Pd Nanoparticles supported on MIL-101: An efficient recyclable catalyst in oxidation and hydrogenation reactions

Article abstract of DOI:10.1166/jnn.2014.8535

Pd nanoparticles supported on the chromium terephthalate metal organic framework MIL-101 (Pd/MIL-101) in different loadings (0.9 and 4.5 wt%) have been successfully prepared through a simple Pd-acetate adsorption and reduction in acetone, and tested as catalyst for selected liquid phase oxidation and hydrogenation reactions. The materials were characterized by XRD, N₂ adsorption-desorption isotherm, TEM, SEM-EDX and ICP analysis. The parent MIL-101 structure was found well preserved after formation of Pd nanoparticles and after catalytic reaction runs. The present catalyst afforded good activity and selectivity for the oxidation of benzyl alcohol to benzaldehyde with 85% conversion and 97% selectivity using air (1 atm) at 85 °C after 14 h. The catalyst also showed good activity in the hydrogenation of the C C bond in alkenes to corresponding alkanes and also benzaldehyde to benzyl alcohol at room temperature using H₂ (1 atm). Rigorous test results confirmed that Pd-nanoparticles supported on MIL-101 are responsible for the catalytic reactions occurred. Pd/MIL-101 was reusable several times without losing the structural integrity and initial activity, and demonstrated significantly higher catalytic activities than those by a commercial Pd catalyst supported on activated carbon. Copyright

Full text of DOI:10.1166/jnn.2014.8535

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 2546–2552, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Pd Nanoparticles Supported on MIL-101: An Efficient
Recyclable Catalyst in Oxidation and
Hydrogenation Reactions
∗
Samiran Bhattacharjee, Jun Kim, and Wha-Seung Ahn
Department of Chemical Engineering, Inha University, Incheon, 402-751, Republic of Korea
Pd nanoparticles supported on the chromium terephthalate metal organic framework MIL-101
(Pd/MIL-101) in different loadings (0.9 and 4.5 wt%) have been successfully prepared through a sim-
ple Pd-acetate adsorption and reduction in acetone, and tested as catalyst for selected liquid phase
oxidation and hydrogenation reactions. The materials were characterized by XRD, N adsorption–
2
desorption isotherm, TEM, SEM-EDX and ICP analysis. The parent MIL-101 structure was found
well preserved after formation of Pd nanoparticles and after catalytic reaction runs. The present
catalyst afforded good activity and selectivity for the oxidation of benzyl alcohol to benzaldehyde
ꢀ
with 85% conversion and 97% selectivity using air (1 atm) at 85 C after 14 h. The catalyst also
showed good activity in the hydrogenation of the C C bond in alkenes to corresponding alka-
nes and also benzaldehyde to benzyl alcohol at room temperature using H (1 atm). Rigorous test
Delivered by Publishing Technology to: York University
2
results confirmed that Pd-nanoparticles supported on MIL-101 are responsible for the catalytic reac-
IP: 130.63.180.147 On: Mon, 11 Aug 2014 13:07:35
tions occurred. Pd/MIL-10 C1 ow pa ys ri rge hu t s: aAb ml eesr ei cv ae rna lS t ci mi e ens ti fwi ci t hP o uu bt l li os sh i en gr sthe structural integrity and
initial activity, and demonstrated significantly higher catalytic activities than those by a commercial
Pd catalyst supported on activated carbon.
Keywords: Metal Organic Frameworks (MOFs), Pd/MIL-101, Oxidation, Hydrogenation.
1
. INTRODUCTION
in the oxidation of alcohols. Pd nanoparticles supported
on alumina also exhibited good catalytic activity for the
aerobic oxidation of alcohols. Supported Pd catalysts are
1
7
The selective oxidation of alcohol to aldehyde, in particu-
lar, benzaldehyde from benzyl alcohol is one of the most
versatile and important class of organic reactions for lab-
oratory and industrial scale. Much attention has been
given for the development of ecologically acceptable cat-
alytic systems using molecular oxygen or air as an oxidant,
also known to be good for liquid phase hydrogenation
1
–4
18ꢀ19
reactions.
