Functionalized graphene oxide as an efficient adsorbent for CO2 capture and support for heterogeneous catalysis

Article abstract of DOI:10.1039/c6ra13590k

We have designed new imine-functionalized graphene oxide (IFGO) through post synthetic modifications involving co-condensation of 3-aminopropyltriethoxysilane with graphene oxide basal plane containing hydroxyl and epoxy functional groups, followed by Schiff base condensation reaction with 2,6-diformyl-4-methylphenol and impregnation of copper(ii) to it through covalent attachment (Cu-IFGO). Powder X-ray diffraction, N₂ sorption analysis, FT-IR, HR-TEM, FE-SEM and TGA/DTA analysis are employed to characterize the materials. The IFGO material exhibits good CO₂ storage capacity of 8.10 mmol g^-1 (35.64 wt%) and 2.10 mmol g^-1 (9.24 wt%) at 273 K and 298 K temperature, respectively, up to 3 bar pressure, suggesting its potential application in environmental clean-up. Also, Cu-IFGO showed high catalytic activity in microwave-assisted one-pot three-component C-S coupling reactions for a diverse range of aryl halides with thiourea and benzyl bromide in aqueous medium to obtain aryl thioether products (maximum yield 86%), which are derivatives of natural products. Moreover, having imine and hydroxyl groups in functionalized graphene oxide, the grafted Cu(ii) chelated at the graphene oxide surface so strongly that it could not be leached out from the material during the course of the coupling reaction. Thus, it displayed very small decrease in product yield up to the sixth reaction cycle suggesting a sustainable future of this Cu(ii)-grafted catalyst.

Full text of DOI:10.1039/c6ra13590k

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Functionalized graphene oxide as an efficient
adsorbent for CO capture and support for
2
Cite this: RSC Adv., 2016, 6, 72055
heterogeneous catalysis†
Piyali Bhanja, Sabuj Kanti Das, Astam K. Patra and Asim Bhaumik*
We have designed new imine-functionalized graphene oxide (IFGO) through post synthetic modifications
involving co-condensation of 3-aminopropyltriethoxysilane with graphene oxide basal plane containing
hydroxyl and epoxy functional groups, followed by Schiff base condensation reaction with 2,6-diformyl-
4
-methylphenol and impregnation of copper(II) to it through covalent attachment (Cu-IFGO). Powder X-
ray diffraction, N sorption analysis, FT-IR, HR-TEM, FE-SEM and TGA/DTA analysis are employed to
characterize the materials. The IFGO material exhibits good CO
2
ꢀ
1
2
storage capacity of 8.10 mmol g
ꢀ1
(
35.64 wt%) and 2.10 mmol g (9.24 wt%) at 273 K and 298 K temperature, respectively, up to 3 bar
pressure, suggesting its potential application in environmental clean-up. Also, Cu-IFGO showed high
catalytic activity in microwave-assisted one-pot three-component C–S coupling reactions for a diverse
range of aryl halides with thiourea and benzyl bromide in aqueous medium to obtain aryl thioether
products (maximum yield 86%), which are derivatives of natural products. Moreover, having imine and
hydroxyl groups in functionalized graphene oxide, the grafted Cu(II) chelated at the graphene oxide
surface so strongly that it could not be leached out from the material during the course of the coupling
reaction. Thus, it displayed very small decrease in product yield up to the sixth reaction cycle suggesting
a sustainable future of this Cu(II)-grafted catalyst.
Received 25th May 2016
Accepted 6th July 2016
DOI: 10.1039/c6ra13590k
www.rsc.org/advances
8
of thiols to disuldes and selective aerobic oxidation of alco-
hols into the corresponding aldehydes or ketones. It is perti-
Introduction
9
Graphene, one of the most promising allotropes of carbon, has nent to mention that owing to the presence of a wide range of
gained immense attention in the eld of nanomaterials oxygen-containing functional groups on its basal planes and
research today mainly because of its two important character- edges, GO is hydrophilic in nature, which could facilitate the
1
istics: excellent electrical property and high structural strength.
anchoring of an active metal ion at its surface.
Graphene oxide (GO), which can be easily derived from graphite
Functionalized GO-based materials, although attracting
powder, is one of the graphene congeners, whose nanostructure huge interest in the context of fundamental research, can be
is composed of a basal plane of sheets enriched with hydroxyl equally attractive for various frontline application areas such as
13
10
11
12
and epoxy functional groups as well as carboxylic acid groups solar cells, photocatalysis, gas storage and biosensors.
