Cooper et al. investigated the carbon dioxide adsorption of CTFs prepared by the catalysis of superacids. They revealed an exceptional adsorption capacity up to 4.17 mmol g
−1 for CTFs [
10]. In their work, P6M had the highest adsorption capacity for carbon dioxide even though it was not the polymer with the highest Brunauer-Emmett-Teller (BET) surface area. They also studied the adsorption selectivity over nitrogen, for which P1M had the highest CO
2/N
2 selectivity of 31.2, although it had the lowest surface area. Janiak et al. reported a new adamantine-based CTF in 2013, which was prepared in high yields using either ZnCl
2 or CF
3SO
3H as catalysts. They optimized the synthetic conditions and Lewis acid catalysis conditions. Under the optimized conditions, the highest BET surface of 2235 m
2 g
−1 and an adsorption of 73 cm
3 g
−1 CO
2 were obtained. They also found that the CTF has high gas adsorption selectivity corresponding to 41:1 for CO
2/N
2 and 7:1 for CO
2/CH
4 [
65]. Han et al. designed perfluorinated CTFs (FCTF-1) and showed the introduction of fluorine has an obvious positive effect on CO
2 adsorption and separation [
66]. The FCTF-1 can uptake 1.76 mmol g
−1 CO
2 at low pressure (0.1 bar) and 273 K. They also found that FCTF-1 has a high selectivity of 77 for CO
2/N
2 separation, which is due to its ultramicropores. Interestingly, due to the hydrophobic nature of fluorine, the TCTF-1 is tolerant to water in gas separation. The good gas adsorption ability of CTFs is partially due to the high nitrogen content. Therefore, it is critical to introduce nitrogen in the framework of CTFs. Indeed, Lotsch et al. introduced a series of nitrogen atoms in the CTFs and achieved high CO
2 adsorption and selectivity [
67]. They showed that bipyridine–CTFs have the highest CO
2 uptake up to 5.58 mmol g
−1 at 273 K and have a high CO
2/N
2 selectivity up to 189. They revealed that microporosity was the main factor for high CO
2 uptake, and the N content was the main contributor for high selectivity. Janiak et al. developed a new strategy to synthesize CTFs with two linkers [
68], which showed better CO
2 adsorption and separation performance than the individual linkers [
69]. The higher CO
2 adsorption of mixed-nitrile CTFs was attributed to the higher micropore content and microporous volume. Tuci et al. developed a new
N-doped CTFs with microporosity (CTF-py) showing excellent CO
2 adsorption ability at 0.1 bar, with 2.03 and 1.12 mmol g
−1 at 273 K and 298 K, respectively, which is better than that of TCTF-1. They also showed high CO
2 adsorption at 1 bar, with 5.97 and 4.22 mmol g
−1 at 273 K and 298 K, respectively [
70]. Voort et al. designed N-heteroaromatic structures as polymerization monomers for CTFs and synthesized CTFs by optimization of the catalyst and monomer ratios and polymerization temperatures. They found that CTF-20-400 had the highest CO
2 uptake ability in the series, with a capacity of 3.48 mmol g
−1 at 1 bar and 273 K [
71]. Yu et al. also introduced N-heteroacyclic units (carbazole) for CTFs, which can capture and fix CO
2. They synthesized CTF–CSUs (CSU: Central South University) using carbazole nitriles monomers using a bottom-up strategy, with a highest BET surface area of 982 m
2 g
−1 and a good CO
2 uptake ability of 12.9 wt % at 273 K and 1 bar. They further showed that CO
2 can be fixed into cyclic carbonate with high yields [
72]. Voort et al. recent reported an acetylacetone CTF (acac-CTF), introducing dual N and O sites in the CTFs. The acac-CTF can achieve a BET surface area as high as 1626 m
2g
−1 and can exhibit a high CO
2 upake of 3.3 mmol g
−1 at 273 K and 1 bar. Good selectivity towards N
2 was also shown, with a CO
2/N
2 up to 46 at 298 K [
55]. On the other hand, porous carbon based on CTFs was also developed for CO
2 adsorption. For example, Han et al. reported a series of triazine-based porous carbon materials (TPCs) which were constructed from fluorinated aromatic nitrile building blocks using the ionothermal synthesis method, resulting in triaizne-based porous carbon materials. The highest achievable BET surface area was 1940 m
2g
−1, and the best CO
2 adsorption ability could be up to 4.9 mmol g
−1 at 273 K, 1.0 bar [
73].
Besides heteroatom introduction, adoption of charge units in the frameworks can also be a promising strategy to capture and fix CO
2. To this end, Coskun et al. designed a dicationic viologen structure as a building block for CTFs and prepared CTFs at ionothermal conditions. The highest surface area reached was up to 1247 m
2 g
−1, and the CO
2 uptake could reach 133 mmol g
−1 at 1 bar and 273 K. More interestingly, CO
2 can be fixed to cyclic carbonates [
27]. There are also some other reports of triazine-based porous materials, such as triaizne-based covalent organic framework (COF) with BET surface area of 609 m
2 g
−1, which can uptake 57.07 wt % CO
2 at 273 K and 5 bar [
74].