•A CoRE COF database that covers most of synthesized materials was provided.•Structure-property relationships of COFs for noble gas separation were revealed.•Structural features of COFs with optimal separation properties were suggested.•COFs with good separation performance were identified for industrial cyclic process.•The CoRE COF database provides a good platform for future study in COF field.In this work, a computation-ready, experimental covalent organic framework (CoRE COF) database that nearly covers all the existing COFs was constructed and provided, which contains 187 COFs with disorder-free and solvent-free structures. Using the CoRE COF database established, structure-property relationships of COFs for Kr/Ar, Xe/Kr and Rn/Xe separations were studied. The conditions of industrial vacuum (VSA) and pressure swing adsorption (PSA) processes were considered in the computation. Qualitative rules of COFs for noble gas separations were clarified, and structural features of COFs with excellent separation performance were summarized and suggested. In addition, COFs with good separation performance were identified from our database for industrial cyclic process. The knowledge obtained in this work may give guidance for experimental efforts in seeking advanced materials for noble gas separation, and the CoRE COF database will facilitate the fundamental research of COFs as well as the development of novel functional materials toward practical applications.Download high-res image (110KB)Download full-size image

Effective capture of radioactive iodic contaminants from nuclear wastes is of great importance for public safety as well as the secure utility of nuclear energy. In this work, a computational study was performed to systematically evaluate the performance of 187 experimentally reported covalent organic frameworks (COFs) for gaseous I2 and CH3I adsorption under real industrial conditions. The results show that 3D-COFs present better performance than 2D-COFs for both I2 and CH3I adsorption. 3D-Py-COF was identified with the highest I2 uptake of 16.7 g g−1, outperforming the adsorbents reported to date. In addition, based on the obtained structure–property relationships, a new 3D-COF with an even higher I2 uptake of 19.9 g g−1 was designed. For CH3I adsorption, the pore morphology plays an important role, and 3D-COFs with ctn topology having a pore size of around 9 Å show superiority compared with other COFs. COF-103 was identified as the best material with a CH3I uptake of 2.8 g g−1, which is much higher than those of traditional adsorbents like activated carbons, alumina and zeolites.

Ultrathin films have the intrinsic feature to show high flux, which may become ideal membranes if they are fabricated to also have high selectivity. 2D covalent organic frameworks (COFs) are a class of crystalline materials with well-defined layered porous structures. Utilizing this feature, 2D-COF nanosheets can be stacked into few-layered ultrathin membranes, and thus the newly formed interlayer flow passages can be regulated to tune their separation properties, making them potential candidates for high-performance membranes. In this work, a series of few-layered 2D-COF membranes were constructed to explore their capability for gas separation as well as the underlying gas transport mechanisms by taking CO2/N2 separation as an example. The results showed that various few-layered 2D-COF membranes can be fabricated to show very different separation performances, from nonselective to highly selective, and even to the molecular sieving level. Furthermore, it was revealed that the energetic microenvironment around the narrow interlayer passages plays a crucial role, and introduction of interacting surfaces to generate van der Waals potential sites near these passages by tuning stacking modes can achieve few-layered membranes with both high CO2 flux and high CO2/N2 selectivity, leading to their separation performance far above the Robeson's upper bound. The mechanisms revealed and the design strategies proposed in this work may provide useful guidance for preparing ultrathin membranes with outstanding performance for gas separation.

Ultrathin membranes with intrinsic pores are highly desirable for gas separation applications, because of their controllable pore sizes and homogeneous pore distribution and their intrinsic capacity for high flux. Two-dimensional (2D) covalent organic frameworks (COFs) with layered structures have periodically distributed uniform pores and can be exfoliated into ultrathin nanosheets. As a representative of 2D COFs, a monolayer triazine-based CTF-0 membrane is proposed in this work for effective separation of helium and purification of hydrogen on the basis of first-principles calculations. With the aid of diffusion barrier calculations, it was found that a monolayer CTF-0 membrane can exhibit exceptionally high He and H2 selectivities over Ne, CO2, Ar, N2, CO, and CH4, and the He and H2 permeances are excellent at appropriate temperatures, superior to those of conventional carbon and silica membranes. These observations demonstrate that a monolayer CTF-0 membrane may be potentially useful for helium separation and hydrogen purification.Keywords: covalent triazine framework; density functional theory; diffusion barrier; gas separation; two-dimensional membrane

To reduce the emission of greenhouse gases, the separation of CO2 from flue gases emitted by power plants with combustion of carbon-based fossil fuels is of great importance. Compared with CO2-selective membranes, N2-selective membranes are more promising for such systems with low concentrations of CO2. Using density functional theory (DFT) calculations and molecular dynamic (MD) simulations, we demonstrated in this work that the poly(triazine imide) (PTI) membrane can be efficiently employed to separate N2 from CO2 with a selectivity of 273 and a N2 permeance of 106 GPU, superior to those of most conventional membranes. Furthermore, it was revealed that the presence of H2O has a negligible influence on gas separation performance of the PTI membrane. This experimentally available N2-selective ultrathin membrane may be expected to find practical applications in postcombustion CO2 capture.

