Application of GO/COF as a novel, efficient and recoverable catalyst in the Knoevenagel reaction

Article abstract of DOI:10.1002/aoc.5631

Hybrid composite based on graphene oxide (GO) and covalent organic framework (COF) [GO/COF] was developed via a simple solvothermal method, at which GO was applied as a platform to load COF based on melamine and terephthaldehyde. The synthesized hybrid na

Full text of DOI:10.1002/aoc.5631

Received: 3 January 2020
DOI: 10.1002/aoc.5631
Revised: 21 February 2020
Accepted: 22 February 2020
F U L L P A P E R
Application of GO/COF as a novel, efficient and recoverable
catalyst in the Knoevenagel reaction
Fatemeh Monehzadeh | Zahra Rafiee
Department of Chemistry, Yasouj
Abstract
University, Yasouj, 75918-74831, Islamic
Republic of Iran
Hybrid composite based on graphene oxide (GO) and covalent organic frame-
work (COF) [GO/COF] was developed via a simple solvothermal method, at
which GO was applied as a platform to load COF based on melamine and ter-
ephthaldehyde. The synthesized hybrid nanocomposite was characterized by
FT-IR, XRD, EDX and SEM techniques. Morphological analyses carried out by
SEM confirm the successful growth of COF over GO. Then, the resultant com-
posite was employed as an amazing and cost-effective catalyst in the condensa-
tion of several aldehydes with malononitrile and produced the corresponding
coupling products in high yields (up to 84%) at room temperature under
solvent-free conditions with an amount of catalyst, 15 mg in a very short reac-
tion time of 10 min. The catalyst could be reused without a noteworthy drop
in catalytic activity at least eight times. The use of GO/COF catalyst outcomes
under mild reaction conditions in very short reaction time, exceptional cata-
lytic activity, high recyclability and an easy work-up process for the
Knoevenagel condensation.
Correspondence
Zahra Rafiee, Department of Chemistry,
Yasouj University, Yasouj 75918-74831,
Islamic Republic of Iran.
Email: zahrarafiee2004@yahoo.com; z.
rafiee@yu.ac.ir
Funding information
Yasouj University, Grant/Award Number:
Gryu-89131307
K E Y W O R D S
covalent organic framework, graphene oxide-based nanocomposite, hybrid material,
Knoevenagel reaction
1 | INTRODUCTION
functional components to attain nanocomposite mate-
rials has gained growing attention, as such
Heterogeneous catalysis is a key process in the manu-
facture of a wide range of chemicals and is at the heart
of green chemical processes.[1] The features of the het-
erogeneous catalytic systems including easy product
separation, efficient recyclability of catalysts and mini-
mization of chemical waste, can overcome the limita-
tions of their homogeneous catalysis counterpart.[2]
Composites have proven to be popular heterogeneous
supports for the immobilization of homogeneous
catalysts.[3]
nanocomposites permit graceful amalgamation of chemi-
cal and physical properties, synergistic effects and multi-
ple functions of individual components.[6–8] The carbon-
based composite materials are striking for various appli-
cations because their outstanding chemical, mechanical,
thermal and electric properties.[9,10] The composite mate-
rials based on graphene and its derivatives possess
unique properties such as chemical stability, low toxicity,
biocompatibility, and flexibility for surface func-
tionalization.[11,12] Hence, the surface of these composites
can be grafted with an extensive variety of functional
groups, making further reactions and modification.[13]
Among the various kinds of nanocomposites, Graphene
Recently, nanotechnology with wide diversity of
nanomaterials create a new revolution in science and
especially chemical fields.[4,5] The combination of the
Appl Organometal Chem. 2020;e5631.
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https://doi.org/10.1002/aoc.5631

