Spectroscopic Properties of Inorganic and Organometallic Compounds: Volume 45

by Richard Douthwaite (Editor), Simon Duckett (Editor), Jack Yarwood (Editor)

Overview

This series provides an unequalled source of information on an area of chemistry that continues to grow in importance. Divided into sections mainly according to the particular spectroscopic technique used, coverage in each volume includes: NMR (with reference to stereochemistry, dynamic systems, paramagnetic complexes, solid state NMR and Groups 13-18); nuclear quadrupole resonance spectroscopy; vibrational spectroscopy of main group and transition element compounds and coordinated ligands; and electron diffraction. Reflecting the growing volume of published work in the field, researchers will find this an invaluable source of information on current methods and applications.

Product Details

ISBN-13:	9781782625964
Publisher:	RSC
Publication date:	06/27/2014
Series:	Specialist Periodical Reports - Spectroscopic Properties of Inorganic and Organometallic Compounds , #45
Sold by:	Barnes & Noble
Format:	eBook
Pages:	298
File size:	13 MB
Note:	This product may take a few minutes to download.

About the Author

Professor Jack Yarwood is an emeritus professor at Sheffield Hallam University. Professor Simon Duckett is a research group leader at the University of York, UK. His group is mainly involved in the design, development and implementation of NMR methods, supported by the synthesis of inorganic and organometallic complexes. Dr Richard Douthwaite is at the University of York, UK. His main research interests include molecular and materials chemistry and photocatalysis. Both an EPSRC college member and fellow of the Royal Society of Chemistry, Dr Douthwaite is also on the SCI National Materials Committee.

Read an Excerpt

Spectroscopic Properties of Inorganic and Organometallic Compounds

Techniques, Materials and Applications Volume 45

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By Jose Antonio Carrero, Gorka Arana, Nicolas Bélanger-Desmarais, Kanishka Biswas, Xavier Bourrat, Kristina Chakarova, Sayandev Chatterjee, William B. Connick, Keith Silvio Decurtins, B. Dillon, Anastasia B. S. Elliott, Peter Flewitt, Patrick Franz, Keith C. Gordon, Regis Guegan, Konstantin Hadjiivanov, Christian W. Huck, Elena Ivanova, Igor V. Koptyug, Abdeltif Lahfid, Dong Liu, Juan Manuel Madariaga, Nicolas Maubec, Mihail Mihaylov, Dimitar Panayotov, Stéphanie Poirier, Christian Reber, Camille Sonneville, Holly van der Salm, Guillaume Wille, J. Yarwood, R. Douthwaite, S. B. Duckett

The Royal Society of Chemistry

Copyright © 2014 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-919-1

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CHAPTER 1

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 an NMR spectrum, the position of a particular resonance (the resonance frequency ω) is proportional to the applied static magnetic field B0 and the magnetogyric ratio g of the magnetic nucleus under consideration (ω [varies] γB0). This implies that if, for instance, a container with water is placed in a spatially uniform magnetic field (Fig. 1a), a single NMR resonance will be observed in the NMR spectrum (Fig. 1b). Indeed, in NMR spectroscopy, high magnetic field homogeneity is essential to obtain high spectral resolution. In contrast, in MRI the magnetic field is intentionally made to depend on a spatial coordinate, often in a linear fashion (Fig. 1c). In this example, different volumes of water reside in different static magnetic fields and therefore produce NMR resonances at different resonance frequencies. The resulting "spectrum" is essentially a one-dimensional integral projection of the sample on the direction of the gradient (Fig. 1d). If three orthogonal gradients are available, they can be combined to produce a resulting gradient with an arbitrary orientation in space, rendering an MRI measurement free from any orientational preferences and making possible the acquisition of 2D and 3D images. Gradients are often applied as pulses and are incorporated in an NMR pulse sequence along with radiofrequency pulses and delays. The borderline between MRI and non-imaging NMR applications is rather vague. Indeed, many modern magnetic resonance (MR) experiments combine spatial and spectral coordinates and are best described as experiments in a multidimensional (sub)space of spatial, spectral and temporal coordinates rather than MR spectroscopy or imaging.

NMR/MRI experiments require that a sample under study contains a large number of magnetic nuclei. Their nuclear spins serve as intrinsic probes or tracers which can convey the information about their spatial position, local environments and mobility of the groups of atoms and molecules they belong to, to the NMR detector which is usually placed externally with respect to the object under study. Therefore, an MRI experiment is non-invasive and non-destructive, does not require the introduction of any other tracers or probes into the object under study, and allows one to monitor various dynamic processes in situ, without a need to periodically interrupt them to make a measurement.

