Catalytic Performance of Layered Double Hydroxides (LDHs) Derived Materials in Gas-Solid and Liquid-Solid Phase Reactions

Article abstract of DOI:10.1002/cctc.201900499

Novel layered double hydroxides (LDHs) were prepared by a co-precipitation method and characterised using X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), Thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). As-prepared LDH derived catalysts were first evaluated in the gas-phase conversion of methanol. The results indicate that LDH derived materials act as selective catalysts towards dimethyl ether (DME), methane or light olefins formation, depending on their chemical composition. For instance, CuAlO_x showed a high selectivity in DME up to 88 %, whilst CuCoO_x converted methanol to CH₄, C₃H₈ and DME. NiFeO_x allowed achieving a full methanol conversion selectively into CH₄ during at least 1800 min. Besides, Ni and Cu-LDHs were successfully tested in the liquid phase benzyl alcohol (BzOH) oxidation leading to more than 50 % conversion. NiAl-LDH appeared as the best catalyst for benzaldehyde production with a 71 % conversion and 100 % selectivity.

Full text of DOI:10.1002/cctc.201900499

DOI: 10.1002/cctc.201900499
Full Papers
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Catalytic Performance of Layered Double Hydroxides
(
LDHs) Derived Materials in Gas-solid and Liquid-Solid
Phase Reactions
[a, b]
[b]
[b]
[b]
Liang Huang, [ Cristina Megías-Sayago, Rogeria Bingre, Qianwen Zheng,
a]
[b]
Qiang Wang,* and Benoît Louis*
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Novel layered double hydroxides (LDHs) were prepared by a co-
precipitation method and characterised using X-ray diffraction
(XRD), Fourier transformed infrared spectroscopy (FT-IR), Ther-
mogravimetric analysis (TGA) and scanning electron microscopy
(SEM). As-prepared LDH derived catalysts were first evaluated in
the gas-phase conversion of methanol. The results indicate that
LDH derived materials act as selective catalysts towards
dimethyl ether (DME), methane or light olefins formation,
showed a high selectivity in DME up to 88%, whilst CuCoO
x
converted methanol to CH
, C H and DME. NiFeO
3 8 x
allowed
during
4
achieving a full methanol conversion selectively into CH
4
at least 1800 min. Besides, Ni and Cu-LDHs were successfully
tested in the liquid phase benzyl alcohol (BzOH) oxidation
leading to more than 50% conversion. NiAl-LDH appeared as
the best catalyst for benzaldehyde production with a 71%
conversion and 100% selectivity.
depending on their chemical composition. For instance, CuAlO
x
Introduction
Layered double hydroxides (LDHs) or hydrotalcite-like com-
pounds are a class of ionic lamellar compounds, consisting of
positively charged brucite-like layers, charge compensating
anions and water molecules within the interlayer spaces. The
metal cations occupy the centres of an octahedral structure,
whose vertexes contain hydroxide anions; these octahedra are
Figure 1. Schematic structure of LDH and its change to layered double
oxides (LDO) upon calcination.
[1–3]
connected by sharing edges to form an infinite sheet.
The
·yH O,
2
+
3+
nÀ
m/n
general chemical formula is [M
M
(OH) ] A
x 2
1
À x
2
2
+
2+
2+
2+
3+
3+
3+
3+
where M =Mg , Zn , Ni etc… and M =Al , Mn , Fe
, respectively. A is a non-framework charge compensation
2
+
2À
3+
2À
…
(both Mg À O and Al À O acid-base pairs), and (iii) strong
nÀ
2À
À
À
2À [18]
anion with charge n, A =CO3 , Cl , NO etc., n is normally
Lewis basic sites (O anions). This renders LDOs promising
catalysts for a variety of acid-base reactions.
3
[4–8]
comprised between 0.17 and 0.33, as shown in Figure 1.
Due to their controllable chemical composition, LDHs show
a wide range of structures/properties and present a high
potential for a wide range of applications such as water
Methanol is one of the topmost raw chemical used in
industry attracting more and more attention as it can be easily
obtained from natural resources. For its proper utilisation,
methanol can be selectively transformed to valuable products
[9–11]
[12,13]
decontamination,
flame
retardant,
After thermal treatment,
medicinal
[14,15]
[16,17]
[19,20]
[21,22]
chemistry
and adsorbents.
such
as
olefins,
dimethyl
ether
(DME),
[
23]
[24]
LDH loses its layered structure and turns into layered double
oxides (LDOs) as shown in Figure 1. LDO structure possesses
dimethoxymethane, formaldehyde, etc.
