An ethane-trapping MOF PCN-250 for highly selective adsorption of ethane over ethylene

Metal-Organic Framework Featuring Cubic Caged Structures for One-Step Ethylene Purification from Ethylene/Ethane Mixtures

Separation of C2H6/C2H4 mixtures is of significant importance in the chemical industry but remains a challenge due to the physicochemical similarities of C2H6 and C2H4. Herein, a metal-organic framework (MOF), [Zn4(μ4-O)(PCTF)3]n (Zn-PCTF) (PCTF2-= 5-trifluoromethyl-1H-pyrazole-4-carboxylic), is provided for the removal of C2H6 from C2H6/C2H4 mixtures. Zn-PCTF displays a three-dimensional framework featuring one-dimensional pore channels with periodic bottleneck segments. The well-balanced C2H6 adsorption capacity (79.0 cm3 g-1 at 298 K) and C2H6/C2H4 selectivity (1.8) for Zn-PCTF under ambient conditions boost Zn-PCTF with highly promising potentials for efficient purification of C2H4 from C2H6/C2H4 mixtures, which is verified by the dynamic column breakthrough experiments. The well-matched caged pores and suitable pore chemistry (particularly the presence of abundant Lewis base sites (N, O, and F) on the pore surfaces) for C2H6 account for the high-performance C2H6/C2H4 separation of Zn-PCTF unveiled by computational simulations.

Reaching Quantum Accuracy in Predicting Adsorption Properties for Ethane/Ethene in Zeolitic Imidazolate Framework-8 at Low Pressure Regime

Nanoporous materials in the form of metal-organic frameworks such as zeolitic imidazolate framework-8 (ZIF-8) are promising membrane materials for the separation of hydrocarbon mixtures. To compute the adsorption isotherms in such adsorbents, grand canonical Monte Carlo simulations have proven to be very useful. The quality of these isotherms depends on the accuracy of adsorbate-adsorbent interactions, which are mostly described using force fields owing to their low computational cost. However, force field predictions of adsorption uptake often show discrepancies from experiments at low pressures, providing the need for methods that are more accurate. Hence, in this work, we propose and validate two novel methodologies for the ZIF-8/ethane and ethene systems; a benchmarking methodology to evaluate the performance of any given force field in describing adsorption in the low-pressure regime and a refinement procedure to rescale the parameters of a force field to better describe the host-guest interactions and provide for simulation isotherms with close agreement to experimental isotherms. Both methodologies were developed based on a reference Henry coefficient, computed with the PBE-MBD functional using the importance sampling technique. The force field rankings predicted by the benchmarking methodology involve the comparison of force field derived Henry coefficients with the reference Henry coefficients and ranking the force fields based on the disparities between these Henry coefficients. The ranking from this methodology matches the rankings made based on uptake disparities by comparing force field derived simulation isotherms to experimental isotherms in the low-pressure regime. The force field rescaling methodology was proven to refine even the worst performing force field in UFF/TraPPE. The uptake disparities of UFF/TraPPE improved from 197% and 194% to 11% and 21% for ethane and ethene, respectively. The proposed methodology is applicable to predict adsorption across nanoporous materials and allows for rescaled force fields to reach quantum accuracy without the need for experimental input.

Structural Design and Energy and Environmental Applications of Hydrogen-Bonded Organic Frameworks: A Systematic Review

Hydrogen-bonded organic frameworks (HOFs) are emerging porous materials that show high structural flexibility, mild synthetic conditions, good solution processability, easy healing and regeneration, and good recyclability. Although these properties give them many potential multifunctional applications, their frameworks are unstable due to the presence of only weak and reversible hydrogen bonds. In this work, the development history and synthesis methods of HOFs are reviewed, and categorize their structural design concepts and strategies to improve their stability. More importantly, due to the significant potential of the latest HOF-related research for addressing energy and environmental issues, this work discusses the latest advances in the methods of energy storage and conversion, energy substance generation and isolation, environmental detection and isolation, degradation and transformation, and biological applications. Furthermore, a discussion of the coupling orientation of HOF in the cross-cutting fields of energy and environment is presented for the first time. Finally, current challenges, opportunities, and strategies for the development of HOFs to advance their energy and environmental applications are discussed.

