A charged metal-organic framework for CO2/CH4 and CO2/N2 separation

Article abstract of DOI:10.1016/j.ica.2016.01.014

A microporous metal-organic framework [InL][(CH₃)₂NH₂]·(H₄L = 9-(3,5-dicarboxyphenyl)carbazole-3,6-dicarboxylic acid) (1) was hydrothermally synthesized and structurally characterized by single crystal X-ray dif

Full text of DOI:10.1016/j.ica.2016.01.014

Accepted Manuscript
A charged metal-organic framework for CO2/CH4 and CO2/N2 separation
Ruiqin Zhong, Zhilong Xu, Wenzhu Bi, Songbai Han, Xiaofeng Yu, Ruqiang
Zou
PII:
S0020-1693(16)00025-6
http://dx.doi.org/10.1016/j.ica.2016.01.014
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Inorganica Chimica Acta
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30 December 2015
6 January 2016
Please cite this article as: R. Zhong, Z. Xu, W. Bi, S. Han, X. Yu, R. Zou, A charged metal-organic framework for
CO2/CH4 and CO2/N2 separation, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.01.014
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A charged metal-organic framework for CO2/CH4 and CO2/N2
separation
Ruiqin Zhong,a Zhilong Xu,b Wenzhu Bi,b Songbai Han,c Xiaofeng Yu,a Ruqiang Zou*,b
a State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,
b
China; Department of Materials Science and Engineering, College of Engineering, Peking
c
University, Beijing 100871, China; Neutron Scattering Laboratory, China Institute of Atomic
Energy, Beijing 102413, China.
;

Abstract
microporous
A
metal-organic
framework
[InL][(CH3)2NH2].(H4L=
9-(3,5-dicarboxyphenyl)carbazole-3,6-dicarboxylic acid) (1) was hydrothermally
synthesized and structurally characterized by single crystal X-ray diffraction and
powder X-ray diffraction. The adsorption behavior of 1 has been investigated with
respect to CO2, CH4 and N2, and the separation efficiency has been calculated by the
Henry’s law analysis of isotherm data. The results indicated that the anionic framework
with the inside protonated dimethylamine could enhance its selective adsorption
performance for CO2 through strong adsorbent–adsorbate interaction aroused by a
strong electrostatic field and pore size effect.
Keywords: metal-organic framework; CO2 separation; gas adsorption; selectivity.

1. Introduction
Metal-organic frameworks (MOFs), consisting of inorganic metal centers
connected by organic linkers, are a relatively new family of porous materials that
possess fine-tuning structural diversity, and offer great promise for extensively
potential applications in many areas, such as gas storage,[1] catalysis,[2] sensing,[3]
and magnetism.[4] In recent year, CO2 capture technologies become more and more
significant and continually developed because the removal of CO2 is not only essential
in the fight against climate change, but also greatly beneficial to the promotion of the
energy efficiency and conservation in the industrial field.[5] Compared to conventional
inorganic porous adsorbents (e.g., activated carbon and zeolites), MOFs have shined
brilliantly on the aspect of CO2 adsorption and separation, owing to their intriguing
properties, such as ultra-high surface area, well-ordered pore structures, modifiable
pore shapes, pore sizes and chemical environment inside the pores.[6] Many efforts
have been made to improve the performance of MOFs for selective CO2 adsorption and
separation, and generally the efficient strategies explored include two aspects. One is to
utilize the molecular sieving effect by tuning pore distribution for controllable
separation of adsorbates with different of van der Waals diameters.[7] Another is to
improve the surface chemistry of pores for the enhanced adsorption interactions.
Because of the differences in electronic properties of adsorbate gases such as
quadrupolarmoment and polarization, functionalization of pores or open channels,[8]
utilization of the Lewis acid sites [9] and framework flexibility [10] could increase the
adsorbent-adsorbate interactions. It is worth noting that the MOFs with charged
frameworks and counterion species inside of the pores or channels are good candidates
to meet both so-called parts, because they not only adjust the pore sizes for better
filtering, but also exhibit strong electrostatic interactions with adsorbates in the
pores.[11]
In this work, we report a novel charged MOF with N-contained tetracarboxylate
ligand (H4L =9-(3,5-dicarboxyphenyl)carbazole-3,6-dicarboxylic acid) and Indium
(III) ions formulated as [InL][(CH3)2NH2] and its selective adsorption and separation

