High Loading of Pd Nanoparticles by Interior Functionalization of MOFs for Heterogeneous Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.5b02739

In this report, the issue related to nanoparticle (NP) agglomeration upon increasing their loading amount into metal-organic frameworks (MOFs) has been addressed by functionalization of MOFs with alkyne groups. The alkynophilicity of the Pd²⁺ (or other noble metals) ions has been utilized successfully for significant loading of Pd NPs into alkyne functionalized MOFs. It has been shown here that the size and loading amount of Pd NPs are highly dependent on the surface area and pore width of the MOFs. The loading amount of Pd NPs was increased monotonically without altering their size distribution on a particular MOF. Importantly, the distinct role of alkyne groups for Pd²⁺ stabilization has also been demonstrated by performing a control experiment considering a MOF without an alkyne moiety. The preparation of NPs involved two distinct steps viz. adsorption of metal ions inside MOFs and reduction of metal ions. Both of these steps were monitored by microscopic techniques. This report also demonstrates the applicability of Pd@MOF NPs as extremely efficient heterogeneous catalysts for Heck-coupling and hydrogenation reactions of aryl bromides or iodides and alkenes, respectively.

Full text of DOI:10.1021/acs.inorgchem.5b02739

Article
pubs.acs.org/IC
High Loading of Pd Nanoparticles by Interior Functionalization of
MOFs for Heterogeneous Catalysis
Bappaditya Gole,† Udishnu Sanyal,† Rahul Banerjee,‡ and Partha Sarathi Mukherjee*,†
†Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
‡Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
S
* Supporting Information
ABSTRACT: In this report, the issue related to nanoparticle (NP) agglomeration upon increasing their loading amount into
metal−organic frameworks (MOFs) has been addressed by functionalization of MOFs with alkyne groups. The alkynophilicity of
the Pd2+ (or other noble metals) ions has been utilized successfully for significant loading of Pd NPs into alkyne functionalized
MOFs. It has been shown here that the size and loading amount of Pd NPs are highly dependent on the surface area and pore
width of the MOFs. The loading amount of Pd NPs was increased monotonically without altering their size distribution on a
particular MOF. Importantly, the distinct role of alkyne groups for Pd2+ stabilization has also been demonstrated by performing a
control experiment considering a MOF without an alkyne moiety. The preparation of NPs involved two distinct steps viz.
adsorption of metal ions inside MOFs and reduction of metal ions. Both of these steps were monitored by microscopic
techniques. This report also demonstrates the applicability of Pd@MOF NPs as extremely efficient heterogeneous catalysts for
Heck-coupling and hydrogenation reactions of aryl bromides or iodides and alkenes, respectively.
structure, tunable pore size of the order of molecular
dimension, and chemical tailoring of the inner surface of the
channels and cavities.6 It has been evident from the precedent
literature that the presence of the pore walls helps to prevent
the agglomeration of the NPs, thus resulting in narrow particle
size and size distribution.
INTRODUCTION
■
Catalysts are often used to generate energy carriers like
hydrogen and hydrocarbon fuels in the chemical industry and
also in the conversion of chemical energy from fuel to
electricity.1 The efficiency of metal nanoparticles (MNPs) as
heterogeneous catalysts depends on their nanostructure (e.g.,
size, shape, and crystal orientation) and composition.2 Thus,
nanoparticles with precisely controlled shapes and sizes are the
key requirements to realize the best catalytic activity. Generally,
the nanoparticles are prepared by reducing the suitable metal
precursor(s) in the presence of a reducing agent wherein the
size is primarily achieved by the judicious choice of surfactants
which are mostly the organic capping ligands and/or polymers.3
However, from the catalysis perspective the presence of
surfactant is detrimental because it blocks the surface site
which essentially deactivates the catalyst surface.4 Also organic
capping agents stabilizing NPs often limit their application in
high temperature catalytic reactions. The use of porous
materials in this regard is particularly promising because they
not only provide the surfactant free specific surface site but also
allow nucleation and growth in a confined cavity, thus
stabilizing the particles in nanosize regime. Although porous
materials such as microporous zeolites, mesoporous silica, and
various others inorganic/organic materials have been used,5
metal-organic-frameworks (MOFs) or porous co-ordination
polymers are emerging as the most promising stabilizing agents
from this family owing to their rationally designed framework
Generally, two major synthetic approaches are used to
stabilize MNPs onto the porous architecture of MOFs.7 The
first and most widely used one is initially the immobilization of
a suitable metal precursor inside the cavity of MOFs which
subsequently reduced or decomposed to generate metal
atoms.8 Here, growth of the MNPs is controlled by the pore
dimension of the respective MOFs. Various synthetic
techniques such as chemical vapor deposition (CVD),9 solution
infiltration,10 and solid grinding8c,e are used to immobilze the
metal precursor inside the MOFs cavity efficiently. Though all
these methodologies are easy to handle, most often the
deposition of the particles takes place on the outer surface of
MOF structure. The second and most recent approach
demonstrated polymer capped MNPs initially synthesized,
and then, the MOF structure was allowed to grow around it
using the appropriate chemicals.11 However, Xu and co-workers
used another method, which is completely different from the
previous two approaches, termed as the “double solvent
Received: November 25, 2015
© XXXX American Chemical Society
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Scheme 1. Schematic Illustration of the Synthesis of Pd NPs Supported by MOFs via the Two-Step Mechanism
Subsequently, the reaction vial was capped and placed in a
programmable oven and heated at 90 °C for 2 days and cooled to
room temperature at a rate of 0.15 °C/min. Colorless crystals (2.5 mg,
61%) of the product were collected by filtration and washed with
DMF (3 × 5 mL). IR spectra (υ/cm−1) 3401, 2937, 2865, 1600, 1548,
1436, 1300, 1246, 1218, 1021, 974, 862, 787, 737, 681, 617, 525.
