Ce/MgAl mixed oxides derived from hydrotalcite LDH precursors as highly efficient catalysts for ketonization of carboxylic acid

Article abstract of DOI:10.1039/c9cy01323g

Ketonization of carboxylic acids provides an attractive way for upgrading biomass feedstocks into biofuels. Here, a series of Mg-Al hydrotalcite (HT) catalysts doped with different Ce contents were prepared and used for the ketonization of propionic acid. Mg₃Al_0.9Ce_0.1 displayed the highest ketonization activity (90.6% conversion of propionic acid) under mild reaction conditions (350 °C). By applying SEM, TEM, H₂-TPR, CO₂/NH₃-TPD, XPS, etc., the mechanism for the improvement of catalytic activity and stability was obtained. When the doping content of Ce is low, such as in Mg₃Al_0.9Ce_0.1, cerium oxide is uniformly dispersed on the HT support. At the same time, the strong interaction between the CeO₂ species and Mg-Al hydrotalcite led to the improvement of redox properties and modified acid-base sites, which is beneficial for the ketonization reaction. However, the structure of the HT support is destroyed by the agglomerated cerium oxide when a more doped Ce is employed, and the specific surface area and the surface Ce³⁺ ratio are remarkably lowered, thereby causing a significant decrease in the ketonization activity. By using the temperature programmed surface reaction (TPSR) technique, a deeper insight into the mechanism of reaction pathways over different catalysts is gained. The significant improvement in the reaction stability of the ketonization reaction over Mg₃Al_0.9Ce_0.1 as compared to unmodified catalysts could be ascribed to the strengthening of the M-O bond of the catalyst after the introduction of Ce.

Full text of DOI:10.1039/c9cy01323g

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Ce/MgAl mixed oxides derived from hydrotalcite
LDH precursors as highly efficient catalysts for
ketonization of carboxylic acid†
Cite this: Catal. Sci. Technol., 2019,
9, 6335
*
Yao Yang,
Binbo Jiang,
Zhixiang Xi, Feipeng Lu, Zhengliang Huang,
Jingyuan Sun, Zuwei Liao, Jingdai Wang and Yongrong Yang
Ketonization of carboxylic acids provides an attractive way for upgrading biomass feedstocks into biofuels.
Here, a series of Mg–Al hydrotalcite (HT) catalysts doped with different Ce contents were prepared and
used for the ketonization of propionic acid. Mg3Al0.9Ce0.1 displayed the highest ketonization activity (90.6%
conversion of propionic acid) under mild reaction conditions (350 °C). By applying SEM, TEM, H2-TPR,
CO2/NH3-TPD, XPS, etc., the mechanism for the improvement of catalytic activity and stability was
obtained. When the doping content of Ce is low, such as in Mg3Al0.9Ce0.1, cerium oxide is uniformly
dispersed on the HT support. At the same time, the strong interaction between the CeO2 species and Mg–
Al hydrotalcite led to the improvement of redox properties and modified acid–base sites, which is beneficial
for the ketonization reaction. However, the structure of the HT support is destroyed by the agglomerated
cerium oxide when a more doped Ce is employed, and the specific surface area and the surface Ce3+ ratio
are remarkably lowered, thereby causing a significant decrease in the ketonization activity. By using the
temperature programmed surface reaction (TPSR) technique, a deeper insight into the mechanism of
reaction pathways over different catalysts is gained. The significant improvement in the reaction stability of
the ketonization reaction over Mg3Al0.9Ce0.1 as compared to unmodified catalysts could be ascribed to the
strengthening of the M–O bond of the catalyst after the introduction of Ce.
Received 4th July 2019,
Accepted 4th October 2019
DOI: 10.1039/c9cy01323g
rsc.li/catalysis
improves the energy density of bio-oil.5,8,9 Such reactions
decrease the oxygen contents of bio-oil without adding
1. Introduction
As the worldwide supply of petroleum diminishes, it becomes
urgent to develop a new generation of fuels that can reduce
the unsustainable consumption of fossil fuel. Liquid biofuels
derived from renewable feedstocks such as lignocellulosic
biomass are a promising alternative.1–4 Fast pyrolysis as an
efficient and cheap method is always applied in converting
lignocellulosic biomass to bio-oil that can be further
upgraded into liquid fuels.4 However, bio-oil is rich in small
organic acids like acetic and propionic acid, and these high-
oxygen-containing substances cause the energy density of bio-
oil to be reduced by more than 50% compared to fuel oil.
