Microreactions: Need microspectroscopy

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  • Published: Mar 1, 2014
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Microreactions: Need microspectroscopy

Catalytic quickstep

In Situ IR and X ‑ ray High Spatial-Resolution Microspectroscopy Measurements of Multistep Organic Transformation in Flow Microreactor Catalyzed by Au Nanoclusters (Credit: American Chemical Society, JACS)

In situ infrared and X-ray high spatial-resolution microspectroscopy have been used for the first time to track multistep organic transformations in a flow microreactor in which gold nanoclusters act as the catalyst.

Finding routes to more efficient reaction schemes for pharmaceutical manufacture might involve the use of scalable flow microreactors rather than conventional industrial-scale reaction flasks and chemical engineering approaches. A team of scientists with the US Department of Energy's Lawrence Berkeley National Laboratory and the University of California Berkeley have now found a way to get a closer look inside such a microreactor for the first time and can observe a catalytic reaction from start to finish at high resolution. The results have the potential for improving our understanding of the chemistry underpinning catalytic reactions but they also open up new opportunities for the optimization of such reactions.

Using Berkeley Lab's Advanced Light Source, chemists Dean Toste and Gabor Somorjai and their colleagues focused beams of infrared and X-rays on their microreactor to track the evolution of a catalytic reaction with a spatial resolution of 15 micrometres.

Go with the flow

"The formation of different chemical products during the reactions was analyzed using in situ infrared micro-spectroscopy, while the state of the catalyst along the flow reactor was determined using in situ X-ray absorption microspectroscopy," explains Toste. "Our results show that using infrared microspectroscopy to monitor the evolution of reactants into a desired product could be an invaluable tool for optimizing pharmaceutical-related synthetic processes that take place in flow reactors."

Writing in the Journal of the American Chemical Society along with Elad Gross, Xing-Zhong Shu, Selim Alayoglu, Hans Bechtel and Michael Martin, the pair explain how there are two basic modes for carrying out catalytic reactions - the "batch" mode, in which a final chemical product is produced over a series of separate stages and the "flow" mode, in which chemical reactions run in a continuously flowing stream to yield a final product. The growing use of microreactors offers the chemical and pharmaceutical industries a way to switch from traditional batch mode reactions to flow mode offering greater highly recyclability of reagents, easier separation of products and scalability. There obstacles to the widespread adoption of microreactors, not least the fact that complicated molecules require complicated multi-step syntheses and for pharmaceutical drugs in particular this can often involve different solvent systems at different stages of the process with separations along the way. All of these need to be carefully monitored. Until now, there was no simple way to follow multistep production process in a flow reactor without interrupting the flow to extract samples for testing.

Our method allows us to watch an entire catalytic movie, from reactants into products formation, instead of only snapshots of the catalytic process," explains team member Gross. "In most cases before, chemists had to extrapolate information on the reaction process based on analysis of the final product. With our technique, we don't have to guess what happened in the first scene based on what we saw in the final scene, since now we’re able to directly watch a high-resolution movie of the entire process."

Catalytic support

As proof of principle, the team investigated gold nanoclusters loaded on to a silica support as a heterogeneous catalyst for the production of dihydropyran, a product that is commonly accessed via a multi-step reaction. Each of the reactants involved as well as the products and any by-products all have their own distinct infrared signatures. The infrared microspectroscopy was performed on ALS beamline 1.4.

"ALS beamline 1.4 provides a bright infrared beam with a diameter of less than 10 micrometres," Gross explains. "The small diameter of the beam enabled us to draw a map of the flow reactor with high spatial resolution of up to 15 micrometres. Without this high resolution imaging, we would not be able to track and understand key processes in the catalytic reaction." The team combined the in situ infrared microspectroscopic data with that from X-ray absorption results to map the catalytic reactivity completely.

In following the reaction kinetics step-by-step, the Berkeley researchers discovered that the catalytic reaction they were observing is completed within the first five-percent of the flow reactor's volume, which meant that the remaining 95-percent of the reactor, though packed with catalyst did not contribute to the catalytic process. "Based on this result, we were able to minimize the volume of the flow reactor and the amount of catalyst by an order of magnitude without deteriorating the catalytic reactivity," Gross adds.

The next step is to step up from this one-dimensional mapping approach, which tracks the reaction along the path of the flow, the more realistic three-dimensional space of the microreactor. Gross and Toste along with Michael Martin and Hans Bechtel, beam scientists at the ALS infrared beamline, are now exploring techniques that would permit two- and three-dimensional mapping of catalytic reactions. "Multidimensional imaging will give us the ability to know where exactly inside the volume of the flow reactor the catalytic reaction takes place," Gross explains. "This will provide us advanced tools for better understanding and optimization of the catalytic reaction."

Related Links

J Am Chem Soc, 2014, online: "In Situ IR and X ‑ ray High Spatial-Resolution Microspectroscopy Measurements of Multistep Organic Transformation in Flow Microreactor Catalyzed by Au Nanoclusters"

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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