Synthesis of high surface area mixed metal oxide from the NiMgAl LDH precursor for nitro-aldol condensation reaction

Article abstract of DOI:10.1039/c4nj01332h

We have investigated the effect of divalent metal cations on the structural and catalytic activity of MgAl-layered double hydroxide (LDH), by partially substituting Mg²⁺ with M²⁺ (M²⁺ = Ni²⁺ or Co²⁺)

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Synthesis of high surface area mixed metal oxide from NiMgAl LDH
precursor for Nitro-aldol condensation reaction
Dipshikha Bharalia, Rasna Devia, Pankaj Bharalia and Ramesh C. Dekaa *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
We have investigated the effect of divalent metal cations on structural and catalytic activity of MgAl-
layered double hydroxide (LDH), by partially substituting Mg2+ with M2+ (M2+= Ni2+ and Co2+) keeping
metal to aluminium ratio (M2+/Al3+) =3 and Mg2+/M2+ = 1.5. The LDH precursors were prepared by a
10 coprecipitation method. The products were finally calcined at 450 °C for 4 h to obtain mixed metal
oxides. The catalysts were characterized by powder X-ray diffraction (PXRD), Fourier transform infra-
red spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and
Brunauer-Emmett-Teller (BET) surface area techniques. The catalysts were then tested towards Nitro-
aldol condensation reaction under different reaction conditions. The mixed metal oxide obtained from
15 calcination of precursor NiMgAl LDH was found to be more efficient catalyst for Nitro-aldol
condensation under solvent free condition at room temperature due to its high BET-surface area and pore
volume.
45 extend its application to various fields such as organic
transformations,27-29 photocatalysis etc.30-32 These ternary
compounds can be prepared by changing divalent or trivalent
cation in the precursor LDHs. Calcination of LDHs give mixed
metal oxides which can be used as basic catalysts in various
50 organic transformations along with the advantages of high surface
area and thermal stability.33-37 Due to its simplicity, cost
effectiveness and reusability, application of mixed metal oxides
have drawn great importance in organic transformations.38,39 The
selective synthesis of 2-nitroalkanol by modified LDH has been
55 investigated. However, catalytic performance of the mixed metal
oxides from LDH precursor has not been well examined.
The aim of the present work is to prepare a green and
environmentally benign solid base catalyst by partial substitution
of Mg2+with M2+ (M2+ = Ni2+& Co2+) ion in MgAl LDH. The
60 synthetic method used to prepare LDHs is the co-precipitation
method.40 The synthesized and calcined catalysts were employed
to test the catalytic activity for the nitro-aldol condensation
reaction under solvent free condition at room temperature
changing various reaction conditions.
Introduction
Nitro-aldol condensation, commonly known as Henry reaction is
20 one of the most important CC bond formation reactions and
mostly used in bulk and fine chemical industries.1 The product of
the Henry reaction is also extensively used in many important
syntheses including the synthesis of various biological
compounds.2,3 Usually, the product 2-nitroalkanol is produced by
25 the aldol condensation of carbonyl compounds with nitroalkanes.
The classical methods for the synthesis of this product use
different bases including carbonate and bicarbonates. Formation
of nitro-alkene by the elimination of water molecule from 2-
nitroalkanol is one of the disadvantages of Henry reaction.
30 Therefore, it is very much challenging to develop a green and
economically cheaper solid base catalyst for selective synthesis of
2-nitroalkanol.
Layered double hydroxides (LDHs) are layered compound
with structural similarities to hydrotalcite clays.4-9 These
35 compounds are also known as anionic clay10-13 with the property
of anion exchange due to which these compounds can be widely
used in various fields as catalysts,14-17 catalyst support,18-20 and
65 Experimental Section
adsorbents.21-24 They can be represented by the general formula
3+
x
[M2+
M
1-x
(OH)2]x+ (An-x/n)·mH2O, where M2+ and M3+ are
Materials
40 divalent and trivalent metal ions, x is the ratio of M3+/(M2++M3+),
An- is anion that can be organic or inorganic by which excess
positive charge in the layer can be compensated and m is the
amount of water present in the interlayer.25,26 Ternary LDHs
exhibit better catalytic activity over precursor LDHs and greatly
Mg(NO3)26H2O,
Ni(NO3)26H2O,
Co(NO3)26H2O,
Al(NO3)39H2O, NaOH, Na2CO3 were purchased from Merck,
Mumbai, nitromethane from G.S. Chemicals Testing Lab &
70 Allied Industries, New Delhi; silica gel, iodine and solvents were
purchased from RANKEM, New Delhi.
