Density functional theory is used to study the mechanism of double-bond isomerization and skeletal isomerization of linear butenes catalyzed by a protonated zeolite, which is simulated by a cluster consisting of two Si and one Al tetrahedra. The study includes complete geometry optimization and characterization of reactants, products, reaction intermediates and transition states, and calculation of the activation energies for the different processes involved. It is shown that the double bond isomerization proceeds by a concerted mechanism which does not involve the formation of either ionic or covalent alkoxy intermediates. According to this concerted mechanism, in one step the acid OH group of the zeolite protonates the double bond of adsorbed but-1-ene and the basic neighboring O atom of the cluster abstracts a hydrogen from the olefin, restoring the zeolite active site and yielding adsorbed but-2-ene. However, the mechanism of skeletal isomerization of linear butenes consists of three elementary steps: protonation of adsorbed but-1-ene to give a secondary alkoxy intermediate, conversion of the secondary alkoxy intermediate into a branched primary one through a cyclic transition state in which the transferring methyl group is halfway between its position in the linear and in the branched species, and decomposition of the primary alkoxy intermediate to give adsorbed isobutene. The activation barriers calculated for the two reactions are in good agreement with experimental data.
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