Water Resistant and Flexible MOF Materials for Highly Efficient Separation of Methane from Nitrogen

Evaluating CH4/N2 Separation Performances of Hundreds of Thousands of Real and Hypothetical MOFs by Harnessing Molecular Modeling and Machine Learning

Considering the large abundance and diversity of metal-organic frameworks (MOFs), evaluating the gas adsorption and separation performance of the entire MOF material space using solely experimental techniques or brute-force computer simulations is impractical. In this study, we integrated high-throughput molecular simulations with machine learning (ML) to explore the potential of both synthesized, the real MOFs, and computer-generated, the hypothetical MOFs (hypoMOFs), for adsorption-based CH4/N2 separation. CH4/N2 mixture adsorption data obtained from molecular simulations were used to train the ML models that could accurately predict gas uptakes of 4612 real MOFs. These models were then transferred to two distinct databases consisting of 98 601 hypoMOFs and 587 anion-pillared hypoMOFs to examine their CH4/N2 mixture separation performances using various adsorbent evaluation metrics. The top adsorbents were identified for vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions and examined in detail to gain molecular insights into their structural and chemical properties. Results revealed that the hypoMOFs offered high CH4 selectivities, up to 14.8 and 13.6, and high working capacities, up to 3.1 and 5.8 mol/kg, at VSA and PSA conditions, respectively, and many of the hypoMOFs could outperform the real MOFs. Our approach offers a rapid and accurate assessment of the mixture adsorption and separation properties of MOFs without the need for computationally demanding simulations. Our results for the best adsorbents will be useful in accelerating the experimental efforts for the design of novel MOFs that can achieve high-performance CH4/N2 separation.

Pore Chemical Modification of Bimetallic Coordination Networks for Coal-Bed Methane Purification under Humid Conditions

The recycling of low-concentration coal-bed methane (CBM) is environmentally beneficial and plays a crucial role in optimizing the energy mix. In this work, we present a strategy involving pore chemical modification to synthesize a series of bimetallic diamond coordination networks, namely CuIn(ina)4, CuIn(3-ain)4, and CuIn(3-Fina)4 (where ina = isonicotinic acid, 3-ain = 3-amino-isonicotinic acid, and 3-Fina = 3-fluoroisonicotinic acid). Among these, the amino-functionalized CuIn(3-ain)4 exhibits excellent CH4 adsorption capacity (1.71 mmol g-1) and CH4/N2 selectivity (7.5) due to its optimal pore size and chemical environment, establishing it as a new benchmark material for CBM separation. Dynamic breakthrough experiments confirm the exceptional CH4/N2 separation performance of CuIn(3-ain)4. Notably, CuIn(3-ain)4 demonstrates excellent stability under wet conditions and maintains outstanding separation performance even in high-humidity environments. Additionally, theoretical simulations provide valuable insights into how selective adsorption performance can be fine-tuned by manipulating the pore size and geometry. Regeneration tests and cycling evaluations further underscore the remarkable potential of CuIn(3-ain)4 as a highly efficient adsorbent for the separation of CBM.

Influence of Microwave-Assisted Supercritical Carbon Dioxide Treatment on the Pore Structure of Low-Rank Coal

CO2 injection into coal seams not only enhances coalbed methane (CBM) extraction but also allows for CO2 sequestration. Microwave irradiation is considered to be an effective technology to enhance CBM extraction. In this paper, the effects of microwave irradiation and supercritical CO2 immersion on the pore structure of low-rank coals were investigated by scanning electron microscopy (SEM), mercury-in-pressure (MIP), low-temperature nitrogen adsorption (LTN2GA), and carbon dioxide isothermal adsorption/desorption (CO2IA/D) of coal samples. The results showed that the macropores and micropores of the coal samples were more developed after microwave irradiation. After carbon dioxide immersion, the coal samples showed huge fissures, and the meso- and micropores were reduced. In contrast, microwave-assisted carbon dioxide not only reduced the specific surface area in the meso- and microporous stages and decreased the adsorption sites of methane but also enhanced the pore connectivity in the macroporous stage instead of the appearance of huge fissures. This study illustrates the potential of microwave-assisted supercritical carbon dioxide for enhanced coalbed methane extraction and carbon dioxide sequestration.

Research Progress in Microporous Materials for Selective Adsorption and Separation of Methane from Low-Grade Gas

Given that methane (CH4) and nitrogen (N2) have similar properties, achieving high-purity enrichment of CH4 from nitrogen-rich low-grade gas is extremely challenging and is of great significance for sustainable development in energy and the environment. This paper reviews the research progress on carbon-based materials, zeolites, and MOFs as adsorbent materials for CH4/N2 separation. It focuses on the relationship between the composition, pore size, surface chemistry of the adsorbents, CH4/N2 selectivity, and CH4 adsorption capacity. The paper also highlights that controlling pore size and atomic-scale composition and optimizing these features for the best match are key directions for the development of new adsorbents. Additionally, it points out that MOFs, which combine the advantages of carbon-based adsorbents and zeolites, are likely to become the most promising adsorbent materials for efficient CH4/N2 separation.

