Fine-tuning pore size by shifting coordination sites of ligands and surface polarization of metal-organic frameworks to sharply enhance the selectivity for CO2

Article abstract of DOI:10.1021/ja309992a

Based upon the (3,6)-connected metal-organic framework {Cu(L1) ·2H₂O·1.5DMF}_∞ (L1 = 5-(pyridin-4-yl)isophthalic acid) (SYSU, for Sun Yat-Sen University), iso-reticular {Cu(L2)·DMF}_∞ (L2 = 5-(pyridin-3-yl) isophthalic acid)

Full text of DOI:10.1021/ja309992a

Communication
pubs.acs.org/JACS
Fine-Tuning Pore Size by Shifting Coordination Sites of Ligands and
Surface Polarization of Metal−Organic Frameworks To Sharply
Enhance the Selectivity for CO2
Liting Du,§ Zhiyong Lu,§ Kaiyuan Zheng, Junyi Wang, Xin Zheng, Yi Pan, Xiaozeng You, and Junfeng Bai*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, China
S
* Supporting Information
narrowing by interpenetration;6 and (3) altering the size of the
ligands in those MOFs with iso-reticular structures.7 However,
these strategies generally lead to a decline in surface area and
CO2 uptake capacity. Very recently, we directly inserted
acylamide functional groups into several MOFs and realized a
balance between large storage capacity and high selectivity for
CO2.8
ABSTRACT: Based upon the (3,6)-connected metal−
organic framework {Cu(L1)·2H2O·1.5DMF}∞ (L1 = 5-
(pyridin-4-yl)isophthalic acid) (SYSU, for Sun Yat-Sen
University), iso-reticular {Cu(L2)·DMF}∞ (L2 = 5-
(pyridin-3-yl)isophthalic acid) (NJU-Bai7; NJU-Bai for
Nanjing University Bai group) and {Cu(L3)·DMF·H2O}∞
(L3 = 5-(pyrimidin-5-yl)isophthalic acid) (NJU-Bai8)
were designed by shifting the coordination sites of ligands
to fine-tune pore size and polarizing the inner surface with
uncoordinated nitrogen atoms, respectively, with almost
no changes in surface area or porosity. Compared with
those of the prototype SYSU, both the adsorption
enthalpy and selectivity of CO2 for NJU-Bai7 and NJU-
Bai8 have been greatly enhanced, which makes NJU-Bai7
and NJU-Bai8 good candidates for postcombustion CO2
capture. Notably, the CO2 adsorption enthalpy of NJU-
Bai7 is the highest reported so far among the MOFs
without any polarizing functional groups or open metal
sites. Meanwhile, NJU-Bai8 exhibits high uptake of CO2
and good CO2/CH4 selectivity at high pressure, which are
quite valuable characteristics in the purification of natural
gases.
Herein, we expand our work by using a (3,6)-connected
structure {Cu(L1)·2H2O·1.5DMF}∞ (L1 = 5-(pyridin-4-yl)
isophthalic acid) (SYSU, for Sun Yat-Sen University) as a new
platform that contains no open metal sites and offers low
energy cost in potential industrial applications.7a Based on this
structure, we synthesized iso-reticular {Cu(L2)·DMF}∞ (L2 =
5-(pyridin-3-yl) isophthalic acid) (NJU-Bai7; NJU-Bai for
Nanjing University Bai group) and {Cu(L3)·DMF·H2O}∞
(L3 = 5-(pyrimidin-5-yl) isophthalic acid) (NJU-Bai8) by a
new strategy: shifting the coordination sites of the ligands and
polarizing the inner environment with uncoordinated nitrogen
atoms, respectively. Compared with prototype SYSU, NJU-Bai7
and NJU-Bai8 showed sharply improved CO2 adsorption
enthalpy and selectivity for CO2 over N2 and CH4 at about 0.15
bar (enthalpy of 28.2 kJ/mol compared to 40.5 and 37.7 kJ/
mol and selectivity of 5.1/4.7 compared to 97.1/14.1 and
111.3/40.8 at 273 K and low pressure), without changes in
topology, inner surface area, or porosity. Remarkably, the CO2
adsorption enthalpy of NJU-Bai7 is the highest reported so far
among MOFs with no polarizing functional groups or open
metal sites. Meanwhile, NJU-Bai8 maintains the balance
between high selectivity for CO2 and large capacity for CO2
storage, with the value of 220 v/v at 298 K and 20 bar.
