Water-resistant porous coordination polymers for gas separation

A comprehensive review and perspective research in technology integration for the treatment of gaseous volatile organic compounds

Volatile organic compounds (VOCs) are harmful pollutants emitted from industrial processes. They pose a risk to human health and ecosystems, even at low concentrations. Controlling VOCs is crucial for good air quality. This review aims to provide a comprehensive understanding of the various methods used for controlling VOC abatement. The advancement of mono-functional treatment techniques, including recovery such as absorption, adsorption, condensation, and membrane separation, and destruction-based methods such as natural degradation methods, advanced oxidation processes, and reduction methods were discussed. Among these methods, advanced oxidation processes are considered the most effective for removing toxic VOCs, despite some drawbacks such as costly chemicals, rigorous reaction conditions, and the formation of secondary chemicals. Standalone technologies are generally not sufficient and do not perform satisfactorily for the removal of hazardous air pollutants due to the generation of innocuous end products. However, every integration technique complements superiority and overcomes the challenges of standalone technologies. For instance, by using catalytic oxidation, catalytic ozonation, non-thermal plasma, and photocatalysis pretreatments, the amount of bioaerosols released from the bioreactor can be significantly reduced, leading to effective conversion rates for non-polar compounds, and opening new perspectives towards promising techniques with countless benefits. Interestingly, the three-stage processes have shown efficient decomposition performance for polar VOCs, excellent recoverability for nonpolar VOCs, and promising potential applications in atmospheric purification. Furthermore, the review also reports on the evolution of mathematical and artificial neural network modeling for VOC removal performance. The article critically analyzes the synergistic effects and advantages of integration. The authors hope that this article will be helpful in deciding on the appropriate strategy for controlling interested VOCs.

Anchoring Au nanoclusters into coordination polymers: A novel approach toward ATP detection and its application

Adenosine triphosphate (ATP) is the main source of energy required for all life activities and is used as a biomarker for diseases such as cancer. It is of great significance to design a novel fluorescent probe with favorable performance for monitoring the changes of ATP concentration. Herein, a fluorescence probe named ZnCPs@AuNCs for ATP sensing was designed and fabricated by integrating AuNCs into ZnCPs. The emission intensity of AuNCs was greatly enhanced upon the formation of the ZnCPs@AuNCs nanocomposites, which may be attributed to ZnCPs restricting the molecular motion of AuNCs. Upon the introduction of ATP, the fluorescence intensity at 564 nm of ZnCPs@AuNCs is quenched. According to this phenomenon, a sensitive and reliable ATP sensing platform was established. Moreover, ZnCPs@AuNCs were incorporated into a poly (vinyl alcohol) matrix for the fabrication of fluorescent film, which exhibited solid-state fluorescence. Inspired by the remarkable fluorescent properties of ZnCPs@AuNCs, the fluorescent hydrogel was prepared by mixing ZnCPs@AuNCs with κ-carrageenan, which demonstrated a response to ATP and favorable self-healing ability. This work presents a perspective of ZnCPs@AuNCs in multiple applications such as biosensing, fluorescent film, and hydrogel construction.

Computational screening and functional tuning of chemically stable metal organic frameworks for I2/CH3I capture in humid environments

High chemical stability is of vital significance in rendering metal organic frameworks (MOFs) as promising adsorbents for capturing leaked radioactive nuclides, under real nuclear industrial conditions with high humidity. In this work, grand canonical Monte Carlo (GCMC) and density functional theory (DFT) methods have been employed to systematically evaluate I2/CH3I capture performances of 21 experimentally confirmed chemically stable MOFs in humid environments. Favorable structural factors and the influence of hydrophilicity for iodine capture were unveiled. Subsequently, the top-performing MIL-53-Al with flexible tunability was functionalized with different functional groups to achieve the better adsorption performance. It has been revealed that the adsorption affinity and pore volume were two major factors altered by the functionalization of polar functional groups, which collectively influenced the iodine adsorption properties. In general, this work has screened the chemically stable high-performance MOF iodine adsorbents and provided comprehensive insights into the key factors affecting I2/CH3I uptake and separation in humid environments.

Flexible Microporous Framework for One-Step Acquisition of Ethylene from Ternary C2 Hydrocarbons

One-step purification of ethylene (C2H4) from ternary C2 hydrocarbon mixtures is a crucial task and an enduring challenge because of their similar molecular size and physical properties. Owing to their intriguing structural dynamics, flexible MOFs have attracted more attention for gas adsorption and separation. Herein, we report a flexible MOF FJI-W-66 that exhibits rarely seen "breathing" behaviors for C2 hydrocarbons. Upon activation, the channels of guest-free FJI-W-66a significantly contract to a nearly closed-pore state. FJI-W-66a shows the stepwise adsorption isotherms for C2 hydrocarbons, which suggests the occurrence of structural transformation between less open and more open phases. Breakthrough experiments provide evidence that FJI-W-66a can selectively separate C2H4 from C2H2/C2H4/C2H6 mixtures with different ratios under ambient conditions, realizing the one-step acquisition of C2H4 from ternary C2 hydrocarbons.

