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16 September, 2014 - 21:04 By News Desk

Super sieves to capture greenhouse gases


Cambridge University researchers have developed advanced and powerful molecular ‘sieves’ which could be used to filter carbon dioxide and other greenhouse gases from the atmosphere.

A Cambridge-Kyoto collaboration has been central to developing super-strong polymer membranes to make the filters. 

Cavendish Laboratory researchers, working with a team from Kyoto University, has created an alternative approach to generating the membranes – ‘baking’ them in the presence of oxygen, a process known as thermal oxidation.

They say the discovery provides a greener and more energy-efficient method of separating gases, and can remove carbon dioxide and other greenhouse gases from the atmosphere; The knock-on benefit is that this could significantly reduce the cost of carbon capture.

Cavendish says 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. Details are published in the journal Nature Communications.

Current methods for separating gases are complex, expensive and energy-intensive, the team says. Also conventional polymers, while reliable and inexpensive, are not suitable for large scale applications as there is a trade-off between low permeability levels and a high degree of selective molecular separation, they add.

“The secret is that we introduce stronger forces between polymer chains,” said Dr Qilei Song of the Cavendish Laboratory, the paper’s lead author.

“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 cross-linked structure also makes these membranes more stable than conventional solution-processed PIMs, which have a twisted and rigid structure - like dried pasta - that makes them unable to pack efficiently. Thermal oxidation and crosslinking reinforces the strength of channels while controlling the size of the openings leading into the cavities, which allows for higher selectivity.

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. 

As well as possible uses for separating carbon dioxide from flue gas emitted from coal-fired power plants, the membranes could also be used in air separation, natural gas processing, hydrogen gas production, or could help make more efficient combustion of fossil fuels and power generation with much lower emissions of air pollutants.

The research was supported by the Engineering and Physical Sciences Research Council and the European Research Council.

• PHOTOGRAPH: Polymer molecular sieves with interconnected pores (in green) for rapid and selective transport of molecules. Credit: Qilei Song

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