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Rational Design and Fabrication of Nitrogen-Enriched and Hierarchical Porous Polymers Targeted for Selective Carbon Capture Sachin Mane, Yu-Xia Li, Ding-Ming Xue, Xiao-Qin Liu, and Lin-Bing Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03672 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Industrial & Engineering Chemistry Research

Rational Design and Fabrication of NitrogenEnriched and Hierarchical Porous Polymers Targeted for Selective Carbon Capture Sachin Mane, Yu-Xia Li, Ding-Ming Xue, Xiao-Qin Liu and Lin-Bing Sun* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China. *Corresponding author. E-mail: [email protected].

ABSTRACT: Herein we present a rational design and fabrication of nitrogen-enriched hierarchical porous polymer networks by direct use of polyethylenimine (usually loaded on porous supports) as the building block. The gas adsorption is mainly dependent on the adsorption temperature and pressure. Therefore, adsorption of CO2, CH4, and N2 was performed by varying adsorption temperature (273 and 298 K) and pressure (upto 1 bar). The fabricated polymer networks demonstrate the remarkable CO2 uptake capacity and selective separation of CO2 over N2 and CH4. Owing to specific acid-base interaction of nitrogenenriched polymers with acidic CO2 and molecular sieving effect of hierarchical pore structure in polymer networks, good CO2 uptake (199 mg.g−1) and superior selectivity of CO2/N2 (411) and CO2/CH4 (107) is observed at 273 K. The adsorption capacities and selectivities obtained in the present work are obviously higher than the reported functional benchmark adsorbents PCTF-1 (143.0 mg.g−1), PECONF-3 (145.2 mg.g−1) and UiO-66-NH2 (171.6 mg.g−1) as well as recently reported carbon-based and metal−organic framework (MOF) adsorbents under the

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analogous conditions. Moreover, perfect recyclability is observed without loss in 10 successive cycles with mild regeneration conditions. Thus, good CO2 uptake and superior CO2/N2 and CO2/CH4 selectivities with perfect reusability for successive CO2 capture make hierarchical porous polymers as a promising candidate for selective CO2 separation from various gas mixtures.

1. INTRODUCTION An anthropogenic CO2 is the major contributor of greenhouse effect and ocean acidification, hence the development of highly efficient adsorbent for CO2 capture is the ultimate requirement.1−5 On the one hand, aqueous monoethanolamine (30% MEA) is an attractive absorbent for CO2 separation. On the other hand, corrosion, solvent loss, energy-intensive regeneration, degradation issue, and poisonous smell are the major shortcomings of MEA, which can be minimized to a remarkable extent if solid adsorbents were instead utilized. Recently, there is potential development of solid adsorbents for selective CO2 capture.6 A lot of porous solids, such as porous organic polymers (POPs), carbon materials, zeolites and metal−organic frameworks (MOFs), have attracted intensive research interest in different applications.7−12 In these porous solids, POPs have become competitive adsorbents due to their low skeletal density resulting from the use of non-toxic light weight elements. Specifically, amine-containing porous polymers have potential applications in CO2 capture, catalysis, and metal extraction.13−15 The high CO2 uptake usually does not usually translate into high selectivities. For good CO2 uptake and superior selectivity, two factors (polar functionality and hierarchical pore structure in polymers) are considerably important. First, CO2 selectivity can be enhanced to a remarkable extent by a specific interaction of functional polymers with CO2.16 It was 2 ACS Paragon Plus Environment

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being observed that, CO2 selectivity can be improved by incorporation of the polar functionality into the polymer.17 Undoubtedly, N-containing porous polymers are often superior over carbon-based adsorbents mainly due to desired acid-base (pole-pole) interaction between adsorbent and CO2. For instance, amine functionalized porous polymers showed high CO2/N2

adsorption

selectivity up to

155, indicating the

significance

of

functionalization.18 Second, hierarchical pore structure often improves the selectivity of CO2 over other gases. Lee et al.19 demonstrated the importance of hierarchical pore structure over monotonous pore structure in POPs. They demonstrated that existence of complementary pores such as micro-, meso-, and macropores in one material improved the CO2 adsorption with 40% increase in ideal adsorption solution theory (IAST) selectivity of CO2/N2 compared to the materials without mesopores. This clearly indicates that the efficient transport of gaseous molecules through meso- and macropores toward the microporous bulk surface of adsorbent is an extremely important. Herein, we reported the direct use of polyethylenimine (usually loaded on porous supports) as the building block for cost-effective fabrication of nitrogen-enriched and hierarchical porous polymers by catalyst-free nucleophilic substitution reactions of low-cost monomers, polyethylenimine (PEI), 2,4,6-tris(chloromethyl)-mesitylene (TCM), 4,4'bis(chloromethyl)-1,1'-biphenyl

