Magnetic resonance imaging methods in heterogeneous catalysis

Igor V. Koptyug DOI: 10.1039/9781782621485-00001

Applications of spatially resolved magnetic resonance in heterogeneous catalysis and related fields are considered. The chapter starts with a simple description of the basic principles of MRI and the discussion of the specific features which make MRI a powerful and versatile toolkit capable of providing useful and diverse information about catalysts, reactors and processes within them in a non-invasive manner. Next, practical aspects of constructing an MRI-compatible reactor are presented along with the methods for, and examples of, the structural MRI studies of packed beds, model reactors and related geometries. The basic principles of mass transport studies with NMR and MRI are considered next, and the literature examples of MRI studies of mass transport in model systems are briefly outlined. The rest of the chapter is devoted to the analysis of the studies of model catalytic reactors under operating conditions, and includes MRI studies of distribution and mass transport of fluids, spatially resolved spectroscopic studies of conversion, MRI thermometry of operating catalytic reactors and microreactors, and the use of the emerging techniques for nuclear spin hyperpolarization to boost the sensitivity of NMR and MRI in catalytic applications.

1 Introduction

Nuclear magnetic resonance imaging, abridged to "MRI" to stress its harmless nature, has become one of the most powerful instruments in modern medical diagnostics. In fact, MRI has revolutionized modern medicine by enabling physicians to literally see the state of internal organs in a human body and various processes taking place within it. This ability, coupled with the non-invasive nature of the technique, made it possible to abandon the "black box" approach, in which the diagnosis is often based on superficial observations and a limited number of symptoms which are often similar in many diseased states. The success of medial MRI might seem surprising given that the technique has modest spatial resolution and a number of significant limitations as compared to other modern imaging techniques, e.g. computer assisted (X-ray) tomography (CAT), positron emission tomography (PET), etc. However, the key feature of MRI is that it is best characterized as a versatile toolkit, in contrast to many other techniques which are powerful but specialized tools. In addition to morphological studies, the medical MRI toolkit contains tools for angiography, thermometry, spectroscopy, elastography, functional MRI, and a lot more. The foundation for this tremendous diversity is the versatile nature of image contrast in MRI. The latter is sensitive to a wide range of properties of an object under study and processes taking place within it. Furthermore, image contrast in MRI can be deliberately tailored to the needs of a particular study. As a result, MRI is able to provide a lot more than just structural information. It is this ability which makes MRI so powerful a technique.

Unlike modern medicine, chemical and process engineering practice still largely relies on the "black box" approach, trying to figure out what is happening inside an operating reactor on the basis of a number of external measurements (pressure drop, temperature, chemical composition of the feed and reactor output, transient response curves, etc.). A number of modern imaging techniques are being developed to overcome the existing limitations, but so far our ability to see the inner works of one of the most sophisticated bioreactors – human body – by far exceeds our ability to visualize processes inside, e.g., a packed bed reactor. In fact, biomedical MRI has reached the stage when metabolic processes taking place in the cells of various tissues and organs can be interrogated in live animals, and this possibility is currently being extended to in-human studies.

With all this progress in biomedical MRI applications, it might seem surprising that MRI has not become a routine technique to "diagnose" the behavior of various chemical reactors. One of the obstacles on this way is the feature that makes MRI such a powerful technique – the diversity of image contrast mechanisms, i.e., the sensitivity of the detected signal to a wide range of object properties. As a result, many MRI strategies developed in medical MRI to a state of perfection perform unsatisfactorily when applied to non-biological objects. At present, non-biomedical applications of MRI are still an art rather than routine studies despite the fact that the interest in such studies is clearly on the rise, including applications to problems related to catalysis.

2 The MRI technique

NMR in general and MRI in particular explore the interaction of nuclear spins with static and oscillating magnetic fields. In NMR spectroscopy, one acquires NMR spectra which characterize local magnetic environments of nuclei possessing a non-zero spin (e.g., 1H, 13C, 19F, 31P) and thereby provide information on chemical composition of a sample, its molecular structure and dynamic transformations, etc. In...