Metal-Organic Framework Membranes: Self-Confined Conversion from Metal Hydroxide Nanostrands

Metal-organic framework (MOF) membranes have been extensively employed for a wide range of applications. Fabrication of high-quality MOF membranes is one of the prerequisites to ensure a good performance. Although various kinds of methods such as in situ growth, seeded secondary growth, interfacial synthesis, contra-diffusion synthesis methods, etc. have been developed for MOF membrane fabrication, their further practical application on a large scale is limited by poor control over the MOF membrane formation or complex fabrication procedures/devices. Developing simple methods that are capable of fabricating high-quality MOF membranes is still a big challenge and remains one of the vibrant topics in MOF membranes. Different from these methods, the self-confined conversion method features using insoluble solid matter instead of dissolved metal salt as metal precursors. During MOF growth, the formed MOF layer confines solid precursors from the organic ligand contributing to the formation of well-intergrown MOF membranes. Besides, by taking advantage of the interaction between guest components and solid precursors, different kinds of guest compounds can be feasibly encapsulated into the MOF membrane, further expanding their application field. Compared with other methods, the self-confined conversion method shows several advantages as follows: (i) good control over MOF nucleation and growth; (ii) high affinity between the MOF layer and the substrate; (iii) highly efficient encapsulation of guest components into the membrane; (iv) easy to scale up production. Solid precursors are the key issue in the self-confined conversion method. Among various kinds of solid precursors (e.g., zerovalent metal, metal oxide, layered double hydroxide, and metal hydroxide nanostrands), metal hydroxide nanostrands, a kind of unique metal hydroxide with a small diameter (<2.5 nm), a long length (a few micrometers), and a highly positively charged surface, are found to be an ideal solid precursor for MOF membrane fabrication and show several advantages such as a simple preparation procedure without the tedious solid hydrothermal and chemical vapor deposition operation, easy deposition on different substrates via suck filtration, high reactivity, and facile encapsulation of guest components into the MOF through electrostatic interaction. Therefore, this Account mainly summarizes our recent progress in the fabrication of MOF membranes through the self-confined conversion method using metal hydroxide nanostrands as the solid precursor. Meanwhile, the application of corresponding MOF membranes for gas separation, ion conduction and separation, electronic conduction, white light-emitting diode, derived carbon materials, and catalysis are discussed. Besides, challenges and opportunities for MOF membranes prepared by the self-confined conversion method are prospected to guide the future preparation of multifunctional MOF membranes.