Precisely placed nanoparticles enhance selectivity in catalytic reactions

Researchers from the Harvard John A. Paulson School of and Applied Sciences (SEAS), Harvard Department of Chemistry & Chemical Biology, and Utrecht University have reported on a previously elusive way to improve the selectivity of , adding a new method of increasing the efficacy of for a potentially wide range of applications in various industries including , cosmetics and much more.

The research is published in Nature Catalysis.

The relies on catalysts for over 90% of its processes and nearly all these catalysts consist of dispersed on top of a substrate. Researchers have long suspected that the size of individual nanoparticles and the distance between them play important roles in the speed of, and the products produced in the catalytic reaction, but because nanoparticles are prone to moving around and agglomerating during catalysis, it's been difficult to study exactly how.

Over the past decade, Joanna Aizenberg, the Amy Smith Berylson Professor of and Professor of Chemistry & Chemical Biology, and her lab have been drawing inspiration from nature to build highly ordered, porous materials for a wide range of catalytic reactions.

Inspired by the structure of butterfly wings, the researchers designed a new catalyst platform that partially embeds nanoparticles into the substrate, trapping them so they don't move around during catalysis, while leaving the rest of the nanoparticles' surface exposed, enabling them to perform the catalytic reactions efficiently and without agglomeration.

The researchers found that the distance between particles had a huge impact on the selectivity of the reaction.

“Many industrially-relevant follow a cascade whereby chemical A is turned to chemical B which can then be turned into chemical C and so forth,” said Kang Rui Garrick Lim, a graduate student in the Aizenberg Lab and first author of the study.

“In some catalytic processes, the intermediate chemical, chemical B, is the goal, while in others it is the end product, chemical C. The selectivity of the catalyst refers to whether it favors the production of chemical B or chemical C.”

A good example of this is the production of benzyl , a chemical used in everything from shellacks, paints, and leather production to intravenous medications, cosmetics, and topical drugs.

Benzyl alcohol is the intermediate chemical B, derived from the hydrogenation of benzaldehyde (chemical A), before the reaction creates toluene (chemical C), another commonly used chemical but of lower value. In order to produce benzyl alcohol efficiently, the formation of toluene needs to be suppressed.

Currently, to make the more useful benzyl alcohol, the catalytic is slowed down, or not run to completion, to ensure that the reaction will stop at B and form as little toluene as possible.

“Generally, to make these intermediate chemicals, you make the catalyst less reactive and the overall reaction slower, which is not productive at all,” said Lim. “Catalysts are meant to speed things up, not slow them down.”

Source: Harvard John A. Paulson School of Engineering and Applied Sciences

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