A pair of polymorphous metal-organic frameworks based on an angular diisophthalate linker: Synthesis, characterization and gas adsorption properties

Article abstract of DOI:10.1039/c7dt04087c

The combination of an angular diisophthalate ligand, 5,5′-(naphthyl-2,7-yl)diisophthalate (H₄L), and copper ions under different solvothermal conditions afforded two polymorphous metal-organic frameworks (ZJNU-77 and ZJNU-78) with the same framework composition of [Cu₂(L)(H₂O)₂], providing a platform to investigate the relationship between MOF polymorphism and gas adsorption properties. As determined by single-crystal X-ray diffraction, ZJNU-77 and ZJNU-78 exhibited three-dimensional networks crystallizing in different space groups. Their structural differences were mainly manifested by the ligand's conformation, the level of framework interpenetration and the network's topology. Interestingly, gas adsorption studies showed that the two compounds after desolvation displayed comparable gas adsorption properties with respect to C₂H₂, CO₂ and CH₄, despite their different surface areas and pore volumes. The C₂H₂, CO₂, and CH₄ uptake capacities at 298 K and 1 atm are 120.2, 78.1, and 18.4 cm³ (STP) g^-1 for ZJNU-77, and 122.0, 82.0, and 18.9 cm³ (STP) g^-1 for ZJNU-78, respectively. The IAST adsorption selectivities for the equimolar C₂H₂/CH₄ and CO₂/CH₄ mixtures are 28.6 and 5.7 for ZJNU-77, and 28.4 and 5.9 for ZJNU-78 at 298 K and 1 atm. These results indicate that besides the surface area, the pore size also plays a crucial role in gas adsorption. This work not only represents an intriguing example of MOF polymorphism achieved by controlling solvothermal conditions, but also provides an insight into the correlation between MOF polymorphism and gas adsorption properties.

Full text of DOI:10.1039/c7dt04087c

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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a Zr-metal–organic
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ARTICLE
A Pair of Polymorphous Metal-Organic Frameworks Based on an
Angular Diisophthalate Linker: Synthesis, Characterization and
Gas Adsorption Properties
Received 00th January 20xx,
Accepted 00th January 20xx
Fengli Chen, Dongjie Bai, Yao Wang, Minghui He, Xiaoxia Gao, and Yabing He*
DOI: 10.1039/x0xx00000x
The combination of an angular diisophthalate ligand, 5,5’-(naphthyl-2,7-yl)diisophthalate (H4L), and copper ions under
different solvothermal conditions afforded two polymorphous metal-organic frameworks (ZJNU-77 and ZJNU-78) with the
same framework composition of [Cu2(L)(H2O)2], which afford a platform to investigate the relationship between MOF
polymorphism and gas adsorption property. As determined by single-crystal X-ray diffraction, ZJNU-77 and ZJNU-78
exhibited three-dimensional networks crystallizing in different space groups. Their structural differences were mainly
manifested by the ligand’s conformation, the level of framework interpenetration and the network’s topology.
Interestingly, gas adsorption studies showed that the two compounds after desolvation displayed comparable gas
adsorption properties with respect to C2H2, CO2 and CH4, despite their different surface areas and pore volumes. The C2H2,
CO2, and CH4 uptake capacities at 298 K and 1 atm are 120.2, 78.1, and 18.4 cm3 (STP) g-1 for ZJNU-77, and 122.0, 82.0, and
18.9 cm3 (STP) g-1 for ZJNU-78, respectively. The IAST adsorption selectivities for the equimolar C2H2/CH4 and CO2/CH4
mixtures are 28.6 and 5.7 for ZJNU-77, and 28.4 and 5.9 for ZJNU-78 at 298 K and 1 atm. These results indicate that
besides surface area, the pore size also plays a crucial role in gas adsorption. This work not only represents an intriguing
example of MOF polymorphism achieved by controlling solvothermal conditions, but also provides an insight into the
correlation between MOF polymorphism and gas adsorption property.
www.rsc.org/
activated carbons, MOFs tend to possess ordered crystal
structures that can be determined by single-crystal X-ray
diffraction, which is conducive to the elucidation of the
1. Introduction
Metal-organic frameworks (MOFs, also known as porous
coordination polymers, PCPs) are emerging as a kind of
relatively novel porous materials formed by the self-assembly
of metal ions and multitopic bridging ligands linked through
coordination bonds. By integrating different metal ions and
various organic linkers, a wide range of MOFs can be obtained,
which can incorporate, in a systematic and modular manner,
both tuneable pore size/shape and modifiable pore surface. In
addition, compared to traditional porous materials such as
structure–property relationship as well as the further
structural and functional optimization. All of these attributes
offer MOFs great promise for a variety of applications
including but not limited to gas storage1 and separation,2
heterogeneous catalysis,3 luminescent sensing4 and drug
delivery.5 In terms of gas adsorption for example, MOFs has
been recognized as prospective adsorbents that can provide
potential solutions to the enduring challenges pertaining to
the separation of industrially important gas mixtures such as
7
8
olefin/paraffin,6 C2H2/C2H4 , and C2H2/CO2 , and C4
hydrocarbons9.
