Two interpenetrated metal-organic frameworks with a slim ethynyl-based ligand: Designed for selective gas adsorption and structural tuning

Article abstract of DOI:10.1039/c8ce00779a

In the design and construction of metal-organic frameworks (MOFs), the utilization of slim ligands is usually more inclined to form interpenetrated structures compared with bulky ones. The structural interpenetration can improve the framework stability and in some cases, enhance gas adsorption capacity and selectivity due to confined pores. In order to explore the structural control of MOFs and construct new MOFs with good selective gas adsorption ability, herein, a slim ethynyl-based 4-connected carboxylate acid ligand, 4,4′,4′′,4′′′-(benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid (H₄BTEB), was used to design and construct MOFs, hopefully having interpenetrated structures. Combining with 4-connected paddle-wheel Cu₂(COO)₄ and 8-connected Zr₆O₄(OH)₈(COO)₈ clusters, BTEB^4- ligand led to two new MOFs, [Cu₂(BTEB)(H₂O)₂] (BUT-43) and [Zr₆O₄(OH)₈(H₂O)₄(BTEB)₂] (BUT-44). As expected, the two MOFs have two-fold interpenetrated framework structures with partitioned channels. BUT-43 contains a rare three-dimensional (3D) 4-connected single network with lvt topology, which then interpenetrates. In BUT-44, each Zr₆-based cluster is coordinated with eight BTEB^4- ligands to give a single 3D 4,8-connected scu network, which then doubly interpenetrates to give the first example of interpenetrated 4,8-connected Zr(iv)-MOF. Studies on their stability and gas adsorption properties demonstrate that BUT-44 shows higher stability in NaOH solution of pH 10 and 1 M HCl aqueous solution. More interestingly, both MOFs represent good gas adsorption selectivities for C₂H₂ over CO₂ and CH₄, suggesting potential application in gas separation.

Full text of DOI:10.1039/c8ce00779a

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Two interpenetrated metal-organic frameworks with a slim
ethynyl-based ligand: designed for selective gas adsorption and
structural tuning
Received 00th January 20xx,
Accepted 00th January 20xx
Xiang-Jing Kong, Yong-Zheng Zhang, Tao He, Xue-Qian Wu, Ming-Ming Xu, Si-Nan Wang, Lin-Hua
Xie, Jian-Rong Li*
DOI: 10.1039/x0xx00000x
www.rsc.org/
In the design and construction of metal-organic frameworks (MOFs), the utilization of slim ligands is usually more inclined
to form interpenetrated structures compared with bulky ones. The structural interpenetration can improve the framework
stability, and in some cases enhance gas adsorption capacity and selectivity due to the confined pores. In order to explore
the structural control of MOFs and construct new MOFs with good selective gas adsorption ability, herein a slim ethynyl-
based 4-connected carboxylate acid ligand 4,4',4'',4'''-(benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid
(H4BTEB) was used to design and construct MOFs, hopefully having interpenetrated structures. Combining with 4-
connected paddle-wheel Cu2(COO)4 and 8-connected Zr6O4(OH)8(COO)8 clusters, BTEB4- ligand led to two new MOFs,
[Cu2(BTEB)(H2O)2] (BUT-43) and [Zr6O4(OH)8(H2O)4(BTEB)2] (BUT-44). As expected, the two MOFs have two-fold
interpenetrated framework structures with partitioned channels. BUT-43 contains a rare three-dimensional (3D) 4-
connected single network with a lvt topology, which then interpenetrates. While in BUT-44 each Zr6-based cluster is
coordinated with eight BTEB4- ligands to give a single 3D 4,8-connected scu network, then it doubly interpenetrates to give
the first example of interpenetrated 4,8-connected Zr(IV)-MOF. Studies on their stability and gas adsorption properties
show that BUT-44 is highly stable to withstand pH = 10 NaOH and 1 M HCl aqueous solutions. And more interestingly, both
MOFs represent good gas adsorption selectivities of C2H2 over CO2 and CH4, suggesting potential application in gas
separation.
