Modulation of Topological Structures and Adsorption Properties of Copper-Tricarboxylate Frameworks Enabled by the Effect of the Functional Group and Its Position

Article abstract of DOI:10.1021/acs.inorgchem.1c00753

To push forward the structural development and fully explore the potential utility, it is highly desired but challenging to regulate in a controllable manner the structures and properties of MOFs. In this work, we reported the structural and functional modulation of Cu(II)-tricarboxylate frameworks by employing a strategy of engineering the functionalities and their positions. Two pairs of unsymmetrical biaryl tricarboxylate ligands modified with a methyl group and a pyridinic-N atom at distinct positions were logically designed and synthesized, and their corresponding Cu(II)-based MOFs were solvothermally constructed. Diffraction analyses revealed that the variation of functionalities and their positions furnished three different types of topological structures, which we ascribed to the steric effect exerted by the methyl group and the chelating effect involving the pyridinic-N atom. Furthermore, gas adsorption studies showed that three of them are potential candidates as solid separation media for acetylene (C2H2) purification, with the separation potential tailorable by altering functionalities and their locations. At 106.7 kPa and 298 K, the C2H2 uptake capacity varies from 64.1 to 132.4 cm3 (STP) g-1, while the adsorption selectivities of C2H2 over its coexisting components of CO2 and CH4 fall in the ranges of 3.28-4.60 and 14.1-21.9, respectively.

Full text of DOI:10.1021/acs.inorgchem.1c00753

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Article
Modulation of Topological Structures and Adsorption Properties of
Copper-Tricarboxylate Frameworks Enabled by the Effect of the
Functional Group and Its Position
Shengjie Lin, Ping Zhou, Tingting Xu, Lihui Fan, Xinxin Wang, Lianglan Yue, Zhenzhen Jiang,
Yuanbin Zhang,* Zhengyi Zhang, and Yabing He*
Cite This: Inorg. Chem. 2021, 60, 8111−8122
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ABSTRACT: To push forward the structural development and
fully explore the potential utility, it is highly desired but challenging
to regulate in a controllable manner the structures and properties
of MOFs. In this work, we reported the structural and functional
modulation of Cu(II)-tricarboxylate frameworks by employing a
strategy of engineering the functionalities and their positions. Two
pairs of unsymmetrical biaryl tricarboxylate ligands modified with a
methyl group and a pyridinic-N atom at distinct positions were
logically designed and synthesized, and their corresponding
Cu(II)-based MOFs were solvothermally constructed. Diffraction
analyses revealed that the variation of functionalities and their
positions furnished three different types of topological structures, which we ascribed to the steric effect exerted by the methyl group
and the chelating effect involving the pyridinic-N atom. Furthermore, gas adsorption studies showed that three of them are potential
candidates as solid separation media for acetylene (C2H2) purification, with the separation potential tailorable by altering
functionalities and their locations. At 106.7 kPa and 298 K, the C2H2 uptake capacity varies from 64.1 to 132.4 cm3 (STP) g−1, while
the adsorption selectivities of C2H2 over its coexisting components of CO2 and CH4 fall in the ranges of 3.28−4.60 and 14.1−21.9,
respectively.
their structures. The previous topology design was achieved by
using the geometrically matched molecular building blocks to
replace the network nodes, but this method is restricted by a
limited number of network prototypes.19,20 A turning point is
the evolution of reticular chemistry wherein the single-metal
ions are replaced with the polynuclear clusters, providing
access to a multitude of MOFs with diversified topologies.21
Nonetheless, to fully explore the potential utility and push
forward the structural development, it is still highly desirable to
expand the topological diversity and enhance the structural
complexity of MOFs. In general, the ligand design and
selection are very critical factors in MOF construction. By
employing the ligand elongation/contraction/truncation/de-
symmetry/angle variation strategies, the organic ligands can be
engineered in terms of size, rigidity/flexibility, geometry, shape,
symmetry/unsymmetry, and connectivity. Such a high level of
1. INTRODUCTION
Metal−organic frameworks (MOFs), as a subclass of
coordination polymers, are a new class of porous crystalline
materials made up of a wide range of inorganic (metallic ions/
clusters) and organic (multidentate organic ligands) building
blocks that link via dative bonds to generate ordered periodic
networks featuring modular design and fabrication as well as
mild synthetic conditions, in conjunction with a large surface
area, exceptional porosity, tunable pore size, and designable
framework functionality that are unattainable in the conven-
tional porous solids. By virtue of such a structural superiority,
MOFs have been developing fleetly as a research arena for
myriad applications including but not limited to molecular
capture/separation,1,2 recognition/sensing,3 chemical trans-
formation,4 and drug delivery.5 Specifically, for the separation
of environment/energy-relevant gases, molecular-sieving sepa-
ration,6−9 selectivity-reversing adsorption,10−17 and single-
column multicomponent purification18 of mixed gases with
very similar properties have been realized by using porous
MOF solids as physisorbents, which have the potential to
significantly simplify the separation scheme and process design.
The structural modulation of MOFs is of great theoretical
and practical significance because the physicochemical proper-
ties and applications of MOF materials are intimately related to
Received: March 11, 2021
Published: May 21, 2021
https://doi.org/10.1021/acs.inorgchem.1c00753
© 2021 American Chemical Society
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a
Scheme 1. Biaryl Tricarboxylate Linkers Used to Construct Cu(II)-Based MOFs
a
For H3L1, partial atomic labels are shown.
ligand designability delivers flexibility, diversity, complexity,
and tunability to topological structures of the MOF materials,
which have been successfully demonstrated by some famous
research groups of Zhou, Li, Maspoch, Trikalitis, Eddaoudi,
Matzger, and so on.22−30 For example, by disrupting the ligand
collinearity, Maspoch et al. used zigzag-shaped dicarboxylate
linkers to construct a family of isoreticular Zr-based MOFs
adopting a 8-connected bcu topology instead of a 12-
connected fcu topology typically observed for the linear
dicarboxylate linkers.26 Zhou and Li et al. very recently
prepared a series of rare-earth MOFs derived from
tetracarboxylate ligands with different sizes, rigidities, and
symmetries and disclosed that the topological structure is
heavily dependent on the ligand backbone geometry.24 In
particular, two unprecedented (4, 8)-c net topologies of lxl and
jun were also obtained in their systematic exploration.
Although remarkable progress has been made in MOF
structural development by judicious ligand design, structural
modulation in a synthetically controllable and predictable
manner remains a significant challenge. Furthermore, to
facilitate the future rational design of MOFs, it is necessary
to understand which structural parameter of the ligands plays a
key role in dictating the finial network architectures of the
resultant MOFs.