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials,
College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004,
China. E-mail: heyabing@zjnu.cn
The judicious selection of metal ions and bridging ligands
plays a very crucial role to target a MOF with desired structure
and property. However, the precise prediction and controlled
synthesis of a MOF is still very challenging because the self-
assembly process is frequently influenced by many other
synthetic parameters such as solvent system10, reaction
temperature11, pH value12, and template agent/additive13.
Even for a fixed combination of metal ion and organic ligand,
structural diversity and uncertainty might occur in the
crystallization process. Potentially, it is possible that the self-
assembly of the same inorganic and organic building blocks
under different conditions can give rise to different MOFs with
the same chemical composition but different network
Electronic Supplementary Information (ESI) available: The photographs of the as-
synthesized ZJNU-77 and ZJNU-78 (Fig. S1); PXRD patterns (Fig. S2); TGA curves
(Fig. S3); FTIR spectra (Fig. S4); the consistency, BET and Langmuir plots (Fig. S5-6);
comparison of the pure-component isotherm data for C2H2, CO2, and CH4 in ZJNU-
77 with the fitted isotherms (Fig. S7); IAST selectivities for the equimolar C2H2/CH4
and CO2/CH4 gas mixtures in ZJNU-77 at three different temperatures of 278 K,
288 K and 298 K (Fig. S8); comparison of the pure-component isotherm data for
C2H2, CO2, and CH4 in ZJNU-78 with the fitted isotherms (Fig. S9); IAST selectivities
for the equimolar C2H2/CH4 and CO2/CH4 gas mixtures in ZJNU-78 at three different
temperatures of 278 K, 288 K and 298 K (Fig. S10); the isosteric heat of C2H2, CO2
and CH4 adsorption in ZJNU-77 and ZJNU-78 as a function of gas loadings (Fig.
S11), 1H NMR and 13C NMR spectra (Fig. S12); crystal data and structure
refinement for ZJNU-77 and ZJNU-78 (Table S1); Langmuir-Freundich parameters
for adsorption of C2H2, CO2, and CH4 in ZJNU-77 (Table S2); Langmuir-Freundich
parameters for adsorption of C2H2, CO2, and CH4 in ZJNU-78 (Table S3); CCDC No:
1582889 and 1582890. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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structures were fully characterized and their gas adsorption
View Article Online
DOI: 10.1039/C7DT04087C
properties were investigated in detail. The results show that
the two compounds possess different topological structures,
and exhibit the comparable gas adsorption properties with
respect to C2H2, CO2 and CH4 resulting from the complicated
interplay between surface area, pore volume and pore size.
HOOC
COOH
COOH
COOH
Scheme 1 The molecular structure of the organic ligand used to construct ZJNU-77 and
ZJNU-78.
2. Experimental Section
2.1 Materials and general methods
superstructures, which are known as the supramolecular
isomers (sometimes termed polymorphous compounds, if not
considering the guest molecules occupied in the pore). This
2,7-dibromonaphthalene (purity 98%) was purchased from
HWRK chemical reagent Co., Ltd. Dimethyl 5-
(pinacolboryl)isophthalate was made as per the previously
described literature procedure.22k All other chemicals, reagents
and solvents were used as received from commercial suppliers
without further purification. Water as a solvent and co-solvent
referred to deionized water. Column chromatography was
performed in silica gel (100–200 mesh, Qingdao Haiyang
Chemical Co., Ltd.). Thin layer chromatography (TLC) was
conducted on aluminium sheets pre-coated with silica gel 60
F254 purchased from Merck, and the spots were visualized
phenomenon,
dubbed
“supramolecular
isomerism
(polymorphism)”, is
a
very interesting subject in
supramolecular chemistry and crystal engineering, and is
attracting increasing interest since it may facilitate the
fundamental understanding of the influencing factors on
crystal growth and allow for the derivation of structure-
property relationships. The topic has been well reviewed by
Zaworotko14, Chen15, Zhou16 and Pombeiro17 et al. However,
rational realization of MOF supramolecular isomerism system
is still extremely challenging and remains elusive, although
some methods such as controlling the reactant concentration18,
and using template agent19 have been established, and
advanced synthetic techniques such as ultrasonication have
been employed.20 To date, the example of MOF polymorphism
is still particularly scarce.11-13, 18-19, 21
Isophthalate/m-benzenedicarboxylate moiety is a powerful
organic subunit exhibiting the strong coordination ability and
propensity to form polyhedral cages during the self-assembly
process, thus endowing the resulting MOFs with promising
potential in gas storage and separation.22 In this context,
continuing effort in our Lab has been devoted to explore MOF
1
with UV light (254 nm). H and 13C NMR (Nuclear Magnetic
Resonance) spectra were taken on a Bruker AV400 or AV600
instruments at ambient temperature using CDCl3 or DMSO-d6
as solvents. Chemical shifts (δ) were reported in parts per
million (ppm) relative to the residual undeuterated solvent
peaks, and coupling constants (J) were reported in Hertz (Hz).
