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Charged Covalent Triazine Frameworks for CO2 Capture and Conversion Onur Buyukcakir, Sang Hyun Je, Siddulu Naidu Talapaneni, Daeok Kim, and Ali Coskun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16769 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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ACS Applied Materials & Interfaces

Charged Covalent Triazine Frameworks for CO2 Capture and Conversion †

†

†

†

Onur Buyukcakir , Sang Hyun Je , Siddulu Naidu Talapaneni , Daeok Kim and Ali Coskun*

†,‡

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Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea. ‡ Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea. KEYWORDS: hierarchical porosity, ionothermal synthesis, ionic networks, charged porous polymers, CO2 fixation ABSTRACT: The quest for the development of new porous materials addressing both CO2 capture from various sources and its conversion into useful products is a very active research area and also critical in order to develop more sustainable and environmental-friendly society. Here, we present the first charged covalent triazine framework (cCTF) prepared by simply heating nitrile functionalized dicationic viologen derivatives under ionothermal reaction conditions using ZnCl2 as both solvent and trimerization catalyst. It has been demonstrated that the surface area, pore volume/size of cCTFs can be simply controlled by varying the synthesis temperature and the ZnCl2 content. Specifically, increasing the reaction temperature led to controlled increase in the mesopore content and facilitated the formation of hierarchical porosity, which is critical to ensure efficient mass transport within porous materials. The resulting cCTFs showed high specific surface areas up to 1247 m2 g-1, and high physicochemical stability. The incorporation ionic functional moieties to porous organic polymers improved substantially their CO2 affinity (up to 133 mg g-1, at 1 bar and 273 K) and transformed them into hierarchically porous organocatalysts for CO2 conversion. More importantly, the ionic nature of cCTFs, homogenous charge distribution together with hierarchical porosity offered a perfect platform for the catalytic conversion 0f CO2 into cyclic carbonates in the presence of epoxides through an atom economy reaction in high yields and exclusive product selectivity. These results clearly demonstrate the promising aspect of incorporation of charged units into the porous organic polymers for the development of highly efficient porous organocatalysts for CO2 capture and fixation.

■ INTRODUCTION There is no doubt that the anthropogenic emission of carbon dioxide (CO2) from the large point sources into the atmosphere is one of the major contributors to the global warming.1-2 In the meanwhile, ever-increasing energy demand coupled with the lack of mature technologies for the widespread use of renewable energy still marks fossil fuels as a primary energy source, thus pointing to the fact that the development of highly efficient Carbon Capture and Utilization (CCU) technologies are rather important.3-4 In this sense, the integration of porous materials in gas separation systems offers new opportunities to tackle global warming fueled by the combustion of non-renewable fossil fuels. As such, there has been a growing interest for porous materials to be used in CO2 capture and separation processes.5-7 To date, several different classes of porous materials have been developed including zeolites, activated carbons, and metal-organic frameworks (MOF), and their performance have been investigated for selective CO2 capture under various conditions as an alternative to the currently used amine-scrubbing or cryogenic cooling technologies.8-12 In addition to these materials, porous organic polymers have emerged as an alternative class of materials in recent

years for gas adsorption, heterogeneous catalysis, chemical separation and energy storage.13-19 When the enormous scale of CO2 emission is taken into account along with the limitations of sequestration, the usage of CO2 as a renewable carbon source for the production of useful and valuable chemicals could contribute to more sustainable and environment-friendly low carbon society.20-21 In this regard, porous organic polymers have shown great potential in not only CO2 capture but also its conversion into industrially important, value-added products due to their high surface areas, synthetic versatility, high chemical and thermal stabilities.22-24 With these unique properties/functions, porous organic polymers certainly go far beyond the amine-based scrubbing systems. So far, a wide variety of porous polymers, e.g., polymers of intrinsic microporosity (PIMs),25 conjugated microporous polymers (CMPs),26-28 hyper cross-linked polymers (HCPs),29-31 covalent organic frameworks (COFs),32-33 porous aromatic frameworks (PAFs)34-35 and porous cage frameworks36-38 have been widely explored as physical/chemical adsorbents and/or heterogeneous catalyst for the capture and conversion of CO2. Recently, covalent triazine frameworks (CTFs), first

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Scheme 1. The synthetic route for the preparation of charged cov alent triazine frameworks (cCTFs).