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Synthesis of LDHs
temperature and amount of catalysts. The reactions were
monitored via thin layer chromatography and % conversions were
LDHs were prepared by co-precipitation of an aqueous solution
1
obtained from H and 13C NMR analyses of the crude reaction
of
metal
nitrates-
Ni(NO3)26H2O,
Co(NO3)26H2O,
mixture. To evaluate the catalytic activity more clearly, turnover
60 frequency (TOF) of the catalysts were calculated using the
general equation,
Mg(NO3)26H2O and Al(NO3)39H2O with an aqueous solution of
NaOH and Na2CO3 at a constant pH of (10±0.2) with M2+/M3+
ratio of 3:1. After the formation of the precipitate, the gels were
aged at 80 °C for 24 h. The solid product was then centrifuged,
washed several times with distilled water until pH of the filtrate
was 7 and then dried in the oven at 80 °C for 12 h. In MgAl LDH,
5
10 Mg2+ was partially substituted by M2+ (M2+= Ni2+ and Co2+)
keeping metal ratio M2+/M3+ = 3 and of Mg2+/M2+ = 1. The
samples were calcined at 450 °C for 6 h in flowing air and the
heating rate of 2 C min1.
where, TON refers to turn over number. For the turn over number
calculations, number of active or basic sites was calculated by
basicity measurements. In this method, a suspension of the 15 mg
65 catalyst in a 2 ml toluene solution of phenolphthalein (0.1mg/ml)
was stirred for 30 minutes and titrated with a toluene solution of
benzoic acid (0.01M). The aqueous soluble amount of basic sites
was determined by acid-base titration method reported
previously.42
Characterization
15 The powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku Multiflex instrument using nickel-filtered CuKα (0.15418
nm) radiation source and a scintillation counter detector. The
intensity data were collected over a 2θ range of 5−70°. The
crystallite size was calculated using Scherrer equation:
70 Results and Discussion
Characterization
20
D = Bλ/β1/2 cos θ
where D is the average crystallite size, B is the Scherrer constant
(0.89), λ is wavelength of the X-ray beam, β1/2 is the full-width at
half-maximum (FWHM) of diffraction peak and θ is the
Powder XRD patterns of LDHs are shown in Fig. 1(i). The
formation of hydrotalcite (HT) type layered structure has been
confirmed from the presence of sharp intense peaks at low 2θ
diffraction angle. Elemental analyses for metal ions were 75 angles and comparatively broad, less intense peaks at higher 2θ
angles which match well with previous reports.12,27 In case of all
the synthesized LDHs, the diffraction peaks were observed
approximately at 2θ values 11.6, 23.4, 34.8, 38.8, 46.3, 60.85 and
62.1 that could be assigned to (003), (006), (012), (015), (018),
25 performed using inductively coupled plasma optical emission
spectroscopy (ICP-OES, PerkinElmer, Optima 2100 DV). The
solutions of the samples were prepared by keeping the samples to
dry at 80 °C for 24 h and then dissolving the samples in (1:1) dil.
HCl.41 Infra-red spectra were measured in
a
FTIR 80 (110) and (113) reflections, respectively (JCPDS card no. 38-
30 spectrophotometer, Model Nicolet Impact I-410. Measurements
are performed by pelletising the samples with KBr in the mid-
infrared region. The TGA curves were obtained on a Thermal
Analyzer (Model TGA-50, Shimadzu) instrument. The samples
were heated from ambient temperature to 700 °C under N2 flow.
