Catalytically Active Silicon Oxide Nanoclusters Stabilized in a Metal–Organic Framework

Article abstract of DOI:10.1002/chem.201701902

Post-synthetic modification of the zirconium-based metal–organic framework (MOF) NU-1000 by atomic layer deposition (ALD), using tetramethoxysilane (Si(OMe)₄) as a precursor, led to the incorporation and stabilization of silicon oxide clusters composed of only a few silicon atoms in the framework's pores. The resulting SiO_x functionalized material (Si-NU-1000) was found to be catalytically active despite the inactivity of related bulk silicon dioxide (SiO₂), thus demonstrating the positive effects of having nanosized clusters of SiO_x. Moreover, Si-NU-1000 showed activity greater than that found for aluminum oxide based catalysts—oxides known for their high acidity—such as an aluminum oxide functionalized MOF (Al-NU-1000) and bulk γ-Al₂O₃. X-ray photoelectron spectroscopy and infrared spectroscopy measurements unmasked the electron donating nature of Si-NU-1000, explaining the unusual electronic properties of the nanosized SiO_x clusters and supporting their high catalytic activity.

Full text of DOI:10.1002/chem.201701902

A Journal of
Accepted Article
Title: Catalytically Active Silicon Oxide Nanoclusters Stabilized in a
Metal-Organic Framework
Authors: Martino Rimoldi, Leighanne C. Gallington, Karena W.
Chapman, Keith MacRenaris, Joseph T. Hupp, and Omar
Farha
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To be cited as: Chem. Eur. J. 10.1002/chem.201701902
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Catalytically Active Silicon Oxide Nanoclusters Stabilized in a
Metal–Organic Framework
Martino Rimoldi,[a] Leighanne C. Gallington,[b] Karena W. Chapman,[b] Keith MacRenaris,[a] Joseph T.
Hupp,[a] and Omar K. Farha*[a,c]
Abstract: Post-synthetic modification of the zirconium-based metal–
NU-1000, a high surface area Zr-MOF composed of 1,3,6,8-
tetrakis(p-benzoic acid)pyrene linkers and zirconium oxide
cluster nodes (Zr6O8) (Figure 1),[7] has been shown to be a
promising ALD platform, principally due to its high chemical
stability, mesoporosity, and the availability of hydroxyl groups on
the Zr-oxide node of the framework which can serve as reactive
deposition sites.[7-8]
Given the tunability and the stability of NU-1000 as well as the
versatility of the atomic layer deposition technique, we targeted
MOF modification – more specifically, zirconium-oxide node
modification – to tune its properties such as acidity.
Herein, we report the use of ALD to post-synthetically install low
nuclearity SiOx clusters at the zirconium oxide nodes of NU-1000
and demonstrate how confinement of the clusters within the
small apertures (about 8–10 Å in size) of the framework
preserves their nanoscale nature, conferring unusual electronic
properties as well as unexpected catalytic activity.
organic framework NU-1000 by atomic layer deposition (ALD) using
tetramethoxysilane (Si(OMe)4) as
a
precursor led to the
incorporation and stabilization of silicon oxide clusters composed of
only a few silicon atoms in the framework’s pores. The resulting SiOx
functionalized material (Si-NU-1000) was found to be catalytically
active despite the inactivity of related bulk silicon dioxide (SiO2), thus
demonstrating the positive effects of having nanosized clusters of
SiOx. Moreover, Si-NU-1000 showed activity greater than that found
for aluminum oxide based catalysts – oxides known for their high
acidity – such as an aluminum oxide functionalized MOF (Al-NU-
1000) and bulk -Al2O3. X-ray photoelectron spectroscopy and
infrared spectroscopy measurements unmasked the electron
donating nature of Si-NU-1000 explaining the unusual electronic
properties of the nanosized SiOx clusters and supporting their high
catalytic activity.
Introduction
Well-known heterogeneous acid catalysts such as amorphous
silica-aluminas and aluminas or crystalline aluminosilicate
zeolites can perform a variety of transformations, and currently
find
a wide range of applications from laboratory scale
syntheses to industrial uses.[1]
Recently, zirconium-based metal–organic frameworks (Zr-
MOFs), an exceptionally robust sub-class of MOFs that has
proven promising both as catalysts and catalyst supports,[2] have
been introduced as high surface area and high porosity acid
catalysts.[3] They have been used successfully in a number of
acid catalyzed reactions, mostly taking advantage of the acidity
Figure 1. View of NU-1000 (a) perpendicular to and (b) parallel to the c-axis.
showing the small apertures in the NU-1000 framework where the oxide
clusters are found after atomic layer deposition.
and ease of functionalization of the zirconium oxide node.[4]
[5]
The use of atomic layer deposition (ALD)
– a self-limiting
deposition technique conducted in the vapor-phase that is able
to achieve surface modification with atomic precision – has been
recently applied to hybrid and porous, high-surface-area
materials,[6] in order to achieve selective and controlled
functionalization.
