Gel-Free Secondary Growth of Uniformly Oriented Silica MFI Zeolite Films and Application for Xylene Separation

Exceptionally Fast Separation of Xylene Isomers with Zeolitic Nanotube Array Membranes

The separation of xylene isomers is of vital importance in chemical industry but remains challenging due to their similar structure and overlapping physiochemical properties. Membrane-based separations using the zeolite MFI, graphene oxide, and metal-organic frameworks have been intensively studied for this application, but the performance is limited by the well-known rule that the filtrate permeance scales inversely with the membrane thickness. We propose a novel membrane design that is capable of breaking this rule, based on an array of recently discovered zeolite nanotubes. Each zeolite nanotube possesses a 3.6-nm-wide central channel, connecting to dense, uniform 0.8-nm-wide holes on its wall that act as selective pores. Comprehensive molecular dynamics simulations show that this membrane exhibits permeance exceeding current state-of-the-art membranes by at least an order of magnitude while simultaneously maintaining an acceptable selectivity. In particular, a thicker membrane featuring longer zeolite nanotubes exhibits a higher permeance due to the presence of more selective pores. The proposed membrane design is expected to be broadly applied to other gas separations and even desalination as long as zeolitic nanotubes with customized pores are available.

Two-Dimensional MFI-Type Zeolite Flow Battery Membranes

Vanadium flow battery (VFB) is one of the most reliable stationary electrochemical energy-storage technologies, and a membrane with high vanadium resistance and proton conductivity is essential for manufacturing high-performance VFBs. In this study, a two-dimensional (2D) MFI-type zeolite membrane was fabricated from zeolite nanosheet modules, which displayed excellent vanadium resistance (0.07 mmol L-1 h-1 ) and proton conductivity (0.16 S cm-1 ), yielding a coulombic efficiency of 93.9 %, a voltage efficiency of 87.6 %, and an energy efficiency of 82.3 % at 40 mA cm-2 . The self-discharge period of a VFB equipped with 2D MFI-type zeolite membrane increased up to 116.2 h, which was significantly longer than that of the commercial perfluorinated sulfonate membrane (45.9 h). Furthermore, the corresponding battery performance remained stable over 1000 cycles (>1500 h) at 80 mA cm-2 . These findings demonstrate that 2D MFI-type membranes are promising ion-conductive membranes applicable for stationary electrochemical energy-storage devices.

Tailoring properties and performance of thin-film composite membranes by salt additives for water treatment: A critical review

During the fabrication of thin film composite (TFC) membranes by interfacial polymerization (IP), the utilization of salt additives is one of the effective methods to regulate membrane properties and performance. Despite gradually receiving widespread attention for membrane preparation, the strategies, effects and underlying mechanisms of using salt additives have not yet been systematically summarized. This review for the first time provides an overview of various salt additives used to tailor properties and performance of TFC membranes for water treatment. By classifying salt additives into organic and inorganic salts, the roles of added salt additives in the IP process and the induced changes in membrane structure and properties are discussed in detail, and the different mechanisms of salt additives affecting membrane formation are summarized. Based on these mechanisms, the salt-based regulation strategies have shown great potential for improving the performance and application competitiveness of TFC membranes, including overcoming the trade-off relationship between water permeability and salt selectivity, tailoring membrane pore size distribution for precise solute-solute separation, and enhancing membrane antifouling performance. Finally, future research directions are suggested to focus on the long-term stability assessment of salt-modified membranes, the combined use of different salt additives, and the integration of salt regulation with other membrane design or modification strategies.

Engineering Metal-Organic-Framework (MOF)-Based Membranes for Gas and Liquid Separation

Separation is one of the most energy-intensive processes in the chemical industry, and membrane-based separation technology contributes significantly to energy conservation and emission reduction. Additionally, metal-organic framework (MOF) materials have been widely investigated and have been found to have enormous potential in membrane separation due to their uniform pore size and high designability. Notably, pure MOF films and MOF mixed matrix membranes (MMMs) are the core of the "next generation" MOF materials. However, there are some tough issues with MOF-based membranes that affect separation performance. For pure MOF membranes, problems such as framework flexibility, defects, and grain orientation need to be addressed. Meanwhile, there still exist bottlenecks for MMMs such as MOF aggregation, plasticization and aging of the polymer matrix, poor interface compatibility, etc. Herein, corresponding methods are introduced to solve these problems, including inhibiting framework flexibility, regulating synthesis conditions, and enhancing the interaction between MOF and substrate. A series of high-quality MOF-based membranes have been obtained based on these techniques. Overall, these membranes revealed desired separation performance in both gas separation (e.g., CO₂, H₂, and olefin/paraffin) and liquid separation (e.g., water purification, organic solvent nanofiltration, and chiral separation).

