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Hollow molecules selectively extract cyclohexane for greener hydrocarbon separation

Hollow, pumpkin-shaped molecules can efficiently separate valuable hydrocarbons from crude oil, KAUST researchers have shown. These "molecular sieves," known as cucurbiturils, could enable a more sustainable approach to producing raw materials for the chemicals industry.

The findings in the journal Nature Sustainability.

Crude oil is a complex mixture of hydrocarbons vital for almost every aspect of life, from fuels to plastics. Cyclohexane, for example, is used in nylon production, but isolating it at sufficient purity typically involves multiple energy-intensive fractional distillation steps.

The KAUST team has now developed an alternative separation strategy based on cucurbiturils, named for their resemblance to pumpkins of the plant family Cucurbitaceae. These come in various sizes and have spherical cavities that can trap other molecules.

When the researchers dissolved a cucurbituril called CB[7] in water and mixed it with crude oil at room temperature and pressure, CB[7] pulled cyclohexane and a couple of closely related hydrocarbons into its cavity. The oil and water naturally separated into two layers so that the water could be removed along with CB[7] and its cargo. These hydrocarbons were then washed out of CB[7] using a common solvent.

Because the hydrocarbons and solvent have significantly different boiling points, they could be easily separated through simple and relatively energy-efficient distillation processes.

"It requires much less energy than crude oil distillation, which involves many components with similar boiling points and complex interactions," says Niveen M. Khashab, who led the research.

The process worked well under harsh chemical conditions—including acid, alkali, and salt—and the CB[7] and solvent could be recycled many times.

"Thanks to the good recyclability of the aqueous CB[7] system, net water usage remains low," adds team member Gengwu Zhang.

Distillation of crude oil is not the only method used to produce cyclohexane. Some cyclohexane is produced through a chemical reaction involving benzene, but separating them is challenging due to their similar boiling points. When the KAUST team tested CB[7] on a 50:50 mixture of benzene and cyclohexane, they found it could deliver cyclohexane of more than 99.6% purity after a single extraction.

They also tested CB[7] on a mixture called crude oil distillate, which includes about 2.5% cyclohexane alongside many other hydrocarbons with comparable boiling points. Again, the cucurbituril provided cyclohexane with a purity of over 99% .

"CB[7] binds cyclohexane so selectively because its spherical cavity closely matches the size and shape of the cyclohexane molecule, enabling strong and specific host-guest interactions," explains Zhang.

CB[7] is also easily synthesized from inexpensive ingredients and is highly stable, allowing it to be reused many times.

The researchers estimate that their CB[7]-based separation method could use approximately 57% to 82% less energy than conventional extractive distillation, making it a more sustainable and potentially cost-effective alternative. They now hope to tune the cavity size and binding properties of other hollow molecules to extract a broader range of components of crude oil.

"We are currently working on scaling up the process and exploring collaborations with industry," says Khashab.