MIT chemists have designed a new type of photoredox catalyst that could facilitate the incorporation of light-driven reactions into continuous-flow manufacturing processes. Polymer catalysts could be used to coat tubes and perform chemical transformations into reagents as they flow through the tube, as imagined in this digital artwork. Credit: Richard Liu
When coated on plastic tubes, the catalysts could act on the chemicals flowing in them, helping to synthesize drugs and other compounds.
A new type of photoredox catalyst, designed by MIT chemists, can facilitate the incorporation of light-driven reactions into continuous-flow manufacturing processes. The key is their insolubility, which allows them to be used over and over again.
Light-driven chemical reactions provide a powerful tool for chemists to develop new methods for producing pharmaceuticals and other important molecules. Harnessing this light energy requires photoredox catalysts, which can absorb light and transfer energy to a chemical reaction.
Now, MIT chemists have designed a new type of photoredox catalyst that could facilitate the incorporation of light-driven reactions into manufacturing processes. Unlike most existing photoredox catalysts, the new class of materials is insoluble, so they can be used over and over again. These catalysts could be used to coat tubes and perform chemical transformations into reagents as they flow through the tube.
A catalyst is a substance that accelerates a chemical reaction. Catalysis is the process of accelerating a reaction by using a catalyst. Photoredox catalysts work by absorbing photons and then using this light energy to drive a chemical reaction.
“Being able to recycle the catalyst is one of the biggest challenges to overcome in terms of the possibility of using 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 make photoredox catalysis on a larger scale, “says Richard Liu, an MIT postdoctoral fellow and lead author of the new study.
The new catalysts, which can be adjusted to carry out many different types of reactions, could also be incorporated into other materials, such as textiles or particles.
Timothy Swager, professor of chemistry John D. MacArthur at MIT, is the lead author of the article, which was published on May 27, 2022 in the journal Nature Communications. Sheng Guo, an MIT research scientist, and Shao-Xiong Lennon Luo, an MIT graduate student, are also the authors of the paper.
Hybrid materials
Photoredox catalysts work by absorbing photons and then using this light energy to drive a chemical reaction, similar to how chlorophyll from plant cells absorbs energy from the sun and uses it to build sugar molecules.
Chemists have developed two main classes of photoredox catalysts, which are known as homogeneous and heterogeneous catalysts. Homogeneous catalysts are usually made up of light-absorbing organic dyes or metal complexes. These catalysts are easy to adjust to carry out a specific 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 3D sheets or structures. These materials do not dissolve, so they can be used more than once. However, these catalysts are more difficult to adjust to achieve the desired reaction.
To combine the advantages of these two types of catalysts, the researchers decided to embed the dyes that form the homogeneous catalysts in 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, researchers showed that they could incorporate a dozen different homogeneous catalysts into their new hybrid material, but they believe it could work longer and many more.
“These hybrid catalysts have the recyclability and durability of heterogeneous catalysts, but also the precise tuning of homogeneous catalysts,” says Liu. “You can incorporate the dye without losing its chemical activity, so you can choose more or less from the tens of thousands of 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 to be more efficient. One reason is that the reagent molecules can remain in the pores of the polymer, ready to react. In addition, light energy can easily travel along the polymer to find the waiting reagents.
“The new polymers bind the molecules in the solution together and effectively preconcentrate them for the reaction,” says Swager. “In addition, excited states can migrate rapidly throughout the polymer. The combined mobility of the excited state and the partitioning of the reactants into the polymer allows for faster and more efficient reactions than are possible in pure solution processes.”
Greater efficiency
The researchers also showed that they could adjust the physical properties of the backbone of the polymer, including its thickness and porosity, depending on the application for which they want to use the catalyst.
As an example, they demonstrated that they could make fluorinated polymers that would adhere to fluorinated tubes, which are often used for continuous flow fabrication. During this type of manufacturing, chemical reagents flow through a series of tubes as new ingredients are added, or other steps such as purification or separation are performed.
It is currently a challenge to incorporate photoredox reactions into continuous flow processes so that the catalysts are depleted rapidly, so that they must be continuously added to the solution. The incorporation of new catalysts designed by MIT into the tubes used for this type of fabrication could allow photoredox reactions to take place during continuous flow. The tube is transparent, allowing the light from an LED to reach the catalysts and activate them.
“The idea is for the catalyst to cover a tube, so that you can flow your reaction through the tube while the catalyst stays in place. That way, you’ll never get the catalyst to end up in the product and you can also get much higher efficiency, ”says Liu.
The catalysts could also be used to coat magnetic beads, facilitating the extraction of a solution once the reaction is complete, or to coat reaction vials or textiles. Researchers are now working on the incorporation of a wider variety of catalysts into their polymers and on the engineering of polymers to optimize them for different possible applications.
Reference: “Solution-processable microporous polymer platform for heterogenization of various photoredox catalysts” by Richard Y. Liu, Sheng Guo, Shao-Xiong Lennon Luo and Timothy M. Swager, May 27, 2022, Nature Communications.DOI: 10.1038 / s41467 -0222-0 -29811-6
The research was funded by the National Science Foundation and the KAUST Sensor Initiative.