Porous materials such as metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are highly promising candidates for technical separation tasks of the modern world. These materials are either consisting of metal-nodes and linker molecules for MOFs, or contain completely organic units making up crystalline polymers in case of COFs. There is an almost unlimited variety to these frameworks, which enables a tailoring of the properties for the specific, desired use-case. The inner surface areas and free volumes of the pores enable specific interactions with guest molecules, such as molecular sieving and competitive adsorptive, which can be used to selectively separate molecular mixtures or sense target molecules. Therefore, MOFs and COFs can be used for carbon capture and storage (CCS), a big topic right now, as well as for the “green production”. To explain the latter, looking only at the big gas separation processes in industry, 5% of the world energy consumption is used in cryogenic separation, which could be replaced with a membrane module. This would safe up to 90% energy on separation tasks and reduce CO2 emission by making the production “green”. MOFs and COFs can exhibit various interactions, from electrostatic adhesion and repulsion to Van-der-Waals and hydrogen bonds up to covalent bond formation by post-synthetic reactions. By using different metals and linkers the chemical environment of the MOFs can be different. This is not only interesting for separation, but can also be used for monitoring the changes of the framework. Measuring electrical, electrochemical, optical (e.g. lumineszent) and other types of responses of the lattice leads to different types of sensors.
Stimuli Responsive Metal-Organic Frameworks
External stimuli, here direct current electric fields of 500 V/mm applied during in-situ gas permeation experiments, can produce polarized polymorphic phases of zeolitic imidazolate framework 8 (ZIF-8), which show increased molecular sieving performance in the separation of propylene from propane.
Figure 1. External stimuli influencing the gas transport through MOF membranes. On the left, an electric field can be applied to a membrane. Middle to right: SEM and corresponding EDX mapping for two different ZIF-8 membranes with 1 µm (middle) and 10 µm (right) scalebar. From: Knebel et al., Science, 358, 6361, pp. 347-351.
Luminescent Structures and Composites
Hierarchical heterostructures of Ln- and Ac-based MOFs allow the preparation of highly luminescent, porous films. These materials reach high quantum yields and can be utilized for optical sensor devices. Embedding these structures into conductive polymers makes flexible LEDs.
Figure 2. Luminescent SURMOFs containing Ln-metal ions.
Covalent Organic Framework Membranes
Covalent organic frameworks usually have larger pores than MOFs and are not very suitable for gas separation. They are commonly used for waste water treatment, however, can be tricked into gas separation applications, e.g. by MOF-to-COF conversion concepts.
Figure 3. Stacking bilayers of COF materials leads to pore diameter confinement in the interlayer. This enhanced molecular sieving. Taken from: H. Fan, A. Mundstock, A. Feldhoff, A. Knebel, J. Gu, H. Meng, J. Caro et al., J. Am. Chem. Soc., 2018, 140, 32, 10094-10098.
Porous Liquids, Polymers and Composite Membranes
In combination with MOFs and COFs, polymers lead to highly desired properties for several use cases. Be it for electroluminescence for diodes, using luminescent structures embedded into conductive polymers, or in mixed-matrix membranes for gas separations. A novel approach is the use of liquids with internal porosity for perfect embedding of MOFs and COFs into the polymeric matrix.
The conversion of thin films of SURMOFs (surface anchored MOFs) into polymers (SURGELs) is highly interesting because of the transfer of the unique properties of MOFs into polymeric materials. But also making polymers in the pores of MOFs is an interesting approach towards narrow Mw distributions, due to the synthesis is confined pore space. Through conversion of SURMOFs it should be possible to get to SURCOFs, highly desired for making novel structures and maintain high crystallinity.
Figure 4. The process of making SURGELs or even SURCOFs from SURMOFs. First, a secondary cross-linker is
introduced via click-chemistry. In the end, the metal-ions are removed from the cross-linked SURMOF, yielding a fullorganic structure. The OH-functions are capable of taking up other ions or act in gas separation, or else, can transport ions.