Palladium nanoparticles supported on mixed-linker metal-organic frameworks as highly active catalysts for heck reactions

Article abstract of DOI:10.1002/cplu.201100021

Well-dispersed palladium nanoparticles (Pd NPs) supported on amine-functionalized, mixed-linker metal-organic frameworks (MIXMOFs) based on MIL-53(Al) were prepared by using the ion-exchange method. Pd NPs were characterized by powder X-ray diffraction (X

Full text of DOI:10.1002/cplu.201100021

DOI: 10.1002/cplu.201100021
Palladium Nanoparticles Supported on Mixed-Linker
Metal–Organic Frameworks as Highly Active Catalysts for
Heck Reactions
[
a]
Yuanbiao Huang, Shuiying Gao, Tianfu Liu, Jian Lꢀ, Xi Lin, Hongfang Li, and Rong Cao*
Well-dispersed palladium nanoparticles (Pd NPs) supported on
amine-functionalized, mixed-linker metal–organic frameworks
spectroscopy (ICP-AES), and X-ray photoelectron spectroscopy
(XPS). The mean diameter of Pd NPs is approximately 3.2 nm. It
was found that the Pd NPs supported on amine-functionalized
MIXMOFs are stable at high temperature. The Pd NPs exhibit
efficient catalytic activity for Heck reaction and can be easily
recovered and reused.
(MIXMOFs) based on MIL-53(Al) were prepared by using the
ion-exchange method. Pd NPs were characterized by powder
X-ray diffraction (XRD), N adsorption, transmission electron mi-
2
croscopy (TEM), inductively coupled plasma atomic emission
Introduction
The palladium-catalyzed Heck reaction between aryl halides
and olefins is one of the most important reactions for the for-
mation of carbon–carbon bond in synthetic organic chemis-
Recently, mixed-linker metal–organic frameworks (MIXMOFs),
which incorporate different functionalities on linking groups
[34–42]
by the mixing linkers, have attracted substantial attention.
[
1]
try. In traditional Heck reactions, with palladium salts or or-
ganometallic complexes with special ligands (such as phos-
phines) that are sensitive to moisture and air, are generally
The MIXMOFs would constitute new materials that contain a
tunable number of functional sites and would inevitably lead
[34]
to a complex chemical environment.
Therefore, they may
[
1]
used, and the homogeneous systems often suffer difficulty in
provide opportunities for the preparation of materials with un-
usual properties, such as thermal stability, gas adsorption, and
[
2]
separation and recycling of expensive Pd catalyst. Therefore,
it is desirable to develop heterogeneous catalysts for Heck re-
[34–42]
catalysis.
MIL-53(Al) (Al(OH)[O C–C H –CO ]) is one of the
2 6 4 2
[
3]
actions to overcome such problems. Heterogeneous catalysts
are distinguished from homogeneous catalysts in the industrial
application for their advantages of easy of separation of prod-
uct and the recovery of expensive catalyst. However, Pd NPs—
an important kind of heterogeneous catalysts—are prone to
aggregate and lose their catalytic activity without the use of a
most outstanding materials that has been extensively used in
[43–46]
gas storage and separation science.
Amine-functionalized
MIL-53(Al)-NH2 (Al(OH)[H N-BDC], where H N-BDC=2-amino-
2
2
terephthalic acid), is an MIL-53(Al) analogue that contains free
[25,47–51]
amine group.
It not only drastically enhances the affinity
for CO and basic catalysis, but can also be used for postmodi-
2
[
4]
[49]
stabilizer. Therefore, they need to be supported on solid ma-
terials (such as carbon frameworks, zeolites, silica, polymers,
fication of amine groups with a set of cyclic anhydrides. Re-
cently, Kleist, Baiker, and co-workers reported that amine-func-
tionalized, mix-linker MIL-53(Al) materials exhibit higher ther-
[
1–14]
Al O ) to be useful as catalyst for Heck reactions.
In addi-
2
3
[40–41]
tion, in many cases, the catalytic active sites may be lost due
to the leaching of palladium in supported systems which
mostability than MIL-53(Al)-NH2.
As an extension of our
previous work on the preparation of Pd NPs supported on
amine-functionalized MIL-53(Al)-NH2 and their application in
[3]
makes the catalyst difficult to recovered and reused. There-
fore, a suitable supports for Pd NP catalysts are important for
heterogeneous catalysis.
