28 April 2011
The most chemically and thermally stable metal-organic frameworks (MOFs) yet have been made by a team in the US. The MOFs could surpass zeolites as industrial catalysts.
Natural zeolites are porous alumina-silicate rocks used as catalysts in industrial processes. However, their pore sizes and surface functionalisations are difficult to alter, which limits their performance. MOFs - made by joining up metal oxide clusters with linking organic molecules - have similar structures to zeolites and are therefore of interest as alternatives. Until now, they have not been robust enough to withstand the conditions that zeolites undergo during industrial processes. Traditionally, MOFs have only been stable in temperatures up to 500 degrees Celsius, have low chemical stability and some even fall apart in water.
Jeffrey Long from the University of California, Berkeley, and colleagues have made MOFs that can withstand temperatures of 510 degrees Celsius and a pH range of 2 to 14. They made the MOFs by reacting a metal salt, such as nickel chloride or nickel nitrate, with trispyrazolylbenzene. The organic molecules were deprotonated and the functional groups were bound to the metal to create a three-dimensional network. 'We made four different compounds based on pyrazolate-type ligands,' says Long. 'They have very strong metal-ligand bonds, which make the porous solids extremely robust. Stability is important for real-world applications. You'd like to be confident that the compounds will survive for long periods of time under extreme conditions.'
The MOFs were made by reacting metal salts with trispyrazolylbenzene
To test the MOFs' ability to retain their structures under extreme conditions, the team heated them to over 500 degrees Celsius and boiled them in either a strong acid or a strong base for two weeks. 'The nickel compound was particularly robust. It was stable for pH 2-14, which hadn't been demonstrated before,' says Long.
'There are a couple of MOFs out there that are stable when you heat them, but the thing that's exciting about Long's compounds is that they have exposed metal sites,' says Russell Morris, who studies MOFs at the University of St Andrews, UK. Exposed metal sites make the reactive site more accessible, explains Morris.
'For catalysis, surface area is important because it dictates the number of catalytic sites you can have on your material,' says Long, who adds that his MOFs have higher surface areas than zeolites. He goes on to say that the MOFs could be used to improve catalytic processes, molecular separations and for storing compressed gas in high density.
Morris believes that the MOFs will be used to catalyse different reactions to zeolites. By putting a metal catalyst into a framework, it might change the way catalysis is done, he says. 'You've also got pores, which could have an effect on shape and size selectivity, which is interesting,' he adds.