Adsorption of a Catalytically Accessible Polyoxometalate in a Mesoporous Channel-type Metal-Organic Framework

Article abstract of DOI:10.1021/acs.chemmater.7b00750

A Keggin-type polyoxometalate (H₃PW₁₂O₄₀) was incorporated into a mesoporous Zr-based MOF (NU-1000) via an impregnation method in aqueous media, resulting in the hybrid material, PW₁₂@NU-1000. The POM@MOF composite was characterized by a suite of physical methods, indicating the retention of crystallinity and high porosity of the parent MOF. The hybrid material was also stable to leaching in aqueous media at varying pH. Finally, the material was tested as a heterogeneous catalyst for the oxidation of 2-chloroethyl ethyl sulfide using hydrogen peroxide as the oxidant. PW₁₂@NU-1000 was shown to have a higher catalytic activity than either of the individual constituents alone.

Full text of DOI:10.1021/acs.chemmater.7b00750

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Article
Adsorption of a Catalytically Accessible Polyoxometalate
in a Mesoporous Channel-type Metal–Organic Framework
Cassandra T. Buru, Peng Li, B. Layla Mehdi, Alice Dohnalkova, Ana E. Platero-
Prats, Nigel D. Browning, Karena W Chapman, Joseph T. Hupp, and Omar K. Farha
Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 24 May 2017
Downloaded from http://pubs.acs.org on May 25, 2017
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Chemistry of Materials
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Adsorption of a Catalytically Accessible Polyoxometalate in a
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Mesoporous Channel-type Metal–Organic Framework
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Cassandra T. Buru, Peng Li, B. Layla Mehdi, Alice Dohnalkova, Ana E. PlateroꢀPrats, Nigel D.
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┴
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†
†‡*
Browning, Karena W. Chapman, Joseph T. Hupp, Omar K. Farha
†
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA
Physical and Computational Science Directorate, PNNL, Richland, WA 99352, USA
§
∆
┴
#
Environmental Molecular Sciences Laboratory, PNNL, Richland, WA, 99352, USA
Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA
Xꢀray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439ꢀ4858, USA
‡
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
KEYWORDS polyoxometalate, metal–organic frameworks, phosphotungstic acid, NU–1000, POM@MOF, catalysis, sulfide
oxidation
ABSTRACT: A Kegginꢀtype polyoxometalate (H PW O ) was incorporated into the mesoporous Zrꢀbased MOF (NU–1000) via
3
12 40
an impregnation method in aqueous media, resulting in the hybrid material, PW @NU–1000. The POM@MOF composite was
12
characterized by a suite of physical methods, indicating the retention of crystallinity and highꢀporosity of the parent MOF. The hyꢀ
brid material is also stable to leaching in aqueous media at varying pH. Finally, the material was tested as a heterogeneous catalyst
for the oxidation of 2ꢀchloroethyl ethyl sulfide using hydrogen peroxide as the oxidant. PW @NU–1000 was shown to have a
12
higher catalytic activity than either of the individual constituents alone.
directly adding preformed POM to a MOF. This method neꢀ
cessitates large apertures in the MOF for POM diffusion.
Though straightforward, the impregnation method has a numꢀ
ber of limitations when used to obtain a potential catalytic
material: low maximum loading (~0.2 POM/node), low homoꢀ
geneity, occupation of large cavities limiting diffusion, and
INTRODUCTION
Polyoxometalates (POMs) are anionic metal oxide clusters,
mainly composed of high valent earlyꢀtransitionꢀmetal ions
bridged by oxygen atoms. Their modular composition and
size give rise to many examples of POMs. Owing to their
acidꢀbase and redox properties, POMs have found many appliꢀ
1
–3
4
,5
27–30
leaching.
The encapsulation method, also known as the
6
7–9
cations in chemical catalysis , such as water oxidation, alꢀ
‘bottle around ship method’, involves building the MOF in the
presence of POM. Such a technique has been useful for
HKUSTꢀ1 and other CuꢀBTC frameworks (BTC = 1,3,5ꢀ
benzeneꢀtricarboxylate, these POM@MOFs are also known as
10,11
12,13
kene epoxidation,
and phosphate hydrolysis.
The reꢀ
covery and reusability of POMs in these catalytic systems is
hindered by their high solubility, low specific area, and poor
stability under catalytic conditions.
