Preferential Adsorption of CO2 in an Ultramicroporous MOF with Cavities Lined by Basic Groups and Open-Metal Sites

Article abstract of DOI:10.1021/acs.inorgchem.8b00304

Here, we present a new ultramicroporous Cu₂ paddlewheel based MOF. This ultramicroporous MOF has most of the features such as porosity (BET surface area = 945 m²/g), CO₂ capacity (3.5 mmol/g at ambient temperature and pressure), CO₂/N₂ selectivity (sCO₂/N₂ = 250), and fast CO₂ diffusion kinetics (D_c = 2.25 × 10^-9 m²/s), comparable to some of the other high-performing ultramicroporous MOFs, with strong binding sites. Typically, such MOFs exhibit strong CO₂-framework interactions (evidenced from a heat of adsorption ≥ 38 kJ/mol). However, the MOF explained here, despite having channels lined by the amine and the open-metal sites, possesses only a moderate CO₂-framework interaction (HOA = 26 kJ/mol). Using periodic DFT, we have probed this counterintuitive observation.

Full text of DOI:10.1021/acs.inorgchem.8b00304

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Preferential Adsorption of CO2 in an Ultramicroporous MOF with
Cavities Lined by Basic Groups and Open-Metal Sites
Shyamapada Nandi,† Rahul Maity,† Debanjan Chakraborty,† Hemkalyan Ballav,†
and Ramanathan Vaidhyanathan*,†,‡
†Department of Chemistry, Indian Institute of Science Education and Research, Dr. Homi Bhabha Rd., Pashan, Pune, 411008, India
‡Centre for Energy Science, Indian Institute of Science Education and Research, Dr. Homi Bhabha Rd., Pashan, Pune, 411008, India
S
* Supporting Information
ABSTRACT: Here, we present a new ultramicroporous Cu2 paddle-
wheel based MOF. This ultramicroporous MOF has most of the features
such as porosity (BET surface area = 945 m2/g), CO2 capacity (3.5
mmol/g at ambient temperature and pressure), CO2/N2 selectivity
(sCO2/N2 = 250), and fast CO2 diffusion kinetics (Dc = 2.25 × 10−9 m2/
s), comparable to some of the other high-performing ultramicroporous
MOFs, with strong binding sites. Typically, such MOFs exhibit strong
CO2−framework interactions (evidenced from a heat of adsorption ≥ 38
kJ/mol). However, the MOF explained here, despite having channels
lined by the amine and the open-metal sites, possesses only a moderate
CO2−framework interaction (HOA = 26 kJ/mol). Using periodic DFT,
we have probed this counterintuitive observation.
energetic framework−CO2 interactions are amine−CO2
interaction,8c,12a−c open-metal−CO2 interaction,12d,e aromatic
π cloud−CO2 interaction,11b,12f and organic functional group−
CO2 interaction.12g,h Understanding the adsorption sites of
high-performing MOFs is vital for the future design of superior
sorbents. De facto, MOFs provide an excellent crystalline
platform for understanding such critical gas−solid interactions.
Herein, we report a 2D Cu based ultramicroporous MOF
(IISERP-MOF20) built from 2-(4-carboxyphenyl)-1H-benzo-
[d]imidazole-5-carboxylate. It shows moderate CO2 uptake as
well as CO2/N2 selectivity for flue gas compositions at room
temperature. Also, this MOF exhibits an optimal Heat of
Adsorption (HOA) for CO2 which further ensures the low
regeneration energy demand. However, the observed HOA is
quite low considering that the channels are lined with both
basic groups and bare metal sites, which are known to interact
strongly with CO2. To investigate this oddity, we simulated the
different binding sites in the MOF using Grand Canonical
Monte Carlo (GCMC) methods. Notably, some unique CO2−
metal interactions are identified, which deviate from the
previously observed ones.
