Graphene based material as a base catalyst for solvent free Aldol condensation and Knoevenagel reaction at room temperature

Article abstract of DOI:10.1016/j.molcata.2014.06.038

Graphene oxide (GO) acts as a highly active heterogeneous base catalyst for a wide variety of reactions. Here we have described the catalytic activities of GO in the condensation reaction of various substituted benzaldehydes with acetophenone (aldol condensation) and with active methylene compound malononitrile (Knoevenagel reaction) at room temperature under solvent free condition. GO is characterized by powder X-ray diffraction (XRD), UV-visible spectra, Fourier transform infrared spectroscopy (FT-IR) and AFM. The experimental results showed that the GO had higher catalytic activity and it can be recycled without significant loss of its activity.

Full text of DOI:10.1016/j.molcata.2014.06.038

Accepted Manuscript
Title: Graphene based material as a base catalyst for solvent
free Aldol condensation and Knoevenagel reaction at room
temperature
Author: S.M. Islam Anupam Singha Roy Ram Chandra Dey
Sumantra Paul
PII:
S1381-1169(14)00289-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.06.038
MOLCAA 9179
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
22-10-2013
21-6-2014
28-6-2014
Please cite this article as: S.M. Islam, A.S. Roy, R.C. Dey, S. Paul, Graphene based
material as a base catalyst for solvent free Aldol condensation and Knoevenagel
reaction at room temperature, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.06.038
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Graphene based material as a base catalyst for solvent free Aldol
condensation and Knoevenagel reaction at room temperature
S. M. Islam,* Anupam Singha Roy,‡ Ram Chandra Dey,‡ Sumantra Paul
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
‡These authors contributed equally to this manuscript.
Abstract
Graphene oxide (GO) acts as a highly active heterogeneous base catalyst for a wide variety of
reactions. Here we have described the catalytic activities of GO in the condensation reaction
of various substituted benzaldehydes with acetophenone (aldol condensation) and with active
methylene compound malononitrile (Knoevenagel reaction) at room temperature under
solvent free condition. GO is characterized by powder x-ray diffraction (XRD), UV-visible
spectra, Fourier transform infrared spectroscopy (FT-IR) and AFM. The experimental results
showed that the GO had higher catalytic activity and it can be recycled without significant
loss of its activity.
Keywords: Graphene oxide; Heterogeneous catalysis; Aldol condensation; Knoevenagel
reaction; room temperature.
*Authors to whom correspondence should be addressed.
Dr. Sk. Manirul Islam, Department of Chemistry, University of Kalyani, Kalyani, Nadia,
741235, W.B., India. Phone: +91-33-2582-8750, Fax: +91-33-2582-8282, E-mail:
manir65@rediffmail.com
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1. Introduction
The aldol reaction is one of the most powerful methods of forming carbon–carbon
bonds and it has tremendous synthetic applications [1]. Biological systems have perfected this
stereospecific transformation by using enzymes aldolases. Despite their lack of large-scale
compatibility and their narrow substrate specificity, aldolases represent a great source of
inspiration for the development of catalysts. In recent years, several aldol reactions using acid
or base catalysts have been reported including iron-catalyzed cross-aldol reactions [2],
proline-loaded catalysts for stereoselectivity [3], aldol-type reactions of lithium enolate [4]
and silyl enolate [5, 6]. Currently, due to low toxicity, operational simplicity and efficiency,
compared to traditional metal-based catalysts, there is much interest in organocatalysts [7].
Organocatalysis is the acceleration of chemical reactions with a sub-stoichiometric amount of
an organic compound, which does not contain a metal atom. Organocatalysts that can
promote aldol reactions have been reported by research groups [8, 9]. However; their
drawbacks also have been realized. The major limitations using organocatalyst catalyzed
reactions are high catalyst loadings and the difficulty of recovering the catalyst [10, 11]. An
alternative strategy is to design recyclable and subsequently reusable versions of
organocatalysts [12].
