Magnetic covalent hybrid of graphitic carbon nitride and graphene oxide as an efficient catalyst support for immobilization of Pd nanoparticles

Article abstract of DOI:10.1016/j.ica.2018.12.048

For the first time a magnetic carbon based hybrid catalyst, Pd@g-C₃N₄-Fe-GO, is prepared through covalent conjugation of magnetic graphitic carbon nitride and graphene oxide followed by incorporation of Pd nanoparticles. First, the formation of the catalyst was confirmed via XRD, TG, BET, TEM, FTIR, ICP and VSM analyses and then its catalytic activity for promoting Suzuki and Sonogashira coupling reactions under mild reaction condition was investigated. To elucidate whether hybridization of two carbon materials could improve the catalytic activity, the catalytic activity of the catalyst was compared with the control catalysts (Pd@g-C₃N₄ -GO, Pd@g-C₃N₄-Fe, Pd@ Fe-GO, Pd@g-C₃N₄, Pd@GO and the GO/g-C₃N₄ physical hybrid). Moreover, the role of magnetic nanoparticles in the catalytic performance was confirmed. Notably, the catalytic activities of the catalyst and the control sample prepared via physical hybridization of two carbon materials were compared to confirm the effect of covalent conjugation on the catalytic activity. Moreover, the study of the substituent effect of p-substituted phenyl iodides was considered by Hammett plot, which revealed a beneficial effect of electron-withdrawing side groups for the C–C coupling reaction. Finally, the recyclability of Pd@g-C₃N₄-Fe-GO as well as leaching of Pd and magnetic nanoparticles was studied.

Full text of DOI:10.1016/j.ica.2018.12.048

InorganicaChimicaActa488(2019)62–70
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Research paper
Magnetic covalent hybrid of graphitic carbon nitride and graphene oxide as
an efficient catalyst support for immobilization of Pd nanoparticles
⁎
⁎
Samahe Sadjadia, , Masoumeh Malmirb, Majid M. Heravib, , Fatemeh Ghoreyshi Kahangic
a Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemicals Institute, PO Box 14975-112, Tehran, Iran
b Department of Chemistry, School of Science, Alzahra University, PO Box 1993891176, Vanak, Tehran, Iran
c Department of Chemistry, University Campus 2, University of Gilan, Rasht, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
For the first time a magnetic carbon based hybrid catalyst, Pd@g-C3N4-Fe-GO, is prepared through covalent
conjugation of magnetic graphitic carbon nitride and graphene oxide followed by incorporation of Pd nano-
particles. First, the formation of the catalyst was confirmed via XRD, TG, BET, TEM, FTIR, ICP and VSM analyses
and then its catalytic activity for promoting Suzuki and Sonogashira coupling reactions under mild reaction
condition was investigated. To elucidate whether hybridization of two carbon materials could improve the
catalytic activity, the catalytic activity of the catalyst was compared with the control catalysts (Pd@g-C3N4 -GO,
Pd@g-C3N4-Fe, Pd@ Fe-GO, Pd@g-C3N4, Pd@GO and the GO/g-C3N4 physical hybrid). Moreover, the role of
magnetic nanoparticles in the catalytic performance was confirmed. Notably, the catalytic activities of the
catalyst and the control sample prepared via physical hybridization of two carbon materials were compared to
confirm the effect of covalent conjugation on the catalytic activity. Moreover, the study of the substituent effect
of p-substituted phenyl iodides was considered by Hammett plot, which revealed a beneficial effect of electron-
withdrawing side groups for the CeC coupling reaction. Finally, the recyclability of Pd@g-C3N4-Fe-GO as well as
leaching of Pd and magnetic nanoparticles was studied.
Graphitic carbon nitride
Graphene oxide
Magnetic catalyst
Coupling reactions
Pd nanoparticles
1. Introduction
results summarized in that review article confirmed that hybridization
of graphene and g-C3N4 can be considered as a solution for cir-
Graphitic carbon nitride (g-C3N4), the most stable carbon nitride
allotrope, is a graphite-like metal-free polymeric carbon nanomaterial
containing potent CeN bonds in its layers [1]. To date, g-C3N4 has been
used in diverse range of applications including (photo) catalysis, de-
tection of hazardous gases, adsorbents, fuel cells, purification of con-
taminants, etc [2–6]. Despite outstanding thermal and optical proper-
ties, g-C3N4 suffers from some drawbacks such as low specific surface
area, low electronic conductivity and wide band gap [7] that limit its
applications in the catalysis.
cumventing the drawbacks of these two carbon materials.
