Selective palladium-loaded MIL-101 catalysts

Article abstract of DOI:10.1002/chem.201101004

Palladium nanoparticles (NPs) of different mean particle size have been synthesized in the host structure of the porous coordination polymer (or metal-organic framework: MOF) MIL-101. The metal-organic chemical vapor deposition method was used to load MIL-101 with the Pd precursor complex [(η⁵-C₅H₅)Pd(η³-C ₃H₅)]. Loadings higher than 50 wt.% could be accomplished. Reduction of the Pd precursor complex with H₂ gave rise to Pd NPs inside the MIL-101 (Pd@MIL-101). The reduction conditions, especially the temperature, allows us to make size-conform (size of the Pd NPs correlates with the size of the cavities of the host structure of MIL-101) and undersized Pd NPs. The Pd@MIL-101 samples were characterized by X-ray diffraction, IR spectroscopy, Brauner-Emmett-Teller (BET) analysis, elemental analysis, and transmission electron microscopy (TEM). Catalytic studies, hydrogenation of ketones, were performed with selected Pd@MIL-101 catalysts. Activity, selectivity, and recyclability of the catalyst family are discussed.

Full text of DOI:10.1002/chem.201101004

FULL PAPER
DOI: 10.1002/chem.201101004
Selective Palladium-Loaded MIL-101 Catalysts
[
a]
Justus Hermannsdçrfer and Rhett Kempe*
Abstract:
Palladium
nanoparticles
rise to Pd NPs inside the MIL-101
(Pd@MIL-101). The reduction condi-
tions, especially the temperature,
allows us to make size-conform (size of
the Pd NPs correlates with the size of
the cavities of the host structure of
MIL-101) and undersized Pd NPs. The
Pd@MIL-101 samples were character-
ized by X-ray diffraction, IR spectros-
copy, Brauner–Emmett–Teller (BET)
analysis, elemental analysis, and trans-
mission electron microscopy (TEM).
Catalytic studies, hydrogenation of ke-
tones, were performed with selected
Pd@MIL-101 catalysts. Activity, selec-
tivity, and recyclability of the catalyst
family are discussed.
(
NPs) of different mean particle size
have been synthesized in the host
structure of the porous coordination
polymer (or metal–organic framework:
MOF) MIL-101. The metal–organic
chemical vapor deposition method was
used to load MIL-101 with the Pd pre-
Keywords: chemical vapor deposi-
5
3
cursor
complex
[(h -C H )Pd ACHNUTRTGENNUN(G h -
5 5
tion
metal–organic frameworks · nano-
AHCTUNGTREGpNNUN articles · palladium
· heterogeneous catalysis ·
C H )]. Loadings higher than 50 wt.%
3
5
AHCTUNGTRENNUNG
could be accomplished. Reduction of
the Pd precursor complex with H gave
2
Introduction
host for growth inhibition of particle size are rather low.
Very recently the successful incorporation of size-conform
Au NPs was demonstrated within the hosts ZIF-8 and ZIF-
[
1]
Porous coordination polymers (PCP) or metal–organic
[
2]
[5]
frameworks (MOF) have experienced an enormous devel-
opment since their discovery. Today, more than a decade
later, numerous fields of scientific interest have occurred
90.
The synthesis of the metal@MOF or PCP systems is usual-
ly performed in two steps. Firstly, the sublimation or infiltra-
tion of a metal complex precursor into the porous network,
and secondly, the consequent reduction of the imbedded
precursor to metal NPs with hydrogen or another reducing
agent. The MOCVD loading with metal complex precursors
can proceed in a highly controlled way, even in a single-crys-
[3]
and advanced significantly. The incorporation of noble-
metal nanoparticles (NPs) within these porous materials by
means of metal–organic chemical vapor deposition
(
MOCVD) initially developed by Fischer and co-workers is
a promising way of developing heterogeneous catalysts.
[4]
[12]
The limiting pore dimensions of the crystalline network dis-
play excellent conditions for the generation of small metal
NPs, the surfaces of which are not blocked by strongly bind-
ing ligands. Furthermore, immobilizing NPs in or on a
macroscopically visible host structure allows easy separation
of liquid reaction mixtures from the catalyst. In this regard
several PCP/MOFs have been loaded with metal NPs by ap-
plying different synthetic methods. In addition to solution
tal to single-crystal reaction. Hence, this step most likely
is not causing the problems in terms of generating over-
sized particles. Therefore, we thought that the conditions of
the reduction step are crucial in terms of particle formation
and became interested in understanding this key step better.
