Synthesis and Exfoliation of a New Layered Mesoporous Zr-MOF Comprising Hexa- A nd Dodecanuclear Clusters as Well as a Small Organic Linker Molecule

Article abstract of DOI:10.1021/jacs.0c06978

A new layered mesoporous Zr-MOF of composition [Zr30O20(OH)26(OAc)18L18] was synthesized by employing 5-acetamidoisophthalic acid (H2L) using acetic acid as the solvent. The new MOF, denoted as CAU-45, exhibits a honeycomb structure of stacked layers which comprise both hexa- A nd dodecanucelar zirconium clusters. Its structure was solved from submicrometer-sized crystals by continuous rotation electron diffraction (cRED). Liquid phase exfoliation and size selection were successfully performed on the material.

Full text of DOI:10.1021/jacs.0c06978

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Article
Synthesis and Exfoliation of a New Layered Mesoporous Zr-MOF Comprising
Hexa- and Dodecanuclear Clusters as well as a Small Organic Linker Molecule
Sebastian Leubner, Viktor E.G. Bengtsson, Kevin Synnatschke, Jonas Gosch, Alexander
Koch, Helge Reinsch, Hongyi Xu, Claudia Backes, Xiaodong Zou, and Norbert Stock
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c06978 • Publication Date (Web): 21 Aug 2020
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Synthesis and Exfoliation of a New Layered Mesoporous Zr-MOF
Comprising Hexa- and Dodecanuclear Clusters as well as a Small
Organic Linker Molecule
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Sebastian Leubner, a Viktor E.G. Bengtsson, b Kevin Synnatschke, c Jonas Gosch, a Alexander Koch, a
Helge Reinsch, a Hongyi Xu, b Claudia Backes, c* Xiaodong Zou b* and Norbert Stock a*
a Institute of Inorganic Chemistry, University of Kiel, Max-Eyth Strasse 2, 24118 Kiel (Germany).
b Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm (Sweden).
c Institute of Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg (Germany).
ABSTRACT: A new layered mesoporous Zr-MOF of composition [Zr30O20(OH)26(OAc)18L18] was synthesized employing
5-acetamidoisophthalic acid (H2L) in acetic acid as the solvent. The new MOF, denoted as CAU-45, exhibits a honeycomb structure
of stacked layers which comprise both hexa- and dodecanucelar zirconium clusters. Its structure was solved from submicron-sized
crystals by continuous rotation electron diffraction (cRED). Liquid phase exfoliation and size selection were successfully performed
on the material.
Introduction
of the product. They occupy coordination sites on the IBUs
which would be otherwise saturated by linker molecules,
leading to uncommon network topologies for Zr-MOFs.[26]
Metal-organic frameworks (MOFs) are potentially porous
coordination networks. They are assembled from inorganic
nodes, also called inorganic building units (IBUs), which are
interconnected by organic linker molecules.[1,2] MOFs are
promising candidates for multiple applications such as gas
storage[3] and separation,[4] catalysis,[5,6] water harvesting,[7] heat
transformation,[8,9] drug carriers,[10,11] sensors[12–14] or imaging
materials.[15] For some of these applications, like catalysis or
drug delivery, mesoporous MOFs are especially valuable due to
their large pores.[16,17]
Layered MOFs can be exfoliated to sheet stacks via top-down
approaches. The individual layers are held together by weak
interactions like van der Waals forces, hydrogen bonds or
dielectric interactions and can potentially be separated into 2D
nanosheets if these interactions are weaker than the in-plane
bonding.[29] An increasingly popular approach to produce
colloidally stable 2D nanosheet inks is liquid phase exfoliation
(LPE).[30,31] This process has been applied to various inorganic
van der Waals crystals[32,33] and very recently also to organic 2D
structures.[34] The nanosheets from this approach are
polydisperse in size and thickness. This enables the selection of
defined sizes by centrifugation-based techniques,[35] for
example liquid cascade centrifugation (LCC),[36] and thus the
isolation of nanomaterials tailored to the needs of specific
applications. Especially, uses in catalysis, sensors or
nanomedicine can profit from nano-sized MOF particles, due to
