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7 High-Temperature Electron Spin Resonance and NMR Methods Applied to Coal 1
Yuzo Sanada and Leo J . Lynch
2
1
Faculty of Engineering, Hokkaido University, Sapporo, 060, Japan Commonwealth Scientific and Industrial Research Organisation (CSIRO), Division of Coal and Energy Technology, P.O. Box 136, North Ryde, New South Wales 2113, Australia 2
The applications of in situ electron spin resonance (ESR) and NMR spectroscopic methods to study thermally induced transformations of coals and related materials are addressed. Brief background statements are given on the underlying principles, and the focus is on the particular methodologies developed at Hokkaido University and CSIRO during the past decade. Designs of both high-temperature and high-temperature—high-pressure ESR and NMR probes and spectrometer configurations for their use are outlined. The high-temperature in situ ESR technique is illustrated by its application to Argonne Premium coals. Applications of NMR techniques to derive chemical-shift information on coal and pitch materials during pyrolysis and liquefaction processes, and techniques that monitor molecular dynamics during pyrolysis are presented Monitoring molecular dynamics is demonstrated to be an effective method of thermal analysis.
J L H E R M A L L Y I N D U C E D C H A N G E S in coals and related materials can
be studied in situ by adapting electron spin resonance ( E S R ) and nuclear 0065-2393/93/0229-0139$09.50/0 © 1993 American Chemical Society
In Magnetic Resonance of Carbonaceous Solids; Botto, Robert E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
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magnetic resonance (NMR) spectroscopic methods. The thermal transformation of coal and other organic solids by pyrolysis—carbonization, combustion, or liquefaction can be investigated by equilibrium methods whereby the properties of the original and product materials at different stages of the process are assessed. A n advantage of this approach is that measurement time is not a constraint to obtaining precise and detailed data. However, in situ or "reaction time" methods of observation are required to measure transient, nonequilibrium intermediate states that occur in these processes. Phenomena of particular significance for coal include thermally induced free radical reactions (1-4) and the so-called "thermoplastic" state obtained in the early stages of pyrolytic decomposition of some coals of bituminous rank (5). A n in situ measurement technique is an effective method of thermal analysis if it has adequate time resolution to detect transient phenomena and thereby to monitor physical transitions and chemical changes in real time. A range of established thermal analysis techniques has been used to study coal materials and their reactions. These techniques are based on a variety of measurements, including some that sense heat flow [differential scanning calorimetry (DSC) and differential thermal analysis (DTA)], changes in mass [thermogravimetric analysis (TGA)], and changes in dimension [thermomechanical analysis (TMA)]. Other in situ measurement techniques involve analysis of evolved products and include such methods as evolved gas analysis (EGA), combined gas chromatography-mass spectrometry (GCMS), and emission and adsorption Fourier transform infrared spectroscopy. The adaptation of magnetic resonance spectroscopy techniques for in situ or reaction time studies at elevated temperatures is made difficult by the complexity of the measurement procedures and the uncertain time resolution that it is possible to achieve. A n effective method of N M R thermal analysis requires that the detected parameters that reflect the physical and chemical properties of the substance be recorded and monitored as a function of temperature or time as the substance is subjected to a controlled heating program. Ideally the measurement should be instantaneous in order to capture the information. The extent to which this measurement can be adequately approximated in practice depends on the rate of change associated with the phenomenon of interest and the time necessary for the magnetic resonance measurements to be completed. The rate of measurement can be limited by the inherent and therefore changing nature of the specimen; therefore, close attention must be given to the procedures of measurement.
High-Temperature ESR Methods The Spectral Parameters of ESR Spectroscopy. ESR spectroscopy, sometimes called electron paramagnetic resonance (EPR) specIn Magnetic Resonance of Carbonaceous Solids; Botto, Robert E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
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troscopy, is a means of detecting direct transitions between electron Zeeman levels. The phenomenon of electron spin resonance is observed only for atomic or molecular systems having net electron spin angular momen tum, that is, materials containing one or more unpaired electrons. Materi als that meet this criterion include organic free radicals, molecules in their triplet states, some types of charge-transfer complexes, transition metal ions, semiconductors, metals, inorganic molecules such as oxygen that con tain partially filled molecular orbitals, and crystals having certain point defects. The initiation of chemical changes in organic substances by thermal energy, gamma irradiation, and mechanical forces usually involves the gen eration of free radicals. The formation of free radicals in such reactions is due to homolytic scission of a single covalent bond between two different atoms as follows: A -
Β -> Α· + B-
Detection of an unpaired electron, as in a free radical, can provide impor tant information about the pathways of reactions that have already occurred or are in progress. Typical free radical reactions are chain reac tions that occur in three steps: (1) formation; (2) propagation including atom transfer, addition, rearrangement, and fragmentation reactions; and (3) termination including combination of two radicals and disproportionation. 7r-radicals are long-lived, having lifetimes of several minutes or greater, in contrast to the transient, short-lived (i.e., highly reactive) nature of σ-radicals. π-radicals occur typically in aromatic or conjugated molecules in which all the electrons occupy sp-type orbitals. The πelectron orbitals in such molecules overlap to form molecular orbitals with a discrete band of energy levels that can be described as linear combina tions of the aromatic 2p orbitals. This derealization of the unpaired elec tron results in the stabilization of the ττ-radical. Hence ^--radicals are readily detected in conventional (equilibrium) ESR spectroscopy, and σradicals are generally difficult to detect. Therefore, the much greater capability of in situ ESR spectroscopy to detect short-lived σ-radicals is of great advantage in studies of coal reactivity. The three most useful parameters that can be extracted from ESR spectra are the spectral intensity, g value, and spectral line width; these parameters provide information on spin concentration, spin types, and the molecular environment. Other parameters, such as the electron relaxation times, can sometimes be measured or estimated, but have had only limited use in coal research (3, 6). Since the first successful ESR experiments in 1945, many investiga tions on coal structure and reactivity have been made with commercially available ESR spectrometers. The aspects of ESR spectrometry necessary In Magnetic Resonance of Carbonaceous Solids; Botto, Robert E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.
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for a basic understanding of its applications in coal research are discussed in several excellent treatises (1-3).
High-Temperature ESR Spectroscopy. Ingram et al. (4) and Ubersfeld et al. (7) were the first to detect stable free radicals in coal by ESR measurements. Since then, many ESR studies on carbonaceous materials such as kerogen, coal, pitch, and their derivatives have been conducted, and the signal parameters of the naturally formed free radicals have been used for coal rank determination. The ESR parameters of spin concentration, spectral line width, and g value have provided details of coal structure and of coal utilization reactions such as extraction with solvent, liquefaction, and gasification (3, 8). The detection of thermally formed free radicals in heated coal was reported earlier (2—4). However, these studies were of samples heattreated outside the ESR cavity prior to measurements at room temperature. The information obtained in such measurements is limited because transient radicals quenched during the course of sample preparation are not observed. More recently the usefulness and importance of ESR in in situ hightemperature spectroscopy have been recognized and adopted by several investigators (9—11) because of its capability of monitoring thermally generated free radicals as they are formed. Extensive studies at the Faculty of Engineering Laboratory, Hokkaido University, using in situ high-temperature ESR spectroscopy are directed at a better understanding of the chemistry of free radicals in coal materials (12-15). High-Temperature—High-Pressure ESR Methods. Realization of the potential of in situ ESR spectroscopy to study coal pyrolysis and liquefaction reactions requires the availability of high-temperature or pressure-regulated ESR instrumentation. However, commercially available ESR cavities have a limited temperature range (
