Pd anchored on C3N4 nanosheets/reduced graphene oxide: An efficient catalyst for the transfer hydrogenation of alkenes

Article abstract of DOI:10.1039/c8nj00947c

In this work, a porous g-C₃N₄ nanosheets/reduced graphene oxide (rGO) composite was synthesized via the hydrothermal co-assembly of GO and g-C₃N₄ nanosheets (g-C₃N₄ NS). Compared with g-C₃N₄ NS, rGO and bulk g-C₃N₄/rGO, the g-C₃N₄ NS/rGO supported Pd nanocatalyst displayed a remarkable catalytic activity for the hydrogenation of alkenes with formic acid and formates as the hydrogen source at atmospheric pressure. Among all the as-prepared Pd-g-C₃N₄ NS/rGO catalysts, the optimized Pd-g-C₃N₄ NS/rGO₂₀ exhibited the highest turnover frequency of 133 mol mol^-1 Pd h^-1, which is among the highest value reported in documents. 99% conversion and 99% selectivity were achieved after 30 min reaction at 30 °C for the hydrogenation of nitrobenzene. In addition, Pd-g-C₃N₄ NS/rGO₂₀ exhibited an excellently high stability after five successive cycles without significant loss of its catalytic activity.

Full text of DOI:10.1039/c8nj00947c

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Pd anchored on C N nanosheets/reduced graphene oxide:
3
4
an efficient catalyst for the transfer hydrogenation of
alkenes
Received 00th January 20xx,
Accepted 00th January 20xx
‡
‡
Jie Li , Saisai Cheng , Tianxing Du, Ningzhao Shang*, Shutao Gao, Cheng Feng, Chun Wang*, Zhi
DOI: 10.1039/x0xx00000x
Wang
www.rsc.org/
College of Science, Hebei Agricultural University, Baoding 071001, China
In this work, porous gꢀC N4 nanosheets/reduced graphene oxide (rGO) composite was synthesized via
3
hydrothermal coꢀassembly of GO and gꢀC N nanosheets (gꢀC N NS). Compared with gꢀC N NS, rGO and
3
4
3
4
3
4
bulk gꢀC N /rGO, the gꢀC N NS/rGO supported Pd nanocatalyst displayed a remarkable catalytic activity for
3
4
3
4
the hydrogenation of alkene with formic acid and formates as hydrogen source at atmospheric pressure.
Among all the asꢀprepared PdꢀgꢀC N NS/rGO catalysts, the optimized PdꢀgꢀC N NS/rGO exhibited the
3
4
3
4
20
ꢀ
1
ꢀ1
highest turnover frequency of 133 mol mol Pd h , which is among the highest value reported in the
documents. 99% of conversion and 99% of selectivity were achieved after 30 min reaction at 30 C for the
o
hydrogenation of nitrobenzene. In addition, the PdꢀgꢀC N NS/rGO exhibited an excellently high stability
3
4
20
after five successive cycles without significant loss of its catalytic activity.
considerable interest in the area of sustainable chemistry due to its
properties of high energy density, nonꢀtoxicity and excellent
stability. FA has been used in transfer hydrogenation of nitro
1
. Introduction
Hydrogenation of unsaturated carbonꢀcarbon bonds (C=C) is an
important reaction in the chemical industry, particularly in the
synthesis of flavor, fragrance chemistry and pharmaceuticals.
Generally, conventional process for alkene hydrogenation requires
the use of pressurized molecular hydrogen (H ) in combination with
1
16
17
compounds, fructose to gꢀvalerolactone,
and alkene . For
1
8
1
example, Basset et al. have developed palladium catalyst supported
on fibrous silica nanospheres (KCCꢀ1) for the hydrogenation of
alkenes and α,βꢀunsaturated carbonyl compounds within 6ꢀ24 h at
2
o
19
2
, 3
1
00 C using of FA as hydrogen source. Du et al. synthesized
various metal catalysts. Although 100% conversion of alkene can
pyridine Nꢀdoped carbon immobilized Pd nanoparticles catalyst. The
resulting catalyst Pd@CN exhibited enhanced efficiency and activity
be achieved using H , however, gaseous hydrogen is dangerous and
2
4
ꢀ6
difficult to handle due to its hazardous properties . Therefore, from
o
7
, 8
for alkene hydrogenation at 90 C within 12 h using FA as
both sustainability
and security perspective, exploring safer
17
sustainable H2 source. Zhao fabricated Pd encapsulated triazine
functionalized porous organic polymer catalyst and used for catalytic
hydrogen sources for hydrogenation of unsaturated bonds is
9
ꢀ11
desirable
.
transfer hydrogenation of alkene in the presence of FA and Et N, the
reactions were performed at 25 C for 4ꢀ15 h. Chen et al. reported
Recently catalytic transfer hydrogenation (CTH) has been
considered as a promising method for the hydrogenation of alkene,
in which hydrogen is replaced by a hydrogen donor, such as formic
3
o
1
the application of gꢀC N4 supported Pd nanoparticles (Pd/CN), a
3
1
2
MottꢀSchottky catalyst, in the hydrogenation of C=C compounds at
room temperature. The reactions were conducted in an aqueous
solution of formic acid within 3 h using 7.5 mol% Pd. Despite recent
progress, the reported catalytic systems still have one or more
acid , hydrazine, and isopropanol. In this case, the hydrogenation
reaction can be performed under mild conditions (e.g. room
temperature, atmospheric pressure), which represents a more costꢀ
13ꢀ15
effective and greener approach
. Among the hydrogen donor
2
0ꢀ22
shortcomings
time and high catalyst dosage.
heterogeneous catalysts for the hydrogenation of alkene is still
, such as harsh reaction conditions, long reaction
employed in CTH, FA, a promising hydrogen source, has attracted
23ꢀ25
Therefore, exploring highly active
College of Sciences, Hebei Agricultural University, Baoding 071001, P. R. China.