their high internal and external surface areas as well as
controllable functionality.[29]
Zirconium based MOFs (Zr-MOFs) usually exhibit high
thermal and chemical stability[18] and even mesoporous
materials can be obtained by employing very large organic
linker molecules. The size of these linkers intrinsically enables
the formation of large pores, but they also tend to increase
susceptibility towards framework collapse.[17] In general, the
stability of Zr-MOFs arises from strong zirconium-oxygen
bonds between the Zr4+ centers and the ligands as well as from
the high connectivity of the IBUs.[18,19]
The most frequently observed IBU in Zr-MOFs is the
hexanuclear [Zr6(µ3-O)4(µ3-OH)4]12+ cluster.[19,20] Recently,
dodecanuclear [Zr12(µ3-O)8(µ3-OH)8(µ-OH)6]18+ clusters were
discovered, comprising two hexanuclear clusters which are
condensed via three shared edges.[21–24] If these are only
connected via one shared edge, an infinite one dimensional (1D)
In this work, we expanded our acetic acid based synthetic
approach further to a small functionalized linker molecule with
bent geometry (H2L: C8H5O4-NHOAc, Fig. 1) and present the
new layered mesoporous Zr-MOF [Zr30O20(OH)26(OAc)18L18],
denoted as CAU-45, together with a suitable liquid phase
exfoliation and size selection route.
IBU (1
∞
{[Zr6(µ3-O)4(µ3-OH)4(µ-OH)2]}) can be obtained, as
observed in CAU-22.[25] Moreover, whole {ZrO8} polyhedra
can be shared, leading to a 1D IBU (1
{[Zr5(µ3-O)4(µ3-OH)4]})
∞
Results and Discussion
with an even higher degree of condensation, as demonstrated
with CAU-27 in one of our latest studies.[26] In that approach,
we described the solvothermal synthesis of new dicarboxylate
zirconium MOFs via the utilization of acetic acid as a green
alternative solvent.[27,28] Acetic acid is available in such high
excess, that acetate ligands are incorporated into the structure
CAU-45 with low crystallinity was first discovered in a high
throughput[37]
linker
screening
upon
reacting
5-aminoisophthalic acid with zirconium acetate in glacial acetic
acid at 160 °C under solvothermal conditions (see SI section
1.3, Tab. S1.3.1). 5-aminoisophthalic acid was transformed to
5-acetamidoisophthalic acid during this procedure, which
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represents a limiting step, since the actual linker molecule
required for the formation of CAU-45 turned out to be the latter.
Therefore, an optimized route was developed. It comprises the
direct use 5-acetamidoisophthalic acid under otherwise similar
conditions (Fig. 1). Synthetic details are given in the supporting
information.
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Figure 1. Optimized synthesis procedure for CAU-45.
The bulk powder sample of CAU-45 consists mostly of tapered
rod-shaped crystals with lateral sizes of ca. 0.3 - 3 µm and
diameters of up to ~0.7 µm (Fig. 2A). Continuous rotation
electron diffraction (cRED) was used to determine the structure
of CAU-45, which shows CAU-45 crystalizes in the hexagonal
space group P63/mmc with a = 33.802(4) and c = 29.096(5) Å
and the c-axis is aligned along the rod direction. In order to
reduce the electron beam damage, a fast hybrid Timepix
electron detector in combination with a fast crystal rotation
speed (1.13s-1) was used. The data collection time for a cRED
dataset was less than 1.5 min and the data resolution is better
than 0.8 Å. Figure 2B presents a typical crystal selected for
cRED measurements. Figures 2C and D show the reciprocal
lattice reconstructed from typical cRED data. In order to
increase the data completeness and quality, cRED data sets
from multiple crystals were combined.[38] Hierarchical cluster
analysis was performed on data sets from 29 crystals. 18 of
them show the best agreement in terms of both the unit cell
parameters and reflection intensities, and were therefore
merged for the final structure determination.[39] The structure
was solved by direct methods and all non-H atoms could be
located directly either from the structure solution or from the
difference Fourier maps in the subsequent structure refinement.
