Selective CO2 Sorption Using Compartmentalized Coordination Polymers with Discrete Voids**

Carbon capture and storage with porous materials is one of the most promising technologies to minimize CO₂ release into the atmosphere. Here, we report a family of compartmentalized coordination polymers (CCPs) capable of capturing gas molecules in a selective manner based on two novel tetrazole-based ligands. Crystal structures have been modelled theoretically under the Density Functional Theory (DFT) revealing the presence of discrete voids of 380 ų. Single gas adsorption isotherms of N₂, CH₄ and CO₂ have been measured, obtaining a loading capacity of 0.6, 1.7 and 2.2 molecules/void at 10 bar and at 298 K for the best performing material. Moreover, they present excellent selectivity and regenerability for CO₂ in mixtures with CH₄ and N₂ in comparison with other reported materials, as evidenced by dynamic breakthrough gas experiments. These frameworks are therefore great candidates for separation of gas mixtures in the chemical engineering industry.     

A computational study of CO2, N2, and CH4 adsorption in zeolites

The adsorption properties of CO₂, N₂ and CH₄ in all-silica zeolites were studied using molecular simulations. Adsorption isotherms for single components in MFI were both measured and computed showing good agreement. In addition simulations in other all silica structures were performed for a wide range of pressures and temperatures and for single components as well as binary and ternary mixtures with varying bulk compositions. The adsorption selectivity was analyzed for mixtures with bulk composition of 50:50 CO₂/CH₄, 50:50 CO₂/N₂, 10:90 CO₂/N₂ and 5:90:5 CO₂/N₂/CH₄ in MFI, MOR, ISV, ITE, CHA and DDR showing high selectivity of adsorption of CO₂ over N₂ and CH₄ that varies with the type of crystal and with the mixture bulk composition.     

Multiscale Computational Study on the Adsorption and Separation of CO2/CH4 and CO2/H2 on Li+‐Doped Mixed‐Ligand Metal–Organic Framework Zn2(NDC)2(diPyNI)

The quantum mechanics (QM) method and grand canonical Monte Carlo (GCMC) simulations are used to study the effect of lithium cation doping on the adsorption and separation of CO₂, CH₄, and H₂ on a twofold interwoven metal–organic framework (MOF), Zn₂(NDC)₂(diPyNI) (NDC=2,6‐naphthalenedicarboxylate; diPyNI=N,N′‐di‐(4‐pyridyl)‐1,4,5,8‐naphthalenetetracarboxydiimide). Second‐order Moller–Plesset (MP2) calculations on the (Li⁺–diPyNI) cluster model show that the energetically most favorable lithium binding site is above the pyridine ring side at a distance of 1.817 Å from the oxygen atom. The results reveal that the adsorption capacity of Zn₂(NDC)₂(diPyNI) for carbon dioxide is higher than those of hydrogen and methane at room temperature. Furthermore, GCMC simulations on the structures obtained from QM calculations predict that the Li⁺‐doped MOF has higher adsorption capacities than the nondoped MOF, especially at low pressures. In addition, the probability density distribution plots reveal that CO₂, CH₄, and H₂ molecules accumulate close to the Li cation site. The selectivity results indicate that CO₂/H₂ selectivity values in Zn₂(NDC)₂(diPyNI) are higher than those of CO₂/CH₄. The selectivity of CO₂ over CH₄ on Li⁺‐doped Zn₂(NDC)₂(diPyNI) is improved relative to the nondoped MOF.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

Adsorption separation of carbon dioxide, methane and nitrogen on monoethanol amine modified B-zeolite

A new type of composite adsorbents was synthesized by incorporating monoethanol amine (MEA) into β-zeolite. The parent and MEA-functionalized β-zeolites were characterized by X-ray diffraction (XRD), N₂ adsorption, and thermogravimetric analysis (TGA). The adsorption behavior of carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂) on these adsorbents was investigated at 303 K. The results show that the structure of zeolite was well preserved after MEA modification. In comparison with CH₄ and N₂, CO₂ was preferentially adsorbed on the adsorbents investigated. The introduction of MEA significantly improved the selectivity of both CO₂/CH₄ and CO₂/N₂, the optimal selectivity of CO₂/CH₄ can reach 7.70 on 40 wt% of MEA-functionalized β-zeolite (MEA(40)-β) at 1 atm. It is worth noticing that a very high selectivity of CO₂/N₂ of 25.67 was obtained on MEA(40)-β. Steric effect and chemical adsorbate-adsorbent interaction were responsible for such high adsorption selectivity of CO₂. The present MEA-functionalized β-zeolite adsorbents may be a good candidate for applications in flue gas separation, as well as natural gas and landfill gas purifications.     

