A novel bifunctional Pd-ZIF-8/rGO catalyst with spatially separated active sites for the tandem Knoevenagel condensation-reduction reaction

Article abstract of DOI:10.1039/c7cy01725a

A novel bifunctional catalyst with spatially separated active sites was prepared by the immobilization of Pd nanoparticles (NPs) via covalent interaction and coordination of a zeolitic imidazolate framework (ZIF-8) on the surface of graphene oxide (GO), respectively, which was used as an efficient catalyst for the Knoevenagel condensation-reduction tandem reaction. The results of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) demonstrated that Pd and ZIF-8 were successfully immobilized on the surface of GO, and the GO was reduced to reduced graphene oxide (rGO) using NaBH₄ as the reductant in the preparation of Pd-ZIF-8/rGO. The textural properties and morphology of Pd-ZIF-8/rGO were characterized by N₂ adsorption-desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Pd-ZIF-8/rGO shows excellent catalytic performance in the tandem reaction with 100% benzaldehyde conversion and 98.3% selectivity to benzylmalononitrile. The excellent catalytic performance of Pd-ZIF-8/rGO in the tandem reaction is due to the high catalytic activities of spatially separated Pd NPs and ZIF-8 active sites and concentrated reactants on the surface of Pd-ZIF-8/rGO due to the π-π interaction between rGO and the reactants. The anchoring and stabilization effects of oxygenated groups of GO inhibit the aggregation and leakage of active sites, leading to good catalytic recyclability with almost unchanged catalytic activity for more than eight cycles in the tandem reaction.

