Synthesis and Characterization of Porous Benzimidazole-Linked Polymers and Their Performance in Small Gas Storage and Selective Uptake

Porous organic polymers containing nitrogen-rich building units are among the most promising materials for selective CO₂ capture and separation which can have a tangible impact on the environment and clean energy applications. Herein we report on the synthesis and characterization of four new porous benzimidazole-linked polymers (BILPs) and evaluate their performance in small gas storage (H₂, CH₄, CO₂) and selective CO₂ binding over N₂ and CH₄. BILPs were synthesized in good yields by the condensation reaction between aryl-<i>o</i>-diamine and aryl-aldehyde building blocks. The resulting BILPs exhibit moderate surface area (SA_BET = 599–1306 m² g^–1), high chemical and thermal stability, and remarkable gas uptake and selectivity. The highest selectivity based on initial slope calculations at 273 K was observed for BILP-2: CO₂/N₂ (113) and CO₂/CH₄ (17), while the highest storage capacity was recorded for BILP-4: CO₂ (24 wt % at 273 K and 1 bar) and H₂ (2.3 wt % at 77 K and 1 bar). These selectivities and gas uptakes are among the highest by porous organic polymers known to date which in addition to the remarkable chemical and physical stability of BILPs make this class of material very promising for future use in gas storage and separation applications

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

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

Systematic Postsynthetic Modification of Nanoporous Organic Frameworks for Enhanced CO₂ Capture from Flue Gas and Landfill Gas

Controlled postsynthetic nitration of NPOF-1, a nanoporous organic framework constructed by nickel(0)-catalyzed Yamamoto coupling of 1,3,5-tris­(4-bromophenyl)­benzene, has been performed and is proven to be a promising route to introduce nitro groups and to convert mesopores to micropores without compromising surface area. Reduction of the nitro groups yields aniline-like amine-functionalized NPOF-1-NH₂ that has a micropore volume of 0.48 cm³ g^–1, which corresponds to 71% of the total pore volume and a Brunauer–​Emmett–​Teller surface area of 1535 m² g^–1. Adequate basicity of the amine functionalities leads to modest isosteric heats of adsorption for CO₂, which allow for high regenerability. The unique combination of high surface area, microporous structure, and amine-functionalized pore walls enables NPOF-1-NH₂ to have remarkable CO₂ working capacity values for removal from landfill gas and flue gas. The performance of NPOF-1-NH₂ in CO₂ removal ranks among the best by porous organic materials

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

Copper(I)-Catalyzed Synthesis of Nanoporous Azo-Linked Polymers: Impact of Textural Properties on Gas Storage and Selective Carbon Dioxide Capture

A new facile method for synthesis of porous azo-linked polymers (ALPs) is reported. The synthesis of ALPs was accomplished by homocoupling of aniline-like building units in the presence of copper­(I) bromide and pyridine. The resulting ALPs exhibit high surface areas (SA_BET = 862–1235 m² g^–1), high physiochemical stability, and considerable gas storage capacity especially at high-pressure settings. Under low pressure conditions, ALPs have remarkable CO₂ uptake (up to 5.37 mmol g^–1 at 273 K and 1 bar), as well as moderate CO₂/N₂ (29–43) and CO₂/CH₄ (6–8) selectivity. Low pressure gas uptake experiments were used to calculate the binding affinities of small gas molecules and revealed that ALPs have high heats of adsorption for hydrogen (7.5–8 kJ mol^–1), methane (18–21 kJ mol^–1), and carbon dioxide (28–30 kJ mol^–1). Under high pressure conditions, the best performing polymer, ALP-1, stores significant amounts of H₂ (24 g L^–1, 77 K/70 bar), CH₄ (67 g L^–1, 298 K/70 bar), and CO₂ (304 g L^–1, 298 K/40 bar)

Nitrogen-Rich Porous Polymers for Carbon Dioxide and Iodine Sequestration for Environmental Remediation

The use of fossil fuels for energy production is accompanied by carbon dioxide release into the environment causing catastrophic climate changes. Meanwhile, replacing fossil fuels with carbon-free nuclear energy has the potential to release radioactive iodine during nuclear waste processing and in case of a nuclear accident. Therefore, developing efficient adsorbents for carbon dioxide and iodine capture is of great importance. Two nitrogen-rich porous polymers (NRPPs) derived from 4-bis-(2,4-diamino-1,3,5-triazine)-benzene building block were prepared and tested for use in CO₂ and I₂ capture. Copolymerization of 1,4-bis-(2,4-diamino-1,3,5-triazine)-benzene with terephthalaldehyde and 1,3,5-tris­(4-formylphenyl)­benzene in dimethyl sulfoxide at 180 °C afforded highly porous NRPP-1 (SA_BET = 1579 m² g^–1) and NRPP-2 (SA_BET = 1028 m² g^–1), respectively. The combination of high nitrogen content, π-electron conjugated structure, and microporosity makes NRPPs very effective in CO₂ uptake and I₂ capture. NRPPs exhibit high CO₂ uptakes (NRPP-1, 6.1 mmol g^–1 and NRPP-2, 7.06 mmol g^–1) at 273 K and 1.0 bar. The 7.06 mmol g^–1 CO₂ uptake by NRPP-2 is the second highest value reported to date for porous organic polymers. According to vapor iodine uptake studies, the polymers display high capacity and rapid reversible uptake release for I₂ (NRPP-1, 192 wt % and NRPP-2, 222 wt %). Our studies show that the green nature (metal-free) of NRPPs and their effective capture of CO₂ and I₂ make this class of porous materials promising for environmental remediation