Modern NMR spectroscopy is performed with virtually all nuclei possessing a non-zero spin. In principle, an image can also be obtained using any nucleus with a non-zero spin. In practice, however, the majority of the MRI studies are performed using 1H NMR signal detection, with a much smaller number of studies reported where other nuclei were addressed. The reason is that the sensitivity (signal-to-noise ratio, SNR) available in an experiment represents one of the major limitations for achieving better spatial resolution, implementing faster imaging strategies and developing more informative MRI applications. At the same time, the multinuclear capability of MRI is one of the highly promising directions for future developments, and the number of such applications is growing. The sensitivity achievable with various nuclei, and thus the practical feasibility of using these nuclei in an MRI experiment, are largely governed by the concentration of the nuclear isotopes in question, their magnetogyric ratios and their nuclear spin relaxation times in the objects under study. For instance, since 12C and 16O isotopes are spinless, NMR studies of these very important atoms require the use of 13C and 17O isotopes characterized by a much lower natural abundance. This greatly complicates the MRI experiments with such nuclei, because low natural abundance leads to very low sensitivity and/or requires the use of expensive isotope-enriched samples. Nuclei with very low magnetogyric ratios are usually characterized by low sensitivity, too, and therefore are seldom used in MRI.

Nuclear spin relaxation times also determine the achievable sensitivity. Furthermore, relaxation times govern spin dynamics in MRI experiments, determine image acquisition times and are important image contrast parameters. Transverse (or spin-spin) relaxation proceeds with a characteristic relaxation time T2 and governs the decay of an observed signal after a single radiofrequency (rf) pulse in a uniform magnetic field. In practice, the apparent signal decay rate is even faster and proceeds with a characteristic time T2*< T2 (or T2* [much less than] < T2). This extra decay is caused by the differences in local magnetic fields experienced by different spins which imply different precession frequencies and gradual dephasing of the ensemble of spins in a non-uniform magnetic field. However, this extra decay caused by the distribution of local fields in the sample can be at least partially reversed by means of spin echo techniques, and thus is fundamentally different from the irreversible loss of signal characterized by T2. After any perturbation, the system tends to return to thermal equilibrium as spins realign with respect to the static field B0. This longitudinal (or spin-lattice) relaxation process is often exponential with a characteristic recovery time denoted as T1.

Many imaging techniques literally scan an object to produce its image. In contrast, modern NMR and MRI techniques (as well as many other techniques such as X-ray crystallography, FT IR, etc.) use Fourier transform to obtain spectroscopic or spatial data. In MRI, data acquisition takes place in the space of wave vector k (k-space) defined as ki = γGit/2π (i = x,y,z), where Gi is the magnitude of the magnetic field gradient along coordinate i and t is the gradient duration. The original data set detected in an experiment comprises a multidimensional array of NMR signal intensity values sampled for various values of k, achieved by varying the magnitude, the duration or the number of the applied gradients. Multidimensional Fourier transform of this data set is used to recover the true image of the same dimensionality.

As 'scanning' is done in k-space and not in the space of Cartesian coordinates, usually the entire data set in k-space has to be acquired before an image can be reconstructed. One-dimensional (1D) imaging requires an acquisition of a single line of sampled signal values in k-space and in principle can be performed very rapidly, within a millisecond or so. For samples which are nearly axially symmetric, acquisition of 1D axial projections can be a very efficient way to study fast dynamic processes. Higher image dimensionality usually increases the minimum imaging time substantially. The reason is that before the next line of samples in k-space can be acquired, one often has to wait (3-5)T1 to allow the spin system to return to thermal equilibrium. As an example, for 1H nuclei of water with T1 = 2.7 s, a 3D image with 128 x 128 x 128 volume elements (voxels) needs 128 x 128 lines in k-space to be detected, which translates into an image acquisition time of more than 36 hours (128 x 128 x 3 x T1). At the same time, for a sample with T1 = 10 ms the same experiment can be performed in 8 min. In practice, however, the sensitivity is often an issue, and N accumulations of each line in k-space are performed to improve SNR in the final image by a factor of N0.5. This makes image acquisition time N times longer and makes this approach of sensitivity improvement impractical for large values of N.

