Two Solvent-Induced In(III)-Based Metal-Organic Frameworks with Controllable Topology Performing High-Efficiency Separation of C2H2/CH4 and CO2/CH4

For pure acetylene manufacturing and natural gas purification, the development of porous materials displaying highly selective C₂H₂/CH₄ and CO₂/CH₄ separation is greatly important but remains a major challenge. In this work, a plausible strategy involving solvent-induced effects and using the flexibility of the ligand conformation to make two In(III) metal-organic frameworks (MOFs) is developed, showing topological diversity and different stability. The X-shaped tetracarboxylic ligand H₄TPTA ([1,1':3',1″-terphenyl]-4,4',4″,6'-tetracarboxylic acid) was selected to construct two new heteroid In MOFs, namely, {[CH₃NH₃][In(TPTA)]·2(NMF)} (MOF 1) and {[In₂(TPTA)(OH)₂]·2(H₂O)·(DMF)} (MOF 2). MOF 1 is a (4, 4)-connected net showing a pts topology with a large channel that is not conducive to fine gas separation. By contrast, with the reduction of SBU from uninucleated In to an {In-OH-In}_n chain, MOF 2 has a (4, 6)-connected net with the fsc topology with an ∼5 Å suitable micropore to confine matching small gas. The permanent porosity of MOF 2 leads to the preferential adsorption of C₂H₂ over CO₂ with superior C₂H₂/CH₄ (332.3) and CO₂/CH₄ (31.2) separation selectivities. Meanwhile, the cycling dynamic breakthrough experiments showed that the high-purity C₂H₂ (>99.8%) capture capacities of MOF 2 were >1.92 mmol g^-1 from a binary C₂H₂/CH₄ mixture, and its separation factor reached 10.

Defective Hierarchical Pore Engineering of a Zn-Ni MOF by Labile Coordination Bonding Modulation

The construction and modulation of hierarchical pore structure in metal-organic frameworks (MOFs) has become a hot topic owing to the advantages of hierarchical pore MOFs (HP-MOFs) in matter storage and mass transfer related applications. Herein, we report the engineering of crystalline defect in a bimetallic MOF for the construction and tuning of HP-MOF. A microporous MOF system showing metal-center-dependent water stability, namely, {[M₃F(bdc)₃ tpt] (solvents)}_n (M = Zn²⁺ and Ni²⁺, H₂bdc = 1,4-benzenedicarboxylic acid, tpt = 2,4,6-tris(4-pyridyl)triazine), was utilized as a platform for the construction of HP-MOF. By tuning the Zn²⁺/Ni²⁺ ratio in the reactant, a bimetallic MOF with a highly tunable Zn²⁺/Ni²⁺ ratio could be obtained. The relatively labile Zn²⁺-based coordination bonding in the bimetallic MOF could be readily and targeted broken through water treatment for the engineering of crystalline defects-based hierarchical pore structure. The resultant HP-MOF reveals a dramatically increased pore volume with the presence of mesopore and macropore. In addition, the anionic framework of HP-MOF could be utilized for the selective adsorption of a cationic dye methylene blue, and a relatively high capacity (250 mg·g^-1, five times compared with the pristine microporous MOF) could be achieved.

Self-Interpenetrated Water-Stable Microporous Metal-Organic Framework toward Storage and Purification of Light Hydrocarbons

Storage and purification of light hydrocarbons are very meaningful for their high-purity requirements and safety utilization in the fields of industry and clean energy. It is a simple and effective way to achieve this goal utilizing the physical adsorption properties of stable porous metal-organic frameworks (MOFs). In this work, a stable self-interpenetrated three-dimensional MOF with a new 3,4-connected topology, {[Zn₂(tpda)₂(4,4'-bpy)]·4DMF}_n (NKM-101; H₂tpda = 4,4'-[4-(4H-1,2,4-triazol-4-yl)phenyl]dibenzoic acid, 4,4'-bpy = 4,4'-bipyridine, and DMF = N,N-dimethylformamide), has been successfully constructed based on a triazole-carboxyl ligand. The dense functional active sites existing on the inner walls of one-dimensional channels of NKM-101 are beneficial to enhancement of the binding affinities between the framework and specific molecules (CO₂, C₂-C₄). Therefore, the selective adsorption and separation performance of the material on CO₂/CH₄ and C₂-C₄/CH₄ are effectively improved. In addition, NKM-101 also exhibits excellent water stability, making it possible to be a practical material for the storage and purification of light hydrocarbons.

