Synthesis of Triazine-Based Porous Organic Polymers Derived N

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Synthesis of Triazine-Based Porous Organic Polymers Derived N-enriched Porous Carbons for CO2 Capture Lishu Shao, yong li, Jianhan Huang, and You-Nian Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04533 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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Industrial & Engineering Chemistry Research

Synthesis of Triazine-Based Porous Organic Polymers Derived N-enriched Porous Carbons for CO2 Capture

Lishu Shao, Yong Li, Jianhan Huang*, You-Nian Liu*

College of Chemistry and Chemical Engineering, Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha, 410083, China

________________ * Corresponding authors. Tel/Fax: 86-731-88879616. E-mail address: [email protected] (J. Huang); [email protected] (Y.N. Liu).

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ABSTRACT: Porous carbon with both high CO2 uptake and CO2/N2 selectivity is desired for reducing the cost of carbon capture. Here, we report the preparation of N-enriched porous carbons (NPCs) derived from the low-cost triazine-based porous organic polymers using KOH as the activating agent under N2. The results indicate that the nitrogen content and textural properties of the NPCs can be effectively adjusted by the polymer precursors and the carbonization temperature. Impressively, the NPCs have an enriched N content (5.56-11.33 wt%) and abundant porosity (BET surface area: 394-1873 m2/g, pore volume: 0.27-1.56 cm3/g), endowing them with high CO2 uptake (120-207 mg/g at 273 K and 1.0 bar) and acceptable CO2/N2 selectivity (Henry’s law: 14.3-16.8). In particular, the ultra micropore volume (d≤0.8 nm) is proven a key factor for the CO2 uptake, while both the ultra micropore volume and N content contribute the CO2/N2 selectivity. Our described work will provide a strategy to initiate developments of rationally designed porous carbons for various potential applications.

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1. INTRODUCTION Carbon dioxide (CO2) emissions due to combustion of fossil fuels are becoming one of the greatest environmental concerns nowadays, and carbon capture and storage (CCS) is generally recognized as an effective method to reduce the CO2 emissions.1-4 As compared with traditional amine-based solvent as currently most mature and commercial technology,5-7 adsorption by various porous materials is proven promising for CO2 capture due to its low energy consumption and simple operation.8,9 Porous organic polymers (POPs) such as hyper-cross-linked polymers (HCPs),10,11 conjugated microporous polymers (CMPs),12 intrinsic microporous polymers (IMPs),13 porous aromatic frameworks (PAFs),14 and covalent organic frameworks (COFs)15 are potential adsorbents for CO2 capture due to their high surface area, tunable pore size, and tailored functionality, and hence have been attracting increasing attention in recent years. However, many polymers involve the costly and toxic starting materials (e.g., CMPs), expensive catalysts (e.g., CMPs, PAFs), and harsh synthetic condition (e.g., PAFs), which make their large-scale preparation challenging. Alternatively, because of their facile synthesis, tunable pore dimensions, and flexible heteroatom doping, porous carbons have gained continuous interest as the huge potential adsorbents for CO2 capture. Nitrogen (N) incorporation on porous carbons is known as one of the remarkable strategies to enhance CO2 uptake and CO2/N2 selectivity.16,17 Kim et al.18 reported that the N-enriched porous carbons (NPCs) impregnated with sodium could adsorb 4.48 mmol/g of CO2 at 298 K and 1.0 bar, and its IAST CO2/N2 selectivity reached 36:1. More recently, a kind of novel hierarchical NPCs with extremely high CO2/N2 selectivity (Henry’s law of 124:1) at 298 K and 1.0 bar was reported, and it was synthesized by assembling 4-(1H-pyrrol-1-yl) butanoic acid and segmented

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copolymer (P123) followed by KOH activation.1 However, many NPCs are commonly prepared from the high-cost N-containing polymers, sometimes expensive catalysts or high temperatures are involved.19,20 Hence, the facile preparation of the low-cost polymer precursors is significant and remains a challenge. El-Kaderi et al.21,22 reported another way for generation of N-doped porous carbon, which is using organic monomers such as pyrazole and benzimidazole, and the process is feasible and cost-effective. Additionally, triazine-based covalent organic polymers (COPs) are becoming the important porous organic frameworks due to high N content and easy synthesis process.23,24 Patel et al.25,26 fabricated the inexpensive COPs from cyanuric chloride and piperazine, and the as-prepared COPs showed high CO2 uptake. In addition, construction of ultra micropores is also desired to enhance the CO2 uptake and CO2/N2 selectivity. Nugent et al.27 reported the porous adsorbents with high portion of ultra micropores which revealed high CO2 capacity and CO2/N2 selectivity due to the particularly strengthened van der Waals force fields from the narrowing of the adjacent ultra micropore walls. As a result, the aforementioned discussions suggest that the CO2 uptake and CO2/N2 selectivity of the porous carbons can be sufficiently improved by tuning the micropore structure and modifying surface properties. In this work, we firstly synthesized a series of triazine-based porous organic polymers by a facile nucleophilic substitution reaction between cyanuric chloride and the aminating agents. The substitution reaction easily occurred at a mild temperature of 65oC in absence of any catalysts. After that, the as-prepared polymer precursors were carbonized using KOH as the activating agent under N2, and hence a series of NPCs was obtained. The CO2 adsorption was comparatively studied for the NPCs using the polymer precursors as the references, and the CO2 isosteric heat of adsorption and the CO2/N2 selectivity were calculated. Meanwhile, the influence of

