For membranes consisting of neat metal-organic framework (MOFs) layers or for mixed-matrix-membranes (MMMs) containing MOFs as filler materials, a reduction of the layer thickness and filler size is often important to meet industrial requirements. The main advantage of hierarchical MOF membranes is the additive adsorption, which improves specific separation tasks in neat MOF membranes, as well as in MMMs.

Alexander Knebel, Leibniz University Hannover, Germany, and colleagues have synthesized hierarchically grown nano-heterostructures composed of ZIF-8 on ZIF-67 (two very similar MOFs) and applied them as gas separation membranes. The MOF-on-MOF structures were prepared by self-assembly, either as thin layers or nanoparticles. The supported thin-layer membranes and free-standing MMMs composed of ZIF-8-on-ZIF-67 outperform the pure ZIF-8, ZIF-67, and comparable MOF materials in several separation tasks, especially in H₂/CO₂ separation. The ZIF-8-on-ZIF-67 nano-heterostructures show enhanced fluxes and separation values, raising the possibility of industrial application.

According to the researchers, the MOF-on-MOF topology is capable of producing high-end materials. Their properties are boosted through the addition of positive effects of the single materials, leading to new heights in gas separation and purification.

• Hierarchical Nanostructures of Metal-Organic Frameworks Applied in Gas Separating ZIF-8-on-ZIF-67 Membranes,
  Alexander Knebel, Paul Wulfert-Holzmann, Sebastian Friebe, Janet Pavel, Ina Strauß, Alexander Mundstock, Frank Steinbach, Jürgen Caro,
  Chem. Eur. J. 2018.
  https://doi.org/10.1002/chem.201705562