Using ligand regulation, metal replacement, and ligand doping strategies on Zr-FUM to improve methane separation from coalbed gas

Developing adsorbents suitable for industrial applications that can effectively enhance the separation of methane (CH4) from nitrogen (N2) in coalbed gas is crucial to improve energy recovery and mitigate greenhouse gas emissions. In this study, three modification strategies were implemented on Zr-FUM, including ligand regulation, metal replacement, and ligand doping, to synthesize Zr-FDCA, Al-FUM, and Zr-FUM-FA, with the aim of improving the performance of CH4/N2 separation under humid conditions. The results demonstrated that the promotion of robust orbital overlap and strengthened electrovalent bonding on adsorbents can selectively enhance CH4 adsorption. As a result, Zr-FUM-FA achieved a saturated CH4 adsorption capacity of 1.37 mmol/g, a CH4 working window of 307 s, and a CH4/N2 sorbent selection parameter (Ssp) of 47.31, exceeding the performance of most reported adsorbents. Analyses of the pore structure, surface morphology, and functional groups revealed that the presence of an ultramicropore proximity to CH4, reduced static resistance, and enhanced electrovalent bond were key factors for CH4 separation. Grand Canonical Monte Carlo and Density Functional Theory studies indicated that the introduction of -C-H- in FA played a crucial role in enhancing CH4 adsorption. Optimization of adsorption parameters using the Aspen adsorption package showed that in a dual-adsorbent bed system, the recovery and purity of CH4 in Zr-FUM-FA reach 99.5% and 97.3%, respectively, providing important theoretical support for the improvement of CH4 recovery in the pressure swing adsorption process from coalbed gas.

Influence of Tectonically Deformed Coal-Based Activated Carbon and Its Surface Modification on Methane Adsorption

A series of coal-based activated carbons (CACs) were synthesized from mylonitized fat coal, a type of tectonically deformed coal (TDC) and symbiotic primary structural coal (PSC), followed by oxidative modification. The pore structure, surface oxygen-containing functional groups, and their influence on methane adsorption by CAC as the simplified coal model were investigated by using low-temperature nitrogen adsorption, Fourier transform infrared spectroscopy, Boehm titration, and X-ray photoelectron spectroscopy. The results showed that tectonic deformation fostered smaller pores, particularly ultramicropores in TDC, dominating methane adsorption. Acid-modified TDC-based activated carbons (ACs) showed higher pore parameters and oxygen-containing functional groups than those of PSC-based ACs. Nitric acid introduced abundant carboxyl groups concurrently increasing the pore volume and specific surface area (SSA), while sulfuric acid-ammonium persulfate treatment resulted in increased lactone groups and a partial collapse/blockage of nanopores. Methane adsorption experiments confirmed the importance of micropores and revealed a significant decrease in capacity owing to increased oxygen-containing functional groups as the primary role, with pore wall corrosion playing a secondary role. Thus, the study highlights the surface effects of TDC on methane adsorption and the potential for producing high-performance methane storage materials from China's tectonic coal resources.

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.

Design and application of metal organic frameworks for heavy metals adsorption in water: a review

The growing apprehension surrounding heavy metal pollution in both environmental and industrial contexts has spurred extensive research into adsorption materials aimed at efficient remediation. Among these materials, Metal-Organic Frameworks (MOFs) have risen as versatile and promising contenders due to their adjustable properties, expansive surface areas, and sustainable characteristics, compared to traditional options like activated carbon and zeolites. This exhaustive review delves into the synthesis techniques, structural diversity, and adsorption capabilities of MOFs for the effective removal of heavy metals. The article explores the evolution of MOF design and fabrication methods, highlighting pivotal parameters influencing their adsorption performance, such as pore size, surface area, and the presence of functional groups. In this perspective review, a thorough analysis of various MOFs is presented, emphasizing the crucial role of ligands and metal nodes in adapting MOF properties for heavy metal removal. Moreover, the review delves into recent advancements in MOF-based composites and hybrid materials, shedding light on their heightened adsorption capacities, recyclability, and potential for regeneration. Challenges for optimization, regeneration efficiency and minimizing costs for large-scale applications are discussed.

One Scalable and Stable Metal-Organic Framework for Efficient Separation of CH4/N2 Mixture

Separating CH4 from coal bed methane is of great importance but challenging. Adsorption-based separation often suffers from low selectivity, poor stability, and difficulty to scale up. Herein, a stable and scalable metal-organic framework [MOF, CoNi(pyz-NH2)] with multiple CH4 binding sites was reported to efficiently separate the CH4/N2 mixture. Due to its suitable pore size and multiple CH4 binding sites, it exhibits excellent CH4/N2 selectivity (16.5) and CH4 uptake (35.9 cm3/g) at 273 K and 1 bar, which is comparable to that of the state-of-the-art MOFs. Theoretical calculations reveal that the high density of open metal sites and polar functional groups in the pores provide strong affinity to CH4 than to N2. Moreover, CoNi(pyz-NH2) displays excellent structural stability and can be scale-up synthesized (22.7 g). This work not only provides an excellent adsorbent but also provides important inspiration for the future design and preparation of porous adsorbents for separations.

