Tailoring the pore chemistry in porous aromatic frameworks for selective separation of acetylene from ethylene

The separation of acetylene from ethylene is a crucial process in the petrochemical industry, because even traces of acetylene impurities can poison the catalysts of ethylene polymerization. Herein, we synthesize a new family of 3D porous aromatic frameworks (PAFs), non-functionalized PAF-28, carbene-functionalized PAF-28 (cPAF-28) and imidazolium-functionalized PAF-28 (iPAF-28), via Sonogashira coupling reactions. These PAFs show high porosity and good thermal stability. Both cPAF-28 and iPAF-28 are proved to be good candidates for C₂H₂ adsorption, demonstrated by C₂H₂/C₂H₄ selectivity of 12.2 and 15.4, and C₂H₂ capacity of 48 cm³ g⁻¹ and 57 cm³ g⁻¹, which are significantly higher than those of non-functionalized PAF-28 (1.8, 37 cm³ g⁻¹). Furthermore, the cPAF-28 and iPAF-28 display good breakthrough performance and remarkable recyclability for the separation of the C₂H₂/C₂H₄ gas mixture. In addition, the C₂H₂/C₂H₄ adsorption sites are revealed by DFT calculations. This work sheds a new light on gas molecular recognition by tailoring the pore chemistry of PAFs.

A family of PAFs with 3D topology was developed through tailoring the pore structures. By anchoring carbene or imidazolium groups in PAF-28, the selectivity for separating acetylene from ethylene was greatly boosted from 1.8 to 15.4.     

Targeted synthesis of novel porous aromatic frameworks with selective separation of CO_2/CH_4and CO_2/N_2

Novel porous aromatic frameworks（PAF-53 and PAF-54） have been obtained by the polymerization of amino compound（p-phenylenediamine and melamine） and cyanuric chloride. They display a certain amount of CO2 adsorption capacity and highly selective separation of CO2/CH4 and CO2/N2 as 18.1 and83 by Henry Law respectively. They may be applied as ideal adsorbents to separate and capture CO2.     

CO2 adsorption in mono-, di- and trivalent cation-exchanged metal-organic frameworks: a molecular simulation study

A molecular simulation study is reported for CO(2) adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations (Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), and Al(3+)). The isosteric heat and Henry's constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) < Al(3+)). At low pressures, cations act as preferential adsorption sites for CO(2) and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO(2) adsorption. Furthermore, the adsorption selectivity of CO(2)/H(2) mixture increases as Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) ≈ Al(3+). At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H(2)O, the selectivity decreases drastically because of the competitive adsorption between H(2)O and CO(2), and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.     

A superacid-catalyzed synthesis of porous membranes based on triazine frameworks for CO2 separation

A general strategy for the synthesis of porous, fluorescent, triazine-framework-based membranes with intrinsic porosity through an aromatic nitrile trimerization reaction is presented. The essence of this strategy lies in the use of a superacid to catalyze the cross-linking reaction efficiently at a low temperature, allowing porous polymer membrane architectures to be facilely derived. With functionalized triazine units, the membrane exhibits an increased selectivity for membrane separation of CO(2) over N(2). The good ideal CO(2)/N(2) selectivity of 29 ± 2 was achieved with a CO(2) permeability of 518 ± 25 barrer. Through this general synthesis protocol, a new class of porous polymer membranes with tunable functionalities and porosities can be derived, significantly expanding the currently limited library of polymers with intrinsic microporosity for synthesizing functional membranes in separation, catalysis, and energy storage/conversion.     

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.     

Metalloporphyrin-based covalent organic frameworks composed of the electron donor-acceptor dyads for visible-light-driven selective CO2 reduction

The visible-light-driven photocatalytic CO_2 reduction with high efficiency is highly desirable but challenging. Herein, we present porphyrin-tetraphenylethene-based covalent organic frameworks(MP-TPE-COF, where M=H_2, Co and Ni;TPE=4,4′,4″,4?-(ethane-1,1,2,2-tetrayl) tetrabenzaldehyde; COF=covalent organic framework) as ideal platforms for understanding photocatalytic CO_2 reduction at molecular level. Experimental and theoretical investigations have demonstrated crucial roles of metalloporphyrin units in selective adsorption, activation and conversion of CO_2 as well as in the separation of charge carriers and electron transfer, thus allowing for flexible modulation of photocatalytic activity and selectivity. Co P-TPE-COF exhibits high CO evolution rate of 2,414 μmol g~(-1) h~(-1) with the selectivity of 61% over H_2 generation under visible-light irradiation, while Ni P-TPE-COF provides CO evolution rate of 525 μmol g~(-1) h~(-1) and 93% selectivity with superior durability.Moreover, the photocatalytic system is feasible for the simulated flue gas, which provides CO evolution rate of 386 μmol g~(-1) h~(-1) and selectivity of 77%. This work provides in-depth insight into the structure-activity relationships toward the activation and photoreduction of CO_2.     

