Boehmite - An efficient and recyclable acid-base bifunctional catalyst for aldol condensation reaction

Article abstract of DOI:10.1166/jnn.2018.15205

In this work boehmite was used as an acid-base bifunctional catalyst for aldol condensation reactions of aromatic aldehydes and ketones. The catalyst was prepared by simple sol-gel method using Al(NO₃)₃ · 9H₂O and NH₄OH as precursors. The catalyst has been characterized by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM), UV-visible spectroscopy (DRS), BET surface area analyses. Boehmite is successfully applied as catalyst for the condensation reaction between 4-nitrobenzaldehyde and acetone as a model substrate giving α,β-unsaturated ketones without any side product. The scope of the reaction is extended for various substituted aldehydes. A probable mechanism has been suggested to explain the cooperative behavior of the acidic and basic sites. The catalyst is environmentally friendly and easily recovered from the reaction mixture. Also the catalyst is reusable up to 3 catalytic cycles.

Full text of DOI:10.1166/jnn.2018.15205

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 4270–4275, 2018
Copyright © 2018 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Boehmite—An Efficient and Recyclable Acid-Base
Bifunctional Catalyst for Aldol Condensation Reaction
∗
P. C. Rajan Reshma, Sekar Vikneshvaran, and Sivan Velmathi
Organic and Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India
In this work boehmite was used as an acid-base bifunctional catalyst for aldol condensation reac-
tions of aromatic aldehydes and ketones. The catalyst was prepared by simple sol–gel method using
Al(NO ꢀ · 9H O and NH OH as precursors. The catalyst has been characterized by X-ray diffrac-
3
3
2
4
tion (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM), UV-visible
spectroscopy (DRS), BET surface area analyses. Boehmite is successfully applied as catalyst for
the condensation reaction between 4-nitrobenzaldehyde and acetone as a model substrate giving
ꢁ
ꢂꢃ-unsaturated ketones without any side product. The scope of the reaction is extended for vari-
ous substituted aldehydes. A probable mechanism has been suggested to explain the cooperative
behavior of the acidic and basic sites. The catalyst is environmentally friendly and easily recovered
from the reaction mixture. Also the catalyst is reusable up to 3 catalytic cycles.
Keywords: Aldol Condensation, Boehmite, Bifunctional Catalyst, Sol–Gel Method.
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Copyright: American Scientific Publishers
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1
. INTRODUCTION
has been extensively used as catalyst, optical material,
cosmetic products, vaccine adjuvant, pillared clays and
sweep-flocculation for fresh water treatment and adsor-
bent, due to its unique properties such as good porosity and
In recent years, boehmite with nanoscale dimensions and
morphological specificity has attracted enormous interest
from both fundamental and practical viewpoints. More-
over, boehmite, which is an aluminium oxyhydroxide
2
4
thermal stability, high specific surface area (>120 m /g).
Another important application of boehmite is producing
-Al O by calcination at high temperature. Furthermore,
(
AlO(OH)) contains terminal OH groups, that render high
reactivity to the surface and provide a hydrophilic envi-
ronment improving the homogeneity of the medium being
for all these reasons a promising support. On the other
hand as a typical oxyhydroxy compound, boehmite is
also extensively used as absorbents, catalysts, fillers in
membrane, optical materials and composite reinforcement
ꢁ
2
3
boehmite has excellent properties such as chemical resis-
tance, good mechanical strength, good conductivity, high
hardness, transparency, low cost, high abrasive and cor-
rosion resistance, excellent biocompatibility and control-
lable synthesis. The chemical and physical properties of
boehmite nanoparticles are dependent on the experimen-
tal factors of its synthesis, such as the nature of Al(III)-
salt, pH, temperature, time of aging and etc. Therefore,
numerous methods have been reported for the prepara-
tion of boehmite nanoparticles such as hydrothermal (HT),
sol–gel and hydrolysis of aluminium. Also effect of tem-
perature, ultrasonic waves, time and Al/OH molar ratio
on the synthesis of boehmite has been studied by Rajabi
1
–3
material in ceramics.
