Metalloporphyrins in coal. 1. Gallium porphyrins in bituminous coals

Abstract

The isolation and characterisation of a homologous series of gallium polyalkylporphyrins from a large scale extraction of a bituminous coal (Daw Mill) from the Carboniferous is described. About 0.2 µg/g of metalloporphyrin fraction is obtained. The homologous series extends from C₂₆ to C₃₂. It is proposed that the homologues arise by fragmentation (thermal cracking, protodesubstitution on a mineral surface) of etioporphyrin III or its metal complexes during the coalification process.

Evidence in support of this is provided by a survey of bituminous coals of increasing rank, where the predominating carbon number decreases in the range C₃₀ to C₂₆ as rank increases. The amount of metal-loporphyrins extracted also decreases with increasing rank : only traces are detected in anthracites.