One-step Ethylene Purification from an Acetylene/Ethylene/Ethane Ternary Mixture by Cyclopentadiene Cobalt-Functionalized Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.202100782

The separation of ethylene (C₂H₄) from a mixture of ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) at normal temperature and pressure is a significant challenge. The sieving effect of pores is powerless due to the similar molecular size and kinetic diameter of these molecules. We report a new modification method based on a stable ftw topological Zr-MOF platform (MOF-525). Introduction of a cyclopentadiene cobalt functional group led to new ftw-type MOFs materials (UPC-612 and UPC-613), which increase the host-guest interaction and achieve efficient ethylene purification from the mixture of hydrocarbon gases. The high performance of UPC-612 and UPC-613 for C₂H₂/C₂H₄/C₂H₆ separation has been verified by gas sorption isotherms, density functional theory (DFT), and experimentally determined breakthrough curves. This work provides a one-step separation of the ternary gas mixture and can further serve as a blueprint for the design and construction of function-oriented porous structures for such applications.

Full text of DOI:10.1002/anie.202100782

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 11350–11358
International Edition:
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doi.org/10.1002/anie.202100782
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Gas Separation
Hot Paper
One-step Ethylene Purification from an Acetylene/Ethylene/Ethane
Ternary Mixture by Cyclopentadiene Cobalt-Functionalized Metal–
Organic Frameworks
Yutong Wang, Chunlian Hao, Weidong Fan, Mingyue Fu, Xiaokang Wang, Zhikun Wang,
Lei Zhu, Yue Li, Xiaoqing Lu, Fangna Dai, Zixi Kang, Rongming Wang, Wenyue Guo,
Songqing Hu, and Daofeng Sun*
Abstract: The separation of ethylene (C2H4) from a mixture of
ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) at
normal temperature and pressure is a significant challenge.
The sieving effect of pores is powerless due to the similar
molecular size and kinetic diameter of these molecules. We
report a new modification method based on a stable ftw
topological Zr-MOF platform (MOF-525). Introduction of
a cyclopentadiene cobalt functional group led to new ftw-type
MOFs materials (UPC-612 and UPC-613), which increase the
host-guest interaction and achieve efficient ethylene purifica-
tion from the mixture of hydrocarbon gases. The high
performance of UPC-612 and UPC-613 for C2H2/C2H4/
C2H6 separation has been verified by gas sorption isotherms,
density functional theory (DFT), and experimentally deter-
mined breakthrough curves. This work provides a one-step
separation of the ternary gas mixture and can further serve as
a blueprint for the design and construction of function-oriented
porous structures for such applications.
lene by catalytic hydrogenation or solvent extraction, finally
obtaining polymerized grade C2H4 through a distillation
tower.[9] This requires a large-scale separation tower, con-
sumes much energy, accounting for about 0.3% of the global
energy consumption.[10] Consequently, it is necessary to
develop a new separation method or improve the separation
efficiency to achieve removal of C2H2 and C2H6 from the
ternary mixture of C2H2, C2H4, and C2H6 in a single step, and
obtaining the polymerized grade C2H4.
Metal-organic frameworks (MOFs) with their control-
lable pore size and modifiable pore environment have been
developed as environmentally acceptable, energy-saving and
efficient gas separation materials. Many experiments have
confirmed that MOFs as adsorbents can effectively separate
C2H2/C2H4, C2H4/C2H6, C2H4/C3H6 and C3H6/C3H8 mix-
tures.[11–15] Current research suggests that purification of
ethylene with MOFs involves two strategies: (1) Precise
control of the pore size to achieve a molecular sieving effect;
(2) Selective increase of the interaction between the MOFand
light hydrocarbon gases to enhance the host-guest interaction.
For example, Chen et al.[16] used a rigid square acid and
calcium nitrate to limit the pore size to 3.8 Š, which caused
C2H6 to be the first to elute through the bed from C2H4/C2H6.