Fine-tuning the pore environment of ultramicroporous three-dimensional covalent organic frameworks for efficient one-step ethylene purification

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C2H6) from ethylene (C2H4), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C2H4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C2H6 adsorption capacity (110.4 cm3 g-1 at 293 K and 1 bar) and good C2H6/C2H4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C2H6. To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C2H4 directly from a C2H6/C2H4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation.

Efficient C2H6/C2H4 adsorption separation by a microporous heterometal-organic framework

Purification of ethylene (C2H4) is an essential and energy-intensive process in the petrochemical industry. Adsorption separation using ethane (C2H6)-selective porous adsorbents is a highly efficient and straightforward method for obtaining polymer-grade C2H4 from a binary C2H6/C2H4 mixture. However, the design and construction of C2H6-selective adsorbents are very challenging tasks. Herein, we demonstrate a microporous heterometal-organic framework, CuIn(ina)4, can preferentially enrich C2H6 than C2H4. Experimental results revealed that CuIn(ina)4 exhibited remarkable separation performance for the C2H6/C2H4 mixture with a high C2H6 loading capacity (3.3 mmol/g), high IAST selectivity (2.3) and separation potential (1578 mmol/L for equimolar C2H6/C2H4 mixture) under ambient conditions. The effectiveness of CuIn(ina)4 for C2H6/C2H4 adsorption separation was confirmed by theoretical calculations and breakthrough experiments.

Multi-Modular Design of Stable Pore-Space-Partitioned Metal-Organic Frameworks for Gas Separation Applications

Pore space partition (PSP) is an effective materials design method for developing high-performance small-pore materials for storage and separation of gas molecules. The continued success of PSP depends on broad availability and judicious choice of pore-partition ligands and better understanding of each structural module on stability and sorption properties. Here, by using substructural bioisosteric strategy (sub-BIS), a dramatic expansion of pore-partitioned materials is targeted by using ditopic dipyridyl ligands with non-aromatic cores or extenders, as well as by expanding heterometallic clusters to uncommon nickel-vanadium and nickel-indium clusters rarely known before in porous materials. The dual-module iterative refinement of pore-partition ligands and trimers leads to remarkable enhancement of chemical stability and porosity. Here a family of 23 pore-partitioned materials synthesized from five pore-partition ligands and seven types of trimeric clusters is reported. New materials with such compositionally and structurally diverse framework modules reveal key factors that dictate stability, porosity, and gas separation properties. Among these, materials based on heterometallic vanadium-nickel trimeric clusters give rise to the highest long-term hydrolytic stability and remarkable uptake capacity for CO2 , C2 H2 /C2 H4 /C2 H6 , and C3 H6 /C3 H8 hydrocarbon gases. The breakthrough experiment shows the potential application of new materials for separating gas mixtures such as C2 H2 /CO2 .

Fe-MOF Catalytic Nanoarchitectonic toward Electrochemical Ammonia Production

Electrochemical reduction of nitrate into ammonia has lately been identified as one among the promising solutions to address the challenges triggered by the growing global energy demand. Exploring newer electrocatalyst materials is vital to make this process effective and feasible. Recently, metal-organic framework (MOF)-based catalysts are being well investigated for electrocatalytic ammonia synthesis, accounting for their enhanced structural and compositional integrity during catalytic reduction reactions. In this study, we investigate the ability of the PCN-250-Fe3 MOF toward ammonia production in its pristine and activated forms. The activated MOF catalyst delivered a faradaic efficiency of about 90% at -1 V vs RHE and a yield rate of 2.5 × 10-4 mol cm-2 h-1, while the pristine catalyst delivered a 60% faradaic efficiency at the same potential. Theoretical studies further provide insights into the nitrate reduction reaction mechanism catalyzed by the PCN-250-Fe3 MOF catalyst. In short, simpler and cost-effective strategies such as pretreatment of electrocatalysts have an upper hand in aggravating the intrinsic material properties, for catalytic applications, when compared to conventional material modification approaches.

Thiazole functionalized covalent triazine frameworks for C2H6/C2H4 separation with remarkable ethane uptake

A highly stable thiazole functionalized covalent triazine framework, namely CTF-BT-500, was developed for C2H6/C2H4 separation, which exhibits a record-high ethane uptake (99.7 cm3 g-1) among all reported COFs at 298 K and 1 bar. This work not only presents an excellent C2H6-selective adsorbent, but also provides guidance for the construction of robust adsorbents for value-added gas purification.

