NMR hyperpolarization methods and their applications to catalytical reactions

Overview & Synopsis

Magnetic resonance methods, i.e. nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), have proven to be powerful tools for analytical chemistry, catalysis and biomedical applications due to their versatility and non-invasiveness. In catalysis NMR has found applications for characterization of both heterogeneous and homogeneous catalysts, reaction products and intermediates in gas, liquid and solid state, while MRI allows to visualize reactants and products flow inside the catalyst bed, obtain spatially resolved conversion information and monitor the process of catalyst preparation. In biomedicine NMR is used for proteins structure determination, drug discovery and metabolites and MRI is routinely used for diagnostic of a wide range of pathologies.However, magnetic resonance suffers from low signal intensity, resulting from low nuclear spin polarization at thermal conditions. The sensitivity problem is further exacerbated by inhomogeneity of magnetic field, associated with the presence of solid catalysts beads or even catalytic reactors inside the spectrometer. Furthermore, industrial catalysts are often subjected to high temperatures – meaning that many catalytic reactions proceed in gas phase, where spin density is significantly reduced.In this thesis it will be demonstrated how several NMR hyperpolarization techniques – parahydrogen-induced polarization, spin-exchange optical pumping and dynamic nuclear polarization – can overcome these limitations, provide new information about heterogeneous catalytical hydrogenation processes and open up new perspective applications. Parahydrogen will be used in hydrogenation of propene and propyne over various carbides (Rh2C and two-dimensional Mo2C) in attempt to better understand the origin of pairwise hydrogen addition over heterogeneous catalysts – the mechanism that was once thought of as impossible and that still escapes a simple explanation. Next, it will be shown how 129Xe, hyperpolarized by spin-exchange optical pumping, can serve as a sensitive non-invasive temperature sensor in a working catalytical reactor. In the last part, versatile and easily filtrable hyperpolarizing solids are introduced, used to generate pure hyperpolarized solutions by dissolution dynamic nuclear polarization, compatible with virtually any existing polarizer. These materials feature increased pore size combined with lower radical concentrations, leading to improved cross-polarization and hyperpolarization preservation upon dissolution. Future coupling of dissolution dynamic nuclear polarization with magnetic resonance at zero- to ultralow field – a technique that opens the door for studies under conditions, challenging for conventional NMR (for example, in metallic containers and directly in porous media) – will also be discussed.