Polycrystalline zeolite and metal-organic framework membranes for molecular separations

Regulating the Layered Stacking of a Covalent Triazine Framework Membrane for Aromatic/Aliphatic Separation

Membrane separation of aromatics and aliphatics is a crucial requirement in chemical and petroleum industries. However, this task presents a significant challenge due to the lack of membrane materials that can endure harsh solvents, exhibit molecular specificity, and facilitate easy processing. Herein, we present a novel approach to fabricate a covalent triazine framework (CTF) membrane by employing a mix-monomer strategy. By incorporating a spatial monomer alongside a planar monomer, we were able to subtly modulate both the pore aperture and membrane affinity, enabling preferential permeation of aromatics over aliphatics with molecular weight below 200 Dalton (Da). Consequently, we achieved successful all-liquid phase separation of aromatic/aliphatic mixtures. Our investigation revealed that the synergistic effects of size sieving and the affinity between the permeating molecules and the membrane played a pivotal role in separating these closely resembling species. Furthermore, the membrane exhibited remarkable robustness under practical operating conditions, including prolonged operation time, various feed compositions, different applied pressure, and multiple feed components. This versatile strategy offers a feasible approach to fabricate membranes with molecule selectivity toward aromatic/aliphatic mixtures, taking a significant step forward in addressing the grand challenge of separating small organic molecules through membrane technology.

MOF-Based Membranes for Remediated Application of Water Pollution

Membrane separation plays a crucial role in the current increasingly complex energy environment. Membranes prepared by metal-organic framework (MOF) materials usually possess unique advantages in common, such as uniform pore size, ultra-high porosity, enhanced selectivity and throughput, and excellent adsorption property, which have been contributed to the separation fields. In this comprehensive review, we summarize various designs and synthesized strategies of free-standing MOF and composite MOF-based membranes for water treatment. Special emphases are given not only on the effects of MOF on membrane performance, removal efficiencies, and elimination mechanisms, but also on the importance of MOF-based membranes for the applications of oily and micro-pollutant removal, adsorption, separation, and catalysis. The challenges and opportunities in the future for the industrial implementation of MOF-based membranes are also discussed.

Solution processing of crystalline porous material based membranes for CO2 separation

The carbon emission problem is a significant challenge in today's society, which has led to severe global climate issues. Membrane-based separation technology has gained considerable interest in CO2 separation due to its simplicity, environmental friendliness, and energy efficiency. Crystalline porous materials (CPMs), such as zeolites, metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, and porous organic cages, hold great promise for advanced CO2 separation membranes because of their ordered and customizable pore structures. However, the preparation of defect-free and large-area crystalline porous material (CPM)-based membranes remains challenging, limiting their practical use in CO2 separation. To address this challenge, the solution-processing method, commonly employed in commercial polymer preparation, has been adapted for CPM membranes in recent years. Nanosheets, spheres, molecular cages, and even organic monomers, depending on the CPM type, are dissolved in suitable solvents and processed into continuous membranes for CO2 separation. This feature article provides an overview of the recent advancements in the solution processing of CPM membranes. It summarizes the differences among the solution-processing methods used for forming various CPM membranes, highlighting the key factors for achieving continuous membranes. The article also summarizes and discusses the CO2 separation performance of these membranes. Furthermore, it addresses the current issues and proposes future research directions in this field. Overall, this feature article aims to shed light on the development of solution-processing techniques for CPM membranes, facilitating their practical application in CO2 separation.

Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.

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.

