Vapor-Phase Cyclohexene Epoxidation by Single-Ion Fe(III) Sites in Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.0c03364

Heterogeneous catalysts supported on metal-organic frameworks (MOFs), which possess uniform porosity and crystallinity, have attracted significant interest for recent years due to the ease of active-site characterization via X-ray diffraction and the subs

Full text of DOI:10.1021/acs.inorgchem.0c03364

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Vapor-Phase Cyclohexene Epoxidation by Single-Ion Fe(III) Sites in
Metal−Organic Frameworks
Ken-ichi Otake,∥ Sol Ahn,∥ Julia Knapp, Joseph T. Hupp, Justin M. Notestein, and Omar K. Farha*
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ABSTRACT: Heterogeneous catalysts supported on metal−organic frameworks
(MOFs), which possess uniform porosity and crystallinity, have attracted
significant interest for recent years due to the ease of active-site characterization
via X-ray diffraction and the subsequent relation of the active site structure to the
catalytic activity. We report the syntheses, structures, and oxidation catalytic
activities of single-ion iron catalysts incorporated into the zirconium MOF NU-
1000. Single-ion iron catalysts with different counteranions were anchored onto
the Zr node through postsynthetic solvothermal deposition. Crystallographic
characterization of the resulting MOFs (NU-1000-Fe-Cl and NU-1000-Fe-NO3)
revealed that, while both frameworks have similar Fe coordination, the distance
between Fe and the Zr6 node differs significantly between the two. The product rate profiles of the two catalysts for vapor-phase
cyclohexene epoxidation demonstrate different initial rates and product formations, likely originating from the different Fe−O
distances.
environmental toxicity. As with many heterogeneous catalysts,
the investigation of a structure−activity relationship in most
iron catalysts is challenging.27 One possible solution is to
generate single-site iron catalysts, which can lead to an
atomically precise understanding of the catalyst design.
Consequently, we have focused on synthesizing single-ion
iron catalysts supported on MOFs.
In this study, we incorporated single-ion-based iron catalysts
incorporated in the highly crystalline and porous MOF NU-
1000 ([Zr6(μ3-O)4(μ3-OH)4(OH)4(H2O)4(TBAPy)2]∞
(TBAPy = 1,3,6,8-(p-benzoate)pyrene), which has Zr6-type
oxide cluster nodes and high porosity (Figure 1).21,28 Because
NU-1000 contains spatially oriented −OH groups pointing
into its mesopores and c pores, various catalytic species have
been installed on NU-1000 to investigate their catalytic
performance.17,22,29−31 Herein, we deposited iron species
inside this MOF using SIM with two different iron precursors,
Fe(NO3)3 and FeCl2, and thoroughly characterized the species
using a combination of analytical and spectroscopic
techniques. In the resulting MOFs (denoted as NU-1000-Fe-
NO3 and NU-1000-Fe-Cl hereafter), single-ion iron(III)
species were observed on the Zr6 node with different Fe−O
distances. Their catalytic properties were investigated through
INTRODUCTION
■
As a new class of crystalline porous materials, porous
coordination polymers (PCPs) or metal−organic frameworks
(MOFs)1,2 have been extensively investigated for a wide range
of applications such as gas storage,3,4 purification,5,6 proton
conduction,7,8 sensing,9,10 drug delivery,11,12 and heteroge-
neous catalysis.13,14 MOFs have a crystalline porous network
structure composed of metal ions/clusters bridged by organic
ligands. Through judicious choices of structural components,
the structural and physical properties of MOFs can be fine-
tuned.15,16 Due to these advantages, MOFs have emerged as
prominent candidates for catalytic supports.17 Typically, the
characterization of deposited species upon conventional
catalyst supports, such as metal oxides, tends to be challenging
due to the nonuniform surface and pore structures of the
support. In contrast, the crystalline nature of MOFs enables
visualization of the catalytically active species within the
framework, which leads to a detailed characterization of active
catalytic sites and provides insight into structure−activity
relationships.18 From this perspective, several groups have
been investigating newly installed catalytic species in MOFs,
which can be introduced using postsynthetic modification
methods such as solvothermal deposition in MOFs (SIM) and
atomic-layer deposition in MOFs (AIM).19−23
Separately, alkene oxidation catalysts have been of significant
importance for decades, due to their potential applications in
direct conversion of low-cost feedstocks, such as products
obtained from the steam cracking of natural gas and naphtha,
to valuable oxygenated products.24−26 Among several
candidate catalysts, iron catalysts have attracted particular
interest owing to their low cost, high availability, and low
Received: November 12, 2020
Published: January 26, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03364
© 2021 American Chemical Society
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Figure 1. Crystal structures of nonfunctionalized NU-1000. (a) View of the ab plane. Mesopores with a channel size of ∼31 Å and so-called
“triangle pores” with a channel size of 10 Å are shown. (b) View along the c axis. Pores with a channel size of 10 Å are shown, which are the so-
called “c-pores”. (c) Structures of the inorganic nodes and the organic linkers.