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.

In Situ Conversion of Ligand to a Coordination Polymer via a Core@Shell Crystal: A Multi-Step Phase-Dependent Structural Transformation

Two pseudopolymorphic 1D coordination polymers of the formulas [Cd(3,3'-pytz)(CH3OH)2(ClO4)2]n (1) and [Cd(3,3'-pytz)(CH3CN)2(ClO4)2]n (2) have been prepared using the electron-deficient 3,6-bis(pyridin-3-yl)-1,2,4,5-tetrazine (3,3'-pytz) ligand and cadmium perchlorate in the chloroform/methanol and chloroform/acetonitrile solvent system, respectively. It was observed that compounds 1 and 2 experienced one-step (CPreagent → CPproduct) single-crystal-to-powder structural transformation to the pure water-coordinated compound [Cd(3,3'-pytz)(H2O)2(ClO4)2]n (3) by absorbing water vapor from air (solid-gas phase transformation). Interestingly, compounds 1, 2, and 3 undergo a different transformation path and show an in situ unique three-step (CPreagent → CPproduct → Ligandintermediate → CPproduct) single-crystal-to-single-crystal (SCSC) structural transformation process through soaking in deionized water (solid-liquid phase transformation). In this fascinating transformation, we report for the first time the direct conversion of a ligand into a coordination polymer by a rare core-shell pathway in a solid-liquid phase transformation. In this process, we obtained compound {[Cd(3,3'-pytz)(H2O)4](3,3'-pytz)2(ClO4)2(H2O)6}n (4) (single-crystal = S, crystal = C, or microcrystal = P) as mixed compounds of core-shell L@4C and 4S or core-shell L@4P and 4P for compounds (1 and 2) and 3, respectively.

Correlation in Structural Architecture toward Fabrication of Schottky Device with a Series of Pyrazine Appended Coordination Polymers

Energy is the center of importance for the survivability of civilization. Use of fossil fuel is going to be suspended, and renewable energy is technologically costlier. In the quest for new energy sources and to minimize fuel expenditure, the design of energy efficient devices is one of the solutions. Toward this objective, highly delocalized π-acidic N-hetreocycle pyrazine bridged Cd(II)-based coordination polymers (CPs), [Cd(tppz)(adc)(MeOH)] (1), [Cd(tppz)(trep)] (2), and [Cd(tppz)(2,6-ndc)] (3; tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) are synthesized in combination with π-accessible dicarboxylato linkers (acetylene dicarboxylic acid (H2adc), terephthalic acid (H2trep), and 2,6-naphthalene dicarboxylic acid (2,6 H2ndc)). The structures of the compounds, 1-3, have been confirmed by single-crystal X-ray diffraction measurements. Analysis of electrical property demonstrates that light irradiation enhances the conductivity and follows the order 3 > 2 > 1; compound 3 possesses the highest conductivity (1.93 × 10-3 (light), 1.12 × 10-4 S m-1 dark)), than 2 (1.80 × 10-4 (light), 1.10 × 10-4 S m-1 (dark)) and 1 (5.06 × 10-5 (light), 4.72 × 10-5 S m-1 (dark)). This light-induced electrical conductivity can pave the way toward fabrication of an active electronic device by using the discussed materials.

Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications

Covalent organic frameworks (COFs) are porous crystalline polymeric materials formed by the covalent bonding of organic units. The abundant organic units library gives the COFs species diversity, easily tuned pore channels, and pore sizes. In addition, the periodic arrangement of organic units endows COFs regular and highly connected pore channels, which has led to the rapid development of COFs in membrane separations. Continuous defect-free and high crystallinity of COF membranes is the key to their application in separations, which is the most important issue to be addressed in the research. This review article describes the linkage types of covalent bonds, synthesis methods, and pore size regulation strategies of COFs materials. Further, the preparation strategies of continuous COFs membranes are highlighted, including layer-by-layer (LBL) stacking, in situ growth, interfacial polymerization (IP), and solvent casting. The applications in separation fields of continuous COFs membranes are also discussed, including gas separation, water treatment, organic solvent nanofiltration, ion conduction, and energy battery membranes. Finally, the research results are summarized and the future prospect for the development of COFs membranes are outlined. More attention may be paid to the large-scale preparation of COFs membranes and the development of conductive COFs membranes in future research.

