Highly Selective Capture of the Greenhouse Gas ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Highly Selective Capture of the Greenhouse Gas CO2 in Polymers Lin-Bing Sun,* Ying-Hu Kang, Yao-Qi Shi, Yao Jiang, and Xiao-Qin Liu State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China S Supporting Information *

ABSTRACT: Owing to their high physicochemical stability and low skeleton density, polymers are highly promising for capturing the greenhouse gas CO2. However, complicated monomers, expensive catalysts, and/or severe conditions are usually required for their synthesis, which makes the process costly, tedious, and hard to scale up. In this paper, a facile nucleophilic substitution reaction is developed to synthesize polymers from low-cost monomers, namely chloromethylbenzene and various diamines. Due to the appropriate reactivity of monomers, the polymerization takes place at a low temperature of about 60 °C in the absence of any catalysts. A series of polymers containing plentiful secondary amines are successfully fabricated; these secondary amines provide a proper adsorbate−adsorbent interaction from the viewpoints of selective capture of CO2 and energy-efficient regeneration of adsorbents. Moreover, the materials possess well-defined micropores with the dimension close to the size of adsorbate molecules and subsequently, exhibit the molecule sieving effect. As a result, these materials are active in selective adsorption of CO2 and show high CO2/N2 and CO2/CH4 selectivities. More importantly, the adsorbents can be completely regenerated under mild conditions, and no loss in activity is detected after eight cycles. KEYWORDS: Polymer, Secondary amine, CO2 capture, Selective adsorption, Energy-saving regeneration

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INTRODUCTION As an important environmental problem, global warming has attracted great attention worldwide, and CO2 is considered to be a predominant greenhouse gas. Since 2010, atmospheric CO2 concentration has been kept at a high level, over 390 ppm, which has become an urgent environmental issue. Carbon capture and sequestration (CCS) is an effective way to control the concentration of atmospheric CO2.1−6 For the combustion of fossil fuels, the content of CO2 in flue gas is around 15%, and others are primarily composed of N2. In the case of natural gas, the main components are CH4, CO2, N2, and a small amount of hydrocarbons. The presence of CO2 can cause corrosion of relevant pipelines and reduction in heat capacity. Hence, separation of CO2 from N2 (postcombustion for flue gas) and from CH4 (precombustion for natural gas) is of growing interest.7−10 The traditional method to separate CO2 is chemical absorption by using aqueous alkanolamines such as monoethanolamine (MEA), which shows good performance with regard to both capacity and selectivity of CO2.11 Nevertheless, this method possesses some drawbacks such as high regeneration costs, unpleasant poisonous smell of amine compounds, and erosion of equipment. Among various techniques for CO2 capture, adsorption using porous materials is regarded as one of the most promising alternatives.12−16 For the past few years, metal−organic frameworks (MOFs) emerged as a novel category of porous materials showing high capacity in CO2 capture due to their large surface areas and pore volumes.17−22 Unfortunately, most MOFs are © 2015 American Chemical Society

unstable in high-temperature, moisture, and other harsh environments, making it difficult to meet strict industrial requirements. Porous polymer networks (also known as covalent organic frameworks,23 covalent triazine-based frameworks,24 porous aromatic frameworks,25 hyper-cross-linked polymers,26 conjugated microporous polymers,27 etc.), another type of adsorbents with analogous porosity, exhibit much better physicochemical stability as a result of the covalent bonding of the network construction. These materials are thus highly competitive in CO2 capture, and much attention has been given to their preparation.28−31 Through condensation reactions of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) with the respective dianhydride building blocks in m-cresol, Liebl et al.24 synthesized several triazine-based porous polyimide (TPI) polymer networks. The resultant TPI polymer networks showed good chemical and thermal stability as well as high CO2 uptakes. However, the monomer TAPT was prepared from the quite preliminary precursor 4-bromobenzonitrile through a series of tedious organic reactions. By Yamamoto-type Ullmann reactions of monomers containing quadricovalent Si and Ge, Ben et al.32 reported the preparation of two porous aromatic frameworks (PAFs). These materials exhibited high surface areas and excellent adsorption ability for CH4 and CO2. It should be stated that an expensive catalyst, bis(1,5-cyclooctadiene) nickel, Received: June 17, 2015 Revised: November 6, 2015 Published: November 15, 2015 3077