Elemental analysis of activated sample: Anal. Calcd for
C23H18O5N2Cd; [Cd(pip) (bpe)] C, 53.66; H, 3.52; N, 5.44.
Found: C, 53.75; H, 3.34; N, 5.62.
Synthesis of Metal Nanoparticles@MOFs. Activated MOFs were
used for the preparation of MOF supported MNPs. A wet chemical
method was employed to generate MNPs. In the case of MOF-3, the
MNP loading amount was gradually increased starting from 5 wt %
until it got saturated. For others MOFs, a constant amount of metal
ion precursor in methanol was used, and the amount of Pd adsorbed
into those was estimated from ICP-MS. The maximum loading
amount was reproduced for bulk scale preparation.
5 wt % Pd@MOF-3. To 5 mg of the activated MOF-3 in 4 mL of
methanol, 0.42 mg (2.4 × 10−3 mmol) of PdCl2 in 1 mL of methanol
was added slowly. The mixture was kept stirring at room temperature
for 30 min to adsorb Pd2+ ions into the MOF. During this time, the
dark orange color of the supernatant (due to PdCl2) turned colorless,
and the MOF turned to brown from colorless. Subsequently, 0.45 mg
(1.2 × 10−2 mmol, 5 equiv with respect to PdCl2) of NaBH4 was
added with constant stirring for another 1 h. During this period, the
mixture turned black indicating the formation of Pd NPs. Finally, the
mixture was filtered and washed several times with fresh methanol.
The solid material was then dried under vacuum.
Thirty wt % Pd@MOF-3. This was also prepared in a similar
method as described above using 2.5 mg (1.4 × 10−2 mmol) of PdCl2
added in 5 mg of MOF in methanol. Subsequently, 2.7 mg (7.05 ×
10−2 mmol, 5 equiv with respect to PdCl2) of NaBH4 was added for
the reduction of Pd2+ ions.
approach” to prepare the ultrafine Pt nanoparticles inside the
MIL-101 pore.12
Although considerable attention has been paid in this
direction, a facile methodology is still important to provide
the selective immobilization of different metal precursors inside
the MOF cavities and control their nucleation and growth.
Another important issue which has not been much highlighted
in the recent past is the amount of loading of metal
nanoparticles without aggregation. Uniform distribution of
MNPs with small sizes was realized only for the lower loading
amount due to the negligible interaction with metal ions or
MNPs and MOFs. Functionalization of MOFs is one promising
way which could enhance metal loading by strong electronic
interaction with the metal ions.
Herein, we report the functionalization of building ligand
with a bare ethynyl moiety and its successful use in the
synthesis of MOFs. As anticipated, the single crystal structures
of the Zn-MOFs of the corresponding ligand showed that the
ethynyl moieties are pointed toward the pore. It is known that
the noble metal ions can strongly interact with the ethynyl
functionality through its distinct π-donation and π-acceptor
characteristics.13 This tends to stabilize noble metal ions
preferentially and thus can facilitate significantly high metal
loading into MOFs (Scheme 1). Because of the presence of
alkyne moieties, initially, Au and Pd metal ions were
immobilized uniformly throughout the MOF matrix via the
solvent impregnation method. Subsequent reduction of those
produced MNPs, wherein alkyne group acts as a stabilizer to
control the growth of the particles and thus result in narrow-
size particles. We have demonstrated earlier the exceptional
Control Experiments. 5 wt % Pd@MOF-A. To 5 mg of activated
MOF-A [Zn(bip)(dpb)] (where bip corresponds to 5-(benzyloxy)-
isophthalate) in 4 mL of methanol, 0.42 mg (2.4 × 10−3 mmol) of
PdCl2 in 1 mL of methanol was added slowly. The mixture was kept
stirring at room temperature for 30 min. Subsequently, 0.45 mg (1.2 ×
10−2 mmol, 5 equiv with respect to PdCl2) of NaBH4 was added with
constant stirring for another 1 h. During this period, the mixture
turned dark gray due to the formation of Pd nanoparticles. Finally, the
mixture was filtered and washed several times with fresh methanol.
The solid material was then dried under vacuum.
General Procedure for the Heck-Coupling Reactions of Aryl
Iodides. To a two-necked 50 mL round-bottomed flask, an aryl iodide
(2 mmol), an alkene (4 mmol, 2 eqv.), tetrabutylammonium acetate
(TBAA, 0.4 mmol, 0.2 eqv.), triethylamine (6 mmol, 3 eqv.), and Pd@
MOF-3 (10 mg, 1.25 mol % Pd) were added. The reaction vessel was
degassed three times and subsequently filled with nitrogen gas. To this,
anhydrous 1-butanol (10 mL) was added through a syringe. Then, the
resulting suspension was stirred under nitrogen atmosphere at 80 °C
for 24 h. After cooling to room temperature, the catalyst was separated
by filtration, and then, the solvent was evaporated under reduced
pressure. The residue obtained was purified by column chromatog-
raphy over silica gel eluting with hexane/ethyl acetate to afford the
corresponding products.
loading of Au NPs (∼50 wt %, 1.85
0.3 nm) into MOFs
without altering their monodispersity and particle size.14
Herein, we report a systematic study on the stabilization of
Pd NPs in different porous MOFs with interior alkyne
functionalization because of the high interest in the chemical
industry for heterogeneous catalysts for many organic reactions.
It is also demonstrated here that the loading amount can be
monitored depending upon the porosity of the MOFs. Finally,
the supported Pd NPs were used for heterogeneous Heck-
coupling reactions and efficient hydrogenation of alkenes with
high reusability.
EXPERIMENTAL SECTION
■
Synthesis and General Characterizations. MOF-1 [Zn(pip)-
(bpy)], MOF-2(Zn) [Zn(pip)(bpe)], and MOF-3 [Zn(pip)(dpb)]
were synthesized according to our previously published report and
characterized thoroughly.14 The ligands: H2pip, bpy, bpe, and dpd have
their usual names, 5-(prop-2-yn-1-yloxy)isophthalic acid, 4,4′-bipyr-
idine, 1,2-di(pyridin-4-yl)ethane, and 1,4-di(pyridin-4-yl)benzene,
respectively.
