Micromesoporous Nitrogen-Doped Carbon Materials Derived from

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Micromesoporous Nitrogen-Doped Carbon Materials Derived from Direct Carbonization of Metal−Organic Complexes for Efficient CO2 Adsorption and Separation Ani Wang, Xinxin Pi, Ruiqing Fan,* Sue Hao, and Yulin Yang*

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/02/19. For personal use only.

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China S Supporting Information *

ABSTRACT: Metal−organic complexes (MOCs) are considered as excellent precursors to prepare carbon materials, due to the fact that heteroatoms and functional groups can be naturally reserved in the resulting carbon materials through the carbonization. Herein, micromesoporous nitrogen-doped carbons MPNC-1 and MPNC-2 are successfully obtained by direct carbonization (800 °C, KOH activation) of metal− organic complexes DQA-1 and DQA-2. MPNC-1 and MPNC-2 exhibit high BET surface area (2368.9 and 2327.6 m2 g−1), pore volume (1.95 and 1.89 cm3 g−1), and N contents (17.2% and 12.3%). At 25 °C and 1 bar, MPNC-1 and MPNC-2 show high CO2 adsorption of 7.53 and 6.58 mmol g−1, the estimated CO2/N2 selectivity are 20.5 and 22.6, indicating excellent promise for practical CO2 adsorption and separation applications. Theoretical calculation indicates carbon surfaces with pyridinic-N, pyrrolic-N, and graphitic-N coexistence could strongly change the local electronic distribution and electrostatic surface potential, enhancing the CO2 adsorption with adsorption energy of −58.96 kJ mol g−1. Theoretical calculation also highlights that CO2 adsorption mechanism is electrostatic interaction with a large green isosurface between CO2 molecules and the carbon surface.

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INTRODUCTION CO2 capture is of crucial importance for human beings to minimize anthropogenic CO2 emissions and mitigate global warming.1−3 Porous carbons materials have caught sustained attention of researchers for their potential applications in CO2 adsorption and separation, catalysis, and energy storage.4−6 For CO2 adsorption and separation, porous carbons materials possess a number of advantages, such as high surface area, pore dimensions, flexibility for surface functionalization, and tunability of pore geometries, as well as chemical stability.7−11 However, the main disadvantage of porous carbons materials are their low CO2/N2 selectivity, which will limited their practical applications at scale. Preparation of the porous carbons can be realized through the following several methods, for instance, nanocasting with hard-templates, pyrolysis followed by chemical or physical activation, and carbonization of metal−organic complexes/ organic matter/polymeric aerogels.12−16 In particular, direct carbonization of metal−organic complexes is an inspiring strategy to obtain porous carbons materials,17−21 excellent structure or partial functional groups of their parent metal− organic complexes precursor can be inherited to the final carbon materials.22−24 In recent years, although a few of the research groups have reported the preparation of N-doped porous carbons through carbonizing the N-containing metal− © XXXX American Chemical Society

organic complexes, most of them focused on the novelty of the variable morphologies, structures, and pore volume for these N-doped porous carbons, the exact assessment of functions for different types of N group on CO2 adsorption are still ambiguous.25,26 Herein, two micromesoporous nitrogen-doped carbons (MPNC-1 and MPNC-2) were prepared by carbonization of DQA-1 and DQA-2 (Scheme 1). The obtained carbon materials MPNC-1 and MPNC-2 show high BET surface area (2368.9 and 2327.6 m2 g−1), pore volume (1.95 and 1.89 cm3 g−1), and N contents (17.2% and 12.3%); therefore, the MPNC-1 and MPNC-2 show a significant amount of CO2 adsorption, 7.53 and 6.58 mmol g−1. After 5 cycles, the value of the CO2 adsorption capacity still can reach as high as 7.15 mmol g−1, occupying approximately 95% of the original adsorption capacity. Theoretical calculation indicates the pyridinic-N, pyrrolic-N, and graphitic-N codoping into carbon layers structure are more beneficial to change internal electronic structure (for instance, the local electronic density, electrostatic surface potential) for enhancing the CO 2 adsorption. Received: February 20, 2019

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DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. MPNCs Prepared by Carbonization of two MOCs and CO2 Adsorption Mechanism

Figure 1. Structure of DQA-1: (a) Asymmetric unit of DQA-1. (b) Dimer of DQA-1 and (c) 1D chain with the seven coordination pentagonal dipyramidal configuration of DQA-1. (d) 3D supramolecular structure with the micropores structure of DQA-1. (e) 3D bnn topology of DQA-1.

