Benchmark Acetylene Binding Affinity and Separation through Induced Fit in a Flexible Hybrid Ultramicroporous Material

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.

Isoreticular Contraction of Cage-like Metal-Organic Frameworks with Optimized Pore Space for Enhanced C2H2/CO2 and C2H2/C2H4 Separations

The C2H2 separation from CO2 and C2H4 is of great importance yet highly challenging in the petrochemical industry, owing to their similar physical and chemical properties. Herein, the pore nanospace engineering of cage-like mixed-ligand MFOF-1 has been accomplished via contracting the size of the pyridine- and carboxylic acid-functionalized linkers and introducing a fluoride- and sulfate-bridging cobalt cluster, based on a reticular chemistry strategy. Compared with the prototypical MFOF-1, the constructed FJUT-1 with the same topology presents significantly improved C2H2 adsorption capacity, and selective C2H2 separation performance due to the reduced cage cavity size, functionalized pore surface, and appropriate pore volume. The introduction of fluoride- and sulfate-bridging cubane-type tetranuclear cobalt clusters bestows FJUT-1 with exceptional chemical stability under harsh conditions while providing multiple potential C2H2 binding sites, thus rendering the adequate ability for practical C2H2 separation application as confirmed by the dynamic breakthrough experiments under dry and humid conditions. Additionally, the distinct binding mechanism is suggested by theoretical calculations in which the multiple supramolecular interactions involving C-H···O, C-H···F, and other van der Waals forces play a critical role in the selective C2H2 separation.

Optimizing Iodine Enrichment through Induced-Fit Transformations in a Flexible Ag(I)-Organic Framework: From Accelerated Adsorption Kinetics to Record-High Storage Density

Efficient capture and storage of radioactive I2 is a prerequisite for developing nuclear power but remains a challenge. Here, two flexible Ag-MOFs (FJI-H39 and 40) with similar active sites but different pore sizes and flexibility are prepared; both of them can capture I2 with excellent removal efficiencies and high adsorption capacities. Due to the more flexible pores, FJI-H39 not only possesses the record-high I2 storage density among all the reported MOFs but also displays a very fast adsorption kinetic (124 times faster than FJI-H40), while their desorption kinetics are comparable. Mechanistic studies show that FJI-H39 can undergo induced-fit transformations continuously (first contraction then expansion), making the adsorbed iodine species enrich near the Ag(I) nodes quickly and orderly, from discrete I- anion to the dense packing of various iodine species, achieving the very fast adsorption kinetic and the record-high storage density simultaneously. However, no significant structural transformations caused by the adsorbed iodine are observed in FJI-H40. In addition, FJI-H39 has excellent stability/recyclability/obtainability, making it a practical adsorbent for radioactive I2 . This work provides a useful method for synthesizing practical radioactive I2 adsorbents.

Insight into the Gas-Induced Phase Transformations in a 2D Switching Coordination Network via Coincident Gas Sorption and In Situ PXRD

Switching coordination networks (CNs) that reversibly transform between narrow or closed pore (cp) and large pore (lp) phases, though fewer than their rigid counterparts, offer opportunities for sorption-related applications. However, their structural transformations and switching mechanisms remain underexplored at the molecular level. In this study, we conducted a systematic investigation into a 2D switching CN, [Ni(bpy)2(NCS)2]n, sql-1-Ni-NCS (1 = bpy = 4,4'-bipyridine), using coincident gas sorption and in situ powder X-ray diffraction (PXRD) under low-temperature conditions. Gas adsorption measurements revealed that C2H4 (169 K) and C2H6 (185 K) exhibited single-step type F-IVs sorption isotherms with sorption uptakes of around 180-185 cm3 g-1, equivalent to four sorbate molecules per formula unit. Furthermore, parallel in situ PXRD experiments provided insight into sorbate-dependent phase switching during the sorption process. Specifically, CO2 sorption induced single-step phase switching (path I) solely between cp and lp phases consistent with the observed single-step type F-IVs sorption isotherm. By contrast, intermediate pore (ip) phases emerged during C2H4 and C2H6 desorption as well as C3H6 adsorption, although they remained undetectable in the sorption isotherms. To our knowledge, such a cp-lp-ip-cp transformation (path II) induced by C2H4/6 and accompanied by single-step type F-IVs sorption isotherms represents a novel type of phase transition mechanism in switching CNs. By virtue of Rietveld refinements and molecular simulations, we elucidated that the phase transformations are governed by cooperative local and global structural changes involving NCS- ligand reorientation, bpy ligand twist and rotation, cavity edge (Ni-bpy-Ni) deformation, and interlayer expansion and sliding.

