Tetrazole-Functionalized Zirconium Metal-Organic Cages for Efficient C2H2/C2H4 and C2H2/CO2 Separations

Article abstract of DOI:10.1002/anie.202102585

Isoreticular functionalization is a well-elucidated strategy for pore environment tuning and the basis of gas separation performance in extended frameworks. The extension of this approach to discrete porous molecules such as metal-organic cages (MOCs) is conceptually straightforward but hindered by synthetic complications, especially stability concerns. We report the successful isoreticular functionalization of a zirconium MOC with tetrazole moiety by bottom-up synthesis. The title compound (ZrT-1-tetrazol) shows promising C₂H₂/CO₂ and C₂H₂/C₂H₄ separation performance, as demonstrated by adsorption isotherms, breakthrough experiments, and density functional theory calculations. The design analogy between MOFs and highly stable MOCs may guide the synthesis of novel porous materials for challenging separation applications.

Full text of DOI:10.1002/anie.202102585

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Accepted Article
Title: Tetrazole-Functionalized Zirconium Metal−Organic Cages for
Efficient C2H2/C2H4 and C2H2/CO2 Separations
Authors: Weidong Fan, Shing Bo Peh, Zhaoqiang Zhang, Hongye
Yuan, Ziqi Yang, Yuxiang Wang, Kungang Chai, Daofeng
Sun, and Dan Zhao
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Tetrazole-Functionalized Zirconium Metal−Organic Cages for
Efficient C2H2/C2H4 and C2H2/CO2 Separations
Weidong Fan,[a] Shing Bo Peh,[a] Zhaoqiang Zhang,[a] Hongye Yuan,[a] Ziqi Yang,[a] Yuxiang Wang,[a]
Kungang Chai,[a] Daofeng Sun,[b] and Dan Zhao*[a]
Abstract: Isoreticular functionalization is a well-elucidated strategy
for pore environment tuning and the basis of gas separation
performance in extended frameworks. The extension of this approach
to discrete porous molecules such as metal-organic cages (MOCs) is
conceptually straightforward but hindered by synthetic complications,
especially stability concerns. We report the successful isoreticular
functionalization of a zirconium MOC with tetrazole moiety by bottom-
up synthesis. The title compound (ZrT-1-tetrazol) shows promising
C2H2/CO2 and C2H2/C2H4 separation performance, as demonstrated
by adsorption isotherms, breakthrough experiments, and density
functional theory calculations. The design analogy between MOFs
and highly stable MOCs may guide the synthesis of novel porous
materials for challenging separation applications.
The acetylene (C2H2) gas market is poised to grow by 105.13
million tons during 2020-2024, corresponding to a compound
annual growth rate (CAGR) of 3% during the forecast period.[1]
High-purity C2H2 is one of the most commonly used chemical raw
materials for synthetic chemicals, such as synthetic α-ethynyl
Figure 1. (a) Structural regulation of MOCs (ZrT-1, ZrT-1-NH2, and
alcohols and acrylic acid derivatives.[2] C2H2 is mainly produced
ZrT-1-tetrazol). (b) ESI-TOF-MS spectra of ZrT-1-tetrazol in
from the cracking of hydrocarbons or through the partial
combustion of methane, coexisting with ethylene (C2H4) or carbon
dioxide (CO2).[3] However, based on the structure of unsaturated
carbon-carbon and carbon-oxygen bonds, their molecular sizes,
boiling points, and electronegativity are very close. Therefore, the
separation of C2H2 from C2H2-containing mixtures (e.g.,
C2H2/C2H4, C2H2/CO2) is an important but challenging industrial
separation task.[4] Prevailing technologies for C2H2 removal
typically rely on cryogenic rectification or liquid absorbents, with
high energy consumption and cost.[5] In contrast, non-thermally
driven approaches such as adsorption or membrane separation
may provide alternatives with lower energy penalties.
acetonitrile/water solutions with various pH values.
