Facile synthesis of aminated indole-based porous organic polymer for highly selective capture of CO₂ by the coefficient effect of π–π-stacking and hydrogen bonding

Abstract

A new aromatic aminated indole-based porous organic polymer, PIN-NH₂, has been successfully constructed, and it was demonstrated that the coefficient effect endows this porous material with outstanding CO₂ absorption capacity (27.7 wt%, 1.0 bar, 273 K) and high CO₂/N₂ (137 at 273 K and 1 bar) and CO₂/CH₄ (34 at 273 K and 1 bar) selectivity.

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Today, one of the most serious environmental problems is climate change, such as global warming and sea-level rises, which are caused by increased concentrations of carbon dioxide (CO₂) in the atmosphere.^1–3 As we all know, CO₂ mainly arises from fossil-fuel combustion in power plants, and the flue gas is always mixed with other gases including nitrogen (N₂), methane (CH₄) and so on. Therefore, it is necessary to design materials for selectively separating and adsorbing CO₂ from these industrial and energy-related sources to improve the environmental problems.^4–6 Aqueous amine solutions are the most common adsorbents for CO₂ separation and capture,⁷ however, not only do these adsorbents degrade over time and are corrosive, toxic, and volatile, but also the regeneration process is highly energy demanding for these systems due to the chemical capture mechanism. As alternatives, porous organic polymers (POPs)^8–10 relying on physical adsorption have become the research focus due to their low density, large specific surface area, good thermal stability, and narrow pore size distribution, but the low uptake capacity, and especially, the poor selectivity are two urgent issues that need to be addressed that seriously restrict the commercialization of POP adsorbents.¹¹ Hence, in the past few years, many methods have been developed to improve the POP performance including increasing the surface area and adjusting the pore size.^12,13

Recently, based on the rapid development of supramolecular interactions^14,15 and the unique advantage of POP materials, i.e., the structure designability, researchers found that introducing special active sites into the framework, such as heteroatoms and diverse organic groups, is a simple and effective way to ameliorate the adsorption performance by the formation of some special non-covalent interactions and various functional groups have been explored.^16–18 Recently, Chang et al.¹⁹ have designed and prepared an novel aerogel (PINAA) that contains both amide and indole groups and they demonstrated that the CO₂ can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area via strong π–π-stacking interactions, and then, the desorbed CO₂ molecule can be captured by an adjacent amide group because of “electrostatic in-plane” interaction. This synergistic effect of electrostatic in-plane and dispersive π–π-stacking interactions of amide and indole with CO₂ endows the resulting aerogel enhanced CO₂ adsorption capacity and CO₂/CH₄ and CO₂/N₂ selectivity. Inspired by this fascinating study, we hypothesized that when the indole group is aminated, the CO₂ can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area via strong π–π-stacking interaction (Fig. 1a), after that, the hydrogen bonding interactions between the O of the CO₂ and –NH of the aniline group would make the CO₂ to further form a stable conformation with the aminated indole system (Fig. 1b), as a result, the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would ensure the high CO₂ adsorption capacity and further enhanced CO₂/CH₄ and CO₂/N₂ selectivity.

Fig. 1 Schematic representation showing the heteroaromatic ring of indole adsorbing CO₂ via π–π-stacking interactions (a) and the CO₂ molecule is further stabled via the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions (b). (c) Synthetic route of PIN-NH₂ aerogel.

