Simultaneous Control of Flexibility and Rigidity in Pore-Space-Partitioned Metal-Organic Frameworks

Metal-organic frameworks for NH3 adsorption and separation

Ammonia (NH3) is not only an air pollutant but also a versatile and favourable chemical with widespread applications in human life. As a key component of nitrogen fertilizers, it plays a crucial role in improving crop yields. Additionally, NH3 serves as a hydrogen carrier and working fluid, contributing to the energy transition process. Given the diverse roles of NH3 and the varying requirements for adsorbents across different application scenarios, the rational design and selection of adsorbent materials are paramount. Metal-organic frameworks (MOFs) have emerged as promising adsorbent candidates due to their highly tunable structure and functionality, which can precisely match the characteristics required for NH3 adsorbents in multiple application scenarios. This review provides a comprehensive evaluation of NH3 adsorbents and delves into the stability characterization of MOFs under NH3 atmospheres and the underlying adsorption/degradation mechanisms. Additionally, we discuss the existing methods used to probe the host-guest interactions between MOFs and NH3. Finally, this study systematically summarizes the latest advancements of MOFs as NH3 adsorbents and classifies them according to the different requirements imposed by the varying roles of NH3. This review provides theoretical support for the design of more efficient NH3 adsorbents in the future.

Heterometallic Aluminum Metal-Organic Frameworks

From spinel gemstone (MgAl2O4) to layered double hydroxides, nature has long relied on combinations between charge-complementary metal ions such as divalent metal ions (M2+) and Al3+ to create diverse valuable materials. However, for metal-organic frameworks (MOFs), heterometallic combinations such as Mg-Al are conspicuously absent. Here, we report a breakthrough in the synthesis of heterometallic Al-MOFs containing M2+/Al3+ trimeric clusters (M = Mg, Mn, Co, Ni). The synergistic effect between M(II) chlorides and aluminum lactate plays a critical role in the cooperative crystallization of M2+ and Al3+ into pore-space-partitioned MOFs (partitioned acs topology) with fast crystallization kinetics (about 3 h). New M2+/Al3+ MOFs exhibit highly tunable porosity and extraordinarily high uptakes for CO2 and small hydrocarbon molecules (112 cm3/g for CO2, 176 cm3/g for C2H2, 156 cm3/g for C2H4, and 163 cm3/g for C2H6) at 298 K and 1 bar. The high uptake capacity coupled with high selectivity (up to 8.5 for C2H2/CO2, 10.8 for C2H2/C2H4) gives rise to efficient separations of either C2H2/CO2 or C2H2/C2H4 gas mixtures, as confirmed by experimental breakthrough experiments.

Purification of Xe and SF6 through Adaptive Contractions in a Flexible Metal-Organic Framework

Overcoming the trade-off effects between adsorption capacity, adsorption selectivity, and adsorption enthalpy of an adsorbent is very important but remains a huge challenge. Here, we report a flexible metal-organic framework (FJI-H36); it can selectively adsorb Xe from Xe/Kr mixtures with high adsorption capacity but very low adsorption enthalpy. Structural analyses show that such excellent adsorption performances come from the adaptive contraction of the flexible framework; pore shrinkage can enhance the interactions between adsorbed Xe and the framework and offset some of the adsorption heats. For SF6/N2 mixtures, FJI-H36 can also enhance the adsorption performance of SF6 through adaptive contraction, resulting in both high adsorption selectivity and low adsorption enthalpy. This not only provides a new adsorbent for the purification of Xe/Kr/SF6 but also offers a potential solution to overcome the trade-offs among adsorption capacity, adsorption selectivity, and adsorption enthalpy of a specific adsorbent.

Hopping Diffusion in Wiggling Nanopore Architecture of MOF Enabling Synergistic Equilibrium-Kinetic Separation of Fluorinated Propylene and Propane

The separation of octafluoropropane (C3F8) from hexafluoropropylene (C3F6) is an industrially important yet challenging process due to their similar physicochemical properties and stringent purity demands in industrial applications. Herein, we address this task through precise pore architecture in a zirconium-based metal-organic framework (Zr-PMA), which exhibits unique "wiggling nanopores" with narrow windows and large cavities. The narrow windows act as diffusion barriers, selectively restricting C3F8 transport, while the large cavities provide strong adsorption sites for C3F6, enabling an equilibrium-kinetic synergistic separation. This dual functionality results in a ∼450-fold difference in diffusion rates and exceptional kinetic selectivity for C3F6 over C3F8, as demonstrated by adsorption isotherms, time-resolved kinetics, and dynamic breakthrough experiments. Theoretical calculations coupled with in situ spectroscopy elucidate the pore geometry-dependent hopping diffusion mechanism responsible for the separation. This work establishes wiggling pore geometry as a versatile paradigm for advanced adsorbents targeting energy-efficient separations of structurally similar fluorocarbon mixtures.

Pore Space Partition Enabled by Lithium(I) Chelation of a Metal-Organic Framework for Benchmark C2H2/CO2 Separation

Adsorptive separation of acetylene (C2H2) from carbon dioxide (CO2) offers a promising approach to purify C2H2 with low-energy footprints. However, the development of ideal adsorbents with simultaneous high C2H2 adsorption and selectivity remains a great challenge due to their very small molecular sizes and physical properties. Herein, we report a lithium(I)-chelation strategy for pore space partition (PSP) in a microporous MOF (Li+@NOTT-101-(COOH)2) to achieve simultaneous high C2H2 uptake and selectivity. The chelation model of Li+ ions within the framework was visually identified by single-crystal X-ray diffraction studies. The immobilized Li+ ions were found to have two functions: (1) partitioning large pore cages into smaller ones while maintaining high surface area and (2) providing specific binding sites to selectively take up C2H2 over CO2. The resulting Li+@NOTT-101-(COOH)2 exhibits a rare combination of a simultaneous high C2H2 capture capacity (205 cm3 g-1) and C2H2/CO2 selectivity (13) at ambient conditions, far surpassing that of NOTT-101-(COOH)2 (148 cm3 g-1 and 3.8, respectively) and most top-tier materials reported. Theoretical calculations and gas-loaded SCXRD studies reveal that the chelated Li+ ions combined with the segmented small cages can selectively bind with a large amount of C2H2 through the unique π-complexation, accounting for the improved C2H2 uptake and selectivity. Breakthrough experiments validated its excellent separation capacity for actual C2H2/CO2 mixtures, providing one of the highest C2H2 productivities of 118.9 L kg-1 (>99.5% purity) in a single adsorption-desorption cycle.

