Significant gas uptake enhancement by post-exchange of zinc(ii) with copper(ii) within a metal-organic framework

Article abstract of DOI:10.1039/c2cc34921c

Coordination of a tetratopic nitrogen-rich linker with Zn²⁺ under solvothermal conditions afforded a Zn-based metal organic framework (MOF), where the Zn(ii) ions could be exchanged by Cu(ii) ions in a single-crystal-to-single-crystal fashion leading to a new Cu-based MOF. Such transformation induces a significant enhancement in gas sorption along with a high selectivity towards CO₂ over N₂ and CH₄.

Full text of DOI:10.1039/c2cc34921c

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Electronic Supplementary Information for
Significant Gas Uptake Enhancement by Post-Exchange of Zinc(II) with
Copper(II) within a Metal-Organic Framework
Xiao-Jun Wang,a Pei-Zhou Li,a Lei Liu,a Quan Zhang,a Parijat Borah,a Jinye Daniel
Wong,a Xiu Xiang Chan,a Ganguly Rakesh,a Yongxin Lia and Yanli Zhao*,a,b
aDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, 21 Nanyang Link, Singapore 637371
bSchool of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang
Avenue, Singapore 639798
To whom correspondence should be addressed
Professor Yanli Zhao
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
21 Nanyang Link
Singapore 637371
E-mail: zhaoyanli@ntu.edu.sg
Phone: +65-63168792
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General method and materials
All chemicals are commercially available and were used as received unless specifically
mentioned. Dimethyl 5-azidoisophthalate (3)S1 and dimethyl 5-ethynylisophthalate (4)S2 were
prepared according to the literature procedures. IR spectra were recorded as KBr-pellet on a
Perkin-Elmer 1760X FT-IR spectrometer. NMR spectra were taken on either a Bruker AV 300 or
a Bruker AV400 at room temperature. ESI-MS experiments were carried out on a
ThermoFinnigan LCQ Fleet MS. Elemental analyses (C, H, and N) were obtained from
EuroVector Euro EA Elemental Analyzer. Thermogravimetric analysis (TGA) was carried out on
a TGA-Q500 thermoanalyzer with a heating rate of 5 °C/min under nitrogen atmosphere. The
powder X-ray diffraction (PXRD) measurements were taken on a Bruker D8 diffractometer using
Cu-Kα radiation (λ = 1.5418 Å) at room temperature. The metal ion concentrations were analyzed
by using Thermo Fischer iCAP 6000 inductively coupled plasma optical emission (ICP-OES)
spectrometer.
Synthesis
Scheme S1. Synthetic route for the preparation of H4-1.
Tetramethyl 5,5’-(1H-1,2,3-triazole-1,4-diyl)diisophthalate (2). Dimethyl 5-azidoisophthalate
(3, 0.24 g, 1.10 mmol) and dimethyl 5-ethynylisophthalate (4, 0.26 g, 1.10 mmol) were added into
a mixture of THF/H2O (50 mL/20 mL), and stirred at room temperature under Ar for 3 days. Then,
the mixture was poured into water (50 mL) and extracted with CH2Cl2 (100 mL × 2). The
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combined organic layers were dried over Na2SO4 and the solvent was evaporated under vacuum.
The crude product was purified by silica gel flash column chromatography (CH2Cl2/CH3OH,
100/1) to give 2 as a white solid (0.46 g, 1.01 mmol, yield: 87%). 1H NMR (300 MHz, CDCl3) δ
13
8.79-8.78 (m, 3H), 8.70-8.69 (m, 3H), 8.49 (s, 1H), 4.03 (s, 6H), 4.00 (s, 6H). C NMR (101
MHz, CDCl3) δ 166.0, 165.0, 147.3, 137.3, 132.7, 131.6, 130.9, 130.8, 130.6, 125.1, 118.5, 53.1,
