Pugmire, RJ

2000

A Global Free-Radical Mechanism for Light Gas Nitrogen Release from Coal during Devolatilization

Perry, S.T.; Fletcher, T.H.; Pugmire, R.J. and Solum, M.S.
Energy & Fuels, 14, 1094-1102 (2000).
Contact: Fletcher

Solid-State C-13 NMR Characterization of Matched Tars and Chars From Rapid Coal Devolatilization

Perry, S.T.; Hambly, E.M.; Fletcher, T.H.; Solum, M.S. and Pugmire, R.J.
Proceedings of the Combustion Institute, 28 (in press, 2000).
Contact: Fletcher

1999

Predicting C-13 NMR Measurements of the Chemical Structure of Coal Based on Elemental Composition and Volatile Matter Content

Genetti, D., Fletcher, T.H. and Pugmire, R.J.
Energy & Fuels, 13:60-68 (1999).

A model that predicts the amount and distribution between tar and light gas of nitrogen released during devolatilization has been developed and incorporated into the Chemical Percolation Devolatilization (CPD) model. This work represents the first volatile nitrogen release model developed based on C-13 NMR measurements of coal structure. This work also represents the first volatile nitrogen release model evaluated by comparing model predictions with chemical structural features of the char (determined by C-13 NMR spectral analyses). The model is limited to nitrogen release during primary pyrolysis, and assumes that all light gas nitrogen is HCN. Model predictions of nitrogen release compare well with measured values for most coals and devolatilization conditions tested.

1998

Nitrogen Transformation in Coal During Pyrolysis

Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-73 (1998).

X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the changes in organically bound nitrogen forms present in the tars and chars of coals after pyrolysis. For fresh coal, pyrrolic nitrogen is the most abundant form of organically bound nitrogen, followed by pyridinic, quaternary, and amino types. Some of the quaternary nitrogen species initially present in coal are lost upon mild pyrolysis, prior to hydrocarbon devolatilization. These quaternary species are attributed to pyridinic or basic nitrogen species associated with hydroxyl groups from carboxylic acids or phenols. A portion of the quaternary nitrogen species is lost at the very earliest stage of pyrolysis. Upon devolatilization, the resultant tar and char contain mostly pyrrolic and pyridinic forms; however, a portion of the quaternary nitrogen initially present in the coal appears in the coal char and tar. The relatively strong bonding interactions associated with these quaternary species suggests that there may be other quaternary nitrogen, in addition to protonated pyridines, in low-rank coal. For low-rank coal, amino groups are preferentially released and concentrate in the tar. XPS analysis of chars and tars produced during rapid heat-up (10^14 deg/s) pyrolysis show similar trends. However, severe pyrolysis of the devolatilized char results in the appearance of an asymmetric carbon (1s) line shape indicative of very large polynuclear "graphitic-like" units. This transformation is accompanied by a rise in the relative number of quaternary nitrogen forms and occurs over a relatively narrow temperature range. Quaternary and pyridinic nitrogen forms become the dominant forms in severely pyrolyzed chars. The relatively low level of quaternary nitrogen in the rapid heat-up chars indicates that very large polynuclear aromatic structures are not fully developed under these pyrolysis conditions.

1997

Dynamic Nuclear Polarization of Nitrogen-15 Benzamide

Hu, J.Z.; Zhou, J.; Yang, B.; Li, L.; Qiu, J.; Ye, C.; Solum, M.S.; Wind, R.A.; Pugmire, R.J. and Grant, D.M.
SOLID STATE: Nuclear Magnetic Resonance, 8:129-37(1997). Funded by US Department of Energy/Basic Energy Services and ACERC.

A N-15 dynamic nuclear polarization (DNP) experiment is reported in which a N-15 DNP enhancement factor of approximately 2.6 x 10² is obtained on free radical doped samples of 99% N-15 labeled benazmide. The free radicals BDPA (1:1 complex of alpha, gamma-bisdiphenylene-beta-phenylallyl with benzene) and DDPH (2,2 -Di (4 - tert - octylphenyl) -1-picrylhydrazyl) are used as dopants and the spin relaxation effects of adding these dopants are studied by means of changes in proton and nitrogen T1 values of the samples. The combination is solids of a very low natural abundance, 0.37%, a small gyromagnetic ration, and a log spin - lattice relaxation time for N-15 nuclei create severe sensitivity problems that, in large part, are ameliorated by the signal enhancement observed in the N-15 DNP experiment on samples containing free electrons.

Solid State N-15 and C-13 NMR Study of Several Metal 5-, 10-, 15-, 20-Tetraphenylporphyrin Complexes

Strohmeir, M.; Orendt, A.M.; Facelli, J.C.; Solum, M.S.; Pugmire, R.J.; Pery, R.W. and Grant, D.M.
Journal of the American Chemical Society, 119:7114-120(1997). Funded by National Institutes of Health, US Department of Energy.

The principal values both the C-13 and N-15 chemical shift tensors are reported in the Zn, Ni, and Mg 5, 10, 15, 20- tetraphenylporphyrin (TTP) complexes. The principal values of the 15N chemical shift tensors were obtained from static powder patterns of N-15 enriched samples. Due to overlap between the powder patterns of the different carbons, the C-13 values were obtained using the recently developed magic angle turning (MAT) 2D experiment on unenriched materials. The measured principals values are presented along with theoretical calculations of the chemical shift tensors and a discussion of the effects that the metal bonding has on the chemical shift tensors in these compounds. Both the isotropic chemical shift and the principal values of the N-15 chemical shift tensor are nearly identical for the Mg and Zn complexes. The N-15 isotropic chemical shift for the NiTPP, however, changes by nearly 80 ppm relative to the Mg and Zn values, with large changes observed in each of the tree principal values. Calculations show that the differences between the N-15 chemical shifts are almost entirely determined by the metal-nitrogen separation. In addition, both the experimental data and the calculations show only very minor differences in the 13C chemical shift tensor components as the metal is changed.

A High-Resolution 3D Separated-Local-Field Experiment by Means of Magic-Angle Turning

Hu, J.Z.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 126:120-26(1997). Funded by US Department of Energy.

A 3D separated-local-field (SLF) experiment based on the 2D PHORMAT technique is described. In the 3D experiment, the conventional 2D SLF powder pattern for each chemically inequivalent carbon is separated according to their different isotropic chemical shifts. The dipolar coupling constant of a C-H pair, hence the bond distance, and the relative orientation of the chemical-shift tensor tot eh C-H vector can all be determined for the protonated carbons with a single measurement. As the sample turns at only about 30 Hz in a MAT experiment, the SLF patterns obtained approach those of a stationary sample, and an accuracy in the measurement similar to that obtained on a stationary sample is expected. The technique is demonstrated on 2,6-dimethoxynaphthalene, where the C-13-H-1 separated-local-field powder patterns for the six chemically inequivalent carbons are clearly identified and measured. The observed dipolar coupling for the mothoxy carbons is effectively reduced by the fast rotation of the group about its C3 symmetry axis. The average angle between the C-H bond direction and the C3 rotation axis in the OCH3 group is found to be about 66°.

Nitrogen Transformations in Coal During Pyrolysis

Kelemen, S.R.; Gorbaty, M.L.; Kwiatek, P.J.; Fletcher, T.H.; Watt, M.; Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 12:159-72(1997). Funded by ACERC, Exxon and Federal Energy Technology Center.

X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the changes in organically bound nitrogen forms present in the tars and chars of coal after pyrolysis. For fresh coal, pyrrolic nitrogen is the most abundant form of organically bound nitrogen, followed by pyridinic, quaternary, and amino types. Some of the quaternary nitrogen species initially present in coal are lost upon mild pyrolysis, prior to hydrocarbon devolatilization. These quaternary species are attributed to pyridinic or basic nitrogen species associated with hydroxyl groups from carboxylic acids or phenols. A portion of the quaternary nitrogen species is lost at the very earliest stage of pyrolysis. Upon devolatilization, the resultant tar and char contain mostly pyrrolic and pyridinic forms; however, a portion of the quaternary nitrogen initially present in the coal appears in the coal char and tar. The relatively strong bonding interactions associated with theses quaternary species suggests that there may be other quaternary nitrogen, in addition to protonated pyridines, in low-rank coal. For low-rank coal, amino groups are preferentially released and concentrate in the tar. XPS analysis of chars and tars produced during rapid heat-up (104 deg/s) pyrolysis show similar trends. However, sever pyrolysis of the devolatilized char results in the appearance of asymmetric carbon (1s) line shape indicative of very large polynuclear "graphitic-like" units. This transformation is accompanied by a rise in the relative number of quaternary nitrogen forms and occurs over a relatively narrow temperature range. Quaternary and pyridinic nitrogen forms become the dominant forms in severely pyrolyzed chars. The relatively low level of quaternary nitrogen in the rapid heat up chars indicates that very large polynuclear aromatic structures are not fully developed under these pyrolysis conditions.

N-15 CPMAS NMR of the Argonne Premium Coals

Solum, M.S.; Pugmire, R.J; Grant, D.M.; Kelemen, S.R.; Gorbaty, M.L. and Wind, R.A.
Energy & Fuels, 11:491-94(1997). Funded by ACERC and US Department of Energy.

N-15 NMR data are reported for the Argonne Premium Coals. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Delectability of different types of nitrogens is discussed in terms of cross-polarization dynamics together with effects of the large chemical shift anisotropy that is found in different types of nitrogen functional groups. Data on acid-treated coals confirm the presence of pyridinic type nitrogens that were not observed in a previous N-15 NMR study of coals.

N-15 NMR Spectroscopy of Coals and Pyrolysis Products

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Fletcher, T.H. and Wind, R.A.
Proceedings of the 9th International Conference on Coal Science, Essen, Germany, September 7-12, 1997. Funded by ACERC, US Department of Energy/University Coal Research and New Energy and Industrial Technology Development Organization.

N-15 NMR spectra are reported for a number of coals from Pacific Rim countries. Arguments are presented to explain discrepancies between observations and conclusions obtained from NMR experiments and those obtained by XPS and XANES techniques. Detection of different types of nitrogen species is discussed in terms of cross polarization dynamics together with the effects of the large chemical shift anisotropy that are found in different types of nitrogen functional groups. Significant differences are observed in the types of nitrogen present in these coals and these variations are associated with coal rank as has been in a previous study of the Argonne Premium coals. We have also begun to examine pyrolysis char and tar samples. The NMR data indicate that significant differences exist between the types of nitrogen structures observed in coal, char and tar samples. These differences suggest that different mechanisms may exist for nitrogen release from tar and char samples.

1996

Effects of Hydrogen Bonding in the Calculation of N-15 Chemical Shift Tensors: Benzamide

Facelli, J.C.; Pugmire, R.J. and Grant, D.M.
Journal of the American Chemical Society, 118(23):5488-5489, 1996. Funded by National Institutes of Health and US Department of Energy/Basic Energy Services.

In a recent paper the principal values of the N-15 chemical shift tensor in benzamide have been reported. The tensor principal values have been measured using the dynamic nuclear polarization (DNP) experiment and also by cross polarization (CP) in static sample of N-15-benzamide at room temperature. Both experimental approaches produce very similar principal values. The values obtained by fitting the CP static spectrum with the Powder method are the following: delta11= -176 ppm, d22= -321 ppm, and d22= -326 ppm. These values are referenced to an external sample of nitromethane and yield and isotropic shift value of -274 ppm, which is close to the -271.5 ppm MAS (Magic Angle Spinning) value and the liquid values of -278.4 ppm in DMSO and -282 ppm in CDC13, from the literature. Because the measurements were done in a disordered sample no information was available on the orientation of the principal components and only reliable quantum chemical calculations of the tensor would allow these assignments to be made.

The results presented in this communication highlight the importance of including HB and, in general, intermolecular interactions in the calculation on N-15 chemical shifts tensors. This has been observed before in a previous study of the N-15 chemical shifts tensors of uracil but it appears that the magnitude of the effect is greater for an amide nitrogen than for the aromatic nitrogens in uracil. Further evidence of the importance of including intermolecular interactions in the calculation of N-15 chemical shifts tensors can be found in a recent study of these tensors in a series of heterocycles. For these compounds the RMS between experimental and calculated values, which do not include intermolecular effects, is approximately 30-40 ppm. The large RMS value is comparable with the values obtained here when the HB is neglected and almost one order of magnitude larger than the RMS obtained when the HB is included in the calculations. This may indicate that the findings presented here are not limited to benzamide, but are a general feature in the calculation of N-15 chemical shift tensors. The extreme sensitivity observed in the tensor components from HB indicates that the study of N-15 tensor components may be used to complement the determination of the tertiary structures of molecules of biological significance in solid state samples and/or in frozen solutions. It is well known that the HB is the principal governing factor in the tertiary components to these interactions makes N-15 tensors premier indicators of biomolecular structure and portends high value for these NMR methods. Finally, due to the high sensitivity and specificity of tensors, this work emphasizes some of the advantages of measuring tensor components instead of the traditional isotropic.

Carbon-13 Chemical Shift Tensors and Molecular Conformation of Anisole

Facelli, J.C.; Orendt, A.M.; Jiang, Y.J.; Pugmire, R.J. and Grant, D.M.
The Journal of Physical Chemistry, 100(20):8268-8272, 1996. Funded by ACERC and US Department of Energy/Basic Energy Services.