which allow construction of a vast spectrum of novel functional Recent studies on GO suggested that transition metal/metal ion
2
groups attached at the GO surface. Owing to a unique nano- nanoparticles can be easily supported over GO nanosheets and
architecture of a one-atom-thick layer of carbon atoms the resulting material can serve as an efficient heterogeneous
arranged in a honeycomb lattice, GO can act as a template/ catalyst. For example,
building block for efficient production of two-dimensional composite (Au/Fe –GO) showed notably higher catalytic
porous nanohybrid materials like graphene/metal oxide activity in reducing nitroaniline in comparison to simple gold
a mixed metal–metal oxide–GO
O
2 3
3
4
5
14
composites, carbon nitride, metal suldes, functionalized nanoparticles. Furthermore, various transition metal/metal
6
nanoporous silica etc. with high surface area. Today graphene oxide/sulde nanoparticles supported on GO/reduced gra-
has been identied as a promising carbon-based material with phene oxides like AuPd–GO/TiO
a huge prospect in industrial chemistry for use in various CdS/RGO are successfully employed in several efficient
catalytic reactions like hydration of various alkenes, oxidation catalytic/photocatalytic reactions. Optimization of a chemical
15
16
2
,
3 4
PtAu/TEGO and g-C N /
17
7
reaction together with the development of an eco-friendly
chemical process can be achievable through the involvement
Department of Materials Science, Indian Association for the Cultivation of Science,
of a porous material support bearing suitable functional groups
Jadavpur, Kolkata-700 032, India. E-mail: msab@iacs.res.in
at the pore surface that can offer fascinating technological and
energy-related advancements.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra13590k
1
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28
In the late 20th and early 21st centuries excessive use of fossil predominant architectural motifs for these compounds. Many
fuel, rapid deforestation and explosive urbanization/ demanding drug molecules consisting of aryl sulde units in
industrialization has led to an exponential rise of atmospheric their core structure are now being utilized in several deadly
18
CO2. This increased concentration of CO will have an adverse diseases like Alzheimer's, various types of cancers, HIV and
2
effect in forthcoming centuries due to the imbalance of natural some inammatory diseases, and also exhibited successful
29
eco-system as it is one major reason for global warming and pharmacological results in many cases. However, metal-
19
climate change. Therefore, environmentally sound and safe catalyzed S-arylation has been investigated to a lesser extent
method of CO capture and storage has always been an attractive than that of the O-arylation and N-arylation reactions like C–N,
eld for researchers in order to control this greenhouse gas to C–C and C–O due to S–S homo-coupling reaction. Among
2
30

31
32
33
34
reverse the eco-balance in the near future. Nowadays, signicant various transition metal (Pd, Ni, Co, Cu )-mediated cata-
experimental activity has been devoted towards synthesis of CO2 lytic materials, copper graed catalysts have more potential and
sorbent materials with good surface basicity, microporosity and effectiveness in S-arylation of different substituted aryl halide
2
0
good recycling ability such as zeolitic imidazole framework,
substrates due to their easy availability and eco-friendly reac-
tion conditions. However, in spite of having huge potential in
21
22
graphene analogue,
functionalized mesoporous silica,
23
24
porous organic polymer, metal organic framework, porous environmental clean-up, like CO
carbon and so on. Sui et al. reported the adsorption capacity catalysis for one-pot three component coupling reactions over
2
storage, heterogeneous
25
26
for carbon dioxide (11.2 wt% at 1.0 bar and 273 K) of 3D mes- functionalized GO surface has not been explored so far.
27
oporous functionalized GO. Chandra et al. mentioned CO2
uptake capacity on N-doped carbon produced by chemical materials, IFGO and Cu-IFGO, where copper(II) has been graf-
activation of polypyrrole-functionalized graphene sheets. ted on imine-functionalized GO material through post synthetic
Herein, we demonstrate two new functionalized GO-based
Recently, innovation in sound and simple methods for C–S functionalization of GO. Surface functionalization strategies are
bond formation reactions has attracted considerable attention employed to prepare amine-functionalized GO via co-
for the production of many pharmaceuticals, value-added ne condensation and followed by Schiff base condensation reac-
chemicals, natural products and agrochemicals where sulfur- tion. Covalent graing of copper(II) at the surface of GO-
containing organic compounds (diaryl suldes) are the supported imine-functionalized product leads to copper-graed
Scheme 1 Schematic illustration for the synthesis of copper-grafted imine-functionalized graphene oxide (Cu-IFGO) via the formation of AFGO
firstly and then IFGO.
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imine-functionalized GO material (Scheme 1), which exhibited Synthesis of imine-functionalized graphene oxide (IFGO)
excellent catalytic activity in one-pot three-component C–S
coupling reaction involving thiourea, benzyl bromide and various
functional group-substituted aryl halides under microwave-
assisted heating and irradiation conditions. Further, the cata-
lyst can be easily recycled and reused for up to six consecutive
reaction cycles without signicant loss in catalytic activity. Also,
the presence of electron-donating imine N-sites at the surface of
the high surface area IFGO material could facilitate the strong
To prepare 2,6-diformyl-4-methylphenol (DFP), hexamethy-
lenetetramine (2.82 g; 0.02 mol) and paraformaldehyde (3 g; 0.1
mol) were added to a mixed solution of p-cresol (1.08 g; 0.01
mol) and acetic acid (5 mL). The reaction mixture was stirred at
ꢁ
0–80 C for 2 h to get a light brown viscous solution. The
7
system was cooled down to room temperature followed by
careful addition of concentrated H SO (1 mL). Then the solu-
2
4
tion was reuxed for 30 min and a light yellow precipitate was
observed aer treating with deionized water (40 mL). It was
adsorption of Lewis acidic CO molecules.
2
ꢁ
allowed to stand at 4 C overnight in a refrigerator to get more
yellow precipitate. The product was collected through simple
Experimental section
Materials

ltration technique and washed with very little amount of cold
dry methanol. Recrystallization of this product was carried out
Commercial graphite powder (99.5% mesh), sodium nitrate in toluene to obtain pure product and this yellow colored
ꢀ
1
1
13
(
NaNO
potassium permanganate (KMnO
hydrogen peroxide (H
3
; M ¼ 84.99 g mol ), concentrated sulfuric acid (H
2
SO
4
), product was characterized by H and C mass NMR and FT-IR
ꢀ
1
; M ¼ 158.03 g mol ), spectroscopy (ESI: sections S1–S3†).