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the micropore structure and N-doping on CO2 uptake as well as CO2/N2 selectivity was clarified in detail.

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2. MATERIALS AND METHODS 2.1. Materials. Cyanuric chloride (CC), ethylene diamine (EDA), diethylene triamine (DETA), triethylenetetramine (TETA), and p-phenylenediamine (p-PEDA) were purchased from Tokyo Chemical Industry (TCI), and they were used as received. Tetrahydrofuran (THF) and potassium hydroxide (KOH) were obtained from Gray West Chengdu Chemical Co. Ltd. All other agents were purchased from Yongda Chemical Company, and they were analytical agents and used without further purification. 2.2. Synthesis of the Triazine-Based Porous Organic Polymers. Scheme 1 shows the synthetic procedure for the triazine-based porous organic polymers, and the nucleophilic substitution reaction was performed between CC and different organic amine.28 In a typical process, the aminating agents (45 mmol) was dissolved in THF (50 mL), and adding NaOH as acid-binding agent (90 mmol) into the solution under N2 protection. CC (30 mmol) was also dissolved in THF (100 mL) and they were dropwise added into the flask under vigorously stirring. The obtained solution was then heated to reflux under stirring for 24 h. After cooling down to room temperature, the reaction mixture was filtered to obtain the solid products, and it was repeatedly washed with THF, deionized water, and ethanol until the filtrate became neutral, and dried in a vacuum oven at 60 °C for 48 h, and hence the triazine-based porous organic polymer was synthesized. As EDA, DETA, TETA, and p-PEDA were selected as the aminating agents, the corresponding polymer precursors were referred as TPOP-1, TPOP-2, TPOP-3, and TPOP-4, respectively. (Scheme 1 can be inserted here) 2.3. Preparation of the NPCs. 0.6 g as-prepared polymer precursors was firstly pre-carbonized at 200oC for 80 min in the air. After that, the product was immersed in

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30 mL 2 mol/L KOH-ethanol solution until ethanol was completely volatilized. The obtained dark orange residue was dried at 60oC under vacuum for 12 h, and then it was placed in a ceramic boat and inserted within a tubular furnace. The furnace was purged with N2 and heated to 500oC at a rate of 5oC/min, and retained at this temperature for 1 h. After cooling down to room temperature, the products were thoroughly washed with 3 mol/L HCl aqueous solution and deionized water until the filtrate was neutral. The obtained porous carbons were dried at 80oC under vacuum for 24 h. The achieved porous carbons were named NPC-1-500, NPC-2-500, NPC-3-500, and NPC-4-500 for TPOP-1, TPOP-2, TPOP-3, and TPOP-4, respectively. Correspondingly, the yield was 9.8%, 8.6%, 7.3% and 32.5%, respectively. 2.4. Characterization. Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Nicolet 6700 Fourier transform infrared spectrophotometer (Thermo Scientific Co., USA) with KBr in the range of 4000-400 cm-1. The pore structure of the samples was measured by N2 adsorption-desorption isotherms at 77 K by Micromeritics ASAP 2020 surface area and porosity analyzer. Before the measurement, the polymers were degassed at 90oC and the NPCs were degassed at 200oC under vacuum for 10 h. The surface area was calculated using the Brunauer-Emmett-Teller (BET) model ranged P/P0=0.05-0.30, the total pore volume (Vtotal) was calculated from the isotherm at P/P0=0.99, and the pore size distribution (PSD) was determined by the non-local density functional theory (NLDFT) method. The quantitative CHNS/O elemental analysis was detected using the elemental analyzer (EA, Vario Micro cube, Germany). Thermogravimetric analysis (TGA) was performed using a thermobalance (STA-499C, NETZSCH). The crystal structure of the samples was characterized by X-ray diffraction (XRD) performed on Rint-2000 diffractometer with Cu Kα radiation (λ=1.5418 Å (2θ=5-800)). The Raman spectra of