Interaction-selective molecular sieving adsorbent for direct separation of ethylene from senary C2-C4 olefin/paraffin mixture

Olefin/paraffin separations are among the most energy-intensive processes in the petrochemical industry, with ethylene being the most widely consumed chemical feedstock. Adsorptive separation utilizing molecular sieving adsorbents can optimize energy efficiency, whereas the size-exclusive mechanism alone cannot achieve multiple olefin/paraffin sieving in a single adsorbent. Herein, an unprecedented sieving adsorbent, BFFOUR-Cu-dpds (BFFOUR = BF4-, dpds = 4,4'-bipyridinedisulfide), is reported for simultaneous sieving of C2-C4 olefins from their corresponding paraffins. The interlayer spaces can be selectively opened through stronger guest-host interactions induced by unsaturated C = C bonds in olefins, as opposed to saturated paraffins. In equimolar six-component breakthrough experiments (C2H4/C2H6/C3H6/C3H8/n-C4H8/n-C4H10), BFFOUR-Cu-dpds can simultaneously divide olefins from paraffins in the first column, while high-purity ethylene ( > 99.99%) can be directly obtained through the subsequent column using granular porous carbons. Moreover, gas-loaded single-crystal analysis, in-situ infrared spectroscopy measurements, and computational simulations demonstrate the accommodation patterns, interaction bonds, and energy pathways for olefin/paraffin separations.

Water vapour induced structural flexibility in a square lattice coordination network

Herein, we introduce a new square lattice topology coordination network, sql-(1,3-bib)(ndc)-Ni, with three types of connection and detail its gas and vapour induced phase transformations. Exposure to humidity resulted in an S-shaped isotherm profile, suggesting potential utility of such materials as desiccants.

Optimizing Sieving Effect for CO2 Capture from Humid Air Using an Adaptive Ultramicroporous Framework

Excessive CO2 in the air can not only lead to serious climate problems but also cause serious damage to humans in confined spaces. Here, a novel metal-organic framework (FJI-H38) with adaptive ultramicropores and multiple active sites is prepared. It can sieve CO2 from air with the very high adsorption capacity/selectivity but the lowest adsorption enthalpy among the reported physical adsorbents. Such excellent adsorption performances can be retained even at high humidity. Mechanistic studies show that the polar ultramicropore is very suitable for molecular sieving of CO2 from N2 , and the distinguishable adsorption sites for H2 O and CO2 enable them to be co-adsorbed. Notably, the adsorbed-CO2 -driven pore shrinkage can further promote CO2 capture while the adsorbed-H2 O-induced phase transitions in turn inhibit H2 O adsorption. Moreover, FJI-H38 has excellent stability and recyclability and can be synthesized on a large scale, making it a practical trace CO2 adsorbent. This will provide a new strategy for developing practical adsorbents for CO2 capture from the air.

Crossover Sorption of C2 H2 /CO2 and C2 H6 /C2 H4 in Soft Porous Coordination Networks

