Polar Ketone-Functionalized Metal-Organic Framework Showing a High CO2 Adsorption Performance

Article abstract of DOI:10.1021/acs.inorgchem.6b02660

The incorporation of various functionalities into porous metal-organic frameworks (MOFs) represents an efficacious strategy to improving their gas adsorption properties. In this work, a carbonylated tetracarboxylic acid ligand (5,5′-carbonyldiisophthalic acid) was synthesized, and a ketone-functionalized MOF with exposed metal sites based on this ligand was formed successfully. Structural analysis reveals that the new MOF possesses channels decorated by the carbonyl groups and rhombicuboctahedral cages, with open Cu^II sites pointing toward the cage center. The framework exhibits exceptionally high CO₂ (46.7 wt % at 273 K and 1 bar) and H₂ (2.8 wt % at 77 K and 1 bar) uptake. Furthermore, it displays high selectivities of CO₂ adsorption over N₂ and CH₄ at 298 K.

Full text of DOI:10.1021/acs.inorgchem.6b02660

Communication
pubs.acs.org/IC
Polar Ketone-Functionalized Metal−Organic Framework Showing a
High CO2 Adsorption Performance
Genfeng Feng,† Yuxin Peng,† Wei Liu,‡ Feifan Chang,† Yafei Dai,‡ and Wei Huang*,†
†State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, Jiangsu Province 210093, P. R. China
‡School of Physics Science & Technology and Jiangsu Key Laboratory for NSLSCS, Nanjing Normal University, Nanjing, Jiangsu
Province 210023, P. R. China
S
* Supporting Information
amine, hydroxyl, acylamide, nitryl, cyano, halide, and sulfo
groups.1b,10
ABSTRACT: The incorporation of various functionalities
into porous metal−organic frameworks (MOFs) repre-
In one of our previous reports, polar PO bonds were
sents an efficacious strategy to improving their gas
adsorption properties. In this work, a carbonylated
tetracarboxylic acid ligand (5,5′-carbonyldiisophthalic
acid) was synthesized, and a ketone-functionalized MOF
with exposed metal sites based on this ligand was formed
successfully. Structural analysis reveals that the new MOF
possesses channels decorated by the carbonyl groups and
rhombicuboctahedral cages, with open CuII sites pointing
toward the cage center. The framework exhibits excep-
tionally high CO2 (46.7 wt % at 273 K and 1 bar) and H2
(2.8 wt % at 77 K and 1 bar) uptake. Furthermore, it
displays high selectivities of CO2 adsorption over N2 and
CH4 at 298 K.
successfully inserted into a [2 + 3] organic molecular cage
displaying selective CO2 capture.11 As a part of the acylamide
group, the polar carbonyl group (CO), however, is paid less
attention.12 In this work, a rigid tetracarboxylic acid ligand, 5,5′-
carbonyldiisophthalic acid with an inserted polar CO unit
(H4cdip in Scheme 1), is selected to fabricate the corresponding
Scheme 1. Strategy of Polar Ketone Incorporation
MOFs, where the polarity and rigidity of the H4mdip ligand13 is
increased by replacing the sp3 methylene moiety with a polar sp2
CO group. As a result, a three-dimensional (3D) ketone-
functionalized Cu-MOF 1 is obtained with open Cu2(COO)4
sites, and it shows anticipated high CO2 (46.7 wt % at 273 K and
1 bar) and H2 (2.8 wt % at 77 K and 1 bar) uptake as well as high
selectivities for CO2 over N2 and CH4 at room temperature. The
presence of channels decorated by the carbonyl groups and
rhombicuboctahedral cages, with open CuII sites pointing toward
the cage center, is believed to play important roles in
exceptionally high CO2 and H2 adsorption.
