Increasing the CO2/N2 Selectivity with a Higher Surface Density of Pyridinic Lewis Basic Sites in Porous Carbon Derived from a Pyridyl‐Ligand‐Based Metal–Organic Framework

The development of functional porous carbon with high CO₂/N₂ selectivity is of great importance for CO₂ capture. In this paper, a type of porous carbon with doped pyridinic sites (termed MOFC) was prepared from the carbonization of a pyridyl‐ligand based MOF. Four MOFCs derived from different carbonizing temperatures were prepared. Structural studies revealed high contents of pyridinic‐N groups and nearly the same pore‐size distributions for these MOFCs. Gas‐sorption studies revealed outstanding CO₂ uptake at low pressures and room temperature. Owing to the high content of pyridinic‐N groups, the CO₂/N₂ selectivity on these MOFCs exhibits values of about 40–50, which are among the top values in carbon materials. Further correlation studies demonstrated that the CO₂/N₂ selectivities show a positive linear relationship with the surface density of pyridinic‐N groups, thus validating the synergistic effect of the doped pyridinic‐N groups on CO₂ adsorption selectivity.     

Ligand and Metal Effects on the Stability and Adsorption Properties of an Isoreticular Series of MOFs Based on T‐Shaped Ligands and Paddle‐Wheel Secondary Building Units

The synthesis of stable porous materials with appropriate pore size and shape for desired applications remains challenging. In this work a combined experimental/computational approach has been undertaken to tune the stability under various conditions and the adsorption behavior of a series of MOFs by subtle control of both the nature of the metal center (Co²⁺, Cu²⁺, and Zn²⁺) and the pore surface by the functionalization of the organic linkers with amido and N‐oxide groups. In this context, six isoreticular MOFs based on T‐shaped ligands and paddle‐wheel units with ScD_0.33 topology have been synthesized. Their stabilities have been systematically investigated along with their ability to adsorb a wide range of gases (N₂, CO₂, CH₄, CO, H_2, light hydrocarbons (C₁–C₄)) and vapors (alcohols and water). This study has revealed that the MOF frameworks based on Cu²⁺ are more stable than their Co²⁺ and Zn²⁺ analogues, and that the N‐oxide ligand endows the MOFs with a higher affinity for CO₂ leading to excellent selectivity for this gas over other species.     

Engineering porous organic polymers for carbon dioxide capture

As atmospheric CO₂ levels rise, the development of physical or chemical adsorbents for CO₂ capture and separation is of great importance on the way towards a sustainable low-carbon future. Porous organic polymers are promising candidates for CO₂ capture materials owing to their structural flexibility, high surface area, and high stability. In this review, we highlight high-performance porous organic polymers for CO₂ capture and summarize the strategies to enhance CO₂ uptake and selectivity, such as increasing surface area, increasing interaction between porous organic polymers and CO₂, and pore surface functionalization.     

Synthesis and evaluation of N,O‐doped hypercrosslinked polymers and their performance in CO2 capture

A series of N, O‐doped hypercrosslinked microporous polymers (HCPs) with high surface area and rich microporosity were constructed via facile Friedel‐Crafts alkylation promoted by anhydrous ferric chloride (FeCl₃), which used benzyl alcohol (BA) and 2‐phenylimidazole (PID) as the basic building blocks. The effects of structural composition on the pore properties and gas adsorption properties of prepared HCPs were systematically investigated. The results show that by adjusting the ratio of PID to BA, the Brunauer–Emmett–Teller (BET) specific surface area and pore volume of the prepared HCPs can be controlled, and the BET specific surface area of the polymers range from 732–992 m²/g. Gas uptake experiments indicate that the obtained HCPs exhibit very high CO₂ adsorption capacity up to 3.55 mmol/g at 273 K/1.0 bar. Simultaneously, these N, O‐doped HCPs also show quite high isosteric heat of CO₂ sorption at the low coverage (up to 33 kJ/mol). In addition, the prepared HCPs have excellent selective adsorption performance, and the optimal selective adsorption of CO₂/N₂ is 62. These results demonstrate that these prepared HCPs are potential candidates for applications in CO₂ capture.     

