Chemical mapping of chemical conversion in heterogeneous catalytic reactors using MRI

Magnetic Resonance Imaging (MRI) is an emerging measurement technique for the study of chemical reactions in situ within reactor environments. Whilst MRI is now well established as a tool to image gas, liquid and even some solids flows in reactors, the ability to perform spatially resolved spectroscopic measurements is still in its infancy. Here we introduce the basic concepts of MRI and then introduce two pulse sequences which are finding increasing use in application to studying catalytic processes. First, we outline the Rapid Acquisition with Relaxation Enhancement (RARE) pulse sequence which provides sub-second image acquisition times thereby enabling unsteady-state processes to be imaged. Second, Single-Point Ramped Imaging with T ₁-Enhancment (SPRITE) is described; this technique is particularly useful for studying systems characterised by short nuclear spin relaxation times, and has been used to map coke distribution in reactors. The four main strategies for obtaining chemical resolution in MRI data are then outlined, these methods being relaxation time contrast, chemical shift imaging, volume selective spectroscopy and natural abundance ¹³C imaging. Examples of all these techniques in application to catalytic systems are given. In particular, the advent of natural abundance ¹³C imaging is identified as a method which may open up substantial opportunities for chemical mapping of a wide range of heterogeneous catalytic conversions in the reactor environment.