construction perspective, the ligand is a very significant building
block because of the fact that its geometry and functionality will
make a direct influence on the output of related MOFs, as well as
Introduction
Metal-organic frameworks (MOFs), emerging as a new class of
versatile porous materials that are constructed from metal-based
nodes connecting with organic linkers through coordination bonds,
have received considerable attention in the past two decades due
to their high crystallinity, extraordinary porosity, and diverse
functionality.1 The unique crystalline periodic networks of MOFs
with tunable pore sizes and functionalities enable them to be
promising materials with extensive applications such as in gas
storage and/or separation, chemical sensing, drug delivery, and
heterogeneous catalysis.2
generated metal clusters in some cases.3 In general, in the
construction of MOFs, the utilization of slim (long and thin) ligands
is more inclined to form interpenetrated structures with improved
stability compared with bulky ones, and the non-interpenetrating
frameworks based on big ligands are usually fragile and vulnerable
to collapse.4 Simultaneously, interpenetrated frameworks also
feature the additional advantage of enhanced size- and shape-
selective effects toward guest molecules, and may thus enhance gas
selective adsorption of resulting MOFs due to the confined pores.5
In addition, it has been demonstrated that the incorporation of
functional groups, such as organic amine, hydroxyl, and C≡C triple
The rational design of organic linkers and precise choice of
metal-containing nodes are of great importance in order to obtain
desired MOFs with specific structures and properties. From a
2-
2- 7
bonds6 and inorganic SiF6 and Cr2O7
, into MOFs is able to
increase the interactions between gas molecules and framework
through van der Waals forces and/or π–π interactions as well as
electrostatic interactions, thereby efficaciously tuning their gas
adsorption property. As a special case, the introduction of ethynyl
into the backbone of the polycarboxylate ligands could thus not
only provide C≡C triple bonds as electron-donors but also enlarge
a Beijing Key Laboratory for Green Catalysis and Separation and Department of
Chemistry and Chemical Engineering, College of Environmental and Energy
Engineering, Beijing University of Technology, Beijing 100124, P. R. China.
E-mail: jrli@bjut.edu.cn.
† Electronic Supplementary Informaꢀon (ESI) available: 1H NMR, FT-IR, TGA and
crystal data (CCDC No.: 1842449 and 1842448). See DOI: 10.1039/x0xx00000x
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the linkers in size, which is in favour of the formation of thermogravimetric analyzer at a heating rate of 10 oC min-1 under
interpenetrated frameworks. air atmosphere. The powder X-ray diffraction (PXRD) patterns were
Beside of organic linkers, the structure and functionality of recorded on a Bruker D8-Focus Bragg-Brentano X-ray powder
MOFs can also be largely affected by the compositions and diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at
structures of their metal-containing nodes with different elements, room temperature (RT). Gas adsorption/desorption isotherms were
sizes, shapes, coordination numbers, and coordination modes.8 recorded in a Micrometrics ASAP 2020 surface area and pore
Square paddle-wheel Cu2(COO)4 (as Cu2 cluster) and dodecahedral analyzer. All the gases used were of 99.999% purity.
Zr6O8(COO)12 clusters are among common secondary building units
Synthesis and characterization of H4BTEB ligand
(SBUs) used in constructing a variety of MOFs, which sometimes
exhibit intriguing structures and properties.9 Particularly, the
As shown in Scheme S1, the H4BTEB was synthesized on a two-
step procedure including Sonogashira coupling and hydrolysis
unique geometry and high linkage number of the latter enable it to
reaction:
serve as various kinds of nodes in resultant networks by reducing
the connectivity (as Zr6O8(COO)n, n = 3, 6, 8, 9, 10, or 12, called Zr6
cluster), which could be achieved through using different ligands
and/or tuning synthetic conditions.10 Cu(II) paddle-wheel based
MOFs have been widely investigated mainly for their mild synthetic
conditions and accessible open metal sites on Cu(II) ions, which are
favorable to gas storage, separation, and catalysis.11 Zr(IV)-MOFs
with unsaturated Zr6 nodes have attracted enormous research
interest, among of which the 8-connected Zr6 cluster is most
popular, due to their good stability and fascinating properties,
thereby great application potential.12 Thus, both Cu2 and Zr6
clusters are excellent SBU platforms for the rational design and
structural tuning of MOFs in terms of given applications, such as for
gas selective adsorption and separation. Nevertheless, it should be
pointed out that the construction of desired MOFs with robust
structures and suitable pores to discriminate different gases is
somewhat a complicated process, for which both the smart strategy
and abundant efforts are required for ever.
