Highly selective CO₂ adsorption performance of carbazole based microporous polymers

Abstract

Non-coplanar shaped carbazole based monomers were used to synthesize microporous polycarbazole materials utilizing an inexpensive FeCl₃ catalyzed reaction. The reactions proceed through direct oxidative coupling and extensive crosslinking polymerization routes. The obtained porous networks exhibit a maximum Brunauer–Emmett–Teller specific surface area of 946 m² g⁻¹ with a total pore volume of 0.941 cm³ g⁻¹, and display a high carbon dioxide uptake capacity (up to 13.6 wt%) at 273 K and 1 atm. Selective adsorption of CO₂ over N₂ calculated using the ideal adsorbed solution theory (IAST) shows that these networks display enhanced selectivity with a maximum value of 155 at 298 K. Remarkably, in contrast to other materials, this value is significantly higher than the selectivity values (102–107) obtained at 273 K. Introduction of the electron rich carbazole structure into the aromatic system and pore geometry contribute to higher adsorption enthalpy which in turn leads to high selective adsorption values. These polymeric networks also show a high working capacity with reasonably high regenerability factors. The combination of a simple inexpensive synthesis approach and high selective adsorption make these materials potential candidates for CO₂ storage, selective gas adsorption, and other environmental applications.

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Introduction

Porous organic polymers (POFs) have been investigated owing to their potential in important fields such as catalysis, sensing, gas storage and separation.^1–4 POFs are characterized by their remarkably high surface area, extraordinary physicochemical stability, flexible pore size, and chemical composition tunability, which collectively control the structure versus function relationship. The chemical heterogeneity of the pores in the microporous structure is a significant parameter in selective gas binding and separation. Among the currently available CO₂ capture technologies, the use of POFs is a possibility due to their high CO₂ storage capacities and low cost regeneration.⁵ The selective capture of CO₂ from flue gas and natural gas has become an urgent issue driven by the concerns of climate change and demands in natural gas upgrading.⁶ To achieve enhanced selectivity, current efforts have been devoted to increasing the binding energy of gas molecules with frameworks by anchoring Lewis basic sites.⁷ In this respect, a wide variety of POFs have been regarded as highly promising for practical CO₂ capture.^8–11 In order to enhance CO₂ separation performance, various porous polymer networks were synthesized involving expensive catalysts which may restrict their bulk scale synthesis for CO₂ capture.^12–16 As such easy-cost effective synthesis procedures, excellent gas separation and sorption properties are very important factors to completely realize the potential of POFs for industrial scale applications in the post-combustion capture of CO₂.

The gas storage capacity and separation capability of microporous organic polymers can be controlled by certain factors such as material nature, specific surface area, pore structure, and material's morphology.¹⁷ In this concern, non-coplanar monomers are of primary importance as the polymers they form have inherent porous properties. Furthermore, incorporation of bulky organic linkers with high connectivity can effectively prevent network interpenetration which leads to materials with enhanced texture properties. In this way, by designing molecules with certain porous geometries such as microporous organic polymers afford good performance for the gas storage and separation. Carbazole has got much attention and a lot of carbazole based polymeric materials have been synthesized already.^18–22 By means of these new strategies, aromatic heterocyclic POFs with CO₂-philic moieties and good porosity can be prepared without using harsh conditions. The carbazole based polymers have reasonably good gas uptake but have a drawback of poor gas selectivity. Recently, carbazole group has been reported to enhance the selectivity of traizines.¹⁸ In order to enhance the gas selectivity, we introduced carbazole on the selected building blocks with non-coplanar geometry molecule. In this way, by designing molecules with certain porous geometries such as microporous organic polymers afford good performance for the gas storage and separation. To the best of our understanding, no such carbaozle containing materials with remarkably high CO₂/N₂ selectivity at 298 K have been reported to date. Herein, we present the synthesis of new POFs for energy-efficient CO₂ capture with favorable CO₂ uptake and sufficiently high CO₂/N₂ selectivity.