Structural features of binary mixtures of supercritical CO2 with polar entrainers by molecular dynamics simulation

Computer simulations of supercritical carbon dioxide and its mixtures with polar cosolvents: water, methanol, and ethanol (concentration, 0.125 mole fractions) at T = 318 K and ρ = 0.7 g/cm³ are performed. Atom-atom radial distribution functions are calculated by classical molecular dynamics, while the probability distributions of relative orientation of CO₂ molecules in the first and second coordination spheres describing the geometry of the nearest environment of CO₂ molecules and the trajectories of cosolvent molecules are found using Car-Parrinello molecular dynamics. Based on the latter, the conclusions regarding structure and interactions of polar entrainers in their mixtures with supercritical CO₂ are made. It is shown that the microstructure of carbon dioxide varies only slightly upon the introduction of cosolvents.     

Spatial disposition of square-planar mononuclear nodes in metal–organic frameworks for C2H2/CO2 separation

The efficient separation of acetylene (C₂H₂) from its mixture with carbon dioxide (CO₂) remains a challenging industrial process due to their close molecular sizes/shapes and similar physical properties. Herein, we report a microporous metal–organic framework (JNU-4) with square-planar mononuclear copper(ii) centers as nodes and tetrahedral organic linkers as spacers, allowing for two accessible binding sites per metal center for C₂H₂ molecules. Consequently, JNU-4 exhibits excellent C₂H₂ adsorption capacity, particularly at 298 K and 0.5 bar (200 cm³ g⁻¹). Detailed computational studies confirm that C₂H₂ molecules are indeed predominantly located in close proximity to the square-planar copper centers on both sides. Breakthrough experiments demonstrate that JNU-4 is capable of efficiently separating C₂H₂ from a 50 : 50 C₂H₂/CO₂ mixture over a broad range of flow rates, affording by far the largest C₂H₂ capture capacity (160 cm³ g⁻¹) and fuel-grade C₂H₂ production (105 cm³ g⁻¹, ≥98% purity) upon desorption. Simply by maximizing accessible open metal sites on mononuclear metal centers, this work presents a promising strategy to improve the C₂H₂ adsorption capacity and address the challenging C₂H₂/CO₂ separation.

C₂H₂/CO₂ separation is a challenging industrial process. We report a microporous MOF (JNU-4a) with square-planar mononuclear copper(ii) centers, allowing for a record high C₂H₂ capture capacity from an equimolar C₂H₂/CO₂ mixture.     

Cooperative effect of temperature and linker functionality on CO2 capture from industrial gas mixtures in metal-organic frameworks: a combined experimental and molecular simulation study

In this work, the cooperative effect of temperature and linker functionality on CO(2) capture in metal-organic frameworks (MOFs) was investigated using experimental measurements in combination with molecular simulations. To do this, four MOFs with identical topology but different functional groups on the linkers and three important CO(2)-containing industrial gas mixtures were adopted. The interplay between linker functionality and temperature was analyzed in terms of CO(2) storage capacity, adsorption selectivity, working capacity of CO(2) in temperature swing adsorption (TSA) processes, as well as sorbent selection parameter (S(ssp)). The results show that the effect of linker functionality on CO(2) capture performance in the MOFs is strongly interconnected with temperature: up to moderate pressures, the lower the temperature, the larger the effect of the functional groups. Furthermore, the modification of a MOF by introducing more complex functional groups can not only improve the affinity of framework for CO(2), but also reduce the free volume, and thus may contribute negatively to CO(2) capture capability when the packing effect is obvious. Therefore, when we design a new MOF for a certain CO(2) capture process operated at a certain temperature, the MOF should be designed to have maximized affinity for CO(2) but with a negligible or small effect caused by the reduction of free volume at that temperature and the corresponding operating pressure.     

A new approach towards tetrahedral imidazolate frameworks for high and selective CO2 uptake

Successful development of a new synthetic approach towards tetrahedral imidazolate frameworks (TIFs) via combining an auxiliary uninegative linker into the zinc-imidazolate tetrahedral assembly leads to new TIF materials (TIF-A1 to TIF-A3) with distinct structural topologies and high CO(2) uptake capacity.     