DOI: 10.1021/acssuschemeng.5b00544 ACS Sustainable Chem. Eng. 2015, 3, 3077−3085

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ACS Sustainable Chemistry & Engineering

Figure 1. Monomers, schematic structures, and digital photos of the resultant polymers. followed by the addition of ED (0.180 g, 3 mmol). The obtained solution was then heated in a closed system at 63 °C for 24 h. After cooling to room temperature, the reaction mixture was centrifuged to remove solvent, and the precipitate was treated with an ethanol/water (20 mL/20 mL) solution of KOH (1.008 g) at 45 °C for 12 h. The material was then washed with an ethanol/water solution 3 times and dried at room temperature. The obtained powder was denoted as NUT-1 (NUT means Nanjing Tech University). In a similar process, other polymers NUT-2, NUT-3, and NUT-4 were synthesized using BD, CHD, and PD as the diamine monomers, respectively. Characterization. Fourier transform infrared (IR) spectra were performed on a Nicolet Nexus 470 spectrometer with KBr wafer. The proportion of samples and KBr is 1:150. X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 advance diffractometer with Cu Kα radiation in the 2θ range from 5° to 60° at 40 kV and 40 mA. Solid state 13C cross-polarization (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were measured on a Bruker AVANCE 400 spectrometer, a Bruker 4 mm MAS probe was used to acquire 13C CP MAS NMR spectra at 12 kHz spinning. Elemental analyses (C, H, and N) were carried out on an Elementar Vario EL elemental analyzer, before measurement samples were dried to remove water. Thermogravimetric (TG) analysis was performed using a thermobalance (STA-499C, NETZSCH). About 10 mg of sample was heated from room temperature to 800 °C in a flow of N2 (20 mL·min−1), and the heating rate was 10 °C·min−1. For the analysis of pore structure, two gas probes, N2 and CO2, were employed. The adsorption measurements were undertaken using a Micromeritics ASAP 2020 surface area and pore size analyzer. Prior to the analysis, the samples were evacuated at 120 °C for 6 h, N2 adsorption analysis was carried out at 77 K while CO2 adsorption analysis was carried out at 273 K. The apparent surface areas for N2 and CO2 adsorption were calculated using the Brunauer−Emmett− Teller (BET) model over a relative pressure range of 0.01−0.10. Total pore volumes were calculated from the uptake at a relative pressure of 0.95. Pore size distributions (PSD) and pore volume were calculated from the adsorption isotherms by the Horvath−Kawazoe (HK) method. Scanning electron microscopy (SEM) images were recorded on a Hitachi S4800 electron microscope operating at 20 kV. Adsorption Tests. CO2, CH4, and N2 adsorption isotherms of samples were conducted using a Micromeritics ASAP 2020 analyzer. CO2 (99.999%), CH4 (99.99%), and N2 (99.999%) gases were used for adsorption measurements. Free space was measured using helium (99.999%), Adsorption−desorption isotherms at 273 K were measured

has to be used in the polymerization process. Actually, complicated monomers, expensive catalysts, and/or high temperatures are normally required for the synthesis of most reported porous polymer networks, which makes the synthetic processes complex, costly, and hard to scale up. Although many attempts have been made, synthesis of porous polymer networks from low-cost monomers via a catalyst-free and simple polymerization reaction remains a great challenge until now. In this study, we developed a strategy to fabricate a series of polymers by a facile nucleophilic substitution reaction of two monomers, namely chloromethylbenzene and diamines (Figure 1). Due to the proper reactivity of monomers, the polymerization reactions can occur under mind conditions in the absence of any catalysts. Furthermore, both monomers are inexpensive and readily available. The obtained materials contain plenty of secondary amine groups, and hence provide an appropriate adsorbate−adsorbent interaction as compared with primary and tertiary amines. This is of great importance for both selective adsorption of CO2 and energy-saving regeneration of adsorbents. Our results demonstrate that the obtained materials are active in the adsorption of CO2, whereas N2 and CH4 are scarcely adsorbed, which leads to an ultrahigh selectivity of CO2 over N2 and CH4. More importantly, the adsorption capacity can be easily recovered without any loss.