INTRODUCTION
■
Carbon dioxide (CO2) is a key heat-trapping gas responsible
for global warming through the greenhouse effect.1 Capturing
CO2 from industrial sources will have marked impact on their
global concentrations.2 Pre and postcombustion CO2 capture
are two efficient technologies for such large-scale captures.3
They are carried out using pressure and temperature swing
adsorption (P/TSA) methods,4 which prove to be extremely
energy efficient.5,6 Typically, such capture processes employ
solid sorbents which physisorb CO2.6a However, implementa-
tion of these large-scale CO2 capture technology requires
substantial cost reduction. This can be achieved by modifying
the engineering aspects of the process. But more prospects lie
in developing superior sorbents.6b
Current industrial scrubbers are zeolites with high molecular-
sieving capability.7 In this regard, Metal−Organic Frameworks
(MOFs) have attracted significant attention due to their high
structural tunability, CO2 capacity, selectivity, and faster
kinetics as well as low regeneration/parasitic energy.8−11
MOFs can be chemically manipulated to adsorb CO2 selectively
at low partial pressures, a property that is critical for the
postcombustion CO2 capture. Ultramicroporous MOFs (UM-
MOFs) (pore size < 6 Å) built from rigid short linkers with
basic character are suitable for selective CO2 capture. The high
selectivity arises from enhanced framework−CO2 and cooper-
ative CO2···CO2 interactions.8c Such molecular level inter-
actions define the optimal heat of adsorption, which gets
expressed as the ease of CO2 regeneration.11c Enormous efforts
are being invested in identifying and optimizing the frame-
work−CO2 interactions in MOFs.12 Some of the most
EXPERIMENTAL SECTION
■
The organic linker, with basic character and metal-binding carboxylate
groups, was synthesized via a simple condensation reaction between
aldehyde and 1,2-diamine (Scheme 1). The dicarboxylic acid, 2-(4-
carboxyphenyl)-1H-benzo[d]imidazole-5-carboxylic acid (LH2) was
synthesized by reacting 4-formyl benzoic acid and 3,4-diaminobenzoic
Received: February 3, 2018
© XXXX American Chemical Society
A
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Scheme 1. Schematic Representation of the Synthesis of 2-(4-Carboxyphenyl)-1H-benzo[d]imidazole-5-carboxylic Acid
acid in the presence of acetic acid at 60 °C for 12 h. The LH2 was
obtained as a yellowish off-white powder in over 80% yield.
Cu(L)(H2O)·(DMF)1.5(EtOH)1.5, IISERP-MOF20, was synthe-
sized via a solvothermal reaction between copper nitrate and 2-(4-
carboxyphenyl)-1H-benzo[d]imidazole-5-carboxylic acid in a solution
containing dimethylformamide, ethanol, and water at 80 °C for 36 h.
The product, blue-colored square-shaped platy crystals, was obtained
in ∼74% yield (based on Cu).
RESULTS AND DISCUSSION
■
Single crystal X-ray structure analysis reveals that IISERP-
MOF20 crystallizes in an orthorhombic system (a =
20.8093(11) Å; b = 9.7634(5) Å; c = 25.2583(11) Å; Sp. Gr.
Cmca). The structure consists of typical Cu2 paddlewheel units,
where the Cu-Cu dimers get connected by four different 2-(4-
carboxyphenyl)-1H-benzo[d]imidazole-5-carboxylate ligands
(Figure S5). Each Cu is penta-coordinated. Among the five
coordinations, four are satisfied by oxygen atoms of the
carboxylate unit and the fifth one is satisfied by a water
molecule. These Cu2 paddlewheel nodes are extended by four
ligand units into a 2D layer along the ac plane with large near
square-shaped apertures (16.21 × 16.21 Å, shown with green
lines in Figures 1 and S7). The adjacent layers are shifted by
half-a-unit cell along the a and c directions and are rotated by
90°, giving a staggered ABAB... arrangement (Figure S8). This
positions the five-membered heteroatomic rings of the
Figure 1. (A) Three-dimensional structure of the IISERP-MOF20
formed by the ABAB... stacking of the Cu2 paddlewheel based layers.
The green line is a guide for the eye to see the large square-shaped
benzimidazole unit from the adjacent layer right at the edges
of the square aperture and the Cu2 paddlewheel from the
adjacent layers above and below the center of the square
aperture formed in each layer. Yellow and maroon circles show the
aperture in the middle layer. Now, for each square shown (in
nodes present in the structure. (B) The column of interlayer N−H···N
green) in Figure 1, there are four N-containing five-membered
rings at the edges which hydrogen bond above and below the
plane, thus forming a column of N−H···N hydrogen bonds
(N···H−N = 2.0137 Å). These are vital in holding the 2D layers
together into a stable 3D framework. Also, the Cu2
paddlewheels from alternate layers run as a column along the
b direction. The half-unit cell shift in the adjacent layers thus
compartmentalizes the large aperture in each paddlewheel
derived layer into four smaller pores, creating a rhombic
channel of dimensions 7.63 × 7.63 Å (not factoring the van der
Waals radii, Figure 1 and Figure S7).
In fact, the overall 3D structure is held together by the
covalent bonds between metal and the ligand in the ac plane
and by the hydrogen bond interactions running along the
perpendicular b direction. Removal of the coordinated water
molecules generates two open-metal sites at each paddlewheel
unit, and the open-metal sites from the adjacent A-A layers in
the ABAB stacking line up at a distance of 7.36(3) Å. A
PLATON analysis suggests a 48.7% potential solvent accessible
void present in the structure. A squeeze refinement drastically
improves the R-factor, indicating the presence of disordered
solvent molecules. Unfortunately, a successful modeling of
these electron densities as solvents was not possible. However,
we have assigned the electron counts from the squeeze analysis
to 1.5 DMF and 1.5 EtOH molecules per unit cell, and this
hydrogen bonds between the nitrogens of the five-membered rings of
the benzimidazole units and the column of Cu2 paddlewheels is shown
(Cu···Cu = 7.36 Å).
matches well with the solvent composition calculated from the
TGA analysis.
