Covalent synthesis of dense zeolite LTA membranes on various 3-chloropropyltrimethoxysilane functionalized supports

Small-Pore Zeolite Membranes: A Review of Gas Separation Applications and Membrane Preparation

There have been significant advancements in small-pore zeolite membranes in recent years. With pore size closely related to many energy- or environment-related gas molecules, small-pore zeolite membranes have demonstrated great potential for the separation of some interested gas pairs, such as CO2/CH4, CO2/N2 and N2/CH4. Small-pore zeolite membranes share some characteristics but also have distinctive differences depending on their framework, structure and zeolite chemistry. Through this mini review, the separation performance of different types of zeolite membranes with respect to interested gas pairs will be compared. We aim to give readers an idea of membrane separation status. A few representative synthesis conditions are arbitrarily chosen and summarized, along with the corresponding separation performance. This review can be used as a quick reference with respect to the influence of synthesis conditions on membrane quality. At the end, some general findings and perspectives will be discussed.

In situ constructed zeolite membranes on rough supports with the assistance of reticulated hydrotalcite interlayer

Zeolite membranes with unique physical and chemical properties are emerging as attractive candidates for membrane separation. However, defects in the zeolite layer seriously affect their molecular sieving performance. In this study, a novel strategy for preparing compact zeolite membranes on rough supports with the assistance of a reticulated hydrotalcite layer was developed. The reticulated hydrotalcite layer was grown on the inner surface of a 170 mm length ceramic tube by an in situ hydrothermal method, and a NaA zeolite membrane was prepared on this reticulated layer by the microwave-heating method. The hydrotalcite interlayer could not only improve the smoothness and regularity of the surface of the support but also fix the Si/Al active ingredients using its reticulate structure, finally effectively improving the quality and stability of the zeolite layer. The optimal molar ratio of the synthesis solution for the synthesis of the zeolite membrane was 3Na₂O : 2SiO₂ : Al₂O₃ : 200H₂O. The permeance flux of H₂ through the zeolite membrane synthesized under the optimal conditions was high as 0.47 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, and its permselectivity for H₂ over N₂ was 4.7, which was higher than the corresponding Knudsen diffusion coefficient. This study provides a new idea for the preparation of defect-free membranes on rough supports.

Reticulated hydrotalcite interlayer controls infiltration of active ingredients into the support, improving the quality and stability of the zeolite membrane.

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.

Removal of Particulate Matters with Isostructural Zr-Based Metal-Organic Frameworks Coated on Cotton: Effect of Porosity of Coated MOFs on Removal

Effective removal of particulate matters (PMs) from air is very important for our safe environment, health, and sustainability. In this study, isostructural (with the same topology of fcu) Zr-metal-organic frameworks (Zr-MOFs) such as UiO-66, UiO-67, and DUT-52 (with different porosity) were coated onto cotton and utilized in PM removal from air to understand the contribution of MOFs in improving the performances of air filters. Moreover, UiO-66s with different porosities were also prepared under different conditions from the same reaction mixture. Experiments to remove PMs such as PM2.5 and PM10 showed a critical role of porosity of coated MOFs in the PM removal. Or, the removal efficiency or quality factor increased linearly with the increasing surface area of all the studied MOFs, irrespective of the applied linkers (for synthesizing different MOFs) and synthesis conditions (for different porosities of UiO-66s). Therefore, this work confirms, for the first time, that the porosity of MOF is one of the most important parameters to improve the performance of air filter (to remove PMs) that is modified with coated MOFs. Moreover, we could suggest why porous materials with small pores were effective in capturing PMs (larger in size than pores of porous materials) from air.

Highly Improved Performance of Cotton Air Filters in Particulate Matter Removal by the Incorporation of Metal-Organic Frameworks with Functional Groups Capable of Large Charge Separation

Currently, air contamination, especially with particulate matters (PMs), is severe in several countries. To increase the efficiency of air filters in PM removal, metal-organic frameworks (MOFs, here, Zr-MOFs, especially with functional groups (FGs) such as -NO₂) were coated, after synthesis, onto cotton using covalent bonding for the first time. The removal efficiencies (REs) and quality factors (QFs) of cottons with or without MOFs were in the order: cotton < Zr-MOF/cotton < Zr-MOF-NH₂/cotton < Zr-MOF-NH-SO₃H/cotton < Zr-MOF-NH₃⁺Cl^-/cotton < Zr-MOF-NO₂/cotton. This monotonic increase in the PM removal efficiency or QF could be explained with the order of charge separation or developed charges (total, in absolute value: ∼0 to 2.0) on FGs of MOFs. Importantly, Zr-MOF-NO₂ coating on cotton showed a very high increase in the performance of cotton in PM removal. QF and RE of Zr-MOF-NO₂/cotton were 4.6 times and 6.2 times of the bare cotton, respectively, for PM2.5 removal, even with a very small increase in pressure drop (3 Pa or less) with MOF coating. Based on the research, it can be suggested that coating MOFs on substrates is a promising way to improve the performances of air filters for PM removal, especially when MOFs have FGs that can have large charge separation such as -NO₂. This work may pave a way to utilize a functionalized MOF in the effective removal of PMs from air.