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EXPERIMENTAL SECTION

Chemicals. Mesitylene was purchased from Aladdin Industrial Co. Paraformaldehyde, sodium chloride, zinc chloride, anhydrous magnesium sulfate, hydrochloric acid, dichloromethane, tetrahydrofuran (THF), potassium hydroxide, ethylene diamine (ED), 1,4butanediamine (BD), trans-1,4-diaminocyclohexane (CHD), and pphenylenediamine (PD) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further treatment. Deionized water were used for all experiments. Materials Synthesis. The monomer 2,4,6-tris(chloromethyl)mesitylene (TCM) was prepared according to the previously reported method.33 The polymers were synthesized by a nucleophilic substitution reaction of TCM with diamines. In a typical process, TCM (0.561 g, 2 mmol) was dissolved in THF (50 mL) with stirring, 3078


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ACS Sustainable Chemistry & Engineering in an ice−water bath while isotherms at 298 K were measured in a water bath. The isosteric heats of CO2 adsorption (Qst) were calculated from the CO2 adsorption isotherms at temperatures of 273 and 298 K, the data were simulated with Virial expression composed of parameters ai and bi that are independent of temperature in terms of eq 1. In general, a nonlinear curve was obtained displaying the connection between ln P and adsorption quantity (N), from the fitting parameters results of ai, the Qst was calculated in terms of eq 2.34 In the equations, 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). ln P = ln N +

1 T

m

n

∑ N i + ∑ biN i i=0

i=0

(1) Figure 2. IR spectra of the monomer TCM and the resultant polymers.

m

Q st = − R ∑ aiN i i=0

(2)

The ideal adsorption solution theory (IAST) was used to predict binary gas mixture adsorption in porous materials.35−37 The selectivity is defined as S = (x1/y1)/(x2/y2), where x1 and y1 (x2 and y2) are the molar fractions of component 1 (component 2) in the adsorbed and bulk phases, respectively. In the calculation, a CO2/N2 ratio of 15/85 and a CO2/CH4 of 50/50 were used, which are typical composition of flue gas emitted from coal-fired power plants and general feed composition of natural gas, respectively. The regeneration experiments was carried out on an ASAP 2020 analyzer, the sample was saturated with CO2 up to 1 bar at 0 °C followed by degassing at 60 °C, and the adsorption capacity of regenerated adsorbents were then measured again. The breakthrough curves of polymers were measured in a steel tube column with an internal diameter of 5 mm. The powder sample was packed with a length of 50 mm. The column was placed into an oven and heated at 120 °C with flowing Ar for 4 h prior to test. After the column was cooling down to room temperature, the gas flow was switched to the gas mixture with 2 mL·min−1 at 0.2 MPa. Binary mixture gases were prepared with a CO2/N2 ratio of 15/85 and a CO2/CH4 of 50/50. The complete breakthrough of CO2 and other gases was revealed by the downstream gas composition reaching that of the feed gas.

