Increasing the surface area and CO2 uptake of conjugated microporous polymers via a post-knitting method

Yuchuan Liu , Shun Wang , Xianyu Meng , Yu Ye , Xiaowei Song * and Zhiqiang Liang *
State Key Lab of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, 130012, P. R. China. E-mail: xiaoweisong@jlu.edu.cn; liangzq@jlu.edu.cn

Received 8th March 2021 , Accepted 19th May 2021

First published on 19th May 2021


Abstract

The synthesis of high-surface-area porous organic polymers (POPs) for CO2 capture and storage (CCS) has received significant attention from researchers. However, the construction of POPs with a large surface area still remains challenging in synthetic chemistry, because of the complex formation process of the porous skeletons. Herein, we developed a facile post-knitting method to increase the surface area of conjugated microporous polymers (CMPs) to improve the CO2 adsorption capacity. Four CMPs were knitted using two different cross-linkers via a Friedel–Crafts reaction to obtain eight CMP-based hyper-crosslinked polymers (KCMPs), respectively. These resulting KCMPs exhibit a high Brunauer–Emmett–Teller (BET) surface area and total pore volume up to 2267 m2 g−1 and 3.27 cm3 g−1, which are 2.3 times and 8.8 times higher than the corresponding CMPs, respectively. In addition, these KCMPs show obvious increases in the CO2 uptake with the best-performing KCMP-M4 of 3.98 mmol g−1 (89.2 cm3 g−1) at 1 bar and 273 K, an increase of 122.2% compared to the pristine CMP-4. This post-knitting method can provide more potential porous adsorbents for CCS technologies and could be used to further develop novel methods for the synthesis of high-surface-area POPs.


Introduction

Unprecedented extreme climate events, for example droughts, extraordinarily high temperatures, snowstorms, floods, and so on, have increasingly occurred across the planet over the past decade and have led to significant threats to the homes of humans and wildlife. Human influences on climate change have become an indisputable fact, human-caused global warming may be the crucial factor,1–3 and the carbon dioxide (CO2) produced from the consumption of fossil fuels is the primary source of greenhouse gases. With the existence of a high concentration of CO2 in the atmosphere and inevitable CO2 emissions produced during the development of civilization, many countries and international organizations have committed to control CO2 emissions and are seeking promising CO2 capture and storage (CCS) technologies. Remarkably, the exploration or preparation of efficient solid adsorbing materials, especially porous adsorbents,4–9 for CCS technologies has become a very promising research area, in comparison to traditional energy-intensive methods.

Porous organic polymers (POPs) with large surface areas, good physical/chemical stabilities, and well-developed nanoporous and pre-designed building blocks have brought an endless vitality to CCS technologies. Since the pioneering work on the first crystalline POPs, COF-1 and COF-5, reported by Yaghi's group in 2005,10 many genres of POPs have sprouted up and been used in CCS technologies, including covalent organic frameworks (COFs),11,12 hyper-crosslinked polymers (HCPs),13 conjugated microporous polymers (CMPs),14 polymers of intrinsic microporosity (PIMs),15 covalent triazine frameworks (CTFs),16,17 and porous aromatic frameworks (PAFs).18 In particular, CMPs have attracted widespread concern owing to the unique π-conjugated skeleton and easily modified structure,19 and HCPs have been a focus owing to their facile synthetic method and low cost.20 In the past few years, many CMPs and HCPs with tunable nanopores,21–24 various functional units25–30 and a wide range of raw materials,31–33 have been prepared. Additionally, to further improve the CO2 uptake of these polymers, some commonly modified methods have been developed including carbonization,34–37 post-functionalization38–41 and composite processes.42 However, CMPs and HCPs are still limited by the synthetic issue of the low surface area, which also brings about a large barrier to CCS technologies. To date, only a few CMPs and HCPs have been synthesized with a Brunauer–Emmett–Teller (BET) surface area higher than 2000 m2 g−1, such as carbazole-based CMP (CPOP-1, SBET = 2220 m2 g−1),43 3,3′,5,5′-tetraethynylbiphenyl-based CMPs (PPN-13 and -14, SBET = 3420 and 2160 m2 g−1),44 Ni(COD)2-catalyzed CMP (NPOF-1, SBET = 2062 m2 g−1),45 1,3,5-triphenylbenzene-based HCPs (SHCP-3a and -3b, SBET = 2525 and 3002 m2 g−1),46 5′-(3,5-diethynylphenyl)-3,3′′,5,5′′-tetraethynyl-2′,4′,6′-trimethyl-1,1′:3′,1′′-terphenyl-based CMP (KPOP-3, SBET = 2620 m2 g−1)47 and hyper-crosslinked conjugated polymers (HCCPs, SBET = 2003–3083 m2 g−1),48 and most of these polymers have been constructed using complex monomers or using strict synthetic conditions. Therefore, it is of urgent importance to explore facile approaches to construct a high porosity for the related application.