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.

Application of Hydrogen-Bonded Organic Frameworks in Environmental Remediation: Recent Advances and Future Trends

The hydrogen-bonded organic frameworks (HOFs) are a class of porous materials with crystalline frame structures, which are self-assembled from organic structures by hydrogen bonding in non-covalent bonds π-π packing and van der Waals force interaction. HOFs are widely used in environmental remediation due to their high specific surface area, ordered pore structure, pore modifiability, and post-synthesis adjustability of various physical and chemical forms. This work summarizes some rules for constructing stable HOFs and the synthesis of HOF-based materials (synthesis of HOFs, metallized HOFs, and HOF-derived materials). In addition, the applications of HOF-based materials in the field of environmental remediation are introduced, including adsorption and separation (NH3, CO2/CH4 and CO2/N2, C2H2/C2He and CeH6, C2H2/CO2, Xe/Kr, etc.), heavy metal and radioactive metal adsorption, organic dye and pesticide adsorption, energy conversion (producing H2 and CO2 reduced to CO), organic dye degradation and pollutant sensing (metal ion, aniline, antibiotic, explosive steam, etc.). Finally, the current challenges and further studies of HOFs (such as functional modification, molecular simulation, application extension as remediation of contaminated soil, and cost assessment) are discussed. It is hoped that this work will help develop widespread applications for HOFs in removing a variety of pollutants from the environment.

Efficient CO2 Capture under Humid Conditions on a Novel Amide-Functionalized Fe- Metal-Organic Framework

CO₂ is the main source of the greenhouse gases, and its capture from flue gas under humid conditions is challenging but important for promoting carbon neutrality. Herein, we report a novel soc topology Fe-based metal-organic framework (Fe-dbai) with highly efficient postcombusion CO₂ capture performance by integrating multiple specific functionalities, such as unsaturated metal sites and amide functional groups. The CO₂ adsorption capacity and CO₂/N₂ selectivity of Fe-dbai are high up to 6.4 mmol/g and 64 (298 K, 1 bar), respectively, superior to many other reported MOFs. More importantly, the CO₂ working capacity of Fe-dbai under 60% RH conditions preserves 94% of that under dry conditions in the breakthrough experiments of CO₂/N₂ (15:85, v/v) mixtures. The molecular simulation highlights that the electronegative amide CO- group has a good affinity for CO₂ and can improve the interaction between Fe UMS and CO₂. Although H₂O molecules will occupy a small fraction of the adsorption sites, the confinement effect it produces can enhance the adsorption affinity of the framework for CO₂, which results in Fe-dbai retaining most of the CO₂ adsorption capacity under humid conditions. The excellent CO₂ capture performance makes Fe-dbai a potential candidate for the practical application of CO₂ capture.

Extraordinary Separation of Acetylene-Containing Mixtures in a Honeycomb Calcium-Based MOF with Multiple Active Sites

Acetylene (C₂H₂) separation from multicomponent mixtures is vitally important but industrially challenging for the collection of high-purity C₂H₂. To address this requirement, the reaction between the alkaline-earth Ca²⁺ ions with a dicarboxylate-diazolate linker, 4,6-di(1H-tetrazol-5-yl)isophthalic acid (H₄dtzip), gave rise to a new metal-organic framework (MOF) material [Ca(dtzip)_0.5H₂O]·2H₂O (1). The material presents unique regular tubular channels based on threefolded helical rod-like secondary building units with rich open metal sites and exposed organic hydrogen-bonding N/O acceptors that enhance the interactions with C₂H₂ molecules, endowing significant selectivity for C₂H₂ over C₂H₄ (5.4), C₂H₆ (5.6), CH₄ (30.0), and CO₂ (7.7) at 298 K and 100 kPa. Column breakthrough experiments confirmed the extraordinary C₂H₂ separation performance of the material with the separation time intervals in the range of 18-24 min g^-1 for binary (C₂H₂-C₂H₄, C₂H₂-C₂H₆, C₂H₂-CO₂, and C₂H₂-CH₄) or ternary (C₂H₂-C₂H₄-C₂H₆ and C₂H₂-C₂H₄-CO₂) gas mixtures under dynamic conditions.