Synthesis of MOF-2(Cd) [Cd(pip) (bpe)]. Cd(NO3)2·4H2O (3 mg,
0.01 mmol), H2pip (2.2 mg, 0.01 mmol), and 1,2-di(pyridin-4-
yl)ethane (bpe) (1.8 mg, 0.01 mmol) were taken in a 8 mL
scintillation vial. Three milliliters of dimethylformamide (DMF) was
then added. The mixture was stirred for 10 min to give a clear solution.
General Procedure for the Heck-Coupling Reaction of Aryl
Bromides. The corresponding aryl bromide (2 mmol), alkene (4
mmol, 2 eqv.), tetrabutylammonium acetate (TBAA, 0.4 mmol, 0.2
eqv.), sodium acetate (6 mmol, 3 eqv.), and Pd@MOF-3 (10 mg, 1.25
B
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Scheme 2. Schematic Representation of the Preparation of Different Porous MOFs Depending on the Pillar Height of the Bi-
pyridyl Ligands
mol % Pd) were taken in a 50 mL two-necked round-bottomed flask.
Then, the reaction flask was degassed three times and subsequently
filled with nitrogen gas. To this, anhydrous dimethylformamide (DMF,
10 mL) was added through a syringe. The resulting suspension was
stirred under nitrogen atmosphere at 120 °C for 24 h. After cooling to
room temperature, the catalyst was separated by filtration, and then,
the solvent was evaporated under reduced pressure. The residue
obtained was purified by column chromatography over silica gel
eluting with hexane/EtOAc to afford the corresponding products.
General Procedure for the Alkene Hydrogenation Reaction. To a
hydrogenation flask, an alkene (2 mmol) was dispersed in anhydrous
ethanol (15 mL). To this, 5 mg of Pd@MOF-3 (0.6 mol %) was
added. Subsequently, the mixture was sonicated for a few minutes to
ensure that the catalyst homogeneously dispersed into ethanol. Finally,
the reaction mixture was fixed in a hydrogenation apparatus with
constant shaking at 4 bar pressure and 60 °C temperature. Initially,
pressure, temperature, and catalyst amount were optimized to obtain
the maximum hydrogenation product using styrene as a substrate. The
reaction time was also optimized for styrene with above-mentioned
pressure and temperature for 5 h. The same reaction conditions were
followed for all other starting materials. After the specified time, the
catalyst was separated by filtration, and then, ethanol was removed
under reduced pressure. The conversion/purity of the crude products
was estimated/judged by NMR spectroscopy. Hydrogenetion using
other Pd@MOFs was carried out in a similar way.
Reusability of the Catalyst. Iodobenzene (10 mmol), methyl
acrylate (15 mmol, 1.5 eqv.), tetrabutylammonium acetate (TBAA, 2
mmol, 0.2 eqv.), triethylamine (15 mmol, 1.5 eqv.), and Pd@MOFs
(50 mg) were taken in a 50 mL reaction vessel. The reaction mixture
was degassed several times and finally filled with nitrogen gas. To this,
anhydrous 1-butanol (30 mL) was added through a syringe. The
resulting mixture was stirred under nitrogen at 80 °C for 24 h. After
cooling down to room temperature, the catalyst was filtered, washed
several times with ethyl acetate, and then dried under vacuum. The
recovered catalyst was used again another three times for the same
reaction using the same procedure. Similarly, for hydrogenation the
recovered catalysts were used for five different cycles, and in every
case, the conversion was estimated from NMR spectroscopy. The
stability of the recovered catalysts after reactions was judged by TEM
and PXRD analysis.
isophthalate (pip) ligand has been used for the preparation of
four MOFs along with different dinitrogen linkers bpy, bpe, and
dpb. The pip ligand has two distinct functionalities: isophthalate
(iph) and a functionalized ethynyl moiety. The iph unit was
used for the generation of co-ordination polymer backbone
with metal ion connectors; however, the ethynyl functionality
was introduced with the intention to stabilize noble metal ions
inside the pore through the π-donor and π-acceptor interaction.
Dinitrogen linkers of different heights were used as an
interlinking unit to gradually enhance the porosity in the
three-dimensional (3D) networks. Also, they will certainly
provide variable pore size in the MOFs, which may give an
opportunity to study pore size dependent size distributions of
metal nanoparticles as well as NP loading amount. Thus,
solvothermal reactions of pip along with bipyridyl ligands, such
as bpy, bpe, and dpb, in dimethylformamide (DMF) with
Zn(NO3)2·6H2O at 100 °C for 48 h resulted in MOF-1, MOF-
2(Zn), and MOF-3, respectively. The reaction of pip and bpe
with Cd(NO3)2·4H2O under similar reaction conditions
yielded MOF-2(Cd). All of the MOFs were isolated in highly
crystalline form with excellent yields (Scheme 2). MOF-2(Zn)
and MOF-2(Cd) are isostructural, and also, our preliminary
experiments suggest that their behavior does not vary
considerably for stabilization of Pd NPs. Therefore, in the
present study, MOF-1, MOF-2(Cd), and MOF-3 have been
highlighted for Pd NPs loading and their catalytic applications.
Single Crystal X-ray Structures. The single-crystal X-ray
diffraction analyses revealed that MOF-1, MOF-2(Zn), and
MOF-2(Cd) were crystallized in a monoclinic crystal system
with P21/c (MOF-1) and C2/c (MOF-2) space groups (Figure
1). All of the MOFs form three-dimensional (3D) networks
with channel-like pores. Though as synthesized MOF-3 was
obtained as a highly crystalline material, several attempts
remained unsuccessful to obtain better single crystal data.