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RESULTS AND DISCUSSION

used as the metal source, micron-sized DQA-2 crystals were obtained and the particle size was decreased to around 10−30 μm. The CCDC reference numbers are 1890129 and 1890130, respectively. Structural Description of Metal−Organic Complexes DQA-1. DQA-1, including one Cd2+ ion, one 2,6-diisopropylN-{(quinoline-2-yl)methylene}aniline ligand, an coordinated acetonitrile molecule and two nitrate ions (Figure 1a).27,28 Each unit of DQA-1 is connected through C5−H15A···O2 hydrogen bonding interactions to construct dimer (Figure 1b)

In this work, two metal−organic complexes (named as DQA-1 and DQA-2) were synthesized. Under the self-assembly method, 2-quinolinecarboxaldehye, 2,6-diethylaniline, and metal source in a 1:1:1 in acetonitrile under reflux at 80 °C for 3 h. After reaction, bulk-like DQA-1 and strip-like DQA-2 crystals were formatted. The obtained crystals were collected and washed carefully. When Cd(NO3)2 was used as a metal source, millimeter-sized DQA-1 crystal particles were obtained, with an average size of around 0.5−3 mm. When CdCl2 was B

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Structure of DQA-1: (a) Asymmetric unit of DQA-2. (b) Seven coordination pentagonal dipyramidal [CdN3O4] configuration of DQA-2. (c) Different types of hydrogen bonding interactions in DQA-2. (d) 1D chain. (e) 3D network of DQA-2. (f) 2D sql topology of DQA-2.

and C14−H14A···O6 to format a 1D chain (Figure 1c).29−32 Central Cd2+ metal cation show distorted seven coordination pentagonal dipyramidal with [CdN3O4] configuration (inset of Figure 1c). Between the adjacent 1D chains, exiting the C7− H7A···O3 (2.618 Å), C10−H10A···O5 (2.577 Å), C14− H14A···O6 (2.632 Å), and C23−H23A···O1 (2.618 Å), therefore, constructed a 3D supramolecular structure (Figure 1d).33−35 To simplify, regarding all of the hydrogen bonding interactions (C−H···O) as linkers and each asymmetric unit of DQA-1 as node, DQA-1 was simplified a 3D bnn topology {46· 64} (Figure 1e).41−45 The overall solvent-accessible volume (338.9 Å3) possesses 11.7% percent of the whole cell volume (2906.8 Å3).34,36−40 N2 adsorption−desorption isotherms was studied to characterized the porosity characteristic of DQA-1. As shown in the Figure S1, type I isotherm adsorption of the DQA-1 present the microporous structure of the 3D supramolecular structure. The BET surface area and N2 adsorption capacity of DQA-1 are ca. 512.3 m2 g−1 and 85.6 cm3 g−1, respectively. Structural Description of Metal−Organic Complexes DQA-2. In each asymmetric unit of DQA-2, one Cd2+ ion, one 2,6-diisopropyl-N-{(quinoline-2-yl)methylene}aniline ligand, a coordinated acetonitrile molecule, and two chloridion ions (Figure 2a) were included. The central metal Cd is connected with three N atom and two Cl atom [CdN3Cl2] presenting