various design strategies. In particular, the acidity of the alkyne
hydrogens in C2H2 is an intuitive sorption target, which may be
addressed by the deliberate installation of polar (Lewis-basic)
groups. Differing strengths of the adsorptive interactions form a
basis for appreciable thermodynamic selectivities in the thus-
decorated materials.[10] It should be noted that the intensity of the
introduced active site should be moderate, as overly high
adsorption strength will lead to higher adsorption energy and,
accordingly, higher regeneration energy consumption.[11]
Metal-organic cages (MOCs) are discrete molecules self-
assembled from organic linkers and metal ions or clusters.[12]
MOCs differ from extended porous frameworks, i.e., MOFs and
COFs, in the sense that their extended periodicity arises from a
multitude of weak interactions instead of strong coordination or
covalent bonding. Their molecular nature grants processability
advantages, which are difficult to achieve in their higher-
dimensional counterparts. Meanwhile, owing to their modular
chemistries, MOCs may, in principle, be rationally designed by
well-elucidated reticular chemistry principles toward numerous
applications. However, the application-driven development of
MOCs remains limited by a smaller pool of available structures
meeting specific prerequisites. In particular, for successful
rational design, the MOC platform must (1) remain isoreticular
during the modification process, and (2) maintain stability in the
intended operating conditions. MOC-based membranes have
demonstrated performance in CO2 separation,[13] but MOCs as
adsorbents have not yet achieved a breakthrough in gas
separation, especially for hydrocarbon separation. Previously, we
have reported zirconium-based MOCs, namely ZrT-1 and ZrT-1-
The performance of adsorptive separation is driven to a large
extent by the sorbent properties. Significant progress has been
[6]
[7]
made for C2H2/C2H4 and C2H2/CO2 separations by metal-
organic frameworks (MOFs), as well as other framework-type
sorbents such as covalent organic frameworks (COFs,
assembled by covalent bonds)[8] and hydrogen-bonded organic
frameworks (HOFs, assembled by hydrogen bonds).[9] The
effective separation of C2H2 using these materials leverages
[a]
[b]
Dr. W. Fan, S. B. Peh, Dr. Z. Zhang, Dr. H. Yuan, Z. Yang, Dr. Y.
Wang, Prof. K. Chai, Prof. D. Zhao
Department of Chemical and Biomolecular Engineering, National
University of Singapore, 4 Engineering Drive 4, Singapore 117585,
Singapore
E-mail: chezhao@nus.edu.sg
Prof. D. Sun
School of Materials Science and Engineering, China University of
Petroleum (East China), Qingdao, Shandong 266580, China
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
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Figure 2. Gas sorption and separation performance of ZrT-1-tetrazol: (a) N2 sorption isotherms at 77 K (filled symbols, adsorption; open
symbols, desorption), the inserted is pore size distribution; (b) Single-component adsorption (ads) and desorption (des) isotherms of C2H2,
C2H4, and CO2 at 298 K; (c) IAST calculations for C2H2/C2H4 (50/50, 1/99) and C2H2/CO2 (50/50) mixtures at 298 K; (d) Breakthrough curves of
equimolar C2H2/C2H4 and C2H2/CO2 mixtures at 298 K and 1 bar (CA/C0, outlet concentration/feed concentration); (e) Separation cycling test of
equimolar C2H2/C2H4 mixtures; (f) Breakthrough curves for C2H2/C2H4 (1/99) mixtures.
NH2,[14] demonstrating the requisite stability for efficient synthetic
design. Herein, we report the successful bottom-up synthesis of
tetrazole functionalized derivative, ZrT-1-tetrazol. This
isostructural congener exhibits superior C2H2 recognition
properties relative to its parent MOCs owing to the tetrazole
moiety, as evidenced by gas sorption experiments, density
functional theory (DFT) calculations, Grand Canonical Monte
Carlo (GCMC) simulations, and comparisons to control MOF
samples prepared in this work (UiO-66-tetrazol, CAU-1-tetrazol,
and MIL-101-tetrazol). The results establish Zr-MOCs as a
promising sub-class of molecular porous materials with significant
potential in gas separation applications.