[52]
Suzuki–Miyaura cross-coupling reactions,
herein we will
report the facile preparation of highly active and stable Pd NPs
supported on amine-functionalized, mix-linker MIL-53(Al) mate-
rials and their application in Heck reactions.
Metal–organic frameworks (MOFs) have emerged as very
promising functional materials for gas storage, separation, het-
erogeneous catalysis, sensing, and drug delivery because of
[15–25]
their high surface area, porosity, and chemical tunability.
Although there are several approaches that are used to load
Pd NPs onto porous MOFs by solution infiltration and surface
[
a] Dr. Y. Huang, S. Gao, Dr. T. Liu, Dr. J. Lꢀ, X. Lin, Dr. H. Li, Prof. R. Cao
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Science
155 Yangqiao West Road Fuzhou, Fujian 335002 (P. R. China)
Fax: (+86)591-83796710
[
25–29]
grafting methods,
developing a general and facile method
to incorporate active Pd NPs on MOFs remains a challenge
owing to the easy agglomeration and leaching into solution
[
25]
without the need for protecting groups. Thus, there are a
limited number of Pd NPs supported on MOFs and even fewer
of them can act as catalysts for heterogeneous cataly-
E-mail: rcao@fjirsm.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201100021.
[
15,17,25,30–33]
sis.
1
06
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ChemPlusChem 2012, 77, 106 – 112

Catalysis
Results and Discussion
Preparation and characterization of Pd NPs supported on
MIXMOFs
Compared with MIL-53(Al)-NH2, MIXMOFs 1, 2, and
3
(
Scheme 1) are more stable and economical to synthesize
[
41]
owing to the use of cheap terephthalic acid. Powder X-ray
Scheme 1. Synthesis of MIXMOFs 1 (x=0.1), 2 (x=0.5), and 3 (x=0.9) based
on MIL-53(Al). BDC=benzene-1,4-dicarboxylate, ABDC=2-aminobenzene-
Figure 2. Nitrogen sorption isotherms of MIL-53(Al), MIXMOFs 2, Pd/2, and
MIL-53(Al)-NH2.
1,4-dicarboxylate.
diffraction (XRD) studies indicate that all three compounds are
highly crystalline and of the same structures as in MIL-53(Al)
of the strong base ethylenediamine; an excess amount of
strong base may destroy the frameworks. Meanwhile the
amine-functionalized MIXMOFs based on MIL-53(Al) can be ob-
tained directly by replacing terephthalic acid with aminoter-
ephthalic acid, and thus avoids introducing the strong base
ethylenediamine to the MOFs. Herein, we use the ion-ex-
change approach for preparation of Pd NPs supported on
amine-functionalized MIXMOFs based on MIL-53(Al). The sur-
face amine groups of 1, 2, and 3 (see the Experimental Section
for details) were first neutralized by aqueous HCl, followed by
ionic reactions of the positively charged ammonium groups on
[
41,44,49]
and MIL-53(Al)-NH (Figure 1).
Nitrogen sorption studies
2
show that the Brunauer–Emmett–Teller (BET) surface area of
2
À1
2
À1
MIL-53(Al) (928 m g ) and MIL-53(Al)-NH (654 m g ) are
2
2
À1
lower than that of 2 (997 m g ; Figure 2). The results demon-
strate that the properties of MIXMOFs are not simple linear
sums of those of the pure components, and thus support the
notion that the sequence of functionalities within MIXMOFs
[
34]
can enhance a specific property.
The high surface area, porous structure, and stability in
water and common organic solvent make the amine-function-
alized MIXMOFs attractive catalyst supports. Recently, Fꢂrey
and co-workers reported an anion-exchange method for the
encapsulation of noble metals over the ethylenediamine-
grafted MIL-101(Cr) (Cr (F,OH)(H O) O[(O C)-C H -(CO )] ·nH O
2À [25b,29,52]
the surface with anionic noble metal salts [PdCl ] .
Fi-
4
nally, the obtained material containing palladium salts were re-
duced with NaBH at low temperature.
4
After the loading of palladium, there is no apparent loss of
crystallinity according to X-ray diffraction patterns, thus indi-
3
2
2
2
6
4
2
3
2
[
29]
[52]
(
nꢀ25)). However, it is difficult to control the concentration
cating that the framework of 2 is well maintained (Figure 3).