Multiplicity was reported as follows: s = singlet, d = doublet, t =
triplet, dd = doublet of doublets, and br = broad. Fourier
transform infrared (FTIR) spectra were determined with a
Nicolet 5DX FTIR spectrometer between 400 and 4000 cm-1.
The samples were prepared as KBr pellets, and the absorption
maxima were reported in wavenumbers (cm-1). Elemental
analyses for CHN were performed with a Perkin-Elmer 240
CHN elemental analyser. Thermogravimetric analyses (TGA)
were carried out with a Netzsch STA 449C thermal analyser
under a continuous nitrogen flow with the sample heated in an
alumina crucible at a heating rate of 5 K min-1. Powder X-ray
diffraction (PXRD) patterns were recorded on a Philips
PW3040/60 automated diffractometer using Cu-Ka radiation (λ
= 1.5418 Å). Topological analyses were performed by the
Topos program (version 4.0). Low-pressure gas adsorption
isotherms were collected with an ASAP (Accelerated Surface
chemistry
concerning
isophthalate-containing
multicarboxylate ligands. In our research, it was found that
compared with linear diisophthalate ligands, bent
diisophthalate ones with low symmetry are inherently prone
to yield MOFs with variable topology structures presumably
because of versatile conformations of the bent diisophthalate
ligands adopted in the crystallization process. This feature in
turn might offer rich opportunity for bent diisophthalate
ligands to engender polymorphous MOFs. For example, Zhou
et al realized two isomeric porous frameworks (PCN-12 and
PCN-12’) based on a bent diisophthalate, 5,5’-methylene
diisophthalate (MDIP), by controlling the symmetry of organic
ligands through fine-tuning of reaction temperatures.21o In
PCN-12, the ligands adopted two different conformations (Cs
and C2v symmetrical forms), while only one conformation (C2v
symmetrical form) was observed for the organic ligands in a
PCN-12’. In this work, we utilized an angular diisophthalate
ligand, 5,5’-(naphthyl-2,7-yl)diisophthalate (see Scheme 1 for
the molecular structure), to construct two polymorphous
MOFs, which were achieved through simply modulating
solvothermal conditions. It should be mentioned that the
ligand has been used by Zhou et al to construct a MOF PCN-88
containing CO2 molecular trap.23 The two MOF compounds’
Area
& Porosimetry) 2020HD88 physisorption analyser
supplied by Micromeritics instruments Inc. During the
measurements, a temperature of 77 K was maintained using
liquid nitrogen, while other temperatures were controlled
using a circulating water bath (Julabo F12ED). Highly pure
gases (N2: 99.9999%; C2H2: 99.9%; CO2, 99.999%; CH4: 99.99%)
were used for the measurements.
2.2 Single-crystal X-ray diffraction
Single-crystal X-ray crystallographic data were collected on a
Bruker Apex II diffractometer using graphite-monochromated
Mo-Ka radiation (λ = 0.71073 Å). The structures were solved by
direct methods using SHELXS-97, and the non-hydrogen atoms
were located from the trial structure and then refined
2 | Dalton Trans., 2017, 00, 1-3
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anisotropically with SHELXTL using a full-matrix least squares ZJNU-77: A mixture of the organic linker (5.0 mg, 11.0 μmol)
View Article Online
procedure based on F2 values. The hydrogen atom positions and Cu(NO3)2∙3H2O (15.0 mg, 62.1 μmol)DwOIa:s10d.1is0s3o9/lvCe7DdTi0n4N08,7NC-
were fixed geometrically at calculated distances and allowed dimethylformamide (DMF, 1.5 mL) and placed into a 20-mL
to ride on the parent atoms. For the two compounds, a large scintillation vial. After the addition of 48% HBF4 (0.22 mL), the
void space is present in the framework, which contains a vial was capped tightly and placed at a preheated oven at 363
number of residual electron density peaks. The species in this K for 24 h at static conditions. The resulting blue crystals were
region were too heavily disordered to be satisfactorily collected when the mother liquor was still warm (Fig. S1a). The
modelled and were treated with SQUEEZE routine within the yield is about 31% based on the organic ligand. Anal. calcd. for
PLATON software package.24 The structures were then refined C47H67N7O18Cu2: C, 49.29; H, 5.90; N, 8.56; found: C, 49.16; H,
again using the data generated. A summary of the structural 5.78; N, 8.57; selected FTIR data (KBr, cm-1): 1655, 1437, 1375,
determination and refinement for the two compounds was 1254, 1101, 847, 779, 731, 663, 480.
listed in Table S1 in the supporting information. CCDC 1582889 ZJNU-78: A mixture of the organic linker (5.0 mg, 11.0 μmol)
and 1582890 contain the supplementary crystallographic data and Cu(NO3)2∙3H2O (15.0 mg, 62.1 μmol) was dissolved in DMF
for this paper. These data can be obtained free of charge from (1.5 mL) and placed into a 20-mL scintillation vial. After the
The Cambridge Crystallographic Data Centre.
