Besides that, when the catalytic tests were made under different tempera-
tures, the reduction temperatures were 500 °C.
eration rates. [d] rp formation rates of propylene. [e] ra acetone generation
rates.
The AgCeO catalyst (not shown) presented the same XRD diffrac-
2
tion patterns than those obtained for pure ceria (fluorite), as expected
due to the low amount of Ag [7].
2
.3. Catalytic tests
Table 1 presents the catalysts BET surface areas (SBET). It is possible
to observe that the SBET of each catalyst containing SiO as support has
2
The ethanol conversion to propene in presence of water vapour was
2
−1
approximately the same value (200 m g ), which is much higher than
evaluated in a fixed-bed reactor under atmospheric pressure, at 400 °C,
2
−1
the area of AgCeO
According to previous works, catalysts based on ceria present
mostly basic sites [1], while t-ZrO presents both acid and basic sites
1,7,8]. These properties were related to the isopropanol dehydration/
dehydrogenation reactions [9,10], which are presented for the ZrO
SiO catalysts in Table 1. Basic sites catalyze dehydrogenation, whereas
2
(40 m g ).
using 50 mg of AgCeO
2
and/or 25 mg of ZrO
2
/SiO
2
. Ethanol and water
−
1
vapors were generated by a N
2
flow (50 mLmin ) through two saturators:
2
one at 57.2 °C and the other at 5.3 °C, respectively, leading to partial
pressures of ethanol and water of 1 and 9%, respectively. The reaction
products were analyzed online during 17 h using a GC Agilent HP6890
instrument equipped with FID and TCD detectors. The catalysts were also
[
2
/
2
acidic sites promote dehydration.
evaluated under different temperatures, proportions (AgCeO
2
: (ZrO
2
/SiO ))
2
Supporting zirconia on silica changed its properties, making it less prone
to dehydration, as observed in the isopropanol model reaction [7,9,10],
and WHSVs. The ethanol conversion was defined as the ratio of the moles of
ethanol consumed to the moles of ethanol in the feed. The definition of
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. Considering the propene synthesis reaction, 75% is the
highest selectivity that can be obtained.
where ZrO
but lower r
the highest proportion of ZrO
especially, to a higher ratio between propylene and acetone formation rates
/r ). SiO was inactive for the isopropanol reaction.
Table 1 also depicts the MPV model reaction between ethanol and
acetone. The higher amount of zirconia on SiO brings about higher
rates of formation of both isopropanol (r ) and acetaldehyde (r ), with
similar ratio values. The rates presented in this work are even higher
than those found for t-ZrO [7]. Both AgCeO and SiO (not shown)
were inactive for this model reaction.
Catalytic tests of these oxides were performed in a physical mixture
with AgCeO under different conditions, as depicted in Table 2.
Table 2 depicts catalytic tests under the same conditions for the
2
p
/SiO
2
catalysts presented not only lower activities to propene,
/r
a
ratios as well. Among the ZrO
2
/SiO
2
catalysts, the one with
2
led to a higher propylene formation and,
(r
p
a
2
3
. Results and discussion
2
i
a
The XRD diffractograms of the samples with different zirconia
contents on silica (2.5, 5 and 7.5% ZrO
Fig. 1.
2
) and AgCeO are presented in
2
2
2
2
o
One can see that a peak around 2θ = 30 assigned to zirconia arises
in the XRD diffractograms of impregnated silica with 5% and 7.5% of
this oxide, represented at Fig. 1 by *. Its low intensity indicates the high
dispersion degree of this component; however the crystalline structure
thereof (tetragonal or monoclinic) cannot be identified. It must be
borne in mind that the presence of silica might interfere with the Bragg
peak of zirconia, since low-crystallinity silica may also present a broad
2
three mixtures used in this work and AgCeO
2
. The ZrO
2
/SiO catalysts
2
presented low conversions; therefore, they are not presented separately.
Furthermore, they could only produce acetaldehyde (as the main pro-
o
o
duct) and ethylene. One can see that the mixture containing ZrO
2
/SiO
2
peak from 15 to 30 .
2

C.R.V. Matheus and E.F.S. Aguiar
Catalysis Communications 145 (2020) 106096
Table 2
Catalytic tests of mixtures containing AgCeO
2
+ ZrO
2
/SiO
2
with different amounts of ZrO
2
(2.5, 5.0 and 7.5% w/w). Temperatures = 400, 425 and 450 °C,
−1
m
AgCeO2 = 50 or 200 mg and mZrO2/SiO2 = 25 or 100 mg depending on the tests. Gas flow rate = 50 mLmin
.
Catalyst
T (°C)
Mass (mg)
Conversion (%)
Selectivities (% C)
AgCeO
2
ZrO
2
/ SiO
2
Acetaldehyde
CO
2
Propene
Ethene
Acetone
Isopropanol
AgCeO
AgCeO
AgCeO
AgCeO
AgCeO
AgCeO
AgCeO
2
2
2
2
2
2
2
400
400
400
400
425
450
425
50
50
50
50
50
50
200
0
36.0
36.6
45.3
46.3
59.5
84.9
95.6
4.2
4.3
3.3
3.0
2.7
0.9
0.4
19.3
18.3
18.1
18.3
19.5
20.0
20.4
0.2
4.6
4.9
4.5
4.9
7.6
10.6
7.2
58.1
29.0
31.2
27.3
30.4
32.9
25.5
0.0
7.8
7.3
5.0
3.3
1.5
0
+ 2.5ZrO
2
/ SiO
2
25
25
25
25
25
100
15.0
13.4
18.5
22.4
21.2
28.8
+ 5ZrO
2
/ SiO
2
+ 7.5ZrO
+ 7.5ZrO
+ 7.5ZrO
+ 7.5ZrO
2
2
2
2
/ SiO
2
2
2
2
/ SiO
/ SiO
/ SiO
Fig. 2. Stability test for the AgCeO
2
+ 7.5%ZrO
2
/SiO
2
system under 425 °C, mAgCeO2 = 50 mg, mZrO2/SiO2 = 25 mg and gas flow rate = 50 mLmin−1.
