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Separate xylene isomers with less energy

| By Gerald Ondrey

Xylene isomers are usually derived from the catalytic reforming of crude oil, and require costly methods to separate them from each other, including distillation, fractional crystallization and adsorption in high-temperature and high-pressure environments. “The separation of xylene’s isomers requires much energy and is a difficult process due to the isomers’ overlapping physiochemical properties,” says Abdul-Hamid Emwas, staff scientist in nuclear magnetic resonance (NMR) from the Imaging and Characterization Core Lab (IAC) at the King Abdullah University of Science and Technology (KAUST; Thuwal, Saudi Arabia; “These include identical molecular weights, close boiling points and similar structures.”

KAUST associate professor Niveen Khashab and her research group recently teamed up with IAC and an international group of scientists to find a new and less energy-consuming method to separate and purify the isomers for the petrochemical industry. Their work is described in a recent issue of the journal Chem.

To separate the isomers, the research team took advantage of the properties of cucurbiturils, which are organic macrocyclic molecules made of glycoluril monomers linked by methylene bridges. They are shaped like pumpkins, with their hydrophobic central cavity able to hold smaller molecules.

The researchers used an aqueous solution of cucurbit[7]uril (or CB7), which “has strong and distinctive binding affinity with xylene isomers in water,” says Gengwu Zhang, a postdoctoral fellow and the lead author of the paper. “Using liquid-liquid extraction (at room temperature and pressure), we showed that the hole in the middle of CB7 can selectively host o-xylene from mixtures of xylene isomers,” explains Zhang. “We could separate o-xylene with selectivity of more than 92% after one extraction cycle.”

Because the three isomers have different NMR spectra, NMR was used to perform the study: high-resolution 1-D NMR was used for quantitative analyses, and advanced 2-D NMR experiments were used to probe the separation mechanism that explains the novelty of the separation process using CB7.

The researchers showed that CB7 can separate xylenes from commercial oil samples at scales of up to 0.5 L. Also, laboratory scale-up experiments using commercial xylenes and C8 aromatic fraction of pyrolysis gasoline proved that CB7 is able to separate o-xylene with a selectivity of up to 83%.