Tetramethyl
(ethyne-2,1-diyl))tetrabenzoate (
) (4.9 g, 30.5 mmol), 1,2,4,5-tetrabromobenzene (2.0 g, 5.1
mmol), Pd(PPh3)2Cl2 (0.43 g, 0.6 mmol), PPh3 (0.32 g, 1.2
mmol), and CuI (0.12 g, 0.6 mmol) were stirred in
4,4',4'',4'''-(benzene-1,2,4,5-tetrayltetrakis
2). Methyl 4-ethynylbenzoate
(
1
a
deoxygenated mixture of Et3N (80 mL) and dry THF (80 mL)
under N2 atmosphere. The reaction mixture was heated at 70
ºC for 2 days. After cooling to RT, the mixture was filtered and
the solid residue was chromatographed on a silica column
(CHCl3) to give 2.1 g (58 %) of
2
as a golden yellow solid. 1H
NMR (CDCl3): δ 8.05 (d, 8H), 7.80 (s, 2H), 7.61 (d, 8H), 7.95 (s,
12H) (Fig. S1).
4,4',4'',4'''-(Benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))
tetrabenzoic acid (H4BTEB). Compound 2 (2.1 g, 3.0 mmol) was
refluxed with 1.4 g of sodium hydroxide in a mixture of THF: MeOH:
H2O (v: v: v = 50 mL : 50 mL : 50 mL) at 70 oC for 24 h. After cooling
to RT, the reaction mixture was concentrated to remove organic
solvents by rotary evaporation at reduced pressure. Water (200 mL)
was then added to the residue. To this suspension 2 M HCl was
slowly added and the pH was adjusted to around 3. The precipitate
was filtered and washed with water, and the resulting solid was
With above considerations, in this work, a slim and
ethynyl-containing 4-connected ligand 4,4',4'',4'''-(benzene-
1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic
acid13
(H4BTEB, Fig. 1a) has been used to assemble with Cu2 and Zr6
clusters, attempting to construct interpenetrated MOFs with
enhanced stability and good gas selective adsorption property.
As expected, two new MOFs, [Cu2(BTEB)(H2O)2] (BUT-43, BUT =
o
dried under vacuum at 60 C to give H4BTEB as a yellow solid (1.6g,
83%). 1H NMR (DMSO-d6): δ 8.01 (m, 10H), 7.70 (d, 8H) (Fig. S2).
Synthesis of [Cu2(BTEB)(H2O)2] (BUT-43)
Beijing
University
of
Technology)
and
BUT-43 was synthesized under solvethermal conditions: H4BTEB
(0.11 mmol, 75 mg), Cu(OAc)2·H2O (0.25 mmol, 50.0 mg), and acetic
acid (1.75 mL) were ultrasonically dissolved in 10 mL of N,N-
dimethylformamide (DMF) in a 20 mL Pyrex vial and sealed. The
reaction system was then heated at 100 °C for 12 h in an oven.
After cooling to RT, the resulting blue crystals of as-synthesized
BUT-43 were timely collected by suction filtration, washed with
DMF and acetone (38 mg, 41% based on H4BTEB ligand). PXRD
pattern and N2 adsorption/desorption isotherm of the as-
synthesized BUT-43 are shown in Fig. S3 and S4, FT-IR spectrum and
TGA curve are shown in Fig. S5 and S7, respectively.
[Zr6O4(OH)8(H2O)4(BTEB)2] (BUT-44) were hydrothermally
synthesized, whose 3D interpenetrated structures were
revealed by single crystal X-ray diffractions (SXRD). To our
knowledge, BUT-44 represents the first example of a 4,8-
connected Zr(IV)-MOF with an interpenetrated structure.
Furthermore, the stability and adsorption properties of them
have been examined. The results show that BUT-44 has
outstanding chemical stability, and both MOFs exhibit good
gas adsorption selectivities of C2H2 over CO2 and CH4,
suggesting their potential application in gas separation.
Synthesis of [Zr6O4(OH)8(H2O)4(BTEB)2] (BUT-44)
Experimental
BUT-44 was also synthesized under solvethermal conditions:
H4BTEB (0.11 mmol, 75 mg), ZrOCl2·8H2O (0.4 mmol, 130 mg), and
formic acid (4 mL) were ultrasonically dissolved in 10 mL of NMP in
a 20 mL Pyrex vial and sealed. The reaction system was heated at
135 °C for 24 h in an oven. After cooling to RT, the resulting light
yellow crystals of as-synthesized BUT-44 were collected, washed
with NMP and acetone, and then dried in air (32 mg, 27% based on
Materials and methods
All reagents (AR grade) were commercially purchased and used as
received. 1H NMR data were collected on a Bruker Avance III HD
400 MHz NMR spectrometer. Fourier-transform infrared (FT-IR)
spectra were recorded on an IR Affinity-1 instrument.