Three in One: Superoleophilic, Chemically and Mechanically Resistant ZIF-7 and ZIF-11 Percolation Networks for Selective Permeation of Oils and Chlorinated Solvents

Imbuing superwetting functions to organic-inorganic hybrid networks displaying chemical resistance, self-cleaning ability, and selective permeation of liquids has received increasing attention in recent years. Here we report superhydrophobic ZIF-7 and ZIF-11 on multilayer fluorinated graphene (FG) nanosheets with long-lasting water-repellent features. By exploring the solution processing of these chemically resistant dispersions, superoleophilic FG-ZIF-7 stainless steel mesh (FG-ZIF-7-SSM) and FG-ZIF-11 over cotton cloth (FG-ZIF-11-CC) possessing superior adhesion were fabricated. These permselective oil-liking prototypes were explored toward mesitylene and crude oil pickup from chemically harsh marine conditions such as seawater, acidic water, and alkaline water, with a separation efficiency of 96-94% up to 10 cycles. Furthermore, using an FG-ZIF-11-CC-wrapped glass pipet, upward diffusion of chloroform from sea, acidic, and alkaline water in 45 s was demonstrated with a separation efficacy of 94% up to 20 cycles. In addition to the chemical resistance and reusability, the mechanical stability of FG-ZIF-7-SSM and FG-ZIF-11-CC was investigated through folding, tape peeling, and dragging through sandpaper up to 250 cycles, showing no signs of changes in the hydrophobic responses. This research sheds light on the application of physiochemically resistant percolation coatings based on fluorinated graphene multilayers supporting ZIF-7 and ZIF-11 toward oil/water separation.

Surface Properties of Fluorine-Functionalized Metal-Organic Frameworks Based on Inverse Gas Chromatography

The introduction of the concept of surface properties can help us to better analyze the basic physicochemical property changes of metal-organic framework (MOF) materials before and after fluorine functional group treatment. In this study, several polar and nonpolar probes were selected to determine the surface properties, including surface-dispersive free energy, Lewis acid-base constants of Ni-MOF-74, and perfluoro carboxylic acid-modified Ni-MOF-74-Fn (n = 3, 5, and 7) in the range of 343.15-383.15 K by inverse gas chromatography (IGC). It was observed that the surface energy of the treated Ni-MOF-74-Fn showed a substantial decrease with the growth of the perfluorocarbon alkyl chains and the increase in surface roughness. In addition, Lewis acidic sites exposed by the Ni-MOF-74 material after adopting modification with fluorine functional groups increased with the increase of perfluorinated carboxylic acid chains, and their surface properties changed from amphiphilic acidic to strongly acidic. These results not only enrich the basic physical property data of Ni-MOF-74 but also provide more theoretical basis for the fluorinated functionalized custom-designed MOFs and enrich their applications in the fields of multiphase catalysis, gas adsorption, and chromatographic separation.

A robust and porous titanium metal-organic framework for gas adsorption, CO2 capture and conversion

A robust and porous titanium metal-organic framework (Ti-MOF; LCU-402) has been hydrothermally synthesized through combining a tetranuclear Ti₂Ca₂(μ₃-O)₂(μ₂-H₂O)_1.3(H₂O)₄(O₂C-)₈ cluster and a tritopic 1,3,5-benzene(tris)benzoic (BTB) ligand. LCU-402 shows remarkable stability and permanent porosity for CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆ gas adsorption. Moreover, LCU-402 as a heterogeneous catalyst can smoothly convert CO₂ under a simulated flue atmosphere into organic carbonate molecules by cycloaddition reactions of CO₂ and epoxides, indicating that LCU-402 might be a promising catalyst candidate in practical applications. We are confident that the identification of a persistent titanium-oxo building unit would accelerate the development of new porous Ti-MOF materials.

Biomedically-relevant metal organic framework-hydrogel composites

Metal organic frameworks (MOFs) are incredibly versatile three-dimensional porous materials with a wide range of applications that arise from their well-defined coordination structures, high surface areas and porosities, as well as ease of structural tunability due to diverse compositions achievable. In recent years, following advances in synthetic strategies, development of water-stable MOFs and surface functionalisation techniques, these porous materials have found increasing biomedical applications. In particular, the combination of MOFs with polymeric hydrogels creates a class of new composite materials that marries the high water content, tissue mimicry and biocompatibility of hydrogels with the inherent structural tunability of MOFs in various biomedical contexts. Additionally, the MOF-hydrogel composites can transcend each individual component such as by providing added stimuli-responsiveness, enhancing mechanical properties and improving the release profile of loaded drugs. In this review, we discuss the recent key advances in the design and applications of MOF-hydrogel composite materials. Following a summary of their synthetic methodologies and characterisation, we discuss the state-of-the-art in MOF-hydrogels for biomedical use - cases including drug delivery, sensing, wound treatment and biocatalysis. Through these examples, we aim to demonstrate the immense potential of MOF-hydrogel composites for biomedical applications, whilst inspiring further innovations in this exciting field.

