Tetrazole-based porous metal-organic frameworks for selective CO2 adsorption and isomerization studies

Article abstract of DOI:10.1039/c9dt04068d

Tetrazole-based molecules have numerous bridging coordination modes which afford great synthetic possibilities for the preparation of porous metal-tetrazolate architectures for many applications, such as carbon capture. We reported here three tetrazole-ba

Full text of DOI:10.1039/c9dt04068d

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Tetrazole-based porous metal-organic frameworks for selective
CO2 adsorption and isomerization studies
Rui Zhang,a De-Xian Meng,a Fa-Yuan Ge,b Jv-Hua Huang,a Li-Fei Wang,a Yong-Kai Xv,a Xing-Gui Liu,a
Received 00th January 20xx,
Accepted 00th January 20xx
Mei-Mei Meng,a Hong Yan,c Zhen-Zhong Lu,*a He-Gen Zheng,*b Wei Huang*a,d
DOI: 10.1039/x0xx00000x
Abstract: Tetrazole-based molecules are of numerous bridging coordination modes affording great synthetic possibilities for
the preparation of porous metal-tetrazolate architectures for many applications, such as carbon captures. We reported here
three tetrazole-based MOFs: 1, {[Cu12(ttz)8/3Cl5(H2O)16]
11+·11Cl–}n (H3ttz = N2,N4,N6-tris(4-(1H-tetrazol-5-yl)phenyl)-1,3,5-
triazine-2,4,6-triamine), contains highly positively charged Cu12 clusters and largest mesopores (32 Å) in the reported MOFs
based on tri-topic tetrazole ligand. 2 and 3 are two MOF isomers built by CuII and 2-(1H-tetrazol-5-yl)pyrimidine. 3 contains
nonporous layers, while 2 contains 1D channel and showed high selectivity for adsorbing CO2, which should be attributed to
the high density of free nucleophilic tetrazole N atoms on the pore surfaces. We found that the isomerization between 2
and 3 was caused by the diverse coordination mode of tetrazole-based ligand and can be controlled in the synthesis
processes.
and MOFs showing stable open structures and excellent carbon
capture properties.8,9
Tetrazoles are aromatic five-membered heterocycles
Introduction
Metal-organic frameworks (MOFs) or porous coordination
polymers (PCPs) are a new class of porous solids consisting of
metal-based nodes (single ions or clusters) bridged by organic
linking groups have attracted much recent attention.1,2 One of
the most attractive aspects of MOFs is their porous structure
showing high surface area, diverse pore size, shape and
functionality, which provide MOFs excellent prospects in many
applications, such as carbon capture, molecular separation and
heterogeneous catalysis.3,4 As for building porous MOFs for
carbon dioxide capture, the choice of ligand that can form
strong metal-ligand bonds to sustain porous structures and
provide binding sites for interacting with gaseous CO2
selectively is a key point.5,6
containing four nitrogen atoms, which exhibit more bridging
coordination possibilities, such as deprotonated tetrazole group
can bridge two, three and up to four metal ions.10,11 These
numerous bridging coordination modes afford great synthetic
possibilities for the design and preparation of porous metal-
tetrazolate architectures, while tetrazole-based molecules are
less studied although some very successful works have proven
powerfulness of them in building porous MOFs for carbon
capture and other applications.12,13 We report herein the design
and synthesis of three MOFs built by tetrazole-based ligands
including MOF containing 3D mesopore and MOF containing 1D
micro-channel for selective adsorption of CO2. And the effects
of the coordination diversity of tetrazole-based ligands on the
structure and the formation of MOF isomers are discussed.
