Competitive mechanisms ofn-butane isomerization on sulfated zirconia catalysts

Isotopic analysis ofn-butane isomerization over sulfated zirconium oxide, using double¹³C-labelledn-butane, shows that at low temperature this isomerization is an inter-molecular process. Probably, a C₈ surface intermediate is formed which isomerizes and undergoes -fission; the iso-C₄ fragments are desorbed asi-butane. Previously, the same mechanism was indicated for Fe, Mn promoted catalysts. The isomerization rate at 130°C is drastically lowered by gaseous H₂, because the concentration of the unsaturated species, required for the formation of the C₈ intermediate, is low under such conditions. Whereas₁₃C-scrambledi-butane is a true primary product, isotopic scrambling ofn-butane continues after chemical equilibrium betweenn-butane andi-butane has almost been reached; i.e.¹³C scrambledn-butane is a secondary product. Intra-molecular rearrangement of carbon atoms inn-butane precedes intermolecular scrambling. The similarity of the isomerization mechanism over unpromoted sulfated zirconia and Fe, Mn promoted sulfated zirconia is paralleled by an equal strength of the acid sites in both catalysts. The shift in the FTIR band of CO adsorbed on the Lewis sites indicates that these sites, presumably surface Zr⁴⁺ ions, are weaker acids than A1³⁺ in dehydrated alumina.