Details on the cRED data collection, data analysis and structure
determination are given in the Supporting Information in
section S2.2, Table S2.2.1 and Figures. S2.2.1-2.2.3. The
powder X-ray diffraction (PXRD) pattern simulated from the
structural model of CAU-45 obtained by cRED reveals a good
match to the measured PXRD pattern (Fig. S2.1.3), and thus
confirms a successful structure determination. To the best of our
knowledge, CAU-45 is the first example of a mesoporous MOF
solved by direct phasing of electron diffraction data without the
need of model building.[40,41] The high resolution and high
quality data also make it possible to perform the structure
refinement (R1 = 0.2253 for 8769 reflections and 335
parameters) so that the hydrogen bond network and disorders of
the amide groups can be identified.
Figure 2. Scanning electron microscopy (SEM) micrograph of a
powder sample (A), transmission electron microscopy (TEM)
micrograph of a typical crystal used for cRED measurements (B),
reconstructed 3D reciprocal lattice of
a typical dataset
reconstructed with REDp[42] viewed along the c*-axis (C) and a 0kl
slice from the same reconstruction (D).
The following structure description assumes the presence of H-
atoms and H-bonds. H-atoms in the IBUs are assigned
according to literature data,[19] while the discussion of H-
bonding is based on donor-acceptor distances. CAU-45
contains two different IBUs, the well-known hexanuclear
[Zr6(µ3-O)4(µ3-OH)4]12+ cluster (Fig. 3, A) and the less
frequently observed dodecanuclear [Zr12(µ3-O)8(µ3-OH)8(µ-
OH)6]18+ cluster (Fig. 3, B). To the best of our knowledge, this
is the first example of hexa- and dodecanuclear clusters
occurring simultaneously in a Zr-MOF structure. So far, only
mixed hexanuclear and octanuclear clusters [Zr8O6(OH)8]12+
could be observed in NPF-200 and -201 by Zhang et al. in
2016.[43] Within the CAU-45 structure, both cluster types are
partly capped by acetate ligands. The hexanuclear clusters
exhibit four acetate ligands per cluster ([Zr6(µ3-O)4(µ3-
OH)4(OAc)4]8+) and the dodecanuclear clusters exhibit six
acetate ligands per cluster ([Zr12(µ3-O)8(µ3-OH)8(µ-
OH)6(OAc)6]12+) (Fig. 3, A, B). Therefore, the potential
coordination sites for linker carboxylate groups are reduced
from 12 to 8 and from 18 to 12, respectively. Consequently,
each hexanuclear [Zr6(µ3-O)4(µ3-OH)4]12+ cluster connects to
eight linkers and each dodecanuclear [Zr12(µ3-O)8(µ3-OH)8(µ-
OH)6]18+ cluster connects to twelve linkers.
Three hexanuclear [Zr6(µ3-O)4(µ3-OH)4]12+ clusters form a
trimer, connected by three pairs of the 5-acetamidoisophthalate
linkers via their carboxylate groups. There are two trimers and
two dodecanuclear [Zr12(µ3-O)8(µ3-OH)8(µ-OH)6]18+ clusters in
each unit cell. All are located on the 3-fold axis, i.e. (1/3, 2/3,
±1/4) or (2/3, 1/3, ±1/4). Each dodecanuclear cluster connects
to three trimers and each trimer connects to three dodecanuclear
clusters by six pairs of the linkers via their carboxylate groups
to form a layer with an overall composition of ([{Zr6(µ3-O)4(µ3-
OH)4(OAc)4}3{Zr12(µ3-O)8(µ3-OH)8(µ-OH)6(OAc)6}(C8H3O4-
NHAc)18]) (Fig. 3). There are two layers in each unit cell,
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related by a 63 symmetry, which stack with alternating trimer
isotherm which is characteristic for mesoporous adsorbents
(Fig. S2.5.1). The maximum water adsorption capacity is
330 mg/g (Fig. S2.5.2). Variable-temperature (VT) PXRD and
TGA measurements indicate a thermal stability up to 330 °C in
air (SI section 2.4). 24 h solvent stability tests with common
organic liquids and aqueous solutions of different pH (1-13)
were carried out with CAU-45 (Tab. S2.10.1). The material
remained crystalline in 16 out of 23 tests.