Crystallographic analysis reveals that the Cu-MOF 1 exhibits a
3D framework constructed by dinuclear paddle-wheel
Cu2(COO)4 secondary building units (SBUs) and organic
cdip4− linkers. In every dinuclear SBU (Figure 1a), the
coordination geometry for each CuII ion is described as a five-
coordinate pyramid with the same τ value of zero.14 The apical
Cu−O bond length [2.115(9)−2.148(8) Å] is significantly
longer than the other four Cu−O bond lengths in the basal plane
[1.951(3)−1.956(4) Å], exhibiting a typical Jahn−Teller
distortion. The organic cdip4− linkers are found to adopt two
types of configurations with different ketone-related C−C−C
bond angles [115.1(5) and 101.4(5)°] and dihedral angles
[62.1(5) and 76.0(5)°] between two phenyl rings (Figures S6
he combustion of fossil fuels has caused serious environ-
T
mental problems because CO2 is one of the major
greenhouse gases and anthropogenic CO2 emissions are
accelerating the greenhouse effect. Metal−organic frameworks
(MOFs) could serve as promising candidates for CO2 adsorption
and separation.1 MOFs as porous materials have experienced
rapid growth in chemistry and materials science during the past 2
decades for promising applications in gas storage,2 separation,3
heterogeneous catalysis,4 sensoring,5 and drug delivery.6
Structure−function relationships have been well-established in
the design process of MOFs.7 The synthesis of crystalline MOFs
with remarkable structural tunability is achieved by altering either
inorganic metal ions/clusters or organic linkers. In the gas
adsorption and separation fields of MOFs, great attention has
been paid toward improving the CO2 adsorption capacity of
elaborately designed MOFs. To date, several strategies have been
developed to tune the size, shape, and polarity of the pores within
a MOF because these factors play significant roles in the CO2
adsorption performance.8 Among them, creating open metal
sites is used as an effective approach to constructing MOFs with
improved CO2 adsorption properties because exposed metal
cations as strong Lewis acids are highly polarizing and have
strong interactions with guest CO2 molecules.1b,9 On the other
hand, the introduction of polar functional groups provides
enhanced affinity to CO2 molecules, and hence this strategy has
been widely adopted in the building of versatile MOFs, including
Received: November 5, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02660
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Inorganic Chemistry
Communication
Figure 1. (a) SBU in the Cu-MOF 1 with the removal of axial H2O
molecules. (b) Rhombicuboctahedral cage, with the CuII ions pointing
toward the center in the Cu-MOF 1. (c) Perspective view of the cages
and channels formed in the 3D framework.
Figure 2. (a) N2 sorption isotherms at 77 K for the Cu-MOF 1 (inset:
pore-size distributions). (b) H2 sorption isotherms at 77 K for the Cu-
MOF 1. CO2 (red), CH4 (blue), and N2 (black) sorption isotherms for
the Cu-MOF 1 at 273 (c) and 298 K (d) (filled symbols, adsorption;
open symbols, desorption).
and S7). Overall, the structural porosity of the Cu-MOF 1 is
mainly attributed to the formation of rhombicuboctahedral cages
and peripheral channels among those cages (Figures 1c and S8).
Isophthalate moieties as 24 corners constitute a rhombicubocta-
hedral cage with a 6.0 Å radius taking into account the van der
Waals radii of the atoms (Figures 1b and S9). On the other hand,
dinuclear paddle-wheel Cu2(COO)4 SBUs occupy 12 quad-
rangular faces, and the CuII ions point straight toward the cage
center after removal of the axial aqua ligands. As a result, two
kinds of windows are found in every rhombicuboctahedral cage
involving 6 quadrangular and 8 triangular faces. The former has a
radius of 3.8 Å, while the latter has a 2.0 Å radius, omitting the van
der Waals radii of the atoms (Figure S10). As for the peripheral
channels, they are decorated by the uncoordinated polar
carbonyl groups with a radius of ∼2.5 Å (Figure S13). It is
suggested that such porosity could afford beneficial circum-
stances for more favorable gas access. The total solvent-accessible
volume of the Cu-MOF 1 is calculated to be 14083 Å3 using the
SOLV function of PLATON, which occupies approximately
65.3% of the volume of the whole structure.
Å (inset of Figure 2a), which is close to that observed in the
single-crystal structure. Such a feature clearly manifests the
microporous nature of the Cu-MOF 1.
The unique ketone-functionalized framework with open metal
sites encouraged us to further evaluate its absorbent potential.
The volumetric H2 sorption isotherms for the Cu-MOF 1 were
recorded up to 1 bar at 77 K (Figure 2b), which exhibit good
reversibility with a high uptake capacity of 2.8 wt % (315.2 cm3
g−1). The H2 uptake of the Cu-MOF 1 is lower than that of PCN-
12 (3.05 wt %)13 but is comparable and even significantly higher
than that of many well-known MOFs under the same conditions,
such as SNU-5 (2.87 wt %),15 PCN-14 (2.7 wt %),16 IRMOF-1
(1.32 wt %)17 and ZIF-8 (1.27 wt %)18 (Table S2). Our results
herein demonstrate good potential storage applications for the
attractive hydrogen energy.
The phase purity of the Cu-MOF 1 was confirmed by powder
X-ray diffraction (PXRD) studies, where the PXRD pattern of
the solvated sample is identical with the simulated one (Figure
S14). Thermogravimetric analysis of the freshly prepared Cu-
MOF 1 reveals a weight loss of about 33.85% from 20 to 320 °C,
corresponding to the loss of guest solvent molecules (Figure
S16). Further variable-temperature PXRD patterns suggest that
the Cu-MOF 1 can remain unchangeable as high as 300 °C
(Figure S15), indicative of the high thermal stability of this
sample. In addition, desolvation of the Cu-MOF 1 was
performed by soaking the sample in methanol for solvent
exchange, followed by heating at 110 °C and 10−10 bar for 24 h,
and the PXRD analysis for a fully desolvated framework indicates
that the porous material still retains its crystallinity (Figure S14).