Suzuki and Sonogashira reactions are potent tools for the synthesis
of natural products and biologically active organic compounds [14].
These reactions are classically promoted with homogeneous Pd-based
catalysts in the presence of phosphine ligand and co-catalyst. Despite all
efforts to develop eco-friendly and cost effective protocols for these
reactions, tedious work-up, recovery and use of toxic solvents remain as
the most challenges [15,16]. To furnish a solution to these issue, use of
heterogeneous catalysts has been suggested [17–25].
Graphene that is a two dimensional carbon sheet [8,9] with high
surface area, thermal conductivity, chemical stability and mechanical
strength is extensively utilized for catalytic purposes [10,11]. However,
it is proved that graphene itself exhibits low catalytic activity. Mostly,
to improve the catalytic activity of graphene it is hybridized with other
materials or chemically modified [7,12,13].
In the following of our interest in the synthesis of hybrid catalysts
contain carbon materials [26–28], herein we wish to report a novel
hybrid support, prepared via synthesis and functionalization of g-C3N4
followed by incorporation of magnetic nanoparticles (MNPs) and
covalent conjugation with GO. The support was then utilized for im-
mobilization of Pd nanoparticles and synthesis of Pd@g-C3N4-Fe-GO
catalyst. The catalytic activity and recyclability of the catalyst were also
studied for Suzuki and Sonogashira reactions (Scheme 1). Moreover,
the role of each hybrid component in the catalysis and the effect of
Recently, Qu et al. reviewed the recent advances in the design,
synthesis and utility of graphene/graphitic carbon nitride hybrid sys-
tems for photo-degradation, O2 evolution, water splitting etc. [7]. The
⁎ Corresponding authors.
E-mail addresses: s.sadjadi@ippi.ac.ir (S. Sadjadi), m.heravi@alzahra.ac.ir (M.M. Heravi).
https://doi.org/10.1016/j.ica.2018.12.048
Received 10 November 2018; Received in revised form 27 December 2018; Accepted 28 December 2018
Availableonline29December2018
0020-1693/©2018PublishedbyElsevierB.V.

S. Sadjadi et al.
InorganicaChimicaActa488(2019)62–70
Scheme 1. Model reaction of a) Sonogashira and b) Suzuki coupling reactions.
covalent hybridization was confirmed by comparison of the catalytic
activity of Pd@g-C3N4-Fe-GO with that of Pd@g-C3N4 -GO, Pd@g-C3N4-
Fe, Pd@ Fe-GO, Pd@g-C3N4, Pd@GO and the GO/g-C3N4 physical hy-
brid. Furthermore, the catalyst recyclability and Pd and Fe leaching
were also studied.
2.2.2. Preparation of MNPs
MNPs were prepared according to the previously reported proce-
dure [29–31]. Typically, FeCl3·6H2O (20 mmol) and FeCl2·4H2O
(10 mmol) were dissolved in 100 mL of distilled water into a two-
necked round-bottom flask (250 mL) and heated at 90 °C under N2 at-
mosphere for 1 h. Then, a solution of concentrated aqueous ammonia
(10 mL) was added into a solution. After cooling, the resulting magnetic
particles 2 was collected by using an external magnet, washed with
distilled water and dried in a vacuum oven at 60 °C for 12 h.
