Multifunctional PdAg@MIL-101 for One-Pot Cascade Reactions: Combination of Host-Guest Cooperation and Bimetallic Synergy in Catalysis

Article abstract of DOI:10.1021/cs501953d

Metal nanoparticles (NPs) stabilized by metal-organic frameworks (MOFs) are very promising for catalysis, while reports on their cooperative catalysis for a cascade reaction have been very rare. In this work, Pd NPs incorporated into a MOF, MIL-101, have jointly completed a tandem reaction on the basis of MOF Lewis acidity and Pd NPs. Subsequently, ultrafine PdAg alloy NPs (～1.5 nm) have been encapsulated into MIL-101. The obtained multifunctional PdAg@MIL-101 exhibits good catalytic activity and selectivity in cascade reactions under mild conditions, on the basis of the combination of host-guest cooperation and bimetallic synergy, where MIL-101 affords Lewis acidity and Pd offers hydrogenation activity while Ag greatly improves selectivity to the target product. As far as we know, this is the first work on bimetallic NP@MOFs as multifunctional catalysts with multiple active sites (MOF acidity and bimetallic species) that exert respective functions and cooperatively catalyze a one-pot cascade reaction. (Chemical Presented).

Full text of DOI:10.1021/cs501953d

Research Article
pubs.acs.org/acscatalysis
Multifunctional PdAg@MIL-101 for One-Pot Cascade Reactions:
Combination of Host−Guest Cooperation and Bimetallic Synergy in
Catalysis
Yu-Zhen Chen,† Yu-Xiao Zhou,† Hengwei Wang,† Junling Lu,† Takeyuki Uchida,‡ Qiang Xu,*,‡
Shu-Hong Yu,† and Hai-Long Jiang*,†
†Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Soft Matter Chemistry, Chinese Academy of
Sciences, Collaborative Innovation Center of Suzhou Nano Science and Technology, School of Chemistry and Materials Science,
University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
‡National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
S
* Supporting Information
ABSTRACT: Metal nanoparticles (NPs) stabilized by metal−organic
frameworks (MOFs) are very promising for catalysis, while reports on
their cooperative catalysis for a cascade reaction have been very rare. In this
work, Pd NPs incorporated into a MOF, MIL-101, have jointly completed a
tandem reaction on the basis of MOF Lewis acidity and Pd NPs.
Subsequently, ultrafine PdAg alloy NPs (∼1.5 nm) have been encapsulated
into MIL-101. The obtained multifunctional PdAg@MIL-101 exhibits good
catalytic activity and selectivity in cascade reactions under mild conditions,
on the basis of the combination of host−guest cooperation and bimetallic
synergy, where MIL-101 affords Lewis acidity and Pd offers hydrogenation
activity while Ag greatly improves selectivity to the target product. As far as
we know, this is the first work on bimetallic NP@MOFs as multifunctional
catalysts with multiple active sites (MOF acidity and bimetallic species) that
exert respective functions and cooperatively catalyze a one-pot cascade reaction.
KEYWORDS: metal−organic framework, heterogeneous catalysis, bimetallic nanoparticles, multifunctional catalyst, cascade reaction
host or support metal nanoparticles (NPs) for catalysis over a
broader scope; the crystalline porous structure of MOFs is
expected to limit the migration and aggregation of metal NPs.
In the past five years, there have been intensive studies on
monometallic NP@MOFs,9,10 in which control of the location
of metal NPs inside MOF cages/channels with tiny sizes
remains very challenging. In addition, most of the previous
work about the catalytic functions of metal NPs@MOF was
based on metal NPs as the only active site. The nano-
composites designed for MOF−metal NP bifunctional (acid/
base−metal) cooperative catalysis remain a largely unexplored
field with very limited reports.10 Recently, we and others have
reported several bimetallic NP@MOF examples which exhibit
catalytic activity superior to that of their monometallic
counterparts, ascribed to the modification of the electronic
structures.11 However, as far as we know, multifunctional
catalysis over bimetallic NPs@MOF with three active sites
(acid/base−metal A−metal B), which takes advantage of not
only the catalytic synergy of bimetallic NPs but also the
catalytic activity of the MOF itself, has never been reported.