(BCB),

and

p-dichloroxylene

(DCX)

(Figure

1).

Undoubtedly, facile preparation, low-cost monomers, and catalyst-free synthesis are the distinct advantages of fabricated polymers. It is noteworthy that the typical adsorbent NUT11 (NUT means Nanjing Tech University) obtained by the reaction of PEI and TCM revealed good performance with regards to CO2 uptake (199 mg.g−1) and CO2/N2 selectivity (411). Such an uptake is superior to the reported functional benchmark adsorbents PECONF-3 (145.2 mg.g−1) and UiO-66-NH2 (171.6 mg.g−1) as well as recently reported carbon-based and MOF adsorbents under the similar experimental conditions. Thus, good CO2 uptake and 3 ACS Paragon Plus Environment


superior CO2/N2 and CO2/CH4 selectivity with and perfect reusability for successive CO2 capture makes them unique adsorbents.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis A series of nitrogen-enriched hierarchical porous polymers was fabricated using following chemicals. PEI (M.W: 1800, 99%) was procured from Macklin. TCM, potassium hydroxide and methanol (anhydrous) was procured from Sinopharm Chemical Reagent Co., Ltd. Ethanol was obtained from Wuxi City Yasheng Chemical Co., Ltd. 1,2-tetrahydrofuran (THF, > 99%) was purchased from H. V. Chemical Co., Ltd. BCB (> 98%) and DCX (> 98%) was obtained from Adamas-beta Co., Ltd. Shanghai, China. All chemicals were used without further purification. The nitrogen-enriched and hierarchical polymers were fabricated through catalyst-free nucleophilic substitution reactions20,21 of low-cost monomers. In a typical process, PEI (0.4 g) was dissolved in THF (100 mL) followed by the addition of TCM (2 g) under stirring (PEI:TCM, 1:5 wt/wt ratio). Then the resulting mixture was heated at a comparatively low temperature of 60 oC for 24 h. After completion of a reaction time, the reaction mixture was filtered and washed using KOH (2 g) solution in ethanol/water (40 mL/40 mL) followed by 25 mL of water and finally dried in oven at 60 °C for 8 h. The side-product of the reaction is HCl, therefore to neutralize the polymer as a final product, we washed the polymer by aqueous KOH solution followed by water. The fabricated polymer was abbreviated as NUT11. Similarly, the reaction of PEI with DCX and BCB results into NUT-12 and NUT-13, respectively. The structures of used monomers and fabricated polymers are depicted in Figure 1 whereas digital photos are represented in Figure S1. 4 ACS Paragon Plus Environment

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2.2. Materials Characterization The NUT polymers were synthesized and subsequently characterized by different techniques. The successful fabrication of NUT polymers was confirmed by infrared spectrometer (Nicolet Nexus 470 spectrometer with KBr wafer), solid state 13C nuclear magnetic resonance (Bruker AVANCE 400 spectrometer; a Bruker 4 mm MAS probe was used to acquire 13C CP MAS NMR spectra at 12 kHz spinning), and elemental analysis (Elementar Vario EL elemental analyzer). The thermostability of NUT polymers was examined by a thermobalance (STA-499C, NETZSCH). Moreover, scanning electron microscopy (SEM, Hitachi S4800 electron microscope operating at 5 kV) and X-ray diffractometer (Bruker D8 advance diffractometer with Cu Kα radiation in the 2θ range from 5° to 85°at 40 kV and 40 mA) was used to observe the surface morphology and to examine the amorphous nature of NUT polymers, respectively. The Brunauer–Emmett–Teller (BET) model at 77 K was used to determine the apparent surface areas through N2 adsorption-desorption isotherms. 2.3. Adsorption Tests The high purity (99.999%) gases such as CO2, CH4, and N2 were used to evaluate the gas adsorption performance (Micromeritics ASAP 2020 analyzer). The free spaces were measured using helium (99.999%). Adsorption isotherms were carried out using ice–water bath (273 K) and water bath (298 K). The isosteric heats of adsorption (Qst) were calculated from the CO2 adsorption isotherms determined at temperatures of 273 and 298 K. The ideal adsorption solution theory (IAST) selectivity of CO2/N2 and CO2/CH4 was measured at 273 and 298 K.

