Topology-directed design and synthesis of carbazole-based conjugated microporous networks for gas storage

Abstract

Two topological-directed conjugated microporous networks, P-TPATCz and P-CzPTCz, have been synthesized. The two initial building blocks have a similar chemical constitution but different geometrical configuration (TPATCz: quasi-tetrahedron 3D and CzPTCz: quasi-coplanar 2D structure). Scanning electron microscopy and powder X-ray diffraction indicated that the quasi-tetrahedron structure monomer TPATCz is facile to form columnar crystalline aggregation, whereas the quasi-coplanar monomer of CzPTCz forms amorphous aggregation networks. Thermogravimetric analysis showed that the thermal stability of two networks at high temperature may be affected by the stability of the core in the building blocks. Changing the triphenylamine core of monomer TPATCz to 9-phenyl-9H-carbazole in CzPTCz resulted in an increase in the Brunauer–Emmett–Teller surface area of P-TPATCz (337 m² g⁻¹) to 1315 m² g⁻¹ for P-CzPTCz. The hydrogen isotherms of P-TPATCz and P-CzPTCz showed H₂ storage up to 0.85 and 1.90 wt% at 77 K/1.1 bar, respectively. At 273 K/1.1 bar, the CO₂ uptake capacity of P-CzPTCz was up to 17.0 wt%, which is 5.8 times than that of P-TPATCz. Fine designing and tailoring of the steric configuration of the building block can pre-determine the physicochemical property of the target networks and influence the gas uptake performance.

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Introduction

Conjugated microporous polymers (CMPs) are networks constructed from conjugated building blocks with large surface areas, small pore sizes and low densities. The unique feature of CMPs is that they combine the stiff pore structure and conjugated electron system into a single bulk material. These materials have received increasing research interest because their specific applications were proved to be invaluable tools in the fields of gas storage and separation,¹ catalysts,² water treatment,³ sensor⁴ and photovoltaics.⁵ To date, the main strategy to enhance the performance of CMPs is to link together the secondary building units with different topology structures to form extended networks.^6,7 To achieve better gas storage performance, the porosity and morphology of the CMPs are two key factors that should be taken into account because the polymer rigidity,⁸ free volume,⁹ order¹⁰ or amorphous¹¹ are pre-determined by the topology structures of secondary building units (SBUs).

The SBUs of porous organic materials can be classified as one, two and three dimensions structures, which are the basic and necessary components for the porosity networks. The created pores of low-dimensional (1D) networks originate from the quasi-regular chains, which are normally built up from linear monomers via the covalent polymerization of small building units such as the polymers of intrinsic microporosity (PIMs), which was first reported by the group of McKeown and Budd.¹² Two-dimensional (2D) networks are usually defined crystalline extended topological structures, whose porosity are generated into periodic layer by linear, triangle and square building blocks. The special nature of 2D networks is that they exhibit well-ordered structures with uniform pores and high specific surface areas. Yaghi's group first fabricated the crystalline 2D polymers COF with a regular pore size. To date, the multifunctional COF-366 and COF-66 with high charge carrier mobility have been reported.¹³ The three-dimensional (3D) networks are more complicated than 1D and 2D networks due to the polyhedral reaction sites that can result in more intricate structures.¹⁴ As stated above, the final topology structure of the target networks was pre-determined by the relative geometry of the starting building blocks, and the final porosity structure of the networks is the cornerstone of the gas uptake application. Hydrogen is one of the alternative fuels to decrease both carbon dioxide emissions and the dependency on oil-related products. How to use hydrogen in safely and design a storage material with a high specific surface area, narrow pore distribution and large isosteric enthalpies in the physisorption field is still a challenge.¹⁵

In this study, two carbazole based conjugated networks, P-TPATCz and P-CzPTCz, were topology-directed designed and synthesized, as shown in Scheme 1. This idea would provide an available way for a detailed understanding of the relationship between the geometry configuration of building block and the performance of materials in gas uptake applications. The FeCl₃ oxidative approach uses the single carbazole-based building block without any other spacer inserted, which could avoid the unnecessary impact of other spacers on the morphology of the target polymer. This study shows that changing the geometry configuration of the initial building block directed by topology can make a distinct difference in the polymer thermostability, aggregated morphology, porosity structure parameters and gas storage performance.

