Nitrogen-Doped Porous Carbons Derived from Carbonization of a

Article pubs.acs.org/IECR

Nitrogen-Doped Porous Carbons Derived from Carbonization of a Nitrogen-Containing Polymer: Efficient Adsorbents for Selective CO2 Capture Jiahui Kou†,‡ and Lin-Bing Sun*,†,§ †

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing 210009, China ‡ College of Materials Science and Engineering and §College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China S Supporting Information *

ABSTRACT: Because of their abundant porosity, tunable surface properties, and high stability, N-doped porous carbons (NPCs) are highly promising for CO2 capture. Carbonization of N-containing polymers is frequently used for the preparation of NPCs, while such an approach is hindered by the high cost of some polymer precursors. In the present study, we report for the first time the fabrication of NPCs through the rational choice of the low-cost, N-rich polymer NUT-2 (NUT indicates Nanjing Tech University) as the precursor, which was obtained from polymerization of easily available monomers under mild conditions in the absence of any catalysts. Through carbonization at different temperatures (500−800 °C), NPCs with various porosity and nitrogen contents are obtained. The pore structure and CO2-philic (N-doped) sites are responsible for the adsorption performance, while the highest surface area does not lead to the highest CO2 adsorption capacity. For the sample carbonized at 600 °C (NPC-2-600), the adsorption capacity on CO2 is as high as 164.7 cm3 g−1 at 0 °C and 1 bar, which is much better than that of the benchmarks, such as activated carbon (62.5 cm3 g−1) and 13X zeolite (91.8 cm3 g−1), as well as most reported carbon-based adsorbents. We also demonstrate that the present NPCs can be regenerated completely under mild conditions. The present adsorbents may provide promising candidates for the capture of CO2 from various mixtures, such as flue gas and natural gas.

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porous materials (e.g., silica,10 metal−organic frameworks,11,12 zeolites,13,14 activated carbons15,16) have been developed for CO2 capture. Among them, N-doped porous carbons (NPCs) have attracted much attention because of their high porosity, tunable surface properties, and good chemical and thermal stability.17,18 Meanwhile, nitrogen elements can be doped into carbon frameworks as CO2-philic sites via carbonization of Ncontaining polymers at proper temperatures. The presence of CO2-philic sites can enhance the interaction between CO2 molecules and material surface and subsequently improve the adsorption capacity.19 Unfortunately, the applications of NPCs are hindered by the high cost of some polymer precursors. Complex monomers, expensive catalysts, and/or high temperatures are often demanded for the synthesis of polymers, which makes the synthetic processes complicated, costly, and hard to scale up. A case in point is Liebl and Senker’s report. Several

INTRODUCTION With the rapid development of industry, an increasing amount of CO2 has been released to the atmosphere, and the elevated atmospheric CO2 concentration has exerted influence on climate and environment.1,2 Carbon capture and sequestration is an effective approach to control the concentration of atmospheric CO2.3,4 Traditionally, a chemical absorption method has been widely employed for the capture of CO2 from flue gas or natural gas stream, and the commonly used aqueous amine solutions include triethanolamine (TEA), monoethanolamine (MEA), etc.5 The utilization of a chemical absorption method for CO2 capture is a double-edged sword. On the one hand, the strong chemical interaction between amines and CO2 results in high capacity and selectivity; on the other hand, high regeneration costs are intrinsic to aqueous amine solutions in addition to other disadvantages such as unpleasant poisonous smell and erosion of equipment.6,7 A promising alternative for CO2 capture is adsorption by porous materials. In comparison with amine solutions, these porous materials possess lower heat capacities, are easy to handle, and are free of corrosion issues.8,9 Nowadays, many © 2016 American Chemical Society

Received: Revised: Accepted: Published: 10916

July 26, 2016 September 24, 2016 September 26, 2016 October 4, 2016 DOI: 10.1021/acs.iecr.6b02857 Ind. Eng. Chem. Res. 2016, 55, 10916−10925

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Industrial & Engineering Chemistry Research interesting triazine-based porous polyimide (TPI) polymers were prepared through the condensation of 2,4,6-tris(4aminophenyl)-1,3,5-triazine (TAPT) with the respective dianhydride building blocks in m-cresol.20 It should be stated that the monomer TAPT was synthesized from the very preliminary compound 4-bromobenzonitrile through a series of tedious organic reactions. For the famous porous aromatic frameworks (PAFs) reported by Zhu and co-workers, an expensive catalyst, bis(1,5-cyclooctadiene) nickel, is compulsory in the polymerization process.21,22 When the aim is developing low-cost polymers as carbon precursors, the polymers should be synthesized from simple monomers under mild conditions and the use of catalysts should be avoided as far as possible. Recently, we reported the synthesis of a series of polymers through a facile nucleophilic substitution reaction of two monomers, that is, chloromethylbenzene and diamines.23 Because of the appropriate reactivity of monomers, the polymerization reactions occur under mild conditions in the absence of a catalyst. In addition, both monomers are inexpensive and readily available. A typical one is the polymer NUT-2 (NUT indicates Nanjing Tech University), which can be synthesized from the monomers 2,4,6-tris(chloromethyl)mesitylene (TCM) and 1,4-diaminobutane (DB) at about 60 °C without any catalysts. As a result, the polymer NUT-2 should be an ideal precursor for the preparation of NPCs. In the present study, the polymer NUT-2 was employed for the first time as a precursor and carbonized at different temperatures, leading to the formation of a series of NPCs. It is worth noting that the obtained NPCs are highly efficient in the capture of CO2 with regard to both adsorption capacity and selectivity. The pore structure and CO2-philic (N-doped) sites are responsible for the adsorption performance, while the highest surface area does not lead to the highest CO2 adsorption capacity. For the sample carbonized at 600 °C (NPC-2-600), the surface area is 1274 m2 g−1 and the nitrogen content is 3.45 wt %. The adsorption capacity on CO2 reaches 164.7 cm3 g−1 at 0 °C and 1 bar, which is much better than that of the benchmarks, such as activated carbon (62.5 cm3 g−1) and 13X zeolite (91.8 cm3 g−1), as well as most reported carbonbased adsorbents. We also demonstrated that NPCs, as prospective solid adsorbents, can be regenerated completely under mild conditions and the adsorption capacities can be well maintained after several cycles.