1. INTRODUCTION
In modern synthetic chemistry, multistep reactions are often
required to produce fine chemicals and final products. For
multistep reactions, it is highly desired to obtain targeted
products with high yield and selectivity in a simplified “one-
pot” form over a multifunctional catalyst, which avoids time-
consuming and tedious intermediate purification/separation
processes and thus significantly reduces energy consumption
and the cost of the final products.1 Chemicals bearing a nitro
group are versatile, are readily available by nitration, and are of
particular interest as reactants or building blocks for multistep
organic reactions. For example, nitroarenes can be hydro-
genated to anilines, while further alkylation, reductive
alkylation, or hydroamination transformation affords their
value-added derivatives, including heterocyclic compounds.2
On the other hand, metal−organic frameworks (MOFs) have
attracted great interest in the last two decades due to their
modular assembly, high surface area, tunability, and tailorability
as well as potential applications such as gas storage and
separation, catalysis, sensing, drug delivery, proton conductivity,
etc.3−7 In particular, MOFs have been widely reported as
heterogeneous catalysts, mostly on the basis of the Lewis acid
sites offered by unsaturated metal centers.8 Furthermore, the
cages or channels with tunable sizes in MOFs allow them to
Received: December 5, 2014
Revised: February 8, 2015
© XXXX American Chemical Society
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In this work, we have synthesized monometallic and
bimetallic NP@MOF catalysts with tiny metal NPs inside
MOF cages well-controlled via a double-solvent approach
(DSA). The obtained PdAg alloy NPs show the smallest sizes
among all bimetallic NPs stabilized by MOFs reported to
date.11 The Pd@MOF cooperatively catalyzes a tandem
reaction on the basis of both MOF Lewis acidity and Pd
NPs. Strikingly, the bimetallic PdAg@MOF bearing three active
sites (Lewis acid−Pd−Ag) as a multifunctional catalyst has
successfully realized one-pot multistep cascade reaction for the
synthesis of secondary arylamines, by not only perfect
cooperation between the MOF host and the metal NP guest
but also synergistic catalysis between bimetallic Pd and Ag
species (Scheme 1), in which the MOF affords Lewis acid sites,
series of HAADF-STEM images was taken for tilted samples
with angle steps of 5° using a Tecnai G2 F20 transmission
electron microscope operated at 200 kV. The nitrogen sorption
isotherms were measured by using automatic volumetric
adsorption equipment (Micrometritics ASAP 2020). Prior to
nitrogen adsorption/desorption measurements, MIL-101 and
its nanocomposite samples were evacuated at 160 °C for 12 h.
The X-ray photoelectron spectroscopy (XPS) measurements
were performed by using an ESCALAB 250 high-performance
electron spectrometer using monochromated Al Kα radiation
1
(hν = 1486.7 eV) as the excitation source. H NMR was
recorded on a Bruker AC-400 FT spectrometer (400 MHz)
using TMS as internal reference. The catalytic reaction
products were analyzed and identified by gas chromatography
(GC, Shimadzu 2010 Plus with a 0.25 mm × 30 m Rtx-5
capillary column); GC-MS samples were recorded on a
Shimadzu QP-5050 GC-MS system. The DRIFT spectra of
CO adsorption were obtained on a Nicolet iS10 spectrometer
with an MCT detector and a low-temperature reaction chamber
(Harrick). Before DRIFT characterization, samples were given
complete activation treatment. After pretreatment, an appli-
cable amount of sample powder was loaded into the IR
reflectance cell and purged by flowing helium (20 mL min−1)
for 120 min at 423 K under vacuum to remove the adsorbed
water, and then the cell was cooled to 173 K with liquid
nitrogen. CO adsorption was carried out by generally
introducing 10% CO/He (5 mL min−1) to the IR reflectance
cell for almost 20 min.
Scheme 1. Schematic Illustration Showing the Multistep
Reaction over PdAg@MIL-101 Involving Lewis Acid and
Pd/Ag Sites
2.2. Preparation of Samples. 2.2.1. Preparation of MIL-
101. MIL-101 was synthesized according to a previous report
with modifications.12 Typically, a mixture of terephthalic acid
(332 mg, 2.0 mmol) with Cr(NO3)3·9H2O (800 mg, 2.0
mmol) in the presence of aqueous HF (0.4 mL, 2.0 mmol) and
deionized water (9.5 mL) was reacted at 200 °C for 8 h. The
as-synthesized MIL-101 was refluxed in water, ethanol, and
NH4F solutions, respectively, for over 12 h, washed with hot
water to eliminate the unreacted terephthalic acid trapped in
the giant pores, and then finally dried overnight at 160 °C
under vacuum for further use.
2.2.2. Preparation of Pd@MIL-101, Ag@MIL-101, and
PdAg@MIL-101 via a Double-Solvent Approach (DSA).
Typically, 200 mg of activated MIL-101 was suspended in 40
mL of a hydrophobic solvent of dry n-hexane, and the mixture
was sonicated for around 20 min until it became homogeneous.