To verify our suppose, in this work, we tactfully designed and fabricated an aminated indole-based aerogel PIN-NH₂ via Friedel–Crafts alkylation (Fig. 1c), and its CO₂ adsorption capacity and CO₂/CH₄ and CO₂/N₂ selectivity were immediately investigated. The successful preparation of PIN-NH₂ was confirmed by Fourier transform infrared spectroscopy (FT-IR) and ¹³C solid state cross-polarization magic-angle-spinning nuclear magnetic resonance (¹³C CP/MAS NMR) spectrometer, and the results are in good agreement with the proposed structures (Fig. S1 and S2, ESI†). In the ¹³C CP/MAS NMR spectrum of PIN-NH₂, the peaks at 169–103 ppm are ascribed to the indole group carbons, and the signals located at 35–40 ppm are assigned to the methylene carbons (Fig. S1, ESI†). For FT-IR spectrum (Fig. S2, ESI†), the peak at 3438 cm⁻¹ is attributed to the stretching vibrations of N–H in amine unit and indole amine. The peaks at 2999 cm⁻¹ and 2927 cm⁻¹ are assigned to the stretching vibration of –CH₂– in the polymer network and the peaks at 1630 cm⁻¹ and 1480 cm⁻¹ are ascribed to the vibrations of the aromatic ring skeleton.

The porosity of PIN-NH₂ was quantified by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM) and N₂ adsorption–desorption isotherms at 77 K. As shown in Fig. 2a, the SEM image displays that the PIN-NH₂ consists of aggregated particles with sub-micrometer sizes. And the microporous characteristic can be observed clearly from the TEM image as shown in Fig. 2b, the presence of porous structure provides the essential condition for CO₂ capture and separation. As shown in Fig. 2c, at a low pressure (0–0.1 bar), there is a rapid raise in the N₂ adsorption–desorption isotherm, indicating its microporous nature, and the increase in the N₂ sorption at a relatively high pressure (∼0.9 bar) shows the presence of meso- and macrostructures of the PIN-NH₂. The specific surface area calculated in the relative pressure (P/P₀) range from 0.01 to 0.1 shows that the Brunauer–Emmett–Teller (BET) specific surface area of PIN-NH₂ is up to 480 m² g⁻¹. Additionally, the pore-size distribution (PSD)²⁰ calculation result was shown in Fig. 2d and S3 ESI,† which indicating the pore diameter is about 14 Å and further confirming the microporous feature of the PIN-NH₂. To gain further insight into the microstructural information, the powder wide-angle X-ray diffraction (PXRD) was further performed on the PIN-NH₂ polymer. As shown in Fig. S4, ESI,† only a broad peak at 17.8° 2θ in the PXRD pattern is present, which clearly suggests that the polymer is mainly amorphous in nature. Additional, the thermogravimetric analysis (TGA) show that the microporous material is stable up to 370 °C indicating its potential in post combustion processes operated at high temperatures (Fig. S5, ESI†).

Fig. 2 The microstructures of PIN-NH₂ framework. (a) SEM, (b) TEM, (c) the nitrogen adsorption–desorption isotherms and (d) the pore size distribution of PIN-NH₂ framework.

Owing to the artful structure design and particular preparation method, there is a reserved aniline group on the side of the indole group in the PIN-NH₂ network. It was expected that after the rapidly capture of the CO₂ molecule via the π–π-stacking interactions, the next aniline group would assist to further stabilize the CO₂ molecule via hydrogen bonding interactions, in other words, the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would make this porous organic polymer more efficiently attract CO₂ molecules, which inspires us to investigate the gas uptake capacity. Physisorption isotherms for CO₂ (at 273 K) measured with a pressure more than 1.0 bar indicated that the resulting PIN-NH₂ network exhibited a high carbon dioxide uptake of 27.7 wt% at 1.0 bar, as shown in Fig. 3a. Comparing with most of reported porous materials such as metal–organic frameworks,²¹ activated carbons,²² and microporous organic polymers,^23,24 the porous organic polymer PIN-NH₂ shows an enhanced CO₂ uptake (Table S1, ESI†). The calculation of isosteric heat of adsorption of PIN-NH₂ shows that the heat of adsorption is 35.7 kJ mol⁻¹ (Fig. S6, ESI†), which is higher than that of the reported azo-linked polymers (27.9–29.6 kJ mol⁻¹),²⁵ the acid-functionalized porous polymers (32.6 kJ mol⁻¹).^26,27 and the indole-based porous polymers.^28,29 The high value of the heat of adsorption indicated the strong physisorption effect owing to the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions.

