CRACKING SELECTIVITY PATTERNS IN THE PRESENCE OF CHAIN MECHANISMS - THE CRACKING OF 2-METHYLPENTANE

Catalytic cracking of 2-methylpentane on HY has been studied at 400, 450, and 500°C and a chain reaction mechanism is used to interpret the selectivity of the reaction. The results of a quantitative description of this mechanism show that protolysis of bonds in the feed molecule at 400°C follows the order (Ct-Cs) > (Cs-Cs) > (Ct-Cp or Cs-Cp) > (Ct-H) in the ratio 0.510/0.416/0.054/0.020, but at 500°C the order becomes (Ct-Cs) > (Cs-Cs) > (Ct-H) > (Ct-Cp or Cs-Cp) in the ratio 0.625/0.2l2/0.138/0.025, where p, s, and t refer to primary, secondary, and tertiary carbons. The average reaction chain length is found to be 3.34 at 400°C with isomerization as the dominant reaction in the chain process. The chain length decreases with increasing reaction temperature. This leads to a pronounced decrease in the formation of isomeric products and an increase in the production of C1-C5 paraffins and C2-C5 olefins. β-Cracking of a parent carbenium ion is very small at 400°C, but becomes noticeable at 500°C. The isomerization process consisting of a skeletal rearrangement of the C6H13S+ ion and a subsequent abstraction of hydride from a feed molecule in the gas phase dominates conversion at lower temperatures. At higher temperatures. however, the appearance of β-cracking of the C+6 ion and the enhancement of the desorption of parent carbenium ions inhibit isomerization. Molecular hydrogen is initially formed from both the protolysis of a CH bond in the feed molecule and from the formation of coke. The initial molar probability of coke formation is only 0.010 at 400°C, while the H°C ratio in the initial coke is 1.96, substantially below the 2.33 ratio present in the feed. Rising temperature results in an increase in the production of hydrogen and the formation of a more hydrogenated coke. © 1993 Academic Press, Inc.