Nitrogen-Rich Covalent Triazine Frameworks as High-Performance Platforms for Selective Carbon Capture and Storage

Article abstract of DOI:10.1021/acs.chemmater.5b03330

The search for new efficient physisorbents for gas capture and storage is the objective of numerous ongoing researches in the realm of functional framework materials. Here we present the CO₂ and H₂ uptake capacities of nitrogen rich

Full text of DOI:10.1021/acs.chemmater.5b03330

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Article
Nitrogen-rich covalent triazine frameworks as high-
performance platforms for selective carbon capture and storage.
Stephan Hug, Linus Stegbauer, Hyunchul Oh, Michael Hirscher, and Bettina V. Lotsch
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03330 • Publication Date (Web): 11 Nov 2015
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Nitrogen-rich covalent triazine frameworks as high-performance
platforms for selective carbon capture and storage.
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Stephan Hug,abc Linus Stegbauer, abc Hyunchul Oh,de Michael Hirscher,d Bettina V. Lotsch*abc
a Department of Chemistry, LudwigꢀMaximiliansꢀUniversität München, Butenandtstr. 5ꢀ13, 81377 Munich, Germany
b Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany. Fax: +49 711 689 1612; Tel: +49 711
689 1610, Eꢀmail: b.lotsch@fkf.mpg.de
c Nanosystems Initiative Munich (NIM) and Center for Nanoscience, Schellingstr. 4, 80799 Munich, Germany
d Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany
e Department of Energy Engineering, Gyeongnam National University of Science and Technology, Jinju, South Korea
ABSTRACT: The search for new efficient physisorbents for gas capture and storage is the objective of numerous ongoing researches in
the realm of functional framework materials. Here we present the CO2 and H2 uptake capacities of nitrogen rich covalent triazine
frameworks based on lutidine, pyrimidine, bipyridine and phenyl units, showing superior gas uptakes and extremely high CO2 selectivities
towards N2. The CO2 uptake of a bipyridineꢀCTF synthesized at 600 °C (5.58 mmol gꢀ1, 273 K) is the highest reported for all CTFs so far
and the second highest for all porous organic polymers (POPs). Moreover, the CO2 selectivity towards N2 of a nitrogenꢀrich pyrimidineꢀ
based CTF synthesized at 500 °C (Henry: 189, IAST: 502) is the highest reported for all POPs, and the H2 uptake of CTF1 synthesized at
600 °C at 1 bar (2.12 wt%, 77 K) is the highest found for all CTFs to date as well. With the wide range of sorption data at hand, we carve
out general trends in the gas uptake behavior within the CTF family and nitrogenꢀcontaining porous polymers in general, revealing the
dominant role of the micropore volume for maximum CO2 uptake, while we find that the nitrogen content is a secondary effect weakly
enhancing the CO2 uptake. The latter however was identified as the main contributor to the high CO2/N2 selectivities found for the CTFs.
Furthermore, ambient water vapor sorption has been tested for CTFs for the first time, confirming the highly hydrophilic nature of CTFs
with high nitrogen content.
potential of CTFs for CO2 capture,23ꢀ30 while a systematic
relationship between nitrogen content and CO2 capture capacities
1. INTRODUCTION
Anthropogenic emission of CO2 is the main contributor to global
warming, as stated recently by the Intergovernmental Panel on
Climate Change (IPCC).1 The power sector’s share of the globally
emitted CO2 amounts to around 40%,2 thus attesting carbon capture
and storage (CCS) a high potential for reducing the emissions and,
ultimately, slowing down climate change. The most established
techniques for CCS are amine scrubbing and oxyfuel combustion,3
which however come at the cost of increasing the energy
has not been explicitly treated, since comparison among various
sorbents with different synthesis histories is challenging.
In this work31 we present the CO2 and H2 capture capacities of
the known CTFs bipyꢀCTF32 and CTF120, 22 synthesized at different
temperatures and two new CTFs based on lutidine (lutꢀCTF) and
pyrimidine (pymꢀCTF) building units, and we find CO2 uptakes
higher than in all other CTFs reported to date. We comprehensively
characterize the materials and discuss their sorption capacities and
selectivities as a function of their micropore volume and nitrogen
contents, thus revealing a major dependency of the CO2 uptake on
the micropore volume and only minor relevance of the total
nitrogen contents. In contrast, the high CO2 over N2 selectivities are
largely attributed to the nitrogen content.
5
requirements of a power plant by as much as 25ꢀ40%.4,
Additionally, amine scrubbing uses toxic solvents which are
difficult to dispose and are subject to decomposition and
6
evaporation.5, Therefore physical adsorbents such as zeolites,
metal organic frameworks (MOFs) and porous organic polymers
(POPs) came into focus owing to their high CO2 capture capacities
and low energy requirements for regeneration.3, 4, 7 Especially POPs
combine the advantages of high selectivities, good chemical and
thermal stabilities and high tolerance towards water vapors, which
has been shown in several recent reviews.8ꢀ11 Newer research
2. EXPERIMENTAL DETAILS
Materials and Methods. All reactions were carried out under an
argon atmosphere in flameꢀdried glassware. Anhydrous solvents
and liquid reagents were transferred by syringe or cannula. Unless
otherwise noted, all materials were obtained from commercial
suppliers (see Supporting Information, Table S1) and used without
further purification. Column chromatography was performed using
an Isolera Four (Biotage AB, Sweden) with Biotage SNAP
cartridges (40ꢀ65 µm silica). Tri(2ꢀfuryl)phosphine (tfp),33 5,5’ꢀ
dicyanoꢀ2,2’ꢀbipyridine32 and 5ꢀbromoꢀ2ꢀiodoopyrimidine34 were
synthesized according to published procedures. THF was
revealed that the incorporation of nitrogen scaffolds significantly
12ꢀ18
increases the CO2 adsorption capacities of POPs,7,
mainly
caused by Lewis acidꢀLewis base electrostatic interactions of the
nitrogen atoms with the carbon atoms of the CO2 molecules, which
in turn result from dipoleꢀinduced dipole and dipoleꢀquadropole
interactions.19 Therefore, inherently nitrogen rich materials such as
covalent triazine frameworks (CTFs),20ꢀ22 a subclass of POPs, are
promising candidates for CCS. Several works already discussed the
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continuously refluxed over potassium and freshly distilled from in the microwave for 2 h at 150 °C. The now orange suspension
sodium benzophenoneketyl under argon.
