Microporous organic polymers based on tetraethynyl building blocks with N-functionalized pore surfaces: synthesis, porosity and carbon dioxide sorption

Abstract

A series of microporous organic polymers (MOPs) based on tetraethynyl monomers such as tetrakis(4-ethynylphenyl)methane and tetrakis(4-ethynylphenyl)silane was synthesized via conventional Sonogashira–Hagihara coupling reaction. The resulting MOPs were characterized by thermogravimetric analyses, IR-spectra, scanning electron microscopies, and the Brunauer–Emmett–Teller (BET) method. The incorporation of triphenylamine or azobenzene moieties into the polymer skeleton increases the number of electron donating basic nitrogen sites in the porous frameworks. Thus, these MOPs could exhibit efficient adsorption of Lewis acidic CO₂ molecules and display good CO₂-over-N₂ selectivity. The triphenylamine-based polymer, TEPM-TPA, shows a high BET specific surface area up to 1072 m² g⁻¹ with a moderate CO₂ uptake capacity of 2.41 mmol g⁻¹ at 273 K and 1.13 bar. As for separation of CO₂, both TEPM-Azo and TEPS-Azo exhibit relatively high CO₂-over-N₂ selectivities of 70.8 and 64.7 at 273 K, respectively, due to the N₂-phobic feature of azo-based polymers.

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Introduction

In recent years, global warming has become a major issue in climate change.¹ Overwhelming scientific consensus has correlated global warming to anthropogenic CO₂ emission. Thus, one of the promising technologies to protect the environment is to develop new efficient adsorbents for CO₂ capture and storage (CCS).² Traditional CO₂ capture is based on the chemical adsorption of CO₂ using amine-containing solvents.³ However, because of high energy consumption, solvent regeneration, and the corrosion of the equipment, scientists are seeking new materials for CCS.

Microporous organic polymers (MOPs) have drawn much attention due to their promising applications such as gas separation matrices,^4–6 heterogeneous catalysis,^7–10 supercapacitors,^11–13 and sensing materials.^14–16 MOPs generally possess large surface area, excellent chemical and thermal stability, tunable pore properties, low skeleton density, and synthetic diversity.⁶ These features could make MOPs became strong candidates for post-combustion CCS, and they have been the subject of recent research interests.^4,17–29 The surface modification of MOPs is an effective approach to modulate framework–CO₂ interactions. It has been proved that the introduction of nitrogen functionalities into the backbone of MOPs could enhance CO₂ and energy storage properties owing to Lewis-acid–Lewis-base electrostatic interactions of the nitrogen atoms with CO₂ molecules.^{17,20–23,30–34} Therefore, synthesis of new MOPs using nitrogen-rich monomers has attracted considerable scientific interest in recent years. A range of MOPs have been successfully developed for CCS applications based on various nitrogen-containing building blocks, such as covalent triazine frameworks,^30–32 benzimidazole-linked polymers,^33,34 carbazole-based MOPs,^20,23 and triphenylamine-containing polymers.²¹ A recent study by Patel and coworkers has shown that nitrogen–nitrogen double bonds (azo-bond) in the porous polymer main chain can play important roles in CO₂ storage and separation application.³⁵ These new azo-functionalized MOPs demonstrated that the π-conjugated azo-bond could act as an electron donor, and thus enhance host–guest interactions.^35–37

Cooper and coworkers applied Sonogashira–Hagihara coupling chemistry to prepare a series of MOPs with controllable pore dimensions and surface areas by varying building blocks.³⁸ Impressively, the use of tetraethynyl monomers such as tetrakis(4-ethynylphenyl)methane²⁸ and tetrakis(4-ethynylphenyl)silane²⁹ is prominent for the generation of 3D-porous assemblies. Their strong covalent linkage leads to high chemical and thermal stability. In this work, we have rationally designed and synthesized a series of nitrogen-rich MOPs by copolymerization of tetraethynyl monomers with tris(4-iodophenyl)amine or 4,4′-diiodoazobenzene via Sonogashira–Hagihara coupling reaction. The highly cross-linked porous structure can substantially increase the surface area of the resulting polymer networks and, on the other hand, the incorporation of azobenzene or triphenylamine groups from the building block into the polymer skeleton could enhance the binding affinity between the adsorbent and CO₂ molecules, and thus lead to the increase of CO₂ capture capacity. As we expected, azo-based polymers exhibited excellent CO₂ adsorption selectivity against N₂. This unique property of TEPM-Azo and TEPS-Azo, along with their chemical and thermal stability, makes them ideal candidates for post-combustion CO₂ separation.