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the samples were carried out (Renishaw inVia Reflex Raman microscope with 532 nm, 50 mw laser excitation) to evaluate the bonding state. The X-ray photoelectron spectroscopy (XPS) spectra of the samples were collected on ESCALAB 250Xi by a monochromatized Al Kα X-ray source to analyze the surface chemical states. The morphology of the samples was characterized by a field-emission scanning electron microscope (SEM, Nova Nano SEM 230) operating at 10 kV and transmission electron microscopy (TEM, FEI Titan G2 60-300). High-angle annular dark field scanning TEM (HAADF-STEM) images and HAADF-STEM energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) maps were obtained from the TEM/STEM (FEI Titan G2 60-300, America), which includes corrector technologies enabling resolution of 80 pm. The strong basic exchange capacity of TPOPs is determined by titration method.29 2.5. Adsorption Experiments. The CO2 and N2 adsorption isotherms of the samples were measured using the Micromeritics ASAP 2020 analyzer at 273 and 298 K. Prior to the measurement, each sample was degassed at 90oC (the polymers) and 200oC (the NPCs) for 12 h to remove the guest molecules from the pores, and then the samples were cooled down to room temperature and followed by introduction of CO2 into the system. 3. RESULTS AND DISCUSSION 3.1. Synthesis of the Triazine-Based Porous Organic Polymers. Cyanuric chloride and the aminating agents including EDA, DETA, TETA, and p-PEDA were used as the raw materials, a simple nucleophilic substitution reaction was performed for the two reactants in THF, and hence the triazine-based porous organic polymers were easily synthesized. The synthetic procedure for the polymer precursors is very facile. EDA, DETA, and TETA are strong bases with certain volatility, and they can

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easily absorb water and CO2, which is not beneficial for formation of the polymers. p-PEDA can be easily oxidized in the air, making the yield of the product low. The polymers TPOP-1, TPOP-2, and TPOP-3 are white precipitates, while TPOP-4 is grey. These polymers are synthesized from the low-cost reactants with plentiful N under mild condition and not any catalysts are needed. These polymers were placed in acidic (1 mol/L of HCl), basic (1 mol/L of NaOH), water, and common organic solvents such as THF, dimethyl sulfoxide (DMSO), dichloromethane (DCM), CHCl3, and acetone for a week, it is highly stable. Particularly, these polymers can be used as the precursors for fabrication of the NPCs under KOH-assisting carbonization. 3.2. Textural Characterization of the Triazine-Based Porous Organic Polymers and NPCs. As can be seen from Figure 1 (a), the vibrational bands ascribed to the C=N stretching of the triazine rings are clearly observed at 1571 and 806 cm-1, respectively,30 while the peak related to the C-Cl stretching does not appear at 850 cm-1, which is accordant with the fact in Scheme 1 that the chlorine of cyanuric chloride is completely substituted by the amino groups of the aminating agents.31,32 Notably, the vibrational bands at 1423 and 2938 cm-1 for TPOP-1, TPOP-2, and TPOP-3 are from the sp3 -CH2- bending and C-H stretching, respectively, while these two bands are not present for TPOP-4. The TGA of TPOP-3 and TPOP-4 in Figure S1 of supporting information indicates that they are stable below 300oC. After the carbonization, most of the characteristic bands are greatly weakened (Figure 1 (b)), while another absorption band related to the polycyclic aromatics emerges at 1621 cm-1.33,34 (Figure 1 can be inserted here) The N content of TPOP-4 is determined to be 28.69 wt% by elemental analysis, corresponding to 0.02 mol/g, and indicative of its high hydrophility. The contact angle

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gives the same result (Figure S2, supporting information). Meanwhile, the weak basic exchange capacity is 1.49±0.04 mmol/g for TPOP-1, 2.17±0.03 mmol/g for TPOP-2, 3.25±0.02 mmol/g for TPOP-3, and 0.708±0.001 mmol/g for TPOP-4 by titration method.29 The SEM images of TPOPs are also shown in Figure S3 of supporting information. After KOH carbonization, there appears a sharp decrease of N content, and only 9.71 wt% of N content is measured for NPC-4-600. The XPS spectrum in Figure 2 (a) displays a similar N content (9.7 %). Of course, the N content of all NPCs in this study surpasses 8.4 wt% except NPC-4-700, and the NPC-2-500 has the highest one (11.33 wt%) (Table1). They can be considered as N-enriched porous carbons. The high-resolution N 1s spectrum in Figure 2 (b) can be divided into three representative bands, namely pyridinic, pyrrolic/pyridonic, and quaternary nitrogen atoms, with the peaks at 398.4, 400.2, and 401.5 eV, respectively.35-37 Moreover, the pyrrolic/pyridonic N is the dominant N species (59.1 %) along with 20.1 % of pyridinic N. Meanwhile, KOH is used as the activator during carbonization process, O content (Table S1) of all NPCs are calculated, and the high-resolution O 1s spectrum (Figure S4, supporting information) indicates the existence of three peaks that corresponded to oxide, -C=O and -C-OH appeared in NPC-4-600 with binding energies 531.0, 532.8 and 534.1 eV, respectively. More specifically, the considerable amount of O (10.51 % based on the survey of XPS) is doped into the framework of NPC-4-600, and the oxygen sites facilitate the dipole-quadruple interaction and/or hydrogen bonding with CO2, which may improve CO2 uptake and CO2/N2 selectivity.43 (Figure 2 and Table 1 can be inserted here) The Raman spectrum in Figure S5 of supporting information indicates that two peaks emerge at 1349 and 1593 cm-1, respectively, which can be assigned to D and G