Photo-responsive azo MOF exhibiting high selectivity for CO2 and xylene isomers

A combination of azo and acylamide ligands is used in the preparation of metal–organic frameworks. Light response research reveals that under UV–vis irradiation, the CO₂ adsorption of 1 declines as much as 21.4%. 1 exhibits excellent CO₂ adsorption selectivity over CH₄, O₂, CO, and N₂ gasses with IAST selectivity of 21–580 at 293 K. This MOF also has promising potential in separation of xylene isomers in the liquid phase with the adsorption of p-xylene of 265.15, o-xylene of 101.25 and m-xylene of 0 mg g^?1, respectively.     

Water‐stable Adenine‐based MOFs with Polar Pores for Selective CO2 Capture

Here, we report two novel water‐stable amine‐functionalized MOFs, namely IISERP‐MOF26 ([NH₂(CH₃)₂][Cu₂O(Ad)(BDC)]?(H₂O)₂(DMA), 1 ) and IISERP‐MOF27 ([NH₂(CH₃)₂]_1/2[Zn₄O(Ad)₃(BDC)₂]?(H₂O)₂(DMF)_1/2, 2 ), which show selective CO₂ capture capabilities. They are made by combining inexpensive and readily available terephthalic acid and N‐rich adenine with Cu and Zn, respectively. They possess 1D channels decorated by the free amine group from the adenine and the polarizing oxygen atoms from the terephthalate units. Even more, there are dimethyl ammonium (DMA⁺) cations in the pore rendering an electrostatic environment within the channels. The activated Cu‐ and Zn‐MOFs physisorb about 2.7 and 2.2 mmol g^?1 of CO₂, respectively, with high CO₂/N₂ and moderate CO₂/CH₄ selectivity. The calculated heat of adsorption (HOA=21–23 kJ mol^?1) for the CO₂ in both MOFs suggest optimal physical interactions which corroborate well with their facile on‐off cycling of CO₂. Notably, both MOFs retain their crystallinity and porosity even after soaking in water for 24 hours as well as upon exposure to steam over 24 hours. The exceptional thermal and chemical stability, favorable CO₂ uptakes and selectivity and low HOA make these MOFs promising sorbents for selective CO₂ capture applications. However, the MOF′s low heat of adsorption despite having a highly CO₂‐loving groups lined walls is quite intriguing.     

Enhanced CO2 Adsorption Affinity in a NbO‐type MOF Constructed from a Low‐Cost Diisophthalate Ligand with a Piperazine‐Ring Bridge

A metal–organic framework (NPC‐6) with an NbO topology based on a piperazine ring‐bridged diisophthalate ligand was synthesized and characterized. The incorporated piperazine group leads to an enhanced adsorption affinity for CO₂ in NPC‐6, in which the CO₂ uptake is 4.83 mmol g^?1 at 293 K and 1 bar, ranking among the top values of CO₂ uptake on MOF materials. At 0.15 bar and 293 K, the NPC‐6 adsorbs 1.07 mmol g^?1 of CO₂, which is about 55.1 % higher than that of the analogue MOF NOTT‐101 under the same conditions. The enhanced CO₂ uptake combined with comparable uptakes for CH₄ and N₂ leads to much higher selectivities for CO₂/CH₄ and CO₂/N₂ gas mixtures on NPC‐6 than on NOTT‐101. Furthermore, an N‐alkylation is used in the synthesis of the PDIA ligand, leading to a much lower cost compared with that in the synthesis of ligands in the NOTT series, as the former does not require a palladium‐based catalyst and borate esters. Thus, we conclude that NPC‐6 is a promising candidate for CO₂ capture applications.     

Separation of CO2 and CH4 by a DDR membrane

The permeation of CO₂ and CH₄ and their binary mixtures through a DDR membrane has been investigated over a wide range of temperatures and pressures. The synthesized DDR membrane exhibits a high permeance and maintains a very high selectivity for CO₂. At a total pressure of 101 kPa, the highest selectivity for CO₂ in a 50∶50 feed mixture was found to be over 4000 at 225 K. This is ascribed to the higher adsorption affinity, as well as to the higher mobility for the smaller CO₂ molecules in the zeolite, preventing the bypassing of the CH₄ through the membrane. An engineering model, based on the generalized Maxwell-Stefan equations, has been used to interpret the transport phenomena in the membrane. The feasibility of DDR membranes as applied to CO₂ removal from natural gas or biogas is anticipated.     

CO2 Separation by Imide/Imine Organic Cages

Two novel imide/imine-based organic cages have been prepared and studied as materials for the selective separation of CO₂ from N₂ and CH₄ under vacuum swing adsorption conditions. Gas adsorption on the new compounds showed selectivity for CO₂ over N₂ and CH₄. The cages were also tested as fillers in mixed-matrix membranes for gas separation. Dense and robust membranes were obtained by loading the cages in either Matrimid® or PEEK-WC polymers. Improved gas-transport properties and selectivity for CO₂ were achieved compared to the neat polymer membranes.     