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3. RESULTS AND DISCUSSION 3.1. Synthesis and Structural Characterization The nitrogen-enriched and hierarchical porous polymers were fabricated via catalyst-free nucleophilic substitution reactions of PEI and functional monomers. At the initial stage, PEI was dissolved in THF, which results into a colourless solution. Subsequently, a clear solution of TCM in THF was added to the solution of PEI in THF. Finally, NUT-11 was fabricated with abundant secondary amine. In a similar process, reaction of PEI with DCX and BCB results into NUT-12 and NUT-13, respectively. The successful fabrication of NUT polymers was confirmed by different characterization techniques. Figure 2a shows the infra-red (IR) spectra of the PEI and resultant NUT polymers. IR spectrum of PEI displays a broad peak of –N-H stretching at 3400 cm–1, aliphatic –C-H stretching at 1477 and 2930 cm–1, and aliphatic –C-N stretching at 1110 cm–1. NUT polymers show the peaks of PEI along with some additional confirmative peaks. NUT-11 displays aromatic C=C at 1457 cm–1, methyl –C-H stretching at 2948 cm–1, and methylene –C-H stretching at 2925 cm–1. NUT-12 displays the aromatic C=C at 1460 cm–1, di-substituted aromatic –C-H at 862 cm–1, and methylene –C-H stretching at 2925 cm–1. Moreover, NUT-13 displays the peaks of aromatic C=C at 1458 cm–1, di-substituted aromatic –C-H at 807 cm–1, and methylene –C-H stretching at 2925 cm–1. The structures of PEI and crosslinkers in three NUT polymers are similar which results into similar IR spectra. In addition to IR, the NUT polymers fabricated by nucleophilic substitution reactions were further characterized by 13C nuclear magnetic resonance (NMR). C NMR peaks of the NUT polymers are NUT-11: δ 19.8, 53.4, 135.2 ppm, NUT-12: δ 53.3,

13

133.1 ppm, and NUT-13: δ 53.5, 129.8 ppm as presented in Figure 2b. The presence of 13C NMR peak at δ 53 ppm in each NUT polymer clearly indicates the existence of –CH2– in PEI. In addition to 13C NMR peak of PEI, NUT polymers show the peaks of crosslinkers. NUT-11 demonstrates the peak at δ 19.8 and 135.2 ppm which corresponds to –CH3 and benzene ring 6 ACS Paragon Plus Environment

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carbons, respectively. NUT-12 and NUT-13 reveals the aromatic carbons peak at δ 133.1 and 129.8 ppm, respectively. Notably, the peak of –CH2– attached to benzene ring of TCM, DCX, and BCB at around δ 60 ppm is overlap with –CH2– of PEI. These NMR peaks demonstrate the successful fabrication of NUT polymers. Table 1 shows that the fabricated NUT polymers contain three light weight elements, carbon, nitrogen, and hydrogen. The elemental composition contains carbon (50−54 wt%), nitrogen (14−15 wt%), and hydrogen (8−8.5 wt%). Thus, IR, 13C NMR, and elemental analysis, clearly indicates the successful fabrication of NUT adsorbents. To evaluate the pore structure, NUT polymers were subjected to SEM analysis (4.0 k). SEM images confirm the existence of porous structure. Figure 3 clearly displays more obvious pores indicating well-developed porosity in NUT polymers.22 In hierarchical pore structured adsorbents, micropores contribute to the bulk of the surface area of the material while meso- and macropores offers highways for the gaseous molecules to quickly diffuse allowing to reach the active sites within micropores. Figure S2 represents the X-ray diffraction (XRD) pattern of the fabricates NUT polymers. All three NUT polymers present similar XRD patterns due to the similar polymeric composition. Three NUT polymers show a single broad peak at around 25°2θ degrees which clearly indicate the amorphous framework of the NUT polymers. The observed peak is different than crystalline adsorbents reported in the literature.23 This indicates the amorphous nature of NUT polymers. To evaluate the thermal stability, NUT polymers were subjected to thermal analysis. TG and DTG of NUT polymers were performed from the temperature range of 40−900 oC. In the TG (Figure S3-a) and DTG (Figure S3-b) of NUT polymers, two peaks were observed. This is mainly due to the presence of two polymeric compositions. The first polymer composition is a low molecular weight PEI which demonstrates a TG and DTG peak at around 290 and 287 oC, whereas second predominant peak is mainly due to weight loss of crosslink polymer which 7 ACS Paragon Plus Environment