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.

One-Step Scalable Fabrication of Highly Selective Monolithic Zeolite MFI Membranes for Efficient Butane Isomer Separation

The reproducible fabrication of large-area zeolite membranes for gas separation is still a great challenge. We report the scalable fabrication of high-performance zeolite MFI membranes by single-step secondary growth on the 19-channel alumina monoliths for the first time. The packing density and mechanical strength of the monolithic membranes are much higher for these than for tubular ones. Separation performance of the monolithic membranes toward the butane isomer mixture was comparably evaluated using the vacuum and Wicke-Kallenbach modes. The n-butane permeances and n-butane/i-butane separation factors for the three membranes with an effective area of ∼84 cm² were >1.0 × 10^-7 mol (m² s Pa)^-1 and >50 at 343 K for an equimolar n-butane/i-butane mixture, respectively. We succeeded in scaling up the membrane synthesis with the largest area of 270 cm² to date which has 1.3 times the area of an industrial 1 m long tubular membrane. Monolith supported zeolite MFI membranes show great potential for industrial n-butane/i-butane separation.

Twin-free, directly synthesized MFI nanosheets with improved thickness uniformity and their use in membrane fabrication

Zeolite nanosheets can be used for the fabrication of low-defect-density, thin, and oriented zeolite separation membranes. However, methods for manipulating their morphology are limited, hindering progress toward improved performance. We report the direct synthesis (i.e., without using exfoliation, etching, or other top-down processing) of thin, flat MFI nanosheets and demonstrate their use as high-performance membranes for xylene isomer separations. Our MFI nanosheets were synthesized using nanosheet fragments as seeds instead of the previously used MFI nanoparticles. The obtained MFI nanosheets exhibit improved thickness uniformity and are free of rotational and MEL intergrowths as shown by transmission electron microscopy (TEM) imaging. The nanosheets can form well-packed nanosheet coatings. Upon gel-free secondary growth, the obtained zeolite MFI membranes show high separation performance for xylene isomers at elevated temperature (e.g., p-xylene flux up to 1.5 × 10^-3 mol m^-2 s^-1 and p-/o-xylene separation factor of ~600 at 250°C).

Continuous Porous Aromatic Framework Membranes with Modifiable Sites for Optimized Gas Separation

Continuous microporous membranes are widely studied for gas separation, due to their low energy premium and strong molecular specificity. Porous aromatic frameworks (PAFs) with their exceptional stability and structural flexibility are suited to a wide range of separations. Main-stream PAF-based membranes are usually prepared with polymeric matrices, but their discrete entities and boundary defects weaken their selectivity and permeability. The synthesis of continuous PAF membranes is still a major challenge because PAFs are insoluble. Herein, we successfully synthesized a continuous PAF membrane for gas separation. Both pore size and chemistry of the PAF membrane were modified by ion-exchange, resulting in good selectivity and permeance for the gas mixtures H₂ /N₂ and CO₂ /N₂ . The membrane with Br^- as a counter ion in the framework exhibited a H₂ /N₂ selectivity of 72.7 with a H₂ permeance of 51844 gas permeation units (GPU). When the counter ions were replaced by BF₄ ^- , the membrane showed a CO₂ permeance of 23058 GPU, and an optimized CO₂ /N₂ selectivity of 60.0. Our results show that continuous PAF membranes with modifiable pores are promising for various gas separation situations.

Advances and challenges in the development of nanosheet membranes

The development of highly efficient separation membranes utilizing emerging materials with controllable pore size and minimized thickness could greatly enhance the broad applications of membrane-based technologies. Having this perspective, many studies on the incorporation of nanosheets in membrane fabrication have been conducted, and strong interest in this area has grown over the past decade. This article reviews the development of nanosheet membranes focusing on two-dimensional materials as a continuous phase, due to their promising properties, such as atomic or nanoscale thickness and large lateral dimensions, to achieve improved performance compared to their discontinuous counterparts. Material characteristics and strategies to process nanosheet materials into separation membranes are reviewed, followed by discussions on the membrane performances in diverse applications. The review concludes with a discussion of remaining challenges and future outlook for nanosheet membrane technologies.