After the mixture was stirred for a certain time, 0.30 mL of
hydrophilic aqueous Pd(NO3)2·2H2O and/or AgNO3 solution
with the desired concentrations was added dropwise with a
syringe pump over a period of 20 min with constant vigorous
stirring. Subsequently, the resultant solution was continuously
stirred for 3 h. The harvested sample was further dried followed
by treating in a stream of 20% H2/Ar (40 mL min−1) at 200 °C
for 4 h to yield Pd@MIL-101, Ag@MIL-101, and PdAg@MIL-
101. The Pd2+/(Pd2+ + Ag+) molar ratios were varied (0, 0.5,
0.67, 0.75, and 1.00) to optimize the activity of the resultant
catalysts, while the molar amount of Pd2+ + Ag+ was fixed to be
0.017 mmol for 200 mg of the MIL-101 host matrix.
Pd offers hydrogenation activity, and Ag greatly improves
selectivity toward the desired product. To the best of our
knowledge, this is the first work that demonstrates an MOF
host and the two species in bimetallic NP guest to functionalize
like “three legs of a tripod” in catalysis and cooperatively boost
one-pot multistep cascade reactions.
2. EXPERIMENTAL SECTION
2.1. Materials and Instrumentation. All chemicals were
obtained from commercial sources and used without further
purification. Powder X-ray diffraction patterns (XRD) were
obtained on a Holland X’Pert PRO fixed anode X-ray
diffractometer equipped with graphite-monochromated Cu
Kα radiation (λ = 1.54178 Å). The UV−vis absorption spectra
were recorded on a UV−vis spectrophotometer (TU-1810,
Beijing Pgeneral, People’s Republic of China) in the wavelength
range of 200−800 nm. The surface acidity of the catalyst was
measured by ammonia temperature-programed desorption
(NH3-TPD); the profile was recorded using a gas chromato-
graph (TECHTEMP GC 7890II) with a TCD detector. The
contents of Pd and Ag in the nanocomposite samples were
quantified by an Optima 7300 DV inductively coupled plasma
atomic emission spectrometer (ICP-AES). The size, morphol-
ogy, and microstructure of Pd@MIL-101 and PdAg@MIL-101
were investigated by using transmission electron microscopy
(TEM) and high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) on JEOL-2010 and
JEOL-2100F instruments with an electron acceleration energy
of 200 kV. To verify whether NPs are really inside the MOF, a
2.2.3. Preparation of Pd2Ag1/MIL-101 via an Incipient
Wetness Impregnation Method. Typically, 200 mg of
activated MIL-101 was dispersed in 20 mL of water and the
mixture was subject to ultrasonication for 20 min at room
temperature. An aqueous solution containing 0.012 mmol of
Pd(NO3)2·2H2O and 0.006 mmol of AgNO3 was added
dropwise to the above solution under vigorous agitation for
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Figure 1. (a) Illustration of the two-step tandem synthesis of 2-(4-aminophenyl)-1H-benzimidazole (D) involving a Lewis acid based catalytic
process and subsequent Pd based hydrogenation. (b) Solution color evolution during the reaction process. (c) UV−vis spectra tracking the catalytic
reaction progress. The intermediate B and product D are pure substances obtained by column chromatographic separation. (d) Representative TEM
image of Pd@MIL-101 catalyst. (e) Recycling performance for the conversion of A and selectivity to the target D after the two-step reaction over
Pd@MIL-101.
about 30 min and was then magnetically agitated at room
temperature for 24 h. The impregnated sample was washed
with water until the filtrate became colorless to give Pd2+Ag+/
MIL-101, followed by further drying at 50 °C for 12 h and
treating under a stream of 20% H2/Ar at 200 °C for 4 h to yield
Pd2Ag1/MIL-101.
2.2.4. Preparation of Pd/Al2O3. A Pd(NO3)2·2H2O (0.009
mmol) aqueous solution of an appropriate volume was slowly
added dropwise to 100 mg of Al2O3 with vigorous agitation.
The slurry was stirred at room temperature for 30 min and then
dried at 50 °C for 12 h under vacuum. Finally, the sample was
reduced at 200 °C for 4 h with 40 mL/min 20% H2/Ar to give
Pd/Al2O3.
2.3. Catalytic Activity Evaluation. 2.3.1. Catalytic
Performance of Pd@MIL-101 and Other Catalysts for the
Two-Step Tandem Reaction. A 40 mg portion of dried Pd@
MIL-101 was dispersed in 5 mL of methanol containing 0.5
mmol of 4-nitrobenzaldehyde and 0.6 mmol of 1,2-phenyl-
enediamine, and the mixture was sonicated to be homogeneous.
