Stable metal-organic framework-supported niobium catalysts

Article abstract of DOI:10.1021/acs.inorgchem.6b02103

Developing structurally well-defined, supported oxide catalysts remains a significant challenge. Here, we report the grafting of Nb(V) oxide sites onto the nodes of the Zr-based metal organic framework (MOF) NU-1000 as a stable, well-defined catalyst support. Nb(V) oxide was deposited with loadings up to 1.6 mmol/g via two postsynthetic methods: atomic layer deposition in a MOF, and solution-phase grafting in a MOF. Difference envelope density measurements indicated that the two synthetic methods resulted in different local structures of the Nb(V) ions within NU-1000. Despite their high Nb(V) loadings, which were equivalent to >60% surface coverage, nearly all Nb(V) sites of the MOF-supported catalysts were active sites for alkene epoxidation, as confirmed by phenylphosphonic acid titration. The MOF-supported catalysts were more selective than the control Nb-ZrO₂ catalyst for cyclohexene epoxidation with aqueous H₂O₂ and were far more active on a gravimetric basis.

Full text of DOI:10.1021/acs.inorgchem.6b02103

Article
pubs.acs.org/IC
Stable Metal−Organic Framework-Supported Niobium Catalysts
Sol Ahn,† Nicholas E. Thornburg,† Zhanyong Li,‡ Timothy C. Wang,‡ Leighanne C. Gallington,§
Karena W. Chapman,§ Justin M. Notestein,*,† Joseph T. Hupp,*,‡ and Omar K. Farha*,‡,∥
†Department of Chemical and Biological Engineering and ‡Department of Chemistry, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208, United States
§X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439,
United States
∥Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
S
* Supporting Information
ABSTRACT: Developing structurally well-defined, supported oxide catalysts remains a significant challenge. Here, we report the
grafting of Nb(V) oxide sites onto the nodes of the Zr-based metal organic framework (MOF) NU-1000 as a stable, well-defined
catalyst support. Nb(V) oxide was deposited with loadings up to 1.6 mmol/g via two postsynthetic methods: atomic layer
deposition in a MOF, and solution-phase grafting in a MOF. Difference envelope density measurements indicated that the two
synthetic methods resulted in different local structures of the Nb(V) ions within NU-1000. Despite their high Nb(V) loadings,
which were equivalent to >60% surface coverage, nearly all Nb(V) sites of the MOF-supported catalysts were active sites for
alkene epoxidation, as confirmed by phenylphosphonic acid titration. The MOF-supported catalysts were more selective than the
control Nb-ZrO2 catalyst for cyclohexene epoxidation with aqueous H2O2 and were far more active on a gravimetric basis.
chemical,19−21 thermal,22,23 and mechanical stability23,24 that is
attributable to the oxophilic nature of the Zr6/Hf6 clusters and
INTRODUCTION
Heterogeneous catalysts based on metal oxides are of
■
paramount interest for industrial chemical processes.1−3 In
Coulombic attraction, manifested as strong ionic bonding,
between these cationic metal nodes and anionic organic linkers.
particular, “single-site” heterogeneous catalysts have received
growing attention. In these materials, nearly all sites are
considered to participate similarly in the catalytic reaction,
making it easier to understand the kinetics and mechanism of
Among the various reported Zr-MOFs, NU-1000 is a
suitable candidate for such application due to its high chemical
and thermal stability.25 In addition, it is readily modifiable
postsynthetically26−28 and provides grafting sites for deposited
the catalysis from both experimental and computational
metals29 as hydroxyl and/or aqua groups are present on Zr6
nodes (see Figure 1).30 Moreover, the lengthy organic linkers
provide spacing between adjacent nodes (>1 nm), which
eliminates surface diffusion as a mechanism for aggregation of
the active metal ions during deposition, postsynthetic treat-
ment, and catalysis.31 Two postsynthetic methods have been
reported for the metalation of these Zr6 nodes in NU-1000: (1)
vapor-phase ALD in a MOF (AIM) using organometallic or
coordination complexes as the metal precursors31−34 and (2)
solution-phase grafting in a MOF (SIM) using organometallic
precursors and anhydrous organic solvents.35
perspectives.4 Strategies to achieve these structurally well-
defined catalyst sites include grafting metal−organic com-
plexes,5 atomic layer deposition (ALD),6 and templated
techniques.7,8
Catalyst supports have been demonstrated to play a
significant role in the activity and stability of single-site
catalysts and in supported oxides in general.9 Metal−organic
frameworks (MOFs), a class of porous materials comprised of
metal nodes and organic linkers, are structurally well-defined
and have found potential applications in, but not limited to, gas
storage and release10−13 and separation,14 sensing,15 and
catalysis.16−18 Because of their high surface areas, MOFs have
been considered as promising candidates as catalytic supports.
Furthermore, Zr- and Hf-based MOFs exhibit relatively high
Received: August 31, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Structure of NU-1000: (a) packing along the c axis; (b) Zr6 node ([Zr6(μ3-O)4(μ3-OH)4(OH)4(H2O)4]8+); (c) organic linker (TBAPy4−).
(tert-Butylimido)tris(diethylamido)niobium(V) (98%) was used as-
received from Strem Chemicals as the niobium precursor for both
ALD and condensed phase grafting. For the synthesis of Nb(V) ALD
in MOF (Nb-AIM), NU-1000 (10 mg, 4.67 μmol Zr6 nodes) was
loaded in an ALD sample holder. The sample was placed in a
Cambridge Nanotech Savannah 100 ALD reactor, and the sample was
dried of physisorbed water under 15 sccm N2 flow for 20 min at 125
°C; this temperature was maintained during the deposition process.
The niobium precursor was heated to 110 °C and then pulsed for 1 s
repeatedly for 400 half-cycles to ensure the saturated metalation of Zr6
node in NU-1000. Each half-cycle comprised of a quasi-static exposure
of the precursor in which the reactor was isolated from rough pumping
for 90 s followed by a 90 s purge under 2 sccm high-purity N2. H2O
was used as the second reactant at room temperature; it was pulsed for
0.02 s repeatedly for 25 half-cycles to remove the ligands from the
grafted Nb(V) precursor. Likewise, each half cycle consisted of
isolating the reactor from rough pumping for 120 s followed by a 120 s
purge under the same flow rate of N2. For Nb(V) on commercial
zirconia (Nb-ZrO2), commercial zirconia (99% trace metals basis,
Sigma-Aldrich) was used as the substrate instead of NU-1000 under
the same ALD condition.