Experimental

Materials

All the reagents, unless otherwise specified, were purchased from Sigma-Aldrich Co., Acros, and Tokyo Chemical Industry Co., Ltd. and were used without further purification. All the solvents were further purified under nitrogen flow. Tetrakis(4-ethynylphenyl)methane (1), tetrakis(4-ethynylphenyl)silane (2) and tris(4-iodophenyl)amine (3) were prepared according to the previously reported method.^28,29,39

Measurements

The NMR spectra were recorded on an Agilent 300-MR NMR spectrometer. Thermogravimetric analysis (TGA) was performed by a differential thermal analysis instrument (Q1000DSC + LNCS + FACSQ600SDT) over the temperature range from 30 to 1000 °C with a heating and cooling rate of 10 °C min⁻¹ under nitrogen. Solid state magic angle spinning ¹³C CP/MAS NMR measurements were carried out on a Varian Infinity-Plus 300 MHz NMR spectrometer at a MAS rate of 10 kHz. The FT-IR spectra were collected in transmission on a Tensor 27 FT-IR spectrometer (Bruker) using KBr disks. The polymer morphology was achieved using a field-emission scanning electron microscopy (SEM) (JSM-7001F, JEOL, Tokyo, Japan). Powder X-ray diffraction measurement (PXRD) was carried out on an X-ray diffractometer (D/Max-3c). The measurements of the surface area and pore-size distributions (PSDs) of the samples were performed on an ASAP 2420-4 (Micromeritics) volumetric adsorption analyzer by means of nitrogen adsorption and desorption at 77.3 K. Samples were degassed at 120 °C for 15 h under vacuum (10⁻⁵ bar) before analysis. The surface areas were calculated in the relative pressure (P/P₀) range from 0.05 to 0.20. PSDs and pore volumes were derived from the adsorption branches of the isotherms using the nonlocal density functional theory (NL-DFT). Gas sorption isotherms were measured on an ASAP 2420-4 as well.

Synthesis

4,4′-Diiodoazobenzene (4). 4-Iodoaniline (6 g, 27.4 mmol), KMnO₄ (10.5 g, 6.26 mmol) and CuSO₄·5H₂O were added to a degassed solution of dichloromethane (160 mL) and stirred at room temperature for 3 days. The reaction mixture was filtered through Celite and the solvent was evaporated under reduced pressure. The crude product was purified on a silica gel column using a petroleum ether/dichloromethane (3 : 1 by volume) mixture as eluent. An orange solid of 4 was isolated in 15% yield (0.89 g). ¹H NMR (300 MHz, CDCl₃), δ (TMS, ppm): 7.84 (d, 4H), 7.61 (d, 4H).

Polymerization

All of the polymer networks were synthesized by palladium-catalyzed Sonogashira–Hagihara cross-coupling polycondensation of arylethynylenes and aryl halides. The molar ratio of ethynyl to halogen functionalities in the monomer feed was set at 1.5 : 1 according to the reported procedure.³⁸ The purification of the polymers was conducted in air with yield of 80–90%. A typical procedure for TEPM-Azo is given below:

Tetrakis(4-ethynylphenyl)methane (81 mg, 0.195 mmol), 4,4′-diiodoazobenzene (113 mg, 0.26 mmol), tetrakis(triphenylphosphine)palladium(0) (15 mg), and copper(I) iodide (10 mg) were dissolved in a mixture of anhydrous DMF (5 mL) and triethylamine (5 mL). The reaction mixture was heated to 90 °C and stirred for 72 h under nitrogen atmosphere. The mixture was cooled to room temperature, and the precipitated polymer networks was filtered and washed four times with dichloromethane, water, methanol, and acetone to remove any unreacted monomer or catalyst residues. The product was dried in vacuum for 24 h at 100 °C. Yield: 80.1%. Apparent BET surface area: 564 cm³ g⁻¹.

