Gas Separation Applications Research Articles

Deep eutectic solvents as reusable catalysts and promoter for the greener syntheses of small molecules: Recent advances

Deep eutectic solvents (DESs) are significant environmentally benign media in the field of green chemistry. They are known not only for their inexpensive nature, biodegradability, non-toxicity, and bioavailability, but also for recyclability and their ease of preparation and purification. DESs are made up of hydrogen bond donor substituents (such as urea, carboxylic acid, glycerol, metals, etc.) and hydrogen bond acceptor substituents such as halide salts; they have a lower melting point than each substituent comprising the mixture. DESs find applications in biotransformation, gas separation, biomass valorization, electrochemistry, extraction, and dissolution process as well as in material chemistry and other assorted fields of chemistry. Interestingly, DESs fulfil a unique dual role as green solvents, promoters and catalysts in organic syntheses. Thus, it is noteworthy that chemical progresses and transformation with external catalysts in cross-coupling reactions, oxidations and reductions are not covered in this overview. This review mostly focuses on leading-edge applications of DES as a reusable catalyst in the expeditious synthesis of assorted small molecules central in the assembly of broad-spectrum pharmaceuticals, and total synthesis of common natural products.

A Cavity-Tailored Metal-Organic Tetrahedral Nanocage and Gas Adsorption Property.

Porous organometallic nanomaterials are a new class of materials based on a three-dimensional structure. They have excellent applications in different fields, but their applications in gas storage and separation have not been fully developed. CO2 adsorption storage and hydrocarbon separation has been a challenging industrial problem. Several typical molecular adsorbents have been used to study the separation, but the problems of long-term stability, high selectivity and synthetic complexity of these adsorbents remain to be solved. Here, we have designed and synthesized tetrahedral metal supramolecular nanocage with custom cavities based on the unique rigid structure of triptycene derivatives. Using the unique discrete porous structure of tetrahedral metal nanocages, the gas adsorption and separation performance of the metal supramolecular nanocage was investigated. By analyzing the adsorption and desorption isotherms and the multi-component competitive adsorption curves, we noticed that the tetrahedral supramolecular nanocages had good CO2 storage capacity and good separation capacity for C2H2/CO2 and C2H2/N2. All these indicate that porous organic metal nanomaterials are expected to be a new energy saving separation material.

Flexibility of Mixed Ligand Zeolitic Imidazolate Frameworks (ZIF-7-8) under CO2 Pressure: An Investigation Using Positron Annihilation Lifetime Spectroscopy.

Fine tuning of the pore architecture and flexibility of zeolitic imidazolate frameworks (ZIFs) is highly crucial for realizing their applications in molecular gas separation. Mixed ligand frameworks (ZIF-7-8) synthesized by mixing 2-methylimidazole (2meIm) and benzimidazole (bIm) ligands show enhanced gas separation performance, attributable to pore and flexibility tuning. In the present study, positron annihilation lifetime spectroscopy (PALS) measurements under CO2 pressure have been used to experimentally investigate the tuning of the pore architecture and flexibility of mixed ligand frameworks ZIF-7-8 having a ZIF-8 structure and similar morphology with varying bIm content up to 18.2%. The aperture and cavity of frameworks begin to open up with an increasing bIm ligand content followed by a decrease at a higher content. On the contrary, flexibility of the frameworks indexed from PALS measurements carried out under CO2 pressure shows a decreasing trend followed by an increase. The present study shows that mixed ligand frameworks having a larger aperture size are less flexible as a result of inherent open configurations of ligands in the framework lattice. On the other hand, frameworks having a comparatively smaller aperture size show higher flexibility as a result of a possibility of twisting of the ligands under CO2 pressure, resulting in aperture opening. The pore-opening phenomenon as a result of lattice flexibility under CO2 pressure is observed to be fully reversible for ZIF-7-8.

