Recent Advancements in Metal-Organic Framework-Based Membranes for Hydrogen Separation: A Review

Metal-organic frameworks (MOFs) are promising porous materials that have huge potential for gas separation when put in the membrane configuration. MOFs have huge potential due to certain salient features of the MOFs such as excellent pore size, ease of tuning the pore chemistry, higher surface area, and chemical and thermal stabilities. MOFs have been explored for various gas separation and storage applications. This review discusses various approaches for fabricating MOFs-based membranes for the separation of H2 gas from a variety of feeds having various gases CO2, CO, N2, and CH4 as impurities. The emphasis has been put on three types of membranes for H2 separation which include MOFs-based hollow fibrous/tubular/disk membranes, MOFs-based mixed matrix membranes (MMMs), and MOFs-based stand-alone membranes. In addition, various challenges such as reducing inhomogeneity between MOFs and polymeric matrices have also been discussed. Similarly, the approaches to successfully decorating MOFs on different supports in different configurations have been explained. The possible ways of improving the MOFs-based membranes for H2 have also been discussed.

Optimizing Trace Acetylene Removal from Acetylene/Ethylene Mixture in a Flexible Metal-Organic Framework by Crystal Downsizing

Improving the gas separation performance of metal-organic frameworks (MOFs) by crystal downsizing is an important but often overlooked issue. Here, we report three different-sized flexible ZUL-520 MOFs (according to the crystal size from large to small, the three samples are, respectively, named ZUL-520-0, ZUL-520-1, and ZUL-520-2) with the same chemical structure for optimizing trace acetylene (C2H2) removal from acetylene/ethylene (C2H2/C2H4) mixture. The three differently sized activated ZUL-520 (denoted as ZUL-520a) exhibited almost identical C2H2 uptake of 4.8 mmol/g at 100 kPa, while the C2H2 uptake at 1 kPa increased with a downsizing crystal. The C2H2 uptake of activated ZUL-520-2 (denoted as ZUL-520-2a) at 1 kPa was ∼55% higher than that of activated ZUL-520-0 (denoted as ZUL-520-0a). The adsorption isotherms and adsorption kinetics validated that gas adsorptive separation is governed not only by adsorption thermodynamics but also by adsorption kinetics. In addition, all three different-sized ZUL-520a MOFs showed high C2H2/C2H4 selectivity. Grand canonical Monte Carlo (GCMC) simulations and dispersion-corrected density functional theory (DFT-D) computations illustrated a plausible mechanism of C2H2 adsorption in MOFs. Importantly, breakthrough experiments demonstrated that ZUL-520a can effectively separate the C2H2/C2H4 (1/99, v/v) mixture and the C2H4 productivity obtained by ZUL-520-2a was much higher than that by ZUL-520-0a. Our work may provide an easy but powerful strategy for upgrading the performance of gas adsorptive separation in MOFs.

Scalable Synthesis of Robust MOF for Challenging Ethylene Purification and Propylene Recovery with Record Productivity

Ethylene (C2H4) purification and propylene (C3H6) recovery are highly relevant in polymer synthesis, yet developing physisorbents for these industrial separation faces the challenges of merging easy scalability, economic feasibility, high moisture stability with great separation efficiency. Herein, we reported a robust and scalable MOF (MAC-4) for simultaneous recovery of C3H6 and C2H4. Through creating nonpolar pores decorated by accessible N/O sites, MAC-4 displays top-tier uptakes and selectivities for C2H6 and C3H6 over C2H4 at ambient conditions. Molecular modelling combined with infrared spectroscopy revealed that C2H6 and C3H6 molecules were trapped in the framework with stronger contacts relative to C2H4. Breakthrough experiments demonstrated exceptional separation performance for binary C2H6/C2H4 and C3H6/C2H4 as well as ternary C3H6/C2H6/C2H4 mixtures, simultaneously affording record productivities of 27.4 and 36.2 L kg-1 for high-purity C2H4 (≥99.9 %) and C3H6 (≥99.5 %). MAC-4 was facilely prepared at deckgram-scale under reflux condition within 3 hours, making it as a smart MOF to address challenging gas separations.

Efficient Ethane and Propane Separation from Natural Gas Using Heterometallic Metal-Organic Frameworks with Interpenetrated Structures

The development of efficient technology for natural gas separation in industrial processes has become imperative. In this regard, the exploration of novel and effective adsorbents has gained significant attention. One promising approach is the metal regulation of metal-organic frameworks (MOFs), particularly heterometallic MOFs, which offer greater potential for gas separation due to their diverse composition. This study presents the synthesis of a series of iron- and vanadium-based heterometallic MOFs (MIL-126), featuring interpenetrated structures, and investigates their adsorption performance for methane (CH4), ethane (C2H6), and propane (C3H8). Experimental results reveal that the choice of metal combinations within the MOF framework significantly influences the adsorption performance of MIL-126. Notably, heterometallic MIL-126(Fe/Ni) exhibits a stronger binding affinity for C3H8, with an impressive uptake of 177 cm3/g. The C3H8/CH4 ideal adsorbed solution theory selectivity of MIL-126(Fe/Ni) surpasses that of MIL-126(Fe) by a factor of 7, reaching a value of 853, second only to the highest reported value. Furthermore, MIL-126(Fe/Ni) exhibits remarkable potential for the recovery of pure CH4 from the equimolar C3H8/CH4 mixture, with the amount of pure CH4 approaching the maximum reported value for MOFs. Insights from isosteric heat at zero loading and Henry's coefficients indicate that the transformation of metal types leads to a change in the interaction energy between C3H8 and the framework. Furthermore, breakthrough experiments validate the effective separation capability of MIL-126(Fe/Ni) for CH4/C2H6/C3H8 mixtures. These findings underscore the remarkable potential of heterometallic MOFs in constructing a wide range of new MOFs with tailorable properties, thereby enhancing their gas separation performance.