14,15
31
29,32–35
To overcome these
the NENU series), MILꢀ100/101 (Fe, Cr, Al),
and UiOꢀ
16,17
36
drawbacks, supports such as mesoporous silica,
67 (see abbreviations section). The cavities inside these
MOFs are large enough to contain the desired POM, while the
apertures are small enough to keep the POM from leaching out
of the pores. This usually leads to reduced activity of the POM
because the pores are blocked, such that the substrate cannot
18,19
20
polymers,
covalent organic frameworks (COFs), and
21,22
metal–organic frameworks (MOFs)
chor POMs to create hybrid heterogeneous materials.
have been used to anꢀ
MOFs are a class of materials consisting of metal nodes
connected by multiꢀtopic organic linkers.
37,29
23,24
easily diffuse through the framework.
Gascon and coworkꢀ
The tunability
ers have demonstrated that one approach to avoid these probꢀ
lems is to use MILꢀ101 to encapsulate 2 POMs per medium
and porosity of MOFs makes them promising heterogeneous
25
catalysts or excellent supports for anchored homogeneous
30
2
6
cavity, leaving the larger cavities open. This material relies
on metal exchange of tungsten with the chromium framework,
potentially limiting the scope of the POMs which can be enꢀ
capsulated into this MOF.
catalysts. Two techniques have been reportedly used to inꢀ
corporate POMs in MOFs (also POM@MOF): impregnation
27
and encapsulation. The first instance of a POM@MOF was
synthesized by Ferey and coworkers using the impregnation
method with MILꢀ101, due to the large apertures and mesoꢀ
2
1
pores in its structure. The impregnation method involves
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To overcome some of these problems of POM and substrate
Scheme 1. General catalytic conditions and reaction path-
ways for 2-chloroethyl ethyl sulfide oxidation with hydro-
gen peroxide
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diffusion, we propose to use a zirconium MOF with microꢀ
and mesoporous channels, NU–1000, as a support for POMs.
NU–1000, an eightꢀconnected MOF, composed of Zr (µ ꢀ
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O) (µ ꢀOH) (H O) (OH) nodes connected by 1,3,6,8ꢀ
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tetrakis(pꢀbenzoate)pyrene linkers (TBAPy),
fulfills these
requirements because it has small triangular channels (12 Å)
which will permit the incorporation of large POMs, while the
larger (31 Å) hexagonal channels provide an auxiliary path for
substrate diffusion (Figure 1). The use of this hierarchical
MOF should ultimately circumvent the previously stated limiꢀ
tations imposed by other MOFs. Additionally, NU–1000 has
been shown to be an excellent host material, supporting guests
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Here we demonstrate the adsorption of a representative
3ꢀ
POM, [PW O ] , within the pores of NU–1000. To our
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from the size of a single atom
or small molecule
to an
knowledge, this is the first channel POM@MOF composite
and achieves the highest POM/node loading attainable via
15,21,28
impregnation as of yet.
Furthermore, we demonstrate the
POMs and MOF nodes in the composite material to be catalytꢀ
ically accessible. For the catalytic sulfide oxidation,
PW @NU–1000 has higher activity than either component
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alone.
RESULTS AND DISCUSSION
Due to the large channels in NU–1000, the direct impregnaꢀ
3ꢀ
tion method of installing [PW O ] was considered. Since
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NU–1000 has anionic character at neutral pH and the POM is
also anionic in water, the incorporation of the POM should
rely upon the acidic nature of the POM precursor
(H PW O ). When the hydroxyl ligands on the MOF nodes
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are protonated at a highly acidic pH, then the MOF will no
longer be anionic and POM uptake should be favorable. To
synthesize PW @NU–1000, NU–1000 was added and susꢀ
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pended in an aqueous H PW O solution (pH 2.5). After 3
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days at room temperature, the solid was washed and rigorously
solvent exchanged with acetone, resulting in PW @NU–
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2
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000. Detailed synthesis and activation of PW @NU–1000 is
Figure 1. The crystal structure of NU–1000, showing the 31 and
2 Å channels. Each layer is separated by 10 Å windows. The
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outlined in the Experimental section.
1
structure for the nodes and linkers are given. The structure and
Characterization. Inductively coupled plasmaꢀoptical emisꢀ
3ꢀ
size of the POM, [PW O ] , is shown.