Lowering Linker Symmetry to Access Zirconium Metal-Organic Frameworks for Inverse Alkane/Alkene Separations

Enriching the structural diversity of metal-organic frameworks (MOFs) is of great importance in developing functional porous materials with specific properties. New MOF structures can be accessed through the rational design of organic linkers with diverse geometric conformations, and their structural complexity can be enhanced by choosing linkers with reduced symmetry. Herein, a series of Zr-based MOFs with unprecedented topologies were developed through a linker desymmetrization and conformation engineering approach. A tritopic carboxylate linker with reduced symmetry and flexible triangular geometry was designed to construct three Zr-based MOFs (denoted as NU-57, NU-58, and NU-59) by modulating synthetic conditions. Notably, the conformational flexibility and reduced symmetry of the linker generated two unprecedented topologies in NU-58 and NU-59. Furthermore, solvent removal in NU-58 via thermal activation process produced missing linker defects. Finally, the adsorption behavior of these MOFs toward alkanes and alkenes was studied to gain insights into their structure-property relationships, which demonstrated that NU-57 and NU-58 exhibit unusual reverse selectivity for alkanes in alkane/alkene separations. Overall, this work highlights the rational design of linkers using a desymmetrization strategy as a powerful method to enrich the structural diversity of MOFs and to access novel MOFs with unique properties.

Architecting Metal-Organic Frameworks at Molecular Level toward Direct Air Capture

Escalating carbon dioxide (CO2) emissions have intensified the greenhouse effect, posing a significant long-term threat to environmental sustainability. Direct air capture (DAC) has emerged as a promising approach to achieving a net-zero carbon future, which offers several practical advantages, such as independence from specific CO2 emission sources, economic feasibility, flexible deployment, and minimal risk of CO2 leakage. The design and optimization of DAC sorbents are crucial for accelerating industrial adoption. Metal-organic frameworks (MOFs), with high structural order and tunable pore sizes, present an ideal solution for achieving strong guest-host interactions under trace CO2 conditions. This perspective highlights recent advancements in using MOFs for DAC, examines the molecular-level effects of water vapor on trace CO2 capture, reviews data-driven computational screening methods to develop a molecularly programmable MOF platform for identifying optimal DAC sorbents, and discusses scale-up and cost of MOFs for DAC.

Isoreticular Tolerance and Phase Selection in the Synthesis of Multi-Module Metal-Organic Frameworks for Gas Separation and Electrocatalytic OER

Although metal-organic frameworks are coordination-driven assemblies, the structural prediction and design using metal-ligand interactions can be unreliable due to other competing interactions. Leveraging non-coordination interactions to develop porous assemblies could enable new materials and applications. Here, we use a multi-module MOF system to explore important and pervasive impact of ligand-ligand interactions on metal-ligand as well as ligand-ligand co-assembly process. It is found that ligand-ligand interactions play critical roles on the scope or breakdown of isoreticular chemistry. With cooperative di- and tri-topic ligands, a family of Ni-MOFs has been synthesized in various structure types including partitioned MIL-88-acs (pacs), interrupted pacs (i-pacs), and UMCM-1-muo. A new type of isoreticular chemistry on the muo platform is established between two drastically different chemical systems. The gas sorption and electrocatalytic studies were performed that reveal excellent performance such as high C2H2/CO2 selectivity of 21.8 and high C2H2 uptake capacity of 114.5 cm3/g at 298 K and 1 bar.

Self-adaptive Coordination Evolution Mediated Pore-Space-Partition in Metal-Organic Frameworks for Boosting SF6/N2 Separation

The controllable and precise structural regulation of metal-organic frameworks (MOFs) based on isoreticular chemistry is an effective strategy for creating functional material platforms, such as efficient porous adsorbents. Herein, for the first time, mediated by an unprecedented self-adaptive coordination evolution (SACE) on pseudo-D2h-symmetric [M4(μ3-O)2(COO)6] (M=Mn/Fe) clusters, two pore space partitioned MOFs (CTGU-47-Mn/Fe, CTGU=China Three Gorges University) have been successfully constructed. Owing to the more confined adsorption space and dense binding sites produced by pore space partitioning (PSP), the CTGU-47-Mn/Fe exhibit significantly enhanced performance in the capture or recovery SF6 (greenhouse/electronic specialty gas) from SF6/N2 mixture compared to their non-partitioned homologous structures (CTGU-46-Mn/Fe) with adsorption selectivity increased from 37/72 to 634/157 (v/v, 10/90, 100 kPa). The theoretical calculations also elucidated that the implementation of PSP within CTGU-47-Mn/Fe leads to dramatically strengthened binding affinity for SF6 over N2 through extra multiple F⋅⋅⋅H interactions. This study represents a valuable advance in crystal engineering field: the SACE of polynuclear metal clusters is expected to be useful in the structural regulation of MOFs and the fabrication of advanced porous adsorbents.

Linker Installation in a Metal-Organic Framework for Enhanced Quantitative Redox Species Recognition

Linker installation has proven to be an effective strategy for introducing diverse functional groups into metal-organic frameworks (MOFs). Reductants and oxidants are found in various environments, but their accumulation poses a danger due to their high reactivity, necessitating prompt monitoring instantly, particularly in natural environments and industrial processes. In this study, a series of redox-active dyes were successfully incorporated into a flexible Zr-based MOF, PCN-700, through linker installation strategy. Notably, both the reduction of PCN-700-Resazurin towards oxidants and oxidation of PCN-700-Amplex Red towards reductants result in the formation of PCN-700-Resorufin, which exhibit visible color changes and largely elevated luminescent intensities, giving rise to the quantitative recognition of reductants and oxidants. This work presents a straightforward approach to developing efficient luminescent sensors for the rapid qualitative and quantitative detection of reductants and oxidants, extending the sensing mechanism from common energy transfer to electron transfer and demonstrating the MOF's versability as a highly designable sensing material for target analytes.

