Skeletal Isomerization of 1-Butene over Ferrierite Zeolite: A Quantum Chemical Analysis of Structures and Reaction Mechanisms

Abstract

The monomolecular skeletal isomerization of 1-butene to isobutene on H-FER zeolite was theoretically studied by using the ONIOM(MP2/6-311+G(2df,2p):M08-HX/6-311+G(2df,2p))//ONIOM(MP2/6-31G(d,p):M08-HX/6-31G(d,p)) approach. A full quantum cluster of 37T (T means tetrahedral of Si or Al atoms) was used to represent the confinement effect from the FER zeolite. The model correctly predicted adsorption energies in favor of 1-butene to isobutene, −18.0 and −14.8 kcal/mol, respectively, indicating the unfavorable steric hindrance of the branched isomer with the medium pore H-FER zeolite. The monomolecular mechanism of skeletal isomerization of 1-butene was found to involve transformation of adsorbed 1-butene through secondary linear butoxide, primary isobutoxide, and tert-butyl cation intermediates. The rate-determining step is the conversion of isobutoxide to isobutene in which the reaction has to proceed through the primary isobutyl cation transition state. The primary isobutyl cation is stabilized by the interactions with the zeolite framework. Its energy relative to the isolated 1-butene and zeolite cluster is 16.9 kcal/mol, which is comparable to the experimental apparent activation energy of 14 kcal/mol. The shape selectivity due to the “nano-confinement” effect of the zeolite framework on the adsorption, the stabilities of intermediates, as well as the skeletal isomerization reaction mechanism of 1-butene were clearly demonstrated.