Nickel Complex Sorts Out Xylenes

Exposing a crystalline nickel complex to xylene vapor is a simple yet promising method for separating xylene’s three structural isomers, according to Matteo Lusi and Leonard J. Barbour of the University of Stellenbosch, in South Africa (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201109084). Xylenes, or dimethylbenzenes, are derived together from petroleum and used collectively or separately as solvents or as commodity feedstocks. However, the isomers are difficult and expensive to separate by conventional methods, which include filtration using porous solids, distillation, and crystallization. Lusi and Barbour found that a nickel complex bearing phenylpyridine and isothiocyanate ligands (shown) readily absorbs the isomers independently via host-guest complexation to form clathrate compounds. But in comparative experiments they found that the complex thermodynamically favors o-xylene over m-xylene and p-xylene, and m-xylene over p-xylene. For instance, when the material is exposed to a vapor of all three isomers, it binds o-xylene. Once the o-xylene is desorbed by heating, and the nickel complex is reexposed to the remaining vapor, the material then binds m-xylene. The researchers believe that with fine-tuning the material could be useful for industrial-scale separation of xylenes.