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 (Q_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.