SYSU was first reported by Su’s group,9a as shown in Figure
1a. The whole structure is topologically a (3,6)-connected net,
he past two decades have witnessed the rapid develop-
T
ment of metal−organic frameworks (MOFs) due to their
fantastic structures and various potential applications, such as in
strategic storage and separation of gases, catalysis, and chemical
sensing.1 Most interestingly, compared with traditional porous
materials (zeolite, active carbon, etc.), many MOFs can be used
as platforms, such as MOF-5, ZIFs, HKUST-1, etc., and their
structures can be further modified systematically toward higher
performance.2
with point (Schlafli) symbol of (4·6 ) (4 ·6 ·8 ). There exist
̈
2
2
2
10
3
wide channels of 6.3 × 6.3 Å2 along the a axis (defined by the
diameters of the inserted interior contact atoms). All Cu-
paddlewheel unsaturated metal sites are occupied by pyridyls
from the ligands, resulting in no specific active sites for the
adsorption of CO2 in the channels. This MOF offers an
opportunity to tune the pore size through the rotation of
pyridine rings along the C4−C7 bonds. Thus, we substituted
pyridin-4-yl with pyridin-3-yl and obtained NJU-Bai7 with the
same topology as SYSU. As shown in Figure 1f, shifting the
Selective capture of CO2 is of paramount importance for the
purification of natural gases and the reduction of CO2 emission.
A proper material that offers high selectivity for and uptake of
CO2, especially at relatively low pressures (∼0.15 bar, the
partial pressure of CO2 in postcombustion flue gases)3a and at
pressures from 0 to 17 bar (the partial pressure in natural gas
streams),4a and requires minimal energetic input to liberate the
captured CO2, is essential to make the process practical. MOFs
are of great interest as new materials in this respect.3 Several
strategies have been introduced to improve the selective
adsorption of CO2: (1) rational decorations in the pore with
nitrogen bases or some other polarizing groups;5 (2) pore size
Received: October 14, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja309992a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 2. (a) Separated channels of NJU-Bai7 viewed along the a axis.
(b) A dumbbell-like channel of NJU-Bai8 viewed along the a axis.
(c,d) Ligands for NJU-Bai7 and NJU-Bai8. (e,f) Coordination modes
of NJU-Bai7 and NJU-Bai8, respectively.
Figure 1. (a) A channel of SYSU viewed along the a axis. (b)
Separated channels of NJU-Bai7 viewed along the a axis. (c,d) Ligands
for SYSU and NJU-Bai7. (e,f) Coordination modes of SYSU and NJU-
Bai7, respectively.
coordination sites leads to structure distortion. In planes
perpendicular to the a axis (Figure 1b), neighboring Cu-
paddlewheels rotate in opposite directions, such that all
pyridine rings stretch into the channel aligned along the a
direction, dividing the channel into two equal parts. The size of
the resulting channel is about 3.4 × 3.4 Å2, which is quite close
to the kinetic diameter of carbon dioxide (3.3 Å). To further
polarize the inner surface, uncoordinated nitrogen atoms were
introduced, to give NJU-Bai8 (Figure 2). The pyrimidine rings
make that section of the channel look like a dumbbell, with the
size of the bell heads about 4 × 3.3 Å2. The structure of NJU-
Bai8 is a little different from that of NJU-Bai7, but after being
soaked in acetone for a few hours, its PXRD pattern became
almost the same as that of NJU-Bai7 (see Supporting
Information, Figure S7d).
The permanent porosity of SYSU, NJU-Bai7, and NJU-Bai8
is confirmed by N2 sorption isotherms at 77 K (Figure S6). The
type-I isotherms indicate that they are microporous materials.
The Brunauer−Emmett−Teller (BET) surface area and
Langmuir surface area of SYSU are calculated to be 1100 and
1216 m2/g, while those of NJU-Bai7 are about 1155 and 1238
m2/g, and those of NJU-Bai8 are 1103 and 1196 m2/g. The
total pore volumes of SYSU, NJU-Bai7, and NJU-Bai8 are
estimated to be 0.436, 0.439, and 0.431 cm3/g, respectively.
