Scalable Room-Temperature Synthesis of Highly Robust Ethane-Selective Metal-Organic Frameworks for Efficient Ethylene Purification

Article abstract of DOI:10.1021/jacs.1c02108

The development of new techniques and materials that can separate ethylene from ethane is highly relevant in modern applications. Although adsorption-based separation techniques using metal-organic frameworks (MOFs) have gained increasing attention, the relatively low stability (especially water resistance) and unscalable synthesis of MOFs severely limit their application in real industrial scenarios. Addressing these challenges, we rationally designed and synthesized two new C2H6-selective MOF adsorbents (NKMOF-8-Br and-Me) with ultrahigh chemical and thermal stability, including water resistance. Attributed to the nonpolar/hydrophobic pore environments and appropriate pore apertures, the MOFs can capture C2 hydrocarbon gases at ambient conditions even in high humidity. The single-crystal structures of gas@NKMOF-8 realized the direct visualization of adsorption sites of the gases. Both the single-crystal data and simulated data elucidate the mechanism of selective adsorption. Moreover, the NKMOF-8 possesses high C2H6 adsorption capacity and high selectivity, allowing for efficient C2H6/C2H4 separation, as verified by experimental breakthrough tests. Most importantly, NKMOF-8-Br and-Me can be scalably synthesized through stirring at room temperature in minutes, which confers them with great potential for industrial application. This work offers new adsorbents that can address major chemical industrial challenges and provides an in-depth understanding of the gas binding sites in a visual manner.

Full text of DOI:10.1021/jacs.1c02108

pubs.acs.org/JACS
Article
Scalable Room-Temperature Synthesis of Highly Robust Ethane-
Selective Metal−Organic Frameworks for Efficient Ethylene
Purification
Shubo Geng, En Lin, Xia Li, Wansheng Liu, Ting Wang, Zhifang Wang, Debobroto Sensharma,
Shaza Darwish, Yassin H. Andaloussi, Tony Pham, Peng Cheng, Michael J. Zaworotko, Yao Chen,
and Zhenjie Zhang*
Cite This: J. Am. Chem. Soc. 2021, 143, 8654−8660
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sı Supporting Information
ABSTRACT: The development of new techniques and materials
that can separate ethylene from ethane is highly relevant in modern
applications. Although adsorption-based separation techniques using
metal−organic frameworks (MOFs) have gained increasing
attention, the relatively low stability (especially water resistance)
and unscalable synthesis of MOFs severely limit their application in
real industrial scenarios. Addressing these challenges, we rationally
designed and synthesized two new C H -selective MOF adsorbents
2
6
(NKMOF-8-Br and -Me) with ultrahigh chemical and thermal
stability, including water resistance. Attributed to the nonpolar/
hydrophobic pore environments and appropriate pore apertures, the
MOFs can capture C2 hydrocarbon gases at ambient conditions
even in high humidity. The single-crystal structures of gas@NKMOF-8 realized the direct visualization of adsorption sites of the
gases. Both the single-crystal data and simulated data elucidate the mechanism of selective adsorption. Moreover, the NKMOF-8
possesses high C H adsorption capacity and high selectivity, allowing for efficient C H /C H separation, as verified by
2
6
2
6
2
4
experimental breakthrough tests. Most importantly, NKMOF-8-Br and -Me can be scalably synthesized through stirring at room
temperature in minutes, which confers them with great potential for industrial application. This work offers new adsorbents that can
address major chemical industrial challenges and provides an in-depth understanding of the gas binding sites in a visual manner.
INTRODUCTION
Scheme 1. Molecular Structures and Physical Properties of
Ethane and Ethylene
■
a
As an important chemical feedstock, ethylene (C H ) is widely
2
4
1
2,3
used in the chemical industries and in agriculture with
increasing demand leading to significant consequences for
society. In industry, ethylene is mostly produced via the
4
cracking of hydrocarbons. The major byproduct, ethane
(
C H ), needs to be removed to produce polymer-grade
2 6
5
,6
ethylene (≥99.95%). This process is usually performed in a
very large distillation tower (120−180 trays) under energy-
intensive and high-cost conditions with low temperature and
7
a
high pressure. Therefore, developing alternative separation
b.p. = boiling point.
techniques with lower-energy consumption is highly desired,
8
,9
yet largely unexplored. Recently, adsorption-based separa-
tion techniques using porous materials as adsorbents have
attracted increasing attention for their high efficiency, easy
10−12
Received: February 23, 2021
Published: June 2, 2021
operation, and low-energy consumption.