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sion spectroscopy (ICPꢀOES) data suggest that approximately
3ꢀ
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0.8 [PW O ] per Zr node is the maximum POM loading in
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enzyme. Intermediateꢀsized charged guests, like POMs,
have yet to be studied in NU–1000. Previous studies have
shown that NU–1000 readily adsorbs small oxyanions (seleꢀ
NU–1000 after thorough washing.* This loading can be obꢀ
tained from soaking the MOF in 2 or more equivalents of
POM per node and is equivalent to 52 wt%. The loading of
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nites/selenates, sulfates ) via displacement of hydroxide ions
3ꢀ
[
PW O ] , however, can be modulated by soaking the MOF
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on the Zr node for charge balancing. We hypothesized that
6
in lower concentrations of POM. Further characterization of
PW @NU–1000 refers to the maximum 0.8 PW /Zr loadꢀ
guest molecules could be extended to larger oxyanions, nameꢀ
ly POMs.
12
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6
ing.
H PW O was selected as an archetypal POM based on
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Scanning electron microscopy (SEM) images show that the
size and morphology of NU–1000 crystallites are maintained
during POM installation (Figure 2, Figure S1). Energy disperꢀ
sive Xꢀray spectroscopy (EDS) mapping suggests these POMs
are located uniformly within, not just on the surface of, the
MOF, except in the center where the crystal is known to have
size and demonstrated catalytic activity for the oxidation of
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organosulfides in the presence of peroxide.
On the other
hand, the nodes on NU–1000 have a structure analogous to
3ꢀ
zirconia, which can act as a counterion for [PW O ] to preꢀ
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40
vent the Keggin unit from leaching once the nodes are protoꢀ
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nated.
Zirconia can also catalyze the oxidation of
Therefore, if the POMs and MOF nodes are acꢀ
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blocked pores * (Figure 2).
sulfides.
cessible, then the proposed composite material should be acꢀ
tive in the oxidation of 2ꢀchloroethyl ethyl sulfide (CEES), a
mustard gas simulant (Scheme 1). Two oxidation products are
possible, a singly oxidized sulfoxide (CEESO) and a doubly
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4
oxidized sulfone (CEESO2). In the case of mustard gas (a
lethal chemical warfare agent), the sulfoxide product is conꢀ
sidered nonꢀtoxic, while the sulfone has toxicity similar to the
*
NU–1000 crystallites have an apparently fully 12ꢀconnected secondary
framework in the center of the crystallites which accounts for ~20% occuꢀ
pancy crystallographically. These defects have been quantified via acidꢀ
base titration (see references 38 and 56).
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parent agent. Thus, selectivity is important when designing a
catalyst for mustard gas detoxification.
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caused by sample preparation or the nature of the flexible orꢀ
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ganic linkers. The orthogonal direction shows a clear cubic
arrangement of the nodes (Figure S4b). Importantly, no elecꢀ
tron density within the hexagonal channels is consistent with a
distribution like Figure 4a. However, further analysis is needꢀ
ed to pinpoint the exact location of the POM directly from the
images.
To further elucidate the location of the POM, difference enꢀ
velope density (DED) was collected for. This technique generꢀ
ates a surface envelope between regions of high and low elecꢀ
tron density which can be extracted from low order, intense,
independent reflections in a high resolution PXRD pattern.
Using differential analysis with a parent framework can yield
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a coarse map of electron density attributed to the incorporation
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Figure 2. SEMꢀEDS of NU–1000 after [PW O ] incorporation.
of guest molecules.
DED (Figure S5) of PW @NU–1000
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40
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EDS legend: Light Blue=W, Green=Zr, Purple=Zn (baseline)
localizes the electron density in the small channels in the plane
of the nodes, akin to Figure 3c.This orientation maximizes the
hydrogen bonding interaction between the oxo groups on the
POM and the hydroxyl groups on the nodes.
The unit cell of PW @NU–1000, as seen by the Bragg
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peak positions on powder Xꢀray diffraction (PXRD, Figure
S2), resembles that of NU–1000. However, locating the POM
is not straightforward. Though the large mesopores in NU–
000 are large enough to accommodate the POM (Figure 4b),
this possibility can be excluded by comparing the simulated
and experimental PXRD of PW @NU–1000 (Figure S3). The
diffraction pattern matches well with a predicted pattern where
the POM is located between two nodes in the windows (Figure
a), but does not definitively prove the location of these
POMs.
