How smart organic cages enable the active separation of dyes in a switchable

By: Esli Diepenbroek

When we ask your typical chemical engineer what would be an all-encompassing subfield within their expertise, combining both aspects of process and materials engineering, membranes are mentioned plenty. The general principle of a membrane is well-known: separations based on a physical boundary and through a difference in size, affinity, or both. In the current membrane engineering field, a widespread range of materials are used and investigated, both of organic and inorganic nature. What is a common factor, however, is that the separations are mostly passive and with separation characteristics determined upon production. This exact aspect is being thrown out the window by Andrew Livingston, Andrew Cooper and colleagues from the University of Liverpool, who were driven to achieve the far opposite [1].

In their recently published paper, they report a tunable membrane that consists of CC3 porous organic cages (POCs) ordered on a polyacrylonitrile (PAN) substrate [2]. The CC3 cages have two different configurations, referred to as the alpha and gamma phase, which differ in both crystalline orientation as well as the pore aperture size. Reversible switching between the two states is possible through exposure to either water (αCC3) or methanol (γCC3).

The dynamic nature of the membranes was demonstrated with the use of dyes with different cut-off sizes. In the alpha configuration, all dyes larger than 600 g/mol were rejected, whereas the gamma configuration showed an increased cut-off value of 1400 g/mol [2]. While the proof of concept does not hold any strong application field(s), the work of Livingston and Cooper provides a hopeful glimpse towards the rise of active membrane separations, which could tremendously ease the separation of complex liquids [1].