Rational Fabrication of Polyethylenimine-Linked Microbeads for

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Rational Fabrication of Polyethyleniminelinked Microbeads for Selective CO2 Capture Sachin Mane, Zhen-Yu Gao, Yu-Xia Li, Xiao-Qin Liu, and Lin-Bing Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04212 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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Industrial & Engineering Chemistry Research

Rational Fabrication of Polyethylenimine-linked Microbeads for Selective CO2 Capture Sachin Mane, Zhen-Yu Gao, Yu-Xia Li, Xiao-Qin Liu and Lin-Bing Sun* State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China. *Corresponding author. E-mail: [email protected].

ABSTRACT: A series of polyethylenimine (PEI)-linked microporous organic polymers were obtained using glycidyl methacrylate-polyethylenimine monomer (GMA-PEI) and different crosslinking agents. Owing to well-defined micropores and CO2-philic plentiful secondary amines, porous organic polymers (POPs) demonstrate high CO2 uptake and excellent selectivity over other permanent gases like CH4 and N2. The adsorption capacity of POPs reaches 92 mg.g−1 at 273 K and 1 bar, which is higher than that of recently reported polymers supported on various porous materials such as MCM-41 (33 mg.g−1), alumina (50 mg.g−1), and SBA-15 (60 mg.g−1) under the analogous conditions. The high CO2/N2 and CO2/CH4 selectivity is observed and reaches 308 and 39, respectively. It is noteworthy that POPs can be entirely regenerated under mild conditions and no loss in activity is detected after four cycles. Notably, no side product is obtained during the fabrication of POPs. Thus, good adsorption capacity, high selectivity, and energy-saving regeneration of POPs make them promising in selective CO2 capture from flue gas and natural gas.

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1. INTRODUCTION Increasing attention has been paid to green-house gases due to their higher concentration in atmosphere than the permissible limit, which becomes a global issue.1 The most common green-house gases are CO2, CH4, NOX, and fluorinated gases. However, CO2 is a main anthropogenic green-house gases contributor to the climate change. With the increasing consumption of fossil fuels (i.e. coal, oil, and natural gas) for energy production, transportation, industry, and burning of agricultural waste and forestry contributes to the rising level of CO2 which results in global warming.2–4 Over the past few years, the concentration of CO2 in the atmosphere has increased from about 310 ppm to 380 ppm.5–7 Therefore, CO2 management is becoming increasingly important and is one of the most challenging issues of this century. Adsorption process is a promising technology for carbon capture, aiming to achieve a meaningful reduction in CO2 emission. It offers possible energy savings compared to other well-established CO2 controlling technologies.8 The development of new adsorbents with superior properties for CO2 capture is a great opportunity.9–11 Aqueous monoethanolamine (30% MEA) is considered the most viable and benchmark solvent for CO2 capture.8,12 The low cost of solvent and effective performance are the advantages of MEA. Unfortunately, corrosion, degradation issues, unpleasant poisonous smell, and energy-intensive adsorbent regeneration are the major drawbacks of MEA. To decrease the energy penalty, amine-containing solid adsorbents are superior alternatives to aqueous solutions of MEA. Moreover, a wider working temperature range and less waste in cycles are other advantages of solid adsorbents. Over the past few years, porous organic polymers (POPs) have been used in many diverse applications such as gas storage and separation,13–15 catalysis,16 and as conducting polymers.17 It is noteworthy that, humid stability,18,19 micropores,20,21 and desired functional group22 in adsorbent are considerably important in selective CO2 adsorption and separation.23 N-containing adsorbents are superior