As we know, ligand functionalization has been frequently
applied to engineer the pore surface, thus endowing the MOFs
with varied properties including selective adsorption, frame-
work flexibility,31 and stability against water31 and desolva-
tion.32,33 In principle, the incorporation of accessible polarized
functional groups can strengthen the framework affinity toward
some specific gas molecules with large quadrupole moments
such as C2H2 and carbon dioxide (CO2) and thus improve
their adsorption properties.34−37 Besides, ligand functionaliza-
tion also has a complicated influence on the formation and
structure of MOFs. Upon coordination of the installed
functionality with the metal ions, the isoreticulation and even
the formation of MOFs might be prohibited. Even though the
functional group remains uncoordinated, it is also capable in
some cases of altering the conformation of an organic ligand,
which further offers an opportunity for the design and
discovery of unusual structures deviating from the default
topology.38 Zhou39 and Farha40 et al. respectively reported the
structural tuning of Zr-tetracarboxylate frameworks toward
diverse topologies by ligand functionalization controlling linker
conformation. Recently, by introducing the methyl group into
ligand skeleton to fix the ligand conformation, we successfully
targeted the fabrication of a Cu(II)-triisophthalate framework
with a novel topology instead of the usually observed rht
type.41 Considering that UMCM-150,42 a well-known MOF
based on a biphenyl tricarboxylate ligand featuring symmetri-
cally unequivalent carboxylate groups, displays excellent
performance in gas-phase and liquid-phase adsorption,43 we
in this work aimed to enrich structural diversities and tune the
gas adsorption properties of the UMCM-150-based system by
employing a ligand functionalization strategy. By introducing
the methyl group and pyridinic-N atom at the different
positions of the biphenyl scaffold, we designed and synthesized
two pairs of tricarboxylate linkers (Scheme 1) and constructed
their corresponding Cu(II)-based MOFs under suitable
solvothermal conditions. Their structural elucidations via X-
ray diffraction studies and performance evaluations via single-
component isotherm measurements and IAST selectivity
predictions were systematically conducted. The results showed
that engineering the substituents and their positions provides
access to three different kinds of topological structures and
enables facile modulation of selective gas adsorption properties
related to C2H2 separation and purification.
2. EXPERIMENTAL SECTION
2.1. Materials and Characterization Details. Dimethyl 5-
(pinacolboryl)isophthalate was prepared by following our own
published protocol.44 All of the other starting materials are
commercially available and used as received without further
depuration. All of the solvents used are analytical grade. Deionized
water (H2O) was used throughout this research. Silica gel (100−200
mesh) purchased from Qingdao Haiyang Chemical Lt. Co. was used
for column chromatography purification. Solution-state 1H/13C NMR
(nuclear magnetic resonance) spectra were recorded on a Bruker
AV400/600 NMR instrument, with the samples dissolved in CDCl3
(for trimethyl ester intermediates) or DMSO-d6 (for target ligands).
The proton and carbon chemical shifts (δ) were referenced to the
residual solvent peaks. The δ values were reported in parts per million
(ppm), and coupling constants (J), in Hertz (Hz). FTIR (Fourier
transform infrared) spectral data were gathered in transmission mode
on a Nicolet 5DX FTIR spectrometer in the wavenumber range of
4000−400 cm−1 with the sample dispersed in a KBr pellet (sample/
KBr = 1:100 by mass). The CHN contents were assayed on a
PerkinElmer 240 CHN elemental analyzer. Thermal behaviors were
characterized by thermogravimetric analyses (TGA), which were
conducted on a Netzsch STA 449C thermal analyzer in a flow of
dinitrogen (N2) at a heating rate controlled to 5 K min−1. PXRD
(powder X-ray diffraction) investigations were accomplished on a
Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418
Å) over an angular range of 5−45° (2θ). Predicted powder patterns
were generated from single-crystal structures using Mercury software.
Gas adsorption isotherm data were collected on a Micrometrics ASAP
(accelerated surface area and porosimetry) 2020 HD88 instrument.
The dead space estimation is based on helium (He, 99.999% purity).
The gases used for adsorption measurements have the following
purities (C2H2, 99.9%; CO2, 99.99%; methane (CH4), 99.999%; and
N2, 99.9999%). Before the isotherm data collection, the sample
activation was carried out according to the protocol below. The guest
species in as-made ZJNU-111 and ZJNU-114 were exchanged with
anhydrous acetone, and then the exchanged samples were heated at a
rate of 1 K min−1 to a temperature of 373 K under a dynamic vacuum.
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For ZJNU-112 and ZJNU-113, the as-synthesized solids were
subsequently guest-exchanged with acetone, dichloromethane, and
n-hexane and heated at a rate of 1 K min−1 to a temperature of 333 K
under a dynamic vacuum. The samples were considered to be
activated when the degassed rate of 2 μm Hg min−1 was approached.
The PXRD measurements revealed that their framework structures
were retained after the above activation treatments (Figure S1).
During the gas adsorption measurement, the samples were
refrigerated at 77 K by immersing the sample tube in a liquid-N2
bath and maintained at the other temperatures by using a circulating
H2O bath.
2.2. Single-Crystal X-ray Diffraction. Diffraction intensity data
for ZJNU-111-114 were collected with a Bruker D8 Venture Photon
II diffractometer with Cu Kα radiation (λ = 1.54178 Å). A
Cryostream 800 system (Oxford Cryosystems) and a SADABS
program were used for temperature regulation and absorption
correction, respectively. The direct method and full-matrix least-
squares method on F2 were used to solve and refine all of the
structures, with anisotropic thermal parameters for all non-hydrogen
(H) atoms. The H atoms were placed in calculated positions with
fixed isotropic thermal parameters. The lattice guest molecules were
treated as the diffuse contribution to the overall scattering by
SQUEEZE/PLATON.45 The details on crystallographic data and
refinement parameters were listed in Table S1 in the Supporting
Information.
methylbenzoate. The ester precursor was purified by recrystallization
in toluene. The overall yield is approximately 25%. For the precursor
of trimethyl ester, 1H NMR (CDCl3, 400.1 MHz) δ (ppm): 9.037 (d,
J = 2.4 Hz, 1H), 8.781 (t, J = 1.6 Hz, 1H), 8.507 (d, J = 1.6 Hz, 2H),
8.285 (d, J = 8.0 Hz, 1H), 8.133 (dd, J = 8.0 Hz, 2.4 Hz, 1H), 4.078
(s, 3H), 4.023 (s, 6H); for the target ligand H3L3, 1H NMR (DMSO-
d6, 600.1 MHz) δ (ppm): 13.537 (br, 3H), 9.087 (s, 1H), 8.523 (s,
1H), 8.476 (s, 2H), 8.365 (d, J = 7.2 Hz, 1H), 8.149 (d, J = 7.8 Hz,
1H); 13C NMR (DMSO-d6, 150.9 MHz) δ (ppm): 166.764, 166.425,
148.260, 148.202, 137.657, 137.233, 136.274, 132.864, 132.293,
130.421, 125.409; selected FTIR (KBr, cm−1): 1728, 1593, 1454,
1387, 1240, 1151, 1080, 1034, 906, 872, 800, 758, 723, 681, 536, 438.
2.3.4. 5-(5-Carboxypyridin-2-yl)isophthalic Acid (H3L4). The
synthesis protocol of H3L1 was adopted to prepare target ligand
H3L4 except that methyl 4-bromo-2-methylbenzoate was substituted
with methyl 6-bromonicotinate. The overall yield is approximately
1
43%. For the precursor of trimethyl ester, H NMR (CDCl3, 400.1
MHz) δ (ppm): 9.345 (dd, J = 2.0 Hz, 0.8 Hz, 1H), 8.957 (d, J = 1.6
Hz, 2H), 8.801 (t, J = 1.6 Hz, 1H), 8.434 (dd, J = 8.0 Hz, 2.0 Hz,
1H), 7.967 (dd, J = 8.0 Hz, 0.8 Hz, 1H), 4.021 (s, 6H), 4.017 (s, 3H).
1
For target ligand H3L4, H NMR (DMSO-d6, 600.1 MHz) δ (ppm):
13.510 (br, 3H), 9.182 (s, 1H), 8.882 (s, 2H), 8.544 (s, 1H), 8.362
(d, J = 8.4 Hz, 1H), 8.249 (d, J = 8.4 Hz, 1H); 13C NMR (DMSO-d6,
150.9 MHz) δ (ppm): 166.804, 166.505, 157.721, 150.992, 138.971,
138.899, 132.562, 131.957, 131.473, 126.274, 120.925; selected FTIR
(KBr, cm−1): 1911, 1720, 1605, 1450, 1414, 1375, 1240, 1134, 1082,
1036, 930, 903, 864, 804, 787, 756, 725, 677.