However, the
separation of ethylene and ethane is extremely challenging due
13
to their similar physical properties and molecular size (3.28
3
3
×
4.18 × 4.84 Å for C H and 3.81 × 4.08 × 4.82 Å for
2 4
C H ) (Scheme 1). Currently, adsorbents used for C H /C H
2
6
2
6
2
4
separation mainly consist of two types: C H -selective and
2
4
©
2021 American Chemical Society
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J. Am. Chem. Soc. 2021, 143, 8654−8660
8
654

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C H -selective adsorbents. In the first case, separation of
RESULTS AND DISCUSSION
2
6
■
C H /C H can be achieved via selecting adsorbents that
2
6
2
4
The reaction of the Br-substituted 4,5-dicyanoimidazole with
CuI in acetonitrile solution at 100 °C for 1 day afforded
colorless crystals of NKMOF-8-Br. White crystals of NKMOF-
-Me were obtained by a slow evaporation method. Single-
crystal X-ray diffraction data revealed that these two
isostructural MOFs crystallized in the Pnna space group
preferentially adsorb C H4 over C H . However, in this
2
2
6
method, C H has to be subsequently harvested in the
2
4
desorption process which increases the complexity of such a
8
system while reducing energy efficiency due to the higher
14,15
temperature or vacuum conditions required.
Separation
performance may also be hampered in the event of
(
Table S1). In the structures, each Cu(I) atom coordinates
16
coadsorption of C H residues in the adsorbent. By contrast,
2
6
with four N atoms from four ligands, two N atoms from the
imidazole groups, and another two from the cyano group. Each
ligand links four Cu(I) atoms to generate a 3-dimensional
(3D) MOF with a pts topology. NKMOF-8-Br and -Me show
narrow square channels along the a direction with pore
apertures of 6.15 Å × 7.11 and 6.21 Å × 6.96 Å, respectively
(Figure 1a). Notably, NKMOF-8-Br and -Me can be
C H -selective adsorbents forego many of these obstacles with
2
6
pure C H gas being readily obtained via a one-step process. At
2
4
present, C H -selective adsorbents are far less reported than
2
6
C H -selective adsorbents as installing C H -binding sites (e.g.,
2
4
2
4
open metal sites (OMSs)) in adsorbents is far more readily
achieved than for C H -binding sites. Additionally, the
2
6
influence of water vapor requires further study in this field as
humidity is ubiquitous in real world applications and often acts
1
7,18
as a competitor for binding sites.
Hence, an ideal
adsorbent for C H /C H separation should preferentially
2
6
2
4
adsorb C H and possess poor water adsorptive ability to
2
6
minimize the effect of humidity on separation capacity.
Metal−organic frameworks (MOFs) with high porosity,
ordered structures, and tunable pore size have emerged as
promising candidates to address the challenge of gas
1
9−24
separation.
To design the MOF platforms for C H -
2 6
selective separation, OMSs are undesirable as they preferen-
tially adsorb water molecules and unsaturated hydrocarbons
such as C H , therefore dampening the adsorption capacity
2
4
25
and selectivity for C H . Introducing and modifying ligand
2
6
functional groups can be a feasible approach to regulate MOF
pore environments to endow specific binding affinities for
2
6−29
C H .
C H molecules possess a larger quadrupole
2
6
2 4
−
26
2
moment (1.50 × 10 esu cm ) than that of C H (0.65 ×
2
6
−
26
2
1
0
esu cm ), and thus MOF materials with nonpolar pore
environment can often selectively adsorb C H rather than
2
6
3
0−32
C H .
The C−H···π interactions between MOFs and gas
molecules are often taken into account when designing this
kind of MOF. For example, in MUF-15, the C H molecule
2
4
3
3
Figure 1. (a) Synthetic routes and structures of NKMOF-8-Br and
2
6
-
Me. (b) Demonstration of the large-scale synthesis of NKMOF-8-Br
was capable of interacting with three adjacent phenyl rings of
34
(10 g) and -Me (12 g). (c) PXRD patterns of NKMOF-8-Br (left)
and -Me (right) after various treatments and large-scale synthesis.
the isophthalic acid ligands via C−H···π interactions. In
addition, ideal MOF adsorbents for C H /C H separation
2
6
2
4
should possess both high adsorption capacity and selectivity,
which play a vital role in ethylene purity and productivity.