Then the resultant mixture was stirred at 80 °C in an air
atmosphere for 5 h and then transferred into a 10 mL Teflon-
lined pressure vessel without any separation or extraction. The
vessel was then purged with H2 ten times and pressurized with
H2 to 2 bar for the reaction at room temperature (25 °C) for 8
h. After the reaction, the catalyst was separated by
centrifugation, thoroughly washed with methanol, and then
reutilized as the catalyst in subsequent runs under identical
reaction conditions. The yield of the product was analyzed by
GC. For comparison, 40 mg of MIL-101, 4 mg of commercial
10% Pd/C, or no catalyst was used for the reaction while all
other reaction parameters and processes were kept the same.
2.3.2. Catalytic Performance of PdAg@MIL-101 and Other
Catalysts for the Multistep Cascade Reaction. The catalytic
reaction was performed in a 10 mL Teflon-lined stainless steel
autoclave equipped with a pressure gauge and a magnetic
stirrer. A 20 mg portion of dried PdAg@MIL-101 or Pd@MIL-
101 catalyst was dispersed in 4 mL of ethanol containing 1
mmol of nitrobenzene and 1 mmol of benzaldehyde or
acetophenone, and the mixture was sonicated to be
homogeneous. The vessel was then purged with H2 10 times
and pressurized with H2 to 2 bar for the reaction. The reaction
was conducted at room temperature (25 °C) and 110 °C for
benzaldehyde and acetophenone, respectively, as substrates.
After the reaction, the catalyst was separated by centrifugation,
thoroughly washed with ethanol, and then reutilized as catalyst
in subsequent runs under identical reaction conditions. The
yield of the product was analyzed by GC with an internal
standard (dodecane). For comparison, 20 mg of Pd2Ag1/MIL-
101, 20 mg of Pd/Al2O3, 20 mg of MIL-101, a physical mixture
of Ag@MIL-101 and Pd@MIL-101 (with the same contents of
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Figure 2. (a) Powder XRD profiles and (b) N2 sorption isotherms at 77 K of as-synthesized MIL-101 and PdAg@MIL-101 catalysts before reaction.
Filled and open symbols represent adsorption and desorption branches, respectively.
Pd and Ag as those in Pd2Ag1@MIL-101), 2 mg of commercial
10% Pd/C, or no catalyst was used for the reaction while all
other reaction parameters and processes were kept the same.
the second step of catalytic hydrogenation, respectively. The
solution color becomes gradually more faint as the time
increases in the cascade reaction. In addition, UV−vis spectra
were used to track the catalytic reaction progress of the two-
step tandem synthesis of D based on Pd@MIL-101 catalyst.
The peak has an evident blue shift during the reaction (Figure
1c). In situ FT-IR spectra of CO adsorption for both MIL-101
and Pd@MIL-101 at 158 K showing a band at ∼2190 cm−1
could be ascribed to CO coordinated at open Cr3+ Lewis acid
sites (Figure S3 in the Supporting Information).9i The NH3
temperature-programmed desorption (TPD) for MIL-101 and
Pd@MIL-101 shows a broad peak in the range of 186−420 °C,
which could be assigned to NH3 adsorption on the open Cr3+
Lewis acid sites at <300 °C and partial decomposition of MIL-
101 at higher temperature (Figure S4 in the Supporting
Information), further validating its Lewis acidity. The
intermediate 2-(4-aminophenyl)-1H-benzimidazole (B), the
intermediate byproduct C, and the target product D were
3. RESULTS AND DISCUSSION
The MIL-101 framework, Cr3X(H2O)2O(BDC)3·nH2O (BDC
= benzene-1,4-dicarboxylate, X = F, OH, n ≈ 25),12 one of the
representative MOFs, was employed as a host. MIL-101
features a three-dimensional (3D) network with two types of
giant cages with diameters of 2.9 and 3.4 nm and high
physicochemical stability as well as a large surface area (BET,
over 3600 m2/g), very suitable for encapsulating tiny metal NPs
and hosting catalytic reactions. Moreover, MIL-101 can be
activated upon heating to afford exposed Cr(III) centers
serving as Lewis acid sites for catalysis.
To prevent the formation of metal NPs on the external
surface of MOF and avoid their aggregation, a DSA synthetic
approach was adopted to rationally introduce Pd or PdAg NPs
to afford Pd@MIL-101 and PdAg@MIL-101, respectively (see
the Experimental Section).11e,f Given the huge cages with
hydrophilic environments in MIL-101, the quantitative metal
precursor aqueous solution should be fully incorporated into
the MOF pores on the basis of capillary force and hydrophilic
interactions during this process. The resultant metal precursors
@MIL-101 were further reduced by H2 to generate tiny metal
NPs confined in MIL-101.