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bands.38 The powder XRD analysis suggests its amorphous structure (Figure S6, supporting information), and the diffraction peaks at 26.4o and 18.6o imply a slight crystal structure. The SEM images in Figure 3 show that the carbons are composed of discrete particles with diverse morphology (bulk, sheet), and many nanoparticles agglomerate to form abundant pores with a rough surface. The TEM in Figure 4 confirms its dominant amorphous feature, and the carbon particles have an alternately dark and bright microstructure, implying its abundant porosity. HAADF-STEM-EDS maps studies indicate that NPC-4-600 contain uniformly distributed N and O content in the structure (Figure 4). (Figure 3 and Figure 4 can be inserted here) The N2 adsorption-desorption isotherms of the polymers at 77 K is a typical Type II character (Figure S7 (a), supporting information), indicative of the macroporous structure. The PSD curves in Figure S7 (b) of supporting information can give the same result. The polymers have a very low SBET ( NPC-2-500 (0.14 cm3/g) > NPC-3-500 (0.11 cm3/g) > NPC-1-500 (0.10 cm3/g). Moreover, as the carbonization temperature increases, the Vultra firstly increases and then decreases, while the Vultra/Vtotal and Vultra/Vmicro sharply decrease. This result suggests that a higher carbonization temperature does not favor formation of ultra micropores for the NPCs, although it can increase the SBET and Vtotal. El-Kaderi et al.42,43 also discussed the impact of temperature and KOH/polymer ratio on textural properties and chemical composition of porous carbons, and similar results are given in the literature. (Figure 5 can be inserted here) 3.3. CO2 Adsorption. The CO2 adsorption isotherms of the polymers as well as the NPCs were measured at 273 and 298 K with the relative pressure at 0-1.0 bar (Figure 6 and Figure S8 of supporting information), and Table 1 summarizes the CO2 uptake at 273 K and 0.15/1.0 bar. It is obvious that the polymers with a much lower SBET and

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Vtotal possess a much lower CO2 uptake (2.8-41 mg/g). The CO2 uptakes of the NPCs from different polymer precursors follow an order of NPC-4-500 (170 mg/g) > NPC-2-500 (145 mg/g) > NPC-3-500 (125 mg/g) > NPC-1-500 (120 mg/g), and NPC-4-500 exhibits the highest CO2 uptake. In addition, for the same polymer precursor of TPOP-4, with increasing the carbonization temperature from 500 to 700oC, the CO2 uptake firstly increases and then decreases, and NPC-4-600 with the carbonization temperature of 600oC has the highest CO2 uptake (207 mg/g). The CO2 uptake of NPC-4-600 is among the best previously described porous carbons (Table S2, supporting information). 1,44-53 (Figure 6 can be inserted here) CO2 uptake at 0.15 bar is also very important for practical application because this pressure is close to CO2 partial pressure in flue gas. At 273 K and 0.15 bar, it is observed that NPC-4-500 with BET surface area of 798 m2/g and N content of 9.05 wt% has the largest CO2 uptake (74 mg/g), while NPC-4-700 with the highest surface area of 1873 m2/g and the lowest N content of 5.56 wt% has the lowest CO2 uptake (45 mg/g). This result suggested that the N content is a predominant factor governing CO2 adsorption at a low pressure (0.15 bar). Similar results are reported by some other researchers.54 To understand the relationship between the CO2 uptake and the pore structure as well as chemical structure of the NPCs, the CO2 uptake of the NPCs at 273 K and 1.0 bar with the SBET, Vtotal, and Vmicro was firstly plotted. For most NPCs, Figure S9 of supporting information displays that the CO2 uptake increases almost linearly with increment of their SBET, Vtotal, and Vmicro with the exception of NPC-4-700. NPC-4-700 with a higher SBET, Vtotal, and Vmicro than NPC-4-600 has a relative lower CO2 uptake than the latter. Compared with the other samples, NPC-4-700 has a much lower N