More critically, these small organic acids will also lead to
equipment corrosion during subsequent processes.5–7
hydrogen and do not produce polluting by-products, thus
providing an attractive route for carboxylic acid removal by
upgrading carboxylic acids into fuels and chemicals.
Metal oxides have been extensively used as active catalysts
for ketonization.9–12 Alkali and alkaline earth metal oxides
(e.g., Na2O, MgO, CaO, etc.) with high basicity or low lattice
energies formed bulk carboxylate salts in ketonization
reactions due to the strong interaction between the catalysts
and acids. In contrast, over metal oxides with high lattice
energies such as CeO2, ZrO2, TiO2, and so forth, the reaction
proceeds via the so-called surface ketonization pathway,
because the reaction only takes place on the catalyst surface.
Besides, it is possible that both the above mechanisms are
simultaneously operative on a given metal oxide catalyst.
Mekhemer et al.13 reported that the ketonization reaction of
acetic acid over MgO occurs via either bulk or surface
reactions. It is noticed that MgO is converted into bulk
magnesium at room temperature while surface acetate species
formed at higher temperatures. However, the reaction
mechanism is still under debate due to the complexity of
numerous catalysts and a variety of reaction conditions in
different kinds of studies.
Ketonization can efficiently convert bio-derived carboxylic
acids into non-corrosive ketones and simultaneously increase
the C–C bond length, which eliminates corrosion and
Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering
Manufacture Technology, College of Chemical and Biological Engineering,
Zhejiang University, Hangzhou 310027, China. E-mail: huangzhengl@zju.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cy01323g
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Previous studies demonstrated that the catalyst basicity 2. Materials and methods
might play a vital role in the ketonization reaction.14 This
point of view was proved by co-feeding experiments during
ketonization of hexanoic and acetic acids.15,16 The results
showed that co-feeding CO2 obviously decreased the
ketonization reaction rates, giving evidence that the basic
sites on the mixed oxides were the active sites for
ketonization. Besides, the catalyst acid sites have also been
thought to be important in the ketonization reaction. Hasan
et al.17 investigated the catalytic ketonization behaviors of
CeO2, TiO2, and Al2O3 by FTIR. It was concluded that the
reducible Lewis acid sites on the catalyst surface were
significantly important for the formation of surface reaction
intermediates in the ketonization mechanism. However,
favorable catalyst properties for ketonization are still
unknown.
Mg–Al mixed oxides, as typically solid acid–base catalysts,
can be obtained through using hydrotalcites (HTs) or
layered double hydroxides (LDHs) as precursors. Parida
et al.18 reported that Mg/Al could find application in the
ketonization reaction. The sample with a Mg/Al ratio of 4 : 1
displayed the highest acetic acid conversion, whereas the
catalytic stability was quite poor. It can be seen that Mg–Al
mixed oxides derived from hydrotalcites are promising as
ketonization catalysts due to their appreciable number of
basic sites, high thermal resistance and high specific
surface area. Besides, MgAl hydrotalcites were often
2.1. Catalyst preparation
The hydrotalcite samples were prepared according to co-
precipitation procedures at controlled pH (10), in which the
atomic ratio of Mg/Al was 3. In a typical procedure, an
aqueous solution
A
was prepared by dissolving the
appropriate amounts of AlIJNO3)3·6H2O and MgIJNO3)2·6H2O in
distilled water, and an alkaline solution B was prepared with
0.35 mol NaOH and 0.15 mol Na2CO3. A peristaltic pump was
used to introduce both solutions A and B simultaneously into
the recipient containing 200 mL water at 25 °C under
vigorous stirring. The pH value was maintained at 10 by
controlling the pump speed of basic solution B. After
precipitation, this suspension was kept at room temperature
for 15 h at 80 °C under vigorous stirring. Then, the
precipitate was washed repeatedly with deionized water and
dried at 100 °C overnight. For the HT samples containing
cerium, an appropriate amount of CeIJNO3)2·6H2O was added
to solution A to reach the atomic composition of Mg3Al1−xCex
(x = 0.03, 0.06, 0.1, 0.15, 0.2, 0.4). The as-synthesized Ce-
modified MgAl-hydrotalcite samples are denoted as xCe-HT
(x = 0.03, 0.06, 0.1, 0.15, 0.2, 0.4). To obtain the mixed oxides,
the precursor was calcined at 500 °C for 5 h in air at a
heating rate of 5 °C min−1. The Ce-modified mixed oxides are
denoted as Mg3Al1−xCex and x denotes the molar ratio of Ce/
(Ce + Al) (x = 0.03, 0.06, 0.1, 0.15, 0.2, 0.4). The raw chemicals
used in this study were purchased from Sinopharm Chemical
Reagent Co. Ltd., China and were of analytical reagent grade.
modified by incorporation of
a third atom into their
structure, which may lead to the modification of acid–base
sites and improvement of other catalytic properties.19–21 La-
and Y-modified HTs were found to exhibit larger surface
areas and smaller particles sizes, resulting in higher specific
amounts of medium/strong basic sites.22 Rare-earth metal
modified mixed oxides displayed higher activity in aldol
condensation of acetone and better water tolerance.
