Tandem catalysis by palladium nanoclusters encapsulated in metal-organic frameworks

Article abstract of DOI:10.1021/cs5006635

A bifunctional Zr-MOF catalyst containing palladium nanoclusters (NCs) has been developed. The formation of Pd NCs was confirmed by transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS). Combining the oxidation activity of Pd NCs and the acetalization activity of the Lewis acid sites in UiO-66-NH₂, this catalyst (Pd@UiO-66-NH₂) exhibits excellent catalytic activity and selectivity in a one-pot tandem oxidation-acetalization reaction. This catalyst shows 99.9% selectivity to benzaldehyde ethylene acetal in the tandem reaction of benzyl alcohol and ethylene glycol at 99.9% conversion of benzyl alcohol. We also examined various substituted benzyl alcohols and found that alcohols with electron-donating groups showed better conversion and selectivity compared to those with electron-withdrawing groups. We further proved that there was no leaching of active catalytic species during the reaction and the catalyst can be recycled at least five times without significant deactivation.

Full text of DOI:10.1021/cs5006635

Research Article
pubs.acs.org/acscatalysis
Tandem Catalysis by Palladium Nanoclusters Encapsulated in Metal−
Organic Frameworks
Xinle Li,‡,†,§ Zhiyong Guo,‡,†,§ Chaoxian Xiao,†,§ Tian Wei Goh,†,§ Daniel Tesfagaber,†,§
and Wenyu Huang*,†,§
†Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
§Ames Laboratory, USDOE, Ames, Iowa 50011, United States
S
* Supporting Information
ABSTRACT: A bifunctional Zr-MOF catalyst containing palladium
nanoclusters (NCs) has been developed. The formation of Pd NCs
was confirmed by transmission electron microscopy (TEM) and
extended X-ray absorption fine structure (EXAFS). Combining the
oxidation activity of Pd NCs and the acetalization activity of the Lewis
acid sites in UiO-66-NH2, this catalyst (Pd@UiO-66-NH2) exhibits
excellent catalytic activity and selectivity in a one-pot tandem
oxidation-acetalization reaction. This catalyst shows 99.9% selectivity
to benzaldehyde ethylene acetal in the tandem reaction of benzyl
alcohol and ethylene glycol at 99.9% conversion of benzyl alcohol. We
also examined various substituted benzyl alcohols and found that
alcohols with electron-donating groups showed better conversion and
selectivity compared to those with electron-withdrawing groups. We
further proved that there was no leaching of active catalytic species
during the reaction and the catalyst can be recycled at least five times without significant deactivation.
KEYWORDS: tandem synthesis, acetalization, bifunctional catalysts, UiO-66, acetal, solid acid, selective oxidation
surface areas, well-defined porosities, and chemical tenability.9
1. INTRODUCTION
MOFs have been utilized for many applications including gas
With the pressing need for catalysis technologies that could
address the energy crisis and environmental pollution
storage, separations, chemical sensing, and drug delivery.10−13
In recent years, MOFs have demonstrated their potential
application as heterogeneous catalysts, especially those using
MOFs as a supporting matrix for metal nanoparticles (known
as metal@MOFs).14−23 Despite the many investigations of the
MOFs’ role in catalysis, there are only a limited number of
studies of bifunctional metal@MOFs systems for tandem
catalysis. Fischer et al. and Haruta et al. reported Au@ZIF-8
and Au@MOF-5 as catalyst in the synthesis of methyl benzoate
from the aerobic oxidation of benzyl alcohol, respectively;24,25
Corma et al. developed Pd@MIL-101 as a catalyst for one-pot
synthesis of menthol from citronellal.26 Li’s group prepared
Pd@MIL-101 for the one-pot synthesis of methyl isobutyl
ketone from acetone.27 Recently, Li’s group also prepared a
bimetallic MOF catalyst (Au−Pd@MIL-101) and used it for
the one-pot synthesis of aromatic esters from alkyl
aromatics.28,29 Tang et al. designed a core−shell Pd@
IRMOF-3 and used it as a multifunctional catalyst for a
cascade condensation-hydrogenation reaction.30
confronting our societies, a great deal of research effort has
been devoted to finding novel and efficient multifunctional
catalysts for one-pot tandem reactions.1−8 Tandem catalysis, in
which multistep chemical transformations are catalyzed with a
multifunctional catalyst, has attracted increasing research
attention because selective multifunctional catalysts will make
a multistep reaction go to the desired products in one reactor
without the need for separation, purification, and transfer of
intermediates produced in each step. The development of
multistep catalytic processes using multifunctional catalysts can
also minimize the amount of byproducts/waste produced
during multistep chemical reactions and achieve atomic
efficiency, one of the focuses of green chemistry. Conversely,
traditional multistep catalytic processes involve a series of
conversions in different reactors with intermediate purification
steps to achieve a desired product. Therefore, traditional
multistep catalytic processes usually require a high-energy input
for the separation of intermediates and cause low yield of the
final products due to the losses of intermediates within each
individual step of the processes.
