Nanoporous amide networks based on tetraphenyladamantane for selective CO2 capture

Article abstract of DOI:10.1039/c6ta01457g

Reduction of anthropogenic CO₂ emissions and CO₂ separation from post-combustion flue gases are among the imperative issues in the spotlight at present. Hence, it is highly desirable to develop efficient adsorbents for mitigating climate change with possible energy savings. Here, we report the design of a facile one pot catalyst-free synthetic protocol for the generation of three different nitrogen rich nanoporous amide networks (NANs) based on tetraphenyladamantane. Besides the porous architecture, CO₂ capturing potential and high thermal stability, these NANs possess notable CO₂/N₂ selectivity with reasonable retention while increasing the temperature from 273 K to 298 K. The quantum chemical calculations also suggest that CO₂ interacts mainly in the region of polar amide groups (-CONH-) present in NANs and this interaction is much stronger than that with N₂ thus leading to better selectivity and affirming them as promising contenders for efficient gas separation.

Full text of DOI:10.1039/c6ta01457g

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PAPER
Nanoporous Amide Networks Based on Tetraphenyladamantane
for Selective CO2 Capture†
Received 00th February 2016,
Accepted 00th February 2016
Sonia Zulfiqar,*ab Daniele Mantione,b Omar El Tall,c Muhammad Ilyas Sarwar,ad Fernando
Ruipérez,b Alexander Rothenbergere and David Mecerreyesbf
DOI: 10.1039/x0xx00000x
www.rsc.org/MaterialsA
Reduction of anthropogenic CO2 emissions and CO2 separation from post-combustion flue gases are among the imperative
issues in spotlight at present. Hence, it is highly desired to develop efficient adsorbents for mitigating climate change with
possible energy savings. Here, we report the design and facile one pot catalyst-free synthetic protocol for the generation
of three different nitrogen rich nanoporous amide networks (NANs) based on tetraphenyladamantane. Besides the porous
architecture, CO2 capturing potential and high thermal stability, these NANs possess notable CO2/N2 selectivity with
reasonable retention while increase in temperature from 273K to 298K. The quantum chemical calculations also suggest
that CO2 interact mainly in the region of the polar amide groups (–CONH–) present in NANs and this interaction is much
stronger than with N2 thus leading to better selectivity and affirming them as promising contenders for efficient gas
separation.
paramount importance together with gas uptake ability from
practical CO2 capture application perspective. One of the limitations
associated with physisorption is the absence of chemical bond
formation that leads to lack of CO2 selectivity, although many
Introduction
Climate change due to rising carbon dioxide (CO2) level in the
atmosphere is undoubtedly an issue of major concern.1 It is crucial to
mitigate anthropogenic CO2 emissions from the environmental
viewpoint for the remediation of CO2 concentration in the
atmosphere. Actual technological solutions involve the capture of
CO2 using liquid absorption, solid adsorption, membrane and
cryogenic separations.2 Owing to the easy handling and
regeneration; solid adsorbents are generally more renowned, in
particular focusing on physisorption as compared to energy intensive
chemisorption processes. CO2/N2 selectivity is a parameter of
attempts have been carried out to enhance selectivity by tuning the
structure and introducing various functional groups, but often
resulted in lowering of CO2 sorption values. However, N2 phobicity
of materials and introduction of CO2 philic groups can help in
improving the CO2 selectivity.2,3
Recently, porous polymer
networks (PPNs), 3 gained significant attention as gas adsorbents due
to their facile synthesis, high physicochemical stability and surface
area, low framework density and tunable porosity. Other wellꢀknown
sorbents that have shown potential for CO2 capture includes
4
polymers of intrinsic microporosity (PIMs), covalent organic
frameworks (COFs), 5 conjugated microporous polymers (CMPs), 6
7
hypercrosslinked polymers (HCPs), porous aromatic frameworks
(PAFs) 8 and others. It is established that the presence of numerous
heteroatoms, in particular nitrogen and oxygen in combination with
an intrinsic mesoꢀnanoporosity in the polymer networks led to
superior CO2 capture ability owing to increased dipole–quadrupole
interactions between the CO2 molecules and polymer framework.9
Therefore, the curiosity in exploiting mildly basic amide
functionality possessing binary interaction sites (NꢀH and C=O) for
CO2 capture is indeed of particular concern.
