Tetraphenylethylene-based microporous organic polymers: insight into structure geometry, porosity, and CO₂/CH₄ selectivity

Abstract

Tetraphenylethylene-based microporous organic polymers with tunable porosities can be synthesized from several acetyl-modified tetraphenylethylenes with mannitol or pentaerythritol. The employment of building blocks with different chemical structures accompanied by a linkage geometry engineering approach have led to the formation of eight polymers (PTPOP-1–4 and MTPOP-1–4) presenting different Brunauer–Emmett–Teller specific surface areas and total pore volumes. In addition, different carbon dioxide adsorption capabilities of the resulting polymers are studied, and their CO₂/CH₄ selectivities are assessed. These results help shed light on designing porous organic polymers with controllable porosities and gas adsorption properties.

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Introduction

Porous organic polymers (POPs), constructed from multiple organic building blocks, are rising as a new kind of material with applications in need of porosity. The applications of POPs in gas separation and storage¹ have been explored extensively, while other fields such as sensing,² catalysis,³ and membrane separation⁴ have also attracted considerable attention. Quite a number of POPs, including covalent organic frameworks (COFs),⁵ polymers of intrinsic microporosity (PIMs),⁶ conjugated microporous polymers (CMPs),⁷ porous aromatic frameworks (PAFs),⁸ covalent triazine-based frameworks (CTFs),⁹ porous polymer networks (PPNs),¹⁰ and hypercrosslinked polymers (HCPs)¹¹ have been obtained through various condensation reactions. Recently, a series of acetal- or ketal-linked POPs have been reported by our group.¹² The polymers were synthesized through the condensation of pentaerythritol or mannitol with some aromatic acetal/ketal derivatives. The obtained polymers exhibit encouraging carbon dioxide sorption capacities and CO₂/CH₄ selectivities. The economical and environmentally friendly polymerization makes it a promising methodology for constructing POPs.

The various functionalizations of POPs offer opportunity for controlling over both chemical structure and gas adsorption performance. With optimum designed surface area or pore size, POPs can be tailored for a broader range of applications. However, as amorphous POPs without ordered alignment, there are no certain relevance between their structure and function. Therefore, huge challenges exist in porosity control and rational design of functionalized POPs. Up to present, intense research efforts have been devoted to controlling the surface area and porosity of POPs, including refining the organic building blocks,¹³ altering the reaction conditions (time/temperature/solvent),¹⁴ and introducing suitable template.¹⁵ Particularly, elaboration of monomers was one of the earliest attempts.^13a A series of poly(aryleneethynylene) (PAE) networks constructed from monomer struts with differing lengths perform fine-tuned porosities.

Carbon dioxide is one of the acknowledged greenhouse gases in the atmosphere that speed up global warming. Moreover, it is also considered to be an undesired composition of natural gas. As chemically and thermally stable materials, POPs generally exhibit highly accessible internal pore surfaces and a variety of chemical functionalities, which make them promising candidates for carbon dioxide (CO₂) capture and separation. The ability to carbon dioxide affinity of POPs can also be tuned by choosing building blocks with different chemical composition, and this method has favorable application foreground in carbon capture and storage (CCS) and gas separation. Varying the linkage geometry to realize the control of the porosity of POPs opens up new prospect on monomer engineering.¹⁶

Herein, we report the strategy of combining different structural linkage and variant geometry to get insight into the tuning of porosities and related vital properties for POPs. The copolymerizations of polyol (pentaerythritol or mannitol) and tetraphenylethylene derivatives afford eight separate microporous organic polymers with differing geometries, denoted as PTPOP-1–4 and MTPOP-1–4. The resulting POPs exhibit tunable surface area as well as carbon dioxide adsorption capability and CO₂/CH₄ selectivity. Considering the current challenge on controlling over the porosity and property of porous polymers, the porosity engineering reported here contributes to elucidating the general rules for the purposive design of POPs.