Hydrogenation of unsaturated hydrocarbons
or aldehyde is an important organic transformation for the
preparation of fine chemicals. A group of scientist reported
5
–8
which only produces water as by-product.
hydrogenation of 1-hexene over Pd/SiO and Pd/MCM-41,
2
Recently, heterogeneous catalytic systems having noble
metals (Pd, Au, Pt) were reviewed by Hutchings, and
and found that Pd/MCM-41 exhibited higher hydro-
genation activity than Pd/SiO2. Pinna and co-workers
9
20
among these noble metals, Pd-based catalysts have been
more extensively investigated. Surveying on the litera-
tures dealing with alcohol oxidation by Pd, it appears that
the catalytic activity and selectivity can be controlled by
the size of active phase and also by the nature of the
support. Thus several strategies involving formation of
described the hydrogenation of benzaldehyde to benzyl
alcohol over various Pd supported catalysts over activated
2
1
carbon, silica and alumina. They found that Pd/C showed
2
1
the highest activity for benzaldehyde hydrogenation.
Rode et al. reported the effect of the nature of the sup-
port and the size of Pd nanoparticles on the activity
1
0
11
12
22
Pd nanoparticles on hydroxyapatite, carbon, zeolite,
and selectivity of alkyne hydrogenation. Supported Pd
1
3–15
TUD-1 or SBA-16 mesoporous silica,
and carbon
nanoparticles showed higher activity than that of the bulk
nanotube16 were reported to afford high catalytic activity
22
Pd catalyst in the butyne hydrogenation.
Metal-organic frameworks (MOFs), a new type of crys-
talline porous materials formed via coordinative bonding
∗
Author to whom correspondence should be addressed.
2
546
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 3
1533-4880/2014/14/2546/007
doi:10.1166/jnn.2014.8535

Bhattacharjee et al.
Pd Nanoparticles Supported on MIL-101: An Efficient Recyclable Catalyst
of metal ions or clusters with polyfunctional organic
molecules to form an infinite one-, two- or three-
dimensional frameworks, have received much interest in
recent years for potential applications in gas storage, sep-
aration, heterogeneous catalysis, and as a template for
(5 wt% Pd) (Aldrich), styrene (99%, Aldrich), cyclooctene
(TCI), benzaldehyde (99%, Sigma-Aldrich) and benzyl
alcohol (99%, Sigma-Aldrich) were obtained commer-
cially and used without further purification.
2
3–29
new materials.
In particular, the robust framework of
2.2. Catalyst Preparation
MIL-101 was hydrothermally prepared from chromium
the chromium terephthalate MOF, MIL-101 developed by
3
0
Férey et al. has shown very high gas storage capacity for
(
III) nitrate nonahydrate, hydrofluoric acid and 1,4-
3
1
CO and CH , and also were effective as a catalyst or
30
2
4
benzene dicarboxylic acid by the literature method.
Dried MIL-101 (0.01 g) was pre-treated in 2 cm of ace-
3
2–36
catalyst support
due to its high surface area, large pore
3
windows (12 Å and 16 Å×14.5 Å), mesopores (29 Å and
tone for 30 min. Pd(O CMe) (0.011 g) was then dissolved
2
2
3
4 Å), accessible coordinative unsaturated sites (CUS),
3
in 3 cm of acetone and added to the above mixture
with stirring. The resulting mixture was sealed tightly in
a capped vial and heated at 45 C for 24 h. The super-
3
7
and excellent chemical and solvent stability.
Reports dealing with Pd nanoparticles on MOFs as cat-
ꢀ
3
6ꢀ38–41
alytic species, however, are rather limited.
