Competitive coordination strategy for the synthesis of hierarchical-pore metal-organic framework nanostructures

Article abstract of DOI:10.1039/C6SC02272C

Metal-organic frameworks (MOFs) usually have micropores smaller than 2 nm, which may restrict their applications in some cases. Hierarchical-pore MOFs (H-MOFs) are a new family of MOF materials, possessing both micro- and mesopores to address this problem. Here we demonstrate a competitive coordination strategy for the synthesis of H-MOF nanostructures, such as two-dimensional (2D) H-MOF nanosheets and H-MOF nanocubes, evolving through an etching process tuned by a competitive ligand. The as-synthesized 2D H-MOF nanosheets can serve as a substrate to in situ immobilize Pd nanoparticles to achieve a surfactant-free Pd catalyst, by means of a simple soaking method of Pd²⁺ precursors. Combined with the unique structure and gas adsorption capacity of H-MOF-5, the Pd-H-MOF-5 catalyst exhibits superior catalytic performance.

Full text of DOI:10.1039/C6SC02272C

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DOI: 10.1039/C6SC02272C
Chemical Science
ARTICLE
Competitive Coordination Strategy for the Synthesis of
Hierarchical-pore Metal-organic Framework Nanostructures
Su Hea, Yifeng Chenb, Zhicheng Zhanga, Bing Nia, Wei Heb, and Xun Wanga
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Metal-organic frameworks (MOFs) usually have micropores smaller than 2 nm, which may restrict their applications in
some cases. Hierarchical-pore MOFs (H-MOFs), are a new family of MOF materials, possessing both micro- and mesopores
to address this problem. Here we demonstrate a competitive coordination strategy to synthesis of H-MOF nanostructures,
such as two-dimensional (2D) H-MOF nanosheets and H-MOF nanocubes, evolving through an etching process tuned by a
competitive ligand. The as-synthesized 2D H-MOF nanosheets can serve as a substrate to in situ immobilize Pd
nanoparticles to achieve surfactant-free Pd catalyst, by means of a simple soaking method of Pd2+ precursors. Combining
with the unique structures and gas adsorption capacity of H-MOF-5, the Pd-H-MOF-5 catalyst exhibits a superior catalytic
performance.
www.rsc.org/
hierarchical-pore MOFs (H-MOFs) would be advantageous in
utterly utilizing of materials22, 23. The micropores provide
exceptionally high surface areas and large pore volumes,
Introduction
Constructed by inorganic nodes and organic linkers with two
or more ligation sites, metal-organic frameworks (MOFs) have
received tremendous attention in recent years1-3
whereas the mesopores facilitate diffusion and accessibility.
Many efforts have been made to reach the aim. The most
commonly adopted one is the ligand-extension method24 - by
using longer organic ligands to achieve the mesoporosity - and
quite a few MOFs with ultralarge pores have been elaborately
designed and synthesized25. However, it is usually limited by
unstability and possible interpenetration as the size extends,
along with the high cost and difficulty in synthesizing larger
organic ligands. Besides, surfactant-templating method22,
microwave-assisted method26, CO2-expanded liquid route27,
ionic liquid system28, post-synthetic method29 are also
alternative to create mesopores. Nonetheless, these
innovative methods require either a template or complicated
techniques, and are difficult to extend to other systems. So
new versatile methods are imperative to establish stable and
high-quality H-MOFs.
. Their
glamorous diversities in structures could be tailored by rational
design of metal-based inorganic nodes and organic linkers with
judicious geometrical and functional selections. Traditional
structure studies of MOFs have long since developed, with
more and more new MOFs burgeoning4, 5, and they have
shown widespread potential applications in gas storage6,
adsorption7, 8, molecular separation9, catalysis10-12, imaging13
and so on. Notably, two-dimensional (2D) MOFs, with many
highly active sites on their surfaces like other 2D materials14-16
,
have attracted increasing interest recently17-19. Besides, due to
their large surface areas, guest molecules could effectively
pass through the micropores18, which could benefit gas-
involved catalytic reactions.