synthesized by Thomas and Antonietti39 in 2008 under io nothermal reaction conditions, have attracted significant i nterest due to their facile and scalable synthesis and struc tural tunability. One of the most popular routes for the pr eparation of CTFs is the ionothermal trimerization reactio n of aromatic nitriles in the presence of ZnCl2 working as both catalyst and reaction medium at high temperatures t o form triazine linkages.40-42 More recently, acidcatalyzed43 and elemental sulfur mediated44 chemical rout es have also been introduced as alternative synthetic rout es. In addition to their high surface area and thermal stabi lity, the high nitrogen content of CTFs makes them CO2philic scaffolds for the selective CO2 capture and separatio n.45 Lately, CO2 capture capacity of CTFs has been further improved by the usage of CO2-philic groups such as Nheterocyclic or fluorine rich moieties.4652 Furthermore, because of their extraordinary chemical a nd physical stabilities, CTFs have also been used as catalyt ic supports for metal particles to catalyze several different reactions including CO2 reduction.53 When environment al concerns and relatively high cost of precious metal cata lysts are considered, carbon-based metalfree catalysts could serve more efficient and lowcost solution for the conversion of CO2 into useful produc ts. More recently, Thomas and coworkers demonstrated the usage of basic nitrogen sites of triazine groups of CTF1 as catalytic centers for the conversion of CO2 into cyclic carbonates in the presence of corresponding epoxide.54 Ne vertheless, while it merely showed desirable catalytic activ ity for the conversion of CO2 to cyclic carbonates at high t emperature and pressure, it suffered from low conversion yield and product selectivity.

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Recently, the incorporation of charged (or ionic) functional moieties to porous organic polymer scaffolds as organic zeolites have shown remarkable properties especially for the separation and catalysis applications.55-60 Their charged nature with exchangeable counterions transformed them into a selective gas adsorption platform as wells as a porous organocatalyst.61-62 Charged porous organic polymers showed exceptionally high CO2 affinity owing to their additional electrostatic interactions between CO2 molecules and charged centers, leading to significantly higher CO2 uptake capacity compared to their neutral counterparts.63 In addition, these charged networks were also investigated as porous heterogeneous catalyst for the simultaneous capture and conversion of CO2 to cyclic carbonates with high yield and selectivity.6467 In spite of their above-mentioned attractive features, the synthetic challenges arising from the low solubility and chemical stability of charged monomers limit their synthesis. Moreover, high microporosity of these porous organic polymers also limits mass transport kinetics significantly, thus decreasing their efficiency as heterogeneous catalysts. It has also been shown that the diffusion of large substrates significantly hampered by the narrow pore size distribution mainly in the micropore range.64, 68 Therefore, it is highly desirable to form charge porous organic polymers possessing hierarchical porosity. In this respect, CTFs can provide a viable, alternative pathway to construct desired charged networks, that is “organic zeolites”, with high surface areas, hierarchical porosity and physicochemical stability. Here, we present (Scheme 1) the first charged covalent triazine framework (cCTF) using cyanophenyl substituted viologen dication as a monomer through ionothermal trimerization reaction using ZnCl2 as both catalyst and reaction medium. The textural properties of cCTFs showed strong dependence to the synthesis temperature. While we observed mostly micropores at 400oC, a gradual increase in the reaction temperature up to 500oC resulted in the formation of well-developed mesopores along with micropores, thus allowing us to form a hierarchical network structure. cCTFs showed surface areas of 744, 861, 1247 m2 g-1, at 400, 450 and 500oC, respectively. Their CO2 affinity have been investigated through CO2 adsorption measurements carried out at three different temperatures up to 1 bar. cCTFs revealed significant CO2 uptake (up to 133 mg g-1, at 1 bar) along with high isosteric heats of adsorption (Qst) values for CO2 exceeding 40 kJ mol-1 at zero loading. The integration of charged sites coupled with the hierarchical porosity cCTFs provide a noteworthy enhancement in catalytic activity compared to CTFs and microporous cationic polymers for the conversion of CO2 into cyclic carbonates in high yields with exclusive product selectivity. Importantly, the improved conversion efficiency for the substrates with large kinetic diameters showed the impact of hierarchical porosity. These result clearly demonstrate the potential of CTFs for the incorporation charged sites, thus extending the scope of these materials into various environmental and energy related applications.