An aliphatic MOF with a molecular sieving effect for efficient C2H2/C2H4 separation

A metal-organic framework, SDMOF-1, with rigid pores of about 3.4 Å, which is appropriate for accommodating C2H2 molecules, exhibits high C2H2 adsorption capacity and great separation capability of the C2H2/C2H4 mixture. This work provides a new method to design aliphatic MOFs with a molecular sieving effect to realize efficient gas separation.

Prediction of Xe/Kr Separation in Metal-Organic Frameworks by a Precursor-Based Neural Network Synergistic with a Polarizable Adsorbate Model

Adsorption and separation of Xe/Kr are significant for making high-density nuclear energy environmentally friendly and for meeting the requirements of the gas industry. Enhancing the accuracy of the adsorbate model for describing the adsorption behaviors of Xe and Kr in MOFs and the efficiency of the model for predicting the separation potential (SP) value of Xe/Kr separation in MOFs helps in searching for promising MOFs for Xe/Kr adsorption and separation within a short time and at a low cost. In this work, polarizable and transferable models for mimic Xe and Kr adsorption behaviors in MOFs were constructed. Using these models, SP values of 38 MOFs at various temperatures and pressures were calculated. An optimal neural network model called BPNN-SP was designed to predict SP value based on physical parameters of metal center (electronegativity and radius) and organic linker (three-dimensional size and polarizability) combined with temperature and pressure. The regression coefficient value of the BPNN-SP model for each data set is higher than 0.995. MAE, MBE, and RMSE of BPNN-SP are only 0.331, -0.002, and 0.505 mmol/g, respectively. Finally, BPNN-SP was validated by experiment data from six MOFs. The transferable adsorbate model combined with the BPNN-SP model would highly improve the efficiency for designing MOFs with high performance for Xe/Kr adsorption and separation.

Active Sites Decorated Nonpolar Pore-Based MOF for One-step Acquisition of C2 H4 and Recovery of C3 H6

Herein, a 2-fold interpenetrated metal-organic framework (MOF) Zn-BPZ-TATB with accessible N/O active sites in nonpolar pore surfaces was reported for one-step C2 H4 purification from C2 H6 or C3 H6 mixtures as well as recovery of C3 H6 from C2 H6 /C3 H6 /C2 H4 mixtures. The MOF exhibits the favorable C2 H6 and C3 H6 uptakes (>100 cm3  g-1 at 298 K under 100 kPa) as well as selective adsorption of C2 H6 and C3 H6 over C2 H4 . The C3 H6 - and C2 H6 -selective feature were investigated detailedly by experimental tests as well as sorption kinetic studyies. Molecular modelling revealed the multiple interactions between C3 H6 or C2 H6 molecules and methyl groups as well as triazine rings in pores. Zn-BPZ-TATB not only can directly generate 323.4 L kg-1 and 15.4 L kg-1 of high-purity (≥99.9 %) C2 H4 from C3 H6 /C2 H4 and C2 H6 /C2 H4 mixtures, but also provide a large high-purity (≥99.5 %) C3 H6 recovery capacity of 60.1 L kg-1 from C3 H6 /C2 H4 mixtures. More importantly, the high-purity C3 H6 (≥99.5 %) and C2 H4 (≥99.9 %) with the productivities of 38.2 and 12.7 L kg-1 can be simultaneously obtained from C2 H6 /C3 H6 /C2 H4 mixtures through a single adsorption/desorption cycle.

Design of Pore Properties of an Al-Based Metal-Organic Framework for the Separation of an Ethane/Ethylene Gas Mixture via Ethane-Selective Adsorption

A series of Al-based isomorphs (CAU-10H, MIL-160, KMF-1, and CAU-10pydc) were synthesized using isophthalic acid (ipa), 2,5-furandicarboxylic acid (fdc), 2,5-pyrrole dicarboxylic acid (pyrdc), and 3,5-pyridinedicarboxylic acid (pydc), respectively. These isomorphs were systematically investigated to identify the best adsorbent for effectively separating C₂H₆/C₂H₄. All CAU-10 isomorphs exhibited preferential adsorption of C₂H₆ over that of C₂H₄ in mixture. CAU-10pydc exhibited the best C₂H₆/C₂H₄ selectivity (1.68) and the highest C₂H₆ uptake (3.97 mmol g^-1) at 298 K and 1 bar. In the breakthrough experiment using CAU-10pydc, 1/1 (v/v) and 1/15 (v/v) C₂H₆/C₂H₄ gas mixtures were successfully separated into high-purity C₂H₄ (>99.95%), with remarkable productivities of 14.0 L_STP kg^-1 and 32.0 L_STP kg^-1, respectively, at 298 K. Molecular simulations revealed that the exceptional separation performance of CAU-10pydc originated from the increased porosity and reduced electron density of the pyridine ring of pydc, leading to a relatively larger decrease in π-π interactions with C₂H₄ than in the C-H···π interactions with C₂H₆. This study demonstrates that the pore size and geometry of the CAU-10 platform are modulated by the inclusion of heteroatom-containing benzene dicarboxylate or heterocyclic rings of dicarboxylate-based organic linkers, thereby fine-tuning the C₂H₆/C₂H₄ separation ability. CAU-10pydc was determined to be an optimum adsorbent for this challenging separation.