Meanwhile, MOFs, owing to their microporosity (pore size <
2 nm), benefit the adsorption and separation of small
molecules. However, the small pore size in microporous MOFs
in turn limits fast mass diffusion and restrains larger molecules
from entering into the center through MOF micropores, hence
Herein, we developed a competitive coordination strategy
to synthesis of H-MOF nanostructures, possessing both micro-
and mesopores in the network. The as-synthesized 2D H-MOF
nanosheets and H-MOF nanocubes evolved through an etching
process tuned by
a competitive ligand as modulator.
restricts the potential practical applications in some cases20, 21
.
Furthermore, the 2D H-MOF nanosheets could be used as a
substrate to in situ form and immobilize Pd nanoparticles
without any surfactant (denoted as Pd-H-MOF), by means of a
simple soaking method of Pd2+ precursors. Taking advantages
of the gas adsorption capacity of H-MOFs, the surfactant-free
Pd catalyst exhibited an excellent catalytic performance.
Therefore, enlarging the pore size or creating some larger
pores in primitive MOF structures is necessary, however,
challenging at present. Once introducing mesopores into the
previously microporous MOF structures, the as-achieved
Results and discussion
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demonstrates that the sizes of pores inside are not so uniform
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and that the pores form a network of muDltOi Il:a1y0e.1r0s3l9ik/Ce6aSCs0p2o2n72gCe
rather than one single layer with pores on it like a paper-cut.
The thickness of 2D porous nanosheets was determined to be
approximately 19 nm, using atomic force microscope (AFM)
(Figure S2).
As for the composition, elemental mapping by energy-
dispersive X-ray spectroscopy (EDX) shows that the elements C,
O, N, Zn are uniformly distributed throughout the whole
nanosheet (Figure 1e). And X-ray photoelectron spectrum (XPS)
also identifies the composition (Figure S3), where the
existence of N is attributed to the residues of solvent molecule
and pyrrolidone rings of PVP. Fourier Transform Infrared (FT-IR)
spectrum proves the accomplished coordinative interaction
between Zn4O clusters and carboxylic group of deprotonated
H2BDC, as disclosed by a red shift in the C=O stretching
frequency from 1680 cm-1 - the characteristic C=O stretching
frequency of uncoordinated free H2BDC30 - to 1603 cm-1 (Figure
S4). The powder X-ray diffraction (PXRD) pattern of the
product, which was obtained after evacuation of solvent
molecules at 100oC under vacuum for 6 h, corresponds well
with that of the simulated MOF-5 XRD patterns31 (Figure S5),
while the difference of relative peak intensities might originate
from preferential orientation.
The 2D H-MOF-5 nanosheets exhibited high thermal stability
(>400oC) evidenced by the thermogravimetric (TG) curve
(Figure S6) taken in air (nitrogen). The weight loss below 100oC
could be ascribed to the solvent liberation or the loss of guest
molecules, while the significant weight loss in the range from
400oC to 480oC arises from the framework collapse20, 31. The
nanosheets also showed permanent porosity after evacuation
of solvent molecules. N2 sorption data at 77 K displayed a type
I isotherm at relatively low P/P0 pressures for a micropore
structure and a typical type IV isotherm at relatively high
P/P027 (Figure 1f), which is related to the capillary condensation
associated with mesopore channels32, suggesting an additional
presence of mesopores. The mesopore diameter distribution
was calculated using the Barrett-Joyner-Halenda (BJH) method
(inset of Figure 1f), indicating a broad range of mesopore
diameters, from 6 nm to 30 nm, which could also be confirmed
visually by TEM and SEM images. The micropore diameter
distribution was also calculated using the Saito-Foley (SF)
method32 (Figure S7), revealing the existence of micropores,
which are the inherent characteristic of the framework of
MOF-51. Due to the hierarchically porous structures, BET
specific surface area of the sample could reach up to 1669 m2
g-1.