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content (5.56 wt%). This suggests that the N content also influences the CO2 uptake in addition to the pore structure. The N-doping can greatly improve the surface polarity of the porous carbons, inducing an enhanced CO2 adsorption by acid-basic force and dipole-quadrupole interaction. Numerous computational studies have reported that N-doping into the internal walls of porous materials enhanced CO2 binding and could dramatically improve selective adsorption of CO2.40,55-57 The ultra micropores (d≤0.8 nm) are proven very important for CO2 capture. These ultra micropores have a strong affinity to CO2 since they have a deep well potential that soaks CO2.58 Thus, the CO2 uptake was considered as a function of the Vultra, and it is interesting that the CO2 uptake has the same tendency as their Vultra (Figure 7 (a)). That is, NPC-4-600 with the highest Vultra (0.20 cm3/g) exhibits the highest CO2 uptake (207 mg/g), while NPC-1-500 owning the lowest Vultra (0.10 cm3/g) has the lowest CO2 uptake (120 mg/g). In particular, plotting of the CO2 uptake of the four NPCs on the dependence of Vultra derived from the four polymer precursors (NPC-1-500, NPC-2-500, NPC-3-500, and NPC-4-500) has a very high correlation coefficient (R2=0.9958, Figure S10 of supporting information), implying that Vultra is a key factor to the CO2 uptake. Even if all NPCs were taken into consideration (Figure 7 (b)), the R2 is still acceptable (0.9118), which further confirms that the Vultra plays a critical role in the CO2 adsorption. Of course, the slightly lower R2 may be from the much lower N content of NPC-4-700 (5.56 wt%) due to the fact that the N content also plays a role in the CO2 adsorption. (Figure 7 can be inserted here) To clarify the interaction strength between CO2 and the polymers as well as the NPCs, the isosteric heat of adsorption (Qst) was calculated from the CO2 adsorption isotherms at 273 and 298 K (Figure S11, supporting information) on the basis of the

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Clausius-Clapeyron equation (the calculation is shown in the supporting information). As depicted in Figure S12 of supporting information and Figure 8, the Qst for the polymers are ranged from 45.4 to 74.7 kJ/mol at low loading, whereas those for the NPCs are in the range of 25.7-38.2 kJ/mol. The carbonization evidently reduce the Qst, suggesting the surface interactions between CO2 and the basic nitrogen functionalities was weaker due to the lower N content in NPCs (9.71 wt%) than TPOPs (28.69 wt%). The Qst of NPCs are below the energy of the chemical bonding formation (>40 kJ/mol), indicating a physical adsorption nature. For these TPOPs, the Qst decreases rapidly with increasing the CO2 uptake, implying the heterogeneity of the adsorbents. CO2 molecules prefer to be adsorbed on the stronger binding sites at first, which makes the Qst greater. The following CO2 molecules have to be adsorbed on the weaker binding sites at a higher CO2 loading, inducing a much less Qst. Relatively, the Qst of the NPCs are mild and then keep approximately equal at a higher CO2 loading, suggesting that the adsorption sites are relatively uniform with a homogenous surface. Additionally, with increasing the carbonization temperature, the Qst reduced at a higher CO2 loading, and NPC-4-700 had the least Qst among NPC-4-500, NPC-4-600, and NPC-4-700. This suggests the lack of strong interaction between CO2 and NPC-4-700, which may be from the lowest N content of NPC-4-700. The regenerability and cycle ability are also investigated by five adsorption-desorption cycles as displayed in Figure 9 and Figure S13 of supporting information. The results suggest these NPCs possessed excellent cycles use ability, no discoverable loss of CO2 adsorption capacity on NPC-4-(500/600/700) after five cycles, and relative mild conditions (degassed at 60 OC, 1 h) were executed for the regeneration. (Figure 8 and Figure 9can be inserted here) The N2 adsorption isotherms at 273 K of TPOPs and NPCs are shown in Figure S14