Aromatic polyamides belong to a class of high performance
materials owing to their outstanding mechanical strength, thermal
stability and chemical resistance.10 The structural rigidity and strong
intermolecular hydrogen bonding developed via amide functionality
results in dense chain packing thus rendering them ideal for spinning
† Electronic supplementary information (ESI) available: Detailed experimental section
including synthetic procedures of the monomers and NANs, Reaction schemes, FT-IR
spectra, CP/MAS 13C NMR spectra, BET linear plots, Cartesian coordinates of CO2, N2,
NANs, NANs/CO2 and NANs/N2 complexes, Calculation methods, Isosteric heat of N2
adsorption, IAST fitting curves and parameters. Comparison of CO2 adsorption data and
CO2/N2 selectivity with other porous polymers reported in literature. See DOI:
10.1039/x0xx00000x
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Journal of Materials Chemistry A
into high performance fibers, tailoring bullet proof vests and flameꢀ Hydrochloric acid (∼37%), chloroform (99.8%), sulfuric acid
resistant garments. These polymers have well known industrial (>99%), dimethyl sulfoxide (99.9%), iꢀpentane (>99%), potassium
relevance predominantly in defense, automobile and aerospace cyanide (>97%), ethylene glycol (>99.9%) acetone (>99%) and extra
applications.11 CO2 capture ability of organic amide polymers have pure methanol were provided by Fisher Scientific. Potassium
rarely been studied so far with few exceptions.12ꢀ14
hydroxide (90%) was purchased from Scharlab and used as received.
Cycloaliphatic hydrocarbons like adamantane possess a diamondꢀ
like cage structure. The most intriguing aspect of this tecton is its Characterization methods
highly symmetrical tetrahedral shape along with huge molecular
volume and stiffness rendering it a superb building block for The elemental analysis of monomers and NANs was performed on a
generating supramolecular architectures. Adamantane structure LECO CHNSꢀ932 microanalyzer. Fourier transform infrared (FTIR)
consists of six secondary and four tertiary carbons. The hydrogens spectra were recorded over the range 4000−400 cm−1 at a resolution
attached to the tertiary carbons can be readily converted into desired of 4 cm−1 using Bruker FTIR spectrometer AlphaꢀP. 1HNMR
functional groups because of their high reactivity.15ꢀ17 The arylation measurements of the monomers were carried out on a Bruker
of adamantane afford tetraphenyladamantane (TPA) consisting of AVANCE 400 spectrometer. The samples were dissolved in DMSOꢀ
adamantane core surrounded by four peripheral phenyl rings that can d6 and CDCl3, and were measured with tetramethylsilane as an
be easily functionalized and can spread out further.18ꢀ22 Shen and coꢀ internal reference. Solid state NMR spectroscopic analysis of NANs
workers have synthesized TPA based microporous polycyanurate was performed on a Bruker 400 AVANCE III WB spectrometer
and polyimide networks and showed promising results for CO2 9.40T. Crossꢀpolarization magicꢀangle spinning (CP/MAS) 13C
adsorption.18 Li and fellow colleagues also thoroughly investigated NMR spectra were recorded using a 4mm MASDVT probe at a
the gas sorption properties of microporous poly(Schiff base) and spinning of 12 KHz, using the cross polarization pulse sequence, at
22
polyaminal networks constructed from TPA.21,
The remarkable 100.63 MHz, a time domain of 2K, a spectral width of 29 KHz, a
molecular structure of TPA also motivated us to synthesize amide contact time of 1.5ms and an interpulse delay of 5s. Xꢀray diffraction
networks exclusively from these units. The inclusion of bulky and pattern of powdered NANs were recorded by a BRUKER D8
stiff TPA moiety not only ensures the pore formation but also offer ADVANCE powder diffractometer at 40 kV and 30 mA.