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Figure 1. XPS survey, C1s, N1s and Cl2p spectra of cCTF-500 (a-d). e) Powder X-ray diffraction (PXRD) patterns of cCTFs, the broad diffraction peak at 2θ = 25.9o assigned to 001 reflection pointing to existence of 2D layers in frameworks with interlayer distance ~3.4 Å. (f) Raman spectra of cCTFs, the dominant G band located at about 1590 cm-1 indicates the presence of porous 2D-honeycomb structure.

■ RESULT AND DISCUSSION The synthesis of cCTFs was carried out under ionothermal reaction conditions using ZnCl2 as both reaction medium and catalyst as previously established by Antonietti and Thomas.39 The mixture of 1,1’-bis(4cyanophenyl)-[4,4’-bipyridine]-1,1’-diium dichloride (M2) and anhydrous ZnCl2 was heated in a sealed ampoule at three different temperatures (400, 450 and 500oC) for 48 h to afford desired cCTFs (Scheme 1). Notably, the addition of 5 eq. of ZnCl2 was found to be ideal condition for high surface areas and desired textural properties (Figure S5). The resulting powders were initially soaked in 1 M HCl solution to remove excess ZnCl2 and then subsequently washed with distilled water, THF and methanol prior to their activation under vacuum at 120oC. cCTFs were found to be insoluble in common organic solvents such as DMF, DMSO, DCM or THF pointing to the formation of covalently crosslinked networks. cCTFs were characterized using various analytical techniques including elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), powder X-ray spectroscopy (PXRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray absorption spectroscopy (EDS). The formation of polymer networks was verified by using FT-IR (Figure S6). While cCTFs showed broad signals, the absence of characteristic nitrile stretching band located at 2237 cm-1 suggested the complete consumption of monomer (M2) and the formation of triazine linkages. The chemical composition of cCTFs were analyzed by elemental analysis (EA). EA results of

cCTFs revealed some deviation from calculated theoretical values due to entrapped residual solvents and gases in the pores (Table S1). It is also important to note that raising the reaction temperature led to lower hydrogen and nitrogen contents as well as higher C/N ratio compared to the theoretical values. This deviation is common for most reported CTFs and can be attributed to the nitrile decomposition along with partial carbonization of frameworks.42, 69 We also carried out the X-ray photoelectron spectroscopy (XPS) to clarify the nature of chemical bonding in cCTFs. As shown in the Figure 1a, the survey spectrum of cCTF-500 exhibited the peaks of only C1s, N1s, O1s and Cl2p, thus demonstrating the purity of the samples. The C1s core-level spectrum denotes that there are three components with binding energies of 284.8, 286.8 and 289.02 eV assigned to C-C, C=N, and C=O, respectively (Figure 1b). The N1s spectra for cCTF500 could be deconvoluted (Figure 1c) into three peaks with the binding energies of 398.5, 400.5 and 402.5 eV. The peaks at 398.5 and 400.5 eV were assigned to the nitrogen atoms of triazine linkages. On the other hand, the peak located at 402.5 eV originated from the nitrogen atoms of dicationic viologen moieties. Interestingly, as a consequence of the X-ray irradiation during XPS analysis, some reduced viologen species (N+.) also formed and contributed to the peak 400.5 eV. The Cl2p core-level spectrum was fitted to Cl- 2p3/2 and Cl- 2p1/2 at 197.3 and 198.5 eV, respectively (Figure 1d). This result is particular ly interesting as it indicates the redox-activity of viologen moieties. The other assigned peaks at 200.3 (2 p1/2) and 201.9 (2 p3/2) eV might indicate the presence of C-Cl bonding which can be result of chlorine insertion to aromatic rings at high temperatures. The counter anion


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Figure 2. (a) Argon adsoption/desorption isotherms of cCTFs measured at 87 K, filled and empty symbols represent adsorption and desorption, respectively. (b) Pore size distribution of cCTFs calculated from Ar isotherms calculated using NLDFT. CO2 adsorption isotherms of cCTFs collected up to 1 bar at (c) 273, (d) 298 and (e) 323 K. (f) The isosteric heat of adsorption (Qst) plots for CO2.