2.3 Synthesis and characterization of the organic ligand
addition of 48% HBF4 (0.42 mL), the glass vial was capped
tightly and placed at a preheated oven at 363 K for 96 h at
static conditions. The formed blue crystals were collected
when the mother liquor was still warm (Fig. S1b). The yield is
about 53% based on the organic ligand. Anal. calcd for
C35H39N3O14Cu2: C, 49.29; H, 4.61; N, 4.93; found: C, 49.12; H,
4.64; N, 4.84; selected FTIR data (KBr, cm-1): 1664, 1632, 1585,
1437, 1375, 1097, 847, 779, 731, 663.
A mixture of 2,7-dibromonaphthalene (1.00 g, 3.50 mmol),
dimethyl 5-(pinacolboryl)isophthalate (2.46 g, 7.70 mmol),
cesium
carbonate
(3.42
g,
10.49
mmol)
and
tetrakis(triphenylphosphine)palladium (0.20 g, 0.17 mmol) was
added to a 250-mL round-bottomed flask equipped with a
Teflon-coated magnetic stirring bar and a refluxing condenser.
The flask was vacuumed and back-filled with nitrogen. After
the process was repeated three times, dry dioxane (80 mL)
was added via syringe and the resulting mixture was stirred
vigorously at 393 K for 72 h. After cooling to ambient
temperature, the organic solvent was evaporated, and the
residue was extracted with dichloromethane. The combined
organic phase was washed with brine, dried over anhydrous
magnesium sulphate and filtered. Subsequently, evaporations
of the solvents yielded a crude product which was purified
with silica gel column chromatography with petroleum
ether/dichloromethane (1:4, v/v) as eluent. The product was
obtained as a pale yellow solid in 46% yield (0.83 g, 1.62 mmol).
1H NMR (CDCl3, 600.1 MHz) δ (ppm): 8.740 (t, J = 1.2 Hz, 2H),
8.651 (d, J = 1.2 Hz, 4H), 8.258 (d, J = 1.8 Hz, 2H), 8.039 (d, J =
8.4 Hz, 2H), 7.877 (dd, J = 8.4 Hz, 1.8 Hz, 2H), 4.041 (s, 12H).
The tetramethyl ester intermediate (0.83 g, 1.62 mmol) was
dissolved in a mixed solvent of THF (20 mL) and MeOH (20 mL),
and then 20 mL of 6 M NaOH was added. The resulting
solution was refluxed overnight, and then allowed to cool to
room temperature. After removal of the volatile solvents, the
residue was dissolved with water, and filtered through Celite.
The filtration was acidified with 6 M HCl under ice-water bath,
and the resulting precipitation was collected by filtration,
washed with water and dried over 343 K under vacuum,
affording the desired ligand as an off-white solid in almost
3. Results and Discussion
3.1 Synthesis and characterization
The organic ligand, 5,5’-(naphthyl-2,7-yl)diisophthalate, was
easily synthesized by Suzuki cross-coupling reaction of
dimethyl
5-(pinacolboryl)isophthalate
and
2,7-
dibromonaphthalene followed by hydrolysis and acidification,
1
and fully characterization by H NMR, 13C NMR and FTIR.
Combination of the organic ligand and copper nitrate
trihydrate in DMF at 363 K for 24 h with a little amount of HBF4
gave rise to a copper-based MOF termed ZJNU-77. However,
with a fixed metal-to-ligand stoichiometry and the same
1
quantitative yield (0.73 g, 1.60 mmol). H NMR (DMSO-d6,
600.1 MHz) δ (ppm): 8.593 (d, J = 1.8 Hz, 4H), 8.585 (d, J = 1.8
Hz, 2H), 8.523 (t, J = 1.8 Hz, 2H), 8.139 (d, J = 8.4 Hz, 2H), 7.984
(dd, J = 8.4 Hz, 1.8 Hz, 2H); 13C NMR (DMSO-d6, 150.9 MHz) δ
(ppm): 168.133, 140.426, 137.385, 134.967, 134.220, 132.341,
131.071, 129.674, 128.994, 126.731, 125.793; selected FTIR
(KBr, cm-1): 1701, 1597, 1439, 1400, 1308, 1248, 1070, 899,
837, 766, 754, 714, 677.
Fig. 1 Single-crystal X-ray diffraction structure of ZJNU-77. (a) The conformation and
coordination environment of the organic ligand; (b-f) five different types of metal-
organic polyhedral cages (A-E) in the framework whose voids are represented by
different coloured spheres; and (f) 3D network structure viewed along the
crystallographical c axis. The hydrogen atoms and terminal water molecules were
omitted for clarity.
2.4 Synthesis and characterization of ZJNU-77 and ZJNU-78
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temperature and solvent, increasing the amount of HBF4
employed and elongating the reaction time led to the
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DOI: 10.1039/C7DT04087C
formation of additional copper-based MOF termed ZJNU-78
.
Their structures were characterized by singe-crystal X-ray
diffraction, and the phase purities were verified by comparing
the experimental and simulated PXRD patterns (Fig. S2).