Fortunately, the PXRD pattern indicated that the as-
synthesized MOF-3 has structural arrangement similar to that
of MOF-1 and MOF-2 (Figure 2a).
RESULTS AND DISCUSSION
The crystal structures of MOF-2(Zn) and MOF-2(Cd)
revealed that they are isostructural to each other and possess
similar porosity. It was also observed that in every MOF the
void channels are occupied by guest solvent molecules such as
■
Synthesis. In general, a well-defined functionalized pore or
channel of MOFs offers potential application for sensing and
separation.15 In this context, the 5-(prop-2-yn-1-yloxy)-
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by four O atoms of three different iph units. In every case, the
ethynyl moieties resided above and below the 2D layered
networks (Figure 1c and d). These 2D layers are
interconnected to each other by either bpy (MOF-1) (Figure
1) or bpe (MOF-2) or dpb (MOF-3) linkers via coordination at
the axial positions of the Zn2+ or Cd2+ ions to render 3D MOFs
with huge porous channels. The representative crystal
structures of MOF-1 and MOF-2(Cd) are given in Figure 1.
Interestingly, as anticipated the ethynyl moieties are pointed
toward the interior of the MOFs pores (Figure 1a or b). It was
observed that in MOF-1 the ethynyl moieties are present in
every channel; however, in MOF-2 those are located in
alternate channels.
Powder X-ray Diffraction (PXRD) and Thermogravimetric
Analysis (TGA). Powder X-ray diffraction (PXRD) and
thermogravimetric analysis (TGA) were carried out to
understand the phase purity and thermal stability of the
MOFs. A comparison of the simulated PXRD patterns of
MOF-1, MOF-2, and MOF-3 with the as-synthesized material
indicated that all of the MOFs were prepared in single phase in
highly pure form (Supporting Information). Moreover, a
comparison of the PXRD patterns of as-synthesized MOF-3
with that of MOF-A indicated that MOF-3 also adopts
molecular structure similar to it (Supporting Information).
Although MOF-1, MOF-2, and MOF-3 have similar structures,
considerable shifts in the PXRD patterns were mainly due to
the increase of unit cell dimension from MOF-1 to MOF-2 to
MOF-3 depending upon pillars. The PXRD patterns of the
activated (desolvated) samples indicated that the MOFs’
networks are extremely stable even after the removal of the
solvent molecules from the pores.
Figure 1. (a and b) Crystal structures of MOF-1 and MOF-2(Cd); the
extended three-dimensional network of the MOFs showing channel
like pores. For MOF-1, each pore is decorated by functionalized
ethynyl moieties highlighted by yellow balls. In MOF-2(Cd), the
ethynyl moieties are located in alternative pores shown as yellow and
green balls. (c and d) Highlighted two-dimensional layered network
involving iph units of pip and Zn2+ or Cd2+ ions. The ethynyl moieties
are highlighted in pink.
Thermogravimetric analysis (TGA) revealed an almost
constant weight loss for all of the MOFs up to 200 °C,
which was attributed to the removal of guest water and DMF
(Figure 2b). All of the MOFs have significant thermal stability
up to ∼250 °C. The stability of the activated samples had been
demonstrated by TGA, which displayed their thermal stabilities
similar to that of as-synthesized samples. Since one of the
requirements of heterogeneous catalysts is their high thermal
stability, these could be useful several times. Thus, the high
thermal stability of the current MOFs meets the demands of
potential application in heterogeneous catalysis.
Loading of Pd Nanoparticles. Here, we have adopted a
solvent infiltration method to load MNPs into MOFs in
stepwise manner starting from corresponding metal ion
precursors. The NP preparation involves two distinct steps.
First, adsorption of metal ions into MOFs interior second,
reduction of metal ions by NaBH4 to their respective NPs
(Scheme 1). The MOFs were initially activated to remove all of
the solvent molecules from the pores. The supported Pd NPs
were synthesized from the PdCl2 precursor in methanol by wet
chemical reduction using NaBH4. To establish the two-step
mechanism of the Pd NPs formation, the reactions were carried
out stepwise. Initially, the metal ion precursor in methanol was
added to the activated MOF-3 crystals. Interestingly, the bright
orange color of the PdCl2 solution disappeared gradually during
2 h of time, and simultaneously, the colorless MOF-3 turned
brown indicating the adsorption of Pd2+ ions into the MOF
(Figure 3). The adsorption process was significantly facilitated
by stabilization of Pd2+ ions inside the MOF’s pores through
the Pd-ethynyl interaction. To get a clear idea about this
adsorption process, SEM energy dispersive X-ray (EDX)
mapping was carried out using Pd2+ adsorbed MOF-3. Figure
Figure 2. (a) PXRD patterns of as-synthesized MOFs-1-3. (b) TGA
plots of MOFs 1−3 along with their activated forms.
DMF and H2O. In all of the frameworks, as expected the iph
moieties of the ligands formed a two-dimensional layered
network connecting distorted octahedral Zn2+ or Cd2+ ions.
The equatorial sites of each Zn2+ or Cd2+ ion are coordinated
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Figure 3. Synthesis of Pd NPs in two sequential steps. (a) MOF-3, (b) PdCl2 solution in methanol, (c) MOF-3 and PdCl2 mixed together, and (d)
visual color change after 2 h. The supernatant becomes colorless from orange, and MOF-3 crystals turned brown. (e) Reduction of Pd2+ ions to Pd
NPs by NaBH4. (f) SEM-EDX mapping of PdCl2 adsorbed MOF-3.
3f indicates that the Pd2+ ions are adsorbed homogeneously
throughout the entire MOF-3. Subsequent reduction of Pd2+
ions by NaBH4 yielded Pd NPs. It is worth mentioning here
that unlike HAuCl4 adsorption reported earlier by us,14 the
same concentration of PdCl2 adsorbed at a much slower rate
compared to that of the HAuCl4 solution. Also, the adsorption
reached a steady-state value at a much lower concentration of
Pd2+ compared to that of Au3+ ions which is mainly due to the
fact that Au3+ ions have stronger electronic interaction with
ethynyl compared to that of the Pd2+ ion.13a This fact was
reflected in the lower loading amount of Pd into MOF-3
compared to that of Au. In this case, it was possible to load only
∼27 wt % Pd NPs without any agglomeration in contrast to 50
wt % Au NPs using the same MOF.