Figure 1. Synthetic route of the N-containing porous polymer NUT-2 and its carbonized derivative NPCs.

stirring. After heating at 60 °C, a brown residue was obtained. Carbonization of the brown residue was carried out under a highly pure nitrogen atmosphere (99.99%) at the temperature range from 500 to 800 °C increasing at the rate of 3 °C min−1 and holding at the final temperature for 1 h. After carbonization, the dark solids were treated with 2 N HCl to remove residual KOH, followed by washing with deionized water four times. The resultant materials are donated NPC-2-x, where x represents the carbonization temperature, which varied from 500 to 800 °C. To examine the effect of KOH, a reference sample NPC-2-600r was also prepared by carbonization of the polymer at 600 °C; the procedure was identical to that of NPC2-600 except that no KOH was added. Material Characterization. Fourier transform infrared (IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr wafer. The ratio of a sample to KBr was 1:150. X-ray powder diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer with Cu Kα at 100 kV and 40 mA. Elemental analysis was carried out on an Elementar Vario EL elemental analyzer. Transmission electron microscopy (TEM) images of the materials were captured in a JEM-2010 UHR electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis were performed using an ESCALAB-220I-XL (Thermo-Electron, VG Company) device. N2 adsorption−desorption isotherms were measured using Micromeritics ASAP 2020 surface area and pore size analyzer at 77 K. The samples were degassed at 150 °C for 4 h before analysis. The Brunauer−Emmett−Teller (BET) surface areas were calculated with the relative pressure ranging from 0.01 to 0.10. The fitting results are shown in Figure S1. The total pore volumes were derived from the uptake at a relative pressure of 0.95. The pore size distributions were calculated from the adsorption isotherms by using density functional theory (DFT). Adsorption Tests. Static adsorption experiments of CO2, N2, and CH4 can be measured by ASAP 2020 analyzer. CO2 (99.999%), N2 (99.999%), and CH4 (99.99%) gases were used for the adsorption measurements. The free space was determined using helium (99.999%), assuming that helium could not be adsorbed at the temperatures investigated. The adsorption isotherms of CO2, N2, and CH4 at 0 °C were measured in an ice−water bath, and isotherms at 25 °C were

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EXPERIMENTAL SECTION Material Synthesis. The two monomers, namely, TCM and DB, were used for synthesis of the polymer NUT-2 (Figure 1).23 In a typical process, TCM (1.22 g, 4 mmol) and DB (0.6 mL, 6 mmol) were dissolved in tetrahydrofuran (THF, 100 mL), and the obtained solution was heated under stirring in a closed flask at 63 °C for 24 h, After cooling to room temperature, the reaction mixture was centrifuged to remove the solvent; the obtained white precipitate was treated with an ethanol/water (50 mL/50 mL) solution of KOH (2.016 g) at 45 °C for 12 h. Then, the white precipitate was washed with an ethanol/water solution four times and dried at room temperature, and the resultant white powder was named NUT-2. The resultant polymer NUT-2 was precarbonization at 200 °C for 80 min under an air atmosphere. The purpose of precarbonization is to avoid the quick collapse of the polymer structure at high temperatures. The precarbonized product was immersed in a KOH (1/3, mass ratio) ethanol solution with 10917

DOI: 10.1021/acs.iecr.6b02857 Ind. Eng. Chem. Res. 2016, 55, 10916−10925

Article

Industrial & Engineering Chemistry Research measured in a water bath. The isotherms were fitted with the double Langmuir (DL) model as shown in eq 1 q = qc

kcp k ip + qi 1 + kcp 1 + k ip

(1)

where qc, qi, kc, and ki are the DL model parameters with the subscripts c and i denoting the channels and intersections, respectively. p is the system pressure, and q is the adsorption amount. To further understand the interaction between CO 2 molecules and adsorbents, the isosteric heats of CO2 adsorption (Qst) were calculated using the Clausius−Clapeyron equation from CO2 adsorption isotherms collected at 0 and 25 °C; Qst is determined using eq 2. ⎛ ∂lnp ⎞ ⎟ Q st = −RT 2⎜ ⎝ ∂T ⎠q

Figure 2. IR spectra of the polymer NUT-2 and the NPCs. The numerals in parentheses indicate the magnification of the spectrum.

