Troger's base functionalized covalent triazine frameworks for CO₂ capture

Abstract

A series of covalent triazine frameworks containing Troger's base segments (CTF-TBs) were designed and prepared by dynamic trimerization under ionothermal conditions, based on the novel dicyano monomer of dicyano Troger's base (DCTB). The CTF-TBs were all amorphous and exhibited surface areas between 294 to 732 m² g⁻¹. The effect of reaction conditions including ratio of catalyst/monomer and temperature on the pore properties was discussed. Meanwhile, the adsorption capacity of CO₂ for all the CTF-TBs was studied and the CO₂ uptake capacity was up to 16.84 wt% at 273 K and 1.10 bar for CTF-TB-3.

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Introduction

Microporous organic polymers (MOPs) are promising candidates for carbon capture and storage (CCS).^1–3 Recently, due to the urgent demand for highly efficient CO₂ adsorption materials with low density and high thermal stability, many CO₂-philic functional groups, such as azo,^4–6 Troger's base (TB),^7–12 benzimidazole,^13–15 amino,¹⁶ carbazole,¹⁷ and porphyrin,¹⁸ have been incorporated into MOPs. Most of the CO₂-philic groups contain heteroatoms (e.g. nitrogen, phosphorus, and sulfur), which can form Lewis acid–base interactions between the heteroatoms and CO₂ molecules. TB segments with double tertiary N atoms have also been designed as active sites for catalysis in organic synthesis,^12,19,20 selective adsorption of CO₂,^7,8 and gas separation.^10,21–25 In general, there are two pathways reported for the introduction of TB segments. One is the polymerization of monomers containing TBs. For example, conjugated microporous polymers and microporous polyimides have been prepared using the monomers of diiodotroger's base and diaminotroger's base, respectively.^19,26 While in the other pathway, TB segments are formed directly during the polymerizations, for instance, polymers with intrinsic microporous (PIM-EA-TB) prepared by N. B. McKeown et al.²⁵ and 3D microporous polymers (TB-MOP) prepared by S. Dai and coworkers.⁷ In these MOPs, TB segments have enhanced their CO₂ uptakes effectively, proved the TBs are promising candidates in design of MOPs for CO₂ adsorption.

On the other hand, covalent triazine frameworks (CTFs) have high nitrogen content and high surface areas, and their preparation procedure is adapted to a large scale.^27–32 Nowadays, A. Thomas et al. developed a novel two-step procedure to prepare CTFs, in which an amorphous, non-porous polymer network was converted into an ordered, microporous covalent organic framework under ionothermal conditions.³³ This method makes the CTF synthesis easier, safer and scalable. Although with high nitrogen loading of about 21.8 wt%, the CO₂ uptake of CTF-1 derived from 1,4-dicyanobenzene (DCB) is not so high. Further modification of CTFs by other CO₂-philic groups, e.g. F atoms,³⁴ pyridine groups,³⁵ and benzimidazole groups,³⁶ have been proved their effectiveness in enhancement of CO₂ adsorption.

In the present work, we combined CTF and TB together to prepare MOPs with high CO₂ adsorptions. CTFs with TB segments are designed and prepared by the ionothermal polymerization procedure. We expect that the CO₂-philic TB segments can enhance the binding affinity between the CTF-TBs and CO₂, and thus increase the CO₂ adsorption. The synthesis and properties of the CTF-TBs, the effect of reaction conditions on the porosity, CO₂ adsorption ability and the selectivity of CO₂/N₂, have been studied in detail.

Experimental

Materials

4-Cyanoaniline, trifluoroacetic acid (TFA) and trifluorosulfonic acid (TFSA) were purchased from J&K Scientific Ltd. (Beijing) and used without further purification. Paraformaldehyde was purchased from Sinopharm Chemical Reagent Co., Ltd. and used directly. DCTB was synthesized in our laboratory according to the literature.^26,37 Zinc chloride (ZnCl₂, anhydrous, 98%) was purchased from J&K Scientific Ltd. (Beijing), stored in a glove box and used as received.