Scheme 1 Synthetic routes of the carbazole-based networks P-TPATCz and P-CzPTCz.

Experimental section

Materials

All reagents and solvents, unless otherwise specified, were obtained from J&K, Aldrich and Acros Chemical Co. and used without additional purification. Anhydrous tetrahydrofuran (THF) and chloroform were distilled over sodium/benzophenone and calcium hydride under N₂ prior to use.

Synthesis of P-TPATCz¹⁶. A solution of monomer TPATCz (200 mg, 0.37 mmol) dissolved in 30 mL of anhydrous chloroform was transferred dropwise to a suspension of ferric chloride (786 mg, 2.94 mmol) in 20 mL of anhydrous chloroform. The solution mixture was stirred for 24 h at room temperature under nitrogen protection, and 100 mL of methanol was then added to the abovementioned reaction mixture. The resulting precipitate was collected by filtration and washed with methanol and concentrated hydrochloric acid solution. After extraction in a Soxhlet extractor with methanol for 24 h and then with tetrahydrofuran for another 24 h, the desired polymer was collected and dried in a vacuum oven overnight at 80 °C. Yellowish powder, yield in 95%. Anal. calcd for C₅₄H₃₆N₄: C, 87.54; N, 7.56; H, 4.90. Found: C, 84.20; N, 7.12; H, 3.94.

Synthesis of P-CzPTCz. This polymer was synthesized by the same method described above for P-TPATCz using CzPTCz as the initial monomer. Yellowish powder, yield in 98%. Anal. calcd for C₅₄H₃₄N₄: C, 87.78; N, 7.58; H, 4.64. Found: C, 84.34; N, 7.28; H, 3.92.

Methods

¹H and ¹³C NMR spectra were obtained on a Bruker AV 600M spectrometer with tetramethylsilane (TMS) as the internal reference. FT-IR spectra were acquired in the attenuated total reflection (ATR) mode on a Thermo Nicolet 6700 FT-IR spectrometer. Scanning electron microscopy (SEM) spectra were obtained using a Hitachi S-4800 with acceleration voltage 3.0 kV and working distance of 8.5 mm. Samples were coated on a thin layer of Au before investigation. Thermogravimetric analysis (TGA) was carried out using a TA Instruments Q-5000IR series thermal gravimetric analyzer with samples held in 50 μL platinum pans under an atmosphere of nitrogen (heating rate 5 °C min⁻¹). Powder X-ray diffraction data were collected on a Bruker D8 ADVANCE. The samples were mounted on a sample holder and measured using Cu Kα radiation with a 2θ range of 5°–50°. Solid-state ¹³C cross-polarization magic angle spinning (CP-MAS) NMR spectra for solid samples were obtained on a Bruker AVANCE-III. Gas adsorption test: all the gas adsorption isotherms were tested on a Quantachrome instruments Autosorb-iQ-MP-VP volumetric adsorption analyzer using the same degassing procedure. In a typical experiment, both networks were degassed at 120 °C for 800 min under vacuum before analysis. BET surface areas and pore size distributions were measured by nitrogen adsorption–desorption at 77 K. H₂ isotherms were measured at 77 K, 87 K up to 1.1 bar. CO₂, CH₄ and N₂ isotherms were measured at 273 K and 298 K up to 1.1 bar.

Results and discussion

Synthesis and characterization of P-TPATCz and P-CzPTCz networks

Two carbazole-based conjugated networks, P-TPATCz and P-CzPTCz, were synthesized by FeCl₃ oxidative coupling polymerization at room temperature, as shown in Scheme 1. FeCl₃ oxidative coupling polymerization was first reported to synthesize microporous polycarbazole by Bao-Hang Han's group, which is a facile and effective way to prepare CMPs.¹⁶ Monomer TPATCz and CzPTCz were prepared by the cuprous oxide catalytic Ullmann reaction.^17,18 The two initial building blocks have different geometrical configurations. The core of TPATCz is the triphenylamine unit, which is a typical quasi-tetrahedron and the core of CzPTCz is a planar unit 9-phenyl-9H-carbazole. The outside of the two secondary building units TPATCz and CzPTCz was constructed by three carbazole molecules to form six polymeric binding sites. The rigid conjugated backbone of carbazole is beneficial for the formation of porous networks with permanent porosity stability, and the well-known highly reactive oxidation coupling in the 3, 6-position of carbazole made this backbone a potential building block to construct conjugated networks.¹⁹ The building blocks constitute rigid cores and outside carbazole topology configurations could avoid the pore structure from collapsing at the dry state. P-TPATCz and P-CzPTCz networks have an identical chemical constitution that provides a platform for fine investigations of the relationship between the monomer structure and gas storage.