MOFs assembled from C3 symmetric ligands: structure, iodine capture and role as bifunctional catalysts towards the oxidation-Knoevenagel cascade reaction

Three new NiII/CoII-metal organic frameworks were self-assembled by the reaction of C3 symmetric 1,3,5-tribenzoic acid (H3BTC) and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (4-TPT) ligands and NiII/CoII salts under solvothermal conditions. Isomorphous MOF1 and MOF2 exhibit a 3D pillar-layer framework based on binuclear M2(OH)(COO)2 units connected by tritopic BTC3- and 4-TPT ligands with a novel (3,5)-connected topology net. MOF3 displays a 3-fold interpenetrated 3D network exhibiting a (3,4)-connected topology net. The porous MOF3 can reversibly take up I2. The activated MOFs contain both Lewis acid (NiII center) and basic (uncoordinated pyridyl or carboxylic groups) sites, and act as bifunctional acid-base catalysts. The catalytic measurements demonstrate that the activated MOF3 exhibits good activities for benzyl alcohol oxidation and the Knoevenagel reaction and can be recycled and reused for at least four cycles without losing its structural integrity and high catalytic activity. Thus, the catalytic properties for the oxidation-Knoevenagel cascade reaction have also been studied.

R-Substituent induced structural diversity, synergistic effect and highly selective luminescence sensing for Fe3+ detection by post-synthetically modified Cd-MOFs

Six Cd₂(OCO)₂ cluster based MOFs involving 5-substituted isophthalic acid and n-TPT ligands have been characterized. Cd-MOF-4 can be functionalized by Eu³⁺ ions by PSMs, which shows highly selective and sensitive to Fe³⁺ ion in aqueous solution.

Rational Construction of Breathing Metal-Organic Frameworks through Synergy of a Stretchy Ligand and Highly Variable π-π Interaction

The synergy of a stretchy ligand and highly variable π-π interaction has been proposed as a rational strategy for the construction of breathing metal-organic frameworks (MOFs). Based on this strategy, a breathing MOF, {[Cd₂(AzDC)₂(TPT)₂](DMF)₃} _n, was successfully constructed with stretchy 4,4'-diazene-1,2-diyldibenzoate acid (H₂AzDC) and 2,4,6-tris(4-pyridyl)triazine (TPT) as a source of the π-π interaction. The MOF features structure transformation upon stimulation with solvent guests and varied temperatures, which is straightforwardly characterized by single-crystal structures. Moreover, the solvent-free framework shows breathing behaviors in response to light hydrocarbon (C₂H₄, C₂H₆, C₃H₆, and C₃H₈) sorption, which was verified by stepwise sorption isotherms and in situ powder X-ray diffraction. Additional investigation of the sorption selectivity of C3/C2 systems indicated that the selectivity can be regulated by the modulation of the dynamic breathing behaviors, which can be used for the selective separation of C3/C2 light hydrocarbons.

Pore Space Partition within a Metal-Organic Framework for Highly Efficient C2H2/CO2 Separation

The pore space partition (PSP) approach has been employed to realize a novel porous MOF (FJU-90) with dual functionalities for the challenging C2H2/CO2 separation under ambient conditions. By virtue of a triangular ligand (Tripp = 2,4,6-tris(4-pyridyl)pyridine), the cylindrical channels in the original FJU-88 have been partitioned into uniformly interconnected pore cavities, leading to the dramatically reduced pore apertures from 12.0 × 9.4 to 5.4 × 5.1 Å2. Narrowing down the pore sizes, the resulting activated FJU-90a takes up a very large amount of C2H2 (180 cm3 g-1) but much less of CO2 (103 cm3 g-1) at 298 K and 1 bar, demonstrating it to be the best porous MOF material for this C2H2/CO2 (50%:50%) separation in terms of the C2H2 gravimetric productivity. IAST calculations, molecular modeling studies, and simulated and experimental breakthrough experiments comprehensively demonstrate that the pore space partition strategy is a very powerful approach to constructing MOFs with dual functionality for challenging gas separation.

Design Strategy for the Controlled Generation of Cationic Frameworks and Ensuing Anion-Exchange Capabilities

Cationic frameworks are an emerging class of exceptional solid adsorbents capable of encapsulating highly toxic and persistent anionic pollutants. The controlled generation of cationic frameworks, however, lags behind the abundant design strategies devised to control the structures and topologies of neutral frameworks. In this regard, we report a rational approach that allows the conversion of the synthetic approach toward constructing a neutral framework into one allowing for the synthesis of a cationic one without incurring any changes to the overall topology or the selected metal ion. We demonstrate that the replacement of a functional group on an organic linker that promotes a similar coordination mode, but bearing one less negative charge, can yield the systematic generation of cationic frameworks. Moreover, we confirm the cationic nature of the metal-organic frameworks through preliminary anion-exchange experiments and propose a method to retain permanent porosity in cationic frameworks through the use of strongly binding anions. Altogether, these results show great promise for the construction of tunable nanoporous frameworks capable of carrying out anion-exchange processes.

