KINETICS AND MECHANISMS OF THE PYROLYSIS OF n-BUTANE: PART I. THE UNINHIBITED DECOMPOSITION

Abstract

The kinetics of the pyrolysis of n-butane have been studied a t temperatures from 520' to 590" C, and a t pressures from 30 to 600 mm Hg; the rate was followed from pressure changes and by gas chron~atography. The reaction was accurately of the three-halves order; the activation energy was found t o be 59.9 kcal mole-', and the frequency factor 3.24X1015 CC''~ mole-u2 sec-l. The reaction is sensitive to surface; packing the vessel and 'conditioning' it usually led to a decrease in rate and an increase in activation energy. The reaction is concluded to be largely homogeneous, and to occur almost entirely by a free-radical mechanism; the initiation reaction is considered to be the dissociation of a butane molecule into two ethyl radicals, in its first-order region, and termination is believed to be the second-order combination of ethyl radicals. The mechanism proposed is shown to account satisfactorily for the observed behavior. The surface effect is attributed to a certain amount of initiation by abstraction, by a surface atom, of a hydrogen atom from butane, and to surface catalysis of the recombination of ethyl radicals. ISTRODUCTIONRecent papers from this laboratory have dealt with the kinetics and mechanisms of the thermal decomposition of ethane (1) and propane (2). The n-butane decomposition presents an interesting contrast in that hydrogen atoms are no longer predominant as chain carriers; the free-radical mechanisms are therefore of a different character. A considerable amount of recent work has been done on the pyrolysis of n-butane, but certain features of the mechanism have still remained obscure; in particular may be mentioned the orderaf the initiating reaction, the nature of the main chain-ending step, and the role of the surface. A reinvestigation of the reaction was therefore considered to be worthwhile, and was carried out in conjunction with the study of the inhibited reaction described in the following paper (Part 11).The main features of the previous work on this reaction may be summarized as follows. Pease (3) and Pease and Durgan (4) established that the reaction is largely homogeneous and approximately of the first order; the overall reaction could be represented mainly by the equationsSteacie and Puddington (5) found that the reaction products could initially be represented by the equationThey reported the first-order rate constant as Echols and Pease (6) found that the order was closer to 3/2 than to unity.The presence of free radicals in the reaction system was demonstrated by Rice, Johnston, and Evering (7), and Rice and Rice (8) proposed a free-radical mechanism which has been modified by Steacie (9), Semenov (lo), and Benson (11). The latter proposed termination steps of the pp type, leading to 3/2-order kinetics.

Kinetics and Mechanisms of the Pyrolysis of Dimethyl Ether: I. The Uninhibited Reaction

“…It should be noted that reachion [4] is almost certainly in its second-order region, but this has no effect on the overall behavior. As discussed earlier, it is not certain whether reaction [3] is first order or second order, and this again makes no difference as far as the overall order is concerned.…”

“…As a result -there has been a revival of interest in the problem of establishing the overall mechanisms for such reactions, and in confirming them quantitatively. Recent work of this kind in these laboratories has been concerned with the thermal decompositions of propylene ( I ) , ethane ( 2 ) , propane (3), and butane (4). In all cases it has been found that the reactions occur predo~ninantly by free-radical mechanisms, the molecular mechanisms being of minor importance.…”

Kinetics and Mechanisms of the Pyrolysis of Dimethyl Ether: Ii. The Reaction Inhibited by Nitric Oxide and Propylene

“…Detailed mechanisms have now been worked out for the decompositions of ethane (4), propane (5), butane (6), and other substances, and a specific scheme has been proposed for the dimethyl ether decomposition (1). The present paper is concerned with an experimental study of the effects of nitric oxide and propylene on the decomposition of dimethyl ether, and with the formulation of reaction mechanisms.…”

Kinetics of the pyrolysis of n-butane

“…Reactions [3] and [5] and reactions [2] and [4] are consecutive. Either one of these reaction pairs can complete a chain cycle converting a molecule of butane to products.…”

“…The rates of reactions [4] and [5] are pressure dependent since n-butane is one of the reactants. The rates of reactions [2] and [3] are pressure independent. A higher pressure, therefore, produces a higher butyl radical concentration relative to that of the methyl or ethyl radicals.…”