We have been interested in PCP or MOF synthesis for
[5]
[13]
many years
and succeeded recently in the synthesis of
[11]
Pt@MOF-177 by means of MOCVD. Pt@MOF-177 shows
an unusually high room-temperature hydrogen-storage ca-
pacity and is an efficient catalyst for the oxidation of alco-
hols using oxygen as an oxidant. Yet massive problems of
catalyst stability encountered. Since water is formed as a by-
product in these oxidation reactions we addressed the insta-
bility of the MOF-177 host to the presence of water and
[
6,17]
[7]
infiltration techniques,
solid grinding, microwave irra-
[8]
[9]
diation, and surface grafting, MOCVD displays a poten-
tial method for the generation of homogeneously confined
metal NPs, such as Pt, Au, Pd, and Ru. In addition, it allows
[10,11]
very high loadings (>30 wt.%).
Problematic for all
methods has been so far the generation of NPs within the
confined dimensions of the porous system. Many examples
explored so far show metal NPs beyond the size of the pore
diameters, which means the energy barriers provided by the
[
14]
switched to the relatively water stable MIL-101.
been investigated as a host for metal NPs by a variety of
groups.
It has
[6,8,9,15]
The structure of MIL-101 is very complex:
The connector consists of a chromium oxo trimer being con-
nected to six other connectors through doubly deprotonated
terephthalic acid linkers. The resulting tetrahedron is assem-
bled into the so-called “super tetrahedrons” leading to two
different cavities with pore dimensions of about 2.9 and
3.4 nm and pore windows of 1.2–1.6 nm (Figure 1).
[
a] J. Hermannsdçrfer, Prof. Dr. R. Kempe
Lehrstuhl fꢀr Anorganische Chemie II
Universitꢁt Bayreuth, Universitꢁtsstrasse 30
9
5440 Bayreuth (Germany)
Fax : (+49)921-55-2157
E-mail: kempe@uni-bayreuth.de
Chem. Eur. J. 2011, 17, 8071 – 8077
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8071

Figure 1. Construction of the pores and the cavities of MIL-101.
Figure 2. The initial weight ratio Pd precursor/MIL-101 is plotted against
the Pd wt.% of Pd@MOF. A good accordance of the measured (squares)
and the calculated values (line) was observed. The loading of the MIL-
Herein, we report on the selective loading of MIL-101
with palladium NPs by means of MOCVD and the depen-
1
01 system with the Pd precursor is almost quantitative.
ACHTUNGTRENNUNGd ence of the particle size on the reduction conditions. We
were able to show that MIL-101-cavity-conform Pd NPs and
also smaller ones can be generated by altering the reaction
conditions. The latter is especially interesting, since bi- and
multimetallic NPs might be grown starting from these
under-sized particles. Furthermore, the behavior of the re-
sulting Pd@MIL-101 hybrid materials as a catalyst in ketone
hydrogenation is discussed.
MIL-101 system with the Pd precursor. When applying a dy-
namic vacuum, no difference to the usually applied static
[6,10]
vacuum
could be noticed. Both synthesis steps, the load-
5
3
ing of MIL-101 and the hydrogenolysis of [(h -C H )Pd
A( h -
C
H
T
U
N
G
T
R
E
N
N
U
N
G
5
5
C H )]@MIL-101, can be performed with up to 98% yield.
3
5
The conditions during the reduction step are listed in
Table 1. The comparison of the calculated values of the Pd
loading with the observed values (Figure 2) nicely demon-
strates the good adjustability of the metal content.
Results and Discussion
The IR spectra (Figure 3) of the different Pd@MIL-101
systems indicates the intact structure of MIL-101 for all re-
duction protocols beside that of VHTP-25. For the VHTP-
25 protocol, additional peaks are visible, showing degrada-
Selective Pd loading of MIL-101—IR, X-ray diffraction, and
Brauner–Emmett–Teller (BET) studies: The inductively
coupled plasma optical emission spectrometry (ICP-OES)
measurements of the loaded Pd@MIL-101 (Table 1 and
Figure 2) nicely show the almost quantitative loading of the
Table 1. Synthesized Pd@MIL-101 systems with initial weight ratio of Pd
precursor and MIL-101 as well as conditions during reduction of the Pd
precursor@MIL-101 systems. Abbreviation scheme: first two letters indi-
2
cate temperature and H pressure condition during reduction, H=high,
L=low; the following two digits indicate reduction time in hours and the
final two digits the wt.% Pd loading. Longer reduction times were used
for the LL-series due to milder reduction conditions.