Considering the modified skeleton of the CMPs and the easy crosslinking process of the HCPs, CMPs could be used as the raw materials for preparing CMP-based HCPs with reconstructed nanopores. To address the challenge of the low surface area, a post-knitting method was adopted to explore the concept of CMP-based HCPs to increase the porosity and CO2 uptake. Herein, the post-knitting of CMPs is achieved by using external cross-linkers via the Lewis acid catalyzed Friedel–Crafts reaction, four dimethoxymethane (FDA) knitted KCMP-Fs and four dichloromethane (DCM) knitted KCMP-Ms were obtained with increased surface areas and CO2 uptakes.

Experimental

Materials

All raw materials were purchased from reagent suppliers and no further purification treatments were performed. 1,4-Phenylenediboronic acid (PDB, 99%) and [1,1′-biphenyl]-4,4′-diyldiboronic acid (BPDB, 99%) were purchased from Soochiral Chemical Science & Technology Co., Ltd. 1,3,5-Tribromobenzene (TBB, 99%), 2,4,6-tribromoaniline (TBA, 99%) and Pd(PhCN)2Cl2 (99%) were purchased from Energy Chemical. Dimethoxymethane (FDA, 98%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Anhydrous AlCl3 (99%) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd. Anhydrous FeCl3 (97%) was purchased from Sinopharm Chemical Reagent Co., Ltd. N,N-Dimethylformamide (DMF, 99%), tetrahydrofuran (THF, 99%), ethanol (EtOH, 99%), dichloromethane (DCM) and 1,2-dichloroethane (DCE, 99%) were purchased from Beijing Chemical Works. 3,4′,5-Tribromobiphenyl (TBBP) was synthesized using a method previously reported by our group and the details are given in the ESI.[thin space (1/6-em)]49

Synthesis of CMPs

The CMPs were synthesized using the method previously reported by our group.48,49
CMP-1. To a solution of TBB (1.574 g, 5.0 mmol) and PDB (1.243 g, 7.5 mmol) in DMF (160 mL), Pd(PhCN)2Cl2 (67 mg, 0.175 mmol) and K2CO3 (4.140 g, 30.0 mmol) were added. Then, the mixed solution was stirred and heated at 120 °C for 3 d under an N2 atmosphere. After that, the reaction was quenched using water, and the solid product was washed with H2O, DMF, THF, EtOH and DCM several times under reflux, except for DMF, which was performed at 100 °C. The resulting product was dried in a vacuum at 120 °C for 12 h (926 mg).

The detailed synthetic processes for CMP-2–4 are given in the ESI.

Synthesis of KCMP-Fs

KCMP-F1 . To a solution of CMP-1 (100 mg) and FDA (86 mg, 1.1 mmol) in 1,2-dichloroethane (DCE, 15 mL), dry FeCl3 (1.000 g, 6.2 mmol) was added. The mixture was then heated and stirred at 85 °C for 16 h under an N2 atmosphere. After that the mixture was cooled to room temperature and the solid product was washed with H2O, DMF, THF, EtOH and DCM several times under reflux, except for DMF which was performed at 100 °C, respectively. The resulting product was dried in a vacuum for 12 h at 120 °C (129 mg).
KCMP-F2–4 . Using the same synthetic conditions of KCMP-F1, FDA was used as the cross-linker and CMPs-2–4 were used as the precursor materials, respectively. The detailed synthetic processes are given in the ESI.