(Cl-) content of cCTFs was also investigated through energy dispersive X-ray adsorption spectroscopy (EDS) by probing chlorine atoms (Figure S7), which also further supported the successful incorporation of charged viologen functionalities into the CTF backbone. The bulk scale morphology of cCTFs was characterized by means of scanning electron microscopy (SEM). cCTFs revealed relatively uniform spherical morphology with particle size ranging from 40 to 80 nm (Figure S8). Thermal stability of the charged frameworks was probed using thermogravimetric analysis (TGA) under both air and nitrogen environment. cCTFs showed excellent thermal stability without any decomposition up to 500oC (Figure S9). Furthermore, cCTFs also exhibited high stability towards moisture as well as acidic or basic aqueous environments.70 PXRD analysis was carried out in order to verify the crystallinity of cCTFs (Figure 1e). Although they exhibited amorphous nature, the broad diffraction peak located at 2θ = 25.9o, which is assigned to the 001 reflection, suggested the existence of graphitic layers with interlayer distance of ~3.4 Å. The electronic structure of cCTFs was further confirmed by Raman spectroscopy (Figure 1f). The observed G band positioned at 1585 cm-1 indicated the presence of porous 2Dhoneycomb structure constructed from benzene, triazine and viologen rings. It is noteworthy to mention that increase in the ratio of peak intensities ID/IG (0.70, 0.88 and 0.94 for cCTF-400, cCTF-450 and cCTF-500, respectively) with increasing temperature suggested the decrease in long-range order for cCTFs. This trend in ID/IG ratio can be attributed to increase in defect density within the network structure at high temperatures. In order to investigate the permanent porosity of cCTFs,

argon adsorption-desorption isotherms were measured at 87 K. As shown in Figure 2a, cCTF-400 showed typical type I adsorption isotherm representing the presence of well-developed micropores. On the other hand, cCTF-45o revealed a combination of type I and type IV isotherms. The rapid uptake at lower relative pressure followed by a moderate uptake at higher relative pressure can be ascribed to the coexistence of micropores and mesopores. In the same manner, cCTF-500 showed similar bimodal micro- and mesoporosity. However, type IV becomes more prominent for cCTF-500 compared to cCTF-450. This is a definite sign of increasing mesoporosity for cCTF-500 by the help of temperature. While a minor hysteresis was observed for cCTF-400, the desorption pattern of cCTF-450 and cCTF-500 were well-matched with H4 type of hysteresis. This type of hysteresis is common for porous materials having both micropores and mesopores. The specific surface areas of cCTFs were estimated using Ar adsorption isotherms via BrunauerEmmett-Teller (BET) model (Figure S10), in which the valid pressure ranges obtained from the Rouquerol plots (Figure S11). The BET surface areas of cCTF-400, cCTF-450 and cCTF-500 were found to be 744, 861 and 1247 m2 g-1, respectively (Table 1). As typical for almost all CTFs, cCTFs also displayed similar trend and showed an increase in surface area with rising temperature. Notably, the surface area of cCTF-500 is one of the highest values reported to date for the charged porous organic polymers, thus demonstrating the potential CTF backbone for the incorporating charged units. In order to further investigate the textural properties of cCTFs, pore size distributions were calculated from argon adsorption/desorption isotherms by using nonlocal


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Table 1 Porosity parameters, CO2 uptakes and heat of adsorption values of cCTFs for CO2 [a]

cCTFs

Surface Area BET, m2 g-1

[b]

[c] [d] Surface Area Micropore External Langmuir Surface Area BET, Surface Area BET, 2 -1 2 -1 m g m g m2 g-1

[e]

Average Pore Diameter nm

[f]

[g]

Total Pore Volume cm3 g-1

CO2 adsorption, mg g-1

[h]

Qst for CO2, kJ mol-1

273 K

298 K

323 K

cCTF-400

744

1115

677

67

0.65

0.36

126

83

52

49 (25)

cCTF-450

861

1755

509

352

1.15

0.59

99

62

38

46 (26)

1247 3026 327 920 1.64 1.04 133 80 47 43 (20) cCTF-500 [a] Surface area of cCTFs calculated from Ar isotherms measured at 87 K using the BET model, in the range of relative pressure determined by the Rouquerol plots. [b] Surface area of cCTFs calculated from argon isotherms measured at 87 K through the Langmuir model. [c] and [d] calculated from t-plots. [e] and [f] determined from nonlocal density functional theory (NLDFT). [g] CO2 uptake values of cCTFs at 1 bar. [h] The isosteric heat of adsorption values (Qst) for CO2 at zero coverage and the values inside the parenthesis refers to heat of adsorption at high loadings.