•Preparation of new mixed matrix membranes (MMMs) based on PSF and NH2-MIL-125(Ti).•Simultaneously enhanced gas permeability and separation factor with adding the MOF.•The MMMs can maintain the performance at high pressure for CO2/CH4 separation.High performance mixed matrix membranes arise from a targeted selection of constituent materials, especially the choice of fillers. An amine-functionalized metal-organic framework (MOF), NH2-MIL-125(Ti), was used in this work to prepare polysulfone-based mixed matrix membranes (MMMs). Permeation measurements on CO2/CH4 gas mixture demonstrate that the incorporation of NH2-MIL-125(Ti) particles can significantly improve the CO2 permeability compared to the pure polymer membrane, along with slightly enhanced CO2/CH4 separation factor. This work also shows that the separation factor at high pressures can remain almost unchanged for the prepared MMMs with the MOF loadings up to 20 wt%. The obtained results may provide useful information in facilitating the applications of promising MOF-containing MMMs for the practical membrane-based natural gas purification.

The separation of H2S/CH4 mixture was computationally examined in the composites of ionic liquids (ILs) supported on metal–organic frameworks (MOFs) at room temperature. Cu-TDPAT was selected as supporter for four types of ILs combined from identical cation [BMIM]+ with different anions ([Cl]−, [Tf2N]−, [PF6]−, and [BF4]−). The results show that introducing ILs into Cu-TDPAT can greatly enhance the adsorption affinity toward H2S compared to the pristine MOF, and the strongest enhancement occurs in the composite containing the anion [Cl]− with the smallest size. The H2S/CH4 adsorption selectivities of each composite are significantly higher than those of the pristine Cu-TDPAT within the pressure range examined, and the selectivity generally shows an increasing trend with increasing the loading of the IL. By further taking the H2S working capacity into account, this work also reveals that the [BMIM][Cl]/Cu-TDPAT composite exhibits the best separation performance in both VSA and PSA processes. These findings may provide useful information for the design of new promising IL/MOF composites applied for H2S capture from natural gas.

The highly flexible hybrid nanoporous MOF MIL-53(Cr) was evoked as a potential medium to store mechanical energy via a structural switching from an open to a close pore form under moderate applied external pressures. Herein, we show that the inclusion of a low concentration of either polar or apolar molecules into the pores can finely tune the structural and energetic behaviour of this solid under compression–decompression. This allows a modulation of the material's storage abilities in the form of not only nano-springs/dampers but also shock adsorbers by confining n-alkane and water–alcohol, respectively. Predicting and further understanding the impact of each guest on the mechanical properties of this MOF are achieved by molecular dynamics simulations based on a refined version of a flexible force field able to accurately capture the breathing of the framework. A careful analysis of the host–guest interactions and the preferential conformations of the confined molecules validated by in situ X-ray diffraction and microcalorimetry data shed light on the microscopic mechanisms at the origin of the singular mechanical behaviour of each guest loaded material.

With the aid of multi-scale computational methods, a diverse set of 46 covalent organic frameworks (COFs), covering the most typical COFs synthesized to date, were collected to study the structure–property relationship of COFs for CO2 capture. For this purpose, CO2 capture from postcombustion gas (CO2–N2 mixture) under industrial vacuum swing adsorption (VSA) conditions was considered as an example. This work shows that adsorption selectivity, CO2 working capacity and the sorbent selection parameter of COFs all exhibit strong correlation with the difference in the adsorbility of adsorbates (ΔAD), highlighting that realization of large ΔAD can be regarded as an important starting point for designing COFs with improved separation performance. Furthermore, it was revealed that the separation performance of 2D-layered COFs can be greatly enhanced by generating “splint effects”, which can be achieved through structural realignment to form slit-like pores with suitable size in the structures. Such “splint effects” in 2D-COFs can find their similar counterpart of “catenation effects” in 3D-COFs or MOFs. On the basis of these observations, a new design strategy was proposed to strengthen the separation performance of COFs. It could be expected that the information obtained in this work not only will enrich the knowledge of the structure–property relationship of COFs for separation, but also will largely facilitate their future applications to the fields related to energy and environmental science, such as natural gas purification, CO2, NOx and SOx capture, etc.