Methods

Solvents, reagents and chemicals were purchased form Aldrich and TCI America. All chemicals were used without any further purification. Cross-polarization (CP) magic angle spinning (MAS) ¹³C NMR measurements were performed on a Agilent VNMRS600 spectrometer at a frequency of 600 MHz. Measurements were made with a double resonance solid state probe (1.9 mm MAS rotor) spinning at 25 kHz. Chemical shifts were externally referenced to tetramethylsilane (δ = 0 ppm) using methyl resonance of hexamethyl benzene (17.5 ppm relative to tetramethylsilane). Fourier transformed infrared (FTIR) spectra were collected in KBr pellets using a Bruker FTIR. Scanning electron microscopy (SEM) images of the product were taken on a field emission scanning electron microscope (FESEM, JEOL, FEG-XL 30S) operating at an accelerating voltage of 5.0 kV. X-ray diffraction patterns (XRD) were recorded from 5° to 80° on a Rigaku, Japan, RINT 2500V X-ray diffractionmeter using Cu Kα irradiation (λ = 1.5406 Å). A Seiko thermogravimetric/differential thermal analyzer-6300 was used to collect thermogravimetric analysis (TGA) data by heating the samples at 5 °C min⁻¹ to 800 °C in a nitrogen atmosphere.

Gas adsorption measurements were performed using a Belsorp mini II (Micromeritics, Japan) device. Before each measurement, a weighed sample was heat treated at 150 °C under vacuum for 16 hours. Brunauer–Emmett–Teller (BET) surface area was investigated using N₂ adsorption–desorption isotherms measured at 77 K. Pore size distributions were calculated using non local density functional theory. CO₂, CH₄ and N₂ storage adsorption isotherms were measured at 273 and 298 K and up to 1 atmosphere.

Experimental details

Synthesis

Carbazole based polymers synthesized by knitting approach. To a solution of 2,7-dicarbazolyl-9,9′-spirobi[9H-fluorene] (SP, 20 mmol) and dimethoxymethane (DMM, 40 mmol) in anhydrous 1,2-dichloroethane (DCE, 20 ml), anhydrous ferric chloride (FeCl₃, 40 mmol) was added under a nitrogen atmosphere. The mixture was heated at 80 °C for 24 h. After cooling down to room temperature, the solid product was collected by filtration and washed with methanol, dilute HCl, distilled water and methanol. The crude polymer was collected and thoroughly washed by Soxhlet extraction with methanol for 24 h. The product was then dried at 120 °C under vacuum to give polymer designated as cross linked SP (CL-SP) (yield ∼ 92%) as a brown solid. 9-(6-(9H-Carbazol-9-yl)-9-phenyl-9H-carbazol-3-yl)-9H-carbazole (NP) was also used in a similar manner to give product named as cross linked NP (CL-NP) as a light brown solid (yield ∼ 90%).

Carbazole based polymers synthesized by direct oxidative coupling approach. To a solution of (SP, 20 mmol) in anhydrous chloroform (CHCl₃, 20 ml), anhydrous ferric chloride (FeCl₃, 40 mmol) was added under a nitrogen atmosphere. The mixture was stirred at room temperature for 24 h. The precipitates were collected by filtration and washed with methanol, dilute HCl, distilled water, and methanol. The brown crude polymer was further washed by Soxhlet extraction with methanol for 24 h to remove unreacted monomers. The solid was then dried at 120 °C under vacuum to give coupled SP (C-SP) (yield ∼ 85%). The same procedure for NP was used to synthesize the coupled NP (C-NP) (yield ∼ 83).

