Mesoporous nickel-aluminum mixed oxide: A promising catalyst in hydride-transfer reactions

Article abstract of DOI:10.1002/ejic.201000732

The design and synthesis of a new nanostructured material that can efficiently catalyze selective reduction reactions in an eco-friendly way is an active area of research today. Here a mesoporous Ni-Al mixed oxide material has been synthesized hydrotherma

Full text of DOI:10.1002/ejic.201000732

FULL PAPER
DOI: 10.1002/ejic.201000732
Mesoporous Nickel–Aluminum Mixed Oxide: A Promising Catalyst in
Hydride-Transfer Reactions
[a]
[a]
[a]
Manidipa Paul, Nabanita Pal, and Asim Bhaumik*
Keywords: Reduction / Capping agents / Hydrides / Mesoporous materials / Nanostructures
The design and synthesis of a new nanostructured material
that can efficiently catalyze selective reduction reactions in
an eco-friendly way is an active area of research today. Here
transmission electron microscopy (HRTEM), scanning elec-
tron microscopy/energy-dispersive spectroscopy (SEM-EDS),
FTIR, and thermogravimetric/differential thermal analysis
a mesoporous Ni–Al mixed oxide material has been synthe- (TG-DTA) tools. The mesoporous Ni–Al mixed-oxide material
sized hydrothermally by using lauric acid as capping agent.
The mesoporosity observed in the material is mainly origi-
nated from the interparticle voids created due to the self-
assembly of the nanoparticles (NPs) in the presence of cap-
ping agent used during the synthesis. The material has been
showed a very high Brunauer–Emmett–Teller (BET) surface
2
–1
area (337 m g ) and excellent catalytic activity in a selective
liquid-phase hydride-transfer reduction reaction of nitroar-
enes to their corresponding anilines in the presence of 2-pro-
panol as hydride source.
2
characterized by powder XRD, N sorption, high-resolution
sol–gel[17] techniques. Tai and Guo have synthesized hexag-
Introduction
onal nickel–aluminum oxide by the homogeneous precipi-
Due to their unique properties such as exceptionally high
surface areas, uniform and tunable nanoscale pore dimen-
sions, and large pore volumes, mesoporous materials have
maintained an outstanding position in the field of materials
science since the first report of M41S materials.[ With the
passage of time and advancements in the field, the attention
is now focused on nonsiliceous mesoporous materials,
[18]
tation method.
But the material is amorphous, and its
catalytic potential has not been explored. Besides that, the
self-organization of nanoparticles (NPs) is another major
[19]
area of research since it has many practical applications.
1]
Because of their special size, a large number of particles are
present at the surface compared to that of the bulk. For
this reason, nanostructured materials have found potential
[
2]
[3]
mainly phosphates and oxides instead of silica-based or-
[20]
utility in a wide range of catalytic reactions.
[
4]
ganic–inorganic hybrid materials. Among these materials,
mesoporous metal oxides have come to occupy a distinct
position owing to their potential application in different
In this context it is pertinent to mention that the aro-
matic amines are an important class of compounds that are
used in large-scale industrial processes for the manufacture
of dye-stuffs, pharmaceuticals, agrochemicals, photographic
chemicals, polymers, chelating agents, and so on. Different
[
3]
frontier areas. Compared to simple metal oxides, mixed
metal oxides such as pervoskites[ and spinels have wider
scopes because of their tunable structural features through
compositional variations and huge potential applications
5]
[6]
methods such as metal/HCl systems, H gas, sulfides and
2
polysulfides, and so forth are commonly utilized as reduc-
ing agents for the conversion of nitro compounds to their
respective amine derivatives. However, on account of envi-
ronmental constraints, particular attention has been paid to
finding a safer alternative strategy for the reduction of nitro
groups in this context. Catalytic hydride-transfer reduction
[7]
[8]
that range from adsorption and catalysis to electrode
[
9]
materials.