2.3. Synthesis and Characterization of the Organic Ligands.
2.3.1. 3′-Methylbiphenyl-3,4′,5-tricarboxylic Acid (H3L1). Into a 500
mL Schlenk flask were placed dimethyl 5-(pinacolboryl)isophthalate
(2.31 g, 7.22 mmol), methyl 4-bromo-2-methylbenzoate (1.50 g, 6.55
mmol), anhydrous Cs2CO3 (3.20 g, 9.82 mmol), Pd(PPh3)4 (0.19 g,
0.164 mmol), and dry 1,4-dixoane (150 mL), and the resultant
mixture was stirred gently under reflux and a N2 atmosphere for 48 h.
After the usual workup, chromatography purification on a silica gel
column with the eluent consisting of petroleum ether/ethyl acetate
(4/1 v/v) afforded the trimethyl ester precursor as an off-white solid
2.4. Synthesis and Characterization of the MOFs.
2.4.1. ZJNU-111. To a 20 mL glass vial preloaded with CuCl2·
2H2O (10.0 mg, 58.7 μmol) and H3L1 (5.0 mg, 16.7 μmol) were
added 1.5 mL of DMF (N,N-dimethylformamide), 0.2 mL of
deionized H2O, and 20 μL of 6 mol·L−1 HCl aqueous solution.
The vial was screw-capped tightly and placed in a preheated oven at
the temperature of 353 K for 48 h. During this period, the green, thin,
plate-shaped crystals of ZJNU-111 were generated, which were
isolated by decanting off the mother liquor and washing with fresh
DMF several times. The yield is about 67% based on H3L1. Selected
FTIR (KBr, cm−1): 3433, 1657, 1439, 1389, 1365, 1254, 1099, 775,
729, 663; elemental analysis for C165H229Cu9N23O66, calcd: C,
47.61%, H, 5.55%, N, 7.74%; found: C, 47.69%, H, 5.58%, N, 7.66%.
2.4.2. ZJNU-112. To a 20 mL glass vial containing CuCl2·2H2O
(15.0 mg, 88.0 μmol) and H3L2 (5.0 mg, 16.7 μmol) were added N-
methylformamide/tetrafluoroboric acid (NMF/HBF4, 1.5 mL/0.1
mL). The vial was screw-capped tightly and placed in an isothermal
oven at 353 K. After the vial was held at this temperature for 72 h
under autogenous pressure, it was removed from the oven. The blue,
thin, plate-shaped crystals of ZJNU-112 formed from the solution
were isolated by decanting off the mother liquor and washed
thoroughly with fresh DMF several times. The yield is about 54%
based on H3L2. Selected FTIR (KBr, cm−1): 3433, 1664, 1585, 1541,
1439, 1414, 1369, 1095, 918, 777, 758, 737, 723, 661, 492. Elemental
analysis for C64H74Cu4N6O26. Calcd: C, 48.12%, H, 4.67%, N, 5.26%;
found: C, 48.27%, H, 4.69%, N, 5.33%.
2.4.3. ZJNU-113. A mixture of Cu(NO3)2·3H2O (5.0 mg, 20.7
μmol) and H3L3 (5.0 mg, 17.4 μmol) was dispersed in 1.0 mL of
DMA (N,N-dimethylacetamide), 50 μL of deionized H2O, and 50 μL
of 3 mol·L−1 HCl, which was added to a 20 mL glass vial. The vial was
screw-capped tightly and placed in a preheated oven at 363 K. After
an incubation period of 96 h under autogenous pressure, the blue
crystals of ZJNU-113 thus formed were isolated by decanting off the
mother liquor and washed with fresh DMA several times. The yield is
about 49% based on H3L3. Selected FTIR (KBr, cm−1): 3446, 3082,
2794, 2482, 1653, 1605, 1574, 1360, 1265, 1043, 769, 685; elemental
analysis for C32H36CuN4O15. Calcd: C, 49.26%, H, 4.65%, N, 7.18%;
found: C, 49.49%, H, 4.68%, N, 7.22%.
1
with a yield of 24% (0.54 g, 1.58 mmol). H NMR (CDCl3, 600.1
MHz) δ (ppm): 8.710 (t, J = 1.8 Hz, 1H), 8.500 (d, J = 1.8 Hz, 2H),
8.056 (d, J = 9.0 Hz, 1H), 7.551−7.564 (m, 2H), 4.013 (s, 6H), 3.952
(s, 3H), 2.722 (s, 3H).
Target ligand H3L1 was quantitatively obtained by 6 mol L−1
NaOH-mediated saponification of the trimethyl ester precursor and
1
subsequent acidification with concentrated HCl. H NMR (DMSO-
d6, 600.1 MHz) δ (ppm): 13.280 (br, 3H), 8.485 (s, 1H), 8.414 (s,
2H), 7.958 (d, J = 7.8 Hz, 1H), 7.690 (s, 1H), 7.654 (d, J = 7.8 Hz,
1H), 2.510 (s, 3H). 13C NMR (DMSO-d6, 150.9 MHz) δ (ppm):
168.794, 166.891, 141.743, 140.711, 140.478, 132.623, 131.916,
131.739, 130.509, 130.399, 129.825, 124.789, 21.779. Selected FTIR
data (KBr, cm−1): 1736, 1695, 1610, 1460, 1423, 1308, 1284, 1261,
1209, 1186, 1147, 1074, 768, 719, 673, 652, 447.
2.3.2. 2′-Methylbiphenyl-3,4′,5-tricarboxylic Acid (H3L2). The
synthesis procedure of H3L2 is similar to that of H3L1, except that
methyl 4-bromo-3-methylbenzoate was used in place of methyl 4-
bromo-2-methylbenzoate. The trimethyl ester precursor was purified
by silica gel column chromatography with the petroleum ether/ethyl
acetate (10/1 v/v) as an eluent. The overall yield is approximately
1
36%. For the precursor of trimethyl ester, H NMR (CDCl3, 600.1
MHz) δ (ppm): 8.725 (t, J = 1.2 Hz, 1H), 8.221 (d, J = 1.2 Hz, 2H),
8.008 (s, 1H), 7.953 (d, J = 7.8 Hz, 1H), 7.326 (d, J = 7.8 Hz, 1H),
3.992 (s, 6H), 3.972 (s, 3H), 2.325 (s, 3H). For the target ligand
1
H3L2, H NMR (DMSO-d6, 600.1 MHz) δ (ppm): 13.290 (br, 3H),
8.507 (s, 1H), 8.102 (s, 2H), 7.926 (s, 1H), 7.861 (d, J = 7.8 Hz, 1H),
7.407 (d, J = 7.8 Hz, 1H), 2.509 (s, 3H); 13C NMR (DMSO-d6, 150.9
MHz) δ (ppm): 167.572, 166.851, 144.106, 141.544, 135.793,
133.869, 132.058, 131.883, 130.821, 130.356, 129.446, 127.619,
20.393; selected FTIR (KBr, cm−1): 1740, 1689, 1458, 1429, 1323,
1296, 1273, 1196, 1138, 1111, 1051, 924, 899, 881, 849, 762, 719,
673, 656, 650, 467.