X-ray Photoelectron Spectroscopy (XPS). XPS measurements
were carried out with a Thermo Scientific ESCALAB 250 Xi
instrument (Al Kα radiation, 1486.6 eV) operated at 14.6 kV and 11.5
mA. An electron flood gun was utilized prior to the scans. Samples
were ground and spread on conductive carbon adhesive tape attached
to sample holders. The carbon 1s binding energy of graphite in the
carbon tape (284.6 eV) was used to calibrate binding energies. Curve-
fitting analyses were performed with XPS PEAK4.1 software by using
a combination of Gaussian and Lorentzian line shapes.
alkene oxidation reactions with hydrogen peroxide. We found
that they initially exhibit different catalytic behavior, which
could be associated with the difference in the Lewis acidity of
the Fe catalytic sites, but ultimately achieve the same level of
catalytic activity. Our study of the structure−property
relationship of isostructural MOF-based catalysts opens the
door for the rational design of MOF-based catalysts through
structural fine tuning.
Single-Crystal X-ray Crystallography (SCXRD). X-ray crystal
diffraction data were carried out using a Bruker Kappa APEX II CCD
detector equipped with a Mo Kα (λ = 0.71073 Å) IμS microfocus
source with MX optics. The single crystals were mounted on
MicroMesh (MiTeGen) with Paratone oil. The structure was solved
by intrinsic phasing methods (SHELXT-2014/5)32 and refined by
full-matrix least-squares refinement on F2 (SHELXL-2017/1)33 using
the Yadokari-XG software package.34 Refinement results are
summarized in Table S1. Crystallographic data in CIF format have
been deposited with the Cambridge Crystallographic Data Centre
(CCDC) under deposition numbers CCDC 2036844 and 2036845.
The data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.).
Catalysis. Reaction tests were performed at 120 °C in a 1/1 ratio
of reactant (cyclohexene) and oxidant (H2O2), with both reactants in
the vapor phase. A detailed description of the reactor was reported in
prior work from our team.35 Catalysts (10 mg) were diluted in 200
mg of quartz sand, and the catalyst beds were supported on quartz
wool in a quartz tube reactor. Cyclohexene (Sigma-Aldrich, ≥99.0%)
was introduced by flowing He (5 mL/min, 99.999%, Airgas) through
a quartz bubbler at 25 °C, giving a saturated vapor pressure of 11.9
kPa. He (20 mL/min) was added to set the cyclohexene partial
pressure at 3 kPa. Four molar H2O2 in acetonitrile solution, previously
dried over magnesium sulfate,36 was loaded in a plastic syringe and
injected directly into the reactor with a rate of 0.2 mL/h,
corresponding to 3 kPa of H2O2 partial pressure. The H2O2 solution
only contacted polypropylene and perfluorinated materials to
minimize decomposition. C6 products (cyclohexene oxide, trans-1,2-
cyclohexanediol, 2-cyclohexen-1-ol) and the reactant were detected
and separated using a gas chromatograph (GC) (Agilent 7890) with
an HP-INNOWAX (50 m length, 0.2 mm diameter, 0.4 μm film)
column, and signal intensities were calibrated using authentic
standards. Concentrations of cis-1,2-cyclohexanediol and 2-cyclo-
hexen-1-one, if present, were below the detection limit of 0.001 mol
%.
EXPERIMENTAL METHODS
■
Chemicals. Reagents and solvents were purchased from Fisher
Scientific Co., Ltd., and Sigma-Aldrich Chemical Co., Ltd., and used
without further purification.