Immobilization of the Polar Group into an Ultramicroporous Metal-Organic Framework Enabling Benchmark Inverse Selective CO2/C2H2 Separation with Record C2H2 Production

One-step harvest of high-purity light hydrocarbons without the desorption process represents an advanced and highly efficient strategy for the purification of target substances. The separation and purification of acetylene (C₂H₂) from carbon dioxide (CO₂) by CO₂-selective adsorbents are urgently demanded yet are very challenging owing to their similar physicochemical properties. Here, we employ the pore chemistry strategy to adjust the pore environment by immobilizing polar groups into an ultramicroporous metal-organic framework (MOF), achieving one-step manufacture of high-purity C₂H₂ from CO₂/C₂H₂ mixtures. Embedding methyl groups into prototype stable MOF (Zn-ox-trz) not only changes the pore environment but also improves the discrimination of guest molecules. The methyl-functionalized Zn-ox-mtz thus exhibits the benchmark reverse CO₂/C₂H₂ uptake ratio of 12.6 (123.32/9.79 cm³ cm^-3) and an exceptionally high equimolar CO₂/C₂H₂ selectivity of 1064.9 at ambient conditions. Molecular simulations reveal that the synergetic effect of pore confinement and surfaces decorated with methyl groups provides high recognition of CO₂ molecules through multiple van der Waals interactions. The column breakthrough experiments suggest that Zn-ox-mtz dramatically achieved the one-step purification capacity of C₂H₂ from the CO₂/C₂H₂ mixture with a record C₂H₂ productivity of 2091 mmol kg^-1, surpassing all of the CO₂-selective adsorbents reported so far. In addition, Zn-ox-mtz exhibits excellent chemical stability under different pH values of aqueous solutions (pH = 1-12). Moreover, the highly stable framework and excellent inverse selective CO₂/C₂H₂ separation performance showcase its promising application as a C₂H₂ splitter for industrial manufacture. This work paves the way to developing reverse-selective adsorbents for the challenging gas separation process.

Tuning the Trade-Off between Ethane/Ethylene Selectivity and Adsorption Capacity within Isoreticular Microporous Metal-Organic Frameworks by Linker Fine-Fluorination

The pore dimension and surface property directly dictate the transport of guests, endowing diverse gas selective adsorptions to porous materials. It is highly relevant to construct metal-organic frameworks (MOFs) with designable functional groups that can achieve feasible pore regulation to improve their separation performances. However, the role of functionalization in different positions or degrees within framework on the separation of light hydrocarbon has rarely been emphasized. In this context, four isoreticular MOFs (TKL-104-107) bearing dissimilar fluorination are rationally screened out and afforded intriguing differences in the adsorption behavior of C₂ H₆ and C₂ H₄ . Ortho-fluoridation of carboxyl allows TKL-105-107 to exhibit enhanced structural stabilities, impressive C₂ H₆ adsorption capacities (>125 cm³ g^-1 ) and desirable inverse selectivities (C₂ H₆ over C₂ H₄ ). The more modified ortho-fluorine group and meta-fluorine group of carboxyl have improved the C₂ H₆ /C₂ H₄ selectivity and adsorption capacity, respectively, and the C₂ H₆ /C₂ H₄ separation potential can be well optimized via linker fine-fluorination. Meanwhile, dynamic breakthrough experiments proved that TKL-105-107 can be used as highly efficient C₂ H₆ -selective adsorbents for C₂ H₄ purification. This work highlights that the purposeful functionalization of pore surfaces facilitates the assembly of highly efficient MOF adsorbents for specific gas separation.

Highly Productive C3H4/C3H6 Trace Separation by a Packing Polymorph of a Layered Hybrid Ultramicroporous Material

Ultramicroporous materials can be highly effective at trace gas separations when they offer a high density of selective binding sites. Herein, we report that sql-NbOFFIVE-bpe-Cu, a new variant of a previously reported ultramicroporous square lattice, sql, topology material, sql-SIFSIX-bpe-Zn, can exist in two polymorphs. These polymorphs, sql-NbOFFIVE-bpe-Cu-AA (AA) and sql-NbOFFIVE-bpe-Cu-AB (AB), exhibit AAAA and ABAB packing of the sql layers, respectively. Whereas NbOFFIVE-bpe-Cu-AA (AA) is isostructural with sql-SIFSIX-bpe-Zn, each exhibiting intrinsic 1D channels, sql-NbOFFIVE-bpe-Cu-AB (AB) has two types of channels, the intrinsic channels and extrinsic channels between the sql networks. Gas and temperature induced transformations of the two polymorphs of sql-NbOFFIVE-bpe-Cu were investigated by pure gas sorption, single-crystal X-ray diffraction (SCXRD), variable temperature powder X-ray diffraction (VT-PXRD), and synchrotron PXRD. We observed that the extrinsic pore structure of AB resulted in properties with potential for selective C₃H₄/C₃H₆ separation. Subsequent dynamic gas breakthrough measurements revealed exceptional experimental C₃H₄/C₃H₆ selectivity (270) and a new benchmark for productivity (118 mmol g^-1) of polymer grade C₃H₆ (purity >99.99%) from a 1:99 C₃H₄/C₃H₆ mixture. Structural analysis, gas sorption studies, and gas adsorption kinetics enabled us to determine that a binding "sweet spot" for C₃H₄ in the extrinsic pores is behind the benchmark separation performance. Density-functional theory (DFT) calculations and Canonical Monte Carlo (CMC) simulations provided further insight into the binding sites of C₃H₄ and C₃H₆ molecules within these two hybrid ultramicroporous materials, HUMs. These results highlight, to our knowledge for the first time, how pore engineering through the study of packing polymorphism in layered materials can dramatically change the separation performance of a physisorbent.

