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High and Selective Carbon Dioxide Capture in Nitrogen-Containing Aerogels via Synergistic Effects of Electrostatic In-Plane and Dispersive π−π-Stacking Interactions Li Yang,† Guanjun Chang,*,† and Dapeng Wang*,‡,§ †

State Key Laboratory Cultivation Base for Nonmetal Composite and Functional Materials and School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, P. R. China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China § Department of Chemical and Biological Engineering, University of ColoradoBoulder, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: A new strategy for CO2 capture is reported based on the synergistic effect of electrostatic in-plane and dispersive π−π-stacking interactions of amide and indole with CO2. Density functional theory illustrated that the amide group can have an increased ability to capture CO2 molecules that were just desorbed from an adjacent indole unit. We used this strategy to fabricate a microporous aerogel that exhibited a superior CO2 capture performance in both dry and wet conditions. The proposed synergistic effect is expected to be a new rationale for the design of CO2 capture materials. KEYWORDS: nitrogen-containing aerogels, CO2 capture, synergistic effect, in-plane and π−π-stacking interactions

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and careful adjustments of the ingredient ratio. It still remains a great challenge to make a facile fabrication of materials that capture CO2 efficiently and selectively. In the application of CO2 capture, these microporous materials act as capable storage media due to physisorption that involves an electron donor−acceptor mechanism between a heteroatom nitrogen (N2) and CO2 on the inner surface of nitrogen-containing microporous materials.17−20 A recent firstprinciples study indicated that nitrogen-containing heteroaromatic groups can form strong physical interactions with CO2 via electrostatic “in-plane” and “dispersive π−π-stacking” mechanisms.21 Specifically, indole and CO2 show strong π−πstacking interactions with each other, whereas they cannot form an in-plane conformation.21,22 On the other hand, formamide can only form an in-plane conformation with CO2. Both indole and formamide have strong binding energies with CO2 (−ΔEe = 16.7 and 20.7 kJ mol−1) while likely having different adsorption kinetics and nonspecific adsorption to other gases.21 Inspired by this fascinating study, we hypothesized that the

lobal warming caused by increased concentrations of carbon dioxide (CO2) in the atmosphere is one of the most serious environmental problems today.1,2 Flue gas arising from fossil-fuel combustion in power plants is one of the main sources for CO2 emission. Moreover, natural gas, one of the most abundant fossil fuels, often mixes with concentrated CO2, which is eventually vented to the atmosphere upon extraction from wells. It is necessary to separate CO2 from these industrial and energy-related sources. Therefore, increased efforts have been made to design materials for CO2 capture, separation, and storage.3−5 Because CO2 was industrially captured using aqueous ammonia,6 the initial work of nitrogen-containing macromolecules for CO2 adsorption evolved.7 Then, subsequent works were published using nitrogen-containing macromolecules, e.g., covalent organic frameworks,8,9 metal−organic frameworks,10,11 and many kinds of nitrogen-containing microporous polymers,12−16 showed high CO2 adsorption capacity by rationally tuning surface functionalization, morphology, porosity, and surface area.17 The structure and CO2 adsorption have complicated relationships.18 For example, there is an optimal surface area for CO2 adsorption capacity, while the selectivity decreases sharply as a function of the surface area.18 Therefore, the design of high-performance CO2 capture materials often involves sophisticated molecular design © XXXX American Chemical Society

Received: February 13, 2017 Accepted: April 27, 2017 Published: April 27, 2017 A

DOI: 10.1021/acsami.7b02077 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