Na+-gated water-conducting nanochannels for boosting CO2 conversion to liquid fuels

Robust, gas-impeding water-conduction nanochannels that can sieve water from small gas molecules such as hydrogen (H₂), particularly at high temperature and pressure, are desirable for boosting many important reactions severely restricted by water (the major by-product) both thermodynamically and kinetically. Identifying and constructing such nanochannels into large-area separation membranes without introducing extra defects is challenging. We found that sodium ion (Na⁺)-gated water-conduction nanochannels could be created by assembling NaA zeolite crystals into a continuous, defect-free separation membrane through a rationally designed method. Highly efficient in situ water removal through water-conduction nanochannels led to a substantial increase in carbon dioxide (CO₂) conversion and methanol yield in CO₂ hydrogenation for methanol production.

Inorganic Membranes: Preparation and Application for Water Treatment and Desalination

Inorganic membrane science and technology is an attractive field of membrane separation technology, which has been dominated by polymer membranes. Recently, the inorganic membrane has been undergoing rapid development and innovation. Inorganic membranes have the advantage of resisting harsh chemical cleaning, high temperature and wear resistance, high chemical stability, long lifetime, and autoclavable. All of these outstanding properties made inorganic membranes good candidates to be used for water treatment and desalination applications. This paper is a state of the art review on the synthesis, development, and application of different inorganic membranes for water and wastewater treatment. The inorganic membranes reviewed in this paper include liquid membranes, dynamic membranes, various ceramic membranes, carbon based membranes, silica membranes, and zeolite membranes. A brief description of the different synthesis routes for the development of inorganic membranes for application in water industry is given and each synthesis rout is critically reviewed and compared. Thereafter, the recent studies on different application of inorganic membrane and their properties for water treatment and desalination in literature are critically summarized. It was reported that inorganic membranes despite their high synthesis cost, showed very promising results with high flux, full salt rejection, and very low or no fouling.

Synthesis of NaA zeolite membrane by maintaining pressure difference between the two sides of the support

Pressure difference assists more Si/Al active ingredient migration to the surface of the support and thus facilitates the formation of uniform and dense zeolite membranes.

LTA zeolite membranes: current progress and challenges in pervaporation

Linde Type A (LTA) zeolite-based membranes have demonstrated excellent selectivity in pervaporation due to their unique structural framework and interaction with water.

Halloysite nanotube-based electrospun ceramic nanofibre mat: a novel support for zeolite membranes

Some key parameters of supports such as porosity, pore shape and size are of great importance for fabrication and performance of zeolite membranes. In this study, we fabricated millimetre-thick, self-standing electrospun ceramic nanofibre mats and employed them as a novel support for zeolite membranes. The nanofibre mats were prepared by electrospinning a halloysite nanotubes/polyvinyl pyrrolidone composite followed by a programmed sintering process. The interwoven nanofibre mats possess up to 80% porosity, narrow pore size distribution, low pore tortuosity and highly interconnected pore structure. Compared with the commercial α-Al₂O₃ supports prepared by powder compaction and sintering, the halloysite nanotube-based mats (HNMs) show higher flux, better adsorption of zeolite seeds, adhesion of zeolite membranes and lower Al leaching. Four types of zeolite membranes supported on HNMs have been successfully synthesized with either in situ crystallization or a secondary growth method, demonstrating good universality of HNMs for supporting zeolite membranes.

Methylcellulose-assisted synthesis of a compact and thin NaA zeolite membrane

The addition of methylcellulose (MC) in synthesis solution helped to patch the defects and retard the thickening of NaA zeolite membrane prepared via microwave heating.

Mussel-inspired polydopamine modification of supports for the facile synthesis of zeolite LTA molecular sieve membranes

Inspired by bio-adhesive ability of marine mussel, a versatile and effective synthesis strategy was developed to prepare dense zeolite LTA membranes.