Adsorption and diffusion of the H2/CO2/CO/MeOH/EtOH mixture into the ZIF-7 using molecular simulation

The adsorption and diffusion of synthesis gas components (methanol, ethanol, H₂, CO_2, and CO molecules) in ZIF-7 by grand canonical Monte Carlo and molecular dynamics simulation were investigated. The initial diffusion coefficient at the beginning of the process depends on the kinetic diameter of the guest molecules. Also, the diffusion coefficient at equilibrium conditions probably depends on the interaction between the guest molecules with the ZIF-7 framework. The radial distribution function results indicate that the distribution of guest molecules in the framework is affected by the interaction between the guest molecules. These results indicate that the CO, CO₂, and H₂ guest molecules are adsorbed on both the Zn metal atom and the organic linker (especially the C₁ atom). In contrast, the organic linker is the most favorable adsorption site for methanol and ethanol guest molecules. In addition, the diffusion coefficient of guest molecules in binary mixtures is related to the attraction or repulsion between the guest molecules.

High-Performance Adsorbent for Ethane/Ethylene Separation Selected through the Computational Screening of Aluminum-Based Metal-Organic Frameworks

The development of a high-performance ethane (C₂H₆)-selective adsorbent for the separation of ethane/ethylene (C₂H₆/C₂H₄) gas mixtures has been investigated for high-efficiency adsorption-based gas separation. Herein, we investigated Al-based metal-organic frameworks (MOFs) to identify an efficient C₂H₆-selective adsorbent (CAU-11), supported by a computational simulation study. CAU-11 exhibited numerous advantageous properties (such as low material cost, structural robustness, high reaction yield, and high C₂H₆/C₂H₄ selectivity) compared to other Al-based MOFs, indicating immense potential as a C₂H₆-selective adsorbent. CAU-11 exhibited preferential C₂H₆ adsorption in single-component gas adsorption experiments, and its predicted ideal adsorption solution theory selectivity of C₂H₆/C₂H₄ was over 2.1, consistent with the simulation analysis. Dynamic breakthrough experiments using representative compositions of the C₂H₆/C₂H₄ gas mixture confirmed the excellent separation ability of CAU-11; it produced high-purity C₂H₄ (>99.95%) with productivity values of 0.79 and 2.02 mol L^-1 while repeating the cyclic experiment with 1:1 and 1:15 v/v C₂H₆/C₂H₄ gas mixtures, respectively, at 298 K and 1 bar. The high C₂H₆/C₂H₄ separation ability of CAU-11 could be attributed to its non-polar pore environment and optimum pore dimensions which strengthen the interaction of its pores (via C-H···π interactions) with C₂H₆ to a greater extent than with C₂H₄.

Ultramicroporous material based parallel and extended paraffin nano-trap for benchmark olefin purification

Selective paraffin capture from olefin/paraffin mixtures could afford high-purity olefins directly, but suffers from the issues of low separation selectivity and olefin productivity. Herein, we report an ultramicroporous material (PCP-IPA) with parallel-aligned linearly extending isophthalic acid units along the one-dimensional channel, realizing the efficient production of ultra-high purity C₂H₄ and C₃H₆ (99.99%). The periodically expanded and parallel-aligned aromatic-based units served as a paraffin nano-trap to contact with the exposed hydrogen atoms of both C₂H₆ and C₃H₈, as demonstrated by the simulation studies. PCP-IPA exhibits record separation selectivity of 2.48 and separation potential of 1.20 mol/L for C₃H₈/C₃H₆ (50/50) mixture, meanwhile the excellent C₂H₆/C₂H₄ mixture separation performance. Ultra-high purity C₃H₆ (99.99%) and C₂H₄ (99.99%) can be directly obtained through fixed-bed column from C₃H₈/C₃H₆ and C₂H₆/C₂H₄ mixtures, respectively. The record C₃H₆ productivity is up to 15.23 L/kg from the equimolar of C₃H₈/C₃H₆, which is 3.85 times of the previous benchmark material, demonstrating its great potential for those important industrial separations.