was quenched by the addition of water (150 mL) and a saturated
aqueous solution of NaHCO3 (150 mL). The water layer was
extracted with dichloromethane (3 × 300 mL), the combined
organic layers were dried over Na2SO4 and concentrated in vacuo.
The crude product was purified by two times flash column
chromatography (first: CHCl3/Hexane 9:1, second: EtOAc/Hexane
1:4) to give pyrimidineꢀ2,5ꢀdicarbonitrile as colorless crystals
Argon, carbon dioxide, water vapour and nitrogen
adsorption/desorption measurements were performed at 87, 273,
298 and 313 K with an AutosorbꢀiQ surface analyzer with vapour
option (Quantachrome Instruments, USA). Samples were outgassed
in vacuum at 120ꢀ300 °C for 6ꢀ12 h to remove all guests. Poreꢀsize
distributions were determined using the calculation model for Ar at
87 K on carbon (slit pore, QSDFT equilibrium model) or for CO2 at
273 K on carbon (NLDFT model) of the ASiQwin software (v2.0)
from Quantachrome. For BET calculations pressure ranges of the
Ar isotherms were chosen with the help of the BET Assistant in the
1
(372 mg, 2.86 mmol, 72%). H NMR (400 MHz; CDCl3): δ 9.71
9
(2 H, s, Ar); 13C{1H} NMR (68 MHz; CDCl3): δ 160.6, 146.3,
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114.8, 112.7, 111.8.
2,6-Dimethylpyridine-3,5-dicarbonitrile.36
ASiQwin software. In accordance with the ISO recommendations 2,6ꢀDimethylpyridineꢀ3,5ꢀdicarbonitrile was synthesized in
a
multipoint BET tags equal or below the maximum in V · (1 ꢀ P/P0) modified literature procedure.37 3ꢀAminocrotonitrile (1.64 g,
were chosen. The isosteric heats of adsorption were calculated 20 mmol) and hafnium trifluoromethanesulfonate (775 mg,
from the CO2 adsorption isotherms based on the Clausiusꢀ 1 mmol) were put in a microwave vial and degassed three times.
Clapeyron equation using the Quantachrome software AsiQwin Chloroform (16 mL) and ethyl orthoformate (3.29 mL, 20 mmol)
(v2.0). The water used for vapour sorption was of Millipore® were added and the vial was sealed. The mixture was heated in the
quality, and degassed for five minutes in vacuo before use. microwave for 10 min at 120 °C. The dark orange solution was
Different temperatures were controlled by a thermostat using water quenched with a saturated aqueous solution of NaHCO3 (200 mL)
as coolant. At a temperature of 298 K a homebuilt heating cable and the water layer was extracted with dichloromethane
was used to heat the glass tube to 50ꢀ60 °C above the thermostatꢀ (3 × 200 mL). The combined organic phases were dried over
heated sample, such that no condensation of water vapor can occur. K2CO3 and the solvent was evaporated in vacuo. The crude product
was purified by flash column chromatography (Hexane/EtOAc 4:1)
Highꢀpressure hydrogen adsorption/desorption measurements
to give 2,6ꢀdimethylpyridineꢀ3,5ꢀdicarbonitrile as yellow crystals
were performed on an automated Sievert’s type apparatus
(778 mg, 5.0 mmol, 50%). 1H NMR (270 MHz; CDCl3): δ 8.08
(PCTProꢀ2000) with
a
soꢀcalled microꢀdoser (MD) from
(1 H, s, Ar), 2.81 (6 H, s, Me); 13C{1H} NMR (68 MHz; CDCl3): δ
HyEnergy. The original setup was upgraded by a heating and
cooling device to regulate the sample temperature. The adsorption
and desorption isotherms (0ꢀ25 bar) were measured at various
165.0, 143.5, 115.2, 107.3, 24.3.
Synthesis of Covalent Triazine Frameworks. In a typical CTF
temperatures (77 to 298 K) in a sample cell volume of ≈ 1.3 mL synthesis a Duran ampoule (1 x 12 cm) was charged with the
using ultra high purity hydrogen gas (99.999%). Samples were dinitrile (100 mg) and ZnCl2 (1ꢀ10 equivalents, see Table S2)
outgassed in vacuum (4.5 · 10ꢀ6 mbar) at 200 °C for 6 h to remove within a glove box. The ampoule was flame sealed under vacuum
all guests. The isosteric heat of adsorption is calculated from the and was subjected in a tube oven to temperatures between 300ꢀ
absolute adsorbed hydrogen according to a variant of the Clausiusꢀ 600 °C (for temperature programs, see Table S3). After cooling to
Clapeyron equation (see ESI).
ambient temperature, the ampoule was opened and its content
ground thoroughly. The crude product was stirred in H2O (50 mL)
for 1 h, filtered, and washed with 1 m HCl (2 × 50 mL). The
mixture was then stirred at 90 °C in 1 m HCl (50 mL) over night,
filtered, and subsequently washed with 1 m HCl (3 × 30 mL), H2O
Infrared (IR) spectroscopy measurements were carried out on a
Perkin Elmer Spektrum BX II (Perkin Elmer, USA) with an
attenuated total reflectance unit.