Synthesis of the KCMP-Ms

KCMP-M1 . To a solution of CMP-1 (188 mg) in CH2Cl2 (DCM, 20 mL), dry AlCl3 (400 mg, 3.0 mmol) was added. The mixture was stirred under reflux for 16 h under an N2 atmosphere. Then, upon cooling to room temperature the solid product was washed with H2O, DMF, THF, EtOH and DCM several times under reflux, with the exception of DMF, which was performed at 100 °C. The resulting product was dried in a vacuum for 12 h at 120 °C (268 mg).
KCMP-M2–4 . Using the same synthetic conditions used for KCMP-M1, DCM was used as the cross-linker and CMPs-2–4 as the precursor materials, respectively. The detailed synthetic processes are given in the ESI.

Results and discussion

Synthetic procedures and characterization

It is worth mentioning that this synthetic inspiration was derived from the molecular expansion (ME) strategy, in which the surface area and pore size of the pristine CMPs can be significantly increased.48 However, the larger pore size is not always constructive to CO2 adsorption at low pressure owing to the weak confining ability.4 To improve the CO2 adsorption capacity, smaller cross-linkers would be more beneficial during the ME process. Meanwhile, the aromatic building blocks can also be used for constructing HCPs using the knitting method.31,46 Considering that the ME strategy and the knitting method share the same synthetic principle, a post-knitting method with FDA and DCM as cross-linkers was developed to reconstruct the skeletons of CMPs. According to the expected results of this method, the generated methylene can promote the pore size towards that of a micropore, and the surface area can be increased owing to the expansion effect. Therefore, the resulting polymers could be promising adsorbents for CO2 adsorption. In this step-by-step synthetic process (Scheme 1), firstly, four CMPs with rigid networks were synthesized using a palladium-catalyzed Suzuki coupling reaction with simple monomers, according to our previously published work.48,49 Then, these CMPs were post-knitted using FDA as a cross-linker to obtain KCMP-F1–4 and DCM was used directly as a solvent and cross-linker to obtain KCMP-M1–4 with different porous properties, respectively.
image file: d1qm00371b-s1.tif
Scheme 1 (a) Diagram representing the post-knitting method for the synthesis of CMPs. (b) The structure of the cross-linkers. (c) The synthetic routes for the KCMPs: step-1 is palladium-catalyzed Suzuki coupling reaction, and step-2 is a Lewis acid catalyzed Friedel–Crafts reaction.

These KCMPs, which have a yellow appearance, were different from the light brown powders of the pristine CMPs, and morphological images were observed using scanning electron microscopy (SEM) images. As shown in Fig. S1–S4 (ESI), the KCMPs showed irregular micron-sized particles with amorphous features, while their sizes were obviously smaller than the corresponding CMPs. The appearance of these smaller particles may be due to the gradual growth and isolation of the polymeric nuclei during the crosslinking process, and this phenomenon can be also observed in some knitted HCPs using rigid aromatic building blocks.31

