1,3-Diyne-Linked Conjugated Microporous ... - ACS Publications

Jun 20, 2018 - Shi-Bin Ren*†§ , Pei-Xian Li† , Andrew Stephenson‡ , Linjiang Chen‡ , Michael E. Briggs‡ , Rob Clowes‡ , Ammar Alahmed‡ ...
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A 1,3-Diyne-Linked Conjugated Microporous Polymer for Selective CO2 Capture Shi-Bin Ren, Pei-Xian Li, Andrew Stephenson, Linjiang Chen, Michael E. Briggs, Rob Clowes, Ammar Alahmed, Kang-Kai Li, Wen-Ping Jia, and De-Man Han Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01401 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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A

1,3-Diyne-Linked

Conjugated

Microporous

Polymer for Selective CO2 Capture Shi-Bin Ren,*a, c Pei-Xian Li,a Andrew Stephenson,b Linjiang Chen,b Michael E. Briggs,b Rob Clowes, b Ammar Alahmed,b Kang-Kai Li,a Wen-Ping Jiaa and De-Man Han*a a School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 317000, China; b Materials Innovation Factory and Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K; c State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. KEYWORDS: 1,3-Diyne-linked; Conjugated microporous polymer; Selective CO2 capture; Breakthrough

ABSTRACT: In this work, we demonstrate that the introduction of pyrene and alkyne groups into a highly conjugated microporous polymer (LKK-CMP-1) can be achieved via oxidative homocoupling of 1,3,6,8-tetraethynylpyrene (L). The 1,3-diyne-linked conjugated microporous polymer not only has good thermal stability but also small pore sizes. These small pores within

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LKK-CMP-1 facilitate the interactions between CO2 and its pore walls, evidenced by the high isosteric heats of adsorption of CO2 and also confirmed by IAST calculations based on singlecomponent-isotherm data. Therefore, despite moderate CO2 adsorption uptake (9.78 wt%, 273 K and 1 bar) and isosteric heat of adsorption (35.0 kJ•mol-1) for CO2, LKK-CMP-1 still exhibits excellent selectivities of CO2/N2 (44.2) or CO2/CH4 (8.2) at a molar ratio of 15:85 or 20:80 respectively. In addition, breakthrough experiments have also shown that LKK-CMP-1 is an effective material for CO2/N2 and CO2/CH4 separations under flowing conditions.

1. Introduction Within the scientific community, CO2 emissions are almost universally considered as a key contributor to global climate change.1-4 The development of CO2 adsorbent materials could efficiently reduce CO2 emissions from fixed point sources and enable the adsorbed CO2 to be reused as a useful raw material.1-4 Currently, aqueous solutions of amines are used to capture CO2 from flue gas mixtures although they are seriously restricted in commercial applications owing to the large energy penalty for CO2 recovery and the highly corrosive nature of the solutions. Solid porous materials can serve as ideal candidates for CO2 capture and separation via the reversible physical sorption of CO2 into the pore network.5-7 There is considerable interest in CO2 capture and separations using metal-organic frameworks (MOFs), but weak coordination bonds within MOFs can result in low stability and high water affinity.8 Wholly organic covalent polymers such as conjugated microporous polymers (CMPs) generally exhibit high thermostability, large surface areas, well-tuned pore structures, and can be readily functionalized.9-12 CMPs generally possess impressive stability since they are built from light elements (H, B, C, N, and O) via strong covalent bonds.9-14 Additionally, weak intramolecular

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interactions, like hydrogen bonds and π-π stacking, can also enhance the stability of the polymer, as well as gas uptake and gas selectivity.15-17 Over the past decades, tremendous attention has been paid to the design and synthesis of CMPs for gas capture, drug delivery, heterogeneous catalysis, photoluminescence, photocatalysis for hydrogen evolution and energy storage.18-26 To date, CO2 capture and separation has been one of the most intensively studied applications of CMPs15,18 as they can be easily functionalized by the introduction of specific CO2-philic moieties to enhance CO2 capture and separation.27-30 Heteroatom incorporation often includes N groups, such as amino, carbazole, dicarboximide, 1,3,5-triazine, azobenzene, pyridine, imidazole, and tetrazole, etc.31-36 In most cases, the interactions between these functional groups and CO2 are restricted to Lewis acid-Lewis base interactions between polarizable CO2 and nitrogen-rich porous materials. The interaction of the quadrupole moment of CO2 with localized dipoles generated by heteroatom incorporation are typically responsible for the enhanced CO2 adsorption.37 However, to the best of our knowledge, the use of pyrene and alkyne functionalized conjugated microporous polymer featuring a fully SP2/SP3 hybridization C-C skeleton for CO2 selective capture has not been reported so far.