Tel.: +86 312 7528291. Eꢀmail: chunwang69@126.com;ningzhao611@126.com.
26, 27
highly desirable
.
‡
†
:
These authors contributed equally to this work.
The graphiteꢀlike carbon nitride (gꢀC N ) possesses many super
3
4
Electronic Supplementary Information (ESI) available:
physicochemical properties, such as nontoxic, excellent thermal and
chemical stability, which make gꢀC N ꢀbased materials a new class
The SEM images of gꢀC
3
N
4
NS/rGO20, the TEM images and Pd particleꢀsize distribution
3
4
in the PdꢀgꢀC
3
4
N NS. The nitrogen adsorptionꢀdesorption isotherms of prepared catalyst.
of multifunctional nanoplatforms for electronic, catalytic and energy
FTꢀIR spectra and the XRD pattern and the nitrogen adsorptionꢀdesorption isotherms and
the the TEM image and the Pd particleꢀsize distribution of the reused catalyst. Various
reported catalyst tested for hydrogenation of alkenes. See DOI: 10.1039/x0xx00000x
28, 29
applications.
Unfortunately, the performance of pristine gꢀC N
3 4
is restricted due to its small surface area and high recombination rate
of electronꢀhole pairs. To address the problems, tremendous efforts
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have been made to manipulate gꢀC
appropriate textural porosity, metal oxide,
3
N
4
, such as designing an 2.2 Characterization
3
0
31ꢀ33
34
nonꢀmetal doping
The BrunauerꢀEmmettꢀTeller (BET) surface areas were determined
12, 35, 36
and coupling with carbon materials.
Graphene, a 2ꢀ
dimensional (2D) sp ꢀconjugated carbon atoms packed in
honeycomb lattice, has received much attention because of its
2
from the N adsorption at 77 K using VꢀSorb 2800P (Jinaipu,
2
a
China). The Xꢀray diffraction (XRD) patterns of the samples were
recorded with a Rigaku D/max 2500 Xꢀray diffractometer (Dandong,
China) using Cu Kα radiation (30 kV, 25 mA) in the range 2θ = 2°ꢀ
3
7ꢀ39
extraordinary chemical, physical and catalytic properties.
The
coupling rGO with gꢀC may induce interesting synergetic
3
N
4
8
0°.The Xꢀray photoelectron spectroscopy (XPS) measurements
were performed on an ESCA Lab 250 (Thermo) spectrometer
Shanghai, China) with a monochromatic Al Kα source (1486.6 eV).
40ꢀ42
43
effects.
graphene/gꢀC
oxide. The GCN demonstrated high visibleꢀlight photoactivity
towards CO reduction at ambient conditions, exhibiting a 2.3ꢀfold
enhancement over pure gꢀC
dimentional (3D) architecture composite with 1D gꢀC
In a recent work, Yong at al. fabricated sandwichꢀlike
3
N
4
(GCN) nanocomposites by urea and graphene
(
A Hitachi S4800 field emission electron microscope (SEM, Tokyo,
Japan) operated at 30 kV and transmission electron microscopy
2
44
3
4
N . Qu at al. synthesized a three
(
TEM) using a JEOL model JEMꢀ2011 (Tokyo, Japan) at 200 kV
3
N
4
were used for determining the morphology of the samples. The Pd
loading in the materials was analyzed by a T. J. A. ICPꢀ9000 type
inductively coupled plasma atomic emission spectroscopy (ICPꢀ
AES) instrument. The FTIR spectra were recorded using a Bruker
IFS 66v/S FTIR spectrometer. GC analyses were carried out on an
Agilent 7820A series gas chromatograph (Agilent Technologies,
CA, USA) equipped with a flame ionization detector (FID) and a
split/splitless injector. All the separations were performed on a HPꢀ5
capillary column (30 m × 0.32 mm i.d. × 0.25 ꢁm film thickness)
nanoribbons and 2D graphene by a simple oneꢀstep hydrothermal
method. The catalyst exhibited efficient electrocatalytic ability for
the hydrogen evolution reaction with low over potential and
extremely large exchange current density. To the best of our
3 4
knowledge, using graphene/gꢀC N composite as catalyst or catalyst
support for catalytic organic reaction still remains in their infancy.
4
5
3 4
Liu prepared uniform graphene/gꢀC N composite, which provides
efficient electron transfer interfaces to boost its catalytic oxidation
activity for cycloalkane with relatively high yield, good selectivity,
and reliable stability. Up to now, there has been no report on the use
(
Agilent J&W Scientific, CA, USA).
.3 Catalyst preparation
3 4
Synthesis of bulk g-C and g-C N NS
of the gꢀC
hydrogenation alkene reaction.