Shengpeng Mo et al.23 reported that Ce modified MnAl
oxide catalysts significantly improve the catalytic activity of
benzene, which was caused by their lower-temperature
reducibility, abundant surface lattice oxygen (Olatt), and the
synergetic effect between Mn4+ and Ce3+/Ce. Nevertheless,
the effect of introducing a third atom into Mg–Al mixed
oxides on the ketonization reaction and their interaction
with the catalyst support have not been studied. Meanwhile,
systematic studies on the application of Mg–Al mixed oxides
in ketonization reactions are very lacking.
In this study, we synthesized a series of Ce-doped MgAl
LDHs and systematically investigated the effects of Ce3+
content doping into MgAl LDHs. The catalytic performance
of Ce-doped MgAl mixed oxides was investigated in the
ketonization of propionic acid. The Ce-doped MgAl mixed
oxides improved the catalytic activities. The catalytic
performances of the as-prepared samples were correlated
with the observed structural evolution of the catalysts. In
particular, the temperature programmed surface reaction
(TPSR) method was used to gain a deeper insight into the
mechanism of reaction pathways.
2.2. Catalyst characterization
The powder X-ray diffraction (XRD) patterns of the samples
were recorded on a Philips PW3050/60 vertical goniometer
using Ni-filtered Cu Kα1 radiation (λ = 1.5406 Å). BET surface
area, pore volume, and pore size distribution measurements
were performed using
a
Quantachrome Autosorb-iQ
instrument. Temperature programmed reduction (TPR) was
conducted using a thermal conductivity detector and a
reduction gas of 10% H2 in Ar set at a flow rate of 40 mL
min−1. The samples were pre-treated at 450 °C for 60 min, the
sample cell was purged with a H2/Ar mixture for 30 min and
the analysis was run from 50 °C to 800 °C at a heating rate of
10 °C min−1. H2 consumption was quantified by comparison
to the reduction of CuO. X-ray photoelectron spectroscopy
(XPS) measurements were performed using an Escalab 250Xi
with a monochromatized Al K source (1486 eV).
The nature and concentration of base sites were studied
using TPD of pre-adsorbed CO2 and desorbed CO2 on a
Quantachrome instrument. Approximately 100 mg samples
were outgassed and dried at 450 °C for 30 min under an Ar
flow (40 mL min−1, purity >99.999% vt%), cooled to room
temperature, and then treated with a flow of 10% vt% CO2/Ar
(40 ml min−1) for 120 min. The sample cell was then
evacuated for 15 min, and a He flow at 40 ml min−1 was
introduced. This continued for 60 min before the start of data
collection. The sample was heated from room temperature to
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500 °C at 10 °C min−1. Desorption of CO2 was quantified by
comparing the areas under the curve with samples of known
amounts of CO2 using a thermal conductivity detector.
2.3. Catalyst tests
The gas-phase ketonization of carboxylic acids was carried
out in a down-flow quartz fixed-bed reactor (9 mm i.d.) under
atmospheric pressure. The external temperature of the
heating furnace was measured using a K-type thermocouple
attached to the reactor and controlled with
a 708P
temperature controller (Yudian Automation Technology Co.,
Ltd). An HPLC pump was used to introduce the liquid feed
solution into the reactor. The ketonization reaction was
conducted in the desired temperature range of 250 °C to 400
°C. Typically, 0.3 g of the as-prepared catalysts (40–60 mesh)
were placed in the middle region of the reactor between two
plugs of quartz wool and fused SiO2 granules and the
reaction was carried out at a liquid acetic acid flow rate of
1.8 mL h−1 with an N2 flow rate of 40 mL min−1. The feed
and gas-phase reaction mixtures were analyzed with an
online GC equipped with two columns in parallel and two
detectors to obtain the reactant conversion and product
selectivity. The outlet line was heated to 150 °C together with
the injection valve of the gas chromatograph to avoid product
condensation. For organic compounds, an FFAP capillary
column (30 m × 0.32 mm × 0.5 μm) was connected to a flame
ionization detector, while for gas products, a TDX-01 column
was connected to a TCD. Prior to the catalytic test, each
sample was pretreated under a nitrogen flow at the lowest
reaction temperature in the stream of reactants for 2 h. The
selectivity was defined as the percentage of carboxylic acid
converted into a particular product taking into account the
reaction stoichiometry; thus, 100% ketone selectivity would
mean that 1 mol of carboxylic acid was converted to form 0.5
moles of ketone. The mean absolute percentage error in
conversion and selectivity was ≤10% and the carbon balance
was maintained within 95%.