Results and Discussion
Amongst
other
available
molecular
compounds,[9]
tetramethoxysilane (Si(OMe)4) has been found to be the most
suitable precursor for silicon oxide ALD in NU-1000, in
combination with water as a second reactant to displace the
unreacted methoxy groups after Si(OMe)4 deposition (the
parameters of the process are reported in the Supporting
Information).
The integrity of the framework after ALD has been preserved. In
fact, Si-NU-1000 maintained its high surface area and
mesoporosity as demonstrated by nitrogen physisorption
isotherms and pore size distribution analyses (Figures S1 and
S2). Si-NU-1000 was also found to retain crystallinity as
demonstrated by its powder X-ray diffraction pattern (Figure S3).
Additionally, scanning electron microscopy-energy dispersive
[a]
[b]
[c]
M. Rimoldi, K. MacRenaris, J. T. Hupp, and O. K. Farha
Department of Chemistry, Northwestern University, 2145 Sheridan
Road, Evanston, Illinois 60208, United States.
E-mail: o-farha@northwestern.edu
L. C. Gallington, K. W. Chapman
X-ray Science Division, Advanced Photon Source Argonne National
Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United
States.
O. K. Farha
Department of Chemistry, Faculty of Science, King Abdulaziz
University, Jeddah 21589, Saudi Arabia.
Supporting information for this article is given via a link at the end of
the document.
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spectroscopy (SEM-EDS) showed retention of crystal
morphology and a homogeneous distribution of the silicon
incorporated through the NU-1000 crystallite (Figure S4). Diffuse
reflectance infrared Fourier transform (DRIFT) spectra showed,
after dehydration, the appearance of broad peaks at 3712 cm-1
Catalytic tests were conducted in a vertical tubular gas flow
reactor and the reaction temperature was set to 250 °C.[12] To
evaluate the catalyst, we determined turn-over frequencies
(TOFs) of the catalytic dehydration reaction.
We obtained a TOF of 5.18 ± 0.05 h-1 for Si-NU-1000, whereas
and 3733 cm-1, attributed to silanol features (Figures S5 and S6). SiO2 (bulk silicon dioxide), which was run for comparison
The 29Si MAS NMR spectrum of Si-NU-1000 (Figure S7) reveals
purpose, was found to be inactive in isopropanol dehydration.
Under the adopted experimental conditions, bare NU-1000
showed only minimal activity with a TOF, obtained from a single
point determination, of 0.05 h-1. This value falls within the
conversion experimental error and therefore it can be
reasonably considered as a negligible background reaction.
Therefore we can exclude any contribution from the zirconium
oxide node when evaluating the activity of the ALD-modified NU-
1000.
the absence of Q4 sites (Si-(OSi)4 sites, commonly at – 110 [10]
)
suggesting the occurrence of small, bulk-free SiOx units.
Additionally, difference envelope density (DED) analyses of the
powder diffraction data reveals an increase in electron density
between Zr6 nodes aligned along the c-axis following silicon
oxide deposition, as similarly observed with other examples of
ALD in NU-1000.[8b-d] The distribution of new species from ALD
throughout the framework is shown in Figure 2.
It is remarkable that the SiOx-functionalized MOF is catalytically
active despite the inactivity of the related bulk oxide (silicon
dioxide). We speculate that the unusual catalytic performance of
Si-NU-1000 can be ascribed to the nanosized nature of the SiOx
units, whose electronic properties and local structure possibly
differs from those of bulk silica.
To further support and understand the origin of the unexpected
high activity of Si-NU-1000 we performed Si2p X-ray
photoelectron spectroscopy (XPS). Si2p binding energies (BEs)
of SiO2 have commonly been reported between 103.5 eV and
103.3 eV.[13] Accordingly we obtained a BE of 103.3 eV for SiO2,
but on the other hand we found the Si2p binding energy of Si-
NU-1000 at 102.5 eV, significantly shifted with respect to bulk
SiO2 (Figure 3).
Figure 2. View of the lattice of NU-1000 (left) perpendicular to and (right)
parallel to its c-axis. A difference envelope indicating extra electron density
present in Si-NU-1000 is represented in purple. Zr6O8 clusters are represented
in cyan.