The characteristic peak of Pd (111) at 2q=40.18 is in-
distinguishable owing to the low Pd loading and
[15]
small diameter Pd NPs. The Pd/2 exhibits a dimin-
2
À1
ished BET surface area of 982 m g , whereas that of
2
À1
MIXMOFs 2 is 997 m g (Figure 2). The decrease of
surface area indicates that the cavities of 2 may be
occupied and/or blocked by the palladium NPs locat-
[15]
ed at the surface.
The TEM images of Pd/2
(
Figure 4) show that the Pd NPs are well dispersed on
the outer surface of the support and no aggregation
is observed, which are similar to the Pd NPs support-
[52]
[15]
ed on MIL-53(Al)-NH2 and MOF-5. The mean di-
ameter of Pd NPs is approximately 3.2 nm (Figure 4),
meanwhile the palladium nanoparticles supported on
[52]
the amino-free MIL-53(Al) aggregate immediately.
The presence of amino group on the functionalized
linker has been proven to be beneficial for the stabili-
[39]
zation of Pd species. It may not be precluded that
there exists coordination interactions between palla-
dium nanoparticles and amino groups. X-ray energy-
2
Figure 1. Powder XRD patterns of the MIL-53(Al), 1, 2, 3, and MIL-53(Al)-NH .
ChemPlusChem 2012, 77, 106 – 112
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www.chempluschem.org
107

R. Cao et al.
Figure 3. Powder XRD patterns of the MIXMOF 2 containing 50% of ABDC,
Pd/2, and Pd/2 after 5th cycle.
Figure 5. XPS measurement of Pd/2 (0.96 wt% of Pd).
0
dispersive spectroscopy (EDS) further confirm the presence of
peaks of Pd appear at 336.0 and 341.3 eV, respectively, and no
2
+
the Pd in the Pd/2 samples (Figure 4). In the X-ray photoelec-
obvious peak of Pd is observed, thus indicating that palladi-
5/2
3/2
[30]
tron spectroscopy (XPS) trace (Figure 5), the 3d and 3d
um is in the reduced form.
Heck reactions catalyzed by
Pd NPs
The Heck reactions were first
performed using a low amount
of palladium (a molar ratio: aryl
halide/Pd=2000) and an excess
of olefin with respect to aryl
halide (molar ratio=1.5) to
avoid of the formation of homo-
[3]
coupling by-products.
Pure
coupling products were ob-
tained by column chromatogra-
phy and characterized by NMR
spectroscopy. The compounds
were selectively obtained with
trans configuration.
Initially, we used Pd/MIL-
[52]
53(Al)-NH2
to investigate the
effects of base and solvent for
the Heck reaction of bromben-
zene and styrene at 1208C
(
Table 1). The results showed
that organic base triethylamine
gave higher yields than that of
inorganic bases such as Na CO ,
2
3
K PO , and NaOAc (Table 1, en-
3
4
tries 1–4). No product was ob-
tained in the absence of the
base (Table 1, entry 5). The sol-
vent also had a significant influ-
ence on the catalyst activity.
Compared with NMP and
Figure 4. TEM and HRTEM images of a) and b) Pd/2 (0.96 wt% of Pd) before the reaction, c) and after five cycles.
d) Size distribution of Pd NPs; mean size 3.2 nm, standard deviation 0.69. Inset in (a) is the EDS pattern of Pd/2.
1
08
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ChemPlusChem 2012, 77, 106 – 112

Catalysis
Table 1. Influences of the base and solvent on the Heck reaction of
brombenzene (1 mmol) with styrene (1.5 mmol).
Table 2. Influence of the supports on the Heck reaction of brombenzene
(1 mmol) with styrene (1.5 mmol).