Fig. 1 XRD patterns of LDH precursors (A) and calcined samples (B):
(a) HT, (b) 0.03Ce-HT, (c) 0.06Ce-HT, (d) 0.1Ce-HT, (e) 0.15Ce-HT, (f)
0.2Ce-HT, and (g) 0.4Ce-HT.
deposited on the HT surface. Lattice constants a and c were
calculated and the results are listed in Table 1. The a
parameter stands for the average cation–cation distance in
the brucite layer while the c parameter relates to the HT
interlayer distance.20,22 It can be found that parameter a
increases with the doping amount of cerium, which is due to
the electronegativity difference as compared to the Al3+
cation. Ce prefers to be located in the interlayer gallery of HT
as agglomerated cerium oxide species due to its high ionic
radius (0.102 nm). The unit cell parameter c increases with
the addition of the cerium cation, indicating an increase in
the electrostatic forces of attraction between the layers and
the interlayer anions for Ce doped HT samples.24
In the absence of the catalyst, no conversion of acetic acid
was observed. However, when SiO2 granules were used, a
background conversion of 2–3% was detected under our
typical reaction conditions.
3. Results and discussion
3.1. Textural characterization of the catalysts
Table 1 Textural properties of the HT precursor and calcined samples
Fig. 1 shows the XRD patterns of the LDH precursors
prepared according to supersaturation conditions. The HT
sample presents the typical patterns of a pure layered double
hydroxide structure (JCPDS 37-0630). The reflections
Lattice
Pore
parametersa
BET
volume
Sample
a (Å)
c (Å)
2θ{003}/°
(m2 g−1
)
(cc g−1
)
HT
3.063
3.071
3.070
3.066
3.074
3.079
—
23.460
23.568
23.542
23.537
23.572
23.761
23.867
11.28
11.24
11.24
11.27
11.24
11.16
—
258
215
233
215
208
204
190
0.843
0.662
0.826
0.669
0.676
0.643
0.430
¯
corresponded to a hexagonal lattice with R3m rhombohedral
0.03Ce-HT
0.06Ce-HT
0.1Ce-HT
0.15Ce-HT
0.2Ce-HT
0.4Ce-HT
symmetry. However, in the XRD patterns of Ce-modified
MgAl-HT, the diffraction peaks of cerium oxide are detected.
So, it can be concluded that cerium cations with a twofold
larger ionic radius than Al3+ cannot be introduced in the
brucite-like layers completely. Some Ce3+ cations will
generate the corresponding agglomerated metal oxide species
a
Lattice parameters a = 2 × d110 and c = 3/2 (d003 + 2d006).
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In order to obtain composite metal oxide samples having
shows the H2-TPR profiles of calcined Ce-HT samples. The
reduction temperature of pure CeO2 is about 510 °C, which is
assigned to the reduction of oxides on the surface,28 while in
a larger specific surface area, different amounts of Ce-
modified hydrotalcites were calcined at 500 °C to obtain an
XRD diffraction peak of the sample as shown in Fig. 1(B).
The XRD patterns of unmodified Mg(Al)O exhibits only the
MgO crystal phase, which is substituted into the octahedral
structure due to the isomorphous substitution of Al3+ to
cause defects. For the Ce-modified Mg(Al)O sample, a distinct
single crystal of ceric dioxide appeared, and as the amount of
Ce doping increased, the crystallinity of CeO2 gradually
increased. For the 0.4Ce-HT sample, mainly in the main
crystalline phase of CeO2, it can be speculated that with the
increase of Ce content, MgO may gradually enter the CeO2
lattice to form a new solid solution structure.25
The specific areas of calcined Ce-doped MgAl-HT catalysts
range from 190–230 m2 g−1 and are slightly lower than that of
Mg(Al)O (260 m2 g−1) (Table 1). All the obtained catalysts
display type IV nitrogen adsorption/desorption isotherms
with H3-type hysteresis loops characteristic of mesoporous
materials at high relative pressures of 0.5–1 (Fig. S1†),
suggesting the presence of slit-like mesopores, associated
with aggregates of plate-like particles.
the H2-TPR profile of Ce-HT samples (x
< 0.15), the
temperature of the first hydrogen consumption peak is about
425 °C. The decrease of the CeO2 reduction temperature is
ascribed to the CeO2 particles strongly interacting with the
Mg(Al)O mixed oxides by the uniform dispersion on this
support. However, the H2 reduction peak moves to the higher
temperature scope when the amount of doping cerium
cations increases, indicating that the CeO2 particles
agglomerate and the reducibility becomes poor. Moreover,
the H2 consumption increases with the Ce addition into the
HT catalysts (Table 2), which is related to the layer structure
variation caused by a large amount of Ce cations replacing
the position of Al cations.
