Nitrogen-doped Microporous Carbon Derived from Pyridine-ligand

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Functional Nanostructured Materials (including low-D carbon)

Nitrogen-doped Microporous Carbon Derived from Pyridine-ligand Based Metal–Organic Complexes as High Performance SO2 Adsorption Sorbents Ani Wang, Ruiqing Fan, Xinxin Pi, Yuze Zhou, Guangyu Chen, Wei Chen, and Yulin Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12739 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Materials & Interfaces

Nitrogen-doped Microporous Carbon Derived from Pyridine-ligand Based Metal–Organic Complexes as High Performance SO2 Adsorption Sorbents Ani Wang,a Ruiqing Fan,*,a Xinxin Pi,b Yuze Zhou,a Guangyu Chen,a Wei Chena and Yulin Yang,*,a

KEYWORDS:

Metal–Organic

Complexes,

Carbonization,

Nitrogen-doped

Microporous Carbon, High surface area, SO2 adsorption

a

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, P. R. of China b

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin,

150001, China.

Corresponding Author: * Ruiqing Fan and Yulin Yang E-mail: [email protected] and [email protected] 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Heteroatom-doped porous carbons are emerging as platforms for gas adsorption. Herein, N-doped microporous carbon (NPCs) materials have been synthesized by carbonization of two pyridine-ligand based Metal–Organic Complexes (MOCs) at high temperatures (800, 900, 1000 and 1100 °C). For the NPCs (termed NPC-1-T and NPC-2-T, T represents the carbonization temperature), micropore is dominant, pyridinic-N and other N atom of MOC precursors mostly retained, the N content reaches as high as 16.61 %. They all show high BET surface area and pore volume, in particular, NPC-1-900 exhibits the highest surface areas and pore volumes, up to 1656.2 m2 g-1 and 1.29 cm3 g-1, high content of pyridinic-N (7.3 %), bring out considerable amount of SO2 capture (118.1 mg g-1). Theoretical calculation (int=ultrafine m062x) indicates that pyridinic-N act as the leading active sites contributing to high SO2 adsorption and higher content of pyridinic-N doping into graphite carbon layer structure could change the electrostatic surface potential, as well as the local electronic density, which enhanced SO2 absorption on carbon edge positions. The results show great potential for preparation of microporous carbon materials from pyridine-ligand based Metal–Organic Complexes for effective SO2 adsorption.

Introduction Sulfur dioxide (SO2) emission has become a serious environmental issue due to its great threat to the environment and human health.1-2 Development of green 2


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absorbents that can efficiently, economically and reversibly capture SO2 to replace the traditional Flue gas desulfurization (FGD) technologies is highly desirable, FGD with its inherent drawbacks such as a large amount of waste water solid wastes (CaSO4), or some other byproduct generated from limestone scrubbing, which cause serious secondary pollution to the environment.3-7 Design and synthesis of highly efficient SO2 adsorption sorbents requires identification of active sites, but the most important active site favor high SO2 adsorption capacity is under continuous hot debate.8-9 The center point for SO2 adsorption sites of N-doped carbons is pyridinic-N sites act as the most active rather than other types of N groups.10 However, preparation of N-doped carbon materials containing a high density of pyridine N sites stabilized on the surface to enhance SO2 adsorption has never been reported.11-12 Preparing carbon materials from Metal–Organic Complexes (MOCs)13-15 becoming an emerging way for scientists to successfully prepare functional carbon in recent years.16-20 Since carbon materials can inherit part of the structure and function of the MOC precursor, it provides a very simple strategy for preparing carbon with the desired structure and function.21-23 For SO2 adsorption adsorbent of carbon material, the MOCs provide an ideal platform due to the fact that active sites can be rather conveniently incorporated into the resulting carbon.9,

24-27

Although several

N-doped carbons have been fabricated through carbonization of the N-containing MOCs in recent years, most of these N-doped porous carbons are studied focused on their novelty, different pore structures and variable morphologies, the exact functions of pyridine-N group are still ambiguous.28-29 3