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to other reported adsorbents due to the specific interaction24–26 which can be seen from recently reported N-containing adsorbents.27,28 Therefore, N-containing POPs can be considered as a promising adsorbent for selective CO2 capture. The development of N-containing POPs without the formation of hazardous gases or toxic liquids as side-products is a great opportunity. On the one hand, porous polymer networks (PPNs), hypercrosslinked polymers (HCPs), conjugated microporous polymers (CMPs), porous aromatic frameworks (PAFs), and polymers of intrinsic microporosity (PIMs) are widely fabricated and potentially used for CO2 capture and storage. On the other hand, some of these porous polymers fabricated using methods which generates hazardous gases or toxic liquids as side-products which pollutes the clean environment.29,30 Therefore, the fabrication of POPs through a more facile and greener method attracts growing interest. In this study, we report the fabrication of three different PEI-linked POPs without formation of a side product. In contrast to traditional adsorbents, POPs obtained in the present work have high physicochemical stability due to the crosslinked network structure.31 PEIlinked microbeads are obtained by suspension polymerization using glycidyl methacrylatepolyethylenimine as a monomer and ethylene dimethacrylate, trimethylolpropane triacrylate, and pentaerythritol tetraacrylate as crosslinking agents at a relatively low temperature (70 oC). The fabricated POPs are predominantly microporous with enhanced nitrogen/carbon ratio. Our results reveal the high selectivity of the POPs toward CO2 whereas other gases such as CH4 and N2 are adsorbed barely. Recycle efficiency demonstrates that no loss in CO2 adsorption capacity of adsorbents after four cycles. Thus, the fabricated POPs might be promising adsorbents for CO2 capture from flue gas and natural gas.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis



The following chemicals were used without further purification. Polyethylenimine (PEI), ethylene dimethacrylate (EDMA), and poly(vinylpyrrolidone) (PVP, molecular weight = 1,200 mol/g, K 29-32) were purchased from Macklin Co. Ltd. Glycidyl methacrylate (GMA) was received from Adamas-beta Co. Ltd. Trimethylolpropane triacrylate (TMPTA) and pentaerythritol tetraacrylate (PETRA) were purchased from Aldrich. Anhydrous methanol was procured from Sinopharm Co. Ltd. Chlorobenzene was obtained from Shanghai Lingfeng Chemicals Co. Ltd. and 2,2'-azobisisobutyronitrile (AIBN) was received from Aladdin Co. Ltd. A two-step reaction approach was used for the synthesis of POPs.32 A series of POPs were fabricated through the synthesis of a GMA-PEG monomer followed by suspension polymerization with different crosslinking agents. GMA-PEI monomer was synthesized using a method available in the literature.33 To the round bottom flask, 6 g of GMA and 2 g of PEI (17 wt% methanolic solution) was added under stirring. The reaction was carried out at room temperature for 15 h. After reaction, the resultant mixture (GMA-PEI) was dried at 65 oC for 1 h. Subsequently, GMA-PEI monomer was further used for polymerization with different crosslinking agents. The GMA-PEI monomer, crosslinker (EDMA), initiator (2,2'-azobisbutyronitrile), and porogen (chlorobenzene) were used for the suspension polymerization.34–36 The polymerization was conducted in a glass reactor with a diameter of 14 cm and height of 20 cm equipped with overhead stirrer, condenser, and thermostat to the reactor. Aqueous and organic phases were prepared before polymerization. Aqueous (continuous) phase containing 4 wt% (25 mL) aqueous solution of PVP was prepared. Further, the organic (discontinuous) phase was prepared by mixing GMA-PEI (2 g), EDMA (0.5 g), AIBN (0.1 g), and chlorobenzene (5 mL). Subsequently, the organic phase was slowly added to the reactor containing the aqueous phase under stirring (400 rpm) in a nitrogen overlay. After addition of


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the organic phase to the aqueous phase, the temperature of reactor was raised to 70 °C and maintained for 3 h. The reaction mixture was filtered and washed with deionized water and finally with methanol to remove unreacted materials (protective colloid and porogen) and further subjected to drying at 100 °C. The same procedure was employed to obtain the crosslinked polymers from TMPTA and PETRA. The polymers obtained from GMA-PEI monomer and crosslinking agents such as EDMA, TMPTA, and PETRA were abbreviated as NUT-8, NUT-9, and NUT-10, respectively (NUT means Nanjing Tech University). Figure 1 indicates the synthesis of the GMA-PEI monomer and NUT polymers by suspension polymerization.