Several catalysts have already been tested in the methanol
À
three kinds of active sites: (i) weak Brønsted basic sites (OH
conversion, such as zeolites, perovskites, TiO , etc. Among
2
groups on the surface), (ii) medium strength sites Lewis sites
widely used catalysts, several types of zeolites, like ZSM-5,
[
20]
SAPO-34, SSZ-13, have been deeply investigated. Peng et al.
prepared highly crystalline SAPO-34 with different Si/Al ratios
for the Methanol-To-Olefins reaction (MTO) and found that the
zeolite possessing the lowest Brønsted acid sites density
provided the highest catalyst stability as well as the highest
[
a] L. Huang, Prof. Q. Wang
Environmental Functional Nanomaterials Laboratory
College of Environmental Science and Engineering
Beijing Forestry University
P.O. Box 60, 35 Qinghua East Road, Haidian District, Beijing 100083 (P.R.
China)
[
22]
selectivity towards ethylene and propylene. Migliori et al.
E-mail: qiangwang@bjfu.edu.cn
compared the catalytic performance in methanol to DME
conversion over MOR, FER and MFI zeolites. They highlighted
drastic differences in stability. MOR was vulnerable against coke
deposition, because of its 1D large pore structure. In contrast,
channel intersections present in FER or MFI hindered carbon
deposits within the structure and therefore induced better
resistance to coke, thus improved catalyst stability.
[b] L. Huang, Dr. C. Megías-Sayago, R. Bingre, Q. Zheng, Dr. B. Louis
Energy and Fuels for a Sustainable Environment Team
Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé,
UMR 7515 CNRS-ECPM
Université de Strasbourg
25 rue Becquerel, 67087 Strasbourg cedex (France)
E-mail: blouis@unistra.fr
French Conference on Catalysis 2
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As composition tailorable multifunctional materials, LDHs
based catalysts for methanol conversion were scarcely reported.
The work reported herein aims to present a comprehensive
study of catalytic behaviour of different kinds of LDHs. LDHs
were prepared by a conventional co-precipitation method with
different M (Mg, Cu, Ni, Zn) and M (Al, Fe, Co) cations. All
synthesized LDHs were thoroughly characterised using X-ray
diffraction (XRD), thermogravimetric analysis (TGA), scanning
electron microscopy (SEM), Fourier transform infrared spectro-
scopy (FT-IR) and nitrogen adsorption-desorption analyses.
Finally, their catalytic performances were evaluated in the
methanol conversion into hydrocarbons but also in the liquid-
phase oxidation of benzyl alcohol to assess the versatility of
those materials in different reaction environments.
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Figure 3. FTIR spectra of as-prepared LDH samples.
Results and Discussion
Characterisation
The FT-IR spectra of the samples are shown in Figure 3. It is
noteworthy that most of the materials exhibit the characteristic
peaks of LDH. All spectra contain a broad and strong absorption
In order to perform an in-depth investigation of the influence of
2
+
the cation, present in the LDHs structure, on its activity, M Al-
type LDHs with various divalent cations, like Mg, Ni, Cu and Zn,
and CuCo, CuFe, NiFe and NiFe were synthesized. All the
samples were prepared by conventional co-precipitation meth-
od, and the pH of the solution was kept at 10. Figure 2 presents
À 1
band between 3383 and 3566 cm being associated with the
superposition of hydroxyl stretching band (À OH), arising from
the hydroxyl groups of brucite-like layers, and hydrogen-
[
27,28]
bonded interlayer water molecules.
A weak vibration band
À 1
at approximately 1631 cm can also be observed in the
samples, attributed to the vibration of interlayer water
molecules. It can be clearly seen that CuFe did not exhibit
[29]
À 1
any vibration at 1631 cm thus suggesting the absence of
,
interlayer water molecules. This result confirms that CuFe failed
to crystallise in the LDH structure, as suggested by XRD data
À 1
(Figure 2). In addition, the vibration of 1382 cm can be
attributed to the charge compensation carbonate anions, whilst
for CuFe, no carbonate species could be detected, confirming
2
À
the absence of CO3 in this sample.