Herein, we investigate for the first time that two different N-content pyridine-ligand Metal–Organic Complexes (MOC-1 and MOC-2) as precursors to prepare the N-doped porous carbons (noted as NPC-1-T and NPC-2-T, T represents the carbonization temperature) through the carbonization process in this work. At different carbonization temperatures (800, 900, 1000 and 1100 °C), the surface areas and pore volumes reach as high as 1656.2 m2 g-1 and 1.29 cm3 g-1, the N content reaches up to 16.61 % for resulting N-doped porous carbons, especially, the content of pyridinic-N up to 7.3 %, therefore, the NPC-1-900 shows a significant amount of SO2 adsorption, over 118.1 mg g-1 at 1 bar and 25 °C. Theoretical calculation indicated that pyridinic-N doping into carbon lattice could effect on electrostatic surface potential and the local electronic density, thus more active sites were created on carbon edge positions, and pyridinic-N act as the leading active sites for enhanced SO2 adsorption. The synthesis, crystallization, carbonization and SO2 adsorption process of MOC-1 as a representative is schematically shown in Scheme 1.

Scheme 1. The synthesis, crystallization, carbonization and SO2 adsorption process of porous carbons NPC-1 derived from MOC-1.

4


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Results and discussion It has been found that porous carbon materials modified with heteroatom dopant (e.g., N, O, S, P, Co, Fe) can impact the SO2 adsorption,30 in order to prepare heteroatom-doped porous carbon materials with exceptional SO2 adsorption capability, the choice of the heteroatom-doped carbon source is a significant step. Herein,

different

N-doped

porous

carbon

materials

were

obtained

using

pyridine-ligand based Metal–Organic Complexes (MOCs) as the precursor. In our study, the two MOCs were synthesized for the first time. As shown in Scheme 1, in the

case

of

MOC-1,

using

one-pot

self-assembled

method,

2-carboxaldehyde-1,10-phenanthroline, 4-methy-o-phenylenediamine and Zn(NO3)2 in a 1:1:1 in anhydrous acetonitrile under stirring for 1h then refluxed at 90 °C for 12 h. Subsequently, the mixture was cooled, filtered, slowly evaporated in air, resulting 5



ultimately in the formation of bulk-like crystals, the CCDC reference number: 1855761 and 1855762, respectively.

Structural description of MOC-1 Each asymmetric unit of MOC-1 (Figure 1a) including one Zn2+ ion, one ligand and two nitrate ions, in addition, there is an uncoordinated acetonitrile molecule.31-32 In particular, introducing of the Zn(II) ion come into being two five-membered rings, enhanced the extended conjugate and resulting in coplanar with intrinsic phenanthroline ring.33-38 The independent unit of MOC-1 is linked through C15–H15A···O4 (Figure 1b) to construct dimer and C4–H4A···O4 hydrogen bonding interactions to format 1D chain (Figure 1c), H15A···O4 and H4A···O4 distances are 2.842 Å and 2.517 Å, respectively. Every Zn2+ cation is slightly distorted octahedron coordination geometry [ZnN3O3] (inset of Figure 1c). The 1D chain were further linked adjacent chains through C1–H1A···O5, C17–H17A···O1 and C18–H18A···O2 hydrogen bonding interactions, a 3D network structure was constructed naturally (Figure 1d). PLATON calculation indicated that the overall solvent-accessible volume is 153.0 Å3, which possess 13.2% percent of the cell volume (1159.6 Å3) (Figure 3e).33-38 Furthermore, regarding the independent unit of MOC-1 as 6-connected node, and C–H···O hydrogen bonding interactions as linkers, MOC-1 was simplified a 3D pcu topology {412·63} (Figure 1f).39-42 The permanent porosity of MOC-1 was confirmed by performing the N2 adsorption experiments at 77 K. As displayed in Figure S1, the N2 adsorption of the activated sample MOC-1 presents the type I adsorption isotherm, showing the microporous nature of the framework, which is consistent with the observed channels in the crystal structure analysis. The N2 adsorption amount of MOC-1 is about 72.3 cm3 g-1, and the Brunauer−Emmett−Teller (BET) surface area is 6


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calculated to be ca. 409.2 m2 g−1.

Figure 1. (a) Structure of MOC-1. (b) Dimer and (c) 1D chain with the distorted octahedron coordination geometry [ZnN3O3] of MOC-1. (d) 3D network of MOC-1. (e) Micropores structure and (f) 3D pcu topology of MOC-1. Some atoms omitted for clarity. Red lines represent the C– H···O hydrogen bonding interactions. Some H atoms that not involved in forming hydrogen bonds omitted for clarity.

