Hyperpolarized Zero-to-Ultralow-Field Nuclear Magnetic Resonance

本文提出将核磁共振（NMR）技术中的两个最新进展--零场至超低场（ZULF）NMR和超极化样品NMR--联合收割机结合起来，建立一个研究催化过程的新平台，为化工过程的优化提供现实条件下的催化研究。常规的高场NMR光谱和成像在这方面是有用的工具，因为它们同时提供关于样品的化学信息以及对具有化学特异性的质量流或传输进行成像的能力。然而，由于磁化率增宽，化学特异性常常丧失，并且有限的射频穿透深度排除了金属容器的使用。需要施加强磁场也限制了便携性，并限制了可以研究的系统的大小。相比之下，ZULF NMR不受敏感性影响，这意味着即使样品是非均相或两相的，也能保持化学特异性。此外，与传统NMR相比，信号频率显著降低，因此可以“看穿”包括金属容器在内的导电材料。我们将探索ZULF NMR作为操作反应监测的方法。为了解决ZULF NMR的低信号强度，我们采用基于仲氢的超极化。氢气可以容易地在非平衡核自旋状态（仲氢）下制备，如果探针分子通过与仲氢的化学反应形成或可逆地与金属中心上的活化仲氢分子接触，则这导致探针分子的显著增强的（超极化的）NMR信号。增强的信号允许在ZULF NMR条件下研究探针分子。我们将使用这些超极化探针开发ZULF NMR作为一种方法来监测化学反应在均相溶液中，以及研究超极化分子与活性中心的多孔催化剂和吸附剂的表面上的相互作用。最后，在这项研究中，我们将使用仲氢不仅作为超极化源，而且还作为化学反应物和机械探针。氢化反应是在工业规模上进行的，如果更好地理解催化机制，这些过程的效率可以大大提高。仲氢仅在成对添加时导致增强的信号，这可用于提供对潜在反应机制的洞察，以及非均相催化剂的优化，其中具有更好定义和结构化活性中心的单位点氢化是一个重要目标。因此，我们将通过应用本项目的先进方法，将ZULF NMR与基于仲氢的自旋超极化的信号增强相结合，研究固体负载金属催化剂上的氢化反应机理。

We propose to combine two recent developments in nuclear magnetic resonance (NMR) spectroscopy and imaging — zero- to ultralow-field (ZULF) NMR and NMR with hyperpolarized samples — to establish a new platform for studying catalytic processes.For the optimization of chemical industrial processes, it is desirable to study catalysis under realistic conditions. Conventional high-field NMR spectroscopy and imaging have been useful tools in this regard, as they simultaneously provide chemical information about a sample and the ability to image mass flow or transport with chemical specificity. However, chemical specificity is often lost due to magnetic-susceptibility broadening, and the limited radiofrequency penetration depth precludes the use of metal containers. The need to apply a strong magnetic field also limits portability and constrains the size of systems that can be investigated. In contrast, ZULF NMR is free from susceptibility effects, which means that chemical specificity is retained even when the sample is heterogeneous or biphasic. Furthermore, the significantly lower signal frequencies compared to conventional NMR make it possible to ‘see through’ conductive materials Including metal containers.We will explore ZULF NMR as a method to perform operando reaction monitoring. To address the low signal strength of ZULF NMR, we employ parahydrogen-based hyperpolarization. Hydrogen gas can be readily prepared in a nonequilibrium nuclear spin state (parahydrogen), which leads to dramatically enhanced (hyperpolarized) NMR signals of a probe molecule if it is either formed by chemical reaction with parahydrogen or brought reversibly in a contact with activated parahydrogen molecule on a metal centre. The enhanced signals allow the probe molecules to be studied under ZULF NMR conditions. We will use these hyperpolarized probes to develop ZULF NMR as a method to monitor chemical reactions in homogeneous solutions, as well as studying the interaction of hyperpolarized molecules with active centres on surfaces of porous catalysts and sorbents.Finally, in this study we will use parahydrogen not only as the hyperpolarization source but also as the chemical reactant and a mechanistic probe. Hydrogenation reactions are carried out on an industrial scale, and if the catalytic mechanisms were better understood, the efficiency of these processes could be vastly improved. Parahydrogen only leads to enhanced signals when the addition is pairwise, which can be utilized to provide insight into the underlying reaction mechanisms, as well as the optimization of heterogeneous catalysts, for which single-site hydrogenation with better-defined and structured active centres is an important goal. We will therefore investigate hydrogenation reaction mechanisms on solid supported metal catalysts by applying the advanced approach of this project which combines ZULF NMR with signal enhancement by parahydrogen-based spin hyperpolarization.