(621eu) Magnetic Resonance Physiochemical Tomography for in Situ Studies of Heterogeneous Reactions

Optimizing catalytic reactions requires knowledge of thermodynamic quantities such as temperature, pressure, velocity, and chemical sensing. We present magnetic resonance imaging (MRI) spectroscopic methods for operando monitoring of heterogeneous reactions in situ. With high spatial resolution (from 1 mm down to tens of microns), non-invasive MRI has long been an excellent tool for the diagnosis and monitoring of various diseases. However, MRI is not limited to biomedical usage; with the ability to retain chemical information through the chemical shift of nuclei, MRI is a potentially transformative tool for operando spectroscopy of heterogeneous reactions. This chemical specificity distinguishes MRI from other non-invasive spectroscopic and imaging techniques, and allows us to follow a substrate through the course of the reaction: from educt to intermediate to product. In addition to chemical sensing, MRI can visualize the coupling of chemistry between flow and catalyst-packing heterogeneity. Spatial encoding can provide tomographic images the gradients in velocity, pressure, diffusion, chemical potential, and temperature throughout the reactor bed. Of particular value are temperature maps, not only because temperature control is essential for optimal reactor operation, but also because thermal gradients contain information about the energetics of a reaction. One technique we present is an MRI method that monitors the temperature of the reactive species in mixed-phase gas/solid reactions. Traditional MRI thermometry techniques are impractical for gas-phase reactions due to weak temperature-dependence and rapid signal attenuation due to diffusion. Our thermometry technique has a strong temperature dependence and exploits self-diffusion, allowing us to produce thermal maps of gases in heterogeneous reactions. We demonstrate the technique by non-invasively studying gas temperatures of propylene hydrogenation in a catalyst-packed reactor.