Rapid Formation of Metal–Organic Frameworks (MOFs) Based Nanocomposites in Microdroplets and Their Applications for CO₂ Photoreduction

A copper-based metal–organic framework (MOF), [Cu₃(TMA)₂(H₂O)₃]_<i>n</i> (also known as HKUST-1, where TMA stands for trimesic acid), and its TiO₂ nanocomposites were directly synthesized in micrometer-sized droplets via a rapid aerosol route for the first time. The effects of synthesis temperature and precursor component ratio on the physicochemical properties of the materials were systematically investigated. Theoretical calculations on the mass and heat transfer within the microdroplets revealed that the fast solvent evaporation and high heat transfer rates are the major driving forces. The fast droplet shrinkage because of evaporation induces the drastic increase in the supersaturation ratio of the precursor, and subsequently promotes the rapid nucleation and crystal growth of the materials. The HKUST-1-based nanomaterials synthesized via the aerosol route demonstrated good crystallinity, large surface area, and great photostability, comparable with those fabricated by wet-chemistry methods. With TiO₂ embedded in the HKUST-1 matrix, the surface area of the composite is largely maintained, which enables significant improvement in the CO₂ photoreduction efficiency, as compared with pristine TiO₂. In situ diffuse reflectance infrared Fourier transform spectroscopy analysis suggests that the performance enhancement was due to the stable and high-capacity reactant adsorption by HKUST-1. The current work shows great promise in the aerosol route’s capability to address the mass and heat transfer issues of MOFs formation at the microscale level, and ability to synthesize a series of MOFs-based nanomaterials in a rapid and scalable manner for energy and environmental applications

Enhanced Carbon Dioxide Capture from Landfill Gas Using Bifunctionalized Benzimidazole-Linked Polymers

Tuning the binding affinity of small gases and their selective uptake by porous adsorbents are vital for effective CO₂ removal from gas mixtures for environmental protection and fuel upgrading. In this study, an amine-functionalized benzimidazole-linked polymer (BILP-6-NH₂) was synthesized by a combination of pre- and postsynthetic modification techniques in two steps. Presynthetic incorporation of nitro groups resulted in stoichiometric functionalization (1 nitro/phenyl) in addition to noninvasive functionalization, where more than 80% of the surface area maintained compared to BILP-6. Experimental studies presented enhanced CO₂ uptake and CO₂/CH₄ selectivity in BILP-6-NH₂ compared to BILP-6, which are governed by the synergetic effect of benzimidazole and amine moieties. DFT calculations were used to understand the interaction modes of CO₂ with BILP-6-NH₂ and confirmed the efficacy of amine groups. Encouraged by the enhanced uptake and selectivity in BILP-6-NH₂, we have evaluated its performance in landfill gas separation under vacuum swing adsorption (VSA) settings, which resulted in very promising working capacity and sorbent selection parameters outperforming most of the best solid adsorbent in the literature

Effective Approach for Increasing the Heteroatom Doping Levels of Porous Carbons for Superior CO₂ Capture and Separation Performance

Development of efficient sorbents for carbon dioxide (CO₂) capture from flue gas or its removal from natural gas and landfill gas is very important for environmental protection. A new series of heteroatom-doped porous carbon was synthesized directly from pyrazole/KOH by thermolysis. The resulting pyrazole-derived carbons (PYDCs) are highly doped with nitrogen (14.9–15.5 wt %) as a result of the high nitrogen-to-carbon ratio in pyrazole (43 wt %) and also have a high oxygen content (16.4–18.4 wt %). PYDCs have a high surface area (SA_BET = 1266–2013 m² g^–1), high CO₂ <i>Q</i>_st (33.2–37.1 kJ mol^–1), and a combination of mesoporous and microporous pores. PYDCs exhibit significantly high CO₂ uptakes that reach 2.15 and 6.06 mmol g^–1 at 0.15 and 1 bar, respectively, at 298 K. At 273 K, the CO₂ uptake improves to 3.7 and 8.59 mmol g^–1 at 0.15 and 1 bar, respectively. The reported porous carbons also show significantly high adsorption selectivity for CO₂/N₂ (128) and CO₂/CH₄ (13.4) according to ideal adsorbed solution theory calculations at 298 K. Gas breakthrough studies of CO₂/N₂ (10:90) at 298 K showed that PYDCs display excellent separation properties. The ability to tailor the physical properties of PYDCs as well as their chemical composition provides an effective strategy for designing efficient CO₂ sorbents