Metal−organic frameworks (MOFs), a class of porous
materials assembled from metal ions/clusters and organic
ligands, have been investigated extensively because of their high
Received: May 14, 2014
Revised: August 22, 2014
Published: August 25, 2014
© 2014 American Chemical Society
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As pointed out by recent reviews, the field of developing
tandem catalysis using MOFs remains relatively unexplored and
will have a large impact on current industrial processes.31
Herein, we developed a bifunctional tandem heterogeneous
catalyst by encapsulating Pd nanoclusters (NCs) in the cavities
of UiO-66-NH2 (Scheme 1), which could catalyze the oxidation
66-NH2, zirconium(IV) chloride (0.40 g, 1.71 mmol; 98%,
Acros Organics) and 2-aminoterephthalic acid (NH2−H2BDC)
(0.31 g, 1.71 mmol; >99%, Sigma-Aldrich) were dissolved in
100 mL of N, N′-dimethylformamide (DMF; ACS grade,
Macron) at room temperature, and then 140 μL of water was
added. For UiO-66, terephthalic acid (0.29 g, 1.71 mmol; 98%,
Sigma-Aldrich) was used as the organic linker, and no water
was added during the synthesis. The obtained mixture was
sealed and placed in a preheated oven at 120 °C for 24 h. The
solid MOFs were washed with fresh DMF, chloroform, and
methanol three times every 12 h. Then it was activated at 150
°C under vacuum (30 mTorr) for 6 h.
Scheme 1. Synthesis of the Heterogeneous Tandem Catalyst
by Encapsulating Pd NCs Inside the Cavities of UiO-66-NH2
Synthesis of Pd NCs Inside the Cavities of UiO-66-NH2 and
UiO-66. UiO-66-NH2 (100 mg) was dispersed in 12 mL of
methylene chloride (Fisher, Optima grade). After sonication for
1 h, a methylene chloride solution of palladium acetate (4.32
mg Pd(OAc)2 in 5 mL of methylene chloride, Kawaken Fine
Chemicals) was added dropwise to the MOF solution under
vigorous stirring. After 24 h of stirring at room temperature, the
as-prepared Pd2+ infiltrated MOF was washed three times for
12 h each time with fresh methylene chloride. The reduction
process of the precursor loaded UiO-66-NH2 was performed
under a 50 mL/min flow of 10% H2/Ar at 200 °C for 2 h. The
same method was used to synthesize Pd NCs inside UiO-66.
2.2. Instrumentation. BET, PXRD, TEM, XPS, and TGA.
Surface area analysis of the catalyst was performed by nitrogen
sorption isotherms in a Micromeritics Tristar 3000 surface area
analyzer at 77 K. Powder X-ray diffraction (PXRD) patterns of
the samples were obtained by a STOE Stadi P powder
diffractometer using Cu Kα radiation (40 kV, 40 mA, λ =
0.1541 nm). The size and morphology of 2.0 wt % Pd@UiO-
66-NH2 were investigated by using transmission electron
microscopy (TEM) and high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) images
recorded on a Tecnai G2 F20 electron microscope equipped
with an energy-dispersive X-ray (EDX) detector (Oxford INCA
EDS). X-ray photoelectron spectroscopy (XPS) measurements
were performed using a PHI 5500 Multitechnique system
(Physical Electronics, Chanhassen, MN) with a monochrom-
atized Al Kα X-ray source (hν = 1486.6 eV). Thermogravi-
metric analysis (TGA) was carried out using STA 6000
simultaneous thermal analyzer (PerkinElmer). In the TGA
experiment, 5.7 mg of Pd@UiO-66-NH2 was analyzed under
N2 flow with a heating rate of 10 °C/min from room
temperature to 900 °C.