Elimination of Grain Boundary Defects in Zeolitic Imidazolate Framework ZIF-95 Membrane via Solvent-Free Secondary Growth

Metal-organic framework membranes are usually prepared by in situ or secondary growth in a solution/hydrogel. The use of organic solvents may cause safety and environmental problems and produce solvent-induced defects. Here, highly oriented and permselective ZIF-95 membranes are prepared for the first time via a solvent-free secondary growth method. The solvent-free growth is not only helpful to control the membrane microstructure and thickness, but also to reduce the intercrystalline defects. In case of solvent-free growth, a perfectly oriented structure leads to an outstanding reduction of intercrystalline defects and transport resistances. For the separation of equimolar binary gas mixtures by using the highly oriented ZIF-95 membrane at 25 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ are 184 and 140, respectively, with H₂ permeance of over 1.9×10^-7  mol m^-2  s^-1  Pa^-1 which are much higher than those of the randomly oriented ZIF-95 membrane.

Synthesis of silicalite-1 zeolite membranes for vapor-permeation separation of dichlorobenzene Isomers

Silicalite-1 zeolitic membranes have been successfully fabricated on the porousα-Al₂O₃support by templated and template-free protocol, respectively, for vapor permeation separation of dichlorobenzene (DCB) isomers. After proving the high quality of the membranes by single gas permeation (He and SF₆) performance, the vapor-permeation of DCB isomers over two types of the silicalite-1 membrane was then investigated. The separation results clearly indicated that under the lower partial pressure and higher temperature, the effect of DCB isomer adsorption on the permeance could be kept at a sufficiently low level and sharp selectivity become more important. Thus, highp-DCB selectivity could be achieved. Comparatively, the template-free silicalite-1 zeolite has a much higher p-DCB selectivity due to the relatively fewer inter-crystalline gaps. Under certain separation conditions, the highest selectivity ofp-DCB forp-/m-DCB andp-/o-DCB binary systems could reach 165 and 113, respectively.

Organic molecular sieve membranes for chemical separations

Molecular separations that enable selective transport of target molecules from gas and liquid molecular mixtures, such as CO2 capture, olefin/paraffin separations, and organic solvent nanofiltration, represent the most energy sensitive and significant demands. Membranes are favored for molecular separations owing to the advantages of energy efficiency, simplicity, scalability, and small environmental footprint. A number of emerging microporous organic materials have displayed great potential as building blocks of molecular separation membranes, which not only integrate the rigid, engineered pore structures and desirable stability of inorganic molecular sieve membranes, but also exhibit a high degree of freedom to create chemically rich combinations/sequences. To gain a deep insight into the intrinsic connections and characteristics of these microporous organic material-based membranes, in this review, for the first time, we propose the concept of organic molecular sieve membranes (OMSMs) with a focus on the precise construction of membrane structures and efficient intensification of membrane processes. The platform chemistries, designing principles, and assembly methods for the precise construction of OMSMs are elaborated. Conventional mass transport mechanisms are analyzed based on the interactions between OMSMs and penetrate(s). Particularly, the 'STEM' guidelines of OMSMs are highlighted to guide the precise construction of OMSM structures and efficient intensification of OMSM processes. Emerging mass transport mechanisms are elucidated inspired by the phenomena and principles of the mass transport processes in the biological realm. The representative applications of OMSMs in gas and liquid molecular mixture separations are highlighted. The major challenges and brief perspectives for the fundamental science and practical applications of OMSMs are tentatively identified.

An Extrinsic-Pore-Containing Molecular Sieve Film: A Robust, High-Throughput Membrane Filter

MFI type zeolites with 10-membered-ring pores (ca. 0.55 nm) have the ability to separate p-xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non-zeolitic micropores (ca. 0.6-1.5 nm) and mesopores (ca. 2-7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non-zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p-xylene perm-selectivity and molar flux. Specifically, compared to a conventional, crack network-containing MFI membranes of similar thickness (ca. 1 μm), the mesoporous MFI membranes showed almost double p-xylene permeance (ca. 1.6±0.4×10^-7  mol m^-2  s^-1  Pa^-1 ) and a high p-/o-xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non-zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

Scalable crystalline porous membranes: current state and perspectives

Crystalline porous materials (CPMs) with uniform and regular pore systems show great potential for separation applications using membrane technology. Along with the research on the synthesis of precisely engineered porous structures, significant attention has been paid to the practical application of these materials for preparation of crystalline porous membranes (CPMBs). In this review, the progress made in the preparation of thin, large area and defect-free CPMBs using classical and novel porous materials and processing is presented. The current state-of-the-art of scalable CPMBs with different nodes (inorganic, organic and hybrid) and various linking bonds (covalent, coordination, and hydrogen bonds) is revealed. The advances made in the scalable production of high-performance crystalline porous membranes are categorized according to the strategies adapted from polymer membranes (interfacial assembly, solution-casting, melt extrusion and polymerization of CPMs) and tailored based on CPM properties (seeding-secondary growth, conversion of precursors, electrodeposition and chemical vapor deposition). The strategies are compared and ranked based on their scalability and cost. The potential applications of CPMBs have been concisely summarized. Finally, the performance and challenges in the preparation of scalable CPMBs with emphasis on their sustainability are presented.