IR spectra of some NPCs were magnified 5−20 times. One can observe the vibration ascribed to phenyl ring skeleton for NPCs, while the IR bands are still quite weak for the samples carbonized at high temperatures. The vibrations of phenyl ring skeleton are generally quite weak and can be enhanced when there are substituents on the ring. During carbonization, phenyl rings are converted to polycyclic aromatics, leading to the decline of substituents on the ring. As a result, higher temperatures result in aromatization but decreasing intensity of phenyl ring in IR spectra. The XRD patterns of the materials are shown in Figure S2. Both NUT-2 and NPCs reveal similar shapes without a sharp peak, which indicates the amorphous properties of these samples. Notably, when the carbonization temperatures are higher than 600 °C, the intensity of two broad peaks at 25 and 43° 2θ is enhanced, which are typical of carbonaceous materials with an amorphous structure.27 Figure 3A shows N2 adsorption−desorption isotherms at 77 K of the polymer NUT-2 and its carbonized derivatives. As shown in the adsorption branches, the N2 uptake of the polymer NUT-2 is low because of its low porosity. However, N2 uptakes of NPCs rise rapidly especially at low relative pressures, and the isotherms become smooth at relative pressures beyond 0.1 for most NPCs, which are typical isotherms of type I for micropore materials. For the sample NPC-2-800, the N2 uptake rises gradually with the increase of relative pressures. This indicates the existence of some larger pores in NPC-2-800 in addition to micropores. As displayed in Table 1, for the polymer NUT-2, the surface area is just 16 m2 g−1, and microporous volume and total volume are 0.01 and 0.06 cm3 g−1, respectively. Upon carbonization, significant improvement can be found in both surface area and pore volume. Also, surface area and pore volume can be further enhanced by increasing the carbonization temperatures. The surface area varies from 680 m2 g−1 (NPC-2-500) to 2282 m2 g−1 (NPC-2-800), whereas pore volume ranges from 0.29 cm3 g−1 (NPC-2-500) to 1.06 cm3 g−1 (NPC-2-800). Figure 3B shows the pore diameter distribution of different samples. For most NPCs, the pore diameters are mainly less than 1 nm. Some larger pores are observed with increasing carbonization temperatures especially for the sample NPC-2-800, which is caused by the collapse of the framework under hightemperature conditions. In comparison with the reference sample NPC-2-600r prepared in the absence of KOH (with a surface area of 334 m2 g−1 and a pore volume of 0.14 cm3 g−1), the counterpart NPC-2-600 has an apparently higher surface area (1274 m2 g−1) and a larger pore volume (0.46 cm3 g−1). This demonstrates that the KOH as activator plays an

(2)

The ideal adsorption solution theory (IAST) has been reported for predicting binary gas mixture adsorption in solid adsorbent.24,25 The adsorption selectivity has been defined according to eq 3

S = (xi /yi )/(xj/yj )

(3)

where xi and yi (xj and yj) are the molar fractions of component 1 (component 2) in the adsorbed and bulk phases, respectively.26 In the calculation, the ratio of CO2/N2 is 15/ 85 and the ratio of CO2/CH4 is 50/50, which are typical composition of flue gas and natural gas, respectively. The regeneration experiment was carried out with and ASAP 2020 analyzer; the samples were saturated with CO2 up to 1 bar at 0 °C. The recovered adsorbents were degassed at 30 °C for 60 min prior to each measurement.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. The Ncontaining polymer NUT-2 can be synthesized from the monomers TCM and DB at the temperature of about 60 °C in the absence of any catalysts. At the initial stage, both monomers are dissolvable in THF, leading to the formation of a clear colorless solution. At the end of the reaction, the white polymer NUT-2 was produced. This polymer obtained from low-cost monomers with abundant nitrogen content is interesting for use as the precursor for NPCs. Via the KOH-assisted carbonization of NUT-2, a series of NPCs could be prepared. Various methods were then employed to characterize the NPCs. Figure 2 shows the IR spectra of the polymer NUT-2 and NPCs. In the IR spectrum of the polymer NUT-2, two strong bands at 2915 and 2945 cm−1 can be ascribed to −CH2− stretching vibrations; meanwhile, the bands at 1103 and 3427 cm−1 originate from C−N and N−H stretching vibrations, respectively. The spectrum also presents bands at 820, 1241, and 1597 cm−1, which are caused by the stretching vibration of benzene rings. These IR bands demonstrate that the polymer NUT-2 was successfully fabricated through the nucleophilic substitution reaction. After carbonization at different temperatures, most of the characteristic bands weaken obviously. At a carbonization temperature of 500 °C, the bands at 1597 cm−1, corresponding to the phenyl ring skeleton, are still visible, while the intensity declines with increasing temperatures; the bands almost disappear at 800 °C. To display the spectra more clearly, 10918


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Figure 3. (A) N2 adsorption−desorption isotherms at 77 K and (B) pore size distributions of the polymer NUT-2 and the NPCs.

Table 1. Textural Properties and Elemental Contents of the Polymer NUT-2 and NPCs elemental content (wt %)

a

sample

SBETa (m2 g−1)

Vtotb (cm3 g−1)

Vmicroc (cm3 g−1)

N

C

H

NUT-2 NPC-2-500 NPC-2-600 NPC-2-600r NPC-2-700 NPC-2-800

16 680 1274 334 2088 2282

0.06 0.29 0.46 0.14 0.84 1.06

0.01 0.27 0.43 0.13 0.71 0.87

11.3 5.9 3.5 6.0 2.2 1.8

79.1 86.5 85.8 81.0 88.4 92.7

6.2 3.3 2.8 2.7 2.6 1.4

BET surface area. bTotal pore volume. cMicropore volume.

important role in the generation of pores. TEM images confirm the existence of micropores with wormhole-like arrangement in NPCs (Figure 4). In comparison with NPC-2-600r, the sample NPC-2-600 displays more obvious and clear pores, which confirms that NPC-2-600 possesses more developed porosity. SEM images were also taken and are shown in Figure S3. Plentiful large pores can be observed on the sample NPC-2600. This indicates that the present NPCs possess hierarchical pore structure, in which small pores are connected by large pores. The hierarchical pore structure of adsorbents benefits the diffusion of adsorbates. The enhanced surface area and pore volume can be attributed to the KOH-assisted carbonization. On the one hand, KOH functions as a chemical activator, which promotes the formation of micropores; on the other hand, KOH acts as a hard template to prevent the collapse of the framework. KOHassisted carbonization is a well-known way to promote the generation of pores in the carbon framework. At elevated temperatures, KOH reacts with carbon to produce K2CO3, elemental potassium, and hydrogen. The emission of hydrogen promotes the formation of micropores, while the formed potassium element and K2CO3 can be washed away by diluted hydrochloric acid, which leads to the formation of additional pores.28−30 At low carbonization temperatures, the smaller pores come from KOH activation and/or pore shrinkage. With the increase of temperature, some larger pores are generated owing to the collapse of the framework. The data of elemental analysis are listed in Table 1. With the increase of carbonization temperatures, the nitrogen content in

Figure 4. TEM images of (A) NPC-2-600 and (B) NPC-2-600r.