2.2. Materials Characterization Fourier transform infrared (IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr wafer. The proportion of samples and KBr was 1:150. Elemental analysis was carried out on an Elementar Vario EL elemental analyzer. The thermogravimetric (TG) analysis of the NUT polymers was performed on a thermobalance (STA-499C, NETZSCH). Approximately 6 mg of the sample was heated from 40–1000 °C in a flow of N2 (20 mL·min– 1

) with a heating rate of 10 °C·min–1. Scanning electron microscopy (SEM) images of the

NUT polymers were recorded (200× magnification) on a Hitachi S4800 electron microscope operating at 5 kV. The apparent surface areas of NUT polymers for CO2 adsorption were calculated using the Brunauer–Emmett–Teller (BET) model over a relative pressure range of 0.01−0.10 at 273 K. Total pore volumes were calculated from the uptake at a relative pressure of 0.95. Pore size distributions was calculated from the adsorption isotherms by the Horvath– Kawazoe (HK) method. The gases CO2, CH4, and N2, were employed for gas adsorption which was performed using a Micromeritics ASAP 2020 surface area and pore size analyzer. Prior to the analysis, the samples were evacuated at 130 °C for 100 min.



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2.3. Gas Adsorption Tests The adsorption of three different gases (i.e. CO2, CH4, and N2) was carried out using a Micromeritics ASAP 2020 analyzer. The high purity gases, CO2 (99.999%), CH4 (99.99%), and N2 (99.999%), were used for adsorption measurements whereas free space was measured using helium (99.999%). The adsorption isotherms were measured at 273 K (ice–water bath) and 298 K (water bath). To understand the interaction between CO2 molecule and adsorbent, isosteric heat of CO2 adsorption (Qst) was calculated from the CO2 adsorption isotherms at temperatures of 273 and 298 K. The data were simulated with Virial equation composed of parameters ai and bi that are independent of temperature in terms of eq 1.

(1) A nonlinear curve was obtained displaying the connection between lnP and adsorption quantity (N), from the fitting parameters results of ai, the Qst was calculated in terms of eq 2.

(2) where, P is pressure, N is adsorption amount, T is temperature, and m and n represent the number of parameters a and b (where m ≤ 5 and n ≤ 2), respectively. The ideal adsorption solution theory (IAST) was used to predict selectivity of gas adsorption in NUT polymers. The selectivity has been defined according to eq 3. S = (x1/y1)/(x2/y2)

(3)

where x1 and y1 (x2 and y2) are the molar fractions of component 1 (component 2) in the adsorbed and bulk phases, respectively. The selectivity of CO2 was measured with respect to N2 and CH4 at 273 and 298 K. The recycle efficiency was carried out using ASAP 2020


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analyzer, the sample was saturated with CO2 up to 1 bar at 273 K followed by degassing at 60 °C for 60 min, and further the same sample was subjected to CO2 adsorption.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structural Characterization A series of NUT polymers (NUT-8, NUT-9, and NUT-10) were fabricated from GMA-PEI monomer and different crosslinking agents (EDMA, TMPTA, and PETRA) by the suspension polymerization37 at relatively low temperature (70 oC). NUT polymers were obtained in the form of spherical microbeads with low skeletal density and high physicochemical stability. It is noteworthy that no side-product was obtained in the fabrication of NUT polymers. The obtained NUT polymers were extensively characterized by IR, thermal analysis, SEM, porosity, elemental analysis, and were further employed for the adsorption of CO2, CH4, and N2. IR spectra of GMA, GMA-PEI, and three different NUT polymers (NUT-8, NUT-9, and NUT-10) were recorded with KBr wafer after drying the sample at 80 °C for 6 h and shown in Figure 2. IR spectrum of GMA gives the peak at 907 and 840 cm−1 which assigned to the epoxy stretching of C–O bond.38,39 Moreover, the band at 1731 cm−1 corresponds to −COO− stretching, and those at 2990–2854 cm−1 are assigned to the aliphatic −CH stretch. The IR spectrum of GMA-PEI shows 3628 and 795 cm−1 ascribed to N–H stretching, 1149 cm−1 assigned to C–N stretching along with the absence of epoxy stretching peak which confirms the successful modification of the GMA. Moreover, NUT-8, NUT-9, and NUT-10 demonstrate the similar peaks along with absence and presence of some confirmative peaks. Due to the presence of similar structural units, IR spectra of all the NUT polymers are similar with difference in absorption intensity. It is interesting to note the absence of epoxy stretching (907 and 840 cm−1) and the presence of amine stretching (3628 and 795 cm−1), 7 ACS Paragon Plus Environment