Structural description of MOC-2 The structure of MOC-2 is very similar to that of MOC-1. For each asymmetric unit of MOC-2, one Zn2+ ions, one pyridine-ligand and two nitrate ions (Figure S2) was included. The independent unit of MOC-2 are firstly constructed into 1D chain, 2D layer and then construct 3D network through C–H···O hydrogen bonding (Figure S3, S4 and S5). Detailed analysis of the supramolecular structure of MOC-2 shows that each MOC-2 molecule is connected with 8 adjacent MOC-2 molecules through noncovalent bonding. Regarding the MOC-2 asymmetric unit as 6-connected node, 7



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and the C–H···O hydrogen bonding interactions as connection lines, MOC-2 exhibits 3D pcu topology with the point symbol of {412·63} (Figure S6). Structural refinement and crystallographic data 43-45 for the three MOCs are listed in Table S1. The selected (Zn–N, Zn–O, Zn–N–O) bond lengths and bond angles of MOC-1 and MOC-2 are shown a clear statement of account in Table S2 (Supporting Information). The detailed data of C–H···O hydrogen bonds for MOC-1 and MOC-2 are listed in Table S3. Carbonization and characterization of MOC-1 and MOC-2 Subsequent the two MOCs crystals (MOC-1 and MOC-2) was heated to specified temperature (800, 900, 1000 and 1100 °C) at a heating rate of 10 °C min-1 and was kept at specified temperature under N2 atmosphere for 2 hours, following removal of the Zn metal with HF acid solution (10 mL, 3 M) obtained the N-doped microporous carbons (termed NPC-1-T and NPC-2-T, T represents the carbonization temperature). The carbonization mechanism and process for porous carbons (NPC-1-900 and NPC-2-900) derived from MOC-1 and MOC-2 is schematically presented in Figure 2 and Figure S7 through thermogravimetric analysis. As the formation of NPC-1-900 representative, the first mass-loss, 0–160 °C, is classified as the vaporization of uncoordinated acetonitrile (CH3CN). The second mass-loss, 160–360 °C, is due to skeleton collapse accompanied by the release of carbonaceous gas products (a small amount of CxHy hydrocarbon mixtures and mostly CO, CO2, C6H6) and metaloxides (ZnO) is formed. The third mass-loss seen in porous carbons (NPC-1) derived from MOC-1 starting at 360 °C is attributed to further release of CO and CO2 equation (1)

and equation (2) as follows: ZnO + C → Zn(g) + CO 8


(1)

46

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CO + O2 →CO2

(2)

Figure 2. Carbonization mechanism and process for porous carbons (NPC-1-900) derived from MOC-1. The plot represents the thermogravimetric analysis of MOC-1. Inset: Topological structure of MOC-1 represents the possible framework structures and the plausible carbonization process.

After carbonization, the NPCs completely changed the crystallite morphology and surface structures of their MOCs precursors to some extent, as shown in the digital photographs and SEM images presented in Figure 3a-3f. It is also noted that the NPCs derived at specified carbonization temperatures resulting in the parent MOCs crystallites shrink after carbonization. Furthermore, the transmission electron microscopy (TEM) images shown in Figure 3g and 3h clearly reveal the highly defective, randomly oriented graphenic type nano-sheets. Such surface morphology and structure can effectively enhance the contact area between SO2 with NPCs, which is beneficial for the SO2 adsorption. 9



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Figure 3. Top: (a, b) Schematic representation of MOC-1 and MOC-2 crystals. Middle: (c, d) SEM images of carbonized crystals materials. (e, f) SEM images of carbonized graphenic type slice layer. (g, h) TEM images of the highly defective, randomly oriented graphenic type nano-sheets.