hydrogen from μ2-OH and the loss of counterion (Cl-). To be
specific, the ion peaks at m/z of 1766.7993, 1178.2098, and
883.9152 correspond to [M-4Cl-2H]2+, [M-4Cl-H]3+, and [M-4Cl]4+
ions, respectively. After m/z deconvolution, the average
measured molecular weight of the component is 3677 Da,
comparable to that of ZrT-1-tetrazol (3677 Da). Compared with
the molecular formula of ZrT-1-NH2, only the functional group on
the ligand is different in ZrT-1-tetrazol. The molecular weight
difference between the two MOCs is 318 Da (3677-3359), which
exactly corresponds to the molecular weight difference of the
functional groups on the six ligands in MOC (53 Da, [M(N4CH)-
M(NH2)]), further confirming the structure similarity between ZrT-
1-tetrazol and ZrT-1-NH2.
Starting from the 2-aminoterephthalic acid ligand of ZrT-1-
NH2,[14] the tetrazole-appended ligand 2-(1H-tetrazol-1-
yl)terephthalic acid (BDC-tetrazol) was synthesized by
heterocyclic reaction (Scheme S1 and Figure S1,2). The one-pot
combination of BDC-tetrazol and zirconocene dichloride
(Cp2ZrCl2) in N,N’-dimethylacetamide (DMA) and H2O yielded
crystals of ZrT-tetrazol (Figure 1a). Although we have obtained
large single crystals (Figure S3,4), the structure analysis failed
due to weak X-ray diffraction. Therefore, the structure and
molecular formula of ZrT-1-tetrazol were confirmed by powder X-
ray diffraction (PXRD) and high-resolution electrospray ionization
mass spectrometry (ESI-TOF-MS). PXRD shows that the peak
positions of ZrT-1-tetrazol and ZrT-1-NH2 (simulated and
experimental) are consistent, indicating that they have the same
framework structures (Figure S9). The ESI-TOF-MS spectra
verified the existence of cage molecules in the solution (Figure 1b
and Figure S10,11). The continuous change of the charge state
peaks from +1 to +4 is attributed to the continuous loss of
Although ZrT-1, ZrT-1-NH2, and ZrT-1-tetrazol have similar
structures and the same cavity size (7.05 Å), the aperture size
gradually decreases (4.01, 3.73, and 3.65 Å for ZrT-1, ZrT-1-NH2,
and ZrT-1-tetrazol, respectively, Figure S5), and the polarity of the
pore surface gradually increases. This feature allows the gradual
tuning of the host-guest interactions between the framework and
gas molecules, thereby facilitating gas separation. ESI-TOF-MS
analysis indicates that ZrT-1-tetrazol remains stable in
acetonitrile/water solutions with variable pH values between 2.0
and 10.0. The excellent stability of ZrT-1-tetrazol originates from
the strong Zr-O bonds, promising for multitudinous applications,
including C2H2 storage and separation.
To clarify the pore properties of ZrT-1-tetrazol, we obtained
gas sorption isotherms with the optimum activated porous
samples. N2 (77 K) sorption measurement gives a type I isotherm,
typical for microporous solids. The adsorption capacity is 207.1
cm3 g-1 (1 bar, Figure 2a). The Brunauer–Emmett–Teller (BET)
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Figure 3. DFT-calculated binding configurations of (a) C2H2, (b) C2H4, and (c) CO2 in ZrT-1-tetrazol (green dotted lines represent the interaction
between gas molecules and the cage; purple dotted lines represent the interaction between gas molecules).
surface area was calculated to be around 636.7 m2 g-1 by utilizing
the non-local density functional theory (NLDFT) based on the N2
sorption isotherm. The distribution of pore size localizes between
5.9 and 11.8 Å, corresponding to the intrinsic porosity and the
extrinsic porosity resulting from crystalline packing, respectively
(Figure S6). The BET and pore size distribution did not change
after breakthrough stability testing, indicating that the internal and
external porosity is stable (Figure S56).