Suppression of CO2-plasticization by semiinterpenetrating polymer network formation

CO₂-induced plasticization may significantly spoil the membrane performance in high-pressure CO₂/CH₄ separations. The polymer matrix swells upon sorption of CO₂, which accelerates the permeation of CH₄. The polymer membrane looses its selectivity. To make membranes attractive for, for example, natural gas upgrading, plasticization should be minimized. In this article we study a polymer membrane stabilization by a semiinterpenetrating polymer network (s-ipn) formation. For this purpose, the polyimide Matrimid 5218 is blended with the oligomer Thermid FA-700 and subsequently heat treated at 265°C. Homogeneous films are prepared with different Matrimid/Thermid ratios and different curing times. The stability of the modified membrane is tested with permeation experiments with pure CO₂ as well as CO₂/CH₄ gas mixtures. The original membrane shows a minimum in its permeability vs. pressure curves, but the modified membranes do not indicating suppressed plasticization. Membrane performances for CO₂/CH₄ gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The modified membrane clearly shows suppression of the undesired methane acceleration. It was also found that just blending Matrimid and Thermid was not sufficient to suppress plasticization. The subsequent heat treatment that results in the s-ipn was necessary to obtain a stabilized permeability. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1547–1556, 1998     

Adsorption of CO<Subscript>2</Subscript>-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for CO<Subscript>2</Subscript> separation

Adsorption of CO₂, N₂, CH₄ and H₂ on triamine-grafted pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41) was investigated at room temperature in a wide range of pressure (up to 25 bar) using gravimetric measurements. The material was found to exhibit high affinity toward CO₂ in comparison to the other species over the whole range of pressure. Column-breakthrough dynamic measurements of CO₂-containing mixtures showed very high selectivity toward CO₂ over N₂, CH₄ and H₂ at CO₂ concentrations within the range of 5 to 50%. These conditions are suitable for effective removal of CO₂ at room temperature from syngas, flue gas and biogas using temperature swing (TS) or temperature-pressure swing (TPS) regeneration mode. Moreover, TRI-PE-MCM-41 was found to be highly stable over hundreds of adsorption-desorption cycles using TPS as regeneration mode.     

CO2 Capture,Separation and Reduction on Boron-Doped MoS2, MoSe2 and Heterostructures with Different Doping Densities: A Theoretical Study

Designing high-performance materials for CO₂ capture and conversion is of great significance to reduce the greenhouse effect and alleviate the energy crisis. The strategy of doping is widely used to improve activity and selectivity of the materials. However, it is unclear how the doping densities influence the materials’ properties. Herein, we investigated the mechanism of CO₂ capture, separation and conversion on MoS₂, MoSe₂ and Janus MoSSe monolayers with different boron doping levels using density functional theory (DFT) simulations. The results indicate that CO₂, H₂ and CH₄ bind weakly to the monolayers without and with single-atom boron doping, rendering these materials unsuitable for CO₂ capture from gas mixtures. In contrast, CO₂ binds strongly to monolayers doped with diatomic boron, whereas H₂ and CH₄ can only form weak interactions with these surfaces. Thus, the monolayers doped with diatomic boron can efficiently capture and separate CO₂ from such gas mixtures. The electronic structure analysis demonstrates that monolayers doped with diatomic doped are more prone to donating electrons to CO₂ than those with single-atom boron doped, leading to activation of CO₂. The results further indicate that CO₂ can be converted to CH₄ on diatomic boron doped catalysts, and MoSSe is the most efficient of the surfaces studied for CO₂ capture, separation and conversion. In summary, the study provides evidence for the doping density is vital to design materials with particular functions.     

A MOF Glass Membrane for Gas Separation

Metal–organic framework (MOF) glasses are promising candidates for membrane fabrication due to their significant porosity, the ease of processing, and most notably, the potential to eliminate the grain boundary that is unavoidable for polycrystalline MOF membranes. Herein, we developed a ZIF‐62 MOF glass membrane and exploited its intrinsic gas‐separation properties. The MOF glass membrane was fabricated by melt‐quenching treatment of an in situ solvothermally synthesized polycrystalline ZIF‐62 MOF membrane on a porous ceramic alumina support. The molten ZIF‐62 phase penetrated into the nanopores of the support and eliminated the formation of intercrystalline defects in the resultant glass membrane. The molecular sieving ability of the MOF membrane is remarkably enhanced via vitrification. The separation factors of the MOF glass membrane for H₂/CH₄, CO₂/N₂ and CO₂/CH₄ mixtures are 50.7, 34.5, and 36.6, respectively, far exceeding the Robeson upper bounds.     