For a practical large-scale industrial CO2 capture process, the
sorbents are required in tonnes. Hence, even in a laboratory
scale, it is worthwhile to demonstrate the MOF synthesis in
grams. In this regard, making MOFs using a single metal and a
single ligand can be advantageous (Scheme S1). Here, the
IISERP-MOF20 was synthesized in gram scale by merely
scaling up the constituents from the mg scale synthesis, and the
purity was confirmed via Powder X-ray Diffraction (PXRD)
analysis as presented in Figures 2A and S9−S10. From TGA,
the MOF was found to be stable up to 260 °C (Figure S12).
The N2 adsorption at 77 K confirms the permanent porosity of
the MOF (Figure 2B). A Brunauer−Emmett−Teller (BET) fit
obtained using the adsorption branch of the 77 K N2 isotherm
yields a surface area of 945 m2/g (Figure S14), which is quite
high for an ultramicroporous MOF.8c,f,11a Nonlocalized Density
Functional Theory (NLDFT) fit yields a pore diameter of 5.85
Å, which matches extremely well with the pore size (inset,
Figure 2B and Figure S15). Notably, this ultramicroporous
MOF adsorbs a good amount of CO2 (3.5 mmol/g) at 1 bar
B
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Figure 2. (A) A Pawley fit to the PXRD of the desolvated sample showing the bulk purity (the as-synthesized sample shows additional solvent
sensitive peaks; see Figure S9). (B) The 77 K N2 isotherm of IISERP-MOF20. The inset shows the pore size distribution. (C) CO2 sorption
isotherms of IISERP-MOF20 at different temperatures. (D) CO2/N2 selectivity of IISERP-MOF20 calculated using IAST model employing a
nominal composition of 15CO2:85N2.
and 298 K (Figure 2C) and has a saturation CO2 capacity of 9.0
mmol/g (195 K CO2 isotherm). However, it shows less N2
uptake at room temperature. The Ideal Adsorption Solution
Theory (IAST) calculation performed using the isotherms
collected at 273 and 298 K, employing a nominal composition
of 15CO2:85N2, yields a CO2/N2 selectivity of 220 and 250,
respectively (Figures 2D and S16). Although the selectivity
values are not as high as those of the recently reported UM-
MOFs,8f,11a these are sufficient to achieve the benchmarked
99% purity during separation.13
The HOA calculated using both virial and DFT models
employing the 298, 283, 273, and 263 K CO2 isotherms was 26
kJ/mol at zero loading. At higher loadings, the HOA reaches
values of 20−22 kJ/mol (Figures 3A and S17). Such moderate
HOA (25−35 kJ/mol) suggests facile regeneration.11c A cycling
experiment further confirmed this.
be 2.25 × 10−9 m2/s (Figure S19). This diffusion coefficient is
comparable to what was observed for the recently reported 1D
porous Ni-PyC MOF11a and is higher than that observed in
some of the other MOFs with similar or larger pores such as
MOF-5 (1.17 × 10−9 m2/s) or MOF-177 (1.17 × 10−9 m2/s).14
It is worth mentioning that the Dc observed for IISERP-
MOF20 is at least 2 orders of magnitude higher than that for
zeolite-13X,14c the current industrial CO2 scrubber.
Considering that the activated MOF consists of appropriately
oriented open-metal sites and basic secondary amine groups,
the observed moderate HOAs were puzzling. We have used
simulations to probe this. For this, the number of CO2
molecules (saturation capacity from 195 K adsorption) for a
2 × 2 × 2 cell was calculated to be 200. These many CO2’s
were allowed to diffuse into the cell freely. From which, the
most probable positions for CO2 inside the structure were
identified using the simulated annealing method embedded in
the Materials Studio program (see the Supporting Informa-
tion). The observed CO2 positions provide some useful
information. As shown in Figure 4A, even in the lowest energy
configurations, the majority of the CO2 molecules preferred the
middle of the pore despite having open-metal sites and
secondary amine groups lining the channels. Also, the adjacent
CO2 molecules were separated by distances in the range of
3.4−4.7 Å (Figures 4B and S21). Weak intermolecular
dispersive interactions operate at such distances.8c Some of
the CO2 molecules arrange themselves into a typical T-shaped
configuration where the δ− oxygen of one CO2 interacts with
the δ+ carbon of another CO2 molecule (Figures 4B and S21).
This T-shaped configuration among CO2 molecules is quite
common in solid phase CO2.15 All of these results are
consistent with our previous reports.8c But the surprising
observation comes from the orientation of the CO2 molecules
that reside proximal to the open-metal sites (Figure 4C).