Programmed Polarizability Engineering in a Cyclen-Based Cubic Zr(IV) Metal-Organic Framework to Boost Xe/Kr Separation

Efficient separation of xenon (Xe) and krypton (Kr) mixtures through vacuum swing adsorption (VSA) is considered the most attractive route to reduce energy consumption, but discriminating between these two gases is difficult due to their similar properties. In this work, we report a cubic zirconium-based MOF (Zr-MOF) platform, denoted as NU-1107, capable of achieving selective separation of Xe/Kr by post-synthetically engineering framework polarizability in a programmable manner. Specifically, the tetratopic linkers in NU-1107 feature tetradentate cyclen cores that are capable of chelating a variety of transition-metal ions, affording a sequence of metal-docked cationic isostructural Zr-MOFs. NU-1107-Ag(I), which features the strongest framework polarizability among this series, achieves the best performance for a 20:80 v/v Xe/Kr mixture at 298 K and 1.0 bar with an ideal adsorbed solution theory (IAST) predicted selectivity of 13.4, placing it among the highest performing MOF materials reported to date. Notably, the Xe/Kr separation performance for NU-1107-Ag(I) is significantly better than that of the isoreticular, porphyrin-based MOF-525-Ag(II), highlighting how the cyclen core can generate relatively stronger framework polarizability through the formation of low-valent Ag(I) species and polarizable counteranions. Density functional theory (DFT) calculations corroborate these experimental results and suggest strong interactions between Xe and exposed Ag(I) sites in NU-1107-Ag(I). Finally, we validated this framework polarizability regulation approach by demonstrating the effectiveness of NU-1107-Ag(I) toward C₃H₆/C₃H₈ separation, indicating that this generalizable strategy can facilitate the bespoke synthesis of polarized porous materials for targeted separations.

Adsorption and membrane separation for removal and recovery of volatile organic compounds

Volatile organic compounds (VOCs) are a crucial kind of pollutants in the environment due to their obvious features of severe toxicity, high volatility, and poor degradability. It is particularly urgent to control the emission of VOCs due to the persistent increase of concentration and the stringent regulations. In China, clear directions and requirements for reduction of VOCs have been given in the "national plan on environmental improvement for the 13th Five-Year Plan period". Therefore, the development of efficient technologies for removal and recovery of VOCs is of great significance. Recovery technologies are favored by researchers due to their advantages in both recycling VOCs and reducing carbon emissions. Among them, adsorption and membrane separation processes have been extensively studied due to their remarkable industrial prospects. This overview was to provide an up-to-date progress of adsorption and membrane separation for removal and recovery of VOCs. Firstly, adsorption and membrane separation were found to be the research hotspots through bibliometric analysis. Then, a comprehensive understanding of their mechanisms, factors, and current application statuses was discussed. Finally, the challenges and perspectives in this emerging field were briefly highlighted.

Porous functional metal–organic frameworks (MOFs) constructed from different N-heterocyclic carboxylic ligands for gas adsorption/separation

This review mainly summarizes the recent progress of MOFs composed of N-heterocyclic carboxylate ligands in gas sorption/separation. This work may help to understand the relationship between the structures of MOFs and gas sorption/separation.

Biomass Nanoarchitectonics for Supercapacitor Applications

Nanoarchitectonics integrates nanotechnology with numerous scientific disciplines to create innovative and novel functional materials from nano-units (atoms, molecules, and nanomaterials). The objective of nanoarchitectonics concept is to develop functional materials and systems with rationally architected functional units. This paper explores the progress and potential of this field using biomass nanoarchitectonics for supercapacitor applications as examples of energetic materials and devices. Strategic design of nanoporous carbons that exhibit ultra-high surface area and hierarchically pore architectures comprising micro- and mesopore structure and controlled pore size distributions are of great significance in energy-related applications, including in high-performance supercapacitors, lithium-ion batteries, and fuel cells. Agricultural wastes or natural biomass are lignocellulosic materials and are excellent carbon sources for the preparation of hierarchically porous carbons with an ultra-high surface area that are attractive materials in high-performance supercapacitor applications due to high electrical and ion conduction, extreme porosity, and exceptional chemical and thermal stability. In this review, we will focus on the latest advancements in the fabrication of hierarchical porous carbon materials from different biomass by chemical activation method. Particularly, the importance of biomass-derived ultra-high surface area porous carbons, hierarchical architectures with interconnected pores in high-energy storage, and high-performance supercapacitors applications will be discussed. Finally, the current challenges and outlook for the further improvement of carbon materials derived from biomass or agricultural wastes in the advancements of supercapacitor devices will be discussed.