For condensed phase grafting (Nb-SIM), NU-1000 (50 mg, 23.3
μmol Zr6 nodes) was introduced to a solution containing the Nb(V)
precursor (35 μL, 93.2 μmol) dissolved in 3 mL of heptane (99%,
anhydrous, Sigma-Aldrich) in an Ar-filled glovebox. The reaction was
allowed to take place over a 24 h period with occasional agitation of
the solution. The mixture was decanted and then washed three times
with 3 mL of heptane. At this point, the material was removed from
the glovebox and subsequently washed in air three times with 5 mL of
acetone.35 For comparison Nb-SiO2 was synthesized by grafting at a
loading of 0.27 mmol/g following a procedure previously established
by some of us.36
Separately, we and others have reported that supported
niobium oxide materials are effective catalysts in selective
oxidation.36−46 Highly dispersed Nb(V) oxide supported on
silica can act as a strong Lewis acid catalyst and is demonstrated
to be more active for alkene epoxidation using hydrogen
peroxide as an oxidant than the benchmark Lewis acid catalyst,
titanium oxide on silica.36,47−50 In this reaction, the epoxidation
with aqueous hydrogen peroxide is well-known for its
sustainability, since water is the only byproduct from this
oxidant.51 Recently, some of us have demonstrated the
importance of bulky precursors for achieving a preponderance
of under-coordinated and highly active Nb(V)-silica sites for
alkene epoxidation with H2O2.52
In this study, we synthesize highly active and isolated metal
oxide catalysts consisting of NbOx on NU-1000. Nb(V) oxides
were supported onto NU-1000 via both AIM and SIM
methods, and we compare their physicochemical properties
via powder X-ray diffraction (PXRD), inductively coupled
plasma atomic emission spectroscopy (ICP-AES), nitrogen
adsorption isotherms, and difference envelope density (DED)
measurements. Additionally, we examined their active site by
titration with phenylphosphonic acid during cyclooctene
epoxidation. Finally, we evaluated their reactivity and selectivity
as heterogeneous catalysts for the epoxidation of cyclohexene at
65 °C with H2O2 as an oxidant.
EXPERIMENTAL METHODS
■
Synthesis of Catalysts. The catalytic support, NU-1000
(nominally 5 μm length crystallites), was synthesized and activated
following a modified procedure in the literature.53 Smaller NU-1000
crystallites (2.5 μm length) were synthesized by controlling both node-
to-linker ratio and reaction time, according to a literature protocol
previously established by our team.54
Characterization of Catalysts. PXRD was used to verify the
integrity of the structure of the materials using a Rigaku SmartLab
diffractometer with a Cu Kα radiation source (45 kV, 160 mA tube
B
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
voltage and current). Diffraction patterns were measured over the 2θ
range of 2−15° with a 0.05° step width and a 5°/min scan speed.
Niobium (309.418, 316.340, 319.498, and 322.548 nm) and
zirconium (327.305, 339.198, 343.832, and 349.621 nm) contents
were measured using inductively coupled plasma atomic emission
spectroscopy (ICP-AES, iCAP 7600, Thermo Scientific), which are
compared to the ICP standard solutions. NU-1000-supported Nb(V)
catalysts (ca. 1 mg) were dissolved in 0.75 mL sulfuric acid (ACS
reagent, 95−98%, Sigma-Aldrich) and 0.25 mL of 30 wt % H2O2
(aqueous, Sigma-Aldrich) mixture, followed by microwave digestion
using Initiator+ (Biotage) at 150 °C for 5 min. Nb-ZrO2 were fused in
a lithium metaborate and a lithium tetraborate mixture, and then Zr
and Nb contents were measured by ICP-OES (Galbraith Laboratories,
Inc., 2323 Sycamore Drive, Knoxville, TN 37921).
Nitrogen adsorption isotherms were measured at 77 K using a
Micromeritics Tristar II instrument, and DFT pore size distributions
were obtained from the desorption branch of the resulting isotherms.
Prior to analysis, samples were pretreated under dynamic vacuum at
120 °C for 18 h using a Micromeritics Smart VacPrep.
Difference Envelope Density Measurement. PXRD patterns
were collected at ambient temperature at beamline 17-BM-B at the
Advanced Photon Source. X-rays (17.04 keV, 0.72768 Å) were used
with an amorphous silicon area detector, allowing for rapid acquisition
of data. Powdered samples were loaded into polyimide capillaries,
which were placed into a sample changer cassette. Patterns were
collected at a nominal distance of 1 m. Calibration of detector to
sample distance, beam center, detector tilt, rotation, and reduction of
the acquired data were performed using GSAS-II.55
Le Bail whole pattern fitting was utilized to extract lattice
parameters and peak intensities from powder diffraction data.56 The
previously reported P6/mmm crystal structure for NU-1000 was used
as a starting point for these analyses.32 Lattice and pseudo-Voigt
parameters were refined using a 2θ range of 1−5°.57 Structure
envelopes (SEs) were generated from the extracted peak intensities
from powder diffraction data.58 SEs of NU-1000 were subtracted from
those of both Nb-AIM and Nb-SIM to generate DED59 to estimate the
distribution of electron density associated with deposited Nb(V).
Active Site Titration and Cyclohexene Epoxidation. An
aqueous solution of 50 mM of phenylphosphonic acid (PPA, Sigma-
Aldrich) was used as a titrant to probe Lewis acid sites at 65 °C.60
MOF-supported catalysts (4 mg, 5.5 μmol based on Nb(V) content),
cis-cycoloctene (550 μmol, 75 μL, 95%, Sigma-Aldrich), 1,2-
dichlorobenzene (internal standard, 1.3 mmol, 60 μL, 98%, Alfa
Aesar), and acetonitrile (reaction solvent, 4.5 mL, >99%, spectro-
photometric grade, Alfa Aesar) were mixed in a 20 mL septum-capped
glass vial. This vial was shaken on a Glas Col heated vortexer at 700
rpm, for each titration. Discrete amounts of PPA were added from 0 to
1.5 equiv (0, 0.25, 0.50, 0.75, 1.0, and 1.5) of PPA relative to total
Nb(V) as determined by ICP-AES. After 1 h of equilibration at 65 °C,
aqueous H2O2 (1.65 mmol, 100 μL, 50 wt %, Sigma-Aldrich) was
added to initiate the titration. The initial rate of each titration was
calculated from the epoxide concentration versus time data over the
first 15 min of reaction.