Full text of DOI:10.1039/c7cy01725a

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PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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Catalysis Science & Technology
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DOI: 10.1039/C7CY01725A
Catalysis Science & Technology
PAPER
A novel bifunctional Pd-ZIF-8/rGO catalyst with spatially
separated active sites for the tandem Knoevenagel
condensation-reduction reaction
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
a
a
a
a
a
Hefang Wang, Yansu Wang, Aizhong Jia, Cunyue Wang, Luming Wu, Yongfang Yang*
www.rsc.org/
a
and Yanji Wang*
A novel bifunctional catalyst with spatially separated active sites was prepared by the
immobilization of Pd nanoparticles (NPs) via covalent interaction and coordination of zeolitic
imidazolate framework (ZIFꢀ8) on the surface of graphene oxide (GO), respectively, which
was used as an efficient catalyst for the Knoevenagel condensationꢀreduction tandem reaction.
The results of Fourier transform infrared spectroscopy (FTꢀIR), Xꢀray diffraction (XRD),
Raman spectra, UVꢀVis spectra, Xꢀray photoelectron spectroscopy (XPS) and
thermogravimetric analysis (TGA) demonstrated that Pd and ZIFꢀ8 were successfully
immobilized on the surface of GO, and the GO was reduced to reduced graphene oxide (rGO)
using NaBH as reductant in the preparation of PdꢀZIFꢀ8/rGO. The textural and morphology of
4
PdꢀZIFꢀ8/rGO were characterized by N
2
adsorptionꢀdesorption, scanning electronic
micrograph (SEM) and transmission electron microscope (TEM). PdꢀZIFꢀ8/rGO shows
excellent catalytic performance in the tandem reaction with 100% benzaldehyde conversion
and 98.3% selectivity to benzylmalononitrile. The excellent catalytic performance of PdꢀZIFꢀ
8
/rGO in the tandem reaction is due to the high catalytic activities of spatially separated Pd
NPs and ZIFꢀ8 active sites and concentrated reactants on the surface of PdꢀZIFꢀ8/rGO due to
the πꢀπ interaction between rGO and the reactants. The anchor and stabilization effects of
oxygenated groups of GO inhibit the aggregation and leak of active sites, leading to good
catalytic recyclability with almost unchanged catalytic activity for more than eight times in the
tandem reaction.
1
. Introduction
Tandem reactions, which enable multistep reactions in
one pot, increase the economic competitiveness in the
production of target compounds and the greenness of the
process by reducing capital investment, being effective in
the minimization of wastes and energy consumption, and
School of Chemical Engineering and Technology, Hebei University of Technology,
Tianjin 300130, China
E-mail address: wuyuyang123456@163.com (Y. Yang), yjwang@hebut.edu.cn (Y.
Wang)
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1
,2
2,26
optimizing the use of solvent. Benzylmalononitrile is an
important pharmaceutical intermediate, which can be
strategy.
However, bifunctional catalysts usually
exhibited inferior catalytic activity and durability due to
the interaction between separated active sites. Therefore,
it is highly desirable to develop a suitable support, which
can be used for the high dispersion of the active species
and inhibit the interaction between separated active sites
effectively. GO is an attractive material because of its
unique properties, such as the strong πꢀπ interaction
between GO and reactants, various oxygenated groups
such as hydroxyl, epoxy or carboxyl groups and high
3
used in the synthesis of antimalarials. Oneꢀpot synthesis
of benzylmalononitrile by tandem Knoevenagel
condensationꢀreduction reaction, as an attractive synthetic
method for improving overall process efficiency, has
raised extensive interest of researchers. Hantzsch 1,4ꢀ
4
5
dihydropyridine or NaBH
4
were used as reductant with
6
8% and 87% yield in the above process, respectively.
However, the use of NaBH had the disadvantages of
4
2
27
high reaction cost and the negative impact of borate on
the aqueous environment.
surface area (2000 m /g). The surface oxygenated
groups and defects on GO can act as nucleation sites to
28ꢀ30
2+
Benzylidenemalononitrile was synthesized efficiently
from benzaldehyde and malononitrile by Knoevenagel
condensation using base catalysts, such as layered double
anchor the Pd NPs
and can also coordinate with Zn
31,32
ions , which are the precursors of ZIFꢀ8. The anchor
and stabilization effects of the oxygenated groups and
defects on GO could prevent the aggregation and leak of
Pd NPs and ZIFꢀ8. A 2D structure of GO also facilitates
the diffusion/transportation of reactant and product and
the utilization of active site in the Knoevenagel
condensationꢀreduction tandem reaction. Therefore, GO
6
7
hydroxides (LDHs), amine modified graphene oxide,
8
amine functionalized K10 montmorillonite, graphitic
9,10
11
carbon nitride (gꢀC N ),
zeolites and Metal Organic
3
4
12
13
Frameworks (MOFs) (e.g. IRMOFꢀ3, MILꢀ101 and
14ꢀ18
ZIFꢀ8
). Among them, ZIFꢀ8 has drawn more attention
2
+
33ꢀ35
owing to its outstanding characteristics, such as Zn as
its acidic site and the nitrogen atom of the 2ꢀ
methylimidazole ligand as its basic site, large surface
area, wellꢀdefined pore structure, high thermal and
can be used as an ideal support for ZIFꢀ8 and Pd NPs.
Herein, PdꢀZIFꢀ8/rGO was prepared by the
immobilization of Pd NPs via covalent interaction and
coordination of ZIFꢀ8 on the surface of GO respectively
and the PdꢀZIFꢀ8/rGO was used as catalyst in the
Knoevenagel condensationꢀreduction tandem reaction
(Scheme 1). As a heterogeneous catalyst, PdꢀZIFꢀ8/rGO
can be easily recovered and reused without leak of Pd and
ZIFꢀ8.
1
4ꢀ18
chemical stability.
Previous studies showed that the
catalytic activity of ZIFꢀ8 increased significantly with the
decrease of the particle size due to almost all active sites
located at the external surface of ZIFꢀ8. However, the
nanoparticles (NPs) of ZIFꢀ8 were difficult to be
19,20
recovered and reused after reaction.
Pd is usually used as the catalytic center for the
reduction of C=C bond in α, βꢀunsaturated compounds.
Pd NPs supported on ZIFꢀ8 showed good catalytic
2
. Experimental
2
.1 Materials
2
1ꢀ25
activity in the reduction.
However, the diameter of Pd
All the reagents were analytical purity and were used as
received without further purification. Natural flake
graphite with an average particle size of 40 ꢁm (99%)
was purchased from Qingdao Guangyao Graphite Co. Ltd