It is more difficult to control the pore size precisely than to
modify the host-guest interaction. Long et al.[17] used exposed
metal sites to adsorb C2H4 from C2H4/C2H6 thus achieving the
separation of mixed gases. Practically, an industrial separation
process is more useful if C2H4 can breakthrough first. Li
et al.[18] found that the introduction of iron(III)-peroxo sites
into Fe2(dobdc) (dobdc4À: 2,5-dioxido-1,4-benzenedicarbox-
ylate) would provide it with ultra-high adsorption capacity for
ethane and thus preferential access to C2H4. For C2H2/C2H4
mixtures, efficient separation also depends on host-guest
interactions. By introducing inorganic anions with their strong
hydrogen bonding capabilities into a framework, Xing et al.[19]
enhanced the specific identification of acetylene molecules
and realized the efficient separation of ethylene and acety-
lene.
Introduction
Ethylene, the core of the petrochemical industry is an
important chemical with which to measure the national level
of a petrochemical industry.[1,2] Three main methods to
produce ethylene are ethane cracking, naphtha cracking,
and light diesel cracking.[3] Among these, naphtha cracking is
currently the most industrialized but its products contain
many impurities, including H2, CH4, C2H2, C2H6, and C3H6.[4]
It is relatively easy to separate light hydrocarbons with
different carbon numbers[5–8] but the separation of C2 gases is
the most difficult, because of the very similar molecular sizes
and boiling points of C2H2, C2H4, and C2H6. At present, the
common separation method is removal of ethane by cryo-
genic and high-pressure technology, then removal of acety-
[*] Y. T. Wang, C. L. Hao, Dr. W. D. Fan, M. Y. Fu, X. K. Wang,
Dr. Z. K. Wang, Dr. L. Zhu, Y. Li, Prof. X. Q. Lu, Prof. F. N. Dai,
Z. X. Kang, Prof. R. M. Wang, Prof. W. Y. Guo, Prof. S. Q. Hu,
Prof. D. F. Sun
Such separation processes in industrial production are
multicomponent and Zaworotko et al.[20] mixed three kinds of
adsorbents to separate and purify ethylene from the quater-
nary (CO2/C2H2/C2H6/C2H4) mixtures. It is however very
important, but challenging to find a single adsorption material
that can achieve multi-component gas separation. While Hao
et al.[21] reported a new material, TJT-100, which has a strong
School of Materials Science and Engineering, College of Science,
China University of Petroleum (East China)
Qingdao, Shandong 266580 (China)
E-mail: dfsun@upc.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100782.
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interaction with C2H2 and C2H6, thus realizing the separation
the guest molecules pass through the cage, they must enter
of C2H4 from the ternary gas mixture of C2H2/C2H6/C2H4,
there has been no systematic study on the selective adsorption
of C2H2 and C2H6 by a single adsorbent through a pore
environment modification. Moreover, stability is another
important factor that realizes the industrialization of MOF
materials. Hence, it remains a challenge to discover a single
material with high stability which can achieve ethylene
purification in one-step from multi-component gases.
through the cage window. They then make irregular move-
ments in the cavity, finally leaving through another cage
window. Therefore, the cage can be modified to increase the
force between the framework and the C2H2 and C2H6. The
traditional modification methods are to expose the metal
sites, for example by removal of the coordinated solvent by
activation[27,28] or to directly introduce the metal sites into the
linker.[29] It is relatively simple to introduce metal ions into the
porphyrin center (Figure 2b), but the disadvantage is that this
significantly enhances the effect on C2H4.[30] It has been shown
that C2H6 can be stabilized in the pores through intermolec-
ular dipolar and supramolecular interactions with the aro-
matic ligands.[31–33] Herein, we describe a new modification,
introduction of cyclopentadiene cobalt into the linker based
on the stable ftw topology platform (Figure 1b and 2c),
together with a layer of cyclopentadiene wrapped on the
outer side of cobalt to selectively increase the force associated
with C2H2 and C2H6. On one hand, cyclopentadiene-cobalt
can increase the weak interaction with C2H2 and C2H6 and on
the other hand, it can further divide the space in the cage to
form a sphere-like force surface (Figure 2 f). This provides the
possibility of one-step purification of C2H4 by the selective
and exclusive adsorption of C2H2 and C2H6.