Boehmite structure consists of
double sheets of octahedral with aluminium ions at their
centers and the sheets themselves are composed of octahe-
dral chains and has a cubic orthorhombic structure. Also,
boehmite nanoparticles was not moisture or air sensitive
and was synthesized in H O at room temperature with-
out inert atmosphere using available materials such as
2
Al(NO ꢀ · 9H O and NH OH. Boehmite has important
3
3
2
4
applications in the preparation of catalysts in petrochemi-
cal and petroleum refining processes.
5
et al. Despite several studies on morphology, properties
and preparation of boehmite, there are few reports on the
modification of nano boehmite surface to act as heteroge-
neous catalyst. But, boehmite as an acid-base bifunc-
tional catalyst has not been reported. Therefore here we
Boehmite is also used for building units and directing
template in the preparation of core/shell materials, which
6
–9
∗
Author to whom correspondence should be addressed.
4270
J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 6
1533-4880/2018/18/4270/006
doi:10.1166/jnn.2018.15205

Reshma et al.
Boehmite—An Efficient and Recyclable Acid-Base Bifunctional Catalyst for Aldol Condensation Reaction
made an attempt to apply this acid-base bifunctionality of
boehmite on aldol condensation reactions.
make the process selective for the synthesis of cross aldol
products.
1
0
The design and preparation of multifunctional materials
containing different types of active sites for many poten-
tial applications have attracted more and more attentions.
In this paper we report the use of boehmite as a bifunc-
tional catalyst for the aldol condensation reaction. The cat-
alyst was synthesised and characterised by various spectro
analytical techniques. Typically, various reaction parame-
ters were optimised to get the maximum yield for the cross
aldol reaction of 4-nitrobenzaldehyde with acetone. The
recyclability of the catalysts also checked and substrate
scope was extended to different substituted aldehydes.
1
1
Not only bifunctional catalysis of different active centers
on separate solid surfaces, but also co-operative effect of
two functions coexisting on a solid surface are also attrac-
tive protocol for creation of novel catalysis. Especially,
acid-base bifunctional catalyst plays a pivotal role in dou-
ble activation of electrophiles and nucleophiles, respec-
tively. The cooperative interactions occurring between two
different catalytic centers and a given reaction transi-
tion state enable transformations that are not possible,
or are done less efficiently, by using each catalyst sepa-
rately. Recently, heterogeneous catalysts having acid base
2
. EXPERIMENTAL SECTION
2
.1. Materials
The aluminium salt used for the synthesis of the catalyst
was Al(NO ꢀ · 9H O and the alkali solution used were
3
3
2
ammonia. For the catalysis reaction, 4-nitrobenzaldehyde,
1
2
bifunctional surfaces have been reported. They can effi-
ciently catalyze several reactions to occur in one-pot
cascade reaction without isolation of intermediates and
self-quenching of catalysts. Therefore, multifunctional het-
erogeneous catalysts are required in the field of fine
chemical synthesis due to the low-cost, step-saving and
2
-nitrobenzaldehyde, 2-methoxybenzaldehyde, 4-methyl-
benzaldehyde, benzaldehyde, cinnamaldehyde, salicy-
laldehyde, pyridine-2-carboxaldehyde, furfuraldehyde,
2
-napththaldehyde, 9-anthraldehyde and acetone were
obtained from Merck. All chemicals were used without
further purification.
1
3–15
environmentally friendly properties.
Aldol condensa-
tion reaction is one of the main synthetic tools for the
construction of C–C bonds, both in nature and in synthetic
chemistry. The resulting ꢂ-hydroxy aldehydes are found in
many important synthetic targets, including natural prod-
2
.2. Catalyst Preparation
Catalyst was prepared by following the literature
procedure.