2.3.3. 5-(6-Carboxypyridin-3-yl)isophthalic Acid (H3L3). H3L3
was synthesized according to the protocol for H3L1 except that
methyl 5-bromopicolinate was used instead of methyl 4-bromo-2-
2.4.4. ZJNU-114. DMF (1.5 mL) and deionized H2O (0.2 mL)
were added to a 20 mL glass vial containing CuCl2·2H2O (10.0 mg,
58.7 μmol) and H3L4 (5.0 mg, 17.4 μmol). The vial was screw-
capped tightly and placed in a preheated oven at 358 K. After 48 h
under autogenous pressure, the blue, thin, plate-shaped crystals of
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Figure 1. SCXRD structure of ZJNU-111. Views of three different types of (a−c) Cu(II)-carboxylate clusters, (d−f) tricarboxylate anions featuring
varied torsional angles, and (g−i) Cu(II)-organic polyhedral cages. Spheres of different colors highlight the cage voids. (j) A view of the 3D
network along the crystallographical c direction. Color codes of the atoms: C, gray; O, red; N, blue; Cu, pink. For clarity, hydrogen atoms were
deleted.
ZJNU-114 thus acquired were harvested by decanting off the mother
liquor and washing with fresh DMF several times. The yield is about
44% based on H3L4. Selected FTIR (KBr, cm−1): 3421, 1655, 1439,
1387, 1296, 1279, 1254, 1103, 771, 727. Elemental analysis for
C46H62Cu3N8O22. Calcd: C, 43.51%, H, 4.92%, N, 8.83%; found: C,
43.59%, H, 4.89%, N, 8.94%.
Single-crystal X-ray diffraction characterization shows that
compounds ZJNU-111 and ZJNU-114 are isostructural but
crystallize in different space groups: Ama2 for ZJNU-111 and
P63/mmc for ZJNU-114. The structural details are illustrated
below by taking ZJNU-111 as a representative. The asym-
metrical unit consists of three fully deprotonated ligands
connecting to six crystallographically unique divalent Cu(II)
ions axially coordinated by five H2O molecules and one DMF
molecule. The occupancy is 0.5 for half of the Cu(II) ions
(Cu4, Cu5, and Cu6) and H2O molecules (H2O(1−5),
H2O(1−6), and H2O(1−7)) and 1.0 for the remaining species.
All of the Cu(II) centers are pentacoordinated by four oxygen
atoms coming from four carboxylate groups and one oxygen
atom provided by the terminal species, displaying square-
pyramidal geometry. The Cu−OCOO bond lengths range from
1.879 to 2.007 Å. As commonly observed in Cu(II)-
carboxylate frameworks, bridging a pair of adjacent Cu(II)
ions by four carboxylate groups from isophthate units yields
the ubiquitous [Cu2(COO)4] paddlewheel-shaped diCu(II)
cluster acting as a secondary building unit (SBU). It is worth
noting that there exist two crystallographically distinct
paddlewheel units dubbed SBU-I (Cu2−Cu3, Figure 1a) and
SBU-II (Cu1−Cu1, Figure 1b), with intrapaddlewheel Cu···Cu
distances of 2.664 and 2.673 Å, respectively. The H2O and
DMF molecules serving as terminal ligands are binding to the
axial locations of SBU-I and SBU-II, respectively. The diCu(II)
paddlewheel unit is connected to four tricarboxylate ligands
and thus can be taken as a square-planar 4-connected node.
SBU-III consists of three neighboring Cu(II) ions bridged by
six carboxylate groups from methylbenzoate units, providing a
trigonal-prism-shaped 6-connected node by linking to six L13−
anions (Figure 1c). As shown in Figure 1d−f, three
crystallographically unique ligands assume different conforma-
tions, with the torsional angles between the two central
benzene rings of 29.1, 30.5, and 29.3° for λ-L1, τ-L1, and ω-L1,
3. RESULTS AND DISCUSSION
The unsymmetrical tritopic biaryl tricarboxylate linkers
investigated in this work were facilely obtained in a two-step
organic synthesis with decent to good isolated yields. Briefly,
starting with dimethyl 5-(pinacolboryl)isophthalate and the
corresponding brominated methyl esters as starting materials, a
palladium-catalyzed cross-coupling reaction produced the
respective trimethyl ester precursors, which underwent de-
esterification under basic conditions and acidification with HCl
1
to furnish the target tricarboxylate ligands. H and 13C NMR
measurements (Figure S19) and FTIR spectroscopic character-
izations (Figure S3) unequivocally verified their chemical
structures. Subsequently, the targeted ligands were solvother-
mally coassembled with the Cu(II) cation, affording crystalline
solids whose structures can be established by single-crystal X-
ray diffraction. For the convenience of the following
description, the resulting compounds were termed ZJNU-
111, ZJNU-112, ZJNU-113, and ZJNU-114 corresponding to
linkers H3L1, H3L2, H3L3, and H3L4, respectively. The final
chemical formulas of ZJNU-111, ZJNU-112, ZJNU-113, and
Z J N U - 1 1 4 w e r e b e s t e s t a b l i s h e d t o b e
[Cu9(L1)6(H2O)7(DMF)2]·21DMF, [Cu4(L2)3(H2O)]·
MeNH3·5DMF·2H2O, [Cu(HL3)2]·2Me2NH2·3H2O, and
[Cu3(L4)2(H2O)3]·6DMF·H2O according to single-crystal
structural analyses, the SQUEEZE results, TGA (Figure S2),
and microanalyses. As shown in Figure S1, the experimental
and predicted PXRD patterns were coincident, affirming the
homogeneity of the as-synthesized bulk materials.
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Figure 2. SCXRD structure of ZJNU-112. Views of (a, b) two different kinds of metallic SBUs and (c) the organic ligand with a torsional angle of
86.7° between the two central benzene rings. Views of (d, e) two different kinds of Cu(II)-organic polyhedral cages and (f) the 3D network
structure along the c axis. The color codes of the atoms are the same as those in Figure 1. The hydrogen atoms are omitted for clarity.
Figure 3. SCXRD structure of ZJNU-113. Views of (a) the coordination modes and environments of Cu(II) ions and (b, c) the ligands with the
different backbone torsional angles. (d) A side view of a metal−organic square flanked with two ligands. Views of (e) the 2D layer along the bc
plane and (f) the 3D supramolecular network along the b axis, with hydrogen bonds indicated by blue broken lines. The color code of the H atom
is white, while those of the other atoms are the same as those in Figure 1.
respectively. All of the ligands connect to two diCu(II) clusters
and one triCu(II) cluster, serving as a 3-connected node. By
means of the assembling fashion above-mentioned, a three-
periodic (3,4,6)-connected three-nodal network was generated
with the agw topological type and the {63}6{64·82}3{66·83·106}
point symbol, as calculated with Topos software. A careful
inspection of the entire structure revealed three different types
of polyhedral cages denoted as A, B, and C in a 1:1:1 ratio,
whose voids were highlighted with different colors in Figure
1g−i. Cage A consists of 6 ligands (2 λ-L1 + 2 τ-L1 + 2 ω-L1)
connected to 6 diCu(II) clusters (4 SBU-I + 2 SBU-II) and 3
triCu(II) clusters. Cage B is formed by 6 diCu(II) clusters (4
SBU-I + 2 SBU-II) and 2 triCu(II) clusters bridged by 6
ligands (2 λ-L1 + 2 τ-L1 + 2 ω-L1). Cage C is encircled by 6
diCu(II) clusters (4 SBU-I + 2 SBU-II), 3 triCu(II) clusters,
12 ligands (4 λ-L1 + 4 τ-L1 + 4 ω-L1), and 6 isophthalate
units. The maximum diameters of the spheres embedded in
cages A−C are estimated to be 11.3, 9.8, and 12.2 Å,
respectively, taking the van der Waals radii into account. As
estimated by PLATON calculations, the potential solvent-
accessible pore volume is as high as 16643 Å3 after removal of
all of the guest species and coordinated solvent molecules,
which is equivalent to 70.5% of the unit cell volume,
highlighting its significant structural porosity.