Experimental measurements have been combined with molecular simulations to investigate the adsorption and separation of aniline/phenol mixtures from aqueous solutions by the aluminum terephthalate MIL-53. The results show that the framework flexibility of this material plays a crucial role in the adsorption process and thus can greatly enhance the separation of the aniline/phenol mixture from their solutions. Compared with the conventional adsorbents, MIL-53(Al) shows the best performance for such systems of interest, from the points of view of both the adsorption capacities and the selectivities for aniline. The findings obtained in this work may facilitate more investigations in connection with the application of flexible nanoporous materials for the separation of organic compounds from liquid-phase environments.

The concentration dependence of the self-diffusivity of short-chain linear alkanes in the narrow window type metal–organic framework (MOF) UiO-66(Zr) has been studied by means of quasi-elastic neutron scattering (QENS) measurements combined with molecular dynamics (MD) simulations. These computations employ a force field to describe the host/guest interactions which was preliminarily validated on the adsorption data obtained for the system of interest via gravimetry and microcalorimetry measurements. The QENS-measured self-diffusivity profile presents a nonmonotonic tendency as the alkane loading increases, with the existence of a maximum that depends on the size of the alkane. The comparison with the simulated results obtained using either a flexible or a rigid framework highlights that the consideration of the flexibility is of prime importance when exploring the diffusion of ethane molecules in porous materials. The self-diffusivities subsequently calculated for propane and n-butane corroborate the results obtained for ethane, leading to a similar form for the plots of self-diffusion coefficient vs loading. The global microscopic diffusion mechanism is further shown to involve a combination of intracage motions and jump sequences between the tetrahedral and octahedral cages of the framework. The self-diffusion coefficients which decrease with increasing molecular size, and thus increasing confinement, are further compared to the values previously reported for MOFs with pore networks of different dimensions.

In this work, molecular simulations combined with experimental measurements were conducted to screen candidate metal–organic frameworks (MOFs) for aniline recovery from aqueous solution. The structure–property relationships that can correlate the recovery performance of the MOFs to their physicochemical features were analyzed. The results show that the interplay between the isosteric heat of adsorption of aniline and the free volume of the material is crucial to the aniline recovery capability of MOFs at low concentrations. In contrast, the aniline uptake at high concentrations is dominated by the material's free volume as usually expected. In addition, the requirements of the best MOF candidates for aniline recovery were suggested in this study. Based on the conclusions drawn from the screening route, several novel MOFs were further computationally designed, which possess high aniline uptake within the whole range of concentrations examined. The obtained information may be helpful for in-depth research to broaden the applications of MOFs in mixture separation under liquid phase conditions.

In this work, nonequilibrium molecular dynamics simulations were performed to investigate the dispersion and spatial distribution of spherical nanoparticles (NPs) in polymer matrix under oscillatory shear flow. We systematically analyzed the influences of four important factors that consist of NP–polymer interfacial strength, volume fraction of NPs, shear conditions, and polymer chain length. The simulation results showed that the oscillatory shear can greatly improve the dispersion of NPs, especially for the polymer nanocomposites (PNCs) with high NP–polymer interfacial strength. Under specific shear conditions, the NPs can exhibit three different spatial distribution states with increasing the NP–polymer interfacial strength. Interestingly, at high interfacial strength, we observed that the NPs can be distributed on several layers in the polymer matrix, forming the PNCs with sandwich-like structures. Such well-ordered nanocomposites can exhibit a higher tensile strength than those with the NPs dispersed randomly. It may be expected that the information derived in present study provides a useful foundation for guiding the design and preparation of high-performance PNCs.

Highly diverse structures and pore sizes make metal–organic frameworks (MOFs) good candidates for the fabrication of gas separation membranes. The synthesis of continuous MOF membranes still remains a challenge. In this work, an integrated Cu-BTC membrane was successfully prepared on the novel potassium hexatitanate support for the first time by in situ solvothermal growth. This kind of support was found to be more suitable for the growth of Cu2+-containing MOF membranes than other traditional supports, such as a porous alumina support. The permeation results of Cu-BTC membranes obtained in this work show moderate separation selectivities of helium over other small gas molecules, including CO2, N2, and CH4. Compared to other MOF membranes, the Cu-BTC membrane exhibits higher ideal selectivity for helium under the condition of similar helium permeance, while it has higher helium permeance with similar ideal selectivity.