corresponds to TG and DTG at around 580 and 680 oC, respectively. In a first peak of TG, it is observed that the initial and end degradation temperature is 240 and 400 oC, respectively. In a second peak, NUT polymers start to degrade at around 410 oC and end degradation temperature is at around 680 oC. The fabricated adsorbents demonstrate the identical TG and DTG plots, which is due to the presence of similar composition with slight difference in crosslinking agents. The N2 adsorption–desorption isotherms (77 K) is often useful for the determination of micro-, meso-, and macropores in the hierarchical porous adsorbents.24,25 Figure 4a shows type II adsorption–desorption isotherms. These isotherms demonstrate that the nitrogen uptake rises gradually with increasing pressures upto 0.8 bar followed by exponential increase in nitrogen uptake upto 1 bar. Figure 4b shows the corresponding pore size distribution of NUT polymers, indicating the existence of micro-, meso-, and macropores. The textural properties of the fabricated NUT polymers are reported in Table 1. NUT 11−13 demonstrates the BET surface areas of 598, 458, and 404 m2.g−1, and pore volumes of 4.1, 3.3, and 2.8 cm3.g−1, respectively. Figure 4b clearly indicates that NUT polymer contains a relatively large proportion of micro- and mesopores whereas quite a few pore widths that fall in the macropores range. At low pressure (P/P0 < 0.8 bar), N2 adsorption demonstrates the presence of abundant micro- and mesopores while exponential raising of N2 uptake at high pressure (P/P0 > 0.8 bar) indicates the presence of few macropores. Micropores effectively improve CO2 uptake, whereas the meso- and macropores facilitate the efficient transport of gas. The average pore diameters of NUT polymers are in the range of about 28–29 nm with hierarchical pore structure. The resulting NUT polymers have the benefits of hierarchical pore structure with enhanced diffusion properties, since micropores contribute to the bulk of the surface area whereas meso- and macropores allow to quick diffusion to the active sites within micropores. 8 ACS Paragon Plus Environment

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3.2. Gas Adsorption Performance The gas uptake performance of NUT polymers was examined using high-purity gases, CO2, CH4, and N2. The adsorption experiments were conducted at realistic temperatures 273 and 298 K. The CO2 adsorption conducted at 273 and 298 K may demonstrates the practical applicability of adsorbents. The gas adsorption is mainly dependent on adsorption temperature and pressure. Therefore, adsorption of CO2, CH4, and N2 was studied by varying temperature and pressure and using static adsorption conditions. Figure 5 revealed that NUT 11−13 possess the CO2 uptake of 199, 183, and 166 mg.g−1, respectively (273 K and 1 bar). At 298 K and 1 bar, these NUT polymers possess the CO2 uptake of 98, 81, and 60 m2.g−1. The fabricated NUT polymers demonstrate higher CO2 uptake whereas CH4 and N2 uptake is extremely low. Probably this is due to the appropriate acid-base interaction between plentiful amines in NUT polymers and acidic CO2 whereas such type of interaction is absent in amine in NUT polymers and non-acidic CH4 and N2. The gases, CH4 and N2 adsorb barely on fabricated NUT polymers at 273 and 298 K. However, each gas adsorption is slightly higher at 273 K over 298 K. Thus, CO2 uptake of NUT-11 is higher compared to NUT-12 and NUT13. The adsorption capacity of various carbon-based and N-containing adsorbents are reported in Table 2. Our NUT polymers exhibit remarkable CO2 uptake as compared with reported adsorbents. It is worth noting that, NUT-11 displays superior CO2 uptake of 199 mg.g−1 (273 K, 1 bar) over functional benchmark adsorbents PCTF-1 (143.0 mg.g−1), PECONF-3 (145.2 mg.g−1), and UiO-66-NH2 (171.6 mg.g−1) (Table 2). Moreover, our adsorbents may work effectively at higher temperatures also due to enough thermostability and plentiful amine functionality. In addition, CO2 selectivity over other gases like N2 and CH4 is considerably important.26 A well-known IAST model was employed to estimate the CO2 selectivity over N2 and CH4. As displayed in Figure 6, NUT polymers demonstrate superior CO2 selectivity 9 ACS Paragon Plus Environment