Nitrogen-containing heterocyclic groups-based molecules are
good choices, the nucleophilic nitrogen is able to form stable
metal–N bond to construct porous MOFs and also able to
interact with CO2 strongly.7 For example, imidazole- or pyrazole-
based ligands formed many zeolitic imidazolate frameworks
Experimental
Materials and methods
All reagents were used as received from commercial suppliers
without further purification. 2-(1H-tetrazol-5-yl)pyrimidine was
synthesized following reported procedures.14 Analyses for C, H
and N were carried out on a CE-440 elemental analyzer (EAI
Company). Powder X-ray diffraction data (PXRD) were collected
over the 2θ range 4-50o on a Bruker Advance D8 diffractometer
using Cu-Kα1 radiation (λ = 1.54056 Å, 40 kV/40mA). CO2 and N2
sorption isotherms were recorded on an ASAP-2020 system
under ultrahigh vacuum. The CO2 and N2 gases used were of
ultrapure research grade (99.99%).
a. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing
211800, China. E-mail: iamzzlu@njtech.edu.cn, iamwhuang@nwpu.edu.cn
b. State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Collaborative Innovation Center of Advanced
Microstructures, Nanjing University, Nanjing 210023, P. R. China. E-mail:
zhenghg@nju.edu.cn
c. 23rd Middle School of Daqing city, No. 385 Jingsan street, Sartu District, 163000,
Daqing city, China
d. Institute of Flexible Electronics (IFE), Northwestern Polytechnical University (NPU),
127 West Youyi Road, Xi'an 710072, China
Electronic Supplementary Information (ESI) available: Experimental details for
ligand synthesis, X-ray crystallographic files, further descriptions of the structures.
CCDC 1956834-1956836. See DOI: 10.1039/x0xx00000x
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Synthesis of ligand N2,N4,N6-tris(4-(1H-tetrazol-5-yl)phenyl)-1,3,5-
triazine-2,4,6-triamine (H3ttz):
View Article Online
DOI: 10.1039/C9DT04068D
Synthesis of 4,4',4''-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))-
tribenzonitrile: cyanuric chloride (1.84 g) and 4-
aminobenzonitrile (3.89 g) were added to 75 mL glacial acetic
acid. The reaction mixture was stirred at reflux for 24 hours,
white solid was formed and collected by vacuum filtration,
washed with 300 mL hot water and vacuum dried (3.16g, yield:
1
74%) and used for next step without further purification. H
Figure 1. (a) Side view and (b) Top view of the Cu12 cluster in 1 (Cu,
light blue; C, black; H, grey; N, dark blue; O, red; Cl, green, water
molecules are omitted for clarity in (b)).
NMR (CDCl3): δ 10.01 (s, br, 3H), 7.73 (d, J=7.58 Hz, 6H), 7.43(d,
J=7.58 Hz, 6H). 13C NMR (CDCl3): δ 163.1, 141.6, 133.3, 117.6,
116.4, 103.5. MS: 429.33.
1, {[Cu12(ttz)8/3Cl5(H2O)16]11+·11Cl–}n, crystalizes in cubic space
group Fm-3c, and contains a [Cu12(tetrazole)8Cl5(H2O)16]11+
cluster including twelve CuII atoms, eight tetrazole groups, five
chloride anions and sixteen coordinated water molecules
(Figure 1a). Four CuII atoms in the center of the Cu12 clusters
(Cu1, Figure 1b) are bridged by eight tetrazole groups through
N atoms at 2, 3-positions, eight CuII atoms on the periphery of
the Cu12 clusters (Cu2, Figure 1b) are bridged by the eight
tetrazole groups through N atoms at 1, 4-positions. The Cl- atom
in the center of the cluster connects four ‘central’ Cu1 atoms in
a μ4 mode, and the rest four Cl- atoms bridge the ‘periphery’
Cu2 atoms in a μ2 mode. Cu1 is in an octahedral coordination
environment defined by two Cl- in the axial direction and four
tetrazole N atoms in the equatorial plane (Cu1–Cl1 = 2.546(2),
Cu1–Cl2 = 2.477(2), Cu1–N = 2.060(1) Å). Cu2 is five-coordinated
by two tetrazole N atoms, one Cl- and two water molecules. The
Cu2–Cl2 bond distance (2.592(2) Å) is comparable to those of
4,4',4''-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))-tribenzonitrile
(1.43 g), NaN3 (1.95 g), NH4Cl (1.0 g) and LiCl (0.4 g) were added
to 40 mL anhydrous DMF, the reaction mixture was stirred at
130oC for 24 hours. After removing DMF by rotary evaporation,
the solid obtained was added into 20 mL sodium hydroxide
aqueous solution (2M) and stirred for 30 minutes, white gel-like
precipitation was formed by adjusting pH to 4 with hydrochloric
acid (2M), which were collected by vacuum filtration, washed
with water and ethanol and vacuum dried. H3ttz was obtained
as white solid (0.57 g, yield: 31%). 1H NMR (CDCl3): δ 9.72 (s, br,
3H), 9.32 (m, 6H), 8.65 (m, 6H), 8.61 (s, br, 3H). 13C NMR (CDCl3):
δ 159.3, 155.1, 140.9, 127.8, 120.6, 119.9.