The structure evolution of each material as a function of the
temperature raise was examined by TGA, as shown in Figure 4.
Figure 2. XRD patterns of as-prepared LDH samples.
the XRD patterns of as-prepared samples. For ZnAl and MgAl,
pure LDHs phase were successfully obtained with characteristic
peaks at 11.7°, 23.7°, 34.7° and 39.3°, corresponding to the
[25]
basal planes of (003), (006), (009) and (015) reflections. NiAl,
NiFe, CuCo and CuAl also exhibited the typical reflections at
2
θ=11° and 23°, ascribed to LDH structure with poor
crystallinity. In contrast, CuFe failed to form LDH under those
co-precipitation conditions and appeared to produce Cu(OH)2
and Fe O ·H O phases (JCPDS 13-0420, JCPDS 13-0092). This
2
3
2
result is in line with CuFe LDH reported formation at lower
Figure 4. TGA analysis of as-prepared LDH samples. The y-axis represents the
relative weight of the different materials.
[26]
pH=4.6.
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All the samples present an obvious two-stage weight loss in the
temperature range from 50°C to 700°C, which is typical for
LDHs. The initial weight loss stopped between 190°C and
exhibits a typical spheroidal sand rose morphology (Figure 5h)
with a size of spheroidal roses around 400–500 nm and a
thickness of the petals around 15 nm, similar to the size of ZnAl
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[30]
[32]
2
50°C, due to the loss of interlayer water.
Again, CuFe
LDH reported elsewhere.
presented a different first stage of weight loss that stopped
around 300°C, probably due to the loss of crystalline water in
Fe O ·H O. For the majority of as-synthesized samples, the initial
Since the LDH derived catalysts need to be activated upon
thermal treatment, XRD patterns were also recorded after
calcination. It is known that LDH usually loses its layered
structure, turning into layered double oxides (LDOs) which
possess three types of active sites. The structure and composi-
tion changes are the most important features for further
catalytic purposes. As shown by TGA analysis (Figure 4), LDH
first lost its interlayer water during calcination. Afterwards,
when the temperature reached 250°C, dehydroxylation of the
octahedral layers and decarbonation of interlayer charge
compensating carbonate anions process started and lasted up
to 450°C. Based on this inwardness, all the samples were
calcined at 450°C for 1 h. As shown in Figure 6, all the materials
2
3
2
weight losses were between 15 and 20 wt%, in agreement with
[31]
former reports.
In contrast for NiAl LDH, the decrease in
weight upon heating from 40 to 200°C was limited to 3%. Its
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second weight loss appeared in the temperature range 250–
4
50°C, probably due to dehydroxylation of the octahedral
layers, as well as decarbonation of interlayer charge compensat-
ing anions. CuFe sample exhibited a relatively higher thermal
stability, with its second weight loss ending at 550°C. For all
remaining LDHs, this latter loss ended at around 400°C.
The morphologies of LDH samples were observed by SEM
Figure 5). CuFe, CuAl, CuCo, NiAl, and ZnFe exhibit rather
(
Figure 6. XRD patterns of calcined samples. (&) CuO, (*) NiO, (
◆
)
Cu0.92Co2.08
O
4
(JCPDS 37-0878), (~) MgAl
2
O
4
(JCPDS 33-0853), (★) ZnO and
(n)Fe O .
2
3
lost their original structure and changed to a mixed-metal oxide
phase. CuCo, CuAl and CuFe exhibited well-crystallised CuO
structure according to the sharp peaks at 35.5° and 38.7°. In
addition, depending on the composition of the different mixed-
metal oxides such as Al O , Fe O and Cu0.92Co2.08O , character-
2
3
2
3
4
istic peaks could be observed, respectively.
For NiAl, NiFe and ZnNi, ZnFe, XRD patterns indicate the
presence NiO or ZnO as main phases with neither aluminium
nor iron oxides detected, suggesting a well dispersion of
amorphous aluminium and iron oxides within the samples. In
contrast, MgAl LDH turned into MgAl O as main phase with
Figure 5. SEM image of as-prepared LDHs. (a) CuFe, (b) CuAl, (c) CuCo, (d)
NiAl, (e) NiFe, (f) ZnAl, (g) ZnFe and (h) MgAl.