It is necessary to compare the modification methods with
the same cage diameter and to further verify the feasibility of
the modification method by changing the cage diameter.
Consequently, TCPP-Co was prepared by cobalt addition to
TCPP as a template. H4L-L and H4L-S modified with
cyclopentadiene cobalt were also obtained. The linker sizes
of TCPP, TCPP-Co, H4L-L, and H4L-S were found to be
13.2 Š, 13.2 Š, 15.3 Š, and 9.5 Š, respectively. The specific
synthesis steps of ligands are detailed in Figure S1–S12 in the
SI.
In this work, we sought to develop a new method of pore
environment modification based on the stable ftw topological
Zr-MOF platform (MOF-525) to achieve efficient purifica-
tion of C2H4 from C2H2/C2H6/C2H4 mixtures at room temper-
ature and atmospheric pressure. To this end, we introduced
cyclopentadiene-cobalt group into the H4L-L and H4L-S
linker in an effort to enhance and increase the host-guest
interactions. Through modular synthesis, two new functional-
ized Zr-MOFs, UPC-612 and UPC-613 with ftw topology
were obtained (Figure 1a). Adsorption tests show that the
adsorption capacities of UPC-612 for C2H2 and C2H6 are
larger than for C2H4. The adsorption enthalpy (Qst) at zero
coverage is C2H2 (23.9 kJmolÀ1) > C2H6 (22.4 kJmolÀ1) >
C2H4 (16.9 kJmolÀ1), indicating that the modified UPC-612
has a stronger interaction with C2H2 and C2H6 than with C2H4.
UPC-612 differs from MOF-525 and MOF-525(Co) modified
only by cobalt ions. It was shown experimentally that
modified UPC-612 can produce > 99.99% pure C2H4 from
the mixtures of C2H2/C2H4 (50/50), C2H6/C2H4 (50/50) and
C2H2/C2H6/C2H4 (1/1/1). The C2H4 productivity of UPC-612
from C2H2/C2H6/C2H4 (1/1/1) reaches 0.47 mmolgÀ1, which is
a new published record. Density functional theory (DFT)
calculations from first principles reveal that the important
adsorption sites are mainly on the corners of the cube cages
and in the cyclopentadiene ring of linkers. A modified
material, UPC-613 has a similar separation effect, even
though the cage diameter is reduced, and this further confirms
the success of our new modification strategy in improving the
separation performance of multicomponent mixture gases.
The reaction of H4L-L and ZrCl4 in the presence of
benzoic acid for 48 h yielded UPC-612 as large pink crystals
with cubic morphology (Figure S13). Single crystal X-ray
diffraction (SC-XRD) analysis shows that UPC-612 crystal-
ꢀ
lizes in the cubic space group Pm3m (Table S1). Crystallo-
graphically, it contains a 12-connected Zr6 cluster [Zr6(m3-
OH)4(m2-O)4(O2C-)12] and a functional tetracarboxylic linker.
