Hierarchically Structured Graphene Coupled Microporous Organic

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Hierarchically structured graphene coupled microporous organic polymers for superior CO2 capture Fa-Qian Liu, Li-Li Wang, Guo-Hua Li, Wei Li, and Chao-Qin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11492 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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ACS Applied Materials & Interfaces

Hierarchically structured graphene coupled microporous organic polymers for superior CO2 capture Fa-Qian Liu,* a Li-Li Wang, b Guo-Hua Li, a Wei Li,a and Chao-Qin Li a a

Engineering Research Center of High Performance Polymer and Molding Technology, Ministry

of Education, Qingdao University of Science and Technology, Qingdao 266042, China. E-mail: [email protected] b

College of Automation and Electronic Engineering, Qingdao University of Science and Technology, Qingdao 266042, China.

KEYWORDS:

CO2 capture, microporous organic polymers, 3-D graphene, hierarchical

structure, Schiff-base chemistry

ABSTRACT: Hierarchically porous materials containing interconnected macro-/meso/micropores are promising for energy storage, catalysis, and gas separation. Here, we present an effective approach for synthesizing three-dimensional (3-D) sulfonated graphene coupled microporous organic polymers (SG-MOPs). The resulting SG-MOPs possess uniform macropores with an average size of ca. 350 nm, abundant mesopores, and micropores with an

ACS Paragon Plus Environment

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average size of ca. 0.6 nm. The SG supported adsorbents exhibit high nitrogen content (more than 38.1 wt %), high adsorption capacity (up to 3.37 mmol CO2 g-1), high CO2/N2 selectivity from 42 to 51, moderate heat of adsorption, as well as good stability because of the hierarchical porous structure and excellent thermal conductivity of SG scaffold. Thus, these nitrogenenriched adsorbents allow the overall CO2 capture process to be promising and sustainable.

Introduction As the steady increase of carbon dioxide (CO2) concentration in the atmosphere, postcombustion carbon capture technologies have aroused people’s great concern during the past decade.1 The proven aqueous alkanolamine, which represents state-of-the-art for CO2 capture, is utilized for large-scale separation of CO2 from flue gas streams.2 However, the high energy penalty necessary for the regeneration, high net consumption of solvent and severe corrosion problems caused by degradation of the amine, are the major bottlenecks for its applications. materials, such as zeolites,

5

porous carbons,

6

MOFs,

7

and COFs,

8

3,4

Porous

have been reported as

promising candidates for applications in CO2 capture. However, the nexus of minimal energy penalty and high-speed capture with high selectivity still makes carbon capture one of true grand challenges for today’s scientists. Designing high-performance CO2 adsorbents requires a number of design criteria that take both thermodynamical and kinetic factors into account. Thermodynamically favorable adsorbent interacts with CO2 molecules moderately to achieve strong but still reversible adsorptiondesorption. In this regards, physical binding through polarizing forces requires lower energy to release the gas than chemisorption, such as carbon, zeolites, COFs/MOFs/ZIFs. However, in


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spite of their high porosities, they can’t afford strong adsorbate-adsorbent interactions to hold CO2 molecules at ambient conditions. As a result, the CO2 adsorption capacity drops rapidly with the increase of temperature.9 As for the most widely used amino-containing chemisorption technique, which distorts symmetric and linear CO2 molecules to the capture medium through strong C–N covalent bonds, requires extensive energy input in desorption and compression process.

10

Kinetically favorable adsorbent requires rapid and reversible capture and release of

CO2, which rely on hierarchical structures at three discrete length scales. The first scale corresponds to the structure of micropores which dominate low-pressure adsorption and offer high CO2/N2 selectivity. The second scale involves the structure of mesopores, which provides paths to the micropores and deeply embedded adsorption sites. The third macroporous scale in the range of hundreds of nanometers, is responsible for the efficient mass transport (CO2 flow rate). Overall, both thermodynamically and kinetically favorable adsorbent for CO2 capture should possess a moderately strong affinity to CO2, a large amount of micropores, preferably tailored hierarchical structures, and long-term stability to provide reversible and fast adsorption and desorption kinetics. Recently, microporous organic polymers (MOPs) formed by organic polymerization method have been prepared as potential candidates for CO2 capture because of their high specific surface area, large pore volume, tunable pore sizes, and prominent physical properties.

