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

微波加热作为一项重要的使能技术继续增长，这主要是因为微波加热已被证明有能力加快化学反应的速度。我们大多数人都会将MW与使用家用微波烤箱进行烹饪联系在一起，在这种烤箱中，当MW与(水)分子相互作用时，食物会迅速加热。然而，令人惊讶的是，关于MWs如何加热液体和固体的确切分子解释仍然知之甚少。例如，众所周知，微波加热在许多情况下比传统加热(依靠相对缓慢和低效的传导和对流换热原理)更好，但尚不清楚有益的加热效果是否仅限于微波辐射。由于越来越多地使用微波反应器来提高化学反应速率(即特定于微波的反应速率增强效应)，因此轻描淡写这一点至关重要。这一点在催化中尤其相关，在某些反应中，使用微波加热可以实现至少一个数量级的速率提高。微波加热催化的另一个关键优点是样品几乎瞬间快速加热(随后快速冷却)。这种快速加热的一个直接好处是，化学反应(形成的产物)的结果可以通过竞争反应的动力学和热力学选择性而改变；快速加热可以导致大量热力学上不受欢迎的产物的形成。因此，虽然微波加热在提高反应速率方面非常重要，特别是在催化方面，但我们对微波特定增强或加热效应的理解很少。同时，也可以利用快速加热化学体系的能力来研究反应机理。大多数化学反应涉及一个平衡过程，正向反应和反向反应的速度控制着任何给定时间点的反应物和产物的总体浓度。当施加应力时，化学平衡或构象平衡很容易被扰乱并向任一个方向移动。这种压力可能涉及浓度、压力或温度的变化。从旧的平衡到新的平衡的变化速度将取决于正向和反向反应或构象变化的速率常数，因此对该速率的分析在化学动力学和动力学中具有极其重要的意义。重要的是，微扰的应用比松弛时间更快，并且通常在比所涉及的混合时间更快的时间尺度上。TJ是一种用于研究化学动力学和反应机理的松弛方法。因此，利用合适的谐振器进行微波快速加热(产生TJ)可以作为研究反应动力学和动力学的一种新手段。因此，在本项目中，我们将开发一种独特的基于双模电子顺磁共振(EPR)的反应堆谐振器。EPR是一种利用微波来检测顺磁性物种的光谱技术。两个不同的微波频率将以共振模式引入反应堆-谐振器中，其中一个频率将用于EPR检测顺磁物种，而第二个频率将用于加热样品。我们将专门建造该装置，以展示其在研究微波加热如何影响一系列均相和非均相催化反应(涉及顺磁物种)中的速率和产物分布的基本性质，潜在地跟踪反应路径如何因温度的快速上升而改变(T跳跃加热)，从根本上了解微波特有的效应如何导致磁场中光生自由基寿命的延长，以及间接了解微波加热液体和固体是如何发生的。

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.