Synthesis and Gas Adsorption Properties of Tetra‐Armed Microporous Organic Polymer Networks Based on Triphenylamine

Two novel tetra‐armed microporous organic polymers have been designed and synthesized via a nickel‐catalyzed Yamamoto‐type Ullmann cross‐coupling reaction or Suzuki cross‐coupling polycondensation. These polymers are stable in various solvents, including concentrated hydrochloric acid, and are thermally stable. The homocoupled polymer YPTPA shows much higher Brunauer–Emmet–Teller‐specific surface area up to 1557 m² g⁻¹ than the copolymer SPTPA (544 m² g⁻¹), and a high CO₂ uptake ability of 3.03 mmol g⁻¹ (1.13 bar/273 K) with a CO₂/N₂ sorption selectivity of 17.3:1. Both polymers show high isosteric heats of CO₂ adsorption (22.7–26.5 kJ mol⁻¹) because the incorporation of nitrogen atoms into the skeleton of microporous organic polymers enhances the interaction between the pore wall and the CO₂ molecules. The values are higher than those of the porous aromatic frameworks, which contain neither additional polar functional groups nor nitrogen atoms, and are rather close to those of previously reported microporous organic polymers containing the nitrogen atoms on the pore wall. These data show that these materials would be potential candidates for applications in post‐combustion CO₂ capture and sequestration technology.

     

Directing the Structural Features of N2‐Phobic Nanoporous Covalent Organic Polymers for CO2 Capture and Separation

A family of azo‐bridged covalent organic polymers (azo‐COPs) was synthesized through a catalyst‐free direct coupling of aromatic nitro and amine compounds under basic conditions. The azo‐COPs formed 3D nanoporous networks and exhibited surface areas up to 729.6 m² g^?1, with a CO₂‐uptake capacity as high as 2.55 mmol g^?1 at 273 K and 1 bar. Azo‐COPs showed remarkable CO₂/N₂ selectivities (95.6–165.2) at 298 K and 1 bar. Unlike any other porous material, CO₂/N₂ selectivities of azo‐COPs increase with rising temperature. It was found that azo‐COPs show less than expected affinity towards N₂ gas, thus making the framework “N₂‐phobic”, in relative terms. Our theoretical simulations indicate that the origin of this unusual behavior is associated with the larger entropic loss of N₂ gas molecules upon their interaction with azo‐groups. The effect of fused aromatic rings on the CO₂/N₂ selectivity in azo‐COPs is also demonstrated. Increasing the π‐surface area resulted in an increase in the CO₂‐philic nature of the framework, thus allowing us to reach a CO₂/N₂ selectivity value of 307.7 at 323 K and 1 bar, which is the highest value reported to date. Hence, it is possible to combine the concepts of “CO₂‐philicity” and “N₂‐phobicity” for efficient CO₂ capture and separation. Isosteric heats of CO₂ adsorption for azo‐COPs range from 24.8–32.1 kJ mol^?1 at ambient pressure. Azo‐COPs are stable up to 350 °C in air and boiling water for a week. A promising cis/trans isomerization of azo‐COPs for switchable porosity is also demonstrated, making way for a gated CO₂ uptake.     

Mediating Order and Modulating Porosity by Controlled Hydrolysis in a Phosphonate Monoester Metal–Organic Framework

A crystalline and permanently porous copper phosphonate monoester framework has been synthesized from a tetraaryl trigonal phosphonate monoester linker. This material has a surface area over 1000 m² g^?1, as measured by N₂ sorption, the highest reported for a phosphonate‐based metal–organic framework (MOF). The monoesters result in hydrophobic pore surfaces that give a low heat of adsorption for CO₂ and low calculated selectivity for CO₂ over N₂ and CH₄ in binary mixtures. By careful manipulation of synthetic conditions, it is possible to selectively remove some of the monoesters lining the pore to form a hydrogen phosphonate while giving an isomorphous structure. This increases the affinity of the framework for CO₂ giving higher ambient uptake, higher heat of adsorption, and much higher calculated selectivity for CO₂ over both N₂ and CH₄. Formation of the acid groups is noteworthy as complexation with the parent acid gives a different structure.     