2
4
small peaks corresponding to Al O .
2
3
Nitrogen adsorption-desorption isotherms for all LDOs were
obtained after activation at 450°C for 1 h and outgassing at
220°C under vacuum overnight. The specific surface area was
aggregated nanoparticles. NiFe LDH grew in a “stone”-like
morphology with no obvious pore structure. ZnAl exhibits
another typical LDH morphology, aggregating into flake-like
particles. The thickness of a plate is roughly 20 nm and its size
is around 100 nm. As a representative hydrotalcite, MgAl LDH
calculated by the Brunauer-Emmett-Teller (BET) method. The
Pore volume and pore size distributions were determined by
the Barrett-Joyner-Halenda (BJH) method. The textural proper-
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Methanol conversion
Table 1. BET specific surface area, pore size, and pore volume of calcined
samples.
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À 1
The catalytic performance of each calcined LDHs was then
evaluated. Figure 7 presents the conversion of methanol over
Samples
BET SSA
BJH pore size
[Å]
BJH pore volume [cm g
]
2
À 1
[
m g
]
Al-based LDO catalysts at 450°C. For CuAlO , the initial
x
CuAlO
CuFeO
CuCoO
x
122
30
26
197
108
57
10.2
19.7
15.9
7.1
4.9
10.9
8.7
0.37
0.15
0.10
0.38
0.19
0.13
0.09
0.74
methanol conversion taken after 5 min on stream was 100%.
The methanol has therefore been entirely converted into DME
x
x
NiAlOx
and CH . Afterwards, the conversion gradually diminished from
4
NiFeO
ZnAlO
ZnFeO
MgAlOx
x
100% to 80% after 2 h, and so did the methane yield
x
x
48
224
(Figure 7a). The DME yield increased along with the reaction
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13.3
time and achieved its highest value at 60 min (79%). Unfortu-
nately, a fast decline in the conversion was observed probably
due to coke formation.
In stark contrast, the methanol conversion and DME yield
remained very low over MgAlO catalyst (Figure 7b), probably
ties are presented in Table 1. It is obvious that the LDH
composition greatly affects the specific surface area (SSA), pore
x
size and pore volume. MgAl LDH derived MgAlO showed the
due to the absence of acid sites. Surprisingly, the methanol
conversion remained complete and stable during 1500 min
x
2
À 1
largest BET surface area of 224 m g
(
and pore volume
3
À 1
0.74 cm g ) owing to its “sand rose” microstructure which
allows the presence of numerous accessible pores between
petals” and maintains a high SSA upon thermal treatment.
over NiAlO and DME presence could not be detected. Indeed,
x
methanol converted selectively into CH over NiAlO . For an
industrial point of view, the reverse reaction CH to methanol
would be economically interesting. However, according to the
micro-reversibility principle in catalysis, a catalyst can be
potentially used in both sides of a chemical reaction. One may
therefore expect the possible design of NiAlOx catalyst to
convert methane into methanol. For ZnAlO (Figure 7d), a rapid
decline in the conversion could be observed, from 40% (initial
conversion) down to 7% after 3 h on stream. However, the
4
x
“
4
Besides, CuFe and CuCo, ZnAl and ZnFe, NiAl and NiFe
exhibited similar SSA values amongst all as-prepared LDHs.
However, Al-containing LDHs exhibited the largest SSA values.
It can therefore be concluded that the composition has a great
effect on the morphology and surface area of LDHs which may
induce an influence on the catalytic performance of these LDO
catalysts.
x
Figure 7. Methanol conversion over LDO catalysts as well as selectivities towards the different products [%] obtained over (a) CuAlO
and (d) ZnAlO
x x x
, (b) MgAlO , (c) NiAlO
x
.
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Figure 8. Methanol conversion as well as selectivities towards the different product obtained over LDO catalysts. (a) CuCoO
x x x
, (b) CuFeO , (c) NiFeO and (d)
ZnFeO
x
.
initial DME yield remained higher than over MgAlO catalyst.
exhibited bad performance in the methanol conversion, prob-
ably because of its large amount of basic sites. Active LDH
catalysts for this gas-phase reaction probably exhibit a sufficient
acidity for catalysing the methanol dehydration at 450°C.