Furthermore, scalable room-temperature synthesis of MOF
adsorbents is highly desirable toward real industry applica-
conveniently obtained on a large scale with a high yield
(>90%) via a convenient, energy-saving approach: stirring
ligands, CuI, and triethylamine (TEA) in acetonitrile at
ambient conditions for as short as a few minutes (Figure
1b). Then the white product was obtained by filtration, washed
with acetonitrile three times, and dried in an oven at 80 °C.
The high purity of the gram-scale NKMOFs was verified by
powder X-ray diffraction (PXRD) (Figure 1c) and scanning
electron microscope (SEM) measurements (Figure S1).
Notably, the acetonitrile solution can be readily recycled via
filtration and then reused in the following reaction cycles
without further purification, as verified by PXRD (Figure S2).
Given the mild reaction conditions and facile scale-up, the
fabrication of NKMOF-8-Br and -Me can be considered a
Green synthesis, which is desirable for enabling MOFs for
industrial application. PXRD results (Figure 1c) and
Brunner−Emmet−Teller (BET) tests (Figure S3) reveal that
NKMOF-8-Br and -Me maintain high stability and crystallinity
after various harsh treatments. NKMOF-8-Br exhibited
structural stability even in boiling water, base solution (pH =
13), or 5 M HCl aqueous solutions for over 2 weeks at room
3
5
tions. Currently, the discovery of efficient C H -selective
2
6
MOF adsorbents fully realizing all the above requirements is
yet to be achieved.
With the above considerations in mind, we reasoned that
metal−organic building blocks without OMSs (e.g., tetrahedral
+
2+
2+
MN building blocks, M = Cu , Zn , and Co ) and
4
nonpolar/inert pore surfaces (e.g., small conjugated ligands)
are preferred for the construction of C H -selective adsorbents.
2
6
Therefore, a green synthesis reaction of two aromatic
+
imidazole derivative ligands with Cu ions afforded two new
C H -selective MOF adsorbents, NKMOF-8-Br and -Me.
2
6
Attributed to the aromatic imidazole derivative, these MOFs
were enriched with nonpolar surfaces and pore channels with a
strong affinity to C H molecules through C−H···π inter-
3
6
2
6
actions, thus enabling the selective adsorption of C H over
2
6
C H . Moreover, NKMOF-8-Br and -Me displayed high C H
2
4
2
6
adsorption capacity and selectivity, making them among the
best adsorbents for ethylene purification.
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temperature. Thermogravimetric analysis showed that
NKMOF-8-Br and -Me have no weight loss before 400 °C
under a N atmosphere (Figure S4). NKMOF-8-Br and -Me
2
maintain their structural integrity when heated for an hour at
3
00 and 270 °C, respectively. The ultrahigh chemical and
thermal stability of NKMOF-8-Br and -Me surpass most of the
reported C H -selective MOF adsorbents (Table S2). The
2
6
ultramicroporous structure, green synthesis method, and
ultrahigh stability make NKMOF-8-Br and -Me ideal plat-
forms for gas separation applications.
The activated MOF adsorbents were prepared directly via
heating at 100 °C under a dynamic vacuum without solvent
exchange treatment (Figure S5). Single-component equili-
brium adsorption behaviors of the activated MOFs were then
investigated in detail. Argon gas sorption isotherms at 87 K
showed type I sorption curves for both MOFs, indicating their
microporous structures. The BET and Langmuir surface areas
2
2
were 352 and 515 m /g and 655 and 907 m /g for NKMOF-8-
Br, and -Me, respectively. Pore volumes, calculated by the
3
7
3
PLATON program, were 0.21 and 0.30 cm /g for NKMOF-
38
8
-Br and -Me, respectively. According to the literature, gas
molecules can reorder in the pores of MOFs with a larger pore
volume, hence leading to the increased argon uptake. Overall,
the larger pore volume and lower density (1.803 and 1.369 g/
3
cm for NKMOF-8-Br, and -Me, respectively) make the
surface area of NKMOF-8-Me higher than that of NKMOF-8-
Br. A similar case (ZIF-8-Br and ZIF-8-Me) has been
39
Figure 2. (a,b) Ar adsorption/desorption isotherms of NKMOF-8-Br
and -Me at 87 K. Pore size distribution of NKMOF-8-Br and -Me.