In-situ interfacial crosslinking of NH2-MIL-53 and polyimide in MOF-incorporated mixed matrix membranes for efficient H2 purification

The key issue to avoiding the degradation of gas separation performance due to the formation of non-selective defects in mixed matrix membranes (MMMs) is to promote the interfacial compatibility of fillers with the continuous polymer matrix. In this work, we report an efficient and simple method to prepare MMMs by crosslinking metal organic frameworks (MOF) and polyamic acid (PAA) (the precursor of polyimide), in which NH2-MIL-53 particles are thermally crosslinked with the PAA during the imidization reaction, forming hydrogen bonds and amide bonds at the interface. The resulting enhanced interfacial force greatly improves the interfacial compatibility of the membrane, which is also more conducive to the separation of H2/CO2. Among the as-prepared samples, PI-NH2-MIL-53 MMMs loaded with 30 wt% MOF exhibit excellent H2 permeability (384.1 Barrer) and H2/CO2 permeation selectivity (16.7), which are significantly higher than the 2008 Robeson upper bound. The membranes also display enhanced mechanical characteristics and long-term stability. Here, building strong interactions on MOF/polymer MMMs using an in-situ cross-linking strategy to enhance membrane performance provides a new opportunity to develop advanced membranes for industrial gas separation applications.

Hafnium(IV) Chemistry with Imide-Dioxime and Catecholate-Oxime Ligands: Unique {Hf5} and Metalloaromatic {Hf6}-Oxo Clusters Exhibiting Fluorescence.

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal-organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H3pidiox) and 2,3-dihydroxybenzaldehyde oxime (H3dihybo) to synthesize the first ever reported pentanuclear {Hf5/H3pidiox} and hexanuclear {Hf6/H3dihybo} clusters (HfOCs). The {Hf6} clusters adopt unique core structures [Hf6IV(μ3-O)2(μ-O)3] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV-vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) 1H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters' band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf3} metallic component revealed by its NICSzz scan curve calculated by means of density functional theory (DFT). The NICSzz(1) value of -44.6 ppm is considerably higher than the -29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H3pidiox ligand, whereas the {Hf6} 3 shifts the violet-emitting light of the H3dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that HfIV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear HfIV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices.

Porosimetry of zeolitic imidazolate frameworks using positron annihilation lifetime spectroscopy

Zeolitic Imidazolate Frameworks (ZIFs) owing to their crystalline morphology have well designed pore architecture. ZIFs show great potential for applications in gas separation, gas storage, catalysis and sensors because of their high surface area, pore volume and well defined pore network. In order to utilize ZIFs for aforementioned applications, investigation of pore architecture (pore size, pore size distribution and interconnectivity) of ZIFs in powder or film form is highly essential. Conventional gas adsorption techniques are found to be incapable for providing complete pore architecture of ZIFs due to ultra-microporosity and pressure dependent framework flexibility. In recent years, positron annihilation lifetime spectroscopy (PALS) has been used to investigate the porosity of few ZIFs. In order to establish the applicability of PALS for pore architecture investigation of ZIFs in general, a systematic study for a number of ZIFs (ZIF-67, ZIF-8, ZIF-7l, ZIF-7n ZIF-62, ZIF-90, ZIF-70.52-8, Zn0.25Co0.75-ZIF-8, and ZIF-8A) have been carried out. These frameworks have been synthesized at room temperature or using solvothermal methods. PALS measurements of these ZIFs confirm the presence of different types of interconnected pores in these frameworks. The estimated pore sizes are compared with the pore sizes determined from other conventional techniques. The present study suggests that PALS is a powerful technique for the investigation of tuning of the pore architecture of all type of ZIFs. The determined pore sizes are the factual average pore size of desolvated bulk ZIF samples free from artifacts. These pore sizes could be correlated with the pore size dependent separation performance of ZIFs.

Cellulose acetate mixed-matrix membranes doped with high CO2 affinity zeolitic tetrazolate-imidazolate framework additives

Introducing CO2 affinity additives into membrane is one of significant approaches to improve the CO2 permeability of membranes for natural gas sweetening. In current work, high performance mixed-matrix membranes were successfully obtained using the cellulose acetate (CA) and the zeolitic tetrazolate-imidazolate frameworks (ZTIFs) with highly CO2 affinity as matrix and additive respectively. The interaction between CA matrix and ZTIFs was verified by the FT-IR. The SEM, EDS and XRD were used to evaluate the morphology properties of these membranes. The CO2 permeability of CA/ZTIFs mixed-matrix membranes which is closely correlated to the loading of high CO2 affinity ZTIF additive were in a range of 9.59–15.85 Barrer, implying the indispensable role of the ZTIF additive in changing the CO2 dissolution and diffusion behavior inside the membranes. The ideal compatibility and gas selectivity proved the great potential of the ZTIFs additives for the gas separation application.