The formation of the pore in the NPCs is understandable from their well-ordered supramolecular framework structure of MOCs precursors. During the process of carbonization, via the reduction of ZnO, considerable mass loss of C and O from the framework structures, resulting in the evolution of Zn, CO and CO2, which naturally leave more hollow or defective carbon networks. This is a self-activated process for carbon

materials,

analogous

to

the

chemical

or

physical

activation

of

carbon-containing precursors produced using KOH or CO2 to achieve porous activated carbons.47-49 10


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Nitrogen adsorption–desorption isotherms of the obtained NPCs is shown in Figure 4a and S8, values of BET surface area and pore volume are listed in Table 1. The Langmuir or type-I isotherms represents dominant microporosity in NPCs (Figure 4b). Based on the previous literature, it can be concluded that the micropores are the best pore structure for the SO2 adsorption by carbonaceous materials.50-51 The pore size distributions of the NPCs obtained at different carbonization temperatures show almost no significant difference but micropores structures, which is beneficial for the SO2 capture. The BET surface area is in the range of 773.8–1656.2 m2 g-1. It is noted that the high surface area of NPC-1-900, up to 1656.2 m2 g-1, which is the relatively high values to date reported in the existing literature for carbons materials.46 Such a high surface area and microporous structure are favourable for SO2 adsorption.28 At lower temperatures, carbonization is insufficient and some gases are not completely released, thus exhibiting a low specific surface area. When the temperature is too high, tending to cause sintering into a block, which is not conducive to the formation of high specific surface area. Therefore, 900 °C is the best carbonization temperature. Table 1. BET surface areas, pore volumes and pore sizes of N-doped porous carbons NPCs. Sample

SBET (m2 g-1)

Smic (m2 g-1)

Vt (mL g-1)

Vmic (mL g-1)

Pore Size (nm)

NPC-1-800

773.8

764.30

0.68

0.5981

0.9266

NPC-1-900

1656.2

1403.51

1.29

1.0365

1.0325

NPC-1-1000

1635.2

1436.24

1.25

1.0221

0.9853

NPC-1-1100

1613.5

1543.62

1.23

0.9923

0.9956

NPC-2-800

931.6

913.02

0.73

0.5264

1.0568

NPC-2-900

1452.3

1426.54

1.23

0.7425

1.0652

NPC-2-1000

1398.3

1365.32

1.15

0.7432

0.9653

11



NPC-2-1100

1335.2

1286.23

1.03

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0.7532

0.6521

The X-ray photoemission spectroscopy (XPS) spectra of these NPCs are shown in Figure 4c and S9, indicating that the NPCs are graphite structure in nature with a majority of sp2 carbons. C1s, N1s and O1s peaks were observed in the full scan spectra and around 284 eV, 399 eV and 532 eV, and no Zn 2p core level XPS spectra is detected. The carbon and nitrogen content of the N-doped microporous carbons are listed in Table S4. Notably, the carbonization temperature has a significant effect on the N content, with increasing the carbonization temperature, the total N contents of NPCs show a conspicuous downward trend. The total N-content percentage of the NPC-1-800, NPC-1-900, NPC-1-1000 and NPC-1-1100 is 16.61 %, 15.91 %, 14.19 % and 13.70 %, NPC-2-800, NPC-2-900, NPC-2-1000 and NPC-2-1100 is 15.23 %, 14.66 %, 13.62 % and 12.12 %, respectively), O contents of NPCs exhibit no significant difference and it occupy a small amount, which can be clearly observed from Table S4. Four peaks appeared at 398.7, 399.5, 401.2 and 402.8 eV of the high-resolution N 1s survey spectra are corresponding to pyridinic-N, pyrrole-N, graphitic-N and N-oxide, respectively (as shown in Figure 4d). When MOCs is carbonized at 800 and 900 °C, the pyridinic-N groups are mostly retained by resulting carbon (accounting for 65.9 % and 63.29 % of the total N content). When the temperature was increased to 1000 and 1100 °C, some of the pyrrole-N and pyridinic-N were converted into N-oxide and graphite-N groups. Among the constituents of these N in each form, the pyridinic-N occupied 40.2–65.9 % of all nitrogen atoms, which is considered as the most efficient active site. The total content 12