4
In order to execute the reaction between amino groups of 3-
2
2
O ; 30%), 3-aminopropyltriethoxysilane
ꢀ
1
(
3-APTES; M ¼ 221.36 g mol ) and copper acetate (Cu(OAc)2; APTES and aldehyde groups of DFP, 300 mg of AFGO nano-
ꢀ1
M ¼ 170.48 g mol ) were purchased from Sigma-Aldrich, India. sheets were dispersed in a solution of anhydrous dime-
Organic polar and non-polar solvents were used without further thylformamide (DMF; 24 mL) and DFP (483 mg; 2.94 mmol) and
purication.
2
it was reuxed for 12 h under N atmosphere. Aer completion
of reaction, DMF and ethanol were used as washing solvents to
remove unreacted DFP from the material surface. The material
has been designated as IFGO and was dried in vacuum over-
night as the next step.
Synthesis of graphene oxide (GO)
The synthesis of GO was performed by using a modied
35
Hummers' method. In a typical synthesis, concentrated
sulfuric acid (46 mL) was added to the graphite mesh (1 g) in
a cleaned 250 mL round-bottom ask containing sodium
nitrate (1.2 g). This reaction mixture was kept in an ice bath
under stirring condition for 15–20 min to reduce the tempera-
ture. Then potassium permanganate (6 g) was added gradually
to the reaction mixture and it was transferred to a preheated
Synthesis of copper-graed imine-functionalized graphene
oxide (Cu-IFGO)
Finally, IFGO nanosheets (300 mg) were dispersed in 20 mL
ethanol containing copper acetate (60 mg) for 10 min, and it was
ꢁ
heated for 12 h at 80 C. Washing of the resulting product was
ꢁ
repeated three to four times to get rid of non-reacted copper
acetate from the material surface. Finally, the black solid
product was named as Cu-IFGO and characterized thoroughly.
This material was used as a heterogeneous catalyst for one pot
C–S cross coupling reaction in aqueous medium under micro-
wave irradiation.
water bath of 40 C temperature for 24 h. Then 80 mL deionized
water was slowly poured into it, followed by increasing the
temperature of the oil bath to 95–98 C and the reaction mixture
was allowed to stir continuously for 24 h. Aer completion of
reaction, the suspension was treated with 5 mL 30% H O fol-
ꢁ
2
2
lowed by 50 mL deionized water. Then the brown colored solid
was ltered and washed with deionized water until the nal pH
ꢁ
was 6. The collected solid material was dried in an oven at 75 C CO
sorption measurement
2
overnight and the material is designated as GO.
To measure the CO
2
gas adsorption property of the IFGO
ꢁ
material, at rst the sample was degassed at 120 C for 12 h
under vacuum. Helium gas was purged into it for the dead
volume measurement. Then the CO2 adsorption/desorption
isotherm of the material was estimated by using a Bel Japan
Inc. Belsorp-HP at two different temperatures of 273 and 298 K
Synthesis of amine-functionalized graphene oxide (AFGO)
1.0 g of the black solid GO was dispersed in a 100 mL two-neck
round-bottom ask containing dry toluene (40 mL) by an
ultrasonication process for 2 h. Then 509 mg (2.3 mmol) 3-
APTES was added dropwise into the dispersion and allowed to
reux under nitrogen atmosphere for 24 h. During this func-
tionalization, 3-aminopropyl groups are graed on the GO
surfaces utilizing their hydroxyl and carboxyl groups. Then the
2
up to 3 bar pressure region. The CO adsorption/desorption
isotherms were analyzed by employing BELMaster™ soware.
C–S coupling reaction catalyzed by Cu-IFGO
solid product was ltered and washed thoroughly with dry In a typical experiment under microwave irradiation, we carried
toluene four times to get rid of unreacted 3-APTES and dry out the coupling reaction in 5 mL deionized water contained in
ethanol was used for nal washing. The dark black solid was a MW glass vial (10 mL) using three organic components,
dried under vacuum overnight, designated AFGO, where the namely 4-bromotoluene (188 mg, 1.1 mmol); thiourea (91 mg,

nal yield was 1.024 g.
1.2 mmol); benzyl bromide (187 mg, 1 mmol), and mild base
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2 3
K CO (580 mg, 4.2 mmol) with 25 mg of Cu-IFGO and also
without any catalyst. Before starting the reaction a small
magnetic needle was inserted in the MW vial to make the
reaction mixture homogeneous as the reaction progresses. The
sealed glass vial was introduced into the microwave for 15 min
and it was allowed to cool down to room temperature for
product extraction using ethyl acetate and water mixture. Aer
the reaction the catalyst was ltered from the reaction mixture.
The extracted pure product was identied by the readily avail-
able thin layer chromatography method, and nally was char-
1
13
acterized using H and C NMR spectroscopy (ESI: section 4†).
Characterization techniques
Powder X-ray diffraction patterns of the functionalized materials
were recorded with a Bruker D8 Advance SWAX diffractometer,
operated at 40 kV voltage and 40 mA current. The instrument
was calibrated by standard silicon sample, using Ni-ltered Cu
Fig. 1 Wide angle powder XRD patterns of (a) GO, (b) AFGO, (c) IFGO
and (d) Cu-IFGO.
