Rajesh Munirathinam

Wall-coated polymer brushes as catalytic support for organic reactions in continuous-flow microreactors

Promotion date: December 18.

Promotor: Prof. dr. ir Jurriaan Huskens

Assistant Promotor: Dr. Wim Verboom

A microreactor platform has been realized as an enabling tool to perform synthetic organic reactions. The platform provides advantages over batch reactions, thus allowing: reactions with a potentially explosive reagent, the execution of heterogeneous catalysis by immobilizing catalysts onto the interior of a microchannel surface, and the performance of controlled formation of supramolecular nanoparticle clusters.

In the first part, the regioselective ring opening of various types of aromatic and aliphatic epoxides with hazardous sodium azide to give vicinal azido alcohols, was studied in a microreactor with and without pillars in the channels. The reactions performed in microreactors with pillars displayed better conversions compared to analogous lab scale reactions.

In the second part, polymer brush layers with thicknesses of a few hundred nanometers were used as a support for catalyst immobilization on the interior of microchannel walls. Polymer brushes support higher catalyst loadings as compared to monolayers. Different types of catalysts were implemented inside the microreactor. Their catalytic efficiency was studied for different reactions. In the last part, self-assembly of supramolecular nanoparticle clusters (SNPCs) has been demonstrated in a microfluidic device, by controlling the diffusive mixing of the constituting supramolecular building blocks.

Microreactors are a valuable tool to study homogeneous and heterogeneous reactions in a safe and efficient way. Polymer brushes have proven to be a robust platform to immobilize various catalysts on the interior of a microchannel for performing supported catalysis. In addition, microreactors provide a unique environment for the controlled fabrication of nanomaterials compared to lab-scale equipment. So, microreactors have the potential to become a common experimental device for chemists in various disciplines.

Which advantages are present in the type of microreactors you studied in your thesis project?

The use of microreactors has been going on for more than two decades already. Recent developments show even more advantages when compared to flask-based chemical reactions. In microreactors the heat and mass transfer is far more optimal, the diffusion processes are faster, the surface to volume ratios are higher, and one can continuously run reactions in parallel reactors increasing the throughput of the product.

In my thesis project, microreactors are used for three different purposes. Firstly, explosive reagents that pose severe danger in the macro world can be handled in a safe manner utilizing the closed environment present in microreactors. Secondly, exploiting the very high surface to volume ratio omnipresent in microreactors, various catalysts are immobilized onto the microchannel walls using polymer brushes to perform organic reactions. This methodology provides high catalytic loading as compared to monolayers. Thirdly, using the well defined laminar flow and controlled diffusive mixing characteristics of miccroreactors, I have studied the size-tunable self-assembly of supramolecular nanoparticles. This can be done in a reproducible way in microreactors, leading to long-term stable results as compared to batch reactors.

My research was part of the POAC and NanoNext NL program, especially the research related to the use of catalysts in microreactors.

Do you recall some special moments occurring during your thesis project?

Supporting different kinds of catalysts onto the microchannel walls using polymer brushes, was an exciting part of the project, as it took me quite some time and effort to find the right conditions to anchor the catalysts. I was ecstatic upon observing conversions using the developed catalytic microreactors.

In what magazines were you able to publish your results?

Six articles appeared in Beilstein Journal of Organic Chemistry, Journal of Flow Chemistry, ACS Applied Materials and Interfaces and Material Horizons. One more article in Advanced Synthesis and Catalysis is in press.

In what way did you develop personally, as a research and scientist?

Though performing experiments is still fascinating to me, I am not only driven by shear fascination anymore. My work approach is much more methodical now, questioning all research steps in a critical manner throughout the process.

My working method is more analytical, and I always have a backup plan in case experiments turn out inoperable or if disappointing results are obtained. By narrowing down on the problems concerning the experiments, the efficiency of the reactions gets better.

Also my communication skills have improved. In writing I am able to present my results in a precise and appealing manner, so referees are more easily convinced of the special results I present.

What are your future plans?
Right now I am working as a post-doc at the Institute of French Petroleum: IFPEN. I like the mix of industrial and academic research of the topic I am working on: hydro-treatment methods to remove sulphur from refinery products. A strategic research position in industry appeals to me as a future job. In my opinion catalyst research will be of prime importance in the near future, leading to appealing business opportunities.

What, in your opinion, is important for Mesa+ to stay successful in future?
I like the way colleagues and experts within Mesa+ bring in their scientific ideas to a wide domain of projects. The working atmosphere is really open.

PhD students leave after four years of working within a Group. With their departure the Group does lose quite some hands-on experience. In order to get more continuity it would be better if the university provides continuous funding for at least one research within the group. In this way a solid knowledge foundation can be build up within the Group, and hands-on expertise is better preserved.