Thermogravimetric analyse (TGA) data were obtained on a TGA-50
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H4BTEB ligand). PXRD pattern and N2 adsorption/desorption on the slim feature of the used ligand. By using acetic acid as the
isotherm of the as-synthesized BUT-44 are shown in Figure 3, FT-IR competing reagent, block single crystals of BUT-43 suitable for
spectrum and TGA curve are shown in Figures S6 and S8, SXRD were obtained from the reaction between H4BTEB and
respectively.
Cu(OAc)2·H2O in DMF. SXRD structure analysis reveals that BUT-43
crystallizes in the monoclinic crystal system of the /c space group.
C
2
As expected, the classical Cu2 cluster exists in the structure of BUT-
43, in which two neighboring Cu(II) atoms are bridged by four
bimonodentate carboxylate groups from four different BTEB4−
ligands (the axis sites of the cluster are occupied by two solvent
molecules). The lengths of Cu–O bond are in the range of 1.95-1.97
Å, and the Cu···Cu separation is 2.64 Å, all similar to those in
reported MOFs based on the Cu2 cluster.19 Each Cu2 cluster
connects with four BTEB4− ligands, and each ligand links to four Cu2
clusters to give a single 3D framework. Topologically, the single
framework has a lvt net with the point symbol of {42.84} when
considering both the Cu2 cluster and the BTEB4− ligand as 4-
connected nodes (Fig. S9). Due to the large voids in single
frameworks and the slight steric hindrance of the slim skeleton of
BTEB4− ligand, two such identical and independent single
frameworks interpenetrate with each other to form the final two-
fold interpenetrated framework of BUT-43 (Fig. 1c). As a result of
the framework interpenetration, the large 1D rhombic channels
(the diagonal distances of the pores are 19.8 and 32.7 Å) in the
single framework along the a-axes are divided into smaller rhombic
channels of two sizes of 13.6 × 19.6 and 7.3 × 8.6 Å2, and rhomboid
channels (edge distances are 6.4 and 11.6 Å). The total solvent-
accessible volume in BUT-43 framework was estimated to be 71.3%,
by using PLATON.
Crystal structure determination
The diffraction data of as-synthesized BUT-43 and -44 were
collected in an Agilent Supernova CCD diffractometer equipped
with a mirror monochromated enhanced Cu K
α radiation (λ =
1.54184 Å) at 100 K. The datasets were corrected by empirical
absorption correction using spherical harmonics, implemented in
the SCALE3 ABSPACK scaling algorithm.14 The structures were
solved by direct methods and refined by full-matrix least-squares on
F2 with anisotropic displacement using the SHELXTL software
package.15 Hydrogen atoms of ligands were calculated in ideal
positions with isotropic displacement parameters. For BUT-44,
hydrogen atoms in coordinated water and hydroxyl groups were
not added but were calculated into molecular formula of the crystal
data. For these compounds, the volume fractions of disordered
solvents in the pores could not be modelled in terms of atomic sites
and were treated by using the MASK routine in the Olex2 software
package.16 The topologies of BUT-43 and -44 were calculated with
ToposPro17 and Systre.18 Crystal parameters and structure
refinement are summarized in Table S1 and S2. Crystallographic
data of BUT-43 and -44 have been deposited on the Cambridge
Crystallographic Data Center (CCDC No. 1842449 and 1842448).
Sample activation and gas adsorption
Before gas adsorption measurements, about 80 mg samples of BUT-
43 or BUT-44 crystals were soaked in 15 mL of DMF for 1 day at RT.
The samples were collected by decanting and then soaked in 15 mL
of acetone for another 3 days, when fresh solvents were exchanged
every day. After solvent exchange, the samples were loaded in a
sample tube and further activated under high vacuum at an
optimized temperature of 150 °C for BUT-43 and 60 °C for BUT-44
for 6 h, respectively. Then, the gas adsorption tests were conducted
at 77 K in a liquid nitrogen bath, at 273 K in an ice-water bath, and
at 298 K in a water bath.