The rigid-flexible networks of the KCMPs were characterized using Fourier transform infrared spectroscopy (FTIR). As shown in the FTIR spectra, the knitted alkyl bonds (–CH2–) present on the networks of KCMPs were confirmed using strong C–H stretching vibrations at 2920 cm−1 (Fig. 1).31,46 Moreover, the vibrational stretches of the phenyl C–Hs at around 3050 cm−1 tended to weaken after the post-knitting process, indicating the partial aromatic carbons were substituted by methylene. The direct aryl–aryl bonds can also be generated between the benzene rings by the AlCl3 catalyzed Scholl reaction. For example, some POPs have been synthesized using a C–C cross coupling reaction between the benzene rings under similar synthetic conditions to the KCMPs.50,51 In addition, the characteristic peaks of the NH2-groups could easily remain on the skeleton of KCMP-M3 at 3400 cm−1, but a marked change occurred on the NH2-groups of KCMP-F3.52,53 The skeletons of KCMPs were further confirmed using solid state 13C cross-polarization magic angle spinning (CP/MAS) NMR. As shown in Fig. 2, there are two main chemical shift regions at 30–40 ppm for the alkyl carbons and 120–145 ppm for the aromatic carbons, and these are consistent with the FTIR analysis. Notably, all KCMPs showed higher ratios for the substituted aromatic carbon at approximately 135 ppm compared to the non-substituted aromatic carbon at approximately 125 ppm and that of the pristine CMPs, and this provided persuasive evidence that the alkyl knitting bridges formed among the phenyl rings. As shown in Fig. S4–S8 (ESI), owing to the flexible bridges on the KCMPs, it was observed that their thermal stability was poorer than the corresponding CMPs. Owing to the strong knitting activity of the DCM solvent, the KCMP-Ms exhibited a greater stability than the corresponding KCMP-Fs. KCMP-F3 showed the highest weight loss, close to 12 wt% below 300 °C, and this loss could be caused by the strong affinity between the NH2-groups and the adsorbed solvent or water. In other words, these resulting KCMP-Fs can remain stable close to 300 °C, and the KCMP-Ms can remain stable up to 400 °C. In addition, the resulting polymers were treated with different solvents, such as DMF, THF, DCM, MeOH, EtOH and water, during the activation processes, meaning they can remain stable in common organic solvents. These excellent stabilities are very significant for the related performances.


image file: d1qm00371b-f1.tif
Fig. 1 FTIR spectra of the resulting polymers.

image file: d1qm00371b-f2.tif
Fig. 2 Solid state 13C CP/MAS NMR of the resulting polymers.