Ka (l ¼ 0.15406 nm) radiation. For HR-TEM analysis, 10 mg The small peak at 2q ¼ 26.12ꢁ could be attributed to some
material (Cu-IFGO) was dispersed into dry ethanol with 5–10 minor amount of unreacted graphite powder. For the alkylated
min of sonication. Then one drop of the dispersed solution was graphene nanosheets upon graing with 3-APTES the former
dropped onto a carbon-coated copper grid and dried before TEM peak showed diminished intensity and larger peak width. The
analysis. Nitrogen adsorption/desorption isotherms were ob- disappearance of the former sharp peak in Fig. 1b could be
tained by using a Quantochrome Autosorb 1C surface area attributed to the formation of a bridge between the amine
analyzer at 77 K. Prior to gas adsorption, samples were degassed functional group and GO nanosheets. The wide-angle X-ray
for 10–12 hours at 453 K under high vacuum. The non-local diffraction pattern of IFGO material is shown in Fig. 1c, which
density functional theory (NLDFT) pore size distributions were suggested the retention of the structure of parent GO material.
determined from the sorption isotherm using the carbon slit- Cu-IFGO displayed diffractions at similar peak positions
cylindrical pore model. FT-IR spectra of the samples were together with further reduced peak intensities compared with
recorded using a Perkin Elmer Spectrum 100. To investigate the GO and AFGO as seen in Fig. 1d. This result suggests that Cu-
morphology and particle size of the samples, a JEOL JEM 6700 IFGO has retained the nanostructure of GO.
eld emission scanning electron microscope (FE SEM) was used.
Electron paramagnetic resonance (EPR) spectrum was recorded
for the solid sample at room temperature using a JESFA200ESR
spectrometer (JEOL). The X-ray photoelectron spectroscopy
Surface area and porosity measurement
Nitrogen adsorption/desorption isotherms of the three samples,
i.e. GO, IFGO and Cu-IFGO, are shown in Fig. 2A. As seen from
these isotherms, each of them is a type I adsorption/desorption
isotherm without any hysteresis loop. In each case, the adsorp-
tion isotherm with large uptake in the relatively low pressure
(XPS) technique has been performed using an Omicron Nano-
technology XPS 0571 spectrometer using an Mg X-ray source.
Thermogravimetric analysis (TGA) and differential thermal
analysis (DTA) of the samples were performed with a TGA
thermal analyzer (TA-SDTQ-600) under air ow. The carbon,
hydrogen and nitrogen contents were determined with a Vario
EL III elemental analyzer. The copper content in the nal
material was determined with a Varian AA240 atomic absorption
spectrophotometer (AAS). All the catalytic reactions were carried
out using an Anton Paar Monowave 300 microwave reactor.
region (P/P
0
¼ 0–0.1) suggested good microporosity in the
materials. The Brunauer–Emmett–Teller surface areas of GO,
2
ꢀ1
IFGO and Cu-IFGO materials are 252, 190 and 123 m
g ,
respectively. Using the NLDFT method, the pore size distribution
plots have been obtained which are given in Fig. 2B. The pore
size distributions of the three materials are estimated as 1.69,
1.64 and 1.60 nm, respectively. The pore volumes of GO, IFGO
and Cu-IFGO materials are obtained from those corresponding
3
Results and discussion
X-ray diffraction studies
N sorption isotherms which are 0.1196, 0.0960 and 0.0593 cm
g
2
ꢀ
1
, respectively. Here, the decrease in surface area, pore size
and pore volume with progressive functionalization of GO
conrms the successful graing of copper to Schiff base immo-
bilized on the GO surface similar to that observed for stepwise
The powder X-ray diffraction patterns of GO, AFGO and Cu-
IFGO materials are shown in Fig. 1. The diffraction pattern of
Fig. 1a shows the original sharp diffraction peak at 2q ¼ 11.67ꢁ
corresponding to (001) plane of GO with d-spacing of 0.75 nm. It
is noticed that the d-spacing value of these layers is increased
37
functionalization at the surface of mesoporous silica materials.
Spectroscopic analysis
˚
˚
from 3.4 A in the pristine graphite powder to 7.5 A in GO due to
the presence of oxygen-containing functional groups like The presence of different functional groups in copper-
36
carboxylic acid and hydroxyl on basal plane of graphite moiety.
immobilized imine-functionalized GO and the intermediate
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Fig. 3 FT-IR spectra of GO (a), AFGO (b), IFGO (c) and Cu-IFGO (d)
materials.
conrming the proper graing of Cu to the imine group. The
ꢀ
1
peaks at 1124 and 1032 cm could be attributed to the vibra-
tional stretching of Si–O–Si and Si–O–C, respectively, for the
three materials, i.e. AFGO, IFGO and Cu-IFGO. So FT-IR spectra
reveal the successful functionalizations at the GO surface.
Morphology and nanostructure
The eld emission scanning electron microscopic images of IFGO
and Cu-IFGO materials are shown in Fig. 4. As seen from the
gure, the two materials have almost similar type of crumpling
and folded morphology but especially the copper-graed organ-
ically functionalized GO exhibited folded morphology with
granules. However, SEM images reveal extended sheets of lateral
dimensions in the length range from 0.5 to 10 mm, approximately.
On the other hand, high resolution transmission electron
microscopic images reveal the nanoscopic features of IFGO and
Cu-IFGO samples and these are shown in Fig. 5. IFGO displays
two-dimensional crumpled nanosheets with a few stacked layers
and these are shown in Fig. 5a and b. Also, Cu-IFGO shows
similar type of crumpling and layer-like morphology in Fig. 5c
and d, with an agglomerated phase, which conrms the
successful incorporation of copper at the surface of IFGO.