In a typical cyclohexene epoxidation catalytic reaction, cyclohexene
(11 mmol, 1.1 mL, >99%, Sigma-Aldrich), and 1,2-dichlorobenzene
(1.3 mmol, 150 μL), were dissolved in acetonitrile (9 mL), to which
Nb-AIM or Nb-SIM (8 mg, 11 μmol based on Nb(V) content) was
added. After 1 h of mixing, 4.0 M H2O2 in an acetonitrile solution (1.1
mmol, 270 μL) was added to initiate the reaction. This low-water
H2O2 solution was prepared by diluting aqueous H2O2 (10 mL, 30 wt
%) in acetonitrile (20 mL), followed by drying over anhydrous MgSO4
(7 g, >99%, Sigma-Aldrich), which was then centrifuged and
decanted.52 The amount of Nb-AIM and Nb-SIM was adjusted to
the following molar ratio, cyclohexene/H2O2/Nb(V) = 1000:100:1.
Aliquots (0.1 mL) from both titration and epoxidation were
collected into gas chromatograph (GC) sampling vials with silver
powder (ca. 0.5 mg, >99%, <250 μL, Sigma-Aldrich) to quench excess
H2O2 to prevent oxidation during GC analysis.36 Products were
identified using GC-MS (Shimadzu QP2010 CG/MS, Zebron ZB 624
capillary column) and quantified using GC-FID (Shimadzu QP2010
GC-FID, TR-1 capillary column). For all reactions, mass balances
calculated from GC-FID peak areas were ≥90%.
RESULTS AND DISCUSSION
Synthesis and Characterization. Table 1 summarizes the
Nb(V) loadings of the different catalysts examined in this study.
■
Table 1. Summary of Catalysts
Nb
Nb surface
density
[Nb/Zrsurf]
b
MOF crystal Nb per Zr6
content
a
b
catalyst
ZrO2
size [μm]
node [-]
[mmol/g]
-
-
-
-
-
NU-1000
Nb-ZrO2
Nb-SIM
5
-
-
c
-
-
0.19
1.41
1.35
1.62
1.50
0.30
d
5
3.7
3.6
4.4
4
0.62
e
d
Nb-SIM(H)
Nb-AIM
2.5
5
0.58
d
0.73
e
d
Nb-AIM(H)
2.5
0.67
a
b
From SEM, controlled by synthesis conditions. From ICP-AES.
c
61,62 d
Using Zr surface density of zirconia is 7.34 Zr/nm.2
Assuming
e
every Zr is presented on the surface. H: half crystal size (2.5 μm
length) of MOF.
The Nb(V) content in Nb-AIM can be controlled by the
number of precursor pulses in ALD process. In this study, we
found that 400 pulses gave 4 Nb(V) per Zr6 node and that the
crystallinity of the NU-1000 was maintained (Figure 2). Higher
Figure 2. PXRD patterns of NU-1000, Nb-AIM, and Nb-SIM as
synthesized and after cyclohexene epoxidation.
numbers of pulses increased the Nb(V) loading but excessively
degraded the MOF crystallinity and were not studied further.
SIM occurred with quantitative grafting of the Nb(V)
precursor, and Nb-SIM was synthesized to match the Nb(V)
loading of Nb-AIM. No change in the crystallinity of the NU-
1000 support was observed in the synthesis of Nb-SIM as well
as after cyclohexene epoxidation reaction with Nb-SIM (Figure
2).
Finally, Nb-ZrO2 was synthesized under the same ALD
conditions (number of pulses) as Nb-AIM, and it gave a similar
Nb(V) surface density when expressed as Nb(V) per surface Zr
atoms. Although surface densities are typically given per
Brunauer−Emmett−Teller surface area, this value is not
particularly meaningful for MOF-supported catalysts, since
much of the surface area originates from the linkers, rather than
the nodes onto which the precursors graft. The Nb(V) content
in MOF-supported catalysts is approximately an order of
magnitude higher than that on zirconia (Table 1), as expected,
since commercial zirconia has a significantly lower density of
grafting sites when compared to NU-1000.
C
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
■
▲
●
Figure 3. (a) N2 isotherms and (b) DFT pore size distributions for NU-1000 (black ), Nb-SIM (blue ), and Nb-AIM (red ).
Figure 4. DED map of Nb-AIM (a) views along the c axis and (b) a axis and of Nb-SIM (c) views along the c axis and (d) a axis.
Nitrogen isotherms (Figure 3a) show that the surface areas
of both Nb-SIM (1470 m2/g) and Nb-AIM (470 m2/g)
decrease compared to the parent NU-1000 (2180 m2/g); the
loss of surface area is consistent with our previous studies.32
Interestingly, a reduction in the mesopore size of Nb-SIM is
observed in Figure 3b, as opposed to no apparent change in the
mesopore size with Nb-AIM. Combined with the differences in
surface area, this gives a preliminary indication that the two
synthetic methods may result in different local structures of the
Nb(V) ions on the NU-1000 node or within the crystallite.