29–31
Typically, 22.5 g of Al(NO ꢀ · 9H O was
3
3
2
dissolved in 120 mL of deionized water under stirring at
IP: 185.89.100.19 On: Tue, 20 Mar 2018 12:10:50
ucts. The dehydrated ꢁꢃꢂ-unsaturated aldehyde derivatives
Copyright: American S rc oi eo mn tif ti ec mPp ue br al i tsu hr ee .r s4 0 mL of 25% ammonia solution is
are present in important biologically active compounds and
Delivered b yd iI sns go el vne t da in 160 mL of water.
find important applications in the fields of pharmaceutical,
1
20 mL each of the above solutions are mixed in 1 L
1
6
fragrance, plasticizer, detergent, and cosmetic chemistry.
ꢀ
beaker and kept on a hot plate at 90 C. The hydrolysis
reaction is performed for 60 minutes by maintaining the
pH of the medium at 7.0. The milky white colloidal pre-
cipitate was recovered by filtration and dried at 200 C for
4
In a typical aldol condensation reaction, a carbonyl com-
pound having no ꢁ-H is reacted with another carbonyl
compound having at least one ꢁ-H with the help of acid
or base catalysts resulting into cross aldol product. The
drawbacks of conventional methodology include the use
of liquid alkalies (NaOH, KOH, etc.) in more than sto-
ichiometric amounts, hazardous and corrosive nature of
alkalies, non-reusability of catalyst, requirement of post
reaction work up etc. Several solid acid as well as base cat-
alysts have been explored for replacement of homogeneous
catalysts to develop eco-friendly route for the synthesis of
ꢀ
8 hrs in a vacuum oven with enforced airflow.
2
.3. Catalyst Characterization
X-ray diffraction was conducted on a diffractometer with
CuKꢁ (ꢄ = 1ꢅ5404 Å) radiation. Scans were made in
ꢀ
ꢀ
the 2ꢆ range 5–70 with a scan rate of 2 /min (wide
angle diffraction). BET surface area and pore size distri-
butions were measured by N adsorption/desorption using
Micromeritics Gemini V-2380 surface area analyzer. Prior
to the analysis samples were degassed at 300 C for 1 hr.
2
1
7–27
cross aldol products.
But, they were monofunctional
and homogeneous materials. So the difficulty in separation
and recyclability is remaining as the major problems. The
cross aldol condensation between two different carbonyls
usually results in formation of significant amount of self
aldol product along with the desired cross product, which
is produced by self aldol condensation of two similar car-
bonyl compounds (having ꢁ-H) and it reduces the yield
ꢀ
UV-visible diffuse reflectance was measured at room tem-
perature in air on a SHIMADZU UV-2600 UV-visible
spectrophotometer over the range from 200–800 nm. SEM-
EDX data were recorded in Hitachi SU6600 scanning
electron microscope. TPD-NH3 analysis was done on
a Chemisorb 2750 unit. FTIR spectrum were obtained
with a Nicolet IS 5 FTIR spectrometer (KBr pressed
disc method). GC-MS data were obtained from the SHI-
MADZU (Rtx-17, 60 m×25 mm ID, film thickness (df =
2
8
of cross aldol product. Therefore, it is of great impor-
tance to study the various elements of the reaction (like
catalyst surface characteristics, substrates nature, reaction
parameters, solvent medium etc.) controlling the self and
cross products selectivity in aldol reactions in order to
−1
ꢀ
ꢀ
0ꢅ25 ꢇm), column flow 1 mL min , 80 C to 240 C at
ꢀ
−1
10 C min rise) GC-Mass Spectrometer.
J. Nanosci. Nanotechnol. 18, 4270–4275, 2018
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Boehmite—An Efficient and Recyclable Acid-Base Bifunctional Catalyst for Aldol Condensation Reaction
.4. Product Analysis
Reshma et al.