Using Artificial Neural Networks to Predict Physical Properties of Membrane Polymers

AbstractMembrane polymers are a promising technology for use in many challenging gas separation applications. The techniques of computer‐aided molecular design can be used to search through the massive molecular space of heteropolymers and develop a set of likely candidate repeat units matching specific physical property targets. However, reasonably accurate property prediction algorithms are needed, but these algorithms must be very fast in order to be combined with an optimization framework. Artificial neural networks (ANNs), a branch of machine learning, are applied in this work to predict the physical properties of polymers. All of the physical properties investigated were found to be predicted by ANNs with R2 scores exceeding 0.82.

Percolation-assisted coating of metal-organic frameworks on porous substrates

In the past two decades, significant advancements have been made toward thin film fabrication of metal-organic frameworks (MOFs) on porous solid substrates for gas separation and catalysis applications. To enhance the physical and chemical stability of the films, various top-down and bottom-up approaches have been implemented. While both approaches have some advantages, they are mostly limited due to a lack of adhesion with the substrate, cracking of the films, and non-uniform coverages. Therefore, there is a need for improvement in the coating processes for the fabrication of robust, uniform, and scalable films. Here, we develop a novel percolation-assisted coating (PAC) process that combines top-down and bottom-up approaches in a continuous-flow microfluidic device to deposit HKUST-1 on a porous substrate, yielding controlled film thicknesses and mass loading. The PAC process is optimized using a Multiphysics modeling approach to design the microfluidic reactor, which can be readily scaled up and deployed to fabricate MOF films on various porous substrates. The desired thickness in the range of 10–200 μm of the MOF film can be achieved by controlling the residence time and temperature of the reaction mixture. The synthesized film is characterized for physical adhesion using sonication, film coverage using scanning electron microscopy, and porosity using a porosimeter. The performance of synthesized films is benchmarked for the effective separation of 50 vol% CH4–H2 gas mixture with the separation factor of 4. The microfluidic device is also applicable to synthesize films of various MOFs like MOF-5, MOF-505 (also reported here), UIO-66, etc., over a wide range of porous substrates. Lastly, we propose a multi-chamber design of the microfluidic device for high-throughput screening of thin-film growth using the PAC process.

Modified CARDO-Based Copolyimides with Improved Sour Mixed-Gas Permeation Properties

Chemical modification is an interesting tool to introduce functional groups into polymers’ backbones to tailor their properties in a desired fashion. Polymers used in membrane-based gas separation applications need to possess high gas permeability and selectivity during mixed-gas separation under harsh operational conditions of pressure and temperature. Herein, we report the modification of three copolyimides containing the moiety 9,9-bis(4-aminophenyl)fluorine (CARDO) through Friedel–Crafts alkylation to introduce bulky tert-butyl groups into their backbones. Membranes prepared from the modified polymers were evaluated using pure- and sweet mixed-gas under aggressive feed pressures. Based on the promising results obtained, a selected copolyimide was subjected to sour mixed-gas separation under feed pressures up to 500 psi. Mixed-gas separation studies, especially those containing hydrogen sulfide, are of great interest to develop membranes for use in sour natural gas reserve treatment. For example, the H2S and CO2 permeability coefficients of 6FDA-Durene/6FDA-CARDO(t-Bu) at 500 psi were recorded as 456 and 238 Barrer, respectively, where the H2S/CH4 and CO2/CH4 are 20.4 and 10.6, respectively. The obtained results are very promising and even superior to many glassy polymers reported previously. This work represents an example of the benefit of chemical modification on polymers to tailor their structure–property relationship. Future works will be focused on the fabrication and testing of asymmetric or composite hollow fibers to imitate the membrane modules used industrially.

