Butene Isomerization and Double-Bond Migration on the H-ZSM-5 Outer Surface: A Density Functional Theory Study

Isomerization of trans-but-2-ene to cis-but-2-ene and double bond migration of trans-but-2-ene to but-1-ene have been investigated by means of density functional theory calculations on a suitable model of H-ZSM-5 surface. The study has been afforded on outer surface sites by considering the hydroxyl group of either a SiO2(OH)2 or a AlO2H(OH)2 moiety. On these outer surface sites, one alkoxide species occurs as a stable intermediate both along the isomerization and double bond migration pathways. The latter process could also occur via a single-step mechanism, which involves a six-center transition state. The energy barriers of the outer surface processes above do not take any advantage by the occurrence of Al↔Si substitutions on the surface, resulting the considered butene transformations negatively affected, indeed. This occurrence can be explained since the reactions on the external surface do not involve acidic hydrogen atoms of the AlO2H(OH)2 moiety.