Scanning transmission electron microscopy (STEM) images
were taken of PW @NU–1000 in the attempt to locate the
The BrunauerꢀEmmettꢀTeller (BET) surface area calculated
from the N isotherm of PW @NU–1000 significantly deꢀ
1
2
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2
creases from 2100 to 850 m /g relative to bare NU–1000 (Figꢀ
ures S6 and S7), which is consistent with POM incorporation
in NU–1000. Volumetrically, however, they achieve similar
1
2
2
3
BET surface areas (1020 to 850 m /cm for NU–1000 and
PW @NU–1000 respectively) (Figure 4). The isotherm of
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PW @NU–1000 shows a type IV feature, similar to that of
12
NU–1000 which is associated with the mesoporous channel,
which is also present in the pore size distribution (Figure 4).
The calculated average pore width indicates a decrease in the
length of the large hexagonal pores from 31 Å to 25 Å comꢀ
pared to NU–1000, but not as significant decrease as if the
POMs were located inside the hexagonal channels. This effect
was also observed when smaller metal ions were deposited
1
2
POMs. STEM images have previously been used to determine
5
7
dispersion of POMs in carbon nanotubes. Figure S4 shows
two Zꢀcontrast images obtained from orthogonal directions
where the node distribution in a representative area of the
MOF is observed. The standard hexagonal distribution of
nodes is clearly observed in the top half of Figure S4a. The
orientation of the sample changes slightly across the image
(bending) and the hexagonal pattern becomes distorted in the
lower half of the image. This distortion could have been
solvothermally in the windows between the channels of NU–
40–42
1
000.
More strikingly, the large decrease in the differential
3 3
pore volume of the micropores (3.8 to 1.0 cm /cm ) could
indicate that the POMs are located in the small channels.
Figure 3. Visual representations (top and side view) of the proposed structures for PW @NU–1000, where the POM is located
12
inside the windows between the channels(a), in the hexagonal channels (b), or in the small triangular channels (c). DED mapping of
PW @NU–1000 has led to the conclusion that (c) is most representative of the material.
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Figure 5. DRIFTS spectra of the H PW O , NU–1000, and the
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PW @NU–1000. The nonꢀhydrogen bonded OꢀH stretches, carꢀ
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boxylate stretches, and PꢀO/WꢀO stretches are highlighted.
reported and caused by either different amounts of protons
63,64
associated with each POM
or distortion of the POM when
65
interacting with the support. Diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS) shows that both
POM and MOF components are present in the composite maꢀ
terial, as highlighted in the boxes in Figure 5. The features in
the 750 to 1100 cmꢀ1 range correspond to WꢀO and PꢀO of the
POM (893, 984, 1080 cmꢀ1), and the bands between 1300 and
1
700 cmꢀ1 correspond to carboxylate stretches in NU–1000
ꢀ
1
ꢀ
1
Figure 4. Volumetric N isotherm (above) and pore size distribuꢀ
2
(1420, 1520, 1610 cm ). The sharp OꢀH stretch at 3670 cm
tion (below) of NU–1000 and PW @NU–1000
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2
from the nonꢀH bonded –OH and H O on the NU–1000 node
2
is noticeably absent in the composite. The Hꢀbonded H O and
2
Thermal gravimetric analysis (TGA) shows a mass loss of
0 % for bare NU–1000 between 25 and 700 °C (Figure S8).
39
ꢀ1
–OH peak at 2551 and 2745 cm are also absent. Based on
these observations, it is tempting to conclude that the nonꢀ
hydrogenꢀbonded aqua and hydroxyl ligands are quantitatively
displaced by the POM. Caution is called for, however, as the
spectrum is complicated by the presence of physisorbed water;
5
The majority of this mass loss is due to the carbon decomposiꢀ
tion at 500 °C. The TGA trace of the POM, H PW O ,
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matches well with those reported in the literature, losing 13
of its initial mass due to water. The corresponding
PW @NU–1000 loses 20 % of its initial mass, consistent with
%
ꢀ
1
its broad resonance (from ~2500 to ~3700 cm ) might obꢀ
scure, or partially obscure, residual features at 3670, 2551 and
1
2
a greater percentage of inorganic composition compared to
NU–1000 which is not lost below 700 °C. The two features
seen in the TGA profile indicate physisorbed water loss from
the POM below 100 °C and carbon decomposition from NU–
1000 around 500 °C (Figure S8). The composite material is
highly hygroscopic, corroborated by TGAꢀmass spectrometry
ꢀ
1
2
745 cm .