both cycloaliphatic and aromatic constituents in the NANs. To the Measurements were performed for 2θ in the range of 2° to 80° with a
best of our knowledge, this is the first ever report on the synthesis, step size of 0.05° and scan speed of 0.6°/min and scan time of 5s at
characterization, gas sorption and CO2/ N2 selectivity of nanoporous 25°C. Thermogravimetric analysis (TGA) on NANs was performed
nitrogen rich amide networks based on tetraphenyladamantane. A using a TA instruments Q500 device. The samples were heated from
series of amide networks have been synthesized by combination of room temperature to 800°C with a heating rate of 10°C/min under N2
TPA with diꢀ, triꢀ and tetra functional aromatic units. Moreover, atmosphere with a gas flow rate of 10 mL/min. The surface
different factors affecting gas sorption properties such as chemical morphology of NANs was determined using FEI Quanta 250 FEG
structure, nanoporosity, temperature and pressure were investigated. and Magellan scanning electron microscope (SEM). NANs were also
The quantum chemical calculations suggest that CO2 interacts examined in a transmission electron microscope, TECNAI G2 20
mainly in the region of polar amide groups present in the amide TWIN (FEI), operating at an accelerating voltage of 200 kV in
networks and this interaction is much stronger than N2 resulting in a brightꢀfield image mode. Sample preparation for TEM involved the
noteworthy CO2/N2 selectivity. The CO2 philic amide functionality deposition of a solution drop on lacey carbon film copper grid which
of NANs provides high Nꢀcontent whereas the presence of has previously been hydrophilized by a glow discharge process. To
mesopores 23 also assists in the strong binding with CO2 thus making determine the porosity and Brunauer–Emmett–Teller (BET) surface
NANs as promising contenders for post combustion CO2 area of NANs, N2 adsorption isotherms at 77 K were obtained using
separations.
Micromeritics ASAP 2020 physisorption analyzer. CO2 and N2
adsorptionꢀdesorption isotherms of NANs were recorded using
Micromeritics ASAP 2050 sorption analyzer at 273K and 298K.
The samples were degassed at 393K for 24h prior to analysis.
Experimental
Materials
Synthesis of amide networks based on tetraphenyladamantane
2ꢀBromoꢀ2ꢀmethylpropane(96%),
1ꢀbromoadamantane
(99%),
potassium nitrate (99%+), acetic anhydride (99%+), glacial acetic Tetraphenyladamantane based three nitrogenꢀrich nanoporous amide
acid (99.8%), palladium on activated carbon,10% Pd, unreduced, networks (NANꢀ1, NANꢀ2 and NANꢀ3) were synthesized using one
iodine (99.5%), copper cyanide (99%), [bis(trifluoroacetoxy)iodo] pot catalystꢀfree polycondensation reaction under basic conditions.
benzene (98%), N,Nꢀdimethylformamide, anhydrous (>99.9%), The detailed step by step protocol for the synthesis and
dichloromethane, anhydrous (>99.8%) and oxalyl chloride (98%) characterization of tetraꢀamine monomer 1,3,5,7ꢀtetrakis(4ꢀ
were procured from Acros Organics.
Anhydrous aluminium aminophenyl) adamantane (TAPA) and tetraꢀacid chloride monomer,
chloride (≥99.9%), benzene (>99%), triethylamine (≥99.5%), 1,3,5,7ꢀtetrakis(4ꢀ(chlorocarbonyl)phenyl)ꢀadamantane (TCPA) is
tetrahydrofuran, anhydrous (≥99.9%), sodium metabisulfite (≥99%), described in ESI (Scheme S1 and Scheme S2†). A typical synthesis
heptane (99%) and sodium sulphate, anhydrous (≥99.0%) were of NANs involved the reaction of TAPA with terephthaloyl chloride
supplied by the courtesy of SigmaꢀAldrich. Terephthaloyl chloride (TPC), trimesoyl chloride (TMC) and TCPA (Scheme 1) in the
(≥99%) and trimesoyl chloride (98%) were obtained from Aldrich. presence of triethylamine (TEA) to quench the hydrochloric acid
2 | J. Mater. Chem. A, 2016, 00, 1-8
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(HCl) generated as a byꢀproduct during the polymerization reaction
in inert atmosphere. THF was used as a solvent medium for the
synthesis of NANs and the reaction was carried out initially at 0°C
to avoid any side reactions because the permanent porosity in
polymer networks is very much reliant on the high conversion rate
during the polymerization reaction. The stirring was continued at
room temperature for 24h and afterwards the precipitates of NANs
were collected by filtration and washed several times with THF,
water, and acetone to remove hydrochloride salt and organic
oligomeric impurities. The resultant NAN powders were then
immersed in acetone for two days during which time the acetone was
decanted and freshly replenished three times and dried under
vacuum for 24h (see detailed procedure in the ESI†).