trigonal bipyramidal coordination geometry (Figure 2b). DQA-2 are connected into 1D chain and then 2D layer through hydrogen bonding interactions (C−H···Cl) (Figure 2c−e). Structure analysis of the 2D layer indicating that each DQA-2 molecule is linked with four neighboring DQA-2 molecules. Regarding DQA-2 as node, and C−H···Cl interactions as linkers, DQA-2 was predigested into a 2D sql topology {46·62} (Figure 1f). Detailed crystallographic information45−47 of the DQA-1 and DQA-2 is shown in the Table 1. Bond lengths (Cd−N, Cd−O) and bond angles (Cd− N−Cl, Cd−N−O) of DQA-1 and DQA-2 are shown in Table S1. Synthesis of Micromesoporous Nitrogen-Doped Carbon Sorbent. The metal−organic complexes crystals DQA-1 and DQA-2 were heated to 350 °C with 5 °C min−1 rate under N2 atmosphere at the first stage. The part structure of crystal was slowly decomposed according to the thermogravimetric analysis (TGA; Figure S2), which is attributed to the removal of coordinated CH3CN molecules. From 350 to 800 °C, the heating rate was changed to 10 °C min−1, subsequently, the samples were kept at 800 °C for 2 h. With the temperature increasing, DQA-1 and DQA-2 further decomposed and converted to more thermodynamically stable materials. Due to the rearrangement of the carbon layers to a more ordered structure, the surface area and pore volume would increase. For C

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

K2CO3.8,48 In order to further improve the carbon structure, residual Cd was removed by hydrofluoric acid (HF) solution (3 M) and two nitrogen-doped hierarchical activated carbon materials were obtained, denoted as MPNC-1 and MPNC-2. The surface morphology (Figure 3a−c) of DQA-1, DQA-2 crystals, MPNC-1 and MPNC-2 were examined by scanning electron microscopy (SEM). After carbonizing DQA-1, DQA-2 at 800 °C under a N2 atmosphere, the result products MPNC1 and MPNC-2 showed obvious shrinkage and deformation, accompanied by the formation of irregularity ellipsoidal carbon materials. The DQA-1 and DQA-2 crystal size distribution were around 0.5−3 mm and 0.8−1.2 μm, respectively. According to the corresponding elemental mapping images (Figure 3d−f), C, N, and O elements are found to be uniformly dispersed throughout the whole carbon materials. On the other hand, it is found that MPNC-2 derived from DQA-2 (Figure 3g) is composed of irregular sheets with some degree of uniformity (Figure. 3h). As shown in Figure 3i, the rough surface is made up of many small globular carbon particles, with a range of 20−120 nm. Such an irregularity ellipsoidal morphology structure is beneficial for the CO2 adsorption due to the fact that it can provide effective contact area between CO2 with MPNCs. The porous structures of MPNC-1 and MPNC-2 are confirmed by N2 adsorption, as shown in Figure 4a. A sharp increase at low relative pressure (P/P0 < 0.1) can be classified as abundant micropores in MPNC-1 and MPNC-2. Moreover, a hysteresis loop belonging to the capillary condensation in mesopores occurs in a N2 adsorption−desorption isotherm at the range of 0.5−1.0 (P/P0 = 0.5−1.0).49,50 Such micromesoporous carbon materials endow MPNC-1 and MPNC-2 material with high specific surface areas of 2368.9 and 2327.6 m2 g−1 and pore volume of 1.95 and 1.89 cm3 g−1, respectively.51 The little bit higher surface area and pore volume of MPNC-1 may be due to the fact that a larger crystal size of precursor DQA-1 than that of DQA-2, leaves more void

Table 1. Crystallographic and Structural Data for DQA-1 and DQA-2 CCDC No. formula Mr crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) volume (Å3) Z Dc (g·cm−3) μ (mm−1) F (000) Θ range (°) h range k range l range data/restraints/params GOF R1, wR2 [I > 2σ(I)]a R1, wR2[all data]a Δρmax, Δρmin (e·Å−3)

DQA-1

DQA-2

1890129 C24H27O6N5Cd 593.91 monoclinic P21/n 9.115(2) 22.817(6) 14.232(5) 90 100.890(4) 90 2906.8(14) 4 1.357 0.793 1208 2.44−27.38 −5 ≤ h ≤ 11 −27 ≤ k ≤ 28 −17 ≤ l ≤ 18 6280/0/326 0.898 0.0674, 0.1721 0.1238, 0.2178 1.449, −0.585

