Palladium nanoparticles supported on amino functionalized metal-organic frameworks as highly active catalysts for the Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.catcom.2011.07.004

Well dispersed Pd nanoparticles supported on amino functionalized metal-organic frameworks MIL-53(Al)-NH₂ (Al(OH)[H₂N-BDC], H₂N-BDC = 2-aminoterephthalic acid, MIL = Materials of Institut Lavoisier) were prepared using a direct anionic exchange approach and subsequent reduction with NaBH₄. The Pd/MIL-53(Al)-NH₂ catalyst exhibitted high activity and good stability for Suzuki-Miyaura cross-coupling reaction.

Full text of DOI:10.1016/j.catcom.2011.07.004

Catalysis Communications 14 (2011) 27–31
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Palladium nanoparticles supported on amino functionalized metal-organic
frameworks as highly active catalysts for the Suzuki–Miyaura
cross-coupling reaction
⁎
Yuanbiao Huang, Zhaoliang Zheng, Tianfu Liu, Jian Lü, Zujin Lin, Hongfang Li, Rong Cao
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Well dispersed Pd nanoparticles supported on amino functionalized metal-organic frameworks MIL-53(Al)-
NH2 (Al(OH)[H2N-BDC], H2N-BDC=2-aminoterephthalic acid, MIL=Materials of Institut Lavoisier) were
prepared using a direct anionic exchange approach and subsequent reduction with NaBH4. The Pd/MIL-53
(Al)-NH2 catalyst exhibitted high activity and good stability for Suzuki–Miyaura cross-coupling reaction.
© 2011 Elsevier B.V. All rights reserved.
Received 26 March 2011
Received in revised form 18 June 2011
Accepted 5 July 2011
Available online 19 July 2011
Keywords:
Metal-organic frameworks
Palladium nanoparticles
Amine
Suzuki–Miyaura coupling reaction
1. Introduction
protecting groups [14]. Therefore, there are a relatively limited
reports on Pd NPs embedded into MOFs [11–14] and even fewer of
Palladium-catalyzed Suzuki–Miyaura coupling reactions of aryl
halides with arylboronic acids are of great importance as a versatile
route to construct biaryl units in organic synthesis [1]. Although
homogeneous palladium catalysts have been extensively investigated,
their practical applications in pharmaceutical industry remain
challenging because the catalysts are expensive and difficult to
separate from the products for re-use [1,2]. By contrast, heteroge-
neous palladium catalysts have been proven to be promising
alternatives to avoid of these problems. A number of solid materials,
such as carbon structures [3], polymers [4], mesoporous silica [5] and
zeolites [6] have been employed as supports for palladium catalysts.
However, the catalytic activities of many supported palladium
catalysts are usually lower than the homogeneous ones [7].
Recently, metal-organic frameworks (MOFs) have emerged as a
new class of attractive functional materials that can be constructed by
metal centers and organic ligands [8–15]. Due to their extraordinarily
high surface area and well defined pore structure, MOFs can be used
for the stabilization of metal nanoparticles with adjustable sizes [14].
Although there have been several approaches for loading palladium
nanoparticles (Pd NPs) onto porous MOFs using solution infiltration
and surface grafting methods [14,16–20], development of a general
and facile method to obtain Pd NPs supported on MOFs remains
challenging due to its easy agglomeration and leach without
them as catalysts towards C–C coupling reactions [21–27].
MIL-53(Al)-NH2 (Al(OH)[H2N-BDC], H2N-BDC =2-aminoter-
ephthalic acid), which has uncoordinated amine groups, contains 1D
channels with different pore sizes depending on the nature of the
inserted molecules [28–31]. The MIL-53(Al)-NH2 is high thermal (up
to 400 °C) and chemical stability to water and common organic
solvents. The introduction of amino functional groups into MIL-53
(Al)-NH2 drastically enhances its affinity to CO2, resulting in a very
large selectivity in CO2/CH4 separations [29]. More importantly, these
functional groups can be used in Knoevenagel condensation reac-
tion [31] and postsynthetic modification [28,30]. Herein, we choose
amino functionalized MIL-53(Al)-NH2 to support palladium NPs as a
highly active catalyst for the Suzuki–Miyaura cross-coupling reaction.
