Introduction to Coal Photochemistry

Abstract

Invariably, coal extracts and solutions of coal liquids are not true solutions at all but contain, besides simple molecules, a range of much larger molecules and even small particles. Consequently, despite their aromaticity, such materials do not give ultraviolet absorption spectra, rather they scatter light. Information about the chemistry of the aromatic nuclei can be obtained, however, from the fluorescence of extracts, notably from their excitation spectra. Measurements of fluorescence commonly examine light emitted at right angles to the incident beam; the problems of scattering are thereby overcome. Even suspensions of 1 μm coal particles in water or chloroform emit fluorescence and the excitation spectra show mono, di and polyaromatic structures to be present.

The lifetimes of fluorescing states lie in the range 0.1–10 ns. This is shorter than the fluorescence lifetimes of simple aromatic molecules but akin to the fluorescence lifetimes of aromatic polymers. Thus the fluorescence behaviour of coal extracts and suspensions appear to be dominated by the ready transfer — both radiative and non radiative — of energy from one aromatic structure to the next. Energy is transferred ever more readily as the aromatic structures come closer together. It is an exothermic process in the sense that it is inevitably accompanied by loss of energy due to the vibration of molecules. Consequently energy emitted by monoaromatic nuclei becomes absorbed by diaromatics, reemitted at longer wavelengths and reabsorbed by polyaromatics and so on, until the energy becomes trapped in a structure from which further energy transfer is impossible and fluorescence occurs. This is what happens when solid coals fluoresce and it is the reason why petrologists observe fluorescence from polished surfaces of coals only at long wavelengths not characteristic of the major aromatic species present. The highly aromatic inertinites and vitrinites transfer energy more efficiently and therefore fluoresce less than the less aromatic exinites.

The intensity and wavelengths of the molecular fluorescence from coal suspensions and coal extracts can be used to monitor the reactions of the aromatic nuclei which they contain. These nuclei exhibit the expected aromatic photochemistry and photooxidation, photohalogenation and photoreduction of coals have been studied. The reactions are limited by the extent to which the light and the reagents can penetrate the surface of the coal. There is evidence that particles of different coals have different morphologies.

The rates of surface reactions of coal systems will obviously depend on the surface areas present. Coal extracts contain species having a wide range of molecular sizes and usually species having a large surface area will react more rapidly than species having smaller surface areas. A good example of this is the reaction of photo excited coal extracts with acetone, the kinetics of which may be described by postulating that the species present in the extract possessed a Gaussian distribution of surface areas.