2. Experimental section
2.1. Materials and instruments
2.2.3. Synthesis of magnetic g-C3N4
Pd@g-C3N4-Fe-GO was synthesized by using the following reagents
and solvents: GO, FeCl3·6H2O, FeCl2·4H2O, K2Cr2O7, H2SO4 ammonia,
PdCl2, urea, thiosemicarbazide, HCl, MeOH, EtOH, NaBH4, all pur-
chased from Sigma-Aldrich and used as received. To investigate the
catalytic activity of Pd@g-C3N4-Fe-GO, two types of CeC coupling re-
actions, i.e. Suzuki and Sonogashira coupling reactions were performed
by using acetylene, aryl boronic acid, halobenzene, distilled water and
EtOH. All the reagents were of analytical grade and provided from
Sigma-Aldrich. Monitoring of the coupling reactions was carried out by
using TLC method on commercial aluminum-backed plates of silica gel
60 F254, visualized by using ultraviolet light.
In order to synthesize magnetic porous g-C3N4 nano sheet, Fe3O4
(1 g) was suspended in a 50 mL of HCl solution (6 M). Then, the re-
sulting mixture was sonicated at ambient temperature. The temperature
of the reaction mixture was kept constant by using icy water bath. After
0.5 h, g‐C3N4 (1 g) was introduced into the mixture and the resulting
suspension was sonicated for further 1 h. Then, the reaction mixture
was transferred to the 150 mL autoclave and heated at 220 °C over-
night. Upon completion of the hydrothermal process, the as‐prepared 3
was collected magnetically, washed several times with EtOH/H2O and
dried at 80 °C for 12 h in oven.
Synthesize of the catalyst was accomplished by using the following
apparatus: autoclave of 150 mL and ultrasonic instrument (Bandelin HD
3200 with output power of 150 W and tip TT13). The structure of Pd@
g-C3N4-Fe-GO was confirmed by using METTLER TOLEDO thermo
gravimetric analysis instrument (heating rate of 10 °C min−1, under
inert (N2) atmosphere), PERKIN-ELMER- Spectrum 65 instrument, ICP
analyzer (Varian, Vista-pro), CM30300Kv field emission transmission
electron microscope, Siemens D5000 (for XRD analysis), vibrating
sample magnetometer (VSM, Lakeshore 7407) at room temperature and
BELSORP Mini II instrument (degassing of the sample for 3 h at 423 K).
2.2.4. Synthesis of amine functionalized magnetic g-C3N4
The amine functionalized magnetic g-C3N4 was obtained by dissol-
ving thiosemicarbazide (0.8 g) in the stirring suspension of magnetic g-
C3N4 (1.5 g) in MeOH (50 mL) followed by refluxing overnight. Upon
completion, the resulting solid was filtered off, washed with hot MeOH
repeatedly and dried at 100 °C overnight.
2.2.5. Hybridization of magnetic g-C3N4 with graphene oxide: Synthesis of
g-C3N4 –Fe-GO
The reaction mixture was formed by adding dispersed magnetic g-
C3N4 nano sheet (1.3 g in 25 mL water) into GO solution (0.7 g in 25 mL
water) under sonication. After sonicating the mixture for 20 min, it was
heated to 90 °C and stirred for 6 h vigorously. Upon completion of the
reaction, the resultant solid was separated magnetically, washed with
hot EeOH repeatedly and dried at 60 °C overnight.
2.2. Catalyst preparation
2.2.1. Synthesis of porous g-C3N4 nano sheet: g-C3N4-OX
Initially, the g‐C3N4 nano‐sheet was prepared through urea pyrolysis
[26,27]. Typically, 10 g of urea was grounded into powder, then heated
and calcined at 450 °C for 4.5 h with a heating rate of 1.6 °C/min. The
resulting light yellow product was then oxidized via the chemical oxi-
dation with K2Cr2O7/H2SO4. In detail, 1 g of the g-C3N4 nano sheet was
dissolved in the solution of K2Cr2O7 (20 g in 100 mL of H2SO4) and then
stirred thoroughly for 2 h at room temperature. Next, the mixture was
slowly poured into 400 mL of deionized water and cooled to room
temperature. After centrifugation at 10000 rpm, the resulting light
yellow solid was dialyzed in a dialysis bag to remove all residual acids,
and dispersed in water for 1 h. Finally, the obtained cream suspension 1
was dried at 80 °C overnight.
2.2.6. Immobilization of Pd NPs on the g-C3N4 –Fe-GO: Pd@g-C3N4 –Fe-
GO
To immobilize Pd nanoparticles on g-C3N4 –Fe-GO, g-C3N4 –Fe-GO
(1.5 g) was dispersed in EtOH (40 mL) by sonication for 0.5 h. Then, a
solution of PdCl2 (0.15 g) in 10 mL deionized water was added into the
suspension under stirring condition overnight. Subsequently, a solution
of NaBH4 (0.2 M, 10 mL) was added into the mixture in a drop wise
manner. Subsequently, the resulting mixture was stirred for 3 h. Then,
the product 5 was magnetically collected, washed with MeOH/H2O for
three times and dried at 60 °C for 12 h (Fig. 1).