Porous sorbents are materials that are used for various applications, including storage and separation. Typically, the uptake of a single gas by a sorbent decreases with temperature, but the relative affinity for two similar gases does not change. However, in this study, we report a rare example of "crossover sorption," in which the uptake capacity and apparent affinity for two similar gases reverse at different temperatures. We synthesized two soft porous coordination polymers (PCPs), [Zn2 (L1)(L2)2 ]n (PCP-1) and [Zn2 (L1)(L3)2 ]n (PCP-2) (L1= 1,4-bis(4-pyridyl)benzene, L2=5-methyl-1,3-di(4-carboxyphenyl)benzene, and L3=5-methoxy-1,3-di(4-carboxyphenyl)benzene). These PCPs exhibits structural changes upon gas sorption and show the crossover sorption for both C2 H2 /CO2 and C2 H6 /C2 H4 , in which the apparent affinity reverse with temperature. We used in situ gas-loading single-crystal X-ray diffraction (SCXRD) analysis to reveal the guest inclusion structures of PCP-1 for C2 H2 , CO2 , C2 H6 , and C2 H4 gases at various temperatures. Interestingly, we observed three-step single-crystal to single-crystal (sc-sc) transformations with the different loading phases under these gases, providing insight into guest binding positions, nature of host-guest or guest-guest interactions, and their phase transformations upon exposure to these gases. Combining with theoretical investigation, we have fully elucidated the crossover sorption in the flexible coordination networks, which involves a reversal of apparent affinity and uptake of similar gases at different temperatures. We discovered that this behaviour can be explained by the delicate balance between guest binding and host-guest and guest-guest interactions.

Interlayer Symmetry Control in Flexible-Robust Layered Metal-Organic Frameworks for Highly Efficient C2H2/CO2 Separation

Removal of the CO2 impurities from C2H2/CO2 mixtures is an essential process to produce high-purity C2H2. Fabricating an adsorbent capable of discriminating these species, which have close kinetic diameters, is critical for developing advanced adsorption processes. Herein, we demonstrate a strategy to exploit the tunability of interlayer and intralayer spaces of two-dimensional (2D) layered metal-organic frameworks to achieve high performance for C2H2/CO2 separation. This indicates that interlayer symmetrical control can achieve more efficient packing of C2H2 into Ni(4-DPDS)2CrO4, with a high C2H2 capacity of 45.7 cm3·g-1 at 0.01 bar and a selectivity of 67.7 (298 K, 1 bar), which strikes a good balance between working capacity and separation selectivity compared to other isostructural Ni(4-DPDS)2MO4 (M = Mo, W). Crystallographic studies and DFT-D calculations reveal that such a C2H2-selective adsorbent possesses strong binding interactions due to the tailored pore confinement provided by the angular anions and rich electronic environment. Experimental breakthrough results comprehensively demonstrate the efficient C2H2/CO2 separation performance of this unique material.

Multivariate Metal-Organic Frameworks Prepared by Simultaneous Metal/Ligand Exchange for Enhanced C2-C3 Selective Recovery from Natural Gas

Recovering light alkanes from natural gas is a critical but challenging process in petrochemical production. Herein, we propose a postmodification strategy via simultaneous metal/ligand exchange to prepare multivariate metal-organic frameworks with enhanced capacity and selectivity of ethane (C2H6) and propane (C3H8) for their recovery from natural gas with methane (CH4) as the primary component. By utilizing the Kuratowski-type secondary building unit of CFA-1 as a scaffold, namely, {Zn5(OAc)4}6+, the Zn2+ metal ions and OAc- ligands were simultaneously exchanged by other transition metal ions and halogen ligands under mild conditions. Inspiringly, this postmodification treatment can give rise to improved capacity for C2H6 and C3H8 without a noticeable increase in CH4 uptake, and consequently, it resulted in significantly enhanced selectivity toward C2H6/CH4 and C3H8/CH4. In particular, by adjusting the species and amount of the modulator, the optimal sample CFA-1-NiCl2-2.3 demonstrated the maximum capacities of C2H6 (5.00 mmol/g) and C3H8 (8.59 mmol/g), increased by 29 and 32% compared to that of CFA-1. Moreover, this compound exhibited excellent separation performance toward C2H6/CH4 and C3H8/CH4, with high uptake ratios of 6.9 and 11.9 at 298 K and 1 bar, respectively, superior to the performance of a majority of the reported MOFs. Molecular simulations were applied to unravel the improved separation mechanism of CFA-1-NiCl2-2.3 toward C2H6/CH4 and C3H8/CH4. Furthermore, remarkable thermal/chemical robustness, moderate isosteric heat, and fully reproducible breakthrough experiments were confirmed on CFA-1-NiCl2-2.3, indicating its great potential for light alkane recovery from natural gas.