Unfortunately, NiAlOx and NiFeOx did not exhibit significant
methanol conversion at 280°C, neither methane nor DME could
x
Along with the reaction time, both methanol conversion and
DME yield diminished to low levels, accompanied by the
appearance of light olefins (mainly butylenes).
As aforementioned, CuAlOx exhibited high selectivity to
DME after 60 min when compared to other catalysts. NiAlOx
showed high selectivity and stability towards CH . For a better
be detected.
4
understanding, CuCo, CuFe, NiFe and ZnFe LDHs were also
tested and the results are presented in Figure 8. CuCoOx
exhibited a 63% conversion after 30 min into a wide distribu-
TGA was performed under air for NiFeO and NiAlO LDO
x
x
samples. Figure 9 shows the relative weight versus temperature
for the later NiAl material (both materials exhibited the same
profile). The first weight decrease corresponds to the water loss
(samples were kept for several days before analysis); then the
weight increase is probably due to Ni oxidation. Importantly,
the last weight decrease can be attributed to coke oxidation
into carbon dioxide (above 400 C). The amount of carbon
present on the catalyst surface could be estimated to 4%wt.
after 25 h on stream.
tion of products: CH , C H , DME and C fraction. As the
6+
4
3
6
reaction time increased from 5 to 30 min, CH fraction at the
4
reactor outlet diminished from 43% to 1.4% at the benefits of
C6+ gasoline fraction which was enhanced from 6% to 43%. By
comparison with CuAlO , DME remained lower over CuCoO ,
x
x
being in the range of 16–18%. CuFeO almost did not exhibit
x
any catalytic activity (Figure 8b). Likewise, to NiAlO , NiFeOx
x
catalyst led to a complete and stable methanol to CH4
conversion: i.e.; CH yield and methanol conversion maintained
4
1
00% even after 1800 min on stream (Figure 8c). To investigate
Benzyl Alcohol Oxidation
the effect of Fe in the matrix, ZnFeO was tested under the
x
same conditions (Figure 8d). Behaving significantly differently
than ZnAlO , the methanol converted mainly into CH rather
Selective liquid phase oxidation of benzyl alcohol was studied
over different LDH samples (Table 2). Scheme 1 presents the
possible reactions involved while reacting benzyl alcohol with
oxygen in the Parr reactor.
x
4
than DME. However, ZnAlO and ZnFeO exhibited both poor
x
x
stability and conversion during the progress of the reaction. In
2
spite of having the highest SSA (224 m /g, Table 1), MgAlOx
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It should also be highlighted that whatever the catalyst used,
the selectivity towards benzaldehyde remained always superior
to 97%.
In general, Ni and Cu-containing samples exhibit high
activity (>50% conversion) in comparison with ZnAl sample
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(entry 5) which only led to reach 15% conversion. Ni-samples
demonstrated higher activity than Cu-samples (entries 1 vs 3
3
+
and 6 vs 7), regardless the nature of the trivalent cation (Al
/
3
+
Fe ), NiAl-LDH remaining the most suitable sample for a
selective benzaldehyde production at an appreciable 71%
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conversion. Regarding Ni-containing samples (entry 1 vs 6), the
3
+
presence of Al led to achieve a higher BzOH conversion in
3
+
comparison with Fe . Similar behaviour was observed over Cu-
samples (entries 3, 7 and 8). The observed catalytic performance
followed the order: CuAl-LDH�CuCo-LDH>CuFe-LDH. Finally,
it has to be said that benzoic acid could not be detected after
the reaction, which accounts for a high efficiency of these
materials in benzaldehyde production and, therefore, for their
versatility in different processes.
Figure 9. TGA under air of aged NiAlOx LDO catalyst after 25 h on stream at
4
50°C. The y-axis represents the relative weight of the different materials.
A possible structural effect, transformation of LDHs into
LDOs, was kept in mind, thus the catalytic performances of LDH
samples were compared with those of previously calcined Conclusions
samples (400°C/4 h in static air). The latter experiments were
carried out over NiAl and CuAl LDHs (entries 1 vs 2 and 3 vs 4 in
Table 2). It is noteworthy, that the differences are not important
as a priori expected, suggesting that the structural trans-
formation of LDHs took place under the reaction environment.