Inductively Coupled Plasma Atomic Emission Spectrosco-
py (ICP-AES). To determine the loading amount of palladium in
UiO-66-NH2, ICP-AES (Optima 2100 DV) was performed.
Because the zirconium in UiO-66-NH2 has similar emission
with palladium in ICP-AES, it is difficult to detect the palladium
amount directly. Instead, we measured the amount of Pd that
was washed away during the washing steps and calculated the
actual Pd remaining in Pd@UiO-66-NH2. When loading the 2.0
wt % Pd in UiO-66-NH2, the exact initial weight of the
palladium precursor (palladium acetate) was recorded first.
Every time after washing, the washing solution that contained
the palladium acetate residue was collected. Finally, ICP-AES
was performed for the washing solution, and the palladium
content was calculated. The difference between added and
removed Pd (during washing) is the actual palladium amount
that was loaded in UiO-66-NH2. The same method was also
used to determine the actual Pd amount in Pd@UiO-66.
of benzyl alcohol and the subsequent acetalization of
benzaldehyde with ethylene glycol. To the best of our
knowledge, this is the first example of applying MOF-based
heterogeneous catalysts in tandem oxidation-acetalization
reaction in liquid phase. One-pot tandem reaction through
oxidation of alcohols followed by acetalization of the carbonyl
compounds using heterogeneous catalysts in liquid phase has
rarely been studied so far. Considerable research aiming at one-
step gas phase conversion of methanol to 1,1-dimethoxy-
methane over a variety of heterogeneous catalytic systems can
be found in the literature, that is, molybdenum-based
catalysts,32 heteropoly acids (HPAs),33 oxides of ruthenium
and rhenium,34−37 as well as vanadium-based catalysts.38−40
With regard to one-pot higher acetals synthesis, we only found
limited work related to the direct synthesis from ethanol via
photocatalytic oxidation or H2O2 oxidation catalyzed by Fe
(III) complexes.41−43 Two combined catalytic systems, Pd-
(OAc)2/Cu(OAc)2/p-toluenesulfonic acid44 and manganese
dioxide/trialkyl orthoformate/indium triflate,45 also showed
activity in tandem oxidation/acetalization reactions.
UiO-66-NH2 is built up from [Zr6O4(OH)4(CO2)12] clusters
linked with 2-aminoterephthalic acid.46 UiO-66-NH2 possesses
uncoordinated zirconium sites, which could serve as Lewis acid
sites for catalysis. These Lewis acid sites have been used to
catalyze the cross-aldol condensation, cyclization of citronellal,
and acetalizaton.47−49 By combining the oxidation activity of Pd
NCs and the acetalization activity of the Lewis acid sites on
UiO-66-NH2, we developed this tandem Pd@UiO-66-NH2
catalyst, which showed 99.9% conversion of benzyl alcohol in
the one-pot tandem reaction of benzyl alcohol and ethylene
glycol with 99.9% acetal selectivity. Acetals are a class of
chemicals that can be used for the protection of carbonyl
compounds. They are also widely utilized as organic solvents,
fuel additives, steroids, besides being important in the
pharmaceutical and fragrance industries.50,51 Generally, acetal
synthesis is carried out by the condensation of alcohols with
aldehydes using mineral acid catalysts.52 However, these
catalysts are toxic, corrosive, and difficult to be removed after
the reaction.
2. EXPERIMENTAL SECTION
2.1. Materials and Synthesis. Synthesis of UiO-66-NH2
and UiO-66. The MOF was synthesized and purified according
to a procedure reported by Lillerud and co-workers.53 For UiO-
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Extended X-ray Absorption Fine Structure (EXAFS). The
EXAFS spectra were measured in transmission mode (Pd K
edge = 24.350 keV) at 20-BM-B and 9-BM-B beamlines of the
Advanced Photon Source at Argonne National Laboratory.