over N2 and CH4. Figures 6, a,b (CO2/N2) and Figures 6, c,d (CO2/CH4) represents the selectivities at 273 and 298 K, respectively. The magnitude of both the selectivities (CO2/N2 and CO2/CH4) are in the sequence of NUT-11 > NUT-12 > NUT-13. Thus, NUT-11 presents superior CO2/N2 selectivity (411 at 273 K) over recently published benchmarks (Table 2). The magnitudes of the selectivity of NUT-11 with respect to CO2/N2 (411) and CO2/CH4 (107) (273 K, 1 bar) are quite marked. Among the fabricated adsorbents, NUT-11 exhibits excellent selectivities of CO2 over both N2 and CH4 due to the existence of plentiful amines. The excellent performance of NUT polymers was compared with recently published adsorbents (Table 2). These results indicate that the fabricated NUT polymers have potential applicability in selective CO2 separation from various gas mixtures. The Clausius–Clapeyron equation was employed to investigate the adsorbateadsorbent interaction. A Virial type equation was used for the determination of nonlinear curve fitting (Figure S4). Figure 7 shows that Qst values of NUT polymers decrease rapidly, indicating that the CO2 adsorption process is uniform. The Qst gets decreased with increase in CO2 uptake and is due to the continuous occupation of active sites of NUT polymers. The Qst of fabricated NUT polymers is in the range of 46−49 kJ.mol−1 (Figure 7). The high Qst at zero loading of NUT polymers is mainly due to the existence of plentiful amine functionality and hierarchical pore structure in NUT polymers. In addition to adsorbent capacity and selectivity, recyclability is of great importance for practical applicability. Therefore, recycle performance and regeneration conditions of NUT polymer was examined. Notably, NUT-11 saturated with CO2 (273 K, 1 bar), was entirely regenerated under quite mild regeneration conditions (50 °C, 60 min). The appropriate adsorbent−adsorbate interaction attributes for energy-saving regeneration. The regeneration of NUT-11 (50 °C) is more energy-saving than recently reported PPN-610 ACS Paragon Plus Environment

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CH2DETA tethered with primary amines (100 oC),18 PEI-modified fibrous adsorbent (120 o

C),49 microporous carbon doped with nitrogen (150 oC),50 and polystyrene based ion-

exchange resin (150 oC)51 and they were employed stricter desorption conditions. Thus, NUT11 possessing perfect recyclability with ease of regeneration (50 °C, 60 min, and under vacuum) has potential practical applicability in energy-saving carbon capture processes. NUT-11 was entirely regenerated without a detectable loss in CO2 uptake for ten successive cycles (Figure 8). Thus, NUT polymer with cost-effective synthesis, good CO2 uptake, superior selectivity, with energy-saving regeneration demonstrates the promising candidature of NUT polymers in selective CO2 separation from various gas mixtures.