The Effect of Pendent Groups upon Flexibility in Coordination Networks with Square Lattice Topology

Gas or vapor-induced phase transformations in flexible coordination networks (CNs) offer the potential to exceed the performance of their rigid counterparts for separation and storage applications. However, whereas ligand modification has been used to alter the properties of such stimulus-responsive materials, they remain understudied compared with their rigid counterparts. Here, we report that a family of Zn2+ CNs with square lattice (sql) topology, differing only through the substituents attached to a linker, exhibit variable flexibility. Structural and CO2 sorption studies on the sql networks, [Zn(5-Ria)(bphy)]n, ia = isophthalic acid, bphy = 1,2-bis(pyridin-4-yl)hydrazine, R = -CH3, -OCH3, -C(CH3)3, -N=N-Ph, and -N=N-Ph(CH3)2, 2-6, respectively, revealed that the substituent moieties influenced both structural and gas sorption properties. Whereas 2-3 exhibited rigidity, 4, 5, and 6 exhibited reversible transformation from small pore to large pore phases. Overall, the insight into the profound effect of pendent moieties of linkers upon phase transformations in this family of layered CNs should be transferable to other CN classes.

Programmed fluorine binding engineering in anion-pillared metal-organic framework for record trace acetylene capture from ethylene

Porous physisorbents are attractive candidates for selective capture of trace gas or volatile compounds due to their low energy footprints. However, many physisorbents suffer from insufficient sorbate-sorbent interactions, resulting in low uptake or inadequate selectivity when gases are present at trace levels. Here, we report a strategy of programmed fluorine binding engineering in anion-pillared metal-organic frameworks to maximize C2H2 binding affinity for benchmark trace C2H2 capture from C2H4. A robust material (ZJU-300a) was elaborately designed to provide multiple-site fluorine binding model, resulting in an ultrastrong C2H2 binding affinity. ZJU-300a exhibits a record-high C2H2 uptake of 3.23 millimoles per gram (at 0.01 bar and 296 kelvin) and one of the highest C2H2/C2H4 selectivity (1672). The adsorption binding of C2H2 and C2H4 was visualized by gas-loaded ZJU-300a structures. The separation capacity was confirmed by breakthrough experiments for 1/99 C2H2/C2H4 mixtures, affording the maximal dynamic selectivity (264) and C2H4 productivity of 436.7 millimoles per gram.

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.

Control over Phase Transformations in a Family of Flexible Double Diamondoid Coordination Networks through Linker Ligand Substitution

In this work, we present the first metal-organic framework (MOF) platform with a self-penetrated double diamondoid (ddi) topology that exhibits switching between closed (nonporous) and open (porous) phases induced by exposure to gases. A crystal engineering strategy, linker ligand substitution, was used to control gas sorption properties for CO₂ and C3 gases. Specifically, bimbz (1,4-bis(imidazol-1-yl)benzene) in the coordination network X-ddi-1-Ni ([Ni₂(bimbz)₂(bdc)₂(H₂O)]_n, H₂bdc = 1,4-benzenedicarboxylic acid) was replaced by bimpz (3,6-bis(imidazol-1-yl)pyridazine) in X-ddi-2-Ni ([Ni₂(bimpz)₂(bdc)₂(H₂O)]_n). In addition, the 1:1 mixed crystal X-ddi-1,2-Ni ([Ni₂(bimbz)(bimpz)(bdc)₂(H₂O)]_n) was prepared and studied. All three variants form isostructural closed (β) phases upon activation which each exhibited different reversible properties upon exposure to CO₂ at 195 K and C3 gases at 273 K. For CO₂, X-ddi-1-Ni revealed incomplete gate-opening, X-ddi-2-Ni exhibited a stepped isotherm with saturation uptake of 3.92 mol·mol^-1, and X-ddi-1,2-Ni achieved up to 62% more gas uptake and a distinct isotherm shape vs the parent materials. Single-crystal X-ray diffraction (SCXRD) and in situ powder X-ray diffraction (PXRD) experiments provided insight into the mechanisms of phase transformation and revealed that the β phases are nonporous with unit cell volumes 39.9, 40.8, and 41.0% lower than the corresponding as-synthesized α phases, X-ddi-1-Ni-α, X-ddi-2-Ni-α, and X-ddi-1,2-Ni-α, respectively. The results presented herein represent the first report of reversible switching between closed and open phases in ddi topology coordination networks and further highlight how ligand substitution can profoundly impact the gas sorption properties of switching sorbents.

