Palladium nanoparticles encapsulated inside the pores of a metal-organic framework as a highly active catalyst for carbon-carbon cross-coupling

Article abstract of DOI:10.1039/c4ra10065d

Well dispersed palladium nanoparticles were successfully encapsulated in the cages of a metal-organic framework, MIL-101, by using a "double solvents" method coupled with subsequent reduction with NaBH₄. Such a fabrication method can effectively avoid the aggregation of palladium nanoparticles on the external surfaces of MIL-101. The as-prepared catalyst, Pd@MIL-101, was characterized by transmission electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, N₂ adsorption and inductively coupled plasma atomic emission spectroscopy. The Pd@MIL-101 composite exhibited high catalytic activity toward the Suzuki-Miyaura and Heck cross-coupling reactions. The catalyst can be readily recovered and reused in at least 5 consecutive cycles without significant loss its catalytic activity. This journal is

Full text of DOI:10.1039/c4ra10065d

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Palladium nanoparticles encapsulated inside the
pores of a metal–organic framework as a highly
active catalyst for carbon–carbon cross-coupling†
Cite this: RSC Adv., 2014, 4, 54487
*
Ningzhao Shang, Shutao Gao, Xin Zhou, Cheng Feng, Zhi Wang and Chun Wang
Well dispersed palladium nanoparticles were successfully encapsulated in the cages of a metal–organic
framework, MIL-101, by using a “double solvents” method coupled with subsequent reduction with
NaBH4. Such a fabrication method can effectively avoid the aggregation of palladium nanoparticles on
the external surfaces of MIL-101. The as-prepared catalyst, Pd@MIL-101, was characterized by
transmission electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, N2
adsorption and inductively coupled plasma atomic emission spectroscopy. The Pd@MIL-101 composite
exhibited high catalytic activity toward the Suzuki–Miyaura and Heck cross-coupling reactions. The
catalyst can be readily recovered and reused in at least 5 consecutive cycles without significant loss its
catalytic activity.
Received 9th September 2014
Accepted 17th October 2014
DOI: 10.1039/c4ra10065d
www.rsc.org/advances
Currently, metal–organic frameworks (MOFs) have received
signicant attention in recent years as a novel class of nanoporous
Introduction
materials mainly due to their tunable pore sizes, high specic
surface areas, the possibility to functionalize, and designable
framework structures modularly built from transition-metal
clusters as nodes and organic ligands as struts.13–15 Due to their
outstanding features, they have found potential applications in
hydrogen storage, drug delivery, gas separation, sensing, and
catalysis.16–21 MOF materials as host matrices to support Pd NPs
for catalyzed C–C coupling reactions have been reported
recently.22–29 It is well known that the size and location of sup-
ported palladium NPs have an important effect on their catalytic
activity. Loading of metal NPs inside the porous matrices of MOF
is of current interest.30–33 Various techniques, such as impregna-
tion, chemical vapor deposition (CVD), solution inltration, and
solid grinding, have been adopted to encapsulate metal NPs
within MOF pores.34–37 Jiang and co-workers prepared Pd/MIL-101
by impregnation method and subsequent molecular H2 reduction
and which was used as catalyst for the Suzuki coupling reac-
tions.24 The reactions were carried out at 80 ꢀC under an inert
atmosphere. In 2011, Cao et al. used amino-functionalized MIL-
53(Al)-NH2 supported Pd NPs as a catalyst for the Suzuki coupling
reaction.25 As an extension, they also reported the preparation of
Pd NPs supported on amine-functionalized mix-linker MIL-53(Al)
materials and their application for Heck reaction.26 In addition,
The palladium-catalyzed carbon–carbon coupling reactions,
such as Suzuki–Miyaura and Heck reactions, are of great
importance as very versatile routes to construct biaryl units in
organic chemistry and industrial processes.1–4 Traditional
Suzuki and Heck reactions are performed mostly with homo-
geneous palladium salts or organometallic complexes with
special ligands (such as phosphine complexes). Although
homogeneous catalytic systems are known to exhibit better
activity than heterogeneous systems, for large scale applications
in liquid phase reactions, they bring about concerns of high
cost and environmental issues. Conversely, heterogeneous
catalysts can be easily separated from the reaction systems and
reused. However, Pd nanoparticles (NPs), important kind of
heterogeneous catalysts, are prone to aggregate and lose their
catalytic activity without the use of a stabilizer.5 Therefore, the
suitable stabilizers and/or supports for Pd NPs catalysts are
generally required. A number of solid materials, such as
carbon,6,7 zeolites,8 mesoporous silica9–11 and polymers,12 have
been used as catalysts for C–C coupling reactions. However,
these materials have one or more limitations in terms of
stability, catalytic efficiency, and recyclability. Therefore, it is
still attractive to explore efficient heterogeneous Pd catalyst for
the coupling reactions.