PdCl2(PPh3)2/CuI DMF, N, N-diisopropylethylamine, 70 °C for 24 h

L LKK-CMP-1

Scheme 1 Synthesis route of LKK-CMP-1

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Herein, the conjugated microporous polymer (LKK-CMP-1), bifunctionalized with pyrene and 1,3-diyne, was synthesized by the oxidative coupling of the terminal alkynes on the planar tetrafunctional alkyne (L=1,3,6,8-tetraethynylpyrene) using Pd(II)-Cu(I) catalysts (Scheme 1). 1,3-diyne is used as a linker for the construction of LKK-CMP-1 with small pores, which can contribute to the CO2 uptake (9.78 wt%, 273 K and 1 bar) and IAST selectivities of CO2/N2 (44.2) and CO2/CH4 (8.2) at a molar ratio of 15:85 or 20:80 respectively. In addition, breakthrough curves have shown that a fixed bed of LKK-CMP-1 is capable of separating CO2/N2 and CO2/CH4 mixtures at the same compositions. This is important as high IAST selectivities do not always translate to efficient separation performance under breakthrough conditions, which are more representative of real world applications. 2. Experimental section 2.1 Preparation of the materials 1,3,6,8-Tetraethynylpyrene (L) was prepared according to the reported method.38 All other reagents and solvents were of analytical grade and used without further purification. Preparation of LKK-CMP-1 Under N2 atmosphere, the reaction mixture of L (1.617 g, 5.43mmol), PPh3 (0.284 g, 1.08 mmol), Pd(PPh3)2Cl2 (0.245 g, 0.35 mmol), and CuI (0.067 g, 0.35 mmol) was added into a 500 mL three-necked round-bottomed flask. DMF (100 mL) and N, N-diisopropylethylamine (200 mL) were added and the stirred reaction mixture was heated at 70 °C for 24 hours. After cooling to room temperature, the solid was collected by filtration and sequentially extracted with

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chloroform, tetrahydrofuran, and methanol in a Soxhlet apparatus for 24 hours per solvent. Finally, the dark brown product (0.566 g, 35%) was dried in a vacuum oven (60 °C, 12 hours). Anal. Calcd for (C24H6) n: C, 98.64; H, 1.36 ; Found C, 92.51; H, 3.38; Pd, 1.7; Cu, 0.08. 2.2 Characterizations Fourier transform infrared (FT-IR) spectra of the samples were collected on a Bruker Vector 22 spectrophotometer with KBr pellets in the range of 4000–400 cm-1. Solid-state 13CP/MAS NMR spectra were measured on a Bruker Advance II WB 400MHz NMR spectrometer using a 4mm DVT CP/MAS probe at a MAS rate of 10 KHz. Thermal analyses (TGA) were performed in a nitrogen stream using Seiko Extar 6000 TG/DTA equipment at a heating rate of 10 °C/min. Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 ADVANCE X-ray diffractometer with Cu-Kα radiation. The morphology and microstructure were observed on a field emission scanning electron microscope (FESEM, Hitachi S-4800). The solid state UV-visible absorption spectra were measured on a Shimadzu UV-2550 UV-Vis spectrometer. 2.3 Gas sorption and breakthrough measurements Gas sorption isotherms were measured on an ASAP 2020 HD88 instrument after the samples were degassed under dynamic vacuum at 423 K for 12 hours. The specific surface area was