Stability test
Stability is one of the primary concerns for the practical application
of MOFs. To examine the chemical stability of BUT-43 and -44, two
as-synthesized samples (100 mg for each) were soaked in water at
RT for 72 h as well as in pH = 10 NaOH and 1 M HCl aqueous
solutions at RT for 24 h, respectively. Then the treated samples
were collected by decanting, washed by water and acetone for
PXRD and N2 adsorption measurements. To assess their thermal
stability, the as-synthesized and activated samples of BUT-43 and -
44 were characterized by TGA analysis, respectively.
Results and Discussion
Synthesis and crystal structures
Fig. 1 Structures of ligand BTEB4− (a), 4-connected Cu2 cluster (b),
and BUT-43 MOF with a two-fold interpenetrated framework (c). H
atoms are omitted for clarity.
As mentioned above, a new MOF composing of ligand BTEB4− (Fig.
1a) and paddle-wheel Cu2(COO)4 cluster (Fig. 1b) was originally
desired, in which a structural interpenetration was expected relying
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Encouraged by the successful trial of BUT-43 with this slim kinds of rotation angles between the central and four peripheral
BTEB4− ligand based on the typical 4-connected paddle-wheel Cu2 benzene rings (Φ1 = 46.222o and Φ2= 55.124o). These two rotation
node, we hope to construct another MOF with Zr6 cluster of high patterns of the ligand BTEB4−, capable of matching the linkage
symmetry and connectivity, which can usually contribute to geometries of Cu2(COO)4 and Zr6O4(OH)8(COO)8 clusters, have thus
robust/stable frameworks.9b Simultaneously, an interpenetrated given rise to BUT-43 and -44, respectively. Furthermore, the less-
structure is also expected when the versatile Zr6 cluster encounters steric BTEB4− ligand and the large hollow pores in single frameworks
this slim ligand. Then, the reaction between H4BTEB and ZrOCl2 was of both BUT-43 and -44 provide an opportunity for them to
performed. At first, we conducted the experiment by using DMF as generate framework interpenetration, giving rise to the partitioned
the solvent in the presence of acetic acid. Only small rod-like pores.
crystals were generated. It is regrettable that all attempts to
characterize the structure of these tiny crystals through SXRD have
failed in our hands so far, due to the too small crystals. Afterwards,
we varied the reaction conditions by changing solvents and
modulators. Fortunately, bigger needle-like crystals suitable for
SXRD were formed in NMP solvent with formic acid as the
competing reagent.
SXRD structure analysis shows that BUT-44 crystallizes in the
orthorhombic crystal system of Cmcm space group. Its framework is
based on a hexanuclear Zr6O4(OH)8(H2O)4(COO)8 cluster, which is
formed by eight bimonodentate carboxylate groups of different
ligands assembling six Zr(IV) atoms (Fig. 2a and 2b); and twelve
H2O/OH− entities complete remaining coordination and account for
the charge balance. After careful analysis, the symmetry of this 8-
conneced Zr6 core in BUT-44 is found to be different from those in
PCN-222,12a NU-1000,12b and BUT-1212c. A slight deformation occurs
in the Zr6 octahedron, with the symmetry reducing from the usual
D4h to C4. The deformation of this Zr6 cluster is mainly reflected in
that the perpendicular distances from the two crystallographically-
independent Zr atoms (Zr1 and Zr2) located on two vertexes of the
octahedron to the equatorial plane of the cluster (formed by four
Zr3 atoms) (Fig. S10) are not the same. Each BTEB4− ligand thus links
to four such 8-connected Zr6 clusters to form an overall 3D
structure with 1D rhombic channels along the a-axes. From the
topological viewpoint, the BTEB4− ligand can be seen as a 4-
connected linker and the Zr6 cluster serves as an 8-connected node,
the 3D structure of BUT-44 can thus be simplified as a 4,8-
connected scu net with the point symbol of {416.612}{44.62}2 (Fig.
Fig. 2 Structures of ligand BTEB4- (a), 8-connected Zr6 cluster (b),
and BUT-44 MOF with a two-fold interpenetrated framework (c). H
atoms are omitted for clarity.
S11). Similar to that in BUT-43,
a two-fold framework
Stability and porosity
interpenetration also occurs in BUT-44, which represents the first
example of 4,8-connected Zr(IV)-MOF with an interpenetrated
structure. The large 1D rhombic channels (diagonal distances are
16.4 and 36.2 Å) in the single framework are divided into smaller
ones of two sizes (13.1 × 18.2 and 10.1 × 10.5 Å2), and rhomboid
pores (edge distances are 7.8 and 11.6 Å), respectively (Fig. 2c). The
total solvent-accessible volume in the framework of BUT-44 is
estimated to be 55.5%, by PLATON.