which are the most significant evidences confirming the successful fabrication of NUT polymers. In addition, IR spectra of monomers (GMA and GMA-PEI) display sharp peaks whereas NUT polymers show blunt peaks which is another confirmation of successful fabrication of NUT polymers. The thermal stability is detected by TG. Figure 3a displays an obvious weight loss at around 440 °C, which indicates the thermal stability of NUT polymers. The similar TG curves of three NUT polymers are mainly due to similar chemical structural composition. Figure 3b displays corresponding DTG curves which give some precise information and minor difference can be observed. All three NUT polymers demonstrate similar DTG curves due to similar structural aliphatic composition. All NUT polymers represent a DTG peak at around 410 oC with very slight difference. It is worth noting that weight loss in TG and DTG is slightly higher in NUT-8 followed by NUT-9 and NUT-10. In fact, despite the organic composition, NUT polymers reveal the remarkable thermal stability. The absence of peak at around 100 oC clearly indicates the hydrophobicity of NUT polymers.18,19 Notably, even after polar amine groups, there is no adsorbed water in NUT polymers. This is the advantage of POPs over metal-organic frameworks. Thus, the fabricated NUT polymers have high thermal stability and hydrophobicity similar to recently reported polymers used for CO2 capture.40 The morphology of NUT polymers was examined by SEM (Figure 4). NUT polymers obtained in the present work have spherical and rigid morphology. The spherical polymer beads used in the literature shows less efficiency in CO2 capture than spherical microbeads fabricated in the present work.41,42 The pore structure of NUT polymers was evaluated by CO2 adsorption at 273 K. The CO2 adsorption−desorption isotherms (Figure 5a) of NUT-8, NUT-9, and NUT-10 reveal that, with the increase of relative pressures, the CO2 uptake raises sharply showing type I adsorption isotherm. It is worth noting that CO2 adsorption at 273 K offers a useful technique 8 ACS Paragon Plus Environment

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to analyze the micropore structure of NUT polymers.43 The kinetic diameter of CO2 (3.30 Å) is smaller than that of N2 (3.64 Å), indicates that CO2 molecules can easily access micropores while the access to N2 is difficult.44 The higher temperature (273 K) used for textural properties determination provides a significant kinetic energy to the CO2 molecules, enabling them to enter into the micropores.45 Therefore, CO2 adsorption was used to estimate the surface area and textual properties of NUT polymers at low relative pressures. The CO2 uptake on NUT-10 is obviously higher than that on other NUT polymers and decreases in the order of NUT-10 > NUT-9 > NUT-8. A similar pore size at about 3.6 Å is demonstrated by all of the NUT polymers (Figure 5b and Table 1). Table 1 illustrates the textural parameters of NUT polymers. NUT-8, NUT-9, and NUT-10 reveal the BET surface areas of 64.9, 78.3, and 100.7 m2.g−1, pore volume of 0.058, 0.070, and 0.090 cm3.g−1, and pore size of 3.60, 3.59, and 3.58 Å, respectively. This clearly indicates that NUT polymers are predominantly microporous. Labreche et al.46 reported the silica based PEI for CO2 adsorption which shows the surface area in the range of 24–110 m2.g–1 by N2 physisorption at 77 K. However, PEIlinked microporous polymer beads obtained in the present work demonstrated the surface area of 100 m2.g−1 with high pore volume and microporous textural properties. The data of elemental analysis of different NUT polymers are listed in Table 1. NUT polymers consist of C, H, and N. The nitrogen content varies from 10−15 wt%, demonstrating the successful fabrication PEI-linked NUT polymers.47 Notably, results clear that, NUT-10 has the high nitrogen content which was decreased in NUT-9, and further decreased in NUT-8. The high nitrogen in NUT-10 is mainly due to four double bonds in the PETRA crosslinker. The nitrogen content in NUT-9 and NUT-8 is decreased due to the presence of three and two double bonds in TMPTA and EDMA crosslinker, respectively.

