Comments on the paper “gas‐phase pyrolysis of 2,2,3,3‐tetramethylbutane using a wall‐less reactor”

Abstract

The article on the above subject by Taylor and Milazzo [ 1) reports an energy of only 42,400 cal/mol versus about 69,000 reported by Tsang [2]. T h e discrepancy is blamed on the use of toluene in Tsang's experiments. Still 42,400 seems a rather low value compared with the heat of breaking the central carbon-carbon bond in tetramethylbutane (the initiation reaction). Using heat of formation and heat capacity data given by Benson [:(I, 1 estimate this to be about 62,000 cal/mol in the temperature range studied. Perhaps there are other possible explanations.We recently discussed one such possibility in connection with the pyrolysis of propane, n-butane, and isobutane. When rates of highly product-inhibited reactions are compared a t variable temperatures, and constant reaction times, inhibition is higher at the higher temperatures (more product), and E,,, comes out low (typically about 46,000 cal/mol for the large number of paraffins we looked at). Study the reaction a t constant decomposition and variable temperatures, and much higher activation energies-comparable to the zero decomposition value -are obtained. In the case we studied the ratio E,,t (const. decomposition)lE,,, (const. time) was 1.68. It was constant enough for us to derive algebraically a useful kinetic expression. The effect appears to be independent of concentration f i r product-inhibited first-order pyrolyses. The concentration of the chain carrier, H or CH:,, is not a function of concentration, and inhibition depends only on the ratio of inhibiting products to substrate, not on the absolute concentration. Thus the finding may apply to Taylor and Milazzo's data, since their comparison was made at essentially constant reaction time.More