Carbohydrate-Based Metal-Organic Frameworks Purify Petrochemicals

Of the chemical riches obtained when refining crude oil, the mixture of benzene, toluene, ethylbenzene, and xylene isomers known as BTEX poses a particular challenge. Because of their similar aromatic characteristics, the BTEX molecules can be tough to separate. But individually, the molecules are valuable feedstocks. p-Xylene, for example, is a precursor to the monomers that make polyethylene terephthalate, a common plastic. Researchers led by Northwestern University’s J. Fraser Stoddart report an environmentally friendly way to separate the BTEX components, using cyclodextrin-based metal-organic frameworks, or CD-MOFs (J. Am. Chem. Soc. 2015, DOI: 10.1021/ja511878b). The CD-MOFs, which are extended porous networks of γ-cyclodextrins and alkali metal cations, separate BTEX components based on their shapes. For example, interactions between cyclodextrin units and o-xylene tend to make that molecule stick to the CD-MOF longer, while p-xylene slides through more easily. The CD-MOF can be produced on the kilogram scale, so Stoddart’s team thinks its strategy offers a greener and economical alternative to current industrial BTEX separations, such as simulated moving bed technologies.