It is of great importance to establish a quantitative structure–property relationship model that can correlate the separation performance of MOFs to their physicochemical features. In complement to the existing studies that screened the separation performance of MOFs from the adsorption selectivity calculated at infinite dilution, this work aims to build a QSPR model that can account for the CO2/N2 mixture (15:85) selectivity of an extended series of MOFs with a very large chemical and topological diversity under industrial pressure condition. It was highlighted that the selectivity for this mixture under such conditions is dominated by the interplay of the difference of the isosteric heats of adsorption between the two gases and the porosity of the MOF adsorbents. On the basis of the interplay map of both factors that impact the adsorption selectivity, strategies were proposed to efficiently enhance the separation selectivity of MOFs for CO2 capture from flue gas. As a typical illustration, it thus leads us to tune a new MOF with outstanding separation performance that will orientate the synthesis effort to be deployed.

A novel separation concept based on the stepped behaviors of adsorption in nanoporous materials was brought out in this work. Molecular simulations were performed to investigate the effect of such stepped phenomena on the separation of CO2/N2 gas mixture in metal−organic frameworks (MOFs). The simulation results show that the stepped behaviors occurring in the isotherms of CO2 can significantly enhance the adsorption selectivity of CO2 from the mixture. The underlying mechanism examined at the molecular level reveals that the stepped phenomenon is mainly caused by the electrostatic interactions between CO2 molecules. In addition, this work shows that the lithium-modified MOFs can greatly reduce the pressure at which the stepped behavior occurs and also enhance the selectivity of CO2 from CO2/N2 gas mixture.

A computational study was firstly performed in this work to examine the applicability of an acid-functionalized metal-organic framework (MOF), UiO-66(Zr)-(COOH)2, in membrane-based H2S/CH4 separation. The results show that this MOF could be potentially interesting when being used as the pure membrane material for the separation of the mixture with low H2S concentration. Further, the performance of 10 different mixed matrix membranes (MMMs) on the basis of the MOF was predicted by combing the molecular simulation data and the Maxwell permeation model. The results indicate that using this MOF as filler particles in MMMs can significantly enhance the permeation performance of pure polymers. The findings obtained in this work may be helpful in facilitating the application of this promising MOF for practical desulfurization process of fuel gas.A computational study was performed to examine the applicability of UiO-66(Zr)-(COOH)2 in membrane-based H2S/CH4 separation. The results reveal that the MOF exhibits a good performance at low H2S concentrations when being used as pure membrane material. In addition, this MOF can be considered as promising nanoporous stuff for the fabrication of MMMs with a significant enhancement on the permeation performance of the polymer membranes.Download full-size image

•Computationally exploring CO2/CH4 separation in 151 diverse MOFs via TSA.•Thermal regeneration energy combined with three common criteria for evaluation.•The difference of adsorbility of adsorbates can be used for preliminary screening.•MOFs with strong adsorption sites for CO2 are preferred materials.Molecular simulations were performed to investigate the performance of 151 metal–organic frameworks (MOFs) with large chemical and topological diversity on CO2/CH4 separation via temperature swing adsorption (TSA) process. The thermal regeneration energy was adopted in this work as an evaluation criterion and combined with other three commonly used ones (CO2 working capacity, adsorption selectivity and regenerability) to explore the structure–property relationships for the separation of the target system. The results show that the four evaluation criteria exhibit intimate correlations with the difference of adsorbility (ΔAD) of adsorbates but with non-concerted changing tendency. With a certain range of this parameter, it can be used as a good indicator for the preliminary screening of MOFs and the tailoring of new materials. Furthermore, from the structural database considered in current study, Cu-TDPAT with strong CO2 adsorption sites was found to possess the best performance by taking the thermal and water-stable properties into account.