Figure 1. (a,b) TEM images, (c) SEM image, (d) HAADF-STEM
images, (e) EDX elemental mapping images and (f) nitrogen
adsorption (▪) and desorption (◦) isotherms measured at 77K of
2D H-MOF-5 nanosheets. Inset in (f): the corresponding pore
size distribution.
MOF-5 was first selected as an example to prove the
conception of competitive coordination strategy. The 2D H-
MOF-5 nanosheets were synthesized via a simple solvothermal
reaction
benzenedicarboxylic acid (H2BDC), lauric acid (LA), and
Polyvinylpyrrolidone, K-30 (PVP, K-30) in N,N-
of
zinc
acetate
(Zn(OAc)2·2H2O),
1,4-
dimethylacetamide (DMAC). The porous morphology and
structure were identified by diverse characterizations. The
transmission electron microscopy (TEM) images (Figure 1a, 1b
and Figure S1) show
a typical shape of 2D H-MOF-5
The formation of these 2D H-MOF-5 nanosheets underwent
a morphologic evolution. Small solid square nanosheets came
into being at the early stage. Then an etching process took
place, with more and more visible pores emerging both inside
and on the edges, until they sprawled over the entire
nanosheets (Figure 2). The formation of MOF-5 was
accomplished at thevery beginning, as confirmed by PXRD
(Figure S8) and FT-IR results (Figure S9), and no obvious
compositional changes happened during the etching process.
nanosheets. All the nanosheets spread flatly without curls or
wrinkles and the length and width could reach to micron-scale.
The porosity of the nanosheets was further confirmed more
distinctly by the scanning electron microscopy (SEM) image
(Figure 1c) and high-angle annular dark-field scanning TEM
(HAADF-STEM) image (Figure 1d), where numerous visible
mesopores are throughout the whole nanosheets, even on the
edges. High-magnification HAADF-STEM image (inset of Figure
1d) provides an access to better observe the mesopores, which
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DOI: 10.1039/C6SC02272C
Figure 2. TEM images of the competitive coordination etching
process. (a) Initial solid MOF-5 nanosheet, (b) Intermediate I
with several mesopores, (c) Intermediate II with more
mesopores, (d) Final 2D H-MOF-5 nanosheets.
Figure 3. Schematic illustration of the competitive
coordination etching process.
Figure 4. (a) TEM, (b) SEM images of IRMOF-3 nanocages; (c)
TEM, (d) SEM images of hierarchical-pore 2D IRMOF-8
hierarchical-pore nanosheets.
LA could coordinate with carboxyl group to surface-exposed
Zn4O sites, acting as a diffusion limiter for both the metal ions
and organic ligands, hence controlling crystal growth7. We
proposed a competitive coordination mechanism to explain
the evolutionary process, in which LA could serve as a
competitive ligand or modulator in the coordination to Zn4O
clusters33. Deprotonated H2BDC and LA were both capable of
coordinating to Zn4O clusters in the initial stage, though the
coordination of deprotonated LA was much weaker, because
LA had one single carboxyl group while H2BDC had two to form
the stable MOF networks. As the reaction proceeded, the
coordination of deprotonated H2BDC predominated, while the
weak bonding of deprotonated LA to Zn4O clusters dissociated
to release LA from the framework. The migration of LA could
provide copious sites for further etching and afterwards
enable the creation of mesopores in the nanosheets (Figure 3).
