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

微波（MW）加热作为一项重要的使能技术继续增长，主要是由于MW加速化学反应速率的能力得到证实。我们大多数人都把微波与家用微波炉联系在一起，在微波炉中，由于微波与（水）分子相互作用，食物被迅速加热。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，众所周知，毫瓦加热在许多情况下比传统加热（它依赖于相对缓慢和低效的传导和对流传热原理）更好，但目前尚不清楚有益的加热效果是否仅限于毫瓦辐射。由于越来越多地使用微波反应器来提高化学反应速率（即，特定于毫瓦的反应速率提高效应），因此了解这一点至关重要。这在催化方面尤其重要，在催化方面，使用毫瓦加热可以使某些反应的速率提高至少一个数量级。兆瓦加热催化的另一个关键优势是样品几乎瞬间和快速加热（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而改变；快速加热可导致形成相当比例的热力学上不利的产物。因此，虽然毫瓦加热在提高反应速率方面非常重要，特别是在催化方面，但我们对毫瓦特定的增强或加热效应的理解却很少。同时，快速加热化学系统的能力也可以用于研究反应机理。大多数化学反应都涉及一个平衡过程，正反反应的速率控制着任何给定时间点的反应物和生成物的总浓度。当施加压力时，化学平衡或构象平衡很容易被打乱，并向任何一个方向移动。这种压力可能包括浓度、压力或温度的变化。从旧平衡到新平衡的变化率将取决于正反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信息。重要的是，施加扰动的速度要比弛豫时间快，而且通常在一个比所涉及的混合时间快的时间尺度上。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，使用合适的谐振器通过微波快速加热（产生TJ）可以作为研究反应动力学和动力学的新手段。因此，在这个项目中，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是一种利用微波探测顺磁物质的光谱技术。在谐振模式下，将两个不同的mw频率引入反应器-谐振器，其中一个频率将用于EPR检测顺磁性物质，而第二个频率将用于加热样品。我们将专门建造该装置，以证明其在研究毫瓦加热如何影响一系列均相和非均相催化反应（涉及顺磁物质）的速率和产物分布的基本性质方面的实用性，以潜在地跟踪反应途径如何因温度的快速上升而改变（t跳加热）。从根本上了解毫瓦特异性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毫瓦加热是如何发生的。

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.