8, 11-16

Thermodynamically, MOPs are favorable because of the weak physical binding nature to CO2. As an improvement technique, nitrogen-enriched MOPs were developed for their enhanced affinity of nitrogen atoms to CO2.17 For this method, an energy penalty must be paid back during the desorption process, similar to that observed in aqueous amine solutions. Nevertheless, nitrogen-enriched adsorbents could offer significant benefits over conventional MOPs if the


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adsorbents exhibit sufficiently stability upon cycling. Furthermore, the large exothermic heat of absorption will cause the reduction of adsorption capacity. This problem could be solved if we couple the MOPs with excellent thermal conductor, such as graphene, which can efficiently transfer the heat out of the adsorbent matrix and finally prevent the rise of temperature. Kinetically, since the kinetic diameters of CO2 (0.330 nm) and N2 (0.364 nm) are very close, micropores less than 1 nm are desirable because of their high kinetic selectivity for the subatmospheric CO2 adsorption at ambient conditions.1 So, hierarchically structured materials possessing micro-, meso- and macropores simultaneously can be ideal supports to disperse MOPs at different length scales of pores and shorten the CO2 diffusion paths, finally improve the accessibility of inner amine groups. As new porous carbon materials, three-dimensional graphene materials can be used as ideal templates for building up hierarchically porous architectures since they possess 3-D interconnected macroporous structures, mesoporous graphene sheets, unique mechanical characteristics, excellent thermal conductance, and large surface area-to-volume ratios.10, 18-19 As such, three-dimensional graphene coupled microporous organic polymers possessing micropores originating from MOPs, meso- and macropores originating from 3-D graphene represent one strategy for achieving the maximum utilization of adsorption sites at different length scales and can be an ideal candidate for CO2 capture. In continuation with our effort to develop high-performance adsorbents for CO2 capture,19-21 herein, we report the synthesis of hybrid nanocomposites of three-dimensional sulfonated graphene (3-D SG) and nitrogen-enriched MOPs, where the MOPs are synthesized through Schiff-base chemistry using commercially available and inexpensive monomers, namely, isophthalaldehyde (IA), and melamine (MA). Ascribed to the hierarchically porous architectures,


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high nitrogen content, excellent thermal conductor of SG, three-dimensional sulfonated graphene coupled microporous organic polymers (SG-MOPs) exhibit high capture capacity, fast CO2 adsorption-desorption, as well as excellent good stability over multiple adsorption-desorption cycles. Experimental Reagents and materials All purchased chemicals were reagent grade at least and were used without further purification. Sulfonated graphene with thickness of 1.0-1.7 nm (3-5 layers) was bought from Suzhou Graphene-Tech. Monodispersed sulfonated polystyrene latex (PS, 5 % solid latex, diameter 500 ± 5 nm) was obtained from Nanjing Janus New-materials. Isophthalaldehyde and melamine were purchased from Aladdin Reagent. Synthesis of 3-D macroporous sulfonated graphene (3-D SG) Typically, 20 mg sulfonated graphene and 3 mL DI water were added to a 10 mL vial. Then 1 mL of sulfonated PS suspension (500 ± 5 nm) was dropped into the vial, followed by a continuously sonication for 1 h. The resulting mixture was dropped onto a glass slide. After drying at R.T. overnight, the PS templates were removed by calcination at 450 °C for 2 h in a nitrogen atmosphere with a heating rate of 1 °C/min. Synthesis of 3-D sulfonated graphene supported microporous organic polymers (SGMOPs)