The structure and topology are the same as in the template
MOF-525 (Figure 1). The functionalization of linkers reduced
the size of the cage to 14.8 Š, and functional cyclopentadiene
cobalt is introduced only on one side. During the process of
self-assembly, it is uncertain in which cage each cyclopenta-
diene cobalt group is located. The modified functional groups
showed extreme disorder in the single-crystal data. Clear pore
size is important for the analysis of gas adsorption and
separation. Speculation is that there can be seven different
cage structures in UPC-612, as shown in Figure S17. Accord-
ing to the nitrogen adsorption at 77 K, the distribution of pore
size in UPC-612 is calculated to be 1.4 nm and 2.0 nm
(Figure S15), corresponding to 6 and 0 functional groups in
the cubic cage. As shown in Figure 3, the inner space of
a cubic cage with six functional groups can be divided into two
regions by filling the cage with a solid. A sphere-like force
surface was formed by cyclopentadiene in the cage of UPC-
612 (Figure 2 f and 3d). Here we compare the nearly over-
Results and Discussion
Design and Synthesis
With their high thermal and chemical stability, Zr-MOFs
are promising candidates for gas separation.[22–24] In partic-
ular, the MOF-525 reported by Yaghi et al. is a three-dimen-
sional ftw topological structure with a (4,12)-connected net,
constructed with a stable Zr6 cluster and a rigid tetracarbox-
yphenyl porphyrin (TCPP) network (Figure 1a).[25] A 12-
connected Zr6 cluster has all coordination sites of the metal
clusters occupied by carboxylate group, which, according to
soft and hard acid-base theory[26] improves its structural
stability. In this framework, there are irregular cubes sur-
rounded by six linkers (cubic cages, Figure 1a), and the
diameter of the cage can reach about 2 nm, which can allow
modifications. With a single cage as an example, the metal
cluster is placed at the vertex position, and the linker acts as
the face (Figure 2d). Distinct from a pore structure, in which
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Figure 1. Illustration of ftw topology platform in Zr-MOFs based on different linkers: a) The interpretation of ftw topology from compressed space,
the node, and the polyhedron; b) The progressive process of linker modification; c) Three-dimensional view of unit cubic cages assembled by
different linkers. The yellow sphere indicates the available pore space.
lapping electron cloud to a spherical force surface (Fig-
ure S18).
15.7 Š. The pore size of UPC-613 is calculated to be 0.7 nm
and 1.2 nm by the nitrogen adsorption isotherm at 77 K
(Figure S16). The feasibility of the new method is further
verified by the control variable method, where the effect of
modification group on separation performance can be
determined by comparing UPC-612 with MOF-525, and the
effect on cage size is determined by comparing UPC-612 with
UPC-613.
To verify the new modification method, we introduced the
same functional group into a smaller isomorphic ligand. H4L-
S with a size of 9.5 Š was obtained by the method similar to
that used for H4L-L and then assembled with a Zr6 cluster to
obtain the framework material, UPC-613 with a smaller cage
size and ftw topology, which exhibits a structure similar to that
of UPC-612 but with a cage diameter reduced to 7.2 and
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Figure 2. Design of the modification method. a) Unmodified TCPP template; b) TCPP modified by cobalt (TCPP-Co); c) Modified cyclopentadiene
cobalt; d) Schematic of MOF-525 single cubic cage; e) Schematic of the force in the cage after MOF-525 is modified with metals; f) Schematic of
sphere like force surface formed by the modification of cyclopentadiene cobalt.
Figure 3. Structural illustration of UPC-612 with cage filling. a) Cubic cage filling of a basic unit in UPC-612; b) Space division of cubic cage;
c) Internal space allocation of UPC-612 after partition; d) Cubic cage filling profile and a sphere like force surface.
Stability Studies
Gas Sorption
The UPC-612 and UPC-613 crystals were solvent-ex-
Stability, especially thermal and chemical stability, is
a prerequisite for materials. The PXRD patterns show that
both UPC-612 and UPC-613 have excellent stability due to
the retained crystallinity after the breakthrough experiment
(Figure S31–S33). Even if the crystal is immersed in an
aqueous solution with pH values of 1–12 for 24 hours, it still
maintains a good crystallinity (Figure S34,35). The thermal
stability of UPC-612 and UPC-613 was measured by ther-
mogravimetric analysis (TGA, Figure S38,S39). The initial
weight loss before reaching 1208C is attributed to the removal
of the solvent molecules in the cage. The decomposition of
UPC-612 and UPC-613 begins at 3508C, and leads to the
removal of organic linkers and structural collapse(Fig-
ure S36,37).
