Thermally induced migration of a polyoxometalate within a metal-organic framework and its catalytic effects

Article abstract of DOI:10.1039/c8ta02562b

The polyoxometalate (POM), H₃PW₁₂O₄₀, was postsynthetically incorporated into the metal-organic framework (MOF), NU-1000. The POM@MOF composite, PW₁₂@NU-1000, was activated under mild conditions, resulting in a material whose diffraction pattern and spectroscopic properties differ from the same material heated at elevated temperatures. These discrepancies, corroborated by difference envelope density analyses, were attributed to the POM residing either in the mesoporous or microporous channels of NU-1000. As a testament to the importance of catalyst accessibility, the POM's locational change also induced a change in the composite's rate and selectivity toward oxidizing 2-chloroethyl ethyl sulfide.

Full text of DOI:10.1039/c8ta02562b

Journal of
Materials Chemistry A
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COMMUNICATION
Thermally induced migration of a polyoxometalate
within a metal–organic framework and its catalytic
effects†
Cite this: DOI: 10.1039/c8ta02562b
Received 19th March 2018
Accepted 10th April 2018
a
b
a
Cassandra T. Buru,
Mercouri G. Kanatzidis,
Ana. E. Platero-Prats, ‡ Daniel G. Chica,
a
b
ac
Karena W. Chapman
and Omar K. Farha*
DOI: 10.1039/c8ta02562b
rsc.li/materials-a
The polyoxometalate (POM),
3 40
H PW12O ,
was postsynthetically
Oen sensitive to its environment and plagued with low
incorporated into the metal–organic framework (MOF), NU-1000. The surface area, polyoxometalates (POMs) are one such class of
POM@MOF composite, PW12@NU-1000, was activated under mild catalyst species that greatly benet from dispersion on solid
13–15
conditions, resulting in a material whose diffraction pattern and supports.
POMs are highly soluble anionic metal oxide clus-
spectroscopic properties differ from the same material heated at ters composed of group V or VI transition metals, which can be
elevated temperatures. These discrepancies, corroborated by differ- partially substituted with virtually any element from the periodic
1
6–19
ence envelope density analyses, were attributed to the POM residing table.
Their ability to undergo multielectron redox trans-
20
either in the mesoporous or microporous channels of NU-1000. As formations allows for applications in redox catalysis. Solid
a testament to the importance of catalyst accessibility, the POM's supports, including activated carbon,
locational change also induced a change in the composite's rate and metal/metal oxide surfaces,
2
1,22
23,24
mesoporous silica,
25,26
27
covalent-organic frameworks,
28
29,30
selectivity toward oxidizing 2-chloroethyl ethyl sulfide.
zeolites, and metal–organic frameworks (MOFs),
have been
used to stabilize POMs under catalytic conditions. The latter four
benet from crystalline structures, thereby permitting rapid
structure determination of the POM/support composite. MOFs,
1
Active catalysts, ranging from single atoms to molecular
2
3
complexes to large nanoparticles, are oen immobilized on
heterogeneous supports in order to improve their reactivity by
preventing aggregation and subsequent deactivation, while
concurrently facilitating their separation from substrate and
product, allowing for reuse of the catalyst material. Deposition
on these supports is most commonly accomplished using elec-
trostatic interactions, encapsulation, adsorption, precipitation,
or covalent tethering. Dependent on the nature of the support
and catalyst, deposited species can adopt several positions and
orientations. Precise knowledge and control over the three
dimensional location of these active species on/in a support is
essential for understanding the role of the support, as location
composed of metal ion or metal oxide nodes connected by
31,32
organic linkers,
are exceptional support materials as their
tunability gives rise to several desirable properties, such as crys-
tallinity, permanent porosity, high surface area, large apertures,
4–6
33
and high chemical and thermal stability.
Targeted assembling strategies toward POM/MOF compos-
ites oen use
a
“bottle around ship” encapsulation
7–9
34–36
37,38
approach
or ion exchange impregnation method
to
direct a catalyst to a desired location; however, these systems
lack the ability to further modify the POM's location post-
synthetically. For example, MIL-101 has a hierarchical pore
˚
˚
˚
structure containing 29 A and 34 A pores connected via 12 A or
10–12
can control reactivity and selectivity in some systems.