Recently,
natant was colorless. The solid was filtered off, washed
with acetone and treated with N,N-dimethylformamide
Fischer and co-workers reported Pd-loaded catalysts on
MOF-54
2–44
and MOF-177 via metal organic chemical
45
3
ꢀ
(
DMF, 5 cm ) at 100 C for 12 h. The solid was filtered
vapor deposition (MOCVD) and subsequent decomposi-
tion/reduction by photo-assisted method. The sample was
ꢀ
off, washed with DMF and dried under vacuum at 140 C
for 4 h. The metal loading achieved was 4.53 wt% deter-
mined by ICP analysis. The Pd sample with weight load-
ing of 1.0% was also prepared by the same method.
The Pd loading on the sample was 0.91 wt% based on
4
3
only weakly active in the hydrogenation of cyclooctene.
Kaskel et al. also prepared a Pd-loaded MOF-5 catalyst via
the incipient wetness and subsequent reduction by hydro-
ꢀ
38
gen at 150–200 C. The prepared Pd-loaded catalysts
ICP analysis. The samples are denoted as Pd /MIL-101 in
n
3
8
were tested in hydrogenation reaction of styrene and
this work, where n stands for the actual Pd wt% of the
catalyst.
4
5
ethyl cinnamate. However, the reported catalysts were
4
3
not stable in air. Clearly, stability and robustness of the
MOF should be considered in selecting a support for cat-
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alytically active species.
Among the Pd-supported MOF ca tCa loy ps yt sr,ig Ph dt /: MA ImL e- 1r 0i c 1a n Scientific Publishers
X-ray powder diffraction patterns were recorded on a
Rigaku diffractometer using CuKꢁ (ꢂ = 1ꢃ54 Å) at
had demonstrated high stability against thermal treatment,
ꢀ
0
.5 /min. The BET surface areas were determined on a
chemical agent and moisture for several reactions such as
ꢀ
Micromeritics ASAP 2020 surface analyzer at −196 C.
3
6
34
Heck coupling, styrene hydrogenation, Suzuki-Miyaura
coupling,39 and synthesis of methyl isobutyl ketone.
These findings prompted us to test the efficacy of Pd-
supported MIL-101 catalyst for oxidation of alcohols using
air at atmospheric pressure as an oxidant. For this purpose,
we have successfully prepared a Pd-supported MIL-101
catalyst by a simple method of Pd-acetate adsorption and
reduction/decomposition without using H2.
Prior to the measurement, the samples were degassed at
40
ꢀ
1
50 C under vacuum for 12 h. TEM micrographs were
obtained on a JEOL JEM-2100F instrument and the sam-
ples were dispersed in ethanol in an ultrasonic bath, and
a drop of supernatant suspension was placed onto a holey
ꢀ
carbon coated grid and dried at 60 C. SEM-EDX data
were obtained on a Hitachi S-4200 instrument. Pd and Cr
contents in MOF were measured using inductively coupled
plasma spectrometry (ICP-OES, Optima 7300DV).
In this paper, we describe the one-step preparation of Pd
nanoparticles on MIL-101 and report their catalytic activ-
ity in benzyl alcohol oxidation using air at atmospheric
pressure. The catalyst was also tested for hydrogenation of
several industrially important alkenes and aldehyde using
2.4. Catalytic Reaction
The oxidation reaction was carried out in a one-necked
round bottom glass reactor equipped with a condenser. In a
H at atmospheric pressure. We also conducted a rigor-
2
3
typical run, 1 mmol of benzyl alcohol, 3 cm of o-xylene
ous test on catalyst stability, and the catalytic activity of
Pd/MIL-101 was compared with a commercially available
Pd-supported catalyst on activated carbon.