To test the stability towards leaching, PW12@NU–1000 was
exposed to aqueous solutions of HCl at pH ranging from 2ꢀ7
Table S1). Zrꢀbased MOFs are known for their chemical staꢀ
(
66
bility (pH 1ꢀ11). However, stability at higher pH was not
investigated because of known decomposition of the POM in
(MS). Water makes up approximately 10 wt% of the asꢀ
67
basic solutions. After one week under near neutral conditions
pH 6), the material lost only 13 % of POMs by ICPꢀOES. At
synthesized PW @NU–1000 mass prior to any catalytic studꢀ
12
(
ies.
lower pH values less POM was lost, demonstrating that the
POMs are not easily displaced by competing chloride ions.
Cyclic voltammetry (CV) shows that the redox activity of
the POM in the MOF is retained (Figures S9 and S10). The
slight positive shift in formal reduction potentials is indicative
Catalytic Studies. POM@MOFs typically have reduced catalytic
3
6,61
of the POM interacting with the support.
Additionally, the
solid state P magic angle spinning (MAS) NMR spectrum of
H PW O has two resonances (Figure S11), ascribed to a
activity due to the blocking of pores and subsequent inhibition of
31
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substrate diffusion.
In order to determine if PW @NU–1000
12
3
12 40
can overcome limitations, the composite was tested for CEES
oxidation (Scheme 1). Activity of H PW O for the oxidation of
wellꢀcrystallized anhydrous phase (ꢀ13.9 ppm) and an amorꢀ
3
12 40
62
4
8–
phous phase (ꢀ15.3 ppm). A phosphoric acid impurity can be
sulfides in the presence of peroxide has been well documented.
Zirconia, with composition analogous to the Zr MOF nodes,
31
50
seen above 0 ppm. The P MAS NMR spectrum of the comꢀ
6
posite PW @NU–1000 shows a slight downfield chemical
also has the ability to oxidize sulfides in the presence of peroxꢀ
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shift and much broader signal compared to the POM alone.
Based on the shape, the observed signal is likely a combinaꢀ
tion of three peaks (ꢀ13.3, ꢀ14.2, ꢀ15.9 ppm), which has been
ides.
To com pare the catalysts, the loadings were normalized
to the amount of active clusters, including both the POM and the
Zr nodes. In a typical run, the catalyst was suspended in 1 mL of
6
acetonitrile (MeCN). CEES and hydrogen peroxide
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000 showed preference to the doubly oxidized CEESO (enꢀ
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try 6). The MOF reaction in entry 10 was limited by the
amount of peroxide, and in the presence of excess peroxide the
reaction goes to completion with CEESO as the only product
2
(entry 7). PW @NU–1000 showed faster initial rate and
12
higher conversion than the POM or the MOF alone (entry 8).
Since the amount of catalyst was normalized to the sum of the
POM and Zr nodes, the enhanced activity can be attributed to
6
the cooperative effect of the POM and the MOF node. The
sulfoxide selectivity suggests that both POM and MOF are
active in the catalysis because CEESO and CEESO are preꢀ
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sent in roughly equal amounts (entry 9).
To determine if the POMs are leaching out of the MOF
during catalysis, the PW @NU–1000 reaction mixture was
12
filtered at 2 min and the reaction process was further moniꢀ
tored (Figure S13). The conversion was approximately 10 %
after 1 hour, which is only 5 % above the background reaction.