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The MOPs were synthesized via the Schiff-base reaction on the 3-D SG surfaces according to the previous report 22 with minor modification. Typically, SG (1 equiv. by weight) was immersed in dry DMSO in a 250 mL three-neck round-bottom flask, followed by the addition of isophthalaldehyde and melamine (19, 9, and 5.67 equiv. by weight for SG-MOP-5, SG-MOP-10, SG-MOP-15 respectively; the molar ratio of melamine to isophthalaldehyde is 2:1). After being purged by dry N2 bubbling, the mixture was gently stirred for 24 h at room temperature. Then the mixture was heated to 180 °C and kept at N2 atmosphere for 72 h. Next the resultant mixture was cooled to room temperature. The resulting precipitates were collected by filtration and washed with copious acetone, THF, and CH2Cl2 to remove non-reacted reactants and the other byproducts not bound to the composites. Finally, the resultant composites were dried in vacuum to remove the remaining solvent. Instruments and characterization The SEM images were obtained from a field-emission scanning electron microscope (SEM, JSM-7500F). TEM images were obtained from A JEOL 100CX transmission electron microscope (TEM). The nitrogen content of the SG-supported adsorbents and MOPs were measured from Flash EA1112 (Thermo Finnigan Inc. Italy). Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27 spectrometer using the KBr pellet method. X-ray diffraction (XRD) patterns were obtained on a Scintag diffractometer. The solid-state 13C crosspolarization magic angle spinning (CP-MAS) NMR spectrum was obtained on a Bruker DSX300 instrument. Argon adsorption was measured by a Quantachrome Autosorb-1 analyzer operating at 87 K. The specific surface area was calculated based on the Brunauer-Emmett-Teller (BET) model in the relative pressure (P/P0) range of 0.05-0.3, and the pore volume was determined at the relative pressure (P/P0) of 0.99. Pore size distribution was calculated by the non-local density


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functional theory (NLDFT) method assuming slit-shaped pores. Prior to the measurement, all the samples were degassed at 383 K for 12 h. X-ray photoelectron spectroscopy (XPS) spectra were collected on an ESCALAB MK II instrument. CO2 adsorption measurements. The CO2 and N2 adsorption isotherms at different temperatures were obtained using a static volumetric analyzer Micromeritics ASAP 2020. Before the experiment, all samples were pretreated in vacuum at 383 K for 12 h to remove the CO2 and moisture adsorbed from the air. The gas purity for CO2 was 99.9995%. The sample loading was about 250 mg. The CO2 regeneration experiment was performed using a TGA/DSC 2 thermogravimetric analyzer under anhydrous conditions. First, about 20 mg of the as-synthesized adsorbent was pre-treated in vacuum at 383 K for 12 h. Then, the sample was cooled to the expected temperature until the weight change was lower than 0.002 mg min-1. Finally, the sample was exposed to a mixture flow containing 15% CO2 and 85% N2 for 60 min with a feed flow of 40 mL/min. To regenerate the adsorbent, the feed gas stream was switched to a He flow of 30 mL/min for 60 min. Results and discussion The overall schematic depiction for the synthetic procedures of three-dimensional (3-D) sulfonated graphene coupled microporous organic polymers (SG-MOPs) is shown in Scheme 1. Self-supported 3-D SG scaffold was prepared by a facile one-pot procedure using negatively charged sulfonated graphene (SG, thickness = 1.0-1.7 nm, 3-5 layers, zeta potential = -17.1 ± 3.5 mV) mixed with negatively charged sulfonated PS beads suspension (zeta potential = -18.2 ± 5.7


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mV) at room temperature. The electrostatic expulsion allows the very thin SG sheets to diffuse and fill up the interstitials among the PS beads.23 After dry overnight and calcination at 450℃ under N2 atmosphere, a 3-D interconnected macroporous nanoarchitecture with good mechanical stability was formed. 23-24 Then, MOPs were tethered onto the inner and outer surfaces of the 3-D SG scaffold surfaces by Schiff-base chemistry. It was reported that the specific surface area can be improved by decreasing the monomer linkage length for melamine based conjugated porous polymers.

22, 25

So, to get a high specific surface area, high nitrogen content and abundant

micropores less than 1 nm, isophthalaldehyde was selected as starting materials for the synthesis of melamine-based MOPs. The as-prepared adsorbents were denoted as SG-MOP-X, where X denotes the weight content of SG.


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Scheme 1. Schematic design for fabricating 3-D sulfonated graphene-microporous organic polymers (SG–MOPs). Figure 1a shows the SEM image of the 3-D porous SG scaffold prepared from 500 nm PS spheres as sacrificial template. The mono-dispersed and disordered macropores with an average inner diameter size of ca. 350 nm in the top layer are apparently origin from the decomposion of