exhibits a network structure distinctly different with that of
its corresponding counterpart, ZJNU-111 (Figure 2f). ZJNU-
112 crystallizes in the trigonal space group of R32:H, with
lattice parameters of a = b = 18.3673(19) Å and c = 36.460(6)
Å. In addition to the disordered guest species, the content of
the asymmetrical unit includes two independent Cu(II) cations
(Cu1 and Cu2) with occupancies of 1 and 1/3 for Cu1 and
Cu2, one fully deprotonated ligand anion, and one H2O
molecule coordinated to a Cu2 ion, thus leading to a negatively
charged framework with a chemical formula moiety of
Cu4(L2)3(H2O), which is balanced by the disordered
methanaminium in situ generated by NMF decomposition
during solvothermal transformation. The Cu1 ion is
surrounded by five carboxylate oxygen atoms originating
from five different ligands, adopting square-pyrimidal coordi-
nation geometry, while the Cu2 ion displays an octahedral
coordination geometry completed by three carboxylate oxygen
atoms from three separate linkers and three terminal water
molecules. The adjacent Cu1 ions are bridged by four
carboxylate groups from isophthalate units to create a diCu(II)
paddlewheel-shaped cluster with the axial positions occupied
by carboxylate oxygen atoms from methylbenzoate units
(Figure 2b, SBU-IV), which is distinct with the diCu(II)
cluster in its counterpart ZJNU-111, wherein the axial position
is accompanied by a H2O or DMF molecule. Besides a
diCu(II) cluster, a mononuclear {CuO4} SBU is formed
When the methyl substituent is shifted from the 3′-position
to the 2′-position, the resulting compound, ZJNU-112,
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around a Cu2 ion (Figure 2a, SBU-V), which is rarely observed
in the literature MOFs. The diCu(II) cluster is linked to six
organic ligands, thus serving as a 6-connected node, while the
mononuclear SBU is connected to three organic ligands, which
can be treated as a 3-connected node. Different from ZJNU-
111, the tricarboxylate ligand incorporated in ZJNU-112
adopts only one conformation with a torsional angle of 86.7°
between the two central phenyl rings (Figure 2c) and connects
to six Cu(II) ions through its three carboxylate groups. The
carboxylate groups at the 3,5-position and 4′-position assume
cis−cis and trans−cis μ2:η1η1 bridging coordination modes,
respectively. The organic ligand is connected to three diCu(II)
clusters and one mononuclear ion and therefore can be
regarded as a 4-connected node. In such an assembly fashion, a
(3,4,6)-connected 3-nodal network is generated with a new
topology type and the {42·64}6{44·68·83}3{63}2 point symbol, as
revealed by the Topos calculation. Two different types of
Cu(II)-organic polyhedral cages are also observed in the entire
network, which are illustrated in Figure 2d,e. If the metallic
SBUs are taken as the polyhedral vertices, then cage D is a
hexagonal bipyramidal cage composed of two mononuclear
SBUs, six diCu(II) SBUs, and six organic ligands (Figure 2d),
while cage E can be described as a bicapped triangular
antiprism consisting of six diCu(II) SBUs and two
mononuclear SBUs connected to six ligands (Figure 2e).
The two kinds of cages are packed alternatively along the
crystallographic c axis. The sizes of cages D and E were about
8.8 and 5.3 Å, respectively. The potential solvent-accessible
pore volume equals 50.0% of the unit cell volume, as estimated
by PLATON software.
hydrogen bonding and π−π interactions to form a 2D layer
(Figure 3e), which are further packed together by hydrogen
bonding (H4−1···O1−2 distance = 1.690 Å) to generate a 3D
+
supramolecular network with the void filled by Me2NH2 and
H2O forming hydrogen bonds with the framework carboxylate
oxygen atoms (Figure 3f).
From the structural description above, it can be seen that the
functionality and its position in the ligand backbone have a
very important influence on the finial network architecture. To
uncover the underlying reason responsible for the structural
discrepancy, we first compare the structures of each pair of
MOFs produced by using the same functional group grafted at
different positions. When MOFs ZJNU-111 and ZJNU-112,
with the ligand modified by the methyl group at different
positions, are considered, it can be observed that the methyl
group, as a noncoordinated group, is capable of sterically
regulating the torsional angle of the ligand skeleton, depending
on the position at which it is grafted. As expected, because of
the steric hindrance effect of the methyl group adjacent to the
isophthalate unit, the 2′-methyl-decorated ligand backbone
displays a larger torsional angle than the 3′-methyl-modified
counterpart. Accordingly, different torsional angles correspond
to different ligand conformations. As such, the ligands with
different conformations can function as organic nodes with
different connectivities and connecting modes. Furthermore, to
meet the specific requirement of ligand conformation and
asymmetry, diversified inorganic SBUs might be formed in the
coassembly process. Indeed, apart from the typical diCu(II)
cluster, uncommon kinds of inorganic SBUs of trinuclear
Cu3(COO)6 and mononuclear Cu(COO)3(H2O)3, which are
extremely rare for Cu(II) ions, are observed in the structures of
ZJNU-111 and ZJNU-112, respectively. As a result, ZJNU-
111 and ZJNU-112 display distinctly different network
architectures. From the above analyses, it can be concluded
that the structural difference between ZJNU-111 and ZJNU-
112 mainly originates from the steric effect of methyl
functionality regulating ligand conformations. Looking at
another pair of MOFs with the ligands decorated with the
pyridinic N atom, namely, ZJNU-113 and ZJNU-114, we
notice that compared to the methyl group, the pyridinic N
atom possesses one pair of lone electrons and therefore in
principle can serve as a coordinating site. In ZJNU-113, the
pyridinic N atom and carboxylate group are situated in
adjacent locations and thus are capable of simultaneously
binding to the same metallic ion. As a consequence, the
thermodynamically favored chelating effect is yielded, sig-
nificantly limiting the formation of the 3D coordination
network. When the pyridinic N atom is far away from the
carboxylate group, it remains free and the corresponding
chelating effect does not occur. Obviously, whether a pyridinic
N atom is involved in the chelating effect is the main reason for
the structural discrepancy between ZJNU-113 and ZJNU-114.
Next, we analyzed the structures of MOFs with the ligands
decorated with different functional groups but at the same
position. It can be observed that although the methyl group
and the pyridinic N atom are introduced at the same location,
the resulting MOFs possess distinctly different network
architectures, indicating that the positional effect of the
functional group varies from functionality to functionality.
The above results demonstrate that engineering the function-
ality and its position is a potential synthesis strategy employed
to modulate the MOF structures.