Result and discussion

Synthesis and characterization

The synthesis of 4,4′-diiodoazobenzene (monomer 4) is shown in Scheme 1. Starting from the commercially available 4-iodoaniline, 4,4′-diiodoazobenzene was prepared by the oxidative coupling of two equivalents of 4-iodoaniline in the presence of CuSO₄·5H₂O and KMnO₄ under nitrogen atmosphere in dichloromethane.⁴⁰ The polymer networks in this study were synthesized by palladium-catalyzed Sonogashira–Hagihara coupling polycondensation. The polymerization reactions were performed at a fixed reaction temperature (90 °C) and the reaction time (72 h). The molar ratio of ethynyl to iodine functionalities in the monomer feed was fixed at 1.5 : 1 since maximum surface areas was observed at this ratio based on the previous reports.³⁸ The general synthetic routes to the polymer networks are shown in Scheme 1. The polymer networks are insoluble in common organic solvents due to their highly cross-linked structures and the rigid skeleton. The FT-IR spectra revealed no residual trace of the specific vibration bands related to terminal alkynes at around 3280 cm⁻¹,²⁸ therefore suggesting high conversion of the starting monomers (Fig. S1†). The ¹³C solid-state NMR (CP/MAS) measurements were carried out to study the local structures of these polymers (Fig. S2†). All of the carbon signals with a chemical shift exceeding 110 ppm are related to aromatic carbon atoms of the building phenylene groups in the framework. The peak at around 91 ppm in the NMR spectra is assigned to the sp carbons in alkyne bonds, thus suggesting that this cross-coupling reaction is effective for the synthesis of these polymer networks.^41,42 A well-resolved peak is detected at 65 ppm associated with the quaternary carbon atom in tetrakis(4-ethynylphenyl)methane, indicating that tetrakis(4-ethynylphenyl)methane is the main building units in the polymer networks TEPM-TPA and TEPM-Azo.^41,42 Additionally, the azo-linkage formation was further confirmed by the presence of characteristic signals for the –C–N=N–C– bond at around 152 ppm in NMR spectra of azo-based polymers.³⁵ TGA measurements were carried out to evaluate the thermal stability of these polymers (Fig. S3†). All the polymer networks exhibit good stability, showing less than 5% weight loss at around 480 °C under nitrogen flow. Powder X-ray diffraction measurements suggested that all the polymer networks are amorphous solid in nature (Fig. S4†). The SEM images of the obtained polymer networks are shown in Fig. 1. For the polymer networks, similar morphology can be observed to be solid submicrometer particles with different size range from 10 to 100 nm.

Scheme 1 Synthetic routes to 4,4′-diiodoazobenzene (4) and the polymer networks.

Fig. 1 SEM images of TEPM-TPA (a), TEPM-Azo (b), TEPS-TPA (c), TEPS-Azo (d), scale bars: 1 μm.

Porosity measurements

In order to study the porosity of these polymer networks, sorption analysis using nitrogen as the sorbent molecules was carried out. Nitrogen adsorption/desorption isotherms of these materials measured at 77.3 K are shown in Fig. 2a. The polymer networks exhibited Type I isotherms with a steep nitrogen gas uptake at low pressure according to the International Union of Pure and Applied Chemistry (IUPAC) classification, suggesting that micropores are dominant in these polymer networks.¹⁸ The gradual increase in N₂ uptake (P/P₀ = 0.05–0.9) at 77.3 K is due to the presence of mesopores in the polymers. The significant hysteresis between adsorption and desorption was observed in the isotherms for all the polymer networks in the whole range of relative pressure, which is in agreement with the fact that all the MOPs contain both meso- and microporosity. As shown in Fig. 2b, similar PSD profiles based on the adsorption branch of the isotherm using nonlocal density functional theory (NL-DFT) were found for the obtained polymer TEPM-TPA and TEPS-TPA. Both polymers show abundant micropore structures with micropore diameters centered at around 1.1 nm. As for the polymer TEPM-Azo and TEPS-Azo, the PSDs are relative broader in the range of 1.1–2.1 nm compared with those of TEPM-TPA and TEPS-TPA, indicating that much more mesopores and/or macropores in these two polymers. Besides, TEPM-Azo and TEPS-Azo show a spot mesopores peaks at around 5 nm. The PSD curves of the polymer networks are in good agreement with the shape of the nitrogen isotherms (Fig. 2a) and suggest that the pores of the polymer networks are dominated by micropores. The contribution of microporosity to the networks can be calculated as the ratio of the micropore volume over the total pore volume. As shown in Table 1, the ratios indicated that polymer networks TEPM-TPA, TEPS-TPA, TEPM-Azo, and TEPS-Azo possess a ratio of 0.63, 0.74, 0.79, and 0.90, respectively. The BET specific surface areas of TEPM-TPA and TEPS-TPA are 1072 and 937 m² g⁻¹, respectively, which are higher than those of TEPS-Azo (564 m² g⁻¹) and TEPM-Azo (598 m² g⁻¹) containing the azobenzene building block. It should be noted that TEPM-TPA and TEPS-TPA exhibit comparable surface area to the most of reported conjugated microporous poly(aryleneethynylene) (PAE) networks by Sonogashira–Hagihara cross-coupling chemistry.²⁶

Fig. 2 (a) Nitrogen adsorption/desorption isotherms (the adsorption branch is labeled with filled symbols and desorption branch is labeled with open symbols); (b) pore size distribution curves calculated by NL-DFT for the polymer networks.