According to the single-crystal X-ray diffraction structural
determination, TGA and microanalyses, ZJNU-77 and ZJNU-78
can be best formulated to be [Cu2L(H2O)2]∙7DMF
∙H2O, and
[Cu2L(H2O)2]∙3DMF H2O, respectively. TGA analyses showed an
∙
appreciable weight loss of 49.4% for ZJNU-77 in the range of
room temperature to 545 K, and 32.1% for ZJNU-78 in the
range of room temperature to 550 K, which correspond to the
departure of the solvent guest molecules trapped in the
framework and the terminal coordinated water molecules (Fig.
S3). The absence of strong absorption in the region of 1690-
1730 cm-1 in FTIR spectra indicates complete deprotonation of
the carboxylate groups of the organic ligands (Fig. S4), which is
consistent with the results of the single-crystal X-ray
diffraction analyses.
3.2 Single-crystal X-ray diffraction analyses
Single-crystal X-ray diffraction analyses show that the
compound ZJNU-77 crystallizes in the cubic space group of Fm-
3m with a = b = c = 39.8231(14) Å. Besides the guest molecules,
the asymmetric unit consists of two independent copper ions
(Cu1 and Cu2, each with occupancy of 0.25), one-quarter of
fully deprotonated organic ligand and two terminal water
molecules (O1w and O2w, each with occupancy of 0.25). Each
copper atom is in a five-coordinated [CuO5] square-pyramidal
geometry completed by four oxygen atoms belonging to four
different carboxylate groups and one oxygen atom coming
from the terminal water molecule binding at the axial
coordination site. The Cu-O bond lengths fall in the region of
1.949 to 1.955 Å, which are comparable to those reported for
the other copper-carboxylate frameworks.25 As expected, two
neighbouring copper atoms are bridged by four individual
carboxylate groups adopting bidentate-bridging coordination
modes (μ2:η1η1) to form a dicopper paddlewheel Cu2(COO)4
inorganic secondary building unit (SBU) with Cu-Cu distance of
2.645 Å, which is further connected by the organic linkers to
Fig. 2 Single-crystal X-ray diffraction structure of ZJNU-78. (a-b) The coordination
environment and conformation of the organic ligands; (c-h) six different types of metal
organic polyhedral cages (A-F) in the single framework whose voids are represented by
different colored spheres, and (i) two-fold interpenetrated 3D network. Hydrogen atoms
and terminal water molecules are omitted for clarity.
the difference lies on that eight isophthalate units are missing
in this cage. Cage B is composed of sixteen dicopper
paddlewheel units and eight organic ligands. Cage C is made
up from twelve dicopper paddlewheel units and eight organic
ligands. Combination of twelve dicopper paddlewheel units,
eight organic ligands and four isophthalate moiteite comprises
cage D, while combination of eighteen dicopper paddlewheel
units and twelve organic ligands leads to the formation of cage
E. Topologically, if we consider dicopper paddlewheel units as
2-connected and 4-connected nodes, and the organic linker as
4-connected node, the Schlꢀfli symbol of the whole network is
{4.83.102}{4.84.10}{4.85}{4}{82.10}{83}{84.102}{85.12}{8}2,
determined by the TOPOS program. To our knowledgement,
the topological structure exhibited by ZJNU-77 is novel and
different with those of copper-based MOFs reported thus far
constructed from bent diisophthalate ligands.22b, 22c, 27
Single-crystal X-ray crystallographic studies reveal that
ZJNU-78 exhibits a 3D network crystallizing in the tetragonal
generate
a three-dimensional (3D) non-interpenetrated
coordination network. Due to the statistical disorder of organic
ligands, we purposefully lower the space group from Fm-3m to
P4/m to better understand the whole network structure.
Particularly interestingly, it was found that some dicopper
paddlewheel units act as 4-connected nodes, while the other
serves as 2-connected nodes. In addition, the organic ligand
only adopts one conformation in which the terminal
isophthalate units are slightly twisted from the central
naphthyl moieties and the carboxylate group is nearly coplanar
with the corresponding conjoint phenyl ring. In the overall
structure, five plausible metal-organic polyhedral cages can be
observed. As shown in Fig. 1, cage A consists of twelve
dicopper paddlewheel units and sixteen isophthalate units.
This cage is similar to the typical MOP-1 formed by the self-
assembly of copper ions and m-benzenedicarboxylic acid,26 but
space group of P42/mnm with a = b = 28.36750(10)
Å, and c =
28.6376(2) . Besides the guest molecules, the asymmetric
Å
unit is composed of six crystallographically independent
copper atoms (Cu1, Cu2, Cu3, Cu4, Cu5 and Cu6, each with
occupancy of 0.5), one and a half fully deprotonated ligands,
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and six terminal water molecules (each with occupancy of 0.5).
As shown in Fig. 2a-b, the organic ligands are fully
deprotonated and display two different types of
conformations, which are different with what is observed in
ZJNU-77. The ligand λ-L is almost coplanar with the average
dihedral angle of 12.7o between terminal isophthalate units
and central naphthyl ring (Fig. 2a), while in the ligand τ-L, the
terminal isophthalate units are severely twisted out of the
central naphthyl plane with a dihedral angle of 55.6o (Fig. 2b).