1
identified by GC-MS spectrometry and H NMR, respectively
(Figures S5 and S6 in the Supporting Information). Almost
100% conversion of the reactant 4-nitrobenzaldehyde in the
acid catalysis step and a >99% yield of D were obtained over
Pd@MIL-101 catalyst bearing dual active sites (Figure 1a). In
contrast, previous yolk−shell Pd@mesoporous aluminosilica
and nonrecyclable catalysts gave lower yields (∼90% and
∼80%, respectively), and even the latter requires time-
consuming procedures.14 In the control experiment, only 30%
reactant was converted to C in the absence of catalyst. When
commercial Pd/C catalyst was used, about 50% of the reactants
was converted to C in the first step due to the lack of acidity,
and therefore the nitro group in both the residual reactant and
C was coreduced to corresponding amino group in the second
step. The reaction was terminated after the first step and
produced only B over MIL-101 catalyst without Pd NPs. The
reusability tests for Pd@MIL-101 catalyst have clearly indicated
that the excellent catalytic activity and selectivity remain high
over threee runs (Figure 1e). TEM observations for Pd@MIL-
101 have also shown that the sizes of Pd NPs are well retained
even after three runs (Figure S2 in the Supporting
Information). All of these results suggest that both the
synergistic effect between Lewis acidity and Pd NPs and the
confinement effect of MIL-101 play important roles in the
catalytic performance of Pd@MIL-101.
No diffraction peak in powder X-ray diffraction (PXRD) was
detected for Pd NPs in Pd@MIL-101, revealing that Pd NPs
could be small (Figure S1 in the Supporting Information). This
assumption is consistent with the results of transmission
electron microscopy (TEM) (Figure 1d), which shows that the
Pd NPs in 1 wt % Pd@MIL-101 are highly dispersed with mean
diameters of 2.5
0.3 nm (Figure S2c in the Supporting
Information). It is reasonably considered that Pd NPs with sizes
smaller than the diameters of MOF cages have been
successfully confined inside the MOF as predesigned.
To demonstrate that Pd@MIL-101 can be used for tandem
catalysis, a reaction route involving acid catalysis and catalytic
hydrogenation steps was chosen for the synthesis of 2-(4-
aminophenyl)-1H-benzimidazole (D) (Figure 1a), which is
biologically active and is a key intermediate in the synthesis of
benzimidazole derivatives for antiviral, antiulcer, and anticancer
ends.13 Figure 1b shows the solution color evolution for the
Pd@MIL-101 catalyzed reaction, recording the progress after 0,
1, 3, and 5 h of the first step of acid catalysis and after 1.5 h of
Encouraged by the extraordinary catalytic activity, selectivity,
and recyclability as well as the great size restriction of Pd NPs
in bifunctional Pd@MIL-101 catalyst in the above tandem
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Figure 3. (a) In situ FT-IR spectra of CO adsorbed on Pd2Ag1@MIL-101 at different temperatures. XPS spectra of (b) Pd 3d and (c) Ag 3d for Pd@
MIL-101, Pd2Ag1@MIL-101, and Ag@MIL-101. (d) TEM and (e) HAADF-STEM images showing the ultrafine PdAg NPs in Pd2Ag1@MIL-101.
reaction, bimetallic PdAg@MIL-101 species with various Pd/
Ag ratios have been synthesized via a similar DSA approach.
Inductively coupled plasma atomic emission spectrometry
(ICP-AES) has confirmed that the actual contents of Pd and
Ag are close to the nominal values and the Pd/Ag ratios match
the predesigned trend (Table S1 in the Supporting
Information). The crystallinity of MIL-101 upon loading Pd/
Ag species has been proven by PXRD results (Figure 2a and
Figure S7 in the Supporting Information). The BET surface
areas of as-synthesized MIL-101, Pd@MIL-101, Pd3Ag1@MIL-
101, Pd2Ag1@MIL-101, Pd1Ag1@MIL-101, Pd1Ag1.4@MIL-
101, and Ag@MIL-101 were 3660, 2784, 2826, 2696, 2465,
2499, and 2322 m2/g, respectively, implying that the cavities of
the host framework are possibly occupied by highly dispersed
metal NPs (Figure 2b). Similar to that of Pd@MIL-101, the in
situ DRIFT IR spectrum of CO adsorbed on PdAg@MIL-101
at 173 K displays a strong peak at 2191 cm−1 attributed to CO
coordinated at open Cr3+ Lewis acid sites (Figure 3a). The peak
significantly decreases with elevated temperature, while the
bands at 2119 and 2170 cm−1 assigned to CO physisorption or