To elucidate the structural changes of Nb-SIM and Nb-AIM
as compared to the parent MOF NU-1000, we utilized a
technique called DED. By subtracting the electron density
contribution from the parent material NU-1000, we can clearly
see that there are deposited Nb(V) atoms in Nb-AIM
uniformly distributed in the lateral cavities between the two
adjacent Zr6 nodes along the c-axis (Figure 4a,b). Detailed
study of this preferential binding of the Nb(V) ions to the small
aperture of NU-1000 is still in progress and will be reported on
in the near future. In contrast, solution-phase grafting results in
deposition in the same cavities, but with a bilobal distribution
of electron density, which may suggest formation of Nb oxide
dimers (Figure 4c,d).63
the number of the active Lewis acidic sites in the MOF-
supported materials. It is important to note that the titration
analysis is restricted to PPA/Nb(V) < 0.75, because PPA
displaces the pyrene linker at high concentrations,64 as
evidenced, in part, in the loss of MOF crystallinity (Figure
S1). However, the data points at sub-stoichiometric PPA-to-
Nb(V) concentrations exhibit a linear relationship (Figure 5)
between the amount of titrant added and the initial rate of
cyclooctene epoxidation (vide infra). Extrapolating the rate
decrease to the baseline gives a maximum of 1.8 mmol/g active
sites in Nb-AIM (111% of the number of Nb(V) atoms) and
1.4 mmol/g active sites in Nb-SIM (99% of the number of Nb
Catalysis. Titration with phenylphosphonic acid (PPA)
during cyclooctene epoxidation reaction has been used to
quantify the number of Lewis acid sites in silica-supported
Group IV and V metal oxide catalysts.52,60 Since cyclooctene
assesses the total reactivity and is highly selective to a single
epoxide product, this is a good model system for the liquid-
phase in situ titration. Here, we employ this technique to count
◆
■
Figure 5. PPA titration of NU-1000 (black ), Nb-SIM (blue ),
●
and Nb-AIM (red ). Extrapolation of the linear fits to the black
dotted line (contribution from the support) or to zero activity gives
the range of kinetically relevant active sites.
D
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
atoms). Bare NU-1000 also possesses mild activity due to the
Brønsted acidity of the zirconium oxide nodes;65 subtracting
this contribution gives an active site count of 1.5 mmol/g
(93%) and 1.3 mmol/g (93%) for Nb-AIM and Nb-SIM,
respectively. Under the reasonable assumptions of 1:1 titrant-
to-active site ratio and the preferential titration of strong Nb(V)
Lewis acid sites over weaker Zr Lewis acid sites, this close
agreement between the number of active sites titrated using
PPA and the Nb(V) content from ICP suggests that nearly all
Nb(V) atoms act as reactive sites. This contrasts with more
typical oxide-supported Nb(V), where less than 65% was
kinetically relevant, even in the best cases.52 However, even
though the catalysts have approximately the same amount of
kinetically relevant Nb(V), apparent rates are different. The
mode of synthesis clearly is a significant factor in controlling
activity. For these well-defined catalysts, the salient difference
with regard to Nb(V) activity presumably is the mode of
arrangement of the catalytic atoms. In the SIM case, Nb(V)
may exist as isolated oxide pairs on each node, while Nb(V)
appears to exist as slightly larger oxide clusters in Nb-AIM.
Motivated to further understand the reactivity differences in
the poisoning experiments for cyclooctene epoxidation, we then
examined cyclohexene epoxidation to study trends in
selectivity. In Table 1, selectivity in cyclohexene oxidation is
assessed as a pathway selectivity for how each catalyst activates
H2O2; conventional product selectivity is given in Table S1.
Scheme 1 describes the proposed reaction network with
catalysts run without apparent deactivation as confirmed by
fitting to a first-order rate law (Figure S3).
The reactivity data for the cyclohexene epoxidation of the
MOF-supported Nb(V) catalysts and control materials are
summarized in Table 2. Unmodified ZrO2 (entry 1) and NU-
1000 (entry 3) both give low initial rates and 5 h yields.
Unmodified ZrO2 gives exclusively oxidation products that arise
from radical decomposition of H2O2, whereas bare NU-1000 is
selective to direct cyclohexene oxidation, albeit with low yields.
Nb-ZrO2 (entry 2) engenders a much higher yield than ZrO2
and ultimately reaches ∼45% pathway selectivity, indicating
that the Nb(V) sites are responsible for the direct cyclohexene
epoxidation observed. All of the 5 μm MOF catalysts (entries 3,
4, and 6) show high pathway selectivity, while the pathway
selectivity decreases somewhat for the smaller crystallites
(entries 5 and 7). Combined, these results suggest that the
radical pathway oxidation products arise from an initial burst of
H2O2 decomposition that occurs on crystallite external surfaces
at the beginning of the reaction; proper catalyst precondition-
ing may mitigate this effect.
The oxide-supported catalyst Nb-ZrO2 and the benchmark
Nb-SiO2 both display high initial TOFs, but all of the MOF-
supported catalysts are faster on a per-gram basis due to their
high Nb(V) loadings. The rates and yields of the MOF-
supported catalysts are both increased significantly by
decreasing the MOF crystallite size (entries 5 and 7), and
both Nb-SIM(H) and Nb-AIM(H) have oxygenate yields
exceeding that of the benchmark Nb-SiO2 catalyst. Decreasing
the crystallite size had a particularly positive effect on Nb-AIM,
consistent with the even distribution of Nb(V) throughout the
pseudo-one-dimensional (1D) pores of the material, as inferred
from the DED analysis (Figure 4a,b) discussed above. This
analysis also indicates that further improvements in TOF may
be possible, potentially approaching that of the oxide-supported
materials, if further size-reduced support can be successfully
utilized.
Scheme 1. Proposed Reaction Network for Cyclohexene
Oxidation
CONCLUSIONS
■
NU-1000-supported Nb(V) catalysts were synthesized with
high Nb(V) content (up to 1.6 mmol/g) via two different
approaches, namely, AIM and SIM. One of the obvious
advantages of the MOF-supported catalysts is that Nb(V)
loadings (per g) can be much higher than for conventional
oxide-supported catalysts, such as Nb-ZrO2 or Nb-SiO2, while
still maintaining high TOF per Nb(V) atom. The MOF
crystallinity and high porosity were preserved after Nb(V)
deposition, as confirmed by PXRD and nitrogen isotherms,
respectively. Of particular interest, DED map studies indicate
that Nb(V) ions are regularly distributed through the
framework with a preference for deposition on the c-axis in
both AIM and SIM cases, but that the local structure of the
Nb(V) ions depends on whether AIM or SIM was used for the
deposition. PPA titrations were also used for the first time to
quantify the amount of active grafted oxide sites in a MOF-
based material, and the result suggests that all Nb(V) sites,
within uncertainty, are active for the electrophilic oxidation.