35 The heating rate in each case was kept at 10 C min−1. To study
the surface topography, SEM analyses were carried out with
“JEOL, JSM Model 6390 LV” Scanning Electron Microscopes,
operating at an accelerating voltage of 15 kV. The Brunauer-
Emmett-Teller (BET) surface areas were determined by N2
40 adsorption using a Quantachrome Instruments (Model: NOVA
1000e). All the BET values in this study were measured within
the precision of ± 5%. The pore size and pore volume is
determined following Barrett-Joyner-Halenda (BJH) method in
the same instrument. The 1H and 13C NMR spectra were recorded
45 on a JEOL JNM-ECS 400 spectrometer taking Me4Si as the
internal standard in CDCl3 and CD3OD solvents. Preparative
column chromatography was carried out with silica gel 60–120
mesh size and thin-layer chromatography (TLC) was carried out
on glass plates with silica gel.
0478).33 It has been observed that partial substitution of Mg2+
ions with Ni2+ and Co2+ affects the crystallization procedure
which is well reflected from the broad and less intense peaks of
both CoMgAl and NiMgAl samples in comparison to MgAl
85 sample. The peaks for (015) and (018) reflections were not
distinct in case of NiMgAl while the reflections at (015), (018),
(110) and (013) were completely disappeared in CoMgAl. This
can be attributed to the larger size of Co2+ (0.88 Å) and Ni2+ (0.83
Å) which could not accommodate well in the brucite layers. The
90 effect can be understood from the calculation of lattice
parameters and crystallite sizes. The value of lattice parameter
„a‟, which is a function of the average distance of metal ions
within the layers can be obtained from the (110) reflection peak
while „c‟ parameter which is related to the thickness of the
95 brucite-like layer and the interlayer distance can be obtained from
the (003) reflection peak.30 The lattice parameters „a‟ (a = 2d110
)
and „c‟ (c = 3d003) are calculated and summarized in Table 1. It is
observed from the table that the „a‟ parameter gradually increases
in case of substituted LDH which can be due to the increased
100 metal-metal distance on partial substitution of metal cations on
MgAl LDH. However, the „c‟ parameter increases in NiMgAl
LDH while it decreases in case of CoMgAl LDH. This difference
in lattice parameter „c‟ can be attributed to the larger ionic radius
of Co2+ (0.88 Å) than Ni2+ (0.83 Å).12,27 Again the crystallite size
105 of NiMgAl and CoMgAl are found to be less compared to that of
MgAl LDH indicating the inclusion of larger cations in LDH
layers resulting in the distortion of layered structure.43
50 Catalytic activity
The nitro-aldol condensation reactions were performed under the
solvent free conditions by taking 1 mmol of the aldehyde, 10
mmol of the nitromethane and 10 mg of the catalysts. For
comparative study all reactions were carried out for 36 h and the
55 reaction conditions were optimized by varying substituent,
2
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Table 1
Chemical analysis, crystallographic data and crystallite size of mixed oxides.
Catalysts
Atomic ratio
Analyzed #
d003
(Å)
d110
(Å)
Cell parameters(Å)
Crystallite size (Å)
c
a
003
110
Starting
MgAl
3:1
3:0.9
7.69
1.52
23.07
3.04
90
134
Ni2+MgAl
Co2+MgAl
1.5:1.5:1
1.5:1.5:1
1.5:1.47:0.9
1.5:1.48:1
7.87
7.68
1.53
1.55
23.61
23.04
3.05
3.09
63.5
54
113
70
# Determined by ICP
5
As shown in Fig. 1(ii), the powder XRD patterns of the mixed 30 from the adsorption of carbon dioxide from air on the surface of
oxides do not possess characteristic peaks corresponding to LDH,
because on calcination, the layered structure gets collapsed and
were converted to corresponding mixed oxides (ESI†).
the metal oxides.27
The Thermogravimetric analysis of the precursor LDHs were
carried out before calcination. From the TGA curve, shown in
Fig. 3, two weight loss steps are observed. The first weight-loss
35 can be observed in the temperature range of 50-200 °C which
corresponds to the removal of water molecules absorbed on the
surface and in the interlayer region of LDHs with weight loss of
~17.8 %. The second weight-loss step observed between 250-400
°C with weight-loss of ~19.5 % is due to the dehydroxylation of
40 the brucite layer as well as the decomposition of the carbonate
anions.44
10 Fig. 1 X-ray powder diffraction patterns of i) LDHs and ii) mixed oxides.
The chemical compositions of LDHs were determined from
the elemental analyses and presented in Table 1. The Table
showed that the molar ratio of M2+Mg/Al was close to 3 and that
of M2+/Mg (M2+ = Ni2+ or Co2+) was 1:1 which nearly same as
15 that of the starting solution.