Incorporated Metal–Organic Framework Hybrid Materials for Gas Separation, Catalysis and Wastewater Treatment

The special features of metal–organic frameworks (MOFs), namely, tunable porosity, exceptional structure, high surface area and high adsorption capability enable them to be widely studied in many applications including carbon capture and storage (CCS), biomedical engineering, catalysis and pollutant treatment. Despite these remarkable properties, MOFs are known to be moisture-sensitive, hardly recyclable and expensive in fabrication cost which limits their breakthrough performance in more efficient uses. Recently, extensive studies have been devoted to counter those shortcomings by embedding MOFs with support materials using various series of synthetic designs to yield incorporated MOF hybrid materials to counter their limitations. In view of this interest, this review summarizes the latest developments of incorporated MOFs with various materials, namely, ionic liquids (ILs), membranes and metal species. Pre-synthetic and post-synthetic synthesis methods are also discussed. This review also aims to highlight the factors associated with incorporated MOF performance such as materials selection and mass ratio which could have favorable effects in gas separation, catalysis and wastewater treatment applications. The data indicate that incorporated MOF hybrid materials exhibit exceptional properties including excellent robustness and stability. Correspondingly, in comparison to pristine MOFs, incorporated MOF hybrid materials significantly improve, among others, the gas selectivity, catalyst activity and dye removal efficiency in gas separation, catalysis and wastewater treatment, respectively. In addition, the challenge related to the utilization of this newly incorporated material is mentioned.

Direct synthesis of nanosized CHA zeolite free of organic template by a combination of cations as structure directing agents

There is an increasing demand for porous materials able to separate the components of natural gas, mainly CH4 and CO2. Flexible small pore zeolites are interesting candidates due to their high sorption capacity and ability to selectively separate molecules of similar sizes. This selectivity comes from their small pores, large cavities, and pore shapes which can be controlled by the introduction of different extra-framework cations such as Na+, K+, Cs+. Furthermore, the extra-framework cations selectively provide access to the pores and cages of the zeolite depending on the nature of the molecules present. The strategy to prepare CHA-type zeolites across a range of particle sizes, from micron-sized dimensions to nanocrystals with a size of 50 nm, was investigated. The quantities of extra-framework cations and the hydrothermal conditions were optimised in order to avoid the formation of secondary structural phases and selectively target the CHA phase. The CHA-type zeolites were characterised by electron microscopy, elemental analysis, MAS NMR and FTIR spectroscopy, and PXRD. The Si/Al ratio of the obtained CHA zeolite was between 1.7 and 2.6 which is ideal for gas separation applications. The nanosized as-synthesised CHA zeolite (Si/Al = 2.4) demonstrates a high CO2 storage capacity (4.6 mmol/g) making it an ideal candidate for gas separation applications.

Potential of a pH-Stable Microporous MOF for C2H2/C2H4 and C2H2/CO2 Gas Separations under Ambient Conditions.

Cost-effective adsorption-based C2H2/C2H4 and C2H2/CO2 gas separations are extremely important in the industry. Herein, a pH-stable three-dimensional (3D) metal-organic framework (MOF), IITKGP-25, possessing exposed functional sites is presented, which facilitates such separations with excellent ideal adsorbed solution theory (IAST) selectivity (4.61 for C2H2/C2H4 and 3.93 for C2H2/CO2) under ambient conditions (295 K, 100 kPa, 50:50 gas mixtures) and a moderate affinity toward C2H2 (26.6 kJ mol-1). Interestingly, IITKGP-25 can maintain structural integrity in water and in aqueous acidic/alkaline (pH = 2-10) medium because of the higher coordination numbers around the metal center and the hydrophobicity of the ligand. The adsorption capacity for C2H2 remains unchanged for a minimum of up to five consecutive cycles and 15 days of exposure to 97% relative humidity, which are the prerequisites of an adsorbent for practical gas separation application. Density functional theory (DFT) calculations reveal that the open Cd(II) sites and carboxylate oxygen-coordinated Cd(II) corner of the triangle-shaped one-dimensional (1D) channel are the enthalpically more preferred binding sites for C2H2, which stabilize the adsorbed C2H2 through nonlocal stronger H-bonding and also pπ-dπ and CH-π interactions.

