Effect of Acid-Catalyzed Formation Rates of Benzimidazole-Linked Polymers on Porosity and Selective CO₂ Capture from Gas Mixtures

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 m² g^–1) and BILP-16 (435 m² 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 m² g^–1 and 643 m² g^–1, respectively, only when 2 equiv of HCl (0.22 M) was used. In contrast, the CO₂ binding affinity (<i>Q</i>_st) 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

Exceptional Gas Adsorption Properties by Nitrogen-Doped Porous Carbons Derived from Benzimidazole-Linked Polymers

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

Highly Selective CO₂ Capture by Triazine-Based Benzimidazole-Linked Polymers

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 m² g^–1) and TBILP-2 (1080 m² g^–1). TBILP-2 adsorbed significantly high CO₂ (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 CO₂ over N₂ (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 CO₂/N₂ (40) and CO₂/CH₄ (7) at 298 K. TBILPs showed moderate isosteric heats of adsorption for CO₂: TBILP-1 (35 kJ mol^–1) and TBILP-2 (29 kJ mol^–1) enabling high and reversible CO₂ uptake at ambient temperature. In addition, TBILPs displayed promising working capacity, regenerability, and sorbent selection parameter values for CO₂ capture from gas mixtures under vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions