A molecular orbital approach to a comprehensive cracking mechanism for linear long chain alkanes in heterogeneous acid catalytic conditions through carbenium ion β-cleavage

Abstract

An extensive MINDO/3 theoretical study of the elementary steps of n-heptane cracking has been performed. A mechanism involving a carbenium ion is proposed and then a number of possibilities for further evolution of the different carbenium ions are considered. The energy surfaces for hydride transfer, β-cleavage processes either direct or concerted with 1,2-hydrogen shifts or concerted with a cyclic rearrangement, are explored. The energy barriers so obtained range over ca. 50 kcal mol^–1 and are insensitive to product size. Consequently, the final product distribution cannot be explained in terms of the β-scission of linear n-heptane as the rate-limiting step. The β-scission energy barrier for some branched seven-carbon carbenium ions have been calculated to be ca. 30 kcal mol^–1, an intermediate value between linear β-cracking and branching rearrangement transition energies. A mechanism is proposed that implies a fast achievement of equilibrium between different branched carbocations after carbenium ion generation by hydride transfer and then irreversible β-cracking.