MR thermometry: Catalytic insight

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  • Published: Nov 1, 2013
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: MR thermometry: Catalytic insight

Microreactor measurements

A variation on the magnetic resonance imaging theme has been used by US chemists to investigate the innards of a catalytic reactor to find out how the temperatures of gases within evolve as the reaction proceeds.

The new method allows the researchers to map the temperatures at the gas–solid interface, in which gases are altered through contact with a solid catalyst. The work represents a significant step forward in bridging the gap between experimental, laboratory-based research and catalysts used in the chemical industry; more than 85 percent of industry products are made using a catalyst at some stage. It could help researchers improve the design of catalytic reactors as well as reducing the environmental impact of chemical processing in the manufacture of pharmaceuticals and other chemical products.

Measuring temperature distributions within a reactor is crucial to understanding the energetics of a chemical reaction as reacting gases flow over a catalytic surface and to optimizing reactor design. But until recently, it was not possible to probe gas temperatures in the reaction mixture without disturbing the very flow being studied within the operating reactor. Magnetic resonance thermometry has been used before to measure temperatures in liquids non-invasively, but in gases, say the researchers, the conventional approaches produce signals that tend to be less sensitive or rapidly attenuated by diffusion or that require the addition of contrast agents that can interfere with the reactions being observed.

Non-invasive thermometry

"The new method is important because an overwhelming majority of chemical industry products are made using heterogeneous catalysts functioning at the gas–solid interface," explains the study's lead author, Louis Bouchard who works at the University of California Los Angeles' California NanoSystems Institute. Bouchard and graduate student Nanette Jarenwattananon and their colleagues, including Omar Yaghi of the University of California at Berkeley, Stefan Glöggler, Trenton Otto, Arek Melkonian, William Morris and Scott Burt of Brigham Young University.

"While there are methods to probe temperatures in liquids, few approaches exist for mapping the properties of gases inside a catalytic reactor without perturbing the flow being studied," Bouchard explains. "The inherent non-invasive nature of MRI bypasses this issue and allows us to generate thermal maps of gases during the course of a reaction."

The team exploited parahydrogen to carry out microscale MRI experiments on a micro-reactor. The study utilizes the inverse relationship between NMR line widths and temperature caused by motional averaging in a weak magnetic field gradient, the team reports. The chemical reaction being studied was hydrogenation of propylene to propane. "The ability to follow the reaction in a micro-reactor will help engineers and chemists design better lab-on-a-chip devices," Bouchard adds. Such catalytic micro-reactors are offering a green chemistry alternative to industry with potentially less chemical waste, lower cost and minimal energy waste as well as a modular approach to scale up. MRI might now surmount one of the obstacles hindering their widespread adoption.

Running hot and cold

"These results establish our technique as a non-invasive tool for locating hot and cold spots in catalyst-packed gas–solid reactors, with unprecedented capabilities for testing the approximations used in reactor modeling," the team concludes. "We now hope to apply this technique to other systems, such as multi-step reactions, as well as using it to screen a number of different catalysts in parallel," Bouchard revealed to SpectroscopyNOW.

Related Links

Nature 2013, 502, 537–540: "Thermal maps of gases in heterogeneous reactions"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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