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Reticular chemistry — the linking of well-defined molecular building blocks by strong bonds into crystalline extended frameworks — has enabled the synthesis of diverse metal–organic frameworks (MOFs) and covalent organic frameworks, in which the pore shape, size and functionality can be tailored towards specific applications. Structural design methodologies are based on three main requisites: building blocks, targeted nets and isoreticular chemistry. In this Review, we highlight the well-developed and cutting-edge methodologies in reticular chemistry for the structural design and discovery of periodic solids. We illustrate the diversity of building blocks and delineate the suitable blueprint nets — namely, edge-transitive nets — for the design of MOFs. These edge-transitive nets are classified into three categories to help rationalize existing MOFs and to provide guidelines for the design of new structures. Two emerging topological concepts, namely, the merged-net approach and net -coded building units, are highlighted for their potential in synthesizing intricate or multi-component MOFs. We also consider isoreticular design strategies for the modification, expansion and contraction of building blocks, and identify challenges and opportunities in the assembly of increasingly intricate frameworks. The development of structural design methodologies in reticular chemistry promotes the discovery of periodic solids, such as metal–organic frameworks. In this Review, we highlight the well-developed and cutting-edge structural design methodologies, focusing on the role of building blocks, targeted nets and isoreticular chemistry.

To use natural gas as a feedstock alternative to coal and oil, its main constituent, methane, needs to be isolated with high purity 1 . In particular, nitrogen dilutes the heating value of natural gas and is, therefore, of prime importance for removal 2 . However, the inertness of nitrogen and its similarities to methane in terms of kinetic size, polarizability and boiling point pose particular challenges for the development of energy-efficient nitrogen-removing processes 3 . Here we report a mixed-linker metal–organic framework (MOF) membrane based on fumarate ( fum ) and mesaconate ( mes ) linkers, Zr- fum 67 - mes 33 - fcu -MOF, with a pore aperture shape specific for effective nitrogen removal from natural gas. The deliberate introduction of asymmetry in the parent trefoil-shaped pore aperture induces a shape irregularity, blocking the transport of tetrahedral methane while allowing linear nitrogen to permeate. Zr- fum 67 - mes 33 - fcu -MOF membranes exhibit record-high nitrogen/methane selectivity and nitrogen permeance under practical pressures up to 50 bar, removing both carbon dioxide and nitrogen from natural gas. Techno-economic analysis shows that our membranes offer the potential to reduce methane purification costs by about 66% for nitrogen rejection and about 73% for simultaneous removal of carbon dioxide and nitrogen, relative to cryogenic distillation and amine-based carbon dioxide capture. A metal–organic framework membrane based on fumarate and mesaconate linkers is shown to have a pore aperture shape that enables efficient and cost-effective removal of nitrogen and carbon dioxide from methane.

Direct air capture is regarded as a plausible alternate approach that, if economically practical, can mitigate the increasing carbon dioxide emissions associated with two of the main carbon polluting sources, namely stationary power plants and transportation. Here we show that metal-organic framework crystal chemistry permits the construction of an isostructural metal-organic framework ( SIFSIX -3-Cu) based on pyrazine/copper(II) two-dimensional periodic 4 4 square grids pillared by silicon hexafluoride anions and thus allows further contraction of the pore system to 3.5 versus 3.84 Å for the parent zinc(II) derivative. This enhances the adsorption energetics and subsequently displays carbon dioxide uptake and selectivity at very low partial pressures relevant to air capture and trace carbon dioxide removal. The resultant SIFSIX- 3-Cu exhibits uniformly distributed adsorption energetics and offers enhanced carbon dioxide physical adsorption properties, uptake and selectivity in highly diluted gas streams, a performance, to the best of our knowledge, unachievable with other classes of porous materials. The capture and removal of low-concentration carbon dioxide from air is appealing. Here, the authors report a metal-organic framework with a precisely tuned network of pores and optimal charge density, which is capable of carbon dioxide uptake at very low partial pressures relevant to direct air capture.

The architectural design and fabrication of low-cost and reliable organic X-ray imaging scintillators with high light yield, ultralow detection limits and excellent imaging resolution is becoming one of the most attractive research directions for chemists, materials scientists, physicists and engineers due to the devices’ promising scientific and applied technological implications. However, the optimal balance among X-ray absorption capability, exciton utilization efficiency and photoluminescence quantum yield of organic scintillation materials is extremely difficult to achieve because of several competitive non-radiative processes, including intersystem crossing and internal conversion. Here we introduced heavy atoms (Cl, Br and I) into thermally activated delayed fluorescence (TADF) chromophores to significantly increase their X-ray absorption cross-section and maintaining their unique TADF properties and high photoluminescence quantum yield. The X-ray imaging screens fabricated using TADF-Br chromophores exhibited highly improved X-ray sensitivity and imaging resolution compared with the TADF-H counterpart. More importantly, the high X-ray imaging resolution of >18.0 line pairs per millimetre achieved from the TADF-Br screen exceeds most reported organic and conventional inorganic scintillators. This study could help revive research on organic X-ray imaging scintillators and pave the way towards exciting applications for radiology and security screening.Heavy atoms like Cl, Br and I introduced into thermally activated delayed fluorescence chromophores can increase the X-ray absorption cross-section. Light yield of ~20,000 photons MeV–1, detection limit of 45.5 nGy s−1 and imaging resolution of >18.0 line pairs per millimetre is demonstrated.