crometer sizes (Figure 1A). Moreover, a TEM image (Figure 1C) reveals the porous structure, which is an essential requirement for CO2 capture. As shown in Figure S11, PIN and PAA aerogels have similar porous structures with minor differences in the size and shape of the aggregated particles. The porosity of aerogels was further quantified by sorption analysis using N2 as the sorbate molecule. At a low pressure (0−0.1 bar), the N2 adsorption−desorption isotherm shows rapid uptake at 77 K, reflecting a microporous nature (Figures 2A and S12). At a relatively high pressure (∼0.9 bar), there is an increase in the N2 sorption due to the interparticulate void associated with the meso- and macrostructures of the sample. Calculations over relative pressures (P/P0) ranging from 0.01 to 0.1 yielded Brunauer−Emmett−Teller specific surface areas of PINAA, PIN and PAA aerogels of 1250, 1190, and 1100 m2 g−1, respectively. Additionally, nonlocal density functional theory (DFT) with a cylindrical pore-oxide surface model was used to approximate the pore-size distribution (PSD).23 The calculations yielded an average pore size of 6−8 nm and a sharp peak at ∼1.7 nm. PIN and PAA materials show similar results (Figure S12). The CO2 adsorption capacity and selectivity (CO2/CH4 and CO2/N2) in PINAA, PIN, and PAA aerogels were evaluated by adsorption isotherm measurements. As shown in Figure S13, the PAA aerogel exhibits CO2, CH4, and N2 capture of 3.25, 0.16, and 0.04 mmol g−1, respectively, resulting in an intermediate level of the CO2 adsorption capacity with a very high selectivity, where the adsorption ratios of CO2/CH4 and CO2/N2 were approximately 20 and 81. Under the same condition, the PIN aerogel exhibits a CO2, CH4, and N2 adsorption capacities of 4.92, 0.29, and 0.15 mmol g−1 (Figure S13), with an intermediate level of selectivity (CO2/CH4 = 17 and CO2/N2 = 33), respectively. It is interesting to evaluate the CO2 adsorption capacity and selectivity of the PINAA aerogel that contains both amide and indole. As shown in Figure 2B, the CO2 capture exhibits nearly a linear increase with an increase of the pressure, whereas the amount of CH4 or N2 capture is negligible. The CO2 adsorption capacity of the PINAA aerogel is as high as 5.83 mmol g−1, while the adsorption capacities of CH4 and N2 are only 0.27 and 0.07 mmol g−1, respectively, and the adsorption ratios of CO2/CH4 and CO2/N2 were approximately 22 and 83 (Figure 3A,B). Therefore, we found that both the CO2 adsorption capacity and

CO2 adsorption capacity and CO2/CH4 and CO2/N2 selectivity can be improved by involving both formamide (amide) and indole groups in microporous materials where multiple mechanisms work for CO2 capture. In this work, we designed measurements to test the hypothesis described above. We fabricated an aerogel (PINAA) that contains both amide and indole groups via sol−gel technology followed by CO2 supercritical drying (the material preparation and characterization are detailed in the Supporting Information, SI). The as-prepared PINAA is a yellow, transparent porous ultralight material, as shown in Figure 1B. For comparison, N-polyindole (PIN) and polyamide

Figure 1. Microstructures of the PINAA aerogel: (A) SEM; (B) photograph; (C) TEM.

(PAA) aerogels were prepared as well (Scheme 1 and Figures S1−S7). PINAA, PIN, and PAA microporous aerogels were characterized by Fourier transform infrared and 13C CP/MAS NMR, and the results were in good agreement with the proposed structures (Figures S8 and S9). The thermal properties of PINAA, PIN, and PAA aerogels were evaluated via thermogravimetric analysis (TGA) in nitrogen, and typical TGA results are shown in Figure S10. PINAA and PAA have great thermal stability with high decomposition temperatures because of their rigid skeletal structures, and PIN showed a lower decomposition temperature because of the presence of methylene units. Overall, all PINAA, PIN, and PAA aerogels exhibited high thermal stability with high char yields at 800 °C. We first quantified the porosity of aerogels by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and sorption analysis. A SEM image shows that the PINAA aerogel consists of aggregated particles with submi-

Scheme 1. Chemical Structures of PAA, PIN, and PINAA Microporous Aerogels

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Figure 2. (A) N2 adsorption−desorption isotherms and PSD (inset) of a PINAA aerogel. (B) Adsorption isotherms of a PINAA aerogel for different gases at 273 K. (C) Adsorption isotherms of a PINAA aerogel for different gases with 3% RH of water at 273 K.

Figure 3. (A) CO2 adsorption capacity, (B) selectivity, and (C) isosteric heat of CO2 adsorption for all three types of aerogels.

Figure 4. DFT results to track the full CO2 capture process involving synergistic “electrostatic in-plane” and “dispersive π−π-stacking” interactions. (a) A CO2 molecule is adsorbed on the face of an electron-rich π heteroaromatic ring of an indole group via the dispersive π−π-stacking interaction. (b) The local π−π-stacking structure is given. (c) The desorbed CO2 molecule can be captured by an adjacent amide group because of “electrostatic in-plane” interaction. (d) A stable “electrostatic in-plane” conformation is formed. (e) Another CO2 molecule comes close to the heteroaromatic ring of an indole group. (f) The CO2 molecule is adsorbed on the heteroaromatic ring of an indole group. The gray, white, blue, and red spheres represent carbon, hydrogen, nitrogen, and oxygen atoms, respectively.