Mixed metal oxides that contain transition-metal atoms
can play a significant catalytic role in a large variety of
heterogeneous chemical processes such as catalytic combus-
tion of hydrocarbons,[ CO oxidation,
10]
[11]
selective partial
(
CHTR) has received significant recognition as an alterna-
oxidation, and reduction of organic molecules.[
12–14]
In par-
[21–23]
tive for the clean reduction of a variety of nitroarenes.
ticular, Ni- and Al-based spinels are potential catalysts in
reduction reactions.[ In the past, nickel–aluminum spinel
oxides have been prepared by diverse synthetic routes
Several greener processes are available by using a CHTR
15]
[21]
pathway that utilizes a phase-transfer catalyst,
Zn/am-
[22]
monium salt/ionic liquid,
ultrasound-promoted re-
and so on. But all these methods have limita-
[
16]
through co-precipitation to layered double hydroxides or
[23]
duction,
tions with regards to their complex synthetic route and
cost-effectiveness. Herein, we report a simple synthetic
route for the preparation of mesoporous nickel–aluminum
mixed-oxide material through self-assembled NPs in the
[
a] Department of Materials Science, Indian Association for the
Cultivation of Science,
Jadavpur, Kolkata 700032, India
E-mail: msab@iacs.res.in
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M. Paul, N. Pal, A. Bhaumik
FULL PAPER
presence of an anionic template, lauric acid, which plays the rameter a = 0.8 nm along with the presence of some minor
role of a capping agent for the positively charged metal cat- NiO nanocrystals [asterisk (*) on the peak in the XRD
ions at the surface of the nanoparticles. Our mesoporous pattern]. The particle size calculated by using the Scherrer
nickel–aluminum oxide material shows a nanocrystalline equation from this wide-angle XRD pattern suggests a
NiAl O spinel structure upon high-temperature calci- grain size of around 6 nm, which agrees well with electron-
2
4
nation. Furthermore, this mesoporous nickel-aluminum ox- microscopic studies (see below). However, the main objec-
ide shows good regioselectivity in the reduction of nitro- tive of our present study is to synthesize nickel–aluminum
benzenes to anilines in an environmentally safer route by mixed-oxide material with maximum mesoporosity and a
means of hydride transfer from a sacrificial secondary large surface area. Although the material heated at higher
alcohol.
temperature (sample 4) exhibits better crystalline features,
this material showed a very small surface area (see below).
Indeed, the sample 3 (heated at 773 K) shows moderate
crystalline features along with high porosity and large sur-
face area, and this sample has shown better catalytic ac-
tivity in the reduction of nitroarenes.
Results and Discussion
Chemical Composition
The chemical composition of the calcined mesoporous
nickel–aluminum oxide sample 3 [obtained from energy-dis-
persive spectroscopy (EDS) analysis] is given in Table 1.
The data reveal that the ratio of nickel/aluminum is approx-
imately 0.5 in the sample, although during synthesis of this
mesoporous nickel–aluminum oxide we used an Ni/Al mo-
lar ratio of 1:1 in the synthesis gel of our first batch. Excess
amounts of nickel were leached out during hydrothermal
synthesis. Later we used a 1:2 molar ratio of Ni/Al in the
synthesis batch, and the final product also followed a sim-
ilar chemical composition. An excess amount of O (Table 1)
could be attributed to the surface defects and the presence
of adsorbed water molecules. Thus, the chemical formula
for our mesoporous nickel–aluminum oxide can be written
as NiO–Al O .
2
3
Figure 1. Wide-angle XRD pattern of mesoporous Ni–Al oxide
samples 4 (a) and 3 (b).
Table 1. EDS results for the mesoporous Ni–Al oxide sample 3 cal-
cined at 773 K.
Element
[keV]
Mass-%
Atom-%
N -Sorption Measurement
2
Ni K
Al K
O K
7.471
1.486
0.525
23.14
21.58
55.29
100.00
8.48
17.2
74.32
100.00
Nitrogen adsorption/desorption isotherms of mesopo-
rous nickel–aluminum oxide samples are shown in Figure 2.
