Ligand Conformation Fixation Strategy for Expanding the Structural Diversity of Copper-Tricarboxylate Frameworks and C2H2 Purification Performance Studies

Structural and functional expansion of metal-organic frameworks (MOFs) is fundamentally important because it not only enriches the structural chemistry of MOFs but also facilitates the full exploration of their application potentials. In this work, by employing a dual-site functionalization strategy to lock the ligand conformation, we designed and synthesized a pair of biphenyl tricarboxylate ligands bearing dimethyl and dimethoxy groups and fabricated their corresponding framework compounds through coordination with copper(II) ions. Compared to the monofunctionalized version, introduction of two side groups can significantly fix the ligand conformation, and as a result, the dual-methoxy compound exhibited a different network structure from the mono-methoxy counterpart. Although only one almost orthogonal conformation was observed for the two ligands, their coordination framework compounds displayed distinct topological structures probably due to different solvothermal conditions. Significantly, with a hierarchical cage-type structure and good hydrostability, the dimethyl compound exhibited promising practical application value for industrially important C₂H₂ separation and purification, which was comprehensively demonstrated by equilibrium/dynamic adsorption measurements and the corresponding Clausius-Clapeyron/IAST/DFT theoretical analyses.

Two Structurally Similar Co5 Cluster-Based Metal-Organic Frameworks Containing Open Metal Sites for Efficient C2H2/CO2 Separation

To reasonably design and synthesize metal-organic frameworks (MOFs) with high stability and excellent adsorption/separation performance, the pore configuration and functional sites are very important. Here, we report two structurally similar cluster-based MOFs using a pyridine-modified low-symmetry ligand [H₄L = 2,6-bis(2',5'-dicarboxyphenyl)pyridine], [(NH₂Me₂)₂][Co₅(L)₂(OCH₃)₂(μ₃-OH)₂·2DMF]·2DMF·2H₂O (1) and [Co₅(L)₂(μ₃-OH)₂(H₂O)₂]·2H₂O·4DMF (2). The structures of 1 and 2 are built from Co₅ clusters, which have one-dimensional open channels, but their microporous environments are different due to the different ways in which ligands bind to the metals. Both MOFs have extremely high chemical stabilities over a wide pH range (2-12). The two MOFs have similar adsorption capacities of C₂H₂ (144.0 cm³ g^-1 for 1 and 141.3 cm³ g^-1 for 2), but 1 has a higher C₂H₂/CO₂ selectivity of 3.5 under ambient conditions. The difference in gas adsorption and separation between the two MOFs has been compared by a breakthrough experiment and theoretical calculation, and the influence of the microporous environment on the gas adsorption and separation performance of MOFs has been further studied.

Engineering ligand conformation by substituent manipulation towards diverse copper-tricarboxylate frameworks with tuned gas adsorption properties

To expand the structural diversity and optimize the material performance, it is essential but challenging to regulate MOF structures in a predictable and controllable manner. In this work, by manipulating the substituents to engineer the ligand conformations, we designed and synthesized two asymmetric tricarboxylate ligands, and used them to successfully target two copper-tricarboxylate frameworks with diversified topologies depending on the ligand conformations. Besides, the ligand asymmetry induced the formation of two uncommon kinds of copper-carboxylate clusters, thus greatly expanding the library of copper-carboxylate secondary building units. Furthermore, the two compounds also displayed tunable gas adsorption properties pertinent to C2H2 separation and purification. At 298 K and 1 atm, the uptake capacity of C2H2 varies from 79.5 to 104.6 cm3 (STP) g-1, while the adsorption selectivities of C2H2 with respect to CO2 and CH4 are in the range of 2.3-3.8 and 15.3-21.6 for the equimolar components, respectively. Compared to the nitro counterpart, the methoxy MOF features higher C2H2 uptake capacity, larger C2H2/CO2 and C2H2/CH4 adsorption selectivities, and lower regeneration energy. This work demonstrates that simple ligand modification can be used to engineer the structures and tune gas adsorption properties of the resulting MOFs.

An aromatic-rich cage-based MOF with inorganic chloride ions decorating the pore surface displaying the preferential adsorption of C2H2 and C2H6 over C2H4

An aromatic-rich chloride-embedded nanocage-based MOF displayed an unusual adsorption relationship towards C₂ hydrocarbons, with the potential for C₂H₄ separation and purification application.

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.

Metal–organic frameworks for C2H2/CO2 separation

A summary of important advancements in C₂H₂/CO₂ separation using metal-organic frameworks (MOFs) is presented in this perspective article. The structure-activity relationship of MOFs has been discussed in detail.

AnN-oxide-functionalized nanocage-based copper-tricarboxylate framework for the selective capture of C2H2

AnN-oxide-functionalized copper-tricarboxylate framework displayed promising potential for use in C₂H₄and C₂H₂purification.