A comprehensive mechanistic pathway for n-butane isomerization on sulfated zirconia

This paper proposes a comprehensive mechanistic model for n-butane isomerization on sulfated zirconia (SZ) that accounts for the apparent contradictory results reported in the literature. The use of nonspecific olefins as molecular probes in n-butane isomerization has played a central role leading to this mechanistic proposal. The contradictory results in the literature have led various authors to conclude that the reaction occurs via a bimolecular mechanism, while others have suggested that a monomolecular pathway is dominant. The presence of butene is well known to lead to increased isobutane formation, suggesting a predominantly bimolecular route. However, we recently showed that the addition of other olefins (ethylene, propylene, isobutene, and 1-pentene) also promotes the reaction rate and modifies the induction period. These results indicate that not only C4 olefins, but also any olefin with the ability to form carbenium ion species on the catalyst surface, can promote catalytic activity. Taking into account our latest experimental evidence, a reaction mechanism is proposed involving a bimolecular pathway with the characteristics of a monomolecular pathway (dual-nature mechanism) using “olefin-modified sites” as the main centers of reaction. The major observations made for the isomerization of n-butane (i.e., isotopic scrambling, nonspecific olefin activity promotion, high isobutane selectivity, and catalyst deactivation) are discussed in light of the proposed molecular pathway, and the seeming duality of the mechanism is addressed.