evaluated over four cycles. As seen in Figure S15A, the adsorption capacities for the four cycles are nearly identical, and this indicates that PINAA has a good recyclability. The captured CO2 is desorbed after the gas carrier has been switched from CO2 to N2 (Figure S15B). The capture−release cycle was repeated four times. We did not find any noticeable changes in the CO2 adsorption and desorption kinetics over the four cycles, indicating a great recyclability of PINAA. In real industrial applications, the flue gas from a power plant is a mixture of CO2, water vapor, and others. It is known that water vapor tends to prevent CO2 adsorption.24 Here, we quantified the CO2 capture performance under a wet condition (Figure 2C). The CO2 adsorption capacity of PINAA decreased from 5.83 to 3.96 mmol g−1 (1.0 bar, 273 K) in the presence of water vapor [relative humidity (RH) = 3%]. The presence of water vapor does not influence CH4 and N2 capture in the

selectivity of the PINAA aerogel are close to the highest ones in all CO2 capture materials.8,12,13,15 Additionally, PINAA aerogels have great CO2 adsorption capacities and selectivities at 291 and 308 K as well, as shown in Figure S13. A comparison of all three types of aerogels indicates that PINAA aerogels combine the high adsorption capacity of indole with the high selectivity of amide, yielding CO2 adsorption capacity and selectivity greater than the simple sum of each functional part. A so-called synergistic effect occurs, playing out as a new physical mechanism for CO2 capture. The selectivity levels at 298 K were also validated by the ideal adsorbed solution theory (IAST), which predicts the adsorption selectivity for gas mixtures based on pure-component gas isotherms. The results from the IAST calculations are consistent with the selectivity levels reported above (Figure S14). Additionally, the reversibility of CO2 adsorption on PINAA at 273 K was C


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the presence of the sulfone group on the polymer backbone. This is probably the reason why even PAA can have a great CO2 adsorption capacity. However, because these three types of microporous polymer materials contain similar fractions of the sulfone group, the effect of the sulfone group cannot be responsible for the difference of the CO2 adsorption capacities and selectivities among these three polymers. Furthermore, in order to study the CO2−sulfone interaction and the potential synergistic effect of sulfone with indole, we performed DFT simulation to calculate the desorption energy and correlation distance between CO2 and sulfone and the probability of a CO2 molecule jumping from an indole to an adjacent sulfone. The DFT results are shown in Figure S17. The desorption energy between CO2 and sulfone is relatively weak, about 10 kJ mol−1, with a correlation distance at 3.06 Å. This value is lower than that of refs 29 and30, likely because of the presence of a “push− pull” π-electron mode from indole to sulfone groups (Figure S18), thereby decreasing the adsorption capacity of sulfone to CO2. In addition, the distance between indole and sulfone is 8.93 Å, much longer than that between indole and amide (3.69 Å). Taken together, the DFT result indicated that the sulfone group cannot effectively capture the CO2 molecules desorbed from the adjacent indole group; that is, the synergistic effect between sulfone and indole cannot occur. In summary, we designed measurements to show that microporous aerogels that consist of indole and amide can efficiently capture CO2 with high adsorption capacity (5.83 mmol g−1) and selectivity ratios (CO2/N2 = 83 and CO2/CH4 = 22) at 1.0 bar and 273 K. Moreover, upon exposure to moisture (RH = 3%), CO2 capture of the PINAA aerogels is still highly efficient and selective, with only minor decreases in the CO2 adsorption capacity (3.96 mmol g−1) and selectivity (CO2/N2 = 57 and CO2/CH4 = 16). DFT calculations were used to study the mechanism in detail. In line with a previous first-principles study, indole and CO2 show strong π−πstacking interactions, while the amide has only two atomistic sites to bind CO2 via an in-plane conformation. We found that the amide group can have an increased ability to capture CO2 molecules that were just desorbed from an adjacent indole unit. This effect speeds up formation of the CO2−amide complex, while the high selectivity of amide for CO2 over other gases is not influenced. Different capture mechanisms allow indole and amide to work synergistically to capture CO2 efficiently and selectively, greater than the sum of each part. This allows one to facilely fabricate CO2 capture materials without sophisticated molecular design. The CO2 capture performance of the PINAA aerogels based on the synergistic mechanism is comparable to the state-of-the-art CO2 capture technique. The proposed synergistic 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.