observed in a previous study. Pore size distributions obtained
by applying nonlocal density functional theory (NLDFT) for
the Ar isotherms showed narrow pores of 6.6 and 5.9 Å, for
NKMOF-8-Br and NKMOF-8-Me, respectively (Figure 2a
and 2b). Subsequently, adsorption of C H and C H was
(
c,d) The C H and C H adsorption isotherms of NKMOF-8-Br and
2 6 2 4
-
Me at 298 K. (e) Qst curves for NKMOF-8-Br and -Me. (f)
Comparison of IAST selectivities of the C H /C H (1:1, v/v)
2
6
2
4
2
6
2
4
mixture and C H adsorption of NKMOF-8-Br and -Me with
2
6
measured at 273 and 298 K. As shown in Figure 2c and 2d,
previously reported C H -selective MOFs at 298 K and 1 bar.
2 6
NKMOF-8-Br and -Me adsorb more C H than C H in the
2
6
2
4
full pressure region. NKMOF-8-Br and -Me adsorb 82.27
NKMOF-8-Br could possibly be due to the presence of
favorable adsorbate−adsorbate interactions at higher loadings.
This behavior is also observed in the literature (e.g., MUF-
3
3
cm /g (3.67 mmol/g) and 104.82 cm /g (4.67 mmol/g) of
C H , respectively, at 298 K and 1 bar, while the C H
6
2
4
2
40
41
42
adsorption of NKMOF-8-Br and -Me at 298 K and 1 bar
15, ZIF-7, and NUP-1 ). To verify the high binding
affinity of NKMOF-8-Br and -Me, the active MOF crystals
were exposed to C H or C H gas via a gas balloon at room
3
3
reaches 95.01 cm /g (4.22 mmol/g) and 108.12 cm /g (4.82
mmol/g), respectively. Notably, the C H6 uptake for
2
2
6
2
4
NKMOF-8-Me is higher than most reported C H -selective
temperature. Interestingly, both C H and C H molecules
2
6
2 6 2 4
MOFs (Figure 2f). Additionally, we conducted kinetic sorption
could be captured in the channels of NKMOF-8-Br or
NKMOF-8-Me as verified by Fourier transform infrared
spectroscopy (FTIR). The characteristic stretching vibration
measurements of C H and C H for NKMOF-8-Br and -Me
2
4
2
6
(
Figure S6). Upon increasing pressure from 0 to 1 bar,
NKMOF-8-Br reached saturation after 83 s (105.6 mg/g) and
4 s (88.9 mg/g) for C H and C H , respectively. Similarly,
−
1
assigned at 2956 and 2846 cm and bending vibration at 1462
−
1
9
cm of C H were detected in C H @NKMOF-8-Br (i.e.,
2
6
2
4
2 6 2 6
for NKMOF-8-Me, the time to reach saturation for C H and
C H trapped in NKMOF-8-Br). Stretching vibrations at 2943
2 6
2
6
−
1
−1
C H (139.5 mg/g and 113.6 mg/g) was 85 and 112 s,
and 2843 cm and a bending vibration at 1460 cm of C H
2
4
2 4
respectively. These results indicated that both NKMOF-8-Br
and -Me exhibited faster kinetic adsorption for C H and
were also found in C H @NKMOF-8-Br (i.e., C H trapped
2 4 2 4
43
2
6
in NKMOF-8-Br) (Figure S9). Notably, gas@NKMOF-8-
Br still possessed excellent crystallinity. Thus, the crystals of
NKMOF-8-Br after adsorbing C H and C H molecules were
slower kinetic adsorption for C H . Desorption kinetic profiles
2
4
confirmed that the adsorption of both gases is reversible on a
very short time scale under vacuum.
