The chemistry of sediment formation during the storage of diesel fuel

Abstract

This thesis describes the identification of chemical species responsible for sediment formation during storage of an unstable diesel fuel. It is currently not possible to reliably predict the storage stability characteristics of diesel fuels, ie. to distinguish between stable diesel fuels, which remain chemically and physically unchanged during storage, and unstable fuels, which deteriorate under the same conditions to produce insoluble sediment. This study was carried out in order to assist with the development of chemical tests to predict diesel fuel storage stability characteristics. An unstable diesel fuel was stored under ambient conditions for two years. The sediment produced during this period was collected and analysed by TLC, IR, MS, pyrolysis/MS and pyrolysis/GC/MS. Alkyl derivatives of indole were found to be important constituents of the sediment. The diesel fuel itself was analysed using LC, GC, TLC and MS. Alkylindoles and compounds, confirmed by synthesis to be alkyl derivatives of indolylphenalene and bis(indolyl)phenalene, were detected in the fuel. It has been shown that indolylphenalenes and bis(indolyl)phenalenes are converted by acid into material whose analytical characteristics are almost identical to those of a major constituent of sediment. Phenalenone and alkylphenalenones have also been detected in the fuel. These compounds are thought to arise in fuel from autooxidation of the corresponding phenalenes. The observed increase in concentration of indolylphenalenes and bis(indolyl)phenalenes during fuel storage was attributed to reaction of alkylindoles with phenalenone and alkylphenalenones. Addition of phenalene and 2-methylindole to an otherwise stable fuel, followed by storage for 6 months at 25°C, resulted in the formation of phenalenone, 2-methylindolylphenalene, bis(2-methlyindolyl)phenalene and sediment, with analytical characteristics consistent with those of sediment formed during storage of the unstable fuel. It is concluded that sediment formation in the unstable fuel was caused by acid promoted reactions of alkylindoles with phenalenones.