n-Butane conversion on sulfated zirconia: in situ 13C MAS NMR monitoring of the kinetics of the 13C-label scrambling and isomerization

The kinetics of the conversion of 13C-labeled n-butane adsorbed on sulfated zirconia (SZ) were monitored by in situ 13C MAS NMR spectroscopy. Rate constants of n- to isobutane isomerization and of the 13C-isotope scrambling from the primary to the secondary carbon atoms in n-butane were determined. The monomolecular scrambling of the 13C-label in adsorbed n-butane has an activation energy of 17 ± 3 kcal  mol−1 and occurs faster than the bimolecular process of n-butane isomerization which has an activation energy of 15.1 ± 0.2 kcal  mol−1. The transfer of the selective 13C-label from the primary to the secondary carbon atom in the adsorbed n-butane seems to consist of two reaction steps: (i) a hydride abstraction by SZ leading to the formation of sec-butyl cations and (ii) a label scrambling in the sec-butyl cations. This two-step process with the formation of sec-butyl cations as intermediate increases the apparent activation energy for the 13C-label scrambling, which is almost twice as large compared with the activation energy for carbon scrambling of sec-butyl cations in a superacidic solution.