products NUT-1, NUT-2, and NUT-3 give two new bands at 1103 and 3427 cm−1, which can be attributed to C−N and N− H stretching vibrations, respectively.39 IR spectra of the monomer and products clearly reveal the scission of C−Cl bonds, the augmentation of −CH2− groups, as well as the formation of C−N and N−H bonds, which prove the formation of networks via nucleophilic substitution reactions. In contrast to NUT-1, the materials NUT-2 and NUT-3 possess intenser bands at 2915 and 2845 cm−1, because they contain more −CH2− groups. Owing to the cyclic structure of CHD in NUT-3, the band at 1010 cm−1 ascribed to linear carbon chain diminishes. It is noticeable that NUT-4 gives an IR spectrum a little different with others. Due to the absence of −CH2− groups in the monomer PD, the bands at 2915 and 2845 cm−1 are weaker than that in other polymers. In addition, the IR spectrum show the bands at 820, 1240, and 1597 cm−1, which correspond to the stretching vibrations of benzene rings from the monomer PD.40 Figure 3 shows the solid state 13C NMR spectra of polymers. The four polymers present similar spectra despite that different

RESULTS Synthesis and Characterization of Materials. Polymers were synthesized through the nucleophilic substitution reaction of TCM with four diamines. Figure 1 shows the structure of these diamines, which contain alkyls with different length, cycloalkyl, or phenyl. All of the monomers can dissolve in the solvent THF, leading to the formation of clear colorless solutions at the beginning of reactions. After reactions, powders with different colors can be obtained. NUT-1 and NUT-2 synthesized from ED and BD are white, whereas NUT-3 and NUT-4 derived from CHD and PD are maple and brown, respectively. The morphology of polymers was examined by SEM. As shown in Figure S1, a sphere-like morphology was observed. Various methods including IR, XRD, solid state 13C NMR, TG, and elemental analysis were then employed to characterize these polymers. As displayed in Figure 2, the IR spectrum of TCM displays intense bands at 600−800 cm−1 due to the C−Cl stretching vibrations. In the IR spectra of NUT-1, NUT-2, and NUT-3; however, these bands diminish to an undetected level, suggesting the scission of C−Cl bonds in the process of polymerization.28 Two weak bands at 2915 and 2845 cm−1 originated from the stretching vibrations of −CH2− can be observed in the spectrum of TCM.38 After polymerization their intensity is greatly enhanced, which is due to the connection of −CH2− groups from the monomer diamine. In addition, the

Figure 3. Solid state 13C NMR spectra of the resultant polymers.

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diamine monomers are used. All of the spectra present a peak at 134.9 ppm, which is assigned to the sp2 C in benzene rings. For the carbon atoms of methyl directly connected to benzene rings, they present a peak at 14.8 ppm. In the case of NUT-1, there is a peak at 44.9 ppm stemmed from the carbon atoms connected to alkyl carbon and nitrogen. In the spectrum of NUT-2, a weak peak at 29.7 ppm ascribed to the carbon atoms in linear chain is also observed. Table 1 gives the results of elemental analysis for different materials. The materials mainly consist of three elements, that is, C, N, and H. The nitrogen contents vary from 10.37 to 12.29 wt %, demonstrating the 3079


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ACS Sustainable Chemistry & Engineering Table 1. Textural Properties and Elemental Analysis of Polymers

elemental analysis (wt %)

a

sample

SBETa (m2·g−1)

Vpa (cm3·g−1)

Dpa (Å)

N

C

H

NUT-1 NUT-2 NUT-3 NUT-4

99.9 70.0 42.4 26.9

0.073 0.064 0.042 0.025

3.6 4.3 4.1 4.0

12.14 10.97 10.37 12.29

71.77 74.67 74.03 74.12

9.22 9.20 9.04 6.74

The parameters were measured by CO2 adsorption at 273 K.