Preparation of NU-1000. NU-1000 was prepared by the
published procedures using diethylformamide as the solvent.28 A 60
mg portion of as-synthesized NU-1000 was then suspended in 20 mL
of N,N-dimethylformamide (DMF) and 0.5 mL of 12 M HCl and
heated at 120 °C for at least 12 h to remove acids and the synthetic
modulators on the node.
Syntheses of Catalysts. For the synthesis of NU-1000-Fe-NO3,
60 mg of NU-1000 was immersed in 20 mL of a 0.05 M Fe(NO3)3
solution in methanol for 2 days at room temperature and then washed
three times with 20 mL of fresh methanol. A dark yellow powder was
obtained. For the synthesis of NU-1000-Fe-Cl, 60 mg of NU-1000
was immersed in 20 mL of a 0.03 M FeCl2 solution in DMF for 2 days
at room temperature and then washed three times with fresh DMF
and three times with fresh acetone. An orange powder was obtained.
Inductively Coupled Plasma−Optical Emission Spectrosco-
py (ICP-OES). ICP-OES spectra were recorded using a iCAP 7600
ICP-OES analyzer calibrated with standard solutions. Samples (ca. 1
mg) were digested in nitric acid (70%, 0.75 mL) and hydrogen
peroxide (35% in H2O, 0.25 mL) at 150 °C for 5 min in a microwave
reactor.
Diffuse Reflectance Ultraviolet−Visible Spectroscopy (UV−
vis). Diffuse reflectance UV−vis spectra were collected using a
Shimadzu UV-3600 instrument with a Harrick Praying Mantis diffuse
reflectance accessory. CaF2 powder was used as a reflector for baseline
measurements, and samples were diluted in CaF2 powder for the
measurements. The obtained reflectance spectra were converted to
absorption spectra according to the Kubelka−Munk function F(R∞).
Diffuse Reflectance Infrared Fourier Transform Spectros-
copy (DRIFTS). DRIFT spectra were recorded from a sample diluted
in KBr powder on a Nicolet 6700 instrument equipped with KBr
optics and a Praying Mantis diffuse reflectance accessory.
Sorption Study. N2 sorption isotherms were collected on a
Micromeritics Tristar II 3020 instrument at 77 K. Prior to the
measurement, the sample was activated on a SmartVacPrep port by
heating at 120 °C under vacuum for at least 12 h. Pore-size
distributions were calculated from these isotherms using a DFT
method, based on a molecular statistical approach.
The product yield was calculated as per eq 1 and then was used to
obtain the conversion rate.
molproduct
product yield =
molreactant + ∑ molproduct
(1)
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RESULTS AND DISCUSSION
of NU-1000-Fe-NO3 and NU-1000-Fe-Cl, respectively. In the
DRIFT spectra (Figure S2), red shifts and decreases in
intensity of the peaks associated with terminal and bridging
−OH stretches on the Zr6 node at 3670 cm−1 were observed
for both NU-1000-Fe-NO3 and NU-1000-Fe-Cl. These
features suggest chemisorption of the iron species onto the
node in both cases. In addition, for NU-1000-Fe-NO3, a
relatively strong peak at 1270 cm−1 was observed, which was
attributed to the stretching vibration mode of nitrate anions,
suggesting that one or more nitrate ions remain coordinated to
Fe after SIM. To verify the porosity of NU-1000-Fe-NO3 and
NU-1000-Fe-Cl, N2 sorption isotherm measurements were
conducted (Figure 3). Decreases in Brunauer−Emmett−Teller
(BET) area from 2120 m2/g (bare NU-1000) to 1480 m2/g
for NU-1000-Fe-NO3 and 1300 m2/g for NU-1000-Fe-Cl
were observed. In addition, density functional theory (DFT)
calculated average pore size distributions showed only small
decreases in the mesopore and c-pore volumes, implying that
the framework porosity remained intact. XPS measurements
were completed to determine the oxidation states of the
incorporated iron species in the MOFs (Figure S3). The Fe 2p
core-level spectra of NU-1000-Fe-NO3 and NU-1000-Fe-Cl
showed single-component peaks with binding energies of 712.2
eV (2p3/2; satellite at ∼719 eV) and 725.3 eV (2p1/2; satellite
at ∼733 eV) for NU-1000-Fe-NO3; for NU-1000-Fe-Cl, these
peaks were present at 711.8 eV (2p3/2; satellite at ∼719 eV)
and 724.9 eV (2p3/2; satellite at ∼733 eV). These observed
binding energies are similar to those of reported Fe(III)
compounds,37 and so the oxidation state of Fe in both
frameworks was assigned as III. The result indicated that the
Fe(II) precursor of NU-1000-Fe-Cl was oxidized to Fe(III)
when it was incorporated on the nodes of MOFs, probably due
to air exposure.