[
a]
Pd/Cr
Conditions of reduction
Pd-content
[wt.%]
[
8C]
A
H
U
G
R
N
N
[bar]
[h]
ACHTUNGTRENNUNG
HH2609
HH2610
HH1545
HH2656
LL6012
LL4842
LL6058
postmod
0.20
0.25
1.68
2.11
0.25
1.68
2.13
0.50
70
70
70
70
21
21
21
21
70
70
70
70
5
5
5
5
30
110
26
26
15
26
60
48
60
20
24
1
9.3
9.9
44.5
55.6
12.4
41.6
57.5
[
b]
Figure 3. IR Spectra of Pd@MIL-101 (selected runs). From top to
bottom: HH-series (HH1545); Postmode; LL-series (LL4832); VHPT-25;
MIL-101. The typical vibrations of MIL-101 are still existent in the
loaded systems. Yet one can see additional peaks for VHTP-25, assuming
a degradation of the host structure (1537 and 925 cm marked by
arrows).
50
17.5
24.9
[
c]
VHTP-25
0.81
220
[
a] Pd precursor to Cr ratio. [b] Post-modified: back-to-back two step re-
À1
duction. After 20 h pressure and temperature were increased. [c] Very
high temperature and pressure.
8072
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Chem. Eur. J. 2011, 17, 8071 – 8077

Heterogeneous Catalysis
FULL PAPER
tion of the host structure, which can be explained by the
very high temperature applied during the reduction step.
The XRD analysis (Figure 4) of the Pd@MIL-101 systems
displays the characteristic MIL-101 pattern in the region
structures clearly show a smaller specific surface area. It is
due to occupying of some cavities by Pd NPs, but also due
to the fact that Pd increases the mass of the systems (espe-
cially with loadings up to 58 wt.%). While systems with low
loadings maintain at least 40% of their former specific sur-
face area, systems with higher loadings drop to merely 10%
of their former specific surface area. In the case of VHTP-
2
5 no significant pore volume and surface area can be de-
tected indicating the destruction of the host structure.
Selective Pd loading of MIL-101—TEM studies: During the
TEM measurements a massive deformation and destruction
of the host structure by operating at high electron intensity
could be observed. Thus we tried to measure at the lowest
possible intensity to obtain reproducible results. For each of
the different reduction protocols (except for VHTP-25) we
could see the typical octahedron shaped crystallites of MIL-
1
01 in the size regime of 100 nm–5 mm.
Reduction protocol—low temperature: The generation of
Pd@MIL-101 at low temperatures (218C) and low H pres-
2
sure (5 bar) led to highly ordered structures. TEM micro-
graphs of LL6012 and LL6058 show a very high regularity
of the trapped Pd NPs (Figure 5A and B). The example
with low metal loadings (LL6012) shows very small Pd NPs
Figure 4. XRD pattern of Pd@MIL-101 (selected runs). From top to
bottom: HH2656; LL6058; HH2609; LL6012; MIL-101; * Palladium.
Beside the fcc characteristic Pd pattern at 2q=40, 46.6, and 68.28, one
can clearly see the intact structure of MIL-101. The Pd reflections
become more intense with higher metal loadings.
(
ꢀ2.7 nm) with a narrow size distribution. The mean parti-
cle size slightly increases with increasing metal loading (to
about 2.9 nm), yet keeping a narrow size distribution.
2
q<208. In addition, an intensified noise at higher loadings
of Pd is observed. It can be explained by the absorption of
diffracted intensities by palladium. The face-centered cubic
Reduction protocol—high temperature: Adjusting the condi-
tions to elevated temperature (708C) and higher pressure
(70 bar) led to narrowly, but differently distributed Pd NPs
within the pores/cavities of MIL-101 as shown in Figure 5.
The NP distribution is again very narrow and the mean par-
ticle size (about 1.7 nm) is well below the dimensions of
both cavities, the size being more in the size regime of the
cavity windows (1.2–1.6 nm), albeit the particles are on aver-
age a little larger. When proceeding to higher loadings, the
saturation of the cavities increases. Again a similarly narrow
Pd NP size distribution is observed with its mean particle
size at 1.7 nm (Figure 5C and D).
