Nanoporous Membranes With Tunable Molecular Separation

Novel membranes with selectivity that can be switched dynamically with the help of light have been developed by researchers at Karlsruhe Institute of Technology (KIT) and Universität Hannover.

Azobenzene molecules were integrated into membranes made of metal-organic frameworks (MOFs). Depending on the irradiation wavelength, these azobenzene units in the MOFs adopt a stretched or angular form.

This makes it possible to dynamically adjust the permeability of the membrane and the separation factor of gases or liquids.

Metal-organic Frameworks

Metal-organic frameworks, MOFs for short, are highly porous crystalline materials, consisting of metallic nodes and organic linkers. They can be tailored to many different applications.

Among others, they have an enormous potential as membranes for efficient separation of molecules according to various parameters. By modifying pore sizes and chemical properties of the pore walls, static selectivity of the membranes can be adapted to the respective requirements.

Azobenzene molecules are used as remote-controlled photoswitches. They consist of two phenyl rings each, which are linked by a nitrogen double bond.

Two different configurations exist.

There is a stretched trans-configuration and an angular cisconfiguration. Irradiation with light causes the molecule to reposition.

Under visible light the molecule stretches, under UV light it bends. Repositioning is reversible, can be repeated as often as desired, and does not affect the crystalline structure of the MOFs.

Membrane Technology Breakthrough

Precise control of the ratio between cis- and trans-azobenzene by e.g. a precisely adjusted irradiation time or simultaneous irradiation with UV light and visible light enables dynamic tuning of membrane permeability and of separation efficiency of gaseous or liquid sub-stance mixtures.

says Dr. Lars Heinke, Head of the IFG Group Dynamic Processes in Porous Systems.

Functioning of the novel smart membranes was demonstrated by the separation of a hydrogen-carbon dioxide gas mixture. The scientists succeeded in dynamically tuning the separation factor be-tween three and eight.

The concept is also suited for separating other gas mixtures, such as nitrogen-carbon dioxide mixtures. It might also be feasible to use MOF membranes with photoswitches to control accessibility of catalyst or sensor surfaces or release of encapsulated medical substances.

Original Study: Tunable molecular separation by nanoporous membranes

Image: Alexander Knebel/Universität Hannover and Lars Heinke/KIT

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