4. DISCUSSION In the present work, NUT polymers were successfully constructed by formation of new C−N bonds through nucleophilic substitution reactions. Herein, we have reported the fabrication of cost-effective NUT polymers possessing plentiful amine functionality with hierarchical pore structure. In Figure 4a, the adsorption at lower pressures (P/P0 < 0.8 bar) reveals the presence of plentiful micropores, while the adsorption at relatively higher pressure (P/P0 > 0.8 bar) demonstrates the existence of few meso- and macropores in the fabricated polymers. It was observed that, the gas uptake is higher in NUT-11 which slightly decreases for NUT-12 and NUT-13. NUT-11 shows highest porosity (as revealed by surface area and pore volume) and highest nitrogen content, thus exhibits the best adsorption capacity. Further, NUT-12 and NUT-13 shows comparatively less amine content with comparatively large-sized hierarchical pore structure and perhaps is the main cause of their relatively less adsorption performance. Notably, the obtained superior selectivities are mainly due to two predominant factors, existence of plentiful amine functionality and hierarchical porous structures in NUT 11 ACS Paragon Plus Environment


adsorbents. On the one hand, CO2-philic amine groups are ideal building blocks and often build appropriate acid−base interactions between nitrogen-enriched NUT polymers and CO2 molecules52−55 while such type of interaction is absent between nitrogen-enriched NUT polymers and CH4/N2 which results into lowering of CH4/N2 uptake. On the other hand, hierarchical pore structure should be taken into consideration. In hierarchical pore structure, meso- and macropores offer highways for gas molecules whereas micropores contribute to the bulk of the surface area for selective CO2 capture.56 The gases CO2, CH4, and N2 possesses the dynamic diameter of 3.30, 3.80, and 3.64 Å, respectively. Owing to molecular sieving effect, small-sized CO2 easily enters into the hierarchical pore structured NUT polymers, whereas other gases like N2 and CH4 are difficult to enter mainly due to their large sizes.57 Under the analogous conditions of amine content and similar pore size, adsorbent with the highest surface area, possess the enhanced CO2 uptake. Thus, the combination of plentiful nitrogen content, hierarchical pore structure, and high surface areas attributes for good CO2 uptake. Among NUT adsorbents, NUT-11 demonstrates the good CO2 uptake and superior selectivity of CO2/N2 and CO2/CH4. However, adsorption performance was slightly decreased for NUT-12 and NUT-13 due to slightly decrease in amine content and surface area in the respective NUT polymers. This is the main of cause of slightly less adsorption efficiency of NUT-12 and NUT-13. The fraction of CO2 is dependent on the type of mixture. For flue gases, the typical ratio of CO2-to-N2 is about 0.15; at this ratio, the CO2/N2 selectivity is about 350 at 273 K and 75 at 298 K (Fig. 6). We think this selectivity is also quite high in comparison with those reported in literature (Table 2). The Qst is higher for low CO2 adsorption which with increase in CO2 adsorption. This is because initially large number of active sites of NUT polymers are available for CO2 adsorption while active sites decreased as CO2 adsorption proceed. Owing to good CO2 uptake and superior selectivity of CO2/N2 and CO2/CH4, NUT-11 was further subjected to test the recycle performance. Remarkably, 12 ACS Paragon Plus Environment

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perfect recyclability was observed for 10 successive CO2 capture cycles with mild regeneration conditions (50 °C, 60 min). Thus, the excellent performance of NUT polymers for selective CO2 capture is the abundant amine content, hierarchical pore structure, and high surface area of NUT polymers which makes the NUT polymers as promising adsorbents.

5. CONCLUSIONS In conclusion, cost-effective NUT polymers were successfully constructed through catalystfree nucleophilic substitution reactions of low-cost monomers. In the present work, good CO2 uptake and superior CO2/N2 and CO2/CH4 selectivity was observed. This is mainly due to the appropriate acid-base (pole-pole) interaction of nitrogen-enriched NUT polymers and CO2. In addition to this, hierarchical pore structure in NUT polymers also attributes to enhance CO2 uptake and selectivity due to molecular sieving effect. Thus, nitrogen-enriched and hierarchical pore structured adsorbent play a pivotal role in selective CO2 separation. NUT11 demonstrated good CO2 uptake (199 mg.g-1) over recently reported functional benchmark adsorbents like PECONF-3 (145.2 mg.g−1) and UiO-66-NH2 (171.6 mg.g−1) as well as recently reported carbon-based and MOF adsorbents under similar adsorption conditions. In selectivity, NUT-11 demonstrated superior selectivity of CO2/N2 (411) and CO2/CH4 (107) which is remarkably higher than reported benchmarks under similar adsorption conditions. Further, NUT-11 was regenerated using very mild conditions, without a detectable loss for ten successive cycles. Overall, the fabricated NUT polymers have good CO2 uptake, superior selectivity of CO2/N2 and CO2/CH4 with perfect reusability for successive CO2 capture which make them an ideal adsorbent for CO2 separation from various gas mixtures.