To determine the maximum loading capability of the MOFs
without altering the monodispersity and particle size of the
NPs, the MNP loading was increased gradually starting from 5
wt % using MOF-3 until the particles start to agglomerate. In
this process, up to ∼27 wt % Pd (inductively coupled plasma
Figure 4. Twenty-seven wt % Pd loading in MOF-3. (a) Bright-field
TEM image showing that the Pd NPs are homogeneously loaded in
MOF-3. (b) Corresponding Pd NPs size distribution histogram. (c)
STEM-HAADF image of the Pd@MOF-3. (d) HRTEM image.
mass spectroscopy (ICP-MS) loading was achieved (the exact
loading is 26.9 1.2 wt %). In a similar procedure, about ∼10
and 12.5 wt % of Pd NPs (ICP-MS experiment indicated the
exact amounts were 9.8
1.1 wt % and 12.5
1.6 wt %,
respectively) loaded into MOF-1 and MOF-2(Cd), respec-
tively.
into MOF-1 and MOF-2(Cd) with mean particle diameters 1.5
0.2 nm and 2.1 0.3 nm, respectively.
The TEM characterization of the Pd@MOF-3 is depicted in
Figure 4. It is evident from the TEM bright field (TEM-BF)
image that Pd NPs are homogeneously dispersed in MOFs with
a mean particle diameter of 2.8 0.3 nm as estimated from
particle size distribution. Considering the pore diameter of the
MOF-3, it can be concluded that the bigger particles with a
diameter more than 2.77 nm are presumably located in a
distorted MOF-3 surface or at the outer surface of the MOF
crystals but stabilized by the surface ethynyl groups. Similar
observations are also reported in the preparation of MNPs in
zeolitic materials or in other reported MOFs.16 We found it
extremely difficult to carry out HRTEM measurements on the
small sized (<3 nm) particles due to sample charging under the
electron beam. Thus, comparatively larger particles were
selected for the HRTEM image. The presence of Pd NPs in
MOF-3 was thus confirmed by the HRTEM image (Figure 4d)
as the lattice spacing d = 2.24 Å of (111) plane of fcc Pd is
consistent with the reported value of Pd(0) NPs. Also in the
STEM dark field image, the brighter spots are attributed to the
Pd NPs, which are homogeneously dispersed into the MOF
matrix. Similarly, Pd@MOF-1 and Pd@MOF-2(Cd) were also
characterized by the TEM techniques which are compiled in
the Supporting Information. Importantly, as observed in the
Pd@MOF-3, the particles are also homogeneously distributed
PXRD has been an excellent tool to confirm the NPs
formation and stability of MOFs after NP loading. In this case,
representative PXRD patterns of PdCl2@MOF-1 and Pd@
MOF-1 are given in Figure 5 compared with that of activated
MOF-1. The crystallinity of MOF-1 remained intact even after
the loading of PdCl2 and its reduction to Pd NPs, although
Figure 5. Comparison of PXRD patterns of MOF-1, PdCl2@MOF-1,
and Pd@MOF-1 along with their visual color change after each step.
E
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Figure 6. Five wt % Pd loading in MOF-A. (a) Bright-field TEM image showing the polydispersity of Pd NPs and their location mostly at the surface
of the MOF-A. (b) STEM bright field and (c) STEM-HAADF images of the Pd@MOF-A.
there was a shift of the peaks after the adsorption of PdCl2.
Along with their PXRD patterns, the visual color change of the
MOF-1 crystals after each step was also provided in Figure 5.
The colorless MOF-1 changes to golden yellow after the
adsorption of PdCl2, while retention of crystallinity was clearly
evident. Finally, the color changes to black after reduction
indicating the formation of Pd NPs. The sharp diffraction peaks
at low angle regions are due to MOF-1 itself, while the high
angle broad peaks are due to Pd NPs. Particularly, peaks at
39.8, 46.3, 67.6, 81.2, and 85° in the PXRD pattern are
characteristics of (111), (200), (220), (311), and (222) planes,
respectively, corresponding to fcc Pd NPs. The formation of
Pd° NPs was also confirmed by XPS analysis. The peaks at
340.7 and 335.3 eV are due to the binding energy corresponds
to 3d3/2 and 3d5/2 levels, respectively, which indicates the
formation of Pd° NPs (Supporting Information).
Control Experiment. To establish whether the ethynyl
moieties have any direct influence or the framework itself is
responsible to stabilize Pd NPs, a structurally similar MOF
(MOF-A) containing a functionalized phenyl moiety instead of
the ethynyl group has been used to stabilize the same metal
NPs.17 Interestingly, in the controlled experiments, loading of
only 5 wt % Pd into MOF-A in a similar process indicated that
MOF-A cannot stabilize either metal ions or the metal NPs. In
Figure 6, the TEM images of Pd@MOF-A show that the Pd
NPs are mostly agglomerated and located on the surface of
MOF-A crystals. The STEM images also showed the
agglomerated nature of Pd NPs and their location on the
surface of MOF-A. However, loading of NPs did not cause any
decomposition of the MOF-A, which was established by the
PXRD analysis. Thus, the above results indicate that the ethynyl
functionality had a substantial role in the stabilization of Pd
NPs.
and MOF-3 (Table 1). However, the mean particle size
observed in the former two MOFs are not in similar to their
Table 1. Pd Loading Amount and Their Corresponding Sizes
When Those Are Loaded into Different MOFs
Pd@MOFs
loading amount
size of the Pd NPs
Pd@MOF-1
9.7 1.1 wt %
12.5 1.6 wt %
26.9 1.2 wt %
1.5 0.2 nm
2.1 0.3 nm
2.8 0.3 nm
Pd@MOF-2(Cd)
Pd@MOF-3
pore diameter, which is smaller than the particle size. The
particles smaller than pore diameters are expected to be mainly
located inside the pore, and the bigger particles are most
probably stabilized on the surface by ethynyl groups.