Analogously, dicopper paddlewheel unit constitutes its
inorganic SBU, which is further connected by the organic
linkers to form a 3D network. Due to the large void, two
identical but individual networks were interpenetrated,
resulting in a doubly-fold interpenetrated network with
reduced pore sizes (Fig. 2i). In the single network, there exist
six different types of metal-organic polyhedral cages. As shown
in Fig. 2c, cage A consists of two dicopper paddlewheel units
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DOI: 10.1039/C7DT04087C
and four organic ligands (λ-L), forming a molecular trap as
observed in PCN-88.28 The cuboctahedral cage (cage B) is
composed of twelve dicopper paddlewheel units and twenty-
four isophthalate units (Fig. 2d). Cage C is formed by eight
dicopper paddlewheel units and four organic ligands (
2e). Cage D is surrounded by sixteen dicopper paddlewheel
units and eight organic ligands ( -L) (Fig. 2f). Sixteen organic
ligands (8 λ-L and 8 -L) and eight dicopper paddlewheel units
comprise cage E (Fig. 2g), while the combination of twelve
dicopper paddlewheel units and ten organic ligands (4 -L and
-L) leads to the formation of cage F (Fig. 2h).
τ-L) (Fig.
λ
τ
Fig. 3 (a) N2 adsorption-desorption isotherms at 77 K, and (b) pore size distributions of
ZJNU-77 and ZJNU-78. The solid and open symbols represent adsorption and
desorption, respectively. STP = standard temperature and pressure.
λ
6
τ
Interpenetration in ZJNU-78 results in an arrangement
whereby the cage A, cage B, and cage C of one net are
respectively located within cage F, cage E, and cage D of the
second net. The PLATON calculation revealed that the effective
solvent accessible void volume is 52.5% (12109.8 Å3 of the unit
cell volume 23045.1 Å3) which is filled with crystallographically
disordered solvent molecules that cannot be located in the
electron density map. From the topological point of view, if the
dicopper paddlewheel unit is simplified as a 4-connected node,
and organic ligand is taken as 4-connected node, the overall
topology of one set of framework is a 4-connected 4-nodal net
sterically crowded. As such, ZJNU-77 and ZJNU-78 represent
two polymorphous MOF compounds with the same formula
but different topological structures, affording an opportunity
to investigate the relationship between MOF polymorphism
and gas adsorption property.
3.3 Permanent porosity
To characterize pore textural properties, we measured the N2
adsorption-desorption isotherms at 77 K. Prior to gas
adsorption measurements, the as-synthesized samples were
activated by multiple guest exchange with acetone and
dichloromethane followed by evacuation at 373 K under
dynamic vacuum. As shown in Fig. 3a, ZJNU-78 displayed a
typical Type-I adsorption isotherm, indicating a microporous
with
the
point
(Schlꢀfli)
symbol
of
{4.62.83}2{4.64.8}2{42.62.82}{46}. The topological structure is the
same as that of PMOF-329 and UHM-6.30
material, while a two-step isotherm was observed for ZJNU-77
,
From the above structural analyses, it can be seen that two
MOF compounds have the same inorganic and organic building
blocks, but their different self-assembly ways result in
completely different structures, which are manifested by
different space groups, unit cell parameters, ligand’s
conformation, and the level of framework interpenetration.
The structural differences lead to significant differences in
pore size, accessible pore volume, surface area and crystal
density, which might exert significant effect on gas adsorption.
Although the exact formation mechanism is unclear, we
postulated that the pH value and reaction time are the two key
factors which should be responsible for the formation of the
two polymorphs. Increasing the amount of HBF4 reduces the
reaction rate, and elongating reaction time may convert the
nearly coplanar ligand to the nonplanar one which is less
which is attributed to the existence of both meso- and micro-
porous cages in the structure. Based on the N2 adsorption
isotherms, the BET (Brunauer–Emmett–Teller) and Langmuir
surface areas were estimated to be 2432 and 3401 m2 g-1 for
ZJNU-77, and 1311 and 1456 m2 g-1 for ZJNU-78, respectively
(Fig. S5 and S6). The BET surface area of ZJNU-77 is among the
highest reported for copper-based MOFs assembled from bent
22j, 28b, 29, 31
diisophthalate ligands.21o,
Due to network
interpenetration, the Langmuir surface area of the
interpenetrated ZJNU-78 is 57% lower than that of non-
interpenetrated ZJNU-77. The pore volume calculated from
the maximum N2 adsorption amount is 1.185 cm3 g-1 for ZJNU-
77, and 0.521 cm3 g-1 for ZJNU-78. The experimental total pore
volumes correspond well to the ones calculated from the
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Dalton Transactions
single-crystal structures using PLATON software, iVniedwicAartticinlegOntlhinee
DOI: 10.1039/C7DT04087C
samples are of high quality and fully activated. Fig 3b shows
the pore size distributions of these two compounds, which
were estimated by the density functional theory (DFT) based
on the N2 adsorption isotherms collected at 77 K. It can be
seen that compared to ZJNU-78, ZJNU-77 has a wider and
larger pore size, which is consistent with single-crystal X-ray
diffraction analyses.
