Gravity-purified MOFs

Metal-organic frameworks (MOFs) can now be quickly and easily purified via a simple solvent-based method that takes advantage of minor density differences between the porous materials and any impurities, according to a study by Northwestern University's Omar K. Farha, Karen L. Mulfort, Alison M. Thorsness, and Joseph T. Hupp (J. Am. Chem. Soc., DOI: 10.1021/ja803097e). The technique could substantially benefit researchers involved in the discovery and production of MOFs, Hupp says. MOFs are highly porous, low-density materials made from metal ions or clusters joined by organic linker groups. Their large internal surface areas make them amenable for chemical separations, selective catalysis, and storing gases such as H₂ and CO₂. MOFs are typically synthesized in one-pot reactions, but chemists must painstakingly discover reaction conditions that result in ultrapure products because purifying the solids is not feasible by typical methods such as recrystallization. Hupp's team, taking advantage of a known solvent-based separation concept, started by floating MOFs on the surface of CH₂BrCl, a high-density solvent. By adding miscible cosolvents the researchers lowered the density of the solvent system to a point where the densest solid material—either the product or the impurities—sank and the remaining solid kept floating, thereby effectively separating the mixtures.