Engineering Pore Environments of Sulfate-Pillared Metal-Organic Framework for Efficient C2 H2 /CO2 Separation with Record Selectivity

Engineering pore environments exhibit great potential in improving gas adsorption and separation performances but require specific means for acetylene/carbon dioxide (C₂ H₂ /CO₂ ) separation due to their identical dynamic diameters and similar properties. Herein, a novel sulfate-pillared MOF adsorbent (SOFOUR-TEPE-Zn) using 1,1,2,2-tetra(pyridin-4-yl) ethene (TEPE) ligand with dense electronegative pore surfaces is reported. Compared to the prototype SOFOUR-1-Zn, SOFOUR-TEPE-Zn exhibits a higher C₂ H₂ uptake (89.1 cm³ g^-1 ), meanwhile the CO₂ uptake reduces to 14.1 cm³ g^-1 , only 17.4% of that on SOFOUR-1-Zn (81.0 cm³ g^-1 ). The high affinity toward C₂ H₂ than CO₂ is demonstrated by the benchmark C₂ H₂ /CO₂ selectivity (16 833). Furthermore, dynamic breakthrough experiments confirm its application feasibility and good cyclability at various flow rates. During the desorption cycle, 60.1 cm³ g^-1 C₂ H₂ of 99.5% purity or 33.2 cm³ g^-1 C₂ H₂ of 99.99% purity can be recovered by stepped purging and mild heating. The simulated pressure swing adsorption processes reveal that 75.5 cm³ g^-1 C₂ H₂ of 99.5+% purity with a high gas recovery of 99.82% can be produced in a counter-current blowdown process. Modeling studies disclose four favorable adsorption sites and dense packing for C₂ H₂ .

Insights into the thermodynamic-kinetic synergistic separation of propyne/propylene in anion pillared cage MOFs with entropy-enthalpy balanced adsorption sites

Propyne/propylene (C₃H₄/C₃H₆) separation is an important industrial process yet challenged by the trade-off of selectivity and capacity due to the molecular similarity. Herein, record C₃H₄/C₃H₆ separation performance is achieved by fine tuning the pore structure in anion pillared MOFs. SIFSIX-Cu-TPA (ZNU-2-Si) displays a benchmark C₃H₄ capacity (106/188 cm³ g^-1 at 0.01/1 bar and 298 K), excellent C₃H₄/C₃H₆ IAST selectivity (14.6-19.3) and kinetic selectivity, and record high C₃H₄/C₃H₆ (10/90) separation potential (36.2 mol kg^-1). The practical C₃H₄/C₃H₆ separation performance is fully demonstrated by breakthroughs under various conditions. 37.8 and 52.9 mol kg^-1 of polymer grade C₃H₆ can be produced from 10/90 and 1/99 C₃H₄/C₃H₆ mixtures. 4.7 mol kg^-1 of >99% purity C₃H₄ can be recovered by a stepped desorption process. Based on the in situ single crystal analysis and DFT calculation, an unprecedented entropy-enthalpy balanced adsorption pathway is discovered. MD simulation further confirmed the thermodynamic-kinetic synergistic separation of C₃H₄/C₃H₆ in ZNU-2-Si.

Substituent Engineering-Enabled Structural Rigidification and Performance Improvement for C2/CO2 Separation in Three Isoreticular Coordination Frameworks

Construction of porous solid materials applied to the adsorptive removal of CO₂ from C₂ hydrocarbons is highly demanded thanks to the important role C₂ hydrocarbons play in the chemical industry but quite challenging owing to the similar physical parameters between C₂ hydrocarbons and CO₂. In particular, the development of synthetic strategies to simultaneously enhance the uptake capacity and adsorption selectivity is very difficult due to the trade-off effect frequently existing between both of them. In this work, a combination of the dicopper paddlewheel unit and 4-pyridylisophthalate derivatives bearing different substituents afforded an isoreticular family of coordination framework compounds as a platform. Their adsorption properties toward C₂ hydrocarbons and CO₂ were systematically investigated, and subsequent IAST and density functional theory calculations combined with column breakthrough experiments verified their promising potential for C₂/CO₂ separations. Furthermore, the substituent engineering endowed the resulting compounds with simultaneous enhancement of uptake capacity and adsorption selectivity and thus better C₂/CO₂ separation performance compared to their parent compound. The substituent introduction not only mitigated the framework distortion via fixing the ligand conformation for establishment of better permanent porosity required for gas adsorption but also polarized the framework surface for host-guest interaction improvement, thus resulting in enhanced separation performance.

Optimization of Pore-Space-Partitioned Metal–Organic Frameworks Using the Bioisosteric Concept

Pore space partitioning (PSP) is methodically suited for dramatically increasing the density of guest binding sites, leading to the partitioned acs (pacs) platform capable of record-high uptake for CO₂ and small hydrocarbons such as C₂H_x. For gas separation, achieving high selectivity amid PSP-enabled high uptake offers an enticing prospect. Here we aim for high selectivity by introducing the bioisosteric (BIS) concept, a widely used drug design strategy, into the realm of pore-space-partitioned MOFs. New pacs materials have high C₂H₂/CO₂ selectivity of up to 29, high C₂H₂ uptake of up to 144 cm³/g (298 K, 1 atm), and high separation potential of up to 5.3 mmol/g, leading to excellent experimental breakthrough performance. These metrics, coupled with exceptional tunability, high stability, and low regeneration energy, demonstrate the broad potential of the BIS-PSP strategy.

Desolvation-Degree-Induced Structural Dynamics in a Rigid Cerium-Organic Framework Exhibiting Tandem Purification of Ethylene from Acetylene and Ethane

Due to the industrial requirements for high production and high quality of ethylene, efficient purification of ethylene from acetylene and ethane is of prime importance but challenging. Dynamic metal-organic frameworks (MOFs) have demonstrated intriguing structural dynamics and diverse applications recently. Among them, although a few flexible ones have exhibited interesting ethylene purification capability, rigid ones were yet barely investigated for such purpose. In this regard, a cerium(III)-based MOF was solvothermally synthesized, which is rigid and assembled from rod molecular building blocks associated with coordinative N,N-dimethylformamide (DMF) molecules. After liberating different degrees of DMF ligands via heating under vacuum or acetone exchange, both partially desolvated compounds of Ce-MOF-1 and Ce-MOF-2 were concertedly isolated in a fashion of single-crystal to single-crystal transformation. Although both newly generated materials crystallize in the same space group, they exhibit dissimilar unit cell parameters and slightly distinct ultramicropore sizes and pore microenvironments, thanks to the discrepancy in the desolvation degree. Consequently, Ce-MOF-1 and Ce-MOF-2 individually demonstrate C₂H₂- and C₂H₆-selective adsorption behavior, resulting in the potential tandem separation of C₂H₄ from C₂H₂ and C₂H₆ mixtures. The above results were successfully supported by not only single gas adsorption isotherms but also grand canonical Monte Carlo (GCMC) calculation studies and dynamic breakthrough experiments. The present work may pave the way for rigid MOFs aiming at advancing applications via solid-state structural dynamics.