However, in the case of MOF-3, because of larger pillar
height and comparatively higher surface area, it was expected
that the particles are located mainly inside the pore. Therefore,
nitrogen gas adsorption was performed for MOF-3 and Pd@
MOF-3 at 77 K to find out their exact surface area and pore
diameter. The surface area reduced significantly from 54.8 m2/g
to 21.7 m2/g upon loading of Pd NPs into MOF-3 (Supporting
Information). Probably, the pores of the MOF-3 are blocked by
Pd NPs which causes significant reduction of the surface area.
Although the mean particle diameter of the Pd NPs (2.8 nm) at
MOF-3 is bigger than the pore dimension of MOF-3, it is
expected that smaller particles are mainly loacated inside the
pores and that the bigger particles are mainly outside the pore.
MOF-1 and MOF-2 do not show any nitrogen adsorption
behavior, which indicates that these two MOFs have less
porosity compared to that of MOF-3.
The effect of pore size and surface area of the MOFs are also
reflected on the Pd loading amount. Initially, 30 wt % Pd was
attempted to load into MOF-1, MOF-2(Cd), and MOF-3 via
the two-step mechanism as mentioned earlier. The amount of
Pd NPs loaded into the MOFs (calculated amount from ICP-
MS) are ∼10 wt %, ∼ 12.5 wt %, and ∼27 wt %, respectively,
which is in accordance with the surface area of the respective
MOFs. The more porous MOFs (MOF-3) can stabilize larger
amounts of NPs without agglomeration compared to the less
porous MOFs. Therefore, the current studies reveal that the NP
sizes and loading amounts are highly dependent on the porosity
of the MOFs. Table 1 summarizes Pd NP loading amounts and
sizes on different MOFs.
Pillar Height Dependent Particle Size. Recently, enormous
attention has been paid to the synthesis of MOF supported
metal NPs. However, the systematic studies on the role of
MOF pore size on metal NPs are extremely rare. More
importantly, size dependent catalytic activities of the metal NPs
were also not explored well. The present MOFs offer an ample
opportunity to study this fact. Porosity as well as solvent
accessible volume of the MOFs was increased from MOF-1 to
MOF-3 as observed from the single crystal X-ray structure
depending upon the interlinking dinitrogen pillar.
Also, the NP size distribution as obtained from TEM
experiments is in accordance with the porosity of the MOFs.
The mean particle diameters are 1.5, 2.1, and 2.8 nm,
respectively, when they are loaded into MOF-1, MOF-2(Cd),
Heck-Coupling Reactions. Recently, Pd-catalyzed C−C
coupling reactions such as Suzuki−Miyaura, Ullmann, Heck,
and Sonogashira reactions have evolved as the most important
F
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Article
and useful reactions for traditional organic synthesis.18
Particularly, Heck-coupling reactions were widely carried out
using homogeneous Pd-catalysts, but their potential applica-
tions in the chemical industry are still limited due to difficulties
related to their efficient recovery from the reaction mixtures.
However, heterogeneous Pd-catalysts could be a promising
solution to this problem. In this current study, we anticipated
that the high loading and small size of the Pd NPs into MOFs
could be ideal for catalysts for the Heck-coupling reactions.
Therefore, several Heck-coupling reactions were performed in
the presence of Pd@MOFs involving aryl iodides or bromides
and alkenes.
Initially, all of the reactions were performed using
representative 1.25 mol % of the Pd@MOF-3 catalyst. Several
electron-donating or electron-withdrawing aryl iodides and
bromides afforded cross-coupling products with excellent
yields. Cross-coupling products involving acryl esters and
styrene are separately listed in Tables 2 and 3, respectively.
Table 3. Catalytic Heterogeneous Heck-Coupling Reactions
with Aryl Halaides and Styrene Using Pd@MOF-3
entry no.
1 or 2 (R1, R2, R3)
5
6
yield [%]
1
2
1 (H, H, H)
5
5
5
5
5
5
5
5
5
5
6a
6b
6c
6d
6e
6f
98
94
97
95
98
94
92
88
82
87
2 (H, H, H)
3
1 (Me, H, H)
4
2 (Me, H, H)
5
1 (H, Me, Me)
2 (H, Me, Me)
1 (OMe, H, H)
2 (OMe, H, H)
1 (H, CO2Me, CO2Me)
1 (H, Me, CO2Me)
6
7
6g
6h
6i
8
9
10
6j
Table 2. Catalytic Heterogeneous Heck-Coupling Reactions
with Aryl Halaides and Acrylic Esters Using Pd@MOF-3
Table 4. Selective Catalytic Heterogeneous Heck-Coupling
Reactions Using Pd@MOF-3 for Reactants Containing Both
Iodo- and Bromo-Groups
entry no.
1 or 2 (R1, R2, R3)
3 (R4)
4
yield [%]
1
1 (H, H, H)
3 (Me)
3 (Et)
4a
4b
4c
4d
4e
4f
99
99
99
95
98
94
98
98
97
93
95
92
87
91
2
1 (H, H, H)
3
1 (H, H, H)
3 (tBu)
3 (Me)
3 (Me)
3 (Me)
3 (Me)
3 (Et)
4
2 (H, H, H)
5
1 (Me, H, H)
entry no. condition Ar−X 8 (R1) (4 eqv.)
