3D porous metal-organic framework for selective adsorption of methane over dinitrogen under ambient pressure

Exceptional capture of methane at low pressure by an iron-based metal-organic framework

The selective capture of methane (CH4) at low concentrations and its separation from N2 are extremely challenging owing to the weak host-guest interactions between CH4 molecules and any sorbent material. Here, we report the exceptional adsorption of CH4 at low pressure and the efficient separation of CH4/N2 by MFM-300(Fe). MFM-300(Fe) shows a very high uptake for CH4 of 0.85 mmol g-1 at 1 mbar and 298 K and a record CH4/N2 selectivity of 45 for porous solids, representing a new benchmark for CH4 capture and CH4/N2 separation. The excellent separation of CH4/N2 by MFM-300(Fe) has been confirmed by dynamic breakthrough experiments. In situ neutron powder diffraction, and solid-state nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modelling, reveal a unique and strong binding of CH4 molecules involving Fe-OH⋯CH4 and C⋯phenyl ring interactions within the pores of MFM-300(Fe), thus promoting the exceptional adsorption of CH4 at low pressure.

Non-CO2 greenhouse gas separation using advanced porous materials

Global warming has become a growing concern over decades, prompting numerous research endeavours to reduce the carbon dioxide (CO2) emission, the major greenhouse gas (GHG). However, the contribution of other non-CO2 GHGs including methane (CH4), nitrous oxide (N2O), fluorocarbons, perfluorinated gases, etc. should not be overlooked, due to their high global warming potential and environmental hazards. In order to reduce the emission of non-CO2 GHGs, advanced separation technologies with high efficiency and low energy consumption such as adsorptive separation or membrane separation are highly desirable. Advanced porous materials (APMs) including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), etc. have been developed to boost the adsorptive and membrane separation, due to their tunable pore structure and surface functionality. This review summarizes the progress of APM adsorbents and membranes for non-CO2 GHG separation. The material design and fabrication strategies, along with the molecular-level separation mechanisms are discussed. Besides, the state-of-the-art separation performance and challenges of various APM materials towards each type of non-CO2 GHG are analyzed, offering insightful guidance for future research. Moreover, practical industrial challenges and opportunities from the aspect of engineering are also discussed, to facilitate the industrial implementation of APMs for non-CO2 GHG separation.

Dynamical and electronic properties of anion-pillared metal-organic frameworks for natural gas separation

The increasing demand for natural gas as a clean energy source has emphasized the need for efficient gas separation technologies. Metal-organic frameworks (MOFs) have emerged as a promising class of materials for gas separation, with anion-pillared MOFs (APMOFs) gaining attention for their fine-tuned pore design and shape/size selectivity. In this study, we investigate the dynamical and electronic properties of three APMOFs, SIFSIX-3-Cu, SIFSIX-2-Cu-i, and SIFSIX-2-Cu, for the separation of methane from ethane, ethene, propane, propene, and N using computational simulations. Our simulations employ Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) techniques combined with Density Functional Theory (DFT) calculations. We find that that all three APMOFs exhibit promising separation capabilities for methane from propane and propene based on both thermodynamics and kinetics parameters. In addition, we use Noncovalent Interaction (NCI) analysis to investigate intermolecular interactions and find that the fluorine atoms in the MOF can polarize gas molecules and establish electrostatic interactions with hydrogen atoms in the molecule. Finally, we show that SIFSIX-2-Cu-i is a potential candidate for separating N2/CH4 due to its interpenetration.

Influence of ligands within Al-based metal-organic frameworks for selective separation of methane from unconventional natural gas

Efficient CH₄/N₂ separation from unconventional natural gas is vital for both energy recycling and climate change control. Figuring out the reason for the disparity between ligands in the framework and CH₄ is the crucial problem for developing adsorbents in PSA progress. In this study, a series of eco-friendly Al-based MOFs, including Al-CDC, Al-BDC, CAU-10, and MIL-160, were synthesized to investigate the influence of ligands on CH₄ separation through experimental and theoretical analyses. The hydrothermal stability and water affinity of synthetic MOFs were explored through experimental characterization. The active adsorption sites and adsorption mechanisms were investigated via quantum calculation. The results manifested that the interactions between CH₄ and MOFs materials were affected by the synergetic effects of pore structure and ligand polarities, and the disparities of ligands within MOFs determined the separation efficiency of CH₄. Especially, the CH₄ separation performance of Al-CDC with high sorbent selection (68.56), moderate isosteric adsorption heat for CH₄ (26.3 kJ/mol), and low water affinity (0.1 g/g at 40% RH) was superior to most porous adsorbents, which was attributed to its nanosheet structure, proper polarity, reduced local steric hindrance, and extra functional groups. The analysis of active adsorption sites indicated that hydrophilic carboxyl groups and hydrophobic aromatic ring were the dominant CH₄ adsorption sites for liner ligands and bent ligands, respectively. The methylene groups with saturated C-H bonds enhanced the wdV interaction between ligands and CH₄, resulting in the highest binding energy of CH₄ for Al-CDC. The results provided valuable guidance for the design and optimization of high-performance adsorbents for CH₄ separation from unconventional natural gas.

