Methylation of indole during catalytic tests for hydrodenitrogenation (HDN) reactions

Hydrodenitrogenation (HDN) is a catalytic process aimed at reducing the nitrogen content of petroleum crudes. We studied the reactivity of organic nitrogen compounds, aniline and indole, representative of those present in the crudes. The catalytic tests were carried out in the presence of H2S in the reacting gas phase. It was generated in situ by adding sulphur compounds which decomposed easily to H2S and hydrocarbon. The use of CS2 as H2S precursor led to the occurrence of side reactions: two methylindole isomers were identified in the indole reaction products. These side reactions were not observed with other H2S precursors like dimethyldisulphide or diethyldisulphide. We established that the formation of the methylindole isomers results from a reaction between (undecomposed) CS2 and the five-membered hetero-ring of indole or indoline and we proposed a two-step reaction mechanism.