3.2. Gas Adsorption Performance



In order to investigate the adsorption behavior of NUT polymers, the adsorption performance on CO2, CH4, and N2 was systematically examined. Figure 6 indicates that NUT-8, NUT-9, and NUT-10 have the adsorption capacity of 70, 89, and 92 mg.g−1, respectively at 1 bar and 273 K. Similarly, the same NUT polymers have the adsorption capacity of 44, 54, and 78 m2.g−1 at 1 bar and 298 K. Thus, CO2 uptake of NUT-10 is higher compared to NUT-9 and NUT-8. All three NUT polymers show higher adsorption for CO2 whereas CH4 and N2 adsorbed barely. For NUT-10, uptake of CH4 and N2 is only 4.4 and 2.2 mg·g−1 whereas NUT-8 and NUT-9 demonstrate further decrease in adsorption of these gases at 1 bar and 273 K. Adsorption of each gas is higher at low temperature (273 K) which decreases at high temperature (298 K).48 Recently reported PEI-containing adsorbents such as MCM-41 (32.9 mg.g−1),49 carbon black (63.8 mg.g−1),50 PMCM-41 (74.8 mg.g−1),51 alumina (49.8 mg.g−1),52 SBA-15 (59.8 mg.g−1),53 fumed silica (74.8 mg.g−1),54 and MCF (46.2 mg.g−1)55 have the lower adsorption capacity than NUT-10 (92 mg.g−1). It is worth noting that, porous carbon adsorbents like PAF-36 (61.3 mg.g−1),56 PAF-52 (88.0 mg.g−1),57 PAA-MCF-41 (68.2 mg.g−1),55 and PEIBr-MCF-39 (79.2 mg.g−1)55 also displayed low CO2 adsorption capacity than NUT-10 (92 mg.g−1) under the analogous conditions. In addition, collected data of MOFs reported in the literature44 has displayed the small CO2 adsorption capacity. This clearly indicates that high adsorption capacity of NUT-10 than some recently reported adsorbents like PEI-containing adsorbents and some porous carbon materials. This is mainly due to the presence of high amine functionality and microporous structure of NUT-10 polymer. The adsorption isotherms of CO2, CH4, and N2 were collected to examine the adsorption selectivity. Figure 7 display that all NUT polymers exhibit high adsorption selectivity for CO2 over N2 and CH4. At 273 K, the IAST selectivity of CO2/N2 on NUT-10 reaches 308, which is higher than NUT-8 and NUT-9. NUT-10 presents much higher selectivity of CO2/N2


