US scientists have synthesised an organic liquid that can capture sulfur dioxide. The resulting reversible zwitterionic liquid, the first of its kind, could be used to prevent acid rain.

The new compound can reversibly bind sulfur dioxide

David Heldebrant and colleagues, at Pacific Northwest National Laboratory, Richland, made a molecule called N,N-dibutylundecanolamine, which consists of a tertiary amine base and an alcohol separated by a long alkyl chain. When they exposed it to sulfur dioxide, it bound the gas as a liquid zwitterionic alkylsulfite salt.

Sulfur dioxide released from power plants is one of the primary causes of acid rain. Most power plants have scrubbers, which are reaction towers containing slurries of caustic soda or lime that bind the sulfur dioxide. But the binding is irreversible, explains Heldebrant, and the resulting sulfurous compounds mostly end up in landfill sites.

Heldebrant's zwitterionic liquid binds reversibly to sulfur dioxide - the reactants are regenerated by heating the liquid to 70°C in a vacuum. 'We're doing a chemical capture that allows a reversible release so you could do something else with the sulfur dioxide later,' he says - the wine and cement industries both use the gas.

Edward Maginn at the University of Notre Dame, US, is an expert in sulfur dioxide capture and says the researchers have made 'an important discovery'. 'This work opens the door for the development of new types of solvents tailored to capture sulfur dioxide,' he adds.

The group intend to use what they have learnt with sulfur dioxide to produce a carbon dioxide-binding zwitterionic liquid. Maginn suggests that these processes could one day be combined. 'It may be possible to develop hybrid systems capable of capturing sulfur and carbon dioxides with either parallel or simultaneous regeneration,' he says.

Anna Roffey

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