gaseous CO present gradually higher intensity with an increase
in temperature to 253 and 298 K.15 The X-ray photoelectron
spectroscopy (XPS) results show that both Pd and Ag are in
the reduced states and the 3d peaks for Pd(0) and Ag(0) in
Pd2Ag1@MIL-101 shift to lower binding energies by ∼0.2 and
∼0.8 eV, respectively (Figure 3b,c), in comparison to those in
monometallic Pd@MIL-101 and Ag@MIL-101. Moreover, the
3d peaks for Ag(0) shift gradually to lower binding energies
with decreasing contents of Ag, and the 3d5/2 peak of Pd(0)
also gradually shifts to lower binding energies with decreasing
contents of Pd in PdAg@MIL-101. Such observed shifts for
both Pd 3d and Ag 3d may be attributed to the flow of electron
density from Ag to Pd (gaining electrons from Ag), indicative
of the presence of an interaction between Ag and Pd and the
formation of PdAg alloy NPs (Figure S8 and Table S2 in the
Supporting Information), in good agreement with previous
reports.11d,g,16 The formation of PdAg alloy NPs is reasonable
because the two soluble metal salts have close reduction
Unexpectedly, very tiny PdAg NPs with sizes of 1.5 0.3 nm
were formed on the basis of TEM and high-angle annular dark-
field scanning transmission electron microscopy (HAADF-
STEM) observations (Figure 3d,e and Figure S9 in the
Supporting Information). In addition, a series of HAADF-
STEM images was taken for tilted Pd2Ag1@MIL-101 with angle
steps of 5° and the results clearly demonstrated that PdAg NPs
were trapped inside MIL-101 (see videos in the Supporting
Information), revealing the successful inducement of Pd2+ and
Ag+ into MOF cages via the DSA approach and the ideal
confinement effect of MIL-101 for PdAg NPs. To our
knowledge, this is the smallest size of bimetallic NPs that
have been confined inside MOFs reported thus far.11 It is
proposed that Ag effectively reduces the aggregation of Pd NPs,
thus resulting in the smaller PdAg NP sizes in comparison to
those of Pd NPs in Pd@MIL-101. This is a very interesting
while unusual finding that alloying allows even smaller particle
sizes confined in a MOF under similar synthetic conditions.
The ultrafine PdAg NPs are expected to offer excellent
performance in heterogeneous catalysis. The coexistence of
Lewis acid sites in MIL-101 and bimetallic Pd and Ag species
may render PdAg@MIL-101 a multifunctional catalyst that is
suitable for multistep cascade reactions. Therefore, a one-pot
multistep conversion of nitroarene to value-added secondary
arylamine has been explored over PdAg@MIL-101. The overall
three-step process involves nitroarene hydrogenation, reductive
amination of aldehydes or ketones, and selective hydrogenation
to secondary arylamine. The results of the reaction over
different PdAg@MIL-101 catalysts, PdAg/MIL-101, Pd/Al2O3,
a physical mixture of Pd@MIL-101 and Ag@MIL1-101, or
commercial Pd/C or in the absence of catalyst are shown in
Table 1. The monometallic Pd@MIL-101 has high catalytic
activity and rapidly completes the conversion with low
selectivity of the main product (entry 1). Significantly, the
alloying of Pd with Ag allows the catalyst to complete the
conversion in a slightly longer time but affords higher selectivity
to the desired product of secondary arylamine (entries 2−5).
As the ratio of Ag/Pd increases, a clear trend of decreased
activity with increased selectivity is observed. Evidently, Pd NPs
have intrinsic hydrogenation activity while Ag greatly improves
the selectivity of the target product. Given the Pd−Ag
2+
+
potentials (E°Pd
= +0.915 eV vs SHE; E°Ag /Ag = +0.80 eV
/Pd
vs SHE). Upon heating under an H2/Ar flow, both Pd2+ and
Ag+ would be simutaneously reduced to afford PdAg alloys.
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nitrobenzene and benzaldehyde (entry 10). The reaction does
not occur with MIL-101 or in the absence of catalyst (entries
11 and 12). The results perfectly demonstrate the significance
of the triply synergistic effect of acidity and Pd and Ag species,
with which the one-pot multistep reaction can be achieved with
good activity and selectivity under very moderate conditions
(25 °C, 0.2 MPa of H2). To optimize the reaction parameters,
both ethanol and toluene were employed as solvents, and the
results show that the former is slightly better (Table 1 and
Table S3 in the Supporting Information).