[
a]
[a]
[
b]
À1
[b]
[c]
À1
Entry
Base
Na CO
Solvent
Yield [%]
TOF [h
]
Entry
Catalyst
Yield [%]
TOF [h ]
1
2
3
4
5
6
Pd/MIL-53(Al)
26
76
93
86
81
49
trace
87
254
310
287
270
164
–
1
2
3
4
2
3
DMF
DMF
DMF
DMF
DMF
NMP
o-xylene
DMF
DMF
DMF
DMF
65
73
42
81
trace
77
69
trace
59
28
217
244
140
270
–
257
230
–
1, Pd/MIL-53(Al) and 10% NH
2, Pd/MIL-53(Al) and 50% NH
3, Pd/MIL-53(Al) and 90% NH
2
2
2
K
3
PO
4
NaOAc
NEt
–
3
[c]
Pd/MIL-53(Al)-NH
2
5
6
7
8
9
1
Pd/C
–
NEt
NEt
NEt
NEt
NEt
NEt
3
3
3
3
3
3
[
d]
7
[
[
d]
e]
[
[
0
a] Reaction conditions: NEt
.05 mol% of Pd. [c] Yield of isolated product. The products have trans
configuration based on NMR spectroscopy. [d] Without Pd catalyst.
3
(1.5 mmol), DMF (3 mL), 6 h. [b] Used
983
933
138
f]
0
[
g]
11
83
[a] Reaction conditions: Pd/MIL-53(Al)-NH
2
(0.05 mol% of Pd), base
(1.5 mmol), solvent (3 ml), 6 h. [b] Yield of isolated product. The products
have trans configuration based on NMR spectroscopy. [c] Without base.
[
d] Only MIL-53(Al)-NH
2
(0 mol% of Pd). [e] 0.01 mol% of Pd.
groups on the functionalized linker proved to be beneficial for
the immobilization of Pd species, which prevent the agglomer-
ation of Pd NPs. However, the strong interaction between the
excess amine groups and Pd species may lead to weakening of
[f] 0.005 mol% of Pd. [g] 0.1 mol% of Pd. DMF=N,N-dimethylformamide,
NMP=N-methylpyrrolidone.
[53,54]
o-xylene (Table 1, entries 6 and 7), higher TOF ((mol of prod)
per (mol of Pd per hour)) was obtained in DMF (Table 1,
entry 4). It was noted that no Pd black was observed under
such rigorous conditions during the reaction. Therefore, the
amino-stabilized Pd NPs are air-stable up to 1208C and their
catalytic activity is virtually independent of the presence of air.
The amount of catalyst is also very important for the reaction.
The reaction could not happen in the presence of only MIL-
the palladium activation for the substrate.
It may be that
the presence of distinct sequences of functionalities in the
MIXMOFs leads to a complex chemical environment for cataly-
[34]
sis.
Next, Pd/2 as the most efficient catalyst was applied to ex-
amine the scope of different substrates in the Heck reaction. A
variety of aryl halides were coupled with different olefins in
[6]
the presence of Pd at low loading (0.05 mol% of Pd) under
the optimized reaction conditions (Table 3). The Heck reaction
of styrene with iodobenzene proceeded easily at 1208C result-
ing in trans-stilbene in excellent yields after 30 minutes. Owing
to the higher bond energy of the CÀBr and CÀCl bonds, aryl
bromide and chlorobenzene compounds are difficult to acti-
vate. Therefore, the reactions of aryl bromides (or chloroben-
zene) with different olefins required harsh conditions and ex-
tended reaction times. Interestingly, as seen from Table 3, the
Heck reactions of a variety of aryl bromide derivatives with sty-
rene could also proceeded smoothly at 1208C and gave the
corresponding products in high yields after 6 h (Table 3, en-
5
3(Al)-NH . Although the use of 0.005 mol% and 0.01 mol% of
2
Pd catalyst gave higher TOF, it gave only 28% (Table 1,
entry 10) and 59% (Table 1, entry 9) yield of the desired prod-
ucts, respectively. It is very interesting that the bromobenzene
can be activated effectively by only 0.05 mol% of Pd catalyst
and very high yield was obtained (81%; Table 1, entry 4). The
product yield was only 83% upon increasing the Pd catalyst to
0
.1 mol% (Table 1, entry 11).
Because the type of support is important for Pd NPs cataly-
sis, Pd NPs supported on different supports (MIL-53(Al), 1, 2,
and 3) as well as commercially available Pd/C were used in
Heck reactions (Table 2). Notably, no product was obtained
without the use of Pd catalyst (Table 2, entry 7). The Pd NPs
supported on amine-functionalized MOFs (1, 2, 3, and MIL-
tries 2, 7–13). The electron-withdrawing groups (-NO , -CN,
2
-CF , and -COCH ) in the para position relative to bromine can
3
3
[3]
weaken the CÀBr bond, so these substituted bromobenzene
compounds reacted more rapidly than bromobenzene (Table 3,
entries 7–10). Reactions between styrene and a variety of elec-
tron-rich substrates such as 4-methy- and 4-methoxyl-substi-
tuted aryl bromides, also proceeded smoothly and gave the
coupling products in good yields (Table 3, entries 11 and 12).