3.3. Ce surface status
The XPS results of Ce3d in all catalysts are shown in Fig. 4.
The surface Ce3+ ratio (Table 2) was calculated by Ce3+/(Ce3+
+
Ce4+) according to the Ce3d spectra. The Mg3Al0.90Ce0.1
catalyst shows the highest values of Ce3+/Cetotal, suggesting
that the CeO2 species strongly interact with the HT support,
which is in accordance with the XRD, SEM, TEM and H2-TPR
results. More importantly, it is beneficial for Ce3+ to lose an
electron to be oxidized to Ce4+, improving the lattice oxygen
defects and the redox transformation between Ce3+ and
Ce4+.23
The TG profiles of Ce-HT samples indicates (Fig. S2†) three
main weight losses at 40–210, 210–310 and 400–700 °C. The
first stage is attributed to the removal of adsorbed water, the
second stage is due to the loss of bound water or carbonate in
the interlayer of the LDH, and the last step was due to the
further removal of hydroxyl groups.26 The total weight loss
decreases with the added amount of cerium, indicating that
the doping cerium reduces the yield of hydrotalcite.
3.4. Acid–base properties
Fig. 2(a)–(h) show the micrographs of the as-prepared Ce-
modified Mg–Al-HT. A ball-flower shape with a size of 1.5–2.5
μm can be clearly seen for fresh HT samples. Under high-
resolution, a well-developed layered and platelet structure
could be identified, and the interparticle space formed by
accumulation of platelets became larger with the increase of
cerium content (0.1Ce-HT and 0.2Ce-HT), which is consistent
with those results detected in N2-adsorption (shown in
Table 1). This interparticle space can be considered as the
source of mesopores.27 In contrast, the 0.4Ce-HT sample
showed indistinct morphologies of layered and platelet
structures, which indicated that the lamellar structure would
partially collapse by the adequate addition of cerium cations
during catalyst preparation. Fig. S3† shows the TEM image of
Ce-modified Mg–Al-HT. It can be seen that small CeO2 grains
present a uniform distribution on the HT surface for the 0.1Ce-
HT sample. In contrast, the CeO2 grains agglomerate into large
particles for 0.4Ce-HT. In conclusion, the Ce doping amount
has a great influence on the HT structure, resulting in the
different states of CeO2 species on the HT surface.
The CO2-TPD profiles of the samples are presented in Fig. 5.
A broad desorption band is observed in the temperature
range from 50 °C to 500 °C, which can be divided into three
main peaks, corresponding to the weak, moderate and strong
basic sites on the catalyst surface.29 The number of basic
sites with different strengths was calculated and listed in
Table 3. It is supposed that the weak basic sites in the mixed
oxides are the surface OH− groups, while the strong basic
sites are related to the coordinately unsaturated O2−
anions.29,30 It can be found that Mg–Al mixed oxides show
more medium basic sites that are associated with the oxygen
in both Mg–O and M–O pairs.31 Obviously, the base strength
distribution of the Ce-doped Mg–Al mixed oxides varies as a
function of the doping amount. The Mg3Al0.90Ce0.1 catalyst
presents the highest number of basic sites (0.34 mmol g−1)
among all the samples, which can be attributed to the higher
base strength of the medium basic sites. Therefore, it can be
speculated that the suitable doping amount of Ce cations
and the strong interaction between the CeO2 species and HT
support favor the formation of more medium basic sites.
The NH3-TPD profiles of Ce-doped Mg–Al mixed oxides
are shown in Fig. 6. The total number of acid sites is
calculated according to the desorbed amount of NH3 and
shown in Table 3. It can be obviously seen that the total
number of acid sites decreases with the introduction of Ce
3.2. H2-TPR
Different reducibilities can be anticipated for the Ce-HT
catalysts because of the different natures of the transition
metal cations and the crystallographic phases present. Fig. 3
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Fig. 2 SEM images of HT precursor samples: (a and b) HT, (c and d) 0.1Ce-HT, (e and f) 0.2Ce-HT, (g and h) 0.4Ce-HT.