Solid state magic angle spinning ¹³C CP/MAS NMR spectra and the corresponding networks with the assignment of the resonances are shown in Fig. 1a. In the networks, P-TPATCz, the characteristic resonance signal peaks at 141, 142 and 145 ppm correspond to carbon binding with a carbazole nitrogen atom and the core of triphenylamine nitrogen atom. The difference between the networks of P-TPATCz and P-CzPTCz is the resonance signal at 121 ppm assigned to the carbon signal of the core 9-phenyl-9H-carbazole. A more detailed analysis of the structures of the polymers was confirmed by the FT-IR spectra collected in attenuated total reflection (ATR) mode, as shown in Fig. S1,† in which we can find the apparent carbazole characteristic peaks of P-TPATCz and P-CzPTCz. The signals at 744, 1224, 1469 and 1608 cm⁻¹ are assigned to the characteristic bands of carbazole. TGA traces show that both networks have high thermal stability, and the thermal decomposition temperature (T_d) is up to ca. 474 °C for P-TPATCz and 525 °C for P-CzPTCz at a 5% weight loss, as shown in Fig. 1b. The two polymers showed better stability than some reported CMPs such as PDCzBT, which exhibited decomposition at 350 °C, and O-TTPP and S-TTPP.^20,21 When the temperature was increased to more than 480 °C, the TGA curve of P-TPATCz exhibited a sharp decrease, and at 592 °C, the loss weight reached to 50%, which can be ascribed to the decomposition of the core of triphenylamine. Interestingly, during the whole temperature range (T < 800 °C), the P-CzPTCz networks showed ideal thermal stability with no evidence of a distinct glass transition below the thermal decomposition temperature due to the nature of its high cross-linking structure. The Powder X-ray Diffraction (PXRD) results for the two networks are shown in Fig. 1c. The P-CzPTCz networks reveal the conventional amorphous nature of the polymer, as commonly observed in other materials.²² However, there are many strong sharp peaks observed in the P-TPATCz networks, providing a sound proof that P-TPATCz was obtained in a quasi-crystalline phase.²³ in addition, scanning electron microscopy (SEM) confirmed again that P-TPATCz and P-CzPTCz (Fig. 1d and e) have remarkably different aggregation morphologies and P-CzPTCz networks consist of regular spherical particles. However, in the P-TPATCz image a certain number of the columnar crystalline aggregation morphologies which correspond to the result of the P-XRD can be seen. Both the building blocks were employed in the same polymerization process, but the monomer TPATCz formed a large long-range ordered 3D crystalline morphology, which is similar to the COF networks (more SEM images of different magnification are shown in Fig. S2†). Based on the analysis mentioned above, it can be concluded that subtle changes in the geometry configuration of the building blocks or starting monomers will not only alter the thermal stability of the target polymer but also determine the aggregation morphology, even though both the polymers have the same chemical composition.

Fig. 1 (a) Solid state magic angle spinning ¹³C CP/MAS NMR spectra. (b) Thermal decomposition temperature curves (T_d). (c) Powder X-ray diffraction. (d) P-TPATCz and (e) P-CzPTCz field-emission scanning electron microscopy images.