Multi-Stage Optimization of Pore Size and Shape in Pore-Space-Partitioned Metal-Organic Frameworks for Highly Selective and Sensitive Benzene Capture

Compared to exploratory development of new structure types, pushing the limits of isoreticular synthesis on a high-performance MOF platform may have higher probability of achieving targeted properties. Multi-modular MOF platforms could offer even more opportunities by expanding the scope of isoreticular chemistry. However, navigating isoreticular chemistry towards best properties on a multi-modular platform is challenging due to multiple interconnected pathways. Here on the multi-modular pacs (partitioned acs) platform, we demonstrate accessibility to a new regime of pore geometry using two independently adjustable modules (framework-forming module 1 and pore-partitioning module 2). A series of new pacs materials have been made. Benzene/cyclohexane selectivity is tuned, progressively, from 4.5 to 15.6 to 195.4 and to 482.5 by pushing the boundary of the pacs platform towards the smallest modules known so far. The exceptional stability of these materials in retaining both porosity and single crystallinity enables single-crystal diffraction studies of different crystal forms (as-synthesized, activated, guest-loaded) that help reveal the mechanistic aspects of adsorption in pacs materials.

Ligand Circuit Concept for Developing Gas Separation Materials from Pore-Space-Partitioned Metal-Organic Frameworks

Isoreticular chemistry is among the most powerful strategies for designing novel materials with optimizable pore geometry and properties. Of great significance to the further advance of isoreticular chemistry is the development of broadly applicable new concepts capable of guiding and systematizing the ligand-family expansion as well as establishing correlations between dissimilar and seemingly uncorrelated ligands for better predictive synthetic design and more insightful structure and property analysis. Here ligand circuit concept is proposed and its use has been demonstrated for the synthesis of a family of highly stable, high-performance pore-space-partitioned materials based on an acyclic ligand, trans, trans-muconic acid. This work represents a key step toward developing highly porous and highly stable pore-space-partitioned materials from acyclic ligands. The new materials exhibit excellent sorption properties such as high uptake capacity for CO2 (81.3 cm3 g-1) and C2H2 (165.4 cm3 g-1) by CPM-7.3a-NiV. CPM-7.3a-CoV shows C2H6-selective C2H6/C2H4 separation properties and its high uptakes for C2H4 (134.0 cm3 g-1) and C2H6 (148.0 cm3 g-1) at 1 bar and 298 K contribute to the separation potential of 1.35 mmol g-1. The multi-cycle breakthrough experiment confirms the promising separation performance for C2H2/CO2.

Highly Water-Stable 2D MOF as Dual Sensor for the Ultra-Sensitive Aqueous Phase Detection of Nitrofuran Antibiotics and Organochlorine Pesticides

Misuse of antibiotics and pesticides has led to hazardous effects on human health, livestock, agriculture, and aquaculture, which urges researchers to find simple, rapid, efficient, and cost-effective methods for quick on-site analysis of these organic pollutants with functional materials. Herein, a 2D chemically robust MOF: IITKGP-71, {[Cd(MBPz)(2,6-NDC)]·2H2O}n is strategically developed with ease in scalability and exploited as dual sensors toward the toxic antibiotic and pesticide detection via luminescence quenching in aqueous medium. The framework displays exceptional chemical robustness in water for 3 months, in an open atmosphere over 2 months, and wide range of aqueous pH solution (pH = 3-12) for a day. IITKGP-71 can selectively quench the nitrofuran antibiotics (NFZ and NFT) and organochlorine pesticide DCN while remaining unaffected by other interfering antibiotics and pesticides, respectively. An excellent trade-off between high effectivity (high Ksv) and high sensitivity (low LOD) was achieved for the targeted analytes. The easy scalability, high chemical stability, fast responsivity, multi-responsive nature, recyclability with outstanding structural stability made this framework viable in playing a crucial role in safeguarding aquatic ecosystems and public health from the hazardous effects of antibiotics and pesticides.

Sequential Linker Installation in Metal-Organic Frameworks

ConspectusMetal-organic frameworks (MOFs) represent a sophisticated blend of inorganic and organic components, promoting the development of coordination chemistry greatly and offering a versatile platform for tailored functionalities. By combining various metal nodes, organic linkers, and functional guests, MOFs provide numerous pathways for their design, synthesis, and customization. Among these, sequential linker installation (SLI) stands out as a novel and crucial strategy, enabling the precise integration of desired properties and functions at the atomic scale. SLI enhances structural diversity and stability while facilitating the meticulous construction of robust frameworks by leveraging open metal sites and functional organic linkers at targeted locations. Compared to the direct synthesis of MOFs, postsynthetic modification methods allow for precise regulation of their structures and corresponding properties. While unlike conventional postsynthetic modification methods, SLI requires the careful selection of linkers and framework design to ensure precise positioning for installation, which gives rise to the well-designed and ordered positions for the installed linkers, confirmed directly by X-ray diffraction technology.Recent advancements in MOF synthesis have led to the creation of increasingly tailored flexible matrix structures, particularly due to the diverse connection modes of multicore metal clusters, especially for the Zr6 cluster. The spatial hindrance of certain ligands has resulted in the formation of unsaturated metal clusters and various missing linker pockets. Examples of these advanced MOFs include PCN-606, PCN-608, PCN-609, PCN-700, and PCN-808, which feature specific open metal sites and certain framework flexibility conducive to SLI. Strategically positioned open metal sites within these frameworks serve as predetermined anchor points for desired functional molecules, while the frameworks' flexibility can accommodate molecules of varying sizes to a certain extent, enlarging the scopes of application greatly. This precise positioning of functional groups enables the creation of tailored sites for enhanced applications, such as adsorption, catalysis, and recognition.In this Account, we delve into the intricate process of designing and synthesizing MOFs with appropriate missing-linker pockets for the aforementioned applications. We discuss the meticulous selection of functional linkers and the methods used to insert them into the corresponding missing-linker pockets within the MOFs. Additionally, we explore the diverse properties and functionalities of the resulting MOFs, focusing on their adsorptive, catalytic, and recognition performance. Furthermore, we provide insights into the future trajectory of SLI methods, complemented by our recent works. This Account not only reviews the evolution of the SLI method but also underscores its practical applications across various functional domains, paving a rational pathway for the future development of advanced multifunctional MOFs through this method.