The Knoevenagel reaction of aldehydes with compounds containing activated
methylene groups has been widely employed in the synthesis of several fine chemicals,
intermediates of anti-hypertensive drugs and calcium antagonists [13-16]. Cinnamic acid, its
ester and carboxylic functional derivatives, very important components in flavors, perfumes,
synthetic indigo and pharmaceuticals, are synthesized by Knoevenagel condensation. This
reaction is conventionally catalyzed by alkali metal hydroxides or by organic bases like
primary, secondary and tertiary amines under homogeneous conditions with attendant
difficulties in catalyst recovery and recycling [17-19]. Over the last few years, a wide range
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of solid catalysts have been investigated for these reactions such as amino-functionalized
mesoporous silica [20], diamine-functionalized mesopolymers [21], amine-functionalized
mesoporous zirconia [22], Mg–Al mixed oxide on hexagonal mesoporous silica [23],
biguanide-functionalized mesoporous silica [24], acid–base bifunctional mesoporous MCM-
41 silica [25], amine-functionalized superparamagnetic nanoparticles [26], chitosan hydrogel
[27], acrylic resin immobilized lipase [28], organic–inorganic hybrid silica materials
containing imidazolium and dihydroimidazolium salts [29], IRMOF-3 [30] and ZIF-8 [31].
Nowadays, Graphene oxide (GO) is a hot topic in the fields of materials science and
synthetic chemistry due to its unique features. Due to the presence of various functionalities
on the surface of graphene oxide, it shows huge catalytic activity. In addition, it has large
specific surface area [32-34], good biocompatibility and high adsorption capacity [35, 36].
This large specific surface area and presence of various functional groups are responsible for
its enormous catalytic activity in synthetic chemistry. Also high thermal and electrical
conductivity [37] and excellent mechanical strength have found enormous applications in the
development of composite materials and catalyst. These composites can be used over a wide
range of applications, including memory devices [38], energy storage [39], catalysis [40-44],
solar cells [45], molecular imaging [46] and drugs delivery [47]. In a recent precedent,
sulfated graphene obtained by hydrothermal sulfation of reduced graphene oxide with fuming
0
sulfuric acid at 180 C was tested as catalyst for the esterification of acetic acid, the
Pechmann condensation and hydration of propylene oxide [48]. The difference between the
material used in this prior work and the present one is that we are using here directly the GO
samples obtained by the modified Hummers method followed by exfoliation in aqueous
solution without any subsequent treatment. Our GO is, therefore, more conveniently prepared
and does not require additional and hazardous treatments. The activity of GO as a base
catalyst was tested for the direct aldol reactions using aldehydes and acetophenone as
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reacting substrates and Knoevenagel condensation of aldehydes and active methylene
compounds at room temperature under solvent free condition.
2. Experimental
2.1. Materials and reagent
All reagents and solvents were purchased from commercial sources and were further purified
by standard procedures. Graphite powder (<20 micron), hydrogen peroxide (30 %) were
purchased from Aldrich and used as received. Other reagents were purchased from Merck and
used as received.
UV-visible spectra were measured using Shimatzu UV- 2550 spectrophotometer with
a quartz cell of 1 cm path length. Fourier transform infrared (FTIR) spectra were measured
with Nicolet 6700 (Thermo Scientific) instrument using KBr plates. Diluted graphene oxide
(GO) was dropped onto silicon substrate and dried at room temperature to prepare specimens
for X-ray diffraction measurements. X-Ray diffraction pattern was measured at room
temperature using Shimadzu, XRD-6000, 40 kV, 20 mA with Cu K radiation, λ = 1.5418 Å.
Diluted GO was deposited on glass slide and then AFM was measured using VEECO DICP
1
II autoprobe (model AP 0100). H NMR spectra were recorded at room temperature on
Bruker Avance DPX-400 spectrometers using TMS as internal standard.
2.2. Preparation of colloidal graphene oxide (GO)
Graphene oxide (Fig. 1) was synthesized via a modified Hummer’s method. In brief
200 mg graphite powder, 100 mg sodium nitrate and 5 mL concentrated H2SO4 were mixed
and cooled to 0 0C. The solution was kept under vigorous stirring followed by addition of 600
0
mg KMnO4 in a stepwise manner keeping the temperature below 20 C. Next, the
0
temperature of the solution was slowly raised to 35 C and kept at this condition for 30 min.