2
Fig. 2 (A) The N adsorption/desorption isotherms of GO (black), IFGO
(red) and Cu-IFGO (blue) materials. (B) The corresponding pore size
distribution plots for GO (black), IFGO (red) and Cu-IFGO (blue) materials,
estimated by the NLDFT (non-local density functional theory) method.
materials has been assigned from the FT-IR spectra as shown in
ꢀ
1
Fig. 3. One intense and broad peak is observed at 3433 cm due
to the presence of –O–H (hydroxyl) groups at the surface of GO,
which is major binding site for amine-functionalized silane
ꢀ
1
material. The vibrational bands at 1724 and 1626 cm could be
attributed to the carbonyl group of carboxylic acid and
stretching vibration of –C]C group of aromatic graphene
Electron paramagnetic resonance
In order to investigate the oxidation state of the metal center
ꢀ1
moiety, respectively. Bands at 1369, 1237 and 1016 cm indi-
(copper) in the catalyst, solid state EPR spectroscopic analysis
cated the presence of C–OH, C–O–C (epoxy) and –C]C groups,
38
respectively. Further, APTES-coated GO shows many vibra-
tional peaks in its FT-IR spectrum (Fig. 3b). The vibration peaks
ꢀ
1
at 3404 and 1573 cm could be assigned to –NH
2
group, which
revealed the successful graing of APTES to the GO. The two set
ꢀ1
of bands at 2934 and 2860 cm corresponding to the methy-
lene and methyl groups in APTES moiety are also seen. Further,
in the FT-IR spectrum in Fig. 3c, the C]N stretching frequency
ꢀ
1
is observed at 1606 cm , which suggested the formation of
37
Schiff base. In Fig. 3d, the two absorption bands centered at
ꢀ1
1547 and 1411 cm
are attributed to asymmetric and
Fig. 4 Scanning electron microscopic images of (a) IFGO and (b) Cu-
IFGO samples.
symmetric vibrations of copper-coordinated acetate ion,
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material, XPS analysis has been carried out. Fig. 7d represents
the full range XPS spectrum, which conrms the presence of
desired elements like copper, silicon, carbon, nitrogen and
oxygen without any other impurity elements. From Fig. 7a, the
narrow range XPS spectrum of Cu reveals two binding energy
values at 932.8 eV and 952.7 eV with a satellite peak at around
943–944 eV, and these are attributed to spin–orbit splitting
components of Cu 2p3/2 and Cu 2p1/2 in the +2 oxidation state of
copper in Cu-IFGO material and this result agrees very well with
41
the literature data. Furthermore, the satellite peak is a char-
2
+
acteristic feature of 2p / 3d satellite peak of Cu oxidation
state in Cu-IFGO material. In the case of the Cu-IFGO sample
the intensity ratio of satellite and Cu 2p3/2 peak is estimated to
2
+
be 0.52 which agrees well with that of pure and uniform Cu
ions. In the case of the Cu-IFGO composite, the narrow range
XPS spectrum of C 1s is shown in Fig. 7c, where a broad peak
appears from 282 eV to 289 eV. However, no separated sharp
peaks are observed like in the C 1s XPS spectrum of bare GO for
the non-oxygenated ring C atoms, C–OH, C–O–C and C atoms in
carboxyl groups, suggesting that successful surface functional-
42
ization has been achieved.
Fig. 5 HR-TEM images of (a and b) IFGO and (c and d) Cu-IFGO
samples.
Thermal stability and composition analysis
has been performed using the fresh catalyst at two different To explore the thermal stability of the three materials GO, IFGO
temperatures of 298 and 77 K (Fig. 6). The EPR pattern of Cu- and Cu-IFGO, TGA has been performed under air ow at
ꢁ
IFGO material clearly suggested a hexa-coordinated Cu(II) temperature ramp of 10 C per min from room temperature to
ꢁ
complex with octahedral geometry and corresponding g values 800 C for each material. Fig. 8A and B represent the TGA and
are g
x
¼ 2.45, g
y
¼ 2.06 and g
z
¼ 1.84 with average value of 2.11, DTA proles, respectively, for the three materials. As seen from
ꢁ
and corresponding magnetic elds are noticed at A ¼ 274, B ¼ Fig. 8A(a), the rst very signicant weight loss up to 105 C
39
3
27 and C ¼ 366 mT, respectively. However, it is pertinent to could be assigned to the evaporation of physically adsorbed
mention that such a hexa-coordinated Cu(II) site with octahe- water molecules from the material surface and the second
9
ꢁ
dral geometry and d electronic conguration will undergo weight loss aer 175 C is attributed to the decomposition of GO
40
Jahn–Teller distortion to minimize the energy of the system.
surface containing functional groups like hydroxyl, epoxy and
carboxyl groups. The last weight loss beyond 248 C could be
ꢁ
attributed to the decomposition of residual very stable fraction
X-ray photoelectron spectroscopy
of the material. In the case of IFGO (Fig. 8A(b)), similarly a rst
To determine the elemental composition as well as for evalua-
tion of the oxidation state of the metal center in Cu-IFGO
ꢁ
weight loss is observed up to 107 C due to moisture evapora-
tion from the material surface and a second weight loss up to
ꢁ
2
18 C is attributed to the cleavage of organic functional groups
present in the sample. Further, as seen from Fig. 8A(c), the rst
ꢁ
weight loss of Cu-IFGO material is observed up to 101 C for
vaporization of adsorbed moisture from the material surface
containing pores, and the second weight loss region from 268 to
ꢁ
4
60 C corresponds to cleavage of oxygen-bearing organic
functional groups of the material. The third weight loss above
ꢁ
4
60 C could be attributed to the decomposition of the
remaining part of the copper imine complex. This result
suggests the materials IFGO and Cu-IFGO have considerably
good thermal stability under aerobic conditions.