and 0.020 g of catalyst (4.53 wt% Pd) were heated at
ꢀ
8
5 C under open air at atmospheric pressure. The hydro-
genation reaction was carried out under atmospheric pres-
sure of hydrogen at room temperature. The reaction vessel
was refilled with hydrogen three times. In a typical run,
2
2
. EXPERIMENTAL DETAILS
.1. Materials
Chromium (III) nitrate nonahydrate (99%, Sigma-Aldrich),
,4-benzene dicarboxylic acid (99%, Sigma-Aldrich),
hydrofluoric acid (Duksan, 48%), palladium (II) acetate
98%, Sigma-Aldrich), Pd-supported activated carbon
3
1 mmol of substrate, 2 cm of methanol and 0.010 g of
catalyst (4.53 wt% Pd) were stirred at room temperature,
and hydrogen at atmospheric pressure was introduced to
the reaction vessel. After completion of the reaction, the
catalyst was filtered off and the conversion was measured
1
(
J. Nanosci. Nanotechnol. 14, 2546–2552, 2014
2547

Pd Nanoparticles Supported on MIL-101: An Efficient Recyclable Catalyst
Bhattacharjee et al.
using a GC (Acme 6000, Younglin, Korea) fitted with a
high performance HP-5 capillary column and a FID.
A hot filtering experiment was carried out by separating
the catalyst quickly from the reaction mixture after 8 h
Pd0
(a)
ꢀ
reaction time, and the filtrate was then maintained at 85 C
for an additional 6 h.
(b)
3. RESULTS AND DISCUSSION
3
.1. Synthetic Aspect of Pd /MIL-101
n
Earlier, Pd nanoparticles-supported catalysts on MIL-101
were usually prepared in a two-step process; for example,
Pd impregnated into MOF from a Pd(NO ) containing
(c)
3
2
DMF solution at room temperature followed by reduction
ꢀ
39
10
20
30
40
in H at 200 C. An alternative synthetic procedure of
2
2
theta (degree)
introducing Pd nanoparticles onto MIL-101 in liquid phase
was applied for the first time in this work. Dilute ace-
tone solution of Pd(O CMe) was added into the MIL-101
Figure 1. X-ray powder diffraction patterns of (a) MIL-101, (b) Pd0ꢃ9
MIL-101 and (c) Pd4ꢃ5/MIL-101.
/
2
2
ꢀ
and heated at 45 C for 24 h, followed by treatment with
DMF to produce an active Pd /MIL-101. The experimen-
tal observation indicated that the reduction of Pd to Pd
n
total pore volume) were lower than those of fresh MIL-
II
0
2
3
1
01 (3100 m /g of BET surface area and 1.88 cm /g of
total pore volume). The average pore size of Pd/MIL-101
17 Å) also became narrower than that of MIL-101 (21 Å),
took place due to the presence of framework functionalities
and acetone as reaction medium; we found that the color
of Pd(O CMe) gradually disappeared in the presence of
(
2
2
due to incorporated Pd nanoparticles as shown in the inset
of Figure 2.
MOF and acetone. DMF was used just as a washing sol-
vent. The resultant material was stable in air or in organic
solvents at high temperature. Similar thermal reduction of
The TEM images of Pd/MIL-101 before the catalytic
reaction are shown in Figure 3. The images show that Pd
0.13847 On: Mon p, a1 r 1t i cAl eu sga 2r e0 1 w4 e 1l l3 :d0 i s7 p: 3e r5s ed over the MOF surface in a
ꢀ
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Pd(O CMe) impregnated MOF-5 in vacuum was reported
2
2
IP: 130.63.18
earlier but at substantially higher 150 to 200 C.
Copyright: American S cn i ae nn ot mi fi ce t eP rus bi zl i es hr ae nr gs e of 2–10 nm, and the size distribution
remained almost constant after the catalytic reaction runs.