Modulated Hydrothermal Chemistry of Metal–Organic Frameworks

ConspectusMetal–organic frameworks (MOFs) are infinite networks composed of inorganic metals and organic linkers, and have demonstrated superior advantages over traditional porous materials in gas storage, separation, catalysis, etc., because of possessing ultrahigh surface areas and tunability in pore sizes, topologies, shapes, and functionalities. Although tremendous progress has been made in the fundamental research and applications of MOFs, developing a green and large-scale approach to synthesizing MOFs with repeatably high quality remains a key step for their massive production and industrial applications. Traditional solvothermal and hydrothermal approaches usually synthesize MOFs on a small scale with challenges in quality control. In addition, these synthetic processes typically require high temperatures, high pressures, and sometimes large amounts of environmentally unfriendly organic solvents. In this Account, we elaborate on the recent progress of the modulated hydrothermal (MHT) chemistry developed by us, which helps synthesize zirconium/hafnium-based MOFs in green (aqueous solutions), mild (≤120 °C, 1 atm), and scalable fashion. The MHT chemistry has already served as a solid foundation for the design and scale-up synthesis of multifunctional MOF materials by tuning the types and concentrations of ligands and modulators and their ratios. It not only applies to the classical UiO-66-type and MOF-808-type MOFs, but also suits the design of new MOFs, such as NUS-6, NUS-8, and NUS-36. In addition, the MHT-enabled MOFs have demonstrated promising applications in gas separation and storage, Lewis/Brønsted acid-catalyzed oxidation reactions/biomass conversion, and piezoelectric/ferroelectric devices. This Account further prospects the evolution of MHT chemistry in developing new functional MOFs composed of different metals, such as titanium, zirconium, hafnium, cerium(IV), etc., on a large scale and sustainable paradigm.

Branched Tröger's base polymer membranes for gas separation

Here we report the synthesis of branched Tröger's base polymers (x-BTB) with different degree of tetraphenylmethane branch structure by incorporating varied amounts of tetrakis(4-aminopheny)methane into the main chains of Tröger's base polymer (TB). We show that branching increases porosity of x-BTB, leading to improved gas permeability. Specifically, the BET surface area and pore volume of x-BTB polymers increase continuously from 410 to 544 m2 g−1 and 0.584–0.607 cm3 g−1, respectively, for branched polymers containing progressively more branch structure. For CO2/N2, CO2/CH4, and O2/N2 binary gas separation, a significant improvement in the gas permeability without obvious compromising the membrane selectivity is observed on the x-BTB membranes with higher branch degree. In particular, for CO2/N2 binary gas separation, the CO2 permeability of 5 wt%-BTB membrane is 145 Barrers, creating an enhancement of 64.8% as compared to TB membrane. Moreover, x-BTB membranes are much more resistant to physical aging than TB membrane. These results suggest that introduction of tetraphenylmethane branch structure is an effective strategy to fabricate polymeric membranes with high performance suitable for gas separation applications.

Ti3C2 MXene Membranes for Gas Separation: Influence of Heat Treatment Conditions on D-Spacing and Surface Functionalization.

Two-dimensional (2D) MXene materials have recently been the focus of membrane research due to their unique properties, such as their single-atomic-layer thickness, flexibility, molecular filtration abilities and microstructural similarities with graphene, which is currently the most efficient precursor material for gas separation applications. In addition, the potential to process nanoscale channels has motivated investigations of parameters which can improve membrane permeability and selectivity. Interlayer spacing and defects, which are still challenging to control, are among the most crucial parameters for membrane performance. Herein, the effect of heat treatment on the d-spacing of MXene nanosheets and the surface functionalization of nanolayers was shown regarding its impact on the gas diffusion mechanism. The distance of the layers was reduced by a factor of over 10 from 0.345 nm to 0.024 nm, the defects were reduced, and the surface functionalization was maintained upon treatment of the Ti3C2 membrane at 500 °C under an Ar/H2 atmosphere as compared to 80 °C under vacuum. This led to a change from Knudsen diffusion to molecular sieving, as demonstrated by single-gas permeation tests at room temperature. Overall, this work shows a simple and promising way to improve H2/CO2 selectivity via temperature treatment under a controlled atmosphere.

Polysiloxane-Based Polyurethanes with High Strength and Recyclability.

Polysiloxanes have attracted considerable attention in biomedical engineering, owing to their inherent properties, including good flexibility and biocompatibility. However, their low mechanical strength limits their application scope. In this study, we synthesized a polysiloxane-based polyurethane by chemical copolymerization. A series of thermoplastic polysiloxane-polyurethanes (Si-TPUs) was synthesized using hydroxyl-terminated polydimethylsiloxane containing two carbamate groups at the tail of the polymer chains 4,4′-dicyclohexylmethane diisocyanate (HMDI) and 1,4-butanediol as raw materials. The effects of the hard-segment content and soft-segment number average molecular weight on the properties of the resulting TPUs were investigated. The prepared HMDI-based Si-TPUs exhibited good microphase separation, excellent mechanical properties, and acceptable repeatable processability. The tensile strength of SiTPU-2K-39 reached 21.5 MPa, which is significantly higher than that of other flexible polysiloxane materials. Moreover, the tensile strength and breaking elongation of SiTPU-2K-39 were maintained at 80.9% and 94.6%, respectively, after three cycles of regeneration. The Si-TPUs prepared in this work may potentially be used in gas separation, medical materials, antifouling coatings, and other applications.