Upon shifting the pyridinic N atom from the 2′-position to
the 3′-position, the obtained compound ZJNU-113 exhibits a
network structure distinctly different with that of its
corresponding counterpart, ZJNU-114. ZJNU-113 is a discrete
̅
binuclear complex that crystallizes in the P1 space group. The
asymmetric unit is composed of one Cu(II) ion and two
+
partially deprotonated HL3 ligands, two Me2NH2 cations
generated by in situ DMF decomposition, and one lattice water
molecule. The Cu(II) center displays square-pyramidal
geometry, with the coordinating donors provided by two
trans pyridinic N atoms and two trans carboxylate oxygen
atoms from two chelating pyrdine carboxylate units located in
the equatorial plane and one carboxylate oxygen atom
occupying at the axial position (Figure 3a). Because of the
Jahn−Teller effect, the separation of 2.587 Å between the
Cu(II) cation and the carboxylate oxygen atom at the apical
position is larger than the corresponding values between the
Cu(II) ion and the carboxylate oxygen atoms in the equatorial
plane falling in the range of 1.931−1.938 Å. The Cu−Npyrdine
distance varies from 1.967 to 1.979 Å. As shown in Figure 3b,c,
the two independent organic ligands are almost coplanar with
slightly different torsional angles, which are 2.7 and 9.8° for α-
L3 and β-L3, respectively. Besides different conformations,
they also exhibit different coordination modes toward Cu(II)
ions. The α-L3 ligand is connected to one Cu(II) ion through
the pyridine carboxylate group adopting a bidentate chelating
mode, while the β-L3 ligand is linked to two Cu(II) ions
through its pyridine carboxylate group and isophthalate
carboxylate group, which respectively assume bidentate
chelating and monodentate coordination modes. Two Cu(II)
ions with a separation of 9.787 Å are bridged by two oppositely
aligned ligands to form a molecular square flanked by the two
ligands (Figure 3d). The molecular squares are interlinked via
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Table 1. Summary of the Textural Parameters and Gas Adsorption Properties of ZJNU-111, ZJNU-112, ZJNU-114, and
a
UMCM-150
b
b
gas uptake (cm3 g−1, STP)
IAST adsorption selectivity
MOFs
S
BET/SLangmuir (m2 g−1
)
Vp (cm3 g−1
)
C2H2
CO2
CH4
C2H2/CH4
C2H2/CO2
ZJNU-111
ZJNU-112
ZJNU-114
UMCM-150
2900/3109
728/823
3042/3375
3020/3375
1.1089
0.2990
1.2111
1.2140
123.6
64.1
132.4
123.0
65.9
43.1
66.6
63.1
16.1
15.6
15.6
15.3
18.8
14.1
21.9
19.5
4.02
3.28
4.60
4.31
a
b
S
BET/SLangmuir: SSAs calculated according to BET and Langmuir equations. Vp: Total pore volume. At a temperature of 298 K and a pressure of
106.7 kPa.
Figure 4. Gas adsorption properties of ZJNU-111, ZJNU-112, and ZJNU-114. (a) 77 K N2 isotherms. (b) DFT-based pore size distributions.
C2H2, CO2, and CH4 isotherms at (c) 298, (d) 288, and (e) 278 K. (f) Isosteric heat of C2H2, CO2, and CH4 adsorption as a function of gas
loadings. The filled and unfilled symbols represent adsorption and desorption data. The vertical line in (f) indicated the error bar. STP stands for
standard temperature and pressure.
It has been reported that a given coordinating group can be
obliged to tilt away from its preferred orientation through the
deliberate incorporation of steric hindrance in its ortho
position, leading to a twisted ligand that in turn provides
access to topologies that are generally not easily accessible.46
However, in ZJNU-111, the steric hindrance of the methyl
group does not induce a significant torsional angle between the
carboxylate group and the benzene ring to which it is attached.
Furthermore, the biphenyl backbone is almost coplanar, thus
allowing the L1 ligand to display a comparable conformation
to that of the undecorated tricarboxylate ligand used to
construct its parent compound UMCM-150.42 As a conse-
quence, ZJNU-111 is isostructural with UMCM-150. Similarly,
the introduction of the pyridinic N atom in the 2′-position
does not induce a significant distortion of the ligand skeleton,
and thus ZJNU-114 is also isostructural with its parent
compound, UMCM-150. Future work will focus on the
introduction of substituents much bulkier than the methyl
group in the ortho position of the 4′-carboxylate group to
develop uncommon topological structures of Cu(II)-tricarbox-
ylate frameworks.
in Table 1. As depicted in Figure 4a, ZJNU-113 displayed a
negligible amount of N2 adsorption, which is reasonable due to
its nonporous supramolecular structure as revealed above by
single-crystal X-ray diffraction. The nonporous nature of
ZJNU-113 was also confirmed by the negligible uptakes of
C2H2, CO2, and CH4 at 298 K (Figure S8). In contrast, the
other three compounds instead adsorbed significant amounts
of N2, and according to their IUPAC classification, they
exhibited typical type-I adsorption isotherms with a sharp
uptake increase in the relatively low pressure region, indicative
of their microporous structures. Employing BET (Brunauer−
Emmet−Teller)/Langmuir equations with N2 adsorption data
gave the corresponding specific surface areas (SSAs) of 2900/
3109 m2 g−1 for ZJNU-111, 728/823 m2 g−1 for ZJNU-112,
and 3042/3375 m2 g−1 for ZJNU-114, respectively (Figures
S4−S7). The relative pressure range used for BET SSA
calculations was based on three consistency criteria defined by
Rouquerol et al.47 An estimation of the total pore volumes
from the saturated amounts of N2 adsorbed furnished the
respective results of 1.1089 cm3 g−1 for ZJNU-111, 0.2990 cm3
g−1 for ZJNU-112, and 1.2111 cm3 g−1 for ZJNU-114. As
shown in Figure 4b, the DFT (density functional theory)-
based pore size distribution analyses showed that the pore
diameter was mainly centered at 11.8 Å for ZJNU-111 and
ZJNU-114 and was in the range of 5.0−8.0 Å for ZJNU-112,
An evaluation of their pore textural properties was
conducted by volumetric measurements of N2 sorption
isotherms at 77 K. Their textural parameters, together with
gas adsorption properties as discussed below, are summarized
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Figure 5. IAST adsorption selectivities of ZJNU-111, ZJNU-112, and ZJNU-114. Equimolar component (a) C2H2/CH4 and (b) C2H2/CO2
adsorption selectivities as a function of the bulk gas-phase pressures at 298 K. Influence of the measurement temperature on the equimolar-
component (c) C2H2/CH4 and (d) C2H2/CO2 adsorption selectivities at a pressure of 106.7 kPa. Influence of the mixture composition on (e)
C2H2/CH4 and (f) C2H2/CO2 adsorption selectivities at 298 K and 106.7 kPa. The connected lines were used as guides for the eyes.
which is consistent with the structural analyses above.
Compared to their isostructural parent compound UMCM-
150, ZJNU-114 has a comparable surface area and pore size,
while ZJNU-111 exhibits much less data due to the methyl
substituent occupying the partial pore volume and the terminal
DMF molecules residing in a section of diCu(II) clusters.
Compared to ZJNU-111 and ZJNU-114, ZJNU-112 displays a
smaller surface area and pore size mainly as a result of different
topological structures.