Results and discussion

To build a porous architecture, polymerization of building blocks with non-coplanar geometry is employed to form 3-D porous organic frameworks. Keeping this in mind, the SP and NP monomers having non-coplanar geometry and rigid conjugated backbone were selected. For the synthesis of these polymers two methods were selected due to their simplicity and much resemblance. Each free carbazoyl group in these monomers has two reaction sites for the oxidative polymerization by which the bi-substituted porous polymers C-NP and C-SP were obtained. However, by using an extensive linking approach, a knitting polymer is formed by random linking of crosslinkers on the carbazole and aromatic rings that certainly facilitates in developing the nanoporous structures CL-NP and CL-SP (Scheme 1). After Soxhlet extraction and purification, light to dark brown solid powders were obtained. All porous networks were insoluble in most organic solvents including DMF, THF, toluene, chlorobenzene, acetonitrile and chloroform at room temperature, and were also found to be chemically stable in dilute NaOH and HCl. FESEM images show the morphology of these materials, where CL-NP, C-SP and CL-SP consist of aggregated particles of various sizes, while C-NP exhibits a fibrous structure (Fig. S1†). There is no clear distinct peak in XRD measurements which suggest the amorphous nature of these materials (Fig. S2†). However, the broad peak centered at ∼20° in the respective X-ray diffraction (XRD) patterns corresponds to the average intersegmental distance of the polymer chains.²³

Scheme 1 Reaction of the polymer synthesis.

The structures of the polymers were characterized at the molecular level by ¹³C solid state NMR. The ¹³C CP-MAS NMR spectrum of the obtained polymer with the assignments of the resonances is shown in Fig. S3.† The representative spectra of C-SP, CL-SP, C-NP and CL-NP exhibit four signals for aromatic carbons at 100–150 ppm. The peak at about 139.9 ppm corresponds to the substituted phenyl carbons binding with carbazole nitrogen atom. The high-intensity peak for the other substituted phenyl carbons is located at about 125 ppm. The signal peak at about 120.8 and 109.8 ppm are attributed to the unsubstituted phenyl carbons. Additionally, for the CL-SP and CL-NP materials the presence of the methylene carbon signal at about 40 ppm confirms that the linkers have participated in the formation of the cross-linking porous polymers.

In a comparison of the FTIR spectra of the polymers, four main peaks are observed as shown in Fig. S4.† The peaks at about 1610 and 1510 cm⁻¹ are attributed to aromatic ring skeletal vibrations, which were consistent with the structure of the corresponding monomers. The weak absorption peaks at about 2900–3000 cm⁻¹ correspond to (C–H band stretching vibrations), while the additional broad peak at about 3450 cm⁻¹ (O–H band stretching vibrations) may be ascribed to the incomplete removal of methanol even after drying.

TGA shows that all polymers decompose between 360–400 °C, indicating impressive thermal stability (Fig. S5†). This higher thermal stability of the polymers is desired in order to withstand application in the harsh power plant conditions.

The porosity of the polymers were evaluated by using nitrogen adsorption–desorption isotherms measured at 77 K (Fig. 1a), in which isotherms show rapid uptake at low relative pressures reflecting a permanent microporous nature of the networks. The isotherms for CL-NP and C-SP can be classified broadly as type I according to the IUPAC classification,²⁴ while C-NP and CL-SP exhibit mixed type I and type II behavior. For all polymers, the isotherms show a continuous increase after the adsorption at low relative pressure (P/P₀ < 0.01), indicating formation of monolayer followed by multi layers. The increase in the nitrogen adsorption at a high relative pressure (P/P₀ > 0.9) may arise from the interparticulate porosity associated with the meso/macro-structures of the samples.²⁵ Hysteresis is observed apparently for CL-SP and C-NP over the entire pressure range, which might be attributed to the swelling effects.²⁶ The BET surface areas for the polymers calculated over a relative pressure (P/P₀) range from 0.01 to 0.1 are 573–946 m² g⁻¹ (Table 1). C-NP and CL-SP show a higher surface area than CL-NP and C-SP, which is comparable to other recently developed microporous polymers.^17,27 C-NP shows the largest micropore volume (0.504 cm³ g⁻¹) of all the polymer networks, followed by CL-SP (0.465 cm³ g⁻¹), C-SP (0.369 cm³ g⁻¹) and CL-NP (0.319 cm³ g⁻¹). The pore size distribution (PSD) curves (Fig. 1b) calculated using non-local density functional theory further suggest that the polymers have a narrow pore size distribution with the pore widths centered around 0.6 nm. These PSD curves are in good agreement with the shape of the nitrogen isotherms (Fig. 1a) strengthening the evidence that all the polymers have the dominant microporous character. Overall the textural properties observed for CL-SP are superior due to the presence of excessive crosslinking compared to the corresponding product C-SP. However, a reverse trend was obtained in the case of CL-NP when compared to C-NP. This observation may be due to the specific geometry of the NP which suggests that besides the selection of synthesis route, the non-coplanar geometry of the monomer is significantly important in obtaining a higher porous character.