Dynamic transient breakthrough experiments were conducted
under ambient conditions to assess the actual separation
performance of the C2H2/CO2 and C2H2/C2H4 mixtures in ZrT-1-
tetrazol. In these experiments, the gas mixture flowed through the
packed column under a total flow rate of 2 mL min−1. Figure 2d
implies that the ZrT-1-tetrazol exhibits a highly efficient separation
of equimolar C2H2/C2H4 and C2H2/CO2 mixtures. C2H4 and CO2
eluted first because of their weak affinity toward the adsorbent,
whereas C2H2 retained in the column for a remarkable period until
its saturated sorption and breakthrough. The C2H2 retention time
of ZrT-1-tetrazol for C2H2/C2H4 (50/50) reached 26 min g-1 and
without any noticeable change in the separation performance
after six adsorption/desorption cycles, indicating its good
recyclability and stability (Figure 2e). The easy desorption of C2H2
and C2H4 suggests that the adsorbent can be facilely regenerated
(Figure S23). In addition, C2H2 can retain in the column for 48 min
g-1, and the productivity of highly pure C2H4 (> 99.99%) can reach
4.24 mol kg−1, calculated based on the C2H2/C2H4 (1/99)
breakthrough curve, indicating that ZrT-1-tetrazol can capture
trace amounts of C2H2 to obtain high purity C2H4 (Figure 2f).
First-principle DFT calculations and GCMC simulations were
conducted to elucidate the host-guest interactions and to better
understand the adsorption behaviors of C2H4, C2H2, and CO2 in
ZrT-1-tetrazol at the molecular level.[18] The optimized structures,
as well as the corresponding adsorption energies, are presented
in Figure 3a. The highly polar hydrogen of C2H2 is bound with the
electronegative or basic tetrazole N sites on the cage surface
through strong C=C–H···N electrostatic interactions with very
short distances between 2.38 and 2.75 Å, implying strong
interactions between C2H2 and the cage. Each ZrT-1-tetrazol
cage contains six exposed N atoms and can bind three C2H2
molecules. The distance between neighboring adsorbed C2H2
molecules is ideal for them to synergistically interact with each
other through multiple Cδ-···H δ+ dipole-dipole interactions, further
enhancing the adsorption energy. The calculated adsorption
energy (ΔE) of C2H2 is −44.67 kJ mol-1, higher than that of C2H4
(−29.72 kJ mol-1) and CO2 (−31.64 kJ mol-1, Figure 3b,c),
indicating weaker interactions of C2H4−ZrT-1-tetrazol and
CO2−ZrT-1-tetrazol compared with that of C2H2−ZrT-1-tetrazol.
GCMC simulation results show that the main preferred gas
sorption sites of ZrT-1-tetrazol are the internal surface of the cage
(tetrazole groups and benzene rings), and the density
The permanent porosity and weakly basic pore surface of ZrT-
1-tetrazol prompted us to study its storage and separation of
acidic C2H2 against its typical separands (C2H4 and CO2, Figure
2b). The uptake of ZrT-1-tetrazol for C2H2 is 57.7 cm3 g-1 (298 K,
1 bar), which is 130% higher than that of ZrT-1-NH2. Meanwhile,
the uptakes of C2H4 and CO2 increased by 21.4% and 54.6%,
respectively, upon tetrazole functionalization. This is because
C2H2 is more acidic than C2H4 and CO2.[15] At the same time, the
C2H2 adsorption of ZrT-1-tetrazol is higher than some of the
adsorbents studied for C2H2/CO2 and C2H2/C2H4 separations,
such as PAF-110 (49.9 cm3 g-1),[8a] PAF-120 (50.8 cm3 g-1),[8b]
HOF-1a (57.1 cm3 g-1 ),[9a] HOF-3a (47 cm3 g-1),[9b] HOF-21a (44.4
