Leaf-Inspired Photomicroreactor Could Power A Step Change In The Chemical Industry
Article Jan 16, 2017
Dr. Tim Noël’s group at the Technische Universiteit, Eindhoven, have developed a novel photomicroreactor that can harness solar energy to catalyse chemical reactions.
The technology is fully scalable and cheap to build and implement. Like the leaves that inspired its production, the sustainable energy source and pleasing aesthetic of this technology could revolutionise the Chemical Industry, replacing boring grey pipes and resource-heavy production that are synonymous with today’s chemical plants
In this blog, we speak to Dr Tim Noël as he describes the journey taken in developing this exciting technology.
AT: Your recent paper in the ASAP section of Angewandte Chemie describes a leaf-inspired photomicroreactor. Could you tell us more about how you developed this technology?
TN: We are very excited about this work and we would like to take you behind the scenes on how we conceived this idea, got the first results and finally made the required breakthrough that led to this publication.
AT: That sounds great, when and why did you start looking into photoredox chemistry?
TN: Since the start of my independent academic career in 2012, my group has been intrigued by visible light photoredox catalysis. Photoredox catalysis provides neat solutions for previously elusive organic transformations (broad scope, high functional group tolerability, mild reaction conditions). However, one of the biggest hurdles of this chemistry was its scalability and we have worked on continuous-flow microreactor solutions to overcome these challenges.
AT: Given the challenges faced, what made you want to pursue this line of investigation?
TN: One of the major selling points of photoredox catalysis is that – at least theoretically – sunlight can be used to drive these reactions forward. However, when scrolling through the photoredox literature (given its popularity, we can assure you this is a tremendous effort), sunlight was almost never used. And, when used, the reaction times became unrealistically long. We realized that continuous-flow microreactors could be beneficial here to ensure that the entire reaction mixture was irradiated equally. However, such flow reactors can only be useful when solar energy is abundant and focused towards the reactor (there is a recent review on this topic if you are interested in it). We are living in The Netherlands, not really the most sun-rich region on this planet, so forget it – this approach will only work on sunny, cloudless days but they are far too scarce! There is even more: typical photoredox catalysts can only absorb light in a narrow wavelength-window (typically around their absorption maximum), which means that only a small amount of the solar energy is harvested to enable the chemical conversion. So photoredox catalysis on its own can never be energy efficient.
AT: How did you overcome these issues of photoredox inefficiency?
TN: To address these issues, we looked at Nature’s tree leaf. It uses various antenna pigment molecules which allow harvesting of solar energy. This harvested energy is subsequently transferred to the reaction center where CO2 and H2O are converted into sugars. So the question is, can we come up with a solution to mimic the behavior in the tree leaf?
A potential solution came when I was discussing my idea with Michael Debije. He is one of the leading experts in so-called luminescent solar concentrators (LSC). When he showed me the material, we both had immediately the Eureka-feeling. Luminescent solar concentrators have been used in the past to improve the efficiency of photovoltaics. These polymeric plates contain fluorescent dyes which absorb the light and, due to internal reflection, the light is guided towards the edges. What is more, by changing the dye, you can also change the color of the polymeric material.
So I believe you can already see where we are heading? The idea is to carve channels into this luminescent solar concentrator AND to match the emission of the embedded dye with the absorption maximum of the photocatalytic system flowing in the microchannels (Fig. 1). As such, at least in theory, we would be able to mimic the principle of the tree leaf.
In 2014, I interviewed Dario Cambié, and I was immediately convinced that he was the right person for the challenge. He had a pharmacist background but he was very broadly educated and was genuinely interested. So Dario came to Eindhoven and started to work on the LSC-based photomicroreactors project. While the project is fairly complicated (chemistry, material science, and engineering), we felt that we had assembled the right competencies to tackle this problem.
Fig. 3. (A) One of the first prototypes of the LSC-photomicroreactor. Note the shining edges demonstrating the waveguiding potential of the device. (B) Using cell phone light results in a strong wavelength conversion as is evident by the red halo.
At that point, Dr. Fang Zhao (a highly skilled chemical engineer) joined the team and we could increase our efforts to prove the efficacy of our device. In a series of fundamental experiments, we proved the light converting and the wave guiding ability of the device. Most of these experiments were carried out in the laboratory with LEDs or solar simulators.
TN: Critical scientists might say that it is easier to use a photovoltaic and then use the current to power an LED strip of the right color. True, this is an option but it requires a couple of energy transformations which means that you will lose a lot (Second Law of Thermodynamics). For the photovoltaic/LED option, we calculated a 2.5 % overall efficiency (from solar energy to chemical conversion). However, with our device, you can skip a few steps and we come to an overall efficiency of 10%.
TN: We can make this device essentially in any shape you want at almost no cost (material cost of our devices are below 1 euro). Our leaf design works as well as the normal square design. Moreover, the color makes it aesthetically appealing and indeed such materials have been used in architecture, e.g. the Musac museum in Léon (Spain), the new biochemistry building at the University of Oxford (UK), and the Palais des congrès de Montréal (Canada). It would be cool to see the boring-greyish piping of chemical plants of today being replaced with our colorful, solar driven photochemical photoreactors. Both the nice color and the use of a sustainable energy source would help to alter the “negative” public image of the chemical industry.
TN: We hope that this design will find its way into solar-driven photochemical transformations. Currently, we are working on exploring the potential of this LSC photomicroreactor in wide series of different photocatalytic transformations powered by solar light. These results will be published in due course! So keep an eye on our website.