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InorganicaChimicaActa488(2019)62–70
Fig. 1. The procedure for the synthesis of Pd@g-C3N4-Fe-GO.
2.3. C-C coupling reactions
accordance with the previous reports and shows the characteristic
bands [32] at 3000–3392 cm−1 (NeH and OeH groups) and
1247–1633 cm−1 (CeN group). In the FTIR spectrum of g-C3N4-OX, not
only the characteristic bands of g-C3N4 can be observed, but also an
additional band at 1691 cm−1 is detectable that can be due to the eC]
O functionalities. As illustrated, the characteristic bands of MNPs are
appeared at 1620 cm−1, 3424 cm−1, 633 cm−1 and 599 cm−1. The
2.3.1. Typical procedure for Sonogashira reaction
Halobenzene (1.0 mmol), acetylene (1.0 mmol), Pd@g-C3N4-Fe-GO
(1.5 mol%) and K2CO3 (2.0 mmol in 5.0 mL water) as a base were mixed
in water (5.0 mL). The resulting mixture was then heated at 45 °C for
appropriate reaction time. Upon completion of the reaction (traced by
TLC, ratio of n-hexane/ethyl acetate: 8/2), Pd@g-C3N4-Fe-GO was
collected magnetically, washed with EtOH three times and dried in
oven at 100 °C overnight. On the other hand, the filtrate, was cooled to
room temperature and the organic layer was extracted with diethyl
ether (10 mL). The resulting coupling product was purified by column
chromatography over silica gel (Scheme 1a).
FTIR spectrum of GO exhibited the bands at 1622 cm−1, 1711 cm−1
,
3418 cm−1 and 1041 cm−1. The FTIR spectrum of the catalyst ex-
hibited the characteristic bands of MNPs, g-C3N4 and GO. Notably, as
the most of the characteristic bands of these components overlapped,
FTIR solely cannot confirm the formation of the catalyst.
In Fig. 3, the thermograms of GO, g-C3N4 and the catalyst are de-
picted. The observed mass loss in the thermogram of g-C3N4 (at
∼670 °C) can be attributed to the degradation of g-C3N4 sheet [33]. In
the thermogram of GO a mass loss can be detected at ∼170 °C that can
be assigned to the detachment of oxygen-containing functional groups
from the surface of GO. Comparing the thermogram of the catalyst with
that of GO and g-C3N4, it can be concluded that the thermal stability of
the catalyst is higher than GO and lower than g-C3N4. More precisely,
two mass losses can be observed in the thermogram of the catalyst, the
first one, observed at ∼175 °C is due to the detachment of oxygen-
containing functional groups from the surface of GO and the next one,
occurred at 590 °C, is stemmed from g-C3N4 degradation.
2.3.2. Typical procedure for Suzuki reaction
Aryl halide (1.0 mmol) and aryl boronic acid (1.2 mmol) were
mixed and dissolved in water/EtOH (2.5:2.5, 10 mL) and then K2CO3
(2.0 mmol) and Pd@g-C3N4-Fe-GO (2.5 mol %) were added into a
mixture. The flask containing reaction mixture was then placed in an oil
bath (120 °C) and stirred for appropriate time. At the end of the reaction
(traced by TLC, ratio of n-hexane/ethyl acetate: 7/3), the mixture was
cooled to room temperature and the catalyst was separated magneti-
cally and washed with EtOH three times and dried in oven at 100 °C
overnight. Finally, the solvent was removed under reduced pressure
and the product was extracted with hexane (10 mL). The resulting
coupling product was purified by column chromatography over silica
gel (Scheme 1b).
In Fig. 4 the XRD pattern of the catalyst is illustrated. As depicted, a
broad halo can be observed at 2θ = 17–27 that is representative of the
g-C3N4. Moreover, the observed peaks at 40°, 46.4°, 69.2° and 81°
(JCPDS, Card No. 46-1043) can also be assigned to Pd nanoparticles
[34]. According to the literature, he XRD pattern exhibit the char-
acteristic peaks of hematite (ICDD Card NO. 33–0664) [35,36]. The
peaks appearing at 2θ range of 24.1°, 33.1°, 36°, 40.7°, 50.2°, 54.1°,
57.42°, 64.3° and 65.7° can be attributed to the 012, 104, 110, 113,
024, 116, 018, 214 and 300 crystalline structures corresponding to pure
ɑ-Fe2O3 nanoparticles [35,36].