Figure 4. (A) CO2 adsorption−desorption isotherms at 273 K and (B) corresponding pore size distributions.

calculated from CO2 adsorption. Take NUT-1 as an example, the surface area and pore volume is 99.9 m2·g−1 and 0.073 cm3· g−1, respectively. On the basis of these results, it is clear that N2 is difficult to access the pores of materials, while the accessibility of CO2 is much high, which suggests that the present polymers have great potential for selective adsorption of CO2 from N2. The thermal stability of polymers is detected by TG. As can be seen from Figure S4, the weight losses at about 100 °C ascribed to the desorption of adsorbed water are negligible, which indicates the hydrophobicity of polymers. The hydrophobicity/hydrophilicity of polymers was also investigated by the static water contact angle. The contact angle of the typical polymer NUT-1 is 64° as shown in Figure S5. Because of surface hydrophilicity in the case of inorganic materials, the water droplet can readily penetrate into the bulk during the measurement of contact angle, and disappears immediately. Nevertheless, the contact angles are generally larger than 90° for organic materials. The existence of hydrophilic groups (e.g., amines, alumina, and acrylic acid) is able to decline the contact angle as reported previously.48,49 Hence, it is understandable that our materials exhibit a contact angel higher than inorganic materials but lower than pure organic polymers. An obvious weight loss appears at around 400 °C, giving evidence of the thermal stability of these materials. Corresponding DTG curves give some precise information, and minor difference can be identified. NUT-4 presents a DTG peak at about 420 °C, which is higher than that of NUT-3 at 410 °C as well as that of NUT1 and NUT-2 at 380 °C. Remarkably, NUT-4 shows a total weight loss of less than 60%, which is far lower in contrast to other materials (more than 80%). This is because PD with a benzene ring is used as the monomer for the synthesis of NUT4. The high weight remained in thermal treatment suggests that NUT-4 is a good precursor for the preparation of porous carbon materials, and this work is in progress in our laboratory. Adsorption Performance of Materials. The adsorption behavior of CO2, CH4, and N2 on different materials was systematically investigated. As shown in Figure 5, CO2 uptakes

successful introduction of amine groups. The XRD patterns shown in Figure S2 give broad peaks regardless of materials. This indicates the amorphous characteristic of the frameworks, which is similar to porous polymer networks reported in literature.41−43 The IR spectra, together with the results of 13C NMR and elemental analysis, demonstrate the successful construction of polymers via nucleophilic substitution reactions. The pore structure of polymers was evaluated by N2 adsorption at 77 K (Figure S3) and CO2 adsorption at 273 K (Figure 4). The N2 adsorption−desorption isotherms of NUT-1, NUT-2, NUT-3, and NUT-4 reveal that the uptakes at relative pressures lower than 0.8 are quite low. With the increase of relative pressures, the uptake increase obviously, which indicates that the adsorption of N2 mainly occurs on the outer surface and/or pores between aggregated particles. Due to the swelling effect of polymers in the process of N2 adsorption or the restricted access of N2 molecules within narrow micropores, N2 adsorption may underestimate the porosity.44 It is worth noting that CO2 adsorption at 273 K offers a useful technique to analyze the pore structure of materials, especially those with small micropores.40,45 There are two main differences between N2 and CO2 adsorption measurements. On the one hand, the kinetic diameter of CO2 (3.30 Å) is smaller than that of N2 (3.64 Å), so that the CO2 molecules can access narrow micropores while the entrance of N2 is difficult.46 On the other hand, the higher temperature used for CO2 adsorption imparts a significant kinetic energy to the molecules, enabling them to enter into the narrow pores.47 Hence, CO2 adsorption was employed to estimate the surface area and pore volume of the present materials. Unlike N2 adsorption, a large amount of CO2 can be adsorbed at low relative pressures. Hence, some valid textual parameters can be obtained from the CO2 adsorption isotherms. The CO2 uptake on NUT-1 is obviously higher than that on other materials and decreases in the order of NUT-1 > NUT-2 > NUT-3 > NUT-4. A similar pore size at about 4.0 Å is determined for all of the materials (Figure 4B and Table 1). Table 1 also lists other textual parameters 3080


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Figure 5. Adsorption isotherms of CO2, CH4, and N2 on polymers at 273 and 298 K.