˚
39
1
6 ꢀ 15 A apertures. When Keggin-type POMs are incorpo-
rated into MIL-101 via impregnation, POMs are located in the
a
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, larger mesopore only. On the other hand, when POM@MIL-101
Illinois 60208, USA. E-mail: o-farha@northwestern.edu
is synthesized via encapsulation, the POMs are directed not only
to the larger pores, but also to the smaller pores, which have
apertures small enough to prevent the POM from leaching. The
location of the POMs was found to have slight effects on the
rates of reactions like acetaldehyde–phenol condensation and
b
X-ray Science Division, Advanced Photon Source, Argonne National Laboratory,
Argonne, Illinois 60439-4858, USA
c
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah
2
1589, Saudi Arabia
†
Electronic supplementary information (ESI) available: Materials, methods and
40,41
the acetalization of benzaldehyde.
In hopes of synthesizing a more dynamic system, we recently
instrumentation, synthesis details, SEM-EDS, in situ PXRD, DED, TGA, DSC,
NMR, CV, total DRUV-vis, catalytic procedure, selectivity, 1 mol% reaction
proles, leach test, post-catalysis PXRD. See DOI: 10.1039/c8ta02562b
3ꢁ
reported the incorporation of the Keggin-type POM, [PW12O ] ,
40
‡
Current address: Department of Inorganic Chemistry, Universidad Aut ´o noma
into the microporous channels of the csq-net MOF, NU-1000
de Madrid, 28049 Madrid, Spain.
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4
2
ꢂ
(
PW12@NU-1000, Fig. 1). In the report, the material was acti-
Both PW12@NU-1000-scCO
2
and PW12@NU-1000-120
C
ꢂ
vated at 120 C under vacuum prior to characterization, and the have a maximum loading of 0.8 POMs per Zr node by induc-
6
resulting material's crystallinity resembled that of the parent NU- tively coupled plasma optical emission spectroscopy (ICP-OES)
43
1
000. Slight differences between the MOF and POM@MOF measurements of the acidic piranha-digested materials. Scan-
powder X-ray diffraction (PXRD) patterns were analyzed using ning electron microscopy (SEM) images indicate that the
difference envelope density (DED) analyses, which concluded integrity and morphology of the crystallites are maintained
that the electron density corresponding to the POM guest lied during POM incorporation, and energy dispersive spectroscopy
˚
within the 12 A microporous channels of NU-1000 and not the (EDS) line scans show a near uniform distribution of POM
˚
3
1 A mesoporous channels. The hierarchical framework allowed throughout the framework, except in the center of the crystal,
43,48
substrate diffusion and enhanced sulde oxidation relative to the where known defects occur (Fig. S1†).
POM or MOF alone, while the interactions between the frame-
work and POM were stable to leaching.
The powder X-ray diffraction (PXRD) pattern of PW @NU-
12
1000-120 C resembles that of the parent NU-1000 (Fig. 2).
ꢂ
In the present work, we report the mild activation of However, the PXRD pattern of PW12@NU-1000-scCO
2
has peaks
ꢂ
PW12@NU-1000 and the movement of the incorporated POM at similar d-spacing to PW12@NU-1000-120 C, but very
during heat exposure. The inuence of the POM location on different intensities. Assuming no preferred orientation of MOF
catalytic rate and selectivity of the composite are also investi- crystallites and retention of MOF structure, the peak intensities
gated. To our knowledge, this is the rst system where the indicate major differences in guest location within the same
location of POMs within a MOF can be monitored and unit cell. To understand the structural changes, in situ variable
controlled postsynthetically.
temperature PXRD patterns were measured (Fig. S2†). The
ꢂ
2,44
NU-1000 and PW @NU-1000-120 C were synthesized via PW @NU-1000-scCO sample was loaded into a rotating
1
2
12
2
4
ꢂ
previously published procedures.
