A bendy polymer that can recognise and separate aromatic hydrocarbons from aliphatic mixtures has been developed by Chinese scientists. 

Yong Cui and colleagues from Shanghai Jiao Tong University built the porous 3D polymer using a flexible 1D polymer made from metal units bound to salen ligands. The resulting structure bends and twists when its polymer chains stretch, triggered by guest molecules - the aromatic compounds - entering or leaving the structure. The structure's flexibility and the host-guest interactions within its hydrophobic channel allow it to bind to and separate these compounds from aliphatic mixtures.

'In the refinery process, separating aromatic hydrocarbons from aliphatic hydrocarbon mixtures is challenging,' explains Cui. This is because the hydrocarbons have similar boiling points or form azeotropes (mixtures of two or more chemicals that can't be separated by simple distillation). 'Although azeotropic and extractive distillation can be used for this separation, both processes suffer from high operational costs,' says Cui.

Cui adds that his polymers are readily tunable, so polymers with different functional surfaces in the channel - hydrophobic, hydrophilic or amphiphilic - could be developed. Darren Bradshaw, an expert in metal-organic frameworks at the University of Liverpool, UK, says that Cui's polymer framework 'could be an effective design strategy for future applications. What is also interesting is that many transition metal salen complexes are catalytic, so Cui's material and its derivatives may also be useful selective heterogeneous catalysts.'

Cui says that in the future, he hopes to balance his polymer's flexibility and stability to generate a semi-flexible porous framework that can be recycled without adsorption and selectivity losses.

Emma Shiells

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