This step is important to control the graphene oxide size since longer oxidation leads to
smaller graphene oxide sheets. Next, 10 mL water was added to the whole solution leading to
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0
a rise in temperature to ~100 C and this temperature was maintained for 15 min. The whole
solution was then mixed with 30 mL water followed by addition of 500 μL 3% H2O2 that
reduces the residual permanganate. The light yellow precipitate was washed with warm water
7–8 times. Finally, this was dispersed in 20 mL distilled water with 15 min sonication at a
concentration of 1.5 mg mL-1. Longer sonication should be avoided to control the size of
graphene oxide. The solution was centrifuged at 3000 rpm to remove large particles and the
solution was used as colloidal graphene oxide.
Fig. 1. A schematic model of a single sheet of graphene oxide
2.3. General procedure for the aldol condensation
A reaction mixture containing aldehyde (1.2 mmol), acetophenone (1 mmol) and GO
in water (0.5 mL) were stirred at room temperature for appropriate time. The completion of
the reaction was monitored by TLC. After completion of the reaction, the reaction mixture
was diluted with dichloromethane followed by subsequent separation of organic and aqueous
layers by separating funnel. Organic layer was dried over Na2SO4 and concentrated under
reduced pressure and obtained crude product was purified by column chromatography using
EtOAc/hexane as eluent. All the yields were calculated from isolated products.
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2.4. General procedure for Knoevenagel condensation reaction
The organocatalyst GO was stirred with 2 mmol of manolonitrite for 30 min at room
temperature in 25 mL RB flask. To this mixture corresponding aldehyde (1 mmol) was added
and the resulting mixture was stirred for appropriate time. After completion of the reaction,
the reaction mixture was diluted with dichloromethane followed by subsequent separation of
organic and aqueous layers by separating funnel. The organic layer was dried over Na2SO4.
After removal of solvents, the crude product was purified by column chromatography using
EtOAc/hexane as eluent. All the yields were calculated from isolated products.
3. Results and discussion
3.1. Characterization
Here, Colloidal graphene oxide was prepared first using Hummer’s method. The
presence of various chemical functionalities on graphene oxide nanosheets and their
dispersion in water were examined by XRD, FTIR, UV-Vis Spectroscopy. Fig. 2 shows the
XRD patterns obtained for graphene oxide powder. A sharp peak at 2θ = ~ 10.60 for graphene
oxide, corresponding to an interlayer spacing of ~0.9 nm (d spacing). The oxidation of
graphite powder leads to the introduction of various functional groups. These functional
groups are bonded on both basal planes and edges of graphitic layers, as well as the presence
of trapped water molecules between these layers, expanding the interlayer spacing in
graphene oxide [50].
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Fig. 2. XRD spectra of graphene oxide
Further, the nature of the chemical functionalities was characterized by FT-IR (Fig.
3). An intense and broad peak appeared at 3408 cm-1, attributed to the stretching mode of an
O–H bond, reveals the abundance of hydroxyl groups in graphene oxide. The band at around
2915 cm-1 appears due to C-H stretching (symmetric and asymmetric) vibrations. The band at
1709 cm-1 (νC=O) represents carboxylic acid and carbonyl groups. Another band at 1591 cm-
1 represents C=C stretching vibration in grapheme oxide. Furthermore, the bands at 1230 cm-1
and 1079 cm-1 are attributed to the presence of C–OH and C–O groups, respectively, in
graphene oxide.
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Fig. 3. FT-IR spectra of graphene oxide
The UV-Vis spectrum of graphene oxide (Fig. 4) exhibits a strong absorption peak at
230 nm, which is attributable to the π-π* transition of graphitic C–C bonds and a shoulder at
~300 nm is assigned to the n-π* transitions of C=O bonds. Thus, similar to the XRD and
FTIR, UV-vis spectra provided evidence of the presence of an ample number of oxygen
functionalities, such as hydroxyl, epoxide, carboxylate and carbonyl on graphene oxide. AFM
study was conducted for as synthesized GO (Fig. 5). It shows that as synthesized graphene
oxide has a typical thickness of ~1 nm and length is around ~1 μm.This result reveals that as
synthesized GO is of a single layer and the obtained GO is of micron size.