This contrasts with more typical oxide-supported Nb(V), where
less than 65% was kinetically relevant, even in the best cases.52
Cyclohexene epoxidation reaction was examined, and MOF-
supported catalysts are more selective and, due to the much
higher Nb(V) loadings, much more active than the chemically
similar Nb-ZrO2 on a per-gram basis. Finally, the apparent rate
of the Nb-AIM catalyst increased by nearly threefold when the
competing radical and nonradical oxidation pathways.66−68
Pathway selectivity is calculated from eq 1:
pathway selectivity(%) = (molepoxide + moldiol − (molenol − molenone))
/molall C6products × 100
(1)
which combines epoxide and its hydrolysis product cyclo-
hexanediol and subtracts cyclohexenol to account for
epoxidation via cyclohexenyl hydroperoxide. Note that epoxide
hydrolysis tends to increase with conversion for all catalysts
(Table S1) due to the simultaneous formation of H2O from
H2O2 utilization;69 this does not impact the pathway selectivity.
Figure 6 shows a representative concentration time-course plot
for this reaction and a fit to a first-order rate law. All of these
E
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. (a) Representative product yields vs time for cyclohexene epoxidation at 65 °C with Nb-SIM and (b) a fit to a first-order rate law with no
evidence of deactivation in the rate of cyclohexene consumption.
a
Table 2. Summary of Kinetic Data
b
c
initial rate
initial TOF
−1
[mol mgcatalyst min−1
]
[mol molNb min−1
]
5 h
−1
d
d
e
d
entry
catalyst
ZrO2
total
direct
total
n/a
2.53
n/a
direct
total yield [%]
direct pathway selectivity [%]
1
2
3
4
5
6
7
8
0.14
0.47
0.06
0.8
0
n/a
0
34
71
14
87
93
59
95
75
0
45
76
72
57
88
64
Nb-ZrO2
NU-1000
Nb-SIM
0
0.02
0.61
0.31
0.40
0.63
0.47
n/a
0.57
0.68
0.32
0.95
1.89
0.43
0.23
0.25
0.42
1.75
Nb-SIM(H)
Nb-AIM
0.92
0.52
1.43
0.51
Nb-AIM(H)
Nb-SiO2
93
a
b
c
Cyclohexene epoxidation at 65 °C with molar ratio of C6/H2O2/Nb(V) = 1000:100:1. Product formation on a catalyst weight basis. Product
formation on a total number of Nb(V) moles basis. Relative rate contribution to direct (nonradical) pathways.66−68 Yield (%) = (mMall C6 products)/
d
e
(103 mMH2O2,0) × 100. Limited by H2O2.
support particle size was reduced, indicating significant rate
limitations due to reactant diffusion within individual particles.
This observation is consistent with the observed uniform
distribution of Nb(V) sites along pseudo-1D pores of this
material. This result suggests that by using nanosized NU-1000,
Nb-AIM catalysts displaying turnover frequencies approaching
those of the oxide-supported catalysts, but with rates per gram
nearly an order of magnitude higher, could potentially be
realized. Further studies are currently in progress involving this
and other catalyst modifications, as well as additional detailed
characterization to elucidate structure/activity relationships.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
O.K.F., K.W.C., and J.T.H. gratefully acknowledge the financial
support from the Inorganometallic Catalyst Design Center, an
EFRC funded by the U.S. Department of Energy (DOE),
Office of Basic Energy Sciences (DE-SC0012702). J.M.N. and
N.E.T. acknowledge funding from the Dow Chemical
Company. This work made use of the J. B. Cohen X-ray
Diffraction Facility supported by the MRSEC program of the
National Science Foundation (DMR-1121262) at the Materials
Research Center of Northwestern Univ. This work also made
use of the EPIC facility (NUANCE Center-Northwestern
University), which has received support from the MRSEC
program (NSF DMR-1121262) at the Materials Research
Center; the International Institute for Nanotechnology (IIN);
and the State of Illinois, through the IIN. This research used
resources of the Advanced Photon Source, a DOE Office of
Science User Facility operated for the DOE Office of Science
by Argonne National Laboratory under Contract No. DE-
AC02-06CH11357.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications Web site at DOI: http://pubs.acs.org/doi/
abs/10.1021/acs.inorgchem.6b02103.
Additional characterization data and kinetic data (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
REFERENCES
*E-mail: j-notestein@northwestern.edu. (J.M.N.)
*E-mail: j-hupp@northwestern.edu. (J.T.H.)
*E-mail: o-farha@northwestern.edu. (O.K.F.)
■
(1) Flytzani-Stephanopoulos, M.; Gates, B. C. Atomically Dispersed
Supported Metal Catalysts. Annu. Rev. Chem. Biomol. Eng. 2012, 3,
545−574.
Author Contributions
(2) Bartholomew, C. H.; Farrauto, R. J. Fundamentals of Industrial
Catalytic Processes, 2nd ed.; Wiley: Hoboken, NJ, 2006.
(3) Thomas, J. M.; Thomas, W. J. Principles and Practice of
Heterogeneous Catalysis; Wiley VCH: New York, NY, 2005.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
F
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(4) Thomas, J. M.; Raja, R.; Lewis, D. W. Single-Site Heterogeneous
Catalysts. Angew. Chem., Int. Ed. 2005, 44, 6456−6482.
(5) Notestein, J. M.; Iglesia, E.; Katz, A. Grafted Metallocalixarenes as
Single-Site Surface Organometallic Catalysts. J. Am. Chem. Soc. 2004,
126, 16478−16486.
(23) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;
Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building Brick
Forming Metal Organic Frameworks with Exceptional Stability. J. Am.
Chem. Soc. 2008, 130, 13850−13851.
(24) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.;
Yildirim, T.; Zhou, W. Unusual and Highly Tunable Missing-Linker
Defects in Zirconium Metal−Organic Framework UiO-66 and Their
Important Effects on Gas Adsorption. J. Am. Chem. Soc. 2013, 135,
10525−10532.
(25) Howarth, A. J.; Liu, Y.; Li, P.; Li, Z.; Wang, T. C.; Hupp, J. T.;
Farha, O. K. Chemical, thermal and mechanical stabilities of metal−
organic frameworks. Nat. Rev. Mater. 2016, 1, 15018.