The Brunauer–Emmett–Teller (BET) surface area of sample
Total
2
–1
3
is 337 m g , whereas sample 4, which calcined at 1073 K,
Characterizations
Powder XRD
The wide-angle powder XRD patterns of the mesopo-
rous nickel–aluminum oxide samples calcined at 773 and
1073 K are shown in Figure 1. It is interesting to note that
no peak for either nickel or nickel oxide (NiO) is observed
for the sample calcined at 773 K. This pattern is similar to
the XRD pattern of alumina (Al O ). This anomaly arises
2
3
II
at this stage due to the fact that the Ni sites remain highly
dispersed in the mesoporous alumina matrix,[
24,25]
and the
calcination temperature employed here (773 K) is insuf-
ficient for the complete crystallization of the NiAl O4
2
phase. But when the sample is heated further to 1073 K, the
XRD pattern matches well with that of a nickel aluminate
Figure 2. N
3 (a) and 4 (b) at 77 K. Pore-size distribution of sample 3 is shown
space group Fd3m with cubic symmetry and a lattice pa- in the inset.
2
adsorption (᭹)/desorption (᭺) isotherms of samples
spinel structure (NiAl O , JCPDS card no. 10-0339) in
2
4
5130
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Eur. J. Inorg. Chem. 2010, 5129–5134

Mesoporous Nickel–Aluminum Mixed Oxide
2
–1
shows a very small BET surface area of 21 m g . From
the difference of these data, it is clear that four repeating
extractions followed by calcination at 773 K are able to re-
move the template lauric acid molecules and generate me-
soporosity. But when this sample is heated to very high tem-
perature (1073 K), the self-assembly of the NPs then col-
lapses, and the material crystallizes completely into an Ni-
Al O spinel structure with no mesoporosity. Pore-size dis-
2
4
tribution (PSD) of sample 3 estimated by the nonlocal den-
sity functional theory (NLDFT; Figure 2, inset) suggests
that the size of the pores is nearly 8.4 nm.
Nanostructure and Morphology
In Figure 3, TEM images of sample 3 are shown. From
the TEM image (left) it is evident that the porosity of the
material arises mainly from the interparticle separations. A
close look at the image also reveals that the material is com-
posed of many tiny close-packed particles of around 6–7 nm
in size and that the particles are separated from each other
at a distance of around 5–8 nm. It is possible that the tem-
plate lauric acid molecules employed in the synthesis play
the role of a capping agent, which restricts the particle-size
growth. The high-resolution transmission electron micro-
scopy (HRTEM) image (Figure 3, lower right) shows lattice
fringes that correspond to (400) planes with a distance of
Figure 4. SEM image of the sample 3.
Optical Study
UV/Vis spectroscopy can give us valuable information
about the symmetry and coordination of the surface species
of a catalyst. Figure 5 shows the UV/Vis spectra of sample
0.2 nm between two planes, which agrees well with the
3. The absorption is mainly due to ligand-to-metal charge-
wide-angle d400 value of nickel aluminate spinel. The se-
lected area electron diffraction (SAED) pattern (Figure 3,
upper right) suggests that the wall of this mesoporous
nickel–aluminum oxide is semicrystalline and composed of
several (311), (400), and (440) planes of the nickel–alumi-
nate spinel lattice crystal. Textural properties and particle
size of the mesoporous material have been investigated by
scanning electron microscopy (SEM) images. From Figure 4
we can see that the material is composed of tiny spherical
particles, which self-assembled among themselves to form
large clusters.