Experimental section

Materials

Tetraphenylethylene, p-toluenesulfonic acid monohydrate (TSA), pentaerythritol, mannitol, acetyl chloride, and o-dichlorobenzene were purchased from the Beijing Chemical Reagent Company. Dichloromethane, acetone, ethanol, ethyl acetate, petroleum ether, and other organic solvent were analytical reagent used without further purification. Acetyl-substituted aromatic monomers 1,1-bis(4-acetylphenyl)-2,2-diphenylethylene (M1), trans-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (M2), 1,1,2-tris(4-acetylphenyl)-2-phenylethylene (M3), and 1,1,2,2-tetrakis(4-acetylphenyl)ethylene (M4) were synthesized according to the reported methods, respectively.¹⁷ All condensation reactions were conducted using the standard Schlenk-line technique.

Instrumental characterization

The solid-state ¹³C cross-polarization/magic-angle-spinning nuclear magnetic resonance (CP/MAS NMR) spectra were recorded on a Bruker Avance III 400 spectrometer (Bruker, Germany). Thermogravimetric analysis (TGA) were carried out on a Pyris Diamond thermogravimetric/differential thermal analyzer (PerkinElmer Instruments Co. Ltd., USA) by heating the samples to 800 °C under nitrogen atmosphere at a heating rate of 10 °C min⁻¹. Fourier transform infrared (FT-IR) spectra were taken on a Perkin Elmer Spectrum One FT-IR spectrometer (PerkinElmer Instruments Co. Ltd., USA). The IR samples were prepared by dispersing polymers in KBr and compressing the blend to get transparent films. Field emission scanning electron microscopy (FE-SEM) images were obtained with a Hitachi S-4800 microscope (Hitachi, Ltd., Japan) operating at an accelerating voltage of 6.0 kV. The samples were prepared by dropping ethanol suspensions onto silicon wafer and coated with gold after drying. High resolution transmission electron microscopy (HRTEM) images were recorded on a Tecnai G2 F20 U-TWIN microscope (FEI, USA) at an accelerating voltage of 200 kV. TEM samples were prepared by dropcasting the ethanol suspension onto copper grids coated with a carbon film. Nitrogen sorption isotherms were measured with an ASAP 2020M + C surface area and porosimetry analyzers (Micromeritics Instrument Corporation, USA) at 77 K. The specific surface area and micropore surface area were calculated based on the obtained sorption isotherms using the Brunauer–Emmett–Teller (BET) model and t-plot method, respectively. The pore size distribution (PSD) profiles of polymers were calculated from the nitrogen adsorption isotherms by the nonlocal density function theory (NLDFT) method, the model is N₂ at 77 K on carbon (slit/cylindr. pores, QSDFT adsorption branch). Total pore volumes were estimated from nitrogen adsorption isotherms at P/P₀ = 0.99. Both carbon dioxide and methane adsorption isotherms under low pressure (0–1.06 bar) were collected on a TriStar II 3020 accelerated surface area and porosity analyzer (Micromeritics Instrument Corporation, USA). Before the measurement, all the samples were degassed for 12 hours at 120 °C under vacuum.

Preparation of PTPOPs

A mixture of M1 (54.1 mg, 0.13 mmol), pentaerythritol (17.7 mg, 0.13 mmol), and TSA (500 mg, 2.90 mmol) was suspended in o-dichlorobenzene (4.00 mL) in a Pyrex tube. After ultrasonication (160 W) for 30 min, the mixture was degassed by three freeze–pump–thaw cycles and then the tube was flame-sealed. After heating at 180 °C for 72 h, the precipitate was filtered and washed with water, ethanol, acetone, and dichloromethane, respectively. PTPOP-1 was obtained as black powder after drying under vacuum at 70 °C for 12 h (53.0 mg, 79% isolated yield).

Similar to the preparation of PTPOP-1, M2 (54.1 mg, 0.13 mmol), M3 (39.9 mg, 0.087 mmol), or M4 (32.5 mg, 0.065 mmol) was used to react with pentaerythritol (17.7 mg, 0.13 mmol) respectively to give PTPOP-2 (67% isolated yield), PTPOP-3 (92% isolated yield), or PTPOP-4 (98% isolated yield).

Preparation of MTPOPs

A mixture of M1 (54.1 mg, 0.13 mmol), mannitol (23.7 mg, 0.13 mmol), and TSA (500 mg, 2.90 mmol) was suspended in o-dichlorobenzene (4.00 mL) in a Pyrex tube. After ultrasonication (160 W) for 30 min, the mixture was degassed by three freeze–pump–thaw cycles and then the tube was flame-sealed. After heating at 180 °C for 72 h, the precipitate was filtered and washed with water, ethanol, acetone, and dichloromethane, respectively. MTPOP-1 was obtained as black powder after drying under vacuum at 70 °C for 12 h (53.0 mg, 72% isolated yield).

