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Studies in the thermal decomposition of the isomeric hexanes Chrysochoos, John

Abstract

An investigation has been made of the pyrolysis of n-hexane, S-methyl-pentane, 3-methyl-pentane, 2, 8-dimethyl-butane and 2,3-dimethyl-butane in a static system at temperatures between 490° and 530°C. The normal pyrolysis and also the pyrolysis inhibited by nitric oxide have been studied. The effect of variation of initial hydrocarbon pressure on the distribution of products and on energies of activation and frequency factors was determined for the uninhibited reactions. The effect of variation in surface-to-volume ratio on rates and on product distribution has also been investigated. A study of factors governing overall reaction rates has provided information on the kinetics of chain termination processes. All pyrolyses at low pressures are significantly inhibited by reactions occurring on the surface of the quartz vessel. Mechanisms have been proposed to account qualitatively for observed decomposition products and the kinetics of pyrolysis of each of the isomers. In the reactions inhibited by nitric oxide, the effect of the pressure of nitric oxide on the rates and product distributions has been examined. The effect of an increase in surface-to-volume ratio on reaction rates and on product distributions has been investigated. The pressure of NO required for maximum inhibition appears to be independent of the partial pressure of the hydrocarbon and also of the nature of the latter. The kinetic and analytical evidence indicates that nitric oxide acts "both homogeneously and heterogeneously as an inhibitor. Significant consumption of nitric oxide was found for all branched isomers. The reduction in inhibition observed as the branching of the isomers increases is attributed in part to increased heterogeneous initiation reactions, from an increase in the extent of adsorption of the more branched isomers relative to the less-branched isomers. A mechanism is proposed for the inhibited pyrolysis of each of the isomers, which accounts qualitatively for the experimental results. A major conclusion drawn from the present study is that nitric oxide acts as an inhibitor through reactions occurring on the surface of the reaction vessel as well as through reactions occurring in the gas phase.

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