Spectrophotometric Determination of Trace Amount of Total FeII/FeIII and Live Cell Imaging of a Carboxylato Zn(II) Coordination Polymer

The coordination polymer, (Zn(II)-CP, 1), {[Zn(2,6-NDC)(4-Cltpy)](H₂O)₄} (1) (2,6-H₂NDC = 2,6-naphthalene dicarboxylic acid and 4-Cltpy = 4'-chloro-[2,2';6',2″]terpyridine) is structurally characterized by single crystal X-ray diffraction measurement and other physicochemical studies (PXRD, FTIR, thermal analysis, microanalytical data). 4-Cltpy acts as end-capping ligand, and NDC^2- is a carboxylato bridging motif to constitute ZnN₃O₂ distorted trigonal bipyramid core that propagates to construct 1D chain. The coordination polymer, 1, detects total iron (Fe³⁺ and Fe²⁺) in aqueous solution by visual color change, colorless to pink. Absorption spectrophotometric technique in aqueous medium measures the limit of detection (LOD) 0.11 μM (Fe²⁺) and 0.15 μM (Fe³⁺), and binding constants (K_d) are 6.7 × 10⁴ M^-1 (Fe³⁺) and 3.33 × 10⁴ M^-1 (Fe²⁺). Biocompatibility of 1 is examined in live cells, and intracellular Fe²⁺ and Fe³⁺ are detected in MDA-MB 231 cells. Zn(II) substitution is assumed upon addition of Fe^III/Fe^II solution to the suspension of the coordination polymer, 1, in water-acetonitrile (41:1) (LZn^II + Fe^III/II → LFe^III + Zn^II, where L is defined as coordinated ligands), which is accompanied by changing from colorless to pink at room temperature. The color of the mixture may be assumed to the charge transfer transition from carboxylate-O to Cltpy via Fe(II/III) bridging center (carboxylate-O-Fe-CltPy). The product isolated from the reaction is finally characterized as Fe(III)@1-CP. It is presumed that product Fe(II)@1-CP may undergo fast aerial oxidation to transform Fe(III)@1-CP. The Fe^III exchanged framework (Fe(III)@1-CP) has been characterized by PXRD, IR, TGA and energy dispersive X-ray analysis (EDX)-SEM. The MTT assay calculates the cell viability (%), and the tolerance limit is 100 μM to total Fe²⁺ and Fe³⁺.

Green Preparation of Antimicrobial 1D-Coordination Polymers: [Zn(4,4'-bipy)Cl2]∞ and [Zn(4,4'-bipy)2(OAc)2]∞ by Ultrasonication of Zn(II) Salts and 4,4'-Bipyridine

We report on the green preparation of one-dimensional metal coordination polymers by sonochemical approach. The spacer ligand 4,4'-bipyridine was ultrasonicated with chloride or acetate zinc salts to obtain [Zn(4,4'-bipy)Cl₂]_∞ and [Zn(4,4'-bipy)₂(OAc)₂]_∞, respectively. Benign solvents such as ethanol and water were selected as reaction media, and the synthesis took place in a few minutes-a very short time compared to conventional methods where some days' synthesis is required. X-ray powder diffraction, Fourier transform infrared spectroscopy, thermal analysis (thermogravimetric and differential scanning calorimetry), and CHN techniques investigated the influence of using different reaction solvents on the chemical, structural, and thermal properties of the final products. The 1D [Zn(4,4'-bipy)Cl₂]_∞ and [Zn(4,4'-bipy)₂(OAc)₂]_∞ polymers, in agreement with the structures reported in the literature, were obtained in the form of nanocrystals with an average crystal size around 100 nm. As a proof of concept, a set of Gram-positive (Staphylococcus aureus) and Gram-negative bacteria (Klebsiella pneumoniae), and three yeast strains (Candida albicans, Candida krusei, Candida glabrata) were tested to evaluate the antimicrobial activity of the coordination polymers, following the Kirby-Bauer procedure and microplate dilution method. Thus, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and minimal biofilm inhibitory concentration (MBIC) were evaluated. Except for Candida krusei, the compounds showed an appreciable antimicrobial and antibiofilm activity against these strains grown in the liquid medium.

Ligand as Buffer for Improving Chemical Stability of Coordination Polymers

Chemical stability is one of the key concerns in coordination polymers (CPs). However, technologies to protect CPs against acidic or alkaline aqueous environments have yet to be implemented. Herein we demonstrate an approach for improving the pH stability by utilizing the ligand salt as buffering site to modify the unsaturated coordination sites of CPs. For the selective one-dimensional CP Eu-d-DBTA (d-H₂DBTA = d-O,O'-dibenzoyltartaric acid) with a pH stability range of 6-8, the introduction of the ligand salt Na-d-DBTA extends the pH stability interval from 3 to 11. Crystallographic structure data reveal the formation of a Eu/Na-d-DBTA dynamic structure with Na-d-DBTA buffer sites on the Eu-O cluster of the Eu-d-DBTA skeleton. Benefiting from the dynamic single-crystal-to-single-crystal transformation, the buffer sites protect the skeleton from the impact of the acidic or alkaline aqueous environment. In addition, Eu/Na-d-DBTA produces stable photoluminescence properties and selective responses toward l-tryptophan (l-Trp) and further toward l-lysine (l-Lys) over the whole buffer capacity range of 3-11. Noticeably, other Ln/Na-d-DBTA CPs and star metal-organic frameworks also exhibit pH stability improvement when the ligand-as-buffer technology is used, which is significant for developing advanced inorganic-organic hybrid materials with superior functionality.