Fig. 1 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution for the polymeric networks.

Table 1 Texture properties, gas uptakes, binding affinities and selectivities (CO₂ : N₂ and CO₂ : CH₄) for the polymer networks

Network	SA^a	Vt^b	V0.1^c	PD^d	CO2^e		CH4^e		Selectivity (initial slope)^f		Selectivity (IAST)^g
					273 K (298 K)	Qst	273 K (298 K)	Qst	CO2 : N2	CO2 : CH4	CO2 : N2	CO2 : CH4
a Specific surface area (m^2 g ^−1) calculated from the nitrogen adsorption isotherm using the BET method.b Total pore volume (cm^3 g^−1) at P/P0 = 0.995.c Micropore volume (cm^3 g^−1).d Average pore diameter (nm).e Gas uptake at in (wt%) and the isosteric enthalpies of adsorption (Qst) in kJ mol^−1.f Selectivity was calculated from initial slope calculations at 273 K and (298 K).g Selectivity was calculated from IAST for 15/85 gas mixtures for CO2/N2 and 5/95 gas mixtures for CO2/CH4 at 273 K (298 K).
C-NP	946	0.941	0.504	3.98	13.6 (8.7)	28.8	1.86 (1.09)	20.3	29 (23)	9.8 (10)	102 (102)	14.7 (14.2)
CL-NP	573	0.433	0.319	3.03	9.5 (5.5)	27.2	1.18 (0.64)	19.9	33 (36)	10.2 (11.6)	103 (121)	15.1 (16.4)
C-SP	660	0.523	0.369	3.17	10.7 (5.8)	26.2	1.26 (0.69)	20.1	31 (39)	10.3 (8.4)	117 (127)	16.0 (16.7)
CL-SP	865	0.735	0.465	3.40	13.1 (8.0)	28.4	1.50 (0.94)	19.2	36 (41)	10.6 (11.3)	107 (155)	16.3 (18.3)

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Low-pressure gas sorption measurements for CO₂ were collected in order to investigate the impact of the narrow pores on gas storage and the preferential binding of CO₂ over CH₄ and N₂ at 273 and 298 K. Remarkably, C-NP and CL-SP show CO₂ uptakes of 13.6 and 13.1 wt% at 273 K, respectively (Fig. 2a and Table 1). The CO₂ adsorption capacity of these polymers is comparable to the many previously reported POFs.^{6,12,17,27–30} This further corroborates that both knitting and oxidative synthesis approaches are simple routes for the large scale manufacturing of polymer based CO₂ adsorbents. On the basis of the CO₂ adsorption isotherms, we found that the CO₂ uptake capacity increases with an increase in the specific surface area. To investigate the impact of microporosity and the chemical nature of the pore surfaces of the as synthesized polymers on the CO₂ uptake, we measured CO₂ adsorption isotherms of polymers at 298 K. Utilizing this data we were able to calculate isosteric heats of adsorption by applying the Clausius–Clapeyron equation. At zero coverage, the isosteric heat of adsorption (Q_st) calculated for these polymeric materials lie in 26–29 kJ mol⁻¹, as shown in Fig. 2b. These values are in comparison to those reported for many other POFs.^12,27–30 Higher Q_st values for polymers indicate strong interactions of the polarizable CO₂ molecules through dipole–quadrupole interactions with the N sites of framework due to the inherent microporosity of polymers. Additionally, the presence of narrow pores provides favorable multiple wall interactions that raise the adsorption potential of CO₂. The change in Q_st with an increase in CO₂ loading may be attributed to the differences in textural and chemical properties of the polymers. Although nitrogen sites of arylamines are less reactive than alkyl amines (i.e. porous polymer network containing frameworks),¹⁶ they show a reasonably high affinity for CO₂ which is evident from the observed large Q_st values. We can say that high Q_st values are due to the integrated basic effects of arylamines and pore geometry. The readily reversible adsorption–desorption process and moderate Q_st values (below the chemisorptive process limit) indicate that CO₂ interactions with pore walls are weak enough to allow for material regeneration without applying heat.³¹