Synthesis and Characterization of Proton-Conducting Composites Prepared by Introducing Imidazole or 1,2,4-Triazole into AlPO-5 and SAPO-5 Molecular Sieves

The present work concerns proton-conducting composites obtained by replacing the water molecules present in aluminophosphate and silicoaluminophosphate AFI-type molecular sieves (AlPO-5 and SAPO-5) with azole molecules (imidazole or 1,2,4-triazole). Both the introduction of azoles and the generation of Brønsted acid centers by isomorphous substitution in aluminophosphate materials were aimed at improving the proton conductivity of the materials and its stability. In the presented study, AlPO-5 and several SAPO-5 materials differing in silicon content were synthesized. The obtained porous matrices were studied using PXRD, low-temperature nitrogen sorption, TPD-NH3, FTIR, and SEM. The proton conductivity of composites was measured using impedance spectroscopy. The results show that the increase in silicon content of the porous matrices is accompanied by an increase in their acidity. However, this does not translate into an increase in the conductivity of the azole composites. Triazole composites show lower conductivity and significantly higher activation energies than imidazole composites; however, most triazole composites show much higher stability. The different conductivity values for imidazole and triazole composites may be due to differences in chemical properties of the azoles.

Single-Molecule Fluorescence Investigations of Solute Transport Dynamics in Nanostructured Membrane Separation Materials

Many materials used for membrane separations are composed of nanoscale structures such as pores and domains. Such nanostructures often control the solute permeability and selectivity of the separation membranes. Thus, for future development of highly efficient separation membranes, it is important to understand the structural and chemical properties of these nanostructures and also their influences on solute transport dynamics. For the last two decades, single-molecule fluorescence techniques have been used to measure the detailed dynamics of solute molecules diffusing in various nanostructured materials, giving valuable insights into molecular transport mechanisms influenced by nanoscale material heterogeneity. This Perspective discusses recent single-molecule fluorescence studies on solute diffusion in materials relevant to membrane separations, including dense polymer films and nanoporous materials. These studies have revealed the formation and properties of nanostructures and unique transport dynamics of solute molecules manipulated by their confinement and partitioning to the nanostructures, which play key roles in membrane separations. This Perspective will also point out scientific challenges toward a thorough understanding of molecular-level mechanisms in membrane separations.

Continuous-imprinted-layer nanofiber membrane with MXene-based precise-designed nanocages for high-accuracy recognition and separation of shikimic acid

Ti₃C₂T_x (MXene) has attracted extensive attention from scholars at home and abroad due to its rich surface termination functional groups and two-dimensional multilayer structure. In this work, MXene was introduced to the membrane by vacuum-assisted filtration processes, and the formed interlayer channel facilitated the construction of recognition sites and molecular transmission. In this paper, PDA@MXene@PDA@SiO₂-PVDF dual-imprinted mixed matrix membrane (PMS-DIMs) were developed by the cooperative dual-imprinting strategy, which was used for the adsorption of shikimic acid (SA). Firstly, SiO₂-PVDF nanofiber basement membrane were prepared by electrospinning method and the first Polydopamine (PDA)-based imprinted layer was constructed on the membrane. PDA not only realized the imprinting process, PDA modification was used to give MXene nanosheets better antioxidant properties and to confer the SiO₂-PVDF nanofiber membrane the interface stability. After that, the second-imprinted sites were constructed on the stacked MXene nanosheets surface as well as between the layers. The SA dual-imprinted sites significantly increased the efficiency of the selective adsorption efficiency, when the template molecule passed through the membrane, the cooperative dual-imprinting strategy enabled multiplex recognition and adsorption of template molecules. As a consequence, which greatly improving the rebinding ability(262.17 g m^-2), and mselectivity factors (β_Catechol/SA, β_P-HB/SA, β_P-NP/SA were 2.34, 4.50 and 5.68). High stability proved the potentials of the PMS-DIMs for practical application. Precise SA-recognition sites were constructed on the PMS-DIMs, PMS-DIMs not only exhibit excellent selective rebinding properties but also have high permeability.