The FTIR spectra of LDHs are shown in Fig. 2 (i). The bands
at 3478.2 cm1 and 1662.9 cm1 can be assigned to the stretching
and bending vibrations of -OH group of LDH layers and
interlayer water molecule.27,30 The sharp band observed at 1388.5
20 cm1 is due to asymmetric stretching of CO3 ion and broad
2
Fig. 3 Thermogravimetric curves of LDHs.
peaks observed below 1000 cm1 (500-800 cm1) are corresponds
to M-O vibrations. 30
The SEM images of LDHs are presented in Fig. 4. Flat plate-
45 like shapes are observed in case of MgAl LDH. Whereas, in case
of NiMgAl and CoMgAl LDH, crystallites with large irregular
particle size are observed which are due to agglomeration taking
place on partial substitution of Mg2+ by M2+ (M2+= Ni2+ and
Co2+) in MgAl LDH.
50
The porosity of the mixed oxides obtained by calcination of
LDH precursors were investigated by nitrogen adsorption-
desorption measurements. The Nitrogen adsorption-desorption
curve showed that NiMgAl mixed oxide exhibit type IV isotherm
with hysteresis loop H1 (Fig. 5) which is the characteristic of
55 mesoporous material with regular pore structure 45 and along with
an average pore size around 49.6 Å (inset of Fig. 5). However, in
case of MgAl and CoMgAl mixed oxides, isotherm of type IV
with combination of hysteresis loop H1 and H341,46 was observed
Fig. 2 FT-infrared spectra of i) LDHs and ii) mixed oxides.
25
In case of mixed oxides as shown in Fig. 2 (ii), the bands
obseved at near 558.6, 672.4 and 811.9 cm1could be attributed to
the vibration of metal-oxygen (M-O) bond. Moreover, the band
at near 1376.9 cm1 is due to the carbonate which may be arises
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seen that good to excellent conversion to the product was
25 obtained (84-94 %) for all the uncalcined catalysts but the time
taken to complete the reaction was more (8-10 h) [Table 2].
Besides, we observed that partial substitution of metal ions on
MgAl LDH further improved the catalytic activity. This increase
in catalytic activity on partial substitution of metal ions can be
30 explained on the basis of increased cell parameter „a‟ and „c‟ in
case of NiMgAl and CoMgAl LDH (Table 1). The increased d
value in case of NiMgAl LDH (d003 = 7.87) can be attributed to
the superior catalytic activity providing increased basal spacing
for the reaction to takes place between the two layers. However,
35 performing the reaction under the same reaction conditions, it
was observed that the reaction with calcined LDHs was much
faster than the uncalcined LDHs and showed improvement of
time giving 95-99 % conversion of the product within 2-6 hours
with 100 % selectivity. Therefore, calcined catalysts were
40 preferred over the uncalcined ones and all reactions were carried
out further by mixed oxides. We have also observed from the
table that in both the cases (uncalcined and calcined), NiMgAl
LDH was found to be more efficient catalyst compared to that of
MgAl and CoMgAl LDHs (entries 2 and 5, Table 2). The high
45 surface area and large pore volume of the NiMgAl mixed oxide
(Fig. 5) results in the superiority in catalytic activity over the
other two mixed oxides for Nitro-aldol condensation reaction.
Therefore, NiMgAl mixed oxide were considered as the best
catalyst in this study.
Fig. 4 Scanning electron micrographs of a) MgAl, b) CoMgAl and c)
NiMgAl LDHs.