Fig. 2 (a) CO₂ adsorption (closed)–desorption (open) isotherms at 273 K and (b) isosteric heat of adsorption of CO₂ for the polymers.

CH₄ gas is being considered as an alternative fuel for automotive applications because of its abundance and cleaner burning nature. We also studied CH₄ uptake in order to evaluate the impact of narrow pores on the binding affinity of CH₄. Following a similar trend to that observed for CO₂ adsorption, C-NP (1.10 wt%) and CL-SP (0.92 wt%) show higher CH₄ uptake than CL-NP (0.66 wt%) and C-SP (0.84 wt%) networks at 273 K. The CH₄ Q_st values (Fig. S6†) calculated for these polymers remain as 19–21 kJ mol⁻¹ which agree with the values obtained for most reported polymers.^12,31 Additionally, the lower Q_st values of CH₄ for these polymers suggest that there are reduced interactions between CH₄ and the pore walls due to its non-polar nature.

Effective CO₂ adsorbents for gas separation applications are expected to have high selectivity for CO₂ over other gases such as N₂ (∼75% in flue gas) and CH₄ (∼95% in natural gas) along with high CO₂ capacity. Therefore, we carried out CO₂/N₂ and CO₂/CH₄ selectivity studies for these polymers to examine their gas separation abilities (Fig. 3 and Table 1). The initial slope ratios estimated from Henry's law constants for single-component adsorption isotherms were used to determine the adsorption selectivity. The maximum CO₂/N₂ and CO₂/CH₄ selectivities using the ratio of the initial slopes of adsorption isotherms for the polymers were calculated to be 36 and 10.6 at 273 K and 1 atm, respectively (see Fig. S7†). Importantly, enhanced CO₂/N₂ and CO₂/CH₄ selectivity values of 41 and 11.6 at 298 K and 1 atm were obtained for the polymers at 298 K and 1 atm. To predict CO₂/CH₄ and CO₂/N₂ binary mixture selectivity, an IAST calculation, coupled with a dual-site Langmuir simulation, was employed on the basis of single-component isotherms. Several studies show that the IAST model can accurately predict gas mixture adsorption of porous materials.^16,31,32 Fig. 4 and S8† show the predicated selectivities for CO₂/CH₄ and CO₂/N₂ as a function of pressure when the gas phase mole fractions are 5/95 and 15/85, which are the typical feed compositions for natural gas and flue gas, respectively. Almost in all cases, selectivities gradually decrease with an increase in the pressure. It is worth noticing that high CO₂/N₂ selectivity values of 102–117 are obtained at 273 K and 1 atm (Table 1), which is similar to other reported materials.³¹ Surprisingly, at 298 K, which is closer to actual power plant conditions, CL-SP, C-SP and C-NP show enhanced CO₂/N₂ selectivity values of 155, 127 and 121, respectively. The initial trend in the selectivity is commonly observed for POFs. These values are sufficiently greater than any other carbazole porous material reported to date.^17–20 These results suggest that CL-SP could be an excellent candidate for the selective sorption of CO₂ from flue gas. The initial slope calculations for CO₂/N₂ selectivity resulted in lower values than those calculated by IAST method. However, the IAST selectivities were in reasonable agreement with the values obtained from Henry's constant ratios when the gas feed was set CO₂/N₂: 50 : 50 showing the ideality of the results. Additionally, we showed that at 273 K, CL-SP (16.3) shows higher CO₂/CH₄ selectivity than C-SP (16), CL-NP (15.1) and C-NP (14.7). As for CO₂/N₂ selectivity, these materials also show enhanced CO₂/CH₄ selectivity values at 298 K, i.e., CL-SP (18.3), C-SP (16.3) and CL-NP (16.4). These values are larger or comparable to other reported materials.^33,34 Importantly, it should be noted that the enhancement in the CO₂/N₂ and CO₂/CH₄ selectivity is due to the reduced uptake of CH₄ and N₂ at 298 K in these polymers. This increased selectivity can be ascribed to the enhanced dipole–quadrupole interaction between the large quadrupole moment of CO₂ molecules (13.4 × 10⁻⁴⁰ cm²) and the polar N-doped sites.¹⁰ Importantly, it should be noted that the enhancement in the CO₂/N₂ selectivity may be due to the integrated effects of aromatic carbazole system coupled with narrow pores in these polymers that attribute to the high interaction strength of CO₂ with the frameworks.¹⁸