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Fig.
Mg3Al0.97Ce0.03, (b) Mg3Al0.94Ce0.06, (c) Mg3Al0.90Ce0.1, (d) Mg3Al0.85
Ce0.15, (e) Mg3Al0.8Ce0.2, and (f) Mg3Al0.6Ce0.4
3 H2-TPR profiles of Ce-doped Mg–Al mixed oxides: (a)
Fig. 5 CO2-TPD profiles of Ce-doped Mg–Al mixed oxides: (a) Mg3Al,
-
(b) Mg3Al0.94Ce0.06
,
(c) Mg3Al0.90Ce0.1
,
(d) Mg3Al0.85Ce0.15
,
(e)
.
Mg3Al0.8Ce0.2, and (f) Mg3Al0.6Ce0.4
.
Table 2 H2 consumption and Ce3+ ratios of Ce-doped mixed oxides
will destroy the HT structure and this structure distortion
may also lead to a decrease of acid strength.
b
Sample
H2 consumptiona (μmol g−1
)
Ce3+/Cetotal
Mg3Al
—
—
Mg3Al0.94Ce0.06
Mg3Al0.9Ce0.1
Mg3Al0.85Ce0.15
Mg3Al0.8Ce0.2
Mg3Al0.6Ce0.4
22.2
25.8
32.0
39.5
58.3
18.1
22.9
20.8
19.6
15.6
3.5. Effect of the Ce doping amount on the catalyst
performance
The catalytic performances of all the catalysts for the
ketonization of propionic acid are investigated from 315 to
380 °C (Fig. 7A and B). As shown in Fig. 7, the seven catalysts
exhibit very different ketonization activities, and the order is
a
The H2 consumption was calculated from the integration of the
TPR area. Ce3+/Cetotal values are percentage contributions of Ce3+
b
cations.
as follows: Mg3Al0.9Ce0.1 > Mg3Al0.94Ce0.06 > Mg3Al0.85Ce0.15
>
Mg3Al0.8Ce0.2 > Mg3Al0.6Ce0.4 ≈ Mg3Al > CeO2. Mg3Al0.9Ce0.1
shows nearly 90% acid conversion at 350 °C, whereas Mg3Al
shows low activity with a 60% acid conversion and pure CeO2
even exhibits the lowest activity among all the catalysts with
the acid conversion of only 27%. The ketonization activity of
Mg3Al0.9Ce0.1 is greatly higher than those of the Mg–Al mixed
oxide or pure CeO2 sample, indicating that the introduction
of Ce ions into the Mg–Al catalysts can significantly enhance
the catalytic performance. Moreover, the Mg–Al catalysts
presented very high ketone selectivity (Fig. 7B), manifesting
that the ketonization reaction of carboxylic acid favored this
kind of catalyst.
cations into Mg–Al mixed oxides and this is mainly due to
the decrease in the percentage of strong acid sites. Actually,
the Lewis acid sites are assigned to the Al–O–Mg species
located within a MgO structure and containing the Al3+
cations predominantly in octahedral sites.29,32 With the
increasing amount of doping Ce cations, the Al3+ cations in
the lattice are replaced by some Ce species, resulting in the
loss of acid sites. Besides, the excess doping of Ce cations
Table 4 shows the catalytic activity for carboxylic acid
ketonization on the catalysts prepared in this study and
catalysts reported in the literature.7,31–33 It can be clearly seen
that the pentanone yield and the turnover frequency (TOF) of
the Mg3Al catalyst after Ce modification are significantly
increased, and Mg3Al0.9Ce0.1 exhibits excellent catalytic
activity for carboxylic acid ketonization at lower reaction
temperatures (350 °C) compared to the catalysts reported in
the literature.
According to the above characterization results and
catalytic activity, it can be concluded that the improvement
of ketonization activity on Ce-doped Mg–Al mixed oxides is
dependent on the interaction between Ce species and the HT
support. For the Mg3Al0.9Ce0.1 catalyst, the CeO2 grains were
uniformly distributed on the HT structure and interacted
Fig.
Mg3Al0.94Ce0.06
Mg3Al0.8Ce0.2, and (e) Mg3Al0.6Ce0.4
4
Ce 3d XPS spectra of Ce-doped Mg–Al mixed oxides: (a)
,
(b) Mg3Al0.90Ce0.1 (c) Mg3Al0.85Ce0.15 (d)
,
,
.