Pore-Window Partitions in Metal-Organic Frameworks for Highly Efficient Reversed Ethylene/Ethane Separations

The development of paraffin-selective adsorbents is desirable but extremely challenging because adsorbents usually prefer olefin over paraffin. Herein, a new pore-window-partition strategy is proposed for the rational design of highly efficient paraffin-preferred metal-organic framework (MOF) adsorbents. The power of this strategy is demonstrated by stepwise installations of linear bidentate N-donor linkers into a prototype MOF (SNNU-201) to produce a series of partitional MOF adsorbents (SNNU-202-204). With continuous pore-window partitions from SNNU-201 to SNNU-204, the isosteric heat of adsorption can be tuned from -34.4 to -19.4 kJ mol^-1 for ethylene and from -25.5 to -20.7 kJ mol^-1 for ethane. Accordingly, partitional MOFs exhibit much higher ethane adsorption capacities, especially for SNNU-204 (104.6 cm³ g^-1), representing nearly 4 times as much ethane as the prototypical counterpart (SNNU-201; 27.5 cm³ g^-1) under ambient conditions. The C₂H₆/C₂H₄ ideal adsorbed solution theory selectivity, dynamic breakthrough experiments, and theoretical simulations further indicate that pore-window partition is a promising and universal strategy for the exploration of highly efficient paraffin-selective MOF adsorbents.

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures

Light hydrocarbon separation is considered one of the most industrially challenging and desired chemical separation processes and is highly essential in polymer and chemical industries. Among them, separating ethylene (C₂H₄) from C2 hydrocarbon mixtures such as ethane (C₂H₆), acetylene (C₂H₂), and other natural gas elements (CO₂, CH₄) is of paramount importance and poses significant difficulty. We demonstrate such separations using an Al-MOF synthesised earlier as a non-porous material, but herein endowed with hierarchical porosity created under microwave conditions in an equimolar water/ethanol solution. The material possessing a large surface area (793 m² g⁻¹) exhibits an excellent uptake capacity for major industrial hydrocarbons in the order of C₂H₂ > C₂H₆ > CO₂ > C₂H₄ > CH₄ under ambient conditions. It shows an outstanding dynamic breakthrough separation of ethylene (C₂H₄) not only for a binary mixture (C₂H₆/C₂H₄) but also for a quaternary combination (C₂H₄/C₂H₆/C₂H₂/CO₂ and C₂H₄/C₂H₆/C₂H₂/CH₄) of varying concentrations. The detailed separation/purification mechanism was unveiled by gas adsorption isotherms, mixed-gas adsorption calculations, selectivity estimations, advanced computer simulations such as density functional theory (DFT), grand canonical Monte Carlo (GCMC) and ab initio molecular dynamics (AIMD), and stepwise multicomponent dynamic breakthrough experiments.

Industrially important C₂H₄ purification from multi-component hydrocarbon mixtures.

Evaluation of Iron-Based Metal-Organic Framework Activation Temperatures in Acetylene Adsorption

One of the major issues regarding long-term human space exploration is the need for a breathable atmosphere. A major component toward achieving this goal is both the removal of exhaled carbon dioxide (CO₂) and the generation or recovery of oxygen (O₂). NASA's current technology only operates at about 50% efficiency due to the need to vent the methane that is produced during the CO₂ reduction process. One method of improving the efficiency of this process is through plasma pyrolysis, wherein the methane is pyrolyzed to produce hydrogen and various dehydrogenated carbon byproducts. In this process, acetylene is one of the main components of this byproduct stream. Unfortunately, while the concentration of this effluent is generally high in hydrogen (>90% typically), the presence of the acetylene waste product can act as a poison for the ruthenium-alumina catalyst used in the CO₂-reducing Sabatier process, requiring a removal step. Metal-organic frameworks (MOFs) represent a valuable method for removing these unsaturated hydrocarbons due to their high tunability, particularly through the incorporation of open metal sites. In this study, two common iron-based MOFs, MIL-100 and PCN-250, were studied for their ability to adsorb acetylene. A combination of gas adsorption analysis and density functional theory calculation results shows the ability of these materials to undergo a thermal-induced reduction event, which results in an improvement in gas adsorption performance. This improvement in gas performance appears to be at least partially due to the increased presence of π-backbonding toward the acetylene molecules.