was prepared in a similar manner to PW12@NU-1000-120 C, PW12@NU-1000-scCO
only differing in the method of solvent evacuation. Briey, NU- 120 C pattern was observed. This change was not reversible; the
PW12@NU-1000-scCO
2
capillary and heated to and held at 120 C. The evolution of the
ꢂ
2
sample pattern to the PW12@NU-1000-
ꢂ
ꢂ
1000 was soaked in an aqueous POM solution for three days. material retained the PW12@NU-1000-120 C PXRD pattern
The composite was washed rigorously with water and anhy- upon cooling to room temperature.
drous ethanol before activation by supercritical CO drying Inspired by the observed transformation in PXRD patterns,
PW @NU-1000-120 C can also be differential envelope density (DED) analyses were performed.
2
45–47
ꢂ
(details in the ESI†).
1
2
synthesized from PW @NU-1000-scCO by heating the mate- DED analysis uses high intensity, low angle diffraction peaks
1
2
2
ꢂ
rial in a 120 C oven for at least one hour.
generated by a synchrotron X-ray source to create an electron
envelope. If the envelope of the parent material is known, its
subtraction from the composite material results in a map of
electron density corresponding to the guest molecules within
49,50
a known structure.
By applying this technique to PW12@NU-
1
000 composites, the electron density corresponding to the
˚
POMs were located close to the c-pore, the 8 ꢀ 10 A windows in
Fig. 1 which connect the large and small channels, and in the
Fig. 1 Structural representations (left) of the PW12@NU-1000 previ-
ꢂ
ously published, also referred to as PW12@NU-1000-120 C. Structures
3ꢁ
of the corresponding POM [(PW12
O
40
)
], MOF node [(Zr
], and MOF linker [1,3,6,8-tetrakis(p-benzoate)
pyrene, (TBAPy) ] given (right). Light blue prisms ¼ WO , pink prisms Fig. 2 PXRD patterns for NU-1000 and PW12@NU-1000 as-synthe-
PO , and
, green ¼ Zr, red ¼ O, gray ¼ C, hydrogen atoms omitted for sized without solvent removal, activated by supercritical CO
6 3 4
(m -O)
8+
3 4 2 4 4
(m -OH) (H O) (OH) )
4
ꢁ
5
¼
4
2
ꢂ
activated at 120 C.
clarity.
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hexagonal mesoporous channels of PW12@NU-1000-scCO
2
(
Fig. 3 and S3†), instead of residing in triangular microporous
ꢂ
42
channels like in PW @NU-1000-120 C. In both cases, the
1
2
POMs resided on the same plane as the MOF nodes. Of note, the
electron density in Fig. S3† has six equally spaced areas around
the mesopore corresponding to the POM guests' locations. The
POMs likely are disordered over different orientations centered
at these six sites. Additionally, if each of these sites were 100%
occupied, then the expected POM loading would be 2
POMs/node. Because ICP-OES measurements calculated only
a 0.8 POM/node loading, the electron clouds represent
approximately 40% POM occupancy (idealized in Fig. 3).
This location change manifests in other characterization
techniques as well. Volumetric N
density functional theory (DFT) pore size distributions (Fig. 4)
reveal that PW12@NU-1000-scCO has a reduced mesopore
2
sorption isotherms and
2
ꢂ
volume, while PW12@NU-1000-120 C has reduced micropore Fig. 4 (a) Volumetric N
2
isotherms collected at 77 K, (b) DFT-calcu-
lated pore size distribution. (c) Diffuse reflectance UV-vis spectra, and
volume. These observations are in agreement with partial
occupation of the mesopores or micropores in the respective
samples. The Brunauer–Emmett–Teller (BET) areas of these
materials are 1020, 850, and 700 m cm
PW12@NU-1000-120 C, and PW12@NU-1000-scCO
(d) emission spectra at 380 nm excitation of NU-1000, PW12@NU-
ꢂ
1
2
000-scCO , and PW12@NU-1000-120 C.