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MONEHZADEH AND RAFIEE
oxide (GO) ones have received interesting attention due
to their unique properties.[14]
recovery and recycling of catalyst. Various solid-
supported catalysts have been applied to this
reaction.[31–33] However, to the best of our knowledge,
the Knoevenagel condensation catalyzed by GO/COF
hybrid was not previously mentioned in the literature. In
this work, we wish to report the utilization of GO/COF
hybrid material as an efficient heterogeneous catalyst for
the Knoevenagel reaction. High activity was observed
and the GO/COF catalyst was easily isolated from the
reaction mixture by simple filtration and reused without
significant degradation in activity.
GO, as an oxidized derivative of graphene and a
single-layered material has attracted noteworthy interest
as one of the most interesting and potential materials due
to its extremely large surface area, low cost, and excellent
mechanical and thermal properties.[15] In particular, its
good hydrophilic properties owing to the presence of
oxygen-containing
hydrophilic
groups,
including
hydroxyl, carbonyl, and carboxyl groups, not only makes
GO readily dispersible in water to produce stable colloi-
dal suspension, but also facilitates the preparation of GO-
based composites in solution. Furthermore, GO sheets
show remarkable improvements in chemical and physical
properties when incorporated in composite materials
with potential applications in many areas, such as solar
2 | EXPERIMENTAL
2.1 | Materials
cell,
resonators,
electronic
devices,
and
supercapacitors.[16,17]
The reagents and chemicals including graphite powder,
potassium permanganate, hydrogen peroxide (30%), sul-
furic acid (95–97%), hydrochloric acid (37%),
Covalent organic frameworks (COFs), an emerging
class of crystalline porous materials fabricated from
purely organic building blocks linked by reversible cova-
lent bonds have attracted much attention due to their
highly ordered structure and accessible distinct
pores.[18,19] The fascinating features of tunable pore size,
perpetual porosity, large surface area, low density, and
plentiful functional groups on the channel walls render
COFs great potential in an extensive variety of applica-
tions, including gas separation,[20] gas storage,[21] chemi-
cal sensors,[22] optoelectronics,[23] drug delivery[21] and
catalysis.[24] However, COFs are generally irregular in
morphology and it is hard to control their growth ratio-
nally.[25] Consequently, shapeless bulk materials are pro-
duced. Moreover, the light density makes COF materials
difficult to precipitate from the matrix. These unexpected
disadvantages bring troublesomeness and impediment in
their further application. Hence, their composites with
other materials is of substantial significance in promoting
extensive applications of COFs.[26–28] Incorporation of
the merits of COFs and GO to construct a novel class of
composites with both enhanced functionality and large
surface area is of great significance and interest.[29] In
particular, the GO sheets can offer a center for COF
growth.
3-aminopropyltriethoxy
silane
(APTES,
KH550,
H2N(CH2)3Si(OCH2CH3)3), melamine, terephthaldehyde,
dimethyl sulfoxide (DMSO), chloroform, ethanol, benzal-
dehyde,
4-chlorobenzaldehyde,
4-methoxybenzaldehyde,
malononitrile,
4-nitrobenzaldehyde,
2-chlorobenzaldehyde,
4-methylbenzaldehyde,
2-hydroxybenzaldehyde were purchased from Merck
Company (Darmstadt, Germany) and used without fur-
ther purification.
2.2 | Apparatus
Fourier transform infrared (FT-IR) spectra of the samples
were recorded with a Jasco-680 spectrometer (Japan) in
the range of 4000–400 cm−1 using KBr pellet. The X-ray
diffraction (XRD) patterns of the samples were recorded
in the reflection mode using a Bruker, D8 Advance dif-
fractometer. The surface morphology of the resulting
materials was investigated using field emission-scanning
electron microscopy (FE-SEM; EM10C-ZEISS, 80 KV,
Zeiss Co., Germany, Oberkocken). The ultrasonic bath
(Tecno-GAZ SPA Ultrasonic system, Italy, Parma) was
used at a frequency of 60 Hz and power of 130 W.
The Knoevenagel condensation of aldehydes with
compounds containing activated methylene groups is one
of the most useful and widely employed methods for
carbon–carbon bond formation with numerous applica-
tions in the synthesis of fine chemicals as well as hetero-
cyclic compounds of biological significance.[30]
Conventionally, this reaction is catalyzed by weak bases
like primary, secondary, and tertiary amines under
homogeneous conditions, which could require high
amount of catalyst with the attendant difficulties in
2.3 | Fabrication of GO
GO was synthesized according to modified hummer's
technique.[34] Typically, 2.0 g of graphite powder, 1.0 g of
sodium nitrate and 6 g of potassium permanganate were
added to 60 ml of sulfuric acid (98%) and stirred in an ice
bath for 90 min. Then, the reaction mixture was placed