Results and discussion
The formation of NANs was verified by FTIR, Elemental analysis
and CP/MAS 13C NMR (see ESI†). The characteristic amide NꢀH
stretching bands were observed in the range of 3344 to 3356 cmꢀ1 in
the FTIR spectra of NANs. The band of CꢀH stretch in NANs
appeared in the range of 2926 to 2928 and at 2852 cmꢀ1. The typical
amide carbonyl C=O stretch was observed at 1653 cmꢀ1 for NANꢀ1
and NANꢀ2 while NANꢀ3 showed the appearance of C=O stretching
band at relatively higher wavenumber, 1688 cmꢀ1 illustrating less
conjugation and hydrogen bonding within the framework as
compared to NANꢀ1 and NANꢀ2, owing to the presence of more
bulky cycloaliphatic adamantane units in the structure respectively
(Figure S1†). Elemental analysis further validated the successful
formation of amide networks and the calculated chemical
compositions of NANs are also in good agreement with the
measured values. The CP/MAS 13C NMR spectra of NANs showed
carbonyl signals in the range 160.9 ppm to 166.1 ppm respectively
(Figure S2†). The peaks in the range of 125 to 153.7 ppm are
attributed to the aromatic carbons of phenyl rings in the amide
networks, while the chemical shifts at 38.4 to 38.9 and 45.5 to 47.5
ppm are ascribed to secondary and tertiary carbon atoms of
adamantane in NANs respectively.
Powder Xꢀray diffraction (PXRD) of NANs reveals broad
patterns suggesting amorphous nature of all amide networks due to
the lack of long range order similar to the previously reported
tetraphenyladamantane based porous polymer networks 18a, 28 (Figure
1A). All NANs show a broad maxima and a shoulder in the Xꢀray
diffractograms. The broad band around 2θ ∼ 20° probably signifies
the segmental distance within the polymer networks whereas the
shoulder around 2θ ∼ 42° is perhaps may be due to πꢀπ stacking of
amide functionality and phenyl rings in the ordered domains.29
Thermal stability of adsorbents is an imperative factor for finding
possible application in post combustion processes at higher
temperatures and CO2 scrubbing operations.30 Keeping in view this
crucial aspect, thermal stability of NANs was recorded using
thermogravimetric analysis (TGA) under nitrogen atmosphere
(Figure 1B). NANꢀ1 and NANꢀ2 amide networks are thermally very
stable and show the decomposition above 500°C which is
considerably beyond the lowest thermal stability limit for a CO2
cyclic operation (min. 200°C).31 The incredible stability of these
NANs is ascribed to the covalent linkages, presence of heat resistant
amide groups and aromatic units in the chemical structure that
increases the durability of these materials for gas separation.13
Computational Details
All the density functional calculations (DFT) have been carried out
with the Gaussian 09 suite of programs.24 The geometrical
optimizations were performed using the longꢀrange corrected
functional ωB97XD,25 which includes dispersion corrections,
together with the 6ꢀ31+G(d,p) basis set. Harmonic vibrational
frequencies were obtained, at the same level of theory, by analytical
differentiation of the gradients in order to determine whether the
structures were minima or transition states. All structures showed
real frequencies for all of the normal modes of vibration. These
frequencies were then used to evaluate the zeroꢀpoint vibrational
energy (ZPVE) and the thermal corrections, at T = 298 K, in the
harmonic oscillator approximation. The electronic energy was
refined by singleꢀpoint calculations using the 6ꢀ311++G(2df,2p)
basis set. The binding energy of gas (CO2 or N2) with the model of
NANs was estimated as follows:
ꢄ
ꢌ
ꢄ
ꢌ
ꢄ
ꢌ
∆ꢀꢁꢂ = ꢀꢃ ꢅꢆꢇꢈꢉꢊꢋ − ꢀꢃ ꢍꢎꢍꢏ − ꢀꢃ ꢐꢑꢏ
where E0 refers to the electronic energy, along with the zeroꢀpoint
energy correction of the NANs/gas (CO2 or N2) complexes and the
individual molecules. ꢁECP is the binding energy including the
correction of the basis set superposition error (BSSE) by means of
the counterpoise method. 26, 27
100
80
60
40
20
0
NAN-1
NAN-2
NAN-3
NAN-1
NAN-2
NAN-3
10
20
30
40
50
60
70
80
100
200
300
400
500
600
700
2θ
(
degree)
Temperature (°C)
Scheme 1 Synthetic route to NANs.
Fig. 1 (A) PXRD patterns of NANs (B) TGA curves of NANs.
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On the contrary, NANꢀ3 show less thermal stability compared to
NANꢀ1 and NANꢀ2 owing to the introduction of more cycloaliphatic
adamantane units in the network that may reduce the degree of
crosslinking leading to more flaws in the structure, where the initial
weight loss is due to the entrapped solvent and moisture in the pores.
