Water-stable metal-organic frameworks (MOFs): rational construction and carbon dioxide capture.

Metal-organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

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.

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.

Two Magnetic Orderings and a Spin-Flop Transition in Mixed Valence Compound Mn3O(SeO3)3.

A mixed-valence manganese selenite, Mn3O(SeO3)3, was successfully synthesized using a conventional hydrothermal method. The three-dimensional framework of this compound is composed of an MnO6 octahedra and an SeO3 trigonal pyramid. The magnetic topological arrangement of manganese ions shows a three-dimensional framework formed by the intersection of octa-kagomé spin sublattices and staircase-kagomé spin sublattices. Susceptibility, magnetization and heat capacity measurements confirm that Mn3O(SeO3)3 exhibits two successive long-range antiferromagnetic orderings with TN1~4.5 K and TN2~45 K and a field-induced spin-flop transition at a critical field of 4.5 T at low temperature.

Induction of Chirality in a Metal-Organic Framework Built from Achiral Precursors.

Induction of chirality from an achiral assembly system remains a huge challenge related to the origin of life. Here, induction of chirality in a metal-organic framework (MOF) built from achiral precursors has been realized. Assembling achiral H3 BTB ligands and ZnII /CdII clusters leads to a 2D coordination polymer (FJI-H16), while introduction of achiral pyridine into such assembly system leads to a 3D chiral MOF (FJI-H27 (M) or (P)). The driven force for chiral generation has been proved to be a pyridine participated kinetic-control assembly process, which can be controlled by changing the amount of pyridine and temperature, from no induction to partial induction to complete induction. The chiral generation process has been identified in detail through a pyridine-involved key intermediate (FJI-H28). The targeted modification of pyridine can selectively lead to FJI-H27 (M) or FJI-H27 (P), making the chiral orientation and distribution of bulk FJI-H27 samples can be controlled. Our work not only represents a new chiral induction process that may relate to the chiral origin in nature, but also firstly reveals how achiral external stimuli generate chirality from achiral precursors, and offers a guide for rational preparation of chiral MOFs.