Herein, porous gꢀC nanosheets/reduced graphene oxide (rGO)
composite was synthesized via hydrothermal coꢀassembly of GO and
3 4
N /rGO nanocomposite as supported catalyst for
2
3
N
4
3
N
4
gꢀC
3
N
4
nanosheets (gꢀC
3
N
4
NS) (Scheme 1). The Pd nanoparticles Tapically, 5 g melamine was transferred to an alumina crucible
o
were immobilized on gꢀC
3
N
4
NS/rGO material by an impregnation covered with a lid and heated at 550 C for 4 h at a ramp rate of 2.3
o
ꢀ1 46
method, which was employed to catalyze the transfer hydrogenation
of alkenes with HCOOH and ammonium formate as hydrogen donor grinded to powders. gꢀC
for the first time. The asꢀprepared PdꢀgꢀC NS/rGO20 displays oxidation of bulk gꢀC
excellent catalytic properties, including high catalytic activity and gꢀC powers were dispersed in concentrated
reliable stability, which is superior to the commercial PdꢀC hybrids.
C min . A yellowish solid was obtained (bulk gꢀC
3 4
N ), and was
3
N
4
NS was synthesized via the chemical
4
7
3
N
4
3
N
4
with concentrated H
2
SO
4
.
The obtained
SO at
3
N
4
H
2
4
a
ꢀ
1
o
concentration of 300 mg·mL at 100 C for 2 h. Then the yellow
liquid was slowly poured into 100 mL of deionized water and
ultrasonicated for 1 h for exfoliation. The temperature of the
suspension increased rapidly, and the mixture color changed from
yellow to milkꢀwhite. Then, the obtained suspension was subjected
to 10 min of centrifugation at 3000 rpm to remove any unexfoliated
gꢀC
3 4
N . After centrifugation at 8000 rpm, the obtained milkꢀwhite
Scheme 1 Schematic illustration of the preparation process of the Pdꢀ solid was washed thoroughly with deionized water to remove the
o
gꢀC
3
N
4
NS/rGO catalyst.
residual acid, and finally dried at 80 C in oven overnight. The
synthesis milkꢀwhite solid were named as gꢀC NS.
3
N
4
2
. Experimentals
.1 Chemicals
Preparation of g-C
gꢀC NS/rGO (x stand for the mass percentage ratio of rGO to gꢀ
NS were prepared by the following method. First, 100 mg
), methanol, GO was added to the 50 mL H O and the mixture solution was
, 98%), anhydrous ultrasonicated for 1 h to obtain a homogeneous aqueous dispersion.
), ammonium hydroxide (NH ·H O, Then, 500 mg gꢀC NS was added into the mixture and
5%) and sodium borohydride (NaBH , 96%) were purchased from ultrasonicated for 30 min, and then the solution was stirred for 30
Huaxin Co., Ltd (Baoding, China). Palladium chloride (PdCl , AR), min at room temperature. Subsequently, the solution was sealed in a
ꢀmethylfurane (C O, 99%), furfuryl alcohol (C CH OH, 98%) Teflonꢀlined autoclave and heated at 180 °C for 12 h, After cooling
and all of the alkenes compounds were obtained from Aladdin down naturally to room temperature, the obtained black product was
Chemicals Co Ltd. (Shanghai, China). collected by filtration, washed with distilled water, and freeze dried
3 4 x 3 4
N NS/rGO and g-C N NS/rGO20(M)
2
3
N
C N
3 4
4
x
4
8
Melamine, dichloromethane (CH
HCOOH, 88%), ammonium formate (HCOONH
ethanol ， concentrated sulfuric acid (H SO
magnesium sulfate (MgSO
2
Cl
2
), rGO and GO, formic acid
(
4
2
2
4
4
3
2
3 4
N
2
4
2
2
5
H
6
4
H
3
2
3 4 3 4 3 4
to obtain the gꢀC N NS/rGO20. gꢀC N /rGO20, gꢀC N NS/rGO10
2
| J. Name., 2012, 00, 1-3
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and gꢀC
except that 500 mg bulk gꢀC
added, respectively. In the process of hydrothermal reduction for shown in Fig.1, the TEM images clearly show the gꢀC
3
N
4
NS/rGO30 were synthesized using the above procedure The nanostructures and morphologies of the asꢀprepared gꢀC
, 50 mg GO and 150 mg GO were NS/rGO20 architectures were first examined by SEM and TEM. As
NS/rGO20
3 4
N
3
N
4
3
N
4
synthesizing composite materials, water is used as reducing agent, with the lamellar structures and a uniform nanosheet structure
and the preparation method is green and sustainable, which is better without obvious aggregation. The similar selected area electron
49, 50
than other preparation methods
. For comparison, gꢀC
3
N
4
3 4
diffraction (SAED) patterns (insets in Fig. 1A) of the gꢀC N
NS/rGO20(M) was prepared by physical mixture of 100 mg rGO and NS/rGO20 demonstrate the homogeneous distribution of rGO sheets
500 mg gꢀC
3
N
4
NS.
in the layered gꢀC
modification on the layered structure of gꢀC
3 4
N structure, implying an ignorable effect of rGO
51
3
4
N NS. As shown in
Fig. 1C, fine Pd nanoparticles are uniformly distributed throughout
the material. Besides, the highꢀresolution TEM image displays clear
lattice fringes, suggesting good crystallinity. Fig. 1B shows the
crystal plane spacing was measured as 0.224 nm, which is the (111)
plane of faceꢀcentered cubic (fcc) Pd (0.224 nm). The HRTEM
observation indicates that the average size of Pd nanoparticles in the
A
B
0
.224 nm
5
2
PdꢀgꢀC
of a count of more than 100 individual nanoparticles via TEM
analysis (Fig. 1D). However, The Pd nanoparticles in the PdꢀgꢀC
NS possess an average particle size of 3.56 nm ± 0.02 nm (Fig. S1D,
see Supporting Information), indicating that gꢀC NS coupling
with rGO can effectively reduce the size of Pd nanoparticles, which
3 4
N NS/rGO20 samples is approximately 1.52 nm on the basis
3 4
N
C
D
3
N
4
5
3, 54
is benefit for enhancing their catalytic performance.