(ESI†, Fig. S1) and an average pore size around 88 and 107 Å
(ESI†, Fig. S2). The BET surface areas and pore volumes of all
the three mixed oxides were calculated and summarized in Table
S1 (ESI†). From the table it can be observed that NiMgAl mixed
oxide possessed high BET surface area and pore volume of 753
m2/g and 279 cm3/g, respectively. While in case of MgAl and
5
50
Table 2
Nitroalkylation of 4-nitrobenzaldehyde with nitromethane catalyzed by
mixed oxides at room temperature.a
Entry
Catalysts
Time (h)
Conversion (%)b
1
MgAl LDH
12
86
2
3
NiMgAl LDH
CoMgAl LDH
8
91
84
10
4
5
6
MgAl (O)
NiMgAl (O)
CoMgAl (O)
6
2
4
95
99
97
aReactions were carried out with 1:10 molar ratio in 1 mmol scale of 4-
55 nitrobenzaldehyde and nitromethane using 10 mg catalyst
bDetermined from 1H NMR data of crude mixture
10 Fig. 5 Nitrogen sorption isotherms of NiMgAl mixed oxide; the inset
shows pore size distribution.
After finding the best catalyst from our observation, we then
studied the effect of various reaction parameters to optimize the
60 reaction conditions for Nitro-aldol condensation reaction. At first
we studied the temperature effect on the reaction by varying
temperature from 25 °C (RT) to 60 °C (Table 3). It can be
observed that on increasing the temperature upto 60 °C, the
reaction time decreases from 2 h to 30 minutes. However,
65 temperature does not have great effect over the conversion and
we observed almost similar type of conversion in all cases.
Therefore, it is convenient to use the greener path. So, the further
studies were performed at room temperature under solvent free
CoMgAl mixed oxides; the observed surface area and pore
volume were 561 m2/g, 401 m2/g, 0.974 cm3/g and 0.937 cm3/g,
respectively. The high BET surface area and pore volume of the
15 mixed oxides are due to the removal of adsorbed and interlayer
water molecule and carbonate ions from the LDH layers at high
temperature.
Catalytic performance
To test the catalytic activity of the synthesized LDHs and mixed
20 oxides, we first performed the reaction under solvent free
condition by taking 1:10 molar ratio of 4-nitrobenzaldehyde and
nitromethane in mmol scale and 10 mg of the catalyst at room
temperature. From thin layer chromatography observation it was
condition.
70
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Table 3
45 nickel (II) complexes were subjected as catalysts in the nitroaldol
(Henry) reaction by employing various aldehydes with
nitromethane in an ionic liquid medium. The reaction was carried
out at room temperature for 5 h and the yield was observed to be
about 52 %. However, on using ionic liquid as solvent medium,
50 slight improve in the yield was observed from 52 to 64 %. The
maximum yield of 92 % was observed in 12 h at 3.0 µmol
concentration of the catalyst in 2 mL of methanol. M. Sutradhar
et al.49 reported the synthesis of a new cyclic binuclear Ni(II)
complex, [Ni2(H2L)2]·4MeOH using the Schiff base N′1,N′3-
55 bis(2-hydroxybenzylidene) malonohydrazide (H4L). A maximum
conversion of ca. 93 % was obtained at 60 °C, for a 24 h reaction
time, with 1 mol % loading of catalyst with the good syn:anti
molar ratio of 72:28 was achieved.
Effect of temperature on Nitro-aldol condensation reaction of 4-
nitrobenzaldehyde and nitromethane catalyzed by NiMgAl mixed oxidea
Entry
Time (h)
Conversion (%)b
Temperature (C)
1
2
3
RT
40
60
2
1
99
98
99
30 min
aReactions were carried out with 1:10 molar ratio in 1 mmol scale of 4-
nitrobenzaldehyde and nitromethane using 10 mg catalyst
5
bDetermined from 1H NMR data of crude mixture
We next investigated the effect of amount of catalyst on
reaction time and percentage conversion (Table 4). On increasing
60 Table 5
10 the catalysts amount from 5 to 15 mg, the % conversion increases
along with decrease in reaction time. Further on increasing
catalyst amount to 20 mg, it was observed that the reaction
completed within 1h while conversion (%) was almost similar.
So, we have selected 10 mg catalyst as our optimum amount for
15 further studies at room temperature under solvent free condition.