with the highest amount of zirconia is the most selective catalyst for the
propene synthesis. It is interesting to notice that increasing the amount
of zirconia on silica elevated the selectivity to propene of the mixture,
as the catalyst becomes more active for both the isopropanol dehy-
dration and the MPV reaction. Nevertheless, one cannot see an increase
in the activity of the system. This would be observed should the MPV
reduction be a limiting reaction, since it consumes ethanol. Hence, one
mixture for 17 h. One can see that, although there is some deactivation,
once the catalysts stabilize (4–5 h), both conversion and selectivity
remain approximately the same for the following hours.
Although ZrO
2
/SiO catalysts perform better in the MPV model
2
reaction, they were not so active for the propene production when
employed in the physical mixture. This demonstrates that although
MPV is an important reaction, without which one cannot expect to
obtain propene [3,7], the use of catalysts more active for MPV reaction
does not necessarily lead to better results in the propene production.
This may happen for two reasons: 1 – if the catalyst is more dehydrating
(acidic), it may dehydrate ethanol to ethylene; 2 – if the catalyst is more
dehydrogenating (basic), it may lead to the isopropanol dehydrogena-
tion after its formation via MPV reaction. This is reinforced by the high
selectivity to acetone in the catalytic tests, even when compared to less
active MPV catalysts, which should lead to lower acetone amounts.
can assume that although MPV reaction is important, the r
p
a
/r ratio,
translated as the competition between dehydration/ dehydrogenation
reactions, is more relevant whenever the selectivity to propene is con-
cerned.
In an attempt to test how different temperatures would affect the
propene yield, the best mixture was evaluated at three temperatures:
4
00, 425 and 450 °C. As expected, higher temperatures accelerate the
reactions and, thus, lead to higher conversions. Interestingly, as ob-
served in previous work [7] raising the temperature positively influ-
ences the dehydration reaction, which leads to better propene yields.
Nevertheless, since the conversion of ethanol to ethene is also a dehy-
dration reaction, it becomes more significant as the temperature in-
creases, thereby affecting negatively the propene production at 425 °C.
Table 2 also depicts how the catalytic mixture behaves under dif-
ferent WHSVs for the best system at 425 °C, which was the best tem-
perature among those tested. As expected, higher contact times lead to
higher conversions and selectivities to the final product. Nevertheless,
as discussed in another paper [7] results ought to be carefully analyzed.
It must be borne in mind that this is a rather complex reaction system,
which cannot be represented by a simple consecutive reaction system.
Indeed, acetaldehyde formed via the MPV reaction returns as a reactant
for the acetone formation. Hence, higher conversions decrease the
amount of ethanol present for the MPV reaction and, thus, decrease the
activity for this reaction which generates propene.
Fig. 3. The influence of MPV and isopropanol model reactions on the propene
synthesis from ethanol. Adapted from Matheus et al., (2018).
Fig. 2 presents the stability test for the AgCeO
2
+ 7.5%ZrO
2
/SiO
2
3

C.R.V. Matheus and E.F.S. Aguiar
Catalysis Communications 145 (2020) 106096
Table 3
Results from this and other studies, under different conditions (temperature, masscat/ flowgas and % ethanol and water in the feed stream) for the propene production
from ethanol..
Catalyst
T (°C)
Masscat/ Flowgas (gscm−3) x 103
% ethanol
% water
Propene
Ethene
Propene/ ethene
Refetence
AgCeO
2
2
+ 7.5ZrO
+ t-ZrO
2
/ SiO
2
425
400
500
430
360
1
9
29
50
30
32
7.2
17.5
10
4.0
This work
[7]
AgCeO
2
360
1
9
2.9
Sc/In
2
O
3
9375
9375
30
30
8.5
30
3.0
[11]
Y
2
O
3
− CeO
2
38
0.84
[3]
Hence, a lower acidity may be desired in order to yield less ethene.
However, high basicity brings about the dehydrogenation reaction,
which produces acetone from isopropanol, thereby consuming ethanol
without leading to the desired product (Fig. 3).
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
Table 3 compares the present results with data published in the
literature. Although the propene selectivity from this study was not the
highest one, it is comparable with most results reported in the literature
The authors acknowledge the DICAP/ INT team for their assistance
in the experimental work. Additionally, we are grateful to Saint-Gobain
NorPro for supplying the silica samples, as well as for the financial
support of CAPES.
(
~30%). Nevertheless, the greatest advance here presented is the
highest propene/ ethene ratio, achieving values as high as 4. This may
be due to two factors: the high MPV capacity of the ZrO
2
/SiO catalyst
2
and its lower acidity – dehydration capacity – when compared to the
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Declaration of Competing Interest
2 3
O
The authors declare that they have no known competing financial
4