Enhancing CH4 Capture from Coalbed Methane through Tuning van der Waals Affinity within Isoreticular Al-Based Metal-Organic Frameworks

Efficient separation of the CH₄/N₂ mixture is of great significance for coalbed methane purification. It is an effective strategy to separate this mixture by tuning the van der Waals interaction due to the nonpolar properties of CH₄ and N₂ molecules. Herein, we prepared several isoreticular Al-based metal-organic frameworks (MOFs) with different ligand sizes and polarities because of their high structural stability and low cost/toxicity feature of Al metal. Adsorption experiments indicated that the CH₄ uptake, Q_st of CH₄, and CH₄/N₂ selectivity are in the order of Al-FUM-Me (27.19 cm³(STP) g^-1, 24.06 kJ mol^-1 and 8.6) > Al-FUM (20.44 cm³(STP) g^-1, 20.60 kJ mol^-1 and 5.1) > Al-BDC (15.98 cm³(STP) g^-1, 18.81 kJ mol^-1 and 3.4) > Al-NDC (10.86 cm³(STP) g^-1, 14.89 kJ mol^-1 and 3.1) > Al-BPDC (5.90 cm³(STP) g^-1, 11.75 kJ mol^-1 and 2.2), confirming the synergetic effects of pore sizes and pore surface polarities. Exhilaratingly, the ideal adsorbed solution theory selectivity of Al-FUM-Me is higher than those of all zeolites, carbon materials, and most water-stable MOF materials (except Al-CDC and Co₃(C₄O₄)₂(OH)₂), which is comparable to MIL-160. Breakthrough results demonstrate its excellent separation performance for the CH₄/N₂ mixture with good regenerability. The separation mechanism of Al-FUM-Me for the CH₄/N₂ mixture was elucidated by theoretical calculations, showing that the stronger affinity of CH₄ can be attributed to its relatively shorter interaction distance with adsorption binding sites. Therefore, this work not only offers a promising candidate for CH₄/N₂ separation but also provides valuable guidance for the design of high-performance adsorbents.

Ultrahigh Size Exclusion Selectivity for Carbon Dioxide from Nitrogen/Methane in an Ultramicroporous Metal-Organic Framework

Separations based on molecular size (molecular sieving) are a solution for environmental remediation. We have synthesized and characterized two new metal-organic frameworks (MOFs) (Zn₂M; M = Zn, Cd) with ultramicropores (<0.7 nm) suitable for molecular sieving. We explore the synthesis of these MOFs and the role that the DMSO/H₂O/DMF solvent mixture has on the crystallization process. We further explore the crystallographic data for the DMSO and methanol solvated structures at 273 and 100 K; this not only results in high-quality structural data but also allows us to better understand the structural features at temperatures around the gas adsorption experiments. Structurally, the main difference between the two MOFs is that the central metal in the trimetallic node can be changed from Zn to Cd and that results in a sub-Å change in the size of the pore aperture, but a stark change in the gas adsorption properties. The separation selectivity of the MOF when M = Zn is infinite given the pore aperture of the MOF can accommodate CO₂ while N₂ and/or CH₄ is excluded from entering the pore. Furthermore, due to the size exclusion behavior, the MOF has an adsorption selectivity of 4800:1 CO₂/N₂ and 5 × 10²⁸:1 CO₂/CH₄. When M = Cd, the pore aperture of the MOF increases slightly, allowing N₂ and CH₄ to enter the pore, resulting in a 27.5:1 and a 10.5:1 adsorption selectivity, respectively; this is akin to UiO-66, a MOF that is not able to function as a molecular sieve for these gases. The data delineate how subtle sub-Å changes to the pore aperture of a framework can drastically affect both the adsorption selectivity and separation selectivity.