1890130 C24H27Cl2N3Cd 540.80 triclinic P1̅ 7.4593(7) 8.7289(7) 19.3530(17) 80.013(7) 83.740(7) 87.916(7) 1233.41 2 1.456 1.117 548 3.22−27.56 −9 ≤ h ≤ 7 −11 ≤ k ≤ 11 −25 ≤ l ≤ 25 5630/0/272 0.822 0.0420, 0.1168 0.0557, 0.1319 0.415, −0.525

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w (F02 − Fc2)2]/∑[ w (F02)2]]1/2.

a

the chemical activation by doping KOH during the carbonization process, the involved mechanism can be described as 6KOH + 2C ↔ 2K + 3H2 + 2K2CO3, followed by the reaction of carbon with K/K2CO3/CO2 and decomposition of

Figure 3. Surface morphology of DQA-1, DQA-2 crystals, MPNC-1, and MPNC-2. (a) SEM images of the parent DQA-1 and (b) MPNC-1 derived at 800 °C. (c) Magnified SEM images of ellipsoidal carbon materials MPNC-1. (d) C, (e) N, and (f) O element mapping images of MPNC-1. (g) SEM images of the parent DQA-2 and (h) MPNC-2. (i) Magnified SEM images of irregular sheets carbon materials MPNC-2. D

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) N2 adsorption−desorption isotherms of MPNC-1 and MPNC-2 and (b) pore distribution of MPNC-1 and MPNC-2. (c) XPS spectra and (d) the high-resolution N 1s spectra of MPNC-1 and MPNC-2. (e) PXRD patterns of MPNC-1 and MPNC-2 and (f) Raman spectroscopy of MPNC-1 and MPNC-2.

Table 2. BET surface areas, pore volumes, and C, N, O, and pyridinic-N content of MPNCs sample

SBET (m2 g−1)

pore volume (cm3 g−1)

C (%)

N (%)

O (%)

pyridinic-N (%)

MPNC-1 MPNC-2

2368.9 2327.6

1.95 1.89

81.17 86.32

17.2 12.3

1.63 1.38

8.33 5.02

space.50 The pore size distribution curves of MPNC-1 and MPNC-2 materials are shown in Figure. 4b, which indicate that the micropores distributed mainly in the range of less than 1 nm and the mesopores are center at around 2.7 and 3.7 nm. This hierarchical structure is very beneficial for CO 2 adsorption, in which micropores play a key role in improving CO2 adsorption capacity, while the mesopores promote the

diffusion of CO2 in the pores because of capillary condensation to reach CO2 adsorption sites.52,53 The analysis results of X-ray photoelectron spectroscopy (XPS) indicate that MPNC-1 and MPNC-2 are graphite-like structures (sp2 carbons), as shown in Figure 4c. Both XPS spectra for MPNC-1 and MPNC-2 displayed C 1s peaks at 283 and 284 eV, N 1s peaks at 399 and 400 eV, and O 1s peaks at 541 and 540 eV, which are consistent with the elements E

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. CO2 adsorption isotherms for MPNC-1 and MPNC-2 materials and the parent DQA-1 and DQA-2 measured at (a) 25 and (b) 0 °C. (c) Cycling stability of adsorbent MPNC-1. (d) CO2/N2 selectivity of MPNC-1 and MPNC-2 (25 °C, 1 bar).

Figure 7. Image of the electrostatic surface potential for (a) carbon surface only containing pyridinic-N model, (b) carbon surface only containing pyrrolic-N model, (c) carbon surface only containing graphitic-N model, and (d) carbon surface model with graphitic-N, pyrrolic-N, and graphitic-N coexistence.