52.7. ESI-MS: m/z calcd for C22H19N3O8: 453.4, found: 454.2 [M+H]+.
5,5’-(1H-1,2,3-triazole-1,4-diyl)diisophthalic acid (H4-1). The ester 2 (0.56 g, 1.24 mmol) was
hydrolyzed with excess LiOH (0.51 g, 21.3 mmol) in the mixture of THF (60 mL) and H2O (30
mL) at room temperature overnight. Then, H2O (50 mL) was added into the above solution and
washed by Et2O (100 mL × 2). The aqueous layer was acidified with HCl to pH = 1. The solid
was collected by filtration, rinsed several times with distilled water, and dried to give H4-1 (0.48 g,
1.21 mmol, yield: 98%). 1H NMR (300 MHz, DMSO) δ 13.59 (br, 4H), 9.96 (s, 1H), 8.77 (dd, J =
4.8, 1.4 Hz, 4H), 8.54 (t, J = 1.4 Hz, 1H), 8.47 (t, J = 1.5 Hz, 1H). 13C NMR (101 MHz, DMSO) δ
166.4, 165.7, 146.1, 137.1, 133.3, 132.2, 131.1, 130.0, 129.5, 129.4, 123.8, 121.0. ESI-MS: m/z
calcd for C18H11N3O8: 397.3, found: 398.2 [M+H]+.
NTU-101-Zn. H4-1 (40 mg, 0.10 mmol) and Zn(NO3)2·6H2O (65 mg, 0.22 mmol) were dissolved
in N,N-dimethylformamide (DMF, 12 mL). The solution was placed in a tightly capped 20 mL
vial and heated at 75°C in an oven for 3 days. The colorless block crystals were collected after
cooling down to room temperature. These crystals are insoluble in water and common organic
solvents such as MeOH, EtOH, MeCN, chloroform, acetone, toluene, DMF, and DMSO. The
crystals were washed by fresh DMF for 3 times and dried under vacuum. The obtained crystals
were subjected to elemental analysis and TGA analysis in order to estimate the empirical formula
as Zn2(1)(DMF)3·DMF·3H2O (25.6 mg, yield: 26% based on H4-1). Elemental analysis calcd (%)
for C30H41N7O15Zn2: C 41.39, H 4.75, N 11.26; found: C 41.42, H 4.79, N 11.23. IR (KBr) (cm-1):
1635(s), 1589(m), 1438(w), 1370(s), 1106(m), 1063(m), 777(m), 724(w), 667(w).
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100
15.0%
cacld. 14.6% for 1DMF & 3H2O
80
60
40
24.7%
cacld. 25.2% for 3DMF
150
300
450
600
Temperature / oC
Fig. S1. TG curve of NTU-101-Zn.
H4-1
NTU-101-Zn
4000
3000
2000
Wavenumbers (cm-1)
1000
Fig. S2. FT-IR spectra of H4-1 and as-synthesized NTU-101-Zn.
Metal-ion exchange of NTU-101-Zn.S3
The as-synthesized crystals of NTU-101-Zn (around 50 mg) were initially washed by fresh
DMF at least three times, and then added into a solution of Cu(NO3)2 (0.1 M, 15 mL) in DMF
without stirring at room temperature for 3 weeks. The color of these crystals gradually changed
from colorless to blue. During this period, the solution was replaced with fresh Cu(NO3)2 solution
twice. After decanting the solvent, the cation-exchanged crystals (NTU-101-Cu) were washed
thoroughly with DMF and suspended in fresh DMF at room temperature for one week in order to
remove the excess metal ions from the channels of the framework, during which time the DMF
was replaced three times, and then dried under vacuum. ICP analysis indicates that ~80% of Zn(II)
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was replaced by Cu(II) and no significant variation was observed with further extension of
exchange time. The elemental analysis and TGA analysis were carried out to estimate the
empirical formula of NTU-101-Cu as Cu1.6Zn0.4(1)(DMF)3·3DMF·7H2O. Elemental analysis
calcd (%) for C36H63N9O21Cu1.6Zn0.4: C 39.85, H 5.85, N 11.62; found: C 39.92, H 5.83, N 11.02.
IR (KBr) (cm-1): 1658(s), 1586(m), 1438(w), 1371(s), 1103(m), 1062(m), 770(m), 727(w),
663(w).
Fig. S3. Zoomed-in PXRD patterns of NTU-101-Zn (bottom) and NTU-101-Cu (top). The positions of main
peaks (1-11) between the patterns well match by each other. The peak (6’) in NTU-101-Cu may be merged into
peaks (6 or 7). The small shifts of the peaks (12, 13, and 14) are probably caused by the little distortion of the
framework as well as the fact that Cu(II) and Zn(II) are different elements.