density functional theory (NLDFT). cCTF-400 showed pores mainly located at micropore regime, below 2 nm.On the other hand, cCTF-450 and cCTF-500 exhibited broader pore size distribution in the range of 0.5 and 7 nm, suggesting their bimodal micro- and mesoporosity. It is important to remark that the increase in polymerization temperature leads to an increase not only in surface area but also mesoporosity of frameworks, thus promoting the formation of hierarchical network structure. Without a doubt, higher temperatures tend to favor the breakage of covalent bonds through opening of triazine rings, which could enable crosslinking via C-C bond formation, thus leading to the local expansion of the framework.42 Owing to their high surface area, ionic and nitrogenrich nature, we investigated the affinity of cCTFs towards Table 2 Catalytic activity of cCTF-500 for the conversion of CO2 to cyclic carbonates in the presence of four different epoxides (1-4)

O

R

Entry

O

CO2 / cCTF-500 (4 wt%)

O

1 MPa / 90 oC / 12h

Epoxides

Cyclic

O R

Time (h)

Yield (%)[a]

12

99

Carbonates O

O

1

O

O

H 3C

CH3

O 2

O Cl

O

O

12

95

12

36

12

85

Cl O

O

O

3

O

O O

4

[a]

O

O

The products were characterized by 1H NMR and the yields refer to isolated products.

CO2. In order to evaluate their performance for CO2 capture, we carried out temperature dependent CO2 uptake measurements at 273, 298 and 323 K up to 1 bar (Figure 2c-e). Because of its higher surface area compared to other cCTFs, cCTF-500 showed the highest CO2 uptake capacity of 133 mg g-1 at 273 K. In spite of its much lower surface area, cCTF-400 showed similar CO2 uptake capacity of 126 mg g-1 at 273 K and slightly higher CO2 uptake of 83 and 52 mg g-1 at 298 and 323 K, respectively, which can be ascribed to its higher CO2 affinity originating from its predominantly microporous structure (Table 1). Remarkably, these uptake capacities of cCTFs are higher than those of previously reported CTFs having similar surface areas and nitrogen contents, PCTFs-5 (1183 m2 g-1, 113 mg g-1), CTF-1 (746 m2 g-1, 22 mg g-1), CTF-1-600 (1553 m2 g-1, 36 mg g-1), FCTF-1-600 (1535 m2 g-1, 62 mg g-1), PCTF-2 (784 m2 g-1, 82 mg g-1), MCTFs@400 (1060 m2 g-1, 104 mg g-1).46-47, 52 These results clearly demonstrated the positive impact of cationic moieties in increasing the CO2philicity of cCTFs. The isosteric heats of adsorption, Qst, values for CO2 were found to be 49, 46, 43 kJ mol-1 for cCTF-400, cCTF-450 and cCTF-500, respectively, at zero coverage (Figure 2f, Table 1). The measured sharp decrease with increasing loading amount at low uptake region clearly indicates the presence of ionic moieties for CO2 interaction. Importantly, the Qst values of CO2 adsorption for cCTFs decreased with increasing temperature suggesting the loss of interaction strength between adsorbent and CO2 molecules due to not only decreasing nitrogen composition but also increasing mesoporosity of frameworks. The incorporation of ionic/charged functionalities into cCTFs not only enhanced their CO2 uptake capacity, but it also transformed them into highly efficient porous organocatalysts for the conversion of CO2 into industrially important chemical feedstock’s such as cyclic carbonates (Table 2). Several different heterogeneous catalysts for CO2 conversion have been reported so far including ionic liquids, silica supported or non-supported amines, silica supported amines, metal oxides and complexes.71 There are, however, still limited number of porous frameworks, which could target both capture and conversion of CO2 into cyclic carbonates. Metal-free nature of cCTFs is particularly important as it eliminates activity loss associated with the leaching of metal-catalysts. Moreover, the presence of hierarchical porosity is also important in order to ensure efficient mass transport into the catalytic


