The Influence of Reaction Temperature on the Cracking Mechanism of 2-Methylhexane

The cracking of 2-methylhexane on USHY has been studied in the temperature range 400-500°C. It was found that this reaction leads to the formation of hydrogen, paraffins, olefins, and aromatics ranging from C1 to C10. Of these only hydrogen, C1-C7 compounds, and coke were found to be primary products. Mechanistic considerations indicate that two main processes take place during 2-methylhexane conversion on USHY: (1) initiation by protolysis on pristine Brønsted sites; (2) chain processes involving isomerization, hydrogen transfer, and disproportion. At low temperatures, conversion of 2-methylhexane proceeds to a significant extent via both mechanisms, while at higher temperatures protolytic cracking is the dominant process by far. We find that protolysis accounts for 67, 83, and 94% of total conversion of 2-methylhexane at 400, 450, and 500°C, respectively. The average activation energy for protolytic cracking of 2-methylhexane on USHY was found to be 159 kJ/mol. The unexpectedly low activation energy for protolysis vis-à-vis the comparable value in 2-methylpentane cracking (246 kJ/mol) is discussed in terms Of temperature effects on active site densities and in terms of the compensation effect in protolysis. Hydride abstraction from gas phase 2-methylhexane by C6H+13 and C7H+15 ions leads to the formation of the paraffinic C6 and C7 skeletal isomers found in the primary products. In addition to hydride transfer, the set of active bimolecular chain reactions involves some but not all possible disproportionations between feed molecules and carbenium ions in the range C2H+5 and C5H+11. The reasons for this specificity in disproportionation are discussed. The probability of initial coke formation was found to decrease with increasing temperature, suggesting a diminished rate of bimolecular reaction between adjacent carbenium ions at higher temperatures. We explain this as being the result of lower surface coverage by carbenium ions at elevated temperatures.