Metals and metal complexes in coal

Abstract

Some of the metal compounds that occur in coal are soluble in organic solvents and are extracted in coal liquefaction processes. The material made by the extraction of coal with hydrogenated anthracene oil has been fractionated by sequential Soxhlet extraction with low-boiling solvents, and the distribution of the metallic elements in the various fractions has been determined. Extraction of Daw Mill coal (92 kg) with acidic methanol furnishes 17.8 mg of a mixture of gallium complexes of homologous porphyrins (C 27 -C 32 ). Similar metalloporphyrin concentrates are obtained from a variety of British bituminous coals, the amount detected being about 1 pg/g, but falling off as coal rank increases. Various lignites and a range of Polish coals have also been surveyed: here, iron porphyrins and (in one case) manganese porphyrins have been observed. The iron porphyrins tend to be confined to coals of lower rank : in the lignites, metal-free tetrapyrroles are also detected. The metalloporphyrins are thought to be derived from the chlorophylls and haems of the biological precursors. Indeed, it is possible that the iron porphyrins are derived directly (i.e. without demetallation-metallation) from biological and microbiological haem compounds.