9 (yield)
10 (yield)
6
2 (Me, H, H)
1
2
3
4
(1)
(2)
(1)
(2)
7
7
7
7
8 (CO2Me)
8 (CO2Me)
8 (Ph)
9a (95%)
9a (0%)
9b (93%)
9b (0%)
10a (2%)
10a (96%)
10b (3%)
10b (87%)
13 (yield)
7
1 (H, Me, Me)
1 (H, Me, Me)
1 (H, Me, Me)
2 (H, Me, Me)
1 (OMe, H, H)
2 (OMe, H, H)
1 (H, CO2Me, CO2Me)
1 (H, Me, CO2Me)
4g
4h
4i
8
9
3 (tBu)
3 (Me)
3 (Me)
3 (Me)
3 (Me)
3 (Me)
8 (Ph)
10
11
12
13
14
4j
entry no. condition Ar−X 8 (R1) (4 eqv.) 12 (yield)
4k
4l
1
2
3
4
(1)
(2)
(1)
(2)
11
11
11
11
8 (CO2Me)
8 (CO2Me)
8 (Ph)
12a (92%)
12a (0%)
12b (88%)
12b (0%)
13a (3%)
13a (94%)
13b (4%)
13b (85%)
4m
4n
8 (Ph)
Interestingly, two different reaction conditions were employed
for aryl iodides and bromides depending on their reactivity.
Although the less reactive aryl bromide generally required
somewhat drastic conditions, its conversion remained excellent
(90−99%) under the present reaction conditions using the
Pd@MOF-3 catalyst. The yields of the reactions were
estimated from the NMR spectra. It is noted that the reaction
rate is highly dependent on the catalyst amount (mol % of Pd);
however, in the presence of a small catalyst amount the
reactions proceeded smoothly but needed longer time for full
conversion.
Selectivity between Iodide and Bromide. The different
reactivities of aryl iodides and bromides gave an opportunity to
convert selectively the former in the presence of the latter. To
establish this fact, two reactants containing both iodide and
bromide were chosen (Table 4). When the reactions were
carried out at 80 °C in n-butanol for 24 h, only iodides
underwent the cross-coupling reaction. However, when similar
reactions were performed at 120 °C in DMF for 24 h, both
iodides as well as bromides gave coupling products. Thus, these
catalysts provide an opportunity to tune the product selectivity
by tuning a simple reaction parameter, which is essential for
selective organic transformation.
Effect of the Size of the Nanoparticles. To understand the
role of NP size on the catalytic activity, a representative Heck-
coupling reaction was carried out using iodobenzene and
methyl acrylate in the presence of Pd NPs at different MOFs
(Table 5) with fixed catalyst amount (in terms of mol %). Size
of the Pd NPs was highly dependent on the porosity of the
MOFs. In the present case, the Pd NP size increased from 1.5
to 2.8 nm in MOF-1 to MOF-3, respectively, depending on the
porosity of the MOFs. It is expected that size of the NPs should
have a role in the conversion rate of the catalytic reaction if the
reactions are monitored with time. In this particular case, even
though the final yields of the reactions were similar after a
longer time period (24 h), a higher reaction rate was noted
during the initial period (6 h) in the case of smaller sized
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Inorganic Chemistry
Article
Pd@MOF Catalyzed Hydrogenation. Stabilized Pd NPs
have been used for heterogeneous catalytic hydrogenation of
unsaturated organic molecules in the recent past.19 However,
the activity and efficiency of these materials are highly
dependent on their size, loading amount on the substrate,
and the nature of the stabilizer. Ideally, the high loading
amount of Pd NPs into MOFs increases the high adsorption of
hydrogen gas, which can facilitate the facile hydrogenation of
alkenes. Initially, the reaction was performed under mild
conditions, that is, at room temperature in chloroform using
Pd@MOF-3. Because of the low conversion rate, finally the
reaction was optimized, and all the other reactions were carried
out at 60 °C and 4 atm pressure in ethanol for 5 h (Table 7).
Table 5. Catalytic Reactions Using Pd NPs @ Various MOFs
Having Different Particle Sizes
time [%]
entry
no.
size of the
NPs
6 h
24 h
(yield)
Pd@MOF
Ar−X (yield)
1
2
3
Pd@MOF-1
1.5 nm
2.1 nm
2.8 nm
Ph-I 16
(74%)
16 (99%)
16 (99%)
16 (99%)
Pd@MOF-2(Cd)
Pd@MOF-3
Ph-I 16
(72%)
Ph-I 16
(62%)
Table 7. Catalytic Hydrogenation of Alkenes Using the Pd@
MOF-3 Catalyst under Hydrogen Gas
particles as catalysts. This could be rationalized by the fact that
smaller particles possess a higher fraction of surface atoms (and
thus the catalytically active site) compared to that of their larger
counterpart.
Reusability of the Catalyst. The recyclability of the Pd@
MOF-3 catalyst was examined by carrying out a similar reaction
using iodobenzene and methyl acrylate (Table 6). The results
entry no.
17 (R)
17 (H)
17 (R1, R2, R3)
17 (H, H, H)
18
yield [%]
1
18a
18b
18c
18d
18e
18f
99
99
99
95
98
94
98
98
97
93
95
92
87
Table 6. Recyclability Test of the Pd@MOF-3 Catalyst in the
Heck-Coupling Reaction Involving Iodobenzene and
Methylacrylate
2
17 (H)
17 (H, Me, H)
17 (H, Cl, H)
17 (H, H, H)
3
17 (H)
4
17 (Ph)
5
17 (Ph)
17 (H, Me, H)
17 (Me, H, Me)
17 (H, Br, H)
17 (Br, H, H)
17 (H, H, H)
6
17 (Ph)
7
17 (Ph)
18g
18h
18i
8
17 (Ph)
9
17 (COOMe)
17 (COOMe)
17 (COOMe)
17 (COOMe)
17 (COOMe)
cycles
Ar−X
15
(16) yield [%]
10
11
12
13
17 (Me, H, Me)
17 (H, Br, H)
17 (Br, H, H)
17 (CO2Me, H, CO2Me)
18j
1
2
3
4
Ph-I
Ph-I
Ph-I
Ph-I
15
15
15
15
99
98
96
90
18k
18l
18m
Under this particular reaction condition, the conversion was
excellent, and more than 90% yield was achieved. Irrespective
of reaction conditions, in polar protic solvent such as ethanol
the reactions were completed faster. The rate of the
representative catalytic hydrogenation of styrene remained the
same irrespective of Pd NPs sizes when they were loaded into
three different porous MOFs described here. This is most
probably due to close particle size distribution which did not
affect the reaction rate much.
