Two chemical reactions key to producing future fuels can be linked together in a single membrane-based reactor to increase their efficiency, say Chinese chemists. 

A team of scientists led by Haihui Wang at the South China University of Technology have created an experimental reactor that combines the splitting of water - to make hydrogen - with the production of syngas (carbon monoxide and hydrogen) from methane. Syngas is converted into liquid hydrocarbon fuels via the Fischer-Tropsch reaction.

Previous studies have shown that the water splitting reaction can be run by heating water within a tube made of an oxygen-permeable membrane called MIECM (mixed oxygen ion and electron conducting membrane). As the water is split into hydrogen and oxygen - an equilibrium reaction - the oxygen is lost through the membrane, driving the reaction towards the products.

The selective membrane gives a two-for-one reaction © Wiley-VCH

Wang has now shown that the oxygen that passes through the membrane can be reacted with methane to produce syngas. The syngas-forming reaction keeps the oxygen concentration on the outside of the membrane low, maintaining the oxygen concentration gradient. This gradient ensures oxygen continues to pass through the membrane, speeding up the water splitting reaction.

The overall process - producing carbon monoxide and hydrogen from water and methane - looks like the well-known steam reforming of methane. However, splitting the reaction into two separate product streams is beneficial, says Wang, because it produces pure hydrogen, plus syngas with the ideal hydrogen to carbon monioxide ratio - two to one - to be used in the subsequent Fischer-Tropsch step.

'I think it's a very nice idea to combine the two reactions,' says Henny Bouwmeester, who researches oxygen permeable membranes at the University of Twente in the Netherlands. 'It's very smart to use two gases which are widely available, and convert them simultaneously into two potentially useful products. The one problem could be the stability of the membrane materials - industry wants membranes that operate over extended times, you can't be replacing them every month.'

Wang says that lab tests show the membrane is stable for 10 days, but he is collaborating with German engineering company Uhde to test longer term operation. 'We hope to develop this technology for industrial use, and a lot of work - including making the module - will be done in the near future,' Wang told Chemistry World.

James Mitchell Crow

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