Similar to the preparation of MTPOP-1, M2 (54.1 mg, 0.13 mmol), M3 (39.9 mg, 0.087 mmol), or M4 (32.5 mg, 0.065 mmol) was used to react with mannitol (23.7 mg, 0.13 mmol) respectively to give MTPOP-2 (62% isolated yield), MTPOP-3 (89% isolated yield), or MTPOP-4 (96% isolated yield).

Results and discussions

Microporous organic polymers PTPOP-1–4 were synthesized from the condensation of pentaerythritol and four acetyl-substituted tetraphenylethylenes (TPE) with different geometries. To further investigate the controlling porosities in POPs by varying types of linkage, we prepared mannitol-based MTPOP-1–4 as a comparison. The corresponding synthesis routes are illustrated in Fig. 1. The TPE precursors were readily¹⁷ obtained according to the reported methods. We used the general sealed-tube polymerization method to obtain POPs with desired properties according to previous work.¹² The oxygen-free enclosed environment is favorable for high degree of polymerization. The acetyl-substituted TPEs and pentaerythritol (or mannitol) were dispersed in o-dichlorobenzene and TSA as a catalyst was added into the reaction system. After polymerization at 180 °C for 3 days, eight polymers (PTPOP-1–4 and MTPOP-1–4) were obtained in quantitative yields. The condensation reaction adopted here is economical and environmentally friendly compared to the traditional metal-catalyzed polymerization. In most cases, the ketal-linkage in this kind of polymerization is formed through a connection between an acetyl group and two hydroxyl groups in adjacent carbon atoms. Meanwhile, an acetyl group reacts with hydroxyl groups from two different monomers is also possible (Fig. S1, ESI†), which will cause the coexistence of two kinds of structures in PTPOP-1–4 and MTPOP-1–4. All of the prepared POPs are stable and insoluble in most common organic solvents, such as tetrahydrofuran, dichloromethane, and N,N-dimethylformamide, indicating their high degree of cross-linked structures.

Fig. 1 (a) Molecular structures of M1–M4. (b) Schematic description of synthesis route to PTPOPs. (c) Schematic description of synthesis route to MTPOPs.

The thermal stabilities of all the obtained polymers were investigated utilizing TGA measurement from 30 to 800 °C under nitrogen atmosphere (Fig. S2, ESI†). The minor weight loss in the initial stages (below 200 °C) of heating is due to the evaporation of trace of water or gas molecules trapped in the organic porous network. All of the polymers are thermally stable up to 300 °C in the nitrogen atmosphere. The residual mass varies from 70% to 60% for all the polymers. The morphologies of all the prepared polymers were studied by the FE-SEM measurement, and the relatively uniform nanoparticles can be observed in the SEM images (Fig. S3 and S4, ESI†). The intrinsic microporous structures were further confirmed by high-resolution TEM characterization of the networks (Fig. S5 and S6, ESI†).

FT-IR spectra and solid-state CP/MAS ¹³C NMR spectra were collected to characterize the obtained polymer networks. In FT-IR spectra, aromatic C=C vibration bands at 1600 cm⁻¹ are observed for all polymers indicating well retained TPE skeleton (Fig. S7 and S8, ESI†). The intensities of the characteristic C–H band at 2854 cm⁻¹ and C=O band at 1690 cm⁻¹ are substantially attenuated, the hydroxyl stretching of pentaerythritol at 3337 cm⁻¹ and of mannitol around 3400 cm⁻¹ are also reduced, suggesting the cross-linking between acetyl and hydroxyl groups takes place. Furthermore, the intense new band appeared at 1700 cm⁻¹ can be assigned to vibrations of the cyclic acetyl-linkage (–C(O)₂–). The characteristic aliphatic C–H and C–O absorption peaks derived from pentaerythritol or mannitol at around 2900 and 1100 cm⁻¹ can also confirm the prospected structures. All these results demonstrate the successful cross-linking between acetyl monomers and pentaerythritol or mannitol. The solid-state CP/MAS ¹³C NMR spectra for PTPOP-1–4 (Fig. S9, ESI†) contain signals at 128 and 140 ppm that correspond to part of carbon atoms on aromatic ring. The weak peak observed for all PTPOPs at 80 ppm is assigned to the carbon atoms connected to the oxygen atoms in the ketal-linkage. A peak at 19 ppm is assigned to the methyl group derived from the acetyl monomers. Meanwhile, weak chemical shift for the quaternary carbon atoms originating from pentaerythritol are detected at ca. 52 ppm. Similar CP/MAS ¹³C NMR spectra can be observed for MTPOP-1–4 on account of analogous polymer structures (Fig. S10, ESI†). Overall, the FT-IR and solid-state ¹³C NMR spectra can well prove the formation of PTPOP-1–4 and MTPOP-1–4 with assumed linkage in the structures.

