Chemical reactions powered by light provide a powerful tool for chemists devising new ways to manufacture drugs and other useful compounds. Harnessing this light energy requires photoredox catalysts, which can absorb light and transfer the energy into a chemical reaction.
MIT chemists have now designed a new type of photoredox catalyst that could make it easier to incorporate light-driven reactions into manufacturing processes. Unlike most existing photoredox catalysts, the new class of materials are insoluble, so they can be used over and over again. Such catalysts could be used to coat tubes and perform chemical transformations on reactants as they flow through the tube.
“Being able to recycle the catalyst is one of the biggest challenges to overcome when it comes to using photoredox catalysis in manufacturing. We hope that by applying flow chemistry with an immobilized catalyst, we can create a new way to do photoredox catalysis on a larger scale,” said Richard Liu, an MIT postdoc and the joint lead author of the new study.
The new catalysts, which can be tuned to carry out many different types of reactions, can also be incorporated into other materials, including textiles or particles.
Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT, is the lead author of the paper, which appears today in nature communication† Sheng Guo, an MIT researcher, and Shao-Xiong Lennon Luo, an MIT graduate student, are also authors of the paper.
Hybrid materials
Photoredox catalysts work by absorbing photons and then using that light energy to drive 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 easily tuned to carry out a specific reaction, but the disadvantage is that they dissolve in the solution where the reaction is taking place. This means that they cannot be easily removed and reused.
In contrast, heterogeneous catalysts are solid minerals or crystalline materials that form plates 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 both types of catalysts, the researchers decided to embed the dyes that make up homogeneous catalysts into a solid polymer. For this application, the researchers adapted a plastic-like polymer with tiny pores that they had previously developed for performing gas separations. In this study, the researchers showed they could incorporate about a dozen different homogeneous catalysts into their new hybrid material, but they think it could work for a lot 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 sort of pick from the tens of thousands of photoredox reactions that are already known and get an insoluble equivalent of the catalyst you need.”
The researchers found that incorporating the catalysts into polymers also helped them become more efficient. One reason is that 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 the waiting reactants.
“The new polymers bind molecules from solution and effectively concentrate them for the reaction,” Swager says. “Also, the excited states can migrate rapidly through the polymer. The combined mobility of the excited state and the distribution of the reactants in the polymer allows for faster and more efficient reactions than are possible in pure solution processes.”
Higher efficiency
The researchers also showed that they could tailor the physical properties of the polymer backbone, including its thickness and porosity, to the application they want to use the catalyst for.
For example, they showed that they could make fluorinated polymers that would stick to fluorinated tubes, which are often used for continuous power production. During this type of production, chemical reactants flow through a series of tubes as new ingredients are added or other steps such as purification or separation are performed.
Currently, it is challenging to incorporate photoredox reactions in continuous flow processes because the catalysts are used up quickly, so they must be continuously added to the solution. By incorporating the new MIT-designed catalysts into the tubes used for this type of production, photoredox reactions can be performed during continuous flow. The hose is clear, allowing the light from an LED to reach and activate the catalytic converters.
“The idea is to have the catalyst coat a tube so you can let your reaction flow through the tube while the catalyst stays put. That way you never get the catalyst into the product and you can also get a lot higher efficiency” , says Liu.
The catalysts can also be used to coat magnetic beads, making them easier to pull from solution once the reaction is complete, or to coat reaction vessels or textiles. The researchers are now working on incorporating a wider variety of catalysts into their polymers and engineering the polymers to optimize them for different potential applications.
The research was funded by the National Science Foundation and the KAUST Sensor Initiative.
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