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of pyridine-N is in the range of 4.3-8.2 %, which ensures its excellent SO2 adsorption performance relative to N-doped carbon from other methods. The PXRD patterns of the carbonization materials with two peaks around 24 and 44° (Figure 4e and S10), which were attributed to graphite structure of the (002) and (101) planes, no peaks of ZnO or other forms of Zn metaloxides can be observed in PXRD patterns at specified carbonization temperature or after acid treatment, indicating the disordered orientation tiny graphite fragments in the structures. It can be seen that all N-doped carbons NPCs show broadened (002) peaks. According to the literature, widest full width at half-maximum (FWHM) of (002) plane can directly influence

the

stacking

height

(Lc)

of

graphitic

structure

(Lc

=

0.89λ/(FWHM002cosθ002)), and higher values of the FWHM represents more disordered packing structures.30 For NPCs, FWHM of (002) plane show an increasing trend with increased carbonization temperatures. NPC-1-1100 (13°) and NPC-2-1100 (12°) exhibit the widest FWHM of 13° and 12°, higher than that of NPC-1-1000 (11°), NPC-2-1100(11°), NPC-1-900 (9°), NPC-2-900 (10°), NPC-1-800 (9°), NPC-2-800 (8°). Therefore, it is note that the NPC-1-1100 and NPC-2-1100 show the highest disordered packing structures. Raman spectroscopy of the different NPCs was further investigated (Figure 4f and S11). Two distinct broad peaks near 1355 and 1596 cm-1 can be designated as disordered carbon and graphitic carbon (D band and G band), respectively. The crystalline and disordered structures of the carbon materials are commonly estimated by the intensity ratio of the ID to IG (ID/IG) and the shift of the peak position.52 13



Furthermore, according to the previous research, the upward shift of D band and G band is due to bond distances difference of C–N and C–C caused from the structural distortion. The data of ID/IG for NPC-1 and NPC-2 following the order that NPC-1/2-1100 > NPC-1/2-1000 > NPC-1/2-900 > NPC-1/2-800, that is to say, NPC-1/2-1100 shows the highest upward shift of D band and G band. The detailed data for NPC-1-1100, NPC-1-1000, NPC-1-900 and NPC-1-800 is 0.918, 0.907, 0.892 and 0.888, NPC-2-800, NPC-2-900, NPC-2-1000 and NPC-2-1100 is 0.916, 0.908, 0.897 and 0.882, respectively.

Figure 4. (a) The nitrogen adsorption–desorption isotherms and (b) Pore volume values for the 14


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NPCs. (c) X-ray photoemission spectroscopy (XPS) spectra in the full scan spectra of NPCs and (d) The high-resolution N 1s survey spectra of NPC-1. (e) PXRD patterns and (f) Raman spectroscopy of the as-synthesized NPCs.

Combination the results of Nitrogen adsorption, XPS, PXRD and Raman analysis, it is found that carbonization temperatures have a remarkable influence on, BET surface area, pore volume, the N content, pyridinic-N content, the stacking height (Lc), upward shift of D band and G band. The NPCs at 900 °C show the highest high BET surface area, the total N contents and pyridinic-N content of NPCs exhibit a significant decreasing trend with increased carbonization temperatures, however, FWHM of (002) plane and Lc show an increasing trend with increased carbonization temperatures. This result motivates us to explore which factors are the dominant factors to effect SO2 adsorption performance.

SO2 adsorption activities of NPCs materials As mentioned above, N-doped microporous carbon with various N-content and microporosity are synthesized by self-assembled and followed by carbonization process at different temperatures. Due to the as-synthesized NPCs materials exhibit the above unique characteristics, which can be used as an ideal platform for studying SO2 gas adsorption performance. As shown in Figure 5, the NPC-1-900 shows the highest SO2 adsorption capacity among these NPCs samples (118.1 mg g-1), which is one of the considerable high adsorption capacities for SO2. SO2 adsorption capacities following the order: for NPC-1, NPC-1-900 (118.1 mg g-1) > NPC-1-800 (107.2 mg g-1) > NPC-1-1000 (88.9 mg g-1) > NPC-1-1100 (80.5 mg g-1), for NPC-2, NPC-2-900 15