The first direct measurement of the ortho steric effect of the methoxy group on the C-13 chemical shifts in anisole is reported. The ortho steric effect on the isotropic C-13 chemical shifts was obtained from a low-temperature MAS spectrum, and the effect on both the isotropic and the tensor principal components was determined from a low-temperature 2D magic angle turning (MAT) experiment. Form the low-temperature MAS spectrum, the C-13 chemical shift of the ortho carbon cis to the methoxy carbon is found to be 7.0 ppm from the low-temperature MAT experiment, a 6.8 ppm decrease in the chemical shift is observed in the isotropic chemical shift, while the effects on the difference (cis minus trans) between the individual tensor components are measured to be -9 ppm in delta1, 1 ppm delta22 and -14 ppm in delta33, in reasonable agreement with the results of a previous linear regression substituent analysis on several di- and trimethoxybenzenes. Comparison of the experimental results with calculations, including thermal averaging considerations, further demonstrates that at room temperature the methoxy group in anisole undergoes stochastic jumps between the two equivalent planar configurations. This work demonstrates the feasibility of using the low-temperature MAT experiments at low temperature to measure the principal values of the C-13 chemical shift tensors in molecules that are liquids at room temperature.

Glass and Crystal Formation in Binary Aromatic Mixtures: A Mechanism for Reducing Spin-Lattice Relaxation Times

Wang, W.; Pugmire, R.J. and Grant, D.M.
J.Phys. Chem (in press), 1996. Funded by US Department of Energy/Pittsburgh Energy Technology Center and US Department of Energy/Basic Energy Services.

The phase diagram constructed form differential scanning calorimetry data indicated that the binary mixture of dibenzofuran (DBF) and hexamethylbezene (HMB) forms a simple eutectic system. Comparative studies of proton T1 Values for the annealed and quenched samples show the annealed material can be best described as a two-phase mixed crystals, while a rapidly quenched sample is a combination of a metastable one-phase glass and two-phased mixed crystals. It is found that glass formation is the key to T1 reduction of DBF in the HMB doping technique reported previously. The interesting rends in the TA and the relative spin population of DBG is explained with the competition between glass formation and crystalline phase separation.

1994

Coal Structuer from Solid State NMR

Pugmire, R.J.
Encyclopedia of NMR, J. Wiley & Sons, Ltd., 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.

Coal has been used as an important source of fuel for thousands of years, but it is a complex, heterogeneous fuel that is difficult to burn or process without serious environmental implications. Coals can vary significantly among different geographic areas in important properties such as rank, ash, sulfur and nitrogen content, mineral impurities, and maceral constituents. Substantial worldwide attention is being focused on more efficient and cleaner methods for utilization of this important energy resource.

The wide heterogeneity of coal has made it difficult to characterize and to correlate its structure. Evidence exists that differences in geologic histories and maceral constituents affect coal chemistry and technological applications of coals since coals of the same apparent rank but of different geologic history can exhibit a variety of physical and chemical properties. Coal structure varies with recognizable geological and geochemical features that affect its reactivity. The variations in the geochemical history and the complexities of the macromolecular structure of coal further confound a clear understanding of the complex nature and interrelationships among coals.

Our understanding of the details of coal structure has improved markedly over the last decade. Coal is no believed to be a heterogeneous mixture composed of a macromolecular network of varying degrees of cross-linking within the macromolecular phase with a smaller molecular phase imbibed or associated with this network. A major portion of the credit fir the present state of knowledge of the general characteristics of coal structure can be traced to studies that employed NMR spectroscopy. It is interesting to observe that NMR was first applied to the study of coals in 1955, only 9 years following the discovery of the NMR phenomenon in bulk matter. These first NMR observations employed broad line H-1 techniques that, in the absence of means to reduce the large proton dipole-dipole interactions, produced very broad bands in fossil fuel related materials. In 1968 Haeberlen and Waugh demonstrated that the hydrogen dipole-dipole interaction could be significantly reduced by averaging in spin space via a multiple-pulse approach rather than attempting to mechanically spin the sample at very high MAS rates in order to achieve the same results. In the mid-1970's Schnable and Gerstein, et.al. demonstrated that if one simultaneously spins the sample about the magic angle then the chemical shift anisotropy (CSA) as well as the inhomogeneous heteronuclear dipolar interactions are simultaneously averaged. Hence, much narrower peaks are obtained with this experiment which Gerstein named CRAMPS. Gerstein first applied the proton CRAMPS experiment to coals in 1981. The CRAMPS experiments have been especially useful as a probe to study the "mobile phase" present in coal structure wherein solvents such as perdeuterated pyridine are imbibed into the parent coal. The reduction in proton NMR line widths is attributed to motional narrowing and to reduction of bulk and molecular susceptibility anisotropies by partial mobilization of certain structural moieties in coal caused by disruption of hydrogen bonds and other non-covalently bonded structural units.

As early as 1966 the first C NMR spectra appeared of materials derived from coal. Broad line C NMR spectroscopy was first applied to whole coals in 1971 to confirm the high aromaticity of anthracite. Significant improvements in resolution began to emerge 5 years later with the application of the cross-polarization technique followed by applications of magic angle spinning to coal. In 1979 Opella introduced the concept of the dipolar dephasing experiment that discriminates between nonprotonated and protonated carbons by means of the effective C-H dipole-dipole interaction. Alemany studied characteristics of the dipolar dephasing experiment on model compounds and then demonstrated the utility of this experiment in a careful study of an Illinois #6 coal. Work by Wilson, Murphy, and Gerstein have verified the value of dipolar dephasing experiments in the study of coal. The data derived from the techniques described, together with others to be described in subsequent sections, have been extremely valuable in probing the general structural features of coals and have greatly enhanced the usable knowledge base for improved coal utilization.

Magic Angle Turning and Hopping Experiments

Hu, J.Z.; Wang, W. and Pugmire, R.J.
Encyclopedia of NMR, J. Wiley & Sons, Ltd., 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.

It is well known that in a solid, the chemical shift of a nucleus is a function of molecular orientation with respect to the external magnetic field. This phenomenon is described as the chemical shift anisotropy (CSA), and it is directly related to the local electronic structure of the nucleus. The principal values of the CSA as well as their orientation in the molecular frame can be obtained through a single crystal study (see "Chemical Shift Measurement by Single Crystal Techniques"). However, for the majority of compounds, the difficulties associated with growing a single crystal of sufficient size limits the application of current single crystal methods. In a powder sample the orientation information is lost because of the random distribution of the crystallites. However, the principal values, which are very useful in characterizing the structure of a molecule, are still available in the powder pattern obtained from a stationary or slowly spinning sample when the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the spectral separation necessary for individual identification and measurement.

In an effort to address this problem of spectral overlap, many 2-D techniques have been developed to obtain a 2D spectrum with an isotropic shift projection along one dimension and a stationary or slow-spinning-sideband powder pattern along the other (see "Chemical Shift Tensors"). One of the first techniques developed was the magic angle hopping (MAH) experiment of Bax et. al. By successively "hopping" the sample 120° about an axis at the magic angle, an isotropic shift dimension is obtained since the average of the resonance frequencies at the three orthogonal positions is the isotropic shift.

An analog of the MAH experiment employing continuous slow rotation of the sample has recently been demonstrated by Gan. Gan's elegant technique uses pulses spaced at 1/3 of the rotor period to obtain the isotropic shift evolution. We call Gan's experimental technique the Magic Angle Turning (MAT) experiment because of the very slow rotation involved. Significant improvements in the experimental details have been made to optimize these two experiments.

In this article, a simple theory is given to describe the MAH experiment and the most recent version of the MAT experiments together with typical experimental results included to show the basic principals and the power of these two related methods.

Magic Angle Turning Experiments for Measuring Chemical-Shift-Tensor Principal Values in Powdered Solids

Hu, J.Z.; Wang, W.; Liu, F.; Solum, M.S.; Alderman, D.W.; Pugmire, R.J. and Grant, D.M.
J. Magnetic Resonance, 1994 (in press). Funded by ACERC and US Department of Energy/Pittsburgh Energy Technology Center.

The magic-angle-turning (MAT) technique introduced by Gan employs slow (ca. 30 Hz) rotation of a powdered sample at the magic angle, in concert with pulses synchronized at 1/3 of the rotor period, to obtain isotropic-shift information in one dimension of a 2D spectrum. The other dimension displays a slow-spinning-sideband powder pattern that, at the low rotor frequencies employed, resembles the stationary-sample powder pattern. The MAT method is very effective for measuring chemical-shift principal values in compounds where spectral overlap precludes the use of 1D methods. Previous MAT implementations are reviewed, and it is shown how a new phase correct MAT (PHORMAT) pulse sequence overcomes many of their limitations. This new pulse sequence produces a spinning-sideband-free isotropic-shift spectrum directly as a projection onto the evolution axis with no spectral shearing. Only two purging operations are employed, resulting in a higher signal-to-noise ratio. Pure absorption-absorption phased 2D spectra are produced. Flat 2D base planes result from an echo sequence which delays acquisition until after probe ring-down and receiver recovery. The technique used for synchronizing the pulses to 1/3 the rotor period without relying on absolute rotor-frequency stability is described. The PHORMAT spectrum of methyl a-D-glucopyranoside is presented. The data are analyzed with an emphasis on the quantitative accuracy of the experiment in measuring chemical shift tensor principal values and determining the relative number of spins of each type present. The FID data from the spectrometer acquisition are fitted with numerical simulations that employ a banded-matrix method for calculation spinning sideband amplitudes. The chemical shift principal values, measured in methyl a-D-glucopyranoside with the PHORMAT method, are compared with those from a single-crystal determination of the full chemical shift tensors. The two measurements differ by an rms average distance of only 0.57 ppm.

Solid State C-13 NMR, X-Ray and Quantum Mechanical Studies of the Carbon Chemical Shift Tensors of P-Tolyl Ether

Facelli, J.C.; Hu, J.Z.; Orendt, A.M.; Arif, A.M.; Pugmire, R.J. and Grant, D.M.
Journal of Physical Chemistry, 1994 (in press). Funded by US Department of Energy and ACERC.

This paper presents a detailed study of the principal components of the C-13 chemical shift tensors in p-tolyl ether. The tensor components of a relative large number of carbon atoms are measured by using the two-dimensional magic angle turning (MAT) technique that allows for the determination of the principal components of the chemical shift tensors in powders. Theoretical calculations of the C-13 chemical shieldings, using the X-ray molecular geometry, are used to assign the NMR resonances to individual carbon nuclei. The principal values of the chemical shift tensors permit assignments that would be unreliable if only the isotropic shift information is used. The chemical shift tensors of the carbons directly attached to the oxygen atom are very sensitive to the structural and electronic properties of the ether linkage. The combination of the C-13 MAT experiment and theoretical chemical shieldings proves to be important in the study of electronic properties and molecular structure.

Measurement of C-13 Chemical Shift Tensor Principal Values with a Magical Angle Turning Experiment

Hu, J.Z.; Orendt, A.M.; Alderman, D.W.; Pugmire, R.J.; Ye, C. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 5:181, 1994. Funded by US Department of Energy and ACERC.

The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of C-13 chemical shift tensors in powdered solids. A large-volume MAT prove with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively, can enhance measurements of the principal values of chemical shift tensors in complex compounds. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the C-13 principal values in 2,3-dimethlnaphthalene and Pocohontas coal are reported at typical examples.

Chemical Structure Changes of Coal, Char, and Tar During Devolatilization

Fletcher, T.H. and Pugmire, R.J.
Proceedings of the ACS Spring Meeting, San Diego, CA, March 1994, ACS Preprints, Division of Fuel Chemistry, 39:108-112. Funded by ACERC, Brigham Young University and University of Utah.

Enormous progress has been made in coal pyrolysis research during the last decade. Models of coal devolatilization have progressed from simple rate expressions based on total mass release to empirical relationships based on the elemental composition of the parent coal to models that attempt to describe the macromolecular network of the coal. Measurements of particle temperature during devolatilization have eliminated much of the controversy regarding overall rates of devolatilization. In the last several years, advancements in chemical analysis techniques have allowed quantitative investigations of the chemical structure of both coal and its pyrolysis products, including the nature of the resulting char. A prominent research goal is to accurately predict the rates, yields, and products of devolatilization from measurements of the parent coal structure. This goal necessitates modeling the reaction processes on the molecular scale, with activation energies that relate to chemical bond breaking rather than release of products from the coal. C-13 and H-1 NMR spectroscopy have proven particularly useful in obtaining average values of chemical structure features of coal, char and tar. This paper reviews experimental data regarding chemical structure features of coal, char, and tar during rapid devolatilization, and how these data have impacted the development and input parameters for devolatilization models. In particular, the relationship between pyridine extract yields and extract yields predicted purely from NMR chemical structure data discussed.

1993

Coal Characteristics, Structure, and Reaction Rates

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Chapter 3, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

The purposes of this chapter are to document, correlate, synthesize, integrate and relate the structural characterization and reaction rates of the suite of ACERC coals. The focus has been on research projects sponsored by ACERC. However, related research work outside of ACERC has also been considered. This chapter (1) reviews the selection of the suite of research coals, (2) reviews the origin of coal which gives rise to the various structural moieties in coal, (3) reviews coal characterization programs and documents the structure and characteristics of the research coals, (4) reviews coal reaction mechanisms, (5) explores the relationships of coal structure to devolatilization and char oxidation reaction rates, and (6) considers the models being developed which predict reaction characteristics based on structurally dependent parameters. Research programs in the field are still very active, the models are still in the formative states, reaction rates for the selected research coals are being measured, and the reaction processes for these coals have yet to be fully explored using the structurally dependent models. Further results will undoubtedly be forthcoming. This chapter is a condensed version of a larger work to characterize the structure and conversion processes of the research coals.

The Structure and Reaction Processes of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Publishing Corp., The Netherlands, 1993 (in press). Funded by ACERC.

This new ACERC book documents and integrates the current understanding of the organic and inorganic structure of coal and its reaction processes. Work in ACERC forms the foundation while the book attempts to include pertinent worldwide results. The book cites more than eight hundred references, almost all within the past decade, while the large majority are from various researchers around the world.

