Methyl pentenes, pyrolysis

The purpose of this paper is to describe, briefly, several interrelated variables that affect the course of methyl pentene pyrolysis Initially, mechanistic aspects are described for the decomposition of methyl substituted pentenes to isoprene. Next, the high temperature degradation of isoprene and related by-product dienes will be discussed. Finally, the heterogeneous effects associated with reactor metallurgy are detailed. 

Kinetic studies have shown that methyl pentene pyrolysis occurs via a free radical process having chain lengths in the range of 10 (1 ) As a consequence, product distribution depends on the types of propagating radicals in the system as well as the parent methyl pentene structiare. 

Since isoprene was the principal desired product from methyl pentene pyrolysis it was essential to determine its stability under reaction conditions. Isoprene degradation proceeds via second-order rate processes. Isoprene dimer and related compounds formed during the pyrolysis are dependent on the isoprene partial pressure in the reaction system. 

Figure 6.1.27. Result fora Py-GC/MS analysis of a high M sample of poly(4-methyl-1-pentene). Pyrolysis done on 0.4 mg material at 600° C in He, with the separation on a Carbowax type column.

Derivation (1) From cracked products of heavy petroleum oils (2) dehydrogenation of isopentene (3) pyrolysis of methyl pentene or of isobutylenefor-maldehyde condensation products (4) dehydration of methyl butenol. 

The pyrolysis of propylene dimers to isoprene was investigated as part of the cortimercialization of the Goodyear Isoprene Process (ls2 3)> A significant amount of experimentation has been done to characterize some of the many reaction variables that affect the formation of isoprene during pyrolysis of various methyl pentenes. 

Inspection of 2-methyl-l-pentene pyrolysis products indicates that fragmentations res ilting from hydrogen atom addition are increased over those... 

Propylene Dimer. The synthesis of isoprene from propjiene (109,110) is a three-step process. The propjiene is dimeri2ed to 2-methyl-1-pentene, which is then isomeri2ed to 2-methyl-2-pentene in the vapor phase over siUca alumina catalyst. The last step is the pyrolysis of 2-methyi-2-pentene in a cracking furnace in the presence of (NH 2 (111,112). Isoprene is recovered from the resulting mixture by conventional distillation. 

Isoprene (melting point -146°C, boiling point 34°C, density 0.6810) may be produced by the dehydrogenation of iso-pentane in the same plant used for the production of butadiene. However, the presence of 1,3-pentadiene (for which there is very little market) requires a purification step. One method produces isoprene from propylene. Thus, dimerization of propylene to 2-methyl-1-pentene is followed by isomerization of the 2-methyl-1-pentene to 2-methyl-2-pentene, which upon pyrolysis gives isoprene and methane. 

The results of pyrolysis of polypropylene in air depends on the pyrolysis heating rate because the pyrolysis process competes with the oxidation [108], By heating between 120° C and 280° C in air, polypropylene is reported to generate ethene, ethane, propene, propane, isobutene, butane, isobutane, pentadiene, 2-methyl-1-pentene, 2,4-dimethyl-1-pentene, 5-methyl-1-heptene, dimethylbenzene, methanol, ethanol, 2-methyl-2-propene-1-ol, 2-methylfuran, 2,5-dimethylfuran, formaldehyde, acetaldehyde, acrolein, propanal, methacrolein, 2-methylpropanal, butanal, 2-vinylcrotonaldehyde, 3-methylpentanal, 3-methylhexanal, octanal, nonanal, decanal, ethenone, acetone, 3-buten-2-one, 2-butanone, 1-hydroxy-2-propanone, 1-cyclopropylethanone, 3-methyl-2-buten-2-one, 3-penten-2-one, 2-pentanone, 2,3-butanedione [109]. 

Kinetics of Product Formation in the H2S-Promoted Pyrolysis of 2-Methyl-2-pentene... 

Thermal Decomposition of 2-Methyl-2-Pentene(2MP2) The following mechanism can be written for the formation of isoprene from the pyrolysis of 2-methyl-2-pen-tene. 

Pyrolysis of cis and trans-lj.-methyl-2-pentene does not yield isoprene. The i. -methyl-2-pentenyl radical decomposes primarily to cis- and trans-l,3-pentadiene ... 

Heterogeneous surface effects on the pyrolysis of 2-methyl-2-pentene have been defined for a number of systems. The origin of these effects appears to be related to the ability of the reactor surface to catalyze olefin (or diolefin) decomposition to form a carbon-iron matrix capable of catalyzing further hydrocarbon decomposition. 

Hutchings, David A., Freeh, Kenneth J., Hoppstock, Frederic H., The Kinetics of Product Formation in the H23-Promoted Pyrolysis of 2-Methyl-2-Pentene , presented April 6, 1975, Philadelphia Meeting,... 

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