High‐Performance Polymers for Membrane CO2/N2 Separation

This Concept examines strategies to design advanced polymers with high CO₂ permeability and high CO₂/N₂ selectivity, which are the key to the success of membrane technology for CO₂ capture from fossil fuel‐fired power plants. Specifically, polymers with enhanced CO₂ solubility and thus CO₂/N₂ selectivity are designed by incorporating CO₂‐philic groups in polymers such as poly(ethylene oxide)‐containing polymers and poly(ionic liquids); polymers with enhanced CO₂ diffusivity and thus CO₂ permeability are designed with contorted rigid polymer chains to obtain high free volume, such as polymers with intrinsic microporosity and thermally rearranged polymers. The underlying rationales for materials design are discussed and polymers with promising CO₂/N₂ separation properties for CO₂ capture from flue gas are highlighted.     

Gas transport properties of poly(ether‐b‐amide) segmented block copolymers

The permeation properties of H₂, N₂, and CO₂ were determined at 35 °C and pressures up to 15 atm in phase‐separated polyether‐b‐polyamide segmented block copolymers. These polymers contain poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO] as the rubbery polyether phase and nylon‐6 [PA6] or nylon‐12 [PA12] as the hard polyamide phase. Extremely high values of polar (or quadrupolar)/nonpolar gas selectivities, coupled with high CO₂ permeability coefficients, were observed. CO₂/H₂ selectivities as high as 9.8 and CO₂/N₂ selectivities as high as 56 were obtained in polymers with CO₂ permeability coefficients of approximately 220 × 10⁻¹⁰ cm³(STP) cm/(cm² s cmHg). As the amount of polyether increases, permeability increases. Gas permeability is higher in polymers with less polar constituents, PTMEO and PA12, than in those containing the more polar PEO and PA6 units. CO₂/N₂ and CO₂/H₂ selectivities are higher in polymers with higher concentrations of polar groups. These high selectivity values derive from large solubility selectivities in favor of CO₂. Because CO₂ is larger than H₂ and has, therefore, a lower diffusion coefficient than H₂, the weak size‐sieving ability of the rubbery polyether phase, which is the locus of most of the gas permeation, also contributes to high CO₂/H₂ selectivity. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2051–2062, 2000     

Covalent and electrostatic incorporation of amines into hypercrosslinked polymers for increased CO2 selectivity

Two methods of incorporating functional groups rich in nitrogen into low cost microporous hypercrosslinked polymers (HCPs) have been evaluated and the effects on the carbon dioxide CO₂/N₂ IAST selectivity were measured. Electrostatic incorporation of an ammonium salt into a sulfonic acid-containing HCP polymer afforded a static CO₂ uptake of 2.5 mmol g⁻¹ with a CO₂/N₂ IAST selectivity of 42:1 at 1 bar and 298 K. Using column breakthrough measurements with a 15:85 CO₂/N₂ mixture at 298 K and 1 bar, a selectivity of 17:1 was obtained. However, varying the counterion resulted in polymers with lower CO₂/N₂ selectivity values. Decoration of the parent polymer with CO₂-philic imidazole followed by electrostatic ammonium salt incorporation blocked some of the micropores reducing the selectivity which re-emphasizes the role and importance of pore width for CO₂/N₂ selectivity. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2513–2521     

Preparation of mixed matrix membranes based on polyimide and aminated graphene oxide for CO2 separation