Nickel-Based Metal-Organic Frameworks for Coal-Bed Methane Purification with Record CH4 /N2 Selectivity

The enrichment and purification of coal-bed methane provides a source of energy and helps offset global warming. In this work, we demonstrate a strategy involving the regulation of the pore size and pore chemistry to promote the separation of CH₄ /N₂ mixtures in four nickel-based coordination networks, named Ni(ina)₂ , Ni(3-ain)₂ , Ni(2-ain)₂ , and Ni(pba)₂ , (where ina=isonicotinic acid, 3-ain=3-aminoisonicotinic acid, 2-ain=2-aminoisonicotinic acid, and pba=4-(4-pyridyl)benzoic acid). Among them, Ni(ina)₂ and Ni(3-ain)₂ can effectively separate CH₄ from N₂ with top-performing performance because of the suitable pore size (≈0.6 and 0.5 nm) and pore environment. Explicitly, Ni(ina)₂ exhibits the highest ever reported CH₄ /N₂ selectivity of 15.8 and excellent CH₄ uptake (40.8 cm³  g^-1 ) at ambient conditions, thus setting new benchmarks for all reported MOFs and traditional adsorbents. The exceptional CH₄ /N₂ separation performance of Ni(ina)₂ is confirmed by dynamic breakthrough experiments. Under different CH₄ /N₂ ratios, Ni(ina)₂ selectively extracts methane from the gaseous blend and produces a high purity of CH₄ (99 %). Theoretical calculations and CH₄ -loading single-crystal structure analysis provide critical insight into the adsorption/separation mechanism. Ni(ina)₂ and Ni(3-ain)₂ can form rich intermolecular interactions with methane, indicating a strong adsorption affinity between pore walls and CH₄ molecules. Importantly, Ni(ina)₂ has good thermal and moisture stability and can easily be scaled up at a low cost ($25 per kilogram), which will be valuable for potential industrial applications. Overall, this work provides a powerful approach for the selective adsorption of CH₄ from coal-bed methane.

Bis(pyrazol-1-yl)methane-4,4′-dicarboxylic Acid

The molecular structure of bis(pyrazol-1-yl)methane-4,4′-dicarboxylic acid (H2bpmdc) was determined by single crystal X-Ray diffraction analysis. The compound crystallizes in a monoclinic crystal system; the unit cell contains four formula units. The molecules of H2bpmdc are linked into zig-zag chains by intermolecular carboxyl–carboxyl hydrogen bonds. Other types of supramolecular interactions, namely, CH···N and CH···O short contacts, CH–π interactions and carbonyl–carbonyl interactions were detected in the crystal structure.

Separation and Purification of Hydrocarbons with Porous Materials

This Minireview focuses on the developments of the adsorptive separation of methane/nitrogen, ethene/ethane, propene/propane mixtures as well as on the separation of C8 aromatics (i.e. xylene isomers) with a wide variety of materials, including carbonaceous materials, zeolites, metal-organic frameworks, and porous organic frameworks. Some recent important developments for these adsorptive separations are also highlighted. The advantages and disadvantages of each material category are discussed and guidelines for the design of improved materials are proposed. Furthermore, challenges and future developments of each material type and separation processes are discussed.

Zr-MOFs for CF4/CH4, CH4/H2, and CH4/N2 separation: towards the goal of discovering stable and effective adsorbents

Zirconium metal-organic frameworks (MOFs) can be promising adsorbents for various applications as they are highly stable in different chemical environments. In this work, a collection of Zr-MOFs comprised of more than 100 materials is screened for CF₄/CH₄, CH₄/H₂, and CH₄/N₂ separations using atomistic-level simulations. The top three MOFs for the CF₄/CH₄ separation are identified as PCN-700-BPDC-TPDC, LIFM-90, and BUT-67 exhibiting CF₄/CH₄ adsorption selectivities of 4.8, 4.6, and 4.7, CF₄ working capacities of 2.0, 2.0, and 2.1 mol kg^-1, and regenerabilities of 85.1, 84.2, and 75.7%, respectively. For the CH₄/H₂ separation, MOF-812, BUT-67, and BUT-66 are determined to be the top performing MOFs demonstrating CH₄/H₂ selectivities of 61.6, 36.7, and 46.2, CH₄ working capacities of 3.0, 4.1, and 3.4 mol kg^-1, and CH₄ regenerabilities of 70.7, 82.7, and 74.7%, respectively. Regarding the CH₄/N₂ separation, BUT-67, Zr-AbBA, and PCN-702 achieving CH₄/N₂ selectivities of 4.5, 3.4, and 3.8, CH₄ working capacities of 3.6, 3.9, and 3.5 mol kg^-1, and CH₄ regenerabilities of 81.1, 84.0, and 84.5%, in successive order, show the best overall separation performances. To further elucidate the adsorption in top performing adsorbents, the adsorption sites in these materials are analyzed using radial distribution functions and adsorbate density profiles. Finally, the water affinities of Zr-MOFs are explored to comment on their practical use in real gas separation applications. Our findings may inspire future studies probing the adsorption/separation mechanisms and performances of Zr-MOFs for different gases.