Eleven U.S. coals of various rank are emphasized in the book. These commonly used and highly characterized eleven coals form the research coals for ACERC and include all eight coals of the Argonne National Laboratory's Premium Coal Sample Bank. Altogether, the book contains six chapters. After an introduction, Chapter 2 documents the selection and characteristics of the suite of eleven coals, and relates them to various national databases. Chapter 3 deals with the geochemical history of coal and its macromolecular structure. Chapter 4 describes advanced analytical methods for measuring organic and inorganic structure of coal and documents results for the eleven coals. Chapters 5 and 6 treat the reaction processes of coals and chars. Recent model developments that relate fuel structure to yields and reaction rates are presented and compared to rate and yield data. Important measurements from the coal suite and other coals are reported and related to coal structure.

Laying a foundation for the future, this book has been written at a time when progress in this area is dramatic. The authors acknowledge that new results will be published at a rapid rate. What we have sought to accomplish through the writing of this manuscript is to promote increasing cooperative focus regarding the understanding of coal structure and its reaction and conversion processes. From this perspective, the book is thought to be the first of its kind.

The Structure and Reaction Processes of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1993 (in press). Funded by ACERC.

This book characterizes the properties and reaction rates of the eleven US coals selected for emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.

An Overview of ACERC Reserach in Fuel Characterization and Reaction Mechanisms

Pugmire, R.J. and Fletcher, T.H.
Energy & Fuels, 7 (6):700-703, 1993. Funded by ACERC.

A major objective of the Advanced Combustion Engineering Research Center is the development and verification of data on fuel characterization and reaction mechanisms and rates that can be incorporated into submodels for use in the comprehensive combustion codes. As Technology has advanced, the levels of analytical sophistication has also advanced, making it possible to augment the existing body of information with new data. From this new data it is possible to draw new insights regarding the complex nature of coal and the various processes associated with combustion. The ACERC program has made it possible to bring different disciplines together to work on an integrated research program that is targeted at a few key strategic issues. The overall program has been divided into six areas of research, designated as Thrust Areas. The principal areas of focus in Thrust Area 1 have been in delineation of coal structure and those key factors that are important in developing fundamental knowledge of devolatilization and char oxidation processes. This article discusses the objectives, accomplishments, and plans of ACERC-sponsored research in these areas.

Improvements to the Magic Angle Hopping Experiment

Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Solid State Nuclear Magnetic Resonance, 2:235-243, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.

Several improvements to the magic angle hopping experiment first introduced by Bax et al. (J. Magn. Reason., 52 (1983) 147 are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120º sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the C-13 chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.

Chemical Structural Features of Pyridine Extracts and Residues of the Argonne Premium Coals Using Solid State C-13 NMR Spectroscopy

Fletcher, T.H.; Bia, S.; Pugmire, R.J.; Solum, M.S.; Woods, S. and Grant, D.M.
Energy & Fuels, 7 (6):734-742, 1993. (Presented at the Spring Meeting of the Western States Section of the Combustion Institute, Salt Lake City, UT, March 1993.) Funded by ACERC.

Soxhlet extractions were performed on the eight Argonne Premium coals using pyridine purged with argon and followed by a novel washing procedure to remove the pyridine. Mass closure (extracts plus residues) on duplicate experiments accounted for 94-96% of the original coal, repeatable to within 2%. Chemical structural features determined from C-13 NMR analyses of the extracts and residues showed more attachments per aromatic cluster for the residues, indicating a greater degree of covalent bonding in the residue than in the extract. H-1 NMR analysis of the extracts showed a gradual increase in the hydrogen aromaticity with rank, along with a maximum in the percentage of a-hydrogen in the high-volatile bituminous coals. Composite chemical features constructed from weighted averages of the features of the residues and extracts agree with many of the features of the parent coal. Chemical structural features of the extracts determined from H-1 NMR analyses agree with similar data reported previously for early coal tars during devolatilization at rapid heating rates.

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
Fuel, 72:587-597, 1993. Funded, in part, by ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanisms in coal pyrolysis. However, recent years have seen a number of new experimental and theoretical approaches that shed new light on the subject. This paper considers the recent progress on kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the 'two-component model of coal structure.' Recent experiments that measured coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. These rates also agree with those derived from experiments at low heating rates. In tar formation and transport, a consensus is being reached on the central role of the volatility of tar molecules in explaining the variation with operating conditions (pressure, heating rate, particle size, etc.) of the amounts and molecular weight distribution of tars. Progress in the quantitative prediction of tar and char yields is being made through recently developed models for the fragmentation of the macromolecular coal network. These models, which provide quantitative descriptions of the relations between the chemical structure of the coal and the physical and chemical properties of the pyrolysis products (gas, tar, soot, and char), are an exciting advance in the understanding of the pyrolysis process. Such models are linking the occurrence of the plastic phrase with the 'liquid' fragments formed during pyrolysis. On the subject of retrogressive cross-linking reactions, both solvent swelling and NMR measurements confirm important rank-dependent differences in reaction rates: these appear to be related to the oxygen functionalities. Reasonable agreement is also seen for variations with coal rank of kinetics rates derived from measurements at low heating rates. Experiments suggest that the recently revived 'two-component' hypothesis of coal structure has application to low-rank coals which are mixtures of two distinct components: polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. Although experiments can distinguish loosely and tightly bound fractions these fractions appear to consist of similar materials and are differentiated primarily in their molecular weight and degree of connection to the network. These coals appear to behave in a manner that is described by the network decomposition models.

Measurement of C-13 Chemical-Shift Anisotropy in Coal

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Carbonaceous Solids, Advances in Chemistry Series #229, 22:419-439, 1993. Funded by US Department of Energy, Basic Energy Services and ACERC.

The methods of available in NMR spectroscopy to obtain the principal values of the chemical-shift tensor are discussed. Applications to coal and to compounds with model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical-shift powder patterns is compared to results obtained by cross-polarization with magic-angle spinning and dipolar dephasing.

An Isotropic Chemical Shift-Chemical Shift Anisotropy Magic-Angle Slow-Spinning 2-D NMR Experiment

Hu, J.Z.; Alderman, D.W.; Ye, C.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 31: 472, 1993. Funded by US Department of Energy and Pittsburgh Energy Technology Center.

High-speed magic-angle spinning has become a ubiquitous method for obtaining high-resolution spectra in polycrystalline and amorphous solids. MAS reduces a chemical-shift-anisotropy powder pattern to a single line at the isotropic shift when the sample spinning speed is larger than the anisotropy. While the isotropic chemical shift is useful in characterizing chemical structure, the three principal values of the tensor are even more valuable. These principal values are available in the powder pattern obtained from a stationary or slowly spinning sample provided the molecule has few enough unique nuclei that the spectrum can be interpreted. Unfortunately, overlap of several broad powder patterns often prevents the separation necessary for their individual identification and measurement.

Recognizing that an equivalent of the hopping experiment could be achieved without stopping the sample, Gan has recently demonstrated an elegant technique that employs pulses spaced at one-third of the rotor period to produce 2D spectra with an isotropic shift projection along the evolution dimension and an undistorted slow-spinning-sideband powder pattern in the acquisition dimension. The pulse sequence produces an isotropic shift dimension completely free of sidebands even in the slow-spinning regime. Because it requires only a slow continuous rotation of the sample, Gan's experiment is much easier to perform than the hopping experiment. However, in the process of projecting the magnetization onto the longitudinal axis twice during the rotor period, only one-fourth of the magnetization is retained and the sensitivity of the experiment is reduced accordingly. Described here is a technique which uses the same slow rotation of the sample about an axis at the magic angle, but instead applies five pi (180º) pulses to the magnetization Precessing in the transverse plane. The pulse sequence produces a result similar to that of Gan's experiment without the sacrifice of any magnetization to projections out of the transverse plane. This "5p" pulse method can be viewed simply as a constant-time version of Gan's experiment, with resultant advantages and disadvantages to be discussed.

Chemical Structural Features of Coal Chars, Tars, and Char Extracts During Rapid Pyrolysis Using C-13 and H-1 NMR Spectroscopy

Fletcher, T.H.; Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Bai, S.; Ma, J. and Woods, S.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by ACERC.

Structural characteristics have been determined for parent coals and for chars collected at different stages of pyrolysis. Recent work has focused on trying to understand the relationship between chemical structural features of the unreacted coal and the devolatilization and char oxidation phenomena. Models of coal devolatilization have recently related devolatilization behavior to the structure of the parent coal and the initial amount of pyridine extracts. Fong et al. used pyridine extraction methods to quantify the amount of metaplast formed during pyrolysis of a Pittsburgh #8 coal. These experiments demonstrated that under moderate heating conditions (~500 K/s to 873 K), as much as 80% of the initial coal was transformed into a combination of extractable material and volatiles. The work presented here is an examination of the pyridine extraction procedure of the Argonne Premium coal samples and the detailed study of the carbon skeletal structure of the extracts and the extraction residues from these coals. This is the first stage of an experimental program to examine the yield and chemical features of extracts of coal chars collected as a function of time during pyrolysis.

The Measurement of C-13 Chemical Shift Tensors in Complex Polycyclic Aromatic Compounds and Coals by an Extremely Slow Spinning MAS Experiment

Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
7th International Conference on Coal Science, Banff, Alberta, Canada, September 1993 (in press). Funded by US Department of Energy and Pittsburgh Energy Technology Center.

The chemical shift of a C-13 spin in a solid sample varies with the change of the relative orientation of the nuclei (or molecule) to the external magnetic field. This orientational dependence produces the well-known chemical shift anisotropy (CSA). The tensor values of the CSA provides a wealth of information about subtle differences in the electronic environment of the nuclei, such as the type of bond, the effects of electron devolatilization and the bond conformation as well as the dynamics of the nuclei. The principal values of the CSA can be obtained in a straightforward way when the molecule has only a few, e.g., 2-3 unique nuclei. In most substances, unfortunately, the overlap of the several broad powder patterns prevents the spectral separation necessary for individual carbon resolution and identification.

The Use of Solid State C-13 NMR Spectroscopy to Study Pyridine Extracted and Extraction Residues in the Argonne Premium Coals

Pugmire, R.J.; Solum, M.S.; Bai, S.; Fletcher, T.H.; Woods, S. and Grant, D.M.
Proceedings of the 205th ACS National Meeting, 38, no. 2, Denver, CO, March 1993. Funded by ACERC.

The relationship between coal structure and combustion behavior is a matter of on-going research in our laboratories. A great deal of effort has gone into obtaining data that is used for modeling studies of devolatilization behavior. We have also carefully studied the process of char formation. Our past work has focused on trying to understand the relationship between coal/char/tar formation as they relate to the devolatilization and char oxidation phenomena. The formation of metaplast during pyrolysis was studied by Fong and Howard in terms of extractable material obtained at different stages of the devolatilization process. We have recently turned our attention to metaplast formation in devolatilization and plan to conduct a series of experiments that will help define the formation and chemical structure of metaplast in coals of different rank.

The Magic Angle Turning Experiment. A New NMR Tool for Studying Coal Structure

Pugmire, R.J.; Hu, J.Z.; Alderman, D.W.; Orendt, A.M.; Ye, C. and Grant, D.M.
10th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 1993. Funded by US Department of Energy and ACERC.

The C-13 CP/MAS experiment has proven to be a powerful technique for obtaining high resolution spectra in complex solids such as coal. MAS narrows the chemical shift anistropy (CSA) to its isotropic shift when the sample spinning speed is greater than the anistropy. While the isotropic chemical shift is useful in characterizing chemical structure, the principal values of the chemical shift tensor provide even more information. These principal values are available from the powder pattern obtained from a stationary or slowly spinning sample. Unfortunately, the overlap of many broad powder patterns in a complex solid often prevents the measurement of the individual principal values.

Coal and Char Structural Parameters Derived from Solid State C-13 NMR Studies

Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
The 5th Australian Coal Science Conference, Melbourne, Australia, March 1993. (Also presented at the 5th Australian Coal Science Conference, Melbourne, Australia, November, 1992.) Funded by ACERC.

In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.

1992

C-13 NMR Techniques for Structural Studies of Coals and Coal Chars

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Chapter 10, Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1992. Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.

Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
Fuel, 1992 (in press). Funded by US Department of Energy and ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanism in coal pyrolysis. The last several years have, however, provided a number of new experimental and theoretical approaches that shed new light on the subject. This paper will consider the recent progress on the topics of: kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the "two-component model of coal structure." In kinetics, recent experiments that measure coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. The rates also agree with those derived from low heating rate experiments. In tar formation and transport, a consensus is being reached on the central role of the tar molecule's volatility in explaining the variation with operating parameters (pressure, heating rate, particle size, etc.) in the tar's amount and molecular weight distribution. Progress in the quantitative prediction of tar and char is being made by recently developed models for the fragmentation of the macromolecular network. These models, which provide quantitative description of the relationship between the chemical structure of the coal and the physical and chemical properties of the resultant pyrolysis products (gas, tar, soot, and char), are an exciting advancement in the understanding of the pyrolysis process. Such models are linking the occurrence of the coal's plastic phase with the "liquid" fragments formed during pyrolysis. On the subject of retrogressive crosslinking reactions, both solvent swelling and NMR measurements confirm important rank dependent differences in reaction rates. These appear related to the oxygen functionalities. Reasonable agreement is also seen for rank variations of kinetics rates derived from low heating rate experiments. Experiments suggest that the recently revived "two-component hypothesis" of coal structure has application to low rank coals that are a mix of polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. These coals appear to behave in a manner that is described by the network decomposition models. The presentation will provide a brief report on these topics.

