Density functional theory studies on the skeletal isomerization of 1-butene catalyzed by HZSM-23 and HZSM-48 zeolites

Abstract

The reaction mechanism of the skeletal isomerization of 1-butene to isobutene on 10-membered ring zeolites HZSM-23 and HZSM-48 was investigated using the ONIOM(B3LYP/6-31G(d,p):UFF) method. It is demonstrated that the skeletal isomerization follows a monomolecular process, which involves the formation of two important intermediates: 2-butoxide and butoxide. The active centers on both zeolites are identified to involve two Brønsted acid sites and three exposed vertex O atoms of the aluminum–oxygen tetrahedron on the pore surface. We further find that the pore size exhibits a significant confinement effect that affects the energetics of each intermediate's formation on both zeolites. Considering the free energy pathways at 700 K, the rate-determining steps are found to be the transformation of 2-butoxide to butoxide on HZSM-23 and the formation of 2-butoxide on HZSM-48, respectively. Our work provides mechanistic insights on the elementary processes of skeletal isomerization on zeolites.