High CO2/N2 and CO2/CH4 selectivity in a chiral metal-organic framework with contracted pores and multiple functionalities

Article abstract of DOI:10.1039/c4cc00334a

A metal-organic framework with chiral pores and decorated imidazole and methyl groups was constructed. Upon activation, this MOF undergoes a pore contraction and shows very high selectivity for CO₂/N₂ and CO₂/CH₄, indicating a synergistic effect of dynamic pores and functional groups. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00334a

ChemComm
View Article Online
View Journal
COMMUNICATION
High CO2/N2 and CO2/CH4 selectivity in a chiral
metal–organic framework with contracted pores
and multiple functionalities†
Cite this:DOI: 10.1039/c4cc00334a
Received 15th January 2014,
Accepted 2nd April 2014
Xiaoxia Lv,ab Liangjun Li,*a Sifu Tang,b Chao Wangb and Xuebo Zhao*ab
DOI: 10.1039/c4cc00334a
www.rsc.org/chemcomm
A metal–organic framework with chiral pores and decorated imidazole them very attractive in the fields of molecular recognition and
and methyl groups was constructed. Upon activation, this MOF selective gas sorption. MIL-53 is the most famous MOF that
undergoes a pore contraction and shows very high selectivity for exhibits dynamic behaviours towards CO2 and water molecules.9
CO2/N2 and CO2/CH4, indicating a synergistic effect of dynamic C. J. Sumby et al. synthesized a MOF with ultrahigh CO2/N2
pores and functional groups.
selectivity induced by the contraction of the rhombus pores.10
Based on these materials, the contributions of structural transition
The selective capture of CO2 from flue gas and natural gas has to gas sorption selectivity were elucidated. However, the combina-
become the most urgent issue driven by the concerns of climate tional effect of pore contraction/expansion and multiple function-
change and demands in natural gas upgrading.1 As an emerging alities on CO2 selectivity, to the best of our knowledge, has not been
type of crystalline porous material, metal–organic frameworks reported yet. It is expected to be possible to further enhance the
(MOFs) have been proven to be very promising materials for selectivity for CO2 under the synergism of these factors.
the selective capture of CO2 due to their high surface area and
In this paper, a new MOF (compound 1) with incorporated
tuneable pore environments.2 To achieve better selectivity, current imidazole and methyl functional groups in 1D rectangular
efforts have been devoted to increasing the binding energy of gas channels was synthesized by using a rigid imidazole-carboxylate
molecules with frameworks by introducing a high density of ligand, 4,40-(5,6-dimethyl-1H-benzo[d]imidazole-4,7-diyl)-dibenzoic
open metal sites3 or by anchoring Lewis basic sites.4 Moreover, acid (H3L), and Zn(II) ions. Upon activation, a structural transition
theoretical and experimental studies revealed that CO2 selectivity occurred and a new structure with contracted pores was generated
in MOFs could also be enhanced by introducing functional due to the high flexibility of the coordination spheres.11 Sub-
groups, such as methyl5 or hydroxyl groups,6 by increasing the sequent to structural characterization, the single-component
field potential of the pores. While the conventional strategies gas sorption properties and ultrahigh gas selectivities towards
are focusing on tuning the pore surface with homogeneous CO2/N2 and CO2/CH4 on the activated sample were studied.
functional groups, anchoring mixed or multiple functional
Colorless columnar crystals of [Zn(L)ꢀH2O]ꢀDMA (1) were
groups in a single MOF would also be intriguing.7 These groups obtained via a solvothermal reaction of H3L and Zn(NO3)2ꢀ6H2O
could function cooperatively when assembled in a single MOF, in a DMA–H2O (1/1) solution at 100 1C for 48 hours (see experi-
which could have some benefits in improving the CO2 selectivity. mental details in the ESI,† S1). Single crystal X-ray analysis reveals
Recently, a type of MOF with flexible networks has attracted that 1 crystallizes in the chiral orthorhombic space group P212121.