changed with dry DMF to remove the benzoic acid template
from the cage. The samples were then equilibrated three
times each with chromatography grade methanol and di-
chloromethane for 8 h. The samples were then degassed at
298 K for 12 h and then at 373 K for 10 h, until the outgas rate
was 5 mmHgminÀ1, to produce the activated UPC-612 and
UPC-613. The N2 gas sorption curves of UPC-612 and UPC-
613 at 77 K indicate their permanent porosities. UPC-612 and
UPC-613 show a typical type I N2 adsorption isotherm with
a Brunauer-Emmett-Teller (BET) surface area of 2016 m2 gÀ1
for UPC-612 and 853 m2 gÀ1 for UPC-613.
Compared with MOF-525, the BET specific surface area
of UPC-612 is slightly decreased. The BET specific surface
area decreased from 3116 m2 gÀ1 to 2016 m2 gÀ1, which is
consistent with the structure. The results show that the
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modification of functional groups in the cage with the same
size reduced the cage space.[34,35] Pore size distributions were
determined by nonlocal DFT calculations based on the N2
adsorption isotherms. The pore sizes of UPC-612 are mainly
distributed between 1.5 and 2.0 nm, which is consistent with
the crystallographic data.
C2H2, C2H4 and C2H6, and the affinity for C2H4 is the lowest.
As shown in Figure S23b, a similar trend of the affinity with
the order of C2H6 > C2H2 > C2H4 is provided by UPC-613.
The results show that the affinity of cyclopentadiene cobalt to
C2H6 and C2H2 is higher than to C2H4, which is useful in the
separation and purification of C2H4. This is different from
many adsorbents (Table S10). Theoretically, C2H2 and C2H6
are preferentially adsorbed from the gas mixture, while C2H4
can pass through the separation material successfully, and
thus the purified ethylene can be obtained from the mixed gas
in a single step. Compared with UPC-613, all Qst values of
UPC-612 are lower, which assists the regeneration of the
adsorbent.
Measurements of the C2H2, C2H4, and C2H6 gas adsorption
capacities of activated UPC-612 were performed. For com-
parison, the adsorption isotherms of MOF-525 and MOF-
525(Co) were also measured. The single-component adsorp-
tion isotherms of C2H2, C2H4, and C2H6 up to 1 atm were
measured at 273, 283 and 298 K, respectively (Figure S19,20
and Table S3). As expected, the amount of adsorbed gas in
UPC-612 decreased with increasing temperature. In addition,
the adsorption capacity of UPC-612 for C2H4 is lower than
those for C2H2 and C2H6 at two temperatures (Fig-
ure S20e,S20f), implying the distinct binding affinity of
UPC-612 for C2H2 and C2H6. At room temperature, the
adsorption capacities of UPC-612 are 67.44, 62.58, and
80.11 cm3 gÀ1 for C2H2, C2H4 and C2H6, respectively (Ta-
bles S3), dramatically higher than those of MOF-525 (59.3,
47.3, and 60.7 cm3 gÀ1) and MOF-525(Co) (58.7, 42.9, and
49.7 cm3 gÀ1). Compared with UPC-612, the adsorption ca-
pacities of UPC-613 with reduced cage diameter for C2H2,
C2H4, and C2H6 decreased to 63.4, 51.7, and 57.1 cm3 gÀ1,
respectively but are still higher than MOF-525 and MOF-
525(Co) with unmodified large cage diameters. This indicates
that the overall adsorption capacity of C2 gas is enhanced
after modification, but the adsorption trend remains the same.
At a given cage size, a higher adsorption amount represents
stronger interaction, while UPC-613 with a smaller size still
shows higher adsorption capacity, which further proves the
effect of the modification. Generally, a large adsorption
capacity is a prerequisite for good separation ability of
materials.
