Recent MRI Studies on Heterogeneous Catalysis

Abstract

Abstract Applications of magnetic resonance imaging to the studies of heterogeneous catalytic processes are relatively rare. At the same time, magnetic resonance toolkit is able to provide valuable insight into the key dynamic processes in an operating reactor which govern its efficiency and safety, such as heat and mass transfer, phase transitions and chemical transformations. Thanks to the versatile nature of image contrast in MRI, it can provide valuable parameter maps with a rich and diverse information content. Achieving a reasonably high spatial and temporal resolution requires high NMR detection sensitivity and is thus particularly challenging in MR studies of gases because their spin density and relaxation times are considerably smaller than those of liquids. Remarkably, the reaction that so far was addressed with MRI most, the heterogeneous hydrogenation of unsaturated compounds such as liquid and gaseous olefins and alkynes provides a useful solution to the sensitivity problem. By simply utilizing parahydrogen instead of normal hydrogen in the reaction, one can hyperpolarize nuclear spins of reaction products and intermediates, thereby achieving a several orders of magnitude enhancement of their NMR signals. Another interesting yet challenging development is the MRI/MRS of operating microreactors where conventional NMR/MRI instruments and techniques fail to provide adequate sensitivity. An elegant solution to boost the sensitivity in such studies is the use of the remote detection NMR approach, either alone or in combination with hyperpolarization techniques. The examples we discuss are intended to demonstrate the current achievements of this research area and the challenges that need to be addressed before such applications can become routine and widespread.