Furthermore, the detailed SEM morphology of a wellꢀdefined and
interconnected 3D hierarchical structure can be clearly observed in
the Fig. S1A and Fig. S1B. The content of Pd in the PdꢀgꢀC
3 4
N
Fig. 1 TEM images of gꢀC
SAED patterns (inse (A, PdꢀgꢀC
particleꢀsize distributionin the PdꢀgꢀC
3 4
N NS/rGO20 and their corresponding NS/rGO20 sample is determined by ICPꢀAES and amounted to 8.38
3
N
4
NS/rGO20 (B, C) and Pd wt%.
4
NS/rGO20 (D).
3
N
A
B
Preparation of supported Pd nanocatalysts
The supported Pd nanocatalysts were prepared by the impregnation
method. Generally, 100 mg of catalyst support was dispersed into
water (10 mL) by sonication. Then, 7.5 mL H
2
PdCl
4
solution (2
ꢀ
1
mg·mL ) was added into the mixture and stirred overnight and 1
ꢀ
1
4
mol L NaBH (1 mL) was added dropwise to the suspension at 25
o
C without the assistance of other stabilizers. After 2 h, the obtained
mixture was separated by centrifugation and washed thoroughly with
o
distilled water and ethanol, then dried at 60 C for 6 h.
Fig. 2 FTꢀIR spectra of GO, gꢀC
3
N
4
NS/rGO, gꢀC
3
N
4
NS (A) and the
2
.4 General procedure for the reduction of nitroarenes via the
3 4 3 4 3 4
XRD pattern of gꢀC N NS, gꢀC N NS/rGO, PdꢀgꢀC N NS/rGO
(B).
catalytic transfer hydrogenation reaction
In a typical catalytic transfer hydrogenation experiment, the Pdꢀgꢀ
C
3
N
4
NS/rGO20 (5 mg) and the alkenes compound (0.25 mmol) were
wellꢀmixed in 5 mL ethanol in a 25 mL roundꢀbottom flask. Next,
HCOONH (2.5 mmol) and FA (2.5 mmol) were added into the
4
o
resultant mixture and the reactions were stirred at 30 C for a certain
time. The advancement of the reaction was monitored by thin
layered chromatography and GC. After accomplishment of the
reaction, the catalyst was recovered by filtration and repeatedly
washed by methanol for three times. Next, the catalysts were
o
allowed to dry in a vacuum at 60 C overnight for the further
catalytic uses. The product was extracted with dichloromethane (3
×
10 mL) and dried over an anhydrous MgSO
4
. The yield of the
Fig. 3 The nitrogen adsorptionꢀdesorption isotherms of gꢀC
3 4
N
product was analyzed by GC.
NS/rGO20, PdꢀgꢀC NS/rGO20
3
N
4
.
3. Results and discussion
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Fig. 2A shows the FTIR spectra of GO, gꢀC
Journal Name
45
3
N
4
NS and gꢀC
3
N
4
3 4
(N3), respectively. The C 1s peak of gꢀC N NS/rGO composite
NS/rGO. The FTIR spectrum of GO showed the presence of polar can also be deconvoluted into three different components (Fig. 4D).
oxygenꢀcontaining functionalities at 1046 (CꢀOꢀC stretching), 1722 The two main peaks located at 284.8 and 288.5 eV, corresponding to
ꢀ
1
2
2
(
C=O stretching vibrations in carbonyl groups) and 3407 cm
sp CꢀC bonds (C1) and sp ꢀbonded carbon in Nꢀcontaining aromatic
(
structural OꢀH groups). It is worthy of noting that the intensity of rings (NꢀC=N) (C3), respectively. The latter peak is supposed as the
4
5
3 4 3 4
most of the oxygenated groups in the gꢀC N NS/rGO significantly major carbon species of gꢀC N NS. The weak new peak at 286.4
decreased or diminished after hydrothermal treatment, indicating that eV is assigned to the carbon in CꢀN (C2).
most of the oxygen functional groups in GO have been partially
58
reduced. For gꢀC
range of 1200ꢀ1650 cm was derived from the skeletal stretching of
CꢀN heterocycles with peaks positioned at 1600 and 1456 cm . The
broad bands in the 3000ꢀ3700 cm region can be assigned to NꢀH
stretching vibration modes and the sharp band at 805 cm was
originated from the breathing vibration of triꢀsꢀtriazine units. All the
3 4
N NS, several strong absorption bands in the
ꢀ
1
A
B
ꢀ
1
ꢀ
1
ꢀ
1
gꢀC
features to the gꢀC
NS remained intact after the incorporation with rGO.