The aminated graphene oxide (GO) was prepared by the functionalization of pristine GO with ethylenediamine and then dispersed into the poly(amic acid) (the precursor of polyimide [PI]) solution followed by the chemical imidization to successfully fabricate the PI/amine‐functionalized GO mixed matrix membranes (MMMs) using in‐situ polymerization method. Chemical structure and morphology of the GO before and after amine modification were characterized by scanning electron microscopy, Raman spectrum, Fourier transform infrared, and X‐ray photoelectron spectroscopy. Scanning electron microscopy indicated that fine dispersion of GO throughout PI matrix was achieved, which indicates that the in‐situ polymerization approach can enhance the interfacial interaction between the GO and the PI matrix, and then improve the dispersion of carbon material in the polymer matrix. Compared with the conventional solution mixture method, the MMMs prepared with in‐situ polymerization method showed excellent CO₂ permeability and CO₂/N₂ selectivity. The MMMs doped with 3 wt.% aminated GO exhibited maximum gas separation performance with a CO₂ permeability of 12.34 Barrer and a CO₂/N₂ selectivity of 38.56. These results suggest that the amino groups on GO have strong interaction with the CO₂ molecules, which can significantly increase the solubility of polar gas. Our results provide an easy and efficient way to prepare MMMs with good mechanical behavior and excellent gas separation performance.     

Synthesis of nitrogen doped activated carbon/polyaniline material for CO2 adsorption

The equilibrium adsorption isotherms of carbon dioxide and nitrogen on the nitrogen doped activated carbon (NAC) prepared by the chemical activation of a pine cone‐based char/polyaniline composite were measured using a volumetric technique. CO₂ and N₂ adsorption experiments were done at three different temperatures (298, 308, and 318 K) and pressures up to 16 bar, and correlated with the Langmuir, Freundlich, and Sips models. The Sips isotherm model presented the best fit to the experimental data. The N‐doped adsorbent showed CO₂ and N₂ adsorption capacity of 3.96 mmol·g⁻¹ and 0.86 mmol·g⁻¹, respectively, at 298 K and 1 bar. The selectivity predicted by ideal adsorbed solution theory (IAST) model was achieved 47.17 for NAC at 1 bar and y_N2 = 0.85 which is a composition similar to flue gas. The results showed that NAC adsorbent has a high CO₂‐over‐N₂ selectivity in a binary mixture. The relatively fast sorption rate of CO₂ on NAC compared to N₂ indicates the stronger affinity between CO₂ and amine groups. The isosteric heat of adsorption of CO₂ by the NAC demonstrated the physico‐chemical adsorption of CO₂ on the adsorbent surface. These data showed that prepared NAC could be successfully applied in separation of CO₂ from N₂.     

Tuning the adsorption and fluorescence properties of aminal‐linked porous organic polymers through N‐heterocyclic group decoration

Aiming at tuning the adsorption and fluorescence properties of targeted porous organic polymer, four new aminal‐linked porous organic polymers (NAPOPs) were synthesized through the reaction of 1,4‐Bis(4,6‐diamino‐s‐triazin‐2‐yl) benzene (BATB) with four kinds of aldehydes substituted with different N‐heterocyclic groups. Among the polymers, NAPOP‐3 decorated with 5‐phenyl‐tetrazole group shows the largest CO₂ adsorption capacity (2.52 mmol g^?1 at 273 K and 100 kPa) because of its relative large surface area, while NAPOP‐1 decorated with piperazine groups shows relative large CO₂/N₂ adsorption selectivity (77 at 273 K and 100 kPa), attributable to its large CO₂ adsorption heats and cabined pore (<4 Å). Meanwhile, NAPOP‐1 and ?3 exhibit high adsorption rate toward iodine with a high capacity (>240 wt %). In addition, different luminescence emissions were also observed for NAPOPs, indicating different intramolecular charger transfer occurred inside polymer networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1724–1730     

Novel Approach to Fabricate Carbon Molecular‐Sieve Membranes Based on Consideration of Interpenetrating Networks

Summary: Carbon molecular‐sieve membranes (CMSMs) have shown great potential for gas separation. They exhibit high selectivity by permitting effective size‐ and shape‐separation between gas molecules of similar molecular dimensions. Hence, the control of their pore size is very important. While previous studies have focused on the conditions of pyrolysis and its effect on CMSM properties, a novel approach is reported here whereby the precursor polymer is chemically modified prior to pyrolysis and the resultant CMSM was investigated for its gas separation properties. Pyrolysis of chemically crosslinked and uncrosslinked Matrimid® resulted in a change in d‐spacing from 5.6 to 3.6 and 3.7 Å, respectively. The crosslinked CMSM also exhibited greater ordering in its packing. The Matrimid‐derived CMSMs exhibited excellent separation properties for CO₂/CH₄.

