Theoretical study of two pathways of double-bond isomerization of pentene catalyzed by zeolites

The double-bond isomerization of 1-pentene over zeolites has been investigated by using density functional theory at the B3LYP/6-31G(d,p) level. The calculated results indicate that the double-bond isomerization may proceed via either a stepwise or a concerted reaction pathway. The stepwise reaction consists of two elementary steps: firstly, the formation of an alkoxy intermediate by addition of a proton from zeolites, and then the decomposition of this alkoxy intermediate to yield an adsorbed 2-pentene. The calculated activation barriers are 17.9 and 31.5 kcal/mol for the first and second step, respectively. The concerted reaction occurs via one-shift proton transfer, which avoids the formation of highly stable alkoxide species. The resulting activation energy, 20.5 kcal/mol, is close to that of the first step of the stepwise reaction. From an energetic point of view, it is expected that at low temperatures, the concerted reaction mechanism dominates the overall isomerization reaction due to the lower activation energy, while at higher temperatures, the two reaction pathways compete against each other because the alkoxy formation will occur relatively easily. The results can well explain the experimental phenomena observed.