The porosity of all the obtained polymers was investigated by nitrogen adsorption–desorption measurements at 77 K (Fig. 2). Rapid nitrogen uptakes observed for PTPOP-1–4 at low relative pressure (P/P₀ < 0.10) indicate their permanent microporosity. The sorption hysteresis between adsorption and desorption branches can generally be interpreted as kinetic desorption barrier of nitrogen molecules due to the narrow pore opening.¹⁸ The isotherms for all polymers show gradually increasing within the range of medium relative pressure (P/P₀ = 0.05–0.90), and a steep rise of the adsorption isotherm at the high relative pressure (P/P₀ > 0.90). The explanation for these phenomena is the presence of macropores that probably arise from the inter-particle porosity. The BET specific surface area results were calculated from the nitrogen sorption branch in the range of P/P₀ = 0.01–0.10 and found to be 380, 400, 610, and 600 m² g⁻¹ for PTPOP-1, PTPOP-2, PTPOP-3, and PTPOP-4, respectively (Fig. S11, ESI†). Obviously, TPEs with more than two acetyl substitutions lead to the construction of porous material with more extended skeletons. Monomer M3 or M4 can theoretically form better interpenetrated polymer networks compared to M1 or M2 due to their densely distributed substitution geometry. Both BET specific surface area and pore volume data well support the theoretical assumption. Meanwhile, MTPOP-1–4 seem follow the similar rule according to the corresponded BET specific surface area and pore volume (Table 1 and Fig. S12, ESI†), which further validate the geometry oriented porosity control speculation. It is worth noting that PTPOPs possess higher BET specific surface area results than MTPOPs with similar structures owing to more contorted pentaerythritol linkage. Pore size distribution (PSD) profiles were evaluated by utilizing the NLDFT model. The majority of micropores for PTPOP-1–4 are populated at the range of 0.68–1.26 nm (Fig. S13, ESI†). PTPOP-4 exhibits a narrow PSD and substantial micropores without distribution in the mesoporous or macroporous range, which match well with its nitrogen adsorption isotherm showing no significant rise at the high relative pressure. The polymers with less cross-linked networks (PTPOP-1 and PTPOP-2) display peaks at mesopore and macropore regions in PSD profiles as well as lower surface area. It should be noted that MTPOP-1 exhibit the lowest BET surface area due to the mannitol linkage with flexible chains and the linear structure originated from the configuration of M1. In comparison, MTPOP-2 with a zig-zag structure shows a higher BET surface area than MTPOP-1. In short, these results demonstrate that the porosity of POPs can be fine-tuned by exploiting linkage with different geometry distribution and configuration.

Fig. 2 Nitrogen sorption isotherms of PTPOP-1–4 (a) and MTPOP-1–4 (b) measured at 77 K.

Table 1 Porosity properties of PTPOP-1–4 and MTPOP-1–4

Polymers	SBET^a (m^2 g^−1)	Smicro^b (m^2 g^−1)	Vmicro^c (cm^3 g^−1)	Vtotal^d (cm^3 g^−1)
a Specific surface area calculated from the nitrogen adsorption isotherm using the BET method in the relative pressure (P/P0) range from 0.01 to 0.10.b Micropore surface area calculated from the nitrogen adsorption isotherm using the t-plot method.c Micropore volume calculated from the nitrogen adsorption isotherm using the t-plot method.d Total pore volume at P/P0 = 0.99.
PTPOP-1	380	260	0.11	0.44
PTPOP-2	400	280	0.11	0.56
PTPOP-3	610	390	0.16	0.61
PTPOP-4	600	450	0.18	0.31
MTPOP-1	100	80	0.03	0.10
MTPOP-2	310	210	0.08	0.30
MTPOP-3	520	310	0.13	0.44
MTPOP-4	470	330	0.13	0.50