Powder Xꢀray diffraction (XRD) was measured on a BRUKER
D8 Avance (Bruker AXS, USA) in BraggꢀBrentano geometry.
(12 × 30 mL),
THF
(2 × 30 mL),
and
dichloromethane
(1 × 30 mL). Finally, the powder was dried over night in a
desiccator.
Elemental analysis (EA) was carried out with an Elementar vario
EL (Elementar Analysensysteme, Germany).
3. RESULTS AND DISCUSSION
Magic angle spinning (MAS) solidꢀstate nuclear magnetic
resonance (ssNMR) spectra were recorded at ambient temperature
on a BRUKER DSX500 Avance NMR spectrometer (Bruker
Biospin, Germany) with an external magnetic field of 11.75 T. The
Synthesis and Characterization. The synthesis of CTFs is
carried out through the Lewis acid catalyzed trimerization of
nitriles. The two main procedures generally used include an
ionothermal approach using ZnCl2 at temperatures above 300 °C
1
operating frequencies are 500.1 MHz and 125.7 MHz for H and
13C, respectively, and the spectra were referenced relative to TMS.
The samples were contained either in 2.4 or 4 mm ZrO2 rotors.
and
the
super
acid
mediated
synthesis
using
trifluoromethanesulphonic acid at 0ꢀ120 °C.20, 22, 29 In both cases the
Lewis acid acts as both solvent and catalyst. In this work we
followed the ionothermal procedure using pyrimidineꢀ2,5ꢀ
Solutionꢀstate NMR spectroscopy was performed on a JEOL
DELTA NMR (JEOL, Japan) by single pulse experiments. The
spectra were referenced against CDCl3 (δ(1H) 7.26 ppm,
δ(13C{1H}) 77.16 ppm).
dicarbonitrile
(pymꢀCTF)
and
2,6ꢀdimethylpyridineꢀ3,5ꢀ
dicarbonitrile (lutꢀCTF) as two new precursors, and 5,5’ꢀdicyanoꢀ
2,2’ꢀbipyridine32 (bipyꢀCTF) and 1,4ꢀdicyanobenzene20, 22 (CTF1)
as known precursors to produce a series of CTFs (Scheme 1).
Microwave reactions were carried out in a Biotage Initiator
(Biotage AB, Sweden) in 10ꢀ20 mL microwave vials from Biotage.
Pyrimidine-2,5-dicarbonitrile.35 A microwave vial was charged
with dry DMF (20 mL), 5ꢀbromoꢀ2ꢀiodoopyrimidine (1.14 g,
4 mmol), Zn(CN)2 (517 mg, 4.4 mmol), Pd(PPh3)4 (462 mg,
0.4 mmol) and 1,5ꢀbis(diphenylphosphino)pentane (182 mg,
0.4 mmol) and the vial was sealed. The yellow mixture was heated
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Scheme 1. Schematic synthesis of the CTFs discussed in this work:
CTF1 (top), pymꢀCTF (middle), lutꢀCTF (bottom left) and bipyꢀ
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CTF (bottom right).
The resulting black powders are not soluble in water, acids,
bases and organic solvents indicating that fully condensed
networks are formed. Previous works revealed that CTFs
synthesized at low temperatures (300ꢀ400 °C) have rather wellꢀ
defined local structures, but at the same time low surface areas
(SA). Syntheses at higher temperatures show a dramatic increase in
the SAs accompanied by different extents of structural degradation.
As increased SAs can lead to higher adsorption capacities for CO2,
a series of temperatures was tested for the syntheses of the new and
known CTFs, which are displayed in Table S2.
Figure 1. IR spectra of bipyꢀCTF300ꢀ600 (top left), CTF1ꢀ400ꢀ600
(top right), lutꢀCTF300ꢀ600 (bottom left) and pymꢀCTF300ꢀ600
(bottom right).
The IR spectra of the materials synthesized at higher
temperatures (500ꢀ600 °C) are rather featureless, indicating
carbonization of the systems. It is noteworthy that the IR spectra of
all CTFs obtained at temperatures of 500 °C or higher look alike
with only minor differences in the band positions, irrespective of
the building blocks used. Therefore, we assume that the overall
compositions and local structures of the different CTFs become
gradually similar at higher temperatures, even if their microꢀ and
nanostructures may differ significantly.
The synthesized materials were tested with respect to their
crystallinity by Xꢀray powder diffraction (Figure S1). As expected,
only CTF1ꢀ400 showed little degrees of crystallinity, which is
comparable to previous results,20 whereas the other materials were
found to be amorphous. IR spectroscopy was used to survey
whether trimerization of the samples was completed and the
organic linkers were still intact. In Figure 1 the IR spectra of the asꢀ
synthesized materials are presented. Nevertheless, the assigned
signals are very broad and weak in the lutꢀCTFs and pymꢀCTFs and
therefore just a weak indication for the existence of triazine rings in
those samples. For all CTFs the signal of the nitrile group
(~2200 cmꢀ1) is absent, which suggests that complete conversion of
the monomers via trimerization has occurred. At low synthesis
temperatures (300ꢀ400 °C) the materials show more defined bands
and the triazine moieties can be assigned to the bands around 1500
and 1350 cmꢀ1 (highlighted green).
