Synthesis of Highly Porous Coordination Polymers with Open Metal Sites for Enhanced Gas Uptake and Separation

Metal-containing amorphous microporous polymers are an emerging class of functional porous materials in which the surface properties and functions of polymers are dictated by the nature of the metal ions incorporated into the framework. In an effort to introduce coordinatively unsaturated metal sites into the porous polymers, we demonstrate herein an aqueous-phase synthesis of porous coordination polymers (PCPs) incorporating bis(o-diiminobenzosemiquinonato)-Cu(II) or -Ni(II) bridges by simply reacting hexaminotriptycene with CuSO₄·5H₂O [Cu(II)-PCP] or NiCl₂·6H₂O [Ni(II)-PCP] in H₂O. The resulting polymers showed surface areas of up to 489 m² g^-1 along with a narrow pore size distribution. The presence of open metal sites significantly improved the gas affinity of these frameworks, leading to an exceptional isosteric heat of adsorption of 10.3 kJ·mol^-1 for H₂ at zero coverage. The high affinities of Cu(II)- and Ni(II)-PCPs toward CO₂ prompted us to investigate the removal of CO₂ from natural and landfill gas conditions. We found that the higher affinity of Cu(II)-PCP compared to that of Ni(II)-PCP not only allowed for the tuning of the affinity of CO₂ molecules toward the sorbent, but also led to an exceptional CO₂/CH₄ selectivity of 35.1 for landfill gas and 20.7 for natural gas at 298 K. These high selectivities were further verified by breakthrough measurements under the simulated natural and landfill gas conditions, in which both Cu(II)- and Ni(II)-PCPs showed complete removal of CO₂. These results clearly demonstrate the promising attributes of metal-containing porous polymers for gas storage and separation applications.

Site Partition: Turning One Site into Two for Adsorbing CO2

We propose the concept of site partition to explain the role of guest molecules in increasing CO2 uptake in metal-organic frameworks and to design new covalent porous materials for CO2 capture. From grand canonical Monte Carlo simulations of CO2 sorption in the recently synthesized CPM-33 MOFs, we show that guest insertion divides one open metal site into two relatively strong binding sites, hence dramatically increasing CO2 uptake. Further, we extend the site partition concept to covalent organic frameworks with large free volume. After insertion of a designed geometry-matching guest, we show that the volumetric uptake of CO2 doubles. Therefore, the concept of site partition can be used to engineer the pore space of nanoporous materials for higher gas uptake.

Covalent Organic Frameworks for CO2 Capture

As an emerging class of porous crystalline materials, covalent organic frameworks (COFs) are excellent candidates for various applications. In particular, they can serve as ideal platforms for capturing CO2 to mitigate the dilemma caused by the greenhouse effect. Recent research achievements using COFs for CO2 capture are highlighted. A background overview is provided, consisting of a brief statement on the current CO2 issue, a summary of representative materials utilized for CO2 capture, and an introduction to COFs. Research progresses on: i) experimental CO2 capture using different COFs synthesized based on different covalent bond formations, and ii) computational simulation results of such porous materials on CO2 capture are summarized. Based on these experimental and theoretical studies, careful analyses and discussions in terms of the COF stability, low- and high-pressure CO2 uptake, CO2 selectivity, breakthrough performance, and CO2 capture conditions are provided. Finally, a perspective and conclusion section of COFs for CO2 capture is presented. Recent advancements in the field are highlighted and the strategies and principals involved are discussed.

Covalent organic frameworks based on Schiff-base chemistry: synthesis, properties and potential applications

Covalent organic-frameworks (COFs) are an emerging class of porous and ordered materials formed by condensation reactions of organic molecules.

Tunable porosity of nanoporous organic polymers with hierarchical pores for enhanced CO2 capture

We present a simple and convenient way to engineer the porosity of NOPs utilizing two crosslinkers with different length and various types of building blocks. The obtained polymers display hierarchical pore structures, remarkablely high CO₂ uptake capacities and sorption selectivity for CO₂/N₂.

Channel-wall functionalization in covalent organic frameworks for the enhancement of CO2 uptake and CO2/N2 selectivity

Two structurally similar groups with one being CO₂-philic but the other not were anchored into the channel walls of 2D COFs. The decreased surface area of COFs undoubtedly decreased CO₂ adsorption if too many functional groups were introduced.

An azine-linked hexaphenylbenzene based covalent organic framework

We report an azine linked covalent organic framework based on hexaphenylbenzene monomer functionalized with aldehyde groups (“HEX-COF 1”, avg. pore size = 1 nm, surface area >1200 m² g⁻¹, sorption capability at 273 K, 1 atm = 20 wt% for CO² and 2.3 wt% for CH₄).

A unique 3D ultramicroporous triptycene-based polyimide framework for efficient gas sorption applications

A novel 3D ultramicroporous triptycene-based polyimide framework with high surface area (1050 m² g⁻¹) and high CO₂ sorption capacity (3.4 mmol g⁻¹ at 273 K and 1 bar), good CO₂/N₂ (45) and CO₂/CH₄ (9.6) selectivity was synthesized and characterized.