transfer transitions. The band observed near 396 nm could
be attributed to the presence of octahedrally coordinated
2+
2–
2+
Ni (O Ǟ Ni charge transfer), whereas the peaks ob-
served in the vicinity of 681 and 738 nm could be attributed
2+
to the tetrahedral Ni ions in this nickel–aluminum mixed-
oxide material. For the bulk NiO/γ-Al O samples with dif-
2
3
II
ferent Ni loadings, two absorption bands at 590 and
35 nm were attributed to tetrahedral Ni ions, and a
2+
6
broad absorption band in the vicinity of 390 nm was attrib-
2+ [26]
uted to octahedral Ni . A moderate shift of the absorp-
tion bands from 390 to 396 nm and a very large shift of the
5
90 and 635 nm bands to 681 and 738 nm could be attrib-
[
27]
uted to the surface-defect states
that originated during
hydrothermal synthesis in the presence of the capping
agent, lauric acid molecules. As there is no sharp peak near
[
28]
720 nm,
we can conclude that NiO crystallites are not
present at the NiO–Al O surface, which also agrees well
2
3
with the powder XRD result. It has been previously ob-
served that when the nickel-ion loading is far below the
dispersion capacity, the supported nickel ions preferentially
incorporate into the tetrahedral vacancies of γ-Al O ; and
2
3
with increased nickel loading, the ratio of Ni2 ions that
+
incorporate into the octahedral vacancies of γ-Al O in-
2
3
[
26]
creases. Since our mesoporous Ni–Al mixed-oxide mate-
II
rial contains a very high concentration of Ni , both tetra-
hedral and octahedral coordination sites of Ni2 are present
+
in this material. NiAl O is a largely inverse spinel that
2
4
shows randomization of the distribution of Ni and Al
atoms between the octahedral and tetrahedral cation sites in
the framework with a change in temperature. This cation
Figure 3. TEM image (left), HRTEM image showing the lattice
fringes (lower right), and selected-area electron-diffraction pattern
[
29]
(
upper right) of mesoporous NiO–Al
2
O
3
sample 3.
Eur. J. Inorg. Chem. 2010, 5129–5134
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Paul, N. Pal, A. Bhaumik
FULL PAPER
distribution (and inversion degree) in the mesoporous Ni– the adsorbed water molecule from the surface. The second
Al mixed oxide can play a crucial role in its surface proper- step (623–773 K) is related to a sharp weight loss of around
ties.
51%, thereby resulting in the removal of the organic cap-
ping agent (lauric acid) moiety from the material. A further
weight loss of around 2.5% at 773–873 K associated with
an exothermic peak in the DTA profile could be an indica-
tion of the transformation of mesoporous Ni–Al mixed ox-
ide into an NiAl O spinel structure.
2
4
Catalytic Hydride-Transfer Reduction
We carried out the liquid-phase hydride-transfer reaction
with three different nitroarenes that contain activating and
deactivating groups in the aromatic ring, and the results
are summarized in Table 2. For all these nitroarenes, the
Figure 5. UV/Vis diffuse reflectance spectrum of mesoporous NiO– mesoporous Ni–Al mixed-oxide catalyst 3 shows moder-
Al
2
O
3
sample 3.
ately good activity for the reduction of NO groups. Selec-
2
tivity for the reduction to the NH group is exclusive in all
2
the cases. For nitrobenzene, the conversion to aniline is
49.6%, whereas it is 57.8% for 4-chloronitrobenzene to 4-
Metal–Template Interaction
chloroaniline. A relatively low conversion (36.2%) has been
achieved for m-dinitrobenzene to 3-nitroaniline by using
this mesoporous nickel–aluminum oxide material. As seen
from the table, the conversions of these nitroarenes follow
the decreasing yield from the more activated aromatic ring
of 4-chloronitrobenzene (+R effect) to the deactivated aro-
matic ring of m-dinitrobenzene. In all the cases, we can see
that the catalyst is highly regioselective, and no further re-
duction of the benzene ring could occur. When the reaction
is carried out in the absence of any catalyst, the reaction
does not proceed at all (Table 2, Entry 4), thereby suggest-
ing the catalytic role played by mesoporous Ni–Al mixed-
oxide material. In the presence of pure alumina catalyst, the
conversion is very low (Table 2, Entry 5), thereby suggesting
FTIR spectra often give us information about metal–
template interactions. In the FTIR spectra of the as-synthe-
sized mesoporous Ni–Al mixed-oxide materials (not
shown), two peaks are observed at around 2850 and
–
1
2922 cm , which are assigned to the C–H stretching fre-
quencies from the lauric acid molecules. These peaks are
absent in sample 3, which indicates that, after heat treat-
ment of the extracted sample, it is possible to remove the
template completely, which also supports our sorption re-
sult. Long-chain fatty acid molecules are frequently used as
capping agents to stabilize metal/metal oxide nanopar-
[
30]
ticles. The interaction between the precursor metal oxide
species and the lauric acid molecules can be indirectly
proved from the thermal analysis results. In Figure 6, the
thermogravimetry (TG) and differential thermal analysis
II
that the presence of Ni at the active sites is mainly respon-
sible for the above reduction reaction. Moreover, when the
reaction was carried out by using catalyst 4 (high-tempera-
ture calcined sample) the product conversion comes down
to only 28% (Table 2, Entry 6). This could be attributed
to the fact that, after high-temperature calcination of the
mesoporous Ni–Al mixed-oxide material, the porous frame-
work collapses to a large extent. Hence the surface area
diminishes and the amount of adsorbed reactant species
does likewise.