The permanent porosity of the activated Cu-MOF 1 was
confirmed by gas adsorption measurements. As shown in Figure
2a, the Cu-MOF 1 displays representative type I sorption
behavior with a saturated uptake of 522.1 cm3 g−1 for N2 at 77 K
and 1 bar. The Langmuir and Brunauer−Emmett−Teller surface
areas are calculated to be 2259 and 1539 m2 g−1 based on N2
adsorption at 77 K, respectively. The pore volume of the Cu-
MOF 1 is 0.81 cm3 g−1. The pore-size distribution analysis based
on the N2 isotherms at 77 K by using the Horvath−Kawazoe
method reveals that the pore size of the Cu-MOF 1 is ca. 5.0−6.0
The successful construction of special dual-functional sites in
the Cu-MOF 1 also prompts us to explore its potential gas
sorption properties. Low-pressure adsorption isotherms of CO2,
CH4, and N2 were carried out at 273 and 298 K up to 1 bar,
respectively. Our results imply that none of them displays
hysteresis upon adsorption and desorption. As illustrated in
Figure 2c,d, the Cu-MOF 1 has high CO2 uptake capacities of
238.1 cm3 g−1 (46.7 wt %) at 273 K and 111.3 cm3 g−1 (21.8 wt
%) at 298 K. The former value at 273 K is among the highest
reported valuess with excellent performance under the same
experimental conditions (Table S3), such as Mg-MOF-74 (44.8
wt %),19 Cu-TDPAT (44.5 wt %),20 and Cu-TPBTM (42.6 wt
%).21 The latter value at 298 K is also impressive because there
are few MOFs exhibiting CO2 uptake of more than 20.0 wt %
under ambient conditions.22 More interestingly, only small
amounts of CH4 and N2 uptake can be observed at 273 and 298 K
and 1 bar compared with the high mass of CO2 uptake, indicating
that the Cu-MOF 1 could selectively adsorb CO2 over CH4 and
N2. To further confirm the separation ability of the Cu-MOF 1,
ideal adsorbed solution theory was applied to calculate the CO2/
N2 and CO2/CH4 selectivities at room temperature with 15:85
and 50:50 molar ratios. As shown in Figure S19, the selectivities
for CO2 over CH4 and N2 are in the ranges of 4.9−5.4 and 14.7−
19.7, respectively, and the calculated values are comparable with
B
DOI: 10.1021/acs.inorgchem.6b02660
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
ORCID
some MOFs modified by other polar groups including NOTT-
125 with oxamide, ZJNU-40 with benzothiadiazole, and ZJU-8a
with amino.23
Wei Huang: 0000-0002-1071-1055
Notes
Isosteric heat of adsorption (Qst) was calculated to reveal the
CO2 adsorption strength with the Cu-MOF 1 by analyzing the
isotherms at 273 and 298 K. An initial Qst value of 31.3 kJ mol−1
was obtained, and then it decreases slowly to 29.7 kJ mol−1 at 111
cm3 g−1 coverage (Figure S20). Qst of the Cu-MOF 1 at zero
coverage is moderately high, reflecting strong interactions
between the porous material and CO2, and it is comparable
with MOFs functionalized by some other polar groups, such as
Cu-TPBTM (26.3 kJ mol−1)21 with acylamide, NOTT-125 (25.4
kJ mol−1)23a with oxamide, and IRMOF-3 (19 kJ mol−1)24 with
amine. It is suggested that the strong interactions between CO2
and the Cu-MOF 1 are ascribed to the presence of open metal
sites and coordination-free ketone moieties within the frame-
work. In the rhombicuboctahedral cage of the Cu-MOF 1, the
open CuII sites are found to point toward the center of the cage
cavity, which is believed to be more favorable for selective CO2
adsorption.9,13 On the other hand, the channels with accessible
space decorated by the carbonyl groups are regarded as an
additional beneficial factor for CO2 adsorption. Some support
from first-principles calculations has been found to elucidate
these speculations (Figure S21). The binding energy of CO2
adsorbed around the ketone group is −9.34 kJ mol−1, which is
very comparable with that of an open CuII metal site (−9.99 kJ
mol−1). In contrast, if the ketone group is removed, the CO2
binding energy is calculated to be only −1.27 kJ mol−1. These
comparisons imply that both open CuII metal sites and ketone
groups can act as important roles in CO2 adsorption of the Cu-
MOF 1.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Major State Basic
Research Development Programs (Grant 2013CB922101), the
National Natural Science Foundation of China (Grant
21171088), the Project of Scientific and Technological Support
Program in Jiangsu Province (Grant BE2014147-2), and
Program B for Outstanding Ph.D. Candidate of Nanjing
University.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02660.
Synthetic details and characterization data (PDF)
X-ray crystallographic data in CIF format for CCDC
1514521 CIF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: whuang@nju.edu.cn.
C
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