3
.2. Characterization of Catalyst
Pd particles tended to appear along the edges of the MOF
crystallites with near spherical morphology. The sizes of
Pd nanoparticles shown in Figures 3(a) and (c) are uniform
MIL-101 before and after Pd deposition was characterized
by XRD, N adsorption–desorption isotherm, TEM, SEM-
2
EDX and ICP-OES. XRD patterns of both MIL-101 and
(ca. 10 nm) and larger than the pore diameter of MIL-
Pd /MIL-101 (Fig. 1) show identical peaks at 2ꢄ values
n
101, which indicates that Pd particles are formed on the
ꢀ
of 4–15 to previously reported characteristic peaks of
external surface of MIL-101. However, closer inspection of
MIL-101, confirming that the structure of MIL-101 was
maintained with high phase purity even after the post-
3
6
synthetic Pd deposition process. In addition, the XRD
of Pd/MIL-101 having 4.53 wt% (Fig. 1(c)) exhibited a
1.0
0.8
(a)
0.6
0.4
ꢀ
0
new weak broad peak at 2ꢄ = 39ꢃ5 , corresponding to Pd .
2000
0
0
.2
.0
0
(b)
Similar observation was reported earlier for Pd -containing
(a)
1
5
10
mesoporous silica or carbon, which was interpreted as
a consequence of small and relatively well-dispersed Pd
Pore size (nm)
1
1
500
000
4
6
nanoparticles formed. Pd /MIL-101, on the other hand,
0
ꢃ9
0
(b)
did not show any additional reflection of Pd (Fig. 1(c))
due to its low Pd concentration.
3
8
500
N adsorption–desorption isotherms at 77 K for fresh
2
MIL-101 and Pd/MIL-101 are shown in Figure 2.
The isotherms of both samples show a type I pattern.
The characteristic two-step adsorption–desorption isotherm
0
0.0
0.2
0.4
0.6
0.8
1.0
of MIL-101 in relative pressure range of 0ꢃ1 < P/P < 0ꢃ3
Relative pressure (P/P0)
0
is due to the two-types of microporous windows (12 Å, and
Figure 2. N2 adsorption–desorption isotherms of (a) MIL-101 and
b) Pd4ꢃ5/MIL-101: adsorption (filled) and desorption (blank). The inset
shows the BJH pore-size distribution curves of (a) MIL-101 and
(b) Pd4ꢃ5/MIL-101.
1
6 Å × 14.5 Å) connected to mesoporous cages of 29 Å
(
3
7
and 34 Å. The textural property values of Pd/MIL-
2
3
1
01 (2400 m /g of BET surface area and 0.66 cm /g of
2
548
J. Nanosci. Nanotechnol. 14, 2546–2552, 2014

Bhattacharjee et al.
Pd Nanoparticles Supported on MIL-101: An Efficient Recyclable Catalyst
(
a)
(b)
(a)
1
00 nm
20 nm
(d)
(
c)
10 µm
(
b)
100 nm
20 nm
(
e)
Figure 4. SEM-EDX of fresh Pd4ꢃ5/MIL-101.
100 nm
Table I. SEM-EDX analysis of fresh Pd4ꢃ5/MIL-101.
Element
Weight%
Atomic%
Figure 3. Transmission electron micrographs of ((a), (b)) fresh
C K
O K
Cr K
25ꢃ69
47ꢃ48
19ꢃ39
38ꢃ54
53ꢃ48
6ꢃ72
Pd4ꢃ5/MIL-101, ((c), (d)) reused Pd4ꢃ5/MIL-101 catalyst after fifth run and
(
e) fresh Pd0ꢃ9/MIL-101.