We used ssNMR spectroscopy to obtain more detailed information
on the local structure of the new lutꢀCTFs and pymꢀCTFs. The
spectra of the lutꢀCTFs are shown in Figures 2 and S2. In contrast
to the IR spectra the ssNMR spectra reveal an intact lutidine unit in
lutꢀCTF300 and lutꢀCTF350, along with signals in the nitrile region
(100ꢀ120 ppm). Although the signals in the spectrum of
lutꢀCTF300 are more distinct, both spectra feature a signal at
21 ppm corresponding to the methyl groups, and signals between
100ꢀ170 ppm, which are attributed to the aromatic carbons.
Although no clear triazine peak is visible, which would be expected
at 160ꢀ170 ppm, a shoulder in the signal at 156 ppm in lutꢀCTF300
Figure 2. 13C MAS ssNMR spectra of lutꢀCTF300 (top left), lutꢀCTF350 (top middle), lutꢀCTF400 (top right), pymꢀCTF300 (bottom left),
pymꢀCTF350 (bottom middle) and pymꢀCTF400 (bottom right).
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is observed, which may relate to the existence of triazine units in order and lower nitrogen content.22, 25, 32, 38 Therefore, argon and
the material. The lutꢀCTFs synthesized above 350 °C do not exhibit carbon dioxide physisorption measurements were performed,
a methyl peak and aromatic signals weaken with synthesis which confirm this trend.
temperature, indicating ongoing carbonization of the systems.
Table 1. Elemental analysis of the CTFs obtained at different
Among the pymꢀCTFs only pymꢀCTF300, pymꢀCTF350 and
temperatures.
pymꢀCTF400 show signals in the ssNMR measurements (Figure 2).
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8
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In line with the IR measurements, the spectra of pymꢀCTF300 and
pymꢀCTF350 look similar, while the spectrum of pymꢀCTF400
exhibits rather weak and broad signals in agreement with the
carbonization of the sample. The most prominent signals found for
pymꢀCTF300 and pymꢀCTF350 at 159 ppm likely relate to the
carbons at positions 4 and 6 in the pyrimidine ring, while the
signals around 130 ppm can be assigned to the remaining two
carbons in the ring. Again, in contrast to the IR spectra, there are
indications for residual nitrile groups based on the signals between
100ꢀ120 ppm. Taken the IR and ssNMR measurements together,
we conclude that the pyrimidine unit withstands the synthesis
conditions up to 350 °C, but unequivocal proof for the presence of
triazine units is elusive in all pymꢀCTFs as well as lutꢀCTFs.
Sample
N
C
H
C/N
1.29
1.34
1.33
1.42
1.41
1.87
3.43
3.77
6.17
7.63
2.57
2.70
3.19
3.85
4.96
2.57
3.05
3.33
3.84
4.95
6.41
pym theory
pymꢀCTF300
pymꢀCTF350
pymꢀCTF400
pymꢀCTF500
pymꢀCTF600
CTF1 theory
CTF1ꢀ400
43.06
33.74
34.82
34.04
33.81
27.77
21.86
18.60
12.39
10.37
27.17
23.77
20.14
16.42
13.61
26.73
18.56
19.05
17.46
14.25
11.64
55.39
45.31
46.19
48.26
47.54
52.03
74.99
70.20
76.45
79.16
69.90
64.09
64.28
63.14
67.53
68.78
56.55
63.42
66.98
70.50
74.57
1.55
3.28
3.16
3.16
2.78
1.89
3.15
3.30
2.06
1.34
2.93
3.19
3.71
2.67
2.01
4.49
4.70
3.62
2.83
2.52
1.39
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CTF1ꢀ500
CTF1ꢀ600
To obtain a more detailed picture of the extent of degradation in
the CTFs as a function of temperature, elemental analysis was
used. As can be seen in Table 1 the C/N ratio of the materials
synthesized at low temperatures are close to the theoretical C/N
ratios. The overall lower absolute nitrogen and carbon content
compared to the theoretical values is likely caused by residual
water adsorbed in the pores, as well as metal salts (see Table S8)
entrapped in nonꢀaccessible pores, which cannot be removed even
by extensive washing (see Supporting Information for details).With
rising synthesis temperature the C/N ratio increases dramatically.
Remarkably, the nitrogen content of the samples never decreases
below 10 wt% corresponding to ≈ 40% of the theoretical
composition of the intact frameworks, leaving nitrogen
functionalities as possible binding sites for CO2 in the materials.
Interestingly, the pymꢀCTFs show very high nitrogen contents at all
temperatures, while there is no evidence for intact pyrimidine,
triazine or nitrile units at synthesis temperatures above 400 °C. It
has been shown before that the increase in synthesis temperature
yields materials with higher SAs, which however show less local
bipy theory
bipyꢀCTF300
bipyꢀCTF400
bipyꢀCTF500
bipyꢀCTF600
lut theory
lutꢀCTF300
lutꢀCTF350
lutꢀCTF400
lutꢀCTF500
lutꢀCTF600
Figure 3. Argon adsorption (filled symbols) and desorption (open symbols) isotherms (top) and pore size distributions (bottom) of bipyꢀCTF,
CTF1, lutꢀCTF and pymꢀCTF. The curves of the pore size distributions are shifted vertically in steps of 1 cm3 nmꢀ1 gꢀ1 for clarity.