EXAFS of reference samples were collected using pure finely
ground powders homogeneously dispersed on polyimide
Kapton tap, whereas the MOF samples were pressed into a
pellet fit to a hole embedded in a Teflon substrate. A palladium
foil spectrum was acquired simultaneously with each measure-
ment for energy calibration. Multiple scans (8−10 scans) were
collected for each sample, and merged into one data set for
EXAFS analysis. For the analysis, the software at the beamlines
was used to perform dead-time correction, and energy
calibration. The Athena program, which is an interface to
IFEFFIT package, was used for glitch removal, pre-edge
subtraction, postedge normalization, and conversion to k-
space.54,55 The EXAFS data were fitted in R-space using the
Artemis program from the same package, with theoretical
models constructed from FEFF6.56−58 All the EXAFS data
fitting were performed with a k3 weighting in R-space. The k
ranges (Å−1), R-ranges, independent points in the EXAFS
spectrum, the number of variables determined in the models,
and the fitting quality are given in Supporting Information,
Table S2.
After separating the MOF powder by centrifugation, we dried
the powder under vacuum at 80 °C and then reduced the
Pd(II) loaded MOF under 50 mL/min flow of 10% H2/Ar at
200 °C to form Pd NCs (Scheme 1). The successful loading of
Pd in UiO-66-NH2 was confirmed by EDX spectroscopy. As
shown in Figure S1 (Supporting Information), a clear Pd peak
was resolved when we probed selected area 1 on the Pd loaded
MOF. We also observed a similar EDX spectrum when we
probed area 2. The loading of Pd in UiO-66-NH2 is 2.0 wt %
measured by inductively coupled plasma atomic emission
spectroscopy (ICP-AES).
The transmission electron microscopy (TEM) image of Pd@
UiO-66-NH2 is shown in Figure 1a, in which Pd NCs are well
2.3. Catalysis. The tandem reaction was carried out under
reflux in a 10 mL round-bottom flask equipped with a
condenser whose temperature was maintained at 5 °C using a
recirculating chiller (NESLAB, R134A). In a typical catalytic
measurement, 10.5 mg of benzyl alcohol (0.1 mmol, Fisher
Scientific), 100 μL of ethylene glycol (Fisher Scientific), 5.0 mg
of 2.0 wt % Pd@UiO-66-NH2 (1 mol %), and 7 mg of
mesitylene (internal standard, 99%, Acros Organics) were
added into 2 mL of cyclohexane in the round-bottom flask. An
oxygen balloon at the top of the condenser was used to supply
oxygen for the reaction. The flask was loaded in an oil bath that
was preheated to 90 °C. The reaction was kept under reflux
with magnetic stirring at 600 rpm for designated time. After the
reaction was finished, the products were analyzed using a gas
chromatograph (GC) equipped with a HP-5 capillary column
(30 m × 0.32 mm × 0.25 μm) and a flame ionization detector.
The products were identified by a GC-MS (SHIMADZU
5050A) equipped with a HP-5ms capillary column (30 m ×
0.25 mm × 0.25 μm).
Recycling Test. The catalyst was isolated at the end of the
reaction and the liquid was removed. The solid catalyst was
reused in the second run. The catalytic activity and acetal
selectivity of the catalyst did not show significant loss in the
second run. PXRD of the used sample showed that the
structure of the catalyst remained intact during the catalytic
cycles. The Pd@UiO-66-NH2 catalyst did not show any
deactivation within five cycles of the reaction.
Figure 1. (a) TEM and (b) HAADF-STEM images of 2.0 wt % Pd@
UiO-66-NH2. The Pd@UiO-66-NH2 was synthesized by loading Pd
precursor, palladium(II) acetate, into UiO-66-NH2 in CH2Cl2. After
separating and drying the powder, we reduced it under 50 mL/min
flow of 10% H2/Ar at 200 °C to form Pd NCs.
dispersed with a mean diameter less than 1.2 nm (the diameter
of the large octahedral cages in UiO-66-NH2). To increase the
contrast of the Pd NCs against the MOF support, we also
measured high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) images of Pd@UiO-66-
NH2 as shown in Figure 1b. We could not find any particle that
has a diameter larger than 1.2 nm, which indicates that the
formed Pd NCs are well-dispersed inside the cavities of the
MOF. From the EDX mapping of selected area on the Pd@
UiO-66-NH2 (the rectangle indicated by the dashed line in
Supporting Information, Figure S2a), Pd was found to exist
along with Zr as shown in Supporting Information, Figure
S2b,c, indicating Pd was effectively trapped by the MOF. The
dispersion was uniform, and no sign of aggregation was
observed. We are aware that the 2D TEM images cannot
exclusively determine whether the Pd NCs are located inside or
on the surface of the MOF. Therefore, we tried to rotate the
TEM sample and track the same Pd NCs to confirm that these
NCs are located inside the MOF. Using this method, we were
able to confirm that Pt NCs are encapsulated in UiO-66-NH2.59
However, to follow the same Pd NC is extremely hard during
the TEM measurement because of the small difference in
electron beam contrast for Pd NCs and [Zr6O4(OH)4(CO2)12]
clusters in the MOF.