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The permanent microporosity of PTPOP-1–4 enlightens us to study their gas uptake capacity. The carbon dioxide adsorption performance for the polymer networks was measured up to 1.0 bar at 273 and 298 K, respectively (Fig. 3). The carbon dioxide uptake of MTPOP-4 at 273 K and 1.0 bar is the highest (11.8 wt%) among all the obtained polymers, though PTPOP-4 possesses the highest BET specific surface area. This result might be attributed to the hydroxyl-rich structure of MTPOP-1–4, which enhances the dipole–quadrupole interactions between carbon dioxide and the hydroxyl functional groups of MTPOP-1–4. From this perspective, linkage engineering also plays a vital role in construction POPs with high carbon dioxide uptake capacity. The steric hindrance of the propeller-like TPE can effectively prevent the π–π packing of the obtained polymers as demonstrated in the reported work.¹⁹ As a result, the TPE building blocks in our polymers are well constructed. It should be noted that the rotation or stretching of the TPE units might afford some hidden micropores that further contribute to the CO₂ adsorption of the polymers. Especially, the CO₂ sorption value of PTPOP-4 (11.8, 273 K) is considerable higher than other reported acetal-linked POPs such as APOP-1–5 (10.4–11.4, 273 K) and MKPOP (6.3–11.5, 273 K). To further determine the binding affinity of the obtained polymers for carbon dioxide, the corresponding isosteric heat (Q_st) of carbon dioxide was calculated with the Clausius–Clapeyron equation by utilizing the adsorption data recorded from 0 to 1.06 bar at 273 and 298 K (Table 2, Fig. S14 and S15, ESI†).²⁰ All the polymer networks present reversible adsorption–desorption behavior and moderate Q_st of carbon dioxide from 16.3 to 23.9 kJ mol⁻¹ at zero coverage. From the above-mentioned Q_st values along with the porosity data, it appears that the interaction of carbon dioxide with the PTPOP-1–4 and MTPOP-1–4 is closely ties to the varied linkages in the porous skeleton. The average Q_st value of all polymers are comparable to that of some reported POPs such as metallic PAFs (15.6–19.2 kJ mol⁻¹)^1a and nitrogen rich BILPs (19.7–26.5 kJ mol⁻¹).^1b Compared with few MOFs with extremely high Q_st,²¹ the moderate enthalpies of adsorption for carbon dioxide reported here is favorable for carbon dioxide escaping from the pores due to the nature of physical adsorption, which greatly reduces the cost of regeneration. This energy-efficient treatment in both pre- and post-combustion carbon dioxide capture is highly preferable because of the zero energy consumption during the regeneration of adsorbent.

Fig. 3 Carbon dioxide adsorption isotherms for PTPOP-1–4 at (a) 273 and (b) 298 K, and for MTPOP-1–4 at (c) 273 and (d) 298 K.

Table 2 Corresponding data of CO₂ adsorption and CO₂/CH₄ separation for PTPOP-1–4 and MTPOP-1–4

Polymers	CO2 at 1.0^a bar			CO2/CH4 selectivity
	273 K	298 K	Qst	Initial slope^b	IAST^c	IAST^d
a CO2 uptake in wt% and the isosteric enthalpies of adsorption (Qst) in kJ mol^−1.b Selectivity (mol mol^−1) was calculated by initial slope method at 273 K.c Selectivity (mol mol^−1, 1.0 bar) was calculated by IAST method at mole ratio of 5/95 for CO2/CH4 (natural gas model) at 273 K.d Selectivity (mol mol^−1, 1.0 bar) was calculated by IAST method at mole ratio of 50/50 for CO2/CH4 (shale gas model) at 273 K.
PTPOP-1	5.9	4.5	23.9	9.0	8.5	2.9
PTPOP-2	6.9	5.0	21.3	9.4	8.6	3.0
PTPOP-3	7.5	5.0	19.6	8.2	8.1	3.2
PTPOP-4	8.6	6.1	20.5	13.0	11.3	4.2
MTPOP-1	5.0	2.7	16.3	5.4	5.2	2.8
MTPOP-2	8.1	6.2	19.2	13.4	11.2	4.0
MTPOP-3	9.5	7.0	19.2	7.2	8.9	3.5
MTPOP-4	11.8	8.8	18.4	9.7	9.8	3.8