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of supporting information. The CO2/N2 selectivity acquired from Henry’s law and the ideal adsorption solution theory (IAST) was determined (the detail calculation and parameters are shown in the supporting information).59 Figure 10, Figure S15 of supporting information, and Table 1 indicate that the polymers exhibit a relatively higher CO2/N2 selectivity (Henry’s law: 25.5-47.6; IAST: 81.9-231.6) than the NPCs (Henry’s law: 14.3-16.8; IAST: 19.2-160.4), and the higher N content of the polymers should be the main reason. The discrepancy between the Henry’s law and IAST values can be attributed to the adsorption site heterogeneity.60 The CO2/N2 selectivity of TPOP-3 (Henry’s law: 47.6) is comparable to most other polymers such as DA-CMPs (44),61 SCMP@1-3 (24.8-35.7),62 MKPOP-2 (60),63 and HCP-Cl/(HCP-Cl-1/2) (35-61).64 After the carbonization, there appears a sharp drop of the CO2/N2 selectivity, which may be from the decreased CO2-philic sites (N), leading to the weaker interaction between CO2 and the NPCs. Certainly, the CO2/N2 selectivity of the NPCs is still acceptable, and superior to some other porous carbons such as NGA (12),65 PC (12.6),66 and CPC-700 (16).42 (Figure 10 can be inserted here) It is reported that the CO2/N2 selectivity strongly depends on Vultra and N content, and increasing the accessible Vultra and N incorporation are generally the two effective strategies to further enhance the CO2/N2 selectivity of the carbon materials.1,67 However, the influence of N incorporation on the CO2/N2 selectivity is a controversial topic with conflicting conclusions reported by various studies.47,68 To expound the possible parameters that influence the CO2/N2 selectivity, the relationship of the CO2/N2 selectivity based on the Henry’s law with Vultra and N content was plotted and the results are shown in Figure S16 of supporting information. What is interesting is that the CO2/N2 selectivity of the NPCs has a similar trend with Vultra except

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NPC-1-500 and NPC-2-500 (Figure S16 (a), supporting information), implying that a higher Vultra will result in a higher CO2/N2 selectivity. NPC-2-500 with a relative lower Vultra than NPC-4-600 has a higher CO2/N2 selectivity (16.8) than the latter. NPC-2-500 owns the highest N content among all NPCs, implies that the N content should also have a positive effect on the CO2/N2 selectivity. Figure S16 (b) of supporting information indicates that the CO2/N2 selectivity has an increased trend with increasing the N content except NPC-1-500, and plotting of the CO2/N2 selectivity as a function of the N content on all NPCs has an acceptable R2 (0.8705, Figure 11 (a)), which confirms that the N functionality plays an important role in the CO2/N2 selectivity. The pyrrolic/pyridonic N is the dominant nitrogen species (59.1%) along with 20.1% pyridinic N, and hence both pyrrolic/pyridonic and pyridinic N is responsible for the CO2/N2 selectivity. Meanwhile, for the NPCs from the same polymer precursors with different carbonization temperature (NPC-4-500, NPC-4-600, and NPC-4-700), Figure 11 (b) indicates that the CO2/N2 selectivity is not only relevant with Vultra, but also the N content, and R2 from the two linear regressions are very high (0.9992 and 0.9883). Within the ultra micropores, the CO2 adsorption potential energy is dramatically enhanced due to the superposition of the strengthened van der Waals force fields from the narrowing of the adjacent pore walls,69 and the higher N content takes effect by acid-basic force and dipole-quadrupole interaction. (Figure 11 can be inserted here) 4. CONCLUSIONS In summary, we reported a facile method for the preparation of a series of NPCs from the triazine-based porous organic polymers. The NPCs developed in this study possess adjustable porous structure and high N-doping content via controlling the polymer precursors and carbonization temperature. The porous carbon namely NPC-4-600 has

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the BET surface area of 1518 m2/g, pore volume of 1.11 cm3/g, and ultra micropore volume of 0.20 cm3/g, and the N content of 9.71 wt%, it exhibits high CO2 uptake of 207 mg/g, and the CO2/N2 selectivity arrives at 34.5 at 273 K and 1.0 bar. Moreover, the ultra micropore volume is more dominant on the CO2 uptake than the N content, and the CO2/N2 selectivity mainly depends on many factors, especially ultra micropore volume as well as the N content in our case. The present NPCs may provide promising candidates for CO2 capture. ACKNOWLEDGMENTS The National Natural Science Foundation of China (Nos. 21636010 and 51673216) and the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007) are gratefully acknowledged for the financial supports. ASSOCIATED CONTENT SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publication website. Additional tables (Table S1-S3) and figures (Figure S1 to S16) as described in the text, including TGA curves, Contact angle images, Raman spectra, X-ray diffraction, N2 adsorption-desorption isotherms of polymers, CO2 adsorption isotherms of polymers and NPCs, the relationship between CO2 adsorption and pore parameters, Qst of polymers, N2 adsorption isotherms on polymers and NPCs at 273 K, CO2/N2 selectivity by IAST model of polymers, Textual properties and CO2 capture performance of various adsorbents (PDF). REFERENCES (1) To, J.W.; He, J.J.; Mei, J.G.; Haghpanah, R.; Chen, Z.; Kurosawa, T.; Chen, S.C.; Wilcox, J.; Bao, Z.N. Hierarchical N-Doped Carbon as CO2 Adsorbent with High CO2 Selectivity from Rationally Designed Polypyrrole Precursor. J. Am. Chem.