This indicates that minimal POMs are leaching under these
conditions, if at all. Solution phase NMR spectroscopy of the
filtered reactions shows no phosphorous signal, reaffirming
that the material does not leach. Upon recycling, PW @NU–
Figure 6. Reaction profile of CEES oxidation for the H PW O ,
3
12 40
NU–1000, and PW @NU–1000 with standard deviation over
1
2
three trials
were added stepwise to the reaction vial, and the reaction was
kept at 45 °C in a sand bath. Since NU–1000 has been reported
to oxidize CEES in the presence of UV LED irradiation and
1
2
6
8
1000 was shown to be reusable. The retention of selectivity
and reactivity upon recycling reaffirm that the material is staꢀ
ble under these catalytic conditions (Figure S14). Additionalꢀ
O2, the sand bath was necessary to block light and suppress
any background reaction. Aliquots were taken at different time
intervals, and analyzed via gas chromatographyꢀflame ionizaꢀ
tion detector (GCꢀFID) or solution NMR (see Experimental
section). The average reaction profile is shown in Figure 6.
The initial turnover frequencies (TOF) from the graph are 3.1
ꢀ1
31
ly, the PXRD, DRIFTS, and P MAS NMR spectroscopy
results show that the integrity of the composite material reꢀ
main unchanged (Figure S15, S16, and S17).¹⁵
Interestingly, the three broad overlapping peaks observed in
ꢀ
1
ꢀ1
min for NU–1000, 9.3 min for H PW O4 , and 10.4 min
31
3
12
0
P CPꢀMAS NMR spectrum prior to any catalysis become
for PW @NU–1000 (Table 1 entries 4, 6, and 8). H PW O4
12
3
12
0
more distinct postꢀcatalysis.
goes to full conversion after 90 minutes, while PW @NU–
12
These catalytic studies indicate that active sites in the MOF
are accessible after the installation of [PW O ] . Importantꢀ
1000 takes only 20 minutes under the same conditions. The
Experimental section has the TOF formula and a detailed proꢀ
cedure on calculating yield and conversion from NMR (Figure
S12).
3ꢀ
12 40
ly, the selectivity of the composite material suggests that the
MOF nodes are also active, meaning that they are not comꢀ
pletely blocked by the POM. Having access to all potential
catalytic sites is important in the development of a robust cataꢀ
lyst. This study has demonstrated that with careful selection of
MOF, we can incorporate POMs while maintaining accessibilꢀ
ity and high sur face area of the parent MOF. Future directions
are aimed at improving composite selectivity by passivating
the exposed Zrꢀnode in NU–1000 and using oxygen as an oxiꢀ
dant.
Table 1 shows yield and selectivity under different catalytic
conditions. All trials with reported error were performed in at
least triplicate. Background reactions without catalyst or perꢀ
oxide, entries 1ꢀ3, showed minimal activity. The homogeneous
H PW O showed selective partial oxidation of the sulfide
3
12 40
to CEESO (entry 4). Even in the presence of excess peroxide
(entry 5), CEESO is the only observed product. Bare NU–
Table 1. Summary of catalytic data.
Loading
Initial TOF
Sulfoxide Seꢀ
lectivity (%)
ꢀ
1 a
Catalyst
ꢀꢀ
(mol%)
Temp. (°C)
Time (min)
Oxidant (eq)
(min )
Yield (%)
5
1
2
3
4
5
6
7
8
ꢀꢀ
2
45
45
45
45
45
45
45
45
180
180
180
90
1.5
0
ꢀꢀ
ꢀꢀ
ꢀꢀ
b
H PW O
0.02
7
3
12 40
b
NU–1000
2
0
0.01
5
ꢀꢀ
d
H PW O
2
1.5
5
9.3
98 (2)
100 (0)
77 (4)
99 (1)
98 (2)
100 (0)
98 (2)
23 (1)
0 (0)
57 (1)
3
12 40
c
H PW O
2
60
1.6
3
12 40
d
NU–1000
2
90
1.5
5
3.1
c
NU–1000
2
60
1.3
d
PW @NU–1000
1.1
20
1.5
10.4
1
2
a
General: 1 mL MeCN, 10 µL CEES, 5 µL internal standard (1ꢀbromoꢀ3,5ꢀdifluorobenzene), H O as the oxidant. TOF=moles prodꢀ
2
2
b
c
d
uct/(moles of total catalytic clusters × amount of time) time=180 min time=30 min time=2 min
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CONCLUSIONS
Moles of product are equal to the sum of CEESO and
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CEESO . The moles of catalytic clusters (POM or MOF node)
2
We have synthesized and fully characterized a POM@MOF
material, PW @NU–1000. This composite achieves the highꢀ
were constant 1.7 µmol.
