Ultramicroporous iron-isonicotinate MOFs combining size-exclusion kinetics and thermodynamics for efficient CO2/N2 gas separation

Mixed-valence metal-organic frameworks: concepts, opportunities, and prospects

Owing to increasing global demand for the development of multifunctional advanced materials with various practical applications, great attention has been paid to metal-organic frameworks due to their unique properties, such as structural, chemical, and functional diversity. Several strategies have been developed to promote the applicability of these materials in practical fields. The induction of mixed-valency is a promising strategy, contributing to exceptional features in these porous materials such as enhanced charge delocalization, conductivity, magnetism, etc. The current review provides a detailed study of mixed-valence MOFs, including their fundamental properties, synthesis challenges, and characterization methods. The outstanding applicability of these materials in diverse fields such as energy storage, catalysis, sensing, gas sorption, separation, etc. is also discussed, providing a roadmap for future design strategies to exploit mixed valency in advanced materials. Interestingly, mixed-valence MOFs have demonstrated fascinating features in practical fields compared to their homo-valence MOFs, resulting from an enhanced synergy between mixed-valence states within the framework.

Recent Advancements in Electrochemical Sensors Based on MOFs and Their Derivatives

Metal-organic frameworks (MOFs) are composed of metal nodes and organic linkers that can self-assemble into an infinite network. The high porosity and large surface area of MOFs facilitate the effective enrichment and mass transfer of analytes, which can enhance the signal response and improve the sensitivity of electrochemical sensors. Additionally, MOFs and their derivatives possess the properties of unsaturated metal sites and tunable structures, collectively demonstrating their potential for electrochemical sensing. This paper summarizes the preparation methods, structural properties, and applications of MOFs and their derivatives in electrochemical sensing, emphasizing sensors' selectivity and sensitivity from the perspectives of direct and indirect detection. Additionally, it also explores future directions and prospects for MOFs in electrochemical sensing, with the aim of overcoming current limitations through innovative approaches.

A polymeric form of basic iron(III) acetate with an acetic acid ligand

A new crystalline compound, catena-poly[hexa-μ-acetato-(acetic acid)-μ3-oxido-triangulo-triiron(III)]-μ-acetato], [Fe3(C2H3O2)7O(C2H4O2)]n, incorporating the basic ferric acetate unit, has been obtained from an acetic anhydride solution of hydrated iron(III) nitrate. The crystals have the composition Fe3O(OAc)7(HOAc) (HOAc is acetic acid) and include the well-known [Fe3O(OAc)6]+ unit, in which the FeIII centres are linked to a central coplanar μ3-oxido ligand. Acetate ions provide bridges between pairs of FeIII centres. These individual [Fe3O(OAc)6]+ units are linked by additional bridging acetate anions to form zigzag chains. The bridging acetate ions coordinate to a position trans to the oxido group on two of the FeIII centres. Remarkably, the trans site on the third FeIII centre is occupied by the carbonyl group of an acetic acid molecule. This is the first reported case of an acetic acid molecule coordinating to an FeIII centre. Not surprisingly, the acetic acid molecule is only weakly coordinating, resulting in a short Fe-O(oxido) bond trans to the carbonyl group. The trans influence apparent in this structure provides an interesting contrast with the structurally similar MnIII analogue, in which the corresponding pair of trans bonds are both elongated because of the Jahn-Teller effect.

Recent Advances of Carbon Capture in Metal-Organic Frameworks: A Comprehensive Review

The excessive emission of greenhouse gases, which leads to global warming and alarms the world, has triggered a global campaign for carbon neutrality. Carbon capture and sequestration (CCS) technology has aroused wide research interest as a versatile emission mitigation technology. Metal-organic frameworks (MOFs), as a new class of high-performance adsorbents, hold great potential for CO2 capture from large point sources and ambient air due to their ultra-high specific surface area as well as pore structure. In recent years, MOFs have made great progress in the field of CO2 capture and separation, and have published a number of important results, which have greatly promoted the development of MOF materials for practical carbon capture applications. This review summarizes the most recent advanced research on MOF materials for carbon capture in various application scenarios over the past six years. The strategies for enhancing CO2 selective adsorption and separation of MOFs are described in detail, along with the development of MOF-based composites. Moreover, this review also systematically summarizes the highly concerned issues of MOF materials in practical applications of carbon capture. Finally, future research on CO2 capture by MOF materials is prospected.

Selective Separation of C8 Aromatics by an Interpenetrating Metal-Organic Framework Material

O-xylene (OX) is an important chemical raw material, but it is often produced in mixtures with other C8 aromatics. Similar physicochemical properties of the C8 isomers make their separation and purification very difficult and energy intensive. There is an unmet need for an adsorbent that would be effective for the separation of OX from the other C8 isomers. This work reports a three-dimensional interpenetrated metal-organic framework, SYUCT-110, that interacts with each of the single-component C8 isomers to form. The selectivity of C8 aromatic hydrocarbons was determined through liquid-phase batch uptake experiments. The results revealed that the selectivity order was OX > PX > MX > ethylbenzene (EB). The selectivity values were found to be 2.63, 1.58, 5.51, 3.71, 1.86, and 3.02 for OX/MX, OX/PX, OX/EB, PX/MX, MX/EB, and PX/EB, respectively. The adsorption capacity of OX was 71 mg/g. Grand Canonical Monte Carlo simulations were used to study the C8 adsorption sites, revealing that π···π interactions are the main reason for the observed adsorption selectivity. The adsorption energy calculation results also verified the selectivity of SYUCT-110 for the synthesis of OX.

