Fabrication of a Kagomé-type MOF Membrane by Seeded Growth on Amino-functionalized Porous Al2O3 Substrate

Article abstract of DOI:10.1002/asia.202100507

In this study, we report an efficient fabrication method for the membrane of a metal-organic framework (MOF) (Kgm?OEt) which is one kind of kagomé-type MOF with a two-dimensional (2D) sheet structure having one-dimensional (1D) channels suitable for separ

Full text of DOI:10.1002/asia.202100507

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Accepted Article
Title: Fabrication of a kagomé-type MOF membrane by seeded growth
on amino-functionalized porous Al2O3 substrate
Authors: Xiaoguang Wang, Shinpei Kusaka, Akihiro Hori, and Ryotaro
Matsuda
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Fabrication of a kagomé-type MOF membrane by seeded growth
on amino-functionalized porous Al2O3 substrate
Xiaoguang Wang, Shinpei Kusaka, Akihiro Hori, Ryotaro Matsuda*
Mr. X. Wang, Dr. S. Kusaka, Dr . A. Hori, Prof. R. Matsuda
Department of Chemistry and Biotechnology, School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
E-mail: ryotaro.matsuda@chembio.nagoya-u.ac.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: In this study, we report the efficient fabrication method for
the membrane of a metal-organic framework (MOF) (Kgm-OEt) which
is one kind of kagomé-type MOF with a two-dimensional (2D) sheet
structure having one-dimensional (1D) channels suitable for
separation of H2 from other larger gases. Kgm-OEt seed layer was
created on a Al2O3 substrate using layer-by-layer (LBL) growth, then
a membrane was fabricated by the secondary growth. The membrane
In this study, a new membrane based on [Cu(OEt-ipa)] (Kgm-OEt,
OEt-ipa = 5-Ethoxyisophthalate) was prepared by secondary
growth method on Al2O3 substrate. Kgm-OEt having an infinite
kagomé-type 2D sheet structure.[25-27] Kgm-OEt is formed through
the connection between Cu paddle-wheel units by the OEt-ipa
ligands. The 2D sheets create two kinds of one-dimensional (1D)
infinite channels along the c axis; one channel is triangular and
contains only coordinated water molecules on the paddle wheel
SBUs. Another pore has hexagonal geometry and contains all the
alkoxy groups pointed towards the pore center. The cross-
sectional size of the two channels is around 3.5 Å after the
removal of coordinated solvent molecules. The channel is larger
than the kinetic diameter of H2 (2.89 Å) and smaller than those of
N2 (3.64 Å), CH4 (3.80 Å) and C2H4 (4.20 Å). In this study, we
on
a
3-aminopropyltriethoxysilane (APTEs)-treated substrate
obtained in this method was continuous and defect-free with the
crystal orientation suitable for gas transportation, while the membrane
grown on an unmodified substrate was loosely packed with the
unfavorable crystal orientation.
Metal-Organic Frameworks (MOFs) are a new class of crystalline
porous solid materials for gas storage,[1,2] separations,[3,4]
heterogeneous catalysis,[5] drug delivery[6] and so on. Their
structures are one, two, or three-dimensional periodic networks
formed by the bridging of metal ion-based nodes and organic
ligands. An important essence of MOFs is the large specific
surface area and tunable pore size, structure and functionality.[7-
employed
a combination of surface modification of Al2O3
substrate and LBL seeding method to fabricate seed layer
followed by the secondary solvothermal crystal growth forming
continuous Kgm-OEt membrane (Scheme 1).
9]
In the past few decades, MOF membranes for gas and liquid
separations have been extensively studied.[10-12] However, it
remains challenging to prepare a continuous and high-quality
MOF membrane for practical gas separation applications, owing
to the low affinity between the MOFs and substrates. For example,
the direct growth of MOF membranes by immersing substrates in
the solution of metal ions and organic ligands is the primary
fabrication method.[13] However, it often leads to heterogeneous
nucleation of MOF crystals resulting in the formation of defects,
although chemical modification of substrates sometimes can
improve it.[14,15] Instead of direct growth, the secondary growth
method, where MOF membrane is grown from the substrate with
seed MOF crystals, is regarded as a promising method. In the
secondary growth method, the fabrication of a continuous seed
layer is one of the key processes. Nevertheless, the preparation
of a homogeneous seed layer is not easy due to the poor
adhesion bond between MOF particle and surface of common
substrate materials such as Al2O3. Some methods have been
developed so far to prepare a uniform seed layer.[16-19] 3-
aminopropyltriethoxysilane(APTEs) is used to chemically modify
substrates to enhance the interaction between the substrate
surface and MOF.[20,21] The layer-by-layer (LBL) growth method is
a commonly used technique to fabricate thin MOF films known as
surMOF by immersing the substrate into solutions of ligands and
metal ions repeatedly.[22-24] However, the studies on the
preparation of two-dimensional (2D) MOF membrane by LBL
growth are still limited since 2D MOF has high aspect ratio of
crystals, which often prevents dense packing of crystals.