NPCs decreases from 5.93 wt % (NPC-2-500) to 1.80 wt % (NPC-2-800). This indicates that carbonization leads to a reduction of nitrogen contents because of denitrogenation and aromatization. The existing form of nitrogen was identified by XPS (Figures 5 and S4). The polymer NUT-2 exhibits a single peak at 399.8 eV, which is assigned to amide and/or pyrrolic nitrogen.31 Carbonization at different temperature results in a change in the N 1s peak; the fitting of N 1s peaks are given as follows: 399.8 eV (amide and/or pyrrolic nitrogen), 401.3 eV (quaternary nitrogen), and 402.8 eV (pyridine-N-oxide). During the process of carbonization, nitrogen species change obviously. Amide and/or pyrrolic nitrogen decreases pro10919


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164.7 cm3 g−1 (115.3 cm3 cm−3). When the carbonization temperatures are increased further, the CO2 uptake on NPC-2700 and NPC-2-800 decreases. For NPC-2-500 and NPC-2600 prepared at relative lower carbonization temperatures, their isotherms exhibit a steep rise at low pressures. The samples NPC-2-500 and NPC-2-600 possess abundant micropores and richer CO2-philic sites, which can enhance CO2−framework interaction and dipole−quadrupole interaction, respectively. Meanwhile, the reversible nature of the CO2 isotherms indicates that these NPCs can be easily regenerated by reducing the pressure under ambient temperature conditions. In the case of NPC-2-700 and NPC-2-800, the increasing trend of CO2 uptake is more obvious at relative high pressures because of their larger pore volumes. In order to explore the effect of KOH, the adsorption capacity of NPC-2-600r and NPC-2-600 was compared. It is worth noting that the CO2 uptake on NPC-2-600r is just 69.1 cm3 g−1 (48.4 cm3 cm−3) at 0 °C and 1 bar, which is about one-third that of NPC-2-600 (164.7 cm3 g−1 or 115.3 cm3 cm−3). The CO2 adsorption capacities of some solid adsorbents reported in the literature are listed in Table 2. Our samples exhibit obviously superior

Figure 5. XPS peak fitting of N 1s spectra of the polymer NUT-2 and the NPCs.

Table 2. Uptake of CO2 over Various Adsorbents

gressively and transforms to more stable protonated quaternary nitrogen and pyridine-N-oxide species.32,33 On the basis of the aforementioned results, it is clear that the polymer NUT-2 was transformed to a series of NPCs via carbonization at different temperatures. The resultant NPCs exhibit high surface areas and abundant micropores; meanwhile, some larger pores are generated with increasing carbonization temperatures. The nitrogen contents keep decreasing with increasing carbonization temperatures. When NPC-2-600 is compared with NPC-2-600r, it can be found that the presence of KOH can promote the carbonization process and the formation of developed porosity. The abundant micropores and CO2-philic sites make these NPCs highly promising for selective capture of CO2 as shown below. Adsorption Performance. Figure 6 displays CO 2 adsorption−desorption isotherms at 0 °C for the polymer

SBET (m2 g−1)

adsorption capacity (cm3 g−1)

Ta (°C)

Sadsb

ref

NPC-2-600

1274

164.7/108.0

0/25

22

zeolite 13X AC CS-500-1.5 CEM 700 AHEP PC-2 HMT-80-600 Th-1 CP-2-600 PC680 ATK-2-600 PAC-550/2 bamboo-1-973 PC AC4-PC SK-0.5-700 AS-2-600 NPC-650 P4-700

726 1175 503 1913 418 1479 642 726 1700 1713 1290 2122 930 1220 2151 1060 1260 1033 1745

91.8/78.4 62.5/41.4 62.7/33.6 127.7/85.1 53.8/31.4 123.2/76.2 100.8/51.5 65.0/38.1 89.6/47.0 163.5/98.6 109.8/67.2 135.5/85.8 118.7/89.6 89.6 81.5 95.0 136.6/107.5 118.7/69.4 78.4

0/20 0/25 0/25 0/25 0/25 0/25 0/25 0/25 0/25 0/25 0/25 0/25 0/25 25 25 25 0/25 0/25 25

NA 17 33 NA NA 18 NA 38 12 8 7 26 11 13 NA NA 5 13 NA

this work 35 34 45 18 36 37 38 39 46 47 48 44 44 49 49 50 51 52 53

adsorbent

a Adsorption temperature: a diagonal is used to distinguish different adsorption temperatures that correspond to the CO2 adsorption capacity in the third column in the same row, and the adsorption pressure is 1 bar. bAdsorption selectivity of CO2 over N2 at 25 °C. NA means not available.

capacity to the benchmarks including activated carbon (62.5 cm3 g−1)34 and 13X zeolite (91.8 cm3 g−1),35 as well as most reported carbon-based adsorbents, such as AHEP (53.8 cm3 g−1),36 PC-2 (123.2 cm3 g−1),37 HMT-80-600 (100.8 cm3 g−1),38 and Th-1 (65.0 cm3 g−1).39 To examine the adsorption selectivity, the adsorption isotherms of N2 and CO2 were measured and shown in Figure 7. Despite the high uptake of CO2 on different NPCs, N2 is barely adsorbed at 0 and 25 °C. The N2 uptake on NPC-2-600 is just 17.8 cm3 g−1 at 0 °C and 1 bar, which is much lower than the uptake of CO2 (164.7 cm3 g−1). Similar results are also

Figure 6. CO2 adsorption−desorption isotherms of the polymer NUT-2 and the NPCs at 0 °C.