Overall, the spectroscopic and crystallographic information
suggests that the SiOx introduced to NU-1000 via ALD is present
as SiOx nanoclusters – more precisely consisting of only a few
silicon atoms and below one nanometer in size – with the
clusters localized in the small cavities of the MOF network rather
than homogeneously distributed over the zirconium node. The
confinement in the small apertures (Figure 2) is pivotal to their
stabilization in preventing aggregation phenomena.
Given the unusual (nanosized) nature of the clusters embedded
in the framework, we decided to further investigate Si-NU-1000,
and in particular with respect to its use as an acid catalyst.
Reactions such as dehydration, isomerization, or cracking are
commonly used to benchmark the acidity of solid catalysts. We
opted to evaluate the performance of Si-NU-1000 in isopropanol
dehydration (Scheme 1). Besides being a way to benchmark the
catalyst, isopropanol dehydration also represents a practicable
Figure 3. Si2p XPS measurements of (right) Si-NU-1000 and (left) SiO2.
Binding energies (eV) are referenced to C1s = 284.8 eV.
It is known that the Si2p BEs of aluminum silicates and zeolites
–oxides of well-proven acidity – are shifted to lower values when
compared to silicon dioxide. For instance, Si2p BE values of
zeolites have been reported between 100.0 eV and 102.9 eV
and this feature, although its interpretation is not straightforward,
is commonly ascribed to the higher acidity of the oxide
route to produce propene
feedstocks in the chemical industry
confirmed by a number of recent patents.[11]
–
one of the most important
–
whose interest is
material.[14] XPS data demonstrates
between the electronic properties of the SiOx nanoclusters
incorporated in NU-1000 and bulk silica, suggesting
a tangible difference
a
significantly higher acidity and thus supporting the unexpected
remarkable activity of Si-NU-1000 in an acid catalyzed reaction.
Given that aluminas and aluminates are common acid catalysts,
for a fair and thorough evaluation of the catalytic activity, we
Scheme 1. Acid catalyzed dehydration reaction of 2-propanol.
compared the performance of Si-NU-1000 with Al-NU-1000 [8d]
–
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a structurally related catalyst obtained by aluminum oxide ALD –
and -Al2O3.
To compare the acidity of Si- and Al-NU-1000, as well as NU-
1000, we grafted the Ir(I) carbonyl complex onto the Zr-MOFs
and measured their CO stretching frequencies. Since the Si- and
Al-oxide functionalities were found to be installed only at certain
faces of the zirconium node (Figure 2) we expected the iridium
complex to react with Zr-OH on the node of NU-1000, in addition
to the Si-OH and Al-OH species introduced into the MOF by
ALD. The IR spectra revealed – besides a peak at 1991 cm-1
(asymmetric CO) typical of [Ir(CO)2] grafted onto the zirconium
node of bare NU-1000 – the appearance of new peaks at 2009
cm-1 (and 2001 cm-1) on Si-NU-1000 and at 2003 cm-1 on Al-NU-
1000 (see Supporting Information for more details).
In our catalytic tests we obtained TOFs of 0.73 ± 0.07 h-1 for Al-
NU-1000 (Figure 4 shows a representative residence time vs
conversion plot used to determine the TOFs of Si-NU-1000 and
Al-NU-1000) and of 2.6 ± 0.2 h-1 for -Al2O3, values significantly
lower than those obtained with Si-NU-1000. This comparison
additionally proves the uniqueness of Si-NU-1000 not only with
respect to the related bulk oxide (SiO2) but also an analogous
aluminum oxide-modified MOF and bulk -Al2O3.
The shifting of the CO stretching to higher frequencies indicates
a lower electron density on the metal center of the supported
dicarbonyl species [Ir(CO)2], tentatively associated to an
increase in the acidity of both Si- and Al-NU-1000 with respect to
bare NU-1000. In addition, Si-NU-1000 causes a shift to a higher
frequency with respect to Al-NU-1000, indicative of a lower
electron donor ability of Si-NU-1000. Although unexpected for a
silicon oxide-based material, this outcome is in line with and
supports the catalytic results.
The spectroscopic data, along with the catalytic data, are
[16]
summarized in Table 1 and Figure 5
shows the TOF (h-1)
values vs CO frequency (cm-1) which underlines the enhanced
performance of Si-NU-1000. Overall, the electron donor ability
extrapolated from the carbonyl stretching frequency highlights a
trend that agrees with the catalytic results, and in particular
confirms the superiority of Si-NU-1000.