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Fig. 4. UV-vis spectra of graphene oxide
Fig. 5. AFM image and line analysis of graphene oxide
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3.2. Catalytic activity of Graphene Oxide
The use of metal-free carbons instead of metal supported catalyst in synthetic
chemistry has largely progressed over the last decade. Since GO contains many functional
groups (i,e –OH, -COOH, -CO-, -O-), we have decided to investigate the catalytic activity of
GO in the base catalysis direct Aldol condensation and Knoevenagel condensation reactions,
respectively.
3.2.1. Direct aldol reaction of aldehydes
Initial efforts sought to explore the base catalytic properties inherent to GO as a
means to effect C=C bond forming reaction via aldol condensation followed by the
elimination of water. In organic synthesis, the aldol reaction is one of the most important
carbon-carbon bond forming processes. The α,β-unsaturated carbonyl compounds are
subunits of many natural products and often used as precursors in the preparation of
flavonoids and isoflavonoids. The efficiency of GO as a base catalyst was first tested in aldol
reaction over various electron-withdrawing and electron-donating aromatic aldehydes with
acetophenone under solvent free condition. Initially, we optimized the condition using the
reaction of acetophenone with benzaldehyde under different experimental conditions.
The condensation of acetophenone and benzaldehyde is illustrated in the reaction
Scheme 1. Before attempting a detailed catalytic work, a noncatalytic reaction between
acetophenone and benzaldehyde was examined, and it was observed that at room temperature
only 3% formation of product was detected, indicating that the reaction is not taking place in
the absence of a catalyst (Table 1, entry 1). The condensation of benzaldehydes and
acetophenones was reported to be catalyzed by acid or bases [50, 51].
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H
O
CH3
GO
RT
O
O
Scheme 1. Aldol condensation between acetophenone and benzaldehyde catalyzed by
GO
Table 1. Room temperature aldol reaction between acetophenone and aromatic aldehydes
catalyzed by GO.a
O
H
CH3
GO
RT
O
O
R
R1
R1
R
1
2
3
Entry
R
H
H
H
H
H
H
H
-CH3
-Cl
R1
H
H
Product
Yield (%)b
1c
2d
3
3a
-
3
-
H
3a
3b
3c
3d
3e
3f
3g
89
84
85
87
91
93
92
4
5
6
7
8
9
-CH3
-OCH3
-OH
-Cl
H
H
a Reaction conditions: acetophenone (1 mmol), aldehyde (1.2 mmol), catalyst (0.5 mL),
solvent-free, at room temperature, 4 h. b Isolated yield. c without catalyst. d reaction at 0 0C.
The effect of temperature plays an important role in the aldol condensation reaction. It
was examined in the temperature range between 0 0C and 50 0C, in the absence of solvent and
0
the results are illustrated in Fig. 6. The above reaction did not proceed at 0 C even after
several hours (Table 1, entry 2). Temperature increase leads to a higher yield. The yield of
0
product for a reaction time of 4 h at 10 C is only 32%, whereas at room temperature the
conversion is 89%. For the same time of 4 h, an increase of only 2% is observed when the
temperature is raised from room to 50 0C. For this reason, room temperature was employed as
the ideal temperature to continue with the analysis of other reaction variables. However, the
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catalytic performance of GO as a function of the reaction time in the condensation of
benzaldehyde and acetophenone was studied at three temperatures under solvent-free
conditions (Fig. 6). The catalytic activity for the three temperatures was found to increase
rapidly with the reaction time up to 4 h, and then remains almost constant. The constant value
is probably due to the blockage of the catalytic center by the high concentration of product.
