Fabrication of highly CO2/N2 selective polycrystalline UiO-66 membrane with two-dimensional transition metal dichalcogenides as zirconium source via tertiary solvothermal growth

UiO-66 Metal-Organic Framework Membranes: Structural Engineering for Separation Applications

Metal-organic frameworks (MOFs) have been recognized as promising materials for membrane-based separation technologies due to their exceptional porosity, structural tunability, and chemical stability. This review presents a comprehensive discussion of the advancements in structure engineering and design strategies that have been employed to optimize UiO-66 membranes for enhanced separation performance. Various synthesis methods for UiO-66 membranes are explored, with a focus on modulated approaches that incorporate different modulators to fine-tune nucleation rates and crystallization processes. The influence of preferred orientation, membrane thickness, pore size, pore surface chemistry, and hierarchical structures on the separation performance is concluded. By providing a consolidated overview of current research efforts and future directions in UiO-66 membrane development, this review aims to inspire further advancements in the field of separation technologies.

Mixed-linker strategy for suppressing structural flexibility of metal-organic framework membranes for gas separation

Structural flexibility is a critical issue that limits the application of metal-organic framework (MOF) membranes for gas separation. Herein we propose a mixed-linker approach to suppress the structural flexibility of the CAU-10-based (CAU = Christian-Albrechts-University) membranes. Specifically, pure CAU-10-PDC membranes display high separation performance but at the same time are highly unstable for the separation of CO₂/CH₄. A partial substitution (30 mol.%) of the linker PDC with BDC significantly improves its stability. Such an approach also allows for decreasing the aperture size of MOFs. The optimized CAU-10-PDC-H (70/30) membrane possesses a high separation performance for CO₂/CH₄ (separation factor of 74.2 and CO₂ permeability of 1,111.1 Barrer under 2 bar of feed pressure at 35°C). A combination of in situ characterization with X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, as well as periodic density functional theory (DFT) calculations, unveils the origin of the mixed-linker approach to enhancing the structural stability of the mixed-linker CAU-10-based membranes during the gas permeation tests.

Facile Synthesis of Ultra-microporous Pillar-Layered Metal-Organic Framework Membranes for Highly H2-Selective Separation

Recently, pillar-layered MOF materials have attracted much attention and shown great potential in separation application due to their fine pore size/channel and pore surface chemistry tunability and designability. In this work, we reported an effective and universal synthesis strategy for preparing ultra-microporous Ni-based pillar-layered MOF [Ni₂(L-asp)₂(bpy)] (Ni-LAB) and [Ni₂(L-asp)₂(pz)] (Ni-LAP) (L-asp = L-aspartic acid, bpy = 4,4'-bipyridine, pz = pyrazine) membranes on a porous α-Al₂O₃ substrate with high performance and good stability by secondary growth. Through this strategy, the seed size reduction and screening engineering (SRSE) is proposed to obtain uniform sub-micron size MOF seeds by high-energy ball milling-combined solvent deposition. This strategy not only effectively addresses the issue of obtaining the uniform small seeds being significant for secondary growth but also provides an approach for the preparation of Ni-based pillar-layered MOF membranes where the freedom of synthesizing small crystals is lacking. Based on reticular chemistry, the pore size of Ni-LAB was narrowed by making use of shorter pillar ligands of pz instead of the longer pillar ligand of bpy. The prepared ultra-microporous Ni-LAP membranes exhibited a high H₂/CO₂ separation factor of 40.4 with H₂ permeance of 9.69 × 10^-8 mol m^-2 s^-1 Pa^-1 under ambient conditions and good mechanical and thermal stability. The superiority of the tunable pore structure and the remarkable stability of these MOF materials showed great potential for industrial H₂ purification. More importantly, our synthesis strategy demonstrated the generality for preparation of MOF membranes, enabling the regulation of membrane pore size and surface functional groups by reticular chemistry.

High-Valence Metal-Organic Framework Materials Constructed from Metal-Oxo Clusters: Opportunities and Challenges

Metal-organic framework (MOF), which possesses stable framework structure constructed by highly connected metal-oxo cluster nodes and organic linkers, has shown great promise in gas storage, adsorption, and separation, owing to the high surface areas, tunable pore aperture, and rich functional groups. In this review article, we summarized recent progress made in synthesizing high-valence MOF (e. g., UiO-66, MIL-125, PCN-22, and MIP-207) with metal-oxo cluster as metal source. Of particular note, recent breakthroughs in the preparation of UiO-66 and MIL-125 membranes with the corresponding Zr₆ -oxo and Ti₈ -oxo cluster sources (e. g., Zr₆ O₄ (OH)₄ (OAc)₁₂ and Ti₈ O₈ (OOCR)₁₆ clusters) possessing superior separation performance were highlighted. In the end, an outlook on the preparation of versatile high-valence MOF membranes with the corresponding metal-oxo clusters as metal sources was highlighted.

