The frameworks of metal-organic, MOFs for short, are exceedingly porours crystalline substances, comprising of metallic nodes and organic linkers. They can be customized to numerous distinct applications. Among others, they have a huge potential as membranes for effective separation of molecules according to numerous parameters.
By modifying sizes of pores and chemical properties of the pore walls, stable selectivity of the membranes can be adapted to the respective needs. The researchers for the very first time introduce membranes, whose selectivity can be tuned dynamically. It is done remotely with the aid of light.
Scientists of KIT’s Institute of Functional Interfaces or IFG and Institute of Organic Chemistry, IOC, in association with researchers of Leibniz Universitat Hannover, equipped MOF based membranes with photoswtiches. “In this way, the membranes are offered with small windows that close and open depending on light irradiation,” the Head of the Institute of Functional Interfaces, Lecturer ChristofWoll says.
Azobenzene molecules are utilized as remote-controlled photoswitches. They comprise of two phenyl rings each that are linked through a nitrogen double bond. Two varying configurations exist – A stretched trans-configuration and an angular cisconfiguration. Irradiation with light results the molecules to respost. Under visible light the molecule stretches, under the UV light it bends. Repositioning is reversible and can be repeated as often as considered, and does not affect the crystalline structure of the MOFs.
Accurate control of the proportion between cis and transazobenzene by example an accurately adjusted irradiation time or simultaneous irradiation with the UV light and visible light allows dynamic tuning of membrane permeability and of separation efficacy of liquid or gaseous substance mixtures. “Regulation of such vital properties by external stimuli that is wihout having direct link with the membrane, is a real breakthrough in membrane technology,” says Dr. Lars Heinke, Head of the IFG Group, a dynamic process in porous systems.”
Performance of the new smart membranes was illustrated by the separation of a carbon – hydrogen dioxide gas mixture. The researchers succeeded in dynamically altering the separation factor between eight and three.
This concept is also suitable for separating other gas mixtures, like nitrogen – carbon dioxide mixtures. It might also be feasible to employ MOF membranes with photoswtiches to regulate accessibility of catalyst or sensor surfaces or release of encapsulated medical materials
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