Credit: © 2008 RSC

Metal–organic frameworks (MOFs) have garnered much attention because of their ability to adsorb gases, which can be tailored by tuning the pore size and MOF–adsorbate interactions through careful choice of ligand and metal. An example of such considered MOF synthesis from Randall Snurr and co-workers at Northwestern University, USA, has led1 to a material that could be used to remove CO2 from CO2/CH4 mixtures — an important step in the purification of natural gas.

The classic framework, MOF-5, uses 1,4-benzenedicarboxylic acid ligands to connect zinc clusters. Now Snurr and colleagues have replaced the planar benzene with a rigid and stable 3-dimensional carborane cluster to create a more complex MOF structure that also sees dimethyl formamide molecules coordinate to two of the zinc cluster ions. These solvent molecules can be removed by heating under vacuum to reveal 'open' metal sites within the framework.

The researchers measured the pressure-dependent adsorption properties of both MOF structures — with and without coordinated solvent molecules — for both single-component gases CO2 and CH4. By fitting this isotherm data to a model, the adsorption properties of the MOFs for CO2/CH4 mixtures can then be predicted, revealing that both preferentially adsorbed CO2 over CH4, but that the MOF with no coordinated solvent molecules showed much higher selectivity. It is suggested that the 'open' metal sites enhance the interaction of the framework with quadrupolar CO2 in preference to nonpolar CH4.