China. Sulfuric acid (98%) and hydrochloric acid (37%)
were obtained from Tianjin Institute of Chemical Agents
China. 2ꢀMethylimidazole (99.0%) and zinc nitrate
NPs (2ꢀ50 nm) are larger than the cavity of ZIFꢀ8, which
has the sodalite (SOD) zeolite structure with large
cavities (11.6 Å) and small apertures (3.4 Å). Therefore,
most of Pd NPs are dispersed on the exterior surface of
ZIFꢀ8, which tend to the migration and aggregation of
NPs due to their high surface energy, leading to loss of
the catalytic activity during reactions.
3 2 2
hexahydrate (Zn(NO ) ·6H O) were purchased from Alfa
The controlled synthesis of bifunctional catalysts,
which are active to promote all the individual reactions
involved in the tandem reaction, is an effective
Aesar. Fuming nitric acid (>90%), palladium chloride,
sodium hydroxide, sodium borohydride, polyvinylꢀ
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o
pyrrolidone (PVP) and hydrogen peroxide (30 wt%) were
purchased from Tianjin FengChuan Chem. Co. Ltd
China. Potassium permanganate was purchased from
Hebei Wuluo Pharmaceutical Co. Ltd China. Methanol,
ethyl cyanocaetate and diethyl malonate were obtained
from Tianjin Guangfu Fine Chemical Research Institute
China. Malononitrile, 4ꢀmethylbenzaldehyde, 4ꢀmethoxꢀ
ybenzaldehyde, 4ꢀfluorobenzaldehyde, 4ꢀbromobenzaldꢀ
ehyde and 4ꢀhydroxybenzaldehyde were purchased from
Energy Chem. Co. Ltd China.
digested in 10% (v/v) nitric acid solution at 60 C for 1.0
h. The dispersion of Pd was calculated by the basis of CO
adsorption on AutoChem II 2920 chemical analyzer
(Norcross, GA, USA) equipped with thermal conductivity
detector (TCD) Sample (0.50 g) was first pretreated in 10%
3
o
H
2
/N
2
(50 cm /min) at 250 C for 2.0 h and purged with
3
helium (50 cm /min) for 1.0 h at the same temperature.
o
And then, the catalyst was cooled to 60 C and CO pulses
were injected from a calibrated onꢀline sampling valve.
CO adsorption was assumed to be completed after three
successive peaks showed the same peak areas. A CO/Pd
2
.2 Characterization
2
stoichiometry of 1 was used for calculations. H ꢀTPR
experiments were also performed on AutoChem II 2920
chemical analyzer. A catalyst sample (0.10 g) was
previously heated for 1.0 h at 250 °C under an Ar stream,
and then cooled to 60 °C. Hydrogen consumption was
Fourier transform infrared (FTꢀIR) spectra were collected
using the KBr pellet technique on the transmission
module of a Bruker Vector 22 FTꢀIR spectrometer at 2
ꢀ1
cm resolution and 64 scans. XꢀRay diffraction (XRD)
was performed on a D8 Discover (Bruker AXS Ltd.)
under ambient conditions using a Cu Kα Xꢀray. The
measurements of Raman spectra were performed on a
inVia Reflex (Renishaw) Raman apparatus, using a 532
nm laser beam with a laser power of 1 mW and a detector
data acquisition time of 10 s. The UVꢀvis spectra were
acquired on a UVꢀVisꢀNIR L5 spectrophotometer in the
range of 200ꢀ700 nm. Thermogravimetric analysis (TGA)
was conducted on the TGA/DTA system (TA2000,
calculated in the flow of 10% H /Ar gas mixture at 50
2
o
mL/min and the temperature was raised up to 300 C with
o
a heating rate of 10 C/min.
2
2
.3 Catalysts preparation
.3.1 Preparation of ZIF-8. ZIFꢀ8 was prepared
32
according to the reported procedure. Typically, 744 mg
of Zn(NO ·6H O and 411 mg of 2ꢀmethylimidazole
3
)
2
2
were dissolved in 50 mL of methanol, respectively. Then
the zinc nitrate solution was mixed with the 2ꢀ
DuPont) from 25ºC to 700 ºC in a 100 mL/min air flow
o
and a ramp rate of 10 C/min. N
2
adsorptionꢀdesorption
o
methylimidazole solution and stirred for 1.0 h at 25 C.
was performed at 77
K
on
a
Micromeritics
o
The molar ratios of metal: ligand: MeOH were 1:2:1250.
The resultant mixture was centrifuged and washed three
times with deionized water, then dried for 12.0 h in a
vacuum oven (60 °C, ꢀ0.1 MPa). Finally, the ZIFꢀ8 was
obtained.
ASAP2020M+C. All samples were degassed at 160 C
for 18 h. The specific surface area was calculated by the
BrunauerꢀEmmettꢀTeller (BET) method. The total pore
volume and average pore size were calculated from
nitrogen desorption isotherm by the HorvathꢀKawazoe
method. Xꢀray photoelectron spectroscopy (XPS) was
carried out on a Thermo ESCALAB 250XI spectrometer
with monochromatic Al Kα radiation. Scanning electron
microscopy (SEM) and energy dispersive spectroscopy
2
.3.2 Preparation of ZIF-8/GO. GO was prepared by
oxidation of graphite powder according to the reported
3
6
Hummers method. Firstly, a certain amount of GO was
dispersed in deionized water and sonicated for 2.0 h at 25
°
3 2 2
C. 744 mg of Zn(NO ) ·6H O and 411 mg of 2ꢀ
(
EDS) were obtained on a FEI Nova Nano SEM450.
methylimidazole were dissolved in 50 mL of methanol,
respectively. Secondly, the methanol solution of zinc
nitrate was mixed with solution of GO and stirred at 25
Transmission Electron Microscope (TEM) images were
acquired on the JEMꢀ2100 microscopy. The palladium
content was determined by inductively coupled plasma
emission spectroscopy (ICPꢀAES) on Opfima 7300V
o
C for 1.0 h. Afterwards, the methanol solution of 2ꢀ
methylimidazole was added into the above solution under
(Perkin Elmer). For ICP analyses, the samples were
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stirring at 25 °C for 1.0 h. The resultant mixture was
centrifuged and washed three times with deionized water,
then dried for 12.0 h in a vacuum oven (60 °C, ꢀ0.1 MPa).
Finally, the ZIFꢀ8/GO was obtained.
All the samples have the same Pd loading. For
comparison, the rGO and ZIFꢀ8/rGO were also prepared
with the same procedure. The detailed synthesis
procedures of Pd/ZIFꢀ8 and Pd/rGO are shown in SI.
2
.3.3 Preparation of Pd-ZIF-8/rGO. PdꢀZIFꢀ8/rGO was
prepared by the immobilization of ZIFꢀ8 via coordination
and covalent interaction of Pd NPs on the surface of GO,
2.4 One-pot tandem reaction
The oneꢀpot tandem Knoevenagel condensationꢀreduction
reaction was carried out in a 25 mL threeꢀnecked round
flask equipped with a magnetic stirrer, a reflux condenser
and a hydrogen balloon. Typically, 40 mg of catalyst, 5.0
mL toluene, 1.9 mmol of benzaldehyde and 3.8 mmol
malononitrile were added to the reactor. Then the mixture
2
+
respectively. As illustrated in Scheme 2, firstly, Zn ions
were coordinated with the oxygenꢀcontaining functional
groups of GO. Then, 2ꢀmethylimidazole coordinate with
2
+
Zn to form ZIFꢀ8/GO. Finally, Pd NPs were anchored