Reusability of the Pd@MOFs Catalyst after Hydro-
genation. Reusability of the Pd@MOF-3 catalyst was tested
for the hydrogenation of styrene several times using the
recovered catalyst under similar reaction conditions as
mentioned earlier (Table 8). The results demonstrated that
demonstrated that the catalyst can be recycled at least four
times without significant loss of catalytic activity. The stability
of the catalyst was further confirmed by the TEM imaging and
PXRD of the recovered catalyst (Supporting Information). The
TEM images of the recycled material showed that NPs are still
highly dispersed into MOFs. The PXRD pattern of the
corresponding recovered catalyst also indicated that the
crystallinity and stability of the MOF-3 was maintained after
four different cycles, though the peak intensity gradually
decreased. That might be due to the degradation of structural
regularity caused by repetitive exposure of the Pd@MOF-3
catalyst to the basic conditions. A Pd leaching test was carried
out to get an idea as to whether Pd was coming out of the
MOFs during reactions. In the absence of reactants, a mixture
of Pd@MOF-3 and triethylamine or sodium acetate was heated
with stirring at 80 °C in n-butanol for 24 h. The filtrate of the
hot reaction mixture was used for the catalytic reaction of
iodobenzene and methyl acrylate. There was no conversion of
the reaction which indirectly confirmed that Pd did not leach
out into the filtrate. This study also indicated the strong
interaction between Pd and MOFs (predominantly the Pd-
ethynyl interaction), and it is even quite stable under reaction
conditions.
Table 8. Recyclability Test for the Hydrogenation of Styrene
in Ethanol Using Pd@MOF-3
cycles
pressure (atm), temperature (°C)
time (h)
conversion (%)
1
2
3
4
5
4, 60
4, 60
4, 60
4, 60
4, 60
6
6
6
6
6
99
97
94
93
87
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Inorganic Chemistry
Article
Pd@MOF-3 can be used for at least five different cycles
without much loss of catalytic activity. The TEM images of the
recovered catalyst indicate that the Pd NPs are homogeneously
dispersed even after the catalytic reactions. Also, a sharp PXRD
pattern of the recycled catalyst suggested that MOF-3 remained
intact under the reaction conditions (Supporting Information).
The Pd leaching test was also performed to understand whether
Pd NPs were coming out of the MOFs during hydrogenation
reactions. Typically, after hydrogenation the whole reaction
mixture was filtered, and ICP-MS analysis was carried out to
find the Pd in the filtrate with respect to the standard solutions.
Note that no significant signal was observed corresponding to
Pd. Therefore, Pd was not leaching out of the MOF-3, and it
also established that Pd NPs have a strong interaction with
MOFs.
Heck-coupling and hydrogenation reactions using several
aromatic iodides/bromides and alkenes, respectively. Both
types of reactions were very efficient even in the presence of an
extremely small amount of catalyst. Most importantly, it has
been shown that under a particular reaction condition selective
coupling of aryl iodides and alkene takes place in the presence
of bromide. Under another condition, both iodide and bromide
undergo coupling reactions with alkene. Moreover, these Pd@
MOFs were recycled several times with excellent catalytic
activities. Importantly, these catalysts are also stable at higher
temperature with respect to NP agglomeration and can be used
for the other high temperature catalytic reactions.
ASSOCIATED CONTENT
■
S
* Supporting Information
High Temperature Stability of the Pd@MOFs Catalyst.
Recently, considerable efforts have been devoted toward the
high temperature stability of the catalyst such that those can be
used for organic transformations at elevated temperature.20
Generally, organic capping-agent stabilized NPs are often
limited for high temperature catalytic reactions due to NPs
agglomeration. However, owing to the inherent high thermal
stability of the MOFs, NPs stabilized by those are expected to
have higher thermal stability and therefore can be useful at
elevated temperature. The thermal stability of representative
Pd@MOF-3 was checked by a thermal annealing process at
150 °C followed by TEM analysis. In the typical annealing
process, a sealed sample of Pd@MOF-3 in a glass tube was
heated under vacuum in a programmable oven for 3 h at 150
°C. After cooling down to room temperature, the sample was
prepared for TEM analysis. The TEM-BF images showed that
the Pd NPs remained monodispersed and that particle size did
not change dramatically compared to that of the preannealed
sample. Moreover, thermal decompositions of Pd@MOF-1,
Pd@MOF-2(Cd), or Pd@MOF-3 were compared with that of
bare MOFs by the TGA method up to 800 °C to confirm their
thermal stability (Supporting Information). The stability of the
MOFs remained intact even after loading of the Pd NPs.
Interestingly, the weight differences between MOFs and Pd@
MOFs after decomposition also gave an indication about the
amount of Pd loaded into MOFs. Therefore, the current results
suggest that Pd@MOFs are excellent catalysts for high
temperature catalytic reactions.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02739.
Crystallographic data, additional PXRD, TEM images
XPS, gas adsorption studies, and characterizations of
catalytic products (PDF)
X-ray data (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*Tel: +91-80-2293-3352. Fax: +91-80-2360-1552. E-mail:
psm@ipc.iisc.ernet.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Department of Science and Technology
(DST), India for financial support. We are thankful to Professor
B. R. Jagirder and Mr. Sourav Ghosh for providing the facilities
to conduct the hydrogenation reaction.
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■
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DOI: 10.1021/acs.inorgchem.5b02739
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