extensive interest due to its framework expansion or contrac- The carboxylate groups in this ligand are coordinated by two Zn(II)
tion, triggered by an external stimulus.8 The intriguing property ions in a monodentate form, while one of the nitrogen atoms in
of responding distinctly to different guest molecules makes the imidazole moiety is deprotonated and coordinated by another
Zn(II) atom. The self-assembly of ligands with the tetrahedrally
coordinated Zn(II) ions finally gives a three-dimensional network,
with a series of parallel one-dimensional rectangular channels
(1.1 ꢁ 2.1 nm) running along the a axis (as shown in Fig. 1b). The
methyl groups and the uncoordinated imidazole nitrogen atoms
are fully exposed in the pores, ensuring their interactions with gas
molecules during sorption. By simplifying the ligands and metal
centres into T-shaped linkers and three-connected nodes, the
a Institute of Unconventional Hydrocarbon and New Energy Sources, China
University of Petroleum (East China), Qingdao 266580, China.
E-mail: liliangjun1982@hotmail.com
b Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao, China. E-mail: zhaoxb@qibebt.ac.cn; Fax: +86 0532-80662728
† Electronic supplementary information (ESI) available: Experimental details,
crystallographic data, structure representation, IR and TGA diagrams, calculation
details about Qst and IAST selectivity are provided. CCDC 963443. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc00334a structure of 1 can be viewed as a network composed of the smaller
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.

View Article Online
Communication
ChemComm
Fig. 2 Isotherms at different temperatures for (a) N2 and (b) CO2 (solid
symbols: adsorption; hollow symbols: desorption).
temperatures (195 K or room temperature), the diffusion resis-
tance could be overcome due to the additional thermal energy of
the N2 molecules. At 195 K, N2 adsorption follows a three-step
process: subsequent to a very obscure adsorption at low pressure
(0–0.26 bar), a steep increase is observed on the isotherm,
followed by another significant increase in N2 uptake at 10 bar,
and finally a N2 uptake of 3.5 wt% is reached. The desorption
branch does not trace the adsorption isotherm and therefore leads
Fig. 1 (a) Representation of the coordination modes and the simplified
structure of the ligand; (b) the channels in 1 viewed from the a axis; (c) the
topological network of 1; (d) a simplified structure of the L-helix channel;
(e) a simplified structure of the R-helix chain.
R-helix rectangular chain surrounded by four larger L-helix to a large hysteresis loop. The stepwise hysteresis in N2 isotherm
channels, as shown in Fig. 1c. The combination of the L-helix indicates pore expansion when the N2 molecules get into the pores
channels and the R-helix chains gives rise to a network with a of 1a at 195 K. Interestingly, at 273 K the adsorption–desorption
rare SrSi2 topology.12 Finally, the 1D rectangular channels with curves are reversible with virtually no hysteresis and the uptake of
L-helix homochirality are observed when considering the van N2 is decreased compared to 195 K due to thermodynamic reasons.
der Waals radius. The guest accessible volume of 1 is 45.2%,
CO2 isotherms at 195 K (Fig. S9, ESI†) and room temperature
calculated from the crystallographic data.
(Fig. 2b) show smooth type-I hysteresis isotherms, with the
The good match between the powder X-ray diffraction (PXRD) degree of hysteresis decreasing with increasing temperatures.
