Theoretical studies of pentene cracking on zeolites: C-C beta-scission processes

The C-C bond-breaking step of pentene adsorbed on a model zeolite cluster i s examined using ab initio and density functional theory (DFT/B3LYP) electr onic structure techniques as an example of the p-scission process that aris es in cracking of alkanes and alkenes. After pentene has been protonated by the acid site, the reactant for the cracking process corresponds to a pent yl cation covalently bound to the oxygen of the zeolite, ZO(-)-C5H11+. The product of the C-C bond-breaking process is propene plus an ethyl cation bo und to a neighboring oxygen. The energy of the transition state relative fr om B3LYP calculations is 60 kcal/mol relative to the pentyl cationic reacta nt. For the case of the branched olefin methyl pentene, the transition stat e energy is slightly lower (55 kcal/mol), but the overall reaction energy i s essentially the same as for pentene. The results are compared to the case of the gas phase pentyl carbenium ion. (C) 1999 Elsevier Science B.V. All rights reserved.