Microporous framework membranes for precise molecule/ion separations

Microporous framework membranes such as metal-organic framework (MOF) membranes and covalent organic framework (COF) membranes are constructed by the controlled growth of small building blocks with large porosity and permanent well-defined micropore structures, which can overcome the ubiquitous tradeoff between membrane permeability and selectivity; they hold great promise for the enormous challenging separations in energy and environment fields. Therefore, microporous framework membranes are endowed with great expectations as next-generation membranes, and have evolved into a booming research field. Numerous novel membrane materials, versatile manipulation strategies of membrane structures, and fascinating applications have erupted in the last five years. First, this review summarizes and categorizes the microporous framework membranes with pore sizes lower than 2 nm based on their chemistry: inorganic microporous framework membranes, organic-inorganic microporous framework membranes, and organic microporous framework membranes, where the chemistry, fabrications, and differences among these membranes have been highlighted. Special attention is paid to the membrane structures and their corresponding modifications, including pore architecture, intercrystalline grain boundary, as well as their diverse control strategies. Then, the separation mechanisms of membranes are covered, such as diffusion-selectivity separation, adsorption-selectivity separation, and synergetic adsorption-diffusion-selectivity separation. Meanwhile, intricate membrane design to realize synergistic separation and some emerging mechanisms are highlighted. Finally, the applications of microporous framework membranes for precise gas separation, liquid molecule separation, and ion sieving are summarized. The remaining challenges and future perspectives in this field are discussed. This timely review may provide genuine guidance on the manipulation of membrane structures and inspire creative designs of novel membranes, promoting the sustainable development and steadily increasing prosperity of this field.

Anisotropic Gas Separation in Oriented ZIF-95 Membranes Prepared by Vapor-Assisted In-Plane Epitaxial Growth

Control of the microstructure grain orientation, grain boundaries and thickness are crucial for MOF membranes. We report a novel synthesis strategy to prepare highly c-oriented ZIF-95 membranes through vapor-assisted in-plane epitaxial growth. In a mixed DMF/water vapor atmosphere, in-plane epitaxial growth of a ZIF-95 seeds layer was achieved to obtain an oriented and well-intergrown ZIF-95 membrane with a thickness of only 600 nm. Demonstrated by both experimental and simulation studies, the c-oriented ZIF-95 membrane displayed superior separation performance because a perfectly oriented structure resulted in a notable reduction of intercrystalline defects and transport pathways. For the separation of equimolar binary mixtures at 100 °C and 1 bar, the mixture separation factors of H₂ /CO₂ and H₂ /CH₄ were 32.2 and 53.7, respectively, with an H₂ permeance of over 7.9×10^-7  mol m^-2  s^-1  Pa^-1 , which was 4.6 times higher than that of a randomly oriented ZIF-95 membrane.

Fabrication of a sandwiched silicalite-1 membrane in a 2D confined space for enhanced alcohol/water separation

A dense zeolite layer with a thickness of approximately 500 nm was demonstrated by a confined-space strategy in a sandwiched mode of (SiO2)/(silicalite-1)/(SiO2). The gel-free secondary growth methodology bypasses the post-calcination step, avoiding excess energy consumption and possible film damage. Significantly enhanced pervaporation separation was observed with separation factors of 136 and 113, and fluxes of 2.3 and 2.2 kg m-2 h-1 for ethanol/n-butanol aqueous solutions, respectively. In addition, the membrane stability was confirmed by the 14 day pervaporation test.

Highly Ordered Nanochannels in a Nanosheet-Directed Thin Zeolite Nanofilm for Precise and Fast CO2 Separation

Precise molecular and ion separations depend largely on the size and uniformity of the nanochannels in a defect-free microporous nanofilm. Ordered and perpendicular nanochannels with uniform pore size are assembled into a continuous and defect-free film by a "gel nuclei-less" route. The ultrathin (<50 nm) zeolite nanosheets seeding layer induces the formation of defect-free zeolite nanofilms (500-800 nm) with preferential [100] orientation well-aligned to the transport pathway. The large-area and thin silicoaluminophosphate-34 (SAPO-34) nanofilm consisting of uniform and straight nanochannels shows a milestone CO₂ permeance of ≈1.0 × 10^-5 mol (m² s Pa)^-1 and high CO₂ /CH₄ and CO₂ /N₂ selectivities of 135 and 41 in equimolar binary mixtures at room temperature and 0.2 MPa feed pressure, respectively. These results suggest that highly oriented and thin SAPO-34 nanofilms prepared from nanosheets might have great potential for CO₂ capture from natural gas, biogas, and flue gas.