The reduction under (firstly) mild conditions and after-
wards at elevated temperature led to an equally sharp distri-
bution of very small Pd NP (Figure 5E). The results are
comparable to those obtained by the high temperature pro-
tocol. This implies a possible post modification of the NP.
While the first reduction step leads to the formation of NP
being in the size regime of 2–3 nm (similar to the results of
the LL runs) the second step leads to a confinement of the
particles to a mean particle size of around 1.7 nm.
(
(
fcc) Pd peaks at 2q=40, 46.6, and 68.28, assigned to (111),
200), and (220), respectively, are visible for the Pd-loaded
systems. More intensive Pd-based diffraction patterns were
found for the highly Pd-loaded systems. NPs in the size
regime of 1 or 3 nm can be assumed based on the full width
at half maximum of the Pd diffraction patterns, which is in
accordance with the dimensions of the cavities in MIL-101.
The surface area for the different MIL-101 samples was
calculated from the N isotherm using the BET model and
2
2
À1
was found to be around 2200 m g (Table 2). It is definitely
lower than the reported value by Fꢃrey et al., but similar
to those reported by others.
[14]
[
8,18]
The Pd-loaded MIL-101
Table 2. Surface area measurements for the free and metal loaded MIL-
01.
1
2
À1
3
À1
BET surface area [m g
]
Pore volume [cm g
]
[
a]
MIL-101
HH2609
HH2610
HH1545
HH2656
LL6012
LL4842
LL6058
postmod
2180
1314
1339
251
223
1263
197
220
1141
31
1.06
0.63
0.63
0.17
0.11
0.64
0.18
0.11
0.24
0.01
Reduction protocol—very high temperature: If we apply ex-
treme conditions like 2208C and 110 bar (note the thermal
stability of MIL-101 being higher than 2758C), we observe a
partial destruction of the host structure and an agglomera-
tion of particles beyond the range of the cavity dimensions
(Figure 5F). A broad particle size distribution ranged from
VHTP-25
[
a] As synthesized material.
Chem. Eur. J. 2011, 17, 8071 – 8077
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8073

R. Kempe and J. Hermannsdçrfer
Catalytic studies—activity, selectivity, and substrate scope:
The synthesized Pd@MIL-101 samples were investigated as
catalysts in the solvent-free reduction of ketones. Propio-
phenone was used as a substrate for initial studies. As no
catalytic activity was detected with 1 bar H , higher pressure
2
was applied. In-situ IR analysis of the reaction is indicative
of nearly no induction period (Figure 6). Experiments of the
Figure 6. Kinetic studies of reduction of propiophenone by in-situ IR
spectroscopy. The catalyst system HH1545 was used for the reduction of
propiophenone; (708C, 4.8 bar H
28 mg), propiophenone (5 mL)).
2
atmosphere, 750 rpm, 76 h, HH1545
(
catalyst system based on HH1545 and LL4842 (for an ex-
planation of the abbreviation scheme of the catalysts, see
caption to Table 1) with a variety of substrates showed a few
interesting features (Table 3). While aromatic ketones were
readily reduced aliphatic ketones did not show any conver-
sion. Electron-rich aryl residues likewise decrease activity
Figure 5. TEM analysis of Pd@MIL-101. A) LL6012; B) LL6058: The
regular structure is not limited to one layer, but stretches throughout the
whole crystallite (as indicative by the FTT); C) HH2609; D) HH1545:
Narrow particle size distributions centered around 1.7 nm were found; E)
Postmod; F) VHTP-25: The host structure is seriously damaged and sam-
ples show a layer of surface particles larger than 15 nm in diameter.
(
Table 3 entries 7–9). 4-Methylbenzophenone (entry 10) did
not show any conversions for LL4842, but for HH1545 indi-
cating a hampered diffusion into the porous LL system. In
General, system LL4842 shows higher conversions for small-
er substrates (entries 1 and 2) and lower conversions for
bigger substrates (entries 3–6 and 9). The complete filling of
the cavities restricts the accessible metal surface, decreasing
possible interactions with sterically demanding substrates.
With time/temperature, the selectivity for both systems can
be switched to the corresponding alkane (deoxygenation).
For reactions with branched alkyl residues (entry 6), selec-
tivity towards the alcohol stays dominant. Comparative ex-
periments with MIL-101 as catalyst (without Pd NPs) did
not show any conversion, excluding a possible activity of the
MIL-101 himself in terms of ketone reduction.