PINAA aerogel. These results indicate that water may occupy some strong adsorption sites, while the capture of CH4 and N2 is nonspecific. Overall, although the selectivity (CO2/N2 = 57; CO2/CH4 = 16) is decreased under humid conditions, PINAA, to the best of our knowledge, still has the highest CO2 selectivity over other CO2 capture materials in similar conditions.25 The isosteric heats of adsorption (Qst) for PINAA, PAA, and PIN were calculated using the virial equations. The details of the calculation are shown in the SI.26 As shown in Figure 3C, PINAA has a Qst value of 33.1 kJ mol−1 It is well-known that Qst between 30 and 50 kJ mol−1 is the optimum for gas adsorption and separation because of a balance between the reversibility and selectivity. This value is higher and more optimized than those of PIN (27.4 kJ mol−1) and PAA (26.7 kJ mol−1). Additionally, the isosteric heat value is also higher than that of a previously reported indole-based microporous material (28.9 kJ mol−1).27 The higher and more optimized Qst value of the PINAA aerogel can be ascribed to the synergistic effect of amide and indole units arising from different interaction mechanisms. To illustrate the molecular mechanism, we used DFT28 at the M06-2X level with the aug-cc-pVDZ basis set to investigate the interaction of indole and amide with CO2 and to track the CO2 capture process. The calculation is detailed in the SI. Figure 4 shows a series of snapshots for CO2 capture by a model compound where indole and amide work synergistically to adsorb multiple CO2 molecules. The figure shows a minimized geometry of the model compound. The calculated binding energies of CO2 on amide and indole are 18.6 and 20.5 kJ mol−1, respectively (Figure S16). For the CO2−indole complex, the minimum-energy structure is obtained when CO2 lies on the heteroaromatic ring of indole at a bond distance of 3.11 Å to form the π−π-stacking conformation (Figure 4B). The equilibrium conformation of CO2−amide involves two sites: one is the electron-deficient central carbon atom of CO2 to the lone pair of electrons on a carbonyl group via dipole− quadrupole interaction; the other is the lone pair of oxygen atoms on CO2 to a hydrogen atom on the secondary amine via hydrogen bonding (Figure 4D). Neither dipole−quadrupole interaction nor hydrogen bonding can stabilize the CO2−amide complex because of their low binding energies. However, previous work and our calculation indicated that the simultaneous formation of dipole−quadrupole interaction and hydrogen bonding at both sites causes a much more stable inplane conformation of the CO2−amide complex.21 However, the capture of flowing CO2 by amide is difficult because of a small binding area by only two atomistic sites. This explains why PAA showed a low CO2 adsorption capacity along with a high selectivity. On the other hand, CO2 can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area (Figures 4A). Desorption occurs frequently, driven by thermal fluctuation. Upon completion of the desorption, the starting speed should be much slower than the bulk speed, resulting in a high probability to be captured by an adjacent amide. In-plane conformation of the CO2−amide complex is, therefore, formed much more easily and efficiently with the help of an adjacent indole while retaining the high selectivity of CO2 over other gas molecules. This mechanism is in line with a previous first-principles calculation.21 In addition, the sulfone group has been reported as a functional group capable of physically interacting with the CO2 molecule.29,30 We believed that PAA, PAMAA, and PXX all benefited from

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02077. Details of the synthesis and characterization of aerogels, main materials and measurements, simulation methods, TGA curves in nitrogen, N2 adsorption−desorption isotherms, gas selectivities of the aerogels, and isosteric heat of CO2 adsorption (PDF) D