Leaching Test. The solid catalyst (Pd@UiO-66-NH2) was
separated from the hot solution right after reaction for 90 min.
The reaction was continued with the filtrate in the absence of
solid catalyst for an additional 8 h. No further increase in either
the conversion of benzyl alcohol or the selectivity of acetal was
observed, which indicates that the catalytically active sites for
both oxidation and acetalization are on the solid catalyst.
PXRD pattern of as-synthesized UiO-66-NH2 matches well
with the reported patterns of the MOF (pattern b in Figure
2).53,60 UiO-66-NH2 is very stable during the loading of Pd
precursor and the reduction treatment under 50 mL/min flow
of 10% H2/Ar at 200 °C. We do not observe any obvious
changes in the PXRD patterns of Pd@UiO-66-NH2 that
indicates the loss of the MOF’s crystalline structure (pattern c
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization. The Pd@UiO-66-
NH2 catalyst was synthesized by loading Pd precursor,
palladium(II) acetate, into UiO-66-NH2 in methylene chloride.
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peaks (from 330 to 350 eV) and the much higher concentration
of Zr from the MOF compared to Pd, we could not get a good
fitting of the XPS spectra to determine the oxidation state of
the Pd (Figure S4, Supporting Information). Instead, we used
extended X-ray absorption fine structure (EXAFS) spectrosco-
py to study the coordination environment of Pd before and
after reducing the Pd(II)-loaded UiO-66-NH2. As shown in
Figure 4a, the EXAFS spectrum of the sample before reduction
is dominated by the Pd−O scattering. The Pd−O coordination
number determined from fitting the EXAFS spectrum is 3.6
0.3 (Table S2, Supporting Information), which agrees with the
structure of the Pd precursor, palladium acetate, although the
interatomic distance (2.05 0.01 Å) is slightly higher than that
in Pd(OAc)2 (2.00 0.01 Å). After reduction with 10% H2/Ar
at 200 °C, the scattering of Pd−Pd appears in Fourier
transform, as shown in Figure 4b. EXAFS fitting showed that
Pd−Pd scattering has a coordination number of 3.7 0.3 with
interatomic distance of 2.78 0.01 Å, indicating the formation
of Pd NCs.
Figure 2. PXRD patterns of (a) simulated UiO-66, (b) as-synthesized
UiO-66-NH2, (c) 2.0 wt % Pd@UiO-66-NH2 after reduction, (d) 2.0
wt % Pd@UiO-66-NH2 after reaction, (e) 2.0 wt % Pd@UiO-66-NH2
after recycle test.
Additionally, Fourier transform also indicates the presence of
another scattering from Pd−N or Pd−O. The interatomic
distance was found to be ∼2.16−2.19 Å with fitting. This
distance is obviously longer than that in PdO phase usually
found in Pd catalysts (∼2.0 Å).61,62 However, in some catalysts
with strong metal−support interaction, the metal−oxide
distance was reported to be much longer (2.19 Å).63 Besides,
Pd−N bond length could vary between 2.0 and 2.2 Å,
depending on the coordination environment.64,65 Due to the
presence of −NH2 groups in UiO-66-NH2, it is more likely that
the −NH2 group coordinates with Pd NCs after the reduction.