3. Result and discussion
3.1. Catalyst characterization
To verify the formation of the catalyst, it was subjected to various
analyses. First, the FTIR spectrum of Pd@g-C3N4-Fe-GO was recorded
and compared with the spectra of GO, g-C3N4, MNPs and g-C3N4-OX. As
illustrated in Fig. 2, the FTIR spectrum of g-C3N4 is in a good
Notably, in the XRD pattern of the catalyst, the characteristic peaks
of GO (Fig. S1) at 2θ = 10.5° and 42.4° were not observed. This can be
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InorganicaChimicaActa488(2019)62–70
Fig. 2. The FTIR spectra of MNPs, GO, g-C3N4, g-C3N4-OX and Pd@g-C3N4-Fe-GO.
attributed to the reduction of GO in the course of the preparation
process [37].
Further characterization of Pd@g-C3N4-Fe-GO was carried out by
performing BET analysis. The nitrogen adsorption–desorption isotherm
of the catalyst, Fig. S2, showed type IV isotherms [38] with H3 hys-
teresis loops [39]. Using BET data, the specific surface area of the
catalyst was estimated to be 35 m2g−1 that is lower than that of GO
(150 m2g−1) and almost similar to that of g-C3N4 (36 m2g−1).
Metal loading of Pd@g-C3N4-Fe-GO was also measured via ICP
analysis. In this line, sample preparation was performed according to
the literature [26] and ICP analysis was carried out for measuring Fe
and Pd atoms. Following this method, the contents of Fe and Pd was
estimated to be 6 and 1 wt% respectively.
To study the magnetic feature of Pd@g-C3N4-Fe-GO, room tem-
perature vibrating sample magnetometer (VSM) was applied and the
maximum saturation magnetization (Ms) value of the catalyst was es-
timated and compared with that of bare magnetic nanoparticle. The
3.2. Catalytic activity
Initially, to optimize the reaction conditions, two model Suzuki and
Sonogashira reactions, reaction of benzyl iodide and boronic acid and
reaction of phenyl acetylene and benzyl iodide, were selected. Then, the
reaction conditions were optimized by varying the reaction variables,
i.e., reaction temperature, solvent, the required amount of the catalyst
and type of the base (Table S1). Upon disclosing the optimum reaction
condition, it was studied whether hybridization of GO with g-C3N4 and
incorporation of MNPs could improve the catalytic activity. To this
purpose, several control catalysts were prepared and their catalytic
activity for the model coupling reactions were compared with that of
results showed that the Ms value of Pd@g-C3N4-Fe-GO (31.15 emu g−1
)
is lower than that of bare magnetic nanoparticle (47 emu g−1), Fig. 5.
In Fig. 6 the TEM images of g-C3N4 –Fe-GO and Pd@g-C3N4-Fe-GO
are illustrated. As shown, in the TEM image of g-C3N4 –Fe-GO the g-
G3N4 sheet can be detected. Moreover, MNPs can be observed as black
spots on the g-G3N4 sheet. In the case of Pd@g-C3N4-Fe-GO (Fig. 6B),
apart from the fine g-G3N4 sheet, the GO sheet can be observed as
darker sheet.
Fig. 3. The TG analyses of the g-C3N4, GO and Pd@g-C3N4-Fe-GO.
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InorganicaChimicaActa488(2019)62–70
Fig. 4. The XRD analysis of the Pd@g-C3N4-Fe-GO.
Fig. 5. The VSM analyses of the bare magnetic NPs and Pd@g-C3N4-Fe-GO catalyst.
Fig. 6. The TEM images of the g-C3N4 –Fe-GO (A) and Pd@g-C3N4-Fe-GO (B).
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InorganicaChimicaActa488(2019)62–70
Table 1
Comparison of the catalytic activity of the present catalyst with the other prepared catalysts in the coupling reactions.
