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Researchers discover how caged molecules 'rattle and sing'

A team of energy researchers from the University of Minnesota and University of Massachusetts Amherst has discovered that molecular motion can be predicted with high accuracy when confining molecules in small nanocages. Their theoretical method is suitable for screening millions of possible nanomaterials and could improve production of fuels and chemicals. The research is published online in ACS Central Science, a leading open-access journal of the American Chemical Society.

Molecules in the air are free to move, vibrate and tumble, but confine them in small nanotubes or cavities and they lose a lot of motion. The total loss in motion has big implications for the ability to capture CO2 from the air, convert biomass molecules into biofuels, or to separate natural gas, all of which use nanomaterials with small tubes and pores.

Researchers from the Catalysis Center for Energy Innovation headquartered at the University of Delaware arrived at their breakthrough when thinking about squeezing molecules into tight spaces. In the air, molecules can move up, down, and into space (three dimensions), but in a nanotube it was not clear if molecules can only move in one direction (through the tube) or two directions (on the surface of the tube). Similarly, molecules can rotate and spin in three ways, but the tube edges can prevent some or all of this motion. The amount of lost rotation was the unknown quantity.

“Our approach was to separate out molecular tumbling and rotating from movement in position,” said Omar Abdelrahman, a co-author of the study who is a University of Massachusetts Amherst chemical engineering assistant professor and Catalysis Center for Energy Innovation researcher. “We discovered that all molecules when put into nano-cages lose the same amount of movement in position, but the amount of rotating and spinning depended highly on the structure of the nano-cage”.

The team connected molecular motion to the quantity of entropy, which combines all aspects of molecular motion into a single number. Molecules lose different amounts of entropy when they access the inside of nanoporous spaces, but it has not been clear how the structure of those nanospaces impacted the change in motion and loss in entropy.

“It might sound esoteric, but the entropy changes of molecules due to limitations of rotation and movement in position within nanopores decides whether nanomaterials will work for thousands of energy and separation technologies,” said Paul Dauenhauer, a co-author of the study who is a University of Minnesota chemical engineering and materials science associate professor and Catalysis Center for Energy Innovation researcher.

“If we can predict molecular motion and entropy of molecules, then we can quickly determine whether advanced nanomaterials will solve our most pressing energy challenges,” Dauenhauer added.

Excerpt from a news release written by Rhonda Zurn, College of Science and Engineering, and Lacey Nygard, University News Service. Click on the link below to read the full news release.

Related Link: http://z.umn.edu/cagedmolecules

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