Figure 4. Representative Conversion vs W/F (h) (Residence Time) plots of
isopropanol dehydration catalysed by Si-NU-1000 (blue circles) and Al-NU-
1000 (red squares).
Table 1. Carbonyl stretching frequencies (CO
isopropanol dehydration of bare NU-1000 and ALD-modified NU-1000.
) and TOF values in
To further support the activities trend amongst the MOF-based
catalysts and examine the properties of small oxide clusters
within the framework, we turned to investigating the electron
donor properties of the ALD-modified catalysts vs bare NU-1000
(whose activity is exclusively associated with the acidity of the
zirconium-oxide node) by using the iridium(I) complex
[Ir(CO)2(acac)] (acac is acetylacetonate – Scheme 2) as probe
molecule.
CO [Ir(CO)2] (cm-1) [a]
TOF (h-1) (± STD Error) [d]
Si-NU-1000
Al-NU-1000
NU-1000
2009 [b]
2004
5.18 (± 0.05)
0.73 (± 0.07)
0.05 [e]
1990 [c]
[a] Asymmetric carbonyl stretching of the grafted [Ir(CO)2] organometallic
fragment onto Si-NU-1000, Al-NU-1000, and NU-1000 (see ESI for more
details). [b] Peak at higher frequency. [c] Previously reported value (Ref.
15). [d] Turn-over frequency values in isopropanol dehydration (± standard
error). [e] Value obtained from a single point determination.
Scheme 2. [Ir(CO)2(acac)] (left) and schematic representation of the grafted
fragment [Ir(CO)2] (right).
The IR carbonyl stretching of the [Ir(CO)2] fragment grafted onto
a solid support via displacement of the acetylacetonate ligand
upon reaction with the –OH groups (Scheme 2) is indicative of
the electron density on the iridium metal center (in formal
oxidation state +1) which, in turn, probes the electron-donor
ability of the support. This method was proven effective with
materials such as HY zeolite, ZrO2, MgO, and more recently with
zirconium-based MOFs, including NU-1000.[15]
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Figure 5. TOF (h-1) in isopropanol dehydration vs CO of [Ir(CO)2] (cm-1)
fragment grafted on the Zr-oxide node of NU-1000, on Al-NU-1000, and on Si-
NU-1000.
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Conclusion
Atomic layer deposition, a vapor-phase deposition method, has
been used to post-synthetically incorporate SiOx units into a Zr-
based MOF. This approach combines the advantages of
solvent-free reaction conditions with precise atomic control of
the incorporation of desired functional sites. In the present
system, the confinement of the SiOx units in the small windows
of the framework (less than 1 nm in size) is pivotal in preserving
their nanometer structure, a feature we believe plays a relevant
role in conferring their unusual properties. Despite the catalytic
inactivity of bulk silicon oxide, Si-NU-1000 surpasses the activity
in an acid catalyzed reaction of an analogous aluminum-based
MOF and -alumina. The unexpected activity of Si-NU-1000 is
corroborated by X-ray photoelectron spectroscopy data and
determining its electron donating properties. These results also
demonstrate that silicon oxide atomic layer deposition into a Zr-
MOF can be efficiently used to post-synthetically modify a MOF
to enhance its acidity.
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This work was supported as part of the Inorganometallic
Catalysis Design Center, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Basic Energy Sciences under Award #DE-SC0012702. This
work made use of the EPIC, Keck-II, QBIC, and Clean Catalysis
facilities of Northwestern University. This research used
resources of the Advanced Photon Source, a U.S. Department
of Energy (DOE) Office of Science User Facility operated for the
DOE Office of Science by Argonne National Laboratory under
Contract No. DE-AC02-06CH11357. M.R. was supported by the
Swiss National Science Foundation with an “Early
Postdoc.Mobility Fellowship”.
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Keywords: metal-organic framework, nanocluster, atomic layer
deposition, silicon oxide, gas flow catalysis.
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for more details).
[16] The correlation between TOFs and IR stretching of the grafted [Ir(CO)2]
fragment has been previously found to fit an exponential curve,[15]
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Entry for the Table of Contents (Please choose one layout)
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Low nuclearity silicon oxide clusters
introduced into a zirconium metal–
organic framework by atomic layer
deposition enhance its activity as an
acid catalyst.
Martino Rimoldi, Leighanne C.
Gallington, Karena W. Chapman, Keith
MacRenaris, Joseph T. Hupp, and Omar
K. Farha*
Page No. – Page No.
Catalytically Active Silicon Oxide
Nanoclusters Stabilized in a Metal–
Organic Framework
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
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Title
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