Gas-liquid plasma micro-reactors for future chemistry – PLAS4CHEM

- In the first part of the project, we will investigate the potential of the developed plasma microreactors for the oxidation of cyclohexane into K-Oil (proof of concept) and the dehydrogenation of cyclohexane into cyclohexene where several preliminary results have already been obtained. The objective here is to optimize the process in terms of yield and energy consumption.

- In the second part of the project, various organic substrates (cyclohexane, linear alkanes substituted or not, aromatics substituted or not) and various gas mixtures (Ar/O2, Ar/N2/H2 and Ar/CO) will be tested with the objectives to perform oxidation, amination and carbonylation reactions. Amination reactions using N2/H2 mixtures and carbonylation reactions using Ar/CO mixtures are the most challenging part of our proposal, but if successful, this will open several opportunities in chemical industry.

In the different tasks, the performances of the process will be evaluated using NMR to analyse the structure of the different species formed in the liquid and GC/MS or LC/MS to evaluate the selectivity of the process and its conversion depending on the plasma process parameters.

To limit the risks in our project, the production and the reactivity of plasma active species will be thoroughly studied using experimental and theoretical tools. Regarding experimental tools, optical emission spectroscopy measurements will allow to identify the gaseous excited species generated by the plasma whereas with Electron Spin Resonance measurements, the quality and the quantity of radical species in the liquid phase will be determined. Finally the modeling tools will be also used to predict theoretically the quantity and quality of gaseous reactive species generated by the plasma discharge and their fate in the plasma gas-liquid microreactor.

The fabrication and the study of hydrodynamics in the plasma-liquid microreactor have been realized in the first place. Robust glass microreactors have been fabricated. The transport phenomena in the microreactor has been studied using numerical modeling (Comsol Multiphysics). Preliminary studies have been carried out for amination reactions. These results have been published in 2020.

To better understand the physical and chemical processes in the plasma microreactor, especially the interactions between the plasma and potential organic solvents, as the solvent molecule may also evaporate into the gaseous phase and interact with the plasma. First of all, various solvents have been introduced into the plasma microreactor as liquid substrates. It has been observed that the breakdown voltage of plasma discharges varies with the dielectric constant of the solvent: the higher the dielectric constant is, the higher will be the breakdown voltage for the plasma. The oxidation of various heterocycles (for example: piperidine) has been studied as model reaction. Optical, electrical and chemical analysis have been realized to evaluate the reaction efficiency. The presence of organic solvent in the liquid phase introduces new reaction possibilities for the reactor: the solvent molecule in the gas phase may interact with the plasma and functionalize the liquid molecule directly, which means that the chemical structure of the solvent can serve as source of functionalization in this kind of plasma/liquid reactor. The results of this internship give us important insights into the roles of solvent and possible synthetic routes in the plasma-liquid microreactor.

The project is targeting to develop of a totally new process based on the coupling of a microreactor system with non-thermal plasma technology in order to explore new organic synthesis pathways addressing energetic, safety and environmental challenges. The project results will generate a pioneering scientific knowledge concerning the design and the chemical performance of a new type of microreactors for flow chemistry, were the active species are produced by plasma micro-discharges in rapid time scales (0.1-10 ns), and then brought into contact with target molecules in an extreme precision and targeted manner.
This technology will give rise to an unprecedented control of the radical chemistry, particularly in terms of selectivity, that can be used in several areas of chemical industry and more specifically in fine chemistry to provide:
* Clean processes adapted to green chemistry by proposing new chemical synthesis routes that involve safer reagents, less catalysts and consume less solvents.
* Radical chemistry controlled at ambient temperature and atmospheric pressure ensuring a relevant operational safety.
* Low energy consumption: energy efficiency of micro-discharges at atmospheric pressure is optimal when microgeometry is used (Paschen Low).
By performing the functionalization of stable organic molecules (hydroxylation, oxidation, amination, carbonylation or deshydrogenation) in plasma microreactors, the developed technology can rapidly integrate the sector of fine chemical industry that has already begun to use microfluidic technology and Flow Chemistry concepts in its own technology palette (like LONZA, DSM, BASF, SANOFI, SOLVAY, ...) as a part of process intensification strategy particularly to handle exothermic reactions or corrosive or dangerous substances in better safety conditions.

M. Zhang, S. Ognier, N. Touati, I. Hauner, C. Guyon, L. Binet, M. Tatoulian, Plasma Process Polym. doi:10.1002/ppap.201700188.

M. Zhang, S. Ognier, N. Touati, L. Binet, C. Thomas, P. Tabeling, M. Tatoulian, Green Processing and Synthesis, september 2016. doi:10.1515/gps-2016-0086abc

- Amination of Cyclohexane by Dielectric Barrier Discharge Processing in a continuous flow Microreactor: Experimental and Simulation Studies« accepted in Plasma Chemistry and Plasma Processing. (Novembre 2020) Aurelien Lepoetre, Stephanie Ognier*, Mengxue Zhang, Julien Wengler, Safwan Al Ayoubi, Cyril Ollivier, Louis Fensterbank, Xavier Duten, and Michael Tatoulian

The "Plas4chem" project is based on a highly multidisciplinary scientific approach and brings together skills in the fields of microfluidics, plasma processes, chemistry and physics. Our project is based on the development and use of microstructured chemical reactors in order to be able to carry out chemical synthesis reactions without catalyst and without solvent thanks to a precise controlled manipulation of high energy radical species. Microstructured reactors have been attracting interest for several years because of the unprecedented level of control they can bring when conducting chemical reactions and the promise of a rapid scale-up obtained by simple Parallelization. Heat and material transfers as well as mixing processes between reactive fluids are particularly accelerated due to the very small size of the channels. By better controlling the flow and transfer conditions, parasitic side reactions can be suppressed, thus obtaining products with greater selectivity. Thanks to these new type of reactors, it is also possible to explore the potential of alternative sources of activation which have hitherto been difficult to implement in conventional batch reactors. Our major innovation, which was patented in 2015, is based on the association of plasma science, which allows the generation of radical species at pressure and ambient temperature by electron impact, and microfluidics that makes possible the control and transfer of these highly reactive chemical medium with a great precision. The radical species generated by the plasma will be able either to react directly with reactants in the gas phase, the liquid phase serving as both reservoir and extraction phase, or be transferred by diffusion to the liquid phase in order to initiate chemical reactions in the liquid phase.
In our project, we propose to explore the reactivity of model molecules (cyclohexane and benzene) when they are brought into contact with plasma discharges generated in gaseous media of variable composition (O2, H2, N2 etc ...pure or mixed with rare gases if needed) in order to control the selectivity of the process towards oxidation, amination, dehydrogenation or carbonylation reactions.
This type of reactor thus opens up promising prospects for chemists by simplifying the steps leading to the desired molecules and the development of activation techniques involving the plasmas could make it possible to envisage new, selective and cleaner reaction pathways without using solvent and catalyst.