Development of a Dual-Mode Microwave-EPR Reactor-Resonator for Studies of Paramagnetic Catalytic Reactions

Microwave (MW) heating continues to grow as an important enabling technology, primarily owing to the proven capability of MWs to speed up the rate of chemical reactions. Most of us associate MWs with cooking using the domestic MW-oven, in which the food is quickly heated as the MWs interact with the (water) molecules. However, it is surprising that the precise molecular explanation of how MWs heat liquids and solids remains poorly understood. It is known for example that MW heating is in many cases better than conventional heating (which relies on comparatively slow and inefficient conductive and convective heat transfer principles) but it is not clear if the beneficial heating effects are specific to the MW radiation. Understating this is vitally important owing to the growing use of microwave reactors for enhancing the rates of chemical reactions (i.e., the MW-specific reaction rate enhancement effect). This is particularly relevant in catalysis, where a rate enhancement in some reactions of at least one order of magnitude can be achieved using MW-heating. Another key advantage of MW-heating for catalysis is the almost instantaneous and rapid heating (and subsequent rapid cooling) of the sample. One immediate benefit of this rapid heating, is that the outcome of the chemical reaction (the products formed) can be altered through the kinetic and thermodynamic selectivity of competitive reactions; rapid heating can result in the formation of significant proportions of thermodynamically unfavoured products. Therefore, whilst MW-heating is very important in reaction rate enhancement, particularly in catalysis, our understanding of the MW-specific enhancement or heating effects are poorly understood.

At the same time, the ability to rapidly heat a chemical system can also be exploited for the study of reaction mechanisms. Most chemical reactions involve an equilibrium process, with the rate of the forward and reverse reactions controlling the overall concentration of reactants and products at any given point in time. The chemical or conformational equilibrium can be easily perturbed and shifted in either direction, when a stress is applied. This stress may involve a change in concentration, pressure or temperature. The rate of change from the old to the new equilibrium will depend on the rate constant for the forward and reverse reactions or the conformational change, so that analysis of this rate is extremely informative in chemical kinetics and dynamics. It is important that the perturbation is applied more rapidly than the relaxation time, and usually on a time scale that is faster than the mixing times involved. TJ is one such type of relaxation method used to study chemical kinetics and reaction mechanisms. Rapid heating by microwaves (creating a TJ) using a suitable resonator, could therefore be used as a novel means of studying reaction kinetics and dynamics.

Therefore, in this project we will develop a unique dual-mode Electron Paramagnetic Resonance (EPR) based reactor-resonator. EPR is a spectroscopic technique that employs microwaves to detect paramagnetic species. Two separate MWs frequencies will be introduced into the reactor-resonator in resonant mode, such that one frequency will be used to detect the paramagnetic species by EPR, while the second frequency will be used to heat the sample. We will build the device specifically to demonstrate its utility for investigating the fundamental nature of how MW heating can influence the rate and product distribution in a series of homogeneous and heterogeneous catalytic reactions (involving paramagnetic species), to potentially follow how the reaction pathways are altered by a rapid rise in temperature (T-jump heating), to fundamentally understand how MW-specific effects lead to enhancement of photogenerated radical lifetimes in magnetic fields, and to indirectly understand how MWs heating of liquids and solids occurs.

Our project mission is to deliver exceptional, agenda-setting fundamental research through the development of a new dual-mode microwave-EPR reactor-resonator, leading to translational impact in other areas, including knowledge, people, economy and society. The impact of our fundamental research through the knowledge gained in new scientific advances and techniques is already firmly embedded in our research. It is also clear that MW devices are used in many branches of our society from communications to heating to basic research. Although our project will focus on and exploit the tremendous opportunities provided by MW radiation to rapidly heat samples of importance to chemistry and catalysis, the wider implications of our research reach far beyond chemistry as we develop a better understanding of how MWs interact with matter. Therefore, whilst our research lies within the Physical Sciences domain, it must also be recognized that the fundamental knowledge of how MWs interact with liquids and solids remains poorly understood, despite the growing use of MW-radiation in synthetic chemical transformations.

Furthermore, since microwave and RF devices have been identified as a growth area within the EPSRC Balancing our Capabilities programme, and are of enormous potential for the UK as applications arise from improved, miniaturised and efficient microwave technology, more fundamental and interdisciplinary research in technology and applications is required. Despite the fact that MW heating continues to grow as an enabling technology in academia and industry, specific research into the nature of MW heating is incredibly sparse. In homogeneous chemical systems for example, there is considerable controversy over the origins of the rate enhancement, and even the existence of MW-specific effects as opposed to simply 'fast heating' is still debated. Our research will indirectly offer insights into many of these controversies and thereby have considerable impact on both academic and industrial sectors.

The MW induced heating capability of our proposed device is also unique. It therefore offers the potential to study a wide range of chemical systems that involve free radicals, paramagnetic centres or spin labels/probes. Furthermore, the resonator can be commercially exploited as a cheap and easy to use stand-alone controlled and rapid heating accessory in other spectroscopy techniques.

The project will also offer many benefits to the academic community (including other EPSRC funded projects such as CHAMPS and the UK Catalysis Hub), whereby a wide range of paramagnetic systems can be studied, and also the wider catalysis community, since free radical or paramagnetic species are often involved but difficult to study. The ability to investigate paramagnetic systems of importance to chemistry using advanced spectroscopic techniques, would represent a significant step-change in our understanding of these processes making a positive impact in the dissemination profile of this work. The impact also reaches beyond the UK scientific community, and of major significance to the international research community within both academic catalysis fields. The technical breadth and collaborative nature of the project ensures that the employed RA's, PhD student or MChem students (directly or indirectly involved) will be provided with unique opportunities to broaden their skill set and technical knowledge, and thus enhance their future employability. This will ensure our project contributes to the 'people pipeline' of highly trained and skilled scientists and graduates in the field of MW engineering and chemistry.

The primary means of freely making our work accessible to world-wide academia will be through publication in the highest profile international journals. We will also participate in the major international conferences within our subject specific disciplines but also in the wider arenas to more broadly disseminate our work