Designing 'smart' crystalline membranes for graded molecular sieving

a) Graphical representation of membrane structure in water (blue) and methanol (green). b) Crystal structure with its 3-D pore network shown in yellow, found in water. A second structure was formed by soaking the membrane in methanol, which has additional extrinsic solvent-filled channels, shown here in orange, that open up additional porosity in the membrane in response to the methanol solvent. c) Photograph of the crystalline membrane with a diameter of 7.4 cm deposited on polyacrylonitrile support. d) In situ diffraction patterns of the crystalline membrane showing the reversible phase transition between the water and methanol structures.

A team of researchers from the University of Liverpool and Imperial College London aimed to fabricate a crystalline membrane using a porous organic cage molecule (POC). It has been reported that POCs are molecules with cavities that can create porosity in porous liquids, molecular crystals and amorphous solids, and host guest molecules. The researchers hoped the guest-accessible cavity would facilitate selective diffusion through the membrane structure. POCs can also undergo exciting structural transformations in crystalline solids in response to chemical stimuli.      

The first step in their investigation was proving the membrane was crystalline. They then needed to investigate the membrane's dynamic behaviour to determine how it behaved in different experimental conditions, such as during filtration experiments.   

They used Diamond's I11 beamline to perform Powder X-ray Diffraction studies and I07 beamline for Grazing Incidence X-ray Diffraction studies. The data collected at Diamond enabled them to determine the structure and study the dynamic behaviour of the crystalline membrane during in situ measurements. A vital aspect of the study is underpinned by the dynamic behaviour of the membrane and its ability to switch its pore aperture during filtration experiments in response to different chemical environments. At Diamond, the scientists could replicate the conditions used in larger-scale separation processes and study the structure and dynamic behaviour of the membrane.  

The study found a highly ordered crystalline membrane with a switchable phase transition between two crystalline forms with different pore apertures. Both forms showed excellent separation performances. By varying the water/methanol ratio, the film can be switched between the two phases with different selectivities, giving a single, 'smart' crystalline membrane that can perform graded molecular sieving. The team used the dynamic behaviour of the membrane to perform graded molecular sieving experiments to separate a mixture of three organic molecules using a single, smart membrane.  

Smart membranes that perform graded molecular sieving experiments to separate complex mixtures of molecules would create a parallel technology to the widespread and highly effective use of solvent gradients in chromatography. At the same time, membranes with switchable pore apertures could also lead to new applications in triggered drug delivery, biosensors, or fermentation/fractionation processes. Although the present method of synthesis poses a challenge to the scalability and implementation of POC membranes in commercial processes, there is potential for the development of a more scalable production method by utilising the solution processability of these molecular cages. 

In the future, computational techniques, such as crystal structure prediction, will be employed to design POC crystals with desired properties based on first principles.