In situ analysis of gas phase reaction processes within monolithic catalyst supports by applying NMR imaging methods

Measuring spatially resolved concentration distributions in gas phase reaction systems is an important tool to validate simulation calculations, improve the understanding of transport processes within the catalyst, and identify potentials for improvements of monolithic catalyst supports. The commonly used measurement methods for such opaque systems are invasive and, thus, might be misleading due to alteration of the system.

To overcome this issue, a 3D magnetic resonance spectroscopic imaging (MRSI) method was developed and implemented on a 7-Tesla NMR imaging system to map the concentration distributions within opaque monolithic catalysts using the ethylene hydrogenation process as case study. The reaction was catalyzed by a coated sponge packing or a honeycomb monolith within an NMR compatible packed bed reactor. Temperatures at the inlet and the outlet of the catalyst beds were simultaneously determined by analyzing the spectra of inserted ethylene glycol filled glass capsules. Steady state concentration profiles and temperature levels were measured at different reaction conditions. In order to prove the plausibility of the measured spatial distributions of compound concentrations, the experimental results were compared to a 1D model of the reactor based on kinetic data from literature. Furthermore, a comparison with integral concentration measurements using a mass spectrometer demonstrated deviations below 5%. The results show that 3D MRSI is a valuable and reliable tool to non-invasively measure spatially resolved process parameters within optically and/or mechanically inaccessible structured monolithic catalyst supports, even if only standard thermal polarization is exploited and the use of expensive and technically challenging signal enhancement techniques (hyperpolarization) is avoided. We expect that 3D MRSI can pave the way toward deeper insight into the interactions between catalyst, catalyst support, and gas phase.