Rice-Kossiakoff mechanism

The product yields by carbon number are plotted in Figure 6. At all except the most severe conditions studied,the major component in the gas phase was n-butane. This observation is consistent with the a-ring opening and dealkylation mechanism proposed for tetra-and octa-hydrophenanthrene cracking. At the most severe conditions ethane was present in the greatest quantities. This can be explained by side chain cracking of n-butylbenzene according to the Rice-Kossiakoff mechanism or by secondary reactions of the n-butane. 

The liquid products of the pyrolysis of PP contain primarily olefins that resemble the molecular skeleton of PP (i.e. branched hydrocarbons). A distinguishing feature of PP pyrolysis is the predominant formation of a particular C9 olefin in the pyrolysis product. The level of this C9 compound identified as 2,4-dimethylhept-l-ene can be as high as 25%. Also present are C5 olefin, Cs olefin, several C15 olefins and some C21 olefins [2]. The tertiary carbon sites in PP allows for the facile chain cleavage and rearrangements according to the Rice-Kossiakoff cracking mechanism shown in Figure 15.2. The noncondensable gas from PP pyrolysis contains elevated levels of propylene, isobutylene and n-pentane. 

Figure 15.2 Rice-Kossiakoff cracking mechanism for polypropylene showing that the pyrolysis products of PP retain a branched structure... 

Mechanism of Decalin Hydropyrolysis. On the basis of some principles of the Rice-Kossiakoff free-radical theory (7-JO), the compositional changes summarized in Table VI and Figure 10 can be rationalized in terms of the following suggested mechanism for hydropyrolysis of 2 (Scheme 2) ... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

---