Scheme 1. Schematic diagram for the preparation of the membrane and the
structure of Kgm-OEt. Atoms are colored as follows: Cu, green; O, red; C, gray.
The LBL method was applied to synthesize Kgm-OEt seed layer.
Firstly, a layer of APTEs was deposited on an Al2O3 substrate
through the reaction of ethoxysilane of APTEs and hydroxyl
groups on the surface of Al2O3. Secondly, the seed layer was
construted by soaking the substrate into the solutions of H2OEt-
ipa and Cu(NO3)2 repeatedly. After the first seeding cycle, strong
XRD peaks indicate the formation of Kgm-OEt crystals on the
APTEs-modified substrate, while the weak XRD peaks suggest
that almost no crystals were formed on the unmodified Al2O3
substrate (Figure S1). In addition, SEM images show that MOF
crystals can grow on the surface of APTEs-modified substrate,
while only the substrate surface is seen on the unmodified Al2O3
substrate (Figure S2). Optical images of the substrate also show
that the color of the seed layer from the APTEs-modified substrate
was green which was far deeper than that from the unmodified
substrate (Figure S3). These results indicate that APTEs
modification can promoted the crystal nucleation.
1
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the intergrowth structure on the APTEs-modified substrate and
the thickness of the membrane on APTEs-modified substrate was
about 3 µm (Figure 2(c)-(d)), which was about 3 times thinner than
that by unmodified substrate, and the membrane was grown very
compact without observable gaps between the intercrystalline
boundary. It should be noted that the 2D sheet MOF crystals on
the unmodified substrate tend to grow nearly vertical to the
substrate surface after the secondary growth. On the other hand,
most hexagonal particles were parallel to the APTEs-modified
substrate, which can be suitable for gas transportation.
Figure 1. SEM images of Kgm-OEt crystals of (a) powder sample and seed layer
on (b) unmodified and (c) APTEs-modified Al2O3 substrate.
The difference in the growth direction of Kgm-OEt could be
explained by the van der drift growth model.[30,31] The morphology
of these Kgm-OEt crystals is a 2D hexagonal sheet structure with
the order of crystal dimensions Lb = La > Lc, where Li denotes the
crystal length along the i axis (Figure S6). As the crystals grow
larger, they collide with each other, and the orientation of the
crystals change to get sufficient space for the crystal growth. On
As the seeding cycle times increased, the intensity of XRD peaks
in APTEs-modified Al2O3 substrate increased (Figure S4),
indicating that denser Kgm-OEt seed layer was fabricated by LBL
cycles. The intensity of XRD peaks was almost unchanged
between four and five LBL cycles, and the SEM image shows that
the seed layer completely covered the APTEs-modified substrate,
thus four cycles can be enough for the deposition of the seed layer. the unmodified substrate, the a- and b- axes growth of the crystal
In contrast, uncovered Al2O3 is still visible on the unmodified
substrates even after seeding for four cycles (Figure 1). Notably,
APTEs modification not only improves the nucleation rate of Kgm-
OEt, but also results in the generation of particles with smaller
sizes than those on unmodified substrate (400 nm and 1.2 μm,
respectively, Figure S5). The nucleation rate on the APTES-
modified substrate might be faster than that on the unmodified
one due to the higher density of ligand on the surface by –NH2
groups, which may lead to the formation of smaller particles on
the APTES-modified substrate.[28,29] The smaller particle size of
the seed layer would be advantageous to form a continuous MOF
is faster than that of the c-axis, so the a- and b- axes are oriented
to be perpendicular to the substrate. On the APTEs-modified
Al2O3 substrate, as the particle sizes of the membrane were
smaller, the collision of the crystals and the change of the crystal
growth direction were greatly suppressed.
Figure. 3(a) shows the XRD patterns of the simulation of Kgm-
OEt, powder, seed layer, membranes on unmodified and APTEs-
modified substrate. All of the peak positions of membrane
samples well match with the simulation, indicating the no impure
phase. To analyze the orientation of the membranes, we
compared the XRD of the membranes on APTEs-modified and
unmodified substrate (Figure S7). Figure 3(b) shows the crystal
plane of Kgm-OEt. The (001) plane is parallel to the hexagonal
crystal. The 001 peak of membrane on unmodified substrate is
almost absent, indicating the crystal orientation vertical to the
hexagonal plane. For the membrane on APTEs-modified
substrate, the 001 peak is clearly visible, indicating that the
vertical growth is suppressed. EDX mapping analysis further
revealed the boundary between the Kgm-OEt membrane and the
APTEs-modified Al2O3 substrate (Figure 4(a), (b)). Optical images
of the APTEs-modified Al2O3 substrate, seed layer and
membrane also showed in Figure 4(c). In short, we tuned
microstructure by deposition of amino groups on Al2O3 substrate
and successfully prepared high-density and continuous Kgm-OEt
membrane.
membrane after the secondary growth, while larger particles tend
to produce binary gaps between particles.