Pore properties

The pore properties of the resulting polymers were measured and analyzed using N2 adsorption/desorption isotherms at 77 K. As shown in Fig. 3a–d, the DCM-knitted KCMP-Ms showed higher N2 uptakes compared to the corresponding FDA-knitted KCMP-Fs, meanwhile the N2 uptakes of the corresponding CMPs were the lowest. It is obvious that the porosities of the CMPs were improved by the post-knitting method. Their N2 adsorption isotherm types were classified according to the International Union of Pure and Applied Chemistry (IUPAC) classification.54 Among the pristine CMPs, CMP-1, CMP-2 and CMP-4 showed type I isotherms with a type IV character at higher relative pressures. CMP-2 and CMP-3 should exhibit similar topological networks and porosity according to the projected routes, but CMP-3 displayed a much lower N2 uptake. The low N2 uptake of CMP-3 may be caused by the fact that the –NH2 groups occupied part of the nanoporous space, meanwhile, the –NH2 groups can weaken the activity of the C–Br bond in the Suzuki coupling reaction, and the intermolecular hydrogen bonding induced by the –NH2 groups was not conductive to the growth of the porous skeleton.52,55 The obvious N2 uptake increases for the resulting KCMPs at a higher relative pressure (P/P0 > 0.9) and this may be caused by the presence of macropores during the post-knitting process. The pore properties, such as the surface area, pore volume and average pore size, are summarized in Table 1. All of the KCMPs showed higher surface areas compared to the pristine CMPs, while the surface areas of the DCM knitted KCMP-Ms were higher than those of the corresponding KCMP-Fs. From this, it can be summarized that KCMP-M2 displayed the highest SBET up to 2267 m2 g−1 with an increase of 1300 m2 g−1, 134.4% higher than that of the pristine CMP-2, whereas the remaining KCMPs showed an SBET from 494 to 2157 m2 g−1. Among the four KCMP-Fs, SBET follows the order of KCMP-F2 (1632 m2 g−1) > KCMP-F1 (1433 m2 g−1) > KCMP-F4 (1245 m2 g−1) > KCMP-F3 (494 m2 g−1). Meanwhile, the SBET order of the KCMP-Ms is KCMP-M2 (2267 m2 g−1) > KCMP-M4 (2157 m2 g−1) > KCMP-M1 (1845 m2 g−1) > KCMP-M3 (1321 m2 g−1). Among which, KCMP-M4 exhibited the largest increment of 1533 m2 g−1 compared to the pristine CMP-4, and the SBET of KCMP-M3 increased by 1491.6% compared to the pristine CMP-3. Their surface areas can be compared with many HCPs and CMPs.13,14 The pore volumes of the KCMP-Ms were outstanding and much higher than those of the corresponding KCMP-Fs and CMPs, and the highest total pore volume reached 3.27 cm3 g−1 of KCMP-M4, while the highest total pore volume of the pristine CMPs was only 0.77 cm3 g−1 of CMP-2. Meanwhile, the micropore volumes were also increased using this post-knitting method, and the highest micropore volumes of KCMP-F2 and KCMP-M4 can reach up to 0.34 cm3 g−1. Specifically, the surface area and pore volume of the KCMPs can be tailored using the cross-linker and CMPs network types, meaning that the solvent-knitting method and high-surface-area CMP are more beneficial for porosity formation. The pore size distributions (PSDs) of these resulting polymers were calculated using the density functional theory (DFT) model using the N2 adsorption curves as shown in Fig. 3e–h. In these pore size distribution (PSD) curves, the KCMPs retained a distinct peak near 1.5 nm with higher differential pore volumes from 0.40 to 2.11 cm3 g−1, and the cumulative pore volumes can increase up to 1.77 cm3 g−1 of KCMP-M2 before 50 nm (Fig. S9–S12, ESI). The post-knitting only resulted in a slight pore size change in the micropore regions, but their micropore volumes were very obviously increased. In addition, the KCMPs also showed abundant mesopores and macropores with the average pore sizes ranging from 2.9 to 6.9 nm.
image file: d1qm00371b-f3.tif
Fig. 3 (a–d) N2 adsorption/desorption isotherms of the resulting polymers. (e–h) Pore size distributions (PSD) of the resulting polymers.
Table 1 Pore properties of the resulting polymers
Samples S BET (m2 g−1) PIb (%) S L (m2 g−1) APSd (nm) V tot (cm3 g−1) V mic (cm3 g−1) V mic/Vtot (%)
a S BET calculated using the BET equation. b Percentage increase in the SBET of KCMPs to the corresponding CMPs. c S L calculated using Langmuir equation. d Adsorption average pore size calculated using the BET model. e Total pore volume calculated at P/P0 = 0.99. f Micropore volume calculated using a t-plot.
CMP-1 1071 1429 2.5 0.67 0.25 37.3
KCMP-F1 1433 33.8 1923 2.9 1.04 0.32 30.8
KCMP-M1 1845 72.3 2598 3.6 1.66 0.35 21.1
CMP-2 967 1338 3.2 0.77 0.18 23.4
KCMP-F2 1632 68.8 2327 5.1 2.08 0.34 16.3
KCMP-M2 2267 134.4 3295 5.3 3.00 0.31 10.3
CMP-3 83 125 17.9 0.37 0.01 2.7
KCMP-F3 494 495.2 655 5.1 0.64 0.12 18.8
KCMP-M3 1321 1491.6 1764 3.6 1.20 0.30 25.0
CMP-4 624 826 2.4 0.37 0.15 40.5
KCMP-F4 1245 99.5 1775 6.9 2.16 0.20 9.3
KCMP-M4 2157 245.7 3082 6.1 3.27 0.34 10.4