The amine groups of MIL-53(Al)-NH2 can stabilize palladium NPs to
prevent agglomeration [25].
2. Experimental
2.1. Catalyst preparation
2.1.1. Synthesis of MOFs
MIL-53(Al)-NH2[29,31] and MIL-53(Al) [32] were synthesized
according to the literatures. The as-made MIL-53(Al)-NH2 was
washed thoroughly in turn with DMF and EtOH, followed by soaking
in methanol at 70 °C for 24 h. The solid was finally dried overnight at
130 °C under vacuum. The as-synthesized MIL-53(Al) was treated at
330 °C for 12 h to remove the unreacted terephthalic acid in the
⁎
Corresponding author. Tel./fax: +86 591 83796710.
E-mail address: rcao@fjirsm.ac.cn (R. Cao).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.07.004

28
Y. Huang et al. / Catalysis Communications 14 (2011) 27–31
channels. A solution of tetrachloropalladinic acid (H2PdCl4) was
prepared by mixing 0.20 g of PdCl2 into 15 ml of 10% v/v HCl aqueous
solution under stirring at 300 K until complete dissolution. All other
reagents were purchased commercially and used as received.
2.1.2. Synthesis of Pd nanoparticles
The activated MIL-53(Al)-NH2 (0.50 g, 40 ml H2O) was treated
with hydrochloric acid to tune at pH~4. A solution of H2PdCl4
(containing ca.1.0 wt.% of Pd) was added to the above acidulated
slurry under vigorous agitation for 10 min. The mixture was then
stirred for another 8 h. The solid was centrifuged and washed with de-
ionized water and ethanol, respectively. The resulting PdCl42−-MIL-53
(Al)-NH3+ samples (1.12 wt.% Pd based on ICP-AES) were then
reduced by sodium borohydride (NaBH4, 0.04 g) at 273 K for 3 h to
yield Pd/MIL-53(Al)-NH2 (0.97 wt.% Pd based on ICP-AES). A solution
of H2PdCl4 (containing ca.1.0 wt.% of Pd) was added to the activated
MIL-53(Al) (0.50 g) which suspended in water (40 ml H2O) under
vigorous agitation for 10 min. The mixture was then stirred for
another 24 h. The solid was centrifuged and washed with de-ionized
water and ethanol, respectively. The resulting MIL-53(Al) samples
containing palladium salts were then reduced by sodium borohydride
(NaBH4, 0.04 g) at 273 K for 3 h to obtain Pd/MIL-53(Al) (0.19 wt.% Pd
based on ICP-AES).
Fig. 1. Support of Pd NPs in the amino functionalized MIL-53(Al)-NH2 via a two-step
process (see text). (aluminium atoms, olivine; carbon atoms, black; oxygen atoms, red;
nitrogen atoms, blue; hydrogen atoms, white; palladium, and orange).
(Fig. 2). The characteristic peak of Pd (111) at 2θ=40.1° was
indistinguishable due to the low Pd loading and small palladium
nanoparticles [22]. The TEM images (Fig. 3) show that the Pd NPs
were highly dispersed, with a mean diameter of 3.12 nm (Fig. 4). The
results conclude that the Pd nanoparticles are too large to get into the
pores of MIL-53(Al)-NH2 and attached on the surface of the support
materials. However, it could not preclude that smaller palladium
particles deposited in the pores of MIL-53(Al)-NH2[27]. In general, for
small Pd NPs (b1 nm) embedded within the frameworks, charges
tend to accumulate and images become distorted. Therefore, it is
difficult to make HRTEM imaging Pd NPs in the small pore size
(0.86 nm) [22]. The EDS spectrum confirms the presence of the Pd in
the Pd/MIL-53(Al)-NH2 samples (Fig. S8). The decrease in surface area
and pore size indicated that the cavities of MIL-53(Al)-NH2 could be
occupied and/or blocked by the palladium NPs that were located at
the surface (Fig. S8 and Table S1). The XPS studies of Pd/MIL-53(Al)-
NH2 implied that most of Pd NPs were in the reduced form (Fig. S10).