Mechanism of chemical crosslinking modification.     

Postsynthetic amine modification of porous organic polymers for CO2 capture and separation

Porous organic polymers (POPs) constitute an important class of sorbents studied in various adsorption and separation processes. Their unique properties, including high surface areas, adjustable pore sizes, and surface chemistries make them ideal candidates for CO₂ capture. To achieve a high CO₂ adsorption capacity and selectivity, particularly at the low partition pressures required for post-combustion CO₂ capture or direct capture of CO₂ from the atmosphere, incorporating amines onto the polymer frameworks or within the pores has shown much promise. This review provides a comprehensive summary of recent studies on the synthesis and CO₂ capture performance of amine-functionalized POPs. The review also provides a detailed discussion of structure-performance relationships, focusing on how the loading amount and amine type influence CO₂ capture capacity, CO₂/N₂ selectivity, heat of adsorption, sorption kinetics, and recyclability of POPs. Additionally, the authors offer their perspective on the challenges associated with the practical implementation of amine-modified POPs for CO₂ capture.     

Confinement of Ionic Liquids in Nanocages: Tailoring the Molecular Sieving Properties of ZIF‐8 for Membrane‐Based CO2 Capture

Fine‐tuning of effective pore size of microporous materials is necessary to achieve precise molecular sieving properties. Herein, we demonstrate that room temperature ionic liquids can be used as cavity occupants for modification of the microenvironment of MOF nanocages. Targeting CO₂ capture applications, we tailored the effective cage size of ZIF‐8 to be between CO₂ and N₂ by confining an imidazolium‐based ionic liquid [bmim][Tf₂N] into ZIF‐8’s SOD cages by in‐situ ionothermal synthesis. Mixed matrix membranes derived from ionic liquid‐modified ZIF‐8 exhibited remarkable combinations of permeability and selectivity that transcend the upper bound of polymer membranes for CO₂/N₂ and CO₂/CH₄ separation. We observed an unusual response of the membranes to varying pressure, that is, an increase in the CO₂/CH₄ separation factor with pressure, which is highly desirable for practical applications in natural gas upgrading.     

Synthesis,characterization, and gas permeation properties of adamantane‐containing polymers of intrinsic microporosity

A new bis(catechol) monomer, namely, 4,4′‐((1r,3r)‐adamantane‐2,2‐diyl)bis(benzene‐1,2diol) (THADM) was synthesized by condensation of 2‐adamantanone with veratrole followed by demethylation of the formed (1r,3r)‐2,2‐bis(3,4 dimethoxyphenyl)adamantane. Polycondensation of THADM and various compositions of THADM and 5,5,6′,6′‐tetrahydroxy‐3,3,3′,3′‐tetramethylspirobisindane was performed with 2,3,5,6‐tetrafluoroterephthalonitrile (TFTPN) to obtain the homopolymer and copolymers. These polymers demonstrated good solubility in common organic solvents such as dichloromethane, chloroform, and tetrahydrofuran and could be cast into tough films from their chloroform solutions. GPC analysis revealed that number average molecular weights of polymers were in the range 48,100–61,700 g mol⁻¹, suggesting the formation of reasonably high molecular weight polymers. They possessed intrinsic microporosity with Brunauer‐Emmett‐Teller (BET) surface area in the range 703–741 m² g⁻¹. Thermogravimetric analysis of polymers indicated that 10% weight loss temperature was in the range 513–518 °C demonstrating their excellent thermal stability. THADM‐based polymer of intrinsic microporosity (PIM) showed P(CO₂) = 1080, P(O₂) = 232 and appreciable selectivity [α(CO₂/CH₄) = 22.6, α(CO₂/N₂) = 26.7, and α(O₂/N₂)= 5.7]. The gas permeability measurements revealed that with increase in the content of adamantane units in PIMs, selectivity increased and permeability decreased, following the trade‐off relationship. The gas separation properties of PIMs containing adamantane units were located close to 2008 Robeson upper bound for gas pairs such as CO₂/CH₄, CO₂/N₂, H₂/N₂, and O₂/N₂. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 16–24     