The structure of the asꢀprepared materials was characterized by XRD
Fig. 2B). The XRD patterns of gꢀC NS present a strong peak at
7.5°, which corresponds to the (002) crystallographic plane. The
3
N
4
NS/rGO samples revealed almost similar characteristic
3
N
4
NS, verifying that the structural integrity of gꢀ
3 4
C N
C
D
(
2
3 4
N
minor peaks (2θ = 13.1°) was indexed as (100) planes inꢀplanar trisꢀ
sꢀtriazine structural packing motifs. Besides, the weak diffraction
peak of graphene at 2θ = 24.5° was shown due to the low content
5
5
and fairly low diffraction intensity of rGO. Fig. 2B shows no
obvious difference in the composite material compared with gꢀC
NS, which indicated gꢀC NS still retained in all gꢀC NS/rGO Fig. 4 XPS spectra of PdꢀgꢀC
hybrids. No obvious Pd metal peaks can be observed, which can be resolution Pd 3d (B), N 1s (C), and C 1s (D) in PdꢀgꢀC
ascribed to the well dispersion of Pd nanoparticles on gꢀC NS/rGO20
NS/rGO. As shown in Fig. 3, N adsorptionꢀdesorption analysis
indicated that gꢀC NS/rGO20 hybrid with mesoporous and
macroporous has a BET surface area of up to 49.8 m g , which composite has been employed for the hydrogenation reaction. Transꢀ
3
N
4
3
N
4
3
N
4
3 4
N NS/rGO20 architectures (A), highꢀ
3 4
N
3
N
4
.
2
3
N
4
3 4
To explore its potential applications, the PdꢀgꢀC N NS/rGO20
2
ꢀ1
ꢀ1
2
nearly three times larger than that of gꢀC
3
N
4
NS (14.2 m g ) (Fig. stilbene was chosen as model substrate with the purpose of
S2A). This phenomenon can be assigned to the coꢀassembly of rGO optimizing the reaction conditions. Blank experiments indicated that
5
6
and gꢀC
aggregation of gꢀC
NS/rGO20 exhibited higher specific surface area than gꢀC
3 4
N NS, which form 3D architecture and prevent the the reduction of transꢀstilbene cannot proceed in the absence of
3
N
4
NS. Besides, it is found that gꢀC
3
N
N
4
hydrogen donors or a catalyst or Pd nanoparticles (Table 1, entries 1ꢀ
3). To our surprise, the PdꢀgꢀC NS/rGO20 catalyst showed high
3
4
3 4
N
2
ꢀ1
2
ꢀ1
NS/rGO10 (33.1 m g ) and gꢀC
S2B). Anyway, the high specific surface area of the gꢀC
NS/rGO20 is benefit for the well dispersion of Pd nanoparticles, achieving the highest turnover frequency (TOF) of 133 mol Pd mol
3
N
4
NS/rGO30 (24.9 m g ) (Fig. catalytic activity toward the transfer hydrogenation of transꢀstilbene
3
N
4
with FA and HCOONH as hydrogen donor at mild condition,
4
ꢀ
1
ꢀ
1
absorbing more reactant molecules on the surface and providing
more active species, which may lead to a better catalytic activity. Pd mol
The slight decreased surface area of PdꢀgꢀC NS/rGO suggests the used as the hydrogen donors (Table 1, entries 5ꢀ6). Besides, When
h (Table 1, entries 4). A decrease in TOF values to 43 and 55 mol
ꢀ
1
ꢀ1
h
took place when FA:HCOONa and FA:HCOOK were
3
N
4
occupation of Pd nanoparticles inside the channel of gꢀC
during the fabrication process (Fig. 3, Fig. S2B).
3
N
4
NS/rGO FA and Et
3
N were used as hydrogen source for the catalytic
reduction of transꢀstilbene (Table 1, entry 7); we achieved only 57%
17
The Xꢀray photoelectron spectroscopy technique was further conversion after 1 h, which is very different with Zhao’s work.
employed to evaluate the elemental composition and nitrogen Various solvents were screened over PdꢀgꢀC NS/rGO20 (Table 1,
content in gꢀC NS/rGO20 nanocomposites. As shown in Fig. 4A, entries 8ꢀ10) to test the solvent effect. The survey shows that
3
N
4
3
N
4
the Pd, N, C and O peaks are clearly observed. The Pd XPS of the methanol and THF provided the same outcome in the same
catalyst demonstrates a double corresponding to Pd 3d3/2 and Pd 3d5/2 condition, which is worse than ethanol (Table 1, entries 8, 9).
(
Fig. 3B). The Pd 3d5/2 peak and Pd 3d3/2 peak at 338.1 eV 343 eV Besides, DMF is
a
poor solvent for the catalytic transfer
2+
are related to Pd (palladium oxide), while the Pd 3d5/2 peak and Pd hydrogenation of transꢀstilbene (Table 1, entry 10). So, ethanol was
3
0
1
d
3/2 peak at 336.2 eV and 341.3 eV are attributed to Pd (metallic chosen for further studies as it is a more attractive solvent. The
palladium), which indicates metallic Pd as the major phase on the effect of mass ratio of rGO in the catalyst on the hydrogenation of
5
7
support. The N 1s XPS spectra of gꢀC
3 4
N NS/rGO20 can be fitted olefins was also investigated (Table 1, entries 4, 11ꢀ12). The optimal
into three peaks (Fig. 4C) centred at 398.6, 399.9 and 401.5 eV, mass ratio of rGO in the nanocomposite was determined to be 20%.