CO2 adsorption performance

The development of advanced CO2 adsorbing materials for preventing global warming is urgently needed. KCMPs have large surface areas, high pore volumes, well-defined nanopores, cost-effective ingredients, mild synthetic conditions, and good thermal stabilities, and hence, could be used for CCS technologies. In this work, the CO2 adsorption capacity of the resulting KCMPs was evaluated using CO2 adsorption isotherms at 273/298 K. As shown in Fig. 4a–d and Table 2, the CO2 adsorption capacity increased obviously using the post-knitting method, and the DCM-knitted KCMP-Ms showed a higher CO2 uptake compared to the FDA-knitted KCMP-Fs. Among which, the CO2 uptake of KCMP-M4 can reach 3.98 mmol g−1 (89.2 cm3 g−1) at 273 K and 1 bar, and it increased by 122.2% compared to the pristine CMP-4 (1.79 mmol g−1). Other KCMPs displayed CO2 uptakes ranging from 1.59 to 3.77 mmol g−1 under the same conditions. Among these KCMP-Ms, KCMP-M3, with the lowest surface area, showed the highest CO2 uptake increment of 2.53 mmol g−1 compared to the pristine CMP-3 at 273 K and 1 bar. Owing to the NH2-groups, KCMP-M3 exhibited the highest CO2 uptakes at 298 K with values of 0.52 and 2.54 mmol g−1 at 0.15 and 1 bar, respectively. Remarkably, the good performance of KCMP-M3 benefited from the NH2-groups on the pore wall, which exhibited strong dipole-quadrupole interactions with CO2 molecules.56 However, owing to the low surface area and changed NH2-groups, KCMP-F3 showed a very low CO2 uptake. It is remarkable that the CO2 uptake of KCMP-Fs matches well with the surface area and micropore volume orders of KCMP-F2 > KCMP-F1 > KCMP-F4 > KCMP-F3, among which the CO2 uptake of the best-performing KCMP-F2 can reach up to 3.51 mmol g−1 of 48.7% increment compared to the pristine CMP-2 at 273 K and 1 bar. It should be mentioned that the porosity, pore size and functional site all play important roles for CO2 adsorption.4 Specifically, the surface area plays the most important factor in CO2 adsorption at high pressure, while the micropore volume and functional site are more important factors at low pressure among these resulting KCMPs.4 As shown in Table 3, the CO2 adsorption capacity of the KCMPs were comparable to many analogous CMPs,25,57 and HCPs.31,51,52,58 In addition, the CO2 adsorption capacity of the KCMPs was compared to other POPs, such as some crystalline COFs,59 N-rich CTFs,60 and high-surface-area PAFs.61 The KCMPs showed an excellent CO2 adsorption capacity and were promising CO2 adsorbents.
image file: d1qm00371b-f4.tif
Fig. 4 (a–d) CO2 adsorption isotherms of these polymers at 273 K (closed) and 298 K (open). (e–h) Isosteric heats of adsorption for CO2 for these polymers.
Table 2 CO2 uptakes of the resulting polymers
Samples Cross-linkers CO2 uptakesa (0.15/1 bar; mmol g−1) CO2 uptakesb (0.15/1 bar; mmol g−1) Q st (kJ mol−1)
a CO2 uptake at 273 K. b CO2 uptake at 298 K. c CO2 isosteric heats of adsorption (Qst) near the zero-coverage region. The detail computational processes are given in the ESI (Fig. S13–S16).
CMP-1 0.48/2.30 0.21/1.16 23.21
KCMP-F1 FDA 0.58/2.70 0.29/1.48 14.81
KCMP-M1 DCM 0.88/3.77 0.42/2.09 15.43
CMP-2 0.56/2.36 0.25/1.27 26.53
KCMP-F2 FDA 0.86/3.51 0.40/1.93 26.53
KCMP-M2 DCM 0.80/3.69 0.39/2.02 21.50
CMP-3 0.33/1.08 0.14/0.56 33.59
KCMP-F3 FDA 0.42/1.59 0.21/0.91 15.70
KCMP-M3 DCM 0.97/3.61 0.52/2.54 27.92
CMP-4 0.45/1.79 0.20/0.98 30.12
KCMP-F4 FDA 0.46/2.14 0.22/1.14 25.87
KCMP-M4 DCM 0.90/3.98 0.42/2.15 24.37


