New light-powered catalysts could help manu

Catalytic coating

image: MIT chemists have designed a new type of photoredox catalyst that could make it easier to integrate light-induced reactions into continuous-flow manufacturing processes. Polymer catalysts could be used to coat the tubes and perform chemical transformations on the reactants as they pass through the tube, as imagined in this digital illustration.
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CAMBRIDGE, MA — Light-triggered chemical reactions offer a powerful tool for chemists who are designing new ways to make pharmaceuticals and other useful compounds. Harnessing this light energy requires photoredox catalysts, which can absorb light and transfer the energy to a chemical reaction.

MIT chemists have now designed a new type of photoredox catalyst that could make it easier to incorporate light-induced reactions into manufacturing processes. Unlike most existing photoredox catalysts, the new class of material is insoluble, so it can be used over and over again. Such catalysts could be used to coat the tubes and perform chemical transformations on the reactants as they flow through the tube.

“Being able to recycle the catalyst is one of the biggest challenges in being able to use photoredox catalysis in manufacturing. We hope that by being able to do flow chemistry with an immobilized catalyst, we can provide a new way to do photoredox catalysis on a larger scale,” says Richard Liu, postdoctoral fellow at MIT and co-lead author of the new study.

The new catalysts, which can be tuned to perform many different types of reactions, could also be incorporated into other materials, including textiles or particles.

Timothy Swager, John D. MacArthur Professor of Chemistry at MIT, is the lead author of the paper, which appears today in Nature Communication. MIT researcher Sheng Guo and MIT graduate student Shao-Xiong Lennon Luo are also the authors of the article.

Hybrid materials

Photoredox catalysts work by absorbing photons and then using that light energy to fuel a chemical reaction, analogous to how chlorophyll in plant cells absorbs energy from the sun and uses it to build sugar molecules.

Chemists have developed two main classes of photoredox catalysts, known as homogeneous and heterogeneous catalysts. Homogeneous catalysts usually consist of organic dyes or light-absorbing metal complexes. These catalysts are easy to tune to perform a specific reaction, but the downside is that they dissolve in the solution where the reaction is taking place. This means that they cannot be easily removed and reused.

Heterogeneous catalysts, on the other hand, are solid minerals or crystalline materials that form sheets or 3D structures. These materials do not dissolve, so they can be used more than once. However, these catalysts are more difficult to tune to achieve a desired reaction.

To combine the advantages of these two types of catalysts, the researchers decided to incorporate the dyes that make up the homogeneous catalysts into a solid polymer. For this application, the researchers adapted a plastic-like polymer with tiny pores that they had previously developed to perform gas separations. In this study, the researchers demonstrated that they could incorporate about a dozen different homogeneous catalysts into their new hybrid material, but they think it could work more.

“These hybrid catalysts have the recyclability and durability of heterogeneous catalysts, but also the precise tunability of homogeneous catalysts,” says Liu. “You can incorporate the dye without losing its chemical activity, so you can more or less choose from the tens of thousands of already known photoredox reactions and get an insoluble equivalent of the catalyst you need.”

The researchers found that incorporating the catalysts into the polymers also helped them become more efficient. One of the reasons is that the reactant molecules can be retained in the pores of the polymer, ready to react. Additionally, light energy can easily travel along the polymer to find waiting reactants.

“The new polymers bind molecules in solution and effectively preconcentrate them for the reaction,” says Swager. “Furthermore, excited states can migrate rapidly throughout the polymer. The combined mobility of the excited state and the separation of reactants in the polymer allows for faster and more efficient reactions than is possible in pure solution processes.

Superior efficiency

The researchers also showed that they could adjust the physical properties of the polymer backbone, including its thickness and porosity, depending on the application for which they wanted to use the catalyst.

As an example, they showed they could make fluoropolymers that would stick to fluorotubes, which are often used for continuous flow manufacturing. During this type of manufacturing, chemical reagents flow through a series of tubes while new ingredients are added or other steps such as purification or separation are performed.

Currently, it is difficult to integrate photoredox reactions into continuous flow processes because the catalysts are rapidly consumed and therefore must be continuously added to the solution. The incorporation of the new catalysts designed by MIT in the tubes used for this type of fabrication could make it possible to carry out photoredox reactions in continuous flow. The tube is transparent, allowing light from an LED to reach the catalysts and activate them.

“The idea is to have the catalyst encasing a tube, so you can run your reaction through the tube while the catalyst stays put. That way the catalyst never ends up in the product and you can also achieve much higher efficiency,” says Liu.

The catalysts could also be used to coat magnetic beads, making them easier to remove from solution after the reaction is complete, or to coat reaction flasks or textiles. The researchers are now working on incorporating a wider variety of catalysts into their polymers and on engineering the polymers to optimize them for different possible applications.

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The research was funded by the National Science Foundation and the KAUST Sensor Initiative.


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