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PS during calcination. In addition, the 3-D SG scaffold also shows slightly crinkly and flexible features,20 contrary to rigid inorganic nanostructures, such as metal oxides.26 The macropore size can be tuned easily by varying the template size. The cross-sectional SEM image (Figure 1b) shows the 3-D interconnected nature and confirms the omnipresent uniform macroporous structure in the matrix, which would be preferred for shuttling the gases and improving the diffusion rates.26 The TEM image (Figure 1c) further confirmed the 3-D macroporous structure with a bright area at the center of each sphere and a dark contrast at the sphere wall. 3-D SGsupported MOP adsorbents for CO2 capture were prepared by coating the support with melamine-based microporous organic polymers by Schiff-base chemistry according to the previous report.22 Fortunately, the 3-D interconnected macroporous scaffold was inherited almost unchanged after the coating of MOPs (Figure 1d), although the surfaces became rather rough, demonstrating the successful Schiff-base polymerization of MOPs. EDS mapping images, shown in Figure 1e and Figure 1f, gave the uniform distribution of nitrogen and carbon elements in the SG-MOP nanocomposite, further verified that MOPs were homogeneously grown on the surfaces of 3-D SG support. All the XRD patterns exhibited two wide diffraction peaks at the Bragg angle of approximately 22︒ (strong) and 44︒ (weak), revealing that the coated microporous organic polymers are amorphous (Figure S1).


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Figure 1. Morphologies of typical 3-D SG scaffold and SG-MOPs. (a) Top-down SEM image of the macroporous 3-D SG scaffold prepared from 500 nm sulfonated PS beads. (b) Crosssectional SEM image showing that the 3-D interconnected porous morphology is omnipresent. (c) TEM image of SG before coating of MOPs. (d) TEM image of SG-MOP-10 showing the structure after the coating of MOPs. (e) EDS nitrogen mapping of SG-MOP-10 and (f) EDS carbon mapping of SG-MOP-10 nanocomposites. The successful polymerization of MOPs on the SG surfaces was further confirmed by Fourier transform infrared (FT-IR) spectroscopy, 13C cross-polarization magic angle spinning (CP-MAS) NMR spectrum and X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, The bands that can be assigned to the NH2 stretching at 3470 and 3419 cm-1 and NH2 deformation at 1652cm -1 of melamine as well as to the C–H stretching at 2870 cm-1 and C=O stretching at 1690


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cm-1 of the aldehyde groups of isophthalaldehyde are absent in the spectra of SG-MOPs.

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25

However, the signals corresponding to the quadrant and semicircle stretching of the triazine ring at 1563 and 1491 cm

-1

are present in the spectra of SG-MOPs, indicating the successful

dehydration condensation reaction of MOPs onto the surfaces of 3-D SG scaffold.22 Also, we found no characteristic bands attributed to C=N stretching vibration of azomethine at 1650-1630 cm-1, suggesting that aromatic imine groups (Ar-C=N-) does not exist on the surfaces of SGMOPs.22 the absence of imine groups was also verified by the appearance of characteristic stretching of free N-H at 3393 cm-1 as well as the C-N stretching of secondary amine (-NH-) at 1150 cm-1. The 13C cross-polarization magic angle spinning (CP-MAS) NMR spectrum (Figure 2b) further confirmed the absence of imine bonds on the surface of SG-MOPs with no C=N resonance at 160 ppm.

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The strong resonances at 166.9 ppm can be assigned to the aromatic

carbons of the triazine ring. The signal at 120-140 ppm corresponds to the CH aromatic carbons of the benzene, whereas the 40-70 ppm region can be correlated with the carbon atoms present in methoxy functions and carbon atoms in aminal-linked units.28 The results are in agreement with FT-IR analyses.


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Figure 2. Surface characteristics of the SG–MOP nanocomposites. (a) FT-IR spectra of melamine, isophthalaldehyde, and SG-MOP-10. (b) Solid state 13C MAS NMR of SG–MOP-10. (c) Full survey XPS spectra of SG and SG-MOP-10. (d) Ar adsorption/desorption isotherms of SG-MOP-5. The pore size distribution plot (inset) was calculated by non-localized density functional theory (NLDFT). XPS data can nicely support the FT-IR data and NMR spectrum and reveal the surface characteristics of SG before and after the polymerization. The full survey XPS analysis of SG shows four peaks at 168.5, 285.9, 398.9 and 532.3 eV corresponding to the existence of S, C, N and O elements, respectively (Figure 2c). 22 After the Schiff-base reaction, the signals of S2p and


