Kalkunte S. Seshadri

Application of carbon-13 nuclear magnetic resonance spectrometry to coal chemistry: Calculation of transferable hydrogen

Abstract A 13 C Fourier-transform nuclear magnetic resonance approach has been employed to calculate the level of hydroaromatics and transferable hydrogen present in coal-liquefaction solvents and products. The optimum conditions for data acquisition were determined by examining a synthetic mixture and five methods are critically examined. The method with minimum percentage difference between the calculated and observed values is that with the reverse gating sequence. However, this method is enormously time-consuming and, therefore, impractical. The methods in which the decoupler is on during data acquisition and with 5 s or less interval between pulses result in large errors in intensity measurement. A 10 s interval between pulses has been chosen, thus keeping the experimental time within reasonable limit and ensuring acceptable accuracy. The synthetic mixture has also been studied with the relaxation agent, Cr(AcAc) 3 . However, in process solvents, the relaxation agent broadens the resonance absorption considerably and its use is not advisable. By selecting a tip angle of 42 °, acquisition time of 1.024 s and 10 s interval between pulses, results have been obtained on several raw and recycled solvents. Typical values are included.

Structural aspects of sub-bituminous coal deduced from solvation studies. 2. Hydrophenanthrene solvents

Abstract In order better to delineate the contribution of coal molecules to liquefaction products, donor solvents of discrete structure and narrow molecular-weight range have been satisfactorily employed. Partially hydrogenated phenanthrenes are ideally suited for these studies. This ring system is 1. (a) prevalent in all coals 2. (b) thermally stable, and 3. (c) capable of functioning ideally in a redox environment. Coal depolymerization can take place in the absence of a catalyst and hydrogen, provided that the level of hydroaromatics, especially octahydrophenanthrene, is sufficiently high to ensure that the coal free-radicals can be quenched before they recombine. Solvation (liquefaction) levels reach a maximum (>98%) with octahydrophenanthrene, making the indirect study of coal possible with a minimum of solvent ‘background’. The role of catalyst, if used during solvation, appears to be that of maintaining the proper level of hydroaromatics and of upgrading the liquids produced. Sub-bituminous coal appears primarily to consist of two- and three-condensedring structures linked by heteroatoms, mainly oxygen.

Carbon-13 nuclear magnetic resonance spectra of model hydroaromatic hydrocarbons and solvent

Abstract Carbon-13 chemical shifts are reported for tetralin, hydrophenanthrenes and hydropyrenes and their alkyl substituted derivatives. Mono- and di- plus tri-aromatic fractions of hydrogenated phenanthrene and pyrene were also examined by carbon-13 n.m.r. and, wherever possible, the components in them identified.

Characterization of coal liquids by 13C n.m.r. and FT-i.r. spectroscopy — fractions of oils of SRC-I and asphaltenes and preasphaltenes of SRC-I and SRC-II

Abstract Coal-derived products of SRC-I liquefaction of Blacksville coal and vacuum tower bottoms (VTB) of SRC-II liquefaction of Powhatan mine coal (both bituminous and from the Pittsburgh Seam) were separated into fractions by solvent extraction. The SRC-I was first extracted with ethylacetate, and solubles were subsequently separated into oils and a mixture of asphaltenes and preasphaltenes (APA). The VTB was Soxhlet extracted with pentane to remove any residual oils, followed by tetrahydrofuran to recover APA. The APA portions were then separated by sequential elution solvent chromatography (SESC) into fractions differing in chemical functionality, and then examined by 13C n.m.r. and FT-i.r. spectroscopic techniques. APA are intermediates and end products of coal liquefaction. SRC-II APA are of higher molecular weight than the APA of SRC-I. The lower numbered fractions of APA of SRC-I in SESC separation have the same functional groups as the corresponding fractions of middle and heavy distillates. However, the higher numbered fractions are rich in oxygen, which is mainly in carbonyl groups. Part of the carbonyl groups are in esters which cross-link aromatic clusters. Therefore, APA and the coal itself are ‘oligomeric’ in structure, with aromatic clusters linked by carbon bridges with different functional groups. The nature of carbonyl groups in APA has been analysed in detail.

Structural aspects of sub-bituminous coal deduced from solvation studies. 1. Anthracene-oil solvents

Abstract In order to study coal structure indirectly and the role of hydrogen donors, an investigation of the major parameters involved in the solvation of a Wyoming sub-bituminous coal has been made. This study utilized the catalytic and non-catalytic hydrogenation of anthracene oil and coal-solvent slurries. Indirect evidence concerning major structural units in the coal was obtained and the net contribution of coal to liquefaction products was estimated. The significance of each parameter to the degradation of coal molecules was also estimated. Data support the concept that coal liquefaction follows a solid → asphaltene → resin → oil route. This stepwise dissociation of the solid is directly related to the breaking of CO, CN, and to a lesser degree CC bonds, resulting in the formation of free radicals of relatively low molecular weight. These free radicals are stabilized by hydrogen transfer from hydroaromatic solvent molecules. A lack of significant quantities of high-molecular-weight hydrocarbons derived from coal solvation implies the prevalence of small molecular units in the coal structure.

Characterization of needle coke feedstocks by magnetic resonance spectroscopy

Abstract Decant oil is the traditional petroleum-derived feedstock for the production of needle coke. However, with the dwindling supply of high-quality decant oil and potential availability of coal-derived liquids, alternative feedstocks are receiving attention. One of these is ethylene pyrolysis tar. The new materials are less suitable needle coke precursors and may need to be upgraded by structural alterations. Therefore, the structure of these materials is examined at the molecular level. The average molecular parameters for the aromatic and asphaltene fractions of decant oil and pyrolysis tar are reported, along with a method for calculating these parameters which is slightly different from the previously available procedures. These parameters indicate that the average molecule in the aromatic and asphaltene fractions of decant oil and pyrolysis tar differ in certain structural units. This difference possibly may be responsible for their different behaviour on pyrolysis.

Structural characterization of fractions of petroleum pitch and ethylene pyrolysis tar by 1H and 13C n.m.r. spectroscopy

A petroleum pitch and > 288 °C fraction of an ethylene pyrolysis tar are separated by sequential solvent extraction into fractions differing in average molecular weight. Average molecular parameters for each fraction are obtained using their H and 13C n.m.r. spectra. Average molecular structures which correlate with the observed data are drawn. The data presented here suggest that the average molecule of the fractions of both petroleum pitch and pyrolysis tar can be represented by an oligomeric structure in which small aromatic clusters are joined by aliphatic bridges and/or biaryl linkages. This contradicts the accepted assumption that the aromatic ring system in petroleum-derived products is fully condensed. Although the average molecular structure of the fractions of pitch and ethylene pyrolysis tar are basically similar, they differ in the number and types of ring-saturated carbons.