2
6
2
4
characterized by single-crystal X-ray diffraction analysis. The
crystal structures of gas@NKMOF-8-Br (gas = C H or C H )
Isosteric enthalpies of adsorption (Q ) for the NKMOF-8
st
2
6
2
4
MOFs were then calculated by the virial equation based on the
isotherms collected at 273, 298, and 308 K (Figures S7 and
S8). The fitted parameters are provided in Table S3 and Table
S4. As shown in Figure 2e, the initial Q values of C H for
(Figure 3a and 3b) enabled direct visualization of the gas
adsorption sites. In the crystal structures, the chemical
occupancies for each C H or C H molecule were
2
6
2
4
approximately 0.5, which correlates with the observation
from gas adsorption isotherms at 298 K and 1 bar. As shown
in Figure 3c, in the crystal structure of C H @NKMOF-8-Br,
st
2
6
NKMOF-8-Br and -Me are 40.8 and 38.4 kJ/mol, respectively,
and are higher than those of C H (33.6 and 37.6 kJ/mol,
2
4
2
6
respectively). This demonstrates that NKMOF-8-Br and -Me
have a stronger thermodynamic affinity toward C H than
there were two binding sites between each C H molecule and
the pore wall. Binding sites 1 and 2 were formed by C−H···π
interactions (a distance of 3.53 Å) between hydrogen atoms of
2 6
2
6
C H . The increasing trend in the Q for both gases in
2
4
st
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and -Me, respectively, at 90% RH) could be absorbed within
NKMOF-8-Br and -Me (Figure 4a). In addition, we found that
Figure 4. (a) Water sorption isotherms for NKMOF-8-Br and -Me at
2
98 K. (b) FTIR spectra of MIL-142A exposed to the C H /C H
2 6 2 4
mixture under 90% RH (shaded area corresponding to the infrared
absorption peak of water). (c,d) FTIR spectra of NKMOF-8-Br and
Figure 3. (a) Single crystal structure of C H @NKMOF-8-Br viewed
2
6
along the a axis (left) and the 1-D channel (right). (b) Single crystal
structure of C H @NKMOF-8-Br viewed along the a axis (left) and
-
9
Me exposed to C H , C H , and a C H /C H (1:1, v/v) mixture in
0% RH (shaded and boxed areas corresponding to the infrared
2 4 2 6 2 4 2 6
2
4
the 1-D channel (right). (c) Binding sites of C H (left) and C H
2
6
2
4
absorption peaks of C H and C H ).
2
4
2
6
(
right) determined by the crystal structure of C H @NKMOF-8-Br
2 6
and C H @NKMOF-8-Br. (d) Binding sites of C H (left) and C H
2
4
2
6
2
4
(
right) in NKMOF-8-Me determined by GCMC simulations.
NKMOF-8-Br and -Me can maintain their crystallinity after
the water vapor sorption experiment (Figure S13). We also
studied the dynamic sorption of water vapor at 298 K for
NKMOF-8-Br and -Me (Figure S14). When the RH rose to
90%, the mass change only increased 0.92% and 0.70% for
NKMOF-8-Br and -Me, respectively, further validating their
low water vapor uptakes. These low water uptakes could be
attributed to the introduction of hydrophobic groups, i.e.,
C H and CN double bonds from imidazole groups. On the
2
6
contrary, there were also two adsorption sites between each
C H molecule and pore wall. The C−H···π interactions (a
distance of 3.90 Å) existed between hydrogen atoms of C H
and CN triple bonds from the cyano groups. The binding
sites in the crystal structures of gas@NKMOF-8-Br (gas =
C H or C H ) also agreed well with the results of grand
2
4
2
4
45,46
bromine and methyl, into the pores.
The combined effect
2
6
2
4
canonical Monte Carlo (GCMC) simulations (Figure S10).
Moreover, gas adsorption sites in NKMOF-8-Me were verified
by GCMC simulations (Figure S11). Similarly, two binding
sites (sites 1′ and 2′) were formed between the hydrogen
atoms of C H and CN double bonds from imidazole
of hydrophobic pores and a lack of OMS sites allow for
NKMOF-8-Br and -Me to act as C H -selective materials that
2
6
operate under high humid conditions. To verify this prediction,
FTIR was applied to track the signal of water molecules under
high humid conditions. The tested MOFs were separately
exposed to C H , C H , or C H /C H (1:1, v/v) mixture
2
6
groups with a distance of 3.66 Å. Two binding sites (sites 1′
and 2′) between hydrogen atoms of C H and CN triple
2
6
2
4
2
6
2
4
under 90% RH, respectively. The characteristic peak (3548−
2
4
−
1
bonds from cyano groups were formed with a distance of 3.94
Å (Figure 3d). For NKMOF-8-Br and -Me, the shorter C−
H···π interaction indicated a stronger affinity between C H
3300 cm ) assigned to water molecules was absent, while the
−
1
−1
characteristic peaks (2956−2843 cm and 1462−1460 cm )
assigned to C H and C H appeared, indicative of water-
2
6
2
6
2
4
and the MOFs than that with C H . Furthermore, static
repellent effect (Figure 4c and 4d). By contrast, the reported
2
4
47
adsorption energies of these binding sites were also revealed by
GCMC simulations (Table S5) that showed the same trend as
C H -selective MOFs, MIL-142A, yielded an opposite result
2 6
to the NKMOF-8 platforms. An obvious infrared peak of water
molecules was present in MIL-142A after exposure to humid
conditions (Figure 4b). These results demonstrate that
NKMOF-8-Br and -Me could potentially operate with
excellent C H /C H separation capacity even under high
the calculated Q results.