patterns of the as-synthesized sample and the simulated pattern The smooth hysteresis isotherm for CO2 is rare in the literature, and
from the crystallographic data confirms the iso-structure and purity may be attributed to the small pore windows and pore expansion in
of the bulk sample (as shown in the ESI,† Fig. S7). In order to fully this MOF, which is also observed in [Cu(bcppm)H2O]-ac.10 At higher
activate the sample, the as-synthesized MOF was solvent exchanged temperatures, the thermal vibrations of the framework and gas
with acetone in a Soxhlet extractor for 48 h, followed by heating at molecules are intensified and lead to lower molecular retention, and
353 K for 12 h under ultrahigh vacuum (10ꢂ7 mbar). The shift of the thus give the smaller hysteresis loops observed in the CO2 isotherms
peaks in the PXRD pattern of the activated sample indicates that the at higher temperatures. The adsorption capacity of CO2 on 1a is
framework of this MOF has undergone a structural transition from 0.93 mmol gꢂ1 at 273 K and 1 bar, which is relatively low compared
its original structure to a new phase (termed 1a). However, this kind to those MOFs with high adsorption capacities,14 but is consistent
of transition is reversible, as indicated by the structural transition with the limited surface area observed in 1a. Using the CO2 isotherm
from 1a to the original phase of 1 when soaking the activated MOF at 273 K in the pressure range of 0–20 bar, the pore volume and pore
in DMA–H2O (1/1, v/v) solution for 24 hours. After several cycles of width of the 1a can be determined as 0.0558 cm3 gꢂ1 and 0.42 nm,
adsorption and desorption, the structure and crystallinity of this respectively, based on the Dubinin–Radushkevich (DR) analysis15
MOF remain unchanged. Furthermore, the structure of this MOF is (see details in the ESI,† S4). The pore volume and pore width
well retained when exposing the activated MOF to moisture or values of 1a are largely reduced compared to the as-synthesized
soaking the activated MOF in water for 24 hours, suggesting good MOF, supporting the conclusion that the pores of this MOF
structural and water stability of this MOF (see the PXRD patterns have contracted upon activation. A CH4 isotherm measured at
in the ESI,† Fig. S8).
room temperature shows similar isotherms to CO2 except for
To investigate the gas sorption properties of 1a, N2, CO2 and lower uptakes and less obvious hysteresis loops (as shown in
CH4 sorption measurements were conducted on an Intelligent the ESI,† S4), indicating the weaker interactions of the CH4
Gravimetric Analyzer (IGA-001). As shown in Fig. S9 (ESI†), molecules with the framework.
nearly no uptake of N2 is observed at 77 K, but a significant
The abundant imidazole and methyl groups pending in the
amount of N2 is adsorbed at 195 K (3.5 wt%) and 273 K (1.5 wt%) pores and the difference in the structural response towards
(see Fig. 2a). These results indicate that the as-synthesized CO2, CH4 and N2 encouraged us to investigate the CO2 selectivity
sample has undergone a contraction upon the solvent exchange over CH4 and N2. To predict CO2/CH4 and CO2/N2 binary mixture
and evacuation. The pore size of 1a may be close to the kinetic selectivity, an ideal adsorbed solution theory (IAST) calculation,
diameter of N2 (3.64 Å),13 and the large diffusional resistances coupled with a dual-site Langmuir–Freundlich simulation,
led to the non-adsorption of N2 at 77 K. However, at higher was employed on the basis of single-component isotherms
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
was employed for the analysis of CO2/CH4 and CO2/N2 adsorption
separation on 1a. As shown in Fig. 3c and d, the experimental
mixed gas isotherms for CO2/CH4 (yCO = 0.429) and CO2/N2
2
( yCO = 0.433) gas mixtures are consistent with the mixed gas
2
isotherms predicated by IAST calculations at the same feed
compositions and separation conditions (as shown in Fig. 3c
and d). This indicates the good reliability of the results of the IAST
calculations and the high CO2/CH4 and CO2/N2 selectivity of this
MOF. The minor differences between experimental and predi-
cated isotherms could be attributed to the non-ideality of the gas
mixtures deviating from an ideal solution.