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Page 28 of 43

Scheme 1 Preparation procedure of the triazine-based porous organic polymers and NPCs. NH N N H

NH2

H2N

N N

TPOP-1

N H

HN

H2N

Cl N Cl

N N

TPOP-2

NH

H N

N

H N

NH2

N H

THF

+

N N

H N

N H

500~700 0C

Reflux

Cl

2 eqKOH, 1 h

NH H N

H2N

N H

NH2

0.5 µm

HN TPOP-3

NH N H H2N

NPC-4-600

N

H N

N H

N N

H N

N H

NH2 TPOP-4 HN N N H

N N

N H

28


N H

Page 29 of 43 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


Table 1 Textural parameters and CO2 adsorption performance of the triazine-based porous organic polymers and NPCs. Sultrac SBET a

Smicro

Vultrac Vtotal d

b

CO2 uptake

Vmicrob

1.0 bar

e

N2

Henry’s

uptake

law

N IAST

0.15 bar e

SCO2/N2f

content SCO2/N2g

h

TPOP-1

2

0

0

0

0

0

2.8

1

/

/

/

/

TPOP-2

35

0

0

0.13

0

0

32

13

0.8

33.9

81.9

/

TPOP-3

36

0

0

0.14

0

0

41

18

0.8

47.6

231.6

/

TPOP-4

33

0

0

0.14

0

0

35

16

1.2

25.5

86.1

28.69

NPC-1-500

486

229

89

0.42

0.19

0.10

120

48

6.0

16.8

64.3

9.33

NPC-2-500

650

455

275

0.53

0.26

0.14

145

62

7.4

16.8

37.6

11.33

NPC-3-500

394

229

147

0.27

0.16

0.11

125

61

5.9

15.6

160.4

8.48

NPC-4-500

798

282

53

0.54

0.34

0.19

170

74

8.9

15.9

67.8

9.05

NPC-4-600

1518

825

342

1.11

0.54

0.20

207

70

11.0

16.6

34.5

9.71

NPC-4-700

1873

685

106

1.56

0.56

0.15

167

45

9.4

14.3

19.2

5.56

29


Industrial & Engineering Chemistry Research 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

a

Calculated using the BET model with the unit of m2/g.

b

Cumulative micropore volume or micropore area with the pore width less than 2 nm using NLDFT model with the unit of cm3/g or m2/g.

c

Cumulative ultra micropore volume or micropore area with the pore width less than 0.8 nm using NLDFT model with the unit of cm3/g or m2/g.

d

Calculated at P/P0=0.99 with the unit of cm3/g.

e

CO2 or N2 uptake for the isotherms with the unit of mg/g at 273 K and 1.0/0.15 bar.

f

Henry’s law for CO2/N2 selectivity at 273 K and 1.0 bar.

g

IAST CO2/N2 selectivity for the mixture including 85 wt% of N2 and 15 wt% at 273 K and 1.0 bar.

h

Determined using the elemental analyzer in (wt%).

30


Page 30 of 43

Page 31 of 43

Figure 1 FT-IR spectra of (a) the triazine-based porous organic polymers; and (b) NPCs.

1571cm

(a)

-1 -1

1423cm -1 806cm Transmittance (%)

TPOP-4 TPOP-3 TPOP-2

TPOP-1

4000 3500 3000 2500 2000 1500 1000 500 -1

Wavenumbers (cm )

(b) -1

1072cm Transmittance (%)

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


NPC-4-500 NPC-3-500 NPC-2-500 NPC-1-500 -1

1621cm

4000 3500 3000 2500 2000 1500 1000 -1

Wavenumbers (cm )

31


500


Figure 2 XPS spectra of NPC-4-600 (a) Survey, and (b) N 1s.

(a)

Intensity (a.u.)

C 1s

O 1s N 1s

0

200

400

600

800

Binding energy (eV)

(b)

Intensity (a.u.)

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

Page 32 of 43

Pyrrolic/Pyridonic N 400.2 eV

Pyridinic N 398.4 eV

Quaternary N 401.5 eV

392

396

400

404

Binding energy (eV)

32


408

412

Page 33 of 43 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


Figure 3 SEM images of NPC-4-600.

33



Figure 4 (a-c) The TEM images (d) The HAADF-STEM image, (e–h) HAADF-STEM-EDS maps of NPC-4-600.

34


Page 34 of 43

Page 35 of 43

Figure 5 (a) N2 adsorption-desorption isotherms at 77 K; and (b) pore size distribution of the NPCs (determined by NLDFT method using a carbon slit pore model).

