Controlled synthesis of conjugated polycarbazole polymers via structure tuning for gas storage and separation applications

A series of conjugated microporous polymers (CMPs) based on 1,3,6,8-tetrabromocarbazole (N4CMP-1–5) is synthesized via Suzuki cross-coupling or Sonogashira polycondensation. The porosity properties and surface area of these polymer networks can be finely tuned by using a linker with different geometries or strut length. These polymers show the Brunauer-Emmett-Tellerthe (BET) surface areas ranging from 592 to 1426 m2 g−1. The dominant pore sizes of the polymers on the basis of the different linker are located between 0.36 and 0.61 nm. Gas uptake increases with BET surface area and micropore volume, N4CMP-3 polymer can capture CO2 with a capacity of 3.62 mmol g−1 (1.05 bar and 273 K) among the obtained polymers. All of the polymers show high isosteric heats of CO2 adsorption (25.5–35.1 kJ mol−1), and from single component adsorption isotherms, IAST-derived ideal CO2/N2 (28.7–53.8), CO2/CH4 (4.6–5.2) and CH4/N2 (5.7–10.5) selectivity. Furthermore, N4CMPs exhibit the high CO2 adsorption capacity of 542–800 mg g−1 at 318 K and 50 bar pressure. These data indicate that these materials are a promising potential for clean energy application and environmental field.

group (-NH) from carbazole unit, which might promote the interaction between the solid adsorbent and acidic CO 2 molecules; (3) conjugated property of rigid carbazole unit is beneficial for formation of porous structure with permanent porosity and stability; (4) the linker monomers possess different steric configuration, such as linear type, triangle, and tetrahedral, which can exhibit different flexibility to form the microporous volume. Therefore, introduction of the carbazole unit into the polymer skeleton makes the porous materials electron-rich to enhance CO 2 uptake of porous polymers, and the steric configuration of linker monomer can effectively construct and adjust the microporous volume and reach outstanding gas storage and separation capacity [31][32][33] .
With these considerations in mind, herein, we prepared a series of CMPs (Fig. 1, N 4 CMP-1-5) based on 1,3,6,8-tetrabromocarbazole as the basic buliding block via Pd-catalyzed Suzuki cross-coupling or Sonogashira polycondensation. The porosity properties and surface area in the kind of amorphous CMPs can be finely controlled by the strut length, size or geometry of linker. The obtained carbazole-based CMPs possess large specific surface area, high micropore volume, narrow pore size distribution, high gas uptake capacity, and good selectivity toward CO 2 over N 2 or CH 4 . Meanwhile, N 4 CMP polymer networks also exhibit high CO 2 adsorption capacity at high pressure condition.

Results
Synthesis and characterization. All of the polymer networks were synthesized by palladium (0)-catalyzed cross-coupling polycondensation of 1,3,6,8-tetrabromocarbazole and a number of benzeneboronic monomers or ethynyl monomers. All the reactions were carried out at a fixed reaction temperature and reaction time (120 °C/48 h). The general synthetic route towards N 4 CMPs polymers is shown in Scheme 1. Our aim is to explore the effect of structure and connecting position of linker on pore properties of the resulting porous polymers. The insoluble polymers were filtered and washed with water, tetrahydrofunan, chloroform and methanol, respectively, in order to remove the inorganic salts, organic monomers, residual catalyst, and oligomers. All of these polymers are insoluble in common organic solvents because of their highly cross linked structures.
The structures of N 4 CMPs polymers were further characterized by Fourier transform Infrared (FT-IR) spectroscopy (Fig. 2). All the polymer networks show the characteristic C=C stretching band at 1600 cm −1 and aromatic C-H stretching frequencies up to 3000 cm −1 . All the spectra exhibit intense characteristic bands of N-H at about 3450 and 1390 cm −1 31-33 . The band around 1500 cm −1 is assigned to the stretching vibration of C-N-C in the five-membered NC 4 ring for all the samples [31][32][33] . The primary bromo group of 1,3,6,8-tetrabromocarbazole at about 590 cm −1 are absent in the polymer networks. The typical C≡C stretching mode at about 2200 cm −1 is also observed in the N 4 CMP-2, N 4 CMP-3, N 4 CMP-4, and N 4 CMP-5, respectively. These results suggested that a hyper-cross-linked structure was successfully obtained. The structures of N 4 CMPs polymers were characterized at the molecular level by solid state 13 C cross-polarization magic-angle spinning (CP/MAS) NMR (ESI, Figure S1). In general, N 4 CMP-1-5 have the similar broad peaks. The resolved resonance peak at about 137 ppm arises from the carbons of the carbazole rings. Two peaks at about 130, 123, and 110 ppm can be ascribed to the other carbons in the aromatic rings. The small peaks at about 93 ppm correspond to acetylene carbons from acetylene monomers. These peaks are perfectly consistent with previous works [31][32][33] .