The high values of CO2 selectivity over CH4 and N2 could be
rationalized by the high adsorption enthalpy (Qst) for CO2 but
moderate values for CH4 on 1a (see Qst plots in the ESI,† S6). The
Qst at zero surface coverage (Qst(n=0)) for CO2 is 43.7 ꢃ 3.1 kJ molꢂ1
which is higher than many common MOFs and similar to those
MOFs with Lewis basic groups or open metal sites,3b,19 indicating
the strong interaction strength of CO2 molecules with the frame-
work. In contrast, CH4 shows a much weaker interaction strength
with the framework, with a Qst(n=0) value of 16.4 ꢃ 1.3 kJ molꢂ1
which is within the normal range of Qst of CH4 adsorption on
porous materials.22 The combinational effects of imidazole and
methyl functional groups coupled with specific confined pores in
1a may attribute to the large initial interaction strength of CO2 with
the frameworks, but these factors have much less effect on CH4 or
Fig. 3 (a) Selectivities of CO2 over CH4 and N2 predicted by IAST calcula-
tions at 273 K. (b) Selectivities of CO2 over CH4 and N2 as a function of gas
phase compositions at 1 bar and 273 K predicated by IAST calculations. (c)
Comparison between the mixed gas isotherms predicated by IAST calcu-
lations (hollow) (yCO = 0.429) and experimental mixed gas isotherms
2
(solid) (yCO = 0.429) for CO2/CH4. (d) Comparison between the mixed gas
2
isotherms predicated by IAST calculations (hollow) (yCO = 0.433) and
2
experimental mixed gas isotherms (solid) for CO2/N2 (yCO = 0.433).
2
(see details in the ESI,† S5). It has been verified by many studies N2 molecules due to their nonpolar moments. Therefore, the
that the IAST model can predict gas mixture adsorption accu- synergistic effects of both surface chemistry and different structural
rately on many zeolites and MOF materials.16 Fig. 3a shows the transitions for different gas molecules could account for the very
predicated selectivities for CO2/CH4 and CO2/N2 as a function of high selectivities for CO2/CH4 and CO2/N2 at low pressures. At
pressure when the gas phase mole fractions are 5/95 and 15/85, higher pressures, the Qst of CO2 gradually decreases with increasing
which are the typical feed compositions for natural gas and flue loading, indicating that the preferable adsorption sites are occu-
gas, respectively. It is worth noticing that very high selectivities pied gradually. However, the gas selectivities for CO2/CH4 and CO2/
for both the CO2/CH4 and CO2/N2 are obtained. The maximum N2 lie in the range of 30–151 and 51–252, respectively, which are
values of selectivities for CO2/CH4 and CO2/N2 are 513 and 867, still very high selectivity values in MOFs.
respectively, at low pressure (0.1 bar). At 1 bar, the corresponding
In summary, a 3D MOF with 1D rectangular chiral channels,
values are 151 and 252, respectively, which are significantly decorated with imidazole and methyl functional groups, was
higher than those MOFs with a homogenous amine17 or a methyl synthesized. PXRD patterns combined with gas adsorption con-
functional group18 and are comparable to the MOFs with highest firmed the pore contraction and the following pore expansion when
selectivity reported in the literature.6a,16b When the pressure is accommodating guest molecules. The different extents of hysteresis
increased, the selectivities decrease. When the pressure is 20 bar, in the isotherms of CO2, CH4 and N2 indicate the different responses
the selectivities for CO2/CH4 and CO2/N2 are 30 and 51, respec- of the framework towards different guest molecules. The synergistic
tively. These results suggest that 1a has very high selectivity for effects of imidazole and methyl functional groups coupled with
CO2/CH4 and CO2/N2 and could be an excellent candidate for the contracted pores enable this MOF to have a very high selectivity for
selective sorption of CO2 from natural gas and flue gas under CO2/CH4 and CO2/N2, suggesting a promising candidate for selective
variable working conditions.
CO2 capture from flue gas and purification of natural gas.