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As can be seen in Table 2, the BET surfaces of the materials wider pore size distribution with an increasing amount of
increase dramatically when synthesized at temperatures above supermicropores and small mesopores with diameters between 1
400 °C. The highest SA was found for lutꢀCTF600 (2815 m2 gꢀ1), and 3 nanometers emerges, which is best visible for the materials
ranging in the upper field of all CTF materials reported to date. It bipyꢀCTF600, CTF1ꢀ600 and lutꢀCTF600.
should be mentioned that there were only sorption measurements in
the BET range performed for pymꢀCTF300ꢀ400, pymꢀCTF500 and
bipyꢀCTF300 (Figure S3), since pymꢀCTF300ꢀ400 showed a very
The pore size distributions of the materials suggest the presence
of pores that are smaller than 0.5 nm (ultramicropores), the
detection limit for Ar physisorption experiments. Therefore, we
low SA and the latter ones were not measureable because of
performed CO2 physisorption measurements at 273 K, which allow
abnormally high equilibrium times. The sluggish equilibration
us to analyze pores down to ≈ 0.35 nm. Figure 4 depicts the CO2
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times suggest the presence of pores with sizes just accessible to Ar
sorption isotherms of the synthesized CTFs. The isotherms feature
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atoms. In Figure 3 (top) the Ar adsorption and desorption isotherms
a prototypical shape and are fully reversible, except the ones of
of the remaining samples are shown. The materials bipyꢀCTF400,
pymꢀCTF300, pymꢀCTF350 (see Figure S12) and pymꢀCTF400,
CTF1ꢀ400, lutꢀCTF300, lutꢀCTF350 and lutꢀCTF400 show typical
which exhibit a significant hysteresis over the whole pressure
range. Here, the value of adsorbed CO2 increases with increasing
synthesis temperature of the CTFs. Interestingly, the pore size
distributions of lutꢀCTF, pymꢀCTF, bipyꢀCTF and CTF1 are similar
shapes of a type I isotherm, featuring rapid uptake of argon at low
relative pressures (p p0ꢀ1 < 0.05), which is characteristic for
micropores.39 The isotherms of bipyꢀCTF500, bipyꢀCTF600,
lutꢀCTF500 and pymꢀCTF600 show similar shapes, but cannot be
with three distinct peaks around 0.35, 0.6 and 0.8 nm, which are
described as type I because of continuous adsorption of argon at
indicative of similar pore structures in all investigated CTFs, both
higher pressures, indicating additional mesoporosity with a broad
at lower and higher synthesis temperatures, respectively
(Figure S15).
mesopore size distribution.39 Notably, the isotherm of bipyꢀCTF600
shows a hysteresis of type H4 spanning the range p p0ꢀ1 = 0.3ꢀ0.9,
CO2 uptake and selectivity. To further investigate the CO2
which is characteristic of systems with slitꢀshaped pores.39 Finally,
physisorption characteristics of the presented CTFs, we
the isotherms of CTF1ꢀ500, CTF1ꢀ600 and lutꢀCTF600 show a
additionally performed temperatureꢀdependent gas uptake
combination of type I and IV isotherms, where the micropore
measurements at 298 and 313 K (Figures S13ꢀ14) and calculated
filling is followed by mesopore filling. The hystereses around
the heats of adsorption (Qst). The quantities of adsorbed CO2 at
p p0ꢀ1 = 0.4 are of type H2, describing systems with rather illꢀ
273 K shown in the previous section are remarkably high and are
defined pore sizes and shapes, which is often observed for
summarized together with the measured values at 298 and 313 K in
amorphous materials.
Table 2 and S6. At 273 K the highest CO2 uptake was found for
(5.58 mmol gꢀ1,
223 mg gꢀ1),
followed
by
The pore size distributions (Figure 3, bottom) from QSDFT
bipyꢀCTF600
bipyꢀCTF500 (5.34 mmol gꢀ1, 214 mg gꢀ1). The lutꢀCTFs and
CTF1s show high adsorptions as well, most of them exceeding
uptakes of 4 mmol gꢀ1.
calculations clearly depict the pore size evolution as a function of
the synthesis temperatures. Almost all samples show distinct pore
sizes around 0.5 and 1 nm. With higher synthesis temperature a
Table 2. BET surface areas, CO2, H2 and N2 uptake behaviour, heats of CO2 adsorption and CO2/N2 selectivities of the presented
CTFs.
