Engineering Metal-Organic Frameworks with Tunable Colors for High-Performance Wireless Communication.

Metal-organic frameworks (MOFs) have emerged as excellent platforms possessing tunable and controllable optical behaviors that are essential in high-speed and multichannel data transmission in optical wireless communications (OWCs). Here, we demonstrate a novel approach to achieving a tunable wide modulation bandwidth and high net data rate by engineering a combination of organic linkers and metal clusters in MOFs. More specifically, two organic linkers of different emission colors, but equal molecular length and connectivity, are successfully coordinated by zirconium and hafnium oxy-hydroxy clusters to form the desired MOF structures. The precise change in the interactions between these different organic linkers and metal clusters enables control over fluorescence efficiency and excited state lifetime, leading to a tunable modulation bandwidth from 62.1 to 150.0 MHz and a net data rate from 303 to 363 Mb/s. The fabricated color converter MOFs display outstanding performance that competes, and in some instances surpasses, those of conventional materials commonly used in light converter devices. Moreover, these MOFs show high practicality in color-pure wavelength-division multiplexing (WDM), which significantly improved the data transmission link capacity and security by the contemporary combining of two different data signals in the same path. This work highlights the potential of engineered MOFs as a game-changer in OWCs, with significant implications for future high-speed and secure data transmission.

Mixed Matrix Membranes with Surface Functionalized Metal-Organic Framework Sieves for Efficient Propylene/Propane Separation.

Membrane technology, regarded as an environmentally friendly and sustainable approach, offers great potential to address the large energy penalty associated with the energy-intensive propylene/propane separation. Quest for molecular sieving membranes for this important separation is of tremendous interest. Here, a fluorinated metal-organic framework (MOF) material, known as KAUST-7 (KAUST: King Abdullah University of Science and Technology) with well-defined narrow 1D channels that can effectively discriminate propylene from propane based on a size-sieving mechanism, is successfully incorporated into a polyimide matrix to fabricate molecular sieving mixed matrix membranes (MMMs). Markedly, the surface functionalization of KAUST-7 nanoparticles with carbene moieties affords the requisite interfacial compatibility, with minimal nonselective defects at polymer-filler interfaces, for the fabrication of a molecular sieving MMM. The optimal membrane with a high MOF loading (up to 45 wt.%) displays a propylene permeability of ≈95 barrer and a mixed propylene/propane selectivity of ≈20, far exceeding the state-of-the-art upper bound limits. Moreover, the resultant membrane exhibits robust structural stability under practical conditions, including high pressures (up to 8 bar) and temperatures (up to 100 °C). The observed outstanding performance attests to the importance of surface engineering for the preparation and plausible deployment of high-performance MMMs for industrial applications.

Advances in metal-organic framework-based membranes.

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

Metal-Organic Frameworks in Mixed-Matrix Membranes for High-Speed Visible-Light Communication.

Mixed-matrix membranes (MMMs) based on luminescent metal-organic frameworks (MOFs) and emissive polymers with the combination of their unique advantages have great potential in separation science, sensing, and light-harvesting applications. Here, we demonstrate MMMs for the field of high-speed visible-light communication (VLC) using a very efficient energy transfer strategy at the interface between a MOF and an emissive polymer. Our steady-state and ultrafast time-resolved experiments, supported by high-level density functional theory calculations, revealed that efficient and ultrafast energy transfer from the luminescent MOF to the luminescent polymer can be achieved. The resultant MMMs exhibited an excellent modulation bandwidth of around 80 MHz, which is higher than those of most well-established color-converting phosphors commonly used for optical wireless communication. Interestingly, we found that the efficient energy transfer further improved the light communication data rate from 132 Mb/s of the pure polymer to 215 Mb/s of MMMs. This finding not only showcases the promise of the MMMs for high-speed VLC but also highlights the importance of an efficient and ultrafast energy transfer strategy for the advancement of data rates of optical wireless communication.

Ultralarge Free-Standing Imine-Based Covalent Organic Framework Membranes Fabricated via Compression.

Demand continues for processing methods to shape covalent organic frameworks (COFs) into macroscopic objects that are needed for their practical applications. Herein, a simple compression method to prepare large-scale, free-standing homogeneous and porous imine-based COF-membranes with dimensions in the centimeter range and excellent mechanical properties is reported. This method entails the compression of imine-based COF-aerogels, which undergo a morphological change from an elastic to plastic material. The COF-membranes fabricated upon compression show good performances for the separation of gas mixtures of industrial interest, N2 /CO2 and CH4 /CO2 . It is believed that the new procedure paves the way to a broader range of COF-membranes.

Pressure-Responsive Two-Dimensional Metal-Organic Framework Composite Membranes for CO2 Separation.