Reusability of porous materials is an essential condition
for industrial applications. Ten adsorption-desorption cycles
of UPC-612 and UPC-613 for C2H2, C2H4, and C2H6 were
recorded without reactivation prior to each cycle. There was
only 2.5%, 3.4%, and 2.8% loss in adsorption capacities of
C2H2, C2H4, and C2H6 after ten cycles for UPC-612 (Fig-
ure S21).
It is well-known that the magnitude of the adsorption
enthalpies of porous materials reveals the affinity of the pore
surface toward adsorbates and determines the adsorptive
selectivity. To evaluate the affinity of C2 hydrocarbons with
MOF-525, MOF-525(Co), UPC-612 and UPC-613, the ad-
sorption enthalpy (Qst) for different C2 hydrocarbons was
calculated using the Clausius-Clapeyron equation. The Qst
values of C2H2, C2H4, and C2H6 are 15.81, 16.74, and
19.95 kJmolÀ1 in MOF-525, 18.57, 23.35, and 20.73 kJmolÀ1
in MOF-525(Co), 23.94, 16.94, and 22.39 kJmolÀ1 in UPC-
612, and 30.38, 28.51, and 31.83 kJmolÀ1 in UPC-613 at zero
coverage, respectively (Figure S22,S23 and Table S4). The
results of these calculations show that MOF-525 has the
strongest affinity for C2H6, and is followed by C2H4 and
finally, C2H2. The Qst values of C2H4 and C2H6 in MOF-
525(Co), modified by cobalt ions were significantly increased.
The modified UPC-612 has an overall increased affinity for
Density Functional Theory (DFT) Calculations
Theoretical simulation is a powerful tool to enable us to
unveil the adsorption mechanisms and provide information
concerning adsorption sites. To provide the basic insight into
the adsorbate-adsorbent interaction, DFT simulations were
performed to calculate the stable adsorption configurations
and energies of C2H2, C2H4, and C2H6 in MOF-525, MOF-
525(Co), UPC-612 and UPC-613 with the Dmol3 module[36]
from Materials Studio. In view of the size of the task to
complete the DFT calculations using a whole MOF unit cell,
we used fragmented cluster models cleaved from unit cells
representing the actual situations as accurately as possible;
the cleaved bonds at cluster boundaries were saturated by
protons (Figure 4). The Perdew-Burke-Ernzerhof (PBE)
function under the generalized gradient approximation
(GGA) was used to complete all-electron spin-unrestricted
DFT calculations. DFT-D calculations were performed using
Grimmeꢀs standard parameters set for the van der Waals
correction of gas adsorption. The double numerical basis set
including polarization (DNP) was chosen for all atoms. The
adsorption energy (DEads) is expressed as DEads = Eads + Efram
À Eads+fram, where Eads, Efram and Eads+fram are the total energy
of the adsorbate molecule, adsorbent framework, and adsor-
bate-framework adsorption system, respectively.
The density distributions of the mass centers of C2H2,
C2H4 and C2H6 molecules within the structure of UPC-612 at
298 K and 1 atm were analyzed. It was found that the gas
molecules were concentrated mainly at the angle of the cube
cage, then at the linker-modified cyclopentadienyl rings.
Subsequently, the optimal adsorption sites and adsorption
energies of C2H2, C2H4, and C2H6 on TCPP, TCPP-Co, H4L-L,
and H4L-S linkers and metal clusters were obtained. As
shown in Figure 4a–c, the adsorption sites of C2H2, C2H4, and
C2H6 are concentrated in the center of TCPP, and the
corresponding adsorption energies are 26.84, 29.37, and
28.56 kJmolÀ1, respectively. In the TCPP-Co modified with
cobalt, the adsorption energies were significantly increased to
54.72, 63.11, and 45.196 kJmolÀ1, respectively (Figure 4d–f).