Optimization of MIL-178(Fe) and Pebax® 3533 loading in mixed matrix membranes for CO2 capture

Global warming is considered as a consequence of extensive use of fossil fuels. Post combustion CO2 capture is an interesting and alternative solution where mixed matrix membranes (MMMs) can be an exciting candidate. This research focuses on the optimization of MMM composition consisting of Pebax® 3533 as the polymer matrix and porous coordination polymer (PCP) MIL-178(Fe) as a filler for gas separation application. MIL-178(Fe) characterized with SEM, TEM and TGA were applied to compare bare polymer and MMM. Optimum composition of the MMM obtained was 5 wt.% MIL-178(Fe) in Pebax® 3533. Average thickness of the optimized dense MMM was 116 ± 8 µm. Such MMM showed CO2 permeability and CO2/N2 selectivity of 312 ± 5 Barrer and 25.0 ± 0.5, respectively, 12% and 25% improved regarding the bare membrane. Additionally, optimum MMM was applied for CO2/CH4 separation and successfully compared in terms of improved CO2 permeability and CO2/CH4 selectivity.

Molecular insights into the role of O2 in reversed C2H6/C2H4 separation on metal–organic frameworks

Previous experimental studies have shown that C2H4-selective metal–organic frameworks (MOFs) can be reversed to C2H6-selective MOFs by introducing an oxygen molecule (O2) into the MOF. The O2 functionalization strategy provides a new insight into one-step C2H4 purification; however, the molecular-level understanding of the reversed separation mechanism remains theoretically and experimentally unclear, which limits further material refinement. In this study, we explored the influence of O2 on the C2H6/C2H4 selectivity of a Cr-BTC MOF via density functional theory (DFT) calculations and grand canonical Monte Carlo simulations within the newly DFT-derived force fields. Results suggest that O2 was antiferromagnetically coupled to the open Cr site, with an end-on pattern forming the superoxo site. The presence of superoxide adducts decreased the dipole moment of the open-metal site by 5.95 debye. Furthermore, the altered gas selectivity could be attributed to the superoxo site blocking the donation and back-donation interactions between Cr and C2H4. This removes the highly polarized C2H4-selective site, while enhancing the pore channel environmental inertness for preferential C2H6 adsorption. To assess the performance of the reversed separation strategy under humid air conditions, ab initio molecular dynamics with metadynamics were used to determine the water stabilities of Cr-BTC and oxidized conjugates. We revealed that O2 introduction weakened the metal-linker bond and reduced the free-energy barrier for the initial framework decomposition process, which may hinder the application of gas separation in further practical environments. These findings provide theoretical support for oxidized MOFs with reversed separation performance and facilitate intentional post-synthetic MOF modifications for paraffin-selective adsorbent design, a critical need for energy applications and the petroleum industry.

Fast hydrogen purification through graphitic carbon nitride nanosheet membranes

Two-dimensional graphitic carbon nitride (g-C3N4) nanosheets are ideal candidates for membranes because of their intrinsic in-plane nanopores. However, non-selective defects formed by traditional top-down preparation and the unfavorable re-stacking hinder the application of these nanosheets in gas separation. Herein, we report lamellar g-C3N4 nanosheets as gas separation membranes with a disordered layer-stacking structure based on high quality g-C3N4 nanosheets through bottom-up synthesis. Thanks to fast and highly selective transport through the high-density sieving channels and the interlayer paths, the membranes, superior to state-of-the-art ones, exhibit high H2 permeance of 1.3 × 10−6 mol m−2 s−1 Pa−1 with excellent selectivity for multiple gas mixtures. Notably, these membranes show excellent stability under harsh practice-relevant environments, such as temperature swings, wet atmosphere and long-term operation of more than 200 days. Therefore, such lamellar membranes with high quality g-C3N4 nanosheets hold great promise for gas separation applications.