Figure 6. Adsorption models and adsorption energy of (a) carbon surface only containing the pyridinic-N model, (b) carbon surface only containing the pyrrolic-N model, (c) carbon surface only containing the graphitic-N model, and (d) carbon surface model with graphitic-N, pyrrolic-N, and graphitic-N coexistence.

sample not only shows a higher content of nitrogen, but also a higher content of pyridinic-N (8.33%), as compared with the 5.02% for MPNC-2 sample. In subsequent studies, the effect of oxygen on CO2 adsorption was not considered because of the low oxygen content. Powder X-ray diffraction patterns (PXRD) of MPNC-1 and MPNC-2 materials indicate two peaks around 25° and 43° (Figure 4e). Noteworthy, MPNC-1 and MPNC-2 exhibit

mapping results. C, N, and O content of the obtained carbon materials MPNC-1 and MPNC-2 are summarized in Table 2, which indicate that the nitrogen content for MPNC-1 and MPNC-2 are 1 7.2% and 12.3%, respectively. As shown in Figure. 4d, it is found that three peaks at about 398.7, 400.0, and 400.7 eV belong to pyridinic-N, graphitic-N, and pyrrolicN, respectively. However, one may note that the MPNC-1 F

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Local electron density of LUMO and HOMO of carbon surface only containing pyridinic-N model, carbon surface only containing pyrrolic-N model, carbon surface only containing graphitic-N model, and carbon surface model with graphitic-N, pyrrolic-N, and graphitic-N coexistence.

Figure 9. BLYP/DEF2-SVP gradient isosurfaces for the CO2 adsorption of (a) carbon surface only containing pyridinic-N model, (b) carbon surface only containing pyrrolic-N model, (c) carbon surface only containing graphitic-N model, and (d) carbon surface model with graphitic-N, pyrrolic-N, and graphitic-N coexistence.

CO2 Adsorption Performance of MPNC-1 and MPNC2. As mentioned above, two nitrogen-doped carbons with micromesoporous and different N-content are obtained by a carbonization process at 800 °C. In consideration of the MPNCs materials exhibit the above excellent properties, CO2 adsorption for MPNC materials and the parent DQA-1 and DQA-2 were measured at 0 and 25 °C under the ambient pressure. The MPNC-1 and MPNC-2 samples exhibit a comparatively high CO2 adsorption of 9.56 and 8.02 mmol g−1 at 0 °C (Figure. 5a), while the CO2 adsorption capacity decreases to 7.53 and 6.58 mmol g−1 at 25 °C and 1 bar (Figure. 5b). Notably, MPNC-1 and MPNC-2 materials both show the higher CO2 adsorption capacity compared with the reported porous carbon materials.4,7,56,57 The temperaturedependent adsorption at 25 and 0 °C indicates that the CO2 adsorption of MPNC material is consistent with physical adsorption accompanied by the exothermic characteristics and is a thermodynamic control process. On the other hand, at high temperatures, the decrease in CO2 adsorption manifests that MPNC-1 and MPNC-2 materials can be recycled under mild conditions. Moreover, at both 0 and 25 °C, CO2 uptake capacities of the micromesoporous nitrogen-doped carbon MPNC-1/MPNC-2 and the parent DQA-1/DQA-2 crystal are following the order MPNC-1 > MPNC-2 > DQA-1 > DQA-2. That is to say, MPNC-1 (17.2%), with higher nitrogen content, presents higher CO2 adsorption capacity than that of MPNC-2

almost the same data of full width at half-maximum (fwhm) about 12°. According to the literature, fwhm (002) can indicate the stacking height (Lc) of the carbon layers (Lc = 0.89 λ/(fwhm002 cos θ002)). With the data of fwhm increasing, the stacks of carbon layers are more disordered.54 Raman spectra of the MPNC-1 and MPNC-2 are shown in Figure 4f, with two broad peaks centered around 1592 and 1357 cm−1 (G and D bands), which belong to graphitic carbon and disordered carbon, respectively. According to the reported research on graphitic carbon (G band) and disordered carbon, the upward shift of the disordered carbon band and graphitic band (ID/IG) is due to the structural distortion rooting of C−C and C−N bonds.55 ID/IG shows the similar data for MPNC-1 and MPNC-2 of 0.982 and 0.981, respectively, which indicate that there is almost no C−N and C−C bond distance differences for MPNC-1 and MPNC-2. From the above analysis results, it is found that MPNC-1 and MPNC-2 have an almost similar BET surface area, with the difference less than 50 m2 g−1 and almost the same data of fwhm and ID/IG data, but MPNC-1 shows a higher content of nitrogen and a higher content of pyridinic-N than that of the MPNC-2 sample. Such results inspire us to investigate the effect of nitrogen-doping for CO2 adsorption performance. Important, an exact assessment of functions for different N groups (pyridinic-N, pyrrolic-N, and graphitic-N) on CO2 adsorption is of great significance in the following research. G

