Reactions of alkanes on iridium and iridium-gold catalysts

Abstract

The formation of benzene and cyclohexane from n-hexane above 300°C on films of iridium and of iridium—gold is believed to come about by ring enlargement of methylcyclopentane intermediate as well as by the direct 1,6-cyclization route previously proposed. Evidence for the former is two-fold. First, cyclization of both n-pentane and 3-methylpentane (showing feasibility of 1,5-cyclization) and skeletal rearrangement of both isobutane and n-pentane (showing feasibility of ring enlargement), in all cases in the presence of hydrogen, take place catalytically on films of iridium (above 250°C) and of iridium—gold (above 300°C). Secondly, methylcyclopentane has been detected among the products of n-hexane conversion on supported Ir and Ir—Au catalysts in a flow reactor. Study of the exchange reactions between methane and deuterium and between neopentane and deuterium lead to the conclusion that 1,1-diadsorbed alkane entities can form reversibly on iridium at relevant temperatures. This suggests that a carbene-type intermediate is possible for the 1,5-cyclization.