Fig. 3 Gas adsorption isotherms of PIN-NH₂ for CO₂ at 273 K (a), adsorption and desorption isotherms of PIN-NH₂ for different gases at 273 K (b), a CO₂ molecule is adsorbed on the heteroaromatic ring of indole via π–π-stacking interaction (c) and the CO₂ molecule is further stabled via the coefficient effect of π–π-stacking and hydrogen bonding interactions (d), adsorption isotherms of PIN-NH₂ for different gases with 3% RH of water at 273 K (e), reversibility of the PIN-NH₂ polymer in CO₂ capture measured by TGA at 273 K (f).

The application in CO₂ separation and adsorption field of the traditional POPs is limited in a great degree by the poor gas selectivity as the flue gas and natural gas are both mixed gas. Here, we believed that the CO₂ can be easily attracted by the heteroaromatic ring via the π–π-stacking interactions and then stabled with the assist of hydrogen bonding interactions, which leading an enhanced gas selectivity. Therefore, we urgently evaluated the selective gas uptake of the PIN-NH₂ network for small gases (CO₂/CH₄, CO₂/N₂). In the calculation, the ratio of CO₂/N₂ is 15/85 and the ratio of CO₂/CH₄ is 5/95, which is the typical composition of flue gas and natural gas, respectively, the test results were shown in Fig. 3b. It can be found there is a rapid increase for the CO₂ uptake while there is a negligible increase for the CH₄ and N₂ uptake with the increase of the pressure, which maybe due to the unique local dipole–π interactions between the porous organic framework PIN-NH₂ and CO₂ molecule. The test results shows that the CO₂ uptake of PIN-NH₂ is up to 5.92 mmol g⁻¹ at a pressure of 1.0 bar and a temperature of 273 K while the CH₄ and N₂ uptake of PIN-NH₂ is only 0.18 and 0.04 mmol g⁻¹, respectively. The estimated ideal CO₂/CH₄ and CO₂/N₂ adsorption selectivities are up to 34 and 137, respectively. Additionally, the selectivities of PIN-NH₂ toward CO₂ over CH₄ and N₂ at 291 and 303 K were also investigated, respectively, and the results indicated that the resulting polymer PIN-NH₂ still exhibited good selectivity at higher temperatures (Fig. S7 and S8, ESI†).

The high gas selectivities of this microporous framework may attribute to the strong affinity for CO₂ compared with N₂ and CH₄ arising from the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions between the sorbent and CO₂ guest molecule, to further attest the above surmise, we used density functional theory (DFT)³⁰ at the M06-2X level with the aug-cc-pVDZ basis set to investigate the interaction of aminated indole system with CO₂ and the details of the calculation is shown in the ESI.† Fig. 3c and d shows the snapshot for CO₂ capture by a model compound. The calculation result shows that owing to the electron-rich and large binding area, the CO₂ was very easily attracted by the indole plane at a distance of 3.706 Å, and the computational binding energy was 13.23 kJ mol⁻¹ (Fig. 3c). Soon, the balance structure was changed, the CO₂ molecular was moved towards the amino group till the distance between the amino group and CO₂ molecular was 3.037 Å, indicating a hydrogen bonding interaction was formed in this system. As a result, the distance between the indole plane and CO₂ molecular decreased to 3.257 Å from 3.706 Å, and the computational binding energy increased to 42.15 kJ mol⁻¹, which meaning a more steady system was formed (Fig. 3d). In the sense of computational chemistry, the expected strong coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would favor the uptake of CO₂ of the PIN-NH₂ network. Additional, The DFT result also indicated that the interaction energy between CO₂ and the imine group of indole is relatively weak with a correlation distance at 4.041 Å.

