A microporous metal-organic framework with basic sites for efficient C2H2/CO2 separation

Developing Water-Stable Pore-Partitioned Metal-Organic Frameworks with Multi-Level Symmetry for High-Performance Sorption Applications

A new perspective is proposed in the design of pore-space-partitioned MOFs that is focused on ligand symmetry properties sub-divided here into three hierarchical levels: 1) overall ligand, 2) ligand substructure such as backbone or core, and 3) the substituent groups. Different combinations of the above symmetry properties exist. Given the close correlation between nature of chemical moiety and its symmetry, such a unique perspective into ligand symmetry and sub-symmetry in MOF design translates into the influences on MOF properties. Five new MOFs have been prepared that exhibit excellent hydrothermal stability and high-performance adsorption properties with potential applications such as C₃ H₆ /C₂ H₄ and C₂ H₂ /CO₂ selective adsorption. The combination of high stability with high benzene/cyclohexane selectivity of ≈13.7 is also of particular interest.

A Hydrolytically Stable Cu(II)-Based Metal-Organic Framework with Easily Accessible Ligands for Water Harvesting

Water scarcity is a critical issue in desert and arid regions, and atmospheric water harvesting is a potential solution. The challenge is lacking ideal adsorbents that can efficiently capture water from low-humidity air and be regenerated readily. Herein, we report a hydrolytically stable metal-organic framework (MOF), [Cu₂(AD)₂(SA)] (Cu-AD-SA), with excellent performance in water harvesting. More importantly, this material can be facilely prepared from two easily accessible ligands adenine (HAD) and succinic acid (H₂SA). Cu-AD-SA has a three-dimensional (3D) framework structure with the crs topology and intersecting channels of ∼5 Å in diameter. The channel surface is decorated by uncoordinated aromatic N atoms, amine groups, and alkyl moieties. Interestingly, Cu-AD-SA shows a high water adsorption capacity of 0.16 g g^-1 at low pressure of 0.2 P/P₀ and 25 °C. Furthermore, dynamic water adsorption-desorption cycling experiments demonstrated a stable working capacity of 0.13 g g^-1 for uptaking water from a low-humidity air (water partial pressure: 0.85 kPa, 20% RH at 30 °C, 5.3% RH at 55 °C) at 30 °C and desorption at 55 °C. The water adsorption mechanism was also studied by analyzing its single-crystal structure after water loading. The results indicated the existence of strong H-bonding interactions between water molecules and uncoordinated N atoms and amine groups on the framework, which should play an important role in the high adsorption at low pressure. All the above features suggest great potential of Cu-AD-SA for water harvesting in arid regions.

Highly Robust Tetranuclear Cobalt-Based 3D Framework for Efficient C2H2/CO2 and C2H2/C2H4Separations

A novel noninterpenetrated tetranuclear cobalt(II)-based metal-organic framework, (NH₄)₂·[Co₄(μ₃-OH)₂(ina)₂(pip)₃]·4EtOH·H₂O (simplified as NbU-10·S), constructed by mix linkers was synthesized by a hydrothermal method. Interestingly, the presence of a hydrophobic benzene ring in the organic linker makes NbU-10·S exhibit high stability in high temperature and even in aqueous solution over a wide pH range of about 4-13. Magnetic studies showed that the tetranuclear cobalt(II) units in NbU-10·S show dominant antiferromangetic properties. However, in the absence of Lewis basic functional sites and open metal sites in the material, NbU-10 still displays high C₂H₂/CO₂ and C₂H₂/C₂H₄ selectivity in ideal adsorbed solution theory calculations and dynamic breakthrough experiments. Moreover, density functional theory calculations were performed to identify the adsorption characteristics of different gas molecules.

Crystal engineering of porous coordination networks to enable separation of C2 hydrocarbons

Crystal engineering, the field of chemistry that studies the design, properties, and applications of crystals, is exemplified by the emergence over the past thirty years of porous coordination networks (PCNs), including metal-organic frameworks (MOFs) and hybrid coordination networks (HCNs). PCNs have now come of age thanks to their amenability to design from first principles and how this in turn can result in new materials with task-specific features. Herein, we focus upon how control over the pore chemistry and pore size of PCNs has been leveraged to create a new generation of physisorbents for efficient purification of light hydrocarbons (LHs). The impetus for this research comes from the need to address LH purification processes based upon cryogenic separation, distillation, chemisorption or solvent extraction, each of which is energy intensive. Adsorptive separation by physisorbents (in general) and PCNs (in particular) can offer two advantages over these existing approaches: improved energy efficiency; lower plant size/cost. Unfortunately, most existing physisorbents suffer from low uptake and/or poor sorbate selectivity and are therefore unsuitable for trace separations of LHs including the high volume C2 LHs (C₂H_x, x = 2, 4, 6). This situation is rapidly changing thanks to PCN sorbents that have set new performance benchmarks for several C2 separations. Herein, we review and analyse PCN sorbents with respect to the supramolecular chemistry of sorbent-sorbate binding and detail the crystal engineering approaches that have enabled the exquisite control over pore size and pore chemistry that affords highly selective binding sites. Whereas the structure-function relationships that have emerged offer important design principles, several development roadblocks remain to be overcome.

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.