NUT-2 and NPCs. The polymer NUT-2 shows a low CO2 uptake of 33.4 cm3 g−1 (20.0 cm3 cm−3) at 0 °C and 1 bar. It is worth noting that after carbonization, the CO2 uptake on the NPCs can be significantly enhanced. Under the same conditions, the CO2 uptake on NPC-2-600 is as high as 10920


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Figure 7. CO2 and N2 adsorption isotherms of (A) the polymer NUT-2, (B) NPC-2-500, (C) NPC-2-600, (D) NPC-2-600r, (E) NPC-2-700, and (F) NPC-2-800 at 0 and 25 °C.

Figure 8. IAST selectivity of CO2/N2 on the NPCs at (A) 0 °C and (B) 25 °C.

°C. For example, the CO2/N2 selectivity on NPC-2-600 is 54 at 0 °C and 1 bar, which is higher than that at 25 °C and 1 bar (22). To further understand the adsorbate−adsorbent interaction, the heat of adsorption (Qst) was calculated using the Clausius− Clapeyron equation from the adsorption isotherms collected at 0 and 25 °C. As displayed in Figure 9, at zero loading, the heat of adsorption of the polymer NUT-2 (75 kJ mol−1) is higher than those of NPCs (38 kJ mol−1 for NPC-2-500, 56 kJ mol−1 for NPC-2-600, 39 kJ mol−1 for NPC-2-600r, 24 kJ mol−1 for NPC-2-700, and 26 kJ mol−1 for NPC-2-800). The high heat of

observed on NPCs obtained at different carbonization temperatures. IAST theory has been widely used to predict adsorption selectivity of gas mixtures. In this calculation, the ratio of CO2/N2 is 15/85, which is the typical component of flue gas. Fitting parameters of DL model are listed in Tables S1−S6, and the IAST selectivity results are shown in Figure 8. At 25 °C and 1 bar, the IAST selectivity of CO2/N2 on the sample NPC-2-600 reaches 22, which is lower than that of NPC-2-500 (56) and NPC-2-600r (55) and higher than that of NPC-2-700 (19) and NPC-2-800 (9). In addition, the IAST selectivity on different samples at 0 °C is higher than that at 25 10921


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°C, 60 min) were executed for the regeneration. No discoverable loss of CO2 adsorption capacity on the sample NPC-2-600 takes place after several cycles, which indicates the excellent recyclability of these materials. It is worth noting that the saturated adsorbents can be regenerated at 30 °C for 60 min; such mild regeneration conditions can recover adsorption capacity thoroughly, which should be ascribed to the proper interaction strength between CO2 molecule and adsorbent surface. In addition to the separation of CO2 from N2 (postcombustion for flue gas), the separation of CO2 from CH4 (precombustion for natural gas) also receives great attention.40,41 The existence of CO2 not only affects the heat value during combustion but also causes the corrosion of relevant equipment.42,43 Therefore, the capture of CO2 from natural gas has recently received growing interest. Figure 11 displays the adsorption isotherms of CH4 at 0 and 25 °C. For the sample NPC-2-600, the respective CH4 uptake on NPC-2-600 is 50.9 and 33.6 cm3 g−1 at 0 and 25 °C, 1 bar, while CO2 uptakes on NPC-2-600 are 164.7 and 108.0 cm3 g−1 under the same conditions. Similar results can be observed on other NPCs, indicating the selective adsorption of CO2 in the presence of CH4. To estimate the selectivity of CO2 over CH4, the fitting parameters are shown in Tables S1−S6 and adsorption selectivity results are shown in Figure S6. The ratio of CO2/ CH4 is 50/50 in the calculation, which is typical of components of natural gas. The NPCs obtained from carbonization at 500 and 600 °C exhibit higher selectivity in contrast to other samples; these materials might be promising candidates for the purification of natural gas. The good performance of NPCs on selective CO2 adsorption NPCs can be attributed to several factors. First, abundant micropores with pore diameter less than 1 nm should be taken into consideration, because this kind of pore can enhance the interaction between CO2 molecules and pore walls.44 Second, nitrogen sites in the NPCs are another factor affecting the adsorption capacity. The presence of nitrogen in the framework is beneficial to the adsorption of CO2 because of the large quadrupole moment of CO2 molecules and the strong pole− pole interactions between CO2 and CO2-philic nitrogen sites.32 Of course, the surface area of an adsorbent is also responsible for the adsorption capacity. Under the same conditions, an adsorbent with a higher surface area will adsorb more CO2. Nevertheless, the sample NPC-2-800, which has the largest surface area, does not possess the highest adsorption capacity. This means that the surface area is not the predominant factor in comparison with microporosity and nitrogen content. For other nonacidic gases (such as N2 and CH4), there are no pole−pole interactions between gas molecules and adsorbent surface, and the uptake of nonacidic gases on NPCs is obviously lower than that of the acidic gas. This suggests that NPCs perform well in adsorbing CO2 from flue gas and natural gas selectively.

