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

微波（MW）加热作为一种重要的使能技术继续增长，主要是由于MW被证明有能力加快化学反应的速率。我们大多数人将MW与使用家用MW烤箱烹饪联系起来，在家用MW烤箱中，由于MW与（水）分子相互作用，食物被快速加热。然而，令人惊讶的是，MW如何加热液体和固体的精确分子解释仍然知之甚少。例如，已知MW加热在许多情况下优于常规加热（其依赖于相对缓慢且低效的传导和对流热传递原理），但不清楚有益的加热效果是否是MW辐射特有的。由于越来越多地使用微波反应器来提高化学反应的速率（即，MW-特异性反应速率增强效应）。这在催化中是特别相关的，其中在某些反应中至少一个数量级的速率提高可以使用MW加热来实现。微波加热用于催化的另一个关键优势是样品的几乎瞬时和快速加热（以及随后的快速冷却）。这种快速加热的一个直接好处是，化学反应的结果（形成的产物）可以通过竞争性反应的动力学和热力学选择性来改变;快速加热可以导致形成显著比例的化学上不利的产物。因此，虽然微波加热是非常重要的反应速率的提高，特别是在催化，我们的理解的微波特定的增强或加热效果是知之甚少。同时，快速加热化学体系的能力也可以用于研究反应机理。大多数化学反应涉及平衡过程，其中正向和反向反应的速率控制在任何给定时间点的反应物和产物的总浓度。当施加应力时，化学或构象平衡可以容易地被扰动并在任一方向上移动。这种应力可能涉及浓度、压力或温度的变化。从旧平衡到新平衡的变化速率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是非常有用的。重要的是，扰动比弛豫时间更快地施加，并且通常在比所涉及的混合时间更快的时间尺度上施加。TJ是一种用于研究化学动力学和反应机理的弛豫方法。因此，使用合适的谐振器通过微波快速加热（产生TJ）可以用作研究反应动力学和动力学的新手段。因此，在本项目中，我们将开发一种独特的基于双模式电子顺磁共振（EPR）的反应器-谐振器。EPR是一种利用微波检测顺磁物质的光谱技术。两个单独的MW频率将被引入到共振模式的反应器-共振器中，使得一个频率将用于通过EPR检测顺磁物质，而第二个频率将用于加热样品。我们将建立专门的设备，以证明其实用性，调查的基本性质，如何微波加热可以影响速率和产品分布在一系列均相和非均相催化反应（涉及顺磁性物质），以潜在地跟踪反应途径如何被温度的快速升高所改变（T-跳跃加热），从根本上了解特定MW效应如何导致磁场中光生自由基寿命的增强，并间接了解液体和固体的MW加热如何发生。

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.