Nitrogen adsorption

The porosity of the networks P-TPATCz and P-CzPTCz was measured by adsorption analysis using nitrogen as the probe molecule at 77 K. Fig. 2 shows the N₂ adsorption–desorption isotherms for the two networks. The sorption branch of P-CzPTCz exhibited a steep rise of uptake at a relative pressure (P/P₀) less than 0.01, indicating the presence of more substantial micropores structure than P-TPATCz. At relatively high pressures, both the networks showed a large increasing course due to the existence of macroporosity structure, which was caused by the collapse of the loose nanoparticulate stockpiling, yielding small and ill-defined interstitial voids in the obtained networks.²⁴ When the Brunauer–Emmett–Teller (BET) model was adopted to calculate the apparent surface area, the surface areas of P-TPATCz and P-CzPTCz were 337 and 1315 m² g⁻¹, respectively. The BET area of the P-CzPTCz network is 3.9 times more than P-TPATCz. A comparison of pore size distribution (PSD) using the quenched solid density functional theory (QSDFT) is shown in Fig. S3.† QSDFT is a multicomponent DFT, in which the networks is treated as one of the components of the adsorbate–adsorbent system and this model has demonstrated that it can improve the accuracy of pore distribution significantly than NLDFT (nonlocal density functional theory).²⁵ The dominant pore widths of P-TPATCz and P-CzPTCz center at about 1.5–5.5 nm and 0.5 nm, 0.9 nm, respectively. The steric configuration of quasi-coplanar building block CzPTCz could be more facile to form smaller porosity than the quasi-tetrahedron monomer, TPATCz. The pore total volume of both networks was 1.02 and 1.09 cm³ g⁻¹ at relative pressure of P/P₀ = 0.99 for P-TPATCz and P-CzPTCz, respectively. N₂ adsorption analysis and pore size distribution show the relationships among the initial building block, polymer aggregation morphology and porosity structural parameters. The building block of TPATCz with a quasi-tetrahedron geometrical configuration may be facile to yield long-range ordered crystalline aggregation during the solution polymerization process, which leads to a low appearance surface area and large pore width. In contrast, the quasi-planar structure of CzPTCz prefers to form amorphous spherical particle aggregation, and this type of aggregation can be more effective to increase the BET areas and reduce the pore width.

Fig. 2 Nitrogen adsorption–desorption isotherms of the P-TPATCz and P-CzPTCz networks were measured at 77 K.

Hydrogen storage

The hydrogen adsorption isotherms of P-TPATCz and P-CzPTCz networks are shown in Fig. 3a, and the adsorption enthalpies that were calculated based on the isotherms of 77 K and 87 K are shown in Fig. 3b. In the low pressure range, the P-CzPTCz networks show much higher adsorption capacity than P-TPATCz, suggesting the importance of higher achievable adsorption enthalpies. One can observe that the adsorption enthalpies of both the networks can be up to 9.74 and 10.22 kJ mol⁻¹ at zero loading. This can be ascribed to the structure of conjugated electron-rich backbone, and plenty of electric charges in the networks can promote strong interactions of H₂ molecules with the surfaces.²⁶ It should be noted that P-TPATCz and P-CzPTCz have an identical chemical constitution and were prepared using the same polymerization process; moreover, the difference between the two enthalpies may be caused mainly by the different degrees of π-conjugation and pore width. Increasing the surface charge density and reducing the pore width are useful strategies for increasing the adsorption enthalpies, which has been predicted by theory²⁷ and demonstrated by experiments.^28,29 At the medium and high pressure range, P-CzPTCz still exhibits larger H₂ uptake capacity than P-TPATCz, and during this period, the pore volume may be the dominant factor for the hydrogen adsorption capacity. At 77 K and 1.1 bar, P-CzPTCz obtains a H₂ uptake of 1.90 wt%, which is higher than P2 (1.36 wt%)³⁰ and PPN-3 (1.58 wt%),³¹ and comparable to BILP-1 (1.90 wt%).³² It is noticeable that the uptake of P-CzPTCz is 2.35 times that of the P-TPATCz (0.81 wt%) networks. No saturation was observed in the isotherms, suggesting that a higher H₂ capacity can be achieved under a higher pressure condition.

Fig. 3 (a) H₂ adsorption isotherms of P-TPATCz and P-CzPTCz networks at 77 K and 87 K. (b) Variation of the H₂ isosteric enthalpies with the amount adsorbed.