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calculated using the Brunauer-Emmett-Teller (BET) method and pore size distribution (PSD) was evaluated by non-local density functional theory (NLDFT). The breakthrough curves were measured using an automated breakthrough analyzer (manufactured by Hiden Isochema, Warrington, U.K.). Full details are in the SI. 3. Results and discussion 3.1 The synthesis and characterization 1,3,6,8-Tetraethynylpyrene (L) was successfully homocoupled using a Pd(II)-Cu(I) catalysts to afford LKK-CMP-1 as a dark brown powder that was insoluble in common solvents (Scheme 1). To gain further insight into its structural details, LKK-CMP-1 was characterized by FT-IR and solid-state 13C CP NMR, TGA, PXRD, SEM, and UV-Vis spectroscopy. The FT-IR spectrum of LKK-CMP-1 showed the disappearance of alkynyl C–H bands visible in L and the shift of intense C≡C bands from 2011 to 2169 cm-1, indicative of the dimerization reaction of alkyne groups in LKK-CMP-1 (Figure S1). The formation of LKK-CMP-1 was also confirmed by 13C CP-NMR (Figure 1). The signals at ca. 117.5 and 128.3 ppm are attributed to the sp2 carbons of pyrene in LKK-CMP-1, while the signals at ca. 82.1 and 94.8 ppm are attributed to the –Cpyr–C≡C– and –C≡C–C≡C– sites based on 1,3-diyne carbons and 1,6-diyne linkages.39-40

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Figure 1 Solid-state 13C CP/MAS NMR spectrum of LKK-CMP-1. SEM imaging (Figure S2) found that LKK-CMP-1 has fluffy woolen morphology. Thermogravimetric analysis (TGA) shows that LKK-CMP-1 possesses high thermal stability and decomposition temperatures of over 600 °C under N2 atmosphere (weight loss < 20% ) (Figure S3). The mass loss below 100 °C is due to the release of solvent inside its pores. It is worth noting that good thermal stability is beneficial to the industrial application of this porous material in gas adsorption and separation. The above experiment results are consistent with the generation of the cross-linked structure of LKK-CMP-1. As indicated by the X-ray powder diffraction measurement (Figure S4), the highly conjugated polymer was amorphous. In addition, L and LKK-CMP-1 were dispersed in DMF to obtain UV/Vis spectra at room temperature (Figure S5). Compared with L, LKK-CMP-1 exhibit a rather broad absorption band ranging from 300 to 570 nm with a large bathochromic shift of about 100 nm, which indicates the formation of the highly conjugated polymer.

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Figure 2 (A) Nitrogen sorption isotherms at 77 K (filled circle: adsorption, open circle: desorption) and (B) Pore size distribution of LKK-CMP-1. 3.2 Gas Adsorption Properties To determine the accessible surface area and pore size distributions, LKK-CMP-1 was extracted with a range of solvents before being activated under dynamic vacuum at 150 ºC for 12 hours. Sorption isotherms for N2 at 77 K were measured (Figure 2A), and show a type I isotherm indicating a microporous structure.41 Based on the N2 adsorption isotherm, the apparent BET surface area was calculated to be 467 m2 g-1, while the total volume (P/P0 = 0.99) was calculated to be 0.371 cm3 g-1. The pore size distribution was determined by fitting the uptake branch of the N2 isotherm with the non-local density functional theory (NLDFT) method implementing a hybrid kernel based on a zeolite-silica model containing cylindrical pores.42 The pore size distribution of LKK-CMP-1were found to be centered around 0.59 nm, just larger than CO2, which therefore promotes close interactions between it and the pore walls of LKKCMP-1 (Figure 2B). Similar to other polymer networks, LKK- CMP-1 showed mild hysteresis in N2 desorption, possibly due to slight swelling effect in the liquid nitrogen.43