•The capability of functionalized graphene for gas separation was computationally investigated.•The novel mass-transport mechanism of the graphene membranes were revealed.•The graphene membranes have great potential applications in the flue gases and natural gas processing.Graphene has enormous potential as a membrane-separation material with ultrahigh permeability and selectivity. The understanding of mass-transport mechanism in graphene membranes is crucial for applications in gas separation field. We computationally investigated the capability and mechanisms of functionalized nanoporous graphene membranes for gas separation. The functionalized graphene membranes with appropriate pore size and geometry possess excellent high selectivity for separating CO2/N2, CO2/CH4 and N2/CH4 gas mixtures with a gas permeance of ∼103–105 GPU, compared with ∼100 GPU for typical polymeric membranes. More important, we found that, for ultrathin graphene membranes, the gas separation performance has a great dependence not only with the energy barrier for gas getting into the pore of the graphene membranes, but also with the energy barrier for gas escaping from the pore to the other side of the membranes. The gas separation performance can be tuned by changing the two energy barriers, which can be realized by varying the chemical functional groups on the pore rim of the graphene. The novel mass-transport mechanism obtained in current study may provide a theoretical foundation for guiding the future design of graphene membranes with outstanding separation performance.We demonstrated theoretically that the functionalized graphene membranes are far superior to the typical polymeric membranes and have great potential applications in the flue gases and natural gas processing. It was further revealed that, for ultrathin graphene membranes, the gas separation performance has a great dependence not only with the energy barrier for gas getting into the pore of the graphene membranes, but also with the energy barrier for gas escaping from the pore.Download high-res image (168KB)Download full-size image

The highly flexible hybrid nanoporous MOF MIL-53(Cr) was evoked as a potential medium to store mechanical energy via a structural switching from an open to a close pore form under moderate applied external pressures. Herein, we show that the inclusion of a low concentration of either polar or apolar molecules into the pores can finely tune the structural and energetic behaviour of this solid under compression–decompression. This allows a modulation of the material's storage abilities in the form of not only nano-springs/dampers but also shock adsorbers by confining n-alkane and water–alcohol, respectively. Predicting and further understanding the impact of each guest on the mechanical properties of this MOF are achieved by molecular dynamics simulations based on a refined version of a flexible force field able to accurately capture the breathing of the framework. A careful analysis of the host–guest interactions and the preferential conformations of the confined molecules validated by in situ X-ray diffraction and microcalorimetry data shed light on the microscopic mechanisms at the origin of the singular mechanical behaviour of each guest loaded material.

With the aid of multi-scale computational methods, a diverse set of 46 covalent organic frameworks (COFs), covering the most typical COFs synthesized to date, were collected to study the structure–property relationship of COFs for CO2 capture. For this purpose, CO2 capture from postcombustion gas (CO2–N2 mixture) under industrial vacuum swing adsorption (VSA) conditions was considered as an example. This work shows that adsorption selectivity, CO2 working capacity and the sorbent selection parameter of COFs all exhibit strong correlation with the difference in the adsorbility of adsorbates (ΔAD), highlighting that realization of large ΔAD can be regarded as an important starting point for designing COFs with improved separation performance. Furthermore, it was revealed that the separation performance of 2D-layered COFs can be greatly enhanced by generating “splint effects”, which can be achieved through structural realignment to form slit-like pores with suitable size in the structures. Such “splint effects” in 2D-COFs can find their similar counterpart of “catenation effects” in 3D-COFs or MOFs. On the basis of these observations, a new design strategy was proposed to strengthen the separation performance of COFs. It could be expected that the information obtained in this work not only will enrich the knowledge of the structure–property relationship of COFs for separation, but also will largely facilitate their future applications to the fields related to energy and environmental science, such as natural gas purification, CO2, NOx and SOx capture, etc.

Ultrathin films have the intrinsic feature to show high flux, which may become ideal membranes if they are fabricated to also have high selectivity. 2D covalent organic frameworks (COFs) are a class of crystalline materials with well-defined layered porous structures. Utilizing this feature, 2D-COF nanosheets can be stacked into few-layered ultrathin membranes, and thus the newly formed interlayer flow passages can be regulated to tune their separation properties, making them potential candidates for high-performance membranes. In this work, a series of few-layered 2D-COF membranes were constructed to explore their capability for gas separation as well as the underlying gas transport mechanisms by taking CO2/N2 separation as an example. The results showed that various few-layered 2D-COF membranes can be fabricated to show very different separation performances, from nonselective to highly selective, and even to the molecular sieving level. Furthermore, it was revealed that the energetic microenvironment around the narrow interlayer passages plays a crucial role, and introduction of interacting surfaces to generate van der Waals potential sites near these passages by tuning stacking modes can achieve few-layered membranes with both high CO2 flux and high CO2/N2 selectivity, leading to their separation performance far above the Robeson's upper bound. The mechanisms revealed and the design strategies proposed in this work may provide useful guidance for preparing ultrathin membranes with outstanding performance for gas separation.