The surface morphology of NANs was examined using scanning
NAN-1 Adsorption
NAN-1 Desorption
NAN-2 Adsorption
NAN-2 Desorption
NAN-3 Adsorption
NAN-3 Desorption
250
200
150
100
50
0.5
0.4
0.3
0.2
0.1
0.0
1
10
100
Pore Diameter (nm)
electron microscopy (SEM). NANꢀ1 image display the presence of
13
0
agglomerated spherical particles
in contrast to NANꢀ2 which
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure
(
P/Po)
shows relatively uniform and interconnected particulate structure.
NANꢀ3 reveals the formation of granulated structure with interꢀ
connected grains forming larger particles at micro scale. Such sized
particles are frequently used for fluidized bed adsorption
applications at industrial level (Figure 2AꢀC). Highꢀresolution
transmission electron microscopy (HRꢀTEM) images of amide
networks exhibit continuous, interconnected and extended porous
network structures (Figure 2DꢀF).32 It is established that the
formation of strong hydrogen bonds in amide polymers promotes
extended three dimensional architectures. 12
The porosity of NANs was determined using N2 adsorptionꢀ
desorption isotherms measured at 77K (Figure 3). Amide networks
illustrate a typical reversible sorption isotherm. NANꢀ3 possesses
relatively higher surface area (SABET: 85 m²/g) as compared to
NANꢀ1 and NANꢀ2 (Table 1, Figure S3†) and other amide
functionalized porous organic polymers,13,14 owing to the inherent
porosity generated from both the monomers having tetrahedral
tecton thus forming a giant extended structure. Low surface area of
NANꢀ1 and NANꢀ2 may also be due to the random orientation of
pores in the network. Appearance of narrow hysteresis and pore size
distribution of NANs signifies the presence of mesopores in the
networks. No considerable N2 uptake into micropores was noticed
for NANs. However, at higher relative pressures, some uptake was
observed illustrating existence of interparticle void spaces which is
also evident from SEM micrographs (Figure 2 AꢀC).9
Fig.3 N2 gas isotherms for NANs measured at 77K (Inset: Differential
pore size distribution curves from BJH method).
adsorption (Table 1).12 To determine the sorption profile of any
sorbent at 298K is very crucial because it is closer to the conditions
34
required for postꢀcombustion CO2 capture
especially from the
practical application perspective.35 Apparently, surface area of
NANꢀ3 is superior than NANꢀ1 and NANꢀ2 but surprisingly its CO2
sorption capacity is the lowest of all NANs suggesting that CO2
sorption performance of sorbents not only depends on the surface
area and porosity 20 but also influenced by the presence of nitrogen
and oxygen rich CO2ꢀphilic groups.36,37 Interestingly, the polar –
CONH– amide functionality is the key unit in all amide networks
which is interacting with CO2 and NANꢀ3 possesses less amide
groups in the network relative to NANꢀ1 and NANꢀ2. Additionally,
NANꢀ3 network contains large number of bulky adamantane
moieties that create hindrance for CO2 molecules to interact with
amide groups present in the network. On the contrary, the presence
of small phenyl rings in NANꢀ1 and NANꢀ2 facilitates the CO2
molecules to approach the amide groups easily for interaction. The
narrow hysteresis of adsorptionꢀdesorption curves imply weak
interactions between CO2 molecules and NANs thus allowing less
energy intensive adsorbent regeneration.38
CO2 and N2 adsorptionꢀdesorption isotherms of NANs were
investigated at 273K and 298K (Figure 4 and Figure 5). Among all
amide networks, NANꢀ2 reveals the highest CO2 sorption capacity of
65.58 mg/g followed by NANꢀ1 (55.78 mg/g) and NANꢀ3 (40.27
mg/g) at 1 bar and 273K respectively. These values for CO2 uptake
are much higher than for other porous amide polymers reported in
literature.12, 14, 33 The CO2 uptake of NANs at 298 K are apparently
lower than the values at 273 K illustrating the exothermic nature of
80
60
40
20
0
50
40
30
20
10
0
NAN-1 Adsorption
NAN-1 Desorption
NAN-2 Adsorption
NAN-2 Desorption
NAN-3 Adsorption
NAN-3 Desorption
NAN-1 Adsorption
NAN-1 Desorption
NAN-2 Adsorption
NAN-2 Desorption
NAN-3 Adsorption
NAN-3 Desorption
273K
298K
B
1.0
A
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
Pressure (bar)
Pressure (bar)
Fig. 4 CO2 gas adsorption-desorption isotherms of NANs at (A) 273K
and (B) 298K.