st
In real-world scenarios, water vapor present in gas mixtures
4
4
is an inevitability. Currently, most of the reported C H -
2
6
selective MOFs possess poor water stability and cannot work
under humid conditions (Table S2). In the NKMOF-8
platform, each Cu(I) metal center is tetra-coordinated, and
no OMS exists to competitively coordinate water molecules. As
tested by PXRD, NKMOF-8-Br and -Me maintain their
structural integrity for at least 20 days (Figure S12) under 90%
RH (relative humidity). Adsorption isotherms of water vapor
were carried out at 298 K, showing that only a small amount of
2
6
2
4
humid conditions, making them more applicable to realistic
industrial environments.
Attributed to the nonpolar pore environments and
appropriate pore sizes, NKMOF-8-Br and -Me exhibited
differences in adsorption between C H and C H . The ideal
2
6
2
4
adsorbed solution theory (IAST) was used to calculate their
C H /C H gas mixture selectivities. The fitted parameters are
2
6
2
4
3
3
water vapor (15.6 cm /g and 17.8 cm /g for NKMOF-8-Br
provided in Table S6. The initial adsorption selectivities of the
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C H /C H (1:1, v/v) mixture at 298 K were 2.7 and 1.9 for
through experiments of NKMOF-8-Br and -Me for a C H /
2 6
2
6
2
4
NKMOF-8-Br and -Me, respectively (Figure S15). Notably,
NKMOF-8-Br possessed superior comprehensive performance
for C H /C H separation. The selectivity of the C H /C H
C H (1:9, v/v) mixture with He as the carrier gas (70%, vol
2 4
%) (total flow rate 3.0 mL/min, 298 K). For NKMOF-8-Br
and -Me, C H first passed the packed column and was
2
6
2
4
2
6
2
4
2
4
(
1:1, v/v) mixture at 298 K for NKMOF-8-Br outperformed
detected in the outlet gas with a high purity of >99.99%, and
then after 85 and 60 min, respectively, C H eluted through
most C H -selective MOFs under the same conditions (Figure
2
Cu(Qc) (3.4). However, NKMOF-8-Br possessed a higher
absorption capacity for C H (4.22 mmol/g) than Fe (O )-
2
6
2
6
48
f) while only being lower than Fe (O )(dobdc) (4.4) and
the packed column (Figure 5b). NKMOF-8-Br and -Me
exhibited excellent regeneration and reusability. NKMOF-8-Br
and -Me could be recycled for at least five cycles in a dynamic
C H /C H column breakthrough test without any deterio-
2
2
4
9
2
2
6
2
2
(
dobdc) (3.40 mmol/g) and Cu(Qc) (1.85 mmol/g) at 298
2
2
6
2
4
K and 1 bar. To further confirm the separation performance,
breakthrough experiments of NKMOF-8-Br and -Me for a
C H /C H (1:1, v/v) mixture with He as the carrier gas
ration in separation performance (Figure 5c, 5d, and S17). For
49
comparison, control breakthrough experiments of Cu(Qc) ,
2
50
47
PCN-250, and MIL-142A were also examined using the
2
6
2
4
(
70%, vol %) were purged into a packed column with a total
same device under the same testing conditions (C H /C H
2
6
2
4
inlet flow rate of 3.0 mL/min at 298 K. It was found that
C H /C H separation was well achieved by the NKMOF-8
(1:1, v/v) mixture with He as the carrier gas (70%, vol %)).