´
Martın-Matute group followed this idea and immobilized Pd NPs
into an amino-functionalized MOF, MIL-101Cr-NH2. Good yields
were obtained for a wide scope of substrates under remarkably
mild conditions.28 [Pd(C3H5)(C5H5)]@MOF-5 catalyst was
prepared by a CVD method. The resulting supported Pd catalyst
showed good initial activity for Suzuki reaction in the solvent of
dimethylformamide (DMF) with K2CO3 as the base.27 However,
College of Sciences, Agricultural University of Hebei, Baoding 071001, Hebei Province,
P. R. China. E-mail: chunwang69@126.com; Fax: +86-312-7528292
† Electronic supplementary information (ESI) available: EDX data and N2 sorption
isotherms of Pd@MIL-101, size distribution of Pd NPs. See DOI:
10.1039/c4ra10065d
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aqueous hydrouoric acid (HF, 40%), sodium borohydride
(NaBH4, 99%), terephthalic acid (HO2CC6H4CO2H), palladium
chloride (PdCl2), aryl boronic acid, aryl halides were purchased
from Aladdin Reagent Limited Company. Potassium carbonate,
n-hexane, anhydrous ethanol, DMF, and ether were all obtained
from Chengxin Chemical Reagents Company (Baoding, China)
and used as received.
The size and morphology of the nanoparticles were observed
by transmission electron microscopy (TEM) using a JEOL model
JEM-2011(HR) at 200 kV. The X-ray diffraction (XRD) patterns of
the samples were recorded with a Rigaku D/max 2500 X-ray
diffractometer using Cu Ka radiation (40 kV, 150 mA) in the
range 2q ¼ 10o–80ꢀ. X-ray photoelectron spectroscopy was per-
formed with a PHI 1600 spectroscopy using Mg Ka X-ray source
for excitation. The Pd content was determined by means of
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) on Thermo Elemental IRIS Intrepid II. The Brunauer–
Emmett–Teller (BET) surface areas measurements were per-
formed with N2 adsorption/desorption isotherms at liquid
nitrogen temperature (77 K) aer dehydration under vacuum at
150 ꢀC for 12 h using a V-Sorb 2800P volumetric adsorption
equipment (Jinaipu, China). The pore volume was calculated by
a single point method at P/P0 ¼ 0.99.
Scheme 1 Schematic representation of immobilization of Pd NPs to
a MIL-101 matrix and the C–C coupling reactions catalyzed by
Pd@MIL-101.
with these methods, the precursor compounds can actually
diffuse out of the pores of MOFs to form the aggregated nano-
particles on the external surfaces of MOFs, which would decrease
the catalytic activity and reusability of the catalyst.38 So it is always
a down-to-earth question of how to avoid the formation of metal
NPs on the external surfaces of MOFs.
Recently, double solvents method (DSM) derived from
incipient wetness impregnation techniques has been developed
by Xu et al., which could greatly minimize the deposition of
metal precursors on the outer surface.39 However, the immo-
bilization of Pd nanoparticles in the pores of MIL-101 and their
catalytic performance for different organic transformations are
still untouched.
Catalyst preparation
MIL-101 were synthesized and puried according to the method
described by Ferey et al.40 Terephthalic acid (3.76 g),
´
Cr(NO3)3$9H2O (6.0 g), 40% HF (0.75 mL) and de-ionized water
(75 mL) were placed in a 100 mL Teon-liner autoclave and
heated at 220 ꢀC for 8 h. Aer cooling, the mixture was ltered
to remove the unreacted crystals of terephthalic acid, and then
MIL-101 (Cr3F(H2O)2O[(O2C)C6H4(CO2)]3$nH2O, n z 25) is a
ꢀ
40
further puried by solvothermal treatment in ethanol at 80 C
´
Cr-based MOF synthesized by Ferey et al., the interconnected
for 24 h. The resulting green solid was soaked in NH4F (1 mol Lꢁ1
)
three-dimensional hydrophilic zeotypic cavities accessible
through small pore windows could be an appropriate host for
nucleation and growth of metal NPs as such systems allow the
metal precursors to inltrate the cavities through the small pore
windows and prevent metal nanoparticles from escaping or
agglomerating from cavities due to the size limitations to cross
the windows.