(102.6 mg g-1) > NPC-2-800 (92.5 mg g-1) > NPC-2-1000 (86.2 mg g-1) > NPC-2-1100 (78.3 mg g-1). However, in view of the BET surface area, pore volume, disordered packing structures and upward shift of D band and G band of NPC-1000 and NPC-1100 is higher than that of NPC-800, but SO2 breakthrough time and the adsorption capacity of NPC-800 higher than that of NPC-1000 and NPC-1100. Combination of the XPS, the N-content and pyridinic-N content for of NPC-800 and NPC-900 is higher than that of NPC-1000 and NPC-1100, which displays higher SO2 adsorption capacity. These results indicate that the total N-content and pyridinic-N content in the porous carbon play an important role to enhance SO2 adsorption. Additionally, as control experiments, SO2 adsorption of the MOC-1 and MOC-2 were evaluated. As shown in Figure 5, SO2 adsorption efficiencies of NPC-1-900 (118.1 mg g-1) and NPC-2-900 (102.6 mg g-1) is 80 and 90 times higher than that of MOC-1 (21.3 mg g-1) and MOC-2 (19.5 mg g-1). Up to now, a number of carbon materials have been successfully applied for SO2 adsorption, which is summarized in Table S4 (A20 series of carbons). From the comparison with the reported literatures, the amount of SO2 adsorption is higher than those in previously reported.10 Recently, the reported literatures reveal that pyridinic-N group is the most favorable site for SO2 capture due to its high Lewis basicity.53 Also, Lewis basic characteristic of pyridinic-N is conducted by using DFT calculations combined with local scanning tunneling microscopy/spectroscopy (STM-STS).54 The results show that the carbon atoms near pyridine-N exhibit a local density of states near the Fermi level and thus exhibit a Lewis base. Therefore, pyridinic-N is indeed as a Louis base 16


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property of the carbon surface, which is beneficial for the SO2 adsorption. In order to further explore the effect of pyridinic-N groups on the adsorption SO2, TPD was used for temperature-programmed desorption of SO2 to saturated samples, as shown in Figure S12. Combined with the results of XPS, the total pyridine-N content show slowly decrease with increasing the temperature, some of the pyridine-N and pyrrole-N were converted into N-oxide and graphite-N groups. It can be seen that among these NPC-1 samples, NPC-1-800 with the most pyridinic-N content has the highest SO2 chemical adsorption. However, NPC-1-1100 with the least N content exhibit the lowest chemical adsorption. That is to say, with increasing of the pyridinic-N content, chemical adsorption gradually enhanced, which indicate that the pyridinic-N content is beneficial for the SO2 chemical adsorption. NPCs adsorbents can be generated by heat treatment at 250 °C for 20 min, as shown in the Figure S13, after 10 cycles of thermal regeneration, the adsorption performance remains almost unchanged, which indicate their applicable stability and durability.

Figure 5. The SO2 adsorption efficiencies and saturation uptake capacities of (a, b) NPC-1 and (c, 17



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d) NPC-2 (25 °C, 1 bar).

SO2 adsorption mechanism of NPCs materials on theoretical calculation level As mentioned above that the total content of pyridinic-N in the porous carbon can effectivity enhance the SO2 adsorption capacity, for further research this behaviors of NPCs materials, the SO2 adsorption mechanism on theoretical calculation level was conducted.28, 55 Higher content of pyridinic-N graphite carbon layer structure and Lower content of pyridinic-N graphite carbon layer structure were adopted to investigate the SO2 adsorption behaviors. Moreover, the pyridinic-N doping influence on adsorption patterns and adsorption positions (plane/edge) were investigated by using B3LYP functional and 6-31G (d, p) basis (Gaussian 09 software). Optimized molecular structures, electrostatic surface potential, electron density distributions, energy levels of HOMO and LUMO of the higher content of pyridinic-N graphite carbon layer structure and lower content of pyridinic-N graphite carbon layer structure before and after SO2 adsorption were obtained. The superposition based on geometric correction and dispersion correction is considered. Use density-fitting approximations to accelerate calculations. Adsorption energy (Ead) is obtained through calculations about single point energy of higher content of pyridinic-N graphite carbon layer structure and lower content of pyridinic-N graphite carbon layer structure with int=ultrafine m062x level. SO2 adsorption energy equation (3) is as follows: Ead = Ecomplex – ESO2 – Esurface

(3)

where Ecomplex, ‫ܧ‬ௌைమ and Esurface represent the single point energies of the higher 18