Multi-Level Computational Screening of in Silico Designed MOFs for Efficient SO2 Capture

SO₂ presence in the atmosphere can cause significant harm to the human and environment through acid rain and/or smog formation. Combining the operational advantages of adsorption-based separation and diverse nature of metal-organic frameworks (MOFs), cost-effective separation processes for SO₂ emissions can be developed. Herein, a large database of hypothetical MOFs composed of >300,000 materials is screened for SO₂/CH₄, SO₂/CO₂, and SO₂/N₂ separations using a multi-level computational approach. Based on a combination of separation performance metrics (adsorption selectivity, working capacity, and regenerability), the best materials and the most common functional groups in those most promising materials are identified for each separation. The top bare MOFs and their functionalized variants are determined to attain SO₂/CH₄ selectivities of 62.4-16899.7, SO₂ working capacities of 0.3-20.1 mol/kg, and SO₂ regenerabilities of 5.8-98.5%. Regarding SO₂/CO₂ separation, they possess SO₂/CO₂ selectivities of 13.3-367.2, SO₂ working capacities of 0.1-17.7 mol/kg, and SO₂ regenerabilities of 1.9-98.2%. For the SO₂/N₂ separation, their SO₂/N₂ selectivities, SO₂ working capacities, and SO₂ regenerabilities span the ranges of 137.9-67,338.9, 0.4-20.6 mol/kg, and 7.0-98.6%, respectively. Besides, using breakdowns of gas separation performances of MOFs into functional groups, separation performance limits of MOFs based on functional groups are identified where bare MOFs (MOFs with multiple functional groups) tend to show the smallest (largest) spreads.

High-Throughput Computational Exploration of MOFs with Open Cu Sites for Adsorptive Separation of Hydrogen Isotopes

Effective separation of hydrogen isotopes still remains one of the extremely challenging tasks in industry. Compared to the present methods that are energy- and cost-intensive, quantum sieving technology based on nanostructured materials offers a more efficient alternative approach, where metal-organic frameworks (MOFs) featuring open metal sites (OMS) can serve as an ideal platform. Herein, a combination of periodic density functional theory (DFT) with dispersive correction and high-throughput molecular simulation was employed from thermodynamic viewpoints to explore the D₂/H₂ separation properties of 929 experimental MOFs bearing a copper-paddlewheel unit. The DFT calculations showed that there is a negligible rotational energy barrier for the molecule adsorbed at the OMS, and the movement of the Cu atoms along the Cu-Cu axis vector almost has no influence on the interaction energy. On the basis of the DFT results, a new force field with a proposed cutoff scheme was developed to accurately describe the strong isotope-OMS interaction. Under practical conditions (40 K and 1.0 bar), large-scale computational material screening demonstrated that the OMS interaction plays a more important role in highly selective materials and ignoring such interactions can lead to completely wrong identification of the most promising materials. Using the adsorption selectivity and adsorbent performance score as evaluation metrics, this work demonstrated that the materials with sql topology notably outperform many benchmark adsorbents reported so far.

Shell-like Xenon Nano-Traps within Angular Anion-Pillared Layered Porous Materials for Boosting Xe/Kr Separation

Adsorptive separation of xenon (Xe) and krypton (Kr) is a promising technique but remains a daunting challenge since they are atomic gases without dipole or quadruple moments. Herein we report a strategy for fabricating angular anion-pillared materials featuring shell-like Xe nano-traps, which provide a cooperative effect conferred by the pore confinement and multiple specific interactions. The perfect permanent pore channel (4-5 Å) of Ni(4-DPDS)₂ MO₄ (M=Cr, Mo, W) can host Xe atoms efficiently even at ultra-low concentration (400 ppm Xe), showing the second-highest selectivity of 30.2 in Ni(4-DPDS)₂ WO₄ and excellent Xe adsorption capacity in Ni(4-DPDS)₂ CrO₄ (15.0 mmol kg^-1 ). Crystallography studies and DFT-D calculations revealed the energy favorable binding sites and angular anions enable the synergism between optimal pore size and polar porosity for boosting Xe affinity. Dynamic breakthrough experiments demonstrated three MOFs as efficient adsorbents for Xe/Kr separation.

Comprehensive Pore Tuning in an Ultrastable Fluorinated Anion Cross-Linked Cage-Like MOF for Simultaneous Benchmark Propyne Recovery and Propylene Purification

Propyne/propylene (C₃ H₄ /C₃ H₆ ) separation is an important but challenging industrial process to produce polymer-grade C₃ H₆ and recover high-purity C₃ H₄ . Herein, we report an ultrastable TiF₆ ^2- anion cross-linked metal-organic framework (ZNU-2) with precisely controlled pore size, shape and functionality for benchmark C₃ H₄ storage (3.9/7.7 mmol g^-1 at 0.01/1.0 bar and 298 K) and record high C₃ H₄ /C₃ H₆ (10/90) separation potential (31.0 mol kg^-1 ). The remarkable C₃ H₄ /C₃ H₆ (1/99, 10/90, 50/50) separation performance was fully demonstrated by simulated and experimental breakthroughs under various conditions with excellent recyclability and high productivity (42 mol kg^-1 ) of polymer-grade C₃ H₆ from a 1/99 C₃ H₄ /C₃ H₆ mixture. A modelling study revealed that the symmetrical spatial distribution of six TiF₆ ^2- on the icosahedral cage surface provides two distinct binding sites for C₃ H₄ adsorption: one serves as a tailored single C₃ H₄ molecule trap and the other boosts C₃ H₄ accommodation by cooperative host-guest and guest-guest interactions.