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sites and also for the removal of products from the catalyst. In this sense, cCTFs could serve as an environmentally-friendly and bifunctional platforms for the metal-free production of cyclic carbonates. In particular, cCTF-500 was identified as the catalyst of choice owing to its high surface area and meso/micropore ratio. In order to demonstrate the catalytic performance of cCTFs-500 for the formation of cyclic carbonates through an atom-economy reaction between CO2 and epoxides, we tested (Table 2) its conversion efficiency using four different epoxides under 1 MPa of CO2 at 90oC for 12 h. The conversion yields were calculated according to the 1H NMR analysis of crude products (Figure s12-15). cCTF-500 showed high conversion yields of 99 and 95 % for propylene oxide (1) and epichlorohydrin (2), respectively, and also exclusive product selectivity. Notably, cCTF-500 also recycled (Figure S12) up to four times for the conversion of propylene oxide under the same reaction conditions without any loss of catalytic activity and conversion efficiency thanks to its metal-free nature. Interestingly, in spite of the mild reaction conditions, a substantial improvement have been observed for the conversion yields of bulky substrates such as styrene oxide (3) and (2,3-epoxypropyl)benzene (4), compared to previously reported cationic microporous polymers.64-65, 68 The possible mechanism for the formation cyclic carbonates using cCTFs could be the activation of epoxide through hydrogen bonding interactions between its oxygen atom and α-protons of bipyridinium moiety and a subsequent nucleophilic attack by Cl- anion to form an oxy ion intermediate. This active intermediate then reacts with CO2 to form corresponding cyclic carbonates (Figure S16).72-75 Therefore, the nucleophilic nature of counteranion plays a crucial role for the catalytic conversion. Our previous studies have also shown that the counteranions with low nucleophilic character such as BF4 and PF6 substantially hampered the catalytic activity of polymers compared to the ones with Clcounteranions.64 The enhanced catalytic activity of cCTFs for the conversion of bulky substrates can be attributed to the presence of mesopores, since cCTF-500 has a bimodal micro- and mesoporosity. The introduction of cationic catalytic sites also improved the catalytic activity significantly compared to CTF-1, which showed relatively low conversion efficiencies along with poor product selectivity, thus clearly demonstrating promising aspect of cationic porous organic polymers for simultaneous capture and conversion of CO2 into value-added products. ■ CONCLUSION We presented the design and synthesis of the first charged covalent triazine framework with tunable surface area and hierarchical porosity depending on the synthesis temperature. In addition to the high surface area, thermal and chemical stability, the ionic nature of cCTFs significantly boosted their CO2 uptake capacities owing to the additional electrostatic interactions between CO2 molecules and charge centers of viologen units. The incorporation of charged units along with hierarchical

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porosity transformed CTFs into highly efficient porous organocatalysts for the conversion of CO2 into cyclic carbonates. Considering the graphitic nature of cCTFs along with their exchangeable counteranions and redoxactive viologen units, we believe that the applications of cCTFs can also be extended to environmental remediation of anionic pollutants, energy storage and gas storage, separation applications. Importantly, these results set a useful example for the incorporation of charged units into the existing/new porous organic polymers in order to impart unique functions into the porous materials and also demonstrate the potential of charged porous organic polymers for environmental applications.

ASSOCIATED CONTENT Supporting Information. Additional spectroscopic data and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors and all authors have given approval to the final version of the manuscript.

Funding Sources National Research Foundation of Korea and KAISTKUSTAR institute.

ACKNOWLEDGMENT This research was supported by the KUSTAR-KAIST Institute, Korea, under the R&D program supervised by the KAIST. This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (NRF-2014R1A4A1003712) and BK21 PLUS.

REFERENCES 1. Monastersky, R. Global Carbon Dioxide Levels Near Worrisome Milestone. Nature 2013, 497, 13-14. 2. Solomon, S.; Plattner, G.-K.; Knutti, R.; Friedlingstein, P. Irreversible Climate Change due to Carbon Dioxide Emissions. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 17041709. 3. Boot-Handford, M. E.; Abanades, J. C.; Anthony, E. J.; Blunt, M. J.; Brandani, S.; Mac Dowell, N.; Fernandez, J. R.; Ferrari, M.-C.; Gross, R.; Hallett, J. P.; Haszeldine, R. S.; Heptonstall, P.; Lyngfelt, A.; Makuch, Z.; Mangano, E.; Porter, R. T. J.; Pourkashanian, M.; Rochelle, G. T.; Shah, N.; Yao, J. G.; Fennell, P. S. Carbon Capture and Storage Update. Energ. Environ. Sci. 2014, 7, 130-189. 4. Yuan, Z.; Eden, M. R.; Gani, R. Toward the Development and Deployment of Large-Scale Carbon Dioxide Capture and Conversion Processes. Ind. Eng. Chem. Res. 2016, 55, 3383-3419. 6


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Charged Covalent Triazine Frameworks for CO2 ... - ACS Publications

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