12
est POM/node loading for a POM@MOF synthesized via imꢀ
pregnation, while still maintaining high porosity. PW @NU–
ASSOCIATED CONTENT
Supporting Information. Materials, Instrumentation, Additional
1
2
1
000 has been shown to have enhanced catalytic activity for
Characterization (SEM, PXRD, DED, TGA, Gravimetric Isoꢀ
the oxidation of sulfide by hydrogen peroxide than either the
POM or MOF under the same conditions. Future studies will
explore other catalytically interesting POMs to be incorpoꢀ
rated in NU–1000 and its family.
3
1
therm, Gravimetric Pore Size Distribution, CV, P CPMAS
NMR, Leach study) and Catalytic Studies (NMR, Conversion and
31
Selectivity Graph, DRIFTS, PXRD, P CPMAS NMR). This
material is available free of charge via the Internet at
http://pubs.acs.org
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
EXPERIMENTAL
Synthesis of PW @NU–1000. In
a
centrifuge tube,
AUTHOR INFORMATION
1
2
H PW O (303 mg, 0.09 mol) was dissolved in 20 mL deionꢀ
3
12 40
Corresponding Author
ized water. The solution (pH about 2.5) was divided into two
0 mL quantities. To one solution, NU–1000 (50 mg, 0.023
1
* Eꢀmail: oꢀfarha@northwestern.edu (O.K.F.).
mmol) was added and suspended by sonicating for about 1
minute. The suspension was shaken periodically. To monitor
uptake of POM, the solutions with and without NU–1000 were
analyzed by ICPꢀOES and compared to each other. After 3
days, the solid was washed with water three times. Then, the
solid was washed with acetone. PW @NU–1000 was allowed
to sit in acetone overnight before washing two more times
with acetone to ensure adequate removal of water. Samples
were dried at 80 °C for 4 hours under vacuum prior to activaꢀ
tion on the Smart VacPrep. The solid material was subject to
ICPꢀOES to determine the final POM loading.
Author Contributions
All authors have given approval to the final version of the manuꢀ
script.
ACKNOWLEDGMENTS
1
2
O.K.F. and J.T.H. gratefully acknowledge support by the Army
Research Office (project no. W911NFꢀ13ꢀ1ꢀ0229). C.T.B.
acknowledges Martino Rimoldi for help with solidꢀstate NMR
and Nicolaas Vermeulen for helpful discussions. Visual structural
representations were generated using CrystalMaker®: a crystal
and molecular modelling program for Mac and Windows, Crysꢀ
talMaker Software Ltd, Oxford, UK (www.crystalmaker.com).
Metal analysis was performed at the Northwestern University
Quantitative Bioꢀelement Imaging Center. This work made use of
the J. B. Cohen XꢀRay Diffraction Facility supported by the
MRSEC program of the National Science Foundation (DMRꢀ
1121262) at the Materials Research Center of Northwestern Uniꢀ
versity. This work made use of the EPIC facility of Northwestern
University’s NUANCE Center, which has received support from
the Soft and Hybrid Nanotechnology Experimental (SHyNE)
Resource (NSF NNCIꢀ1542205); the MRSEC program (NSF
DMRꢀ1121262) at the Materials Research Center; the Internationꢀ
al Institute for Nanotechnology (IIN); the Keck Foundation; and
the State of Illinois, through the IIN. This work made use of the
IMSERC at Northwestern University, which has received support
from the NSF (CHEꢀ1048773 and DMRꢀ0521267); Soft and Hyꢀ
brid Nanotechnology Experimental (SHyNE) Resource (NSF
NNCIꢀ1542205); the State of Illinois and International Institute
for Nanotechnology (IIN). Work at Pacific Northwest National
Laboratory (PNNL) was supported by the Chemical Imaging Iniꢀ
tiative under the Laboratory Directed Research and Development
Program. PNNL is a multiꢀprogram national laboratory operated
by Battelle for the U.S. Department of Energy (DOE) under Conꢀ
tract DEꢀAC05ꢀ76RL01830. A portion of the research was perꢀ
formed using the Environmental Molecular Sciences Laboratory