Table 1. Synthesis of Secondary Arylamines through
Hydrogenation of Nitrobenzene and Reductive Amination of
a
Benzaldehyde
To understand the role of the Lewis acid sites in MIL-101 for
the reaction, a control experiment was conducted with a
pyridine probe. The reaction immediately stops upon the
introduction of pyridine (100 μL) into the system when the
reaction has proceeded for 7 h. In contrast, for the Pd/C-
catalyzed system, the reaction has no influence when pyridine
was introduced, revealing that pyridine is able to terminate the
reaction by poisoning of the Lewis acid sites of MIL-101 while
not by capping Pd or PdAg NPs (Figure 4). The phenomenon
b
selectivity (%)
b
time
(h)
conversn
(%)
entry
catalyst
Pd@MIL-101
J
I
G
1
2
3
4
5
6
7
8
3
6
100
100
99
56
77
85
90
66
1
43
23
14
7
Pd3Ag1@MIL-101
Pd2Ag1@MIL-101
Pd1Ag1@MIL-101
Pd1Ag1.4@MIL-101
Ag@MIL-101
0
1
10
28
40
99
3
40
23
11
Pd2Ag1@MIL-101
Pd/Al2O3
24
3
99
67
70
63
53
61
1
20
4
36
27
35
c
9
Pd@MIL-101 + Ag@
MIL-101
10
10
11
12
Pd/C
MIL-101
no
3
100
31
0
69
a
Reaction conditions: substrates E (1.0 mmol) and F (1.0 mmol), 0.17
mol % catalyst (based on n(Pd+Ag)) in 4 mL of ethanol at room
b
temperature, 0.2 MPa of H2. Conversion of E and selectivity were
analyzed by GC, and n-dodecane was used as the internal standard.
c
The catalytic result was based on a physical mixture of Pd@MIL-101
and Ag@MIL-101, in which the contents of Pd and Ag are the same as
those in Pd2Ag1@MIL-101.
Figure 4. Recycling and filtration tests as well as the control
experiment with introduction of pyridine during the reaction process
over Pd2Ag1@MIL-101 catalyst.
interaction reflected by the XPS data (Figure S8 in the
Supporting Information) and the electronic effect of Ag making
the reaction proceed preferably toward the target product, it is
proposed that the addition of Ag will slow down the reduction
rate of benzaldehyde to generate alcohol and lead to higher
conversion of benzaldehyde to secondary arylamine.17 As a
result, higher selectivity to the desired product was obtained
when increasing the ratio of Ag in PdAg alloy, while Ag
contents that are too high will lead to sluggish reaction rates
because Ag species are inactive for the conversion of nitroarene
to aminoarene (entry 6). For comparison, Pd2Ag1/MIL-101
was prepared by incipient wetness impregnation, which
exhibited only 63% selectivity to the target product after 24
h, possibly due to the lack of sufficient confinement effects for
the PdAg NPs (entry 7). Similarly, the Pd/Al2O3 presented
lower catalytic activity than Pd@MIL-101 and also quite low
selectivity, although in the presence of Lewis acidity (entry 8).
In addition, the lower catalytic activity and selectivity over a
physical mixture of Pd@MIL-101 and Ag@MIL-101 in
comparison to those of Pd2Ag1@MIL-101 indicated the
possible formation of Pd−Ag bimetallic NPs in MIL-101
(entry 9). Commercial Pd/C has high activity similar to that of
Pd@MIL-101 catalyst but even lower selectivity (30%),
reflecting the critical role of acidity in the reaction between
can be readily explained with the poisoning of the Lewis acid
sites by strong interactions with pyridine.5f This result offers
solid evidence to support the presence of Lewis acid active sites
in MIL-101 and demonstrate its significant influence on the
catalytic performance. In addition, the catalytic activity and
selectivity over Pd2Ag1@MIL-101 are well preserved even after
three cycles (Figure 4 and Figure S10 in the Supporting
Information), suggesting its good recyclability and durability.
No significant loss of crystallinity and no identifiable peaks
for metal NPs in the PXRD patterns occurs after three runs or
in the presence of pyridine. The results reveal the retained
integrity of the MIL-101 framework and the nonexistence of
metal NP agglomeration (Figure S7 in the Supporting
Information), which finds further evidence in TEM and
HAADF-STEM images that PdAg NPs have well retained
sizes after three cycles (Figure 5 and Figure S9 in the
Supporting Information), demonstrating the great confinement
effect of MIL-101 and structural stability of the catalyst.
On the basis of the above results, the Lewis acidity in both
Pd@MIL-101 and PdAg@MIL-101 catalysts is supported by
the following evidence. (1) Both CO FT-IR and NH3-TPD
spectra show the available unsaturated Cr(III) centers in MIL-
101 and reveal the presence of Lewis acid sites, as
2067
DOI: 10.1021/cs501953d
ACS Catal. 2015, 5, 2062−2069

ACS Catalysis
Research Article
Table 2. Synthesis of Secondary Arylamines through
Hydrogenation of Nitrobenzene and Reductive Amination of
a
Acetophenone
Figure 5. (a) TEM and (b) HAADF-STEM images for Pd2Ag1@MIL-
101 catalyst after three runs of the reaction.