As a continuation of our effort to develop heterogeneous
transitional metal catalysts and new functional materials,41–44
herein, the ultrane Pd nanoparticles were encapsulated in the
pores of MIL-101 (Pd@MIL-101) without aggregation on the
external surface of the host framework by using the double
solvents method for the rst time. To investigate the catalytic
performance of the as-prepared Pd@MIL-101 catalyst, Suzuki
and Heck reactions were selected as the model reactions
(Scheme 1). The results demonstrated that the as-obtained
catalyst exhibited high catalytic activity toward the Suzuki–
Miyaura and Heck cross-coupling reactions.
solution at 70 ꢀC for 24 h to eliminate the terephthalic acid inside
the pores of MIL-101 and immediately ltered, washed with water
several times, nally dried overnight at 150 ꢀC under vacuum to
obtained activated MIL-101 powder.
For preparing Pd encapsulated by MIL-101, 100 mg of acti-
vated MIL-101 powder was suspended in 20 mL of anhydrous n-
hexane as hydrophobic solvent and the mixture was sonicated
for 15 min until it became homogeneous. Aer stirring for 1 h,
0.16 mL of aqueous PdCl2 solution (20 mg mLꢁ1) as the
hydrophilic solvent was added dropwise over a period of 15 min
with constant vigorous stirring. The resulting solution was
continuously stirred for 2 h. Aer ltration, the green powder
was dried in air at room temperature. These synthesized
samples were further dried at 150 ꢀC under vacuum for 12 h. An
overwhelming reduction approach was employed by using 3.5
mL freshly prepared 0.6 mol Lꢁ1 NaBH4 aqueous solution while
vigorous stirring, resulting in the generation of catalysts as a
dark green suspension. The synthesized samples were collected
by centrifuging, and used for the catalytic reactions.
Experimental section
Materials and methods
General procedure of Suzuki reactions
All chemicals were commercial and used without further puri- A mixture of aryl bromide or iodide (0.5 mmol), phenylboronic
cation. Chromic nitrate nonahydrate (Cr(NO3)3$9H2O, 99%), acid (0.6 mmol), potassium carbonate (1.5 mmol), 4 mL
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EtOH–H2O (1 : 1, v/v) and the catalyst Pd@MIL-101 (3 mg, 0.15
mol%) was stirred at room temperature for a desired reaction
time. Aer the reaction was complete, the mixture diluted with
10 mL of H2O and extracted with ethyl acetate (3 ꢂ 10 mL). The
organic layers were combined, dried over anhydrous MgSO4 and
ltered. The ltrate was concentrated by vacuum. The pure
products were obtained by ash chromatography on silica gel
with petroleum ether/ethyl acetate as the eluent.
Characterization of the catalysts
The synthesized Pd@MIL-101 catalyst was characterized by
transmission electron microscopy (TEM), energy dispersive X-ray
spectroscopy (EDX), powder X-ray diffraction (PXRD), X-ray
photoelectron spectroscopy (XPS), N2 adsorption and induc-
tively coupled plasma atomic emission spectroscopy (ICP-AES).
The TEM images (Fig. 1a and b) show that the Pd nanoparticles
were highly dispersed. The average sizes of the Pd NPs were 2.4
nm (Fig. S1, see ESI†), which are small enough to be accommo-
dated in the mesoporous cavities of MIL-101. The corresponding
carbon, oxygen, chromium and palladium element mappings
were shown in Fig. 1c–f, respectively. Fig. 1f clearly demonstrated
the uniform distribution of ultrane Pd nanoparticles
throughout the Pd@MIL-101 composite. The EDX data (Fig. S2,†)
further conrm the presence of the Pd in the Pd@MIL-101
composite. The palladium content in Pd@MIL-101 was deter-
mined by means of ICP-AES and amounted to 2.7 wt%.