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.     

Investigation into the adsorption of CO2, N2 and CH4 on kaolinite clay

Investigating the adsorption characteristics of CO₂, N₂ and CH₄ on kaolinite clay is beneficial for enhanced shale gas recovery by gas injection. In this paper, the experiments of CO₂, N₂ and CH₄ adsorption at 288 K, 308 K and 328 K on kaolinite clay were conducted, and the thermodynamics analysis of adsorption of three gases was performed. The findings reveal that the order of the uptakes of three gases on kaolinite clay is as follows: N₂ < CH₄ < CO₂. Reducing temperature enlarges the separation coefficients of CO₂ over CH₄ (α_CO2/CH4), CO₂ over N₂ (α_CO2/N2), and CH₄ over N₂ (α_CH4/N2). The value of α_CO2/CH4 greater than one validates that CO₂ is capable to directly replace the pre-adsorbed CH₄. The spontaneity of CO₂ adsorption is the highest, while N₂ has the lowest adsorption spontaneity. Injecting N₂ into gas-bearing reservoir can cause CH₄ desorption by lowering the spontaneity of CH₄ adsorption. Adsorbed CO₂ molecules form a most ordered rearrangement on kaolinite surface. The decrease rate of entropy loss for N₂ adsorption is higher than those for CO₂ and CH₄ adsorption.     

Decorated Traditional Zeolites with Subunits of Metal–Organic Frameworks for CH4/N2 Separation

Metal–organic frameworks (MOF) materials are promising materials for gas separation, but their application still faces various challenges. A strategy is now reported for introducing subunits of MOFs into traditional zeolite frameworks to obtain applicable adsorbents with advantages of both zeolites and MOFs. The subunits of ZIFs were introduced into zeolite Y and zeolite ZSM‐5 for CH₄/N₂ separation. Both the molecular simulation and experimental results validated that the IAST CH₄/N₂ selectivity of the resulting samples greatly improved (above 8, at 100 kPa and 25 °C) with the incorporation of ZIF subunits into zeolites structure, and the selectivities were obviously higher than that of zeolites and even better than that of ZIFs. This strategy not only gave rise to an efficient adsorbent for CH₄/N₂ separation but also provided ideas for design of other adsorption and separation materials.     

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.     

Zeolites modified by CuCl for separating CO from gas mixtures containing CO2

Although zeolites such as NaY and 13X adsorb CO₂ much more than CO, the adsorption amount of CO₂ and CO can be reversed if the zeolites are modified with CuCl. When zeolite NaY or 13X is mixed with CuCl and heated, high CO adsorption selectivity and capacity can be obtained. Isotherms show the adsorbents have CO capacity much higher than CO₂. This is because CuCl has dispersed onto the surface of the zeolites to form a monolayer after the heat treatment and the monolayer dispersed CuCl can provide tremendous Cu(I) to selective adsorb CO and inhibit the CO₂ adsorption. The monolayer dispersion of CuCl is confirmed by XRD and EXAFS studies. The loading of CuCl on the zeolites has a threshold below which the CuCl forms monolayer after heating and crystalline phase of CuCl can not be detected by XRD. An adsorbent of CuCl/NaY with CuCl content closed to the monolayer capacity shows very high CO selective adsorbability for CO₂, N₂, H₂ and CH₄. At temperature higher than room temperature, the adsorbent has even better CO selectivity for CO₂. Using the adsorbent, a single-stage 4 beds PSA process, working at 70°C and 0.4 MPa to 0.013 MPa, can obtain CO product with purity >99.5% and yield >85%.     

The effect of gas molecule affinities on CO2 separation from the CO2/N2 gas mixture using inorganic membranes as investigated by molecular dynamics simulation

Applying molecular dynamics simulation and computer graphics methods we have investigated the dynamic behavior of the separation process of CO₂ from the CO₂/N₂ gas mixture in inorganic membranes at high temperatures. We have demonstrated that the permeation dynamics follows the Knudsen diffusion mechanism in our model system that has a slit-like pore of 6.3 Å. We have analyzed the effect of affinities of gas molecules for the membrane wall on the permeation to predict the optimal affinity strength for high selectivity of CO₂. Our results indicate that in the model with the 600 K and 200 K affinities for CO₂ and N₂, respectively, we can obtain a high selectivity of CO₂ even if the temperature is 1073 K. It is also shown that there is an optimal range for the CO₂ affinity for the membrane wall to achieve good separation, which was estimated as the range of 400–600 K in our system, if the affinity of N₂ is always weaker than that of CO₂.