Figure 9. CO2 isosteric heat of adsorption of the polymer NUT-2 and the NPCs.

adsorption at zero loading of NUT-2 is considered to be caused by two factors. First, there are abundant basic amino groups in NUT-2, which can interact strongly with acidic CO2. Second, pores do exist in the polymer, although the pore size is very small. It is difficult for N2 molecules to enter the pores, while CO2 molecules can enter, because the dynamic diameter of CO2 (3.30 Å) is smaller than that of N2 (3.64 Å). As a result, the polymer can adsorb a certain amount of CO2, whereas negligible amount of N2 is adsorbed (see Figure 7). These small pores will increase the interaction between pore walls and CO2 molecules. Hence, the abundant basic amino groups, in combination with the small pores, are responsible for the high heat of adsorption of NUT-2. The Qst of the polymer NUT-2 decreases rapidly with the increasing CO2 uptake, which results from the continuous occupation of CO2-philic sites with growing CO2 uptake. However, the Qst downward tendency of NPCs is relatively mild and then keeps almost stable at high CO2 uptakes. This suggests that the adsorption of CO2 on NPCs is relatively uniform. The Qst value depends on the balance of nitrogen content and microporous structure. Although nitrogen content keeps decreasing with increasing carbonization temperature, microporous structure can be improved in the meanwhile. Recyclability of adsorbents is of significant importance from the viewpoint of practical applications; therefore, regeneration of NPCs was examined. The regeneration of adsorbents was conducted using the ASAP 2020 analyzer. After an adsorbent was saturated with CO2 up to 1 bar, the adsorbent was recovered. Then the recovered adsorbent was degassed at 30 °C for 60 min and used for adsorption again. The adsorption of CO2 was carried out for six cycles. As displayed in Figures 10 and S5, after each adsorption cycle, quite mild conditions (30

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CONCLUSIONS A series of NPCs were fabricated through direct carbonization of the low-cost N-containing polymer NUT-2 for the first time. The resultant NPCs exhibit abundant micropores with a diameter less than 1 nm. Nitrogen is successfully doped in the framework of carbon during carbonization, while the nitrogen content is dependent on the carbonization temperature. The abundant micropores and nitrogen sites endow the present NPCs with high activity in selective adsorption of CO2.

Figure 10. Adsorption isotherms of CO2 during cycling regeneration experiment over the adsorbent NPC-2-600 at 0 °C. 10922


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Figure 11. CO2 and CH4 adsorption isotherms of (A) the polymer NUT-2, (B) NPC-2-500, (C) NPC-2-600, (D) NPC-2-600r, (E) NPC-2-700, and (F) NPC-2-800 at 0 and 25 °C.

National Natural Science Foundation of China (21576137 and 51303079), the project on the Integration of Industry, Education and Research of Jiangsu Province (BY2015005-16), the Distinguished Youth Foundation of Jiangsu Province (BK20130045), and the Fok Ying-Tong Education Foundation (141069).

Moreover, the adsorbents can be completely regenerated under mild conditions, and no loss is detected after six cycles. Our adsorbents may provide promising candidates for the capture of CO2 from various mixtures, such as flue gas and natural gas.

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ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02857. XRD, SEM, and XPS data as well as additional adsorption and fitting data (PDF)

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REFERENCES

(1) Haszeldine, R. S. Carbon Capture and Storage: How Green Can Black Be? Science 2009, 325, 1647−1652. (2) Wickramaratne, N. P.; Jaroniec, M. Activated Carbon Spheres for CO2 Adsorption. ACS Appl. Mater. Interfaces 2013, 5, 1849−1855. (3) Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal−Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869−932. (4) Sun, L.-B.; Li, A.-G.; Liu, X.-D.; Liu, X.-Q.; Feng, D.; Lu, W.; Yuan, D.; Zhou, H.-C. Facile Fabrication of Cost-Effective Porous Polymer Networks for Highly Selective CO2 Capture. J. Mater. Chem. A 2015, 3, 3252−3256. (5) Filburn, T.; Helble, J. J.; Weiss, R. A. Development of Supported Ethanolamines and Modified Ethanolamines for CO2 Capture. Ind. Eng. Chem. Res. 2005, 44, 1542−1546. (6) Seema, H.; Kemp, K. C.; Le, N. H.; Park, S.-W.; Chandra, V.; Lee, J. W.; Kim, K. S. Highly Selective CO2 Capture by S-doped Microporous Carbon Materials. Carbon 2014, 66, 320−326. (7) Feng, S.; Li, W.; Shi, Q.; Li, Y.; Chen, J.; Ling, Y.; Asiri, A. M.; Zhao, D. Synthesis of Nitrogen-Doped Hollow Carbon Nanospheres for CO2 Capture. Chem. Commun. 2014, 50, 329−331.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This contribution was identified by Prof. Virender Sharma (Texas A&M University) as the Best Presentation in the session “Division of Industrial and Engineering Chemistry” of the 2016 ACS Spring National Meeting in San Diego, CA. The authors acknowledge the financial support of this work by the 10923


