Reaction Mechanism of Pentene Skeletal Isomerization on Zeolites

The microcosmic interaction mechanism of pentene skeletal isomerization on zeolites was studied by the density functional theory at the B3LYP/6-31G (d, p) level. The reaction trajectories were determined by the intrinsic reaction coordinate (IRC) methods. The results indicate that the skeletal isomerization of pentene can proceed by two kinds of mechanisms: the alkoxide intermediate mechanism and methylcyclopropane-like intermediate mechanism. The alkoxide intermediate mechanism involves two reaction pathways: methyl shift and ethyl shift. Accordingly, the overall skeletal isomerization of pentene has three reaction pathways. Both the methyl and ethyl shift mechanisms consist of three elementary steps. The rate determining steps are the shift of the methyl group and the shift of the ethyl group, respectively. The corresponding activation barriers are nearly equivalent (206.17 and 207.31 kJ.mol(-1), respectively), indicating that two reaction pathways compete between each other. The methylcyclopropane-like intermediate mechanism includes two elementary steps: the torsion of the carbon chain and the methyl shift. This intermediate has highly ionic character and is a high energy species. The rate determining step is the torsion of the carbon chain, and its activation barrier is 147.93 kJ.mol(-1). This value obviously is lower than those of the methyl and ethyl shift process, implying that the methylcyclopropane-like intermediate pathway occurs more easily.