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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the CO2 adsorption mode and adsorption position. All the calculations were conducted using Gaussian 09 programs with the BLYP level.25,61−63 According to the XPS results, five carbon surface models without or with different typical N-containing functional groups were built, including a pure carbon surface model (Figure S4), carbon surface only containing the pyridinic-N model (Figure 6a), carbon surface only containing the pyrrolicN model (Figure 6b), carbon surface only containing the graphitic-N model (Figure 6c), and carbon surface model with pyridinic-N, pyrrolic-N, and graphitic-N coexistence (Figure 6d), which was also constructed as a comparison. Calculation results indicate that the adsorption position of CO2 on the basal plane with weak interactions and the adsorption energy (Ed) for the pure carbon surface is −39.33 kJ mol−1. Three N-containing theoretical models, carbon surface only containing the pyridinic-N, carbon surface only containing the pyrrolic-N model, and carbon surface only containing the graphitic-N model show adsorption energies of −47.96, −45.71, and −40.53 kJ mol−1, respectively, indicating stronger affinity for CO2 compared with −39.33 kJ mol−1 on the pure carbon basal plane; moreover, this demonstrates that pyridinic-N and pyrrolic-N contribute to stronger CO2 molecules adsorption capacity. Remarkably, the carbon surface model with pyridinic-N, pyrrolic-N, and graphitic-N coexistence exists a comparable CO2 adsorption energy of −58.96 kJ/mol, larger than that of another four models. Interestingly, in the initial construction of the theoretical model, the CO2 molecules were placed at arbitrary positions (carbon edge region or carbon basal plane), and the final optimization found that the CO2 molecules were adsorbed on the carbon basal plane rather than at other locations. That is to say, the CO2 adsorption positions can be attributed to the basal plane adsorption. According to the analysis results of the electrostatic surface potential and the LUMO and HOMO electron density (Figures 7, 8, S5, and S6), it is shown that the carbon surface model with pyridinic-N, pyrrolic-N, and graphitic-N coexistence does change the internal electronic structure (for instance, the local electronic density, electrostatic surface potential) for enhancing the CO2 adsorption. Noncovalent interaction (NCI) of the adsorption mechanism between of pure carbon models and CO2 molecules are clearly detected in Figures 9 and S7, a large green isosurface between CO2 molecules and the carbon surface indicate the electrostatic interaction mechanism. A similar large green isosurfaces are also found in the case of the pyridinic-N, pyrrolic-N, and graphitic-N basal plane carbon basal plane, which claims that electrostatic interaction between carbon materials and CO2 molecules for the CO2 adsorption.