100
100C
90
room
500C
80
70
60
50
40
30
20
10
0
2
4
6
8
Time (h)
Fig. 6. Effect of reaction temperature and time on the catalytic performance of GO in
aldol condensation reaction between benzaldehyde and actophenone.
To show the generality of the process, we applied the optimized conditions to
reactions of acetophenone with a variety of aryl-benzaldehydes and the results of this study
are summarized in Table 1. The results showed that, in general, the reactions were clean and
products were isolated by liquid column chromatography in pure form without further
purification. Finally, we studied the influence of the substituting groups on the aromatic ring
of acetophenones and benzaldehydes. The reactions of aldehydes bearing electron-donating
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groups such as –Me, –OMe and –OH gave yields similar to those of the corresponding
reaction with unsubstituted benzaldehyde (Table 1, entries 4-6). The presence of electron-
withdrawing groups in acetophenone such as –Cl gave a comparable rate of condensation,
even when the aldehyde contains an electron-withdrawing group such as –Cl, presumably due
to ease of formation of the enolate anion in the first step (Table 1, entries 7 and 9). Then we
tested several aldol reactions between various aromatic aldehydes and acetone using GO as
the catalyst. As shown in Table 2, the yields of reactions are kept moderate by carrying the
reaction at room temperature.
Table 2. Room temperature aldol reaction between acetone and aromatic aldehydes catalyzed
by GO.a
H
O
O
GO
RT
O
R
R
1
4
5
Entry
R
Product
Yield (%)b
1
2
3
4
5
H
5a
5b
5c
5d
5e
72
67
65
69
78
-CH3
-OCH3
-Br
-Cl
a Reaction conditions: acetophenone (1 mmol), aldehyde (1.2 mmol), catalyst (0.5 mL),
solvent-free, at room temperature, 4 h. b Isolated yield
3.2.2. Knoevenagel condensation reaction
The catalytic activity of the GO was tested for the Knoevenagel condensations of
benzaldehyde with malononitrile to produce benzylidene malononitrile as the major product
(Scheme 2). Romero [52] reported amine-functionalized SBA-15 giving 98% conversion at
0
105 C towards the Knoevenagel condensation reaction. However, at room temperature
amine-functionalized MCM-41 showed 81% conversion as reported by Sugi [53]. The
inherent disadvantages associated with these reports were either higher temperature or longer
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reaction time. However, in this study we got better results due to the co-operative effect of
the acidic and basic sites present in the GO.
CHO
CN
CN
GO
CN room temperature
CN
Scheme 2. Knoevenagel reaction of benzaldehyde with malononitrile using the GO catalyst.
As motivated by the principles of green chemistry, it was decided to carry out the
reaction under room temperature and solvent free conditions. Initially it was found that no
reaction occurred in the absence of GO, whereas GO effectively catalyzed Knoevenagel
condensation reactions producing benzylidene malononitrile in high yields. This may be due
to the presence of basic sites in GO, which is responsible for the activity.
With such results in hand, we then decided to investigate the effects of various
reaction parameters on the condensation of benzaldehyde with malononitrile to produce
benzylidene malononitrile using GO as a catalyst. The effect of a benzaldehyde to
malononitrile molar ratio on the reaction conversion was studied by carrying out the reaction
at molar ratios of 1:4, 1:3, 1:2, and 1:1 respectively (Fig. 7). At a benzaldehyde: malononitrile
molar ratio of 1:2, the catalyst gave the best conversion of 97% in 4 h. The selectivity
towards benzylidene malononitrile was 99%. However, on further increasing the
malononitrile concentration, though the conversion remains almost the same, the product
selectivity decreased. This suggests that the self-condensation of malononitrile takes place
when its concentration in the reactant mixture increases.
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100
95
90
85
80
75
70
100
96
92
88
84
80
1:1
1:2
1:3
1:4
Molar ratio (benzaldehyde:malononitrile)
Fig. 7. The effect of molar ratios of benzaldehyde: malononitrile towards Knoevenagel
condensation.