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Formation and Dispersion of Cuprous Sites. J. Mater. Chem. A 2014, 2, 3399−3406. (27) Sevilla, M.; Fuertes, A. B. Direct Synthesis of Highly Porous Interconnected Carbon Nanosheets and Their Application as HighPerformance Supercapacitors. ACS Nano 2014, 8, 5069−5078. (28) Coromina, H. M.; Walsh, D. A.; Mokaya, R. Biomass-Derived Activated Carbon with Simultaneously Enhanced CO2 Uptake for Both Pre and Post Combustion Capture Applications. J. Mater. Chem. A 2016, 4, 280−289. (29) Hilton, R.; Bick, P.; Tekeei, A.; Leimkuehler, E.; Pfeifer, P.; Suppes, G. J. Mass Balance and Performance Analysis of Potassium Hydroxide Activated Carbon. Ind. Eng. Chem. Res. 2012, 51, 9129− 9135. (30) Lillo-Ródenas, M. A.; Cazorla-Amorós, D.; Linares-Solano, A. Understanding Chemical Reactions Between Carbons and NaOH and KOH: An Insight into the Chemical Activation Mechanism. Carbon 2003, 41, 267−275. (31) Weidenthaler, C.; Lu, A.-H.; Schmidt, W.; Schüth, F. X-ray Photoelectron Spectroscopic Studies of PAN-Based Ordered Mesoporous Carbons (OMC). Microporous Mesoporous Mater. 2006, 88, 238−243. (32) Hao, G.-P.; Li, W.-C.; Qian, D.; Lu, A.-H. Rapid Synthesis of Nitrogen-Doped Porous Carbon Monolith for CO2 Capture. Adv. Mater. 2010, 22, 853−857. (33) Pietrzak, R. XPS Study and Physico-Chemical Properties of Nitrogen-Enriched Microporous Activated Carbon from High Volatile Bituminous Coal. Fuel 2009, 88, 1871−1877. (34) Zhang, C.; Song, W.; Ma, Q.; Xie, L.; Zhang, X.; Guo, H. Enhancement of CO2 Capture on Biomass-Based Carbon from Black Locust by KOH Activation and Ammonia Modification. Energy Fuels 2016, 30, 4181. (35) Lee, J.-S.; Kim, J.-H.; Kim, J.-T.; Suh, J.-K.; Lee, J.-M.; Lee, C.-H. Adsorption Equilibria of CO2 on Zeolite 13X and Zeolite X/Activated Carbon Composite. J. Chem. Eng. Data 2002, 47, 1237−1242. (36) Zhang, Z.; Wang, K.; Atkinson, J. D.; Yan, X.; Li, X.; Rood, M. J.; Yan, Z. Sustainable and Hierarchical Porous Enteromorpha Prolifera Based Carbon for CO2 Capture. J. Hazard. Mater. 2012, 229−230, 183−191. (37) Wang, J.; Heerwig, A.; Lohe, M. R.; Oschatz, M.; Borchardt, L.; Kaskel, S. Fungi-Based Porous Carbons for CO2 Adsorption and Separation. J. Mater. Chem. 2012, 22, 13911−13913. (38) Liu, L.; Deng, Q.-F.; Hou, X.-X.; Yuan, Z.-Y. User-Friendly Synthesis of Nitrogen-Containing Polymer and Microporous Carbon Spheres for Efficient CO2 Capture. J. Mater. Chem. 2012, 22, 15540− 15548. (39) Luo, Y.; Li, B.; Wang, W.; Wu, K.; Tan, B. Hypercrosslinked Aromatic Heterocyclic Microporous Polymers: A New Class of Highly Selective CO2 Capturing Materials. Adv. Mater. 2012, 24, 5703−5707. (40) Raaijmakers, M. J. T.; Ogieglo, W.; Wiese, M.; Wessling, M.; Nijmeijer, A.; Benes, N. E. Sorption Behavior of Compressed CO2 and CH4 on Ultrathin Hybrid Poly(POSS-imide) Layers. ACS Appl. Mater. Interfaces 2015, 7, 26977−26988. (41) Saleh, M.; Lee, H. M.; Kemp, K. C.; Kim, K. S. Highly Stable CO2/N2 and CO2/CH4 Selectivity in Hyper-Cross-Linked Heterocyclic Porous Polymers. ACS Appl. Mater. Interfaces 2014, 6, 7325− 7333. (42) Lu, W.; Verdegaal, W. M.; Yu, J.; Balbuena, P. B.; Jeong, H.-K.; Zhou, H.-C. Building Multiple Adsorption Sites in Porous Polymer Networks for Carbon Capture Applications. Energy Environ. Sci. 2013, 6, 3559−3564. (43) Mota, J. P. B.; Rodrigues, A. E.; Saatdjian, E.; Tondeur, D. Dynamics of Natural Gas Adsorption Storage Systems Employing Activated Carbon. Carbon 1997, 35, 1259−1270. (44) Li, K.; Tian, S.; Jiang, J.; Wang, J.; Chen, X.; Yan, F. Pine Cone Shell-Based Activated Carbon Used for CO2 Adsorption. J. Mater. Chem. A 2016, 4, 5223−5234. (45) Chen, T.; Deng, S.; Wang, B.; Huang, J.; Wang, Y.; Yu, G. CO2 Adsorption on Crab Shell Derived Activated Carbons: Contribution of

