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

Advancing Electrically Conductive Metal-Organic Frameworks for Photocatalytic Energy Conversion

ConspectusPhotocatalytic energy conversion is a pivotal process for harnessing solar energy to produce chemicals and presents a sustainable alternative to fossil fuels. Key strategies to enhance photocatalytic efficiency include facilitating mass transport and reactant adsorption, improving light absorption, and promoting electron and hole separation to suppress electron-hole recombination. This Account delves into the potential advantages of electrically conductive metal-organic frameworks (EC-MOFs) in photocatalytic energy conversion and examines how manipulating electronic structures and controlling morphology and defects affect their unique properties, potentially impacting photocatalytic efficiency and selectivity. Moreover, with a proof-of-concept study of photocatalytic hydrogen peroxide production by manipulating the EC-MOF's electronic structure, we highlight the potential of the strategies outlined in this Account.EC-MOFs not only possess porosity and surface areas like conventional MOFs, but exhibit electronic conductivity through d-p conjugation between ligands and metal nodes, enabling effective charge transport. Their narrow band gaps also allow for visible light absorption, making them promising candidates for efficient photocatalysts. In EC-MOFs, the modular design of metal nodes and ligands allows fine-tuning of both the electronic structure and physical properties, including controlling the particle morphology, which is essential for optimizing band positions and improving charge transport to achieve efficient and selective photocatalytic energy conversion.Despite their potential as photocatalysts, modulating the electronic structure or controlling the morphology of EC-MOFs is nontrivial, as their fast growth kinetics make them prone to defect formation, impacting mass and charge transport. To fully leverage the photocatalytic potential of EC-MOFs, we discuss our group's efforts to manipulate their electronic structures and develop effective synthetic strategies for morphology control and defect healing. For tuning electronic structures, diversifying the combinations of metals and linkers available for EC-MOF synthesis has been explored. Next, we suggest that synthesizing ligand-based solid solutions will enable continuous tuning of the band positions, demonstrating the potential to distinguish between photocatalytic reactions with similar redox potentials. Lastly, we present incorporating a donor-acceptor system in an EC-MOF to spatially separate photogenerated carriers, which could suppress electron-hole recombination. As a synthetic strategy for morphology control, we demonstrated that electrosynthesis can modify particle morphology, enhancing electrochemical surface area, which will be beneficial for reactant adsorption. Finally, we suggest a defect healing strategy that will enhance charge transport by reducing charge traps on defects, potentially improving the photocatalytic efficiency.Our vision in this Account is to introduce EC-MOFs as an efficient platform for photocatalytic energy conversion. Although EC-MOFs are a new class of semiconductor materials and have not been extensively studied for photocatalytic energy conversion, their inherent light absorption and electron transport properties indicate significant photocatalytic potential. We envision that employing modular molecular design to control electronic structures and applying effective synthetic strategies to customize morphology and defect repair can promote charge separation, electron transfer to potential reactants, and mass transport to realize high selectivity and efficiency in EC-MOF-based photocatalysts. This effort not only lays the foundation for the rational design and synthesis of EC-MOFs, but has the potential to advance their use in photocatalytic energy conversion.

Oriented Metal-Organic Framework Membranes for Molecular Separations

Metal-organic framework (MOF) membranes, which are recognized as state-of-the-art platforms applied in various separation processes, have attracted widespread attention. Nonetheless, to overcome the trade-off between permeability and selectivity, which is crucial for achieving efficient separation, it is important to rationally design and manipulate MOF membrane structure. Given remarkable advances in the past decade, a timely summary of recent advancement in this field has become indispensable. This review introduces major strategies for fabricating oriented MOF membranes, including in situ growth, contra-diffusion method, interface-assisted approach, and laminated nanosheet assembly. New insights into their updated progress and potential are elucidated. Of particular note, recent development and emerging applications of oriented MOF membranes, illustrating their potential to address environmental and energy challenges, are highlighted. Finally, remaining challenges facing their bath production and practical applications are discussed.

Design and Machine Learning Prediction of In Situ Grown PDA-Stabilized MOF (UiO-66-NH2) Membrane for Low-Pressure Separation of Emulsified Oily Wastewater

Significant progress has been made in designing advanced membranes; however, persistent challenges remain due to their reduced permeation rates and a propensity for substantial fouling. These factors continue to pose significant barriers to the effective utilization of membranes in the separation of oil-in-water emulsions. Metal-organic frameworks (MOFs) are considered promising materials for such applications; however, they encounter three key challenges when applied to the separation of oil from water: (a) lack of water stability; (b) difficulty in producing defect-free membranes; and (c) unresolved issue of stabilizing the MOF separating layer on the ceramic membrane (CM) support. In this study, a defect-free hydrolytically stable zirconium-based MOF separating layer was formed through a two-step method: first, by in situ growth of UiO-66-NH2 MOF into the voids of polydopamine (PDA)-functionalized CM during the solvothermal process, and then by facilitating the self-assembly of UiO-66-NH2 with PDA using a pressurized dead-end assembly. A stable MOF separating layer was attained by enriching the ceramic support with amines and hydroxyl groups using PDA, which assisted in the assembly and stabilization of UiO-66-NH2. The PDA-s-UiO-66-NH2-CM membrane displayed air superhydrophilicity and underwater superoleophobicity, demonstrating its oil resistance and high antifouling behavior. The PDA-s-UiO-66-NH2-CM membrane has shown exceptionally high permeability and separation capacity for challenging oil-in-water emulsions. This is attributed to numerous nanochannels from the membrane and its high resistance to oil adhesion. The membranes showed excellent stability over 15 continuous test cycles, which indicates that the developed MOFs separating layers have a low tendency to be clogged by oil droplets during separation. Machine learning-based Gaussian process regression (GPR) models as nonparametric kernel-based probabilistic models were employed to predict the performance efficiency of the PDA-s-UiO-66-NH2-CM membrane in oil-in-water separation. The outcomes were compared with the support vector machine (SVM) and decision tree (DT) algorithm. This efficiency includes various metrics related to its separation accuracy, and the models were developed through feature engineering to identify and utilize the most significant factors affecting the membrane's performance. The results proved the reliability of GPR optimization with the highest prediction accuracy in the validation phase. The average percentage increase of the GPR model compared to the SVM and DT model was 6.11 and 42.94%, respectively.

Hetero-Polycrystalline Membranes with Narrow and Rigid Pores for Molecular Sieving

Molecular sieving membranes have great potential for energy-saving separations, but they suffer from permeability-selectivity trade-off limitation. In this report, simultaneous hetero-crystallization and hetero-linker coordination of metal-organic framework (MOF) hollow fiber membranes through one-pot synthesis for precise gas separation is reported. It is found that the hetero-polycrystalline membranes consist of 2D and 3D MOF phases and are defect-free and roughly orientated, hetero-linker exchange of 3D phase by larger geometric ones can narrow transport pathway, and framework rigidification occurs and thus fixes MOF channels. The prepared membranes are robust and reproducible, and exhibit substantially improved performance, with H₂ /CO₂ , H₂ /N₂ , and H₂ /CH₄ selectivities up to 361, 482, and 541, respectively, accompanied by high H₂ permeance over 1100 gas permeation units, which can easily outclass trade-off upper bounds of state-of-the-art membranes.