Characterization

Fourier transform infrared (FT-IR) spectra were recorded on IFS 66v/S Fourier transform spectrophotometer. The powder wide-angle X-ray diffraction (XRD) was conducted on an X'Pert PRO X-ray diffractometer (PANalytical) with Cu/K-α1 radiation, operated at 60 kV and 55 mA. Thermogravimetric analysis (TGA) was recorded on a Netzsch thermal analysis system (STA 449C), in nitrogen, at a heating rate of 10 °C min⁻¹. Solid-state ¹³C NMR spectra were performed on a Bruker Avance III Spectrometer operating at a frequency of 100 MHz and using cross-polarization/magic angle spinning (CP/MAS). The magic angle spinning rate was set at 5.0 kHz to minimize spinning sideband overlap. Combustion elemental analysis was conducted on Vario EL III Elemental Analyzer System. X-ray photoelectron spectroscopy (XPS) was performed on ESCALAB 250Xi (ThermoFisher Scientific). SEM images were obtained with a field emission scanning electron microscope (JSM-5600LV, JEOL, Japan). The powder samples were coated with gold by sputtering prior to observation. TEM images were obtained with A JEM-2010 transmission electron microscopy (TEM), operating at an accelerating voltage of 200 kV. Nitrogen sorption experiments and micropore analysis were conducted at 77.3 K using Micromeritics ASAP 2020 HD88. Before sorption measurements, the samples were degassed in vacuum overnight at 150 °C. The surface areas were calculated from multipoint BET plot, and the pore volume was determined by nonlocal density functional theory (NLDFT). Carbon dioxide and nitrogen sorption isotherms at 273 K and 303 K were all obtained with a Micromeritics ASAP 2020 HD88 analyzer at the required temperature. The temperature during adsorption and desorption was kept constant using a circulator. Before the adsorption measurements, the samples were activated in situ by increasing the temperature to 150 °C at a heating rate of 10 °C min⁻¹ under vacuum and the temperature and vacuum was maintained for 5 hours.

Synthesis of 2,8-dicyano-6H,12H-5,11-methanodibenzo[1,5]diazocine (DCTB)

In a 250 mL three-necked flask with a magnetic stir and N₂ inlet, 5.91 g (50 mmol) 4-cyanoaniline and 3.15 g (105 mmol, 2.1 equiv.) paraformaldehyde were dissolved in 143 g (1.25 mol, 25 equiv.) trifluoroacetic acid (TFA), under ice-bath. The reaction mixture turned dark-brown gradually and was stirred for three days at nitrogen atmosphere under room temperature. Then the mixture was poured into 200 mL pure water under continuously stirring giving a light yellow precipitate. KOH solution (6 mol L⁻¹) was gradually added to this suspension adjusting pH to 8.5–9. The precipitate was filtered off and washed several times to neutral. The crude product was then purified by column chromatography with hexane/AcOEt to give light yellow powders 3.4 g, yield 50%.

Synthesis of the CTF-TBs

The CTF-TBs were synthesized according to a reported ionothermal procedure.²⁷ The synthesis of CTF-TB-1 was given as an example (Scheme 1). The DCTB monomer (136.2 mg, 0.50 mmol) and ZnCl₂ (68.2 mg, 0.50 mmol) were first mixed well in agate mortar in glove box and then transferred into a pyrex ampoule (3 × 18 cm, about 25 mL). The ampoule was evacuated to vacuum, sealed and heated to 400 °C in a muffle furnace and kept there for 40 h. The ampoule was then cooled down to room temperature and opened carefully. The black reaction mixture was subsequently grounded in an agate mortar and then washed thoroughly with diluted HCl to remove most of the ZnCl₂. Then the powder was extracted in Soxhlet extractor by deioned water and THF for 24 h each and drying in vacuum at 150 °C. The yield was about 82%. The other CTF-TBs were synthesized in a similar procedure, while the reaction conditions, including the molar ratio of monomer to ZnCl₂ and reaction temperatures, were set according to Table 1.

Scheme 1 Schematic representation of the synthesis of CTF-TBs.

Table 1 Overview of the reaction conditions

Polymer code	DCTB (mmol)	ZnCl2 (mmol)	Molar ratio of DCTB/ZnCl2	Reaction conditions	Yield (%)
CTF-TB-1	0.50	0.50	1 : 1	400 °C/40 h	82
CTF-TB-2	0.50	1.0	1 : 2	400 °C/40 h	80
CTF-TB-3	0.50	2.5	1 : 5	400 °C/40 h	80
CTF-TB-4	0.50	5.0	1 : 10	400 °C/40 h	84
CTF-TB-5	0.50	10.0	1 : 20	400 °C/40 h	84
CTF-TB-6	0.50	2.5	1 : 5	350 °C/40 h	84
CTF-TB-7	0.50	2.5	1 : 5	450 °C/40 h	74
CTF-TB-8	0.50	2.5	1 : 5	500 °C/40 h	71
CTF-TB-9	0.50	2.5	1 : 5	550 °C/40 h	65