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content of pyridinic-N graphite carbon layer structure and lower content of pyridinic-N graphite carbon layer structure after SO2 adsorption, the configuration optimized SO2 molecule and the higher content of pyridinic-N doping graphite carbon layer structure and lower content of pyridinic-N doping graphite carbon layer structure, respectively. As shown in Figure 6a-6d, result of the theoretical calculation indicate that SO2 adsorption of the higher content of pyridinic-N doping graphite carbon layer structure, the adsorption energy (Ed) is equal to 37.26 kJ/mol, and that of lower content of pyridinic-N doping graphite carbon layer structure is 54.37 kJ/mol, indicating that the higher content of pyridinic-N doping graphite carbon layer structure shows higher adsorption energy than that of the corresponding lower content of pyridinic-N doping graphite carbon layer structure. That is to say, SO2 adsorption of the pyridinic-N doping carbons greatly promotes SO2 absorption at the carbon edge locations. In addition, SO2 adsorption of the nitrogen doping carbons is inclined to pyridinic-N not other types of N atoms.

19



Figure 6. (a) and (b) Lower and higher content of pyridinic-N doping graphite carbon layer structure model and (c, d) for SO2 absorption. Inset, top: the lowest unoccupied molecular orbital (LUMO), bottom: the highest occupied molecular orbital (HOMO).

Most importantly, one interesting phenomenon of the calculations is when we place SO2 molecules near carbon edge positions or basal plane, geometry optimization of SO2 molecule adsorbs in carbon edge positions not on surface area and not directly interacted with the nitrogen atom. That is to say, SO2 adsorption of the nitrogen doping carbons is inclined to edge position. This demonstrates that the enhancement of the SO2 absorption due to the whole N-doped carbon atoms saturated models serve as active site rather than nitrogen atom itself and SO2 adsorption on the edge position greatly improved by N doping. Combining the electrostatic surface potential (Figure 7a-7d) and LUMO and HOMO local electronic density (Inset of Figure 6), it is indicated that the role of higher content of pyridinic-N doping into graphite carbon 20


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layer structure could change the electrostatic surface potential, as well as the local electronic density. These effects could increase more chance for SO2 to interaction on carbon edge positions.

Figure 7. (a) Electrostatic surface potential of lower and (b) higher content of pyridinic-N doping graphite carbon layer structure models (c) Electrostatic surface potential of lower and (d) higher content of pyridinic-N doping graphite carbon layer structure models for SO2 absorption

Conclusions A series of different N-doped microporous carbons were synthesized by carbonization of pyridine-ligand based MOCs. The NPCs are sp2-bonded graphite carbon layer in nature, effectively enhancing the contact area between SO2 with NPCs. The NPCs shows the considerable SO2 adsorption capacity about 118.1 mg g−1 at 1 21



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bar and 25 °C, which is due to the high BET surface area, pore volume, the N content, pyridine-N content. In addition, the high stacking height and upward shift of D band and G band also play the important role in enhancing the SO2 adsorption. Finally, we note that the N doping into graphite carbon layer could remodel the electrostatic surface potential and the local electronic density for SO2 adsorption. This study provide a clear direction for preparation of N-doped carbon materials containing a high density of pyridine N sites stabilized on the surface and demonstrate that pyridine N acts as the most important active site to enhance SO2 adsorption.

Experimental Section MOC-1 A mixture of 2-carboxaldehyde-1,10-phenanthroline (108.1 mg, 2 mmol), 4-Methy-o-phenylenediamine (224.3 mg, 1 mmol), Zn(NO3)2·6H2O (297.49 mg, 1 mmol) was dissolved in CH3CN (30 mL) and stirred for 50 min, then heated in a sealed vial at 85 °C for 5 h. Yellow rectangular block crystals of MOC-1 were obtained. Yield: 69%. Anal. Calcd (%) for C20H13O6N6Zn (M = 539.68 g mol−1): C, 48.95; H, 2.99; N, 18.16. Found: C, 48.93; H, 2.97; N, 18.15. FT-IR (KBr, cm–1): 3344(w), 1629(w), 1612(w), 1586(w), 1481(s), 1436(w), 1378(w), 1353(w), 1280(s), 1015(s), 999(m), 969 (m), 948(w), 917(w), 875(w), 823(w), 807(w), 786(w), 760(w), 746(w), 639(w), 619(w), 578(w), 558(w), 536(w), 527(w), 501(w), 485(w), 469(w), 436(w). MOC-2 The

complex

MOC-2

was

synthesized

by

dissolving

2-carboxaldehyde-1,10-phenanthroline (216.2 mg, 2 mmol), o-phenylenediamine (208.3 mg, 1 mmol), Zn(NO3)2·6H2O (297.49 mg, 1 mmol) in 20 mL anhydrous 22