The adsorption energy of C2H4 after the modification which
placed cobalt in the center of porphyrin reached
63.11 kJmolÀ1, which is similar to the chemical adsorption
energy and is detrimental to the separation and purification of
ethylene. The modified H4L-L and H4L-S showed the same
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Figure 4. Preferential C2H2 (left), C2H4 (middle), and C2H6 (right), adsorption sites and corresponding adsorption energies in the linker of MOF-
525 (a–c), MOF-525(Co) (d–f), UPC-612 (g–i), and UPC-613 (j–l) obtained from first-principles calculations.
tendency for the adsorption of C2H2, C2H4, and C2H6, the
adsorption energy of C2H4 being the smallest. The adsorption
energies of C2H2, C2H4, and C2H6 in H4L-L are 33.3, 32.38, and
34.64 kJmolÀ1, and the corresponding adsorption energies in
H4L-S are 42.76, 42.56, and 43.22 kJmolÀ1, respectively. This
follows the same trend as the Qst calculated from the
adsorption isotherm and can be attributed to the stronger
van der Waals force exerted by the modified cyclopentadiene
ring on C2H2 and C2H6. The adsorption energies of C2H2,
C2H4, and C2H6 on the metal clusters are also given, as shown
in Figure 5 and S29, and are generally greater than those on
linkers. It is also normal that the unsaturated coordination
sites on the metal clusters provide an increased weak
interaction for the gas. It should be noted that the adsorption
energy of C2H4 for Zr6-Clusters is less than that of C2H2 and
C2H6 in UPC-612, but higher in MOF-525 and MOF-525(Co).
This indicates that the electron cloud and space effect of
linkers affect the adsorption sites of the zirconium clusters. It
was proved theoretically that the new modification method
provides the possibility for the material useful for the
purification of ethylene.
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C2H4/C2H2 (1/1/1) mixtures flowed over a packed bed at
a total flow rate of 2 mLminÀ1 at 298 K (Figure 6 and
Table S7,8). With C2H6/C2H4 (50/50) as an example, C2H4
was first to elute through the bed, before it was contaminated
with undetectable amounts of C2H6, resulting in a high
concentration of C2H4 feed with a purity of ꢀ 99.99%. After
a period of time, C2H6 broke through after the adsorbent
became saturated, and then the outlet gas stream quickly
reached equimolar concentrations. Replacing the gas with
C2H6/C2H4 (10/90), C2H2/C2H4 (50/50), and C2H2/C2H4 (10/90)
led to similar results, and the difference between them lies in
the separation time. To make the systematic comparison for
the C2H4 separation performance in the selected MOFs, C2H4
purity and productivity were calculated from their break-
through curves (Table S8). For UPC-612, 0.56 mmolgÀ1 of
C2H4 with ꢀ 99.99% purity can be recovered from the C2H4/
C2H6 (50/50) mixture in a single breakthrough operation; this
value is nearly three times that for the benchmark material
MAF-49 (0.28 mmolgÀ1),[37] and slightly lower than for Fe2-
(O2)(dobdc) (0.79 mmolgÀ1).[10] The productivity of UPC-612
is nearly ten times that of MOF-525 (0.05 mmolgÀ1) and
MOF-525(Co) (0.07 mmolgÀ1), which have the same struc-
ture platform. However, the separation performance of UPC-
613 (0.47 mmolgÀ1) with the same modification group did not
change with the cage size, and the productivity was similar to
that of UPC-612. UPC-612 also showed excellent perfor-
mance in the separation of C2H2/C2H4 (50/50), the recovery
Figure 5. Adsorption energies of C2H2, C2H4, and C2H6 on linker and
metal cluster.