Entry
Catalyst
Sonogashira coupling reactiona
Suzuki coupling reactionb
Yieldg (%)
Time (min)
TOF (h−1
)
Yieldh (%)
Time (min)
TOF (h−1
)
1
2
3
4
5
6
7
Pd@g-C3N4-Fe-GO
Pd@g-C3N4-GOc
Pd@g-C3N4-Fe-GO*d
Pd@Fe-GOe
92
85
50
60
75
40
30
70
5287
4885
2873
3448
4310
888
94
80
68
55
75
50
45
120
120
120
120
120
180
180
1880
1600
1360
1100
1500
666
70
70
70
Pd@g-C3N4-Fef
Pd@g-C3N4
70
180
180
Pd@GO
666
600
a
Reaction condition: aryl halide (1.0 mmol), terminal alkyne (1.2 mmol), catalyst (1.5 mol%), K2CO3 (2.0 mmol) in water (5.0 mL) at 45 °C.
b
c
d
e
f
Reaction condition: aryl halide (1.0 mmol), boronic acid (1.2 mmol), Pd@g-C3N4-Fe-GO (2.5 mol%), K2CO3 (2.0 mmol) in water:EtOH (2.5:2.5, 10 mL) at 120 °C.
Non-magnetic catalyst (without Fe2O3).
Without thiosemicarbazide.
Palladium catalyst without g-C3N4.
Palladium catalyst without GO.
Isolated yield.
g
h
Isolated yield.
that the unpaired electrons on the GO can facilitate electron transfer to
metal or other active centers [40].
Table 2
Pd@g-C3N4-Fe-GO catalyzed Sonogashira reaction of various halides with
terminal alkynes.a
Entry
Then, the contribution of MNPs in the catalysis was investigated. In
this line, Pd@g-C3N4 –GO was prepared and applied as a catalyst for the
model coupling reactions. The comparison of the catalytic activity of
Pd@g-C3N4-Fe-GO with Pd@g-C3N4 –GO confirmed that MNPs could
improve the catalytic activity to some extent. Notably, the main role of
MNPs is in catalyst recovery. Notably, it can be assumed that the in-
teraction between Pd and Fe may affected the surface diffusion of Pd
atoms during immobilization, which led to the formation of fine hybrid
system [41].
X
R1
R2
Time
Yield (%) TOF
(h−1
(min)
)
1
I
H
Ph
70
92
80
90
80
85
85
95
95
95
80
90
88
73
65
5287
2
I
p-Me
Ph
255
195
220
195
120
90
1254
1846
1457
1743
2833
4222
4222
3166
2792
4800
3534
2162
1575
3
I
p-OMe
p-COMe
p-NO2
1-naphtalene
H
Ph
4
I
Ph
5
I
Ph
6
I
Ph
7
I
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
Ph
Next, it was investigated whether the covalent conjugation of GO
and g-C3N4 could affect the catalytic activity. To this purpose, a control
catalyst Pd@g-C3N4-Fe-GO* was prepared through immobilization of
Pd nanoparticles on the physical hybrid of g-C3N4 and GO (see SI).
Interestingly, this control catalyst exhibited lower catalytic activity
compared to Pd@g-C3N4-Fe-GO, implying the remarkable role of
covalent conjugation of Go and g-C3N4 in the catalysis.
8
I
p-OMe
p-Me
90
9
I
120
115
75
10
11
12
13
14
I
p-COMe
p-NO2
1-naphtalene
H
I
I
100
135
165
Br
Cl
H
Ph
Pd@g-C3N4-Fe-GO catalyzed Suzuki-Miyaura reaction of various halides with terminal
Confirming the role of each component of the hybrid system in the
catalysis, the scope of the developed protocols for Sonogashira and
Suzuki coupling reaction was studied by synthesis of various coupling
derivatives, Table 2. As illustrated, Pd@g-C3N4-Fe-GO could promote
the reaction of various substrates with different electronic and steric
features and furnish the corresponding products in high yields. As ex-
pected, the less active aryl halides, i.e. aryl chlorides and aryl bromides
led to the corresponding products in longer reaction times and lower
yields. Moreover, use of aryl halides with electron withdrawing groups
was more successful and higher yields of products were achieved. No-
tably, the steric substrates, 1-naphtalene, could also tolerated the re-
action to afford the corresponding product in 85% yield.