and stabilized by oxygenated groups of GO with the
protection of PVP during the reduction of NaBH4.
o
was stirred and heated at 60 C for 4.0 h under air
Typically, the PdCl
was then added under stirring until the pH=7 obtaining
the Na PdCl solution (0.01 M). Then, 11.29 mg of
2
was dissolved in HCl, and NaOH
atmosphere. Subsequently, the reactor was purged with
H to remove the air for five times and then the reactor
2
2
4
was sealed. The reaction mixture was further stirred for
polyvinylꢀpyrrolidone (PVP) was added to the PdCl
2
12.0 h at the same temperature under H
2
atmosphere.
o
solution (2.56 mL, 0.01 M) at 25 C. After stirring for 1.0
h, 100 mg of the dried ZIFꢀ8/GO was added into the
above solution and the mixture was stirred for 2.0 h.
After the reaction, the remaining H was removed by a
2
careful venting process, then the mixture was separated
by centrifugation. The organic phase was confirmed by a
Then, 1.28 mL of NaBH solution (0.1 M) was added
4
Thermo
Trace
DSQ
gas
chromatographꢀmass
dropwise into the reaction mixture with vigorous stirring
spectrometer and product was analyzed by a SPꢀ3420A
gas chromatograph equipped with a KBꢀWax fcolumn
o
at 0 C. After further stirring for 5.0 h, the resultant
mixture was isolated by filtration and washed three times
with deionized water. Then, the obtained product was
dried for 12.0 h in a vacuum oven (60 °C, ꢀ0.1 MPa). The
product is named as PdꢀZIFꢀ8/rGO due to the fact that
GO was reduced during the preparation process.
(30 m, 0.32 mm id, 0.25 ꢁm film thickness) with decane
as internal standard. The recovered catalysts were washed
with ethanol and deionized water and then dried for 12.0
o
h in a vacuum oven (60 C, ꢀ0.1 MPa) before reuse.
2
.3.4 Preparation of Pd/(ZIF-8+rGO). For comparsion,
3
. Results and discussion
Pd/(ZIFꢀ8+rGO) was prepared by immobilization of Pd
particles onto the physical mixture of ZIFꢀ8 and GO.
Typically, 75 mg of the dried ZIFꢀ8 was added into the
GO dispersion (5.0 mg/mL) and the mixture was stirred
for 2.0 h (ZIFꢀ8+GO). Then, the above mixture was
3
.1 Catalyst characterization
3
.1.1 Compositional and structural information of Pd-
ZIF-8/rGO. The FTꢀIR spectra of rGO, ZIFꢀ8, ZIFꢀ8/GO
and PdꢀZIFꢀ8/rGO samples are shown in Fig. 1a. The FTꢀ
added into the Na
1.29 mg of polyvinylꢀpyrrolidone (PVP) at 25 °C and
the mixture was stirred for 2.0 h. Afterwards, 1.28 mL of
2 4
PdCl solution (2.56 mL, 0.01 M with
ꢀ1
IR spectrum of rGO shows two peaks at 1050 cm and
1
ꢀ
1
1
390 cm , corresponding to the CꢀO stretching and OꢀH
2
8,30
bend vibration, respectively,
which indicates
NaBH
4
solution (0.1 M) was added dropwise into the
o
significant reduction of all oxygen containing groups by
above mixture with vigorous stirring at 0 C for 5.0 h.
The resultant mixture was isolated by filtration, washed
three times with deionized water and dried for 12.0 h in a
vacuum oven (60 °C, ꢀ0.1 MPa).
NaBH4. Similar results were also obtained by
37
Wojtoniszak. For the spectrum of ZIFꢀ8, the adsorption
ꢀ1
peak at 1578 cm is associated with the C=N stretching
vibrations of the imidazole, whereas the intense and
ꢀ1
convoluted peak at 1421 cm is ascribed to the entire ring
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3
8
ꢀ1
stretching. The peaks at 1306, 1145 and 998 cm are
characteristic diffraction peaks of Pd are observed in the
XRD patterns of PdꢀZIFꢀ8/rGO, which is mainly due to
the high dispersion of the ultrasmall Pd NPs (~2.8 nm) in
the composite, which was also proved by the results of
TEM and CO adsorption. Similar conclusion was also
obtained in case of the immobilization of Pt NPs on the
ZIFꢀ8/GO support via a facile liquid impregnation
attributed to the inꢀplane bending of the ring, while the
ꢀ1
39
peak at 750 cm is attributed to outꢀofꢀplane bending.
ꢀ1
In addition, the peak at 421 cm may be related to the
4
0,41
ZnꢀN stretch mode.
The above characteristic peaks of
ZIFꢀ8 are obviously observed in the FTꢀIR spectra of
ZIFꢀ8/GO and PdꢀZIFꢀ8/rGO, indicating that the
immobilization of GO and Pd does not disturb the
coordination of 2ꢀmethylimidazole linker to the zinc(II)
centers and thus the formation of ZIFꢀ8. However, the
vibration peaks related to functional groups of rGO are
not observed in PdꢀZIFꢀ8/rGO, which is probably due to
51
method. Additionally, the intensity of the diffraction
peaks of PdꢀZIFꢀ8/rGO is weaker than that of ZIFꢀ8 and
ZIFꢀ8/GO, which could be due to the changes in the
charge distribution and electrostatic fields as the result of
the existence of Pd NPs on the surface and interaction of
37
52
the weak absorption peaks and high dispersion of rGO.
The powder XRD patterns of asꢀsynthesized GO,
their electrophilic surface with framework atoms.
Raman spectroscopy is a widely used and reliable
technique to characterize the structural changes that occur
before and after the reduction of GO. Raman spectra of
the GO, ZIFꢀ8 and PdꢀZIFꢀ8/rGO are presented in Fig.
rGO, ZIFꢀ8, ZIFꢀ8/GO and PdꢀZIFꢀ8/rGO samples are
shown in Fig. 1b. For GO, the peak located at 12.5° (d =
0
.71 nm) is attributed to (002) crystal plane of the layered
ꢀ1
structure of GO. The large interlayer distance is attributed
1c. The peaks at 685, 1142, 1184, 1460 and 1509 cm in
to the formation of hydroxyl, epoxy and carboxyl groups,
the spectra of ZIFꢀ8 are assigned to the vibrational mode
42ꢀ44
53
which increase the distance between the layers.
of the 2ꢀmethylimidazolate ligand.
The Raman
However, the disappearance of the peak at 12.5° and the
presence of a new broad diffraction peak at 23.7° (d =
spectrum of GO exhibits two feature peaks at 1337 and
ꢀ1
1596 cm , which are attributed to the D and G bands,
4
0
0
.37 nm) in the spectra of rGO indicate the reduction of
respectively. The G band represents the vibration of
45,46
2
GO to rGO.
For the spectrum of ZIFꢀ8/GO, no
graphitic (sp carbon) lattice, while the D band indicates
characteristic diffraction peaks of GO are observed,
which is attributed to that the high intensity of ZIFꢀ8 peak
at 12.4° overlapped the characteristic peak of GO at
the defects and disordered regions of the lattice caused by
54,55
oxidation reaction.
The intensity ratio of D band to G
band (I
D
/I
G
) indicates the degree of structural defects and
47
56
12.5°.
For the spectrum of PdꢀZIFꢀ8/rGO, no
a quantitative measurement of edge plane exposure. For
PdꢀZIFꢀ8/rGO, no obvious peaks of the ZIFꢀ8 bands are
observed in Raman spectra except for the appearance of