To confirm the mechanism and disclose the details, a series of
control experiments were conducted (Figure S10-S12). It turns
out that the final morphology is sensitive to initial
concentration and reaction temperature, so these parameters
need to be reconsidered when extending to other MOFs. All
the reactants are indispensable. Zn(OAc)2·2H2O acts as metallic
nodes and H2BDC as organic linkers to construct the
elementary framework of MOF-5. PVP determines the
formation of 2D nanosheets, and the molecular weight of PVP
plays an important role in directing the morphology (Figure
S13). While LA, as mentioned before, directs the etching
process and mesoporous structure. Without LA, merely solid
nanosheets could be prepared (Figure S12a), because no
competitive coordination was involved to provide empty sites
for further etching process and the formal coordination led to
complete structures. Notably, other long-chain organic acid
with 10-18 carbon atoms could function as well (Figure S14) to
achieve 2D H-MOF-5 nanosheets, whereas short-chain organic
acid or alkane could only result in solid nanosheets (Figure
S15).
This competitve coordination strategy is also effective to
direct other H-MOFs. By the expansion of MOF-5 structure,
other hierarchical-pore IRMOFs, an isoreticular series sharing
the same topology with MOF-5, could likewise be achieved.
For IRMOF-3, functinalized with amino, H-MOF nanocubes
could be prepared through competitive coordination. And for
IRMOF-8, extending from the original ligand of the one
phenylene ring to two, 2D H-MOF nanosheets could be
realized similiar with previous result of MOF-5 (Figure 4). They
were further characterized (Figure S16, S17).
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Figure 5. (a) TEM image, (b,c) HRTEM images, (d) HAADF-STEM corresponding products (Figure 6c), with boVtihew AertlieclcetOrnolinne-
image of Pd-H-MOF-5 nanosheets. (e) EDX elemental mapping withdrawing (2b, 2c) and –donating (2d, D2OeI,:210f,.120g39,/2Ch6)SCg0ro22u7p2sC,
results of the selected area marked by the white dashed line in and besides, the reaction was even extended to pyridyl system
(d). (c) is the magnified HRTEM image of the selected area (2i). All the results proved that the Pd-H-MOF nanosheets can
marked by the white dashed line in (b).
be a good candidate for reduction of nitroarene reactions. The
The 2D H-MOF-5 nanosheets can be used to confine the excellent catalytic performance originates from the unique
growth of surfactant-free Pd nanoparticles through a simple structures and gas adsorption capacity of H-MOF, combined
soaking method with Pd2+ precursor in the mixed solvent of with the surfactant-free immobilized Pd nanoparticles (Figure
DMAC and EtOH. Up to now, many reports have discussed 6d).
about the loading of various noble metal nanoparticles into
MOF nanostructures, which requires extra efforts to
synthesize noble metal nanoparticles alone in advance, where
the introduction of surfactants is mostly difficult to avoid34. In
a typical experiment, we chose PdCl2 as the Pd2+ precursor.
Due to the reducibility of DMAC, the reduction of Pd2+ was
easy to achieve. Pd nanoparticles with the size of sub-10 nm
were uniformly distributed in the nanosheets (Figure 5d),
without obvious agglomeration. Meanwhile, the 2D porous
morphology and crystal structure of H-MOF-5 could be
maintained, as demonstrated by PXRD results (Figure S18). The
morphology, size and composition of the Pd-H-MOF-5 were
also illustrated by TEM, HRTEM, EDX, EDX mapping images and
inductively coupled plasma optical emission spectrometry (ICP-
OES) (Figure 5, Figure S19). Additionally, Pd acetylacetonate
(Pd(acac)2) could also serve as Pd2+ precusor to acquire similar
results (Figure S20), although the reduction was much more
difficult and required more soaking time. But no Pd
Figure 6. (a) Reduction of nitroarene. Reaction conditions: 1
nanoparticles could be obtained without 2D H-MOF-5
(0.5 mmol), Pd-H-MOF-5 nanosheets (0.01 equiv), H2 (balloon)
in EtOH at 60oC, GC yields. (b) Conversion (%) as a function of
nanosheets in the mixed solvent of DMAC and EtOH, even
soaking for two days. This indicates that the nanosheets may
time in the reduction reaction over Pd-H-MOF-5 nanosheets
serve as a substrate to provide appropriate nucleation sites for
(catalyst A), Pd-H-MOF-5 post-treated with H2O (catalyst B),
Pd nanoparticles to form and grow easily, and the solvent
DMAC could effectively assist the reduction of Pd2+.