PXRD analyses were employed to check the phase purity and
stability of BUT-43 and -44. The results show that the diffraction
patterns of their fresh samples are in good agreement with the
simulated ones from single crystal structure data, indicating their
pure phases (Fig. 3a and 3b). To examine the chemical stability of
BUT-43 and -44, their samples were treated in water for 72 h, pH =
10 NaOH, and 1 M HCl aqueous solutions for 24 h at RT. After
immersed in these solutions for given times, the samples were re-
collected for further structural characterizations. The measured
PXRD patterns of BUT-44 samples after treatments show retained
structure, demonstrating its excellent stability (Fig. 3b). By contrast,
BUT-43 decomposes in all tests probably due to the weaker Cu−O
bonds vulnerable to water molecules, but its framework can remain
intact after activation and degassing under above mentioned
conditions (Fig. 3a). TGA curves show that the framework of BUT-43
and -44 can be stable up to 300 and 350 °C, respectively (Fig. S7 and
S8).
Carefully analyzing the ligand geometry in BUT-43 and -44, the
BTEB4− is found to have a “smart” model, where the introduction of
ethynyl not only stretches the ligand to a slim one, but also allows
the free rotation of four peripheral benzoates (Fig. 1a and 2a).
These intrinsic features enable it to well adapt the coordination
requirement of metal ions/clusters with different symmetry and
connectivity, thereby forming intriguing structures. In BUT-43, the
dihedral angles between central benzene ring and four peripheral
benzene rings are different (Φ1 = 27.337o, Φ2 = 38.395o, Φ3
=
33.541o, and Φ4 = 14.766o), while in BUT-44, there only exist two
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In order to assess their permanent porosity, N2 adsorption 326 cm3 g−1 (STP) at 1 atm, and the evaluated BET surface area of
measurements were conducted on BUT-43 and -44 samples pre- 1021 m2 g−1 (Fig. 3d). The CO2 sorption at 195 K on both materials
activated by solvent exchange and heating under high vacuum (see (Fig. S3) has also been performed to examine the rationality of
more details in the Experimental Section). As shown in Fig. 3c, results observed in N2 adsorption at 77 K, which shows good
stepwise N2 adsorption/desorption isotherms are observed for BUT- agreement. Furthermore, N2 adsorption isotherms of BUT-44
43, which implies the framework flexibility of this interpenetrated samples treated under different conditions were also measured,
MOF.20 The N2 uptake is 340 cm3 g−1 (STP) at 1 atm, and the and the results show almost the same uptakes as that of the
evaluated Brunauer-Emmett-Teller (BET) surface area is 1124 m2 g−1. pristine sample (Fig. 3d), which further confirms the framework
For BUT-44, the N2 adsorption/desorption at 77 K represents a type stability of BUT-44 under neutral, acidic, and weak basic aqueous
I isotherm of microporous materials with the saturated uptake of solutions.
Fig. 3 PXRD patterns of BUT-43 (a) and BUT-44 (b) samples treated under different conditions, and N2 adsorption/desorption isotherms of
pristine BUT-43 (c) and BUT-44 (d) samples treated under different conditions.
Gas selective adsorption
outstanding selective adsorption of C2H2 over CO2 and CH4. These
results indicate the potential applications of BUT-43 and -44 for
important C2H2/CO2 and C2H2/CH4 separations. In order to evaluate
the feasibility of these MOFs for separations, the adsorption
selectivities for the binary C2H2/CO2 and C2H2/CH4 in the two MOFs
were evaluated by the ideal adsorbed solution theory (IAST)
method22. As shown in Fig. 4c and 4d, the estimated C2H2/CO2
selectivities for an equimolar gas mixture in BUT-43 and -44 at 298
K are in ranges of 3.9–8.4 and 2.7–5.8, respectively. Using the same
method, the selectivities of the two MOFs for C2H2/CH4 binary
mixture at 298 K were also calculated, to be in ranges of 19.5–35.2
and 18.9–33.5, respectively. At 273 K, the selectivities of BUT-43 for
C2H2/CO2 and C2H2/CH4 are 4.3–8.9, 22.2–41.1, and of BUT-44 are
3.7–6.3, 23.5–40.4, higher than those at 298 K (Fig. S12c and S12d).