Notably, the electron-donating and sterically hindered 2-me-
thoxyl-substituted bromobenzene also gave good yields
(Table 3, entry 13). However, the most challenging, yet readily
accessible, chlorobenzene had much lower reactivity even
after 24 hours (Table 3, entry 3). In addition, various olefin de-
rivatives with different ester substituents were applied to this
5
3(Al)-NH ) showed significantly higher reactivity (up to 76%
2
yield for the brombenzene substrates) than that of Pd/MIL-
3(Al) and Pd/C. These results might be attributed to the
5
[
29]
Pd NPs agglomeration (Figure S1). Interestingly, the different
amount of amines in the supports also affected the activity of
the Pd catalyst. By increasing of the amount of the amine
groups, the activity of the catalyst increases first (up to 93%
yield with Pd/2, 50% NH ; Table 2, entry 3), and then declines
81% yield with MIL-53(Al)-NH ; Table 2, entry 5). It may not
2
(
2
preclude that there exists coordination interactions between
[
39,52]
palladium and amino groups.
A certain number of amino
ChemPlusChem 2012, 77, 106 – 112
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
109

R. Cao et al.
[
a]
Table 3. Results for the Heck reaction catalyzed by Pd/2.
Table 4. Reusability of the Pd/2 catalyst in the Heck reaction of 1-bromo-4-
[
a]
methoxyenzene and styrene
.
1
2
[b]
À1
Entry
R
X
R
Yield [%]
TOF [h ]
[
c]
[b]
[b]
1
2
3
4
5
6
7
8
9
1
H
H
H
H
H
H
4-NO
4-CN
4-CF
4-COCH
4-Me
4-OMe
2-OMe
H
I
Ph
Ph
Ph
CO
CO
CO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
97
93
11
95
94
91
98
97
95
96
86
81
66
3880
310
9
Cycle
1st
Yield [%]
Cycle
Yield [%]
Br
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
[
d]
81
80
82
4th
5th
78
76
2
3
nd
rd
2
2
2
CH
nBu
tBu
3
317
313
303
327
323
317
320
287
270
220
216
306
[
(
[
a] Reaction conditions: 1-bromo-4-methoxyenzene (1 mmol), styrene
1.5 mmol), Pd/2 (0.05 mol% of Pd), NEt (1.5 mmol), DMF (3 mL), 6 h.
3
b] Yield of isolated product.
2
3
0
3
1
1
12
13
14
15
[30]
gible, thus resulting in the preservation of catalytic activity.
[
[
e]
g]
[f]
65 (67)
92
Moreover, the Pd catalysts supported on amine-functionalized
MOFs was comparably active after being stored in air for three
months (Table 3, entry 15). These features are obvious improve-
ments from existing air- and moisture-sensitive homogeneous
H
[
a] Reaction conditions: aryl halide (1 mmol), olefin (1.5 mmol), Pd/MIX-
MIL-53(Al) and 50% of NH (0.05 mol% of Pd), NEt (1.5 mmol), DMF
3 mL), 6 h. [b] Yield of isolated product. [c] 30 min. [d] 24 h, yield base on
GC analysis. [e] The catalyst was removed after 3 h. [f] The filtrate was
kept stirring for 24 h. [g] After being stored in air for three months.
2
3
(
[10]
palladium-phosphine catalysts.
Conclusion
In conclusion, amine-functionalized MIXMOFs based on MIL-
3(Al) drastically enhance N adsorption, thus suggesting that
reaction (Table 3, entries 4–6). It was found that the cross-cou-
pling of bromobenzene with these ester-substituted olefins af-
forded the corresponding trans-configured products in excel-
lent yields. Interestingly, the efficiency of the tert-butyl acetate-
substituted olefin was not affected by its steric hindrance and
gave high yield (Table 3, entry 6).
5
2
the properties of MIXMOFs are not simple linear sums of those
of the pure components. Moreover, we have developed a
facile approach for the preparation of Pd NPs catalysts sup-
ported on amine-functionalized MIXMOFs based on MIL-53(Al).