Breakthrough Experiments
To evaluate the effect of gas selectivity, breakthrough
experiments were performed in
a
typical process
(Scheme S3). The breakthrough experiments were performed
on MOF-525, MOF-525(Co), UPC-612 and UPC-613, with
C2H6/C2H4 (10/90, 50/50), C2H2/C2H4 (10/90, 50/50), and C2H6/
Figure 6. Breakthrough experiments. a) Experimental column breakthrough curves for C2H6/C2H4 (50/50) mixtures absorber bed packed with
MOF-525, MOF-525(Co), UPC-612, and UPC-613, respectively; b) Experimental column breakthrough curves for C2H2/C2H4 (50/50) mixtures
absorber bed packed with MOF-525, MOF-525(Co), UPC-612, and UPC-613, respectively; c) Experimental column breakthrough curves for C2H6/
C2H4/C2H2 (33/33/33) mixtures and an absorber bed packed with UPC-612; d) Experimental column breakthrough curves for C2H6/C2H4 (10/90)
mixtures absorber bed packed with MOF-525, MOF-525(Co), UPC-612, and UPC-613, respectively; e) Experimental column breakthrough curves
for C2H2/C2H4(10/90) mixtures absorber bed packed with MOF-525, MOF-525(Co), UPC-612, and UPC-613, respectively; f) Experimental column
breakthrough curves for C2H6/C2H4/C2H2 (33/33/33) mixtures absorber bed packed with UPC-613. (Description: In (a) and (d), all filled spheres
represent ethylene and all open spheres represent ethane; In (b) and (e), all filled spheres represent ethylene and all open spheres represent
acetylene.)
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Research Articles
Chemie
value (0.35 mmolgÀ1) is nearly twice that for MOF-525
Acknowledgements
(0.15 mmolgÀ1), six times that for MOF-525(Co)
(0.06 mmolgÀ1), and similar to UPC-613 (0.38 mmolgÀ1).
UPC-612 has the highest separation potential for recovering
the pure C2H4 from C2H6/C2H4 (50/50) and C2H2/C2H4 (50/50)
mixtures during the adsorption process (Figure 6a and b).
Even when the concentrations of C2H6 and C2H2 decrease to
10% (Figure 6d and e), UPC-612 maintains the highest
separation potential (Table S8), which makes it the most
promising material for the separation of C2H4 from C2H6/
C2H4 and C2H2/C2H4 mixtures. These excellent breakthrough
results encouraged us to further evaluate the separation
performance of ternary gas mixtures closer to industrial
applications, and to expand the gas ratio to 1/1/1 for C2H2/
C2H4/C2H6. As shown in Figure 6c, C2H2, C2H4, and C2H6 are
simultaneously adsorbed in UPC-612. After a period of time,
C2H4 first breaks through the adsorbent to produce pure
C2H4, then the adsorbent reaches saturation, and C2H2 and
C2H6 breakthrough at the same time. The separation time was
16 mingÀ1, and the productivity was calculated to be
0.47 mmolgÀ1. This is the first case of a single adsorbent for
one-step C2H4 purification from a large proportion (1/1/1)
ternary mixture of C2H2/C2H4/C2H6. The cycling and regen-
eration capabilities of UPC-612 were further studied by
breakthrough cycle experiments (Figure S30), which showed
no noticeable decrease in the mean residence times for both
C2H4, C2H6 and C2H4 within five continuous cycles under
ambient conditions. The UPC-612 material retained its
stability after the breakthrough cycling test (Figure S32).
The same experiment was carried out on UPC-613, and the
similar results obtained confirmed the feasibility of this new
modification method. Therefore, it is a good choice for
functional modification and selective separation of mixed
gases.
This work was financially supported by the National Natural
Science Foundation of China (NSFC Grant Nos. 21875285),
Taishan Scholar Foundation (ts201511019). Key Research
and Development Projects of Shandong Province
(2019JZZY010331), Fundamental Research Funds for the
Central Universities(20CX05010A).
Conflict of interest
The authors declare no conflict of interest.
Keywords: cobalt ·ethylene purification ·metal–
organic frameworks (MOFs) ·one-step purification ·
ternary gas mixture
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Manuscript received: January 18, 2021
Accepted manuscript online: March 4, 2021
Version of record online: March 30, 2021
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