A distinctly combinatorial approach

As energy costs go up, will we be able to leverage the power of the sun to produce chemicals? Meet Dr Timothy Noël, who last year scooped the DECHEMA award for his work on photochemistry in microfluidic reactors… ­

You work in microfluidics... a field that has been on the verge of revolutionising many areas of technology for a while. Can it live up to the hype?

I believe in the power of microfluidics and flow chemistry in general. However, it is true that we should be careful not to hype the field too much. Therefore, it is important for researchers to always very carefully look at their results, to do the required control experiments and to not oversell these results. Especially at the outset of this field, effects were claimed which were not entirely correct. This actually does a lot of damage and gives ammunition to sceptics to ridicule the entire field. So we should be very critical and claim only the facts.

You have combined photochemistry and microfluidics - how important was a multi-disciplinary approach?

I think it is crucial to have a fundamental understanding of both the chemistry and the engineering principles behind microfluidics. A lot of the microfluidics research is very inter- and multi-disciplinary. If you understand the different disciplines, you can get so much more out of this technology. More specifically, for photochemistry, we see that the chemistry benefits a lot from microscale effects. This has to do with the Lambert-Beer law which basically says that your light intensity exponentially decays. As a consequence, you cannot easily scale photochemical transformations by using a large reactor vessel. Your reaction time will go up exponentially with the consequence that you have to over-irradiate your reaction mixture leading often to the formation of by-products. However, by using micro flow, you can accelerate and scale your photochemistry quite easily. Furthermore, you can tune the residence/reaction time carefully and stop the reaction right at the sweet spot, giving rise to excellent selectivity.

What was your inspiration for the development?

I started to get interested in photochemistry in flow when I was doing my postdoc at MIT. I got familiar with flow chemistry and worked on cross coupling chemistry. During my spare time, I read a lot of literature and got inspired – as many others working in organic synthetic chemistry – by the photoredox catalysis work of MacMillan, Yoon and Stephenson. I immediately realised that this chemistry could not be scaled easily in conventional batch vessels. So my first proposals in my independent career were all on flow photochemistry. This culminated into our most recent work on the use of solar light for photoredox catalysis. When we studied the literature, we saw a lot of great examples but we felt that these solutions were very technologically complex and were not going to be used by many chemists. In addition, photoredox catalysis is chromoselective, so a lot of the solar light is actually not absorbed and thus lost. Nature has found some pretty nice solutions to cope with that. They use antennae complexes which allow them to harvest different colors and to deliver that energy to the actual photosynthesis complexes. So we wanted to find a material which could do something similar. These materials are called luminescent solar concentrators. They are pieces of plastic which are doped with dyes. The dye absorbs solar light and re-emits at a lower energy wavelength. If we match the color of the material with the absorption maximum of the photocatalyst, you are actually converting useless colors into useful energy for the photocatalytc transformation. Furthermore, due to the high refractive index of the material, light is actually waveguided to the reaction channels. In short, this material is our light converter and harvester.

Potentially this could have incredibly important applications... do you think it will become mainstream for process industries, chemical engineering and biotech?

I really believe in the potential of our reactors to scale photochemistry. Owing to the popularity of photoredox catalysis for the synthesis of pharmaceuticals, I expect that photochemistry finally will be used in the large scale synthesis of these valuable compounds. So our technology will be very useful here. Now the question is, will solar energy be used in the near future for the production of chemicals? That remains to be seen actually. At the moment, the energy derived from fossil fuels is still too cheap to justify the direct use of solar energy. However, I do expect that the story for solar photochemistry could be similar to the story of photovoltaics. Despite the fact that they were developed a few decades ago, photovoltaics are only now becoming mainstream. Similarly, if energy becomes even more expensive, solar photochemistry will be more of interest. Basically, the energy is free and the reactor is extremely cheap to make. And, instead of storing solar energy temporally in batteries, using it directly for driving reactions forward is by far the most energy efficient manner.

Congratulations on the award! What is it that awards like can do for an early career scientist?

I believe it is really a recognition for the entire team and our collaborators. It shows that our research is of interest to a broader audience and that we are on the right track. It gives us a lot of energy to keep going! Because, research is not a 9-to-5 job, so a lot of effort has gotten into this research by a lot of people. Our research is really a team effort and I am grateful to be able to work with so many talented young people.

Dr Timothy Noël obtained his a PhD at the Laboratory for Organic and Bioorganic Synthesis at Ghent University, then moved to MIT as a Fulbright Postdoctoral Fellow. He currently holds a position as an associate professor in the Micro Flow Chemistry & Process Technology group at Eindhoven University of Technology. His research on photochemistry in microfluidic reactors was awarded the DECHEMA award 2017