The tandem oxidation-acetalization reactions of benzyl
alcohol and ethylene glycol were carried out with 2.0 wt %
Pd@UiO-66-NH2 and 1.9 wt % Pd@UiO-66 under 0.1 MPa O2
at 90 °C in cyclohexane. As shown in Table 1 (entry 1), Pd@
UiO-66-NH2 was an active catalyst for this one-pot tandem
reaction and 99.9% conversion of benzyl alcohol with 99.9%
selectivity to benzaldehyde ethylene acetal was achieved. On
the contrary, UiO-66-NH2 alone only gave 1.5% conversion of
benzyl alcohol, which confirmed the oxidation of benzyl alcohol
to benzaldehyde is catalyzed by Pd NCs. To confirm that the
acetalization of benzaldehyde and ethylene glycol is catalyzed
by UiO-66-NH2, we tested the activity of UiO-66-NH2 in the
acetalization reaction using benzaldehyde and ethylene glycol as
the reactants. We found that the reaction reached 99.9%
conversion and 99.9% selectivity after 6 h (Table 1, entry 3),
which confirmed that UiO-66-NH2 is a highly active and
selective catalyst in the acetalization reaction. We also tested
two control catalysts, 5.0 wt % Pd/C and 1.9 wt % Pd@UiO-
66. We could not achieve high selectivity using Pd/C under the
same reaction conditions (Table 1, entry 5). Using Pd@UiO-
66, we obtained 91.0% conversion of benzyl alcohol and 95.0%
selectivity to acetal (Table 1, entry 4), which are slightly lower
than the respective values acquired on Pd@UiO-66-NH2.
These catalysis results show the superiority of using MOFs as
supports and also suggest that both Pd NCs and Lewis acid
sites on MOFs are essential for the efficient tandem oxidation-
acetalization reaction.
in Figure 2). Furthermore, we do not find any X-ray diffraction
peaks from Pd nanocrystal, which suggests that the
encapsulated Pd particles are very small. Thermogravimetric
analysis (TGA) indicates that Pd@UiO-66-NH2 is thermally
stable below ∼280 °C (Figure S3, Supporting Information).
The surface areas and pore volumes of the samples were
measured by N2 physisorption at −196 °C. Figure 3 shows the
Figure 3. Nitrogen adsorption isotherms of the as-synthesized UiO-
66-NH2 and 2.0 wt % Pd@UiO-66-NH2.
N2 adsorption−desorption isotherm profiles of UiO-66-NH2,
and 2.0 wt % Pd@UiO-66-NH2. All adsorption−desorption
isotherms show a type I shape, a characteristic of microporous
materials. As shown in Supporting Information, Table S1, the
BET surface area and micropore volume of UiO-66-NH2 were
calculated to be 1194 m2 g−1 and 0.44 cm3 g−1, respectively,
which are close to the reported values.47 Compared to UiO-66-
NH2, the BET surface area and pore volume of 2.0 wt % Pd@
UiO-66-NH2 decreased to 936 m2 g−1 and 0.36 cm3 g−1,
respectively, mainly because of the occupation of the cages of
UiO-66-NH2 by Pd NCs.16
We also investigated the effect of solvent and found that
cyclohexane is the best solvent for this tandem reaction (entries
1, 6, and 7 in Table 1). The high selectivity to benzaldehyde
ethylene acetal in cyclohexane can be attributed to the low
solubility of water in this nonpolar solvent. Because water is
one of the products in the acetalization reaction, the low
We also tried to determine the oxidation state of Pd in UiO-
66-NH2 after reduction using X-ray photoelectron spectroscopy
(XPS). However, due to the overlap between Pd 3d and Zr 3p
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Figure 4. Fourier transform (FT) of EXAFS data extracted from Pd K edge (blue empty dot) and the computed fits (red line) in R-space for 4.8 wt
% Pd@UiO-66-NH2 samples: (a) before and (b) after reduction under 50 mL/min flow of 10% H2/Ar at 200 °C. FT peak positions are not
corrected for phase shifts. The individual theoretical paths (Pd−Pd and Pd−N) used to fit the experimental data were also shown in (b). For
comparison, only their magnitudes were shown.
Table 1. Performance of Different Catalysts in the Oxidation of Benzyl Alcohol and the Subsequent Acetalization with Ethylene
Glycol
f
g
entry
sample
substrate
solvent
t (h)
conv. (%)
alde (%)
54.1
acetal (%)
ester (%)
a
1
Pd@UiO-66-NH2
UiO-66-NH2
UiO-66-NH2
Pd@UiO-66
benzyl alcohol
benzyl alcohol
benzaldehyde
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
ethylene glycol
toluene
22
22
6
99.9
1.5
99.9
45.9
99.9
95.0
59.7
59.2
54.1
b
2
b
,c
3
4
5
6
7
99.9
91.0
99.0
99.9
98.7
a
22
22
3
5.0
40.3
6.2
d
e
Pd/C
Pd@UiO-66-NH2
Pd@UiO-66-NH2
30.6
e
22
45.9
a
Reaction conditions: 90 °C, 0.1 MPa O2, benzyl alcohol (0.1 mmol), ethylene glycol (100 μL, 1.8 mmol), cyclohexane (2 mL), 5 mg 2.0 wt % Pd@
b
c
d
e
MOF, metal/substrate = 1:100. 4.9 mg of UiO-66-NH2 was used. 0.1 mmol benzaldehyde was used. 2 mg of 5%Pd/C was used. 2 mL of solvent
was used. Benzaldehyde ethylene acetal. 2-hydroxyethyl benzoate.