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The moderate surface area and pore volume together with the alternating arrays of the pyrene and alkyne moieties of LKK-CMP-1 inspired us to further investigate other gas uptake capacities. The CO2 adsorption isotherms of LKK-CMP-1 were measured at 273 K and 298 K, and also showed slight hysteresis on desorption (Figure 3A). LKK-CMP-1 demonstrated a considerable CO2 uptake at 1 bar of 9.78 wt% and 6.08 wt% at 273 K and 298 K, respectively. The value is higher than some reported materials at 273 K, such as microporous organic polymers MOP-3 (8.0 wt%, SABET = 509 m2g-1),44 porous imine-linked networks PIN-1 (7.92 wt%, SABET = 458 m2 g-1),41 covalent organic frameworks COF-102 (8.6 wt%, SABET = 3620 m2g-1),46 and iminelinked covalent organic frameworks ILCOF-1 (6.0 wt%, SABET = 2723 m2 g-1)47 even though LKK-CMP-1 possesses relatively low surface areas compared with the above materials. It is worth noting that the low-pressure gas adsorption isotherm is far from saturation, indicating that LKK-CMP-1 could absorb additional amounts at higher pressure conditions. The considerable CO2 uptake of LKK-CMP-1 could be attributed to the highly conjugated structure and small pore sizes of LKK-CMP-1. Due to the presence of these narrow pores, a large amount of CO2 can have great access to the pore walls of the highly conjugated polymer via dispersion forces. In addition, CH4 uptake of LKK-CMP-1 up to 1.0 bar was 1.14 wt% and 0.57 wt% at 273 K and 298 K, respectively (Figure S6A). This value was comparable to some other porous organic polymers, such as ILCOF-1 (0.9 wt%),47 nanoporous organic frameworks NPOF-4-NH2 (1.2 wt%),48 benzimidazole-linked polymers BILP-13 (1.2 wt%),49 and 2-fold higher than PIN-1 (0.46 wt%).45

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Figure 3 (A) CO2 uptake at 273 K (square) and 298 K (circle), (B) Isosteric heat values of CO2 adsorption of LKK-CMP-1. Adsorption (filled) and desorption (empty). To provide a better understanding of the gas adsorption properties, the isosteric heat (Qst) of adsorption for CO2 and CH4 were calculated. As displayed in Figure 3B, the low-coverage Qst value of CO2 was 35.0 kJ mol-1 for LKK-CMP-1, which is in the optimal range for CO2 absorbents. Moreover, the Qst value over the entire loading range is also comparable with other porous materials like hypercrosslinked polymers (HCPs) (21.2–23.5 kJ mol-1),50borazine-linked polymer (BLPs) (20.2−28.3 kJ mol-1),51microporous polyimides (MPIs) (30.4–34.8 kJ mol-1),52 and CMPs (26.8–32.6 kJ mol-1).53 The considerable CO2 uptake of LKK-CMP-1 could account for the high Qst value, which is also due to strong interactions between CO2 and the pore walls of LKK-CMP-1. In addition, the CO2 adsorption capability of LKK-CMP-1 can still be completely recovered after reactivated under vacuum at room temperature. As shown in Figure S6B, the Qst of CH4 ranges from 22 to 27 kJ mol-1 for LKK-CMP-1, which was slightly higher than some other polymers.44-47 It is worth noting that the Qst for CO2 is much greater than that for CH4, which is likely due to CO2 being more polarizable than CH4.

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60 IAST Selectivity (298K)

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CO2/N2

B

CO2/CH4

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (bar)

Figure 4 (A) Adsorption isotherms of CO2, CH4 and N2 at 298 K for LKK-CMP-1 (B) IAST adsorption selectivities of CO2/N2 at the ratio of 15:85 or CO2/CH4 at the ratio of 20:80. 3.3 Selectivity and breakthrough of gas adsorption Ideal adsorption solution theory (IAST) was used to predict selectivity of CO2/N2 or CO2/CH4 binary mixtures based on their single component adsorption isotherms collected at 298 K. The CO2/N2 selectivity for a 15:85 binary mixture at 298 K was calculated to be 44.2, while the CO2/CH4 selectivity for a 20:80 binary mixture at 298 K is 8.2 (Figure 4B). These values are superior or at least close to those for some other CMPs.54-59 Breakthrough measurements are a more realistic assessment of the separation ability of a material than IAST calculations as they use mixed gas pairs and are under kinetic flowing conditions as opposed to single gas sorption isotherms under equilibrium.

