Hyperpolarized Zero-to-Ultralow-Field Nuclear Magnetic Resonance

点击翻译按钮获取中文摘要

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.