Synthesis of single crystals of 1, {[Cu12(ttz)8/3Cl5(H2O)16]11+·11Cl–}n
A mixture of Cu(NO3)2·3H2O (0.096 g), H3ttz (0.056 g), DMF (10.0
mL), acetonitrile (0.5 mL) and hydrochloric acid (0.1 mL, 1.0 M)
was transferred into a 23 mL Teflon lined autoclave and sealed.
Reaction took place at 90oC for 3 days, and dark green
octahedron-shaped crystals were formed (yield: ~ 80% based on
ligand). The crystals of 1 was preserved in acetone, which
cracked into small pieces and became amorphous when put in
air for several minutes.
Synthesis of single crystals of 2, Cu3(pmtz)6·DMF
A mixture of Cu(NO3)2·3H2O (0.241 g), pmtz (0.075 g), DMF (5.0
mL) and nitric acid (0.1 mL, 1.0 M) was transferred into a 23 mL
Teflon lined autoclave and sealed. Reaction took place at 50oC
for 1 day, and afforded dark green prism-shaped crystals, which
were collected by filtration, washed with fresh DMF and then
acetone (yield: 90% based on ligand). Elemental analysis (%) for
2, C33H25Cu3N38O: Calcd: C, 34.16; H, 2.17; N, 45.87. Found: C,
34.02; H, 2.15; N, 45.76.
Synthesis of single crystals of 3, Cu(pmtz)2
A mixture of CuSO4·5H2O (0.250 g), pmtz (0.075 g) and DMF (5.0
mL) was transferred into a 23 mL Teflon lined autoclave and
sealed. Reaction took place at 50oC for 1 day, and afforded dark
green prism-shaped crystals, which were collected by filtration,
washed with fresh DMF and then acetone (yield: 70% based on
ligand). Elemental analysis (%) for 3, C10H6CuN12: Calcd: C, 33.57;
H, 1.69; N, 46.98. Found: C, 33.49; H, 1.62; N, 46.59.
Figure 2. (a) 3D framework in 1 (CuII shown as blue polyhedrons).
(b) The topology of ‘the’ of 1 (Cu12 cluster and ttz ligand are shown
as 8- and 3-connected red balls). (c) Octahedral cage formed by six
Cu12 clusters and eight ttz ligands (the Cl- counter-anions are
shown as green balls). (d) Truncated cube-shaped cage with
square-shaped windows formed by packing of octahedrons.
Results and discussion
2 | J. Name., 2012, 00, 1-3
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Cu1–Cl1, while Cu2–N (2.230(2) and 2.264(2) Å) are much
longer than Cu1–N.
