Hierarchical porous materials are promising for catalyst, separation and sorption applications. A ligand-assisted etching process is developed for template-free synthesis of hierarchical mesoporous MOFs as single crystals and well-intergrown membranes at 40 °C. At 223 K, the hierarchical porous structures significantly improve the CO₂ capture capacity of HKUST-1 by more than 44 % at pressures up to 20 kPa and 13 % at 100 kPa. Even at 323 K, the enhancement of CO₂ uptake is above 25 % at pressures up to 20 kPa and 7 % at 100 kPa. The mesoporous structures not only enhance the CO₂ uptake capacity but also improve the diffusion and mass transportation of CO₂. Similarly, well-intergrown mesoporous HKUST-1 membranes are synthesized, which hold the potential for film-like porous devices. Mesoporous MOF-5 crystals are also obtained by a similar ligand-assisted etching process. This may provide a facile way to prepare hierarchical porous MOF single crystals and membranes for wide-ranging applications.

Nanofibrous materials have been extensively investigated and used as building blocks for various nanodevices, due to their unique one-dimensional structures. Recently, novel membranes constructed by using nanofibers have been reported by various techniques. Here, we will give a critical review of our recent research on the general solution processed unique sub-3 nm thin metal hydroxide nanofibers and their application for constructing ultrathin separation membranes via filtration technique. The superior separation performances of these membranes hold the promising future for pressure-driven membrane separation processes.

Ultrathin β-MnOOH nanofibers can be produced on a large scale via a green-chemical method using an aqueous solution of very dilute Mn(NO₃)₂ and aminoethanol at room temperature. High-magnification electron microscopy demonstrates that the β-MnOOH nanofibers are 3–5 nm thin and up to 1 micrometer long and the nanofibers are parallel assembled into bundles with an average diameter of 25 nm. By a filtration process, ultrathin mesoporous membranes with strong mechanical, thermal, and chemical stabilities are prepared from the β-MnOOH nanofiber bundles. The membranes can separate 10-nm nanoparticles from water at a flux of 15120 L m⁻²·h⁻¹·bar⁻¹, which was 2–3 times higher than that of commercial membranes with similar rejection properties. Based on the Young-Laplace equation, β-MnOOH nanofiber/polydimethylsiloxane composite membranes are developed through a novel downstream-side evaporation process. From nanoporous to dense separation membranes can be achieved by optimizing the experimental conditions. The membranes show desirable separation performance for proteins, ethanol/water mixtures, and gases. The synthesis method of β-MnOOH nanofibers is simple and environmentally friendly, and it is easily scalable for industry and applicable to other metal oxide systems. These composite membranes constitute a significant contribution to advanced separation technology.