Direct evidence of intramolecular rearrangement in skeletal isomerization of n-butane over bifunctional catalysts

Abstract

Mechanisms of skeletal isomerization of n-butane over bifunctional catalysts, Pt–Cs2.5H0.5PW12O40 and Pt-sulfated ZrO2, as well as the corresponding solid acids were studied using 1,4-13C2-n-butane. The isotopic distributions of the reactant and product were analyzed with field ionization mass spectrometry, by which the parent peak patterns were obtained. It was found that 1,4-13C2-n-butane was selectively isomerized to 13C2-isobutane over these catalysts in the presence of H2 at 423–523 K, while the corresponding solid acids gave isobutane with binomial distributions of 13C. These results clearly demonstrate that the skeletal isomerization of n-butane proceeded mainly via a monomolecular path with intramolecular rearrangement on both the bifunctional catalysts, while it occurred through a bimolecular path with intermolecular rearrangement on the solid acids. This difference in reaction mechanism is reflected on that in the selectivity to isobutane.