Further, from the AAS analysis, the copper content in freshly
prepared Cu-IFGO material has been estimated as 0.463 mmol
ꢀ1
g
. Since XPS analysis is regarded as the most authentic one for
the determination of the composition of the functionalized GO
materials, we have estimated the composition of Cu-IFGO
material based on XPS chemical analysis data. Using the
Vamas les of the XPS prole the elemental composition of Cu-
Fig. 6 Electron paramagnetic resonance spectra of Cu-IFGO sample
at two different temperatures: 298 K (black) and 77 K (red).
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Fig. 7 The XPS spectra of Cu-IFGO material for (a) copper, (b) silicon, and (c) carbon elements and (d) full range.
Fig. 8 TGA (A) and DTA (B) plots of three materials: (a) GO, (b) IFGO and (c) Cu-IFGO.
IFGO is calculated as (atomic%): C 40.74, O 31.36, N 2.44, Si calculated the loading of ligand which comes out to be 0.585
ꢀ
1
ꢀ1
1
6.45, Cu 9.01. Based on the fact that two imine moieties are mmol g . From the AAS analysis we observed 0.463 mmol g
bound per Cu(II) center in Cu-IFGO (Scheme 1) we have or 2.94 wt% Cu in the Cu-IFGO material. So compared to the Cu
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Fig. 9 (A) CO
2
adsorption/desorption isotherms of IFGO material at two different temperatures, 273 K and 298 K, up to 3 bar pressure region. (B)
2
Isosteric heat of adsorption (Qst) plot of IFGO material as a function of CO gas molecules adsorbed.
NaOH and (M/100) oxalic acid solution. This high surface
basicity of the IFGO material has motivated us to explore its
application in CO storage.
2
CO
The CO
different temperatures of 298 and 273 K up to 3 bar pressure.
Fig. 9A demonstrates the CO adsorption/desorption isotherms
at two different temperatures, which shows that with increasing
pressure up to 2.0 bar, the amount of CO uptake increases. In
2
adsorption analysis
2
adsorption/desorption study was performed at two
2
2
the high-pressure region (2.0 to 3.0 bar), the rate of CO2
adsorption onto the material surface gradually decreases, but
nevertheless the increasing nature of the adsorption isotherm
indicates that with further increasing pressure, the material can
2
Fig. 10 Reusability of IFGO material for CO uptake at 273 K.
2
have more CO uptake ability. On the other hand, the desorp-
ꢀ
1
tion isotherm indicates that the CO2 molecules have been
adsorbed physically, called physisorption. Since physisorption
is associated with a decrease in free energy and entropy of the
system, the process is exothermic. As seen from the gure, the
loading of 0.463 mmol g almost 79% of the ligand sites are
coordinated with Cu(II) centers.
Surface basicity measurement
total amounts of CO uptake for the IFGO sample are 8.10 mmol
2
ꢀ1
ꢀ1
For surface basicity measurement of the IFGO material, acid–
g
(35.64 wt%) and 2.14 mmol g (9.24 wt%) at 273 and 298 K,
base titration has been performed using phenolphthalein as an respectively. It is clearly noticeable that the amount of CO
indicator. The total surface basicity of the IFGO material is uptake is more for the ice-cold condition than for room
2
ꢀ
1
found to be 1.187 mmol g , which is notably high. The basicity temperature because the desorption process is more feasible
43
of polymer has been measured by taking (M/100) standardized with increasing temperature. By employing the Clausius–
Scheme 2 Schematic outline of one-pot three-component C–S cross coupling reaction in the presence of Cu-IFGO catalyst.
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Table 1 One-pot three-component C–S coupling reactions over Cu-IFGO
c
Entry no.
Aryl halide
Time (min)
C–S coupling product
Yield (%)
TON
74.3
a
1
15
86
84
2
15
72.5
3
4
12
15
85
81
73.4
70.0
5
6
7
8
9
12
12
15
15
15
83
73
70
79
78
71.7
63.0
60.4
68.2
67.3
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Table 1 (Contd.)
c
Entry no.
Aryl halide
Time (min)
C–S coupling product
Yield (%)
TON
53.5
b
1
0
10
62
a
Reaction was performed using 1 mmol 4-bromotoluene, 1.2 mmol thiourea and 1.1 mmol benzyl bromide over 25 mg Cu-IFGO catalyst at the same
ꢁ
b
reaction temperature of 140 C, for 15 min, with H
other reaction parameters. TON (turnover number) is the number of moles of substrate converted per mole of active Cu(II) sites (0.463 mmol g
present in the catalyst.
2
O (5 mL) and K
2
CO
3
(580 mg, 4.2 mmol). Reaction was carried out for 10 min without changing
c
ꢀ1
)
Clapeyron equation, the temperature dependence of sustainable and recyclable CO
adsorption/desorption equilibrium pressure can be described mental clean-up.
which provides the isosteric heat of adsorption (Qst) of the
2
storage material for environ-
ꢀ1
material, found to be 43.9 kJ mol . This result suggested
strong interaction between adsorbent and adsorbate species.
Catalysis
However, the value of heat of adsorption (Q ) is sufficiently less The functionalized Cu-IFGO material has been employed as
st
than the covalent bond energy which is also indicative of a heterogeneous catalyst for one pot C–S cross coupling reaction
a physisorption process. To check the reusability of the mate- using different aryl halides with thiourea and benzyl bromide
ꢁ
rial, the CO
2
adsorption/desorption study was carried out ve under microwave (MW) heating conditions at 140 C in water
uptake of 2.3 medium (Scheme 2). Under these microwave heating conditions
times, resulting in a very small decrease in CO
2
wt% (from 35.64 to 33.2 wt%) aer completion of the last cycle. no desired product was formed in the absence of any catalyst.