Room-Temperature Preparation of Covalent Organic Framework Membrane for Nanofiltration

The uniquely tunable nature of covalent organic frameworks (COFs), whose pore size and stability can be controlled by choosing diverse organic building blocks and linkage types, makes COFs potential candidates for the membrane separation. Therefore, the preparation of membranes with effective separation efficiency based on COFs has aroused great interest among researchers. Although solvothermal approach has been the most popular method for the preparation of COF membranes, fabricating COF membranes at room temperature (RT) will provide a simple and captivating strategy for separation membranes. Herein, a P-COF membrane on porous alumina substrate at RT, showing 99.7% rejection of rhodamine B and excellent water permeance up to 52 L m^-2 h^-1 bar^-1 , which can effectively purify wastewater is successfully obtained. P-COF is directly grown on alumina to form the composite membrane, which enhances the mechanical strength of COF membrane and avoids the risk of damaging the membrane structure during the transfer process of self-standing membrane. Moreover, P-COF membrane is grown at RT, which is more energy efficient than the conventional solvothermal method. Thus, it is of great significance to obtain COF membranes with excellent nanofiltration performance in a simple and mild condition to alleviate environmental and energy concerns.

Fluorinated metal-organic frameworks for gas separation

Fluorinated metal-organic frameworks (F-MOFs) as fast-growing porous materials have revolutionized the field of gas separation due to their tunable pore apertures, appealing chemical features, and excellent stability. A deep understanding of their structure-performance relationships is critical for the synthesis and development of new F-MOFs. This critical review has focused on several strategies for the precise design and synthesis of new F-MOFs with structures tuned for specific gas separation purposes. First, the basic principles and concepts of F-MOFs as well as their structure, synthesis and modification and their structure to property relationships are studied. Then, applications of F-MOFs in adsorption and membrane gas separation are discussed. A detailed account of the design and capabilities of F-MOFs for the adsorption of various gases and the governing principles is provided. In addition, the exceptional characteristics of highly stable F-MOFs with engineered pore size and tuned structures are put into perspective to fabricate selective membranes for gas separation. Systematic analysis of the position of F-MOFs in gas separation revealed that F-MOFs are benchmark materials in most of the challenging gas separations. The outlook and future directions of the science and engineering of F-MOFs and their challenges are highlighted to tackle the issues of overcoming the trade-off between capacity/permeability and selectivity for a serious move towards industrialization.

Tunable rare-earth metal-organic frameworks for ultra-high selenite capture

Linkers and clusters with various conformations present challenges for the design and prediction of highly porous and stable rare-earth metal-organic frameworks (RE-MOFs) for trapping toxic ions in aqueous solutions. Herein, we designed and synthesized a series of RE-MOFs based on a malleable ligand to explore the effects of ligands, clusters, and configurations on structural stability. The results showed that the nonanuclear high-connected UPC-183 exhibited better stability than the hexanuclear low-connected RE-MOF (UPC-181/182 series). Due to the syngenetic effect of chemi- and physisorption, the adsorption capacity of UPC-183-Eu for selenite (SeO₃^2-) is as high as 308.39 mg/g, recorded one of the highest ever reported for MOFs. Furthermore, we accurately analyzed the adsorption site of UPC-183-Eu for SeO₃^2- through single-crystal structure and theoretical simulation. The ultra-high selenite adsorption capacity and removal efficiency endow UPC-183-Eu an excellent porous adsorbent for removing pollutants.

Applications of metal-organic framework-based bioelectrodes

Metal-organic frameworks (MOFs) are an emerging class of porous nanomaterials that have opened new research possibilities. The inherent characteristics of MOFs such as their large surface area, high porosity, tunable pore size, stability, facile synthetic strategies and catalytic nature have made them promising materials for enormous number of applications, including fuel storage, energy conversion, separation, and gas purification. Recently, their high potential as ideal platforms for biomolecule immobilization has been discovered. MOF-enzyme-based materials have attracted the attention of researchers from all fields with the expansion of MOFs development, paving way for the fabrication of bioelectrochemical devices with unique characteristics. MOFs-based bioelectrodes have steadily gained interest, wherein MOFs can be utilized for improved biomolecule immobilization, electrolyte membranes, fuel storage, biocatalysis and biosensing. Likewise, applications of MOFs in point-of-care diagnostics, including self-powered biosensors, are exponentially increasing. This paper reviews the current trends in the fabrication of MOFs-based bioelectrodes with emphasis on their applications in biosensors and biofuel cells.