Metal-Organic Framework Based Hydrogen-Bonding Nanotrap for Efficient Acetylene Storage and Separation

The removal of carbon dioxide (CO₂) from acetylene (C₂H₂) is a critical industrial process for manufacturing high-purity C₂H₂. However, it remains challenging to address the tradeoff between adsorption capacity and selectivity, on account of their similar physical properties and molecular sizes. To overcome this difficulty, here we report a novel strategy involving the regulation of a hydrogen-bonding nanotrap on the pore surface to promote the separation of C₂H₂/CO₂ mixtures in three isostructural metal-organic frameworks (MOFs, named MIL-160, CAU-10H, and CAU-23, respectively). Among them, MIL-160, which has abundant hydrogen-bonding acceptors as nanotraps, can selectively capture acetylene molecules and demonstrates an ultrahigh C₂H₂ storage capacity (191 cm³ g^-1, or 213 cm³ cm^-3) but much less CO₂ uptake (90 cm³ g^-1) under ambient conditions. The C₂H₂ adsorption amount of MIL-160 is remarkably higher than those for the other two isostructural MOFs (86 and 119 cm³ g^-1 for CAU-10H and CAU-23, respectively) under the same conditions. More importantly, both simulation and experimental breakthrough results show that MIL-160 sets a new benchmark for equimolar C₂H₂/CO₂ separation in terms of the separation potential (Δq_break = 5.02 mol/kg) and C₂H₂ productivity (6.8 mol/kg). In addition, in situ FT-IR experiments and computational modeling further reveal that the unique host-guest multiple hydrogen-bonding interaction between the nanotrap and C₂H₂ is the key factor for achieving the extraordinary acetylene storage capacity and superior C₂H₂/CO₂ selectivity. This work provides a novel and powerful approach to address the tradeoff of this extremely challenging gas separation.

Improving Ethane/Ethylene Separation Performance of Isoreticular Metal-Organic Frameworks via Substituent Engineering

The preferential capture of ethane (C₂H₆) over ethylene (C₂H₄) presents a very cost-effective and energy-saving means applied to adsorptive separation and purification of C₂H₄ with a high product purity, which is however challenged by low selectivity originating from their similar molecular sizes and physical properties. Substituent engineering has been widely employed for selectivity regulation and improvement, but its effect on C₂H₆/C₂H₄ separation has been rarely explored to date. In this work, four isoreticular coordination framework compounds based on 5-(pyridin-3-yl)isophthalate ligands bearing different substituents were rationally constructed. As revealed by isotherm measurements, thermodynamic studies, and IAST computations, they exhibited promising utility for C₂H₆/C₂H₄ separation with moderate adsorption heat and a high uptake amount at a relatively low-pressure domain. Furthermore, the C₂H₆/C₂H₄ separation potential can be finely tuned and optimized via purposeful substituent alteration. Most remarkably, functionalization with a nonpolar methyl group yielded an improved separation efficiency compared to its parent compound. This work offers a good reference value for enhancing the C₂H₆/C₂H₄ separation efficiency of MOFs by engineering the pore microenvironment and dimensions via substituent manipulation.

Ligand Bent-Angle Engineering for Tuning Topological Structures and Acetylene Purification Performances of Copper-Diisophthalate Frameworks

To enrich structural chemistry and widen the application prospects of MOFs (metal-organic frameworks), the development of a synthetic strategy to realize structural and functional modulation is highly demanded. By implementation of the linker bent-angle engineering strategy, three banana-like diisophthalate linkers with distinct bent angles were designed and synthesized. The inclusion of the targeted linkers into MOFs through solvothermal assembly with CuCl₂·2H₂O under identical conditions yielded three crystalline solids featuring diversified topological structures as revealed by X-ray crystallographic studies. Furthermore, functional explorations indicated that they are promising solid adsorbents for acetylene (C₂H₂) purification application with structurally dependent separation potentials. The results reported in this study illustrated a rare example of modulating the topological structures and separation efficiencies of MOFs by engineering the ligand bent angles.

Interpenetration symmetry control within ultramicroporous robust boron cluster hybrid MOFs for benchmark purification of acetylene from carbon dioxide

The separation of C2H2/CO2 is an important process in industry but challenged by the trade-off of capacity and selectivity owning to their similar physical properties and identical kinetic molecular size. Herein, we report the first example of symmetrically interpenetrated dodecaborate pillared MOF, ZNU-1, for benchmark selective separation of C2H2 from CO2 with a high C2H2 capacity of 76.3 cm3 g-1 and record C2H2/CO2 selectivity of 56.6 (298 K, 1 bar) among all the robust porous materials without open metal sites. Single crystal structure analysis and modelling study indicated that the interpenetration shifting from asymmetric to symmetric mode provided optimal pore chemistry with ideal synergistic "2+2" dihydrogen bonding sites for tight C2H2 trapping. The exceptional separation performance was further evidenced by simulated and experimental breakthroughs with excellent recyclability and high productivity (2.4 mol/kg) of 99.5% purity C2H2 during stepped desorption process.