The X-ray powder diffraction patterns of our synthesized
samples of MIL-101 (Fig. 2a) matched with those reported in the
literature.40 Furthermore, no signicant diffraction peak char-
acteristic of Pd species is detected from the wide-angle XRD
pattern for Pd@MIL-101 (Fig. 2b), which can be ascribed to the
encapsulated of Pd nanoparticles into the pores of MIL-101, low
Pd loading and small well dispersed palladium nanoparticles.
This observation is very similar to the results reported by Zhou
et al.22
General procedure of Heck reactions
Aryl halide (0.5 mmol), vinyl substrate (0.75 mmol), K2CO3
(0.75 mmol), TBAB (0.5 mmol) and Pd@MIL-101 (3 mg,
0.15 mol%) was added to DMF (2 mL). The reaction mixture was
stirred at 120 ꢀC for a desired time. Aer the reaction was
complete, the solution was centrifuged and washed with ethyl
acetate for three times. The organic phase was subsequently
washed with brine, dried over anhydrous MgSO4 and ltered.
The ltrate was concentrated by vacuum. The pure products
were obtained by ash chromatography on silica gel with
petroleum ether/ethyl acetate as the eluent.
Hot ltration test
In the case of the Suzuki reaction, reaction mixture of bromo-
benzene (0.5 mmol), phenylboronic acid (0.6 mmol), potassium
carbonate (1.5 mmol), EtOH–H2O (4 mL, 1 : 1, v/v) and the
catalyst Pd@MIL-101 (3 mg, 0.15 mol%) was stirred at room
temperature. Aer 0.5 h, the catalyst was separated by centri-
fugation, the reaction solution was stirred at ambient temper-
ature for another 2 h, and the progress of the reaction was
examined by ash chromatography on silica gel with petroleum
ether/ethyl acetate as the eluent.
The XPS spectra (Fig. 3) demonstrate that the Pd species in the
sample Pd@MIL-101 was present in the metallic state with the
bond energy about 335.5 and 340.98 eV in the Pd 3d5/2 and 3d3/2
Results and discussion
For the DSM, the double solvents, a hydrophilic solvent (water)
and a hydrophobic solvent (n-hexane), was used in this paper.
The former containing the metal precursor (PdCl2) with a
volume set equal to or less than the pore volume of the MOF
could be absorbed within the hydrophilic pores of the MOF. The
latter, in a large amount, was used to suspend the adsorbent
and facilitate the impregnation process. Because the inner
surface area of MIL-101 is much larger than the outer surface
area, the small amount of PdCl2 aqueous solution could go
inside the hydrophilic pores by capillary force, which make the
deposition of PdCl2 on the outer surface of the MOF reduce
greatly. In contrast, a large amount of solvent containing the
metal precursor is used in the conventional single-solvent
impregnation process, some of which will be deposited on the
outer surfaces of the MOF aer drying, generating aggregated
MNPs on the outer surfaces.45 At the same time, an over-
whelming reduction approach with
NaBH4 solution was carried out for avoiding Pd nanoparticles
aggregation on external surfaces of the MOF.
a high-concentration
Fig. 1 (a and b) TEM images of Pd@MIL-101, (c–f) the corresponding
carbon (c), oxygen (d), chromium (e), and palladium (f) element
mappings, respectively.
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were summarized in Table 1 (entries 4–6), with the amount of
catalyst xed as 0.15 mol%, an excellent yield was obtained
when the inorganic base K2CO3 was used.
Further experiments shows that 0.15 mol% amount of
Pd@MIL-101 was sufficient to guarantee a clean and complete
conversion. The reaction does not take place in the absent of
catalyst (Table 1, entry 8). The yield of biphenyl increased from
51% to 97% when the amount of catalyst increased from 0.05
mol% to 0.15 mol% (Table 1, entries 4, 9). When the amount of
the catalyst was larger than 0.15 mol%, the yield of biphenyl no
longer increased greatly (Table 1, entry 10).
