| 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 C6 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. |