f
g
solubility of water in the reaction solvent could shift the
equilibrium of the reaction to the acetalization product.66
Furthermore, it is noteworthy that we did not detect any
benzoic acid or the corresponding ester after the designated
reaction time when we used cyclohexane as the reaction
solvent, which shows the superior selectivity of Pd@UiO-66-
NH2 under appropriate reaction conditions. The inhibition of
the further oxidation of benzaldehyde could also be attributed
to the low solubility of water in cyclohexane. The oxidation of
alcohols to carboxylic acid normally proceeds through an
aldehyde hydrate (R−CH(OH)2), which can be further
oxidized to the carboxylic acid.67,68 Because water is expelled
from the cyclohexane, carboxylic acid cannot be formed. To
explore the effect of Pd loading on the activity and selectivity of
the tandem reaction, we synthesized Pd@UiO-66-NH2 with
1.0, 2.0, and 4.0 wt % Pd. At the same metal to substrate ratio
(1 mol %), the catalysts with higher Pd loading show higher
conversion (Table S3, Supporting Information), which means
the catalysts with higher Pd loading are more active and give
higher turnover frequencies.
Figure 5. Recycling test with 2.0 wt % Pd@UiO-66-NH2 catalyst.
Reaction conditions: Cyclohexane (2 mL), benzyl alcohol (0.1 mmol),
ethylene glycol (100 μL), 5 mg of 2.0 wt % Pd@UiO-66-NH2, metal/
substrate = 1:100, 0.1 MPa O2, reflux at 90 °C for 6 h.
The reusability of the Pd@UiO-66-NH2 and Pd@UiO-66
catalyst were also examined. After an initial reaction, the solid
catalyst was isolated from the liquid phase by centrifugation and
then reused in additional runs. The Pd@UiO-66-NH2 catalyst
could be recycled at least five times (Figure 5). However, Pd@
UiO-66 deactivated in the third recycle run (Figure S6,
Supporting Information). We also observed more severe
aggregation of Pd NCs in UiO-66 (Figure S7, Supporting
Information) after reaction compared to those in UiO-66-NH2
(Figure S5, Supporting Information), which indicates UiO-66-
NH2 is a better support for Pd NCs. The conversion was
controlled at below 70% because complete conversion could
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a
Table 2. Tandem Oxidation-Acetalization Reaction of Various Substituted Benzyl Alcohols Catalyzed by Pd@UiO-66-NH2
a
Reaction conditions: 90 °C, 0.1 MPa O2, substituted benzyl alcohol (0.1 mmol), ethylene glycol (100 μL, 1.8 mmol), cyclohexane (2 mL), 5 mg of
2.0 wt % Pd@UiO-66-NH2, metal/substrate = 1 mol %.
mask the deactivation of the catalyst.69 The BET surface area of
the used Pd@UiO-66-NH2 decreased from 936 m2 g−1 to 828
m2 g−1 (Figure S8, Supporting Information), which may be due
to the residual reactants or products trapped inside the UiO-66-
NH2. Furthermore, we measured the PXRD pattern of the used
catalysts and found the crystalline structure of the UiO-66-NH2
was still maintained after the fifth cycle (pattern e in Figure 2).
TEM images of used Pd@UiO-66-NH2 catalysts only show
slight aggregation of Pd NCs (Figure S5, Supporting
Information), which does not affect the activity of the catalysts
within five recycle runs.