Experimental and molecular simulation studies of CO2 adsorption on zeolitic imidazolate frameworks: ZIF-8 and amine-modified ZIF-8

ZIF-8 has been rapidly developed as a potential candidate for CO₂ capture due to its low density, high surface area, and robust structure. Considering the electron-donating effect of amino functional groups, amino-modification is expected to be an efficient way to improve CO₂ adsorption of ZIF-8. In this work, grand canonical Monte Carlo (GCMC) simulation was performed to study the CO₂ adsorption isotherm based on ZIF-8, ZIF-8-NH₂, and ZIF-8-(NH₂)₂. ZIF-8 was synthesized and CO₂ adsorption isotherms based on ZIF-8 was measured. The experimental surface area, pore volume, and CO₂ adsorption isotherm were used to validate the force field. Adsorptive capacity of ZIF-8-NH₂, and ZIF-8-(NH₂)₂ were first estimated. The GCMC simulation results indicated that the order of increasing CO₂ capacity of the ZIF-8 in the lower pressure regime is: ZIF-8 < ZIF-8-NH₂ < ZIF-8-(NH₂)₂, and in the high pressure is: ZIF-8 < ZIF-8-(NH₂)₂ < ZIF-8-NH₂. New adsorption sites can be generated with the existence of-NH₂ groups. In addition, for non-modified and amino-modified ZIF-8, it was the first time to use density functional theory (DFT) calculations to investigate their CO₂ adsorption sites and CO₂ binding energies. The present work indicates that appropriate amine-functionalized can directly enhanced CO₂ capacity of ZIF-8.     

Synthesis of porous aromatic framework with Friedel–Crafts alkylation reaction for CO_2 separation

A novel porous aromatic framework, PAF-8, derived from tetraphenylsilane as basic building unit, was successfully synthesized via Friedel–Crafts alkylation reaction. This PAF material had high thermal stability as well as high surface area(785 m~2g~(-1)) calculated from the Brunauer–Emmett–Teller(BET)model. Meanwhile, PAF-8 possessed high performances in gas sorption and especially for CO_2 separation.     

Diffusion and separation of CO2 and CH4 in silicalite, C168 schwarzite, and IRMOF-1: a comparative study from molecular dynamics simulation

Recently we have investigated the storage and adsorption selectivity of CO(2) and CH(4) in three different classes of nanoporous materialssilicalite, IRMOF-1, and C(168) schwarzite through Monte Carlo simulation (Babarao, R.; Hu, Z.; Jiang, J. Langmuir, 2007, 23, 659). In this work, the self-, corrected, and transport diffusivities of CO(2) and CH(4) in these materials are examined using molecular dynamics simulation. The activation energies at infinite dilution are evaluated from the Arrhenius fits to the diffusivities at various temperatures. As loading increases, the self-diffusivities in the three frameworks decrease as a result of the steric hindrance; the corrected diffusivities remain nearly constant or decrease approximately linearly depending on the adsorbate and framework; and the transport diffusivities generally increase except for CO(2) in IRMOF-1. The correlation effects are identified to reduce from MFI, C(168) to IRMOF-1, in accordance with the porosity increasing in the three frameworks. Predictions of self-, corrected, and transport diffusivities for pure CO(2) and CH(4) from the Maxwell-Stefan formulation match the simulation results well. In a CO(2)/CH(4) mixture, the self-diffusivities decreases with loading, and good agreement is found between simulated and predicted results. On the basis of the adsorption and self-diffusivity in the mixture, the permselectivity is found to be marginal in IRMOF-1, slightly enhanced in MFI, and greatest in C(168) schwarzite. Although IRMOF-1 has the largest storage capacity for CH(4) and CO(2), its selectivity is not satisfactory.     

CO(2) as a separation switch for ionic liquid/organic mixtures

A novel technique to separate ionic liquids from organic compounds is introduced which uses carbon dioxide to induce the formation of an ionic liquid-rich phase and an organic-rich liquid phase in mixtures of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C4mim][PF6]). If the temperature is above the critical temperature of CO2 then the methanol-rich phase can become completely miscible with the CO2-rich phase, and this new phase is completely ionic liquid-free. Since CO2 is nonpolar, it is not equipped to solvate ions. As the CO2 dissolves in the methanol/[C4mim][PF6] mixture, the solvent power of the CO2-expanded liquid is significantly reduced, inducing the formation of the second liquid phase that is rich in ionic liquid. This presents a new way to recover products from ionic liquid mixtures and purify organic phases that have been contaminated with ionic liquid. Moreover, these results have important implications for reactions done in CO2/ionic liquid biphasic mixtures.     