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 .

Selective sorption of oxygen and nitrous oxide by an electron donor-incorporated flexible coordination network

Incorporating strong electron donor functionality into flexible coordination networks is intriguing for sorption applications due to a built-in mechanism for electron-withdrawing guests. Here we report a 2D flexible porous coordination network, [Ni₂(4,4'-bipyridine)(VTTF)₂]n(1) (where H₂VTTF = 2,2'-[1,2-bis(4-benzoic acid)-1,2ethanediylidene]bis-1,3-benzodithiole), which exhibits large structural deformation from the as-synthesized or open phase (1α) into the closed phase (1β) after guest removal, as demonstrated by X-ray and electron diffraction. Interestingly, upon exposure to electron-withdrawing species, 1β reversibly undergoes guest accommodation transitions; 1α⊃O₂ (90 K) and 1α⊃N₂O (185 K). Moreover, the 1β phase showed exclusive O₂ sorption over other gases (N₂, Ar, and CO) at 120 K. The phase transformations between the 1α and 1β phases under these gases were carefully investigated by in-situ X-ray diffraction, in-situ spectroscopic studies, and DFT calculations, validating that the unusual sorption was attributed to the combination of flexible frameworks and VTTF (electron-donor) that induces strong interactions with electron-withdrawing species.

A Robust Molecular Porous Material for C2 H2 /CO2 Separation

A molecular porous material, MPM-2, comprised of cationic [Ni₂ (AlF₆ )(pzH)₈ (H₂ O)₂ ] and anionic [Ni₂ Al₂ F₁₁ (pzH)₈ (H₂ O)₂ ] complexes that generate a charge-assisted hydrogen-bonded network with pcu topology is reported. The packing in MPM-2 is sustained by multiple interionic hydrogen bonding interactions that afford ultramicroporous channels between dense layers of anionic units. MPM-2 is found to exhibit excellent stability in water (>1 year). Unlike most hydrogen-bonded organic frameworks which typically show poor stability in organic solvents, MPM-2 exhibited excellent stability with respect to various organic solvents for at least two days. MPM-2 is found to be permanently porous with gas sorption isotherms at 298 K revealing a strong affinity for C₂ H₂ over CO₂ thanks to a high (ΔQ_st )_AC [Q_st (C₂ H₂ ) - Q_st (CO₂ )] of 13.7 kJ mol^-1 at low coverage. Dynamic column breakthrough experiments on MPM-2 demonstrated the separation of C₂ H₂ from a 1:1 C₂ H₂ /CO₂ mixture at 298 K with effluent CO₂ purity of 99.995% and C₂ H₂ purity of >95% after temperature-programmed desorption. C-H···F interactions between C₂ H₂ molecules and F atoms of AlF₆ ^3- are found to enable high selectivity toward C₂ H₂ , as determined by density functional theory simulations.

Optimizing Acetylene Sorption through Induced-fit Transformations in a Chemically Stable Microporous Framework

Developing practical storage technologies for acetylene (C₂ H₂ ) is important but challenging because C₂ H₂ is useful but explosive. Here, a novel metal-organic framework (MOF) (FJI-H36) with adaptive channels was prepared. It can effectively capture C₂ H₂ (159.9 cm³  cm^-3 ) at 1 atm and 298 K, possessing a record-high storage density (561 g L^-1 ) but a very low adsorption enthalpy (28 kJ mol^-1 ) among all the reported MOFs. Structural analyses show that such excellent adsorption performance comes from the synergism of active sites, flexible framework, and matched pores; where the adsorbed-C₂ H₂ can drive FJI-H36 to undergo induced-fit transformations step by step, including deformation/reconstruction of channels, contraction of pores, and transformation of active sites, finally leading to dense packing of C₂ H₂ . Moreover, FJI-H36 has excellent chemical stability and recyclability, and can be prepared on a large scale, enabling it as a practical adsorbent for C₂ H₂ . This will provide a useful strategy for developing practical and efficient adsorbents for C₂ H₂ storage.

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.