Table 1 Summary of pore properties for the polymer networks

Polymer	SBET^a (m^2 g^−1)	SMicro^b (m^2 g^−1)	VMicro^c (cm^3 g^−1)	VTotal^d (cm^3 g^−1)	SMicro/SBET (%)	VMicro/VTotal (%)
a Surface area calculated from N2 adsorption isotherm in the relative pressure (P/P0) range from 0.05 to 0.20.b Micropore surface area calculated from the N2 adsorption isotherm using the t-plot method based on the Harkins–Jura equation.c The micropore volume derived from the t-plot method.d Total pore volume at P/P0 = 0.988.
TEPM-TPA	1072	901	0.446	0.708	84.05	62.99
TEPM-Azo	598	552	0.267	0.338	92.3	78.99
TEPS-TPA	937	787	0.396	0.536	83.99	73.88
TEPS-Azo	564	489	0.236	0.261	86.7	90.42

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Gas uptake of CO₂, H₂, and CH₄

Because of the considerable porosity and nitrogen-rich features, we were interested in assessing their performance in gas uptake. The CO₂ uptake of the polymer networks were measured up to 1.13 bar at 273, 283, and 298 K, respectively (Fig. 3a and b, S5,† and Table 2). It can be seen that the CO₂ uptake capacity enhances monotonically with increasing CO₂ pressure. TEPM-TPA exhibited the highest CO₂ uptake at both 273 and 298 K, adsorbing 2.41 and 1.39 mmol g⁻¹ at 1.13 bar among the resulting four polymers. When azobenzene is copolymerized with tetrakis(4-ethynylphenyl)methane, the absolute CO₂ uptake capacity of the polymer decreases from 2.41 mmol g⁻¹ for TEPM-TPA to 1.78 mmol g⁻¹ for TEPM-Azo. This could be explained by the decrease of surface area and pore volume for the polymer networks as mentioned above (Table 1). The CO₂ uptake of 2.41 mmol g⁻¹ for TEPM-TPA is comparable to that of many reported MOPs produced by Sonogashira–Hagihara coupling reaction at the same conditions, such as the conjugated microporous PAE networks without nitrogen-containing building blocks and their metalized derivatives,^41,43–47 tetraphenylmethane-based hypercrosslinked porous aromatic frameworks (2.27 mmol g⁻¹ for PAF-32-OH),⁴⁸ the tri(4-ethynylphenyl)amine-based porous aromatic frameworks (1.19–2.50 mmol g⁻¹),⁴² the triazine-functionalized CMPs (2.62 mmol g⁻¹ for TNCMP-2),⁴⁹ the phenolsulfonephthalein-containing CMPs (2.77 mmol g⁻¹ for BFCMP-2),⁵⁰ but still lower than that of other tetrakis(4-ethynylphenyl)methane-based MOPs (3.77 mmol g⁻¹, 5.07 mmol g⁻¹, and 4.28 mmol g⁻¹ for F-MOP-1, F-MOP-2, and HEX-POP-3, respectively),^51,52 the imine-linked porous polymer frameworks (e.g. 6.1 mmol g⁻¹ for PPF-1),⁵³ the benzimidazole-incorporated porous polymer network of PPN-101 (5.1 mmol g⁻¹),⁵⁴ the microporous polycarbazole of CPOP-1 (4.8 mmol g⁻¹),⁵⁵ and the nanoporous azo-linked polymer of ALP-1 (5.36 mmol g⁻¹).³⁶ To determine the binding affinity between the polymer network and CO₂ molecules, the isosteric heats of adsorption (Q_st) were calculated from the CO₂ adsorption isotherms at 273, 283, and 298 K by using the Clausius–Clapeyron equation. As illustrated in Fig. 4, the Q_st values of the obtained MOPs are in the range from 22 to 33 kJ mol⁻¹ at the near zero coverage. TEPM-Azo shows the highest Q_st of 33 kJ mol⁻¹ among the polymers, which could be attributed to the high nitrogen content from azo groups in the skeleton of MOPs leading to a strong induce-dipole force with CO₂ molecules.^35–37 TEPS-Azo shows somewhat lower Q_st values than TEPM-Azo. We suspect that even though the azo framework is same in both, TEPS-Azo with tetrakis(4-ethynylphenyl)silane units in the structure generated diminish basicity in the nitrogen species compared with those in TEPM-Azo with no Si (silicon is acidic in nature).⁵⁶ As for triphenylamine-containing polymers, the Q_st value of TEPS-TPA is lower than that of TEPM-TPA, which is further support the above results in the case of azo-based polymers.

