A 13C 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)s. 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.
One of the most important parameters in coal-liquefaction processes is the level of hydroaromatics present in the solvent. These hydroaromatics are the effective hydrogen donors, and a method to quantify them in coal-liquefaction solvents is necessary. Initially', a minimum estimate was obtained by separating the solvent into various fractions, one of which (monoaromatics) contained exclusively hydroaromatic molecules when the solvent was coalderived (anthracene oil or recycle solvent). This was a minimum estimate since hydroaromatic structures in the other fractions could not be quantified. This deficiency can be eliminated, in part, by examining a total aromatic fraction by 13C n.m.r. This technique examines the carbon skeleton of molecules, and differentiates and quantifies the carbons according to their environment.
This information can then be used to calculate the amount of transferable hydrogen present in the sample.
In a previous paper', carbon-13 chemical shift values were reported for a series of hydroaromatic hydrocarbons. This information has been utilized to determine the abundance of hydroaromatic carbons, C,, and from this, the abundance of transferable hydrogen, H,.