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acetonitrile solutions under stirring at room temperature for 1.5 h then refluxed for 2 h. The mixture was then cooled and filtered. The filtrate was allowed to stand at room temperature in air. Quality yellow bulk single crystals MOC-2 were obtained by slow evaporation after 2 days. Complex MOC-2 was stable in the solid state under further exposure to air. Yield: 71%. Anal. Calcd (%) for C19H12O6N6Zn (M = 485.74 g mol−1): C, 48.02; H, 2.75; N, 17.76. Found: C, 48.00; H, 2.74; N, 17.74. FT-IR (KBr, cm–1): 3541(w), 2950(w), 1686(w), 1607(m), 1575(m), 1468(s), 1414(w), 1294(s), 1214(w), 1179(w), 1164(w), 1150(w), 1099(w), 1088(w), 1065(m), 1051(s), 1032(m), 1014(s), 994(m), 951(w), 738(w), 682(w), 617(w), 597(w), 563(w), 540(w), 517(w), 476(w), 439(w). Synthesis of Nitrogen-doped Microporous Carbon (NPCs) The synthesized MOCs crystals sample was carefully and uniformly placed in a quartz boat, and then placed in a programmable tube furnace with flowing into an inert gas (N2). The furnace was set at a heating rate of 10 °C min-1 and heated to the specified temperature (900 °C) and kept at a high temperature for 2 hours. After carbonization is completed, the furnace is naturally cooled to room temperature. The resulting black powder was stirred in an HF solution (10 mL, 3 M) at 60 °C for 12 hours in order to remove the remaining metal oxide in the carbon. The black precipitate was then filtered and washed several times with deionized water. The resulting carbon was dried at 100 °C for 12 hours. (Note: Treatment of carbon with HF solution and the following filtration should be performed in a fume hood to prevent poisoning). SO2 adsorption 23



The reaction was studied in a differential quartz reactor at constant temperature using the experimental system shown in Figure S12. The temperature of the entire adsorption and desorption process is controlled by the electric furnace heating. In each adsorption test, 0.1 g of the prepared carbon material was mixed with 1 g of quartz sand (70 mesh) and placed in a quartz reactor (length 100 cm, diameter 0.4 cm). First, flowing N2 was used to flush the reactor for 10 min to remove residual air. Subsequently, when the temperature in the reaction zone reached the desired value and kept a steady state, SO2 gas (2000 ppm, balance N2) was then introduced into the reactor. The outlet stream from the reactor was diluted with N2 by mass flowmeters (D07-7B and D07-19B, Beijing Sevenstar Electronics Co., Ltd.). Here, the concentration of SO2 was detected by the portable FTIR from Finland Gasmet Company(GASMET-DX4000).

Int=ultrafine m062x calculations Int=ultrafine m062x calculations were carried out in Gaussian 09 software by employing the classical carbon-based cluster models. The geometries considered are fully optimized and the nature of stationary points characterized by the frequency calculations at B3LYP/6-31G (d, p) level. Single point energy of the optimized geometry was calculated at B3LYP/6-311+G (d, p) level. The adsorption energy Ead = Ecomplex – ESO2 – Esurface , where Ecomplex, ESO2 and Esurface represent the single point energies of the N-doped graphite carbon layer structure and un-doped large carbon layer structure after SO2 adsorption, the configuration optimized SO2 molecule and the graphite carbon layer structure and un-doped graphite carbon layer structure, respectively. Supporting Information 24


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The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org: Selected bond distances (Å) and angles (°) for MOC-1 and MOC-2. Crystal structures of MOC-2. FT-IR patterns, PXRD patterns and Raman spectroscopy of MOC-1 and MOC-2. Nitrogen adsorption–desorption isotherms of MOC-1 and MOC-2.

Confict of interest The authors declare no competing financial interest.

Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 21873025, 21571042 and 51603055).

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Table of Contents graphic

35


Nitrogen-doped Microporous Carbon Derived from Pyridine-ligand

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