■
Syntheses and Characterization. We synthesized NU-
1000-Fe-NO3 and NU-1000-Fe-Cl using solution-phase
postsynthetic metalation. NU-1000 was exposed to a solutions
of either Fe(NO3)3 or FeCl2 in dimethylformamide at room
temperature. Clear color changes from the characteristic yellow
of NU-1000 to yellow-orange (NU-1000-Fe-NO3) or to
orange (NU-1000-Fe-Cl)) were observed after treatment
(Figure 2a), visually indicating the successful incorporation
Figure 2. (a) Photographic images of pristine NU-1000 (left), NU-
1000-Fe-NO3 (middle), and NU-1000-Fe-Cl (right). (b) PXRD
patterns (after activation) of NU-1000 (black), NU-1000-Fe-NO3
(blue), and NU-1000-Fe-Cl (red).
To obtain further structural insights into the incorporated
iron species in the MOFs, single-crystal X-ray diffraction
analyses on NU-1000-Fe-NO3 and NU-1000-Fe-Cl were
conducted (Figure 4, Figures S4−S6, and Table S1). Following
the refinement of the underlying NU-1000 framework, the
residual electron densities were calculated to identify and refine
the Fe locations and occupancies. We used ShleXLe38 and
PLATON39 software to visualize the electron densities that are
assignable to Fe species (Figure 4 and Figure S5).
Interestingly, the structures of the deposited iron ions were
found to be different between NU-1000-Fe-NO3 and NU-
of iron species inside the MOFs. The PXRD patterns of both
frameworks showed that the underlying NU-1000 framework
remained crystalline after SIM (Figure 2b). Consistent with
the color changes, the diffuse reflectance UV spectra showed
ligand to metal charge-transfer (LMCT) peaks centered at
around 3.0 and 4.0 eV (Figure S1). ICP-OES measurements
indicated Fe loadings of 0.5 0.1 and 2.2 0.2 per Zr6 node
Figure 3. (a) N2 adsorption (filled symbols) and desorption isotherms (open symbols) of NU-1000 (black squares), NU-1000-Fe-NO3 (blue
diamond), and NU-1000-Fe-Cl (red circles) (b) DFT-calculated pore size distributions of NU-1000 (black squares), NU-1000-Fe-NO3 (blue
diamonds), and NU-1000-Fe-Cl (red circles).
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Figure 4. Fo − Fc contoured Fourier maps around the Zr6 node of NU-1000-Fe-NO3 calculated before assigning iron species. Shown are (a) slicing
planes, (b) the contoured map in the (1−10) plane, and residual densities observed (c). The contoured map in the plane shifted 1.2 Å from the
(1−10) plane; the residual electron densities due to iron species are visualized, (d) Contoured map in the (110) plane. (e) Contoured map in the
(001) plane. The contours are from −1.80 to 0.90 e Å−3 in steps of 0.10 eÅ−3 for (b), from −0.80 to 1.80 e Å−3 in steps of 0.10 e Å−3 for (c), from
−1.00 to 1.40 e Å−3 in steps of 0.20 e Å−3 for (d), fand rom −1.50 to 2.50 e Å−3 in steps of 0.10 e Å−3 for (e). PLATON software was used for the
calculations.
Figure 5. Crystal structures of NU-1000-Fe-NO3 and NU-1000-Fe-Cl (100 K): (a) node structure of NU-1000-Fe-NO3; (b) 3D structure of NU-
1000-Fe-NO3 viewed on the ab plane; (c) node structure of NU-1000-Fe-Cl; (d) 3D structure of NU-1000-Fe-Cl viewed along the c axis.
Hydrogen atoms and nitrate and chloride anions are omitted for clarity.
