Molecular sieve promises cheaper way to capture carbon

UK researchers have created a molecular sieve that could help cut the cost of removing carbon dioxide from the atmosphere.

The scientists from Cambridge and Manchester universities have developed a method of baking polymer membranes to create specifically shaped channels in them that only allow certain molecules to pass through.

They claim this could be a cheaper and more energy-efficient of separating carbon dioxide and other greenhouse substances from the other gases in exhaust streams or the atmosphere than existing technologies.

‘The secret is that we introduce stronger forces between polymer chains,’ said Dr Qilei Song from Cambridge’s Cavendish Laboratory, lead author of a paper on the research in the journal Nature Communications.

‘Heating microporous polymers using low levels of oxygen produces a tougher and far more selective membrane which is still relatively flexible, with a gas permeability that is 100 to 1,000 times higher than conventional polymer membranes.’

The synthetic membranes, made of materials known as polymers of intrinsic microporosity (PIMs), mimic the hourglass-shaped protein channels found in biological membranes in cells. The tiny openings in these molecular ‘sieves’ – just a few billionths of a metre in size – can be adjusted so that only certain molecules can pass through.

Inducing a thermal oxidation reaction – essentially by baking in the presence of oxygen – in the PIMs causes the loosely-packed long chains of polymer molecules to form into a cross-linked network structure, with hourglass-shaped cavities throughout.

This structure not only results in a membrane which is more selective to gas molecules, but also the size of necks and cavities can be tuned according to what temperature the PIMs are ‘baked’ at.

Current methods for separating gases, such as passing them through liquid solvents, are more complex, expensive and energy-intensive, the researchers said.

However, conventional inexpensive polymers are not suitable for separating gases because the better they are at targeting specific molecules, the slower those molecules can pass through the membrane.

The new membrane is twice as selective for separation carbon dioxide as conventional polymer membranes, but allows carbon dioxide to pass through it a few hundred times faster.

It is also more stable than conventional solution-processed PIMs, which have a twisted and rigid structure – like dried pasta – that makes them unable to pack efficiently.

The researchers said that as well as separating carbon dioxide from air or power-station flue gas, the membranes could be used in natural gas processing, hydrogen gas production and even to making fossil-fuel combustion more efficient and less polluting.

‘This new way of modifying PIMs brings the prospect of large-scale, energy-efficient gas separation a step closer,’ said Manchester’s Prof Peter Budd, one of the inventors of PIMs materials.

This research was supported by the Engineering and Physical Sciences Research Council (EPSRC) and the European Research Council (ERC).