bands associated with GO owing to the strong intensity of
GO. The Raman spectra of PdꢀZIFꢀ8/rGO show higher
characteristic diffraction peaks of rGO are observed,
which is probably due to the high dispersion of rGO in
48
the composite. The diffraction patterns of ZIFꢀ8 are in
4
6,49
agreement with the previous reports,
confirming the
formation of pure crystalline ZIFꢀ8 phase. The typical
peaks of ZIFꢀ8 in the XRD patterns of ZIFꢀ8/GO and Pdꢀ
ZIFꢀ8/rGO indicate that ZIFꢀ8 is successfully
immobilized onto the surface of GO and its structure is
well preserved. According to the Scherrer equation, the
average ZIFꢀ8 particle sizes of pure ZIFꢀ8, ZIFꢀ8/GO and
PdꢀZIFꢀ8/rGO are 94, 53 and 56 nm, respectively, which
could be probably ascribed to that ꢀOH and ꢀCOOH
groups inhibit the growth of large crystals of ZIFꢀ8
D G
I (1.22) than that of GO (0.99), indicating that the GO
/I
is reduced to rGO during the information of PdꢀZIFꢀ
5
4
8/rGO using NaBH
4
as reductant. The result is
consistent with the results of FTꢀIR and XRD.
The UVꢀVis absorption spectra are used to study the
interaction between ZIFꢀ8 and GO. Fig. 1d shows the
UVꢀVis absorption spectra of GO, ZIFꢀ8, ZIFꢀ8/GO and
PdꢀZIFꢀ8/rGO dispersed in deionized water. The
absorption peak of GO at 233 nm and a shoulder peak at
about 300 nm could be assigned to the π→π transitions of
aromatic CꢀC bonds and the n→π transitions of C=O
bonds, respectively. The results are in agreement with the
31,50
through coordination modulation.
Small particle size
of ZIFꢀ8 anchored on GO could have good catalytic
activity in Knoevenagel condensation. In addition, no
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5
7
58
results of Zhang and Jahan . The UVꢀVis absorption
spectrum of pristine ZIFꢀ8 has an absorption band at 204
nm, while the UVꢀVis absorption spectrum of ZIFꢀ8/GO
and PdꢀZIFꢀ8/rGO composites shows a slight shift in
absorption band at 205 nm and 206 nm compared with
characteristic peaks at 340.5 eV and 335.3 eV for Pd 3d,
0
0
which are attributed to Pd (3d3/2) and Pd (3d5/2),
respectively. Meanwhile, peaks at 343.2 eV and 338.2 eV
2
+
2+
are attributed to Pd (3d ) and Pd (3d ), respectively.
3
2+
/2
5/2
0
The area ratio of Pd : Pd is about 7.3:1, demonstrating
59
2+
that of pristine ZIFꢀ8. This red shift in the absorption
band could be due to the charge or energy transfer
interaction between the polyaromatic scaffold in GO and
that most of Pd on the surface of the PdꢀZIFꢀ8/rGO was
30,44
reduced.
Graphene oxide is known to interact with supported
metal components affecting the catalyst’s properties and
subsequently its catalytic performance. Therefore, H2ꢀ
TPR of PdꢀZIFꢀ8/rGO catalyst can provide information
on the interactions between the Pd and rGO anchored
58,59
ZIFꢀ8,
suggesting the existence of the coordination
interaction between the GO and ZIFꢀ8 in the in situ
synthesized ZIFꢀ8/GO and PdꢀZIFꢀ8/rGO composites,
which is consistent with the results of XRD.
XPS was performed to further investigate the
interaction between active sites (ZIFꢀ8 and Pd) and rGO
in PdꢀZIFꢀ8/rGO. The XPS wide scan spectrum shows
that PdꢀZIFꢀ8/rGO is composed of C, N, O, Zn and Pd
2
with ZIFꢀ8. Fig. 3 shows the H ꢀTPR profiles of Pd/ZIFꢀ
8, Pd/(ZIFꢀ8+rGO) and PdꢀZIFꢀ8/rGO carried out under
identical operating conditions, indicating the reduction
behavior of incorporated Pd NPs. The H
2
ꢀTPR profile of
o
(
Fig. 2a). The existences of Zn and Pd indicate that ZIFꢀ8
Pd/ZIFꢀ8 shows a broad peak (centered at 176 C)
65
and Pd are successfully immobilized on the surface of
rGO. The C 1s XPS spectrum of GO display four
characteristic peaks at 284.8 eV, 286.6 eV, 287.0 eV and
corresponds to the reduction of PdO particles. However,
the reduction temperature of PdO in Pd/(ZIFꢀ8+rGO) and
o
PdꢀZIFꢀ8/rGO decreases to 168 C. The observed
288.2 eV (Fig. 2b), attributed to CꢀC/C=C, CꢀO, C=O and
decrease in the reduction temperature is attributed to the
OꢀC=O, respectively, which are in agreement with the
smaller PdO particles of Pd/(ZIFꢀ8+rGO) and PdꢀZIFꢀ
6
0ꢀ62
66
results reported previously.
For the XPS spectra of
8/rGO, which are more easy to be reduced. As observed
ZIFꢀ8, the C 1s spectra can be deconvoluted into two
from the H ꢀTPR profiles in Fig. 3, the quantification of
2
peaks at 284.6 eV and 285.4 eV (Fig. 2c), corresponding
consumed hydrogen of PdꢀZIFꢀ8/rGO (2.96 mmol H
is higher than that of Pd/ZIFꢀ8 (1.29 mmol H /g) and
Pd/(ZIFꢀ8+rGO) (1.87 mmol H /g). These results suggest
2
/g)
19
to the CꢀC bond and CꢀN bond, respectively. The C 1s
peaks of ZIFꢀ8 and the C=O bond of GO are observed in
the C 1s XPS spectrum of ZIFꢀ8/GO, however, other
functional groups (CꢀC/C=C, CꢀO and OꢀC=O) disappear
in comparison with those of GO, which is attributed to
the chemical interaction between ZIFꢀ8 and GO. The
2
2
that PdꢀZIFꢀ8/rGO shows strong covalent interaction
between the Pd NPs and the surface carbon atom of GO
6
7
in PdꢀZIFꢀ8/rGO. This strong interaction is essential for
the dispersion of the Pd NPs and consequently the high
catalytic activity and recyclability of the PdꢀZIFꢀ8/rGO
catalysts.
4
0
similar results were also obtained by Luanwuth. As
shown in Fig. 2e, a strong peak at 284.7 eV (C=C/CꢀC) is
dominant and the peak intensity of C=O decreases
significantly compared with those of ZIFꢀ8/GO,
indicating that the GO component is reduced to rGO by
The TGA curves of rGO, ZIFꢀ8, ZIFꢀ8/rGO and Pdꢀ
ZIFꢀ8/rGO are shown in Fig. 4. The TGA curve of rGO
shows two weight loss steps. The first weight loss of
about 44.5% between 25 °C and 250 °C corresponds to
NaBH . Moreover, a new C 1s peak at 291.4 eV
4
68
corresponding to PdꢀC bond is observed in comparison
with that of ZIFꢀ8/GO, indicating the existence of
covalent interaction between the Pd NPs and the surface
the removal of oxygenꢀcontaining functional groups.
The second weight loss of about 55.5% between 250 °C
and 500 °C is due to the decomposition of the carbon
63
54,68
carbon atom of GO in PdꢀZIFꢀ8/rGO. Similar results
skeleton.
For ZIFꢀ8, a small weight loss of about
64
were reported by Yan and coworkers, which confirmed
covalent interaction between the Pd NPs and graphene by
IR spectroscopy for the first time. Fig. 2f shows the
7.10% up to 250 °C corresponds to the removal of
adsorbed water and other absorbed unreacted molecules,
and the weight loss of 61.6% from 250 to 650 °C is
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attributed to the structural degradation and decomposition