(12.3%). As we all know, CO2 physisorption is closely related to BET surface areas and pore volume of adsorbents. However, in this work, MPNC-1 and MPNC-2 possess almost the same BET surface areas and pore volume, with a similar high BET surface area of ∼2300 m2 g−1 and a similar high pore volume of ∼1.90 cm3 g−1, but showing the different CO2 adsorption capacities of 9.56 and 8.02 mmol g−1. The above results fully proved that rich N-content plays an important role in CO2 adsorption It is worth noting that MPNC-1 and MPNC-2 materials both show the high CO2 adsorption capacity, which is first due to the nitrogen-containing groups (pyridinic-N, pyrrolic-N, and graphitic-N) in MPNC-1 and MPNC-2, which act as Lewis bases and can interact with the CO2 molecules (acidic molecules); in addition, the micromesoporous structure also makes a significant contribution for CO2 adsorption, owing to the fact that the micropores enhance the interaction between MPNC-1/MPNC-2 carbon adsorbent and CO2 molecules, and at the same time, mesopores improve the utilization efficiency of basic sites and surface area and accelerate the diffusion of CO2 in the pore channels. The cycling stability of the CO2 adsorption adsorbent is also an important factor in practical applications. Figure 5c displays multicycle CO2 adsorption isotherms for MPNC materials at 25 °C. After five cycles, the value of CO2 uptakes is 7.12 mmol g−1, which is approximately 95% of the original adsorption capacity. These results show that MPNC materials are highly stable CO2 adsorbents and can be regenerated without any evidence of loss of CO2 adsorption performance. In practical CO2 gas adsorption applications, the carbon material adsorbent should have high selectivity for CO2 over N2 in addition to excellent CO2 adsorption performance. CO2 and N2 selectivity adsorption experiments were conducted together at 25 °C and 1 bar for samples MPNC-1 and MPNC2. An obvious phenomenon was observed that the CO2 adsorption capacity is much higher than that of N2 adsorption. The estimated CO2/N2 selectivity of MPNC-1 and MPNC-2 materials are 20.5 and 22.6, respectively. Although the observed CO2/N2 selectivity of ∼20−23 is moderate, not outstanding, compared with the microporous crystals of zeolites or metal organic frameworks, it is valuable for porous carbon materials.58,59 The highly selective adsorption of CO2 by MPNC-1 and MPNC-2 porous carbon materials can be attributed to the interaction CO2 (CO2 as acid gas) with basic nitrogen-containing heteroatoms on the carbon surface (high CO2 absorption in the initial stage of the isotherm), which, on the other hand, may be due to the different adsorption modes of N2 and CO2 on the carbon surface. These results indicate that MPNC materials can be used for highly selective adsorption and separation of CO2 and N2. Subsequently, breakthrough experiments of CO2/N2 selectivities were carried out on MPNC-1 and MPNC-2 using a flow of 15:85 CO2/N2 at 298 K (Figures 5d and S3). It can be seen that N2 can be detected at the outlet quickly, but CO2 out flows very late, meaning that MPNC-1 and MPNC-2 adsorb a large amount of CO2 and barely adsorbs N2.60 CO2 Adsorption Mechanism of MPNC-1 and MPNC-2 Materials. The above experimental studies clearly show that MPNC carbon materials exhibit excellent CO2 adsorption properties, and high adsorption characteristics are closely related to N content. Further research was conducted by theoretical calculation (DFT) to reveal the interaction between the N-doped carbon surface and the CO2 molecule and obtain

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CONCLUSIONS In summary, micromesoporous nitrogen-doped carbon (MPNC-1 and MPNC-2) materials are synthesized through direct carbonization of two metal−organic complexes (DQA-1 and DQA-2). MPNC-1 and MPNC-2 exhibit micromesoporous structures with high BET surface area and pore volume. MPNC-1 and MPNC-2 materials with high N contents possess excellent CO2 adsorption and CO2/N2 selectivity. Theoretical calculations indicate that pyridinic-N, pyrrolic-N, and graphitic-N coexistence in carbon materials contribute to stronger CO2 molecules adsorption capacity and could change internal electronic structure (for instance, the local electronic density, H

DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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electrostatic surface potential) for enhancing the CO 2 adsorption. Moreover, theoretical calculations also demonstrate electrostatic interaction mechanism for CO2 adsorption with a large green isosurface between CO2 molecules and the carbon surface. This study provide good strategy to prepare micromesoporous rich nitrogen-doped carbon materials for enhancing CO2 adsorption.

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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/acs.inorgchem.9b00500. Crystal structures of DQA-2. N2 adsorption−desorption isotherms of DQA-1. Bond distances (Å) and angles (°) of DQA-1 and DQA-2. Mechanism involved in the carbonization process of DQA-1. Adsorption models and adsorption energy of pure carbon surface model. Electrostatic surface potential of pure carbon surface model. The local electron density of LUMO and HOMO of pure carbon surface model. BLYP/DEF2SVP gradient isosurfaces for the CO2 adsorption of pure carbon surface (DOCX) Accession Codes

CCDC 1890129−1890130 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ruiqing Fan: 0000-0002-5461-9672 Yulin Yang: 0000-0002-2108-662X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 21873025, 21571042, and 51603055).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00500 Inorg. Chem. XXXX, XXX, XXX−XXX