The amino groups in the linker of MIL-53(Al)-NH2 immobilize the
H2PdCl4 anions to form MIL-53(Al)-NH3+-PdCl42−. Therefore it can
enhance the Pd loading on the supports (0.97 wt.%) compared to that
unfunctionalized MIL-53(Al) (only 0.19 wt.%). The Pd NPs supported
on MIL-53(Al)-NH2 were well dispersed on the outside surface and no
obvious aggregation was observed, which utilizes electrostatic
attraction for uniform NPs formation on a large area scale, whereas
Pd/MIL-53(Al) aggregated immediately (Fig. S6). It may not preclude
2.2. Catalyst characterization
The as-synthesized materials were characterized by means of the
XRD, ICP-AES, SEM, HRTEM, EDS, XPS and BET. The detailed
procedures were described in the Supplementary data.
2.3. Catalytic reactions
Typically, aryl halide, phenylboronic acid, Na2CO3, and palladium
catalyst were added to a mixing solvent of water and ethanol
(4 ml/2 ml). The reaction mixture was stirred at a desired tempera-
ture under air atmosphere. The solution was centrifuged and washed
with ethyl acetate. The above solution was subsequently extracted
with ethyl acetate (3×10 ml), dried over Na2SO4, and concentrated
under vacuum. The crude product was quantified firstly by GC-MS
analysis and then purified by silica gel chromatography (mixture of
light petroleum and ethyl acetate as eluent). The identification was
conducted by GC-MS analysis and 1H NMR and 13C NMR
measurement.
To study the catalysis of Pd leaching during the reaction, after
reaction, the mixture was hot filtrated under vacuum. The solid was
washed with water (5 ml) and ethanol (5 ml), and the liquid phase
was analyzed by ICP-AES. Moreover, a hot-filtration experiment was
also run to investigate whether the reaction proceeded in a
heterogeneous or homogeneous fashion. After 20 min, the catalyst
was separated by hot-filtration and the filtrate was further stirred for
another 12 h.
3500
3000
2500
2000
1500
1000
500
c
For the recyclability experiment, the catalyst was recovered at the
end of catalytic reaction of brombenzene and phenylboric acid, washed
with water and ethanol, respectively, followed by drying at 60 °C under
vacuum. The dried catalyst was then reused in the next run.
b
3. Results and discussion
a
MIL-53(Al)-NH2 was synthesized and purified according to the
reported procedure [29,31]. The chloride anions in the acidulated MIL-
53(Al)-NH2 were firstly exchanged with [PdCl4]2− precursors.
Supported Pd NPs were then obtained by reduction with NaBH4 at
273 K (Fig. 1). After the loading of palladium (0.97 wt.% based on ICP-
AES), there is no significant loss of crystallinity in X-ray diffraction
patterns, and no supplementary Bragg peaks appears, indicating that
the integrity of the MIL-53(Al)-NH2 framework was maintained
0
5
10
15
20
25
30
35
40
45
50
2 Theta (degree)
Fig. 2. Powder XRD patterns of MIL-53(Al)-NH2 samples: (a) activated MIL-53(Al)-
NH2; (b) Pd/MIL-53(Al)-NH2, before catalysis reaction; (c) Pd/MIL-53(Al)-NH2, after
five circles catalysis.

Y. Huang et al. / Catalysis Communications 14 (2011) 27–31
29
Fig. 3. TEM (a, b) and HRTEM (c) and dark-filed TEM (d) images of Pd NPs supported on MIL-53(Al)-NH2 (0.97 wt.% Pd).
that there exists coordination interactions between palladium and
amino group. The amino group at the functionalized linker proved to
be beneficial for the immobilization of Pd species [14].