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Table 3 The distribution of acid–base sites of Ce-doped Mg–Al mixed oxides
Total number of basic sites
Total number of acidic sites
Sample
(mmol g−1
)
(μmol m−2
)
Wa
Ma
Sa
(mmol g−1
)
(μmol m−2
)
Mg3Al
0.31
0.30
0.34
0.33
0.29
0.27
1.19
1.29
1.54
1.58
1.42
1.40
0.06
0.08
0.06
0.06
0.09
0.08
0.14
0.17
0.20
0.18
0.16
0.10
0.11
0.05
0.08
0.09
0.04
0.10
0.40
0.35
0.27
—
0.25
0.21
1.55
1.52
1.26
—
1.24
1.08
Mg3Al0.94Ce0.06
Mg3Al0.9Ce0.1
Mg3Al0.85Ce0.15
Mg3Al0.8Ce0.2
Mg3Al0.6Ce0.4
a
W, weak basic site; M, medium basic site; S, strong basic site, mmol g−1
.
strongly with the Mg–O–Al bonds. This strong interaction
leads to the formation of coordinately unsaturated surface
Ce3+ cations, which will facilitate the absorption and
activation of acid molecules. Besides, the introduction of Ce
cations in Mg–Al mixed oxides can modify the acid–base
properties of the mixed oxides. The abundant basic sites and
suitable acid sites of the Mg3Al0.9Ce0.1 catalyst should be
beneficial for the ketonization of propionic acid. In contrast,
the Mg3Al0.6Ce0.4 sample shows a weak interaction between
Ce species and the HT support due to the destruction of the
uniform structure and the agglomeration of CeO2 grains.
Fig. 9 presents the XRD patterns of fresh and spent
catalysts. It can be found that some new XRD diffraction peaks
appeared for the spent Mg3Al catalyst and these peaks may be
assigned to the formation of the carboxylate phase. Meanwhile,
the XRD pattern of the spent Mg3Al0.9Ce0.1 catalyst is nearly the
same as that of the fresh one, which gives the evidence that the
introduction of Ce increases the M–O bond strength of the
Mg–Al mixed oxides. Therefore, it is difficult for the M–O
bonds in the Mg3Al0.9Ce0.1 sample to break and to form the
carboxylate species, which explains why its catalytic stability
enhanced as compared to the Mg3Al catalyst.
3.6. Stability of Ce doped Mg–Al mixed oxides
The stability test for the Mg3Al0.9Ce0.1, Mg3Al0.6Ce0.4 and Mg3-
Al catalysts is shown in Fig. 8. It can be seen that the Mg3Al
catalyst presents a decrease of the ketonization activity at 350
°C with the time on stream. The Mg3Al0.9Ce0.1 catalyst shows
a higher activity and better stability as compared with the
Mg3Al catalyst, indicating that the doping of Ce cations can
improve the catalytic stability of the ketonization reaction on
Mg–Al mixed oxides. However, a decrease in activity is
observed for the Mg3Al0.6Ce0.4 catalyst which suggests that
the excess doping of Ce cations in Mg–Al mixed oxides will
be adverse to maintaining the ketonization activity.
Fig. 7 Propionic acid conversion (A) and 3-pentanone selectivity (B) as
a function of temperature for Ce-doped Mg–Al mixed oxides (reaction
Fig. 6 NH3-TPD profiles of Ce-doped Mg–Al mixed oxides: (a) Mg3Al,
(b) Mg3Al0.94Ce0.06
,
(c) Mg3Al0.9Ce0.1
,
(d) Mg3Al0.8Ce0.2
,
and (e)
conditions: WHSV = 6.7 h−1, feed rate = 0.5 mL min−1, N2: 40 mL min−1
TOS = 1 h).
,
Mg3Al0.6Ce0.4
.
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Table 4 Catalytic activity for carboxylic acid ketonization on different catalysts
Entry
Catalyst
Conv/%
Select/%
Yield/%
TOFa/h−1
Ref.
1
2
2
3
4
5
6
7
Mg3Al0.9Ce0.1
Mg3Al
MnOx/CeO2
CM-MCM-41d
CeZrOx
CeIJ9)/ZrO2
Ce0.5Fe0.2Al0.3Ox
Zn–Cr (10 : 1)
91
71
80
73
50
98
97
89
69
215
184
130
88
This workb
This workb
32c
32c
7e
90
45
78
91
70
6
33f
33f
34g
178
232
—
98
100
70
TOF (h−1) = (mol acid/mol basic sites per h). Reaction conditions: 350 °C, WHSV = 6.0 h−1, propionic acid as feed. Reaction conditions:
a
b
c
410 °C, WHSV = 4.0 h−1, propionic acid as feed. CM-MCM-41 stands for MCM-41 supported Ce–Mn oxides. Reaction conditions: 270 °C,
d
e
f
g
WHSV = 0.6 h−1, 20 wt% acetic acid in water. Reaction conditions: 400 °C, WHSV = 2.0 h−1, acetic acid as feed. Reaction conditions: 350 °C,
W/F = 4 h·g·mol−1, propionic acid as feed.