performance for CO2 over CH4 and N2.
2. Experimental section
2.1 General
All chemicals applied in the syntheses were commercial available and used without
1
further purification. HNMR spectra were recorded on a Bruker Advance DMX500
(500 MHz) spectrometer using tetramethysilane (TMS) as an internal standard. Powder
X-ray diffraction (PXRD) was performed on a Rigaku Dmax/2400 X-ray
diffractometer operating at 40 kV and 100 mA, using Cu-Kɑ radiation (λ=1.5406 Å).
Thermo gravimetric analysis (TGA) was carried out under nitrogen atmosphere on a
Q600 SDT TGA-DTA-DSC thermal analyzer from room temperature to 800 ºC with a
heating rate of 10 ºC min-1. Gas adsorption measurements were carried out with
QUANTACHROME AUTOSORB-iQ gas adsorption analyzer. The N2 sorption
isotherms for surface area calculation were collected in the pressure range from 0.01 to
0.99 P/P0 at 77 K in a liquid nitrogen bath. CO2, CH4 and N2 adsorption isotherms were
collected in the pressure range from 0.01 to 0.99 P/P0 at 273K in a circulator with
temperature controller with a mixture of 50 v% water and 50 v% ethylene glycol.
2.2. Synthesis of the ligand
Synthesis of the ligand (H4L) was started with 5-(amino)isophthalic acid (1’) and
followed the reaction routes shown below. The detailed synthesis procedures are
1
provided in the Supporting Information. H NMR (500 MHz, DMSO-d6), [ppm]:
13.23-13.25(m, 3H), 8.99(t, 3H), 8.65(t, 1H), 8.36(d, 2H), 8.10-8.12(q, 2H),
7.48-7.50(d, 2H). 13C{1H} NMR (125 MHz, DMSO-d6), [ppm]: 196.97, 165.68, 143.31,
136.39, 133.63, 131.32, 130.57, 129.55, 127.09, 122.97, 122.82, 109.75. Anal. Calcd
for C22H13NO8: C, 63.01; H, 3.12; N, 3.34 %. Found: C, 63.40; H, 3.13; N, 3.62 %.
( Insert Scheme 1)

2.3. Synthesis of [InL][(CH3)2NH2] (1)
A mixture of H4L (0.012 g, 0.03 mmol), In(NO3)3·4.5 H2O (0.015 g, 0.053 mmol) in
10 mL of N,N’-dimethylmethanamide (DMF) and 2 mL of deionized water was placed
in a 25 mL Teflon-lined steel autoclave, and the mixture was stirred at room
temperature until the solid was completely dissolved. The autoclave was sealed, heated
up to 160 °C and kept at this temperature for 4 days. Block-shaped yellow crystals were
obtained. The product was filtered out, washed by DMF, soaked in CHCl3 for 2 days,
and then dried under the vacuum. Yield: 29% (based on H4L). IR (cm–1): 3430s,
2918m, 1660m, 1465m, 1393s, 1280m, 1228w, 1099m, 1022m, 812w, 777m, 750w,
719w, 668w, 536w. W:weak, vs=very strong, s=strong, m=medium, sh=shoulder,
br=broad. It is noted that the peaks at 3430 and 1469 cm-1 correspond to the stretching
vibration of N–H and C–N bonds in DMA+, respectively, which further demonstrates
its existence inside the framework. [11]
2.4. Single-crystal X-ray crystallography
The crystal data were collected on Agilent SuperNava single-crystal X-ray
diffractometer at room temperature with Cu-Kα radiation (λ = 1.54 Å). The structures
were solved by direct methods using the SHELXS program of the SHELXTL package
and refined by full-matrix least-squares methods with SHELXL [12]. The metal atoms
in each complex were located from the E-maps and other non-hydrogen atoms were
located in successive difference Fourier syntheses, which were refined with anisotropic
thermal parameters on F2. The hydrogen atoms of the ligands were generated
theoretically onto the specific atoms and refined isotropically with fixed thermal factors.
Solvent molecules in the structure were randomly dispersed, and thus their positions
were impossible to refine using conventional discrete-atom models. The crystal and
refinement data are collected in Table 1. Selective bond distances and angles are
given in Table S1, (Supporting Information).
(Insert Table 1)