Challenges in Developing MOF-Based Membranes for Gas Separation

Metal-organic frameworks (MOFs) are promising candidates for membrane gas separation. MOF-based membranes include pure MOF membranes and MOF-based mixed matrix membranes (MMMs). This Perspective discusses the challenges for the next stage of the development of MOF-based membranes based on research conducted in the past decade. We focused on three major issues associated with pure MOF membranes. First, some MOF compounds have been overstudied, despite the availability of numerous MOFs. Second, gas adsorption and diffusion in MOFs are often independently investigated. The correlation between adsorption and diffusion has seldom been discussed. Third, we identify the importance of characterizing the gas distribution in MOFs to understand the structure-property relationships for gas adsorption and diffusion in MOF membranes. For MOF-based MMMs, engineering the MOF-polymer interface is essential for achieving the desired separation performance. Various approaches to modify the MOF surface or polymer molecular structure have been proposed to improve the MOF-polymer interface. Herein, we present defect engineering as a facile and efficient approach for engineering the MOF-polymer interfacial morphology and its extended application for various gas separations.

Complete twin suppression in oriented NH2-MIL-125 film via facile coordination modulation

Complete suppression of twin crystal formation in oriented metal-organic framework (MOF) film remains a great challenge. In this study, we successfully avoided the twin generation in c-oriented NH₂-MIL-125 film through simple competitive metal ion-based coordination modulation. Simultaneously, relevant mechanism associated with twin suppression was elucidated.

Inorganic Pillar Center-Facilitated Counterdiffusion Synthesis for Highly H2 Perm-Selective KAUST-7 Membranes

Fluorinated metal-organic framework materials (NbOFFIVE-1-Ni, also referred to as KAUST-7) have attracted widespread attention because of their high chemical stability and thermal stability, outstanding tolerance with water and H₂S, and high CO₂-adsorption selectivity over H₂ and CH₄. KAUST-7 was expected to be a new membrane material candidate for H₂/CO₂ separation because of the hindered permeation of CO₂ resulting from the interaction between CO₂ and (NbOF₅)^2- of the KAUST-7 framework. A highly H₂ perm-selective KAUST-7 membrane was first achieved using a novel strategy of inorganic pillar center-facilitated counterdiffusion (IPCFCD) proposed by us. The IPCFCD method not only effectively avoided the corrosion of hydrofluoric acid to α-Al₂O₃ tubes in the process of preparing KAUST-7 membranes, but also better reduced grain boundary defects because of the faster nucleation rate and resultant high crystallinity. The KAUST-7 membrane exhibited a high H₂/CO₂ separation factor (SF) of 27.30 for the 1:1 H₂/CO₂ binary gas mixture with a high H₂ permeance of 5.30 × 10^-7 mol m^-2 s^-1 Pa^-1 under ambient conditions and a slight decrease of the H₂/CO₂ SF with increasing operation temperature and presence of steam. This study highlighted the importance of pre-synthesizing inorganic pillar centers (NiNbOF₅ intermediate) and the innovation of a membrane formation process for synthesizing polycrystalline KAUST-7 membranes. Most important of all, our study provided a novel approach to overcome the challenge in fabricating metal-organic framework membranes containing corrosive reactants for the corresponding supports.

Seed assisted synthesis of anionic metal organic framework membrane for selective and permeable hydrogen separation

Hydrogen selective metal organic framework (MOF) membranes with excellent performances are still in high demand. Here, we are developing an anionic MOF material of CPM-5 into a membrane for H2...

A multifunctional UiO-66@carbon interlayer as an efficacious suppressor of polysulfide shuttling for lithium-sulfur batteries

Restraining the shuttle effect in lithium-sulfur (Li-S) battery is crucial to realize its practical application. In this work, a UiO-66@carbon (UiO-66@CC) interlayer was developed for Li-S battery by growing a continuous UiO-66 film on carbon cloth. The continuous UiO-66 crystal layer contributes to provide sufficient adsorptive and catalytic sites for efficient adsorption and catalytic conversion towards polysulfides. Moreover, the hydrophilic property of UiO-66 material ensures the full infiltration of electrolyte and accelerates the transportation of lithium ions. Profiting from the above advantages of the proposed interlayer, the shuttle effect is effectively inhibited and a fast redox kinetic is also realized. Accordingly, the Li-S battery using UiO-66@CC delivers a specific capacity of 1228.9 mAh g^-1at 0.2 C with a nearly 100% capacity retention after 100 cycles, and the first specific capacity is 1033.1 mAh g^-1at 1.0 C with a decay rate of 0.07% over 600 cycles. Meanwhile, UiO-66@CC interlayer also has an excellent rate performance with a specific capacity of 535.9 mAh g^-1at 5.0 C and a high area capacity of 6.2 mAh cm^-2at increased sulfur loading (8.15 mg cm^-2).