Gallium porphyrins in bituminous coal

Abstract

Metalloporphyrins are minor constituents in crude oil and its relatives¹ but their presence in coal is not clearly established. This is surprising as such complexes are thought to arise from chlorophyll, and coal is largely of plant origin. Although coal samples were among the earliest examined in this context^2,3, most later workers^1,4 followed Treibs⁵ in mainly considering crude oils, asphaltenes and shales. Weigelt and Noack² presented early evidence for chlorophyll derivatives in fossilised leaves from brown coal, while Treibs³ found trace amounts of metal-free porphyrins in various coals. Uspenskii and Gorskaja⁶ confirmed the latter finding, but observed that the porphyrin content of sapropelic coals such as cannel coal was much higher than that of hard coals. Metalloporphyrins in fossil plant material are important because they provide information on the geochemical transformations of chlorophyll. Moreover, metal complexes with organic ligands (such as the metalloporphyrins) are of particular interest in contemporary coal chemistry because they are probably extracted by the organic solvent in the conditions used in certain coal liquefaction processes, and hence may modify the activity of catalysts used in subsequent chemical reactions with the coal extract. We report here the isolation of a metalloporphyrin fraction from a British bituminous coal, and show that the principal metal present in the isolated material is gallium.