Mechanism of butane skeletal isomerization on sulfated zirconia

A kinetic model of the skeletal isomerization of n-butane and isobutane on sulfated zirconia in the absence of Pt is presented. The skeletal isomerization of butane on sulfated zirconia has been shown to be initiated by oxidative dehydrogenation of the alkane. This is followed by the formation of alkoxy groups/carbenium ions at the surface, induced by strong Brønsted acid sites. The isomerization of the sec-butyl carbenium ion occurs mono-molecularly, as suggested by the 100% selectivity for isomerization extrapolated to zero conversion. With increasing conversion, the selectivity decreased linearly, leading to propane and pentanes up to 40% of conversion. The lower selectivity for isomerization is qualitatively explained by reactions of isobutene present in small concentrations in the reactor at higher conversions. Transient experiments show conclusively that the isomerization of the carbenium ion and not the hydride transfer from the alkane to the carbenium ion is the rate-determining step.