With the optimized reaction conditions in hand, the scope of
catalytic utility of Pd@MIL-101 in Suzuki reaction was explored
with various aryl halides and arylboronic acids. Those reactions
were carried out in the presence of K2CO3 using EtOH–H2O
(1 : 1, v/v) as environmentally benign solvents. It is known that
the yields of the Suzuki coupling reactions depend on the type
of halide element, the positions of the substitutional groups
and their electron withdrawing/donating capabilities. As shown
in Table 2, excellent yield was achieved in the reactions of aryl
iodides with phenylboronic acid (Table 2, entries 1–3). For the
less reactive bromobenzene, the coupling reactions could also
proceeded well and the reactions could be completed in 1–6
hours at ambient temperature with good to excellent yields
(Table 2, entries 4–9). Among them, the electron donating
groups on aryl bromides needed relatively long reaction time (6
h). However, when those reactions were carried out at 60 ꢀC, the
coupling reaction can be completed within 1.5 h with yields of
95–99% (Table 2, entries 10–13). Unfortunately, this catalytic
system was less effective for the reaction of inactive aryl chlo-
Fig. 2 PXRD spectrum of MIL-101 (a) and Pd@MIL-101 (b) samples.
core level. The presence of PdO species in Pd@MIL-101 was
conrmed by tted the high energy shoulder on the metallic Pd
lines at bond energy of about 336.95 and 342.6 eV.46 The
production of oxidized Pd species could be due to the oxidation
of metallic Pd le in an oxygen-containing environment.47,48
The N2 physisorption–desorption isotherms of the Pd@MIL-
101 were shown in Fig. S3.† The experimental multipoint Bru-
nauer–Emmett–Teller (BET) surface areas and the total pore
volume of the MIL-101 and Pd@MIL-101 were calculated to be
3664.6 m2 gꢁ1and 1.79 cm3 gꢁ1, 1986.0 m2 gꢁ1and 1.0 cm3 gꢁ1
,
respectively. The great decrease in the amount of N2 adsorption
and the pore volume of Pd@MIL-101 indicated that the pores of
the MIL-101 were occupied by the highly dispersed Pd
nanoparticles.
The nature of base in Suzuki coupling reaction greatly affects
the efficiency of the reaction. In general, the reactivities of
different bases for the coupling reactions of aryl iodꢁides and
rides (Table 2, entries 14, 15) because of the strength of the C–C
bond, whose bond dissociation energy was 96 kcal molꢁ1
.
52
In order to evaluate the efficiency of the catalyst Pd@MIL-101
for the Suzuki–Miyaura reaction, the present catalytic system
was compared with other reported Pd nanoparticles catalysts in
terms of the yield of the product, the dosage of catalyst, reaction
bromides decreased in the order: OHꢁ > CO3 > AcO .28 High
2ꢁ
concentrations of base can lead to the formation of unreactive
boronates (ArB(OH)3ꢁ) and thus, the [OHꢁ]/[ArB(OH)2] ratio
needs to be controlled.49,50 Moreover, MIL-101 was not stable
enough under these strongly basic conditions.51 For this reason,
carbonates were preferred because the release of hydroxy anions
into the aqueous medium was relative slow. Carbonates, K2CO3,
Na2CO3, and NaHCO3 were used as the base and the results
Table 1 Optimization of the reaction conditions for the Suzuki reac-
tion of bromobenzene with phenylboronic acida
Entry Solvents
Bases
Pd/mol% Time/h Yield/%
1
2
3
4
5
6
7
8
MeOH
EtOH
H2O
K2CO3
K2CO3
K2CO3
0.15
0.15
0.15
0.15
0.15
2
2
2
2
2
2
2
2
2
2
0.5
1
3
15
17
24
97
84
50
—
—
51
98
41
75
99
43b
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) Na2CO3
EtOH–H2O (1 : 1) NaHCO3 0.15
EtOH–H2O (1 : 1)
—
0.15
—
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (1 : 1) K2CO3
9
0.05
0.25
0.15
0.15
0.15
0.15
10
11
12
13
14
2
a
Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid
b
(0.75 mmol), base (1.5 mmol), solvents (4 mL), 25 ꢀC. The catalyst
was removed aer 0.5 h, the ltrate was stirred for another 2 h.
Fig. 3 XPS measurement of Pd@MIL-101.
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Table 2 Pd@MIL-101-catalyzed Suzuki coupling reaction
The Heck reaction is also an important method oen used
for the formation of C–C bonds by coupling aryl halides with
activated alkenes. First, the coupling of iodobenzene with
styrene was chosen as model reaction. The reactions were
carried out with Pd@MIL-101 as a catalyst and DMF as a solvent
under air atmosphere (Table 4). It was found that K2CO3 worked
the best compared with Na2CO3, NaHCO3, KOH and NaOAc.