On the Role of Flexibility for Adsorptive Separation

Chemical separations, mostly based on heat-driven techniques such as distillation, account for a large portion of the world's energy consumption. In principle, differential adsorption is a more energy-efficient separation method, but conventional adsorbent materials are still not effective for many industry-relevant mixtures. Porous coordination polymers (PCPs), or metal-organic frameworks (MOFs), are attractive for their well-defined, designable, modifiable, and flexible structures connecting to various potential applications. While the importance of the structural flexibility of MOFs in adsorption-based functions has been demonstrated, the understanding of this special feature is still in its infancy and mostly stays at the periodic structural transformation at the equilibrium state and the special shapes of single-component adsorption isotherms. There are many confusions about the categorization and roles of various types of flexibility. This Account discusses the role of flexibility of MOFs for adsorptive separation, mainly from the thermodynamic and kinetic points of view.As the classic type of framework flexibility, guest-driven structural transformations and the corresponding adsorption isotherms can be thermodynamically described by the energies of the host-guest system. The highly guest-specific pore-opening action showing contrasting single-component adsorption isotherms is regarded as a strategy for achieving molecular sieving without the need for aperture size control, but its effect and role for mixture separation are still controversial. Quantitative mixture adsorption/separation experiments showed that the common periodic (cooperative) pore-opening action leads to coadsorption of molecules smaller than the opened aperture, while the aperiodic (noncooperative) one can achieve inversed molecular sieving under a thermodynamic mechanism.The energy barrier and structure in the nonequilibrium state are also important for flexibility and adsorption/separation. With suitable energy barriers between metastable structures, new types of framework flexibility such as aperture gating can be realized. While kinetically controlled gating flexibility is usually ignored because of the difficulty of characterization or considered as disadvantageous for separation because of the variable aperture size, it plays a critical role in most kinetic separation systems, including adsorbents conventionally regarded as rigid. With the concept of gating flexibility, the meanings of aperture and guest sizes for judging molecular sieving need to be reconsidered. Gating flexibility depends on not only the host itself but also the guest, the host-guest interaction, and the external environment such as temperature, which can be rationally tuned to achieve special adsorption/separation behaviors such as inversed temperature dependence, molecular sieving, and even inversed thermodynamic selectivity. The comprehensive understanding of the thermodynamic and kinetic bases of flexibility will give a new horizon for next-generation separation materials beyond MOFs and adsorbents.

Fluorinated metal-organic frameworks for gas separation

Fluorinated metal-organic frameworks (F-MOFs) as fast-growing porous materials have revolutionized the field of gas separation due to their tunable pore apertures, appealing chemical features, and excellent stability. A deep understanding of their structure-performance relationships is critical for the synthesis and development of new F-MOFs. This critical review has focused on several strategies for the precise design and synthesis of new F-MOFs with structures tuned for specific gas separation purposes. First, the basic principles and concepts of F-MOFs as well as their structure, synthesis and modification and their structure to property relationships are studied. Then, applications of F-MOFs in adsorption and membrane gas separation are discussed. A detailed account of the design and capabilities of F-MOFs for the adsorption of various gases and the governing principles is provided. In addition, the exceptional characteristics of highly stable F-MOFs with engineered pore size and tuned structures are put into perspective to fabricate selective membranes for gas separation. Systematic analysis of the position of F-MOFs in gas separation revealed that F-MOFs are benchmark materials in most of the challenging gas separations. The outlook and future directions of the science and engineering of F-MOFs and their challenges are highlighted to tackle the issues of overcoming the trade-off between capacity/permeability and selectivity for a serious move towards industrialization.

Pore-Structure Control in Metal-Organic Frameworks (MOFs) for Capture of the Greenhouse Gas SF6 with Record Separation

In the electronics industry, the efficient recovery and capture of sulfur hexafluoride (SF₆ ) from SF₆ /N₂ mixtures is of great importance. Herein, three metal-organic frameworks with fine-tuning pore structures, Cu(peba)₂ , Ni(pba)₂ , and Ni(ina)₂ , were designed for SF₆ capture. Among them, Ni(ina)₂ has perfect pore sizes (6 Å) that are comparable to the kinetic diameter of sulfur hexafluoride (5.2 Å), affording the benchmark binding affinity for SF₆ gas. Ni(ina)₂ exhibits the highest SF₆ /N₂ selectivity (375.1 at 298 K and 1 bar) and ultra-high SF₆ uptake capacity (53.5 cm³  g^-1 at 298 K and 0.1 bar) at ambient conditions. The remarkable separation performance of Ni(ina)₂ was verified by dynamic breakthrough experiments. Theoretical calculations and the SF₆ -loaded single-crystal structure provided critical insight into the adsorption/separation mechanism. This porous coordination network has the potential to be used in industrial applications.