Results showing the influence of reaction time on conversion and product selectivity
in the condensation reaction over the GO catalyst are presented in Fig. 8. As can be seen from
the figure, the conversion and selectivity are increasing with time up to 4 h. Then the reaction
became slower and reached a plateau. The conversion and selectivity values show no
appreciable change with further rise in reaction time.
The GO was employed in the reaction, where the catalyst activity is mainly due to the
structural basicity. In order to demonstrate the versatility of GO for Knoevenagel
condensation, we submitted various substituted aldehydes and substrates containing active
methylene groups with the molar ratio 1:2. In the case of substituted benzaldehyde containing
electron donating groups (–CH3), the conversion decreased while the electron withdrawing
substituents (-NO2) on the aromatic ring substantially increased the conversion (Table 3).
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100
90
80
70
60
50
40
30
100
95
90
85
80
0
1
2
3
4
5
6
7
Time (h)
Fig. 8. Influence of reaction time on Knoevenagel condensation
Table 3 Knoevenagel condensation of different aldehydes with malononitrile catalyzed by
GO.a
CHO
CN
CN
GO
RT
CN
CN
R
R
Entry
R
Time (h)
Product
Yield (%)b
1
H
4
CN
CN
97
CN
CN
2
4-NO2
3
98
O2N
O2N
3
4
3-NO2
4-Cl
3
3
97
96
CN
CN
CN
CN
Cl
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CN
CN
5
4-CH3
4
88
H3C
CN
CN
6
7
8
4-OEt
4-OH
4
4
4
79
84
83
EtO
HO
CN
CN
CN
CN
4-OMe
MeO
a Reaction conditions: malononitrile (2 mmol), aldehyde (1 mmol), catalyst (0.5 mL),
solvent-free, at room temperature,. b Isolated yield
In the case of electron withdrawing groups the possibility of attack of the carbanion
(generated from the active methylene group) on the carbonyl carbon is more when compared
to that of electron donating groups. Condensation of various benzaldehydes with various
substrates possessing active methylene groups is shown in Table 4. The substituents with
electron withdrawing nature stabilize the carbanion (due to resonance) compared to those
having electron donating nature. So malononitrile displays the maximum conversion among
all other substrates.
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Table 4. Effect of various active methylene groups towards Knoevenagel condensation.a
CHO
X
GO
X
H2C
RT
Y
Y
R
Entry
R
Y
COOEt
CN
X
Yield (%)b
90
1
2
H
H
COOEt
CN
97 (99)c
94 (100/0)d
93 (100/0)d
92 (100/0)d
95 (100/0)d
94 (100/0)d
90c
3
H
CN
COOEt
COOEt
COOEt
COOEt
COOEt
Cl
4
OCH3
Cl
CN
5
CN
6
NO2
OH
H
CN
7
CN
8
CN
9
H
CN
COOH
Ph
89c
10
H
CN
84c
a Reaction conditions: active methylene compound (2 mmol), aldehyde (1 mmol), catalyst
(0.5 mL), solvent-free, at room temperature, 4 h. b Isolated yield. cYield determined by GC
analysis. dE/Z ratio, Determined by 1H NMR of the product.
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3.3. Comparison with other reported system
Base catalyzed Aldol condensation of benzaldehyde with acetophenone or
Knoevenagel condensation of benzaldehye with malononitrile under heterogeneous
conditions over a variety of catalysts has been studied. Table 5 provides a comparison of the
results obtained for our present catalytic system with those reported in the literature [20, 29,
54-58]. From Table 5, it is seen that present catalyst exhibited higher yields compared to the
other reported system [20, 29, 54-58]. Reactions conducted at room temperature, shorter
reaction time was required for these reactions and most importantly above coupling reactions
occurred under solvent free condition using GO catalyst.