2–11 nm is observed. In addition, surface particles being
larger than 15 nm were found.
Despite the fact that TEM is an invasive analysis and the
particle sizes can be modified by the electron beam, a clear
correlation between the reduction protocol and the resulting
particle size can be observed. The reduction temperature
seems to play an important role. In case of MIL-101 higher
reduction temperatures lead to smaller particles. Especially
with MIL-101 is the presence of cavities, which are rather
large in comparison to the size of its connecting windows
(
Figure 1). It is conceivable that at high (708C), but not too
Experiments in the reduction of alkyl residue varied ke-
tones showed higher activity for sterically less hindered mol-
ecules, which are (most likely) able to access the active sites
better (Figure 7). These observations are indicative of size
selectivity as well. Hexanophenone shows a conversion of
high, temperatures the Pd NPs are “dynamic” enough and
the window size of the MIL-101 has a larger influence on
the particle size than the cavity sizes and hence undersized
particles can be obtained.
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Chem. Eur. J. 2011, 17, 8071 – 8077

Heterogeneous Catalysis
FULL PAPER
Table 3. Reduction of various aryl alkyl ketones to the corresponding alcohol and/or arylalkane at 39–808C (pressure 20 bar: time: 24 h); left: LL4842,
1
2
right: HH1545 (0.1 mol% Pd); R =aryl residue, R =aliphatic residue.
LL4842
HH1545
508C
conv (sel
Ketone
808C
conv (sel
508C
conv (sel
808C
conv (sel
398C
conv (sel /sel )
1 2
[
a]
[a]
[a]
[a]
[a]
1
/sel
2
)
1
/sel
2
)
1
/sel
2
)
1
/sel
(90/8)
(92/6)
(97/2)
(99/<1)
(98/<1)
(97/2)
(89/10)
(20/79)
(4/95)
(95/4)
2
)
1
2
3
4
5
6
7
8
9
0
acetophenone
propiophenone
valerophenone
heptanophenone
isopropylphenylketone
2,2-dimethylpropiophenone
4’-fluoropropiophenone
4’-methylpropiophenone
4’-methoxypropiophenone
4-methylbenzophenone
100
100
98
100
100
99
100
100
100
0
A
T
N
T
E
N
N
100
100
57
23
72
22
100
96
30
0
A
T
G
R
N
U
G
100
99
A
T
N
T
E
N
N
99
98
A
T
N
T
E
N
N
90
95
68
70
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
A
H
U
T
E
N
N
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
A
H
U
G
E
N
N
100
100
100
100
100
100
100
99
A
T
N
T
E
N
N
100
100
100
100
100
100
100
100
A
T
N
T
N
U
G
ACHTUNGTRENNUNG
A
H
N
R
N
U
G
A
H
U
G
E
N
N
A
H
U
T
N
U
G
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
A
H
U
T
N
U
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
A
H
U
G
E
U
G
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
A
H
N
T
E
U
G
ACHTUNGTRENNUNG
A
H
U
T
E
N
N
A
H
U
T
N
U
G
ACHTUNGTRENNUNG
A
H
U
G
R
N
U
G
A
H
N
T
E
N
N
ACHTUNGTRENNUNG
1
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
[
1 2
a] conv=conversion [%], sel =selectivity for the corresponding alcohol (1) [%], sel =selectivity for the corresponding alkenes (2) [%].
Figure 8. Recyclability test of HH1545 catalyst in the reduction of propio-
phenone. No significant decrease of activity after six runs was observed.
Reaction conditions: 508C, 40 bar H atmosphere and 750 rpm, 9 h,
2
HH1545 (18 mg), propiophenone (4 mL), dodecane (1 mL).
Figure 7. Plotting of conversion against the chain length of the alkyl resi-
due. (508C, 20 bar H atmosphere, 750 rpm, 16 h, 0.09 wt.% Pd, LL4842
2
catalyst).
23%, which is a third of the conversion of acetophenone at
equal reaction conditions.