b
selectivity (%)
demonstrated in previous reports.8a,9i,10a (2) For the first step
in the tandem reaction, almost 100% conversion of the 4-
nitrobenzaldehyde in the acid catalysis step and >99% yield of
the desired product (D) were obtained over Pd@MIL-101
catalyst. However, for the commercial Pd/C catalyst, only
about 50% yield was obtained. The results indicated the
presence of Lewis acidity in Pd@MIL-101. (3) The second
reaction between nitrobenzene and benzaldehyde presents
good selectivity to the target product (J) with acidity-involved
catalyst. In contrast, the benzaldehyde is prone to be reduced in
preference to benzyl alcohol in the absence of acidity, although
the reaction between aniline and benzaldehyde proceeds
slightly, as supported the fact that commercial Pd/C catalyst
gives only 31% selectivity to target J. The comparison results
among Pd/MIL-101, Pd2Ag1/MIL-101, and Pd/C catalysts
reflect the critical role of the acidity in MIL-101 for the
reaction. (4) Pyridine has been introduced into both reaction
systems in the presence of Pd2Ag1/MIL-101 and Pd/C for
comparison. The results clearly show that the pyridine as a
Lewis base terminates the reaction toward the production of
target J by poisoning of the Lewis acid sites of MIL-101 while
not by capping Pd or PdAg NPs.
Given the great performance of Pd2Ag1@MIL-101 catalyst
for the cascade nitroarene hydrogenation−reductive amination
of aldehydes, and to further illustrate the Lewis acid role in the
catalyst, the aldehyde has been replaced by the more inert
acetophenone in the second reaction, and then the reaction
requires higher acidity.2b The commercial Pd/C catalyst gives
88% conversion after 60 h while the product was exclusively
phenylethanol without even traces of the imine or secondary
amine, due to the lack of required acidity. In sharp contrast,
with strong Lewis acidity, the Pd2Ag1@MIL-101 catalyst
effectively promotes the reaction to convert 65% acetophenone
in 30 h with 71% selectivity toward the target secondary amine
(Table 2). The much higher selectivity of the title catalyst
toward the secondary amine with respect to Pd/C evidences
the advantages of using MIL-101, which not only acts as a
porous host for incorporating active PdAg NPs but also actively
participates in the reaction with Lewis acid sites.
b
catalyst
Pd/C
Pd2Ag1@MIL-101
time (h) conversn (%)
O
L
M
60
30
88
65
73
14
27
15
71
a
Reaction conditions: substrates K (1.0 mmol), and F (1.0 mmol),
0.46 mol % catalyst (based on n(Pd+Ag)) in 4 mL of ethanol at 110
°C, 0.2 MPa of H2. Conversion of E and selectivity were analyzed by
b
GC and GC-MS.
1H-benzimidazole. Following a similar approach, ultrafine PdAg
alloy NPs (∼1.5 nm) have been encapsulated into MIL-101 and
the resultant multifunctional catalyst, for the first time, has
fulfilled a one-pot multistep cascade conversion of nitroarene to
secondary arylamine through the combination of host−guest
cooperation and guest bimetallic synergy, in which MOF
affords Lewis acid sites and Pd offers hydrogenation activity
while Ag greatly improves selectivity toward the target
secondary amine. The facile and rational preparation as well
as excellent activity, selectivity, and recyclability of PdAg@MIL-
101 in this work might open a new avenue to MOF-based
multifunctional catalysts, which has great potential for broad
applications in one-pot cascade reactions for the synthesis of
organic chemicals in the future.
ASSOCIATED CONTENT
* Supporting Information
■
S
The following files are available free of charge on the ACS
Publications website at DOI: 10.1021/cs501953d.
Experimental details, NH3-TPD profiles, FT-IR, GC-MS,
and H NMR spectra, TEM images, XRD patterns, and
1
Tables S1−S3 (PDF)
HAADF-STEM images for tilted Pd2Ag1@MIL-101
(AVI)
HAADF-STEM images for tilted Pd2Ag1@MIL-101
(AVI)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Q.X.: q.xu@aist.go.jp.
■
*E-mail for H.-L.J.: jianglab@ustc.edu.cn.
4. CONCLUSION
Notes
In summary, a MTN zeolite-type MOF, MIL-101, with large
cages has been employed to confine Pd NPs with sizes of ∼2.5
nm on the basis of a DSA encapsulation followed by H2
reduction. The obtained Pd@MIL-101 behaves as a bifunc-
tional catalyst and presents excellent performance in tandem
catalysis, a reaction route involving acid catalysis and a catalytic
hydrogenation process for the synthesis of 2-(4-aminophenyl)-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully thank the reviewers for their valuable suggestions.
This work was supported by the NSFC (21371162 and
51301159), the 973 program (2014CB931803), the NSF of
2068
DOI: 10.1021/cs501953d
ACS Catal. 2015, 5, 2062−2069

ACS Catalysis
Research Article
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