The addition of tetrabutylammonium bromide (TBAB)
enhanced the yield of the product, presumably due to the
stabilization effect of TBAB for the Pd(0) species.56 The inuence
of solvent on the catalytic activity was also investigated with the
optimized base. The catalytic activity of the catalyst in DMF was
higher than that in H2O, ethylene glycol, and ethanol (Table 4,
entries 1, 6–8). For the reaction of iodobenzene with styrene in
DMF nearly quantitative conversion was obtained at 120 ꢀC for
2 h. We next tested the effects of the amount of catalyst on the
yields of the products. From the results listed in Table 3
(Table 4, entries 1, 9–11), we can see that 0.15 mol% amount of
catalyst was sufficient to guarantee a clean and complete
conversion. When the amount of the catalyst was reduced to
0.05 mol% under otherwise same conditions, the yield
decreased to 49%. Moreover, the reaction can not occur in the
absence of Pd catalyst (Table 4, entry 9).
Under the optimized reaction conditions, the scope of the
Pd@MIL-101 catalyzed Heck reactions were investigated by
employing a variety of substrates. These results are summarized
in Table 5. The Heck reactions of aryl iodobenzene with styrene
proceeded easily with excellent yield at 120 ꢀC (Table 5, entries
1–3). In addition, various olen derivatives were applied to this
reaction, resulting in the formation of cinnamic esters in yields
of 95–99% (Table 5, entries 4–6). Owing to the higher bond
energy of the C–Br bonds, aryl bromide compounds are difficult
to activate. Therefore, the reactions of bromobenzene with
different olens required relative long reaction time and the
yields were ranged in 46–84% (Table 5, entries 7–10).
To verify the heterogeneity of the catalyst, a hot ltration
test was performed for the Suzuki–Miyaura reaction of bro-
mobenzene with phenylboronic acid. Aer 0.5 h, the Pd@MIL-
101 catalyst was separated though centrifugation and the
ltrate was monitored under identical reaction conditions for
another 2 h. It was noticed that no further conversion was
detected aer the removal of the catalyst (Table 1, entry 14).
The inductively coupled plasma atomic emission spectroscopy
analysis of the supernatant solution of the reaction mixture
thus collected by ltration also conrmed the absence of
palladium ions in the liquid phase. These results clearly
demonstrate that Pd nanoparticles were not being leached out
from the MIL-101 during the cross-coupling reactions, which
can be ascribed to the encapsulated of Pd nanoparticles into
the pores of MIL-101.
The recyclability of the catalyst was further investigated
because the recycling of the heterogeneous catalysts plays an
important role in practical applications. The heterogeneous
catalyst, Pd@MIL-101, can be separated easily via ltration.
The Suzuki coupling reaction between bromobenzene and
phenylboronic acid and the Heck reaction of iodobenzene with
styrene under the optimized reaction conditions were used for
Entry
Aryl halides
Time/h
Products
Yielda/%
1
2
1
2
95
97
3
4
5
6
7
1
2
1
6
6
99
97
99
91
92
8
6
90
9
6
95
98
96
99
10b
11b
12b
1.5
1.5
1.5
13b
14
1.5
12
95
Trace
Trace
15
12
a
Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid
(0.75 mmol), K2CO3 (1.5 mmol), EtOH–H2O (1 : 1) (4 mL), 25 ꢀC. b 60 ꢀC.
temperature and time (Table 3). The data in Table 3 indicated
that the as-prepared Pd@MIL-101 can efficiently catalyze the
coupling reaction with comparable or higher yield, low catalyst
loading and relative short reaction time. Especially the reaction
catalyzed by Pd@MIL-101 can be performed efficiently at room
temperature without using toxic organic solvent. It has been
known that the catalytic activity generally increases with
decreasing metal NP size, as smaller particles possess higher
surface areas available for reactants. The high catalytic activity
of Pd@MIL-101 could be attributed to the well dispersion of Pd
nanoparticles in the pores of MIL-101 and the connement
effect. The presence of monodisperse pores in MIL-101 has a
connement effect on the growth and gathering of Pd particles
in the cages, which would avoid the aggregation of Pd particles
before and aer catalysis.
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Table 3 Suzuki coupling reaction of aryl halide with phenylboronic acid catalyzed by different catalysts
Aryl halide
Catalyst
Reaction conditions solvent/temp./time/Pd loading
Yield/%
Ref.
