Reaction paths in donor solvent coal liquefaction

The classic studies of Sol Weller and his collaborators1,2 provided the working model for most American investigations of donor solvent coal liquefaction. The investigations, particularly at what is now the Pittsburgh Energy Research Centre of ERDA, at what is now Conoco Coal Development Company, at Pittsburg and Midway Coal Company and at the University of Utah, added notably to Weller's picture. Recently, however, new details have emerged that contradict the common interpretation of Weller's model, viz. that oils appear only after the coaly matter has traversed a sequence of reactions, coal --> asphaltenes --> oil. University of Utah studies at short residence times showed that coal can sometimes be converted very quickly to oil species, whereas asphaltenes hydrogenate to oils only slowly.3 With interest in the conversion of coal to a clean [`]solvent refined coal' for electricity generation,4 researchers have taken interest in the portion of liquefaction product that is not soluble in benzene. Earlier, this portion had been implicitly regarded as [`]unreacted coal' and held little interest, but it was found to be soluble in pyridine5 and, as we shall see, differs from coal in important respects. We owe the new information to a sharp increase in research activity, both at the older, established centres and at newly recruited institutions which have responded to opportunities afforded by the RANN Program ([`]Research Applied to National Needs') of the US National Science Foundation, by the Energy Research and Development Administration (ERDA) and by the Electric Power Research Institute (EPRI). The increased activity has led to greater, in some cases pioneering, applications to coal science of sophisticated new tools for examining materials, such as advanced chromatographic separation procedures and pulsed Fourier transform 1H and 13C nuclear magnetic resonance spectroscopy. To a large extent, the recent additions of knowledge arise from new attention to the first few minutes of the sequence of reactions by which a donor solvent solubilises coal.