APPLICATION OF GO/COF AS A NOVEL, EFFICIENT AND RECOVERABLE CATALYST
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ꢀ
in a 40 C water bath and stirred for 2 hr, followed by
adding 150 ml of deionized water. Consequently, the
temperature was elevated to 98 ꢀC and stirred for 30 min.
ultrasonic bathꢀ. Then APTES (1 wt%) was added and
refluxed at 75 C for 12 hr. The obtained product was
washed with ethanol to remove additional APTES and
dried at 60 ꢀC for 10 hr.
ꢀ
After the temperature reduced to 60 C, 10 ml of H2O2
(30%) was added and stirred for another 2 hr. The
resulting product was filtered and washed with
hydrochloric acid (5%) and deionized water until the pH
of the washing water becomes neutral, and afterward
dried at 60 ꢀC for 12 hr.
2.5 | Preparation of GO/COF
0.20 g of GO-NH2, 0.5 g (3.96 mmol) of melamine, 0.5 g
(3.73 mmol) of terephthaldehyde and 25 ml of DMSO
were mixed and transferred to a 150 ml Teflon-lined
ꢀ
2.4 | Preparation of GO-NH2
autoclave, sealed and heated in an oven at 180 C for
12 hr. After cooling, the resulting solid was collected by
filtration, washed by ethanol several times, and dried
overnight at room temperature. Schematic illustration of
the all steps for the synthesis of GO/COF hybrid material
is shown in Figure 1.
ꢀ
GO was placed in a 110 C oven for 12 hr to remove the
adsorbed water. GO was modified with APTES
(H2N(CH2)3Si(OCH2CH3)3) as follows: 20 mg of GO was
dispersed in 20 ml of ethanol for 15 min using an
FIGURE 1 Schematic representation of the preparation of GO/COF

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MONEHZADEH AND RAFIEE
2.6 | General procedure for the
Knoevenagel condensation using GO/COF
as a catalyst
A mixture of various aldehydes (1 mmol), malononitrile
(1.5 mmol) and GO/COF (15 mg) was stirred at room
temperature. TLC was utilized to monitor the progress of
the reaction. After the completion of the reaction, the cat-
alyst was centrifuged. Then, the catalyst was recycled,
washed several times with ethanol and water, and reused
after drying. Then, the recovered catalyst was reused in
eight runs under similar conditions as the first run to
exhibit the recyclability and stability of the prepared
catalyst.
3 | RESULTS AND DISCUSSION
3.1 | Fabrication of GO/COF
GO/COF hybrid material was prepared using the
solvothermal technique. After the preparation of GO, it
was functionalized with APTES, to form amine-
functionalized GO. Then, COF was synthesized on the
surface of GO-NH2 by covalently linking melamine and
terephthaldehyde through the condensation reaction to
form the GO/COF hybrid material (Figure 1).
FIGURE 2 FT-IR spectra of GO (a), GO-NH2 (b) and
GO/COF (c)
1500, and 1357 arising from the C=N, C=C, and C-N
bonds, respectively, were observed showing condensation
and tautomerization.
The XRD patterns of GO and GO/COF hybrid are
shown in Figure 3. The diffraction peak of GO is
observed at 10.2ꢀ revealing of the (001) plane of GO. The
diffraction pattern of GO/COF hybrid is indicated the
three peaks at 9.0, 26.0ꢀ and 43.0ꢀ. Compared with the
pure GO peak, there are no diffraction peaks belonging
to the GO sheets in GO/COF. This result is ascribed to
the formation of COF between the layers of GO, which
3.2 | Characterization of GO/COF
Figure 2 indicated the FT-IR spectra of GO, GO-NH2 and
GO/COF, the absorption band around 3430 cm−1 in GO
is contributed to O-H stretching vibration and the absorp-
tion bands appeared at 1708 and 1039 cm−1 correspond
to C=O and C-O stretching vibrations in carboxyl and
epoxy groups. An absorption peak at 1623 cm−1 is con-
tributed to C=C skeletal vibrations. In the FT-IR spec-
trum of GO-NH2, two additional absorption bands at
2940 and 2880 cm−1, revealing of the asymmetric and
symmetric stretching vibrations of C-H bonds. The addi-
tional absorption bands at 1547 and 797 cm−1 are assign-
able to the symmetric stretching and out-of-plane
bending vibrations of N-H band, respectively. The two
absorption bands observed at 1116 and 1030 cm−1 corre-
spond to the Si-O-Si and Si-O-C stretching vibrations.
Besides, the intensity of the absorption band around
3400 cm−1 that is characteristic of the surface OH groups
considerably decreased, signifying that the silylation hap-
pened through reaction with surface OH groups. From
the obtained spectral data, it can be realized that APTES
has been grafted on GO. The FTIR spectrum of GO/COF
revealed a series of new stretching vibrations at 1562,
FIGURE 3 XRD patterns of GO (a) and GO/COF (b)