As discussed above, both BUT-43 and -44 have interpenetrated 3D
framework. The structural interpenetration leads to their decreased
pore size and improved network stability to some extent, making
them excellent for gas selective adsorption and separation.
Since the separation of C2H2 from CO2 or CH4 is very important
in industrial productions,21 we investigated the performances of
BUT-43 and -44 in the adsorptive separation of these gases. The
adsorption isotherms of pure C2H2, CO2, and CH4 in the evacuated
BUT-43 and -44 were measured, respectively, and the results are
shown in Fig. S12 (at 273 K) and Fig. 4 (at 298 K). As can be seen
from these isotherms, the two MOFs have a maximum C2H2 uptake
of 85.6 and 83.1 cm3 g−1, CO2 uptake of 51.3 and 52.0 cm3 g−1, and
CH4 uptake of 16.3 and 14.3 cm3 g−1 at 273 K and 1 atm, respectively
(Fig. S12a and S12b). At 298 K, there are slight decreases of the
adsorbed amounts for C2H2 down to 67.7 and 61.9 cm3 g-−1, CO2 to
42.9 and 42.0 cm3 g−1, and CH4 to 12.7 and 9.5 cm3 g−1, respectively
(Fig. 4a and 4b). It was observed that the uptakes of C2H2 are
significantly higher than those of CO2 and CH4, suggesting the
These data suggest that both BUT-43 and -44 exhibit
moderately high selectivities of C2H2 over CO2 and CH4 at 298 K
comparable to some reported MOFs,23 which could be ascribed to
the confined/suitable pores segmented by the network
interpenetration and the higher density of acetylenic bonds on the
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limited exposed surface of the framework. Compared with BUT-44, calculated Qst values of BUT-43 are higher, indicating the stronger
the selectivities of BUT-43 (Fig. 4c) are slightly higher, presumably interaction between its framework and guest molecules.
attributing to the different pore size in two MOFs resulting from
It should be pointed out that initially we want to explore the
their different degree of framework interpenetration and rich open
specific effect of framework interpenetration on gas selective
metal sites in the latter.
adsorption of MOFs through designing and constructing BUT-43 and
In addition, to evaluate the interaction between gas molecules -44. Regrettably, their non-interpenetrated counterparts have not
and the host frameworks, the isosteric heats of adsorption (Qst) for been obtained even after a lot of attempts. Therefore, data of the
C2H2, CO2, and CH4 in BUT-43 and -44 were calculated by using the evidential comparison between their selective gas adsorption
Clausius–Clapeyron equation22a, based on their pure component capacities is not available now. Further exploration is going on in
isotherms at 273 and 298 K (Fig. S13). The calculated Qst values for our lab.
C2H2, CO2 and CH4 of BUT-43 are in ranges of 17.1–25.5, 14.9–17.2,
and 13.8–14.8, respectively. While those of BUT-44 are 17.1–23.1,
9.0–13.6, and 12.2–13.1, respectively. Among these two MOFs, the
Fig. 4 C2H2, CO2, and CH4 adsorption isotherms recorded at 298 K for BUT-43 (a) and BUT-44 (b); IAST C2H2/CH4 and C2H2/CO2 selectivities in
BUT-43 (c) and BUT-44 (d) at 298 K.
exhibit moderately high C2H2/CO2 and C2H2/CH4 adsorption
selectivities owing to the confined pores resulting from the
Conclusions
framework interpenetration and the C≡C groups on their pore
surfaces, suggesting potential application in gas separation.
In summary, we have successfully constructed two
interpenetrated MOFs, by employing a slim ethynyl-based
ligand. With typical 4-connected Cu2 and 8-connected Zr6
Conflicts of interest
metal-containing clusters, the rational tuning on structures
There are no conflicts to declare.
and gas adsorption performance of resulting MOFs was
achieved. The Cu(II)-based MOF with higher porosity and large
pores could remain intact after removing guest molecules.
While the Zr(IV)-based one presents the first example of
interpenetrated Zr(IV)-MOF with a 4,8-connected framework
structure and can withstand pH = 10 NaOH and 1 M HCl
aqueous solutions, being quite stable. Moreover, both MOFs
Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation (21576006 and 21771012) and the
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Structural tuning and selective gas adsorption of two interpenetrated metal-organic frameworks
with a slim ethynyl-based ligand was identified.