Carbohydrate-Based Metal-Organic Frameworks Purify Petrochemicals | Chemical & Engineering News
Volume 93 Issue 14 | p. 23 | Concentrates
Issue Date: April 6, 2015

Carbohydrate-Based Metal-Organic Frameworks Purify Petrochemicals

Porous compounds offer an environmentally friendly route to sorting out xylene isomers and other aromatic hydrocarbons
Department: Science & Technology
News Channels: Organic SCENE, Materials SCENE, JACS In C&EN
Keywords: xylenes, BTEX, cyclodextrin, metal organic framework, MOF
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.
Credit: J. Am. Chem. Soc.
Ortho-xylene molecules (blue) get trapped in a metal-organic framework (red), while a para-xylene molecule (black) slips through. Methyl groups are yellow.
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.
Credit: J. Am. Chem. Soc.

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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Robert Buntrock (April 10, 2015 4:54 PM)
The crucial separation is of the xylene isomers since benzene and toluene can be efficiently separated by fractional distillation which is not effective for separation of the xylene isomers. The established processes for that separation, specifically for the more valuable p-xylene, involve cryogenic crystallization since the freezing point for p-xylene is significantly higher than the other isomers. However, this requires the world's largest chemical plants since the process is only about 25% efficient per pas and the recycle volume is large. Even at that, a p-xylene plant is still smaller than the smallest oil refinery.

Bottom line: scale up of this interesting process to produce the necessary volumes of p-xylene will be very difficult. Also not stated is what happens to the m-xylene.

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