(26) Deria, P.; Mondloch, J. E.; Tylianakis, E.; Ghosh, P.; Bury, W.;
Snurr, R. Q.; Hupp, J. T.; Farha, O. K. Perfluoroalkane
Functionalization of NU-1000 via Solvent-Assisted Ligand Incorpo-
ration: Synthesis and CO2 Adsorption Studies. J. Am. Chem. Soc. 2013,
135, 16801−16804.
(27) Klet, R. C.; Tussupbayev, S.; Borycz, J.; Gallagher, J. R.; Stalzer,
M. M.; Miller, J. T.; Gagliardi, L.; Hupp, J. T.; Marks, T. J.; Cramer, C.
J.; Delferro, M.; Farha, O. K. Single-Site Organozirconium Catalyst
Embedded in a Metal−Organic Framework. J. Am. Chem. Soc. 2015,
137, 15680−15683.
(28) Rimoldi, M.; Nakamura, A.; Vermeulen, N. A.; Henkelis, J. J.;
Blackburn, A. K.; Hupp, J. T.; Stoddart, J. F.; Farha, O. K. A metal-
organic framework immobilised iridium pincer complex. Chem. Sci.
2016, 7, 4980−4984.
(29) Yang, D.; Odoh, S. O.; Wang, T. C.; Farha, O. K.; Hupp, J. T.;
Cramer, C. J.; Gagliardi, L.; Gates, B. C. Metal−Organic Framework
Nodes as Nearly Ideal Supports for Molecular Catalysts: NU-1000-
and UiO-66-Supported Iridium Complexes. J. Am. Chem. Soc. 2015,
137, 7391−7396.
(30) Planas, N.; Mondloch, J. E.; Tussupbayev, S.; Borycz, J.;
Gagliardi, L.; Hupp, J. T.; Farha, O. K.; Cramer, C. J. Defining the
Proton Topology of the Zr6-Based Metal−Organic Framework NU-
1000. J. Phys. Chem. Lett. 2014, 5, 3716−3723.
(31) Li, Z.; Schweitzer, N. M.; League, A. B.; Bernales, V.; Peters, A.
W.; Getsoian, A. B.; Wang, T. C.; Miller, J. T.; Vjunov, A.; Fulton, J. L.;
Lercher, J. A.; Cramer, C. J.; Gagliardi, L.; Hupp, J. T.; Farha, O. K.
Sintering-Resistant Single-Site Nickel Catalyst Supported by Metal−
Organic Framework. J. Am. Chem. Soc. 2016, 138, 1977−1982.
(32) Mondloch, J. E.; Bury, W.; Fairen-Jimenez, D.; Kwon, S.;
DeMarco, E. J.; Weston, M. H.; Sarjeant, A. A.; Nguyen, S. T.; Stair, P.
C.; Snurr, R. Q.; Farha, O. K.; Hupp, J. T. Vapor-Phase Metalation by
Atomic Layer Deposition in a Metal−Organic Framework. J. Am.
Chem. Soc. 2013, 135, 10294−10297.
(33) Kung, C.-W.; Mondloch, J. E.; Wang, T. C.; Bury, W.; Hoffeditz,
W.; Klahr, B. M.; Klet, R. C.; Pellin, M. J.; Farha, O. K.; Hupp, J. T.
Metal−Organic Framework Thin Films as Platforms for Atomic Layer
Deposition of Cobalt Ions To Enable Electrocatalytic Water
Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 28223−28230.
(34) Peters, A. W.; Li, Z.; Farha, O. K.; Hupp, J. T. Atomically
Precise Growth of Catalytically Active Cobalt Sulfide on Flat Surfaces
and within a Metal−Organic Framework via Atomic Layer Deposition.
ACS Nano 2015, 9, 8484−8490.
(35) Liu, T.-F.; Vermeulen, N. A.; Howarth, A. J.; Li, P.; Sarjeant, A.
A.; Hupp, J. T.; Farha, O. K. Adding to the Arsenal of Zirconium-
Based Metal−Organic Frameworks: the Topology as a Platform for
Solvent-Assisted Metal Incorporation. Eur. J. Inorg. Chem. 2016, 2016,
4349−4352.
(6) Stair, P. C. Synthesis of Supported Catalysts by Atomic Layer
Deposition. Top. Catal. 2012, 55, 93−98.
(7) Lu, P.; Campbell, C. T.; Xia, Y. A sinter-resistant catalytic system
fabricated by maneuvering the selectivity of SiO2 deposition onto the
TiO2 surface versus the Pt nanoparticle surface. Nano Lett. 2013, 13,
4957−62.
(8) Bo, Z.; Eaton, T. R.; Gallagher, J. R.; Canlas, C. P.; Miller, J. T.;
Notestein, J. M. Size-Selective Synthesis and Stabilization of Small
Silver Nanoparticles on TiO2 Partially Masked by SiO2. Chem. Mater.
2015, 27, 1269−1277.
́
(9) Boudart, M.; Djega-Mariadassou, G. Kinetics of Heterogeneous
Catalytic Reactions; Princeton University Press: Princeton, N.J., 1984.
(10) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen storage in metal-
organic frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314.
̈
(11) Farha, O. K.; Ozgur Yazaydın, A.; Eryazici, I.; Malliakas, C. D.;
̈
Hauser, B. G.; Kanatzidis, M. G.; Nguyen, S. T.; Snurr, R. Q.; Hupp, J.
T. De novo synthesis of a metal−organic framework material featuring
ultrahigh surface area and gas storage capacities. Nat. Chem. 2010, 2,
944−948.
(12) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi,
̈
E.; Yazaydin, A. O.; Snurr, R. Q.; O’Keeffe, M.; Kim, J.; Yaghi, O. M.
Ultrahigh Porosity in Metal-Organic Frameworks. Science 2010, 329,
424−428.
(13) Grunker, R.; Bon, V.; Muller, P.; Stoeck, U.; Krause, S.; Mueller,
U.; Senkovska, I.; Kaskel, S. A new metal-organic framework with
ultra-high surface area. Chem. Commun. 2014, 50, 3450−3452.