Cavity-Partitioned Self-Assembled Cage for Sequential Separation in Aqueous Solutions

The concept of pore space partition has emerged as an effective strategy for developing improved coordination-based supramolecular porous materials with exceptional performance. Herein, we report that a water-soluble self-assembled tetrahedral cage 1 with a partitioned cavity shown excellent performance as a multifunctional extractant. The results show that this unique partitioned cavity can efficiently separate halogenated adamantanes, adamantane isomers, and polycyclic aromatic hydrocarbons. Furthermore, the influence of cavity-partitioned cage 1 on the electrochemical properties of redox-active molecules and electrochemically driven reversible host-guest process has also been demonstrated. The findings offer valuable insights into the design and development of new type of materials with controlled phase separation and tailored electrochemical properties.

Advancing Pore-Space-Partitioned Metal-Organic Frameworks with Isoreticular Cluster Concept

Trigonal planar M3(O/OH) trimers are among the most important clusters in inorganic chemistry and are the foundational features of multiple high-impact MOF platforms. Here we introduce a concept called isoreticular cluster series and demonstrate that M3(O/OH), as the first member of a supertrimer series, can be combined with a higher hierarchical member (double-deck trimer here) to advance isoreticular chemistry. We report here an isoreticular series of pore-space-partitioned MOFs called M3M6 pacs made from co-assembly between M3 single-deck trimer and M3x2 double-deck trimer. Important factors were identified on this multi-modular MOF platform to guide optimization of each module, which enables the phase selection of M3M6 pacs by overcoming the formation of previously-always-observed same-cluster phases. The new pacs materials exhibit high surface area and high uptake capacity for CO2 and small hydrocarbons, as well as selective adsorption properties relevant to separation of industrially important mixtures such as C2H2/CO2 and C2H2/C2H4. Furthermore, new M3M6 pacs materials show electrocatalytic properties with high activity.

Tailorable Multi-Modular Pore-Space-Partitioned Vanadium Metal-Organic Frameworks for Gas Separation

Currently, few porous vanadium metal-organic frameworks (V-MOFs) are known and even fewer are obtainable as single crystals, resulting in limited information on their structures and properties. Here this work demonstrates remarkable promise of V-MOFs by presenting an extensible family of V-MOFs with tailorable pore geometry and properties. The synthesis leverages inter-modular synergy on a tri-modular pore-partitioned platform. New V-MOFs show a broad range of structural features and sorption properties suitable for gas storage and separation applications for C2H2/CO2, C2H6/C2H4, and C3H8/C3H6. The c/a ratio of the hexagonal cell, a measure of pore shape, is tunable from 0.612 to 1.258. Other tunable properties include pore size from 5.0 to 10.9 Å and surface area from 820 to 2964 m2 g-1. With C2H2/CO2 selectivity from 3.3 to 11 and high uptake capacity for C2H2 from 65.2 to 182 cm3 g-1 (298K, 1 bar), an efficient separation is confirmed by breakthrough experiments. The near-record high uptake for C2H6 (166.8 cm3 g-1) contributes to the promise for C2H6-selective separation of C2H6/C2H4. The multi-module pore expansion enables transition from C3H6-selective to more desirable C3H8-selective separation with extraordinarily high C3H8 uptake (254.9 cm3 g-1) and high separation potential (1.25 mmol g-1) for C3H8/C3H6 (50:50 v/v) mixture.

Metal-Organic Framework with Space-Partition Pores by Fluorinated Anions for Benchmark C2H2/CO2 Separation

The efficient separation of C2H2 from C2H2/CO2 or C2H2/CO2/CH4 mixtures is crucial for achieving high-purity C2H2 (>99%), essential in producing contemporary commodity chemicals. In this report, we present ZNU-12, a metal-organic framework with space-partitioned pores formed by inorganic fluorinated anions, for highly efficient C2H2/CO2 and C2H2/CO2/CH4 separation. The framework, partitioned by fluorinated SiF62- anions into three distinct cages, enables both a high C2H2 capacity (176.5 cm3/g at 298 K and 1.0 bar) and outstanding C2H2 selectivity over CO2 (13.4) and CH4 (233.5) simultaneously. Notably, we achieve a record-high C2H2 productivity (132.7, 105.9, 98.8, and 80.0 L/kg with 99.5% purity) from C2H2/CO2 (v/v = 50/50) and C2H2/CO2/CH4 (v/v = 1/1/1, 1/1/2, or 1/1/8) mixtures through a cycle of adsorption-desorption breakthrough experiments with high recovery rates. Theoretical calculations suggest the presence of potent "2 + 2" collaborative hydrogen bonds between C2H2 and two hexafluorosilicate (SiF62-) anions in the confined cavities.

Mechanical bonding of rigid MORFs using a tetratopic rotaxane

The preparation of highly rigid cobalt(II)- and copper(II)-organic frameworks incorporating a tetralactam [2]rotaxane as a ligand is described. The interlocked ligand is functionalized with two pairs of carboxylate groups placed at each counterpart, thus limiting its dynamics within the crystal. The solid structure of the metal-organic rotaxane frameworks showed different, unprecedented polycatenation modes of grids, depending on the employed metal, providing great rigidity to the structures. This rigidity has been evaluated by using single crystal X-ray diffraction analyses of the cobalt(II)-organic frameworks embedded in different solvents, observing that the lattices remain unchanged. Thus, this research demonstrates that rigid and robust materials with permanent porosity can be achieved using dynamic ligands.

Chiral Linker Installation in a Metal-Organic Framework for Enantioselective Luminescent Sensing

Linker installation is a potent strategy for integrating specific properties and functionalities into metal-organic frameworks (MOFs). This method enhances the structural diversity of frameworks and enables the precise construction of robust structures, complementing the conventional postsynthetic modification approaches, by fully leveraging open metal sites and active organic linkers at targeting locations. Herein, we demonstrated an insertion of a d-camphorate linker into a flexible Zr-based MOF, PCN-700, through linker installation. The resultant homochiral MOF not only exhibits remarkable stability but also functions as a highly efficient luminescent material for enantioselective sensing. Competitive absorption and energy/electron transfer processes contribute to the sensing performance, while the difference in binding affinities dominates the enantioselectivity. This work presents a straightforward route to crafting stable homochiral MOFs for enantioselective sensing.

