IV. Three-dimensional porous fused aromatic networks for high performance gas and iodine
4.3 Results and discussion
Three-dimensional (3D) porous organic networks (PONs) with fused aromatic structures were prepared by double condensation between tetrapodal octamine (4,4′,4′′,4′′′- methanetetrayltetrakis(benzene-1,2-diamine)) and pyrenetetraketone (PTK, pyrene-4,5,9,10-tetraone) or hexaketocyclohexane (HKH, cyclohexane-1,2,3,4,5,6-hexaone), respectively, to yield PTK-based PON (P-PON) and HKH-based PON (H-PON) (Figure 4.1). To exclude unexpected results, the resulting PONs were completely worked-up prior to characterizations.
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Figure 4.1. Schematic illustration of the synthesis of fused aromatic networks (PONs): P-PON formed by double condensation between tetrapodal octamine and pyrenetetraketone (PTK). H-PON formed by double condensation between tetrapodal octamine and hexaketocyclohexane (HKH).
Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used to investigate the chemical structures of the resultant PONs. Both the P-PON and H-PON showed quite similar 13C-NMR spectra with a peak at 65 ppm (Figure 4.2), which corresponds to quaternary non-aromatic carbon, confirming the presence of a tetrapodal skeleton (pink dot) in their structure. The aromatic carbon atoms associated with phenyl and pyrazine rings appear in the range of 128.39 to 151.24 ppm. The carbonyl carbon at the edges of P-PON and H-PON appeared at 194.62 and 197.56 ppm, respectively.
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Figure 4.2. Solid state CP-MAS 13C-NMR spectra of 3D PONs. (a) P-PON. (b) H-PON. * indicates side band peak.
Powder X-ray diffraction (PXRD) patterns indicated that both PONs had poor long-range ordering (Figure 4.3a). Their high molecular weights (~∞) and 3D structural complexities were driven by extremely high thermodynamic energy gains, and fast network formation by the kinetically controlled reaction.30 The results suggest that the structural formation of both P-PON and H-PON can mainly be attributed to kinetic control, which results in less ordered structures but extremely higher molecular weights than those of thermodynamically driven products.134
The chemical environment and bonding nature of the samples were resolved using X-ray photon spectroscopy (XPS). Both PONs revealed three major peaks, belonging to C 1s, N 1s and O 1s (Figure 4.3b). The C 1s peak of P-PON was further deconvoluted into three peaks at 284.0 (C-C sp2), 285.0 (C- C sp3) and 285.5 (C-N sp3) (Figure 4.3c). The deconvoluted N 1s peak was further resolved into two major peaks located at 398.30 (pyridinic N=C) and 399.5 (pyridinic N-C) (Figure 4.3d).
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Figure 4.3. Structural characterization of PONs. (a) Powder X-ray diffraction (PXRD) patterns. (b) XPS survey spectra. (c) High resolution C 1s spectrum of P-PON. (d) High resolution N 1s spectrum of P-PON.
The deconvoluted O 1s peak consists of three peaks positioned at 531.30 (C=O), 532,20 (C-O sp3), 533.80 (moisture/air) (Figure 4.4a). Similarly, the C 1s peak of H-PON was also deconvoluted into three main peaks at 283.89 (C-C sp2), 285.06 (C=N), 285.68 (C-N) (Figure 4.4b). The N 1s peak at high resolution showed two peaks at 398.6 (pyridinic N) and 400.1(tertiary N) (Figure 4.4c). The O 1s peak contained two peaks at 531.30 (O=C-C), 532,20 (O=C-N) (Figure 4.4d).
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Figure 4.4. Structural characterization of PONs. (a) High resolution O 1s spectrum of P-PON. (b) High resolution C 1s spectrum of H-PON (c) High resolution N 1s spectrum H-PON. (d) High resolution O 1s spectrum of H-PON.
The elemental contents of the PONs were calculated based on structural repeating units, excluding edge contributions. The experimental results from three different techniques, XPS, energy dispersive X- ray spectroscopy (EDXS) and elemental analysis (EA) were compared, and the most reliable EA results matched well with the theoretical values (Table 4.1, 4.2).
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Table 4.1. Elemental composition of P-PON network obtained from different techniques
Method C H N O Total
Theoretical (wt%) 76.67 3.61 12.55 7.17 100
EA (wt%) 76.36 2.95 12.72 5.55 97.87
XPS (at%) 83.67 - 10.93 4.45 99.05
SEM (at%) 78.65 - 13.74 7.60 99.99
SEM (wt%) 75.04 - 15.29 9.66 99.99
Table 4.2. Elemental composition of H-PON network acquired from different techniques
Method C H N O Total
Theoretical (wt%) 66.89 3.40 18.91 10.80 100
EA (wt%) 62.52 3.47 16.44 17.19 99.62
XPS (at%) 77.59 --- 11.68 10.14 99.41
SEM (at%) 63.48 --- 20.89 15.63 100
SEM (wt%) 58.42 --- 22.42 19.16 100
The thermal stability of the PONs were probed by thermogravimetric analysis (TGA) in both air and nitrogen atmospheres (Figure 4.5a, b). It was found that both PONs had a decomposition temperature higher than 450 °C in nitrogen (Figure 4.5c), suggesting that they are thermally stable thanks to their all aromatic network structures. The weight losses (up to 5%) in the low temperature region in both PONs were associated with trapped air-moisture. They are hygroscopic due to the presence of polar heteroatoms (N and O) in their structures.