Gelless Secondary Growth of Zeolitic Aluminophosphate Membranes on Porous Supports with High Performance in CO2 /CH4 Separation

Zeolitic aluminophosphate, a three-dimensional microporous material, with an average pore size of 0.38 nm is good candidate for molecular sieve application in CO₂ gas separation. The separation of CO₂ /CH₄ gas mixtures for precombustion processes is desirable from the standpoint of both environmental concerns and energy efficiency. This study concerns an environmentally friendly method to synthesize zeolitic aluminophosphate thin films on various configurations and low-cost kaolin porous substrates with high performance in the separation of CO₂ /CH₄ mixtures. The membranes are prepared by a gelless seed growth method that uses lower amounts of chemicals, forms no liquid gel, chemical waste, or byproducts and generates no washing water. The obtained membranes show very high selectivity for CO₂ with a CO₂ /CH₄ separation factor above 1000 in the separation of CO₂ /CH₄ gas mixtures.

Uniform hierarchical MFI nanosheets prepared via anisotropic etching for solution-based sub-100-nm-thick oriented MFI layer fabrication

Zeolite nanosheets have shown unprecedented opportunities for a wide range of applications, yet developing facile methods for fabrication of uniform zeolite nanosheets remains a great challenge. Here, a facile approach involving anisotropic etching with an aqueous solution of tetrapropylammonium hydroxide (TPAOH) was developed for preparing uniform high-aspect ratio hierarchical MFI nanosheets. In addition, the mechanism associated with the formation of MFI nanosheets was proposed. In the next step, a dynamic air-liquid interface-assisted self-assembly method and single-mode microwave heating were used for b-oriented MFI nanosheets monolayer deposition and controlled in-plane solution-based epitaxial growth, respectively, ensuring the formation of well-intergrown b-oriented MFI layers with sub-100-nm thickness. Moreover, our study indicated that b-oriented ultrathin MFI layers could be fabricated on diverse substrates demonstrating excellent anticorrosion capacity, ionic sieving properties, and n-/i-butane isomer separation performance.

Effects of Silica-Particle Coating on a Silica Support for the Fabrication of High-Performance Silicalite-1 Membranes by Gel-Free Steam-Assisted Conversion

Silicalite-1 membranes with high pervaporation performance were prepared successfully on a silica-particle-coated tubular silica support using a gel-free steam-assisted conversion (SAC) method. The effects of the silica-particle layer formed on the top surface of the silica support and the physical properties of the silica particles themselves on the membrane-formation process were investigated. The silica particles coated served as the additional silica source for growing the silicalite-1 seed crystal layer into the silicalite-1 membrane. As a result, it was possible to form a dense and continuous membrane even under gel-free conditions. Furthermore, it was found that the properties of the silica particles, such as their primary particle diameter, had a determining effect on their solubility during the steam treatment, that is, on the supply rate of the silica source. The silicalite-1 membrane obtained using the spherical-silica-particle-coated support had an approximately 9-μm-thick separation layer and showed very high pervaporation performance, exhibiting a separation factor of 105 and a flux of 3.72 kg m⁻² h⁻¹ for a 10 wt % ethanol/water mixture at 323 K. Thus, the gel-free SAC method can be used with a silica support coated with silica particles to readily prepare high-performance membranes without producing any chemical waste.

Zeolitic Imidazolate Framework Membranes for Light Olefin/Paraffin Separation

Propylene/propane and ethylene/ethane separations are performed by energy-intensive distillation processes, and membrane separation may provide substantial energy and capital cost savings. Zeolitic imidazolate frameworks (ZIFs) have emerged as promising membrane materials for olefin/paraffin separation due to their tunable pore size and chemistry property, and excellent chemical and thermal stability. In this review, we summarize the recent advances on ZIF membranes for propylene/propane and ethylene/ethane separations. Membrane fabrication methods such as in situ crystallization, seeded growth, counter-diffusion synthesis, interfacial microfluidic processing, vapor-phase and current-driven synthesis are presented. The gas permeation and separation characteristics and membrane stability are also discussed.