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Pd L
7ꢃ44
1ꢃ26
Figures 3(b) and (d) also show smI Pa l :l e1r 3u 0n . i6f o3 r. m1 8 P0 d. 1n 4a 7n o Op an r: -Mon T, o 1t a 1l s Aug 2014 13:07:35100ꢃ00
Copyright: American Scientific Publishers
ticles (ca. 2 nm) that can be accommodated inside the
pore system. TEM images thus indicate that the metallic
Pd particles of dual size distribution are formed during
the synthesis, while maintaining the host crystalline struc-
ture of MIL-101. Decrease of Pd loading in Pd /MIL-
alcohol was converted to benzaldehyde with 85% conver-
sion and 97% selectivity after 14 h reaction. Aerobic ben-
zyl alcohol oxidation over Pd catalysts can be explained
by the dehydrogenation mechanism; the oxidation of the
alcohol by dehydrogenation, which results in the formation
of the aldehyde and adsorbed hydrogen on the Pd surface,
0
ꢃ9
1
01 resulted in a similar size distribution. H reduction of
2
Pd(O CMe) on MIL-101, on the other hand, was reported
2
2
3
4
to produce smaller Pd particles of ca. 1.5 nm. We deter-
mined the metal loading values of the supported catalysts
by SEM-EDX (Fig. 4) and ICP analysis. The values of
Cr/Pd ratio observed from SEM-EDX (Table I) and the bulk
ICP were in agreement and 5.33 and 5.70, respectively.
80
3
.3. Catalytic Performances Over the
Pd/MIL-101 Catalyst
60
To test the catalytic performance of Pd/MIL-101 catalyst
in alcohol oxidation, we carried out the oxidation of
benzyl alcohol using open air at atmospheric pressure.
The results are summarized in Table II. Blank test reac-
tions were carried out under the same reaction conditions
in the presence of parent MIL-101 and in the absence
of Pd/MIL-101. No reaction products were occurred in
both cases, indicating that the presence of active metal
supported on MIL-101 is essential for the oxidation reac-
tion. As expected, benzyl alcohol conversion increased
with time, and Figure 5 shows the conversion values of
4
2
0
0
0
Filtration of catalyst
0
3
6
9
12
15
Time (h)
ꢀ
Figure 5. Benzyl alcohol oxidation using air (1 atm) at 85 C, catalyst
amount 20 mg in o-xylene: (ꢀ) with Pd4ꢃ5/MIL-101 catalyst and (ꢁ) fil-
trate (catalyst filtering off after 8 h reaction).
ꢀ
benzyl alcohol at different reaction time. At 85 C, benzyl
J. Nanosci. Nanotechnol. 14, 2546–2552, 2014
2549

Pd Nanoparticles Supported on MIL-101: An Efficient Recyclable Catalyst
Bhattacharjee et al.
Table II. Summary of the catalytic oxidation/hydrogenation activity of
Pd-supported catalysts.
100
Time Conv. Select.
Reaction run
Catalysts
Substrate
(min) (%)
(%)
75
a
a
b
1
2
5
Pd4ꢃ5/MIL-101 Benzyl alcohol
840
84ꢃ8 97ꢃ1
84ꢃ7 97ꢃ1
84ꢃ7 97ꢃ0
50
25
0
1
Pd5ꢃ0/C
Benzyl alcohol
840
50ꢃ3 96ꢃ7
Filtration of catalyst
c
d
1
2
5
Pd4ꢃ5/MIL-101 Styrene
60 100ꢃ0 100ꢃ0
100ꢃ0 100ꢃ0
100ꢃ0 100ꢃ0
c
c
1
1
Pd0ꢃ9/MIL-101 Styrene
135 100ꢃ0 100ꢃ0
120 100ꢃ0 100ꢃ0
Pd5ꢃ0/C
Styrene
c
e
f
1
5
Pd4ꢃ5/MIL-101 Cyclooctene
240 100ꢃ0 100ꢃ0
100ꢃ0 100ꢃ0
0
10
20
30
40
50
60
Time (min)
c
1
5
Pd4ꢃ5/MIL-101 Benzaldehyde
600
84ꢃ5 96ꢃ0
84ꢃ5 96ꢃ0
Figure 6. Conversion profiles for the hydrogenation of styrene: (ꢀ)
fresh Pd4ꢃ5/MIL-101 and (ꢁ) filtered Pd /MIL-101 after 20 min reaction
4ꢃ5
a
3
time.
