Vera P. Shmachkova

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

Abstract Using 13 C MAS NMR, conversion of selectively 13 C-labeled n -butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13 C label in the n -butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13 C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the 13 C-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.

Acidic Properties of Sulfated Zirconia

IR spectroscopy of adsorbed probe molecules (CO, pyridine) is used to characterize the acidic properties of sulfated zirconia derived from zirconium oxide and hydroxide. Their acidic properties are found to be similar. The strength of the Lewis and Brönsted site measured by the frequency shift of adsorbed CO is lower than that in zeolites. It is concluded that sulfated zirconia have no superacid Brönsted and Lewis sites. Brönsted sites capable of protonating pyridine vanish when calcining the catalysts at temperature above 773 K, but the strength and concentration of the Lewis acid sites (LAS) do not change.