of organic ligands. The final white solid residue was
identified as ZnO by XRD according to the report of
particle size of ~90 nm. Fig. 6c and d show SEM images
of ZIFꢀ8/GO and PdꢀZIFꢀ8/rGO, in which the ZIFꢀ8 can
be observed as irregular hexagons with ~50 nm size
covered the GO sheets. Pd NPs in PdꢀZIFꢀ8/rGO can not
be observed in the SEM image due to their small size.
Fig. 6e shows TEM image of PdꢀZIFꢀ8/rGO, in which the
curled and corrugated layered sheets of GO and spherical
morphology of ZIFꢀ8 are clearly observed. Furthermore,
Pd NPs also can be observed obviously as black dots with
an average size of approximately 2.8 nm calculated from
a statistical evaluation of 100 particles, suggesting that Pd
NPs are effectively immobilized onto the GO sheets (Fig.
6g). No significant aggregation of Pd NPs is observed in
PdꢀZIFꢀ8/rGO, which is mainly attributed to the fact that
PVP was a protecting agent in the course of preparation.
In addition, the diameter of Pd NPs is larger than the
diameter of the channel or the cavity inside the ZIFꢀ8 (the
large pores with diameters of 1.16 Å). Therefore, these
large Pd NPs cannot enter the pores of ZIFꢀ8. Fig. 6f
shows that the Pd NPs are crystalline with a spacing of
0.224 nm, corresponding to the Pd (111) interꢀplanar
4
0
Luanwuthi. The TGA curves also show that ZIFꢀ8/rGO
and PdꢀZIFꢀ8/rGO composites have excellent chemical
o
stability between 25 to 200 C with the small weight loss
about 4.05% and 2.12%, respectively. According to the
above results, the Pd, ZIFꢀ8 and rGO content in PdꢀZIFꢀ
8
/rGO are about 11.8%, 78.4% and 7.63%, respectively.
Additionally, Pd loading determined by ICPꢀAES is about
12.5%, which is similar to the result of TGA.
3
.1.2 Morphology and textural property of GO, ZIF-8,
2
ZIF-8/GO and Pd-ZIF-8/rGO. The N adsorptionꢀ
desorption isotherms and the corresponding pore size
distribution curves (PSD curves) of ZIFꢀ8, ZIFꢀ8/GO and
PdꢀZIFꢀ8/rGO are shown in Fig. 5. A typical Type I
isotherm with a steep increase at low relative pressures (<
0
.01) indicates that all ZIFꢀ8 composites are
4
6
microporous.
The hysteresis loops indicating the
existence of mesoporous structures corresponding to the
central cavities in ZIFꢀ8 composites and inevitable voids
32,52,69
72
between neighboring ZIFꢀ8 nanocrystals.
The
spacing. In addition, the EDS spectra of the PdꢀZIFꢀ
corresponding PSD curves of ZIFꢀ8, ZIFꢀ8/GO and Pdꢀ
ZIFꢀ8/rGO also show that the samples exhibit a dominant
pore diameter of approximately 0.66 nm (Fig. 5 inset).
The specific surface area, the micropore volume and the
8/rGO composite clearly show that the material consists
of C, O, N, Zn and Pd, further indicating that ZIFꢀ8 and
Pd are successfully immobilized on the surface of rGO
(Fig. 6h).
2
3
pore size of ZIFꢀ8 are 1417 m /g, 0.58 cm /g and 0.66
nm, respectively (Table 1), which are very close to the
The above characterization results indicate that ZIFꢀ8
and Pd NPs were successfully immobilized on the surface
of rGO by the coordination and covalent interaction,
respectively. In addition, ZIFꢀ8 and Pd NPs are wellꢀ
distributed on the rGO, the unique structure of which can
provide high surface area and high catalytic activity.
6
0,70
results in the previous reports.
For ZIFꢀ8/GO, the
decreases in BET surface area and pore volume compared
with those of ZIFꢀ8 are due to the introduction of GO to
ZIFꢀ8 (shown in Table 1). For PdꢀZIFꢀ8/rGO, a slight
decrease in BET surface area compared with that of ZIFꢀ
8
/GO is observed, which might be due to the introduction
3.2 Catalytic Activity
7
1
of Pd NPs to ZIFꢀ8/GO (Table 1). However, the
incorporation of Pd NPs does not alter the poreꢀsize
distribution of the ZIFꢀ8/GO, consistent with the fact that
the Pd NPs are anchored on the surface of rGO (Fig. 5
inset).
PdꢀZIFꢀ8/rGO was used as catalyst for oneꢀpot synthesis
of benzylmalononitrile (Scheme 1). The tandem reaction
involves two separate steps: 1) Knoevenagel
condensation between benzaldehyde (A1)
and
malononitrile to benzylidenemalononitrile (B ) and 2) the
1
The morphology of GO, ZIFꢀ8, ZIFꢀ8/GO and Pdꢀ
ZIFꢀ8/rGO was observed by SEM and TEM (Fig. 6). The
SEM image of GO exhibits curled and corrugated layered
structure. Fig. 6b shows SEM image of ZIFꢀ8, in which
the ZIFꢀ8 exhibits a hexagonal shape with an average
reduction of benzylidenemalononitrile (B
1
)
to
benzylmalononitrile (C ) under H
1
2
.
The first Knoevenagel condensation step could be
very important for the Knoevenagel condensationꢀ
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reduction tandem reaction. For comparison, GO, ZIFꢀ8,
ZIFꢀ8+GO (a physical mixture of GO and ZIFꢀ8) and
ZIFꢀ8/GO were used as catalysts in Knoevenagel
condensation (Fig. 7). The conversions of benzaldehyde
are 11.0%, 35.3%, 31.2% and 80.1% over GO, ZIFꢀ8,
ZIFꢀ8+GO and ZIFꢀ8/GO in 2.0 h, respectively (Fig. 7).
The low conversion of benzaldehyde over GO primarily
due to its weak acidity of GO. The conversion of
benzaldehyde reached nearly 100% over ZIFꢀ8/GO
within 4.0 h. Whereas, the conversion of benzaldehyde
reached nearly 100% over ZIFꢀ8 and ZIFꢀ8+GO after 6.0
h and 8.0 h under the same conditions, respectively. A
physical mixture of GO and ZIFꢀ8 can also promote the
Knoevenagel condensation reaction, but it was not as
efficient as the ZIFꢀ8 and ZIFꢀ8/GO catalyst probably due
to the increased diffusion limitations. The results
demonstrate that ZIFꢀ8/GO exhibits higher catalytic
activity than that of GO, ZIFꢀ8 and ZIFꢀ8+GO in the
Knoevenagel condensation reaction. The better
condensation activity of the ZIFꢀ8/GO was due to the the
πꢀπ interaction between GO and reactants (benzaldehyde
and malononitrile) and a good dispersion of ZIFꢀ8 on GO.
Then Pd NPs were immobilized onto the ZIFꢀ8/GO to
form a bifunctional PdꢀZIFꢀ8/rGO catalyst for the tandem
synthesis of benzylmalononitrile. Also, for comparison,
Pd/ZIFꢀ8, Pd/rGO and Pd/(ZIFꢀ8+rGO) were prepared by
than Pd loading for PdꢀZIFꢀ8/rGO was due to the strong
covalent interaction between the Pd NPs and the surface
carbon atom of GO, which facilitated good dispersion of
Pd NPs and also promote the reduction reaction
effectively.
The results of CO adsorption indicate that the surface
of GO anchored with ZIFꢀ8 favors the anchor of Pd NPs.
The effects of reaction conditions such as reaction time,
temperature and catalyst amount on the catalytic activities