Mechanistical Insights into the Ultrasensitive Detection of Radioactive and Chemotoxic UO22+ Ions by a Porous Anionic Co-Metal-Organic Framework

Development of a simple, cost-efficient, and portable UO22+ sensory probe with high selectivity and sensitivity is highly desirable in the context of monitoring radioactive contaminants. Herein, we report a luminescent Co-based metal-organic framework (MOF), {[Me2NH2]0.5[Co(DATRz)0.5(NH2BDC)]·xG}n (1), equipped with abundant amino functionalities for the selective detection of uranyl cations. The ionic structure consists of two types of channels decorated with plentiful Lewis basic amino moieties, which trigger a stronger acid-base interaction with the diffused cationic units and thus can selectively quench the fluorescence intensity in the presence of other interfering ions. Furthermore, the limit of detection for selective UO22+ sensing was achieved to be as low as 0.13 μM (30.94 ppb) with rapid responsiveness and multiple recyclabilities, demonstrating its excellent efficacy. Density functional theory (DFT) calculations further unraveled the preferred binding sites of the UO22+ ions in the tubular channel of the MOF structure. Orbital hybridization between NH2BDC/DATRz and UO22+ together with its significantly large electron-accepting ability is identified as responsible for the luminescence quenching. More importantly, the prepared 1@PVDF {poly(vinylidene difluoride)} mixed-matrix membrane (MMM) displayed good fluorescence activity comparable to 1, which is of great significance for their practical employment as MOF-based luminosensors in real-world sensing application.

Stimulus-Induced Dynamic Behavior Regulation of Flexible Crystals through the Tuning of Module Rigidity

Introducing dynamic behavior into periodic frameworks has borne fruit in the form of flexible porous crystals. The detailed molecular design of frameworks in order to control their collective dynamics is of particular interest, for example, to achieve stimulus-induced behavior. Herein, by varying the degree of rigidity of ditopic pillar linkers, two isostructural flexible metal-organic frameworks (MOFs) with common rigid supermolecular building bilayers were constructed. The subtle substitution of single (in bibenzyl-4,4'-dicarboxylic acid; H2BBDC) with double (in 4,4'-stilbenedicarboxylic acid; H2SDC) C-C bonds in pillared linkers led to markedly different flexible behavior of these two MOFs. Upon the removal of guest molecules, both frameworks clearly show reversible single-crystal-to-single-crystal transformations involving the cis-trans conformation change and a resulting swing of the corresponding pillar linkers, which gives rise to Flex-Cd-MOF-1a and Flex-Cd-MOF-2a, respectively. Strikingly, a more favorable gas-induced dynamic behavior in Flex-Cd-MOF-2a was verified in detail by stepwise C3H6/C3H8 sorption isotherms and the corresponding in situ powder X-ray diffraction experiments. These insights are strongly supported by molecular modeling studies on the sorption mechanism that explores the sorption landscape. Furthermore, a consistency between the macroscopic elasticity and microscopic flexibility of Flex-Cd-MOF-2 was observed. This work fuels a growing interest in developing MOFs with desired chemomechanical functions and presents detailed insights into the origins of flexible MOFs.

Isoreticular Contraction of Cage-like Metal-Organic Frameworks with Optimized Pore Space for Enhanced C2H2/CO2 and C2H2/C2H4 Separations

The C2H2 separation from CO2 and C2H4 is of great importance yet highly challenging in the petrochemical industry, owing to their similar physical and chemical properties. Herein, the pore nanospace engineering of cage-like mixed-ligand MFOF-1 has been accomplished via contracting the size of the pyridine- and carboxylic acid-functionalized linkers and introducing a fluoride- and sulfate-bridging cobalt cluster, based on a reticular chemistry strategy. Compared with the prototypical MFOF-1, the constructed FJUT-1 with the same topology presents significantly improved C2H2 adsorption capacity, and selective C2H2 separation performance due to the reduced cage cavity size, functionalized pore surface, and appropriate pore volume. The introduction of fluoride- and sulfate-bridging cubane-type tetranuclear cobalt clusters bestows FJUT-1 with exceptional chemical stability under harsh conditions while providing multiple potential C2H2 binding sites, thus rendering the adequate ability for practical C2H2 separation application as confirmed by the dynamic breakthrough experiments under dry and humid conditions. Additionally, the distinct binding mechanism is suggested by theoretical calculations in which the multiple supramolecular interactions involving C-H···O, C-H···F, and other van der Waals forces play a critical role in the selective C2H2 separation.

Thiadiazole-Functionalized Th/Zr-UiO-66 for Efficient C2H2/CO2 Separation

Adsorptive separation technology provides an effective approach for separating gases with similar physicochemical properties, such as the purification of acetylene (C2H2) from carbon dioxide (CO2). The high designability and tunability of metal-organic framework (MOF) adsorbents make them ideal design platforms for this challenging separation. Herein, we employ an isoreticular functionalization strategy to fine-tune the pore environment of Zr- and Th-based UiO-66 by the immobilization of the benzothiadiazole group via bottom-up synthesis. The functionalized UPC-120 exhibits an enhanced C2H2/CO2 separation performance, which is confirmed by adsorption isotherms, dynamic breakthrough curves, and theoretical simulations. The synergy of ligand functionalization and metal ion fine-tuning guided by isoreticular chemistry provides a new perspective for the design and development of adsorbents for challenging gas separation processes.

A molecular sieve with ultrafast adsorption kinetics for propylene separation

The design of molecular sieves is vital for gas separation, but it suffers from a long-standing issue of slow adsorption kinetics due to the intrinsic contradiction between molecular sieving and diffusion within restricted nanopores. We report a molecular sieve ZU-609 with local sieving channels that feature molecular sieving gates and rapid diffusion channels. The precise cross-sectional cutoff of molecular sieving gates enables the exclusion of propane from propylene. The coexisting large channels constituted by sulfonic anions and helically arranged metal-organic architectures allow the fast adsorption kinetics of propylene, and the measured propylene diffusion coefficient in ZU-609 is one to two orders of magnitude higher than previous molecular sieves. Propylene with 99.9% purity is obtained through breakthrough experiments with a productivity of 32.2 L kg-1.