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and dodecanuclear cluster along the c-axis.
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Figure 4. Two stacked layers of CAU-45 viewed along [110] (left)
and [001] (right). The layers are held together by hydrogen bonds
between the N-H protons and C=O oxygen atoms of neighboring
linker amide groups. Since proton positions could not be located,
hydrogen bonds are represented as dashed green lines between
amide nitrogen and oxygen atoms. Linker carbon atoms are
displayed in black and acetate carbon atoms in pink. Zirconium:
bronze, oxygen: red, nitrogen: dark blue, hydrogen: not displayed.
Zirconium-oxygen clusters: blue.
Figure 3. Inorganic building units (IBUs) of CAU-45 and
interconnection to a layer motive via 5-acetamidoisophthalate ions.
A: Hexanuclear clusters. B: Dodecanuclear clusters. Linker carbon
atoms are displayed in black and acetate carbon atoms in pink.
Zirconium: bronze, oxygen: red, nitrogen: dark blue, hydrogen: not
displayed. Zirconium-oxygen clusters: blue.
The layered structure should enable exfoliation. To demonstrate
this, inks of delaminated CAU-45 containing different sizes of
nanosheets were prepared using LPE in aqueous surfactant
solution coupled with centrifugation-based size selection
according to established protocols as detailed in the SI. As a
first approach, the structural integrity upon exfoliation was
tested. A stock dispersion was prepared by ultrasonication and
incompletely exfoliated material was removed at low
centrifugal acceleration (400 g; expressed in multiples of the
earth’s gravitational force, g). The dispersion was subsequently
concentrated by sedimenting the nanosheets at 74k g and
characterized via PXRD (dried dispersion) as well as Raman
measurements (drop-cast dispersion). Since the comparison of
bulk and exfoliated material (Fig. S2.9.1) does not show any
differences with respect to the characteristic features, the
structure of the MOF can be considered intact after the
exfoliation. The nanosheet yield after removal of the mostly
unexfoliated material after centrifugation at 50 g was
determined to be 38.0% by gravimetric filtering. However, due
to the strong interlayer interactions, the nanomaterial isolated is
thicker than observed for other 2D-materials of the same lateral
dimensions. If the material sedimented at speeds up to 400 g
(i.e. <T> ≤ 80 nm) is considered as nanosheets, the yield of the
exfoliation would decrease to 4.3%. We would like to point out,
that the yield of the exfoliation mostly depends on what is
considered as nanomaterial.
The layers are held together by a network of hydrogen bonds,
being formed between neighboring linker amide groups (Fig. 4,
left). Since neighboring amide groups within the structure of
CAU-45 never originate from the same layer, a relatively strong
layer-layer interaction is established. It is likely that the
hydrogen bonding is crucial for the formation of CAU-45, since
the MOF could not be obtained with other related linker
molecules, like isophthalic acid for example (Tab. S1.3.1). The
overall CAU-45 network forms large hexagonal-shaped
channels (Fig. 4, right). Per pore, each layer is connected to the
next layer by 18 hydrogen bonds. Six acetate ligands per
formula unit, which exclusively originate from hexanuclear
clusters, point directly into the pore channels, opening up
possibilities for postsynthetic modifications. Examples would
be ligand exchange or removal, which have been already
realized for various Zr-MOFs.[26,44–48] The widest section of the
channels can accommodate a non-colliding sphere with a
diameter of ~22 Å (Fig. S2.2.9), while the narrowest (limited
by the steric demand of the acetate ligands) has a diameter of
~17 Å. A more in-depth structure description is given in the SI.