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O1s spectra of SG-MOPs decrease greatly due to the coverage of MOPs on the sulfonated graphene surfaces. The successful reaction was further verified by the high resolutions XPS spectra. The high-resolution C1s peak of SG-MOP-10 can be fitted into three peaks at 284.6, 285.6 and 287.5 eV, which are attributed to the sp2 carbon in benzene ring (-C=C-), sp3 carbon atoms bonded to the more electronegative N atoms (-N-C-N-), and sp2 carbon in triazine (-C=N-), respectively (Figure S2). Also, the high-resolution N1s peak of SG-MOP-10 demonstrates three overlapped peaks at 398.1, 398.9, and 399.8 eV corresponding to nitrogen in triazine ring (-C=N), nitrogen in -NH2 group and –NH- group, respectively (Figure S3). The chemical nature of SGMOPs was further examined by elemental analysis (Table S1). The nitrogen content gradually increases from 25.7 to 38.1 wt % with the increase of isophthalaldehyde and melamine content. These nitrogen-enriched adsorbents, which serve CO2 molecules as ‘docking sites’ through dipole-quadrupole interactions, making them significantly advantageous for CO2 capture and separation. The permanent porosity of the as-synthesized SG scaffold before and after the coating of MOPs was studied by argon physisorption measurements. The argon adsorption/desorption isotherms and the pore-size distribution (PSD, inset) of SG-MOP-5, SG, SG-MOP-10, and SGMOP-15 are show in Figure 2d, and Figure S4~S6. The porosity parameters are summarized in Table 1. The surface areas were calculated to be 807, 765, and 693 m2 g-1 using the BrunauerEmmett-Teller (BET) model for SG-MOP-5, SG-MOP-10, and SG-MOP-15, respectively. Figure S4 shows a fundamental type II isotherm, and exhibits a H3-type hysteresis loop around 1.0 P/P0, indicating the existence of abundant macropores in the SG scaffold.26 After the polymerization of MOPs, the argon adsorption isotherms demonstrate a combination of type I and type IV profile, indicating the presence of hierarchically porous structures (Figures 2d and


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Figure S5-S6). The abundance of permanent micropores is verified by the rapid gas uptake at low pressures (0-0.1 atm). The PSD curves calculated by non-localized density functional theory (NLDFT) further confirm the presence of well-developed micropores at ca. 0.6 nm arising from the melamine-based MOPs. (Figure 2d and Figure S5-S6). Also, we observed the existence of mesopores from the wide hysteresis loops at relative pressures above 0.3. The PSD curves show that the nanocomposites are rich in mesopores larger than 3 nm. The existence of macropores after the polymerization is verified by the upward trend at high pressures (P/P0 > 0.95) as well as the TEM image described before (Figure 1d). Table 1 The total specific surface areas and pore volumes

Sample

SBET

Vpore

Vpore

Vmicro

Dpore

[m2g-1]

[cm3g-1]a

[cm3g-1]b

(cm3g-1)c

(nm)d

SG

221

0.76

0.53

0.06

3.7

SG-MOP-5

807

2.8

1.94

0.23

0.60

SG-MOP-10

765

2.1

1.71

0.21

0.58

SG-MOP-15

693

1.6

1.42

0.18

0.58

a

Total pore volume obtained at p/p0 = 0.99.

b

Pore volume calculated from NLDFT.

c

Micropore volume calculated from NLDFT.

d

Pore width calculated from NLDFT.


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The CO2 capture properties of 3-D HG based adsorbents were evaluated by measuring adsorption isotherms using the volumetric method （Figure S7）. The CO2 adsorption amounts at 273, 298, 323, and 348K versus nitrogen contents of SG-MOPs are shown in Figure 3a and summarized in Table 2. The adsorption capacity of CO2 increases with the increase of the MOPs content (the increase of amine loading). The maximum adsorption amounts for SG-MOP-5, SGMOP-10 and SG-MOP-15 at 273K and 1 atm are 3.37, 3.02, and 2.86 mmol CO2 g-1, respectively (Figure 3a), which are among the highest level of MOP based adsorbents. i.e., 2.56 mmol g-1 for melamine-based microporous organic polymers. 11As shown in Figure 3a, the 3-D SG based adsorbents demonstrated a CO2 capture capacity higher than that of pristine MOPs in the temperature range of 273~348 K, indicating that the HG scaffold not only increased the BET surface areas but also expanded the molecular diffusion/transportation channels into the inner micropores, and finally enhanced the capture capacity. 29 As temperature increases from 273K to 348K, the CO2 adsorption amount decreases as expected as a result of exothermic adsorption reaction (van't Hoff behavior), which favours low adsorption temperature. Thanks to the 3-D interconnected macropores and omnipresent mesopores, the adsorption was strongly dominated by the thermodynamic factors rather than kinetic diffusion.20 We also noticed that the capture capacity decrease rate of SG-based adsorbents is lower than that of pristine MOPs. When the temperature increased from 273 K to 348K, SG-MOP-15 retained 38% (1.10 mmol g-1) of the initial capacity, whereas pristine MOPs preserved only 26% (0.71 mmol g-1). The improved capture properties at elevated temperatures can be attributed to the excellent thermal conductivity of the 3-D SG scaffold, which can efficiently transfer the heat out of the adsorbent matrix and finally prevent the rise of temperature.20 The N2 adsorption isotherms of SG-MOPs and MOP at