View Article Online
DOI: 10.1039/C9DT04068D
The Cu12 clusters are linked by ttz ligands into a highly porous
3D framework (Figure 2a). The overall topology of the
framework is ‘the’ following the classification from RCSR
(reticular chemistry structure resource)15 with Cu12 cluster and
ttz ligand acting as 8- and 3-connector, respectively (Figure 2b).
The topology of ‘the’ are often observed in MOFs based on tri-
topic carboxylate ligands16 and much less observed in tetrazole-
based MOFs.17
It is interesting to notice that six Cu12 clusters are linked by
eight ttz ligands into an octahedral cage (Figure 2c), the inner-
space of one cage is as large as 2727 Å3 calculated by PLATON.18
The charge of one Cu12 cluster is +11, so the overall 3D
framework is highly positively charged and a large number of
counterions are needed. There are 33 uncoordinated Cl–
observed in one octahedral cage (Figure 2c, the shortest Cl–···Cl–
separation is 3.92 Å), which are exactly adequate to maintain
the charge balance of the framework. Besides Cl– counterion,
30 acetonitrile molecules were also observed in one octahedral
cage.
Figure 4. (a) Coordination environment of CuII in 3. (b) 2D layer of
distorted Cu4(pmtz)4 tetragons in 3.
high stability to the overall framework since more metal-ligand
bonds are formed, which might combine metal clusters and
ligands tighter. While powder X-ray diffraction (PXRD) result
showed that the porous framework of 1 collapsed when solvent
guests removed, which probably because the ttz ligand is not
rigid enough to support such a highly porous and positively
charged structure, similar cases were observed in other MOFs
with large pores.19
2, Cu3(pmtz)6·DMF, was obtained by solvothermal reaction
of Cu(NO3)2 with a much smaller tetrazole-based ligand 2-(1H-
tetrazol-5-yl)pyrimidine (pmtz). 2 crystalizes in space group P-1.
The pmtz ligand is in a chelating and bridging mode (Figure 3a),
and the CuII atom is in an octahedral coordination environment
defined by four chelating N atoms in the equatorial plane and
two N atoms in the axial direction. Cu–N bonds in the equatorial
plane (1.958 ~ 2.091 Å) are much shorter than those in the axial
direction (2.394 ~ 2.422 Å) due to the Jahn-Teller effect.
2D layers containing Cu3(pmtz)3 triangles and Cu6(pmtz)6
hexagons are formed in 2 (Figure 3b). The Cu6(pmtz)6 hexagon
is large enough for incorporating a DMF molecule, while the
Cu3(pmtz)3 triangle is too small to incorporate any guests
(Figure 3b, c). The 2D layers are packed parallelly forming a
hexangular channel, all of the pyrimidine rings are involved in
forming π···π interactions between adjacent layers with
c.g.pyrimidine···c.g.pyrimidine distances of 3.809(2) and 3.839(2) Å
(Figure 3c). Although the individual π···π interaction is not
strong as indicated by the relatively long c.g.···c.g. separations,
the synergy effect of many π···π interactions between adjacent
layers make the porous structure very stable as proved by PXRD
and gas adsorption tests.
As shown in Figure 2d, truncated cube-shaped cage with
square-shaped windows is formed by the packing of
octahedrons, the separation between octahedrons is as large as
32 Å and the square-shaped windows is 13.6 × 13.6 Å taking van
der waals radii into consideration (Figure S1). Therefore, 3D
interconnected mesopores are formed, which corresponds to
70 % of the total crystal volume (the inner space of the
octahedral cages was uncounted). The pore size of 1 is the
largest in the reported MOFs based on tri-topic tetrazole
ligand.17 We expected that the large Cu12 cluster would afford
3, Cu(pmtz)2, an isomer of 2, crystalizes in space group P 21/c.
