Bigger particles are first through the net

Scientists have reported new nano-engineered membranes that could enable a new generation of efficient and selective separation devices for applications in petroleum, chemicals and fuel production.

Scientists have reported new nano-engineered membranes that could enable a new generation of highly efficient, highly selective separation devices for applications in petroleum, chemicals and fuel production, environmental cleanup, and perhaps even biomolecule purification.

The new ‘Ultrapermeable, Reverse-selective Nanocomposite Membranes’ are reported by Research Triangle Institute (RTI), NC State University, and industry scientists in Science magazine, 19 April-2002.

The research team found a way to make membranes through which large molecules permeate faster than small molecules do. Researchers achieved this by using nanoparticles to modify the molecular structure of plastic membranes.

The first applications, already under development, include producing purified hydrogen for uses such as fuel cells, and cleaning pollution-causing chemicals out of fossil fuels.

Previous attempts to use particles in industrial gas separation membranes ran into a catch-22 situation. If the membrane became more permeable to molecules, then it also became less selective.

Tim Merkel, first author of the paper and his colleagues, found a way to increase the selectivity of the membrane and at the same time make it more permeable.

‘Usually, people add particles to plastics to make them less permeable,’ Merkel said. ‘For example, additives in plastic wrap might be used to allow less oxygen through, keeping your food fresh longer. In our case, however, the particles work at a molecular level as ‘nanospacers,’ opening the membrane and making it more permeable’

Merkel is heading up a basic research project to figure out how to make the particles active.

‘Right now, the particles just change the physical structure of the membrane,’ said Raghubir Gupta, who oversees RTI’s energy technology laboratory ‘But what if we make them chemically active? Then we could fine-tune a membrane to pass or block specific chemicals. For example, we could use enzymes and selectively separate proteins from mixtures in biochemical processes.’

While the basic research on activated particles is said to offer exciting possibilities, Merkel and colleagues at RTI emphasise the more immediate promise of a cheaper way to purify hydrogen.

‘The existing processes to purify hydrogen are expensive and complicated,’ he said. ‘They do have the advantage of being well established: people at, for example, oil refineries know how to make these processes work. But ours would be so much cheaper and simpler that you could even start to think about recovering hydrogen from hydrocarbon streams that refineries currently just burn as waste.’

Cheap hydrogen also could diversify the sources from which the US makes gasoline and diesel fuel.

‘Cheap hydrogen, combined with a cost-effective way to remove pollution-causing chemicals, could be the key to a practical way to make motor fuels using coal as the starting point, rather than crude oil,’ Gupta said.