2
ꢁ3
for NU-1000,
, respec-
ꢂ
2
tively. The differences in surface area between the two POM@- assigned to the loss of water that is hydrogen bonded to the
51
MOF samples are attributed to the additional 5 wt% water acidic protons of the POM. Around the same temperature in
present in the supercritical CO activated sample, determined the differential scanning calorimetry (DSC) curve a slight
by thermogravimetric analysis (TGA) (Fig. S4a†). In the exothermic change in the heat ow rate of PW @NU-1000-
2
1
2
PW @NU-1000-scCO sample, a small mass loss occurs around scCO could correlate to the POM movement from the meso-
1
2
2
2
ꢂ
70 C, which has been observed with free H PW O , could be pore to the micropore (Fig. S4b†). This exothermicity is
3 12 40
1
consistent with POM positioned at an intrinsically favored
binding site, possibly due to increased van der Waals interac-
tions, as the POM is surrounded by only one pyrene linker in the
mesopore and three pyrene linkers in the micropore. Interest-
ingly, these interactions appear to be strong enough to immo-
bilize the POM in the MOF, while also weak enough to allow
POM movement.
Based on these observations, we believe the POMs sit in the
mesopores close to the c-pore, when synthesized and remain
ꢂ
there if no heat is applied (up to 80 C). POM movement to the
micropores is facilitated by elevated temperature coupled with
partial removal of the POM's waters of hydration. Because of the
consistent distribution of POM in the crystallite and its ability to
change location in the absence of solvent, we propose that the
POM migrates through the c-pore of NU-1000 to the more
thermodynamically favorable micropore upon application of
sufficient heat.
The different interactions of the POM and MOF as a function
3
1
of location were also investigated spectroscopically. The
P
MAS NMR spectrum of PW12@NU-1000-scCO is identical to
2
3
1
free H
3
PW12O
40 (Fig. S6†). The P signal shis and become
broader in PW12@NU-1000-120 C, indicating loss of symmetry
around the phosphorous and suggesting a strong interaction
ꢂ
52–54
between the POM and the support.
In the cyclic voltam-
mograms, the reduction events shi to more positive potentials
ꢂ
from H PW O to PW @NU-1000-120 C to PW @NU-1000-
3
12 40
12
12
Fig. 3 Structural representation of one possible POM conformation in
PW12@NU-1000-scCO
inferred by DED analysis. Light blue prism ¼
WO , green ¼ Zr, red ¼ O, gray ¼ C, hydrogen atoms
, pink prism ¼ PO
omitted for clarity.
scCO (Fig. S7†), again indicating strong interaction of the POM
with the pyrene since the composite is easier to reduce than the
2
2
5
4
29,55
POM alone;
bare NU-1000 has no redox activity in the
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window scanned. Contrasting to the spectrum of bare NU-1000, product (Fig. S8†). Meanwhile under identical conditions,
62–64
a charge transfer band emerges in the diffuse reectance UV-vis H PW O was likewise active
with a half-life of 5 min and
3
12 40
(
DRUV-vis) spectra of both PW @NU-1000 samples (Fig. 4c). In preferred selectivity for the singly-oxidized product. The
1
2
ꢂ
conrming the charge transfer mechanism, by uorescence composite PW @NU-1000-120 C, normalized to the total
1
2
ꢂ
emission spectroscopy, PW12@NU-1000-120 C and PW12@NU- number of active clusters (POMs and MOF nodes), decreased
000-scCO quenched the pyrene's uorescence 95% and 99%, the half-life of the reaction to 3 min with an intermediate
further reduces
1
2
respectively, aer excitation at 380 nm (Fig. 4d). These data selectivity of 59 ꢃ 7%. PW12@NU-1000-scCO
2
suggest an efficient electron transfer from the excited pyrene- the half-life of CEES conversion to 1 min with a greater selec-
based linkers to the POM similar to what was observed when tivity (90 ꢃ 5%) for the preferred singly oxidized product.