Interzeolite Conversion-Synthesized Sub-1 μm NaA Zeolite Membrane for C2H2/C2H4 Selective Separation

Zeolite membranes with reduced thickness and high continuity are of paramount importance for accelerating selective gas separation for resemblant molecules, and the synthesis of such membranes remains a grand challenge. Herein, we developed an interzeolite conversion synthesis approach to grow NaA zeolite membranes on NaX. The conversion of NaX into NaA proceeded via mild hydrothermal treatment of a dilute synthesis solution, preferentially forming a continuous polycrystalline NaA layer on the surface of NaX, which was precrystallized on a porous alumina support. The thickness of the NaA zeolite membrane was successfully controlled to the submicron scale (500 nm). The synthesized NaA membrane functioned as a selective separator for C₂H₂ and C₂H₄ gases. Taking the traditionally in situ grown membrane as a reference, the interzeolite-derived membrane exhibited a 3.5-fold separation factor and ∼4.0 times C₂H₂ permeance. This approach provides an alternative synthesis option for zeolite membranes with advanced properties, and high efficiency in terms of superior gas selectivity and permeability is promising in precise gas separation.

Hetero-lattice intergrown and robust MOF membranes for polyol upgrading

Metal-organic framework membranes are frequently used in gas separations, but rare in pervaporation for liquid chemical upgrading, especially for separating water from polyols, due to lack of highly compact and robust micro-architecture. Here, we report hetero-lattice intergrown membranes in which amino-MIL-101 (Cr) particles embedded into the micro-gaps of MIL-53 (Al) rod arrays after secondary growth. By means of high-resolution TEM and two-dimensional topologic simulation, the connection between these two distinct MOF lattices at molecular-level and their crystallographic geometry harmony is identified, which leads to a close-knit structure at crystal boundaries of membranes. Typically, the membrane shows a separation factor as high as 13,000 for 90/10 ethanediol/water solution in pervaporation, yields polymer-grade ethanediol, and saves ca. 32% of energy consumption vs. vacuum distillation. It has a highly robust micro-architecture, with great tolerance to high pressure, durability against ultrasonic therapy and long-term separation stability over 600 h.

Multivariate Polycrystalline Metal-Organic Framework Membranes for CO2/CH4 Separation

Membrane technology is attractive for natural gas separation (removing CO₂, H₂O, and hydrocarbons from CH₄) because of membranes' low energy consumption and small environmental footprint. Compared to polymeric membranes, microporous inorganic membranes such as silicoaluminophosphate-34 (SAPO-34) membrane can retain their separation performance under conditions close to industrial requirements. However, moisture and hydrocarbons in natural gas can be strongly adsorbed in the pores of those membranes, thereby reducing the membrane separation performance. Herein, we report the fabrication of a polycrystalline MIL-160 membrane on an Al₂O₃ substrate by in situ hydrothermal synthesis. The MIL-160 membrane with a thickness of ca. 3 μm shows a remarkable molecular sieving effect in gas separation. Besides, the pore size and environment of the MIL-160 membrane can be precisely controlled using reticular chemistry by regulating the size and functionality of the ligand. Interestingly, the more polar fluorine-functionalized multivariate MIL-160/CAU-10-F membrane exhibits a 10.7% increase in selectivity for CO₂/CH₄ separation and a 31.2% increase in CO₂ permeance compared to those of the MIL-160 membrane. In addition, hydrophobic MIL-160 membranes and MIL-160/CAU-10-F membranes are more resistant to water vapor and hydrocarbons than the hydrophilic SAPO-34 membranes.

A porous anionic zinc(ii) metal–organic framework for gas adsorption, selective uptake of dyes and sensing of Fe3+ by Tb3+ ion encapsulation

A 3D anionic, porous MOF exhibits selective adsorption of cationic dyes and can be used as a fast-response fluorescence sensor for the detection of Fe3+ ions by Tb3+ ion encapsulation.