A
B
C
3.0
1.2
NAN-1 Adsorption
NAN-1 Desorption
NAN-2 Adsorption
NAN-2 Desorption
NAN-3 Adsorption
NAN-3 Desorption
NAN-1 Adsorption
NAN-1 Desorption
NAN-2 Adsorption
NAN-2 Desorption
NAN-3 Adsorption
NAN-3 Desorption
273K
298K
2.5
2.0
1.5
1.0
0.5
0.0
0.8
0.4
0.0
B
1.0
A
1.0
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
D
E
F
Pressure (bar)
Pressure (bar)
Fig. 2 SEM images of NAN-1 (A), NAN-2 (B) and NAN-3 (C). HR-TEM
Fig. 5 N2 gas adsorption-desorption isotherms of NANs at (A) 273K
images of NAN-1 (D), NAN-2 (E) and NAN-3 (F)
and (B) 298K.
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Table 1 Surface area, pore size, CO2 and N2 adsorption data at 1 bar, Isosteric heat of adsorption (Qst) and CO2/N2 selectivty of NANs.
Pore
size
CO2 Uptake (mg/g)
N2 Uptake (mg/g)
T = 273K T = 298K
1.78 0.89
CO2/N2 Selectivity
SA BET
(m2/g)
SA Langmuir
(m2/g)
Qst, CO2
Qst, N2
NANs
(kJ/mol)
(kJ/mol)
(nm)
T = 273K
T = 298K
T = 273K
T = 298K
NAN-1
NAN-2
NAN-3
30
56
85
47
87
18.9
55.78
65.58
40.27
35.02
39.57
26.49
27.94
31.52
22.46
12.32
21.77
18.55
74.0
72.7
57.8
56.4
66.2
50.1
22.7
16.2
2.28
1.43
0.96
0.69
135
BET and Langmuir surface area from N2 adsorption isotherms, Average pore size from BJH adsorption, Isosteric heat of adsorption (Qst) obtained from CO2
and N2 isotherms data at 273K and 298K using Clausius-Clapeyron equation, CO2/N2 selectivity calculated by IAST method for CO2:N2 gas mixture of
0.15:0.85.
A
B
C
Fig. 6 Molecular models for (A) NAN-1/CO2, (B) NAN-2/CO2 and (C) NAN-3/CO2 complexes.
stabilization of the complex. Thus, the C=O bond which interacts
Theoretical calculations were also carried out to verify our with the amide hydrogen is stretched (1.169 Å vs 1.165 Å of free
experimental findings of CO2 capture ability of NANs. In order to
calculate the interaction of a CO2 molecule with NANs, three models CO2) and the other one is shortened (1.161 Å). As in the previous
have been defined for NANꢀ1, NANꢀ2 and NANꢀ3 (Figure 6). The complex, the bond angle is hardly modified (178.3°). Finally,
results of quantum chemical calculations and geometrical parameters regarding NANꢀ3ꢀCO2 complex, the CO2 molecule is located a bit
of CO2 molecule in the complexes are presented in Table 2 (see farther from the amide moiety, at 3.26 Å from the nitrogen and 4.98
Table S1 to S4 for more details †). It is observed that the binding Å from the carbonyl oxygen, approximately (Figure 6). In this
energy of CO2 with NANꢀ2 (ꢀ16.19 kJ/mol) is greater than the model, the extra stabilization accomplished by a second amide
corresponding NANꢀ1 (ꢀ14.72 kJ/mol) and NANꢀ3 (ꢀ13.36 kJ/mol), moiety, as in NANꢀ2, is absent, so that the binding energy is very
implying that NANꢀ2 exhibits the highest affinity for CO2, similar to the one of NANꢀ1. Although the interaction with CO2 in
supporting the experimental results. The interaction takes place this model is slightly weaker than in NANꢀ1 (ꢀ13.36 vs ꢀ14.72
mainly in the region of the polar –CONH– amide groups, as it was kJ/mol), the particular arrangement of CO2 with respect to the amide
expected.32 The magnitude of this interaction with the three models moiety makes the geometry of this molecule to be slightly more
of NANs is reflected in the deviation from the linear geometry of distorted. Thus, in NANꢀ1 the CO2 is located in a parallel plane and
CO2 and in the stretching of C=O bond as well. In NANꢀ1ꢀCO2 the bond lengths are hardly modified, however, in NANꢀ3, CO2 is
complex, the CO2 molecule locates in a parallel plane over the amide oriented at a greater extent towards the –NꢀH bond, and the bond
moiety of NANꢀ1 (Figure 6) at around 3.07 Å from the carbonyl distance of the C=O bond closer to the hydrogen is stretched (1.168