Catalytic studies—recyclability: The recyclability of the cat-
alyst system HH1545 was tested in six consequent runs in
which the reaction was stopped after ꢀ50% conversion
(
9 h). After six runs no significant decrease of activity could
be detected (Figure 8). The detection of the formed byprod-
ucts stayed continuously low. A second experiment involved
the examination of the system LL4842. Again no decrease
of activity after 11 runs could be detected (Figure 9). Yields
of 1-phenyl-1-propanol were around 90%. A higher activity
of LL4842 in comparison to HH1545 was observed if pro-
piophenone is used. To search for possible leaching effects
we investigated the hydrogenation activity of the separated
reaction mixture. No further conversion was detected. We
therefore assume that Pd leaching is rather low. ICP-OES
measurements of the catalyst after 11 runs did not show a
reduction of the Pd loading, confirming our assumption.
Figure 9. Recyclability test of LL4842 catalyst in the reduction of propio-
phenone. After eleven runs no significant decrease of activity could be
detected. Reaction conditions: 508C, 40 bar H atmosphere, 750 rpm, 8 h,
2
LL4842 (4 mg), propiophenone (2 mL), dodecane (0.113 mL).
In contrast to these experiments, which were stopped
after relatively short reactions times, 8 or 9 h, experiments
after prolonged catalysis showed a deactivation of the cata-
lyst. This was accompanied by a coloring of the reaction
mixture after 76 h that cannot be attributed to any product
or educt (Figure 10).
Chem. Eur. J. 2011, 17, 8071 – 8077
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8075

R. Kempe and J. Hermannsdçrfer
Experimental Section
Analytical and spectroscopic methods: Elemental analysis was performed
by standard protocols employing digestion in HNO /HCl/H and in-
3
2 2
O
ductively coupled plasma optical emission spectrometry (ICP-OES) using
a Varian, Vista-Pro radial. GC analyses were performed by using an Agi-
lent 6890N gas chromatograph equipped with a flame ionization detector
(
FID) and an Agilent 19091 J-413 FS capillary column using dodecane as
internal standard. All X-ray powder diffractograms were recorded by
using a STOESTADI-P-diffractometer (CuKa radiation, 1.54178 ꢄ) in q–
2
q geometry and with a position sensitive detector. All powder samples
were introduced into glass capillaries (ø=0.7 mm, Mark-tubes Hilgen-
berg No. 10) in a glove box and sealed prior to the measurements. FTIR-
measurements were performed at a Perkin–Elmer FTIR-Spectrum 100
À1
over a range of 4400 to 650 cm . In-situ IR measurements were carried
out by using a ReactIR 4000 (Mettler Toledo) with an adamantine
À1
window over a range of 4400 to 650 cm . The nitrogen physisorption iso-
therms were measured at 77 K by using a Quantachrome Autosorb 1 ap-
paratus. A pre-degassed sample (25–80 mg) was transferred to a quartz
À3
cell and consequently degassed again at 1008C, 10 mbar for 24 h. Spe-
cific surface areas were calculated by using five points by BET. The spe-
cific total pore volume was measured by the DFT calculations. Transmis-
sion electron microscopy (TEM) was carried out by using a Varian LEO
9220 (200 kV) instrument. The sample was suspended in chloroform and
sonicated for 5 min. Subsequently a drop of the suspended sample was
placed on a grid (Plano S 166–3) and allowed to dry.
Figure 10. XRD pattern and TEM images of the catalyst system based on
HH1545 after prolonged catalysis, reduction of propiophenone. Change
of particle size after 48 h and 76 h catalysis time. Reaction conditions:
5
2
08C, 20 bar H atmosphere, 76 h, 750 rpm.
Reactants and solvents: Terephthalic acid, propiophenone as well as the
other ketones were purchased from Acros Organics. 1-Phenyl-1-propanol
was purchased from Sigma Aldrich chemicals. Chromium
ACHTUNGTRENNGNU( III) nitrate
nonahydrate (Cr(NO ·9H O) and allylpalladium(II) chloride dimer
were purchased from ABCR. All manipulations and chemical reactions
were conducted under an inert atmosphere (Schlenk-technique (Ar) and/
A
H
U
G
R
N
N
2
)
3
2
The XRD pattern after 76 h of catalysis shows a narrow-
ing of the fcc Pd signals indicating a growth of particle sizes.
TEM images of the samples confirm an increase of the
mean particle size of Pd beyond the confinement of the
pore dimensions. These results seem to be in contrast to the
recyclability tests earlier, yet this instability is relevant to
prolonged catalysis only. TEM investigations after 48 h pro-
longed catalysis revealed the formation of bigger, 24 nm par-
ticles (Figure 10). They do not consist out of one big particle
or glove box technique (N
vents were dried with sodium/benzophenone ketyl and halogenated sol-
vents with CaH . Deuterated solvents were obtained from Cambridge
2 2 2
, H O, O <0.1 ppm)). Non-halogenated sol-
2
Isotope Laboratories, degassed, dried with molecular sieves and distilled
prior to use.