Chemical Structure of Char in the Transition from Devolatilization to Combustion

Fletcher, T.H.; Solum, M.S.; Grant, D.M. and Pugmire, R.J.
Energy & Fuels, 6(5):643-650, 1992. [Also included in the ACS Division of Fuel Chemistry Preprints, 37(2):677-684, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by Sandia National Laboratories, Brigham Young University and ACERC.

Coal devolatilization experiments are generally conducted separately from char oxidation experiments, and the relationship between the chars generated in the two types of research is often ignored. However, char is one of the most important products of coal devolatilization and must be characterized as a function of temperature and heating rate in a manner similar to that for gaseous behavior. In this work, the chemical structures of chars from five coals of different rank are examined, and implications on char reactivity are discussed. Chars were obtained as a function of residence time in a devolatilization experiment (1250 K, 2 x 104 J/s in nitrogen) and just subsequent to devolatilization in a laminar flame-fired experiment (1500 K, 5 x 104 K/s). Quantitative measurements of chemical structure were performed on the coals and chars using C-13 nuclear magnetic resonance (NMR). Results show that the chemical structures of fully devolatilized chars are very similar, even though a wide diversity is seen in the parent coal structures. For example, the average cluster molecular weights of the chars span a range of only 50 amu, and the side-chain molecular weights of the chars span a range of only 6 amu. The similarity in chemical structure of fully devolatilized coal chars suggests that differences in measured heterogeneous char reactivities are caused by differences in the physical structure of the char.

Spectroscopic Studies of Coal Maceral Depolymerization Effected by an Iron-Based Catalyst

Wang, H.P.; Lo, R.; Sommerfeld, D.A.; Huai, H.; Pugmire, R.J.; Shabtai, J.S. and Eyring, E.M.
Fuel, 71(7):723-729, 1992. Funded by US Department of Energy.

Demineralized Hiawatha (Utah) coal was divided into narrow density fractions of resinite and vitrinite macerals using a density gradient centrifugation technique. The distribution of an iron-based catalyst in the coal maceral matrix was studied using an electron probe microanalyser. The data indicate that the iron catalyst is evenly dispersed inside the vitrinite. This uniform dispersion is preserved upon mild (275ºC) hydrotreatment and attendant partial depolymerization of the vitrinite maceral. However, the iron catalyst does not completely penetrate into the resinite maceral matrix due to a lack of microporosity. The small amount of dispersed iron catalyst in the resinite causes some depolymerization of maceral components, e.g. dimers, trimers, or higher polymers, into monomers. Due to the insufficient dispersion of the iron catalyst in the resinite a competing thermal breakdown of the maceral occurs. This apparently involved a partial dehydrogenation reaction that may be favored during the hydrotreatment of resinite.

Selective Saturation and Inversion of Multiple Resonances in High-Resolution Solid-State C-13 Experiments Using Slow Spinning CP/MAS and Tailored Dante Pulse Sequences

Hu, J.Z.; Pugmire, R.J.; Orendt, A.M.; Grant, D.M. and Ye, C.
Solid State Nuclear Magnetic Resonance, 1:185-195, 1992. [Also included in ACS Division of Fuel Chemistry Preprints, 37(2):646-659, 1992 (203rd ACS National Meeting, San Francisco, CA, April 1992)]. Funded by US Department of Energy and Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction.

Taking advantage of the long C-13 T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in C-13 CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90% DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of his method, including the simplification of the measurement of the principal values of the C-13 chemical shift tensor under slow MAD conditions are described. The determination of the aromaticity of coal using a relatively slow MAS spinning rate is also described.

High-Resolution Chromatographic Characterization of Depolymerized Coals of Different Rank: Aliphatic and Aromatic Hydrocarbons

Carlson, R.E.; Critchfield, S.; Vorkink, W.P.; Dong, J.-Z.; Pugmire, R.J.; Bartle, K.D.; Lee, M.L.; Zhang, Y. and Shabtai, J.S.
Fuel, 71(1):1-29, 1992. Funded by US Department of Energy, Gas Research Institute and ACERC.

A selective, low temperature depolymerization procedure has been applied to four Argonne coals of different rank to produce products that are representative of the original coal macromolecular structure, and that are amenable to chromatographic analysis. The products of this depolymerization procedure retained most of the original aromatic and functional group structures of the original coals. A comparison of liquid C-13 NMR spectra of the products and solid-state C-13 NMR spectra of the original coals showed only minor changes in the aromaticities of two of the coals, and some loss of the carbonyl carbons in all of the coals.

Tetrahydrofuran pre-extracts of the four coals and their depolymerized products were separated into chemical classes by adsorption chromatography. Two of these fractions, which contained aliphatic hydrocarbons and polycyclic aromatic hydrocarbons were analyzed using gas chromatography/mass spectrometry. Structural identifications were based on a combination of chromatographic retention and mass spectral fragmentation data. For the lower rank coals, the compositions of the pre-extracts were quite different from the corresponding depolymerized products, and they contained an abundance of molecular biological markers. The compositions of the pre-extracts became more similar to the depolymerized products as rank increased.

The Revised Structure of Bistramide A (Bistratene A): Application of a New Program for the Automated Analysis of 2D INADEQUATE Spectra

Foster, M.P.; Mayne, C.L.; Dunkel, R.; Pugmire, R.J.; Grant, D.M.; Kornprobst, J.-M.; Verbist, J.-F.; Biard, J.-F. and Ireland, C.M.
Journal of the American Chemistry Society, 114(3):1110-1111, 1992. Funded by National Institutes of Health and US Department of Energy.

The two-dimensional structure of bistramide A (a.k.a. bistratene A) has been revised. The bis-amide polyether was isolated from a Lissoclinum sp. ascidian collected in the Fiji islands. The structure was established by a combination of modern NMR methods including 2D INADEQUATE, PS-DQF-COSY, HMQC and HMBC experiments. Critical to establishing the carbon-carbon backbone was CCBond, a new program for automated analysis of 2D INADEQUATE spectra. Bistramide A demonstrated in vitro cytotoxicity towards human colon tumor HCT-116 and murine leukemia L1210 cell lines with an IC50 of 0.1 µg/mL.

Carbon-13 Chemical Shift Tensors in Aromatic Carbons. 4. Substituted Naphthalenes

Orendt, A.M.; Sethi, N.K.; Facelli, J.C.; Horton, W.J.; Pugmire, R.J. and Grant, D.M.
Journal of the American Chemistry Society, 114(8):2832-2836, 1992. Funded by US Department of Energy and ACERC.

The principal values of the C-13 chemical shift tensor were measured for the aromatic carbons of 1,4,5,8-tetramethylnaphthalene, 1,2,3,6,7,8-hexadropyrene, and pyracene using the variable angle spinning technique. Ab initio calculations of the complete shielding tensors are also reported and provided the orientation of the principal values in the molecular frame. Good agreement between theory and experiment is obtained. The chemical shift principal values and orientations of these substituted naphthalenes correspond to those found in naphthalene; the observed differences can be rationalized in terms of the strain introduced by the alkyl substituent. In the naphthalenic derivatives studied, the bridgehead carbons exhibit relatively extensive pi-electron delocalization not found in some bridgeheads in more highly condensed aromatic systems.

An Efficient Large Sample Volume System for Solid State NMR

Pugmire, R.J.; Jiang, Y.J.; Solum, M.S. and Grant, D.M.
Bulletin of Magnetic Resonance, 14:144, 1992. (Presented at ISMAR 92, Vancouver, B.C., July 1992.) Funded by ACERC.

An efficient large sample volume system has been developed to carry out MAS solid-state NMR experiments. The system components are primarily zirconia and macor and no background C-13 is observed. The stator design employs separate air bearing and drive systems and is run using dry air at a bearing pressure of about 32 psi. the rotor can be spun in a stable manner from less than one hundred Hz (with a driving pressure of 5 psi) to 4.3 KHz (24 psi driving pressure). This low gas pressure feature makes the system easy to operate. The volume of the rotor is 1.8 cm³ and it can hold 1.1 g of HMB. The S/N ratio obtained is a factor of 4.6 better than the rotor previously designed and used in our laboratory (volume 0.6 cm³ : 0.28g HMB). This increased sample size allows us to obtain the same S/N ratio in a MAS spectrum with a factor of 21 saving in spectrometer time. The time saving achieved with this rotor system is extremely useful in obtaining data on biological samples and polymers, and is especially useful when experiments on fossil fuels require the use of the Bloch delay technique. Examples of relevant applications will be discussed.

Improvements in the Computerized Analysis of 2D INADEQUATE Spectra

Dunkel, R.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
Analytical Chemistry, 64 (24):3133-3149, 1992. Funded by US Department of Energy, Pittsburgh Energy Technology Center and ACERC.

The carbon skeleton of a molecule can be determined by using the powerful 2D INADEQUATE experiment, but the method suffers from very poor sensitivity at natural carbon-13 abundance. A computer program, described previously, has been significantly improved in its ability to recognize AB spectral patterns corresponding to carbon-carbon bonds which makes it possible to evaluate reliably spectra with rms S/N ratio as low as 2.5, i.e., nearly 1 order of magnitude below the level required for routine manual interpretation. Application of the INADEQUATE experiment to samples containing as little as 20 µmol of a compound of interest is now possible. The method is described in detail and critically evaluated by means of examples and simulations.

Applications of the Improved Computerized Analysis of 2D INADEQUATE Spectra

Dunkel, R.; Mayne, C.L.; Foster, M.P.; Ireland, C.M.; Li, D.; Owen, N.L.; Pugmire, R.J. and Grant, D.M.
Analytical Chemistry, 64:3150-3160, 1992. Funded by US Department of Energy, Pittsburgh Energy Technology Center and ACERC.

This paper illustrates the use of the program CCBond to determine the carbon skeletons of bioorganic molecules in low concentration samples. Discussed is the structure elucidation of bistramide A, a compound extracted from a Fijian Lissocllnum sp. and cholesterol in 71- and 20-µmol samples, respectively. The detection limit of the automated bond extraction is shown to be improved compared to the manual interpretation of 2D INADEQUATE spectra.

Coal and Char Structural Parameters Derived from Solid-State C-13 NMR Studies

Pugmire, R.J.; Solum, M.S.; Fletcher, T.H. and Grant, D.M.
Fifth Australian Coal Science Conference, Melbourne, Australia, November 1992. Funded by ACERC.

In contrast to previous efforts where coal-general devolatilization model input parameters describing chemical structure are adjusted to force agreement between predicted and measured tar and total volatiles yields, coal-dependent chemical structure coefficients for the Chemical Percolation Devolatilization (CPD) model developed by the authors and others are taken directly from C-13 NMR analyses of parent coals. This procedure, outlined in the paper eliminates most adjustable parameters from the model, and predictions of tar and total volatiles yields become true tests of the model and the NMR data, rather than mere results of curve fitting. Resulting model predictions of tar and total volatiles yields as a function of coal type, temperature, heating rate, and pressure comparable with available experimental data, showing the value of both the model and the NMR chemical structure data.

The Use of C-13 NMR Structural Information for Predicting Coal Devolatilization Behavior

Pugmire, R.J.; Solum, M.S.; Grant, D.M. and Fletcher, T.H.
Second International Conference on Coal Structure, Krakow, Poland, September 1992. Funded by ACERC.

Recent coal devolatilization models have attempted to incorporate coal structural information to predict product yields and composition. The chemical structure of the parent coal directly affects devolatilization behavior. The parent coal structure also has implications for pyrolysis char structure and reactivity. C-13 NMR data have been used to obtain coal-dependent chemical structural data that is used as input parameters to the chemical percolation devolatilization (CPD) model. The CPD model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Data will be presented on 20 different coals and the value of NMR data in predicting gas and tar yields will be discussed. The use of C-13 NMR data will also be used to describe the relationship between chemical structure features of the parent coals and the resultant pyrolysis chars.

1991

C-13 NMR Techniques for Structural Studies of Coals and Coal Chars

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, (H.L.C. Meuzelaar, ed.), Plenum Publishing Corp., New York, 1991 (in press). Funded by Pittsburgh Energy Technology Center/Consortium for Fossil Fuel Liquefaction, US Department of Energy and ACERC.

Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
Fuel, 1991 (in press). Funded by US Department of Energy and ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanism in coal pyrolysis. The last several years have, however, provided a number of new experimental and theoretical approaches that shed new light on the subject. This paper will consider the recent progress on the topics of: kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the "two-component model of coal structure." In kinetics, recent experiments that measure coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. The rates also agree with those derived from low heating rate experiments. In tar formation and transport, a consensus is being reached on the central role of the tar molecule's volatility in explaining the variation with operating parameters (pressure, heating rate, particle size, etc.) in the tar's amount and molecular weight distribution. Progress in the quantitative prediction of tar and char is being made by recently developed models for the fragmentation of the macromolecular network. These models, which provide quantitative description of the relationship between the chemical structure of the coal and the physical and chemical properties of the resultant pyrolysis products (gas, tar, soot, and char), are an exciting advancement in the understanding of the pyrolysis process. Such models are linking the occurrence of the coal's plastic phase with the "liquid" fragments formed during pyrolysis. On the subject of retrogressive crosslinking reactions, both solvent swelling and NMR measurements confirm important rank dependent differences in reaction rates. These appear related to the oxygen functionalities. Reasonable agreement is also seen for rank variations of kinetics rates derived from low heating rate experiments. Experiments suggest that the recently revived "two-component hypothesis" of coal structure has application to low rank coals that are a mix of polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. These coals appear to behave in a manner that is described by the network decomposition models. The presentation will provide a brief report on these topics.