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In comparison, the super acid TFSA catalyzed polymerization of DCTB in solution was also performed as following. In a 50 mL round bottom flask equipped with a magnetic stir, 20 mL of anhydrous DCM and DCTB (272.3 mg, 1.0 mmol) were added. Under ice bath, TFSA (1.5 g, 10 mmol) was dropped into the mixture slowly. The color turned to deep red finally and some brownish powders precipitated. The mixture was stirred at room temperature for 3 days and it was poured into methanol. The brown powders were collected by filtration and washed several times to neutral and drying in vacuum at 150 °C. The sample was denoted as CTF-TB-S.

Results and discussion

Synthesis and characterization of the CTF-TBs

The monomer DCTB was synthesized by the dimerization of 4-cyanoaniline catalyzed by TFA with a good yield. In the FT-IR spectra of DCTB and 4-cyanoaniline (Fig. S1†), the absorption bands at 3375 cm⁻¹ and 3488 cm⁻¹ disappeared, indicating the aminos had been consumed. The new absorption bands at around 2813 cm⁻¹ and 2892 cm⁻¹ could be assigned to the methylene groups in TB segments.⁷ Meanwhile, the characteristic absorption bands of nitrile groups can also be seen at 2226 cm⁻¹. In the TGA curve (Fig. S4†) of DCTB under N₂, the first/main weight loss at about 350 °C indicating the decomposition of TB segments, in well accordance with the endo thermal peaks shown in the DSC curve at the same temperature range (Fig. S5†).

The CTF-TBs were synthesized by ionothermal polymerization in sealed tube. In order to find the relationship between the reaction conditions and the porosity properties, a set of reaction conditions was listed in Table 1. The molar ratios of DCTB to ZnCl₂ were set as 1 : 1, 1 : 2, 1 : 5, 1 : 10, and 1 : 20, respectively. The reaction temperature was set as 350 °C, 400 °C, 450 °C, 500 °C and 550 °C. All the reaction time was set as 40 hours. In all cases, after polymerization, the light yellow powdery reactants turned black dust in the sealed glass tube. For CTF-TB-1 to CTF-TB-6, there were no positive pressures when the sealed tubes were opened. In comparison, for CTF-TB-7, CTF-TB-8 and CTF-TB-9 reacted at 450 °C, 500 °C and 550 °C, there was obviously positive pressure, which should be paid more attention. All black dusts were grounded in agate mortar to get fine powders. The black powder were all washed and extracted thoroughly to remove the residual ZnCl₂. The final yields were in the range of 65–84%, which were a little lower than the reported results on CTF-1. This can be mainly attributed to the TB segments, which were relatively fragile to thermal treatment, especially at elevate temperatures (Fig. S4†). For comparison, the polymerization catalyzed by TFSA in solution was also carried out at room temperature. The formation of CTF-TBs was characterized by FT-IR, PXRD, solid-state ¹³C NMR spectra, and thermogravimetric analysis (TGA).

Representative FT-IR spectra of polymer (CTF-TB-3, CTF-TB-S) and DCTB were shown in Fig. 1a. After polymerization, the nitrile groups at about 2226 cm⁻¹ were totally disappeared, indicating a high degree of reaction. The characteristic peaks of triazine rings were not very strong, which were located at 1430 cm⁻¹ and 870 cm⁻¹, may be covered by the nearby peaks. At the same time, maybe because the TB segments were fragile to high temperature, the characteristic peaks at around 2800 cm⁻¹ were very weak. The adsorption peaks of aliphatic C–H vibrations were all relatively weak in the other CTF-TBs (Fig. S2 and S3†). As the reaction temperature rose up to 550 °C, the adsorption peaks of TB and triazine segments became even weaker (Fig. S3†), due to the partially decomposition or fragmentation of both TB segments and triazine groups.^38,39 In the solid-state ¹³C NMR spectra shown in Fig. 1b, the chemical signals located at 107.5 ppm should be assigned to the asymmetric nitrile carbons,^40,41 which almost disappeared completely after reaction. The nitriles first trimerized to triazine groups and then partially decomposed under high temperatures. Accordingly, the peaks of TB segments disappeared completely after reaction due to the fragile aliphatic methylene, in well accordance with the results by FT-IR. CTF-TB-7 exhibited remarkable thermal stability up to 800 °C in N₂ (Fig. S4†). The initial weight loss before 100 °C could be attributed to the absorbed moisture. The slightly weight loss from 300 °C should be attributed to the further degradation of organic frameworks. Therefore, they have similar thermal stability compared with the reported MOPs with TB segments.^7,8,24 There was no observable glass transition temperature within the range of 30–500 °C, except the endothermal peak according to the evaporation of adsorbed moisture below 100 °C. The PXRD patterns of CTF-TB-3 and CTF-TB-7 (Fig. S6†) indicated no long-range structures. They were all amorphous with broad diffraction peaks at about 24.8°, which was in well accordance with the reported MOPs containing TB segments.^8,22,26 The morphology was also characterized by FESEM as shown in Fig. 1c and S7,† which revealed aggregated particles. In the HR-TEM images shown in Fig. 1d and S8,† the alternating areas of light and dark contrast revealed their disordered porous structural natures.