BET SAa CO2 uptakeb [mmol gꢀ1]
H2 uptakec [wt%]
25 bar
N2 uptaked
[mmol gꢀ1]
ꢀ
Qst [kJ molꢀ1]
Maxe Min
Selectivityf
Henry IASTg
Sample
[m2 gꢀ1]
273 K 298 K 313 K 1 bar
pymꢀCTF300
pymꢀCTF350
pymꢀCTF400
pymꢀCTF500
pymꢀCTF600
CTF1ꢀ400
ꢀ*
0.28
0.33
0.45
2.75
3.34
2.83
4.26
4.36
1.87
3.08
5.34
5.58
3.63
4.06
4.55
5.04
4.99
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ*
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ*
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
208
689
610
1830
2557
360
753
1548
2479
486
635
968
1680
2815
1.77
2.15
1.52
2.23
2.21
0.98
1.78
3.07
2.95
2.14
2.41
2.72
2.58
2.52
1.26
1.44
0.98
1.47
1.39
0.61
1.18
1.77
1.84
1.51
1.59
1.80
1.71
1.66
ꢀ
ꢀ
0.13
0.16
0.08
0.19
0.23
0.05
0.10
0.25
0.28
0.14
0.16
0.20
0.23
0.22
40.5
37.4
33.7
35.3
31.6
33.3
35.2
34.2
34.4
36.6
37.4
37.5
38.2
33.3
39.0
30.5
30.2
27.0
24.6
30.7
28.0
31.0
27.0
31.4
33.9
30.8
30.0
24.9
189
126
59
36
26
47
50
61
37
69
76
63
27
26
502
124
45
29
17
41
40
42
24
57
66
53
27
23
ꢀ
ꢀ
ꢀ
ꢀ
CTF1ꢀ500
1.70
2.12
ꢀ
3.36
4.34
ꢀ
CTF1ꢀ600
bipyꢀCTF300
bipyꢀCTF400
bipyꢀCTF500
bipyꢀCTF600
lutꢀCTF300
lutꢀCTF350
lutꢀCTF400
lutꢀCTF500
lutꢀCTF600
ꢀ
ꢀ
1.63
2.10
ꢀ
2.71
4.00
ꢀ
1.22
1.36
1.60
2.00
1.79
2.09
2.90
4.18
aFrom Ar sorption. bAt 1 bar. cAt 77 K. dAt 1 bar and 298 K. eAt zero coverage. fCO2/N2 at 298 K. gA CO2:N2 ratio of 15:85 was used. * BET fit
yields less than 5 m² gꢀ1. Errors assumed to be in the range of ± 20 m2 gꢀ1.
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Only the pymꢀCTFs perform in the lower class of CTFs in
terms of CO2 sorption capacity, most likely due to their low
SAs.Compared to the literature, the value measured for
bipyꢀCTF600 is even slightly higher than that obtained for the
fluorinated CTF FCTFꢀ600 (5.53 mmol gꢀ1)40 and therefore
represents the highest value reported for all CTFs so far and
the second best value among all POPs, where bipyꢀCTF600 is
outperformed only by the porous polymer framework PPFꢀ1
(6.12 mmol gꢀ1), an imineꢀlinked polymer.41 Notably, other
high performance CO2 sorbent materials such as
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benzimidazoleꢀlinked
polymers
azoꢀlinked polymers (ALPs; 3.52ꢀ
(BILPs;
2.91ꢀ
16, 18, 42
5.34 mmol gꢀ1),14,
5.37 mmol gꢀ1)17 and aminalꢀlinked porous organic polymers
(APOPs; 2.27ꢀ4.45 mmol gꢀ1)43 show lower CO2 uptakes
compared to bipyꢀCTF600. As can be seen in Table 2 and
Figure 5 (top right), the CO2 uptake does not directly correlate
with the BET SAs of the materials. bipyꢀCTF600 has a BET
SA just less than CTF1ꢀ600, but takes up significantly more
CO2 and additionally outperforms lutꢀCTF600, which has a
significantly higher SA. A correlation between CO2 uptakes
and pore sizes has previously been revealed in porous carbons,
suggesting increased CO2 uptake with increasing pore volumes
below 1.5 nm.44 We found a similar trend for our CTFs, which
can be seen in Figure 5 (top left); however in our case several
outliers are present. As mentioned above, the incorporation of
nitrogen in the materials has been demonstrated to lead to
higher CO2 adsorption in POPs,7, 12ꢀ18 which is in line with our
finding that the integration of pyridine units (bipyꢀCTF and
lutꢀCTF) leads to higher CO2 uptakes compared to CTF1 with
a purely carbonꢀbased linker (see Figure S42).
Figure 5. Dependencies of CO2 uptake on the micropore volume
(≤ 1.5 nm, top left) and BET SA (top right), and of the IAST
selectivity on the nitrogen content (bottom left) and CO2 uptake
(bottom right). lutꢀCTFs (black squares), pymꢀCTFs (red circles)
bipyꢀCTFs (blue triangles) and CTF1s (orange diamonds).
The heats of adsorption (Qst) can be considered as a rough
estimate of these interaction energies. The calculated curves
are shown in Figure S16 and the maxima and minima are
listed in Table 2. Qst values of 30ꢀ50 kJ molꢀ1 are preferred for
CO2 capture materials since in principle higher values lead to
higher adsorption and selectivities, while materials with Qst
values exceeding 50 kJ molꢀ1 need high energies for desorption
and, hence, regeneration.45 The Qst values at zero coverage of
the presented CTFs all exceed 30 kJ molꢀ1 and even at high
loading the values of most of the materials do not drop below
that mark. This finding indicates relatively strong interactions
of the sorbent and sorbate, thus giving a rationale to the
observed overall high CO2 capacities of the materials.
Interactions between CO2 and basic nitrogen sites would
indeed explain the higher amount of CO2 adsorbed, therefore
we assume that the nitrogen content of the CTFs is a
secondary effect weakly enhancing the CO2 uptake. While
small micropores are the main contributing factor for the
observed high CO2 uptakes, they also contribute to a higher
degree of accessible nitrogen sites and are therefore “vehicles”
to efficiently utilize the nitrogen sites present in the sample.
