A permanently porous single molecule H-bonded organic framework for selective CO2 capture

Article abstract of DOI:10.1039/c6cc02964g

Permanent porosity has been realized in a hydrogen bonded framework formed by a single tripodal tricarboxylic acid molecule. The presence of three phenyl rings linked to a flexible sp³ nitrogen centre renders a near-propeller shape to the molec

Full text of DOI:10.1039/c6cc02964g

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DOI: 10.1039/C6CC02964G
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Permanently Porous Single Molecule H-Bonded Organic
Framework for Selective CO2 Capture
Shyamapada Nandi, a Debanjan Chakrabortya and Ramanathan Vaidhyanathanab*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
directed weak interactions and different types of π–π
ABSTRACT: Permanant porosity has been realized in a hydrogen
bonded framework formed by a single tripodal tricarboxylic acid
molecule. The presence of three phenyl rings linked to a flexible
interactions.4a,9,10 A major and common feature in such HOFs
include the directional H-bonds,1,10 for example carboxylic acid
dimers have been extensively employed owing to their highly
predictable and directional H-bond connectivity.1,10b-d As much
as they are advantageous in improving the stability of the HOF,
owing to their highly planar nature, these carboxylate dimers
tend to limit the ability of propagating the HOFs into a three-
dimensional frameworks. In several, bi-component systems a
key design strategy towards improving the HOFs stability and
introducing porosity is by tuning the geometric orientation or
sp3 nitrogen centre renders
a near-propeller shape to the
molecule generating an unusual ‘non-planar’ 3-D framework
formed by highly directional planar –COOH…HOOC- hydrogen
bonds, propagating in all three directions. The material shows
exceptional hydrolytic, acid and thermal stability and has surface
area of 1025 m2/g. Importantly, it shows preferential adsorption
of CO2 over N2 with very high selectivities (350:1 @ 1bar, 303K).
DFT modeling shows the presence of stable T-shaped CO2...CO2 packing between the hydrogen bonding components. In many
cases, the solvent play a key role in extending the h-bond
connectivities. But due to the high degree of rotational,
translational and torsional freedom such solvent participation
makes predictive designing very complex. On the contrary, if
the supramolecular hydrogen bonding assembly of the
molecule can be controlled from atomic manipulation within
the molecule it would be highly predictable.
dimers within the channels suggesting favorable co-operativity
between them.
Hydrogen bonded frameworks (HOFs) forming well-defined
open-frameworks have been known for a long time.1 In recent
times, they have gained enormous interest owing to their
rational designability.(2,3) Among porous materials, molecular
crystals bring advantageous features in terms of synthesis,
solution processability and facile regeneration via
recrystallization. However, generally they are made up of
flexible components and the organic molecules can have many
packing possibilities giving rise to unpredictable crystal
structures. Moreover, many of them have not been known for
their material properties as a sorbent due to their tendency to
collapse upon solvent removal. Only few HOFs possess
permanent porosity.(2-4) Meanwhile, there has been a surge in
the design and synthesis several uni- or binary-component H-
bonded frameworks and in some cases applications such as
gas separation,3-6 chiral separation7 and ion sensing8 have
been demonstrated. In general, HOFs are linked via a range of
non-covalent interactions such as hydrogen bonds, halogen-
Herein, we show a very simple design strategy wherein a
simple variation of the sp2 core by sp3 core in a tricarboxylic
acid enables the enhancement of porosity in a homo-
molecular HOF. Recently, Rowsell and co-workers9 reported a
simple planar tricarboxylic acid molecule which get assembled
via six-membered -COOH...HOOC- H-bond interactions into a
hexagonal sheet structure. This highly symmetrical hexagonal
sheets crystallize with high degree of interpenetration.3b,9 We
realized that one potential route to preventing
interpenetration of these rigid H-bonded planar frameworks
would be to break the planarity of the layers itself. For this, we
designed a similar tricarboxylic acid molecule with high
propensity for H-bonding, but replaced the planar sp2
nitrogens with flexible sp3 nitrogen, which generally favour
pyramidal or pseudo-tetrahedral orientation. Being crystallized
from acetic acid, the HOF shows exceptional acid and
a.Department of Chemistry, Indian Institute of Science Education and Research, Dr.