2
3 4
corresponding to sp N atoms in triazine rings (N1), bridging N The reaction can be completed after 30 min catalyzed by PdꢀgꢀC N
48
atoms in Nꢀ(C)3 (N2) and amino functional groups (CꢀNꢀH) NS/rGO20. The maximum TOF value of the reaction was estimated
4
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ꢀ
1
ꢀ1
to be 133 mol mol Pd h , surpassing majority heterogeneous provided 80% conversion after 1h, suggesting that our developed
catalysts using N ·H O, FA or iꢀPrOH as hydrogen source for catalyst is superior to it. The phenomenon may come from the fact
hydrogenation of alkenes (Table S1). Furthermore, the PdꢀgꢀC that our PdꢀgꢀC NS/rGO catalysts can provide many more
NS/rGO20 (M) catalyst shows lower catalytic activity than Pdꢀgꢀ catalytic sites in comparison to this conventional Pd@C catalyst.
NS/rGO20 composite (Table 1, entry 14). Therefore, it can be Moreover, the component of the underlying material also plays a
concluded that the improvement of the catalytic activity for Pdꢀgꢀ crucial role in determining the catalytic performance of the final
NS/rGO20 is indeed attributed to synergistic effect between materials. For comparison, PdꢀgꢀC NS, PdꢀbulkꢀgꢀC /rGO20
2
H
4
2
3
N
4
3 4
N
3 4
C N
C
3
N
4
3
N
4
3 4
N
PdꢀrGO
59
rGO and gꢀC
3
N NS. To better clarify the role of the support, we and
performed control experiments with commercial Pd@C. The Pd@C
[
a]
Table 1 Reduction of alkene under various conditions.
Time Conversion
Entry
Catalysts
ꢀ
Hydrogen donors Solvents
Selectivity (%)
TOF
(
h)
3
(%)
ꢀ
1
2
3
4
5
6
FA:HCOONH
FA:HCOONH
ꢀ
4
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
methanol
THF
ꢀ
ꢀ
ꢀ
gꢀC
3
N
4
NS/rGO20
4
3
ꢀ
ꢀ
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
PdꢀgꢀC
3
3
3
3
3
3
3
3
3
3
N
4
N
4
N
4
N
4
N
4
N
4
N
4
N
4
N
4
N
4
NS/rGO20
NS/rGO20
NS/rGO20
NS/rGO20
NS/rGO20
NS/rGO20
NS/rGO20
NS/rGO20
NS/rGO10
NS/rGO30
3
ꢀ
ꢀ
ꢀ
FA:HCOONH
FA:HCOONa
FA:HCOOK
4
0.5
1
>99
64
82
57
78
79
15
87
100
>99
>99
>99
6
>99
>99
>99
>99
>99
>99
>99
100
98
133
43
55
38
26
26
5
1
[b]
7
FA:Et
3
N
1
8
9
FA:HCOONH
FA:HCOONH
FA:HCOONH
FA:HCOONH
FA:HCOONH
FA:HCOONH
4
4
4
4
4
4
2
2
1
0
1
2
3
4
5
6
DMF
2
1
1
1
1
1
1
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
1
58
65
33
66
66
2
1
PdꢀbulkꢀgꢀC
3
N
4
/rGO20
2
100
100
100
>99
>99
PdꢀgꢀC N NS/rGO (M)
FA:HCOONH4
FA:HCOONH4
FA:HCOONH4
FA:HCOONH4
1
3
4
20
PdꢀgꢀC N NS
1
3
4
PdꢀrGO
Pd@C
2
[
c]
1
7
1
80
53
o
[
[
a] Reaction: transꢀstilbene (0.25 mmol, 45 mg). Catalyst (5 mg, 1.5 mol% Pd). FA : Formate (2.5 mmol : 2.5 mmol). Solvent (5 mL), 30 C.
b] FA : Et N (2.5 mmol : 2.5 mmol). [c] 1.5 mol% Pd.
3
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ARTICLE
were prepared. The transfer hydrogenation of alkene over Pdꢀbulkꢀgꢀ of gꢀC N NS could enhance the catalytic activity of rGO efficiently.
3
4
C N /rGO , PdꢀgꢀC N NS and PdꢀrGO were investigated under the It may be due to addition of carbon nitride nanosheets can
3
4
20
3
4
6
0, 61
same reaction conditions (Table 1, entries 13, 15ꢀ16). The full effectively prevent the agglomeration of graphene.
conversions of transꢀstilbene catalyzed by PdꢀbulkꢀgꢀC N /rGO indicated that the nature of the support played important roles in the
and PdꢀgꢀC N NS needed 2 h and 1 h, respectively. In addition, a transfer hydrogenation of alkene reaction. The results further
These results
3
4
20
3
4
low transꢀstilbene conversion of 6% was produced after 2 h in the confirm the application potential of the hybrid materials in chemical
31
presence of the PdꢀrGO catalyst, which shows that the introduction synthesis.
[
a]
Table 2 Hydrogenation of various alkenes by using PdꢀgꢀC N NS/rGO catalyst.