oxygen and 3.40 Å from the nitrogen. Since the interaction is weak, Å vs 1.165 Å of free CO2) while the other bond is shortened (1.162
the C=O bond lengths remain almost unchanged compared to the Å), similar as in the NANꢀ2 model. The bond angle, nevertheless,
CO2 free molecule (R = 1.165 Å) and the O=C=O angle deviates remains almost unchanged (179.0°). Likewise, N2 uptake of NANs
slightly from linearity (177.7°). In case of NANꢀ2ꢀCO2 complex, the also follows the decreasing order NANꢀ2 > NANꢀ1 > NANꢀ3 at both
CO2 molecule locates along the –NꢀH bond line at 2.91 Å (Figure 6). the temperatures (273K and 298K), but the values are negligible
Besides, the molecular structure of NANꢀ2, the presence of a second relative to CO2 uptake owing to weak interaction of NANs with N2
amide moiety located at around 3.27
Å
provides an extra which was further validated by our quantum chemical calculations
(Table 2, Figure S4†). In NANꢀ1, the interaction takes place mainly
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Journal of Materials Chemistry A
with the aromatic ring, and the N2 locates on a plane parallel over it, containing 15% CO2 and 85% N2, a typical postꢀcombustion flue gas
whereas in NANꢀ2, the interaction is similar as in the case of CO2, composition. The CO2/N2 selectivity of NANꢀ1, NANꢀ2 and NANꢀ3
and two amide groups stabilizes the N2 and higher affinity is
Table 2 Binding energy of CO2 and N2 with NANs (ΔECP), in kJ/mol,
C=O bond length (Ri), in Å, and O=C=O angle (α), in degrees, of the
CO2 molecule, N≡N bond length (R), in Å. Data obtained at the
ωB97XD/6-31+G(d,p)//ωB97XD/6-311++G(2df,2p) level of theory.
at 1 bar is 74.0, 72.7 and 57.8 at 273K and 56.4, 66.2 and 50.1 at
40
35
30
25
20
15
10
250
200
150
100
50
NAN-1
NAN-2
NAN-3
NAN-1 (273K)
NAN-1 (298K)
NAN-2 (273K)
NAN-2 (298K)
NAN-3 (273K)
NAN-3 (298K)
Q
CO2
R1
N2
A
NANs
0.0
0.2
0.4
0.6
0.8
1.0
0
5
10
15
20
25
30
35
40
45
Pressure (bar)
CO2 Uptake
(
mg/g
)
ΔECP
R2
α
ΔECP
-6.68
-10.33
-6.49
R
Fig. 7 (A) Isosteric heat of CO2 adsorption for NANs. (B) CO2/N2
selectivity calculated by IAST method for CO2:N2 gas mixture of
0.15:0.85.
NAN-1
NAN-2
NAN-3
-14.72
-16.19
-13.36
1.166
1.169
1.168
1.164
1.161
1.168
177.7
178.3
179.0
1.102
1.102
1.102
298K respectively (Table 1 and Figure 7B). Keeping in view the
conditions required for postꢀcombustion capture, it is imperative to
determine CO2/N2 selectivity at 298K while some studies were only
focused on 273K.22 These NANs display CO2/N2 selectivity with
reasonable retention on changing temperature from 273K to 298K.
Noticeably at 298K, NANꢀ2 performed certainly well in terms of
selectivity (66.2) followed by NANꢀ1 and then NANꢀ3, duly
complementing the same sequence for CO2 uptake capacity
determined through experimental as well as theoretical estimations.
Interestingly, these NANs exhibit higher CO2/N2 selectivity,
surpassing the selectivity for many porous polymers reported in
literature at these temperatures 40, 41 even having much larger surface
area correspondingly (Table S6†). 42ꢀ48 The CO2 selectivity of NANs
is due to the extensive interactions between CO2 and amide
functional groups present in the networks thus, proving the
superiority of using amide link in the NANs architecture.12 In fact,
the basic amide functionality possesses binary interaction sites (N–H
and C=O) and a strong Lewis acidꢀbase interaction exists between
CO2 molecules and oxygen of amide carbonyl moiety. Likewise, this
interaction in NANs is evident from reasonably high CO2 uptakes
and marked CO2/N2 selectivities owing to the CO2 affinity of Nꢀ
observed. While similar to CO2, N2 locates over the benzene ring
and the extra stabilization of the second amide is absent in NANꢀ3
(Figure S4†).