Synergistic binding sites in a hybrid ultramicroporous material for one-step ethylene purification from ternary C2 hydrocarbon mixtures

One-step separation of C₂H₄ from ternary C₂H₂/C₂H₄/C₂H₆ hydrocarbon mixtures is of great significance in the industry but is challenging due to the similar sizes and physical properties of C₂H₂, C₂H₄, and C₂H₆. Here, we report an anion-pillared hybrid ultramicroporous material, CuTiF₆-TPPY, that has the ability of selective recognition of C₂H₄ over C₂H₂ and C₂H₆. The 4,6-connected fsc framework of CuTiF₆-TPPY exhibits semi-cage-like one-dimensional channels sustained by porphyrin rings and TiF₆^2- pillars, which demonstrates the noticeably enhanced adsorption of C₂H₂ and C₂H₆ over C₂H₄. Dynamic breakthrough experiments confirm the direct and facile high-purity C₂H₄ (>99.9%) production from a ternary gas mixture of C₂H₂/C₂H₆/C₂H₄ (1/9/90, v/v/v) under ambient conditions. Computational studies and in situ infrared reveal that the porphyrin moieties with large π-surfaces form multiple van der Waals interactions with C₂H₆; meanwhile, the polar TiF₆^2- pillars form C-H•••F hydrogen bonding with C₂H₂. In contrast, the recognition sites for C₂H₄ in the framework are less marked.

One-Step Ethylene Purification by an Ethane-Screening Metal-Organic Framework

Efficient purification of ethylene (C₂H₄) from ethane (C₂H₆) is a crucial but daunting task for the chemical industry given their similar physical natures and molecular dimensions. Reversed capture of C₂H₆ from C₂H₆/C₂H₄ dual-mixtures can be expected to directly yield high-purity C₂H₄ through a one-step separation unit, but it remains a daunting challenge. Here, we skillfully target an unusual "electrostatic-driven linker microrotation" (EDLM) in a Zr-MOF through coupling dual-ligands having electron-withdrawing/donating groups (e.g., F and CH₃ motifs). EDLM triggered microrotation of linker geometry and screening sites not only enhanced structural rigidity and hydrophobic nature, etc., but also effectively purified C₂H₄ through reversely trapping C₂H₆. Under ambient conditions, 1 kg of activated 2 adsorbents directly produces 7.2 L of C₂H₄ with over 99.9%+ purity in a single breakthrough operation starting from the equimolar C₂H₆/C₂H₄ cracked mixtures. Geometrical models and simulations have revealed that EDLM-derived H-bonding interaction and microrotation of linker geometry, synergistically customized C₂H₆-selective screening sites and pore inert for reversed C₂H₆ capture and improved surface hydrophobicity. Adsorption isotherms, modeling simulations, and breakthrough tests based on pressure swing adsorption (PSA) conditions have jointly elucidated the underlying separation properties for C₂H₄ purification. The enhanced hydrophobic nature, cycling durability, and separation property awarded 2 a new benchmark adsorbent to purify the olefin/paraffin mixtures.

Bottom-Up Synthesis of 8-Connected Three-Dimensional Covalent Organic Frameworks for Highly Efficient Ethylene/Ethane Separation

Developing cost-/energy-efficient separation techniques for purifying ethylene from an ethylene/ethane mixture is highly important but very challenging in the industrial process. Herein, using a bottom-up [8 + 2] construction approach, we rationally designed and synthesized three three-dimensional covalent organic frameworks (COFs) with 8-connected bcu networks, which can selectively remove ethane from an ethylene/ethane mixture with high efficiency. These COF materials, which are fabricated by the condensation reaction of a customer-designed octatopic aldehyde monomer with linear diamino linkers, possess high crystallinity, good structural robustness, and high porosity. Attributed to the well-organized micro-sized pores with a nonpolar/inert pore environment, these COFs display high ethane adsorption capacity and good selectivity over ethylene, making them among the best ethane-selective adsorbents for ethylene purification. Their excellent ethylene/ethane separation performance is validated by dynamic breakthrough experiments with high-purity ethylene (>99.99%) produced through a single adsorption process. The separation performance surpasses all reported C₂H₆-selective COFs and even some benchmark metal-organic frameworks. This work provides important guidance for the design of new adsorbents for value-added gas purification.