As we all know that the CO₂ adsorption property will be affected in a great degree for porous polymers in the presence of water.³¹ In real industrial applications, the flue gas from a power plant is a mixture of CO₂, water vapor, and others. As a result, it has very important practical significance to study the CO₂ capture performance under humid condition. Here, the CO₂ capture property of PIN-NH₂ was studied at a relative humidity of 3% RH, as shown in Fig. 3e, the CO₂ adsorption capacity of PIN-NH₂ decreased from 5.92 to 4.88 mmol g⁻¹ (1.0 bar, 273 K), however, the uptake of CH₄ and N₂ does not affected by the water. These results indicate that adsorption of water diminishes the CO₂ capture. Although the selectivity (CO₂/N₂ = 104, CO₂/CH₄ = 21) is decreased under humid condition, PIN-NH₂, to the best of our knowledge, still has very good CO₂ selectivity over other CO₂ capture materials in similar conditions.³² Moreover, the CO₂ adsorption process is fully reversible (Fig. 3f). Herein, the new aminated indole-based aromatic porous organic polymer PIN-NH₂ synthesized from easily available starting materials demonstrated not only remarkable CO₂ capture capacity, but also prominent CO₂/N₂ and CO₂/CH₄ selectivities. Further, the dynamic breakthrough separation experiments of gas mixture at 298 K using a fixed-bed column packed with PIN-NH₂ was carried out to evaluate the performances of PIN-NH₂ aerogel in an actual adsorption-based separation process. The details of the experiment process were described in ESI.† As shown in Fig. S9 and S10, ESI,† the CH₄ and N₂ penetrated through the bed firstly with a retention time for only 6.5 and 3.4 min, respectively, while PIN-NH₂ column can retain CO₂ until above 23 min, which means the high CO₂ adsorption capacity and selectivity of the PIN-NH₂ adsorbent in actual application.

Conclusions

In this work, we have designed and synthesized a novel aromatic aminated indole-based porous organic polymer PIN-NH₂ via Friedel–Crafts alkylation of 4-aminoindole with formaldehyde dimethyl acetal. FTIR and ¹³C CP/MAS NMR characterizations were performed to study the structural information and confirmed the successful formation of the resulting porous organic polymer PIN-NH₂. The nitrogen adsorption–desorption test shows that the Brunauer–Emmett–Teller (BET) specific surface area of PIN-NH₂ is up to 480 m² g⁻¹ and the thermal analysis indicates that the PIN-NH₂ possesses good thermal stability. More interestingly, we proved that the CO₂ adsorption capacity (27.7 wt%, 1.0 bar, 273 K) and selectivities (CO₂/N₂ = 137, CO₂/CH₄ = 34) could be significantly improved may owing to the presence of coefficient effect of π–π-stacking interactions and hydrogen bonding interactions between the sorbent and CO₂ guest molecule, making it a promising material for potential application in gas separation. In addition, upon exposure to moisture (RH = 3%), CO₂ capture of the PIN-NH₂ is still highly efficient and selective, with only minor decreases in the CO₂ adsorption capacity and selectivity. Moreover, we demonstrated that the CO₂ adsorption process is fully reversible. The above advantages make the porous organic polymer PIN-NH₂ a outstanding candidate for CO₂ separation material, more importantly, the proposed coefficient effect is expected to be a new rationale for the design and fabrication of CO₂ capture materials for applications in natural gas purification, greenhouse gas reduction, etc.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (No. U1233202 and No. 51175434). And the Youth Research Foundation of the Civil Aviation Flight University of China (No. Q2019-106), the Laboratory Research Foundation for the State Key Laboratory of Environment-friendly Energy Materials (No. 17kffk03).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of materials, measurements and gas adsorption tests; synthesis and characterizations of PIN-NH₂ aerogel; thermal properties, XRD and gas selectivities at 291 and 303 K and isosteric heat of CO₂ adsorption of PIN-NH₂ aerogel; the dynamic breakthrough separation curves; the details of the simulation calculation. See DOI: 10.1039/c9ra01532a

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