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273 K are shown in Figure S8. Compared with the adsorption of CO2, the adsorption amounts of N2 on SG-MOPs are fairly low, indicating the preferential adsorption of CO2 over N2. To elucidate the nature of adsorbent-adsorbate interactions, the isosteric heats of adsorption (Qiso) were calculated from the CO2 adsorption isotherms measured at 273 and 298 K (Figure S9). The adsorption data were modeled with virial-type expression.

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The Qiso values at zero

coverage were listed in Table 2. The Qiso values of SG-MOPs are all above 38 kJ/mol, higher than that of pristine MOPs (32.09), supporting that the 3-D SG scaffold provides more paths to the micropores and deeply embedded adsorption sites. The high adsorption enthalpies also mean that the interaction between SG-MOPs and CO2 molecules, to some extent, are chemical. 31 The existence of chemical absorption mechanism was also supported by the CO2 adsorption isotherms. As shown in Figure S7, The amine functionalized groups dominated the fast surface chemisorption at low pressures. Then, a relatively slow uptake controlled by the physisorption between CO2 molecules and the unfunctionalized micro- and mesopores accumulates with the increase of the pressure.31 The Qiso values decreases gradually with the increase of adsorbed CO2, indicating that the CO2 adsorption on the surfaces is energetically heterogeneous. At zero coverage, the Qiso values of SG-MOPs are higher than the values reported for melamine-based MOPs (24.1-32.2 kJ mol−1) 11 and amine functioned COFs (35.0 kJ mol−1). We also noticed that the Qiso of SG-MOP-5 was higher than other SG-MOPs or MOP, partly because it’s high surface area and pore volume, which allow efficient interactions of CO2 to the deeply embedded adsorption sites. However, the values are still fall in the preferred range for flue gas separation (30-50 KJ·mol-1), 32 which allows strong but still reversible CO2 adsorption in the pressure swing adsorption (PSA) processing. Table 2. CO2 adsorption characteristics of SG-MOPs


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Sample

N[wt%]

Adsorption enthalpies

Amount adsorbed [mmol CO2 g-1]

a

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CO2/ N2 b

(kJ mol-1) a

273K

298K

323K

348K

SG-MOP-5

38.1

3.37

2.42

1.72

1.15

39.94

49

SG-MOP-10

31.3

3.02

2.16

1.55

1.09

39.11

51

SG-MOP-15

25.7

2.86

2.02

1.46

1.10

38.33

42

MOP

49.1

2.73

1.92

1.15

0.71

32.09

38

The Qiso values were calculated from the CO2 adsorption isotherms measured at 273 and 298

K (at zero coverage).

b

estimated by Ideal Adsorption Solution Theory (IAST) at 273 K and 1

atm.