3. Results and discussion
3.1. Structural Description
X-Ray crystallographic analysis indicates that 1 crystallizes in an orthogonal
P212121 space group, consisting of anionic [InL]- framework and protonated
dimethylamine (DMA+) in 1:1 stoichiometric ratio. The host framework is composed
of an independent In(III) coordinated to a tetragonal L ligand in an asymmetric unit.
The L ligand has a disturbed tetrahedronal configuration with the four carboxylate
groups on the vertexes and N atom locating in the center (Fig. 1a). Each carboxylate
group of L ligand exhibits a unique ŋ2 coordination mode. The In(III) center adopts an
8-coordinated mode to four carboxylate groups from separate L ligands with the In-O
bond lengths falling into the range of 2.227-2.335 Å (Fig. 1b). The anionic framework
leads a three-dimensional (3D) network with two types of irregular quadrangular
one-dimensional channels with pore size less than 1 nm, respectively. The
corresponding protonated DMA+ are trapped in the molecular space between two
channels (Fig. 1c). Notably, the adjacent carbazole rings of two L ligand lack any π-π
stacking interactions. The solvent-accessible volume in 1 is 2916.3 Å3 that constitutes
as high as 59.8% volume per unit cell calculated by PLATON routine,[13] indicating
its significant porous nature.
To better understand such a complicated structure, the topology of the anion
framework of 1 was studied by using Topos 4.0 software.[14] As shown in Fig. 1d,
the In(III) center and L ligand are both regarded as four-connected nodes, and
therefore 1 leads to a 3D 4-c network with point symbol {66}, which is a classic unc
topological type based on the search from RCSR database (http://rcsr.net/nets).[14a]
(Insert Fig. 1)
3.2. Thermal stability
The thermal stability of 1 was studied by thermogravimetric analysis (TGA). As
shown in Fig. 2, two steps of weight losses were observed in the TGA curve. The first

weight loss step of 1 starts from room temperature and does not end until 120 °C. The
corresponding weight loss should be attributed to the loss of guest CHCl3 molecules
after solvent exchanged with DMF. From 120 to 400 °C, the TGA curve of 1 passes
through a slight decline, which could be attributed to the partial removal of the
decomposed DMA+ product with temperature.[11a] The second weight loss step from
400 °C to 800 °C is probably caused by the decomposition of organic ligand.
(Insert Fig. 2)
3.3. Gas adsorption properties
The presence of well-defined channels and the retention of the overall framework
structure after removal of the lattice solvent molecules make the complex a potential
candidate for gas storage. To account for the porosity of 1, N2 adsorption
measurements were performed at 77 K on the sample that dried under a dynamic
vacuum after soaked in CHCl3. Before the gas adsorption, the as-synthesized 1 was
soaked in CHCl3 for 2 days to exchange the nonvolatile DMF solvents included in the
channels, and then was further filtered and dried under a dynamic vacuum to remove
CHCl3 molecules. The PXRD patterns of as-synthesized sample and the vacuum-dried
sample are in agreement with the simulated pattern based on the crystal structure,
indicating that the host framework of 1 remains crystallinity and integrity of the overall
framework in both samples (Fig. S4). The observed slight differences in intensity could
be due to preferred orientation of the powder samples. Unfortunately, the specific
surface area of activated samples of 1 is calculated to be only 20 m2g-1 using the
Brunauer–Emmett–Teller (BET) model based on N2 sorption isotherm. The possible
reason is that the protonated DMA+ cations are highly disordered in the channels and
prevent the entrance of N2 molecules. We further studied the sorption behavior of CO2,
CH4 and N2 at 273 K. As shown in Fig. 3, the largest uptake amount of CO2 is 32.8 cm3
g-1 at 1 atm, while those of CH4 (7.2 cm3 g-1) and N2 (2.2 cm3 g-1) are quite low under
the same conditions.