This work was supported by grants from the National Natural
Moreover, the CO2/CH4 and CO2/N2 selectivities as a func-
tion of feed composition at 1 bar and 273 K were investigated Science Foundation of China (NSFC) (grant No. 21073216 and
(as shown in Fig. 3b). Both the CO2/CH4 and CO2/N2 selectivities 21173246) and the ‘‘Hundred-Talent Project’’ (KJCX2-YW-W34)
increase significantly with the increasing gas phase mole fractions of the Chinese Academy of Sciences.
of CH4 and N2. In the case of yCH = 0.5, the selectivity for CO2/CH4
4
is 52. The selectivity for CO2/N2 is 134 at 1 bar when yN = 0.5.
These values are in the top ranks of selectivities for CO2/CH4 and
2
Notes and references
CO2/N2 in MOF materials.19 To investigate the separation perfor-
mance of this MOF on real CO2/CH4 and CO2/N2 gas mixtures,
a gravimetric sorption experiment, a technique established by
Van Ness20 and validated by many gas separation experiments,21
1 (a) J. R. Li, J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869;
(b) S. Xiang, Y. He, Z. Zhang, H. Wu, W. Zhou, R. Krishna and
B. Chen, Nat. Commun., 2012, 3, 954.
2 (a) J. Liu, P. K. Thallapally, B. P. McGrail, D. R. Brown and J. Liu,
Chem. Soc. Rev., 2012, 41, 2308–2322; (b) P. K. Thallapally,
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.

View Article Online
Communication
ChemComm
R. K. Motkuri, C. A. Fernandez, B. P. McGrail and G. S. Behrooz, 11 C. A. Fernandez, P. K. Thallapally, R. K. Motkuri, S. K. Nune,
Inorg. Chem., 2010, 49, 4909–4915. J. C. Sumrak, J. Tian and J. Liu, Cryst. Growth Des., 2010, 10, 1037–1039.
3 (a) S. R. Caskey, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 12 Y.-G. Huang, B. Mu, P. M. Schoenecker, C. G. Carson, J. R. Karra,
2008, 130, 10870–10871; (b) D. Britt, H. Furukawa, B. Wang, Y. Cai and K. S. Walton, Angew. Chem., Int. Ed., 2011, 50, 436–440.
T. G. Glover and O. M. Yaghi, Proc. Natl. Acad. Sci. U. S. A., 2009, 13 Y.-S. Bae, O. K. Farha, J. T. Hupp and R. Q. Snurr, J. Mater. Chem.,
106, 20637–20640. 2009, 19, 2131–2134.
4 (a) X. Si, C. Jiao, F. Li, J. Zhang, S. Wang, S. Liu, Z. Li, L. Sun, F. Xu, 14 (a) P. L. Llewellyn, S. Bourrelly, C. Serre, A. Vimont, M. Daturi,
Z. Gabelica and C. Schick, Energy Environ. Sci., 2011, 4, 4522–4527;
(b) L. Li, S. Tang, C. Wang, X. Lv, M. Jiang, H. Wu and X. Zhao, Chem.
Commun., 2014, 50, 2304–2307.
L. Hamon, G. De Weireld, J.-S. Chang, D.-Y. Hong, Y. Kyu Hwang,
´
S. Hwa Jhung and G. r. Ferey, Langmuir, 2008, 24, 7245–7250;
(b) L. Li, S. Tang, X. Lv, M. Jiang, C. Wang and X. Zhao, New
J. Chem., 2013, 37, 3662–3670.
5 (a) H. Liu, Y. Zhao, Z. Zhang, N. Nijem, Y. J. Chabal, H. Zeng and
J. Li, Adv. Funct. Mater., 2011, 21, 4754–4762; (b) Y. T. Huang, 15 (a) J. Garrido, A. Linares-Solano, J. M. Martin-Martinez, M. Molina-Sabio,
W. P. Qin, Z. Li and Y. W. Li, Dalton Trans., 2012, 41, 9283–9285.
6 (a) H.-L. Jiang, D. Feng, T.-F. Liu, J.-R. Li and H.-C. Zhou, J. Am.