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Figure 3. CO2 capture performances of as-prepared adsorbents. (a) CO2 capture capacities at 273, 298, 323, and 348K versus N contents of the as-prepared adsorbents. (b) The CO2/N2 selectivity of as-prepared adsorbents estimated by ISAT at 273 K. (c) Pressure swing cyclic stability of SG-MOP-5 and SG-MOP-10. (d) The CO2 adsorption/desorption cyclic stability of SG-MOP adsorbents and MOP. Experimental conditions for Figure 3c and 3d: Adsorption in simulated flue gas (15% CO2 balanced with N2) at 273K in a flow of 40 mL/min for 60 min and desorption at 273K in a He flow of 30 mL/min for 60 min. The CO2/N2 selectivity of these as-prepared adsorbents at 273 K and 1 atm was estimated by Ideal Adsorption Solution Theory (IAST) from the experimental single-component isotherms, which has been reported to predict binary adsorption in many porous materials accurately.33 As shown in Figure 3b and Table 2, SG supported adsorbents exhibit high CO2/N2 selectivity from 42 to 51 under simulated flue gas (15% CO2 and 85% N2). It was reported that the CO2 selectivity increases with the CO2 molar fraction.11 So it is expected that higher selectivity value could be obtained with the increase of CO2 molar fraction in CO2-N2 mixture. Thus, these materials are promising for industrially applicable carbon capture applications. Full regeneration of solid adsorbents by pressure swing adsorption (PSA) is preferred in industrial settings due to the low energy consumption. Based on the fast mass transfer and moderate adsorbate-adsorbent interaction (38.33-39.94 kJ mol−1), SG-MOPs allows for fast and strong but still reversible CO2 adsorption. Furthermore, the desorption can be achieved by only purging with a He flow rather than simultaneously vacuum the system or raising the temperature. Therefore, pressure-driven regeneration mode was selected to assess the reversible adsorption behaviour using simulated flue gas (15% CO2).19 The recyclability was performed at 273 K. The samples were first exposed to the 15% CO2 for 60 min to get an equilibrium adsorption. Then


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these adsorbents were fully regenerated in a He flow for 60 min. As shown in Figure 3c, After 30 cycles, no obvious capacity loss was observed for SG-MOP-5 and SG-MOP-10, indicative of constant adsorption capacities upon repeated regeneration. The CO2 adsorption/desorption profile of SG-MOP-5 was shown in Figure S10. The excellent cyclic performance of SG-MOP-5 and SG-MOP-10 can be assigned to the hierarchical structure and high thermal conductivity of sulfonated graphene. First, the 3-D interconnected hierarchical structure simultaneously allows fast mass transfer of gases into the densely packed adsorbents through mesopores and macropores, and eases the interactions of CO2 molecules with the amino groups in micropores. Second, the 3-D interconnected SG scaffold with excellent thermal conductivity is beneficial for the heat transfer in the adsorbent matrix. As verified in Figure 3d, Fast capacity loss was observed for conventional MOP adsorbent after 10 cycles, while the capture capacities of SGbased adsorbents were very stable.20 Conclusions In conclusion, a 3-D interconnected hierarchical SG-MOP nanocomposite, in which melamine-based microporous organic polymers (MOPs) were uniformly coated onto the surfaces of 3-D macroporous sulfonated graphene (SG) scaffold, has been prepared via the Schiff-base reaction of melamine and isophthalaldehyde. The resultant SG-MOPs exhibit large specific surface areas up to 807 m2 g-1, high nitrogen contents up to 38.1 wt %, high CO2/N2 selectivity, moderate heat of adsorption, and good stability in the PSA process. The hierarchical structure significantly increases the CO2 capture capacity by increasing the BET surface areas and expanding the molecular diffusion/transportation channels into the inner micropores. The SG scaffold efficiently transfers the heat out of the adsorbent matrix and guarantees good recyclability. These merits of SG-MOPs make them promising adsorbents in the application of


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CO2 capture. Furthermore, considering their uniform porous nature and high nitrogen content, we predict that SG-MOPs can also serve as precursors for yielding nitrogen-enriched porous carbon nanosheets,25 which can find their potential application in lithium batteries, supercapacitors, catalysts, and gas storage/separation and sensing. AUTHOR INFORMATION Corresponding Author * Corresponding Author, Email: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT FL acknowledges the support from the National Natural Science Foundation of China (Nos. 51773106 and 21371105) and the Scientific Development Plan of Qingdao (14-2-4-41-jch). Supporting Information Available: XRD patterns, High-resolution XPS spectra, Elemental analysis results, Argon adsorption/desorption isotherms, CO2 adsorption isotherms,

N2

adsorption

isotherms,

Isosteric

heats

of

adsorption,

and

CO2

adsorption/desorption profile. This material is available free of charge via the Internet at http://pubs.acs.org.

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Entry for the Table of Contents

FULL PAPER Fa-Qian Liu,* Li-Li Wang, Guo-Hua Li, Wei Li, and Chao-Qin Li Page No. – Page No. Hierarchically structured graphene coupled microporous organic polymers for superior CO2 capture

Hierarchically structured graphene coupled microporous organic polymers prepared by Schiffbase chemistry exhibits superior CO2 capture behavior.


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Hierarchically Structured Graphene Coupled Microporous Organic

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