3.7. Reaction route
At higher temperatures (>350 °C), the steady ketonization
should be regarded as the pure surface catalytic reaction.
Fig. 10 shows the ketonization reaction behavior of propionic
acid over the Mg3Al0.9Ce0.1 catalyst as observed
chromatographically in a flow reactor. The amounts of
propionic acid and its detectable products (pentanone and
CO2) are graphically presented as a function of temperature
in Fig. 10. Separation and estimation of water among the
products were not feasible with the analytical techniques
applied. Fig. 10 reveals three different temperature-
dependent behaviors: firstly, a sudden consumption of the
amount of acid is observed when it is allowed to flow over
the catalyst below 250 °C, without detecting any products;
secondly, a steep, considerable consumption of the acid
appeared at 280–350 °C; thirdly at >350 °C, a steady
As
a
comparison,
the
temperature-programmed
ketonization reactions of pure MgO and Al2O3 were also
conducted as shown in Fig. S4.† The Al2O3 catalyst displays a
very different behavior as compared with the other samples.
The sudden acid consumption which occurred at <250 °C
disappeared, which can be caused by the strong acidity of the
Al2O3 catalyst without suitable basic sites. The product of the
ketonization reaction appeared at >350 °C and reaches the
highest selectivity at almost 450 °C. However, Al2O3 shows a
sharp decrease in the selectivity to the ketone product with
further
increasing
temperature,
indicating
that
decarboxylation occurred instead of the ketonization reaction.
The temperature-dependent behavior of the MgO sample is
similar to that of the Mg3Al0.9Ce0.1 catalyst. The sharp but
large consumption of acid at <250 °C can be observed, which
is due to the strong reactive base sites of the MgO catalyst.
In summary, the Mg3Al0.9Ce0.1 catalyst shows different
TPSR profiles in contrast to MgO and Al2O3. The Mg3Al0.9Ce0.1
catalyst with modified acid–base sites follows a surface
ketonization route under the reaction conditions in this
paper. Although the reaction temperature has a significant
consumption of the acid, and
a corresponding steady
production of Ac and CO2 can be found. As reported by
Mekhemer et al.,13 the ketonization reaction over the MgO
catalyst occurred via overlapping catalytic and pyrolytic
routes which are based on the reaction conditions. The
sudden consumption of the acid at <250 °C could be due to
the adsorption by the catalyst surface and bulk. Then, the as-
formed bulk acetate together with surface acetates in the
lower temperature range decomposed rapidly at 280–350 °C.
Fig. 8 Propionic acid conversion of Mg3Al0.9Ce0.1, Mg3Al0.6Ce0.4 and
Mg3Al at 350 °C as a function of time on stream (reaction conditions:
WHSV = 6.7 h−1, feed rate = 0.5 mL min−1, N2: 40 mL min−1).
Fig. 9 XRD patterns of post-reaction catalysts (reaction conditions:
WHSV = 6.7 h−1, feed rate = 0.5 mL min−1, N2: 40 mL min−1, TOS = 500
min).
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for the Ce doped Mg–Al mixed oxide catalysts and the
introduction of Ce enhanced the strength of the M–O bond
of the catalysts, which are beneficial for stabilizing the
catalyst structures. Hence, the reaction stability of the
ketonization reaction over Mg3Al0.9Ce0.1 is significantly
improved as compared to that over the Mg3Al catalyst. It is
shown that Ce–MgIJAl)O catalysts have broad application
prospects due to their excellent carboxylic acid ketonization
properties.
Conflicts of interest
There are no conflicts to declare.
Fig. 10 Profiles of the temperature-programmed ketonization
reaction of propionic acid over Mg3Al0.9Ce0.1
.
Acknowledgements
The authors gratefully acknowledge the support and
encouragement of the National Natural Science Foundation of
China (U1663222) and the National Natural Science
Foundation of China (21525627).
influence on the reaction mechanism, the Ce-doped Mg–Al
mixed oxides perform a transformation from a bulk to
surface ketonization route at a relatively lower reaction
temperature in comparison with MgO. This should be
another reason for the improvement of the catalytic stability
for Ce-doped Mg–Al mixed oxides.
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