Table 3 Comparison of the CO2 uptake of some representative POPs at 1 bar
Polymer genres Building blocks S BET (m2 g−1) CO2 uptakes (mmol g−1) Ref.
273 K 298 K
CMP 1,3,5-Triethynylbenzene 837 1.2 25
HCP Benzene 1391 3.1 31
Biphenyl 815 3.1
1,3,5-Triphenylbenzene 1059 3.6
HCP 1,3,5-Triphenylbenzene 2435 5.9 3.6 51
Hexaphenylbenzene 1790 4.5 2.7
Biphenyl 1555 4.0 2.7
HCP Tetraphenylmethane 1679 1.7 52
CMP 1,3,5-Benzene 1018 2.1 1.2 57
1,3,5-Triazine 963 2.4 1.3
HCP Fluoranthene 1788 5.6 3.4 58
Binaphthalene 1702 4.6 2.7
Naphthalene 1227 4.0 2.4
Phenanthrene 978 4.1 2.6
COF –OH/porphyrin 1284 1.4 0.8 59
–COOH/porphyrin 364 4.0 1.7
CTF Tetraphenylethylene/triazine 2235 1.7 1.0 60
Adamantane/triazine 1183 1.3 0.8
PAF Tetraphenylmethane 4023 1.2 (295 K) 61


To further study the interaction between the porous skeleton and CO2 molecules, the CO2 isosteric heats of adsorption (Qst) of these resulting polymers were calculated using the virial-type expression using two different temperatures as shown in Fig. 4e–h, and the detailed calculation processes are given in the ESI.[thin space (1/6-em)]62 Based on the higher micropore ratio, almost all pristine CMPs displayed higher Qst values than the corresponding KCMPs at low coverage (Table 2). Specially, CMP-3 with CO2-polarising groups showed the highest Qst of 33.59 kJ mol−1, which was 113.9% and 20.3% higher than that of the corresponding KCMP-F3 and KCMP-M3, respectively. The lower CO2Qst of KCMPs may be caused by the flexible methylene bridges with larger pore size, indicating that the high CO2 capacity of KCMPs benefited from the high surface area and micropore volume.

Given the highest CO2 uptake of KCMP-M3 was obtained at 298 K, this was used to evaluate the separation performance of CO2 over CH4 and N2. As shown in Fig. S17a (ESI), the CH4 and N2 adsorption isotherms of KCMP-M3 were measured at 298 K, and the CO2 uptake (2.54 mmol g−1) of KCMP-M3 was much higher than those of CH4 and N2 (0.55 and 0.11 mmol g−1) at 1 bar. The ratios of the initial slopes of these gas adsorption isotherms were used to estimate the CO2/CH4 and CO2/N2 adsorption selectivities (Fig. S17b, ESI).63 The adsorption selectivities of KCMP-M3 were up to 5.4 for CO2/CH4 and 29.1 for CO2/N2, respectively. This result clearly confirmed the potential separation capacity of the KCMPs for CO2.

Conclusion

In conclusion, we have developed a facile post-knitting method to reconstruct the porous skeletons of CMPs. Compared to pristine CMPs, the surface areas and CO2 uptakes of the KCMPs are highly increased using the tailored methods. The aromatic skeletons of CMPs can be cross-linked and expanded using methylene bonding with different cross-linkers. The highest surface area and CO2 uptake of these KCMPs can reach up to 2267 m2 g−1 and 3.98 mmol g−1 (273 K and 1 atm), increasing by 134% and 122% compared to the corresponding CMPs, respectively. Additionally, this implies that novel KCMPs could be designed and synthesized by adjusting the types of CMPs and cross-linkers, and this method can probably be applied to improve the application performances of POPs in contaminant capture, heterogeneous catalysts, energy storage, drug release and membrane separation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 21871104 and 21621001), and the 111 project of China (Grant No. B17020) for supporting this work.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Synthetic processes, TGA, and cumulative pore volume distributions. See DOI: 10.1039/d1qm00371b

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