Homi Bhabha Rd. Pashan, Pune, 411008.
hydrolytic stability. Importantly, the trapped solvent acetic
acid molecules could be removed without collapse of the
framework. This provides a permanently porous framework
capable of preferentially adsorbing CO2 over N2. Another drive
for the study comes from the fact that HOFs can be visualized
as being MOFs without the metal. In which case, what type of
b.Centre for Energy Science, Indian Institute of Science Education and Research, Dr.
Homi Bhabha Rd. Pashan, Pune, 411008.
Electronic Supplementary Information (ESI) available: Electronic supplementary
information (ESI) available: Experimental procedures, IR spectra, TGA plots, and
PXRD data and computational details. CCDC 1472884-1472885. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/x0xx00000x.
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adsorption characteristics to expect, will there be well-defined molecules.1c,3b,9 There are two types of intermolecular H-bonds
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binding sites within the pores, how polarizing are the pores,
these are some worthwhile questions that prop up. To gain
insights, here, we have carried out a combined experimental
and computation study of the adsorption characteristics of a
HOF (IISERP-HOF1) built from a non-planar molecules. We
have carried out GCMC and DFT simulations to gain good
estimate of the CO2-framework and CO2...CO2 interactions.
between the carboxylic acid as shown inDtOheI: 1f0ig.1u0r3e9/1C.6TChCe02t9w6o4G
individual tricarboxylic acid molecules themselves are
arranged as dimers (highlighted by yellow circle in figure 1B)
and such dimers propagate along the a-axis into a column and
such columns are stacked along bc-plane and tend to interact
with each other via inter-column H-bonds. This gives rise to
uniform 1-D channels (9.4 x 9.1 Å) along the a-axis which are
lined mostly by the carboxylate groups and the aromatic
phenyl rings (Figure 1A). Topologically these channels have a
triangular shape and alternate channels propagate as base-up
and base-down triangles (Figure 1A). An alternate stick
representation of the molecular packing is given in figure S1.
IISERP-HOF1 could be synthesized as pure phases in bulk
(Figure 2A) and the simple single molecule nature of this
material enables a facile preparation even up to 20 gm (See
Supporting Information). The TGA analysis indicated
exceptional thermal stability for an organic framework built
entirely from H-bonds (>280 °C) (Figure S2 and S4). There was
the loss of water and acetic acid from the pores at around 90-
150oC, which was characterized by a sharp smell of acetic acid.
However, this loss does not trigger the framework collapse
suggesting a mere templating role of the acetic acid. To
confirm this we carried out the TGA by holding it at 120oC for
an hour (b.p. of acetic acid = 118oC) which gives rise to a sharp
weight loss corresponding to acetic acid and the pxrd of this
acetic acid removed sample indicated complete retainment of
crystallinity. This was further confirmed by variable
temperature PXRD,(Figure 2B) which showed that the material
retains crystallinity well above 280oC (Figure 2B). Having had
been grown from a solvothermal reaction employing acetic
acid as solvent, the IISERP-HOF1 exhibits excellent stability
towards acid. In fact, a sample soaked in 3N HCl for 3 days,
shows slight improvement in crystallinity (FWHM decreases).
Since no obvious dissolution was noticed, this improvement in
crystallinity could be due to the framework going through
subtle recrystallization which helps removal of any molecular
defects. IR spectra contains the characteristic peaks due to the
symmetric and asymmetric carboxylate stretch (1590 and 1670
cm-1) and the O-H stretching frequencies from acid were
observed in the 2500-3100 cm-1 region (Figure S5). A squeeze
analysis of the structure, using PLATON,11 revealed the
presence of ~34 % solvent accessible void in the framework.
Figure 1. (A) Connolly representation of the 1-D channels running
along the a-axis (9.4 x 9.1 Å). (B) A single triangular-shaped channel
with -COOH groups arranged in a dimeric fashion. Yellow circle
highlights the dimeric arrangement of two organic molecules. (C) The
different type of intermolecular -COOH...HOOC- H-bonding motifs
The adsorption measurements were carried out on the
IISERP-HOF1 using N2 and CO2. The 77K N2 isotherm showed
that the HOF1 had good N2 uptake (8.8 mmol/g, Figure 2C) and
a non-localized density functional theory (NLDFT) fit obtained
using a carbon model showed the presence of bimodal
present in the IISERP-HOF1. Note:  has molecules symmetrically
arranged in stacks, while  has adjacent molecules rotated by ~45o.
distribution with significant concentration of micropores of
dimension 11 Å and some larger micropores of dimension18 Å.