56
nickel bis(dicarbollide) was installed in NU-1000. Of note, the Because the initial rate of reaction using 2 mol% catalyst was
evidence of interactions with the linker and the POM's position too fast for time points with low conversion to be collected
does not rule out potential interactions with the zirconia-like reliably, initial turnover frequencies (TOFs) were therefore
25,57
MOF nodes.
determined using 1 mol% catalyst (Fig. S9†). The TOF of
With the knowledge that different activation procedures site PW12@NU-1000-scCO
the POM in either the mesopores or micropores and affect the than that of PW12@NU-1000-120 C.
2
was found to be about 3 times higher
ꢂ
electronic structure, we attempted to determine if the location
The difference in selectivity has been attributed to the
impacted a substrate's accessibility to the catalytic sites. As different mechanisms which occur on the MOF node or the
6
0,65,66
a model reaction, we chose to study the oxidation of 2-chloroethyl POM.
The intermediate selectivity when using PW @NU-
12
ꢂ
ethyl sulde (CEES) to 2-chloroethyl ethyl sulfoxide (CEESO) and 1000-120 C could indicate that both the POM and MOF are
2
-chloroethyl ethyl sulfone (CEESO ) (Fig. 5a). CEES is a simulant accessible. The micropores are blocked so access to the POM
2
of the chemical warfare agent (CWA) mustard gas (HD or sulfur requires diffusion through the windows connecting the channels,
mustard), and one possible pathway for detoxication of HD which has been observed and calculated in other NU-1000-based
58
67,68
involves oxidation of the central thioether to a sulfoxide.
systems.
When the POMs are situated in the mesopores,
However, over-oxidation to the sulfone yields another toxic diffusion of the substrate to POM is no longer hindered; and
59
compound. Therefore, selectivity is paramount when designing therefore, the sulde readily reacts with the POM to produce the
materials for HD detoxication via oxidation. singly-oxidized product almost exclusively. Since these reactions
42
ꢂ
Our previous ndings showed that in a 45 C acetonitrile are normalized to the number of active clusters, the increased
solution using H O as an oxidant, NU-1000 nodes, with activities of the composites compared to the individual compo-
2
2
60,61
a structure similar to zirconia,
catalyzed the oxidation of nents alone are attributed to the stabilization of in the POM on
CEES with a half-life (time to 50% conversion) of 13 min the MOF. Leach tests conrm these reactions occur heteroge-
(Fig. 5b) and preferred selectivity toward the doubly-oxidized neously (Fig. S9†) and post-catalysis PXRD patterns indicate the
POMs do not move during catalysis (Fig. S10†).
Conclusions
PW @NU-1000 has been synthesized by postsynthetic incor-
1
2
3
ꢁ
poration of [PW O ] in NU-1000. The composite material
was activated by using supercritical CO and by 120 C under
1
2 40
ꢂ
2
vacuum. Differences in the diffraction patterns of these mate-
rials suggested that the POMs are located in the mesopores
when supercritical CO is used to evacuate the pores of solvent
2
molecules and migrate to the micropores when heated to
ꢂ
1
20 C; these structural changes were corroborated by sorption
and spectroscopic properties. PW @NU-1000-scCO displayed
1
2
2
a faster rate of reaction and higher product selectivity in the
partial oxidation of CEES, a mustard gas simulant than the
ꢂ
same material activated at 120 C. To our knowledge, this is the

rst system where a POM catalyst can be monitored and
controlled postsynthetically within a MOF. These ndings
highlight the importance of knowing and controlling catalyst
location to engender favorable synergistic effects between
a catalyst and support. Current efforts aim to identify other
systems with similar control over catalyst location.
Fig. 5 (a) Scheme of the reaction pathway for the oxidation of CEES to
CEESO and CEESO
Reaction profiles for the reaction in (a) using the catalysts: NU-1000,
2
under the conditions presented in this work. (b) Conflicts of interest
ꢂ
H PW12O
3
40, PW12@NU-1000-120 C, and PW12@NU-1000-scCO
2
.
There are no conicts to declare.
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Wiley & Sons, Inc., Hoboken, NJ, 2013, pp. 263–319.
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