These values demonstrate that the surface area and porosity of
NJU-Bai7 and NJU-Bai8 are almost unchanged after the
channels are narrowed and polarized.
To systematically evaluate the effect of narrowing channels
and the inner environment on adsorption of gases, the ability of
SYSU, NJU-Bai7, and NJU-Bai8 to adsorb CO2, CH4, and N2
was investigated at 273 and 298 K. At 298 K, as shown in
Figure 3a, NJU-Bai7 and NJU-Bai8 perform much better than
SYSU (8.0 wt% for NJU-Bai7 and 5.4 wt% for NJU-Bai8 vs 3.6
Figure 3. (a) CO2 adsorption isotherms for SYSU, NJU-Bai7, and
NJU-Bai8 at 298 K. (b) CO2 adsorption enthalpy of SYSU, NJU-Bai7,
and NJU-Bai8.
wt% for SYSU) at a relatively low pressure (∼0.15 bar). The
CO2 uptake for NJU-Bai7 at 298 K and 0.15 bar is larger than
those of many well-known MOFs with open metal sites or
functional groups, such as bio-MOF-119b (5.4 wt%), ZIF-785c
(3.3 wt%), MIL-53(Al)9e (3.1 wt%), en-Cu-BTTri5a (2.3 wt%),
and MOF-17710b (0.6 wt%). The obvious enhancement from
SYSU to NJU-Bai7 and NJU-Bai8 at low pressure could be
mainly attributed to the narrow channels created in NJU-Bai7
and NJU-Bai8, which facilitate the interaction between CO2
and the channel walls.9b This interaction is further confirmed
by the adsorption enthalpies of CO2 for these three MOFs
(Figure 3b). The zero-coverage adsorption enthalpies of CO2 in
SYSU, NJU-Bai7, and NJU-Bai8 are 28.2, 40.5, and 37.7 kJ/
B
dx.doi.org/10.1021/ja309992a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
mol, respectively. Notably, the CO2 adsorption enthalpy of
NJU-Bai7 is the highest reported so far among the MOFs with
no polarizing functional groups or open metal sites,10c and is
quite comparable to those of famous compounds with open
metal sites such as Mg/DOBDC3c and MIL-1019d (47 and 44
kJ/mol, respectively).
The separation ratios of CO2 versus CH4 and N2 were
calculated from the ratio of the initial slopes based on the
isotherms,3d,5c,9b as listed in Table 1. Compared with SYSU, the
Figure S12. After activation, both NJU-Bai7 and NJU-Bai8
contract, but to a larger extent for NJU-Bai8 (Figure S7d). As a
result, the uncoordinated nitrogen atoms in NJU-Bai8 are
blocked. Thus, the CO2 uptake amount at low pressure and
zero-coverage adsorption enthalpy of CO2 for NJU-Bai8 are a
bit lower than those for NJU-Bai7. As the pressure increases,
the channels in NJU-Bai8 extend to reveal uncoordinated
nitrogen atoms that can easily capture CO2 molecules, causing a
steeper increase in isotherms than for NJU-Bai7. Although
theoretical studies have predicted that uncoordinated nitrogen
atoms in five- or six-membered heteroarene rings could
strengthen the adsorption of CO2,11 and some research results
have shown this effect in five-membered heteroarene
rings,4b,11c,d to our best knowledge, this is the first experimental
substantiation of the theoretical conclusions on six-membered
heteroarene rings. Besides, varying the inner environment
caused a repulsive effect that resulted in highly unfavorable
conditions for nonpolar CH4 molecules.12 As a synergistic
result, NJU-Bai8 exhibits good CO2/CH4 selectivity and large
CO2 uptake capacity.