The fit obtained using the NLDFT model was excellent (Figures
S9 and S10) and gave a surface area of 405 m2/g. However, the
theoretical surface area based on the single crystal structure
was much higher, 1030 m2/g (calculated with a probe radius of
1.4Å, Materials Studio). This mismatch could most probably be
due to the presence of ill-defined mesopores. Meanwhile, the
195K CO2 isotherm (Figure 2C) showed a typical type-I
The crystals of IISERP-HOF1 was obtained by solvothermally
heating microcrystalline powder of tricarboxytriphenylamine
in acetic acid at 150°C for 3 days. Structure of the HOF was
determined from single crystal x-ray diffraction. The structure
is made up of strong intermolecular -COOH...HOOC- hydrogen
bonds. The presence of sp3 nitrogen centre directs each
organic molecule to take non-planar shape, which disposition
each of the phenyl carboxylic arms along all three directions.
This generates a framework entirely different from the
hexagonal rosette type structure built from planar
adsorption profile with a sharp uptake in the low pressure
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region. A BET model could be fit to this CO2 isotherm and this
gave a surface area of 1025 m2/g, which matches very well
with the theoretical surface area (Figure S14). The pore
volume estimated from the N2 adsorption isotherm (0.231
cm3/g) was quite comparable to the value estimated from the
273K CO2 adsorption isotherm (0.263 cm3/g) using non-local
density functional theory modeling.
77K N2 isotherm yielded BET and Langmuir surface areas 412
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and 659 m2/g, respectively (Figure S7 anDdOSI8: 1).0.I1n0t3e9r/eCs6tCinCg0l2y9,64G
when the 77K N2 isotherm was carried out on a sample that
was treated with 3N HCl over 3 days, washed with water and
then activated, the isotherm displayed a typical type-I
behaviour. This suggest that some inhomogeneities in the pore
sizes could be resulting from disorders in the molecular
assemblies which seem to repair upon subtle recrystallization
in acid. This is in agreement with the improvement in
crystallinity upon acid treatment (Figure 2A). In fact, the BET
and Langmuir surface areas for this acid-treated sample
increase to 512 and 681 m2/g, respectively (Figures S12 and
S13). However, still the surface area estimated from this 77K
N2 isotherm was significantly lower than the theoretical
surface area. This indicates that the acid treatment for these
class of acid-stable HOFs could be an effective way to activate
these samples. Considering that the research on developing
HOFs as candidate porous materials is in its infancy these
observations could be of enormous value.
Encouraged by the excellent apparent selectivity for CO2
observed from the pure component isotherms, we decided to gain
further understanding on the CO2 adsorption sites in the HOF.
Earlier we had used GCMC and DFT methods to simulate the CO2
isotherms and to determine their positions and orientations within
the ultra-micropores of MOFs13 as well as porous polymers.12,14
Here we have employed a similar routine using Materials Studio
(Accelrys) to determine the CO2 positions and interactions within
the channels of the IISERP-HOF1. First, the 195K CO2 isotherms
were simulated using the GCMC. Considering the CO2 being a small
probe with no torsional degree of freedom, a Metropolis algorithm
was employed. The simulated isotherm matched closely with the
experimental one (Figure 3A). This yielded the most probable
densities of CO2, which were found to be around the framework
walls. More importantly it suggested the presence of different
orientations between the CO2 molecules. To obtain more specifics
on the CO2 positions and orientations, we carried out tight binding
density functional theory (DFT-TB) calculations using a 2 x 1 x 1 cell.
For this the framework atoms were kept fixed and the CO2 positions
were allowed to optimize with zero torsional degree of freedom.