(14) McDonald, T. M.; Mason, J. A.; Kong, X.; Bloch, E. D.; Gygi,
D.; Dani, A.; Crocella, V.; Giordanino, F.; Odoh, S. O.; Drisdell, W. S.;
Vlaisavljevich, B.; Dzubak, A. L.; Poloni, R.; Schnell, S. K.; Planas, N.;
Lee, K.; Pascal, T.; Wan, L. F.; Prendergast, D.; Neaton, J. B.; Smit, B.;
Kortright, J. B.; Gagliardi, L.; Bordiga, S.; Reimer, J. A.; Long, J. R.
Cooperative insertion of CO2 in diamine-appended metal-organic
frameworks. Nature 2015, 519, 303−308.
(15) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne,
R. P.; Hupp, J. T. Metal−Organic Framework Materials as Chemical
Sensors. Chem. Rev. 2012, 112, 1105−1125.
(16) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.;
Hupp, J. T. Metal-organic framework materials as catalysts. Chem. Soc.
Rev. 2009, 38, 1450−1459.
(17) Ma, L.; Abney, C.; Lin, W. Enantioselective catalysis with
homochiral metal-organic frameworks. Chem. Soc. Rev. 2009, 38,
1248−1256.
(18) Miner, E. M.; Fukushima, T.; Sheberla, D.; Sun, L.;
Surendranath, Y.; Dinca, M. Electrochemical oxygen reduction
catalysed by Ni3(hexaiminotriphenylene)2. Nat. Commun. 2016, 7,
10942.
(19) Jiang, H.-L.; Feng, D.; Wang, K.; Gu, Z.-Y.; Wei, Z.; Chen, Y.-P.;
Zhou, H.-C. An Exceptionally Stable, Porphyrinic Zr Metal−Organic
Framework Exhibiting pH-Dependent Fluorescence. J. Am. Chem. Soc.
2013, 135, 13934−13938.
́
(20) Furukawa, H.; Gandara, F.; Zhang, Y.-B.; Jiang, J.; Queen, W. L.;
Hudson, M. R.; Yaghi, O. M. Water Adsorption in Porous Metal−
Organic Frameworks and Related Materials. J. Am. Chem. Soc. 2014,
136, 4369−4381.
(36) Thornburg, N. E.; Thompson, A. B.; Notestein, J. M. Periodic
Trends in Highly Dispersed Groups IV and V Supported Metal Oxide
Catalysts for Alkene Epoxidation with H2O2. ACS Catal. 2015, 5,
5077−5088.
(37) Tanabe, K. Application of niobium oxides as catalysts. Catal.
Today 1990, 8, 1−11.
(38) Wachs, I. E.; Jehng, J. M.; Deo, G.; Hu, H.; Arora, N. Catalytic
Properties of Niobium Materials and Related Subjects (Proceedings of
the Second International Symposium on Niobium Compounds)Redox
properties of niobium oxide catalysts. Catal. Today 1996, 28, 199−205.
(21) Mondloch, J. E.; Katz, M. J.; Planas, N.; Semrouni, D.; Gagliardi,
L.; Hupp, J. T.; Farha, O. K. Are Zr6-based MOFs water stable? Linker
hydrolysis vs. capillary-force-driven channel collapse. Chem. Commun.
2014, 50, 8944−8946.
(22) Jakobsen, S.; Gianolio, D.; Wragg, D. S.; Nilsen, M. H.; Emerich,
H.; Bordiga, S.; Lamberti, C.; Olsbye, U.; Tilset, M.; Lillerud, K. P.
Structural determination of a highly stable metal-organic framework
with possible application to interim radioactive waste scavenging: Hf-
UiO-66. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 125429.
G
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(39) Nowak, I.; Ziolek, M. Niobium Compounds: Preparation,
Characterization, and Application in Heterogeneous Catalysis. Chem.
Rev. 1999, 99, 3603−3624.
(56) Le Bail, A.; Duroy, H.; Fourquet, J. L. Ab-initio structure
determination of LiSbWO6 by X-ray powder diffraction. Mater. Res.
Bull. 1988, 23, 447−452.
̌ ̌ ̌
(57) Petrícek, V.; Dusek, M.; Palatinus, L. Crysallographic
(40) Gao, X.; Wachs, I. E.; Wong, M. S.; Ying, J. Y. Structural and
Reactivity Properties of Nb-MCM-41: Comparison with That of
Highly Dispersed Nb2O5/SiO2 Catalysts. J. Catal. 2001, 203, 18−24.
(41) Aronne, A.; Turco, M.; Bagnasco, G.; Ramis, G.; Santacesaria,
E.; Di Serio, M.; Marenna, E.; Bevilacqua, M.; Cammarano, C.; Fanelli,
E. Gel derived niobium−silicon mixed oxides: Characterization and
catalytic activity for cyclooctene epoxidation. Appl. Catal., A 2008, 347,
179−185.
Computing System JANA2006: General features. Z. Kristallogr. -
Cryst. Mater. 2014, 229, 345.
(58) Yakovenko, A. A.; Reibenspies, J. H.; Bhuvanesh, N.; Zhou, H.-
C. Generation and applications of structure envelopes for porous
metal-organic frameworks. J. Appl. Crystallogr. 2013, 46, 346−353.
(59) Yakovenko, A. A.; Wei, Z.; Wriedt, M.; Li, J.-R.; Halder, G. J.;
Zhou, H.-C. Study of Guest Molecules in Metal−Organic Frameworks
by Powder X-ray Diffraction: Analysis of Difference Envelope Density.
Cryst. Growth Des. 2014, 14, 5397−5407.
(60) Eaton, T. R.; Boston, A. M.; Thompson, A. B.; Gray, K. A.;
Notestein, J. M. Counting Active Sites on Titanium Oxide−Silica
Catalysts for Hydrogen Peroxide Activation through In Situ Poisoning
with Phenylphosphonic Acid. ChemCatChem 2014, 6, 3215−3222.
(61) Nawrocki, J.; Rigney, M.; McCormick, A.; Carr, P. W.
Chemistry of zirconia and its use in chromatography. J. Chromatogr.
A 1993, 657, 229−282.
(62) Kouva, S.; Honkala, K.; Lefferts, L.; Kanervo, J. Review:
monoclinic zirconia, its surface sites and their interaction with carbon
monoxide. Catal. Sci. Technol. 2015, 5, 3473−3490.
(63) James, A. M.; Kowalczyk, P.; Fournier, R.; Simard, B. Electronic
spectroscopy of the niobium dimer molecule: Experimental and
theoretical results. J. Chem. Phys. 1993, 99, 8504−8518.