The reusability of the IFGO material has been demonstrated We have calculated the turnover numbers (TONs) of Cu-IFGO
using a bar diagram (Fig. 10), suggesting it to be a highly for the production of different diarylsulde products and
these are shown in Table 1. The product yields are much higher
Scheme 3 Probable reaction mechanism for production of thioether products over Cu-IFGO catalyst.
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than that of homogeneous catalysts. Very little amount of Table 2 Dependence of thiourea product yield and TON value on
different solvents and temperatures in one-pot three-component
coupling reaction over microporous Cu-IFGO catalyst
product (yield 27%) was obtained when 4-bromotoluene is
replaced by n-propyl chloride, suggesting aliphatic halides are
a
not suitable for this C–S coupling reaction. A possible reaction
mechanism for the C–S coupling reaction over Cu-IFGO as
a heterogeneous catalyst has been proposed and is shown in
Scheme 3. In the presence of a base, thiourea reacts with benzyl
bromide via an electrophilic reaction pathway to produce (S)-
alkylisothiouronium salt, followed by the hydrolysis of this salt
to form thiolate anions and urea. Then under treatment of
microwave heating, urea can be converted to ammonia (g) and
carbon dioxide (g) during the course of the reaction. Thiolate
anions could be involved in nucleophilic reaction with an
intermediate produced from oxidative addition step of aryl
halides with Cu(II)-graed material to provide organocopper
sulde as an intermediate. Upon reductive elimination of the
intermediate, various substituted diarylsuldes bearing
electron-donating and electron-withdrawing groups are ob-
tained. In the cross coupling reaction, temperature, loading of
the active metal in the catalyst and solvent play crucial roles for
product yield, and these are discussed below.
ꢁ
b
Entry no.
Solvent
Temperature ( C)
Isolated yield (%)
TON
1
2
3
4
5
6
7
8
9
H
H
H
H
H
2
2
2
2
2
O
O
O
O
O
110
120
130
140
150
140
140
140
140
34
48
69
86
86.5
45
12
52
55
29.3
41.4
59.6
74.3
74.7
38.8
10.3
44.9
47.5
EtOH
THF
CH
3
CN
DMF
a
b
Reaction conditions are similar to those of Table 1. Isolated product
yields (entries 1–5) are obtained from C–S coupling reaction over Cu-
IFGO catalyst by varying reaction temperature only in aqueous medium.
2 2
CuCl $2H O as a homogeneous catalyst where the product yield
was 46% only. Maximum product yield (86%) was obtained
when that cross coupling reaction was performed using Cu-
IFGO material as a heterogeneous catalyst. So, it is concluded
that catalytic activity originates from the reactive Cu(II) species
at the surface of the Cu-IFGO material. More active Cu(II) site as
well as microporosity in Cu-anchored Schiff base-immobilized
GO material have crucial roles in increasing the diarylsulde
product yields. In order to standardize the catalyst loading
amount during the course of coupling reaction, we have per-
formed some control experiments using the former three
components as a representative example. For this reaction, we
have chosen 20, 25, 30 and 40 mg of the catalyst. It has been
observed that 25 mg of catalyst loading is a more appropriate
amount for effectively carrying out this coupling reaction than
the other amounts. Further increase in the catalyst loading
beyond 25 mg does not affect the product yield signicantly.
The dependence of thioether product yields on catalyst loading
amount is listed in Table 3. Moreover, we have varied the nature
of the substituent groups in the aryl halide. As seen in the
Effect of reaction temperature, time and solvents
In the three-component coupling reaction between 4-bromoto-
luene, thiourea and benzyl bromide over Cu-IFGO, temperature
and reaction time play important roles under microwave heat-
ing conditions. It is observed that with increasing reaction
temperature the yield of product increases, but the maximum
ꢁ
yield is observed at 140 C. By employing the thin-layer chro-
matographic technique the conversion of the starting material
to product has been conrmed aer completion of the reaction.
It is pertinent to mention that with further increasing the
temperature, the intensity of product spot does not change
signicantly. Also, by varying the reaction time, we have
checked the thin-layer chromatogram of the reaction mixture to
analyze the maximum thioether product yield in each case of
aryl halide. In Table 2 the temperature and solvent dependence
of the product yield (%) is shown. We have used ve different
solvents which are H O, EtOH, THF, CH CN and DMF for the
2
3
solvent optimization in this one-pot three-component thio-
esterication reaction. It is noticed that the product yield ob-
Table 3 Dependence of isolated product yield obtained over various
tained in the aqueous medium is more than that in other catalysts as well as amount of catalyst loading in one-pot three-
ꢁ
a
solvents due to the high solubility of thiolate ion in water. Thus, component coupling reaction at 140 C
under optimum reaction conditions for this C–S coupling
reaction, use of Cu-IFGO led to 86.0% thioether yield at 140 C
in aqueous medium under microwave-assisted heating.
ꢁ
Time
(min)
Catalyst
loading (mg)
Product
yield (%)
b
c
Entry no.