It is interesting to notice that 3 also contains layers of
Cu4(pmtz)4 tetragons, while the tetragon is too distorted to
incorporate any guest molecules (Figure 4). The coordination
mode of pmtz ligand and the coordination environment of CuII
atom are both similar to those in 2 (Figure 4a), and the Cu–N
bond distances in the equatorial plane (1.985 ~ 2.055(1) Å) are
also comparable to those in 2, while the Cu–N bond distances in
axial direction (2.518 Å(1)) are much longer than those in 2.
In order to study the isomerization between 2 and 3, we
summarized all the reported MOFs based on pmtz and
transition metals (Table S3).20-24 M(pmtz)2 (M = ZnII, CoII, FeII or
NiII, we named them as 4R hereafter, R is for reported)
represent a serial of isostructural MOFs, in which the
Figure 3. (a) Coordination environment of CuII in 2. (b) 2D layer of
Cu3(pmtz)3 triangles, Cu6(pmtz)6 hexagons and 1D channels. (c) The
π···π interactions between pyrimidine rings of adjacent layers (the
pyrimidine and tetrazole groups of different layers are shown in
different color). (d) A perspective view showing the positions of
free tetrazole N atoms on the inner-pore surfaces. (C/H, grey; N,
blue; O, red; Cu, purple)
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J. Name., 2013, 00, 1-3 | 3
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coordination mode of pmtz ligands and coordination
environment of metal centers are both quite similar to those in
2 and 3. 4R contains 2D layers of M4(pmtz)4 tetragons, but the
adjacent pmtz molecules in the tetragons are orthogonal to
each other (Figure S4). The M–N bond lengths in the equatorial
plane and axial direction are in a very narrow range (such as Zn–
N bonds are in the range of 2.114 ~ 2.169 Å for Zn(pmtz)2, Table
S4), in a sharp contrast to the highly elongated octahedral
coordination environment of CuII in 2 or 3.
View Article Online
DOI: 10.1039/C9DT04068D
Scheme 1. (a) The in-plane and (b) out-of-plane mode of pmtz
ligand (Mb and Mc denote bridged and chelated metal atoms by
pmtz ligand, respectively).
From structure point of view, pmtz ligands in 2, 3, and 4R are
all in the chelating and bridging coordination mode, the
chelated metal atom (Mc, Scheme 1) is in the same plane of
pmtz molecule, but the bridged metal atom (Mb, Scheme 1) can
be in the plane or out of the plane of pmtz. In 4R, pmtz ligands
are all in the in-plane mode, so that a 2D layer containing
regular Cu4(pmtz)4 tetragons is formed with pmtz molecules
orthogonally arranged (Figure S5); In 3, pmtz ligands are all in
the out-of-plane mode leading to a layer containing highly
distorted Cu4(pmtz)4 tetragons; In 2, there are 1/3 pmtz ligands
in the in-plane and 2/3 pmtz ligands in the out-of-plane mode,
which should account for the formation of a more complicated
layer containing a combination of Cu3(pmtz)3 and Cu6(pmtz)6
grids. More importantly, the CuII showing highly distorted
octahedral environment (elongated Cu–N bonds in the axial
direction) meet the request of the out-of-plane coordination of
pmtz in 2 and 3.
We found that the isomerization between 2 and 3 can be
controlled in the synthesis processes, and phase-pure samples
of both 2 and 3 can be obtained directly. A good majority of
MOF isomers were obtained as a mixture of phases, and phase-
pure samples were obtained by further separations, such as
separation based on density differences. Phase-pure samples of
both 2 and 3 were obtained directly by using different CuII salts
as starting materials (see experiment section), and the phase
purity were confirmed by PXRD results (Figure 5a).
Figure 5. (a) PXRD patterns for 2, 3, and 4R. (b) TG result for 2.
molecules. So that 2 should be a very good candidate for carbon
capture.
TG result of 2 showed a weight loss of 4 wt% below 80oC
corresponding to the loss of weakly bonded solvent molecules,
and a weight loss of 10 wt% starting at 200oC corresponding to
the loss of adsorbed DMF guests, and a sharp weight loss of 62
wt% at 310oC should be ascribed to the loss of pmtz and collapse
of the porous structure (Figure 5b).