NPC-1-500 NPC-2-500 NPC-3-500 NPC-4-500 NPC-4-600 NPC-4-700

1200 1000 800

(a)

600 400 200 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Differential pore volume (cm3/g)

Volume adsorbed (cm3/g STP)

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


(b) NPC-4-700 NPC-4-600 NPC-4-500 NPC-3-500 NPC-2-500 NPC-1-500 10

100 Pore size (Å)

35



Figure 6 CO2 adsorption isotherms of the NPCs at 273 K (a) and 298 K (b) with the relative pressure at 0-1.0 bar.

250

CO 2 uptake (mg/g)

200 150


(a)

100 50 0 0.0

160 140 CO 2 uptake (mg/g)

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

Page 36 of 43

120 100

0.2

0.4 0.6 0.8 Pressure (bar)

1.0


(b)

0.2

1.0

80 60 40 20 0 0.0

0.4 0.6 0.8 Pressure (bar)

36


Page 37 of 43

Figure 7 (a) The relationship of CO2 uptake on the NPCs at 273 K and 1.0 bar with the Vultra (b) Plotting of CO2 uptake on the NPCs as a function of the Vultra.

220 (a) 200 180

0.16

3

Vultra /(cm /g)

0.18

160

0.14 0.12

140

0.10

120 NPC-3-500 NPC-1-500 NPC-2-500 NPC-4-500 NPC-4-600 NPC-4-700

220 (b) 200 180 160 140 2

y=746.1x+44.85, R =0.9118 120 100

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 3

Vultra (cm /g)

37


CO2 uptake /(mg/g)

0.20

CO2 uptake (mg/g)

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



Figure 8 Qst of CO2 of NPCs as the function of the CO2 uptake.

40

30 Qst (KJ/mol)

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

Page 38 of 43

20

10

0

NPC-1-500 NPC-2-500 NPC-3-500 0

20

40

60

80

NPC-4-500 NPC-4-600 NPC-4-700 100

CO2 uptake (mg/g)

38


120

140

Page 39 of 43

Figure 9. Recycling performance of NPC-4-600 for CO2 uptake at 273 K and 0-1.0 bar.

200 160 CO2 uptake (mg/g)

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


120 80 40 0 1

2

3 4 Number of cycles

39


5


Figure 10 CO2/N2 selectivity by IAST model of the NPCs at 273 K.

180 160 140 Selectivity

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

Page 40 of 43

120 100


80 60 40 20 0 0.0

0.2

0.4 0.6 Pressure (bar)

40


0.8

1.0

Page 41 of 43

Figure 11 (a) Plotting of CO2/N2 selectivity as a function of (a) Vultra for the NPCs at 273 K and 1.0 bar; and (b) Vultra and N content for NPC-4-500, NPC-4-600, and NPC-4-700 at 273 K and 1.0 bar.

17.5 (a)

CO2/N2 selectivity

17.0 16.5 16.0 15.5 2

y=11.76x+0.4754, R =0.8705

15.0 14.5 14.0

5

6

7

8

9

10

11

12

3

Vultra/(cm /g)

3

Vultra /(cm /g) 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 17.5 17.0 (b) 16.5 CO2/N2 selectivity

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


16.0

2

y=7.90x=42.5, R =0.9992

15.5 15.0 14.5 14.0

2

y=11.36x+0.523, R =0.9883

13.5 13.0

0

1

2

3

4

5

6

7

N Content (wt%)

41


8

9

10


Graphical abstract

NPC-4-600

0.5 µm

3

Vultra /(cm /g) 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 17.5 17.0 (b)

220 (a) 200

16.5 180

CO2/N2 selectivity

CO 2 uptake (mg/g)

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

Page 42 of 43

160 140

2

y=746.1x+44.85, R =0.9118 120

16.0

2

y=7.90x=42.5, R =0.9992

15.5 15.0 14.5 14.0

2

y=11.36x+0.523, R =0.9883

13.5 100

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

13.0

0

1

2

3

3

4

5

6

7

N Content (wt%)

Vultra (cm /g)

42


8

9

10

Page 43 of 43

Graphical abstract

NPC-4-600

0.5 μm

3

Vultra /(cm /g) 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 17.5 17.0 (b)

220 (a) 200 180 160 140

2

y=746.1x+44.85, R =0.9118

CO2/N2 selectivity

16.5 CO 2 uptake (mg/g)

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


120

16.0

2

y=7.90x=42.5, R =0.9992

15.5 15.0 14.5 14.0

2

y=11.36x+0.523, R =0.9883

13.5 100

13.0

0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

0

1

2

3

3

4

5

6

7

N Content (wt%)

V ultra (cm /g)

1


8

9

10

Synthesis of Triazine-Based Porous Organic Polymers Derived N

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