Bromobenzene
p-Bromoanisoles
Bromobenzene
Bromobenzene
Bromobenzene
p-Bromoanisoles
Bromobenzene
Pd/MIL-53-NH2
Pd@MIL-101-NH2
Pd@MOF-5
IRMOF-3-PI-Pd
Pd/PMA-MIL-101
MOF-253-PdCl2
Pd@MIL-101
EtOH–H2O/40 ꢀC/0.5 h/0.5 mol%
EtOH–H2O/23 ꢀC/0.5 h/3 mol%
DMF/90 ꢀC/20 h/2 mol%
94
95
81
98
50
90
97
25
28
27
53
54
55
EtOH–H2O/80 ꢀC/2 h/0.024 mol%
DMF/130 ꢀC/24 h/1.3 mol%
DMF–EtOH/100 ꢀC/14 h/0.23 mol%
EtOH–H2O/25 ꢀC/1–6 h/0.15 mol%
This study
Table 4 Optimization of the reaction conditions for Heck reaction of
iodobenzene with styrene catalyzed by Pd@MIL-101a
Entry Solvent
Base
Catalyst/mol% Time/h Yields/%
1
2
3
4
5
6
7
8
DMF
DMF
DMF
DMF
DMF
H2O
K2CO3
Na2CO3 0.15
NaHCO3 0.15
KOH
NaOAc
K2CO3
0.15
2
2
2
2
2
2
2
2
2
2
2
98
62
37
55
21
5
9
7
—
49
82
0.15
0.15
0.15
0.15
0.15
—
Ethylene glycol K2CO3
Ethanol
DMF
DMF
DMF
K2CO3
K2CO3
K2CO3
K2CO3
9
10
11
0.05
0.1
Fig. 4 Reusability of the Pd@MIL-101 catalyst for the Suzuki and Heck
reactions.
a
Reaction conditions: iodobenzene (0.5 mmol), styrene (0.75 mmol),
base (0.75 mmol), TBAB (0.5 mmol), solvent (2 mL).
Table 5 Results for the Heck reaction catalyzed by Pd@MIL-101a
Conclusions
In conclusion, we have successfully encapsulated the ultrane
Pd in the pores of MIL-101 by the double solvents method in
combination with the overwhelming reduction approach, which
could minimize the aggregation of the NPs on the external
surface of the host framework. Those well-dispersed Pd NPs
show high catalytic activity for the Suzuki and Heck coupling
reactions. Moreover, the Pd@MIL-101 catalyst exhibit high
stability which can be recycled and reused easily without
reduction of catalytic activity. The present study might highlight
the development of high performance heterogeneous catalysts
by using functionalized cavities of MOFs as hosts for ultrane
metal NPs.
Entry
R1
X
R2
Time/h
Yields/%
1
2
3
4
5
6
7
8
9
H
I
I
I
I
I
I
Br
Br
Br
Br
Ph
Ph
Ph
CO2Me
CO2Et
CO2Bu
Ph
CO2Me
CO2Et
CO2Bu
2
2
2
2
2
2
24
24
24
24
98
97
92
95b
99
97
46
81b
80
84
4-COCH3
4-OCH3
H
H
H
H
H
H
H
10
Acknowledgements
a
Reaction conditions: aryl halide (0.5 mmol), Pd@MIL-101 (0.15
mol%), olen (0.75 mmol), K2CO3 (0.75 mmol), TBAB (0.5 mmol),
DMF (2 mL), 120 ꢀC. b 2 equiv. of methyl acrylate was used.
This work was nancially supported by the National Natural
Science Foundation of China (no. 31171698, 31471643), the
Innovation Research Program of Department of Education of
Hebei for Hebei Provincial Universities (LJRC009), Natural
Science Foundation of Hebei Province (B2011204051) and the
Natural Science Foundation of Agricultural University of Hebei
(LG201404).
reusability experiments. As can be seen from Fig. 4, aer being
recycled for ve successive runs, the catalyst still exhibits an
excellent activity. Inductively coupled plasma atomic emission
spectroscopy analysis showed that the leaching of Pd into the
reaction solution is negligible, which could account for the
preservation of the catalytic activity of Pd@MIL-101
composite.
Notes and references
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RSC Adv., 2014, 4, 54487–54493 | 54493