(c)
(a)
(b)
Membrane on APTEs-modified substrate
100
a
b
*
200
210
*
420
001
Membrane on unmodified substrate
Figure 2. Top and cross-sectional view SEM images of Kgm-OEt membranes
synthesized on the (a, b) unmodified and the (c, d) APTEs-modified substrate
at different magnifications.
(210)
Seed layer
Powder
Simulation of Kgm-OEt
(001)
5
10
15
20
25
30
35
Kgm-OEt membrane was grown from the seed layer by the
secondary growth at room temperature. Top view SEM image of
the membrane on the unmodified substrate shows that the
crystals are loosely packed and poorly intergrown (Figure 2(a)).
The SEM cross-sectional images indicate that the thickness of the
membrane prepared on the unmodified substrate was about 10
µm. The continuity of the membrane was relatively poor, and
small gaps between MOF particles were observed (Figure 2(b)).
On the other hand, the surface was densely covered with
hexagonal plate crystals merged tightly with each other forming
2 Theta (degree)
Figure 3. (a) XRD patterns of the simulation of Kgm-OEt, powder, seed layer
and membranes. The asterisks (*) indicate the peaks derived from Al2O3
substrate. (b) Crystal plane of Kgm-OEt.
The single gas permeances of the Kgm-OEt membrane were
measured at 10 kPa differential pressure and 25 °C using a
Wicke-Kallenbach setup (Figure S8). As-prepared Kgm-OEt
2
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membranes were activated by N2 sweeping at 100 °C prior to gas
permeation experiments. The activation temperature was
determined by TGA curve (Figure S9). The results are shown with
the kinetic diameter of the gas molecules (Figure 5(a)). The
membrane exhibits the highest permeance for H2 (2.73 × 10-8 mol
m-2 s-1 Pa-1), and the permeances decrease with an order of H2 >
CO2 > N2 > CH4 > C2H4, which corresponds to their kinetic
diameters except for CO2. The calculated ideal selectivities for
H2/CO2, H2/N2, H2/CH4 and H2/C2H4 are 5.2, 4.4 ,4.6 and 6.5,
respectively. The experimental separation factors for H2 over CO2,
CH4, N2 and C2H4 obtained by 1:1 binary-gas permeation tests are
5.0, 4.1, 4.4, and 6.2. All of the separation factors superpass
Knudesen constant and C2H4 molecules have the lowest
permeability, indicative of the sieving effect of the membrane. The
stronger CO2 adsorption of Kgm-OEt (Figure S10) can reduce the
CO2 mobility so that the membrane shows lower permeance of
CO2 than N2 and CH4.[32,33] H2/CO2 selectivity is also higher than
those of polymer membranes based on Robeson (2008) and
Robeson (1991) (Figure 5(b)).[34]
improve the growth of Kgm-OEt on the Al2O3 substrate to form a
defect-free membrane.
In summary, a continuous and defect-free MOF Kgm-OEt
membrane has been successfully synthesized by the secondary
growth approach on the APTEs-modified porous Al2O3 substrate
surface. We applied an LBL method to construct the seed layer
for this kagomé-type of MOF. It is shown that APTEs-modified
Al2O3 substrate can form a fully covered seed layer. The particle
size of the seed is smaller on the APTEs-modified substrate than
the unmodified substrate. The microstructure of the membrane
grew in the APTEs-modified substrate is greatly improved
compared with the unmodified substrate. Our work would give a
convenient method for the preparation of various types of MOF
membranes, especially those based on metal carboxylates, which
cause defects by the ordinary methods.
Acknowledgements
This work was supported by the PRESTO (Grant No.
JPMJPR141C) and CREST (Grant No. JPMJCR17I3) of the
Japan Science and Technology Agency (JST), and JSPS
KAKENHI (Grant No. JP18K14043, JP19H02734, JP20H02575,
and JP20K20564). X. Wang acknowledges the financial support
from China Scholarship Council (No. 201806060138).
Keywords: Metal-organic framework 1 • MOF membrane 2 •
Gas permeation 3 •
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4
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Entry for the Table of Contents
A simple chemical modification for Al2O3 substrate facilitates the preparation of more compact and better-oriented Kgm-OEt
membrane.
5
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