The supported Pd/MIL-53(Al)-NH2 catalyst described above was
applied to the Suzuki–Miyaura coupling reaction (Tables 1 and 2). The
effect of the catalyst amount was first investigated on brombenzene
with phenylboronic acid using Na2CO3 as base in water/ethanol (2:1)
at 40 °C. It was found that the amount of catalyst affected the yields of
the reaction obviously (entries 2–4, Table 1) [2]. The use of only
0.05 mol% Pd of catalyst gave low yield (23%, entry 3, Table 1). Good
yields could be obtained in presence of 0.5 mol% Pd of catalyst (92%,
entry 2, Table 1). Different benzene halides also significantly
influenced the reaction yields. When iodobenzene and brombenzene
were used, very high yields of biphenyl were obtained in 30 min
under air atmosphere (entries 1 and 2, Table 1). We next tried to
extend the substrate to chlorobenzene. However, only trace amounts
of cross-coupling products were formed for chlorobenzene under the
similar conditions (entry 5, Table 1). It is well known that the
activation of C–Cl bond is much more difficult than that of C–Br and C–
I bonds, resulting in much harsher reaction conditions in heteroge-
neous catalysis system [23]. Increasing temperature or extending
reaction time enhanced the yields significantly (entries 6 and 7,
Table 1). Interestingly, When the reaction was carried out using
NaOMe as base under N2[23], the higher yield was obtained as well as
64% for chlorobenzene (TON=128, entry 8, Table 1). However,
unfortunately, the support MIL-53(Al)-NH2 degraded slowly at the
prolonged time in the presence of the strong base NaOMe. To
investigate if there did exist the Ullmann reaction and homocoupling
reaction of phenylboronic acid, we chose the reactions in the absence
of arylboronic acids or aryl bromides. The results showed that there
was hardly any product of Ullmann reaction and homocoupling
reaction of phenylboronic acid in our system (entries 9 and 10,
Table 1) [23].
50
Mean size 3.12 nm
40
SD 0.69
30
20
10
0
To investigate the scope of aryl bromides that could be tolerated in
the Suzuki–Miyaura coupling reaction with phenylboronic acid,
different aryl bromides were employed in the reactions (Table 2).
The reactions proceeded extraordinarily well with a wide range of aryl
0
1
2
3
4
5
6
Diameter / nm
Fig. 4. Particle size distribution of the Pd/MIL-53(Al)-NH2 (0.97 wt.% Pd).

30
Y. Huang et al. / Catalysis Communications 14 (2011) 27–31
97
Table 1
96
97
96
100
95
Suzuki–Miyaura coupling reactions of aryl halides and phenylboronic acid over Pd/MIL-
53(Al)-NH2.a
80
60
40
20
0
Pd/MIL-53(Al)-NH2
X
B(OH)2
H2O/EtOH
Entry
X
Yield (100%)b
TON [(molprod)./(molPd)]
1
2
I
94 (99)
92 (97)
23 (29)
69 (74)
Trace
198
194
580
740
–
Br
Br
Br
Cl
Cl
Cl
Cl
Br
–
3c
4d
5
6e
7f
8g
9h
10i
21 (25)
31 (36)
59 (64)
Trace
50
72
128
–
1
2
3
4
5
Run
Trace
–
a
Reaction conditions: Aryl halide (1 mmol), phenylboronic acid (1.2 mmol), Na2CO3
(1.5 mmol), Pd/MIL-53(Al)-NH2 (0.5 mol% Pd), H2O/EtOH (4 ml/2 ml), 40 °C, 30 min.
Fig. 5. Recycling experiment of Pd/MIL-53(Al)-NH2 catalyst towards the Suzuki–
Miyaura reaction of brombenzene and benzoboric acid for 30 min.
b
Yield of isolated product (GC yield in parenthesis).
Pd/MIL-53(Al)-NH2 (0.05 mol% Pd).
Pd/MIL-53(Al)-NH2 (0.1 mol% Pd).
80 °C.
80 °C, under N2, 24 h.
80 °C, NaOMe (1.5 mmol), under N2, 24 h.
Only brombenzene.
c
d
A commercial Pd/C catalyst (5 wt.% palladium) was also tested under
the same conditions. Only 44% yield of cross-coupled product was
obtained using Pd/C in the coupling of brombenzene with phenyl-
boronic acid (TON=88, entry 11, Table 2).
e
f
g
h
i
Only phenylboronic acid.