The morphology information was collected by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The typical SEM images of N 4 CMPs indicate that all the polymers have irregular shapes with sizes of about several micrometers (Fig. 3). The TEM images revealed that they are porous structures of the materials, which is similar to some reported amorphous microporous organic materials (ESI, Figure S2) [34][35][36] . The UV/vis diffuse reflectance spectra of the N 4 CMP are displayed in Figure S3. Both polymers showed a broad absorption range. All the five polymers displayed a broad absorption with wavelengths ranging from 375 to 428 nm. Although the shapes of the spectra are similar, they do display differences in terms of the onset of the absorption, which correlates with the band gap of the conjugated system. The TGA results verify that the polymers have a good thermal stability, and the thermal degradation temperature is up to ca. 360 °C (ESI, Figure S4). The weight loss below 100 °C is generally attributed to the evaporation of adsorbed water and gas molecules trapped in the micropores. Powder X-ray diffraction (PXRD) measurements indicated that all the polymers are amorphous in nature (ESI, Figure S5), as most other reported CMP networks 12 .
The porosity of the polymer networks was investigated by nitrogen adsorption/desorption experiments at 77 K, in which the fully reversible isotherms exhibit rapid nitrogen uptake at low relative pressures (P/P 0 < 0.01), indicates that the five polymers belong to microporous materials ( Fig. 4a and Figure S6). Meanwhile, all the polymer networks gave rise to type I nitrogen gas sorption isotherms according to the IUPAC classification 21 . N 4 CMP-1, N 4 CMP-2 and N 4 CMP-3 show little hysteresis upon desorption, suggesting that adsorption and desorption are almost equally facile. Nevertheless, N 4 CMP-4 and N 4 CMP-5 networks show an evident hysteresis loop, which is partly attributed to the swelling in a flexible polymer network, as well as mesopore contribution 34,35 . In fact, the described low-pressure hysteresis is common phenomenon in a variety of microporous polymer networks 18,19,[36][37][38][39][40][41] . The presence and magnitude of this hysteresis may be an indication of the softness of the material. Besides, a sharp rise in the high pressure region (P/P 0 > 0.8) is also observed in the sorption isotherms of N 4 CMP-1, N 4 CMP-2 and N 4 CMP-3, respectively, suggesting that the materials possess some macropores, which is attributed to the nitrogen condensation in interparticular voids formed by the aggregation of polymer microspheres observed in the SEM images, which is similar to the previous reports 42, 43 . The apparent BET surface areas for the networks (Table S1) were calculated over a relative pressure range P/P 0 = 0.015-0.1, which was found to give a positive value of C in the BET equation. Polymer N 4 CMP-3 shows the highest BET surface areas with a value of 1426 m 2 g −1 , while N 4 CMP-2 represents the lowest value of 592 m 2 g −1 among the five polymer networks. As a general trend, when the similar synthetic methodology was used, polymers prepared from longer linkers showed lower surface areas than those from shorter linkers, the similar results were reported by the Cooper and Thomas group 23,24,36 . For example, N 4 CMP-1 had a BET surface area of 650 m 2 g −1 , which decreased to 592 m 2 g −1 for N 4 CMP-2. This phenomenon is similar to that of reported previously, in which CMPs constructed   with longer connecting struts have lower BET surface areas 23,36 . Beyond this, Jiang et al. reported the linkage geometry between the connecting nodes could affect pore properties, conjugation and surface area for CMP networks 37 . Their experiment results demonstrated that the meta-linkage is superior to the ortho-and para-linkages for the construction of porous skeletons. This phenomenon is also observed in this system, for example, polymer N 4 CMP-3 containing the meta-linkage 1,3,5-triethylbenzene possesses the highest surface area (1426 m 2 g −1 ), which is higher than that of polymers N 4 CMP-4 (995 m 2 g −1 ) and N 4 CMP-5 (1347 m 2 g −1 ) with the ortho-and para-linkage. In addition, the polymer N 4 CMP-5 shows a higher surface area in the contrastion of N 4 CMP-4, possible reason is that the effect of steric hindrance from tetrakis(4-bromophenyl)methane improved the further growth of the molecular weight of the polymer during polymerization, which leads to the increase of surface area. The micropore volume was calculated from the adsoprtion branch of the nitrogen adsorption-desorption isotherm using the t-plot method (ESI, Figure S7). A summary of the BET specific surface area and porous properties is shown in Table S1.