APPLICATION OF GO/COF AS A NOVEL, EFFICIENT AND RECOVERABLE CATALYST
5 of 9
might destroy the regular layer stacking of GO and exfoli-
ate the GO nanosheets, resulting in the disappearance of
the GO peak.
Figure 4 shows FE-SEM images of GO (a) and
GO/COF (b). FE-SEM image of GO/COF shows the puffy
nanoparticles growth of COF on GO surfaces. Further-
more, the puffy nanoparticles of COF are located
between GO plates, which prevent from collapsing of the
GO plates and provide a suitable surface area. In addi-
tion, the EDX analysis of the prepared catalyst confirms
the presence of carbon, nitrogen, oxygen, and Si
(Figure 5).
FIGURE 5 EDX spectrum of GO/COF
3.3 | Catalytic activity test
bonding between active protonic sites of GO/COF and
these solvents which declines the catalytic efficiency.
Hence, the use of 15 mg of the catalyst under the solvent-
free conditions at room temperature is designated as opti-
mum conditions. After the optimization of the condi-
tions, the generality and the scope of this system are
examined using several aromatic aldehydes bearing both
electron-donating and electron-withdrawing groups.
Based on the obtained results all substrates give the
corresponding products in relatively high yield in very
short reaction time as reveal in Table 2. The benzalde-
hyde derivatives containing electron-withdrawing groups
such as -NO2, and -Cl are transformed to the
corresponding products with a high yield (Table 2, entries
2–4), whereas the benzaldehyde derivatives possessing
electron-donating moieties including -OCH3, -CH3, and -
OH provide the lower yields (Table 2, entries 5–7). This
result proves that the benzaldehyde derivatives having
electron-withdrawing groups are slightly reactive com-
pared to those with electron-donating moieties.
The catalytic application of GO/COF was examined in
the Knoevenagel reaction under diverse conditions
(Table 1). For the optimization of the reaction conditions,
the reaction between malononitrile with benzaldehyde in
the presence of GO/COF as a catalyst was designated as a
test reaction. The reaction was performed at 5, 10, 15 and
20 mg of GO/COF loading. With increasing of catalyst
loading from 5 to 15 mg, the yield was enhanced and the
best result in an appropriate time was obtained using
15 mg of catalyst (Table 1, entry 4). By increasing the cat-
alyst loading to 20 mg no change in the yield of the prod-
uct is observed (Table 1, entry 5). The effect of different
solvents such as H2O, EtOH, CH2Cl2 and CH3CN and
solvent-free conditions is also investigated and the results
reveal that the yields in H2O and EtOH solvents are
lower than solvent-free conditions (Table 1, entries 6 and
7). It might be attributed to the solvation of the active
functional groups by these solvents and hydrogen-
FIGURE 4 SEM images of GO (a) and GO/COF (b)

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MONEHZADEH AND RAFIEE
TABLE 1 Effect of catalyst loading and solvent in the
Knoevenagel condensation of malononitrile with benzaldehyde
Catalyst
Entry (mg)
Time
Solvent (min.)
Yield
(%)
1
2
3
4
5
5
6
7
8
-
5
-
240
10
10
10
10
10
10
10
10
-
75
89
98
96
84
91
38
43
-
10
15
20
15
15
15
15
-
-
-
H2O
EtOH
CH2Cl2
CH3CN
Reaction conditions: benzaldehyde (1 mmol), malononitrile (1.5 mmol),
catalyst (15 mg), room temperature.
TABLE 2 The Knoevenagel condensation of aldehydes with
malononitrile in the presence of GO/COF
Entry
R
Time (min.)
Yield (%)
1
2
3
4
5
6
7
8
9
H
90
45
28
55
60
60
60
50
30
98
100
99
4-NO2
4-Cl
2-Cl
4-MeO
4-Me
4-OH
2-OH
4-Br
93
89
87
84
86
98
Reaction conditions: aldehyde (1 mmol), malononotrile (1.5 mmol), catalyst
(15 mg), room temperature and solvent-free.
FIGURE 6 Reusability of the GO/COF

APPLICATION OF GO/COF AS A NOVEL, EFFICIENT AND RECOVERABLE CATALYST
7 of 9
In order to prove the heterogeneity of the catalyst, the
filtration experiment of GO/COF was performed. The cat-
alyst was separated from the reaction mixture after
5 min, and the resulting filtrate was further stirred for
5 min. The conversion yield of the benzaldehyde remains
unchanged for the filtrate even at extended time, indicat-
ing that the catalytic process is heterogeneous and there
is not any progress for the reaction in the homogeneous
phase.
ORCID
Zahra Rafiee https://orcid.org/0000-0002-4296-8760
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ACKNOWLEDGEMENTS
We are grateful to Yasouj University (grant number
Gryu-89131307) for financial assistance.

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Rafiee Z. Application of GO/COF as a novel,
efficient and recoverable catalyst in the
Knoevenagel reaction. Appl Organometal Chem.
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