These well-dispersed Pd NPs show high activity and selectivity
for Heck reactions. The Pd NPs catalysts exhibit high stability
which can be recycled and reused easily.
Notably, one of the disadvantages related to the use of sup-
ported Pd catalysts in the Heck reaction is leaching of the pal-
ladium into the solution, and results in contamination of the
product with metal that is difficult to remove along with loss
[
3–5,55]
of the expensive Pd catalyst.
Therefore, the leaching of
Experimental Section
the metal from Pd/2 catalyst was examined. After the workup,
inductively coupled plasma (ICP) analysis showed that the
amount of Pd leached into the reaction mixture was very low
[44]
[49,50]
MIL-53(Al),
MIL-53(Al)-NH2,
and amine-functionalized MIX-
MOFs Al(OH)(BDC)1 (ABDC) based on MIL-53(Al) (BDC=benzene-
Àx
x
1
0
,4-dicarboxylate, ABDC=2-aminobenzene-1,4-dicarboxylate, x=
.1; 0.5; 0.9, denoted 1, 2, 3, respectively; Scheme 1) were synthe-
sized according to literature procedures. The as-synthesized MIL-
(0.1 ppm). Pleasingly, the low amount of Pd satisfies specifica-
tions required by the pharmaceutical industry regarding the
[40]
[
3]
final purity of the products (Pd <2 ppm). A hot-filtration ex-
periment clearly indicated that the reaction stopped when the
filtrate was further subjected to the same reaction (Table 3,
entry 14).
5
3(Al) was treated at 3308C for 12 h to remove unreacted tereph-
thalic acid. The obtained MIXMOFs and MIL-53(Al)-NH2 were
washed with DMF, and subsequently soaked in methanol at 708C
for 24 h. The solid was finally dried overnight at 1308C under
vacuum. All other reagents were commercial available and used as
received.
Advantages of the Pd/2 material were that the catalytic reac-
tions could be conveniently carried out in air, and the separa-
tion of this heterogeneous catalyst could be achieved easily by
filtration. Less active 4-methoxy-1-bromobenzene and styrene
were used for reusability experiments. The results indicate that
after being recycled for five runs, the Pd catalyst still showed a
remarkable activity (a reduction from 81% to 76% yield;
Table 4). The TEM image of the reused catalyst (Figure 5) re-
veals that the mean diameter of the nanoparticles is 3.4 nm,
which is very similar to that before the catalysis reaction. The
results illustrate that the loss of palladium active sites is negli-
The activated MOFs MIL-53(Al)-NH , 1, 2, and 3 (0.50 g) in H O
2
2
(40 mL) were treated with hydrochloric acid to give a pH value of
. Then the solution of H PdCl4 (containing 1.0 wt% of Pd) was
4
2
added drop-wise to the above acidulated solution under vigorous
stirring for 10 min and then stirring for 8 h. The resulting solid was
centrifuged and washed with deionized water and ethanol. The
samples were reduced by NaBH (0.04 g) at 273 K for 3 h to obtain
Pd NPs (denoted Pd/MIL-53(Al)-NH , Pd/1, Pd/2, and Pd/3). For
4
2
comparison, a solution of H PdCl (containing ca. 1.0 wt% of Pd)
was added to the activated MIL-53(Al) (0.50 g) which was suspend-
2
4
110
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Catalysis
ed in H O (40 mL) under vigorous agitation for 10 min and the mix-
3586; Angew. Chem. Int. Ed. 1998, 37, 3387; h) A. F. Littke, C. Dai, G. C.
Fu, J. Am. Chem. Soc. 2000, 122, 4020; i) A. F. Littke, G. C. Fu, J. Am.
Chem. Soc. 2001, 123, 6989; j) M. R. Netherton, G. C. Fu, Angew. Chem.
2
ture was then stirred for 24 h. The solid was centrifuged and
washed with deionized water and ethanol. The resulting MIL-53(Al)
samples containing palladium salts were then reduced with NaBH4
2
002, 114, 4066; Angew. Chem. Int. Ed. 2002, 41, 3910; k) J. H. Kirchhoff,
C. Dai, G. C. Fu, Angew. Chem. 2002, 114, 2025; Angew. Chem. Int. Ed.
(
0.04 g) at 273 K for 3 h to obtain Pd/MIL-53(Al) (0.19 wt% of Pd
2
2
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