The nitrogen content and type of nitrogen sites present should
influence the interaction energy between the material and CO2
The flue gas of coalꢀfired power plants consists of
8
approximately 15% CO2, 5% H2O, 5% O2 and 75% N2.4,
sorptive as
interactions.
a consequence of Lewis acidꢀLewis base
Therefore, the sorption selectivity of CO2 over N2 is
particularly relevant in order to evaluate the CTFs as potential
flue gas sorbents. We calculated the selectivities by the ratios
of the initial slopes in the Henry region and the ideal adsorbed
solution theory (IAST) at 298 K (see Supporting Information).
The values listed in Table 2 show a discrepancy between the
two different theories with higher selectivities found by Henry
calculations. Since the Henry calculations only take the initial
slopes into account, while the IAST calculations consider the
whole pressure range, it may be concluded that the materials
show high selectivities towards CO2 in the low pressure range,
which decreases with higher working pressures, as shown in
Figure S25. The highest selectivities (Henry: 189, IAST: 502)
were found for pymꢀCTF500 which to the best of our
knowledge outperforms all POPs measured so far, including
PPNꢀ6ꢀCH2DETA (IAST: 442)13, TPI@ICꢀ2 (IAST: 151)46
and PCBZL (IAST: 137).47 Overall, the selectivities decrease
with increasing adsorption capacity (Figure 5, bottom right),
which is a general trend in POP chemistry.8 An even better
correlation is found between the nitrogen content and the
selectivity as can be seen in Figure 5 (bottom left). The best
Figure 4. Carbon dioxide adsorption (filled symbols) and
desorption (open symbols) isotherms of bipyꢀCTFs, CTF1s, lutꢀ
CTFs and pymꢀCTFs measured at 273 K.
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compromise of high CO2 uptakes and high selectivities was
2.71 wt%, 10 bar), while a direct comparison is difficult due to
found for lutꢀCTF350 (Henry: 76, IAST: 66), lutꢀCTF400
(Henry: 63, IAST: 53) and bipyꢀCTF500 (Henry: 61, IAST:
42), which besides good selectivities show uptakes above
4 mmol gꢀ1. For comparison PPFꢀ1, the best POP for CO2
uptakes reported to date, shows low IAST selectivities of 15 at
273 K.41
the different pressure conditions.58ꢀ62 The highest values for
POPs were found so far for the highly porous 3D COFs (6.98ꢀ
7.16 wt%, sat. pressure),57 PAFs (4.2ꢀ7.0 wt%)63, 64 and PPNs
(8.34 wt%, 55 bar),13 which however were measured at higher
pressures. Other CTFs were only studied with respect to their
H2 capacities up to 1 bar, where CTFꢀ1 adsorbs 1.55 wt%,20
PCTFꢀ1 1.86 wt%24 and PCTFꢀ2 0.9 wt%.24 CTF1ꢀ600
(2.12 wt%), bipyꢀCTF600 (2.10 wt%) and lutꢀCTF600
(2.00 wt%) studied here substantially outperform these
materials and show even higher adsorptions than flꢀCTF400
(1.95 wt%) at that pressure. In contrast to the CO2 uptakes, the
H2 uptakes at 77 K and 25 bar strictly depend on the BET SAs:
The higher the total SA, the higher the observed H2
adsorption. This observation supports our previous
observations, namely that the adsorption of CO2 is promoted –
at least to some extent – by the accessible nitrogen sites due to
electrostatic interactions, which seems to be irrelevant for H2
adsorption.
Water vapor sorption. CTFs with their high surface areas
and pore volumes are potential candidate for atmospheric
water capture, which is a promising technology for drinking
water supply in arid climates. To date, there are no studies
about the water sorption characteristics of CTFs, which is
inherently related to the surface polarity and framework
porosity. We have therefore explored the potential of two
CTFs described herein for ambient vapour harvesting by
measuring their water vapor sorption isotherms. The two
samples bipyꢀCTF500 and pymꢀCTF500, which show high
CO2 uptake and selectivity, respectively, were investigated as
model systems. Each material offers a distinct water vapour
sorption isotherm (Figure 6). While bipyꢀCTF500 shows a
gradual rise in water uptake at low pressures reminiscent of
porous carbons, which indicates a rather hydrophobic nature,48
pymꢀCTF500 shows a more zeoliteꢀlike behaviour.49 The
rather steep uptake at low pressures observed for pymꢀCTF500
attests to its more hydrophilic nature, which likely is a direct
consequence of its almost doubled nitrogen content compared
to bipyꢀCTF500. The observed uptake behaviour of pymꢀ
CTF500 puts it amongst the most hydrophilic porous carbon
known:48 At p p0ꢀ1 = 0.1, pymꢀCTF500 shows an uptake of
115 cm3 gꢀ1, which is higher than that observed for the
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4. CONCLUSION
Two new covalent triazine frameworks – lutꢀCTF and
pymꢀCTF – were obtained by ionothermal synthesis at
different temperatures and their composition, porosity as well
as CO2 and H2 uptakes were analyzed and compared to those
of bipyꢀCTF and CTF1. While the heterocyclic building
blocks show varying levels of thermal stability, all materials
were found to be microporous with high surface areas up to
2815 m2 gꢀ1 and show very high CO2 uptakes. The best uptakes
were found for the pyridineꢀbased materials bipyꢀCTF600
(5.58 mmol gꢀ1),
bipyꢀCTF500
(5.34 mmol gꢀ1)
and
53
mesoporous CMKꢀ350, PCCꢀ1,51 activated charcoals,52,
lutꢀCTF500 (5.04 mmol gꢀ1), with bipyꢀCTF600 being the
second best performing material of all POPs and the best
performing CTF reported to date. Our work reveals a general
trend according to which the CO2 uptake capacities
predominantly scale with the micropore volume, rather than
the total BET SA, while the nitrogen content is a secondary
effect weakly enhancing the CO2 uptake. The latter however
was found to be the main contributor to the high CO2/N2
selectivities found for the CTFs. In fact, the porous material
with the highest nitrogen content, pymꢀCTF500, showed the
highest selectivity (Henry: 189, IAST: 502) found for all POPs
to date. In addition, water vapour sorption has been tested for
CTFs for the first time, showing the great potential of fineꢀ
tuning the hydrophilicity of carbonꢀbased materials by
adjusting the nitrogen content. Finally, H2 sorption
measurements reveal high uptake capacities at ambient and
high pressures, rendering the presented CTFs interesting
candidates as prospective CCS and hydrogen storage
materials.