2
1
13
The H NMR and C NMR spectra were recorded on
1
a Bruker AV-500 spectrometer (500 MHz for H and
1
3
1
25 MHz for C) (Bruker, Switzerland).
3
. RESULTS AND DISCUSSION
The XRD pattern of the catalyst shows crystallization as
a single phase boehmite. The calculation of the average
crystallite size from the XRD pattern using the Scherer
equation showed the particle size as 4.54 nm. As seen
in Figure 1, the boehmite phase was identified from the
XRD patterns by the peak positions at 14.40 (0 2 0), 28.41
(
1 2 0), 38.55 (0 3 1), 46.45 (1 3 1), 49.55 (0 5 1),
5
6
1.94 (2 0 0), 56.02 (1 5 1), 59.35 (0 8 0), 65.04 (2 3 1),
5.56 (0 0 2), 68.09 (1 7 1), and 72.38 (2 5 1), the main
peaks of the XRD pattern are consistent with the char-
3
2
Figure 3. NH -TPD profile of boehmite.
3
acteristic peaks of the orthorhombic unit. SEM images
of boehmite were shown in Figure 2, which reveals that
the particle size of synthesized catalyst are in nanometer
range. The BET surface area of the boehmite nanoparticles
temperature programmed desorption (TPD-ammonia) tech-
nique, the total amount of acid sites of the catalyst is
2
2
33
was 226.49 m /g with a pore volume of 0.174 cm /g. The
acid strength of prepared catalyst was investigated using
2.021 mmol/g (Fig. 3).
−1
In the IR spectrum (Fig. 4), peak at 3294 and 3081 cm
corresponds to O–H stretching frequency. The peak at
−1
1
068 cm has been assigned for Al–O stretching fre-
−1
−1
quency. Other peaks at 1068 cm , 746 cm
and
22 cm corresponds to Al–O stretching and bending
−1
6
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3
.1. Optimization of Reaction Conditions
The synthesized boehmite acid-base bifunctional catalyst
was used for the aldol condensation reaction between
aromatic aldehydes and ketones. The reaction between
4
-nitrobenzaldehyde and acetone is taken as the model
reaction. Schematic representation of the reaction is given
in Scheme 1. In order to get effective results, the reac-
tion variables such as time, temperature and amount of
catalyst were optimized. The completion of the reaction
is monitored through GC-MS. The reaction was studied at
Figure 1. XRD pattern of boehmite.
Figure 2. (a–d) SEM images of boehmite.
Figure 4. FT-IR spectrum of boehmite.
4272
J. Nanosci. Nanotechnol. 18, 4270–4275, 2018

Reshma et al.
Boehmite—An Efficient and Recyclable Acid-Base Bifunctional Catalyst for Aldol Condensation Reaction
Table II. Extension of substrate scope of aldol condensation using
boehmite.
Entry
1
R1
H
R2
Product
Yield (%)
a
b
NO2
83 (91 ꢀ
Scheme 1. Aldol reaction using boehmite.
different time, temperature and catalytic amount (Table I).
In order to get good yield, the time for reaction is
increased and the catalytic amount has decreased. As can
be seen from the Table I, the yields of ꢁꢃꢂ-unsaturated
ketones improved initially when catalyst load decreased
from 1 mol% to 0.5 mol% and then decreased with cata-
lyst loading decreased from 0.5 to 0.25 mol%. It revealed
that the best conditions were 24 hr and 30 mg (0.5 mol%)
of boehmite catalyst (entry 11, Table I).
After optimal conditions were established, this method
was extended to aldol condensations of other aromatic
aldehydes with acetone. As shown in Table II, aldol
condensations of benzaldehyde and substituted benzalde-
hyde proceeded smoothly. Both electron-withdrawing and
electron-donating substituents on the aldehyde aryl ring
were tolerated well. Methyl-, methoxy-, nitro-, benzalde-
hydes, cinnamaldehyde, napthaldehyde and anthraldehyde
produced the desired products in high yields (Scheme 2).