The pore size distribution (PSD) of polymers N 4 CMPs was calculated from the related adsorption branch of the isotherms by the NLDFT method. As shown in Fig. 4b, the N 4 CMPs polymers based on different building blocks showed the dominant pore sizes range between 0.36 and 2.0 nm. Polymers N 4 CMP-1 and 2 possessing same core structures were synthesized by bis-substituted monomers with the increasing length (biphenylene, diethynylbenzene), and N 4 CMP-3-5 were prepared with different geometry linkage nodes. For instance, N 4 CMP-3 has smaller micropore diameter (0.36 nm) and higher surface area compared to other four polymers, this result could be explained by the fact that the meta-linkage is superior to the ortho-and para-linkages for the construction of porous skeletons, similar results were observed for other reported CMPs 24,31,37 . Meanwhile, from the N 2 sorption isotherm, the ratio of micropore volume to total pore volume (V Micro /V Total ) can be calculated which describes the degree of microposity. All the five polymers exhibited the V Micro /V Total values above 0.55, ranging from 0.57 to 0.79, which indicates that a high fraction and dominance of micropores in these networks. These results implyed that the BET surface area and porosity porosities in the CMP networks could be finely controlled by using either linker with different strut length or different geometries. The key structural properties of polymers derived from the corresponding isotherm are listed in Table S1, such as the BET specific surface area, micropore surface area, pore volume, micropore volume, dominant pore size, gas uptake, and selectivity.

Discussion
Gas uptake capacity and separation. It has been proved that compare to other most porous polymers, carbazole-based polymer networks can efficiently adsorb gas (hydrogen and carbon dioxide) under the same conditions 32,38-41 . Therefore, gas uptake capacities of the nitrogen-rich polymers were investigated. The CO 2 uptakes of the polymer networks were studied at 1.05 bar and different temperature (273 K and 298 K) ( Fig. 5a and Figure S8). We find that an increasing trend in the carbon dioxide loading capacity (from 2.05 mmol g −1 to 3.62 mmol g −1 ) with increasing micropore surface area and pore volume (Table S1). N 4 CMP-3 with the highest micropore surface area and micropore volume, shows the highest CO 2 uptake of 3.62 mmol g −1 at 273 K and 1.05 bar among the resulting polymer networks, followed by N 4 CMP-5 (3.18 mmol g −1 ), N 4 CMP-4 (2.49 mmol g −1 ), N 4 CMP-1 (2.24 mmol g −1 ), and N 4 CMP-2 (2.05 mmol g −1 ) ( Table S2 in the Supporting  Information). From this result, we found that N 4 CMP-1 and N 4 CMP-2 have similar micropore surface area, however, N 4 CMP-1 shows the relatively higher CO 2 uptake, possible reason is that N 4 CMP-1 has high micropore volume (Table S1). Interestingly, these CO 2 uptake values are not only significantly higher than many microporous materials with a similar specific surface area but also comparable to the reported large surface area of porous aromatic frameworks under the same conditions, such as PAF-1 (2.1 mmol g −1 , S BET = 5640 m 2 g −1 ) 44 , COF-102 (1.56 mmol g −1 , S BET = 3620 m 2 g −1 ) 45 , and PP-CMP@mmm (2.52 mmol g −1 , S BET = 1928 m 2 g −1 ) 37 , but lower than TSP-2 (4.1 mmol g −1 , S BET = 913 m 2 g −1 ) 40 , ALP-1 (5.4 mmol g −1 , S BET = 1235 m 2 g −1 ) 46 and PPF-1 (6.1 mmol g −1 , S BET = 1740 m 2 g −1 ) 47 , implying that the surface area of porous polymers is not a sole factor in determining the capacity of CO 2 uptake. The superior adsorption properties of the N 4 CMP polymers can be ascribed to the enhanced dipole-quadrupole interactions and/or the strong interactions of the polarizable CO 2 molecules through hydrogen bonding (from the N-H group), similar results were also observed for previously reported carbazole-based CMPs 23 . The recycling is an important parameter for their practical application. We tested the reusability of N 4 CMP-3, N 4 CMP-4 and N 4 CMP-5 at 1.05 bar and 273 K, and found the samples could be efficiently recycled and reused for four cycles without significant loss of CO 2 uptake (ESI, Figure S9). Furthermore, a high pressure CO 2 sorption properties of the polymers were also investigated at 50 bar and 318 K. As seen in Fig. 5b, N 4 CMP-2 and N 4 CMP-3 produce a type IV isotherm according to IUPAC classifications 48 . N 4 CMP-1, N 4 CMP-4 and N 4 CMP-5 show a nearly linear increase with the increasing pressure no obviously turning point. All the polymers show similar growth trend in the capture of carbon dioxide gas (from 542 mg g −1 to 800 mg g −1 ) with increasing surface area and pore volume under high pressure condition. N 4 CMP-3 exhibits the highest CO 2 capture capacity of 800 mg g −1 . N 4 CMP-1, N 4 CMP-2, N 4 CMP-4, and N 4 CMP-5, exhibit the CO 2 uptake of 605, 542, 556, and 695 mg g −1 , respectively. These results indicate that electron-rich polymer backbone, porosity property and pressure play a positive role in the increase of gas capature capacity.