High CO2‐Capture Ability of a Porous Organic Polymer Bifunctionalized with Carboxy and Triazole Groups

A new porous organic polymer, SNU‐C1 , incorporating two different CO₂‐attracting groups, namely, carboxy and triazole groups, has been synthesized. By activating SNU‐C1 with two different methods, vacuum drying and supercritical‐CO₂ treatment, the guest‐free phases, SNU‐C1‐va and SNU‐C1‐sca , respectively, were obtained. Brunauer–Emmett–Teller (BET) surface areas of SNU‐C1‐va and SNU‐C1‐sca are 595 and 830 m²g^?1, respectively, as estimated by the N₂‐adsorption isotherms at 77 K. At 298 K and 1 atm, SNU‐C1‐va and SNU‐C1‐sca show high CO₂ uptakes, 2.31 mmol g^?1 and 3.14 mmol g^?1, respectively, the high level being due to the presence of abundant polar groups (carboxy and triazole) exposed on the pore surfaces. Five separation parameters for flue gas and landfill gas in vacuum‐swing adsorption were calculated from single‐component gas‐sorption isotherms by using the ideal adsorbed solution theory (IAST). The data reveal excellent CO₂‐separation abilities of SNU‐C1‐va and SNU‐C1‐sca , namely high CO₂‐uptake capacity, high selectivity, and high regenerability. The gas‐cycling experiments for the materials and the water‐treated samples, experiments that involved treating the samples with a CO₂‐N₂ gas mixture (15:85, v/v) followed by a pure N₂ purge, further verified the high regenerability and water stability. The results suggest that these materials have great potential applications in CO₂ separation.     

Synthesis,characterization, and KOH activation of nanoporous carbon for increasing CO2 adsorption capacity

Ordered nanoporous carbons (ONCs) were prepared using a soft-templating method. To improve the CO₂ adsorption efficiency, ONCs were chemically activated to obtain high specific surface area and micro-/mesopore volume with different KOH amounts (i.e., 0, 1, 2, 3, and 4) as an activating agent. The prepared nanoporous carbons (NCs) materials were analyzed by low-angle X-ray diffraction for confirmation of synthesized ONCs structures. The structural properties of the NCs materials were analyzed by high-angle X-ray diffraction. The textural properties of the NCs materials were examined using the N₂/77 K adsorption isotherms according to the Brunauer–Emmett–Teller equation. The CO₂ adsorption capacity was measured by CO₂ isothermal adsorption at 298 K/1 bar. From the results, the NCs activated with KOH showed that the increasing specific surface areas and total pore volumes resulted in the enhancement of CO₂ adsorption capacity.     

Synthesis of Porous Covalent Quinazoline Networks (CQNs) and Their Gas Sorption Properties

The development of different classes of porous polymers by linking organic molecules using new chemistries still remains a great challenge. Herein, we introduce for the first time the synthesis of covalent quinazoline networks (CQNs) using an ionothermal synthesis protocol. Zinc chloride (ZnCl₂) was used as the solvent and catalyst for the condensation of aromatic ortho‐aminonitriles to produce tricycloquinazoline linkages. The resulting CQNs show a high porosity with a surface area up to 1870 m² g^?1. Varying the temperature and the amount of catalyst enables us to control the surface area as well as the pore size distribution of the CQNs. Furthermore, their high nitrogen content and significant microporosity make them a promising CO₂ adsorbent with a CO₂ uptake capacity of 7.16 mmol g^?1 (31.5 wt %) at 273 K and 1 bar. Because of their exceptional CO₂ sorption properties, they are promising candidates as an adsorbent for the selective capture of CO₂ from flue gas.