alkynesb
15
16
17
18
19
20
21
22
I
H
120
160
140
135
120
180
280
330
94
80
86
92
90
85
82
80
1880
1130
1476
1635
1800
1133
703
I
p-Me
I
p-OMe
p-CO2Me
p-NO2
1-naphtalene
H
I
I
I
Br
Cl
H
581
a
Reaction condition: aryl halide (1.0 mmol), terminal alkyne (1.2 mmol),
Pd@g-C3N4-Fe-GO (1.5 mol%), K2CO3 (2.0 mmol) in water (5.0 mL) at 45 °C.
b
Reaction condition: aryl halide (1.0 mmol), boronic acid (1.2 mmol), Pd@
g-C3N4-Fe-GO (2.5 mol%), K2CO3 (2.0 mmol) in water:EtOH (2.5:2.5, 10 mL) at
120 °C.
We further obtained a Hammett plot to measure quantitatively the
effects of electronic changes in phenyl iodide on the rates of
Sonogashira and Heck reaction. The relative rates of coupling of p-
substituted phenyl iodide (p-H, p-NO2, p-Me, p-OMe) were investigated.
A linear relationship was found between log (kX/kH) and σp – (Figs. 7
and 8). The slope of the plots show reaction constant ρ values of 0.33
and 0.27 for the studied phenyl iodide derivatives for Sonogashira and
Heck coupling reactions. These results suggest development of a ne-
gative charge at the reaction center.
Pd@g-C3N4-Fe-GO, Table 1.
First, Pd@Fe-GO and Pd@ Fe-g-C3N4 were prepared (see SI) and
their catalytic activities for both Sonogashira and Suzuki model reac-
tions were studied. As tabulated in Table 1, the yields of the model
products was lower than that of Pd@g-C3N4-Fe-GO, indicating that
hybridization of GO and g-C3N4 could effectively improve the catalytic
activity. The incorporation of GO can improve the specific surface area
of the catalyst. In other word, the specific surface area of g-C3N4 (with
no metal loading) was 36 m2g−1 that can be significantly reduced upon
incorporation of Fe and Pd nanoparticles. However, in the hybrid
system of g-C3N4 and Go, the specific surface area of the catalyst after
metal loading was measured to be 35 m2g−1. This indicate the role of
GO in enhancing the specific surface area. Moreover, it can be assume
3.3. Catalyst recyclability
Considering the importance of recyclability of heterogeneous cata-
lysts from the industrial point of view, the recyclability of Pd@g-C3N4-
Fe-GO for both Suzuki and Sonogashira model reactions were
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InorganicaChimicaActa488(2019)62–70
Fig. 7. The Hammett plot for the competitive couplings of p-substituted phenyl iodide with phenyl acetylene.
Fig. 8. The Hammett plot for the competitive couplings of p-substituted phenyl iodides with boronic acid.
Fig. 9. Recyclability of the Pd@g-C3N4-Fe-GO in the A) Sonogashira and B) Suzuki coupling reactions.
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InorganicaChimicaActa488(2019)62–70
Fig. 10. The FTIR spectra of fresh and recycled Pd@g-C3N4-Fe-GO.
investigated. As described in the experimental section, at the end of the
reaction, the catalyst was magnetically separated from the reaction
mixture, washed, dried and subjected to the next run of the model re-
actions under exactly similar reaction condition. After each reaction
runs, the yield of the model coupling product was estimated and
compared with that of the fresh Pd@g-C3N4-Fe-GO. The comparison of
the yield of fresh and recycled catalysts, Fig. 8, confirmed high re-
cyclability of the catalyst. As depicted in Fig. 9, the recyclability of the
catalyst for Sonogashira reaction was slightly higher than Suzuki cou-
pling reaction. More precisely, only 12% loss of the catalytic activity
was observed for Sonogashira reaction, while the yield of the Suzuki
model product showed 19% decrease upon recycling for 10 times.
To further study the recycled catalyst, it was subjected to the FTIR
spectroscopy. The comparison of the FTIR spectrum of the fresh and
recycled catalyst after 10 Sonogashira reaction time, Fig. 10, confirmed
that Pd@g-C3N4-Fe-GO was structurally stable in the course of recycling
and the FTIR spectrum of the recycled catalyst contained the char-
acteristic bands of the fresh ones.
Acknowledgements
The authors appreciate partial financial supports from Iran Polymer
and Petrochemical Institute and Alzahra University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2018.12.048.
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