cm3 g-1),[9c] and ECUT-HOF-30a (43.7 cm3 g-1).[9d] To evaluate the
affinity of ZrT-1-tetrazol to gas molecules, Clausius–Clapeyron
equation was used to calculate the heat of adsorption (Qst). The
C2H2 Qst in ZrT-1-tetrazol is 33.27 kJ mol-1 at zero coverage,
higher than that of C2H4 (29.87 kJ mol-1) and CO2 (29.89 kJ mol-
1), indicating that the framework of ZrT-1-tetrazol possesses a
higher affinity to C2H2 than C2H4 and CO2. Notably, the C2H2 Qst
of ZrT-1-tetrazol is lower than that of PAF-110 (38.4 kJ mol-1),[8a]
PAF-120 (37.5 kJ mol-1),[8b] SIFSIX-2-Cu-i (41.9 kJ mol-1),[16] HOF-
1a (58.1 kJ mol-1),[9a] and NKMOF-1-Ni (60.3 kJ mol-1),[4a]
indicating easy regeneration due to reversible physisorption. To
evaluate the separation performance of ZrT-1-tetrazole, we used
the ideal adsorbed solution theory (IAST) to calculate the
selectivities for C2H2/C2H4 (50/50, 1/99) and C2H2/CO2 (50/50)
mixtures of ZrT-1-tetrazol at 298 K (Figure 2c). The selectivity of
ZrT-1-tetrazol to C2H2/C2H4 is 4.05, which is comparable to that of
PAF-110 (3.9)[8a] and PAF-120 (4.1),[8b] but higher than CTF-
PO71 (2.8),[8c] M'MOF-2a (1.93),[17] NOTT-300 (2.17),[6b] and Fe-
MOF-74 (2.08).[6a] The selectivity of ZrT-1-tetrazol to C2H2/CO2 is
2.83, higher than that of M'MOF-2a (1.89).[17] The low C2H2 Qst,
high C2H2 uptake, and moderate separation selectivity highlight
that ZrT-1-tetrazol is a promising candidate for C2H2/C2H4 and
C2H2/CO2 separations with low regeneration energy.
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can improve the selectivity toward C2H2. During the breakthrough
experiments, the C2H2 retention time of UiO-66-tetrazol, CAU-1-
tetrazol, and MIL-101-tetrazol for equimolar C2H2/C2H4 reached
29.2, 33.3, and 25.4 min g-1, respectively; and the C2H2 retention
time for equimolar C2H2/CO2 reached 27.1, 81.8, and 14.0 min g-
1, respectively (Figure 4b, d, and f). It is worth noting that CAU-1-
tetrazole has a C2H2 adsorption capacity of 126 cm3 g-1 for
equimolar C2H2/CO2 separation, which is higher than its static
uptake (97.5 cm3 g-1). On the other hand, the C2H2 uptake
capacity of CAU-1-tetrazole obtained from the single-component
breakthrough test (94.9 cm3 g-1) is consistent with the static
uptake (Figure S55), suggesting that the difference in C2H2
capacity of the mixed gas breakthrough test may originate from
competitive adsorption. These data further confirm the actual
separation performance of the tetrazole functionalized MOFs
toward equimolar C2H2/C2H4 and C2H2/CO2 mixtures. The
selectivity and breakthrough performance of ZrT-1-tetrazol is
similar, if not superior, to some of these stable MOFs. We
anticipate that further exploration of reticular chemistry with stable
MOCs may expand the relevance of these molecular materials in
other demanding applications. In particular, performance
enhancements to competitive levels may bring light to other
valuable attributes such as materials processability.
In conclusion, we used isoreticular chemistry to synthesize a
tetrazole-functionalized MOC (ZrT-1-tetrazol) with the same
structure of ZrT-1 and ZrT-1-NH2. The adsorption capacity of ZrT-
1-tetrazol for C2H2 is increased by 130% (298 K) compared with
ZrT-1-NH2, and the IAST separation selectivities for C2H2/CO2
and C2H2/C2H4 are increased to 2.83 and 4.08, respectively.