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Figure 5. Dry and wet breakthrough curves of CO2/ N2 (15:85) through a fixed bed of LKKCMP-1. Total flow rate 5 mL min-1, pressure 1 bar, 298 K. In order to test the potential for practical separations using LKK-CMP-1, we conducted breakthrough measurements using CO2/N2 (15:85) and CO2/CH4 (20:80). The CO2/N2 breakthrough curve (Figure 5) shows that LKK-CMP-1 has the ability to separate the two gases under flowing conditions with CO2 breaking through around three minutes after N2. The kinetic uptake capacity (from breakthrough curves) of each gas was calculated to be about 0.3 mmol/g while the static capacity of CO2 and N2 was measured to be 0.46 and 0.01 mmol/g, respectively. The selectivity from breakthrough measurements is 5.5, far lower than IAST calculations predicted (Figure 4B). This is probably caused by the weak interactions between CO2 and LKKCMP-1 not being strong enough under the kinetic conditions of the breakthrough measurements, meaning gas uptake is lower than by conventional gas sorption. However the weak interactions can also have some advantages in that desorption of the CO2 is rapid and complete within 30 minutes with a gentle helium purge (5 mL min-1 flow rate). The material was also shown to be very stable and recyclable without a decrease in capacity. Four repeat breakthrough measurements with a simple helium purge in between all gave near-identical breakthrough curves (Figure S8). The material is also tolerant to humid conditions, essential for practical

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applications such as the capture of CO2 from flue gas. The CO2/N2 selectivity is barely reduced under the wet conditions, with CO2 breaking through marginally faster (Figure 5). In addition, we also measured breakthrough curves for a 20:80 CO2/CH4 gas mixture (Figure S9). The CH4 breaks through fastest with CO2 breaking through around 3 minutes later. The selectivity calculated from the breakthrough curve for CH4/CO2 is 4.3, again lower than IAST calculations for the same reasons as suggested earlier. 4. CONCLUSIONS LKK-CMP-1 has been successfully prepared via the dimerization of terminal alkynes on planar tetrafunctional alkynes. The alternating arrays of pyrene and alkyne groups give a highly conjugated structure which is stable up to 600 °C. In addition, LKK-CMP-1 shows considerable uptakes as well as excellent selectivities of CO2/N2 and CO2/CH4 at ambient temperature. Breakthrough measurements have shown that LKK-CMP-1 has the ability to separate CO2/N2 and CO2/CH4 under flowing conditions, important for any potential practical applications. The performance of LKK-CMP-1 can be attributed to various factors including its highly conjugated structure, bifunctionalization with pyrene and alkyne groups, and its narrow pores. The above features may provide a guideline for the design of a large conjugated carbon material with enhanced gas uptakes and highly selective CO2 adsorption. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxx. The characterization of LKK-CMP-1 and L, gas adsorption and breakthrough experiments

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AUTHOR INFORMATION Corresponding Author * . Tel.: +86-576-83137082. * Fax.: +86-576-83137082. Email: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (21471110, 21575097 and 21375092), the Science and Technology project of Zhejiang Province (2015C33224), and the open fund of the State Key Laboratory of Coordination Chemistry (SKLCC1608). ABBREVIATIONS CMPs, conjugated microporous polymers; L, 1,3,6,8-tetraethynylpyrene; MOFs, metal-organic frameworks; COFs, covalent organic frameworks; LKK-CMP-1, 1,3-diyne-linked conjugated microporous polymer REFERENCES (1) Du, N.; Park, H. B.; Robertson, G. P.; Dal-Cin, M. M.; Visser, T.; Scoles, L.; Guiver, M. D. Polymer nanosieve membranes for CO2-capture applications. Nature mater. 2011, 10, 372. (2) Cooper, A. I. Materials chemistry: cooperative carbon capture. Nature 2015, 519, 294.