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Figure 4.5. Thermal gravimetric analysis and porosity of PONs. (a) TGA thermograms obtained with a ramping rate of 10 °C min–1 in air. (a) P-PON. (b) H-PON. (c) TGA curves obtained with a ramping rate of 10 °C min−1 in nitrogen atmosphere. (d) Nitrogen (N2) adsorption-desorption isotherms calculated at 77 K.
The specific surface areas (SBET) and permanent porosities of the PONs were determined by nitrogen (N2) adsorption-desorption isotherms at 77 K. Both PONs showed steep increases in the relatively low- pressure region (P/Po = 0 − 0.1), suggesting that the isotherms correspond to type-1 behavior with a microporous nature (Figure 4.5d). The SBET values of P-PON and H-PON were approximately 873.43 and 741.3 m2 g–1, respectively. Their pore size distributions were calculated using non-local density functional theory (NLDFT). The values were approximately 1.29 and 0.78 nm, respectively (Fig. 6a, b).
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Figure 4.6. Pore size distributions calculated from NLDFT: (a) P-PON; (b) H-PON.
Due to their intrinsic microporous nature, PONs have attracted huge interest as sorbent materials for hydrogen (H2), carbon dioxide (CO2), methane (CH4) and iodine (I2). To achieve enhanced gas uptakes, PONs should have a large specific surface area with suitable pore size and strong interaction with gas molecules. The introduction of polar heteroatoms, such as nitrogen (N) and oxygen (O), is known to be an effective way to enhance sorption capacity, by inducing interactions between adsorbents and adsobates.31 Taking advantage of their outstanding physicochemical stability, porosity and nitrogen rich sites, the PONs were evaluated as sorbent materials.
Growing CO2 associated environmental problems32-33 have motivated the scientific community to develop new technologies that might help to mitigate global warming, by capturing CO2 at its emission point. The two main strategies for carbon capture and sequestering that have been develop so far are chemisorption (where CO2 is passed through an amine solution) and physisorption on porous materials, such as activated carbon,34 silicates, zeolites,35 metal organic frameworks (MOFs)36-37 and COFs.38
In this study, the adsorption isotherms for CO2 were investigated at two different temperatures (Figure 4.7a, d). The low-pressure CO2 uptake capacities were tested at 273 K and 298 K. The CO2
uptakes of P-PON were found to be 14.57 (273 K) and 8.77 wt% (298 K). The values were 17.20 (273 K) and 10.57 wt% (298 K) for H-PON. The results indicated that H-PON had the higher CO2 uptakes at both low and high temperatures. Because H-PON has a richer nitrogen content per volume compared to P-PON (see Figure 4.1), it has stronger dipole-dipole interactions with CO2 which results in high CO2 uptake.39
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Some special features, such as high energy density, nontoxicity and clean combustion, makes hydrogen an ideal energy carrier, especially for vehicular applications.40 Storage and transportation are two of the main hurdles to the wider use of hydrogen as an energy carrier.41 Recently, new attention has been paid to carbon materials for hydrogen storage, and given their features, PONs were also tested as candidate hydrogen storage materials. The H2 isotherms were calculated at 77 K and 87 K with a pressure range of 1.00 bar (Figure 4.7b, e). Because of their weak van der Waals interactions with hydrogen, both PONs showed very similar H2 uptakes, 1.25 (77 K) and 1.00 wt% (87 K) for P-PON and 1.26 (77 K) and 1.01 wt% (87 K) for H-PON.
Natural gas is considered one of the world’s cheaper and important sources of energy. Methane comprises approximately 90% of natural gas, but it has a low energy density.42 As a result, its transportation and storage requirements limit its utility in practice. Storage of CH4 involves compression and liquification, but both need bulky and expensive equipment. Adsorption on porous material is an alternative approach for methane storage, and the methane uptake behaviors of PONs at low pressure range were studied. As shown in (Figure 4.7c, f), both PONs displayed similar methane uptakes, of 1.68 (273 K) and 1.00 wt% (298 K) for P-PON and 1.79 (273 K) and 1.02 wt% (298 K) for H-PON.
The adsorption isotherms obtained at different temperatures were used to further investigate the isosteric heat of adsorption (Qst), which reveals the interaction between adsorbate molecules and polymer networks. The CO2 and CH4 isotherms were calculated at 273 and 298 K, whereas the H2
isotherm was conducted at 77 and 87 K. As shown in the insets of Figure 4.7 a, d, the Qst values (CO2) for P-PON and H-PON were, respectively, 36.81 and 42.43 KJ mol–1 at zero-coverage. The higher Qst
value of H-PON indicates it had a more favorable physical interaction with CO2 molecules.
