Clay catalysis of aromatic hydrogen-exchange reactions

The exchange of aromatic hydrogen in the presence of clay catalysts has been studied by measuring the rates of detritiation of some tritiated naphthalene derivatives on homoionic bentonites. The rate of reaction is influenced by the position of tritium on the ring, the presence of a methoxysubstituent on the ring and the acidity of the clay catalyst. It is inferred that the mechanism of exchange involves an adsorbed species similar to the arenium ion intermediate of electrophilic aromatic substitutions in homogeneous systems. In some cases exchange of hydrogen between acidic clay surfaces and naphthalene derivatives could be detected at temperatures as low as 23°C, and in aqueous slurries at 70°C. This observed reactivity suggests that these reactions could occur in sedimentary environments where organic matter, such as petroleum hydrocarbons, is in contact with clay surfaces. The hydrogen isotopic composition of aromatic hydrogen in petroleum may therefore be related to that of the acidic water adsorbed on clay surfaces with which the petroleum has been in contact.