The pyrolysis of methane, ethane and n -butane on a platinum filament

Abstract

A mass spectrometer of high sensitivity and low resolving power was constructed and used to examine the nature of the reactions of methane, ethane and n-butane on an incandescent platinum filament at pressures of the order of 10 “5 mm. Free radicals evolved from the filament could be directly detected by ionizing them with low-energy electrons. The primary dissociation of methane on platinum at about 1000° C gave methyl radicals, but no methylene radicals could be detected. About 1 in 300 of the collisions of the methane on the platinum led to reaction. Further reactions of radical recombination and hydrogen transfer gave ethane, propane and butane, and part of the ethane was dehydrogenated on the platinum to ethylene. The extent of this dehydrogenation was the same as that observed with pure ethane. No free methyl or ethyl radicals could be detected in the dehydrogenation of ethane to ethylene at 950° C. Higher hydrocarbons and methane were not formed. The presence of methyl radicals from methane in the ethane did not bring about any further dehydrogenation of ethane. This reaction was concluded to be a molecular dehydrogenation. Ethyl radicals were detected from the pyrolysis of n-butane at 1050° C. The main products were ethane and ethylene and traces of unidentified higher hydrocarbons. No C 3 hydrocarbons and only a trace of methane were observed. Thus the butane underwent a selective fission at the central carbon-carbon bond. Consideration of bond dissociation energies in butane shows that this selective fission very probably arises from the influence of the platinum catalyst and not from a property of the butane molecule itself.