Approaches to the Synthesis of Dicarboxylic Derivatives of Bis(pyrazol-1-yl)alkanes

Carboxylation of bis(pyrazol-1-yl)alkanes by oxalyl chloride was studied. It was found that 4,4'-dicarboxylic derivatives of substrates with electron-donating methyl groups and short linkers (from one to three methylene groups) can be prepared using this method. Longer linkers lead to significantly lower product yields, which is probably due to instability of the intermediate acid chlorides that are initially formed in the reaction with oxalyl chloride. Thus, bis(pyrazol-1-yl)methane gave only monocarboxylic derivative even with a large excess of oxalyl chloride and prolonged reaction duration. An alternative approach involves the reaction of ethyl 4-pyrazolecarboxylates with dibromoalkanes in a superbasic medium (potassium hydroxide-dimethyl sulfoxide) and is suitable for the preparation of bis(4-carboxypyrazol-1-yl)alkanes with both short and long linkers independent of substitution in positions 3 and 5 of pyrazole rings. The obtained dicarboxylic acids are interesting as potential building blocks for metal-organic frameworks.

Selective CO2 or CH4 adsorption of two anionic bcu-MOFs with two different counterions: experimental and simulation studies

Two new bcu-MOFs with counterions tuned from Li(H₂O)₄⁺ to DMA⁺ have been successfully synthesized and their selective CO₂ or CH₄ adsorption over N₂ gas has been systematically investigated in-depth by both experimental and simulation studies.

Facile Synthesis of 3-(Azol-1-yl)-1-adamantanecarboxylic Acids-New Bifunctional Angle-Shaped Building Blocks for Coordination Polymers

For the first time, orthogonally substituted azole-carboxylate adamantane ligands were synthesized and used for preparation of coordination polymers. The angle-shaped ligands were prepared by the reaction of 1-adamantanecarboxylic acid and azoles (1H-1,2,4-triazole, 3-methyl-1H-1,2,4-triazole, 3,5-dimethyl-1H-1,2,4-triazole, 1H-tetrazole, 5-methyl-1H-tetrazole) in concentrated sulfuric acid. Variation of the solvent and substituents in azole rings allowed to prepare both 1D and 2D copper(II) and nickel(II) coordination polymers, [Cu₂(trzadc)₄(H₂O)_0.7]∙DMF∙0.3H₂O, [Cu(trzadc)₂(MeOH)]∙MeOH, [Ni(trzadc)₂(MeOH)₂] and [Cu₂(mtrzadc)₃(MeOH)]⁺NO₃^– (trzadc-3-(1,2,4-triazol-1-yl)-adamantane-1-carboxylic acid; mtrzadc-3-(3-methyl-1,2,4-triazol-1-yl)-adamantane-1-carboxylic acid) which were structurally characterized by single crystal X-ray diffraction. Complex [Cu(trzadc)₂(MeOH)]∙MeOH was shown to act as a catalyst in the Chan-Evans-Lam arylation reaction.

A Metal-Organic Framework Based Methane Nano-trap for the Capture of Coal-Mine Methane

As a major greenhouse gas, methane, which is directly vented from the coal-mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano-trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal-organic framework (MOF). The alkyl MOF-based methane nano-trap exhibits a record-high methane uptake and CH₄ /N₂ selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single-crystal X-ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF-based nano-trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF-based methane nano-trap is a new benchmark for CH₄ /N₂ separation, thereby providing a new perspective for capturing methane from coal-mine methane to recover fuel and reduce greenhouse gas emissions.

Torsion Angle Effect on the Activation of UiO Metal-Organic Frameworks

We present a systematic investigation of the factors influencing the surface area of zirconium-based UiO-type metal-organic frameworks (MOFs), revealing an important relationship between factors including the conformation of the organic linker in the MOF, surface tension of the guest molecules (solvent), and the stability of MOFs toward activation (removal of guest molecules). The results obtained demonstrate how the structure of the linkers forming the isostructural series of UiO MOFs with fcu topology could alter the resistance and stability of the MOF frameworks toward capillary force-driven structural degradation governed by the solvent during activation.