were investigated (Fig. S2). The optimized reaction
conditions are reaction time 4+12 h (4.0 h for
Knoevenagel condensation and 12.0 h for reduction),
reaction temperature at 60 °C and catalyst amount of 40
mg. Then under the optimized reaction conditions, the
oneꢀpot synthesis of A
1
over the above catalysts using
was
toluene as solvent were investigated (Fig. 8). No C
1
formed in the presence of Pd/rGO due to no active sites
for Knoevenagel condensation. As expected, PdꢀZIFꢀ
8/rGO showed the best catalytic activity towards the
Knoevenagel condensationꢀreduction reaction with ~99%
benzaldehyde conversion, which is much higher than
those of both Pd/ZIFꢀ8 (68.5%) and Pd/(ZIFꢀ8+rGO)
(78.4%). The high conversion of benzaldehyde over Pdꢀ
ZIFꢀ8/rGO in the tandem reaction is attributed to the
good catalytic performance of ZIFꢀ8 and Pd NPs in the
Knoevenagel condensation and reduction, respectively.
Moreover, the good dispersion and small sizes of ZIFꢀ8
and Pd NPs also promote the tandem reaction effectively.
the same procedure using NaBH
loading determined by ICPꢀAES are 12.3%, 11.9%,
3.2% and 12.5% for Pd/rGO, Pd/ZIFꢀ8, Pd/(ZIFꢀ8+rGO)
4
as reductant. The Pd
1
1
In addition, the high selectivity to C (98.3%) over Pdꢀ
and Pd/ZIFꢀ8/rGO, respectively. The dispersion of Pd
NPs that calculated on the basis of CO adsorption is
summarized in Table 2. The degree of Pd dispersion of
PdꢀZIFꢀ8/rGO is higher than that of Pd/ZIFꢀ8 and
Pd/(ZIFꢀ8+rGO), which are also in accordance with the
ZIFꢀ8/rGO towards the Knoevenagel condensationꢀ
reduction reaction is due to the acidic sites of the rGO
and ZIFꢀ8. The same results were also obtained by
73
Zhang. Therefore, the above results suggest that the Pdꢀ
ZIFꢀ8/rGO could be used as the desired bifunctional
catalyst with spatially separated active sites for both
Knoevenagel condensation and reduction reaction in a
oneꢀpot reactor.
1
total selectivity towards the products C . Moreover, no
apparent aggregation is found for the Pd NPs with
uniform dispersion in PdꢀZIFꢀ8/rGO (Fig. 8), however,
there is obvious aggregation of the Pd NPs in Pd/(ZIFꢀ
In order to determine the general applicability and
efficiency of the PdꢀZIFꢀ8/rGO in the tandem reactions
with different substrates, various benzaldehyde
derivatives (A) and active methylene compounds were
investigated (Table 3). Introduction electron donor groups
at the paraꢀposition of the benzaldehyde did not affect the
high activities (Table 3, entries 2, 3 and 4) with
8+rGO) (Fig. S1). Thus the aggregation could be
contributed to the degree of Pd dispersion. The Pd NPs
sizes of PdꢀZIFꢀ8/rGO are smaller than those of Pd/ZIFꢀ8
and Pd/(ZIFꢀ8+rGO), which are attributed to the strong
covalent interaction between the Pd NPs and the surface
6
6
carbon atom of GO. Therefore, the higher Pd dispersion
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appropriate prolonged reaction time. However, when the
electron withdrawing group substituted at the paraꢀ
position, A was efficiently converted to the corresponding
products (B) in only 3.0 h (Table 3, entries 5 and 6),
which indicated that the electron withdrawing group was
tandem reaction. Combined with the results of catalytic
performance of the reused catalyst, leaking test, FTꢀIR
and XRD, PdꢀZIFꢀ8/rGO catalyst is recoverable and
recyclable. The anchor and stabilization effects of the
surface oxygen functional groups and defects on GO
could prevent the aggregation and leak of Pd NPs and
ZIFꢀ8, facilitating the recyclability and the
heterogenization of the active sites.
1
4,74
benefit for the Knoevenagel reaction.
On the other
hand, when the condensation reactant malononitrile was
substituted by ethyl cyanocaetate or diethyl malonate, the
yield for C decreased moderately (Table 3, entries 7 and
8), because malononitrile is the most active methylene
4
. Conclusions
compound among the three used condensation
1
9,75
A bifunctional PdꢀZIFꢀ8/rGO with spatially separated
active sites was prepared by a facile impregnation method
via coordination of ZIFꢀ8 and covalent interaction of Pd
on the surface of GO, respectively. PdꢀZIFꢀ8/rGO
showed good catalytic activity in the Knoevenagel
condensationꢀreduction tandem reaction of benzaldehyde
and its substituted derivatives, which is due to the high
catalytic activities of spatially separated Pd NPs and ZIFꢀ
substrates.
3
.3 Catalyst recycling
An important issue associated with the performance of a
heterogeneous catalyst is its recovery and reusability. The
PdꢀZIFꢀ8/rGO catalyst could be easily separated by
centrifugation after reaction. To investigate the
recyclability of the PdꢀZIFꢀ8/rGO catalyst, the catalyst
was recycled eight times. As summarized in Fig. 9A, no
obvious decrease of the benzaldehyde conversion and the
selectivity to benzylmalononitrile during the eight times
reuse. In order to confirm that the active species are not
the ZIFꢀ8 and Pd leaking from PdꢀZIFꢀ8/rGO catalyst in
the Knoevenagel condensation and reduction reaction, hot
filtration experiments were carried out. For Knoevenagel
condensation, after 0.5 h reaction time, the catalyst was
filtered, and reaction solution was kept on stirring under
the same reaction conditions. The conversion of
benzaldehyde almost remained constantly during the
period of 0.5 h to 4.0 h (Fig. 9B). For reduction reaction,
after 5.0 h reaction time, the catalyst was filtered, and
reaction solution was kept on stirring under the same
8
active sites and the enriched reactants on the surface of
PdꢀZIFꢀ8/rGO due to the πꢀπ interaction between rGO
and the reactants. The present approach provides a
promising and universal strategy for the construction of
hybrid catalysts based on MOFs, metal and GO materials
for the tandem reaction.
Acknowledgements
This work was financially supported by the National
Natural Sciences Foundation of China (Nos.21776058
and 21236001), Natural Sciences Foundation of Hebei
province (No.B2017202226 and E2015202306) and the
Tianjin Municipal Natural Science Foundation
(15JCYBJC21000) and Foundation of Education Bureau
of Hebei Province (QN2016152). We thank to Prof. J.
Zhao for helpful comments.
reaction
conditions.
The
conversion
of
benzylidenemalononitrile almost remained unchanged
during the period of 5.0 h to 16.0 h (Fig. 9C).
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Journal Name ARTICLE
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Catal. Sci. Technol., 2017, 00, 1-11 | 11