Table 5. Comparison of catalytic activity of the GO catalyst in the Aldol condensation and
Knoevenagel condensation with other reported systems
Reaction
Catalyst
Reaction Conditions
Yield (%) Ref.
aminopropyl-functionalized 1.3 mmol benzaldehyde, 1
87
54
Aldol
condensation
silica sol–gel
mmol acetophenone, 100 mg
catalyst, 140 0C, 4 h, solvent
free
Fe3O4@Fe(OH)3
0.2 mmol aldehyde, 1 mmol
acetophenone, 128 mg catalyst,
toluene, RT, 20 h
38
89
55
GO
1.2 mmol benzaldehyde, 1
mmol acetophenone, 0.5 mL
catalyst (1.5 mg/mL), RT, 4 h,
solvent free
This
study
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Knoevenagel
condensation
amino-functionalized
mesoporous silica
1 mmol benzaldehyde, 1 mmol
malononitrile, 20 mg catalyst,
RT, 6 h, ethanol
90
93
20
29
M3
4.61 mmol benzaldehyde, 5.24
mmol malononitrile, 0.0297 g
catalyst, 1000C, 6 h, solvent
free
HAP-γ-Fe2O3
2 mmol benzaldehyde, 2 mmol
malononitrile, 0.025 g catalyst,
300C, 1 h, water
64
97
56
GO
1 mmol benzaldehyde, 2 mmol
malononitrile, 0.5 mL catalyst
(1.5 mg/mL), RT, 4 h, solvent
free
This
study
mp-PBI
1 mmol benzaldehyde, 1 mmol
malononitrile, 50 mg catalyst,
RT, 20 h, acetonitrile
100
67
57
58
mpg-C3N4-tBu
1 mmol benzaldehyde, 1 mmol
malononitrile, 50 mg catalyst,
700C, 2 h, acetonitrile
3.4. Recycling of the catalyst
Furthermore, we checked the recycling of the graphene oxide catalyst by choosing the
reaction of benzaldehyde with acetophenone or with malononitrile as a model reaction. After
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completion of the reaction, the catalyst could easily be recovered by simple extraction of the
reaction mixture with dichloromethane, the as-obtained aqueous layer containing catalyst
used for the subsequent experiments (5 runs). The results of these experiments are shown in
the Fig. 9. As shown, the yield of the product was found to be similar during these
experiments, establishing the efficient recycling of the catalyst, as well as the heterogeneous
nature of the developed methodology.
Fig. 9. Results of recycling tests of GO.
4. Conclusion
In summary, we have synthesized a grapheme oxide through a modified Hummer’s
method, which can effectively catalyse the aldol reaction as well as knoevenagel
condensation reaction. This organocatalyst shows excellent catalytic activity towards direct
aldol reactions of different substituted benzaldehydes with acetophenone and acetone at room
temperature. GO showed high reactivity with 97% conversion and 99% product selectivity in
8 h towards the Knoevenagel condensation reaction at room temperature under solvent free
21
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condition. Furthermore, the organocatalyst GO can be recovered and reused as the catalyst in
the subsequent runs without any loss of activity. In a broader perspective, these results
establish GO’s potential for use in synthetic chemistry, and it could open new catalytic
applications especially in the development of metal free methodologies in future.
Acknowledgement
SMI acknowledges Council of Scientific and Industrial Research (CSIR) and Department of
Science and Technology (DST), New Delhi, India for funding. ASR acknowledges CSIR,
New Delhi, India for his senior research fellowship. We thank the Indian Association for the
Cultivation of Science, Kolkata for providing the instrument support.
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Graphene based material as a base catalyst for solvent free Aldol
condensation and Knoevenagel reaction at room temperature
S. M. Islam,* Anupam Singha Roy,‡ Ram Chandra Dey,‡ Sumantra Paul
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
‡These authors contributed equally to this manuscript.
Research Highlights
•
•
•
•
•
Graphene oxide (GO) was prepared by the modified Hummers method.
The activity of GO as a base catalyst was tested for the Aldol reaction.
GO was also used as catalyst for Knoevenagel condensation.
The activity of GO was tested at room temperature under solvent free condition.
GO can be reused upto five times.
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Graphical Abstract
Graphene based material as a base catalyst for solvent free Aldol
condensation and Knoevenagel reaction at room temperature
S. M. Islam,* Anupam Singha Roy,‡ Ram Chandra Dey,‡ Sumantra Paul
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India
‡These authors contributed equally to this manuscript.
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