S9). Interestingly, the three compounds exhibited distinctly
different adsorption behavior toward C2H2, CO2, and CH4
enabled by the effect of functionality and its position. For
example, at 106.7 kPa and 298 K, the C2H2, CO2, and CH4
uptake capacities were recorded as 123.6, 65.9, and 16.1 cm3
(STP) g−1 for ZJNU-111, 64.1, 43.1, and 15.6 cm3 (STP) g−1
for ZJNU-112, and 132.4, 66.6, and 15.6 cm3 (STP) g−1 for
ZJNU-114. Not surprisingly, as a result of exothermic physical
adsorption, the uptake values increased with the decrease in
temperature. Of the three compounds, ZJNU-112 exhibited
the lowest adsorption amounts of C2H2 and CO2, presumably
as a result of its lowest specific surface area. Compared to its
isostructural compound ZJNU-114, ZJNU-111 exhibited
comparable CO2 and CH4 uptake capacities despite a lower
specific surface area. However, ZJNU-114 adsorbed a 7.1%
larger amount than ZJNU-111 with respect to C2H2, which is
mainly attributed to the larger specific surface area as well as
the Lewis base pyridinic-N atom capable of yielding a
hydrogen-binding interaction with the C2H2 molecule
adsorbed. More intriguingly, each of the three compounds
exhibited a higher adsorption capacity toward C2H2 than CO2
and CH4 in the temperature range investigated. The uptake
ratios of C2H2 vs CO2 and CH4 at 298 K and 106.7 kPa are
1.88 and 7.68 for ZJNU-111, 1.49 and 4.11 for ZJNU-112, and
1.99 and 8.49 for ZJNU-114. The C2H2/CO2 uptake ratio of
ZJNU-114 is comparable to and even higher than that of some
literature MOFs published for C2H2/CO2 separation such as
DICRO-4-Ni-I (1.87),48 MFM-160 (1.86),49 SIFSIX-Cu-TPA
(1.73),50 FeNi-M′MOF (1.58),51 UTSA-74 (1.53),52 NbU-10
(1.46),53 ZJNU-13 (1.35),54 ZJU-74 (1.29),55 and NKMOF-1-
Ni (1.19).56 Apart from a higher C2H2 uptake capacity, each of
them also displayed a larger isotherm profile slope with respect
to C2H2 than CO2 and CH4 in the low-pressure region,
indicating a much stronger binding strength toward C2H2 than
toward CO2 and CH4. To quantity the gas-framework
interaction, the isosteric heat of adsorption (Qst) was
C2H2 is regarded as a strategic resource used as not only an
energy carrier but also a versatile organic building block for
synthesizing various valuable chemicals. Despite wide
applications in daily life and the chemical industry, one issue
to be solved is associated with C2H2 separation and
purification involving the recovery of C2H2 from unconverted
CH4 and CO2 impurities because CO2 is inevitably formed in
the manufacturing process of C2H2 by oxidative coupling of
CH4. Because of the simple operational process as well as the
environmentally friendly, cost-efficient, and energy-saving
nature, a promising alternative to the traditional separation
methods of solvent extraction and cryogenic distillation is
based on the physisorption technology, which heavily relies on
the development of porous materials. Burgeoning MOF
materials have been recognized to be promising candidates
as efficient separation adsorbents. The establishment of
permanent porosity stimulated us to evaluate their utility as
solid adsorbents for C2H2 separation and purification. As a
starting point, measurements of the equilibrium adsorption
data with respect to C2H2, CO2, and CH4 were undertaken
under a pressure of up to 106.7 kPa and at three different
temperatures of 278, 288, and 298 K. As shown in Figure 4c−
e, the adsorption−desorption process is fully reversible for
each of the isotherms, as indicated by the overlapping of the
adsorption and desorption branches. They displayed good
cyclic stability, as revealed by consecutive isotherm measure-
ments showing no apparent loss of uptake capacity (Figure
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calculated from the isotherm data measured at 278, 288, and
298 K by the implementation of the Clausius−Clapeyron
method, and the loading dependences of Qst for C2H2, CO2,
and CH4 are presented in Figure 4f. Indeed, each compound
displayed higher Qst values for C2H2 than for CO2 and CH4
during the whole adsorption process. In particular, the near-
zero-coverage Qst values for C2H2 are less than 35 kJ mol−1 for
all three compounds, implying relatively low energy con-
sumption required for adsorbent regeneration. Taken together,
the higher uptake capacity and adsorption affinity toward C2H2
than toward CO2 and CH4 indicated that they can be
considered to be prospective adsorption agents employed for
C2H2/CO2 and C2H2/CH4 separations.
(Figure 5e,f). Collectively, the above results indicated that
similar to the uptake capacity, the adsorption selectivity can
also be tuned by the effect of functional groups and their
locations.
4. CONCLUSIONS
We elaborately designed and synthesized two pairs of
unsymmetrical biaryl tricarboxylate ligands that were modified
with the methyl and pyridinic N atom aligned at different
positions and used them to solvothermally fabricate the
corresponding Cu(II)-based MOFs. Structural determinations
showed that each couple of MOFs displayed different
topological structures, depending on the functionality location.
Also, when the functional group is changed from a methyl to a
pyridinic N atom even at the same position, the resulting
topological structure is not identical, revealing that the
positional effect of the functional group varies from
functionality to functionality. Besides topological structures,
the gas adsorption properties can also be modulated by
changing the functionality and its disposition, as revealed by
systematic single-component isotherm measurements and
comprehensive IAST adsorption selectivity analyses. This
work not only reported a family of Cu(II)-tricarboxylate
frameworks with diverse topologies and the promising
potential for C2H2 separation and purification but more
importantly revealed that engineering the functionality and its
position is an effective strategy employed for regulation on the
MOF structures and properties. The availability of various
substituents not only facilitates structural design for the
discovery of various and even unique topological structures but
also enables access to tailored properties by pore surface
modification. The application of such a design strategy to the
other MOF systems is undergoing.
To evaluate the adsorptive separation efficiency, we
predicted the adsorption selectivity from unary isotherm data
by employing widely used IAST (ideal adsorbed solution
theory) methodology.57 Before the IAST calculations were
started, we correlated the isotherms data at three different
temperatures simultaneously with the single-site Langmuir−
Fredunlich equation, q = qsatbpv/(1 + bpv), with T-dependent
parameter b, b = b0 exp(E/RT), yielding the corresponding
fitting parameters listed in Tables S3−S5, which were used as
input data in selectivity computations (Figures S10−S12).
Figure 5a,b showed the calculated equimolar-component
C2H2/CH4 and C2H2/CO2 adsorption selectivities as a
function of the bulk gas-phase pressures at a temperature of
298 K. It can be seen that the adsorption selectivity of C2H2
over CH4 and CO2 exhibited a declining trend in the pressure
range of 1−109 kPa. Specifically, at the atmospheric pressure,
the C2H2/CH4 and C2H2/CO2 adsorption selectivities reached
18.8 and 4.02 for ZJNU-111, 14.1 and 3.28 for ZJNU-112, and
21.9 and 4.60 for ZJNU-114. Both C2H2/CH4 and C2H2/CO2
adsorption selectivities followed the hierarchy of ZJNU-114 >
ZJNU-111 > ZJNU-112, which is also consistent with the
priority order of C2H2 uptake capacity described above. The
maximum values of uptake capacity and adsorption selectivity
correspond to ZJNU-114 featuring the maximum specific area
and uncoordinated pyridinic-N-functionalized site, meaning
that ZJNU-114 performed the best among the three
compounds for the recovery of C2H2 from CO2 and CH4.