Isosteric heat of adsorption (Qst CO2) was calculated to determine
the binding affinity between NANs and CO2 molecules from the CO2
adsorption isotherms measured at 273K and 298K using Clausiusꢀ
Clapeyron equation (Figure 7A). This parameter performs a key role
in determining the adsorption selectivity and energy required for
CO2 release during regeneration phase. Qst CO2 for NANs are in the
range of 22.46
̶ 31.52 kJ/mol which is distinctly less than expected
for chemisorption (> 40 kJ/mol), signifying a physisorption process
similar to other porous polymers.36 Qst,CO2 values exhibit a noticeable
decreasing trend with the adsorbed amount of CO2, suggesting
strong interactions of the polarizable CO2 molecules through dipoleꢀ
quadrupole interactions with the polar amide groups in NANs rather
than the gas–gas interaction and aggregation of the CO2 molecules
themselves. Isosteric heat of N2 adsorption for NANs (Qst,N2) was
also calculated and values appeared in the physisorption range
(Figure S5†).
12
containing amide groups. Interestingly, mesoporosity might also
High selectivity of CO2 over N2 is one of the essential features for
an adsorbent to be exploited for postꢀcombustion CO2 capture. The
foremost concern in CO2/N2 gas separation is the minute difference
in properties of both the gases which is clear from the kinetic
diameters of CO2 (3.30 Å), and N2 (3.64 Å). Conversely, their
quadrupolar moments and polarizabilities exhibit some differences,
where CO2 possesses a large quadrupole moment of 13.4 × 10−40
Cm2 as compared to N2 (4.7 × 10−40 Cm2) and a polarizability of
26.3 × 10−25 cm3 (17.6 × 10−25 cm3 for N2). Accordingly, the
variation in chemical reactivity of gases can assist control at the
molecular level resulting in strong interactions of the sorbents with
the surface of pores and increased selectivity.39 In addition to high
CO2 uptake, high CO2/N2 selectivity of adsorbents is equally crucial
from practical application perspective. To evaluate the performance
of NANs in CO2 separation from gas mixtures, Ideal Adsorbed
Solution Theory (IAST) was used (Table S5 and Figure S6†). For
this purpose, CO2 and N2 single component isotherms were fitted by
Langmuir dual sites or Langmuir single site models to calculate
CO2/N2 selectivity by IAST method for CO2:N2 binary gas mixture
be another factor responsible for this decent CO2/N2 selectivity
23
together with contribution from CO2 philic amide groups. Porous
polymers 49 in this respect, specially containing azo linkages have
already been exploited for selective CO2 capture. 47, 48 Another study
50
reveals that an ideal sorbent should possess high CO2/N2
selectivity at low CO2 pressures (<0.2 bar) which is in good
agreement with our results displaying the decreasing pattern of
CO2/N2 selectivity with the increase in pressure owing to the
pressureꢀdriven poreꢀfilling mechanism, irrespective of CO2ꢀphilic
moieties present in the networks (Figure 7B).
Conclusions
We presented first ever study on the nanoporous nitrogen rich amide
networks based on tetraphenyladamantane using one pot facile
strategy. These thermally stable NANs also possess reasonable CO2
sorption capacity and notable CO2/N2 selectivity (74.0 at 273K for
NANꢀ1). A combined impact emanating from the presence of CO2
philic Nꢀcontaining amide groups and mesoporosity in the
6 | J. Mater. Chem. A, 2016, 00, 1-8
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their simple and catalyst free synthetic protocol. Therefore, the
current effort open up further avenues to tailor and design new
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Acknowledgments
Dr Sonia Zulfiqar is highly thankful for the financial support
provided by Marie Curie IIF grant “NABPIL” (No. 629050) from
the European Commission under the 7th Framework Programme
(FP7ꢀPEOPLEꢀ2013ꢀIIF) and also appreciate the cooperation of Dr.
José Ignacio Santos for Solid State NMR facility, Iñigo Pérez
Miqueo for BET measurements and Dr. Monica Moreno for
administrative support from UPV/EHU, San Sebastian, Spain. The
authors would like to thank for technical and human support
provided by IZOꢀSGI SGIker of UPV/EHU and European funding
(ERDF and ESF). Daniele Mantione is highly grateful for financial
support provided through EU project FP7ꢀPEOPLEꢀ212ꢀITN
316832ꢀOLIMPIA.
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Table of Contents
This paper reveals the potential of nanoporous amide networks based on tetraphenyladamantane for selective CO2 capture
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