100
31.5%
80
cacld. 31.8% for 3DMF & 7H2O
60
40
20
21.2%
cacld. 20.2% for 3DMF
150
300
450
600
Temperature / oC
Fig. S4. TG curve of NTU-101-Cu.
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The metal ion exchange of NTU-101-Zn by other divalent metal ions, such as Ni2+ and Co2+,
was conducted similarly in order to investigate the selectivity of metal ions. ICP analysis shows
that there were 3.4% of Ni2+ (metal percentage) and 5.8% of Co2+ (metal percentage) in the
corresponding metal-ion exchanged samples, indicating that the metal ion exchange of
NTU-101-Zn is selective towards Cu2+ over Ni2+ and Co2+.
−
Fig. S5. FT-IR spectra of NTU-101-Zn, NTU-101-Cu, and NO3 .
−
The absence of NO3 stretching bands at 1385 and 825 cm-1 in the FT-IR spectrum of
NTU-101-Cu indicates that no Cu2+ ions were absorbed in the pores of framework and/or
coordinated with the triazole moiety. A similar IR spectrum to the parent NTU-101-Zn and the
band shift from 1635 to 1658 cm-1 in NTU-101-Cu demonstrate that both of MOFs share a
similar framework.
In addition, other Cu salts with different anions, including Cu(CF3SO3)2, CuSO4, Cu(ClO4)2,
and CuCl2, were used to carry out the metal ion exchange in NTU-101-Zn for one week,
respectively. The ICP analysis for the exchanged samples indicated that the Zn ions in the parent
MOF could be exchanged with Cu ions, although the Cu(II) contents in the exchanged samples
are different (the contents of Cu(II) in the exchanged samples: Cu(CF3SO3)2: Cu% = 20%; CuSO4:
Cu% = 69%; Cu(ClO4)2: Cu% = 57%; CuCl2: Cu% = 93%). The Powder XRD and FT-IR studies
demonstrated that the exchanged samples share similar framework with the parent MOF (Fig.
S6-S8).
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Fig. S6. Powder XRD patterns of NTU-101-Cu prepared with different Cu salts. The similar Powder XRD
patterns suggest that the exchanged MOFs share the similar framework with the parent MOF.
Fig. S7. FT-IR spectra of NTU-101-Cu prepared with different Cu salts. The similar FT-IR spectra suggest that
the exchanged MOFs share similar framework with the parent MOF.
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Fig. S8. FT-IR spectra of different Cu salts and NTU-101-Cu prepared from these Cu salts. The absence of
typical stretching bands of corresponding anions demonstrates that the triazole rings in NTU-101-Cu are
coordinatively free.
Kinetic studies of Cu(II)-ion exchange process
Since the as-synthesized crystals present quite different sizes, a ground sample was employed
to study the kinetic process of Cu(II) ion exchange as shown below.
Ground crystals of NTU-101-Zn were immersed in a solution of Cu(NO3)2 (0.1 M, 20 mL) in
DMF. A small amount of sample was taken out under regular intervals, washed thoroughly with
DMF, and then kept in DMF for one week (DMF was replaced three times to remove the excess
Cu2+) followed by drying the sample under vacuum before subjected to the ICP analysis.
The Cu(II) ion exchange process using the ground sample was rapid and facile. Almost 80% of
Zn(II) from the framework was replaced by Cu(II) within 6 h and no significant change was
detected in the following days, as shown in Figure S9. For freshly prepared crystals, it will take
approximately 2 weeks to achieve a similar amount of Zn(II) exchange. The metal ion exchange
from Cu(II) back to Zn(II) is irreversible, even in a more concentrated DMF solution containing
Zn(NO3)2 (0.3 M) and under long exchange time (1 month), evidenced by no crystal color change
and no obvious variation of nCu/nZn ratio based on the ICP results before and after the exchange.
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0.8
0.6
0.4
0.2
Cu
Zn
0
20
40
Time / h
60
80
100
Fig. S9. Kinetic profile of the metal-ion exchange of Zn(II) by Cu(II) in NTU-101-Zn.
Attempted direct synthesis of NTU-101-Cu
H4-1 (40 mg, 0.10 mmol) and Cu(NO3)2·2.5H2O (48 mg, 0.21 mmol) were dissolved in
N,N-dimethylformamide (DMF, 12 mL). The solution was placed in a tightly capped 20 mL vial
and heated at 75°C in an oven for 3 days. Light blue powder was obtained at the bottom of the
vial, and the single crystal determination was unsuccessful under the current conditions. The
powder XRD analysis indicates that the prepared sample has different phases from NTU-101-Cu,
suggesting that the Cu(II)-ion exchanged MOF cannot be synthesized directly.