The regulation of permeance and selectivity in membrane systems may allow effective relief of conventional energy-intensive separations. Here, pressure-responsive ultrathin membranes (≈100 nm) fabricated by compositing flexible two-dimensional metal-organic framework nanosheets (MONs) with graphene oxide nanosheets for CO2 separation are reported. By controlling the gas permeation direction to leverage the pressure-responsive phase transition of the MONs, CO2 -induced gate opening and closing behaviors are observed in the resultant membranes, which are accompanied with the sharp increase of CO2 permeance (from 173.8 to 1144 gas permeation units) as well as CO2 /N2 and CO2 /CH4 selectivities (from 4.1 to 22.8 and from 4 to 19.6, respectively). The flexible behaviors and separation mechanism are further elucidated by molecular dynamics simulations. This work establishes the relevance of structural transformation-based framework dynamics chemistry in smart membrane systems.

Two-Dimensional Membranes: New Paradigms for High-Performance Separation Membranes.

Two-dimensional (2D) materials with atomic thicknesses have aroused great interest as promising building blocks for the preparation of ultrathin 2D membranes. These 2D membranes can exhibit unprecedentedly high separation permeance owing to their ultrasmall membrane thicknesses and superior selectivity because of their size-selective nanopores and/or nanochannels. Until now, a large number of 2D membranes with good performance have been reported, highlighting the potential of these novel membranes for efficient liquid and gas separations. Summarized in this review are the latest advances in 2D membranes, with a special focus on industrially attractive separation processes, fabrication methods of laminar membranes, choices of membrane materials, designs of membrane structures, and unique membrane transport properties. Opportunities and challenges of 2D membranes for commercial applications are also briefly discussed.

Cluster nuclearity control and modulated hydrothermal synthesis of functionalized Zr12 metal-organic frameworks.

The synthesis of chemically stable metal-organic frameworks (MOFs) in benign solvents is a key step towards their scalable production and commercialization. Extending the modulated hydrothermal (MHT) synthesis approach for zirconium (Zr) MOFs, we herein demonstrate cluster nuclearity control over the secondary building units (SBUs) to target Zr6-based and Zr12-based phases for the Zr terephthalate (Zr-BDC) system. Different functional groups (-NH2, -NO2, -Br, -F4) can be incorporated into Zr12-BDC, providing an avenue to further tune the chemical properties of these promising MOFs.

Enhanced Polymer Crystallinity in Mixed-Matrix Membranes Induced by Metal-Organic Framework Nanosheets for Efficient CO2 Capture.

The design and fabrication of novel mixed-matrix membranes (MMMs) with simultaneously enhanced gas permeability and selectivity are highly sought for the industrial deployment of membrane technology for large-scale CO2 capture and storage. Conventional isotropic bulky particle fillers often exhibit limited interfacial compatibility that eventually leads to significant selectivity loss in MMMs. Here, we report the incorporation of chemically stable metal-organic framework (MOF) nanosheets into a highly permeable polymer matrix to prepare defect-free MMMs. MOF nanosheets are homogeneously dispersed within the polymer matrix, owing to their high aspect ratios that improve the polymer-filler integration. The strong hydrogen bonding and π-π interactions between the two components not only enhance the interfacial compatibility but also favor the efficient polymer chain packing along the surface of MOF nanosheets, leading to enhanced polymer crystallinity as well as size-sieving capability of the membranes. The as-prepared MMMs demonstrate high CO2-selective separation performance, good antipressure, and antiaging abilities, thus offering new opportunities in developing advanced membranes for industrial gas separation applications.

In Situ Formation of Micropore-Rich Titanium Dioxide from Metal-Organic Framework Templates.

Phase and porosity control in titanium dioxide (TiO2) is essential for the optimization of its photocatalytic activity. However, concurrent control over these two parameters remains challenging. Here, a novel metal-organic framework templating strategy is demonstrated for the preparation of highly microporous anatase TiO2. In situ encapsulation of Ti precursor in ZIF-8 cavities, followed by hydrolysis and etching, produces anatase TiO2 with a high Brunauer-Emmett-Teller surface area of 335 m2·g-1 and a micropore surface area ratio of 48%. Photocatalytic hydrogen generation catalyzed by the porous TiO2 can reach a rate of 2459 µmol·g-1·h-1. The measured photocatalytic activity is found to be positively correlated to the surface area, highlighting the importance of porosity control in heterogeneous photocatalysts.

Luminescent Metal-Organic Frameworks for the Detection and Discrimination of o-Xylene from Xylene Isomers.

Differentiation of xylene isomers remains as one of the most important challenges in the chemical industry, mainly due to the similar molecular sizes and boiling points of the three xylene isomers. Fluorescence-based chemical sensors have attracted wide attention due to their high sensitivity and versatile applications. Here, we report a novel fluorescent metal-organic framework named NUS-40, which is able to selectively detect and discriminate o-xylene from other xylene isomers. Suspension of NUS-40 in o-xylene produces a distinct red shift in the fluorescence emission compared to that in either m-xylene or p-xylene. Moreover, the extent of peak shift is dependent on the concentration of o-xylene in xylene isomer mixtures, and the observed linear correlation between fluorescence intensity and o-xylene concentration is beneficial for quantitative detection. The possible mechanism of such responsive fluorescence behavior was investigated by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and vapor sorption experiments. In addition, facile metalation of the porphyrin centers with metal ions provides an additional route to fine-tune the sensing properties.