The composition of CAU-45 was confirmed with various
characterization techniques such as thermogravimetric (TGA)
1
and elemental analysis (EA) as well as H nuclear magnetic
resonance (NMR), Infrared (IR) and Raman spectroscopy
(details are given in the SI).
Nitrogen and water adsorption/desorption experiments were
carried out in order to investigate sorption properties of the
material. According to the IUPAC (International Union of Pure
and Applied Chemistry) guidelines for the physisorption of
gases,[49] CAU-45 exhibits a type IV(b) nitrogen adsorption
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(Fig. 5 C-D, Fig. S2.9.4). Mean size and thickness of the
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(A)
(B)
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2
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nanosheets follow the typical power law dependence with
respect to the midpoint of the centrifugation speed (central
RCF) between two steps (Fig. 5 E-F).[32,36,50]
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The mean lateral size can be varied between 300 and 700 nm,
with thicknesses ranging from 60-150 nm and
a
0.5 µm
0.5 µm
length/thickness aspect ratio centered around 6 which is similar
to layered hydroxides.[32]
20
10
15
10
5
This clearly demonstrates both successful exfoliation, as well as
size selection. Systematic size dependent changes are observed
in absorbance (Fig. 5G and Fig. S2.9.6), extinction (Fig. S2.9.6)
and fluorescence measurements (Fig. 5 H and Fig. S2.9.7). This
behavior is typical for 2D nanomaterial dispersions[34,36,51–53]
and commonly attributed to a combination of light scattering[53]
(in extinction spectra), edge[34,51] as well as dielectric screening
and confinement effects.[51,52] Each effect changes spectra in a
characteristic way. In the case of CAU-45, we attribute the
spectral changes shown mostly due to edge effects. It allows to
develop spectroscopic metrics for the mean size of the
nanosheets in the ink which is useful for future work to assess
the size dependent application potential. For a detailed
discussion see SI.
50-100 g
50-100 g
(C)
(D)
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Conclusion
The discovery and characterization of the thermally (up to
330 °C in air) and chemically stable layered mesoporous
zirconium MOF CAU-45 [Zr30O20(OH)26(OAc)18L18] is
reported. The compound was synthesized by reacting zirconium
acetate with the small bent-geometry linker molecule
5-acetamidoisophthalic acid (H2L) under solvothermal
conditions in acetic acid, a green solvent. Continuous rotation
electron diffraction (cRED) was used for the structure
determination of CAU-45. Its structure features hexa- and
dodecanuclear zirconium-oxygen clusters as IBUs and large
hexagonal pores with a diameter of ca. 17 Å in its narrowest and
22 Å in its widest sections. Per formula unit, a total of 18 acetate
ligands are capping otherwise free coordination sites on the
IBUs, whereof six are directly pointing towards the center of
the pore, opening up possibilities for postsynthetic
modifications. The layers of the material are held together by a
large number of hydrogen bonds, which adds up to 18 per pore
channel at the layer intersection. These bonds are established
between neighboring linker amide groups via their N-H protons
and C=O oxygen atoms to form a circular connection scheme
along the pore wall, perpendicular to the crystallographic
c-axis. In spite of this relatively strong out of plane bonding, we
demonstrate that exfoliation by LPE is possible which allows to
produce inks of structurally intact CAU-45 nanosheets that can
be size-selected by centrifugation. The inherent solution
processability in combination with the tuneability of size are the
prerequisite to test CAU-45 in various application areas for
example catalysis, gas and ion separation or sensing in future
work.