An Azine-Linked Covalent Organic Framework: Synthesis, Characterization and Efficient Gas Storage

A azine-linked covalent organic framework, COF-JLU2, was designed and synthesized by condensation of hydrazine hydrate and 1,3,5-triformylphloroglucinol under solvothermal conditions for the first time. The new covalent organic framework material combines permanent micropores, high crystallinity, good thermal and chemical stability, and abundant heteroatom activated sites in the skeleton. COF-JLU2 possesses a moderate BET surface area of over 410 m(2)  g(-1) with a pore volume of 0.56 cm(3)  g(-1) . Specifically, COF-JLU2 displays remarkable carbon dioxide uptake (up to 217 mg g(-1) ) and methane uptake (38 mg g(-1) ) at 273 K and 1 bar, as well as high CO2 /N2 (77) selectivity. Furthermore, we further highlight that it exhibits a higher hydrogen storage capacity (16 mg g(-1) ) than those of reported COFs at 77 K and 1 bar.

Direct On-Surface Patterning of a Crystalline Laminar Covalent Organic Framework Synthesized at Room Temperature

We report herein an efficient, fast, and simple synthesis of an imine-based covalent organic framework (COF) at room temperature (hereafter, RT-COF-1). RT-COF-1 shows a layered hexagonal structure exhibiting channels, is robust, and is porous to N2 and CO2 . The room-temperature synthesis has enabled us to fabricate and position low-cost micro- and submicropatterns of RT-COF-1 on several surfaces, including solid SiO2 substrates and flexible acetate paper, by using lithographically controlled wetting and conventional ink-jet printing.

A 2D azine-linked covalent organic framework for gas storage applications

A new azine-linked covalent organic framework, ACOF-1, was synthesized by condensation of hydrazine hydrate and 1,3,5-triformylbenzene under solvothermal conditions. ACOF-1 has a high surface area and a small pore size, and it can store up to 177 mg g(-1) of CO2, 9.9 mg g(-1) of H2, and 11.5 mg g(-1) of CH4, at 273 K and 1 bar, with high selectivity towards CO2 over N2 and CH4.

Two-dimensional covalent organic frameworks for carbon dioxide capture through channel-wall functionalization

Ordered open channels found in two-dimensional covalent organic frameworks (2D COFs) could enable them to adsorb carbon dioxide. However, the frameworks' dense layer architecture results in low porosity that has thus far restricted their potential for carbon dioxide adsorption. Here we report a strategy for converting a conventional 2D COF into an outstanding platform for carbon dioxide capture through channel-wall functionalization. The dense layer structure enables the dense integration of functional groups on the channel walls, creating a new version of COFs with high capacity, reusability, selectivity, and separation productivity for flue gas. These results suggest that channel-wall functional engineering could be a facile and powerful strategy to develop 2D COFs for high-performance gas storage and separation.

The microwave-assisted solvothermal synthesis of a crystalline two-dimensional covalent organic framework with high CO2 capacity

A covalent organic framework was synthesized by using a rapid microwave-assisted solvothermal method via a Schiff base reaction with a high CO₂ capacity.

Covalent organic polymer framework with C–C bonds as a fluorescent probe for selective iron detection

Iron detection has just gone heterogeneous, thanks to the selective quenching of fluorescence by the nanoporous polymers that are tuned for optimal processability.

Novel ferrocene-based nanoporous organic polymers for clean energy application

A novel ferrocene-based nanoporous organic polymer has been prepared by coupling 1,1′-ferrocene-dicarboxaldehyde with melamine.

Graphene-like single-layered covalent organic frameworks: synthesis strategies and application prospects

Two-dimensional (2D) nanomaterials, such as graphene and transition metal chalcogenides, show many interesting dimension-related materials properties. Inspired by the development of 2D inorganic nanomaterials, single-layered covalent organic frameworks (sCOFs), featuring atom-thick sheets and crystalline extended organic structures with covalently bonded building blocks, have attracted great attention in recent years. With their unique graphene-like topological structure and the merit of structural diversity, sCOFs promise to possess novel and designable properties. However, the synthesis of sCOFs with well-defined structures remains a great challenge. Herein, the recent development of the bottom-up synthesis methods of 2D sCOFs, such as thermodynamic equilibrium control methods, growth-kinetics control methods, and surface-assisted covalent polymerization methods, are reviewed. Finally, some of the critical properties and application prospects of these materials are outlined.

A novel azobenzene covalent organic framework

The first example of a novel Azo-linked 2D COF with a hexagonal skeleton, high crystallinity and permanent porosity. The trans-to-cis photoisomerization can lead to the decline of Azo-COF crystallinity but cannot impact the pore size of Azo-COF. The current results will provide a strategy for designing smart COF materials.