Thermal Reaction of Propylene Mechanism Taiseki Kunugi, Kazuhiko Soma, and Tomoya Sakai Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Tokyo, Japan
A mechanism for the thermal reaction of propylene is based on kinetic evaluation of the relevant elementary reactions a t 1 1 00°K. and atmospheric pressure. At low conversion of propylene around a few mole per cent, initiation is: C4Hs-1 -+ CH3. C3H5'; propagation: CH3. C3H6=sec-C4Hg.,sec-C?Hg' C4Hs-1 H sec-CdHg. -+ trans- or cis-C1Hs-2 H *, sec-CdHg. CH.L=CH-CH-CH~ Hz, CH:=CH-CH-CH3-+ C4H6-1,3 H e , CH3. C3H6 e isoC4Hg., isoCdH9. isoC4Hs H . , CH3. C3H6 --+ CH4 C3H5*, H . -In-C3H7., n-C3H7. C2H4 4- CH3., n-C3H7. HZ C3H5., H . C3H6 Hz C Q H ~ C3Hs. ., C3H6 -+ methylcyclopentene H . ; termination: C3H5' C4H7. + C3H6 c4H13-1~3.The initiator, 1 -butene,
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was one of the main products. This mechanism suggested that the amount of ethylene formed is equal to the sum of those of methane, butenes, and butadiene, that the selectivities of formation of the main products do not depend on the conversion of propylene a t low conversion region, and that the selectivities of the main products are independent of temperatures from 750" to 850°C. This was consistent with the experimental results. The rate of propylene disappearance was increased considerably in the presence of about 3 mole % of 1 -butene.
MECHANISMS
of the thermal reaction of propylene have been proposed by many authors (Amano and Uchiyama, 1963; Ingold and Stubbs, 1951; Kallend et al., 1967; Laidler and Wojciechowski, 1960; Szwarc, 1949). However, few mechanisms seem to account for the observed fact a t 700" to 85OOC. and atmospheric pressure. The relatively long induction period suggests that the initiation reaction must not be a reaction of propylene, as proposed by many authors (Amano and Uchiyama, 1963; Benson, 1964; Laidler and Wojciechowski, 1960; Marshal et al., 1966; Szwarc, 1949), but a reaction of one of the products. Possible initiation reactions of propylene and its products are t o be examined in more detail. A reaction mechanism was deduced from the kinetic evaluation of the relevant elementary reactions and compared with experimental results (Kunugi et al., 1970). Elementary Reactions
The thermal reaction of propylene seems to proceed substantially by a free radical mechanism. Szwarc (1949) observed the increase in the rate of propylene disappearance on the addition of 1.0 and 4.0 mole yoof diallyl. The homogeneity of the reaction was demonstrated by packing the reaction vessel with silica wool, which increased the surface by a factor of 6. I n our study, possible elementary reactions are divided into three groups-initiation, propagation, and termination. The elementary reactions are evaluated as in previous work (Kunugi et al., 1969), based upon kinetic parameters, to select the important ones which comprise the reaction scheme. The standard reaction condition for the evaluation is presumed to be 1100OK. and 1 atm. absolute. Kinetic parameters are taken from references or estimated from thermodynamic and kinetic points of view. Initiation Reactions. Possible initiation reactions are listed in Table I : elementary reactions, their -4 factor