The Revised Structure of Bistramide A (Bistratene A): Application of a New Program for the Automated Analysis of 2D INADEQUATE Spectra

Foster, M.P.; Mayne, C.L.; Dunkel, R.; Pugmire, R.J.; Grant, D.M.; Kornprobst, J.-M.; Verbist, J.-F.; Biard, J.-F. and Ireland, C.M.
Journal of the American Chemistry Society, 1991 (in press). Funded by National Institutes of Health and US Department of Energy.

The two-dimensional structure of bistramide A (a.k.a. bistratene A) has been revised. The bis-amide polyether was isolated from a Lissoclinum sp. ascidian collected in the Fiji islands. The structure was established by a combination of modern NMR methods including 2D INADEQUATE, PS-DQF-COSY, HMQC and HMBC experiments. Critical to establishing the carbon-carbon backbone was CCBond, a new program for automated analysis of 2D INADEQUATE spectra. Bistramide A demonstrated in vitro cytotoxicity towards human colon tumor HCT-116 and murine leukemia L1210 cell lines with an IC50 of 0.1 µg/mL.

Carbon-13 Chemical Shift Tensors in Aromatic Carbons. 4. Substituted Naphthalenes

Orendt, A.M.; Sethi, N.K.; Facelli, J.C.; Horton, W.J.; Pugmire, R.J. and Grant, D.M.
Journal of the American Chemistry Society, 1991 (in press). Funded by US Department of Energy and ACERC.

The principal values of the C-13 chemical shift tensor were measured for the aromatic carbons of 1,4,5,8-tetramethylnaphthalene, 1,2,3,6,7,8-hexadropyrene, and pyracene using the variable angle spinning technique. Ab initio calculations of the complete shielding tensors are also reported and provided the orientation of the principal values in the molecular frame. Good agreement between theory and experiment is obtained. The chemical shift principal values and orientations of these substituted naphthalenes correspond to those found in naphthalene; the observed differences can be rationalized in terms of the strain introduced by the alkyl substituent. In the naphthalenic derivatives studied, the bridgehead carbons exhibit relatively extensive pi-electron delocalization not found in some bridgeheads in more highly condensed aromatic systems.

Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 70:414-423, 1991. Funded by Pittsburgh Energy Technology Center and ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.

The Chemical Structure and Petrology of Resinite from the Hiawatha "B" Coal Seam

Crelling, J.C.; Pugmire, R.J.; Meuzelaar, H.L.C.; McClennen, W.H.; Huai, H. and Karas, J.
Energy & Fuels, 5:688-694, 1991. Funded by US Department of Energy and ACERC.

The objective of the present study is to examine the chemical structure and composition of Utah Wasatch Plateau coal resinite. Macerals were separated from the coal matrix by hand picking, sink-float treatments, and/or density gradient centrifugation (DGC). DGC separation was found to produce highly purified resinite fractions. Resinite-rich Wattis Seam coal samples were collected from fresh mine faces and, after varying degrees of concentration, subjected to C-13 magic angle spinning NMR, Curie-point pyrolysis MS, and Curie-point pyrolysis GC/MS in addition to petrographic analysis and fluorescence microscopy as well as conventional (e.g., ultimate analysis) characterization methods. The data obtained confirm recent findings indicating that the abundant blue/green fluorescing resinite component is a polymeric substance composed of sesquiterpenoid repeat units with a median size of 204 Da, corresponding to the empirical formula C15H24. The monomeric sesquiterpenoid units obtained during pyrolysis appear to represent different degrees of unsaturatization ranging from C15H26 sesquiterpenes to the aromatic C15H18 cadalene. Clearly, sesquiterpenoids constitute the bulk of extractable resinite in Wasatch Plateau field coal and are likely to be important precursors of the abundant extractable alkynaphthalene moieties in such coal.

Predicting Vapor Pressures of Tar and Metaplast During Coal Pyrolysis

Fletcher, T.H.; Grant, D.M. and Pugmire, R.J.
ACS Division of Fuel Chem. Preprints, 36(1):250-257, 1991 (201st ACS National Meeting, Atlanta, GA, April 1991). Funded by Pittsburgh Energy Technology Center and ACERC.

Models of coal pyrolysis have progressed from simple one or two step empirical Arrhenius expressions that correlate total mass release during devolatilization, as reviewed by Anthony and Howard, to detailed descriptions of hydrocarbon chemistry and mass transport. These models describe the yields and compositions of pyrolysis products from coal under a wide range of heating conditions and ambient pressures. During pyrolysis of softening coals, a liquid phase appears that is referred to as metaplast. Release of pyrolysis gases and tar vapors inside the particle cause bubble formation in the softened coal particle, followed by swelling (increase in the particle diameter) with large internal voids (cenosphere formation). The softened state is followed by crosslinking or repolymerization which solidify the char matrix. As the coal particle is heated to sufficiently high temperatures, the light species in the metaplast are released as hydrocarbon vapors, along with light gases. Coal tar is generally defined to consist of those species which are released from the coal during pyrolysis which condense at room temperature and pressure. Low rank coals and lignite generally give low tar yields, and do not exhibit much softening or swelling behavior; this non-softening behavior may be caused by early crosslinking reactions. High rank coals (i.e., anthracites and low volatile bituminous coals) contain low amounts of volatile matter, and hence coal particles remain relatively intact during pyrolysis unless fragmentation occurs.

Mass transport affects coal pyrolysis in two ways: (1) as the ambient pressure increases, the tar yield decreases, and (2) as particle size increases, the tar yield decreases. However, there seem to be regions where the two mass transport effects are not controlling. For instance, in vacuum, the small pressure generated inside the pyrolyzing coal particle from the release of light gases and tar vapors may control the process. Also, total volatiles yields from a lignite were observed to remain constant with increasing ambient pressure, although this is probably due to the low tar yield of the lignite. Changes in coal pyrolysis yields as a function of particle size for diameters less than 200 µm are small.

Comparison of C-13 Chemical Shielding Anisotropy in Model Compounds and Coals with Theoretical Values

Pugmire, R.J.; Orendt, A.M.; Facelli, J.C. and Grant, D.M.
International Conference on Coal Science, University of Newcastle-upon-Tyne, U.K., 1991. Funded by US Department of Energy and ACERC.

The measurement of C-13 chemical shift tensor components in organic compounds has been shown to be valuable in gaining insight into the relationship between structure and the individual shift components, in a manner similar to earlier work on correlations between structural features and average or solution chemical shifts. In this work, the tensor components are measured for the aromatic carbons in several polycyclic aromatic hydrocarbons (PAHs); the compounds were chosen due to their incorporation of structural features that are believed to be important in the aromatic clusters in coals. The aromatic carbons in these compounds, as in coals can be divided into four groups: protonated, alkyl substituted, oxygen substituted, and bridgehead or condensed.

While there have been many solid state C-13 NMR measurements on both coals and PAHs using the anisotropy averaging technique of magic angle spinning (MAS), which measured only the average or isotropic chemical shift values, only a limited amount of data exists on the tensor components on either group of samples. For aromatic carbons, the majority of available data is on substituted benzenes; the only polycyclic aromatic compounds for which tensorial data exists in the literature are naphthalene (I), pyrene (V), coronene, and most recently buckminsterfullerene (C60). Therefore, before any correlations between the tensorial components and structure can be explored, much more data on this class of compounds must be obtained. Recently, the shielding data on 1,4,5,8-tetramethynaphthalene (II), 1,2,3,6,7,8-hexahydropyrene (III), pyracene (IV) (the alpha protonated carbons only), and triphenylene (VII) have been obtained on our laboratory, and the results are discussed below.

The chemical shielding tensor is a measure of the effect that the electronic environment has on changing the strength of the magnetic field experienced at the nucleus. Depending on the orientation a given molecule has with respect to the applied magnetic field, the observed resonance frequency changes. Therefore, the chemical shielding tensor is a reflection of the three dimensional electronic environment of the nucleus and, as such, it contains much potential information about structure and bonding. If one studies a powder sample, (where all possible orientations of the molecule can exist), a broad pattern is obtained from which the three principal values of the shielding tensor can be extracted. Each of the four classes of aromatic carbons has distinctive lineshape reflective of the differences in their electronic surroundings.

Experimentally, there are a number of methods that can be applied to obtain these tensor components. For the results presented below, measurement was either made on a static sample or the technique of variable angle sample spinning (VASS) was used. In the case of phenanthrene, isotopic C-13 labeling was used in the positions 1, 4, and 9, in order to obtain the tensor data on the three protonated carbons with very similar shielding tensors. Quantum mechanical calculations of the complete shielding tensor were also completed on these and other aromatic molecules of interest. These calculations are essential in providing the orientations of the tensor components in the molecular frame, information not obtained experimentally. Past experience has indicated that the calculated orientations are quite reliable (e.g. within ±7 º) in the cases where orientations have been determined experimentally either by single crystal or dipolar spectroscopy.

Network Changes During Coal Pyrolysis: Experiment and Theory

Solomon, P.R.; Charpenay, S.; Yu, Z.-Z.; Serio, M.A.; Kroo, E.; Solum, M.S. and Pugmire, R.J.
8th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, October 1991. Funded by US Department of Energy and ACERC.

Coal pyrolysis is a complicated combination of chemical and physical processes in which coal is transformed at elevated temperatures to produce gases, tar, and char. These processes are described in the Functional Group - Depolymerization, Vaporization, and Crosslinking (FG-DVC) model of coal pyrolysis. An important aspect of this model is that crosslinking is rank dependent. This is based on solvent swelling experiments on chars made from coals of different rank. Low rank coals start to loose their solvent swelling ability prior to significant depolymerization at temperatures as low as 200ºC. Including such crosslinking in the FG-DVC model leads to predictions for low rank coals of a highly crosslinked network (exhibited by low solubility and low fluidity in chars) and low tar amounts.

While the model is in good agreement with a variety of data, it is difficult to find experiments to validate the predicted behavior of the network. In this paper we have used CP-MAS, NMR with dipolar dephasing and other techniques to examine the chars changing functional group and network characteristics. The changes in the char composition have been modeled using the FG-DVC model and the results compared with the data for Pittsburgh Seam bituminous coal and Zap lignite.

Progress in Coal Pyrolysis

Solomon, P.R.; Fletcher, T.H. and Pugmire, R.J.
8th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, October 1991. Funded by US Department of Energy and ACERC.

The heterogeneous nature of coal and the complexity of the pyrolysis process has made it very difficult to perform unambiguous experiments to determine the rates and mechanism in coal pyrolysis. The last several years have, however, provided a number of new experimental and theoretical approaches that shed new light on the subject. This paper will consider the recent progress on the topics of: kinetics, the formation of volatile products, network models, crosslinking, rank effects, and the "two-component model of coal structure." In kinetics, recent experiments that measure coal particle temperatures at high heating rates provide reasonable agreement on kinetic rate constants. The rates also agree with those derived from low heating rate experiments. In tar formation and transport, a consensus is being reached on the central role of the tar molecule's volatility in explaining the variation with operating parameters (pressure, heating rate, particle size, etc.) in the tar's amount and molecular weight distribution. Progress in the quantitative prediction of tar and char is being made by recently developed models for the fragmentation of the macromolecular network. These models, which provide quantitative description of the relationship between the chemical structure of the coal and the physical and chemical properties of the resultant pyrolysis products (gas, tar, soot, and char), are an exciting advancement in the understanding of the pyrolysis process. Such models are linking the occurrence of the coal's plastic phase with the "liquid" fragments formed during pyrolysis. On the subject of retrogressive crosslinking reactions, both solvent swelling and NMR measurements confirm important rank dependent differences in reaction rates. These appear related to the oxygen functionalities. Reasonable agreement is also seen for rank variations of kinetics rates derived from low heating rate experiments. Experiments suggest that the recently revived "two-component hypothesis" of coal structure has application to low rank coals that are a mix of polymethylenes and a more aromatic network. Bituminous coals, however, appear far more homogeneous. These coals appear to behave in a manner that is described by the network decomposition models. The presentation will provide a brief report on these topics.

Measurement of C-13 Chemical Shielding Anisotropy in Coal

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, (R.E. Botto and Y. Sanada, eds.), 1991 (in press). Funded by US Department of Energy and ACERC.

The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).

1990

C-13 NMR Techniques for Structural Studies of Coals and Coal Chars

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
Advances in Coal Spectroscopy, Meuzelaar, H.L.C., Editor, Plenum Publishing Corp., New York, 1990 (In press). Funded by ACERC and US Department of Energy.

Techniques in C-13 nuclear magnetic resonance spectroscopy applied in the study of coal and coal chars are discussed along with details of the analysis of the spectral results. The results are compared for various methods of analysis: cross polarization with magic angle spinning (CP/MAS), dipolar dephasing (DD), MAS with block decays (BD), and chemical shielding anisotropy (CSA) measurements. Results of the CP/MAS and DD experiments on the Argonne premium coals as well as other coals and coal chars are reported in terms of twelve structural parameters, including aromaticity. Methods used to determine average cluster size and molecular weight are discussed. Models of coal structure and devolatilization processes are presented along with an analysis of the information obtained from the C-13 NMR experiments.

Chemical Percolation Model for Devolatilization: 2. Temperature and Heating Rate Effects on Product Yields

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 4 (54), 1990. Funded by Pittsburgh Energy Technology Center, US Department of Energy, National Science Foundation and ACERC.

The chemical percolation devolatilization (CPD) model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side-chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained-flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.

Computerized Analysis of 2D INADEQUATE Spectra

Dunkel, R.; Mayne, C.L.; Curtis, J.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 90, 290-302, 1990. Funded by ACERC and US Department of Energy.