Three novel MOFs constructed from 1,3,5-tris(1-imidazolyl)benzene and dicarboxylate ligands with selective adsorption for C2H2/C2H4 and C2H6/CH4

The urothermal reactions of Co(II)/Zn(II) salts with the diverse carboxylic acid and 1,3,5,-tris(1-imidazolyl)benzene(tib) ligands afforded three novel MOFs, namely, [Co₃(tib)₂(abdc)₂(ox)]₂·6H₂O (1), [Co₃O(tib)(abdc)₂(DMI)]·2DMI·2H₂O (2) and [Zn₃I₂(tib)₂(napdc)₂]·2DMI·2H₂O (3), (H₂abdc = 5-amino-1,3-benzenedicarboxylate, H₂napdc = 1,4-naphthalene dicarboxylic acid and DMI = 1,3-dimethyl-2-imidazolidinone). In compounds 1 and 2, the Co(II) atoms are connected by polycarboxylate ligands to form two-dimensional (2D) layers that are pillared by tib ligands leading to the formation of 3D porous frameworks. In compound 3, the Zn(II) atoms are linked by tib ligands to form one-dimensional ribbon-like chains which are further connected by polycarboxylate ligands, making a 3D framework possible. Compound 1 can selectively adsorb unsaturated hydrocarbon molecules (C₂H₂ and C₂H₄) and saturated hydrocarbon molecules (C₂H₆ and CH₄). Specifically, compound 1 has high IAST selectivity for acetylene and methane (0.50 : 0.50, v/v) at 273 K and 1 bar. DFT calculations reveal that the π-conjugated hexagonal carbon ring may be the primary adsorption site because there are π-π interactions between the unsaturated hydrocarbon molecules (C₂H₂ and C₂H₄) and the π-conjugated hexagonal carbon ring in the framework of compound 1.

Preferential Adsorption Performance of Ethane in a Robust Nickel-Based Metal-Organic Framework for Separating Ethane from Ethylene

Development of an ethane-selective adsorbent to separate ethane from ethylene is a challenging issue with great significance for ethylene purification. The adsorptive separation technique based on physical adsorption holds a great promise to address this issue. Herein, we report a robust ethane-selective metal-organic framework, Ni(BODC)(TED), and investigate its separation performance on C₂H₆/C₂H₄. The as-synthesized Ni(BODC)(TED) exhibits excellent water vapor stability and high capacity of C₂H₆ molecules with an uptake of 3.36 mmol/g at 298 K and 100 kPa, higher than those of many adsorbents reported in recent years. Its C₂H₆/C₂H₄ selectivity predicted by the ideal adsorbed solution theory (IAST) model reaches 1.79. A molecular simulation is applied to unveil the preferential adsorption mechanism of ethane. Calculation shows that five strong C-H···H interactions are formed between C₂H₆ and the framework of Ni(BODC)(TED), and the isosteric heat of ethane on Ni(BODC)(TED) is 27.02 kJ/mol, higher than that of ethylene, resulting in preferential adsorption of ethane. Ni(BODC)(TED) would become a promising member of the family of ethane-selective materials for the industrial separation of ethane from ethylene.

Computational Screening of Metal-Organic Frameworks for Ethylene Purification from Ethane/Ethylene/Acetylene Mixture

Identification of high-performing sorbent materials is the key step in developing energy-efficient adsorptive separation processes for ethylene production. In this work, a computational screening of metal-organic frameworks (MOFs) for the purification of ethylene from the ternary ethane/ethylene/acetylene mixture under thermodynamic equilibrium conditions is conducted. Modified evaluation metrics are proposed for an efficient description of the performance of MOFs for the ternary mixture separation. Two different separation schemes are proposed and potential MOF adsorbents are identified accordingly. Finally, the relationships between the MOF structural characteristics and its adsorption properties are discussed, which can provide valuable information for optimal MOF design.

A Microporous Metal-Organic Framework with Channels Constructed from Nonpolar Aromatic Rings for the Selective Separation of Ethane/Ethylene Mixtures

The separation of ethane and ethylene is an important segment in the purification of chemical raw materials in industrial production. However, due to their similar physical and chemical properties, the separation of C2 H6 /C2 H4 is challenging. Herein, we report the selective adsorption of ethane over ethylene by a microporous metal-organic framework with nonpolar aromatic rings constructed channels, [Co1.5 (TATB)(H2 O)0.5 ] ⋅ 5DMA ⋅ 3H2 O (Co-TATB, H3 TATB=4,4',4''-(s-triazine-2,4,6-triyl) tribenzoic acid). This compound showed a higher ethane capacity than that of ethylene, and a low adsorption enthalpy of ethane only of 19.4 kJ mol-1 . Further, the dynamic breakthrough experimental confirmed that Co-TATB can selectively adsorb ethane from ethane/ethylene 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.