Fig. 3 (a) CO₂ adsorption isotherms collected at 273 K up to 1.13 bar for the polymer networks; (b) CO₂ adsorption isotherms collected at 298 K; (c) volumetric H₂ sorption curves collected at 77.3 K; (d) CH₄ adsorption isotherms collected at 273 K.

Fig. 4 Isosteric heats of adsorption for CO₂ calculated from the adsorption isotherms collected at 273, 283, and 298 K.

H₂ and CH₄ are important potential clean fuels for automotive technology; therefore, we also investigated the uptake capacities of these two gases by volumetric methods. The hydrogen sorption performance for the polymers networks was measured at 77.3 K up to a pressure of 1.13 bar. All adsorption isotherms show a gradual rise and reached a maximum of 0.88–1.4 wt% for hydrogen uptake (Fig. 3c). The H₂ uptake sorption properties of polymer networks can be correlated with the micropore volume and micropore surface area, which was also observed in other MOPs. Triphenylamine-based polymer networks exhibit higher H₂ adsorption capacity than azo-based polymer networks under the same conditions because of their lager surface areas and narrower pore sizes. The CH₄ uptakes at 273 K up to 1.13 bar (Fig. 3d) were also conducted. As expected, TEPM-TPA having the highest micropore volume and highest surface area shows the largest CH₄ uptake of 0.81 mmol g⁻¹.

Selective CO₂ capture over N₂ and CH₄

We also investigated the CO₂ sorption selectivity over N₂ and CH₄ to assess their potential application in gas separation. The CO₂, N₂, and CH₄ adsorption isotherms were measured at 273 K up to 1.13 bar (Fig. S6†). The selectivity of the polymer networks was estimated using the ratios of the Henry's law constant calculated from the initial slopes of the single-component gas adsorption isotherms collected at 273 K at low pressure coverage less than 0.15 bar, which is typical partial pressure of CO₂ in flue gas. As shown in Table 2, TEPM-Azo and TEPS-Azo exhibit relative high CO₂/N₂ selectivity ratios of 70.8 and 64.7 at 273 K, whereas TEPM-TPA and TEPS-TPA show poor selectivities of 26.3 and 20.5, respectively. These results suggested that the azo-based polymer is efficient for CO₂/N₂ separation as expected because of the “N₂-phobicity” of the azo-linkage.^35–37 The differences in CO₂/N₂ selectivity between azo- and triphenylamine-based polymer networks indicate that in addition to the function of the surface area and pore size, the linkage is very important factor that affect CO₂ uptake and the CO₂/N₂ selectivity. The calculated CO₂/N₂ selectivities by both TEPM-Azo and TEPS-Azo are significantly higher than those of TEPM-TPA, TEPS-TPA, and some other MOPs such as tetraethynylspirobifluorene-based porous organic polymers POP (6.7–11.8 at 298 K),⁵⁷ porous hypercrosslinked aromatic polymers (PHAPs) using tetrahedral precursors (29.3–34.2 at 273 K),⁵⁶ tetraphenylsilane-containing porous covalent triazine polymer networks PCTP-2 (31.6 at 298 K),⁵⁸ tetra(4-(N-carbazolyl)phenyl)silane-based polycarbazole microporous polymer P-TCzPhSi (24.8 at 273 K),⁵⁹ the nanoporous azo-bridged polymers (30–43 at 273 K),³⁶ the carbazolic porous organic frameworks (19–37 at 273 K),⁶⁰ the imine-linked porous polymer frameworks (14.5–20.4 at 273 K),⁵³ the triazine-functionalized CMPs (9.0–25.2 at 298 K),⁴⁹ and the amide-functionalized CMPs (8.5–12 at 298 K) under the similar conditions.⁶¹ Moreover, they are also comparable to the best azo-polymers ever reported like azo-COPs,³⁷ tri(4-ethynylphenyl)amine-containing polymers (72 and 63.9 for PAF-34 and PAF-34-OH, respectively),⁴² tetrakis(4-ethynylphenyl)methane-based porous aromatic frameworks PAF-18-OH, PAF-26-COOH, and their metalized derivatives (16–73 at 298 K).^41,46 As for separation of CH₄, the calculated CO₂/CH₄ adsorption selectivity was about from 3.9 to 4.6 for the four polymers at 273 K. The results are comparable to that of most other reported porous organic polymers, such as tetrakis(4-ethynylphenyl)methane-based porous materials PAF-26 (4–9 at 298 K),⁴¹ tetraethynylspirobifluorene-containing porous organic polymers POP (3.4–4.8 at 298 K),⁵⁷ the carbazole-based microporous organic polymer of Cz-POF (4.4–7.1 at 273 K),⁶⁰ the nanoporous covalent triazine-based frameworks (5–7 at 273 K),⁶² the tetra-armed triphenylamine-containing MOPs (3.5–4.3 at 273 K),⁶³ and the azo-containing polymer networks ALP (8 at 273 K)³⁶ under the same conditions. The CO₂/CH₄ selectivity of the resultant polymers is considerably lower than that of CO₂/N₂ because of the higher polarizability of CH₄ than that of N₂.⁶⁰

