Spatially Resolved NMR Spectroscopy for Operando Studies of Heterogeneous Hydrogenation with Parahydrogen

Magnetic resonance imaging (MRI) is a unique tool for operando studies owing to its non-invasive manner of signal detection. MRI can provide information about structure of the reactor, distribution of the reagents and products in the reactor, and heat and mass transport processes. However, the heterogeneous solid phase of a catalyst in a reactor largely distorts the static magnetic field of an MRI instrument, which leads to a major loss in spectroscopic resolution and measurement sensitivity. On top of that, many chemical reactions involve gases, so that the reduced spin density compared to liquids is yet another complication in such studies. To overcome these challenges, a proper choice of model catalytic reactors for NMR-based experiments is required. In this study, the configuration of model catalytic reactors was varied to explore its effect on the spatially resolved ¹H NMR spectra acquired during heterogeneous hydrogenation of propene to propane with parahydrogen over several supported metal catalysts. The results demonstrate that a judicial choice of a reactor geometry in combination with signal enhancement provided by parahydrogen makes such studies feasible and informative.