Chem. Soc., 2012, 134, 14690–14693; (b) Z. X. Chen, S. C. Xiang,
H. D. Arman, J. U. Monda, P. Li, D. Y. Zhao and B. L. Chen, Inorg.
F. Rodriguez-Reinoso and R. Torregrosa, Langmuir, 1987, 3, 76–81;
´
˜
(b) D. Cazorla-Amoros, J. Alcaniz-Monge and A. Linares-Solano, Langmuir,
´
´
1996, 12, 2820–2824; (c) D. Lozano-Castello, D. Cazorla-Amoros and
A. Linares-Solano, Carbon, 2004, 42, 1233–1242.
Chem., 2011, 50, 3442–3446; (c) Z. X. Chen, S. C. Xiang, H. D. Arman, 16 (a) Y.-S. Bae, K. L. Mulfort, H. Frost, P. Ryan, S. Punnathanam,
P. Li, D. Y. Zhao and B. L. Chen, Eur. J. Inorg. Chem., 2011,
2227–2231.
7 X. Kong, H. Deng, F. Yan, J. Kim, J. A. Swisher, B. Smit, O. M. Yaghi
and J. A. Reimer, Science, 2013, 341, 882–885.
L. J. Broadbelt, J. T. Hupp and R. Q. Snurr, Langmuir, 2008, 24,
8592–8598; (b) P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns,
R. Luebke, K. Forrest, T. Pham, S. Ma, B. Space, L. Wojtas, M. Eddaoudi
and M. J. Zaworotko, Nature, 2013, 495, 80–84.
8 (a) Y. Sakata, S. Furukawa, M. Kondo, K. Hirai, N. Horike, 17 J. Duan, Z. Yang, J. Bai, B. Zheng, Y. Li and S. Li, Chem. Commun.,
Y. Takashima, H. Uehara, N. Louvain, M. Meilikhov, T. Tsuruoka, 2012, 48, 3058–3060.
S. Isoda, W. Kosaka, O. Sakata and S. Kitagawa, Science, 2013, 339, 18 H. Jasuja and K. S. Walton, J. Phys. Chem. C, 2013, 117, 7062–7068.
193–196; (b) S. Yang, L. Liu, J. Sun, K. M. Thomas, A. J. Davies, 19 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch,
M. W. George, A. J. Blake, A. H. Hill, A. N. Fitch, C. C. Tang and
Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724–781.
¨
M. Schroder, J. Am. Chem. Soc., 2013, 135, 4954–4957; 20 H. C. Van Ness, Ind. Eng. Chem. Fundam., 1969, 8, 464–473.
(c) P. K. Thallapally, J. Tian, M. Radha Kishan, C. A. Fernandez, 21 (a) A. L. Myers, C. Minka and D. Y. Ou, AIChE J., 1982, 28, 97–102;
S. J. Dalgarno, P. B. McGrail, J. E. Warren and J. L. Atwood, J. Am.
Chem. Soc., 2008, 130, 16842–16843.
9 S. Bourrelly, P. L. Llewellyn, C. Serre, F. Millange, T. Loiseau and
(b) F. Karavias and A. L. Myers, Chem. Eng. Sci., 1992, 47, 1441–1451;
(c) R. Van Der Vaart, C. Huiskes, H. Bosch and T. Reith, Adsorption,
ˆ
2000, 6, 311–323; (d) L. Hamon, E. Jolimaıtre and G. D. Pirngruber,
´
G. Ferey, J. Am. Chem. Soc., 2005, 127, 13519–13521.
Ind. Eng. Chem. Res., 2010, 49, 7497–7503.
10 W. M. Bloch, R. Babarao, M. R. Hill, C. J. Doonan and C. J. Sumby, 22 T. A. Makal, J.-R. Li, W. Lu and H.-C. Zhou, Chem. Soc. Rev., 2012, 41,
J. Am. Chem. Soc., 2013, 135, 10441–10448.
7761–7779.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014