The mechanism of catalytic cracking of n‐alkenes on ZSM‐5 zeolite

Abstract The catalytic cracking of n ‐alkenes on ZSM‐5 zeolite at 405°C can occur both by a monomolecular mechanism and a bimolecular process. In the latter, cracking is preceeded by dimerization. We show that pentenes are cracked exclusively by the bimolecular process. The dominant cracking mechanism for n ‐hexenes also requires initial dimerization, although a small proportion (<19%) of the total cracking may proceed by a monomolecular process. Cracking of n ‐heptenes is predominantly monomolecular, with only 13% of the total occurring via an initial dimer formation. The cracking of n ‐octenes and n ‐nonenes can be interpreted by assuming a monomolecular mechanism only. Thus it appears that at 405°C olefins smaller than C 6 are stable with respect to direct cracking and must dimerize before a species is formed which is unstable enough to crack. No molecular hydrogen was produced in any of the cracking reactions reported here in the range of conversions studied.