The final configuration for the CO2@IISERP-HOF1 at 1 bar has been
presented in figure 3 C and D. Few interesting observations were
found on the mutual orientation of CO2 molecules. Given the
triangular shape of the channels (Figure S21), the CO2 molecules
closer to the apex end were all vertically oriented with respect to
the base of the triangle whereas the ones closer to the base were
lying parallel to the basal plane. This made the oxygens of the ones
closer to the apex point at the electropositive carbon centers of the
CO2 molecules lying on the base. Some of the CO2 molecules with
intermolecular proximities (O(-)...C(-) in the range of 3.3-3.5Å
orient into T-shaped dimers (Figure 3D and S22). Such T-shaped
dimers are typically found in highly ordered CO2 molecules present
in solid phases of CO2.15 Meanwhile, the distances between the
framework atoms and the CO2 molecules (~4.3-5Å) suggest lack of
any strong interactions. But the triangular shape of the channels
certainly seem to have a role in deciding the orientation of the CO2
molecules. These observations indicate the significant polarization
of the CO2 molecules in the channels, which facilitates favorable
CO2...CO2 co-operative interactions. This is quite comparable to
what was observed directly from single crystal structures of CO2
trapped in a ZnAtzOx MOF.13a However, when their HOAs are
compared IISERP-HOF1's is at least 12 kJ/mol lower. This can be due
Figure 2. (A) PXRD plots showing the bulk purity of IISERP-HOF1 and
the stability of it to steam, water and acid. (B) A variable temperature
PXRD showing the complete retainment of crystallinity even at 573K
and no phase change is observed. This reflects the strong
supramoleuclar hydrogen bonding. (C) N2 and CO2 isotherms of IISERP-
HOF1. The subtle hysteresis in the N2 isotherm was highly
reproducible. (D) The IAST plots of IISERP-HOF1 for a 85%N2:15%CO2
composition. The inset shows the HOA plot calculated using virial
methods.
The saturation CO2 uptake (6.3 mmol/g) at 195K in IISERP-
HOF1 is the second highest among all HOFs and SOFs (Table
S3). It is remarkable that a rigid single molecule based HOF
with 1-D channels is able to show such high CO2 uptakes. Only
HOF1a constructed from much larger and highly flexible
tetrahedral amine based molecules, show CO2 uptakes higher
than this (9.8mmol/g @ 195K), which in fact is achieved via a
gate-opening.4c The 273K and 303K CO2 isotherms displayed
uptakes of 4.8 and 2.9mmol/g, respectively. Interestingly,
negligible uptake was observed for N2 at room temperature.
Encouraged by this we calculated the selectivity for CO2 over
N2 using Ideal Adsorption Solution Theory (IAST) and found that
the HOF exhibited a remarkable selectivity for CO2 at room
temperature and the selectivity increases with increasing
pressure in the 0-1 bar range. The selectivity at low pressures
start from 240:1 and then end up at 350:1 at 1 bar (Figure 2D).
Such increase in selectivity with increasing pressure is quite
unusual but has been observed in porous polymers.12 This can
be rationalized from the fact that with the increasing pressures
the CO2 uptake increases, but the N2 uptake does not show
any significant change. Also, a single site Langmuir model could
be fitted extremely well for the 273 and 303K CO2 isotherms,
suggesting presence of homogeneous adsorption sites. The
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Figure 3. (A) Comparison of the experimental and simulated 195K CO2
isotherms of IISERP-HOF1 (GCMC algorithm). (B) Probability densities for
CO2 @ 1bar within the channels of IISERP-HOF1 obtained from GCMC
simulations. (C) The positions and orientation of CO2 molecules within the
channel obtained from DFT-TB geometry minimizations. (D) A lateral view of
the CO2 in the channels showing the T-shaped CO2 dimers at optimal
distances (3.3 to 3.5Å).
The zero-loading HOA (31kJ/mol) observed for IISERP-HOF1 is quite
comparable to those found in supramolecular organic frameworks
(SOFs) and some of the HOFs 3a, 5a but is much lower than what is
found for the HOF3 (~40kJ/mol). 5c Importantly, Schroder et al.
carried out a similar GCMC based investigation to determine the
interactions of CO2 in a multifunctional SOF7 with pores decorated
with amine and amide groups .3a The binding energies they found
for CO2 in these pockets were typically in the order of 35-30kJ/mol,
which were attributed mainly to amine-CO2 and amide-CO2
interactions. In our case since we do not have accessible basic
amine sites the observed HOA are much lower.
6
7
8
Also, the highest probable densities for the CO2 in the channels
estimated from the GCMC calculations do suggest the presence of
slightly different orientations between the CO2 molecules in the
adjacent channels. These channels could be interpreted to be
behaving as adsorption sites of different energy. However, unlike
our recently reported ZnAtzOx based MOF,16 here these do not give
rise to marked steps in adsorption isotherms. Which suggests that
the CO2's in IISERP-HOF1 all posses adsorption sites of similar
energies. This corroborates with lack of any abrupt change in the
HOA values with increasing CO2 uptakes (Figure 2D).
9
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We believe, due to their facile synthesis and crystallization, HOFs
present a potential platform for a 'by design' assembly of active
adsorption pockets in a periodic framework. For this, synthesis of
highly amine-functionalized HOFs with the basic sites laced along
the channel walls are being pursued in our group.
1
, e1500421.
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