Notes: Reaction conditions: 1 mmol substrate, 0.020 g catalyst, 3 cm o-xylene,
open air (1 atm) and 85 C; Benzaldehyde; Reaction conditions: 1 mmol sub-
strate, 0.010 g catalyst, 2 cm methanol, H2 (1 atm) and 25 C; Ethyl benzene;
ꢀ
b
c
3
ꢀ
d
e
f
Cyclooctane; Benzyl alcohol.
cyclooctane with 100% conversion after 220 min under
the identical reaction conditions (Table II). The results
indicated that the external C C bond in styrene hydro-
genation over Pd4ꢃ5/MIL-101 catalyst proceeded much
faster than in cyclooctene. Alkene conversions with H2
over Pd catalysts has been extensively studied and gener-
ally described by the Horiuti-Polanyi mechanism, which
followed by hydrogen removal from the Pd surface by oxi-
4
7
dation with oxygen.
The reaction was also carried out using a commer-
cially available Pd-supported activated carbon (5 wt% Pd),
Pd /C, under the identical reaction condition as applied
5ꢃ0
proceeds through a series of successive hydrogenation-
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for Pd/MIL-101. The results are listed in Table II. Pd /
4
ꢃ5
4
9
IP: 130.63.180.147 On: Mon d, e1 h 1y dA r uo gg e 2n 0a t1i o4 n1 s3t e: 0p 7s .: 35
MIL-101 catalyst showed higher activity than Pd /C.
5
ꢃ0
Copyright: American Scientific Publishers
The heterogeneity and reusability of Pd /MIL-101
4
ꢃ5
The origin of the high activity in Pd /MIL-101 is not
4
ꢃ5
catalyst in oxidation and hydrogenation reactions were
investigated by performing repeated reaction cycles and
hot filtering experiments, respectively. In the recycling test,
the catalyst was recovered by filtration at the end of each
reaction cycle, and washed with solvent, dried, and reused.
The results are also included in Table II. The conversion
and selectivity were virtually identical, irrespective of the
number of reaction cycles performed. No leached Pd was
detected in the filtrate by ICP measurements.
clear at this stage, but may be associated with the uni-
4
8
formly dispersed Pd particles in MIL-101, and also sig-
nificantly larger pores and pore volume of the support
material MIL-101 than in activated carbon, which can con-
tribute towards reducing the mass transfer resistance of
reactant/product to the smaller Pd particles located inside
the pores.
A series of experiments were also performed for the
hydrogenation of alkenes and aldehyde over both Pd /
0
ꢃ9
The morphology of the reused catalyst in the benzyl
alcohol oxidation was also examined by TEM (Figs. 3(c)
and (d)). The original near spherical Pd nanoparticles were
retained nearly the same as that of the fresh catalyst with
good crystalline MIL-101 structure after fifth run. Pd and
other nanoparticles supported on MOFs can be classified
as three kinds:
MIL-101 and Pd /MIL-101 catalysts using 1 atm H in
4
ꢃ5
2
methanol at room temperature. The results are also listed
in Table II. Blank test reactions were carried out under the
same reaction conditions in the presence of parent MIL-101
and in the absence of Pd/MIL-101. No reaction products
were occurred in both cases. Figure 6 shows the conver-
sion values of styrene at different reaction time over Pd /
4
ꢃ5
(
(
1) those formed inside the pores,
2) those deposited preferentially on the external sur-
MIL-101. The conversion of styrene gradually increased
with time and reached 100% conversion after 60 min.
The reaction was then carried out over Pd /MIL-101 under
face, and
0
ꢃ9
(
3) both formed inside and on the external surface.41
the identical conditions. At 100% conversion, styrene was
converted to ethyl benzene within 135 min (Table II).