The extraction of carbon-carbon bond information from two-dimensional INADEQUATE spectra is both time consuming and complex due to the low sensitivity of the method, the incomplete suppression of single-quantum signals, and the large size of the data sets. A computerized analysis technique is introduced which detects bonds through a nonlinear regression analysis of carefully chosen subsets of the spectral data. A quantitative one-dimensional carbon spectrum is used to establish initial values for the regression and to determine the data subsets to be used. Using statistical analysis techniques, bonds are detected with reliability and sensitivity comparable to those of careful manual interpretation.

Structural Evolution of Matched Tar/Char Pairs in Rapid Pyrolysis Experiments

Pugmire, R.J.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Fletcher, T.H.
Fuel, 1990 (In press). Funded by ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the relationship between chemical structures of coal and the char particles and condensed tar vapors produced from coals of various ranks at rapid heating conditions. The C-13 NMR analysis of the coal chars indicate that significant amounts of aliphatic material is released from the coal during devolatilization with little initial change to the aromatic cluster size or number of cross links per cluster. The evolution of the char structure following tar release is a function of the time/temperature history of the char. The structures of the primary tars are compared to the parent coal and the gas phase evolution of the tar structure is followed with time.

The Chemical Structure and Petrology of Resinite from the Hiawatha "B" Coal Seam

Crelling, J.C.; Pugmire, R.J.; Meuzelaar, H.L.C.; McClennen, W.H.; Huai, H. and Karas, J.
Energy & Fuels, 1990 (In press). Funded by ACERC and US Department of Energy.

Although the maceral resinite occurs in most U.S. coals, it is particularly abundant in the coal seams of the Wasatch Plateau coalfield in central Utah. The high resinite content of the coals of central Utah has long been known and commercially exploited but little work has been reported on the elucidation of the chemical composition of this material. Details of the chemical structure of the micropetrographically defined maceral resinite have generally been lacking because it is noncrystalline and is only partially soluble in organic solvents. In contrast with the abundance of spectroscopic and chromatographic data available on some of the better known fossil resin types, e.g., Baltic amber, Utah coal resins appear to have generated relatively little interest among coal scientists. The overall objective of the present study was to examine the structure of Utah Wasatch Plateau coal resinite macerals that have been separated from the coal matrix, purified, and most important, carefully characterized by fluorescence spectral analysis. The samples were then analyzed by means of CP/MAS C NMR and by Curie-point pyrolysis in direct combination with mass spectrometry (Py-MS) or via preseparation by gas chromatography (Py-GC/MS).

High Resolution Chromatographic Characterization of Depolymerized Coals of Different Rank: Aliphatic and Aromatic Hydrocarbons

Carlson, R.E.; Critchfield, S.; Vorkink, W.P.; Dong, J.-Z.; Pugmire, R.J.; Bartle, K.D. and Lee, M.L.
Fuel, 1990 (In Press). Funded by US Department of Energy, Gas Research Institute and ACERC.

A selective, low temperature depolymerization procedure has been applied to four Argonne coals of different rank to produce products that are representative of the original coal macromolecular structure, and that are amenable to chromatographic analysis. The products of this depolymerization procedure retained most of the original aromatic and functional group structures of the original coals. A comparison of liquid C-13 NMR spectra of the products and solid-state C-13 NMR spectra of the original coals showed only minor changes in the aromaticities of two of the coals, and some loss of the carbonyl carbons in all of the coals.

Tetrahydrofuran pre-extracts of the four coals and their depolymerized products were separated into chemical classes by adsorption chromatography. Two of these fractions, which contained aliphatic hydrocarbons and polycyclic aromatic hydrocarbons were analyzed using gas chromatography/mass spectrometry. Structural identifications were based on a combination of chromatographic retention and mass spectral fragmentation data. For the lower rank coals, the compositions of the pre-extracts were quite different from the corresponding depolymerized products, and they contained an abundance of molecular biological markers. The compositions of the pre-extracts became more similar to the depolymerized products as rank increased.

A Chemical Percolation Model of Coal Devolatilization: 3. Direct Use of C-13 NMR Data to Predict Effects of Coal Type

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J.; Solum, M.S. and Grant, D.M.
Polycyclic Aromatic Compounds, 1:251-264, 1990. Funded by Gas Research Institute and ACERC.

The chemical percolation devolatilization (CPD) model describes the devolatilization behavior of rapidly heated coal based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe the generation of tar precursors of finite size based on the number of cleaved labile bonds in the infinite coal lattice. The chemical percolation devolatilization model described here includes treatment of vapor-liquid equilibrium and a cross-linking mechanism. The cross-linking mechanism permits reattachment of metaplast to the infinite char matrix. A generalized vapor pressure correlation for high molecular weight hydrocarbons, such as coal tar, is proposed based on data from coal liquids. Coal-independent kinetic parameters are employed. Coal-dependent chemical structure coefficients for the CPD model are taken directly from C-13 NMR measurements, with the exception of one empirical parameter representing the population of char bridges in the parent coal. This is in contrast to the previous and common practice of adjusting input coefficients to precisely match measured tar and total volatiles yields. The CPD model successfully predicts the effects of pressure on tar and total volatiles yields observed in heated grid experiments for both bituminous coal and for lignite. Predicted tar molecular weights are consistent with size-exclusion chromatography (SEC) data and field ionization mass spectrometry (FIMS) data. Predictions of average molecular weights of aromatic clusters as a function of coal type agree with corresponding data from NMR analyses of parent coals. The direct use of chemical structure data as a function of coal type helps justify the model on a mechanistic rather than an empirical basis.

Mass Spectrometric Studies of the Chemical Composition of Coal Tars Produced in a Laminar Flow Reactor

Lo, R.; Pugmire, R.J.; Fletcher, T.H. and Meuzelaar, H.L.C.
Preprints for Papers Presented at the 200th ACS National Meeting, 35 (3), 697-704, Washington, D.C., 1990. Funded by ACERC and Consortium for Fossil Fuel Liquifaction Science.

Curie-point desorption in combination with Gas Chromatography/Mass Spectrometry (GC/MS) and, alternatively, with direct Low Voltage Mass Spectrometry (LV-MS) was used to investigate the chemical composition and structure of condensed tar vapors produced during rapid devolatilization (heating rate ~10,000 K/sec) of carefully sized coal particles representing the Beulah Zap, Big Blue, Illinois #6, Pittsburgh #8, and Pocahontas #3 seams, respectively, using the laminar flow reactor described by Fletcher et al at two gas temperatures (1050 K and 1250).

Tar samples were collected by means of a special probe at different points downstream of and corresponding to residence times between 70 and 250 ms. GC/MS analyses of the corresponding tars indicate that the degree of aromaticity increased rapidly as a function of residence time at the 1250 K gas temperature condition. Moreover, at 1250 K devolatilization is complete within 70 ms and beginning secondary gas phase reactions of tar vapors (viz. marked increases in PNAH content and corresponding decreases in phenolic components) are observed within less than 100 ms. However, at 1050 K the coal devolatilization process appears to be barely complete after 250 ms and little or no evidence of secondary gas phase reactions is found.

Measurement of C-13 Chemical Shielding Anisotropy in Coal

Orendt, A.M.; Solum, M.S.; Sethi, N.K.; Hughes, C.D.; Pugmire, R.J. and Grant, D.M.
Magnetic Resonance of Solid Carbonaceous Fuels, ACS Symposium Series, Botto, R.E. and Sanada, Y., Editors, 1990 (In press). Funded by ACERC and US Department of Energy.

The methods available in nuclear magnetic resonance to obtain the principal values of the shielding tensor are discussed. Applications to coal and to compounds that model structures that might be important in coal are presented. The composition of aromatic carbons in coal as determined by chemical shielding powder patterns is compared to results obtained by cross polarization with magic angle spinning (CP/MAS) and dipolar dephasing (DD).

Solid-State C-13 and H-1 NMR Studies of the Evolution of the Chemical Structure of Coal Char and Tar During Devolatilization

Fletcher, T.H.; Solum, M.S.; Grant, D.M.; Critchfield, S. and Pugmire, R.J.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, France, 1990 (In press). Funded by US Department of Energy and ACERC.

Solid-state C-13 and H-1 nuclear magnetic resonance (NMR) spectroscopy techniques are used to investigate the chemical structure of char particles and condensed tar vapors produced as pyrolysis products from an Illinois #6 coal at rapid heating conditions (~104 K/s) at two gas conditions (maximum gas temperatures of 1250 K and 1050 K). The temperature history of particles in the flow reactor is determined using a unique infrared sizing-pyrometry system. The C-13 NMR analyses of the coal chars indicate that significant amounts of aliphatic material are released from the coal during devolatilization, with little change to the aromatic cluster size or number of attachments per cluster. At long residence times, and at higher temperatures, small increases in the cluster size in the char are observed. The H-1 NMR analyses indicate that thermal decomposition of tar vapor occurs at the 1250 K gas condition, as evidenced by increases in the aromaticity and decreases in the peripheral aliphatic groups, such as methyl groups and aliphatic bridge material.

1989

C-13 Solid-State NMR of Argonne Premium Coals

Solum, M.S.; Pugmire, R.J. and Grant, D.M.
Energy & Fuels, 3, 187, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Eight Argonne Premium Coal samples and three other oxidized coals have been investigated by the C-13 CP/MAS NMR technique. Spin-lattice relaxation, variable contact time, and dipolar-dephasing experiments were performed on each coal. The proton spin-lattice relaxation time, T1H, the proton spin-lattice relaxation in the rotating frame time, T1pH, and the cross-polarization time, TCH, are reported along with the dipolar-dephasing time constants, TG, and TL, for both the aromatic and aliphatic regions of the 11 coals. These data, together with normal CP/MAS integrations over selected chemical shift ranges, have been used to derive 12 parameters relating to the carbon skeletal structure, including the aromaticity. From the mole fraction of bridgehead or inner carbons as measured by NMR, the aromatic cluster size may be estimated for each coal.

Chemical Percolation Model for Devolatilization: II. Temperature and Heating Rate Effects on Product Yields

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Accepted for publication by Energy & Fuels, 1989. Funded by US Department of Energy and ACERC (National Science Foundation and Associates and Affiliates).

The CPD model previously developed to describe the devolatilization behavior of rapidly heated coal was based on the chemical structure of the parent coal. Percolation lattice statistics are employed to describe generation of finite tar clusters as labile bonds are cleaved in the infinite coal lattice. The model is used here to describe effects of heating rate and temperature on tar and gas release from coal. Coefficients for the net rate of competition between char formation and side chain formation are generated from heated screen data performed at five different heating rates. The model also compares well with heated screen data obtained at 1000 K/s and different hold times at the final temperature as well as with data from entrained flow reactors obtained at higher heating rates (104 K/s) where particle temperatures have been measured. Results indicate that the CPD model predictions yield good agreement with published data for a wide range of coals and particle heating rates.

Investigation of the Molecular Structure of Organic Sulfur in Coal by XAFS Spectroscopy

Huffman, G.P.; Huggins, F.E.; Mitra, S.; Shah, N.; Pugmire, R.J.; Davis, B.H.; Lytle, F.W. and Greegor, R.B.
Energy & Fuels, 3, 200, 1989. Funded by US Department of Energy.

X-ray absorption fine structure (XAFS) spectroscopy has been used to investigate the molecular structure of organic sulfur in a suite of maceral separates and in several biodesulfurized and extracted coal specimens. For most samples, the X-ray absorption near-edge structure (XANES) exhibits sharp peaks just above the absorption edge that are characteristic of s Æ p transitions of compounds containing an aromatically bound sulfur atom and a broad, structured maximum at somewhat higher energies. The latter maximum is believed to arise from resonant backscattering of photoelectrons by carbon atoms 3.5-4.1 Å from the sulfur atom and possibly from s Æ p transitions of sulfur bonded to oxygen. The radial structure functions derived by Fourier analysis of the EXAFS exhibit peaks at distances that are compatible with the first three neighbor shells surrounding an aromatically bound sulfur atom.

Distortion in Six-Membered Saturated Rings by Natural Abundance Deuterium NMR

Curtis, J.; Grant, D.M. and Pugmire, R.J.
J. Am. Chem. Soc., 111, 7711, 1989. Funded by National Institutes of Health.

Distortions in six-membered aliphatic ring systems were examined using deuterium chemical shifts and multiple linear regression analysis. A new set of regression parameters has been developed that successfully interprets not only spectra for those methylcyclohexanes with normal chair conformations but also spectra for cis- and trans-decalin and for three highly distorted trimethylcyclohexanes with skew-boat type distortions to their equilibrium conformations. The origin of deformations in the cyclohexane ring and the effect of distortions on ring hydrogen/deuterium chemical shifts are discussed in terms of gauche interactions between vicinal C-C and C-D bonds. The fits for 115 deuterium shifts in these cyclic alkanes exhibited a multiple R = 0.9816 and s = 0.0623 ppm for an overall shift range of about 1.5 ppm. As the 14 structural parameters obtained in this study are very sensitive to distortions and conformational features, the results indicate that ²H NMR methods should be very powerful in those cyclic paraffins in which highly second-order banding is observed in the corresponding H-1 NMR spectrum.

Solid-State C-13 NMR Studies of Coal Char Structure Evolution

Solum, M.S.; Pugmire, R.J.; Grant, D.M.; Fletcher, T.H. and Solomon, P.R.
Fuel Fiv. Preprints, 34 (4), 1337-1346, 198th ACS National Meeting, Miami, 1989. (Also presented at the Western States Section, The Combustion Institute Spring Meeting, Pullman, Washington, 1989.) Funded by ACERC (National Science Foundation and Associates and Affiliates).

Solid state C-13 NMR techniques have been used to study the evolution of char structure during pyrolysis processes. The effects of residence time, heating rate, and final char temperature are observed. The NMR data demonstrates that extensive loss of aromatic ring bridge material precedes significant change in aromatic cluster size.

