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Materials

Carbohydrate-Based Metal-Organic Frameworks Purify Petrochemicals

Porous compounds offer an environmentally friendly route to sorting out xylene isomers and other aromatic hydrocarbons

by Bethany Halford
April 6, 2015 | A version of this story appeared in Volume 93, Issue 14

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Credit: J. Am. Chem. Soc.
o-Xylene molecules (blue with yellow methyl groups) get trapped in a CD-MOF (red), while p-xylene (black with yellow methyl groups) glides through.
Ortho-xylene molecules (blue) get trapped in a metal-organic framework (red), while a para-xylene molecule (black) slips through. Methyl groups are yellow.
Credit: J. Am. Chem. Soc.
o-Xylene molecules (blue with yellow methyl groups) get trapped in a CD-MOF (red), while p-xylene (black with yellow methyl groups) glides through.

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.

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