Carbon dioxide and methane adsorption

The carbazole-based networks of P-TPATCz and P-CzPTCz showed a large difference in porosity, structure and H₂ adsorption capacity. To better understand the relationship of building blocks, topology structure and performance among the two networks, we further investigated the CO₂ and CH₄ uptake at 273 K and 298 K, up to 1.1 bar. The adsorption isotherms and gas isosteric enthalpies are shown in Fig. 4. CO₂ physisorbed process is reversible in all cases because the desorption branches are overlapped with the adsorption ones. At 273 K and 1.1 bar, the CO₂ uptake capacity of P-CzPTCz can be up to 17.0 wt%, which is 5.8 times that of the networks of P-TPATCz (2.9 wt%). The capacity of P-CzPTCz also exceeds other reported conjugated polymers, such as CMP Network 1-4 (7.59 wt%, 298 K, 1.13 bar) and³³ CMP-1-COOH (7.04 wt%).³⁴ At 298 K, the capacities of CO₂ uptake were 2.0 and 9.4 wt% for P-TPATCz and P-CzPTCz, respectively. The isosteric enthalpies Qst of the two networks were calculated from the adsorption isotherms based on the Clausius–Clapeyron equation and the values for P-TPATCz and P-CzPTCz are 40.3 and 30.7 kJ mol⁻¹ at zero loading, as shown in Fig. 4b. As the CO₂ loading was increased, the Qst of P-TPATCz sharply dropped to 25 kJ mol⁻¹ and then slowly decreased to 20 kJ mol⁻¹. However, the networks of P-CzPTCz can retain the Qst around 28 kJ mol⁻¹ at the whole pressure range with increasing amount of adsorbed CO₂. The stable Qst can enhance the CO₂ capture despite the low Qst at zero loading. The CH₄ adsorption isotherms of P-TPATCz and P- CzPTCz were collected at 273 K, 298 K and 1.1 bar, as shown in Fig. 4c. The CH₄ uptake of P-TPATCz is 0.5 wt% at 273 K and 0.3 wt% at 298 K; P-CzPTCz is 2.5 wt% at 273 K and 1.2 wt% at 298 K. At the whole pressure range (273 K and 298 K), P-CzPTCz networks exhibit steep adsorption isotherms and show no saturation station, which means that it could absorb more CH₄ under higher pressure conditions. It can be noted that the isosteric heat of CH₄ uptake can be up to 38.2 kJ mol⁻¹ at a zero loading, which was calculated from adsorption experiment at 273 K and 298 K (Fig. 4d). The property of high isosteric heats is advantageous because it allows the possibility of greater storage capacities for materials with comparable micropore sizes and volumes. The lower BET surface area, larger pore width and rapidly decreasing adsorption enthalpies gave the P-TPATCz networks a poorer gas uptake capacity compared to P-CzPTCz. It can be concluded that the geometry configuration of the building blocks or starting monomers may be the dominant factor determining the gas uptake performance of CMPs with the similar chemical constitution.

Fig. 4 (a) CO₂ and (c) CH₄ adsorption isotherms of P-TPATCz and P-CzPTCz networks at 273 K and 298 K. (b) CO₂ and (d) CH₄ variation of the gas isosteric enthalpies with the amount adsorbed.

Conclusions

Two carbazole based building blocks, TPATCz and CzPTCz, were designed and prepared as conjugated networks, P-TPATCz and P-CzPTCz, through FeCl₃ oxidative polymerization. SEM, P-XRD and TGA indicated that the geometry configuration of the monomer could pre-determine the morphology and influence the thermal stability of the networks. Gas adsorption investigation shows that subtle changes in the building block core can make a great difference in pore structures and gas capacities of CMPs. The BET surface areas of the two networks were increased from 337 to 1315 m² g⁻¹, and the H₂ (77 K), CO₂ (273 K), and CH₄ (273 K) storage of P-CzPTCz can be up to 1.9, 17.0 and 2.5 wt% at 1.1 bar, respectively. The results provide an approach to better understand the relationship between topological structure design and the gas uptake performance in clean energy applications and environmental fields, and the findings would be helpful for designing new efficient materials for the relative community.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21274161, 51173199 and 51303197) and the Ministry of Science and Technology of China (2010DFA52310).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of synthesis, IR spectrum, additional SEM images and pore size distribution, Fig. S1, S2 and S3. See DOI: 10.1039/c5ra09854h

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