How natural oils can be hydrogenated without making unhealthy trans fats

UC Riverside researchers illustrate how controlling shape of heterogeneous catalysts can promote desired chemical reaction

28-Jan-2009 - USA

To prolong the shelf life of foods, manufacturers often add hydrogen to natural oils, a process called hydrogenation. But hydrogenation also results in the production of trans fats, which have adverse health effects such as raising bad cholesterol and increasing the risk for coronary heart disorders. Now UC Riverside chemists have designed a catalyst that allows hydrogenated oils to be made while minimizing the production of trans fats.

In their experiments, the researchers, led by Francisco Zaera , a professor of chemistry, used platinum, a common catalyst for these processes. By controlling the shape of the platinum particles, the Zaera group was able to make the catalyst more selective. Zaera's lab found that the platinum catalyst performed most selectively when its particles assumed tetrahedral shapes, with the atoms arranged in a hexagonal honeycomb lattice. Particles with these shapes allow for the preservation of the harmless cis configuration in the hydrogenated fats. Other lattices, the researchers found, favor the production of trans fats.

Platinum catalysts such as those used by the Zaera group are considered heterogeneous because they exist in a different phase (solid) than the reactants (liquid or gas). Compared with homogeneous catalysts, where the catalyst is in the same phase (liquid) as the reactants, heterogeneous catalysts have the advantages of easy preparation, handling, separation from the reaction mixture, reuse, high stability, and low cost.

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But their main disadvantage is that, unlike homogeneous catalysts, which tend to be molecular, heterogeneous catalysts must be dispersed as small particles in a high surface-area support in order to optimize their use. This typically results in catalysts with surfaces of ill-defined structures.

The research by Zaera and his colleagues is a breakthrough also because it shows for the first time that it is possible to achieve selectivity with heterogeneous catalysts like platinum by controlling the structure of their surfaces.

"The more control we can exert on how we prepare catalysts, the more we can control the catalytic selectivity of a particular chemical process," Zaera said. "Our work shows that it is possible to make heterogeneous catalysts that afford us more control on selectivity. This opens the door, we hope, for chemists to think about achieving selectivity for other reactions via the design of specific heterogeneous catalysts with specific shapes."

Zaera explained that heterogeneous catalysts tend to be more practical in terms of manipulation, but are harder to control.

Original publication: Nature Materials 2009.

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