Alkyl decorated metal-organic frameworks for selective trapping of ethane from ethylene above ambient pressures

The trapping of paraffins is beneficial compared to selective olefin adsorption for adsorptive olefin purification from a process engineering point of view. Here we demonstrate the use of a series of Zn2(X-bdc)2(dabco) (where X-bdc2- is bdc2- = 1,4-benzenedicarboxylate with substituting groups X, DM-bdc2- = 2,5-dimethyl-1,4-benzenedicarboxylate or TM-bdc2- = 2,3,5,6-tetramethyl-1,4-benzenedicarboxylate and dabco = diazabicyclo[2.2.2.]octane) metal-organic frameworks (MOFs) for the adsorptive removal of ethane from ethylene streams. The best performing material from this series is Zn2(TM-bdc)2(dabco) (DMOF-TM), which shows a high ethane uptake of 5.31 mmol g-1 at 110 kPa, with a good IAST selectivity of 1.88 towards ethane over ethylene. Through breakthrough measurements a high productivity of 13.1 L kg-1 per breakthrough is revealed with good reproducibility over five consecutive cycles. Molecular simulations show that the methyl groups of DMOF-TM are forming a van der Waals trap with the methylene groups from dabco, snuggly fitting the ethane. Further, rarely used high pressure coadsorption measurements, in pressure regimes that most scientific studies on hydrocarbon separation on MOFs ignore, reveal an increase in ethane capacity and selectivity for binary mixtures with increased pressures. The coadsorption measurements reveal good selectivity of 1.96 at 1000 kPa, which is verified also through IAST calculations up to 3000 kPa. This study overall showcases the opportunities that pore engineering by alkyl group incorporation and pressure increase offer to improve hydrocarbon separation in reticular materials.

Optimal Pore Chemistry in an Ultramicroporous Metal-Organic Framework for Benchmark Inverse CO2 /C2 H2 Separation

Isolation of CO₂ from acetylene (C₂ H₂ ) via CO₂ -selective sorbents is an energy-efficient technology for C₂ H₂ purification, but a strategic challenge due to their similar physicochemical properties. There is still no specific methodology for constructing sorbents that preferentially trap CO₂ over C₂ H₂ . We report an effective strategy to construct optimal pore chemistry in a Ce^IV -based ultramicroporous metal-organic framework Ce^IV -MIL-140-4F, based on charge-transfer effects, for efficient inverse CO₂ /C₂ H₂ separation. The ligand-to-metal cluster charge transfer is facilitated by Ce^IV with low-lying unoccupied 4f orbitals and electron-withdrawing F atoms functionalized tetrafluoroterephthalate, affording a perfect pore environment to match CO₂ . The exceptional CO₂ uptake (151.7 cm³  cm^-3 ) along with remarkable separation selectivities (above 40) set a new benchmark for inverse CO₂ /C₂ H₂ separation, which is verified via simulated and experimental breakthrough experiments. The unique CO₂ recognition mechanism is further unveiled by in situ powder X-ray diffraction experiments, Fourier-transform infrared spectroscopy measurements, and molecular calculations.

Cyclotetrabenzoin Acetate: A Macrocyclic Porous Molecular Crystal for CO2 Separations by Pressure Swing Adsorption*

A porous molecular crystal (PMC) assembled by macrocyclic cyclotetrabenzoin acetate is an efficient adsorbent for CO₂ separations. The 7.1×7.1 Å square pore of PMC and its ester C=O groups play important roles in improving its affinity for CO₂ molecules. The benzene walls of macrocycle engage in an apparent [π⋅⋅⋅π] interaction with the molecule of CO₂ at low pressure. In addition, the polar carbonyl groups pointing inward the square channels reduce the size of aperture to a 5.0×5.0 Å square, which offers kinetic selectivity for CO₂ capture. The PMC features water tolerance and high structural stability under vacuum and various gas adsorption conditions, which are rare among intrinsically porous organic molecules. Most importantly, the moderate adsorbate-adsorbent interaction allows the PMC to be readily regenerated, and therefore applied to pressure swing adsorption processes. The eluted N₂ and CH₄ are obtained with over 99.9 % and 99.8 % purity, respectively, and the separation performance is stable for 30 cycles. Coupled with its easy synthesis, cyclotetrabenzoin acetate is a promising adsorbent for CO₂ separations from flue and natural gases.

Deep learning combined with IAST to screen thermodynamically feasible MOFs for adsorption-based separation of multiple binary mixtures

The structures of metal-organic frameworks (MOFs) can be tuned to reproducibly create adsorption properties that enable the use of these materials in fixed-adsorption beds for non-thermal separations. However, with millions of possible MOF structures, the challenge is to find the MOF with the best adsorption properties to separate a given mixture. Thus, computational, rather than experimental, screening is necessary to identify promising MOF structures that merit further examination, a process traditionally done using molecular simulation. However, even molecular simulation can become intractable when screening an expansive MOF database for their separation properties at more than a few composition, temperature, and pressure combinations. Here, we illustrate progress toward an alternative computational framework that can efficiently identify the highest-performing MOFs for separating various gas mixtures at a variety of conditions and at a fraction of the computational cost of molecular simulation. This framework uses a "multipurpose" multilayer perceptron (MLP) model that can predict single component adsorption of various small adsorbates, which, upon coupling with ideal adsorbed solution theory (IAST), can predict binary adsorption for mixtures such as Xe/Kr, CH₄/CH₆, N₂/CH₄, and Ar/Kr at multiple compositions and pressures. For this MLP+IAST framework to work with sufficient accuracy, we found it critical for the MLP to make accurate predictions at low pressures (0.01-0.1 bar). After training a model with this capability, we found that MOFs in the 95th and 90th percentiles of separation performance determined from MLP+IAST calculations were 65% and 87%, respectively, the same as MOFs in the simulation-predicted 95th percentile across several mixtures at diverse conditions (on average). After validating our MLP+IAST framework, we used a clustering algorithm to identify "privileged" MOFs that are high performing for multiple separations at multiple conditions. As an example, we focused on MOFs that were high performing for the industrially relevant separations 80/20 Xe/Kr at 1 bar and 80/20 N₂/CH₄ at 5 bars. Finally, we used the MOF free energies (calculated on our entire database) to identify privileged MOFs that were also likely synthetically accessible, at least from a thermodynamic perspective.