Hydrazone-based covalent organic frameworks for Lewis acid catalysis.

Hydrazone-based covalent organic frameworks (COFs) are rich in functional groups but have very limited variety and applications. Herein, two hydroxy-rich hydrazone-based COFs are synthesized in pure water and postsynthetically incorporated with CoII, exhibiting Lewis acid catalytic activity towards cyanosilylation of various aldehydes with size selectivity.

Advanced Porous Materials in Mixed Matrix Membranes.

Membrane technology has gained great interest in industrial separation processing over the past few decades owing to its high energy efficiency, small capital investment, environmentally benign characteristics, and the continuous operation process. Among various types of membranes, mixed matrix membranes (MMMs) combining the merits of the polymer matrix and inorganic/organic fillers have been extensively investigated. With the rapid development of chemistry and materials science, recent studies have shifted toward the design and application of advanced porous materials as promising fillers to boost the separation performance of MMMs. Here, first a comprehensive overview is provided on the choices of advanced porous materials recently adopted in MMMs, including metal-organic frameworks, porous organic frameworks, and porous molecular compounds. Novel trends in MMMs induced by these advanced porous fillers are discussed in detail, followed by a summary of applying these MMMs for gas and liquid separations. Finally, a concise conclusion and current challenges toward the industrial implementation of MMMs are outlined, hoping to provide guidance for the design of high-performance membranes to meet the urgent needs of clean energy and environmental sustainability.

Process-Tracing Study on the Postassembly Modification of Highly Stable Zirconium Metal-Organic Cages.

Metal-organic cages (MOCs) are discrete molecular assemblies formed by coordination bonds between metal nodes and organic ligands. The application of MOCs has been greatly limited due to their poor stability, especially in aqueous solutions. In this work, we thoroughly investigate the stability of several Zr-MOCs and reveal their excellent stability in aqueous solutions with acidic, neutral, and weak basic conditions. In addition, we present for the first time a process-tracing study on the postassembly modification of one MOC, ZrT-1-NH2, highlighting the excellent stability and versatility of Zr-MOCs as a new type of molecular platform for various applications.

Mechanoassisted Synthesis of Sulfonated Covalent Organic Frameworks with High Intrinsic Proton Conductivity.

It is challenging to introduce pendent sulfonic acid groups into modularly built crystalline porous frameworks for intrinsic proton conduction. Herein, we report the mechanoassisted synthesis of two sulfonated covalent organic frameworks (COFs) possessing one-dimensional nanoporous channels decorated with pendent sulfonic acid groups. These COFs exhibit high intrinsic proton conductivity as high as 3.96 × 10(-2) S cm(-1) with long-term stability at ambient temperature and 97% relative humidity (RH). In addition, they were blended with nonconductive polyvinylidene fluoride (PVDF) affording a series of mixed-matrix membranes (MMMs) with proton conductivity up to 1.58 × 10(-2) S cm(-1) and low activation energy of 0.21 eV suggesting the Grotthuss mechanism for proton conduction. Our study has demonstrated the high intrinsic proton conductivity of COFs shedding lights on their wide applications in proton exchange membranes.

Efficient CO2 capture by functionalized graphene oxide nanosheets as fillers to fabricate multi-permselective mixed matrix membranes.

A novel multi-permselective mixed matrix membrane (MP-MMM) is developed by incorporating versatile fillers functionalized with ethylene oxide (EO) groups and an amine carrier into a polymer matrix. The as-prepared MP-MMMs can separate CO2 efficiently because of the simultaneous enhancement of diffusivity selectivity, solubility selectivity, and reactivity selectivity. To be specific, MP-MMMs were fabricated by incorporating polyethylene glycol- and polyethylenimine-functionalized graphene oxide nanosheets (PEG-PEI-GO) into a commercial low-cost Pebax matrix. The PEG-PEI-GO plays multiple roles in enhancing membrane performance. First, the high-aspect ratio GO nanosheets in a polymer matrix increase the length of the tortuous path of gas diffusion and generate a rigidified interface between the polymer matrix and fillers, enhancing the diffusivity selectivity. Second, PEG consisting of EO groups has excellent affinity for CO2 to enhance the solubility selectivity. Third, PEI with abundant primary, secondary, and tertiary amine groups reacts reversibly with CO2 to enhance reactivity selectivity. Thus, the as-prepared MP-MMMs exhibit excellent CO2 permeability and CO2/gas selectivity. The MP-MMM doped with 10 wt % PEG-PEI-GO displays optimal gas separation performance with a CO2 permeability of 1330 Barrer, a CO2/CH4 selectivity of 45, and a CO2/N2 selectivity of 120, surpassing the upper bound lines of the Robeson study of 2008 (1 Barrer = 10(-10) cm(3) (STP) cm(-2) s(-1) cm(-1) Hg).