Taking into account the hydrogen storage capacity of MOF-56
Pd-H-MOF-5 nanosheets with surfactants (catalyst C), Pd@bulk
MOF-5 (catalyst D) and without catalyst. (c) Products and
yields of isolated products from the reduction of nitroarene
and the activity of surfactant-free Pd nanoparticles, we
catalyzed by Pd-H-MOF-5 nanosheets for 4 h. (d) Schematic
envisioned that the Pd-H-MOF nanosheets should integrate
representation of the reduction of nitroarene over Pd-H-MOF-
these advantages and show an excellent catalytic performance.
5 nanosheets.
A Pd-catalyzed reduction of nitroarene using hydrogen as a
reductant is an important industrial process (Figure 6a), and
was chosen to demonstrate the structural advantage. In order
Conclusions
to confirm the effectiveness of Pd-H-MOF-5 nanosheets
Foregoing results demonstrate the availability and versatility
(catalyst A), three other kinds of Pd catalysts were synthesized
of the competitive coordination strategy to synthesis of H-
as well for comparison (Figure S21). Catalyst B was Pd-H-MOF-
MOF nanostructures, such as 2D H-MOF nanosheets and H-
5 nanosheets post-treated with water to destroy the MOF
MOF nanocubes. All the unique H-MOF nanostructures were
nanostructures, while the Pd nanoparticles maintained pristine
evolved through an etching process tuned by a competitive
structures. Catalyst C had a similar shape to Pd-H-MOF
ligand. Using the 2D H-MOF nanosheets as a substrate through
nanosheets, except for the synthetic procedure involving
a simple soaking method, surfactant-free Pd nanoparticles
surfactants, to ascertain the virtue of surfactant-free Pd as
could be in situ formed and immobilized. This could integrate
catalysts. Catalyst D was bulk MOF-5 immobilized with Pd
the advantages of H-MOF and surfactant-free Pd nanoparticles,
nanoparticles (Pd@bulk MOF-5) using the similar soaking
utilizing the hydrogen adsorption properties and large surface
method, to illustrate the benefit of H-MOF-5 nanosheets.
area of 2D H-MOF-5 nanosheets, as well as the surfactant-free
Figure 6b exhibits the kinetic curves for the reduction of
merit of Pd nanoparticles. We believe, such a competitive
nitroarene reactions on these three catalysts, where the
coordination strategy could provide an opportunity to design
excellent catalytic activity of catalyst A was uncovered. The
and synthesize other various MOF nanostructures and
results demonstrate the structural advantages of H-MOF-5 and
promote the precise morphologic control of MOFs for further
surfactant-free Pd nanoparticles. In addition, substrates with
applications.
different functional groups were likewise tested to obtain the
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29 Y. Kim, T. Yang, G. Yun, M. B. Ghasemian, J. Koo, E. Lee, S. J.
Cho and K. Kim, Angew. Chem. Int. Ed., 2015, 54, 13273-
Acknowledgements
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DOI: 10.1039/C6SC02272C
13278.
This work was supported by NSFC (21431003, 21521091) and
China Ministry of Science and Technology under Contract
of 2016YFA0202801.
30 Z. Zhang, Y. Chen, X. Xu, J. Zhang, G. Xiang, W. He and X.
Wang, Angew. Chem. Int. Ed., 2014, 126, 439-443.
31 Z. Zhang, Y. Chen, S. He, J. Zhang, X. Xu, Y. Yang, F. Nosheen,
F. Saleem, W. He and X. Wang, Angew. Chem. Int. Ed., 2014,
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