(64) Deria, P.; Bury, W.; Hod, I.; Kung, C.-W.; Karagiaridi, O.; Hupp,
J. T.; Farha, O. K. MOF Functionalization via Solvent-Assisted Ligand
Incorporation: Phosphonates vs Carboxylates. Inorg. Chem. 2015, 54,
2185−2192.
(65) Klet, R. C.; Liu, Y.; Wang, T. C.; Hupp, J. T.; Farha, O. K.
Evaluation of Bronsted acidity and proton topology in Zr- and Hf-
based metal-organic frameworks using potentiometric acid-base
titration. J. Mater. Chem. A 2016, 4, 1479−1485.
(42) Nowak, I. Frontiers in mesoporous molecular sieves containing
niobium: From model materials to catalysts. Catal. Today 2012, 192,
80−88.
(43) Tiozzo, C.; Bisio, C.; Carniato, F.; Marchese, L.; Gallo, A.;
Ravasio, N.; Psaro, R.; Guidotti, M. Epoxidation with hydrogen
peroxide of unsaturated fatty acid methyl esters over Nb(V)-silica
catalysts. Eur. J. Lipid Sci. Technol. 2013, 115, 86−93.
(44) Gallo, A.; Tiozzo, C.; Psaro, R.; Carniato, F.; Guidotti, M.
Niobium metallocenes deposited onto mesoporous silica via dry
impregnation as catalysts for selective epoxidation of alkenes. J. Catal.
2013, 298, 77−83.
́
(45) Feliczak-Guzik, A.; Wawrzynczak, A.; Nowak, I. Selective
catalytic oxidations of cyclohexene, thioether and geraniol with
hydrogen peroxide. Sensitivity to the structure of mesoporous
niobosilicates. Microporous Mesoporous Mater. 2015, 202, 80−89.
(46) Turco, R.; Aronne, A.; Carniti, P.; Gervasini, A.; Minieri, L.;
Pernice, P.; Tesser, R.; Vitiello, R.; Di Serio, M. Influence of
preparation methods and structure of niobium oxide-based catalysts in
the epoxidation reaction. Catal. Today 2015, 254, 99−103.
(47) D’Elia, V.; Dong, H.; Rossini, A. J.; Widdifield, C. M.;
Vummaleti, S. V. C.; Minenkov, Y.; Poater, A.; Abou-Hamad, E.;
Pelletier, J. D. A.; Cavallo, L.; Emsley, L.; Basset, J.-M. Cooperative
Effect of Monopodal Silica-Supported Niobium Complex Pairs
Enhancing Catalytic Cyclic Carbonate Production. J. Am. Chem. Soc.
2015, 137, 7728−7739.
́
(66) Morlanes, N.; Notestein, J. M. Grafted Ta−calixarenes: Tunable,
selective catalysts for direct olefin epoxidation with aqueous hydrogen
peroxide. J. Catal. 2010, 275, 191−201.
(67) Fraile, J. M.; García, J. I.; Mayoral, J. A.; Vispe, E. Optimization
of cyclohexene epoxidation with dilute hydrogen peroxide and silica-
supported titanium catalysts. Appl. Catal., A 2003, 245, 363−376.
(68) Fraile, J. M.; García, J. I.; Mayoral, J. A.; Vispe, E. Catalytic sites
in silica-supported titanium catalysts: silsesquioxane complexes as
models. J. Catal. 2005, 233, 90−99.
(69) Morlanes, N.; Notestein, J. M. Kinetic study of cyclooctene
́
epoxidation with aqueous hydrogen peroxide over silica-supported
calixarene−Ta(V). Appl. Catal., A 2010, 387, 45−54.
(48) Jehng, J.-M.; Wachs, I. E. Catalytic Conversion With Niobium
Materials Proceedings of the Niobium and Catalysts Symposium at the
1989 International Chemical Congress of Pacific Basin SocietiesThe
molecular structures and reactivity of supported niobium oxide
catalysts. Catal. Today 1990, 8, 37−55.
(49) Nowak, I.; Kilos, B.; Ziolek, M.; Lewandowska, A. Epoxidation
of cyclohexene on Nb-containing meso- and macroporous materials.
Catal. Today 2003, 78, 487−498.
(50) Tiozzo, C.; Bisio, C.; Carniato, F.; Gallo, A.; Scott, S. L.; Psaro,
R.; Guidotti, M. Niobium-silica catalysts for the selective epoxidation
of cyclic alkenes: the generation of the active site by grafting
niobocene dichloride. Phys. Chem. Chem. Phys. 2013, 15, 13354−62.
(51) Sheldon, R. A. Synthetic and mechanistic aspects of metal-
catalysed epoxidations with hydroperoxides. J. Mol. Catal. 1980, 7,
107−126.
(52) Thornburg, N. E.; Nauert, S. L.; Thompson, A. B.; Notestein, J.
M. Synthesis-structure-function relationships of silica-supported
niobium(V) catalysts for alkene epoxidation with H2O2. ACS Catal.
2016, 6, 6124−6134.
(53) Wang, T. C.; Vermeulen, N. A.; Kim, I. S.; Martinson, A. B. F.;
Stoddart, J. F.; Hupp, J. T.; Farha, O. K. Scalable synthesis and post-
modification of a mesoporous metal-organic framework called NU-
1000. Nat. Protoc. 2015, 11, 149−162.
(54) Li, P.; Klet, R. C.; Moon, S.-Y.; Wang, T. C.; Deria, P.; Peters, A.
W.; Klahr, B. M.; Park, H.-J.; Al-Juaid, S. S.; Hupp, J. T.; Farha, O. K.
Synthesis of nanocrystals of Zr-based metal-organic frameworks with
csq-net: significant enhancement in the degradation of a nerve agent
simulant. Chem. Commun. 2015, 51, 10925−10928.
(55) Toby, B. H.; Von Dreele, R. B. GSAS-II: the genesis of a
modern open-source all purpose crystallography software package. J.
Appl. Crystallogr. 2013, 46, 544−549.
H
DOI: 10.1021/acs.inorgchem.6b02103
Inorg. Chem. XXXX, XXX, XXX−XXX