To further understand the pore surface properties and the adsorption process, the isosteric heat (Q st ) of polymers N 4 CMP for adsorption CO 2 is calculated based on adsorption isotherms of CO 2 at 273 K and 298 K in terms of the Clausius-Clapeyron equation (Fig. 5c) 49 . The Q st values of CO 2 drop with loading amount, meaning that the interaction between CO 2 and the pore wall is stronger than that between CO 2 molecules. The Q st values of CO 2 adsorption for all the polymer networks range from 25.5 to 35.1 kJ mol −1 at the near zero coverage, which can be comparable to other carbazole-based polymers and higher than those of some reported POPs, such as polycarbazole CPOP-1 (24.5-30.2 kJ mol −1 ) 38,40,41 , HCP materials (20-24 kJ mol −1 ) 50 , polybenzimidazole BILP polymers (26.7-28.8 kJ mol −1 ) 51 . The higher Q st value may be attributed to the appropriate pore structures for CO 2 adsorption, electron-rich polycarbazole network and high charge density at the nitrogen sites of polymers that can enhance the interaction between CO 2 molecules and the pore surfaces of N 4 CMP.
Besides gas storage, the selectivity is very important for potential application in gas separation. Considering the good gas adsorption performance of the obtained polymers, the gas selective adsorption behaviors of N 4 CMP polymers are measured. The sorption experiments of CO 2 , CH 4 , and N 2 were carried out at 273 K and 1.05 bar, respectively. The CO 2 or CH 4 uptake shows a almost linear increase with the increasing pressure, whereas that of nitrogen has no apparent increase trend (ESI, Figure S10). From the available single-gas adsorption isotherms, the selectivity in the adsorption of CO 2 , CH 4 and N 2 from CO 2 -N 2 , CO 2 -CH 4 , and CH 4 -N 2 gas mixtures was estimated by Ideal Adsorption Solution Theory (IAST), which has been widely used to predict gas mixture adsorption behavior in the porous materials 52,53 . Under simulated flue gas streams (typically 15% CO 2 and 85% N 2 ), N 4 CMP-3 exhibits the highest selectivity for CO 2 /N 2 among the five polymers (53.8 at 273 K and 1.05 bar) (Fig. 6a). This value is lower than that of porous benzimidazole linked polymers BILP 1-7 (59-113) 54 and PECONF 1-4 (74-109) 55 , while it is comparable to those of previously reported carbazole-based polymers (29.7-35.4) 31 . In addition, the CO 2 /CH 4 adsorption selectivity for N 4 CMP polymer networks is calculated to be 4.6-5.2 at 273 K and 1.05 bar, which is moderate value (Fig. 6b and Table S1). The CH 4 /N 2 selectivity for N 4 CMP polymers is 5.7-10.5 ( Fig. 6c and Table S1). Furthermore, we employ initial slopes ratio estimated from Henry's law constants for single-component adsorption isotherms, which has been widely adopted by researchers 56 . the CO 2 selectivities of N 4 CMP-1-5 over N 2 and CH 4 were also calculated using initial slopes calculations at 273 K and 1.0 bar, the selectivities of CO 2 /N 2 (46.2-69.1) (ESI, Figure S11, Table S1) and CO 2 /CH 4 (6.4-12.1) (ESI, Figure S12, Table S1). The CH 4 /N 2 adsorption selectivities range from 4.0 to 10.6 (ESI, Figure S13, Table S1). These excellent CO 2 selective capture performances of N 4 CMP samples evaluated by Henry's law are consistent with the results calculated from IAST. Due to their relatively high heats of adsorption and good selectivity of CO 2 / N 2 , CO 2 /CH 4 and CH 4 /N 2 , the N 4 CMPs synthesized in this work could have potential for post-combustion CO 2 capture.

Conclusion
In conclusion, a series of carbazole-containing CMPs have been synthesized by palladium (0) catalyzed cross-coupling polycondensation. The study suggests that the monomer reactivity and the efficacy of chemistry have a pronounced effect on the surface areas and porosity attained in addition to the monomer geometry structure and linker strut length. All of the polymers are microporous with the BET surface areas ranging from 592 to 1426 m 2 g −1 and possess quite narrow pore distributions at around 0.36 and 0.61 nm. The thermally stable polymer N 4 CMP-3 exhibits the highest CO 2 adsorb uptake up to 3.62 mmol g −1 (1.05 bar/273 K) and 800 mg g −1 (50 bar/318 K) among the obtained polymers. All of the CMP networks show high isosteric heats of CO 2 adsorption (25.5-35.1 kJ mol −1 ). We believe that this type of CMPs material would be a promising candidate for clean energy application and environmental-friendly field.