The C2H2/CO2 adsorption selectivity of ZJNU-114 parallels
and even transcends those of some reported MOFs such as
TCuI (5.3),58 JXNU-5 (5.0),59 ZJU-195 (4.7),60 SNNU-45
(4.5),61 UTSA-220 (4.4),62 ZJNU-15 (4.4),63 FJU-90 (4.3),64
JNU-1 (4.0),65 BSF-1 (3.3),66 and Co(btzip)(H2btzip)
(2.5),67 and UPC-200(Al)-F-BIM (3.15)68 but is surpassed
by those of several top-performing MOFs (Table S8). Also, the
C2H2/CH4 selectivity value of ZJNU-114 rivals those of some
literature MOFs, such as pyridine-decorated Zr-MOF Py-UiO-
66 (21),69 BUT-70B (23.3),70 and UTSA-10 (6.2).71
Compared to undecorated compound UMCM-150, methyl-
functionalized MOFs ZJNU-111 and ZJNU-112 exhibited a
lower adsorption selectivity of C2H2 over CO2 and CH4, while
the N-decorated MOF ZJNU-114 displayed higher selectivity
values at 298 K and 106.7 kPa (Figures S16−S18). As shown
in Figure 5c−f, when the analysis temperature and mixture
composition are varied, the selectivity sequence is basically
preserved. With decreasing temperatures, the adsorption
selectivity of C2H2 in preference to CH4 increased (Figure
5c), while that of C2H2 over CO2 displayed different trends for
all three compounds (Figure 5d and Figures S13−S15).
Decreasing C2H2 content in mixed gases leads to increasing
adsorption selectivity of C2H2 over its competing components
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00753.
Additional figures displaying PXRD patterns, TGA
curves, FTIR and NMR spectra, isotherm fitting, and
SSA and IAST selectivity calculations; additional tables
presenting the parameters with respect to structure
refinement and isotherm fitting; and a summary of
adsorption data (PDF)
Accession Codes
CCDC 2068417−2068420 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
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AUTHOR INFORMATION
Corresponding Authors
■
Yuanbin Zhang − Key Laboratory of the Ministry of
Education for Advanced Catalysis Materials, College of
Chemistry and Life Sciences, Zhejiang Normal University,
Jinhua 321004, China; Email: ybzhang@zjnu.edu.cn
Yabing He − Key Laboratory of the Ministry of Education for
Advanced Catalysis Materials, College of Chemistry and Life
Sciences, Zhejiang Normal University, Jinhua 321004,
8119
https://doi.org/10.1021/acs.inorgchem.1c00753
Inorg. Chem. 2021, 60, 8111−8122

Inorganic Chemistry
pubs.acs.org/IC
Article
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Organic Framework for High Sieving Separation of Propylene from
Propane. J. Am. Chem. Soc. 2020, 142, 17795.
China; orcid.org/0000-0002-0094-0591;
Email: heyabing@zjnu.cn
Authors
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R.; Li, J.; Zhou, W.; Chen, B. Molecular Sieving of Ethylene from
Ethane Using a Rigid Metal−Organic Framework. Nat. Mater. 2018,
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H.; Krishna, R.; Zhou, W.; Chen, B.; Ren, Q. Molecular Sieving of
Ethane from Ethylene through the Molecular Cross-Section Size
Differentiation in Gallate-Based Metal-Organic Frameworks. Angew.
Chem., Int. Ed. 2018, 57, 16020−16025.
(10) Gu, Y.; Zheng, J.-J.; Otake, K.-i.; Shivanna, M.; Sakaki, S.;
Yoshino, H.; Ohba, M.; Kawaguchi, S.; Wang, Y.; Li, F.; Kitagawa, S.
Host-Guest Interaction Modulation in Porous Coordination Polymers
for Inverse Selective CO2/C2H2 Separation. Angew. Chem., Int. Ed.
2021, 60, 11688.
(11) Jiang, Z.; Fan, L.; Zhou, P.; Xu, T.; Hu, S.; Chen, J.; Chen, D.-
L.; He, Y. An Aromatic-Rich Cage-Based MOF with Inorganic
Chloride Ions Decorating the Pore Surface Displaying the Preferential
Adsorption of C2H2 and C2H6 over C2H4. Inorg. Chem. Front. 2021, 8,
1243−1252.
Shengjie Lin − Key Laboratory of the Ministry of Education
for Advanced Catalysis Materials, College of Chemistry and
Life Sciences, Zhejiang Normal University, Jinhua 321004,
China
Ping Zhou − Key Laboratory of the Ministry of Education for
Advanced Catalysis Materials, College of Chemistry and Life
Sciences, Zhejiang Normal University, Jinhua 321004, China
Tingting Xu − Key Laboratory of the Ministry of Education
for Advanced Catalysis Materials, College of Chemistry and
Life Sciences, Zhejiang Normal University, Jinhua 321004,
China
Lihui Fan − Key Laboratory of the Ministry of Education for
Advanced Catalysis Materials, College of Chemistry and Life
Sciences, Zhejiang Normal University, Jinhua 321004, China
Xinxin Wang − Key Laboratory of the Ministry of Education
for Advanced Catalysis Materials, College of Chemistry and
Life Sciences, Zhejiang Normal University, Jinhua 321004,
China
Lianglan Yue − Key Laboratory of the Ministry of Education
for Advanced Catalysis Materials, College of Chemistry and
Life Sciences, Zhejiang Normal University, Jinhua 321004,
China
(12) Zhu, B.; Cao, J.-W.; Mukherjee, S.; Pham, T.; Zhang, T.; Wang,
T.; Jiang, X.; Forrest, K. A.; Zaworotko, M. J.; Chen, K.-J. Pore
Engineering for One-Step Ethylene Purification from a Three-
Component Hydrocarbon Mixture. J. Am. Chem. Soc. 2021, 143,
1485−1492.
(13) Yang, H.; Wang, Y.; Krishna, R.; Jia, X.; Wang, Y.; Hong, A. N.;
Dang, C.; Castillo, H. E.; Bu, X.; Feng, P. Pore-Space-Partition-
Enabled Exceptional Ethane Uptake and Ethane-Selective Ethane-
Ethylene Separation. J. Am. Chem. Soc. 2020, 142, 2222−2227.
(14) Lysova, A. A.; Samsonenko, D. G.; Kovalenko, K. A.; Nizovtsev,
A. S.; Dybtsev, D. N.; Fedin, V. P. A Series of Mesoporous Metal-
Organic Frameworks with Tunable Windows Sizes and Exceptionally
High Ethane over Ethylene Adsorption Selectivity. Angew. Chem., Int.
Ed. 2020, 59, 20561−20567.
(15) Ma, D.; Li, Z.; Zhu, J.; Zhou, Y.; Chen, L.; Mai, X.; Liufu, M.;
Wu, Y.; Li, Y. Inverse and Highly Selective Separation of CO2/C2H2
on a Thuliumorganic Framework. J. Mater. Chem. A 2020, 8, 11933−
11937.
Zhenzhen Jiang − Key Laboratory of the Ministry of
Education for Advanced Catalysis Materials, College of
Chemistry and Life Sciences, Zhejiang Normal University,
Jinhua 321004, China
Zhengyi Zhang − Bruker (Beijing) Scientific Technology Co.,
Ltd, Beijing 100192, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00753
Notes
The authors declare no competing financial interest.
(16) Zeng, H.; Xie, X.-J.; Xie, M.; Huang, Y.-L.; Luo, D.; Wang, T.;
Zhao, Y.; Lu, W.; Li, D. Cage-Interconnected Metal−Organic
Framework with Tailored Apertures for Efficient C2H6/C2H4
Separation under Humid Conditions. J. Am. Chem. Soc. 2019, 141,
20390−20396.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the Natural Science
Foundation of China (no. 21771162) and the Natural Science
Foundation of Zhejiang Province, China (LR16B010001).
(17) Liao, P.-Q.; Zhang, W.-X.; Zhang, J.-P.; Chen, X.-M. Efficient
Purification of Ethene by an Ethane-Trapping Metal-Organic
Framework. Nat. Commun. 2015, 6, 8697.
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