Heteroatom-doped porous carbons are emerging as platforms for use in a wide range of applications including catalysis, energy storage, and gas separation or storage, among others. The use of high activation temperatures and heteroatom multiple-source precursors remain great challenges, and this study aims to addresses both issues. A series of highly porous N-doped carbon (CPC) materials was successfully synthesized by chemical activation of benzimidazole-linked polymers (BILPs) followed by thermolysis under argon. The high temperature synthesized CPC-700 reaches surface area and pore volume as high as 3240 m2 g–1 and 1.51 cm3 g–1, respectively, while low temperature activated CPC-550 exhibits the highest ultramicropore volume of 0.35 cm3 g–1. The controlled activation process endows CPCs with diverse textural properties, adjustable nitrogen content (1–8 wt %), and remarkable gas sorption properties. In particular, exceptionally high CO2 uptake capacities of 5.8 mmol g–1 (1.0 bar) and 2.1 mmol g–1 (0.15 bar) at ambient temperature were obtained for materials prepared at 550 °C due to a combination of a high level of N-doping and ultramicroporosity. Furthermore, CPCs prepared at higher temperatures exhibit remarkable total uptake for CO2 (25.7 mmol g–1 at 298 K and 30 bar) and CH4 (20.5 mmol g–1 at 298 K and 65 bar) as a result of higher total micropores and small mesopores volume. Interestingly, the N sites within the imidazole rings of BILPs are intrinsically located in pyrrolic/pyridinic positions typically found in N-doped carbons. Therefore, the chemical and physical transformation of BILPs into CPCs is thermodynamically favored and saves significant amounts of energy that would otherwise be consumed during carbonization processes.

Triazine-based benzimidazole-linked polymers (TBILPs), TBILP-1 and TBILP-2, were synthesized by condensation reaction of 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (TFPT) with 1,2,4,5-benzenetetraamine tetrachloride (BTA) and 2,3,6,7,14,15-hexaaminotriptycene (HATT), respectively. The Ar sorption isotherms at 87 K revealed high surface areas for TBILP-1 (330 m 2 g −1 ) and TBILP-2 (1080 m 2 g −1 ). TBILP-2 adsorbed significantly high CO 2 (5.19 mmol g −1 /228 mg g −1 ) at 1 bar and 273 K. Initial slope selectivity calculations demonstrated that TBILP-1 has very high selectivity for CO 2 over N 2 (63) at 298 K, outperforming all triazine-based porous organic polymers reported to date. On the other hand, the larger surface area of TBILP-2 leads to relatively lower selectivity for CO 2 /N 2 (40) and CO 2 /CH 4 (7) at 298 K. TBILPs showed moderate isosteric heats of adsorption for CO 2 : TBILP-1 (35 kJ mol −1 ) and TBILP-2 (29 kJ mol −1 ) enabling high and reversible CO 2 uptake at ambient temperature. In addition, TBILPs displayed promising working capacity, regenerability, and sorbent selection parameter values for CO 2 capture from gas mixtures under vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions.

A synergistic experimental and theoretical study (DFT) highlights the impact of material design at the molecular and electronic levels on the binding affinity and interaction sites of CO2 with benzimidazole-linked polymers (BILPs); CO2 is stabilized by benzimidazole units through Lewis acid-base (N···CO2) and aryl C-H···O=C=O interactions.

Benzimidazole-linked polymers (BILPs) are emerging candidates for gas storage and separation applications; however, their current synthetic methods offer limited control over textural properties which are vital for their multifaceted use. In this study, we investigate the impact of acid-catalyzed formation rates of the imidazole units on the porosity levels of BILPs and subsequent effects on CO2 and CH4 binding affinities and selective uptake of CO2 over CH4 and N2. Treatment of 3,3'-Diaminobenzidine tetrahydrochloride hydrate with 1,2,4,5-tetrakis(4-formylphenyl)benzene and 1,3,5-(4-formylphenyl)-benzene in anhydrous DMF afforded porous BILP-15 (448 m(2) g(-1)) and BILP-16 (435 m(2) g(-1)), respectively. Alternatively, the same polymers were prepared from the neutral 3,3'-Diaminobenzidine and catalytic amounts of aqueous HCl. The resulting polymers denoted BILP-15(AC) and BILP-16(AC) exhibited optimal surface areas; 862 m(2) g(-1) and 643 m(2) g(-1), respectively, only when 2 equiv of HCl (0.22 M) was used. In contrast, the CO2 binding affinity (Qst) dropped from 33.0 to 28.9 kJ mol(-1) for BILP-15 and from 32.0 to 31.6 kJ mol(-1) for BILP-16. According to initial slope calculations at 273 K/298 K, a notable change in CO2/N2 selectivity was observed for BILP-15(AC) (61/50) compared to BILP-15 (83/63). Similarly, ideal adsorbed solution theory (IAST) calculations also show the higher specific surface area of BILP-15(AC) and BILP-16(AC) compromises their CO2/N2 selectivity.