Nitro-aldol condensation reaction of nitromethane with different
aldehydes.
Conversion (%)a
Entry
Substrate
Time (h)
1
2
3
4
5
6
7
8
4-nitobenzaldehyde
2-nitrobenzaldehyde
3-nitrobenzaldehyde
1-naphthaldehyde
2
99b
98
97
76
70
80
78
91
4
4
Table 4
20
20
36
36
36
Effect of the amount of catalyst (NiMgAl mixed oxide) used in the
conversion (%) of Nitro-aldol condensation reactiona.
Furfuraldehyde
4-hydroxybenzaldehyde
4-methylbenzaldehyde
Chloroaldehyde
Entry
Catalyst (mg)
Time (h)
Conversion (%)b
1
2
3
5
4
2
2
95
99
98
10
15
a Determined by 1H NMR data of crude mixture
b Isolated pure product
4
20
1
99
65
We have also studied the effect of calcination temperature on
catalytic activity with the optimum reaction conditions. We have
calcined the catalyst at a lower (350 °C) and at a higher (550 °C)
calcinaton temperature to the reported one (450 °C). The sample
calcined at 350 °C, showed low catalytic activity with 84 %
20 aReactions were carried out with 1:10 molar ratio in 1 mmol scale of 4-
nitrobenzaldehyde and nitromethane using 10 mg catalyst
bDetermined from 1H NMR data of crude mixture
After getting optimizing conditions for Nitro-aldol
condensation reaction, the reaction was then performed with
25 variety of aldehydes, e.g. simple aromatic aldehydes, heterocyclic
aldehyde and polycyclic aromatic aldehydes (Table 5). It is
observed from the table that with changing the substituents, the
amount of the % conversions are also getting changed. As
expected, substrates with electron withdrawing groups give
30 higher % conversion (97-99 %, entries 1-3, Table 5) while the %
conversion become decreases (60-50 %) in case of the substrate
with electron donating groups (entries 6-8). However it can be
observed from the table that the reaction with polycyclic
aldehydes gives better % conversion of 70-80 (entries 4-5).
70 conversion in 6 h (Table S2, ESI†). This lower calcination
temperature leads to no any characteristic change in the layered
structure. It can be considered as a dehydrated one with its basic
sites occupied by CO32- ions. Consequently, the BET surface area
of the catalyst was relatively lower (584 m2/g) that leads to the
75 lower catalytic activity. On the other hand, the sample calcined at
550 °C, also exhibited lower activity with the 86 % conversion in
8 h (Table S2, ESI†). The high calcination temperature result in
the phase transformation of mixed oxides into crystalline spinel
phases which results in the lower BET surface area (566 m2/g)
80 and hence lower in catalytic activity.7,50-54 The N2 adsorption
desorption graphs of NiMgAl (O) at 350 and 550 °C calcination
temperature also followed type IV isotherm (Fig. S3, ESI†). The
basic sites formed after calcination of LDHs at 450 °C were
considered as the active sites in mixed metal oxides. The
85 presence of different types of basic sites is related to –OH groups,
O2--Mn+ pairs and O2- anions.55-60
35
Comparing our results with the reported one, we have
observed that A. Cwik et al.47 carried out Nitroaldol-reaction of
aldehydes in the presence of non-activated Mg:Al 2:1
hydrotalcite. The reaction was performed at room temperature in
methanol, tetrahydrofuran and nitromethane and best results were
40 observed using nitromethane as solvent (20 mol equivalent).
However, conversion was 95 % in 5 h. R. Manikandan et al.48
Reported the synthesis and spectral characterization of three new
nickel (II) complexes containing pyridoxal N(4)-substituted
thiosemicarbazone ligands with triphenylphosphine. The new
The turnover frequency (TOF) of the mixed metal oxides is
shown in Fig. S4 (ESI†). It can be seen from the figure that the
TOF increases on partial substitution of metal cation with Mg2+ in
90 MgAl mixed oxides and has been found to be maximum in case
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Acknowledgements
of NiMgAl (O) mixed oxide (149/h). While in case of MgAl (O)
and CoMgAl (O), the TOF are 12.5 and 36/h respectively which
are almost 11 and 4 times less than NiMgAl (O). This increase in
TOF reflects the better catalytic performance of NiMgAl (O)
mixed oxides for Nitro-aldol condensation reaction.