Metal-Induced Aminosilica Rigidity Improves Highly Permeable Microporous Membranes via Different Types of Pendant Precursors

In this study, nickel-doped aminosilica membranes containing pendant groups were prepared with 3-aminopropyltriethoxysilane (APTES), trimethoxy[3-(methylamino)propyl]silane (MAPTS), 3 N,N-dimethyl aminopropyltrimethoxysilane (DAPTMS), N-[3-(trimethoxysilylpropyl]ethylene diamine (TMSPED), and 1-[3-(trimethoxysilyl)propyl] urea (TMSPU). Differences in the structures of terminal amine ligands significantly contributed to the formation of a coordinated structural assembly. Ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and N₂ adsorption isotherms revealed that short and rigid pendant amino groups successfully coordinated with nickel to produce subnanopores in the membranes, while an ion-exchange interaction was suggested for longer and sterically hindered aminosilica precursors. Moreover, the basicity of amine precursors affected the affinity of ligands for the development of a coordinated network. A pristine aminosilica membrane showed low levels of H₂ permeance that range from 0.1 to 0.5 × 10^-6 mol m^-2 s^-1 Pa^-1 with a H₂/N₂ permeance ratio that ranges from 15 to 100. On the contrary, nickel coordination increased the H₂ permeance to 0.1-3.0 × 10^-6 mol m^-2 s^-1 Pa^-1 with H₂/N₂ permeance ratios that range from 10 to 68, which indicates the formation of a microporous structure and enlargement of pore sizes. The strong level of coordination affinity between nickel ions and amine groups induced rearrangement of the flexible pendant chain into a more rigid structure.

A Dual Functional 2D MOF Exhibiting Rare Photosalient Effect as well as Selective Pd(II) Sensing in Aqueous Medium

A Zn(II) based two-dimensional metal-organic framework (2D MOF) [Zn₂(suc)₂(4-nvp)₂] (1) [H₂suc = succinic acid and 4-nvp = 4-(1-naphthylvinyl)pyridine] exhibits a "photosalient effect" under UV light as well as sunlight along with the release of a stereoselective cyclobutane ligand, 1,3-bis(4'-pyridyl)-2,4-bis(naphthyl)cyclobutane (rctt-4-pncb). Photolysis of in situ generated MOF in solution also leads to the formation of rctt-4-pncb crystals. Interestingly, compound 1 shows a high selectivity for Pd(II) sensing in aqueous medium.

Rotation configuration control of the sp2 bond in the diimidazole-dicarboxylate linker for the isomerism of porous coordination polymers

Porous isomers constructed from the same building blocks but different topology break the general preferred coordination rule of organic linkers and metal nodes, representing an invaluable opportunity for enriching their pore chemistry. Herein, a new group of porous isomers (termed as NTU-69 and NTU-70) was prepared from a C_2v symmetric diimidazole-dicarboxylate ligand and mononuclear Cu ion. The structural differences arise from the different rotation configuration of the sp² bond in the ligand, leading them to exhibit completely different topologies of unc (NTU-69) and sod (NTU-70) as well as framework rigidness. This rotation configuration of the sp² bond can be controlled by the different acidity of the synthetic solution and the metal/ligand ratio. Gas adsorption and IAST selectivity show that NTU-70 features high potential for CH₄ purification from C₂H₄, C₂H₆, C₃H₆ and CO₂ mixtures at room temperature. The insight from this work establishes a new bridge between the ligand design and controlled construction of porous isomers.

Single-crystal structure determination of nanosized metal-organic frameworks by three-dimensional electron diffraction

Metal-organic frameworks (MOFs) have attracted considerable interest due to their well-defined pore architecture and structural tunability on molecular dimensions. While single-crystal X-ray diffraction (SCXRD) has been widely used to elucidate the structures of MOFs at the atomic scale, the formation of large and well-ordered crystals is still a crucial bottleneck for structure determination. To alleviate this challenge, three-dimensional electron diffraction (3D ED) has been developed for structure determination of nano- (submicron-)sized crystals. Such 3D ED data are collected from each single crystal using transmission electron microscopy. In this protocol, we introduce the entire workflow for structural analysis of MOFs by 3D ED, from sample preparation, data acquisition and data processing to structure determination. We describe methods for crystal screening and handling of crystal agglomerates, which are crucial steps in sample preparation for single-crystal 3D ED data collection. We further present how to set up a transmission electron microscope for 3D ED data acquisition and, more importantly, offer suggestions for the optimization of data acquisition conditions. For data processing, including unit cell and space group determination, and intensity integration, we provide guidelines on how to use electron and X-ray crystallography software to process 3D ED data. Finally, we present structure determination from 3D ED data and discuss the important features associated with 3D ED data that need to be considered. We believe that this protocol provides critical details for implementing and utilizing 3D ED as a structure determination platform for nano- (submicron-)sized MOFs as well as other crystalline materials.