The breakthrough time difference between C H and C H for
2
6
2
4
2
6
2
4
MOFs (Figure 5a). When a C H /C H mixture was passed
Cu(Qc) , PCN-250, and MIL-142A was 68 min, 52 min, and
2
6
2
4
2
1
2 min (Figures S18−S20), respectively, shorter than that of
51
NKMOF-8-Br. Moreover, MCIF-1 constructed by 4,5-
dicyanoimidazole was also examined for comparison. MCIF-
1
possessed a similar structure as NKMOF-8 but different
connectivity (a face-to-face comparison in Figure S21). The
breakthrough results showed no separation effect for MCIF-1
under the same testing conditions (Figure S22).
CONCLUSION
■
In conclusion, we rationally designed and synthesized two new
highly robust ethane-selective MOFs, NKMOF-8-Br and -Me,
which can be conveniently fabricated on a large scale at room
temperature within minutes. These two isostructural MOFs
possess a regular ultramicroporous 3D pts network and
exhibited remarkably high stability against harsh treatments
such as boiling water, acid/base, or heating at 300 °C. The
nonpolar pore environments and appropriate pore apertures
enabled the preferential adsorption of C H over C H . Thus,
2
6
2
4
Figure 5. (a) Experimental breakthrough curves of the C H /C H
NKMOF-8-Br and -Me possessed both high C H /C H
2 6 2 4
2
6
2
4
(
1:1, v/v) mixture with He as the carrier gas (70%, vol %) for
selectivity and C H uptake that surpass most reported
2
6
NKMOF-8-Br and -Me at 298 K and 1 bar. The total flow rate was 3
C H -selective MOFs. Breakthrough experiments verified the
2
6
mL/min. (b) Experimental breakthrough curves of a C H /C H
2
6
2
4
C H /C H separation performance of NKMOF-8-Br and
2
6
2
4
(
1:9, v/v) mixture with He as the carrier gas (70%, vol %) for
-
Me, with C H of high purity >99.99% being produced, and
2 4
NKMOF-8-Br and -Me at 298 K and 1 bar. The total flow rate was 3
mL/min. (c,d) Five cycles of experimental breakthrough curves of the
C H /C H (1:1, v/v) mixture with He as the carrier gas (70%, vol
these two MOFs possessed excellent reusability. NKMOF-8-Br
and -Me were able to selectively capture C2 hydrocarbon gases
even under high humidity conditions due to the presence of
hydrophobic pores. Moreover, the gas adsorption sites can be
observed directly in a visual way through single-crystal
structures of gas@NKMOF-8-Br and are further verified by
simulation results. Therefore, this work provides important
2
6
2
4
%
) for NKMOF-8-Br and -Me at 298 K and 1 bar. The total flow rate
was 3 mL/min.
over the packed column of NKMOF-8-Br, as predicted, C H
2
4
was the first to elute through the packed column, followed by
guidance to fabricate ideal C H -selective adsorbents for
C H 112 min later. Similarly, the breakthrough time
2
6
2
6
C H /C H separation in real industrial scenarios.
difference between C H6 and C H4 was 44 min for
2
6
2
4
2
2
NKMOF-8-Me. Notably, C H of high purity >99.99% was
2
4
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c02108.
produced from the above separation process. We also have
conducted the C H /C H (1:1, v/v) breakthrough measure-
■
2
6
2
4
*
ment without He (Figure S16). As predicted, both NKMOF-8-
Br and -Me can efficiently separate C H /C H . The results
2
6
2
4
showed the C H first passed the packed column and was
2
4
Experimental procedures, simulation details, NMR data,
PXRD, gas sorption data, extra figures, and tables (PDF)
detected from the outlet gas within a high purity >99.99%, and
then after 68 and 22 min, C H eluted through the packed
2
6
C H @NKMOF-8-Br (CIF)
column for NKMOF-8-Br and -Me, respectively. Overall, the
2
4
separation result and trend for the C H /C H (1:1, v/v)
C H @NKMOF-8-Br (CIF)
2 6
2
6
2
4
mixture without He are consistent with those for the C H /
2
6
NKMOF-8-Br (CIF)
NKMOF-8-Me (CIF)
C H (1:1, v/v) mixture with He. To further evaluate the
2
4
performance of NKMOFs, we also conducted the break-
8
658
https://doi.org/10.1021/jacs.1c02108
J. Am. Chem. Soc. 2021, 143, 8654−8660

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AUTHOR INFORMATION
■
Corresponding Author
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