Fig. 1 (a) FT-IR spectra of the monomer DCTB, CTF-TB-S and CTF-TB-3 obtained by KBr pellets, in the range of 4000–400 cm⁻¹; (b) solid-state ¹³C NMR spectra of DCTB, CTF-TB-3, and CTF-TB-7 (100 MHz); (c) FESEM image of CTF-TB-3; (d) HR-TEM of CTF-TB-3.

Porosity properties and gas uptake capacities of the CTF-TBs

The porous character of the CTF-TBs was studied by nitrogen adsorption–desorption experiments at 77.3 K. As shown in Fig. 2a, d, S9, S10 and S12,† except CTF-TB-S and CTF-TB-6, all the isotherms showed a steep increase in adsorbed volume at low relative pressure, suggesting that these CTF-TBs were microporous materials. All the nine CTF-TBs show type I N₂ sorption isotherms with Brunauer–Emmett–Teller (BET) surface areas ranging from 294 to 731 m² g⁻¹ (Table 2). The Langmuir surface areas of CTF-TBs were varied from 431 to 1077 m² g⁻¹. None of them showed remarkable hysteresis at the low pressure region in the N₂ isotherms. Therefore, the dominating pores were micropores, which were in well accordance to the pore size distribution as shown in Fig. 2c, f, S11, and S13.† For CTF-TB-S obtained in the solution polymerization, although the reaction extent was high according to the FT-IR analysis, the surface area was very small (Fig. S9†). Compared with other CTFs catalyzed by TFSA, CTF-TB-S showed much lower surface area, maybe due to the formation of oligomers with low molecular weights.

Fig. 2 Porosity properties of CTF-TBs. (a) Nitrogen adsorption and desorption isotherms at 77.3 K of CTF-TB-3; (b) the influence of molar ratio (DCTB to ZnCl₂) on the surface areas; (c) pore size distributions (PSD) for CTF-TB-3 calculated by the NLDFT method; (d) nitrogen adsorption and desorption isotherms at 77.3 K of CTF-TB-7; (e) the influence of reaction temperature on the surface areas; (f) pore size distributions (PSD) for CTF-TB-7 calculated by the NLDFT method.

Table 2 Porosity data for the CTF-TBs from N₂ isotherms collected at 77.3 K

Polymer code	SABET^a [m^2 g^−1]	SALang^b [m^2 g^−1]	SAmicro^c [m^2 g^−1]	SAext^d [m^2 g^−1]	V0.1^e [m^3 g^−1]	Vtot^f [m^3 g^−1]	V0.1/Vtot
a BET surface area calculated over the pressure range 0.05–0.30 P/P0 at 77.3 K.b Langmuir specific surface area calculated from the nitrogen adsorption isotherm by application of the Langmuir equation.c Micropore surface area calculated from the nitrogen adsorption isotherm using the t-plot method.d The external surface areas calculated using the t-plot method based on the Halsey thickness equation.e V0.1, pore volume at P/P0 = 0.1 at 77.3 K.f Vtot, total pore volume calculated at P/P0 = 0.99 at 77.3 K.g Not detected any reasonable data.
CTF-TB-1	294	431	243	51	0.146	0.155	0.942
CTF-TB-2	446	653	302	144	0.212	0.244	0.869
CTF-TB-3	612	901	338	274	0.283	0.349	0.811
CTF-TB-4	581	858	293	288	0.265	0.329	0.805
CTF-TB-5	495	721	257	238	0.225	0.295	0.763
CTF-TB-6	10	17	17	—^g	0.007	0.009	0.778
CTF-TB-7	731	1077	488	244	0.350	0.415	0.843
CTF-TB-8	689	1014	486	203	0.332	0.373	0.890
CTF-TB-9	625	919	424	202	0.298	0.343	0.868
CTF-TB-S	11	16	4	—^g	—^g	—^g	—^g