Table 2 Summary of gas uptakes values for the polymer networks

Polymer	H2 uptake^a (wt%)	CH4 uptake^b (mmol g^−1)	CO2 uptake^b (mmol g^−1)	CO2 uptake^c (mmol g^−1)	Henry law selectivity^d		IAST selectivity^e
					CO2/N2	CO2/CH4	CO2/N2	CO2/CH4
a Data collected by volumetric H2 sorption method at 77.3 K and 1.13 bar.b Data collected at 273 K and 1.13 bar.c Data collected at 298 K and 1.13 bar.d Adsorption selectivity based on the Henry's law.e Calculated from ideal adsorbed solution theory (IAST) by using the CO2, N2, and CH4 adsorption isotherms collected at 273 K.
TEPM-TPA	1.4	0.81	2.41	1.39	26.3	4.6	36.9	5.1
TEPM-Azo	0.91	0.55	1.78	1	70.8	4.6	121.1	6.3
TEPS-TPA	1.08	0.65	1.97	1.14	20.5	3.9	30.5	4.9
TEPS-Azo	0.88	0.52	1.71	0.99	64.7	4.3	111.9	6.5

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In addition, the gas adsorption selectivity for mixtures of CO₂ : N₂ (0.15 : 0.85) and CO₂ : CH₄ (0.05 : 0.95) were predicted by using ideal adsorbed solution theory (IAST) at a standardized temperature of 273 K. The method allows for the determination of the selectivities as a function of pressure, and has been widely used to predict binary gas mixture adsorption behaviors in many MOPs.^64,65 The adsorption data in Table 2 exhibit that the calculated IAST selectivities are higher than the values obtained from Henry's constant ratios, as observed for most reported porous materials.^17,30,31

Conclusions

In summary, we have synthesized a series of microporous organic polymer networks by using Sonogashira–Hagihara cross-coupling chemistry from two kinds of tetrahedral-shaped monomers of tetrakis(4-ethynylphenyl)methane and tetrakis(4-ethynylphenyl)silane. The resulting polymer networks incorporated nitrogen-rich triphenylamine or azobenzene groups present a high affinity to CO₂ molecules, which has been significantly verified by gas adsorption measurements. TEPM-TPA shows high BET specific surface area up to 1072 m² g⁻¹ with a moderate CO₂ uptake capacity of 2.41 mmol g⁻¹ at 273 K and 1.13 bar, which reinforce the benefits of tetrahedral monomers for the construction of high surface area materials. In addition, TEPM-Azo and TEPS-Azo exhibit relative high CO₂/N₂ selectivity of 70.8 and 64.7 at 273 K, respectively, which compared to that of some other reported polymer networks.

Acknowledgements

This work was supported by Natural Science Foundation of China (Grant No. 51103111, 21304055 and 21574077), Education Ministry of China (Program for NCET-12-0714), Shaanxi Innovative Team of Key Science and Technology (2013KCT-17), the Fundamental Research Funds for the Central Universities (GK201501002), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (2014-skllmd-11), and Graduate Innovative Fund of Wuhan Institute of Technology (Grant No. CX2014017, CX2014026).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20765k

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