carbideꢀderived carbon fibers,54 and carbon nanotubes.55, 56
Figure 6. Water vapor adꢀ (filled symbols) and desorption (open
symbols) isotherms of bipyꢀCTF500 and pymꢀCTF500 measured
at 298 K.
H2 storage. We further performed hydrogen storage
measurements at 77 K, 87 K and 298 K up to 25 bar with the
materials that showed the highest CO2 uptake capacities
(Table 2, Figures 7 and S22ꢀ23). The highest H2 uptake was
found for CTF1ꢀ600 (4.34 wt%), which is comparable to that
reported for flꢀCTF400 (4.36 wt%, 20 bar),25 but higher than
that observed for 2D COFs (1.46ꢀ3.88 wt%, sat. pressure)57
and polymers of intrinsic microporosity (PIMs, 1.45ꢀ
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Figure 7. Hydrogen adsorption (filled symbols) and desorption (open symbols) isotherms of bipyꢀCTFs, CTF1s and lutꢀCTFs measured at
77 K.
frameworks on small gas uptake and selective CO2 capture. J. Mater.
Chem. A 2013, 1, 10259ꢀ10266.
ASSOCIATED CONTENT
For precursor synthesis, PXRD, gas sorption data, ssNMR spectra
and residual metal content, see supporting information. The
Supporting Information is available free of charge via the Internet
at http://pubs.acs.org/.
13.
Lu, W.; Sculley, J. P.; Yuan, D.; Krishna, R.; Wei, Z.;
Zhou, H.ꢀC., PolyamineꢀTethered Porous Polymer Networks for
Carbon Dioxide Capture from Flue Gas. Angew. Chem., Int. Ed. 2012,
51, 7480ꢀ7484.
14.
Rabbani, M. G.; ElꢀKaderi, H. M., TemplateꢀFree Synthesis
9
AUTHOR INFORMATION
of a Highly Porous BenzimidazoleꢀLinked Polymer for CO2 Capture
and H2 Storage. Chem. Mater. 2011, 23, 1650ꢀ1653.
15.
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Corresponding Author
Xie, L.ꢀH.; Suh, M. P., High CO2ꢀCapture Ability of a
Porous Organic Polymer Bifunctionalized with Carboxy and Triazole
Groups. Chem. ꢀ Eur. J. 2013, 19, 11590ꢀ11597.
*B.V. Lotsch. Eꢀmail: b.lotsch@fkf.mpg.de.
16.
Rabbani, M. G.; ElꢀKaderi, H. M., Synthesis and
Notes
Characterization of Porous BenzimidazoleꢀLinked Polymers and
Their Performance in Small Gas Storage and Selective Uptake. Chem.
Mater. 2012, 24, 1511ꢀ1517.
The authors declare no competing financial interest
.
ACKNOWLEDGEMENTS
17.
Arab, P.; Rabbani, M. G.; Sekizkardes, A. K.; Đslamoğlu,
The authors acknowledge financial support by the Max Planck
Society, the Deutsche Forschungsgemeinschaft (DFG; SPPꢀ1362,
LO 1801/2ꢀ1), the Nanosystems Initiative Munich (NIM), the
Center for Nanoscience (CeNS) and the Fonds der Chemischen
Industrie (FCI). H. Oh was supported for this research through a
stipend from the International Max Planck Research School for
Advanced Materials (IMPRSꢀAM). We thank Christian Minke for
ssNMR measurements. We acknowledge Prof. Thomas Bein and
Prof. Wolfgang Schnick for access to the respective measurement
facilities.
T.; ElꢀKaderi, H. M., Copper(I)ꢀCatalyzed Synthesis of Nanoporous
AzoꢀLinked Polymers: Impact of Textural Properties on Gas Storage
and Selective Carbon Dioxide Capture. Chem. Mater. 2014, 26, 1385ꢀ
1392.
18.
Altarawneh, S.; Behera, S.; Jena, P.; ElꢀKaderi, H. M., New
insights into carbon dioxide interactions with benzimidazoleꢀlinked
polymers. Chem. Commun. 2014, 50, 3571ꢀ3574.
19.
Vogiatzis, K. D.; Mavrandonakis, A.; Klopper, W.;
Froudakis, G. E., Ab initio Study of the Interactions between CO2 and
NꢀContaining Organic Heterocycles. Chem. Phys. Chem. 2009, 10,
374ꢀ383.
20.
Kuhn, P.; Antonietti, M.; Thomas, A., Porous, Covalent
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