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(308 at 273 K) than some reported adsorbents such as NPC-500 (118),58 PCN-88 (23),59 NO2PAF-1 (22.1),60 and NPAF-11 (15).61 NUT-10 demonstrates CO2/N2 selectivity of 159 at 298 K which is also higher than BUT-11 (31.5),62 PIN (31),63 UiO-67 (9.4),62 UiO-66(Zr)-NO2 (26.4),64 and ZIF-100 (25).65 In case of CO2/CH4, NUT-10 (10.2, 298 K) also demonstrates higher selectivity than some recently reported adsorbents like as [Cu(bpy-1)2SiF6] (10),66 BUT-11 (9),62 PIN (8),61 ZIF-100 (5.9),65 UiO-66(Zr)-NO2 (5.1),64 and UiO-67(2.7)62 under the analogous conditions. Wang et al.62 collected the recently reported selectivity data of CO2 capture which indicates that selectivity of CO2/N2 and CO2/CH4 in the present work is obviously higher than recently reported MOF adsorbents. In addition, CH4 and N2 are barely adsorbed at 273 and 298 K. The above-mentioned results reveal that NUT polymers are promising adsorbents for selective CO2 separation from the flue gas and natural gas. In order to understand the interaction between the polymers and CO2, the heat of adsorption (Qst) was calculated using the Clausius−Clapeyron equation from the adsorption isotherms collected at 273 and 298 K. For this, nonlinear curve fitting (Figure S1) was calculated by Virial type equation. As presented in Figure 8, at zero loading, the relatively high Qst of NUT-10 (49 kJ mol−1) is mainly due to abundant basic amino groups and micropores exist in the polymer. The basic amino groups interact strongly with acidic CO2 whereas micropores increase the interaction between pore walls and CO2 molecules. Thus, the combination of abundant basic amino groups and the micropores account for high Qst of NUT-10. The Qst of NUT-10 declines rapidly with the increase of CO2 uptake, due to active sites occupied by adsorbed CO2 uptake. However, similar Qst behavior of NUT-8 and NUT-9 is mainly due to the nearly same amine content and pore size in NUT polymers. Similar to adsorption capacity and selectivity, recyclability is also an important issue from the view of practical applicability.67 As a result, we carried out the regeneration and recycle efficiency of the adsorbent. Recycling (adsorption-desorption) was performed by the process



of thermal treatment of NUT-10 under vacuum. CO2 adsorption was measured for four cycles. The four regeneration cycles of NUT-10 was conducted using the ASAP 2020 analyzer. After each adsorption cycle, saturated NUT-10 with CO2 up to 1 bar was regenerated at very mild conditions (60 oC, 60 min). This is due to appropriate pole–pole (acid–base) interaction between amine in NUT and CO2. The recovered NUT-10 was then employed for next cycle. As shown in Figure 9, no loss in adsorption capacity of NUT-10 was observed for each cycle.

4. DISCUSSION A new series of NUT polymers were fabricated by suspension polymerization of GMA-PEI monomer and crosslinkers such as EDMA, TMPTA, and PETRA using AIBN as an initiator at relatively mild conditions. Recently fabricated porous materials have the major drawback of the formation of toxic or hazardous side-products. Herein, we have fabricated NUT polymers without formation of a side-product and were used for selective CO2 capture. The result shows that, the gas adsorption capacity of the NUT polymers is in the sequence of NUT-10 > NUT-9 > NUT-8 and is mainly due to the difference in amine content and textural properties. NUT-10 demonstrate higher amine content as well as greater surface area with microporous structure which results in better CO2 adsorption capacity. On the other hand, NUT-9 and NUT-8 has decreased surface area and amine content which results in relatively low CO2 capture. Based on this it was concluded that CO2 adsorption capacity of NUT polymers depends on the micropores in combination with the nitrogen content and relatively high surface areas. These factors are considerably important for the design of solid adsorbents for CO2 capture and separation. It is worth noting that, excellent selectivity of CO2 over N2 and CH4 was achieved on NUT polymers. The presence of abundant secondary amine groups and appropriate pore sizes of adsorbents play a quite significant role. On the one hand, the selective interaction of amine-