3.4 Selective gas adsorption
As an important strategic gas, acetylene (C2H2) plays a very
important role in daily lives and modern industries. For its
widespread usage, there are two issues that need to be solved.
One issue is regarding its storage which is very challenging
because of its highly explosive nature. Unlike other fuel gases
such as CH4 and H2, C2H2 cannot be stored in steel cylinder
under pressures higher than 2 atm. The current storage
method involves dissolving acetylene in acetone placed in a
steel cylinder. However, this method suffers low acetylene
purity due to the volatile solvent contamination. Another issue
is regarding its production where separation of C2H2 from CO2
and CH4 is needed. In addition, natural gas, as a clean energy
source, is considered as promising candidate for substituting
gasoline and diesel fuels in vehicles. Natural gas is mainly
composed of CH4 but also contain other impurities such as
acidic and easy-freezing CO2. The existence of CO2 will reduce
energy efficiency, corrode pipelines in the presence of water,
and have a risk of blocking the pipeline during transportation.
Consequently, efficient separation of CO2 from CH4 is a key
step in natural gas processing/upgrade. For the above-
mentioned purposes, development of porous materials such as
MOFs exhibiting highly selective adsorption of C2H2 and CO2
from CH4 is highly desired.
Establishment of permanent porosity motivates us to
explore its potential for C2H2/CH4 and CO2/CH4 separations.
Accordingly, the C2H2, CO2 and CH4 adsorption isotherms were
collected at three different temperatures of 278 K, 288 K and
298 K up to 1 atm. As shown in Fig. 4a-c, no obvious hysteresis
between adsorption and desorption cycles was observed for all
isotherms over the temperature ranges examined, indicating a
fully reversible adsorption process. At 298 K and 1 atm, C2H2,
CO2, and CH4 uptake capacities are 120.2, 78.1, and 18.4 cm3
(STP) g-1 for ZJNU-77, and 122.0, 82.0, and 18.9 cm3 (STP) g-1
for ZJNU-78, respectively. When the temperature is lowered to
278 K, the corresponding uptake values increase to 158.5,
117.6 and 26.9 cm3 (STP) g-1 for ZJNU-77, and 157.0, 125.5 and
30.3 cm3 (STP) g-1 for ZJNU-78, respectively. Compared with
those top-performing MOFs such as ZJNU-47 (213 cm3 (STP) g-
1)23, NJU-Bai17 (222.4 cm3 (STP) g-1)22e, FJI-H8 (224 cm3 (STP) g-
1)32, MFM-188 (232 cm3 (STP) g-1)33 and ZJU-12 (244 cm3 (STP)
g-1)34under the same conditions, C2H2 uptakes of these two
compounds are relatively moderate, but their values are still
comparable to and even higher than those of other porous
MOFs such as ZJU-26 (127 cm3 (STP) g-1),35 PCP-33 (121.8 cm3
(STP) g-1),36 MIL-100(Fe) (119 cm3 (STP) g-1),37 PMOF-3 (117.3
cm3 (STP) g-1),38 FJI-C1 (93.8 cm3 (STP) g-1),39 MOF-508 (90 cm3
(STP) g-1),40 MFM-130 (85.9 cm3 (STP) g-1),41 JXNU-2(Sm) (79.1
Fig. 4 C2H2, CO2 and CH4 adsorption isotherms of ZJNU-77 and ZJNU-78 at three
different temperatures of (a) 298 K, (b) 288 K and (c) 278 K. The solid and open symbols
represent adsorption and desorption, respectively. (d) IAST selectivities for the
equimolar C2H2/CH4 and CO2/CH4 mixtures at 298 K in ZJNU-77 and ZJNU-78.
6 | Dalton Trans., 2017, 00, 1-3
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ARTICLE
cm3 (STP) g-1 at 273 K),42 MAF-2 (70 cm3 (STP) g-1),43 SNNU-23 properties of C H and CO relative to CH such as higher
View Article Online
2
2
2
4
(62.6 cm3 (STP) g-1),44 UTSA-10 (43.0 cm3 (STP) g-1)45 and FJU-12 critical temperatures (308.30 K, 304.12DKOI: a10n.d103199/C0.75D6T0K408f7oCr
(19.6 cm3 (STP) g-1)46. As for CO2 adsorption, the uptake value C2H2, CO2 and CH4) and polarizabilities (33.3-39.3×10-25 cm3,
is also comparable to those of the copper-based MOFs derived 29.11×10-25 cm3 and 25.93×10-25 cm3 for C2H2, CO2 and CH4),
from bent diisophthalate linkers.23, 27-28, 47 Interestingly, despite which are favourable for enhancing their interaction with the
different surface areas and pore volumes, ZJNU-77 and ZJNU- frameworks.