Catalysis Procedure. For 2ꢀchloroethyl ethyl sulfide (CEES)
oxidation experiments, the catalyst (1.7 µmol of active site)
was dispersed in 1 mL of acetonitrile in a 2 dram vial. For the
POM and POM@MOF, the water weight was considered, so
1.7 µmol active sites was equivalent to 3.7 mg NU–1000, 5.7
mg H PW O , and 4.7 mg PW @NU–1000. CEES (10 µL,
3
12 40
12
85 µmol) and an internal standard (1ꢀbromoꢀ3,5ꢀ
difluorobenzene, 5 µL) was added to the reaction vial. Then,
hydrogen peroxide (30 wt% in water, 13 µL, 1.5 eq) or tertꢀ
butyl hydroperoxide (70 wt% in water, 18 µL, 1.5 eq) was
added. The vial was placed in a sand bath, which was preꢀ
heated to 45 °C. Variations to this procedure are summarized
in Table 1. Aliquots, approximately 50 µL, were withdrawn
from the vials with a glass pipette at different time points,
filtered, and diluted with dichloromethane for GCꢀFID or deuꢀ
terated acetonitrile for NMR spectroscopy. GCꢀFID was used
to monitor conversion and NMR spectroscopy was used to
determine selectivity. Figure S9 is a representative NMR specꢀ
trum of CEES, CEESO, and CEESO . Conversion was calcuꢀ
2
lated by subtracting the peak at 3.71 ppm from the peaks at
3.93 ppm, and selectivity was calculated by subtracting the
peak at 3.47 ppm from 3.93 ppm. In the event that the peak at
3.93 peak was convoluted, the peak at 3.05 ppm was used
instead. GCꢀMS at the final time point was used to confirm the
(EMSL), a national scientific user facility sponsored by
the DOE’s Office of Biological and Environmental Research at
PNNL. Work done at Argonne was performed using the Adꢀ
vanced Photon Source, a U.S. Department of Energy (DOE) Ofꢀ
fice of Science User Facility operated for the DOE Office of Sciꢀ
ence by Argonne National Laboratory under Contract No. DEꢀ
AC02ꢀ06CH11357.
identity of the products. For leaching studies, PW @NU–
12
1
000 was filtered out at 2 minutes and any remaining reaction
was monitored. To recycle the catalyst, the reaction mixture
was centrifuged and the solvent extracted. The solid material
was washed three times with acetone and dried under vacuum
at 70 °C before reuse.
Turnover frequencies were calculated using the first time
point for each run and the equation:
ABBREVIATIONS
POM, polyoxometalate; COFs, covalent organic frameworks;
MOFs, metal–organic frameworks; HKUST, HongꢀKong Univerꢀ
sity of Technology; BTC, 1,3,5ꢀbenzeneꢀtricarboylate; NENU,
Northeast Normal University; MIL, Material Insitut Lavoisier;
ꢃꢄꢅꢆꢇꢈꢄꢉꢈꢊꢋꢄꢌꢍꢎꢏ
ꢀꢁꢂ =
ꢈ
ꢃꢄꢅꢆꢇꢈꢄꢉꢈꢎꢅꢍꢇꢏꢆꢋꢇ × ꢐꢃꢄꢍꢑꢏꢈꢄꢉꢈꢏꢒꢃꢆꢈ
6
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Chemistry of Materials
UiO, University in Oslo; NU, Northwestern University; TBAPy,
(16) Thompson, D. J.; Zhang, Y.; Ren, T. Polyoxometalate [γꢀ
SiW10O34(H 2O)2]4ꢀ on MCMꢀ41 as catalysts for sulfide
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1,3,6,8ꢀtetrakis(pꢀbenzoate)pyrene; SALI, solvent assisted ligand
incorporation; ICPꢀOES, inductively coupled plasma optical
emission spectroscopy; SEMꢀEDS, scanning electron microscopy
energy dispersive Xꢀray spectroscopy; PXRD, powder Xꢀray difꢀ
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try; BET, Brunauer Emmett Teller; CV, cyclic voltammetry;
MAS NMR, magic angle spinning nuclear magnetic resonance;
DRIFTS, diffuse reflectance infrared Fourier transform spectrosꢀ
copy; CEES, 2ꢀchloroethyl ethyl sulfide; CEESO, 2ꢀchloroethyl
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0
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TOC: A channel-type POM@MOF (PW @NU–1000) was synthesized for the first time. The hybrid
material shows stability and enhanced activity towards the oxidation of organosulfur with hydrogen
peroxide.
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