These results indicate that the availability of active Pd
sites controls the catalytic reaction, which is fundamentally
heterogeneous in nature. We then tested the catalytic activ-
ity of Pd /MIL-101 in the hydrogenation of a model cyclic
Al-Shall et al. also reported 2–6 nm-sized Pd nanoparti-
cles deposited on the external surface of MIL-101 similar
5
0
to this work. Hydrothermally stable MIL-101 with high
surface area offers preferential nucleation sites over the
coordinative open metal sites, and also expected to bind the
nanoparticles strongly. Particles of ca. 10 nm on MIL-101
4
ꢃ5
ꢀ
alkene, cyclooctene. At 25 C, cyclooctene converted to
2
550
J. Nanosci. Nanotechnol. 14, 2546–2552, 2014

Bhattacharjee et al.
Pd Nanoparticles Supported on MIL-101: An Efficient Recyclable Catalyst
would be expected to have little possibility of leaching
considering their lower radius of curvature than those
among the smaller Pd nanoparticles. Further confirmation
on the heterogeneous nature of Pd/MIL-101 was provided
by hot filtering experiments. The catalyst was filtered-off
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(after 8 h of reaction and conversion reached 49.5% for
(
benzyl alcohol (Fig. 4), and after 20 min of reaction and
conversion reached 55.6% for styrene (Fig. 5)) and the fil-
trates were then stirred for an additional duration. No fur-
ther reaction product was observed after catalyst separation.
These results indicated that catalysis reactions occurred on
the Pd nanoparticles supported on MIL-101 and Pd/MIL-
1
9. S. M. Reddy, K. K. R. Datta, C. H. Sreelakshmi, M. Eswaramoorthy,
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2
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1
01 is behaved truly as a heterogeneous catalyst.
(
2
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4
. CONCLUSIONS
2
2
Pd nanoparticles supported on the chromium terephtha-
late metal-organic framework MIL-101 were prepared
via a simple Pd-acetate adsorption-reduction procedure.
The structure of the hosting MIL-101 was unchanged
after post-synthetic Pd introduction or after catalytic runs.
26. M. Hu, J. Reboul, S. Furukawa, N. L. Torad, Q. Ji, P. Srinivasu,
K. Ariga, S. Kitagawa, and Y. Yamauchi, J. Am. Chem. Soc.
134, 2864 (2012).
2
7. W. Chaikittisilp, K. Ariga, and Y. Yamauchi, J. Mater. Chem. A 1, 14
The Pd /MIL-101 catalyst showed high activity and
4
ꢃ5
(2013).
found to be a true heterogeneous catalyst for oxidation of
alcohols using open air at atmospheric pressure, which also
produced good activity for the hydrogenation of alkenes
and aldehyde. The present Pd-supported MOF catalyst
exhibited higher activity than Pd supported on activated
28. J.-R. Li, J. Sculley, and H.-C. Zhou, Chem. Rev. 112, 869
(2012).
2
9. G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang,
X. Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang,
X. Chen, J. Ma, S. C. Joachim Loo, W. D. Wei, Y. Yang, J. T. Hupp,
and F. Huo, Nat. Chem. 4, 310 (2012).
Delivered by Publishing Technology to: York University
carbon. The catalyst could be separated quantitatively by
30. G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour,
IP: 130.63.180.147 On: Mon, 11 Aug 2014 13:07:35
simple filtration and reused several times without losing
S. Surbli, and J. Margiolaki, Science 309, 2040 (2005).
Copyright: American Scientific Publishers
3
3
3
1. P. L. Llewellyn, S. Bourrelly, C. Serre, A. Vimont, M. Daturi,
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Acknowledgment: This study was supported by the
NRF grant funded by MEST (No. 2013005862) in Korea.
3. J. Kim, S. Bhattacharjee, K. E. Jeong, S. Y. Jeong, and W. S. Ahn,
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Received: 4 January 2013. Accepted: 6 February 2013.
Delivered by Publishing Technology to: York University
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Copyright: American Scientific Publishers
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J. Nanosci. Nanotechnol. 14, 2546–2552, 2014