(8) Hwang, C.-C.; Tour, J. J.; Kittrell, C.; Espinal, L.; Alemany, L. B.; Tour, J. M. Capturing Carbon Dioxide as a Polymer from Natural Gas. Nat. Commun. 2014, 5, 3961. (9) Huck, J. M.; Lin, L.-C.; Berger, A. H.; Shahrak, M. N.; Martin, R. L.; Bhown, A. S.; Haranczyk, M.; Reuter, K.; Smit, B. Evaluating Different Classes of Porous Materials for Carbon Capture. Energy Environ. Sci. 2014, 7, 4132−4146. (10) Monazam, E. R.; Shadle, L. J.; Miller, D. C.; Pennline, H. W.; Fauth, D. J.; Hoffman, J. S.; Gray, M. L. Equilibrium and Kinetics Analysis of Carbon Dioxide Capture Using Immobilized Amine on a Mesoporous Silica. AIChE J. 2013, 59, 923−935. (11) Millward, A. R.; Yaghi, O. M. Metal−Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998−17999. (12) Stavitski, E.; Pidko, E. A.; Couck, S.; Remy, T.; Hensen, E. J. M.; Weckhuysen, B. M.; Denayer, J.; Gascon, J.; Kapteijn, F. Complexity behind CO2 Capture on NH2-MIL-53(Al). Langmuir 2011, 27, 3970− 3976. (13) Fang, H.; Kamakoti, P.; Zang, J.; Cundy, S.; Paur, C.; Ravikovitch, P. I.; Sholl, D. S. Prediction of CO2 Adsorption Properties in Zeolites Using Force Fields Derived from Periodic Dispersion-Corrected DFT Calculations. J. Phys. Chem. C 2012, 116, 10692−10701. (14) Konduru, N.; Lindner, P.; Assaf-Anid, N. M. Curbing the Greenhouse Effect by Carbon Dioxide Adsorption with Zeolite 13X. AIChE J. 2007, 53, 3137−3143. (15) Presser, V.; McDonough, J.; Yeon, S.-H.; Gogotsi, Y. Effect of Pore Size on Carbon Dioxide Sorption by Carbide Derived Carbon. Energy Environ. Sci. 2011, 4, 3059−3066. (16) Qie, L.; Chen, W.-M.; Wang, Z.-H.; Shao, Q.-G.; Li, X.; Yuan, L.-X.; Hu, X.-L.; Zhang, W.-X.; Huang, Y.-H. Nitrogen-Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047− 2050. (17) Chandra, V.; Yu, S. U.; Kim, S. H.; Yoon, Y. S.; Kim, D. Y.; Kwon, A. H.; Meyyappan, M.; Kim, K. S. Highly Selective CO2 Capture on N-doped Carbon Produced by Chemical Activation of Polypyrrole Functionalized Graphene Sheets. Chem. Commun. 2012, 48, 735−737. (18) Xia, Y.; Mokaya, R.; Walker, G. S.; Zhu, Y. Superior CO2 Adsorption Capacity on N-doped, High-Surface-Area, Microporous Carbons Templated from Zeolite. Adv. Energy Mater. 2011, 1, 678− 683. (19) Wang, J.; Senkovska, I.; Oschatz, M.; Lohe, M. R.; Borchardt, L.; Heerwig, A.; Liu, Q.; Kaskel, S. Imine-Linked Polymer-Derived Nitrogen-Doped Microporous Carbons with Excellent CO2 Capture Properties. ACS Appl. Mater. Interfaces 2013, 5, 3160−3167. (20) Liebl, M. R.; Senker, J. Microporous Functionalized TriazineBased Polyimides with High CO2 Capture Capacity. Chem. Mater. 2013, 25, 970−980. (21) Zhao, H.; Jin, Z.; Su, H.; Zhang, J.; Yao, X.; Zhao, H.; Zhu, G. Target Synthesis of a Novel Porous Aromatic Framework and Its Highly Selective Separation of CO2/CH4. Chem. Commun. 2013, 49, 2780−2782. (22) Ben, T.; Li, Y.; Zhu, L.; Zhang, D.; Cao, D.; Xiang, Z.; Yao, X.; Qiu, S. Selective Adsorption of Carbon Dioxide by Carbonized Porous Aromatic Framework (PAF). Energy Environ. Sci. 2012, 5, 8370−8376. (23) Sun, L.-B.; Kang, Y.-H.; Shi, Y.-Q.; Jiang, Y.; Liu, X.-Q. Highly Selective Capture of the Greenhouse Gas CO2 in Polymers. ACS Sustainable Chem. Eng. 2015, 3, 3077−3085. (24) Myers, A. L.; Prausnitz, J. M. Thermodynamics of Mixed-Gas Adsorption. AIChE J. 1965, 11, 121−127. (25) Jiang, W.-J.; Yin, Y.; Liu, X.-Q.; Yin, X.-Q.; Shi, Y.-Q.; Sun, L.-B. Fabrication of Supported Cuprous Sites at Low Temperatures: An Efficient, Controllable Strategy Using Vapor-Induced Reduction. J. Am. Chem. Soc. 2013, 135, 8137−8140. (26) Yin, Y.; Tan, P.; Liu, X.-Q.; Zhu, J.; Sun, L.-B. Constructing a Confined Space in Silica Nanopores: An Ideal Platform for the 10924


Article

Industrial & Engineering Chemistry Research Micropores and Nitrogen-Containing Groups. RSC Adv. 2015, 5, 48323−48330. (46) Sevilla, M.; Valle-Vigón, P.; Fuertes, A. B. N-Doped PolypyrroleBased Porous Carbons for CO2 Capture. Adv. Funct. Mater. 2011, 21, 2781−2787. (47) Li, D.; Tian, Y.; Li, L.; Li, J.; Zhang, H. Production of Highly Microporous Carbons with Large CO2 Uptakes at Atmospheric Pressure by KOH Activation of Peanut Shell Char. J. Porous Mater. 2015, 22, 1581−1588. (48) Sevilla, M.; Fuertes, A. B. CO2 Adsorption by Activated Templated Carbons. J. Colloid Interface Sci. 2012, 366, 147−154. (49) Cai, J.; Qi, J.; Yang, C.; Zhao, X. Poly(vinylidene chloride)Based Carbon with Ultrahigh Microporosity and Outstanding Performance for CH4 and H2 Storage and CO2 Capture. ACS Appl. Mater. Interfaces 2014, 6, 3703−3711. (50) Xing, W.; Liu, C.; Zhou, Z.; Zhang, L.; Zhou, J.; Zhuo, S.; Yan, Z.; Gao, H.; Wang, G.; Qiao, S. Z. Superior CO2 Uptake of N-doped Activated Carbon through Hydrogen-Bonding Interaction. Energy Environ. Sci. 2012, 5, 7323−7327. (51) Sevilla, M.; Fuertes, A. B. Sustainable Porous Carbons with A Superior Performance for CO2 Capture. Energy Environ. Sci. 2011, 4, 1765−1771. (52) Wang, J.; Senkovska, I.; Oschatz, M.; Lohe, M. R.; Borchardt, L.; Heerwig, A.; Liu, Q.; Kaskel, S. Highly Porous Nitrogen-Doped Polyimine-Based Carbons with Adjustable Microstructures for CO2 Capture. J. Mater. Chem. A 2013, 1, 10951−10961. (53) Hu, X.; Radosz, M.; Cychosz, K. A.; Thommes, M. CO2-Filling Capacity and Selectivity of Carbon Nanopores: Synthesis, Texture, and Pore-Size Distribution from Quenched-Solid Density Functional Theory (QSDFT). Environ. Sci. Technol. 2011, 45, 7068−7074.

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