Porous materials are normally solids, with rigid cage structures used for separation and storage technologies, and for catalysis. Porous liquids were first reported in 2015 by Andrew Cooper’s lab at the University of Liverpool and Stuart James at Queen’s University Belfast (Nature, 2015, doi:10.1038/nature16072). They comprise liquids containing organic cages of just the right size to hold single molecules, but keep out solvent molecules.
Now, university spin-out company, Porous Liquid Technologies, is developing ‘porous liquids’ for commercial use, for possible applications in petrochemical, household goods and biomedical industries. ‘Liquids consist of molecules sticking quite close to each other, with spaces only opening up transiently. In our Nature paper, we described a liquid where we create and protect those spaces, using a cage,’ explains Stuart James at Queen’s University.
The new porous liquids offer unique advantages. Usually, gases such as methane and ethane have no acidity or basicity, and dissolve poorly in water. The new porous liquids allow gases to enter and sit inside rigid organic cages in a crown ether solvent, 15-crown-5, a cyclic pentamer of ethylene oxide.
Porous Liquid Technologies, part owned by Queen’s and the University of Liverpool, is now working on problems such as separating ethylene from ethane, and removing excess carbon dioxide from biogas to improve fuel quality. ‘Separating ethylene from ethane requires staggering amounts of energy in the petrochemical industry,’ explains Andrew Cooper, materials chemist at Liverpool.
‘Our idea is that you bubble these gases through our [porous liquid] solvent and the solvent absorbs one component. You could perhaps flow the liquid containing the gas to another area of the plant where the temperature is higher.’ This is less demanding in terms of energy and cooling, compared with a porous solid, which cannot be moved so easily.
The original synthesis of the porous liquid required seven steps, with 7% yield, and involved a toxic solvent. ‘They were quite difficult and expensive to make, and weren’t especially stable,’ says James. ‘We are now patenting some new systems where we use much more conventional types of solvents, which are much more economical to make.’
‘From a purely scientific point of view, this is a fascinating, clever and novel discovery of a new class of materials that deserves considerable future study,’ says Brian Laird, materials chemist at the University of Kansas, US.
He notes that the selectivity of the liquids to capture one molecule and not another, has yet to be revealed. Also, ‘right now the thermochemical data on the adsorption of these materials seems to be lacking,’ says Laird. Pressure or temperature changes to free the gas from its cage would both consume energy.