Novel LDH derived LDO catalysts were prepared and success-
fully tested first in the gas-phase methanol conversion. By using
a conventional co-precipitation method, a series of LDHs were
synthesized and thoroughly characterised. After thermal activa-
tion, all the samples turned into a mixture of metal oxide or
bimetal oxide which exhibited a decent to significant catalytic
activity.
SEM images confirmed drastic changes in the materials
morphology: ZnAl consisted of small nanoplates, MgAl formed
typical sand rose morphology, NiFe grew in a stone-like
morphology and the others rather formed aggregated nano-
particles.
Table 2. Liquid-phase benzyl alcohol oxidation with oxygen.
[b]
Entry
Catalyst
Conversion
%]
Selectivity [%]
BzCHO BnBzO
[
1
2
3
4
5
6
7
8
NiAl-LDH
71
68
64
71
15
60
50
65
100
100
98
98
100
97
0
0
2
2
0
3
1
0
[
a]
NiAlO
CuAl-LDH
x
[a]
x
CuAlO
While CuAlOx demonstrated a high selectivity in DME,
ZnAl-LDH
NiFe-LDH
CuFe-LDH
CuCo-LDH
NiFeO showed a 100% selectivity to CH at full conversion and
x
4
99
100
a high stability (superior to 30 h), suggesting that NiFeO could
x
be a potential candidate for the timely reversible CH4 to
methanol reaction.
[
(
a] Calcined at 400°C/4 h in static air. [b] reaction conditions: BzOH
0.3 mmol), toluene (10 mL), catalyst (0.3 g), 500 rpm, 150°C, 4 bar O
and
2
Finally, those LDH derived materials were also successfully
tested in the liquid-phase oxidation of benzyl alcohol with
oxygen, thus further strengthening their applications in
catalysis and why not for targeting green syntheses of fine
chemicals.
6 h reaction time.
Experimental Section
Preparation of LDHs
2
+
3+
A series of LDHs with different M /M cations composition was
2
+
3+
prepared by using the co-precipitation method, M /M was set
[3c,33]
to 3.
All chemicals were purchased from Acros Organics, VWR
and Sigma-Aldrich. Briefly, an aqueous salt solution containing a
2+
mixture of 0.075 molM precursor [i.e., Mg(NO ) , Cu(NO ) , Ni
(
3
2
3 2
3
+
NO ) , and Zn(NO ) and Al(NO ) ] and 0.025 molM precursor
3 2 3 2 3 3
Scheme 1. Possible reactions occurring during the oxidative conversion of
benzyl alcohol.
[i.e., Al(NO ) , Fe(NO ) , and Co(NO ) ] was added dropwise to an
3
3
3 3
3 3
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alkaline solution (100 mL) containing 0.25 mol Na CO . The pH of Acknowledgements
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the precipitation solution was kept constant at 10 by addition of
NaOH (4 M) solution. The resulting mixture was aged at room
temperature overnight under continuous stirring. The aged mixture
was then filtered and washed with deionized water followed by
drying at 100°C in an oven.
The authors are grateful to the Chinese Science Council and the
French Ministry for Foreign Affairs for funding their Cai Yuanpei
project. RB would like to thank St Gobain-CREE and Region Grand
Est for her PhD grant.
Characterisation of LDH Samples
XRD patterns of the samples were recorded using a Bruker D8 Conflict of interest
instrument in reflection mode with Cu Kα radiation. The accelerat-
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The authors declare no conflict of interest.
À 1
Keywords: LDHs · LDOs · methanol conversion · benzyl alcohol
oxidation · hydrotalcites
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Solid reaction! This article presents
the synthesis and in-depth character-
isation of layered double hydroxides
L. Huang, Dr. C. Megías-Sayago, R.
Bingre, Q. Zheng, Prof. Q. Wang*,
Dr. B. Louis*
(LDHs). Their catalytic performances
1
– 9
have been evaluated in two different
reactions, methanol conversion (gas-
solid) and benzyl alcohol oxidation
Catalytic Performance of Layered
Double Hydroxides (LDHs) Derived
Materials in Gas-solid and Liquid-
Solid Phase Reactions
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(liquid-phase), to assess their
potential implementation as hetero-
geneous catalysts.