Fig. S10. Powder XRD patterns of NTU-101-Cu and the directly synthesized Cu(II)-containing materials.
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Gas sorption
Low-pressure gas sorption measurements were performed by using Quantachrome Instruments
Autosorb-iQ (Boynton Beach, Florida USA) with the extra-high pure gases. The as-synthesized
NTU-101-Zn and NTU-101-Cu crystals were immersed in CH2Cl2 (30 mL) for 1 day, during
which time CH2Cl2 was replaced three times. The samples were then moved into a sample cell
and dried under vacuum at 60 °C by using the “outgasser” function for 12 h before the
measurement. The Brunauer−Emmett−Teller (BET) surface area, pore size distribution, and total
pore volume were calculated from the N2 sorption isotherms at 77 K based on Non-Local Density
Functional Theory (NL-DFT) model in the Quantachrome ASiQwin 2.01 software package. The
isosteric heat of adsorption (Qst) for CO2, defined as
∂ln p
∂T




2 
Qst = RT
(Clausius-Clapeyron equation)
q
was determined by using the CO2 adsorption isotherms at 273 and 298 K (ASiQwin 2.01).
X-ray crystallography.
The single crystal NTU-101-Zn for X-ray analysis was picked up under a microscope from a
freshly synthesized solution and then mounted immediately. The diffraction data were collected at
103 K using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker CCD
APEXII diffractometer. The collected frames were processed with the software SAINTS4. The
data were corrected for absorption by using the SADABS programS5. The structure was solved by
the direct methods and refined by full-matrix least-squares analyses on F2 (SHELXTL-97S6).
There are a lot of disorders for the DMF molecules coordinated to the metal centers. These
disorders were treated as multiple components using constrained geometry. The solvent molecules
in NTU-101-Zn are highly disordered and cannot be modeled (except two solvent DMF
molecules), thus the SQUEEZE routine of PLATON was applied to remove the contributions of
the solvent molecules to the scattering. Crystallographic data for NTU-101-Zn is summarized in
Table S1. CCDC 890896 contains the supplementary crystallographic data for NTU-101-Zn.
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Table S1. Crystal data and structure refinement for NTU-101-Zn.
Empirical formula
Formula weight
Temperature
C117H133N27O47Zn8
3192.62
103(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Orthorhombic
Pccn
Unit cell dimensions
a = 26.4360(4) Å
b = 26.1150(4) Å
c = 26.3473(4) Å
α = 90°.
β = 90°.
γ = 90°.
3
Volume
18189.5(5) Å
4
Z
3
Density (calculated)
1.166 Mg/m
-1
Absorption coefficient
1.130 mm
F(000)
8320
3
Crystal size
0.34 x 0.28 x 0.08 mm
Theta range for data collection
Index ranges
1.73 to 30.11°.
-37<=h<=31, -36<=k<=36, -37<=l<=33
180525
Reflections collected
Independent reflections
Completeness to theta = 30.11°
Absorption correction
Max. and min. transmission
26771 [R(int) = 0.0636]
99.8 %
Semi-empirical from equivalents
0.9151 and 0.7000
2
Refinement method
Full-matrix least-squares on F
Data / restraints / parameters
26771 / 2145 / 1283
1.034
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0633, wR2 = 0.1905
R1 = 0.0957, wR2 = 0.2056
2.000 and -0.969 e.Å
Largest diff. peak and hole
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Fig. S11. The simplified view of PtS-type network of NTU-101-Zn. The [Zn2(CO2)4(DMF)3] secondary
building unit can be viewed as a 4-connected node (purple, tetrahedral SBU), and the organic linker 1 that
coordinates to four [Zn2(CO2)4(DMF)3] units acts as a 4-connected node (green).
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Fig. S12. The packing view of NTU-101-Zn along with axes a (top), b (middle), and c (bottom) with (left) and
without (right) the coordinated DMF molecules. As shown from the left column, the coordinated DMF
molecules are located in the framework channels, blocking the pores (yellow circle). Such channels would be
opened upon the removal of the coordinated DMF molecules as shown from the right column. However, the
removal of the coordinated DMF molecules caused the collapse of the frameworks, resulting in poor gas
adsorption. Hydrogen atoms are omitted for clarity.