Lanthanide-Organic Frameworks Featuring Three-Dimensional Inorganic Connectivity for Multipurpose Hydrocarbon Separation

Implementation of lanthanide-organic frameworks (LOFs) as solid adsorbents has been frequently handicapped by their permanent porosity being difficult to establish owing to the remarkable flexibility and diversity of lanthanide ions in terms of coordination number and geometry. Construction of robust LOFs with permanent porosity for industrially important hydrocarbon separation will greatly expand their application potential. In this work, by distributing N and O donors into an m-terphenyl skeleton, we rationally synthesized a heterofunctional linker, and constructed a pair of isostructural LOFs. Due to the inclusion of a rarely observed three-dimensional metal-carboxylate backbone serving as a highly connected inorganic secondary building unit, their permanent porosities were successfully established by diverse gas isotherms. They can be applied as separating media not only for natural gas purification and removal of carbon dioxide from C₂ hydrocarbons but also more importantly for single-step ethylene (C₂H₄) purification from a three-component C₂H_n mixture during the adsorption process. The latter separation is very challenging and has been less reported in the literature. This work provides a unique example of LOFs featuring three-dimensional inorganic connectivity applied to multipurpose hydrocarbon 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.

Thermal decarboxylation for the generation of hierarchical porosity in isostructural metal-organic frameworks containing open metal sites

The effect of metal-cluster redox identity on the thermal decarboxylation of a series of isostructural metal-organic frameworks (MOFs) with tetracarboxylate-based ligands and trinuclear μ3-oxo clusters was investigated. The PCN-250 series of MOFs can consist of various metal combinations (Fe3, Fe/Ni, Fe/Mn, Fe/Co, Fe/Zn, Al3, In3, and Sc3). The Fe-based system can undergo a thermally induced reductive decarboxylation, producing a mixed valence cluster with decarboxylated ligand fragments subsequently eliminated to form uniform mesopores. We have extended the analysis to alternative monometallic and bimetallic PCN-250 systems to observe the cluster's effect on the decarboxylation process. Our results suggest that the propensity to undergo decarboxylation is directly related to the cluster redox accessibility, with poorly reducible metals, such as Al, In, and Sc, unable to thermally reduce at the readily accessible temperatures of the Fe-containing system. In contrast, the mixed-metal variants are all reducible. We report improvements in gas adsorption behavior, significantly the uniform increase in the heat of adsorption going from the microporous to hierarchically induced decarboxylated samples. This, along with Fe oxidation state changes from 57Fe Mössbauer spectroscopy, suggests that reduction occurs at the clusters and is essential for mesopore formation. These results provide insight into the thermal behavior of redox-active MOFs and suggest a potential future avenue for generating mesoporosity using controlled cluster redox chemistry.

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.

Octanuclear Cobalt(II) Cluster-Based Metal-Organic Framework with Caged Structure Exhibiting the Selective Adsorption of Ethane over Ethylene

A novel metal-organic framework (MOF) of [Co₈(OH)₄(TCA)₄(H₂O)₄]_n (abbreviation: JXNU-9) based on the unique octanuclear Co₈(μ₃-OH)₄ clusters linked by 4,4',4″-nitrilotribenzoate (TCA^3-) ligands featuring small caged structures and one-dimensional channels was prepared and characterized. JXNU-9 shows a high C₂H₆ uptake capacity of 3.60 mmol g^-1 (4.46 mmol cm^-3) at 298 K and 1 atm with a small isosteric heat of adsorption (23.6 kJ mol^-1) and a moderate C₂H₆/C₂H₄ adsorption selectivity of 1.7, resulting in excellent C₂H₆/C₂H₄ separation performance. The pore walls decorated by plenty of aromatic rings provide π-electron-cloud-surrounding environments to accommodate the large polarizable C₂H₆ molecules. The calculations demonstrate that the rich π-systems in JXNU-9 facilitate an adsorption affinity for large C₂H₆ molecules through multiple C-H···π interactions. Additionally, the open metal sites located in the concave pores with a close Co···Co separation (4.21 Å) in octanuclear Co₈(μ₃-OH)₄ clusters make the open metal sites inaccessible for the C₂H₄ molecule with a kinetic diameter of 4.163 Å. Thus, the annihilation of open metal sites in this structure is achieved, which further facilitates the C₂H₆-selective C₂H₆/C₂H₄ separation.