The position of the substituent had no significant effect on
the yield. Literatures based on aldol condensation reactions
are mostly focused on ꢂ-hydroxy substituted products than
a
b
2
3
4
5
H
OCH3
NO2
H
H
H
H
56 (65 ꢀ
a
b
55 (68 ꢀ
a
b
61 (70 ꢀ
a
b
CH
60 (72 ꢀ
3
a
b
6
7
8
52 (65 ꢀ
a
b
50 (67 ꢀ
a
b
60 (73 ꢀ
ꢁ
ꢃꢂ-unsaturated ketones, results oI Pb t :a 1i n 8e 5d . 8f r 9o .m1 0t 0h e. 1 p9 r eOs en n: tTue, 20 Mar 2018 12:10:50
Copyright: American Scientific Publishers
work were compared with already available literatures, val-
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ues in Table III suggests that boehmite is better than other
a
b
9
63 (71 ꢀ
catalysts on ꢁꢃꢂ-unsaturated ketones preparation.34–37
3
.2. Proposed Mechanism
a
b
1
0
1
55 (67 ꢀ
Mechanism for the synthesis of ꢁꢃꢂ-unsaturated ketones
in the presence of the boehmite catalyst is shown in
Scheme 3. In the first step of the reaction, catalyst acts as
a
b
1
62 (73 ꢀ
a
Table I. Optimization of reaction conditions.
ꢀ
Entry Time (hrs.) Catalyst amount (mmol) Temp. ( C) GC yield (%)
Notes: aindicate isolated yield and bGC yield.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
32
24
6
12
18
24
32
6
12
18
24
32
6
12
18
24
32
Without catalyst
55
rt
24
33
35
44
70
73
80
25
63
74
91
76
28
52
58
64
71
1ꢅ0
1ꢅ0
1ꢅ0
1ꢅ0
1ꢅ0
1ꢅ0
0ꢅ5
0ꢅ5
0ꢅ5
0ꢅ5
0ꢅ5
0ꢅ25
0ꢅ25
0ꢅ25
0ꢅ25
0ꢅ25
55
55
55
55
55
55
55
55
55
55
55
55
55
55
55
a base by abstracting a proton from acetone and forms the
carbanion. Then the carbanion attacks the carbonyl carbon
of aldehyde giving an intermediate anion. In the second
step, catalyst acts as an acid by releasing a proton which
results in protonation of the intermediate anion and then
dehydration leads to the formation of ꢁꢃꢂ-unsaturated
ketone.
0
1
2
3
4
5
6
7
Note: a4-Nitrobenzaldehyde (1.5 mmol) and acetone (5 mL).
Scheme 2. Aldol reaction of different substituted aldehydes.
J. Nanosci. Nanotechnol. 18, 4270–4275, 2018
4273

Boehmite—An Efficient and Recyclable Acid-Base Bifunctional Catalyst for Aldol Condensation Reaction
Reshma et al.
Table III. Comparison of present results with available literatures.
Product
Catalyst
Yield (%) literature
Catalyst
Ca(OH)2
concentration
Literature
Present work
(present work)
Reference
[34]
10 mol%
79 (56)
SBA-15-C.A.
10 mol%
75+24 (83)
[35]
L-proline
20 mol%
3.4 mmol
68 (83)
98 (56)
[36]
[37]
Functionalized ionic liquid
89 (60)
8
(83)
3
.3. Recyclability of the Catalyst
4. CONCLUSION
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Recyclability of the catalyst is very important for industrial
In conclusion, acid-base bifunctional boehmite catalyst
Copyright: American Scientific Publishers
application. The reusability of the catalyst was also exam-
ined and presented in Table IV. After each run, DCM was
added, and the product was filtered. The residue (catalyst)
was washed with ethyl acetate and reused. The treatment
with ethyl acetate removed tars more efficiently from the
catalyst surface. The catalyst was reusable up to 3 cat-
alytic cycles, although a gradual decline in its activity was
observed due to the loss of its active sites during treatment
after each catalytic cycle.
was successfully synthesized by sol–gel method. The
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synthesized bifunctional boehmite was used as an effi-
cient catalyst for aldol condensation reaction between aro-
matic aldehydes and ketones to produce ꢁꢃꢂ-unsaturated
ketones in good yield. The major progress of the work
is new approach to C–C bond formation using boehmite
as bifunctional catalyst. This procedure is environmentally
benign, general, efficient, high yielding, safe and opera-
tionally simple.
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1
2
3
4
5
6
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Received: 26 April 2017. Accepted: 14 June 2017.
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4275