3
4
20
Entry
Substrate
Product
Time (min) Conversion (%) Selectivity (%) TOF
1
30
>99
>99
133
2
3
4
5
6
7
120
15
30
30
30
30
>99
100
97
>99
>99
57
33
266
74
Cl
100
100
100
>99
>99
>99
133
133
133
8
9
30
100
>99
133
30
100
100
>99
>99
133
33
1
1
0
1
120
30
98
>99
131
1
1
2
3
30
240
90
100
86
>99
>99
>99
>99
>99
133
14
44
64
63
[b]
1
4
100
96
1
1
5
6
60
60
94
1
7
240
52
>99
9
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[
b]
b]
1
1
8
9
30
45
100
>99
>99
>99
133
89
[
[b]
2
0
45
100
>99
89
o
[
5
a] Reaction: 0.25 mmol Substrate. PdꢀgꢀC
0 C.
3
N
4
NS/rGO20 (5 mg, 1.5 mol% Pd). Ethanol. FA: HCOONH
4
(2.5 mmol : 2.5 mmol). 30 C. [b]
o
Besides of high catalytic activity, the stability and reusability of
the catalyst are also important issues for practical applications of
unique structure catalysts in industry. We thus studied the catalytic
A
B
performance of the used PdꢀgꢀC
3
N
4
NS/rGO20 catalyst by
considering transꢀstilbene as the model substrate. The PdꢀgꢀC
3
N
4
NS/rGO20 catalyst presented tremendous recyclability for at least 5
competitive catalytic cycles without significant loss of catalytic
activity (Fig. 5A). After five recycled times, the catalyst gave
corresponding 1,2ꢀdiphenylethane in 92% conversion at 30 min. Fig. 5 Stability test for the transfer hydrogenation in the presence of
Furthermore, the recovered catalyst was analyzed by FTIR, XRD, PdꢀgꢀC NS/rGO20 catalyst (A) and the filtration test for the
3
N
4
BET and TEM. As showed in Fig. S3, XRD and FTIR results hydrogenation of alkene over PdꢀgꢀC
3 4
N NS/rGO20 (B).
showed that no obvious diffraction peak change can be observed
between fresh and recycled catalyst, indicating good dispersion of
(Table 2, entries 2, 8). The total conversion of styrene to
metal and high durability and stability of the hybrid material. The ethylbenzene was achieved within 15 min (Table 2, entry 3).
results also coincide with the TEM images. The TEM images show Interestingly, for the Clꢀsubstituted styrene, slight dehalogenation
that the Pd nanoparticles still exhibited a wellꢀdispersed distribution process was observed during the reduction, which gave the products
over the support surface and no significant particle agglomeration in 97% conversion and 57% selectivity (Table 2, entry 4). For Fꢀ
(
2.24 nm) (Fig. S3D). In addition, The BET surface area of the substituted styrene, no dehalogenation process was observed during
2
ꢀ1
reused catalyst is 38 m g , which is comparable with the fresh the reduction, which gave the product in 100% yield (Table 2, entry
catalyst. The preservation of BET surface area of the reused catalyst 5). The styrene compounds involving electronꢀrich groups such as pꢀ
signifies high recyclability of the catalyst, revealing no sign of pore methyl and pꢀmethoxy were reduced to the corresponding products
wall destruction and pores clogging (Fig. S3C). In order to further in an efficient fashion with absolute selectivity (Table 2, entries 6,
gain insight into the heterogeneous nature of the catalyst, the hot 7). Cyclohexene and norbornylene, cyclic olefin, reached 100%
leaching test was carried out. After hydrogenation of transꢀstilbene product conversion after 30 min (Table 2, entries 9, 12). A vinyl
3 4
was run for 10 min, the PdꢀgꢀC N NS/rGO20 catalyst was removed substituted polycyclic aromatic hydrocarbon was reduced to the
from the reaction mixture by centrifugation, and the filtrate was corresponding ethyl derivative with 100% conversion at 2 h (Table
allowed to react for 2 h, negligible changes in conversion was 2, entry 10). The hydrogenation of unsaturated C=C bonds was
observed (Fig. 5B). Obvious Pd leaching during the reaction was successfully achieved even in the presence of reducible substituent
also excluded here according to the ICP result (data not shown), with (ester group) (Table 2, entry 11). Surprisingly, very poor conversion
the concentration of leached Pd species in the reaction solution was achieved for cyclooctene (Table 2, entry 13), which may be due
1
7
below the detection limit of the equipment. Those finding to the presence of almost orthogonal allylic CꢀH bond. When the
o
demonstrated that PdꢀgꢀC
heterogeneous catalyst nature. This high performance can be greatly increased and the reaction time for the hydrogenation of
attributed to the strong synergistic effect between rGO and gꢀC cyclooctene greatly decreased (Table 2, entry 14). The
3
N
4
NS/rGO20 exhibits typical reaction temperature rose to 50 C, the cyclooctene conversion
3
N
4
NS. Those results illustrated that the composites can be thus chemoselective transfer hydrogenation of C=C has been achieved for
considered as stable and reusable heterogeneous catalysts for the α,βꢀunsaturated ketone, chalcone (Table 2, entry 18) with 100%
o
practical applications.
conversion at 50 C. What makes us happy is that 4ꢀ
We investigated the generality of the alkene hydrogenation methoxychalcone and 4ꢀnitrochalcone can gain excellent conversion
reactions over PdꢀgꢀC NS/rGO20 with the optimized reaction under the same conditions (Table 2, entries 19ꢀ20). After knowing
parameters. In some cases, slight adjustment of the reaction the excellent activity of the PdꢀgꢀC NS/rGO20 in reducing C=C
conditions was required due to the distinct reactivity of reactants. As bonds with FA and HCOONH , so we thus turned towards the
3
N
4
3
N
4
4
shown in Table 2, various unsaturated C=C bonds were successfully hydrogenation of 2ꢀmethylfuran to 2ꢀmethyltetrahydrofuran and
o
hydrogenated with good yield in ethanol at 30 C. Successful transfer hydrogenation 2ꢀfuranemethanol to 2ꢀtetrahydrofurfuryl
hydrogenation was conducted for monoꢀ and diꢀsubstituted alkenes
alcohol, which is the critical step for increasing the hydrogen/carbon
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ratios and thus transforming the biomass to renewable biofuel. The 12. X. Bai, L. Wang, Y. Wang, W. Yao and Y. Zhu, Appl. Catal.,
experimental results showed that the prepared catalyst has high B, 2014, 152-153, 262ꢀ270.
catalytic performance for the hydrogenation of the two substances 13. A. Mittal, J. Mittal, A. Malviya and V. K. Gupta, J. Colloid
(
Table 2, entries 15, 16).