(DTA) plots for sample 1 are shown. The TG curve consists
of three distinct weight losses. In the first step (i.e., in the
temperature range 303–523 K), there is a weight loss of
around 15%, which could be attributed to the removal of
In this hydride-transfer reduction reaction, nitroarenes
are first adsorbed onto the surface of the catalyst. NaOH
used in the reaction acts as a promoter to stabilize the ad-
sorbed nitro groups present at the catalyst surface. Then the
adsorbed nitroarene takes up hydrogen from the 2-propanol
(it plays the role of hydride source) and forms a hydroxy-
type intermediate, which further rearranges to a nitroso in-
[
31]
termediate.
This could be followed by two consecutive
[
32–35]
steps in which hydride transfer from 2-propanol
to the
nitroso and hydroxylamine species takes place to form the
II
desired aniline. Ni sites present at the catalyst surface sta-
bilize the 2-propanol molecules through adsorption and
Figure 6. TG (a) and DTA (b) curves for as-synthesized sample 1. subsequent hydride transfer from the intermediate species.
5
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Mesoporous Nickel–Aluminum Mixed Oxide
Table 2. Liquid-phase catalytic reduction by using mesoporous Ni–Al mixed oxide.[1]
Substrate
Reaction time [h]
Conversion [%]
Product
Nitrobenzene[a]
-Chloro-1-nitrobenzene
5
5
5
6
6
9
49.6
57.8
36.2
0.4
0.6
28.0
aniline
[
a]
4
4-chloroaniline
3-nitroaniline
aniline
4-chloroaniline
4-chloroaniline
[a]
m-Dinitrobenzene
Nitrobenzene[
b]
[
c]
4
4
-Chloro-1-nitrobenzene
-Chloro-1-nitrobenzene[
d]
[
a] Reaction conditions: substrate (15 mmol), promoter (15 mmol), heated to reflux at 353 K in 2-propanol (15 mL), and sample 3 as
3
catalyst (0.05 gcm ). [b] Reaction was carried out in the absence of catalyst. [c] Reaction was carried out by using pure alumina catalyst.
d] Reaction was carried out by using sample 4 (calcined at 1073 K).
[
In this way, the desired anilines are formed, which then de- 40 mA current and calibrated with a standard silicon sample. Ni-
trogen adsorption–desorption isotherms were obtained with a Bel
sorb from the surface of the catalyst. Thus, the above results
show that the surface Ni sites and the high BET surface
area of our mesoporous nickel–aluminum oxide material
are crucial in this catalytic hydride-transfer reduction reac-
tion.
