Highly porous photoluminescent diazaborole-linked polymers: synthesis, characterization, and application to selective gas adsorption

The formation of boron–nitrogen (B–N) bonds has been widely explored for the synthesis of small molecules, oligomers, or linear polymers; however, its use in constructing porous organic frameworks remains very scarce. In this study, three highly porous diazaborole-linked polymers (DBLPs) have been synthesized by condensation reactions using 2,3,6,7,14,15-hexaaminotriptycene and aryl boronic acids. DBLPs are microporous and exhibit high Brunauer–Emmett–Teller surface area (730–986 m2 g−1) which enable their use in small gas storage and separation. At ambient pressure, the amorphous polymers show high CO2 (DBLP-4: 4.5 mmol g−1 at 273 K) and H2 (DBLP-3: 2.13 wt% at 77 K) uptake while their physicochemical nature leads to high CO2/N2 (35–42) and moderate CO2/CH4 (4.9–6.2) selectivity. The electronic impact of integrating diazaborole moieties into the backbone of these polymers was investigated for DBLP-4 which exhibits green emission with a broad peak ranging from 350 to 680 nm upon excitation with 340 nm in DMF without photobleaching. This study demonstrates the effectiveness of B–N formation in targeting highly porous frameworks with promising optical properties

Covalent organic frameworks

The first members of covalent organic frameworks (COF) have been designed and successfully synthesized by condensation reactions of phenyl diboronic acid C6H4[B(OH)2]2 and hexahydroxytriphenylene C18H6(OH)6. The high crystallinity of the products (C3H2BO)6 (C9H12)1 (COF-1) and C9H4BO2 (COF-5) has allowed definitive resolution of their structure by powder X-ray diffraction methods which reveal expanded porous graphitic layers that are either staggered (COF-1, P63/mmc) or eclipsed (COF-5, P6/mmm). They exhibit high thermal stability (to temperatures up to 500- to 600-C), permanent porosity, and high surface areas (711 and 1590 m2/g, respectively) surpassing those of related inorganic frameworks. A similar approach has been used for the design of other extended structures

Metal-organic and covalent organic frameworks (MOFs and COFs) as adsorbents for environmentally significant gases (H2, CO2, and CH4)

A series of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) possessing various functionalities, pore structures, and surface areas were evaluated for sorption and storage properties of environmentally significant gases (H_2, CO_2, and CH_4). It was concluded that the gas sorption behavior follows a general trend that materials with high surface area show enhanced gas uptake performance. For example, MOF-177 (SA = 5200 m^2/g) captures 7.2 wt% of H_2 at 77 K and 19 wt% of CH_4 at 298 K. In addition, MOF-177 exhibits exceptionally high gravimetric CO_2 uptake up to 120 wt% at 298 K. Similarly, the gas storage capacity for COFs seems to follow the same trend and it is determined by the apparent surface area. The architectural stability of both COFs and MOFs upon high pressure H_2 and CH_4 gas sorption measurements were manifested by isotherms which reach saturation without significant hysteresis

Designed Synthesis of 3D Covalent Organic Frameworks

Three-dimensional covalent organic frameworks (3D COFs) were synthesized by targeting two nets based on triangular and tetrahedral nodes: ctn and bor. The respective 3D COFs were synthesized as crystalline solids by condensation reactions of tetrahedral tetra(4-dihydroxyborylphenyl) methane or tetra(4-dihydroxyborylphenyl)silane and by co-condensation of triangular 2,3,6,7,10,11-hexahydroxytriphenylene. Because these materials are entirely constructed from strong covalent bonds (C-C, C-O, C-B, and B-O), they have high thermal stabilities (400° to 500°C), and they also have high surface areas (3472 and 4210 square meters per gram for COF-102 and COF-103, respectively) and extremely low densities (0.17 grams per cubic centimeter)

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

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

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

Pyrene Bearing Azo-Functionalized Porous Nanofibers for CO2 Separation and Toxic Metal Cation Sensing

This article describes the construction of a novel luminescent azo-linked polymer from 1,3,6,8-tetra(4--aminophenyl)pyrene using a copper(I)-catalyzed oxidative homocoupling reaction

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