An iron-based porous solid that can store hydrogen and capture carbon dioxide could lead to greener energy and cleaner air, say US scientists.

Porous solids known as metal-organic frameworks (MOFs) can have extremely high internal surface areas, making them of interest for use in mobile hydrogen storage systems. However, at room temperature, MOFs aren't good at storing hydrogen because the interactions between the gas and the MOF surface are weak. One way to improve their performance is to generate exposed metal cation sites on the framework surface, which provide strong adsorption sites for hydrogen.

Jeffrey Long, at the University of California, Berkeley, and colleagues made a MOF with accessible Fe²⁺ sites on its surface. Although they had previously made similar structures using manganese and copper cations, they found that the iron ions were better at binding hydrogen. In fact, at room temperature, the iron-based MOF bound hydrogen more strongly than most other reported MOFs, moving hydrogen storage research a step closer to US Department of Energy targets for hydrogen fuel cell-powered cars. The MOF is also good at binding carbon dioxide, says Long, making it of interest for capturing the greenhouse gas from power station waste.

Accessible Fe2+ sites on the MOF surface allow hydrogen storage

Identifying and optimising the synthetic conditions for a new metal-organic framework typically involves the systematic variation of many reaction parameters, including the metal salt, metal-to-ligand ratio, solvent composition, acid/base content, reaction temperature, and reaction time, explains Long. So, to make the process quicker Long developed a high-throughput method that allows rapid dispensing of reagents and solvents into individual reaction vials with up to 96 reactions performed on a single vial plate. The composition of each reaction mixture is programmed through a computer interface to be precisely set and monitored. This could make it much easier to synthesise new MOFs in the future, says Long.

'We have laid out a strategy for using automated instrumentation for the rapid, parallel synthesis of new materials for hydrogen storage and carbon dioxide capture,' says Long. 'We are now working to couple this technology with high-throughput screening instrumentation that will enable us to further speed the pace of discovery of these desperately needed materials.'

Lee Brammer, an expert in MOFs at the University of Sheffield, UK, comments, 'the Fe-based material shows great promise both in hydrogen sorption and in carbon dioxide sequestration under realistic conditions for industrial applications. It will be an important step towards developing a new generation of materials for commercial use.'

Joanne Thomson 

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