Breaking the photoswitch speed limit

The forthcoming generation of materials, including artificial muscles, recyclable and healable systems, photochromic heterogeneous catalysts, or tailorable supercapacitors, relies on the fundamental concept of rapid switching between two or more discrete forms in the solid state. Herein, we report a breakthrough in the "speed limit" of photochromic molecules on the example of sterically-demanding spiropyran derivatives through their integration within solvent-free confined space, allowing for engineering of the photoresponsive moiety environment and tailoring their photoisomerization rates. The presented conceptual approach realized through construction of the spiropyran environment results in ~1000 times switching enhancement even in the solid state compared to its behavior in solution, setting a record in the field of photochromic compounds. Moreover, integration of two distinct photochromic moieties in the same framework provided access to a dynamic range of rates as well as complementary switching in the material's optical profile, uncovering a previously inaccessible pathway for interstate rapid photoisomerization.

Isoreticular Chemistry and Applications of Supramolecularly Assembled Copper-Adenine Porous Materials

The useful concepts of reticular chemistry, rigid and predictable metal nodes together with strong and manageable covalent interactions between metal centers and organic linkers, have made the so-called metal-organic frameworks (MOFs) a flourishing area of enormous applicability. In this work, the extension of similar strategies to supramolecularly assembled metal-organic materials has allowed us to obtain a family of isoreticular compounds of the general formula [Cu7(μ-adeninato-κN3:κN9)6(μ3-OH)6(μ-OH2)6](OOC-R-COO)·nH2O (R: ethylene-, acetylene-, naphthalene-, or biphenyl-group) in which the rigid copper-adeninato entities and the organic dicarboxylate anions are held together not by covalent interactions but by a robust and flexible network of synergic hydrogen bonds and π-π stacking interactions based on well-known supramolecular synthons (SMOFs). All compounds are isoreticular, highly insoluble, and water-stable and show a porous crystalline structure with a pcu topology containing a two-dimensional (2D) network of channels, whose dimensions and degree of porosity of the supramolecular network are tailored by the length of the dicarboxylate anion. The partial loss of the crystallization water molecules upon removal from the mother liquor produces a shrinkage of the unit cell and porosity, which leads to a color change of the compounds (from blue to olive green) if complete dehydration is achieved by means of gentle heating or vacuuming. However, the supramolecular network of noncovalent interactions is robust and flexible enough to reverse to the expanded unit cell and color after exposure to a humid atmosphere. This humidity-driven breathing behavior has been used to design a sensor in which the electrical resistance varies reversibly with the degree of humidity, very similar to the water vapor adsorption isotherm of the SMOF. The in-solution adsorption properties were explored for the uptake and release of the widely employed 5-fluorouracil, 4-aminosalycilic acid, 5-aminosalycilic acid, and allopurinol drugs. In addition, cytotoxicity activity assays were completed for the pristine and 5-fluorouracil-loaded samples.

Postsynthetic Modification of the Nonanuclear Node in a Zirconium Metal-Organic Framework for Photocatalytic Oxidation of Hydrocarbons

Heterogeneous catalysis plays an indispensable role in chemical production and energy conversion. Incorporation of transition metals into metal oxides and zeolites is a common strategy to fine-tune the activity and selectivity of the resulting solid catalysts, as either the active center or promotor. Studying the underlying mechanism is however challenging. Decorating the metal-oxo clusters with transition metals in metal-organic frameworks (MOFs) via postsynthetic modification offers a rational approach to construct well-defined structural models for better understanding of the reaction mechanism. Therefore, it is important to expand the materials scope beyond the currently widely studied zirconium MOFs consisting of Zr6 nodes. In this work, we report the design and synthesis of a new (4,12)-connected Zr-MOF with ith topology that consists of rare Zr9 nodes. FeIII was further incorporated onto the Zr9 nodes of the framework, and the resulting MOF material exhibits significantly enhanced activity and selectivity toward the photocatalytic oxidation of toluene. This work demonstrates a delicate ligand design strategy to control the nuclearity of Zr-oxo clusters, which further dictates the number and binding sites of transition metals and the overall photocatalytic activity toward C-H activation. Our work paves the way for future exploration of the structure-activity study of catalysts using MOFs as the model system.

Guest-Stimulated Nonplanar Porphyrins in Flexible Metal-Organic Frameworks

Nonplanar porphyrins with out-of-plane distortions play crucial roles in many biological functions and chemical applications. The artificial construction of nonplanar porphyrins usually involves organic synthesis and modification, which is a highly comprehensive approach. However, incorporating porphyrins into guest-stimulated flexible systems allows to manipulate the porphyrin distortion through simple ad/desorption of guest molecules. Here, a series of porphyrinic zirconium metal-organic frameworks (MOFs) is reported that exhibit guest-stimulated breathing behavior. X-Ray diffraction analysis and skeleton deviation plots confirm that the material suffers from porphyrin distortion to form a ruffled geometry under the desorption of guest molecules. Further investigation reveals that not only the degree of nonplanarity can be precisely manipulated but also the partial distortion of porphyrin in a single crystal grain can be readily achieved. As Lewis acidic catalyst, the MOF with nonplanar Co-porphyrin exhibits active properties in catalyzing CO2 /propylene oxide coupling reactions. This porphyrin distortion system provides a powerful tool for manipulating nonplanar porphyrins in MOFs with individual distortion profiles for various advanced applications.

Multi-Modular Design of Stable Pore-Space-Partitioned Metal-Organic Frameworks for Gas Separation Applications

Pore space partition (PSP) is an effective materials design method for developing high-performance small-pore materials for storage and separation of gas molecules. The continued success of PSP depends on broad availability and judicious choice of pore-partition ligands and better understanding of each structural module on stability and sorption properties. Here, by using substructural bioisosteric strategy (sub-BIS), a dramatic expansion of pore-partitioned materials is targeted by using ditopic dipyridyl ligands with non-aromatic cores or extenders, as well as by expanding heterometallic clusters to uncommon nickel-vanadium and nickel-indium clusters rarely known before in porous materials. The dual-module iterative refinement of pore-partition ligands and trimers leads to remarkable enhancement of chemical stability and porosity. Here a family of 23 pore-partitioned materials synthesized from five pore-partition ligands and seven types of trimeric clusters is reported. New materials with such compositionally and structurally diverse framework modules reveal key factors that dictate stability, porosity, and gas separation properties. Among these, materials based on heterometallic vanadium-nickel trimeric clusters give rise to the highest long-term hydrolytic stability and remarkable uptake capacity for CO2 , C2 H2 /C2 H4 /C2 H6 , and C3 H6 /C3 H8 hydrocarbon gases. The breakthrough experiment shows the potential application of new materials for separating gas mixtures such as C2 H2 /CO2 .