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.

Tuning the Pore Environment of MOFs toward Efficient CH4/N2 Separation under Humid Conditions

Adsorption separation technology using adsorbents is promising as an alternative to the energy-demanding cryogenic distillation of natural gas (CH₄/N₂) separation. Although a few adsorbents, such as metal-organic frameworks (MOFs), with high performance for CH₄/N₂ separation, have been reported, it is still challenging to target the desired adsorbents for the actual CH₄/N₂ separation under humid conditions because the adsorption capacity and selectivity of the adsorbents might be mainly dampened by water vapor. Except for the high CH₄ uptake and CH₄/N₂ selectivity, the adsorption material should simultaneously have excellent stability against moisture and relatively low-water absorption affinity. Here, we tuned the ligands and metal sites of reticular MOFs, Zn-benzene-1,4-dicarboxylic acid-1,4-diazabicyclo[2.2.2]octane (Zn-BDC-DABCO) (DMOF), affording a series of isostructural MOFs (DMOF-N, DMOF-A₁, DMOF-A₂, and DMOF-A₃). Because of the finely engineered pore size and introduced aromatic rings in the functional DMOF, gas sorption results reveal that the materials show improved performance with a benchmark CH₄ uptake of 37 cm³/g and a high CH₄/N₂ adsorption selectivity of 7.2 for DMOF-A₂ at 298 K and 1.0 bar. Moisture stability experiments show that DMOF-A₂ is a robust MOF with low water vapor capacity even at ∼40% relative humidity (RH) because of the presence of more hydrophobic aromatic rings. Breakthrough experiments verify the excellent CH₄/N₂ separation performances of DMOF-A₂ under high humidity.

Robust Nickel-Based Metal-Organic Framework for Highly Efficient Methane Purification and Capture

Developing energy-efficient alternatives for methane (CH₄) purification from natural gas and methane capture of coal-mine gas is of great significance and challenge in the chemical industry. Herein, we report a robust nickel-based metal-organic framework (MOF), Ni-BPZ, featuring one-dimensional (1D) rhombic channels decorated with abundant pyrazole rings. Ni-BPZ exhibits excellent separation performance toward both C₂H₆/CH₄ and CH₄/N₂ binary mixtures. The C₂H₆/CH₄ selectivity of Ni-BPZ is high, up to 50.2, exceeding those of most MOF adsorbents reported, and it simultaneously possesses a remarkable C₂H₆ uptake of 2.46 mmol/g at 298 K and 0.1 bar. The CH₄/N₂ selectivity of Ni-BPZ reaches 6.6, and its high CH₄ uptake is 1.56 mmol/g, which is also superior to most high-performance CH₄ adsorbents. The molecular simulation reveals that the uniform 1D rhombic channels with abundant pyrazole rings provide a high density of potential adsorption sites for efficient C₂H₆/CH₄ and CH₄/N₂ separations.

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.

Enhanced Adsorption and Separation of Xenon over Krypton via an Unsaturated Calcium Center in a Metal-Organic Framework

Krypton (Kr) and xenon (Xe) are nowadays widely applied in technical and industrial fields. Separating and collecting highly pure Xe from nuclear facilities are necessary and urgent. However, the technology is limited due to the inert nature of Xe and other interferential factors. In this work, a calcium-based metal-organic framework, Ca-SINAP-1, which comprises a three-dimensional microporous framework with a suitable pore width, was researched for xenon and krypton separation through both experimental and theoretical methods. Ca-SINAP-1, synthesized in solvothermal and gamma ray conditions, features accessible open-metal sites, exhibits a high Xe/Kr selectivity of 10.32, and owns a Xe adsorption capacity of 2.87 mmol/g at room temperature (1.0 bar). Particularly, its excellent chemical stability (from pH 2 to 13) and thermal stability (up to 550 °C), as well as radiation-resistance (up to 400 kGy β irradiations), render this material a promising candidate for radioactive inert gases treatment.

UiO-66-NH2 as an effective solid support for quinazoline derivatives for antibacterial agents against Gram-negative bacteria

UiO-66-NH2 is used as an effective drug carrier for the control of quinazoline release for antibacterial agents against Gram-negative bacteria.

Recent progress in environmental applications of metal-organic frameworks

Nanomaterials have aroused the interest of many researchers and become a research hotspot in recent years and metal-organic frameworks (MOFs) included in that are a class of new organic-inorganic hybrid porous materials formed through the self-assembly of organic ligands and inorganic metal ions. MOFs have been attracting increasing attention due to their structural diversification, large specific surface area, high porosity, inerratic pore space framework. These characteristics play their advantages in different fields and make some excellent achievements. This article summarizes the research progress of metal-organic framework in the field of environment especially the remarkable achievements in adsorption and provides a clear help for understanding the research progress and prospects for future research.