Breakthrough experiments confirmed the selective adsorption of
ZrT-1-tetrazol to C2H2 and the excellent separation cycle
performance of C2H2/C2H4. DFT calculation revealed that ZrT-1-
tetrazol exerts a strong influence on C2H2 molecules through
tetrazole N∙∙∙H—C hydrogen bond and multiple Cδ-···H δ+ dipole-
dipole interactions. The selective separations of C2H2/CO2 and
C2H2/C2H4 by UiO-66-tetrazol, CAU-1-tetrazol, and MIL-101-
tetrazol confirmed the versatility of tetrazole functional sites for
selective C2H2 adsorption in MOFs. Our work provides a
benchmark for C2H2/CO2 and C2H2/C2H4 separations of MOCs,
and guides the design and synthesis of other porous materials for
C2H2-selective adsorption.
Figure 4. Single-component adsorption (ads) and desorption (des)
isotherms of C2H2, C2H4, and CO2 for UiO-66-tetrazol (a), CAU-1-
tetrazol (c), and MIL-101-tetrazol (e) at 298 K. Breakthrough curves
of equimolar C2H2/C2H4 and C2H2/CO2 mixtures in an absorber bed
packed with UiO-66-tetrazol (b), CAU-1-tetrazol (d), and MIL-101-
tetrazol (f) at 298 K and 1 bar.
Acknowledgements
This work was supported by the National Research Foundation
Singapore (NRF2018-NRF-ANR007 POCEMON), the Ministry of
Education - Singapore (MOE AcRF Tier 2 MOE2018-T2-2-148,
MOE2019-T2-1-093), and the Agency for Science, Technology
and Research (IRG A1783c0015, IAF-PP A1789a0024).
distributions of C2H2 are stronger than that of C2H4 and CO2
(Figure S27).
We synthesized tetrazole functionalized MOFs (UiO-66-
tetrazol, CAU-1-tetrazol, and MIL-101-tetrazol) to further prove
the role of tetrazole functionalization in improving C2H2 selectivity.
PXRD and BET data confirmed the phase purity and porosity of
the synthesized MOFs (Figure S28,29,30,34,40,46). The C2H2
uptakes of UiO-66-tetrazol, CAU-1-tetrazol, and MIL-101-tetrazol
are 94.3, 97.5, and 77.4 cm3 g-1 (298 K, 1 bar), respectively
(Figure 4a, c, and e), representing increases of 30.6%, 26.6%,
and 11.7% compared with that of UiO-66-NH2, CAU-1-NH2, and
MIL-101-NH2, respectively (Figure S33,39,45). The C2H2/C2H4
selectivities of UiO-66-tetrazol, CAU-1-tetrazol, and MIL-101-
tetrazol are 2.34, 1.68, and 1.96, respectively; and the C2H2/CO2
selectivities are 2.89, 3.78, and 3.17, respectively (Figure
S49,50,51), suggesting that the tetrazole-functionalized MOFs
Conflict of interest
The authors declare no conflict of interest.
Keywords: metal-organic cages; isoreticular chemistry; tetrazole
functionalization; gas separation; DFT calculation
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10.1002/anie.202102585
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
We report using isoreticular chemistry
Weidong Fan, Shing Bo Peh,
Zhaoqiang Zhang, Hongye Yuan, Ziqi
Yang, Yuxiang Wang, Kungang Chai,
Daofeng Sun, and Dan Zhao*
to
synthesize
a
tetrazole-
functionalized metal-organic cage
(ZrT-1-tetrazole) with the same
structure of ZrT-1 and ZrT-1-NH2. The
adsorption capacity of ZrT-1-tetrazol
for C2H2 increases by 130% (298 K)
compared with ZrT-1-NH2, and it
exhibits promising C2H2/CO2 and
C2H2/C2H4 separation performance.
Page No. – Page No.
Tetrazole-Functionalized Zirconium
Metal−Organic Cages for Efficient
C2H2/C2H4 and C2H2/CO2 Separations
This article is protected by copyright. All rights reserved.