78 exhibited comparable gas adsorption properties with
respect to C2H2, CO2 and CH4. The results indicate that besides
surface area and pore volume, the pore size also plays a very
4. Conclusions
crucial role in gas adsorption since the two polymorphous
To conclude,
frameworks
a
pair of polymorphous metal-organic
MOFs have the same chemical composition of Cu2L and the
density of open copper site of 3.45 mmol g-1. In fact, it has
been established in the earlier literature that the high specific
surface area is not the only approach to enhance gas uptake at
ambient conditions.48 To further understand the pore size
effect, the gas adsorption capacity per unit of specific surface
area and the gas storage density were calculated. Take C2H2
gas as example, the C2H2 uptake of 0.093 cm3 (STP) m-2 and
C2H2 storage density of 0.272 g cm-3 at 298 K and 1 atm for
ZJNU-78 is around two times higher than the corresponding
values for ZJNU-77 (0.049 cm3 (STP) m-2 and 0.118 g cm-3),
indicative of more suitable pore space in ZJNU-78 for hosting
C2H2 molecules. In ZJNU-78, the reduction in pore size results
in an increase in the overlapping potential and thus the
interaction energy between the framework and gas molecules,
which offsets the negative effect of surface area reduction. In
addition, from these temperature-dependent isotherms, we
calculated the isosteric heat of adsorption (Qst) according to
Clausius-Clapeyron equation (Fig. S12). At near zero coverage,
the Qst values for C2H2, CO2 and CH4 are 27.2, 24.5 and 15.4 kJ
mol-1 in ZJNU-77, and 36.5 and 25.3 and 16.1 kJ mol-1 in ZJNU-
(
ZJNU-77 and ZJNU-78 were successfully
)
synthesized by reacting a bent diisophthalate ligand with
copper ions under different solvothermal conditions. In terms
of structure, ZJNU-77 possesses
a non-interpenetrated
network in which the organic ligands only adopt one
configuration, while ZJNU-78 is a doubly-interpenetrated
network in which the organic ligands adopt two different types
of conformations. Most strikingly, ZJNU-77 exhibits novel
topological structures which are totally different with those of
copper-based MOFs reported thus far constructed from bent
diisophthalate ligands. In terms of performance, their gas
adsorption properties respect to C2H2, CO2 and CH4 were
systematically investigated, revealing that they possess the
promising potential for selective adsorptive separation of C2H2
and CO2 from CH4 at ambient condition as established by gas
adsorption isotherm measurements and IAST calculations.
Interestingly, despite different surface area and pore volume,
the two compounds exhibit comparable gas adsorption
properties, indicating that besides surface area and pore
volume, the pore size also plays a very crucial role in gas
adsorption. This work not only represents a nice example of
MOF polymorphism realized by adjusting the solvothermal
conditions, but also affords a basic understanding regarding
the relationship between MOF polymorphism and gas
adsorption property.
78, respectively. The Qst values follow the hierarchy: C2H2

CO2 CH4, showing the same sequence as the molecular

polarizabilities. The heats of C2H2 and CO2 adsorption are
comparable to those of some reported copper-
multicarboxylate frameworks22b, 22e, 49
.
The difference in the adsorption behaviours and heats of
adsorption between C2H2/CO2 and methane indicate their
potential for C2H2/CH4 and CO2/CH4 separations. To assess the
adsorption selectivity, we employed the ideal adsorbed
solution theory (IAST) to calculate the multicomponent
adsorption behaviour from the experimental single-
component gas isotherms. The predicted adsorption selectivity
for the equimolar C2H2/CH4 and CO2/CH4 gas mixtures as a
Acknowledgement
This work was supported by the Natural Science Foundation of
Zhejiang province, China (LR16B010001), the National Natural
Science Foundation of China (no. 21301156), and the Qianjiang
Talents Project in Zhejiang province (ZC304015017).
function of pressure was presented in Fig. 4d and Fig. S8 an
d
Conflicts of interest
S10. It can be seen that the two compounds possess comparable
C2H2/CH4 and CO2/CH4 adsorption selectivities at ambient
conditions. At 298 K and 1 atm, the predicted IAST adsorption
selectivities for the equimolar C2H2/CH4 and CO2/CH4 gas
mixtures are 28.6 and 5.7 in ZJNU-77, and 28.4 and 5.9 in
ZJNU-78. The corresponding values increase to 35.7 and 7.4 in
ZJNU-77, and 42.4 and 7.4 in ZJNU-78 when the temperature
is lowered to 278 K. Overall, the impressive uptake capacity
and adsorption selectivity indicate that these two compounds
are promising materials for the separation and purification of
acetylene and natural gas. The selective adsorption of C2H2 and
CO2 over CH4 can be mainly attributed to the different physical
There are no conflicts to declare
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DOI: 10.1039/C7DT04087C
TOC
A pair of polymorphous MOFs derived from a bent diisophthalate ligand was synthesized by
modulating solvothermal conditions, exhibiting comparable gas adsorption properties with
respect to C2H2, CO2 and CH4.