How Reliable Is the Ideal Adsorbed Solution Theory for the Estimation of Mixture Separation Selectivities in Microporous Crystalline Adsorbents?

Microporous crystalline adsorbents such as zeolites and metal-organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity S _ads is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of S _ads, with unary isotherms of the constituent guests as data inputs. There are two basic tenets underlying the development of the IAST. The first tenet mandates a homogeneous distribution of adsorbates within the pore landscape. The second tenet requires the surface area occupied by a guest molecule in the mixture to be the same as that for the corresponding pure component. Configurational-bias Monte Carlo (CBMC) simulations are employed in this article to highlight several scenarios in which the IAST fails to provide a quantitatively correct description of mixture adsorption equilibrium due to a failure to conform to either of the two tenets underpinning the IAST. For CO₂ capture with cation-exchanged zeolites and MOFs with open metal sites, there is congregation of CO₂ around the cations and unsaturated metal atoms, resulting in failure of the IAST due to an inhomogeneous distribution of adsorbates in the pore space. Thermodynamic non-idealities also arise due to the preferential location of CO₂ molecules at the window regions of 8-ring zeolites such as DDR and CHA or within pockets of MOR and AFX zeolites. Thermodynamic non-idealities are evidenced for water/alcohol mixtures due to molecular clustering engendered by hydrogen bonding. It is also demonstrated that thermodynamic non-idealities can be strong enough to cause selectivity reversals, which are not anticipated by the IAST.

Screening Metal-Organic Frameworks for Separation of Binary Solvent Mixtures by Compact NMR Relaxometry

Metal–organic frameworks (MOFs) have great potential as an efficient alternative to current separation and purification procedures of a large variety of solvent mixtures—a critical process in many applications. Due to the huge number of existing MOFs, it is of key importance to identify high-throughput analytical tools, which can be used for their screening and performance ranking. In this context, the present work introduces a simple, fast, and inexpensive approach by compact low-field proton nuclear magnetic resonance (NMR) relaxometry to investigate the efficiency of MOF materials for the separation of a binary solvent mixture. The mass proportions of two solvents within a particular solvent mixture can be quantified before and after separation with the help of a priori established correlation curves relating the effective transverse relaxation times T_2eff and the mass proportions of the two solvents. The new method is applied to test the separation efficiency of powdered UiO-66(Zr) for various solvent mixtures, including linear and cyclic alkanes and benzene derivate, under static conditions at room temperature. Its reliability is demonstrated by comparison with results from ¹H liquid-state NMR spectroscopy.

Realization of Ethylene Production from Its Quaternary Mixture through Metal-Organic Framework Materials

Ethylene production from oxidative coupling of methane is a sustainable and economically attractive alternative to that through traditional hydrocarbon cracking technology. However, efficient ethylene separation from the complex reaction mixture is a daunting challenge that hinders the practical adoption of this technology. Herein, we report the efficient adsorptive separation of the CH₄/CO₂/C₂H₄/C₂H₆ mixture using three representative metal-organic frameworks (MOFs) (UTSA-74, MOF-74, and HKUST-1) with diverse open metal sites. The efficient separation relies on tuning the selectivity through the convergence of characteristics including Lewis acidity of open metal sites, pore space, and cooperative binding behavior. The separation performance of these materials has been evaluated through single-component gas adsorption and dynamic breakthrough experiments. HKUST-1 provides the highest separation potential (4.1 mmol/g) thanks to its simultaneously high ideal adsorbed solution theory (IAST) selectivity and ethylene adsorption capacity, representing a benchmark material for such a challenging quaternary separation.

Simultaneous interlayer and intralayer space control in two-dimensional metal-organic frameworks for acetylene/ethylene separation

Three-dimensional metal−organic frameworks (MOFs) are cutting-edge materials in the adsorptive removal of trace gases due to the availability of abundant pores with specific chemistry. However, the development of ideal adsorbents combining high adsorption capacity with high selectivity and stability remains challenging. Here we demonstrate a strategy to design adsorbents that utilizes the tunability of interlayer and intralayer space of two-dimensional fluorinated MOFs for capturing acetylene from ethylene. Validated by X-ray diffraction and modeling, a systematic variation of linker atom oxidation state enables fine regulation of layer stacking pattern and linker conformation, which affords a strong interlayer trapping of molecules along with cooperative intralayer binding. The resultant robust materials (ZUL-100 and ZUL-200) exhibit benchmark capacity in the pressure range of 0.001–0.05 bar with high selectivity. Their efficiency in acetylene/ethylene separation is confirmed by breakthrough experiments, giving excellent ethylene productivities (121 mmol/g from 1/99 mixture, 99.9999%), even when cycled under moist conditions.

Designing efficient adsorbents for trace gas removal remains a serious challenge. Here, the authors show promise in layered 2D metal−organic frameworks, often overlooked in favor of 3D frameworks, for separating trace acetylene from ethylene with enhanced performance and high stability.

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Microporous crystalline porous materials such as zeolites, metal-organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents. For this purpose, configurational bias Monte Carlo (CBMC) simulations are used to determine the component loadings for adsorption equilibrium of water/methanol and water/ethanol mixtures in all-silica zeolites (CHA, DDR, and FAU) and ZIF-8. Due to the occurrence of strong hydrogen bonding between water and alcohol molecules and attendant clustering, IAST fails to provide quantitative estimates of the component loadings and the adsorption selectivity. For a range of operating conditions, the water loading in the adsorbed phase may exceed that of pure water by one to two orders of magnitude. Furthermore, the occurrence of water-alcohol clusters moderates size entropy effects that prevail under pore saturation conditions. For quantitative modeling of the CBMC, simulated data requires the application of real adsorbed solution theory by incorporation of activity coefficients, suitably parameterized by the Margules model for the excess Gibbs free energy of adsorption.