New light-powered catalysts could help with production

Light-controlled chemical reactions offer a powerful tool for chemists who are proposing new ways to make drugs and other useful compounds. The use of this light energy requires photoredox catalysts that can absorb light and transfer energy to a chemical reaction.

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

“The ability to recycle the catalyst is one of the biggest challenges to be overcome with regard to the possibility of using photoredoxic catalysis in production. We hope that by being able to perform flow chemistry with an immobilized catalyst, we can provide a new way to perform photoredoxic catalysis on a larger scale, ”said Richard Liu, MIT’s postdoctoral fellow and co-author of the new study.

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

Timothy Swager, a professor of chemistry at John D. MacArthur at MIT, is the lead author of an article that appears in Nature communication. The authors of the article are also Sheng Guo, a MIT researcher, and Shao-Xiong Lennon Luo, a MIT graduate student.

Hybrid materials

Photoredoxic catalysts work by absorbing photons and then using this light energy to drive a chemical reaction, just as chlorophyll in plant cells absorbs energy from the sun and uses it to build up 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 can be easily tuned to carry out a particular reaction, but the disadvantage is that they dissolve in the solution where the reaction takes 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 multiple times. However, these catalysts are more difficult to tune to achieve the desired reaction.

To combine the advantages of both types of catalysts, the researchers decided to incorporate the dyes that form homogeneous catalysts into a solid polymer. For this application, the researchers modified a polymer similar to the small-pore plastic they had previously developed to perform gas separations. In this study, researchers have shown that they can incorporate about a dozen different homogeneous catalysts into their new hybrid material, but they believe it could work much 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 photoredox reactions that are already known and get the insoluble equivalent of the catalyst you need.”

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

“The new polymers bind the molecules from solution and efficiently preconcentrate them for the reaction,” says Swager. “Excited states can also migrate rapidly through the polymer.” The combined mobility of the excited state and the distribution of reactants in the polymer allow faster and more efficient reactions than are possible in pure solution processes.

Higher efficiency

Researchers have also shown that they can tune the physical properties of the polymer backbone, including its thickness and porosity, based on the application for which they want to use the catalyst.

As one example, they have shown that they can produce fluorinated polymers that adhere to fluorinated tubing, which is often used for continuous production. During this type of production, the chemical reactants flow through a series of tubes while new additives are added or further steps such as purification or separation are performed.

At present, it is difficult to incorporate photoredoxic reactions into continuous flow processes because the catalysts are consumed quickly, so they must be added to the solution continuously. The incorporation of new catalysts designed by MIT into the piping used for this type of production could allow photoredoxic reactions to be performed during a continuous flow. The piping is clear and allows LED light to penetrate the catalysts and activate them.

“The idea is to cover the tube with catalyst so you can pass your reaction through the tube while the catalyst stays in place.” This way, you never get a catalyst that ends up in the product, and you can also get much higher efficiency, ”says Liu.

The catalysts can also be used to coat magnetic beads, which facilitates their withdrawal from solution once the reaction is complete, or to coat reaction vials or fabrics. Researchers are now working to incorporate a wider range of catalysts into their polymers and to design polymers to optimize them for a variety of possible applications.

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