Interface Sci., 2010, 344, 497ꢀ507.
1
1
4. V. K. Gupta, R. Jain, A. Nayak, S. Agarwal and M.
Shrivastava, Mater. Sci. Eng., C, 2011, 31, 1062ꢀ1067.
5. V. K. Gupta, S. Agarwal and T. A. Saleh, J. Hazard. Mater.,
4
.
Conclusion
In summary, we have demonstrated the successful fabrication of Pdꢀ
decorated architectures built from rGO and graphitic carbon nitride
NS by means of a costꢀeffective and facile coꢀassembly approach.
The hybrid composite showed high catalytic activity and stability in
the alkene hydrogenation reactions with FA and AF as the hydrogen
donors at mild conditions. The superior catalytic performance can be
attributed to the synergistic effect between the highly dispersed Pd
nanoparticles and the unique structure of the gꢀC N NS/rGO20
3 4
support, as well as the high specific surface area with high
adsorption ability of the catalyst for the alkenes. We believe that this
2
011, 185, 17ꢀ23.
1
1
6. L. Qi and I. T. Horváth, ACS Catal., 2012, 2, 2247ꢀ2249.
7. J. Mondal, Q. T. Trinh, A. Jana, W. K. Ng, P. Borah, H. Hirao
and Y. Zhao, ACS Appl. Mater. Interfaces, 2016, 8, 15307ꢀ
1
5319.
1
1
8. Z. S. Qureshi, P. B. Sarawade, M. Albert, V. D'Elia, M. N.
Hedhili, K. Köhler and J.ꢀM. Basset, ChemCatChem, 2015, 7,
6
35ꢀ642.
9. C. Zhang, Y. Leng, P. Jiang, J. Li and S. Du, ChemistrySelect,
017, 2, 5469ꢀ5474.
2
3 4
study could boost the feasibility of gꢀC N NS/rGO as a supporter
2
2
0. V. K. Gupta and A. Nayak, Chem. Eng. J., 2012, 180, 81ꢀ90.
1. S. Karthikeyan, V. K. Gupta, R. Boopathy, A. Titus and G.
Sekaran, J. Mol. Liq., 2012, 173, 153ꢀ163.
for integrating various functional metal particles for broader
application field in organic synthesis under environmentally benign
conditions.
2
2
2. T. A. Saleh and V. K. Gupta, J. Colloid Interface Sci., 2012,
3
71, 101ꢀ106.
5.
Acknowledgments
3. R. Saravanan, E. Thirumal, V. K. Gupta, V. Narayanan and A.
Financial supports from the National Natural Science Foundation of
Stephen, J. Mol. Liq., 2013, 177, 394ꢀ401.
China (21603054) the Natural Science Foundation of Hebei Province 24. M. Ghaedi, S. Hajjati, Z. Mahmudi, I. Tyagi, S. Agarwal, A.
B2015204003, B2016204131, B2018204145), the Young Topꢀ Maity and V. K. Gupta, Chem. Eng. J., 2015, 268, 28ꢀ37.
notch Talents Foundation of Hebei Provincial Universities 25. V. K. Gupta, A. Nayak, S. Agarwal and I. Tyagi, J. Colloid
BJ2016027), and the Natural Science Foundation of Hebei Interface Sci., 2014, 417, 420ꢀ430.
Agricultural University (ZD201613, LG201709, LG201711) are 26. A. Mittal, J. Mittal, A. Malviya, D. Kaur and V. K. Gupta, J.
(
(
gratefully acknowledged.
Colloid Interface Sci., 2010, 342, 518ꢀ527.
2
2
2
3
3
7. A. Mittal, D. Kaur, A. Malviya, J. Mittal and V. K. Gupta, J.
Colloid Interface Sci., 2009, 337, 345ꢀ354.
8. Y. Gong, M. Li, H. Li and Y. Wang, Cheminform, 2015, 17,
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New Journal of Chemistry
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DOI: 10.1039/C8NJ00947C
3 4
Pd anchored on C N nanosheets/ reduced graphene oxide: an efficient
catalyst for the transfer hydrogenation of alkenes
‡
‡
Jie Li , Saisai Cheng , Tianxing Du, Ningzhao Shang*, Shutao Gao, Cheng Feng,
Chun Wang*, Zhi Wang
College of Science, Hebei Agricultural University, Baoding 071001, China
3 4
Pd nanoparticles anchored on g-C N NS/rGO was prepared by means of a
cost-effective and facile co-assembly approach. The hybrid composite showed high
catalytic activity and stability for the hydrogenation of alkenes with FA and
4
HCOONH as the hydrogen donors at mild conditions.