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Fig. 1. FTIR spectra of rGO, ZIF-8, ZIF-8/GO, Pd-ZIF-8/rGO and R-Pd-ZIF-8/rGO (a), XRD patterns of GO, rGO,
ZIF-8, ZIF-8/GO, Pd-ZIF-8/rGO and R-Pd-ZIF-8/rGO (b), Raman spectra of GO, ZIF-8 and Pd-ZIF-8/rGO (c) and
UV-Vis spectra of GO, ZIF-8, ZIF-8/GO and Pd-ZIF-8/rGO (d).
1

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Fig. 2. XPS wide scan spectra of Pd-ZIF-8/rGO (a) and C 1s XPS spectra of GO (b), ZIF-8 (c), ZIF-8/GO (d),
Pd-ZIF-8/rGO (e) and Pd 3d XPS spectra of Pd-ZIF-8/rGO (f).
2

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2
Fig. 3. H -TPR profiles of Pd/ZIF-8 (a), Pd/(ZIF-8+rGO) (b) and Pd-ZIF-8/rGO (c).
3

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Fig. 4. The TGA curves of rGO, ZIF-8, ZIF-8/rGO and Pd-ZIF-8/rGO.
4

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DOI: 10.1039/C7CY01725A
Fig. 5. N
2
adsorption-desorption isotherms of ZIF-8, ZIF-8/GO and Pd-ZIF-8/rGO and the corresponding pore-size
distributions calculated by the Horvath-Kawazoe method (inset).
5

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DOI: 10.1039/C7CY01725A
Fig. 6. SEM images of GO (a), ZIF-8 (b), ZIF-8/GO (c) and Pd-ZIF-8/rGO (d), TEM images of Pd-ZIF-8/rGO (e,
f), the size distribution plot of Pd NPs (g) and EDS image of Pd-ZIF-8/rGO (h)
6

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Page 18 of 26
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Fig. 7. Knoevenagel condensation of benzaldehyde with malononitrile over different catalysts. Reaction conditions:
A (1.9 mmol), toluene (5.0 mL), malononitrile (3.8 mmol), 25 °C, 8.0 h. catalyst: GO 10 mg, ZIF-8 30 mg,
1
physical mixture of ZIF-8 30 mg and GO 10.0 mg and ZIF-8/GO (the weight percentage of ZIF-8 is 75.0%) 40.0
mg.
7

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DOI: 10.1039/C7CY01725A
Fig. 8. The tandem reaction of Knoevenagel-reduction, conversion of A
1
(blue), selectivity to C
1
(pink) and
selectivity to B (green). Reaction conditions: A (1.9 mmol), toluene (5 mL), malononitrile (3.8 mmol), catalyst:
1
1
Pd/rGO 10 mg, Pd/ZIF-8 30 mg, Pd/(ZIF-8+rGO) 40 mg and Pd-ZIF-8/rGO 40 mg. Condensation was carried out
o
o
at 60 C for 4.0 h; reduction was carried out at atmospheric pressure with H
2
and at 60 C for 12.0 h.
8

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Fig. 9. (A) Recyclability test of the Pd-ZIF-8/rGO catalyst; (B) Hot filtration experiments for Knoevenagel
condensation reaction: (a) with the presence of Pd-ZIF-8/rGO; (b) Pd-ZIF-8/rGO was hot filtrated at 0.5 h; (C) Hot
filtration experiments for reduction reaction: (a) with the presence of Pd-ZIF-8/rGO; (b) Pd-ZIF-8/rGO was hot
filtrated at 5.0 h.
9

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DOI: 10.1039/C7CY01725A
Table 1. Textural parameters of ZIF-8, ZIF-8/GO and Pd-ZIF-8/rGO
BET Surface Area
Langmuir
External
Micropore
Pore
Sample
2
3
(
m /g)
Surface Area
Surface Area
Volume (cm /g)
Width
ZIF-8
1417
757
1876
1001
766
146
135
130
0.58
0.31
0.28
0.66
0.66
0.66
ZIF-8/GO
Pd-ZIF-8/rGO
578
1
0

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Table 2. CO characteristics of Pd/ZIF-8, Pd/(ZIF-8+rGO) and Pd-ZIF-8/rGO
a
b
b
Catalyst
Pd loading (wt.%)
Pd dispersion (%)
Particle size (nm)
CO adsorption (µmol/g)
Pd/ZIF-8
11.9
13.2
12.5
7.68
11.2
31.0
12.9
9.81
3.20
85.9
138.9
364.2
Pd/(ZIF-8+rGO)
Pd-ZIF-8/rGO
a
Determined by ICP.
b
Calculated according to CO uptake.
1
1

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Table 3. Knoevenagel condensation-reduction tandem reactions with different substrates over Pd-ZIF-8/rGO
R2
R2
R2
R3
H2
R1
O
R1
R3
R1
R3
A
B
C
Time
Conversion A
Selectivity ( % )
Entry
A
R
2
R
3
(
h)
( % )
C
B
CHO
1
2
3
4
5
6
7
8
CN
CN
CN
4+12
5+12
5+12
5+12
3+12
3+12
6+12
6+12
99.9
99.9
99.9
99.9
99.9
99.9
87.6
78.5
98.3
1.7
CHO
CN
96.7
92.5
93.2
91.2
92.8
92.3
88.6
3.3
7.5
6.8
8.8
7.2
7.7
11.4
HO
CHO
CHO
CN
CN
3
H C
CN
CN
3
H CO
CHO
CN
CN
F
CHO
CN
CN
Br
CHO
CN
COOEt
CHO
COOEt
COOEt
o
Reaction conditions: A (1.9 mmol), toluene (5 mL), 60 C, active methylene compound (3.8 mmol), Pd-ZIF-8/rGO
1
50 mg.
1
2

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DOI: 10.1039/C7CY01725A
CHO
CN
CN
NC
CN
H2
CN
CN
Pd-ZIF-8/rGO
Pd-ZIF-8/rGO
A1
B1
C1
Scheme 1. One-pot tandem Knoevenagel condensation-reduction reaction over Pd-ZIF-8/rGO.
1
3

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Scheme 2. Illustration of the preparation of Pd-ZIF-8/rGO catalyst.
1
4

Catalysis Science & Technology
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Table of contents entry
A novel bifunctional catalyst for Knoevenagel condensation-reduction tandem reaction was
prepared by immobilizing ZIF-8 and Pd on GO, respectively.
1