(Insert Fig. 3)
The selectivities of CO2 over CH4 and N2 at 273 K were further calculated by the
Henry’s law analysis of isotherm data. For an equimolar binary mixture of CO2/CH4
and CO2/N2, the separation factor SCO2/CH4 and SCO2/N2 are 24.97 and 105.09,
respectively, which are quite close to those calculated by ideal adsorbed solution theory
(IAST) method (Fig. S11). It is known that the kinetic diameters of CH4 (3.38 Å) and
N2 (3.64 Å) is larger than CO2 (3.30 Å).[15] Notedly, the pore size suggests that
molecular sieve effect could play an important role when considering the selectivity
of CO2 over other gases.[16] In addition, the existence of DMA+ enhances the
interaction between the host framework and CO2 molecules. In contrast, the near-linear
adsorption isotherms of CH4 and N2 are indicative of their low affinity to the charged
framework, which can be expected from their relatively low polarizability.
4. Conclusions
In summary, a novel 3D charged metal–organic framework has been synthesized and
exhibits selective adsorption ability of CO2 over CH4 and N2. The anionic framework
with the inside DMA+ counterion could have the synergistic effect of pore size effect
and the enhanced adsorbent–adsorbate interaction aroused by a strong electrostatic
field during the CO2 adsorption. Therefore the charged metal-organic framework can
act as an efficient sorbent for selective CO2 separation from CO2/CH4 and CO2/N2
mixtures.
Acknowledgements
We gratefully acknowledge the financial supported from the NSFC (No. 21203249
and No. 11375271) and the Science Foundation of China University of Petroleum,
Beijing (No. 01JB0229).
Appendix A. Supplementary material

Detailed synthesis procedure of the ligand, crystal structure data, PXRD patterns,
N2 isotherm, selectivity calculation method are provided. Crystallographic data for the
structural analysis have been deposited to the Cambridge Crystallographic Data Centre,
CCDC No. 1415547 for 1. The 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 1. Crystallographic Data and Structural Refinement Summary for 1.
Formula
C24H17InN2O8
576.22
Formula weight
T (K)
293(2)
Crystal system
space group
a (Å)
Orthogonal
P212121
12.7580(9)
17.5203(11)
21.8129(11)
4875.7(5)
90
b (Å)
c (Å)
V (Å3)
α (deg)
β (deg)
90
γ (deg)
90
Z
4
ρ (mg/m3)
µ (mm-1)
0.785
0.510
F (000)
1152
Range of h, k, l
Total/independent
reflections
Parameters
R indices [I > 2σ(I)]
Goodness-of-fit on F2
Residuals (e Å-3)
-10/15,-20/13,-26/20
11239 / 7756
313
0.0831, 0.2360
1.064
0.889/-1.283
2
2 2
2 2 1/2
∑||
∑
∑
R1= Fo|- |Fc||/|Fo|. wR2=[ w(Fo - Fc ) / w(Fo ) ] .

Scheme and Figure captions
Scheme 1 Synthesis of H4L. Reagents and conditions: (a) SOCl2, EtOH, 0°C; (b)
NaNO3, HCl, H2O, KI, 0-5°C; (c) K2CO3, Cu, carbazole, 220°C; (d) AlCl3,
CH3COCl, CH2Cl2, 0°C; (e) 1,4-dioxane, NaOH, Br2, 0°C.
Fig. 1 Single crystal structure of 1: (a) tetragonal L ligand, (b) four-coordinated In(III)
center, (c) the 3D InL- porous anionic framework with DMA+ cations, and (d)
topological structure. Hydrogen atoms are omitted for clarity.
Fig. 2 TGA curve of the CHCl3-exchanged sample.
Fig. 3 CO2, CH4 and N2 sorption isotherms of 1 at 273 K.

H3CH2COOC
COOCH2CH3
H3CH2COOC
COOCH2CH3
HOOC
COOH
b
a
I
NH2
NH2
3'
1'
2'
c
H3CH2COOC
COOCH2CH3
H3CH2COOC
COOCH2CH3
HOOC
COOH
e
d
N
N
N
HOOC
COOH
O
O
H4L
5'
4'
Scheme 1

Fig. 1

Fig. 2

35
30
25
20
15
10
5
CO
2
CH
4
N
2
0
0.0
0.2
0.4
0.6
0.8
1.0
p/p
0
Fig. 3

Highlights:
ꢀ
ꢀ
A three-dimensional microporous charged metal-organic framework is reported.
The MOF shows much higher uptake of CO2 than those of CH4 and N2 at 273K
and 1atm.
ꢀ
The separation factor of CO2/N2 and CO2/CH4 of 1 could reach 24.97 and 105.09,
respectively.

Graphical Abstract
A charged metal-organic framework for CO2/CH4 and CO2/N2 separation
Ruiqin Zhong, Zhilong Xu, Wenzhu Bi, Xiaofeng Yu, Ruqiang Zou*
A novel three-dimensional charged metal-organic framework has been synthesized, which
exhibits high selectivities of CO2 over CH4 and N2.