As suggested by TG result, we ‘activated’ as-synthesized
sample of 2 by removing the adsorbed solvents at 240oC under
vacuum, and take the solvent-free sample for gas adsorption
tests. The CO2 uptake capacity are 18.3 and 12.9 cm3·g-1 at 273
and 293K and 1.0 bar, in a sharp contrast to the uptake capacity
for N2, which is 0.7 cm3·g-1 at 273K and 1.0 bar (Figure 6a). The
IAST selectivity for equal-molar mixture of CO2 and N2 is ~110 in
favor of CO2 at low pressure, and decreases to 80 as pressure
increases (Figure 6b). The low-coverage isosteric heat of
adsorption for CO2 is 31 kJ·mol-1, which is comparable to that
for the benchmark MOF HKUST-1 (29 kJ·mol-1, and most MOFs
show isosteric heat of adsorption for CO2 below 29 kJ·mol-1),27,28
and lower than that for Mg2(dobdc) containing very high density
of open metal sites (40 kJ·mol-1)26 and those for alkanolamines
functionalized MOFs (50~100 kJ·mol-1, but the mechanism of
CO2 adsorption in these MOFs fall into the chemisorptive
The pore size of the 1D channels in 2 is 4.5 Å in diameter
(taking van der waals radii into consideration), which is ideal for
CO2 adsorption selectivity.25,26 And there are full of free
nucleophilic tetrazole nitrogen atoms on the inner-pore
surfaces (Figure 3d), which might be strong binding sites for CO2
Figure 6. (a) Isotherms of CO2 and N2 for 2. (b) CO2/N2 IAST selectivity for 2 (calculated for equimolar mixtures at 273 K and 0~1 bar), inset:
variation of isosteric heat of adsorption for CO2.
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regime and more energy is needed to release adsorbed CO2).29
The magnitude of the isosteric heat of adsorption indicates the
affinity of the pore surface toward CO2, which in turn plays a
crucial role in determining the adsorptive selectivity and the
energy required to release the CO2 molecules during
regeneration.30 This high magnitude of isosteric heat of
adsorption of CO2 in 2 is probably attributed to the high density
of free nucleophilic tetrazole nitrogen atoms on the pore
surfaces, which makes 2 interact with CO2 strongly.31
3
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Conclusions
We have synthesized three tetrazole-based MOFs. 1 contains
highly positively charged Cu12 clusters and the largest 3D
mesopores (32 Å) in the reported MOFs based on tri-topic
tetrazole ligand. 2 and 3 are a pair of isomers originated from
the versatile coordination mode of a tetrazole-based ligand. We
managed to control the isomerization between 2 and 3 and
make phase-pure samples for both 2 and 3 directly. More
interestingly, 2 represents the most porous MOFs based on
pmtz ligand, and showed high selectivity for adsorbing CO2 with
low-coverage isosteric heat of adsorption up to 31 kJ·mol-1,
which should be attributed to the very high density of free
nucleophilic tetrazole nitrogen atoms on the pore surfaces.
Further work on studying isomerization effect of MOFs and
building stable porous frameworks with tetrazole or other
nitrogen-containing heterocyclic groups-based molecules is
ongoing.
5
6
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Conflicts of interest
The authors declare no competing financial interests.
Acknowledgements
We thank China Postdoctoral Science Foundation
(2018M640478), Nanjing Tech University, National Natural
Science Foundation of China (No. 21771101), Fundamental
Studies of Perovskite Solar Cells (2015CB932200) and Primary
Research & Development Plan of Jiangsu Province (BE2016770)
for funding.
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DOI: 10.1039/C9DT04068D
Porous MOFs and isomers base on tetrazole-based molecules were synthesized for
adsorbing carbon dioxide showing high selectivity.