Credit: ©2008 National Academy of Sciences, USA

Filtration of harmful chemicals from the environment is most often performed by activated carbons. However, because of the amorphous nature of the carbon network in these systems, it is difficult to control the functionality and capacity of the pores — features that are desirable to ensure toxic chemicals have been efficiently eliminated. Great control is offered by metal–organic frameworks (MOFs), which are networks of metal-oxide units joined together by organic linkers via covalent bonds. The size and functionality of the pores and the shape of the overall structure can be varied by judicious choice of the constituent components. So far, MOFs have only been used to store gases, but now Omar Yaghi and colleagues at the University of California, Los Angeles, have investigated their dynamic adsorption capabilities1 to determine whether they could be used instead of activated carbons.

The researchers chose eight test gases covering a range of uptake in a standard activated carbon (BPL), from those that are poorly taken up, such as ammonia and ethylene hydroxide, to those that have a good uptake, such as chlorine and benzene. Six test MOFs were chosen to provide a variety of pore size, functionality and overall surface area.

In many cases the MOFs were found to have a far greater adsorption capacity than BPL. Although further work is needed to investigate how MOFs would respond to gaseous mixtures and to impurities, this increased uptake, along with the ability to tune their functionality, makes them an attractive proposition for replacing activated carbons.