1000-Fe-Cl (Figure 5a,b). For NU-1000-Fe-NO3, two
crystallographically nonequivalent Fe sites were observed.
The first site, denoted Fe1, resides in the c-pore and is
coordinated to the Zr6 node through both terminal and
bridging oxygens (O5 and O3, respectively). The second site,
denoted Fe2, is found in the hexagonal mesopore and exhibits
very similar binding to the node through O6 (terminal) and
O4 (bridging). The crystallographically estimated loadings of
the Fe sites were 0.32 (Fe1) and 0.40 (Fe2) per Zr6 node,
where the total crystallographically determined iron content
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(0.72 Fe/Zr6) is in good agreement with that obtained from an
hexenol. Mechanistic details have previously been reported
thoroughly.36,43,44 Cyclohexenone is often formed in an
approximately 1/1 ratio with cyclohexenol, but in this study,
we only observed cyclohexenol. Cyclohexenone may have been
present but was at levels below the GC detection limits.
Product rate profiles of the two catalysts present different
behaviors initially but then show both catalysts eventually
reaching steady-state rates (Figure 6 and Figure S7).
Interestingly, neither catalyst fully oxidizes the reactant to
CO2, unlike the case in our previous study, which may be due
to low conversion by both catalysts.35 Additionally, the radical
pathway reaction that eventually yields CO2 may be slower
than decomposition of H2O2 to H2O over these catalysts.
More importantly, NU-1000-Fe-NO3 initially yields direct
products at much higher rates than NU-1000-Fe-Cl, but these
rates rapidly decrease to a steady state by ∼3 h time on stream.
This change in rate and selectivity suggests restructuring of the
active site. Since isolation of the Fe sites from one another can
be speculated, as seen in the ICP results (2.5 and 0.6 per Zr6
node for NU-1000-FeCl and NU-1000-Fe-NO3, respectively),
we speculate that the difference in initial rates is related to the
difference in Fe−O distances. The Fe sites present in NU-
1000-Fe-NO3, as evidenced by their stronger interactions with
electrogenative oxygen, may be stronger Lewis acid sites than
the Fe sites in NU-1000-Fe-Cl.
ICP-OES analysis (0.5
0.1 Fe/Zr6). The observed Fe−O
distances between Fe and the Zr node are 2.02 Å (Fe1−O5),
2.77 Å (Fe1−O3), 2.43 Å (Fe2−O6), and 2.60 Å (Fe2−O4).
These distances point toward strong interactions between the
Fe sites and Zr node for NU-1000-Fe-NO3.40−42 The location
of any nitrate anions could not be determined due to their
disordered nature and low site occupancies. In the case of NU-
1000-Fe-Cl, single-ion Fe species were observed at positions
that were more than 3.0 Å away from the oxygen sites of Zr
nodes (Figure 5c,d). There are two crystallographically
independent Fe sites (Fe1 and Fe2); their occupancies were
estimated to be 0.10 (Fe1, c-pore), 0.15 (Fe2, mesopore),
which correspond to 0.80 and 1.20 irons per Zr6 node. The
total crystallographically determined Fe content (2.0 Fe/Zr6)
is in good agreement with that obtained from an ICP-OES
analysis (2.2
0.2 Fe/Zr6). The observed Fe−O distances
between Fe and the Zr node are 3.13 Å (Fe1−O5), 3.57 Å
(Fe1−O1), 3.50 Å (Fe1−O3), 3.08 Å (Fe2−O6), 3.54 Å
(Fe2−O2), and 3.41 Å (Fe2−O4). Thus, relatively weak
interactions between Fe sites and the Zr node are implied for
NU-1000-Fe-Cl.