Adaptive Pore Opening to Form Tailored Adsorption Sites in a Cooperatively Flexible Framework Enables Record Inverse Propane/Propylene Separation

A proposed low-energy alternative to the separation of alkanes from alkenes by energy-intensive cryogenic distillation is separation by porous adsorbents. Unfortunately, most adsorbents preferentially take up the desired, high-value major component alkene, requiring frequent regeneration. Adsorbents with inverse selectivity for the minor component alkane would enable the direct production of purified, reagent-grade alkene, greatly reducing global energy consumption. However, such materials are exceedingly rare, especially for propane/propylene separation. Here, we report that through adaptive and spontaneous pore size and shape adaptation to optimize an ensemble of weak noncovalent interactions, the structurally responsive metal-organic framework CdIF-13 (sod-Cd(benzimidazolate)2) exhibits inverse selectivity for propane over propylene with record-setting separation performance under industrially relevant temperature, pressure, and mixture conditions. Powder synchrotron X-ray diffraction measurements combined with first-principles calculations yield atomic-scale insight and reveal the induced fit mechanism of adsorbate-specific pore adaptation and ensemble interactions between ligands and adsorbates. Dynamic column breakthrough measurements confirm that CdIF-13 displays selectivity under mixed-component conditions of varying ratios, with a record measured selectivity factor of α ≈ 3 at 95:5 propylene:propane at 298 K and 1 bar. When sequenced with a low-cost rigid adsorbent, we demonstrated the direct purification of propylene under ambient conditions. This combined atomic-level structural characterization and performance testing firmly establishes how cooperatively flexible materials can be capable of unprecedented separation factors.

Functionally Tailored Metal-Organic Framework Coatings for Mediating Ti Implant Osseointegration

Owing to their mechanical resilience and non-toxicity, titanium implants are widely applied as the major treatment modality for the clinical intervention against bone fractures. However, the intrinsic bioinertness of Ti and its alloys often impedes the effective osseointegration of the implants, leading to severe adverse complications including implant loosening, detachment, and secondary bone damage. Consequently, new Ti implant engineering strategies are urgently needed to improve their osseointegration after implantation. Remarkably, metalorganic frameworks (MOFs) are a class of novel synthetic material consisting of coordinated metal species and organic ligands, which have demonstrated a plethora of favorable properties for modulating the interfacial properties of Ti implants. This review comprehensively summarizes the recent progress in the development of MOF-coated Ti implants and highlights their potential utility for modulating the bio-implant interface to improve implant osseointegration, of which the discussions are outlined according to their physical traits, chemical composition, and drug delivery capacity. A perspective is also provided in this review regarding the current limitations and future opportunities of MOF-coated Ti implants for orthopedic applications. The insights in this review may facilitate the rational design of more advanced Ti implants with enhanced therapeutic performance and safety.

Development of a Mixed Multinuclear Cluster Strategy in Metal-Organic Frameworks for Methane Purification and Storage

Metal-organic frameworks (MOFs) have attracted extensive attention in methane (CH4) purification and storage. Specially, multinuclear cluster-based MOFs usually have prominent performance because of large cluster size and abundant open metal sites. However, compared to diverse combinations of organic linkers, one MOF with two or more multinuclear clusters is difficult to achieve. In this paper, we demonstrate a mixed multinuclear cluster strategy, which successfully led to three new heterometallic MOFs (SNNU-328-330) with the same common H3TATB [2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine] tritopic linker and six types of multinuclear clusters ([YCd(COO)4(μ2-H2O)], [YCd2(COO)8], [In3(COO)6(μ3-OH)], [In3Eu2(COO)9(μ3-OH)3(μ4-O)], [Y9(COO)12(μ3-OH)14] and [Y2Cd8(COO)16(μ2-H2O)4(μ3-OH)8]). Three MOF adsorbents all show great potentials to remove the impurities (CO2 and C2-hydrocarbons) in natural gas and show prominent high-pressure methane storage capacity. Among them, the ideal adsorbed solution theory separation ratios of equimolar C2H2/CH4, C2H4/CH4, C2H6/CH4, and CO2/CH4 at 298 K for SNNU-328 reach to 29.7-16.0, 19.1-8.2, 33.2-10.3, and 74.3-8.5, which have surpassed many famous MOF adsorbents. Dynamic breakthrough experiments conducted at 273 and 298 K showed that SNNU-330 can separate CH4 from C2H2/CH4, C2H4/CH4, C2H6/CH4, and CO2/CH4 mixtures with the breakthrough interval times of about 48.2, 17.9, 37.2, and 17.1 min g-1 (273 K, 1 bar, v/v = 50/50, 2 mL min-1), respectively. Remarkably, SNNU-329 exhibits extremely high methane storage performance at 298 K with the total uptake and working capacity of 192 cm3 cm-3 (95 bar) and 171 cm3 cm-3 (65 bar) due to the synergistic effects of high surface area, suitable pore sizes, and multiple open metal sites.