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The influence of the amount of ZnCl₂, as well as reaction temperatures on the properties of the materials was investigated. First, the influence of the DCTB/ZnCl₂ ratio was investigated under standard conditions at 400 °C. It was found that the amount of ZnCl₂ had obvious effect on the pore properties of CTF-TBs, because it played the role of both solvent and catalyst in the reaction. When the ratios increased from 1 : 1 to 1 : 20, the BET surface areas first increased and then decreased gradually, as shown in Fig. 2b, which was in good agreement with that of dicyanobisphenyl (DCBP) and dicyanobenzimidazole (DCBI) under similar conditions.^36,42 For CTF-TB-3, where the ratio of DCTB to ZnCl₂ was 1 : 5, the BET surface area was 612 m² g⁻¹. This value was much lower than that of CTF-1 derived from 1,4-dicyanobenzene (DCB, 1123 m² g⁻¹) and dicyanobenzimidazole (DCBI, 1025 m² g⁻¹),^27,36 under similar conditions. The main reason should be the serious decomposition of the unstable TBs under high temperatures. Further increase of ZnCl₂ amount lead to decrease in BET surface areas may be due to the dilutive effect of excess amount of solvents.

It was revealed that higher reaction temperatures generate mesoporosity in addition to microporosity.²⁷ Therefore, a series of temperatures from 350 °C to 550 °C was checked. When reaction temperatures increased, the BET surface areas first increased gradually and then decreased slightly as shown in Fig. 2e. For CTF-TB-7 obtained at 450 °C, the BET surface area was higher than that of CTF-TB-3 (731 m² g⁻¹ vs. 612 m² g⁻¹). However, compared with the reported CTFs with high surface areas, for example, CTF-0 derived from 1,3,5-tricyanobenzene (TCB, 2011 m² g⁻¹) and CTF-1 derived from DCB (1750 m² g⁻¹) under similar conditions,^28,43 the present value was much lower. Furthermore, higher temperatures lead to some side reactions such as carbonizations, decompositions and ring fragmentations, as evidenced by FT-IR and solid-state ¹³C NMR analysis. In the present system, both the triazine rings and the TBs could decompose under higher temperatures to a relatively high extent, leading to partially collapse of pores. The obviously different tendency of BET surface areas vs. reaction temperatures can be mainly ascribed to the relatively lower thermal stability of TB segments, compared with benzene rings in CTF-1. According to results obtained by A. Thomas, the mesoporous pores mainly derived from a retrotrimerization/recombination pathway.²⁸ If the TB segments were destroyed by the high temperatures, they cannot reorganize to form porous structures, thus leading to decrease in surface areas.