Water-Stable Carborane-Based Eu3+/Tb3+ Metal-Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding

Luminescent lanthanide metal-organic frameworks (Ln-MOFs) have been shown to exhibit relevant optical properties of interest for practical applications, though their implementation still remains a challenge. To be suitable for practical applications, Ln-MOFs must be not only water stable but also printable, easy to prepare, and produced in high yields. Herein, we design and synthesize a series of m CB-Eu _y Tb _1-y (y = 0-1) MOFs using a highly hydrophobic ligand mCBL1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. The new materials are stable in water and at high temperature. Tunable emission from green to red, energy transfer (ET) from Tb³⁺ to Eu³⁺, and time-dependent emission of the series of mixed-metal m CB-Eu _y Tb _1-y MOFs are reported. An outstanding increase in the quantum yield (QY) of 239% of mCB-Eu (20.5%) in the mixed mCB-Eu_0.1Tb_0.9 (69.2%) is achieved, along with an increased and tunable lifetime luminescence (from about 0.5 to 10 000 μs), all of these promoted by a highly effective ET process. The observed time-dependent emission (and color), in addition to the high QY, provides a simple method for designing high-security anticounterfeiting materials. We report a convenient method to prepare mixed-metal Eu/Tb coordination polymers (CPs) that are printable from water inks for potential applications, among which anticounterfeiting and bar-coding have been selected as a proof-of-concept.

Reversible One- to Two- to Three-Dimensional Transformation in CuII Coordination Polymer

A reversible transformation between 1D, 2D, and 3D is demonstrated for the first time in coordination polymers comprising Cu^II ions and bidentate terephthalate (BDC^2- ). 1D uniform chains were reversibly transformed into 2D layers with the construction of Cu-paddlewheels by eliminating water molecules. 2D/3D reversible transformation was achieved by removing/rebinding N,N-dimethylformamide coordinated to the paddlewheels. These dimensional transformations significantly changed chemical and physical properties such as gas sorption and magnetism. Although the uptake in open-framework 1D and 2D Cu-BDC was insignificant, pronounced absorption was observed for 3D Cu-BDC. Drastic difference in magnetic behavior is consistent with their coordination structures; uniform 1D chain of Cu^II in 1D Cu-BDC and 2D sheet based on Cu^II -paddlewheel dimers in 2D Cu-BDC. Ferromagnetic behavior observed in air-exposed 3D Cu-BDC is attributed to the 3D structure formed by the connection of 2D sheets.

Improving Ethane/Ethylene Separation Performance under Humid Conditions by Spatially Modified Zeolitic Imidazolate Frameworks

Gas separation performances are usually degraded under humid conditions for many crystalline porous materials because of the lack of water stability and/or the competition of water vapor toward the interaction sites (e.g., open metal sites). Zeolitic imidazolate frameworks (ZIFs) are suitable candidates for practical applications in gas separation because of their excellent physical/chemical stabilities. However, the limitation of substituent positions in common ZIFs has prevented extensive pore engineering to improve their separation performance. In a type of gyroidal ZIFs with gie topology, the Schiff base moiety provides additional substituent positions, making it possible to modify the spatial arrangement of hydrophobic methyl groups. Herein, a new gyroidal ZIF, ZnBAIm (H₂BAIm = 1,2-bis(1-(1H-imidazol-4-yl)ethylidene)hydrazine), is designed, synthesized, and characterized. The spatially modified ZnBAIm exhibits improved thermal/chemical/mechanical stabilities compared to ZnBIm (H₂BIm = 1,2-bis((5H-imidazol-4-yl)methylene)hydrazine). ZnBAIm can remain intact up to about 480 °C in a N₂ atmosphere and tolerate harsh treatments (e.g., 5 M NaOH aqueous solution at room temperature for 24 h and 190 MPa high pressure in the presence of water). Moreover, the modified pore and window sizes have improved significantly the ethane/ethylene selectivity and separation performance under humid conditions for ZnBAIm. Breakthrough experiments demonstrate efficient separation of a C₂H₆/C₂H₄ (50/50, v/v) binary gas mixture under ambient conditions; more importantly, the C₂H₆/C₂H₄ separation performance is unaffected under highly humid conditions (up to 80% RH). The separation performance is attributed to combined thermodynamic (stronger dispersion interaction with C₂H₆ than with C₂H₄) and kinetic factors (diffusion), determined by density functional theory calculations and kinetic adsorption study, respectively.

