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Environment

Splitting Up Fatty Acids

Nanofiltration membrane technique separates vegetable oil molecules, opening the way to their broader industrial use

by Stephen K. Ritter
February 11, 2013 | A version of this story appeared in Volume 91, Issue 6

SIZED UP AND SPLIT UP
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By converting fatty acids to amine salts, Iowa researchers are able to separate mixtures of fatty acids using polymer nanofiltration membranes.
Structure scheme shows how by converting fatty acids to amine salts, a polymer nanofiltration membrane can be used to separate mixtures of fatty acids.
By converting fatty acids to amine salts, Iowa researchers are able to separate mixtures of fatty acids using polymer nanofiltration membranes.

Vegetable oils derived from soybeans, canola, and other seed crops are already widely used for cooking, to make biodiesel fuel, and as solvents for inks and paints. Chemists could do much more with vegetable oils if they could affordably separate on an industrial scale the various long-chain fatty acids that make up the oils.

Rather than creating a specialized membrane, the researchers turned the problem around and chemically modified the fatty acids instead. They were then able to separate mixtures of fatty acids depending on subtle differences in the shapes of the fatty acid alkyl chains.

“Membranes prepared from polymers and block copolymers are attracting substantial attention worldwide with respect to the efficient separation of species as diverse as gases and proteins,” says Ian Manners, a polymer chemist at the University of Bristol, in England. “The value of this example,” Manners adds, “is underscored by the current interest in using vegetable oils, which are rich in mixtures of fatty acids, as a biorenewable resource.”

Vegetable oils are made up of triglycerides in which three long-chain fatty acids are attached to a glycerol core. The fatty acid chains are readily hydrolyzed to release them from glycerol. Depending on the source, vegetable oils contain five or more different fatty acids—which are 16 to 22 carbon atoms in length, with no double bonds (saturated acids) or one to three double bonds with cis geometry (unsaturated acids). Vegetable oils sometimes also include trans fatty acids made by partial hydrogenation of triglycerides.

When used in chemical processing, the mixture of fatty acids in vegetable oils leads to a mixture of products, which limits industrial applications to examples where mixtures are not a problem, such as solvents or biofuels, Bowden says. Chemists can purify fatty acids for other applications by precipitation, distillation, liquid chromatography, or selective hydrolysis of triglycerides, he notes. But those methods aren’t amenable to scaling up to the millions of tons per year needed for industrial processes.

Because fatty acids in vegetable oils have similar properties, they zip through standard polymer membranes at the same rate, Bowden points out. That makes separating them seemingly impossible. But the Iowa researchers hit on the idea of separating fatty acids by modifying them with bulky trialkylamines to form fatty acid-amine salt pairs.

Adding the amine is like sticking a blob on the end of the fatty acid chain, Bowden explains. It increases the cross-sectional area of the fatty acids and amplifies the differences in the shapes of the alkyl chain tails.

The team made 100-μm-thick nano­porous membranes from highly cross-linked polydicyclopentadiene. Saturated and trans fatty acid-amine salts with their straight chains slide through the membranes fairly easily but at slightly different rates, Bowden says. Cis fatty acid-amine salts with their kinked tails wiggle through more slowly.

VEGGIE OIL CAPTAINS
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Credit: Robert Hougas
Iowa’s Bowden (left) and Gupta invented a nanofiltration separation technique that splits up the multiple fatty acids found in vegetable oils. They hold a lab-scale device they used for the experiments.
Iowa’s Bowden (left) and Gupta invented a nanofiltration separation technique that splits up the multiple fatty acids found in vegetable oils. They hold a lab-scale device they used for the experiments.
Credit: Robert Hougas
Iowa’s Bowden (left) and Gupta invented a nanofiltration separation technique that splits up the multiple fatty acids found in vegetable oils. They hold a lab-scale device they used for the experiments.

By using different amines and several membranes in tandem, the Iowa team is able to isolate all the fatty acids in a mixture. The researchers also show that by applying nitrogen gas pressure to the membrane system they can increase the rate of fatty acid separation to meet the speed needed for an industrial process. The technique does have a couple of liabilities in that it requires the use of amines as auxiliary agents and a relatively large amount of solvent, although those chemicals are recovered and reused.

“This discovery is potentially important because there are few simple and inexpensive methods for separating fatty acids,” says polymer chemist T. Randall Lee of the University of Houston.

“A particularly interesting aspect of this work is that using amines provides a straightforward and rapid method that should allow the separation of new classes of fatty acids beyond those explored in this publication,” Lee continues. “This discovery might have a profound impact in the development of new chemical feedstocks for industrial applications.”

For example, the individual fatty acids could replace petroleum-derived chemicals used to make detergents and polymers, Bowden says. The separation method also could be used to make designer vegetable oils with selected properties, such as leaving out unhealthy trans fatty acids.

“I’m surprised at how well the separations have gone so far,” Bowden says. He adds that the university has filed patents on the technology and is now seeking industrial collaborators.

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