Cucurbituril-Shaped Cd18(triazolate)12 Unit-Based Metal-Organic Framework Exhibiting an C2H2/CO2 Separation Ability

Herein, a metal-organic framework (MOF), {[(Me2NH2)4][Cd(H2O)6][Cd18(TrZ)12(TPD)15(DMF)6]}n (denoted as JXNU-18, TrZ = triazolate), constructed from the unique cucurbituril-shaped Cd18(TrZ)12 secondary building units bridged by 2,5-thiophenedicarboxylic (TPD2-) ligands, is presented. The formation of the cucurbituril-shaped Cd18(TrZ)12 unit is unprecedented, demonstrating the geometric compatibility of the organic linkers and the coordination configurations of the cadmium atoms. Each Cd18(TrZ)12 unit is connected to eight neighboring Cd18(TrZ)12 units through 30 TPD2- linkers, affording the three-dimensional structure of JXNU-18. More interesting is that JXNU-18 displays an efficient C2H2/CO2 separation ability, as revealed by the gas adsorption experiments and dynamic gas breakthrough experiments, which afford insights into the potential applications of JXNU-18 in gas separation. The tubular pores composed of two Cd18(TrZ)12 units bridged by six 2,5-thiophenedicarboxylic linkers provide the suitable pore space for C2H2 trapping, as unveiled by computational simulations.

pH-Stable Zn(II) Coordination Polymer as a Multiresponsive Turn-On and Turn-Off Fluorescent Sensor for Aqueous Medium Detection of Al(III) and Cr(VI) Oxo-Anions

Nowadays, coordination polymers (CPs) are promising candidates as sensory materials for their high sensitivity, improved selectivity, fast responsive nature, as well as good recyclability. However, poor chemical stability often makes their practical usage limited. Herein, employing a mixed ligand approach, we constructed a chemically robust CP, {[Zn2L2(DPA)2]·3H2O}n (IITKGP-70, IITKGP stands for the Indian Institute of Technology Kharagpur), which exhibited excellent framework robustness not only in water but also over a broad range of pH solutions (pH = 3-11). The developed framework displayed high selectivity and sensitivity for the detection of trivalent Al3+ ions and toxic hexavalent Cr(VI)-oxo anions in an aqueous medium. The developed framework exhibited an aqueous medium Al3+ turn-on phenomenon with a limit of detection (LOD) value of 1.29 μM, whereas a turn-off effect was observed for toxic oxo-anions (Cr2O72- and CrO42-) having LOD values of 0.27 and 0.71 μM, respectively. Both turn-on and turn-off mechanisms are speculated via spectroscopic methods coupled with several ex situ studies. Such a multiresponsive nature (both turn-on and turn-off) for aqueous medium detection of targeted cations and anions simultaneously in a single platform coupled with high robustness, ease of scalability, recyclability, and fast-responsive nature makes IITKGP-70 highly fascinating as a sensory material for real-world applications.

Pore Space Partitioning MIL-88(Co): Developing Robust Adsorbents for CO2/CH4 Separation Featured with High CO2 Adsorption and Rapid Desorption

Metal-organic frameworks (MOFs) have attracted significant attention as sorbents for gas separation and purification. Ideally, an industrially potential adsorbent should combine exceptional gas uptake, excellent stability, and a lower regeneration energy; however, it remains a great challenge. Here, by utilizing the pore space partition (PSP) strategy, we develop three isostructural MOF materials (Co-BDC-TPB, Co-DCBDC-TPB, and Co-DOBDC-TPB) based on pristine MIL-88(Co). The three pore-space-partitioned crystalline microporous MOFs have triangular bipyramid cages and segmented one-dimensional channels, and among them, Co-DOBDC-TPB exhibits the highest CO2 uptake capacity (4.35 mmol g-1) and good CO2/N2 (29.7) and CO2/CH4 (6.2) selectivity. The selectivity-capacity synergy endows it with excellent CO2/N2 and CO2/CH4 separation performance. Moreover, Co-DOBDC-TPB can complete desorption within 10 min. The satisfactory CO2 adsorption ability can be attributed to both microporous aperture arising from PSP and modification of the pore surface by the polar hydroxy group, which enhances the interaction between Co-DOBDC-TPB and CO2 molecules significantly. The exceptional regeneration property may be due to its lower CO2 isosteric heat of adsorption (23.6 kJ/mol). The developed pore-space-partitioned MIL-88(Co) material Co-DOBDC-TPB may have potential application to flue gas and natural gas purification.

Cyclobutanedicarboxylate Metal-Organic Frameworks as a Platform for Dramatic Amplification of Pore Partition Effect

Ultrafine tuning of MOF structures at subangstrom or picometer levels can help improve separation selectivity for gases with subtle differences. However, for MOFs with a large enough pore size, the effect from ultrafine tuning on sorption can be muted. Here we show an integrative strategy that couples extreme pore compression with ultrafine pore tuning. This strategy is made possible by unique combination of two features of the partitioned acs (pacs) platform: multimodular framework and exceptional tolerance toward isoreticular replacement. Specifically, we use one module (ligand 1, L1) to shrink the pore size to an extreme minimum on pacs. A compression ratio of about 30% was achieved (based on the unit cell c/a ratio) from prototypical 1,4-benzenedicarboxylate-pacs to trans-1,3-cyclobutanedicarboxylate-pacs. This is followed by using another module (ligand 2, L2) for ultrafine pore tuning (<3% compression). This L1-L2 strategy increases the C2H2/CO2 selectivity from 2.6 to 20.8 and gives rise to an excellent experimental breakthrough performance. As the shortest cyclic dicarboxylate that mimics p-benzene-based moieties using a bioisosteric (BIS) strategy on pacs, trans-1,3-cyclobutanedicarboxylate offers new opportunities in MOF chemistry.

Isoreticular Contraction of Metal-Organic Frameworks Induced by Cleavage of Covalent Bonds

Isoreticular chemistry, in which the organic or inorganic moieties of reticular materials can be replaced without destroying their underlying nets, is a key concept for synthesizing new porous molecular materials and for tuning or functionalization of their pores. Here, we report that the rational cleavage of covalent bonds in a metal-organic framework (MOF) can trigger their isoreticular contraction, without the need for any additional organic linkers. We began by synthesizing two novel MOFs based on the MIL-142 family, (In)BCN-20B and (Sc)BCN-20C, which include cleavable as well as noncleavable organic linkers. Next, we selectively and quantitatively broke their cleavable linkers, demonstrating that various dynamic chemical and structural processes occur within these structures to drive the formation of isoreticular contracted MOFs. Thus, the contraction involves breaking of a covalent bond, subsequent breaking of a coordination bond, and finally, formation of a new coordination bond supported by structural behavior. Remarkably, given that the single-crystal character of the parent MOF is retained throughout the entire transformation, we were able to monitor the contraction by single-crystal X-ray diffraction.