Typically, the CO2 molecules interact with the open-metal sites
via M···O(δ−)C(δ+)O(δ−) (head-on interaction, M···O
Figure 3. (A) HOA as a function of CO2 loading calculated using DFT
and virial models. (B) CO2 adsorption−desorption cycles at room
temperature.
As can be seen from the Figure 3B, the CO2 uptake remains
almost the same even after 14 cycles. Also, the CO2 sorption is
very smooth, suggesting smooth diffusion of CO2 in the MOF.
Diffusion kinetics was determined using a Rate of Adsorption
(ROA) analysis at 273 K. The average diffusion coefficient (Dc)
for CO2 within the pores of IISERP-MOF20 was calculated to
C
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HOA), the metal−metal separation needs to be <7.3 Å and
there have to be cooperative interactions from other directions
within a pore to place the trapped CO2 molecule in such a
perfectly linear pillaring position. In our case, the two
potentially strong interacting sites, the open-metal site and
the basic groups, compete for the same CO2 and their
orientations do not seem to support any mutual cooperativity.
This is similar to one of our earlier observations, where the high
density of amine functional groups becomes mutually
antagonistic, hindering their interactions with the adsorbed
CO2 molecules.17
CONCLUSIONS
■
Here, we have a Cu-MOF with favorable CO2 capture
characteristics. However, despite having pores functionalized
simultaneously with a bare-metal site and basic benzimidazole
functionality, it shows only a moderately strong interaction with
CO2. The positions and the orientations observed for CO2
within these pores indicate that these two strong adsorption
sites compete among them for the same CO2. It emphasizes
that, when it comes to improving the CO2 interactions by
trapping cooperatively aggregated CO2’s via strong binding
functionalities, their mutual orientation becomes crucial to gain
a synergistic advantage.
Figure 4. (A) CO2 positions from GCMC simulations. (B)
Interactions and distances between CO2 molecules in the pore labeled
as I. (C) A specific CO2 sandwiched between the open-metal sites.
Note: The other CO2 molecules in the pore have not been shown for
clarity. (D) Lowest energy configuration from periodic DFT for a
sandwiched/pillaring CO2, calculated without including any other CO2
molecules in the pore.
distance = ∼2.4 Å).16 In contrast, in our case, the CO2
molecules align parallel to the open-metal sites and get
sandwiched between two open-metal sites from the alternate
layers. This enables the CO2 oxygens to interact weakly, but
equally, with both the metal sites (Figure 4C) and
simultaneously with the other proximal CO2 molecules in the
pore.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00304.
The synthetic and experimental details (PDF)
To emphasize the role of CO2···CO2 cooperative interactions
and to evaluate the feasibility of the pillaring CO2 to get
activated at this open-metal sites separation, we carried out
further DFT simulations. For this, as an initial model, we
positioned a CO2 molecule to pillar the Cu2 paddlewheels with
the oxygens of the CO2 interacting in a head-on fashion with
the metal (Cu···OCO = 2.46 Å; OC = 1.22 Å). The
2.46 Å is as per the value available in the literature for the
strongest head-on interaction with the open-metal site.16 No
other CO2 molecules were included. We optimized the
geometry using a periodic DFT (CASTEP program), with a
constraint on the metal−organic layers (to retain the
coordinates from the single crystal XRD, Cu−Cu distance =
7.36 Å).
Accession Codes
CCDC 1577174 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vaidhya@iiserpune.ac.in.
■
ORCID
Ramanathan Vaidhyanathan: 0000-0003-4490-4397
At this separation, this CO2 molecule does not remain
perpendicular; instead, it tilts by an angle of ∼53° (about the
mean plane) and the Cu···OCO distance equilibrates to
2.72 Å and the OC to 1.18 Å (Figure 4D). This explains that,
for the CO2 to be strongly activated, the interlayer separation
needs to be even shorter. But the N−H···N hydrogen bonds
keep the layers away at a distance of 7.36 Å, thus decreasing the
polarization of the CO2 molecules by the open-metal sites. It is
noteworthy that this distance (Cu···OCO = 2.72 Å) is
different from the distance observed from the GCMC routine
as in this case the cooperative effects from the other CO2
molecules in the pores were not included. In fact, considering
the mere moderate HOA (22−26 kJ/mol), the parallel to
mean-plane orientation observed from the GCMC simulations
seems to be the most probable. Also, this orientation most
probably gets stabilized by the cooperative interactions between
the CO2 molecules in the pore. For a CO2 to act as a pillar
between the two open-metal sites and to get activated (high
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge IISER-Pune and the MHRD-FAST program
for the necessary funding. S.N. thanks DST, SERB, and IISER-
Pune for the fellowship.
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