Natural gasmust be dehydrated before it can be transported and used, but conventional drying agents such as activated alumina or inorganic molecular sieves require an energy-intensive desiccant-regeneration step. We report a hydrolytically stable fluorinated metal-organic framework, AlFFIVE-1-Ni (KAUST-8), with a periodic array of open metal coordination sites and fluorine moieties within the contracted square-shaped one-dimensional channel. This material selectively removed water vapor from gas streams containing CO₂, N₂, CH₄, and higher hydrocarbons typical of natural gas, as well as selectively removed both H₂O and CO₂ in N₂-containing streams. The complete desorption of the adsorbed water molecules contained by the AlFFIVE-1-Ni sorbent requires relatively moderate temperature (∼105°C) and about half the energy input for commonly used desiccants.

The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that metal–organic frameworks, a type of highly crystalline porous solid, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known zeolitic imidazolate framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal–organic frameworks and other applications. Solution processability is required for many industrial processes, but metal–organic frameworks are in general not dispersible, hindering their application. Here, a surface modification is reported that allows porous liquid formation and so synthesis of highly loaded and mechanically robust mixed matrix membranes.

Metal–organic frameworks have drawn attention as potential catalysts owing to their unique tunable surface chemistry and accessibility. However, their application in thermal catalysis has been limited because of their instability under harsh temperatures and pressures, such as the hydrogenation of CO 2 to methanol. Herein, we use a controlled two-step method to synthesize finely dispersed Cu on a zeolitic imidazolate framework-8 (ZIF-8). This catalyst suffers a series of transformations during the CO 2 hydrogenation to methanol, leading to ~14 nm Cu nanoparticles encapsulated on the Zn-based MOF that are highly active (2-fold higher methanol productivity than the commercial Cu–Zn–Al catalyst), very selective (>90%), and remarkably stable for over 150 h. In situ spectroscopy, density functional theory calculations, and kinetic results reveal the preferential adsorption sites, the preferential reaction pathways, and the reverse water gas shift reaction suppression over this catalyst. The developed material is robust, easy to synthesize, and active for CO 2 utilization. Here, authors report an inter-site structural heterogeneity induced effect of hierarchical single atom Fe catalysts for robust oxygen reduction. Dynamic evolutions and insights into structure-activity relationship are presented.

Metal–organic frameworks (MOFs) are a promising class of porous materials because it is possible to mutually control their porous structure, composition and functionality. However, it is still a challenge to predict the network topology of such framework materials prior to their synthesis. Here we use a new rare earth (RE) nonanuclear carboxylate-based cluster as an 18-connected molecular building block to form a gea -MOF ( gea -MOF-1) based on a (3,18)-connected net. We then utilized this gea net as a blueprint to design and assemble another MOF ( gea -MOF-2). In gea -MOF-2, the 18-connected RE clusters are replaced by metal–organic polyhedra, peripherally functionalized so as to have the same connectivity as the RE clusters. These metal–organic polyhedra act as supermolecular building blocks when they form gea -MOF-2. The discovery of a (3,18)-connected MOF followed by deliberate transposition of its topology to a predesigned second MOF with a different chemical system validates the prospective rational design of MOFs. It is often difficult to predict or control the topologies of metal–organic frameworks (MOFs) before synthesis. Now, the topology of a MOF has been used as an ideal blueprint for the deliberate design of a related MOF, by substitution of molecular building blocks with supermolecular building blocks. The two MOFs share the same underlying topology but have different chemical compositions.

Membrane-based approaches can offer energy-efficient and cost-effective methods for various separation processes. Practical membranes must have high permselectivity at industrially relevant high pressures and under aggressive conditions, and be manufacturable in a scalable and robust fashion. We report a versatile electrochemical directed-assembly strategy to fabricate polycrystalline metal–organic framework membranes for separation of hydrocarbons. We fabricate a series of face-centred cubic metal–organic framework membranes based on 12-connected rare-earth or zirconium hexanuclear clusters with distinct ligands. In particular, the resultant fumarate-based membranes containing contracted triangular apertures as sole entrances to the pore system enable molecular-sieving separation of propylene/propane and butane/isobutane mixtures. Prominently, increasing the feed pressure to the industrially practical value of 7 atm promoted a desired enhancement in both the total flux and separation selectivity. Process design analysis demonstrates that, for propylene/propane separation, the deployment of such face-centred cubic Zr-fumarate-based metal–organic framework membranes in a hybrid membrane–distillation system offers the potential to decrease the energy input by nearly 90% relative to a conventional single distillation process. Metal–organic framework membranes may be able to separate mixtures of hydrocarbons in an energy-efficient manner, but high-quality robust membranes are difficult to prepare. Here, Zhou et al. fabricate high-performance continuous metal–organic framework membranes using an electrochemical method.

Energy-efficient approaches to propylene/propane separation such as molecular sieving are of considerable importance for the petrochemical industry. The metal organic framework NbOFFIVE -1-Ni adsorbs propylene but not propane at room temperature and atmospheric pressure, whereas the isostructural SIFSIX -3-Ni does not exclude propane under the same conditions. The static dimensions of the pore openings of both materials are too small to admit either guest, signalling the importance of host dynamics for guest entrance to and transport through the channels. We use ab initio calculations together with crystallographic and adsorption data to show that the dynamics of the two framework-forming units, polyatomic anions and pyrazines, govern both diffusion and separation. The guest diffusion occurs by opening of the flexible window formed by four pyrazines. In NbOFFIVE -1-Ni, (NbOF 5 ) 2− anion reorientation locates propane away from the window, which enhances propylene/propane separation. Porous materials acting as molecular sieves for propylene/propane separation are important for the petrochemical industry. Here the authors show an example of how specific guest-host interactions can result in structural changes in the porous host and shut down diffusion of one of the two similar guest molecules.