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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.xxxxxx. Digital photos, XRD pattern, TG, DTG, and non-linear curve fitting of the resultant NUT polymers (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Lin-Bing Sun: 0000-0002-6395-312X Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS We thanks to the Science and Engineering Research Board (SERB) New Delhi, India for providing the Postdoc Fellowship, Award No.: SB/OS/PDF-341/2015-16. In addition, we acknowledge financial support of this work by the National Natural Science Foundation of China (21676138, 21722606, and 21576137) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Figure 1. Fabrication of NUT polymers by nucleophilic substitution reactions.


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Figure 2. (a) IR spectra and (b) solid-state 13C NMR spectra of the resultant NUT polymers.



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Table 1. Textural parameters and elemental analysis of the resultant NUT polymers. SBETa

Vp a

(m2.g–1)

(cm3.g–1)

C

N

H

NUT-11

598

4.1

50.41

14.95

8.41

NUT-12

458

3.3

50.73

14.86

8.49

NUT-13

404

2.8

53.50

14.34

8.24

Adsorbent

Elemental analysis (wt%)

a

The parameters were measured by N2 adsorption at 77 K.


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Figure 3. SEM images of the resultant NUT polymers.



Figure 4. (a) N2 adsorption-desorption isotherms at 77 K and (b) corresponding pore size distributions.


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Figure 5. Adsorption isotherm of CO2, CH4, and N2 on (a) NUT-11, (b) NUT-12, and (c) NUT-13 at 273 and 298 K.



Figure 6. IAST selectivity of (a) CO2/N2 at 273 K, (b) CO2/N2 at 298 K, (c) CO2/CH4 at 273 K, and (d) CO2/CH4 at 298 K.


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Table 2. Comparison of CO2 uptake (static adsorption) and selective performance of different adsorbents. Adsorbent

SBETa (m2.g−1)

Tb (K)

CO2 uptakec (mg.g−1)

SCO2/N2

SCO2/CH4

Ref.

NUT-11

598

273/298

198/96.8

411/91

107/26

this work

NUT-12

458

273/298

184.8/83.6

348/82

98/25

this work

NUT-13

404

273/298

167.2/61.6

305/70

90/23

this work

AC

1175

273

122.8

17

NA

27

BUT-10

3040

298

171.6

19

5

28

PECONF-3

851

273

145.2

77

10

29

PCTF-1

2235

273

143.0

13

5

30

PCTF-3

641

273

101.2

25

6

31

MOF-890

295

298

114.4

63

9

32

ZIF-100

780

298

44.0

25

6

33

PAF-26-COOH

717

298

66.0

20

4

34

Network-1

1980

273

158.4

33

NA

35

Th-1

726

273

132

39

NA

36

Network-A

4077

298

63.8

8.7

NA

37

AHEP

418

273

104.7/61.2

NA

NA

38

UiO-66-NH2

1080

273

171.6

2

7

39

NPC-900

1256

298

113.1

22

NA

40

NO2-PAF-1

610

273

98.1

22

NA

41

SBA-NC

726

298

103.0

31

NA

42

CTF-BI-3

676

273

88

NA

43

CuPor-BPDC

442

273/298

52.0/31.2

NA

6/5

44

SA-3N

444

273/298

120.1/88.9

NA

NA

45

NC-600-1

1023

273/298

246.4/180.4

NA

NA

46

NC-600-2

879

273/298

209.9/161.9

NA

NA

47

RN-400-1

735

273/298

211.2/146.1

NA

NA

48

150.0

a

BET surface areas were measured by N2 adsorption-desorption isotherm at 77 K. bCO2

uptake and selectivity measured temperature. cCO2 uptake at 1 bar. NA: not available.



Figure 7. CO2 isosteric heat of adsorption of the resultant NUT polymers.


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Figure 8. Cycling adsorption of CO2 over the adsorbent NUT-11.



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