Relay Adsorption in Metal-Organic Frameworks for One-Step Helium Purification at Ambient Temperature

One-step He purification from natural gas represents a crucial solution for addressing the global He shortages. The prevailing method to produce high-grade He involves cryogenic distillation and ultralow temperature adsorption processes, which is highly cost- and energy-intensive. Separating and purifying He at ambient temperature is a great challenge because the fundamental limitation lies in the boiling point, polarizability, and kinetic diameters of CH4/N2/He gases. In this study, we seek to implement a relay adsorption strategy using Ni(ina)2 and MIL-100(Cr) metal-organic frameworks (MOFs) to produce high-purity He from ternary mixtures (CH4, N2, and He) at ambient temperature. The CH4/He selectivity in Ni(ina)2 and N2/He selectivity in MIL-100(Cr) both reach record 15.39 and 128.49, respectively, making the relay adsorption for helium purification highly efficient. The breakthrough experiments show that the two MOFs can sequentially adsorb CH4 and N2 in ternary mixtures, producing He with a purity of up to 99.99% in one step. The remarkable separation performance and stability of these MOFs underscore the industrial potential in purifying He at ambient temperature.

Hierarchical porous amorphous metal-organic frameworks constructed from ZnO/MOF glass composites

For the first time, hierarchical porous amorphous metal-organic frameworks (HP-aMOFs) containing ultramicropores, micropores, and mesopores were synthesized by etching a composite of MOF glass (agZIF-76) and ZnO using ammonia. These materials show potential applications in the adsorption of C2 hydrocarbons.

Construction of metal-organic framework/cellulose nanofibers-based hybrid membranes and their ion transport property for efficient osmotic energy conversion

The development of advanced nanofluidic membranes with better ion selectivity, efficient energy conversion and high output power density remains challenging. Herein, we prepared nanofluidic hybrid membranes based on TEMPO oxidized cellulose nanofibers (T-CNF) and manganese-based metal organic framework (MOF) using a simple in situ synthesis method. Incorporated T-CNF endows the MOF/T-CNF hybrid membrane with a high cation selectivity up to 0.93. Nanoporous MOF in three-dimensional interconnected nanochannels provides massive ion transport pathways. High transmembrane ion flux and low ion permeation energy barrier are correlated with a superior energy conversion efficiency (36 %) in MOF/T-CNF hybrid membrane. When operating under 50-fold salinity gradient by mixing simulated seawater and river water, the MOF/T-CNF hybrid membrane achieves a maximum power density value of 1.87 W m-2. About 5-fold increase in output power density was achieved compared to pure T-CNF membrane. The integration of natural nanofibers with high charge density and nanoporous MOF materials is demonstrated an effective and novel strategy for the enhancement of output power density of nanofluidic membranes, showing the great potential of MOF/T-CNF hybrid membranes as efficient nanofluidic osmotic energy generators.

Enhancing CO2/N2 and CH4/N2 separation performance by salt-modified aluminum-based metal-organic frameworks

The energy-saving separation of CO2/N2 and CH4/N2 in the energy industry facilitates the reduction of greenhouse gas emissions and replenishes energy resources, but is a challenging separation process. The trade-off between adsorption capacity and selectivity of the adsorbents is one of the key bottlenecks in adsorption separation technologies' large-scale application in the above separation task. Herein, we introduced a series of fluoroborate or fluorosilicate salts (Cu(BF4)2, Zn(BF4)2 and ZnSiF6) into the open coordination nitrogen sites of aluminum-based metal-organic frameworks (MOF-253) to create multiple binding sites to simultaneously enhance the adsorption capacity and selectivity for the target gas. By the synergistic adsorption effect of metal ions (Cu2+ or Zn2+) and fluorinated anions (BF4- or (SiF6)2-), the single-component adsorption capacity and selectivity of salt-modified MOF-253 (MOF-253@Cu(BF4)2, MOF-253@Zn(BF4)2 and MOF-253@ZnSiF6) for CO2 and CH4 were effectively improved when compared to pristine MOF-253 at 298 K and 1 bar. In addition, the salt-modified MOF-253 has a moderate adsorption heat (<30 kJ mol-1) which could be rapidly regenerated at low energy by evacuation desorption. As confirmed by the ambient breakthrough experiments of MOF-253 and MOF-253@ZnSiF6, the real separation performance for both CO2/N2 (1/4) and CH4/N2 (1/4) was obviously improved. This work provides a feasible post-modification strategy on uncoordinated sites of the framework to improve adsorption separation performance and promote the development of ideal adsorbents with a view to realizing their application in the energy industry.