Figure 6. IAST selectivity of CO2/N2 and CO2/CH4 on the resultant polymers at 298 K.

gases N2 and CH4 are barely adsorbed. At 298 K, the uptake of N2 and CH4 is only 0.13 and 1.45 mg·g−1 on NUT-1. The same tendency is also found on NUT-2, NUT-3, and NUT-4. The uptakes of CH4 and N2 on the present polymers are lower than common adsorbents reported in literature. These results indicate a high selectivity of CO2 over CH4 and N2. The IAST model is employed to estimate the selectivity of CO2/N2 and CO2/CH4. As shown in Figures 6 and S7, all polymers exhibit high adsorption selectivities of CO2 over N2 and CH4. At 298 K and 1 bar, the IAST selectivity of CO2/N2 on NUT-1 can reach 10 140, which is higher than that on NUT-2 (3140), NUT-3 (590), and NUT-4 (110), To the best of our knowledge, NUT-1 and NUT-2 present a CO2/N2 selectivity much higher than reported adsorbents such as SIFSIX-3-Zn (1818)10 and PPN-6-CH2DETA (442)55 as well as benchmarks Mg-MOF-74 (352)58 and 13X zeolite (220).59 Similarly, NUT-1 and NUT-2 exhibit a high selectivity of CO2/ CH4 (ca. 100), which is obviously higher than that of other reported porous solids under the analogous conditions (e.g., 63.2 for PAF-30,9 12.0 for PECONF-4,60 7.9 for PAF-1-450,61 3.7 for activated carbon62). Dynamic breakthrough curves were measured to evaluate the separation performance of the polymers for gas mixtures. The gas mixtures CO2/N2 (15/

on NUT-2, NUT-3, and NUT-4 are 43.6, 28.4, and 17.7 mg· g−1, respectively, at 298 K. Under the same adsorption conditions, the uptake of CO2 on NUT-1 can reach 62.8 mg· g−1, which is obviously higher than that on other samples. At 273 K the uptake is even higher and reaches 82.1 mg·g−1. The adsorption capacity of NUT-1 is comparable to some reported adsorbents such as MOFs like PCN-68 (48.4 mg·g−1),50 ZIF-79 (64.4 mg·g−1),51 and NH2-MIL-53(Al) (50.0 mg·g−1)52 as well as porous polymers like covalent organic polymers COP-1 (60.0 mg·g−1),53 conjugated microporous polymer CMP-1 (52.8 mg·g−1),54 and porous polymer network PPN-6-CH2Cl (56.3 mg·g−1).55 It is worth noting that the adsorption and desorption branches are not reversible well, and the isotherms show hysteresis (Figure 4). This is caused by the strong interaction between adsorbate and adsorbent. The adsorbents are capable of chemisorption of CO2, so that the adsorbed CO2 is not completely desorbed from the active sites. Similar results were also reported in the literature for some amine-containing adsorbents.56,57 In addition to 273 and 298 K, the CO2 adsorption of NUT-1 at 313 K was also examined. As shown in Figure S6, the adsorption capacity decreases with the increase of temperature, and 52.3 mg of CO2 can be captured by per gram of NUT-1. In comparison with CO2, the other 3081


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there was no detectable loss in capacity after each cycle, thus indicating the excellent recyclability of these materials. It is worthy of note that the saturated adsorbents were regenerated at only 60 °C for 100 min such mild conditions are adequate to recover the adsorption capacity entirely. This can be ascribed to the existence of abundant secondary amines that acting as active sites, which provide suitable interactions of CO2 with adsorbents. A porous polymer network tethered with primary amines (i.e., PPN-6-CH2DETA) was reported by Lu et al.55 Their results revealed that the lower limit for the regeneration of adsorbent is 80 °C for 100 min, which is stricter than our materials containing secondary amines. Through chemical activation of polyindole nanofibers, nitrogen-doped microporous carbons were prepared by Saleh et al. and used for the adsorption of CO2.63 During regeneration of adsorbents, a temperature of 150 °C was employed; however, a slight loss of adsorption capacity was also found after ten cycles. The present adsorbents, possessing excellent recyclability as well as mild regeneration conditions, thus have high potential for practical applications in energy-efficient carbon capture processes.