Catalyst
1
2
3
4
5
6
7
8
GO
AFGO
IFGO
Cu-IFGO
Cu-IFGO
Cu-IFGO
Cu-IFGO
20
20
20
15
15
15
15
15
30
30
30
20
25
30
35
25
0
0
0
72
86
87
87
46
Catalyst variation
To understand the role of the graed Cu(II) species in the Cu-
IFGO catalyst, we have carried out the same reaction by
utilizing various catalysts, i.e. GO, AFGO, IFGO, Cu-IFGO and
ꢁ
CuCl
2
$2H
2
O, in aqueous medium at 140 C. No satisfactory
2 2
CuCl $2H O
results were obtained in one-pot three-component reaction
using the rst three catalysts which contain no copper. Thus, we
have carried out the same experiment as a model example with
a
b
Reaction conditions are similar to those of Table 1. All the coupling
c
reactions were performed using various types of catalyst. Product yield
(entries 4–7) was obtained by varying the amount of Cu-IFGO catalyst.
4-bromotoluene, thiourea and benzyl bromide using
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product yields from entries 6 and 7 of Table 1, the desired C–S Leaching and hot ltration test
coupled products from the electron-rich haloarenes are difficult
48
For this cross-coupling reaction a hot ltration experiment
has been carried out to understand whether leaching of Cu(II)
took place from the Cu-IFGO material. In this experiment at
to obtain in high yields compared with the electron-decient
44
haloarenes. But the product yields are also dependent upon
the position of substituent groups in the haloarenes. For
instance as seen from entries 1 and 2 (Table 1), yields are very
close to each other but the para-substituted haloarene provides
considerably more yield compared to the meta-substituted one.
Some reported C–S coupling reactions are mentioned as
follows to compare the product yield with the thioether product
yield obtained using our Cu-IFGO material as catalyst. Feng
et al. reported C–S cross-coupling reaction with high yield of
thioether product using various copper catalysts. Although
most of those catalysts exhibit high catalytic activity, being
homogeneous in nature, their separation from the reaction
mixture as well as reusability of the catalyst are major draw-
backs compared to heterogeneous catalysts. As seen from Table

rst, the reaction is stopped aer 6 h and the reaction mixture
containing the dispersed catalyst is ltered under hot
conditions. Then the same reaction is continued for another 6
h using that ltrate without any change of the reaction
parameters. Aer completion of the reaction almost no
further increase in product yield (49.3% aer 12 h as
compared to 49.1% aer 6 h) is observed, suggesting there
was no leaching of Cu from the material surface during this
coupling reaction. Also, AAS analysis has been carried out to
determine the Cu content in the ltrate; this analysis could
not detect the presence of Cu in the solution. This result
suggested that the Cu-IFGO catalyst is heterogeneous in
nature due to strong binding ability of the Cu(II) sites with the
imine (–N]C) and phenolic (–OH) functional groups present
in the DFP moiety in the material framework. Robustness of
this Cu-anchored functionalized GO material is further
conrmed from the reusability test experiments. However, it
45
1, our Cu(II)-anchored functionalized graphene catalyst showed
comparable catalytic activity to the polycrystalline CuO nanoc-
46
ages and Cu O nanoframes reported by LaGrow et al. and
2
copper nanoparticles stabilized in a porous chitosan aerogel
47
reported by Garcia et al. for the C–S coupling reaction of
different aromatic halides.
0
II
is pertinent to mention that apart from Cu /Cu , few other
catalytic systems including mesoporous polyaniline
composite with porous magnetic Fe O can act efficient
3
4
catalysts for C–S bond formation reaction in water as reported
by Damodara et al.
4
9
Reusability test
To check the reusability of the Cu-graed imine-functionalized
GO catalyst, one-pot C–S coupling reaction has been carried out
successively under the same optimized reaction conditions.
Conclusion
Aer completion of reaction the catalyst was collected from the In summary, we have reported the synthetic strategy for new
reaction mixture by simple ltration and washed thoroughly functionalized GO and Cu(II)-graed immobilized GO mate-
with deionized water to remove the excess K CO3 base and rials via co-condensation as well as Schiff base condensation
2
soluble unreacted thiourea. Then it was washed with ethyl reaction followed by impregnation of copper. The microporous
ꢀ
1
acetate to get rid of the organic reactant molecules. To reuse the IFGO exhibited a CO uptake capacity of 8.10 mmol g (35.64
2
ꢀ1
catalyst for the same coupling reaction in the next run it was wt%) and 2.14 mmol g (9.24 wt%) at 273 K and 298 K,
dried in oven at 75 C for 2 h. The catalytic efficiency of the Cu- respectively. Furthermore, the Cu-graed microporous mate-
ꢁ
IFGO material for different cycles is illustrated in Fig. 11. It is rial revealed high catalytic activity and recycling potency in
noticed that aer completion of ve reaction cycles the total one-pot three-component C–S cross coupling reaction of
yield decreased by about 8.0%. As seen from this bar diagram, various aryl halides, thiourea and benzyl bromide in aqueous
only very little decrease in the product yield occurred aer each medium under microwave-assisted heating conditions. The
reaction cycle and this result suggested that Cu-IFGO is a highly synthesis of value-added C–S coupling products over the Cu-
recyclable heterogeneous catalyst.
graed imine-functionalized microporous GO as described
herein is a very efficient and environmentally friendly pathway.
Thus, this result may motivate researchers to design func-
2
tionalized GO materials and use them as CO storage materials
for environmental clean-up as well as heterogeneous catalysts
in various catalytic reactions, which could offer a sustainable
future.
Acknowledgements
PB and SKD wish to thank CSIR, New Delhi and UGC, New Delhi
for their respective senior and junior research fellowships. AB
wishes to thank DST, New Delhi for use of instrumental facili-
ties through the DST Unit on Nanoscience and DST-UKIERI
project grants.
Fig. 11 Recycling efficiency of Cu-IFGO in the C–S coupling reaction
between benzyl bromide, thiourea and 4-bromotoluene.
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