Two-Dimensional Metal-Organic Framework with Ultrahigh Water Stability for Separation of Acetylene from Carbon Dioxide and Ethylene

Highly selective separation and purification of acetylene (C2H2) from ethylene (C2H4) and carbon dioxide (CO2) are daunting challenges in light of their similar molecule sizes and physical properties. Herein, we report a two-dimensional (2D) stable metal-organic framework (MOF), NUM-11 ([Cu(Hmpba)2]·1.5DMF) (H2mpba = 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid), with sql topology, stacked together through π-π interactions for efficient separation of C2H2 from C2H4 and CO2. The 2D-MOF material offers high hydrolytic stability and good purification capacity; especially, it could survive in water for 7 months, even longer. This stable MOF selectively captures C2H2 from mixtures containing C2H4 and CO2, as determined by adsorption isotherms. The ideal adsorbed solution theory selectivity calculations and transient breakthrough experiments were performed to verify the separation capacity. The low isosteric heat of NUM-11a (desolvated NUM-11) (18.24 kJ mol-1 for C2H2) validates the feasibility of adsorbent regeneration with low energy footprint consumption. Furthermore, Grand Canonical Monte Carlo simulations confirmed that the pore surface of the NUM-11 framework enabled preferential binding of C2H2 over C2H4 and CO2 via multiple C-H···O, C-H···π, and C-H···C interactions. This work provides some insights to prepare stable MOF materials toward the purification of C2H2, and the water-stable structure, low isosteric heat, and good cycling stability of NUM-11 make it very promising for practical industrial application.

Robust and Radiation-Resistant Hofmann-Type Metal-Organic Frameworks for Record Xenon/Krypton Separation

The discovery of high-performance adsorbents for highly efficient separation of xenon from krypton is an important but challenging task in the chemical industry due to their similar size and inert spherical nature. Herein, we report two robust and radiation-resistant Hofmann-type MOFs, Co(pyz)[Ni(CN)₄] and Co(pyz)[Pd(CN)₄] (termed as ZJU-74a-Ni and ZJU-74a-Pd), featuring oppositely adjacent open metal sites and perfect pore sizes (4.1 and 3.8 Å) comparable to the kinetic diameter of xenon (4.047 Å), affording the benchmark binding affinity for polarizable Xe gas. These materials thus exhibit both record-high Xe uptake capacities (89.3 and 98.4 cm³ cm^-3 at 296 K and 0.2 bar) and Xe/Kr selectivities (74.1 and 103.4) at ambient conditions, all of which are the highest among all the state-of-the-art materials reported so far. The locations of Xe molecules within ZJU-74a-Ni have been visualized by single-crystal X-ray diffraction studies, in which two oppositely adjacent metal centers combined with the right aperture size can construct a unique sandwich-like binding site to offer unprecedented and ultrastrong Ni²⁺-Xe-Ni²⁺ interactions with xenon, thus leading to the record Xe capture capacity and selectivity. The excellent separation capacity of ZJU-74a-Pd was verified by breakthrough experiments for Xe/Kr gas mixtures, providing both unprecedentedly high xenon uptake capacity (4.63 mmol cm^-3) and krypton productivity (214 cm³ g^-1).

