ACID-CATALYZED CRACKING OF PARAFFINIC HYDROCARBONS .3. EVIDENCE FOR THE PROTONATED CYCLOPROPANE MECHANISM FROM HYDROCRACKING HYDROISOMERIZATION EXPERIMENTS

The mechanism for acid-catalyzed cracking proposed in the first paper of this series, which assumes a protonated dialkylcyclopropane carbonium ion as intermediate instead of a classical carbenium ion, has been applied to rationalize experimental results obtained in the hydrocracking/hydroisomerization of normal paraffins. The new mechanism is shown to be capable of explaining many characteristics of the hydrocracking process which cannot be understood via the classical theory, such as the virtual absence of methane and ethane as cracked products, the relatively low production of propane as compared to the higher hydrocarbons, and characteristic patterns in the branching of the cracked fragments. The new mechanism also makes it clear why crackability increases so rapidly with increasing number of carbon atoms above n-heptane and why n-hexane and lower hydrocarbons are so difficult to crack. An explanation is also offered for the ease with which high selectivities can be achieved in the isomerization of C5/C6, as contrasted with the difficulty of isomerizing C-7+ without significant cracking.