Effect of Acid-Catalyzed Formation Rates of Benzimidazole-Linked Polymers on Porosity and Selective CO 2 Capture from Gas Mixtures Suha Altarawneh, §,‡ Timur I ̇ slamoğ lu, § Ali Kemal Sekizkardes, § and Hani M. El-Kaderi* ,§ § Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States * S Supporting Information ABSTRACT: Benzimidazole-linked polymers (BILPs) are emerging candidates for gas storage and separation applica- tions; 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 CO 2 and CH 4 binding affinities and selective uptake of CO 2 over CH 4 and N 2 . Treatment of 3,3′-Diaminobenzidine tetrahydro- chloride 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 CO 2 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 CO 2 /N 2 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 CO 2 /N 2 selectivity. 1. INTRODUCTION Acid-catalyzed covalent bond formation plays an important role in organic synthesis. Generally, the rate of bond formation and catalyst (acid) amounts are not crucial for the synthesis of discrete molecules, since they can be isolated and purified from the reaction mixture. However, controlling these parameters during the synthesis of porous organic polymers or frameworks is essential, as they dictate porosity levels and solid-state packing. 1,2 For example, several classes of porous organic polymers (POPs) were synthesized by acid-catalyzed processes that lead to CN bond formation as in the case of imine- linked covalent organic frameworks (COFs) 3-5 and poly- benzimidazole (PBIs). 5-10 In both systems, condensation reactions are facilitated by controlling acid loading and the temperature of the reaction. 11,12 Because water is generated during condensation steps, it can mediate the rate of reversible bond formation as in the case of imine-linked polymers and hence control both porosity and crystalinity of the resulting materials. 13-15 The role of water in such reactions was clearly outlined for COFs generated from boronic acids in self- condensation reactions or copolymerization with aryl-diols. 2 Likewise, the synthesis of PBIs was accomplished by applying acid-catalyzed polycondensation reactions between diamine moieties and aryl-carboxylic acids or aryl-aldehydes and esters in the presence of polyphosphoric acid. 11,16,17 However, the resulting PBIs have very modest surface area (∼200 m 2 g -1 ) due to the lack of controlled polymerization rates. We have recently investigated the use of BILPs in selective CO 2 capture from gas mixtures containing N 2 and CH 4 as in the case of flue gas and natural gas, respectively. BILPs are very promising because of their high porosity, physiochemical stability, and ability to selectivity bind CO 2 over other gases which is mainly due to basic N-functionality within the framework that provides interaction sites for CO 2 . 9,18 In contrast to alcohol amine-solutions (i.e., MEA) which are volatile, thermally unstable, and toxic, POPs bind CO 2 with moderate affinities and thus enable rapid adsorbent regener- ation with a low energy penalty. 19 In this study, we seek to unravel the role of acid concentration and ratio on the textural properties of BILPs. We have synthesized new polymers using our previously reported method, wherein HCl-salts of the amine-containing building units lead to uncontrolled condensation rates and thus surface area. The resulting BILPs were resynthesized using HCl-free amine building units in the presence of diluted acid concentrations and variable acid/amine ratios. The impact of Received: November 25, 2014 Revised: February 11, 2015 Accepted: March 2, 2015 Article pubs.acs.org/est © XXXX American Chemical Society A DOI: 10.1021/es505760w Environ. Sci. Technol. XXXX, XXX, XXX-XXX