Fig. S13. Photographs of as-synthesized (left) and activated (right) NTU-101-Cu. The dark blue color (right)
suggests the open metal sites around the Cu(II) ions and the removal of the solvent molecules. Upon exposure
to air, the dark blue color returned back to its original blue color due to the adsorption of moisture.
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0.10
0.08
0.06
0.04
0.02
500
400
300
200
100
0
S
BET = 2017.43 m2 g-1
BET summary
Slope = 1.725
Intercept = 7.739e-04
Correlation coefficient, r = 0.999979
C constant = 2230.479
0.0
0.2
0.4
0.6
0.8
1.0
0.01
0.02
0.03
0.04
0.05
Relative Pressure P/P
0
Relative Pressure P/P
0
Fig. S14. Nitrogen sorption isotherm at 77K (left) and BET specific surface area plot (right) for activated
NTU-101-Cu.
45
Absorption
Desorption
30
S
BET = 36.87 m2 g-1
30
15
0
25
20
BET summary
Slope = 83.872
Intercept = 1.058e+01
Correlation coefficient, r = 0.998296
C constant= 8.929
0.0
0.2
0.4
0.6
0.8
1.0
0.10
0.15
0.20
0.25
Relative Pressure P/P
0
Relative Pressure P/P
0
Fig. S15. Nitrogen sorption isotherm at 77K (left) and BET specific surface area plot (right) for activated
NTU-101-Zn.
200
NTU-101-Cu
150
100
50
NTU-101-Zn
0
0
200
400
600
800
Absolute Pressure / mmHg
Fig. S16. H2 adsorption isotherms of activated NTU-101-Zn and NTU-101-Cu at 77 K.
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1
Fig. S17. H NMR spectra of the activated NTU-101-Zn (top) and NTU-101-Cu (bottom) digested by a
solution of TFA (10%) in d6-DMSO. Less DMF molecules observed in NTU-101-Cu indicate the efficient
removal of the coordinated DMF molecules as compared with that of NTU-101-Zn. The original blocked
channels by the coordinated DMF molecules in NTU-101-Zn were opened after the Cu(II) exchange process.
Fig. S18. Powder XRD patterns of NTU-101-Zn and NTU-101-Cu after the activation, indicating the
significant collapse of the NTU-101-Zn framework as compared with that of NTU-101-Cu upon the activation.
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30
20
10
0
100
NTU-101-Cu at 273 K
80
60
NTU-101-Cu at 298 K
40
20
NTU-101-Zn at 273 K
0
0
10
20
V (CO
30
40
50
0
200
400
600
800
Absolute Pressure / mmHg
2
) / cm3 g-1
Fig. S19. CO2 adsorption isotherms (left) of activated NTU-101-Zn and NTU-101-Cu. Isosteric heat of CO2
adsorption (right) for activated NTU-101-Cu calculated from the adsorption isotherms at 273 K and 298 K.
References
S1. Z. P. Demko and K. B. Sharpless, Angew. Chem. Int. Ed., 2002, 41, 2110.
S2. G. J. Bodwell, D. O. Miller, and R. J. Vermeij, Org. Lett., 2001, 3, 2093.
S3. (a) S. Das, H. Kim and K. Kim, J. Am. Chem. Soc., 2009, 131, 3814; (b) J. Zhao, L. Mi, J. Hu, H. Hou and Y.
Fan, J. Am. Chem. Soc., 2008, 130, 15222; (c) T. K. Prasad, D. H. Hong and M. P. Suh, Chem. Eur. J., 2010, 16,
14043.
S4. SAINT V6.1, Bruker Analytical X-ray Systems, Madison, WI, 1999.
S5. G. M. Sheldrick, SADABS, Empirical Absorption Correction Program, University of Göttingen, Göttingen,
Germany, 1997.
S6. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112.
S7. A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
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Characterization spectra of key compounds
1H NMR of 2 in CDCl3
13C NMR of 2 in CDCl3
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ESI-MS of 2
1H NMR of H4-1 in DMSO
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13C NMR of H4-1 in DMSO
ESI-MS of H4-1
S19