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containing adsorbent with CO2 is mainly due to the strong acid-base (pole-pole) interactions between them. On the other hand, CH4 and N2 are non-acidic gases and unable to form pole– pole interactions between the gas molecules and the adsorbent surface. In addition to the amine functional groups, microporous properties of an adsorbent is also an important factor attributes for the selectivity. The dynamic diameter of CO2 (3.30 Å) is smaller than N2 (3.64 Å) and CH4 (3.80 Å) whereas pore diameter of NUT polymer is 3.6 Å. It is noteworthy that the dynamic diameter of CO2 is smaller than pore sizes of the NUT polymers, therefore CO2 can easily enter into the pores of adsorbents, while N2 and CH4 with relatively large sizes and difficult to enter into the pores of an adsorbent. This clearly indicates that amine functionality and microporous properties are the most significant factors that affect gas adsorption. As a result, NUT polymers demonstrate the high selectivity of CO2/N2 and CO2/CH4. Adsorption of CO2 was carried out for four cycles and no detectable loss was observed in CO2 adsorption. This indicates the excellent regeneration of NUT-10 polymers. In the present work, mild regeneration conditions (60 °C, 60 min) were utilized to recover entire adsorption capacity of NUT-10 without loss in adsorption capacity. In some published literature, stricter regeneration conditions such as 80 oC for 120 min and 105 oC for 60 min were employed for adsorbent regeneration.68,69 Further, Jung et al.70 reported the PEI containing polymer and carried out five cycles where they observed decreased CO2 adsorption capacity for each cycle. The result clearly indicates that, this mild condition is adequate for the regeneration of NUT10. Thus, NUT-10 has excellent adsorption-desorption tendency under mild regeneration conditions. The good adsorption capacity, high selectivity, and excellent recyclability make the NUT polymers highly promising in selective CO2 capture from flue gas and natural gas. As a result, NUT-10 obtained in the present work has high practical applicability in selective CO2 capture.



5. CONCLUSIONS A series of PEI-linked microbeads were successfully fabricated without the formation of side products. It is fascinating that the resultant NUT polymers contain abundant secondary amines and exhibit plentiful micropores with a pore diameter of around 3.6 Å. The typical adsorbent NUT-10 exhibits higher adsorption capacity than recently reported polymers supported on various porous materials such as MCM-41, alumina, and SBA-15. The CO2/N2 and CO2/CH4 selectivity reaches 308 and 39, respectively at 273 K. Moreover, entire regeneration of NUT-10 is obtained under mild conditions and no loss in CO2 adsorption is detected after four cycles. NUT polymers offer high CO2 uptake and excellent selectivity with energy-saving regeneration at mild conditions, which makes them promising for CO2 capture from flue gas and natural gas. ASSOCIATED CONTENT Supporting Information Nonlinear curve fitting data of the NUT polymers. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *L.-B. S.: E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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Sachin thanks the Science and Engineering Research Board (SERB-Overseas), New Delhi, India for providing the Postdoc Fellowship [Award No. SB/OS/PDF-341/2015-16]. We acknowledge financial research support of this work by the National Natural Science Foundation of China (21722606, 21576137, and 21676138) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES (1)

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Figure 1. (a) Synthesis of the GMA-PEI monomer and (b) fabrication of NUT polymers by suspension polymerization.


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Figure 2. IR spectra of the monomer and the resultant NUT polymers.



Figure 3. (a) TG and (b) DTG curves of the resultant NUT polymers.


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NUT-8

200 μm

NUT-9

200 μm

NUT-10

200 μm Figure 4. SEM images of the resultant NUT polymers.



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Table 1. Porous properties and elemental analysis of the resultant NUT polymers. SBETa

Vpa

Dpa

Elemental analysis (wt%)

(m2.g–1)

(cm3.g–1)

(Å)

C

H

N

NUT-8

64.9

0.058

3.60

53.45

6.51

10.17

NUT-9

78.3

0.070

3.59

52.41

6.54

10.61

NUT-10

100.7

0.090

3.58

56.66

8.97

15.64

NUT

a

The parameters were measured by CO2 adsorption at 273 K.


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Figure 5. (a) CO2 adsorption-desorption isotherms at 273 K of NUT polymers and (b) corresponding pore size distributions.



Figure 6. Adsorption isotherms of CO2, CH4, and N2 on NUT polymers at 273 and 298 K.


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Figure 7. 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 on NUT polymers.



Figure 8. CO2 isosteric heat of adsorption of the resultant NUT polymers.


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Figure 9. Cycling adsorption of CO2 over the polymer NUT-10.



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