Recycling studies were then performed under the optimal reaction
conditions. The supported Pd/MIL-53(Al)-NH2 catalyst can be easily
recovered and reused in the Suzuki–Miyaura coupling reaction of
brombenzene and phenylboronic acid for five cycles without any
visible loss of activity (Fig. 5). XRPD patterns indicated that the
crystalline structure of the catalyst was mostly retained after five
catalytic cycles (Fig. 2). The liquid phase of the reaction mixture was
collected by hot-filtration and analyzed by ICP-AES. Very low Pd
leaching was detected (0.3 ppm) at the end of the reaction, indicating
that the Pd NPs were stabilized by the amine groups of the MIL-53
(Al)-NH2. Moreover, a hot-filtration experiment was also run to
investigate whether the reaction proceeded in a heterogeneous or
homogeneous fashion. After 20 min, the solid catalyst was separated
by hot-filtration and the filtrate was further performed for another
12 h. It was clear that the reaction was completely terminated after
removal of the catalyst. These results suggested that the reaction may
bromides containing electron-withdrawing (entries 1–4, Table 2) and
electron-donating groups (entry 5, Table 2). It should be noted that 2-
bromoanisol, which contains electron-donating group and is sterically
hindered, also gave excellent yield (entry 6, Table 2). Furthermore,
different arylboronic acids were also carried out under air atmo-
sphere. Both of the arylboronic acids with electron-donating and
electron-withdrawing substituents were found to give good yields
(entries 7–8, Table 2). It was found that the yields decreased slightly
for the sterically hindered arylboronic acids (entry 9, Table 2).
For comparison, palladium supported on MIL-53(Al) without
amine groups [32] was also prepared by impregnating PdCl2 on the
vacuum dried MIL-53(Al). Lower activity was observed for palladium-
supported MIL-53(Al) in the coupling of brombenzene with phenyl-
boronic acid under the same conditions (TON=98, entry 10, Table 2).
Table 2
Suzuki–Miyaura coupling reactions of different brombenzene derivatives and arylboronic acid over supported palladium catalysts.a
Entry
R1
R2
Yield (100%)b
TON [(molprod)./(molPd)]
1
2
3
4
5
6
7
8
4-NO2
4-CN
4-CF3
4-COCH3
4-OMe
2-OMe
H
H
H
H
H
H
H
H
H
H
H
97 (99)
95 (99)
93 (99)
94 (96)
91 (94)
88 (92)
87 (90)
95 (97)
91 (94)
45 (49)
37 (44)
198
198
198
192
188
184
180
194
188
98
4-NO2
4-OMe
2-OMe
H
9
10c
11d
H
88
a
Reaction conditions: Aryl halide (1 mmol), Arylboronic acid (1.2 mmol), Na2CO3 (1.5 mmol), Pd/MIL-53(Al)-NH2 (0.5 mol% Pd), H2O/EtOH (4 ml/2 ml), 40 °C, 30 min.
Yield of isolated product (GC yield in parenthesis).
Pd/MIL-53(Al) (0.5 mol% Pd).
b
c
d
Pd/C (0.5 mol% Pd).

Y. Huang et al. / Catalysis Communications 14 (2011) 27–31
31
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proceed over the catalyst surface in a heterogeneous fashion [23].
Although there is still a possibility that re-deposition of Pd leaching
species can occur in coupling reactions [33,34], the good reusability of
this catalyst make it more like a heterogeneous one. The results
illustrated that Pd NPs stabilized by the amine groups of the MIL-53
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4. Conclusions
In summary, palladium NPs supported on amino functionalized
MIL-53(Al)-NH2 were prepared using a simple direct anionic
exchange method and subsequent chemical reduction with NaBH4.
The palladium NPs stabilized by amine groups showed high activity
and good stability for Suzuki–Miyaura coupling reaction under mild
conditions.
Acknowledgments
We acknowledge the financial support from 973 Program
(2011CB932504, 2007CB815303), NSFC (20731005, 2103128 and
91022007), Fujian Key Laboratory of Nanomaterials (2006 L2005),
and the Key Project from CAS.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.07.004.
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