The gas storage properties of the CTF-TBs were tested. There are two kinds of CO₂-philic groups in the CTF-TBs, e.g., the TB segments which contained tertiary amine and the triazine groups. Therefore, it can be predicted that the CTF-TBs would have high CO₂ adsorption ability. We tested the CO₂ adsorption properties at 273 K and a pressure up to 1.10 bar (Fig. 3a and b). The adsorption data were summarized in Table 3, from which we can see that all CTF-TBs showed moderate to high capacity of CO₂ adsorption, consistent with our expecting. The CO₂ adsorptions for all CTF-TBs ranged from 10.67 wt% to 16.84 wt% at 273 K and 1.10 bar. However, the CO₂ adsorptions were not in good agreement with their BET surface areas, which was quite different from the results obtained in the CTF-BIs. CTF-TB-3 had the highest CO₂ adsorption amount at 273 K and 1.10 bar (about 16.84 wt%). This adsorption amount was in the range of the highest values observed for CTFs under similar conditions. For example, CTF-1 (10.87 wt%, 746 m² g⁻¹) and CTF-1-600 (16.81 wt%, 1553 m² g⁻¹) based on DCB,³⁴ CTF-P6M (18.35 wt%, 947 m² g⁻¹) prepared by microwave method,²⁹ MCTF@500 (13.90 wt%, 640 m² g⁻¹) containing metalloporphyrin segments,⁴⁴ PCTF-1 (14.21 wt%, 2235 m² g⁻¹) derived from tetranitrile containing tetraphenyl ethylene segments,⁴⁵ and TPI-1 (10.78 wt%, 809 m² g⁻¹) containing imide groups,⁴⁶ etc. Compared with the above mentioned CTFs, the present values showed comparable or better adsorption ability with relatively lower BET surface areas (CTF-TB-3, 16.2 wt% at 1.0 bar, 612 m² g⁻¹), which can be ascribed to the introduction of CO₂-philic TB segments into the frameworks. However, compared with the recently reported FCTF-1-600 (24.33 wt%, 1535 m² g⁻¹),³⁴ bipy-CTF600 (24.55 wt%, 2479 m² g⁻¹),³⁵ and CTF-BI-11 (21.7 wt%, 1549 m² g⁻¹),³⁶ the present result was still much lower. On the other hand, compared with the reported MOPs containing TBs, the present CTF-TBs showed comparable adsorption ability with relatively lower BET surface area under similar conditions. For instance, TB-MOP (17.8 wt%, 694 m² g⁻¹) with triptycene segments,⁷ and TB-MOP (16.89 wt%, 802 m² g⁻¹) derived from tris(4-aminophenyl)amine.¹² However, compared with TB-COP-1 (22.8 wt%, 1340 m² g⁻¹) derived from tetraanilyladamantane,⁸ the present CTF-TBs didn't show any advantage. The main reason would be ascribed to the partially decomposition and fragmentations of TB segments, which lead to sharply decrease of both surface areas and TB concentrations in the final CTF-TBs.

Fig. 3 (a) CO₂ adsorption isotherms at 273 K of CTF-TBs; (b) CO₂ adsorption isotherms at 303 K of CTF-TBs; (c) the ratios of four N-configurations (N(x)) in CTF-TBs; (d) heat of adsorption for CTF-TB-3 and CTF-TB-7.

Table 3 CO₂ and N₂ capture capacities, CO₂/N₂ selectivities for the CTF-TBs

Polymer code^a	SABET (m^2 g^−1)	CO2 adsorption (cm^3 g^−1)		N2 adsorption (cm^3 g^−1)		CO2/N2^c
		273 K^b	303 K	273 K	303 K	273 K	303 K
a The CO2 adsorptions of CTF-TB-6 and CTF-TB-S were not measured due to the very low BET surface areas.b CO2 adsorption of CTF-TBs at 273 K and 1.10 bar.c Selectivity estimated using the ratios of the Henry law constant calculated from the initial slopes of the single-component gas adsorption isotherms at low pressure coverage (<100 mbar).
CTF-TB-1	294	54.94	33.57	4.07	1.25	44.3	50.0
CTF-TB-2	446	65.92	43.46	5.34	2.56	41.0	52.2
CTF-TB-3	612	85.77	55.13	6.69	2.11	43.4	48.1
CTF-TB-4	581	61.37	37.65	4.77	2.49	34.4	39.0
CTF-TB-5	495	54.35	33.56	2.74	2.10	53.3	39.5
CTF-TB-7	732	78.59	50.62	6.96	3.72	31.0	35.4
CTF-TB-8	689	66.26	39.96	4.34	2.70	42.5	32.5
CTF-TB-9	626	66.47	41.01	5.66	1.77	34.1	41.0