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(12) Rabbani, M. G.; Reich, T. E.; Kassab, R. M.; Jackson, K. T.; ElKaderi, H. M. High CO2 Uptake and Selectivity by Triptycene-Derived Benzimidazole-Linked Polymers. Chem. Commun. 2012, 48, 1141− 1143. (13) Chang, G.; Yang, L.; Yang, J.; Huang, Y.; Cao, K.; Ma, J.; Wang, D. A Nitrogen-Rich, Azaindole-Based Microporous Organic Network: Synergistic Effect of Local Dipole-π and Dipole-Quadrupole Interactions on Carbon Dioxide Uptake. Polym. Chem. 2016, 7, 5768−5772. (14) Saleh, M.; Lee, H. M.; Kemp, K. C.; Kim, K. S. Highly Stable CO2/N2 and CO2/CH4 Selectivity in Hyper-Cross-Linked Heterocyclic Porous Polymers. ACS Appl. Mater. Interfaces 2014, 6, 7325− 7333. (15) Islamoglu, T.; Behera, S.; Kahveci, Z.; Tessema, T.; Jena, P.; ElKaderi, H. M. Enhanced Carbon Dioxide Capture from Landfill Gas Using Bifunctionalized Benzimidazole-Linked Polymers. ACS Appl. Mater. Interfaces 2016, 8, 14648−14655. (16) Saleh, M.; Baek, S. B.; Lee, H. M.; Kim, K. S. Triazine-Based Microporous Polymers for Selective Adsorption of CO2. J. Phys. Chem. C 2015, 119, 5395−5402. (17) Torrisi, A.; Mellot-Draznieks, C.; Bell, R. G. Impact of Ligands on CO2 Adsorption in Metal-Organic Frameworks: First Principles Study of the Interaction of CO2 with Functionalized Benzenes. II. Effect of Polar and Acidic Substituents. J. Chem. Phys. 2010, 132, 044705. (18) Wilmer, C. E.; Farha, O. K.; Bae, Y.; Hupp, J. T.; Snurr, R. Q. Structure−Property Relationships of Porous Materials for Carbon Dioxide Separation and Capture. Energy Environ. Sci. 2012, 5, 9849− 9856. (19) Lü, J.; Perez-Krap, C.; Suyetin, M.; Alsmail, N. H.; Yan, Y.; Yang, S.; Lewis, W.; Bichoutskaia, E.; Tang, C. C.; Blake, A. J.; Cao, R.; Schröder, M. A Robust Binary Supramolecular Organic Framework (SOF) with High CO2 Adsorption and Selectivity. J. Am. Chem. Soc. 2014, 136, 12828−12831. (20) Chen, F.; Huang, K.; Zhou, Y.; Tian, Z.; Zhu, X.; Tao, D.; Jiang, D.; Dai, S. Multi-Molar Absorption of CO2 by the Activation of Carboxylate Groups in Amino Acid Ionic Liquids. Angew. Chem. 2016, 128, 7282−7286. (21) Lee, H. M.; Youn, I. S.; Saleh, M.; Lee, J. W.; Kim, K. S. Interactions of CO2 with Various Functional Molecules. Phys. Chem. Chem. Phys. 2015, 17, 10925−10933. (22) Li, W.; Shi, H.; Zhang, J. From Molecules to Materials: Computational Design of N-Containing Porous Aromatic Frameworks for CO2 Capture. ChemPhysChem 2014, 15, 1772−1778. (23) Vishnyakov, A.; Ravikovitch, P. I.; Neimark, A. V. Molecular Level Models for CO2 Sorption in Nanopores. Langmuir 1999, 15, 8736−8742. (24) Kizzie, A. C.; Wong-Foy, A. G.; Matzger, A. J. Effect of Humidity on the Performance of Microporous Coordination Polymers as Adsorbents for CO2 Capture. Langmuir 2011, 27, 6368−6373. (25) Liu, J.; Tian, J.; Thallapally, P. K.; McGrail, B. P. Selective CO2 Capture from Flue Gas Using Metal-Organic Frameworks―A Fixed Bed Study. J. Phys. Chem. C 2012, 116, 9575−9581. (26) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Calorimetric Heats of Adsorption and Adsorption Isotherms. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on Silicalite. Langmuir 1996, 12, 5888−5895. (27) Chang, G.; Shang, Z.; Yu, T.; Yang, L. Rational Design of a Novel Indole-Based Microporous Organic Polymer: Enhanced Carbon Dioxide Uptake via Local Dipole-π Interactions. J. Mater. Chem. A 2016, 4, 2517−2523. (28) Balzer, C.; Cimino, R. T.; Gor, G. Y.; Neimark, A. V.; Reichenauer, G. Deformation of Microporous Carbons during N2, Ar, and CO2 Adsorption: Insight from the Density Functional Theory. Langmuir 2016, 32, 8265−8274. (29) Neofotistou, E.; Malliakas, C. D.; Trikalitis, P. N. Unprecedented Sulfone-Functionalized Metal-Organic Frameworks and GasSorption Properties. Chem. - Eur. J. 2009, 15, 4523−4527.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.C.). *E-mail: [email protected] or [email protected] (D.W.). ORCID

Guanjun Chang: 0000-0002-9589-8030 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21504073, 21202134, and 11447215), the Sichuan Youth Science & Technology Foundation (Grant 2016JQ0055), and the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (Grant 11zxfk26). D.W. is thankful for support from “The Thousand Talents Plan” for Young Professionals, China. We thank the Southwest Computing Center of the China Academy of Physics Engineering for their help with computer simulation.

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ACS Applied Materials & Interfaces (30) Xydias, P.; Spanopoulos, I.; Klontzas, E.; Froudakis, G. E.; Trikalitis, P. N. Drastic Enhancement of the CO2 Adsorption Properties in Sulfone-Functionalized Zr- and Hf-UiO-67 MOFs with Hierarchical Mesopores. Inorg. Chem. 2014, 53, 679−681.

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