Table 1. Porosities, CO2 Enthalpies, and CO2/N2 and CO2/
CH4 Selectivities of SYSU, NJU-Bai7, and NJU-Bai8 at 273 K
BET
pore volume
(cm3 g‑1)
−Qst,n=0
CO2/
N2
CO2/
CH4
MOF
SYSU
(m2 g−1
)
(kJ mol−1
)
1100
1155
1103
0.436
0.439
0.431
28.2
40.5
37.7
25.5
97.1
4.7
14.1
40.8
NJU-Bai7
NJU-Bai8
111.3
separation ratios of CO2/N2 and CO2/CH4 for NJU-Bai7 are
significantly improved from 25.5 to 97.1 and from 4.7 to 14.1
(almost quadruple). The separated channels in NJU-Bai7 show
a pore size effect preference for CO2 over N2 and CH4, as CO2
possesses a smaller kinetic diameter (3.3 Å, <3.64 Å for N2,
<3.8 Å for CH4). To the best of our knowledge, this is the first
example of enhancing the selectivity for CO2 to such a
significant extent by shifting coordination sites without
changing the inner surface areas or porosities. Combining this
selectivity with the high CO2 uptake at relatively low pressure,
NJU-Bai7 may be a good candidate for carbon capture from
industrial postcombustion flue gases. Furthermore, NJU-Bai8
shows an even higher selectivity for CO2, with separation ratios
of 111.3 for CO2/N2 and 40.8 for CO2/CH4. These values are
larger than those of CAU-13d (CO2/N2, 101; CO2/CH4, 28)
and bio-MOF-119b (CO2/N2, 81) under similar conditions.
Meanwhile, the separation ratios of CO2/N2 and CO2/CH4 for
NJU-Bai8 at 298 K are 58.3 and 15.9 (see Supporting
Information, Table S3), which are higher than those for some
well-known MOFs, such as ZIF-785c (CO2/N2, 50.1; CO2/
CH4, 10.6), Mg-MOF-743c,9c (CO2/N2, 49), and en-Cu-
BTTri5a,10c (CO2/N2, 44). Making it even more attractive,
NJU-Bai8 shows a high saturated CO2 uptake amount of 220 v/
v at 20 bar and 298 K, which is quite comparable to those of
some leading MOFs for CO2 storage (Figure S15 and Table
S2). Obviously, from the type-IV-like CO2 isotherms with
hysteresis at 273 and 298 K, a gate-opening process10d exists in
both NJU-Bai7 and NJU-Bai8, and the gate-opening pressure
for NJU-Bai8 (4 bar at 273 K and 7 bar at 298 K) is much
lower than that for NJU-Bai7 (10 bar at 273 K and no gate
opening observed at 298 K), which indicates a greater
mutability of NJU-Bai8. In contrast to NJU-Bai7, with NJU-
Bai8 the saturated CO2 uptake amount increases while the CH4
amount decreases at both 273 and 298 K. Thus, by qualitative
evaluation, the selectivity of NJU-Bai8 for CO2/CH4 in the high
pressure range is also much higher than that of NJU-Bai7. The
high selectivity of CO2 and large capacity for CO2 storage may
make NJU-Bai8 a versatile material in methane purification and
carbon capture.
In summary, based on a platform of (3,6)-connected MOF
SYSU, we successfully synthesized iso-reticular NJU-Bai7 and
NJU-Bai8 by a new strategy: shifting the coordination sites of
ligands and polarizing the inner environment with uncoordi-
nated nitrogen atoms, respectively. Compared with the
prototype structure, both NJU-Bai7 and NJU-Bai8 exhibit
much higher adsorption enthalpies, uptake amounts, and
selectivities for CO2 at low pressure, which make them
excellent candidates for postcombustion carbon capture.
Additionally, NJU-Bai8 may be a good material for purification
of natural gases because of its high uptake of CO2 and good
selectivity for CO2 over CH4 at high pressure. Our work
presents a new strategy for fine-tuning pore size, and this
strategy may provide a new way to finely tune the size and
properties of MOFs, thus facilitating further exploration of
more amazing MOFs with high CO2 selectivity and uptake
capacity.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures, crystallographic tables, PXRD data,
additional gas adsorption isotherms, and details of the isosteric
adsorption enthalpy. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
bjunfeng@nju.edu.cn
Author Contributions
§L.D. and Z.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Major State Basic Research Development
Programs (2011CB808704), the NSFC (20931004), the
Science Foundation of Innovative Research Team of NSFC
(20721002), and the Fundamental Research Funds for the
Central Universities (1114020501) for support of this work.
The discrepancies in CO2 adsorption behavior mentioned
above between NJU-Bai7 and NJU-Bai8 could mainly be
attributed to their structural differences. Uncoordinated
nitrogen atoms in NJU-Bai8 make the whole structure more
flexible, which can be confirmed by the VT-PXRD patterns in
C
dx.doi.org/10.1021/ja309992a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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Zhou, H. C. Chem. Soc. Rev. 2009, 38, 1477.
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