Improvement of Double-Tuned Probe Circuit for High Field Solid-State NMR

Jiang, Y.J.; Woolfenden, W.R.; Orendt, A.M.; Anderson-Altmann, K.L.; Pugmire, R.J. and Grant, D.M.
30th Experimental NMR Conference, Pacific Grove, California, 1989. Funded by US Department of Energy.

A double tuned probe circuit used in high field solid-state NMR experiments has been evaluated. The idea of improved efficiency in both the decoupling and observe channels has been considered, with emphasis on alleviating the stray capacitance in the proton channel. The results of solid-state NMR experiments are included.

Computerized Analysis of 2D-Inadequate Spectra

Dunkel, R.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
30th Experimental NMR Conference, Pacific Grove, California, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

A phase sensitive two-dimensional INADEQUATE spectrum contains carbon-carbon connectivity information; and, hence, allows one to trace molecular carbon skeletons. When a bond is present between two carbons, there are antiphase doublets centered at the chemical shift of each of the carbons. This AB pattern is displaced in F1 by the double quantum frequency that is the sum of the chemical shifts. For each pair of lines in the 1D spectrum two small regions may be defined which must contain the pattern if a bond is present. Using eight parameters the AB pattern can be modeled as a sum of four two dimensional Lorentzians. A nonlinear least squares fit of this pattern to the experimental data from the two small regions is then performed for each pair of resonances in the 1D spectrum. The best-fit values of the parameters and the covariance matrix are used to decide whether the two carbons are bonded.

The Use of Pattern Recognition With High Resolution H-1 and C-13 NMR Techniques in the Study of Fuels and Coal-Derived Liquids

Pugmire, R.J.; Dunkel, R.; Mayne, C.L. and Grant, D.M.
1989 International Conference on Coal Science, Vol. 2,Tokyo,1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

NMR spectroscopy is a widely used analytical tool for probing molecular structure and dynamics. The growth in the use of this tool for studying coal-derived liquids has been quite dramatic. Spectral analysis of these complex hydrocarbon mixtures, however, has been hampered by the vast amount of data present in any given spectrum. We have embarked on a program to use multiple-pulse, two-dimensional (2D) NMR spectroscopy (1, 2) coupled with computer assisted analysis techniques to extract structural information from these materials. Described herein is a program called CCBOND for the computer aided analysis of 2D-INADEQUATE spectra.(3)

A Chemical Percolation Model for Devolatilization: Temperature and Heating Rate Effects

Fletcher, T.H.; Kerstein, A.R.; Pugmire, R.J. and Grant, D.M.
Fuel Div. Preprint, 34 (4), 1272-1279, 198th ACS National Meeting, Miami, 1989. Funded by US Department of Energy and ACERC (National Science Foundation and Associates and Affiliates).

It is well known that the yield of volatile matter obtained from a pulverized coal is dependent upon the temperature history of the particle. However, the effect of heating rate on volatiles yield is difficult to study independently of final temperature. For example, the volatile yields obtained in an entrained flow reactor study by Kobayashi, et al. increase with both temperature and heating rate, but the independent contribution of heating rate could not be assessed. Heated screen experiments were developed to study devolatilization behavior at different heating rates independently from the final particle temperature. The data of Anthony and Howard show little increase in volatiles yield when particles are heated to the same final temperature on a heated screen at different heating rates. In a more recent study, Gibbins-Matham and Kandiyoti show evidence for small increases in the volatiles yield from a Pittsburgh #8 coal as the heating rate is increased from 1 K/s to 1000 K/s on a heated screen. Coal samples were heated at 5 different heating rates to a final temperature of 700ºC and held for 30 s. Experiments were repeated several times in order to ensure accuracy of the data. The total volatiles yield increases from 41.5% at 1 K/s to 46.8% at 1000 K/s, a relative increase in yield of 13%. This increase in yield with increase in heating rate is small, but is larger than associated experimental errors.

The chemical percolation devolatilization (CPD) model was developed as a means to describe coal devolatilization behavior based upon the chemical structure of the patent coal. Some of the input parameters for this model are obtained from NMR characterizations of the parent coal. Percolation statistics are used to describe the probability of generating finite tar fragments from the infinite coal matrix. Pyrolysis yields of tar, gas, and char for three different types of coal are described using a single set of kinetic parameters: only chemical structure parameters are changed for the different coals. The initial description of the CPD model allowed for a temperature dependence of the competition between side chain formation and char formation. However, this option was not exercised in the initial study in order to demonstrate general utility of the model for one set of devolatilization data on three coals collected over a narrow range of temperatures and heating rates. In the present work, the Gibbins-Matham and Kandiyoti data are used to determine additional coefficients for the CPD model that accurately predict the changes in char and tar yield as a function of heating rate.

1988-1987

Investigation of Structure/Property Relationships of Selected C5 to C10 Hydrocarbons Using Canonical Correlation Analysis of Multisource Data

Hoesterey, B.L.; Meuzelaar, H.L.C. and Pugmire, R.J.
Submitted to Anal. Chem., 1988. 28 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Twelve physico-chemical and thermodynamic properties including molecular weight, boiling point, flash point, density, refractive index, volumetric, gravimetric, and molar heats of combustion, atomic H/C, carbon number, hydrogen number and the fuel-related threshold sooting index for 47 C5 to C10 hydrocarbons were subjected to factor analysis. Two factors with eigenvalues greater than 1.0 were found, accounting for 95% of the variance. The major groups of variables were interpreted as arising from either molecular size (boiling point, flash point, molar heat of combustion, molecular weight and carbon number) or degree of unsaturation (density, refractive index, volumetric heat of combustion, threshold sooting index, H/C and gravimetric heat of combustion). When three factors from the physicochemical and thermodynamic properties factor space were used for canonical correlation analysis with the Wiener topological index, molecular size related papers were found to correlate with it. Canonical correlation analysis of the properties factor space with mass spectrometry/Kavats retention index data showed that characteristic mass spectral variables correlated closely with the degree of unsaturation by differentiating aliphatic from aromatic compounds. The Kovats retention index variable, not unexpectedly, modeled molecular size related parameters such as carbon number.

Quantitative Determination of Different Carbon Types in Fusinite and Anthracite Coals From Carbon-13 Nuclear Magnetic Resonance Chemical Shielding Line-Shape Analysis

Sethi, N.K.; Pugmire, R.J.; Facelli, J.C. and Grant, D.M.
Analytical Chemistry, 60, 1574-1606, 1988. 32 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

C-13 NMR shielding tensors have been determined for two anthracite coals and a fusinite maceral using powder pattern line-shapes that have been analyzed as a superposition of three different bands due to benzene-like, condensed (bridgehead and inner) and substituted carbons. Theoretical calculations on circumcoronene (I) as a model compound support the interpretations of the experimental data. Determination of the ratio of non-protonated to protonated aromatic carbons obtained on the anthracites by the spectroscopic analysis is in excellent agreement with the elemental analysis and previous studies by dipolar dephasing NMR techniques. The method therefore constitutes a valuable way to analyze the structure of high rank coals and should be useful in char characterization. The mole fraction of condensed carbons obtained by this technique is used to estimate the average cluster size in these polycondensed aromatic hydrocarbon materials.

An Integrated Spectroscopic Approach to the Chemical Characterization of Pyrolysis Oils

Hoesterey, B.L.; Windig, W.; Meuzelaar, H.L.C.; Eyring, E.M.; Grant, D.M. and Pugmire, R.J.
Processing of Pyrolysis Oils, 1988, E.J., Soltes (ed.) ACS Symp. Series, in press. Funded by Consortium for Fossil Fuel Liquefaction Science and US Department of Energy.

The hydrocarbon ("oil") fraction of a coal pyrolysis tar prepared by open-column liquid chromatography (LC) was separated into l6 subfractions by a second LC procedure. Low voltage mass spectrometry (MS), infrared spectroscopy (1R), and proton (PMR) as well as carbon-13 nuclear magnetic resonance spectrometry (CMR) were performed on the first 13 subfractions. Computerized multivariate analysis procedures such as factor analysis followed by canonical correlation techniques were used to extract the overlapping information from the analytical data. Subsequent evaluation of the integrated analytical data revealed chemical information that could not have been obtained readily from the individual spectroscopic techniques. The approach described is generally applicable to multisource analytical data on pyrolysis oils and other complex mixtures.

A Chemical Model of Coal Devolatilization Using Percolation Lattice Statistics

Grant, D.M.; Pugmire, R.J.; Fletcher, T.H. and Kerstein, A.R.
ACS Div. of Fuel Chemistry, 33, (2), 322-332, 1988. 10 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

We have developed a model for coal devolatilization that incorporates the diversity of coal structure in such a way that the analytical data obtained from solid state NMR provides the initial input data. Using experimentally determined kinetic rate parameters, it is possible to fit the gas, tar and char production of a lignite and high volatile bituminous coal. We have employed percolation theory to provide analytical expressions for the lattice statistics required in devolatilization modeling. The percolation theory allows one to avoid the more time-consuming Monte Carlo technique with no loss of generality or important statistical features. Percolation theory analytically describes the size distribution of finite clusters of sites joined by intact bridges but isolated from all remaining sites by broken bridges. The theory specifies a critical bridge population, depending only on the site coordination number, above which infinite arrays will coexist with clusters of finite size. It is a simple matter to adapt the structural features of percolation theory to both the tar and gas obtained in coal pyrolysis. The infinite arrays of percolation theory are interpreted as the macroscopic lattice of unreacted coal and/or char while the relatively small tar molecules may be identified with the fine clusters of percolation theory. The details of the model will be discussed together with the results obtained in modeling devolatilization behavior of coals of various ranks.

EXAFS Investigation of Organic Sulfur in Coal

Huffman, G.P.; Huggins, F.E.; Shah, N.; Bhattacharyya, D.; Pugmire, R.J.; Davis, B.H.; Lytle, F.W. and Greegor, R.B.
ACS Div. of Fuel Chemistry, 33, (1), 200-208, 1988. 9 pgs. Funded by US Department of Energy.

EXAFS spectroscopy is shown to be a very promising technique for investigating the molecular structure of organically bound sulfur in coal and coal derivatives. The current paper presents sulfur K-shell EXAFS results for a number of a maceral separates prepared by density gradient centrifugation and for several biodesulfurized coals. Both the near-edge structure and the radial structure functions exhibit some similarities to dibenzothiophene. However, a broad peak occurs in the XANES region of the coal spectra that is not observed for the molecular structures usually ascribed to organic sulfur in coal. This is believed to arise from resonant photoelectron scattering from second and third nearest neighbor carbon shells and from sulfur bonded to oxygen.

The Use of Variable Angle Sampling Spinning C-13 NMR Spectroscopy to Assess Aromatic Cluster Size in Coals, Coal Chars, and Carbonaceous Materials

Pugmire, R.J.; Sethi, N.K.; Solum, M.S.; Facelli, J.C. and Grant, D.M.
Proc. Carbon Conference, 1988. 3 pgs. Funded by US Department of Energy and Basic Energy Sciences.

Magic angle spinning experiments produce narrow lines in solids but valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements [1]. Knowledge of all three frequencies with the isotropic value is a source of information that is indispensable for molecular characterization. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid-state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. We have analyzed both static and variable angle sample spinning line shapes [2, 3] to obtain the CSA tensor values and population factors for the aromatic carbons of coals, macerals and carbonaceous materials from which aromatic cluster size is estimated.

Investigation of the Atomic and Physical Structure of Organic and Inorganic Sulfur in Coal

Huffman, G.P.; Huggins, F.E.; Shah, N.; Bhattacharyya, D.; Pugmire, R.J.; Davis, B.H.; Lytle, F.W. and Greegor, R.B.
Processing and Utilization of High Sulfur Coals II, 3-12, 1987. 10 pgs. Funded by US Department of Energy.

A complete description of the microstructure and molecular state of sulfur in coal can be achieved by combining several experimental techniques. For quantitative analysis of pyrite and its transformation products resulting from oxidative or reductive processes, Fe Mossbauer spectroscopy appears to be the best technique available. Computer-controlled scanning electron microscopy (CCSEM) provides an excellent method of measuring the particle size distributions of pyrite and its transformation products, information that is critical in physical and biological cleaning processes. Finally, it is shown that X-ray absorption fine structure spectroscopy, usually referred to as EXAFS spectroscopy, is capable of determining the atomic structure of organic or other forms of sulfur in coal and coal derivatives. EXAFS data will be presented on organic sulfur in standard compounds, maceral separates, and microbially desulfurized coal.

C-13 Chemical Shielding Anisotropy Studied by Variable-Angle Sample Spinning

Sethi, N.K.; Grant, D.M. and Pugmire, R.J.
Journal of Magnetic Resonance, 71, 476-479, 1987. 4 pgs. Funded by US Department of Energy.

Spinning the sample at angles other than the magic angle is shown to have interesting applications for obtaining chemical shielding anisotropies for systems where severe overlapping makes the analysis of static powder pattern either cumbersome or impossible. Results for 1,3,5-trimethoxybenzene and p-dimethoxybenzene are in excellent agreement with those obtained via single-crystal studies.