Several approaches are used to reduce imaging time in MRI. One possibility is to reduce image dimensionality. In many cases, it suffices to acquire images of a number of carefully chosen 2D slices or even 1D bars of an object. The slices to be imaged can be non-invasively selected using appropriate combinations of gradients and frequency-selective rf pulses. As a result, the observed NMR signal is produced only by the spins residing within the selected slice, eliminating the need to define the slice of interest in any destructive fashion. As most of the image acquisition time is wasted while waiting for the spin subsystem within the imaged slice to return to equilibrium, slice-selective excitation in combination with a carefully chosen interleaved acquisition of k-space lines for different slices allows one to acquire not just one but several 2D slices within essentially the same imaging time. Another efficient strategy widely used in MRI is the implementation of various rapid imaging techniques. One of the approaches is based on the accumulation of multiple k-space lines for each pulse sequence repetition. For instance, single-shot 2D imaging allows one to acquire the entire 2D image in fractions of a second, provided that the sensitivity is sufficient. Another possibility is to repeat the pulse sequence without waiting for the complete recovery of the spin system. Unfortunately, rapid imaging techniques developed and successfully used in medical MRI, work properly for bulk liquids but often fail for samples with short T2 times such as liquids in porous media, gases and solids. At the same time, such materials are characterized by shorter T1 times as well, sometimes allowing faster imaging using standard imaging sequences. In certain cases, T1 times of liquids can be reduced artificially by dissolving paramagnetic species, as it is done in medical MRI when paramagnetic contrast agents are administered.

For any imaging technique, an important feature is its spatial resolution. In contrast to many other techniques, however, the spatial resolution in MRI is sample-dependent. Images with the best resolution achieved to date have voxel sizes of the order of 5 x 5 x 5 µm3, with some studies reporting voxels as small as 3.7 x 3.3 x 3.3 µm3 or a volume of 40 femtoliters which contains 3 x 1012 proton spins of water. However, the resolution of the MRI technique in many cases is limited by SNR, i.e., by the sample rather than the instrument. Indeed, if the voxel size is reduced, the NMR signal intensity associated with each voxel diminishes leading to the reduced SNR in the image. Therefore, the very high spatial resolution could be obtained only using samples with large water content and many hours of imaging time. In most practical applications the resolution will be (much) coarser. The resolution doesn't have to be isotropic, however, and in many cases the slice thickness can be a few mm while the in-plane resolution is from a few to a few hundred microns. The sensitivity limit on the attainable resolution means that the best applications for the MRI technique are those where the ultimate spatial resolution is not required. Higher SNR can be achieved using signal averaging, development of better hardware and more efficient pulse sequences, implementation of more sensitive signal detection methods and polarization of nuclear spins to increase the available nuclear spin magnetization. Nevertheless, one micron appears to be the ultimate limit for conventional imaging schemes which will be difficult to overcome even if a dramatic progress in sensitivity improvement can be achieved in the future. Indeed, there are other factors which limit the attainable resolution to roughly the same value of a few microns, which include diffusive displacements of molecules (especially for gases) and available magnetic field gradients (for materials characterized by short T2 and T2* relaxation times).

(Continues...)

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Excerpted from Spectroscopic Properties of Inorganic and Organometallic Compounds by Jose Antonio Carrero, Gorka Arana, Nicolas Bélanger-Desmarais, Kanishka Biswas, Xavier Bourrat, Kristina Chakarova, Sayandev Chatterjee, William B. Connick, Keith Silvio Decurtins, B. Dillon, Anastasia B. S. Elliott, Peter Flewitt, Patrick Franz, Keith C. Gordon, Regis Guegan, Konstantin Hadjiivanov, Christian W. Huck, Elena Ivanova, Igor V. Koptyug, Abdeltif Lahfid, Dong Liu, Juan Manuel Madariaga, Nicolas Maubec, Mihail Mihaylov, Dimitar Panayotov, Stéphanie Poirier, Christian Reber, Camille Sonneville, Holly van der Salm, Guillaume Wille, J. Yarwood, R. Douthwaite, S. B. Duckett. Copyright © 2014 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents

NIR studies of nano-structured materials;
Raman Measurements of Stress in inorganic films and coatings;
Use of Raman spectroscopy to detect Road Transport Pollution;
Spectroscopic studies of Inorganic Nano-Materials;
Magnetic Resonance Imaging methods in heterogeneous catalysis;
Combined SEM/Raman studies of Inorganic materials