Immobilization of Lewis Basic Sites into a Stable Ethane-Selective MOF Enabling One-Step Separation of Ethylene from a Ternary Mixture

Purification of C₂H₄ from a ternary C₂H₂/C₂H₆/C₂H₄ mixture by one-step adsorption separation is of prime importance but challenging in the petrochemical industry; however, effective strategies to design high-performance adsorbents are lacking. We herein report for the first time the incorporation of Lewis basic sites into a C₂H₆-selective MOF, enabling efficient one-step production of polymer-grade C₂H₄ from ternary mixtures. Introduction of amino groups into highly stable C₂H₆-selective UiO-67 can not only partition large pores into smaller cagelike pockets to provide suitable pore confinement but also offer additional binding sites to simultaneously enhance C₂H₂ and C₂H₆ adsorption capacities over C₂H₄. The amino-functionalized UiO-67-(NH₂)₂ thus exhibits exceptionally high C₂H₂ and C₂H₆ uptakes as well as benchmark C₂H₂/C₂H₄ and C₂H₆/C₂H₄ selectivities, surpassing all of the C₂H₂/C₂H₆-selective materials reported so far. Theoretical calculations combined with in situ infrared spectroscopy indicate that the synergetic effect of suitable pore confinement and functional surfaces decorated with amino groups provides overall stronger multipoint van der Waals interactions with C₂H₂ and C₂H₆ over C₂H₄. The exceptional performance of UiO-67-(NH₂)₂ was evidenced by breakthrough experiments for C₂H₂/C₂H₆/C₂H₄ mixtures under dry and wet conditions, providing a remarkable C₂H₄ productivity of 0.55 mmol g^-1 at ambient conditions.

Hierarchical Co/CoO/FeO/C nanocomplex derived from Co(OH)2@NH2-MIL-88 to aid highly efficient microwave absorption

Facile preparation of a flake-like Co/CoO/FeO/C nanocomplex employing Co(OH)2 on NH2-MIL-88B through one-step pyrolysis. The nanocomplex can efficiently scatter microwaves.

Dense Packing of Acetylene in a Stable and Low-Cost Metal-Organic Framework for Efficient C2 H2 /CO2 Separation

Porous materials for C₂ H₂ /CO₂ separation mostly suffer from high regeneration energy, poor stability, or high cost that largely dampen their industrial implementation. A desired adsorbent should have an optimal balance between excellent separation performance, high stability, and low cost. We herein report a stable, low-cost, and easily scaled-up aluminum MOF (CAU-10-H) for highly efficient C₂ H₂ /CO₂ separation. The suitable pore confinement in CAU-10-H can not only provide multipoint binding interactions with C₂ H₂ but also enable the dense packing of C₂ H₂ inside the pores. This material exhibits one of the highest C₂ H₂ storage densities of 392 g L^-1 and highly selective adsorption of C₂ H₂ over CO₂ at ambient conditions, achieved by a low C₂ H₂ adsorption enthalpy (27 kJ mol^-1 ). Breakthrough experiments confirm its exceptional separation performance for C₂ H₂ /CO₂ mixtures, affording both large C₂ H₂ uptake of 3.3 mmol g^-1 and high separation factor of 3.4. CAU-10-H achieves the benchmark balance between separation performance, stability, and cost for C₂ H₂ /CO₂ separation.

Highly Selective Adsorption of Carbon Dioxide over Acetylene in an Ultramicroporous Metal-Organic Framework

Separating carbon dioxide from fuel gases like hydrocarbons by physical adsorbents is industrially important and more energy-efficient than traditional liquid extraction or cryogenic distillation methods. It is very important while very challenging to develop CO₂ -selective adsorbents, considering CO₂ is less polarizable than light hydrocarbon molecules, particularly those simultaneously with almost identical molecular dimensions and physical properties, such as acetylene. Herein, an ultramicroporous metal-organic framework constructed from copper(II) and 5-fluoropyrimidin-2-olate, termed Cu-F-pymo, is carefully studied under different activations for inverse separation of CO₂ from C₂ H₂ . The partially desolvated Cu-F-pymo can exclusively capture CO₂ over C₂ H₂ with very high selectivity exceeding 10⁵ under ambient conditions, the highest ever reported. Sorption experiments and modeling studies reveal that such molecular sieving effect is attributed to the suppression of C₂ H₂ adsorption from the blockage of the preferential sites for C₂ H₂ by residual water molecules. The inverse separation is further confirmed by column breakthrough studies given that highly pure acetylene (>99.9%) can be directly harvested from the gas mixture. Cu-F-pymo also shows remarkable stability under harsh conditions.