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Based on moderately high specific surface area and different gas sorption behaviors for PTPOP-1–4 and MTPOP-1–4, carbon dioxide over methane adsorption selectivity of these materials was also investigated. The carbon dioxide and methane adsorption isotherms were measured at 273 K for all the polymers. The selectivity for the PTPOP-1–4 and MTPOP-1–4 was firstly estimated using the ratios of the Henry's law constant resulted from the initial slopes of the corresponding isotherms collected at a lower than 0.15 bar pressure coverage (Fig. S16 and S17, ESI†).²² The calculated CO₂/CH₄ adsorption selectivity ranges from 5.4 to 13.4 for all the polymers (Table 2). To test the reliability of the selectivity estimated from Henry's constant ratios, we further applied the ideal adsorbed solution theory (IAST) of Myers and Prausnitz²³ to determine the selectivity of carbon dioxide over methane as a function of pressure. This is one of the most commonly used methods to evaluate gas selectivity for two-component gas based on pure gas isotherms and has been used widely to predict gas selective adsorption performance for many porous materials.²⁴ The carbon dioxide and methane isotherms of PTPOP-1–4 and MTPOP-1–4 are well fitted by employing the dual-site Langmuir model and the single-site Langmuir model, respectively (black lines in Fig. S18 and S19, ESI†). The detailed IAST data for PTPOP-1–4 and MTPOP-1–4 are listed in Table 2, and the calculated CO₂/CH₄ selectivity for binary gas mixtures with different composition ratio as function of pressure are shown in Fig. 4. The calculated IAST selectivity is in good agreement with the values obtained from Henry's constant ratios. When specifying the carbon dioxide and methane gas mixture with a mole fraction ratio of 0.05/0.95 (typical composition of natural gas), the PTPOP-4 exhibits the highest CO₂/CH₄ selectivity (11.3) among all the polymers at 273 K and 1.0 bar. When altering the mole fraction ratio to 0.5/0.5 (shale gas), the IAST selectivities were calculated ranging from 2.8 to 4.2 at 273 K and 1.0 bar. These values are comparable to some of the reported POPs, such as the APOPs²⁵ (5.3–6.7, 273 K, 1.0 bar) and PANs²⁶ (7 and 10, 273 K, 1.0 bar). Notably, PTPOP-4 and MTPOP-4, originated from M4, show preferable CO₂/CH₄ adsorption selectivity behavior in virtue of the high degree of cross-linking networks. Moreover, MTPOPs exhibit superior CO₂/CH₄ adsorption selectivity behavior to PTPOPs on account of the hydroxyl-rich structure of MTPOPs. The results suggest both the linkage structure and geometry have considerable effect on the gas sorption performance.²⁷

Fig. 4 CO₂/CH₄ selectivity of PTPOP-1–4 and MTPOP-1–4 for a molar ratio of 5/95 (a and b) and 50/50 (c and d) at 273 K.

Conclusions

In summary, a series of tetraphenylethylene-based porous organic polymers (PTPOP-1–4 and MTPOP-1–4) with tunable porosities were synthesized from several acetyl-modified tetraphenylethylenes with mannitol or pentaerythritol. The as-prepared polymers possessing linkages with different structure and geometry were obtained. These engineering approaches have led to the formation of eight polymer presenting different porosities and gas adsorption properties. The carbon dioxide adsorption capability of all the resulting polymers have been investigated, and so as their CO₂/CH₄ adsorption selectivity. These results are valuable for designing of porous organic polymers with controllable porosity and gas adsorption performance.

Acknowledgements

We thank the financial support of the National Science Foundation of China (Grants no. 21304022, 21374024, and 21574032) and the Ministry of Science and Technology of China (Grant no. 2014CB932200).

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Footnote

† Electronic supplementary information (ESI) available: Details regarding TGA curves, FT-IR and solid-state ¹³C NMR spectra, SEM images, TEM images, isosteric heats of adsorption for CO₂, and the corresponding data of CO₂ selective capture. See DOI: 10.1039/c6ra09061c

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