II
Japan Inc. Belsorp-HP instrument at 77 K. Samples were degassed
at 423 K for 5 h prior to both the analyses. BET surface area and
pore-size distribution were obtained according to the Brunauer–
Emmett–Teller (BET) and NLDFT methods, respectively. For
TEM analysis, a small amount of the powder sample was first dis-
persed in ethanol, and one drop of the dispersion was transferred
onto a TEM Cu grid. It was allowed to dry in the ambient environ-
ment, and the analysis was carried out with a Jeol JEM 2010 trans-
mission-electron microscope. The morphology of the sample was
recorded by SEM (JEOL JEM 6700F with EDS attachment). UV/
Vis diffuse reflectance spectra were obtained with a Shimadzu UV
Conclusion
Mesoporous nickel–aluminum mixed-oxide material has
been synthesized by a one-step hydrothermal route using
lauric acid as the capping agent. The material showed a
large surface area and high mesoporosity due to the inter-
2401PC spectrophotometer with an integrating sphere attachment
by using a BaSO pellet as background standard. FTIR spectra of
4
particle separation assisted by the long-chain fatty acid mo- these samples were recorded on KBr pellets with a Nicolet
lecules used during the hydrothermal synthesis. Liquid- MAGNA-FT IR 750 series II spectrometer. Thermogravimetry
phase hydride-transfer reactions were carried out for dif- (TG) studies and differential thermal analysis (DTA) were carried
out with a TA Instruments SDT Q-600 thermal analyzer.
ferent nitroarenes by using this mesoporous mixed-metal
oxide as catalyst and 2-propanol as hydride source under Liquid-phase hydride-transfer reactions were performed by using
mild and environment-friendly conditions. The material our various mesoporous NiO–Al O mixed-oxide-based materials,
2 3
showed moderately good catalytic activity in the reduction namely, samples 3 (calcined at 773 K) and 4 (calcined at 1073 K).
The reactions were also carried out with pure alumina and in the
of nitroarenes together with high regioselectivity for the
absence of any catalyst. Here we have chosen three different ni-
corresponding desired anilines.
troarenes as substrate. The reactions were carried out in a two-
necked round-bottomed flask fitted with a reflux condenser and
temperature-controlled oil bath on a magnetic stirrer plate. In a
typical reaction, the substrate (15 mmol) was mixed with NaOH
Experimental Section
For the synthesis of mesoporous NiO–Al
2
O
3
mixed-oxide, nickel
, Merck)
(15 mmol; as promoter). Then 2-propanol (15 mL; it acts as solvent
as well as reducing agent) and catalyst (0.05 g) were added to it.
The temperature of the reaction was maintained at 353 K, and ali-
quots were collected at different time intervals. Finally, the catalyst
was separated by filtration, and the products were analyzed with a
gas chromatograph (Agilent 7890 A) equipped with a flame-ioniza-
tion detector (FID) fitted with a capillary column.
chloride (NiCl , Merck) and aluminum chloride (AlCl
2
3
were used as the precursors for nickel and aluminum, respectively.
Lauric acid (Merck) was used as capping agent. Nickel chloride
(0.02 ) and aluminum chloride (0.02 ) were first dissolved in
water (10 mL) and then blended with lauric acid (0.02 , 20 mL)
solution. The resulting mixture was stirred at 313 K for 2 h to ob-
tain a clear sol. Ammonia solution was added dropwise to this
solution until it formed a thick sky-blue precipitation. The pH of
the mixture was maintained at around 10. The solution was hydro-
thermally treated at 373 K for 2 d, then filtered and thoroughly
washed with water. To remove the capping agent, the as-synthesized
material was treated four times with ethylenediamine/ethanol mix-
ture at room temperature and finally calcined at 773 K in air for
Acknowledgments
A. B. wishes to thank the Department of Science and Technology
(
DST), New Delhi for financial support. M. P. and N. P. are grate-
ful to the Council of Scientific and Industrial Research (CSIR),
New Delhi for their senior research fellowships.
2
h. The sample obtained after calcination at 773 K is designated
as 3, the as-synthesized one as 1, and the fourth extracted one as
. The sample calcined at 1073 K has been designated as sample 4.
The molar composition of the synthesis mixture was 0.02 NiCl
.02 AlCl /0.02 lauric acid.
2
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Eur. J. Inorg. Chem. 2010, 5129–5134
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Received: July 5, 2010
Published Online: September 29, 2010
5134
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 5129–5134