DB acknowledges the financial support from UGC, New Delhi
for providing Rajiv Gandhi National Fellowship. The authors
also thank DST, New Delhi for financial support.
5
After testing the catalytic activity it is important to check the
recyclability of the catalyst for their practical application. To test
the recyclability we carried out the condensation reaction upto
four cycles employing 10 mg of the catalyst and it was found that
60 Notes and references
a
Department of Chemical Sciences, Tezpur University, Napaam-784028,
India. Fax: +91 3712 267005/6; Tel: +91 3712 267008; E-mail:
ramesh@tezu.ernet.in (R.C. Deka).
10 the catalyst become active up to 4 cycles as shown in Table S3
(ESI†). In the first 3 cycles, it took 2-3 h to complete the reaction
with decrease in % conversion from of 96 to 92 %. However,
after 3rd cycle, the time to complete the reaction become
increased upto more than 4 h. Thus we observed that with
15 increasing the number of cycles, the time taken to complete the
reaction gradually increases along with decrease in catalytic
activity. The reason for this decrease in catalytic activity of the
catalyst may be due to the loss of catalyst during separation or
deactivation of the active sites in the catalyst. The leaching of
20 metals during recyclability of catalysts was observed. For this
test, 1 mmol 4-nitrobenzaldehyde, 10 mmol nitromethane and 10
mg of the catalyst were taken in a round bottomed flask and
stirred for 1 h at room temperature without any solvent. The
catalyst was then filtered off and the experiment was continued
25 with the filtrate for upto 24 h. It was observed that the conversion
was remained unchanged indicating the absence of any metal in
the filtrate.
65 †Electronic Supplementary Information (ESI) available: XRD
analysis and textural properties of mixed oxides, effect of
calcinaton temperature on Nitro-aldol reaction, Recyclability test,
N2 adsorption-desorption isotherms and pore size distribution of
MgAl and CoMgAl mixed oxides, N2 adsorption-desorption
70 isotherms of NiMgAl mixed oxide at different calcination
temperature, Turnover frequency (TOF) plot of mixed metal
oxides, 1H and 13C NMR spectra of the pure product. See
DOI: 10.1039/b000000x/
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In summary, we have synthesized high surface area mixed metal
30 oxides from their precursor LDHs and also observed the
influence of divalent metal cations on structural property and
catalytic activity of MgAl LDH. The PXRD of all the as prepared
LDHs revealed the formation of hydrotalcite (HT) type layered
structure. The partial substitution of Mg2+ ions with Ni2+ and Co2+
35 affects the crystallization procedure which is well reflected from
the broad and less intense peaks of both CoMgAl and NiMgAl
samples in comparison to MgAl sample. The formation of mixed
oxides can be revealed from the powder XRD patterns which
possess no any characteristic peaks that corresponding to LDH.
40 The mixed metal oxides were found to be catalytically more
active towards Nitro-aldol condensation reaction under solvent
free condition at room temperature with NiMgAl mixed oxide
being more active. The high catalytic activity of NiMgAl mixed
oxide can be attributed to the high BET surface area (753 m2/g).
45 The catalysts can be reused upto 4th cycle without any
characteristics loss in its activity.
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50 2H), δ 5.60-5.62 (m, 1H), δ 7.64(d, JHH = 8.7 Hz, 2H), 8.27 (d,
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55
6
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DOI: 10.1039/C4NJ01332H
Submitted to New Journal of Chemistry
Graphical Abstract
Synthesis of high surface area mixed metal oxide from NiMgAl LDH
precursor for Nitro-aldol condensation reaction
Dipshikha Bharali, Rasna Devi, Pankaj Bharali and Ramesh C. Deka *
Department of Chemical Sciences, Tezpur University, Napaam-784028, India
NiMgAl mixed oxide catalyzes Nitro-aldol condensation reaction at room temperature under
solvent free condition exhibiting 99 % conversion.