Interestingly, the H-PON’s higher CO2 uptake with high Qst value, compared to P-PON under the same conditions, is attributable to the presence of H-PON’s larger specific number of N atoms per volume and smaller pore diameter.41
The Qst values (H2) for P-PON and H-PON were 7.77 kJ mol−1 and 8.15 kJ mol−1, respectively (insets, Figure 4.7 b, e). The Qst values (CH4) for P-PON and H-PON were 22.20 and 23.17 kJ mol–1, respectively (insets, Figure 4.7 c, f). The moderately higher Qst values for both H2 and CH4 uptakes suggest that H- PON has stronger interactions with the adsorbates.
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Figure 4.7. Gas uptake properties of PONs. P-PON measured up to 1 bar: (a) CO2 adsorption-desorption isotherms at 273 and 298 K, (b) H2 uptakes measured at 77 and 87 K, (c) CH4 adsorption-desorption isotherms at 273 and 298 K. The insets are the Qst values for CO2, H2, CH4 as a function of gas uptake estimated from low pressure isotherms. H-PON measured up to 1 bar: (d) CO2 adsorption-desorption isotherms at 273 and 298 K, (e) H2 uptakes measured at 77 and 87 K, (f) CH4 adsorption-desorption isotherms at 273 and 298 K. The insets are the Qst values for CO2, H2, CH4 as a function of gas uptake estimated from the low-pressure isotherms.
To further evaluate their adsorption performance, iodine (I2), a volatile element emitted in the nuclear fission reaction, was investigated as a guest molecule. PON samples (30 mg) were exposed to I2 vapor and its uptake was measured at 75 °C under ambient pressure. The PON samples were further heated at 60 °C for 120 minutes to remove any weakly adsorbed iodine on their surface. Then, the I2
uptake capacity was gravimetrically measured at different time intervals. Based on the results, the P- PON exhibited a rapid iodine uptake over the first 12 h and then reached saturation level between 12 and 24 h. Similarly, the H-PON also revealed quick adsorption and reached an optimum level in 24 h (Figure 4.8a, b). The theoretical uptake capacities (pore volume in cm3 g × iodine density) of P-PON and H-PON with full possession of their pores for I2 were 2.59 and 2.06 g, respectively. The experimental values were 2.50 and 2.00 g, respectively, which are over 96% of their theoritical maxima.
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Consequently, it can be concluded that the microporus 3D networks are entirely accessible to iodine, leading to their high uptake capacities. TGA measurment was also performed to check the retention behaviour of I2 in the PONs (I@PONs, Figure 4.8c, d). The TGA curves of both samples showed two mass loss steps. The first weight losses are associated with the loaded iodine, in the ranges of 55−220 °C for P-PON and 60- 280 °C for H-PON. The second weight losses are related to the decomposition of the PON skeletons, which is similar to that of pristine PONs (Figure 4.5a, b).
Figure 4.8. Iodine uptake properties of PONs. (a) Gravimetric iodine uptake capacity of PONs with respect to time. (a) P-PON. (b) H-PON. TGA thermogram of I@PONs under nitrogen condition (c) P- PON. (d) H-PON. (c) High resolution XPS spectrum of I@P-PON.(d) Gravimetric iodine uptake capacity of H-PON with respect to time. (e) TGA thermogram of the H-PON under nitrogen condition.
(f) High resolution XPS spectrumof I@H-PON.
The existance of iodine and its chemical binding nature in the PONs was examined by XPS spectroscopy (Figure 4.9a, b). The high resolution XPS spectra for I2 showed four major peaks corresponding to physically adsorped I2 (630.36 and 618.97 eV) and chemically bonded I2 (632.39,
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621.00 eV)43 (Figure 4.9c, d). SEM-EDS element mapings showed that the I2 was uniformly distributed over the entire PONs matrix (Figure 4.10, 4.11).
Figure 4.9. Characterization of I@PONs. (a) XPS survey spectra after iodine uptakes: (a) I@P-PON;
(b) I@H-PON. (c) High resolution XPS spectra of (c) I@P-PON. (d) I@H-PON.
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Figure 4.10. (a) SEM image of I@P-PON. Corresponding SEM-EDS elemental mappings: (b) carbon;
(c) nitrogen; (d) iodine.
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Figure 4.11. (a) SEM image of I@H-PON. Corresponding SEM-EDS elemental mappings: (b) carbon;
(c) nitrogen; (d) iodine.
One of the most important factors when evaluating the performance of an absorbent is its recyclability. As shown in (Figure 4.12a, b), the uptake capacity remained at over 81 % and 89 % after five uses for P-PON and H-PON, respectively. The sharp decrease in uptake capacity of P-PON after the first cycle is due to the presence of covalently bonded iodine, as evident in the XPS. After the first cycle, the remaining cycles stayed almost constant. The recyclability tests suggest that the PONs are stable against oxidative I2 for long time exposures at elevated temperature. These uptake properties of PONs may be attributed to the presence of tetrahedral skeleton, polar group (N) and conjugated units,
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which direct the formation of the 3D porous structure essential to sorption performance. Together, the results suggest that the PONs are potential materials for iodine capture.44
Figure 4.12. Cyclability of the iodine uptakes: (a) P-PON; (b) H-PON.