85) and CO2/CH4 (50/50) were utilized for the measurements. As displayed in Figure S8, for the CO2/N2 mixture, N2 is the first gas to break through the column within 1 min, while the breakthrough of CO2 is 7 min and much later in comparison with N2. Similarly, CH4 is the weak adsorption component for the CO2/CH4 mixture, and can break through the column first at around 1 min. This breakthrough time is apparently earlier than that of CO2 (5 min). In addition, a rollup of N2 is observed in the breakthrough curve of CO2/N2, which is displaced by CO2 when CO2 becomes the most adsorbed gas. Similarly, a roll-up of CH4 can be seen from the breakthrough curve, and is more obvious than that of N2. This is due to the higher content of CH4 in the binary gas mixture. The above-mentioned results reveal that these polymers are highly promising for separating CO2 from the postcombustion flue gas and purifying the precombustion natural gas. To further understand the adsorption behavior, the isosteric heat of adsorption (Qst) is calculated from the isotherms at different temperatures. The nonlinear curve fitting results of isotherms by Virial type equation are shown in Figure S9. As displayed in Figure 7, at zero loading, the heat of adsorption of

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DISCUSSION Through the nucleophilic substitution reactions of the monomer TCM with various diamines, a range of polymers are successfully constructed. The reactions of TCM with primary amines leads to the formation of new C−N bonds; hence, a great deal of secondary amine groups are formed (Figure 1). The networks are comprised of not only rigid groups (benzene rings) but also flexible linkages (C−C and C− N single bonds). Hence, it is difficult to form regular pores due to the random orientation of flexible linkages. This is responsible for the relatively poor porosity compared to some reported microporous polymers. Interestingly, the obtained polymers show relatively uniform pores with the size of 3.6−4.3 Å. Such a size is quite close to the molecular diameter of THF (4.1 Å). That means, THF may act as a molecular template directing the growth of polymers in addition to the function of a reaction medium. Similar templating roles of solvent molecules played in the formation of polymers were also reported previously.64,65 After the removal of solvents, polymers with a pore size similar to the diameter of solvent molecules are yielded. Both monomers are not molecules with a 3D configuration, and the flexible single bonds tend to bend toward different directions. This may lead to the fabrication of polymers lacking ordered structure. The XRD patterns confirm that all polymers are made up of amorphous frameworks rather than crystalline ones. It is worth noting that ultrahigh selectivities of CO2 over N2 and CH4 are achieved on the obtained polymers. Such selectivities stem from two predominant factors, that is, the existence of abundant secondary amine groups in the frameworks and the appropriate pore sizes of adsorbents. On the one hand, secondary amines are typical CO2-philic sites, and are able to build acid−base interactions between CO2 molecules and adsorbents. Nonetheless, this kind of interaction is absent when the adsorbate molecules are CH4 or N2. On the other hand, the dynamic diameter of CO2 (3.30 Å) is smaller than that of N2 (3.64 Å) and CH4 (3.80 Å). It is noteworthy that the pore sizes of polymers are quite close to the diameters of these gas molecules. As a result, the effect of molecule sieving appears in the adsorption system. CO2 with a small molecular size is readily to enter the pores of adsorbents, while N2 and CH4 with relatively large sizes is comparatively difficult. Among

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

NUT-1 and NUT-2 reaches about 75 kJ·mol−1, while that of NUT-3 (54 kJ·mol−1) and NUT-4 (29 kJ·mol−1) is relatively lower. With the increase of CO2 uptake, the heat of adsorption declines, which may be caused by the continuous occupation of active sites with growing uptake. It is known that recyclability is of great importance for the practical application of adsorbents, regeneration of polymers was thus carried out. Cyclic adsorption−desorption was performed via a combination of thermal treatment and vacuum. The adsorption of CO2 was measured for eight cycles; after each adsorption cycle, mild conditions (60 °C, 100 min) was utilized for the desorption of CO2. As displayed in Figure 8,