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As mentioned above, the CO₂ adsorption ability did not show a positive proportional relationship with the BET surface areas. CTF-TB-7 had a higher BET surface area than that of CTF-TB-3 (732 m² g⁻¹ vs. 612 m² g⁻¹), but had a lower CO₂ adsorption amount (15.43 wt% vs. 16.84 wt%). The main reason should be again ascribed to the partially decomposition and fragmentations, as mentioned above. From the elemental analysis results (Table S1†), as the reaction temperature increased, the loadings of N decreased gradually from 11.82 wt% (350 °C) to 8.52 wt% (550 °C). It should be evidence of the ring fragmentation. X-ray photoelectron spectroscopy (XPS) analysis of CTF-TBs were performed in order to further explore the origin of the effect of reaction temperatures on the CO₂ adsorption ability. The N 1s XPS results of DCTB and CTF-TBs (Fig. S14†) reveal that about three to four distinct N configurations exist in the skeletons of CTF-TBs: pyridinic N (397.98–398.40 eV, N(1)), pyrrolic N (399.60–399.99 eV, N(2)), quaternary N (400.86–401.19 eV, N(3)), and N-oxide (402.65–403.94 eV, N(4)).^47,48 For CTF-TB-6 (350 °C) and CTF-TB-3 (400 °C), there were three distinct N configurations while for CTF-TB-7 (450 °C), CTF-TB-8 (500 °C), and CTF-TB-9 (550 °C), there were four, also indicated the decomposition or fragmentation reactions may occur under high temperature conditions. The atomic ratios of the four N-configurations in CTF-TBs were further evaluated from the deconvolution of N 1s peaks, and the results are plotted as a function of synthesis temperature in Fig. 3c. In general, the ratio of N(1) and N(2) tend to decrease when the synthesis temperature increases; while the ratios of N(3) and N(4) increase with increasing synthesis temperature. The high CO₂ uptakes by nitrogen containing polymers were found to arise from strong interactions of the polarizable CO₂ molecules and N atoms with Lewis basic sites. The interactions between CO₂ and quaternary N and N-oxide were much weaker than those of pyridinic and pyrrolic N. Although CTF-TB-7 had a much higher BET surface area, the content of quaternary N and N-oxide were also much higher than that of CTF-TB-3 (24.5% vs. 9.2%, Fig. S14†), thus leading to a lower CO₂ uptakes.

To determine the binding affinity of CTF-TBs for CO₂, we calculated the isosteric heat of adsorption (Q_st) for CO₂ using the Clausius–Clapeyron equation, as shown in Fig. 3d and S15.† The Q_st values of CTF-TBs were in the range of 27.0–30.4 kJ mol⁻¹, which were well below the values expected for a chemisorption process (>40 kJ mol⁻¹). Therefore, the high CO₂ affinity of the CTF-TBs was mainly attributed to the inherent microporosity and the enhanced dipole–quadrupole interactions between the quadrupole moment of the CO₂ molecules and the nitrogen-rich polar binding sites, both from TB and triazine rings.

CO₂ and N₂ sorption properties were measured by volumetric methods at the same conditions of 273 K and 303 K, respectively. The selectivity of CO₂ over N₂ was estimated according to a reported method by using the Henry law constants (Fig. S16†). For all CTF-TBs, the calculated CO₂/N₂ adsorption selectivities were in the range of 31.0–53.3 at 273 K, and 32.5–52.2 at 303 K, respectively (Table 3 and Fig. S17†). The selectivities were comparable to most of reported CTFs, for instance, TPC-1 with F atoms (selectivity for CO₂/N₂ at 273 K: 38),⁴⁹ Cz-POFs with carbazoles (selectivity for CO₂/N₂ at 273 K: 19–37),⁵⁰ fl-CTFs (selectivity for CO₂/N₂ at 273 K: 13–35),⁴⁰ which were calculated by the same method. However, compared with the highly selective MOPs for CO₂ adsorptions, e.g. the azo-COPs with multiple azo groups (selectivity for CO₂/N₂: 73–124 at 273 K, 113–142 at 298 K),⁴ these values herein were much lower.

Conclusions

In conclusion, a novel dicyano monomer containing Troger's base segments (DCTB) was designed and synthesized. A series of CTFs with TB segments (CTF-TBs) were prepared by the dynamic trimerization under ionothermal condition. The CTF-TBs exhibited BET surface areas up to 732 m² g⁻¹. The reaction conditions including molar ratio of ZnCl₂ to DCTB and temperature had significant impact on the BET surface areas. The capacity of CO₂ adsorption for all the CTF-TBs was studied and the CO₂ uptake capacity was up to 16.84 wt% at 273 K and 1.10 bar for CTF-TB-3, obtained at the ratio of DCTB to ZnCl₂ of 1 : 5 at 400 °C for 40 hours. This made these materials potential candidates for applications in CO₂ capture and storage technology.

Acknowledgements

The authors thank the Fundamental Research Funds for the Central Universities (No. lzujbky-2014-72) and National Natural Science Foundation of China (No. 51103168, 21201093) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Structure characterization of CTF-TBs; the nitrogen adsorption and desorption isotherms and pore size distributions of all CTF-TBs; the CO₂ adsorption isotherms, and the CO₂/N₂ selectivities of all CTF-TBs. See DOI: 10.1039/c6ra21196h

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