300
1.5
10
100
RCF (g)
1000
10
100
RCF (g)
1000
1.5
1-50 g
50-100 g
100-400 g
400-1000 g
1000-5000 g
1-50 g
50-100 g
100-400 g
400-1000 g
(G)
(H)
SIZE
1.0
0.5
0.0
1.0
0.5
0.0
1000-5000 g
SIZE
200
300
400
400
500
600
Wavelength (nm)
Wavelength (nm)
Figure 5. Liquid phase exfoliation. AFM images of relatively large
(50-100 g, A) and small (1-5k g, B) drop-cast nanosheets after LPE
and LCC. The images show representative nanosheets of the
respective fractions. (C-D) Histograms of lateral sheet size (C) and
thickness (D) distribution of two above mentioned fractions of size-
selected nanosheets. (E+F) Plot of arithmetic mean nanosheet
length, <L> (E) and thickness, <T> (F) as a function of the
midpoint of the pair of centrifugal accelerations used during size
selection. The data in (E) and (F) is characterized by a power law
scaling as indicated by the dashed line. (G-H) Normalized
absorbance (G) and fluorescence (H) spectra measured with an
excitation wavelength of 325 nm of different sizes of nanosheets.
Size dependent changes in (G-H) are highlighted by a grey arrow.
To achieve size selection, LCC was performed on the stock
dispersion involving consecutive centrifugation steps with
iteratively increasing centrifugation speeds. After each
centrifugation step, the sediment was redispersed at increased
nanosheet concentration and analyzed, while the supernatant
was subjected to centrifugation at higher speeds. In this way,
five fractions were isolated and labelled by the boundaries of
centrifugal accelerations between two steps (Fig. S2.9.2).The
nanosheet dimensions were determined by statistical atomic
force microscopy (AFM) measurements (Fig. 5 A-D,
Fig. S2.9.4 and S2.9.5). From images such as in Figure 5 A-B,
the longest dimensions (length, L), and thickness (T) of each
nanosheet were measured to obtain distribution histograms
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website. It contains the utilized chemicals and
instruments, synthetic procedures, detailed structural analysis by
cRED and structure description, powder X-ray diffraction (PXRD)
and variable temperature (VT) PXRD data, thermogravimetric
(TGA) and elemental analyses (EA), infrared (IR) and RAMAN
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(13) Vikrant, K.; Kumar, V.; Ok, Y. S.; Kim, K.-H.; Deep, A. Metal-
spectra, scanning electron microscopy (SEM) and atomic force
microscopy (AFM) micrographs, nitrogen and water sorption
isotherms, 1H nuclear magnetic resonance (NMR) analysis as well
as UV-Vis and fluorescence measurements (PDF file). The CAU-
45 crystal structure is deposited as CIF file and also available from
the Cambridge Crystallographic Data Centre (CCDC) under entry
number 2012374.
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AUTHOR INFORMATION
Corresponding Authors
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Prof. Dr. N. Stock, Institute of Inorganic Chemistry, University of
Kiel, Max-Eyth Strasse 2, 24118 Kiel (Germany). E-mail:
stock@ac-uni-kiel.de
Prof. Dr. X. Zou, Department of Materials and Environmental
Chemistry, Stockholm University, SE-106 91 Stockholm
(Sweden). E-mail: xzou@mmk.su.se
Dr. C. Backes, Institute of Physical Chemistry, Heidelberg
University, Im Neuenheimer Feld 253, 69120 Heidelberg
(Germany). E-mail: backes@uni-heidelberg.de
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
X.Z. acknowledges support from the Swedish Research Council
(VR, 2017-04321) and the Knut and Alice Wallenberg Foundation
(KAW, 2012-0112). C.B. acknowledges support from the German
Research Foundation (DFG) under grant agreement Emmy-
Noether, BA4856/2-1, and Jana Zaumseil for the access to the
infrastructure at the Chair of Applied Physical Chemistry.
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