The Chemical Structure and Petrology of Resinite from the Hiawatha "B" Coal Seam

Crelling, J.C.; Pugmire, R.J.; Meuzelaar, H.L.C.; McClennen, W.H. and Karas, J.
Submitted to Coal Geology, 1987. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Although the maceral resinite occurs in most US coals, it is particularly abundant in the coal seams of central Utah. The high resinite content of the coals of central Utah has long been known and commercially exploited but little work has been reported on the elucidation of the chemical composition of this material. Indeed, there is little such information on the resinite of any coal. The resinite occurrences have been described by Spieker and Baker, Tomlinson, Theissen and Sprunk, and Buranek and Crawford. An unusual feature of the coal seams in Utah is that most of the resinite occurs in a secondary manner as cleat, fissure, or other void fillings. Similar occurrences have been reported in British coals by Jones and Murchison and Murchison and Jones. They concluded that the metamorphic effects of coalification in the bituminous rank range caused the resinite to be gently mobilized without the more severe manifestations of metamorphism such as vesiculation or increased refectance. Teichmuller, observed that secondary resinite (exudatinite) seemed to be exuded from other coal macerals during coalification in the lower bituminous range.

Entrained gasification tests with a Utah high-volatile bituminous coal were performed at atmospheric pressure to assess the influence of particle size, coal feed rate, steam-coal ratio and oxygen-coal ratio. Independent argon-carbon balanced and ash balance methods were used to evaluate carbon conversion, with good agreement observed between the methods. A higher O2-coal ratio for finer particles increased the carbon conversion. Carbon conversion and hydrogen formation showed little dependence on the amount of steam injected in the secondary stream, indicating minimal steam-coal reaction. When the coal feed rate was varied from 23 to 27 kgh-1, a small increase in carbon conversion was observed with no significant change in the gas composition.

Solid State Nuclear Magnetic Resonance of Fossil Fuels

Solum, M.S. and Pugmire, R.J.
Energy & Fuels, 448, 1987. 1 pg. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Axelson has written a valuable book for those interested in the field of NMR spectroscopy of fossil fuels and polymers. This book is best suited for those who have had some experience with solid-state NMR but not perhaps with application to fossil fuels. The Introduction and Appendix A give a brief description of some of the more common fossil fuel terminology and standard analysis techniques, which was quite helpful. In Chapter 1 a very brief description of line broadening mechanisms in solids is given and cross-polarization and magic angle spinning are introduced. The different carbon and proton relaxation parameters and the pulse sequenced used to measure them are discussed in Chapter 2. This chapter is excellent as a quick reference for help in setting up these experiments. Examples of relaxation times of model organic compounds, polymers, and fossil fuels are given, and their relationship to quantitative analysis of fossil fuels is discussed in Chapter 3. Chapter 4 deals with the removal of instrumental artifacts from the NMR spectrum including spinning sideband suppression methods. Also discussed in this chapter are the causes of many instrumental problems. This chapter is recommended to those who are not experienced in solid-state NMR but are initiating these types of experiments. Line-broadening mechanisms in solid-state NMR arising from both natural and instrumental factors are discussed in Chapter 5. Chapter 6, dealing with resolution enhancement, contains a good section on the dipolar-dephasing experiment and the structural parameters that may be obtained from this experiment. Some less common and more selectively useful NMR pulse sequences are also treated. Chapter 7 provides the fa value for a variety of fossil fuels.

While the treatment of many topics in this book is at an overview level, there is a large selection of references at the end of each chapter where a more in-depth discussion of the topics may be found. This book is recommended as a reference for those interested in solid-state NMR in general and, in particular, in application to coals. The book provides an excellent overview of the work published prior to the appearance of this treatise. For the beginner, the fundamentals of solid NMR techniques are not well developed and one should consult other works in order to obtain the necessary basic foundation. In the main, Axelson has provided a very useful reference for those who work in the fossil energy field.

Structural Variations and Evidence of Segmental Motion in the Aliphatic Region in Coals Observed with Dipolar-Dephasing NMR

Soderquist, A.; Burton, D.J.; Pugmire, R.J.; Beeler, A.J.; Grant, D.M.; Durand, B. and Huk, A.Y.
Energy and Fuels, 1, (1), 50-55, 1987. 6 pgs. Funded by US Department of Energy and Standard Oil Co.

We report the structural variations of a number of coals and coal macerals. Through the time constants associated with dipolar-dephasing techniques, CP/MAS spectral data reveal the presence of segmental motion in certain low-rank coal samples. The motion detailed is in the aliphatic region and is thought to be due to CH2 groups associated with hydroaromatic and/or polymethylene structural units.

An Efficient Double-Tuned C-13/H-1 Probe Circuit for CP/MAS NMR and Its Importance in Linewidths

Jiang, Y.J.; Pugmire, R.J. and Grant, D.M.
Journal of Magnetic Resonance, 71, 485-494, 1987. 10 pgs. Funded by Standard Oil, US Department of Energy, Office of Energy Research and Basic Energy Sciences.

A simple double-tuned C-13/H-1 probe for high-field CP/MAS NMR is described. The tuning and matching can be adjusted separately in each channel and made exactly equal to 50 W. Isolation between H-1 and C-13 channels exceeds 40 dB. The sensitivity (S/N) of the observation channel (C-13) for methyl carbons is close to 47 in a CP/MAS experiment of 10 scans on 70 mg of a sample of hexamethylbenzene for the rotor sample volume of 140 ml. The probe delivers a 2.4 ms 90º proton pulse with only 90 W at 200 NHz and 250 W at 50.3 MHz. Examples are presented of limewidth reduction with higher decoupling power. The effects of anisotropy in the bulk magnetic susceptibility are also discussed.

Efficient Stator/Rotor Assembly for Magic-Angle Spinning NMR

Jiang, Y.J.; Woolfenden, W.R.; Alderman, D.W.; Mayne, C.L.; Pugmire, R.J. and Grant, D.M.
Rev. SCI Instrum, 58, (5), 755-758, 1987. 4 pgs. Funded by US Department of Energy and Office of Basic Energy Sciences.

A newly designed stator assembly for cylindrical spinners used in magic-angle spinning nuclear-magnetic resonance experiments is described. Separate driving and bearing gas chambers allow variable and stable spinning speeds, and this design permits easy starting and stopping of the rotor. Isolation of the chambers is achieved with the application of pressure screws rather than O-rings or glue lines to avoid leakage at high gas pressures. The overall dimensions are optimal to facilitate easy assembly. Some significant modifications have been made to an earlier spinner design. These improvements give better efficiency and concentricity of the spinner. Applications are illustrated with carbon-13 cross polarization/magic angle spinning (CP/MAS) spectra carried out at different rotor spinning rates.

Correlation of Ring Nitrogen Substituents with Carbon-13 Nuclear Magnetic Resonance Data in Azoloazines

Pugmire, R.J.; Smith, J.C.; Grant, D.M.; Stanovnik, B.; Tisler, M. and Vercek, B.
J. Heterocyclic Chem, 24, 805-809, 1987. 5 pgs. Funded by Institute of General Medical Sciences of the National Institutes of Health and by the Council of Yugoslav Academies, the Slovene Academy of Science and Arts, and the US National Academy of Sciences.

Carbon-13 nuclear magnetic resonance data have been acquired on 22 azoloazines. Chemical shifts have been correlated by a step-wise linear multiple regression with nitrogen substituents in both the 5- and 6- membered rings using pyrrolo [1,2-a] pyridine as the reference for chemical shift correlation. The data demonstrate that a highly correlated set of chemical shift parameters exist. Nitrogen substitution in the five-membered ring produces larger cross-ring effects than is observed in the five-membered ring when substitution occurs in the six-membered ring. Within the six-membered ring a constant para-substituent parameter is noted. The meta- and para- parameters are more complex and fall into two groups for each parameter. Within the five-membered ring, a highly regular chemical shift pattern is observed which reflects an attenuated perturbation from nitrogen substitution in the six-membered ring.

Application of High Resolution Solid State NMR to Analysis of the Chemical Shift Anisotropy Tensors in Coals

Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
1987 International Conference of Coal Science, 1987, 41-44, Science, Holland, 1987. 4 pgs. Funded by US Department of Energy, Basic Energy Sciences, and ACERC (National Science Foundation and Associates and Affiliates).

Carbon-13 NMR spectroscopy has become an important tool in the structural determination of coal. The major techniques used are cross polarization magic angle spinning (CP/MAS) and dipolar dephasing (DD/MAS) experiments. The CP/MAS technique has been extensively used on both whole coals and macerals by this laboratory as a direct method for determining the fraction of aromatic carbons, fa, in these samples. The dipolar dephasing experiment (DD/MAS) provides additional definition of the structure of coal by using the C-13-H-1 dipolar coupling to separate the carbons into sub-classes, i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aromatic and aliphatic regions of the spectrum. The application of the technique to coals has been thoroughly described.

MAS experiments produce narrow lines in solids but, in doing so, valuable chemical shift anisotropy (CSA) information is lost as the chemical shift tensors are averaged to their isotropic values. CSA is a second rank tensor and is characterized by three unique resonance frequencies s11, s22 and s33 called the principal values of the tensor elements . Knowledge of all three frequencies in addition to the isotropic value (average of tensor elements) in a source of information which is indispensable for molecular characterization and independent peak assignments. Measurement of the tensor elements is carried out by recording the C-13 NMR spectrum of a finely powdered sample to obtain the solid state line shape which can then be analyzed to determine tensor elements of chemically and structurally different types of carbons. Other noteworthy techniques for extracting such data are analysis of spinning sidebands from slow magic angle spinning experiments and non-magic angle sample spinning referred to in this paper as variable angle sample spinning (VASS). In the present study, we have analyzed both static and variable angle sample spinning line shapes to obtain the CSA tensor values and population factors for the aromatic carbons of coals.

C-13 NMR Structural Determination of the Coals in the Premium Coal Sample Bank

Solum, M.S.; Pugmire, R.J. and Grant, D.M.
ACS division of Fuels, Preprint, 32, (4), 273-279, 1987. 8 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Solid state C-13 cross polarization (CP) magic angle spinning (MAS) NMR experiments have become very useful for the study of the structure of coal and other fossil fuels (1, 2). Solid-state NMR techniques have the advantage over liquid NMR methods because the whole solid coal can be analyzed in a nondestructive manner and no assumption need be made that the soluble portion represents the whole organic portion of the coal. From CP/MAS experiments twelve NMR structural parameters may be determined including the aromaticity, fa, that give information on the carbon skeletal backbone. The variation of the NMR structural parameters for coals of different rank from lignite to anthracite has been demonstrated (3). The coals in the Premium Coal Sample Bank (PCSB) have been included in the set of coals studied at the Advanced Combustion Engineering Research Center. The NMR data obtained on these coals will be used to provide chemical structural parameters for refinement of the devolatilization sub-model of the PCGC-2 combustion model.

This paper reports the twelve structural parameters for two of the PCSB coals, Pittsburgh #8 (HVB) and Blind Canyon (HVB). These data were derived from integrated intensities in a normal CP/MAS experiment along with a complete set of dipolar dephasing experiments. In addition, the proton spin lattice relaxation times for these coals have been determined. From relaxation experiments on the Pittsburgh #8 coal there is some evidence of heterogeneity between different sample vials of the same standard coal.

Non-Magic Angle Spinning NMR - An Approach to Determine the Chemical Shift Tensors in Char Particles

Sethi, N.K.; Pugmire, R.J. and Grant, D.M.
ACS Div. of Chemistry, Preprints, 32, (4), 155-167, 1987. 13 pgs. Funded by Office of Basic Energy Science and US Department of Energy.

Carbon-13 NMR spectroscopy has been widely accepted as a major analytical technique for studying fossil fuels. Axelson and Davidson have reviewed the various solid state NMR spectroscopic techniques for coal studies. Cross polarization/magic angle spinning (CP/MAS) provides information on aromaticity while the dipolar dephasing technique (DD/MAS) provides additional definition of the structure of coal by using the C-13/H-1 dipolar coupling to separate the carbons into subclasses; i.e., those that are strongly coupled to protons from those that are weakly coupled in both the aliphatic and aromatic regions of the spectrum. The combination of these two experimental procedures permits one to derive a carbon skeletal structure of coal samples.

MAS experiments produce narrow lines in solids but valuable structural information is lost; i.e., the chemical shift anisotropy (CSA) which is a manifestation of the three-dimensional shielding of the nucleus by the surrounding electrons. The CSA is a second-rank tensor having three principal elements s11, s22, and s33 characterized by unique resonance frequencies. These three tensor components taken together with the isotropic shielding value (the MAS value which is the average of the three tensor components) provide valuable data regarding the local electric environment. The tensor can be obtained from the C-13 NMR spectrum of a finely powdered sample and the tensor elements are extracted by analysis of the line shape. This technique provides not only tensor components but population values as well. However, the spectral analysis is complicated if more than one tensor is present and unique results are not always achievable. Evenso, Pines, et al. used the technique to analyze the static spectra of several coals. These workers succeeded in differentiating the contributions from aromatic and condensed aromatic carbons. Furthermore, they pointed out that there is little difference in the isotropic chemical shifts between these types of carbons and these shift differences cannot be resolved in a CP/MAS experiment. The DD/MAS experiment has been shown to differentiate between benzene-like (i.e., C-H) and non-protonated aromatic carbons (substituted plus inner, or bridgehead) but the resolution of substituted and inner carbons is not readily attainable with standard MAS experiments. The shielding anisotropies of the three general types of aromatic carbons are quite different and, in principal, should be resolvable in the "non-spinning" experiment.

Only for isolated carbons, or for simple compounds where breakpoints in the powder pattern are discernible, is it possible to obtain unique fits of the line shape. Tensor information can be extracted by other techniques such as analysis of the spinning side bands from slow spinning MAS experiments and variable angle sample spinning (VASS). In this study we have used the static and VASS methods to study pertinent model compounds and have used these data as a guide to analyze line shapes and extract CSA tensor values and population factors in coals. The experimental techniques are applicable to chars as well as coals. The data permit us to estimate the size of the polycondensed aromatic structure that is the main structural component in chars. The aromatic structure of coal chars should provide valuable information regarding their reactivity.