Silver-anchored porous aromatic framework for efficient conversion of propargylic alcohols with CO2 at ambient pressure

The conversion of propargylic alcohols and carbon dioxide (CO₂) into fine chemicals suffers from issues of harsh reaction conditions and difficult catalyst recovery. To achieve efficient CO₂ activation at low energy consumption, a silver-anchored porous aromatic framework catalyst Ag@PAF-DAB with high active phase density and CO₂ adsorption capacity was proposed. Since Ag@PAF-DAB has the dual functions of CO₂ capture and conversion, propargylic alcohols were completely converted into α-alkylidene cyclic carbonate or α?hydroxy ketone as high value-added product under atmospheric pressure (CO₂, 0.1 MPa) and low silver equivalent (0.5 mol%). Notably, Ag@PAF-DAB exhibited broad substrate diversity, high stability, and excellent reusability. By applying FTIR and GC, the key to green synthetic route of α?hydroxy ketone was confirmed to lie in the further hydration of α-alkylidene cyclic carbonate.     

Adsorption technology for CO2 separation and capture: a perspective

The capture of CO₂ from process and flue gas streams and subsequent sequestration was first proposed as a greenhouse gas mitigation option in the 1990s. This proposal spawned a series of laboratory and field tests in CO₂ capture which has now grown into a major world-wide research effort encompassing a myriad of capture technologies and ingenious flow sheets integrating power production and carbon capture. Simultaneously, the explosive growth in materials science in the last two decades has produced a wealth of new materials and knowledge providing us with new avenues to explore to fine tune CO₂ adsorption and selectivity. Laboratory and field studies over the last decade have explored the synergy of process and materials to produce numerous CO₂ capture technologies and materials based on cyclic adsorption processes. In this brief perspective, we look at some of these developments and comment on the application and limitations of adsorption process to CO₂ capture. We identify major engineering obstacles to overcome as well as potential breakthroughs necessary to achieve commercialization of adsorption processes for CO₂ capture. Our perspective is primarily restricted to post-combustion flue gas capture and CO₂ capture from natural gas.     

Adsorptive behavior of CO2, CH4 and their mixtures in carbon nanospace: a molecular simulation study

Using molecular simulation, four types of nanoporous carbons are examined as adsorbents for the separation of CO(2)/CH(4) mixtures at ambient temperature and pressures up to 10 MPa. First, the adsorption selectivity of CO(2) is investigated in carbon slit pores and single-walled carbon nanotube bundles in order to find the optimal pore dimensions for CO(2) separation. Then, the adsorptive properties of the optimized slit pore and nanotube bundle are compared with two realistic nanoporous carbon models: a carbon replica of zeolite Y and an amorphous carbon. For the four carbon models, adsorption isotherms and isosteric heats of adsorption are presented for both pure components and mixtures. Special attention is given to the calculation of excess isotherms and isosteric heats, which are necessary to assess the performance of model nanoporous materials in the context of experimental measurements. From these results, we discuss the impact that variables such as pore size, pore morphology, pressure and mixture composition have on the performance of nanoporous carbons for CO(2) separation.     

Solvation structure and dynamics for passivated Au nanoparticle in supercritical CO2: a molecular dynamic simulation

All-atomic molecular dynamics simulations have been performed to study the interfacial structural and dynamical properties of passivated gold nanoparticles in supercritical carbon dioxide (scCO(2)). Simulations were conducted for a 55-atom gold nanocore with thiolated perfluoropolyether as the packing ligands. The effect of solvent density and surface coverage on the structural and dynamical properties of the self-assembly monolayer (SAM) has been discussed. The simulation results demonstrate that the interface between nanoparticle and scCO(2) solvent shows a depletion region due to the preclusion of SAM. The presence of scCO(2) solvent around the passivated Au nanoparticle can lead to an enhanced extension of the surface SAM. Under full coverage, the structure and conformation of SAM are insensitive to the density change of scCO(2) fluid. This simulation results clarify the microscopic solvation mechanism of passivated nanoparticles in supercritical fluid medium and is expected to be helpful in understanding the scCO(2)-based nanoparticle dispersion behavior.     

Zeolite Rho: a highly selective adsorbent for CO2/CH4 separation induced by a structural phase modification

Zeolite Rho is able to successfully separate CO(2) from CH(4) with the highest selectivity ever observed on the basis of pore diameter and surface polarity. The adsorption of CO(2) provokes structural changes in the zeolite Rho.