Catalysis. Vapor-phase cyclohexene epoxidation with
vaporized H2O2recently developed by our team35was
used to compare the reactivity and selectivity for the two
catalysts. Simplified reaction pathways are given in Scheme 1,
CONCLUSION
■
Scheme 1. Proposed Mechanism of Cyclohexene Oxidation
Two single-ion iron catalysts with different counteranions were
synthesized by postsynthetic metalation of NU-1000 using
Fe(NO3)3 or FeCl2. The resulting MOFs, NU-1000-Fe-NO3
and NU-1000-Fe-Cl, retained the structural characteristics of
bare NU-1000 and exhibited isolated Fe(III) single-ion sites
after deposition. SCXRD studies revealed that both frame-
works had two crystallographically nonequivalent Fe sites
coordinating to the bridging and terminal oxygens of the Zr6
node, with the Fe−O distances of NU-1000-Fe-Cl being much
longer than those of NU-1000-Fe-NO3. Vapor-phase cyclo-
hexene epoxidation by NU-1000-Fe-NO3 yields direct
products initially, whereas NU-1000-Fe-Cl exclusively yields
a mixture of direct products and byproducts; as time passes,
NU-1000-Fe-NO3 achieves a steady state similar to that of
NU-1000-Fe-Cl. This behavior is attributed to the initial
presenting two proposed H2O2 activation mechanisms. In
short, heterolytic activation of H2O2 yields cyclohexene
epoxide, which in turn hydrolyzes rapidly to trans-cyclo-
hexanediol, while homolytic activation of the oxidant produces
the radical oxidation products cyclohexenone and cyclo-
Figure 6. Time on stream C6 product rate profiles of cyclohexene epoxidation with H2O2 at 120 °C, with cyclohexene 3 kPa and H2O2 3 kPa over
(a) NU-1000-Fe-Cl and (b) NU-1000-Fe-NO3.
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difference in Fe−O distances between the frameworks. Further
investigations are in progress to clarify how this initial
difference affects the catalytic behavior. We anticipate that
the observation of different rates and product formation based
on the metal−node distance, as well as the occurrence of
active-site rearrangement, will provide insight into the design
of more selective, stable, and efficient MOF-supported single-
site catalysts.
(ICEP), funded by the DOE, Office of Basic Energy Sciences
(award number DE-FG02-03ER15457), for the synthesis of
the catalysts. K.O. acknowledges Early-Career Scientists
(JP19K15584) from the Japan Society of the Promotion of
Science for financial support. S.A. acknowledges financial
support from the Inorganometallic Catalyst Design Center, an
EFRC funded by the DOE, Office of Basic Energy Sciences
(DE-SC0012702), for the catalysis part. This work made use of
the IMSERC X-ray facility at Northwestern University, which
has received support from the Soft and Hybrid Nano-
technology Experimental (SHyNE) Resource (NSF ECCS-
1542205), the State of Illinois, and the International Institute
for Nanotechnology (IIN). Also, this work made use of the
Keck-II facility of Northwestern University’s NUANCE
Center, which has received support from the Soft and Hybrid
Nanotechnology Experimental (SHyNE) Resource (NSF
ECCS-1542205), the MRSEC program (NSF DMR-
1720139) at the Materials Research Center, the International
Institute for Nanotechnology (IIN), the Keck Foundation, and
the State of Illinois, through the IIN. MOFkey:45
Zr.HVCDAMXLLUJLQZ.MOFkey-v1.csq (NU-1000).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03364.
■
sı
Synthesis and characterization data, including DRIFTS,
UV, and XPS spectra, and crystallographic data and
details of the refinement for NU-1000-Fe-NO3 and NU-
1000-Fe-Cl (PDF)
Accession Codes
CCDC 2036844−2036845 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Omar K. Farha − Department of Chemistry and Department
of Chemical and Biological Engineering, Northwestern
University, Evanston, Illinois 60208, United States;
orcid.org/0000-0002-9904-9845; Email: o-farha@
northwestern.edu
Authors
̈
(4) Farha, O. K.; Ozgur Yazaydın, A.; Eryazici, I.; Malliakas, C. D.;
̈
Ken-ichi Otake − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
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Julia Knapp − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States
Joseph T. Hupp − Department of Chemistry, Northwestern
University, Evanston, Illinois 60208, United States;
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Justin M. Notestein − Department of Chemical and Biological
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60208, United States; orcid.org/0000-0003-1780-7356
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https://pubs.acs.org/10.1021/acs.inorgchem.0c03364
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Author Contributions
∥K.O and S.A. contributed equally to this work.
Notes
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The authors declare the following competing financial
interest(s): O.K.F has financial interest in the start-up
company NuMat Technologies, which is seeking to commerci-
alize metal-organic frameworks.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the Northwestern
University Institute for Catalysis in Energy Processes
■
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