The scope of the tandem catalytic system was investigated by
performing the oxidation-acetalization reaction of various
substituted benzyl alcohols. Table 2 summarizes the results of
these tandem reactions. We found that benzyl alcohols with
electron-withdrawing groups like fluoro and bromo gave low
Figure 6. Leaching test for the tandem reaction of benzyl alcohol with
ethylene glycol over 2.0 wt % Pd@UiO-66-NH2. Reaction conditions:
conversion and/or selectivity, whereas electron-donating
groups (−OCH3 and −CH3) showed moderate to high
conversion and selectivity to the corresponding acetals. We
used Hammett’s equation to obtain the correlation between
substituent constant and reaction rate (Figure S9, Supporting
Information). The negative value of ρ (−4.07) indicates the
tandem reaction is facilitated by the high electron density at the
reaction center. Currently, we are studying bimetallic NCs
encapsulated in this MOF for oxidation of aliphatic alcohols.
In order to confirm the heterogeneity of the catalyst, the hot
filtration test was carried out by stopping the tandem reaction
after 90 min. The reaction reached 57% conversion and 90%
selectivity to benzaldehyde ethylene acetal. The reaction
solution was removed quickly from the solid catalyst and was
transferred to another flask under the same reaction conditions.
No further increase in either the conversion of benzyl alcohol
or the selectivity to benzaldehyde ethylene acetal was observed
in the transferred solution after the solid catalyst was removed
(Figure 6). These results demonstrate that the active sites for
both the oxidation and the acetalization reactions were on the
benzyl alcohol (0.1 mmol), ethylene glycol (100 μL), mesitylene (8
μL), cyclohexane (2 mL), 2.0 wt % Pd@UiO-66-NH2, substrate/metal
= 100, reflux at 90 °C. The solid catalyst was filtered from the reaction
solution after 90 min, whereas the filtrate was transferred to a new vial
and reaction was carried out under identical conditions for an
additional 8 h.
solid catalyst, and there are no leached active species under the
reaction conditions.
4. CONCLUSIONS
We have developed a heterogeneous tandem catalyst for the
oxidation of benzyl alcohol and the subsequent acetalization
with ethylene glycol catalyzed by UiO-66-NH2 supported Pd
nanoclusters. This bifunctional Pd@UiO-66-NH2 catalyst
exhibits high selectivity to give acetal products in this tandem
oxidation-acetalization reaction and can be reused five times
without any loss in activity and selectivity. On the contrary,
Pd@UiO-66 showed deactivation at the third recycle run and
more severe aggregation of Pd NCs was observed. We also
found that using the nonpolar solvent, cyclohexane, is essential
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catalysis. Under appropriate conditions, both high selectivity
and stability of MOF-based catalysts can be achieved. Further
work on exploring new tandem catalytic reactions with these
catalytic systems and optimizing the performance of the
multifunctional MOFs via preparing bimetallic nanoclusters is
currently underway.
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ASSOCIATED CONTENT
* Supporting Information
Autrey, T.; Shioyama, H.; Xu, Q. J. Am. Chem. Soc. 2012, 134, 13926−
■
13929.
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EDX, EXAFS, XPS, nitrogen sorption and TGA measurements;
TEM images for the catalyst after reaction. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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(21) Dhakshinamoorthy, A.; Garcia, H. Chem. Soc. Rev. 2012, 41,
5262−5284.
*E-mail: whuang@iastate.edu.
Author Contributions
(22) Hermannsdoerfer, J.; Kempe, R. Chem.Eur. J. 2011, 17,
‡X.L. and Z.G. contributed equally. The manuscript was written
through contributions of all authors. All authors have given
approval to the final version of the manuscript.
8071−8077.
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Soc. 2011, 133, 11822−11825.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(26) Cirujano, F. G.; Llabres i Xamena, F. X.; Corma, A. Dalton
Trans. 2012, 41, 4249−4254.
We thank Ames Laboratory (Royalty Account) and Iowa State
University for startup funds. This work was also supported by
the Laboratory Research and Development Program of The
Ames Laboratory. The Ames Laboratory is operated for the
U.S. Department of Energy by Iowa State University under
Contract No. DE-AC02-07CH11358. We thank to Dale L.
Brewe, Steve M. Heald, Trudy B. Bolin, Tianpin Wu, and Jeff
Miller for the help during XAS measurement at APS. Use of the
Advanced Photon Source, an Office of Science User Facility
operated for the U.S. Department of Energy (DOE) Office of
Science by Argonne National Laboratory, was supported by the
U.S. DOE under Contract No. DE-AC02-06CH11357. We
thank Robert J. Angelici for his advice during the writing of this
manuscript. We thank Gordon J. Miller for use of PXRD, Brent
Shanks for use of TGA, and Igor I. Slowing for use of gas
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