Figure 8. Cycling adsorption of CO2 over the adsorbent NUT-1. 3082



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CONCLUSIONS A series of polymers are synthesized through a nucleophilic substitution reaction between chloromethylbenzene and various diamines. Due to the proper reactivity of monomers, polymerization can take place under mild conditions without the addition of any catalysts. The resultant materials contain abundant secondary amines as well as well-defined micropores with a size close to the dimension of solvent molecule. These properties endow the materials with excellent capacity in selective adsorption of CO2 over N2 and CH4 as well as in energy-saving regeneration. By judicious choice of monomers, the present polymerization method should enable secondary amines to be incorporated into frameworks with a variety of pore structures, resulting in the fabrication of new porous polymer networks that show high potential for adsorptive applications.

polymers, NUT-1 presents the highest selectivity of CO2/N2 and CO2/CH4. This can be associated with the high density of amine groups, since the small molecule ED is used as the diamine monomer for the fabrication of polymer. With the increase of molecular size of diamine monomer, the density of amine groups in the polymers decreases. Accordingly, the adsorption performance with regard to capacity and selectivity becomes worse over corresponding polymers. When the diamine monomer PD is employed, the basicity of resultant amine groups connected to benzene rings in NUT-4 is weaker as compared with the alkylamines in other polymers. This is believed to be responsible for relatively poor adsorption performance of NUT-4. In brief, the excellent capacity of polymers on selective adsorption of CO2 can be ascribed to the abundant amine groups in the frameworks as well as the appropriate sizes of the pores. Despite great efforts, the development of a low-cost method to synthesize porous polymer networks is still an open question. The low-cost synthesis is judged from monomers, catalysts, and reaction conditions. First, both monomers employed in the present study are cheap and easily available (as shown in Figure 1), which is different from the synthesis of some polymers reported in literature. For example, the complicated monomer 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) was used for the synthesis of porous polyimide polymer;24 such a monomer was prepared from the quite preliminary precursor 4-bromobenzonitrile via several tedious organic reactions. Second, a nucleophilic substitution is designed to fabricate porous polymer networks, in which no catalysts are required. However, catalysts are widely used for the fabrication of porous polymers. For instance, the expensive catalyst, bis(1,5-cyclooctadiene) nickel, has to be employed in the polymerization process for PAFs.32 Third, the polymerization reaction can take place under quite mild conditions of about 60 °C. On the basis of above analysis, it is safe to say that the present study provides the synthesis of porous polymer networks from low-cost monomers via a catalyst-free and simple polymerization reaction. Because of the templating role played by the solvent, the obtained materials exhibit well-defined micropores with dimensions of solvent molecules. It is of great importance that the materials contain abundant secondary amines. It is known that amine groups are frequently used active sites that can create strong interaction between adsorbents and CO2.66−68 Various type of amines have been introduced to the frameworks of porous polymer networks. Primary amines can capture CO2 efficiently through the formation of stable complexes, whereas the regeneration of adsorbents is rather difficult. Tertiary amines can be regenerated easily under mild conditions, but at the expense of adsorption capacity and selectivity. Thus, secondary amines are ideal building blocks for porous polymer networks, and they can take the balance of adsorption performance and energy-saving regeneration.38,39,69,70 As a result, the present polymers contain copious secondary amines are highly active for selective adsorption of CO2, and unusually high CO2/CH4 and CO2/N2 selectivities are realized. Moreover, the materials can be completely regenerated under quite mild conditions, and no loss of capacity is observed after eight cycles. The cost-effective synthesis, excellent selective adsorption capacity, and energysaving regeneration make the present materials highly promising in adsorptive separation of CO2 from mixtures including natural gas and flue gas.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00544. Additional characterization data for the polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support of this work by the National Natural Science Foundation of China (21576137), the Distinguished Youth Foundation of Jiangsu Province (BK20130045), the Fok Ying-Tong Education Foundation (141069), the National Basic Research Program of China (973 Program, 2013CB733504), and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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