Fig. 3 Selective gas adsorption at 273 K for the polymers.

Fig. 4 IAST selectivities of CO₂/N₂ (a) and CO₂/CH₄ (b) for binary gas mixtures of 15/85 and 5/95 molar compositions, respectively.

The potential of these polymeric materials in CO₂ capture from flue gas (CO₂/N₂: 10/90) was evaluated using vacuum swing adsorption at 298 K as suggested by Snurr.³⁵ The working capacities (ΔN₁) were determined by calculating the CO₂ adsorption difference between 1.0 and 0.1 bar. CO₂ uptake under adsorption conditions (N^ads₁) were found to be 0.32, 0.27, 0.32 and 0.28 mol kg⁻¹ for C-NP, CL-NP, CL-SP and C-SP, respectively. They exhibit ΔN₁ of 0.29, 0.23, 0.29 and 0.25 mol kg⁻¹, respectively which is comparable to that of the Co-carborane MOF-4b³⁶ and [Zn₂(tcpb){p-(CF₃)NC₅H₄}₂]³⁶ but lower than TBILP-1,³⁷ BILP-13 (ref. 38) and PEI (40 wt%) ⊂ PAF-5 (ref. 39) materials. Regenerability (R) was determined by the percent ratio of ΔN₁ to the N^ads₁. Both C-NP and CL-SP show highest regenerability (R) of 90 which is also comparable to zeolite-5A,³⁶ TBILP-1,³⁷ BILP-13 (ref. 38) and PEI (40 wt%) ⊂ PAF-5 materials.³⁹ The higher regenerability levels are associated with the secondary amines which have less preferable binding N-sites for CO₂, therefore their regeneration processes is rather smooth than metal organic frameworks. The appreciable N^ads₁, ΔN₁ and R values made these materials a promising candidate for selective gas adsorption.

Conclusions

Non-coplanar carbazole based porous polymers with reasonably good textural properties were synthesized via the oxidative and Friedel–Crafts reactions. The obtained materials display impressive CO₂ uptake abilities as well as high adsorption selectivity for CO₂ over N₂. The CO₂ storage for the C-NP material can reach up to 13.6 wt% at 273 K. Importantly, the selectivity of the polymer materials shows an increased selectivity at higher adsorption temperatures. Moreover, the high selectivity of CL-SP toward CO₂ over N₂ from IAST calculations (155) makes it a promising material for CO₂ separation. These polymeric networks also show high working capacity with reasonably high regenerability factors. The obtained networks and their adsorption characteristics provide us with further understanding towards topological structural design and its effect on adsorbed gas uptake performance for clean energy and environmental applications.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0020414) and the Research Fund of UNIST (Project no. 1.140001.01).

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Footnote

† Electronic supplementary information (ESI) available: This section contains eight figures and one table including SEM images, X-ray diffraction patterns, ¹³C CP-MAS NMR spectra, FTIR spectra, thermogravimetric data, isosteric heat of adsorption for CH₄ and gas selectivity data as well. See DOI: 10.1039/c5ra06767g

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