Three microporous metal-organic frameworks assembled from dodecanuclear {NiLn} subunits: synthesis, structure, gas adsorption and magnetism

Three isostructural heterometallic metal-organic frameworks (MOFs) {[Ln₂Ni(OAc)₅(HL)(L)]·solvent molecules}_n (H₂L = 2-hydroxyimino-N-[1-(2-pyrazinyl)ethylidene]-propanohydrazone, Ln = Dy for 1, Tb for 2 and Gd for 3) were solvothermally synthesized by varying rare-earth metal ions with different electron configurations. Their crystal structures, gas adsorption and magnetic behaviors were fully investigated. The three isomorphous MOFs exhibit three-dimensional microporous frameworks with two different orientated dodecane metallic {Ni^IILn^III(HL)}₆ metallomacrocycles alternately connected by {Ln^III(L)} connectors, in which an empty one-dimensional channel decorated by the basic hydrazone interior is generated. Due to their Ln^III-independent microporous nature, the activated sample of 1 as a representative example has a significant CO₂ uptake up to 42.2 cm³ g^-1 and an unusually high CO₂/N₂ and CO₂/CH₄ adsorption selectivity of up to 98.8 and 16.8 at 298 K and 100 kPa. Magnetically, apparent antiferromagnetic interactions for both 1 and 2 as well as ferromagnetic coupling for 3 are respectively observed at low temperature resulting from the competition of magnetic anisotropy and intermetallic ferromagnetic superexchange. Additionally, 1 with highly anisotropic Dy^III spin shows slow magnetization relaxation under zero dc field, whereas 3 possessing isotropic Gd^III ions displays a significant cryogenic magnetocaloric effect with a maximum entropy change of 26.6 J kg^-1 K^-1 at 3.0 K and 70 kOe. These interesting results can provide valuable information on gas separation-based multifunctional 3d-4f MOF materials.

Enhancement of CO2 Uptake and Selectivity in a Metal-Organic Framework by the Incorporation of Thiophene Functionality

The complex [Zn2(tdc)2dabco] (H2tdc = thiophene-2,5-dicarboxylic acid; dabco = 1,4-diazabicyclooctane) shows a remarkable increase in carbon dioxide (CO2) uptake and CO2/dinitrogen (N2) selectivity compared to the nonthiophene analogue [Zn2(bdc)2dabco] (H2bdc = benzene-1,4-dicarboxylic acid; terephthalic acid). CO2 adsorption at 1 bar for [Zn2(tdc)2dabco] is 67.4 cm3·g-1 (13.2 wt %) at 298 K and 153 cm3·g-1 (30.0 wt %) at 273 K. For [Zn2(bdc)2dabco], the equivalent values are 46 cm3·g-1 (9.0 wt %) and 122 cm3·g-1 (23.9 wt %), respectively. The isosteric heat of adsorption for CO2 in [Zn2(tdc)2dabco] at zero coverage is low (23.65 kJ·mol-1), ensuring facile regeneration of the porous material. Enhancement by the thiophene group on the separation of CO2/N2 gas mixtures has been confirmed by both ideal adsorbate solution theory calculations and dynamic breakthrough experiments. The preferred binding sites of adsorbed CO2 in [Zn2(tdc)2dabco] have been unambiguously determined by in situ single-crystal diffraction studies on CO2-loaded [Zn2(tdc)2dabco], coupled with quantum-chemical calculations. These studies unveil the role of the thiophene moieties in the specific CO2 binding via an induced dipole interaction between CO2 and the sulfur center, confirming that an enhanced CO2 capacity in [Zn2(tdc)2dabco] is achieved without the presence of open metal sites. The experimental data and theoretical insight suggest a viable strategy for improvement of the adsorption properties of already known materials through the incorporation of sulfur-based heterocycles within their porous structures.

Crystal structure of poly[(μ2-1,4-bis((2-ethyl-1H-benzo[d]imidazol-1-yl)methyl)benzene-κ2 N:N′)-(μ2-4,4′-sulfonyldibenzoato-κ2 O:O′)zinc(II)], C40H34N4O6SZn

C40H34N4O6SZn, triclinic P1̅ (no. 2), a = 11.157(2) Å, b = 11.867(2) Å, c = 14.141(3) Å, α = 75.30(3)°, β = 86.09(3)°, γ = 84.91(3)°, V = 1801.8(6) Å3, Z = 2, R gt(F) = 0.0419, ω R ref(F 2) = 0.1063, T = 293(2) K.

Functionalization of MOFs via a mixed-ligand strategy: enhanced CO2 uptake by pore surface modification

Two isostructural MOFs based on rare quaternary SBUs with a new (3,3,4,6)-connected topology have been assembled, which possess high porosity with highly polar pore systems, and also exhibit high uptake for CO₂ and significant selectivity for CO₂ over CH₄.

Structural diversity and catalytic properties of five Co2(COO)4cluster-based coordination polymers modified with R-isophthalic acid (R = H, NO2, CH3, OH andtBu)

By regulation the substituents on 5-position of isophthalic acid, we isolated five Co-based CPs with structural diversity, which all constructed from the Co₂(COO)₄unit. The catalytic properties for Knoevenagel condensation and selective epoxidation of styrene have also been investigated.

Governing metal–organic frameworks towards high stability

Stability of MOFs is a crucial issue for their practical applications, which might be improved by varying their chemical composition and/or structurally tuning them. Several strategies for enhancing the stability of MOFs were provided.