5
3
Starting materials synthesis: The Pd precursor [(h -C
5 5 3 5
H )Pd ACHTUNGTRENNUNG( h -C H )]
was synthesized under exclusion of light following a published proce-
[16]
dure.
lute THF (50 mL) and cooled to À608C. Drop wise addition of NaCp
6 mL, Cp=cyclopentadienyl) under constant cooling and stirring lead to
Allylpalladium(II)chloride dimer (2.5 g) was dissolved in abso-
(
as does the samples after 76 h) but of agglomerated parti-
cles of about 4 nm.
(
a red coloring of the solution, which was stirred for another 15 min at
À208C and for 30 min at RT. The solvent was removed under vacuum
and the residue was dissolved in hexane (50 mL) and separated by cannu-
la filtration. The solvent was removed under vacuum.
Conclusion
MIL-101 was synthesized and washed according to a published proce-
[
17]
In summary, we have presented evidence for a selective
loading of MIL-101 with Pd NPs. The volatile Pd precursor
dure.
2 3 3 2
H BDC (400 mg), Cr ACHTUGNTRNEUNG( NO ) 9H O (640 mg), HF (0.08 mL) and
H
2
O (8 mL) were mixed and sealed in a Teflon-lined hydrothermal auto-
clave. The mixture was heated for 8 h at 2208C, cooled down fast to
608C and slowly to 308C (cooling rate: 2.78Ch ). The resulting green
5
3
complex [(h -C H )Pd ACHTUNGTRNEGNU( h -C H )] was introduced by means
3 5
5
5
À1
1
of MOCVD up to loadings higher than 50 wt.% of Pd. Pd
NPs of different size could be generated by varying the re-
duction conditions especially the temperature. Reduction at
low (room) temperature gave rise to cavity-size-conform
particles and reduction at elevated temperature (708C)
yielded smaller Pd NPs, as given by the host cavities. We see
some potential for these under-sized particles in terms of
loading with a second (or third) metal to make multimetallic
NPs. Catalysis studies—hydrogenation of ketones—indicate
that the Pd@MIL-101 catalysts are size selective. The cata-
lysts can be recycled, but decompose after (very long) reac-
tion times during which the ketone and alcohol concentra-
tion becomes low.
mixture was filtered off over pore 3 filters to eliminate excessive crystal-
lized terephthalic acid. The filtrate was again filtered off using a fine
pore paper filter and washed with water (50 mL). The resulting solid was
refluxed in EtOH for 12 h and filtered off using a fine pore paper filter;
this process was repeated. The resulting solid was soaked in 1m NH F so-
4
lution, stirred for 24 h at 708C and filtered hot; again this process was re-
peated. The resulting green powder was washed with water and evacuat-
À5
ed at 10 bar to remove any solvent. All materials were stored under
argon.
5
3
Infiltration of [(h -C
5
H
5
)Pd
3 5
ACHTUNGTRNENUG( h -C H )] into MIL-101—preparation of
5
3
[
(h -C H )Pd ACHTNUREGTNNUGN( h -C H )]@MIL-101: Freshly evacuated MIL-101 powder
5 5 3 5
5
3
and [(h -C
5
H
5
)Pd
A
H
N
T
E
N
N
(h -C
3
H
5
)] were placed in a two-chamber-tube separated
À4
by a glass frit and were kept at 258C in a 1.4ꢅ10 mbar (diffusion
pump) dynamic vacuum for 3–8 h. The procedure yielded a dark green to
black powder (depending on the loading of Pd), which was immediately
processed in hydrogenolysis to yield Pd@MIL-101.
8076
www.chemeurj.org
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8071 – 8077

Heterogeneous Catalysis
FULL PAPER
5
Preparation of Pd@MIL-101—quantitative hydrogenolysis of [(h -
[5] D. Esken, S. Turner, O. I. Lebedev, G. V. Tendeloo, R. A. Fischer,
Chem. Mater. 2010, 22, 6393–6401.
3
II
0
5 5 3 5
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Parr Instruments steel autoclave (Table 1). To remove traces of the
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4
Received: April 1, 2011
Published online: June 15, 2011
Chem. Eur. J. 2011, 17, 8071 – 8077
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8077