n-Butane conversion on sulfated zirconia: the mechanism of isomerization and 13C-label scrambling as studied by in situ 13C MAS NMR and ex situ GC-MS

Using 13C MAS NMR, conversion of selectively 13C-labeled n-butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13C label in the n-butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the 13C-label scrambling, whereas isomerization into isobutane proceeds through a pure bimolecular mechanism. Further, the intermolecular mechanism of n-butane isomerization is complicated and turns into conjunct polymerization. Besides isobutane, conjunct polymerization gives also the products of butane disproportionation, propane and pentanes, as well as the stable cyclopentenyl cations; the latter may be in charge of catalyst deactivation.