Reversible ammonia uptake at room temperature in a robust and tunable metal-organic framework

Ammonia is useful for the production of fertilizers and chemicals for modern technology, but its high toxicity and corrosiveness are harmful to the environment and human health. Here, we report the recyclable and tunable ammonia adsorption using a robust imidazolium-based MOF (JCM-1) that uptakes 5.7 mmol g⁻¹ of NH₃ at 298 K reversibly without structural deformation. Furthermore, a simple substitution of NO₃⁻ with Cl⁻ in a post-synthetic manner leads to an increase in the NH₃ uptake capacity of JCM-1(Cl⁻) up to 7.2 mmol g⁻¹.

Recyclable and tunable ammonia adsorption with JCM-1 and JCM-1(Cl⁻) at room temperature occurs reversibly without structural decomposition.

Stars and stripes: hexatopic tris(3,2':6',3''-terpyridine) ligands that unexpectedly form one-dimensional coordination polymers

The hexatopic ligands 1,3,5-tris(4,2':6',4''-terpyridin-4'-yl)benzene (1), 1,3,5-tris(3,2':6',3''-terpyridin-4'-yl)benzene (2), 1,3,5-tris{4-(4,2':6',4''-terpyridin-4'-yl)phenyl}benzene (3), 1,3,5-tris{4-(3,2':6',3''-terpyridin-4'-yl)phenyl}benzene (4) and 1,3,5-trimethyl-2,4,6-tris{4-(3,2':6',3''-terpyridin-4'-yl)phenyl}benzene (5) have been prepared and characterized. The single crystal structure of 1·1.75DMF was determined; 1 exhibits a propeller-shaped geometry with each of the three 4,2':6',4''-tpy domains being crystallographically independent. Packing of molecules of 1 is dominated by face-to-face π-stacking interactions which is consistent with the low solubility of 1 in common organic solvents. Reaction of 5 with [Cu(hfacac)2]·H2O (Hhfacac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) under conditions of crystal growth by layering resulted in the formation of [Cu3(hfacac)6(5)] n ·2.8nC7H8·0.4nCHCl3. Single-crystal X-ray diffraction reveals an unusual 1D-coordination polymer consisting of a series of alternating single and double loops. Each of the three crystallographically independent Cu atoms is octahedrally sited with cis-arrangements two N-donors from two different ligands 1 and, therefore, cis-arrangements of coordinated [hfacac]- ligands; this observation is unusual among compounds in the Cambridge Structural Database containing {Cu(hfacac)2N2} coordination units in which the two N-donors are in a non-chelating ligand.

Study on the synthesis and host–guest luminescence properties of a novel Cd(ii)-picolinate coordination polymer

A coordination polymer with guest-molecule-based luminescence is flexible in preparation and shows greater controllability.

Post-synthetic modification of Prussian blue type nanoparticles: tailoring the chemical and physical properties

This review focuses on recent advances in the post-synthetic modification of nano-sized Prussian blue and its analogues and compares them with the current strategies used in metal–organic frameworks to give future outlooks in this field.

Room temperature ferroelectric copper(ii) coordination polymers based on amino acid hydrazide ligands

This work brings into focus the superior coordination properties and facile applicability of amino acid hydrazide ligands for the design of molecular magnets and molecular ferroelectrics.

Rational design of carborane-based Cu2-paddle wheel coordination polymers for increased hydrolytic stability

A new unsymmetric carborane-based dicarboxylic linker provided a 1D Cu₂-paddle wheel coordination polymer (2) with much higher hydrolytic stability than the corresponding 2D Cu₂-paddle wheel polymer (1), obtained from a related more symmetrical carborane-based linker. Both 1 and 2 were used as efficient heterogeneous catalysts for a model aza-Michael reaction but only 2 can be reused several times without significant degradation in catalytic activity.

pH-Dependent formation of three porous In(III)-MOFs: framework diversity and selective gas adsorption

pH-Dependent self-assembly and structural transformation have been observed in a series of porous In(III)-MOFs, H₃O[In₃(pta)₄(OH)₂]·10H₂O (NXU-1), [In(pta)₂]·C₃H₁₀N (NXU-2) and [In(pta)₂]·C₃H₁₀N (NXU-3) (H₂pta = 2-(4-pyridyl)-terephthalic acid). The structural diversities of NXU-1-3 reveal that the pH value of the reaction plays a key role in the assembly of In-MOFs. NXU-1 with excellent stability exhibits highly selective CO₂ adsorption over CH₄ as compared to NXU-2 and NXU-3, owing to the presence of abundant multiple active sites unveiled by theoretical calculations.