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(3) Xie, Y.; Wang, T. T.; Liu, X. H.; Zou, K.; Deng, W. Q. Capture and conversion of CO2 at ambient conditions by a conjugated microporous polymer. Nat. Commun. 2013, 4, 1960. (4) Wang, J.; Senkovska, I.; Oschatz, M. M.; Lohe, R.; Borchardt, L.; Heerwig, A.; Liu, Q.; Kaskel. S.; Highly porous nitrogen-doped polyimine-based carbons with adjustable microstructures for CO2 capture. J. Mater. Chem. A 2013, 1, 10951. (5) Li, B.; Zhang, Z.; Li, Y.; Yao, K.; Zhu, Y.; Deng, Z.; Yang, F.; Zhou, X.; Li, G.; Wu, H.; Nijem, N.; Chabal, Y. J.; Lai, Z.; Han, Y.; Shi, Z.; Feng, S.; Li. J. Enhanced binding affinity, remarkable selectivity, and high capacity of CO2 by dual functionalization of a rht-type metalorganic framework. Angew. Chem. Int. Ed. 2012, 51, 1412. (6) An, J.; Geib, S. J.; Rosi, N. L. High and selective CO2 uptake in a cobalt adeninate metalorganic framework exhibiting pyrimidine- and amino-decorated pores. J. Am. Chem. Soc. 2010, 132, 38. (7) Zhang, X.; Lu, J.; Zhang, J. Porosity Enhancement of Carbazolic Porous Organic Frameworks Using Dendritic Building Blocks for Gas Storage and Separation. Chem. Mater. 2014, 26, 4023. (8) Lu, W.; Sculley, J. P.; Yuan, D.; Krishna, R.; Wei, Z.; Zhou, H. C. Polyamine-tethered porous polymer networks for carbon dioxide capture from flue gas. Angew. Chem. Int. Ed. 2012, 51, 7480. (9) Ding, S. Y.; Wang, W. Covalent organic frameworks (COFs): from design to applications. Chem. Soc. Rev. 2013, 42, 548.

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(10) Xu, Y.; Jin, S.; Xu, H.; Nagai, A.; Jiang, D. Conjugated microporous polymers: design, synthesis and application. Chem. Soc. Rev., 2013, 42, 8012. (11) Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard, W. A. Covalent Organic Frameworks as Exceptional Hydrogen Storage Materials. J. Am. Chem. Soc. 2008, 130, 11580. (12) Lu, W.; Sculley Clowes, J. P.; Bradshaw, D.; Hasell, T.; Chong, S. Y.; Tang, C.; Thompson, S.; Parker, J.; Trewin, A.; Bacsa, J.; Slawin, A. M. Z.; Steiner, A.; Cooper, A. I. Porous organic cages. Nature Mater. 2009, 8, 973. (13) Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W. C.; Xu, J.; Deng, F.; Simmons, J. M.; Qiu, S.; Zhu, G. Targeted synthesis of a porous aromatic framework with high stability and exceptionally high surface area. Angew. Chem. Int. Ed. 2009, 48, 9457. (14) Patel, H. A.; Karadas, F.; Byun, J.; Park, J.; Canlier, E. A.; Jung, Y.; Atilhan, M.; Yavuz, C. T. Highly Stable Nanoporous Sulfur-Bridged Covalent Organic Polymers for Carbon Dioxide Removal. Adv. Funct. Mater. 2013, 23, 2270. (15) Yan, Z.; Yuan, Y.; Tian, Y.; Zhang, D.; Zhu, G. Highly Efficient Enrichment of Volatile Iodine by Charged Porous Aromatic Frameworks with Three Sorption Sites. Angew. Chem. Int. Ed., 2015, 54, 12733. (16) Katsoulidis, A. P.; Kanatzidis, M. G. Phloroglucinol Based Microporous Polymeric Organic Frameworks with −OH Functional Groups and High CO2 Capture Capacity. Chem. Mater., 2012, 24, 471.

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Graphical Abstract and image A 1,3-Diyne-Linked Conjugated Microporous Polymer for Selective CO2 Capture Shi-Bin Ren,* Pei-Xian Li, Andrew Stephenson, Linjiang Chen, Michael E. Briggs, Rob Clowes, Ammar Alahmed, , Kang-Kai Li, Wen-Ping Jia and De-Man Han*

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